Handbook Of Pollution Control And Waste Minimization - Chapter 12 doc

12
Monitoring In-Situ Electrochemical Sensors
Joseph Wang
New Mexico State University, Las Cruces, New Mexico
1 INTRODUCTION
Electroanalytical methods are concerned with the interplay between electricity
and chemistry, namely, the measurements of electrical quantities such as current
or potential, and their relationship to chemical parameters. Electroanalytical
chemistry can play a major role in pollution control and prevention. In particular,
electrochemical sensors and detectors are very attractive for on-site and in-situ
monitoring of priority pollutants. Such devices are highly sensitive, selective
toward electroactive species, fast, accurate, compact, portable, and inexpensive.
Several electrochemical devices, such as oxygen or pH electrodes, have been
widely used for years for environmental analysis. Recent advances in electro-
chemical sensor technology have expanded the scope of electrochemical devices
toward a wide range of organic and inorganic contaminants.
The present chapter reviews recent efforts at the author’s laboratory, aimed
at in-situ monitoring of priority pollutants. Continuous monitoring, effected in the
natural environment, offers a rapid return of the chemical information (with a
proper alarm in case of a sudden discharge), avoids costs and errors associated
with the collection of discrete samples, while maintaining the sample integrity.
The use of remote sensors thus has significant technical and cost benefits over
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
traditional sampling and analysis. Our latest developments of remote electro-
chemical probes will be covered in the following sections.
2 REMOTELY DEPLOYED ELECTROCHEMICAL SENSORS
Remotely deployable submersible sensors capable of monitoring contaminants in
both time and location are advantageous in a variety of applications. These range
from shipboard marine surveys, downhole monitoring of groundwater contami-
nation, to real-time analysis of industrial streams. The development of sub-
mersible electrochemical probes requires proper attention to various challenges,

including the effect of sample pH, ionic strength, dissolved oxygen, or natural
convection, specificity and sensitivity, surface fouling, in-situ calibration, and
miniaturization. By addressing these and other obstacles, we were able to develop
remote sensors for a wide range of inorganic and organic contaminants.
2.1 Remote Monitoring of Metal Contaminants
Metal pollution has received enormous attention due to its detrimental impact on
the environment. The need for continuous monitoring of trace metals in a variety
of matrices has led to the development of submersible sensors based on electro-
chemical stripping analysis (1,2). Stripping analysis has been established as a
powerful technique for determining toxic metals in environmental samples (3,4).
The remarkable sensitivity of stripping analysis is attributed to its unique “built-
in” preconcentration step, during which the target metals are electroplated onto
the surface. Both electrolytic and nonelectrolytic (adsorptive) accumulation
schemes have thus been employed to achieve sub-parts-per-billion detection
limits. The analytical current signal (i), obtained during the subsequent stripping
(potential scanning) step is proportional to the metal concentration (C) and
accumulation time (t
acc
):
i = KC t
acc
(1)
Remote metal monitoring has been realized by eliminating the needs for
mercury surfaces, oxygen removal, forced convection, or supporting electrolyte
(which previously prevented the direct immersion of stripping electrodes into
sample streams). This was accomplished through the development of nonmercury
electrodes, judicious coupling of potentiometric stripping operation, and the use
of advanced ultramicroelectrode technology (1). Compatibility with field opera-
tions was achieved by connecting the three-electrode housing [including a gold
fiber working electrode, in the polyvinyl chloride (PVC) tube], via environmen-

tally sealed three-pin connectors, to a 25-m-long shielded cable. Convenient and
simultaneous quantitation of several trace metal levels (e.g., Pb, Cu, Ag, Hg) has
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thus been realized in connection to measurement frequencies of 20–30/h (based
on deposition periods of 1–2 min).
The in-situ monitoring capability of the remote metal sensor was demon-
strated in studies of the distribution of labile copper in San Diego Bay (CA) (5).
For this purpose, the probe was floating on the side of a small U.S. Navy vessel.
The resulting map of copper distribution reflected the metal discharge and
circulation pattern in the bay. We are currently collaborating with Prof. Daniele’s
group in using the remote probes for assessing the distribution of metal contam-
inants in the canals and lagoon of Venice, Italy (6).
The extension of remote stripping electrodes to additional metals that
cannot be electroplated relies on the adaptation of adsorptive stripping protocol
for a submersible operation (7). Such procedures rely on the formation and
adsorptive accumulation of complexes of the target metals. Accordingly, remote
adsorptive stripping sensors require a new probe design based on an internal
solution chemistry. Such a renewable-reagent adsorptive stripping sensor relies
on the continuous delivery of the ligand, its complexation reaction with the metal
“collected” in a semipermeable microdialysis sampling tube, and transport of the
complex to the working electrode compartment. Such dialysis sampling also
offers extension of the linear range and protection against surface fouling (due to
its dilution and filtration actions).
The new flow-probe format was employed for monitoring trace metals such
as nickel, uranium, or chromium. As desired for effective in-situ monitoring, such
adsorptive stripping probes have the capability to detect rapidly fluctuations in
the analyte concentration continuously. Such ability is indicated from Figure 1,
which displays the response of a chromium probe (8) upon switching from the 5-
to 25 µg/l chromium solutions. Such behavior is attributed to the reversibility of
the accumulation/stripping cycle, with the stripping and subsequent 10-s “clean-

ing” steps completely removing the accumulated complex. In addition, the
FIGURE 1 Response of the remote chromium(VI) probe to alternate expo-
sures to (a) “low” (5 µg/l) and (b) “high” (25 µg/l) chromium(VI) levels.
Accumulation for 30 s at –0.9 V; square-wave voltammetric stripping scan.
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reagent flow continuously replenishes the solution, to “erase” an internal buildup
of chromium.
Other groups have also been involved in the development of remote metal
sensors. For example, Kounaves’s team reported on probes based on mercury-
plated iridium-based microelectrode arrays and square-wave voltammetric strip-
ping detection (9). A solid-state reference electrode that eliminates leakage of
electrolyte to the surrounding low-ionic-strength aquatic environment was em-
ployed. The device developed by Buffle’s group (4,10) has been coupled to a
thick agarose-gel antifouling membrane that facilitates measurements in complex
media. There is no doubt that these and similar developments of submersible
stripping sensors will have a major impact on the surveillance of our water
resources.
2.2 Remote Modified Electrodes and Biosensors
Chemically and biologically modified electrodes (CMEs) have greatly enhanced
the power of electrochemical detectors and devices (11). The ability to deliber-
ately control and manipulate surface properties can lead to a variety of attractive
effects. Electrochemical sensors based on modified electrodes combine the re-
markable sensitivity of amperometry with new chemistries and biochemistries.
Such manipulation of the molecular architecture of the detector surface offers new
levels of reactivity that greatly expand the scope of electrochemical devices, and
enhance the power of in-situ electrochemical probes.
2.3 Biosensors
Biosensors are small devices employing biochemical molecular recognition prop-
erties as the basis for a selective analysis. The major processes involved in any
biosensor system are analyte recognition, signal transduction, and readout. The

remarkable specificity of biological recognition processes has led to the develop-
ment of highly selective electrochemical biosensors. In particular, enzyme elec-
trodes, based on amperometric or potentiometric monitoring of changes occurring
as a result of the biocatalytic process, have the longest tradition in the field of
biosensors. Such devices are usually prepared by immobilizing an enzyme onto
the electrode surface. The integration of these devices with remotely deployed
probes should add new dimensions of specificity to in-situ electrochemical
monitoring of pollutants. In the adaptation of enzyme electrodes to a submersible
operation, one must consider the influence of actual field conditions (pH, salinity,
temperature) on the biocatalytic activity.
The first remotely deployed biosensor targeted phenolic contaminants
in connection to a submersible tyrosinase enzyme electrode (12). The enzyme,
immobilized within a stabilizing carbon paste matrix, converted its phenolic
substrates to easily reducible quinone products. The sensor responded rapidly
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
to micromolar levels of various phenol contaminants, with no carryover (mem-
ory) effects.
We also developed a remote biosensor for field monitoring of organo-
phosphate nerve agents (13). The device relied on the coupling the enzymatic
activity of organophosphate hydrolase (OPH) with the submersible amperometric
probe configuration. Low (micromolar) levels of paraxon or parathion have thus
been measured directly in untreated natural water matrices. The OPH enzyme
obviates the need for lengthy and irreversible enzyme inhibition protocols com-
mon to inhibition-based biosensors.
Finally, hydrogen peroxide and organic peroxides have been monitored at
large instrument–sample distances by incorporating a reagentless peroxidase
bioelectrode into the remote probe assembly (14). A low detection potential
(~0.0 V) accrued from the use (co-immobilization) of a ferrocene co-substrate
allowed convenient monitoring of micromolar peroxide concentrations in un-
treated samples.

2.4 Modified Electrodes
Chemical layers can also be used to enhance the performance of electrochemical
devices. The use of electrocatalytic surfaces can expand the scope of remote
electrodes to pollutants possessing slow electron-transfer kinetics. One example
of the adaptation of modified electrodes for a submersible operation is a remote
sensor for toxic hydrazine compounds, based an electropolymerized films of
3,4-dihydroxybenzadehyde (15). The low-potential detection accrued from this
catalytic action offers convenient measurements of micromolar hydrazine con-
centrations in untreated groundwater or river water samples.
We also developed a submersible probe based on a carbon-fiber working
electrode assembly, connected to a 50 ft-long shielded cable, for the continuous
monitoring of the 2,4,6-trinitortoluene (TNT) explosive in environmental matri-
ces (16). The facile reduction of the nitro moiety allowed convenient and
fast (1–2 s) square-wave voltammetric measurements of parts-per-million levels
of TNT.
3 SUBMERSIBLE ELECTROCHEMICAL ANALYZERS
The ability to perform metal–ligand complexation reactions on a cable platform,
in connection to adsorptive stripping measurements, has led to the development
of submersible electrochemical analyzers (17). As opposed to current in-situ
sensors (which lack sample preparatory steps, essential for optimal analytical
performance), the new on-cable automated microanalyzer will eventually incor-
porate all the steps of the analytical protocol into the submersible device. The new
“lab-on-cable” concept thus involves the combination of sampling, sample pre-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
treatment, separation of components, and detection step (along with self-calibration)
into a single sealed submersible package. The first generation of this submersible
microlaboratory integrates microdialysis sampling, with reservoirs for the re-
agent, waste, and calibration/standard solution, along with the micromump and
necessary fluidic network on a cable platform (Figure 2). The sample and reagent
are thus brought together, mixed, and allowed to react in a reproducible manner.

Future generations will accommodate additional functions (e.g., preconcentration,
filtration, extraction) for addressing the needs of complex environmental samples.
Micromachining technology is being explored for further miniaturization and for
facilitating these in-situ sample manipulations. Proper attention is also being
given to the design of compact, low-powered, automated instrumentation for
unattended operation, “smart” data processing, and signal transmission (via
satellite links). Such a standalone “microlaboratory” can be submersed directly
in the environmental sample, to provide real-time continuous information on a
wide range of priority pollutants. The ability to perform in-situ all the necessary
FIGURE 2 Schematic diagram of the electrochemical “lab-on-cable” system:
(A) cable connection; (B) micropump; (C) reservoirs for reagent and waste
solutions; (D) microdialysis sampling tube and an electrochemical flow
detector.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
steps of the analytical protocol should have an enormous impact on pollution
control and prevention. Preliminary examples based on stripping monitoring of
trace metals or in-situ biosensing of phenols and various enzyme inhibitors are
presented below.
For example, we demonstrated the utility of the “lab-on-cable” probe for
circumventing in-situ problems common to electrochemical stripping analysis
(18). In particular, an internal delivery of an appropriate solution, containing a
ligand, third element, or a conducting salt, was used to minimize errors due to
overlapping peaks, intermetallic compounds, or ohmic distortions, respectively.
Similarly, internal delivery of a strong acid was used for on-cable release of the
metals from “collected” metal complexes, as desired for in-situ monitoring of
the total metal content (19).
We also developed a submersible phenol analyzer, based on an enzymatic
(tyrosinase) bioassay (17). The assay involved microdialysis sampling of the
phenolic compounds, their mixing with the internally delivered tyrosinase solu-
tion, and amperometric monitoring of the quinone product. The internal buffer

solution assured independence of sample conditions such as pH or ionic strength
[which commonly influence the performance of remote biosensors (12)]. Another
enzymatic assay was developed for the in-situ monitoring of cyanide (20). Such
an enzyme inhibition assay relied on the internal delivery of tyrosinase and its
catechol substrate using a flow-injection manifold. The flow probe thus addressed
the challenges to in-situ enzyme-inhibition devices (e.g., the replacement of the
inhibited enzyme and of the consumed substrate).
We envision the integration of multiple techniques and assays onto a single
cable platform, i.e., a complete submersible laboratory. Eventually, we expect to
eliminate the cable platform, and to use microlaboratories on miniaturized boats
or submarines, which would travel across the water stream and provide the
desired spatial and temporal information on target contaminants.
4 CONCLUSIONS AND FUTURE PROSPECTS
Electrochemical sensor technology is still limited in scope and cannot address all
environmental monitoring needs, yet a vast array of electrochemical devices has
been developed in recent years for in-situ monitoring numerous organic and
inorganic pollutants. By providing a fast return of the analytical information in a
timely, safe, and cost-effective fashion, the new, remotely deployed probes would
offer direct and reliable assessment of the fate and gradient of contaminants sites,
while greatly reducing the huge analytical costs. While the concept of “lab-on-
cable” is still at infancy, such a strategy should revolutionize the way of
monitoring priority pollutants, and would have a major impact on field analytical
chemistry. Ongoing commercialization efforts, coupled with regulatory accep-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
tance, should lead to the translation of these research efforts into large-scale
environmental applications.
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy Environmental
Managenemt Science Program (grant DE-FG07-96ER62306) and by the DOE-
WERC program.

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Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.