ELECTROCHEMICAL SENSORS FOR
ENVIRONMENTAL MONITORING:
A REVIEW OF RECENT TECHNOLOGY
by
JOSEPH WANG
Department of Chemistry and Biochemistry
New Mexico State University
Las Cruces, New Mexico 88003
Solicitation No.
LV-94-012
Project Officer
Kim Rogers
EMSL-U.S. EPA
P.O. Box 93478
Las Vegas, NV 89139-3478
National Exposure Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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Notice
The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD),
funded and managed in the compilation of this research review. It has been subjected to the Agency's peer review
and has been approved as an EPA publication. The U.S. Government has the right to retain a non-exclusive,
royalty-free license in and to any copyright covering this article.
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Abstract
Electrochemical sensors are expected to play an increasing role in environmental monitoring. Significant
technological advances during the 1980's and early 19 90's are certain to facilitate the environmental applications
of electrochemical devices. This report surveys important advances in electrochemical sensor technology,
including amperometric or potentiometric biosensors, chemically modified electrodes, stripping-based metal
sensors, and other tools for on-site field testing. Such devices should allow one to move the measurement of
numerous inorganic and organic pollutants from the central laboratory to the field, and to perform them rapidly,
inexpensively, and reliably. Representative environmental applications and future prospects are discussed.
in
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Contents
Notice ii
Abstract iii
Introduction 1
Principles 2
Electrochemical Biosensors 3
Chemically Modified Electrodes for Environmental Monitoring 6
Stripping-based Metal Sensors 8
Ion and Gas Selective Electrodes 11
Conclusions 12
References 13
Tables
Table 1. Typical Environmental Applications of Stripping Analysis 9
Table 2. Examples of Electrochemical Sensors and Biosensors for Environmental Analysis 12
Figures
1. Electrochemical biosensors: biorecognition and signal transduction 3
2. Enzyme (tyrosinase) electrode for monitoring phenolic compounds 4
3. Amperometric immunosensor based electroactive-(A) and enzyme (B) tagged antigen 5
4. Electrocatalysis at modified electrodes; electron transfer mediated reaction between
the target analyte and surface-bound catalyst 7
5. Steps in anodic (A) and adsorptive (B) stripping voltammetry of trace metals, based
on electrolytic and adsorptive accumulation, respectively, of target metal analytes 10
IV
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Introduction
Electroanalytical chemistry can play a very
important role in the protection of our environment. In
particular, electrochemical sensors and detectors are very
attractive for on-site monitoring of priority pollutants, as
well as for addressing other environmental needs. Such
devices satisfy many of the requirements for on-site
environmental analysis. They are inherently sensitive
and selective towards electroactive species, fast and
accurate, compact, portable and inexpensive. Such
capabilities have already made a significant impact on
decentralized clinical analysis. Yet, despite their great
potential for environmental monitoring, broad
applications of electrochemical sensors for pollution
control are still in their infancy.
Several electrochemical devices, such as pH- or
oxygen electrodes, have been used routinely for years in
environmental analysis. Recent advances in
electrochemical sensor technology will certainly expand
the scope of these devices towards a wide range of
organic and inorganic contaminants and will facilitate
their role in field analysis. These advances include the
introduction of modified- or ultramicroelectrodes, the
design of highly selective chemical or biological
recognition layers, of molecular devices or sensor arrays,
and developments in the areas of microfabrication,
computerized instrumentation and flow detectors.
The EPA's Office of Research and Development is
currently pursuing the development of environmental
monitoring technologies which can expedite the
characterization of hazardous waste sites in the U.S.
Relevant to this objective, is the review and evaluation
of currently reported field analytical technologies. The
objective of this report is to describe the principles, major
requirements, prospects, limitations, and recent
applications of electrochemical sensors for monitoring
ground or surface waters. It is not a comprehensive
review of these topics, but rather focuses on the most
important advances and recently reported devices which
hold great promise for on-site water analysis.
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Principles
The purpose of a chemical sensor is to provide
real-time reliable information about the chemical
composition of its surrounding environment. Ideally,
such a device is capable of responding continuously and
reversibly and does not perturb the sample. Such devices
consist of a transduction element covered with a
biological or chemical recognition layer. In the case of
electrochemical sensors, the analytical information is
obtained from the electrical signal that results from the
interaction of the target analy te and the recognition layer.
Different electrochemical devices can be used for the task
of environmental monitoring (depending on the nature of
the analyte, the character of the sample matrix, and
sensitivity or selectivity requirements). Most of these
devices fall into two major categories (in accordance to
the nature of the electrical signal): amperometric and
potentiometric.
Amperometric sensors are based on the detection of
electroactive species involved in the chemical or
biological recognition process. The signal transduction
process is accomplished by controlling the potential of
the working electrode at a fixed value (relative to a
reference electrode) and monitoring the current as a
function of time. The applied potential serves as the
driving force for the electron transfer reaction of the
electroactive species. The resulting current is a direct
measure of the rate of the electron transfer reaction. It is
thus reflecting the rate of the recognition event, and is
proportional to the concentration of the target analyte.
In potentiometric sensors, the analytical information
is obtained by converting the recognition process into a
potential signal, which is proportional (in a logarithmic
fashion) to the concentration (activity) of species
generated or consumed in the recognition event. Such
devices rely on the use of ion selective electrodes for
obtaining the potential signal. A permselective
ion-conductive membrane (placed at the tip of the
electrode) is designed to yield a potential signal that is
primarily due to the target ion. Such response is
measured under conditions of essentially zero current.
Potentiometric sensors are very attractive for field
operations because of their high selectivity, simplicity
and low cost. They are, however, less sensitive and often
slower than their amperometric counterparts. In the past,
potentiometric devices have been more widely used, but
the increasing amount of research on amperometric
probes should gradually shift this balance. Detailed
theoretical discussion on amperometric and
potentiometric measurements are available in many
textbooks and reference works.1"5
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Electrochemical Biosensors
The remarkable specificity of biological recognition
processes has led to the development of highly selective
biosensing devices. Electrochemical biosensors hold a
leading position among the bioprobes currently available
and hold great promise for the task of environmental
monitoring. Such devices consist of two components: a
biological entity that recognizes the target analyte and
the electrode transducer that translates the biorecognition
event into a useful electrical signal. A general schematic
diagram for the operation of electrochemical biosensors
is shown in Figure 1. A great variety of schemes for
implementing the electrochemical biosensing approach,
based on different combinations of biocomponents and
electrode transducers have been suggested. These rely on
the immobilization of enzymes, antibodies, receptors or
whole cells onto amperometric or potentiometric
electrodes. Fundamental aspects of these devices have
been reviewed in the literature.6"8
Enzyme electrodes have the longest tradition in the
field of biosensors. Such devices are usually prepared by
attaching an enzyme layer to the electrode surface, which
monitors changes occurring as a result of the biocatalytic
reaction amperometrically or potentiometrically. Am-
perometric enzyme electrodes rely on the biocatalytic
generation or consumption of electroactive species. A
large number of hydrogen-peroxide generating oxidases
and NAD+-dependent dehydrogenases have been
particularly useful for the measurement of a wide range
of substrates The liberated peroxide or NADH species
can be readily detected at relatively modest potentials
(0.5-0.8V vs. Ag/AgCl), depending upon the working
electrode material. Lowering of these detection
potentials is desired for minimizing interferences from
coexisting electroactive species. Potentiometric enzyme
electrodes rely on the use of ion- or gas-selective
electrode transducers, and thus allow the determination
of substrates whose biocatalytic reaction results in local
pH changes or the formation or consumption of ions or
gas (e.g. NH4+ or CO2). The resulting potential signal
thus depends on the logarithm of the substrate
concentration. Proper functioning of enzyme electrodes
is greatly dependent on the immobilization procedure.
The design of enzyme electrodes is such that the
current or potential measured is proportional to the rate
limiting step in the overall reaction. For reactions
limited by the Michaelis-Menten kinetics, a leveling off
of calibration curves is expected at high substrate
concentrations. Mass-transport limiting membranes can
be used to greatly extend the linear range. This will also
lead to a slower response. The signal may be dependent
also upon the pH of the water sample or its heavy-metal
content that affect the enzymatic activity. Attention
should be given also to the long-term stability of these
devices, due to the limited thermostability of the
biocatalytic layer. Improved immobilization and use of
thermophilic or 'synthetic' enzymes should be useful for
TARGET
INALYTE
AMPEROMETRIC
POTENTIOMETRIC SIGNAL
CONDUCTOMETRIC
ELECTROCHEMICAL SENSOR
Figure 1. Electrochemical biosensors: biorecognition and signal transduction.
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extending the lifetime of enzyme electrodes (particularly
in connection with field applications). Mass producible,
disposable enzyme electrodes can be readily fabricated
(as common for clinical serf-testing of blood glucose),
and used as 'one-shot' throwaway devices.
Several enzyme electrodes have already proven
useful for the task of environmental monitoring. For
example, several groups reported on highly sensitive
amperometric biosensors for phenolic compounds.9"15
Such devices rely on the immobilization of tyrosinase
(polyphenol oxidase) onto carbon- or platinum
transducers, and the low potential detection of the
liberated quinone product (Figure 2). Assays of industrial
wastes or natural water have been documented,12"14
including possible remote phenol sensing13 and
single-use on-site sensing.14'15 Similarly, low potential
biosensing of organic peroxides or hydrogen peroxides
can be accomplished at peroxidase-modified
electrodes.16'17 "Class-selective" enzyme electrodes,
based on tyrosinase or peroxidase, can be used for semi-
quantitative field screening. They can also be used as
detectors for liquid chromatography, hence providing
quantitation of the individual substrates.18 The or-
ganic-phase activity of these enzymes should be useful
not only for chromatographic separations, but also in
connection with rapid solvent extraction procedures.
Other enzymes, such as sulfite oxidase, nitrate reductase,
nitrilase, alcohol dehydrogenase, or formaldehyde
dehydrogenase have been employed for electrochemical
biosensing of environmentally-relevant species such as
sulfite,19 nitrate,20 organonitriles,21 alcohols,22 or
formaldehyde,23 respectively. Most of the above devices
offer low (micromolar) detection limit, good precision
(RSD = 1-3%) and fast (30-60 sec.) response.
In addition to substrate monitoring, it is possible to
employ enzyme electrodes for measuring various toxins
(via the perturbation/modulation of the enzyme activity).
For example, the inhibition of enzymes, such as
cholinesterase, tyrosinase, orperoxidase, has led to useful
biosensors for organophosphates and carbamates
pesticides,24 cyanide,25 or toxic metals.26'27 The resulting
(inhibition) plots thus reflect the enzyme inhibition
kinetics. Such enzyme inhibition devices may thus be
useful as early warning poison detectors. Improved
specificity may be achieved by designing multi-enzyme
arrays that offer a "fingerprint" pattern of the individual
inhibitors. Analogous detection of benzene or herbicide
contaminations and of anionic surfactants can be
accomplished by immobilizing whole cells onto
electrodes and monitoring the modulation in the
microbial activity.28'29'30 Another environmentally
important microbial sensor offers rapid estimate of BOD
(biochemical oxygen demand), hence replacing the long
(5 day) conventional BOD test.31 The use of whole cells
(instead of isolated enzymes) can increase the sensor
stability and allows regeneration of the bioactivity (via
immersion in a nutrient media). Other whole cell
electrodes, relying on plant tissues (such as mushroom or
horseradish) have been used for detecting phenolic and
peroxide substrates (of their tyrosinase and peroxidase
enzymes). While offering prolonged lifetimes, such
tissue electrodes may suffer from side reactions due to the
coexistence of several enzymes.
QUINONE
Figure 2. Enzyme (tyrosinase) electrode for monitoring phenolic compounds.
4
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Affinity electrochemical biosensors, employing
natural binding molecules as the recognition element
should also play a growing role in future environmental
monitoring. In this case the recognition process is
governed primarily by the shape and size of the receptor
pocket and the analyte of interest. Particularly promising
are electrochemical immunosensors due to the inherent
specificity of antibody-antigen reactions.32 Disposable
immunoprobes based on mediated electrochemistry have
been developed.33 In addition to immunosensors, the
environmental arena may benefit from the production of
electrochemical immunoassay test kits. Such assays
commonly rely on labelling the antigen with an elec-
troactive tag (Figure 3 A), or with an enzyme that acts on
a substrate and liberates an electroactive product (Figure
3B). A wide variety of enzymes are suitable (peroxidase,
alkaline phosphatase, etc.), and there is also a wide
choice of substrates for these enzymes. New test kits,
developed for the clinical market, may be readily adapted
for environmental monitoring. Other pro raising concepts
are based on specific binding between
membrane-embedded receptors and target analytes34 or
the hybridization of electroactive markers by surface-
bound DNA.35 Amperometric or potentiometric
transducers are useful to follow these binding events.
Genetic engineering technology is currently being
explored for designing binding molecules for target
analytes.
W.E.
Electroactive Labeled Antigens
W.E..
B
Aa
Enzyme Labeled Antigens
Figure 3. Amperometric immunosensor based electroactive-(A) and enzyme (B) tagged antigen.
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Chemically Modified Electrodes for Environmental Monitoring
Chemical layers can also be used for imparting a
high degree of selectivity to electrochemical transducers.
While conventional amperometric electrodes serve
mainly for carrying the electrical current, powerful
sensing devices can be designed by a deliberate
modification of their surfaces. Basically, the
modification of an electrode involves immobilization (on
its surface) of reagents that change the electrochemical
characteristics of the bare surface. Inclusion of reagents
within the electrode matrix (e.g. carbon paste) is another
attractive approach for modifying electrodes. Such
manipulation of the mole-cular composition of the
electrode thus allows one to tailor the response to meet
specific sensing needs. The new "mercury-free" surfaces
address also growing concerns associated with field
applications of the classical mercury drop electrode.
Theoretical details on modified electrodes can be found
in several reviews.36"38
While sensors based on modified electrodes are still
in the early stages of their lifetime, such preparation of
structured interfaces holds great promise for the task of
environmental monitoring. There are different directions
by which the resulting modified electrodes can benefit
environmental analysis, including acceleration of
electron-transfer reactions, preferential accumulation or
permselective transport.
Electrocatalysis involves an electron transfer
mediation between the target analyte and the surface by
an immobilized catalyst (Figure 4). Such catalytic action
results in faster electrode reactions at lower operating
potentials. Various catalytic surfaces have thus been
successfully employed for facilitating the detection of
environmentally-relevant analytes (with otherwise slow
electron-transfer kinetics). These include the electro-
catalytic determination of hydrazines39 ornitrosamines40
at electrodes coated with mixed-valent ruthenium films,
monitoring of aliphatic aldehydes at palladium-modified
carbon paste,41 sensing of nitrite at a glassy carbon
electrode coated with an osmium-based redox polymer,42
of nitrate at a copper modified screen printed carbon
electrode,43 monitoring of organic peroxides at co-
balt-phthalocyanine containing carbon pastes,44 and of
hydrogen peroxide at a copper heptacyano-
nitrosylferrate-coated electrode.45
Preconcentrating modified electrodes can also be
useful for environmental sensing. In this case an
immobilized reagent (e.g. ligand, ion-exchanger) offers
preferential uptake of target analytes. This approach
enjoys high sensitivity because it is a preconcentration
procedure. A second major advantage lies in the added
dimension of selectivity, which is provided by the
chemical requirement of the modifier-analyte
interactions. Such improvements have been documented
for the measurement of nickel, mercury, or aluminum
ions at dimethylglyoxine,46 crown-ether,47 or alizarin48
containing carbon pastes, respectively, monitoring of
nitrite, chromium, or uranyl ions at ion-exchanger
modified electrodes,49"51 and of copper at an
algaemodified electrode.52 Covalent reactions can be
used for analogous collection/determination of organic
analytes, e.g. monitoring of aromatic aldehydes at
amine-containing carbon pastes.53 Routine
environmental applications of these preconcentrating
electrodes would require attention to competition for the
surface site and the regeneration of an 'analyte-free'
surface.
Another promising avenue is to cover the sensing
surface with an appropriate permselective film.
Discriminative coatings based on different transport
mechanisms (based on analyte size, charge, or polarity)
can thus be used for addressing the limited selectivity of
controlled-potential probes in complex environmental
matrices. The size-exclusion sieving properties of
various polymer-coated electrodes offer highly selective
detection of small hydrogen peroxide or hydrazine
molecules.54'55 In addition, surface passivation (due to
adsorption of macromolecules present in natural waters)
can be prevented via the protective action of these films.
More powerful sensing devices may result from the
coupling of several functions (permselectivity,
preconcentration or catalysis) onto the same surface.
Additional advantages can be achieved by designing
arrays of independent modified electrodes, each coated
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with a different modifier and hence tuned toward a
particular group of analytes. The resulting array
response offers a unique fingerprint pattern of the
individual analytes, as well as multicomponent analysis
(in connection with statistical, pattern-recognition
procedures). Use of different permselective coatings or
catalytic surfaces thus hold great promise for
multiparameter pollution monitoring. The development
of electrochemical sensor arrays has been reviewed
recently.56 Related to this are new molecular devices
based on the coverage of interdigitated microarrays with
conducting polymers.57'58 Eventually we expect to see
molecular devices in which the individual components
are formed by discrete molecules. Modification of
miniaturized screen-printed sensor strips can also be
accomplished via the inclusion of the desired reagent
(e.g. ligand, catalyst) in the ink used for the micro-
fabrication process.
Aox
Ared
Mred
Mox
Figure 4. Electrocatalysis at modified electrodes; electron transfer mediated reaction between the target analyte and
surface-bound catalyst.
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Stripping-based Metal Sensors
The most sensitive electroanalytical technique,
stripping analysis, is highly suitable for the task of field
monitoring of toxic metals. The remarkable sensitivity
of stripping analysis is attributed to its preconcentration
step, in which trace metals are accumulated onto the
working electrode. This step is followed by the stripping
(measurement) step, in which the metals are "stripped"
away from the electrode during an appropriate potential
scan. About 30 metals can thus be determined by using
electrolytic (reductive) deposition or adsorptive
accumulation of a suitable complex onto the electrode
surface (Figure 5). Stripping electrodes thus represent a
unique type of chemical sensors, where the recognition
(accumulation) and transduction (stripping) processes are
temporally resolved. Short accumulation times (of 3-5
min) are thus sufficient for convenient quantitation down
to the sub-ppb level, with shorter periods (1-2 min)
allowing measurements of ppb and sub-ppb
concentrations. The timeconsuming deaeration step has
been eliminated by using modern stripping modes (e.g.
potentiometric or square-wave stripping), that are not
prone to oxygen interferences. Stripping analysis can
provide useful information on the total metal content, as
well as characterization of its chemical form (e.g.
oxidation state, labile fraction, etc.).59 Field
measurements of chromium(VI) represent one such
example.60'61 Overlapping peaks, formation of
intermetallic compounds and surfactant adsorption
represent the most common problems in stripping
analysis.
Various advances in stripping analysis should
accelerate the realization of on-site environmental testing
of toxic metals. New sensor technology has thus replaced
the traditional laboratory-based mercury electrodes and
associated cumbersome operation (oxygen removal,
solution stirring, cell cleaning, etc.). Of particular
significance are new stripping-based tools, such as
automated flow systems for continuous on-line
monitoring,62"64 disposable screen-printed stripping
electrodes for single-use field applications,65 or
remote/submersible devices for down-hole well
monitoring or unattended operation.66'67 Portable and
compact (hand-held), battery-operated stripping
analyzers are currently being commercialized for
controlling these field-deployable devices. In addition to
providing on-site realtime information, such in-situ
devices should minimize errors (due to contamination or
loss) inherent to trace metal measurement in a
centralized laboratory. Stripping analysis has been
extensively used by marine chemists on board ships for
numerous oceanographic surveys.62 Relevant examples
of environmental applications of stripping analysis are
given in Table 1.
In addition to trace metal pollutants, it is possible to
employ new adsorptive stripping procedures for
measuring low levels of organic contaminants that
display surface-active properties (e.g. detergents, oil
components). However, due to competitive adsorption
such schemes usually require a prior separation step.
Another version of stripping analysis, cathodic stripping
voltammetry, can be used for measuring
environmentally-relevant anions (e.g. S"2, I", Br") or
sulfur- or chlorine-containing pollutants (e.g. pesticides)
following their oxidative deposition onto the working
electrode. Additional background information on
stripping analysis and its environmental opportunities
can be found in various books or reviews.77"80
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e
The Accumulation Step
M+n + ne" -»M
Time
e
o
CL
~acc
Accumulation
ML
MLn -» MLn,ads
The Stripping Step
M -^ fvfn + ne"
Stripping
B
Time
Figure 5. Steps in anodic (A) and adsorptive (B) stripping voltammetry of trace metals, based on electrolytic and adsorptive
accumulation, respectively, of target metal analytes.
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Table 1. Typical Environmental Applications of Stripping Analysis
Trace Metal
As
Cd
Cr
Cu
Hg
Mn
Ni
Pb
Se
Tl
U
Matrix
Natural waters
Lakes and Oceans
Seawater
Sediments
Tap water
Seawater
Natural waters
Seawater
Lakes and Oceans,
Sediments
River water
Natural waters
Groundwater Sediments
Electrode
Gold
Mercury film
Mercury drop
Mercury film
Gold
Mercury drop
Mercury drop
Mercury film
Mercury film
Gold
Mercury film
Mercury drop
Stripping Mode
Differential pulse
Differential pulse
Adsorptive
Potentiometric
Differential pulse
Potentiometric
Adsorptive
Differential pulse
Potentiometric
Potentiometric
Differential pulse
Adsorptive
Ref.
68
65,69
61
60
70
71
72
73
65,69
60
74
75
76
10
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Ion and Gas Selective Electrodes
Ion selective electrodes offer direct and selective
detection of ionic activities in water samples. Such
potentiometric devices are simple, rapid, inexpensive and
compatible with on-line analysis. The inherent
selectivity of these devices is attributed to highly selective
interactions between the membrane material and the
target ion. Depending on the nature of the membrane
material used to impart the desired selectivity, ion
selective electrodes can be divided into three groups:
glass, solid, or liquid electrodes. Many ion selective
electrodes are commercially available and routinely used
in various fields.
By far the most widely used ion selective electrode is
the pH electrode. This glass-membrane sensor has been
used for environmental pH measurements for several
decades. Its remarkable success is attributed to its
outstanding analytical performance, and in particular to
its extremely high selectivity for hydrogen ions, broad
dynamic range, and fast and stable response. Various
solid-state crystalline membrane electrodes have been
shown useful for monitoring environmentally-important
ions, such as F, Br, OST, S"2 or Cu.+281 The calcium and
nitrate ion-exchanger sensors represent environmentally
useful liquid membrane electrodes. The synthetic design
of macrocyclic polyether ionophores has led to liquid
membrane electrodes for heavy metals, such as lead or
zinc.82 Anion selective liquid membrane electrodes have
been developed in recent years for sensing of phosphate
or thiocyanate. New technologies of thin film
(dry-reagent) slides or semiconductor chips will certainly
facilitate field monitoring of ionic analytes.83 The
principles and applications of ion selective electrodes
have been reviewed.84"86
The rapid detection of ammonia or oxygen plays a vital
role in pollution control. Gas sensing electrodes are
highly selective devices for monitoring these (and other)
gases. Such sensors commonly incorporate a
conventional ion selective electrode, surrounded by an
electrolyte solution and enclosed by a gas permeable
membrane. The target gas diffuses through the
membrane and reacts with the internal electrolyte, thus
forming or consuming a detectable ionic species. The
ammonia selective probe uses an internal pH glass
electrode in connection with an ammonium chloride
electrolyte. The glass electrode detects the decreased
activity of protons. While most gas sensors rely on
potentiometric detection, the important oxygen probe is
based on covering an amperometric platinum cathode
with a Teflon or silicon rubber membrane. Handheld
and remote oxygen probes are available commercially.87
Potentiometric sensors for other gases (SO2, NO2, HF,
etc.) have been designed by using different membranes
and equilibrium processes.
11
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Conclusions
Electrochemical sensor technology is still limited in
scope, and hence cannot solve all environmental
monitoring needs. Yet, a vast array of electrochemical
sensors have been applied in recent years for monitoring
a wide range of inorganic and organic pollutants (Table
2). We are continuously witnessing the introduction of
new electrochemical sensing devices, based on a wide
range of chemical or biological recognition materials. In
addition, mass production techniques (adapted from the
microelectronic industry) enable the fabrication of
extremely small and reproducible, and yet inexpensive
(disposable), sensing devices. Such devices are being
coupled with light and user-friendly
microprocessor-based instrumentation.
Fast-responding electrochemical sensors are also being
adapted for detection in on-line monitoring or
flow-injection systems (as needed for continuous
monitoring or field screening applications). Other
advances of selective and stable recognition elements,
"smart" sensors and molecular devices, remote
electrodes, multiparameter sensor arrays or
micromachining and nanotechnology, are certain to have
a major impact on pollution control. Additional efforts
should be given to the development of new
immobilization procedures (that increase the stability of
the biocomponent), to the design of new electrocatalysts
(that facilitate the detection of additional priority
pollutants), to the replacement of classical mercury
electrodes with well-defined solid surfaces, to address the
fouling and degradation of electrochemical sensors
during use, to the development of immunoassay-based
electrochemical sensors and of remote electrodes for
unattended operations, and introduction of multi-sensor
systems for simultaneous monitoring of several priority
contaminants. On-going commercialization efforts,
coupled with regulatory acceptance, should lead to the
translation of these and future research efforts into large
scale environmental applications.
Table 2. Examples of Electrochemical Sensors and Biosensors for Environmental Analysis
Analyte
Benzene
Cyanide
Hydrazines
Lead
Mercury
Nickel
Nitrite
Nitrosamines
Peroxides
Pesticides
Phenol
Sulfite
Uranium
Recognition
Modulated microbial activity
Enzyme inhibition
Electrocata lysis
Ion recognition
Preconcentration
Preconcentration
Preconcentration
Electrocatalysis
Biocata lysis
Enzyme inhibition
Biocata lysis
Biocatalysis
Preconcentration
Recognition
Process
Whole cell
Tyrosinase
Ruthenium catalyst
Macrocyclic ionophore
Crown ether
Dimethylglyoxine
Aliquat 336 ion exchanger
Ruthenium catalyst
Peroxidase
Acetylcholinesterase
choline oxidase
Tyrosinase
Sulfite oxidase
Nafion
Transduction
Element
Amperometry
Amperometry
Amperometry
Potentiometry
Voltammetry
Voltammetry
Voltammetry
Amperometry
Amperometry
Amperometry
Amperometry
Amperometry
Voltammetry
Ref.
mode
28
25
39
82
47
46
49
40
16,17
16,17
9-15
19
51
12
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