EPA/600/R-10/081F August 2012 www.epa.gov/research
United States
Environmental Protection
Agency
Nanomaterial Case Study:
Nanoscale Silver in
Disinfectant Spray
c
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Office of Research and Development
National Center for Environmental Assessment
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A rrM Sonm7ntel Protection EPA/600/R-10/081F
United States
I Protection tra/ou
August 2012
Nanomaterial Case Study:
Nanoscale Silver in Disinfectant Spray
August 2012
National Center for Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC
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Disclaimer
This document has been reviewed in accordance with U.S. Environmental Protection Agency
policy and approved for publication. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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Table of Contents
Disclaimer
List of Tables vii
List of Figures vii
Abbreviations viii
Authors, Contributors, and Reviewers xi
Preamble xiv
Chapter 1. Introduction to this Document 1-1
1.1. Background 1-1
1.2. Purpose of this Document 1 -6
1.3. How to Read this Document 1-7
1.4. Terminology 1-8
Chapter 2. Introduction to Silver and Nanoscale Silver 2-1
2.1. Conventional Silver: Uses, Occurrence in the Environment, and U.S. Standards 2-1
2.1.1. Uses of Silver and Silver Compounds 2-1
2.1.2. Occurrence of Silver in the Environment 2-2
2.1.3. U.S. Standards for Environmental Silver 2-4
2.2. Historical and Emerging Uses of Nanoscale Silver 2-6
2.3. Physicochemical Properties of Nanoscale Silver 2-10
2.3.1. Size 2-11
2.3.2. Morphology 2-12
2.3.3. Surface Area 2-13
2.3.4. Chemical Composition 2-13
2.3.5. Solubility 2-14
2.3.6. Surface Chemistry, Reactivity, and Coatings 2-16
2.3.7. Conductive, Magnetic, and Optical Properties 2-17
2.4. Analytical Methods to Characterize Nanoscale Silver 2-18
2.4.1. Methods for Laboratory Research 2-18
2.4.2. Methods to Assess Environmental Occurrence 2-20
2.4.3. Methods to Assess Workplace Occurrence 2-21
2.4.4. Methods for Quantifying Dose and Dose Metrics 2-23
2.5. Summary of Physicochemical Properties and Analytical Methods 2-25
Chapter 3. Life-Cycle Stages 3-1
3.1. Feedstocks 3-1
3.2. Manufacturing 3-2
3.2.1. Synthesis of Silver Nanoparticles 3-3
3.2.2. Manufacturing of Nano-Ag for Disinfectant Sprays 3-7
3.3. Distribution and Storage of Nano-Ag Disinfectant Sprays 3-9
3.4. Use of Nano-Ag Disinfectant Sprays 3-9
3.5. Disposal of Nano-Ag Disinfectant Sprays 3-11
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3.6. Summary of Life-Cycle Staaes
Chapter 4. Transport, Transformation, and Fate Processes in Environmental Media
4.1 . Factors Influencina Transport, Transformation, and Fate Processes of Nano-Aa
4.1.1. Persistence
4.1.2. Particle Clusterina, Deposition, and Sedimentation
4.1.3. Adsorption
4.1.4. Transport/Mobility Potential
4.2. Air
4.2.1. Diffusion
4.2.2. Particle Clusterina
4.2.3. Residence Time
4.2.4. Deposition and Resuspension
4.2.5. Additional Factors
4.3. Terrestrial Systems
4.3.1. Soil
4.3.2. Plants
4.4. Aquatic Systems
4.4.1 . Natural Aquatic Systems
4.4.1.1. Surface Properties
4.4.1 .2. Ionic Aa and Aa Complexes in Water
4.4.1.3. Particle Clusterina
4.4.1.4. Important Environmental Factors
4.4.2. Wastewaters
4.5. Transport, Transformation, and Fate Models
4.6. Summary of Nano-Aa Transport, Transformation, and Fate in Environmental Media
Chapter 5. Exposure, Uptake, and Dose
5.1. Biotic Exposure
5.2. Biotic Uptake and Dose
5.2.1. Bioavailability, Bioconcentration, and Bioaccumulation
5.2.1 .1 . Attributes of an Oraanism that Influence Bioavailability of Nano-Aa and Silver Ions
5.2.1.2. Attributes of the Environment Influencina Bioavailability of Silver Ions and Nano-Aa
5.2.1.3. Bioaccumulation Models
5.2.2. Uptake by Bacteria and Funai
5.2.3. Uptake in Aquatic Ecosystems
5.2.3.1. Uptake by Alaae
5.2.3.2. Uptake by Protozoa
5.2.3.3. Uptake by Bivalve Mollusks
5.2.3.4. Uptake by Aquatic Crustacea
5.2.3.5. Uptake by Vertebrate Eaas
5.2.3.6. Uptake by Freshwater Fish
5.2.3.7. Uptake by Saltwater Fish
5.2.3.8. Bioaccumulation in Aquatic Food Webs
5.2.4. Terrestrial Ecosystems
5.2.4.1 . Uptake by Terrestrial Plants
5.2.4.2. Uptake by Soil Macrofauna
5.2.4.3. Transfer throuah Terrestrial Food Webs
5.3. Human Exposure
5.3.1. General Population Exposure
5.3.1 .1 . Respiratory Exposure
5.3.1.2. Dermal Exposure
5.3.1.3. Oral Exposure
5.3.2. Occupational Exposure
5.4. Aaareaate Exposure to Nano-Aa from Multiple Sources and Pathways
5.4.1 . Human Aaareaate Exposure
3-12
4-1
4-2
4-3
4-4
4-6
4-7
4-7
4-8
4-8
4-9
4-9
4-10
4-10
4-11
4-12
4-13
4-13
4-14
4-14
4-16
4-16
4-18
4-19
4-23
5-1
5-4
5-6
5-7
5-8
5-9
5-13
5-14
5-17
5-19
5-20
5-20
5-21
5-23
5-25
5-27
5-28
5-30
5-30
5-32
5-33
5-35
5-36
5-38
5-39
5-39
5-40
5-44
5-45
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5.4.2. Biotic Aggregate Exposure 5-47
5.5. Cumulative Exposure to Nano-Aa and Other Contaminants
5.5.1 . Nano-Aa By-Products and Transformation Products
5.5.2. Examples of Nano-Aa Facilitatina Absorption of Other Contaminants
5.5.3. Examples of Nanoparticles Facilitatina Absorption of Other Contaminants
5.6. Models to Estimate Exposure
5.7. Human Uptake and Dose
5.7.1. Pharmacokinetics
5.7.1.1. Absorption
5.7.1.2. Distribution
5.7.1.3. Metabolism
5.7.1.4. Excretion
5.7.2. Uptake and Dose by Route
5.7.2.1. Respiratory (Inhalation and Instillation)
5.7.2.2. Dermal
5.7.2.3. Inaestion
5.7.3. Models to Estimate Dose
5.8. Summary of Exposure, Uptake, and Dose
Chapter 6. Characterization of Effects
6.1 . Factors that Influence Ecoloaical and Human Health Effects of Nano-Aa
6.1.1. Physicochemical Properties
6.1. 1.1. Size
6.1.1.2. Morpholoay
6.1.1.3. Surface Chemistry and Reactivity
6.1.2. Test Conditions
6.1.3. Environmental Conditions
6.2. Ecoloaical Effects
6.2.1. Microoraanisms (Excludina Alaae)
6.2.2. Aquatic Oraanisms
6.2.2.1. Alaae
6.2.2.2. Aquatic Invertebrates
6.2.2.3. Aquatic Vertebrates
6.2.2.4. Model to Estimate Toxicity to Aquatic Biota
6.2.3. Terrestrial Oraanisms
6.2.3.1. Terrestrial Plants
6.2.3.2. Terrestrial Invertebrates
6.2.3.3. Terrestrial Vertebrates
6.3. Human Health Effects
6.3.1. In Vitro Studies
6.3.1.1. Reproduction and Development
6.3.1.2. Oxidative Stress
6.3.1 .3. DNA Damaae and Mutation
6.3.1.4. Pro-inflammatory Response
6.3.2. In Vivo Studies
6.3.2.1. Central Nervous System Effects
6.3.2.2. Respiratory System Effects
6.3.2.3. Liver, Kidney, and Urinary System Effects
6.3.2.4. Cardiovascular System Effects
6.3.2.5. Hematoloay
6.3.2.6. DNA Damaae
6.3.2.7. Skin
6.3.2.8. Reproductive/Developmental Effects
6.3.3. Human and Epidemioloaical Studies
6.3.3.1. Medical Use Studies
6.3.3.2. Occupational Studies
5-47
5-48
5-49
5-49
5-50
5-51
5-51
5-52
5-53
5-56
5-56
5-57
5-57
5-60
5-61
5-62
5-62
6-1
6-3
6-4
6-5
6-7
6-8
6-11
6-12
6-15
6-17
6-26
6-27
6-30
6-34
6-42
6-44
6-44
6-46
6-50
6-52
6-52
6-54
6-55
6-56
6-58
6-60
6-61
6-62
6-63
6-64
6-64
6-65
6-65
6-66
6-66
6-67
6-69
6.4. Summary of Ecological and Human Health Effects 6-71
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Chapter 7. Summary 7-1
7.1. Case Study Highlights 7-3
7.1.1. Terminology 7-3
7.1.2. Conventional Silver 7-4
7.1.2.1. Historic and Current Uses of Silver and Silver Compounds 7-4
7.1.2.2. Historical Environmental Silver Levels 7-4
7.1.3. Nanoscale Silver 7-5
7.2. Nanoscale Silver Case Study Summary 7-6
7.2.1. Physical-chemical Properties of Nanoscale Silver 7-6
7.2.1.1. Analytical Methods 7-6
7.2.1.2. Analytical Methods for Laboratory or Occupational Settings 7-7
7.2.1.3. Analytical Methods for Environmental Media 7-7
7.2.1.4. Analytical Methods for Quantifying Dose and Dose Metrics 7-8
7.2.1.5. CEA Workshop Findings on Analytical Methods 7-8
7.2.2. Life Cycle Characterization 7-9
7.2.2.1. Production 7-9
7.2.2.2. Distribution 7-10
7.2.2.3. Use 7-10
7.2.2.4. Disposal 7-11
7.2.2.5. CEA Workshop Findings on Life Cycle Characterization 7-11
7.2.3. Transport, Transformation, and Fate Processes 7-11
7.2.3.1. Factors Influencing Transport, Transformation, and Fate Processes in Environmental Media 7-12
7.2.3.2. Transport, Transformation, and Fate Processes in Air 7-12
7.2.3.3. Transport, Transformation, and Fate Processes in Terrestrial Systems 7-13
7.2.3.4. Transport, Transformation, and Fate Processes in Aquatic Systems 7-13
7.2.3.5. Transport, Transformation, and Fate Models 7-15
7.2.3.6. CEA Workshop Findings on Transport, Transformation, and Fate Processes 7-16
7.2.4. Exposure-Dose 7-16
7.2.4.1. Biotic Exposure and Uptake 7-16
7.2.4.2. Human Exposure and Uptake 7-18
7.2.4.3. Aggregate Exposure in Humans and Biota 7-19
7.2.4.4. Exposure and Uptake Models 7-20
7.2.4.5. CEA Workshop Findings on Exposure and Uptake 7-20
7.2.5. Characterization of Effects 7-20
7.2.5.1. Ecological Effects 7-21
7.2.5.2. Human Effects 7-22
7.2.5.3. CEA Workshop Findings on Effects 7-23
7.3. Role of Case Study in Research Planning and Assessment Efforts 7-23
7.3.1. Workshop Outcomes 7-24
7.3.2. Implications for Research Planning 7-25
7.3.3. Implications for Future Assessment Efforts 7-26
References R-1
Appendix A. Common Analytical Methods for Characterization of Nanomaterials A-1
Appendix B. Summary of Ecological Effects Studies of Nano-Ag B-1
Appendix C. Summary of Human Health Effects Studies of Nano-Ag C-1
Appendix D. Identified Research Priorities (January 2011 Workshop) D-1
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List of Tables
Table 2-1. Selected U.S. studies of silver contamination in the environment. 2-3
Table 2-2. Solubility product constants for various silver solids. 2-15
Table 2-3. Types of common coatings of nano-Ag. 2-17
Table 2-4. Analytical methods for nanomaterials in soil, sediment, and ground water for size fractionation and distribution, surface area,
and phase and structure. 2-21
Table 4-1. Formation constants for silver complexes (Agligand =1:1) with environmentally relevant ligands. 4-15
Table 5-1. Nano-Ag applications and potential routes of human exposure. 5-46
Table 6-1. Experimental parameters and toxicity of nano-Ag in deionized water and natural surface waters. 6-15
List of Figures
Figure 1-1. Comprehensive environmental assessment framework. 1-3
Figure 1-2. Steps in the CEA process. 1-5
Figure 2-1. Silver emissions to the environment by geographical region. 2-5
Figure 4-1. Potential nano-Ag pathways into the environment associated with production, use, and disposal of spray disinfectants
containing nano-Ag. 4-24
Figure 5-1. Absorption and uptake of nanoparticles and transport to the central blood circulatory system. 5-52
Figure 7-1. Comprehensive environmental assessment framework. 7-2
Figure 7-2. Steps in the CEA process. 7-2
VII
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Abbreviations
ACGIH
Ag
Ag(NH3)2+
Ag+, Ag2+, and Ag3+
Ag°
Ag2S
AgsAsSs
AgsSbSs
Ag5SbS4
AgCI
AgCI04
Agl
AgNOs
AgS04
ALP
ALT
AMO
AST
AsM
ATP
ATSDR
BAF
BCF
BLM
BMR
BSA
Ceo
Ca2+
CaCI2
CCC
CEA
ci-
CPB
American Conference of Governmental Industrial Hygienists
Silver
Ionic Diamminesilver
Ionic Silver
Zero-Valent Silver
Silver Sulfide or Argentite
Proustite
Pyrargyrite
Stephanite
Silver Chloride or Cerargyrite
Silver Perchlorate
Silver Iodide
Silver Nitrate
Silver Sulfate
Alkaline Phosphatase
Alanine Aminotransferase
Ammonia Monooxygenase
Aminotransferase
Arsenate
Adenosine Triphosphate
Agency for Toxic Substances and Disease Registry
Bioaccumulation Factor
Bioconcentration Factor
Biotic Ligand Model
Basal Metabolic Rate
Bovine Serum Albumin
Carbon 60, Buckminster Fullerenes, or Buckyballs
Ionic Calcium
Calcium Chloride
Critical Coagulation Concentration
Comprehensive Environmental Assessment
Ionic Chlorine
Cetylpyridine Bromide
VIII
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CPC
CPF
DGGE
DGT
DLS
DOC
EDS
EDTA
EPA
EPS
Fe3+
FIFRA
Gl
GSH
H202
ICP
ICP-MS
ICRP
IL-6, IL-8, IL-p
K+
LOAEL
LSPR
Mg2+
MIC
MIP-2
MLE
MTT
N2
Na+
Na+/K+-ATPase
NaBH4
NaCI
NAG
nano-Ag
nano-Au
Condensation Particle Counter
Chlorpyrifos
Denaturing Gradient Gel Electrophoresis
Diffusive Gradients in Thin Films
Dynamic Light Scattering
Dissolved Organic Carbon
Energy-Dispersive X-Ray Spectroscopy
Ethylenediaminetetraacetic Acid
U.S. Environmental Protection Agency
Exopolymeric Substances
Ferric Iron
Federal Insecticide, Fungicide, and Rodenticide Act
Gastrointestinal
Glutathione
Hydrogen Peroxide
Inductively Coupled Plasma
Inductively Coupled Plasma-Mass Spectroscopy
International Commission on Radiological Protection
Interleukins
Ionic Potassium
Lowest Observed Adverse Effect Level
Localized Surface Plasmon Resonance
Ionic Magnesium
Minimum Inhibitory Concentrations
Macrophage Inhibitory Protein-2
Modified Ludzack-Ettinger
3-(4,5-Dimethylthiazol- 2-YI)-2,5-Diphenyltetrazolium Bromide
Nitrogen
Ionic Sodium
Sodium- and Potassium-Activated Adenosine Triphosphatase
Sodium-Silver Thiosulfate
Sodium Borohydride
Sodium Chloride
N-Acetyl-B-D Glucosaminidase
Nanoscale Silver
Nanoscale Gold
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nanomaterials
nano-Ti02
NH2OH.HCI
NOEC
NOM
OA
OECD
PAA
PBMC
PEC
PEN
PHA
PMFA
PNEC
P043-
PT
PVP
ROS
-S
s2-
SAP
SEM
-SH
SMPS
SC>42-
TEM
TNF-a
U.S. EPA
Nanoscale Materials
Nanoscale Titanium Dioxide
Hydroxylamine Hydrochloride
No Observed Effect Concentration
Natural Organic Matter
Oleic Acid
Organisation for Economic Co-operation and Development
Polyacrylic Acid
Peripheral Blood Mononuclear Cell
Predicted Environmental Concentration
Project on Emerging Nanotechnologies
Phytohaemagglutinin
Probabilistic Material Flow Analysis
Predicted No-Effect Concentration
Phosphate
Permeability Transition
Polyvinylpyrrolidone
Reactive Oxygen Species
Inorganic Sulfide
Sulfide
Scientific Advisory Panel
Scanning Electron Microscopy
Thiol
Scanning Mobility Particle Sizer
Sulfate
Transmission Electron Microscopy
Tumor Necrosis Factor-a
U.S. Environmental Protection Agency
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Authors, Contributors, and Reviewers
U.S. Environmental Protection Agency
Project Leaders:
Primary Authors:
Contributors:
ICF International4
J. Michael Davis, ORD/NCEA (retired); Christina Powers, ORD/NCEA
J. Michael Davis, ORD/NCEA; Patricia Gillespie, ORD/NCEA; Maureen
Gwinn, ORD/NCEA; Christine Hendren, ORISE; Tom Long, ORD/NCEA;
Christina Powers, ORD/NCEA
GenyaDana, ORISE; Meredith Lassiter, ORD/NCEA; Jeff Gift, ORD/NCEA;
Richard Fehir, OCSPP/OPPT; Justin Roberts, OCSPP/OPPT; Amy Wang,
ORD/NCCT
Project Manager: Dave Burch
Primary Authors:
Contributors:
Technical Editor:
Dave Burch, Pamela Hartman, Margaret McVey, Audrey Turley,
Amalia Turner
Rebecca Boyles, Michelle Cawley, Adeline Harris, Whitney Kihlstrom,
Courtney Skuce, Kate Sullivan, Satish Vutukuru, Ronald White
Penelope Kellar
Consultant Reviewer: Jo Anne Shatkin (CLF Ventures)
*This document was prepared by ICF International under EPA Contract No. EP-C-09-009 with
technical direction by the U.S. EPA National Center for Environmental Assessment.
Internal EPA Reviewers
Jim Allen, ORD/NHEERL
Jim Alwood, OCSPP/OPPT
Chris Andersen, ORD/NHEERL
Mary Ann Curran, ORD/NRMRL
Carl Blackman, ORD/NHEERL
Will Boyes, ORD/NHEERL
Lyle Burgoon, ORD/NCEA
Jonathan Chen, OCSPP/OPP
Jed Costanza, OCSPP/OPPT
Dave Demarini, ORD/NHEERL
Steve Diamond, ORD/NHEERL
Timothy Dole, OCSPP/OPP
Kevin Dreher, ORD/NHEERL
Brendlyn Faison, OW/OST
Richard Fehir, OCSPP/OPPT
Greg Fritz, OCSPP/OPPT
Earl Goad, OCSPP/OPP
Karen Hamernik, OCSPP/OSCP
Kathy Hart, OCSPP/OPPT
William Hazel, OCSPP/OPP
Ed Heithmar, ORD/NERL
Mike Hughes, ORD/NHEERL
William Jordan, OCSPP/OPP
Andy Kligerman, ORD/NHEERL
David Lai, OCSPP/OPPT
Anjali Lamba, OCSPP/OPPT
Jennifer McLain, OCSPP/OPP
Connie Meacham, ORD/NCEA
Melba Morrow, OCSPP/OPP
Mian Nguyen, OCSPP/OPPT
Matt Odegaard, ORD/NHEERL
Marti Otto, OSWER/OSRTI
Scott Prothero, OCSPP/OPPT
Kim Rogers, ORD/NERL
Jessica Ryman, OCSPP/OPP
John Scalera, OEI/OIAA
Prabodh Shah, OCSPP/OPPT
Najm Shamim, OCSPP/OPP
Jenny Tao, OCSPP/OPP
Patti TenBrook, Region 9
Nicolle Tulve, ORD/NERL
John Vandenberg, ORD/NCEA
Katrina Varner, ORD/NERL
Debra Walsh, ORD/NCEA
Barbara Wright, ORD/NCEA
Robert Willis, ORD/NERL
Bob Zucker, ORD/NHEERL
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Internal Review Coordinators
Daniel Axelrad, OA/OPEI
Ambika Bathija, OW/OST
Jane Denne, ORD/NERL
Patricia Erickson, ORD/NRMRL
Interagency Reviewers
Karen Hamernik, OCSPP/OSCP
Keith Houck, ORD/NCCT
Carl Mazza, OAR/IO
Ginger Moser, ORD/NHEERL
Marti Otto, OSWER/OSRTI
Nora Savage, ORD/NCER
Parti TenBrook, Region 9
Dennis Utterback, ORD/OSP
Department of Energy
Executive Office of the President
Occupational Safety and Health Administration
Interagency Review Coordinator
Jeff Morris*, EPA/ORD/IOAA, National Program Director for Nanotechnology
*Currently Deputy Director for Programs in EPA Office of Pollution Prevention and Toxics
Public Commenters
David Q. Andrews, Senior Scientist,
Environmental Working Group (EWG) et al.
Anonymous
Samantha Dozier, Nanotechnology Policy
Advisor, Regulatory Testing Division, People
for the Ethical Treatment of Animals (PETA)
Jaydee Hanson, Policy Director, International
Center for Technology Assessment (ICTA)
Ben Horenstein, Chair, Tri-TAC
Peer Reviewers
Paul M. Bertsch, Ph.D., University of Kentucky
Jaclyn Canas, Ph.D., Texas Tech University
Robert I. MacCuspie, Ph.D., NIST
Peter R. McClure, Ph.D., DABT, SRC, Inc.
Organizational Abbreviations
Empa Swiss Federal Laboratories for Materials
Testing and Research Technology
IO Immediate Office
IOAA Immediate Office of the Assistant
Administrator
NCEA National Center for Environmental
Assessment
NCCT National Center for Computational
Toxicology
NCER National Center for Environmental
Research
Jeff Keane, Chief Executive Officer, Noble
Biomaterials, Inc.
Fred Klaessig, Pennsylvania Bio Nano Systems, LLC
Maria Victoria Peeler, Senior Policy Specialist,
Washington State Department of Ecology
Rosalind Volpe, Executive Director, Silver
Nanotechnology Working Group (SNWG)
BerndNowack, Ph.D., Empa
Stig I. Olsen, Ph.D., Technical University of Denmark
James F. Ranville, Ph.D., Colorado School of Mines
NERL National Exposure Research Laboratory
NCER National Center for Environmental
Research
NHEERL National Health and Ecological
Effects Research Laboratory
NIST National Institute of Standards and
Technology
NRMRL National Risk Management Research
Laboratory
OA Office of the Administrator
OAR Office of Air and Radiation
XII
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OCSPP Office of Chemical Safety and
Pollution Prevention
OEI Office of Environmental Information
OIAA Office of Information, Analysis, and
Access
OPEI Office of Policy, Economics, and
Innovation
OPP Office of Pesticide Programs
OPPT Office of Pollution Prevention and
Toxics
ORD Office of Research and Development
ORISE Oak Ridge Institute for Science and
Education
OSCP Office of Science Coordination and
Policy
OSP Office of Science Policy
OSRTI Office of Superfund Remediation and
Technology Innovation
OST Office of Science and Technology
OSWER Office of Solid Waste and
Emergency Response
OW Office of Water
XIII
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Preamble
This document is part of continuing efforts by the U.S. Environmental Protection Agency (EPA) to
understand the scientific issues and information gaps associated with nanotechnology, consistent with
recommendations in the EPA Nanotechnology White Paper (U.S. EPA. 2007b) and EPA Nanomaterial
Research Strategy (U.S. EPA. 2009e). Engineered nanoscale materials (nanomaterials) have been
described as having at least one dimension on the order of approximately 1 to 100 nanometers (nm)
(NSTC. 2011). Such materials often have unique or novel properties that arise from their small size.
The specific type of nanomaterial considered in this document is nanoscale silver (nano-Ag) in a
disinfectant spray application. This "case study" does not represent a completed or even preliminary
assessment, nor is it intended to serve as a basis for near-term risk management decisions on possible uses
of nano-Ag. Rather, the intent is to describe what is known and unknown about nano-Ag in this selected
application as part of a process to identify and prioritize scientific and technical information to support
long-term assessment efforts. The information used to populate this report is up to date as of March 1,
2011, when the last broad literature search to identify new information was conducted. Previous EPA case
studies focused on nanoscale titanium dioxide used in drinking water treatment and in topical sunscreen
(U.S. EPA. 201 Od).
Like the previous case studies, this case study of nano-Ag is based on the comprehensive
environmental assessment approach, which consists of both a framework and a process, the principal
elements of which are illustrated in Figures 1-1 and 1-2 respectively. The organization of this document
reflects the comprehensive environmental assessment framework: After a general introduction (Chapter 1)
and introductory material on silver and nano-Ag (Chapter 2), Chapter 3 highlights stages of the product
life cycle (research and development, feedstock processing, manufacturing, storage, distribution, use,
disposal), followed by Chapter 4 on transport, transformation and fate processes, Chapter 5 on exposure-
dose characterization, and Chapter 6 on ecological and health effects. Chapter 7 summarizes highlights
from preceding chapters and major research issues.
XIV
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Chapter 1. Introduction to this Document
1.1. Background
Nanoscale materials (nanomaterials) have been described as having at least one dimension on the
order of approximately 1-100 nanometers (nm) (NSTC. 2011).' Although this size range is not
universally accepted and continues to evolve, 100 nm is typically used as an upper bound, and this
working definition is used as the size standard in this case study. Engineered nanomaterials are
intentionally made, as opposed to being an incidental by-product of combustion or a natural process such
as erosion, and often have unique or novel properties that arise from their small size. Like all
technological developments, engineered nanomaterials offer the potential for both benefits and risks. The
assessment of such benefits and risks relies on information, and, given the nascent state of
nanotechnology, much remains to be learned about nanomaterials to support such assessments. This
document is part of an endeavor to identify what is known and, more importantly, what is not yet known
that could be of value in assessing the broad environmental implications of certain nanomaterials.
The focus of this document is a specific application of a selected nanomaterial: the use of
engineered nanoscale silver (nano-Ag)2 as an agent in disinfectant spray products. The U.S.
Environmental Protection Agency (EPA) completed similar "case studies" of nanoscale titanium dioxide
(nano-TiO2) used for drinking water treatment and for topical sunscreen (U.S. EPA. 2010d). Such case
studies do not represent completed or even preliminary assessments; rather, they are intended as a starting
point in a process to identify and prioritize possible research directions to support future assessments of
nanomaterials.
Part of the rationale for focusing on a series of nanomaterial case studies is that such materials and
applications can have highly varied and complex properties that make considering them in the abstract or
in generalities quite difficult. Different materials and different applications of a given material could raise
unique questions or issues, as well as some issues that are common to various applications of a given
nanomaterial or even to different nanomaterials. Focusing on only one possible example of a nano-Ag
:Key terminology (e.g., nano-Ag, conventional silver, engineered nanoparticles) used throughout this section is
further defined and clarified in Section 1.4.
throughout this document, the term "nano-Ag" refers to silver nanoparticles that can display a range of properties
and behaviors depending on specific characteristics of the particle, environmental conditions, and other factors.
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application obviously does not represent all ways in which this nanomaterial could be used or all issues
that different applications could raise. By considering this single application of nano-Ag, however,
research directions can be identified that might pertain to nano-Ag in disinfectant spray, as well as to
nano-Ag in general and even more broadly to other nanomaterials. The present case study, along with
previous case studies of nano-TiO2 used for drinking water treatment and for topical sunscreen (U.S. EPA.
2010d) and any future case studies of other nanomaterials, could assist research planning for
nanomaterials at EPA and in the broader scientific community.
The process for selecting specific nanomaterials and particular applications to focus on for each
case study involved individuals representing several EPA program offices, regional offices, and Office of
Research and Development laboratories and centers. During an initial selection process, the individuals
considered several candidate nanomaterials, including nanoscale cerium dioxide, several nanoscale
carbon materials, as well as several metal and metal oxide nanomaterials. They then voted for their
preferences based on, among other things, exposure potential, applicability to human and ecological risk
assessment, and potential relevance of the nanomaterial to EPA programmatic interests. Nanoscale
nano-TiO2, nano-Ag, and single-walled carbon nanotubes were the top candidates based on this voting
process. As discussed above, case studies were then completed on nano-TiO2 and subsequently this
document on nano-Ag was initiated. The choice of a specific application of nano-Ag, disinfectant spray in
this case, was determined by a smaller team directly involved in the production of the case study
document; however, similar to the selection of nanomaterial candidates, several applications were
considered and disinfectant spray was then chosen based on consideration of the factors used to select
candidate materials. This is not to say, however, that the selection of nano-Ag in disinfectant spray
signifies a determination that it presents the greatest potential for exposure, or most relevance to risk
assessment, or is clearly preferable based on any other single factor compared to all possible applications.
Rather, based on information available at the time, it was deemed as the best application to focus thinking
about the types of information that could inform future assessments of the potential ecological and health
implications of nano-Ag.
This case study of nano-Ag, like the first case studies of nano-TiO2 (U.S. EPA. 2010d). is built on
the comprehensive environmental assessment (CEA) approach, which consists of both a framework
and a process, the principal elements of which are illustrated in Figures 1-1 and 1-2 respectively. The
uppermost box of Figure 1-1 lists typical stages of a product life cycle: research and development,
feedstock processing, manufacturing, storage, distribution, use, and disposal (which would include reuse
or recycling, if applicable). Releases to the environment associated with any stage of the product life
cycle lead to what is depicted in the second box, which refers to transport, transformation, and fate
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Product Life Cycle
R&D - Feedstock Processing- Manufacturing -
Storage/Distribution- Use - Disposal/Recycling
VIRONMENTA
ONDITIONS
NVIRONMENTA
ED1A
Transport/Transformation/Fate
Primary and Secondary Substances
biological
Exposure-Dose
Other Biota Abiotic Resources'?
^Natural features, structures, painted surfaces, etc. ]
Impacts
Health Ecological Other *7
Aesthetic, Climate, Energy, Ethical-Legal-Social, Resources, Sustainability, etc.
Source: http://www.epa.gov/nanoscience/files/CEAPrecis.pdf.
Figure 1-1. Comprehensive environmental assessment framework.
The CEA framework is used to systematically organize complex information in evaluations of the environmental implications of selected chemicals, products, or
technologies (i.e., materials). The framework starts with the inception of a material and encompasses the environmental fate, exposure-dose, and impacts.
Notably, the sequence of events is not always linear when, for example, transfers occur between media or via the food web. In addition, a variety of factors
influence each event, including differences in environmental media and the physical, chemical, biological, and social conditions in which the material event
occurs. Details on these influential factors are thus included throughout the framework when possible. The color gradient from top to bottom conveys the
interactions and transfers that can occur between environmental media and conditions throughout the vertical layers of the framework.
processes that can result in secondary as well as primary contaminants spatially distributed in the
environment.
The chains of events represented in the CEA framework occur within multiple environmental
media (air, water, sediment, soil) and under various conditions (physical, chemical, biological, social),
and any or all of these factors can be important in these events and processes. Also of note is that the
single arrows connecting one facet of the CEA framework to the next actually represent numerous
linkages, transfers, and feedback loops that can be far more complex than this simplified figure suggests.
For example, the transfer of material from one organism to another through the food chain would
represent a bidirectional exchange between transport, transformation, and fate and exposure-dose. In an
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effort to convey the numerous types of complex exchanges, however, the framework does not present
these interactions beyond a high-level, linear progression.
The third box in Figure 1-1, exposure-dose, goes beyond characterizing the occurrence of
contaminants in the environment, as exposure refers to actual contact between a contaminant and a
receptor, whether living or nonliving. Living organisms consist of humans and other biota;3 abiotic
receptors can include features of the natural landscape, structures such as buildings and statues, and
painted surfaces of vehicles and other objects. Exposure can involve aggregate exposure across routes
(e.g., inhalation, ingestion, dermal), cumulative exposure to multiple contaminants (both primary and
secondary), and various spatiotemporal dimensions (e.g., activity patterns, diurnal and seasonal changes).
Dose is the amount of a substance that actually enters an organism by crossing a biological barrier or that
deposits on an inanimate object.
As part of a chain of cause-effect events, dose links exposure with potential impacts of various
types, as indicated in the last box of Figure 1-1. Human health effects might result when a delivered dose
reaches a target cell or organ; or, in an ecological context, effects might occur when a stressor is at a level
sufficient to cause an adverse response in biotic or abiotic receptors (e.g., buildings, cars, other
structures). Impacts encompass both qualitative hazards and quantitative exposure-response relationships
and can extend to aesthetic effects (e.g., alterations in visibility, taste, and odor), climate change, energy
consumption, resource depletion, and other types of effects. Although none of these effects are being
excluded a priori from consideration here, their inclusion in a CEA framework would depend on having a
plausible premise for expecting a discernible impact. If such a premise can be articulated for additional
types of effects, the case study can be expanded to encompass their consideration within the CEA
framework. Even economic and societal impacts related to sustainability could be encompassed by the
CEA approach, but the focus in this case study is limited to environmental implications related to human
health and ecological populations.
Not reflected in Figure 1-1 is the role of analytical methods that make possible the detection,
measurement, and characterization of nanomaterials in the environment and in organisms.
Characterization of the substance(s) of interest (e.g., determination of chemical identity, reactivity, purity,
and other properties) is fundamental to any assessment of nanomaterials, but is not included in this
high-level view of the CEA framework for the sake of simplicity.
As previously mentioned, the CEA approach consists of both a framework and a process.
Compiling the information described above into the CEA framework is the first step of the CEA process
(Figure 1-2). Next, a collective judgment process is used to evaluate this information and prioritize it.
3The term biota is used throughout this document to refer to all living organisms other than humans.
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Collective judgment, as it has been applied in the CEA process to date, refers to a formal, structured
procedure that enables a diverse group of individuals to be heard individually and represented in a
transparent record of the collectively reached outcomes. In turn, it supports an essential feature of CEA:
the inclusion of diverse technical and stakeholder perspectives to ensure that a holistic evaluation is
achieved qCF.2011).
Compile Information
in CEA Framewor
Prioritize Risk
Trade-Offs
Prioritize
nformation
Gaps
Develop Risk
Management Plan
Develop
Research Plan
Monitor, Evaluate
Outcomes
Conduct Research
1
Source: http://www.epa.gov/nanoscience/files/CEAPrecis.pdf
Figure 1 -2. Steps in the CEA process.
The CEA process involves a series of steps that result in judgments about the implications of information contained in the CEA framework. Compiling information
in the CEA framework is fundamental for a given material, but is only a first step in the CEA process. Next, the information in the framework is evaluated using a
collective judgment technique (i.e., a structured process that allows the participants representing a variety of technical and stakeholder viewpoints to learn from
one another, yet form their own independent judgments). The result of the collective judgment step is a prioritized list of risk trade-offs and information gaps that
then can be used in planning research and developing adaptive risk management plans. The knowledge gained from these research and risk management
activities feeds back in an iterative process of periodic CEA updates.
Prioritization is a key objective in this holistic evaluation within the CEA process. Depending on
one's objectives and the state of the science surrounding an issue, CEA can be used to prioritize
(1) information gaps leading to development of a research plan that will support future assessment efforts
and (2) risk trade-offs leading to development of an adaptive risk management plan. As depicted in Figure
1-2, these uses of CEA cross over from conducting assessments into management efforts after the initial
identification and prioritization of information. In either instance, CEA is meant to be iterative, and thus
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the results of research and risk management efforts would be used in updating the CEA framework after
some period of time determined by those conducting the CEA process.
At present, the CEA framework and process are focused on helping refine research planning for
nanomaterials, with particular focus on nano-Ag as it might be used in disinfectant spray products.
Although CEA has been exclusively applied to planning research to date, as the knowledge base grows
for nanomaterials, and it becomes feasible to identify and prioritize risk-risk and risk-benefit trade-offs
with more complete information, the path leading to risk management (as shown in Figure 1-2) will be
pursued. Such efforts to extend its application would strive to inform risk management decisions by
providing a systematic approach for organizing and evaluating complex information from a variety of
existing assessment approaches, such as life-cycle assessment, exposure assessments, ecological risk
assessment, and human health assessments.
Other efforts have been made to assess the potential risks of nanomaterials by incorporating a life-
cycle perspective [e.g., (Shatkin. 2008; EnvironmentalDefense - DuPontNanoPartnership. 2007; Thomas
and Sayre. 2005)] or by using collective expert judgment methods [e.g., (Kandlikar et al.. 2007; Morgan.
2005)1. primarily in a risk management context. Although the present document differs somewhat from
these other efforts in its purpose—namely, this document is intended to aid in the development of research
strategies that support individual assessments that subsequently could be used to evaluate nanomaterial
risks—all of these endeavors complement and reinforce one another.
1.2. Purpose of this Document
This document represents the "Compile Information in CEA Framework" step of the CEA process
(Figure 1-2). As such, it has supported the next step of the process, identifying and prioritizing
information gaps about nano-Ag that could be relevant to conducting a CEA of nanomaterials. This
document was externally reviewed prior to the collective judgment step of the CEA process and again
before its final publication here to help ensure that all relevant sources were considered. This document
does not, however, purport to present an exhaustive review of the literature.4 Instead, the document
attempts to illustrate the information available from the literature within each area of the CEA framework
4The information used to populate this report is up to date as of March 1, 2011, when the last broad literature search
to identify new information was conducted. New literature identified from the March 2011 search was incorporated
into this document if the study provided added value (i.e., filled a previous data gap or elucidated a process or
outcome that was previously ambiguous or unclear) to the External Review Draft released in August 2010. In some
cases, information released after March 1, 2011 also was added to this document if the information was explicitly
requested during peer review and provided unique insight to the implications associated with the specific application
of nano-Ag in spray disinfectant.
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to aid research planning that supports future assessment efforts. Thus, this case study is not an actual
assessment and does not provide conclusions on potential ecological or human health impacts related to
nano-Ag.
Further, it must be emphasized that this case study has been developed without a specific
regulatory or policy objective in mind. EPA has the authority to regulate disinfectant spray products
containing nano-Ag under the Federal Insecticide, Fungicide, and Rodenticide Act (7 U.S.C. §136), and
such products might be of interest in various other policy and regulatory contexts. This document,
however, is not intended to serve as a basis for near-term risk management decisions on this specific
nanomaterial use.5 Rather, as stated above, the intent is to use this document to identify scientific and
technical information that could be pertinent for future assessment efforts. The results of future
assessments might, of course, provide input to policy and regulatory decision-making at that time.
When implemented for informing risk management decisions, the CEA approach is meant to be
comparative, examining the relative risks and benefits of different formulation options, for example. The
focus of a comparative CEA would be guided by risk management objectives. For example, nano-Ag
disinfectant spray products might be compared to disinfectants containing conventional silver, or the
comparison might be to a different nanomaterial formulation, such as nano-TiO2, or to a non-spray
product type or some other variable. Given that a number of different options could be of interest to risk
managers, considering every potential option in the present case study is not feasible. Therefore, this
document focuses solely on nano-Ag as it might be used in disinfectant spray, which is also consistent
with the fact that the case study is not intended to be an assessment, but rather is meant to assist in
identifying and prioritizing research related to nano-Ag that would support future assessment efforts.
1.3. How to Read this Document
As discussed above, this document presents a case study of nano-Ag in spray disinfectants with
chapters corresponding to the main elements of the CEA framework (Figure 1-1). First, Chapter 2
5Other EPA efforts are concerned with scientific issues relevant to nanomaterials within a regulatory context. A
notable example is a recent review by a Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory
Panel on hazard and exposure issues related to nano-Ag pesticide products (U.S. EPA. 2010bX As noted in the
review, the Panel's purpose is to provide advice and recommendations to EPA on pesticides regarding "the impact
of regulatory actions on health and the environment" (p. ii). Although this purpose fundamentally differs from that
of this case study, much of the information included in the Panel's review is relevant to this case study (reflected by
the fact that both publications cite many of the same literature sources). Furthermore, the potential data gaps
identified in the workshop based on this case study and discussed in the last chapter of this document are generally
consistent with the research needs the Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel
identified.
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provides an introduction to the physicochemical characteristics, historical context, and current uses of
nano-Ag. Chapter 3 then describes the first level of the CEA framework, product life cycle, and the
potential for release of nano-Ag and its by-products at each life-cycle stage. Chapter 4 describes the
transport, transformation, and fate of nano-Ag in environmental compartments. Chapter 5 examines the
potential for exposure of various receptors to nano-Ag and related contaminants, and Chapter 6
summarizes the ecological and human health effects associated with those exposures. Finally, Chapter 7
provides a high-level summary of the identified information and data gaps and describes how these
findings fit into the remainder of the CEA process. Although organizing individual chapters on the current
knowledge and apparent data gaps related to each main framework element facilitates a logical
organization and sequential presentation of information, it also results in some compartmentalization into
chapters and subsections. The document attempts to bridge such compartmentalization with cross-
references between sections and the inclusion of summaries at the end of each chapter.
The information presented here was obtained from a variety of published and unpublished sources,
including corporate websites and personal communications, and some information is based on inferences
drawn from other materials or applications. Such information sources are used because of the limited
amount of published materials on nano-Ag and its applications in the peer-reviewed literature, coupled
with the limited mechanisms for making manufacturer-specific data publicly available. Given that this
case study is not an assessment but, rather, a means to identify information gaps and research questions,
the use of a range of information sources seems justifiable.
As discussed above and again in the summary, Chapter 7, this document is not an end in itself. The
information presented in the case study served as a starting point for a workshop process during which
participants from diverse technical disciplines and economic sectors identified and prioritized information
gaps. This workshop process was funded by EPA and conducted independently by an EPA contractor. The
focus of this workshop was to gather input on "what might we need to know to be able to conduct a CEA
of nano-Ag in disinfectant spray products?" The prioritized research questions and the themes workshop
participants grouped them into are listed in Appendix D, which is an excerpt of the contractor's Workshop
Summary Report (1CF. 2011). More information on the workshop, its outcomes, and the CEA process as a
whole can be found in Chapter 7 and in the Workshop Summary Report (1CF. 2011).
1.4. Terminology
A number of terms used in the field of nanotechnology have specialized meanings, and definitions
of certain terms could have important legal, regulatory, and policy implications. Not surprisingly, perhaps,
defining such words, including the term nanomaterial itself, has often been a matter of considerable
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interest and debate. For the purposes of this document, however, having a connotative definition that
states the necessary and sufficient conditions that define a nanomaterial is not deemed necessary. Instead,
a denotative approach is used; that is, the term "nanomaterial" denotes something that would generally be
considered (or at least appears to be) an example of a material or a product that incorporates an
engineered nanoscale material, regardless of whether a consensus exists regarding what properties or
characteristics qualify it as such.
Although this case study focuses on "nano-Ag," readers should note that this term encompasses a
variety of materials that might possess a range of physicochemical properties. As a result, not all materials
referred to as nano-Ag will necessarily behave in the same manner and exert the same biological effects.
Thus, caution in extrapolating from one nano-Ag formulation to another when assessing hazards is
appropriate (U.S. EPA. 2010b). Conversely, until more information is available to discern more precisely
how various formulations differ in behavior and effects, pooling information from multiple sources can be
useful for the purpose of this document, namely to identify potential research directions to pursue.
Some other terms used throughout this document are discussed below, primarily to explain how the
terms are used here rather than to attempt to provide a formal definition of them.
• Nano-Ag. This document focuses primarily on engineered nanoscale silver (nano-Ag),
which usually is in the form of particles in the 1- to 100-nm size range. The term
"nano-Ag," as it is used in this document, refers to a variety of formulations containing
silver particles that meet this size-based definition. When reading this document, it is
important to understand that the general use of this term encompasses specific formulations
that can display a range of characteristics and behaviors depending on the properties of the
particle, the experimental or environmental conditions, and other factors.6 Where
information is not specific to nano-Ag, the term silver is used without the "nano" prefix.
• Conventional silver. To make an explicit distinction between the nanoscale material and
other forms of silver not specifically engineered at the nanoscale, the term "conventional"
is used in this document. Even so, materials described as conventional often contain a
range of particle sizes; including some with nanoscale dimensions (see the definition of
"colloid" below). In the scientific and technical literature on silver, the terms "bulk" and
"ionic" also are often used to distinguish conventional from nanoscale silver. Additionally,
terms such as ultrafine, particulate matter less than 0.1 micrometer (urn) diameter, and
micronized grade have been used to denote nanoscale particles, but typically in a particular
context or field of specialization such as aerosols and air pollution.
• Aggregate and agglomerate. As discussed in Chapter 4, in many circumstances primary
nanoscale particles can aggregate or agglomerate into secondary particles with dimensions
greater than 100 nm (a cluster that is sometimes referred to as a colloid, as described in the
next paragraph). Specifically, the terms "aggregate" and "agglomerate" are used in the
literature on nanomaterials and other fields to indicate the clustering of particles into a
6Where sources have provided documentation on size, surface coating, extent of clustering, and other salient
properties or characteristics, this information is included in the case study with sources referenced appropriately.
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single entity of such particles. These two terms can have specific meanings. For example,
the British Standards Institution (BSI. 2007) defines aggregate as a "particle comprising
strongly bonded or fused particles where the resulting external surface area may be
significantly smaller than the sum of calculated surface areas of the individual
components" and notes that "the forces holding an aggregate together are strong forces, for
example, covalent bonds, or those resulting from sintering or complex physical
entanglement." The British Standards Institution defines agglomerate as a "collection of
loosely bound particles or aggregates or mixtures of the two where the resulting external
surface area is similar to the sum of the surface areas of the individual components" and
notes that "the forces holding an agglomerate together are weak forces, for example van
der Waals forces, as well as simple physical entanglement." However, the meanings of
aggregate and agglomerate have sometimes been interchanged, as noted by Nichols et al.
(2002). This difference in meanings across, and sometimes within, the various fields that
contribute to nanomaterials research highlights the emerging and multidisciplinary nature
of the nanotechnology field. The nanotechnology community is an amalgam of
investigators who all study nanoscale materials but whose scientific roots are in a variety
of other, mature fields spanning toxicology, ecology, colloid science, materials science,
and many other disciplines. The customary terminology for aggregates and agglomerates
might be well established within one field, but use of these terms can elicit different
interpretations within another; as a result, the definitions for these terms are not specific,
nor are they consistent. Given this inconsistency in usage and, more importantly, the
frequent lack of adequate information to determine which term might be more
appropriately applied in a particular study or report, the term "cluster" is used in this
document to subsume both aggregates and agglomerates of nanoparticles. This term has
precedent within multiple disciplines and avoids confusion between potentially
inconsistent connotations of the other terms. Note that, in addition to being used as a noun
(as explained above), the word "aggregate" is used as an adjective (primarily in Chapter 5)
to refer to exposure to a given material from multiple sources, pathways, and routes.
Colloid. A "colloid" is a dynamic particle or cluster of particles often defined on the basis
of size (i.e., at least one dimension between 1 and 1,000 nm) and suspended in a given
medium (Stumm and Morgan. 1995); however, the term can be used rather loosely in the
literature specific to nanomaterials. Wijnhoven et al. (2009b), for example, refer to
colloidal silver as comprising silver particles primarily in the range of 250 to 400 nm,
thereby distinguishing nano-Ag and colloidal silver. By contrast, Luoma (2008) describes a
colloidal particle as containing multiple atoms of a substance measuring between 1 and
1,000 nm, and thus a colloid might or might not be a nanoparticle in that context. In this
case study, although the term colloid is used at times to refer to a sub-microscale particle
(especially if a cited publication uses this terminology), either the more specific term
"nanoscale" or a specific size range is used when the particle size is salient to the
discussion. The extent to which the properties of a cluster of primary nano-Ag particles
that exceeds 100 nm are similar to the properties of conventional silver is unclear. Also
unclear is the extent to which changes in conditions might initiate the formation,
decomposition, or dissolution of a cluster, and there is uncertainty as well regarding what
specific factors drive important changes in conditions. As will be discussed in Chapter 4,
disaggregation can occur under some conditions. Given these considerations, this
document does not use 100 nm as the essential and exclusive criterion for considering what
might be relevant to an evaluation of nano-Ag.
Naturally occurring, incidental, and engineered nanoparticles. In addition to
distinctions based on size of particles, The Project on Emerging Nanotechnologies (2009)
divides nanoscale materials into three classes based on the origin of the particles. Naturally
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occurring nanosized particles include, for example, particles that originate from volcanic
explosions, ocean spray, and soil and sediment weathering and biomineralization processes
(which can result in crystals of aluminum and iron oxides with nanometer-scale
dimensions). The second class is incidental nanosized particles, which are generated as
by-products of processes such as combustion, cooking, or welding. The focus of this report
is on the third class of nanoscale materials, engineered nanomaterials. This class comprises
materials purposely generated for a specific function, such as the carbon nanotubes used in
tennis rackets to make them lighter and stronger. In this case study, unless otherwise
specified, references to nano-Ag indicate engineered nanoscale materials. Non-engineered
types of nanosized silver (from the first or second class) are referred to as nanoscale silver,
rather than nano-Ag.
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Chapter 2. Introduction to Silver and
Nanoscale Silver
2.1. Conventional Silver: Uses, Occurrence in the
Environment, and U.S. Standards
Silver (Ag) is rarely found in its pure, free form, but rather is more commonly found as an alloy
with gold and other metals. Silver is also associated with minerals, the predominant form of which is
argentite (Ag2S), followed by cerargyrite (AgCl). Other minerals containing silver, in order of prevalence,
are proustite (Ag3AsS3), pyrargyrite (Ag3SbS3), and stephanite (Ag5SbS4). Silver can exist in its metallic
state (Ag°) and in three cationic states (Ag+, Ag2+, and Ag3+), with Ag° and Ag+ being most common
(Lide. 2000). The most abundant silver compounds in the environment are silver sulfide (Ag2S), silver
nitrate (AgNO3), and silver chloride (AgCl) (Wiberg etal.. 2001).
Silver is naturally released into the environment by wind and water erosion of soils and rocks
containing silver. Earth's crust contains approximately 0.1 part per million (ppm) of silver, and soils
contain approximately 0.3 ppm (Boyle. 1968). Ambient water concentrations as high as 0.2 microgram
per liter (ug/L) in fresh water and 0.25 ug/L in sea water have been reported, and background silver has
also been found in biota at levels of micrograms per gram (ug/g) of tissue, particularly in fish and
shellfish (ATSDR. 1990).
2.1.1. Uses of Silver and Silver Compounds
The use of silver pre-dates 2500 B.C., when the Chaldeans in present-day Iraq and Kuwait had
mastered a mining technique to extract silver from lead ores. In addition to using silver as a durable metal
in the production of jewelry, coins, and utensils, ancient peoples also recognized its potential to keep
liquids sterile (The Silver Institute. 2009a). The Greek historian Herodotus recorded that Cyrus the Great,
King of Persia from 559 B.C. to 530 B.C., had water drawn from a stream and "boiled, and very many
four-wheeled wagons drawn by mules carry it in silver vessels, following the king wherever he goes at
any time" (Herodotus. 1920).
The use of silver compounds for therapeutic purposes has a long history. In the 18th century, silver
nitrate, often called lunar caustic, was molded into pencil-like forms and used to remove granulation
tissue from wounds and to lance abscesses, while powdered silver nitrate was used to kill impurities and
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to dry open wounds (Klasen. 2000). In the 19th century, physicians used silver nitrate to promote healing
of burns and other wounds, and others began to research the bactericidal effects of silver. As early as
1880, physicians used a silver nitrate eye-drop solution to prevent gonococcal conjunctivitis in newborns.
In the early 20th century, surgeons used silver foil, silver sutures, and other silver dressings to prevent
infection in surgical wounds. Today, wound dressings containing silver and silver-based vascular and
urinary catheters are used in healthcare (Chopra. 2007).
In 1839, Louis-Jacques-Mande introduced a photographic technique that used a silver-plated sheet
of copper, sensitized with iodide vapors to make the silver react with light, and a salt solution to
permanently set the image on the sheet (Metropolitan Museum of Art Department of Photography. 2004).
Silver is still used in photography today, but with the advent of digital photography, the percentage of its
total usage continues to decline, dropping from 39% in 1979 to 13% in 2008 (GFMS Limited. 2009).
Silver has the highest thermal and electrical conductivities of any pure metal across a range of
temperatures (Lide. 2000). and is therefore used in industrial applications such as household switches,
switch panels in electrical appliances, batteries, and superconductors (The Silver Institute. 2009b). In
2008, industrial applications of silver accounted for 54% of the total silver used in manufacturing (GFMS
Limited. 2009). Jewelry (19%), silverware (7%), and coins (8%) account for the remaining silver used in
manufacturing (GFMS Limited. 2009). Other industrial uses of silver include as coatings on mirrors and
compact discs, in water purification systems, as antibacterial disinfectants, and in rear window defrosters
in automobiles (The Silver Institute. 2009b).
Silver iodide is used for cloud seeding. This process introduces silver iodide to clouds to induce
contact freezing of supercooled liquid water (colder than 0 °C) so that the amount or type of precipitation
that falls from clouds is altered (StroyprojectLTD. 2009). Supercooled liquid water is chemically unstable
and thus freezes upon contact with silver iodide, an artificial ice nucleus. Freezing releases heat to the
environment, thereby adding energy to the system and possibly increasing the intensity or duration of
precipitation. Silver iodide can be introduced via aircraft or by ground-level dispersion devices (e.g.,
rockets, anti-aircraft guns, generators). The efficacy of cloud seeding is unknown, but the practice is
popular in geographic locations where freshwater supplies are scarce (U.S. NCAR. 2006).
2.1.2. Occurrence of Silver in the Environment
Industrial processes such as smelting and mining, photography, and jewelry manufacture have led
to elevated levels of silver being released into the environment (U.S. EPA. 1987). Typically, areas of
elevated silver concentrations occur near sewage outfalls, electroplating plants, mine waste sites, and
silver iodide-seeded areas (Eisler. 1996). Runoff from silver disposal sites can transport silver farther
from these locations, and subsequent human activities such as dredging and construction can further
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Table 2-1. Selected U.S. studies of silver contamination in the environment.
Location
Anthropogenic source
Medium
Levels measured
Source
Quinnipiac River,
Connecticut
Manufacturing of silver tableware Water
and decorative items
Sediment
Effluent from municipal
wastewater treatment plant
5-500 nanograms per liter (ng/L)
Max = 800 ng/L
15-30 micrograms per gram (ug/g)
Max = 250 ug/g
120-180 ng/L
RozanetaU 19951
San Francisco Bay Municipal wastewater treatment
Lower South Bay plant handling discharge from a
photo processing facility
Surface sediments
Surface water
0.052-1.18 ug/g, dry weight
Mean = 0.388 ug/g, dry weight
<0.1-26.3 ng/L (total dissolved
silver between 1989 and 2005)
Flegal et al. (2007'
Michigan None specified
Surface soil 0.5 ± 0.2 ug/g
0.8 ± 0.5 ug/g
2.3 ± 2.2 ug/g
Sub-surface soil 0.6 ± 0.4 ug/g
0.5 ±1.1 ug/g
2.2 ± 4.0 ug/g
commercial)
residential)
industrial)
commercial)
residential)
industrial)
Murray etal. (20041
transport the silver (Purcell and Peters, 1998). As presented in Table 2-1, silver contamination has been
studied in several U.S. locations including the Quinnipiac River (Connecticut) (Rozan and Hunter. 2001;
Rozan et al.. 1995), San Francisco Bay (Flegal et al., 2007). and various sites in Michigan (Murray et al..
2004).
In the Quinnipiac River, Rozan et al. (1995) and Rozan and Hunter (2001) found that silver
concentrations in the river water peaked at 800 nanograms per liter (ng/L) following rainstorms due to
erosion of the silver-laden soil on the river banks and resuspension of contaminated sediments. Using
cesium-137, Rozan et al. (1995) dated the highest concentrations of silver in the river bank to the 1950s,
corresponding to the peak production period of the local silver industry.
Locally elevated silver concentrations have been observed in surface water near facilities that
manufacture or dispose of products containing silver, including facilities that process photographs and a
silver plating facility (Luoma. 2008). Ecological and toxicological effects have been linked to silver
concentrations in the environment in the range of 10-100 ng/L (or lower) as demonstrated by several field
and dietary toxicity studies summarized by Luoma (2008) and described here. Hornberger et al. (2000)
observed biochemical signs of stress, most notably failure to reproduce, in clam species (Corbula
amurensis) on a mudflat 2 kilometers (km) from a domestic-sewage outfall in South San Francisco Bay.
Over a 30-year period, after the passage of the Clean Water Act in 1972, the amount of silver and other
metals in the waste delivered to this sewage facility decreased, and the facility also improved its treatment
process for wastes containing silver by adding trickle filters, clarifiers, nitrification processes, and
increased retention time, and implementing other specific source controls. With these changes, the
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amount of metals discharged from the sewage facility to the bay decreased. The reproductive capabilities
of the clams subsequently recovered as evidenced by an increased number of months in which mature
gametes were observed and an increased proportion of the population with mature reproductive tissues
(Hornbergeretal.. 2000).
In 1978, an estimated 2.47 million kilograms (kg) of silver was emitted to the environment by the
United States alone (Smith and Carson. 1977). Eighty-two percent of the annual silver loss originated
from anthropogenic activities (42% from the photography industry) (Smith and Carson. 1977). In 1978,
3.7% of released silver entered the atmosphere, 28.5% ended up in the aquatic environment, and 67.8%
entered the terrestrial environment (Smith and Carson. 1977).
More recently, Eckelman and Graedel (2007) characterized the emissions of silver based on data
and estimations of environmental releases of silver in 1997 (Figure 2-1). Their assessment began with
data on silver production, fabrication, and import/export, which were collected and estimated in a material
flow analysis for 64 countries by Johnson et al. (2005). Johnson et al. concluded that their material flow
analysis comprised "well over 90%" of the global silver flow, including mining and production,
fabrication and manufacture, use, and waste management. Eckelman and Graedel (2007) further
characterized the losses of silver by applying a series of assumptions about the percentage of silver lost in
each process. For example, they assumed particulate silver emissions from incineration of municipal solid
waste varied directly with the overall income level of the country, ranging from an emission factor of
0.1% for high-income countries to 0.4% for low-income countries. The variance was due to the type of
furnace and pollution control technology used; the researchers assumed that high-income countries
employ control technologies that are more modern and more efficient than those used in low-income
countries. Thus, in high-income countries, more of the silver is captured before it is emitted to the
environment resulting in a lower emission rate.
Eckelman and Graedel calculated silver emissions for 64 countries, and Figure 2-1 presents their
estimates of silver emissions aggregated by geographical regions worldwide. The reported values do not
account for transport of the silver from one medium (i.e., air, land, or water) to another after initial
emissions. Eckelman and Graedel (2007) estimated 65% of the 2.84 million kg of silver emitted from the
United States enters landfills, 18% is released to the environment via tailings and 13% is contained in
leachate from the mining and productions process.
2.1.3. U.S. Standards for Environmental Silver
The U.S. Environmental Protection Agency's (EPA's) National Secondary Drinking Water
Regulations recommend a guideline of less than 0.10 milligram per liter (mg/L) (or ppm) of total silver in
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5,000,000
• Landfill (land)
D Dissipation (land)
• Dissipation (water)
D Particulate (air)
D Leachate (water)
• Slag (I and)
D Tailings (land)
'« 2,500,000
'E
^ 2,000,000
1
CO 1,500,000
500,000
Landfill (land)
Dissipation (land)
Dissipation (water)
Particulate (air)
Leachate (water)
Slag (land)
Tailings (land)
Total (kg/yr)
North Asia (East .- South _ . _
v Europe East Europe
America South, and SE America
48%
2%
1%
1%
16%
0.1%
33%
4,514,998
34%
29%
9%
2%
8%
0.1%
17%
2,717,772
87%
4%
2%
1%
0.1%
6%
1,939,904
16%
11%
3%
1%
14%
0.1%
55%
1,637,824
42%
5%
3%
1%
5%
0.2%
44%
955,917
Asia (Central
and West)
46%
8%
2%
1%
12%
0.1%
31%
800,734
Oceania
13%
4%
0.2%
1%
22%
0.2%
59%
481,398
Africa
22%
32%
8%
1%
4%
0.1%
32%
372,633
Source: Adapted with permission from Eckelman and Graedel (20071
Figure 2-1. Silver emissions to the environment by geographical region.
The original analysis by Eckelman and Graedel (2007) examined silver emissions in 64 countries. For this report, the data have been aggregated
according to the United Nations' geographical regions (U.N. Statistics Division, 20081.
North America: Canada, Mexico, United States
Asia (East, South, Southeast): China, Hong Kong, India, Indonesia, Iran,
Japan, Malaysia, Philippines, Singapore, South Korea, Taiwan, Thailand
Europe: Austria, Bel-Lux, Denmark, Finland, France, Germany, Greece,
Italy, Netherlands, Norway, Portugal, Spain, Sweden, United Kingdom
South America: Argentina, Bolivia, Brazil, Chile, Colombia, Peru, Venezuela
East Europe: Bulgaria, Poland, Romania, Russia, Ukraine
Asia (Central and West): Israel, Kazakhstan, Saudi Arabia, Turkey, UAE,
Uzbekistan
Oceania: Australia, New Zealand
Africa: Algeria, Cameroon, Egypt, Ethiopia, Ghana, Ivory Coast, Kenya,
Morocco, Namibia and South Africa, Nigeria, Sudan, Tanzania,
Tunisia, Uganda, Zimbabwe
drinking water due to the potential cosmetic (not health) effects of silver ingestion, specifically argyria
(i.e., an accumulation of silver in the skin that causes it to turn blue or bluish-gray) (U.S. EPA. 2009g).
States may decide to enforce this standard, but enforcement is not required. Due to lack of evidence, EPA
has not prescribed ambient water quality criteria for silver for human health (i.e., to protect from exposure
to silver by consumption of contaminated water or organisms).
With respect to chronic water quality criteria to protect aquatic life, a few states have set or
proposed threshold concentrations; for example, the North Carolina Division of Water Quality has
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proposed a criterion of 0.06 ug/L (NC DEHNR, 2007). Oregon has established a 0.12-ug/L criterion in its
Administrative Rules based on chronic toxicity to rainbow trout and minnows in fresh water and to
mysids in salt water (Oregon Department of Environmental Quality. 2004). Texas, New York, and some
regions in California have established similar chronic aquatic life criteria concentrations. Several regions
in California have established maximum contaminant levels for silver between 5 and 10 ug/L. EPA
proposed and then withdrew a chronic toxicity standard in the early 1990s on the grounds that additional
research was needed to support the available database in the development of chronic ambient water
quality criteria (Ford. 2001; U.S. EPA. 1987). The Agency has, however, prescribed maximum acute
concentrations of 3.2 ug/L in fresh water and 1.9 ug/L in salt water, based on acute toxicity of silver to
macroinvertebrates and fish (U.S. EPA. 2009f). which is discussed in detail in Section 6.2.2. These
standards are enforced through the issuance of discharge permits at the state level.
In addition to drinking water and water quality regulations, several other national and state-level
regulations have been established for silver and silver compounds. An oral reference dose for silver of
0.005 milligram per kilogram per day (mg/kg-day) is included in EPA's Health Effects Summary Tables
(U.S. EPA. 2003a) and in the Integrated Risk Information System database (U.S. EPA. 2012). This
reference dose is based on a chronic human exposure study carried out by Gaul and Staud (1935). An
occupational exposure limit of 0.01 milligram per cubic meter (mg/m3) has been derived for silver metal,
compounds, and soluble silver compounds by the Occupational Safety and Health Administration
(OSHA. 2010) and by the American Conference of Governmental Industrial Hygienists (ACGIH. 2010).
The National Institute for Occupational Safety and Health (NIOSH. 2010) has similarly recommended an
8-hour, time-weighted average exposure limit of 0.01 mg/m3 for silver metal dust and soluble compounds.
Silver is also on the list of chemicals subject to Section 313 of the Emergency Planning and Community
Right-to-Know Act of 1986 (U.S. EPA. 2010a).
2.2. Historical and Emerging Uses of Nanoscale Silver
Nanoscale silver is not new. The Lycurgus Cup, a glass and bronze cup on display at the British
Museum, was likely created in the 4th century A.D. (Evanoff and Chumanov. 2005). The glass in the cup
appears green until light is shone through it, shifting the absorption spectrum so that the glass appears to
glow red. Small gold and silver particles incorporated in the glass by the original craftsman cause the shift
in the apparent color of the cup; in the 1990s, researchers at the British Museum determined the average
diameter of the gold and silver particles in the glass to be 70 nanometers (nm). The Lycurgus Cup
therefore represents what is likely one of the first uses of nanoscale silver. Nanoscale silver also has been
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found in stained glass. The unique optical properties of nanoscale silver that prompted these early uses are
discussed further in Section 2.3.7.
The literature suggests that products containing colloidal silver have been available (although not
necessarily registered or supported by science) for use by humans as therapeutic agents for more than 100
years (Bottomry et al., 1909). and colloidal silver suspensions containing some particles with at least one
dimension in the 1- to 100-nm range were likely employed long before their use in these applications was
recorded. For example, Nowack et al. (2011) described several specific silver-based pesticides,
medications, and other applications that have been in use for many decades that contain nano-sized silver
particles, including some products for which particles were intentionally formulated at the nanoscale.
The specific forms of nanoscale silver used over the past 100 years in consumer products and other
applications are not clear, and researchers have made the case that nanoscale particles of silver have likely
been present in many (or perhaps most) of the silver products in use over time (Nowack et al.. 2011). The
ability to control and visualize matter at the nanoscale, however, has advanced considerably in recent
years. The evolution of such techniques has allowed for stricter control over physicochemical properties
of engineered nanoscale silver and greater accuracy in distinguishing single silver nanoparticles from
clusters of silver nanoparticles and larger particles of silver. As a result, when precise engineering,
detection, and identification of nanoscale silver became possible, the term nano-Ag was generally
substituted for the term colloidal silver when referring to relatively monodisperse particles intentionally
engineered at the nanoscale. The term colloidal silver is still used as defined in Section 1.4.7
A report by Hendren et al. (2011) is the only study to date to estimate production volumes of raw
nano-Ag (i.e., nano-Ag not incorporated into products) in the United States. They estimated a range of
nano-Ag production volumes using information from websites and personal communications with
manufacturers explicitly producing nano-Ag (as opposed to colloidal silver), and they filled in gaps with
data extrapolated from proxy parameters (e.g., number of employees, maximum possible order sizes,
annual sales revenues). Hendren et al. (2011) estimated the lower bound of U.S. nano-Ag production at
2.8 tons per year and the upper bound at 20 tons per year. These estimates relied primarily on
extrapolations using the number of employees at manufacturing companies as a proxy parameter from
which to estimate production volumes; uncertainty in this estimated range is therefore high.
Over the past several years, the Woodrow Wilson Center's Project on Emerging Nanotechnologies
(PEN) has compiled an inventory of consumer products reported by their manufacturers to contain
7This report generally uses the nomenclature provided by study authors in the literature. The distinction between
colloidal silver and nano-Ag, as provided here, however, is not used consistently in the literature. And, as mentioned
previously, the distinction in terms might be misleading due to the overlap in size ranges between colloidal silver
and nano-Ag.
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nanomaterials. The products in this inventory were identified through Internet searches for products
reported by their manufacturers or distributors to contain silver nanoparticles. Not all manufacturers'
claims that products listed in the PEN inventory contain engineered nanoparticles have been
independently or scientifically validated. Based on the (unverified) data PEN has collected, however,
nano-Ag is one of the most commonly used nanomaterials in manufactured consumer products,8 second
only to carbonaceous nanomaterials (Quadros and Marr. 2010). Of the more than 1,317 consumer
products included in the March 2011 PEN inventory of nanomaterial-based consumer products, nearly
25% are listed as containing nano-Ag. Consumer products based on nano-Ag represent a significant
fraction of every product category examined for the PEN inventory. From March 2006 to March 2011, the
number of products reported to contain nano-Ag rose from 25 to 313, a more than 10-fold increase.9 The
inventory does not account for products that have since been removed from the market, however, so these
values more accurately reflect the total amount of products that have been introduced to the market over
time rather than the total amount of products currently available on the market.
The PEN Silver Nanotechnology Consumer Product Inventory contains detailed information about
current consumer products reportedly containing nano-Ag and indicates that most nano-Ag products
claim to eliminate bacteria and their related odors (Project on Emerging Nanotechnologies. 2009; Fauss.
2008). For instance, manufacturers have introduced nano-Ag into cooking utensils to prevent bacterial
contamination and to reinforce the strength of the utensils. For similar reasons, nano-Ag also has been
incorporated into materials used to produce clothing, socks, fabrics, and shoe soles. Some products, such
as dietary supplements, laundry detergent, body soap, toothpaste, and wall paint, appear to contain
colloidal nano-Ag. One manufacturer suggests that dietary supplements containing nano-Ag promote a
healthy immune system by inhibiting the growth of and possibly destroying bacteria and viruses in the
digestive tract (ConSeallnternational. 2010). One manufacturer claims that nano-Ag supplements can
defend the body from "colds, flu, and hundreds of diseases (even anthrax)" and that silver has been used
throughout history as a cure for more than 650 diseases, from AIDS to cancer (Melchizedek. 2010). Other
products to which nano-Ag reportedly has been added include home furnishings, cleaning products, food
storage containers, kitchen appliances, curling irons, hair dryers, make-up, burn creams, nasal sprays,
soaps, dish detergents, and medical products, some of which might not be included in the PEN consumer
product inventory (Wrjnhoven et al. 2009b).
Spray disinfectants represent a category of the nano-Ag cleaning products that could become
available for use in the home, garden, or other settings such as hospitals in the United States. Products
8The Project on Emerging Technologies' Consumer Products Inventory website at
http://www.nanotechproject.org/inventories/consumer.
9Ibid.
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intended to disinfect inanimate objects or otherwise control microorganisms, except on or within living
humans or animals, are considered "pesticides," and federal law requires that EPA register such products
before they may lawfully be sold or distributed in the United States (7 U.S.C. § 136). Spray disinfectants
containing nano-Ag particles might be more effective, for example, at killing bacteria than those made
with larger, conventional silver particles due to the higher surface area-to-volume ratios of the smaller
particles, which could result in greater reactivity (see Chapter 6). Some manufacturers have claimed that
aerosol disinfectant sprays containing nano-Ag kill 99% of bacteria on various surfaces and prevent odor
for long periods of time (ConSeallnternational. 2010; Shanghai Huzheng Nanotechnology Co.. 2009).
Theoretically, nano-Ag sprays could also serve as broad-spectrum fungicides, and the sprays could exhibit
antiviral properties (Sun et al.. 2005; Wright etal.. 1999). No sources were identified that compare the
bactericidal or fungicidal properties of disinfectant sprays containing nano-Ag with sprays containing
conventional silver. Several laboratory studies, however, have compared the effects of nano-Ag and ionic
silver in a number of types of microorganisms (See Section 6.2.1).
The expanding use of nano-Ag in the consumer market suggests that, depending on the behavior of
nano-Ag in the environment, background concentrations of silver, in nano and non-nano form, are
anticipated to increase in some environmental media and thus could represent additional sources of long-
term and incremental exposure to both humans and biota. In the home, nano-Ag spray disinfectants might
be applied to walls, tables, beds, and other surfaces in order to kill harmful bacteria, particularly in the
kitchen and bathroom. Outside of the home, spray disinfectants with nano-Ag might be used in hospitals,
nursing homes, airports, and other public places in efforts to protect people from illness and disease.
As described in the list of key terms in Section 1.4 of this case study, materials containing colloidal
silver typically contain silver particles of nanoscale dimensions. Consequently, information on
conventional silver can inform the development of a comprehensive environmental assessment (CEA) for
nano-Ag used in disinfectant sprays, and therefore this case study includes some information on
conventional silver. At the beginning of each effects subsection, for example, is a brief summary of the
conventional silver effects on the organisms of interest for that section. It is important to keep in mind
though that this case study is focused on silver particles that have been specifically engineered to have
nanoscale characteristics and on the fate, exposures, and effects specific to nano-Ag. Therefore, studies in
which the nanoscale properties of the silver were systematically evaluated are particularly emphasized in
this case study. This nuance must be considered in determining the appropriate interpretation and use of
conventional silver data within the context of a nano-Ag-specific CEA.
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2.3. Physicochemical Properties of Nanoscale Silver
The physicochemical properties of nanoparticles determine both their fate in the environment as
well as their beneficial and harmful effects. Although the size of the nanoparticles can be the most
distinguishing property when compared to conventional particles, other unique physical and chemical
properties begin to emerge as particles approach the nanoscale range. For this reason, Auffan et al.
suggest that "below a critical size, it is not possible to simply scale the properties of bulk materials based
on the surface area to predict the properties of nanoparticles" (Auffan et al.. 2009b). Other scientists agree
that "although some material properties, like chemical composition and crystal structure, are the same on
the nanoscale as in the bulk phase, other properties differ ... a nanoparticle retains properties of both
materials in the bulk phase and molecular precursors" (Saves and Warheit. 2009). Indeed some studies
support these assertions (Griffitt et al., 2009); however, other researchers have found that despite the fact
that the physicochemical properties differ, the effects of nano-silver can be similar to those produced by
conventional ionic silver (Pal etal.. 2007). These differing findings led the Federal Insecticide, Fungicide,
and Rodenticide Act Scientific Advisory Panel (201 Ob) to conclude that "comparison[s] of
physicochemical properties of the nano [versus] bulk materials are needed."
Exactly which physiochemical properties of engineered nanoparticles, including nano-Ag, can be
useful for predicting their behavior and interactions in the environment is unclear. Several organizations
and independent researchers have published recommendations on the physicochemical characterization
data that should accompany research findings on transport, transformation, and fate processes and
ecological and human toxicity (U.S. EPA. 2010b: Saves and Warheit. 2009; MINCharlnitiative. 2008;
OECD. 2008; Tiede et al., 2008; Oberdorster et al., 2005a). These recommendations are based on a
synthesis of published, peer-reviewed studies on the behavior and effects of nanoparticles, but the
recommended properties vary by organization and researcher. Some of the recommendations regarding
characterization before, during, and after toxicity studies are further described in Section 6.1.1. In general,
the most prescribed physicochemical properties include:
• Size, including clustering tendencies;
• Morphology, including shape and crystal structure;
• Surface area;
• Chemical composition;
• Surface chemistry and reactivity;
• Solubility; and
• Conductive, magnetic, and optical properties.
Each property, as it relates to nano-Ag and to the other properties identified, is briefly discussed below.
For nano-Ag spray disinfectants, Hansen et al. (2007) suggest that all of the above properties, with the
exception of conductive, magnetic, and optical properties, are relevant to hazard identification. These
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properties, which could affect the efficacy of the final product, also could offer clues as to which types of
nano-Ag might be preferentially commercialized and thus most relevant to study as potential hazards.
Additional details on the state of knowledge of physicochemical properties in relevant environmental
compartments, exposure routes, and effects are provided in the chapters that follow.
2.3.1. Size
In recent years, synthesis methods have been developed to produce nanoparticles, including silver
nanoparticles, of various shape and size distributions. See, for example, Bar-Han et al. (2009). Evanoff
and Chumanov (2005). Khaydarov et al. (2009). and Tolaymat et al. (2010). Relatively monodisperse
particles can be obtained within the size range of 1-100 nm (see Chapter 3 for synthesis methods).
The size distribution of nanoparticles, including silver nanoparticles, however, does not necessarily
remain constant and depends on the chemical and physical environment surrounding the nanoparticles;
silver nanoparticles can agglomerate or aggregate to form larger clusters of nanoparticles, as well as
disperse into smaller particles or dissociate into ionic forms of silver. How rapidly the particles cluster or
disperse in an aqueous medium depends on particle collision frequencies (e.g., Brownian motion and
particle concentration), the energy of the particle collisions, the attractive-repulsive properties of the
particles involved (e.g., repelling surface charges of two positively charged particles), and the interactions
with colloidal materials such as natural organic matter present in the water. Handy et al. (2008b)
summarized that "after collision, particles can remain in aqueous phase as single particles or form
particle-particle, particle-cluster, and cluster-cluster aggregates." The dispersion state describes the extent
to which particles become clustered by interparticle attractive forces. Surface coatings and stabilizing
agents can enhance the stability of the dispersion and maintain the original or intended size distribution.
These phenomena can affect the transport, transformation, and fate of nano-Ag particles in the
environment and in humans and biota. Often, coagulation leads to the formation of larger, less mobile
particle clusters (AFSSET. 2006; Wiesner et al.. 2006; Aitken et al.. 2004). Nano-Ag used in some
products can enter the environment as individual nanoparticles, as small clusters, or potentially dissolve
into ions. In other cases, the nano-Ag incorporated into consumer products as composites or mixtures
could be released into the environment in an encapsulated form (Lowry and Gasman. 2009). The
translocation of particles depends in part on their size; hence, clusters of nano-Ag behave quite differently
compared to single particles (Ma-Hock et al.. 2007). The size of the nano-Ag (i.e., an individual particle
versus a cluster) can determine the likelihood of release of silver ions (Ag+) from the particle and
influence the particle's behavior in the environment (O'Brien and Cummins. 2009). Moreover, size alone
might determine particle mobility in the environment and within the body (Chen and Schluesener. 2008)
and enable nano-Ag to enter cells (Bar-Han et al.. 2009; Morones et al.. 2005). As emphasized by the
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Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel (20K)b), however, the
impact of size on the biological response elicited by nano-Ag particles is less clear. Size-dependent
particle mobility is discussed as it relates to potential biotic and human uptake and dose in Chapter 5, and
size-dependent ecological and human health effects are discussed in Chapter 6.
2.3.2. Morphology
Nano-Ag can be synthesized into various forms, including particles, spheres, rods, cubes, truncated
triangles, wires, films, and coatings (Wijnhoven et al.. 2009b; Pal et al.. 2007). The shape of nano-Ag
particles can affect the kinetics of their deposition and transport in the environment. Depending on its
surface structure and shape, a nano-Ag particle might exhibit different reactivity (Oberdorsteretal..
2005a), as its shape could make it difficult for particles to approach each other. Such shape-related
interactions can be controlled in some situations by adding detergents or surface coatings to the particles
to change their shape or surface charge (Handy et al.. 2008b).
Differences in shape and related changes in nano-Ag interactions with other particles or
surrounding environmental milieu also can influence particle toxicity, as shown by Pal et al. (2007). Pal et
al. (2007) studied the antibacterial activity (using Escherichia coli, or E. coll) of silver nanoparticles of
various shapes. Results indicated that nano-Ag particles of various shapes could kill E. coll, but the
inhibition results differed based on the percent of active facets in the crystal structure10. Specifically,
truncated triangular silver nanoplates with a {111} lattice plane as the basal plane displayed the strongest
biocidal action, compared to the spherical and rod-shaped nano-Ag particles, indicating that increasing
the number of active facets on the surface of a crystalline, or highly ordered, nanoparticle increases its
ability to inhibit bacterial growth.
Notably, particles with similar morphologies (e.g., spherical particles) can be synthesized with
unique crystal structures, a parameter which has been shown to influence the toxicity of nanoscale
titanium dioxide [Sayes et al. (2006); see Johnston et al. (2009) for a review of how crystal structure and
other physicochemical characteristics influence nanoscale titanium dioxide toxicity]. No data were
identified, however, on whether nano-Ag with different crystal structures, but similar shapes, could
exhibit different toxicities.
10Crystal structure refers to the same repeated arrangement, or lattice, throughout the crystal of atoms, molecules, or
ions that compose the crystal (Barronand Smith. 2010). Auguste Bravais, a 19th century mathematician and
physicist, determined that in 3-dimensional space, 14 lattice configurations are possible such that the arrangements
of the points appear identical when viewed from any other point. The crystal structures can be described in terms of
the planes that join the points in the lattice. These planes are denoted using Miller Indices, which are the reciprocals
of the intercepts of the crystal plane with the x, y, and z three-dimensional axes.
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2.3.3. Surface Area
Because of their small size, nano-Ag particles have greater specific surface area and a greater
surface area-to-volume ratio when compared to the same mass of material in larger particles (Auffan et
al., 2009b; Luoma, 2008). Auffan et al. estimate that a 10-nm particle has approximately 35-40% of its
atoms on the surface compared to 15-20% of the atoms on a particle larger than 30 nm in diameter. This
large surface area relative to mass or volume increases the reactivity and sorption behavior of
nanoparticles (U.S. EPA. 201 Ob; Auffan et al., 2009b; Tiede et al., 2008). Large specific surface area
enhances chemical reactivity, which means that smaller silver nanoparticles have more reaction sites
(i.e., sites that can receive electrons) on their surfaces and are more sensitive to oxygen, a natural electron
donor, as compared to larger particles of the same mass (Auffan et al., 2009b). Therefore, smaller
particles could exhibit greater efficacy as biological agents or stressors in ecosystems or on human health,
as discussed in Section 2.3.4.
Surface area also affects the ratio of silver ions on the surface of a silver particle to ions that are
"buried" inside the same particle. This ratio usually increases as particle size decreases. Thus, for larger
particles with smaller surface area-to-volume ratios, most of the silver ions might be unable to interact
with the environment or biological surfaces. This idea is further discussed in Section 6.1.
2.3.4. Chemical Composition
As mentioned previously, silver exists in one of four oxidation states: Ag°, Ag+, Ag2+, and Ag3+, and
the free silver ion under natural conditions is Ag+ (Lide. 2000). Of these, Ag° and Ag+ are the most
commonly occurring oxidation states in the environment. Based on a review of the existing literature,
Wijnhoven et al. (2009b) concluded that environmental and human health studies appear to demonstrate
that forms of conventional silver that release free silver ions are more toxic than other forms of
conventional silver that do not. Speciation would therefore strongly influence how much silver is
available to affect living organisms. To achieve stability, positively charged silver ions will associate with
negatively charged ligands11 (e.g., sulfide in fresh water, sulfide and chloride in salt water) (Luoma.
2008); the reader is referred to a detailed discussion of this process in Section 4.4. The concentrations of
these ligands and the bond strength between the silver ions and the ligands influence the distribution of
silver as free silver ions (its more bioavailable form) and the ligand-bound forms, which exhibit varying
degrees of bioavailability (Luoma. 2008).
11A ligand is a substance (e.g., atom, molecule, radical, or ion) that forms a complex around a central atom (see
Section 4.4).
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Chemical composition also includes the surface coating of the nanoparticle (Saves and Warheit.
2009). Coatings can be used to stabilize the nanoparticles in solution, to prevent clustering, or to add
functionality to the nanoparticle depending on its intended use. Surface coatings can influence the
reactivity of the nanoparticle in various media including surface water, biological fluids, and laboratory
test media (Auffan et al.. 2009b: Cumberland and Lead. 2009).
2.3.5. Solubility
Solubility influences the fate and behavior of nanoparticles in the environment (Wijnhoven et al..
2009b), as well as the dissolution of the nanomaterial and release of silver ions (Auffan et al.. 2009b).
Silver nanoparticles are composed of elemental silver (Ag°), which is not soluble or reactive in pure water
(Wiberg et al., 2001). but is soluble in acidic solutions (i.e., nitric acid); nanoscale particles have been
shown to dissolve completely in less acidic conditions than conventional silver particles (Elzey and
Grassian. 2010). Nano-Ag is also soluble in aqueous solutions under oxidizing conditions. The dissolution
of nano-Ag in aqueous solutions involves two coupled processes: (1) oxidation with release of reactive
oxygen species and (2) proton-mediated release of dissolved silver (Liu et al.. 2010a). The surface
oxidation of nano-Ag results in the formation of highly reactive ionic silver both adsorbed to the surface
of the nanoparticle and released to the surrounding milieu. Colloidal suspensions of nano-Ag will
therefore contain at least three forms of silver: nano-Ag particles, dissolved silver (both ionic silver and
soluble silver complexes), and ionic silver adsorbed to the surface of nano-Ag (Liu and Hurt. 2010).
Silver solids like silver sulfide and silver chloride also will form under certain environmental conditions
following release of nano-Ag to the environment, and these solid precipitates exhibit varying degrees of
solubility (see Table 2-2). Further, although silver chloride solids exhibit low solubility under normal
conditions and silver sulfide is considered to be nearly insoluble (Lide. 2000). unlike nano-Ag, silver salts
require no rate-limiting oxidation step to release silver ions to the surrounding medium (Gammons and
Yu. 1997).
The rate of dissolution also can be considered proportional to particle surface area; therefore, based
on surface area alone, smaller nanoparticles should dissolve faster than larger nanoparticles, and
nanoparticles in general should dissolve faster than conventional-scale materials (O'Brien and Cummins.
2009). Liu et al. (2010a) examined the release rate of soluble silver from nano-Ag of different sizes (4.8
and 60 nm) and from macroscopic silver foil. As expected, the mass-based release rates were inversely
proportional to particle size, with the first-order release rate constant for the 4.8-nm particle five orders of
magnitude higher than that for macroscopic silver. When the data were renormalized by surface area,
however, the variation in rate constants fell from five orders of magnitude to one order of magnitude,
demonstrating that in this scenario, surface area drives soluble silver release rates (Liuetal.. 2010a).
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Table 2-2. Solubility product constants for various silver solids.
Silver compound
Silver
Silver(l) sulfide (a-form)
Silver(l) sulfide ((3-form)
Silver(l) arsenate
Silver(l) iodide
Silver(l) phosphate
Silver(l) bromide
Silver(l) chromate
Silver(l) oxalate
Silver(l) carbonate
Silver(l) chloride
Silver(l) sulfate
Silver(l) bromate
Silver nitrate
Formula
Ag
Ag2S
Ag2S
AgsAsCU
Agl
AgsPCU
AgBr
Ag2Cr04
Ag2C204
Ag2C03
AgCI
Ag2S04
AgBrOs
AgNOs
Solubility product constant (Ksp)
Insoluble
6.69 x 10-50
1.09x10-«
1.03x10-22
8.51 x 10-"
8.88 x 10-"
5.35 x 10-«
1.12x10-12
5.40 x 10-12
8.45 x 10-12
1.77x10-1"
1.20x10-5 ^
5.34 x 10-5
Highly Soluble
Increasing Solubility
'w
Source: Lide (2000)
Particle concentration, surface morphology, surface energy, propensity for clustering, and other
properties are also relevant when considering dissolution at the nanoscale. In general, the dissolution rate
is higher for lower concentrations of nano-Ag; at higher concentrations, key factors that influence
dissolution, like available oxygen and presence of protons (i.e., pH), might be depleted, and high
concentrations of dissolved silver and ligands, which can inhibit surface reactions, might further impede
dissolution (Liu and Hurt. 2010). Liu and Hurt (2010) demonstrated that a low, environmentally relevant
concentration (0.05 mg/L) of citrate-stabilized nano-Ag (2-8 nm) undergoes complete oxidative
dissolution at room temperature in less than 2 weeks, while the highest concentration (2 mg/L) tested took
more than 4 months to completely dissolve under the same conditions.
Nanoparticle clusters might be more or less soluble, depending on how tightly they are clustered.
The equilibrium solubility of the system is inversely proportional to overall particle size. Therefore, if
clustering affects the surface area available to react with dissolved oxygen, the dissolution rate will
decrease, but if the total reactive surface area of the nanoparticles is preserved in the cluster, the
dissolution rate should differ little for the same concentration of homogenously dispersed nano-Ag (Liu
and Hurt. 2010: Borm et al.. 2006a).
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2.3.6. Surface Chemistry, Reactivity, and Coatings
One of the primary features of nano-Ag that distinguishes its behavior from that of ionic silver is
the reactive surface of the particle that enables nano-Ag to interact (i.e., complex) or react with chemical
and biological constituents in the milieu. As discussed in the previous section, oxidative dissolution of
nano-Ag takes place on the surface of the particle, leading to the steady, prolonged release of soluble
silver and reactive oxygen species. Nano-Ag also can undergo surface complexation with some negatively
charged ligands in the environment, which can variably increase or decrease its solubility and
bioavailability (Liu et al., 2010a).
The surface area of nano-Ag particles correlates with the availability of possible reactive sites and
chemical reaction potential, adsorption potential, and the potential to form clusters. The same is true for
surface chemistry, which dictates the important surface reactions (e.g., complexation, oxidative
dissolution) that drive nano-Ag behavior in complex media. Surface charge and the thickness of the
electric double layer on the surface of nanoparticles are important in determining the particle zeta
potential, which is one measure of stability (Nallathamby et al., 2008). These properties, in turn, can
affect the transport, behavior, interaction, distribution, bioavailability, and effects of nano-Ag particles in
the environment (O'Brien and Cummins. 2009; Wiesner et al., 2009). Surface charge also influences
particle stability in dispersions and overall solubility (Saves and Warheit 2009; Tiede etal.. 2008).
Surface charge is not the only factor that can be manipulated to control surface chemistry and
reactivity. Coatings of various chemical compositions can be added to the nanoparticle surface, which in
turn can influence particle behavior, including stability and overall persistence (Handy et al., 2008b;
Tiede etal., 2008). Some examples of coatings commonly applied to nanoparticles are provided in
Table 2-3. Nano-Ag is often coated with a surfactant, polymer, or polyelectrolyte (Lowry and Gasman.
2009). These coatings can impart charge to the particles (positive or negative) and stabilize them against
clustering and deposition (Nowack and Bucheli. 2007; Wiesner et al., 2006). For example, some nano-Ag
particles are engineered to remain in water as single particles, by adding a coating to improve their water
solubility and suspension characteristics, thus making them more water dispersible (Luoma, 2008). The
magnitude of the effect of the coating depends on the type and repulsive forces of the coating. Small-
molecular-weight coatings provide primarily electrostatic stabilization by imparting a surface charge to
the particle. These repulsive forces are fairly weak and are readily blocked by cations in solution. Large-
molecular-weight polymers (uncharged) can provide steric repulsions that stabilize particles against
clustering and enhance transport (Lowry and Gasman. 2009). In a study of nano-Ag disinfectant spray,
Kvitek et al. (2008) found that nano-Ag particles with different surface coatings varied with regard to the
amount of bacterial growth inhibition, based on the surface coating and the bacteria tested. Their findings
imply that the surface coating characteristics can influence bactericidal effectiveness of nano-Ag sprays
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Table 2-3. Types of common coatings of nano-Ag.
Type Coatings
Emulsifiers Bovine serum albumin (BSA)
Polysorbate 80
Polyvinyl alcohol (PVA)
Sodium citrate
Surfactants Cetyltrimethylammonium bromide (CTAB)
Polysorbate 80
Sodium dodecyl sulfate (SDS)
Ligands and Polymers Polyethylene glycol (PEG)
Starches/sugars
Polysaccharides
Polyvinylpyrrolidone (PVP)
Note: The listed coatings are examples mentioned in the literature summarized in Appendices B and C.
(see Section 6.1.1). As discussed in Section 3.2.2, however, the composition of nano-Ag surface coatings
is not always reported by manufacturers.
2.3.7. Conductive, Magnetic, and Optical Properties
Silver nanoparticles have been studied and characterized by material scientists extensively over the
past two decades. In addition to its biocidal effects, nano-Ag, like other noble metals such as copper and
gold, interacts strongly with electromagnetic radiation, which causes nano-silver to take on unique
conductive, magnetic, and optical properties. These properties facilitate the use of nano-Ag in
biomolecular labeling and detection and in other electronic sensor technologies. For example, silver
exhibits the highest surface plasmon resonance band among all metals, and only silver, gold, and copper
display this resonance in the visible spectrum (Wijnhoven et al., 2009b; Evanoff and Chumanov. 2005).
(A plasmon is a quantum of a collective oscillation of charges on the surface of a solid induced by a time-
varying electric field.) A high surface plasmon resonance means that the electrons on the surface of a
particle are highly interactive with electromagnetic fields, and as the surface plasmons resonate, the
energy can be detected, quantified, and, in the case of silver, seen in the visible spectrum. Several studies
have shown that these properties strongly depend on particle size, shape, spatial ordering, composition,
and surface properties (Evanoff and Chumanov. 2005; Temgire and Joshi. 2004; Kamat 2002; Murphy
and Jana. 2002; Jin etal.. 2001; Henglein. 1998).
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2.4. Analytical Methods to Characterize Nanoscale Silver
Accurate analytical methods can help improve understanding of the behavior and properties of
nano-Ag particles in various environmental media and can improve the characterization of exposure and
resulting impacts. The ability to monitor nanoparticles in various media, however, relies on sufficiently
sensitive instrumentation. Although measuring only the physicochemical properties that are relevant from
a risk assessment or risk management perspective might be desired, the mechanistic understanding of
biological effects of nanoparticles is still evolving and this list of physicochemical properties is in flux.
Furthermore, because physicochemical properties of nano-Ag are dynamic and depend highly on the
surrounding media, instrumentation for characterization in various environmental conditions would be
useful (Tiede et al., 2008). Isolating and recovering nano-Ag particles in sample matrices ranging from
animal and plant cells to soil and water could be an important step in the complete characterization of
nano-Ag particles.
A few recently published review articles summarize current techniques available to characterize
engineered nanomaterials (Jiang et al., 2009; Saves and Warheit 2009; Tiede et al., 2008; Mavnard and
Aitken. 2007; Powers et al., 2007; Powers et al., 2006). This section highlights some of the currently
available techniques used specifically in nano-Ag studies; this section, however, is not intended to present
a comprehensive literature review of nanomaterial characterization techniques. The reader is referred to
Appendix A and the review articles cited above for more information on such methods. Appendix A
contains summary tables that present limits of detection for the techniques listed and summarize some of
their advantages and disadvantages. Even so, these tables are not offered as definitive summaries of the
field and should be viewed as illustrations of the complexities in nanomaterial characterization.
2.4.1. Methods for Laboratory Research
Several laboratory methods and types of instrumentation are available to characterize nano-Ag (see
Appendix A). Although many of these methods are considered accurate techniques for characterization of
nano-Ag and incorporate state-of-the-science instrumentation, they are usually resource intensive and
require trained specialists. These methods are generally used to develop new synthesis methods, modify
surface properties, and study biological interactions at the individual particle level. Despite the existence
of numerous possible characterization methods, researchers face challenges in maintaining the
nanoparticles as they occur in products or environmental media during characterization and also in
determining the dose affecting tissues or cells.
Microscopy techniques (sometimes referred to as "single-particle imaging") such as scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) can be applied to study the size,
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shape, and morphology of individual particles or powder samples. These techniques are used in studies of
novel synthesis methods (Siekkinen et al.. 2006; Sun et al.. 2002; Sun andXia. 1991) where nanoparticles
are embedded in a matrix. Microscopic techniques might also be used to study the biocidal properties of
nanoparticles and to detect the presence and any localization of nanoparticles in biological structures. For
example, SEM and TEM studies have shown the presence of nanoparticles within bacterial cells
(Shrivastava et al.. 2007; Baker et al.. 2005; Morones et al.. 2005; Sondi and Salopek-Sondi. 2004).
alveolar macrophages (Carlson et al.. 2008). and HIV (Elechiguerra et al.. 2005). Microscopic analyses
can be conducted in conjunction with energy-dispersive X-ray spectroscopy (EDS) to determine chemical
composition; for convenience, many SEM systems come equipped with EDS.
Aerosolized nanoparticulate systems are often studied using ensemble methods. Whereas
single-particle imaging methods like microscopy analyze characteristics of individual particles in a
sample, ensemble methods convert a signal from the entire sample of particles into size or concentration
distributions. Such methods include laser diffraction (Powers et al.. 2006). dynamic light scattering (DLS)
(Murdock et al.. 2008; McMurry. 2000). centrifugal sedimentation, and impaction. Many ensemble
techniques are used to characterize particulate or aerosol systems and to study ultrafine atmospheric
particles, carbon nanotubes, and other nanoengineered materials.
In addition to analytical instrumentation, the development of characterization protocols also is
relevant so that results are consistent, reproducible, and reliable. Sayes and Warheit (2009) suggest such
protocols emphasize that characterization data for the material should be assessed in the biologically or
environmentally relevant media, in the most dispersed state possible, and using more than one method.
Although single-particle imaging and ensemble methods are important tools in detecting and identifying
nanoparticles, each individual technique has its own advantages and limitations. For example, although
DLS can detect and characterize nanoparticles at environmentally relevant concentrations, including in
aerosol samples, the accuracy of DLS decreases with broader size distributions (Powers et al.. 2006).
Additionally, although electron microscopy methods can definitively identify small nanoparticles in
simple matrices, these techniques only work with concentrations that are higher than those that are
expected to occur in the environment, and they cannot be used for aerosol samples. Multiple or
orthogonal methods are therefore often employed in tandem when evaluating particle characteristics to
combine the respective strengths and to counterbalance the respective weaknesses of different techniques
(Powers et al.. 2006). The selection of which methods to use together is therefore an important step in
characterizing the materials, as different methods can affect measurement results in unique ways
(MacCuspie et al.. 2011). MacCuspie et al. (2011) compared size range distributions for commercially
available nano-Ag measured using multiple techniques and show distinct variation depending on several
factors, including the methods employed, parameterization, and the length of time between dispersing
materials in solution and characterizing the material. The authors recommended the use of multiple
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orthogonal measurement techniques to facilitate interlaboratory comparisons and collaborations to
characterize nanomaterials fully (MacCuspie et al.. 2011).
2.4.2. Methods to Assess Environmental Occurrence
Detecting nanoparticles in the environment (particularly the natural environment) is challenging
because available analytical methods often are not sufficiently sensitive at environmentally relevant
concentrations and cannot distinguish natural materials in the nanoscale size range from manufactured
nanomaterials (Domingos et al., 2009b; Simonet and Valcarcel. 2009). Also, many analytical methods
require sample treatment and solvent evaporation, which could cause nanoparticle clustering and
precipitate formation (Simonet and Valcarcel 2009). Detecting nanoparticles in water or soil is further
complicated by the heterogeneous nature of the sample matrix and the clustering tendencies of the
nanoparticles. Making such measurements in situ would help address physical and other changes in
nanoparticles due to different conditions in the immediate medium, but portable equipment with sufficient
sensitivity has not yet been developed (Simonet and Valcarcel 2009). Although traditional methods to
measure metals in samples (e.g., atomic absorption furnace methods) cannot differentiate between
conventional silver and nano-Ag, these traditional methods can be coupled with other methods to confirm
and quantify the presence of nano-Ag in a sample. Methods also can be coupled to enable detection of
more than one parameter simultaneously. For example, field-flow fractionation can be coupled with
inductively coupled plasma-mass spectrometry for both size and chemical analyses.
To illustrate the variety of methods available to assess nanomaterials in environmental matrices, a
sample of available methods for analyzing nanomaterials in soil, sediment, and ground water is shown in
Table 2-4.
In a study comparing six analytical methods for determining nanoparticle sizes (TEM, atomic force
microscopy, DLS, fluorescence correlation spectroscopy, nanoparticle tracking analysis, and field-flow
fractionation), Domingos et al. (2009a) concluded that the two most commonly used techniques in the
literature (TEM on air-dried samples and DLS) were also the two that appear to be most prone to artifacts.
Their recommendation was to use multiple analytical techniques or multiple preparation techniques, or
both.
Several recent studies have employed multiple methods to characterize crystal structure, particle
size, and morphology of nano-Ag particles in biological matrices. For example, Laban et al. (2009)
coupled TEM with electron diffraction to verify that the particles detected within the embryos of fathead
minnows were nano-Ag particles. Similarly, Asharani et al. (2008) combined TEM analysis with EDS to
confirm the presence and location of nano-Ag particles in zebrafish embryos.
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Table 2-4. Analytical methods for nanomaterials in soil, sediment, and ground water for size
fractionation and distribution, surface area, and phase and structure.
Metric
Analytical method
Notes
Size fractionation Centrifugation
Ultrafiltration—direct-flow ultrafiltration or tangential-flow ultrafiltration (IFF)
Field-flow fractionation (FFF)
Capillary electrophoresis (CE)
Size exclusion chromatography (SEC)
Analyze aquatic colloids and particles
extracted from soil and sediment
samples. Nanoparticles must be in
solution.
Size distribution Transmission electron microscopy (TEM)
Scanning electron microscopy (SEM)
Scanning probe microscopy (SPM)
Dynamic light scattering (DLS)
Laser-induced breakdown detection (LIBD)
Small- and wide-angle X-ray scattering (SAXS/WAXS)
In most cases, samples analyzed by
electron microscopy will be destroyed
and cannot be analyzed by another
method (TiedeetaL 20081.
Surface area BET (Brunauer, Emmett, Teller method of calculating specific surface area)
Calculation from TEM (length and width) and atomic force microscopy (AFM) (height) Only nanoparticles with a regular or
measurements, and particle nanocrystalline geometries pseudo-regular geometry and without
significant porosity
Phase and structure Electron diffraction
X-ray diffraction (XRD)
X-ray absorption spectroscopy (XAS)
Raman spectroscopy
High-resolution transmission electron microscopy (HR-TEM)
XRD and XAS are nondestructive
techniques (TiedeetaL, 20081.
Source: Adapted from U.S. EPA (2009d).
2.4.3. Methods to Assess Workplace Occurrence
The potential for workplace exposure to nano-Ag exists during all manufacturing stages of
nano-Ag and products containing nano-Ag, as well as during certain recycling and disposal stages. The
monitoring of a specific nanomaterial poses several challenges, due to the presence of background
particulate matter generated from other activities that typically occur at industrial sites. Such activities
include combustion processes, metal operations where vapors can condense (e.g., soldering, welding,
smelting), and mechanical processes (e.g., grinding, blending) (Ono-Ogasawara et al.. 2009). Although
standardized protocols exist for monitoring the suspended particulate matter at workplaces, they do not
distinguish between ultrafine particles and nanoparticles.
Analysis of workplace exposure thus far has focused on measuring nanoparticles in the air.
Instruments that can be used for aerosol sampling are available, but most are designed for laboratory use
(Nanosafe. 2008) and lack one or more of the following attributes: portability, ease of use, capacity to
distinguish nanoparticles from non-nanoparticles, different size bins in the 1- to 100-nm range, or ability
to sample personal breathing zones (Ostraat. 2009). Engineered nanoparticles can be measured in the
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workplace using a variety of instrumentation, including condensation particle counters, optical particle
counters, fast mobility particle sizers, scanning mobility particle sizers (SMPS), electrical low pressure
impactors, aerosol diffusion chargers, and tapered element oscillating microbalances.
Several studies have characterized nanoparticles at manufacturing facilities using various analytical
methods. Thus far, however, only one study has been identified that characterized nanoparticles at a
nano-Ag production facility. Park et al. (2009) used an SMPS to measure the size of particles in ambient
air at a manufacturing facility using liquid-phase processes to produce silver nanoparticles. Also,
electrostatic precipitators were used to collect particles on TEM grids to analyze surface morphology.
Because analytical instrumentation and techniques for measuring mass and number concentrations
of other nanomaterials could be used directly or adapted to characterize silver nanoparticles, a few recent
studies characterizing other nanomaterials at manufacturing sites are mentioned here. Fujitani et al.
(2008) characterized fullerenes at a manufacturing facility using an SMPS, optical particle counter, and
SEM during non-work periods, work periods, and an agitation process. Similarly, Demou et al. (2008)
quantified real-time size, mass, and number concentrations using an SMPS and a condensation particle
counter at a pilot plant producing metal oxide nanostructures.
Given the active research in both academic and commercial laboratories to develop new
nanomaterial-based technologies, the potential exists for laboratory workers to be exposed. Tsai et al.
(2009) sampled and characterized the ambient air from laboratory hoods using a fast mobility particle
sizers and an SEM. These analyses were performed during the handling (i.e., pouring or transferring with
a spatula) of nano-alumina and nano-Ag (Tsai et al.. 2009). Inside the fume hood, the researchers
observed a shift in the mean particle diameter of the originally spherical nano-Ag particles from around
60 nm to 150 nm, indicating clustering of the particles during handling. Based on testing with a 100-gram
(g) sample and a 15-g sample of nano-alumina, the researchers concluded that working with smaller
quantities of sample decreases the concentration of particles entering the laboratory space from the fume
hood by approximately 20%.
In recent years, several governmental and environmental organizations have voiced a need for
methods and protocols to monitor nanomaterials in the workplace. For example, the National Institute for
Occupational Safety and Health recently published a document entitled Approach to Safe
Nanotechnology—Managing the Health and Safety Concerns Associated with Engineered Nanomaterials
(NIOSH. 2009) in which sampling and monitoring methods and equipment are discussed. The
Nanoparticle Occupational Safety and Health Consortium, an industry-led consortium of participants
from academia and governmental and nongovernmental organizations, is helping to define best practices
for working safely with engineered nanoparticles (NOSH Consortium. 2008). The Nanoparticle
Occupational Safety and Health Consortium has developed portable air monitoring methods suitable for
daily monitoring in nanoparticle research and development and in manufacturing settings. In 2008, the
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NanoSafe2 project, a European Community-sponsored project for safe production and use of
nanomaterials, released a report that highlighted findings in measurement methodologies for nanoparticle
detection and measurement with various types of online and offline monitoring instruments (Nanosafe.
2008). The report provides examples of new nano-aerosol measurement equipment that is easy to
transport and use. No commercially available equipment, however, is currently available for long-term
monitoring. The report also recommends that monitoring at workplaces include not only personal
sampling and measurements inside the facility, but also measurements of nanomaterials in drains and in
the exhausted air to help ensure protection of the environment. Finally, several companies are developing
or have developed air monitoring devices for nanoparticle detection; the parameters that each device
measures vary (TRS Environmental. 2009; vandenBrink. 2008; Bennett. 2005).
2.4.4. Methods for Quantifying Dose and Dose Metrics
Researchers and risk assessors often quantify dose12 in terms of mass (e.g., in micrograms per cubic
meter [ug/m3] for inhalation exposures, or in mg/kg-day for ingestion exposures), but, for some
substances, mass might not be the physical parameter most closely correlated with biological response.
For example, dose of asbestos fibers is typically characterized by number concentration (i.e., number of
particles in a specific quantity of exposure medium) of fibers of specific shape and composition (Maynard
and Aitken. 2007). For nanoparticles, dose can be measured in terms of number concentration (e.g., the
number of particles inhaled per volume of air) and surface area concentration (e.g., the surface area of the
particles inhaled per volume of air, square meters per cubic meter[ m2/m3]) in addition to mass
concentration.
In some respects, using mass as the primary metric for characterizing dose is an attractive option
for nanoparticles; for example, measuring mass for a pollutant is standard procedure, and instrumentation
for conducting such measurements is widely available (Maynard and Aitken. 2007). Oberdorster et al.
(2005a) suggested that measuring mass concentrations for inhalation or intratracheal instillation studies of
nanomaterials and conducting gravimetric and chemical analyses of filter samples can provide relatively
accurate dose characterization when compared to surface area or particle number metrics. In some cases,
estimating particle surface area or number concentration from measures of mass concentration also might
be possible based on the estimated diameter of the particles; however, when the distribution of particle
sizes is wide or the number of very large particles is great, using mass concentration to calculate number
12The term dose is used in several ways across the risk assessment community. Here, dose refers to "potential dose,"
as defined by the EPA Integrated Risk Information System (IRIS): "The POTENTIAL DOSE is the amount
ingested, inhaled, or applied to the skin." (http://www.epa.gov/ncea/iris/help gloss.htm).
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concentration could be unreliable. Other potential drawbacks to using mass as the primary metric for
characterizing dose are also apparent. Nanoparticles have been shown in many cases to be more toxic
than larger particles with the same chemistry; specifically, researchers have shown that the toxicity of
insoluble materials increases with decreasing particle size on a mass-to-mass basis (Mark. 2007). This
might occur because the increased specific surface area of nanoparticles compared to larger particles
elevates the potential for biological reactions more than mass alone would predict (Oberdorster et al..
2QQ5a).
Evidence suggests that particle number might be highly correlated with health effects and might be
a relevant dose metric. Wittmaack (2007) found for titanium dioxide that particle number is the dose
metric that correlates best with pulmonary inflammation response. Although devices to count particles are
available, even the most complex and powerful detection units are limited to the detection of particles
with diameter of about 10 nm or greater (Maynard and Aitken. 2007). These instruments also tend to be
very expensive, which could preclude the use of this technology to obtain information about particle size
or size distribution. Tsuji et al. (2006) also questioned the value of measuring only particle number and
using this as a dose metric because particle number does not necessarily correlate with health effects as
well as other dose metrics.
Surface area might be another appropriate metric for characterizing dose (Oberdorster et al.. 2007;
Tsuji et al.. 2006; Oberdorster et al.. 2005b). In general, the increased surface area of nano-sized particles
can change their chemical reactivity, bioavailability, and the biological responses they can induce
(Luoma. 2008; Mark. 2007); thus, surface area concentration might be highly correlated with response.
Direct, real-time measurement of particle surface area has become possible in recent years. One device,
the Nanoparticle Surface Area Monitor (TSI Model 3550), filters only particles that deposit in the alveolar
or thoracic region of the respiratory system (Maynard and Aitken. 2007). Such measurement systems,
however, are not yet cost-effective.
Although identifying a single dose metric that best reflects risk might be desirable, the toxicity and
reactivity of nanoparticles appear to be functions of multiple factors, including surface area, number,
shape and size, and composition. For this reason, Maynard and Aitken (2007) suggested that different
metrics (e.g., particle number concentration, surface area concentration, mass concentration, or length
concentration) might be selected for different aerosol sprays depending on factors such as these.
Similarly, Oberdorster et al. (2005a) suggested that mass, surface area, and particle number are essential
dose metrics for nanoparticles and that, when possible, dose should be characterized by all three
measures.
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2.5. Summary of Physicochemical Properties and
Analytical Methods
The size, morphology, surface area, chemical composition, surface chemistry and reactivity, and
solubility of nano-Ag particles are all thought to play a role in determining their use and effectiveness in
commercially available spray disinfectant solutions, behavior in the environment, and human and
ecological exposure potential and toxicity. These properties are interdependent, however, and can change
as nano-Ag particles are dispersed in different solutions, move between environmental compartments, and
are transported within living organisms. Adequate characterization of nano-Ag could help when
evaluating potential risks associated with its use in disinfectant sprays, and the chapters that follow
highlight the physicochemical properties of nano-Ag that might be pertinent at each stage of a CEA.
Sensitive analytical methods underpin characterizing the presence and physicochemical properties
of nano-Ag in the laboratory, natural environment, workplace, and living organisms. Laboratory methods
such as spectroscopy, chromatography, electron microscopy, and spectrometry help researchers
characterize nano-Ag and its interaction in environmental media (e.g., in water, air, sediment, or soil),
within organisms, and in cells. These same methods cannot necessarily be used to detect and characterize
nano-Ag outside of the laboratory for several reasons. Instruments are not easily portable and are
expensive; environmentally relevant nano-Ag concentrations can occur below current method and
instrument detection limits; and the methods cannot yet consistently distinguish between naturally
occurring nanoparticles and engineered nanoparticles such as those used in nano-Ag spray disinfectants.
Additional information on analytical methods is presented in Chapters 4, 5, and 6 regarding specific
methods used to study the transport, transformation, and fate processes of nano-Ag, the potential exposure
of humans and biota to nano-Ag, and the effects of such exposure.
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Chapter 3. Life-Cycle Stages
The first step in a comprehensive environmental assessment is to examine the life-cycle stages of
the nanoscale product. This chapter provides a description of information available about the life cycle of
nano-Ag spray disinfectant products to support the discussions in the chapters that follow about
environmental transport, transformation, and fate processes; potential exposure pathways for humans and
biota; and possible effects resulting from exposure to nano-Ag. As noted in a recent Federal Insecticide,
Fungicide, and Rodenticide Act Scientific Advisory Panel review of issues related to nano-Ag in
pesticides (201 Ob), any use of nano-Ag as a pesticide (including as a disinfectant) could cause nano-Ag or
a related by-product derived from such use to enter the environment. In the environment, the potential
exists for human and biotic exposure. For this case study, by-products comprise material waste from
feedstock processing and manufacturing, secondary pollutants formed through chemical or other
transformations of primary pollutants, and, within organisms, metabolic products derived from primary
toxicants (Davis. 2007).
3.1. Feedstocks
Anthropogenic sources of silver emissions into the environment include:
• mining, smelting, and coal combustion operations;
• manufacturing, use, and disposal or recycling of products containing silver; and
• waste discharges from mining operations, industrial processes, and wastewater treatment
facilities (Purcell and Peters. 1998).
A report commissioned by The Silver Institute (2009b) estimates that 76.6% of the world's 2008 silver
supply of 27,631,905 kilograms (kg) came from mining, with North America and Latin America leading
the world's mining (GFMS Limited. 2009). Approximately 28% of this amount was mined in operations
where the main revenue source is silver. Of the remaining mined supply, 37% was recovered from lead
and zinc mining operations, and the rest from gold (11%), copper (23%), and other metal (2%) mining
operations. These ores are typically mined using open-pit or underground methods and are enriched using
flotation and smelting processes. Silver metal is extracted electrochemically using the Parkes, Moebium,
or Balbach-Thum process. In a silver mass flow analysis conducted by Johnson et al. (2005). the authors
estimated that 20% of the ore mined for silver enters the environment through mine tailings, although the
authors suggest that further research could refine this estimate.
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The remaining silver in the world's 2008 supply came from other net government sales (3.5% in
2008) and the recycling of silver scrap (19.9% in 2008), including silver recovered from jewelry,
photographic chemicals, discarded computers, and other manufactured products that originally contained
silver components (GFMS Limited. 2009). The annual survey of The Silver Institute (2009a) states that
95% of annual silver consumption is for industrial, photographic, and jewelry applications.
What percentage of the total silver feedstock supply is used to produce nano-Ag is not clear.
Mueller and Nowack (2008) state that worldwide nano-Ag production could be as high as 5% of total
silver production, and a "best guess" for the worldwide production of nano-Ag based on this assumption
is 500,000 kg per year (approximately 550 tons per year). To form their estimate, Mueller and Nowack
reviewed data collected by survey and personal communications about the quantity of nano-Ag
manufactured in Switzerland and extrapolated from this estimate to apply to the entire world. As
discussed in Section 2.2, only one study to date—Hendren et al. (2011)—has employed a systematic
approach to estimating a range of current nano-Ag production volumes in the United States, and that
range spans an order of magnitude (2.8-20 tons per year).
3.2. Manufacturing
Manufacturing procedures for nano-Ag are generally proprietary. For example, the Top Nano
Technology company website advertises that the company has "kilogram-scale manufacturing" capability
for nano-Ag, but information about their manufacturing process is not accessible through the U.S. Patent
Office database, the company website, or written inquiries to the company (Chou. 2010). A search of the
U.S. Patent Office database did reveal some patented, company-specific manufacturing processes, so
those processes, and others described in the peer-reviewed literature, are incorporated into the section that
follows. In general, limited data were identified on specific points of release or the quantity of nano-Ag
released as a result of the manufacturing process. This lack of data is consistent with statements included
in the report on nano-Ag from the Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory
Panel (201 Ob).
The Project on Emerging Nanotechnologies (2009) reports that, as of 2009, the companies
producing spray disinfectant solutions that purportedly incorporate nano-Ag include American Biotech
Labs, ConSeal International, Inc., Daido Corporation, GNS Nanogist Co., Lion Corporation, Shanghai
Huzheng Nanotechnology Co. Ltd., Skybright Natural Health, and Top Nano Technology. This list,
however, is not comprehensive, as manufacturing of products potentially containing nano-Ag continues to
expand. Furthermore, the Project on Emerging Nanotechnologies notes that selection of products for
inclusion on the list and the information presented on specific products is based on data that are publicly
available on company websites, and none of the information has been independently verified (2009).
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3.2.1. Synthesis of Silver Nanoparticles
Silver nanoparticles can be synthesized using wet-chemistry methods (chemical reduction), laser
ablation, radiolysis, and vacuum evaporation methods. Krutyakov et al. (2008) and others have published
comprehensive reviews discussing the strengths, drawbacks, and challenges of available nanoparticle
synthesis methods. Tolaymat et al. (2010) searched the scientific literature to collect information about
how nano-Ag is synthesized. Relying on the synthesis methods described in nearly 200 papers, they
concluded that the synthesis of nano-Ag particles almost exclusively produces spherical elemental silver
particles with a diameter of less than 20 nanometers (nm). In the synthesis of nanoparticles, controlling
more than a single factor is difficult without altering other variables. For example, as particle size is
manipulated, other characteristics, such as crystal structure and shape, could be altered unintentionally as
a result.
Chemical reduction of silver ions is the primary method for rapidly producing large quantities of
silver nanoparticles (Zhang et al.. 2007a). and indeed Tolaymat et al. (2010) reached this same conclusion
based on their review of the literature on the synthesis of nano-Ag. The chemical reduction of transition
metal salts to generate zero-valent particles (not necessarily nanoparticles) was first described by Faraday
in 1857 (Bonnemann and Richards, 2001). Following this early discovery, Carey Lea described the
reduction of silver nitrate (AgNO3) in the presence of trisodium citrate (1889). which was subsequently
extended to gold nanoparticles by Turkevich et al. (1951) by reducing chloroauric acid with sodium
citrate.
With the use of a reducing agent, silver ions (Ag+) in solution are reduced from a positive valence
to a zero-valent state (Ag°) (Zhang et al., 2007a). Because zero-valent silver tends to cluster, a primary
challenge is to maintain the nanoparticles in the desired size range. Controlling size range is generally
accomplished by using surface coatings such as surfactants, polymers, or stabilizing ligands (Zhang et al..
2007a). The choice of reducing agent and the order and rate of mixing can alter the rates of nucleation and
particle growth (Bonnemann and Richards. 2001). Thus, particle size and dispersion can be controlled by
altering the synthesis process. Manipulation of laboratory conditions also enables the shape of the
nanoparticles (e.g., rods, wires, disks, spheres) to be controlled (Yu and Yam. 2004; Sun andXia. 2002).
There are many variations on the basic theme of chemical reduction. The common elements,
however, are AgNO3 as a feedstock, an aqueous solvent or a nonpolar solvent, a reducing agent, and a
stabilizing agent, most often a surfactant (Goia and Matijevi. 1998). When AgNO3 is used as a feedstock,
nitrate (NO3) will likely result from the reaction and can then be a by-product of concern (Tolaymat et al..
2010). In their review of nano-Ag synthesis methods reported in the literature, Tolaymat et al. (2010)
found that AgNO3 is the most commonly reported silver salt precursor used in synthesis. Reduced metal
atoms are insoluble and thus tend to cluster, eventually forming solid particles. The driving force behind
the reduction is the difference between the reduction and oxidation (redox) potentials of the two half-cell
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reactions (Goia and Matijevi. 1998). The system achieves greater oversaturation as the redox potential
increases. For instance, sodium borohydride (NaBFL,) is a relatively strong reducing agent compared to
ascorbic acid, and as such, a larger redox potential is attained with NaBFL,. The larger redox potential
results in a more rapid reaction with more nuclei forming, thus resulting in smaller particles, as the
available silver is distributed among many nuclei. If a weaker reducing agent is used, such as ascorbic
acid, the reaction rate can be increased by elevating the temperature. If larger particles are desired, a
weaker reducing agent is used because it will cause a slower reaction and lead to the formation of fewer
nuclei. The available silver will be consumed by the smaller number of particles, resulting in a larger final
particle size. Further growth can occur by continued addition of metal atoms, leading to crystals with a
regular shape and few internal grain boundaries. If aggregation takes place, the resulting particles will be
spherical and polycrystalline, with internal grain boundaries. Both mechanisms can occur in the same
system.
Examples of other mild reducing agents are sodium citrate (Lee and Meisel. 1982) and sugars
(Panacek et al.. 2006; Kvitek et al.. 2005). An advantage of using sodium citrate is that it has low toxicity
compared to stronger reducing agents such as sodium borohydride. A disadvantage of sodium citrate is its
lower reduction activity, which necessitates higher temperatures and results in longer reaction times
(Zhang et al.. 2007a). Kvitek et al. (2005) and Panacek et al. (2006) used sugar as the reducing agent and
ammonia as the complexing agent to form diamminesilver cation [Ag(NH3)2+]. By controlling ammonia
concentration and varying the type of sugar, the researchers could control particle size. The redox
potential for Ag(NH3)2+ is lower than that for silver ions, and the ensuing slower reaction leads to fewer
nuclei and larger particles. Kvitek et al. (2005) obtained particles ranging from 45 to 380 nm. Panacek et
al. (2006) obtained particles in the 25- to 450-nm range.
Leopold and Lendl (2003) used a stronger reducing agent, hydroxylamine hydrochloride
(NH2OH.HC1), combined with a solution of AgNO3. According to the researchers, the advantages of this
method are a rapid reaction rate, a narrow particle size distribution, reliably reproducible results, and the
ability to carry out the process at room temperature. The size distribution of the particles can be controlled
by changing the order and rate of mixing of the reactants. The adjustments of these variables that Leopold
and Lendl (2003) used yielded spherical particles with an average diameter that varied from 23 to 67 nm.
The choice of stabilizing agent also influences the final product. A method by Sun et al. (2005)
using 4-(2-hydroxyethyl)-l-piperazineethanesulfonic acid (i.e., HEPES buffer) at pH 7.4 produced "face-
center-cubic phase" particles between 5 and 20 nm, with a mean diameter of 10 nm. In this study, human
serum albumin was used to stabilize the particles, rendering them suitable for study of their ability to
inhibit HIV growth. Niskanen et al. (2010) developed a method to stabilize silver nanoparticles through
the use of amorphous copolymers of acrylic acid and butyl acrylate-methyl methacrylate. This specific
polymer produced 4-nm particles that were then aged to obtain relatively monodisperse particles of
approximately 12 nm. The acrylic acid was shown to form a hydrophilic layer on the silver nanoparticles,
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thereby promoting dissolution of silver ions while individual nanoparticles remained attached to the
coating via thiol-silver bonds. Hu et al. (2008) developed a method to produce very small nano-Ag
particles by using polyacrylic acid (PAA) as a surfactant. The authors noted that the carboxylic groups in
the PAA bonded well to the silver nanoparticles, effectively limiting their growth. Their method uses
polyols as both the solvent and the reductant, and the synthesis is performed at the boiling temperature of
the solvent. Through careful selection of the PAA chain length and the specific polyol (ethylene glycol,
diethylene glycol, or triethylene glycol), the authors could produce particles <10 nm with good
dispersion.
Repeatability and reproducibility of nano-Ag synthesis can be augmented by other techniques, such
as microwaving or sonication. Yin et al. (2002). for example, developed a method to synthesize nano-Ag
based on the Tollens method for electroless plating. The researchers mixed a solution of AgNO3 with an
activator (sodium hydroxide) and a reducing agent (formaldehyde and sorbitol). This procedure was
carried out in a sonicator (ultrasound bath), which the researchers believe promoted a more uniform
concentration profile, leading to a narrow size distribution in the samples.
Zhang et al. QOOTa) presented a detailed discussion of the micro-emulsion method. This type of
synthesis involves either a mixture of water, surfactant, and oil, or a mixture of water, surfactant,
co-surfactant, and oil. The system can involve oil micelles in water, or water micelles in oil. The
discussion by Zhang et al. (2007a) focused on water-in-oil micro-emulsion, also referred to as "reverse
micelles." The basic principle is that the reduction reaction takes place in water droplets, which are
covered by surfactant molecules. As the particles enlarge, the surfactant molecules then become adsorbed
to the particle surfaces, halting further growth and preventing them from forming clusters. The diameter
and shape of the particles can be controlled by the size and shape of the droplets. This technique can
produce relatively stable, small particles (2-5 nm average diameter), with a narrow size distribution. To
initiate the reaction, two micro-emulsions are mixed—one carrying the silver salt and the other carrying
the reducing agent. The micelles coalesce, and the reactants mix.
In addition to serving as reactors, the reverse micelles act as templates for the shapes of the
nanoparticles (Zhang et al.. 2007a). A low surfactant concentration gives rise to spherical particles. Higher
concentrations can produce rods or columns. In a mixed cationic-anionic surfactant solution, "worm-like"
micelles form, producing nanowires. The reducing agent, water content, AgNO3 concentration, and chain
length of the alkane used as the solvent also influence particle morphology. Despite achieving tight
control of particle size and flexibility in shape, this method has several disadvantages, including
potentially high expense because it requires large amounts of surfactants and solvent. Also challenging is
the removal of the surfactants and solvent from the nanoparticles and nanoparticle products. As
mentioned previously, manufacturers of nano-Ag and the spray products to which they are added often do
not report—and in some cases do not characterize—the composition of the surface-active agents in their
products. As a result, products as-manufactured might contain multiple uncharacterized substances, which
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presents a challenge to evaluating as-manufactured materials systematically. Furthermore, stable
dispersions are obtained only at low concentrations. Because of these disadvantages, the authors noted
that the method is not currently suitable for large-scale manufacturing (Zhang et al.. 2007a).
Wet-chemical methods often leave trace amounts of reducing agents or surfactants on silver
nanoparticle surfaces (Amendola et al.. 2007). Some unconventional methods have therefore been
developed for applications requiring pure nanoparticles. For example, laser ablation of pure silver
immersed in a solvent generated nanoparticles with a logarithmically normal particle size distribution
(Amendola et al.. 2007). In an earlier study, laser ablation of conventional silver in aqueous sodium
dodecyl sulfate solution generated silver nanoparticles for which the size could be increased by using
more radiation power or decreased by adding more surfactant (Mafune et al.. 2000).
Researchers are also investigating the possibility of using plants or other biota to synthesize
metallic nanoparticles, including nano-Ag (Harris and Bali. 2008). Other nontraditional "green" chemistry
techniques for nano-Ag synthesis also have emerged recently, including the use of purified rhamnolipids
from bacteria (Kumar et al.. 2010). incubation of proteobacteria with silver nitrate (Suresh et al.. 2010).
modification of the Tollens process by UV-irradiation, glucose reduction, and use of nontoxic chemicals
(Le et al.. 2010). Organic waste (specifically banana peels) rich in lignin (Bankar et al.. 2010) has been
used, as have flavonoid-rich tea extracts to synthesize nano-Ag particles while protecting cells from
reactive oxygen species (Moulton et al.. 2010).
Several relatively recent patents have been granted for silver nanoparticle synthesis. Although the
information obtained from the patents does not generally indicate which, if any, have been adopted for use
in large-scale manufacturing, the patents do provide perspective on potential developments. Available
descriptions, however, are brief. For example, Oh et al. (2003) proposed a method to produce silver and
silver-alloyed nanoparticles in a surfactant solution. As with other similar methods, the technique involves
using a reducing agent to produce silver nanoparticles from a silver salt solution; the use of the surfactant
in this method presumably controls nanoparticle size. Holladay et al. (2004). who are associated with
American BioTech, a nano-Ag spray disinfectant manufacturer, hold a patent that describes a method for
generating nano-Ag particles between 5 and 20 nm by immersing electrodes (including silver-coated
wires) in a 15-gallon plastic container filled with water. The patent claims that the silver particles are
dispersed evenly through the water by the rotation of an impeller in the container.
Despite the lack of details associated with nano-Ag patents, assuming that the bulk of nano-Ag is
produced using wet-chemistry methods involving liquid-phase materials and processes is reasonable
because of the inherent disadvantages of other synthesis methods. A recently published study
characterized airborne silver nanoparticles inside a manufacturing facility located in Korea (Park et al..
2009). The authors of this study assume that wet-chemistry methods result in less inhalation exposure to
nanoparticles than gas-phase reactions but that inhalation of aerosolized particles from liquid-phase
processing is not negligible. This large manufacturing facility, producing 3,000 kg of silver nanoparticles
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per month, uses AgNO3 as feedstock and employs a chemical reduction method. The manufacturing of
nano-Ag occurs in four stages: The chemical reduction step is followed by filtering, drying, and grinding
stages. Real-time monitoring and sampling of silver nanoparticles using scanning mobility particle sizer
and long differential mobility analyzer techniques at the facility indicated that the highest concentration of
airborne nanoparticles occurred after the reaction stage, when some aerosolized nano-Ag particles were
released into the air. The researchers also noted that particles deposited on the floor and other surfaces
following the release of particles to the air at the end of each stage. The study authors did not attempt to
quantify emissions data on a mass-released-per-mass-produced basis. More details on the potential
occupational exposure scenarios implied by this study are included in Section 5.3.2.
3.2.2. Manufacturing of Nano-Ag for Disinfectant Sprays
The production, characterization, and handling of nano-Ag for use in disinfectant sprays require
specialized technical expertise, and the available data suggest that this expertise might be developed
in-house at companies that manufacture nano-Ag disinfectant sprays (Sawafta et al., 2008; Holladay et al..
2004). Sawafta et al. (2008) described a "nanocomposite" of at least two metals, including silver, in
solution where the metal nanoparticles are created either by "mechanical/physical size reduction
processes" or "co-precipitation processes." Size reduction is further described with steps including
grinding, pulse laser evaporation, sonication, and sorting by centrifugation or magnetic separation.
Alternatively, production of nano-Ag might occur at facilities that exclusively produce nano-Ag and other
engineered nanomaterials in large volumes, which are then sold to manufacturers of spray disinfectants.
When contacted, ConSeal International, Inc. reported that they produce and sell more than 2,500 gallons
annually of their nano-Ag disinfectant spray, NanoSil, which they claim contains nano-Ag (Gilmore.
2010); they did not specify, however, whether the nano-Ag in their disinfectant spray was manufactured
on site or purchased from another source.
Based on available patent data and data presented on company websites (unverified), some major
operations that manufacture spray bottle products containing nano-Ag involve mixing various ingredients,
mechanical or chemical processes to achieve uniform product consistency, filtration processes to remove
impurities, intermediate storage of the prepared bulk spray product in tanks, and finally automated
dispensing into bottles (ConSeallnternational, 2010; Park et al., 2009; Shanghai Huzheng
Nanotechnology Co.. 2009: Sawafta et al.. 2008: Holladav et al.. 2004). Thermal heating or cooling
steps also can be involved, depending on the ingredients, spray formulation, and desired properties.
Although bulk spray liquid can be produced in batches in mixing vessels and transferred to intermediate
storage, dispensing and packaging can be accomplished using continuous, automated processes.
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Individual bottles, sealed after completion of quality control, might be packaged into cardboard cartons
for distribution and retail sales.
A 2008 U.S. patent application (Sawafta et al.. 2008) describes a metallic nanocomposite
synthesized for its biocidal properties. As explained in the application, to create a spray disinfectant, the
nanocomposite can be combined with hypochlorite or another chlorine-releasing compound;
chlorhexidine or another biguanide molecule with the chemical formula C2H7N5; quaternary ammonium
salts commonly used as germicides, disinfectants, and sanitizers; or other ionic liquids, surfactants, soaps,
or detergents. The information included in the patent application supports the likelihood described above
that nano-Ag manufacturing processes will require mixing a variety of ingredients in a potentially wide
range of ways. The composition of coatings, residual impurities from the synthesis process, and
proprietary ingredients in the spray disinfectant product might not be disclosed—or even identified—by
manufacturers. As a result, the degree to which the different nano-Ag powders and suspensions and spray
disinfectant products differ among manufacturers and between batches is unknown.
No data are currently available on nano-Ag releases from the manufacturing of the spray
disinfectant product or on air concentrations in facilities that manufacture these products. During the
manufacture of nano-Ag sprays, releases of nano-Ag, other spray ingredients, or by-products of the spray
could occur. Preliminary handling of large quantities of nano-Ag prior to creating the spray disinfectants,
such as unpacking, sampling for quality control, measuring, and transporting nanoparticles, could lead to
release of nano-Ag to the air or surfaces in the facility. Depending on the quality of packaging and storage
conditions at facilities where manufacturers acquire nano-Ag in large volumes and stockpile the raw
material for extended periods, nano-Ag and associated substances might be released to ambient air.
Similarly, mechanical processes such as mixing, grinding, or agitation of liquids can cause nano-Ag to
escape to the ambient air. As Park et al. (2009) demonstrated, wet-chemistry handling processes also can
emit nano-Ag to the air. Once bulk spray is produced, other activities such as storage, addition of other
ingredients, and dispensing into bottles could result in particle releases to the environment, resulting in
the potential for worker exposure. Available data on exposures are described in Section 5.3.2. In addition
to the potential for exposure during routine manufacturing operations, accidental short-term exposure at
high doses might occur at spray production facilities. These exposures could occur as a result of incidents
ranging from major accidents to medium-scale adverse events, such as a leak or break in process vessels
or pipes, to minor events such as small chemical spills.
Finally, nano-Ag, other spray ingredients, or by-products from disinfectant spray-manufacturing
facilities could enter waste streams including landfill waste and waste water streams by way of fluids
released from flushing and cleaning of processing equipment, improperly treated processing waste, and
cleaning of contaminated surfaces.
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3.3. Distribution and Storage of Nano-Ag Disinfectant
Sprays
Disinfectant sprays are most likely distributed in sealed plastic bottles. The principal method of
retail distribution likely is through the transport of cardboard cartons, each containing several dozen spray
bottles. Although the boxes with spray bottles might be stored at intermediate distribution facilities, they
are apt to be opened only at the retail location where the individual units are ultimately sold to customers.
The possible scenarios for releases during transport include damage to the cartons or leakage from the
bottles as a result of mishandling of cartons, faulty packaging, or improper stacking of cartons in transport
vehicles, or spills that result from accidents involving transport vehicles. If the bottles are sealed properly
and not damaged during transport, releases of product prior to use might be limited to breakage of bottles
or large-volume spills of the liquid spray at retail locations where silver sprays are sold. Product shelf
lives are currently unknown, and potential for release exists during removal of products from shelves and
subsequent disposal.
3.4. Use of Nano-Ag Disinfectant Sprays
Disinfectant sprays can be used on a wide variety of surfaces, including walls, floors, sinks, door
knobs, light-switch covers, telephones, appliances, tables, and chairs (See Section 2.2). Sprays are likely
to be used in both residential and institutional settings, such as hospitals, restaurants, and schools. Nano-
Ag from the use of sprays likely will be found in the air, on the intended surfaces, and on unintended
surfaces contaminated by overspray, including humans, pets, and food, particularly when used in confined
spaces. Spraying of kitchen surfaces with nano-Ag products could result in the transfer of the particles to
food items and to light switches, door knobs, telephones, and other surfaces that are often touched.
Additional activities involving the sprayed surfaces could release more nano-Ag or spray by-products. For
example, subsequent cleaning of the surface with products containing oxidizing agents, such as hydrogen
peroxide (H2O2), could oxidize the nano-Ag and release ionic silver. Concomitant release of other spray
ingredients also would occur and could affect the behavior of nano-Ag. Surfaces sprayed with nano-Ag
products could be wiped down with paper towels, disposable dust cloths, or other single-use products;
these disposable products then are likely to enter municipal waste collection systems and landfills (e.g.,
see Benn and Cavanagh (2010) below).
Nano-Ag disinfectants sprayed on sinks, bathtubs, and toilets could enter wastewater streams or
septic tanks. Similarly, fabric or clothing that is sprayed and then laundered also could release nano-Ag
and by-products into wastewater. As described by Benn and Westerhoff (2008). several clothing
manufacturers have advertised clothing products containing nano-Ag. The processes by which the
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nano-Ag is added to the textile products are generally not available in the literature, but the likelihood that
at least some of these products have been treated with nano-Ag coatings is high, and such applications are
expected to result in similar releases of nano-Ag to the environment as for textiles sprayed with nano-Ag
disinfectant. As a result, the discussion of nano-Ag releases from products that have been coated with a
nano-Ag solution could provide insight to this discussion.
Benn and Westerhoff (2008) found that three of six brands of socks containing nano-Ag leached the
particles, at different rates, during wash simulations using tap water or distilled water with no soap.
Geriano et al. (2009) conducted a follow-up study in which the quantity and form of nano-Ag released
during washing simulations were determined using nine different fabrics with different methods of silver
incorporation into the fibers. They found that, under typical washing conditions (pH 10) the dissolved
concentrations were 10 times lower than in a cycle with pH 7, but that the addition of bleaching products
accelerated the dissolution of Ag. During other washing simulations in which the fabrics were placed in
steel washing containers agitated with steel balls, Geriano et al. found that most of the silver particles that
were released were in the size fraction >450 nm; the authors attributed this to the dominant role of
mechanical stress caused by the agitation and the presence of the steel balls. Authors also noted that a
conventional silver textile did not show any significant difference in the released silver particle size
distribution compared to the nano-Ag products. In a similar experiment, Kulthong et al. (2010)
investigated the release of nano-Ag from antibacterial fabrics into artificial sweat. Using both
commercially produced and laboratory-prepared fabrics containing nano-Ag and four formulations of
artificial sweat, the authors found that silver was released from the various fabrics at rates ranging from 0
to 322 milligrams per kilogram (mg/kg) of fabric weight. Release rates depended on the amount of silver
coating, fabric quality, method of fabric preparation, and components of the artificial sweat. These results
suggest that aspects of the textile manufacturing process, such as incorporation of nanoparticles into
fabrics versus spraying onto fabrics, and specific washing parameters, such as use of bleaching agents and
water pH, can influence the amount of nano-Ag that could enter wastewater streams when products are
laundered or that could come into contact with human skin via leaching into perspiration (Kulthong et al..
2010: Geranio et al.. 2009: Benn and Westerhoff. 2008). Benn and Cavanagh (2010) also analyzed the
amount of silver that could be released by use in the home of several consumer products containing
nano-Ag. The analysis included washing clothing articles containing nano-Ag, release of silver into tap
water from toothpaste and shampoo that include nano-Ag, and release while cleaning with nano-Ag
detergents. The authors calculated that the amount of silver released by the use of these household
products could be as high as 470 micrograms (fig) of silver per day for a single consumer (conservative
assumptions were made regarding the amount of silver released from individual products).
Commercial establishments such as restaurants and hospitals might purchase bulk quantities of
spray solutions containing nano-Ag for use with spray-gun applicators. For example, the usage
instructions for NanoSil from ConSeal International, Inc. (2010) suggest using a spray gun or mop for
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application. Excess product remaining after spraying is likely to be disposed of into municipal wastewater
streams, as would the water used to rinse spray guns or mops after use.
Nano-Ag disinfectant sprays might also be used outdoors; for example, sprays might be used to
disinfect outdoor trash cans, outdoor furniture and children's toys, or boats or other recreational
equipment. From these applications, nano-Ag and by-products might directly enter natural waters or soil,
rather than being processed at wastewater treatment facilities. Kaegi et al. (2010) have reported that about
30% of the silver originally incorporated in outdoor paint as nano-Ag was released to the environment
over the course of one year (primarily during rain events); similar processes could transport silver from
outdoor objects treated with nano-Ag disinfectant sprays.
Shanghai Huzheng Nanotechnology Co. Ltd. (2009) reports on their website that their products will
continue to protect against bacteria for up to 24 hours after application. This claim suggests that particles
will continue adhering to the surfaces to which they are applied for up to a day. Research by Brady et al.
(2003) suggests that a silver disinfectant continues to effectively inhibit bacterial growth on a solid glass
surface despite repeated rinsing under tap water; other non-silver disinfectants did not show the same
effectiveness. A subsequently published letter to the editor of the journal, however, questions the
applicability of the results because the film created by the disinfectant, rather than the silver, could have
prevented bacterial growth (Schuster et al.. 2004). Additionally, the authors of the letter suggested that
incubating the glass tiles under humid conditions between tests rather than at room temperature artificially
increased bacteria growth over normal conditions and overstated the effectiveness of the disinfectant
spray. They also questioned whether the surface disinfectant would be as effective on porous surfaces
such as wood as it purportedly was on glass.
The size of droplets from spray disinfectant products likely will vary based on the formulation of
the liquid used as a delivery medium and the delivery system (Hagendorfer et al.. 2009; Pandis and
Davidson. 1999). By using a propellant gas spray, Hagendorfer et al. (2009) produced nanoscale water
droplets that contained a homogenous nanoparticle distribution. Following application, the liquid
evaporated, leaving a nano-Ag-containing residue. The physical and chemical properties of the droplet
residue after evaporation are likely influenced by the composition of the formulation, including the
physical properties of the solvents used (Hagendorfer et al.. 2009; Pandis and Davidson. 1999)
3.5. Disposal of Nano-Ag Disinfectant Sprays
The most likely scenario for disposal of spray bottles is through household wastes, whether those
containers are taken to a landfill or recycled. Regardless of the pathway, any nano-Ag and associated
substances remaining in the bottles ultimately would enter municipal solid waste streams. If the waste is
incinerated, nano-Ag might be released to the air. If the waste is deposited in landfills, nano-Ag could
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leach into the soil and eventually enter the ground water. Alternatively, if containers are recycled, both
workers and consumers could come in contact with nano-Ag in the manufacturing and use of products
made from recycled materials that previously contained nano-Ag spray. The disposal of bottles that are
unopened or contain unused portions of spray would be an additional source of nano-Ag and other spray
ingredients in municipal solid waste streams. That disinfectant sprays or materials that have come in
contact with them might be disposed of improperly is also possible, for example, in wooded areas, rivers,
or other illegal dumping grounds. In such cases, nano-Ag could directly enter the environment.
3.6. Summary of Life-Cycle Stages
The life cycle of nano-Ag used in spray disinfectants begins with the extraction or recovery of
conventional silver from mining operations. As much as 5% of silver production could be nano-Ag, but
substantial uncertainty is associated with this figure (Mueller and Nowack. 2008). Various methods of
nano-Ag production are reported in the literature and in patent filings, but how many of these are used on
an industrial scale or which are used most frequently in general or in the production of spray disinfectants
is unknown. Results of bench-scale syntheses of nano-Ag suggest that wet chemical processing is more
efficient than other production processes; wet chemical processing is likely to result in lower inhalation
exposures during the manufacturing stage than solid- or vapor-phase processes (Parket al., 2009). No
information specific to releases of nano-Ag and associated substances to the environment during
distribution, use, or disposal of spray disinfectants was identified. At any of these life-cycle stages,
nano-Ag could be released to the air (especially to indoor air during use) or to surfaces within homes and
public spaces. Disposal could result in the release of nano-Ag, other spray ingredients, or nano-Ag
by-products to the environment by way of landfills or wastewater streams.
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Chapter 4. Transport, Transformation, and
Fate Processes in Environmental Media
The production, use, and disposal of engineered nano-Ag eventually will lead to its occurrence in
air, soil, and water (Wiesner et al., 2006). Chapter 4 examines what might happen to nano-Ag after its
release to the environment at various stages of the product life cycle for spray disinfectants. Nano-Ag
released to air, water, or soil then could be transported or transformed through chemical, physical, and
biological processes. Although the transport, transformation, and fate processes of nano-Ag-associated
contaminants, such as waste by-products related to feedstocks and manufacturing, is also of relevance to a
comprehensive environmental assessment, the current insufficiency of information on these associated
contaminants precludes their coverage in this chapter.
Current literature suggests that the fundamental properties governing the environmental fate of
engineered nanoparticles in general are not thoroughly understood,13 and studies on transport,
transformation, and fate processes of nano-Ag, although beginning to emerge, are still relatively few. The
lack of data on the transport, transformation, and fate processes of nano-Ag by-products and waste
produced after disposal precludes a comprehensive discussion in this chapter and represents a potential
data gap for a comprehensive environmental assessment of nano-Ag. This chapter does, however,
summarize what is known about the environmental behavior as well as transport and transformation
processes of engineered nanoparticles (and specifically nano-Ag, when available), the physical-chemical
properties of these particles, and the characteristics of the environmental media that can affect the
behavior of these particles.
Section 4.1 provides a brief discussion of the chemical and physical characteristics and processes
that influence transport, transformation, and fate processes of nano-Ag in environmental media. The
sections that follow provide the available information regarding nano-Ag behavior in indoor and ambient
air (Section 4.2), terrestrial systems (Section 4.3), and aquatic systems (Section 4.4). A discussion of
models that might be used for evaluating the transport, transformation, and fate processes of nano-Ag, or
silver ions released from nano-Ag, in environmental media is provided in Section 4.5.
13The recent Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel review (U.S. EPA.
2010b') came to a similar conclusion.
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4.1. Factors Influencing Transport, Transformation, and
Fate Processes of Nano-Ag
The literature indicates that aerosols of atmospheric nanoscale particles formed by combustion
processes (e.g., from cars, incinerators) have been studied at length; relatively little, however, is known
about aerosols from engineered nanomaterials (Ma-Hock et al.. 2007). Aerosols of engineered
nanoparticles are synthesized in the laboratory to have unique physicochemical properties and certain
functional properties for use in commercial products (Jiang et al.. 2009; Ma-Hock et al.. 2007). These
"intentional" nanoparticles are controlled for size and shape and designed for functionality, and might
have a surface coating or other surface modifications to help increase the product's stability and
persistence after its release (see Section 4.1.1) (Oberdorster et al.. 2005b).
For decades, health-related aerosol exposures have been represented in terms of mass concentration
measurements alone. For assessing exposure to airborne nanoparticles, other factors such as particle
number, particle shape and surface area, surface chemistry (including coatings), and the degree to which
particles agglomerate or aggregate to form clusters14 play a critical role in determining nanoparticle
distribution and fate within the environment and in evaluating their potential health impacts (Jiang et al..
2009: Ma-Hock et al.. 2007: Mavnard and Aitken. 2007).
Once released into the environment, nanoparticles would be expected to behave generally in one or
more of the following ways: (1) stay in suspension as individual particles; (2) form clusters with other
particles (and potentially deposit or undergo facilitated transport); (3) dissolve in a liquid; or
(4) chemically transform based on reactions with natural organic matter (NOM) or other environmental
constituents (Luoma. 2008). Transformation can affect size, shape, and surface chemistry of the particles
and their coatings, and this process will affect their ultimate distribution, persistence, and toxicity in the
environment (Lowry and Gasman. 2009). Transformation can lead to substances that present a very
different hazard than the untransformed material that was originally released (Maynard. 2006).
As described in the following sections, the distribution and fate of nano-Ag within the environment
depends on the physical and chemical processes that occur in the environment and the characteristics of
the environmental system (Boxall et al.. 2007). as well as characteristics of the particles, as described in
detail in Section 2.3. The presence of spray ingredients or materials used in the manufacturing process
14As summarized by Nichols et al. (2002) and discussed in more detail in Chapter 1, the meanings of the terms
"aggregate" and "agglomerate" as they refer to the formation of particle "clusters" are sometimes interchanged in
the literature; thus, the definitions of these terms are neither specific nor consistent. To simplify the discussion for
this case study, the term "cluster" is used throughout this document to indicate an aggregate or agglomerate of
nanoparticles, regardless of the nature or strength of particle cohesion or the mechanisms by which the particles
assemble.
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also can potentially affect the environmental behavior of nano-Ag, although no specific information
regarding this phenomenon was identified during development of this case study.
For the remainder of this chapter, much of the information presented is applicable to engineered
nanoparticles in general, as few fate and transport studies specific to nano-Ag were identified.
4.1.1. Persistence
Although silver, in general, can accumulate in water, sediments, soils, and biological organisms, the
behavior and persistence of silver ions and silver nanoparticles are fundamentally different. Free silver
ions can associate with other ions, but the ion itself is intrinsically persistent, although it can be converted
to other species (i.e., speciate). In contrast, a nano-Ag particle is not necessarily persistent. For example,
particles can dissolve or disassemble (i.e., not persist in the particulate form); sorb (as single particles or
clusters) to soil or sediment, where they can persist long term; undergo direct oxysulfidation to form
nanoscale silver sulfide precipitates that can persist for a long period of time; form complexes with
ligands and organic matter that can variably increase or decrease persistence of the nanoparticle; and
silver nanoparticles also can form (or re-form) from ionic silver in the presence of humic and fulvic acids
(Akaigheetal.. 2011: Maccuspie. 2011: Liuetal.. 2010a: Liu and Hurt. 2010: Salnikovet al.. 2009).
Because these reactions depend on the interplay of multiple environmental factors, the equilibrium
species are expected to change as nano-Ag, dissolved silver, and silver solids are transported through
various conditions.
As introduced in Section 2.3.5, Liu and Hurt (2010) demonstrated that citrate-stabilized nano-Ag
particles (Ag°; 2-8 nanometers [nm]) at low, environmentally relevant concentrations are not expected to
be persistent in complex, aquatic systems containing dissolved oxygen. Complete oxidative dissolution of
nano-Ag particles, however, can take up to several months, during which nano-Ag particles might interact
with and impact receptors. Complex conditions in environmental systems including changes in pH,
temperature, presence of ligands and organic matter, among other factors, also can influence the
persistence of nano-Ag. For example, decreasing temperature, increasing pH, increasing nano-Ag
concentrations, or increasing concentrations of NOM, all within ranges of environmentally relevant
values, have generally been shown to increase nano-Ag stability (Liu and Hurt. 2010).
In complex systems, the equilibrium distribution of nano-Ag and ionic silver will vary with the
environmental conditions (e.g., dissolved oxygen concentrations, presence of organic matter), making
possible a cycle of oxidation/dissolution (decreasing presence of particles) and reduction/formation
(increasing presence of particles), thereby increasing overall persistence of nano-Ag in the colloidal
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system. This dynamic equilibrium can be illustrated by the multiple ways in which organic matter can
stabilize nano-Ag:
• the organic matter could bind to nano-Ag surfaces and block its reaction sites, thus
preventing oxidative dissolution, complexation with other constituents, and silver salt
formation;
• humic and fulvic acids that make up organic matter could act as reductants that convert
Ag+ to Ag°, thus forming silver nanoparticles from dissolved ionic silver; or
• organic matter could compete with Ag° for oxidants such as hydrogen peroxide (H2O2),
which would also limit reactive dissolution (Akaighe et al.. 2011; Liu and Hurt. 2010;
Salnikov et al.. 2009).
Some types of silver nanoparticles in commercial products are engineered to have charged
functional groups or surface coatings to increase stability, and these surface treatments can variably affect
long-term persistence of nano-Ag in environmental media. MacCuspie (2011) examined the effect of
three different capping agents—bovine serum albumin, citrate, and starch—on nano-Ag (10-20 nm)
persistence under a variety of environmentally relevant conditions. In general, the bovine serum albumin-
capped nano-Ag exhibited greater stability in the presence of high electrolyte (i.e., sodium chloride
[NaCl] and calcium chloride [CaCl2]) concentration and a range of pH values (stable at pH 2, 3, 7, and 9)
than citrate-capped nano-Ag (relatively stable only at pH 7 and 9). The starch-capped nano-Ag, however,
exhibited the greatest stability at the widest range of pH values (relatively stable at pH 2-9) (2011). It is
reasonable to expect, however, that surface coatings could be degraded by chemical or biological
reactions, which will further affect the persistence of the nano-Ag over time in ways dependent on both
the presence of a coating and the type of coating used.
4.1.2. Particle Clustering, Deposition, and Sedimentation
The formation of particle clusters, deposition to surfaces from air, and sedimentation from water
are closely related phenomena. As described by Navarro et al. (2008a) and Wiesner et al. (2006).
clustering15 describes the interaction between two mobile objects (transport). Deposition refers to the
settling of a mobile particle or cluster from the air onto a water surface or land, and sedimentation—
another form of deposition—is the process of settling from the water column or aqueous phase of soils
onto aquatic sediments or the particulate phase of soils.
The extent of nanoparticle clustering depends on the properties of the particle (e.g., shape, size,
surface area, surface charge, surface coating) and the characteristics of the environmental system (Tiede et
15See footnote 14.
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al.. 2009; Handy et al.. 2008b). Silver nanoparticles are often coated to reduce the formation of clusters,
thereby maintaining the high surface area-to-volume ratio (and corresponding increase in surface
reactivity) of the dispersed single silver nanoparticles (Kandlikar et al.. 2007). Other spray ingredients
and environmental conditions might also affect nano-Ag clustering by altering the surface coating and
physicochemical characteristics of the original particles (as discussed in more detail in Sections 4.2-4.4)
dowry and Gasman. 2009: Handy et al.. 2008b).
The rate and extent of particle clustering depend also in part on ionic strength and ionic
composition. In general, increasing ionic strength (e.g., additions of salt) and the presence of divalent
cations such as Ca2+ (ionic calcium) and Mg2+ (ionic magnesium) increase the rate and extent of
clustering and can affect the stable size of the clusters formed (Cumberland and Lead. 2009; Lowry and
Gasman. 2009; Handy et al.. 2008b). According to Lee et al. (2007). the presence of a sufficiently high
concentration of salt (NaCl at 100 millimoles per liter [mM]) appears to reduce the thickness of the
electric double layer on the surface of nanoparticles and decrease the zeta potential (a measure of stability
behavior of a colloid) below its critical point, leading to the formation of nanoparticle clusters. As
discussed by Mukherjee and Weaver (2010). competing, similarly charged ions can promote clustering of
nano-Ag to different extents. These authors found that Ca2+ stimulated clustering (i.e., increased the
average cluster size) of nano-Ag to a greater extent than K+ (ionic potassium) and Na+ (ionic sodium).
Li et al. (2010c) tested the early-stage aggregation kinetics of nano-Ag (spherical, monodisperse
particles, outer oxidized layer) in the presence of competing cations in a sodium bicarbonate buffer by
using electrolyte solutions of NaCl, NaNO3, and CaCl2 at neutral pH with and without fulvic acid coating.
For each electrolyte solution, they found that in the early stage of aggregation kinetics, an initial
particle-size decrease (dissolution) preceded an increase in the clustering rate of nano-Ag—a process they
called "dissolution-accompanied aggregation." The adsorption of anions imparts a negative surface
charge that varies with both pH and electrolyte concentration; the electrostatic force stabilizes particles
and prevents clustering while electrolyte addition balances and screens the surface charge-inducing
destabilization and clustering. The critical coagulation concentration (CCC) for nano-Ag—defined as the
concentration at which the maximum clustering rate is achieved—for the three electrolyte solutions were
determined to be 40, 30, and 2 mM for NaCl, NaNO3, and CaCl2, respectively. The CCC highly depends
on the characteristics of the surface coating of the nanomaterial; as a result, the CCC values provided here
are relevant only to this specific context.
The pH of a medium will change the surface charge of particles, thereby affecting the size of
clusters that form (DunphyGuzman et al.. 2006). As pH of the system increases, the number of negatively
charged sites in the system increases, elevating the potential for adsorption of the nanoparticles to
positively charged species (O'Brien and Cummins. 2009). Elzey and Grassian (2010) investigated the size
distribution and fractionation of manufactured nano-Ag in aquatic systems at pH ranges of 0.5-6.5 and
determined that clustering of nano-Ag occurred between pH 6.5 and 3; independent (unclustered)
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nano-Ag particles existed at pH values between 2.5 and 1. A pH of 0.5 resulted in nearly complete (93%)
dissociation of nano-Ag particles into ionic silver.
Mukherjee and Weaver (2010) studied the clustering behavior of metallic nanoparticles including
nano-Ag (prepared using Ag nanopowder <100 nm, 99.5% purity, surfaces organically coated, specific
surface area 5 square meters per gram [m2/g]; Sigma-Aldrich) and claimed that clustering of nanoparticles
increases as the pH approaches the isoelectric point pH (pHiep) for that specific type of nanoparticle. This
pHiep represents the pH at which the surface of the particle carries no net electrical charge and is the point
of a particle's minimum solubility, where it will precipitate from solution. By contrast, the farther away
from the pHiep, the greater the stability of the individual particle (and the increased likelihood that the
particles would stay suspended and become transported). Mukherjee and Weaver (2010) suggested that
the pHiep for nano-Ag is 2.0.
The transport of nanoparticles in general depends largely on their size; for this reason, among
others, clusters of engineered nanoparticles will behave quite differently compared to single engineered
nanoparticles (Ma-Hock et al.. 2007). Generally, nanoparticle clusters are less mobile than individual
nanoparticles in environmental media because they deposit faster and to a greater extent than individual
nanoparticles when suspended in air, in water, or in aqueous phases of soil (Nowack and Bucheli. 2007).
As summarized by Luoma (2008). most nanoparticles that associate with dissolved materials or particles
in water likely will deposit in soils or in aquatic sediments (Luoma. 2008). Changes in aquatic or soil
chemistry and physical disturbance, however, can lead to dissociation of nanoparticles from materials to
which they were attached and resuspension in air or water. The formation of clusters and therefore of
larger particles that are trapped or eliminated through deposition or sedimentation reduces concentrations
of bioavailable engineered nanoparticles in the water column, but uptake by soil- or sediment-dwelling
organisms or filter feeders can still occur (Nowack and Bucheli. 2007).
4.1.3. Adsorption
Adsorption, conceptually similar to both deposition and the formation of clusters as introduced
above, is the binding of molecules or particles to an abiotic or a biotic surface. The potential of the
nanoparticle to adsorb to a surface is influenced by its surface area, surface charge, and degree of
clustering, as well as the presence of surface coatings (O'Brien and Cummins, 2009). Because of their
high surface area-to-volume ratio and surface reactivity, nanoparticles can adsorb pollutants or other spray
ingredients, which might change the transport, solubility, and bioavailability of both the nanoparticles and
the pollutants in the environmental systems and modify their toxic effects (Navarro et al., 2008a).
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4.1.4. Transport/Mobility Potential
Once nanoparticles are released into the environment, their transport is a critical factor in assessing
their impact and ultimate fate in the environment. Generally, nanoparticle transport on the molecular or
particle scale is dominated by Brownian motion (random motion of small particles suspended in a gas or
liquid). Weaker forces such as London or van der Waals are responsible for attachment behaviors that
ultimately determine particle mobility (Biswas and Wu. 2005). Transport or mobility potential of nano-Ag
is affected by the characteristics of the nanomaterial (including those of the material matrix and surface
coatings), associated substances from manufacturing or product formulation, and the environmental
medium (O'Brien and Cummins. 2009).
The greater potential mobility of nanoparticles in the environment relative to the mobility of larger
particles implies a greater potential for exposure because they are dispersed over greater distances and
their effective persistence in the environment increases. The physical movement of a nanoparticle,
however, is restricted by its small size and propensity to adsorb to surfaces (BormetaL 2006b). The
propensity of nanoparticles to adsorb to abiotic and biotic surfaces or to form clusters can particularly
decrease their mobility in porous media such as soil (Borm et al., 2006b; Wiesner et al., 2006).
4.2. Air
Nano-Ag in spray disinfectants could be released into air in several ways:
• During manufacturing of nano-Ag spray products, nano-Ag might be released to indoor air
during standard mechanical operations, including mixing, grinding, or agitation of liquids.
Nano-Ag released in indoor air might subsequently be transported to outdoor air (Quadros
and Marr. 2010: Grassian. 2009: Mueller and Nowack. 2008)
• During manufacturing, storage, and disposal, nano-Ag might be released to indoor or
outdoor air via fugitive emissions, spills, cleaning operations, and other accidental or
unintentional releases.
• The consumer use of spray disinfectants containing nano-Ag will result in direct emissions
of nano-Ag and other substances to indoor air and possibly to outdoor air if used outdoors
or transported from the indoor environment to outdoor air.
• Disposal of spray products or contaminated containers via incineration might release
nano-Ag to ambient air (Blaser et al., 2008).
• Some nano-Ag particles that deposit to soil might experience secondary transport via wind
and become resuspended into ambient air. Once deposited, however, nanoparticles would
be unlikely to resuspend in the air or re-aerosolize given their propensity to cluster or
attach to surfaces (Aitken et al. 2004: Colvin. 2003).
Overall, few studies are available on the transport, transformation, and fate processes of
nanoparticles in indoor and outdoor air. Information obtained from the literature provides a general
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description of the behavior of particles in general and some nanoparticles in air, although information
specific to nano-Ag is limited. As discussed below, available information suggests that particle behavior
will vary based on particle size, the extent of particle clustering, and other parameters such as surface
coatings. Notably, the magnitude of nano-Ag release into air is unknown.
4.2.1. Diffusion
Several processes and factors influence the fate of airborne particles in indoor and outdoor
environments, including size, chemical characteristics, the nature of interactions with other airborne
particles, residence time in the air, and distance traveled prior to deposition (U.S. EPA. 2007b). The fate
of airborne particles outdoors could also be influenced by meteorological factors, including wind,
temperature, and relative humidity (Navarro et al.. 2008a). Nanoparticles might be in the form of single
particles or in clusters that are larger than the primary particle (Grassian. 2009) (see Section 2.3).
Individual nano-sized particles likely will follow the laws of gaseous diffusion when released to air, with
their diffusion rate inversely related to their diameter (i.e., smaller particles will diffuse more quickly)
(U.S. EPA. 2007b). Due to their high diffusion coefficients, nanoparticles (based on size alone) should
diffuse more readily than micrometer-sized particles (Aitken etal.. 2004). The dynamics of airborne
nano-sized particles suggest that they generally will follow airflows and not be influenced by mechanisms
such as gravitational settling and inertial deposition (Mavnard. 2006). They are, however, more likely
than larger particles to deposit to surfaces via diffusion.
Once they are emitted to the indoor atmosphere, nanoparticles will diffuse according to a
concentration gradient, from high-concentration zones to low-concentration zones. Nanoparticles will mix
rapidly through indoor air and quickly disperse, and can be carried by various air movements caused by
differences in temperature, ventilation, or the movement of people or objects. For nanoparticle aerosols
released during discreet uses of a disinfectant spray product, concentrations at the emission source can
therefore drop fairly quickly over time. These aerosols can diffuse over greater distances (e.g., throughout
the interior of a home) and persist for a relatively long time in the indoor environment (AFSSET. 2006;
Aitken et al.. 2004). although diffusive deposition will decrease their airborne concentration (see Section
4.2).
4.2.2. Particle Clustering
Nanoparticles in aerosol sprays are typically in the form of clusters (Biswas and Wu. 2005).
although some commercial products are engineered to have surface coatings to help stabilize the liquid
suspensions against clustering so that they remain dispersed within the gas. Particle clusters have a much
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larger aerodynamic diameter than single nanoparticles and thus disperse and deposit differently. Single
nanoparticles are generally not observed in dispersed nanomaterials in the atmosphere, even when using
relatively high-energy dispersion methods (e.g., through spraying) (Ma-Hock et al.. 2007).
4.2.3. Residence Time
Nanoparticles as single particles have short residence times in air because of their rapid diffusion,
diffusive deposition to surfaces, and association with larger particles. In attaching to larger particles (0.1-
1 micrometer [um]), however, they are likely to persist longer in the atmosphere (AFSSET. 2006; Biswas
and Wu. 2005; Aitken et al., 2004) and thus can diffuse over greater distances (e.g., on a regional scale).
In general, particles in the 0.1- to 10-um range have the longest residence time in the atmosphere (Biswas
and Wu. 2005). Longer residence time in the atmosphere allows more time for the particles to be
mobilized by wind and other forces; therefore, long-range atmospheric transport of nanoparticles is
possible (including transport up to the global scale) if the nanoparticles attach to a larger particle (Wiesner
etal.. 2006).
4.2.4. Deposition and Resuspension
Particles suspended in the indoor air could be removed from the atmosphere and deposited onto
floors, walls, and other surfaces. Single nanoparticles might remain suspended in the air until they are
randomly deposited on a surface due to Brownian motion. Once on a surface, the nano-Ag could sorb to
dust particles and either remain on a particular surface or become resuspended after the surface is
disturbed (e.g., by individuals who touch or clean the surface). Nano-Ag also could form larger clusters or
sorb to dust particles in air; larger, non-nanoscale particles are more susceptible to gravitational settling,
which might remove larger particles from the atmosphere more quickly than nanoscale particles and
deposit them closer to the emission source (AFSSET, 2006; Aitken et al., 2004).
Eventually, all particles in the ambient air are deposited (dry deposition) or washed out (wet
deposition) to aquatic or terrestrial systems (e.g., soil and plants) (Mueller and Nowack, 2008). Some
nano-Ag particles that have become deposited could experience secondary transport via wind and become
resuspended into ambient air and deposited elsewhere. Once deposited, however, nanoparticles likely
would not be resuspended in the air or re-aerosolized (Aitken et al.. 2004; Colvin. 2003). Aerosol particles
that contact one another generally stick together because of attractive London or van der Waals forces and
form clusters (Aitken et al., 2004). London or van der Waals forces also act to keep a particle attached to a
surface. Once clustered (or attached), small particles would be much more difficult to resuspend.
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4.2.5. Additional Factors
As discussed in Section 4.1, nanoparticles for use in commercial spray products are synthesized to
have unique physicochemical properties and certain functional properties (Jiang et al., 2009; Ma-Hock et
al., 2007). They might be engineered to have a surface coating or other surface modifications to help
stabilize them against cluster formation and deposition and to increase the product's persistence after its
release (Wiesner et al., 2006; Oberdorster et al., 2005b). Spray ingredients also might affect the transport
and persistence of nano-Ag in air. In addition to its physical properties, the transport and ultimate fate of a
sprayed product are affected by the environmental or meteorological factors that it encounters (e.g.,
magnitude of air currents, temperature, relative humidity) (Navarro et al., 2008a).
4.3. Terrestrial Systems
Nano-Ag in spray disinfectants can enter terrestrial ecosystems in several ways:
• During manufacturing, distribution, use, or disposal of nano-Ag spray products, nano-Ag
might be transported into ambient air and subsequently deposited or washed out to aquatic
or terrestrial systems (Mueller and Nowack. 2008).
• Disposal of spray products containing nano-Ag might result in nano-Ag release to soil
(Navarro et al.. 2008a). Plastic/polymer containers with bound nano-Ag could be disposed
of in landfills, and nano-Ag could subsequently be released to the surrounding soil at a rate
dependent on environmental conditions at the landfill (e.g., amount of organic matter, pH)
(Reinhart et al.. 2010). Disposal products are either incinerated (whereby nano-Ag could be
released into the air and deposited on soil and plants) or deposited in solid waste landfills.
Land-filled sewage sludge could result in leaching of the nano-Ag into subsoils and ground
water (Blaser etal.. 2008).
• Products containing nano-Ag might wash down a sink or bathtub drain or be discharged
from a washing machine into a wastewater treatment plant. Sewage sludge (separated
during the wastewater treatment process) is sometimes applied as a fertilizer to agricultural
soils (Blaser et al.. 2008). Therefore, nano-Ag might be released into soil via sewage
sludge. Runoff flows along the ground surface could transfer nanoparticles in the sewage
sludge to nearby terrestrial systems or aquatic systems (O'Brien and Cummins. 2009;
Blaser et al.. 2008).
• Some nano-Ag particles that deposit on soil might experience secondary transport via wind
and become resuspended into ambient air and re-deposited into aquatic or terrestrial
systems. As stated previously, however, once deposited, nanoparticles would be unlikely to
resuspend in the air or re-aerosolize given their propensity to cluster or attach to surfaces
(Aitken et al.. 2004; Colvin. 2003).
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Overall, information on the transport, transformation, and fate processes of nanoparticles in
terrestrial systems is limited. Information obtained from the literature provides a general description of
the behavior of nanoparticles in soil and plants; limited information specific to nano-Ag was identified.
4.3.1. Soil
The fate of nanoparticles released to soil is likely to vary depending on the physical and chemical
properties of the nanoparticles, the presence of other spray ingredients, and the complex characteristics of
the soil environment (e.g., redistribution of nanoparticles by biota). Climatic conditions (e.g.,
precipitation, temperature) also can determine how the nanoparticles are physically transferred (e.g., by
runoff, drainage, leaching) (AFSSET. 2006).
Due to their size, nanoparticles are potentially mobile in soils (AFSSET. 2006). Nanoparticles are
small enough to fit into the spaces between soil particles, and therefore might travel farther than larger
particles before becoming trapped in the soil matrix. Alternatively, nanoparticles released to soil can be
strongly sorbed to soil due to their high surface areas, and therefore become immobile. The strength of the
sorption of any nanoparticle to soil will depend on its size, chemistry, applied particle surface treatment,
the presence of other substances, and the conditions under which it is applied (O'Brien and Cummins.
2009; U.S. EPA. 2007b). The propensity of a nanoparticle to adsorb to soil surfaces can make it less
mobile (Borm et al.. 2006b; Wiesner et al.. 2006). Changes in a subsurface soil's solution chemistry (e.g.,
by introducing a rainfall event), however, can result in nanoparticles becoming detached from soil
surfaces and leaching to ground water. The risks associated with nano-Ag contamination of ground water
are not currently understood (Torkzaban et al.. 2010).
Properties of the soil environment (e.g., soil type, soil organic matter, pH, ionic strength, presence
of other pollutants) also could affect nanoparticle transport (O'Brien and Cummins. 2009; U.S. EPA.
2007b). Interactions between nanoparticles and soil organic matter alter the degree of nanoparticle
clustering in soils (see Section 4.1.2). Soil porewater is generally rich in dissolved organic molecules
(e.g., soluble organic matter and humic and fulvic acids) that can enhance colloidal stability of
nanomaterials and increase their mobility (Jaisi and Elimelech. 2009). Changes in pH can affect the size
of the cluster, adsorption potential, and mobility in environmental media, as discussed in Section 4.1.3.
Soils are not static, and changes to their constituents (e.g., the addition of fertilizers or rain) can decrease
soil pH and hence can increase the mobility of contaminants (ATSDR, 1990). As discussed in Section
4.1.4, the presence of salt ions in soil can promote the association of nanoparticles, thus reducing their
bioavailability or physically restraining nanoparticle-organism interactions. Nanoparticles might adsorb
other pollutants and become transport vectors for these pollutants. This resulting complex could change
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the transport, bioavailability, and toxicity of both the nanoparticles and the other pollutants in the soil
(Navarroetal.. 2008a).
4.3.2. Plants
Exposure and uptake of silver and nano-Ag in terrestrial plants are discussed further in Section
5.2.4.1, however, the potential for subsequent transport and transformation in terrestrial plants is briefly
discussed here. Evidence indicates that conventional silver is taken up by some terrestrial plants from
highly contaminated soils and remains largely in the root system after exposure via contaminated soil,
although some distribution to leaves and other plant tissues is possible (Hirsch. 1998a) (See Section
5.2.4.1). As discussed further in Section 5.2.4.1, Harris and Bali (2008) examined transport and fate of
aqueous AgNO3 in two metal-tolerant plants, Medicago sativa (alfalfa) and Brassica juncea (a type of
mustard plant). TEM with energy dispersive X-ray spectroscopy revealed clusters of large numbers of
spherical-shaped silver nanoparticles with a size distribution reportedly centered around 50 nm. The
authors did not report which plant tissues were examined (e.g., root, stem, leaves). The sequestration of
nano-Ag led the authors to recommend using these plant species to synthesize large quantities of nano-Ag
particles.
Plants could be exposed to nano-Ag in air, water, and soil. Airborne nanoparticles could attach to
leaves and other aerial parts of plants (Navarre et al.. 2008a). Once on the leaf surface, nanoparticles
could be translocated to different tissues of the plant. If nano-Ag is present in soils, plant roots could
interact with nano-Ag associated with soil material and in soil pore water. The mobility of nanoparticles
in pore water is an essential condition for interactions with plant roots or fungal hyphae. In the presence
of certain organic compounds, nanoparticles will have improved mobility in soils, and could thus interact
more efficiently with plant roots (Navarro et al.. 2008a).
Nowack and Bucheli (2007) and Ma et al. (2010) hypothesize that nanoparticles in general could
interact with plant roots through several mechanisms, including adsorption onto the surface of roots,
assimilation into root cell walls, and uptake into root cells. Nowack and Bucheli (2007) suggest that
nanoparticles might diffuse into the apoplast (i.e., intercellular space); from this location, they could be
taken up by apoplastic membranes or could enter the xylem at sites of damage. Both groups suggest
possible translocation to shoots and leaves from there. As noted by Nowack and Bucheli (2007), uptake
into roots is likely to depend on the shape, size, and composition of nanoparticles and on plant species
anatomy. Based on observations of the behavior of other metal complexes in plants, nanoparticles might
be transported to plant shoots once they are present in the xylem (Nowack and Bucheli. 2007). Data
specific to nano-Ag is limited but suggests that it can be taken up by terrestrial plants (Yin et al.. 2011;
Maet al.. 2010) (See Section 5.2.4.1). Evaluations of nano-Ag transport and transformation within
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terrestrial plants are also extremely limited but indicate that translocation of silver after nano-Ag exposure
is possible (See Section 5.2.4.1). The extent to which silver and nano-Ag might transfer through terrestrial
plants to other portions of the terrestrial food web is discussed in Section 5.2.4.3.
4.4. Aquatic Systems
Nano-Ag in spray disinfectants could be released into aquatic systems in several ways:
• During manufacturing, nano-Ag might be transported into ambient air and subsequently
deposited or washed out to aquatic or terrestrial systems (Mueller and Nowack, 2008).
Washing of manufacturing equipment or disposing of water used during production could
potentially also result in direct releases to waste water streams.
• During its use, a product containing nano-Ag could be washed down the sink or bathtub
drain, or be discharged from a washing machine and material could be released into the
sewage system, wastewater collection and treatment facilities (U.S. EPA. 2010b). and
eventually to water bodies. Sewage sludge (separated during the wastewater treatment
process) is sometimes applied as a fertilizer to agricultural soils (Blaser et al., 2008).
Therefore, nano-Ag might be released into soil via sewage sludge. Runoff flowing along
the ground surface (which causes erosion) could transfer nanoparticles in the sewage
sludge to nearby waterways (O'Brien and Cummins. 2009; Blaser etal.. 2008).
• After its use, a spray containing nano-Ag might be transported into ambient air and
subsequently deposited in water bodies.
• Disposal of spray products containing nano-Ag might result in release of nano-Ag to soil
(Navarro et al.. 2008a). Disposal of by-products released during the manufacturing process
might result in a similar type of discharge to soil. Land-filled sewage sludge could cause
the silver to leach into subsoil and ground water and to migrate to surface water (U.S. EPA.
2010b:Blaseretal..20Q8).
Overall, few studies are available on the transport, transformation, and fate processes of
nanoparticles in natural aquatic systems. Information obtained from the literature provides a general
description of the behavior of nanoparticles in water and sediments, although information specific to
nano-Ag is limited.
4.4.1. Natural Aquatic Systems
As discussed in Section 6.2.2, aquatic organisms are highly susceptible to silver ion toxicity in
natural waters. Therefore, the behavior of nano-Ag in water will strongly influence whether significant
incidences of exposure and toxicity to aquatic organisms can be expected. The key chemical, physical,
and environmental factors in natural waters that could affect fate and transport behavior of nano-Ag in
aquatic systems are discussed below.
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4.4.1.1. Surface Properties
As described in Section 2.3, the surface properties of nano-Ag are among the most critical
determinants governing its mobility and fate in aquatic systems. Particles in suspension settle at rates that
depend on particle size, density, and shape. Waterborne nanoparticles generally settle more slowly than
larger particles of the same substance. Due to their high surface area-to-mass ratios, however, nano-sized
particles can sorb to sediment particles and become removed from the water column (U.S. EPA. 2007b:
Oberdorster et al.. 2005b). The surface properties of nano-Ag govern its stability and mobility as colloidal
suspensions or their clustering into larger particles and deposition in aquatic systems (Navarre et al..
2008a). Nano-Ag particles can be engineered with surface coatings to improve water solubility and
suspension characteristics (see Section 4.4.1.4).
4.4.1.2. Ionic Ag and Ag Complexes in Water
The mechanisms of action that govern toxicity of nano-Ag particles and ionic silver are the subject
of ongoing research, as investigators seek to determine whether nanoscale silver toxicity is due to the
particles themselves and their intrinsic properties, the particles releasing silver ions, or some synergistic
combination of the two (Lubick. 2008). For this reason, the behavior of ionic silver in the environment is
relevant to understanding potential impacts of disinfectant sprays that include nano-Ag.
The form of silver in the water is governed in part by water chemistry. In studies over the past few
years, a very small proportion of the total dissolved silver in water has been observed to remain as free
silver ions, meaning that other forms predominate in the aquatic environment (Blaser et al.. 2008; Luoma.
2008). The free silver ion has a strong tendency to associate with negatively charged ions (ligands) in
natural waters to achieve stability. Ligands can occur in solution, on particle surfaces, on dissolved
organic matter, or in biological tissues (Luoma. 2008). Spray ingredients and other substances involved in
the manufacturing process also could act as ligands. The distribution of free silver ions and silver
complexes depends on the concentration of silver, concentrations of the different negatively charged
ligands (such as chloride, sulfide, thiosulfate, and dissolved organic carbon), and the strength of the bond
between each ligand and the silver ion (Choi et al.. 2009; Blaser et al.. 2008; Luoma. 2008). Table 4-1
presents the ligands with which most silver is expected to complex in the aquatic environment and the
log-normalized formation constants that quantify the relative affinities of each group of ligands for silver
(see Table 2-2 for solubility constants of solid silver compounds). Although complexation of silver with
some ligands under certain environmental conditions leads to the formation of silver precipitates (Ag2S,
AgCl) that will generally deposit to sediments, other complexes (AgCl2", silver thiolates) will be present
in the dissolved phase under certain conditions.
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Ligands that strongly hold silver are abundant in most sediments; therefore, silver ions tend to bind
readily to particulate matter (Blaser et al.. 2008; Luoma. 2008). The availability of oxygen in sediments,
which is a function of both the depth of the water above the sediment and the depth of a particular
sediment sample, tends to dictate the form of silver bound to the particles. Strong complexes with organic
material predominate at the sediment surface and in sediments in shallow water, where oxygen is usually
present and sulfides typically are not. In deeper sediments and in deeper water sediments, where oxygen
is absent, silver forms stable precipitates with sulfide (Luoma. 2008).
Silver ions form especially strong complexes with free thiol (-SH) ligands and with the sulfide
ligands that are present in NOM dissolved in water. Silver ions also can interact strongly with the chloride
anion (Cl ), although the nature of that reaction differs depending on whether the medium is fresh water
or sea water. In general, concentrations of chloride ions are low in fresh water,
Table 4-1. Formation constants for silver complexes (Ag:l_igand = 1:1) with environmentally
relevant ligands.
Ligand Formation constant with silver (log K)
Inorganic sulfides 14-21
Organic sulfides (thiols) 12-15
Thiosulfate 8.2
Iodide 6.6
Nitrogen(ammonia and amino) 3-6
Chloride 3
Oxygen (carboxylates) <2
Source: Adapted from Andren and Bober (2002)
Note from source: Constants are conditional, usually because of differences in ionic strength. Corrections to actual constants generally should result in less than one order of magnitude
change. Complexes with reduced sulfur compounds are many orders of magnitude more stable than with other ligands, and no realistic background chemistry (pH and competing
cations) would make silver-reduced sulfur complexes unimportant.
but silver ions could react with any chloride ions present to produce silver chloride, most of which
precipitates out of solution under normal conditions. Dissolved sulfides, organic materials, and chloride
ions likely will complex with essentially all the free silver ions in fresh waters (making it unavailable for
uptake by organisms) and drive the free silver ions to very low levels (Luoma. 2008).
In sea water, chloride occurs in very high concentrations. Multiple chloride ions can react with each
silver ion to form soluble silver-chloro complexes that keep silver in solution. Although this silver-chloro
complex dominates in solution in sea water, sulfide complexes also could be present (Luoma. 2008).
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4.4.1.3. Particle Clustering
In water, the physical structure of nanoparticles can be modified, and hence the properties of the
particles can change (AFSSET. 2006). Interactions with some environmental compounds can increase the
stability and thus the bioavailability of nanoparticles, while interactions with others can promote
clustering (Navarre et al.. 2008a). The formation of clusters can significantly affect the transport of
particles in aquatic systems. Clusters of nanoparticles that settle can be expected to accumulate in
sediments (unless disruption of sediments [e.g., through dredging] causes re-mobilization of the sediment
particles). Those that do not settle can travel in the water column from the point of release (Lowry and
Gasman. 2009). Nanoparticle clusters are assumed to be less bioavailable than single nanoparticles
(Navarre et al.. 2008a). The formation of clusters through sedimentation affects the concentrations of
nanoparticles that are bioavailable to organisms. Although clustered or adsorbed nanoparticles are less
mobile, they still can be taken up by sediment-dwelling animals or filter feeders (see Chapter 5) (Nowack
and Bucheli. 2007).
4.4.1.4. Important Environmental Factors
As is true for nanoparticles in general, environmental factors that influence the dispersion and
deposition behavior of nano-Ag include salinity (ionic strength), the presence of surface coatings
(i.e., engineered surface coating or NOM), pH, and water hardness (the concentration of competing
cations, such as Ca2+ and Mg2+). Results of a recent study by Gao et al. (2009) on the behavior of nano-
sized silver in complex natural waters suggested that dissolved organic carbon, pH, and the concentrations
of electrolytes (e.g., Ca2+ and Mg2+) help control the formation of clusters (see Section 4.1.2).
Typical aquatic environments, including rivers, lakes, and estuaries, contain monovalent and
divalent salts, as well as NOM (Saleh et al.. 2008). Particles of all dimensions are more likely to associate
as salinity increases (Luoma. 2008). Thus, nanoparticles will tend to form clusters to a greater degree in
salt water (which has a high ionic strength and higher pH [seawater has a pH of about 7.9]) than in fresh
water (Lowry and Gasman. 2009; Klaine et al.. 2008). Even small increases in salinity above that of fresh
water (-2.5 parts per trillion [ppt]) can cause a rapid loss of colloids through clustering and precipitation
processes (Stolpe and Hassellov. 2007). When the concentration of the chloride anion increases to a
certain point, however, the dominant equilibrium species shift to soluble silver-chloro complexes
including AgCl° (the neutral and most bioavailable complex), AgQ2~, AgCl32~, and AgQ43~, which act to
keep silver in suspension, thus increasing mobility. Moreover, a high percentage of silver adsorbed to
suspended sediments in lower salinities (i.e., in brackish or estuarine water) will desorb from these
suspended particles at high salinities (WHO. 2002).
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Another factor affecting the transport and distribution of nanoparticles in the aquatic environment
is surface coating, either that which is acquired upon release to the environment (e.g., coating by NOM)
or a coating that is engineered onto the nanoparticles (Nowack and Bucheli. 2007). The interactions
between nanoparticles and NOM can influence nanoparticle transport, transformation, and fate processes
in aquatic systems. The formation of larger nanoparticle clusters by high-molecular-weight NOM
compounds might favor deposition of the particles into sediments, likely decreasing their bioavailability.
Solubilization by natural surfactants such as lower molecular-weight NOM compounds, however, might
increase their mobility and their bioavailability to organisms (Navarre et al.. 2008a).
The behavior of conventional silver in aquatic systems, which has been well studied, also could be
relevant to understanding the behavior of nano-Ag in these systems. Silver, with a distribution coefficient
of 1045-106 (based on filtrate and particulate silver concentrations in various aquatic systems), is known
to be an extremely particle-reactive metal (Andren and Bober. 2002). Silver thus has a comparatively
short residence time in aquatic systems; it is quickly scavenged from the water column and ends up in
sediments.
As a ubiquitous component of aquatic systems, NOM can influence the surface speciation and
charge of nanoparticles, thereby affecting their mobility and their propensity to cluster or deposit
(Navarro et al.. 2008a). NOM (containing negatively charged humic and fulvic acids) could coat the
surface of nanoparticles, resulting in particles that tend to stay dispersed rather than form clusters (Handy
et al.. 2008b). As mentioned in Section 4.1.2, however, in the presence of certain electrolyte solutions
containing high CaCl2, adsorbed humic acid on nanoparticles leads to enhanced particle clustering (Chen
and Elimelech. 2007). NOM can stabilize particles against forming clusters in water, which can enhance
transport in aqueous environments and ground water (Lowry and Gasman. 2009).
In addition to NOM, artificially produced organic compounds might be used to stabilize
nanoparticle suspensions (Navarro et al.. 2008a). Some nano-Ag particles are engineered to disperse and
remain as single particles (i.e., not form clusters), increasing the possibility of the persistence and
accumulation of non-associated forms in natural waters. Surface coatings can be added to improve water
solubility and suspension characteristics (Luoma. 2008). Metallic nanoparticles often are coated with
inorganic or organic compounds to maintain their stability and mobility as colloidal suspensions (Navarro
et al.. 2008a). The potential effect of spray ingredients co-occurring with nano-Ag on the stability and
mobility of suspensions is unclear.
The pH of water could influence the rate of nanoparticle clustering, depending on the surface
charge of the particles involved (Handy et al.. 2008b). In general, the mobility of silver increases under
conditions of increased acidification (lowering of pH) (Luoma. 2008). Water hardness will alter the
chemistry that controls particle clustering (and ultimately ecotoxicity; see Section 6.1) (Handy et al..
2008b). Hard water (in contrast to soft water) has a high mineral content. Nanoparticle surface charge
effects could be influenced by the concentrations of competing cations like Ca2+ and Mg2+ that might
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screen off a negatively charged surface. Nanoparticle dispersion in aquatic systems likely will be
influenced by the free cation concentration (Handy et al.. 2008b).
4.4.2. Wastewaters
The formation of particle clusters, surface charge, and surface area of nano-Ag, as well as the
presence of other spray ingredients and the treatment method in use, will affect removal efficiency and
fate of nano-Ag in wastewater (O'Brien and Cummins. 2009). At a treatment facility, sorption processes
and chemical reactions likely would affect nanoparticles. Those nanomaterials that do not sorb during the
primary treatment phase could be removed via settling in the secondary clarifier, after which they might
become entrapped in larger sludge floes. Although wastewater treatment plants can remove much of the
nano-Ag and associated free silver ions from the wastewater, some silver might survive treatment, remain
in the treated water, and ultimately be discharged into water bodies. Additionally, nanomaterials that are
removed in the wastewater treatment process could be released into soil via sewage sludge, which is
sometimes applied as a fertilizer to agricultural soils (Benn and Westerhoff. 2008; Blaser etal.. 2008).
Runoff along the surface of the ground then could transfer nanoparticles in the sewage sludge to nearby
terrestrial systems or waterways (O'Brien and Cummins. 2009; Blaser etal.. 2008). Land-filled sewage
sludge could cause the nano-Ag to leach into subsurface soil and ground water (Blaser et al., 2008).
Tiede et al. (2010) investigated the fate of nano-Ag in activated sludge and found that a significant
fraction of nano-Ag (>90%) in wastewater treatment systems was removed with the sludge solids; that
still would leave some fraction of nano-Ag in wastewater, however, that could be released to aquatic
environments.
Kiser et al. (2010) identified two types of nano-Ag that are found in the greatest quantities in
commercial products that contain nano-Ag—non-functionalized nano-Ag (not associated with a
functional group) and functionalized nano-Ag containing a carboxyl functional group. Both types of
nanoparticles were sorbed to activated sludge and removed during wastewater treatment but to different
extents. Approximately 97% of the non-functionalized nano-Ag was biosorbed and removed from the
wastewater; only 39% of the functionalized nano-Ag was sorbed and removed. The authors believe that
the functionalization of the nano-Ag impeded its interaction with the biomass (i.e., the activated sludge)
and resulted in less removal. These functionalized nanoparticles could persist to a greater extent in
wastewater effluent.
Kim et al. (2010a) examined sewage sludge products from a large-scale wastewater treatment plant
and characterized the nature of the nano-Ag that had accumulated. They discovered that nano-Ag was
transformed into silver sulfide (Ag2S) nanocrystals, with excess sulfur attached to the surface of the Ag2S.
This transformation likely occurred during the sedimentation process in which the surrounding
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environment is anaerobic, reduced, and sulfur rich. These observations are consistent with those reported
by Kaegi et al. (In Press), where enhanced sulfide in nonaerated mixed liquor resulted in the near
complete transformation of nano-Ag into Ag2S. Such discoveries makes considering the properties of
Ag2S essential in understanding the complete life cycle of nano-Ag.
In some rural areas of the United States, formal wastewater or solid-waste collection methods
might not be available as they are in municipal settings. Households in these areas might rely on septic
systems that could be compromised or be otherwise ineffective or might dispose of wastewater through
pipes into a pond or the woods. Additionally, rural environments could be exposed to illegal or
unmonitored disposal of manufacturing waste products (e.g., into informal landfills, onto roadsides, and
into aquatic systems). To the extent that such sources might exist, these exposure pathways should be
considered.
4.5. Transport, Transformation, and Fate Models
Most current models are not appropriate for use in predicting nano-Ag fate and transport through
environmental compartments (U.S. EPA. 201 Ob). Linking adapted models of the dispersive and
convective movement of airborne particles and gases with models of transport, transformation, and fate of
chemicals and particles in surface waters and soils, however, could help predict the environmental
transport, transformation, and fate of nano-Ag and silver ions released from those particles. The potential
influence of other spray ingredients or other substances used in manufacturing on the fate and transport of
nano-Ag also could be incorporated into the model. Such a comprehensive model, however, has yet to be
developed for nano-Ag.
The U.S. Environmental Protection Agency (EPA) and others have used environmental models
widely to simulate diffusive and convective movement of aqueous-phase chemicals through
environmental compartments (e.g., soils, sediments, water) and partitioning of the chemicals between
media (e.g., between solid and aqueous phases). Examples of such models are included in the Models
Knowledge Base compiled by EPA's Council for Regulatory Environmental Modeling.16 Transport of
nano-Ag clusters or nano-Ag sorbed to organic particles could be simulated with particulate matter
transport models for surface waters. Although adapting models designed to predict transport,
transformation, and fate of suspended solids or organic matter of small sizes (e.g., the Particle Tracking
Model, developed by the Army Corps of Engineers) to nanoparticle transport is possible, these models
might need to be adapted to include additional processes, including particle clustering, sorption to
16http://www.epa.gov/crem/knowbase/index.htm
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suspended particles, and possibly colloidal behavior. Evaluation of such models by comparing model
outputs with measured values also could be challenging, given the questionable reliability of analytical
methods to detect nanoparticles at environmentally relevant concentrations (i.e., in the nanogram/liter
[ng/L] range) (Luoma. 2008; Demirbilek et al.. 2005). Models that can be used specifically to estimate the
transport, transformation, and fate of nano-Ag in air and soils have not been developed, but some fate and
transport models have been proposed for evaluating the transport, transformation, and fate of nano-Ag, or
silver ions released from nano-Ag, in water and sediments. These models are described below. In
addition, Gottschalk et al. (2010) recently described a probabilistic material flow model used for assessing
the environmental exposure to engineered nanoscale titanium dioxide particles. A brief discussion of this
modeling approach is also provided below.
Although empirical data on nano-Ag concentrations in the environment are lacking, a recent study
by Mueller and Nowack (2008) used computer modeling to predict nano-Ag concentrations in air, water,
and soil in Switzerland based on simplifying assumptions and a substance flow analysis. The authors
acknowledged that transformation, degradation, and bioaccumulation of nano-Ag are likely important
factors in characterizing substance flow through environmental compartments, but that these factors were
not considered in the analysis. In addition, flows through secondary compartments (e.g., ground water,
sediment) were not modeled due to inadequate data available for these compartments.
The volumes of different environmental compartments for the entire country were calculated as the
surface area multiplied by depth for soil, surface water, and air:
• soil volume = agricultural and nonagricultural surface areas multiplied by 0.2- and
0.05-meter (m) mixing depths, respectively;
• surface water volume = surface area multiplied by 3-m mixing depth; and
• air volume estimated as volume of air within 1 kilometer (km) of ground level across the
country.
Homogeneous and complete mixing within each medium was assumed. Predicted environmental
concentrations (PECs) were calculated for "realistic exposure scenarios" (based on nano-Ag use
worldwide, estimated as 500 tons per year) and for "high exposure scenarios" (based on 1,230 tons
nano-Ag per year, or 5% of the world-wide extraction of 25,620 tons of silver that is not used in jewelry,
photography, or industry) and were compared to calculated predicted no-effect concentrations. Allocation
of worldwide nano-Ag use to Switzerland was based on the country's share of the total population of
industrialized countries (i.e., 0.0068). The investigators estimated that more than 15% of nano-Ag is used
in sprays and cleaning agents, and that most (85%) of that nano-Ag is discharged into wastewater from
wastewater treatment plants. Of the remaining 15%, approximately 5% is discharged to air, 5% is
discharged to soils, and 5% is disposed of in waste incineration plants. For nano-Ag discharged in
wastewaters, the investigators further assumed that 97% of nanoparticles are removed in packed-bed
filters and that, on average, 97-99% of suspended particles are removed during treatment. Finally,
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overflow wastewater discharge during storm events was assumed to be 5-10% of total wastewaters. In the
realistic scenario, the PECs for nano-Ag were 0.0017 microgram per cubic meter (ug/m3), 0.03
microgram per liter (ug/L), and 0.02 microgram per kilogram (fig/kg) for air, water, and soil, respectively.
In the high emission scenario, PECs were 0.0044 ug/m3, 0.08 ug/L, and 0.1 ug/kg for air, water, and soil,
respectively. Authors stated that the risk quotients (calculated as PEC/predicted no-effect concentration)
were notably less than 1.
Blaser et al. (2008) modeled fate and transport of silver ions, instead of nano-Ag particles, in the
Rhine River to assess potential risks from use and disposal of plastic and textile consumer products
containing nano-Ag. They assessed the likely fate of silver ions released to municipal wastewaters from
washing and wearing textiles spun with nano-Ag and contact with water of plastics coated or impregnated
with nano-Ag. Their model estimated silver ion concentrations in the water column and in the top layer of
sediments for three different scenarios ("minimum," "intermediate," and "maximum" emission
scenarios). Silver ion releases were estimated from silver content in biocidal plastics and textiles. The
fraction of wastewater treated was assumed to be 80-90% and the fraction of silver removed by filtration
and treatment was assumed to be 85-99%. Data on release rates of silver ions from different types of
plastics embedded with nano-Ag and from products with surface applications of nano-Ag are sparse, and
measured rates vary substantially among different formulations.
The model, which simulates silver ion fate and transport processes in river waters and sediments,
estimated PEC ranges of 4-40, 10-140, and 30-320 ng/L in river water for the "minimum,"
"intermediate," and "maximum" emission scenarios, respectively. Predicted silver ion concentrations
increase in both the water column and sediments downstream as the river flows through populated areas
with wastewater treatment facilities (Blaser et al.. 2008). In the top layer of the sediment, the PEC ranges
for the scenarios were 0.04-2, 0.1-6, and 0.3-14 milligrams per kilogram (mg/kg), respectively. In the
interstitial waters of the sediments, calculated PECs for the scenarios were 9, 30, and 70 ng/L,
respectively. The investigators reported that the PECs calculated for the river water were generally
consistent with the range of empirical data available for silver concentrations in river waters
(>0.01-148 ng/L). The sediment PECs, however, were generally higher than the range of measured data
for river sediments (0.2-2 mg/kg), but they were well below the value of 150 mg/kg reported for heavily
affected river beds. The proportion of the silver ions that is likely to be bioavailable, however, depends on
the availability of organic and inorganic sulfides and other materials in the river to bind silver ions, as
discussed in Section 4 A.I A and in Chapter 5.
Musee (2011) applied a mathematical model to compute quantities of several nanomaterial
(including nano-Ag) flows from nanoenabled cosmetic products into terrestrial and aquatic ecosystems in
the Johannesburg Metropolitan City of South Africa through four potential release pathways. These
included wastewater from a treatment plant, direct runoff into the environment (i.e., untreated streams),
solid waste (i.e., directly disposed materials after product use), and use of sewage sludge for agricultural
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applications. PECs were reported to depend primarily on wastewater treatment plant efficiency, as well as
product matrix, dilution factor, and other factors. Musee reported PECs for nano-Ag ranging from 2.7* 10"
3 ug/L to 2.7 x 10"1 ug/L in the aquatic environment under a realistic dilution factor of 1 and an assumed
high removal efficiency. Although these values resulted in risk quotients (hazard quotients) <1 for the
terrestrial environment, hazard quotients >1 were reported for the aquatic environment, indicating
potential risk to the aquatic environment (Musee. 2011).
Gottschalk et al. (2010) developed a probabilistic material flow analysis (PMFA) framework to
derive PECs for a range of nanomaterials in environmental media. The PMFA uses a whole life-cycle
perspective and takes into account uncertainty and variability in model inputs to calculate concentrations
of nano-Ag in all "natural" environmental compartments (atmosphere, soil, surface water, sediment, and
ground water) and "technical" environmental compartments (production, manufacturing, use, recycling,
and disposal in waste incineration and sewage treatment plants). Using the PMFA, PECSs (reported as
range of lower quantiles [Q(0.15)] to upper quantiles [Q(0.85)]) were derived for nano-Ag and other
nanomaterials from all anticipated sources in the United States, European Union, and Switzerland either
for 2008 or as the annual increase in concentration (Gottschalk et al.. 2009). Because production volumes
for the nanomaterials evaluated in this study were scaled based on number of inhabitants in each of the
three geographic regions, the PEC ranges for each environmental compartment differed little among these
three areas. As a result, only the data for the United States are presented here.
In general, U.S. PECs were lowest for nano-Ag in air (0.0020-0.0097 nanogram per cubic meter
[ng/m3]), surface water (0.088-0.428 ng/L), and sewage treatment plant effluent (16.4-74.7 ng/L) and
highest in sediments (increase between 153 and 1,638 nanograms per kilogram per year [ng/kg-yr]),
sewage treatment plant sludge (1.29-5.86 mg/kg), and sludge-treated soil (increase between 526 and
2,380 ng/kg-yr). Despite high PECs for sludge-treated soil, the expected contribution of nano-Ag to PECs
in the soil compartment as a whole was quite low (increase between 6.6 and 29.8 ng/kg-yr) because
sludge-treated soils account for only 1% of agricultural soils (Gottschalk et al.. 2009).
Although Gottschalk et al. (2010) believe that the PMFA is applicable to predict concentrations of
compounds in the environment when little information is available concerning environmental fate and
exposure characteristics, the added value of their approach over that of Mueller and Nowack (2008) has
yet to be evaluated by other researchers. Some of the principal assumptions included in the PMFA (e.g.,
homogeneous mixing of material in environmental compartments on a country- or continent-wide scale)
were not actually built into the Monte Carlo simulation and sensitivity analyses. Some results (e.g., total
mass, mass flux) estimated by the PMFA are in the form of ranges extending more than two orders of
magnitude for several environmental compartments and pathways. As also noted by the authors,
additional empirical data are still required to generate useful model input distributions.
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4.6. Summary of Nano-Ag Transport, Transformation, and
Fate in Environmental Media
The important potential pathways of nano-Ag and by-products into the environment associated
with the production (including manufacturing, distribution, and storage), use, and disposal of spray
disinfectants containing nano-Ag are summarized in Figure 4-1. Nano-Ag can be released into air, water,
and soil at various stages of the life cycle. Within these media, nano-Ag can be transported, transformed,
and spatially distributed in the environment. Ultimately, ecological or human receptors could be exposed
to nano-Ag and associated contaminants.
One of the primary pathways for release of nano-Ag in spray disinfectants could occur through
indoor use, where it might be sprayed into the air and onto a surface. Transport of nano-Ag from the
indoor environment to the outdoor environment, where it could partition into the ambient air, water, and
soil, is then possible. Release of nano-Ag in spray disinfectants also might occur during production or as a
result of waste disposal. For example, a product containing nano-Ag that is wiped up with a paper towel
and then discarded in the trash could end up in a landfill, with subsequent leaching into subsoil and
ground water and possible migration to surface water. A product containing nano-Ag that is washed down
a sink or bathtub drain might enter wastewater treatment plants, and treated water containing nano-Ag
subsequently could be released into water bodies. Other spray ingredients and substances involved in the
manufacture of nano-Ag sprays could co-occur with nano-Ag in the environment and potentially modify
its fate and transport behavior, although information regarding this possibility was not identified during
development of this case study.
Either a model focused on the movement of airborne particles and gases or one designed to predict
the fate and transport of chemicals and particles in surface waters and soils (or a combination of these two
model types) could serve as a basis for developing a comprehensive model for predicting environmental
fate and transport of nano-Ag and the associated release of silver ions. Such a comprehensive model,
however, has yet to be developed for nano-Ag.
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Source: Adapted with permission of Elsevier; Nowack and Bucheli (2007).
Figure 4-1. Potential nano-Ag pathways into the environment associated with production, use, and
disposal of spray disinfectants containing nano-Ag.
Waste disposal includes products containing nano-Ag that might be incinerated, washed down a sink or bathtub drain, or discharged from the washing machine
into wastewater treatment plants, land-filled sewage sludge, or sewage sludge used as a fertilizer on agricultural fields. This nano-Ag then could migrate into water
or soil media and be distributed throughout various environmental compartments.
2Dynamic relationships between single nano-Ag particles and clusters exist in indoor air, ambient air, and water and soil compartments.
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Chapter 5. Exposure, Uptake, and Dose
This chapter examines the potential for biota and humans to be exposed to primary and secondary
contaminants associated with nano-Ag in disinfectant spray products. As described in preceding chapters,
nano-Ag and associated materials (e.g., feedstock and manufacturing waste by-products, co-product
ingredients) constitute primary contaminants that might be released to different environmental media at
one or more stages during the life cycle of the material. Once released, nano-Ag and associated materials
might undergo physicochemical and biological transformation processes that result in exposing biota and
humans to various secondary contaminants. From a comprehensive environmental assessment standpoint,
all of these primary and secondary contaminants are of potential relevance. At present, however, attention
is directed first to nano-Ag as the primary contaminant of interest.
As previously discussed, throughout this chapter, the term nano-Ag is used to refer in general to
any type or formulation of engineered silver nanomaterials and might encompass a variety of physical and
chemical properties. As noted in Chapter 1, no clear demarcation between exposure-dose and effects
exists, and thus, some overlap of information in Chapter 5 on exposure and uptake with information in
Chapter 6 on effects is unavoidable. To the extent possible, reference to studies cited in both chapters is
limited in Chapter 5 to discussion of exposure-dose and in Chapter 6 to discussion of effects.
Exposure to a substance requires contact between the substance and the surface of an organism via
one or more environmental media (i.e., water, air, soil). For internal exposure to occur, the substance must
penetrate the organism's cell walls, cell membranes, or other barriers between the organism and its
environment; that is, the substance must be bioavailable to the organism. Transfer of a substance from any
of these environmental media and across exchange boundaries results in an internal dose that is
distributed by the circulatory system to organs.
Several terms are used throughout this chapter in describing the characterization of exposure-dose
(U.S. EPA. 2005, 1992). Exposure is contact of an agent with the outer boundary of an organism.
Exposure concentration is the concentration of a substance in its transport or carrier medium at the point
of contact. Dose is the amount of a substance available for interaction with biological receptors or in
metabolic processes after crossing the outer boundary of an organism. Applied dose is the amount of
substance presented to an absorption barrier and available for absorption (although not necessarily having
yet crossed the outer boundary of the organism). In non-experimental settings, potential dose, a more
general term, is the amount ingested, inhaled, in contact with the skin, flowing past gills, or in contact
with other exchange boundaries (e.g., surface of plant roots). Absorbed dose is the amount crossing a
specific absorption barrier (e.g., gills, digestive tract) through one or more uptake processes. Internal dose
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is a more general term, representing all of the substance that has been absorbed by one or more exchange
boundaries. Finally, delivered dose is the amount of a substance available for interaction with a particular
organ, tissue, or cell. The portal of entry determines the extent to which the substance might be
transformed (e.g., by the liver following ingestion) prior to reaching the general circulation. Where the
exchange boundary (e.g., gills) is the site of toxic action, potential dose might equal delivered dose for
that boundary.
Metrics describing exposure for nano-Ag can either be units of total Ag per unit mass or volume of
exposure medium (e.g., nanogram of silver per liter of water [ng(Ag)/L(water))J or number of particles
per unit volume of exposure medium. In either case, the mean and distribution of sizes and shapes, among
other physicochemical characteristics, of nano-Ag particles need to be reported to adequately describe
exposure. The amount of total silver per unit mass or volume of exposure medium is the measure most
often used and the measure that is most precise. The same concentration of total silver, however, can
result from a few large nanoparticles per unit volume or many smaller nanoparticles per unit volume. For
that reason, the size distribution of particles should be reported along with total silver concentration in an
exposure medium.
Particle size is best described by a distribution (e.g., percentiles), but the common practice in
literature is to specify only a mean and range of particle sizes. In addition, the metric for size is usually
the diameter of spherical particles. If nano-Ag particles are not spherical in shape, the investigators should
report the shape. Because the characteristics of nano-Ag in a starting material in the laboratory can
change after its addition to air, water, or soil (e.g., clustering of particles, dissolution of particles),
characterizing the nano-Ag particles once in an exposure medium also is important. In the discussion
below, the units of exposure are total silver per unit volume, with particle-size distributions characterized
as reported (e.g., mean, range).
Chemicals and substances that cannot be biodegraded beyond inorganic compounds ultimately can
persist and in some cases accumulate in both environmental media and biota. Bioavailability is defined as
the availability of a substance in an environmental medium for absorption by an organism in contact with
that medium (e.g., absorption of a substance inhaled with air or ingested with food). Bioconcentration
refers to the direct uptake and accumulation of a substance from an external medium (e.g., from water
through gills for aquatic organisms), while bioaccumulation occurs from both direct uptake from an
external medium and ingestion of a substance. Both bioconcentration and bioaccumulation require that
some fraction of the substance is bioavailable in environmental media. Bioconcentration of an agent can
occur across cell walls (e.g., plants) or across specialized exchange surfaces (e.g., gills of aquatic
invertebrates or fish), whereas bioaccumulation applies to animals only (U.S. EPA. 2003b). In natural
environments, the ratio of the chemical concentration in an animal to the chemical concentration in its
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environment generally is referred to as a bioaccumulation factor (BAF). Bioavailability is a prerequisite
for toxicity (Luoma. 2008). and therefore, often is assessed indirectly by evaluating the toxicity of an
agent to organisms under specified conditions (Berthet et al., 1992).
Chemicals that sorb to the external surface of organisms, but cannot penetrate the outer layer (e.g.,
epidermis, cell wall) because of large size, surface charge, or other properties are generally not considered
to be bioavailable. Nanoparticles sorbed to an external surface of organisms, however, can in some cases
damage cell walls or cell membranes or provide a steady release of ions that affect the organisms'
performance (e.g., survivorship, growth, reproduction; see Chapter 6). Different coatings and surface
properties of nano-Ag products might enhance or inhibit sorption to gill surfaces or uptake by the
gastrointestinal (GI) tract. A nano-Ag particle that associates with and disrupts essential cell processes at
the external membrane, or delivers silver ions that do so, therefore, is considered bioavailable for
purposes of this document. This approach is consistent with the convention used by Luoma (2008).
Toxic effects following exposure depend not only on delivered dose, but also on the timing and
patterns of exposure to the substance in environmental media, bioavailability of the substance from a
specific medium to particular organisms through their uptake processes, fate of the substance in the
organism, and the sensitivity of the organism to the substance. The disposition of the substances within
the organism includes its metabolism (possibly to more toxic entities), distribution, storage, and excretion.
The behavior of organisms in their environment can play a key role in their exposure profiles. For
example, exposure scenarios for children can differ drastically from those for adults because they spend
more time in contact with floor surfaces and they mouth a variety of objects that adults would not. Other
characteristics of an organism (e.g., age, reproductive status, size, health status, individual exposure to
other agents) can modify its sensitivity to atoxic substance, that is, its response to a given exposure, as
discussed in Chapter 6.
Because limited information is available on the releases of nano-Ag during manufacturing, storage,
use, and disposal of products such as spray disinfectants, quantitative, data-driven estimates of the
potential geographic extent of releases and media concentrations of nano-Ag and associated contaminants
are not yet possible. Thus, conceptual models are currently necessary to identify the most likely
significant exposure pathways and routes of intake. Shatkin (2008) suggests that a risk assessor can "step
through" the life cycle of a specific product to identify points in the manufacturing, storage, distribution,
and application of a new product that might result in releases to the environment and exposures of biota or
humans. Thus, a risk assessor can deduce, from limited information on the product, as well as general
manufacturing and distribution practices for similar products, some likely release points in the life cycle
of a new product and the context in which releases might occur.
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Nano-Ag can be released to the environment at various points in the life cycle of a disinfectant
spray, as described in Chapter 3, and many of these release scenarios could be similar to those for
nano-Ag contained in other end-use products such as fabrics. Human occupational exposures and silver
releases into the environment are possible during the synthesis of nano-Ag particles from feedstocks and
the manufacture of disinfectant spray products that incorporate nano-Ag. Some of the most likely and
significant exposure scenarios can be identified from the potential release scenarios described in
Chapter 3 and existing knowledge of the characteristics and behavior of nano-Ag and associated
substances (e.g., silver ions, transformation products, waste products, by-products) in the environment.
This chapter characterizes current knowledge of measured nano-Ag exposure and uptake and
internal dose (i.e., absorption of nano-Ag by organisms). Exposure and dose data are presented first for
biota in Sections 5.1 and 5.2, respectively. Exposure data then are presented for humans in Section 5.3.
General discussions of the potential for aggregate and cumulative exposures involving nano-Ag are
presented in Sections 5.4 and 5.5, respectively. As will be discussed in Section 5.4, aggregate exposure to
nano-Ag from disinfectant sprays and other products or sources determines the total potential and internal
doses of nano-Ag. Cumulative exposures to nano-Ag from multiple substances and other types of
nanoparticles are examined in Section 5.5. Because limited product formulation data are available, the
evidence for co-exposure to other spray constituents is not evaluated in detail, although such an
evaluation could be relevant in a comprehensive environmental assessment. Section 5.6 includes a brief
discussion of exposure models. Human uptake and dose from exposure to nano-Ag are discussed in
Section 5.7.
Improved methods to measure, monitor, and predict environmental concentrations of nano-Ag
likely will be necessary to assess current and future nano-Ag exposures in human and ecological
populations. Also, information on how exposure to nano-Ag particles translates into internal dose of
either nano-Ag, silver ions, or both, is needed to evaluate possible modes of action17 of nano-Ag in
different groups of organisms.
5.1. Biotic Exposure
As discussed in Chapter 3, uses of nano-Ag in spray disinfectant products could lead to two
potential types of environmental releases: (1) down the drain to wastewater treatment plants, with
subsequent release of treated water to surface waters and (2) land application of sewage sludge in
17"Mode of action" is defined and discussed in greater detail in Section 6.2, footnote 22.
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agricultural areas. These releases are similar to those from nanofunctionalized textiles, as described by
Blaser et al. (2008). Wastewater containing nano-Ag from manufacturing facilities also could reach
wastewater treatment plants, and nano-Ag not removed during the wastewater treatment process could
enter surface waters through effluent. Thus, exposure of aquatic organisms via the water column and
sediments in riverine through coastal environments is possible. Additional exposure of aquatic animals to
silver accumulated in their food also could occur. Luoma (2008) argues that if mass discharges of silver
from nano-Ag applications reach levels equivalent to those during historically high levels of discharge in
the 1980s, silver concentrations in aquatic systems will also mirror those during historical periods of
elevated releases. For environmental concentrations of this magnitude to occur, however, concentrations
entering wastewater treatment plants would have to be much higher than those during the 1980s to
account for improvement in wastewater treatment technologies. But assuming that such elevated silver
mass discharges to environmental compartments are possible, Luoma hypothesized that nano-Ag levels in
the waters of South San Francisco Bay, for example, could reach concentrations of 26-189 ng/L, which is
similar to those previously observed during the 1980s when discharges of conventional silver to the
environment were occurring at a rate of approximately 550 kilograms (kg) per year (Smith and Flegal.
1993). Given this scenario, Luoma further argues that nano-Ag discharges at that rate might increase
concentrations in the sediment of San Francisco Bay by more than an order of magnitude from a 2007
baseline of approximately 0.2 parts per million (ppm) (or 200 micrograms per kilogram [fig/kg] sediment)
to concentrations of approximately 3 ppm (3,000 ug/kg sediment). Blaser et al. (2008) estimated similar
concentrations for the Rhine River following anticipated increases in nano-Ag releases. Note, however,
that these estimated concentrations represent releases of nano-Ag from all sources, not from spray
disinfectants alone. Such theoretical projections are the only estimates of potential environmental
exposures of nano-Ag currently available because monitoring technology for measuring nanoparticle
concentrations in the ng/L range is still under development. Estimating the relative contribution of
nano-Ag from spray disinfectants to total nano-Ag exposures from all sources also is not yet possible.
As discussed in Chapter 4, the anticipated life-cycle stages and behavior of nano-Ag in
environmental compartments suggest that accumulation of nano-Ag in the terrestrial environment
generally will not be as great as in the aquatic environment (particularly in sediments); however, as
discussed above, soil biota and agricultural crops could be exposed to high concentrations of nano-Ag
where sewage sludge is used to amend soils. In addition, sewage sludge or solid waste containing
nano-Ag might be disposed of in landfills, which could result in exposure of terrestrial organisms or
leaching of nano-Ag into ground water. Exposures of reptiles, birds, and mammals could occur through
incidental soil ingestion in agricultural areas, and the sensitivity to ingested silver in these organisms is
expected to be low (as for humans). Herbivores might encounter nano-Ag accumulated in plants, while
insectivores might be exposed to nano-Ag accumulated in their prey. Airborne nano-Ag in outdoor
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environments based on indoor uses is expected to be insignificant; outdoor uses, however, might result in
inhalation exposure of animals and exposure of plants through their foliage.
Localized accidental releases or spills could create "hot spots" of nano-Ag contamination.
Accidental releases to terrestrial or aquatic environments might occur at bulk material storage facilities or
during transport of nano-Ag from one facility to another during the production, manufacture, distribution,
and disposal of products containing nano-Ag (see Chapter 3). Empirical data or modeling results appear
to be lacking regarding accidental releases of nano-Ag throughout the life cycle of nano-Ag sprays and
other products. Risk assessments could require special consideration of accidental release scenarios.
5.2. Biotic Uptake and Dose
Investigations of the absorption, distribution, metabolism, and excretion, as well as
pharmacokinetics, of nanoparticles in general and of nano-Ag in particular have not yet been conducted
on species from many of the major groups of organisms (e.g., algae, macrophytes, higher plants, annelids,
echinoderms, arthropods, mollusks, amphibians, reptiles, birds), although some data exist on aquatic
species, primarily fish (Handy et al., 2008a). In the context of the current case study, the uptake of
nano-Ag in the environment by different groups of organisms would depend on the likelihood of exposure
via different pathways leading from the production, storage, use, and disposal of consumer products
containing nano-Ag.
Few techniques have been developed that can accurately quantify dosimetry for nano-Ag, and
therefore differentiating between exposure and uptake in biota is difficult. The primary dose metric used
for ecological effects studies is an exposure concentration expressed in terms of mass. Depending on the
study, the term "mass" could correspond to the mass of silver in the nanoparticles added, mass of total
silver in solution, mass of silver ions, or mass of free silver in solution, although the form is not always
specified. Some studies have attempted to normalize the dose across various types of nanoparticles by
expressing concentration in terms of moles. Dose-dependent effects have been observed in multiple
organisms at various time scales as detailed in Appendix B, but the mechanisms that allow the penetration
of nano-Ag into and across membranes and into the cells of tissues in biota are not yet well understood.
The rate of uptake, tissue or whole-body concentrations, and the fate of nano-Ag at the cell, tissue, and
organism level are not currently known (Luoma. 2008).
As described in the introduction, metrics describing exposure for nano-Ag can either be units of
total Ag per unit mass or volume of exposure medium (e.g., ng[Ag]/L[water]) or number of particles per
unit volume of exposure medium. In either case, the mean and distribution of sizes and shapes of nano-Ag
particles also need to be reported to adequately describe exposure. In the discussion below, the units of
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exposure are total silver per unit volume, with particle-size distributions characterized as reported (e.g.,
mean, range).
The discussion of biotic uptake and dose of nano-Ag is divided into four major sections. The first
section provides general descriptions of bioavailability, bioconcentration, and bioaccumulation of
nano-Ag and conventional silver in biota (Section 5.2.1) as the terms apply to the remainder of Section
5.2. The subsequent three sections focus on specific aspects of the uptake of nano-Ag, silver ions, and
conventional silver by: bacteria and fungi (Section 5.2.2), aquatic ecosystems (Section 5.2.3), and
terrestrial biota in agricultural settings (Section 5.2.4). Throughout, nano-Ag is distinguished from silver
ions and from other forms of silver to the extent possible. In addition, dose is addressed, wherever
possible, by reporting absorption of nano-Ag or silver ions through the surface of an organism to its
interior, rather than the adsorption of either onto exterior surfaces (e.g., gills, GI tract).
5.2.1. Bioavailability, Bioconcentration, and Bioaccumulation
Chemicals and substances that cannot be biodegraded beyond inorganic compounds ultimately can
persist and in some cases accumulate in both environmental media and biota. The relationship between
external exposure and internal dose is influenced by bioavailability, bioconcentration, and
bioaccumulation, and thus these concepts are described briefly here. Within the organism, the potential
for systemic effects depends on the bioavailability of the nano-Ag at the organism's exchange boundary,
the release of silver ions from the particles, and either or both being absorbed and delivered to target
organs or tissues (Lowry and Gasman. 2009). Chronic effects depend, moreover, on the ability of
organisms to detoxify or excrete nano-Ag or silver ions derived from the particles. Silver, conventional or
nano-sized, that is not excreted can accumulate in an organism (Berthet et al., 1992) and could be passed
up the food chain (Bianchini and Wood, 2008). Bioavailability of nano-Ag is likely to be a function of its
form, the characteristics of the environmental exposure medium, and characteristics of the organism for
several reasons:
• Properties of the nano-Ag such as size, potential to form clusters, or surface properties and
coatings can influence its uptake. Alternatively, each of these considerations can facilitate
or preclude the binding of nano-Ag to other particles that might or might not be available
for uptake.
• Characteristics of the environmental medium (e.g., surface water pH, temperature,
presence of calcium carbonate, sulfates, other salts, dissolved and particulate natural
organic matter [NOM] and other organic materials) can modify the bioavailability of
nano-Ag through the binding of nano-Ag or silver ions in a nonbioavailable form.
• Characteristics of the organism and its route(s) of intake (e.g., acidic environment of the GI
tract, fish gill active transport of silver ions through ionic sodium [Na+] uptake channels)
can affect bioavailability and biocompatibility of nano-Ag and silver ions.
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Processes that readily transport nanoparticles from one environmental medium to another reduce
bioavailability in the first medium. For example, when nano-Ag is released to aquatic systems, many
agents in natural waters can bind to nano-Ag and silver ions, which results in precipitation or
sedimentation of the silver (see Section 4.4.1). Silver that has precipitated out of solution is no longer
directly bioavailable to organisms in the water column; depending on its form, however, it might be more
available to organisms in the sediment.
Attributes of organisms that help to determine bioavailability of silver ions and nano-Ag to them
are discussed in Section 5.2.1.1. Environmental factors that can modify bioavailability of silver ions and
nano-Ag are discussed in Section 5.2.1.2. Use of bioaccumulation models is discussed briefly in Section
5.2.1.3. The potential for bioaccumulation of nano-Ag or silver ions from nano-Ag is examined for
aquatic and terrestrial food webs as part of the broader discussion about uptake in aquatic and terrestrial
organisms in Sections 5.2.3 and 5.2.4, respectively.
5.2.1.1. Attributes of an Organism that Influence Bioavailability of Nano-Ag and Silver
Ions
The type of organism, its structure, and its physiology are key determinants of the mechanisms by
which it might be able to absorb nano-Ag and silver ions from the environment. Uptake of silver ions and
nano-Ag from the environment can depend on whether the organism is a prokaryote or eukaryote, its cell
size, cell wall or membrane construction, type of circulatory system, respiratory physiology, and other
major aspects of its body plan and physiology. At this time, however, data concerning the influence of
basic phylogenetic attributes on absorption or adsorption of nano-Ag are lacking for most groups of
organisms (Choi et al.. 2009; Handy et al.. 2008b). Sections 5.2.2 through 5.2.4 examine potential uptake
routes for silver ions and nano-Ag for different groups of organisms and describe evidence for the degree
of penetration into epithelial and systemic cells and extracellular matrices.
Within a group of organisms, the degree to which an individual absorbs silver ions from its
environment is likely to depend on many aspects of its condition, such as its life stage, reproductive
status, existing body burden of silver, osmotic status, and nutritional and overall health condition. These
conditions as well as an animal's behavior (e.g., filter feeding) also might affect potential exposure to
nano-Ag. Note that if nano-Ag particles adhere to an organism's external surfaces, and if the particles
release silver ions, the concentration of silver ions in the immediate vicinity of the organism would be
expected to be higher than concentrations of silver ions in the surrounding medium. Measuring the silver
ion concentrations in a micro-thin, "unstirred" layer of water or viscous medium (e.g., mucus)
immediately surrounding the organism without mixing, and thereby contaminating, that layer with the
surrounding medium, however, would be technically challenging.
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5.2.1.2. Attributes of the Environment Influencing Bioavailability of Silver Ions and
Nano-Ag
As described in Section 4.1.2, several characteristics of the environment can affect the fate and
transport of nanoparticles in water and in soil. These characteristics are likely to influence the amount of
nano-Ag and silver ions that organisms come in contact with in different exposure media. Characteristics
of the environment that influence the bioavailability of nanoparticles in general and nano-Ag specifically
differ for aquatic and terrestrial systems.
For aquatic systems and metallic nanoparticles, several characteristics of surface waters can affect
particle fate, including salinity (e.g., freshwater, estuarine, marine), hardness, organic matter content, and
pH (see Section 4.4.1.4). Processes that can remove nano-Ag from suspension and silver ions from
solution are noted below (Gao et al.. 2009; Wrjnhoven et al.. 2009b: Luoma. 2008):
• Aggregation (referred to in this case study as the formation of clusters) of nano-Ag
particles with each other into larger particles that settle out of suspension;
• Complexing of nano-Ag with NOM, which might create larger particles that settle out of
the water column; and
• Complexing of nano-Ag or silver ions with inorganic materials, forming insoluble
precipitates.
These processes deposit the silver to the sediments where, depending on the particles and sediment
chemistry, nano-Ag and silver ions might be more or less bioavailable to benthic organisms.
To date, essentially no data have been published that indicate the fate of nano-Ag particles released
to surface waters (Wrjnhoven et al.. 2009b). Data are available, however, concerning the bioavailability of
silver ions introduced to surface water in the form of the highly soluble silver nitrate (AgNO3). Therefore,
factors that affect the bioavailability of silver ions from AgNO3 released to surface waters are discussed
first. Factors that might influence the bioavailability of nano-Ag particles are considered second.
Environmental Factors that Affect Bioavailability of Silver Ions
Many ligands exist for silver ions in natural surface waters (see Section 4.4.1.2). As discussed in
Chapter 4, silver-complexing agents include inorganic ligands (e.g., chloride [Cl], bicarbonate,
thiosulfate), simple organic ligands (e.g., amino acids, ethylenediaminetetraacetic acid [EDTA]), and
complex polydispersed organic ligands such as humic and fulvic acids (Bianchini and Wood. 2008).
In natural fresh waters, concentrations of free silver ions are likely to be very low (Luoma. 2008)
(see Table 4.1 in Chapter 4, which lists the solubility products of common silver compounds in water).
Silver ions bind very strongly to reduced sulfur in natural waters, and sulfide (S2~) concentrations
typically exceed free silver concentrations in the environment by several orders of magnitude (Blaser et
al.. 2008). The general assumption has been that ligands present in the water will reduce bioavailability
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(and toxicity) of metals to aquatic biota by reducing the free metal ion concentrations (Bianchini and
Wood. 2008). Blaser et al. (2008) concluded that in freshwater systems, silver is expected to be bound to
S2~ either in the form of colloidal silver sulfide (Ag2S) solid-phase clusters or as an Ag2S surface complex
on organic matter. As discussed in Section 2.3.5, Ag2S solids are relatively stable and insoluble in water
(Blaser etal.. 2008).
In wastewater treatment plants, silver ions are easily removed because of their strong sorption to
suspended particles. Most silver ions that might reach surface waters should rapidly bind to ligands, settle
out of the water column, and become incorporated into the sediments, although repeated resuspension
back into the water column during scouring by storm events and by bioturbation (animal-sediment
interactions) is possible (Blaser et al.. 2008).
Toxicity tests with aquatic organisms confirm that bioavailability of silver ions is reduced in the
presence of excess sulfides. Reactive S2 , as found in zinc sulfide clusters for example, reduces the acute
toxicity of silver ions to both daphnia (Bianchini et al.. 2002) and rainbow trout (Oncorhynchus mykiss)
(Mann et al.. 2004). With a ratio of sulfide to total silver of 250 to 20, Bianchini et al. (2002) observed no
toxicity in Daphnia magna neonates after 48 hours at a silver concentration as high as 2.1 micrograms per
liter (ug/L) (or 19 nanomoles per liter [nM] x 108 grams per mole [g/mol] of silver), while in the absence
of sulfide (less than 5 nM), the 48-hour LC5018 for neonate D. magna was estimated to be 0.18-0.26 ug/L.
The daphnia accumulated more total silver in the presence of sulfide than in its absence (Bianchini et al..
2002) owing to accumulation of unabsorbed sulfide-bound conventional silver on the gills (on thoracic
appendages) and in the digestive tract, but the sulfide ligand appeared to shield the organism from toxicity
(Bianchini et al.. 2005b).
In fresh waters, Cl also can react with silver ions to form relatively insoluble silver chloride
(AgCl) precipitates, thereby reducing the concentration of silver ions, as demonstrated in several studies
reported by Bielmyer et al. (2008). In seawater, however, the abundance of chloride anions favors the
creation of soluble silver-chloro complexes, of which the circumneutral AgCl0 (i.e., at a pH favoring the
neutral complex instead of ionic disassociation) has been shown to passively enter and accumulate in
rainbow trout (Wood et al.. 2002; Hogstrand and Wood. 1996).
Environmental Factors that Affect Bioavailability of Nano-Ag Particles
Many environmental factors could affect bioavailability of nano-Ag particles in aquatic ecosystems (see
Section 4.4.1.4), including surface water chemistry (e.g., ionic strength, pH, dissolved materials) and
suspended solids. Together with physicochemical properties of particles (e.g., size, shape, surface area),
18Lethal concentration is the chemical concentration at which 50% of the exposed organisms die; this effect level is
commonly used to estimate the toxicity of a substance to a specific group of organisms.
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environmental factors influence dissolution potential, clustering potential, resulting particle surface
properties, and interactions with dissolved and particulate organic matter in surface water (Boxall et al..
2007). Also, the capacity of wastewater treatment plants to remove nano-Ag from sewage (and to
"dispose" of it in sewage sludge) influences the overall amount of nano-Ag likely to reach surface waters.
Metallic nanoparticles released to surface waters tend to become coated with NOM quickly (Gao
et al.. 2009; Navarro et al.. 2008a: Boxall et al.. 2007). In aquatic ecosystems, the organic matter usually
originates from one or more of the following sources (Navarro et al.. 2008a):
• Fulvic compounds from humic substances, primarily from decomposition of plant
materials from terrestrial sources;
• Rigid biopolymers including polysaccharides and peptidogylcans produced by
phytoplankton or bacteria; and
• Flexible biopolymer recombination of decomposed organic materials.
Sorption to large clusters of organic matter or high-molecular-weight materials tends to remove
nanoparticles from solution, depositing them to the sediments (Handy et al.. 2008b; Klaine et al.. 2008).
On the other hand, complexation with low-molecular-weight organic materials might enhance the
particles' ability to stay in suspension (Hyung et al.. 2007). Also relevant to the bioavailability of nano-Ag
are any processes that might sequester free metal ions after their release to water from suspended
nanoparticles, such as complexation with available sulfides. No studies regarding the specific fate and
bioavailability of nano-Ag in the environment were identified for this case study. See Section 6.1 for
additional discussion of factors that might affect the bioavailability of nano-Ag as indicated by toxicity
tests with specified modifications to either the nano-Ag (e.g., coatings) or the test water (e.g., pH, Cl ,
ionic strength).
The rate of release or dissolution of silver ions from nano-Ag particles, and hence the
bioavailability of silver ions, generally decreases with increasing particle size owing to surface-volume
relationships. For example, Ho et al. (2010) examined dissolution of nano-Ag particles due to oxidation in
vitro with bacterial and mammalian cells. Using spherical nano-Ag particles of 5-20 nanometers (nm) in
diameter, they found that the rate of dissolution of nano-Ag particles decreased with increasing particle
size.
Toxicity tests have confirmed that sulfides can reduce the bioavailability of nano-Ag or silver ions
released from the nanoparticles. Choi et al. (2009) compared the toxicity, and by inference the
bioavailability, of nano-Ag (average size 15 ± 9 nm) to an enriched concentration of nitrifying bacteria in
the absence and presence of several possible ligands, including S2~, sulfate (SO42 ), Cl~, phosphate
(PO43 ), and EDTA. The source of the bacteria was a local nitrifying activated sludge plant in Missouri
(Choi etal., 2008). The biomass suspensions were aerated with pure oxygen to maintain a dissolved
oxygen concentration of approximately 20 milligrams per liter (mg/L) before the nano-Ag or silver ions
were added. Sulfide was the most effective ligand in reducing nano-Ag toxicity to the bacteria (80%
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reduction inferred from increased oxygen uptake rates). A back-scattered electron detector coupled with a
secondary electron detector was used to locate nano-Ag particles and Ag2S complexes attached to the
surface of the bacteria. Scanning electron microscopy (SEM) in conjunction with energy dispersive x-ray
analysis identified elemental composition. These techniques revealed that the nano-Ag particles reacted
with the S2~ to form new AgxSy complexes and precipitates that did not oxidize during 18 hours of
aeration. The toxicity of the test medium was enhanced, however, at S2 concentrations higher than
1 mg/L, presumably from inhibition of bacterial metabolism by the free available S2 (Choi et al.. 2009).
Release of silver ions from nano-Ag also is enhanced in oxidative conditions. In in vitro testing of
bacterial and mammalian cells under quasi-physiological conditions, Ho et al. (2010) found increasing
silver ion release rates, increasing toxicities, and therefore increasing bioavailability of silver ions from
nano-Ag particles with increasing concentrations of hydrogen peroxide (H2O2), a reactive oxygen species
(ROS), in the exposure medium.
In saltwater systems, the high ionic strength of seawater and high concentration of Cl~ can lead to
both the formation of nano-Ag clusters, which could precipitate out of the water column, and the
formation of soluble silver-chloro complexes; which of the two processes dominates will depend on the
water chemistry, presence of NOM, and nano-Ag surface treatments (Maccuspie. 2011). Also in saltwater
systems, exopolymeric substances, rich in polysaccharides and anionic colloidal biopolymers, are secreted
by phytoplankton and bacteria. Such substances could either protect the organisms from interactions with
nanoparticles (e.g., by initiating extracellular clustering of nanoparticles or by binding metal ions), or
enhance interactions (e.g., by adsorbing and holding nanoparticles in the immediate vicinity of cell
surfaces) (Miao etal.. 2009).
Using seawater, Miao et al. (2009) demonstrated rapid and complete clustering of
polyvinylpyrrolidone (PVP)-coated nano-Ag (60-80 nm), leaving no detectable particles smaller than 220
nm in solution. The addition of natural organic compounds or thiols greatly enhanced the presence of
nano-Ag in solution. Miao et al. (2009) found further that diatoms exposed to nano-Ag in the seawater
accumulated silver linearly with the estimated free silver ions in solution at higher nano-Ag
concentrations. It was not clear whether the higher accumulation of silver in the diatoms (measured as
microgram Ag per mg diatom carbon [ug/mg]) resulted solely from influx of silver ions into or beyond
the cell wall, or included accumulation of the neutral complex AgCl°, which was a few orders of
magnitude more prevalent in the solution (Miao etal.. 2009).
Although Tiede et al. (2010) reported that approximately 90% of nano-Ag added to sewage will
transfer to the sludge, the form of silver adsorbed to the sludge solids was not characterized in this study.
Two studies investigated the presence of nano-Ag in municipal wastewater treatment plants and reported
only nano-sized Ag2S (Kim etal.. 2010a) and the near complete transformation of nano-Ag to Ag2S
(Kaegi et al.. In Press) (see Section 4.4.2). Their findings suggest that insoluble Ag2S might be formed in
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situ under anaerobic, sulfur-rich conditions during wastewater treatment (Kim et al.. 2010a: Kaegi et al..
In Press).
The bioavailability of nano-Ag (and ionic silver released from nano-Ag) in sewage sludge applied
to terrestrial environments has not been investigated. Soils contain numerous ligands that can complex
nano-Ag and silver ions, and sewage sludge has substantial particulate organic matter to which nano-Ag
could bind depending on its size and surface coating (see Sections 4.1.2 and 4.3.1). The degree to which
nano-Ag might be available in sewage sludge applied to agricultural fields depends on properties of the
particles, properties of the sludge, and the soil medium.
5.2.1.3. Bioaccumulation Models
Models to estimate bioaccumulation of nano-Ag or silver ions released from nano-Ag in aquatic
and possibly terrestrial food webs are relevant because the nano-Ag in spray disinfectants is expected to
be released into wastewaters (see Chapter 3). An existing model that could be adapted for aquatic food
webs for at least the silver ions released from nano-Ag is the U.S. Environmental Protection Agency's
(EPA's) Bioaccumulation and Aquatic System Simulator model. Bioaccumulation and Aquatic System
Simulator is a ligand-binding model for positive ionic inorganic substances that includes toxicokinetic,
physiological, and ecological processes affecting chemical uptake directly from water in fish (Barber.
2008). Models for nano-Ag uptake from food via the GI tract, however, are not yet available.
For terrestrial ecosystems, a first step would be to examine the availability of measured
conventional silver accumulation factors for terrestrial plants and soil invertebrates. Baes et al. (1984) cite
a plant-soil bioconcentration factor (BCF) value of 0.138 for "above-ground" and 0.10 for "below-
ground" terrestrial plant parts consumed by humans, where the plant BCF value is based on total
dry-weight silver concentrations in the plants and soils. Values less than 1.0 generally indicate a very low
concern for bioconcentration (e.g., BCF values <100 are considered of low concern by programmatic EPA
offices), which might explain the paucity of data on conventional silver uptake by plants. If the behavior
of nano-Ag in the soil is similar to the behavior observed for ionic silver in other plant-soil studies,
nano-Ag could accumulate at low levels in metal-tolerant plant root systems (Sections 5.2.4.1 and
5.2.4.3). If such accumulation occurs, small mammals or other biota that consume roots and tubers might
be able to hyper-accumulate nano-Ag. No models were identified that investigate this food pathway in
terrestrial ecosystems. Bioaccumulation of inorganic chemicals from soils by earthworms (a commonly
studied soil organism) has been investigated for conventional silver compounds. The results of these
bioaccumulation studies vary: Some studies suggest that conventional silver does not bioaccumulate in
earthworms (Ratte. 1999) while other studies demonstrate that bioaccumulation from soil or pore water
does occur, but the kinetics of bioaccumulation are not well characterized (Nahmani et al.. 2009; 2007). A
recent study, however, did demonstrate bioaccumulation of both nano-Ag and silver ions in earthworms,
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with BAFs statistically significantly higher for ionic silver than for nano-Ag coated with either PVP or
oleic acid (Shoults-Wilson et al.. 2011a).
5.2.2. Uptake by Bacteria and Fungi
Many bacteria and fungi readily take up conventional silver and nano-Ag, as summarized in this
section by type of organism.
Bacteria. Only oxidized nano-Ag particles, that is, particles with chemisorbed silver ions on the
surface, exert antimicrobial effects (Loketal.. 2007). Such effects might be due to the combination of the
nano-Ag and the silver ions that are tightly adsorbed (via physical or chemical forces) on the particle
surface (Lok et al., 2007). Reduced nano-Ag appears to be unstable and easily oxidized.
Prokaryotes such as bacteria have a cell wall that separates them from their environment. Unlike
the more advanced eukaryotes, bacterial cells cannot perform phagocytosis or endocytosis, the two
processes by which nanoparticles might be absorbed into eukaryotic cells or organisms. Most bacteria
excrete siderophores, which chelate the relatively insoluble Fe3+ (ferric iron) ions in the environment,
forming a soluble complex that the bacteria absorb by active transport (Raymond. 2003). Although other
metal ions (e.g., aluminum) have been reported to complex with siderophores (del Olmo et al.. 2003). no
reports of silver ion chelation were found. In the environment, the ion for which siderophores have the
highest affinity is Fe3+ (Raymond. 2003).
The membrane structures of bacteria are classified into two groups: gram-positive and
gram-negative. The structural differences occur in the key component of the cell wall, peptidoglycan,
located immediately outside the cytoplasmic membrane. The cell wall of gram-positive bacteria (e.g.,
Bacillus, Clostridium, Listeria, Staphylococcus, Streptococcus) includes an ~30-nm-thick peptidoglycan
layer, while the cell wall of gram-negative bacteria (e.g., Escherichia, Salmonella, Pseudomonas)
includes only a thin, ~2-to 3-nm layer of peptidoglycan. The gram-negative cell wall also contains an
additional outer membrane composed of phospholipids and lipopolysaccharides facing the external
environment. The largest pores in the outer membrane of gram-negative bacteria secrete bacterial proteins
out of the cell and can measure almost 10 nm in diameter (Bitter et al., 1998). These pores are likely to
"open" only as needed for protein transport across the membrane (Filloux. 2004). Fixed porins, which
allow diffusion of smaller molecules in both directions across the outer cell membrane, are smaller, with
effective diameters for solutes of 1-2 nm. Yet despite the small pore sizes of both membranes, Xu et al.
(2004) demonstrated that nanoparticles up to 80 nm can enter Pseudomonas aeruginosa cells.
Furthermore, bacteria can then extrude these nanoparticles through the living cell membranes via an
extrusion pump consisting of two outer membrane proteins (MexA and MexB) and one inner membrane
protein (OperM), even though the pores associated with this extrusion pump are more than 50 times
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smaller than the nanoparticles they extrude. Cellular invasion and extrusion mechanisms of these larger
nanoparticles are not well understood.
Studies investigating uptake of nano-Ag by gram-positive bacteria are few (Panacek et al.. 2006).
with most studies focusing on gram-negative bacteria. Sondi and Salopek-Sondi (2004). Morones et al.
(2005). and Hwang et al. (2008) have shown that nano-Ag anchors to and penetrates the cell wall of
gram-negative bacteria. Wijnhoven et al. (2009b) and others have proposed that the physical penetration
changes the structure of the cell membrane (presumably the outer membrane first), which could increase
its permeability and result in uncontrolled transport of materials into and out of the cytoplasm. Others
have suggested the antibacterial mechanism of nano-Ag is the formation office radicals that damage the
membrane (Kim et al.. 2007; Danilczuk et al.. 2006). Hwang et al. (2008) proposed a synergistic toxic
effect of nano-Ag and silver ions from the nano-Ag in producing ROS in two strains of bioluminescent
bacteria (DS1 and DK1) sensitive to oxidative-stress damage. With nano-Ag sorbed to the bacterial cell
wall surface, silver ions can move into the cells and produce ROS inside. Hwang et al. (2008)
hypothesized that the membrane damage caused by the nano-Ag attachment and insertion demonstrated
by Morones et al. (2005) also might disrupt the ion efflux system, thereby preventing expulsion of the
silver ions from the bacterium.
Pal et al. (2007) assessed the influence of nano-Ag shape on toxicity to the gram-negative
Escherichia coll bacterium. They found that truncated, triangular, silver nanoplates exhibited the strongest
antibacterial activity and reported the top "basal plane of truncated triangular silver nanoplates [i.e., a
{111} facet] is a high-atom-density surface" (Pal et al.. 2007). Images obtained with energy-filtering
transmission electron microscopy (TEM) revealed that many of the nano-Ag particles adhered to the cell
surfaces were coincident with pits (depressions) in the cell wall. Using high-angle annular dark-field
scanning transmission electron microscopy, Morones et al. (2005) demonstrated that individual, roughly
spherical, nano-Ag with {111} facets attached directly to the outer cell membrane. In addition, nano-Ag
was found throughout the interior of cells. That physical disruption of cell membrane integrity by
nano-Ag might be the primary cause of antibacterial effects has been proposed, however, with
accumulation of nano-Ag in the cytoplasm occurring as a secondary effect (Neal. 2008).
One difficulty with interpreting literature on the interaction of nano-Ag with gram-negative
bacteria is that investigators do not distinguish the cytoplasmic membrane of the cell wall from the
external membrane (Hwang et al.. 2008; Pal et al.. 2007; Morones et al.. 2005). which together are only
2-3 nm thick. These distinctions are important for gaining a better understanding of the specific site and
mechanism of entry for nano-Ag into gram-negative bacteria.
Nitrifying bacteria. Nitrifying bacteria oxidize inorganic nitrogen compounds for energy
(chemoautotrophic). They can also oxidize ammonium ions to nitrites and nitrates, and are common in
municipal wastewaters. Key enzymes for these processes, including ammonia monooxygenase and nitrite
oxidoreductase, are organized along internal membrane systems. Choi et al. (2008) examined the effects
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of exposing nitrifying bacteria to nano-Ag (average size 14 ± 6 nm), silver ions (from AgNO3), and AgCl
colloids (average size 250 nm). Interactions between microbes and nano-Ag were examined using
environmental SEM, which can image hydrous samples. The images revealed that nominally 10-nm
nano-Ag from a commercial source, when mixed with nitrifying bacteria in suspension, formed clusters in
extracellular polymeric substances (from the bacteria), which resulted in larger particles ranging from 200
nm to a few micrometers. Electron micrographs demonstrated nano-Ag attached to bacterial cells. Choi
and Hu (2008) found that metabolic inhibition of nitrifying bacteria (as inferred from oxygen uptake
measurements) corresponded to the fraction of nano-Ag less than 5 nm in diameter, suggesting that only
small nano-Ag particles penetrate the cell wall and membrane.
Fungi. Fungi are eukaryotic organisms with a cell nucleus and distinct organelles. Nano-Ag is
fungicidal against many common fungi, including the genera Aspergillus, Candida, and Saccharomyces
(Wrjnhoven et al.. 2009b). Yeast is a unicellular fungus. The cell wall, plasma membrane, and periplasmic
space between the wall and membrane together account for approximately 15% of the total cell volume
(Feldmann. 2005). The membrane is selectively permeable, and the cells are capable of both endo- and
exocytosis. Both the cell wall and membrane are involved in budding (reproduction). To investigate
uptake and mode of action of nano-Ag on microfungi, Kim et al. (2009) used a budding yeast Candida
albicans exposed to spherical nano-Ag with an average diameter of 3 nm. TEM revealed that treated
fungal cells exhibited pits and holes in their cell walls and transmembrane pores through which cellular
constituents could leak. Comparisons, such as exposing yeast to surface-coated nano-Ag or to silver ions
from AgNO3, were not provided.
Ionic silver in solution can form nano-Ag in the presence of some fungi. For example, extracellular
nano-Ag particles between 5 and 25 nm in diameter have been produced by exposing the filamentous
fungi Fusarium oxysporum (Ahmad et al.. 2003) and Aspergillus fumigatus (Bhainsa and D'Souza, 2006)
to aqueous silver ions. Nano-Ag 5-15 nm in size is stabilized by proteins secreted by the fungus (Ahmad
etal.. 2003). Given the relatively low concentrations of ionic silver in surface waters, this particular
mechanism of nano-Ag formation is probably without consequence except for green synthesis methods.
Mukherjee et al. (2001) found "intracellular" nano-Ag of 25 ± 12 nm in Verticullium exposed to
aqueous silver ions. Electron microscopy revealed that the nano-Ag particles formed adjacent to the cell
wall surface but external to the plasma membrane, possibly as a result of reduction of the ions reaching
the periplasmic space by plasma membrane enzymes. No toxicity to the fungus was observed; the cells
continued to multiply after exposure and synthesis of nano-Ag. In this case, the fungi were essentially
removing free silver ions from the environment. Sauluo et al. (2010) also found intracellular nano-Ag in
addition to nano-Ag on cell surfaces of yeast (Saccharomyces cerevisiae) exposed for 24 hours to a
nanocomposite film (thin layer of nano-Ag particles, 5-10 nm in diameter, embedded in an organo-silicon
matrix) coating stainless steel. Damage to the cell wall structure and protein configurations inside the
cells was observed, but nucleic acids appeared normal.
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Viruses. Viruses are obligate intracellular parasites that replicate only within a living host. Viruses
themselves are not living cells having independent metabolic processes; they consist of a core of DNA or
RNA encapsulated in a glycoprotein coat. Speshock et al. (2010) found that Tacaribe virus exposed to
nano-Ag particles 10-25 nm in diameter prior to the virus' introduction to and infection of host cells
facilitated virus entry into the cells along with virus-attached nano-Ag. Once inside the cells, the virus
pre-treated with nano-Ag exhibited a significant reduction in viral RNA production and progeny virus
release compared with unexposed virus controls. The investigators concluded that nano-Ag likely was
binding to the virus surface membrane (e.g., to the thiol groups found in cysteine) and entering host cells
via endocytosis. The mechanism by which the nano-Ag deactivates viral replication in the host cells has
not yet been demonstrated. Other investigators have found that nano-Ag particles in the range of 1-10 nm
can bind to the HFV-1 virus, which inhibits the virus from binding to host cells (Elechiguerra et al.. 2005).
5.2.3. Uptake in Aquatic Ecosystems
In general, conventional silver contamination of aquatic ecosystems is thought to be of more
concern than contamination of terrestrial systems because of the high toxicity of silver ions to many
groups of aquatic organisms (Kramer et al.. 2009). Historically, most notable impacts of conventional
silver in the environment have been in the immediate vicinity of silver mines (Ratte. 1999) and in some
estuaries receiving wastewaters containing silver from photographic facilities (Flegal etal., 2007). As a
notable example, evaluations in San Francisco Bay suggest that waste silver originating at a photographic
processing plant was discharged from a regional water quality control plant through the late 1970s and
subsequently accumulated in estuarine sediments to high levels, leading to the Bay's characterization as
the "Silver Estuary" (Flegal etal.. 2007). Even after active discharging of silver wastes ceased, silver
concentrations remained elevated. For example, in intertidal mudflats of the southern reach of the estuary,
or South Bay, concentrations measured during that time were approximately 0.2-0.6 microgram per gram
(ug/g) (i.e., two to six times higher than regional background levels of less than 0.1 ug/g). Concentrations
of silver in the clamMacomapetalum dropped from approximately 100 ug/g in the late 1970s to 2-4 ug/g
by the late 1990s. Silver concentrations in the Asian clam Corbula amurensis dropped from
approximately 4 ug/g to 0.5 ug/g over the same time period. In both species, the drop in body tissue silver
corresponded with improved maturation of gonadal tissues and readiness to spawn. To date, however,
those scenarios appear to be the only real-world, documented situations in which silver contamination has
caused a high accumulation and adverse effects in aquatic biota (in benthic bivalves).
Once nano-Ag reaches an aquatic ecosystem from wastewater treatment facilities, or other sources,
its fate and the likelihood that it will contact aquatic biota depend on many factors. First, properties of the
particles (e.g., size, shape, coatings) and water chemistry (e.g., dissolved organic carbon, ionic strength,
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pH) will influence the extent to which the particles remain in suspension, partition to dissolved organic
carbon in the water column, form clusters with each other, and adsorb to suspended particles and
plankton. As noted in Chapter 4 and Section 5.2.1, the chemistry and content of natural fresh water,
estuarine water, and salt water generally favor formation of silver complexes and clusters that settle out of
the water column to the surface of the sediment bed. In freshwater systems, the formation of Ag2S and S2~
complexes removes silver ions from solution, while in saltwater systems the formation of AgCl or AgCl2"
predominates depending on the water chemistry, leading to both removal (AgCl precipitates) and
retention of silver ions in solution (soluble AgCl2"). Silver nanoparticles, on the other hand, might remain
in suspension if coated to prevent the formation of clusters or complexation with, or adsorption to, other
particles. Laboratory tests of the toxicity of nano-Ag to freshwater aquatic organisms generally use one or
more methods to ensure suspension of the nano-Ag in the water column. The methods generally do not
reflect conditions in natural surface waters. The current consensus is that nano-Ag rarely remains in
suspension in natural ecosystems, although opinions differ as to whether advances in technologies to keep
particles in suspension will alter this particle behavior (Luoma. 2008).
For animals, uptake of nano-Ag from water or pore water in sediments might occur at the gill
surface during respiration, following ingestion with food (e.g., detritus, algae, smaller animals), or dermal
absorption, depending on the form and bioavailability of nano-Ag and silver ions in food, water, and
sediments. Uptake into an organism versus adsorption to its surface depends on the nanoparticulate
chemistry at exterior surfaces, which could cause the particles to form clusters, and the behavior of the
particles inside cells and circulatory fluids (e.g., blood plasma) (Handy et al.. 2008a). Assuming that
nano-Ag particles remain in suspension in the water column, as the particles come in contact with the
surfaces of organisms, the following three scenarios are possible:
• No interaction; nano-Ag drifts away from the organism and continues movement in the
water according to Brownian motion. This option seems most likely for nano-Ag with
some type of surface coating, either a natural one acquired post-release (e.g., dissolved
organic matter) or manufactured (e.g., to keep nano-Ag in suspension until a spray
disinfectant is used).
• Nano-Ag particles adhere (adsorb) to the surface(s) of the organism; for example, to the
cell wall surface of phytoplankton and macrophytes, the carapace and appendages of
crustacean zooplankton, the epidermis of larval forms of invertebrates, the outer surface of
aquatic eggs, and, for larger animals (e.g., mussels, crabs, fish), to the exchange surface of
gills, olfactory receptors, and the lining of the GI tract for ingested particles. Uptake of
silver ions released from the nano-Ag in the vicinity of organisms is expected to the same
degree as uptake of silver ions released from dissolved silver salts, conventional silver
powders, or other sources of silver ions.
• Nano-Ag particles penetrate the surface of an organism. Whether a silver nanoparticle can
penetrate the outer cell wall or membrane of an organism depends on properties of the
particle and characteristics of the organism. Depending on the organism, various uptake
mechanisms are possible for nano-sized particles. Sorption to natural low-molecular-
weight organic materials in the surface water or some manufactured surface coatings might
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facilitate uptake of nano-Ag at the cell wall or cell membrane by increasing the particles'
lipophilicity.
Where nano-Ag particles do penetrate cell walls, cell membranes, circulatory systems, or
interstitial spaces of organisms, they might interfere with cellular structure and function by physical
(mechanical) forces, by providing a continuing source of free silver ions, or by both types of action. How
the route of uptake of nano-Ag (and silver ions from the particles) is influenced by surface modifications
of the particles is currently unknown (Behra and Krug. 2008). Presumably, smaller particles are more
likely to be taken up by endocytosis than larger particles. Routes of uptake for silver ions and nano-Ag
are discussed below by type of aquatic organism and environment: (1) freshwater algae, (2) freshwater
protozoa, (3) mollusks, (4) aquatic Crustacea, (5) fish eggs, (6) freshwater fish, and (7) saltwater fish.
5.2.3.1. Uptake by Algae
Freshwater algae appear to take up silver ions rapidly, presumably through a copper ion transporter
in the cell membrane (Lee et al., 2005). resulting in high BCFs (e.g., 10,000 liters per kilogram [L/kg] wet
weight) (Ratte. 1999). To evaluate the toxicity to and uptake of nano-Ag in green algal cells, Navarro et
al. (2008b) exposed Chlamydomonas reinhardtii for 1 hour to nano-Ag and silver ions with and without
cysteine as a ligand to decrease free silver ions in solution. The nano-Ag particles were evaluated for size
and zeta (Q potential using dynamic light scattering and were visually inspected using TEM. Particle size
ranged from 10 to 200 nm with a median of 40 nm; the diameter of 98% of particles was within 25 ± 13
nm. According to measurements with diffusive gradients in thin films, the maximum concentration of
silver ions (most of the labile silver measured) comprised approximately 0.9-1% of the total silver in
solution, while measurements with a Ag-ion-selective electrode indicated that the silver ion
concentrations were 0.7-1.2% of the total silver. With the addition of algae, silverions dropped to 0.1%
after 1 hour. The inhibition of algal photosynthesis in the presence of AgNO3 was completely eliminated
by the addition of equimolar concentrations of cysteine. Several indirect lines of evidence led the authors
to conclude that the toxicity of nano-Ag to the algae required interaction between the algae and
nanoparticle, but was mediated by silver ions released from the nanoparticles either at the algal interface
or near the algal interface where products of algal metabolism, notably H2O2, might be secreted.
In saltwater systems, the high ionic strength of sea water and high concentration of Cl can favor
either the formation of nano-Ag particle clusters or soluble silver-chloro complexes. Available natural
organic compounds (e.g., humic substances and thiols) can behave as surfactants, binding with nano-Ag
and stabilizing some of the particles in suspension (Hvung et al.. 2007). Exopolymeric substances
secreted by phytoplankton and bacteria (Verdugo et al., 2004) could either protect the organisms from
nanoparticle toxicity or enhance toxicity, as discussed in Section 5.2.1.2 (Wilkinson and Reinhardt 2005).
Algal uptake of nano-Ag in sea water was examined using the diatom Thalassiosira weissflogii
(Miao et al.. 2009). Diatoms are eukaryotic single-celled algae encased in a silica-based rigid "frustule"
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(essentially two glass half shells, or valves, fused together). Nutrients were limited to enhance excretion
of carbohydrates by the cells. Initial solutions of 60- to 80-nm nano-Ag in deionized water were well
dispersed, but when added to sea water, nano-Ag rapidly formed clusters, with no particles detectable in a
less-than-200-nm filtrate. The addition of natural organic compounds (fulvic acid from the Suwannee
River) stabilized some of the nano-Ag in suspension, and the addition of thiols increased nano-Ag particle
concentrations in the less-than-220-nm filtrate by a factor of 100. Because toxicity was not enhanced by
the addition of thiols, Miao et al. (2009) concluded that one or more protective mechanisms occurred:
Such protective mechanisms might be the carbohydrate coating of the silica shell repelled the particles,
the nano-Ag particles were too large to penetrate pores in the shells, or the coating of nano-Ag with
organic material prevented interaction with the diatoms. The exopolymeric coatings excreted by the cells
contain covalently bound proteins that might bind silver ions. The authors concluded further that the
observed toxicity of the nano-Ag was due to the release of free silver ions in the vicinity of the cell;
nano-Ag did not penetrate the cells.
Whether inside the algal cell or sorbed to the surface, nano-Ag can act as a continual, slow-release
source of silver ions (Lubick. 2008). To date, the data are consistent with nano-Ag adsorption to the outer
cell wall and release of silver ions in the immediate vicinity of the cell wall.
5.2.3.2. Uptake by Protozoa
Using the single-celled flagellate protozoan Paramecium caudatum, Kvitek et al. (2009)
demonstrated that nano-Ag particles approximately 30 nm in diameter were not toxic at concentrations as
high as 25 mg/L (paramecia survived for 7 days; the 1-hour LC50 was 39 mg/L), whereas silver ions
caused immediate death of all of the paramecia at concentrations as low as 0.4 mg/L (see Section 6.2.2).
The size and £ potential of the nano-Ag particles were measured by dynamic light scattering, and size was
confirmed with TEM. Modifying the nano-Ag with Tween 80 (1% w/w), a nonionic surfactant, increased
the bioavailability (as reflected in measures of toxicity) of the nano-Ag to the paramecia somewhat, with
a measured 1-hour LC50 of 16 mg Ag/L.
5.2.3.3. Uptake by Bivalve Mollusks
Investigators have noted that surface sediment-dwelling and filter-feeding mollusks are likely to
ingest engineered nanoparticles released to the environment, particularly if the nanoparticles associate
with natural particles (Moore. 2006). Mollusks accumulate conventional pollutants sorbed to suspended
particles and sediment (Galloway et al.. 2002; Livingstone. 2001). Lamellibranch bivalves (e.g., mussels,
scallops, cockles, most clams) filter food from the water passing over their gills, with cilia moving the
food particles to the mouth. Nanoparticles might be trapped and ingested with the phytoplankton and
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suspended detritus that comprise the food ("Wijnhoven et al.. 20091)). Moore (2006) postulated that benthic
marine bivalves such as the blue mussel (Mytilus edulis) might absorb ingested nanoparticles by
endocytosis.
Moore et al. (1997) and Owen (1970) demonstrated uptake of nano-sized particles and their
deposition in the digestive glands of marine mussels (Mytilus edulis) and cockles (Cardium edule),
respectively. In the case of mussels, the nanoparticles were composed of sucrose polyester oil (particle
diameters not reported). In vitro experiments demonstrated that isolated hepatopancreatic digestive cells
from the mussel took up the particles by endocytosis, and the internalized vesicles containing
nanoparticles subsequently attached to and released their contents in the lysosomal degradative
compartment of the cells (Moore et al.. 1997). In the case of cockles, animals were fed nanoparticles
comprising colloidal graphite and iron oxide in vivo, and animals were sacrificed and tissues removed and
fixed at several time intervals. The electron micrographs of digestive cells showed that some phagosomes
in the cells engulfed single particles (phagocytosis) of 50 to 300 nm in diameter. Smaller nanoparticles
appear to have been ingested by pinocytosis (i.e., through pinocytic vesicles with a characteristic granular
outer coat). Both types of vesicles transferred their contents to primary phagosomes in subapical regions
of the digestive cells, which ultimately connected to the lysosomal degradative compartment (Owen.
1970).
Although uptake of nano-Ag by bivalve mollusks has not yet been evaluated, the rate of uptake
likely would depend on the rate at which the animal moves water over its gills and the proportion of
nano-Ag particles large enough to be trapped by the gill lamellae, but small enough not to be ejected from
the food ingestion stream. Water filtration rates would depend on temperature and the size of the mollusk,
as well as reproductive status and other factors.
5.2.3.4. Uptake by Aquatic Crustacea
Uptake of silver ions from water has been examined for several species of Crustacea, with BCFs as
high as 1,100 and >2,200 for Gammarus pulex (Terhaar et al.. 1977) and the freshwater cladoceran
D. magna (Garnier-Laplace et al.. 1992). respectively. Uptake of silver ions is associated with branchial
sodium- and potassium-activated adenosine triphosphatase (Na+/K+-ATPase) in both crayfish (Grosell et
al.. 2002). which are tolerant of conventional silver, and in daphnia (Bianchini and Wood. 2003). which
are sensitive to conventional silver. Uptake of nano-Ag or silver ions from the nanoparticles could occur
in Crustacea through the adhesion of nano-Ag or nano-Ag clusters to the external carapace (e.g.,
planktonic crustacean) or gills (macrocrustacea). In addition, transport of ingested nano-Ag across the gut
epithelium by endocytosis might be possible.
Zooplankton feed by filtering large volumes of water through setae (i.e., structures similar to tiny
combs) on their appendages. The setae collect larger bacteria, algal cells, and possibly nanoparticle
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clusters (Baun etal.. 2008a). Ingestion of nanoscale titanium dioxide (nano-TiO2) clusters has been
verified by their presence in the gut of D. magna (Baun et al.. 2008a). Adhesion of clusters of nano-TiO2
to the exoskeleton and antennae of D. magna and carbon-60 (C6o) to antennae of the marine copepod
Acartia tonsa also have been demonstrated (Baun et al.. 2008a). Nano-Ag particles also could sorb to the
exoskeleton and antennae of planktonic invertebrates.
Zhao and Wang (2010) used a radiotracer method with no mAg to estimate uptake and efflux of
nano-Ag particles (carbonate coated, hydrodynamic diameter 40-50 nm, predominant particle size 20 nm)
in water and in food by D. magna. At low concentrations, the investigators found slower uptake of
nano-Ag than free silver ions (from AgNO3). At higher nano-Ag concentrations in water, uptake increased
disproportionately, possibly due to direct ingestion of the particles by the organism. To estimate dietary
assimilation efficiency, algal cells were pretreated with nano-Ag (with free silver ions removed by
addition of cysteine) for 12 hours and the Daphnia then were allowed to ingest the nano-Ag-treated algal
cells for 15 minutes. The assimilation efficiencies measured for algal associated nano-Ag ranged from
approximately 22% to 45%, with the lower assimilation efficiency associated with a higher nano-Ag
concentration in the algae.
Zhao and Wang (2010) also assessed Daphnia depuration rates after uptake of nano-Ag or free
silver ions (AgNO3) from water only. The percent radioactivity remaining in the cladocerans after transfer
to clean water declined rapidly in the first 2 days, followed by a slower elimination rate. After 4 days
depuration in clean water, retained 110mAg was similar for both the high (500 ug/L) and low (5 ug/L)
nano-Ag exposures (4.6-6.7%), whereas the 4-day retention after exposure to free silver ions was
significantly lower (2%). The investigators did not attempt to determine the distribution of retained
110 mAg in the animals (e.g., along gut lining, in internal tissues, on exoskeleton) or whether the retained
110 mAg from nano-Ag was associated with nano-Ag particles.
For the silver depurated over the 4-day period, most was released to water (approximately
45-73%), with the remainder released with molts of the exoskeleton (approximately 12-15%) and in
feces (10 to almost 40%) and a small fraction (2-4%) in neonates. Elimination in feces was highest for
the high nano-Ag exposure (500 ug/L), presumably because a larger proportion of the nano-Ag moved
straight through the gut without assimilation. The investigators did not discuss possible mechanisms of
elimination to water, nor did they determine whether the amount remaining after 4 days was associated
with nanoparticles.
After developing a model of nano-Ag uptake from both water and food using biokinetic parameter
values for the low-concentration exposures, Zhao and Wang (2010) estimated that approximately 70% of
the uptake of silver from nano-Ag by Daphnia in the environment would be via ingestion with food.
Thus, zooplankton exposure to silver can occur from nano-Ag associated with algae.
Although studies specific to the uptake of nano-Ag by other crustaceans are lacking, multiple
studies have demonstrated the toxicity of nano-Ag to these organisms (see Section 6.2.2.2 and Appendix
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B). Those studies indicate that uptake, or at least sorption to gill and possibly GI epithelia, with
subsequent release of silver ions, does occur.
5.2.3.5. Uptake by Vertebrate Eggs
Several university laboratories are assessing uptake of chemicals and nanoparticles by eggs and
embryos of the freshwater zebrafish Danio rerio. Three have published recently on uptake of nano-Ag in
aqueous suspension (Bar-Han et al.. 2009; Asharani et al.. 2008; Nallathamby et al.. 2008) and all have
reported that nano-Ag can be incorporated in the body of developing embryos. Asharani et al. (2008)
prepared nano-Ag "capped" with either soluble potato starch or bovine serum albumin (BSA) to prevent
cluster formation (both capping agents are considered nontoxic). TEM and surface plasmon resonance
analyses of stock solutions indicated an average size range of 5-20 nm, with a broader distribution of
sizes for the BSA-capped particles. Embryos of an unspecified age (likely less than 6 hours post-
fertilization) were exposed for 72 hours to one of five concentrations of capped nano-Ag or only to the
capping agent or to control water. Concentration-dependent toxicity was observed as described in Section
6.2.2.3 and Appendix B. Tissue analysis revealed significantly higher concentrations of residual total
silver in test organisms than in the controls, although no concentration data were provided in the study
report (Asharani et al.. 2008). TEM examination of older zebrafish embryos showed nano-Ag deposits
throughout the body, including the trunk, tail, skin, heart, and brain, and clusters of nano-Ag throughout
the epidermis. Closer examination of the trunk and tail showed most nano-Ag particles deposited inside
cell nuclei, with fewer in the cytoplasm. In the nuclei, nano-Ag particles were found as distinct clusters.
The investigators noted that nano-Ag appeared as clusters in most organs except the brain, where they
remained dispersed.
Laban et al. (2009) also observed "clumps" of nano-Ag distributed throughout fathead minnow
(Pimephales promelas) embryos after nano-Ag had attached in large quantities to the chorion surface.
Asharani et al. (2008) reported that nanoparticles that enter early embryonic cells (e.g., 4-cell stage) have
a high chance of being distributed throughout the embryo, although they did not state whether any of their
embryo exposures started at that stage (e.g., less than 1.5 hours after fertilization). Nano-Ag particles also
could be transported across the epidermis of later stage embryos.
Smaller nano-Ag particles can be absorbed more readily than larger nano-Ag particles. While
investigating the toxic mode of action of nano-Ag, Bar-Ilan et al. (2009) exposed 4- to 6-hour-old (post
fertilization) sphere-stage embryos to four size groups (3, 10, 50, and 100 nm) and concentrations of
either nano-Ag or nanoscale gold (nano-Au), which is relatively inert. Exposure water was renewed daily
for up to 10 days. The smaller size groups (3 and 10 nm) of nano-Ag produced a higher incidence of
sublethal effects than the larger sizes (50 and 100 nm), although mortality was similar across sizes.
Exposure to nano-Au in the same size categories and series of concentrations produced no measurable
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toxicity. Instrumental neutron activation analysis demonstrated that both nano-Ag and nano-Au adsorbed
to or were taken up by the embryos. The investigators provided no information on whether the size of the
nano-Au influenced the quantity of gold associated with the embryo, although they cited other studies
indicating better "translocation" of smaller sizes. Pent et al. (2010) studied the uptake and distribution of
fluorescent core-shell silica nanoparticles (FSNP) in the early life stages of zebrafish using 60- and 200-
nm particles. They found localization of both sizes of particles mainly in the egg chorion; the embryos did
not exhibit overt embryotoxicity.
The method by which nano-Ag enters the outer egg chorion was elucidated using real-time
visualization of nano-Ag particles traversing the zebrafish chorion (Nallathamby et al.. 2008; Lee et al..
2007). Nano-Ag has the highest quantum yield of Rayleigh scattering of the "noble" metal nanoparticles,
and scattering intensity is proportional to the volume of the particles. The bright particles can be observed
directly using dark-field single-nanoparticle optical microscopy and spectroscopy. Also, the localized
surface plasmon resonance spectra (wavelength/color) show size dependence, which allows a size
category to be estimated for nanoparticles of different visible colors. Using these techniques, Lee et al.
(2007) and Nallathamby et al. (2008) estimated the proportion of nano-Ag used in their initial
experiments to be -75% particles of 5-15 nm diameter, -23% particles of 16-30 nm, and only -1%
particles as large as 31-46 nm. They prepared the particles by reducing silver perchlorate (AgClO4) and
then washing and centrifuging the resulting nano-Ag to obtain highly purified particles without surface
coating. By adjusting the concentration of sodium chloride (NaCl) in the medium to maintain a low ionic-
strength solution, they could keep the £ potential of the particles high, and the particles were stable in
solution (i.e., did not form clusters) for months (Nallathamby et al.. 2008).
Lee et al. (2007) also demonstrated that the single nanoparticles in the test water did not form
clusters and could pass through zebrafish chorionic pores (500-700 nm in diameter) by diffusion
(Brownian motion). Some single particles remained in the pores, but most passed through into the
chorionic fluid where they continued to move by Brownian motion, as demonstrated by real-time video.
Some particles also diffused back out of the egg through the chorionic pores. The Brownian motion in the
chorionic fluid inside the egg was 26 times slower, however, than in the water in which the eggs were
placed, owing to the higher viscosity of the chorionic fluid than water. Few clusters of nano-Ag were
observed in the fluid. Estimated diffusion coefficients for the nano-Ag in the chorionic fluid revealed a
viscosity gradient across the embryo, yolk, and chorion. The nano-Ag that stayed (possibly adsorbed) in
the chorionic pores appeared to serve as nucleation sites for nano-Ag cluster formation, physically
clogging some chorionic pores, which would limit oxygen and waste exchange between the embryo in the
egg and its environment. This laboratory also demonstrated a counterclockwise movement of fluid within
the chorion with a range of viscosity gradients, with slower movement of nano-Ag in the more viscous
portions of the chorionic fluid (Nallathamby et al.. 2008).
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Most zebrafish embryos treated with nano-Ag concentrations less than 0.08 nM at or before the
cleavage stage (8-cell) completed development at 120 hours post-fertilization (Lee et al.. 2007).
Examining these fish using single-nanoparticle optical microscopy and spectroscopy, researchers
identified nano-Ag embedded in multiple organs, notably in the retina, brain, heart, gill arches, and tail.
Thus, some of the nano-Ag particles that entered the egg by diffusion reached the embryo the same way
and were taken up into the body of the developing embryo, possibly by endocytosis. Higher exposure
concentrations resulted in higher incidence of deformed and dead embryos, as discussed in Section
6.2.2.3.
5.2.3.6. Uptake by Freshwater Fish
Fish are exposed to chemicals in solution or in suspension in water at both their gills and GI
epithelia. Between the aquatic milieu and the external surface of the fish is an "unstirred layer," usually
with polyanionic mucus secretions (Handy et al., 2008a). The unstirred layer tends to be more viscous and
move more slowly than bulk water, thereby holding nanoparticles at the external surface of the organism
(Handy et al., 2008b). The various ligands present on the cell surface also are predominantly anionic.
Nanoparticles should generally diffuse across the mucous layer more slowly than single molecules such
as electrolytes and metal ions, and cationic nanoparticles might bind to strands of mucoproteins hindering
their uptake (Handy et al.. 2008a: Handy and Shaw. 2007). Cell surfaces also might present ligands for
nano-Ag (e.g., gill epithelium is predominantly anionic) (Handy and Eddy. 2004).
At the gills, metal cations can move through the epithelial membrane using specialized cation
transporter channels. Most nano-Ag particles, however, are too large to traverse the cation channels,
which have a pore diameter less than Inm. In the gut, where endocytosis is one method by which the
epithelial cells absorb nutrients, nanoparticle uptake through vesicular transport is possible (Handy et al..
2008a).
Nano-Ag, however, need not cross epithelial membranes to affect fish; adsorption to the gill
membranes is sufficient for nano-Ag to deliver silver ions, which are toxic to fish. Investigators have
hypothesized and provided evidence that exposure to silver ions blocks the Na+/K+-ATPase active
transport of Na+ from fresh water across the gills into fish (Bury et al., 1999; Morgan etal.. 1997). Recent
studies suggest that the mechanism of action of silver ions on the gill might be more complex and that
silver ions are absorbed by the basolateral gill membrane into the bloodstream, where they then travel to
and concentrate in the kidneys. Bury (2005) found increased sodium ion efflux rather than decreased
sodium ion uptake in juvenile rainbow trout (O. mykiss) exposed to sublethal concentrations of silver ions
(added as AgNO3). The Na+ ionic balance of the fish was restored by day 21 of the exposure, although
kidney K+-dependent/>-nitrophenol phosphatase activity was reduced and the total silver concentration in
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both gills and kidneys was elevated 20-fold. Indirect in vitro evidence suggested that the gill basolateral
membrane could sequester silver in membrane vesicles. The fish then could absorb the silver or expel it
back into the surrounding water (Bury. 2005).
Many studies measure total silver tissue burden following exposure to nano-Ag, although some
traditional methods used for tissue analysis (such as mass spectrometry) cannot distinguish silver
nanoparticles from silver ion concentrations (Griffitt et al.. 2009). A chronic study (10-38 days)
investigating the effect of nano-Ag on caudal fin regeneration in D. rerio reported that nano-Ag had
penetrated fish organelles, including mitochondria, nuclei, and blood vessels (Yeo and Pak. 2008). The
investigators measured the residual silver concentrations in fish muscle, intestine, and testes following
exposure to nano-Ag, but did not report the specific particle size or the age of the test organism. They did
report, however, that the total silver concentration was highest in the muscle 2 hours post exposure, but
that total silver concentrations in the muscle and testes decreased to nearly zero by about 100 hours post
exposure. Conversely, total silver concentrations in intestinal tissues of zebrafish, both with and without
amputated fins, continued to increase through 140 hours post exposure (Yeo and Pak. 2008).
Farkas et al. (2011) demonstrated that at least some types and small sizes of nano-Ag particles can
penetrate through the fish gill multicellular epithelial layer and enter gill cells by using in vitro cultured
primary gill cells of rainbow trout. Nanoparticle coating was an important factor for whether nano-Ag
entered cells. Citrate-coated nano-Ag particles, 3-40 nm in diameter, with an average of 12 nm, were
readily taken up into gill cells cultured in monolayers, whereas PVP-coated nano-Ag particles, from 1-60
nm with an average size of 7 nm, entered gill cells to a much lesser extent over the 48-hr exposure period.
As seen through light microscopy, the citrate-coated nano-Ag particles appeared to accumulate in light-
dense clusters around, but not in, the nuclei of the gill cells. Using TEM, the particles also could be seen
in as yet unidentified lamellar structures inside the cells. The slightly smaller PVP-coated nano-Ag
particles could not be seen in cells by using light microscopy, but were seen via TEM inside cells as
single particles or in small clusters of particles. No loss of silver was observed over a 48-hour depuration
period, and the authors speculated that elimination of nano-Ag particles from cells was unlikely or
inefficient. Using a multilayer gill epithelium in culture, Farkas et al. (2011) demonstrated that PVP-
coated nano-Ag primarily passes over and between the cells to penetrate the multiple layers comprising
the epithelium, whereas the citrate-coated nano-Ag tended to be absorbed into individual cells. These
results emphasize that the type of coating on nano-Ag particles can influence their behavior and transport
at biological interfaces such as fish gill epithelia. The results also demonstrate uptake of nano-Ag by gill
epithelial cells.
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5.2.3.7. Uptake by Saltwater Fish
Many investigators believe the uptake and effects of nano-Ag in saltwater fish occur by different
mechanisms than in freshwater fish. Freshwater and anadromous fish must be able to maintain body
fluids that are hyperosmotic compared with surrounding fresh waters, and they do so with active ionic
transport. Silver ions released by nano-Ag near or sorbed to the gill membranes are actively absorbed by
the Na+/K+-ATPase ion channels in the gills. Saltwater fish, on the other hand, are somewhat
hypo-osmotic with respect to surrounding sea water, and free silver ions in the water column readily bond
with abundantly available Cl to form AgCl, which precipitates from solution. For example, investigations
of the gulf toadfish (Opsanus beta), which can survive in a wide range of salinities if acclimated, have
indicated silver uptake. Above the isosmotic point of approximately 32% sea water, toadfish drink the
water and absorb Na+, CF, and water across the GI tract and actively excrete Na+ and Cl~ across the gills
and secrete ionic magnesium (Mg2+) into urine (Wood et al.. 2004). At lower salinities, the toadfish
actively takes up Na+ and Cl~ across the gills and retains the ions in the kidneys. Wood et al. (2004)
acclimated toadfish to salinities ranging from 2.5 to 100% sea water, followed by 24-hour exposure to
2.18 ugAg/Las 110mAg-labelledAgNO3. Speciation of silver varied with salinity: The silver chloride
anion AgCl32 dominated at 100% salinity and declined with decreasing salinity, while another silver
chloride anion, AgCl2~, dominated at intermediate salinities (10-60% sea water). Neutral dissolved AgCl0
was negligible at higher salinities but gradually increased with decreasing salinity to a concentration
approximately equal to AgQ2~ at the lowest salinity (2.5%). At all salinities, total silver ion
concentrations in solution decreased over the 24-hour exposure period due to adsorption and precipitation.
Only 5% of the total silver initially present in the test solutions was accounted for by the amount found in
the toadfish. Maximum total silver accumulation in the toadfish occurred at the lowest salinity tested,
2.5% sea water; minimum uptake occurred at 40% sea water.
Wood et al. (2004) also found that silver concentrations in bile were higher at lower sea-water
concentrations. Of the toadfish tissues, the liver showed the highest internal accumulation of silver, while
muscle concentrations were lowest. The authors concluded that silver ions entering the gills were
efficiently absorbed by the blood and distributed to other organs. They attributed the variation in patterns
of total silver accumulation in different tissues and at different salinities to at least two factors:
(1) salinity-dependent changes in the silver speciation and (2) salinity-dependent changes in the
ionoregulatory physiology of the fish (Wood et al.. 2004). The relative importance of ingestion and GI
tract absorption became greater with increasing salinity after the isosmotic point.
Nichols et al. (2006) examined total silver accumulation in gills and plasma of toadfish exposed for
a longer duration, 6 days. The investigators found the same pattern of decreasing silver accumulation with
increasing salinity, which was expected given the lower bioavailability of AgCl complexes formed at
higher salinities. The group also compared total silver uptake in water with and without NOM obtained
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from the Suwannee River. The addition of NOM appeared to reduce gill accumulation of silver in toadfish
only at salinities less than 40% sea water. At higher salinities, the organic matter did not appear to
influence silver accumulation in gills. In contrast, the addition of organic matter appeared to increase
silver concentrations in blood plasma at salinities less than 40% sea water. The investigators hypothesized
that the organic matter helped to keep more silver in solution, facilitating gill uptake of silver (Nichols et
al.. 2006).
Information presented in this section regarding the uptake of conventional silver might be relevant
for this case study; however, no data specific to the uptake of nano-Ag by saltwater fish were identified.
Additional information would be useful to fully assess the potential for exposures for saltwater fish.
5.2.3.8. Bioaccumulation in Aquatic Food Webs
As discussed previously, nano-Ag that sorbs to or is absorbed by organisms is defined as
bioavailable for the purposes of this document. A consumer organism (e.g., herbivore, carnivore) that
feeds on smaller organisms (e.g., phytoplankton, zooplankton, eggs, small fish) will ingest the nano-Ag
and other conventional silver compounds that are on or in their food. The question then becomes to what
extent the nano-Ag on or in the food is bioavailable to the consumer, either via interaction with the gut
epithelial cells or absorption by those cells followed by the particles passing through to the circulatory
system. This question is examined below for water-column and sediment communities. What remains
unknown for all groups is how the route of uptake of nano-Ag (and silver ions from the particles) is
influenced by surface modification of the particles (Behra and Krug. 2008).
Water-Column Organisms
Algae, the primary producers in the water column, show high bioconcentration of silver inside cells
when exposed to free silver ions, with BCF values as high as 10,000 to 100,000 in some studies (Ratte.
1999). Studies of algae exposed to nano-Ag focus on toxicity endpoints (e.g., growth inhibition) and the
contribution of silver ions to toxicity, rather than on calculating BCF values (Navarro et al.. 2008a).
Concentrations of total silver versus nano-Ag on or in the cell walls or in the cell cytoplasm have not been
explored. At a minimum, however, algal cells might concentrate nano-Ag particles relative to particulate
concentrations in the water column by adhesion of nano-Ag to the external cell wall, as has been
demonstrated for bacteria (Morones et al.. 2005).
Bioconcentration or bioaccumulation of free silver ions in filter-feeding zooplankton, the next step
up pelagic food webs, is on the order of a factor of 1000-5000, somewhat lower than for algae (Ratte.
1999). Zooplankton might sorb nano-Ag to setae, cilia, antennae, other appendages, gills, and the GI tract
epithelium (Zhao and Wang. 2010; Baun et al.. 2008a). This sorption would not be considered
bioaccumulation for those organisms, but could lead to bioaccumulation in their consumers.
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Bioaccumulation of nano-Ag would require uptake of the particles after ingestion, possibly by
endocytosis along the consumer's GI tract. Bioconcentration of silver ions could occur if nano-Ag
particles sorbed to gills, and then the water's chemistry promoted the release of silver ions from the
particles. On the other hand, crustacean zooplankton, which shed their carapace at regular intervals, might
facilitate sedimentation of nano-Ag particles sorbed to their exoskeletons (Ratte. 1999).
Some forms of complexed silver are bioaccumulated in aquatic crustaceans without evidence of
toxicity or internal absorption. Several studies have demonstrated bioaccumulation of silver atoms bound
with inorganic (-S) or thiol (-SH) sulfides without evidence of toxicity, presumably owing to a lack of free
silver ions (Kramer et al.. 2009; Bianchini et al.. 2002). In the presence of S2 , complexed silver can be
ingested and accumulated in the digestive tract of D. magna, enhancing the apparent "whole-body" silver
burden even though none is absorbed into the body of the animal (Bianchini et al.. 2005b). Although not
associated with internal accumulation or toxicity of silver ions to the D. magna themselves, the silver is
passed along the food chain to consumers of D. magna (Bianchini and Wood. 2008). For the consumers,
the ingested silver might or might not be adsorbed or absorbed by the GI tract.
Fish exhibit low BCF values for silver ions relative to BCFs reported for algae and zooplankton,
and the potential for bioconcentration or bioaccumulation of nano-Ag, either in solution or in food, has
not been examined. BCF values for free silver ions of approximately 1-350 have been measured for the
body and viscera offish on a wet-weight basis compared with surrounding water (i.e., for Cyprinus
carpio, P. promelas, and O. mykiss) (Ratte. 1999). Measures of bioaccumulation of total silver from the
diet were not found. For ingested nano-Ag that is adsorbed to or in food, the low pH in the gut might
favor formation of AgCl from silver ions released from silver nanoparticles (Panyala et al.. 2008). thereby
favoring dissolution of the nano-Ag in the gut. Larger nano-Ag particles with smaller surface-to-volume
ratios might take longer to dissolve, however, and they could be excreted in feces if not absorbed or
adsorbed by the gut. For mammals, small nano-Ag particles can be absorbed into the blood stream,
accumulated in the liver, and excreted back into the gut lumen in bile through exocytosis (Sadauskas et
al.. 2007). The silver then might be reabsorbed by the gut or excreted in feces (Sadauskas et al.. 2007).
The evidence cited above indicates that in general, bioaccumulation of silver atoms or compounds
appears to decrease with increasing trophic level in water-column food webs. The highest BCF values are
reported for algae (i.e., 10,000 to 100,000), with lower values reported for zooplankton (e.g., 5000) and
fish (i.e., 1 to 350). Fish show limited absorption of silver from their diet or from water in the first place,
and hence limited accumulation (Ratte. 1999). No data are available on potential bioaccumulation of
nano-Ag specifically.
The extent to which water-column biota might alter nano-Ag after coming in contact with
nanoparticles in the environment is another consideration for food chain accumulation (Behra and Krug.
2008); no studies were identified, however, that examined the bioavailability of nano-Ag in various water-
column biota to consumer organisms up the food chain.
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Sediment Organisms
Sediment is likely to be an important sink for nano-Ag clusters, nano-Ag, silver ions sorbed to
particles, and silver ions precipitated in insoluble compounds. Detritus (i.e., decaying plant and animal
materials) and microorganisms form the base of food webs originating in the benthos. Benthic detritivores
and filter-feeders contact and ingest relatively large quantities of detritus, associated microorganisms, and
plankton near the sediment surface. Thus, silver in sediments could enter aquatic food webs through
benthic organisms.
In general, BCF values for small crustaceans exposed to soluble silver salts added to
sediment/water systems are close to or less than 1 (Ratte. 1999). BCF values measured for the same
organisms (e.g., freshwater G. pulex, aquatic Chironomus luridus) in water only, however, can be three
orders of magnitude higher (e.g., 1100) (Ratte. 1999; Garnier-Laplace et al.. 1992). These findings
suggest that much of the silver in sediments is not bioavailable, presumably due to the high availability of
ligands for silver ions in sediments.
One category of benthic organism that can bioaccumulate total silver by a factor of 1,000 or more
is bivalve mollusks (Ratte. 1999). As filter feeders, bivalves can accumulate silver ions directly from
water and from any silver on or in their food near the sediment/water interface. Terhaar et al. (1977)
determined wet-weight BCF values of up to 1,400 for the freshwater bivalve Ligumia spp. In San
Francisco Bay, the Macomapetalum clam accumulated total silver at concentrations five to seven times
higher than in the phytoplankton (Reinfelder et al.. 1998) and acquired between 40 and 95% of the silver
through their diet (Griscom et al.. 2002).
The extent to which silver transfer from the benthos into aquatic food chains might occur depends
on many factors, including bioavailability and the consumer organisms. No studies of nano-Ag
bioaccumulation in benthic invertebrates or their predators were identified.
5.2.4. Terrestrial Ecosystems
The uptake of dissolved silver ions and complexes from soils by terrestrial plants and soil micro-
and macrofauna has been investigated in a few laboratory and field experiments. Uptake of nano-Ag by
soil invertebrates has been investigated in the laboratory.
5.2.4.1. Uptake by Terrestrial Plants
Available evidence indicates that terrestrial plants can take up and accumulate silver ions when
conventional silver is present at high concentrations in the surrounding medium. For instance, sweet corn
(Zea mavs cv. Sundance), lettuce (Lactuca sativa cv. Ithaca), oats (Avena sativa cv. Hercules), turnips
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(Brassica rapa cv. Just Right), and soybeans (Glycine max cv. HT1779) grown on soils amended with
sewage sludge spiked with Ag2S showed below-ground accumulation of silver after exposure for 76, 56,
64, 54, and 93 days, respectively. Total silver concentrations in the roots of these plants after exposure to
soil with 12 or 106 mg/kg silver ranged from 2.0 to 33.8 milligrams per kilogram (mg/kg) dry weight,
which was significantly greater than concentrations in corresponding controls (Hirsch. 1998a). Silver
concentrations in the aboveground parts of the same plants were consistently lower than in the roots of the
same plants and only portions of the stalks of corn and oats had concentrations that were significantly
higher than in corresponding controls (Hirsch. 1998a). In a similar study by the same author with higher
concentrations of silver (68 mg/kg or 155 mg/kg) in the soil, a significantly higher concentration (0.69
mg/kg dry weight) of silver was observed in lettuce leaves compared to corresponding controls, but no
significant difference from control was detected in the leaves of Chinese cabbage (Brassica campestris cv.
China Doll), or spinach (Spinacia oleracea cv. Melody) (roots were not examined) (Hirsch. 1998a). The
ability of plants to take up and accumulate silver ions in plant tissues, however, remains inconclusive as
Ag2S is less soluble than other forms of silver. Concentrations of silver in trees grown in areas subject to
silver iodide (Agl) cloud seeding exhibited total silver concentrations between 1 and 13 mg/kg in
aboveground leaves, twigs, bark, and wood (Klein. 1978). In this case, exposure through deposition on
leaves would have been possible in addition to root exposures.
Harris and Bali (2008) demonstrated "hyper" accumulation of silver ions by Brassica juncea (a
mustard plant) andMedicago sativa (alfalfa), two species known to be metal tolerant. The plants were
grown hydroponically from seeds for 4 weeks in demineralized water, and then moved to Petri dishes
containing aqueous solutions of AgNO3 at concentrations up to 1% silver by weight. The ratios of silver in
the plant tissues (entire plant assays) to the silver concentration in the aqueous growth medium ranged
from 6 to 67 for alfalfa and from 10 to 124 for mustard.
Limited evidence is available comparing uptake and translocation of conventional silver in
terrestrial plants to nano-Ag, however, available data suggest that nano-Ag is taken up by some plants.
Yin et al. (2011) reported that nano-Ag coated with gum arabic were internalized by common grass
(Lolium multiflorum) by either direct uptake in the roots followed by the release of ionic silver species
within the root tissues, or by dissolution of the nano-Ag particles on the root surface followed by
internalization of the ionic species by the roots. The investigators noted that most available evidence
suggests that nano-Ag adsorbs to plant roots and crosses the cell membrane by oxidative dissolution, at
which point internalized silver can be translocated to other plant tissues (Yinet al.. 2011).
Stampoulis et al. (2009) reported total silver concentrations in zucchini shoots exposed by roots to
nano-Ag in hydroponic solution that were almost five times higher than in zucchini shoots exposed to
equivalent concentrations of conventional (powdered) silver. Conversely, Hawthorne et al. (2012)
reported that uptake of Ag was not particle-size dependent (i.e., similar concentrations reported for
nano-Ag and bulk Ag). Ma et al. (2010) reported unpublished findings in their laboratory that show
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nano-Ag particles as large as 40 nm could be taken up at the roots and transported to shoots of the dicot
thale cress (Arabidopsis thaliana), although most of the nano-Ag adhered to the root cap. These studies
suggest that formulations of nano-Ag and conventional silver are taken up by plants and that transport of
at least silver ions to shoots occurs. In support of this hypothesis, Judy et al. (2011) demonstrated that
tobacco plants (Nicotiana tabacum L. cv Xanthi) accumulate gold (Au) following exposure to 100 mg
Au/L nano-Au (5, 10, or 15 nm; coated with tannic acid) in deionized water. Inductively coupled plasma-
mass spectrometry (ICP-MS) analysis detected mean gold concentrations of 40.3, 95.8, and 61.7 mg
Au/kg dry tobacco weight for the 5-, 10-, and 15-nm nano-Au treatments, respectively. Gold was verified
to be present not only on the leaf surface but throughout the plant tissue cross section.
5.2.4.2. Uptake by Soil Macrofauna
The limited available studies suggest that invertebrates might take up nano-Ag and silver ions from
soil. Nematodes, which are multicellular, usually microscopic, organisms in soil communities that feed on
bacteria and detritus, have been shown to take up nano-Ag (Meyer et al.. 2010; Roh et al.. 2009). as have
earthworms (Eisenia fetidd) (Shoults-Wilson et al.. 201 Ib) (see Section 5.2.4.3).
Using genomic, proteomic, and cellular-level endpoints, Roh et al. (2009) demonstrated uptake of
nano-Ag particles in the nematode Caenorhabditis elegans. 3-day-old nematodes cultured on agar growth
medium with E. coll for food were exposed to nano-Ag in water for 24 or 72 hours. The nano-Ag
particles (smaller than 100 nm) were dispersed in deionized water by sonication for 13 hours, stirring for
7 days, and filtering through a cellulose membrane with pore size of 100 nm to remove nano-Ag clusters
in solution. Measures of light scattering using dark-field microscopy indicated uptake of nano-Ag into the
body of the nematodes and clustering of nano-Ag predominantly around the uterine area. The
investigators used physical rather than chemical means to disperse the nano-Ag in aqueous solution to
provide relevance to environmental exposures, and they demonstrated that nano-Ag was absorbed into the
body of the nematode by some unidentified mechanism(s).
Using darkfield microscopy in combination with a CytoViva visible and near-infrared
hyperspectral imaging system, Meyer et al. (2010) also demonstrated ingestion and absorption of
nano-Ag by C. elegans. All three types of roughly spherical particles used (citrate-coated particles with a
mean diameter of 7 nm and PVP-coated particles with mean diameters of 21 and 75 nm) were internalized
by the nematodes. The citrate-coated 7-nm particles caused the nematode to retain developing eggs,
which allowed the investigators to visualize transfer of nano-Ag into the fertilized eggs. In a soil
environment with organic matter capable of sorbing nano-Ag, however, nano-Ag and silver ions might be
largely immobilized and not available for uptake by nematodes.
The study by Judy et al. (2011) using nano-Au offers some insight into whether silver nanoparticles
might be taken up by another soil macroinvertebrate. Judy et al. (2011) reported accumulation of Au in
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the tissue surrounding the gut lumen of tobacco hornworms (Manduca sexto) after consumption of
nano-Au-treated tobacco. These concentrations were detectable by sunchrotron uXRF scans of hornworm
cross sections from each treatment (0 or 100 mg Au/L deionized water; treatment particle diameter size
classes of 5, 10, and 15 nm). ICP-MS also verified that the gold detected in the hornworms was
attributable to nano-Au from the plant material.
5.2.4.3. Transfer through Terrestrial Food Webs
As discussed in Section 4.3, the major pathway by which nano-Ag from indoor uses of spray
disinfectants could reach terrestrial ecosystems is expected to be application of sewage sludge to soils
(e.g., for agriculture). Terrestrial organisms also might be exposed to nano-Ag in contaminated water
from flooding or crop irrigation, or possibly through deposition of nano-Ag suspended in air following
outdoor use of nano-Ag products. With the possible exception of areas near silver mining operations
(Kramer et al.. 1994). bioaccumulation of silver in macroflora and macrofauna of terrestrial ecosystems,
even in areas with silver-spiked sludge applications (Hirsch. 1998a). does not appear to have been
observed. This section discusses uptake of silver ions in solution or in soils by plants, and whether
nano-Ag applied to soils could reach herbivorous species. The potential for nano-Ag applied to soils in
sewage sludge to reach insectivorous wildlife through soil invertebrates in direct contact with soils is
considered. Finally, this section considers the potential for indirect ecological effects of nano-Ag as a
result of inhibition of soil microorganisms (e.g., decomposing bacteria, nitrifying bacteria, nitrogen-fixing
bacteria, fungi), which could disrupt soil nutrient cycling to support food webs. To date, however, no data
on the presence of nano-Ag in soils have been published (Wrjnhoven et al.. 2009b).
Transfer through Terrestrial Plants
A single published study of plants exposed to nano-Ag in soils or solution was found. Investigators
exposed zucchini to nano-Ag in hydroponic solutions and found that some form of silver, possibly silver
ions, was transported to shoots (Stampoulis et al.. 2009). Translocation of gold from roots to other plant
parts was also demonstrated by Judy et al. (2011) following exposure of tobacco plants to nano-Au.
Some studies have demonstrated that most silver ions and complexes dissolved in solution and
taken up by metal-tolerant plants accumulate in the roots, possibly in intercellular spaces (e.g., apoplast)
(Nowack and Bucheli. 2007). and are not transported to other parts of the plant (Ratte. 1999). As noted
previously, some metal-tolerant terrestrial plants have been demonstrated to hyper-accumulate silver ions
from water (Harris and Bali. 2008). Young B. juncea (a mustard plant) andM sativa (alfalfa) plants
exposed to high concentrations of AgNO3 exhibited BCF values greater than 10 (concentration in fresh,
hydrated plants:concentration in water), with silver sequestration occurring in the form of large numbers
of silver nanoparticle clusters.
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As yet, no evidence indicates that nano-Ag in soils with sewage sludge amendments is likely to
accumulate in the foliage, fruits, or vegetables of plants above ground; however, higher concentrations of
nano-Ag in soils, might lead to different conclusions. If nano-Ag or silver ions in soil pore water are taken
up by plant roots, they might be sequestered in the roots, possibly as nanoparticles in extracellular spaces
such as the apoplast. Some silver, probably in the form of silver ions, however, might be transported to
plant shoots and beyond. Whether nano-Ag, and excess silver in any form, in sewage sludge applied to
agricultural fields might accumulate in root and tuber vegetables has not yet been investigated.
Among herbivorous wildlife, only two groups might be highly exposed to total silver in plants due
to their ecology; however, these exposures might still be below the thresholds for toxic effects. Small
burrowing mammals that consume plant roots might ingest quantities of nano-Ag stored in the roots of
metal-tolerant plants, and grazing animals might ingest silver that accumulates in stems of a variety of
herbaceous plants. Herbivorous insects feeding on plant roots and stems also might be exposed to
accumulated silver. The potential for silver exposure via these pathways to affect grazing animals and
insects has not been investigated. The potential bioavailability of nano-Ag particles sequestered in plant
roots of the hyper-accumulating species also has not been examined.
Transfer through Soil Macrofauna
Shoults-Wilson et al. (201Ib) reported the bioaccumulation of nano-Ag in earthworms (Eisenia
fetida) exposed to nominal concentrations of 10 or 100 mg AgNO3/kg dry soil or nano-Ag at
concentrations of 10, 100, or 1,000 mg Ag/kg dry soil (30-50 nm nominal size range; coated with either
oleic acid [OAJor PVP) for 28 days. Authors noted that measured tissue concentrations of silver were
concentration-dependent and that organisms exposed to similar doses accumulated significantly lower
concentrations when exposed to nano-Ag compared to AgNO3 (up to 5.5-fold less accumulation from
nano-Ag-treated soils than from AgNO3-treated soils). Accumulation was noted to be comparable
between the two nano-Ag treatments, OA and PVP, even though the former displayed hydrophobic
behavior while the latter displayed hydrophilic properties (hydrophobicity is often inversely correlated
with bioavailability). Similarly, no significant differences in tissue concentration or BAF between the two
treatments were observed.
Judy et al. (2011) reported measured nano-Au in greater concentrations in the tobacco hornworm
(Manduca sexto) after it fed on treated tobacco (Nicotiana tabacum L. cvXanthi) for 1 week than was
measured in the treated tobacco alone (experimental set-up was designed to disallow access to plant
roots). Authors reported that concentrations in the hornworm tissue exceeded that of the dried tobacco
tissue by mean factors of 6.2, 11.6, and 9.6 for the 5-, 10-, and 15-nm treatments, respectively, suggesting
that some nanometals might accumulate in soil invertebrate tissues following ingestion of lesser
contaminated plants.
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Disruption of Ecological Functions of Soil Microorganisms
Nano-Ag toxicity in soil microorganisms could adversely affect an ecosystem at large. Of
particular concern are the possibilities that silver might inhibit bacteria that fix atmospheric nitrogen (in
symbiosis with legumes and other plant species), disrupt denitrifying bacteria (which convert nitrates to
nitrogen gas), and impair decomposing and lithotrophic bacteria and other microbes that release essential
nutrients from inorganic and organic matter in soils (Panyala et al.. 2008).
Senjen (2007) reported that few studies have been conducted that directly evaluate the
community-level effects of nano-Ag exposure for soil microbial communities. Many laboratory studies,
however, have evaluated the toxicity of nano-Ag to microorganisms that might be found in soils,
particularly bacteria (see Section 6.2.1). In theory, disruption of the nutrient cycling roles of soil
microorganisms could result in a cascade of adverse impacts on the structure and composition of
terrestrial plant communities, and then on the animal communities as well. A risk assessment based on
projections for nano-Ag discharge to the Rhine River in 2010 concluded that most silver released into
wastewater would be collected in sewage sludge, which often is spread on agricultural fields (Blaser et
al.. 2008). Environmental concentrations were predicted for the river water, river sediment, interstitial
water of the sediments, and the wastewater entering sewage treatment plants. Although Blaser et al.
(2008) could not exclude possible risks to benthic organisms in the river, they concluded that the
nitrifying bacteria in the sewage treatment plants were at negligible risk of impairment.
5.3. Human Exposure
One obstacle to measuring or estimating exposure of humans to nanoparticles is the lack of
consensus regarding the particle properties that require characterization and on which metric to use to
express exposure concentrations or dose. Further complicating human exposure assessments is the lack of
broadly applied methods for distinguishing background and incidental exposures from source-specific
contributions to total nanoparticle exposures. For example, atmospheric particle collection methods that
collect ultrafine inorganic particles cannot, without further chemical analyses, separate the fraction
comprising manufactured nanoparticles (SCHER. 2009). Data on nanomaterial use in consumer products
rely on information provided by the manufacturers and do not account for uses that might be "off-label,"
or products that incorporate nanomaterials but are not labeled as such. In addition, independent
investigations of some nano-enabled products have revealed that actual particle size range, concentration,
composition, and purity often differ from what is reported by the manufacturer, and different products
with different formulations would result in exposure to a variety of substances in conjunction with the
nanomaterials added intentionally to the products. This section describes the current methods and data
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available for evaluating human exposure to nano-Ag from spray disinfectant use in homes and institutions
(Section 5.3.1) and during manufacture (Section 5.3.2).
5.3.1. General Population Exposure
Chemical form, shape, concentration, zeta potential, exposure route(s), and media concentration(s)
have been highlighted as important parameters when evaluating human exposures to nanoparticles
(SCHER. 2009; Wijnhoven et al., 2009a). The application scenario (including how much, how long, how
often, and how many people use the product) of a chemical also should be considered. Despite the rapid
penetration of nanomaterials into the consumer market, little information on their content or the content of
other ingredients in consumer products is publicly available. The absence of that information precludes a
complete exposure assessment. At the time of the last literature search for this document, no studies were
identified that empirically examined exposure to nano-Ag from spray products, and only one study
(Hagendorfer et al., 2009) was found that examined the form of nano-Ag released from spray products
that contain nanomaterials (see Section 3.4)19. The results of this study suggest that single silver
nanoparticles and clusters with diameters less than 100 nm can remain as aerosols for at least several
minutes following use of a gas-propellant sprayer delivering a water-based nano-Ag suspension to the air.
Although the nano-Ag released from the spray nozzle is homogenously dispersed in nanoscale water
droplets, the nano-Ag particles and the clusters formed after release can remain suspended in air after the
water evaporates. Conversely, no silver particles of any size were detected in the air following use of a
pump spray mechanism, suggesting that any single silver nanoparticles or clusters were more likely to
settle onto surfaces close to the spray nozzle (the size range of these particles was not investigated in this
study) (Hagendorfer et al., 2009). These findings suggest that consumers might be exposed to nano-Ag
aerosols or to nano-Ag particles that have settled on surfaces during or after use, depending on the spray
delivery mechanism. The degree to which the use of solvents other than water affect dispersion of
nano-Ag in aerosolized liquid droplets, how quickly the liquid solvent evaporates in air and on surfaces,
and the form of the nano-Ag to which consumers are exposed, however, are unclear.
Hansen et al. (2008) recently developed a framework for conducting general population exposure
assessments from products containing nanomaterials. As part of this framework, they categorized the 580
products listed in the 2007 Woodrow Wilson Center consumer product inventory based on whether the
nanoparticles were bound to the surface of the product, suspended in liquid, suspended in solids, or
available as free, airborne particles. Using these characteristics, exposures were categorized as "expected"
19Two additional studies (Nazarenko et al.. 2011) and (Quadros and Marr. 2011) that examine nano-Ag released
from spray products containing nanomaterials became available after the completion of the last literature search and
were not identified during peer-review.
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(e.g., due to direct exposure to liquids containing nanoparticles or free, airborne nanoparticles), "possible"
(e.g., due to release of surface-bound nanoparticles owing to wear and tear), or "not expected" (e.g.,
particles are encapsulated in solids and are not released). Based on the types of products claiming to use
nanomaterials and the likely exposure scenarios for those products, the authors concluded that nano-Ag
has the highest possible consumer exposure of all nanomaterials considered, with roughly 25% of the
products categorized as "possible" exposures and 50% categorized as "expected" exposures (Hansen et
al.. 2008). Most of the exposure scenarios (53) for nano-Ag were for surface-bound products, but a large
proportion of scenarios was for exposure to nano-Ag suspended in liquids (33) or suspended in solids (28)
(Hansen et al.. 2008). For spray disinfectants, inhalation, oral, and dermal exposures could be either to the
wet formulation or to dry nano-Ag particles after the carrier vehicle evaporates.
Expert elicitation has been used to assess potential for exposure to approximately 50 nanomaterials
used frequently in consumer products (Wrjnhoven et al.. 2009a). A panel of seven experts from the
Netherlands National Institute for Public Health and the Environment was assembled to independently
identify the most significant exposure characteristics and to rank consumer products according to
potential exposure. Characteristics of each product and the nanomaterial in the product were used to
generate rankings of potential exposure (high, medium, or low). Six product categories in which
nanomaterials are purportedly used were assigned the rank of "high" potential for exposure: sunblock
cosmetics, oral hygiene products, health products, fuel, coatings and adhesives, and cleaning products.
Cleaning products (the category under which nano-Ag spray disinfectants would fall) were generally
labeled by Wrjnhoven et al. (2009a) as "do-it-yourself," suggesting that products requiring application by
the user might lead to higher exposures. For cleaning products containing nanomaterials, the expert panel
determined that the characteristics of primary concern are the form of the product (e.g., spray, powder,
liquid, suspension, solid, coating), the form of the nanomaterial (e.g., free particles, particles fixed inside
a matrix), the potential for direct versus indirect exposure from application (e.g., direct exposure to
nanoparticles in the product, indirect exposure from particles released from the product), and potential
exposure route (e.g., inhalation, dermal, oral). The characteristics of nano-Ag sprays led them to be
categorized as "high-potential-exposure" products. These characteristics include: the spray form of the
product, in which particles are free and not fixed; the potential for direct exposure through application;
and the potential for exposure through multiple indirect routes.
In one of the few studies that have modeled exposure to nanoparticles, Mueller and Nowack (2008)
analyzed nano-Ag, nano-TiO2, and carbon nanotube use and release into the environment in Switzerland,
albeit using substantially simplified assumptions as discussed in Sections 3.1 and 4.5. They estimated
release rates for products containing nano-Ag and concluded that, although sprays and cleaners account
for only 15% of the nano-Ag in consumer products, the bulk of release (95%) from this use occurs during
application when the consumer is likely to be exposed (Mueller and Nowack. 2008).
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Wardak et al. (2008) created a hypothetical exposure scenario for the use of an air-freshener spray
containing nano-Ag that would also act as a disinfectant. Based on expert elicitation, Wardak et al. (2008)
concluded that the most significant human exposures would occur via inhalation and dermal pathways
and that the most significant environmental exposures would result from water entrainment
(i.e., suspension of nano-Ag in water) after disposal. This approach, which is similar to that used by
Wrjnhoven et al. (2009a). was reported by the authors to be useful for estimating risk when few or no data
on environmental concentrations exist. The authors assumed that the nano-Ag would be contained in a
liquid matrix (Wardak et al.. 2008). Exposures during and after application were considered possible.
Inhalation was estimated to be the primary exposure route, followed by dermal, and to a lesser degree
ingestion. Exposure to workers who manufacture or use nano-Ag spray disinfectant in occupational
settings is discussed in Section 5.3.2.
The potential for secondary human exposures, after nano-Ag particles have been released into the
environment outside of homes and facilities where used or released, has not been investigated.
Environmental nano-Ag can change form in the environment due to processes such as dissolution, cluster
formation, complexation, and other processes that can significantly affect its transport, transformation,
and fate processes and potential biological activity, as described in Section 4.1.1. The silver itself cannot
be degraded and persists indefinitely.
5.3.1.1. Respiratory Exposure
Consumers could be exposed to nano-Ag particles as a result of inhaling spray disinfectants during
application or disposal, particularly in confined spaces (e.g., laundry room or kitchen). Data on spray use
and disposal, and on the concentrations of ingredients other than nano-Ag, such as capping agents and
stabilizers involved in the manufacturing process and disposal of waste by-products, were not identified
in the literature. The concentrations of nano-Ag in end-use products, the application rates, and the use
profiles for these products are not yet known (refer to discussion in Section 2.2), but some hypotheses
have been proposed. Wardak et al. (2008) independently surveyed eight experts from five areas of
expertise (environmental sciences, toxicology, chemistry, material sciences, and technology policy) and
asked them to score (from 1 = low to 5 = high) the exposure and hazard potential of nano-Ag disinfectant
air-freshener sprays. The authors concluded that inhalation by users represented the highest potential
human exposures. The experts believed that nano-Ag spray uses also might result in elimination of useful
bacteria in susceptible populations (Wardak et al.. 2008).
Children in the home might inhale aerosols containing nano-Ag formed when others are spraying
the material; no data were found, however, on the proximity of children to adult disinfectant spraying
activities or how long nano-Ag from the air-freshener sprays would remain airborne. Children might be
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exposed to higher concentrations of airborne nano-Ag near the floor level from original spraying activities
and from resuspension of dust and nanoparticles containing nano-Ag as they crawl or play on the floor.
Again, no information was found to quantify this possibility. Children also have higher metabolic rates
than adults, and therefore must consume more oxygen. With higher inhalation rates relative to body
weight than adults, children will experience higher exposures to air pollutants than adults at the same
ambient air concentrations (Bearer. 1995).
5.3.1.2. Dermal Exposure
Dermal exposure to nano-Ag could result from spray deposition on the skin or by contact with a
surface that has been sprayed or a cleaning accessory used during application (e.g., cleaning rag, paper
towel, sponge). The expert elicitation described by Wardak et al. (2008) for an air freshener scenario
indicated that the potential for exposure via skin contact was medium-high, which was greater than the
potential for ingestion exposure but less than the potential for inhalation exposure. Individuals with cuts
or abrasions of the skin might be more likely to absorb nano-Ag following dermal contact.
Children might be a susceptible population for dermal exposure because the skin of infants and
young children and young adults has been shown to be more permeable to some substances than that of
older adults (Hostynek. 2003). although no data for nano-Ag were found. Furthermore, children often
have cuts and scratches from play activities.
5.3.1.3. Oral Exposure
The potential exists for inadvertent ingestion of nano-Ag through hand-to-mouth contact following
dermal exposure in the home (Drake and Hazelwood. 2005). Wardak et al. (2008) concluded that the
potential for exposure via ingestion was medium-low, below the potential for inhalation and dermal
exposure, and therefore not of primary concern. Inhalation of nano-Ag also could result in exposure via
the GI tract following mucociliary clearance of the lung (SCHER. 2009).
For children (e.g., toddlers), oral exposures could occur from mouthing objects that have been
sprayed with nano-Ag, and touching or handling sprayed objects and then mouthing their hands. Although
no studies were identified that examined children's exposure to nano-Ag sprayed in the home, the
exposure pathways could be pertinent for children in homes where such sprays are used intensively or for
cleaning toys. Children could be more highly exposed than adults to nano-Ag that reaches food and water
in the home because they consume more food and water per unit body weight than adults (Bearer. 1995).
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5.3.2. Occupational Exposure
As the number of products containing nanomaterials in commercial distribution increases, the
number of workers involved in the manufacturing of nanomaterials is also likely to increase. Lack of
knowledge regarding nanomaterial manufacturing processes requires that precautionary assumptions be
made about potential exposure routes and likelihood of exposure during manufacturing and by-product
disposal; therefore, all possible exposure routes (i.e., respiratory, dermal, oral) are currently considered
for occupational settings. The Occupational Safety and Health Administration and the National Institute
for Occupational Safety and Health recommend and set limits on conventional silver concentrations in the
workplace environment at 0.01 milligram per cubic meter (mg/m3) (equivalent to 10 micrograms per
cubic meter [ug/m3]) for an 8-hour workday and 40-hour workweek (U.S. EPA. 2009g). The American
Conference of Governmental Industrial Hygienists (ACGIH) recommends threshold limit values of 0.01
and 0.1 ug/m3 for soluble silver and metallic silver, respectively (U.S. EPA. 2009g).
The differences, sometimes spanning an order of magnitude, between occupational exposure limits
set by different agencies led Drake and Hazelwood (2005) to review the toxicity literature on chronic
exposure to conventional silver. They concluded that the potential effects of chronic exposure to
conventional silver depend on its chemical form. Soluble forms of silver are more readily absorbed by the
body and therefore can more readily cause adverse health outcomes. For example, in an occupational
exposure study of workers employed in silver manufacturing (Armitage et al., 1996). silver reclamation
workers exposed to soluble silver compounds showed the highest blood levels, with an average of
6.8 ug/L (range = 1.3-20 ug/L, n = 19), while jewelry makers exposed to metallic silver had the lowest,
ranging from 0.2 to 2.8 ug/L (« = 9). Blood silver levels ranged from 0.1 to 23 ug/L among workers in all
of the factories (no exposure levels were measured), while 11 of 15 agricultural workers with no
occupational exposure had blood silver concentrations below the detection limit of 0.1 ug/L, and no blood
silver concentrations were higher than 0.2 ug/L.
Workers exposed to conventional silver and to silver fumes and dusts, which might contain
nanoparticles, have displayed clinical symptoms due to dermal, ocular, and inhalation exposures (Panyala
et al., 2008; Drake and Hazelwood. 2005; Rosenman et al.. 1987). The literature commonly reports the
effects of these exposures, but few specifics are reported on measured exposure levels. In one study where
ambient exposure levels were reported, Pifer et al. (1989) evaluated workers exposed to silver via
inhalation. Air sampling indicated an 8-hour time-weighted-average airborne silver concentration of
1-100 ug/m3, with most silver present in insoluble forms. Elevated blood silver concentrations (mean of
0.010 microgram per milliliter [ug/mL] among 80% with detectable blood silver levels) and body burdens
were reported in these workers relative to controls; however, no instances of argyria were reported.
Tsai et al. (2009) measured exposure from the transfer of nano-Ag and nano-alumina powder from
inside fume hoods to the worker breathing zones for three common hood designs. This study is one of the
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few reporting personal levels of exposure associated with the occupational handling of nanomaterials.
Despite the use of common laboratory precautions, the release of airborne nanoparticles into the
laboratory environment and the researchers' breathing zone was substantial. Results indicate that
researchers should not transfer powders in a fume hood because the highest breathing-zone concentrations
resulted from the constant-flow hood. Tsai et al. (2010) then compared the efficacy of a recent hood
design, the air-curtain hood, to the other hood designs using the same procedures, but with aluminum
oxide nanoparticles only 27-56 nm in size. Because the nanoparticles could cluster in the bulk powder,
the concentrations of airborne particles with diameters measuring from 5 nm up to 20,000 nm were
measured in the breathing zone. Release of particles from the hood was negligible for particle clusters
greater than 500 nm in size; but releases of clusters in this size range were again highest for the
constant-flow hood. Furthermore, pouring the nanomaterial manually from one beaker to another in the
constant-flow hood resulted in particle concentrations ranging between approximately 500 and 1,500
particles/cubic centimeter (cm3) in the breathing zone (particle size was primarily 100- to 200-nm
clusters; peak particle concentration was 7,000 particles/cm3). Releases were lower for the air-curtain
hood, and ranged from non-detectable to approximately 500 particles/cm3. These studies suggest that
procedures generally considered adequate to protect workers during handling of harmful substances might
not be sufficient while handling nanomaterials.
Once released in an occupational setting, nanoparticles can be inhaled and might deposit in the
lungs (Kreyling et al.. 2002; Oberdorster et al.. 1995). Only two occupational exposure studies to date
(Lee et al., 2011; Park et al., 2009) have examined workplace exposure specifically to nano-Ag during
the manufacturing process. Park et al. (2009) assessed exposure to nano-Ag in the liquid phase during a
wet chemical process at a commercial production facility in Korea. Although most field studies that have
analyzed nanomaterial exposure in the workplace have concentrated on the gas-phase production process
because of the clear potential for inhalation of powders and aerosol particles, the investigators argue that
the potential remains for exposure to nanomaterials during liquid-phase processes, which are frequently
used to manufacture nano-Ag. In this study, Park et al. (2009) report that the production of nano-Ag at the
facility involves a four-stage process: (1) batch reaction based on wet chemical methods, (2) filtering, (3)
drying, and (4) grinding. Of these stages, the investigators demonstrated that at least three had the
potential for worker exposure (filtering was not explicitly described). During the batch reaction process,
the nano-Ag reaction mixture is allowed to age before the filtration stage. The real-time air-particle
monitor in the reaction room was located 1 meter from the hatch of the main reactor, which was under a
ventilation hood. Nanoparticle concentrations in the room were about 6 x 104, 5 x 104, and 2 x 104
particles/cm3 for particles with average diameters of 100 ± 5 nm, 200 ± 5 nm, and 20 ± 5 nm,
respectively, after about 13 hours. At that point, the reactor hatch was opened for 1 hour to allow
sampling of particles to characterize nano-Ag. During that time, the concentrations of the 20- and 200-nm
nanoparticles in the room air remained relatively stable, but the concentration of 100-nm nanoparticles
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increased to about 9 x 104 particles/cm3. TEM images indicated that the particles collected in the reaction
room were nano-Ag. According to Park et al. (2009). nano-Ag 50-60 nm in diameter formed clusters in
the room air. Concentrations of nano-Ag were estimated by subtracting the background particle
concentrations before operations started from the particle concentrations during the silver processing.
Similar results were observed during the grinding and drying processes. When the dryer door was opened
at the end of the drying period, the number of 60- to 100-nm particles in the air doubled. During the
grinding process (1 hour), irregular increases in nanoparticles in the air were observed as workers
disturbed particles that had settled on the floor. After the ventilation system was turned on, these irregular
increases were reduced. When the grinder hatch was opened, however, the concentration of 30- to 40-nm
particles in the air spiked. These results indicate that nano-Ag in a solution can be aerosolized in the
workplace air, where workers can inhale the nano-Ag particles (Park et al.. 2009) and that the particles
might deposit on exposed skin. Nano-Ag in the air also might settle on clothing or on floors, eventually
resulting in a secondary exposure when the nano-Ag is disturbed during clothing removal, sweeping, or
walking in a room without an adequate ventilation system.
Lee et al. (2011) used a combination of personal air samples, area monitors, and real-time monitors
to examine potential workplace exposures at two different nano-Ag manufacturing facilities in Korea.
Filters were used to collect nano-Ag in worker breathing zones and in work areas; silver mass
concentrations on the filters were analyzed using an inductively coupled plasma (ICP) method, and
nano-Ag particles were identified and characterized using TEM and energy dispersive X-ray analysis.
Real-time aerosol monitoring also was conducted using scanning mobility particle sizers and dust
monitors to capture the particle number concentrations for particles with diameters ranging from 15 to
710.5 nm and 0.3 to 20 micrometers (um), respectively.
The first facility examined by Lee et al. (2011) synthesized nano-Ag in a pilot ICP reactor with an
electric atomizer and utilized no control technologies outside of natural ventilation. Workers fed silver
powder, the precursor material, into the reactor and collected the nano-Ag particles from the collector. All
processes between precursor feeding and nano-Ag collection took place in a closed, negative pressure
system, indicating that under normal circumstances, releases of nano-Ag and precursors should occur
only during feeding and extraction of materials. The highest silver mass concentration (0.00102 mg/m3)
was measured in the personal air sample of one of the two operators of the feeding process. Although no
personal air samples were taken for workers collecting nano-Ag from the reactor, the silver mass
concentration in the area nearest the collector was 0.00034 mg/m3, which was about three times higher
than in the other area air samples. The authors noted that all silver mass concentrations measured in
personal air and area samples were below occupational exposure levels for silver dust (0.1 mg/m3) and
soluble silver compounds (0.01 mg/m3) established by ACGIH. Nano-Ag particle number concentrations
for the 15-710.5 nm size fraction were measured both in the ICP reactor and in workplace air. Although
concentrations in the reactor ranged from about 60,000 to 2.3 million particles/cm3, with a fairly
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consistent average particle size of 20-30 nm, concentrations in workplace air were relatively low (less
than 7,000 particles/cm3) despite the lack of control technologies, and the average particle size was
around 400 nm, suggesting that nano-Ag clustering occurred once the nano-Ag particles escaped from the
reactor into the workplace air (Lee et al.. 2011).
The second nano-Ag manufacturing facility examined by Lee et al. (2011) used an attrition milling
system with ventilation and a fume hood to manufacture nano-Ag solutions from sodium citrate and silver
nitrate. Both chemicals were weighed, milled (i.e., pulverized), added to a wet tank, stirred, and sonicated
before they were mixed together in a reactor. No personal air samples were collected at this facility, but
several area monitors were placed in two laboratories in the facility. Throughout the manufacturing
process, silver mass concentrations in workplace air ranged from 0.00003 to 0.00043 mg/m3, with the
exception of one very short-duration measurement (9.6 minutes compared to 162- to 223-minute sampling
durations for the other areas), which was 0.00118 mg/m3. The conditions that led to this higher
concentration were not described in the study, but the authors reported that all of the measured
concentrations were again below the ACGIH occupational exposure levels for silver dusts and soluble
silver compounds. The scanning mobility particle sizers and dust monitors revealed that concentrations in
the smaller 15- to 710.5-nm size fraction ranged from about 400 to 3,500 particles/cm3, and particle
number concentration in the larger 0.3- to 20-um fraction ranged from about 1,000 to 2,200 particle/cm3.
Peaks in particle number concentrations in both size fractions occurred when sodium citrate was weighed
and mixed with water, when the sodium citrate and silver nitrate were mixed, and when the equipment
was cleaned (Lee et al.. 2011).
Unlike traditional occupational exposure studies like that of Park et al. (2009) and Lee et al. (2011).
which focus on workers involved in extracting or refining the material, exposure to a nano-Ag
disinfectant spray could involve occupational use by janitorial service workers who might be chronically
exposed. No studies measuring or modeling this type of exposure to a nanomaterial were found in the
literature. In addition, the possibility of transport of nano-Ag from workers to their homes warrants
consideration. The risk of secondary exposures could be lowered by using protective uniforms that remain
at the facility where they would be cleaned and by establishing decontamination protocols before workers
return home.
Seipenbusch et al. (2008) studied the release of spherical platinum (Pt) nanoparticle aerosols into a
simulated workplace environment under particle-free conditions and with pre-existing background aerosol
concentrations (simulated by spherical submicron oil droplets or micron-sized silica spheres). The
nano-Pt aerosol had a median particle diameter of 7-8 nm, whereas the background aerosols had particle
sizes in the range of 100-1,000 nm. The study monitored particle-size distributions and total number
concentrations over several hours of release of nano-Pt and found that collision between nanoparticles
within their own size class and with the background aerosol, if present, was the primary mechanism
driving changes in particle size and number concentration. The authors concluded that nanoparticles are
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unlikely to reach receptors in the form of the original, released aerosol. The nanoparticles are likely to
increase in size by homogeneous clustering within their size class or heterogeneous clustering with
background aerosols. Methner et al. (2010) demonstrated the same phenomenon, but across many
different sizes, shapes, and compositions of nanoparticles, many of which are not possible for nano-Ag
(e.g., fibers, tubules, oxides).
Additional information on exposure to nano-Ag and associated substances during the
manufacturing process of nano-Ag and disinfectant sprays and on occupational use of end products would
aid in understanding chronic workplace exposures more fully. Traditionally, such information also has
proved relevant to the study of subchronic exposure because mechanisms and health effects can be
extrapolated to lower exposures in the general population before a complete body of research is available.
Occupational exposures historically have been the first indication of toxic effects that ultimately might be
occurring more broadly or more subtly within the population (e.g., exposure to mercury by hat makers,
exposure to asbestos by shipyard workers, exposure to radium by watch dial painters).
5.4. Aggregate Exposure to Nano-Ag from Multiple Sources
and Pathways
Nano-Ag has been advertised as a constituent in at least 31320 consumer products currently on the
market, although the content of nano-Ag in these products has not been verified. And as described in
Section 1.4, silver nanoparticles can occur naturally in the environment or can be produced
unintentionally. Humans and biota are therefore likely to be exposed to nano-Ag from multiple sources
and through multiple pathways.
The simplified exposure scenarios mentioned below consider only engineered nano-Ag and assume
that the nano-Ag entering the environment would remain in its current form; however, the surface
chemistry of nano-Ag might be significantly altered as a result of "aging" and transformation processes in
complex environmental systems (as discussed in Chapter 4). Chemical and biological transformations of
nano-Ag might occur as a result of reduction and oxidation (redox) reactions, particle dissolution, or
interactions with pollutants or organic matter, which, in turn, might result in changes in particle form,
clustering, transport, and pathways of exposure (Wiesner et al., 2009). The susceptibility of nano-Ag to
transformation and complexation might limit exposure to nano-Ag itself, while increasing exposure to
20The Project on Emerging Technologies' Consumer Products Inventory website at
http://www.nanotechproject.org/inventories/consumer.
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nano-Ag complexes, silver ions released from nanoparticles, and other transformation products (see
Section 5.2).
5.4.1. Human Aggregate Exposure
Potential routes of human exposure for some possible nano-Ag applications are provided in
Table 5-1.
Because nano-Ag might be used in various applications, numerous pathways exist for human and
ecological exposures, some of which could overlap, resulting in aggregate exposures to nano-Ag from
many sources. Many of the nano-Ag applications listed in Table 5-1 could be used by the same
individuals at approximately the same time. For example, an individual might inhale aerosol particles
from a nano-Ag spray used to clean and disinfect surfaces in the home, wear a bandage containing
nano-Ag on the skin, and consume an oral nano-Ag dietary supplement. Also, a single product could lead
to exposure through multiple routes (Wijnhoven et al., 2009b). While using a spray disinfectant, the
nano-Ag solution might be sprayed unintentionally on the skin, the aerosol particles inhaled (and possibly
coughed up and subsequently swallowed), and ingested from foods in contact with the disinfected
surfaces.
As one or more nano-Ag products are used in the home, particularly disinfectant sprays, nano-Ag
could accumulate on surfaces and in airborne dust. Removal mechanisms for airborne nano-Ag are
limited to normal leakage and, perhaps less commonly, slow transfer to outdoor air when windows are
open, transfer through a central vacuum system, or capture of particles on heating and cooling system
filters, which generally have not been designed for this purpose. High efficiency particulate air filters can
remove some proportion of nanoparticles in the air, but are designed to remove only 99.7% of particles
300 nm or larger. Buildup of nano-Ag in carpets, furniture, and floors might lead to higher exposure of
children and pets in particular.
The aggregate exposure of children might be higher than that of adults. Children are more likely to
crawl and play on surfaces sprayed with disinfectants than adults and their inhalation rates are higher than
adults (Bearer. 1995). They also might be exposed through toys to which manufacturers have added
nano-Ag to keep the toys bacteria free. Finally, the skin of young children can be more permeable to some
substances than that of adults, and cuts and scratches from play activities break the dermal barrier to
absorption of most substances.
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Table 5-1. Nano-Ag applications and potential routes of human exposure.
Product category
Food and beverage
Personal care and
cosmetics
Textiles and shoes
Electronics
Household
products/home
improvement
Filtration, purification,
neutralization,
sanitization
Medical products
Product subcategory
Cleaning
Cooking utensils, coatings
Storage
Supplements
Skin care
Oral hygiene
Cleaning
Hair care
Baby care
Over-the-counter products
Clothing
Other textiles
Toys
Personal care
Household appliances
Computer hardware
Mobile devices
Cleaning
Coating
Furnishing
Furnishing/coating
Filtration
Cleaning
Anesthesiology
Neurosurgery, Cardiology
Eye care
Patient care
Orthopedics
Pharmaceuticals
Surgery
Wound care
Product examples
Food product sterilizing sprays
Cutting and chopping boards, kitchenware and tableware, baby-
bottle brushes
Refrigerator fresh boxes, storage bags and containers, baby
bottles, mugs
Colloidal metal in water
Body creams, hand sanitizers, beauty soaps, face masks
Tooth brushes, tooth cleaners, toothpastes
Elimination wipes and sprays
Hair brushes, hair masks
Pacifiers, tooth developers
Foams, condoms
Fabrics and fibers, socks, shirts, caps, jackets, gloves, underwear
Sheets, towels, shoe care, sleeves and braces
Plush toys
Hair dryers, wavers, shavers
Refrigerators, washing machines
Notebooks, (laser) mouse, keyboards
Mobile phones, game systems
Cleaning products for bathroom, kitchen, toilets; detergents, fabric
softeners
Sprays, paint supplements
Pillows
Showerheads, locks, water taps
Air filters, ionic sticks
Disinfectant sprays
Breathing masks, endotracheal tubes
Catheters
Contact lenses
Incontinence materials
Implants, stockings
Dermatitis, acne, ulcerative colitis treatments; HIV-I replication
inhibition
Gowns, face masks, slings for reconstructive surgery
Hydrogel for wound dressing
Expected exposure route
Inhalation, dermal
Dermal, oral
Dermal, oral
Oral
Dermal
Oral
Inhalation, dermal
Dermal
Dermal
Dermal
Dermal, oral
Dermal, oral
Dermal, oral
Dermal
Dermal
Dermal
Dermal
Inhalation, dermal, oral
Inhalation, dermal
Dermal
Inhalation, dermal
Inhalation
Inhalation, dermal
Inhalation
Intravascular, intrathecal,
intravesical, urethral
Ophthalmic
Dermal
Intramedullary, dermal
Oral, dermal
Inhalation/dermal/ intraperitoneal
Dermal
Adapted with permission of Informa Healthcare; Wijnhoven et al. (2009b).
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5.4.2. Biotic Aggregate Exposure
Biota also could be exposed to nano-Ag through multiple sources and routes, but exposure might
be significantly affected by environmental parameters (Luoma. 2008). Although initially released as
nanoparticles, the subsequent transport, transformation, and fate of the particles depend on many
environmental characteristics in the receiving media. Nanoparticles in surface waters might associate to
form microparticles or sorb to other materials and fall out of suspension into sediments. Water conditions
and chemistry might make the nanoparticles either more or less available for uptake by biota (Navarre et
al.. 2008a). Transport, transformation, and fate processes of nano-Ag in the environment are discussed in
Chapter 4 of this document, and bioavailability of nano-Ag in different environmental media is discussed
in Section 5.2.1.
Although exposure of biota to nano-Ag can be mitigated through various environmental processes,
some exposure is likely given that many nanoparticles are engineered to maximize their dispersion in
water (Lee et al., 2007; BaloghetaL 2001). Development of biocidal nano-Ag products for potential use
in the home (e.g., clothes washers, surface sprays, cosmetics) or in occupational settings (e.g., industrial
misters and foggers, architectural coatings, water filters) could lead to the release of nano-Ag during
manufacturing, use, and disposal. Once released into sewer systems, very small nano-Ag particles that
escape filtration-capture during wastewater treatment can be released back into aquatic ecosystems, where
they could impact biota that are particularly susceptible to aggregate exposure via direct uptake from the
water and ingestion of contaminated prey (Navarro et al., 2008a). In cases where sewage sludge from
wastewater treatment is applied to land for soil amendments or for disposal, nano-Ag might be absorbed
by plants, leached to ground water, or contained in runoff to surface waters (Blaser et al.. 2008).
Terrestrial biota then might be exposed to nano-Ag through ingestion of contaminated soil and prey, as
well as through contact with contaminated media.
5.5. Cumulative Exposure to Nano-Ag and Other
Contaminants
Given their high surface area-to-volume ratio and enhanced chemical reactivity, nanoparticles can
modify the bioavailability of other toxic agents, such as manufacturing by-products, transformation
products, waste products, and other contaminants present in the environment. Moreover, given the
possible processes by which nanoparticles sorb to or are absorbed into cell walls and cells (see
Section 5.7), they also might act as carriers of other chemicals or nanomaterials onto or into cells. Thus,
nano-Ag particles serving as carriers could increase exposure of organisms to additional toxic agents.
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Nano-Ag might be coated with agents that exhibit toxicity during manufacturing or adhere to toxic
agents after release into the environment. Navarro et al. (2008a) observed that metallic engineered
nanoparticles often are coated with inorganic or organic compounds or surfactants (e.g., sodium dodecyl
sulfate) to maintain a colloidal suspension of the nanoparticles in the end product. In the future, nano-Ag
might be combined with other materials to enhance certain properties for specific end uses. Cumulative
exposure to other substances released during the manufacturing process and other ingredients of
disinfectant sprays also might be a relevant consideration.
The potential for nano-Ag releases to result in increased exposure to manufacturing by-products or
transformation products in the environment is discussed in Section 5.5.1. Whether nano-Ag might
specifically facilitate absorption of other toxic agents or nanomaterials is discussed in Section 5.5.2.
Evidence that some other types of nanoparticles facilitate absorption of other contaminants by living
organisms is presented in Section 5.5.3.
5.5.1. Nano-Ag By-Products and Transformation Products
At this time, no information suggests that nano-Ag manufacturing processes result in the formation
of hazardous by-products; however, relatively few data on large-scale manufacturing of nano-Ag are
currently available. Information is similarly lacking regarding other materials used in manufacturing other
ingredients of disinfectant sprays. Manufacturing of nano-Ag might therefore result in increased exposure
to hazardous by-products, and nano-Ag might facilitate the absorption of toxic by-products in living
organisms. This consideration is relevant in toxicity testing as well. For example, Samberg et al. (2010)
found that "unwashed" nano-Ag received from a commercial producer of nanomaterials was toxic to
human epidermal keratinocytes in vitro, with significant dose-dependent decreases in viability, whereas
the same nano-Ag product washed five times did not diminish cell survival. The investigators concluded
that the residual formaldehyde solvent and methanol by-product from the production of the silver
nanomaterial were probably responsible for the observed toxicity.
As stated in Chapter 4, transformation of nano-Ag to other silver forms and complexation with
other materials will occur in environmental media following release of nano-Ag from the spray
disinfectant life cycle (see Section 4.1). Although exposures of humans and other biota to these nano-Ag
transformation products are expected to occur, such exposures have yet to be characterized.
Nano-Ag also can sorb to other materials in water or soils, and Navarro et al. (2008a) suggested
that sorption of nanoparticles to low-molecular-weight NOM might increase bioavailability of the
nanoparticles and increase the chances of other contaminants to "hitch a ride" with the nanoparticles into
aquatic organisms, in particular.
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5.5.2. Examples of Nano-Ag Facilitating Absorption of Other
Contaminants
Silver nanoparticles act as carriers of silver ions, possibly delivering them directly to a biological
surface or into cells, where they might interact directly with the cell machinery (Asharani et al.. 2009;
Miura and Shinohara. 2009; Lee et al.. 2007; Hussain et al.. 2005). If nano-Ag particles adhere to cell
surfaces, they can serve as a continuous delivery system for silver ions to the cell. If so, greater human
health and ecological risks can be expected from nano-Ag than from comparable quantities of ionic silver
because not all silver ions that are free in solution will necessarily come in contact with a biological
surface. No reports that nano-Ag facilitates the delivery or entry of other toxic chemicals to or into living
organisms were found in the readily available literature; other nanomaterials, however, have been shown
to facilitate the absorption of other substances (see Section 5.5.3).
5.5.3. Examples of Nanoparticles Facilitating Absorption of Other
Contaminants
Nano-Ag spray disinfectant formulations could contain other chemicals with which nano-Ag might
react, resulting in a synergistic effect that facilitates the uptake of the other contaminants. Although not
yet demonstrated with nano-Ag, studies have shown synergistic uptake of contaminants occurring with
polymer fumes (Johnston et al.. 2000). diesel particulate matter (Wallace et al.. 2007). and some
nanoparticles, which are described further in this section. Furthermore, medical applications are being
developed using nanoparticles as carriers for targeted drug delivery (McNeil. 2009). In general, however,
the capacity of nanoparticles to sorb and facilitate uptake of other contaminants depends on the structure
and composition of the nanoparticle.
Zhang et al. (2007b) found an increase in the accumulation of cadmium in the gills and viscera of
carp in the presence of nano-TiO2. Similarly, Sun et al. (2007) found an increased accumulation of arsenic
in carp exposed to arsenate (As[V]) in the presence of titanium dioxide. The As[V] sorbed quickly to
nano-TiO2 in the water. Both nano-TiO2 and arsenic concentrations were highest in the intestines,
stomach, and gills, and somewhat lower in the liver, muscles, and skin. Much of the internally
accumulated arsenic might have been released from nano-TiO2 at the epithelium of the gills and GI tract.
Some nano-TiO2 reached the liver as well, presumably with sorbed arsenic.
Baun et al. (2008b) evaluated the potential effects of C6o nanoparticles (Buckminster fullerenes, or
buckyballs) on the bioavailability of 13 organic toxicants, as measured by their toxicity to the green alga
Pseudokirchneriella subcapitata and the freshwater invertebrate D. magna. They observed no change in
the toxicity of methyl parathion to the algae or daphnia in the presence of C6o, while the toxicity, and by
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inference the bioavailability and uptake (see Section 5.2.3), of pentachlorophenol was reduced 1.9-fold in
the presence of C6o. On the other hand, the presence of C6o enhanced phenanthrene toxicity to daphnia by
60%. Analysis showed 85% sorption of phenanthrene to C6o clusters (Baun et al.. 2008b). In contrast to
nano-Ag, however, C6o nanoparticles form spherical molecular cages that can carry molecules trapped to
some degree in their interior. This mechanism is not expected for nano-Ag particles.
Moore (1997) reported uptake of sucrose polyester nanoparticles in seawater by the hepatopancreas
of whole mussels. The uptake of sucrose polyester nano-"droplets" increased the uptake (160%) and
cellular toxicity (122%) of anthracene, a polycyclic aromatic hydrocarbon. Anthracene damaged the
lysosomal system (measured as lysosomal membrane stability) in the hepatopancreatic cells, indicating
that although the sucrose polyester was not biodegradable (even in lysosomes), the polycyclic aromatic
hydrocarbon must have been released into the cell (Moore. 2006).
Despite these examples, however, there are no indications that the structure of nano-Ag particles is
likely to facilitate uptake of other contaminants into biota. Clusters of nano-Ag might house other
chemical contaminants in the inter-particle spaces; however, clusters of nano-Ag also might be less likely
to be absorbed because of their larger size than nano-Ag particles.
5.6. Models to Estimate Exposure
Models can be used to provide initial estimates of potential release scenarios, behavior in the
environment, exposure pathways, dosimetry, and toxicity, provided that the attributes of nano-Ag particles
that influence fate, transport, and dosimetry are adequately considered. Modeling focused on tracking
environmental transport, transformation, and fate after release can assist in estimating the potential for
human and biotic exposures, linking release estimates with models of uptake and dose (Shatkin. 2008).
EPA uses various models to estimate exposures for chemical assessments, some of which are described on
the websites for the Council for Regulatory Environmental Modeling (U.S. EPA, 2009c) and the Center
for Exposure Assessment Modeling (U.S. EPA. 2009a). For example, the Exposure and Fate Assessment
Screening Tool Version 2.0 is a publicly available program that EPA uses for screening-level assessments
of conventional industrial chemicals. The tool provides estimates of aquatic, general population, and
consumer exposure based on chemical release data (U.S. EPA. 2007a).
Quantifying exposure or dose using measured environmental or occupational concentrations is not
yet possible because nano-Ag concentrations have not been widely measured in relevant media. Instead,
exposure concentrations can be estimated using a fate and transport model (Section 4.5) with inputs based
on measured or assumed release scenarios (not covered in detail in this case study). Potential and internal
doses can be predicted by models of dosimetry or pulmonary deposition (Section 5.7.3), pharmacokinetics
(Section 5.7.1), and bioaccumulation (Section 5.2.1.3). Mode-of-action models can be used to estimate
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doses delivered to target organs. All models described in these sections use chemical concentrations on a
mass basis (e.g., mg/L, mg/kg) to predict chemical behavior (e.g., diffusion along concentration
gradients). The applicability of this approach for nanomaterials, which exhibit some properties and
behaviors that cannot be attributed strictly to changes in mass concentration, has not yet been determined.
5.7. Human Uptake and Dose
As described previously in this chapter, internal dose is the amount of a substance that enters an
organism by crossing a biological barrier. Quantifying internal dose, or at least administered
(i.e., potential) dose (e.g., quantity inhaled or ingested whether absorbed or not), enables estimation of
individual or population-level risk, or both (U.S. EPA. 1992). Measuring and understanding the
dose-response relationship is integral to predicting the human health impacts resulting from an exposure.
Because nanoparticles possess unique, size-dependent properties that are not necessarily related to mass,
however, their uptake and dose are not understood as well as that of traditional substances, which
typically use mass as a dose metric (Borm et al., 2006a). A summary of the various metrics that can be
used to best characterize nano-Ag dose is presented in Chapter 2. This current section builds on that
summary by presenting information on uptake of nano-Ag and dose levels in laboratory mammals.
Current knowledge on uptake and dose of nano-Ag in humans is also presented when available, but most
information on this topic is inferred from studies involving laboratory mammals.
This section begins by summarizing what is known regarding the internal behavior
(i.e., pharmacokinetics) of nano-Ag. A discussion of uptake and dose in laboratory mammals then
follows. Uptake of nano-Ag through different routes has been investigated predominantly in laboratory
rats. For all terrestrial organisms, including laboratory animals used for toxicological studies and as
models for human health effects, the route of exposure is critical in determining the dose that ultimately
enters the body. Information relevant to nano-Ag uptake and dose to humans is therefore presented here
according to the inhalation, ingestion, and dermal routes of uptake. This section concludes with a brief
discussion of models to estimate nano-Ag dose.
5.7.1. Pharmacokinetics
Pharmacokinetics, the study of the fate of a substance after it has entered a body, encompasses the
absorption, distribution, metabolism, and excretion of a substance. By extension, toxicokinetics focuses
on the fate of a toxic substance once present within the body. Understanding toxicokinetics is essential to
understanding the mechanism of action and resulting toxicity of atoxic substance. Figure 5-1 illustrates
an overview developed by Hagens et al. (2007) of physiological paths by which nanoparticles have been
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confirmed to travel, and other paths that are hypothesized to be relevant. Although this figure was
developed for nanoparticles in general, and thus does not indicate which pathways are more relevant for
nano-Ag, it could serve as a framework within which the pharmacokinetics of nano-Ag could be
represented once adequate data are available.
5.7.1.1. Absorption
Absorption encompasses the events that lead from external exposure of a substance to its uptake
and transport to the central blood circulatory system. The concept of dose involves the absorption or
uptake of a substance, and in vivo studies of dose often focus on the amount of a substance absorbed by
an organism (i.e., how much of the substance moves from the external environment to the internal space
of an organism). Penetration into the body depends on the specific properties of the nanoparticle,
including charge, hydrophobicity, and surface coating, and the physiology of the particular organ. Studies
identified for this case study that examine the absorption of a substance are discussed later in this section
according to exposure route. An evolving concept relevant to the discussion of absorption of a
EXTERNAL
EXPOSURE
I
I
-j
1
Central blood
circulation
ME
(Metabolism, Excretion)
Source: Reprinted with permission of Elsevier; Hagens et al. (20071.
Figure 5-1. Absorption and uptake of nanoparticles and transport to the central blood
circulatory system.
This figure, developed by Hagens (20071, depicts the organs and other parts of the body involved in the absorption, distribution, metabolism, and
excretion (i.e., the pharmacokinetics) of a nanoparticle that enters the body. Solid lines show paths that have been confirmed to pertain to nanoparticles;
dotted lines represent hypothetical routes. Although not all-inclusive, this figure illustrates how nanoparticles might enter and be transported to various
parts of the body.
-------
nanoparticle into the body is its "corona" (Cedervall et al.. 2007). A corona is a layer of biomolecules that
forms on the surface of a substance once it is absorbed into a physiological system. This layer can have
important implications regarding how the substance interacts with the surrounding tissue. The concept of
a corona is not particularly new or unique to colloid science or the study of nanoparticles; for example,
Lynch and Elder (2009) note that researchers using medical devices have been aware of the same
phenomenon. When a nanoparticle contacts extracellular bodily fluids, proteins and other molecules
compete to attach to the particle surface, thereby coating the nanoparticle. Because of the extremely high
surface area-to-volume ratio of a nanoparticle, the absorption potential is significantly greater than for
larger particles. Once the particle is encapsulated by these biomolecules, it is this corona that encounters
the cell surfaces and might determine a cell's initial reaction to the particle (Lynch and Elder. 2009). One
complicating factor in studying this phenomenon is that the composition of the corona is not static.
Instead, its composition is determined largely by competitive binding, with a constant tendency toward
equilibrium between the corona and its surroundings (Cedervall et al.. 2007). In a recent review, a
European Commission scientific committee noted that the composition of the corona is thought to
determine, in part, a particle's ability to cross membranes and enter cells or organelles (SCHER. 2009).
For example, a particle coated with polyethylene glycol polymer was not available for cellular uptake,
thereby increasing the particle's lifetime in the blood, whereas serum albumin (a plasma protein) coatings
increased nanoparticle uptake by macrophages. For the current case study, no information specific to the
effect of protein coatings on nano-Ag particles was identified.
5.7.1.2. Distribution
Nanoparticles apparently can be distributed via blood circulation following absorption into the
body. Anecdotal case reports of medicinal exposures and occupational studies suggest that humans
exposed to conventional silver and nano-Ag through various routes in occupational or medicinal settings
showed elevated levels of silver in their blood and urine (see Section 6.3.3). Few controlled studies
examining the systemic distribution of nano-Ag were identified. Because distribution appears to differ in
accordance with the route of exposure, general observations on distribution are presented briefly here and
are further described in Section 5.7.2 on human uptake and dose by route.
As demonstrated by the studies described in the next section, distribution of silver throughout the
body via blood circulation might be widespread following exposure to nano-Ag and subsequent
absorption. This pattern would be consistent with the general behavior of nanoparticles following
absorption, where they appear to have the potential to distribute to most, if not all, organs throughout the
body (Hagens et al.. 2007). Information presented by these authors suggests, however, that the relative
extent of distribution to various organs is not well understood for nanoparticles in general, and patterns
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among substances might vary. For example, the authors note that how readily nanoparticles in circulation
can cross the blood-brain barrier or how nanoparticles are eliminated from the body is not clear.
Two studies that demonstrated general distribution of silver in the body following exposure to
nano-Ag are summarized here. In a 28-day study of the toxicity and distribution of silver following oral
administration of nano-Ag particles approximately 53-71 nm (60 nm average) in diameter to rats, Kim et
al. (2008) observed that silver distributed to the stomach, liver, kidneys, lungs, testes, brain, and blood,
with dose-dependent accumulation rates reported for all of these tissues (organs are listed in decreasing
order of observed silver concentration). In the kidney, authors observed gender-specific accumulation of
silver, with females accumulating about twice the mass of silver as males. In a similar study conducted by
Sung et al. (2009). systemic distribution of silver in rats was reported for animals exposed via inhalation
to nano-Ag aerosols averaging approximately 18-19 nm in diameter. Statistically significant (p < 0.01)
increases in silver concentrations were reported in the lungs, liver, kidneys, brain (excluding the olfactory
bulb), and whole blood (organs are listed in order of decreasing silver concentration). That the nano-Ag
used in this case was generated by the thermal-condensation method (see International Organization for
Standardization 10801:2010; International Organization for Standardization 10808:2010) should be noted.
Although these studies have been reported in some secondary sources as having demonstrated distribution
of nano-Ag to internal organs (Hussain and Schlager. 2009; Kaluza et al.. 2009). neither of them actually
examined tissues for nano-Ag particles following necropsy. Total silver concentrations in the various
tissues were determined after wet digestion using an atomic absorption spectrophotometer with a Zeeman
graphite furnace in both studies. Light microscopy was used to identify histopathological changes in
various tissues. In their review, Wijnhoven et al. (2009b) noted that no studies have determined whether
silver distributed to various rat tissues following oral, inhalation, or dermal exposure to nano-Ag remains
in nanoparticulate form; all studies measured only total silver concentrations. For substances that are
absorbed by the body into the blood, transfer to the brain is generally restricted by the blood-brain barrier.
Examples of conventional silver crossing the blood-brain barrier have been identified in the literature and
summarized in at least one review (Lansdown. 2007). Consensus on this phenomenon has not been
reached, however, and reports of functional consequences are inconsistent, suggesting that penetration of
this barrier by conventional silver is low (Lansdown. 2007). With respect to silver present as nano-Ag,
one study used SEM to demonstrate nano-Ag particles in brain tissues of rats following subcutaneous
injection of nano-Ag (Tang et al.. 2008). The authors suggested that nano-Ag could penetrate the blood-
brain barrier by transcytosis (i.e., the transport of substances into the interior of a cell by way of vesicles
or intracellular sacs); only brain tissues, however, were examined for nano-Ag particles. A separate
possible route to the central nervous system specific to inhaled substances, however, is via the olfactory
nerve, which connects the nasal cavity with the olfactory bulb in the brain. This potential method for
distribution of nano-Ag is discussed below in the section describing dose of inhalation exposures.
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Lankveld et al. (2010) investigated tissue distribution of silver in rats over time, up to 16 days
following intravenous injection of 20-, 80-, or 110-nm-diameter nano-Ag particles. Regardless of particle
size and injection frequency (one time or once daily for 5 days), the investigators found that silver was
rapidly distributed out of the blood to the liver, spleen, and lungs and to a lesser extent to the brain, heart,
kidneys, and testes. During repeated injections, silver concentrations in the brain, heart, and testes never
exceeded an average of 40 nanograms per gram (ng/g organ), whereas silver concentrations by day 5 in
the spleen, for example, exceeded 5,000 ng/g organ for the 80-nm-Ag injection. By day 17 of the
experiment (12 days after the 5-day repeated injections stopped), silver could not be detected in the blood,
but relatively high levels remained in the lungs (600 ng/g organ for 80-ng-Ag injections, 200 ng/g for
110-nm-Ag injections), liver (1,100 ng/g organ for 80- and 110-nm-Ag injections), and spleen (several
thousand ng/g organ). Remaining silver in all organs was much less for the 20 nm-Ag injection group. In
the brain, by day 17, approximately 20 ng/g of Ag remained from the 110-nm Ag injections, 10 ng/g for
the 80-nm-Ag injections, and 3 ng/g for the 20-nm-Ag injections. The 20-nm particles distributed mainly
to the liver, followed by kidneys and spleen, whereas the larger particles distributed mainly to the spleen,
followed by liver and lung. Tissues were not examined for the presence or distribution of silver in
nano-Ag particles; rather, only total silver content (and concentration) was quantified. Garza-Ocanas et al.
(2010) administered nano-Ag less than 3 nm in diameter coated with BSA via intraperitoneal injection in
rats to study the fate of the particles in organs including the liver, heart, and brain. They reported
significant accumulation of nano-Ag particles (1-2 nm in diameter), as verified by ICP-MS and TEM
techniques, in the liver and heart. Brain tissues showed no evidence of nano-Ag particle content, although
both tissue pathology and ICP-MS indicated that silver permeated the blood-brain barrier, presumably
silver ions released from the nano-Ag elsewhere in the body.
No animal studies identified in the literature describe the distribution of nano-Ag following
controlled dermal application of nano-Ag, but the use of nano-Ag in topical burn treatment might provide
useful information. In one case study, a teenage burn patient was treated with a nano-Ag-coated mesh
applied over the burned skin. After 1 week, liver and kidney effects and skin discoloration from silver
absorption were observed (Trop et al.. 2006). Silver concentrations in plasma (107 ug/kg) and urine
(28 ug/kg) also were elevated. Once the mesh was removed, silver and liver enzyme levels returned to
normal, and clinical symptoms disappeared. In another case, a burn patient developed neurological
problems following a 2-week exposure to a cream containing silver. Upon autopsy 4 months later,
elevated silver concentrations were detected in the brain, indicating that silver must have crossed the
blood-brain barrier, but a complicating factor in this case was a pre-existing kidney condition (Iwasaki et
al.. 1997). As described in Section 5.7.2.2, absorption of nano-Ag through damaged dermal tissues (as
was present for the burn patients in the aforementioned cases) is greater than for healthy tissue (Larese et
al.. 2009).
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5.7.1.3. Metabolism
The liver typically serves a detoxifying function in the body by removing harmful substances from
the blood and metabolizing them to forms that can be excreted more easily from the body. Once nano-Ag
particles enter the GI tract, either as a result of absorption or distribution, the particles would presumably
enter the portal vein for transport to the liver. No evidence of metabolism of nano-Ag by liver enzymes
has been reported ("WJjnhoven et al.. 2009b). This observation is not unexpected, given that inert metals
typically are not transformed by the body into different chemical forms. Wrjnhoven et al. (2009b)
suggested that nano-Ag might bind to metallothioneins, which are proteins that bind to metals and are
involved in metal regulation and transport out of cells; the authors, however, present no specific evidence
for this activity.
5.7.1.4. Excretion
The European Commission's Scientific Committee on Emerging and Newly Identified Health
Risks (2009) suggested that two physiological methods of excretion can occur for nanoparticles that have
been absorbed in the body and are present in the circulatory system. Clearance of nanoparticles by
urination requires that nanoparticles be absorbed by the gut epithelium and undergo glomerular filtration
in the kidneys; the nanoparticles then would be shunted to the bladder and excreted in the urine.
Alternatively, nanoparticles could travel in bile from the liver to the intestine and be excreted in feces
(SCHER. 2009). Other pathways of excretion also might exist, including transport out of the body
through sweat or saliva, but no information was identified regarding these methods. Wrjnhoven et al.
(2009b) have suggested that these other routes might be less important but provided no firm evidence as
to why.
Human studies of occupational exposures have shown that exposure to conventional silver results
in elevated silver levels in feces and urine (Pifer et al.. 1989; Rosenman et al.. 1987). Pifer et al. (1989)
compared fecal silver concentrations in workers exposed for at least five years in positions with high
exposure potential in an Eastman Kodak plant to a control set of employees at the plant in positions with
low exposure. Measured concentrations in indoor air at the facility were reported as 1-100 ug/m3, with
the majority present in insoluble forms. Although no cases of argyria were reported, 80% of silver
workers had detectible blood silver concentrations (mean of 0.010 ug/mL among those with detectable
blood silver levels), and none of the individuals in the low-exposure group had detectible blood silver
levels. Fecal concentration among workers was higher than in the control group (i.e., 15 ug/g, compared
to 1.5 ug/g in controls). Body burdens also were calculated and reported to be 14 ug/kg in workers, which
was seven times the level observed in control samples. In one case study, Trop et al. (2006) reported that
the body can clear silver from the blood once the exposure has been terminated. Cases of argyria and
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argyrosis (accumulation of silver in the eye), which are generally believed to be irreversible conditions,
demonstrate that the body cannot completely clear silver from all organs. How much of an absorbed dose
remains as a residual burden, however, is not well understood (Wrjnhoven et al.. 2009b). Wrjnhoven et al.
(2009b) also did not identify information in their review regarding whether the silver excreted following
exposure to nano-Ag would be released as nano-Ag or as other forms of silver.
5.7.2. Uptake and Dose by Route
5.7.2.1. Respiratory (Inhalation and Instillation)
In contrast to fine particles with diameters in the 1- to 2.5-um range, which are deposited mainly in
the peripheral lung, inhaled nanoparticles can be deposited throughout the respiratory tract: in the oral and
nasal cavities, the tracheobronchial region of the respiratory tract, and the alveolar region of the lung
(Oberdorster et al.. 2005b). Deposition of particles within the respiratory system depends largely on
particle size and chemistry and on breathing force (Wrjnhoven et al.. 2009b).
The International Commission on Radiological Protection (ICRP) developed and continues to
update the Human Respiratory Tract Model for Radiological Protection, which can be used to predict the
fractional deposition of single particles (not clusters) in the human respiratory tract. Oberdorster et al.
(2005b) and Mark (2007) used the ICRP model to estimate size-dependent deposition patterns of
nanoparticles; these studies produced similar results. In general, the ICRP model estimates that very small
nanoparticles (i.e., 1-nm diameter) primarily deposit in the nasopharyngeal region, slightly larger
nanoparticles (i.e., 5-10 nm) deposit about equally in all three regions of the respiratory tract, and larger
nanoparticles (i.e., 20-100 nm) deposit primarily in the alveolar region. Mark (2007) found that the
projected probability of nanoparticles reaching the alveoli peaked at a size of approximately 20 nm, with
lower probabilities of deposition in the alveoli for both smaller and larger nanoparticles. Nanoparticle
deposition, especially for particle sizes of 20 nm and smaller, is governed by Brownian motion and
diffusion, which allows movement of particles into the alveolar region of the lung, where larger particles
(which are transported via bulk air flow) generally are not deposited (Elder et al.. 2009). For nanoparticles
between 20 and 100 nm in size, deposition probability dropped for all three regions of the respiratory tract
(Mark. 2007). Nonetheless, the ICRP model indicated that for nanoparticles measuring between 10 and
100 nm, the highest fractional deposition would occur in the alveolar region (Lynch and Elder. 2009).
Each region of the respiratory tract employs different mechanisms for clearing inhaled particles
from the mucosal surface (Oberdorster et al.. 2005b). Nanoparticles that deposit in any region of the
respiratory tract can, however, undergo chemical clearance through dissolution. Solutes then can bind to
proteins or be absorbed into the blood or lymphatic system and translocated to other parts of the body via
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the circulation system. As discussed previously, an important feature of nano-Ag particles is their ability
to adsorb biomolecules (e.g., lipids, proteins), which alter the particle's surface properties. Biomolecules
adsorbed along the way to the circulatory system could influence the ability of the nanoparticles to
interact with cells and systems they encounter (Lynch and Elder. 2009). Because translocation of the
nanoparticles depends on their physical and chemical properties, nano-Ag deposited in the lung and
translocated to other parts of the body might carry a biological marker of its deposition site until other
biomolecules displace those that are initially adsorbed. The surface corona of proteins and other
biomolecules surrounding a nanoparticle can affect its solubility, clustering, uptake, and distribution in the
body (Lynch and Elder. 2009).
Unlike chemical clearance mechanisms, physical clearance (i.e., translocation) mechanisms differ
somewhat for the three regions of the respiratory tract, and many of these clearance processes appear to
be more effective for some sizes of nanoparticles than for others. Nanoparticles deposited in the alveolar
region of the respiratory tract might be physically cleared from the alveolar region by: (1) macrophage
phagocytosis followed by mucociliary transport along the tracheobronchial tree to the GI tract
(i.e., particles are internalized by cells, gradually moved in a mucus flow by cilia to the trachea,
eventually reaching the esophagus, where they are swallowed); (2) translocation across the alveolar
epithelium; (3) translocation into interstitial sites; and (4) blood circulation (Elder et al.. 2009; Chen and
Schluesener. 2008; Geiser et al.. 2008; Krevling et al.. 2002; Oberdorster. 1988). Nanoparticles deposited
in the tracheobronchial region are generally cleared through similar mechanisms as for the alveolar region
but also through lymphatic drainage and neuronal uptake, and nanoparticles deposited in the
nasopharyngeal region are typically cleared through epithelial translocation and neuronal transport (Elder
et al.. 2009; Chen and Schluesener. 2008; Geiser et al.. 2008; Krevling et al.. 2002; Oberdorster. 1988).
These mechanisms, and their removal efficiencies for different-sized particles, are described in more
detail below (neuronal transport is discussed in the Olfactory Nervous System subtopic at the end of this
section).
From a few studies conducted by others, Elder et al. (2009) concluded that larger particles and
clusters of nanoparticles (especially those greater than 100 nm in size) are more likely to be taken up by
alveolar macrophages than single nanoparticles. Takenaka et al. (2000) examined the fate of 20-nm
diameter nano-Ag in rats at 1, 4, and 7 days after intratracheal instillation. They found the nano-Ag
particles in larger clusters taken up by alveolar macrophages and inside the alveolar walls at all three time
intervals. A small proportion of single particles also was observed. The appearance of clusters of nano-Ag
particles in the macrophages seemed unchanged up to 7 days after instillation, and no substantial changes
in silver concentrations were observed in the lung, liver, and lung-associated lymph nodes over time.
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Liver silver concentrations remained approximately 3% of that in the lungs. Based on these observations,
the investigators suggested that such clusters do not rapidly translocate to other organs.
Because alveolar macrophage phagocytosis appears to be an inefficient clearance mechanism for
nanoscale particles, most physical clearance of nanoparticles from the different regions of the respiratory
tract occurs through epithelial endocytosis, and—in the alveolar and tracheobronchial regions—further
translocation across the epithelial cells to the interstitium (Oberdorster et al.. 2005b). Although
subsequent translocation from epithelial cells and interstitial sites to the circulatory system is not well
understood, such clearance processes appear to be size-dependent, favoring nanoscale particles
(Oberdorster et al.. 2005b).
Nano-Ag is thought to reach the bloodstream after inhalation dosing via three pathways:
(1) ingestion after movement up the mucociliary escalator, (2) passage into the lymph nodes, and (3)
direct entry via alveolar epithelial cells (Ji et al.. 2007). Inhalation studies of rats exposed to nano-Ag
have demonstrated absorption of silver through the lungs into the circulatory system and distribution to
other organs as well (Sung et al., 2009; Takenaka et al., 2001). Takenaka et al. (2001) exposed rats via
inhalation using whole-body exposure chambers and reported a cumulative dose to each rat of
approximately 7.2 micrograms (fig) of nano-Ag particles approximately 15 nm in size. Total silver
concentrations in various organs and biological systems were monitored, and the distribution of silver was
observed. The highest silver concentration and total content were observed in lung tissue. Elevated
concentrations also were reported for the liver and blood, with measurable amounts reported in these
components 7 days after exposure. Lower levels were measured in other organs, including lymph nodes,
kidneys, blood, heart, and brain (listed in order of decreasing concentration).
Researchers in Korea administered differing doses of aerosolized nano-Ag to rats and monitored
total silver concentrations in organs over 28 days. These researchers found that lung concentrations of
silver showed a dose-dependent relationship following exposure (Hvun et al.. 2008; Ji et al.. 2007).
Whether nano-Ag particles are distributed in the bloodstream to other organs or only the silver ions
reach the circulatory system is not yet known. The eventual fate of the inhaled nano-Ag also is unclear at
this time.
Olfactory Nervous System
For inhaled substances, the olfactory nerve represents another pathway to the brain. This pathway
is treated here as a subtopic of inhalation exposure; it is given special attention because it represents a
potential exposure and distribution route to the central nervous system that does not require passage
through the blood-brain barrier.
The olfactory nerve facilitates the sense of smell by extending from the nasal cavity to the olfactory
bulb of the brain, where the sensation of smell is processed. As described in detail in a review by Ilium
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(2000). substances that deposit on the nasal olfactory mucosa and enter into the olfactory nerve can be
transmitted directly to the brain without encountering the blood-brain barrier. Entry of drugs and other
substances to the olfactory bulb of the brain through the olfactory nerve has been demonstrated, and
evidence indicates that this pathway might have an upper size limit of 200 nm (Elder et al.. 2009; Elder et
al.. 2006; Oberdorster et al.. 2004). Oberdorster et al. (2004) noted that results from several studies
(including their own research using radiolabeled carbon nanoparticles) suggest that this nerve serves as a
pathway to the central nervous system for soluble metals and nanomaterials. In that study, approximately
20% of the carbon nanoparticles deposited on the olfactory mucosa of the rats was translocated to the
olfactory bulb in the brain (Oberdorster et al.. 2004). Oberdorster et al. noted that once deposited in the
olfactory bulb, nano-Ag might be able to travel to other areas of the brain, a possibility also noted by
Lynch and Elder (2009). Transfer from the olfactory bulb to other parts of the brain, however, was not
confirmed by these researchers, and no studies confirming this possibility were identified for this case
study. No studies focusing specifically on the transport of nano-Ag to the olfactory bulb via this pathway
were identified for this case study.
5.7.2.2. Dermal
In their review, Elder et al. (2009) summarized evidence regarding the interaction of various
nanoparticles with skin. The results varied, with different degrees of penetration into and through skin
observed in both in vitro and in vivo studies involving human skin and that of other organisms (e.g., rats,
pigs). They noted that nanoparticles of varying composition have been absorbed into the blood in
scenarios involving mechanical flexing of the skin and damaged skin patches as well as passage through
hair follicles (for particles smaller than ~5 nm).
Elder et al. (2009) noted that nanoparticle penetration of the skin is influenced by surface coatings
and geometry of the particles. For example, Monteiro-Riviere and Riviere (2009) reported that skin is
"surprisingly permeable" to some nanoparticles, and in particular quantum dot nanoparticles, which are
readily absorbed. In addition, the formulation of the nanoparticles that contact the skin might also
influence the skin's permeability by altering its barrier properties. For example, dimethyl sulfoxide
facilitates absorption of substances through the skin by removing much of the lipid matrix of the stratum
corneum, leaving holes and shunts (Lehman-McKeeman. 2008). The vehicle in which the nanoparticles
are dissolved or suspended also might influence partitioning between the stratum corneum and the
vehicle. Elder et al. (2009) concluded that dermal absorption of nanoparticles does not appear to occur
readily but can take place under certain conditions, and the factors dictating the extent to which
absorption occurs are varied and complex.
Only a few experiments on dermal penetration of nano-Ag were identified for this case study.
Larese et al. (2009) reported that nano-Ag can pass through normal human skin (i.e., full-thickness
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abdominal skin) in vitro at a rate of 0.46 nanogram per square centimeter (ng/cm2) and through damaged
skin at a rate five times higher. The silver nanoparticles used in this experiment were coated with PVP to
prevent clustering in an aqueous suspension. TEM of skin samples following exposure were reported to
show silver nanoparticles in the stratum corneum and the upper layers of the epidermis "in some slices."
Based on observations from studies using other nanoparticles, Larese et al. (2009) inferred that
nanoparticles between 7 and 20 nm can penetrate into the hair follicle, and particles less than 30 nm can
passively penetrate the deepest skin layers, probably through the intercellular route (Larese et al.. 2009).
Samberg et al. (2010) applied nano-Ag particles 20 and 50 nm in size in solutions ranging from 0.34 to
34.0 ug/mL to the backs of pigs for 14 days. TEM demonstrated the presence of nano-Ag within the
superficial layers of the stratum corneum for the 50-nm particles and on the top layer of the stratum
corneum for the 20-nm particles. Although some of the lower tissue layers exhibited symptoms of chronic
irritation such as focal inflammation (epidermis and dermis) and edema (epidermis), no evidence of
nanoparticle penetration into these lower layers of the skin was found. Samberg et al. (2010) therefore
hypothesized that silver ions released from the nanoparticles in the stratum corneum might translocate to
lower tissue layers and cause the observed lesions.
Receptor characteristics also might affect internal dose following dermal exposure. Certainly the
presence of cuts or scratches would enhance dermal absorption of nano-Ag and other substances. Recent
data indicate possible sexual differences in retention of nanoparticles after dermal absorption. Using
tracer techniques, Gulson et al. (2010) demonstrated that outdoor application of a sunscreen containing
nano-68ZnO (zinc oxide) resulted in absorption of small amounts of Zn, measured as 68Zn in the blood.
Retention of the 68Zn tracer in blood was higher in women than in men, although the sample sizes were
small (n = 17 subjects total).
5.7.2.3. Ingestion
Absorption of conventional silver following ingestion has been reported; for example, Boosalis et
al. (1987) observed that 10-20% of ingested silver metal was absorbed in the GI tract, mainly by the
duodenum and small intestines. Nanoparticles, however, do not appear to be readily absorbed. In separate
review discussions, Mark (2007) and Elder et al. (2009) noted that the few studies investigating the
uptake and deposition of various nanoparticles to the GI tract have typically demonstrated that ingested
particles pass through without absorption and are eliminated quickly. Specific to nano-Ag, Kim et al.
(2008) reported that ingestion of nano-Ag by rats resulted in distribution of silver in a range of tissues,
with dose-dependent accumulation of silver observed in all tissues evaluated. Specifically, in a 28-day
oral administration study of nano-Ag particles approximately 53-73 nm in diameter, silver was detected
in the blood, stomach, brain, liver, kidneys, lungs, and testes, indicating that silver was distributed
systemically (Kim et al.. 2008). Silver uptake in kidneys was observed to be sex-specific, with silver
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accumulation in females twice as high as in males (Kim et al.. 2008). The investigators did not
demonstrate, however, that nano-Ag particles were absorbed by the GI tract. In another recent evaluation
of the ability of nano-Ag to cross the human intestinal wall using an in vitro model, Bouwmeester et al.
(2010) reported limited (0.5%) translocation of nano-Ag across the membrane, with no dependency on
size in the range of 20-112 nm in diameter (results not yet published; year indicates date of release of
preliminary data). As is the case with other routes of exposure, surface properties of nano-Ag present in
the GI system are likely to influence uptake across this biological barrier, especially given the changes in
acidity and the negatively charged mucous layer in the small intestine (Elder et al.. 2009).
5.7.3. Models to Estimate Dose
No models for estimating the pharmacokinetics of nano-Ag were identified for this case study.
Some models for nanoparticle deposition within the body have been developed that, by extension, could
be useful in evaluating dose for nano-Ag. One such model that ICRP developed estimates human and rat
airway particle dosimetry by modeling deposition of nanoparticles based on their size (Price et al.. 2002;
ICRP. 1994). Researchers including Mark (2007). Maynard and Kuempel (2005). and others have used to
this model to predict where in the respiratory tract particles of different sizes are likely to deposit, as
described in more detail above (see Section 5.7.2.1).
5.8. Summary of Exposure, Uptake, and Dose
Currently available data indicate that over 300 consumer products on the market could contain
nano-Ag, suggesting that an understanding of aggregate exposure from numerous sources might be useful
for accurately determining exposure pathways and estimating dose levels. Nano-Ag disinfectant spray use
alone can result in inhalation, ingestion, and dermal exposure to nano-Ag. Through environmental
pathways, nano-Ag might bind to other molecules, which can affect bioavailability to both biota and
humans.
Biotic Exposure and Uptake
Few data exist to determine the extent to which nano-Ag is present in the environment and whether
it is bioavailable to organisms. Most current models for estimating exposure and fate are not suitable for
simulating nanoparticles in general or nano-Ag in particular, and therefore require modification and
additional research. For biota, the aquatic environment is expected to be a greater source of potential
exposure than the terrestrial environment, and sediment also appears to be more a more likely exposure
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pathway given that nano-Ag preferentially accumulates in sediment. Exposure and bioavailability are
strongly affected by environmental factors, such as pH, the presence of other ligands (including sulfides),
other particles, and the nature of the environmental medium in which the nano-Ag is present. Ingredients
of spray formulations might also alter the behavior of nano-Ag or exhibit increased uptake in the presence
of nano-Ag. Some of these factors affect silver in general (e.g., presence of excess sulfides, ligands), and
evaluations in the laboratory have confirmed that they also affect the bioavailability of silver present in
nanoparticle form. Some environmental factors might particularly affect nano-Ag because of specific
properties of this form of silver.
Bacteria and fungi readily take up nano-Ag, which is consistent with the well-known antibacterial
properties of silver. Bioaccumulation by aquatic organisms has been studied to a limited extent and some
organisms (e.g., algae, eggs of vertebrates) readily take up nano-Ag. Other biota, including bivalve
mollusks and aquatic crustaceans, bioaccumulate conventional silver and some nanoparticles, but
nano-Ag bioaccumulation has not been specifically studied in these organisms. Some microorganisms
appear to have the ability to synthesize nano-Ag. Bioaccumulation in fish appears to occur to a limited
extent and is more likely in freshwater than saltwater species. Nano-Ag particles appear to adsorb to fish
gills, which then could serve as a pathway for delivering silver ions to the animal. In embryonic zebrafish,
nano-Ag particles were absorbed and accumulated in tissues, including the brain, and silver entered the
nuclei of cells in diverse organs. Nano-Ag particles can enter via chorion pores. Overall, bioaccumulation
of nano-Ag appears to decrease with increasing trophic level in water-column food webs.
Some terrestrial plants bioaccumulate silver to a limited extent, although conventional silver is
rarely absorbed beyond plant roots. Due to the smaller size and increased surface area of nano-Ag, the
potential exists for greater release, and therefore uptake, of silver ions from nano-Ag compared with ions
released from conventional silver; however, few data on the uptake of silver of any type are available for
terrestrial plants. Limited evidence suggests that invertebrates might absorb nano-Ag that is bioavailable
in soil. Bioaccumulation of nano-Ag in larger terrestrial organisms has not been studied. The possibility
remains, however, that terrestrial ecosystems could be impacted if microorganisms in soil and elsewhere
in terrestrial ecosystems are affected by nano-Ag.
Human Exposure and Dose
With the growing use of nano-Ag (especially in consumer products), elevated human exposures to
nano-Ag through a range of scenarios is increasingly possible. Several expert elicitation and modeling
exercises have concluded that use of nano-Ag in a spray solution is likely to result in consumer exposure
(including potential exposures to sensitive subpopulations) via inhalation and dermal pathways
(Wrjnhoven et al.. 2009a: Wardak et al.. 2008): however, no data focusing on nano-Ag were identified for
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this case study. Persons in consumer households, particularly children, can be exposed orally by hand-to-
mouth behaviors after touching or handling treated surfaces. Occupational exposures to nano-Ag in
powders or solutions used in manufacturing might also result in inhalation and dermal exposures, with the
potential for subsequent ingestion exposures (e.g., from hand-to-mouth or contact with treated surfaces).
These exposures appear to differ from those known for conventional silver, because smaller particles have
a greater potential to become aerosolized or to penetrate the skin. Occupational studies of conventional
silver have not shown clear associations of effects with particular exposures due to small sample sizes and
confounding factors. Perhaps because many of the human studies are retrospective (as described in the
following chapter), few data on exposure characterization are available. Only two occupational exposure
studies specific to nano-Ag were identified for this case study.
With respect to the human uptake of nano-Ag, considerations relevant to understanding uptake and
dose include the properties of particles and the route of exposure. Surface properties, such as charge, and
surface characteristics, such as the coating and presence of biomolecules that sorb to the surface of the
nanoparticle, can affect absorption. Other spray ingredients or materials used in the manufacturing
process might associate with nano-Ag and thereby exhibit increased uptake. Current data suggest that
silver from nano-Ag crosses biological membranes following oral and inhalation exposure, with resulting
accumulation in the lungs, liver, kidneys, stomach, brain, and blood. Whether soluble silver, silver ions, or
nano-Ag particles are entering various tissues after exposure to nano-Ag is unclear. Whether conventional
silver can cross the blood-brain barrier in humans is controversial; no evidence to date indicates that
nano-Ag particles, even particles as small as 1-2 nm, can penetrate the blood-brain barrier in mammals.
No evidence exists regarding the metabolism or transformation of nano-Ag in tissues, nor regarding
urinary or fecal excretion pathways and whether they differ from conventional silver excretion.
Deposition of nano-Ag in the human respiratory tract is expected to differ from that of
conventional silver, but the degree to which this difference in lung deposition quantitatively influences
distribution to other tissues is unclear (Elder et al.. 2009; Lynch and Elder. 2009). In animal studies (Sung
et al.. 2009; Ji et al.. 2007; Takenaka et al.. 2001). the finding of elevated silver concentrations in
extrapulmonary tissues and blood following inhalation exposure to nano-Ag aerosols provides qualitative
evidence of absorption of silver by the respiratory tract, followed by distribution to other tissues. The
finding of very high silver concentrations in the lungs following exposure, compared with other organs,
however, suggests that, at the tested concentrations, translocation to other tissues is not extensive.
Possible routes of translocation to other tissues following deposition of silver nanoparticles in the
respiratory tract include direct translocation to the brain olfactory bulb from the nasal olfactory epithelium
via the olfactory nerve, translocation (of particles and silver ions) to lymph nodes and blood following
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alveolar deposition, and translocation via mucociliary clearance to the digestive tract following
macrophage engulfment of alveolar-deposited particles (Ji et al., 2007).
Treatment of burn wounds in humans has resulted in kidney, liver, and skin accumulation of silver,
and in one case neurological effects were observed, suggesting silver might have entered the brain. Silver
absorption has been demonstrated across healthy human skin samples exposed to suspensions of
nano-Ag, but the degree to which this observation was due to transport of the silver nanoparticles or silver
ions released from those particles in the stratum corneum is unknown. Rates of silver absorption were
five-fold higher in skin samples damaged by abrasion.
Ingestion exposures to nano-Ag appear to result in lower relative absorption and subsequent dose
compared to other exposure pathways. Conventional silver has been demonstrated to cross the intestinal
barrier following ingestion exposure. The limited data for nano-Ag suggest particle characteristics,
including surface modification, affect whether nano-Ag is absorbed or excreted following ingestion.
Models do not currently exist for estimating nano-Ag distribution in the body. Models developed for other
particle types could be applied for nano-Ag if such models adequately consider chemistry and surface
properties.
As expressed by the Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel,
broad data gaps about potential exposures (and toxicity) related to nano-Ag exist (U.S. EPA. 201 Ob).
Attempting to follow the risk assessment paradigm for nano-Ag exposures, Christensen et al. (2010)
concluded that available data relevant to exposures and toxicity are inadequate at this time for use in
regulatory decision-making. When examining nano-Ag as a hypothetical registration under the
Registration, Evaluation, Authorisation and Restriction of Chemicals program in Europe, the Netherlands
National Institute for Public Health and the Environment (Pronk et al., 2009) also identified key data gaps
in particle characterization, exposure, uptake, and toxicity. These gaps were large enough at that time to
prevent implementation of the Registration, Evaluation, Authorisation and Restriction of Chemicals
process for this widely used material.
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Chapter 6. Characterization of Effects
This chapter summarizes the effects of nano-Ag on humans and biota associated with the use of
nano-Ag in spray disinfectants. The preceding chapters in this case study have laid a foundation for this
chapter by providing an exposure context for characterizing such effects. In this chapter, Section 6.1
provides information on the factors that influence the effects of nano-Ag on ecological receptors and
human health. The ecological effects resulting from exposure to nano-Ag are discussed in Section 6.2,
and relevant results from ecotoxicological studies are summarized for microorganisms (Section 6.2.1),
aquatic organisms (Section 6.2.2), and terrestrial organisms (Section 6.2.3). Human health effects
resulting from exposure to nano-Ag are discussed in Section 6.3, and relevant information is presented for
in vitro studies (Section 6.3.1), in vivo studies (Section 6.3.2), and human health and epidemiological
studies (Section 6.3.3). Because nano-Ag releases are likely to result in the formation of silver compounds
and discharges of silver ions, the ecological and human health effects of other silver species also are
discussed, when appropriate. The technology to differentiate the effects of silver nanoparticles from those
of the silver ions released from the nanoparticles is still developing. As a result, determining whether the
observed effect is due to the nanoparticle per se, the silver ions alone, or the silver ions modulated by the
nanoparticle is not always possible. Few ecological and human health effects studies distinguish between
the effects the silver nanoparticle and the silver ions released by the nanoparticle; where this distinction
has been made by investigators, it is presented here.
A few reviews are available on the ecological and human health effects of exposure to silver
compounds and silver ions (Lansdown. 2007; Ratte. 1999); comparatively few studies, however, are
available on the effects of silver nanoparticles. The Scientific Advisory Panel (SAP) for the Federal
Insecticide, Fungicide, and Rodenticide Act (FIFRA) concluded in its 2009 meeting on the evaluation of
hazards and exposure of nano-Ag that data gaps about potential hazards of nano-Ag are broad, and that
the hazard profile for nano-Ag can differ significantly from that for conventional silver and other silver
species (U.S. EPA. 201 Ob). As noted in Chapter 1, the findings presented in this case study generally
support the FIFRA SAP conclusions. Consistent with studies of other nanomaterials (Ostrowski et al.,
2009). most studies of nano-Ag have investigated the ecological or human health effects of various
formulations of silver nanoparticles and silver ions, and relatively few have investigated the effects of
end-use products containing nano-Ag or their life-cycle by-products. Moreover, the term "nano-Ag"
encompasses a variety of materials with a diverse range of physicochemical properties. As a result, not all
materials referred to as nano-Ag will necessarily behave the same or cause the same ecological or human
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health effects. Various members of the scientific community and the FIFRA SAP have cautioned against
extrapolating from one nano-Ag formulation to another when assessing hazards. The current dearth of
information, however, necessitates compiling results from studies using various nano-Ag formulations to
detail the current state of knowledge about the potential toxicological properties of nano-Ag. Therefore,
for the purposes of this case study, available information from all nano-Ag materials is described together,
but individual particle characteristics are noted in parallel.
This chapter focuses on characteristics of the nanoparticle, exposure media, and biological
receptors that might influence the degree to which nano-Ag is toxic to humans and biota. In general,
Section 6.1 focuses on nanoparticle properties and factors of the exposure environment that can influence
nano-Ag toxicity, because these data are relevant to both ecological and human health effects. As noted in
Chapters 1 and 5, there is no sharp demarcation between exposure-dose and effects, so some overlap is
unavoidable between the information on exposure and dose, presented in Chapter 5, and the information
on effects, presented here. To the extent possible, discussion of studies cited in both chapters is limited in
Chapter 5 to discussion of exposure-dose and in this chapter to discussion of effects.
Evidence is growing that nano-Ag in particular forms and under certain testing conditions can be
toxic to bacteria, fungi, algae, aquatic invertebrates, terrestrial invertebrates, and fish, and to mammalian
brain, liver, skin, and stem cells (Kahru and Dubourguier. 2010; Panvala et al., 2008). The breadth of
representative species for which nano-Ag toxicity has been studied and the scope of these studies,
however, are too narrow to draw definitive conclusions regarding the degree to which nano-Ag might
present a threat to environmental and human health (Wijnhoven et al., 2009b). Until recently, the
consensus was that the toxicity of silver in the environment depended mainly on the concentration of free
silver ions to which an organism is exposed (Khavdarov et al., 2009). Results from recent studies,
however, suggest that some adverse effects on biota can be attributed to properties of the silver
nanoparticle itself; furthermore, these effects might be exacerbated by the release of silver ions at the
biological interface (Choi et al. 2009; Laban et al.. 2009; Roh et al.. 2009; Navarre et al. 2008b; Lee et
al.. 2007). These properties are described in more detail in Section 6.1.
The following sections are not meant to be an exhaustive review of the ecological and human
health effects literature for nano-Ag, silver compounds, or silver ions. Instead, this chapter is intended to
highlight recent work on the effects of nano-Ag particles and to identify the information status and gaps
for assessing potential risks of nano-Ag in spray disinfectants.
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6.1. Factors that Influence Ecological and Human Health
Effects of Nano-Ag
Because many variables are associated with synthesis, characterization, and behavior of nano-Ag in
experimental and environmental conditions, identifying the primary property(ies) of nano-Ag that
contribute to an effect is extremely difficult. The complexity of the factors influencing ecological and
human health effects of nano-Ag also makes comparing the respective influence and importance of the
different properties extremely difficult. That many of the novel properties exhibited by nanoparticles
result from their small size is widely accepted, but other factors have been noted as variably contributing
to the effects of nanoparticles on biota and humans (Luoma. 2008).
For example, in a study comparing nano-gold and nano-Ag toxicity to zebrafish embryos,
nano-gold induced minimal sublethal toxic effects at the end of a 120-hour exposure period at the highest
concentration tested, while nano-Ag particles of the same size and at the same concentration resulted in
nearly 100% mortality (Bar-Ilan et al., 2009). This study demonstrated that nanoparticle size alone does
not dictate toxic effects, and it highlights the potential importance of nanoparticle chemical composition
in eliciting a toxic response. In a study comparing the toxicity of nanoparticles of different metals to fish,
aquatic invertebrates, and algae, Griffitt et al. (2008a) concluded by process of elimination that particle
chemistry appears to be the most influential characteristic of the nanoparticle because the investigators
did not identify a relationship between size, surface area, or zeta (Q potential and toxicity. Other
physicochemical properties, such as morphology, surface treatments, and solubility of particles, however,
also might significantly influence the toxicity of nano-Ag (Choi et al.. 2008). Furthermore, any particle
properties that influence bioavailability in general also will influence the effects of nano-Ag on an
organism, as interaction between the nanoparticle and the organism (or at least silver ions released from
the particles) is necessary to induce an effect. Factors that influence all substances in the environment
might also affect the toxicity of nano-Ag to humans and biota, including exposure medium, type of
organism, environmental bioavailability, route(s) of exposure, and the physicochemical properties and
size distribution of the nano-Ag particles. Furthermore, although processes in the environment and within
the body can detoxify harmful substances to a certain degree, the health of some receptors might be
sufficiently compromised such that small concentrations can trigger an adverse effect. This particular
factor, which undoubtedly contributes to the toxicity of nano-Ag in some susceptible or vulnerable
populations, however, has not yet been explored in the literature.
Although nanoparticle synthesis, characterization, and detection techniques have advanced
considerably in recent years, the influence of various physicochemical properties of nano-Ag has not been
fully elucidated using well-characterized nano-Ag under controlled conditions (U.S. EPA. 2010b). Nor
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have experimental or environmental characteristics been the focus of most toxicity studies to date. As a
result, the factors and conditions that drive nano-Ag toxicity in specific exposure scenarios have not yet
been determined, and data on the influence of physicochemical properties and experimental and
environmental conditions are still somewhat limited. This section focuses on factors that have been
demonstrated in the supporting literature to be pertinent to nano-Ag; however, findings related to other
types of nanomaterials are noted when relevant.
6.1.1. Physicochemical Properties
Size, chemical composition, and surface treatment appear to be three of the most critical
nanotoxicity metrics (Bar-Han et al.. 2009). Other physicochemical properties such as shape, doping, and
purity (or impurities) also could influence the outcomes of nano-Ag toxicity tests, but that information is
usually not reported in ecological and human health effects studies. Databases describing detailed
nanoparticle properties and health effects are being developed (Miller et al., 2007); these include the
National Institute for Occupational Safety and Health/Centers for Disease Control and Prevention
Nanoparticle Information Library,21 Rice University/International Council on Nanotechnology
Nanotechnology Environment, Health and Safety Database,22 the Organisation for Economic Co-
operation and Development (OECD)-maintained Database on Research into the Safety of Manufactured
Nanomaterials 23 and Oregon State University's Nanomaterial-Biological Interactions Knowledgebase.24
The need to characterize basic physical and chemical attributes of the nanomaterials used in
toxicity studies has been noted in numerous reports and journal articles (Auffan et al., 2009a; DEFRA.
2007; Powers et al.. 2007; Warheitetal.. 2007b; Powers et al.. 2006). The Minimum Information for
Nanomaterial Characterization Initiative (2008) has provided recommendations for the minimum required
physical and chemical parameters that should be reported for nanomaterials used in toxicological studies.
These parameters would establish generally what the material looks like, what the material is made of,
and what factors affect how the material interacts with its surroundings. The specific attributes for
minimal characterization recommended by the Minimum Information for Nanomaterial Characterization
include particle size or size distribution, agglomeration state or aggregation (i.e., clustering), shape,
overall composition (including chemical composition and crystal structure), surface composition, purity
(including levels of impurities), surface area, surface chemistry (including reactivity and hydrophobicity),
21http://www.cdc.gov/niosh/topics/nanotech/NIL.html
22http://icon.rice.edu/virtualjournal.cfm
23http://webnet.oecd.org/NanoMaterials/Pagelet/Front/Default.aspx
24http://oregonstate.edu/nbi/nanomaterial.php
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and surface charge (MINCharlnitiative. 2008). For more information on nanomaterial physiochemical
properties that could influence ecological and toxicological effects, readers are referred to detailed reports
listing information recommended to include in nanomaterial studies, including publications by OECD
(2008). Taylor (2008). and Warheit et al. (2QQ7a).
Furthermore, methods for establishing the toxic potential of chemicals, in general, have not been
fully standardized internationally. Multiple organizations (e.g., OECD, the U.S. Environmental Protection
Agency [EPA], the European Union) have proposed different sets of standard testing protocols, and
efforts are currently underway to develop a set of harmonized testing guidelines for establishing chemical
toxicity. Because consistent protocols have not yet been developed, not all results from the toxicological
studies described in this chapter (and in greater detail in Appendices B and C) can be compared directly.
Questions also exist about the suitability of current EPA and other standard testing guidelines for
assessing nanomaterials. A general overview of the issues associated with the various guidelines for
assessing physicochemical properties, human health effects, and ecological effects, as they pertain to
nanomaterials, is available in a nano-Ag case study released by the Netherlands National Institute for
Public Health and the Environment (Pronk et al.. 2009).
In the following subparts of this section, key properties affecting toxicity of nano-Ag are discussed,
including size, particle shape and crystal structure, and surface chemical composition and reactivity.
6.1.1.1. Size
Although nanoparticles are defined as particles having at least one dimension in the 1- to
100-nanometer (nm) range, not all nanoparticles within this range exhibit the same novel properties that
distinguish them from their conventional counterparts. It has been argued that the unique size-dependent
properties that necessitate a classification separate from conventional materials occur primarily in
particles 1-30 nm in size, and that larger particles (31-100 nm) generally do not exhibit properties
distinct from particles larger than 100 nm (Auffan et al.. 2009a). The smaller size of nano-Ag might allow
it to enter an organism more easily than its conventional counterpart. For example, for nanoparticles to
penetrate the membrane of zebrafish embryos, they must be small enough to diffuse easily through
transmembrane porins, which are proteins that facilitate passage of small molecules across the membrane,
without attaching to the walls of the chorion25 pore canal. These porins are approximately 0.5-0.7
micrometer (urn) in diameter. Once in the chorionic space, nanoparticles have been shown to penetrate
the amnion and enter the inner mass of the embryo where they can interact directly with cellular
25The chorion is the outermost of two membranes surrounding the embryo; the inner membrane is the amnion.
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organelles and potentially disrupt cellular processes (Lee et al., 2007). As discussed in Chapter 5, Lee et
al. (2007) used optical microscopy to observe the uptake of single silver nanoparticles by zebrafish
embryos in real time. They reported that nanoparticles can enter cells by passive transport (i.e., Brownian
diffusion) through the chorion pore canals and that the diffusion coefficients are inversely proportional to
the radius of the nanoparticles. Although most nanoparticles were observed to penetrate the embryo, some
that entered the chorion pore canals docked at the chorionic surface and formed clusters with other
nanoparticles. Lee et al. (2007) speculated that larger silver nanoparticles (>31 nm) embed in the chorion
pore canals and act as sites where clustering could occur. These clusters might eventually block the
canals, thus inhibiting normal chemical transport between the egg and its environment.
The intrinsic properties of materials in the nanoscale size range, such as enhanced reactivity and
unique surface structures, can result in higher dissolution rates, reduction and oxidation (redox) reactions,
or increased generation of reactive oxygen species (ROS), all of which can in turn affect toxicity in a size-
dependent manner (Auffan et al.. 2009a). As a result, toxicity studies using the same protocol and test
species likely are not directly comparable if the studies do not use nano-Ag within a similar size range or
with the same surface coating (see Section 6.1.1.3). Furthermore, if studies use nano-Ag with average
sizes greater than 30 nm (or if the nano-Ag material is not characterized in experimental conditions), the
studies might not capture the toxic effects related to unique nanoscale properties if the hypothesis
regarding particle size proposed by Auffan et al. (2009a) holds true. For example, Choi et al. (2008)
observed that growth inhibition in nitrifying bacteria correlated strongly with the availability of particles
less than 5 nm in diameter and not with silver nanoparticles that were 10 nm or larger. Morones et al.
(2005) also observed that, although bacteria were exposed to silver nanoparticles with an average size of
21 nm, the average size of nano-Ag penetrating the membranes of Escherichia coll was about 5 nm.
Nano-Ag particle size, however, might be correlated with other properties that affect toxicity, such
as the surface area-to-volume ratio, which in turn affects the ratio of reactive silver ions to unavailable
silver atoms. The ratio of silver ions on the exterior of the particle available to react with a biological
surface to the silver atoms that are "buried" within the interior of the particle and blocked from interaction
might influence nano-Ag toxicity levels. For example, equivalent total silver concentrations (by mass) of
nano-Ag and conventional silver (or even different sizes of nano-Ag) do not contain equivalent amounts
of silver ions available to react with a biological surface and induce toxicity. Most of the silver in the
larger particles is blocked from interacting with the environment or biological surfaces, whereas relatively
more of the silver in nano-Ag will be on the surface and readily available as the size of the nano-Ag
decreases. As a result, the methods used now to assess chemical toxicity might not account for variation
in biological responses related to particle size. Furthermore, several studies have proposed that the
combination of silver nanoparticles and silver ions is more toxic to some receptors than either form of
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silver alone (Bae et al., 2010; Sotiriou and Pratsinis. 2010). Sotiriou and Pratsinis (2010) proposed that
multiple modes of action exist for nano-Ag, and which mode of action dominates under specific
conditions is influenced by whether silver nanoparticles are smaller or larger than 10 nm in diameter.
They posited that silver nanoparticles smaller than 10 nm dissociate into silver ions more readily than
those larger than 10 nm because of higher specific surface area and greater surface curvature. As a result,
the chance that a nanoparticle of less than 10 nm will interact directly with the receptor before that
nanoparticle completely dissociates is small. Silver ions therefore appear to drive the toxicity of nano-Ag
in this size fraction. For silver nanoparticles with diameters greater than 10 nm, however, the release rate
of silver ions relative to silver content is reduced, allowing more time for the nanoparticle to attach to the
receptor surface and disrupt or damage that surface while shedding silver ions in a concentrated area. This
suggests that the particle effect is driving the toxicity of nano-Ag in the nano-Ag size fraction larger than
10 nm. As discussed in Chapter 2, individual nano-Ag particles can form clusters under various
conditions; in turn, particle clustering might influence the particle toxicity. For example, Zook et al.
(2011) report that the size of nano-Ag clusters correlates with hemolytic toxicity where larger clusters
resulted in less toxicity than smaller clusters at the same doses between 13.8 and 55.0 micrograms per
milliliter (ug/mL) (the highest dose, 110 ug/mL, resulted in almost complete hemolysis regardless of
cluster size).
Note that many of the studies investigating nano-Ag effects in humans and biota do not report the
average sizes or the size ranges of the nano-Ag materials used. Where size is reported, some investigators
report only the size distribution provided by the manufacturer, especially in the case of commercial
nano-Ag materials. This size distribution might not be representative of actual particle sizes, given the
potential for nano-Ag to form clusters (see Chapter 4). One example was reported by Miao et al. (2009).
where the manufacturer's information identified the average size of silver nanoparticles in a commercial
powder form as 10 nm, but the investigators experimentally determined the size of the nano-Ag primary
particles to be between 60 and 70 nm.
6.1.1.2. Morphology
Shape and crystal structure also can influence toxicity. A recent review by Auffan et al. (2009a)
examined unique properties at the nanoscale and observed that particles with diameters less than 30 nm
exhibit increased reactivity (e.g., changes in surface reaction rates, redox state, adsorption capacity) on
crystal facets due to size-dependent changes in crystalline structure. It has been shown that {111} facets,
which are high-atom-density surfaces, are more reactive than {100} facets in silver crystals (see Section
2.3.2) (Hatchett and White. 1996). Pal et al. (2007) reported the first comparative study on the
bactericidal properties of silver nanoparticles of different shapes. The study demonstrated that interactions
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with gram-negative E. coll bacteria were shape-dependent, with truncated triangular silver nanoparticles
that have the {111} basal plane exhibiting higher bactericidal activity than spherical and rod-shaped silver
nanoparticles, which are dominated by {100} facets. A dose of 10 micrograms (fig) of truncated triangular
silver nanoparticles added to 100 milliliters (mL) of nutrient broth with a bacterial concentration of 107
colony forming units per milliliter (CFU/mL) completely inhibited growth for at least 24 hours, while 100
ug of silver ions (added as silver nitrate [AgNO3])26 or spherical silver nanoparticles resulted in growth
inhibition only for up to 10 hours post-exposure, after which bacterial colonies appeared to grow at a
normal rate (Pal et al.. 2007). Morones et al. (2005) observed that the silver nanoparticles most likely to
be found on the surface of the bacterial membrane are those having more {111} facets. The nanoparticles
that interact with the cell membrane, such as those with {111} facets, are those that are most likely to
penetrate the cell; interacting with the cell membrane can result in the disruption of membrane processes,
damage to the membrane, and the release of silver ions directly to the membrane surface in high
concentrations. Therefore, morphology can play a key role in conferring toxicity (Morones et al.. 2005).
6.1.1.3. Surface Chemistry and Reactivity
Lok et al. (2007) found that partially oxidized silver nanoparticles were more toxic to E. coll than
freshly prepared zero-valent (reduced) nano-Ag. Oxidation of zero-valent nano-Ag produces ionic silver,
which is likely bound to the surface of the nanoparticle but could become available through desorption or
dissolution. The investigators reported that partially oxidized (i.e., oxidized surface) nano-Ag decreased
adenosine triphosphate (ATP) levels in E. coll cells by 90%, while exposing E. coll to reduced nano-Ag
did not elicit a response different from that of the controls. Additionally, silver nanoparticles synthesized
under an atmosphere of molecular nitrogen (N2), which precludes surface oxidation, exhibited no
antibacterial activity. The investigators also observed that oxidized silver nanoparticles do not appear to
elicit a toxic response in silver ion-resistant E. coll strains, indicating that under these conditions nano-Ag
does not produce a toxic response that is completely independent of silver ion effects (LoketaL 2007).
The surface chemistry of nanoparticles can be changed by coatings that in turn can influence the
particle's toxicity. Nanoparticle surface coatings have been demonstrated to influence cellular uptake,
binding of serum proteins in vivo, ROS generation, and immunosuppression or stimulation response to a
high degree in vertebrates (Bar-Han et al.. 2009). Surface coatings are frequently applied to nanoparticles
to functionalize them for a specific purpose or to stabilize them in suspensions. These coatings can
26Where investigators draw comparisons between the effects of nano-Ag and silver ions added as AgNO3, they
generally have concluded that the nitrate concentrations in these solutions are too low to elicit a toxic effect and that
any observed effects are attributable to the silver ions.
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influence the bioavailability or biocompatibility (i.e., the capability of the nanoparticle to coexist with
biological tissue without causing adverse effects) of the nanoparticle, which in turn can affect toxicity
(Limbach et al., 2005). Surface coatings that are used to ensure stability can facilitate an interaction
between the nanoparticle and the organism. Consequently, such treatments can have a profound effect on
the behavior of nano-Ag in the environment and its bioavailability to humans and biota. Metallic
nanoparticles can be coated with organic or inorganic compounds that prevent the formation of clusters in
solution (Navarre et al.. 2008a) or control the size of clusters (Zooket al.. 2011). both of which can
influence the transport properties of the nanoparticles and maximize the number of individual
nanoparticles in suspension. Stability of nano-Ag in suspension in spray disinfectants, for example, is
important for product efficacy. When used as a bactericide in water, silver nanoparticles must remain
suspended to be effective; therefore, in aquatic environments, coatings that keep nano-Ag in suspension
can result in higher concentrations of nano-Ag in the water column and increased exposure offish and
other aquatic biota. Furthermore, the chemicals used to coat nanoparticles could inherently be toxic to
certain organisms. For example, Stampoulis et al. (2009) reported that in the absence of nano-Ag, the
surfactant sodium dodecyl sulfate significantly inhibited zucchini (Curcurbita pepo) seed germination and
root growth when added to reverse osmosis water. In turn, when added to the nano-Ag solution, the
surfactant appeared to amplify the toxic effect of nano-Ag.
Several examples of the effect of surface-coated nano-Ag on toxicity were identified in the
literature. For example, Ahamed et al. (2008) compared the uptake by mouse embryonic fibroblasts of
nonfunctionalized silver nanoparticles with a nonuniform hydrocarbon surface layer and functionalized
silver nanoparticles coated with the polysaccharide gum arabic. After 24 hours at a nano-Ag concentration
of 50 ug/mL, most of the nonfunctionalized nano-Ag had formed clusters and had not penetrated cell
organelles, while the functionalized (i.e., coated) nano-Ag was distributed throughout the cells. The
investigators also reported higher levels of genotoxicity, as determined by measuring levels of the p53
protein (a molecular marker for DNA damage) and two DNA repair proteins, Rad51 and phospho-H2AX.
They observed that exposure to functionalized nano-Ag resulted in more upregulation of these proteins
than nonfunctionalized nano-Ag, suggesting that the functionalized nano-Ag causes greater genotoxicity
(Ahamed et al.. 2008). Kvitek et al. (2009) investigated the effects of surfactant- and polymer-modified
nano-Ag on the protozoan Paramecium caudatum and found that surface modification using a nonionic
surfactant, Tween 80, increased the materials' toxicity. In contrast, modification with the polymers PVP
360 (polyvinylpyrrolidone with an average molecular weight of 360 kilo-Daltons [kDa]) and PEG 35,000
(polyethylene glycol with an average molecular weight of 35 kDa) did not significantly affect the toxicity
of nano-Ag in those organisms.
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Nano-Ag also can be coated by biological substances released by or contained within organisms.
Kahn et al. (2011) used a nano-Ag-resistant strain of Bacillus pumilis to investigate a mechanism of
bacterial resistance to nano-Ag toxicity and discovered that exopolysaccharides secreted by this resistant
strain of bacteria coated the silver nanoparticles, thus preventing direct contact of the silver nanoparticle
with the bacterial cell. When nano-Ag was coated with these exopolysaccharides and exposed to strains of
E. coll, Staphylococcus aureus, and Mlcrococcus luteus that were not resistant to nano-Ag, growth rates
were still comparable to those of controls, whereas when these strains were exposed to nano-Ag not
coated with exopolysaccharides, growth rates were reduced substantially (statistical significance not
reported). The exopolysaccharide-secretion mechanism of tolerance might be selected for in bacteria
exposed continuously to nano-Ag in an environmental milieu.
Surface coatings also can influence the ^-potential of the silver nanoparticle, which affects the
particle's electrostatic attraction to biological surfaces. Some investigators have claimed that direct
contact between the nanoparticle and a bacterial membrane is required for bactericidal activity (El
Badawy et al.. 2011; Neal. 2008) (refer to Section 6.2.1). El Badawy et al. (2011) examined the toxicity to
Bacillus of four types of nano-Ag representing four different surface-charging scenarios, ranging from
very negative to very positive. The investigators demonstrated that toxicity was correlated with surface
charge (as measured by ^-potential) of nano-Ag, with the most negative surface charges resulting in the
least toxicity. The most negatively charged (-38 millivolt [mV]) citrate-coated nano-Ag was the least
toxic to Bacillus species, which are similarly charged themselves (-37 mV). As the surface charge
changed from negative to positive, the electrostatic repulsion between the bacterial cell and the nano-Ag
decreased, eventually allowing the nano-Ag to overcome the electrostatic barrier surrounding the cell. As
a result, nano-Ag formulations with a smaller negative charge were more toxic. Specifically, PVP-coated
nano-Ag (^-potential of-10 mV) was more toxic than uncoated27 H2-nano-Ag (-22 mV ), which was
more toxic than the citrate-coated nano-Ag (-38 mV). Finally, when the nano-Ag was coated with
positively charged (+40 mV) branched polyethyleneimine, attraction between the positively charged
nano-Ag and the negatively charged bacterial cell resulted in the highest degree of toxicity. Furthermore,
toxicity following exposure to the branched-polyethyleneimine nano-Ag was greater than that of ionic
27Elemental silver nanoparticles are inherently unstable in solution; without the aid of stabilizers or specific types of
preparation, they will immediately form clusters. As a result, the term "uncoated," as used by some investigators and
manufacturers to describe nano-Ag, as supplied or produced, is often misleading. Silver nanoparticles are likely
coated with a by-product of the synthesis process (e.g., OH") or a mild stabilizer (e.g., citrate, hydrocarbon) before a
coating intended to functionalize the nanoparticles (e.g., PVP, polysaccharide) is applied. Although this document
provides information on surface treatments of nano-Ag, as provided by investigators and manufacturers, determining
whether nanoparticles reported as "uncoated" in the literature were actually pre-treated to preserve the presence of
stable nanoscale particles was not possible.
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silver alone, indicating that the physical interaction of the silver nanoparticle with the cell was a key
component of the nano-Ag mode of action, at least in Bacillus. Also in this study, El Badawy et al. (2011)
determined that under these conditions surface charge was more highly correlated with nano-Ag toxicity
to Bacillus than shape and size.
6.1.2. Test Conditions
Experimental study design can significantly influence results obtained in toxicity testing. For
example, because the toxicity of nano-Ag is related in part to the solubility of the nanoparticle (i.e., the
rate at which the nanoparticle releases silver ions), the time allowed for nano-Ag to dissociate can
drastically affect toxicity. One striking example was provided by Ivask et al. (2010) in a study of toxicity
of nano-Ag to several sod-deficient strains of E. coli (i.e., luminescent bacteria that lack a particular
enzyme in oxidative stress response) and wild-type strains of E. coli. The investigators reported that the
ranges of EC50s28 for the different strains of E. coli exposed to nano-Ag were much higher in 30-minute
exposures (5.8-571 milligrams/liter [mg/L]) than in 2-hour exposures (3.11-45.9 mg/L), suggesting that
results from experimental designs in which exposure duration differs by a matter of hours might not be
comparable.
Methods of mixing such as sonication and ultrasound can be used in the preparation of nanoparticle
suspensions to increase stability and the contact of the nanoparticles with the test organism or cells. The
importance of standardizing sonication procedures and reporting them along with experimental results for
reproducibility is described in Taurozzi et al. (2011). Other reports note that when these mixing methods
are a part of aquatic toxicity testing procedures, they can result in an overestimate of toxicity of
nanoparticles compared to results under realistic (natural) conditions (Gao et al.. 2009). For example,
Laban et al. (2009) exposed fathead minnow (Pimephalespromelas) embryos to nano-Ag solutions that
had been either sonicated or stirred. Stirring mimics fin movement by males in natural conditions, while
sonication is not expected to represent any process in the natural environment. The investigators found
that sonicating the nano-Ag solutions from two commercially produced nano-Ag products for 5 minutes
before adding the embryos resulted in LC50s29 that were statistically significantly lower than when
nano-Ag solutions were stirred (Laban et al.. 2009).
28Effective concentration is the chemical concentration at which 50% of the exposed organisms experience a specific
effect; this effect level is commonly used to estimate the toxicity of a substance to a specific group of organisms.
29Lethal concentration is the chemical concentration at which 50% of the exposed organisms die; this effect level is
commonly used to estimate the toxicity of a substance to a specific group of organisms.
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The medium used in experimental studies also can affect the apparent toxicity of nano-Ag. For
example, nano-Ag in a liquid medium might only delay bacterial growth, while equivalent mass
concentrations of nano-Ag added to plated agar appears to inhibit bacterial growth completely, although
results are not always consistent from study to study (Pal et al., 2007; Sondi and Salopek-Sondi. 2004). In
water, damaged microbial cells can release intracellular substances that cause nanoparticles to cluster and
fall out of suspension, ultimately resulting in decreased numbers of silver nanoparticles in the water
(Sondi and Salopek-Sondi. 2004). No such microbial-induced clustering of nanoparticles seems to occur
on agar plates.
The antibacterial effect of nano-Ag also seems to depend in part on initial bacterial density used in
experiments (measured in terms of bacterial CPUs) (Sondi and Salopek-Sondi. 2004). Antibacterial
activity is generally higher at lower bacterial cell concentrations. Because the high CPUs used in many
experiments are rarely found in the environment, the bactericidal effect of nano-Ag in "real-life" systems
might be underestimated using current experimental techniques (Sondi and Salopek-Sondi. 2004).
6.1.3. Environmental Conditions
The characteristics of the environmental medium in which nano-Ag exposure occurs can affect the
properties of nano-Ag that ultimately influence toxicity. For example, changes in the pH, ionic strength,
dissolved oxygen content, temperature, quantity of natural organic macromolecules, light availability, and
quantity of ligands in the environment can significantly affect nano-Ag dissolution, bioavailability, and
reactivity, all of which can affect toxicity (Dasari and Hwang. 2010; Liu and Hurt. 2010; Cumberland and
Lead. 2009; Gao et al.. 2009; Choi and Hu. 2008).
The type of liquid medium and the characteristics of that medium also can affect the behavior of
nano-Ag. For example, nano-Ag released in wastewater can disperse silver ions, form complexes with
ligands, cluster to form larger silver particles, or remain as nanoparticles to varying degrees depending on
the characteristics of the wastewater (see Section 4.4.2). Few studies, however, have examined nano-Ag
effects in complex natural media. For example, one recent study compared the effects of nano-Ag on
bacteria and aquatic invertebrates in natural waters obtained from different locations in a river-estuarine
system (Gao et al.. 2009) (see below; see also Sections 6.2.1 and 6.2.2.2), and another study investigated
the effect of nano-Ag on bacterial diversity in natural estuarine sediments (Bradford et al.. 2009) (see
Section 6.2.1). Most toxicity studies, however, have added nano-Ag to deionized water and other
experimental media purely to establish the maximum toxic potential of the test material outside of natural
systems.
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Kvitek et al. (2008) demonstrated that unmodified nano-Ag in deionized water can remain well-
dispersed, exhibiting "long-term30 stability" in solution. As the pH of the system was lowered and the
solution became acidic, however, Kvitek et al. (2008) observed that the nano-Ag particles slowly formed
clusters, a condition that can influence particle uptake by aquatic organisms. Liu and Hurt (2010) also
demonstrated that citrate-stabilized nano-Ag phase partitioning was highly dependent on pH, and that
silver ion release rates increased at lower pH. They noted, however, that changes in pH affected ion
release kinetics only in the presence of dissolved oxygen. Yet, because Kvitek et al. (2008) did not report
the dissolved oxygen content of the solution in which the unmodified nano-Ag was reportedly stable,
their data are of limited value in determining the effect of pH on nano-Ag properties that influence
toxicity.
Nano-Ag forms clusters in media with high salt content, thereby diminishing its antibacterial
activity (Gan et al.. 2004). Because of the high ionic strength of sea water, particle association can occur
rapidly when nano-Ag solutions are released to estuaries and coastal environments, thus preventing the
large-scale dispersion of nano-Ag in the water column. Liu and Hurt (2010) offered a different
perspective, however, arguing that the inhibition of oxidation (i.e., formation of silver ions on the surface
of nanoparticles) is less dependent on salt content (i.e., ionic strength) and more dependent on the higher
pH of sea water when compared to deionized water. They suggested that the clustering due to increases in
ionic strength results in the formation of larger particle associations, but that the amount of available
surface area with which oxygen can react is preserved. This implies that ionic strength has little effect on
oxidation of nano-Ag in solution, and could have a correspondingly small effect on nano-Ag
environmental effects if the formation of silver ions on the surface of the nanoparticle and subsequent
release are the principal actions conferring nano-Ag toxicity.
Natural organic compounds in sea water can have surfactant and binding qualities that stabilize
nano-Ag suspensions, thus making the particles more available for sorption to or uptake by specific
aquatic organisms (Miao et al.. 2009). Gao et al. (2009) found that increasing dissolved organic carbon
(DOC) content generally decreased nano-Ag toxicity to both bacteria and Ceriodaphnia dubia. Other
natural organic compounds, such as humic substances and carboxylic acids, also adsorb quickly onto
nanoparticle surfaces and stabilize them in suspension by providing an electrostatically charged coating
(Cumberland and Lead. 2009; Fabrega et al.. 2009). The molecular composition of natural substances can
influence toxicity. For example, Dasari et al. (2010) compared the relative toxicity of nano-Ag to natural
aquatic bacterial assemblages in the presence of two types of humic acid (commercially available
terrestrial humic acid vs. Suwannee River water). The study authors discovered that the type of humic
30"Long-term" was not defined.
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acid present in the nano-Ag exposures statistically significantly affected bacterial viability count. Also,
whether samples were exposed to light influenced bacterial viability in the presence and absence of
nano-Ag and both humic acid substances. These results suggest that overall, the presence of light
increases the inhibition of bacterial growth and amplifies the toxicity of nano-Ag and humic acids, but
that the degree of toxicity observed might differ by geographic location due to variability in the molecular
composition of humic substances.
Gao et al. (2009) also experimentally examined the effects of natural surface-water characteristics
on the dispersion, bioavailability, and toxicity of manufactured nanoparticles, including nano-Ag. Toxicity
was examined in the freshwater invertebrate, C. dubia, and in bacteria using a 48-hour bioassay and
METPLATE analysis, respectively. Characteristics of the materials used and toxicity test results are
provided in Table 6-1.
The manufacturer reported the nominal diameter of the nano-Ag to be in the 20- to 30-nm range,
although transmission electron microscopy revealed the average size of the nano-Ag in suspension to be
approximately 80 nm in deionized water, as well as in one river-water sample, and more than 100 nm in
two other river-water samples (Table 6-1) (Gao etal.. 2009). The average size of nano-Ag suspended in
the water sample taken from the river SR3 delta was in the urn range. Note that differences in total silver
measured in solution indicate differences in sorption of some of the silver to inorganic ligands and DOC.
The bacterial bioassay, known to be sensitive to dissolved metal ions, indicated no toxicity at the
two highest DOC concentrations (45.71 and 10.18 mg C/L). The nano-Ag solution prepared in deionized
water was the most toxic to both the bacteria and C. dubia. Gleaning specific conclusions from these
experiments, however, is challenging due to the co-variation among some water chemistry parameters.
Nano-Ag from spray disinfectants might end up in treated and untreated wastewaters;
consequently, other constituents of wastewater might influence nano-Ag toxicity (see Sections 4.4.2. and
5.2.1.2). For example, wastewater often contains an abundance of organic and inorganic ligands with
which nano-Ag and silver ions form strong complexes (Choi et al.. 2009; Blaser et al.. 2008). Information
on how ligands might influence the bioavailability of nano-Ag, which in turn influences the effect of
nano-Ag on organisms, is presented in Section 5.2.1.2. Choi et al. (2009) investigated the influence of
ligands on the toxicity of nano-Ag to nitrifying bacteria and found that a range of ligands, including
chloride, phosphate, H2EDTA2 , and sulfide, reduced toxicity to varying degrees, although
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Table 6-1. Experimental parameters and toxicity of nano-Ag in deionized water and natural surface
waters.
Water characteristics
PH
Alkalinity (mg CaCOs/L)
Ionic strength (mM)
Dissolved organic carbon (mg C/L)
Na*(mM)
Ca2* (mM)
Mg2* (mM)
Agraai(ug/L)
Deionized water
Not reported
-0=
-0=
-0=
-0=
-0=
-0=
-0=
Headwater (SR1)
4.7
6
0.94
45.71
<1
<1
<1
<10»
River-water samples
Midsection (SR2)
7.15
88
3.34
10.18
<1
<1
<1
<10»
Delta (SR3)
7.56
132
475
2.3
31.38
6.61
30.94
<10»
Silver characteristics and toxicity assays
Diameter nano-Ag (nm)
Nominal Ag (mg/L)
Measured total Ag (mg/L)c
MetPLATE bacterial LCso (ug/L)
Ceriodaphnia dubia 48-hr LCso (ug/L)
95% Confidence limits (ug/L) probit analysis (for C.
dubia)
-80
1000
1.67
47.79
0.46
0.45-0.47
-80
1000
0.54
No toxicity
6.18
5.5-6.7
-300
1000
0.043
No toxicity
0.77
0.75-0.80
>1000
1000
0.66
112
0.70
0.66-0.73
a Not measured, but assumed to be approximately 0
b The detection limit for Ag is 10 ug/L
0 After mixing and filtering to remove particles larger than 1.6 urn
Source: Data extracted from Gao et al. (2009).
sulfide was the only ligand to reduce nano-Ag toxicity by more than 40%. At a 1-mg Ag/L concentration
in deionized water, nano-Ag inhibited nitrification by 100%. After sulfide was added to achieve a final
sulfide concentration of 10 micromoles per liter (uM), toxicity decreased by about 80%. Miao (2009)
reported that adding thiols (-SH) to aqueous suspensions of nano-Ag increased the dispersion of nano-Ag
several orders of magnitude beyond levels predicted for the natural environment. No toxicity was
observed in the marine diatom Thalassiosim weissflogii, however, when it was exposed to nano-Ag in the
presence of thiols. The investigators believed this lack of toxicity might have been due to the large size of
the nano-Ag (60-70 nm), the protective layer of natural organic matter around the nano-Ag that prevented
a direct interaction between nano-Ag and the algal cell, the concentrations of nano-Ag used, or a
combination of these factors (Miao et al.. 2009).
6.2. Ecological Effects
In its conventional form, silver can be toxic to fish, aquatic invertebrates, algae, some terrestrial
plants, fungi, and bacteria (U.S. EPA. 1993). The ecological effects of conventional silver have been
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studied extensively; although some data gaps remain, tests and environmental case studies have revealed
that conventional silver can be toxic to biota at aqueous concentrations at or below 50 nanograms per liter
(ng/L) (Wijnhoven et al., 2009b). Some of the organisms most sensitive to conventional silver are
freshwater and marine phytoplankton, freshwater salmonids, and marine invertebrates in early life stages
(Luomaetal.. 1995). Although conventional silver can be extremely toxic to biota, concentrations of free
silver ions are not expected to be high enough in most natural systems to adversely affect these organisms
(Luoma. 2008). Nano-Ag, however, might present a higher risk to ecosystems because it could become
more bioavailable under certain conditions and provide a reservoir of silver ions that could be delivered
directly onto the surface of an organism or to cell constituents. Despite this possibility, relatively few
studies have investigated the effects of nano-Ag on organisms other than bacteria and laboratory rodents.
Moreover, such single-species tests likely do not capture the influence of nano-Ag on structural and
functional complexities at the ecosystem level. In addition, studies have not explored the ecological
effects from actual nano-Ag technologies at the product level, although manufacturers report that these
products are available on the market.
Although in vitro studies dominate the literature investigating nano-Ag ecological toxicity, the
prevalence of in vivo studies has increased in recent publications. Ecological effects studies
predominantly investigate nano-Ag effects associated with acute exposure (generally 96 hours or less),
and only a few studies examine nano-Ag effects over subchronic and chronic exposure periods. Studies of
ecological effects indicate that exposure to nano-Ag could lead to adverse effects on higher level
endpoints such as survival, growth, and reproduction, and on sublethal endpoints such as phenotypic
changes, gene expression, and oxidative stress. Reported indirect effects of nano-Ag include pore
clogging, solubilization of toxic compounds, and production of ROS (Navarro et al., 2008a).
Because the dose in all studies discussed in this section was given as either mass concentration or
nanoparticle number in exposure media, these are the dose metrics provided here. Converting all
concentration data to the same units was not possible due to a lack of information provided by many
study authors on the factors used to define their units of measurement (e.g., for parts per million [ppm],
whether this unit is based on ppm by mass, number, or another metric is not always stated). Nominal
nano-Ag concentrations in the studies were based on total silver, silver ions, free silver, or added nano-Ag
content. This information is provided in the tables in Appendix B that summarize the ecological effects
studies; measured concentrations are also presented when provided in the studies. The studies are
presented in Appendix B in alphabetical order by author for each of the ecological effects sections; the
reader is referred to this appendix for study details not presented in this chapter. The following sections
present the available data on the effects of nano-Ag on non-algal microorganisms (Section 6.2.1), aquatic
organisms (algae in Section 6.2.2.1, invertebrates in 6.2.2.2, and vertebrates in 6.2.2.3), and
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nonmammalian terrestrial organisms (plants in 6.2.3.1, invertebrates in 6.2.3.2, and vertebrates in 6.2.3.3).
For each group of organisms, the discussion is organized into three parts: known effects of conventional
silver exposure, effects of nano-Ag exposure, and nano-Ag mode of action.31 This organization is
intended to capture the potential effects of nano-Ag, silver ions released from the silver nanoparticles, and
common silver complexes.
6.2.1. Microorganisms (Excluding Algae)
The effects of silver ions on microbial communities are well-documented, and nano-Ag could have
similar effects. That the proposed use of nano-Ag in this case study is for disinfectant sprays implies that
a certain level of antimicrobial efficacy is expected and desired at the application site. Once the spray is
used, however, nano-Ag is expected to enter the environment, as discussed in Chapter 2, where effects
might occur in natural microbial communities. Such communities are key to nutrient decay and recycling
processes that support overall ecosystem functioning (Navarro et al.. 2008b); nano-Ag spray use therefore
could result in unintended antimicrobial effects potentially leading to ecosystem cascade effects. In
addition, nano-Ag might disrupt beneficial gut microflora found within the digestive systems of higher
level organisms (Sawosz et al.. 2007). The antimicrobial activity of nano-Ag has been demonstrated in
laboratory tests with isolated prokaryotic species, including E. coll (Hwang et al.. 2008; Pal et al.. 2007;
Morones et al.. 2005; Sondi and Salopek-Sondi. 2004). Pseudomonas aeruginosa (Morones et al.. 2005).
Vibrio cholerae (Morones et al.. 2005). Bacillus subtilis (Yoon et al.. 2007). and nitrifying cultures (Choi
et al.. 2009; Choi et al.. 2008; Choi and Hu. 2008) and in eukaryotic species, including Saccharomyces
cerevisiae (Saulou et al.. 2010). The results from these studies demonstrate that the range of
microorganisms potentially susceptible to nano-Ag toxicity spans gram-positive, gram-negative,
prokaryotic, eukaryotic, autotrophic, heterotrophic, mesophilic, and halophilic species. Studies specific to
nano-Ag effects on microbes, however, remain limited in number and scope compared to those
investigating effects of exposure to other forms of silver. Furthermore, only a handful of studies to date
have attempted to determine nano-Ag bacterial toxicity in more complex natural media (Liang et al..
31Mode of action is defined in the U.S. EPA Cancer Guidelines as "a sequence of key events and processes, starting
with interaction of an agent with a cell, proceeding through operational and anatomical changes and resulting in
cancer formation" (U.S. EPA. 2005). Multiple definitions for mode of action exist within the regulatory context.
For the purposes of this document, mode of action refers to the key steps in the toxic response at the target site that
are responsible for the physiological outcome or pathology of the chemical. Because mode of action is inherently
linked to effects, completely separating the discussion of mode of action from the discussion of adverse effects (e.g.,
physical disruption of cell membrane is both an effect and mode of action for lethality) is sometimes not possible.
Where possible in this chapter, mode of action is discussed in a section following effects to highlight the processes
that might be responsible for the observed adverse effects (e.g., reduced reproductive success, increased mortality).
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2010; ParketaL 2010b; Bradford et al.. 2009; Gao et al., 2009). and even these studies were limited in
scope and not necessarily representative of nano-Ag behavior in highly complex ecosystems. As a result,
the available studies provide limited insight into potential impacts from intentional and unintentional
releases of nano-Ag into the environment.
Known Effects of Conventional Silver Exposure on Microorganisms
Silver is a relatively toxic substance for microbes; for example, when compared to 12 other metals,
silver was identified as the most toxic to microbial soil communities (Cornfield. 1977). Silver can inhibit
microbial growth, affecting sensitive communities such as ammonifying, nitrogen-fixing, and
chemolithotrophic32 bacteria (Albright and Wilson. 1974). The bacterial plasma and cytoplasmic
membrane are important target sites because silver ions cause the release of ionic potassium (K+) from
bacteria (Jung et al.. 2008). Silver exposure also reduces DNA transcription in bacteria, which results in a
delay or complete inhibition of microbial growth. Other evidence suggests that silver exposure to bacteria
and fungi leads to changes in membrane structure and deposition of silver throughout the cell via
formation of electron-dense granules (by combination of silver with cell constituents) (Saulou et al.. 2010;
Feng etal. 2000). The antimicrobial mode of action of conventional silver is only partially understood,
but is believed to be the result of contact of the silver compounds with microbial cell walls, followed by
release of silver ions that combine with -SH groups of enzymes, ultimately leading to the deactivation of
microbial proteins (Yoon et al., 2007; Morones et al., 2005).
Effects Specific to Nano-Ag Exposure on Microorganisms
Examples of recent studies investigating the effects of nano-Ag on microorganisms are presented in
detail in Section B.2 of Appendix B. These studies illustrate that exposure of microorganisms to nano-Ag
frequently results in growth inhibition, inhibition of nitrifying enzymatic processes, arrest in fungal cell
cycles, cell membrane damage (e.g., pitting, perforation), cell membrane process disruption, and ROS
generation.
In experimental conditions, the bacterial cell density and nano-Ag concentrations are generally
high, and contact between nanoparticles and bacteria is generally ensured. In natural systems,
significantly more reactive surfaces are available with which both the nanoparticle and bacteria can
interact, and concentrations of both might be much lower than in experimental settings, which might
result in relatively rare contact between nanoparticles and bacteria (Neal. 2008).
32Chemolithotrophic bacteria are those that derive energy from the oxidation of inorganic materials.
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Sensitivity to nano-Ag varies among phyla, among species, and even among studies using the same
species. Shrivastava et al. (2007) reported that gram-negative bacteria were more sensitive to nano-Ag
than gram-positive bacteria. For the gram-negative species E. coll and Salmonella typhi (drug-resistant
strains), 100% inhibition of growth was observed at a nano-Ag concentration of 25 ug/mL, while no
growth inhibition was noted in the gram-positive species Staphylococcus aureus at the same
concentration. Even at 100 ug/mL, the growth of S. aureus was only partially inhibited (Shrivastava et al..
2007). Kvitek et al. (2008) also reported that E. coll was more sensitive to exposure to unmodified silver
nanoparticles than S. aureus, but further comparisons of six gram-positive bacterial strains to four gram-
negative strains indicated that other factors beyond gram-status influence sensitivity. The difference in
minimum inhibitory concentrations (MICs) appeared to be species-specific, rather than gram-status-
specific, as both the gram-positive and gram-negative bacteria displayed the same range of MICs (1.69-
6.75 ug/mL). In a comparison between mesophilic (i.e., thriving in moderate temperatures) and halophilic
(i.e., thriving in high-salinity environments) bacteria, Sinha et al. (2011) reported that 2 millimoles per
liter (mM) nano-Ag was more toxic to the marine, gram-negative, halophilic Marinobacter species than to
all other bacteria species tested. Nano-Ag had no effect on the growth of the other halophile EMB4,
which is a gram-positive strain. Marinobacter has a thinner protective peptidoglycan layer around the cell
and contains a higher percentage of negatively charged cardiolipins than gram-positive halophiles and
gram-positive and gram-negative mesophiles. These results indicate that some bacteria in marine
ecosystems could be particularly susceptible to nano-Ag exposures, while others might remain unaffected.
Another area of concern resulting from increases in nano-Ag released to the environment is the
potential effect of nano-Ag and associated compounds on bacteria used in wastewater treatment processes
or located in areas where wastewater effluent is discharged (Bradford et al.. 2009; Choi et al.. 2008).
Although many different types of bacteria can be used at different stages of the wastewater treatment
process, nitrifying bacterial communities33 are considered especially vulnerable due to their slow growth
rate and history of sensitivity to other environmental pollutants (Choi et al.. 2009; Choi et al.. 2008; Choi
and Hu. 2008). Furthermore, nitrifying bacteria are critical to processes involving nutrient removal in
wastewater treatment (Neal. 2008). If nitrifying bacterial concentrations were significantly reduced or
eliminated in wastewater treatment bioreactors, chemical nutrients (e.g., ammonia) in the wastewater
would not be removed, which might ultimately result in eutrophication in areas where the wastewater is
33Nitrifying bacterial communities can include a mixture of species responsible for oxidizing ammonia to nitrite or
from nitrite to nitrate, including bacteria in the Nitrospira, Nitrosococcus, Nitrobacter, Nitrospina, and
Nitrosomonas genera. The exact composition of nitrifying communities is often not characterized in experimental
studies.
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discharged (GradyetaL 1999). Eutrophication can cause anoxia and other reductions in water quality,
leading to adverse effects on aquatic biota.
Nitrifying bacteria are sensitive to nano-Ag exposure, but seemingly less so than E. coll.
Nitrification occurs in nitrifying bacterial communities by a two-step process involving three specific
enzymes. Of these three critical enzymes, the enzyme partially responsible for the oxidation of ammonia,
ammonia monooxygenase (AMO), appears to be the most sensitive to nano-Ag exposure. A concentration
of 1 mg/L nano-Ag inhibited nitrification (measured as change in AMO-related oxygen uptake rates) by
100% in a respirometric assay, while silver ions inhibited growth by 83% at this concentration (Choi et
al.. 2009). Choi and Hu (2008) determined that nano-Ag inhibited the growth of nitrifying cultures (EC50
= 0.14 mg/L) more than silver chloride (AgCl) colloids and silver ions (as AgNO3). They also found that
intracellular ROS concentrations increased significantly compared to the controls when bacteria were
exposed to nano-Ag, and that this increase correlated strongly with growth inhibition (R2 = 0.86). Choi et
al. (2008) demonstrated that at 1 mg Ag/L in nitrifying suspension, nano-Ag, silver ions, and AgCl
colloids inhibited respiration by approximately 86%, 42%, and 46%, respectively.
Bradford et al. (2009) examined the effects of nano-Ag concentrations of up to 1 mg/L on bacterial
abundance and diversity in an estuarine microcosm study. The investigators reported that the lowest
concentration used in this study (0.25 mg/L) was an order of magnitude higher than the concentration
expected from the highest estimated release of engineered nanoparticles from nano-enabled products, as
estimated by Boxall et al. (2007). After applying l/20th of the total nano-Ag dose to 20 liters (L) of
estuarine water over ~3.8 kilograms (kg) of estuarine sediment for 20 days (followed by a 10-day period
with no dosing), Bradford et al. determined that nano-Ag exposure at a total concentration of 1 mg/L did
not affect bacterial abundance and had a small statistically significant effect on bacterial diversity on the
sediment surface at the highest concentration tested (1 mg/L) when compared to controls. The authors
argued, however, that this difference was likely due to chance (based on a similarity profile permutation
procedure) arising from the presence of other sediment-dwelling organisms in these samples with their
own associated bacterial communities that differ from those at the sediment surface. To assess bacterial
diversity, the investigators used a nested polymerase chain reaction-denaturing gradient gel
electrophoresis method to amplify the 16S ribosomal RNA fragment in the bacteria species. They
reported that the DNA primers they used were specific to "Bacteria" but which species they included in
this classification is unclear. Nevertheless, multiple studies have used similar methods to establish
nitrifying community structures in wastewater treatment processes and to establish bacterial diversity in
communities not associated with the nitrification process (Mills et al.. 2008; Muhling et al.. 2008). so
nitrifying species among others likely were included in this analysis. Because nitrifying bacteria generally
will be found in areas where wastewater is discharged into the environment due to higher concentrations
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of ammonia in these locations, and because the samples were taken from an area that received discharges
of wastewater effluent, such bacteria likely also were present in the sediment cores. Without reporting
which species comprised the clusters in the estuarine sediment, Bradford et al. (2009) argued that impacts
on bacteria in estuarine sediment likely will be negligible at nano-Ag concentrations expected from
estimated future releases (<1 mg/L). The investigators also reported, however, that the transformation of
nano-Ag into other forms of silver (e.g., Ag+, AgCl, AgCl2", AgCl32") within the experimental tanks was
not studied, but such transformations would be expected to influence the potential impacts of nano-Ag in
estuarine waters (Bradford et al.. 2009).
As discussed in Chapter 5, Kim et al. (2010a) determined that activated sewage sludge from
wastewater treatment plants not located near major industrial sources of silver or photochemical
processors could contain high concentrations of silver in the form of nano-sized silver sulfides. Although
silver sulfide is relatively insoluble in water, some evidence from studies with other nanomaterials
indicates that nano-sized sulfide complexes might dissolve more readily than their conventional
counterparts (Liu et al.. 2009). As a result, some studies have begun to examine the effects of nano-Ag on
bacteria in sewage, activated sewage sludge, and in sediments (Gao etal., 2011; Khanet al., 2011; Liang
et al., 2010). In a study using a Modified Ludzack-Ettinger (MLE) activated sludge treatment system,
Liang et al. (2010) determined that most of the nano-Ag in the reactor strongly adsorbed to activated
sludge. The investigators first compared the effect of nano-Ag to that of silver ions on activated sludge
and then investigated the effects of a shock load of nano-Ag on bacterial activities and community
structure in the MLE reactor. In the first part of the experiment, sludge was spiked with 1 mg Ag/L of
either PVA-capped nano-Ag (average size 21 nm) or ionic silver, and nitrification inhibition was assessed
(measured as specific oxygen uptake rates) using short-term batch extant respirometry. They found that
nano-Ag and ionic silver inhibited nitrification in the sludge samples by about 41.4% and 13.5%,
respectively. In a study examining the effects of 0.5 mg/L nano-Ag (average size 66 nm) on denitrification
(i.e., reduction of nitrate) in river sediments, Gao et al. (2011) reported no significant inhibition of nitrate-
reducing processes, but they did report that acetate degradation, another indicator of nitrogen-cycling
bacterial response, was reduced two-fold when compared to the controls. The authors suggested that
because of the lack of statistically significant inhibition of nitrate reduction at 0.5 mg/L nano-Ag (the
IC5034 for Pseudokirchneriella subcapitata, a sensitive model aquatic organism), denitrifying bacteria are
less sensitive to nano-Ag than some other aquatic organisms; the effect on acetate degradation, however,
34Inhibitory concentration is the chemical concentration at which a given percentage (in this case, 50%) of the
exposed organisms demonstrate a response in a chosen endpoint; this effect level is commonly used to estimate the
toxicity of a substance to a specific group of organisms.
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implies that exposure to nano-Ag at higher concentrations could result in an adverse effect on the broader
nitrogen-cycling process (Gao etal.. 2011).
In the second part of the experiment conducted by Liang et al. (2010). the PVA-capped nano-Ag
was fed into the MLE reactor at a continuous rate for 12 hours, achieving a peak silver concentration of
0.75 mg/L. Autotrophic growth was significantly inhibited, with maximum inhibition (46.5%) occurring
14 days after completion of the shock-load administration. A shift in the community structure of the
bacteria also was observed after the shock loading. The dominant ammonia-oxidizing genus
Nitrosomonas and one of the dominant nitrite-oxidizing genera Nitrospira were significantly reduced
while another dominant nitrite oxidizing genus, Nitrobacter, was completely eliminated from the system
following the nano-Ag shock load. These results suggest that short-term respirometric assays (like the
first part of the experiment) might underestimate the toxicity of nano-Ag to nitrifying bacteria in
continuous flow systems because short-term assays do not account for longer term kinetics of metal
internalization and effects due to continuous exposures. The authors note, however, that inhibition
observed in this study was nearly half that observed in the study by Choi et al. (2008). which used the
same experimental method, but on enriched nitrifying cultures instead of on activated sludge. Liang et al.
(2010) hypothesized that the reduction in toxicity in the activated sludge could be due to the presence of
exopolymeric substances (EPS) secreted by the bacteria that act as biological barriers between bacterial
cells and silver nanoparticles.
Little information is available on the effects of nano-Ag on fungi compared with bacteria. In a
study comparing the antimicrobial efficacy of different nanomaterials in combination with their effect on
mammalian cell viability, Martinez-Gutierrez et al. (2010) reported that nano-Ag MICs for three species
of fungi ranged from 3 to 25 ug/mL. The range of MICs for the 10 species of bacteria exposed to nano-Ag
in the same experiment was 0.4-1.7 ug/mL, suggesting that, overall, fungi might be less sensitive to
nano-Ag than bacteria.
Based on a study of the model fungus Candida albicans, Kim et al. (2009) observed that exposure
to nano-Ag resulted in a loss of membrane potential and an increase of pitting on the cell surface, which
was tentatively linked to the formation of large pores through the cell walls and membranes and
subsequent cell death. The MIC of nano-Ag for this species was 2 ug/mL. Exposure to 40 ug/mL nano-Ag
also resulted in the arrest of the fungal cell cycle, most likely by inhibiting some of the cellular processes
necessary for bud growth. This fungus, however, is commensal with humans and not commonly found in
the "external" environment. Saulou et al. (2010) examined nano-Ag effects on cell composition and
ultrastructure of S. cerevisiae, a yeast that can be found outside of humans. The investigators reported
damage to the cell membrane similar to that reported for C. albicans by Kim et al. (2009) and further
reported changes to intracellular proteins. Adhesion of fungal cells to silver nanoparticles embedded in an
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organosilicon matrix for 24 hours resulted in a 1.4-log reduction of viable cell counts compared to the
control. Saulou et al. (2010) attributed this antifungal activity to a multifactorial nano-Ag mode of action
whereby the nanoparticle both disrupts the cell membrane processes and causes changes in the
intracellular structures.
Hypothesized Nano-Ag Mode(s) of Action in Microorganisms
Silver ions released from silver nanoparticles are often purported to be the primary source of
toxicity from exposure to nano-Ag, particularly for higher level organisms. Several studies have
investigated whether the nanoparticles inherently exacerbate the toxic effects or cause them outright. The
full sequence of events that causes silver nanoparticles to be toxic to bacteria and fungi is still largely
unknown or unconfirmed, although several recent studies have attempted to elucidate such mode(s) of
action (Kim et al.. 2009: Choi and Hu. 2008: Hwang et al.. 2008: Lok et al.. 2006: Morones et al.. 2005).
Several possible modes of action are discussed in the literature concerning nano-Ag effects on
microorganisms. These are (1) membrane disruption through direct attachment of the nanoparticle to the
bacterial membrane, (2) cellular invasion and enzyme disruption by nanoparticles, (3) changes in cell
membrane permeability, (4) interference with cellular S-containing compounds, and (5) intracellular ROS
accumulation (Kim et al.. 2009: Hwang et al.. 2008: Pal et al., 2007: Lok et al.. 2006: Panacek et al..
2006: Morones et al.. 2005: Sondi and Salopek-Sondi. 2004). That several of these events might act
together to result in cell death is probable, but the specific processes and interactions required for toxicity
have not been fully confirmed.
Choi and Hu (2008) present one example of how potential modes of action can work in tandem in a
study assessing the effects of nano-Ag on autotrophic bacteria. Although apoptosis occurred and silver
nanoparticles adsorbed to the surface of the microbial cell walls, cell membrane leakage was not evident.
The investigators believed that cell death occurred partially as a result of intracellular ROS generation by
the silver ions released at the membrane surface, although this was not proven directly. Silver ions
delivered at the membrane surface have been shown to damage DNA and to induce apoptosis without
causing visible damage to the outer bacterial wall or cytoplasmic membrane (Inoue et al.. 2002). The
toxic effects cannot be explained completely by ROS generation, however, because silver ions did not
induce the same level of toxicity at similar intracellular ROS concentrations and total silver mass
concentrations (Choi and Hu. 2008). Thus, the higher degree of toxicity exhibited by nano-Ag might also
have resulted from the presence of particles smaller than 5 nm that were able to cross the cell membrane
and interact directly with cell constituents, releasing silver ions directly to sensitive areas. Choi and Hu
(2008) also postulated that nano-Ag in the cell likely disrupted enzyme function, ultimately causing cell
death.
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As discussed in Section 6.1.1.1, the driving factor in the nano-Ag mode of action might be either
silver ions or nano-Ag particle interactions with the cell wall, depending on the size fraction of the
nano-Ag material in the suspension. But overall, several studies have now suggested that nano-Ag and
silver ions together are more toxic than either silver species alone (Bae etal.. 2010; Sotiriou and Pratsinis.
2010), and inconsistencies in the literature on this point are likely the result of uncharacterized clustering
states, the presence of residual silver ions left over from particle synthesis, and unquantified
characteristics of the exposure medium (El Badawy et al.. 2011; Bae etal.. 2010; Sotiriou and Pratsinis.
2010).
Many studies suggest that heightened antibacterial activity of nano-Ag (compared to conventional
silver or silver ions alone) is related to a physical disruption of membrane function and cellular processes,
most likely due to the direct contact of the nanoparticle with the cell membrane. El Badawy et al. (2011)
reported this physical interaction as the "limiting step" in the nano-Ag mode of action; until the silver
nanoparticle can overcome the electrostatic barrier surrounding the bacterial outer membrane or
peptidoglycan layer, the nanoparticle is not free to interact with, disrupt, and damage the cellular
membrane, and therefore cannot release ionic silver directly to the biological surface or within the cell
itself. Because surface charge of nano-Ag largely depends on the coating (both chemical or biological),
nano-Ag can be negatively or positively charged, as demonstrated by El Badawy et al. (2011) and
discussed in Section 6.1.1.3. When nano-Ag is negatively charged, it naturally repels negatively charged
bacteria, thus establishing an electrostatic barrier. Jin et al. (2010) proposed that this barrier can be
overcome, however, by ion bridges formed by cations present in environmental media. In their study, Jin
et al. (2010) reported that toxicity to gram-negative bacteria increased in the presence of divalent cations
such as Mg2+ (ionic magnesium) and Ca2+ (ionic calcium), which are thought to form a bridge between
negatively charged silver nanoparticles and the lipid polysaccharide layer surrounding the bacterial cell
wall. Such bridges can change cell permeability and facilitate transport of silver nanoparticles across the
outer cell membrane and the peptidoglycan layer of gram-negative species, ultimately leading to
membrane disruption or damage.
Several studies have reported the presence of pits or perforations on microbial surfaces, and Sondi
and Salopek-Sondi (2004) confirmed by energy dispersive X-ray analysis that nano-Ag was incorporated
into E. coll cell membranes. This effect also has been reported in the fungus C. albicans, where exposure
to nano-Ag resulted in pitting of the cell wall and a breakdown of the cell membrane permeability barrier.
The authors postulated that the membrane effects occurred through physical perturbation of the lipid
bilayers on the outer membrane, which causes ion leakage, pore formation, and dissipation in the
electrical membrane potential. The destruction of the membrane integrity also could have inhibited
normal fungal budding processes (Kim et al.. 2009). Saulou et al. (2010) took the analysis one step farther
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in their investigation of structural changes to both the cell wall and the intracellular proteins of S.
cerevisiae. They observed electron-dense granules that were considered likely to be silver based on
transmission electron microscopy images; these granules were distributed along the cell wall and inside
the cell. They also observed disordered secondary structures of proteins, which could lead to deactivation
of enzymes critical to fungal metabolic processes (e.g., cell antioxidant defense mechanisms).
In a proteomic35 analysis, Lok et al. (2006) demonstrated that certain envelope protein expressions
in E. coli were significantly altered after exposure to 0.4- and 0.8-nanomole per liter (nM) bovine serum
albumin-stabilized nano-Ag. This observation suggests that the ATP-dependent preprotein translocase,
which is associated with the inner membrane, had ceased to function. In a typical cell, mature proteins are
translocated to the outer membrane, but if that process is inhibited, protein precursors simply build up in
the cytoplasm. This study also indicated that E. coli exposed to nano-Ag experience a decrease in proton
motive force, which was observed in the near complete loss of intracellular K+. Nano-Ag also decreased
cellular ATP levels, which might also have contributed to cell death (Loketal.. 2006).
To demonstrate the effect of membrane disruption on cell invasion, certain antibiotics have been
used to increase permeability and porosity in bacterial membranes, which allows the ingress of silver
nanoparticles as large as 80 nm (Kvriacou et al., 2004; Xu et al.. 2004). Although many investigators have
observed through optical microscopy that nanoparticles can accumulate in the cytoplasm of bacterial
cells, very little information is available to explain the process by which cell invasion occurs. For
example, Pal et al. (2007) noted that the cell walls of bacteria treated with nano-Ag were significantly
damaged (the type of damage was not specified). They also determined that nanoparticles had
accumulated in the cell walls and membrane and inside the cells, but did not report the mode by which the
nanoparticles were transported into these areas. Lok et al. (2006) observed silver nanoparticles attached to
the surface of E. coli cells and within the cells, but again the mode of transport was not reported. They
did, however, observe perforation of the cell walls. Neal (2008) hypothesized that nano-Ag, like other
metals, might lead to the release of lipopolysaccharide proteins from gram-negative bacteria, causing the
formation of pits in the outer membrane. This morphology change increases permeability and leads to
uncontrolled transport through the plasma membrane, finally resulting in overall cell malfunction and
death.
Bacterial membranes exposed to nano-Ag might be compromised as a result of lipid peroxidation
by ROS, which form as natural by-products of aerobic metabolism but can increase considerably under
conditions of stress (Choi and Hu. 2008). Some support for ROS-mediated adverse cellular effects comes
from a study using stress-specific bioluminescent E. coli (Hwang etal, 2008). The bioluminescent
35A proteomic analysis evaluates the structure, function, interactions, and control of proteins.
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response of the bacteria indicated that superoxide radicals, which are a type of ROS, were generated in
response to nano-Ag exposure, and that protein and membrane damage also occurred, most markedly at a
concentration of 0.4 mg Ag/L. At a concentration of 0.5 mg/L, the reduction in the growth rate of the
E. coli strains was statistically significant. Silver nanoparticles also have been observed to embed in
nitrifying cell floes when added to nitrifying cultures, resulting in toxicity to the membrane-bound AMO
enzyme (Choi et al.. 2009). Choi et al. (2009) speculated that toxicity occurred as the result of small
(<5 nm) silver nanoparticles entering the cell, where they generated ROS or interfered with cellular
S-containing compounds in the respiratory path. Section 5.2.2 discusses the findings of various studies on
membrane disruption and permeability in gram-negative bacteria, suggesting various ways that nano-Ag
could enter bacterial cells.
6.2.2. Aquatic Organisms
Due to the range of products into which nano-Ag is thought to be incorporated and the fact that
wastewater could be one of the most significant release pathways (see Chapter 3), the aquatic
environment might act as a substantial reservoir for nano-Ag and ionic silver discharged from silver
nanotechnologies or silver complexes formed as a result of those discharges.
Ionic silver is the only form of silver that has been broadly tested on aquatic organisms. From these
tests, ionic silver was deemed the second most toxic metal to aquatic organisms, after mercury (Luoma.
2008). Silver ions in the aquatic environment occur only at very low doses, however, suggesting that
under natural conditions, contact between silver ions and aquatic organisms might be relatively rare
(Blaseretal.. 2008). On the other hand, several studies report that nano-Ag functionalized to remain
stable in suspension might pose a significant risk to aquatic species if the nanoparticles are in this state in
the aquatic environment (Kvitek et al.. 2009; Asharani et al.. 2008; Lee et al.. 2007).
Of the available data on nano-Ag effects, most concentrate on effects in either aquatic organisms or
bacteria. Kahru and Dubourguier (2010) compared nano-Ag with six other types of engineered
nanomaterials in a toxicity-grid exercise and determined that nano-Ag was one of the two most toxic to
aquatic organisms. This assessment was based on an LC50 of less than 1 mg/L for D. magna that was
deemed "extremely toxic" relative to all but one of the other nanomaterials tested. Despite evidence
suggesting that lower order aquatic organisms like Daphnia can be highly sensitive to nano-Ag exposure,
most of the research conducted on nano-Ag effects on aquatic organisms has focused on fish, with very
little attention paid to nano-Ag effects on aquatic plants and invertebrates (Griffitt et al.. 2008; Luoma.
2008). At this point, the comparative toxicity of silver nanoparticles and conventional silver to aquatic
organisms is unclear, with studies reporting both enhanced toxicity of nano-Ag to aquatic organisms
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(Chae et al.. 2009) and lower toxicity relative to silver ions (Miao et al., 2009; Griffitt et al., 2008).
Because nano-Ag sheds silver ions, the distinction between nano-Ag toxicity and silver ion toxicity is not
always clear, so some studies have focused explicitly on investigating the effects due to exposure to silver
ions from AgNO3 in comparison to silver ion effects as mediated by the silver nanoparticle (Navarro et
al.. 2008a: Navarre et al.. 2008b).
The silver nanoparticles and silver ions released from nano-Ag are also likely to form complexes
with ligands and other materials in the aquatic environment, as discussed in Chapter 4. Silver complexes
will be most common in the particulate form, both suspended in the water column and deposited to the
sediment; however, no studies have been conducted investigating the effects of silver in the particulate
fraction or for silver thiolates in benthic organisms (Blaser et al.. 2008).
6.2.2.1. Algae
Most studies examining the effects of nano-Ag on biota have focused on bacteria and higher level
organisms, with only a few focused on plants. Even fewer studies are available for marine plants, with the
focus for most aquatic plants centering on freshwater species. At present, algae are the only aquatic plants
for which nano-Ag effects have been investigated.
Algae are primary producers, acting as the food base in aquatic ecosystems, and algae in the oceans
provide much of Earth's oxygen. In addition to being an ecologically important group of organisms, algae
can sometimes act as indicators of aquatic ecosystem change. As such, algal toxicity tests are integral to
the investigation of potential effects on the aquatic environment resulting from the release of nano-Ag.
Known Effects of Conventional Silver Exposure on Algae
Silver ions are highly algicidal, and various silver compounds (e.g., AgNO3, sodium-silver
thiosulfate [NaAgS2O2], silver sulfate [AgSO4]) can cause toxic effects in both freshwater and marine
algae (Ratte. 1999). Exposure to silver has been shown to reduce freshwater growth rates in
Chlamydomonas eugametos, Chlorella vulgaris, Haematococcus capensis, and Scenedesmus accuminata
at concentrations of 0.01 mg/L or less (Hutchinson and Stokes, 1975). Conversely, chronic exposure to
silver (up to 0.05 mg/L) promoted algal growth in Selenastrum capricornutum, but higher silver
concentrations (up to 0.1 mg/L) inhibited growth (Schmittschmitt et al.. 1996).
Effects Specific to Nano-Ag Exposure on Algae
Examples of recent studies investigating the effects of nano-Ag on algae are presented in detail in
Section B.3 of Appendix B. Effects on algae are measured at the population level, for example, in terms
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of population growth. Effects on both freshwater and marine algal species have been investigated, as
described below.
In a comparative study by Griffitt et al. (2008). the freshwater green algaPseudokirchneriella
subcapitata was reported to be more sensitive to nano-Ag (96-hour EC50 = 0.19 mg/L) than fish (48-hour
EC50 = 7.07 and 7.2 mg/L for adult and juvenile zebrafish \Danio rerio], respectively). P. subcapitata was
slightly less sensitive than the aquatic invertebrates that were tested (48-hour EC50 = 0.040 and 0.067
mg/L for Daphniapulex adults and C. dubia neonates, respectively).
Exposure to nano-Ag resulted in significant inhibition of growth, chlorophyll a production, and
photosystem II quantum yield in the marine diatom Thalassiosira weissflogii (Miao et al.. 2009). The
investigators used photosynthetic yield as a toxicity endpoint because of the importance of this process to
aquatic ecosystems. To eliminate the possibility that the direct effect of nano-Ag was being masked by
indirect effects from much higher concentrations office silver ions in the immediate vicinity of the
nanoparticles, Miao et al. (2009) removed the silver ions from solution either by filtration or
complexation with thiols. No significant toxicity to the diatom was observed following silver ion
removal. The authors tentatively concluded that toxicity was mainly due to the release of silver ions,
rather than from the direct interaction of nanoparticles with the diatom. The authors then challenged this
conclusion, however, by pointing out that the silver nanoparticles used might not have been appropriate
for eliciting toxic effects because of their large size (60-70 nm), the wide range of concentrations used, or
the presence of organic compounds in the sea water that complexed with the nanoparticles and made them
less bioavailable. These caveats suggest that further research using different-sized particles and
experimental conditions could be useful to understanding toxic effects of nano-Ag on T. weissflogii and
other diatoms.
In a separate study, photosynthetic yield of the freshwater green alga Chlamydomonas reinhardtii
also was reduced after exposure to nano-Ag (5-hour EC50 = 829 nM [based on total Ag] or 8 nM [based
on free silver ions at the beginning of the experiment]) (Navarro et al.. 2008b). The EC50 values for silver
ions determined in this study were 2-13 times higher than those shown in other studies to inhibit growth
in several algal species, including C. reinhardtii. The exposure duration for this study was only 5 hours,
however, while most algal toxicity studies evaluate effects after exposure durations of 1 or more days.
Navarro et al. (2008b) argued, however, that algal toxicity cannot necessarily be attributed to the
concentration of silver ions in original suspensions. They observed that nano-Ag was more toxic than
AgNO3 to C. reinhardtii based on free silver ion content at the beginning of the experiment. In other
words, the lower free silver ion concentrations measured in the nano-Ag test waters compared with the
free silver ion concentrations measured in AgNO3 solutions could not account for the higher toxicity
observed in the nano-Ag test vessels. Silver nanoparticles appear to continue to release silver ions over
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time, whereas silver ions from AgNO3, which is highly soluble, are released quickly. Thus, the
investigators speculated that the heightened toxicity of silver nanoparticles to algae compared with
soluble silver compounds was due in part to the nanoparticles' ability to act as prolonged sources of silver
ion delivery. Because the release and uptake of silver ions could depend on interaction between the
nanoparticle and the algal cell, assimilation of silver ions into the cell from nano-Ag might be more
efficient (Navarre et al.. 2008b). Whether silver ions form at the algal surface or in the water following
interaction of the nanoparticle with secreted algal products, however, is unclear.
The sensitivity of blue-green algae (i.e., cyanobacteria) in comparison to freshwater green algae has
also been investigated in both a laboratory microcosm experiment and a field enclosure experiment in the
same study (Parket al.. 2010b). The microcosm experiment was conducted in a lab under controlled
conditions using eutrophic lake water. In this experiment, the blue-green algaMicrocystis aeruginosa
UTEX 2388 and the green algal species Ankistrodesmus convolutes and Scenedesmus quadricauda were
exposed to 1 mg/L of two different types of nano-Ag formulations. The first formulation (nano-Ag Fl)
was prepared using reduction of AgNO3 by tannic acid to create nano-Ag ranging in size from 10 to
50 nm, and the other formulation (nano-Ag F2) was prepared by adding AgNO3 to sodium persulfate and
Tween 20, which resulted in a suspension containing silver oxide (Ag404) nanoparticles ranging in size
from 20 to 50 nm. The field enclosure experiment was conducted with the same materials, but in a portion
of a eutrophic lake, where the enclosures were left open at the top, resulting in temporal changes in
sunlight and precipitation. Exposure ofM aeruginosa to nano-Ag Fl and F2 after 10 days resulted in
growth inhibition of 93 and 95%, respectively, in the microcosm experiment and 55 and 64%,
respectively, in the field enclosure experiments. Comparatively little or no inhibitory effect was exhibited
by the freshwater green algal species at this concentration in either experiment.
In addition to the effects observed in the toxicity studies, due to the propensity of nano-Ag to form
clusters and complexes, some speculate that high nano-Ag exposures might lead to increased cell density,
shading, and clogging that produce adverse effects that cannot be attributed to the toxicity of the silver
nanoparticles (Navarro et al.. 2008a). Although this result has not been confirmed for nano-Ag, nanoscale
titanium dioxide was shown to adsorb to algal cell surfaces and increase cellular weight by more than
two-fold (exposure concentration not specified) (Huang et al.. 2005).
Hypothesized Nano-Ag Mode(s) of Action in Algae
Only one study has specifically explored the nano-Ag mode of action in algae. Depending on
whether observed effects are the result of direct nanoparticle effects or indirect effects resulting from the
release of silver ions from the nanoparticle on the surface of the algae, the mode of action might differ.
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In the study by Miao et al. (2009) investigating the effect of nano-Ag on growth and photosynthetic
yield in the marine diatom T. weissflogii under different nutrient conditions, investigators distinguished
between the direct effects of the silver nanoparticle and the indirect effects of released silver ions, and
proposed a potential mode of action. They reported that the silver ions released from the nanoparticles
appeared to be driving the toxic responses, and that the growth endpoint was more sensitive than
photosynthetic yield to silver ion exposure, indicating that the photosynthetic system was not the primary
target of the silver ions. Under nutrient-limited conditions, this diatom seemed to be less susceptible to
adverse effects of nano-Ag and produced significantly higher levels of carbohydrates, which indicates the
generation of polysaccharide-rich EPS. EPS could be involved in processes that regulate the uptake and
subcellular distribution of silver ions, and higher EPS levels might protect algae from oxidative damage
associated with exposure to certain metals. As a result, under nutrient-rich conditions, a potential mode of
action for nano-Ag toxicity in this species might be ROS accumulation leading to oxidative damage.
6.2.2.2. Aquatic Invertebrates
Known Effects of Conventional Silver Exposure on Aquatic Invertebrates
Data for the effects of conventional silver on benthic organisms are highly varied due to the
complex processes occurring in the sediment and differences in experimental designs and species
characteristics. In general, the amphipod Hyalella azteca is believed to be among the most sensitive
benthic invertebrates in the limnic environment, exhibiting a 10-day LC50 as low as 1.6 milligrams Ag per
kilogram (mg/kg) dry weight when exposed to AgNO3. Like most aquatic organisms, H. azteca is
markedly less sensitive to other silver species like silver thiosulfate and silver sulfide; toxicity of these
silver complexes is limited, however, primarily due to significantly lower bioavailability in the benthic
environment (Hirsch. 1998b).
More data are available for silver toxicity to freshwater planktonic invertebrates, such as the water
flea, Daphnia magna, which is also among the most sensitive planktonic invertebrates identified in
laboratory toxicity studies. Acute LC50s for D. magna are as low as 5 micrograms total silver per liter
(ug/L) when unfed organisms are exposed to silver added as AgNO3 (Erickson et al.. 1998). Increased
mortality and decreased growth and reproduction in D. magna also have been reported following chronic
exposure to dissolved silver (IC2o36 = 2.56 ug/L) (Naddya et al.. 2007).
36Inhibitory concentration is the chemical concentration at which a given percentage (in this case, 20%) of the
exposed organisms demonstrate a response in a chosen endpoint; this effect level is commonly used to estimate the
toxicity of a substance to a specific group of organisms.
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A toxic mode of action similar to that in freshwater fish is believed to lead to the toxic effect on
freshwater invertebrates from exposure to conventional silver. That sequence of events, discussed further
in Section 6.2.2.3, involves silver inhibition of branchial ionic sodium/potassium-ATPase (Na+/K+-
ATPase), which ultimately leads to failure in the organism's ability to regulate ionic transport (Bianchini
and Wood. 2003).
Studies on silver toxicity to marine invertebrates are not abundant and often are not comparable to
one another due to differences in experimental procedures. The existing data suggest that juvenile
bivalves are among the most sensitive marine organisms, with toxicity to silver ions occurring in the
<1- to 14-ug/L range. Toxicity endpoints observed in marine invertebrates include increased mortality and
delayed or abnormal development (Ratte. 1999). The primary mode of action dictating silver toxicity to
marine invertebrates is suspected to be different from that in freshwater invertebrates and fish. Silver
toxicity to marine invertebrates is not associated with osmotic or ionoregulatory disruption at the
hemolymph level, but silver still could act on the Na+/K+-ATPase enzymes at the gill level, only
producing different effects (e.g., increased changes in univalent and divalent cations in tissues, change in
intracellular ion concentrations) (Bianchini et al., 2005a).
Effects Specific to Nano-Ag Exposure on Aquatic Invertebrates
Examples of recent studies investigating the effects of nano-Ag exposure on aquatic invertebrates
are presented in detail in Section B.4 of Appendix B. Currently, the toxic endpoints that have been
examined for nano-Ag exposure to aquatic invertebrates include mortality, immobility, and embryonic
development. Mortality and immobility endpoints have been examined for the unicellular eukaryote,
P. caudatum and two species of water flea, D. pulex and C. dubia. Reproductive toxicity and genotoxicity
have been examined in the aquatic midge Chironomus riparius, and embryonic development and adult
reproductive toxicity have been examined in the estuarine oyster Crassostrea virginica.
Exposure of P. caudatum to nano-Ag without surface modification resulted in significantly lower
toxicity (LC50 = 39 mg/L) than that observed for many bacteria (LC50 values from 1.69 to 13.5 mg/L)
(Kvitek et al.. 2009). No toxic effects were observed in P. caudatum at nano-Ag concentrations lower than
25 mg/L, but mortality occurred at silver ion concentrations of 0.4 mg/L, indicating that silver ions are
more toxic to P. caudatum in terms of total silver added.
Interspecies differences and environmental or experimental conditions could affect toxicity to
aquatic invertebrates, as evidenced by the much higher sensitivity of C. dubia to nano-Ag added to natural
waters than that of paramecia exposed to nano-Ag in deionized water. Under different experimental
conditions not using natural water samples, adult D. pulex and C. dubia neonates exhibited significantly
higher sensitivity (Navarre et al.. 2008a) than that noted by Kvitek et al. (2009) in P. caudatum, although
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this observation might be due in part to differences in experimental conditions. A recent study reported an
LD5037 range of 3 to 4 ug/L for D. magna exposed to nano-Ag synthesized using different ratios of silver
nitrate to sodium citrate, resulting in three different sizes (36, 52, and 62 nm) of silver nanoparticles. A
lack of statistically significant differences in toxicity among the three size fractions indicated that toxicity
was not a function of size, at least in sizes above 36 nm (Li et al.. 2010b). As discussed in Section 6.1,
however, several factors can influence the toxicity of nano-Ag to biota. Allen et al. (2010) demonstrated
7 JO \ /
the range of LC50s that can result from small changes to the experimental design of the study. Their study
examined the relative toxicity to D. magna of silver ions (added as AgNO3), commercially available
nano-Ag ("uncoated" and "organically coated" Sigma Aldrich Ag-nanoparticles38 [SA nano-Ag]), and
laboratory-synthesized nano-Ag (coated with coffee or citrate). The investigators also examined
differences in nano-Ag toxicity when D. magna were unfed verses fed and exposed to unfiltered versus
filtered (100 nm) suspensions. The LC50 values for unfed D. magna exposed to unfiltered silver ions and
both the unfiltered coffee- and citrate-coated nano-Ag were comparable (around 1 ug/L), while the LC50
for the uncoated SA nano-Ag was an order of magnitude higher (16.7 ug/L) and the LC50 for the coated
SA nano-Ag was higher still (31.5 ug/L). When organisms were fed, the LC50 for coated SA nano-Ag
jumped to about 176.4 ug/L. In unfed organisms, if the suspensions were filtered, the LC50s for the SA
nano-Ag dropped an order of magnitude, making the results comparable to those for the laboratory-
synthesized nano-Ag and the silver ions, which changed very little when filtered.
Griffitt et al. (2008) proposed that the large difference in nano-Ag toxicities exhibited by various
aquatic organisms could largely depend on feeding strategies. Exposure of the filter-feeding water flea
C. dubia to unspecified concentrations of nano-Ag resulted in significant mortality when added to
samples of headwaters, midsection, and delta waters of the Suwannee River, with lower toxicity observed
for headwaters (Gao et al.. 2009). Daphnia also appear to be more sensitive to nano-Ag than adult and
juvenile zebrafish and algae (Griffitt et al.. 2008). Because daphnia are particulate filter feeders, they
might encounter relatively large numbers of nanoparticles over the course of an acute exposure period.
Nanoparticles also might adhere to invertebrate exoskeletons, interfering with swimming and appendage
movement. Significant changes in mobility and behavior have been observed in D. magna on which
carbon-60 clusters have formed, although these changes were not explicitly linked to particle adhesion
(Lovern et al.. 2007).
37Lethal dose is the chemical dose at which 50% of the exposed organisms die; this effect level is commonly used to
estimate the toxicity of a substance to a specific group of organisms.
38Synthesis and treatment (i.e., stabilizing agents) of "uncoated" nano-Ag were not reported, and "organic coating"
for coated nano-Ag was considered proprietary by the manufacturer.
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One chronic ecotoxicity experiment evaluating the effects of nano-Ag exposure to aquatic
invertebrates was identified in the literature. In this study, Nair et al. (2011) observed effects on pupation
and adult emergence in C. riparius after exposure to nano-Ag (40-70 nm) concentrations >0.2 mg/L for
25 days. The authors also observed a statistically significant change in the sex ratio of exposed midges
relative to controls, showing a greater average number of females in treated groups. A comet assay further
revealed a dose-dependent increase in DNA damage in treated larvae, with effects statistically significant
at the 1-mg/L exposure concentration. Nair et al. (2011) concluded that exposure to nano-Ag could result
in developmental and reproductive failure along with genotoxicity in C. riparius.
One published study on the effects of nano-Ag in benthic marine invertebrates also was identified
in the literature. This study evaluated the effects of nano-Ag on the estuarine oyster C. virginica
(Ringwood et al.. 2010). The investigators examined the toxicity of nano-Ag on reproductive and
embryonic development endpoints because oysters release their gametes into the surrounding water,
where they might be exposed to nano-Ag. Newly fertilized oyster embryos were administered a single
dose of nano-Ag (in seawater, average size 25 nm), with doses ranging from 0.0016 to 1.60 ug Ag/L
nano-Ag, and development was assessed 48 hours after treatment. Adult oysters were subjected to the
same treatment regime at doses ranging from 0.0016 to 16 ug Ag/L, and lysosomal destabilization (an
indicator of gamete viability) was assessed after 48 hours. Statistically significant effects when compared
to controls were observed on embryonic development at a nano-Ag concentration of 1.6 ug Ag/L, and
lysosomal integrity of adult hepatopancreas tissues was statistically significantly affected at 0.16, 1.6, and
16.0 ug Ag/L. The authors were unsure, however, whether the statistically significant response of the
adult C. virginica at 0.16 ug Ag/L also was biologically significant; previous studies by these
investigators have demonstrated that lysosomal destabilization rates that exceed 30-40% result in high
levels of impaired gamete viability (and thus reproductive failure). As such, the 1.6 ug Ag/L-
concentration would be the first level for which both statistically and biologically significant effects have
been confirmed.
Hypothesized Nano-Ag Mode(s) of Action in Aquatic Invertebrates
Very little information specific to nano-Ag on the mode of action causing toxicity to aquatic
invertebrates was located. One study examined differential gene expression in C. riparius exposed to
1 mg/L nano-Ag (Nair et al.. 2011). Results suggested possible mechanisms involving the down-
regulation of ribosomal protein L15, affecting ribosomal assembly (and protein synthesis as a result), and
the up-regulation of GnRHl, a gonadotropin-releasing hormone gene that could lead to reproductive
failure. The authors did not investigate the effects of silver ions on C. riparius, and thus whether these
observed effects were nanoparticle specific is unclear.
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Nano-sized particles have been reported to enter the digestive gland cells of blue mussels and
cockles by endocytosis (Moore. 2006). Once in the cell, nanoparticles could become embedded in cell
constituents and contribute to oxidative damage by preventing the cell from extruding the particles,
although no direct evidence supports this hypothesis. Nano-Ag also might adhere to the surfaces of sperm
cells spawned freely into the water by organisms using this reproductive strategy (e.g., seaweed, mussels,
clams). Adhesion to the sperm cell surfaces might affect fertilization success in aquatic invertebrates, as
was demonstrated in the marine seaweed, Fucus serratus, when exposed to carbon black. But whether
nano-Ag would be available in suspension in sufficient quantities for adhesion to occur is unclear
(Nielsen et al.. 2008). Furthermore, as observed in D. magna exposed to carbon-60, direct particle
adhesion to zooplankton exoskeletons might result in adverse effects on behavior and mobility (Lovern et
al.. 2007).
6.2.2.3. Aquatic Vertebrates
Of the data available on nano-Ag toxicity to aquatic organisms, fish studies are the most abundant.
In addition to fish, only one other group of aquatic vertebrates was examined in the supporting literature
for nano-Ag effects: A single study was available in the published literature for nano-Ag effects on
amphibians. Ongoing research is investigating the effects of nano-Ag on two species of whale (Wise et
al., 2009). but overall, published research on the toxicity of nano-Ag to aquatic mammals, amphibians,
and other aquatic vertebrates is very limited or nonexistent at this time.
Known Effects of Conventional Silver Exposure on Aquatic Vertebrates
Acute silver LC50s for the most sensitive fish species are between 2.5 and 10 ug/L. Chronic no
observed effect concentrations39 (NOECs) and maximum acceptable toxic concentrations were between
0.4 and 0.7 ug/L for sensitive fathead minnows exposed to AgNO3 (Ewell et al.. 1993). Concentrations of
AgNO3 at or above 17 ug/L resulted in premature hatching and 15% reduced growth in rainbow trout
(Salmo gairdneri} fry (Davies et al., 1978). Other silver compounds that are less soluble, such as Ag-
thiosulfate and AgCl, exhibited very little toxicity to developing S. gairdneri (Hogstrand et al., 1996).
Silver toxicity to freshwater fish is believed to result from silver ion interaction at the negatively
charged gill surface, where nano-Ag inhibits the basolateral Na+/K+-ATPase-dependent transport across
the gills. Due to the inhibition of the ionic transport system, normal electrochemical gradients are
39The highest tested concentration at which no adverse effects are observed on the aquatic test organisms at a
specific time of observation.
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disrupted, and fish lose the ability to actively control the transport of ions across the gills, which can
result in a net loss of ions from the blood plasma, osmoregulatory failure, and ultimately in circulatory
collapse causing death (Bar-Ilan et al., 2009). Although fish bioaccumulate silver, the toxic mode of
action in freshwater fish does not appear to be the result of internal silver accumulation, but rather the
accumulation of silver at the gill surface (Ratte. 1999).
One study examined the effects of silver ions on survival and morphology of zebrafish (D. rerio)
embryos and on behavior and development of larval zebrafish (Powers et al.. 2010a). At concentrations as
low as 1 uM silver ions, delayed hatching was observed. Survival decreased at concentrations of 3 uM
and observed dysmorphology increased. Swimming performance was impaired at concentrations below
the thresholds for survival and dysmorphology indicating that long-term survival could be affected (due to
persistent behavioral effects), even at concentrations that appear otherwise nontoxic in studies examining
only physiological endpoints.
The mode of action of silver toxicity to marine fish is not well understood, but appears to be very
different from that of freshwater species. Toxicity is speculated to be equally attributable to processes
taking place at the gill surface and those occurring in the gut of the fish (Grosell and Wood. 2001). Silver
induces ionoregulatory failure in marine fish, although typically at concentrations that are one to two
orders of magnitude higher than in freshwater fish (Pedroso et al.. 2007). Preliminary (unpublished)
research underway at Duke University's Center for Environmental Implications of Nanotechnology
suggests that silver toxicity to Atlantic killifish (Fundulus heteroclitus) embryos and larvae is not due
entirely to exposure to silver ions, as previously thought (Matson. 2010). The investigators reported that
conventional silver toxicity did not follow a linear response curve along an increasing salinity gradient.
Instead, toxicity decreased up to a certain chloride concentration and then increased again at salinities
similar to those in estuarine environments. The reason underlying the U-shaped salinity-toxicity
relationship is unknown (this response is not observed in adult F. heteroclitus under the same conditions),
but the investigators proposed that the observed toxicity might be due to the concentration of total
dissolved silver in solution.
Effects Specific to Nano-Ag Exposure on Aquatic Vertebrates
Examples of recent studies investigating the effects of nano-Ag on fish are presented in detail in
Section B.5 of Appendix B. The only species offish for which nano-Ag toxicity tests have been published
are freshwater zebrafish (D. rerio) and fathead minnow (P. promelas), and the anadromous rainbow trout
(Oncorhynchus mykiss), Japanese medaka (Oryzias latipes), and European perch (Percafluviatilis). One
study also examined the effects of nano-Ag on the American bullfrog (Rana catesbeiand). D. rerio, the
most widely used test organism for investigating the effects of nano-Ag in fish, are gaining popularity as
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model organisms in toxicological studies due to the high degree of homology to the genome of other
vertebrates (including humans) and similarities in physiologic responses to various stressors across
vertebrate species (Postlethwait et al., 2000). They also display rapid ex utero and post-fertilization
development and high fecundity. The transparent embryos, with tissues turning opaque upon cell death,
allow for real-time analysis of developmental effects in addition to real-time monitoring of nanoparticle
transport (Bar-Han et al.. 2009; Lee et al.. 2007). Fish study endpoints can include tissue and whole-body
concentrations of chemicals (e.g., bioaccumulation, as discussed in Section 5.2.3.8), mortality, behavioral
markers (e.g., coughing and abnormal swimming), and morphological malformations (e.g., pericardial
edema, bent spine, small head).
A recent study using O. latipes reported that this species is susceptible to nano-Ag with observed
effects at concentrations at or above 25 ug/L, but that changes in gene expression possibly indicative of
nano-Ag toxicity occurred at concentrations as low as 1 ug/L (Chae et al.. 2009). Moreover, the gene
expression patterns observed in O. latipes exposed to nano-Ag were distinguishable from those observed
following exposure to silver ions (added as AgNO3), indicating a distinct nanoparticle effect. The genes
analyzed were hepatic biomarkers associated with metals detoxification, antioxidant defense, cellular
responses to stressors, toxin binding and transport, catalysis, cell-cycle arrest, apoptosis, DNA repair,
biotransformation and detoxification of endogenous and exogenous compounds, iron metabolism, and
immune system response. Chae et al. (2009) found that nano-Ag at concentrations of 1 and 25 ug/L
significantly affected gene expression in the six genes examined at various time points during the 10-day
exposure period. Although all tested genes responded differently to nano-Ag and silver ions, the largest
statistically significant differences were observed in the heat shock protein HSP70, a stress protein; p53, a
DNA repair and apoptosis-inducing protein; and transferrin, an iron transport protein. As more ecological
toxicity tests attempt to elucidate a mode of action by investigating the induction of such "stress-response
genes," however, keeping in mind that gene induction in response to stressors does not necessarily
indicate an adverse effect at the organism level is important. Genes responding to stress are expected to
provide some resistance to adverse effects up to a certain threshold exposure level (Crawford and Davies.
1994). Furthermore, the impact of changes at the molecular level due to exposure to toxicants has not
been investigated fully or correlated to changes that might occur in dynamic populations of these same
organisms. This caveat is applicable to all toxicogenomic studies discussed hereafter.
In a conference presentation, Wise et al. (2009) reported that nano-Ag is highly cytotoxic (in a
concentration-dependent manner) and genotoxic to O. latipes. Additionally, nano-Ag appears to be more
toxic to this species than silver ions (added as AgNO3). Although the 96-hour LC50 for nano-Ag and silver
ion exposures to O. latipes were comparable (34.6 vs. 36.5 ug/L), significantly more mortality occurred
at higher nano-Ag concentrations than at equivalent concentrations of silver ions from AgNO3, and
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statistically significant gene induction was less common following exposure to AgNO3 (Chae et al.,
2009). Laban et al. (2009) reported that dissolved silver from AgNO3 was more toxic to P. promelas
embryos than dissolved silver released from nano-Ag.40 They reported a AgNO3 LC50 of 15 ug/L, which is
below the level of dissolved silver released by the silver nanoparticles in any treatment group
(18-95 ug/L). The investigators compared the toxicity of nano-Ag solutions that had been briefly
sonicated to those that had only been stirred, and measured the concentrations of dissolved silver in each
type of solution. Although the dissolved silver concentrations were not significantly different between the
stirred and sonicated samples, the sonicated samples were 10 times more toxic than the stirred samples,
suggesting that nano-Ag toxicity cannot be attributed purely to the concentration of dissolved silver
released from the nanoparticle. This result, however, is not always corroborated in other studies. Kennedy
et al. (2010) reported that silver ions were more acutely toxic (LC50 = 5.7 ug/L) to P. promelas than
nano-Ag of various sizes and coatings (LC50s = 6.6-125.6 ug/L) at equivalent total silver concentrations.
But when exposed to equivalent amounts of silver ions and "fractionated" nano-Ag (defined as the
dissolved fraction of the silver nanoparticles), the resulting LC50s for nano-Ag (LC50s = 1.5-5.6 ug/L)
were about the same or statistically significantly lower than that of AgNO3, depending on the nano-Ag
material used. These results suggest that the dissolved fraction (i.e., silver ions) of nano-Ag in suspension
was driving toxicity. The investigators caveat, however, that the filtration method used did not remove
silver nanoparticles with diameters less than 4 nm from the fractionated samples.
Lower sensitivity than described for O. latipes was also observed in O. mykiss exposed to nano-Ag
in the form of Nanocid®, a water-based colloidal suspension designed by Nano Nasb Pars Co. (Tehran,
Iran) for use as a disinfectant in aquaculture. In addition to lethality, reported effects included hypoxia,
lethargy, unusual swimming behavior, elevated gill ventilation, and darkening of the body, although the
concentrations at which these effects occurred were not reported. The 96-hour LC50 was 2.3 mg/L, which
is almost two orders of magnitude higher than that reported for other fish species. Characterization of the
test material was not reported, however, and particle clusters could have formed, reducing the number of
nanoparticles in suspension (Shahbazzadeh et al.. 2009). A more recent study examined the cytotoxic
effects of nano-Ag (PVP- and citrate-coated) on the tissues ofO. mykiss in vitro (Farkas et al.. 2011). The
citrate-coated nanoparticles were more readily taken up by the tissues than the PVP-coated nanoparticles,
but cell viability was reduced in all exposures to all nanoparticle types and sizes (starting at nano-Ag
concentrations of 0.1 mg/L). Higher levels of reduced glutathione (GSH) were reported at exposure levels
>0.1 mg/L citrate-coated particles and >1 mg/L PVP-coated particles. Uptake was also observed by
Scown et al. (2010) of "uncoated" nano-Ag (10 and 35 nm; surface treatment not reported by
40Defmed as ionic silver released from silver nanoparticles and dissolved silver nanoparticles.
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manufacturer) in the tissues of O. mykiss after exposures to 10 ug/L nano-Ag. These exposures resulted in
increased expression of the cypla2 gene in the gill tissue of exposed fish, suggesting a possible increase
in oxidative metabolism, although no effects on lipid peroxidation were observed.
Zebrafish (D. rerio) have been examined in the embryonic, juvenile, and adult life stages for lethal
and sublethal toxic effects, including the influence of nano-Ag on caudal fin regeneration. Bar-Han et al.
(2009) investigated the size-dependent toxicity of nano-Ag on D. rerio embryonic development using
3-, 10-, 50-, and 100-nm silver nanoparticles. The embryos exhibited almost 100% mortality at nano-Ag
concentrations of 250 uM, regardless of the size of the nanoparticle. LC50 values (93.31 uM for 3-nm
particles to 137.26 uM for 100-nm particles) indicated that toxicity is loosely size dependent, although
only at certain concentrations and time points. In a study by Asharani et al. (2008). mortality was higher
in D. rerio embryos exposed during the cleavage period of development (2- to 8-cell stages, up to
approximately 1 hour post-fertilization) than in those exposed after the cleavage period when embryos are
entering epiboly (4 to 6 hours post-fertilization). Embryos exposed earlier were more sensitive, exhibiting
an LC50 of 25 ug/mL, compared to embryos exposed later in development that exhibited an LC50 of 50
ug/mL (Asharani et al., 2008). Developmental effects of nano-Ag were also examined in O. latipes
embryos by Wu et al. (2010). Morphological defects including heart malformations, edema, spinal
abnormalities, and finfold abnormalities were reported at concentrations of nano-Ag (25 nm) at and above
100 ug/L. These effects were observed as a U-shaped dose-response curve, with delayed hatching and
abnormalities in embryos exposed to the lower and higher concentrations of the dose distribution. These
results suggest the need for a greater understanding of the mechanism of developmental toxicity of
nano-Ag.
Bar-Ilan et al. (2009) reported that 3-nm and 10-nm silver nanoparticles seemed to produce the
greatest amount of statistically significant sublethal toxic effects (based on seven quantified effects) in
D. rerio embryos at a concentration of 100 uM nano-Ag when compared to a range of other nano-Ag
concentrations. Overall, 100 uM nano-Ag exposure resulted in embryos having 31-46% smaller heads,
87-119% larger yolk sacs, 31-68% smaller caudal fins, and 38-55% smaller eyes than the controls. In
addition, embryos were 14-25 mm shorter, and had 57i-14?i more axial curvature and 16-64% larger
pericardial sacs, but these were not statistically significantly different from the controls (Bar-Ilan et al..
2009). At 120 hours post-fertilization, the embryos had not depleted their yolk sacs, which were their only
source of food throughout the exposure period. The underdeveloped bodies of the exposed embryos
suggest that nano-Ag exposure inhibited the uptake of nutrients from the yolk sac, although how nutrient
uptake was impaired was not investigated. Lee et al. (2007) reported a threshold for D. rerio of 0.19 nM
nano-Ag, above which no embryos exposed at the eight-cell stage developed normally. At concentrations
higher than 0.19 nM, all embryos were either dead or deformed, and the incidence of deformities
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decreased as the number of dead embryos increased. Mortality ranged from 20% to 90% at nano-Ag
concentrations of 0.05 to 0.72 nM, respectively, and deformities ranged from approximately 2% at
0.05 nM nano-Ag to 42% at 0.19 nM (Lee et al., 2007). Some sublethal toxic effects, such as yolk sac and
pericardial edema, also were observed in P. promelas exposed to nano-Ag concentrations up to 20 mg/L
(Laban et al., 2009). Several studies have demonstrated that nano-Ag exposure contributes to mortality
and sublethal developmental effects on D. rerio embryos in a concentration-dependent manner; the results
from these studies, however, cannot be compared directly due to different experimental designs and
dosing methods (Bar-Ilan et al. 2009; Asharani et al.. 2008; Lee etal.. 2007).
Additional phenotypic and physiological endpoints shown to be affected by D. rerio exposure to
nano-Ag at concentrations greater than 50 ug/mL are decreased heart rate, hatching delay, accumulation
of blood in the blood vessels near the tail, apoptosis, slimy external skin coating, finfold abnormalities,
tail and spinal cord flexure and truncation, cardiac malformation, head edema, eye malformation,
hemorrhaging, blood clots, and distortion of the yolk sac (Bar-Han et al.. 2009; Asharani et al.. 2008; Yeo
and Kang. 2008; Lee etal.. 2007). Laban et al. (2009) observed similar abnormalities in developing P.
promelas embryos exposed to nano-Ag. One concern in the testing of nano-Ag for toxic effects is that
residual silver ions from the nano-Ag feedstock might be present in nano-Ag stock solutions and therefore
be responsible for the observed effects. Asharani et al. (2008) investigated whether exposure to silver ions
alone could result in the effects observed in developing zebrafish embryos following exposure to the
nano-Ag test material. They reported that exposure of D. rerio embryos to concentrations of silver ions
equivalent to the range of nano-Ag concentrations shown to result in gross malformations did not affect
any of the phenotypic endpoints examined. The highest silver ion concentration tested (20 nM) resulted in
10% mortality and hatching delay in 4% of the embryos, but did not significantly affect overall
development of the embryos. The results indicate that the observed sublethal toxic effects were not the
result of residual silver ions in the exposure medium left over from the synthesizing process.
Phenotypic changes such as those described above have been observed in D. rerio embryos
exposed to toxic chemicals other than nano-Ag. For example, Lee et al. (2007) noted that finfold
abnormalities, tail and spinal cord flexure and truncation, cardiac malformation, and yolk sac edema have
all been observed in embryos exposed to dichloroacetic acid and cadmium. The specific eye malformation
(no formation of retina or lens), however, resulting from exposure to nano-Ag has not been reported in
literature describing exposure of D. rerio to any other toxic chemical (Lee etal.. 2007).
Danio rerio can regenerate many body structures, including the spinal cord, optic nerve, heart, and
fins (Yeo and Pak. 2008). The effect of nano-Ag on caudal fin regeneration was investigated by Yeo and
Pak (2008). At a concentration of 4 ppm, nano-Ag significantly inhibited regeneration, and exposure to
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0.4 ppm delayed, but did not completely inhibit, caudal fin regeneration. Exposure to 0.4 and 4 ppm
nano-Ag resulted in defects in regeneration observed within 10 days following amputation.
Griffitt et al. (2009) investigated the effects of the NOEC of nano-Ag on adult D. rerio gill
histology and gene expression, and compared these responses to those elicited by soluble silver
concentrations (through addition of AgNO3) equivalent to those released by the nano-Ag after 48 hours.
Although the dissolved silver concentrations were comparable between the nano-Ag and AgNO3
solutions, gill filament widths were significantly larger in those zebrafish exposed only to the dissolved
silver from the AgNO3 solutions. The gill tissues and whole carcasses of zebrafish exposed to nano-Ag,
however, contained significantly higher concentrations of total silver than those of the zebrafish exposed
only to soluble silver. Another study examining oxygen consumption of P. fluviatilis exposed to nano-Ag
reported evidence supporting that nano-Ag might act on the gills externally, thereby reducing gas
exchange at the gill surface (Bilberg et al.. 2010). The investigators measured the basal metabolic rate
(BMR) and critical oxygen tension (Pcnt) of P. fluviatilis by automated intermittent closed respirometry.
Results showed no effect on the BMR of exposed fish, but did show an increase in Pcnt by 50% after
exposure to 300 ug/L nano-Ag for 24 hours. Exposure to AgNO3 caused an increase in both BMR and Pcnt
at exposure concentrations lower than those of nano-Ag. These results indicate that P. fluviatilis could be
vulnerable to hypoxia following exposures to nano-Ag, albeit at very high exposure levels.
The single study that assessed the effects of nano-Ag on the amphibian Rana catesbeiana
conducted an in vitro cultured tail fin biopsy assay using tadpole tissue (Hinther et al.. 2010).
R. catesbeiana tissues show extreme sensitivity to thyroid hormone action, making them good model
species for assessing potential effects to human health, for which thyroid hormone is essential. The
investigators reported LC50 values of 0.25 mg/L for AgNO3 and 0.95 mg/L for nano-Ag. They also
observed disruption of non-thyroid-hormone-mediated cellular stress response pathways at higher
concentrations of nano-Ag (>2.75 mg/L) and altered thyroid-hormone action at lower concentrations of
nano-Ag (0.6-550 ug/L). Similar responses were not observed after exposure to equivalent concentrations
of silver ions alone. How these stress responses at the subcellular level translate to effects at the organism
level is unclear, but they could represent a sensitive endpoint that has not yet been evaluated in
amphibians (Hinther et al.. 2010).
Because the research presented in this section suggests that aquatic organisms are susceptible to
nano-Ag toxicity, the effects on fish and other aquatic species at the organism level suggest that impacts
from nano-Ag at the population and ecosystem level are possible. Research investigating nano-Ag effects
on sperm whales (Physeter macrocephalus) and North Atlantic right whales (Eubalaena glacialis) (Wise
et al., 2009); Atlantic tomcod (Microgadus tomcod) (Nichols et al.. 2009); and Atlantic killifish
(F. heteroclitus) (Matson. 2010) is currently underway. Preliminary results indicate that nano-Ag is toxic
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to both cells and genetic constituents of cells in both whale species in a concentration-dependent manner,
although whale cells appear to be less sensitive than those of medaka (O. latipes) (Wise etal.. 2009).
These data can be used as an indication of the body of information that might become available in the
next several years.
Hypothesized Nano-Ag Mode(s) of Action in Aquatic Vertebrates
The toxic mode of action of nano-Ag in fish has not been fully elucidated. Although silver
nanoparticles have been observed inside fish embryos and toxic effects have been quantified, these results
serve only to allow speculation concerning the mode of action. Griffitt et al. (2009) examined the
transcriptional profiles of D. rerio adults exposed to nano-Ag and soluble silver (from AgNO3). Despite
having similar concentrations of soluble silver in both exposure groups, the zebrafish exposed to nano-Ag
exhibited a significantly different gene expression profile than those fish exposed to only the soluble
silver, indicating that the nano-Ag mode of action differs significantly from that of silver ions. As
examined by Chae et al. (2009). the independent expression of genes in the O. latipes liver that act as
indicators for carcinogenesis, mutagenesis, DNA repair, and oxidative damage indicates that rapid
biotransformation and detoxification was occurring in the liver. If biotransformation and detoxification
were indeed occurring, this suggests that nano-Ag might induce apoptosis, which is assumed to result
from nano-Ag-generated ROS. No direct evidence, however, is available to link ROS to toxicity in this
case; cytotoxicity from ROS was inferred only from the response of the indicator genes. Furthermore,
Griffitt et al. (2009) did not measure induction of any D. rerio genes currently mapped to oxidative stress
regulation when the fish were exposed to nano-Ag NOEC concentrations, indicating that ROS generation
alone might not completely explain toxic effects from nano-Ag exposure.
Several studies (Laban et al.. 2009; Asharani et al.. 2008; Lee et al., 2007) have demonstrated the
ability offish embryos to take up silver nanoparticles (see Section 5.2.3). Lee et al. (2007) demonstrated
that silver nanoparticles can penetrate the egg chorion through the chorionic pore canals by passive
transport. From there, some nanoparticles penetrated the embryo itself and embedded in several zebrafish
organs (Lee etal.. 2007). Another study showed that nano-Ag deposits on the skin and uniformly within
the body, showing a particular affinity for the cell nucleus (Asharani et al.. 2008). One study also reported
penetration and accumulation in all organelles—including the nucleus—of the gill, muscle, and
regenerated fin tissue of zebrafish (age not specified) (Yeo and Pak. 2008). Because accumulation in the
nucleus can lead to genomic damage, this observation supports other results indicating a genomic
response to nano-Ag exposure in O. latipes. Yeo and Pak (2008) reported upregulation of five genes
involved in apoptosis 50 hours post-fertilization in D. rerio embryos exposed to nano-Ag. Increased
apoptosis in response to nano-Ag might result from increased production office radicals. Additional
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evidence that nano-Ag exposure increases free radicals that could cause DNA damage comes from
observations in a study by Yeo and Kang (2008). in which levels of the enzyme catalase, which is
responsible for removing free radicals, increased in D. rerio exposed to 10 and 20 parts per trillion (ppt)
nano-Ag. Although Rojo et al. (2007) did not observe any developmental effects or mortality in D. rerio
at nano-Ag concentrations up to 5 ppm, they did measure increased expression of genes involved in
detoxification and regulation of the oxidative stress response. The role of oxidative stress, DNA damage,
and apoptosis in the toxicity pathway also has been examined in adult D. rerio (Choi etal.. 2010).
Histological analysis of the liver revealed hepatic cell cords and TUNEL-positive apoptotic changes, and
p53-related genes were upregulated in treated fish, suggesting that oxidative stress and apoptosis are
associated with toxic effects in the livers of adult zebrafish. Hinther et al. (2010) also demonstrated
disruption of cellular stress-response pathways in amphibians at relatively high concentrations of
nano-Ag, as shown through alterations in hsp30 and CAT transcripts. An alternative mode of action for
nano-Ag at low concentrations also was proposed by Hinther et al. (2010) for toxicity to amphibians. At
low doses, nano-Ag seemed to affect thyroid-hormone signaling pathways that are crucial to embryonic
development, as shown through alteration of the thyroid-hormone-induced thyroid hormone receptor beta
(TR(3) and thyroid-hormone-repressed Rana larval keratin type I (RLKI).
Nano-Ag has been observed to interfere with cardiac muscle function, preventing the flow of blood
through the body of the embryo (Asharani et al., 2008). Nano-Ag might inhibit cardiac function directly
by interacting with cardiac cells, disrupting normal cell functioning, and weakening the pumping of the
heart. The weakened pumping of the heart results in restricted blood flow, which might indirectly affect
the ability of the embryo to access vital energy sources contained in the blood. Following a loss of blood
flow, nano-Ag deposits in the brain, which could interfere with signal transduction and other nervous
system processes, leading to a loss in neurological function. A loss of neurological function is supported
by observed insensitivity of the larvae to touch. The silver nanoparticle, by simply attaching to a
biological surface (e.g., a developing zebrafish organ), might act as a foreign body, thereby limiting
functionality of the cell or organ to which it has attached (Asharani et al.. 2008). For example, Handy et
al. (2008a) noted that although engineered nanoparticles are typically too large to exploit direct uptake
channels to nerve cells, they are probably capable of attaching to the epithelium and interacting with
receptors. Such interactions might interfere with olfactory function, such as chemical signaling, leading to
changes in behavior that affect survival.
6.2.2.4. Model to Estimate Toxicity to Aquatic Biota
The computational models that are currently being recommended for use in predicting the toxicity
of nano-Ag and related silver species are limited in number and scope. At this time, no models appear to
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be available for assessing toxicity from nano-Ag particles, but two models can be used to estimate the
toxicity of silver ions released from the particles. The models apply only to the aquatic environment,
however, and no terrestrial toxicity models appear to be available at this time. The models discussed here
are used to estimate predicted no-effect concentrations (PNECs) for aquatic biota in the water column
(Mueller and Nowack. 2008) and to predict metal toxicity to fish (DiToro et al., 2001).
Predicted Water Column No-Effect Concentration Model
Mueller and Nowack's (2008) fate and transport model (described in Section 4.5) estimated a
PNEC for aquatic biota based on a published study by Yoon et al. (2007) of acute nano-Ag toxicity with
two bacteria, B. subtilis andE. coll. The threshold concentrations (equivalent to aNOEC) for these
species in water (20 and 40 mg/L, respectively) were divided by an assessment factor of 1000, in
accordance with the Technical Guidance Document on Risk Assessment published by the European
Chemicals Bureau (Mueller and Nowack. 2008; ECB. 2003). The ratios of the predicted environmental
concentrations (PEC) of nano-Ag in water of 0.03 ug/L for the "realistic scenario" and 0.08 ug/L for the
"high scenario," to the PNEC of 20 ug/L (for bacteria) were orders of magnitude less than 1.0 (i.e., less
than a hazard quotient of 1.0). The investigators did not, however, consider silver ions potentially released
from the nanoparticles. Mueller and Nowack (2008) noted that release of nano-Ag particles is of
secondary importance to the release of silver ions from the nanoparticles given the higher toxicity of ionic
silver, for which the authors cite Blaser et al. (2008). Gottschalk et al. (2009) report a modal PEC for
nano-Ag in surface water of 0.116 ng/L and a Q(0.85) of 0.428 ng/L in the United Sates.
Biotic Ligand Model
A biotic ligand model (BLM) has been developed as a predictive tool to enable estimation of acute
metal toxicity in an aqueous environment when several aspects of the water chemistry are known (DiToro
etal., 2001). The BLM accounts for several water chemistry parameters, including ionic chlorine (CF)
concentration and the amount of dissolved organic matter, to predict the amount of free metal ion
available to bind to the fish gill. For acute toxic effects on fish, the gill is considered a proximate site of
toxic action. The biotic ligand binding site is thought to be one or more sensitive enzyme systems
(carbonic anhydrase or Na+/K+-ATPase) (Bielmyer et al.. 2008). Accumulation at the surface of gills is
relevant to a possible mode of action of nano-Ag because gill transport of silver and other ions also varies
with water chemistry. Notably, however, the BLM was specifically developed to model binding and
toxicity of metal cations, and consequently that this model would adequately predict the behavior and
toxicity of nano-Ag and other metals when present in nanoparticle form is unlikely.
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6.2.3. Terrestrial Organisms
Very few studies have investigated the effects of nano-Ag on terrestrial organisms, with no
investigations of nano-Ag toxicity in soils (Wijnhoven et al.. 2009b). This section summarizes knowledge
regarding conventional silver and nano-Ag effects on several types of terrestrial organisms, including
plants, invertebrates, and vertebrates.
6.2.3.1. Terrestrial Plants
Only recently have some studies emerged that investigate the effects of plant exposure to nano-Ag.
At this point, literature on the effects from environmental exposure to nano-Ag are still lacking, with most
studies relying on experimental procedures that do not mimic natural conditions and endpoints that might
not be relevant to environmental assessment. Nonetheless, these studies provide some insight into the
potential responses of higher order plants to nano-Ag exposure.
Known Effects of Conventional Silver Exposure on Terrestrial Plants
Although data on the effects of silver in higher plants are limited and highly varied, a review by
Ratte (1999) suggests that plants are most sensitive to silver during germination and growth. Lettuce
(Lactuca sativa), radish (Rhaphanus sativas), and maize (Zea mays) seeds exposed to AgNO3 exhibited
reduced germination and growth at or below 7.5 mg/L AgNO3 (Ewell et al.. 1993). Spiking sewage sludge
with 5.2 and 120 mg Ag/kg dry weight, however, did not have a statistically significant effect on growth
or emergence of lettuce, turnips, oats, and soybeans. In fact, the mean fresh weight of the lettuce leaves
increased significantly in the groups exposed to silver, although a change in dry weight was not observed
(Hirsch. 1998a).
Effects Specific to Nano-Ag Exposure on Terrestrial Plants
Summaries of three recent relevant studies investigating the effects of nano-Ag on terrestrial plants
are provided in Section B.6 of Appendix B. These studies suggest that nano-Ag might be toxic to higher
order plants at concentrations above 10 ppm, although statistical significance was not always reported
(Kumari et al.. 2009; Rostami and Shahsavar. 2009; Babu etal. 2008). In a review article, Ma et al.
(2010) described phytotoxicity of nano-Ag to plant seedlings and to plant cells. In their own research, Ma
et al. (2010) found that very low concentrations of nano-Ag (<1 ppm) sized 20-80 nm could be toxic to
seedlings of thale cress (Arabidopsis thaliana) by stunting growth in a concentration- and particle size-
dependent manner. Similarly, Hawthorne et al. (2012) reported that nano-Ag reduced zucchini (Cucurbita
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pepo) plant biomass and transpiration by 49-91% compared to equivalent concentrations of conventional
silver, while shoot silver content did not differ based on particle size or concentration. Nano-Ag also was
found to inhibit seed growth and produce cell damage in grass (Lolium multiforum) (Yin et al., 2011). For
a given mass, smaller nano-Ag particles had a greater impact on plant growth than similar concentrations
of larger particles; however, when doses were expressed in units of specific surface area, the effects were
comparable. Authors suggested that such alterations could be directly attributed either to the nanoparticles
themselves or to the ability of nano-Ag to deliver dissolved silver to critical biotic receptors (Yin et al..
2011).
Rostami and Shahsavar (2009) demonstrated that submerging olive (Olea europea L.) explants, or
the part of the plants used to initiate a culture, in nano-Ag solutions with concentrations ranging from 100
to 400 ppm effectively eliminated bacterial contamination in the explants, but also resulted in a high
percentage of plant mortality. When the investigators added nano-Ag to a prepared medium in lower
concentrations (2-6 ppm) and allowed the explants to grow in the contaminated medium for 30 days, the
nano-Ag exposure seemed to result in more than a 50% decrease in plant mortality up to 4 ppm when
compared to controls (statistical significance not reported); mortality then again increased slightly
(-20%), although this increase was reported as not statistically significant.
In a toxicity assay, Babu et al. (2008) reported that nano-Ag exposure induced a dose- and
duration-dependent mitodepressive and cytotoxic effect on onion (Allium cepa) meristems (root tips).
Exposure of A. cepa meristems to nano-Ag resulted in a reduced frequency of mitotic index, which is a
measure of cell proliferation, and increased frequency of chromosomal aberrations. These results occurred
at all nano-Ag concentrations tested and at every exposure duration, although results were not always
statistically significant for shorter exposure durations (Babu et al.. 2008). Kumari et al. (2009) also
reported a nano-Ag concentration-dependent effect on A. cepa mitotic index, but the decrease was
significantly different from the control group only at concentrations at or above 50 ppm. Babu et al.
(2008) reported a significant reduction in frequency of cell division even in the groups exposed to the
lowest concentration of nano-Ag (10 ppm) for 2 hours, and at higher concentrations (20-50 ppm) within
1 hour. A significant increase in chromosomal aberrations was observed by Babu et al. (2008) in all
treatment groups after a 0.5-hour exposure and by Kumari et al. (2009) at all treatment groups except the
lowest concentration group (25 ppm) at the end of a 4-hour exposure period. Chromosomal aberrations
described in both studies include chromatin bridge, stickiness, disturbed metaphase, and breaks and
fragments (Kumari et al.. 2009; Babu et al.. 2008).
In unpublished research by Cho et al. (2008b) presented at the first meeting of the Asian
Horticultural Congress in 2008, the investigators reported that nano-Ag inhibits the growth and
elongation of lettuce and pak-choi roots in a concentration-dependent manner. Nano-Ag exposure also
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reduced lettuce and pak-choi fresh and dry weights with an increase in concentration above the 0.04-ppm
treatment level. At 0.04 ppm, however, nano-Ag appeared to optimize growth, resulting in 10% and 20%
increases in the weight of lettuce and pak-choi, respectively. This U-shaped concentration-response curve
is similar to that reported by Rostami and Shahsavar (2009) in olive explants, indicating the presence of a
narrow nano-Ag threshold at which plant performance is optimized, but above and below which plant
performance is inhibited.
Hypothesized Nano-Ag Mode(s) of Action in Terrestrial Plants
Although few data exist on the specific mode of action for nano-Ag in terrestrial plants,
comparisons to conventional silver and silver ions suggest that its mode of action differs from its
conventional counterparts. Stampoulis et al. (2009) compared the toxicity of nano-Ag, "bulk" silver, and
ionic silver in zucchini (Curcubita pepo) and determined that 1,000 mg/L nano-Ag resulted in a 69%
greater reduction in biomass when compared to 1,000 mg/L conventional silver. The authors also found
that exposure to 10 mg silver ions/L (from AgNO3) produced an effect similar to that of nano-Ag, but that
exposure to the supernatant containing silver ions released from the 1000-mg/L nano-Ag solution resulted
in significantly more growth when compared to the nano-Ag solution, indicating that toxicity of nano-Ag
is not due entirely to dissolution of the silver ion (Stampoulis et al., 2009).
Some authors have proposed that the reduction of the mitotic index in A. cepa meristems results
from DNA transcription inhibition at S-phase, and that the observed mitotic abnormalities are indications
that mitotic spindle function is impaired, likely due to nano-Ag interactions with tubulin-SH groups
(Kumari et al., 2009; Babu et al.. 2008). Babu et al. (2008) posit that the observed chromosome stickiness
might be the result of "intermingling" chromatin fibers leading to connections between chromosomes at
the sub-chromatic level. They also argue that nano-Ag has a clastogenic effect, as observed in the
induction of the chromosomal breaks and micronuclei, which might result in a loss of genetic material.
6.2.3.2. Terrestrial Invertebrates
Terrestrial invertebrates include those living in soils and aboveground. Few studies have been
conducted on nanomaterial toxicity to terrestrial invertebrates, and even fewer have specifically focused
on nano-Ag. Because the terrestrial environment has not been thoroughly investigated for pathways of
concern, very little information is available on suspected routes of exposure. Nano-Ag in soils is likely to
form complexes with organic matter and thiols, which might render it largely unavailable for uptake, as
discussed in Section 5.2.4.2. If nano-Ag were applied in sprays directly to the surface of plants, plant-
dwelling invertebrates might ingest particles unbound to organic matter. This type of application,
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however, is considered "off-label," or not in accordance with product instructions for indoor spray
disinfectants.
Known Effects of Conventional Silver Exposure on Terrestrial Invertebrates
Silver toxicity has not been studied extensively in terrestrial invertebrates. Some data presented by
Ewell et al. (1993) at the 1st Argentum International Conference on the Transport, Fate, and Effects of
Silver in the Environment indicate that silver is toxic to the earthworm Lumbricus terrestris at
concentrations above 62 mg Ag/kg when chronically exposed to artificial soil contaminated with AgS.
Because the worms did not bioaccumulate silver after the 28-day exposure period, the investigators
determined that direct contact of the dermal tissues with silver in the soil particles resulted in the observed
reduction in growth, although the mode of action has not been elucidated.
Effects Specific to Nano-Ag Exposure on Terrestrial Invertebrates
Few studies were found that explicitly examined nano-Ag toxicity to soil invertebrates; study
details are provided in Section B.7 of Appendix B. Roh et al. (2009) examined the effect of nano-Ag on
DNA transcription, survival, growth, and reproduction in wild type and mutant strains of the soil
nematode Caenorhabditis elegans. Using microarray analysis, the investigators observed that exposure to
nano-Ag resulted in the significant upregulation of 415 gene probes41 and significant downregulation of
1,217 gene probes. The investigators found that survival and growth were not affected by exposure to
nano-Ag in any of the treatment groups for the wild type and mutant C. elegans strains tested.
Reproduction, however, decreased significantly in all strains at all concentrations, with only one
exception: Reproduction was not statistically significantly affected in one C. elegans type evaluated at the
lowest concentration. Exposure to silver ions from AgNO3 did not result in any significant effect on
survival or growth, but did significantly reduce reproduction potential. The degree to which reproduction
potential decreased, however, was greater in C. elegans exposed to nano-Ag than to silver ions. Exposure
to silver ions resulted in a different gene expression pattern than that of nano-Ag. Silver ion
concentrations of 0.1 and 0.5 mg/L resulted in the statistically significant induction of four hsp gene
groups, which are heat shock proteins, but did not result in the upregulation of the gene probes
significantly affected by exposure to nano-Ag (Roh et al.. 2009).
Another study of nano-Ag toxicity to C. elegans did observe growth inhibition after exposure to
PVP-coated and citrate-coated nano-Ag (Meyer et al.. 2010). Statistically significant growth inhibition
41A probe is a specific sequence of single-stranded DNA or RNA, usually labeled with a radioactive atom, that is
designed to bind to, and therefore single out, a particular segment of DNA to which it is complementary.
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was observed after exposure to 5 mg/L of citrate-coated nano-Ag, and 50 mg/L of PVP- and citrate-coated
nano-Ag. The investigators performed a separate assay in which they fed DNA-damaged bacteria to
C. elegans as a negative control to account for the possibility that the observed growth deficiency was
confounded by nano-Ag toxicity to the bacterial food supply. Their results showed that growth inhibition
was not mediated by toxicity to the bacteria and was indeed due to exposure to nano-Ag. Meyer et al.
(2010) also accounted for possible confounding due to toxic effects of the PVP and citrate coatings and
found that exposure to these chemicals alone seemed to stimulate growth above the levels of control.
Measurements of dissolved silver in the exposure media revealed that citrate-coated nano-Ag released
less dissolved silver than PVP-coated nano-Ag. This, coupled with evidence that exposure to PVP
supernatant induced much higher toxicity than exposure to pure citrate supernatant, indicated that the
toxicity observed is due mainly to the release of silver ions (Meyer etal.. 2010).
Reproductive toxicity of the earthworm Eiseniafetida was assessed by Heckmann et al. (2011) in a
limit-test toxicity screening conducted according to the OECD guidelines for assessing earthworm
reproductive toxicity. After exposure to 1,000 mg/kg soil PVP-coated nano-Ag for 28 days, complete
reproductive failure (0% cocoon production) was observed in the earthworms. Survival was statistically
significantly decreased in E. fetida exposed to AgNO3 when compared to controls, but was not affected in
groups treated with nano-Ag. Although the nanoparticles were very thoroughly characterized, authors
were unable to identify any clear correlation between toxicity and specific particle characteristics
(Heckmann et al.. 2011). Similarly, in another reproductive toxicity test, E. fetida exposed to 727.6 or
773.3 mg/kg nano-Ag (with oleic acid and PVP coatings, respectively) showed a significant decrease in
reproduction (Shoults-Wilson et al.. 201 la); the effect of AgNO3 was more pronounced with decreased
reproduction reported at 94.21 mg/kg. Another study involving E. fetida examined avoidance of soils
contaminated with nano-Ag as a behavioral endpoint and found it to be a more sensitive indicator of
toxicity than growth and mortality (Shoults-Wilson et al.. 20lib). Earthworms were exposed to untreated
soil and soil treated with nano-Ag coated with PVP, oleic acid, or citrate. The investigators observed
avoidance behavior in E. fetida after 48 hours of exposure to nano-Ag-treated sandy loam soils at
concentrations of 6.92-7.42 mg/kg-soil, which are comparable to PECs of nano-Ag in sewage sludge (see
Section 4.5). E. fetida avoided soils treated with nano-Ag and AgNO3 at equivalent total silver
concentrations, indicating that under these circumstances and based on silver mass alone, both substances
were equally toxic. The avoidance response to AgNO3 was immediate, however, whereas avoidance of
nano-Ag was observed only after the 48-hour exposure. The investigators estimate that a maximum of
10-15 % of the total silver added as nano-Ag would have dissociated from the nanoparticles after
48 hours. As a result, they argue that these results preclude an association between avoidance of nano-Ag
and concentrations of silver ions released from the nano-Ag. Furthermore, avoidance of soils treated with
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smaller (15-25 nm) citrate-coated particles was statistically significantly higher than in soils treated with
larger (30-50 nm) PVP-coated particles in the same type of soil. The investigators could not, however,
distinguish between effects of coating and effects of size in this case and could not explain why avoidance
of nano-Ag-treated soils was delayed compared to that of AgNO3-treated soils.
Another species of earthworm, Lumbricus terrestris, was examined for mortality and levels of
apoptosis (a more sensitive sublethal endpoint than survival or reproductive capability) in various tissues
after exposure to two types of nano-Ag (average size 20.2 nm and 8.8 nm, respectively) via water, food,
and soil (Lapied et al.. 2010). L. terrestris experienced 40% mortality after exposure to 100 mg/L of
20-nm nano-Ag and 10 mg/L of 8.8-nm nano-Ag in water. No mortality was observed at lower exposure
concentrations in water or in the experiments in which earthworms were exposed to nano-Ag in soil and
food. Apoptotic response (measured as apoptotic cells/square millimeter [mm2]) was statistically
significantly increased when L. terrestris was exposed to 100 mg nano-Ag (20 nm) in 1 L water or 1 kg
soil or food. When L. terrestris was exposed to the 8.8-nm nano-Ag in water, food, and soil, statistically
significant increases in apoptotic response were observed at and above 5 mg/L water, 20 mg/kg food, and
4 mg/kg soil.
Survival, hatchability, and growth of insect larvae exposed to nano-Ag were examined by Sap-lam
et al. (2010) following exposure of the larvae of the mosquito Aedes aegypti to nano-Ag concentrations
ranging from 0.01 to 5 ppm. The investigators reported that mortality increased in a dose-related manner,
increasing from 0 to 12% in larvae exposed to 1 ppm nano-Ag, followed by an increase to 90% mortality
in the 5-ppm exposure group after 3 hours. This effect was even more pronounced in larvae treated with
silver ions; mortality increased to 90% after 17 hours of exposure to 1-ppm silver ions.
Hypothesized Nano-Ag Mode(s) of Action in Terrestrial Invertebrates
The mode of action for nano-Ag on terrestrial invertebrates is not yet known. Roh et al. (2009)
attempted to elucidate part of the mode of action for nano-Ag toxicity by analyzing expression in genes
mapped to specific metabolic processes in the wild type and mutant C. elegans strains. The investigators
reported that the significant induction of the sod-3 gene, which is a superoxide dismutase protein, in
C. elegans exposed to nano-Ag confirms that oxidative stress contributes to nano-Ag toxicity. These gene
expressions are correlated with reproduction; this does not, however, necessarily support a causal
relationship. The investigators also propose that the loss in function of certain genes might improve the
reproductive potential of C elegans when exposed to nano-Ag, possibly related to antioxidant response.
The significant upregulation of these genes could have occurred as a compensatory mechanism in the
absence of this primary antioxidant enzyme gene. The sequence of processes or events by which the sod-3
gene contributes to a decrease in reproductive potential, however, was not explored (Roh et al.. 2009).
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The role of oxidative stress in the nano-Ag toxicity pathway is challenged by the results of Meyer et al.
(2010). Exposure to nano-Ag resulted in comparable growth inhibition of the wild-type N2 C. elegans
and the mutant strains sod-1 and mev-\, which are known to be sensitive to oxidative stress.
Ahamed et al. (2010). however, produced similar results to those of Roh et al. (2009) in a
proteomic analysis of the exposure ofDrosophila melanogaster to 50 and 100 ug/mL of
polysaccharide-coated nano-Ag. The investigators observed statistically significantly higher levels of
malondialdehyde (an end product of lipid peroxidation), and sod and cat (antioxidant enzymes) in
exposed larvae than in controls, showing statistically significant levels of oxidative stress. The
investigators speculate that this generation of free radicals leads to DNA damage (inferred from
observations of upregulated p53 and p38 levels in exposed larvae). These results were dose- and
time-related, showing increases in effect from 24 to 48 hours. This evidence suggests, but does not
confirm, a causal relationship between these toxic endpoints, and suggests a mode of action involving the
induction of free radicals by nano-Ag exposure resulting in DNA damage and ultimate apoptosis of the
cell.
6.2.3.3. Terrestrial Vertebrates
Because silver is not considered a significant risk to higher order organisms, ecotoxicological
studies have traditionally focused on more sensitive and more susceptible (i.e., having greater potential
for exposure) lower order organisms. Many studies investigating human health toxicity rely on
mammalian bioassays from which a human response to nano-Ag is inferred; these studies are covered in
detail in Section 6.3 on human health effects. Non-mammalian terrestrial vertebrates are discussed in this
section, although the available data are limited to avian species.
Known Effects of Conventional Silver Exposure on Terrestrial Vertebrates
No studies were identified that have investigated conventional silver toxicity to non-mammalian
terrestrial vertebrates. The 1992 data call-in for EPA's Silver Reregistration Eligibility Decision for silver
and silver compounds in pesticides, however, required that one avian study be conducted using the
formulated product under consideration for reregistration (U.S. EPA. 1993). As a result, proprietary
studies might exist that have investigated effects on avian species.
Effects Specific to Nano-Ag Exposure on Terrestrial Vertebrates
Only two studies examining nano-Ag toxicity to terrestrial vertebrates were identified, and only
one of these examined the direct effects of nano-Ag. These studies are presented in detail in Section B.8
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of Appendix B. In a study by Grodzik and Sawosz (2006), chicken eggs were injected with nano-Ag to
investigate its effect on the development of chicken embryos. Special attention was paid to the bursae of
Fabricius (lymphoid glands contributing to immune system development) in the embryos. Although
nano-Ag exposure did not affect the weight of the embryos or the weights of the hearts, livers, and eyes of
the chicks, some effects were observed in the bursae of Fabricius. The investigators reported that embryos
exposed to nano-Ag developed fewer and smaller lymph follicles in the bursae of Fabricius than in the
control groups; statistical significance was not reported. They also observed that the surfaces of the
primary and secondary canals extending between the lymphoid follicles in the bursae were larger and
more wrinkled than in the controls (Grodzik and Sawosz. 2006). What effect, if any, the abnormalities
observed in the bursae of Fabricius might have on normal development of the chicks is unclear.
Furthermore, the nano-Ag material used in the study was not well-characterized, so the size, shape,
stability, and other properties that might affect toxicity are not known.
The other study examining nano-Ag effects on terrestrial organisms is primarily a study of the
caecum microflora42 and secondarily an examination of the histological effects of nano-Ag in tissues in
the duodena of 10-day-old quail (Coturnix coturnix japonicd) free-fed nano-Ag in drinking water
(Sawosz et al., 2007). Gut flora are sometimes likened to a virtual organ within an organ because of the
high level of metabolic activity associated with these organisms (O'Hara and Shanahan. 2006). Bacterial
colonization in the gut has been shown to heighten immunological function in animals, and the bacterial
composition in the gut might influence variations in immunological response.
The only significant effect reported in the study by Sawosz et al. (2007) was on the content of the
gut microflora in the quail exposed at the highest nano-Ag concentration. Of the nine bacterial species
included in the microbial caecum profile, four significantly increased in density at the 25-mg/kg nano-Ag
level. Why the concentrations were reported in terms of mg/kg, when the nano-Ag was dispersed in water,
is not clear from the report. The four affected bacterial species were the gram-positive lactic acid bacteria
Lactobacillus salivarius, Lactobacillus fermentum, Leuconostoc lactis, and Actinomyces naeslundii.
Sawosz et al. (2007) could not explain why the densities of these bacteria increased while the other
species remained unchanged when exposed to nano-Ag, a known antimicrobial agent. Furthermore, the
investigators note no currently available data suggest that nano-Ag interacts with constituents of the
digestive tract or that it can be absorbed from the digestive tract. In this experiment, they hypothesized
that nano-Ag successfully penetrated the gastric acid barrier to the stomach and passed through to the
duodenum (Sawosz et al.. 2007).
42The caecum microflora is the natural bacterial population in the gut organs of animals and humans.
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Hypothesized Nano-Ag Mode(s) of Action in Terrestrial Vertebrates
Because significant toxicity to non-mammalian terrestrial vertebrates exposed to nano-Ag has not
been reported, modes of action for toxicity have not been explored.
6.3. Human Health Effects
The bactericidal effects of conventional silver have led to the incorporation of conventional silver
into a range of consumer products, and as described in Chapter 2, the use of nano-Ag in antiseptic
products has increased markedly in recent years. As described in Chapters 2, 4, and 5, and Section 6.1,
differences in the behavior of conventional silver and nano-Ag appear to be attributable to differences in
key properties, including surface area, reactivity, and quantum behavior (ACHS. 2009). This section
examines and summarizes the evidence for nano-Ag-induced health effects from in vitro studies (Section
6.3.1), in vivo studies (Section 6.3.2), and human health and epidemiological studies (Section 6.3.3) as
they pertain to the use of nano-Ag in spray disinfectants. In each section, the effects of conventional silver
are described first, followed by information on the effects relevant to the nano-Ag life cycle specific to
this use scenario. For more comprehensive information regarding the health effects of nano-Ag in general,
the reader is referred to reviews by Wijnhoven et al. (2009b) and Panyala et al. (2008).
6.3.1. In Vitro Studies
Separating the physical properties affecting nano-Ag toxicity from experimental factors has proven
to be an ongoing challenge in the field of nanoparticle exposure. In vitro studies can provide a useful
evaluation of controlled dose and exposure scenarios and material characteristics to help identify the
processes and factors potentially contributing to nano-Ag toxicity. Because testing environments for in
vitro studies, however, are not identical to those for in vivo systems, in vitro studies cannot be compared
directly to real-world exposures. Despite such limitations, in vitro studies can be a useful approach for
exploring possible mechanisms of action at the cellular and molecular levels, as well as a tool in deciding
whether or what further testing is appropriate to pursue.
Nano-Ag properties and relevant effects of nano-Ag exposure on different cell types and endpoints
observed in key in vitro studies are presented in detail in Section C.2 of Appendix C, with studies
presented in alphabetical order by author. Many studies have demonstrated the ability of nanoparticles to
penetrate cells, although the mechanism appears to depend on the cell type, particular particle type, and
exposure method. Additionally, many researchers have reported that exposures can be cytotoxic. Other
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endpoints observed in association with exposure to nano-Ag include oxidative stress, induction of
cytokines and chemokines as markers of inflammation, DNA and molecular damage, growth inhibition,
mitochondrial perturbation, and changes in cellular morphology (see Section C.2 of Appendix C for
citations and study details).
Known Effects of Conventional Silver Exposure In Vitro
The potential destabilizing effect of metal ions, including silver ions, on the mitochondrial electron
transport chain has been well understood for some time. Chappell et al. (1954) demonstrated that using
conventional silver electrodes to pass electricity through rabbit cerebral cortex cells increased ATPase
activity, which could not be replicated with the same currents using different electrodes. Application of
AgNO3 to the cells similarly increased activity, and experiments using pigeon breast muscle mitochondria
resulted in the same effects following exposure to AgNO3. This study indicated that silver ions could
increase mitochondrial respiration.
Almost four decades after the work of Chapell et al. (1954). Almofti et al. (2003) showed that,
when isolated mitochondria from rat liver cells were similarly treated with silver ions, the mitochondria
immediately swelled and metabolism accelerated. These mitochondrial reactions have been shown to be a
preliminary step along the mitochondrial permeability transition (PT) path, a cascade of events resulting
from increased permeability of proteinaceous pores in the inner mitochondrial membrane. Certain
conditions including the presence of calcium and inorganic phosphate increase the likelihood of PT,
which is characterized by the subsequent release of apoptogenic proteins into the cytoplasm. Silver ions
induced proteinaceous pores opening, resulting in the release of the apoptogenic protein cytochrome c and
apoptosis-inducing factor from the mitochondrial intermembrane space, thereby leading to programmed
cell death. Notably, increased respiration and mitochondrial swelling occurred in a dose-dependent pattern
correlated with silver ion concentration, and the effects were more profound in the presence of inorganic
phosphate. The kinetics of silver ion effects on the mitochondria was markedly different from the classical
calcium and inorganic phosphate PT. Mitochondrial respiration and swelling were immediate and
independent of inorganic phosphate concentration, and known inhibitors of classical PT could not block
the effect of conventional silver. The diameter of the pore opened by silver ion-PT was also larger,
although whether this pore was distinct or one associated with classical PT is unclear; the pore, however,
did not remain open as it does in classical PT. Additionally, the conventional silver effect was blocked by
(but could be reversed following) treatment with GSH or dithiothreitol. These substances keep the
sulfhydryl groups from being reduced during oxidative stress, suggesting that silver ions were causing PT
by binding to the sulfhydryl groups on mitochondrial membrane proteins. Taken together, these results
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suggest that silver ions induce nonclassical PT, characterized by increased mitochondrial respiration and
cytochrome c signaling due to binding of silver ions to mitochondrial membrane proteins.
More recent in vitro work has examined the effect of exposure to silver on neurotoxicity endpoints
and reproductive and developmental effects. With regard to neurotoxicity, Powers et al. (201 Ob) examined
effects of silver ions on the viability, division, and differentiation of neuronotypic PC 12 cells in vitro,
using chlorpyrifos (CPF), a known developmental neurotoxicant, as a positive control. After 1 hour of
exposure, DNA synthesis was inhibited more by exposure to 10 [jJVI silver ions than by exposure to
50 uM CPF. Longer exposures to 10 uM silver ions reduced cell viability. With onset of cell
differentiation, DNA synthesis was inhibited even further, and the acetylcholine phenotype was
preferentially expressed over the dopamine phenotype.43 Exposing the PC12 cells to 1 uM silver ions, on
the other hand, enhanced cell numbers by suppressing ongoing cell death and impaired differentiation for
both neurotransmitter phenotypes. In a similar study, Hahn et al. (2010) examined the effects of silver
ions on the viability of PC 12 cells, L929 fibroblasts, and spiral ganglion cells. Results showed that
exposure to 10 mmol/L silver ions resulted in suppression of tissue growth without inhibiting neuronal
cell growth.
Effects Specific to Nano-Ag Exposure In Vitro
Information on health effects specific to nano-Ag that have been observed in vitro is also available.
Information is presented in the following sections according to notable toxicity endpoints, including
reproduction and development endpoints (Section 6.3.1.1), oxidative stress (Section 6.3.1.2), damage to
DNA and mutagenic effects (Section 6.3.1.3), and pro-inflammatory response (cytokine induction)
(Section 6.3.1.4).
6.3.1.1. Reproduction and Development
Li et al. (2010a) evaluated the cytotoxic effects of nano-Ag on embryonic attachment and
outgrowth of mouse embryos at the blastocyst stage. Blastocysts were pre-treated with 25 or 50 uM
nano-Ag (~13 nm) to determine apoptosis, cell proliferation, and developmental potential and results
compared to controls. Cells treated with 50 uM nano-Ag showed clear evidence of apoptosis and
statistically significant inhibition of cell proliferation. Cytotoxic effects observed at this dose level led to
the impaired development of blastocysts. At 50 uM nano-Ag, embryo attachment to fibronectin-coated
43"Phenotype" in this case refers to a distinct behavior profile resulting from interacting neuronal networks
modulated by different nerve centers in the brain.
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culture dishes was higher, and a lower incidence of post-implantation developmental stages was observed.
According to the study authors, these results indicate that nano-Ag affects implantation and the potential
of blastocysts to develop into postimplantation embryos in vitro.
6.3.1.2. Oxidative Stress
Oxidative stress is a state of imbalance between radical-generating and radical-scavenging
activities within a cell's mitochondrial metabolism. During the metabolism of oxygen by the electron
transport chain, the production of ROS occurs. Studies have demonstrated that the ability of nanoparticles
to generate ROS plays a key role in inducing toxicity. Elevated ROS production overpowers the cellular
antioxidant defenses and decreases mitochondrial function. These events enhance oxidative stress,
resulting in cellular damage including mitochondrial apoptosis and necrosis (Xiaetal.. 2006). Nano-Ag
appears to generate ROS by disrupting ion and electron flux across the mitochondrial membrane, thereby
interfering with the electrochemical gradient (Almofti et al.. 2003). ROS can react with critical cellular
molecules (lipids, proteins, nucleic acids, and carbohydrates) and generate additional radicals. Cellular
defense mechanisms such as the production of GSH peroxidase, which scavenges radicals, can counteract
ROS generation, at least to some extent.
Many studies have focused on the effect of nano-Ag on skin cells due to its use in treating wounds.
These studies might be useful in estimating the potential effects of dermal exposure to nano-Ag in
disinfectant spray, despite the differences in the exposure scenarios. Arora et al. (2009) studied dermal
fibroblasts and primary liver cells to examine possible cellular responses following dermal exposure to an
antimicrobial gel for wound treatment. They showed that exposure to spherical nano-Ag particles with
diameters 7-20 nm did not cause cell death despite intracellular incorporation of the particles, and that
cellular antioxidant defenses were upregulated in both primary fibroblasts and primary liver cells. In a
similar model of therapeutic treatment conducted by Asharani et al. (2009). starch-coated nano-Ag caused
mitochondrial damage and dose-dependent ROS damage in lung fibroblast and glioblastoma cell lines.
Separately, Asharani et al. (2009) measured DNA damage, presumably from ROS, and observed G2/M
cell-cycle arrest possibly due to DNA damage repair following exposure to starch-capped globular
particles of nano-Ag 6-20 nm in size. Although cell death was observed by Asharani et al. (2009) but not
by Arora et al. (2009). both studies concluded that nano-Ag resulted in increased ROS production.
Possible explanations for differing toxicities include differences in doses, properties of particles, and
cellular sensitivity.
Three additional in vitro studies involving oxidative stress might be relevant when evaluating
effects from exposure to spray disinfectants containing nano-Ag. First, a study conducted by Liu et al.
(201 Ob), involving exposure of four human cell models (A549, SGC-7901, HepG2 and MCF-7) to
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nano-Ag particles led to damage to the cellular membrane. Particles with diameters of 5, 20, and 50 nm
were tested at the same mass dose (1 milligram per milliliter [mg/mL]). Elevated ROS levels in cells were
measured in affected cells, which were arrested at S phase, and the ROS-generating capability was
observed to be inversely proportional to the particle size.
Additionally, Carlson et al. (2008) and Hussain et al. (2005) observed decreased mitochondrial
function in alveolar macrophages and liver cells, respectively, in response to nano-Ag exposure. Alveolar
macrophages might be vulnerable upon exposure to sprays because their generation is the primary
response in the deep lung following insult (macrophages act to phagocytose or endocytose foreign
matter). The liver is one organ known to be affected in people exposed to conventional silver (Venugopal
and Luckey. 1978). Carlson et al. (2008) used many of the same protocols in their study as Hussain et al.
(2005). making comparisons between these two studies feasible and appropriate.
Carlson et al. (2008) observed that a loss of mitochondrial function was associated with exposure
to a range of sizes of spherical nano-Ag particles (15 nm, 30 nm, and 55 nm in diameter; reported by the
manufacturer), with the greatest effect observed for the 15-nm size. The toxic effect on mitochondrial
function was measured by the degree of mitochondrial reduction of the tetrazolium salt
3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. For smaller particle sizes,
lower doses of nano-Ag were sufficient to achieve a significant decrease compared to controls. In the
same assay, no significant MTT reduction was observed when the cells were exposed to nanoscale
titanium dioxide.
In liver cells, Hussain et al. (2005) observed that nano-Ag 15 nm and 100 nm in diameter showed a
similar ability to impede mitochondrial function, as measured by MTT reduction, and to increase ROS
production. Unlike Carlson et al. (2008). however, no significant difference was observed between the
effects at different particle sizes. These studies also measured levels of the potent antioxidant GSH
because it plays a key role in maintaining oxidation-reduction equilibrium within cells. Both studies
found that GSH was dramatically reduced following increased exposure to nano-Ag, with a size-
dependent trend, in macrophages and independently in the liver cells. Both studies also noted a dose-
dependent loss of cell viability that the authors assumed was due to oxidative stress (Carlson et al.. 2008;
Hussain etal. 2005).
6.3.1.3. DMA Damage and Mutation
As described in Section 6.2 for biota, the antiseptic properties of nano-Ag are in part due to its
ability to bind to and alter the cell membrane and for silver ions to alter the cell's DNA, which in turn
interrupts cell proliferation. Nano-Ag might cause DNA damage within mammalian cells by several
modes of action. By affecting the mitochondria, nano-Ag can cause an increase in ROS (as described in
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the previous section), which can interact with and damage proteins or DNA (Asharani et al., 2009).
Additionally, silver ions have been observed in vitro to bind directly with DNA and RNA (Chi et al.,
2009). Furthermore, DNA repair mechanisms, which operate in normal functioning of the cell, depend on
ATP. Reduction in cellular ATP levels through interference with mitochondrial respiration could hamper
the essential enzymes for DNA repair, leading to damage (Asharani et al., 2009). DNA damage or a
reduction in ATP can interfere with the cell cycle, and thus cellular proliferation (Asharani et al.. 2009;
Sweet and Singh. 1995).
Surface coating is one property of nanoparticles that can influence toxicity (see Section 6.1.1.3);
the genotoxicity of nano-Ag coated with a detergent (such as might occur in an environmental exposure)
was examined by Chi et al. (2009). In this study, the investigators exposed calf thymus DNA to nano-Ag
(20-50 nm, spherical) and demonstrated genotoxicity, but only in combination with a detergent. No effect
was observed when the DNA was exposed to detergent alone or to nano-Ag alone. The researchers
concluded that the detergent cetylpyridine bromide (CPB) formed a complex, reducing the distance
between the nano-Ag particles and the calf thymus DNA. This effect was maintained until the
concentration exceeded 3.3 ug/mL, when the electrostatic repulsive forces between the DNA and the
particle apparently overcame the attraction from the CPB and the genotoxicity decreased.
In an exposure system using mouse embryonic and fibroblast cells, Ahamed et al. (2008) showed
the DNA damage-repair proteins to be upregulated upon exposure to nano-Ag. Exposures were conducted
with both nonfunctionalized and polysaccharide-functionalized nano-Ag. Similar effects on the
expression of DNA damage proteins Rad51 and phosphorylated H2AX were observed for functionalized
and nonfunctionalized particles, including upregulation of the cell-cycle checkpoint protein p53. The
result of p53 activation is cell-cycle arrest, senescence, or apoptosis. Both nonfunctionalized and
functionalized particles induced cell death in mouse embryonic and fibroblast cells, as measured by
annexin V and MTT assays. Annexin V expression was lower in mouse fibroblast cells treated with
functionalized nano-Ag than in those treated with nonfunctionalized nano-Ag. Mouse embryonic cells,
however, did not display much difference when treated with functionalized or nonfunctionalized nano-Ag,
suggesting mouse embryonic cells are more sensitive than fibroblast cells to both types of nano-Ag used
in this study. Despite initiating these common responses, nano-Ag functionalized with the polysaccharide
gum arabic appeared to be more genotoxic than nonfunctionalized nano-Ag (statistical significance was
not reported). The coating (i.e., functionalization) of the particles was believed to prevent the formation of
particle clusters, allowing functionalized nano-Ag to disperse throughout the cell. Nonfunctionalized
nano-Ag tended to form clusters, resulting in exclusion from some organelles such as the nucleus and
mitochondria.
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Activation of p53 has been linked to ROS production; the mechanism by which p53 is upregulated
following nano-Ag exposure, however, is unclear. Some evidence indicates that nano-Ag can directly
interact with DNA and the DNA replication machinery. Yang et al. (2009) demonstrated in a cell-free
assay that nano-Ag (30-50 nm in size) in various forms can influence DNA replication as measured by
polymerase chain reaction fidelity. The authors observed similar results in E. coll in which the particles
became bound to the genomic DNA and influenced replication. Cha et al. (2008) measured a decrease in
DNA content of cells exposed to nano-Ag. Microscale silver also resulted in a decrease, but neither
exposure was observed to affect mitochondria or GSH. ROS production can also lead to genotoxicity, as
described in the previous section. Research investigating whether either DNA damage or p53 activation is
caused directly by nano-Ag or indirectly through ROS generation is lacking.
Potential genotoxicity of nano-Ag also has been evaluated in vitro using the single cell gel
electrophoresis assay (or comet assay) and the chromosome damage assay, with results suggesting that
exposure to nano-Ag can result in DNA damage. Kim et al. (201 Ob) examined genotoxicity in L5178Y
cells and BEAS-2B cells exposed to nano-Ag particles via the comet assay, with and without metabolic
activation. The comet assay indicated that exposure to nano-Ag resulted in genotoxicity, with statistically
significant differences in tail movement values in both cell lines and at all dose levels when compared to
the respective control groups. The genotoxicity of nano-Ag also was evaluated using the comet assay and
chromosomal aberration test in human mesenchymal cells in a study conducted by Hackenberg et al.
(2011). In both tests, DNA damage was observed after 1, 3, and 24 hours at O.lug/mL. In contrast, Lu et
al. (2010) conducted a comet assay, in which human skin HaCaT keratinocytes were exposed to
100 ug/mL of citrate-coated nano-Ag (30 nm). The authors found that the coated particles were not
genotoxic.
The Kim et al. (201 Ob) study also examined the mutagenic potential of nano-Ag particles through
the mouse lymphoma thymidine (tk +/~) assay, with and without metabolic activation in L5178Y cells.
Results showed that although small colonies were slightly more mutagenic than large colonies in plates
with and without metabolic activation, nano-Ag did not cause a statistically significant increase in the
mutation frequency at any concentration. Therefore, exposure to nano-Ag was not considered mutagenic.
6.3.1.4. Pro-inflammatory Response
Several in vitro studies have investigated the effects of nano-Ag on pro-inflammatory cytokine
induction within the cell. Cytokines are considered to be classic indicators of toxic effects and have been
implicated in effects from exposure to other nanoparticles, particularly in the vascular system (Kreyling et
al.. 2006). Lung cell models have shown upregulation of pro-inflammatory cytokines following exposure
to ultrafine (nano-sized) particulate matter (Brown et al.. 2007). Brown et al. (2007) suggested that this
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induction is caused by intracellular ROS generation. On the other hand, chromium and manganese
exposures both cause induction of the pro-inflammatory cytokines Interleukin-8 (IL-8) and Interleukin-6
(IL-6) in airway epithelial cells, effects that are thought to be mediated through the epidermal growth
factor receptor pathway (Pascal and Tessier. 2004). Although several signals could trigger cytokine
production, including ROS, the specific signal resulting from nano-Ag exposure remains unclear.
Carlson et al. (2008) evaluated the production of several cytokines in alveolar macrophages and
found that, at all doses evaluated (5, 10, and 25 ug/mL) for a range of sizes of nano-Ag (15, 30, and
55 nm), the cells produced significantly increased amounts of tumor necrosis factor-a (TNF-a), a central
mediator of immune response; macrophage inhibitory protein-2 (MIP-2), a signal that recruits neutrophils
to sites of inflammation; and Interleukin 1(3 (IL-1(3), a mediator in the inflammatory response. IL-6,
another pro-inflammatory signal, was not induced. Similarly, Greulich et al. (2009) observed that human
mesenchymal stem cells exposed to nano-Ag concentrations ranging from 5 to 50 ug/mL decreased
production of IL-8, a neutrophil attractant, and IL-6. At concentrations less than 5 ug/mL, however,
statistically significant increases in IL-8 production were observed when compared with controls.
Samberg et al. (2010) reported a statistically significant dose-dependent decrease in viability of human
epidermal keratinocytes (HEKs) and statistically significant increases in IL-1 (3, IL-8, TNF- a, and IL-6
production following exposure to 0.34 ug/mL unwashed nano-Ag of various sizes. Nano-Ag from the
same source that was washed several times, however, had no effect on cell viability. The investigators
believe that the toxicity of the unwashed nano-Ag preparation could be attributed to residual
contamination of the nano-Ag with formaldehyde and methanol by-products of the nanomaterial
production.
In a study by Trickier et al. (2010). the interactions between nano-Ag particles (25, 40, and 80 nm
in diameter) and primary rat brain microvascular endothelial cells (rBMEC) were evaluated using an in
vitro blood-brain barrier model to estimate pro-inflammatory mediators, including IL-1 (3, IL-2, TNF-a,
and prostaglandin E2 (PGE2). Pro-inflammatory responses were correlated with increased permeability of
rBMEC and size of nano-Ag particles. The concentration of TNF- a and IL-1 (3 overtime diminished at 40
and 80 nm when compared to the time-release profile at 25 nm. Larger particles (i.e., 80 nm) had less
effect on rBMEC than smaller particles, which induced greater effects on all endpoints at lower
concentrations or shorter times. Study authors noted that nano-Ag particles might interact with the
cerebral microvasculature producing a pro-inflammatory cascade, which, if left unchecked, could further
induce brain inflammation and neurotoxicity.
In a study conducted by Hackenberg et al. (2011). human mesenchymal stem cells were exposed to
0.01, 0.1, 1, or 10 ug/mL of nano-Ag (< 50 nm), and cytokine release of IL-6, IL-8, and vascular
endothelial growth factor (VEGF) was evaluated using the ELISA technique. Statically significant
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increases of IL-6, IL-8, and VEGF release were observed at 1 ug/L, indicating human mesenchymal stem
cell activation.
Shin et al. (2007) obtained peripheral blood mononuclear cells (PBMCs), including lymphocyte
and monocyte cells, from blood drawn from healthy human volunteers. The cells were incubated in the
presence of 1, 5, 10, 15, 20, or 30 ppm nano-Ag for 72 hours. The PBMCs then were resuspended in a
complete medium supplemented with phytohaemagglutinin (PHA, 5 ug/L) in the absence or presence of
1,3,5, 10, or 20 ppm nano-Ag. PHA was used to stimulate the PBMCs to divide and produce cytokine.
Nano-Ag levels greater than 15 ppm were observed to be cytotoxic and inhibited the PHA-induced
cytokine production in a dose-dependent manner. The PHA effect on cell division was not affected.
6.3.2. In Vivo Studies
In vitro studies, discussed above, can improve the understanding of factors potentially contributing
to the effects of nano-Ag. In vivo studies elucidate the whole-animal exposure response. Both in vitro and
in vivo studies can provide insight on possible modes of action that can be used in interspecies
extrapolation, with the goal of better understanding the effect of nano-Ag in a human exposure scenario
(Sutter. 1995). As previously mentioned, however, the recent review conducted by the FIFRA SAP
cautions that differences in the formulation of nano-Ag (e.g., generated in a laboratory vs. commercially
available) reduce the reliability of extrapolating effects from experimental studies to effects of exposures
to commercial products (U.S. EPA. 2010b). This section summarizes the effects of in vivo exposures of
nano-Ag in animal studies. The subsections that follow describe noteworthy points, followed by a
summary of the key studies. Nano-Ag properties and relevant effects of nano-Ag exposure on different
cell types and a range of endpoints observed in key in vivo studies are presented in detail in Section C.3
of Appendix C, with studies presented in alphabetical order by author.
Known Effects of Conventional Silver Exposure In Vivo
The Agency for Toxic Substances and Disease Registry (ATSDR) provided a comprehensive
overview of the known effects of exposure to conventional silver in their 1990 publication, Toxicological
Profile for Silver, citing numerous scientific publications reporting health effects in animals exposed to
conventional silver by inhalation, ingestion, or dermal exposure (ATSDR. 1990). ATSDR determined a no
observed adverse effect level of 181.2 milligrams Ag per kilogram per day (mg/kg-day) based on animal
mortality from a study by Walker et al. (1971) of rats ingesting AgNO3 in drinking water over a 2-week
exposure period. The lowest observed adverse effect level (LOAEL) for this study was 362.4
mg Ag/kg-day. In a chronic 37-week study of rats also ingesting AgNO3 in drinking water, the LOAEL
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associated with decreased weight gain was 222.2 mg Ag/kg-day (Matuk et al., 1981). ATSDR did not
report specific immunological, developmental, reproductive, carcinogenic, or systemic (e.g., respiratory
or GI) effects in animals resulting from ingestion of conventional silver. Oral exposure studies also
demonstrated that conventional silver was deposited in the liver, spleen, bone marrow, lymph nodes, skin,
and kidneys of rats (ATSDR, 1990). Dermal exposure of guinea pigs to conventional silver resulted in a
LOAEL of 137.13 mg Ag/kg-day associated with decreased weight gain compared to unexposed animals;
the treated animals were given 2 mL of 0.239 M AgNO3 solution on an area of skin measuring 3.1 square
centimeters (cm2) for 8 weeks (Wahlberg. 1965).
Effects Specific to Nano-Ag Exposure In Vivo
Health effects specific to nano-Ag exposure and possibly relevant to exposures associated with the
use of spray disinfectants are discussed in this section. The following sections present information on
effects on the central nervous and respiratory systems (Sections 6.3.2.1-6.3.2.2); effects on the liver,
kidney, and urinary system (Section 6.3.2.3); effects on the cardiovascular system (Section 6.3.2.4);
effects on hematology (Section 6.3.2.5); DNA damage (Section 6.3.2.6); effects on skin (Section 6.3.2.7);
and effects on reproduction/development (Section 6.3.2.8).
6.3.2.1. Central Nervous System Effects
Several studies have examined the translocation of nano-Ag particles into the brain and the
potential for effects. In rats, Takenaka et al. (2001) observed accumulation of nano-Ag in the brain 7 days
after an acute (6-hour) inhalation exposure to nano-Ag; however, no adverse pulmonary effects were
reported (see Section 5.7.2.1 for study details). Tang et al. (2008) subcutaneously injected nanoscale and
microscale silver at 62.8 mg/kg into rats and observed that nano-Ag crossed the blood-brain barrier and
accumulated in the brain, while silver microparticles did not. Unlike other studies such as the one
conducted by Takenaka et al. (2001). which simply measured accumulation, neuronal degeneration and
necrosis endpoints were measured as indicated by pyknotic, necrotic neurons. Incidences of electron-
dense globular substances in both normal and pyknotic neurons were observed in vascular endothelial
cells of the rats treated with nano-Ag, but not in those treated with microscale silver. Tang et al. (2008)
concluded that nano-Ag accumulation in neurons overtime increased the incidence of necrosis in these
cells.
Two additional studies were identified that suggest exposure to nano-Ag could result in central
nervous system effects through altered gene expression. Rahman et al. (2009) evaluated the effects of
nano-Ag (25 nm) on gene expression in different regions of the mouse brain following exposure by
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intraperitoneal injection in adult male C57BL/6N mice. After 24 hours of exposure, the caudate nucleus,
frontal cortex, and hippocampus were analyzed using the Mouse Oxidative Stress and Antioxidant
Defense Arrays; positive results were observed, with gene expression varying by brain region. Lee et al.
(2010) also investigated the effects of nano-Ag on gene expression by exposing mice to nano-Ag of
similar size over 2 weeks using a nose-only exposure system. Alterations in gene expression were
observed for genes in the cerebrum and in the cerebellum, including genes associated with motor neuron
disorders, neurodegenerative disease, and immune cell function, indicating potential neurotoxicity and
immunotoxicity.
6.3.2.2. Respiratory System Effects
The most complete analysis of effects of nano-Ag on the respiratory system appears to have been
conducted by Sung et al. (2009; 2008). who observed changes in rat lung function and inflammation
parameters following exposure to nano-Ag (average size 18-19 nm) concentrations ranging from
0.7 x 106 to 2.9 x 106 particles/cubic centimeter (cm3), although statistically significant
(p < 0.01-0.05) changes were observed only at the highest dose. Statistically significant increases in
inflammatory markers (albumin, lactate dehydrogenase, and total protein levels) in the bronchoalveolar
lavage fluid were observed in females at the highest dose. Decreases in tidal volume, minute volume, and
peak inspiration flow in males and increases in incidences of alveolar macrophage inflammation, chronic
alveolar inflammation, and mixed cell perivascular infiltrate in males and females were statistically
significant in the high-dose group, compared with the control group (Sung et al.. 2009; 2008).
Although Ji et al. (2007) and Takenaka et al. (2001) contributed to the knowledge of potential
health effects from inhalation of nano-Ag, they did not specifically examine respiratory endpoints. The
28-day inhalation exposure paradigm and analysis from Ji et al. quantified silver concentration in tissues
but did not report any significant respiratory effects of exposure. In an acute exposure paradigm,
Takenaka et al. (2001) measured silver in the lungs and found that only 4% of the initial silver burden
remained 7 days after exposure; however, the functional effects of this accumulation were not examined.
Takenaka et al. (2001) suggested that the rapid clearance of silver from the lungs could be due to
phagocytosis of fine particles by macrophages or direct access of ultrafine particles in the alveolar wall to
blood capillaries.
Hyun et al. (2008) evaluated the "real-life effects" of repeated exposure to nano-Ag (13-15 nm) on
the nasal respiratory mucosa of rats. In the study, rats were exposed to nano-Ag via an inhalation chamber
to low, mid, and high nano-Ag exposure levels, each quantified using three different metrics:
particles/cm3, square nanometers per cubic centimeter (nm2/cm3), and ug/m3. Mid- and high-dose animals
showed a significant increase in the size and number of goblet cells containing neutral mucins following
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exposure to nano-Ag for 28 days (6 hours per day, 5 days per week). No histopathological changes in the
nasal cavities and lungs in treated animals were observed relative to the control group. A slight increase in
some mucins, including sulfomucins, was observed at the mid- and high-dose levels, but the level of
increase was not associated with a toxicological outcome. Increased mucin production has been shown to
be associated with a variety of respiratory diseases, however, so this endpoint might warrant further
investigation (Jeong et al.. 2006; Rogers. 2003; Jeffery and Li. 1997).
The study of respiratory-related health effects from exposure to nanoparticles has grown out of the
study of ambient ultrafine particulate matter exposure studies. Exposure to ultrafine particles has been
shown to have greater health consequences for susceptible populations, such as those with pre-existing
respiratory disorders (U.S. EPA. 2009b: Pietropaoli et al.. 2004). No studies were identified in the current
literature examining nano-Ag health effects in models of susceptible populations.
6.3.2.3. Liver, Kidney, and Urinary System Effects
Consistent with its role in detoxification, the liver has been shown to accumulate a disproportionate
amount of silver following exposure to nano-Ag (Sung et al.. 2009; Kim et al.. 2008; Sung et al.. 2008).
An oral 28-day toxicology study (Kim et al.. 2008) dosed rats with a 0.5% aqueous carboxymethyl-
cellulose vehicle and low (30 mg/kg), middle (300 mg/kg), and high (1,000 mg/kg) levels of 60-nm
diameter nano-Ag powder suspended in the vehicle. The study was said to have been performed
according to OECD test guideline 407, but the exact method of oral administration was not reported.
Tissue damage was primarily observed in the liver, as indicated by incidences of bile-duct hyperplasia and
inflammatory cell infiltration. Incidences of bile-duct hyperplasia, dilation of the central vein, and
increased foci also were recorded for the kidneys.
In an acute oral dose study by Cha et al. (2008). the livers of mice fed either microscale or
nanoscale silver were examined for RNA upregulation and for pathology. As observed using RNA
analysis, seven genes in the apoptotic pathway and five in the inflammatory pathway were induced in the
livers of nanoparticle-exposed mice. Histopathology of livers exposed to both micro- and nano-Ag
showed infiltration of lymphocyte immune cells, suggesting inflammation. Mice treated with nano-Ag
exhibited additional pathologies that were not observed in those exposed to microscale silver, including
hemorrhages in the heart, lymphocyte infiltration in the intestine, and congestion in the spleen.
Elevated levels, reaching statistical significance, of alanine aminotransferase (ALT), aspartate
aminotransferase (AST), and alkaline phosphatase (ALP) were also reported by Park et al. (2010a)
following administration of 1 mg/kg nano-Ag. Similarly, Twari et al. (2011) observed a statistically
significant increase in these and other liver function enzymes in Wistar rats injected with a solution of
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nano-Ag for 32 weeks. These results suggest that repeated exposures to 1 mg/kg nano-Ag or higher might
result in hepatotoxicity.
Ji et al. (2007) conducted a 28-day inhalation study in which concentrations of spherical
aerosolized nano-Ag were delivered at multiples of 100 micrograms per cubic meter (ug/m3) (the
American Conference of Governmental Industrial Hygienists occupational exposure limit) for dust
generated from conventional silver. The doses represented one-half, one, and five times the limit. The
geometric mean diameters of nano-Ag particles in the low-, mid-, and high-concentration chambers were
reported to be 12.61, 12.60, and 15.38 nm, respectively. Significant effects (vacuolization and hepatic
focal necrosis) in this study were found not to be dose-related. These findings support the conclusions of
the subchronic oral study conducted by Kim et al. (2008) described above and the subchronic inhalation
study conducted by Sung et al. (2009) described in Section 6.3.2.2, in which the authors demonstrated
bile duct hyperplasia in high-dose male and females rats. Christensen et al. (2010) investigated the
feasibility of conducting a human health risk assessment for nano-Ag by conducting a literature review
and similarly concluded that "the liver is expected to be a (or the) major target organ of systemic toxicity
of nano-silver."
6.3.2.4. Cardiovascular System Effects
Only one known study has evaluated effects of nano-Ag on the cardiovascular system. In this
24-hour study, Rosas-Hernandez et al. (2009) exposed rat coronary endothelial cells to 0, 0.1, 0.5, 1, 5,
10, 50, or 100 ug/mL of nano-Ag in aqueous solution. Mitochondrial function decreased at concentrations
at 10 ug/mL or less and was associated with an increase of LDH activity, an indicator of membrane
disruption. At higher concentrations, the investigators observed an increase in cell proliferation that was
dependent on increased production of nitric oxide. At a low concentration (5 ug/mL), nano-Ag had a
vasoconstrictive effect on isolated rat aortic rings, while a vasodilative effect was observed at a high
concentration (100 ug/mL). No effects were observed when the endothelium was removed from the aortic
ring. These findings suggest that cardiovascular effects of nano-Ag target the vascular endothelium and
could have opposite effects depending on particle size.
6.3.2.5. Hematology
Studies suggest that silver is transported to the blood of rats exposed orally (Kim et al.. 2008) and
via inhalation (Sung et al.. 2009) to nano-Ag, but whether nano-Ag particles or just silver ions are
responsible is not known. If nano-Ag is absorbed by the blood, functional consequences to the circulatory
system might result. To evaluate potential medical uses in the treatment of thrombotic disorders, nano-Ag
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was injected into mice to evaluate the behavior of platelets. Shrivastava et al. (2009) found that nano-Ag
inhibited platelet aggregation in a dose-dependent manner in the whole blood. Results from cellular
assays suggested that the particles might have affected the signaling necessary for aggregation. Many
modes of action are possible. Twari et al. (2011) also has reported changes in hematology parameters,
including increased white blood count, decreased platelet count, decreased hemoglobin, and decreased red
blood count, in rats injected intravenously with nano-Ag at dose levels ranging from 4 to 40 mg/kg for
32 weeks.
6.3.2.6. DMA Damage
Although genotoxicity of nano-Ag has been observed in vitro, two studies were identified that
evaluated DNA damage in vivo. In a study conducted by Kim et al. (2008). rats were orally exposed for
1 month to 30, 300, or 1,000 mg/kg-day of nano-Ag at 10 milliliters per kilogram (mL/kg) dosing
volumes. The bone marrow of exposed rats was evaluated for chromosomal damage using a micronucleus
test, and the investigators reported no significant difference compared to controls. An incomplete
description of the statistical procedures was provided in this study, however, and no positive controls were
used to evaluate the production of micronuclei. The data presented by Kim et al. (2008) for the treated
male animals showed a modest trend toward increased micronuclei as a function of dose, using 10
animals per dose and 3 doses plus the control, but no statistical analysis was described to demonstrate
either a statistically significant or nonsignificant trend. The investigators indicated that nano-Ag was
found in the blood, but no information was provided to demonstrate that the nano-Ag was actually
detected in the bone marrow. In summary, the investigators indicate no genetic toxicity in bone marrow in
vivo caused by nano-Ag, but the data might actually show a weak effect.
Another in vivo genotoxicity study was conducted by Twari et al. (2011). in which a comet assay
was conducted on blood cells after Wistar rats were injected intravenously with nano-Ag. Results showed
a statistically significant increase in comet tail length from high to low dose; these results indicate damage
to the DNA strand in the high-dose group.
6.3.2.7. Skin
As part of the study discussed in Section 6.3.1.3 that examined the effects of nano-Ag in primary
human epidermal keratinocytes in vitro, Samberg et al. (2010) examined whether nano-Ag particles
(-20-50 nm in diameter) could penetrate porcine skin in vivo and cause morphological alterations of the
skin cells. Because of its comparable thickness and absorption rates, porcine skin is considered to be a
good model for human skin (Monteiro-Riviere and Riviere. 1996). Pigs were topically dosed daily for
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14 days with nano-Ag concentration ranging from 0.34 to 34 ug/mL, after which skin samples were
examined both macroscopically and microscopically. Although macroscopic observations revealed no
gross edema or erythema at any tested dose, microscopic observations showed dose-dependent increases
in morphological changes at tissue layers below the stratum corneum. These changes included
intracellular and intercellular epidermal edema at 0.34 ug/mL; moderate epidermal edema and focal
epidermal and dermal inflammation at 3.4 ug/mL; and severe epidermal edema with severe dermal
inflammation, epidermal hyperplasia, and parakeratosis at 34 ug/mL. The observed responses were not
affected by particle size or washing and were reported to be typical of irritation following exposure to jet
fuels (Monteiro-Riviere et al.. 2001). As discussed in Section 5.7.2.2, however, no nano-Ag has been
detected in tissue layers below the stratum corneum, prompting Samberg et al. (2010) to hypothesize that
the observed effects resulted from translocation of silver ions to deeper tissues following their release
from nano-Ag in the stratum corneum. Similar effects in pigs exposed dermally also were reported by
Nadwory et al. (2008). with some increased inflammatory cell apoptosis, decreased expression of pro-
inflammatory cytokines, and decreased gelatinase activity observed.
6.3.2.8. Reproductive/Developmental Effects
In a study by Li et al. (2010a). the cytotoxic effects of nano-Ag on implantation of mouse embryos
at the blastocyst stage were evaluated in vivo by embryo transfer. Blastocysts were pre-treated with 25 or
50 uM nano-Ag (-13 nm) and transferred, and the uterine content examined at 13 days post-transfer. At
50 uM nano-Ag, increased resorption of post-implantation embryos and decreased fetal weight were
observed.
6.3.3. Human and Epidemiological Studies
Nano-Ag has long been present as a fraction in colloidal silver, which is considered to be a form of
"conventional silver" for the purposes of this document largely because the broad range of particle sizes
that can fall under the definition of colloid does not allow for the study of the isolated effect of nanoscale
particles, and because many colloidal suspensions have not been well characterized (see Section 2.2 for
more detail). Recent interest in mining the colloidal silver database has yielded some information,
however, suggesting that certain historical colloidal silver products have been relatively well-
characterized and were found to contain relatively monodisperse nanoscale particles (Silver
Nanotechnology Working Group. 2009). For example, Muller (1926) investigated exposure effects from a
colloidal silver medical product, Collargol, and reported that the average particle size was 10-20 nm.
Most historical studies, however, do not characterize the particles in the exposure thoroughly, and the
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specific presence of nano-Ag is often difficult to determine. The following sections are intended as an
overview of the state of the science of health effects from conventional silver (including the conservative
assumption that colloidal silver is a form of conventional silver) exposure only. Particle properties of the
exposures are reported, as available.
Chronic exposure to conventional silver has been shown commonly to result in argyria
(discoloration of the skin) and argyrosis (discoloration of the eyes), due to tissue incorporation of soluble
forms of conventional silver (Wrjnhoven et al.. 2009b). AgNO3 is associated with lowered blood pressure,
diarrhea, stomach irritation, and respiratory irritation. Inhalation or ingestion of conventional silver salts
can result in fatty degeneration of the liver and kidneys, as well as blood cell abnormalities (Venugopal
and Luckey. 1978). Studies have shown that soluble silver compounds can accumulate in organs, muscle
tissue, and the brain, while elemental silver appears to have no known severe health effects (Drake and
Hazelwood. 2005). This difference in absorption is thought to be due to differences in solubility, which is
the reasoning supporting the differing threshold limits for soluble silver (0.01 milligram per cubic meter
[mg/m3]) and metallic silver (0.1 mg/m3) proposed by the American Conference of Governmental
Industrial Hygienists. The following sections summarize several key human studies on the health effects
of exposure to different silver forms (given the limited data available for these studies, and that no
relevant studies specific to nano-Ag were available, a study table appendix was not developed for this
section). No studies were identified that addressed the topic of susceptible populations. Taken together,
they function to provide a context for considering the human health consequences of nano-Ag.
6.3.3.1. Medical Use Studies
Known Effects of Conventional Silver Exposure
Conventional silver has long been used for medicinal purposes. Colloidal silver has been used as a
dietary supplement to treat illness and, in combination with sulfadiazine, as a treatment for burns (Drake
and Hazelwood. 2005). The Nanotechnology Project inventory of commercially available products
reported to contain nanomaterials includes examples of these types of nano-Ag products, although the
presence of nano-Ag in these products has not been verified (Project on Emerging Nanotechnologies.
2009). Germ Slayer by Aluwe, LLC, for example, is a colloidal silver, liquid dietary supplement that is
reported to contain 20 ppm nano-Ag. This product is intended to kill viruses and bacteria while not
harming the body, and the manufacturer recommends that it be taken upon sign of infection.
Ingestion of dietary supplements can lead to extremely high exposures, such as an estimated
70-90 ug/day (Wrjnhoven et al.. 2009b). In vitro studies of burn creams, such as Acticoat, show
cytotoxicity upon exposure (Paddle-Ledinek et al.. 2006; Fraser et al.. 2004; Lam et al.. 2004) and raise
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concerns that these products might cause health effects. Acticoat is described as a nano-Ag-coated, high-
density polyethylene mesh developed by Smith & Nephew, Inc., with approximately 0.2-0.3 milligram
(mg) of silver per mg of mesh (Trop et al., 2006). Although the magnitude of these exposures is unlikely
to occur in the case of a spray disinfectant, the health effects associated with exposure via dietary
supplements and burn creams might illustrate worst-case scenarios for ingestion and dermal contact,
respectively.
Mirsattari et al. (2004) described the case of a 71-year-old man who, upon taking a homemade
colloidal silver dietary supplement for 4 months, developed seizures, followed by coma, and death. His
blood-plasma-silver level was 41.7 nM, as compared to the normal range of 1.0-2.3 nM, and his urinary
level for a 24-hour period was 47.28 nM, as compared to the normal range of 0.0-0.46 nmol/L.
Additionally, silver levels in erythrocytes and in cerebral spinal fluid were extremely high. No other
significant contaminants were found in the body. Blood purification attempts successfully removed silver
from the blood, reducing the plasma-silver level from 41.7 to 1.9 nmol/L over 6 days. The silver
concentration in the brain measured at autopsy, however, was elevated, with 0.068 microgram per gram
(ug/g) reported in the cerebrum (compared to 0.029 ug/g in a control sample). The cellular mechanism of
conventional silver neurotoxicity in this case is not clear (Gvori et al.. 1991; Rungbyetal.. 1987; Rungby
andDanscher. 1983).
Two other clinical cases documenting neurological effects following conventional silver exposure
have been reported by Ohbo et al. (1996) and Iwasaki et al. (1997). The exposure scenario described by
Ohbo et al. (1996) involved a schizophrenic patient who had been addicted to antismoking pills
containing conventional silver and displayed convulsive seizures and argyria. The patient reportedly
ingested more than 20 mg of silver per day for 40 years. Blood-serum -silver levels were elevated at
1.2 micrograms per deciliter (ug/dL) (normal levels are less than 0.05 ug/dL). Levels of silver in blood
plasma observed by Mirsattari et al. (2004) were more than three times those found in this study. Sixty-
three days following treatment, serum -silver levels had been reduced to 0.2 ug/dL and seizure activity
ceased. Iwasaki (1997) studied a case involving a burn patient treated with a silver sulfadiazine cream; the
male patient developed severe neurotoxicity that reduced brain tissue weight (determined post mortem),
decreased mental ability, and increased blood-silver levels. Although the patient eventually died, his renal
function had previously been compromised, which is thought to have contributed to the dramatic toxicity
observed in this case.
Other investigations of asymptomatic general argyria from medical applications are reported by
Kakurai et al. (2003) and Van de Voorde et al. (2005). Exposures in these cases were associated with
silver acupuncture needles and argyrophedrine nose drops, respectively. Argyria is the most common
health effect from exposure to conventional silver in general; as the cases above illustrate, confounding
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and small sample sizes impair the ability to demonstrate statistically significant associations between the
exposures and outcomes (Drake and Hazelwood. 2005).
Effects Specific to Nano-Ag Exposure
Nano-Ag toxicity has not been extensively studied with regard to medical use; several studies of
nano-Ag wound dressings, however, provide insight on the potential effects resulting from dermal
exposure. For example, Trop et al. (2006) documented the case of a previously healthy, 17-year-old
patient who suffered burns over 30% of his body as the result of an accident. After cleaning and debriding
the patient's wounds under anesthesia, the burns were covered with Acticoat and moistened with sterile
water and the areas were wrapped with sterile gauze. The Project on Emerging Technologies'
nanomaterial database reports that the concentration of nano-Ag in Acticoat is 70-100 ppm and the
particle size ranges from 1 to 100 nm (2009).
The patient's Acticoat dressings were changed on days 4 and 6 following surgery. On day 6, the
patient presented with argyria-like skin discoloration, lack of energy, loss of appetite, elevated liver
enzymes, a slightly enlarged liver and spleen, and normal renal function. On day 7, laboratory tests
revealed concentrations of 107 micrograms silver per kilogram (fig/kg) in the blood and 28 ug/kg silver in
the urine. The patient reportedly was not exposed to any other form of silver. The Acticoat dressings were
immediately removed, and the patient's facial discoloration reversed. At 7 weeks, however, the levels of
silver in the blood and urine, although four-fold lower, remained elevated; 10 months following treatment,
silver concentration had returned to within normal levels (0.9 ug/kg in the blood and 0.4 ug/kg in the
urine) (Trop et al.. 2006).
6.3.3.2. Occupational Studies
Known Effects of Conventional Silver Exposure
Although case studies of human exposure to conventional silver are common, no studies were
identified that specifically report effects from exposure during the manufacture or use of nano-Ag spray
disinfectants. A few studies evaluated the effects of exposure to nano-Ag or substances containing
nano-Ag, where the exposure route could be similar to possible exposures to commercial spray
disinfectants. These studies are described below and are also mentioned in Section 5.7.1.4.
Rosenman et al. (1987) conducted a cross-sectional study of workers in a plant producing AgNO3,
silver oxide, AgCl, and silver cadmium oxide powders, as well as silver ingots. Air sampling in the
factory by the Occupational Safety and Health Administration resulted in calculated time-weighted
averages, assuming an 8-hour exposure period, of 0.04-0.35 mg/m3. Particle size and other dosimetric
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factors were not measured. Of the 27 workers in the study, 6 had general argyria and 20 had argyrosis.
Most study participants had high levels of silver in the blood (mean of 1.0 ug/100 mL, with range 0.05-
6.2 ug/100 mL) and in the urine (mean 11.3 ug/L, with range 0.5-52.0 ug/L), and 30% of the workers
complained of nose bleeds and respiratory irritation. Kidney dysfunction, indicated by levels of the
urinary enzyme N-acetyl-B-D glucosaminidase (NAG), was significantly correlated with blood-silver
levels (p < 0.05); yet blood-silver levels were not a significant predictor of NAG levels when normalized
by age. Cadmium was also found in significant levels in the urine of the workers, and this additional
exposure, and exposure to solvents, makes definitive associations difficult.
Pifer et al. (1989) compared workers exposed for at least five years in positions with high exposure
potential in an Eastman Kodak plant to employees at the plant in positions with low potential for
exposure. Air sampling provided 8-hour time-weighted average air-borne silver concentrations of
1-100 ug/m3, with most silver in insoluble forms. Although no cases of argyria were noted in the study,
80% of silver workers had detectible blood-silver levels, while none of the study controls had detectible
levels. No organ function tests were performed as part of the study.
Case reports of individuals occupationally exposed to conventional silver complement the
epidemiology studies by providing a more in-depth view of the response at the individual level. Williams
et al. (1999) described the case of a 51-year-old man who displayed corneal and conjunctival argyrosis
following seven years of employment in a silver refinery. Concentrations of silver in air were between
0.11 and 0.17 mg/m3. No other functional abnormalities were observed. Similarly, Cho et al. (2008a)
described the case of a 27-year-old employee of the mobile telephone industry whose job was to apply
plating to mobile telephone subunits with aerosolized silver. After four years of occupational exposure,
the employee developed general argyria. The employee's blood-silver concentration was 15.44 ug/dL, as
compared to normal values between 1.1 and 2.5 ug/dL, and the urinary silver concentration was
243.2 ug/L, as compared to normal values between 0.4 and 1.4 ug/L. Despite these high internal silver
levels, a complete blood count, chemistry panel, liver function test, and routine urinary analysis did not
demonstrate any adverse functional effects.
Similarly, clinical examinations, both general and neurological, reported by Williams and Gardner
(1995) demonstrated no negative health outcomes in the cases of two conventional silver reclamation
workers with blood-silver levels of 49 and 74 ug/L. One of these workers, a 42-year-old process engineer
exposed for two years, mostly through shoveling insoluble silver halide and silver oxide-containing ash,
showed no argyria or other negative signs of exposure. Personal air samples in different areas of the plant
measured air-silver compound concentrations of 0.0085, 1.03, and 1.36 mg/m3. The other reclamation
worker, a 51-year-old engineer working in a refinery dominated by soluble AgNO3 and metallic silver
species for seven years, presented argyrosis and fingernail discoloration, but no other effects of exposure
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were noted following clinical examination. Personal air sampling in the refinery area measured the
highest concentrations ranging from 0.10 to 0.17 mg/m3 atmospheric silver. Following improved safety
measures to reduce exposure, both men's blood-silver levels decreased as measured at 6, 12, and 18
months.
Effects Specific to Nano-Ag Exposure
No studies investigating nano-Ag toxicity resulting from occupational exposure were identified for
this case study.
6.4. Summary of Ecological and Human Health Effects
As described in earlier chapters, the behavior of engineered nanoparticles is greatly influenced by
the properties of the particles and the composition and chemistry of the surrounding environment. This
influence also extends to the toxicity of nanoparticles, and some evidence suggests that particle size and
surface properties affect nano-Ag toxicity. No conclusive determinations have been made concerning the
degree to which specific particle or environmental properties influence nano-Ag toxicity. Particular
emphasis, however, has been placed on surface coatings, which can affect the degree to which particles
form clusters thereby influencing the level of uptake into the organism and cells, and subsequently
organelles, including the cell nucleus.
Most information available on the effects of conventional silver and nano-Ag on biota is from
bacterial studies. A robust database exists regarding the toxicity of colloidal silver and silver salts in
aquatic organisms, but information on the toxicity of nano-Ag in the aquatic environment is relatively
scarce. Even fewer data are available for terrestrial organisms on the effects of exposure to conventional
silver or nano-Ag.
Although the effects of silver ions from nano-Ag particles are presumed to be similar to effects of
silver ions from conventional silver, the rate of ion release and the proximity of that release to the receptor
surface appear to affect toxicity, as demonstrated in bacterial assays. These studies also have
demonstrated that nano-Ag can result in adverse effects on gram-negative, gram-positive, autotrophic,
heterotrophic, and nitrifying species of bacteria. Microbial assays suggest that nano-Ag also can result in
greater toxicity to bacteria than silver ions alone. Studies on embryonic zebrafish, a freshwater vertebrate,
indicate that nano-Ag can be taken up and can affect development. Studies in which nano-Ag was
sonicated demonstrate higher toxicity, suggesting that the form of the silver (i.e., as particles or ions)
plays a role in inducing a toxic response.
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Silver ions and complexes interfere with ion transport pathways in freshwater invertebrates, and
appear to affect marine organisms by altering ionic tissue concentrations. Differences in the toxic
responses from exposure to ionic silver, silver complexes, and nano-Ag have been observed, and these
responses tend to vary with water quality conditions. Overall, many model aquatic organisms (e.g.,
C. reinhardtii, D. magna, D. rerio) are sensitive to nano-Ag exposure, but they appear to be less sensitive
than bacteria. Some evidence from studies on fish indicates that exposure to nano-Ag activates certain
"stress-response" genes and that the particles can enter key organs and organelles, resulting in physical
and toxic effects. Additional evidence, although limited, indicates that nano-Ag is cytotoxic, inhibits
growth, and alters the genome in some plants and soil macroinvertebrates.
Conventional silver affects mitochondrial and cytochrome c signaling in mammalian cells, which
can cause cell death. Exposure to conventional silver also causes argyria and argyrosis, which are
cosmetic effects associated with the distribution of silver in the body following circulation in the blood.
Gastrointestinal distress, seizures, and neurotoxicity have been reported in humans ingesting very high
levels of colloidal silver. At the cellular level, nano-Ag has been shown to bind to and enter cells, generate
ROS, affect mitochondrial function, and result in genotoxicity.
In vivo mammalian studies with conventional silver suggest systemic toxicity, reported as
decreased weight gain following ingestion and dermal exposure, and cell death following inhalation
exposure. Nano-Ag has been demonstrated to cause upregulation of gene expression pathways for cell
death, inflammation in the liver and kidney following ingestion exposure, and adverse effects in the heart,
intestine, and spleen. Inhalation exposure to nano-Ag also was demonstrated to affect liver, kidney, and
lung function. Nano-Ag has been shown to accumulate in the olfactory bulb and brain following
inhalation exposure, although the toxic effects have not been elucidated fully. Genotoxicity has not been
demonstrated in whole-animal studies.
For spray disinfectants, the potential for human and biotic nano-Ag toxicity depends on the level of
exposure to nano-Ag and related silver compounds from these products, and also aggregate exposure to
nano-Ag from other products containing nano-Ag. Toxicity of nano-Ag is dictated by the abundance and
bioavailability of nano-Ag in environmental compartments, suggesting that factors influencing release
scenarios, transport, transformation, and fate processes, and exposure potential of nano-Ag all influence
toxicity.
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Chapter 7. Summary
This chapter summarizes the information presented in the preceding chapters on nanoscale silver
(nano-Ag) in disinfectant spray and highlights several information gaps identified in this document,
particularly issues that were prioritized through a collective judgment process at a workshop held in
January 2011 (ICF. 2011). The outcomes of that workshop and their relevance to future research and risk
assessment efforts also are described.
As discussed in the Preamble and Chapter 1, this case study makes use of the Comprehensive
Environmental Assessment (CEA) approach, which offers both a. framework for systematically organizing
complex information and a.process of collective judgment to evaluate such information. This document is
structured around the CEA framework (Figure 7-1) with chapters devoted to product life-cycle stages
(feedstocks, production processes, uses, disposal, and other aspects of the life-cycle value chain);
transport, transformation, and fate processes; exposure-dose; and ecological and human health impacts.
Such an extended perspective on the "cradle-to-grave" life-cycle approach ultimately will support a more
holistic understanding of nano-Ag in research planning and risk management efforts. Given the relatively
immature state of the science and limited understanding of many issues surrounding nanomaterials, this
document is focused on research planning rather than attempts to complete an actual assessment. It
therefore does not draw conclusions about environmental, ecological, or human health risks related to
nano-Ag in disinfectant spray. Instead, this case study provided a basis for identifying and prioritizing
research areas to support future assessment efforts and contribute ultimately to policy and regulatory
decision-making.
As summarized in Figure 7-2, the first step of the CEA process is to compile information in the
CEA framework, which is represented by this document. For this nano-Ag case study, the next step of the
process involved having selected reviewers and the general public evaluate the External Review Draft of
this document (dated August 2010) for completeness, accuracy, clarity, and other considerations,
including whether information gaps44 identified throughout the document were fully and adequately
stated. Selected reviewers who participated in a January 4-6, 2011 workshop also were asked to identify
44Information gaps are referred to throughout this document variously as data gaps or research questions or needs.
Not all such "issues" would necessarily be addressed through empirical research (e.g., manufacturing data or
monitoring data would not require experimental studies), but the commonly used term "research needs" is
sometimes applied loosely here. Such "needs" might be more accurately termed "desired data" from a risk
assessment perspective and do not signify requirements based on regulatory or policy determinations.
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cradle Product Life Cycle ». Grave
R&D - Feedstock Processing - Manufacturing -
Storage/Distribution- Use - Disposal/Recycling
Transport/Transformation/Fate
Primaryand Secondary Substances
Exposure-Dose
Humans Other Biota Abiotic Resources'?
Natural features, structures, painted surfaces, etc. 1
Impacts
Ecological Other"?
Source: http://www.epa.gov/nanoscience/files/CEAPrecis.pdf
Figure 7-1. Comprehensive environmental assessment framework.
The CEA framework is used to organize, systematically, complex information in evaluations of the environmental implications of selected chemicals, products, or
technologies (i.e., materials). The framework starts with the inception of a material and encompasses the environmental fate, exposure-dose, and impacts.
Notably, the sequence of events is not always linear when, for example, transfers occur between media or via the food web. In addition, a variety of factors
influence each event, including differences in environmental media and the physical, chemical, biological, and social conditions in which the material event
occurs. Details on these influential factors are thus included throughout the framework when possible.
Compile Information
in CEA Framewor
Develop Risk
Management Plan
Monitor, Evaluate
Outcomes
Source: http://www.epa.gov/nanoscience/files/CEAPrecis.pd f
Figure 7-2. Steps in the CEA process.
The CEA process involves a series of steps that result in judgments about the implications of information contained in the CEA framework. Compiling information
in the CEA framework is fundamental for a given material, but is only a first step in the CEA process. Next, the information in the framework is evaluated using a
collective judgment technique (i.e., a structured process that allows the participants representing a variety of technical and stakeholder viewpoints to learn from
one another, yet form their own independent judgments). The result of the collective judgment step is a prioritized list of risk trade-offs and information gaps that
then can be used in planning research and developing adaptive risk management plans. The knowledge gained from these research and risk management
activities feeds back in an iterative process of periodic CEA updates.
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and prioritize information gaps or research questions on nano-Ag using a specific type of collective
judgment methodology, nominal group technique. Details about the Nanoscale Silver Case Workshop are
included in a summary report (ICF. 2011). and a brief description is provided in Section 7.3.1.
Section 7.3.2 discusses how the information presented in this case study document and compiled
during the workshop can be used in nanomaterials research planning. Finally, Section 7.3.3 considers how
the information in this document, prioritized research directions from the workshop, and emerging
research can be integrated into subsequent assessment efforts guided by the CEA approach.
7.1. Case Study Highlights
This section highlights what is known about nano-Ag in disinfectant spray as it relates to each
major component of the CEA framework, specifically product life-cycle stages (feedstocks,
manufacturing, distribution, storage, use, and disposal/recycling); transport, transformation and fate
processes; exposure-dose characterization for biota and environmental resources and for humans; and
ecological and human health effects. Readers are referred to the detailed discussions and literature
citations on each of these components in the preceding chapters. Some of the discussion and knowledge
gaps identified below focus on nano-Ag in particular, while other parts refer to nanomaterials in general.
The discussion therefore might be useful to those conducting research on nano-Ag and those involved in
designing future research and assessment efforts for nanomaterials in general.
7.1.1. Terminology
Within the field of nanotechnology, several terms, including "nanomaterial" itself, are evolving. As
stated in Chapter 1, this document does not attempt to use a definitive definition for nanomaterial or even
for "nano-Ag." Although nano-Ag generally refers to engineered nanoscale silver particles between 1 and
100 nanometers (nm) in size, the term encompasses a range of formulations with different
physicochemical characteristics. As discussed below, physicochemical characteristics influence
nanomaterial behavior and eventual impact in the environment in ways that are not entirely understood.
Caution is therefore warranted when extrapolating generalizations about nano-Ag from one formulation.
Forms of silver that are not intentionally engineered at the nanoscale are referred to as "conventional
silver." Various conventional silver formulations that consist of different particle sizes exist, including
some with nanoscale dimensions. For other terms related to the field of nanotechnology, refer to
Section 1.4.
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7.1.2. Conventional Silver
In the environment, silver (Ag) rarely exists as a pure metal; instead, it is usually found as a metal
alloy, associated with minerals, or a compound. Common forms of silver include gold-silver alloys,
argentite (Ag2S), cerargyrite (AgCl), silver sulfide (Ag2S), silver nitrate (AgNO3), and silver chloride
(AgCl). These compounds form from one of the three cationic states (Ag+, Ag2+, and Ag3+) that exist in
addition to the metallic state (Ag°). Silver in various forms is released into the environment by wind and
water erosion of soils and rocks containing silver. Levels of silver in the environment on the order of
0.3 part per million (ppm) in soils, 0.2 microgram per liter (ug/L) in fresh water, and 0.25 ug/L in sea
water have been observed. Silver in the microgram-per-gram (ug/g) range has been detected in biota,
particularly fish and shellfish.
7.1.2.1. Historic and Current Uses of Silver and Silver Compounds
Silver has been used for centuries to sterilize liquids, treat wounds, and prevent infection, and its
medicinal use continues today, such as for wound dressings and catheters. Although silver has been used
heavily in photography since the late 1900s, the recent rise in digital photography has led to an
approximately 20% decline in its use in this industry. With the highest thermal and electrical
conductivities of any pure metal over a range of temperatures, Ag has numerous other applications, such
as use in household switches and batteries, as mirror coatings, and in antibacterial disinfectants, jewelry,
silverware, coins, and cloud seeding.
7.1.2.2. Historical Environmental Silver Levels
The variety of silver applications outlined above has led to environmental releases and subsequent
elevations in Ag levels. Several field and oral toxicity studies indicate ecological and toxicological effects
from silver concentrations reaching the nanogram-per-liter (ng/L) range in waters surrounding industrial
facilities. A 1997 data analysis suggests that silver emissions to the environment were highest in North
America (4,500,000 kilograms per year [kg/yr]), followed by Asia and Europe. The same analysis
indicates that, within the United States, approximately 65% of emitted silver enters landfills, 18% enters
the environment via tailings, and 13% derives from leachate associated with mining and production
processes.
Several regulatory agencies have guidelines or restrictions for silver levels in environmental or
occupational settings. The U.S. Environmental Protection Agency's (EPA's) National Secondary Drinking
Water Regulations recommend that drinking water levels not exceed 0.10 milligram per liter (mg/L or
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ppm) of total silver. This guideline is not based on potential health effects, but rather a cosmetic condition
known as argyria, which is characterized by silver accumulation in the skin that leads to a blue or blue-
gray color. EPA has not established an ambient water quality criterion for human health due to insufficient
data and thus each state may choose to enforce the 0.10 mg/L guideline. Some states have established
chronic water quality criteria or maximum contaminant levels, however, to protect aquatic life. The need
for more data led EPA to withdraw its proposed chronic ambient water quality criteria in the early 1990s,
but EPA has established a maximum acute concentration of 3.2 ug/L in fresh water and 1.9 ug/L in salt
water. State agencies are responsible for enforcing these levels/guidelines when issuing discharge permits.
In addition, the Agency established an oral reference dose of 0.0005 milligram per kilogram per day
(mg/kg-day) based on a chronic human exposure study. The Occupational Safety and Health
Administration and the American Conference of Governmental Industrial Hygienists have established an
occupational exposure limit of 0.01 milligram per cubic meter (mg/m3) for silver metal, silver
compounds, and soluble silver compounds (U.S. EPA. 2009g). The National Institute for Occupational
Safety and Health Administration also recommends 0.01 mg/m3 as an 8-hour, time-weighted average
exposure limit for silver metal dust and soluble compounds (U.S. EPA. 2009g). For more information
related to silver exposure limit recommendations and guidelines, refer to Section 2.1.2.
7.1.3. Nanoscale Silver
Although intentionally engineering silver at nanoscales is a relatively recent development, the
unintentional use of nano-Ag in various applications such as colored glass and medicinal products has
occurred for centuries. As mentioned above, colloidal silver suspensions contain some particles in the
nanometer size range, and they likely have been available for more than a century in various products
including pesticides and medications. Recent growth in the intentional use of engineered nano-Ag in
consumer products has led to more than 300 products purportedly using the material according to the
Woodrow Wilson Center's Project on Emerging Nanotechnologies (PEN). Notably, the reported number
of products using nano-Ag grew by more than 10-fold between March 2006 and March 2011. Most of
these products claim to eliminate bacteria and their related odors in a fairly extensive variety of
applications, such as cooking utensils, fabrics and socks, dietary supplements, food storage containers,
appliances, and personal care items such as soap, toothpaste or make-up.
As discussed in Chapter 3 and summarized below, spray disinfectants are another category of
nano-Ag products that might be used in the home, garden, or commercial settings such as hospitals and
schools. Federal law requires EPA to register products designed to act as an antimicrobial or disinfectant
agent in any way other than direct application to humans or pets prior to sale of the product within the
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United States. With a higher surface-to-volume ratio than larger, conventional silver particles, nano-Ag
could confer greater antimicrobial activity to spray disinfectants using this form of silver compared to
conventional silver; several manufacturers have claimed that nano-Ag disinfectant sprays kill 99% of
bacteria on a variety of surfaces and prevents odor for long periods of time. Although disinfectant spray
use represents only one potential way for nano-Ag to enter the environment, it is used throughout this
document as an example of how increased nano-Ag use in consumer products might affect environmental
levels of silver and subsequently lead to exposure and effects in humans and biota. As mentioned above,
colloidal silver products can contain nano-sized particles, and thus information on conventional silver also
is presented here to offer potential insight on engineered nano-silver behavior and effects. Although the
comparability of conventional silver and engineered nano-Ag is not well understood, it should be
considered when interpreting or using information on conventional silver in the context of nano-Ag.
7.2. Nanoscale Silver Case Study Summary
7.2.1. Physical-chemical Properties of Nanoscale Silver
As discussed above, the term nano-Ag encompasses a variety of materials having unique physical
and chemical, or physicochemical, properties. Commonly discussed physicochemical properties related to
nanomaterials include particle size, morphology (shape and crystal structure), surface area, chemical
composition, surface chemistry and reactivity, solubility, conductivity, magnetism, and optical properties,
each of which can influence the behavior and ultimate impact of nanoparticles in the environment. The
relationship between these properties and the particle's interaction with the environment is complex.
Indeed, the current uncertainty surrounding this relationship has led many organizations and individual
researchers to recommend that data on physicochemical properties accompany any report on nanomaterial
behavior or impact in the environment. No consensus on which physicochemical properties need to be
reported with experimental results, however, has been reached. The complexity of the relationship
between physicochemical properties of nano-Ag and its environmental behavior or effects is discussed
briefly below. Section 2.3 presents additional information on the physicochemical properties of nano-Ag.
7.2.1.1. Analytical Methods
Accurate characterization of nanomaterials such as nano-Ag in disinfectant spray is critical to
understanding their potential environmental and human health effects. As discussed in Chapter 2,
nanomaterial physicochemical characteristics, such as size, morphology, chemical composition, and
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surface chemistry, likely are instrumental in the efficacy of the products that contain them and likely play
important roles in their environmental behavior, exposure, and toxicity in ecological and human
populations. Importantly, physicochemical characteristics might change after the nanomaterial is
produced, incorporated into a product, and released into the environment, and moves through
environmental compartments and organisms. Sensitive techniques to detect and characterize nano-Ag
during each stage of its existence are therefore fundamentally relevant to understanding the potential
impacts of a nanomaterial throughout its life cycle.
7.2.1.2. Analytical Methods for Laboratory or Occupational Settings
Techniques to characterize nano-Ag in the laboratory during the research and development phase
include spectroscopy, chromatography, electron microscopy, and spectrometry. Once the production has
begun, detecting nano-Ag in the workplace can be complicated by the presence of background levels of
other nano-sized particles from other ongoing processes in the manufacturing facility, such as combustion
or welding. Detecting engineered nano-Ag in the workplace requires a combination of several
instruments, such as a scanning mobility particle sizer, optical particle counter, and scanning electron
microscope. Currently though, the standardized protocols for monitoring suspended particulate matter in
the workplace do not distinguish between ultrafme and nano-sized particles, and thus the application of
these measurement techniques in tandem has been rather limited.
7.2.1.3. Analytical Methods for Environmental Media
The manufacture, use, and disposal of nanomaterials can lead to their presence in the environment
and thus methods are needed to measure nanomaterials in environmental media such as water and soil.
Similar to measurement efforts in the workplace, measuring nano-Ag in environmental media involves
combining several techniques to characterize different aspects of the nanomaterials simultaneously. For
instance, using both field-flow-fractionation and inductively coupled plasma mass spectrometry can
inform researchers about the size and chemical composition of a nanomaterial in soil. Such measurements
are often difficult to make though, for several reasons: (1) The physical size and cost of many instruments
restrict their utility in the field; (2) the relatively low concentrations of nano-Ag in the environment are
often below instrument detection limits; and (3) distinguishing naturally occurring nano-Ag from
engineered nano-Ag is often difficult. Chapter 2 and Appendix A present a more detailed description of
efforts to overcome these challenges and to characterize nanomaterials in a variety of environmental,
laboratory, and workplace settings.
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7.2.1.4. Analytical Methods for Quantifying Dose and Dose Metrics
Information on physicochemical characteristics of the nanomaterial in any particular setting
influences methods for evaluating the concentration, or potential dose, of the material. For nanomaterials,
distinguishing between different types of the same material is necessary, such as between two distinct size
ranges of nano-Ag in a disinfectant spray. The reporting of nanomaterial concentrations is thus
complicated by disagreement over whether researchers should quantify the nanomaterial based on its total
mass, particle number, or surface area. Although using mass as a primary metric aligns with traditional
risk assessment approaches and measurement techniques, doing so can lead to misinterpretation of results.
For instance, some mass-based findings show greater toxicity from nanoparticles compared to larger sized
particles of the same material; the greater surface area-to-mass ratio of nanomaterials versus larger
materials rather than any intrinsic toxicity of the nanoparticles, however, could explain these results. As
such, particle number and surface area have been suggested as alternative metrics to use when reporting
nanomaterial results at various concentrations, although these metrics also have drawbacks. Several
researchers have therefore suggested using all three metrics when reporting nanomaterial results at
different concentrations.
7.2.1.5. CEA Workshop Findings on Analytical Methods
The challenges in characterizing and quantifying nanomaterials in different environments have
been widely recognized in several efforts to set research goals for nanomaterials. In fact, in the Nanoscale
Silver Case Study Workshop (ICF. 2011), participants identified analytical methods as the top priority.
Other areas related to physicochemical properties and their relationship to nanomaterial behavior or
effects also were included in the top ten priority areas, for example, physical and chemical toxicity and
test method development for effects on humans and the environment. Within these general themes were
more specific ideas such as (1) determining a minimum set of assays in harmonized test guidelines for
nano-Ag human and ecological health effects, (2) identifying standardized methods and characterization
protocols to ensure results are comparable between laboratories, and (3) evaluating which
physicochemical properties are essential to characterize before, during, and after toxicity experiments.
Common to each of these issues is recognizing the importance of characterizing nanomaterials at multiple
stages of the product life cycle and experimental protocols (e.g., before, during, and after toxicity testing),
while acknowledging the practical limitations of intensive characterization procedures. By pursuing the
research directions identified here, a better understanding of which kinds of measurements are
consistently important could be reached overtime.
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7.2.2. Life Cycle Characterization
The two major sources of the world silver supply that could be used as feedstock in nano-Ag
production are mining operations and recycling of scrap silver. Mining comprises 76.6%, recycling
19.9%, and net government sales 3.5% of the world silver supply. How much silver from each source is
used in nano-Ag remains a question, but one estimate is that as much as 5% of total silver production is
nano-Ag production. Based on 2008 estimates, therefore, approximately 500,000 kilograms (kg) of
nano-Ag are produced per year worldwide. A 2011 estimate puts production of nano-Ag in the United
States alone within a range of about 2-20 tons per year.
7.2.2.1. Production
The specifics of nano-Ag production procedures are largely proprietary and, as such, few details
are available on possible release points or concentrations of nano-Ag emitted during the manufacturing
process. Although PEN lists a growing number of companies claiming to produce nano-Ag products, and
spray disinfectants in particular, releasing information on their use of nano-Ag is at each company's
discretion. Furthermore, no system exists to verify the presence of nano-Ag in commercial products listed
by PEN. This dearth of information on private manufacturing methods is countered by a growing body of
literature on nano-Ag synthesis in laboratory settings. Several review papers detail the strengths and
weakness of common synthesis methods including chemical reduction, laser ablation, radiolysis, and
vacuum evaporation. Of these and other available methods, chemical reduction using a silver salt such as
silver nitrate and a reducing agent like sodium borohydride is the most common production technique for
large-scale volumes. Surface coatings such as surfactants, polymers, or stabilizing ligands also are used in
the synthesis process to control the natural clustering of zero-valent silver and to maintain particles in the
desired size range. Although the specific choices of silver salt, reducing agent, and surface coating dictate
particle size and shape, most synthesis procedures yield spherical particles less than 20 nm in size. An
extensive discussion on variations in nano-Ag synthesis methods and resulting particle characteristics is
included in Chapter 3.
Based on information available from patents and company websites, manufacturing nano-Ag
disinfectant sprays requires mixing nano-Ag with several other ingredients such as chlorine-releasing
compounds. Although specific information on the manufacturing process is sparse, procedures generally
might include the following: nano-Ag synthesis, either in-house or by a supplier, followed by mechanical
or chemical processes to ensure uniform consistency of all ingredients in the spray mixture, filtration to
remove impurities, short-term storage of bulk spray product in tanks, and, finally, automated dispensing
into bottles. Individual spray bottles presumably would be sealed after quality control procedures are
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completed and then packaged into cardboard cartons for distribution to retailers. Nano-Ag, other spray
ingredients, and by-products could be released during any manufacturing step but few data are available
to support whether such releases in fact do occur. An exception are the two studies that demonstrated
nano-Ag release into the air of a manufacturing facility during particle synthesis procedures. The
possibility for worker exposure during manufacturing is discussed further in Section 5.3.2. Notably, the
production life stage could also lead to nano-Ag releases into the environment from manufacturing waste
deposited in landfills and wastewater streams after flushing or cleaning processing equipment, improperly
treated processing waste, and cleaning surfaces contaminated with nano-Ag.
7.2.2.2. Distribution
As outlined above, nano-Ag disinfectant sprays are likely distributed in sealed plastic bottles that
are then transported in cardboard cartons containing several dozen spray bottles. The cartons could be
stored at an intermediate storage site, but more likely they would be opened only at retail locations where
individual consumers purchase them. Damage to cartons, leaking bottles, or spills resulting from
accidents involving transport vehicles all offer potential release scenarios during transport. Unless
damage occurs or sealing methods are improper, however, minimal release is expected during transport.
7.2.2.3. Use
After purchasing nano-Ag disinfectant sprays, consumers might use the product, per
manufacturer's recommendations, on a variety of surfaces such as walls, floors, sinks, door knobs,
appliances, and furniture. Consumers are likely to use the spray in both residential and occupational
settings such as restaurants, hospitals, and schools. Use in these settings likely will result in nano-Ag in
air, on intended surfaces, and on other areas contaminated by overspray (e.g., human skin, pets, and food).
Importantly, the conditions present where the spray is used can affect nano-Ag or spray by-products. For
example, using oxidizing agents like hydrogen peroxide (H2O2) in conjunction with the disinfectant spray
could oxidize nano-Ag particles, which would release ionic silver. Other ingredients in the spray also
could affect particle behavior. The duration over which nano-Ag from disinfectant sprays remains on
surfaces likely will vary based on the formulation of the spray and characteristics of the surface, but at
least one company states that its product is effective for up to 24 hours. These factors (length of time the
nano-Ag remains on the surface and the context in which the spray is used) likely will affect the
availability of the product for uptake by humans, other biota, and environmental resources.
Various nano-Ag release scenarios are possible during the product use stage, including one in
which nano-Ag might enter waste streams such as landfills or wastewater. Scenarios include disposing of
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cloths used to wipe down surfaces sprayed with disinfectant, washing cleaning supplies or clothing with
disinfectant on them, or spraying sinks, bathtubs, and other surfaces near drains. Releases also could
occur when products containing nano-Ag, such as socks or shirts, are laundered depending on the
washing conditions and fabric characteristics. In addition, direct release of nano-Ag into the environment
could occur in the disinfection of trash cans, furniture, and children's toys.
7.2.2.4. Disposal
After use, nano-Ag spray disinfectant bottles likely would end up in landfills or recycling centers
and thus ultimately be incorporated into municipal solid waste streams. Incineration of the solid waste
could release nano-Ag into the air, while waste in landfills could release nano-Ag into the surrounding
soil and ground water. Alternatively, recycling the bottles could lead to their use in manufacturing new
products, which could result in nano-Ag exposure to both workers and consumers coming into contact
with these new products. If spray bottles are disposed of before they are empty (e.g., by retailers clearing
off shelf space), more nano-Ag would end up in municipal waste streams. Finally, disposal of products
sprayed with nano-Ag disinfectant is another potential source of nano-Ag in waste sites or other areas
such as illegal dumping grounds.
7.2.2.5. CEA Workshop Findings on Life Cycle Characterization
Participants in the Nanoscale Silver Case Study Workshop (1CF. 2011) suggested several ways to
address the information gaps in the product life cycle for nano-Ag disinfectant sprays. For instance, they
proposed evaluating issues such as (1) the potential exposure vectors for nano-Ag or nano-Ag by-products
during each life-cycle stage, (2) the associated feedstocks and by-products of each life-cycle stage and
how these materials might be released, and (3) ways to engage consumers and workers in conversations
about how nano-Ag sprays are used. Such data-gathering efforts can play a significant role in future
assessments by identifying when and how nano-Ag releases occur in the environment and subsequently
lead to exposure in ecological and human populations.
7.2.3. Transport, Transformation, and Fate Processes
Each stage of the product life cycle for nano-Ag disinfectant spray could result in environmental
releases. The propensity for nanomaterial physicochemical properties to change as the material moves
through different environmental compartments such as air, water, and soil emphasizes the importance of
understanding nano-Ag transformation, transport, and fate processes. Yet, little is known about what
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governs these processes for engineered nanomaterials in general, let alone for nano-Ag. Chapter 4
summarizes what is known about environmental behavior, and key points are highlighted below.
7.2.3.1. Factors Influencing Transport, Transformation, and Fate Processes in
Environmental Media
Upon release into the environment, nanoparticles generally behave in one or more of the following
ways: (1) stay in suspension as individual particles; (2) form clusters with other particles (and potentially
deposit or undergo facilitated transport); (3) dissolve in a liquid; and (4) chemically transform by reacting
with natural organic matter (NOM) or other particles. The degree to which particle behavior follows any
of these patterns depends on particle characteristics, the surrounding environment, and several physical,
chemical, and biological processes. Processes affecting particle behavior include particle dissolution,
during which particles release silver ions; particle clustering and deposition; and particle adsorption,
transport, and transformation. The extent to which each process occurs largely depends on particle
characteristics, such as size, shape, and surface coating, and on environmental conditions, such as oxygen
content, pH, organic matter content, ligand concentration, and temperature. Importantly, although these
processes can alter nano-Ag so that it is no longer in particle form, silver will remain in the system in
other physical and chemical forms, such as free silver ions, associated with other ions, or in another
speciated form. Section 4.1.2 provides a more detailed description of the ways that environmental
conditions can affect nano-Ag behavior.
7.2.3.2. Transport, Transformation, and Fate Processes in Air
As discussed above, nano-Ag can enter indoor and outdoor air environments at multiple life-cycle
stages. Individual nano-Ag particles diffuse at a rate inversely related to their diameter, and their size thus
suggests that they likely will diffuse more readily than micrometer-sized particles of similar composition.
Indoors, the diffusion rate of nanoparticle aerosols can vary with changes in indoor air movement due to
temperature, ventilation, or other factors. In general though, single nanoparticles have short resident times
in air due to rapid diffusion, diffusive deposition on surfaces, nanoparticle clustering, and association with
larger particles. Most aerosol sprays contain nanoparticles in clusters,45 however, which behave
45As summarized by Nichols et al. (2002) and discussed in more detail in Chapter 1, the meanings of the terms
"aggregate" and "agglomerate" as they refer to the formation of particle "clusters" are sometimes interchanged in
the literature; thus, the definitions of these terms are neither specific nor consistent. To simplify the discussion for
this case study, the term "cluster" is used throughout this document to indicate an aggregate or agglomerate of
nanoparticles, regardless of the nature or strength of particle cohesion or the mechanisms by which the particles
assemble.
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differently from individual particles. As clusters, the particles can diffuse over relatively larger distances
(e.g., throughout a residence) and persist for a longer time indoors than smaller particles. Importantly,
individual particles that deposit on walls, floors, and other surfaces could be resuspended if they form
clusters with dust particles or the surface is disturbed, for example, by individuals touching it.
Outdoors (i.e., in ambient air), all particles are eventually deposited (dry deposition) or washed out
(wet deposition) to aquatic or terrestrial systems. Particles between 0.1 and 10 micrometers (um) in size,
however, generally remain in the atmosphere longer and can undergo long-range transport by wind and
other forces. To reach micron size for long-range transport, nanoparticles could associate with larger
particles or with other nano-sized particles through London or van der Walls forces to form clusters. The
behavior of engineered nano-sized particles, such as nano-Ag, in indoor and outdoor environments is
influenced by unique physicochemical properties, such as surface coatings that manufacturers use to
prevent cluster formation and improve their persistence after release. Other ingredients in the spray also
could affect the transport and persistence of nano-Ag in indoor and outdoor air.
7.2.3.3. Transport, Transformation, and Fate Processes in Terrestrial Systems
Similar to the discussion of nano-Ag in air, information on nanoparticle transport, transformation,
and fate processes in terrestrial systems is limited. No studies specifically addressing nano-Ag transport,
transformation, and fate in terrestrial systems were identified. In soil, nanoparticles can be highly mobile
due to their small size, but their large surface areas (relative to size) increase their propensity to sorb to
soil, which could render them relatively immobile. A combination of particle and environmental
characteristics coupled with physical factors, such as temperature and precipitation, thus will influence
the balance between particle movement and sorption in soil. In soils with pore water rich in dissolved
organic molecules, nanoparticle stability is enhanced and in turn particles are more mobile. Changes in
pH due to fertilizer addition or rain events also can increase particle mobility, while salt ions in soil
reduce particle mobility by augmenting the propensity to form clusters. Nano-Ag in air, water, or soil
could result in exposure to leaf surfaces and roots of plants.
7.2.3.4. Transport, Transformation, and Fate Processes in Aquatic Systems
In natural aquatic systems, the sensitivity of many organisms to silver ion means that the release of
the ion from nano-Ag during transport and transformation could have a large influence on exposure and
toxicity. Among the key influences of nanoparticle transport, transformation, and fate processes in aquatic
systems are particle surface properties, dissolution rates, and clustering, and environmental factors such as
salinity, pH, and water hardness. For example, surface properties of nanoparticles dictate how mobile they
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are in aquatic environments by influencing their propensity to cluster with other particles or to deposit in
sediment. Thus, nanoparticles engineered to have surface coatings that improve their solubility and
suspension might be more mobile than non-coated nano-sized or larger particles.
Particle dissolution, or the release of silver ions from nano-Ag particles, also impacts transport,
transformation, and fate processes for a variety of reasons. First, dissolution alters particle size and other
characteristics that might affect the particle's behavior in water. Second, nano-Ag toxicity in aquatic
organisms could be due to nano-Ag, silver ions from the nanoparticles, or a combination of the two; thus,
understanding both particle and silver ion behavior is key. Free silver ions are rarely observed in water
because they have a tendency to associate with other ions. Although the predominant form of silver
ultimately will depend on the characteristics of the aquatic environment, the free ion generally associates
with negatively charged ions or ligands, in solution, on particle surfaces, or on dissolved organic matter.
In areas with a high oxygen content, such as on sediment surfaces or in shallow waters, silver generally
forms strong complexes with ligands in organic matter, whereas in less oxygen-rich areas silver forms
stable complexes with sulfide. Silver also complexes with chloride anions, although the nature of the
reaction depends on whether the metal is in fresh or salt water. Environmental conditions also affect the
behavior of silver in nanoparticle form; increases in salinity or ionic strength lead to greater particle
clustering, which in turn decreases particle mobility. Similarly, nanoparticles can form large clusters with
high-molecular-weight NOM present in many aquatic systems, which also could lead to particle
deposition into sediments; association with lower molecular-weight organic matter, however, can increase
particle mobility. The composition of NOM therefore can be highly influential on nanoparticle behavior,
reinforcing the importance of environmental conditions in understanding transport, transformation, and
fate of these materials. Likewise, pH and water hardness impact nanoparticle clustering and, in general,
lower pH levels and higher mineral content result in greater mobility.
As discussed, nano-Ag could end up in natural waters after flowing through wastewater streams.
Characteristics that influence nano-Ag behavior in wastewater systems include surface charge, formation
of particle clusters, the presence of other spray ingredients, and the treatment method used at the facility.
Although most nano-Ag and associated silver ions are removed during wastewater treatment, some could
remain in treated water and ultimately enter water bodies. Evidence to date shows that most of the
nano-Ag removed during treatment processes ends up in sewage sludge; sewage sludge however, is used
as fertilizer for agricultural soils, and the nano-Ag could end up in soil and ground water runoff that
enters surrounding terrestrial systems and waterways. Alternatively, sewage sludge disposed of in landfills
could lead to leaching of the nano-Ag into subsurface soils and ground water. The degree to which
nano-Ag adheres to activated sludge and is removed in wastewater treatment depends on the surface
properties of the nanoparticles. In turn, adding surface coatings to stabilize particles in suspension also
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might impede their clearance during wastewater treatment. Additionally, the treatment process itself can
alter nanoparticle characteristics and result in transformation products such as silver sulfide as the
dominate form of silver coming out of the process.
7.2.3.5. Transport, Transformation, and Fate Models
The complexity of nano-Ag environmental transport, transformation, and fate processes elucidates
the need for models to predict these processes for nanomaterials. Although no models currently exist to
estimate nanoparticle movement in all environmental compartments, the potential exists to link current
models of airborne particle dispersion and convection with models of particles in surface waters and soils;
such a linkage could create an adapted, holistic model of nanomaterial transport, transportation, and fate
processes. The Models Knowledge Base that EPA's Council for Regulatory Environmental Modeling46
has compiled contains models that might serve as a starting point for development of a nanomaterial flow
model. Important possible adjustments to the models to represent nanoparticles accurately include
accounting for particle clustering, sorption to suspended particles, and the potential for colloidal behavior.
The verification of model predictions is hindered, however, by the limited ability to detect and quantify
nanoparticles reliably under different environmental conditions with current analytical techniques.
Despite this dilemma, a few models are available that predict nano-Ag or silver ion concentrations
in environmental compartments. As discussed in Section 4.5, one of these models uses estimates of
nano-Ag levels in Switzerland to predict levels in air, water, and soil. The model relies on several
simplifying assumptions, including that no nano-Ag transformation, degradation, bioaccumulation, or
flow occurs through secondary compartments. Authors estimated that most nano-Ag is discharged to
wastewater via wastewater treatment plants and that risk quotients (calculated as predicted environmental
concentrations [PECs] divided by predicted no-effect concentrations) were less than 1 for both " realistic"
(500 tons nano-Ag used per year) and "high" (1,230 tons nano-Ag used per year) emissions scenarios. A
second model predicted silver ion fate and transport in rivers and sediments based on release from
biocidal plastics and textiles using nano-Ag. Again, the lack of data made several assumptions necessary
but authors showed that PECs in river waters generally were consistent with empirical data (>0.01-
148 ng/L). Sediment PECs on the other hand generally were higher than measured concentrations (0.2-
2 milligrams per kilogram [mg/kg]), although they were below the level reported in heavily affected river
beds (150 mg/kg). Recent work used the results of the first nano-Ag model described above to develop a
probabilistic material flow analysis of nano-Ag and subsequently derive probability distributions of PECs.
46http://www.epa.gov/crem/knowbase/index.htm
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In the United States, PECs were lowest for nano-Ag in air and sewage treatment plant effluent and highest
in sediments and sewage treatment plant sludge; with the numerous assumptions used in the model,
however, the ranges for some results spanned two orders of magnitude for several environmental
compartments and pathways.
7.2.3.6. CEA Workshop Findings on Transport, Transformation, and Fate Processes
As described above, environmental transport, transformation, and fate of nanomaterials is a
complex issue, which likely will be of critical importance in future risk assessments. Participants in the
Nanoscale Silver Case Study Workshop (ICE 2011) highlighted this topic in several suggested research
themes: Fate and transport, particle dissolution, and kinetics all were identified as top research themes to
pursue. Within these themes, participants suggested specific research worthy of consideration:
(1) determining which nano-Ag physicochemical properties can predict fate and transport in
environmental media, (2) identifying existing information on temporal changes in ionic silver release
from nanoparticles, and (3) evaluating how reactions between nano-Ag and other materials like organic
matter or polymers alter particle properties.
7.2.4. Exposure-Dose
Transport, transformation, and fate processes of nano-Ag eventually lead to exposure in humans
and biota. Exposure occurs when a primary or secondary contaminant, such as nano-Ag or a spray by-
product, comes in contact with an outer barrier of an organism. In contrast, uptake is the process by which
the material crosses a biological barrier to enter the organism, which results in the dose, or amount of
substance available to interact with biological receptors or metabolic processes within the organism.
7.2.4.1. Biotic Exposure and Uptake
In biota, exposure might occur when nano-Ag disinfectant spray ingredients in wastewater
treatment plants and in sewage sludge are applied to agricultural land. Alternatively, disposal of sewage
sludge or solid waste containing nano-Ag in landfills might result in exposure to organisms in the
surrounding soil or ground water. Aquatic species, particularly those dwelling near or in sediments, might
experience higher exposure levels than those in terrestrial environments. Little information exists on how
much nano-Ag use will elevate environmental concentrations of nano-Ag or species of silver, but current
estimates predict that levels will increase (see Section 4.5). Actual exposure levels and related
bioavailable concentrations are heavily influenced by particle characteristics, environmental conditions,
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characteristics of the organism, and route(s) of uptake into the organism. An extensive discussion on the
influence of each factor is included in Chapter 5. For example, particle characteristics such as size can
impact whether nano-Ag passes through an organism's outer layer. On the other hand, environmental
conditions such as low pH can alter particle size, which in turn might result in preferential uptake during a
particular life stage of an organism, or via a specific uptake route (e.g., through the gastrointestinal tract
versus the skin). In addition, other ingredients in the spray could affect nano-Ag behavior or be taken up
to a greater extent in the presence of nano-Ag. Although some of the factors outlined above, such as pH,
affect silver regardless of whether it is in nanoparticle form, others, like changes to surface coatings,
might apply exclusively to nano-Ag.
Understanding the role that particle and environmental factors play in determining the
concentration of nanoparticles taken up by an organism is complicated by the difficulty in measuring such
uptake with current analytical techniques. Data do show, however, that biota, including bacteria, fungi,
algae, and fish readily take up, and in some instances bioaccumulate, nano-Ag. To date, several studies
focused on nano-Ag in fish demonstrate that, although fairly limited, bioaccumulation occurs to a greater
extent in freshwater species. In adult fish, nano-Ag appears to absorb to the gills, which can serve as a
point of entry for both nano-Ag and silver ions; in developing fish, nanoparticles can cross the egg barrier
(chorion) and accumulate in tissues, including the brain. Studies evaluating nano-Ag uptake and
bioaccumulation in other organisms are forthcoming, but data indicate conventional silver bioaccumulate s
in several organisms, including bivalve mollusks and aquatic crustaceans. In general, bioaccumulation of
nano-Ag and silver ions decreases with trophic level in the water column, but statements about this
relationship are tempered by the current lack of evaluations of nano-Ag in higher order species, such as
fish.
Several plants, including agricultural crops such as corn, take up and accumulate conventional
silver in the root system, but rarely in aboveground parts such as leaves. Little evidence is available
regarding whether this pattern is applicable to nano-Ag; initial work in plants is not consistent regarding
whether uptake of nano-Ag is comparable to, greater than, or less than uptake of conventional silver. Soil-
dwelling invertebrates such as worms also take up nano-Ag and can pass them on to the next generation,
but data in this case also are limited to a few laboratory studies. No information was identified on
bioaccumulation of nano-Ag in larger terrestrial organisms, although effects in invertebrates and soil
microorganisms could affect terrestrial ecosystems as a whole.
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7.2.4.2. Human Exposure and Uptake
In humans, the growing number of products containing nano-Ag suggests that exposure is
increasingly likely; methods to measure nanoparticle exposures are generally lacking, however, and thus
information from manufacturers is the main source of information on potential exposure. As discussed in
Section 5.3, the extent of exposure inevitably will depend in part on: (1) nanomaterial characteristics,
such as shape and form; (2) product characteristics, like how the material is incorporated; and (3) the
route of exposure to the nano-Ag in the product. The types of products claiming to contain nanomaterials
and the likely exposure scenarios for those products led one group of authors to suggest that, of all of the
nanomaterials considered, nano-Ag has the highest potential to result in consumer exposure. To that end,
experts agree that use of cleaning products such as nano-Ag disinfectant spray is likely to result in high
exposure levels via inhalation and dermal pathways, particularly to consumers. Initial evidence
characterizing emissions from consumer spray products containing nano-Ag indicates that inhalation
exposure to nanosized particles and larger clusters might occur, although the type of spray application
method and product composition likely will influence these exposures. In both consumer and
occupational populations, exposure also can occur through hand-to-mouth contact from touching or
handling treated surfaces, a behavior that is particularly prevalent in children. Higher metabolic rates and
greater consumption of food and water per body weight also indicate that children could be a susceptible
population to nano-Ag spray use. In occupational settings, current exposure limits for conventional silver
span an order of magnitude, depending on which regulatory guidelines apply. Part of the difference in
exposure limits reflects higher absorption potential of soluble silver compared to non-soluble silver, but
little evidence exists regarding how these limits might relate to nano-Ag exposure levels. This knowledge
gap could be important, given that initial studies evaluating worker exposure in a laboratory setting and
traditional manufacturing facilities found the potential for both inhalation and dermal exposures. Notably,
as a disinfectant spray, nano-Ag exposure could occur in worker populations, such as janitorial staff, who
are not traditionally considered in occupational studies.
The degree to which exposure leads to absorption, distribution, biotransformation (metabolism), or
excretion (clearance) in humans depends on particle properties, the presence of other spray ingredients,
and the route of exposure. Data indicate that nano-Ag is taken up via both inhalation and oral exposure
pathways and subsequently crosses biological barriers to accumulate in tissues, including the liver,
stomach, brain, and blood. Whether the silver in these tissues is ionic, soluble silver, or nano-Ag,
however, is unclear. Particle size is instrumental in how inhaled particles deposit or move through the
body. Studies show that nano-Ag is more likely to enter the alveolar region of the lung than conventional
silver, and some translocation to other tissues can occur in a size-dependent fashion that favors nanoscale
particles. Following inhalation, levels of nano-Ag and other species of silver in the lung are much higher
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than in other organs, suggesting relatively limited potential translocation from the respiratory tract to
other tissues. Oral exposure to nano-Ag generally leads to lower absorption than from other exposure
routes, but still results in silver accumulation in multiple organs. Conventional silver also is taken up
across the intestinal lining after oral exposure. Although conventional silver is likely not taken up after
dermal exposure, data indicate that nano-Ag can cross the upper layers of the stratum corneum under
some circumstances, and silver ions released from these particles might then penetrate into deeper layers
of the skin. Exposure to nano-Ag in burn wounds results in silver accumulation in tissues, and in one case
led to neurological symptoms, but absorption through healthy skin depends on the exposure conditions,
particle size, and other factors. Whether absorption via any of the above pathways leads to silver in any
form passing the blood-brain barrier is controversial, although at least one study shows that nanoparticles
as small as 1-2 nm cannot penetrate this barrier.
Data on metabolism or transformation of nano-Ag in tissues, as well as its excretion, are currently
lacking. Therefore no evidence exists on whether these processes are similar for conventional silver and
nano-Ag. Importantly though, the high surface area-to-volume ratio of nanoparticles results in a dynamic
coating of proteins and other extracellular molecules on the particle surface, which can ultimately impact
the particle's interaction with cells and, in turn, metabolism, transformation, or excretion.
7.2.4.3. Aggregate Exposure in Humans and Biota
Even a single product like nano-Ag disinfectant spray can lead to exposure via multiple routes in
humans and biota. This observation, compounded by the ever-expanding number of products claiming to
contain nano-Ag and the numerous sources of naturally occurring and incidental nanoscale silver
particles, points to the likelihood of aggregate exposures in both human and ecological receptors.
Moreover, the high surface area-to-volume ratio and enhanced chemical reactivity of nanoparticles can
modify the bioreactivity of other contaminants, such as manufacturing by-products or environmental
toxicants; in turn, repeated exposure to nano-Ag via multiple pathways also could lead to greater
cumulative exposures to other contaminants. In this light, models estimating the potential for nano-Ag
exposure, particularly from multiple routes, would be informative; due to the lack of empirical data in
relevant occupational and environmental settings, however, exposure concentrations currently are
estimated with fate and transport models. As mentioned in the previous section, such models require
assumptions and their applicability has yet to be determined, given the unique properties of nanomaterials
compared to conventional chemicals.
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7.2.4.4. Exposure and Uptake Models
The lack of data on environmental or occupational levels of nano-Ag dictates that exposure
estimates are based on the few fate and transport models noted in Section 7.1.3. Although no models
currently exist to estimate nano-Ag absorption, distribution, metabolism, and excretion, some tools are
available to model nanoparticle deposition based on size, including the Human Respiratory Tract Model
for Radiological Protection developed by the International Commission on Radiological Protection. More
information on this model and estimates of particle deposition in the respiratory tract is included in
Section 5.7.2.1.
7.2.4.5. CEA Workshop Findings on Exposure and Uptake
Several reports from national bodies such as the Scientific Advisory Panel for the Federal
Insecticide, Fungicide, and Rodenticide Act and the Netherlands National Institute for Public Health and
the Environment and in the peer-reviewed literature conclude that data on nano-Ag exposure and potential
toxicity are insufficient to reach meaningful conclusions about the material's risk. Participants in the
Nanoscale Silver Case Study Workshop (ICF. 2011) supported this evaluation by highlighting several
research themes that could improve scientific understanding of exposure and uptake to eventually
facilitate risk assessments of nano-Ag. For example, general research themes for the workshop, such as
analytical methods, exposure and susceptibility, surface properties, and sources and release, included
specific ways to address some of the data gaps outlined above. In particular, participants suggested the
following research activities: (1) determining how to detect and characterize nano-Ag exposure in
different environmental media and food, (2) investigating how parameters such as behavior and life stages
of humans and biota affect susceptibility to nano-Ag, (3) identifying any influence that surface properties
have on processes such as uptake and bioaccumulation, and (4) evaluating the potential exposure vectors
through which nano-Ag and nano-Ag products are released to the environment during each product life-
cycle stage.
7.2.5. Characterization of Effects
Nanoparticle effects on ecological and human health also are influenced greatly by particle
properties and environmental conditions. Readers can refer to Chapter 6 for an extensive discussion on
how specific factors, such as particle size, morphology, surface charge, and test or environmental
conditions, can influence nano-Ag effects. One parameter evaluated in several studies is the potential
effect that particle surface coatings have on particle cluster formation, and in turn the concentration of
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particles taken up into organisms and cells. Conclusive data are lacking, however, on the degree to which
specific particle or environmental characteristics impact nano-Ag toxicity. Moreover, few studies have
been able to differentiate between effects from nano-Ag and silver ions released from nanoparticles.
Given this technical difficulty, the discussion below highlights information on both nano-Ag and
conventional silver.
7.2.5.1. Ecological Effects
Studies on conventional silver and nano-Ag effects in ecological receptors largely have focused on
their bactericidal effects and outcomes in a few laboratory animals. Researchers found that similar to
conventional silver, nano-Ag is toxic to many types of bacteria, both gram negative and gram positive.
Data further indicate that nano-Ag is more toxic than silver ions alone, potentially due to greater ion
exposure as a result of silver ions released from nano-Ag particles directly at the cell surface.
A fairly extensive body of work shows that conventional silver is toxic to fungi and bacteria,
aquatic invertebrates, algae, and fish, and to some terrestrial plants. Organisms most sensitive to
conventional silver include freshwater and marine phytoplankton, freshwater salmonids, and early life
stage marine invertebrates. Numerous studies also have found that conventional silver disrupts ion
regulation in fish, particularly freshwater species. Although sensitivity in these aquatic organisms is
counterbalanced by the low bioavailability of free silver ion, nano-Ag might become more bioavailable
under certain conditions, and thus pose a greater concern to these organisms as a reservoir of silver ions.
To date though, relatively few studies are available on nano-Ag effects in aquatic environments. Existing
work does show differences in the toxic responses of freshwater invertebrates exposed to ionic silver,
silver complexes, or nano-Ag, and water conditions influence the differences in these exposures. In
general, model aquatic organisms such as Ceriodaphnia reinhardtii, Daphnia magna, and Danio rerio are
sensitive to nano-Ag, but less so than bacterial populations. Most studies on aquatic organisms have
focused on fish with little to no information available on nano-Ag in aquatic mammals, amphibians, and
other aquatic vertebrates. Within the body of work on fish, zebrafish (D. rerio) are the most widely used
test species. Results indicate nano-Ag uptake and subsequent developmental effects in embryonic
zebrafish. Other studies in fish suggest that nano-Ag toxicity results, at least in part, from its entry into
key organs and organelles followed by changes in cellular signaling pathways and gene expression. As
noted above, the extent to which nano-Ag, silver ions released from nano-Ag, or a combination of the
two, is responsible for nano-Ag toxicity in fish is a topic of debate. The ability to predict toxicity in
aquatic environments from nano-Ag versus conventional silver would help to shed light on this debate,
but no models are currently available to assess nano-Ag toxicity. The Predicted Water Column No-Effect
Concentration model and the biotic ligand model, however, are available to predict the toxicity of silver
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ions released from nanoparticles in aquatic environments. Readers should refer to Chapter 6 for details
these models.
Information on nano-Ag effects in terrestrial organisms is even scarcer than for aquatic species. In
fact, no data were identified on nano-Ag toxicity in soil. Mixed results of conventional silver effects in
terrestrial plants indicate that conventional silver might affect growth and germination. Effects of
nano-Ag in plants are highly dose-dependent, but survival and growth apparently can be altered. Data
suggest that nano-Ag effects are not solely due to silver ion release, but could result in part from
interaction with thiol groups in cellular tubulin. Limited evidence suggests that conventional and nano-Ag
can affect behavior, growth, reproduction, and survival in terrestrial invertebrates, specifically worms.
These effects might be driven in part by free-radical generation followed by DNA damage and subsequent
cell death, but conclusive evidence for this mechanism of action is lacking. The limited data identified for
nano-Ag effects in terrestrial vertebrates indicate minimal toxicity in these animals, but further study is
needed to substantiate these findings.
7.2.5.2. Human Effects
As detailed in Section 6.3, studies to elucidate effects of conventional silver and nano-Ag in
humans have used a variety of in vitro, in vivo, and some epidemiology approaches. In vitro data indicate
that silver ions alter mitochondrial function, resulting in release of apoptogenic signals and subsequent
cell death. Other work shows dose-dependent effects of silver ion on cell replication and other
developmental endpoints in mammalian cells. Nano-Ag also can penetrate cells and result in cytotoxicity,
possibly due to one or a combination of the following observed effects in mammalian cells: oxidative
stress, inflammation response, DNA and molecular damage, growth inhibition, mitochondrial disruption,
and changes in cell morphology. Although specific responses can vary by cell type, nano-Ag elicits a
toxic response from a variety of cell types.
As described in Section 6.3.2, in vivo mammalian studies indicate systemic toxicity from
conventional silver exposure, as demonstrated by decreased weight gain following ingestion and dermal
exposure, and cell death following inhalation exposure. In a 1990 overview of conventional silver effects,
the Agency for Toxic Substances and Disease Registry determined a no observed adverse effect level of
181.2 mg Ag/kg-day). The no observed adverse effect level was based on animal mortality from a study
of rats ingesting AgNO3 in drinking water over a 2-week exposure period. The lowest observed adverse
effect level for this study was 362.4 mg Ag/kg-day due to decreased weight gain. Findings indicate that
nano-Ag exposure via one of several routes (e.g., oral, intravenous) can lead to gene expression changes,
inflammatory response in the liver and kidney, and adverse functional effects in the lungs, heart, intestine,
and spleen. Of these responses, several studies suggest that the liver might be particularly susceptible to
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nano-Ag exposure. Ultimately, the route of exposure and particle characteristics will influence which
specific effects are observed but the multitude of studies in this area suggests that nano-Ag exposure
could result in toxic responses in mammals.
In humans, chronic exposure to conventional silver can lead to argyria, or skin discoloration, and
argyrosis, or discoloration of the eyes, as soluble silver is incorporated in the tissue. Importantly, exposure
to different forms of silver leads to distinct outcomes. Whereas elemental silver exposure is not associated
with health effects, soluble silver is associated with several effects, including lowered blood pressure,
diarrhea, respiratory irritation, and fatty degeneration in the liver and kidneys. A few case studies suggest
that conventional silver exposure also can lead to neurological symptoms, but conclusions cannot be
drawn due to the small sample size. A single case study of nano-Ag exposure in a teenage boy revealed
elevated silver concentrations in blood and urine coupled with several symptoms, such as loss of appetite
and elevated liver enzyme. Blood and urine levels of silver were still elevated 7 weeks later, but returned
to normal by 10 months post-exposure.
7.2.5.3. CEA Workshop Findings on Effects
The above discussion outlines some of the large data gaps related to the effects of nano-Ag on
ecological and human health. The need for better understanding of nano-Ag effects and how they relate to
product life-cycle stages, transport, transformation, and fate processes, and to exposure was recognized by
participants in the Nanoscale Silver Case Study Workshop (ICF. 2011). Eight research themes related to
nano-Ag effects were identified, including: physical and chemical toxicity, toxicity mechanisms, and test
method development for humans and ecological populations. Within these broad themes were more
specific research activities such as (1) evaluating which physicochemical characteristics can help predict
toxicity in humans or biota, (2) distinguishing between nano-Ag and ionic silver effects after nano-Ag
exposure, and (3) measuring biological responses after short- and long-term exposures to nano-Ag in
occupational settings.
7.3. Role of Case Study in Research Planning and
Assessment Efforts
This document is part of a larger process to support research planning that ultimately supports
future assessment and risk management efforts for selected nanomaterials or nanomaterial-enabled
products or both. The purpose of this case study, and others like it (U.S. EPA. 2010dX is to help identify
what data are available and what information needs to be developed to complete assessments of
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nanomaterials. Compiling available information using the CEA framework (see Figure 7-1), as is done in
this case study, is only an initial step in the CEA process (see Figure 7-2). A key aspect of the CEA
process is engaging diverse perspectives in a structured collective judgment procedure to evaluate the
information in a CEA framework (e.g., this case study). Outlined below are the results of such a collective
judgment process for nano-Ag in disinfectant spray and how this information informs research planning
and assessment efforts.
7.3.1. Workshop Outcomes
A variety of methods exist for deriving collective judgments about setting priorities. In this
instance, a specific approach known as nominal group technique was selected and carried out in a 3-day
workshop (ICF. 2011). The 23 participants in the workshop represented a cross-section of technical
disciplines (e.g., manufacturing, environmental fate, exposure, ecology, toxicology, risk management) and
sectors (academia, government, industry, and others). During the workshop, participants used the
information in this case study document in conjunction with their own experience and knowledge to
identify what types of data would be useful for completing assessments of nano-Ag in disinfectant sprays.
To do this, individuals participated in round-robin fashion to describe the type of research or information
they felt would most inform future assessment efforts (e.g., characterization techniques for nano-Ag in
wastewater streams, or determining the half-life of nano-Ag in the environment).
Following the identification of data gaps, participants combined similar issues into broader
research area themes, such as "Analytical Methods" and "Mechanism of Action." Some questions were
similar or overlapped (e.g., "How does surface coating affect toxicity to humans or biota?" and "To what
extent do particle properties determine biological responses to nano-Ag?" are both related to a "Physical
and Chemical Toxicity" Theme). Therefore, combining similar issues was useful provided the participants
in the process agreed that no significant distinction was being overlooked. After combining individual
issues into broader research area themes, a voting process ensued whereby each participant allotted 10
points to the most important research theme, 9 points to the second most critical theme, and so on, down
to 1 point. Combining the points from all participants for each broader theme resulted in a prioritized list
of research directions that would help support assessments of nano-Ag that then could be used to identify
risk-related trade-offs using the CEA approach (see Figure 7-2). More information on the workshop itself
and the prioritized research themes is in a summary report (ICF. 2011). but a brief description of the top
research themes is included below. Appendix D, which is an excerpt of the summary report produced by
the EPA contractor that independently conducted the workshop process, contains the complete list of
prioritized research questions and the themes that participants grouped them into.
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The broader research themes identified as the highest priorities for nano-Ag in disinfectant sprays
focused on improving characterization methods for the materials themselves; exposure scenarios;
transformation, transport, and fate processes; and toxicity in humans and other biota. Specific data gaps
within these themes include, for example: (1) evaluations of nano-Ag kinetics and dissolution, such as
measuring its half-life in different environmental compartments; (2) determining how nano-Ag differs
from conventional silver, for instance, in terms of environmental behavior and effectiveness in consumer
products; and (3) obtaining manufacturing information by, for example, collaborating with other federal
agencies. The workshop process resulted in participants' identifying several issues that were not
considered in the first draft of this case study, such as (1) evaluating exposure scenarios for vulnerable
populations, (2) investigating whether nano-Ag releases could contribute to climate change, and
(3) developing effective communication techniques to convey information on nano-Ag risks and benefits
to the general public. Although some of these research needs specifically relate to nano-Ag research,
others apply more broadly to nanomaterials as a whole. As such, these outcomes can support a research
strategy that addresses both the data needs for individual nanomaterials and those necessary to support a
better understanding of nanomaterials in general.
7.3.2. Implications for Research Planning
This work on nano-Ag and other, similar efforts on different nanomaterials (U.S. EPA. 2010c. d)
are not the first to identify data needs for nanomaterials (U.S. EPA, 2009e; NSTC, 2008; Tsuji et al.,
2006). Nevertheless, the CEA approach to research planning provides a unique perspective in four
important ways: (1) emphasis on prioritization, (2) attention to general and specific nanomaterial research
needs, (3) breadth of perspective, and (4) transparency.
Prioritization. As noted in the description of the Nanoscale Silver Case Study Workshop above,
prioritizing information gaps is a key focus of the CEA approach. As with previous case studies, the
prioritized research gaps that emerge are intended to inform decision-makers in EPA and the broader
scientific community in developing research agendas that support future risk assessment and risk
management goals. Such information would expected to be considered in the context of the particular
focus, budgetary constraints, ongoing research, and other considerations of any organization; however, the
prioritization of potential research areas could make clearer where to focus funding within an agency's or
organization's purview to support future risk management goals.
Attention to General and Specific. As demonstrated by the research needs summarized above, the
case study approach lends itself to developing a research plan pertinent to individual types or applications
of these materials and to nanomaterials in general. Because even small changes in the properties of a
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nanomaterial or its application can influence its behavior and ultimate effects, an approach that balances
research needs for many types of nanomaterials with those for specific materials or applications is
necessary for the field to progress.
Breadth of Perspective. The use of the CEA framework (see Figure 7-1), coupled with input from
diverse perspectives during the collective judgment step, facilitates a strategy that includes research needs
spanning individual technical disciplines and sectors. This approach contrasts with the more narrowed
perspective on research directions that might develop from a single agency or organization.
Transparency. Finally, the CEA approach emphasizes the need for a clear record of what
information was considered and how certain information gaps were identified as high priorities. This case
study and the accompanying Workshop Summary Report and others like it (ICF. 2011; U.S. EPA. 2010c)
illustrate an effort to provide a transparent account of how judgments about research priorities were
reached.
7.3.3. Implications for Future Assessment Efforts
Efforts thus far to assess nanomaterial impacts on environmental and human health demonstrate
that data gaps such as those outlined above currently impede carrying out assessments and generally
restrict evaluations to limited aspects of specific nanomaterials (Aschberger et al., 2011; Walser et al.,
2011; Christensen et al., 2010; Savolainen et al., 2010; O'Brien and Cummins. 2009; Zuin et al.. In Press).
This document, and the CEA approach in general, are intended to help address the lack of data by
identifying and prioritizing research areas using the holistic CEA framework and process to support a
broader understanding of nanomaterials. For example, one of the top priority research themes for nano-Ag
is evaluating how parameters such as nanoparticle characteristics influence exposure. Methods that are
developed to characterize nano-Ag particles in environmental or occupational exposure settings could be
used to improve exposure models for several nanomaterials, and thus reduce uncertainty surrounding
exposure estimates in nanomaterial risk assessments. Subsequent applications of the CEA approach can
then incorporate this research and other results from the research priorities identified here to develop
comparative risk assessments that include life-cycle considerations, a collective decision process, and
possibly additional perspectives, such as sustainability considerations, stakeholders' values, and other
decision-makers' considerations.
Synthesizing information from areas as diverse as ecological and human toxicity, exposure,
environmental fate, and physicochemical properties is a challenge for risk assessment in general, and not
a specific issue for nanomaterials. As outlined above, CEA addresses this challenge by combining a
holistic framework (see Figure 1-1) to organize information on the material under consideration and a
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process that supports the structured evaluation of the available information from a diverse set of
perspectives. Although CEA does not offer the only solution for integrating information from numerous
scientific fields, its emphasis on holistic, transparent, and structured evaluations of such information
should demonstrate an improved approach to addressing complex and difficult issues in research planning
and risk management.
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Appendix A. Common Analytical Methods
for Characterization of Nanomaterials
A-1
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Appendix A. Common Analytical Methods
for Characterization of Nanomaterials
A.1. Introduction
A.2. Aggregation
A.3. Chemical Composition
A. 4. Crystal Structure
A.5. Dissolution
A.6. Heterogeneity
A.7. Mass Concentration
A.8. Melting Point
A.9. Particle Number Concentration
A.10. Porosity
A/11. Shape
A.12. Size
A.13. Size Distribution
A.14. Speciation
A.15. Structure
A.16. Surface Area
A.17. Surface Charge
A.18. Surface Chemistry
A.19. Surface Contamination
Appendix A References
A-4
A-5
A-6
A-7
A-7
A-8
A-8
A-9
A-9
A-9
A-10
A-11
A-12
A-13
A-14
A-14
A-15
A-15
A-16
A-17
A-2
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A-3
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A.1. Introduction
Presented in this appendix is a compilation of analytical methods that have been used to
characterize nanoparticles, including nano-Ag. This information is not intended to be exhaustive in
reporting every applicable method, or to be comprehensive in describing available methods; rather, it is a
summary of relatively common or known methods for characterizing nanoparticles based on the U.S
Environmental Protection Agency's (EPA) experience and knowledge at the time this case study was
developed. Because of the rapid pace at which the field of nanotechnology continues to evolve and grow,
undoubtedly there are additional methods that are not included here, and some information presented
about specific methods might not fully reflect the current state of the science. Furthermore, the most
appropriate methods for characterizing certain nanomaterials or the best methods for specific contexts
could vary, and having such a general compilation cover each of these nuances is not possible.
Nevertheless, this appendix is expected to be useful in that it lists some of the more commonly used
methods and provides general information relevant to evaluating the research needs regarding nano-Ag.
The methods summarized here are grouped into 19 tables, presented alphabetically according to the
properties and characteristics being analyzed. Within each table, information is presented regarding the
approximate detection range and advantages and disadvantages for each technique. Although the
techniques included in each table have not been prioritized according to accuracy, cost, or other
characteristics (methods are listed alphabetically within each table), the most commonly employed
techniques are listed in bold font.
Staff in EPA's Office of Pesticide Programs compiled most of this information. Although citations
are not provided for individual techniques, several important sources used to develop these tables, such as
some websites of manufacturers that produce equipment used to characterize nanomaterials, are cited
here: (AZoNano. 2010; Buchan Lawton. 2010; General Electric Company. 2010; Imaging Technology
Group. 2010; NCEM. 2010; PerkinElmer. 2010; Ouantachromelnstruments. 2010; Shimadzu Scientific
Instruments. 2010; TSI Inc.. 2010; Varian. 2010; Becker. 2008; Tiede et al.. 2008; Zuin et al.. 2007;
Oberdorster et al.. 2005; Coulson et al.. 2002; Tsuda and Tanaka. 1996). Full citations are listed at the end
of this appendix.
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A.2. Aggregation
Technique
Analytical Ultracentrifugation (ANUC)
Atomic Force Microscopy (AFM)
Detection Range
>nm range
>0.1 nm
Advantages
Can be used to analyze dry,
moist, and liquid samples
Disadvantages
Artifacts can result from tip
smearing
Prone to overestimations
Chemical Force Microscopy (CFM)
>0.1 nm
Used in biology
Many modifications to AFM tip
Confocal Laser Scanning Microscopy (CLSM)
Differential Interference Contrast Microscopy
(DIG)
Can be used to analyze unstained
biological samples
High resolution with no artifacts
Calls for transparent specimen with
refractive index similar to its
surroundings
Expensive
Differential Mobility Analyzer (DMA)'
Dynamic Light Scattering (DLS)
Field Emission Scanning Electron Microscopy
(FE-SEM)2
Flow Field-Flow Fractionation (FIFFF)
Fluorescence Microscopy (FLM)
Nuclear Magnetic Resonance (NMR)
Scanning Electron Microscopy (SEM)2
Scanning Transmission Electron
Microscopy (STEM)'
Scanning Tunneling Electron Microscopy (STM)
Size Exclusion Chromatography (SEC)
Small Angle Neutron Scattering (SANS)
Transmission Electron Microscopy (TEM)4
Ultracentrifugation (UC)
X-ray Diffraction (XRD)
Zeta Potential
3 nm-um range
3 nm-um range
1 nm-1 urn
>10nm
mM range
1 nm-1 urn
resolution: <0.1 nm
nm in x, y, and z directions
5 nm-1 00 urn
1 nm-1 urn
>0.1 nm
nm range
1-3 wt%
5 nm-10 urn
Can be used in combination with
many techniques
Allows in situ measurement
Fast and simple
Convenient for analyzing
aggregation
Ultra high resolution
Images secondary electrons with
backscatter detector
Used to analyze solid or liquid
samples
Convenient temperature range
High resolution
Used to analyze low
concentrations (ppm)
Allows three-dimensional
characterization
Good separation efficiency
Used to analyze liquids
Used to characterize structural
details of pores of all types (open,
blind, and closed)
High resolution
Currently used in carbon nanotubes
Possible sample degradation
Dust particles can ruin
measurement
High particle interactions
Requires high vacuum for sample
preparation
Solid-state experiments are more
difficult
Requires high vacuum for sample
preparation
Interactions of solute and solid
phase
Careful data analysis needed
Requires high vacuum for sample
preparation
More qualitative than quantitative
Requires homogeneous sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with X-ray Diffraction (XRD), High Angle Annular Dark-Field Imaging (HAADF), Coherent Electron Nanodiffraction (CEND), Annular Dark Field Imaging
(ADF), Thermophilic Aerobic Digestion (TAD), Analytical Electron Microscopy (AEM), and Convergent Beam Electron Diffraction (CBED).
4Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-5
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A.3. Chemical Composition
Technique
Aerosol Time-of-Flight Mass Spectroscopy
(ATOF-MS)
Analytical Electron Microscopy (AEM)1
Atomic Absorption Spectroscopy (AAS)
Auger Electron Microscopy (AES)2
Chemical Force Microscopy (CFM)
Electron Backscattered Diffraction (EBSD)
Electron Paramagnetic Resonance (EPR)
Field Emission Scanning Electron Microscopy
(FE-SEM)3
Flow Field-Flow Fractionation (FIFFF)
Fourier Transform Infrared Spectroscopy (FT-
IR)
Gel Permeation Chromatography (GPC)
High Performance Liquid Chromatography
(HPLC)
Inductively Coupled Plasma Mass
Spectroscopy (ICP-MS)
Mossbauer Spectroscopy (MS)
Nuclear Magnetic Resonance (NMR)
Raman Spectroscopy
Scanning Electron Microscopy (SEM)3
Secondary Ion Mass Spectrometry (SIMS)
Surface Enhanced Raman Spectroscopy
(SERS)
Transmission Electron Microscopy (TEM)4
Ultraviolet/Visible Spectroscopy (UV/Vis)
X-ray Diffraction (XRD)
X-ray Photoelectron Spectroscopy (XPS)
Detection Range
0.32-1 .8 urn
>0.5 nm
ppm range
1-2 nm
>0.1 nm
20-1 00 nm
mM range
1 nm-1 urn
ppm range
5 nm-1 00 urn
ug/mL range
1 ppt-0.1 ppb
mM range
mM range
ppm range
1 nm-1 |jm
1,01 2-1 ,01 6 atom/cm3
Can detect single molecules
>0.1 nm
mM range
1-3 wt%
>1 urn
Advantages
Electron Energy Loss
Spectroscopy (EELS) can be
used (< Zn)
Used in biology
Many modifications to Atomic
Force Microscopy (AFM) tip
Can be used to analyze
paramagnetic samples
Ultra high resolution
Images secondary electrons with
backscatter detector
Can be used to analyze solid or
liquid samples
Used to determine molecular weight
and distribution of polymers
Can be used to analyze solid or
liquid samples
Good temperature range
Can be used to analyze solid or
liquid samples
High resolution
Small sample size
High resolution
Fast
Reveals atomic composition of
layers (1-10 urn)
Disadvantages
Data interpretation can be difficult
Requires high vacuum for sample
preparation
Sample must be soluble in suitable
solvent
Solid-state experiments are more
difficult
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Possible sample degradation
Sensitive to the surface on which
the experiment is conducted
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1 Can be used in combination with Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Scanning Transmission Electron Microscopy (STEM).
2Can be used in combination with Scanning Electron Microscopy (SEM).
3Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
4Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-6
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A.4. Crystal Structure
Technique
Differential Scanning Calorimetry (DSC)
Electron Paramagnetic Resonance (EPR)
Field Emission Scanning Electron Microscopy
(FE-SEM)1
Fourier Transform Infrared Spectroscopy (FTIR)
Nuclear Magnetic Resonance (NMR)
Raman Spectroscopy
Scanning Electron Microscopy (SEM)1
Thermo-Gravimetric Analysis (TGA)
Transmission Electron Microscopy (TEM)2
X-ray Diffraction (XRD)
Detection Range
mg range
mM range
ppm range
mM range
ppm range
1 nm-1 |jm
mg range
>0.1 nm
1-3 wt%
Advantages
Allows the study of phase transitions
Can be used to analyze
paramagnetic samples
Ultra high resolution
Images secondary electrons with
backscatter detector
Can be used to analyze solid or
liquid samples
Can be used to analyze solid or
liquid samples
Good temperature range
Can be used to analyze solid or
liquid samples
High resolution
Allows the study of weight loss in
samples
High resolution
Disadvantages
Data interpretation can be difficult
Requires high vacuum for sample
preparation
Solid-state experiments more
difficult
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1 Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
2Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A.5. Dissolution
Technique
Cross Flow Ultrafiltration (CFUF)
Dialysis
Diffusive gradients in thin films
Voltammetry
Detection Range
1 nm-1 urn
<5nm
nM-uM range
mM-ppm range
Advantages
High speed
High volume
Low concentration
Simple
Concentrating effect helps lower
detection limits
Disadvantages
Not good for high concentrations
Not well defined size fractionation
Not fully quantitative
Separation is based only on size
Titration limits: micromolarto
millimolar lower limits
Bold font indicates most commonly employed techniques.
A-7
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A.6. Heterogeneity
Technique
Atomic Force Microscopy (AFM)
Differential Mobility Analyzer (DMA)1
Field Emission Scanning Electron Microscopy
(FE-SEM)2
Infrared Spectroscopy (IR)
Nuclear Magnetic Resonance (NMR)
Raman Spectroscopy
Scanning Electron Microscopy (SEM)2
Scanning Tunneling Electron Microscopy
(STM)
Transmission Electron Microscopy (TEM)3
Ultraviolet/Visible Spectroscopy (UV/Vis)
Detection Range
>0.1 nm
3 nm-um range
ppm range
mM range
ppm range
1 nm-1 urn
nm in x, y, and z directions
>0.1 nm
mM range
Advantages
Can be used to analyze dry,
moist, and liquid samples
Can be used in combination with
many techniques
Ultra high resolution
Images secondary electrons with
backscatter detector
Can be used to analyze solid or
liquid samples
Can be used to analyze solid or
liquid samples
Good temperature range
Can be used to analyze solid or
liquid samples
High resolution
Allows three-dimensional
characterization
High resolution
Fast
Disadvantages
Artifacts can result from tip
smearing
Possible sample degradation
Requires high vacuum for sample
preparation
Solid-state experiments more
difficult
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A.7. Mass Concentration
Technique
Analytical Electron Microscopy (AEM)1
Chemical Force Microscopy (CFM)
Gravimetrics
Thermal Analysis
Detection Range
>0.5 nm
>0.1 nm
ppb range
mg range
Advantages
Electron Energy Loss
Spectroscopy (EELS) can be
used (< Zn)
Used in biology
Many modifications to Atomic
Force Microscopy (AFM) tip
Precise measurements
Stable
Inexpensive
Disadvantages
Gravity difference measurements
are site dependent and require
calibration
Less efficient than
spectrophotometry
Bold font indicates most commonly employed techniques.
1 Can be used in combination with Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Scanning Transmission Electron Microscopy (STEM).
A-8
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A.8. Melting Point
Technique Detection Range Advantages Disadvantages
Differential Scanning Calorimetry (DSC) mg range
A.9. Particle Number Concentration
Technique Detection Range Advantages Disadvantages
Condensation Particle Counter (CPC)1 5-1,100 nm
Particle Counter >1 urn Meets clean room standards
Bold font indicates most commonly employed technique.
1Can be used in combination with a Differential Mobility Analyzer (DMA).
A.10. Porosity
Technique Detection Range Advantages Disadvantages
Brunauer Emmett Teller (BET) >1,000 m2/g
Differential Mobility Analyzer (DMA)' 3 nm-um range Can be used in combination with Possible sample degradation
many techniques
Transmission Electron Microscopy OW >0.1 nm High resolution Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques (all three techniques are common).
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-9
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A.11. Shape
Technique
Atomic Force Microscopy (AFM)
Detection Range
>0.1 nm
Advantages
Can be used to analyze dry,
moist, and liquid samples
Disadvantages
Artifacts can result from tip
smearing
Confocal Laser Scanning Microscopy (CLSM)
Differential Interference Contrast Microscopy
(DIG)
Can be used to analyze unstained
biological samples
High resolution with no artifacts
Calls for transparent specimen with
refractive index similar to its
surroundings
Expensive
Differential Mobility Analyzer (DMA)1
Dynamic Light Scattering (DLS)
Field Emission Scanning Electron Microscopy
(FE-SEM)2
Flow Field-Flow Fractionation Static Light
Scattering (FIFFF-SLS)
Fluorescence Microscopy (FLM)
Scanning Electron Microscopy (SEM)2
Scanning Transmission Electron
Microscopy (STEM)3
Scanning Tunneling Electron Microscopy
(STM)
Sedimentation Field-Flow Fractionation
Dynamic Light Scattering (SedFFF-DLS)
Transmission Electron Microscopy (TEM)4
Ultracentrifugation (UC)
3 nm-um range
3 nm-um range
1 nm-1 urn
>10nm
1 nm-1 urn
resolution: <0.1 nm
nm in x, y, and z directions
1 nm-1 urn
>0.1 nm
nm range
Can be used in combination with
many techniques
Allows in situ measurement
Fast and simple
Convenient for analyzing
aggregation
Ultra high resolution
Images secondary electrons with
backscatter detector
High resolution
Can be used to analyze low
concentrations (ppm)
Allows three-dimensional
characterization
High resolution
Currently used in carbon nanotubes
Possible sample degradation
Dust particles can ruin
measurement
Higher particle interactions
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
More qualitative than quantitative
Requires homogeneous sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with X-ray Diffraction (XRD), High Angle Annular Dark-Field Imaging (HAADF), Coherent Electron Nanodiffraction (CEND), Annular Dark Field Imaging
(ADF), Thermophilie Aerobic Digestion (TAD), Analytical Electron Microscopy (AEM), and Convergent Beam Electron Diffraction (CBED).
4Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-10
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A.12. Size
Technique
Atomic Force Microscopy (AFM)
Detection Range
>0.1 nm
Advantages
Can be used to analyze dry,
moist, and liquid samples
Disadvantages
Artifacts can result from tip
smearing
Confocal Laser Scanning Microscopy (CLSM)
Differential Interference Contrast Microscopy
(DIG)
Can be used to analyze unstained
biological samples
High resolution with no artifacts
Calls for transparent specimen with
refractive index similar to its
surroundings
Expensive
Differential Mobility Analyzer (DMA)1
Dynamic Light Scattering (DLS)
Field Emission Scanning Electron Microscopy
(FE-SEM)2
Fluorescence Microscopy (FLM)
Scanning Electron Microscopy (SEM)2
Scanning Transmission Electron
Microscopy (STEM)3
Scanning Tunneling Electron Microscopy
(STM)
3 nm-um range
3 nm-um range
>10nm
1 nm-1 urn
resolution: <0.1 nm
nm in x, y, and z directions
Can be used in combination with
many techniques
Allows in situ measurement
Fast and simple
Convenient for analyzing
aggregation
Ultra high resolution
Images secondary electrons with
backscatter detector
High resolution
Can be used to analyze samples
of low concentrations (ppm)
Allows three-dimensional
characterization
Possible sample degradation
Dust particles can ruin
measurement
Higher particle interactions
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Size Exclusion Chromatography (SEC)
5 nm-100 urn
Can be used to determine molecular
weight and distribution of
polymers
Transmission Electron Microscopy (TEM)4
>0.1 nm
High resolution
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with X-ray Diffraction (XRD), High Angle Annular Dark-Field Imaging (HAADF), Coherent Electron Nanodiffraction (CEND), Annular Dark Field Imaging
(ADF), Thermophilic Aerobic Digestion (TAD), Analytical Electron Microscopy (AEM), and Convergent Beam Electron Diffraction (CBED).
4Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-11
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A.13. Size Distribution
Technique
Detection Range
Advantages
Disadvantages
Atomic Force Microscopy (AFM)
Can be used to analyze dry, moist, and
liquid samples
Artifacts can result from tip
smearing
Cross Flow Ultrafiltration (CFUF)
1 nm-1 urn
High speed
Higher volume
Less clogging than piston filtration or
stirred cells
Potential alterations due to increased
particle concentration
Turbulent flow
Large surface exposure
Size fractionation is not well defined
Cross-Flow Filtration (CFF)
Can be used to separate several
compounds based on size
Differential Mobility Analyzer (DMA)1
3 nm-um range ^an ')e used m combination with many
techniques
Possible sample degradation
Dynamic Light Scattering (DLS)
3 nm-|jm range
Allows in situ measurement
Fast and simple
Convenient for analyzing aggregation
Dust particles can ruin
measurement
Higher particle interactions
Field Emission Scanning Electron Microscopy (FE-
SEM)2
Ultra high resolution
Images secondary electrons with
backscatter detector
Requires high vacuum for sample
preparation
Field Flow Fractionation (FFF)3
Flow FFF: 1 nm-1 urn
Sedimentation FFF:
50 nm-1 urn
Good size range
Direct relation between retention time and
size
Experienced operator needed
Flow Field-Flow Fractionation (FIFFF)
1 nm-1 urn
High Performance Liquid Chromatography (HPLC) ug/mL range
Can be used to separate and analyze
several compounds
Hydrodynamic Chromatography (HOC)4
5-1 ,200 nm
Mobile phase interactions
Scanning Electron Microscopy (SEM)2
1 nm-1 |jm
High resolution
Requires high vacuum for sample
preparation
Scanning Mobility Particle Sizer (SMPS)5
3-1 ,000 nm
Particle Concentration Range 20-
1 0,000,000 particles/cc
Higher resolution than DMPS10
Scanning Transmission Electron Microscopy resolution: <0.1 nm
(STEM)6
Can be used to analyze low
concentrations (ppm)
Scanning Tunneling Electron Microscopy (STM) nm in x, y, and z
directions
Allows three dimensional
characterization
Single Particle Mass Spectrometry (SPMS)5
3-1,000 nm
Particle Concentration Range 20-
10,000,000 particles/cc
Higher resolution than DMPS10
Size Exclusion Chromatography (SEC)7
5 nm-100 urn
Simple; good separation efficiency
Can be used to determine molecular
weight and distribution of polymers
Limited size separation range
Small-Angle X-ray Scattering (SAXS)8
5-25 nm
Averaged particle sizes
Shapes, distribution, and surface-to-
volume ratio can be determined.
Can be used to analyze liquids or solids
Transmission Electron Microscopy (TEM)9
>0.1 nm
High resolution
Requires high vacuum for sample
preparation
Ultracentrifugation (UC)
nm range
Currently used in carbon nanotubes
More qualitative than quantitative
Requires homogeneous sample
preparation
A-12
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A.13. Size Distribution (continued)
Technique
Detection Range
Advantages
Disadvantages
Ultrafine Condensation Particle Counter (UCPC) 2.7-10 nm
X-ray Diffraction (XRD)
1-3 wt%
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with Ultraviolet/Visible Spectroscopy (UVA/is) and Inductively Coupled Plasma Mass Spectrometry (I CP-MS) on line; and Atomic Force Microscopy (AFM)
off line.
4Can be used in combination with Ultraviolet/Visible Spectroscopy (UVA/is) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
5A Differential Mobility Analyzer (DMA) can be used in combination.
6Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
7Can be used in combination with UltravioletA/isible Spectroscopy (UVA/is) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
8Ultra-Small Angle X-ray Scattering (USAXS) can be used in combination.
9Can be used in combination with X-ray Diffraction (XRD), High Angle Annular Dark-Field Imaging (HAADF), Coherent Electron Nanodiffraction (CEND), Annular Dark Field Imaging
(ADF), Thermophilic Aerobic Digestion (TAD), Analytical Electron Microscopy (AEM), and Convergent Beam Electron Diffraction (CBED).
10Differential particle mass Spectrometry (DMPS).
A.14. Speciation
Technique
Size Exclusion Chromatography with
Inductively Coupled Plasma Mass
Spectroscopy (SEC-ICP-MS)
Titration
X-ray Absorption Fine Structure (XAFS)
X-ray Diffraction (XRD)
X-ray Photoelectron Spectroscopy (XPS)
Detection Range
uM-mM range
ppm to ppb range
1-3 wt%
>1 urn
Advantages
Element-specific separation
Simple
Nearly all elements have binding
energies in range of X-rays
Reveals atomic composition of
layers (1-10 urn)
Disadvantages
Prevalent human errors
Sample must be soluble
Inadequate for lighter elements
Bold font indicates most commonly employed techniques.
A-13
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A.15. Structure
Technique
Atomic Force Microscopy (AFM)
Field Emission Scanning Electron Microscopy
(FE-SEM)1
Scanning Electron Microscopy (SEM)1
Scanning Transmission Electron
Microscopy (STEM)'
Scanning Tunneling Electron Microscopy
(STM)
Secondary Ion Mass Spectrometry (SIMS)
Small Angle Neutron Scattering (SANS)
Transmission Electron Microscopy (TEM)3
X-ray Diffraction (XRD)
Detection Range
>0.1 nm
1 nm-1 urn
resolution: <0.1 nm
nm in x, y, and z directions
10'MO16 atom/cm3
nm-um range
>0.1 nm
1-3 wt%
Advantages
Can be used to analyze dry,
moist, and liquid samples
Ultra high resolution
Images secondary electrons with
backscatter detector
High resolution
Can be used to analyze low
concentrations (ppm)
Allows three-dimensional
characterization
Small sample size
Can be used to analyze liquids
High resolution
Disadvantages
Artifacts can result from tip
smearing
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Requires high vacuum, which can
degrade sample
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1 Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
2Can be used in combination with X-ray Diffraction (XRD), High Angle Annular Dark-Field Imaging (HAADF), Coherent Electron Nanodiffraction (CEND), Annular Dark Field Imaging
(ADF), Thermophilie Aerobic Digestion (TAD), Analytical Electron Microscopy (AEM), and Convergent Beam Electron Diffraction (CBED).
3Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A.16. Surface Area
Technique
Atomic Force Microscopy (AFM)
Brunauer Emmett Teller (BET)
Differential Mobility Analyzer (DMA)'
Dynamic Light Scattering (DLS)
Field Emission Scanning Electron Microscopy
(FE-SEM)2
Scanning Electron Microscopy (SEM)2
Scanning Tunneling Electron Microscopy
(STM)
Transmission Electron Microscopy (TEM)3
Ultracentrifugation (UC)
Detection Range
>0.1 nm
>1,000m2/g
3 nm-um range
3 nm-um range
1 nm-1 urn
nm in x, y, and z directions
>0.1 nm
nm range
Advantages
Can be used to analyze dry,
moist, and liquid samples
Can be used in combination with
many techniques
Ultra high resolution
Images secondary electrons with
backscatter detector
High resolution
Allows three-dimensional
characterization
High resolution
Currently used in carbon nanotubes
Disadvantages
Artifacts can result from tip
smearing
Possible sample degradation
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
More qualitative than quantitative
Requires homogeneous sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Electron Spectroscopy (ES), Condensation Particle Counter (CPC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES),
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Aerosol Time-of-Flight Mass Spectroscopy (ATOF-MS).
2Can be used in combination with Auger Electron Microscopy (AES) and Energy-Dispersive X-ray Spectroscopy (EDS).
3Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-14
-------
A.17. Surface Charge
Technique
Detection Range
Advantages
Disadvantages
Capillary Electrophoresis (CE)
25-100 pm diameter
Run time is short
Zeta Potential
5 nm-10 |jm
Bold font indicates most commonly employed technique.
A.18. Surface Chemistry
Technique
Analytical Electron Microscopy (AEM)1
Atomic Force Microscopy (AFM)
Auger Electron Microscopy (AES)2
Chemical Force Microscopy (CFM)
Differential Scanning Calorimetry (DSC)
Electron Paramagnetic Resonance (EPR)
Field Emission Scanning Electron Microscopy
(FE-SEM)3
Detection Range
>0.5 nm
>0.1 nm
1-2 nm
>0.1 nm
mg range
mM range
Advantages
Can be used to analyze dry,
moist, and liquid samples
Used in biology
Many modifications to AFM tip
Allows the study of phase transitions
Can be used to analyze
paramagnetic samples
Ultra high resolution
Images secondary electrons with
backscatter detector
Disadvantages
Artifacts can result from tip
smearing
Data interpretation can be difficult
Requires high vacuum for sample
preparation
Flow Field-Flow Fractionation (FIFFF)
Fourier Transform Infrared Spectroscopy (FT-
IR)
High Performance Liquid Chromatography
(HPLC)
Nuclear Magnetic Resonance (NMR)
Raman Spectroscopy
Scanning Electron Microscopy (SEM)3
Scanning Tunneling Electron Microscopy
(STM)
Secondary Ion Mass Spectrometry (SIMS)
Size Exclusion Chromatography (SEC)4
ppm range
ug/mL range
mM range
ppm range
1 nm-1 urn
nm in x, y, and z directions
1012-1016 atom/cm3
5 nm-1 00 urn
Can be used to analyze solid or
liquid samples
Can separate and analyze several
compounds
Can be used to analyze solid or
liquid samples
Good temperature range
Can be used to analyze solid or
liquid samples
High resolution
Allows three-dimensional
characterization
Small amount of sample
necessary for analysis
Good separation efficiency
Solid-state experiments more
difficult
Requires high vacuum for sample
preparation
Requires high vacuum for sample
preparation
Possible sample degradation
Limited size separation range
Simple
Can be used to determine molecular
weight and distribution of
polymers
Surface Enhanced Raman Spectroscopy
(SERS)
Can detect single molecules
Sensitive to the surface on which
the experiment is conducted
Thermo-Gravimetric Analysis (TGA)
mg range
Allows the study of weight loss in
samples
A-15
-------
A.18. Surface Chemistry (continued)
Technique
Transmission Electron Microscopy (TEM)5
Ultraviolet/Visible Spectroscopy (UV/Vis)
X-ray Photoelectron Spectroscopy (XPS)
Zeta Potential
Detection Range
>0.1 nm
mM range
>1 pm
5 nm-10 |jm
Advantages
High resolution
Fast
Can be used to determine atomic
composition of layers (1-1 0 urn)
Disadvantages
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Transmission Electron Energy Loss Spectroscopy (EELS) (0.1 nm
1-2 nm
mg range
ppm range
mM range
ppm range
nm in x, y, and z directions
mg range
>0.1 nm
mM range
>1 urn
Advantages
Can be used to analyze dry,
moist, and liquid samples
Allows the study of phase transitions
Can be used to analyze solid or
liquid samples
Can be used to analyze solid or
liquid samples
Good temperature range
Can be used to analyze solid or
liquid samples
Allows three-dimensional
characterization
Allows the study of weight loss in
samples
High resolution
Fast
Can be used to determine the
atomic composition of layers
(1-10 |jm)
Disadvantages
Efficiencies decrease as particle
gets smaller
Artifacts can result from tip
smearing
Solid-state experiments more
difficult
Requires high vacuum for sample
preparation
Bold font indicates most commonly employed techniques.
1Can be used in combination with Scanning Electron Microscopy (SEM).
2Can be used in combination with Electron Energy Loss Spectroscopy (EELS) and Energy-Dispersive X-ray Spectroscopy (EDS).
A-16
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Appendix A References
AZoNano. (2010). Atomic force microscopes. Available online at http://www.azonano.com/nanotechnology-
equipment.asp?cat=9 (accessed May 17, 2010).
Becker. JS. (2008). Inorganic mass spectrometry: Principles and applications. West Sussex, England: Wiley-
Interscience.
Buchan Lawton (Buchan Lawton Parent Ltd). (2010). Broker technologies: Our product lines. Available online
at http://www.bruker.com/product.html (accessed May 17, 2010).
Coulson. JM: Richardson. JF: Marker. JH: Backhurst JR. (2002). Coulson and Richardson's chemical
engineering: Particle technology and separation processes (5th ed.). Woburn, MA: Butterworth-
Heinemann.
General Electric Company. (2010). Cross flow filtration systems. Available online at
http://www5.gelifesciences.com/aptrix/upp01077.nsf/Content1)ioprocess~filtrationl~svstems complete
(accessed May 17,2010).
Imaging Technology Group. (2010). Environmental scanning electron microscope (ESEM). Available online at
http://itg.beckman.illinois.edu/microscopv suite/equipment/ESEM/ (accessed May 17, 2010).
NCEM (National Center for Electron Microscopy). (2010). Analytical electron microscopy. Available online at
http://ncem.lbl.gov/frames/aem.htm (accessed May 17, 2010).
Oberdorster. G: Maynard. A: Donaldson. K: Castranova. V: Fitzpatrick. J: Ausman. K: Carter. J: Karn. B:
Krevling. W: Lai. D: Olin. S: Monteiro-Riviere. N: Warheit D: Yang. H. (2005). Principles for
characterizing the potential human health effects from exposure to nanomaterials: Elements of a screening
strategy. Part Fibre Toxicol 2:1-35. http://dx.doi.org/10.1186/1743-8977-2-8
PerkinElmer. (2010). Technologies. Available online at
http://www.perkinelmer.com/Technologies/default/catl/NAV_04 TCH Technologies 001/Country/USA/
Ecommerce/Yes/Dealer/No (accessed May 17, 2010).
Quantachromelnstruments. (2010). Surface area and pore size by gas sorption. Available online at
http://www.quantachrome.com/product listing/surface area analvzers.html?gclid=CPbd9quvjZwCFR2dn
Aodj34CZQ (accessed May 17, 2010).
Shimadzu Scientific Instruments. (2010). Laboratory instruments. Available online at
http://www.ssi.shimadzu.com/products/products main.cfm?maincategorv=Laboratorv%20Instruments
(accessed May 17,2010).
Tiede. K: Boxall ABA: Tear. SP: Lewis. J: David. H: Hassellov. M. (2008). Detection and characterization of
engineered nanoparticles in food and the environment. Food Addit Contam Part A Chem Anal Control
Expo Risk Assess 25: 795-821. http://dx.doi.org/10.1080/02652030802007553
TSI Inc. (Trust Science Innovation Incorporated). (2010). Aerosol time-of-flight mass spectrometers. Available
online at http://www.tsi.com/en-1033/segments/chemical analysis/2194/aerosol time-of-
flightmass spectrometers.aspx (accessed May 17, 2010).
Tsuda. K: Tanaka. M. (1996). Interferemetry by coherent convergent-beam electron diffraction. J Electron
Microsc (Tokyo) 45: 59-63.
Varian (Varian Inc.). (2010). Products. Available online at http://www.varianinc.com/cgi-
bin/nav?products/index&cid=LLPLMOLIFQ (accessed May 17, 2010).
Zuin. S: Pojana. G: Marcomini. A. (2007). Effect-oriented physicochemical characterization of nanomaterials.
In NA Monteiro-Riviere; CL Tran (Eds.), Nanotoxicology: Characterization, dosing, and health effects (pp.
19-57). New York, NY: Informa Healthcare.
A-17
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Appendix B. Summary of Ecological
Effects Studies of Nano-Ag
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Appendix B. Summary of Ecological
Effects Studies of Nano-Ag
B.1. Study Selection Criteria B-4
B.2. Summary of Nano-Ag Effects in Microorganisms (Excluding Algae) B-5
Bae et al. (2010) Bacterial cytotoxicity of the silver nanoparticle related to physicochemical metrics and agglomeration
properties. B-5
Bradford et al. (2009) Impact of silver nanoparticle contamination on the genetic diversity of natural bacterial assemblages in
estuarine sediments. B-6
Choi and Hu (2008) Size dependent and reactive oxygen species related nanosilver toxicity to nitrifying bacteria. B-7
Choi et al. (2008) The inhibitory effects of silver nanoparticles, silver ions, and silver chloride colloids on microbial growth. B-8
Choi et al. (2009) Role of sulfide and ligand strength in controlling nanosilver toxicity. B-9
Dasari and Hwang (2010) The effect of humic acids on the cytotoxicity of silver nanoparticles to a natural aquatic bacterial
assemblage. B-10
El Badawy et al. (2011) Surface charge-dependent toxicity of silver nanoparticles. B-11
Gao et al. (2011) Effects of engineered nanomaterials on microbial catalyzed biogeochemical processes in sediments. B-12
Hwang et al. (2008) Analysis of the toxic mode of action of silver nanoparticles using stress-specific bioluminescent bacteria. B-13
Ivask et al. (2010) Profiling of the reactive oxygen species-related ecotoxicity of CuO, ZnO, Ti02, silver and fullerene
nanoparticles using a set of recombinant luminescent Escherichia coli strains: differentiating the impact of particles and
solubilised metals. B-14
Jin et al. (2010) High throughput screening of silver nanoparticle stability and bacterial inactivation in aquatic media: influence
of specific ions. B-15
Khan et al. (2011) Silver nanoparticles tolerant bacteria from sewage environment. B-16
Kim et al. (2009) Antifungal activity and mode of action of silver nano-particles on Candida albicans. B-17
Kvitek et al. (2008) Effect of surfactants and polymers on stability and antibacterial activity of silver nanoparticles (NPs). B-18
Lok et al. (2006) Proteomic analysis of the mode of antibacterial action of silver nanoparticles. B-19
Martinez-Gutierrez et al. (2010) Synthesis, characterization, and evaluation of antimicrobial and cytotoxic effect silver and
titanium nanoparticles. B-20
Morones et al. (2005) Bactericidal effect of silver nanoparticles. B-21
Pal et al. (2007) Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? A study of the
Gram-negative bacterium Escherichia coli. B-22
Saulou et al. (2010) Synchrotron FTIR microspectroscopy of the yeast Saccharomyces cerevisiae after exposure to plasma-
deposited nanosilver-containing coating. B-23
Shrivastava et al. (2007) Characterization of enhanced antibacterial effects of novel silver nanoparticles. B-24
Sinhaetal. (2011) Interaction and nanotoxic effect of ZnO and Ag nanoparticles on mesophilic and halophilic bacterial cells. B-25
Sondi and Salopek-Sondi (2004) Silver nanoparticles as antimicrobial agent: a case study on E. coli as a model for Gram-
negative bacteria. B-26
Sotiriou and Pratsinis (2010) Antibacterial activity of nanosilver ions and particles. B-27
B.3. Summary of Nano-Ag Effects in Algae B-28
Griffitt et al. (2008) Effects of particle composition and species on toxicity of metallic nanomaterials in aquatic organisms. B-28
Park et al. (201 Ob) Selective inhibitory potential of silver nanoparticles on the harmful cyanobacterium Microcystis aeruginosa. B-29
Miao et al. (2009) The algal toxicity of silver engineered nanoparticles and detoxification by exopolymeric substances. B-30
Navarro et al. (2008) Toxicity of silver nanoparticle to Chlamydomonas reinhardtii. B-31
B.4. Summary of Nano-Ag Effects in Aquatic Invertebrates B-32
Allen et al. (2010) Effects from filtration, capping agents, and presence/ absence of food on the toxicity of silver nanoparticles
to Daphnia magna. B-32
B-1
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Gao et al. (2009) Dispersion and toxicity of selected manufactured nanomaterials in natural river-water samples: effects of
water chemical composition. B-33
Griffitt et al. (2008) Effects of particle composition and species on toxicity of metallic nanomaterials in aquatic organisms. B-34
Li et al. (201 Ob) Comparative toxicity study of Ag, Au, and Ag-Au bimetallic nanoparticles on Daphnia magna. B-35
Kvitek et al. (2009) Initial study on the toxicity of silver nanoparticles (NPs) against Paramecium caudatum. B-36
Nair et al. (2011) Differential expression of ribosomal protein gene, gonadotrophin releasing hormone gene and Balbiani ring
protein gene in silver nanoparticles exposed Chironomus riparius. B-37
Ringwood et al. (2010) The effects of silver nanoparticles on oyster embryos. B-38
B.5. Summary of Nano-Ag Effects in Aquatic Vertebrates B-39
Asharani et al. (2008) Toxicity of silver nanoparticles in zebrafish models. B-39
Bar-Han et al. (2009) Toxicity assessments of multisized gold and silver nanoparticles in zebrafish embryos. B-40
Bilberg et al. (2010) Silver nanoparticles and silver nitrate cause respiratory stress in Eurasian perch (Perca fluviatilis). B-41
Chae et al. (2009) Evaluation of the toxic impact of silver nanoparticles on Japanese medaka (Oryzias latipes). B-42
Choi etal. (2010) Induction of oxidative stress and apoptosis by silver nanoparticles in the liver of adult zebrafish. B-43
Farkas et al. (2011) Uptake and effects of manufactured silver nanoparticles in rainbow trout (Oncorhynchus mykiss) gill cells. B-44
Griffitt et al. (2009) Comparison of molecular and histological changes in zebrafish gills exposed to metallic nanoparticles. B-45
Hinther et al. (2010) Nanometals induce stress and alter thyroid hormone action in amphibia at or below North American water
quality guidelines. B-46
Kennedy et al. (2010) Fractionating nanosilver: importance for determining toxicity to aquatic test organisms. B-47
Laban et al. (2009) The effects of silver nanoparticles on fathead minnow (Pimephales promelas) embryos. B-48
Lee et al. (2007) In vivo imaging of transport and biocompatibility of single silver nanoparticles in early development of
zebrafish embryos. B-49
Scown et al. (2010) Effects of aqueous exposure to silver nanoparticles of different sizes in rainbow trout. B-50
Shahbazzadeh et al. (2009) The effects of nanosilver (Nanocid®) on survival percentage of rainbow trout (Oncorhynchus
mykiss). B-51
Wu et al. (2010) Effects of silver nanoparticles on the development and histopathology biomarkers of Japanese medaka
(Oryzias latipes) using the partial-life test. B-52
Yeo and Kang (2008) Effects of nanometer-sized silver materials on biological toxicity during zebrafish embryogenesis. B-53
Yeo and Pak (2008) Exposing zebrafish to silver nanoparticles during caudal fin regeneration disrupts caudal fin growth and
p53 signaling. B-54
B.6. Summary of Nano-Ag Effects on Terrestrial Plants B-55
Babu et al. (2008) Effect of nano-silver on cell division and mitotic chromosomes: a prefatory siren. B-55
Kumari et al. (2009) Genotoxicity of silver nanoparticles in Allium cepa. B-56
Rostami and Shahstavar (2009) Nano-silver particles eliminate the in vitro contamination of olive 'Mission' explants. B-57
Stampoulis et al. (2009) Assay-dependent phytotoxicity of nanoparticles to plants. B-58
B.7. Summary of Nano-Ag Effects on Terrestrial Invertebrates B-59
Ahamed et al. (2010) Silver nanoparticles induced heat shock protein 70, oxidative stress and apoptosis in Drosophila
melanogaster. B-59
DNA Damage. Levels of p53 and p38 were significantly increased in treated larvae from those of the control in a time- and
dose-related manner, indicating significant DNA damage caused by exposure to nano-Ag. B-59
Apoptosis. Activities of caspase-3 and caspase-9 were significantly increased in treated larvae from the control levels,
suggesting that nano-Ag exposure is involved in the apoptotic pathway. B-59
Heckmann et al. (2011) Limit-test toxicity screening of selected inorganic nanoparticles to the earthworm Eisenia fetida. B-60
Lapied et al. (2010) Silver nanoparticle exposure causes apoptotic response in the earthworm Lumbricus terrestris
(Oligochaeta). B-61
Meyer et al. (2010) Intracellular uptake and associated toxicity of silver nanoparticles in Caenorhabditis elegans. B-62
Roh et al. (2009) Ecotoxicity of silver nanoparticles on the soil nematode Caenorhabditis elegans using functional
ecotoxicogenomics. B-63
Sap-lam et al. (2010) UV irradiation-induced silver nanoparticles as mosquito larvicides. B-64
Shoults-Wilson et al. (2011) Evidence for avoidance of Ag nanoparticles by earthworms (Eisenia fetida). B-65
B-2
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B.8. Summary of Nano-Ag Effects on Non-mammalian Terrestrial Vertebrates B-66
Grodzik and Sawosz (2006) The influence of silver nanoparticles on chicken embryo development and bursa of Fabricius
morphology. B-66
Sawosz et al. (2007) Influence of hydrocolloidal silver nanoparticles on gastrointestinal microflora and morphology of
enterocytes of quail. B-67
Appendix B References B-68
B-3
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B.1. Study Selection Criteria
The process used to select studies for inclusion in the ecological effects tables differed for each
category of organism based on the quantity and quality of available ecotoxicological data. In general,
literature searches were conducted for each category of organism (e.g., bacteria and fungi, aquatic plants,
terrestrial vertebrates). To reflect the most current state of the science, the tables in this appendix include
only studies published in or after 2000. The information presented here is up to date as of March 1, 2011,
when the last broad literature search to identify new information was conducted. For those categories for
which a substantial amount of ecotoxicity data was available (e.g., bacteria, fish), studies examining
relevant endpoints were selected based on the data quality and the relative contribution of the results to
the state of the science (determined largely by examining the number of articles in which the study was
later cited). Also, studies were included if the investigators examined an endpoint for which there was
otherwise little information, used a novel technique to assess toxicity, or compared the relative toxicities
of nano-Ag possessing different sets of characteristics (e.g., nano-Ag of different sizes, surface areas,
shapes). For categories with very little available ecotoxicological information (e.g., terrestrial organisms),
all identified studies were included unless they were judged to be of poor quality.
Information in the tables is organized to take into account the minimum requirements for
physicochemical characterization proposed by the Minimum Information for Nanomaterial
Characterization (MINChar) Initiative (MINCharlnitiative. 2008). The limited understanding of nano-Ag
toxicity and its mechanisms, and the equivocal nature of some studies that give conflicting results,
preclude the direct comparison of results for many studies. To emphasize that caution is warranted in
interpreting the results of the available nano-Ag toxicological studies, these tables are organized in a way
that emphasizes each study's relevant attributes in the context of this case study - especially
characterization of the nano-Ag used in the study - rather than to facilitate direct comparison of results
among studies.
-------
B.2. Summary of Nano-Ag Effects in Microorganisms
(Excluding Algae)
Bae et al. (2010) Bacterial cytotoxicity of the silver nanoparticle related to physicochemical metrics
and agglomeration properties.
Test Species
Escherichia coll (strain ATCC 8739)
Material
Nano-Ag powder (Sigma-Aldrich) prepared as a suspension in an aqueous phase via ultrasonication with deionized water and
tetrahydrofuran.
Shape: Polyhedral (determined using TEM) Solubility: Not reported
Composition: Not reported Surface Area: ca 1,255 nm2 (determined using image
Crystal Structure: Ag (111), (200), and (220) (determined analysis)
using TEM) Surface Treatment: Not reported
Average Size: 38 ± 1.4 nm (Determined using HR-TEM); 43 Surface Charge: Not reported
± 3.2 nm (Determined using DLS)
Size Distribution: <150 nm (reported by manufacturer)
Protocol
Exposure Duration: 30 minutes Exposure Concentrations: 0.2, 0.4, 0.6, 0.8, and 1.0 mg/L.
Endpoints: Activity and morphological abnormalities Exposure Medium: Phosphate-buffered saline
Bacterial Density: 5 xlO5 CFU/mL
Methods: Toxicity was evaluated for three controlled parameters at several ionic ratios, average hydrodynamic particle
diameters, and total Ag and AgNOs and was measured by the extent of inactivation of E. coli by viable cell count. Sensitivity for
toxicity and agglomeration was determined using the principle of the tornado diagram whereby positive and negative changes
relative to the baseline were represented by a bar graph.
Study Outcome
The rate of agglomeration was dependent on particle size rather than total Ag concentration of Ag+ ratio. Agglomeration rate
increased rapidly with particle sizes <50 nm. Total Ag concentration was the most important determinant of toxicity, rather than
size distribution or ionic ratio. Sensitivity to agglomeration was inversely correlated with toxicity.
Microbial Activity. The inactivation of E. coli was found to be dependent on the concentration of nano-Ag and Ag-. 1.0 mg/L
nano-Ag caused 3.9 log inactivation, whereas Ag+caused 3.6 log inactivation at the same total Ag concentration. Inactivation was
also a function of the Ag+ ion ratio. The log inactivation of E. coli increased for an ionic ratio in the range of 5-24% (ionic ratio),
while ionic ratios beyond 24% were associated with depressed toxicity. In terms of particle size, at 0.8 mg/L, log inactivation
increased in the range of 21-323 nm diameter and decreased in the range of 4-21 nm. At this concentration inactivation was
depressed more strongly in relation to ionic ratio than size. At 0.4 mg/L, microbial activity was independent of nano-Ag size.
Depression in inactivation effect was correlated with changes in the agglomeration rate for each of the properties for nano-Ag
(depressed significantly in the case of a high agglomeration rate: a high ionic ratio and small sized particles induced higher
agglomeration rates). In the inactivation test, factors in order of importance were: dosage, ionic ratio, and size.
Morphological Abnormalities. Structural abnormalities included partial loss of the outer membrane, localized or complete
separation of the cytoplasm from the cell wall, and cellular degradation.
B-5
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Bradford et al. (2009) Impact of silver nanoparticle contamination on the genetic diversity of natural
bacterial assemblages in estuarine sediments.
Test Species
Natural bacterial assemblages
Material
Commercial nano-Ag (form not reported) supplied by Sigma-Aldrich (location not reported).
Shape: Assumed to be spherical (not verified experimentally) Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 58.6 ± 18.6 nm (determined using TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 30 days Exposure Media: Estuarine sediment and water from Tamar
Endpoints: Prokaryotic abundance and genetic diversity Estuary in Plymouth Sound ("St John's Lake" mud flats
Exposure Concentrations: 0, 25, and 1,000 ug/L OSGB grid ref SX412539)
Bacterial Density: Not reported (natural assemblage)
Methods: To achieve final concentrations of 25 and 1,000 ug/L in the experimental tanks, daily doses 1/20*1 of the final
concentration were added for 20 days to estuarine water overlying estuarine sediment, followed by 10 days in which no dose was
administered. Prokaryotic abundance in the water column of each of the experimental tanks was determined using a Becton
Dickenson flow cytometer for 10-mL subsamples from experimental tanks. Clades were defined on cytogram plots of side scatter
vs. green fluorescence to define high and low nucleic acid cells. Environmental DNA was extracted from sediment samples and a
two-step nested PCR-denaturing gradient gel electrophoresis (DGGE) approach, using PCR primers specific to the "phylum
Bacteria," was adopted to assess bacterial diversity. Fragments of the 16S rRNAgene were amplified from the environmental
DNA. DGGE profiles of PCR-amplified 16S rRNA gene fragments were converted to binary (presence/absence) data and
analyzed using analysis of similarities.
Study Outcome
Abundance. Flow cytometric analysis of samples from the overlying water revealed that mean prokaryotic cell counts did not
change significantly between treatments over time but were highly correlated (p = 0.725), indicating that bacterial and archaeal
abundance in the water was not affected by the presence of nano-Ag.
Diversity. Independent of the nano-Ag-dosing, there were no changes in bacterial diversity in the surface of the sediment over
the 30-day exposure. DGGE-PCR results for sediment samples taken at the start and finish of the dosing period (Day 1-20) for
the control tank and the 1,000 ug/L tank differed slightly but significantly (p = 0.04). However, similarity profile permutation
analysis revealed that most of the clustering of the bacterial diversity in these samples could have arisen by chance.
B-6
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Choi and Hu (2008) Size dependent and reactive oxygen species related nanosilver toxicity to
nitrifying bacteria.
Test Species
Nitrifying bacteria (species not reported)
Material
Nano-Ag synthesized using AgNOs by varying the molar ratios (R) of Bhk to Ag- due to changes in NaBhU concentration.
Shape: Not reported Size Distribution: 5-70 nm
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 9 ± 5 nm (R = 0.1), 15 ± 9 nm (R = 0.2), Surface Treatment: Capped with polyvinyl alcohol (PVA)
14 ± 6 nm (R = 0.36), 12 ± 4 nm (R = 0.6), or 21 ± 14 nm Surface Charge: Not reported
(R = 1.2) (determined using TEM)
Protocol
Exposure Duration: 30 minutes Exposure Concentrations: 0.05-1 mg Ag/L
Endpoints: Growth and reactive oxygen species (ROS) Exposure Medium: Nitrifying biomass from tank reactor
generation Bacterial Density: Not reported
Methods: Growth inhibition of nitrifying bacteria was inferred from oxygen uptake rates due to ammonia oxidation and measured
using batch extant respirometric assay. ECsos were determined using a saturation-type biological toxicity model. Intracellular
ROS concentrations were determined using fluorescence assays following exposure to nano-Ag with an average size of 15 nm.
Photocatalytic ROS concentrations were determined using the same method but in the absence of nitrifying cultures to determine
exogenous influence of ROS generation.
Study Outcome
Growth Inhibition.
ECso (nano-Ag): 0.14 mg/L ECso (silver chloride [AgCI] colloid): 0.25 mg/L ECso (Ag+): 0.27 mg/L
ROS Generation. Exposure to nano-Ag resulted in an increase of intracellular ROS concentrations, which correlated strongly
with the degree of growth inhibition (R2 = 0.86). Photocatalytic ROS concentrations did not correlate strongly with observed
inhibition and were therefore not deemed a good predictor of growth inhibition by nano-Ag.
B-7
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Choi et al. (2008) The inhibitory effects of silver nanoparticles, silver ions, and silver chloride
colloids on microbial growth.
Test Species
Autotrophic bacteria (nitrifying; species not reported) and heterotrophic bacteria (Escherichia coli PHL628-gfp)
Material
Nano-Ag synthesized through reduction of AgNOs with NaBhU.
Shape: Polydisperse (spherical and ellipsoidal)
Composition: Not reported
Crystal Structure: Not reported
Average Size: 14 ± 6 nm (determined using STEM)
Protocol
Exposure Duration: 24 hours
Endpoint: Growth
Exposure Concentrations: 0.1-1 mgAg/L
Size Distribution: 10-40 nm (determined using STEM)
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Capped with polyvinyl alcohol (PVA)
Surface Charge: Not reported
Exposure Media: Mixed liquor from sludge tank reactor
(autotrophic) or BBL™ medium containing Gelysate peptone
and beef extract (heterotrophic)
Bacterial Density: Not reported
Methods: Antibacterial activity of nano-Ag was assessed using LIVE/DEAD Baclight™ bacterial viability kit. The degree of growth
inhibition in autotrophic bacteria was inferred from oxygen uptake rates due to ammonia oxidation and measured using a batch
extant respirometric assay. The degree of growth inhibition in heterotrophic bacteria was determined using an automated
microtiter assay using hourly fluorescence intensity measurements to derive a microbial growth rate.
Study Outcome
Autotrophic Bacterial Growth. At 1 mg Ag/L in the nitrifying suspension, nano-Ag inhibited growth by 86%, while Ag+ and AgCI
colloids inhibited growth by 42% and 46%, respectively.
Heterotrophic Bacterial Growth. ICso = 4.0 uM. No inhibitory effect was observed at concentrations below 1 uM. The inhibitory
effect of nano-Ag increased to 55% at 4.2 uM (~0.5mg/LAg), while Ag+and AgCI colloids inhibited growth by 100% and 66%,
respectively. At 100 mg Ag/L, nano-Ag inhibited growth completely.
B-8
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Choi et al. (2009) Role of sulfide and ligand strength in controlling nanosilver toxicity.
Test Species
Nitrifying bacteria (species not reported)
Material
Nano-Ag suspension synthesized through reduction of AgNOs with NaBH and capped with polyvinyl alcohol (PVA); in suspension
with Ag+.
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 15 ± 9 nm (method not reported) Surface Treatment: PVA-capped
Surface Charge: Not reported
Protocol
Exposure Duration: 300 seconds (inhibition test) or 8 or 18 Exposure Media: Nitrifying enriched culture to which MOPS
hours (Ag2S stability test) was added
Endpoints: Enzyme inhibition and uptake Bacterial Density: Nitrifying biomass concentrations of
Exposure Concentrations: 1 mg Ag/L 540 mg/L chemical oxygen demand (COD) or 210 mg/L COD
Methods: Following nano-Ag exposure, inhibition of ammonia monooxygenase (AMO), hydroxylamine oxidoreductase (HAD),
and nitrite oxidoreductase (NOR), three critical enzymes involved in nitrification, was calculated by relative oxygen uptake rate
after the addition of aliquots of ammonium, hydroxylamine, and nitrate to the respirometric bottles. Effects of ligands on nano-Ag
toxicity were determined through a one-time addition of Ag-ligand complexes as well as sequential addition of a specific ligand
followed by nano-Ag to the nitrifying cultures. The ligands tested were chloride, sulfate, phosphate, EDTA4~, and sulfide. Toxicity
was determined using a respirometric assay which also determined stability of Ag2S complexes made from nano-Ag and sulfide.
Nano-Ag attachment to cells was determined using a back-scattered electron detector coupled with a secondary electron
detector, and elemental composition of specimens was determined using energy dispersive X-ray spectroscopy.
Study Outcome
Inhibition of Enzyme Activity. AMO was the most sensitive of the three enzymes tested when exposed to both nano-Ag and
Ag- at nitrifying biomass concentrations of 540 mg/L COD.
Influence of Ligand Complexation. At 1 mg Ag/L and nitrifying biomass concentrations of 210 mg/L COD, nano-Ag inhibited
nitrification by 100%. A10 uM concentration of sulfide reduced nano-Ag toxicity by 80%, while the other ligands also reduced
toxicity, but to a lesser degree. Additional phosphate concentrations up to 0.3 mM had little effect on toxicity, while chloride
concentrations of 2.8 mM reduced nano-Ag toxicity up to 20%, and a sulfide concentration of 15 uM reduced nano-Ag toxicity
from 86% to 15%.
Adsorption to Cell Floes. Nano-Ag embeds in cell floes. When sulfide was added prior to nano-Ag exposure, embedding largely
decreased.
B-9
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Dasari and Hwang (2010) The effect of humic acids on the cytotoxicity of silver nanoparticles to a
natural aquatic bacterial assemblage.
Test Species
Natural aquatic bacterial assemblages (Species not determined)
Material
Nano-Ag synthesized using citrate reduction method
Shape: Spherical (determined using TEM)
Composition: Not reported
Crystal Structure: Not reported
Average Size: 15 - 25 nm (determined using TEM)
Protocol
Exposure Duration: 2 hours
Endpoints: Bacterial viability cytotoxicity
Exposure Concentrations: 0, 2.5, and 5.0 pM
Size Distribution: Not reported
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Medium: Standard terrestrial humic acid (HA)
from the Sigma Aldrich (St. Louis, MO); standard Suwannee
river humic acid from International Humic Substances
Society (St. Paul, MN); both in 1 mM sodium phosphate
buffer
Bacterial Density: 0,10, 20, and 40 ppm HA
Methods: HA solutions were prepared in 1 mM sodium phosphate buffer. Nano-Ag was synthesized and centrifuged for spherical
geometry. In a 4-way factorial design experiment the independent variables identified were: HA concentration, nano-Ag
concentration, HA source, and light effect. The HA solutions were prepared and incubated for a total of 12 treatments. TEM
measurements were performed on 10 pL samples that were dried overnight. A cell permanent ROS indicator was used to detect
ROS production in bacterial cells. After ROS results were obtained, silver ion concentration was measured in select groups by
ICP-OES.
Study Outcome
Exposure to concentrations of 2.5 and 5.0 pM nano-Ag alone decreased viability of bacteria by 48-89% and 65-84%,
respectively, when compared to the control group and exposed under darkened conditions. ROS production caused in the dark
was negligible (with the exception of 5.0 pM nano-Ag); therefore, the observed nano-Ag toxicity in the dark was attributed to
mechanisms other than the production of ROS. Bacterial viability was inhibited more in the light exposure groups than in the
darkness exposure groups; shrinkage in the size of cells occurred after they were exposed to sunlight irradiation. Larger
reductions in bacterial viability count were observed in the combined treatment of HA and nano-Ag under light exposures; the
light exposure inhibited viability more than the darkness exposure. There was a statistically significant difference in the influence
on bacterial viability imposed by the type of HA used. When exposed to the terrestrial HA, viability was statistically significantly
affected by each variable (i.e., light/darkness, HA concentration, and nano-Ag concentration) independently, and by some of the
two-way or three-way interactions among variables. When exposed to the Suwannee River HA, viability was also statistically
significantly affected by the independent variable and some two-way interactions, but no three-way interactions were statistically
significant.
B-10
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El Badawy et al. (2011) Surface charge-dependent toxicity of silver nanoparticles.
Test Species
Bacillus spp. (Gram-positive bacteria) obtained from Interlab Supply
Material
Synthesis of uncoated (H2-AgNP), citrate-coated (citrate-AgNP), and branched polyethylene!mine (BPEI)-coated nano-Ag (BPEI-
AgNP) suspensions described in a previous study (El Badawyetal., 2010). Polyvinylpyrrolidone (PVP)-coated nano-Ag (PVP-
AgNP) synthesized by modified Lee and Meisel procedure. All characteristics of synthesized nano-Ag are reported in author's
previous research.
Shape: Spherical (determined using TEM)
Composition: Not reported
Crystal Structure: Not reported
Average Size: 18 nm as prepared/immediately after
purification, 17 nm after 3 weeks (hb-AgNP); 10 nm as
prepared/immediately after purification, 11 nm after 3 weeks
(citrate-AgNP); 12 nm as prepared/immediately after
purification/after 3 weeks (PVP-AgNP); 10 nm as
prepared/immediately after purification/after 3 weeks (BPEI-
AgNP)
Protocol
Exposure Duration: 120 ± 2 hours (02 consumption)
Endpoints: Oxygen consumption and mortality
Exposure Concentrations: Not reported
Size Distribution: Not reported
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: uncoated, citrate-coated, PVP-coated,
or BPEI-coated
Surface Charge: ~ -22.5 mV (uncoated); ~ -39 mV (citrate-
coated); ~ -11 mV (PVP-coated); ~ 40 mV(BPEI-coated)
(determined using a Zetasizer Nanoseries; average
measurements for post-purification and 3 weeks post-
purification)
Exposure Medium: BODs test media
Bacterial Density: Not reported - three replications per test
concentration
Methods: A 5-day oxygen consumption test was carried out on Bacillus spp., and the live/dead technique was used as a rapid
screening method to evaluate toxicity. Oxygen consumption was determined by method 2510 B (Eaton etal., 2005) Dissolved
oxygen was measured by Thermo Scientific Orion DO probe. Microbial live/dead measurements were obtained using BacLight
live/dead kit with two-color fluorescence assay and fluorescence spectroscopy.
Study Outcome
Oxygen Consumption and Live/Dead Test. A direct correlation between toxicity and surface charge was observed. The more
negatively charged citrate-capped nano-Ag particles were the least toxic, whereas the positively charged BPEI-capped nano-Ag
particles were the most toxic (both in terms of oxygen consumption and in the live/dead test). The surface charge of the citrate-
capped nanoparticles was assumed to be less toxic due to the fact that the Bacillus spp. has been shown to have a similar
charge under test conditions (caused by the carboxyl, phosphate, and amino groups on the cellular membrane) leading to
repulsion between the two negative charges. As the negative zeta potential gradually decreased (with different capping agents),
the repulsion, or electrostatic barrier, is reduced causing a higher degree of toxicity. No statistical significance was reported for
this study.
B-11
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Gao et al. (2011) Effects of engineered nanomaterials on microbial catalyzed biogeochemical
processes in sediments.
Test Species
Unspecified sediment microorganisms (nitrate-reducing bacteria)
Material
Nano-Ag from Quantum Spheres, Inc. (Santa Ana, CA) produced by gas-phase condensation. A suspension (initial concentration
200 mg/L) was prepared by shaking in Nanopure water at room temperature for 28 hours followed by filtration.
Shape: Not reported Size Distribution :_Not reported
Composition: Purity >99.9% Solubility: 0.07% of mass dissolved after 48 hours (Griffitt et
Crystal Structure: Not reported al., 2008)
Average Size: Primary particles: 26.6 ± 8.8 nm (determined Surface Area: 14.53 m2/g (Griffitt etal., 2008)
by Coulter LS 13320) (Griffitt etal., 2008): suspended Surface Treatment: Not reported
particles: 66 ± 27 nm (determined by Coulter LS 13320) Surface Charge: -27 mV (Griffitt etal., 2008)
Protocol
Exposure Duration: 17 days Exposure Medium: Freshwater sediment from an urban lake
Endpoints: Changes in acetate oxidation Bacterial Density: Not reported
Exposure Concentrations: 0.5 mg/L (determined using
inductively coupled plasma atomic emission spectrometry
[ICP-AES])
Methods: Sediment slurries were prepared and purged with N2 for 2 hours. The terminal electron acceptors (TEAs) naturally
present, primarily oxygen and nitrate, were allowed to be consumed over time. Sediment slurries were incubated under
anaerobic conditions, and changes in concentrations in sulfate, nitrate, and nitrite were tracked (determined by ion
chromatography) to assess effect on microbial-catalyzed oxidation and nitrate reduction of organic matter as evidenced by
determination of the impacts on the terminal electron accepting process. Aliquots of well-homogenized sediment slurries were
spiked with de-aerated solution of sodium acetate with or without the addition of nano-Ag. Another set of slurries was spiked with
both acetate and nitrate with or without the addition of nano-Ag. The microbial degradation of acetate was monitored.
Study Outcome
Changes in Acetate Oxidation. Decreases in acetate concentrations in control slurries and in those with the addition of nano-Ag
(without the addition of nitrate) were similar. In the second set of studies where both acetate and nitrate were added to the
slurries, acetate degradation occurred at a faster rate, however the addition of nano-Ag did not produce any statistically
significant results compared to control. Parallel and slightly decreasing nitrite concentration trends were observed in both
nano-Ag treated slurries and in the control. The acetate oxidation rate of reaction (Kapp) was comparatively smaller in nano-Ag
spiked slurries (Kapp = -0.24 day1) than in the control (Kapp = 0.44 day1), without a statistically significant inhibitory effect.
B-12
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Hwang et al. (2008) Analysis of the toxic mode of action of silver nanoparticles using stress-specific
bioluminescent bacteria.
Test Species
Wild-type bacteria (Escherichia coli RFM443) and recombinant bioluminescent bacteria (All £. co//'strains: DS1 \yoda::luxCDABE\,
DK1 [katG::luxCDABE\, DC1 [clpB::luxCDABE\, DPD2794 [recA::luxCDABE\)
Material
Commercial nano-Ag purchased from Nanopoly Company (Republic of Korea), and synthesized by the company through
reduction of AgNOs with hydrazine hydrate, formaldehyde, and sodium formaldehydesulfoxylate.
Shape: Not reported Average Size: 10 nm (provided by the manufacturer and
Composition: Pure silver (provided by the manufacturer supported by TEM)
based on X-ray diffraction pattern and Size Distribution: Not reported
thermogravimetric/differential thermal analyzer [TG/DTA] Solubility: Not reported
curves) Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Surface Charge: Not reported
Protocol
Exposure Duration: 2 hours or 60 minutes (RT-PCR Exposure Concentrations: 0.1-1 mg/L
analysis) Exposure Media: Luria-Bertani culture media
Endpoints: Growth, oxidative stress, protein/membrane and Bacterial Density: 0.08 O.D.eoo
DNA damage
Methods: Growth inhibition was determined in the wild-type £ coli based on O.D.eoo. DS1 and DK1 were exposed to incremental
concentrations of nano-Ag and 15 units/ml catalase and superoxide dismutase to measure response to superoxide radicals and
hydroxyl radicals, respectively, based on maximum relative bioluminescence (RBL). DC1 was exposed to incremental
concentrations of nano-Ag to measure response to protein/membrane damage based on RBL. DPD2794 was exposed to
incremental concentrations of nano-Ag to measure response to DNA damage based on RBL and real-time quantitative RT-PCR.
Study Outcome
Growth Inhibition. Growth rates declined significantly after exposure to concentrations above 0.5 mg/L.
Oxidative Stress Damage. Nano-Ag led to production of superoxide radicals but little or no production of hydroxyl radicals.
Protein/Membrane Damage. Nano-Ag induced a bioluminescent response indicative of protein/membrane damage seen most
strongly at the 0.4 mg/L nano-Ag. Greater concentrations led to a reduction in bioluminescence due to toxic conditions (as seen in
growth inhibition test). DC1 did not discriminate between toxicity caused by nano-Ag or Ag+.
DNA Damage. Nano-Ag did not induce a response; therefore, no DNA damage was inferred.
B-13
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Ivask et al. (2010) Profiling of the reactive oxygen species-related ecotoxicity of CuO, ZnO, Ti02,
silver and fullerene nanoparticles using a set of recombinant luminescent Escherichia coll strains:
differentiating the impact of particles and solubilised metals.
Test Species
Escherichia coli. Eight constitutively luminescent recombinant strains (£. coli AB1157 [pSLlux], £ coli JI130 [pSLlux], £ coli
JI131c [SLIux], £ CO//AS393 [pSLlux], £ coli JI132 [pSLlux], £ CO//AS391 [pSLlux], £ coli K12::lux, and £ coli MC1061
[pDNlux]), one superoxide-inducible recombinant luminescent strain (E. coli K12::soxRSsodAlux), and two metal-inducible
recombinant luminescent strains (£. coli MC1061 [pSLzntR/pDNPzntAlux] and £ coli MC1061 [pSLcueR/pDNPcopAlux]).
Material
Nano-Ag purchased from Sigma-Aldrich. Stock solution of nano-Ag (40 g/L) prepared in MilliQ water, sonicated in ultrasonication
bath, and stored in the dark.
Shape: Not reported Solubility: AgNOs reported as reference chemical for
Composition: Not reported solubility effects (100%); 3.3% of nano-Ag reported to be
Crystal Structure: Not reported soluble and released to the test environment as bioavailable
Average Size: <100 nm (reported by the manufacturer; stock metal ions.
contained aggregates, SEM showed suspensions contained Surface Area: Not reported
non-nanosized particles). Surface Treatment: Not reported
Size Distribution: Not reported Surface Charge: Not reported
Protocol
Exposure Duration: 30 minutes or, 2, 5, or 12 hours Exposure Medium: Microplate
Endpoints: ROS-generating potential (toxicity) and Bacterial Density: 5*107 cells/mL (toxicity test), 107 cells/mL
superoxide anion production (induction assay), OD600-0.1 (bioluminescence induction
Exposure Concentrations: 40 g/L nano-Ag prepared in assay)
MilliQ (specific concentration not reported)
Methods: Maximum specific growth rate and ATP content were determined. Superoxide dismutase activity (SOD) was analyzed
using a commercial SOD determination kit. ROS-generated toxicity was evaluated by socf-deficient £. coli strains that were
transformed with luxCDABE genes to build luminescent ROS-sensitive strains, and by the use of a recombinant £. coli strain
specifically induced by superoxide anions. Toxicity was measured by the inhibition of luminescence of £ coli. Luminescence was
continuously recorded during the first 5 seconds of exposure, then once after 30 minutes and 2 hours of incubation using Orion II
plate luminometer. Bioluminescence inhibition was calculated as the percentage of the negative control. Measurements were
repeated in three independent assays. The presence of superoxide anions in bacterial cells after exposure was analyzed with
superoxide anion-inducible £. coli K12::soxRSsodAlux biosensor. The presence of silver ions from nano-Ag was determined
using Ag sensor bacteria £ co//MC1061(pSLcueR/pDNPcopAlux). Nano-Ag (100 pL) was mixed with 100 pL of bacterial
suspension on 96-well microplate, and bacterial bioluminescence was measured after 30 minutes, 2 hours, 5 hours, 7 hours, and
24 hours of exposure.
Study Outcome
ROS Generating Potential. The toxicity of nano-Ag to ROS-sensitive strains was shown to increase with time: EC50s= 571,
331, 485, 329,17.9, and 5.8 mg nano-Ag/L at 30 min for socf wt, sodA~, sodB~, socfC- sodAB-, sodABC-, respectively, and 45.9,
30.9,18.8,19.7, 3.79, and 3.11 mg nano-Ag/L at 2 h for socf wt, sodA~, sodB~, sodC~, sodAB-, sodABC-, respectively. Similar
results were observed for AgNOs (ECsos ranging from 0.1-1,374 mg/L at 30 min exposure and 0.34-1.11 mg/L at 2 h exposure).
In general sodAB- and sodABC- were more susceptible to ROS generation than other strains.
Superoxide Anion Production. Nano-Ag induced bioluminescence of the sensor strain, indicating the presence of intracellular
superoxide anions after 5 hours of exposure. When corrected for solubility, ECsos were 1.52 mg/L in wild-type £ coli with intact
socf genes and 0.1 mg/L in E. coli with sodABC~defective genes. Maximum induction of superoxide dismutase sensor corrected
for solubility seemed to be specific to a nanoparticle effect and not to silver ions alone (ECsos = 0.054 mg/L [AgNOs] and 0.0035
mg/L [nano-Ag]).
B-14
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Jin et al. (2010) High throughput screening of silver nanoparticle stability and bacterial inactivation
in aquatic media: influence of specific ions.
Test Species
Bacillus subtilis and Pseudomonas putida
Material
Nano-Ag powder (QuantumSphere Inc., Santa Ana, CA); silver nitrate (99% pure) (Sigma-Aldrich, St. Louis, MO). Nano-Ag
suspensions were prepared in ultrapure sterile water by ultrasonication.
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 25 nm (reported by manufacturer), 26 ± 8 nm Surface Treatment: Used as received
(determined using TEM) Surface Charge: Approximately -10 to -60 mV (determined
using Zeta-PALS)
Protocol
Exposure Duration: 24 hours Exposure Medium: Model freshwater electrolyte with ionic
Endpoints: Bacterial viability strength maintained at 5.6 mM by replacing specific cation or
Exposure Concentrations: 19 concentrations from 0.2 pg/L anion with Na+ or Cl- (pH varied)
to 50 mg/L Bacterial Density: 108 cells/mL
Methods: Silver concentrations were measured using ICP-OES; samples were centrifuged and filtered to determine the
concentration of dissolved nano-Ag. USGS software PHREEQC was used to calculate the theoretical maximum free Ag+
concentration. The high-throughput bacterial viability assay was based on a Live/Dead Baclight bacterial viability kit; 25 pL of
nano-Ag suspension and 25-pL of bacterial suspensions were dispensed into wells and incubated and stains were added to the
wells. A florescence microreader provided the percentage of live bacteria in each well. Data were fitted by a dose-response
model using a nonlinear regression with four-parameter logistic equation.
Study Outcome
The measured particle size increased significantly, indicating that Ca2- and Mg2- ions enhanced aggregation of nano-Ag in the
matrix containing divalent cations. There was no correlation between aggregate size and zeta potential.
Bacterial Viability. ICso values for the addition of nano-Ag were statistically significantly higher (by several orders of magnitude)
than those values for the addition of AgNOs in the high-throughput assays. In general, Gram positive B. subtilis was less resistant
to the antibacterial activity of nano-Ag than Gram negative P. putida (statistical significance not reported). Both bacteria were
more resistant to nanotoxicity when bicarbonate was in the media.
B-15
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Khan et al. (2011) Silver nanoparticles tolerant bacteria from sewage environment.
Test Species
Bacillus pumilus, Escherichia coli, Staphylococcus aureus, and Micrococcus luteus.
Material
Nano-Ag obtained from Sigma Aldrich (USA).
Shape: Spherical (determined using TEM and SEM) Solubility: Not reported
Composition: Not reported Surface Area: 0.26 m2/g (determined using Smart Sorb 93
Crystal Structure: Not reported Single point BET surface area analyzer)
Average Size: Not reported Surface Treatment: Exopolysaccharide capping
Size Distribution: 10-40 nm (determined using TEM and Surface Charge: Not reported
SEM)
Protocol
Exposure Duration: 30 minutes Exposure Medium: Nutrient agar, Muller Hinton agar, and
Endpoints: Growth rate and viability LB agar (disc diffusion); Unspecified agar (agar-well
Exposure Concentrations: 25,50, and 100 pg/disc (disc diffusion); LB agar (dilution plate count); or LB broth (growth
diffusion method); 10, 25, 50,100, and 200 pg/well (agar- kinetics study)
well diffusion method); 10, 25, 50, 100, and 200 pg/plate Bacterial Density: 10M04 CFU/mL (disc diffusion) 102-103
(dilution plate count method); 10-200 mg/L (growth kinetics CFU/mL(growth kinetics study); densities for other methods
study). not reported.
Methods: Anano-Ag tolerant 8. pumilus strain was isolated and identified by 16S rRNA analysis and lack of diameter of inhibition
zone (DIZ) using disc diffusion and agar well diffusion test. Exopolysaccharides were extracted from 8. pumilus, quantified,
purified, and reacted with nano-Ag; lyophilized nanoparticles were analyzed by XRD. Toxicity of exopolysaccharides-coated
nano-Ag tested on £ Coli, S. aureus, and M. luteus by culturing organisms on LB broth supplemented with coated and uncoated
nano-Ag particles (100 mg/L) was measured by disc diffusion, dilution plate count method, and agar well diffusion test. Growth
kinetics were measured as increases in absorbance at 600 nm using a colorimeter after sonication of nano-Ag (10-200 mg/L) to
prevent aggregation after inoculation with bacterial culture.
Study Outcome
Lyophilized nano-Ag did not give the characteristic peak of silver with XRD analysis due to persistence of exopolysaccharide
capping. Similarly, SPR analysis showed a peak shift for nano-Ag before and after capping possibly due to the repelling action of
the exopolysaccharides-capped nano-Ag.
Growth Rate and Viability. Growth kinetics of bacteria exposed to exopolysaccharides-coated nano-Ag were similar to controls
indicating reduction of toxicity compared to documented growth inhibition zones when bacteria were exposed to uncoated
nano-Ag. The observed bacterial count (mean ± standard error) in the agar plates were 95.0 ± 0.966, 95.2 ± 0.833, 96.0 ± 0.931,
94.7 ± 1.145, 94.8 ± 1.138, and 94.8 ± 0.792 for control, 10, 25, 50,100, and 200 ug of nano-Ag, respectively. When SNPs are
present on the surface of the nutrient agar plates, they could completely inhibit the bacterial growth compared to liquid broth;
however, the growth inhibition was not observed in any of the plates in dilution plate count method. Bacterial tolerance was
suggested to be due to the secretion of exopolysaccharides which inhibit interaction of nano-Ag with the bacterial cell wall.
B-16
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Kim et al. (2009) Antifungal activity and mode of action of silver nano-particles on Candida albicans.
Test Species
Diploid fungus (Candida albicans ATCC 90028)
Material
Nano-Ag synthesized by dissolving solid silver in nitric acid and adding sodium chloride.
Shape: Spherical Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 3 nm (determined using TEM) Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Durations: 2 hours (membrane dynamics and Exposure Concentrations: 20, 40, 60, and 80 ug/mL
released glucose and trehalose), 3 hours (membrane (membrane dynamics); 20 ug/mL (released glucose and
integrity), 8 hours (cell cycle), 24 hours (envelope structure), trehalose), 30 ug/mL (membrane integrity); 40 ug/mL (cell
48 hours (growth inhibition) cycle); and not reported for envelope structure or growth
Endpoints: Growth, membrane damage/disruption, and cell- inhibition tests.
cycle arrest Exposure Media: Yeast extract, peptone, and dextrose broth
for all tests but released glucose and trehalose, which used
phosphate-buffered saline
Bacterial Density: 2 x 104cells/mL (growth inhibition) or
1 xio«cells/mL
Methods: Growth was assayed with a microtiter enzyme-linked immunosorbent assay (ELISA) reader by monitoring absorption at
the 580-nm wavelength. Minimum inhibitory concentrations (MICs) were defined as the lowest concentrations that inhibited 90% of
fungal growth when compared to the control. MICs were determined by a series of 2-fold dilutions. Membrane integrity was
assessed using flow cytometric analysis, and membrane dynamics were determined by steady-state fluorescence anisotropy
using spectrofluorometry. Released glucose and trehalose were measured by weighing dry fungal pellets and measuring color
formations in supernatants. Fungal envelope structure was examined using TEM, and cell cycle arrest was assessed using flow
cytometric analysis.
Study Outcome
Growth Inhibition. MIC: 2 ug/mL
Membrane Damage/Disruption. Membrane depolarization occurred. Plasma membrane 1,6-diphenyl-1,3,5-hexatriene (DPH)
significantly decreased with increasing concentrations of nano-Ag. Nano-Ag-treated cells both accumulated more intracellular and
more extracellular glucose and trehalose than untreated cells. Extracellular glucose and trehalose amounts were 30.3 ug/mg
fungal dry wt. Treated fungal cells showed significant damage characterized by pits in the cell walls and pores in the plasma
membranes.
Arrest of Cell Cycle. The percentage of cells in the gap 2/mitosis (G2/M) phase increased by 15%, while cells in gap 1 (d) phase
significantly decreased by about 20%. This indicates that the budding process was inhibited.
B-17
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Kvitek et al. (2008) Effect of surfactants and polymers on stability and antibacterial activity of silver
nanoparticles (NPs).
Test Species
Gram-positive bacteria (Enterococcus faecalis CCM 4224, Staphylococcus aureus CCM 3953, Staphylococcus aureus MRSA,
Staphylococcus epidermidis [methicillin-susceptible], Staphylococcus epidermidis [methicillin-resistant], Enterococcus faecium
VRE) and Gram-negative bacteria (Escherichia coli CCM 3954, Pseudomonas aeruginosa CCM 3955, Pseudomonas aeruginosa',
Klebsiella pneumonias ESBL).
Material
Nano-Ag synthesized by modified Tollens process, prepared by reduction of the Ag(NHs)2+ with D(+)-maltose monohydrate.
Shape: Spherical Surface Area: Not reported
Composition: Not reported Surface Treatment: Unmodified or modified with anionic
Crystal Structure: Not reported sodium dodecyl sulfate (SDS), nonionic polyoxyethylene-
Average Size: 26 nm with 2.3% polydispersity (determined sorbitan monooleat (Tween 80), or polyvinyipyrrolidone
using DLS and a Zeta plus analyzer) (PVP 360)
Size Distribution: Not reported Surface Charge: -25 mV (( potential in aqueous dispersion)
Solubility: Not reported
Protocol
Exposure Duration: 24 hours Exposure Medium: Mueller-Hinton broth
Endpoint: Growth Bacterial Density: 105-106 CFU/mL
Exposure Concentrations: 0.84 to 54 ug/mL
Methods: Modifiers were added to dispersions of nano-Ag prior to titration in the final amount of 1% (w/w). Stability of unmodified
and modified nano-Ag was tested using serial additions of a destabilizer, cationic polyelectrolyte poly(diallyldimethylammonium)
chloride (PDDA, 20% [w/w] aqueous solution), and was confirmed using DLS and UV/vis absorption spectra.
Study Outcome
Minimum inhibitory concentrations (MICs) in ug/mL listed below for unmodified nano-Ag, and nano-Ag modified with SDS, Tween
80, and PVP 360, respectively.
E. faecalis CCM 4224: 6.75, 3.38, 6.75, 6.75 E. faecium VRE: 6.75, 3.38, 3.38, 3.38
S. aureus CCM 3953: 3.38, 1.69, 3.38, 3.38 E. coli CCM 3954: 1.69, 1.69, 1.69, 3.38
S. aureus MRSA: 3.38,1.69, 3.38,1.69 P. aeruginosa CCM 3955: 3.38,1.69, 3.38,1.69
S. epidermidis (methicillin-susceptible): 1.69, 0.84,1.69,1.69 P. aeruginosa: 3.38, 3.38,1.69,1.69
S. epidermidis (methicillin-resistant): 1.69,1.69,1.69,1.69 K. pneumoniae ESBL: 6.75, 6.75, 3.38, 6.75
B-18
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Lok et al. (2006) Proteomic analysis of the mode of antibacterial action of silver nanoparticles.
Test Species
Wild-type gram-negative bacteria (Escherichia coli K12, MG1655)
Material
Nano-Ag synthesized by borohydride reduction of AgNOs in the presence of citrate as a stabilizing agent.
Shape: Spherical Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Stabilized with bovine serum albumin
Average Size: 9.3 ± 2.8 nm (determined using TEM) (BSA) when in M9 medium, but uncoated when in HEPES
Size Distribution: Not reported buffer
Surface Charge: Not reported
Protocol
Exposure Duration: >600 minutes (growth) or 30 minutes Exposure Medium: M9 defined medium for growth inhibition
Endpoints: Growth, protein expression, membrane and proteomic analyses and sodium or potassium HEPES
damage/disruption buffers containing glucose for membrane analyses.
Exposure Concentrations: 0.4 and 0.8 nM Bacterial Density: 0.15 O.D.eso for growth inhibition and
proteomic analyses and 0.1 O.D.eso for membrane analyses.
Methods: Growth inhibition was assessed by monitoring the O.D.eso. Proteomes were analyzed using two-dimensional
electrophoresis (2-DE) followed by silver staining. Proteins stimulated by nano-Ag were identified using matrix-assisted laser
desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) on the tryptic
digests of protein spots of interest. Expression of cell envelope protein OmpA was examined using immunoblots. Membrane
damage was determined by pretreating E. coli with 1 nM nano-Ag and followed by exposure to 0.1 % sodium dodecyl sulfate
(SDS). Effects of nano-Ag at the minimum inhibitory concentration (MIC: 1 nM) on the cytoplasmic membrane potential were
examined using fluorescence.
Study Outcome
Growth Inhibition. Inhibition became apparent at concentrations of 0.4 nM and 6 uM, for nano-Ag and AgNOS, respectively.
Statistical significance of the results was not reported.
Protein Expression. No global changes in proteomes due to nano-Ag exposure were found. Expressions of "at least" 8 proteins
were stimulated by nano-Ag and by AgNOs. Expressions of a number of cell envelope proteins (OmpA, OmpC, OmpF, and MetQ)
were stimulated by nano-Ag. The 37 kDa band was enhanced by nano-Ag. Nano-Ag resulted in accumulation of precursor forms
of OmpA.
Membrane Damage. Rapid cell lysis occurred in cells pretreated with nano-Ag and subsequently exposed to SDS, where no cell
lysis occurred with only exposure to SDS or nano-Ag alone. DiSCs(5) fluorescence decreased upon addition to E. coli cells and
stabilized. After nano-Ag was added, fluorescence rapidly recovered, indicating a dissipation of membrane potential. Also, an
almost complete loss of potassium from the cell and depletion of ATP was observed after 5 minutes.
B-19
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Martinez-Gutierrez et al. (2010) Synthesis, characterization, and evaluation of antimicrobial and
cytotoxic effect silver and titanium nanoparticles.
Test Species
Escherichia coli, Acinetobacter baumanii, and Pseudomonas aeruginosa (gram-negative); Bacillus subtilis, Mycobacterium
smegmatis, Mycobacterium bovis, and Staphylococcus aureus (gram-positive); Candida albicans, Cryptococcus neoformans, and
Aspergillus niger (fungi); and human-derived monocyte cell lines.
Material
Nano-Ag synthesized from AgNOs (purchased from Sigma-Aldrich, USA) with gallic acid and sodium hydroxide (20-25 nm
particles) or AgNOs with a UV light reactor, and gallic acid (80-90 nm particles). Ti02-Ag nanoparticles were synthesized from Ti02
particles (purchased from Ti-Pure R-902 [Dupont Wilmington, DE] and Degussa P25 [Degussa, Parsippany, NJ], Ti1 and Ti2,
respectively), dispersed by ultrasound in deionized water and the addition of different quantities of AgNOs with NaBhU and NhUOH
using UV light as a reacting agent to produce particles with Ti02-Ag molar ratios of 10:1, 25:1, and 50:1.
Shape: Not reported Solubility: Not reported
Composition: Ag only, Ti02 only, Ti1 Or2-Ag NaBhU, Ti1 or2- Surface Area: Not reported
Ag UV Surface Treatment: Not reported
Crystal Structure: Not reported Surface Charge: Not reported
Average Size: Not reported
Size Distribution: 20-25 nm and 80-90 nm (nano-Ag only);
250-300 nm (Ti02-Ag bimetallic) (determined by TEM)
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 107.8 pg/mL (nano-Ag only); 10
Endpoints: Viability and cytotoxicity Mg/mL (Ti1 Or2-Ag NaBhU or UV 10:1); 0.5 pg/mL (Ti1 Or2-Ag
Bacterial Density: Not reported NaBhU or UV 25:1); 0.35 (Ti1 °r2-Ag NaBhU or UV 50:1)
Exposure Medium: Not reported
Methods: Organisms were exposed to serial dilutions of nanoparticles and viability assessed to calculate MICs. A cytotoxicity
assay, including DNA damage and cell viability, was carried out on monocyte cells. Toxicity was measured by staining with
propidium iodide. The viability of cells was assayed by Trypan blue staining. The effectiveness of the nanoparticles was expressed
as the therapeutic index (TI), which represents the amount of a therapeutic agent that causes a therapeutic effect of 50% in the
population and estimates the extent to which the administration of the agent is safe (TI = LDso/MIC). DNA single-strand breaks and
alkali-labile lesions were detected using the alkaline comet assay and were tested using only nano-Ag (20-25 nm) and Ti2-Ag
NaBhU. The cytotoxicity studies were carried out using 10 different concentrations (dilutions not specified) of nano-Ag, Ti2-Ag
NaBhU nanoparticles, and Ti2-Ag UV particles.
Study Outcome
Viability. The highest antimicrobial activity was observed with the smallest nano-Ag particles (20-25 nm) with MIC averages
between 0.4 -1.7 pg/mL (bacteria) and 3-25 pg/mL (fungi), which is comparable to results obtained using commercial antibiotics.
Ti02-Ag nanoparticles showed fungal-specific activity but resulted in low MIC levels for bacteria (5 - >100 pg/mL for bacteria, and
3-25 |jg/mL for fungi). Nano-Ag (only) was found to possess the highest TI calculated for bacterial strains (13.86 ± 1.31 for gram-
positive, 23.25 ± 0.63 for gram-negative), but the lowest TI value for fungal strains (0.9 ± 0.35) when compared to Ti2-Ag UV 50:1
(1.57, 5.03, and 10.7 for gram-positive, gram-negative, and fungi, respectively), andTi2-Ag NaBhU (2.6, 2.56, and 22.1 for gram-
positive, gram-negative, and fungi, respectively).
Cytotoxicity. The smallest nano-Ag particles (20-25 nm) resulted in a dose-dependent increase in death rate of the monocytes.
Concentrations <2 pg/mL resulted in no significant cytotoxicity (<20% death rate), while concentrations >5 pg/mL resulted in
statistically significant increases in toxicity reaching >50%. The Ti-Ag nanoparticles showed only slight toxicity, with cell death rates
ranging from 15 to 25%, even at the highest concentrations (25 pg/mL). LDsos were calculated as 10 ± 3.4 pg/mL for nano-Ag and
only 55.9 pg/mL and 367.3 pg/mL for the Ti-Ag bimetallic nanoparticles (Ti2-Ag UV 50:1 and Ti2-Ag NaBhU 10:1, respectively).
Results of DNA damage assay showed no evidence of the production of DNA single-strand breaks above the background level.
B-20
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Morones et al. (2005) Bactericidal effect of silver nanoparticles.
Test Species
Gram-negative bacteria (Escherichia coli, Vibrio cholerae, Pseudomonas aeruginosa, and Salmonella typhi)
Material
Commercial nano-Ag powder inside a carbon matrix, supplied by Nanotechnologies, Inc. (location not reported).
Shape: Cuboctahedral and multiple-twinned icosahedral and Size Distribution: Not reported
decahedral Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: {111} lattice plane Surface Treatment: Not reported
Average Size: 16 ± 8 nm (determined using TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 30 minutes Exposure Media: Agar plates with Luria-Bertani medium
Endpoints: Growth and membrane damage broth
Exposure Concentrations: 0, 25, 50, 75, and 100 ug/mL Bacterial Density: 0.5 0.0.595 which corresponds to
~5xl07CFU/mL solution
Methods: Growth inhibition was assessed by monitoring the 0.0.595. Effects on the bacterial membrane were examined using
high angle annular dark field scanning transmission electron microscopy (HAADF-STEM).
Study Outcome
Growth Inhibition. At nano-Ag concentrations above 75 ug/mL, there was no bacterial growth for any species. £ coli and S. typhi
were more sensitive to nano-Ag exposure than P. aeruginosa and V. cholerae.
Membrane Damage. Individual silver nanoparticles were attached to the cell membrane and distributed throughout the cell. Only
particles of the sizes attached to the membrane were observed inside the cell. The mean size of the nano-Ag interacting with
bacteria was 5 nm, even though the mean nano-Ag size in solution was 16 nm.
B-21
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Pal et al. (2007) Does the antibacterial activity of silver nanoparticles depend on the shape of the
nanoparticle? A study of the Gram-negative bacterium Escherichia coll.
Test Species
Gram-negative bacteria (Escherichia co//'ATCC 10536).
Material
Nano-Ag powder reduced from aqueous AgNOs with sodium citrate or produced via large-scale preparation in particle growth
solution containing AgNOs, ascorbic acid, CTAB, silver seeds, and NaOH.
Shape: Rod, truncated triangular plate, or spherical Size Distribution: Not reported
Composition: Pure silver (triangular), composition of other Solubility: Not reported
shapes was not reported Surface Area: Not reported
Crystal Structure: {111} basal lattice plane (triangular) Surface Treatment: CTAB, a cationic quaternary ammonium
Average Size: 133-192 nm (rod edge), 16 nm (rod surfactant, was used in synthesis of truncated triangular and
diameter), 40 nm (triangle edge), 39 nm (spherical) rod-shaped nano-Ag
(determined using EFTEM) Surface Charge: Not reported
Protocol
Exposure Duration: Not reported (nutrient broth) or 24 Exposure Media: Difco nutrient broth or Difco nutrient agar
hours (agar plate) Bacterial Density: 108 CFU/mL (in broth) or 107 and 105
Endpoint: Growth and membrane damage CFU/mL (on agar pDate)
Exposure Concentrations: 1-100 ug (delivered to either
100 ml nutrient broth or onto agar plates; concentrations not
reported)
Methods: Colonies were exposed to concentrations of nano-Ag of different shapes and counted at the end of the exposure period.
Membranes were examined using EFTEM.
Study Outcome
Growth. A dose of 10 ug of triangular particles added to 100 ml of the nutrient broth inhibited growth after 24 hours, and 100 ug of
AgNOs and spherical nano-Ag delayed growth up to 10 hours. At cell concentrations of 107 CFU/mL, almost complete growth
inhibition was observed at a triangular nano-Ag content of 1 ug on the agar plate. An amount of 12.5 ug spherical nano-Ag
reduced bacterial colonies significantly on the agar plate, and 100% inhibition was seen at contents of 50-100 ug. Even at 100 ug,
rod-shaped nano-Ag and AgNOs did not inhibit growth completely on the agar plate. Nano-Ag inhibition of bacterial growth was
also dependent on the initial number of bacterial cells. At cell concentrations of 105 CFU/mL, spherical nano-Ag almost completely
prevented growth at 6 ug, and 12 ug of AgNOs inhibited growth completely on the agar plate.
Membrane Damage. Nano-Ag-treated bacterial cells were significantly changed, and major damage was observed on the outer
membrane, characterized by pitting. Nanoparticles also accumulated in both the membrane and within the cells.
B-22
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Saulou et al. (2010) Synchrotron FTIR microspectroscopy of the yeast Saccharomyces cerevisiae
after exposure to plasma-deposited nanosilver-containing coating.
Test Species
Sessile Saccharomyces cerevisiae BY4741 yeast cells
Material
Nano-Ag in plasma-mediated thin films surrounded by organosilicon matrix (Determined using XPS)
Shape: Granular-type structure with spherical metal clusters Size Distribution: 5-10 nm (TEM)
(determined using SEM) Solubility: 2-mM AgNOs
Composition: Ag: 20.5%, Si: 15.1%, C: 43.5%, 0:20.9% Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Coating mainly composed of C, 0, and
Average Size: Not reported Si
Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours Exposure Medium: Plasma-coated Al SI 316L stainless steel
Endpoints: Cell composition and antifungal properties coupons
Exposure Concentrations: Not reported Bacterial Density: 2.107 CFU/mL
Methods: Stainless steel square (10 mm x 10 mm) coupons were chemically cleaned, coated in an organosilicon matrix, and
used as a substrate. A silver disc RF powered electrode was used in conjunction with an RF generator. The steel coupons were
placed over the bottom of the sample holder electrode in front of the silver target, and silver atoms from the electrode formed
nanoclusters embedded in the organosilicon matrix. A batch preculture of S. cerevisiae BY4741 was prepared, stationary growth
phase was reached, and yeast cells were harvested by centrifugation. A suspension that corresponded to 2.107 CFU/mL was
used for adhesion tests to the organosilicon matrix and the nano-Ag-containing coating. The plasma-coated samples were
immersed in the yeast suspension for a 24-h contact time; sessile cells were removed by sonication. Synchrotron FTIR analysis
was performed on yeast suspensions. TEM observations were performed parallel to the synchrotron FTIR analysis.
Study Outcome
Antifungal Properties. A significant loss of cell viability was observed for the nano-Ag-containing coating; the CFU/total cell
number ratio was decreased by a 1.4 log reduction. This was indicative of the antifungal properties of the nano-composite
coating.
Composition. Significant downshifts of amide I and II bands were observed in the protein absorption region. Following exposure
to silver nitrate, the IR spectrum of the yeast suspension revealed a downshift (30 cm-1) of the peak at 1,655 cm-1, corresponding
to a loss in a-helix structures. The nano-Ag-containing coating displayed a slightly weaker downshift (20 cm-1). This was
indicative of the inhibitory action of silver against S. cerevisiae targeted against mannoproteins and intracellular proteins.
Disordered secondary structures of proteins were observed, marking striking differences in cell composition.
B-23
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Shrivastava et al. (2007) Characterization of enhanced antibacterial effects of novel silver
nanoparticles.
Test Species
Gram-negative bacteria (Escherichia coli ATCC 25922, ampicillin-resistant E. coli, multi- drug-resistant strain of Salmonella
typhus) and Gram-positive bacteria (Staphylococcus aureus ATCC 25923).
Material
Nano-Ag synthesized through reduction of AgNOs with a blend of reducing agents including D-glucose and hydrazine.
Shape: Spherical or polyhedral Size Distribution: Approximately 5-47 nm
Composition: Not reported Solubility: Not reported
Crystal Structure: Face centered cubic Surface Area: Not reported
Average Size: Approximately 10-15 nm (determined using Surface Treatment: Not reported
TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours (agar plates) Exposure Media: Luria-Bertani agar plates or liquid broth
Endpoints: Growth and membrane damage Bacterial Density: 106 CPU per agar plate or 108 CPU per
Exposure Concentrations: 10,25, 35, 50 (only S. aureus), ml liquid broth
and 100 (only S. aureus) ug/mL
Methods: Growth inhibition on the agar plates was assessed by counting the colonies following exposure, and growth rate in the
liquid broth was determined by measuring the optical density of bacterial cultures at 600 nm (O.D. eoo). Bacterial cells were also
cultured for 60 minutes in the presence of nano-Ag, after which they were re-cultured in a fresh medium without nano-Ag. Growth
was then measured in these cells and compared to controls that were not exposed initially to nano-Ag. The effect on bacterial
signal transduction was explored by measuring phosphotyrosine content of proteins. Membrane damage was examined using
transmission electron microphotographs.
Study Outcome
Growth Inhibition. For non-resistant E. coli, 60% growth inhibition was observed at the 5-ug/mL level, which increased to 90%
inhibition at 10 ug/mL and complete inhibition at 25 ug/mL For ampicillin-resistant £ coli and S. typhi, 70-75% inhibition was
observed at 10 ug/mL and complete inhibition at 25 ug/mL. Similar effects were observed in the liquid medium, where lag time
before growth was 8 hours at 25 ug/mL. No reduction in S. aureus growth was observed at 25 ug/mL on the agar plate. Cells that
were re-cultured in fresh medium following exposure to nano-Ag (concentration not reported) exhibited significant retardation in
growth relative to controls. There was very little change in the tyrosine phosphotyrosine profile in S. aureus, but noticeable
dephosphorylation of two peptides (unidentified) occurred in £ coli.
Membrane Damage. Clusters of nano-Ag were observed anchored to bacterial cell wall, which perforated the cell membrane, and
accumulated inside the cell.
B-24
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Sinha et al. (2011) Interaction and nanotoxic effect of ZnO and Ag nanoparticles on mesophilic and
halophilic bacterial cells.
Test Species
Mesophilic and halophilic bacteria. Mesophiles: Enterobacter spp. (Gram negative) and Bacillus subtilis (Gram positive).
Halophiles: Marinobacterspp. (Gram negative) and EMB4 (Gram positive)
Material
Ag nanopowder (Cat. No 576832) from Sigma Aldrich, St. Louis mixed in Milli-Q water and ultrasonicated for 30 minutes; ZnO
nanopowder (Cat. No 544906) from Sigma Aldrich, St Louis; bulk ZnO from Qualigens Fine Chemicals, Mumbai, India; bulk Ag
from Central Drug House Pvt. Ltd New Delhi, India
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: less than 100 nm (reported by
manufacturer)
Protocol
Exposure Duration: Not reported Exposure Medium: Analytical grade (%, w/v) yeast extract
Endpoints: Growth, viability, and membrane structure 0.5, peptone 0.5, dextrose 1.0, NaCI 0.25 and 10.0 (for
Exposure Concentrations: 2 mM, except in B. subtilis tests mesophiles and halophiles, respectively), MgS040.05.
at 10 mM Adjusted to pH 7.0 for mesophiles and pH 8.0 for halophiles.
Bacterial Density: Not reported
Methods: 12-hr culture was prepared and used as inoculum. The inoculated medium was incubated and nanoparticle
suspensions were aseptically added to the nutrient medium. The bacterial growth was monitored by UV- Visible
spectrophotometry. Viable cells were counted and CPUs were calculated. Cultures were centrifuged, fixed with Karnovsky's fluid,
dehydrated, and the SEM micrographs of the dehydrated samples were recorded by SEM. The processed cells were
micrographed in TEM. The samples were analyzed for energy dispersive X-ray analysis.
Study Outcome
Nano-Ag accumulated in the cytoplasm of both Enterobacter and Marinobacter species, but Nano-Ag was more toxic to halophilic
gram negative cells than mesophilic gram negative cells.
Growth and Viability. Nano-Ag caused a substantial decrease in growth rate and viable cell count of Enterobacter. Exposure to
nano-Ag also resulted in changes in morphology and reduction in size of Enterobacter. The growth of the B. subtilis was not
affected by exposure to nano-Ag (acknowledging a marginal reduction at 10mM exposure). There was an 80% reduction in the
growth rate of Marinobacter cells after exposure to nano-Ag. Exposure to nano-Ag particles did not affect EMB4 bacterial growth.
Membrane Structure. The cell membranes of Enterobacter and Marinobacter were disrupted after exposure to nano-Ag, while the
cell membranes of B. subtilis and EMB4 remained intact.
B-25
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Sondi and Salopek-Sondi (2004) Silver nanoparticles as antimicrobial agent: a case study on E. coli
as a model for Gram-negative bacteria.
Test Species
Gram-negative bacteria (Escherichia coli Strain B)
Material
Nano-Ag powder synthesized by reduction of AgNOs with ascorbic acid and Daxad 19, a dispersing and thinning agent.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: 158 m2/g (dried powder)
Crystal Structure: Not reported Surface Treatment: Daxad 19 reportedly removed prior to
Average Size: 12.3 ± 4.2 nm (determined using TEM) bacterial exposure
Size Distribution: 4-29 nm Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours (agar plates) Exposure Media: Luria-Bertani agar plates or liquid broth
Endpoints: Growth and membrane damage Bacterial Density: 105 CPU per agar plate or 107 CPU per
Exposure Concentrations: 10-100 ug/cm3 ml liquid broth
Methods: Growth inhibition was assessed by counting colonies on agar plates following exposure. Growth rate was determined by
measuring O.D.eoo in the liquid broth. SEM and TEM were used to evaluate the surface morphology of the cells. The qualitative
chemical composition of the membranes was assayed by energy dispersive X-ray analysis (EDX).
Study Outcome
Growth Inhibition. Nano-Ag at a concentration of 10 ug/cm3 inhibited growth on the agar plates by 70%. The number of colonies
was reduced when compared to controls and most were located on edges of agar plates at exposure concentrations of 20 ug/cm3
nano-Ag, and concentrations of 50-60 ug/cm3 nano-Ag inhibited growth completely. The number of cells applied to the plate was
related to the degree of antibacterial activity (i.e., less cells resulted in more antibacterial activity). All concentrations of nano-Ag
applied to the liquid medium resulted in delayed growth, with the higher concentrations resulting in longer delays.
Membrane Damage. Nano-Ag-treated cells were significantly changed, showing major damage, characterized by formation of pits
in the cell walls. EDX showed that nano-Ag was incorporated into the bacterial membrane, which was confirmed by TEM. TEM
also revealed penetration of nano-Ag into cells and intracellular substances, in addition to coagulated nano-Ag on the bacterial
surface.
B-26
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Sotiriou and Pratsinis (2010) Antibacterial activity of nanosilver ions and particles.
Test Species
£. co//'
Material
Nano-Ag on nanostructured silica was synthesized using flame spray pyrolysis using silver acetate, 2-ethylhexanoic acid, and
acetonitrile.
Shape: Spherical (TEM)
Composition: Ag, Si, 0
Crystal Structure: Monocrystalline (XRD)
Average Size: 4-16 nm (HRTEM)
Protocol
Exposure Duration: 330 minutes
Endpoints: Bacterial growth (fluorescence)
Size Distribution: 4-16 nm (HRTEM)
Solubility: Not reported
Surface Area: <40 AgSSAE m2/g of Ag (62 pulse
chemisorptions)
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Concentrations: Ag content x in Ag/Si02 wt %:
0-100
Exposure Medium: Dl water
Bacterial Density: 107CFU/mL
Methods: Bacterial growth was monitored by fluorescence intensity in the assessment of relative impacts of particle size, surface
area, and Ag+ content (wt %).
Study Outcome
When exposed to the smaller nano-Ag particles with lower Ag content, a much stronger antibacterial activity was observed
compared to higher Ag-content particles that were larger in size (reduced surface area). By controlling the Ag content in the
treatment (Ag/Si02 particles), the antibacterial was also controlled: x = 1 -10 wt % Ag resulting in little or no bacterial growth,
compared to maximum bacterial growth reached for x= 50 (at C = a mg/L of Ag). When the Ag content within particles was equal,
smaller particles disassociated more rapidly into Ag+ ions, indicating that the antibacterial activity is largely dictated by the ions
themselves rather than the nanoparticles. However, when larger particles associated with low ionic release were tested, the
nano-Ag particles themselves also played an important role in toxicity (when Ag/Si02 particles were removed, bacterial growth was
restored for both Ag mass concentrations of 20 and 30 mg/L at x>90 wt %. When only Ag+ ions were present, the antibacterial
activity was comparable to that in the presence of the larger nanoparticles). Authors conclude that antibacterial activity depends on
size, which increases the amount of ions released into the environment, but that larger particles play a significant role in toxicity
themselves when the ionic release is reduced.
B-27
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B.3. Summary of Nano-Ag Effects in Algae
Griffitt et al. (2008) Effects of particle composition and species on toxicity of metallic nanomaterials
in aquatic organisms.
Test Species
Freshwater green algae (Pseudokirchneriella subcapitata)
Material
Commercial nano-Ag powder produced by gas-phase condensation and coated with a thin layer (2-3 nm) of metal oxide;
supplied by Quantum Sphere (Santa Ana, CA, USA).
Shape: Not reported Solubility: Dissolution 48 hours after resuspension was
Composition: Not reported 0.07% of total mass
Crystal Structure: Not reported Surface Area: 14.53 m2/g (determined using the Brunauer,
Average Size: 26.6 ± 8.8 nm (determined using a laser Emmett, and Teller method)
diffraction particle size analyzer) Surface Treatment: Sodium citrate stabilizer
Size Distribution: Approximately 20-1,000 nm Surface Charge: -27.0 mV (£ potential in moderately hard
freshwater with pH 8.2, determined using a Zeta Reader Mk
21-11)
Protocol
Exposure Duration: 96 hours Exposure Medium: Moderately hard freshwater (dissolved
Endpoint: Growth oxygen 8.5-8.9 mg/L; pH 8.2 ± 1, hardness 142 ± 2 mg/L
Exposure Concentrations: ECso from the ranging-finding CaCOs; conductivity 395 uS; un-ionized ammonia <0.5 mg/L)
test, and concentrations 0.6-, 0.36-, 1.67-, and 2.78-times the Cell Density: Not reported
estimated ECso from the range-finding test
Methods: Algal growth media were prepared to produce a concentration gradient before being inoculated with a similar volume
of algal culture. Algal growth was assessed by measurement of chlorophyll a. These results from exposure to nanometals of
copper, aluminum, and nickel were compared to controls.
Study Outcome
EC5o:0.19mg/L
P. subcapitata was more susceptible to nano-Ag than to any of the other nanometals tested. The reported test concentrations are
nominal; therefore actual concentrations to which the organisms were exposed may be lower or higher than reported.
B-28
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Park et al. (201 Ob) Selective inhibitory potential of silver nanoparticles on the harmful
cyanobacterium Microcystis aeruginosa.
Test Species
Cyanobacteria Microcystis aeruginosa, and green algae A. convolutus, and S. quadricauda
Material
Nano-Ag prepared in two solutions. Solution 1 was prepared by reducing silver nitrate with tannic acid. Solution 2 was prepared
by adding silver nitrate, sodium persulfate, and NaOH with the application of heat and Tween 20 as a dispersing agent (producing
silver oxide nanoparticles).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: 20-50 nm at 200 mg/L for both solutions
(determined by TEM)
Protocol
Exposure Duration: 10 days Exposure Concentrations: 0.001, 0.01, 0.1, or 1 mg/L
Endpoint: Colony growth inhibition; chlorophyll a (solution 1, experiment 1); 1 mg/L (solution 1 and 2,
concentration, community composition, and growth inhibition; experiments 2 and 3)
community composition, and cell count Exposure Medium: Pre-filtered eutrophic lake water
Cell Density: 10 mL in 90 mL lake water
Methods: 10 mL M. aeruginosa culture was added to 90 mL lake water. The inhibition of algal growth was calculated as: algal
inhibition efficiency (%) = [(control - treatment)/control] X100. Cell count was taken by removing 1 mL from the flask and fixing
with Lugol's solution. In a separate experiment, plastic containers filled with 2 L lake water, 100 mL of algal mixture (M.
aeruginosa, A. convolutes, and S. quadricauda at 2:1:1 byvol), plus solutions 1 and 2 nano-Ag (1 mg/L). 10 mL aliquots taken at
various times to determine chlorophyll a concentration spectrometrically. In a third experiment, the enclosure experiment was
carried out in a eutrophic lake with six enclosures receiving 150 L lake water and exposed to precipitation and sunlight
penetration. Solutions 1 and 2 of nano-Ag were added to 1 mg/L, and chlorophyll a concentration and cell numbers were
measured as above.
Study Outcome
Colony Growth: In the first experiment, bacterial growth was inhibited by 87% at 1 mg/L nano-Ag compared to control, and
lower concentrations 0.001, 0.01, 0.1 mg/L inhibited bacterial growth at 2.5,14, and 39%, respectively.
Growth, Community Composition, and Chlorophyll Concentration: In experiment 2, both solutions decreased chlorophyll a
in the algal mixture (84% with solution 1, 73% with solution 2). M. aeruginosa was more sensitive than A. convolutus, and S.
quadricauda to both nano-Ag solutions (decrease from 95.5% of algal composition in control to 49% (solution 1) and 21 %
(solution 2)). M. aeruginosa growth was also inhibited 93% by solution 1 and 95% by solution 2, whereas there was little/no
inhibition for A. convolutus (31 % and 0%, respectively) or S. quadricauda (0% for both solutions) on day 10.
Community Composition and Cell Count: In experiment 3, both solutions inhibited algal growth (55% and 64%, respectively).
Solution 1 reduced cyanobacteria percent composition from 69% (control) to 41% (treatment), and solution 2 reduced bacterial
composition to 37% on day 10. Cell count for green algae was not significantly impacted, while M. aeruginosa cell numbers were
reduced by 85 and 87% (addition of solutions land 2, respectively).
B-29
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Miao et al. (2009) The algal toxicity of silver engineered nanoparticles and detoxification by
exopolymeric substances.
Test Species
Coastal marine diatom (Thalassiosira weissflogii CCMP 1336)
Material
Commercial nano-Ag powder supplied by Nanostructured & Amorphous Materials Inc. (Houston, TX, USA).
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 60-70 nm (determined using TEM)
Size Distribution: Not reported
Solubility: Not reported
Surface Area: 9-11 m2/g (provided by manufacturer)
Surface Treatment: polyvinylpyrrolidone (PVP) surfactant
and suspended with Suwannee River Fulvic Acid
Surface Charge: Not reported
Exposure Media: f/2 Medium with a basis of artificial
seawater containing different nutrient conditions (nutrient-
enriched [+NE], nitrogen-limited [-N], and phosphorous-
limited [-P])
Cell Density: 15,000 to 65,000 cells/ml, with higher cell
density used in nutrient-limited and higher nano-Ag
concentration treatments.
Protocol
Exposure Duration: 48 hours
Endpoints: Growth, quantum yield, and chlorophyll a
production
Exposure Concentrations: Dispersed in the 0.22 uM
fraction as (1) Total Ag: 2.12 x 10-1°, 1.06 x 1Q-9, 5.30 x 10-9,
2.65 x 10-9, and 1.03 x 10-?M; (2) Total dissolved Ag: 2.28 x
ID-9, 4.56 x 10-8, 2.28 x 10-7, 1.14 x 10-6, 5.70 x 10-6, and
2.21 x 10-5 M; or (3) FreeAg+: 1.23 x 10-1", 2.46 x 10-", 1.23
x 10-12, 6.14 x 10-12, 3.07 x 1Q-11, and 1.19 x 1Q-1°M.
Methods: In +NE experiment, diatoms were harvested after acclimating in f/2 medium and arriving at mid-exponential growth
phase. Cells were then suspended in toxicity media to which a range of nano-Ag concentrations had been added. In -N and -P
test, +NE cells further incubated in f/2 medium without any addition of N or P for 2 and 4 days, respectively, before resuspension
in the -N (or -P) toxicity media. Quantum yield was determined by fluorescence induction and relaxation system, and chlorophyll
a content was quantified using a fluorometer. To separate effects of Ag+ from Ag nanoparticles, 4 additional tests were conducted
with +NE culture in f/2 medium. (1) nano-Ag was removed by ultrafiltration through 1 kiloDalton (kDa) membrane to examine
indirect effects on nano-Ag (<1 kDa). (2) Diafiltration was performed to compare photosynthesis, chlorophyll a, and growth in
treatments and control. (3) Glutathione (GSH) and cysteine were added to the nano-Ag stock to assess direct effect s from
nano-Ag. (4) Nano-Ag aggregate toxicity was assessed by mixing 4.5 x 1Q-4 M GSH with 4.63 x 1Q-4 M nano-Ag and adding to
nutrient enriched cells.
Study Outcome
Because the concentration of Ag was so much higher in the <1 kDa fraction than in the <1 kDa-0.22 urn fraction (2.21 x 1Q-5
versus 1.03 x 1Q-7 M), and because the cellular concentration of Ag- was 10-fold higher than maximum possible nano-Ag
concentrations, it was deemed that the direct effects from the nanoparticles were negligible compared to the indirect effects from
the released Ag+. Results are therefore presented in terms of indirect effects from free and cellularly accumulated Ag+.
+NE
ICsofor Free Ag+(M)
-N -P
<1kDa
Growth Inhibition
2.16x10-12 1.02 x10-11 2.14 x10-11 1.03 x 1Q-12
Quantum Yield Inhibition
8.83 xio-11 Not inhibited more than 50% 6.36x10-"
Chlorophyll a Inhibition
5.82x10-12 Not inhibited more than 50% 4.13 x 10-12
ICso for Ag+ Accumulated in the Cell (M)
+NE -N -P <1kDa
Growth Inhibition
3.11x103 1.61x104 2.08 x104 1.37 x 1Q3
Quantum Yield Inhibition
1.84 x 105 Not inhibited more than 50% 1.72 x 10=
Chlorophyll a Inhibition
7.39x103 Not inhibited more than 50% 6.29 x 1Q3
B-30
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Navarro et al. (2008) Toxicity of silver nanoparticle to Chlamydomonas reinhardtii.
Test Species
Freshwater green algae (Chlamydomonas reinhardtii)
Material
Commercial nano-Ag suspension supplied by Nanosys GmbH (Wolfhaldon, Switzerland).
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Carbonate-coated
Surface Charge: -36.6 ± 3.2 mV (£ potential at pH 7.52
determined using DLS with Zeta Sizer)
Exposure Media: MOPS media
Cell Density: 2 xl05cells/mL
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 44 nm (determined using DLS and TEM)
Size Distribution:^ 0-200 nm
Protocol
Exposure Duration: 1-5 hours
Endpoint: Photosynthetic yield
Exposure Concentrations: 10-100,000 nM
Methods: Toxicity of nano-Ag and AgNOs to algal photosynthesis was assessed by dose-response experiments, and
photosynthetic yield was measured periodically. To examine effects of Ag+, cysteine, an amino acid, was added in varying
concentrations to 100 nM AgNOs solution to which algae were exposed for 1 hour, and photosynthetic yield was recorded. The
role of Ag+ in toxicity of nano-Ag was examined by exposing algae for 1 hour to 5 or 10 uM nano-Ag and cysteine concentrations
ranging from 10 to 500 nM, and results were plotted as a function of calculated Ag+. Photosynthetic values were reported as
percent of the controls, and values were plotted as a function of measured values of total Ag and Ag+ to obtain ECsos.
Study Outcome
ECsos for nano-Ag are presented as a function of total Ag content and free Ag- concentrations, respectively, at the beginning of
the experiment. The ECsos for nano-Ag complexed with cysteine are also presented. Based on total Ag concentration, AgNOs
appeared to be more toxic than nano-Ag, but based on Ag+, nano-Ag appeared to be more toxic than AgNOs.
Total Ag
1-hour: 3,300 nM
2-hour: 1,049nM
3-hour: 879 nM
4-hour: 801 nM
5-hour: 829 nM
Free Ag+
1-hour:33nM
2-hour: 10 nM
3-hour: 9 DM
4-hour: 8 nM
5-hour: 8 nM
Nano-Ag + cysteine (expressed as free Ag+)
1-hour (5 uM nano-Ag + cysteine): 57 nM
1-hour (10 uM nano-Ag + cysteine): 61 nM
B-31
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B.4. Summary of Nano-Ag Effects in Aquatic Invertebrates
Allen et al. (2010) Effects from filtration, capping agents, and presence/ absence of food on the
toxicity of silver nanoparticles to Daphnia magna.
Test Species
Daphnia magna
Material
Commercial nano-Ag from Sigma Aldrich (organically coated or uncoated); laboratory synthesized nano-Ag from AgNOs in
combination with citrate or coffee solutions.
Shape: Not reported Surface Area: Not reported
Composition: Not reported Surface Treatment: Proprietary (commercial capping
Crystal Structure: Not reported agent); coffee or citrate (synthesized nanoparticle capping
Average Size: 681.4 and 5,412 nm (commercial uncoated agents)
nano-Ag in moderately hard reconstituted water [MHRW]); Surface Charge: -20.4 mV (commercial, uncoated in
39.39 and 249.8 nm (commercial coated nano-Ag in MHRW); MHRW); -18.3 mV (commercial coated in MHRW); -9.3 mV
101.5 and 773.6 nm (coffee-capped nano-Ag in MHRW); and (coffee-capped in MHRW); and -39.7 mV (citrate-capped in
5.94 and 39.75 nm (citrate-capped nano-Ag in Nanopure) Nanopure)
Solubility: Not reported
Protocol
Exposure Duration: 48 hours Exposure Media: MHRW, PH 7.8-8.0, polished to ASTM
Endpoints: Mortality type II specifications by a Barnstead Nanopure Ultrapure
Exposure Concentrations: Approximately >100,000 ppb (Barnstead Thermolyne) system. Additional characteristics
(commercial uncoated); >100,000 ppb commercial coated; provided in the supplemental information.
215, 800 ppb (coffee capped, 1:10 dilution); 21,580 ppb Organisms per Replicate: Not reported
(coffee capped, 1:100 dilution); 70,000 ppb (citrate capped)
Methods: Toxicity assays exposed D. magna to commercially available nano-Ag (either organically capped or uncapped) and
laboratory synthesized nano-Ag (capped with coffee or citrate). These assays were also carried out for fed and unfed replicates,
as well as replicates using filtered or unfiltered nano-Ag suspensions. X-ray diffraction was used to identify crystalline phases of
nano-Ag. STEM was used to characterize particles and D. magna specimens. Particle size and hydrodynamic diameter were
measured with DLS. Zeta potentials were calculated from micro electrophoresis measurements.
Study Outcome
TEM imaging of D. magna exposed to the LC50 concentrations showed a few silver particles along the membrane of the mid-gut
following exposure to commercial coated nano-Ag, accumulation of silver particles around the gill and gut following exposure to
coffee-capped nano-Ag, accumulation of relatively more particles attaching to the gills and gut following exposure to citrate-
capped nano-Ag. At approximately three times the LCso exposure, silver particles were observed in about the same areas as with
the LCso exposure, but an increased incidence of particles was identified in tissues.
Mortality. Dose-response results spanned an order of magnitude for the various nano-Ag formulations tested. The laboratory
synthesized nano-Ag coated with coffee or citrate resulted in dose-response relationships that were similar to, but slightly higher
than the AgNOs solution (LCsos = 1.0 ug/L [coffee]; 1.1 ug/L [citrate]; 0.7 ug/L [AgNOs]). Commercial nano-Ag was less toxic when
coated than when uncoated (LCsos = 31.5 ug/L [coated]; 16.7 ug/L [uncoated]). Filtered suspensions for both coated and
uncoated commercial nano-Ag were more toxic than when suspensions were unfiltered (LCsos = 4.4 ug/L [coated, filtered]; 31.5
ug/L [coated, unfiltered]; 1.4 ug/L [uncoated, filtered] and 16.7 ug/L [uncoated, unfiltered]). Addition of food resulted in increased
total organic carbon content, and decreased toxicity of unfiltered coated commercial nano-Ag (LCsos = 176.4 ug/L [fed, coated,
unfiltered]; 31.5 ug/L [unfed, coated, unfiltered]).
B-32
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Gao et al. (2009) Dispersion and toxicity of selected manufactured nanomaterials in natural river-
water samples: effects of water chemical composition.
Test Species
Water flea neonates (Ceriodaphnia dubia)
Material
Commercial nano-Ag powder produced by gas-phase condensation and coated with a thin layer (2-3 nm) of metal oxide;
supplied by Quantum Sphere (Santa Ana, CA, USA).
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: -80 nm (Dl water and SR-1 sample), -300
nm (SR-2 sample), >1,000 nm (SR-3 sample), all determined
using DLS)
Size Distribution: 20-30 nm (nominal; provided by
manufacturer), but larger in solution
Protocol
Solubility: The highest analyzed total dissolved and
particulate Ag concentration in the original suspensions was
in Dl water, with concentrations in SR-1 and SR-3 an order
of magnitude lower. The concentrations in SR-2 were an
order of magnitude lower than SR-1 and SR-3.
Surface Area: 14.53 m2/g [reported by Griffitt et al. (2008)1
Surface Treatment: Not reported
Surface Charge: -27.0 mV [( potential in moderately hard
freshwater with pH 8.2, determined using a Zeta Reader Mk
21-11 by Griffitt etal.
Exposure Duration: 48 hours
Endpoint: Mortality/immobility
Exposure Concentrations: Not reported
Exposure Media: Dl water, Suwannee River headwater
sample (SR-1), midsection water sample (SR-2), or delta
water sample (SR-3) diluted with moderately hard water used
as culture medium.
Organisms per Replicate: 5 neonates x 3 replicates
Methods: Neonates were exposed to 5 incremental concentrations of nano-Ag (not reported) and survival was assessed visually
after 48 hours.
Study Outcome
LCso (Dl-water): 0.46 ug/L LCso (SR-1): 6.18 ug/L
LCso (SR-2): 0.771 ug/L
LCso (SR-3): 0.696 ug/L
B-33
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Griffitt et al. (2008) Effects of particle composition and species on toxicity of metallic nanomaterials
in aquatic organisms.
Test Species
Adult water fleas (Daphnia pulex) and water flea neonate (Ceriodaphnia dubia)
Material
Commercial nano-Ag powder produced by gas-phase condensation and coated with a thin layer (2-3 nm) of metal oxide;
supplied by Quantum Sphere (Santa Ana, CA, USA).
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 26.6 ± 8.8 nm (determined using laser
diffraction particle size analyzer)
Size Distribution: Approximately 20-1,000 nm
Solubility: Dissolution 48 hours after resuspension was
0.07% of total mass
Surface Area: 14.53 m2/g (determined using the Brunauer,
Emmett, and Teller method)
Surface Treatment: Sodium citrate stabilizer
Surface Charge: -27.0 mV (( potential in moderately hard
freshwater with pH 8.2, determined using a Zeta Reader Mk
21-11)
Protocol
Exposure Duration: 48 hours
Endpoint: Mortality/Immobility
Exposure Concentrations: The estimated LCso from the
range-finding test and concentrations 0.6-, 0.36-, 1.67-, and
2.78-times the estimated LCso (approximately 0.01, 0.02,
0.04, 0.07, and 0.1 mg/L for D. pulex and 0.02, 0.04, 0.07,
0.1, and 0.2 mg/L for C. dubia)
Methods: Death was assessed by lack of movement or response to gentle prodding. Significance of these results were
determined by comparing nanometal-induced (copper, aluminum, and nickel) effects relative to control.
Study Outcome
Exposure Medium: Moderately hard freshwater (dissolved
oxygen 8.5 to 8.9 mg/L; pH 8.2 ± 1; hardness 142 ± 2 mg/L
as CaCOs; conductivity 395 uS; un-ionized ammonia <0.5
mg/L)
Organisms per Replicate: 5 adult D. pulex or 10 C. dubia
neonates x 4 replicates
LC5o(D. pulex adults): 0.04 mg/L
LC5o(C. dubia neonates): 0.067 mg/L
Daphnia were more susceptible to nano-Ag than to any of the other nanometals tested. The concentration data in this test are
nominal, and the actual concentrations to which the organisms were exposed may be lower than reported, with the nanometals
being correspondingly more toxic.
B-34
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Li et al. (201 Ob) Comparative toxicity study of Ag, Au, and Ag-Au bimetallic nanoparticles on
Daphnia magna.
Test Species
Daphnia magna
Material
Ag nanoparticles synthesized reduction of AgNOs in the presence of different additions of sodium citrate, producing nanoparticles
with different molar ratios of silver ion to citrate. Gold nanoparticles were synthesized in a similar manner. Ag-Au bimetallic
nanoparticles were produced through co-reduction of metal precursor salts and sodium citrate as a reducing and capping agent.
Shape: Spherical, prismatic, and rod-shaped (determined Size Distribution: Not reported
using STEM) Solubility: Not reported
Composition: Nano-Ag with small amounts of sulfur and Surface Area: Not reported
oxygen present (determined using XEDS) Surface Treatment: Sodium citrate-capped
Crystal Structure: Not reported Surface Charge: Not reported
Average Size: 36-66 nm (nano-Ag:citrate ratios 1:1.6-1:4.2,
as prepared); 58-71 nm (nano-Ag:citrate ratios 1:1.6-1:4.2,
in standard synthetic freshwater [SSF] after 0.5 hour); 378-
553 nm (nano-Ag:citrate ratios 1:1.6-1:4.2, in SSF after 24
hours); 41-72 nm (nano-Ag:Au ratios 1:4-4:1, as prepared);
14-73 nm (nano-Ag:Au ratios 1:4-4:1, in SSF after 0.5 hour);
and 17-77 nm (nano-Ag:Au ratios 1:4-4:1, in SSF after 24
hours) (determined using DLS).
Protocol
Exposure Duration: 48 hours Exposure Medium: Standard synthetic freshwater (SSF)
Endpoints: Mortality Organisms per Replicate: 4 organisms per replicate, 4
Exposure Concentrations: 7.8-32 mg/L (determined by replicates per concentration
AAS)
Methods: Nanoparticles were characterized by absorbance measurements, atomic force or electron microscopy, flame atomic
absorption spectrometry, and dynamic light scattering (most morphological details not reported). Acute toxicity tests were
conducted using multi-concentration test including control and ionic silver. Four Daphnia neonates were placed in a centrifuge
tube which was diluted with SSF to obtain the desired concentration of nano-Ag particles. Mortality was assessed at 24 and 48
hours.
Study Outcome
Mortality. The toxicity of all nanoparticles tested was dose- and composition-dependent. Nano-Ag toxicity was found to decrease
with increasing amounts of citrate added in the synthesis process (expressed as ratios of Ag:citrate), which corresponded to an
increase in particle size. Toxicity was not, however, found to be a function of particle size when total silver concentration was
adjusted according to AAS (lethal concentrations ranged from 3 to 4 pg/L for all three particle sizes tested). Authors suggest that
this was due to the large degree of aggregation (analyzed by DLS). Ionic silver was more toxic than nano-Ag in this study. The
LCso for Au nanoparticles was 65-75 mg/L. LCso values for Au-Ag nanoparticles were between those for Au and Ag, but closer to
that of Ag. A 4:1 Ag to Au ratio was less toxic (LCstF 15 pg/L) than pure nano-Ag. A1:4 Ag to Au ratio was more toxic than
anticipated (LCso= 12 pg/L) given the low silver content, but in general, the introduction of gold into silver nanoparticles reduced
the toxicity of the nano-Ag.
B-35
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Kvitek et al. (2009) Initial study on the toxicity of silver nanoparticles (NPs) against Paramecium
caudatum.
Test Species
Unicellular eukaryote (Paramecium caudatum)
Material
Nano-Ag synthesized through a modified Tollens process using AgNOs, NHs, NaOH, and D(+)-maltose monohydrate.
Shape: Spherical Surface Treatment: Unmodified and modified with nonionic
Composition: Not reported surfactant polyethylene (2) sorbitan mono oleate (Tween 80),
Crystal Structure: Not reported polyethylene glycol with molecular weight 35,000 (PEG
Average Size: 27 nm (determined using TEM and UV/vis 35000), and polyvinylpyrrolidone with a molecular weight of
absorption spectra) 360,000 (PVP 360)
Size Distribution: Not reported Surface Charge: -37 mV (£ potential determined using
Solubility: Not reported dynamic light scattering in diluted solution with pH 11.5), 1.23
Surface Area: Not reported mS/cm (conductivity)
Protocol
Exposure Duration: 1 hour Exposure Media: Culture medium (type not reported)
Endpoint: Mortality Organisms per Replicate: 1-5 ml containing 200-300
Exposure Concentrations: Approximately 0.1-100 mg/L organisms/ml x 3 replicates
Methods: A set of about 50 organisms was monitored over an area of 1 cm2. LTso was measured from the moment of the addition
of the nano-Ag into the culture up to the point when 50% of the organisms died. LCsos were then determined from the
dependence of the LTso on nano-Ag concentration.
Study Outcome
LCso (unmodified nano-Ag): 39 mg/L LCso (modified with PEG 35000): -39 mg/L
LCso (modified with Tween 80): 16 mg/L LCso (modified with PVP 360): -39 mg/L
B-36
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Nair et al. (2011) Differential expression of ribosomal protein gene, gonadotrophin releasing
hormone gene and Balbiani ring protein gene in silver nanoparticles exposed Chironomus riparius.
Test Species
Fourth instar larvae of aquatic midge Chironomus riparius (Korea Institute of Toxicology, Daejeon, Korea).
Material
Nano-Ag particles (Sigma-Aldrich, St. Louis, MO).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: 40-70 nm (determined using DLS)
Protocol
Exposure Duration: 24 hours (acute), 25 days (chronic) Exposure Media: Dechlorinated water
Endpoint: Mortality (acute), pupation, emergence failure, Organisms per Replicate: 10 larvae per concentration
reproduction (chronic), and differential gene expression (acute), 50 larvae per concentration (chronic)
Exposure Concentrations: 0.5,1, 2, and 4 mg/L (acute); 0,
0.2, 0.5,1 mg/L (chronic)
Methods: Acute ecotoxicity was assessed in a 24-hour exposure assay using mortality as the endpoint. Chronic ecotoxicity was
determined after a 25-day exposure assay by numbers of successfully emerged adults, pupae, and sex ratio. A Comet assay was
used to determine genotoxicity.
Study Outcome
Mortality. Mortality was observed at acute exposure concentrations of 4 mg/L. LCso was unable to be determined due to absence
of mortality at lower dose levels.
Reproduction. Pupation and adult emergence were statistically significantly inhibited at concentrations >0.2 mg/L. Sex ratio was
also affected by exposure, producing a greater proportion of female midges in all treated groups.
Genotoxicity. DNA damage increased in a dose-related manner; statistically significant results were observed at 1 mg/L.
Gene Expression. CrGnRHI and CrBR2.2 were observed to be significantly upregulated, while CrL15 was observed to be
significantly downregulated.
B-37
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Ringwood et al. (2010) The effects of silver nanoparticles on oyster embryos.
Test Species
Crassostrea virginica
Material
Commercial nano-Ag supplied and characterized by the Wake Forest University Center for Nanotechnology.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 15 ± 6 nm Surface Charge: Not reported
Protocol
Exposure Duration: 48 hours Exposure Media: Natural seawater adjusted to 25 psu
Endpoints: Development, lysosomal destabilization, and Organisms per Replicate: Embryo: 200 per replicate for
metallothionein (MT) gene expression developmental assay, and 4L with 50 embryos/mL for MT
Exposure Concentrations: 0.0016-1.6 ug/L (embryo); assay Acyt rep|icates not reported
0.0016-16 ug/L (adult)
Methods: Embryos were studied in 48-hour developmental assays (range of concentrations) and in tests for sublethal effect on
MT (0.16 ug/L). Adults were tested for effects of lysosomal destabilization of hepatopancreas cells by neutral red assay. Effect of
nano-Ag (0.16 ug/L) on MT gene expression was analyzed in embryos and adults by quantitative real-time PCR.
Study Outcome
Development. Nano-Ag impaired normal embryonic development in a threshold response manner characterized by limited
change compared to control at low doses, and highly significant impairment (approximately 10% normal development rate) at the
highest dose.
Lysosomal Destabilization. In adult oysters, significant adverse effects were noted in a dose-response manner with increasing
dose corresponding with increased percentage of destabilized cells (ranging from approximately 30% to 55% destabilized cells).
MT Gene Expression. Increases in MT mRNA levels were observed in both adults and embryos exposed to 0.16 ug/L. Adults
showed 2.4-fold increase compared to controls, and embryos showed 80-fold increase compared to controls.
B-38
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B.5. Summary of Nano-Ag Effects in Aquatic Vertebrates
Asharani et al. (2008) Toxicity of silver nanoparticles in zebrafish models.
Test Species
Zebrafish embryos (Danio rerio)
Material
Nano-Ag synthesized through reduction of AgNOs with NaBhU.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Capped with soluble potato starch or
Average Size: Not reported bovine serum albumin (BSA)
Size Distribution: 5-20 nm (for both types of surface-treated Surface Charge: Not reported
nano-Ag, determined using TEM)
Protocol
Exposure Duration: 72 hours Exposure Medium: Embryo water (60 mg sea salt per L
Endpoints: Mortality, hatching rate, heart rate, phenotypic ultrapure water)
changes, and apoptosis Organisms per Replicate: 10 embryos x 6 replicates
Exposure Concentrations: 5,10, 25, 50, and 100 ug/mL
Methods: To differentiate between dead and malformed embryos, opaque embryos were transferred to well plates with 4 ml
medium and incubated for 24 hours. Mortality was determined by counting the number dead after 72 hours. Hatching rate was
determined by the number of embryos hatched by 72 hours, and heart rate was recorded using a stopwatch at various stages post-
fertilization. Other phenotypic deformities were also recorded. Embryos were examined using a photographic method. To assess the
level of apoptic cells, acridine orange was added to all embryos exposed to nano-Ag above 50 ug/mL and samples were examined
under a microscope. Embryos at various stages were collected and the chorion poked to aspirate fluid containing unidentified brown
flakes. Flakes were examined using DAPI staining.
Study Outcome
Nano-Ag was uniformly distributed throughout the embryos, on the skin, and in brain and heart cells, showing affinity for the cell
nucleus. Nano-Ag in the brain was well-dispersed, but clumping was observed elsewhere.
Mortality. LCsos varied from 25 to 50 ug/mL, and were dependent on the growth stage (64-128 cell stage) of the embryo, with the
later embryonic stages exhibiting more resistance to nano-Ag.
Hatching Rate and Heart Rate. Delay was observed with increasing nano-Ag concentrations, with 15% of embryos exposed to
BSA-capped nano-Ag and 33% of embryos exposed to starch-capped nano-Ag hatching at the 100 ug/mL level. Heart rate
decreased with increasing nano-Ag concentration, reached an average of 39 beats/minute above a concentration of 50 ug/mL
(versus 150 beats/minute in the controls).
Phenotypic Changes and Apoptosis. Above 50 ug/mL concentrations of BSA-capped nano-Ag and starch-capped nano-Ag, 60-
90% of embryos exhibited severe phenotypic changes characterized by bent and twisted notochord, accumulation of blood in blood
vessels near the tail, low heart rate, pericardial edema, degeneration of body parts, and distorted yolk sacs. About 40-50% of
embryos displayed apoptosis spots all over the body. Decay was observed primarily near the head and tail.
B-39
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Bar-Man et al. (2009) Toxicity assessments of multisized gold and silver nanoparticles in zebrafish
embryos.
Test Species
Zebrafish embryos (Danio rerio)
Material
Nano-Ag of various sizes synthesized using commonly used methods utilizing various strengths and types of reducing agents (no
specific details provided).
Shape: Not reported Size Distribution: <4.5 to >7.5 nm (3 nm group), <12 to
Composition: Not reported >20.1 nm (10 nm group), <31 to >71 nm (50 nm group), and
Crystal Structure: Not reported <85 to >120 nm (100 nm group)
Average Sizes: 5.9 nm (nominally 3 nm), 15.3 nm (nominally Solubility: Not reported
10 nm), 51.2 nm (nominally 50 nm), and 108.9 nm (nominally Surface Area: Not reported
100 nm) (determined using TEM) Surface Treatment: Not reported
Surface Charge: Not reported
Protocol
Exposure Duration: 120 hours (5 days) Exposure Medium: Egg water
Endpoints: Mortality and morphology Organisms per Replicate: 12 embryos x 3 replicates
Exposure Concentrations: 0.25, 2.5, 25,100, and 250 uM
Methods: Dosing solutions were prepared by transferring nano-Ag in reverse osmosis water to egg water, which was changed daily
to prevent destabilization of nano-Ag in solution. Using a scoring system, embryos were evaluated for severity of morphological
defects, survival, and toxic adverse effects. The 100 uM treatment group was used to analyze sublethal toxic effects. At 120-h post
fertilization, embryos exposed to 75 uM nano-Ag were examined by instrumental neutron activation analysis (INAA) to determine
whether nano-Ag were taken up or adsorbed.
Study Outcome
Mortality. Almost 100% mortality occurred in all size groups at 250 uM nano-Ag 120-h post-fertilization.
LCso(3 nm nano-Ag): 93.31 uM LCso(10 nm nano-Ag): 125.66 uM
LCso(50 nm nano-Ag): 126.96 uM LCso(100 nm nano-Ag): 137.26 uM
Phenotypic Effects. Sublethal endpoints that were statistically significant from controls were opaque and nondepleted yolk; small
head; jaw and snout malformations; stunted growth; circulatory malformations, such as hemorrhages and blood clots; tail
malformations; body degradation, such as bubble-like formations on yolk sac and decaying tail tissue; pericardial edema; bent spine;
and not hatching. Preliminary findings suggest that there was embryonic uptake and/or adsorption of nano-Ag
B-40
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Bilberg et al. (2010) Silver nanoparticles and silver nitrate cause respiratory stress in Eurasian perch
(Perca fluviatilis).
Test Species
Eurasian perch (Perca fluviatilis), unspecified age
Material
Commercial nano-Ag powder purchased from NanoAmor (Houston, USA). A water dispersion was prepared by suspending 0.5 g
nano-Ag powder in 100 ml Milli-Q water followed by ultrasonication and filtration. AgNOs pellets were purchased from SigmaAldrich.
Shape: Spherical (reported by manufacturer); elliptical, Size Distribution: 30-40 nm (reported by manufacturer)
multifaceted, and triangular shapes (determined using TEM) Solubility: Not reported
Composition: 99.5% purity (reported by manufacturer) Surface Area: Not reported
Crystal Structure: basic cubic orientation, crystallite size of Surface Treatment: 0.2% PVP coated
-78.1 nm (determined using PXRD) Surface Charge:-28.5 ± 0.8 mV (determined using DLS)
Average Size: 81 ± 2 nm with aspect ratio of 1.2 ± 0.2 nm
(determined using TEM)
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 0, 63,129, and 300 pg/L
Endpoints: Changes in oxygen consumption (Mo2) as (measured by ICP-OES)
expressed by the basal metabolic rate (BMR) and the critical Exposure Medium: Aarhus city tap water
oxygen tension (Pent) (below which the fish cannot maintain Organisms per Replicate: 6 animals per test group (no
aerobic metabolism) replicates)
Methods: Five test groups of six perch (male and female) were exposed to nano-Ag in water. Oxygen consumption (Mo2) was
measured by automated intermittent closed respirometry. Each fish was tested for one day pre-exposure for background BMR and
Pcrit before the 24-hour exposure.
Study Outcome
BMR. Pre-exposure BMR was similar for all groups. The three concentrations of nano-Ag did not have any effect on exposure BMR.
Only the high dose of AgNOs (386 pg/L) resulted in statistically significant elevation in exposure BMR.
Pcrit. Pre-exposure Pcrit was similar for all groups. No significant differences were observed for the two lowest nano-Ag concentration
test groups (63 and 129 pg/L). However, Pcrit statistically significantly increased from 4.8 to 9.2 kPa after exposure to 300 pg/L
nano-Ag, Pcrit also significantly increased for both exposure doses of AgNOs (increased 31 % for low dose and 48% for high dose).
B-41
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Chae et al. (2009) Evaluation of the toxic impact of silver nanoparticles on Japanese medaka (Oryzias
latipes).
Test Species
Japanese medaka d-rR (Oryzias latipes), 4 to 5 months old
Material
Commercial nano-Ag powder purchased from Sigma Aldrich (Korea).
Surface Area: 50,710 nm2 (determined using X-ray
diffraction)
Surface Treatment: Not reported
Surface Charge: 29.9 ± 3.55 mV at pH 7.5 (( potential
determined using electrophoretic light scattering [ELS]
spectrophotometry)
Exposure Medium: Water (pH 7.0-8.0)
Organisms per Replicate: 7-8 fish x 3 replicates
Shape: Cuboctahedral and decahedral
Composition: Not reported
Crystal Structure: Not reported
Average Size: 49.6 nm (determined using TEM)
Size Distribution: Approximately 10-120 nm
Solubility: Not reported
Protocol
Exposure Duration: 24-96 hours and 10 days
Endpoints: Acute toxicity (mortality), gene expression
Exposure Concentrations: 0-50 ug/L total Ag (determined
using inductively coupled plasma-optical emission
spectroscopy [ICP-OES])
Methods: Fish were exposed to a flow-through system (1 L/24 hours to renew 50% of aquaria each day), and lethality was
assessed after 96 hours. After 24, 48, and 96 hours, 2 fish exposed to concentrations of 1 ug/L and 25 ug/L from each replicate were
killed and livers examined. The remaining 2 fish were exposed only to 1 ug/L and livers extracted after 10 days. RNAwas isolated
from livers and gene expression analyzed using real-time RT-PCR method. Genes analyzed were metallothionein (MT), heat shock
protein 70 (HSP70), glutathione S transferase (GST), p53, cytochromep450 1A(CYP1A), and transferrin (TF). The 18S rRNAgene
was the endogenous control.
Study Outcome
LOEC: 25 ug/L
LCso: 34.6 ug/L
Gene Expression. Nano-Ag exposure led to significant induction (6-fold increase) of MT after 24 hours in the 25 ug/L group and
increased levels of mRNA (2.2-fold) after 48 hours, but after 4 days, mRNA returned to the basal level. Statistically significant
induction of HSP70 also occurred at 25 ug/L nano-Ag and mRNA increased 2.5-fold after 24 hours, then decreased by 3.5-fold after
96 hours. 10-day exposure did not result in statistically significant induction of HSP70. At 1 ug/L, GST was significantly induced
initially, with the highest mRNA levels seen at 24 hours (3-fold increase) and 48 hours (2.5-fold increase), but were not significantly
different for 10-day exposure. At 25 ug/L, GST was statistically significantly induced at both 24 hours and 96 hours. CYP1A was
significantly induced after 24 hours in both the 1 and 25 ug/L groups (4- and 3.2-fold increase, respectively), but no statistically
significant induction occurred after 10-day exposure. Significant induction of P53 occurred after 24 hours in both the 1 and 25 ug/L
groups (3.5- and 3.4-fold increase, respectively), but no significant induction occurred after 10-day exposure. TF was significantly
down-regulated in the 25 ug/L group after 48 hours (73.3-fold lower) and 96 hours (35.7-fold lower), and significantly down-regulated
in the 1 ug/L group after 48 hours (8-fold lower) and after 10-day exposure.
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Choi et al. (2010) Induction of oxidative stress and apoptosis by silver nanoparticles in the liver of
adult zebrafish.
Test Species
Adult zebrafish (Danio rerio)
Material
Water-based solution of 1.0 g/L nano-Ag (Nanopoly, Seoul, Korea).
Shape: Spherical Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: 5-20 nm (determined using TEM)
Protocol
Exposure Duration: 24 hours Exposure Medium: Deionized water
Endpoints: Acute toxicity (mortality) and hepatocellular Organisms per Replicate: 5 organisms per treatment (no
toxicity replicates)
Exposure Concentrations: 30,60, and 120 mg/L
Methods: Fish were exposed to various concentrations of nano-Ag deionized by treatment with C ion exchange resin.
Histopathology was performed on liver tissues to identify any cellular alterations.
Study Outcome
Mortality. 24 hour LCso of approximately 250 mg/L.
Hepatocellular Toxicity. Disruption of hepatic cell cords, chromatin condensation, and pyknosis were observed in the livers of all
treated fish. ATUNEL assay confirmed that these changes were due to apoptosis. MT2 mRNA expression significantly increased in
a dose-dependent manner (3.9-, 5.4-. and 7.1-fold at 30, 60, and 120 mg/L, respectively). MDA levels increased at 60 and 120 mg/L
concentrations (1.5-and 1.7-fold, respectively), and GSH levels increased at the highest concentration (120 mg/L). DNA damage
was detected in tissues of fish exposed to 120 mg/L nano-Ag. Authors did not find evidence of altered mRNA levels of the p53
protein.
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Farkas et al. (2011) Uptake and effects of manufactured silver nanoparticles in rainbow trout
(Oncorhynchus mykiss) gill cells.
Test Species
Rainbow trout (Oncorhynchus mykiss) gill cells isolated from juvenile fish with body weights of 150-200 g.
Material
Nano-Ag synthesized by sodium borohydride reduction of AgNOs dissolved in Milli-Q water with citrate as a capping agent. PVP-
coated nano-Ag was obtained from the University of Manchester, UK.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Citrate-capped or PVP-coated
Average Size: 12 nm (citrate-capped); 7 nm (PVP-coated) Surface Charge: -8 mV (citrate-capped) and -4 mV (PVP-
(determined using TEM) coated)
Size Distribution: 3-40 nm (citrate-capped); 1-60 nm (PVP-
coated) (determined using TEM)
Protocol
Exposure Duration: 48 hours (monolayer, cytotoxicity, and Exposure Medium: L15 cell culture media (monolayer,
oxidative stress tests); 48 hours or 24 hours (multilayer) cytotoxicity, and oxidative stress tests); L15 or artificial
Endpoints: Cytotoxicity and oxidative stress freshwater (multilayer)
Exposure Concentrations: 20 mg/L citrate-capped or 10 Organisms per Replicate: Isolated cell density = 5 x 105 to
mg/L PVP-capped (monolayer); 10 mg/L citrate- or PVP- 1 x 1Q6 cells/cm (monolayer); 1.4-2.5 x 106cells/mL
capped (multilayer); 0.1,1, 5, or 10 mg/L (cytotoxicity and (multilayer); 4 individuals (cytotoxicity test)
oxidative stress tests)
Methods: Gill cells were isolated by a modified procedure described by Kelly et al. (2000) and seeded onto culture dishes to
produce monolayers or multilayers. Cultures of cell multilayers were evaluated for transepithelial resistance (TER) by Epithelial
Voltohmmeter. The uptake of nano-Ag into monolayered cells was evaluated by exposing cells to citrate-capped or PVP-capped
nano-Ag and observing the possible clearance of nanoparticles using light microscopy imaging. Monolayer cells were also examined
using TEM and EDX for nano-Ag accumulation from exposure media. The uptake and transport of nano-Ag into multilayered cells
was evaluated in a similar manner. Multilayer cells were sampled by detachment with Trypsin and analyzed for total silver content
using ICP-MS. Multilayer cells were also analyzed for the number of nanoparticles associated with the cell surface. Cytotoxicity was
measured as the reduction of membrane integrity (Schreeretal., 2005) as indicated by fluorescence measurements. Oxidative
stress was measured by the depletion of reduced glutathione (GSH) as indicated by fluorescent probe (monochlorobimane, or
mBCI).
Study Outcome
In monolayered cells, accumulated nanoparticles were observed within the gill cells around the nuclei of citrate-capped nano-Ag-
exposed cells, whereas no accumulated particles were observed in the control cells. No effective clearance of accumulated particles
was observed. No nanoparticle accumulations were observed in PVP-coated treatment groups. TEM and EDX verified citrate-
nano-Ag accumulation within lamellar bodies and single particles present in the cystol and PVP-coated nano-Ag in vesicle-like
structures. In multilayered cells, nano-sized particles were observed in epithelia for both nano-Ag exposures but were too small for
EDX analysis. Silver transported through the epithelium was approximately 19 ng/cnr2 for citrate-capped nano-Ag, and 47 ng/cnr2
for PVP-capped nano-Ag (combined results for both exposure media - when separated, FW showed reduced uptake compared to
L15). Silver transport through epithelia was dependent on TER in all cases.
Cytotoxicity and Oxidative Stress. Cells exposed to citrate-capped nano-Ag or PVP-coated nano-Ag showed statistically
significant dose-dependent reduction in viability as evidenced by reduced membrane integrity at exposures levels > 5 mg/L. Cells
exposed to silver ions significantly reduced viability at levels > 1 mg/L. Significantly higher levels of GSH were found at exposure
levels > 5 mg/L for citrate-capped nano-Ag and for silver ions, while PVP-coated nano-Ag exposure significantly increased GSH at
exposure levels >1 mg/L.
B-44
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Griffitt et al. (2009) Comparison of molecular and histological changes in zebrafish gills exposed to
metallic nanoparticles.
Test Species
Adult female zebrafish (Danio rerio)
Material
Commercial nano-Ag powder produced by gas-phase condensation and coated with a thin layer (2-3 nm) of metal oxide; supplied
by Quantum Sphere (Santa Ana, CA, USA).
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 26.6 ± 8.8 nm (determined using laser
diffraction particle size analyzer)
Size Distribution: Approximately 20-1,000 nm
Protocol
Exposure Duration: 48 hours
Endpoints: Mortality, gill histopathology, and gene
expression
Solubility: After 24 and 48 hours, only 5.1% of initial
nano-Ag dose remained, with the rest aggregating and
settling out. Soluble Ag decreased over time, reaching a
peak concentration of 4.9 ug/L at 2 hours, and decreasing to
0.2 ug/L at 48 hours.
Surface Area: 14.53 m2/g (determined using the Brunauer,
Emmett, and Teller method)
Surface Treatment: Not reported
Surface Charge: -27.0 mV (( potential in moderately hard
freshwater with pH 8.2, determined using a Zeta Reader Mk
21-11)
Exposure Concentration: 1,000 ug/L nano-Ag
(corresponding to 0.014 m2/L if monodispersed)
Exposure Medium: 0.22 urn filtered moderately hard water
Organisms per Replicate: 4 fish x 3 replicates
Methods: Static renewal assays were conducted using the no observed effect concentration (NOEC) of nano-Ag. Gills and whole
carcasses were analyzed for metal concentration at 24 and 48 hours, and lethality was assessed. After 48 hours, gills were
examined for structural changes in filament and lamellae, characterized by increased cellularity in intramellar space. RNAfrom gill
tissue samples was also analyzed by microarray to determine gene response.
Study Outcome
Lethality. NOEC concentrations were used, and no mortality occurred. There were no changes in appearance or behavior.
Gill Histopathology. No significant change in gill filament width resulted from exposure to nano-Ag.
Gene Expression. At 24 hours, nano-Ag-exposed fish exhibited 66 significantly upregulated genes and 82 downregulated. At 48
hours, there were 126 significantly upregulated and 336 downregulated genes.
B-45
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Hinther et al. (2010) Nanometals induce stress and alter thyroid hormone action in amphibia at or
below North American water quality guidelines.
Test Species
Rana catesbeiana tissue.
Material
Commercial nano-Ag supplied by Northern Nanotechnologies (ViveNano, Toronto, Canada).
Shape: Not reported Solubility: Soluble in water
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Carboxyl-functionalized coating
Average Size: Not reported Surface Charge: 41.14 mV
Size Distribution: Approximately 2-10 nm (determined
using TEM and DLS)
Protocol
Exposure Duration: 48 hours Exposure Concentration: 0.06 pg/L-5.5 mg/L
Endpoints: Acute toxicity (cell viability), thyroid hormone Exposure Medium: Not reported
receptor (TR(3) transcript levels, and Rana larval keratin I Organisms per Replicate: 8-16 animals per replicate; 8-10
(RLKI) transcript levels biopsies per animal
Methods: Stress and thyroid hormone signaling were assessed in frog tissue after exposure to nano-Ag using a cultured tail fin
biopsy (C-fin) assay. Nano-Ag effects were assessed in the presence and absence of Ts using a quantitative real-time polymerase
chain reaction technique.
Study Outcome
Cell viability. LCso = 0.95 mg/L
TRp and RLK1. Thyroid-hormone-response gene transcript levels were observed to significantly decrease after exposure to 10nM
nano-Ag and stress-response gene transcript levels to significantly increase after exposure to 5 and 10nM nano-Ag. Catalase
transcript levels were significantly decreased after exposure to 10 nM nano-Ag.
B-46
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Kennedy et al. (2010) Fractionating nanosilver: importance for determining toxicity to aquatic test
organisms.
Test Species
Fathead minnow (Pimephales promelas) larvae (24 hours old).
Material
Nano-Ag dispersions obtained from NanoComposix (Biopure, San Diego, CA), PVP-coated nano-Ag synthesized at Luna
Innovations (Blacksburg, VA), and citrate and EDTAstabilized nano-Ag synthesized at VATech (Blacksburg, VA).
Shape: Mostly spherical with some rods (determined using Size Distribution: 10-50 nm (determined using TEM)
TEM) Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Citrate-, PVP-, or EDTA-coated
Average Size: Not reported Surface Charge: -15 to -34 mV
Protocol
Exposure Duration: 48 hours Exposure Medium: Moderately hard reconstituted water
Endpoints: Mortality Organisms per Replicate: Not reported, 4 replicates
Exposure Concentrations: -6-126 pg/L
Methods: Mortality was measured as an indicator of acute toxicity after 48 hours of exposure to nano-Ag of various sizes and
coatings.
Study Outcome:
LCso (AgNOs): 6.5 ug/L LCso (citrate-coated): 19.2 ug/L LCso (10 nm): 41.0 ug/L
LCso (20 nm): 64.1 ug/L LCso (50 nm): 60.7 ug/L LCso (PVP-coated): >69.9 ug/L
LCso (EDTA-coated): 55.2 ug/L
B-47
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Laban et al. (2009) The effects of silver nanoparticles on fathead minnow (Pimephales promelas)
embryos.
Test Species
Fathead minnow embryos (Pimephales promelas)
Material
Commercial nano-Ag supplied by either NanoAmor (Houston, TX, USA) or Sigma-Aldrich (St. Louis, MO, USA)
Shape: Not reported Solubility: Increasing concentrations of nano-Ag (Sigma-
Composition: Not reported Aldrich) resulted in a decreased percentage of dissolved
Crystal Structure: Not reported silver
Average Size: approximately 31-50 nm (NanoAmor) and Surface Area: Not reported
21-60 nm (Sigma-Aldrich) (determined using TEM) Surface Treatment: Not reported
Size Distribution 29-100 nm (NanoAmor) and 21 to >300 Surface Charge: Not reported
nm (Sigma-Aldrich)
Protocol
Exposure Duration: 96 hours Exposure Medium: Test water with dissolved oxygen
Endpoints: Mortality and morphology content of 7.5 mg/L, pH 8.3-8.5, and water hardness
Exposure Concentrations: 0.625,1.25, 2.5, 5, 7.5,10,15, 215-240 mg/L as CaCOs
20, and 25 mg/L Organisms per Replicate: 10 embryos x 3 replicates
Methods: Static nonrenewal tests were conducted using both commercial nano-Ag products and AgNOs. To assess mortality and
developmental changes, dead and abnormal embryos were counted, and uptake was analyzed by TEM. Toxicity was assessed
using both stirred and sonicated samples.
Study Outcome
LCso (sonicated NanoAmor): 1.25 mg/L LCso (sonicated Sigma-Aldrich): 1.36 mg/L LCso (AgNOs): 15 ug/L
LCso (stirred NanoAmor): 9.4 mg/L LCso (stirred Sigma-Aldrich): 10.6 mg/L
Significant abnormalities included absence of air sac, pericardial and yolk sac edema, hemorrhages to head and pericardial area,
and lordosis (upward bending of vertebral column).
B-48
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Lee et al. (2007) In vivo imaging of transport and biocompatibility of single silver nanoparticles in early
development of zebrafish embryos.
Test Species
Zebrafish embryos (Danio rerio), at the 8-64-cell stages and 0.75-2.25 hours post fertilization
Material
Nano-Ag synthesized by sAgCI04 reduction with sodium citrate and NaBhU.
Size Distribution: Approximately 5-46 nm
Solubility: Stable in egg water
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Medium: Egg water
Organisms per Replicate: 35-40 embryos x 3 replicates
Shape: Spherical
Composition: Not reported
Crystal Structure: Not reported
Average Size: 11.6 ± 3.5 nm (determined using
resolution TEM)
Protocol
Exposure Duration: 120 hours (5 days)
Endpoints: Mortality and morphology
Exposure Concentrations: 0.04, 0.06, 0.07, 0.08, 0.29,
0.38, 0.57, 0.66, and 0.71 nM
Methods: Nano-Ag effects on embryonic development and survival were determined through direct observation. Single nanoparticle
transport was analyzed in vivo in embryos exposed to nano-Ag in real-time using dark-field single-nanoparticle optical microscopy
and spectroscopy (SNOMS).
Study Outcome
Ag nanoparticles were observed embedded in retina, brain, heart, gill arches, and tail. The number of dead and deformed zebrafish
increased with increasing dose, with no normal development occurring after the 0.19 nM concentration level (i.e., all zebrafish were
either dead or deformed at concentrations higher than 0.19 nM). Pinfold abnormality and tail/spinal cord flexure and truncation were
observed at all concentrations. Cardiac malformation and yolk sac edema were observed in the 0.07 to 0.71 nM concentration
range. Head edema and eye deformity were observed only at the higher concentrations ranging from 0.44-0.71 and 0.66-0.71,
respectively. Multiple deformities in a single embryo were observed concentrations higher than 0.38 nM.
B-49
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Scown et al. (2010) Effects of aqueous exposure to silver nanoparticles of different sizes in rainbow
trout.
Test Species
Juvenile female rainbow trout (Oncorhynchus mykiss), mean weight 19.52 g (Hatchlands Trout Farm, Devon, UK)
Material
Commercial nano-Ag provided by Nanostructured and Amorphous Materials Inc. (Houston, TX).
Shape: Not reported
Composition: Not reported
Crystal Structure: Cubic (determined using XRD)
Average Size: 49 nm (small particles Nio) and 114 nm
(medium particles Nss) (determined using TEM)
Size Distribution Not reported
Protocol
Exposure Duration: 10 days
Endpoint: Lipid peroxidation and gene expression
Exposure Concentrations: 10 and 100 pg/L
Solubility: Not reported
Surface Area: 2.0 m2/g (small particles), 2.9 m2/g (medium
particles) (determined using BET)
Surface Treatment: Not reported
Surface Charge: -12.52 mV (small particles), -6.5 mV
(medium particles)
Exposure Medium: Dechlorinated tap water (pH 7.79,
conductivity 189.58 |js, temperature 9-11°C)
Organisms per Replicate: 8 fish per treatment x 2
replicates
Methods: A 10-day exposure assay using AgNOs and two sizes of nano-Ag was conducted. Particle uptake into various tissues was
examined by inductively coupled plasma-optical emission spectrometry. Levels of thiobarbituric-acid-reactive substances (TBARS)
were monitored as indicators of lipid peroxidation. Differential expression of cyp1a2, cyp3a45, hspTOa, gpx, and g6pcf were analyzed
in the gills and liver using real-time PCR.
Study Outcome
Silver amounts were statistically significantly increased in and on the gills and livers of fish treated with 10 pg/L Nio and Nss nano-Ag
and 100 pg/L Nss nano-Ag compared to control.
Lipid Peroxidation. No evidence of lipid peroxidation was observed after analysis of TBARS except in the group treated with 100
|jg/L Nio nano-Ag.
Gene Expression. Expression of gene cyp1a2 was increased threefold compared to control in the gills offish exposed to 100 pg/L
Nio nano-Ag.
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Shahbazzadeh et al. (2009) The effects of nanosilver (Nanocid®) on survival percentage of rainbow
trout (Oncorhynchus mykiss).
Test Species
Rainbow trout (Oncorhynchus mykiss), median weight 1.049 g
Material
Commercial colloidal Ag suspension (Nanocid L-series colloidal product) provided by Nasb Pars Co. (Iran).
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: Not reported
Size Distribution Not reported
Protocol
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Duration: 96 hours
Endpoint: Mortality
Exposure Concentrations: 1.25, 2.5, 5,10, 20, 30-50, and
60-70 ppm
Exposure Medium: Tap water maintained at 16 ± 1°C,
dissolved oxygen >8 mg/L, carbon dioxide <6 mg/L,
ammonia <0.01 mg/L, nitrite <0.1 mg/L, water hardness
<200 mg/L as CaCOs, conductivity 780 uS, pH 7.5-8.4
Organisms per Replicate: 30 fish x o replicates
Methods: Partial mortality data at 24 and 96 hours were used to determine LCso values, and all calculations were based on mean
measured concentrations in the aquarium, rather than nominal concentrations.
Study Outcome
LCso (48-hour): 3.5 mg/L LCso (72-hour): 3 mg/L LCso (96-hour): 2.3 mg/L
B-51
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Wu et al. (2010) Effects of silver nanoparticles on the development and histopathology biomarkers of
Japanese medaka (Oryzias latipes) using the partial-life test.
Test Species
Japanese medaka (Oryzias latipes) aged 7 days postfertilization to 60 days posthatch.
Material
Nano-Ag synthesized through reduction of 16.9% AgNOs with sodium hexametaphosphate, sodium hypophoshite, and PVR
Shape: Spherical (determined using TEM) Size Distribution: 20-37 nm (determined using TEM)
Composition: Not reported Solubility: Not reported
Crystal Structure: Face-centered cubic phase (determined Surface Area: Not reported
using XRD) Surface Treatment: Not reported
Average Size: 25 nm (determined using TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 48 hours (adult acute toxicity); 168 Exposure Concentrations: 0, 0.5,1.0, 2.0, 4.0, and 8.0
hours (embryo acute toxicity); ~ 70 days (developmental mg/L (adult acute toxicity); 0, 0.5,1.0, 2.0, and 4.0 mg/L
toxicity) (embryo acute toxicity); 0,100, 200, 400, 600, 800, and
Endpoints: Mortality (adult/embryo acute toxicity); 1,000 ug/L (developmental toxicity)
developmental retardation, morphological defects, hatching Exposure Medium: Not reported
rate, and histopathology (developmental toxicity) Organisms per Replicate: 16 adult x 2 replicates (adult
acute toxicity); 30 embryos x 2 replicates (embryo acute
toxicity); 95 embryo x 3 replicates (developmental toxicity)
Methods: Mortality was recorded for calculating the LCso based on the Karber method. In the developmental toxicity study, embryos
were exposed to 0-1,000 ug/L until 60 days post-hatch. On Days 5 and 7 postfertilization (dpf), morphological abnormalities were
noted with a stereo dissecting microscope, and the maximum width of the optic tectum (correlating to brain development) was
measured. Embryos were also observed under an inverted fluorescence microscope to evaluate the autofluorescence of
leucophores. Histopathological assessment of optic development was carried out on six 2-day-old larvae (pooled) from each group
using light microscopy. Remaining fish were collected on 60 dph and examined for body weight and length.
Study Outcome
Mortality. In adults, 100% mortality was observed in the 2.0 mg/L group (mortality in groups exposed to higher concentrations is
unclear), while 0% was observed in the 0.5 mg/L group. The adult LCso was determined to be 1.03 mg/L (Karber analysis). In the
embryonic study, 100% mortality was also observed in the 2.0 mg/L group. An LCso for embryos was not reported.
Developmental Retardation. While control embryos developed to stages 34, 35, or 36 on 5 dpf and stages 36, 37, or 38 on 7 dpf,
treated embryos exhibited delayed development. For 5 dpf this trend represented a U-shaped dose-response pattern where more
delayed embryos were observed in groups exposed to 200, 400, and 1,000 ug/L when compared to other treatments. A significant
dose-related increase in heart rate was also observed on 5 dpf for exposure groups 400-1,000 ug/L compared to control.
Pigmentation in exposed groups was inhibited by exposure on 5 dpf. The width of the optic tectum decreased significantly with
increasing exposure concentration, indicating that neural development might be affected by nano-Ag exposure.
Morphological Defects. Observed impairments included sluggish circulation; hemorrhage and hemostasis; circulatory
abnormalities (pericardial edema/tube heart); eye abnormalities (e.g., microphthalmia, anisophthalmia, cyclopia, exophthalmia,
anophthalmia); finfold abnormalities; vertebral abnormalities (e.g., scoliosis, lordosis, upward curvature); edema of the yolk sac,
head, heart, and gallbladder; and, most commonly, skeletal flexure and truncation. Individual abnormalities did not display a
concentration-dependent relationship; however, a significant increase in the occurrence of total abnormalities was observed in low-
dose exposed groups (100-400 ug/L) and in the high-dose group (1,000 ug/L). The total abnormality rate increased to 32.3% at 400
ug/L, decreased to 18.9-23.3% at 600-800 ug/L, and finally increased again to 56.8% at 1,000 ug/L.
Hatching Rate. A dose-dependent reduction in hatching rate was observed. The hatching rate was reduced by 37.62% at 1,000
ug/L compared to control. Embryos exposed to 100-800 ug/L resulted in precocious hatching (hatching time reduced by 1.2-1.7
days). Larval survival rates decreased to 0% at 800 and 1,000 ug/L.
Histopathology. Microscopic changes in the shape and structure of the eyes were observed in exposed medaka. Ganglion cells
were deficient, and the thickness of the inner nuclear cell layer was decreased, while retinal pigment epithelium thickness increased.
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Yeo and Kang (2008) Effects of nanometer-sized silver materials on biological toxicity during zebrafish
embryogenesis.
Test Species
Zebrafish embryos, 64- to 265-cell stages and 2.5 hours post fertilization (Danio rerio)
Material
Commercial nano-Ag supported by titanium oxide purchased from N Corporation (Korea)
Shape: Not reported Solubility: Not reported
Composition: AgsO, Ag4H, and TiO Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Supported by TiO
Average Size: Not reported Surface Charge: Not reported
Size Distribution: Approximately 10-20 nm (determined
using TEM)
Protocol
Exposure Duration: 72 hours Exposure Medium: Carbon-filtered city water
Endpoints: Hatching rate, morphology, and gene expression Organisms per Replicate: 300 embryos x 3 replicates
Exposure Concentrations: 10 and 20 ppt
Methods: Hatching rates were determined based on the number of hatched embryos 72 hours post-fertilization. RNA was isolated
from nano-Ag exposed zebrafish 72 hours post-fertilization and expression of SEL N1 and N2 genes were analyzed by RT PCR.
Study Outcome
Hatching Rate. Hatching rates were significantly decreased in both 10 and 20 ppt treatment groups, and catalase activity increased
significantly in the 20-ppt groups.
Phenotypic Changes. Almost all individuals in the 10- and 20-ppt nano-Ag groups exhibited abnormal properties, with more
observed in the 20-ppt compared to the 10-ppt groups (significance was not reported). Observed phenotypic changes included
abnormal notochord development, undeveloped eyes, weak heartbeats, and edema.
SEL N Gene Expression. SEL N1 and N2 gene expression reduced in a concentration-dependent manner (significance not
reported). SEL N2 gene expression was 38% that of control group.
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Yeo and Pak (2008) Exposing zebrafish to silver nanoparticles during caudal fin regeneration disrupts
caudal fin growth and p53 signaling.
Test Species
Zebrafish (Danio rerio), age not specified
Material
Commercial nano-Ag supported by titanium oxide purchased from N Corporation (Korea).
Shape: Not reported Solubility: Not reported
Composition: AgsO, Ag4H, and TiO Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Supported by TiO
Average Size: Not reported Surface Charge: Not reported
Size Distribution: Approximately 10-20 nm (determined
using TEM)
Protocol
Exposure Duration: 36 days Exposure Medium: Distilled water, supplemented with
Endpoints: Caudal fin regeneration, histology, and gene 0.3 g/L Instant Ocean Sea Salt, filtered through 0.45 urn
expression mesh, denitrified by bacterial filtration, and disinfected by
Exposure Concentrations: 0.4 and 4 ppm ultraviolet light exposure
Organisms per Replicate: 1 fish x 5 replicates
Methods: Regeneration experiments were performed on caudal fins amputated at approximately 50% proximal-distal level.
Phenotypic comparisons were made between fish challenged to regenerate in the presence of nano-Ag when compared to the
controls. Photographs of the regeneration area were taken intermittently. To examine histological effects from exposure to nano-Ag,
tissues were fixed, post-fixed, dehydrated, and then embedded in Embed 812-Araldite 502 resin. Ultra-thin sections were then
mounted, stained, and examined using a field emission TEM. RNA was isolated in nano-Ag-exposed zebrafish 52 hours post-
fertilization, and gene expression profiles were analyzed by microarray analysis.
Study Outcome
Caudal Fin Regeneration. Caudal fin regeneration was significantly inhibited at 4 ppm of nano-Ag while only a delay in
regeneration was observed at 0.4 ppm.
Histological Effects. Nano-Ag penetrated all organelles, including the nucleus, and accumulated in blood vessels in both treatment
groups. Destroyed or swollen mitochondria with empty matrices were observed in fin, gill, and muscle tissue.
Gene Expression. Genes coding for tumor protein p53; bc12-associated X protein; phosphatidylinositol glycan, class C;
phosphatidylinositol glycan, class P; and insulin-like growth factor binding protein 3 were upregulated (range: 2.05- to 3.08-fold).
Gene coding for insulin-like growth factor 1 was significantly downregulated (0.38-fold).
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B.6. Summary of Nano-Ag Effects on Terrestrial Plants
Babu et al. (2008) Effect of nano-silver on cell division and mitotic chromosomes: a prefatory siren.
Test Species
Onion (Allium cepa)
Material
Commercial nano-Ag (source not reported).
Shape: Not reported
Composition: Pure Ag (method not reported)
Crystal Structure: Not reported
Average Size: 2 nm (method not reported)
Size Distribution: Not reported
Protocol
Exposure Duration: 0.5,1, 2, or 4 hours
Endpoints: Cell proliferation and chromosomal damage
Exposure Concentrations: 10,20, 40, and 50 ppm
Methods: Root tips (meristems) from A. cepa bulbs were treated with nano-Ag concentrations of 10, 20, 40, or 50 ppm.
Chromosomal preparations were made and approximately 10,000 cells from 10 root tips and 5 bulbs were analyzed to score the
frequency of mitotic index and chromosomal aberrations.
Study Outcome
Significantly reduced frequency in mitotic index was observed at all concentrations and exposure durations except 10 ppm
concentration for 0.5- and 1-hour exposure periods and 40 and 50 ppm for the shortest exposure period (0.5 hour).
Chromosomal aberrations increased in a dose-and duration-dependent manner, and were significantly different from the controls
for all concentrations and exposure durations. Structural aberrations included C-metaphase, disturbed metaphase, fragments,
sticky metaphase, laggards, anaphasic bridge, disturbed anaphase, and micronuclei.
Solubility: Approximately 80% nano-Ag and 20% ionic Ag in
solution (method not reported)
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Medium: Distilled water
Plants per Replicate: 8 meristems
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Kumari et al. (2009) Genotoxicity of silver nanoparticles in Allium cepa.
Test Species
Onion (Allium cepa)
Material
Commercial nano-Ag purchased from Sigma-Aldrich (USA).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: 5.0 m2/g (provided by the manufacturer)
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: <100 nm (provided by the manufacturer) Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 4 hours Exposure Medium: Deionized water
Endpoints: Cell proliferation and chromosomal damage Plants per Replicate: 4 bulbs and 8 root tips x 3 replicates
Exposure Concentrations: 25,50, 75, or 100 ppm
Methods: Roots 2-3 cm in length were treated with nano-Ag, and mitotic index and aberrant cells were quantified after
microscopic observation.
Study Outcome
There was a concentration-dependent decrease in mitotic index (Ml), with the lowest Ml (27%) occurring in the 100 ppm group.
Ml was significantly different from the controls at the 50-, 75-, and 100-ppm levels. Different chromosomal aberrations occurred at
the different nano-Ag concentrations. At 50 ppm, chromatin bridge, stickiness, and disturbed metaphase were observed; at 75
ppm, chromosomal breaks were observed; complete disintegration of the cell walls was observed at 100 ppm.
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Rostami and Shahstavar (2009) Nano-silver particles eliminate the in vitro contamination of olive
'Mission' explants.
Test Species
Olive (0/ea europea L)
Material
Commercial nano-Ag (L2000 Series) purchased from NanoCid (Iran).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 1 hour (submerged in nano-Ag solution) Exposure Medium: Sterile distilled water or Murashige and
or 30 days (amended media) Skoog half strength media
Endpoints: Decontamination and mortality Organisms per Replicate: 20 explants x 4 replicates
Exposure Concentrations: 100,200, 300, and 400 mg/L
(submerged) or 0, 4, 8, and 16 mg/L (amended media)
Methods: Explants with nodes and shoot apices were submerged in 100 mg/L ascorbic acid and 100 mg/L citric acid to control
phenolic compounds. Explants were then submerged in 70% ethylic ethanol for 0, 0.5, or 1 min, rinsed, then submerged in 0.1%
Clorox solution for 0, 5, or 10 min, then washed. Explants were then submerged in nano-Ag solution for 1 hour, after which they
were placed in uncontaminated media and monitored for 30 days. In a separate experiment, some explants were placed in
Murashige and Skoog half strength media amended with concentrations of 2, 4, or 6 mg/L nano-Ag and grew for 30 days in the
amended media. After 30 days, infected and developed plants were recorded, but the method by which these endpoints were
quantified was not reported.
Study Outcome
Nano-Ag at all concentrations appeared to effectively reduce or eliminate internal bacterial contamination in both submerged
explants and those in contaminated media, though significance was not reported and the method by which contamination was
assessed was not reported. Nano-Ag resulted in increased mortality in explants submerged in nano-Ag solutions in a dose-
dependent manner, with no explants surviving in the 300 and 400 mg/L treatment groups (significance not reported). Mortality
decreased with increasing nano-Ag concentrations in media up to 4 mg/L, after which mortality increased (significance not
reported).
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Stampoulis et al. (2009) Assay-dependent phytotoxicity of nanoparticles to plants.
Test Species
Zucchini (Cucurbita pepo cv Costata Romanesco)
Material
Commercial nano-Ag purchased from Sigma-Aldrich (St. Louis, MO, USA).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Sodium dodecyl sulfate (SDS)
Average Size: <100 nm (provided by the manufacturer) surfactant
Size Distribution: Not reported Surface Charge: Not reported
Protocol
Exposure Duration: 5 days (elongation), 12 days Exposure Medium: Reverse osmosis water (elongation and
(germination), 15 days (biomass), 17 days (transpiration) germination) or Hoagland solution (biomass and
Endpoints: Changes in time to germination, root elongation, transpiration)
biomass, and transpiration volume Organisms per Replicate: 3 seeds x 5 replicates
Exposure Concentrations: 1,000 mg/L (elongation and (germination and elongation) or 1 plant x 6 replicates
germination) or 1.0,10, 50,100, 500, and 1,000 mg/L (biomass and transpiration)
(biomass and transpiration)
Methods: Pre-germinated seeds with radicals of 0.5 mm were selected for the root elongation assay. Root elongation was
measured after a 5-day exposure to nano-Ag. To assess time to germination, seeds were placed on Petri dishes and amended
with 1,000 mg/L nano-Ag with or without 0.2% SDS. A batch hydroponic experiment was conducted to determine the effect of
nano-Ag on biomass of seedlings. 18-day-old seedlings were exposed to 1,000 mg/L nano-Ag in Hoagland solution after which
biomass was monitored for 15 days. In addition, 4-day-old seedlings were exposed to concentrations of nano-Ag ranging from 0
to 1,000 mg/L nano-Ag in Hoagland solution and biomass and transpiration volume (determined by mass change of solution)
were measured over a 17-day exposure period.
Study Outcome
Seed Germination. There was no significant impact on seed germination in the nano-Ag or nano-Ag plus SDS treatment groups
when compared to controls. Nor did conventional Ag affect seed germination.
Root Elongation. There was no significant impact on root growth in the nano-Ag or nano-Ag plus SDS treatment groups when
compared to controls. Nor did conventional Ag affect root growth.
Biomass. Nano-Ag exposure resulted in 69% reduction in plant biomass compared to the control and 1,000 mg/L conventional
Ag groups.
Transpiration. Exposure to 500 and 1,000 mg/L nano-Ag resulted in 51 and 70% reduction in biomass when compared to
controls and conventional Ag, respectively. Transpiration volume decreased significantly at and above 100 mg/L nano-Ag.
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B.7. Summary of Nano-Ag Effects on Terrestrial
Invertebrates
Ahamed et al. (2010) Silver nanoparticles induced heat shock protein 70, oxidative stress and
apoptosis in Drosophila melanogaster.
Test Species
Third instar larvae of wild-type Drosophila melanogaster (Oregon RS, Bloomington Stock Centre, Bloomington, IN).
Material
Nano-Ag particles (Clarkson University, Potsdam, NY) synthesized by reduction of silver ions in a polysaccharide solution (acacia
gum).
Shape: Spherical (determined using TEM) Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 11 nm (determined using TEM), 48 nm Surface Treatment: Coated with polysaccharide
(determined using DLS) Surface Charge: -38.6 (determined using LDV)
Protocol
Exposure Duration: 24, 48 hours Exposure Medium: Standard cornmeal
Endpoints: Oxidative stress, DNA damage, and apoptosis Organisms per Replicate: 25 organisms x 3 replicates
Exposure Concentrations: 50,100 pg/mL
Methods: Larvae were administered nano-Ag for 2 durations of exposure. Oxidative stress and apoptosis were assessed by
levels of malondialdehyde (MDA), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and caspase enzymes.
Levels of DNA-damage markers p53 and p38 were also assessed.
Study Outcome
Oxidative Stress. Levels of hsp70, MDA, and SOD and CAT activity were statistically significantly increased in treated larvae
when compared to controls, while GSH levels were statistically significantly lower in treated larvae, indicating high levels of
biological stress.
DNA Damage. Levels of p53 and p38 were significantly increased in treated larvae from those of the control in a time- and dose-
related manner, indicating significant DNA damage caused by exposure to nano-Ag.
Apoptosis. Activities of caspase-3 and caspase-9 were significantly increased in treated larvae from the control levels,
suggesting that nano-Ag exposure is involved in the apoptotic pathway.
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Heckmann et al. (2011) Limit-test toxicity screening of selected inorganic nanoparticles to the
earthworm Eisenia fetida.
Test Species
Adult earthworm (Eisenia fetida) purchased from Skandinavisk Miljogodning (Grindsted, Denmark).
Material
Nano-Ag (NanoAmor).
Shape: Spherical, multi-faceted, or slightly elongated Size Distribution: Not reported
(determined using TEM) Solubility: Not reported
Composition: Not reported Surface Area: 5-10 m2/g
Crystal Structure: Cubic (determined using PXRD) Surface Treatment: Coated with 0.2% w/w PVP
Average Size: 81.8 nm (determined using TEM) Surface Charge: -28.6 mV (determined using DLS)
Protocol
Exposure Duration: 28 days Exposure Medium: Sandy loam soil (pH 5.8; TOC 1.36%,
Endpoints: Adult survival and reproductive toxicity cly 11.6%, silt 21.4%, and sand 64.7%)
Exposure Concentrations: 1,000 mg/kg-dry soil Organisms per Replicate: 10 organisms x 4 replicates
Methods: Using a limit-test design, ecotoxicological life history trait data including survival and reproduction success were
evaluated in earthworms following exposure to 1,000 mg/kg-soil to nano-Ag. Particles were characterized thoroughly in order to
relate any observed toxicity to particle characteristics.
Study Outcome
Survival. Survival was not statistically significantly affected by nano-Ag exposure.
Reproductive Toxicity. Exposure to nano-Ag caused 100% reproductive failure. Cocoon production in the exposed group (0%)
was significantly different (p < 0.05) from that of the control group (100%). Hatchability and juvenile production were not able to
be assessed due to the lack of cocoon production.
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Lapied et al. (2010) Silver nanoparticle exposure causes apoptotic response in the earthworm
Lumbricus terrestris (Oligochaeta).
Test Species
Depurated adult earthworm (Lumbricus terrestris).
Material
Commercial QSI nano-Ag purchased from Quantum Sphere (Santa Ana, CA).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: None
Average Size: 20.2 nm (determined using TEM) Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 24 hours (water), 8 weeks (diet), 4 Exposure Medium: Water, nonmedicated horse manure,
weeks (soil) and agricultural soil
Endpoints: Mortality and apoptosis Organisms per Replicate: 5 organisms x 3 replicates
Exposure Concentrations: 1,10,100 mg/L (water); 0,1, (water), 10 organisms per treatment (no replicates, diet and
10,100 mg/kg-food (diet); 1,10,100 mg/kg-soil (soil) soil)
Methods: Earthworms were exposed to nano-Ag via water, food, and soil, and were examined for apoptosis in cuticule, circular
musculature, longitudinal musculature, intestinal epithelium, and chloragogenous matrix tissues using TUNEL and Apostain
methods.
Study Outcome
Mortality. 40% mortality after 24 hours was observed in earthworms exposed to 100 mg nano-Ag per L water, though no
mortality was observed in either the dietary exposure experiment or the soil-exposure experiment.
Apoptosis. Highly statistically significant levels of apoptosis were observed in all tissues at exposures of 100 mg nano-Ag per L
water. The number of apoptotic cells per mm2 was lower in both of the exposures in soil, but followed the same dose-related
pattern as exposure in water. Cuticule, intestinal epithelium, and chloragog cells showed significantly higher apoptotic responses
than that of muscular tissue at the highest exposure of each experiment.
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Meyer et al. (2010) Intracellular uptake and associated toxicity of silver nanoparticles in
Caenorhabditis elegans.
Test Species
Wild-type (N2) strains RB877 (nth-1), RB1072 (socf-2), RB864 (xpa-1), JF23 (mtl-2), and TK22 (mev-1) larvae of nematode
Caenorhabditis elegans.
Material
Commercial PVP-coated nano-Ag purchased from NanoAmor (Los Alamos, NM) or citrate-coated nano-Ag synthesized by
combining solutions of silver nitrate and sodium citrate.
Shape: Spherical (determined using TEM) Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Coated with PVP and citrate
Average Size: 3,13, and 76 nm (citrate-coated, small PVP- Surface Charge: -33.0, -22.5, and -23.4 mV (citrate-
coated, large PVP-coated particles, respectively) coated, small PVP-coated, and large PVP-coated particles,
Size Distribution: Not reported respectively)
Protocol
Exposure Duration: 3 days Exposure Medium: Complete K+ medium
Endpoints: Growth Organisms per Replicate: -200 organisms x 24 replicates
Exposure Concentrations: 0.5, 5, and 50 mg/L
Methods: Accounting for confounding due to toxic effects to the bacterial food supply and exposure to the particle coatings, the
growth of C. elegans was measured after exposure to various sizes of nano-Ag particles by 50x magnification imaging.
Study Outcome
Internal retention of developing eggs, also known as "bagging" was observed in nematodes exposed to citrate-coated nano-Ag,
indicating the uptake of significant amounts of nano-Ag particles as the cause of a stress response.
Growth. Growth was inhibited in wild-type nematodes exposed to citrate-coated nano-Ag at concentrations of 5 and 50 mg/L and
to PVP-coated nano-Ag at a concentration of 50 mg/L. Authors ruled out mediation of toxicity due to toxic effects on the food
source of the nematodes or toxic effects of the coatings of the particles.
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Roh et al. (2009) Ecotoxicity of silver nanoparticles on the soil nematode Caenorhabditis elegans
using functional ecotoxicogenomics.
Test Species
Soil nematode (3 days old), wild type and 3 mutants (Caenorhabditis elegans N2 var. Bristol, mtl-2 [gk125], sod-3 [gk235], and
daf-12 [rh286fi
Material
Commercial nano-Ag purchased from Sigma-Aldrich (St. Louis, MO, USA).
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: Approximately 14-20 nm (determined using Surface Treatment: Not reported
DLS) Surface Charge: Not reported
Protocol
Exposure Duration: 24 (gene expression, survival, and Exposure Medium: K-media
growth) or 72 hours (reproduction) Organisms per Replicate: 800-1,000 x 3 replicates (gene
Endpoints: Gene expression, survival, growth, and expression), 10x5 replicates (survival), 150 x 5 replicates
reproduction (growth), and 1 x 5 replicates (reproduction)
Exposure Concentrations: 0.05, 0.1, and 0.5 mg/L
Methods: To determine effects on gene expression following exposure to nano-Ag, RNAwas prepared and analyzed by
microarray analysis and 26 genes were analyzed by real-time RT-PCR. At the end of the exposure period, survival was assessed
by counting live and dead individuals, and growth was assessed in heat-killed samples by measuring body length. Reproduction
was assessed by counting the number of offspring at all developmental stages beyond the egg following exposure of young
adults to nano-Ag.
Study Outcome
Gene Expression. Microarray analysis revealed upregulation of 415 gene probes and downregulation of 1,217 by more than 2-
fold compared to the controls. Thirteen gene ontology categories were significantly represented within upregulated genes and
149 in downregulated genes. Four genes analyzed by PCR were significantly upregulated (M 162.5, nonannotated; mtl-2, stress-
response metallothionein; sod-3, stress-response superoxide dismutase; and daf-12, stress-response abnormal dauer formation
protein).
Survival and Growth. Survival and growth were not significantly affected in either the wild type or mutant C. elegans.
Reproduction. Reproduction decreased significantly in both wild type and mutant strains at all treatment levels except in sod-
3(gk235) mutant exposed to 0.05 mg/L nano-Ag. Wild type and daf-12(rh286) were the most sensitive of the four types tested.
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Sap-lam et al. (2010) UV irradiation-induced silver nanoparticles as mosquito larvicides.
Test Species
Fourth-stage larvae of mosquito (Aedes aegypti) from Chulalongkorn University, Bangkok, Thailand.
Material
Silver nanoparticles synthesized by adding PMA to AgNOs and exposed to low intensity UV lamp for 90 minutes.
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Polymethacrylic acid (PMA)-coated
Surface Charge: -27.7 mV (determined by zetasizer
apparatus)
Exposure Medium: Not reported
Organisms per Replicate: 20 organisms x 8 replicates
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 5-10 nm (determined by TEM)
Size Distribution: Not reported
Protocol
Exposure Duration: 24 hours
Endpoints: Survival and hatchability
Exposure Concentrations: 0.01, 0.1,1, and 5 ppm nano-Ag
Methods: Survival of mosquito larvae was monitored after exposure to various concentrations of nano-Ag over 24 hours.
Morphology of larvae was evaluated microscopically. Hatchability was also assessed.
Study Outcome
Survival. No significant increase in mortality was observed at concentrations of 0.01 and 0.1 ppm. Survival of larvae exposed to
1 ppm decreased to 88% after 24 hours of exposure, and at 5 ppm 90% mortality was observed after 3 hours of exposure.
Morphology. Authors noted dark spots in the abdomens of treated larvae.
Hatchability. A number of eggs hatched in the 5-ppm solution of nano-Ag; however, the mortality of the larvae upon hatching was
immediate.
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Shoults-Wilson et al. (2011) Evidence for avoidance of Ag nanoparticles by earthworms (Eisenia
fetida).
Test Species
Adult earthworm (Eisenia fetida).
Material
Nano-Ag powders purchased from NanoAmor (Houston, TX)
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 10 nm (small PVP-coated particles)
(determined by TEM)
Size Distribution: 15-25 nm (small citrate-coated particles);
30-50 nm (large PVP-coated particles and large oleic acid-
coated particles) (determined by TEM)
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: PVP, oleic acid, and citrate
Surface Charge: Not reported
Exposure Medium: Yeager Sandy Loam natural soil (Estill
County, KY) or artificial soils with high and low pH
Organisms per Replicate: 10 organisms x 5 replicates
Protocol
Exposure Duration: 48 hours
Endpoints: Contaminated soil avoidance
Exposure Concentrations: 9.0,18, 27, 36, and 54 mg/kg-
soil (small PVP-coated particles); 0.3,1.0, 3.0, 9.0, and 27
mg/kg-soil (large PVP-coated particles); 9.0 mg/kg-soil (large
oleic acid-coated and small citrate-coated particles)
Methods: Earthworm soil preference of untreated soils and soils treated with nano-Ag was monitored in an avoidance assay
performed according to standard protocol (ISO 2008). Soil type, soil characteristics, nano-Ag coating, and nano-Ag size were all
tested as potential mediating variables of toxicity.
Study Outcome
Earthworms displayed avoidance of soils treated with nano-Ag after 48 hours, while displaying avoidance to soil treated with
AgNOs immediately. Assays performed in natural soil revealed a significantly higher tendency of earthworms to avoid nano-Ag
treated soil than in artificial soils. Earthworms avoided soils treated with small citrate-coated particles in natural soil significantly
more than soil treated with large PVP-coated particles.
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B.8. Summary of Nano-Ag Effects on Non-mammalian
Terrestrial Vertebrates
Grodzik and Sawosz (2006) The influence of silver nanoparticles on chicken embryo development
and bursa of Fabricius morphology.
Test Species
Ross hen embryos
Material
Commercial hydrocolloidal Ag suspension purchased from NanoTech (Poland) and diluted with NaCI.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 13 days Exposure Medium: Egg interior
Endpoint: Development Organisms per Replicate: 30 eggs x 1 replicate
Dose: 10 ppm administered on incubation days 5,11, and 17
Methods: Eggs were injected with 10 ppm nano-Ag on incubation days 5,11, and 17. Eggs were weighed on day 18, opened
and sacrificed. Embryos' livers, hearts, and eyes were weighed and examined according to Hamburger and Hamilton standard
(1951). Bursae of Fabricius (BF) were collected and treated for examination by microscope.
Study Outcome
Exposure to nano-Ag did not affect embryo weight or weights of liver, heart, or eyes. There was no significant difference in
apoptotic signs between control and nano-Ag groups, and no necrosis was observed. The number and luminal intensity of cell
nuclei in BFs of nano-Ag group were decreased and a slight increase was observed in the population of cells with peripherally
deep staining nuclei (significance not reported).
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Sawosz et al. (2007) Influence of hydrocolloidal silver nanoparticles on gastrointestinal microflora
and morphology of enterocytes of quail.
Test Species
Quail (Coturnixcoturnixjaponica), approximately 10 days old, and quail gut microflora (Escherichia coli, Enterobacter,
Streptococcus bovis, Enterococcus faecium, Bacteroides spp., Actinomyces naeslundii, Lactobacillus salivarius, Lactobacillus
fermentum, and Leuconostoc lactis)
Material
Commercial hydrocolloidal Ag suspension purchased from NanoTech (Poland) and produced by solid-liquid phase discharge
method.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 52% of Ag nanoparticles were 3-4 nm in size. Surface Charge: Not reported
Size Distribution: 2 to >7nm (determined using TEM)
Protocol
Exposure Duration: 12 days Exposure Medium: Drinking water
Endpoints: Bacterial content and tissue damage Organisms per Replicate: 15 birds x 1 replicate
Exposure Concentrations: 0, 5,15, and 25 mg/kg
Methods: Quail were allowed to drink freely from water containing nano-Ag. Following exposure for 12 days, quail were killed
and caeca were opened. The mucosa was scraped and the number of total culturable anaerobic bacteria was enumerated along
with other enterobacteria. Tissues samples of the duodenum were also analyzed.
Study Outcome
No pathological changes or behavioral changes were observed in quail exposed to nano-Ag. Composition of the gut microflora
was significantly altered in the 25 mg/kg nano-Ag group, with significant increases in the Lactobacillus spp., L lactis, and A.
naeslundii. No significant changes were observed in the other bacteria tested. There were no changes in the structure of
enterocytes, glands, or connective tissue of intestinal villi. There was also no change in number of leucocytes.
B-67
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Appendix C. Summary of Human Health
Effects Studies of Nano-Ag
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Appendix C. Summary of Human Health
Effects Studies of Nano-Ag
C.1. Study Selection Criteria C-3
C.2. Summary of Key In Vitro Studies C-4
Ahamed et al. (2008) DMA damage response to different surface chemistry of silver nanoparticles in mammalian cells. C-4
Arora et al. (2009) Interactions of silver nanoparticles with primary mouse fibroblasts and liver cells. C-5
Asharani et al. (2009) Cytotoxicity and genotoxicity of silver nanoparticles in human cells. C-6
Carlson et al. (2008) Unique cellular interaction of silver nanoparticles: size-dependent generation of reactive oxygen species. C-7
Greulich et al. (2009) Studies on the biocompatibility and the interaction of silver nanoparticles with human mesenchymal stem
cells (hMSCs). C-8
Hussain et al. (2005) In vitro toxicity of nanoparticles in BRL 3A rat liver cells. C-9
Kim et al. (2010) Cytotoxicity and genotoxicity of nano-silver in mammalian cell lines. C-10
Li et al. (2010a) Induction of Cytotoxicity and apoptosis in mouse blastocysts by silver nanoparticles. C-11
Liu et al. (2010) Impact of silver nanoparticles on human cells: effect of particle size. C-12
Luetal. (2010) Effect of surface coating on the toxicity of silver nanomaterials on human skin keratinocytes. C-13
Paddle-Ledinek et al. (2006) Effect of different wound dressings on cell viability and proliferation. C-14
Rosas-Hernandez et al. (2009) Effects of 45-nm silver nanoparticles on coronary endothelial cells and isolated rat aortic rings. C-15
Samberg et al. (2010) Evaluation of silver nanoparticle toxicity in vivo and keratinocytes in vitro. C-16
Shin et al. (2007) The effects of nano-silver on the proliferation and cytokine expression by peripheral blood mononuclear cells. C-17
Shrivastava et al. (2009) Characterization of antiplatelet properties of silver nanoparticles. C-18
Trickier et al. (2010) Silver nanoparticle-induced blood-brain barrier inflammation and increased permeability in primary rat
brain microvessel endothelial cells. C-19
C.3. Summary of Key In Vivo Studies C-20
Cha et al. (2008) Comparison of acute responses of mice livers to short-term exposure to nano-sized or micro-sized silver
particles. C-20
Ji et al. (2007) Twenty-eight-day inhalation toxicity study of silver nanoparticles in Sprague-Dawley rats. C-21
Kim et al. (2008) Twenty-eight-day oral toxicity, genotoxicity, and gender-related tissue distribution of silver nanoparticles in
Sprague-Dawley rats. C-22
Lee et al. (2010) Genomics-based screening of differentially expressed genes in the brains of mice exposed to silver
nanoparticles via inhalation. C-23
Li et al. (2010a) Induction of Cytotoxicity and apoptosis in mouse blastocysts by silver nanoparticles. C-24
Park et al. (201 Oa) Repeated-dose toxicity and inflammatory responses in mice by oral administration of silver nanoparticles. C-25
Samberg etal. (2010) Evaluation of silver nanoparticle toxicity in vivo and keratinocytes in vitro. C-26
Shrivastava et al. (2009) Characterization of antiplatelet properties of silver nanoparticles. C-27
Sung et al. (2008) Lung function changes in Sprague-Dawley rats after prolonged inhalation exposure to silver nanoparticles. C-28
Sung et al. (2009) Subchronic inhalation toxicity of silver nanoparticles. C-29
Takenaka et al. (2001) Pulmonary and systemic distribution of inhaled ultrafine silver particles in rats. C-30
Tang et al. (2008) Influence of silver nanoparticles on neurons and blood-brain barrier via subcutaneous injection in rats. C-31
Tiwari et al. (2011) Dose-dependent in-vivo toxicity assessment of silver nanoparticles in Wistar rats. C-32
Appendix C References C-33
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C.1. Study Selection Criteria
The process by which studies were selected for inclusion in the human health effects tables differed
for each category of study (i.e., in vitro, in vivo) based on the quantity and quality of available
toxicological data. In general, literature searches were conducted for specific human and rodent cell types,
and later for specific toxicological endpoints, to identify relevant in vitro studies. Literature searches were
also conducted for exposure pathways (e.g., inhalation, oral, dermal) to identify relevant rodent in vivo
studies. To reflect the most current state of the science, the tables in this appendix include only studies
published in or after 2000. The information presented here is up to date as of March 1, 2011, when the last
broad literature search to identify new information was conducted. For those toxicological endpoints for
which a substantial amount of information was available (e.g., mitochondrial function, reactive oxidative
stress), studies examining relevant endpoints were selected based on the data quality and the relative
contribution of the results to the state of the science (determined largely by examining the number of
articles in which the study was later cited). Studies were also included if the investigators examined an
endpoint for which there was otherwise little information, used a novel technique to assess toxicity, or
compared the relative toxicities of nano-Ag with different sets of characteristics (e.g., nano-Ag of
different sizes, surface areas, shapes). For study categories with very little available toxicological
information, all identified studies were included unless they were judged to be of poor quality. In this case
study, no studies were included for toxicity due to occupational exposure because no occupational effects
studies specific to nano-Ag were identified.
Information in the tables is organized to take into account the minimum requirements for
physicochemical characterization proposed by the Minimum Information for Nanomaterial
Characterization (MINChar) Initiative and others (MINCharlnitiative. 2008). The limited understanding
of nano-Ag toxicity and its mechanisms, and the equivocal nature of some studies that give conflicting
results, preclude the direct comparison of study results. To emphasize that caution is warranted in
interpreting the results of the available toxicological studies of nano-Ag, these tables are organized in a
way that emphasizes each study's relevant attributes in the context of this case study - especially
characterization of the nano-Ag used in the study - rather than to facilitate direct comparison of results
among studies.
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C.2. Summary of Key In Vitro Studies
Ahamed et al. (2008) DMA damage response to different surface chemistry of silver nanoparticles in
mammalian cells.
Test Species
Mouse embryonic stem cells (mES) and mouse embryonic fibroblasts (MEF)
Material
Commercial uncoated nano-Ag, supplied by Novacentrix, Austin, Texas, and polysaccharide-coated nano-Ag, supplied by Clark
University, Potsdam, NY. Nano-Ag dispersions were vortexed.
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 25 nm (reported by manufacturer)
Size Distribution: Not reported
Protocol
Exposure Duration: 4, 24, 48, and 72 hours
Endpoint: Uptake, morphology, viability, and apoptosis
Exposure Concentrations: 50 ug/mL
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Uncoated and polysaccharide-coated
Surface Charge: Not reported
Exposure Medium: Ham's F12 Dulbecco's modified eagle
medium (DMEM)
Cell Density: Not reported
Methods: Uptake and morphology were measured by fluorescence and confocal microscopy. Morphology was also analyzed
with sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-Page) immunoblotting. Cell viability was assessed by 3-
MTT-based cell viability test and by measuring annexin V protein.
Study Outcome
Uptake. The uncoated particles tended to agglomerate, suggesting they may not be found in some organelles (e.g., nucleus and
mitochondria), while the coated particles appeared dispersed.
Membrane Integrity. Exposure to 50 ug/mL of uncoated or coated nano-Ag up-regulated cell cycle checkpoint protein p53 in
mES and MEF cells. After 4 hours, p53 was phosphorylated only by coated nano-Ag, and only in mES cells. After 4 and 24 hours,
both types of nano-Ag resulted in increased levels of the DNA damage repair proteins Rad51 and phosphorylated-H2AX
expression in mES cells. Though significance was not reported, they report that cellular response to coated nano-Ag appears
greater than uncoated nano-Ag.
Mitochondrial Function. Exposure to 50 ug/mL of uncoated and coated nano-Ag induced mES and MEF cell death (annexin V
protein expression and MTT assay). The annexin V expression was higher in MEF cells treated with coated nano-Ag.
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Arora et al. (2009) Interactions of silver nanoparticles with primary mouse fibroblasts and liver cells.
Test Species
Primary mouse fibroblasts and liver cells
Material
Nano-Ag synthesized by photo-assisted reduction of Ag+ to metallic nanoparticles.
Shape: Spherical Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 16.6 nm (determined using DLS) Surface Charge: Not reported
Size Distribution: 6.5-43.8 nm (determined using DLS)
Protocol
Exposure Duration: 24 hours Exposure Medium: Colloidal aqueous solution
Endpoint: Morphology, viability, antioxidant defense, and Cell Density: 2 x 104 cells/2 ml growth medium/plate
apoptosis (morphology and apoptosis); 1 x 1Q4 cells/200 ml growth
Exposure Concentrations: 0, 0.78,1.56, 3.12, 6.25,12.5, medium/well (cytotoxicity)
25, 50, 100, 150, 200, 250, 300, 350, 400, and 500 ug/mL
Methods: Cell morphology was examined by phase contrast microscopy and cell viability was assessed by XTT-based cell
viability test. Glutathione (GSH) reduction and antioxidant defense were used in the estimation of enzyme activity. Apoptosis was
measured by using a colorimetric assay and fluorescence microscopy.
Study Outcome
Morphology. Cells less polyhedric, more fusiform, and shrunken with treatment concentration from 50 ug/mL to 100 ug/mL.
Primary liver cells displayed no changes relative to control cells at concentrations of 100 ug/mL and below. At 200 ug/mL and
above, primary liver cells displayed damaged irregular cell membranes.
Membrane Integrity. ICso: 61 ug/mL primary fibroblasts; 449 ug/mL primary liver cells; spherical assemblages were found inside
the mitochondria of both treated fibroblasts and liver cells, also in the vacuoles of liver cells.
Superoxide Dismutase (SOD). In primary fibroblasts, changes in SOD levels were statistically insignificant. In primary liver cells,
SOD levels increased from 9.2 micromoles per milligram (uM/mg) protein in untreated cells to 13 uM/mg protein in treated cells, a
factor of 1.4.
Lipid Peroxidation. In primary fibroblasts, lipid peroxidation decreased from 0.31 uM/mg protein in untreated cells to 0.22
uM/mg protein in treated cells, a factor of 1.4. In primary liver cells, changes in lipid peroxidation were statistically insignificant.
GSH Reduction. In primary fibroblasts, GSH levels increased from 0.82 uM/mg protein in untreated cells to 0.95 uM/mg protein
in treated cells, a factor of 1.2. In primary liver cells, GSH levels increased from 72.3 uM/mg protein in untreated cells to 79
uM/mg protein in treated cells, a factor of 1.1.
Apoptosis. Nano-Ag induced apoptosis at concentrations in the ranges 3.12-50 ug/mL and 12.5-400 ug/mL for primary
fibroblasts and primary liver cells, respectively. For primary fibroblasts, 69% live cells, 24% apoptotic cells, and 7% necrotic cells
were observed at nano-Ag concentration 30 ug/mL (~ 1/2 ICso) whereas, at 4-fold higher nano-Ag concentration (~2 x ICso), 37%
live cells, 17% apoptotic cells, and 46% necrotic cells were observed. In the case of primary liver cells, 71% live cells, 24%
apoptotic cells, and 5% necrotic cells and 32% live cells, 14% apoptotic cells, and 54% necrotic cells for -1/2 ICso and ~2 x iCso,
respectively.
C-5
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Asharani et al. (2009) Cytotoxicity and genotoxicity of silver nanoparticles in human cells.
Test Species
Normal human lung fibroblast cells (IMR-90) and human glioblastoma cells (U251)
Material
Nano-Ag synthesized by reducing AgNOs solution using NaBhU
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 6 to 20 nm (determined using TEM and UV Surface Treatment: Coated with starch
absorption) Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 24, 48, and 72 hours Exposure Media: Dulbecco's modified eagle medium
Endpoint: Morphology, viability, reactive oxygen species (DMEM) with 10% fetal bovine serum (FBS) and 1% penicillin
(ROS) generation, cell cycle, apoptosis, genotoxicity, and streptomycin (U251); glutamine with 15% FBS, 1 % each of
uptake penicillin streptomycin, nonessential amino acids, vitamins,
Exposure Concentrations: 0, 25, 50,100, 200, and 400 and 2% essential amino acids
ug/mL Cell Density: 1 x 104 cells
Methods: Morphology was assessed by TEM and STEM; ATP concentration was measured using Cell-liter glow luminescent
cell viability assay; mitochondrial function was analyzed using a CellTiter blue cell viability assay; ROS generation was assessed
using DCF-DAand DHE staining methods; apoptosis was measured using Annexin-V staining; genotoxicity was analyzed using
comet assay and cytokinesis-blocked micronucleus assay; and uptake was studied using TEM.
Study Outcome
Cell Morphology. Treated cells were clustered with few cellular extensions and displayed restricted spreading patterns.
ATP Concentration. ATP assays demonstrated a concentration- and time-dependent decrease in luminescence intensity in both
IMR-90 and U251 cells, with ATP content declining statistically after 48 hours.
Mitochondrial Function. CellTiter blue cell viability assays demonstrated a concentration-dependent decrease in mitochondrial
activity in both IMR-90 and U251 cells.
ROS Generation. Statistically significant increase in hydrogen peroxide and superoxide production in cells treated with 25 and
50 ug/mL; however no significant increase was observed beyond 100 ug/mL.
Cell Cycle. Possible mechanism of toxicity is proposed which involves disruption of the mitochondrial respiratory chain by Ag-NP
leading to production of ROS and interruption of ATP synthesis, which in turn cause DNA damage. It is anticipated that DNA
damage is augmented by deposition, followed by interactions of nano-Ag to the DNA leading to cell cycle arrest in the gap-
2/mitosis (G2/M) phase.
Apoptosis. Apoptotic cells populations increased from 25 to 100 ug/mL in IMR-90 cells. Late apoptosis and necrosis caused
16% (±5) of cell death.
Chromosomal Aberrations. Chromosomal breaks were observed in cells treated with nano-Ag.
DNA Damage. A concentration-dependent increase was observed in tail momentum. A concentration-dependent increase in DNA
damage was observed in U251 cells, while DNA damage did not increase beyond 100 ug/mL in IMR-90 cells.
Uptake. Treated cells displayed endosomes containing many nanoparticles near the cell and nuclear membrane, suggesting
nanoparticles more likely entered through endocytosis than diffusion. Nanoparticles were found throughout the cytoplasm, as well
as inside lysosomes, mitochondria, nucleolus, and nucleus.
C-6
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Carlson et al. (2008) Unique cellular interaction of silver nanoparticles: size-dependent generation of
reactive oxygen species.
Test Species
Rat alveolar macrophages (NR8383 CRL-2192)
Material
Commercially-available hydrocarbon-coated silver (NovaCentrix, formerly Nanotechnologies, Inc.).
Shape: Spherical (confirmed by SEM) Solubility: Insoluble
Composition: Not reported Solubility: Insoluble
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 15, 20, and 50 nm (reported by Surface Treatment: Hydrocarbon coated (~2 nm thick) to
manufacturer); 15, 30, and 55 nm (determined using prevent sintering during plasma synthesis and maintain
microscopy) constant coating in aqueous solutions
Size Distribution: Not reported Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 0, 5,10, 25, 50, and 75 ug/mL
Endpoint: Uptake, morphology, viability, inflammatory Exposure Medium: Ham's Nutrient Mixture F12K
response, reactive oxygen species (ROS) generation Cell Density: 80% confluency
Methods: Following 24-hour incubation of treated samples, uptake, and morphology examined by light microscopy and TEM
(uptake only). Cell viability assessed by MTT-based cell viability test, membrane integrity assessed using fluorescence to
measure lactate dehydrogenase (LDH) released, and mitochondrial membrane potential assay measured by fluorescence.
Glutathione (GSH) levels and inflammatory responses measured by GSH assay and enzyme-linked immunosorbent assay
(ELISA), respectively. ROS generation determined using dichlorofluorescein diacetate (DCFH-DA).
Study Outcome
Uptake. Nanoparticles of all sizes taken up into cells.
Cell Morphology. Cells dosed with 15-nm particles (only 0, 25, and 75 ug/mL tested) appeared shrunken and lacked defined
plasma membrane. Agglomerated nano-Ag observed inside and outside cells dosed with 30-nm particles, and macrophages
appeared larger. 55-nm particles agglomerated and were not observed inside the cells.
Mitochondrial Function. ECso: 27.87 ± 12.23 ug/mL 15-nm particles; ECso: 33.38 ±11.48 ug/mL 30-nm particles; ECso greater
than 75 ug/mL 55-nm particles; tests with silver nitrate (AgNOs) produced results similar to 15-nm nano-Ag.
Mitochondrial Membrane Potential (MMP). Statistically significant loss of MMP observed with increasing dose for 15- and 30-
nm nano-Ag. Loss of MMP may be size dependent and warrants further study to determine if cell death is result of mitochondrial-
instigated apoptosis.
Membrane Integrity. Statistically significant, dose-dependent decrease in cell viability for 15-nm and 30-nm nano-Ag at
concentrations between 10 and 75 ug/mL. Decrease in cell viability as compared to control not statistically significant until 75
ug/mL dose for samples treated with 55-nm nano-Ag.
GSH Reduction. GSH levels decreased with increasing dose for both 15-nm and 30-nm, and the GSH level was undetectable at
50 ug/mL. However, the same responses were not seen for the 55-nm doses.
ROS Generation. For 15-nm particles, statistically significant increases in ROS with increasing dose from 10 ug/mL to 50 ug/mL.
Results for 30- and 55-nm particles not statistically different from control.
Inflammatory Response. Statistically significant increases in the cytokines, TNF-a, MIP-2, and IL-1(3 at all doses (5,10, and 25
ug/mL tested), but responses showed no trend with size or concentration. No increases occurred in IL-6 cytokine, but this was
unexplained in the study.
C-7
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Greulich et al. (2009) Studies on the biocompatibility and the interaction of silver nanoparticles with
human mesenchymal stem cells (hMSCs).
Test Species
Human mesenchymal stem cells (hMSC)
Material
Nano-Ag particles prepared by polyol process using AgNOs in C2He02 and polyvinylpyrrolidone (PVP).
Shape: Spherical Solubility: Not reported
Composition: Pure silver Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: PVP
Average Size: 100 nm (determined using DLS) Surface Charge: Not reported
Size Distribution: 35 nm to 350 nm with median of 100 nm
(determined using DLS)
Protocol
Exposure Duration: 7 days Exposure Medium: Cell culture medium RPMI1640 with
Endpoint: Morphology, viability, inflammatory response, and 10% fetal calf serum
chemotaxis Cell Density: Not reported
Exposure Concentrations: 0, 0.05, 0.5,1, 2.5, 3, 3.5, 4, 5,
and 50 ug/mL
Methods: hMSCs treated with nano-Ag/ultrapure water solution or silver acetate/ultrapure water solution. Cell viability assessed
by fluorescence staining and microscopy after 7 days of incubation. Morphology of cells in same samples observed by phase-
contrast microscopy. Chemotaxis analyzed by transwell assay using peripheral blood mononuclear cells as chemoattractants.
Inflammatory response assessed via enzyme-linked immunosorbent assay (ELISA).
Study Outcome
Preliminary experiment with nano-Ag in different fluids and cell media showed no agglomeration of nano-Ag particles in cell
culture media with fetal calf serum but agglomeration when cells were incubated in phosphate-buffered saline or culture medium
alone.
Viability. No viable cells detected in samples treated at concentrations between 3.5 and 50 ug/mL nano-Ag particles and
between 2.5 and 50 ug/mL silver acetate.
Chemotaxis. Observed decreasing chemotactic response with increasing silver concentration for both nano-Ag particles and
silver acetate solution. Response statistically significant compared to control for nano-Ag doses of 3.5, 4, and 5 ug/mL and for
silver acetate doses of 2.5, 3, 3.5, 4, and 5 ug/mL.
Inflammatory Response. Observed decrease in the release of interleukin-6 (IL-6), interleukin-8 (IL-8), and vascular endothelial
growth factor (VEGF) from cells for Ag particle and Ag ion doses of 0.05-50 ug/mL. Release of interleukin-11 (IL-11) in adherent
cells was not affected following silver treatment; however IL-11 release resembled that of IL-6 or VEGF when silver was added
during cell seeding.
C-8
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Hussain et al. (2005) In vitro toxicity of nanoparticles in BRL 3A rat liver cells.
Test Species
BRL 3A (ATCC, CRL-1442) immortalized rat liver
Material
Commercial nano-Ag, supplied by Air Force Research Laboratory, Brooks AFB, Texas.
Shape: Not reported Solubility: Ag-100 nm was not homogeneously suspended
Composition: Not reported in solution
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 15 and 100 nm (reported by manufacturer) Surface Treatment: Not reported
Size Distribution: Not reported Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 0, 2.5, 5,10, 25, 50 and 150
Endpoint: Morphology, viability, and reactive oxygen species ug/mL
(ROS) generation Exposure Medium: Culture media with 5% fetal bovine
serum
Cell Density: Confluent
Methods: After exposure, morphology was observed by microscopy. Membrane integrity was measured by measuring lactate
dehydrogenase (LDH) leakage, cell viability was assessed spectrophotometrically by MIT-based cell viability test, ROS
generation was determined using dichlorodihydrofluorescein diacetate (hb DCFDA), mitochondrial membrane potential was
determined by the uptake of rhodamine 123, reduced glutathione (GSH) measured by GSH assay.
Study Outcome
Cell Morphology. Cells began to shrink and became irregular in shape with increasing doses of nano-Ag.
Membrane Integrity. LDH ECso: 24 ± 9.25 ug/mL 100-nm particles; LDH ECso: 50 ± 10.25 ug/mL 15-nm particles; statistically
significant concentration-dependent decrease in cell viability for 100-nm and 15-nm particles at concentrations between 10 and
50 ug/mL, with 100-nm particles demonstrating higher toxicity at 25 and 50 ug/mL.
Mitochondrial Function. MTT ECso: 19 ± 5.2 ug/mL 100-nm particles; MTT ECso: 24 ± 7.25 ug/mL 15-nm particles; Ag exposure
demonstrated a significant cytotoxicity from 5 to 50 ug/mL.
GSH Reduction. Statistically significant decrease of GSH (70%) at 25 ug/mL, relative to controls.
ROS Generation. Statistically significant concentration-dependent increase in ROS generation from 10 ug/mL, with an
approximately 10-fold increase in ROS generation at 25 and 50 ug/mL over control levels.
Mitochondrial Membrane Potential (MMP). Statistically significant decrease (80%) of MMP at 25 and 50 ug/mL
C-9
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Kim et al. (2010) Cytotoxicity and genotoxicity of nano-silver in mammalian cell lines.
Test Species
Mouse lymphoma L5178Y cells and human bronchial epithelial BEAS-2B cells
Material
Silver nanopowder purchased from Aldrich, USA.
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Medium: Cell culture medium RPMI-1640 with
1 mM sodium pyruvate, 0.1% pluronic, 10% heat-inactivated
horse serum, and S-9 mixture (comet assay); THMG medium
and S-9 mixture (mouse lymphoma assay)
Cell Density: Not reported
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: Not reported
Size Distribution: <100 nm (reported by the manufacturer)
Protocol
Exposure Duration: 3 hours
Endpoint: DNA damage, cytotoxicity, mutation frequency
Exposure Concentrations: 0, 942.38,1,884.77, 3,769.53
ug/mL (mouse lymphoma cells, with S-9), 449.22, 898.44,
1,796.88 ug/mL (mouse lymphoma cells, without S-9),
292.97, 585.94,1,171.88 ug/mL (human bronchial epithelial
cells, with S-9), 190.43, 380.86, 761.72 ug/mL (human
bronchial epithelial cells, without S-9) (comet assay); 0, 313,
625,1,250, 2,500, 3,750 (mouse lymphoma assay).
Methods: DNA damage and cytotoxicity were determined by a comet assay involving both mouse lymphoma cells and human
bronchial epithelial cells with and without metabolic activation. Genotoxicity was assessed in a mouse lymphoma assay
performed with and without metabolic activation.
Study Outcome
DNA Damage. Nano-Ag significantly induced a higher incidence of DNA damage at all exposure levels with and without the
addition of S-9 mixture in the comet assay.
Cytotoxicity. Exposure to nano-Ag increased cytotoxicity in a dose-dependent manner without metabolic activation in the mouse
lymphoma assay, with pronounced but not significant drops in RS and RTG.
Mutation Frequency. Mutation frequencies were comparable to historical controls. Exposure to nano-Ag did not significantly
affect the level of mutation frequencies at any dose level with or without metabolic activation by S-9 fraction.
C-10
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Li et al. (201 Oa) Induction of cytotoxicity and apoptosis in mouse blastocysts by silver
nanoparticles.
Test Species
ICR mouse blastocytes
Material
Nano-Ag prepared by the polyol process (reduction of AgNOs with C2He02 in the presence of polyvinylpyrrolidone [PVP]).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Coated with a shell of polymer to reduce
Average Size: 13 nm (determined using TEM) cluster formation
Size Distribution: 6-24 nm (determined using TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 0, 25, and 50 uM
Endpoint: Cytotoxicity, embryonic developmental potential, Exposure Medium: BSA-free IVh medium with 0.1 % PVP
and cell proliferation Cell Density: Not reported
Methods: DNAfragmentation, a characteristic of cell apoptosis, was assessed using a Terminal deoxynucleotidyl transferase
dUTP nick end labeling (TUNEL) assay. Cell proliferation and assessment of whether inner cell mass (ICM) or thymic epithelial
(TE) cells were most affected by exposure was performed following differential staining and cell counting. Embryonic
developmental potential was tested by comparing the ratio of morulas to blastocysts in treated and untreated cells.
Study Outcome
Cytotoxicity. A nine-fold increase in incidence of apoptosis was observed in cells treated with 50 uM nano-Ag.
Embryonic Developmental Potential. The ratio of morulas to blastocysts was significantly lower in cells treated with 50 uM
nano-Ag than that of the control cells. The rate of embryo attachment to culture dishes was higher, and the number of post-
implantation developmental milestones lower, in blastocysts treated with 50 uM nano-Ag.
Cell Proliferation. Significantly fewer ICM and TE cells were observed in cells treated with 50 uM nano-Ag than in control cells.
These cells also contained a higher incidence of apoptotic cells.
C-11
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Liu et al. (2010) Impact of silver nanoparticles on human cells: effect of particle size.
Test Species
Human lung adenocarcinoma epithelial (A549) cells, human stomach cancer (SGC-7901) cells, human hepatocellular carcinoma
(HepG2) cells, and human breast adenocarcinoma (MCF-7) cells
Material
5- and 20-nm nano-Ag particles purchased from Huzheng Nano Technology Limited Company, Shanghai, China; 50-nm particles
synthesized by reduction of AgNOs.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: 5- and 20-nm particles coated with PVP
Average Size: 5.9, 23.8, and 47.5 nm (determined using Surface Charge: Not reported
TEM)
Size Distribution: 2.6-9.2 nm, 17.1-30.5 nm, 25.4-69.6 nm
(determined using TEM)
Protocol
Exposure Duration: 24 hours Exposure Medium: Unspecified medium containing 10%
Endpoint: Cytotoxicity, cell morphology, cellular uptake, fetal bovine serum
oxidative stress, cell cycle progression Cell Density: 1 x 105 cells per dish
Exposure Concentrations: 0, 0.01, 0.1, 0.5,1, 2.5, 5,10,
25, 50, and 100 ug/mL
Methods: Cytotoxicity was evaluated in an MTT assay and morphological changes evaluated visually. Cellular uptake of
particles, cellular oxidative stress, and cell cycle progression were also assessed.
Study Outcome
Cytotoxicity. Dose-dependent toxicity was observed in all cell types. Toxicity was size-dependent, with the smaller particles
producing the most and the larger particles producing the least toxic reaction. An increased fraction of apoptotic cells was
observed in cells exposed to nano-Ag.
Cell Morphology. Dose-related morphological alterations were observed in HepG2 cells.
Cellular Uptake. 0.004-0.031% of silver entered the cells in a potentially size-dependent manner.
Oxidative Stress. Strongly fluorescent cells were observed in treated groups, indicating that reactive oxygen species were
evoked by silver nanoparticles.
Cell Cycle Progression. Changes in proportion of cells in G1 and S phases from treated cells to controls suggest that the cell
cycle was disturbed by exposure to nano-Ag.
C-12
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Lu et al. (2010) Effect of surface coating on the toxicity of silver nanomaterials on human skin
keratinocytes.
Test Species
Human skin HaCaT keratinocytes
Material
Silver nanoparticles synthesized according to the conventional citrate reduction method.
Shape: Spheres and prisms (characterized by TEM) Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Coated with citrate ions and PVP
Average Size: 30 nm (determined using TEM) Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 3 weeks Exposure Medium: Modified Eagle's Medicum with 10%
Endpoint: Cell viability, cytotoxicity, genotoxicity FBS
Exposure Concentrations: 10 to 100 ug/mL Cell Density: Not reported
Methods: HaCaT cell viability was assessed after exposure to spherical and prism-shaped silver nanoparticles in the comet
assay. A separate assay compared the toxicity of particles coated with citrate ions or PVP after exposure to sunlight.
Study Outcome
Viability and Cytotoxicity. No difference in cell viability was observed between treated and untreated cells with colloidal
nanoparticles in the presence or absence of sunlight. Cell viability was significantly decreased when exposed to powder form of
the citrate-coated particles that had been dried under direct sunlight. This decrease in viability was not observed in cells treated
with the powder form of PVP-coated particles.
Genotoxicity. Citrate-coated colloidal nanoparticles at 100 ug/mL were not genotoxic.
C-13
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Paddle-Ledinek et al. (2006) Effect of different wound dressings on cell viability and proliferation.
Test Species
Human keratinocytes
Material
Acticoat (nanocrystalline Ag/Polyethylene mesh), manufactured by Smith & Nephew; Aquacel-Ag (Ag ions/cm cellulose),
manufactured by ConvaTec; Avance (Ag/Polyurethane foam), manufactured by SSL; and Constreet-H (Ag/Hydrocolloid/alginate),
manufactured by Coloplast.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: Not reported Surface Charge: Not reported
Size Distribution: Not reported
Protocol
Exposure Duration: 40 hours Exposure Medium: Basal keratinocyte serum-free medium
Endpoint: Viability, proliferation, morphology supplemented with L-glutamine, bovine pituitary extract, and
Exposure Concentrations: Acticoat (109 mg/100 cm2); recombinant epidermal growth factor
Aquacel-Ag(19.7 mg/100 cm2); Avance (1.6 mg/100 cm2); Cell Density: 6,000 cells/well
Constreet-H (31-32 mg/100 cm2)
Methods: Cell viability was assessed by estimation of mitochondrial ability to reduce MTT and cell proliferation was estimated by
measuring the incorporation of bromodeoxyuridine (Br-dU) into nuclear DNA, both using a microplate reader. Phase contrast
microscopy was used to assess cell morphology.
Study Outcome
Viability and Proliferation. Percent MTT reduction and percent Br-dU incorporation listed below.
Percent MTT Reduction Percent Br.du incorporation
% Control (mean ± SE) % Control (range) % Control (mean ± SE) % Control (range)
Acticoat 1.3 ±0.3 °-5-2-5 8.5 ±0.8 5.2-11.4
Acuacel-Ag °-6±0-1 °-3-1-4 5-6±1-6 °-°-8-7
Avance 15.2 ±6.3 1.9-46.0 12.5 ±1.8 4.7-18.4
Contreet-H 1.0 ±0.2 0.2-1.9 9.8 ±3.0 0-34.2
Cell Morphology. In no extracts was there any evidence of a monolayer, and cell apposition did not follow the normal pattern. All
treated cells lacked conspicuous nuclei, had missing nucleoli, and cultures displayed particulate and irregularly shaped cell
debris. Cells treated with Contreet-H and Avance extracts were three times larger than control cells and displayed abundant
polygonal cytoplasm with many cells showing cytoplasmic projections. Cells treated with Aqualcel-Ag and Acticoat extracts had a
round to ovoid cytoplasm.
C-14
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Rosas-Hernandez et al. (2009) Effects of 45-nm silver nanoparticles on coronary endothelial cells
and isolated rat aortic rings.
Test Species
Rat coronary endothelial cells (CECs)
Material
Commercial nano-Ag, supplied by Novacentrix Inc., Austin, Texas.
Shape: Irregular spheres Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 45 nm (reported by manufacturer); Surface Charge: Not reported
35.75 ± 13.1 nm (determined using TEM)
Size Distribution: 10-90 nm (determined using TEM)
Protocol
Exposure Duration: 24 hours Exposure Medium: Dulbecco's modified eagle medium
Endpoint: Proliferation, nitric oxide production, cytotoxicity, (DMEM) for cultured cells; Krebs-Henseleit (K-H) for rat aortic
aortic ring vascular tone ring
Exposure Concentrations: 0, 0.1, 0.5,1, 5,10, 50, and 100 Cell Density: Not reported
ug/mL
Methods: Proliferation was measured by MTT assay, and nitric oxide production was measured using the Griess reaction to
determine the ratio of total nitrites to total nitrates. Cytotoxicity was assessed using a cytotoxicity assay kit to detect the level of
lactate dehydrogenase (LDH). Aortic ring vascular tone was analyzed by pre-contracting rat aortic vessels with 5 and 100 ug/mL
nano-Ag.
Study Outcome
Proliferation. Mitochondrial function was significantly decreased at concentrations of 1, 5, and 10 ug/mL and significantly
increased at concentrations of 50 and 100 ug/mL.
Nitric Oxide Production. Concentrations of 10, 50, and 100 ug/mL were associated with significantly increased production of
nitric oxide.
Cytotoxicity. LDH activity was significantly increased at concentrations of 1, 5, and 10 ug/mL.
Aortic Ring Vascular Tone. Rat aortic rings were constricted at 5 ug/mL and relaxed at 100 ug/mL.
C-15
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Samberg et al. (2010) Evaluation of silver nanoparticle toxicity in vivo and keratinocytes in vitro.
Test Species
Primary neonatal human epidermal keratinocytes (HEKs)
Material
Three sizes (reported by manufacturer as 20, 50, and 80 nm in diameter) of commercial unwashed/uncoated and
washed/uncoated nano-Ag in deionized water and two sizes (reported by manufacturer as 25 and 35 nm in diameter) of
dried/carbon-coated nano-Ag powder, all supplied by nanoComposix, San Diego, CA. Unwashed and washed nano-Ag
synthesized by manufacturer using ammonium hydroxide-catalyzed growth onto 5-nm gold seed particles. Carbon-coated
nano-Ag synthesized by manufacturer using pulsed plasma reactor and coated with polyaromatic graphitic carbon.
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: Unwashed: 30.8 ± 0.6, 47.7 ± 0.5, and Surface Treatment: None (unwashed and washed),
75.5 ± 1.0 nm (determined using DLS) or 22.4 ± 2.6, polyaromatic graphite carbon (carbon-coated)
49.4 ± 6.2, and 79.2 ± 8.0 nm (determined using TEM). Surface Charge: Unwashed: -29.7 mV (~20-nm size),
Washed: 25.5 ± 0.4, 43.7± 1.1, and 79.9 ± 28.0 nm -27.8 mV (~50-nm size), and -33.2 mV (~80-nm size).
(determined using DLS) or 21.4 ± 3.1, 50.0 ± 5.9, and Washed; -46.0 mV (~20-nm size), -44.3 mV (~50-nm size),
77.0 ± 6.0 (determined using TEM). Carbon-coated: 149.0 ± and -43.7 mV (~80-nm size). Carbon-coated: -24.0 mV
89 and 167.0 ± 110 nm (determined using DLS) or (~25-nm size) and -29.0 mV (~35-nm size) (expressed as
27.2 ± 10.3 and 37.0 ± 11.6 nm (determined using TEM). (potential in deionized water)
Protocol
Exposure Duration: 24 hours Exposure Medium: Keratinocyte Growth Medium-2 (KGM-2)
Endpoint: Viability and cytokine release Cell Density: Initial density of 12,500 cells/well grown over
Exposure Concentrations: 0.000544 to 1.7 jig/mL for all 18-24 hours to 80% confluency
nano-Ag types, and up to 45 jig/mL for washed and carbon-
coated nano-Ag
Methods: HEKs were exposed to either KGM-2 (control) or to serial dilution of each of the eight types of nano-Ag. To assess
viability, HEKs were exposed to nano-Ag diluted in KGM-2 at concentrations ranging from 0.000544 to 1.7 ng/mL for 24 hours
using MTT, alamarBlue (aB), and Celltiter 96 Aqueous One (96AQ) assays. Concentrations up to 42.5 jig/mL of the washed and
carbon-coated nano-Ag particles were also tested. To determine if the supernatants or washing permeates resulting from the
synthesis and washing of the unwashed/uncoated and washed/uncoated particles, respectively, were contributing to observed
cytotoxicity, HEKs were treated with either the supernatant or permeates for 24 hours at concentrations ranging from 0.068 to 1.7
jig/mL. For those nano-Ag concentrations resulting in cytotoxicity, cytokine analysis was conducted by assessing release of
interleukin (IL)-8, IL-6, tumor necrosis factor-cr (TNF- cr), IL-10, and IL-1(3. Results were presented with those from an in vivo
porcine skin test conducted as part of the same study. These results are presented in presented in Section C.3 of this appendix.
Study Outcome
Viability. Exposure of HEKs to unwashed/uncoated nano-Ag resulted in a dose-dependent decrease in cell viability in all three
assays, with significant (p < 0.05) decreases occurring at the 0.34-jig/mL level in the -20- and ~50-nm groups using the aB and
96AQ assays and at the 1.7-jig/mL level for these size groups using the MTT assay. The ~80-nm group resulting in a significant
decrease in viability at the 0.34-jig/mL level using all three assays. No significant decrease in viability was observed in any assay
for the washed/uncoated or carbon-coated nano-Ag samples. The unwashed nano-Ag supernatant contained 5.55 mg/mL
formaldehyde solvent and methanol by-product from synthesis. Exposure to this supernatant resulted in significantly decreased
viability at the 0.34-jig/mL level using the aB and MTT assays and at the 1.7-jig/mL level using the 96AQ assay. Exposure to the
washing permeates from the washed and uncoated samples did not result in any loss in viability.
Cytokine Release. Following exposure to 0.34 jig/mL unwashed nano-Ag of all sizes, significant (p < 0.05) increases were
observed in IL-1 (3, IL-6, IL-8, and TNF- cr.
C-16
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Shin et al. (2007) The effects of nano-silver on the proliferation and cytokine expression by
peripheral blood mononuclear cells.
Test Species
Human peripheral blood mononuclear cells (PMBCs)
Material
Nano-Ag colloidal solution synthesized from AgNOs.
Shape: Not Reported
Composition: Not Reported
Crystal Structure: Not Reported
Average Size: 1.3 nm
Protocol
Exposure Duration: 72 hours
Endpoint: Cytotoxic effects on PBMCs
Size Distribution: 1-2.5 nm
Solubility: Not Reported
Surface Area: Not Reported
Surface Treatment: Not Reported
Surface Charge: Not Reported
Exposure Concentrations: 0,1,3, 5,10, 20, 30 ppm
nano-Ag
Exposure Medium: Not reported
Cell Density: 2 x 106 cells/mL
Methods: PBMCs from healthy human volunteers were stimulated with 5 ug/ml phytohaemagglutinin (PHA) in the presence of
varying concentrations of nano-Ag. PBMC proliferations were measured using an aqueous cell proliferation assay kit and
supernatants were analyzed using enzyme-linked immunosorbent assays.
Study Outcome
Inflammatory Response. Proliferation of PBMCs showed cytotoxicity levels of nano-Ag over 15 ppm. At low levels nano-Ag
decreased cytokine production (PHA-induced IL-5: at 10 ppm, interferon (INF)-gamma and tumor necrosis factor (TNF)-alpha at
3 ppm).
C-17
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Shrivastava et al. (2009) Characterization of antiplatelet properties of silver nanoparticles.
Test Species
Human primary platelets
Material
Nano-Ag synthesized fromAgNOs using deionized water, NaOH, and NHs.
Shape: Spherical Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Face centered cubic Surface Treatment: Not reported
Average Size: 10-15 nm (determined using diffraction) Surface Charge: Not reported
Size Distribution: Monodispersed
Protocol
Exposure Duration: 10 minutes Exposure Medium: Aqueous
Endpoint: Morphology and membrane integrity Cell Density: Not reported
Exposure Concentrations: 0, 25, 50,100,150, 200, 250,
and 500 uM
Methods: Morphology was assessed by labeling cells with ANS and recording fluorescence emission spectra. Membrane
integrity was measured by assaying lactate dehydrogenase (LDH) activity from the decrease in reduced NADH absorbance.
Study Outcome
Morphology. Fluorescence intensity declined in a dose-dependence manner, suggesting nano-Ag contributes significantly to
platelet membrane disorder.
Viability. Nano-Ag did not cause significant release of LDH from platelet cytosol. Platelet membrane integrity was not
compromised nor did cell lysis occur.
C-18
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Trickier et al. (2010) Silver nanoparticle-induced blood-brain barrier inflammation and increased
permeability in primary rat brain microvessel endothelial cells.
Test Species
Sprague-Dawley rat brain microvessel endothelial (rBMEC) cells
Material
Nano-Ag, gift from Nanocomposix, San Diego, CA.
Shape: Spherical
Composition: Not reported
Crystal Structure: Not reported
Average Size: 28.3, 47.5, and 102.2 nm (determined using
TEM)
Size Distribution: 20-40, 40-55, and 60-140 nm
(determined using TEM)
Protocol
Exposure Duration: 24 hours
Endpoint: Cytotoxicity and proinflammatory mediators
Exposure Concentrations: 6.25-50 ug/cm3
Methods: Cytotoxicity was assessed by the mitochondrial dependent conversion of XTT reagent. Release of proinflammatory
mediators TNF, IL-1B and PGE2 were also monitored.
Study Outcome
Cytotoxicity. Significantly decreased cell viability was observed in cells treated with 25 and 40 nm particles at 25 and 50 ug/cm3,
and in cells treated with 80 nm particles at only 50 ug/cm3. Significant morphological changes were observed to be size- and
dose-dependent.
Proinflammatory Mediators. Association with nano-Ag particles was observed to increase the release of proinflammatory
mediators, suggesting that morphological changes in rBMEC cells are associated with increased permeability.
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: PVP coating
Surface Charge: -44.2, -46.0, and -29.5 mV (determined
using LDV)
Exposure Medium: rBMEC complete media
Cell Density: Not reported
C-19
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C.3. Summary of Key In Vivo Studies
Cha et al. (2008) Comparison of acute responses of mice livers to short-term exposure to nano-
sized or micro-sized silver particles.
Test Species
Balb/c mouse (male), 7 weeks old
Material
Nano-Ag, synthesized by the reduction of AgNOswith NaBhU
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 13 nm, or 2-3.5 urn (determineD usingD TEM)
Protocol
Size Distribution: Not reported
Solubility: Not reported
Surface Area: Not reported
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Duration: 24 hours Exposure Concentrations: 2.5 g
Endpoint: Acute liver effects Exposure Media/Route: Gastric intubation
Methods: Livers, hearts, intestines, and spleens from test species were obtained 3 days after treatment, fixed in 10% formalin,
and subjected to histopathological analysis. A total RNA isolation kit was used to isolate the RNA from the livers and results were
confirmed by a semi-quantitative RT-PCR.
Study Outcome
Nanoparticle and microparticle-treated livers showed lymphocyte infiltration, which indicated inflammation. Nonspecific focal
hemorrhages in the heart, focal lymphocyte infiltration in the intestine, and nonspecific medullary congestion in the spleen were
also observed in mice treated with nano-Ag. RNA microarray analysis of livers showed altered apoptosis and inflammatory
responses in the livers of nanoparticle-exposed mice, as indicated by the up-regulation of seven genes in the apoptotic pathway
and five in the inflammatory pathway gene expression.
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Ji et al. (2007) Twenty-eight-day inhalation toxicity study of silver nanoparticles in Sprague-Dawley
rats.
Test Species
Sprague-Dawley rat, 40 males and females (10 rats per dosing group), 10 weeks old at start of experiment (~283-g males,
~169-g females)
Material
Nano-Ag; particles generated by evaporation/condensation a using small ceramic heater within a quartz tube case
Solubility: Not Reported
Surface Area: 1.41 x W, 9.68 x W, and 1.32 x 10?
nm2/cm3 in the high-, middle-, and low-concentration
chambers, respectively
Surface Treatment: Not Reported
Surface Charge: Not Reported
Shape: Spherical
Composition: Not Reported
Crystal Structure: Not Reported
Average Size: 16 nm (determined using differential mobility
analyzer and a condensation particle counter)
Size Distribution: 1.98-64.9 nm; geometric mean diameter
and geometric standard deviation of 15.38 nm and 1.58,
12.60 nm and 1.53, and 12.61 nm and 1.52 in the high-,
middle-, and low-concentration chambers, respectively
Protocol
Exposure Duration: Subchronic 28-day inhalation Exposure Concentrations: 1.32 * 106 particles/cm3 (high
Endpoint: Inflammatory response dose), 1.27 * 105 particles/cm3 (middle dose), and 1.73 * 104
particles/cm3 (low dose) for 6 hours/day, 5 days/week
Exposure Media/Route: Whole-body inhalation chamber
Methods: Exposure-related effects including respiratory, dermal, behavioral, nasal, and genitourinary changes suggestive of
irritancy were studied daily on weekdays. Body weights were measured after purchase, grouping, weekly during exposure, and at
study termination. Hematological analysis was conducted 24 hours after study termination. Organ weights were then measured,
followed by a complete histopathological analysis and determination of tissue silver using an atomic absorption
spectrophotometer.
Study Outcome
Despite deposition of nano-Ag particles in the liver, olfactory bulb, and brain, no significant exposure-related adverse health
effects were observed.
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Kim et al. (2008) Twenty-eight-day oral toxicity, genotoxicity, and gender-related tissue distribution
of silver nanoparticles in Sprague-Dawley rats.
Test Species
Sprague-Dawley rat, 40 males and females (10 rats per dosing group), 6 weeks old at start of experiment (~283-g males, ~192-g
females)
Material
Commercial nano-Ag, micro-sized Ag; purchased from NAMATECH Co., Ltd.
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Not reported
Average Size: 60 nm Surface Charge: Not reported
Size Distribution: 52.7-70.9 nm
Protocol
Exposure Duration: 28 days Exposure Concentrations: Vehicle control (0.5%
Endpoint: Effects on hematology and blood biochemistry; carboxymethylcellulose), low-dose group (30 mg/kg), middle-
genotoxic effect on rat bone marrow; and Ag distribution in dose group, (300 mg/kg), and high-dose group (1,000
tissue. mg/kg).
Exposure Media/Route: Daily oral gavage
Methods: After exposure, the blood biochemistry and hematology were investigated using a blood cell counter, along with a
histopathological examination and Ag distribution study using spectrophotometry.
Study Outcome
Hematology and Blood Biochemistry. Dose-dependent changes in serum alkaline phosphatase, with a significant increase in
high- and middle-dose male rats and high-dose female rats. Similarly, high-dose male rats and high- and middle-dose female rats
displayed significant increases in cholesterol. Total protein significantly decreased in high-dose male rats. Mean corpuscular
volume significantly increased in high-dose male rats, while the red blood cell count, hemoglobin content, and hematocrit
significantly increased in high- and middle-dose female rats.
Inflammation. Dose-dependent bile duct hyperplasia with increase in inflammatory cells.
Genotoxic Effect on Rat Bone Marrow. Nano-Ag did not induce genetic toxicity in bone marrow.
Ag Accumulation in Tissues. All tissues revealed significant dose-dependent accumulation of Ag in tissues. Two-fold increase
in the female kidneys compared with the male kidneys.
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Lee et al. (2010) Genomics-based screening of differentially expressed genes in the brains of mice
exposed to silver nanoparticles via inhalation.
Test Species
28 Male C57BL/6 mice (20-25 g), 7 per group
Material
Nano-Ag produced using a nanoparticle generator.
Shape: Not reported
Composition: Not reported
Crystal Structure: Not reported
Average Size: 22.18 nm (determined using SEM)
Size Distribution: 20.46-23.9 nm (determined using SEM)
Protocol
Solubility: Not reported
Surface Area: 1.46 x 1010 nrrvYcm3
Surface Treatment: Not reported
Surface Charge: Not reported
Exposure Duration: 6 hours/day, 5 days/week for 2 weeks
Endpoint: Gene expression and morphology
Exposure Concentrations: 1.91 x 107 particles/cm3
Exposure Media/Route: Inhalation, nose-only exposure
system
Methods: Total RNA isolated from brain tissues of exposed mice was assessed by subjection to hybridization. Real-time
polymerase chain reaction (PCR) procedures were used to analyze whole blood samples. Brain tissues were also examined
histopathologically for structural and functional changes.
Study Outcome
Gene Expression. 468 genes in the cerebrum and 952 genes in the cerebellum of treated rats were observed to have been
affected by nano-Ag exposure in a significant manner. Those genes associated with signal transduction were the most affected,
along with genes involved in protein metabolism, developmental processes, and nucleic acid metabolism. Five genes in the
whole blood analysis were downregulated in response to nano-Ag exposure.
Morphology. No structural changes were observed in tissues after hematoxylin and eosin staining.
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Li et al. (201 Oa) Induction of cytotoxicity and apoptosis in mouse blastocysts by silver
nanoparticles.
Test Species
ICR mouse blastocytes
Material
Nano-Ag prepared by the polyol process (reduction of AgNOs with C2He02 in the presence of polyvinylpyrrolidone [PVP]).
Shape: Not reported Solubility: Not reported
Composition: Not reported Surface Area: Not reported
Crystal Structure: Not reported Surface Treatment: Coated with a shell of polymer to reduce
Average Size: 13 nm (determined using TEM) cluster formation
Size Distribution: 6-24 nm (determined using TEM) Surface Charge: Not reported
Protocol
Exposure Duration: 24 hours Exposure Concentrations: 0, 25, and 50 uM
Endpoint: Blastocyst development Exposure Media/Route: Implantation of pre-treated
blastocysts
Methods: Blastocysts pretreated with 0, 25, or 50 uM nano-Ag were transferred to female recipient ICR mice and uterine content
was examined 13 days after transfer.
Study Outcome
Developmental Potential. The implantation ratio of blastocysts pretreated with 50 uM nano-Ag was significantly lower than that
of the control cells. The proportion of successfully implanted embryos that failed to develop normally was significantly higher in
the group pretreated with 50 uM nano-Ag. Overall fetal weight was lower in the group pretreated with 50 uM nano-Ag.
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Park et al. (201 Oa) Repeated-dose toxicity and inflammatory responses in mice by oral
administration of silver nanoparticles.
Test Species
Male and female ICR mice, 6 weeks old, 5 mice per group in 14-day study, 6 mice per group in 28-day study
Material
Nano-Ag particles (Sigma-Aldrich, USA) suspended with sonication in tetrahydrofuran (THF) and evaporated.
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: 22, 42, 71, 323 nm (14-day study); 42 nm (28- Surface Treatment: Not reported
day study) (determined using DLS) Surface Charge: Not reported
Protocol
Exposure Duration: 14, 28 days Exposure Concentrations: 0,1 mg/kg (14-day study), 0,
Endpoint: Tissue distribution, cell function, cell phenotype, 0.25, 0.5,1 mg/kg (28-day study)
body weight (14-day study), serum biochemistry, lymphocyte Exposure Media/Route: Daily oral gavage in deionized
phenotype, histopathological changes (28-day study) water
Methods: Mice were administered equal doses of varying sizes of nano-Ag particles in a size-differentiated toxicity test, and the
same size particle in different doses in a repeated-dose toxicity test. Toxicity was examined by determination of blood
biochemistry, nanoparticle distribution in the body, concentration of cytokines, blood serum levels, and histopathology.
Study Outcome
Tissue Distribution. In the 14-day study, nano-Ag was observed to significantly accumulate in dose groups administered 22, 42,
or 71 nm particles, but not in the group administered 323 nm particles.
Cell Function. Small particles (22, 42, and 71 nm) significantly increased the level of TGF-(3 in the 14-day study.
Cell Phenotype. The 14-day study revealed increased distribution of NK and B cells in the groups treated with 22, 42, or 71 nm
particles. The ratio of T-cell subtype was also noticeably decreased in these dose groups.
Body Weight. Body weight and organ-to-body-weight ratios were not affected by administration of various sizes of nano-Ag
particles administered for 14 days.
Serum Biochemistry. In the 28-day study, significantly increased levels of AST and ALP were observed in males and females at
the 1 mg/kg dose level. ALT was also increased in female mice at the 1 mg/kg dose level.
Cytokine Levels. Pro-inflammatory cytokine levels increased in a dose-dependent manner in the 28-day study. Levels were not
detectable in control mice, and increased to 8.8 ± 0.70 pg/mL at 1 mg/kg.
Histopathology. No significant histopathological findings were observed in the 28-day study.
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Samberg et al. (2010) Evaluation of silver nanoparticle toxicity in vivo and keratinocytes in vitro.
Test Species
Pig, two weanling females (20-30 kg)
Material
Two sizes (reported by manufacturer as 20 and 50 nm in diameter) of commercial unwashed/uncoated and washed/uncoated
nano-Ag in deionized water, both supplied by nanoComposix, San Diego, CA. Unwashed and washed nano-Ag synthesized by
manufacturer using ammonium hydroxide-catalyzed growth onto 5-nm gold seed particles.
Shape: Not reported Size Distribution: Not reported
Composition: Not reported Solubility: Not reported
Crystal Structure: Not reported Surface Area: Not reported
Average Size: Unwashed: 30.8 ± 0.6, 47.7 ± 0.5, and Surface Treatment: None
75.5 ± 1.0 nm (determined using DLS) or 22.4 ± 2.6, Surface Charge: Unwashed: -29.7 mV (~20-nm size),
49.4 ± 6.2, and 79.2 ± 8.0 nm (determined using TEM). -27.8 mV (~50-nm size), and -33.2 mV(~80-nm size).
Washed: 25.5 ± 0.4, 43.7± 1.1, and 79.9 ± 28.0 nm Washed; -46.0 mV (~20-nm size), -44.3 mV (~50-nm size),
(determined using DLS) or 21.4 ± 3.1, 50.0 ± 5.9, and and -43.7 mV (~80-nm size).
77.0 ± 6.0 (determined using TEM).
Protocol
Exposure Duration: 14 days Exposure Concentrations: 0.34 to 34 jig/mL
Endpoint: Morphological alterations Exposure Media/Route: Daily dermal application
Methods: Pigs were topically dosed once per day for 14 days at 14 sites on back skin with nano-Ag concentrations ranging from
0.34 to 34 jig/ml. The Draize system was then used to evaluate the skin for erythema and edema. Microscopic observations
were taken on harvested skin samples after pigs were euthanized on Day 14. Samples were analyzed for intercellular and
intracellular edema, dermal edema, and inflammation. Results were presented with those from an in vitro test using human
epidermal keratinocytes conducted as part of the same study. These results are presented in Appendix C.2.
Study Outcome
Macroscopic observations of porcine skin exposed topically with nano-Ag of all types and sizes revealed no gross erythema or
edema. Microscopic observations of the exposed skin samples revealed a concentration-dependent response that was not
related to particle size or washing. Effects observed following exposure to the washed ~20-nm nano-Ag solutions were
intracellular and intercellular epidermal edema (all doses), focal epidermal and dermal inflammation (mid dose), epidermal
hyperplasia (high dose), parakeratosis (high dose), and extension of rete pegs into the superficial papillary layer of the dermis
(high dose). Effects observed following exposure to the unwashed ~20-nm nano-Ag solutions were intracellular epidermal edema
(all doses), intercellular edema (mid and high dose), and focal areas of intraepidermal infiltrates and superficial papillary dermal
inflammation (high dose).
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Shrivastava et al. (2009) Characterization of antiplatelet properties of silver nanoparticles.
Test Species
AKR and PARKES mouse, 50 males (25 from each strain), 7-8 weeks
Material
Nano-Ag synthesized fromAgNOs using deionized water, NaOH, and NHs.
Shape: Spherical Size Distribution: Monodispersed
Composition: Not reported Solubility: Not reported
Crystal Structure: Face centered cubic Surface Area: Not reported
Average Size: 10-15 nm (determined using TEM) Surface Treatment: Not reported
Surface Charge: Not reported
Protocol
Exposure Duration: 10 minutes Exposure Concentrations: 2, 4, 6, and 8 mg/kg body
Endpoint: Hematology and mortality weight
Exposure Media/Route: Intravenous injection into tail veins
Methods: Mice were divided into 10 groups of 5 animals each. Aggregation was measured in whole blood by electronic
impedance. Tail bleeding was monitored at 15-minute increments until no blood was observed on the filter paper.
Study Outcome
Hematology. Intravenous nano-Ag (2-8 mg/kg) inhibited platelet aggregation in whole blood. No adverse effect on bleeding time
was observed.
Mortality. Survival was unaffected.
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Sung et al. (2008) Lung function changes in Sprague-Dawley rats after prolonged inhalation
exposure to silver nanoparticles.
Test Species
Sprague-Dawley rat, 40 males and females (10 rats per dosing group), 8 weeks old at start of experiment (~253-g males, ~162-g
females)
Material
Nano-Ag; particle generation described in Ji et al. (2007)
Shape: Spherical; non-aggregated/agglomerated forms with Size Distribution: 1.98-64.9 nm
diameters under 55 nm (determined using TEM)
Composition: Not Reported
Crystal Structure: Not Reported
Average Size: 18-19 nm (determined using differential
mobility analyzer and a condensation particle counter)
Protocol
Exposure Duration: Subchronic 90-day inhalation
Endpoint: Inflammatory response; pulmonary function
changes
Solubility: Not Reported
Surface Area: 1.08 x W, 2.37 x W, and 6.61 x W
nrrWcm3 in the low-, middle-, and high-concentration
chambers, respectively
Surface Treatment: Not Reported
Surface Charge: Not Reported
Exposure Concentrations: 0.7 x 106 particles/cm3 (low
dose), 1.4 x 106 particles/cm3 (middle dose), and 2.9 x
particles /cm3 (high dose) for 6 hours/day
Exposure Media/Route: Whole-body inhalation chamber
Methods: The lung function was measured every week after the daily exposure, and the animals were sacrificed after the 90-day
exposure period. Cellular differential counts and inflammatory measurements, such as albumin, lactate dehydrogenase (LDH),
and total protein, were also monitored in the acellular bronchoalveolar lavage (BAL) fluid of the rats.
Study Outcome
Lung Function. Tidal volume and minute volume showed a statistically significant decrease during the 90 days of nano-Ag
exposure.
Inflammatory Response. Although BAL fluid cellular differential counts were not found to be statistically significant, increased
inflammation measurements were observed in the high-dose female rats. Dose-dependent increases in lesions related to
nano-Ag exposure, such as infiltrate mixed cell and chronic alveolar inflammation, including thickened alveolar walls and small
granulomatous lesions.
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Sung et al. (2009) Subchronic inhalation toxicity of silver nanoparticles.
Test Species
Sprague-Dawley rat, 40 males and females (10 rats per dosing group), 8 weeks old at start of experiment (~253-g males, ~162-g
females)
Material
Nano-Ag; particle generation described in Ji et al. (2007)
Shape: Spherical; non-aggregated/agglomerated forms with
diameters under 55 nm (determined using TEM)
Composition: Not Reported
Crystal Structure: Not Reported
Average Size: 18-19 nm (determined using differential
mobility analyzer and a condensation particle counter)
Protocol
Exposure Duration: Subchronic 13-week inhalation
Endpoint: Inflammatory response; pulmonary function
changes; liver toxicity; andAg distribution in tissue
Size Distribution: 1.98-64.9 nm
Solubility: Not Reported
Surface Area: 1.08 x 109, 2.37 x w, and 6.61 x w
nrrWcm3 in the low-, middle-, and high-concentration
chambers, respectively
Surface Treatment: Not Reported
Surface Charge: Not Reported
Exposure Concentrations: 0.6 x 106 particles/cm3; 49
ug/m3 (low dose), 1.4 x 106 particles/cm3; 133 ug/m3 (middle
dose), and 2.9 x 106 particles /cm3; 515 ug/m3 (high dose) for
6 h/day, 5 days/week
Exposure Media/Route: Whole-body inhalation chamber
Methods: At the end of the study, the rats were subjected to a full necropsy, blood samples were collected for hematology and
clinical chemistry tests, and the organ weights were measured.
Study Outcome
NOAEL: 100 ug/m3.
Inhalation Toxicity. Dose-dependent increases in lesions related to nano-Ag exposure, including mixed inflammatory cell
infiltrate, chronic alveolar inflammation, and small granulomatous lesions.
Liver Toxicity. Dose-dependent bile duct hyperplasia in liver.
Ag Distribution in Tissue. Statistically significant dose-dependent increases in Ag concentration in lung tissue. Dose-dependent
increase in the Ag concentration in the blood. Dose-dependent increase in the liver Ag concentration. Ag concentration in the
olfactory bulb was higher than in brain, and increased in a dose dependent manner in both genders (p < 0.01). Ag concentrations
in the kidneys showed a gender difference, with female kidneys containing 2-3 times more Ag accumulation than in male
kidneys.
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Takenaka et al. (2001) Pulmonary and systemic distribution of inhaled ultrafine silver particles in
rats.
Test Species
Female Fischer 344 rat, (150-200 g)
Material
Nano-Ag; particles generated by spark discharging through an argon atmosphere.
Shape: Spherical Size Distribution: Not Reported
Composition: Not Reported Solubility: Not Reported
Crystal Structure: Not Reported Surface Area: Not Reported
Average Size: 14.6 nm (determined using differential Surface Treatment: Not Reported
mobility analyzer) Surface Charge: Not Reported
Protocol
Exposure Duration: Acute 6-hour inhalation; intratracheal Exposure Concentrations: Inhalation mass concentration of
instillation 133 ug Ag/m3 and a particle number concentration of 3 x 106
Endpoint: Accumulation in the lungs and brain cm3 for 6 hours; injected with 150 uL aqueous solution of 7
ug AgNOs (4.4 ug Ag) or 150 uL aqueous suspension of 50
ug elemental Ag.
Exposure Media/Route: Whole-body inhalation chamber;
intratracheal instillation
Methods: Rats were sacrificed 0,1, 4, and 7 days after exposure or intratracheal instillation for morphology and elemental
analysis. The ultrastructure of nano-Ag particles was examined using a transmission electron microscope.
Study Outcome
After inhalation exposure, silver accumulation was largest in the lungs, followed by the nasal cavities, in particular the posterior
portion, and the lung-associated lymph nodes. Only 4% of the initial body burden in the lungs remained on day 7. Low
concentrations of silver were observed in the brain at day 0 and day 1, but no data were collected for the brain on days 4 and 7.
Following intratracheal instillation, particles were observed within the alveolar walls, and the rate of clearance was seen to be
much slower than following inhalation.
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Tang et al. (2008) Influence of silver nanoparticles on neurons and blood-brain barrier via
subcutaneous injection in rats.
Test Species
Wistar rat, 90 females (110-130 g), divided into 3 groups (control, nano-Ag, micro-sized Ag)
Material
Commercial nano-Ag, micro-sized Ag; obtained from Sigma-Aldrich (USA).
Shape: Globular (nano-Ag; determine by TEM); irregular Solubility: Not Reported
cubes (micro-sized Ag; determined by SEM) Surface Area: Not Reported
Composition: Not Reported Surface Treatment: Not Reported
Crystal Structure: Not Reported Surface Charge: Not Reported
Average Size: Not Reported
Size Distribution: 50-100 nm (nano-Ag; determined by
TEM)); 2-20 urn (micro-sized Ag; determined by SEM)
Protocol
Exposure Duration: Single injection Exposure Concentrations: 1 ml suspension, 62.8 mg/kg
Endpoint: Accumulation in the brain and blood-brain barrier Exposure Media/Route: Subcutaneous injection
effects
Methods: Rat brains were obtained for ultra-structural observation and Ag level detection. Five rats from each group were
sacrificed at weeks 2, 4, 8,12,18, and 24 to obtain brain tissue. The remaining brain tissue was digested to measure Ag levels.
Study Outcome
Ag levels were significantly higher in the nano-Ag group than in the micro-Ag and control groups. Results show that micro-sized
Ag did not traverse into the brain, whereas nano-Ag did and accumulated in the brain at least 24 weeks. Nano-Ag can induce
neuronal degeneration and necrosis by accumulating in the brain over a long period of time.
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Tiwari et al. (2011) Dose-dependent in-vivo toxicity assessment of silver nanoparticles in Wistar
rats.
Test Species
Male and female Wistar rats (8-10 weeks old, 200-225 g), divided into groups of six animals
Material
Silver nanoparticles dispersed in ethylene glycol obtained from Sigma-Aldrich (USA).
Shape: Rounded (characterized by TEM) Solubility: Not Reported
Composition: Not Reported Surface Area: Not Reported
Crystal Structure: Not Reported Surface Treatment: Not Reported
Average Size: Not Reported Surface Charge: Not Reported
Size Distribution: 15-40 nm
Protocol
Exposure Duration: 32 days Exposure Concentrations: 0, 4,10, 20, 40 mg/kg
Endpoint: Analysis of enzymes related to liver and kidney Exposure Media/Route: Intravenous injection administered
function in phosphate buffer
Methods: Rats were exposed intravenously to silver nanoparticles over 32 days. Blood samples were collected each week and
analyzed for levels of AST, ALT, GGTP, ALP, total protein, and bilirubin.
Study Outcome
Levels of AST, ALT, and ALP significantly increased after exposure to nano-Ag particles at doses of 20 and 40 mg/kg. No
significant changes were observed in the 4 and 10 mg/kg groups with respect to control. GGTP also showed a significant
increase in the 40 mg/kg group. Increased levels of bilirubin along with correlated decreases in hemoglobin indicated changes in
liver function.
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Appendix D. Identified Research Priorities
(January 2011 Workshop)
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D-1
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Appendix D. Identified Research Priorities
(January 2011 Workshop)
D.1. Introduction D-4
D.2. Prioritized Research Themes and Specific Questions from Collective Judgment Workshop D-5
Appendix D References D-8
D-2
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D-3
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D.1. Introduction
As discussed in Chapters 1 and 7, the external review draft of this case study served as the starting
point for identifying and prioritizing research gaps that, if pursued, could inform future assessments of
nano-Ag in disinfectant spray. The workshop process, which was funded by EPA and independently
conducted by an EPA contractor, and outcomes are summarized in Section 7.3.1 and explained in more
detail in a summary report produced by the EPA contractor (ICF. 2011). After identifying priority research
questions participants grouped similar questions into themes. A voting process then ensued whereby each
participant allotted 10 points to the most important research theme, 9 points to the second most critical
theme, and so on, down to 1 point. Combining the points from all participants for each broader theme
resulted in a prioritized list of research directions. This list, which is an excerpt of the summary report
produced by the EPA contractor conducting the workshop process (ICF, 2011). is included in Section D.2
below.
D-4
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D.2. Prioritized Research Themes and Specific Questions
from Collective Judgment Workshop
Research Theme
Specific Question
Points (Votes)
1. Analytical Methods
120(19)
Do adequate analytical methods exist to detect and characterize exposure to
nano-Ag via soil, water, and air?
Do adequate analytical methods exist to detect and characterize nano-Ag in
environmental compartments and in biota?
Are there standard nano-Ag reference materials that can be used in exposure and
effects testing to aid in comparison of results among investigators?
Are available methods adequate to characterize nano-Ag concentrations and
associated exposure via relevant matrices such as:
a. air?
b. water?
c. food?
At a minimum, what assays could be considered in a harmonized test guideline for
determination of the human health effects of nano-Ag?
2.
Exposure and
Susceptibility
120 (17)
How should dose and exposure be characterized and how do the following
parameters affect it: (1) physiological characteristics, (2) behavior, (3) lifestages,
and (4) susceptibility factors?
What are the relevant susceptibility factors in terms of exposure?
What kinds of exposure do these populations have, including physicochemical
characteristics?
Do particular species of biota and particular human populations have greater
potential for exposure to nano-Ag through the life cycle?
Which source, pathways, and routes offer the greatest exposure potential to
nano-Ag for humans?
What is the distribution of exposure intensities and frequencies of such exposures
among homemakers, children, and maintenance personnel, and are these of
concern for acute and or chronic health effects?
3. Physical and Chemical
Toxicity
115(16)
What physicochemical properties of nano-Ag can be used to predict toxicity to
humans or biota?
How does surface coating affect toxicity to humans or biota?
To what extent do particle properties (e.g., size, shape, chemical composition,
surface treatments) determine biological responses to nano-Ag?
Which physicochemical properties of nano-Ag are most essential to characterize
before, during, and after toxicity experiments?
4.
Kinetics and
Dissolution
98 (15)
What is the half life of nano-Ag in the environment?
D-5
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Research Theme
Specific Question
Points (Votes)
5. Surfce Characteristics
81 (14)
How does surface coating affect the physicochemical properties of nano-Ag?
Do explosion risks exist for dried nano-Ag powders or nano-Ag powders modified
with certain types of surface coatings?
What effect, if any, do surface treatments of nano-Ag particles have on:
a. uptake?
b. biopersistence?
c. bioaccumulation?
d. biomagnification?
What effect, if any, do surface treatments of nano-Ag particles have on human
exposures and uptake?
6. Sources and Releases
76(15)
How effectively is nano-Ag removed from sewage and industrial process water by
wastewater treatment technology, and can information on the removal of
conventional silver be applied to nano-Ag removal?
What are the potential exposure vectors by which nano-Ag or nano-Ag by-products
could be released to the environment at the various life-cycle stages?
What are the associated feedstocks and by-products (and, of these feedstocks and
by-products), which might be released, in what quantities, and via which pathways?
What are the release rates of all sources of nano-Ag into the environment?
7. Mechanisms of
Nanoscale Silver
Toxicity
72(11)
What are the fundamental biological responses to and associated mechanisms of
nano-Ag exposure at the cell, organ, and whole-animal levels?
Are the effects observed for exposure to nano-Ag due to silver ion release or the
presence of nanoparticles? Can this be distinguished?
8.
Test Methods—
Mammals/ Humans
67 (11)
At a minimum, what assays could be considered in a harmonized test guideline for
determination of the human health effects of nano-Ag?
What standardized test methods or characterization protocols are necessary to
ensure that research results generated in multiple laboratories are consistent,
reproducible, and reliable?
Are the current tests for regulatory acceptance relevant to nano-Ag?
Can nano-Ag have impacts on the F-1 (next) generation via changes in gene
expression patterns?
9.
Ecotoxicity Test
Methods
59 (10)
At a minimum, what assays could be considered in a harmonized test guideline for
determination of the ecological effects of nano-Ag?
What standardized test methods or characterization protocols are necessary to
ensure that research results generated in multiple laboratories are consistent,
reproducible, and reliable?
Are the current tests for regulatory acceptance relevant to nano-Ag?
Can nano-Ag have impacts on the F-1 (next) generation via changes in gene
expression patterns?
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Research Theme
Specific Question
Points (Votes)
10. Is New Nano Unique?
59(10)
Does nano-Ag form the same strong complexes with anions as conventional silver,
and if so, is it also effectively mobilized in aquatic environments?
What are the phys-chem properties of currently available and historic silver
products?
Do nano-Ag products actually offer more efficacy than products currently on the
market?
Do the properties of nano-Ag that differ from those of well-characterized colloidal
silver, if any, cause them to behave differently in aquatic, terrestrial, and
atmospheric environmental compartments?
a. If they do differ, how do they differ?
b. Can information about how colloidal silver behaves in these environments be
used to understand how nano-Ag behaves?
11. Biological Effects
56 (10)
What are the most sensitive ecological endpoints to nano-Ag exposure?
What are relevant susceptibility factors (for biological response)?
What are the short-term and long-term biological responses observed at current
nano-Ag occupational exposure levels as well as consumer exposure levels?
Many effects of emerging substances are not known until many years after their
introduction and use in commerce. What are the chronic and subchronic effects of
nano-Ag, and how can we accelerate our understanding of them? Can nano-Ag
have impact on F-1 (next) generation via changes in gene expression patterns?
12. Ecological Effects
Required for Risk
Assessment
43(9)
What are the most sensitive ecological endpoints to nano-Ag exposure? Are there
sufficient data/analytical techniques to determine how sensitive specific endpoints
and organisms are to nano-Ag exposure, including:
a. Benthic invertebrates;
b. Marine invertebrates; and
c. Freshwater invertebrates?
Is the available ecological effects evidence adequate to support ecological risk
assessment for nano-Ag? If no, what research is needed to make an assessment
possible?
12. Communication,
Engagement, and
Education
43(9)
How do we effectively communicate risk/benefit information for nano-Ag to the
general public?
How do we engage citizens and workers in discussions about how nano-Ag sprays
are being used?
How do we educate people about the risks, benefits, and safety related to nano
products?
We need an integrated holistic approach to nano risk assessment. How can we do
this?
14. Fate and Transport of
Nano-Ag
39(12)
What physicochemical properties of nano-Ag can be used to predict fate and
transport in environmental media?
How could existing models applicable to conventional silver be used to adequately
predict the transport and fate of nano-Ag through environmental compartments, or
how could they be modified to do so?
14. Adequacy of Current
Data
39(6)
Do current publications describing the health effects of nano-Ag particles and
laboratory-generated nano-Ag particles accurately depict the toxicity of
commercially available nano-Ag materials?
Are there any parallels between health effects of conventional silver and those in
emerging studies on nanosilver?
D-7
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Research Theme
Specific Question
Points (Votes)
16. Dissolution
36(9)
What information exists on the temporal changes in the release of ionic silver by
nanoparticles physicochemical and environmental characteristics?
What are the rates of dissolution of nano-Ag into the environment?
Does particle size of nano-Ag affect the rate of release of silver ions in
environmental compartments?
17. Information from
Manufacturers
35 (10)
Has the database and risk assessment methodology used by FDA during approval
of nano-Ag medical devices been integrated with EPA's database and risk
assessment processes?
What are realistic strategies for collecting data on production quantities and product
characteristics given that much of this information is proprietary?
17. Adapative Tolerance /
Resistance
35(8)
The majority of toxicity studies with conventional silver were conducted over a
decade ago. Are more studies needed that utilize state-of-the-art technology for
comparing its mode of toxicity to that of nano-Ag? In other words, can we
accurately say that nano-Ag and conventional silver have different modes of toxicity
if most of the studies available for conventional silver were not conducted using
current methods?
Is the nano-Ag harmful to the beneficial organisms in wastewater treatment?
19. Metrics
33(7)
How should dose and exposure be characterized for human exposures?
For the purpose of assessing potential risk, what metrics are most informative for
quantifying exposure and dose of nano-Ag?
20. Kinetics II
22(5)
Does nano-Ag react with materials (i.e., organic matter, other metals, polymers) and
alter properties such as REDOX potential or leached metal ion rates?
What changes occur to the physicochemical properties of nano-Ag throughout the
life-cycle stages, either as a function of process and product engineering or as a
function of incidental encounters with other substances and the environment?
Does the release of nano-Ag contribute to climate change?
21. Benefits
9(5)
Do nano-Ag products actually offer more efficacy than products on the market?
22. Incentivize Research
for CEA
8(1)
How can we incentivize researchers to focus in on the most critical questions and
best methods for CEA?
How urgent is the need for the benefits offered by the candidate
application/material?
23. CEA Framework
1(1)
How can CEA framework be improved to ensure passive or active
consumer/occupational exposure research is completed for nano-Ag and for other
nanomaterials?
Appendix D References
ICF (ICF International). (2011). Nanomaterial case study workshop: Developing a comprehensive
environmental assessment research strategy for nanoscale silver - Workshop report. Research Triangle
Park, NC: U.S. Environmental Protection Agency.
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