Electrochemical sensors for environmental monitoring: design ...
Electrochemical sensors for environmental monitoring: design, development and applications Grady Hanrahan,*a Deepa G. Patila and Joseph Wang*b a
Department of Chemistry & Biochemistry, California State University at Los Angeles, Los Angeles, CA 90032, USA. E-mail: [email protected]; Fax: +1 323-343-6490 b Department of Chemistry & Biochemistry, New Mexico State University, Las Cruces, NM 88003, USA. E-mail: [email protected]; Fax: +1 505-646-6033 Received 16th March 2004, Accepted 18th May 2004 First published as an Advance Article on the web 28th June 2004
The advancement in miniaturization and microfabrication technology has led to the development of sensitive and selective electrochemical devices for field-based and in situ environmental monitoring. Electrochemical sensing devices have a major impact upon the monitoring of priority pollutants by allowing the instrument to be taken to the sample (rather than the traditional way of bringing the sample to the laboratory). Such devices can perform automated chemical analyses in complex matrices and provide rapid, reliable and inexpensive measurements of a variety of inorganic and organic pollutants. Although not exhaustive due to the vast amounts of new and exciting electrochemical research, this review addresses many important advances in electrochemical sensor design and development for environmental monitoring purposes. Critical design factors and development issues including analytical improvements (e.g. detection limits), microfabrication and remote communication are presented. In addition, modern environmental applications will be discussed and future perspectives considered.
1. Introduction
DOI: 10.1039/b403975k
Traditional environmental monitoring approaches are based upon discrete sampling methods followed by laboratory analysis. These approaches do not improve our understanding of the natural processes governing chemical species behavior, their transport and bioavailability, or the relationship between anthropogenic releases and their long-term impact on aquatic systems. The stability of natural water samples during longterm storage is questionable, as they are subject to various biological, chemical and physical affects.1–4 Furthermore, discrete sampling methods and analyses are expensive, time consuming and do not provide the high resolution data needed to truly study chemical species dynamics in aquatic systems. In view of the limitations of discrete sample collection and subsequent laboratory analysis, real-time, continuous analytical methods capable of detecting chemical species with high temporal and spatial resolution, are desirable. Grady Hanrahan is an Assistant Professor of Chemistry at California State University, Los Angeles. His interests include the design and deployment of miniature analytical sensors for in situ aquatic monitoring and the development of laboratorybased flow injection-capillary electrophoresis (FI-CE) techniques for the analysis of bimolecular interactions, microscale reactions, and metals in natural waters. Dr. Hanrahan is also involved in the use of chemometrics for experimental design/ optimization and data analysis.
Grady Hanrahan
Joseph Wang is a Regents Professor of Chemistry at New Mexico State University (NMSU). He holds a Manasse
Electrochemical sensors represent an important subclass of chemical sensors in which an electrode is used as the transduction element, and are highly qualified for meeting the size, cost, and power requirements of on-site environmental monitoring.5–7 Characteristics of electrochemical sensing systems include high sensitivity and selectivity, a wide linear range, minimal space and power requirements, and low-cost instrumentation. Table 1 gives specific applications and quantitative details of advances in selected electrochemical sensing systems in recent years. Such devices have found a vast range of important applications in the fields of clinical, industrial, environmental, and agricultural analyses. Electrochemical devices have been used for several decades for field monitoring of a variety of water quality parameters (e.g. conductivity, dissolved oxygen or pH). These have led to a wider range of environmental applications including the measurement of trace metals in natural waters,8,16–22 carcinogen monitoring (e.g. N-nitroso compounds or aromatic amines),23,24 the Chair at NMSU and serves also as the Chief Editor of Electroanalysis. The research interests of Dr. Wang include the development of microfluidic (‘‘Lab-on-Chip’’) devices, DNA and protein recognition and diagnostics, bioelectronic, nanomaterials-based sensors, counterterrorism-detection, nanobiotechnology, the development and characterization of new surfaces for electroanalysis, sensor/recognition coatings, electrochemical sensing devices for environmental, security, and clinical monitoring, microfabrication and miniaturization, remote sensing, the development of techniques for ultratrace measurements and the design of on-line flow detectors. Joseph Wang
This journal is ß The Royal Society of Chemistry 2004
J. Environ. Monit., 2004, 6, 657–664
657
15 A gas phase biosensor using formate dehydro genase as the formic acid selective enzyme
10
DPV at NEE allowed the determination (with no preconcentration) of trace amounts of FA+ Field-deployable instrument Reagentless transduction of molecular recognition at gold electrode surfaces Lower detection limits achieved by partially replacing primary ions at the inner membrane surface Remote monitoring of enzyme inhibitors
14
9 —
13
8 Submersible probe for freshwater, estuarine and marine systems
11 12
Reference Notes
development of biosensors for the detection of organic pollutants (e.g. pesticides, phenols) in ground water,25,26 and environmental protection and clean energy conversion.27,28 Increased security concerns have led to the development of remote/submersible electrochemical sensors for monitoring explosives29 and nerve-agents.30 By providing a fast return of the analytical information in a timely, safe and cost effective fashion, such devices offer direct and reliable monitoring (including assessment of the fate and gradient of the target analytes). This paper critically examines the role electrochemical sensors play in current environmental monitoring efforts. It must be noted, however, that other classes of sensors (e.g. optical) are unique in their own right and often compliment the field of electrochemical sensors.31,32 We have focused our attention on recent advances in electrochemical sensor technology in terms of microfabrication, analytical improvements and remote communication capabilities. Recent applications and future trends in electrochemical sensor technology will also be reviewed, including microfluidic integration and submersible devices for remote, continuous monitoring.
658
0.03 mg m23
2 6 1026 M
1028.5 M
0.5 mg L21 15 pM
0.02 mM
23 pM (Cd2+) 1.13 nM (Cu2+) 23 pM (Pb2+) 1 6 1027 mol L21
Detection limit (unit as reported)
Electroanalytical sensors are concerned with the interplay between electricity and chemistry, namely the measurements of electrical quantities, such as current, potential or charge and their relationship to chemical parameters. Most of the electrochemical devices used for environmental monitoring fall within three categories and ultimately depend upon the specific analyte, nature of the sample matrix and the sensitivity and selectivity requirements.33
Enzyme-inhibition biosensor with amperometric detection Screen printing to deposit carbon paste, Ag/AgCl paste and insulating ink on polyester sheets
Polymeric ion-selective membrane electrode
Anodic stripping voltammetry (ASV) Protein scaffold-based electrochemical biosensor
The use of a potential applied between a reference and a working electrode causing the oxidation or reduction of an electroactive species. The applied potential thus serves as the driving force for the electron-transfer reaction. The resulting current is a direct measure of the rate of the electron transfer reaction and proportional to the target analyte concentration. The most common example is the oxygen Clark electrode that has been routinely used for monitoring the level of oxygen in water column and sediment pore water. Potentiometry In potentiometric sensors (primarily ion-selective electrodes), the analytical information is obtained by converting an ionrecognition event into a potential signal. A local equilibrium is established across the recognition membrane, leading to a change in the membrane potential. The analytical information is obtained from the potential difference between the ionselective electrode and a reference electrode. Potentials are a function of species activity, not concentration. Typical examples are potentiometric devices for in situ monitoring of pH, pCO2 or pS22. Conductimetry Conceptually the simplest of the electroanalytical techniques but inherently non-specific. The concentration of the charge is obtained through measurement of solution resistance.
J. Environ. Monit., 2004, 6, 657–664
Formic acid
CN2
Ca2+
3. Design and fabrication criteria As(III), As(IV) Avidin
DPV with gold nanoelectrode ensembles (NEEs)
N-Nitrosocompounds (e.g. N-nitroso-Nmethylanilines) FA+PF62(Ferrocenylmethyl)trimethylammonium hexafluorophosphate
Agarose, Hg-plated, Ir-based microelectrode (mAMMIE) by square wave anodic stripping voltammetry (SWASV) Differential pulse voltammetry (DPV) Cd2+, Cu2+, Pb2+
Sensing system
Amperometry and voltammetry
Measured species
Table 1
Applications and quantitative details of advances in selected electrochemical sensing systems in recent years
2. Principles
Table 2 lists a number of critical design criteria that should be considered when designing and developing robust electrochemical sensors for environmental monitoring. These are especially true for the development of submersible sensors where microfabrication, portability, analytical response, sensitivity,
Table 2 Criteria for the design and development of electrochemical sensors for environmental monitoring Macro vs. miniaturized fabrication design Overall cost, simplicity/complexity of design Robustness, reliability Sensitivity and selectivity Reversibility and stability Speed Artifact minimization Speciation capabilities Automation, data acquisition Single vs. multicompound analysis capabilities Low power consumption
descriptions of this technology and relevant environmental applications can be found in recent studies.8,22 This innovative, micromachined technology has been an important improvement from traditional approaches, especially in terms of field use and overall portability. It also provides significant advantages in terms of modern environmental efforts (i.e., metal speciation measurements and temporal resolution of the data). Moreover, micromachined devices allow for the use of smaller sample quantities (mL) as compared to mL quantities used in traditional electrochemical cells as well as lower overall power usage.
4. In situ applications selectivity, biofouling, reversibility and power consumption issues are of major concern. The type of fabrication used is ultimately dependent upon such factors as monitoring necessities, techniques employed and cell configuration. Traditionally, bulky electrodes and ‘‘beaker-type’’ cells have been employed.5 The advent of microfabrication allows the replacement of traditional electrochemical cells and bulky electrodes with easy-to-use sensing devices and has led to significant advances in the development of miniaturized electrochemical sensors and sensor arrays, especially in regards to remote monitoring devices.34 Recent studies have used innovative techniques such as thick and thin film technology, silicon-based techniques and photolithography in designing electrochemical sensors for environmental monitoring. Silicon-based techniques, for example, have recently been used in the development of micromachined stripping electrochemical sensors for trace metal analysis in natural waters.20 Stripping analysis technology such as this has remarkable sensitivity, with detection limits of 1.5 6 1028 M nickel and 4.2 6 1028 M uranium obtained following 5 and 20 min adsorption times. Novel thin-film, gel integrated Ir microelectrode arrays for square-wave anodic stripping voltammetry (SWASV) have also been developed for the analysis of trace elements in fresh and sea waters.35 The addition of the agarose gel allows the diffusion of dissolved metal ions and small complexes to the electrode surface, whilst acting as a barrier to unwanted colloids and macromolecules (e.g. humic and fulvic acids), helping eliminate fouling problems experienced with traditional devices.17 A schematic of a voltammetric in situ probe employing this technology is shown in Fig. 1. More detailed
The development of in situ electrochemical devices requires proper attention to major issues including reversibility (carry over), long-term stability (surface fouling, drift), specificity (overlapping signals caused by co-existing compounds), and changes in natural conditions (such as oxygen or convection) that may affect the response of interest. For example, prolonged direct operations in organic-rich natural-water systems may be complicated by surface fouling by co-existing surface-active substances. High levels of organic surfactants can lead to suppressions and distortions of the analyte response during extended monitoring operations. Coating the transducer with a permselective film has been shown to be useful for imparting the necessary stability by exclusion from the surface of unwanted macromolecules leading to its passivation.36 Such coating can also greatly enhance the selectivity by rejecting interferences from naturally-occurring electroactive substances and hence minimizing overlapping signals. The use of ultramicroelectrodes (with diameter smaller than 20 mm) has been employed for minimizing errors associated with fluctuations in the natural convection. Such relative independence of microelectrode sensors from convective flow reflects the larger natural convection boundary layer compared to the Nernst layer. In addition, the decreased ohmic distortions at ultramicroelectrodes allow direct electrochemical measurements to be made in aquatic systems (e.g. inland water) of low ionic strength. This also obviates the need for supporting electrolyte therefore minimizing possible impurities. For example, Brendel and Luther demonstrated the utility of a voltammetric microelectrode for obtaining depth profiles of dissolved iron, manganese, oxygen and S22 in marine environments.37 Stripping-based electrochemical sensors and applications
Fig. 1 Voltammetric in situ probe incorporating gel integrated Ir microelectrode arrays (based on ref. 8 with permission).
Growing concerns about heavy metal contamination has led to increasing needs to monitor trace metals in a variety of environmental matrices. Stripping analysis has been established as a powerful technique for determining toxic metals in environmental samples.17,38 Its remarkable sensitivity is attributed to the ‘built-in’ preconcentration step, during which the target metals are accumulated onto the working electrode. Whilst electrolytic deposition has been traditionally applied for trace heavy metals, non-electrochemical preconcentration schemes (based on adsorptive accumulation of metal complexes) have been developed for over two dozen environmentallyrelevant trace metals that cannot be readily deposited (e.g. Cr, Al, U, Fe, Ti, V, Mo).39 Stripping analysis is highly qualified to provide important information on metal speciation.40 The stripping protocol can be adjusted to measure the total or labile metal or for measuring different oxidation states (through control of the solution or deposition conditions). Remotelydeployable submersible sensors are capable of monitoring metal contaminants with respect to both time and location in shipboard marine surveys. Besides the continuous monitoring capability, such in situ measurements obviate errors common to laboratory-based trace metal analyses (due to metal contamination or loss). J. Environ. Monit., 2004, 6, 657–664
659
Fig. 2 Remote sensor for trace metals based on stripping potentiometry (based on ref. 19 with permission).
Submersible trace-metal analyses commonly rely on electrochemical stripping analysis.19,41 Such 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 judicious coupling of gold surfaces, potentiometric stripping operation, and ultramicroelectrode technology. The remote sensor assembly consisted of the gold-fiber, silver, and platinum working, reference, and counter electrodes, respectively (Fig. 2) and operated in the stripping potentiometric mode.19 Huang and Dasgupta11 described a compact and light lap-top controlled ASV analyzer for the measurement and speciation of trace arsenic in potable water. Such developments allow to move the measurements of trace metals to the field, and to perform them more rapidly, reliably and inexpensively. The recent development of ‘‘green’’ bismuth film electrodes, with a performance comparable to that of toxic mercury electrodes,42 should further facilitate the development of such in situ metal sensors. In situ voltammetric measurements of trace metals in coastal waters have been reviewed recently.43 Submersible modified electrodes 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 on electropolymerized films of 3,4-dihydroxybenzaldehyde.44 The low-potential detection accrued from this catalytic action offers convenient measurements of micromolar hydrazine concentrations in untreated groundwater or river water samples. A submersible carbon-fiber working electrode assembly has also been developed, connected to a 50 ft-long shielded cable, for the real-time monitoring of the 2,4,6-trinitortoluene (TNT) explosive in natural water.29 The facile reduction of the nitro moiety allowed convenient and rapid square-wave voltammetric measurements of sub-ppm levels of TNT (Fig. 3). Lower (ppb) concentrations have been detected using a background subtraction operation. Such high sensitivity is coupled to good selectivity and stability, and an absence of carry-over effects. The latter reflects the absence of recognition/binding events. The voltammetric sensor has been integrated onto an Unmanned Underwater Vehicle (UUV). This integration was a part of the US-Navy program aimed at demonstrating the effectiveness, in the field environment, of advanced technology sensors for detecting explosives in coastal regions of the ocean. 660
J. Environ. Monit., 2004, 6, 657–664
Fig. 3 Remote TNT voltammetric sensor response to seawater samples.
With such a sensor an UUV can be used to detect and prosecute plumes containing trace amounts of explosives that leach into the sea water from man-made objects, such as unexploded ordnance.
Electrochemical biosensors There have been a number of published reviews45–48 in the area of electrochemical biosensors each providing an overview of the most recent literature. Table 3 provides a selected compilation of biosensor performance data from published work relating to environmentally important analytes. More detailed examples and applications are discussed below. The remarkable specificity of biological recognition events has led to the development of highly selective electrochemical biosensors.54,55 In particular, enzyme electrodes, 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.5,56 The integration of these devices with the remotely-deployed probes should add new dimensions of specificity into in situ electrochemical monitoring of contaminants. In the adaptation of enzyme electrodes to such in situ operation one must consider the influence of actual field conditions (e.g. salinity, pH, temperature) upon the biocatalytic activity. The first remotely-deployed biosensor targeted phenolic pollutants in connection to a submersible tyrosinase enzyme electrode.26 The enzyme, immobilized within a carbon paste matrix, converted its phenolic substrates to easily reducible quinone products. The sensor responded rapidly to micromolar levels of various phenol pollutants, with no apparent memory effects, and high stability (e.g. RSD of 2.4% for 50 runs over a 10 hour period). A remote biosensor for field monitoring of organophosphate pesticides has also been developed.30 The device relies on the coupling of the biocatalytic 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 common to inhibition-based biosensors. Hydrogen peroxide and organic peroxides have been monitored at large instrument–sample distances by incorporating a reagentless peroxidase bioelectrode into the remote probe assembly.26 A low detection potential (y0.0 V) accrued from the use (coimmobilization) of a ferrocene cosubstrate, allowed
NO32 biosensor containing immobilized denitrifying bacteria with a built in electrochemical transducer for N2O
53
30 ng L ppb detectable range
Chemically modified SPEs Amperometric detection using an enzyme/gas diffusion electrode Biosensor consisting of a N2O microelectrode
1 mM
21
51 52
25 nM Potentiometric assay of b-galactosidase activity
50
49
Intercalative or electrostatic collection of aromatic amines onto an immobilized dsDNA or ssDNA layer followed by chronopotentiometric analysis Electrochemical biosensing for online and in situ monitoring of gene expression in response to Cd2+ Coupled biosensing device with flow injection analysis. Use of tyrosinase enzyme 0.05 mM 0.2 mM DNA coated screen printed electrodes (SPEs)
Potentiometric sensors for environmental monitoring Ion-selective electrodes are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H+ concentration in a solution. The remarkable performance of the pH ISE, coupled with the importance of pH measurements, make it extremely attractive for field monitoring. Other parameters that can be measured by ISE include fluoride, bromide, nitrate and cadmium and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. A compilation of linear ranges, interferences, selectivity coefficients and limits of detection for commercially available ISEs for common analytes are shown in Table 4. Commercially available ISEs provide a convenient way to effectively measure environmentally important parameters at a relatively low cost. Early ISEs have been limited to the micromolar range owing to leaking of the primary ion from the internal electrolyte solution. By choosing an internal electrolyte with low activity of the primary ion and preventing its leak it is possible to greatly lower the detection limits by up to 6 orders of magnitude down to the picomolar range.57,58 ISEs have thus been recently applied to trace metal measurements in relevant environmental matrices. For example, a neutral carrier ISE has been applied for trace (sub-nM) measurements of lead and lead speciation in various natural waters.59 Adjusting a lead-containing sample to various pH values allowed reliable measurements of the fractions of uncomplexed lead (with a good correlation to the theoretical expectations). Continuous environmental monitoring efforts have also been performed using state-of-the-art ISE technology. Automatic continuous monitoring of nitrate in rivers, for example, has recently been performed using a novel ion-selective electrode.60,61 This nitrate-ISE was constructed using immobilized ion exchangers in a rubbery membrane and achieved detections limits on the order of 0.007 mg L21. Pioneering studies have shown the effectiveness of ISEs in the in situ monitoring of freshwater and marine sediments.62,63 In the latter study, a micro-profiling module incorporating up to four oxygen microelectrodes and two pH microelectrodes was developed for insertion into the sediment to measure pore water concentrations at very high spatial resolution (y50 mm intervals). Recent studies have also used ISEs in the determination of anionic surfactants in river water and wastewater64 as well as seawater under shipboard conditions.65 In addition, sensor units housing multielement ISEs have been developed for the determination of environmentally relevant NO32, Cl2, Ca2+, K+ and Na+ ions.66
NO32
Formaldehyde 4-Chlorophenol
Automation and wireless communication advances
Cd2+
1,2-Diaminoanthraquinone2Anthramine
Reference Notes Detection limit (unit as reported) Sensing system Measured species
Table 3 Selected biosensor performance data in relation to environmentally important analytes
convenient monitoring of micromolar peroxide concentrations in untreated environmental samples. In addition to direct enzymatic substrate measurements it is possible to exploit enzyme inhibition processes for remote measurements of toxins. For example, continuous monitoring of cyanide can be accomplished using a renewable-reagent flow probe, involving internal delivery of the tyrosinase enzyme and its substrate, along with microdialysis sampling of the toxin and downstream amperometric measurements.14
Recent technological advances enable sensors to be fully automated and integrated with flow-based delivery systems and wireless communication technology (thus freeing them from being physically attached to a base station). Recent advances in flow injection (FI) technology coupled with electrochemical detection, for example, have led to the development of automated sensors for the continuous detection of a variety of compounds. Such applications include the development of a J. Environ. Monit., 2004, 6, 657–664
661
Table 4 Commercially available ion-selective electrodes and analytical figures of merit for common analytes Analyte
Type
Concentration range (M)
Lower limit (ppm)
26
1–10
0.02 0.02
NH3
Gas permeable membrane
NH4+
Liquid membrane
1021–1026
Ba2+
Liquid membrane
1021–1025
Br2
Solid state
1025–1026
0.4
Cd2+
Solid state
1021–1026
0.2
2+
25
10
27
0.02
Ca
Liquid membrane
10 –10
F2 I2
Solid state Solid state
1025–1027 1–1027
0.01 0.02
Pb2+
Solid state
1021–5 6 1026
1.0
NO32
Liquid membrane
1–5 6 1026
0.08
K+
Liquid membrane
1–1026
0.04
Ag+ Na+
Solid state Glass sensing membrane
1–1027 Saturated–1026
0.01 1 ppb
S22
Solid state
1–1027
micro flow biosensor for the determination of organophosphorus pesticides,67 a laccase-based FI biosensor for the detection of phenolic compounds68 and a FI electrochemical system for pentachlorophenol assays.69 In addition, technology such as Micro-Electro-Mechanical Systems (MEMS) have made it possible to construct functional ‘‘smart sensors’’ and to connect a multitude of sensors for distributed measurement applications.70 Aqueous sensor networks incorporating an array of sensor nodes have also been developed which can be randomly distributed throughout a lake or drinking water reservoir for long-term in situ monitoring.71 Ostensibly, each node of the sensor network acts as a data router and contains sensors to measure environmentally relevant parameters. Host nodes on land are physically connected to a personal computer and incorporate an rf transceiver for wireless communication with the sensors of interest (Fig. 4).
0.003
Interferences and selectivity ratios Hydrazine = 5 6 1022 Aliphatic amines = 0.1–0.5 Na+ = 2.0 6 1023 Mg2+ = 2.0 6 1024 K+ = 1.2 6 1021 Na+ = 4 6 1024 K+ = 9.0 6 1023 Ca2+ = 2.5 6 1022 I2, S22, and CN2 must be absent OH2 = 3 6 1025 Cl2 = 2.4 6 1023 Ag+, Hg2+, Cu2+ v 1027 M Lead and ferric ion interfere at high levels Mg2+ = 2.5 6 1024 Ba2+ = 3 6 1023 Pb2+ = 0.1 Zn2+ = 1.0 Na+ = 1.5 6 1024 OH2 = 1021 S22 w1027M CN2 = 1.0 S22, Ag+, Hg2+ should be absent Cd2+, Cu2+, Fe3+ interfere Cl2 = 1022 NO22 = 3 6 1022 Br2 = 5 6 1022 SO422 = 3.5 6 1023 F2 = 1026 ClO32 = 20 ClO42 = 16.2 Na+ = 2.6 6 1023 Ca2+ = 2.5 6 1023 Rb+ = 1.9 Mg2+ = 1.9 6 1023 Cs+ = 0.38 NH4+ = 0.30 S22 and Hg2+ must be absent Li+ = 2 6 1022 K+ = 1 6 1023 NH4+ = 3 6 1023 Ag1 should be absent Hg2+ and Ag+ must be absent
detection capabilities. A vast array of electrochemical devices has been developed in recent years for in situ monitoring of numerous inorganic and organic contaminants. There is, however, a realization that electrochemical sensor technology cannot address all environmental monitoring needs. Yet, we are continuously being introduced to electrochemical devices based on new technology with future efforts in the areas of ‘‘smart’’ sensors and molecular devices, multiparameter sensor arrays and remote electrodes. ‘‘Smart’’ devices, capable of ‘‘switching’’ between ‘‘screening/warning’’ and ‘‘detailedanalysis’’ modes of operation, are desired for various environmental scenarios. Electrochemical research is progressing in many unique directions. Ultimately, the push towards further miniaturization will bring together such fields as chemistry, engineering and physics. The integration of electrochemical technology into
5. Conclusions The combination of modern electrochemical techniques with breakthroughs in microelectronics and miniaturization allows the introduction of powerful and reliable electrical devices for effective process or pollution control. The consequence of these developments is that major considerations are now given to on-site and real-time electrochemical measurements. Electrical sensors allow remote deployment with near-real time monitoring capability, along with highly sensitive and selective 662
J. Environ. Monit., 2004, 6, 657–664
Fig. 4 Aqueous sensor setup incorporating an rf transceiver and acoustic transducers.
microfluidic platforms, for example, would further facilitate on-site and in situ environmental monitoring efforts. In addition, network communication refinement and commercialization advances will play major roles in large scale efforts to address today’s environmental monitoring needs.
21 22
Acknowledgements JW acknowledges financial support from the US EPA (Award RD830900). GH acknowledges K.A. Howell for her helpful comments.
23
24
References 1 I. Heninger, M. Potin-Gautier, I. de Gregori and H. Pinochet, Storage of aqueous solutions of selenium for speciation at trace level, Fresenius’ J. Anal. Chem., 1997, 357, 600–610. 2 A. Aminot and R. Kerouel, Assessment of heat treatment for nutrient preservation in seawater samples, Anal. Chim. Acta, 1997, 351, 299–309. 3 P. Gardolinski, G. Hanrahan, E. Achterberg, M. Gledhill, A. Tappin, A. House and P. Worsfold, Comparison of sample storage protocols for the determination of nutrients in natural waters, Water Res., 2001, 35, 3670–3678. 4 D. A. Polya, P. R. Lythgoe, F. Abou-Shakra, A. G. Gault, J. R. Brydie, J. G. Webster, K. L. Brown, M. K. Nimfogpoulus and K. M. Michailidis, IC-ICP-MS and IC-ICP-HEX-MS determination of arsenic speciation in surface and groundwaters: preservation and analytical issues, Mineral. Mag., 2003, 67, 247–261. 5 J. Wang, Analytical Electrochemistry, John Wiley & Sons, New York, 2nd edn., 2000. 6 M. Taillefert, G. W. Luther and D. B. Nuzzio, The Application of Electrochemical Tools for in situ Measurements in Aquatic Systems, Electroanalysis, 2000, 12, 401–412. 7 E. Bakker and M. Telting-Diaz, Electrochemical sensors, Anal. Chem., 2002, 74, 2781–2800. 8 K. Howell, E. P. Achterberg, C. B. Braungardt, A. D. Tappin, D. R. Turner and P. J. Worsfold, The determination of trace metals in estuarine and coastal waters using a voltammetric in situ profiling system, Analyst, 2003, 128, 734–741. 9 S. D. Collyer, J. J. Butler and S. P. Higson, The electrochemical determination of n-nitrosamines at polymer modified electrodes via an adsorptive stripping voltammetric regime, Electroanalysis, 1997, 9, 985–993. 10 L. M. Moretto, N. Pepe and P. Ugo, Voltammetry of redox analytes at trace concentrations with nanoelectrodes ensembles, Talanta, 2004, 62, 1055–1060. 11 H. Huang and P. K. Dasgupta, A field-deployable instrument for the measurement and speciation of arsenic in potable water, Anal. Chim. Acta, 1999, 380, 27–37. 12 S. D. Jhaveri, J. M. Mauro, H. M. Goldston, C. L. Schauer, L. M. Tender and S. A. Trammell, A reagentless electrochemical biosensor based on a protein scaffold, Chem. Commun., 2003, 338–339. 13 T. Sokalski, A. Ceresa, M. Fibbioli, T. Zwicki, E. Bakker and E. Pretsch, Lowering the detection limit of solvent polymeric ionselective membrane electrodes. 2. Influence of composition of sample and internal electrolyte solution, Anal. Chem., 1999, 71, 1210–1214. 14 J. Wang, B. Tian, J. Lu, J. MacDonald, J. Wang and D. Luo, Renewable reagent enzyme inhibition sensor for remote monitoring of cyanide, Electroanalysis, 1998, 10, 1034–1043. 15 K. J. M. Sandsrto¨m, A.-L. Sunesson, J.-O. Levin and A. P. F. Turner, A gas-phase biosensor for environmental monitoring of formic acid: laboratory and field validation, J. Environ. Monit., 2003, 5, 477–482. 16 G. E. Bately, Electroanalytical Techniques for the Determination of Heavy Metals in Seawater, Mar. Chem., 1983, 12, 107–117. 17 M.-L. Tercier and J. Buffle, In situ voltammetric measurements in natural waters: future prospects and challenges, Electroanalysis, 1993, 5, 187–200. 18 M.-L. Tercier and J. Buffle, Antifouling membrane-covered voltammetric microsensor for in situ measurements in natural waters, Anal. Chem., 1996, 68, 3670–3678. 19 J. Wang, D. Larson, N. Foster, S. Armalis, J. Lu, X. Rongrong, K. Olsen and A. Zirino, Remote electrochemical sensor for trace metal contaminants, Anal. Chem., 1995, 67, 1481–1485. 20 J. Wang, J. Lu, D. Luo, J. Wang, M. Jiang and B. Tian,
25
26 27 28 29 30 31 32
33 34 35
36
37
38 39 40 41 42 43 44 45
Renewable-reagent electrochemical sensor for monitoring trace metal contaminants, Anal. Chem., 1997, 69, 2640–2645. J. Wang, In situ electrochemical monitoring: from remote sensors to submersible microlaboratories, Lab. Rob. Autom., 2000, 12, 178–182. M. Tercier-Waeber, F. Confalonieri, G. Riccardi, A. Sina, F. Graziottin and J. Buffle, Multi physical-chemical profiler for real-time automated in situ monitoring of specific fractions of trace metals and master variables, J. Phys. IV, 2003, 107, 1927–1300. J. Barek and J. Zima, Electrochemistry of environmentally important organic substances, in Electrochemistry for Environmental Protection, ed. K. Sˇtulı´k and R. Kalvoda, UNESCO, Venice, 1996, Technical Report 25. J. Barek, J. Cvacˇka, A. Much, V. Quaiserova´ and J. Zima, Electrochemical methods for monitoring of environmental carcinogens, Fresenius’ J. Anal. Chem., 2001, 369, 556–562. G. K. Budnikov, G. A. Evtyugin, E. P. Rizaeva, A. N. Ivanov and V. Z. Latypova, Comparative assessment of electrochemical biosensors for determining inhibitors-environmental pollutants, J. Anal. Chem., 1999, 54, 864–871. J. Wang and W. Chen, Remote Electrochemical Biosensor for Field Monitoring of Phenolic Compounds, Anal. Chim. Acta, 1995, 312, 39–44. F. C. Walsh, Electrochemical technology for environmental treatment and clean energy conversion, Pure Appl. Chem., 2001, 73, 1819–1837. L. J. J. Janssen and L. Koene, The role of electrochemistry and electrochemical technology in environmental protection, Chem. Eng. J., 2002, 85, 137–146. J. Wang, R. Bhada, J. Lu and D. MacDonald, Remote electrochemical sensor for monitoring TNT in natural waters, Anal. Chim. Acta, 1998, 361, 85–92. J. Wang, L. Chen, A. Mulchandani and W. Chen, Remote Biosensor for in situ Monitoring of Organophosphate Compounds, Electroanalysis, 1999, 11, 866–890. G. Gauglitz, Optical sensors, Biosensors for Environmental Monitoring, ed. U. Bilitewski and A. P. F. Turner, Harwood Academic Publishers, The Netherlands, 2000. G. Whitenett, G. Stewart, K. Atherton, B. Culshaw and W. Johnstone, Optical fibre instrumentation for environmental monitoring applications, J. Optics A: Pure Appl. Optics, 2003, 5, S140–S145. C. M. A. Brett, Electrochemical sensors for environmental monitoring. Strategy and examples, Pure Appl. Chem., 2001, 73, 1969–1977. W. Bourgeois, A.-C. Romain, J. Nicolas and R. M. Stuetz, The use of sensor arrays for environmental monitoring: interests and limitations, J. Environ. Monit., 2003, 5, 852–860. C. Belmont-He´bert, M.-L. Tercier, J. Buffle, G. C. Fiaccabrino, N. F. de Rooij and M. Koudelka-Hep, Gel-integrated microelectrode arrays for direct voltammetric measurements of heavy metals in natural waters and other complex media, Anal. Chem., 1998, 70, 2949–2956. S. Sasso, R. Pierce, R. Walla and A. Yacynych, Electropolymerized 1,2-diaminobenzene as a means to prevent interferences and fouling and to stabilize immobilized enzyme in electrochemical biosensors, Anal. Chem., 1990, 62, 1111–1117. P. Brendel and G. Luther, Development of a gold amalgam voltammetric microelectrode for the determination of dissolved Fe, Mn, O2, and S(-II) in pore-waters of marine and freshwater sediments, Environ. Sci. Technol., 1995, 29, 751–761. J. Wang, Stripping Analysis, VCH Publishers, New York, 1985. M. Paneli and A. Voulgaropoulos, Applications of adsorptive stripping voltammetry in the determination of trace and ultratrace metals, Electroanalysis, 1993, 5, 355–373. T. M. Florence and G. Batley, Scheme for classification of heavy metal species in natural waters, Anal. Chem., 1980, 52, 1962–1963. J. Wang, B. Tian, J. Lu, J. Wang, D. Luo and D. MacDonald, Remote Electrochemical Sensor for Monitoring Trace Mercury, Electroanalysis, 1998, 10, 399–402. J. Wang, J. Lu, S. Hocevar and B. Ogorevc, Bismuth-coated screen-printed electrodes for stripping voltammetric measurements of trace lead, Electroanalysis, 2001, 13, 13–17. K. A. Howell, E. Achteberg, C. Braungardt, A. Tappin, P. J. Worsfold and D. Turner, Voltammetric in situ measurements in coastal waters, Trends Anal. Chem., 2003, 22, 828–835. J. Wang, Q. Chen and G. Cerpia, Electrocatalytic modified electrode for remote monitoring of hydrazines, Talanta, 1996, 43, 1387–1391. A. J. Baeumner, Biosensors for environmental pollutants and food contaminants, Anal. Bioanal. Chem., 2003, 377, 434–445. J. Environ. Monit., 2004, 6, 657–664
663
46 V. Saxena and B. D. Malhotra, Electrochemical biosensors, Adv. Biosens., 2003, 5, 63–100. 47 H. Nakamura and I. Karube, Current research activity in biosensors, Anal. Bioanal. Chem., 2003, 377, 446–468. 48 C. Fan, Z Genxi and D. Zhu, Recent progresses in immobilized enzyme-based reagentless electrochemical biosensors, Curr. Top. Anal. Chem., 2002, 3, 233–251. 49 G. Chiti, G. Marrazza and M. Mascini, Electrochemical DNA biosensor for environmental monitoring, Anal. Chim. Acta, 2001, 427, 155–164. 50 I. Biran, R. Babai, K. Levcov, J. Rishpon and E. Z. Ron, Online and in situ monitoring of environmental pollutants: electrochemical biosensing of cadmium, Environ. Microbiol., 2000, 2, 285–290. 51 Y. Herschkovitz, I. Eshkenazi, C. E. Campbell and J. Rishpon, An electrochemical biosensor for formaldehyde, J. Electroanal. Chem., 2000, 491, 182–197. 52 J. Sˇvitel and S. Miertusˇ, Development of tyrosinase-based biosensor and its application for monitoring of bioremediation of phenol and phenolic compounds, Environ. Sci. Technol., 1998, 32, 828–832. 53 L. H. Larsen, T. Kjaer and N. P. Revsbech, A microscale NO32 biosensor for environmental applications, Anal. Chem., 1997, 69, 3527–3531. 54 J. E. Frew and H. A. Hill, Electrochemical Biosensors, Anal. Chem., 1987, 59, 933A–939A. 55 A. P. F. Turner, Biochemistry: biosensors-sense and sensitivity, Science, 2000, 290, 1315–1318. 56 J. Kulys, Amperometric enzyme electrodes in analytical chemistry, Fresenius’ Z. Anal. Chem., 1989, 335, 86–91. 57 E. Bakker, P. Buhlmann and E. Pretsch, Polymer membrane ionselective electrodes - What are the limits?, Electroanalysis, 1999, 11, 915–922. 58 M. Pu¨ntener, T. Vigassy, E. Baier, A. Ceresa and E. Pretsch, Improving the lower detection limit of potentiometric sensors by covalently binding the ionophore to a polymer backbone, Anal. Chim. Acta, 2004, 503, 187–194. 59 A. Ceresa, E. Bakker, B. Hattendorf, D. Gunther and E. Pretsch, Potentiometric polymeric membrane electrodes for measurement of environmental samples at trace levels: New requirements for selectivities and measuring protocols, and comparison with ICPMS, Anal. Chem., 2001, 73, 343–348. 60 T. Le Goff, J. Braven, L. Ebdon and D. Scholefield, Automatic
664
J. Environ. Monit., 2004, 6, 657–664
61
62
63 64
65
66
67 68
69 70 71
continuous river monitoring of nitrate using a novel ion-selective electrode, J. Environ. Monit., 2003, 5, 353–358. J. Braven, L. Ebdon, N. C. Frampton, T. Le Goff, D. Scholefield and P. G. Sutton, Mechanistic aspects of nitrate-selective electrodes with immobilised ion exchangers in a rubbery membrane, Analyst, 2003, 128, 1067–1072. B. Muller, K. Buis, R. Stierli and B. Wehrli, High spatial resolution measurements in lake sediments with PVC based liquid membrane ion-selective electrodes, Limnol. Oceanogr., 1998, 43, 1728–1733. K. S. Black, G. R. Fones, O. C. Peppe, H. A. Kennedy and I. Bentaleb, An autonomous benthic lander, Cont. Shelf Res., 2001, 21, 859–877. S. Martinez-Barrachina, J. Alonso, L. Matia, R. Prats and M. del Valle, Determination of trace levels of anionic surfactants in river water and wastewater by a flow injection analysis system with on line preconcentration and potentiometric detection, Anal. Chem., 1999, 71, 3684–3691. O. V. Stepanets, G. Y. Solov’eva, A. M. Mikhailova and A. I. Kulapin, Rapid determination of anionic surfactants in seawater under shipboard conditions, J. Anal. Chem., 2001, 56, 290–293. T. Taboada-Castro, A. Dieguez, B. Lopez and A. Paz-Gonzalez, Comparison of conventional water testing methods with ionselective electrodes technique for NO32, Cl2, Ca2+, K+, and Na+, Commun. Soil Sci. Plant Anal., 2000, 31, 1993–2005. T. Neufeld, I. Eshkenzai, E. Cohen and J. Rishpon, A micro flow injection electrochemical biosensor for organophosphorus pesticides, Curr. Sep., 2001, 19, 94–100. R. S. Freire, N. Duran and L. T. Kubota, Development of a laccase-based flow injection electrochemical biosensor for the determination of phenolic compounds and its application for monitoring remediation of Kraft E1 paper mill effluent, Anal. Chim. Acta, 2002, 463, 229–238. K. B. Male, C. Saby and J. H. T. Luong, Optimization and characterization of a flow injection electrochemical system for pentachlorophenol assay, Anal. Chem., 1998, 70, 4134–4139. J. Zhou and A. Mason, Communication buses and protocols for sensor networks, Sensors, 2002, 2, 244–257. X. Yang, K. G. Ong, W. R. Dreschel, K. Zeng, C. S. Mungle and C. A. Grimes, Design of a wireless sensor network for longterm, in situ monitoring of an aqueous environment, Sensors, 2, 455–472.
Department of Chemistry & Biochemistry, California State University at Los Angeles, Los Angeles, CA 90032, USA. E-mail: [email protected]; Fax: +1 323-343-6490 b Department of Chemistry & Biochemistry, New Mexico State University, Las Cruces, NM 88003, USA. E-mail: [email protected]; Fax: +1 505-646-6033 Received 16th March 2004, Accepted 18th May 2004 First published as an Advance Article on the web 28th June 2004
The advancement in miniaturization and microfabrication technology has led to the development of sensitive and selective electrochemical devices for field-based and in situ environmental monitoring. Electrochemical sensing devices have a major impact upon the monitoring of priority pollutants by allowing the instrument to be taken to the sample (rather than the traditional way of bringing the sample to the laboratory). Such devices can perform automated chemical analyses in complex matrices and provide rapid, reliable and inexpensive measurements of a variety of inorganic and organic pollutants. Although not exhaustive due to the vast amounts of new and exciting electrochemical research, this review addresses many important advances in electrochemical sensor design and development for environmental monitoring purposes. Critical design factors and development issues including analytical improvements (e.g. detection limits), microfabrication and remote communication are presented. In addition, modern environmental applications will be discussed and future perspectives considered.
1. Introduction
DOI: 10.1039/b403975k
Traditional environmental monitoring approaches are based upon discrete sampling methods followed by laboratory analysis. These approaches do not improve our understanding of the natural processes governing chemical species behavior, their transport and bioavailability, or the relationship between anthropogenic releases and their long-term impact on aquatic systems. The stability of natural water samples during longterm storage is questionable, as they are subject to various biological, chemical and physical affects.1–4 Furthermore, discrete sampling methods and analyses are expensive, time consuming and do not provide the high resolution data needed to truly study chemical species dynamics in aquatic systems. In view of the limitations of discrete sample collection and subsequent laboratory analysis, real-time, continuous analytical methods capable of detecting chemical species with high temporal and spatial resolution, are desirable. Grady Hanrahan is an Assistant Professor of Chemistry at California State University, Los Angeles. His interests include the design and deployment of miniature analytical sensors for in situ aquatic monitoring and the development of laboratorybased flow injection-capillary electrophoresis (FI-CE) techniques for the analysis of bimolecular interactions, microscale reactions, and metals in natural waters. Dr. Hanrahan is also involved in the use of chemometrics for experimental design/ optimization and data analysis.
Grady Hanrahan
Joseph Wang is a Regents Professor of Chemistry at New Mexico State University (NMSU). He holds a Manasse
Electrochemical sensors represent an important subclass of chemical sensors in which an electrode is used as the transduction element, and are highly qualified for meeting the size, cost, and power requirements of on-site environmental monitoring.5–7 Characteristics of electrochemical sensing systems include high sensitivity and selectivity, a wide linear range, minimal space and power requirements, and low-cost instrumentation. Table 1 gives specific applications and quantitative details of advances in selected electrochemical sensing systems in recent years. Such devices have found a vast range of important applications in the fields of clinical, industrial, environmental, and agricultural analyses. Electrochemical devices have been used for several decades for field monitoring of a variety of water quality parameters (e.g. conductivity, dissolved oxygen or pH). These have led to a wider range of environmental applications including the measurement of trace metals in natural waters,8,16–22 carcinogen monitoring (e.g. N-nitroso compounds or aromatic amines),23,24 the Chair at NMSU and serves also as the Chief Editor of Electroanalysis. The research interests of Dr. Wang include the development of microfluidic (‘‘Lab-on-Chip’’) devices, DNA and protein recognition and diagnostics, bioelectronic, nanomaterials-based sensors, counterterrorism-detection, nanobiotechnology, the development and characterization of new surfaces for electroanalysis, sensor/recognition coatings, electrochemical sensing devices for environmental, security, and clinical monitoring, microfabrication and miniaturization, remote sensing, the development of techniques for ultratrace measurements and the design of on-line flow detectors. Joseph Wang
This journal is ß The Royal Society of Chemistry 2004
J. Environ. Monit., 2004, 6, 657–664
657
15 A gas phase biosensor using formate dehydro genase as the formic acid selective enzyme
10
DPV at NEE allowed the determination (with no preconcentration) of trace amounts of FA+ Field-deployable instrument Reagentless transduction of molecular recognition at gold electrode surfaces Lower detection limits achieved by partially replacing primary ions at the inner membrane surface Remote monitoring of enzyme inhibitors
14
9 —
13
8 Submersible probe for freshwater, estuarine and marine systems
11 12
Reference Notes
development of biosensors for the detection of organic pollutants (e.g. pesticides, phenols) in ground water,25,26 and environmental protection and clean energy conversion.27,28 Increased security concerns have led to the development of remote/submersible electrochemical sensors for monitoring explosives29 and nerve-agents.30 By providing a fast return of the analytical information in a timely, safe and cost effective fashion, such devices offer direct and reliable monitoring (including assessment of the fate and gradient of the target analytes). This paper critically examines the role electrochemical sensors play in current environmental monitoring efforts. It must be noted, however, that other classes of sensors (e.g. optical) are unique in their own right and often compliment the field of electrochemical sensors.31,32 We have focused our attention on recent advances in electrochemical sensor technology in terms of microfabrication, analytical improvements and remote communication capabilities. Recent applications and future trends in electrochemical sensor technology will also be reviewed, including microfluidic integration and submersible devices for remote, continuous monitoring.
658
0.03 mg m23
2 6 1026 M
1028.5 M
0.5 mg L21 15 pM
0.02 mM
23 pM (Cd2+) 1.13 nM (Cu2+) 23 pM (Pb2+) 1 6 1027 mol L21
Detection limit (unit as reported)
Electroanalytical sensors are concerned with the interplay between electricity and chemistry, namely the measurements of electrical quantities, such as current, potential or charge and their relationship to chemical parameters. Most of the electrochemical devices used for environmental monitoring fall within three categories and ultimately depend upon the specific analyte, nature of the sample matrix and the sensitivity and selectivity requirements.33
Enzyme-inhibition biosensor with amperometric detection Screen printing to deposit carbon paste, Ag/AgCl paste and insulating ink on polyester sheets
Polymeric ion-selective membrane electrode
Anodic stripping voltammetry (ASV) Protein scaffold-based electrochemical biosensor
The use of a potential applied between a reference and a working electrode causing the oxidation or reduction of an electroactive species. The applied potential thus serves as the driving force for the electron-transfer reaction. The resulting current is a direct measure of the rate of the electron transfer reaction and proportional to the target analyte concentration. The most common example is the oxygen Clark electrode that has been routinely used for monitoring the level of oxygen in water column and sediment pore water. Potentiometry In potentiometric sensors (primarily ion-selective electrodes), the analytical information is obtained by converting an ionrecognition event into a potential signal. A local equilibrium is established across the recognition membrane, leading to a change in the membrane potential. The analytical information is obtained from the potential difference between the ionselective electrode and a reference electrode. Potentials are a function of species activity, not concentration. Typical examples are potentiometric devices for in situ monitoring of pH, pCO2 or pS22. Conductimetry Conceptually the simplest of the electroanalytical techniques but inherently non-specific. The concentration of the charge is obtained through measurement of solution resistance.
J. Environ. Monit., 2004, 6, 657–664
Formic acid
CN2
Ca2+
3. Design and fabrication criteria As(III), As(IV) Avidin
DPV with gold nanoelectrode ensembles (NEEs)
N-Nitrosocompounds (e.g. N-nitroso-Nmethylanilines) FA+PF62(Ferrocenylmethyl)trimethylammonium hexafluorophosphate
Agarose, Hg-plated, Ir-based microelectrode (mAMMIE) by square wave anodic stripping voltammetry (SWASV) Differential pulse voltammetry (DPV) Cd2+, Cu2+, Pb2+
Sensing system
Amperometry and voltammetry
Measured species
Table 1
Applications and quantitative details of advances in selected electrochemical sensing systems in recent years
2. Principles
Table 2 lists a number of critical design criteria that should be considered when designing and developing robust electrochemical sensors for environmental monitoring. These are especially true for the development of submersible sensors where microfabrication, portability, analytical response, sensitivity,
Table 2 Criteria for the design and development of electrochemical sensors for environmental monitoring Macro vs. miniaturized fabrication design Overall cost, simplicity/complexity of design Robustness, reliability Sensitivity and selectivity Reversibility and stability Speed Artifact minimization Speciation capabilities Automation, data acquisition Single vs. multicompound analysis capabilities Low power consumption
descriptions of this technology and relevant environmental applications can be found in recent studies.8,22 This innovative, micromachined technology has been an important improvement from traditional approaches, especially in terms of field use and overall portability. It also provides significant advantages in terms of modern environmental efforts (i.e., metal speciation measurements and temporal resolution of the data). Moreover, micromachined devices allow for the use of smaller sample quantities (mL) as compared to mL quantities used in traditional electrochemical cells as well as lower overall power usage.
4. In situ applications selectivity, biofouling, reversibility and power consumption issues are of major concern. The type of fabrication used is ultimately dependent upon such factors as monitoring necessities, techniques employed and cell configuration. Traditionally, bulky electrodes and ‘‘beaker-type’’ cells have been employed.5 The advent of microfabrication allows the replacement of traditional electrochemical cells and bulky electrodes with easy-to-use sensing devices and has led to significant advances in the development of miniaturized electrochemical sensors and sensor arrays, especially in regards to remote monitoring devices.34 Recent studies have used innovative techniques such as thick and thin film technology, silicon-based techniques and photolithography in designing electrochemical sensors for environmental monitoring. Silicon-based techniques, for example, have recently been used in the development of micromachined stripping electrochemical sensors for trace metal analysis in natural waters.20 Stripping analysis technology such as this has remarkable sensitivity, with detection limits of 1.5 6 1028 M nickel and 4.2 6 1028 M uranium obtained following 5 and 20 min adsorption times. Novel thin-film, gel integrated Ir microelectrode arrays for square-wave anodic stripping voltammetry (SWASV) have also been developed for the analysis of trace elements in fresh and sea waters.35 The addition of the agarose gel allows the diffusion of dissolved metal ions and small complexes to the electrode surface, whilst acting as a barrier to unwanted colloids and macromolecules (e.g. humic and fulvic acids), helping eliminate fouling problems experienced with traditional devices.17 A schematic of a voltammetric in situ probe employing this technology is shown in Fig. 1. More detailed
The development of in situ electrochemical devices requires proper attention to major issues including reversibility (carry over), long-term stability (surface fouling, drift), specificity (overlapping signals caused by co-existing compounds), and changes in natural conditions (such as oxygen or convection) that may affect the response of interest. For example, prolonged direct operations in organic-rich natural-water systems may be complicated by surface fouling by co-existing surface-active substances. High levels of organic surfactants can lead to suppressions and distortions of the analyte response during extended monitoring operations. Coating the transducer with a permselective film has been shown to be useful for imparting the necessary stability by exclusion from the surface of unwanted macromolecules leading to its passivation.36 Such coating can also greatly enhance the selectivity by rejecting interferences from naturally-occurring electroactive substances and hence minimizing overlapping signals. The use of ultramicroelectrodes (with diameter smaller than 20 mm) has been employed for minimizing errors associated with fluctuations in the natural convection. Such relative independence of microelectrode sensors from convective flow reflects the larger natural convection boundary layer compared to the Nernst layer. In addition, the decreased ohmic distortions at ultramicroelectrodes allow direct electrochemical measurements to be made in aquatic systems (e.g. inland water) of low ionic strength. This also obviates the need for supporting electrolyte therefore minimizing possible impurities. For example, Brendel and Luther demonstrated the utility of a voltammetric microelectrode for obtaining depth profiles of dissolved iron, manganese, oxygen and S22 in marine environments.37 Stripping-based electrochemical sensors and applications
Fig. 1 Voltammetric in situ probe incorporating gel integrated Ir microelectrode arrays (based on ref. 8 with permission).
Growing concerns about heavy metal contamination has led to increasing needs to monitor trace metals in a variety of environmental matrices. Stripping analysis has been established as a powerful technique for determining toxic metals in environmental samples.17,38 Its remarkable sensitivity is attributed to the ‘built-in’ preconcentration step, during which the target metals are accumulated onto the working electrode. Whilst electrolytic deposition has been traditionally applied for trace heavy metals, non-electrochemical preconcentration schemes (based on adsorptive accumulation of metal complexes) have been developed for over two dozen environmentallyrelevant trace metals that cannot be readily deposited (e.g. Cr, Al, U, Fe, Ti, V, Mo).39 Stripping analysis is highly qualified to provide important information on metal speciation.40 The stripping protocol can be adjusted to measure the total or labile metal or for measuring different oxidation states (through control of the solution or deposition conditions). Remotelydeployable submersible sensors are capable of monitoring metal contaminants with respect to both time and location in shipboard marine surveys. Besides the continuous monitoring capability, such in situ measurements obviate errors common to laboratory-based trace metal analyses (due to metal contamination or loss). J. Environ. Monit., 2004, 6, 657–664
659
Fig. 2 Remote sensor for trace metals based on stripping potentiometry (based on ref. 19 with permission).
Submersible trace-metal analyses commonly rely on electrochemical stripping analysis.19,41 Such 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 judicious coupling of gold surfaces, potentiometric stripping operation, and ultramicroelectrode technology. The remote sensor assembly consisted of the gold-fiber, silver, and platinum working, reference, and counter electrodes, respectively (Fig. 2) and operated in the stripping potentiometric mode.19 Huang and Dasgupta11 described a compact and light lap-top controlled ASV analyzer for the measurement and speciation of trace arsenic in potable water. Such developments allow to move the measurements of trace metals to the field, and to perform them more rapidly, reliably and inexpensively. The recent development of ‘‘green’’ bismuth film electrodes, with a performance comparable to that of toxic mercury electrodes,42 should further facilitate the development of such in situ metal sensors. In situ voltammetric measurements of trace metals in coastal waters have been reviewed recently.43 Submersible modified electrodes 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 on electropolymerized films of 3,4-dihydroxybenzaldehyde.44 The low-potential detection accrued from this catalytic action offers convenient measurements of micromolar hydrazine concentrations in untreated groundwater or river water samples. A submersible carbon-fiber working electrode assembly has also been developed, connected to a 50 ft-long shielded cable, for the real-time monitoring of the 2,4,6-trinitortoluene (TNT) explosive in natural water.29 The facile reduction of the nitro moiety allowed convenient and rapid square-wave voltammetric measurements of sub-ppm levels of TNT (Fig. 3). Lower (ppb) concentrations have been detected using a background subtraction operation. Such high sensitivity is coupled to good selectivity and stability, and an absence of carry-over effects. The latter reflects the absence of recognition/binding events. The voltammetric sensor has been integrated onto an Unmanned Underwater Vehicle (UUV). This integration was a part of the US-Navy program aimed at demonstrating the effectiveness, in the field environment, of advanced technology sensors for detecting explosives in coastal regions of the ocean. 660
J. Environ. Monit., 2004, 6, 657–664
Fig. 3 Remote TNT voltammetric sensor response to seawater samples.
With such a sensor an UUV can be used to detect and prosecute plumes containing trace amounts of explosives that leach into the sea water from man-made objects, such as unexploded ordnance.
Electrochemical biosensors There have been a number of published reviews45–48 in the area of electrochemical biosensors each providing an overview of the most recent literature. Table 3 provides a selected compilation of biosensor performance data from published work relating to environmentally important analytes. More detailed examples and applications are discussed below. The remarkable specificity of biological recognition events has led to the development of highly selective electrochemical biosensors.54,55 In particular, enzyme electrodes, 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.5,56 The integration of these devices with the remotely-deployed probes should add new dimensions of specificity into in situ electrochemical monitoring of contaminants. In the adaptation of enzyme electrodes to such in situ operation one must consider the influence of actual field conditions (e.g. salinity, pH, temperature) upon the biocatalytic activity. The first remotely-deployed biosensor targeted phenolic pollutants in connection to a submersible tyrosinase enzyme electrode.26 The enzyme, immobilized within a carbon paste matrix, converted its phenolic substrates to easily reducible quinone products. The sensor responded rapidly to micromolar levels of various phenol pollutants, with no apparent memory effects, and high stability (e.g. RSD of 2.4% for 50 runs over a 10 hour period). A remote biosensor for field monitoring of organophosphate pesticides has also been developed.30 The device relies on the coupling of the biocatalytic 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 common to inhibition-based biosensors. Hydrogen peroxide and organic peroxides have been monitored at large instrument–sample distances by incorporating a reagentless peroxidase bioelectrode into the remote probe assembly.26 A low detection potential (y0.0 V) accrued from the use (coimmobilization) of a ferrocene cosubstrate, allowed
NO32 biosensor containing immobilized denitrifying bacteria with a built in electrochemical transducer for N2O
53
30 ng L ppb detectable range
Chemically modified SPEs Amperometric detection using an enzyme/gas diffusion electrode Biosensor consisting of a N2O microelectrode
1 mM
21
51 52
25 nM Potentiometric assay of b-galactosidase activity
50
49
Intercalative or electrostatic collection of aromatic amines onto an immobilized dsDNA or ssDNA layer followed by chronopotentiometric analysis Electrochemical biosensing for online and in situ monitoring of gene expression in response to Cd2+ Coupled biosensing device with flow injection analysis. Use of tyrosinase enzyme 0.05 mM 0.2 mM DNA coated screen printed electrodes (SPEs)
Potentiometric sensors for environmental monitoring Ion-selective electrodes are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H+ concentration in a solution. The remarkable performance of the pH ISE, coupled with the importance of pH measurements, make it extremely attractive for field monitoring. Other parameters that can be measured by ISE include fluoride, bromide, nitrate and cadmium and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. A compilation of linear ranges, interferences, selectivity coefficients and limits of detection for commercially available ISEs for common analytes are shown in Table 4. Commercially available ISEs provide a convenient way to effectively measure environmentally important parameters at a relatively low cost. Early ISEs have been limited to the micromolar range owing to leaking of the primary ion from the internal electrolyte solution. By choosing an internal electrolyte with low activity of the primary ion and preventing its leak it is possible to greatly lower the detection limits by up to 6 orders of magnitude down to the picomolar range.57,58 ISEs have thus been recently applied to trace metal measurements in relevant environmental matrices. For example, a neutral carrier ISE has been applied for trace (sub-nM) measurements of lead and lead speciation in various natural waters.59 Adjusting a lead-containing sample to various pH values allowed reliable measurements of the fractions of uncomplexed lead (with a good correlation to the theoretical expectations). Continuous environmental monitoring efforts have also been performed using state-of-the-art ISE technology. Automatic continuous monitoring of nitrate in rivers, for example, has recently been performed using a novel ion-selective electrode.60,61 This nitrate-ISE was constructed using immobilized ion exchangers in a rubbery membrane and achieved detections limits on the order of 0.007 mg L21. Pioneering studies have shown the effectiveness of ISEs in the in situ monitoring of freshwater and marine sediments.62,63 In the latter study, a micro-profiling module incorporating up to four oxygen microelectrodes and two pH microelectrodes was developed for insertion into the sediment to measure pore water concentrations at very high spatial resolution (y50 mm intervals). Recent studies have also used ISEs in the determination of anionic surfactants in river water and wastewater64 as well as seawater under shipboard conditions.65 In addition, sensor units housing multielement ISEs have been developed for the determination of environmentally relevant NO32, Cl2, Ca2+, K+ and Na+ ions.66
NO32
Formaldehyde 4-Chlorophenol
Automation and wireless communication advances
Cd2+
1,2-Diaminoanthraquinone2Anthramine
Reference Notes Detection limit (unit as reported) Sensing system Measured species
Table 3 Selected biosensor performance data in relation to environmentally important analytes
convenient monitoring of micromolar peroxide concentrations in untreated environmental samples. In addition to direct enzymatic substrate measurements it is possible to exploit enzyme inhibition processes for remote measurements of toxins. For example, continuous monitoring of cyanide can be accomplished using a renewable-reagent flow probe, involving internal delivery of the tyrosinase enzyme and its substrate, along with microdialysis sampling of the toxin and downstream amperometric measurements.14
Recent technological advances enable sensors to be fully automated and integrated with flow-based delivery systems and wireless communication technology (thus freeing them from being physically attached to a base station). Recent advances in flow injection (FI) technology coupled with electrochemical detection, for example, have led to the development of automated sensors for the continuous detection of a variety of compounds. Such applications include the development of a J. Environ. Monit., 2004, 6, 657–664
661
Table 4 Commercially available ion-selective electrodes and analytical figures of merit for common analytes Analyte
Type
Concentration range (M)
Lower limit (ppm)
26
1–10
0.02 0.02
NH3
Gas permeable membrane
NH4+
Liquid membrane
1021–1026
Ba2+
Liquid membrane
1021–1025
Br2
Solid state
1025–1026
0.4
Cd2+
Solid state
1021–1026
0.2
2+
25
10
27
0.02
Ca
Liquid membrane
10 –10
F2 I2
Solid state Solid state
1025–1027 1–1027
0.01 0.02
Pb2+
Solid state
1021–5 6 1026
1.0
NO32
Liquid membrane
1–5 6 1026
0.08
K+
Liquid membrane
1–1026
0.04
Ag+ Na+
Solid state Glass sensing membrane
1–1027 Saturated–1026
0.01 1 ppb
S22
Solid state
1–1027
micro flow biosensor for the determination of organophosphorus pesticides,67 a laccase-based FI biosensor for the detection of phenolic compounds68 and a FI electrochemical system for pentachlorophenol assays.69 In addition, technology such as Micro-Electro-Mechanical Systems (MEMS) have made it possible to construct functional ‘‘smart sensors’’ and to connect a multitude of sensors for distributed measurement applications.70 Aqueous sensor networks incorporating an array of sensor nodes have also been developed which can be randomly distributed throughout a lake or drinking water reservoir for long-term in situ monitoring.71 Ostensibly, each node of the sensor network acts as a data router and contains sensors to measure environmentally relevant parameters. Host nodes on land are physically connected to a personal computer and incorporate an rf transceiver for wireless communication with the sensors of interest (Fig. 4).
0.003
Interferences and selectivity ratios Hydrazine = 5 6 1022 Aliphatic amines = 0.1–0.5 Na+ = 2.0 6 1023 Mg2+ = 2.0 6 1024 K+ = 1.2 6 1021 Na+ = 4 6 1024 K+ = 9.0 6 1023 Ca2+ = 2.5 6 1022 I2, S22, and CN2 must be absent OH2 = 3 6 1025 Cl2 = 2.4 6 1023 Ag+, Hg2+, Cu2+ v 1027 M Lead and ferric ion interfere at high levels Mg2+ = 2.5 6 1024 Ba2+ = 3 6 1023 Pb2+ = 0.1 Zn2+ = 1.0 Na+ = 1.5 6 1024 OH2 = 1021 S22 w1027M CN2 = 1.0 S22, Ag+, Hg2+ should be absent Cd2+, Cu2+, Fe3+ interfere Cl2 = 1022 NO22 = 3 6 1022 Br2 = 5 6 1022 SO422 = 3.5 6 1023 F2 = 1026 ClO32 = 20 ClO42 = 16.2 Na+ = 2.6 6 1023 Ca2+ = 2.5 6 1023 Rb+ = 1.9 Mg2+ = 1.9 6 1023 Cs+ = 0.38 NH4+ = 0.30 S22 and Hg2+ must be absent Li+ = 2 6 1022 K+ = 1 6 1023 NH4+ = 3 6 1023 Ag1 should be absent Hg2+ and Ag+ must be absent
detection capabilities. A vast array of electrochemical devices has been developed in recent years for in situ monitoring of numerous inorganic and organic contaminants. There is, however, a realization that electrochemical sensor technology cannot address all environmental monitoring needs. Yet, we are continuously being introduced to electrochemical devices based on new technology with future efforts in the areas of ‘‘smart’’ sensors and molecular devices, multiparameter sensor arrays and remote electrodes. ‘‘Smart’’ devices, capable of ‘‘switching’’ between ‘‘screening/warning’’ and ‘‘detailedanalysis’’ modes of operation, are desired for various environmental scenarios. Electrochemical research is progressing in many unique directions. Ultimately, the push towards further miniaturization will bring together such fields as chemistry, engineering and physics. The integration of electrochemical technology into
5. Conclusions The combination of modern electrochemical techniques with breakthroughs in microelectronics and miniaturization allows the introduction of powerful and reliable electrical devices for effective process or pollution control. The consequence of these developments is that major considerations are now given to on-site and real-time electrochemical measurements. Electrical sensors allow remote deployment with near-real time monitoring capability, along with highly sensitive and selective 662
J. Environ. Monit., 2004, 6, 657–664
Fig. 4 Aqueous sensor setup incorporating an rf transceiver and acoustic transducers.
microfluidic platforms, for example, would further facilitate on-site and in situ environmental monitoring efforts. In addition, network communication refinement and commercialization advances will play major roles in large scale efforts to address today’s environmental monitoring needs.
21 22
Acknowledgements JW acknowledges financial support from the US EPA (Award RD830900). GH acknowledges K.A. Howell for her helpful comments.
23
24
References 1 I. Heninger, M. Potin-Gautier, I. de Gregori and H. Pinochet, Storage of aqueous solutions of selenium for speciation at trace level, Fresenius’ J. Anal. Chem., 1997, 357, 600–610. 2 A. Aminot and R. Kerouel, Assessment of heat treatment for nutrient preservation in seawater samples, Anal. Chim. Acta, 1997, 351, 299–309. 3 P. Gardolinski, G. Hanrahan, E. Achterberg, M. Gledhill, A. Tappin, A. House and P. Worsfold, Comparison of sample storage protocols for the determination of nutrients in natural waters, Water Res., 2001, 35, 3670–3678. 4 D. A. Polya, P. R. Lythgoe, F. Abou-Shakra, A. G. Gault, J. R. Brydie, J. G. Webster, K. L. Brown, M. K. Nimfogpoulus and K. M. Michailidis, IC-ICP-MS and IC-ICP-HEX-MS determination of arsenic speciation in surface and groundwaters: preservation and analytical issues, Mineral. Mag., 2003, 67, 247–261. 5 J. Wang, Analytical Electrochemistry, John Wiley & Sons, New York, 2nd edn., 2000. 6 M. Taillefert, G. W. Luther and D. B. Nuzzio, The Application of Electrochemical Tools for in situ Measurements in Aquatic Systems, Electroanalysis, 2000, 12, 401–412. 7 E. Bakker and M. Telting-Diaz, Electrochemical sensors, Anal. Chem., 2002, 74, 2781–2800. 8 K. Howell, E. P. Achterberg, C. B. Braungardt, A. D. Tappin, D. R. Turner and P. J. Worsfold, The determination of trace metals in estuarine and coastal waters using a voltammetric in situ profiling system, Analyst, 2003, 128, 734–741. 9 S. D. Collyer, J. J. Butler and S. P. Higson, The electrochemical determination of n-nitrosamines at polymer modified electrodes via an adsorptive stripping voltammetric regime, Electroanalysis, 1997, 9, 985–993. 10 L. M. Moretto, N. Pepe and P. Ugo, Voltammetry of redox analytes at trace concentrations with nanoelectrodes ensembles, Talanta, 2004, 62, 1055–1060. 11 H. Huang and P. K. Dasgupta, A field-deployable instrument for the measurement and speciation of arsenic in potable water, Anal. Chim. Acta, 1999, 380, 27–37. 12 S. D. Jhaveri, J. M. Mauro, H. M. Goldston, C. L. Schauer, L. M. Tender and S. A. Trammell, A reagentless electrochemical biosensor based on a protein scaffold, Chem. Commun., 2003, 338–339. 13 T. Sokalski, A. Ceresa, M. Fibbioli, T. Zwicki, E. Bakker and E. Pretsch, Lowering the detection limit of solvent polymeric ionselective membrane electrodes. 2. Influence of composition of sample and internal electrolyte solution, Anal. Chem., 1999, 71, 1210–1214. 14 J. Wang, B. Tian, J. Lu, J. MacDonald, J. Wang and D. Luo, Renewable reagent enzyme inhibition sensor for remote monitoring of cyanide, Electroanalysis, 1998, 10, 1034–1043. 15 K. J. M. Sandsrto¨m, A.-L. Sunesson, J.-O. Levin and A. P. F. Turner, A gas-phase biosensor for environmental monitoring of formic acid: laboratory and field validation, J. Environ. Monit., 2003, 5, 477–482. 16 G. E. Bately, Electroanalytical Techniques for the Determination of Heavy Metals in Seawater, Mar. Chem., 1983, 12, 107–117. 17 M.-L. Tercier and J. Buffle, In situ voltammetric measurements in natural waters: future prospects and challenges, Electroanalysis, 1993, 5, 187–200. 18 M.-L. Tercier and J. Buffle, Antifouling membrane-covered voltammetric microsensor for in situ measurements in natural waters, Anal. Chem., 1996, 68, 3670–3678. 19 J. Wang, D. Larson, N. Foster, S. Armalis, J. Lu, X. Rongrong, K. Olsen and A. Zirino, Remote electrochemical sensor for trace metal contaminants, Anal. Chem., 1995, 67, 1481–1485. 20 J. Wang, J. Lu, D. Luo, J. Wang, M. Jiang and B. Tian,
25
26 27 28 29 30 31 32
33 34 35
36
37
38 39 40 41 42 43 44 45
Renewable-reagent electrochemical sensor for monitoring trace metal contaminants, Anal. Chem., 1997, 69, 2640–2645. J. Wang, In situ electrochemical monitoring: from remote sensors to submersible microlaboratories, Lab. Rob. Autom., 2000, 12, 178–182. M. Tercier-Waeber, F. Confalonieri, G. Riccardi, A. Sina, F. Graziottin and J. Buffle, Multi physical-chemical profiler for real-time automated in situ monitoring of specific fractions of trace metals and master variables, J. Phys. IV, 2003, 107, 1927–1300. J. Barek and J. Zima, Electrochemistry of environmentally important organic substances, in Electrochemistry for Environmental Protection, ed. K. Sˇtulı´k and R. Kalvoda, UNESCO, Venice, 1996, Technical Report 25. J. Barek, J. Cvacˇka, A. Much, V. Quaiserova´ and J. Zima, Electrochemical methods for monitoring of environmental carcinogens, Fresenius’ J. Anal. Chem., 2001, 369, 556–562. G. K. Budnikov, G. A. Evtyugin, E. P. Rizaeva, A. N. Ivanov and V. Z. Latypova, Comparative assessment of electrochemical biosensors for determining inhibitors-environmental pollutants, J. Anal. Chem., 1999, 54, 864–871. J. Wang and W. Chen, Remote Electrochemical Biosensor for Field Monitoring of Phenolic Compounds, Anal. Chim. Acta, 1995, 312, 39–44. F. C. Walsh, Electrochemical technology for environmental treatment and clean energy conversion, Pure Appl. Chem., 2001, 73, 1819–1837. L. J. J. Janssen and L. Koene, The role of electrochemistry and electrochemical technology in environmental protection, Chem. Eng. J., 2002, 85, 137–146. J. Wang, R. Bhada, J. Lu and D. MacDonald, Remote electrochemical sensor for monitoring TNT in natural waters, Anal. Chim. Acta, 1998, 361, 85–92. J. Wang, L. Chen, A. Mulchandani and W. Chen, Remote Biosensor for in situ Monitoring of Organophosphate Compounds, Electroanalysis, 1999, 11, 866–890. G. Gauglitz, Optical sensors, Biosensors for Environmental Monitoring, ed. U. Bilitewski and A. P. F. Turner, Harwood Academic Publishers, The Netherlands, 2000. G. Whitenett, G. Stewart, K. Atherton, B. Culshaw and W. Johnstone, Optical fibre instrumentation for environmental monitoring applications, J. Optics A: Pure Appl. Optics, 2003, 5, S140–S145. C. M. A. Brett, Electrochemical sensors for environmental monitoring. Strategy and examples, Pure Appl. Chem., 2001, 73, 1969–1977. W. Bourgeois, A.-C. Romain, J. Nicolas and R. M. Stuetz, The use of sensor arrays for environmental monitoring: interests and limitations, J. Environ. Monit., 2003, 5, 852–860. C. Belmont-He´bert, M.-L. Tercier, J. Buffle, G. C. Fiaccabrino, N. F. de Rooij and M. Koudelka-Hep, Gel-integrated microelectrode arrays for direct voltammetric measurements of heavy metals in natural waters and other complex media, Anal. Chem., 1998, 70, 2949–2956. S. Sasso, R. Pierce, R. Walla and A. Yacynych, Electropolymerized 1,2-diaminobenzene as a means to prevent interferences and fouling and to stabilize immobilized enzyme in electrochemical biosensors, Anal. Chem., 1990, 62, 1111–1117. P. Brendel and G. Luther, Development of a gold amalgam voltammetric microelectrode for the determination of dissolved Fe, Mn, O2, and S(-II) in pore-waters of marine and freshwater sediments, Environ. Sci. Technol., 1995, 29, 751–761. J. Wang, Stripping Analysis, VCH Publishers, New York, 1985. M. Paneli and A. Voulgaropoulos, Applications of adsorptive stripping voltammetry in the determination of trace and ultratrace metals, Electroanalysis, 1993, 5, 355–373. T. M. Florence and G. Batley, Scheme for classification of heavy metal species in natural waters, Anal. Chem., 1980, 52, 1962–1963. J. Wang, B. Tian, J. Lu, J. Wang, D. Luo and D. MacDonald, Remote Electrochemical Sensor for Monitoring Trace Mercury, Electroanalysis, 1998, 10, 399–402. J. Wang, J. Lu, S. Hocevar and B. Ogorevc, Bismuth-coated screen-printed electrodes for stripping voltammetric measurements of trace lead, Electroanalysis, 2001, 13, 13–17. K. A. Howell, E. Achteberg, C. Braungardt, A. Tappin, P. J. Worsfold and D. Turner, Voltammetric in situ measurements in coastal waters, Trends Anal. Chem., 2003, 22, 828–835. J. Wang, Q. Chen and G. Cerpia, Electrocatalytic modified electrode for remote monitoring of hydrazines, Talanta, 1996, 43, 1387–1391. A. J. Baeumner, Biosensors for environmental pollutants and food contaminants, Anal. Bioanal. Chem., 2003, 377, 434–445. J. Environ. Monit., 2004, 6, 657–664
663
46 V. Saxena and B. D. Malhotra, Electrochemical biosensors, Adv. Biosens., 2003, 5, 63–100. 47 H. Nakamura and I. Karube, Current research activity in biosensors, Anal. Bioanal. Chem., 2003, 377, 446–468. 48 C. Fan, Z Genxi and D. Zhu, Recent progresses in immobilized enzyme-based reagentless electrochemical biosensors, Curr. Top. Anal. Chem., 2002, 3, 233–251. 49 G. Chiti, G. Marrazza and M. Mascini, Electrochemical DNA biosensor for environmental monitoring, Anal. Chim. Acta, 2001, 427, 155–164. 50 I. Biran, R. Babai, K. Levcov, J. Rishpon and E. Z. Ron, Online and in situ monitoring of environmental pollutants: electrochemical biosensing of cadmium, Environ. Microbiol., 2000, 2, 285–290. 51 Y. Herschkovitz, I. Eshkenazi, C. E. Campbell and J. Rishpon, An electrochemical biosensor for formaldehyde, J. Electroanal. Chem., 2000, 491, 182–197. 52 J. Sˇvitel and S. Miertusˇ, Development of tyrosinase-based biosensor and its application for monitoring of bioremediation of phenol and phenolic compounds, Environ. Sci. Technol., 1998, 32, 828–832. 53 L. H. Larsen, T. Kjaer and N. P. Revsbech, A microscale NO32 biosensor for environmental applications, Anal. Chem., 1997, 69, 3527–3531. 54 J. E. Frew and H. A. Hill, Electrochemical Biosensors, Anal. Chem., 1987, 59, 933A–939A. 55 A. P. F. Turner, Biochemistry: biosensors-sense and sensitivity, Science, 2000, 290, 1315–1318. 56 J. Kulys, Amperometric enzyme electrodes in analytical chemistry, Fresenius’ Z. Anal. Chem., 1989, 335, 86–91. 57 E. Bakker, P. Buhlmann and E. Pretsch, Polymer membrane ionselective electrodes - What are the limits?, Electroanalysis, 1999, 11, 915–922. 58 M. Pu¨ntener, T. Vigassy, E. Baier, A. Ceresa and E. Pretsch, Improving the lower detection limit of potentiometric sensors by covalently binding the ionophore to a polymer backbone, Anal. Chim. Acta, 2004, 503, 187–194. 59 A. Ceresa, E. Bakker, B. Hattendorf, D. Gunther and E. Pretsch, Potentiometric polymeric membrane electrodes for measurement of environmental samples at trace levels: New requirements for selectivities and measuring protocols, and comparison with ICPMS, Anal. Chem., 2001, 73, 343–348. 60 T. Le Goff, J. Braven, L. Ebdon and D. Scholefield, Automatic
664
J. Environ. Monit., 2004, 6, 657–664
61
62
63 64
65
66
67 68
69 70 71
continuous river monitoring of nitrate using a novel ion-selective electrode, J. Environ. Monit., 2003, 5, 353–358. J. Braven, L. Ebdon, N. C. Frampton, T. Le Goff, D. Scholefield and P. G. Sutton, Mechanistic aspects of nitrate-selective electrodes with immobilised ion exchangers in a rubbery membrane, Analyst, 2003, 128, 1067–1072. B. Muller, K. Buis, R. Stierli and B. Wehrli, High spatial resolution measurements in lake sediments with PVC based liquid membrane ion-selective electrodes, Limnol. Oceanogr., 1998, 43, 1728–1733. K. S. Black, G. R. Fones, O. C. Peppe, H. A. Kennedy and I. Bentaleb, An autonomous benthic lander, Cont. Shelf Res., 2001, 21, 859–877. S. Martinez-Barrachina, J. Alonso, L. Matia, R. Prats and M. del Valle, Determination of trace levels of anionic surfactants in river water and wastewater by a flow injection analysis system with on line preconcentration and potentiometric detection, Anal. Chem., 1999, 71, 3684–3691. O. V. Stepanets, G. Y. Solov’eva, A. M. Mikhailova and A. I. Kulapin, Rapid determination of anionic surfactants in seawater under shipboard conditions, J. Anal. Chem., 2001, 56, 290–293. T. Taboada-Castro, A. Dieguez, B. Lopez and A. Paz-Gonzalez, Comparison of conventional water testing methods with ionselective electrodes technique for NO32, Cl2, Ca2+, K+, and Na+, Commun. Soil Sci. Plant Anal., 2000, 31, 1993–2005. T. Neufeld, I. Eshkenzai, E. Cohen and J. Rishpon, A micro flow injection electrochemical biosensor for organophosphorus pesticides, Curr. Sep., 2001, 19, 94–100. R. S. Freire, N. Duran and L. T. Kubota, Development of a laccase-based flow injection electrochemical biosensor for the determination of phenolic compounds and its application for monitoring remediation of Kraft E1 paper mill effluent, Anal. Chim. Acta, 2002, 463, 229–238. K. B. Male, C. Saby and J. H. T. Luong, Optimization and characterization of a flow injection electrochemical system for pentachlorophenol assay, Anal. Chem., 1998, 70, 4134–4139. J. Zhou and A. Mason, Communication buses and protocols for sensor networks, Sensors, 2002, 2, 244–257. X. Yang, K. G. Ong, W. R. Dreschel, K. Zeng, C. S. Mungle and C. A. Grimes, Design of a wireless sensor network for longterm, in situ monitoring of an aqueous environment, Sensors, 2, 455–472.
