Anal. Chem. 2000, 72, 4497-4520
Dynamic Electrochemistry: Methodology and Application James L. Anderson*
Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556 Louis A. Coury, Jr.
Bioanalytical Systems Inc., 2701 Kent Avenue, West Lafayette, Indiana 47906-1382 Johna Leddy
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Review Contents Review Articles and Books Reviews Books Mass Transport General Mass Transport Theory Finite Difference Models Finite Element Models Boundary Element Models Monte Carlo Methods Mixed Modes of Mass Transport External Magnetic or Electric Fields Electrode Arrays Analytical Voltammetry Reviews Sensor Concepts Sensing in Small Spatial Domains Sensor Functionality Enhancements Kinetic Studies Electrode Materials SECM for Kinetic Studies Frequency Domain Methods Advances in Kinetic Theory Unusual Applications Surface Electrochemistry Reviews Double-Layer Characterization Approaches Interfacial Structural Insights Modified Electrodes Architecture Conducting Polymers Electroactive Materials Monolayers Ion-Exchange Matrixes Nonconducting Polymers General Theories Bioelectrochemistry In Vitro Biosensors Analytical Systems That Mimic Nature Novel Bioanalytical Sensing Strategies Electrochemical Studies of Biomolecules Characterization of Redox Reactions
10.1021/ac0007837 CCC: $19.00 Published on Web 08/03/2000
© 2000 American Chemical Society
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Spectroelectrochemistry UV-Visible Absorption Reflectance and Attenuated Total Reflection (ATR) Fourier Transform Infrared Spectrometry Raman Spectrometry Specialized Optical Techniques Electrochemical Mass Spectrometry Electron Paramegnetic Resonance (EPR) Methods Nuclear Magnetic Resonance (NMR) Methods X-ray Methods Instrumentation Remote Sensors Instruments Electrodes and Cells Spatially Localized Multifunctional Systems Failure Modes Auxiliary Experimental Functions Literature Cited
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Dynamic electrochemistry generally refers to that subset of electrochemical systems in which a clear equilibrium is not established. Topics such as cyclic voltammetry, amperometry, coulometry, pulse voltammetry, and electrochemical impedance spectroscopy fall naturally into this category. These techniques share the common feature of involving the imposition of a potential or current program, resulting in a response (transient and/or steady state) by the system that may be measured and recorded. Although the areas covered by this review are similar to those covered by the review two years ago (A1), the number of references cited has decreased dramatically. The tradition of publishing review articles in Analytical Chemistry surveying the major subdisciplines predates the widespread availability of electronic literature-searching capabilities, and (obviously) the Internet. Clearly, there is little value in presenting thousands of citations for recent papers on electrochemical topics, when readers can execute their own specific electronic searches at will. Instead, this review attempts to provide a snapshot of those topics that were new, exciting, and unusual at the end of the 1990s. To repeat, absolutely no attempt has been made to provide comprehensive coverage of the literature in this area! Analytical Chemistry, Vol. 72, No. 18, September 15, 2000 4497
To further characterize the nature of recent electrochemical literature in broad terms, the following statistical information may be of interest. A Chemical Abstracts search was conducted to find articles pertaining to electrochemistry published during the period extending roughly from October 1997 through October 1999. A general and inclusive set of keywords returned approximately 14 000 citations. The languages in which the papers found were published had the following distribution: 71.4% English, 15.3% Japanese, 7.3% Chinese, 2.3% German, 2.0% Russian, 0.5% French, 0.5% Korean, 0.3% Spanish, 0.2% Portuguese, 0.1% Polish, 0.1% Romanian, and 0.2% all other languages. Approximately 20% of the abstracts received were of patents. Of the latter, about half were Japanese (∼2.3× as many as English language patents), accounting for more than 80% of all of the Japanese language articles. Finally, as a tip for readers contemplating conducting electronic searches of their own, 7% of abstracts retrieved dealt with welding technology (because they referred to “electrodes”)! REVIEW ARTICLES AND BOOKS Reviews. Most areas within the discipline of electrochemistry benefited from at least one review article during the past two years. Table 1 lists one example review article for each of a variety of electrochemical topics and the number of references cited in each review. Absolutely no attempt has been made to generate a comprehensive listing. Books. A comprehensive list of all books published on subjects related to electrochemistry is available on the Website called ESTIR, the Electrochemical Science and Technology Information Resources (URL: http://electrochem.cwru.edu/estir/). Readers interested in electrochemical monographs should refer to that site. During the period covered by this review, new volumes of the series Advances in Electrochemical Science and Engineering, Bioelectrochemistry of Biomacromolecules, Electroanalytical Chemistry, and Modern Aspects of Electrochemistry appeared. Detailed reviews of new volumes in these series appear in The Journal of the American Chemical Society and the Journal of Electroanalytical Chemistry. Some chapters appearing in these volumes have been listed in Table 1. Due to space constraints, only four recent books will be singled out for special mention here. A new edition of volume 1 of Modern Electrochemistry by Bockris and Reddy was published in 1998 (A46). A general text on electrochemistry by Hamnett, Hamann, and Vielstich also appeared (A47). Brett and Brett contributed a concise text called Electroanalysis to the Oxford Chemistry Primers series (A48). And finally, Rajeshwar and Ibanez authored a unique book entitled Environmental Electrochemistry: Fundamentals and Applications in Pollution Abatement (A49). MASS TRANSPORT Interesting developments in the treatment of mass transport continue apace. Initially, 384 papers were identified from an initial selection of approximately 1000 abstracts related to mass transport as particularly interesting and potentially citable. The citations discussed here were selected from this list. Clearly many interesting papers had to be excluded to meet the editorially imposed constraints. Important trends include the increasing use of more sophisticated numerical methods to model electrochemical problems, including several flavors of implicit finite difference, finite element, 4498
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Table 1. Electrochemical Review Articles topic adiabatic electrode processes adsorptive stripping voltammetry batteries biogenic amines in neurotransmission biosensors for clinical and therapeutic drug monitoring biosensors and electroanalysis capillary electrophoresis and electrochemical detection carbohydrates, electrochemical detection carbon-halogen bond cleavage in aromatic compounds catalysts for electroreduction of O2 to H2O channel electrodes chemically modified electrodes, terminology clay-modified electrodes coordination compounds diamond electrodes, boron-doped, thin film Ebonex electrodes electrogenerated chemiluminescence enzymes in cast biomembrane-like films free radicals, redox properties frequency-domain methods fullerenes and derivatives impedance methods in organic electrode kinetics infrared spectroscopy in electrochemistry; UHV science ion-selective electrodes ionic strength, low microemulsions microparticles immobilized on electrode surfaces molecular dynamics simulations molecular recognition neuronal microenvironments noise measurements in corrosion studies nonlinear phenomena organic compounds, studying by CV to SECM pulsed electrochemical detection, antibiotics quantum mechanical treatments in electrode kinetics SECM, studies of heterogeneous processes simulations of electrochemical interfaces solid-state voltammetry sol-gel solids, electrochemistry in spectroelectrochemistry, in situ spectroscopy theoretical electrochemistry thin-layer chromatography, electrochemical detection transmission line models for modified electrodes zeolites and molecular sieves, modified, electrochemistry
no. of citations
ref
138 159 87 52 35
A2 A3 A4 A5 A6
144 54
A7 A8
81 36
A9 A10
46 132 17 121 66 31 59 79 41 61 86 50 71
A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23
112
A24
215 124 40 116
A25 A26 A27 A28
215 44 202 88 67 252 93 131
A29 A30 A31 A32 A33 A34 A35 A36
80 285 115 92 100 151 30
A37 A38 A39 A40 A41 A42 A43
63
A44
118
A45
and boundary element methods. This reflects the increasing complexity of problems that are being tackled, involving both steady-state and time-dependent problems in multiple spatial dimensions and frequently involving stiff differential equations, where one variable may be changing on a very small dimensional scale while another variable is changing on a much larger dimensional scale. Adaptive or expanding spatial grids are being used with increasing frequency to improve the efficiency of such simulations. These problems frequently make analytical treatments very difficult or approximate, although a significant number of analytical solutions (often with simplifying assumptions) were reported. The continuing importance of very small electrodes or pairs or ensembles of electrodes gives rise to the need to treat multidimensional spatial geometries, which tax numerical methods
such as explicit finite difference. Increasingly, multiple modes of mass transport must be explicitly treated simultaneously, particularly migration, for situations where the supporting electrolyte concentration is not in large excess relative to the analyte, or for very small electrodes or fast times, where the dimensions of the diffusion layer may overlap the dimensions of the diffuse double layer. This is increasingly essential for electrodes with at least one dimension in the low micrometer range or smaller. Hydrodynamic voltammetric methods are increasingly popular, including flow channel, wall jet, and rotating disk electrode methods. These methods also frequently require heavy-duty simulation methods for efficient execution with complex geometries and experimental constraints. Interacting electrodes remain of interest, including dual generator/collector and interdigitated array geometries, as well as the traditional ring-disk electrode. Scanning electrochemical microscopy is becoming increasingly popular and used by an increasing number of research groups. This review will treat only a few examples of the mass transport aspects of scanning electrochemical microscopy. Additional reviews can be found in the Scanning Probe Microscopy review by Lillehei and Bottomley (B1). Several authors have treated multilayer systems, which may involve a series of different diffusion coefficients and, in some cases, a variable diffusion coefficient within the same medium. For example, Galceran et al. treated the complex case of an inlaid disk microelectrode in a multilayered medium, as might be observed for a membrane-covered sensor. The treatment used a double integral equation approach which accounts for edge effects traversing the boundary between the multilayered phases. Approximations enabled simple, accurate analytical solutions (B2). Lacroix et al. modeled moving front phenomena in a conducting polymer film, as the development of the mass transport process changes the conductivity and other key properties of the medium spatially as the interface between the conducting and insulating zones of the polymer moves across the film with the reaction, leading to a moving front that is not treated in elementary texts of electrochemistry, with complex effects on the mass transport process (B3). Unwin and co-workers have used scanning electrochemical microscopy (SECM) to probe partitioning of electroactive solutes between two immiscible phases. The SECM tip electrolytically perturbs the partition equilibrium and induces current flow, allowing noncontact determination of either the diffusion coefficient or the concentration of an electroactive analyte as a function of spatial position adjacent to the interface (B4). In collaboration with Bard, they also pointed out that modeling of SECM results in such systems needs to consider diffusion of the redox-active component in both phases, rather than using the approximation of constant composition in the second phase which has commonly been used (B5). Interest in enhanced mass transport is also increasing, taking several orthogonal directions. Work on sonically enhanced mass transport continues (e.g., sonovoltammetry), including characterization of the phenomenology and effects on mass transport as well as development of models to predict behavior. Mikkelsen and Schroder demonstrated that low-frequency sound in the audible range was effective in enhancing mass transport by diffusion layer thinning, with a ∼3-fold enhancement of signal for
stripping voltammetry relative to quiet solution. They suggested that the approach may have merit as an alternative to mechanical stirring and can be used during the analytical stripping step as well as the preconcentration step because it is far more reproducible and imparts far lower noise to the current signal (B6). Compton and co-workers were able to simulate ultrasonically enhanced limiting currents with the commercially available software Digisim, assuming a planar diffusion layer, for an electrocatalytic reaction, suggesting that the approach is extensible to other problems with coupled homogeneous chemical steps (B7). Compton and co-workers also compared ultrasound-assisted sonovoltammetry with laser ablation voltammetry with application to electroanalysis of ascorbic acid. While each approach showed particular advantages of continuous cleaning and activation of the electrode surface, sonovoltammetry showed a ∼15-fold enhancement of limiting current due to the enhanced mass transport afforded by cavitation (B8). Several interesting papers have appeared on the topic of magnetohydrodynamic enhancement of mass transport in the presence of strong magnetic fields. This field appears to have particularly interesting potential for studies at very small electrodes, where the impact of the magnetic field can be dramatic. Several papers have also considered the effect of thermal gradients on mass transport (e.g., thermal diffusion), either at a continuously heated electrode or as a result of ohmic heating by high current densities. An interesting variation on this theme is the consideration of temperature jump experiments as an analogous alternative to potential step experiments. Voss et al. described a new method, which they termed temperature pulse voltammetry, that affords peak-shaped voltammograms analogous to differential pulse voltammograms, recorded at nominally ambient temperature by applying localized temperature pulses at a heated electrode They demonstrated the concept for ferrocene at a gold layer electrode screen-printed onto a low-temperature ceramic substrate (B9). Although related studies have been reported in the past, there are hints that this approach is being considered by a larger group. In addition to all of these new directions, development of new boundary problems and revisiting of old boundary value problems for all of the traditional electrochemical methods continues unabated. General Mass Transport Theory. A number of papers have had a focus on calculation methods in mass transport. The interested reader can consult the Electrochemical Science and Technology Information Resource Website, which maintains information on a number of shareware simulation software resources (URL: http://electrochem.cwru.edu/estir/), and several commercial sites, including Bioanalytical Systems, which distributes the popular simulator, DigiSim. Alden and Compton have reviewed the application of a versatile fully implicit finite difference simulator with a multigrid subspace method to accelerate computation, which can handle a wide variety of electrode geometries and reaction mechanisms in steady-state flow channels (B10). They discussed the Compton group’s Web service (URL: http://physchem.ox.ac.uk:8000/wwwda), which enables users to evaluate experimental data for flow channel voltammetry for a number of common reaction mechanisms. Alden et al. also discussed a simulator that enables users to perform data analysis Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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for steady-state voltammetry at wall-jet electrodes and allows assessment of the effects of radial diffusion, using the same Website (B11). Reviews. Bieniasz and Speiser discussed the use of sensitivity analysis to obtain the best model fit to experimental data for cyclic voltammetric investigation of quasi-reversible electron transfer with potential-dependent transfer coefficient. They advocated this approach for improving the insight obtained from digital simulations of kinetic models (B12). Unwin reviewed the use of dynamic electrochemical methods to probe interfacial processes, for a wide variety of techniques and applications, including various flow channel methods, and scanning electrochemical microscopy, including key issues relating to mass transport (B13). Williams and Macpherson reviewed hydrodynamic modulation methods and their mass transport issues (B14). Ciszkowska and Stojek reviewed voltammetry at microelectrodes in solutions of low ionic strength, where coupled diffusion and migration are important factors and the treatment of mass transport is quite complex (B15). Eklund et al. reviewed cyclic voltammetry, hydrodynamic voltammetry, and sonovoltammetry for assessment of electrode reaction kinetics and mechanisms, with significant discussion of mass transport modeling issues (B16). Galvez reviewed the theoretical treatment of pulse voltammetric methods, including numerical and analytical solutions (B17). He compared and contrasted particularly the relative advantages of the Laplace transform and the dimensionless parameter series solution methods among analytical treatments and suggested some unsolved problems that still need to be addressed. Senda et al. treated the theory underlying amperometric ion-selective electrodes, based on ionophore-assisted transfer across an immiscible solvent interface, with emphasis on normal pulse and cyclic voltammetric methods (B18). Finite Difference Models. A variety of implicit models are being increasingly used for reasons already stated. Examples will be given for methods based on either steady-state or dynamic systems. The steady-state systems are easier to solve and less computationally demanding because they involve one fewer dimension. Steady State. Bond and Mahon applied the fast quasi-explicit method to model steady-state mass transport to a microdisk electrode and used the Oldham-Zoski steady-state equation to determine heterogeneous electrode kinetics with potential-dependent electron-transfer coefficient (B19). Compton and coworkers evaluated and critically compared the kinetic discrimination (time scale and effective range of the working curve) of steady-state voltammetry at a number of popular electrode configurations (microdisk, hemispherical, rotating disk, channel microband, wall-jet, and wall-tube), using primarily the fully implicit method (B20). Transient. Amphlett and Denuault simulated both transient and steady-state amperometric tip responses for scanning electrochemical microscopy using the alternating direction implicit method with expanding space and time grids (B21). They evaluated the topographical sensitivity and found that tip currents have a much greater dependence on tip geometry than previously reported. Strutwolf and Williams compared the use of a fully implicit extrapolation method with the alternating direction implicit method for treatment of two-dimensional mass transport at a 4500
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microband electrode (B22). They concluded that the extrapolation technique had greater stability, making it more applicable to stiff relaxation problems at short times. Ball and Compton treated square wave anodic stripping voltammetry at planar mercury film electrodes under hydrodynamic conditions, using the timedependent fully implicit method (B23). They compared the hydrodynamic results with predictions based on semi-infinite diffusion as a function of diffusion layer thickness and hydrodynamic conditions. Gavaghan discussed in detail strategies for implementing an exponentially expanding grid for fast and efficient simulation of transient diffusion processes at microdisk electrodes and compared the fully implicit method with the alternating direction implicit method for cyclic voltammetric simulation, with suggestions for when to use each method (B24). Finite Element Models. The finite element method, which has heretofore seen rather limited use in electrochemistry, appears to be on the ascent. Orthogonal collocation, a special case of the finite element method, has been used for years by a few groups, notably that of Speiser, but until recently finite element methods were avidly avoided by electrochemists. The complexity of the method inhibited its use for many years, but the increasingly complex multidimensional diffusion problems at microelectrodes are making it increasingly competitive with implicit methods due to the strong dependence of computational time on the dimensionality of the problem. Nann and Heinze gave a nice tutorial on the application of the finite element method to electrochemical problems (B25). Stevens and Fisher applied the finite element method to simulate transient voltammetry under hydrodynamic conditions at both micro- and macroelectrodes (B26). Bartlett and Taylor simulated diffusion to recessed microdisk electrodes using the finite element method (B27). Girault and co-workers used the finite element method to simulate an electrosynthetic reactor based on interdigitated planar band microelectrodes as a function of electrode spacing and flow rate (B28). Boundary Element Models. Despite the increased efficiency of the finite element method over the finite difference method, it still suffers from the need to compute concentrations at a number of interior grid points in the reaction volume, leading to long computations for complex geometries, multidimensional diffusion, or stiff systems. An alternative approach is the boundary element method, which is based on an integrating transformation of the data by multiplying by a function that causes the function to vanish everywhere except at the boundaries. (The Laplace transform method is based on a similar trick.) This has the effect of dramatically reducing the number of points that must be calculated, confining them to the boundaries rather than the interior of the reaction zone. Fulian and Fisher presented a very useful tutorial on the method, noting dramatic reductions in computational time for three-dimensional simulations with complex and irregular electrode geometries (B29). With Denuault, they presented an application of the method to simulate a scanning electrochemical microscopy line scan experiment (B30). Monte Carlo Methods. Fransaer and Penner modeled the growth of metal nanocrystal ensembles using a Brownian Dynamics simulation model, which involves the random displacement of a large ensemble of particles to simulate the mass transport process (B31). They also obtained an approximate analytical solution and observed that the explicit form of the spatial
distribution of particles on the surface leads to variations in the time-dependent response. These results may be usefully compared with more conventional diffusional models based on statistically controlled nucleation and growth, such as the work of Vilaseca et al., which uses an alternative to the usual Poisson-distributionbased Avrami model for deposit growth with interaction and coalescence of diffusion fields (B32). Mixed Modes of Mass Transport. Amatore et al. developed an analytical solution for the effect of migrational flux contributions in cases of successive electron-transfer reactions in solutions of low ionic strength (B33). They demonstrated that treatment of the migrational component is essential to achieve even qualitatively accurate prediction of the current response. Amatore and co-workers also developed a simplified model to show that convection can have a substantial impact on uncompensated ohmic potential drop for steady-state voltammetry at low ionic strength because convection precludes establishment of infinite diffusion layers (B34). Qiu et al. used an integral equation numerical boundary element method procedure to model current density governed by combinations of diffusion, migration, and convection simultaneously (B35). Oldham and Myland pointed out that interruption of a steady-state current leads to a charging current effect as a result of the continuing flux of ions needed to discharge the double-layer capacitance of the electrode (B36). External Magnetic or Electric Fields. The magnetohydrodynamic effect has been known for a long time, but interest in this phenomenon has picked up recently as a result of some elegant work based on the interaction of microelectrodes with magnetic fields. White and co-workers have presented some fascinating work on this topic, which has significant elements in common with earlier investigations of mixed modes of diffusive and migrational mass transport in systems of low ionic strength, where the external field is electrical and induced by the electrochemical reaction itself. White and co-workers have investigated the interaction of a microelectrode with a large, nonuniform magnetic field and showed that there are significant contributions from both an external magnetohydrodynamic force due to the applied magnetic field and an internally generated magnetic force due to movement of paramagnetic ions in the external magnetic field gradient. They illustrated dramatically different effects depending on the orientation of the electrode with respect to the field, the homogeneity of the magnetic field, and the presence or absence of paramagnetic species. They showed that order of magnitude theoretical estimates of these effects were in qualitative accord with experimental data (B37). Ragsdale and White also presented images of magnetohydrodynamic flows in nanoliter volumes adjacent to a microelectrode, by means of scanning electrochemical microscopy. They showed with the aid of color images that the magnetic field can be used to focus and control the spatial distribution of molecules in the field (B38). In favorable cases, the external magnetic field strength can be used to cancel gravitational forces which normally would promote natural convection. The small size of microelectrodes lends itself very well to demonstrating these concepts. It is quite conceivable that this approach could ultimately be exploited for control of mass transport in specialized miniature micromachined devices (e.g., micro total analysis systems) to carry out desired tasks with greater spatial control than heretofore feasible.
Leventis and Gao (B39) and Ngo Boum and Alemany (B40) have also numerically simulated magnetohydrodynamic phenomena with good success. Leventis and Gao examined the dependence of steady-state voltammetric response at stationary electrodes of larger cross section on the electron balance of the faradaic process in a magnetic field, modeling the behavior in terms of a moving-boundary diffusion layer model (B39). Ngo Boum and Alemany have numerically simulated mass transport in channel flows electromagnetically induced by an external magnetic field for the case of high supporting electrolyte, with results consistent with available experimental data (B40). Orlik et al. have investigated the mechanism of electrohydrodynamic convection in the absence of any external electrical or magnetic fields, induced by electrochemical current flow in a thinlayer electrochemical cell. They simulated numerically the coupled diffusional and migrational mass transport of electroactive and supporting electrolyte species and sketched a comparison of theory with experimental data which visualized the convective patterns by means of the electrochemiluminescence of rubrene (B41). Electrode Arrays. Ensembles of Coupled or Independent Electrodes. Compton et al. demonstrated the utility of observing the steady-state current response at an array of individual microband electrodes in a flow channel to probe mechanism and kinetics for a variety of mechanisms, when current is measured as a function of electrode length and flow rate for a series of electrodes with length in the direction of flow ranging from submicrometer to millimeter (B42). The variation of electrode length in this case has benefits similar to varying the time scale of observation in conventional transient experiments (e.g., scan rate in cyclic voltammetry). Girault and co-workers demonstrated that an array of independent microholes covered with a polymer film at the interface of immiscible liquids such as water and nitrobenzene can exhibit planar diffusion to the whole ensemble when the time scale is sufficiently long that diffusion layers at each microhole overlap and that the ensemble can afford a flow rate-independent response when used in a flow-through electrochemical cell at sufficiently high flow rates relative to the microhole dimensions and spacing (B43). Interacting Electrode Pairs and Arrays. Phillips and Stone presented a general method for calculating the generator/collector response between electrodes of arbitrary size and spacing without solving the detailed transport problem, by using a form of Green’s theorem to calculate the total current at the collector as an integral over the surface of the generator multiplied by a weighting function that can be determined by solving a reactive diffusion problem for the collector alone. The method was applied for three different geometrical configurations, in the presence or absence of coupled homogeneous chemical reactions (B44). The results indicate that collection efficiencies are relatively weakly dependent on the current distribution at the generator electrode. Stockgen and Heusler approached the related problem of collection efficiency at a rotating ring-disk electrode by using a deconvolution method to compensate for the transfer time between the generator disk and the collector ring (B45). Baur et al. studied the interaction between two independent carbon fiber electrodes using SECM in the feedback mode to measure the distance between the electrodes. Diffusional interaction was successfully modeled Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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by convolution of the concentration generated at the generator electrode with an impulse response function for a spherically divergent source (B46). The method was successfully applied for various excitation waveforms applied to the generator electrode. Fisher’s group exploited the boundary element method to handle the complexities of generation/collection at a dual microelectrode in generator/collector mode, under conditions where proper treatment of current distribution required solution of the Laplace equation for potential distribution simultaneously with the diffusion equations (B47). The reduction of the problem from three dimensions to two dimensions by means of the boundary element approach made the problem much more computationally tractable and greatly reduced simulation times. Gooding treated an analogous problem involving a biosensor based on only one detector electrode, but with upstream generation of an analyte by an enzyme reactor. Treatment of the collection efficiency in this system enabled tuning of the design of the reactor for optimum performance (B48). ANALYTICAL VOLTAMMETRY A wide variety of applications of voltammetric and related electrochemical methods was reported during the review period. An initial set of papers numbering near 1000 was initially culled to 509 references which appeared significant and relevant. The papers covered here are an eclectic sampling of a subset of the most interesting papers, selected on the basis of novelty of application and the personal biases of the reviewer. Reviews. Kulesza and Cox reviewed the analytical prospects of solid-state voltammetric analytical sensors based on selective solid-state interactions with a variety of selective membrane materials in the absence of a liquid solution phase (C1). They discussed future prospects for such devices, which can be used for applications such as gas sensors, among many others. Umezawa and co-workers reviewed chemical sensors for indirect detection of electroinactive analytes based on modified electrodes that mimic gating at biomembranes containing ion channel receptors (C2). They discussed binding of the analyte in an inclusion complex with a receptor to block conduction of an artificial channel or to electrostatically or sterically control the redox chemistry of a reporter species. Wang et al. reviewed new developments and future prospects for stripping voltammetry, emphasizing the growing roles of microfabricated sensor strips, micromachined hand-held instruments, and remote field analytical probes (C3). Fogg and Wang reviewed terminology and conventions for stripping analysis (C4). Wang also reviewed the present state of amperometric enzyme biosensors for clinical and therapeutic drug monitoring (C5). Kalvoda reviewed the effects of adsorption of surface-active substances on voltammetric environmental measurements. He stressed that, in addition to complications, adsorption can lead to significant enhancements of detection limits in some cases of strong adsorption (C6). Jadhav and Bakker outlined the theory, principles, and operational concepts of voltammetric and amperometric polymeric membrane-based ion sensors and demonstrated both the feasibility of sensing both cations and anions at the same membrane interface in the voltammetric mode and continuous long-term monitoring of a specific anion by means of a repetitive pulsed amperometric mode to ensure continuous restoration of 4502
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the membrane state (C7). Speiser presented a review of the range of electroanalytical tools available to characterize organic compounds, materials, and reaction chemistry (C8). Sensor Concepts. Okochi et al. developed a very simple and potentially universal allergen detector based on the observation that serotonin is secreted during an allergic reaction. The allergen was incubated with a 20-µL droplet of whole blood for 40 min, followed by application of the blood droplet to a Nafion-coated microelectrode array electrode with oxidative amperometric detection (C9). Winquist et al. described an “electronic tongue”, a solution-phase analogue of the recently popular electronic nose array sensor. They used pulse voltammetry at a double gold/ platinum working electrode with principal component chemometric analysis to classify fruit juices and milk and to follow aging processes in these beverages (C10). In a similar vein, Cammann and co-workers developed a multianalyte sensor for glucose, L-lactate, and uric acid, based on a microelectrode array consisting of sets of polymer-encapsulated microelectrodes modified with the corresponding enzymes (C11). Mallouk and co-workers demonstrated a combinatorial method for screening a large number of electrocatalytic electrode materials simultaneously with high throughput, using a fluorescent pH indicator with visual detection to provide visual confirmation of the most active catalyst for methanol oxidation (C12). As many as 645 wells could be monitored, for 5 different electrocatalysts singly and in various combinations. Creager and co-workers described a new, high-sensitivity signal amplification approach for amperometric detection, based on a sacrificial electron donor (or acceptor) in solution with the analyte whose role is to regenerate the initial oxidation state of the analyte, and a selective permeable coating on the electrode that allows facile electron transfer of the analyte but blocks reaction of the sacrificial reagent. The scheme affords very high amplification, which may reach 1000-fold due to multiple recycling and counting of the same analyte molecule many times, with detection limits in the range of as few as 60 000 molecules, corresponding to concentrations of 1 × 10-11 M or lower depending on injection volume for flow injection oxidative detection of hydroxymethylferrocene with ferrocyanide as the sacrificial electron donor (C13). Kobayashi and Martin described a microtubule membrane consisting of gold nanotubules with inside diameters comparable to molecular dimensions, fabricated in a membrane filter template. They explored two modes of analyte sensing: either via changes in transmembrane current in the presence of analyte or changes in membrane potential, with detection limits as low as 10-11 M (C14). Brewster and Mazenko described an immunoelectrochemical sensor for rapid assay of Escherichia coli bacteria labeled by an enzyme-antibody conjugate, captured on a filter, which was then placed on an electrode and incubated with the enzyme substrate, enabling detection of as few as 50 bacteria (C15). Abdel-Hamid et al. developed a semiautomated flow-through amperometric immunosensor with a disposable immunoelectrode for rabbit immunoglobulin (IgG), which afforded detection limits in the 10-11 M range for a 17-min analysis time (C16) Uto et al. demonstrated the utility of ferrrocenyl derivatization of a primer for polymerase chain reaction (PCR) amplification of DNA to detect the muscular dystrophin gene by liquid chromatography with electrochemical
detection (LCEC) (C17). Hsueh et al. developed an electrochemiluminescent microcell method for DNA determination with detection limits of 40 fmol of DNA (C18). Lam et al. described an electrochromatography column for separation of proteins, consisting of gold-plated metal particles coated with a selective ion-exchange or metal-affinity coating. The retention of RNase could be controlled by varying the potential applied to the column packing over a range of (0.3 V vs SCE (C19). Sensing in Small Spatial Domains. Kashyap and Gratzl discussed electrochemistry in very small sample domains, such as picoliter- or nanoliter-volume droplets, individual biological cells, or tissue domains, and deviations from conventional bulk measurements due to depletion of the very small sample volume (C20). Lu and Gratzl investigated the drug resistance of individual cancer cells by microvoltammetrically monitoring the efflux of the anticancer drug doxorubicin from the cells. The approach offers the prospect of exploring the heterogeneity of individual cancer cell drug resistance on a scale previously not attainable (C21). Xin and Wightman demonstrated the simultaneous detection of catecholamine exocytosis and calcium ion release from single bovine chromaffin cells with a dual amperometric/calciumselective dye-bound fluorescence sensor and demonstrated that catecholamine and calcium release were temporally and spatially correlated (C22). Luther et al. reported the in situ spatial microprofiling of the pore water of ocean sediments for dissolved oxygen, ionic manganese and iron, and sulfide within 4 cm of the water/sediment interface, with a microelectrode voltammetric sensor deployed on a remotely operated vehicle (C23). Wang et al. used adsorptive stripping chronopotentiometry at a vibrating electrode in microliter sample volumes to detect nanogram to picogram amounts of nucleic acids (C24). Dong and co-workers used a cyclic voltammetric technique as a function of pH to titrate and determine the pKa of a self-assembled monolayer of 3-mercaptopropionic acid on a gold electrode (C25). Sensor Functionality Enhancements. Cullison et al. reported the detection of catecholamines below the 5-fg level by using capillary liquid chromatography at a very low flow rate and an optimized dual-electrode detector with a very thin channel gap between electrodes, which allowed substantial signal amplification by cycling the analyte numerous times between the two electrodes so that each molecule was detected on average more than 10 times (C26). Wang et al. reported significant benefits of performing anodic stripping voltammetry of trace metals at lithographically fabricated pyrolytic carbon interdigitated microelectrode arrays. Collection by rereduction at one set of electrodes of the soluble metal released by oxidation at the other set allows improved performance under steady-state conditions with improved baseline due to diminished charging current (C27). In a similar vein, Tomcik et al. used an interdigitated microelectrode array to electrogenerate hypobromite and titrate analytes such as iodide and ammonia in the gap between microelectrodes for quantitation (C28). Aldrich and Van den Berg reported a catalytic cathodic stripping method for determination of iron in seawater with remarkably low detection limits as low as 8 × 10-11 M after an adsorption time as low as 30 s (C29). Buffle and co-workers reported that coating an array of mercury-plated iridium micro-
electrodes with a 300-600-µm agarose gel membrane significantly improves its immunity to fouling and convection and its performance for amperometric and anodic stripping voltammetry of heavy metals in natural waters and other complex media (C30). Wang and Lu showed that oxidase enzyme electrodes, which require stable oxygen levels to function properly, can work well for prolonged periods in an oxygen-free test solution if the enzyme is initially immobilized under aerobic conditions in carbon paste with a Kel-F oil pasting liquid, due to the high affinity of the Kel-F oil for oxygen and its ability to store it long-term (C31). BrajterToth and colleagues demonstrated that fast-scan voltammetry (scan rate >500 V/s) with a bare carbon fiber electrode can overcome the common interference of ascorbic acid with uric acid determination in biological fluids without need for a permselective membrane such as Nafion to achieve discrimination, based on differences in electrode kinetics at high scan rates (C32). Wightman and co-workers discussed ways to overcome nonidealities of carbon fiber electrodes for sensing in biological media, including response to pH variations often encountered in vivo, and variation in sensitivity according to electrode pretreatment (C33). Johnson and co-workers investigated the oxidative electrochemical incineration of 4-chlorophenol, with mass spectrometric identification of reaction products, with the ability to identify products in the ppb to ppm range (C34). Popovic and Johnson also investigated the oxidative amperometric detection of a range of biologically significant sulfur compounds at Bi(V)-doped PbO2 electrodes. They concluded that these doped electrodes appear to be more effective than undoped electrodes due to their promotion of anodic discharge of water to produce adsorbed hydroxyl radicals, which serve as a source of O atoms, which must be transferred to the compounds to enable their detection (C35). KINETIC STUDIES In preparation for writing this section of the review, a search of the Chemical Abstracts database for articles published between October 1997 and October 1999 on the subject of electrochemical kinetics was executed. Keywords used for the search included the following: electrochemical, electrode, heterogeneous, interfacial, kinetics, homogeneous, rate, quasi-reversible, irreversible, and transfer coefficient. (Various logical connectors were used and some stemming was allowed.) This search returned 1940 abstracts fitting the criteria listed. In what follows below, a few brief remarks will be made on 10 of these papers (∼0.5%). Obviously, a great many interesting and important papers will not be mentioned due to space constraints. Electrode Materials. Numerous electrode materials useful for electroanalysis have been investigated over the years. At this point, the vast majority of electroanalytical chemistry experiments is conducted using carbon electrodes. Some might argue this point, particularly those routinely employing stripping analysis in their daily activities. However, considering that glassy carbon electrodes are the material of choice for electrochemical HPLC detectors, and screen-printed carbon electrodes are used in most in vitro blood glucose measurements, the preeminence of carbon is clear. Recently, reliable methods for doping diamond films to prepare conductive phases have become available. The characterization Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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of these electrodes has given insight into the surface chemistry of common carbon materials such as glassy carbon and graphite. Interested readers are directed to a paper by Swain and colleagues (as well as to references cited in their paper) comparing kinetic studies of anthraquinone-2,6-disulfonate conducted using conventional glassy carbon, hydrogenated glassy carbon, highly ordered pyrolytic graphite, and diamond electrodes (D1). SECM for Kinetic Studies. Electrochemical instrumentation and techniques for kinetic studies are often viewed as mature areas. However, the scanning electrochemical microscope has recently been applied to kinetic studies at liquid-liquid interfaces instead of for imaging experiments. Two recent studies along these lines will be highlighted here. Work by Unwin, Bard, and colleagues describes and tests a numerical model for irreversible electron-transfer processes at the interface between two immiscible electrolyte solutions (D2). In their experiment, a redox probe is generated at an ultramicroelectrode tip located in one liquid phase and positioned close to the interface formed with another immiscible phase. This second phase contains the complementary half of another redox couple. If an electron-transfer reaction occurs between the two redox probes at the interface, regeneration of the probes occurs, thereby enhancing the current measured. Liu and Mirkin report the results of another fascinating SECM experiment at a liquid-liquid interface (D3). Their study represents the first report of the effects of solvent dynamics on the rate of electron transfer at a liquid-liquid interface. Using an uncharged zinc porphyrin in one phase and Ru(CN)63- in the other phase, they measure electron-transfer rates that do not depend on the potential drop across the interfacial boundary. This study also illustrates the precision and sensitivity with which kinetic parameters may be measured with the SECM. Frequency Domain Methods. The efforts of the late Donald Smith are bearing fruit in the electroanalytical community. This is perhaps due to the relatively recent widespread availability of control and modeling software for ac experiments. Another possibility is that the direction taken by electroanalytical research in the past few years has led to the investigation of systems that demand the sophistication and complexity of frequency domain experiments. Of the many, many examples of work in this area that could have been chosen for this review, reports from the Bond and Baranski groups will be used to highlight this active area. Bond and co-workers report the development of instrumentation that allows for both trace analytical studies and electrode kinetics investigations (D4). Their system applies a multifrequency excitation signal at each step along a dc staircase. The Fourier transform of the response yields frequency-related amplitude and phase information and effectively performs both analytical ac voltammetry and electrochemical impedance spectroscopic measurements. Charging current is discriminated against by using the impedance information generated in an experiment to correct the real part of the admittance data for uncompensated solution resistance. Detection limits of 50 nM are demonstrated. Baranski and Winkler present results of ac voltammetric studies of fast charge-transfer processes in benzene solutions using microelectrodes (D5). Because the supporting electrolyte was less than 1% dissociated into “free” ions, this paper illustrates the use of ac methods for separating kinetic information from 4504
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resistive distortion of the data. Measurements were made on two kinetically demanding redox probes: ferrocene and p-dicyanobenzene. Their results show that standard rate constants in benzene are similar to those obtained in solvents with higher dielectric constants (e.g., DMF). Advances in Kinetic Theory. Two papers discussing kinetic concepts of general interest to the electrochemical community are discussed next. Evans discusses an issue described as “the kinetic burden of potential inversion”, which relates to the fact that, for given values of the standard heterogeneous electrontransfer rate constants, potential inversion will cause an apparent decrease in the reversibility of the electrode reaction (D6). This observation leads to discussion of the conditions under which the transfer of two electrons in a stepwise versus simultaneous fashion may be distinguished. A paper by Lewis and co-workers illustrates recent work on kinetic theory for semiconductor electrodes (D7). In their study, Frumkin corrections for semiconductor electrodes were considered for both depletion and accumulation conditions. Under the conditions considered, the Frumkin correction was considered to be negligible at depleted semiconductor electrodes compared to metallic electrodes at the same applied potential relative to the potential of zero charge. The situation was found to be somewhat different for accumulation conditions, but under all conditions considered, the correction to the driving force used to evaluate the heterogeneous rate constant did not result in a change of more than a factor of 2, relative to the uncorrected rate constant. Unusual Applications. Finally, this section concludes by mentioning three recent, interesting kinetic studies. There were easily a few hundred other papers that could have been highlighted here. The reader is asked to accept these choices for what they are: a few interesting papers that exemplify the variety and complexity of kinetic measurements that have been reported over the past two years. The Murray group reports kinetic studies for cobalt complexes over a range of conditions for which both the physical self-diffusion coefficients (D) of the complexes and their heterogeneous rate constants (k0) vary by a factor of 1011 (D8). To access such a wide range of conditions, measurements were made in a roomtemperature molten salt composed of undiluted redox probes, which were bipyridyl complexes functionalized with poly(ethylene) or poly(propylene) side chains. Remarkably, a linear correlation was found between the D and k0 values, which the authors attribute to solvent dynamics control of electron-transfer rates. The effect of temperature on electrode kinetics is well understood and studied for many different types of electrochemical systems. Much less work has been done to experimentally demonstrate the effects of pressure, due largely to the experimental difficulties in implementing such experiments. Fu and Swaddle report kinetic results at high pressures for a family of cyanometalate complexes in aqueous solutions (D9). Experimental verification of the “50% rule” is shown, indicating that the volumes of activation measured for heterogeneous reactions are half those for the homogeneous (bimolecular) self-exchange reaction of the same couple. A mechanism based on these results is proposed for electron-transfer reactions of cyanometalates in which counterions mediate the electron-transfer process.
Last, a paper from the Save´ant group will be highlighted. Any paper published from these laboratories could be chosen as a prime example of the application of fundamental electrochemical principles to the elucidation of complex chemical reactions. One of their recent studies describes the generation of a series of radical species through reduction of the corresponding chlorides with electrons photoinjected by laser pulses (D10). In interpreting the results, the effects of radical dimerization are separated from follow-up reactions of the carbanions with acidic species present. Changes in reactivity across the series of reactants are discussed using density functional theory calculations. SURFACE ELECTROCHEMISTRY Some very elegant work is being carried out these days on the characterization of electrochemical surfaces, both in situ and ex situ, using a wide variety of electrochemical and spectroscopic tools. The papers emphasized here deal with the detailed characterization of the structure of either electrochemical interfaces or in some cases vacuum-phase models of such interfaces, with an emphasis on increasing our understanding of the interaction between an adsorbent and the electrode, the structure of the electrode/solution interface, or the nature of the electrical double layer adjacent to an electrode. The papers cited here were culled from a list of 224 papers identified in the literature search as highly relevant. Reviews. Lipkowski reviewed surface electrochemistry and the nature of ionic adsorption at gold single-crystal electrodes (E1). You and Nagy reviewed the use of synchrotron X-ray scattering for the study of electrochemical interfaces (E2). Morallon et al. reviewed the use of voltammetric studies coupled with FT-IR to investigate the oxidation and adsorption of species such as methanol, ethanol, and simple amino acids on platinum single-crystal electrodes (E3). Miranda and Shen reviewed the use of the sum frequency generation technique of nonlinear optics for characterization at the molecular level of liquid interfaces with solids, liquids, and gases (E4). Double-Layer Characterization Approaches. Hecht and Strehblow treated the use of X-ray photoelectron spectrometry and ion scattering spectrometry to investigate the structure of the electrical double layer on silver electrodes (E5). Shingaya and Ito discussed model double-layer structures on a series of metallic (111) single-crystal electrodes, using LEED and FT-IR spectrometry of water, bisulfate, and sulfuric acid molecules adsorbed on a model metal interface under variable low-temperature UHV conditions and compared with the double-layer structure in the liquid environment (E6). Hanken and Corn discussed the measurement of electric fields inside self-assembled multilayer films by means of surface plasmon resonance (E7). Bohn and coworkers coupled second-harmonic generation, surface plasmon resonance, and ac impedance studies to characterize full and partial monolayers of alkanethiolates at silver and gold interfaces with aqueous solutions (E8). Weaver and co-workers used surface-enhanced Raman spectrometry to characterize the chemical nature of oxide film formation, including adsorption by water, hydroxide, or molecular oxygen species on copper electrodes and the formation of Cu(II) and Cu(I) oxides at the copper/water interface in noncomplexing aqueous solutions. They noted that the SERS enhancement is still
detectable with as many as 15-20 monolayers separating the species of interest from the metallic surface responsible for the SERS gain phenomenon (E9). Chen and Lipkowski used FT-IR spectrometry (SNIFTIRS) to obtain analogous information for the adsorption of hydroxide ion at Au(111) electrodes (E10). Weaver et al. made use of the potential dependence of vibrational frequencies of nitric oxide and carbon monoxide monolayers measured by FT-IR reflection-absorption spectrometry (IRAS) at low-index platinum-group (Pt, Rh, Ir, Pd) metal/acidic aqueous interfaces to characterize the potential-dependent nature of chemisorptive bonding to the interface and the electrical double layer (E11). They compared in situ results with results derived from UHV measurements and discussed the insights accessible by such complementary approaches. Fawcett and co-workers followed the specific adsorption of nitrate ion at gold (111) electrode surfaces by means of SNIFTIRS, obtaining insight about the symmetry and interaction of nitrate ion with the gold surface as well as with other components in the electrical double layer (E12). They found evidence of potentialdependent water chemisorption and could use the width of the FT-IR nitrate band to estimate the potential of zero charge, which shifted negative by ∼0.1 V/pH unit due to specific adsorption of nitrate ion. Futamata treated the nature of water adsorption and interaction with gold electrode surfaces, based on ATR infrared absorption measurements, and identified three distinct water species in the vicinity of the interface (E13). Interfacial Structural Insights. Abrun ˜a and co-workers compared the dependence of the structure of underpotentially deposited copper layers on the metal electrode lattice structure for two different single-crystal electrode materials (Au(111) and Pt(111)) in the presence of bromide, using grazing incidence X-ray diffraction to characterize the structure (E14). They attributed the observed differences in behavior primarily to geometrical constraints arising from differences in lattice dimensions. Mukerjee and McBreen used in situ X-ray absorption spectrometry to characterize the effect of tin addition on the electrocatalytic activity of carbon-supported platinum materials (E15). Shannon and co-workers illustrated the utility of resonance Raman spectrometry to characterize thin CdS films grown on gold electrode substrates by either electrochemical atomic layer epitaxy (ordered films) or chemical bath deposition (disordered films consisting of spherical particles) as a function of deposition conditions (E16). They could readily characterize electronic band structure and the extent of charge carrier trapping. Zou and Weaver characterized a similar system consisting of very thin (up to 10 atomic layers) cadmium chalcogenide compound semiconductor films (CdSe, CdS) formed on a gold electrode substrate by electrochemical atomic layer epitaxy, using SERS spectrometry. They could readily observe evidence of quantum confinement and could detect differences in the film spectral properties depending on whether Cd or the chalcogenide was deposited first (E17). Lucas used in situ X-ray diffraction from localized sites as a function of applied potential to monitor the change in the surface structure of the metal/electrolyte interface, as a result of phenomena such as surface reconstruction, metal growth, and formation of oxide phases. He termed a display of X-ray response vs applied potential an X-ray voltammogram (E18). Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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Feldberg et al. determined the surface pKa of a self-assembled mercaptoundecanoic acid layer on vapor-deposited gold films by a temperature jump method (E19). Ray and McCreery used resonance Raman spectrometry of a covalently bound fluorescein derivative on a glassy carbon surface to investigate the relative abundance of surface hydroxyl vs carbonyl surface sites as a function of surface treatment of the glassy carbon (E20). Morris and co-workers used extended focus reflectance microscopy to assess the fractal dimension of roughened silver surfaces and demonstrated that a threshold fractal dimension needed to be exceeded to observe SERS spectral enhancement, independent of the nature of the adsorbed molecule under study (E21). Plieth and co-workers used locally resolved confocal Raman microscopy in conjunction with a double potential pulse method to prepare and characterize single silver clusters of homogeneous size and spatial distribution (E22). MODIFIED ELECTRODES Modified electrodes include electrodes where the surface has been deliberately altered to impart functionality distinct from the base electrode. Here, deliberately constructed electrode architecture is described, as well as more traditional classes of materials including electron conducting polymers, electroactive materials, monolayers, ion-exchanging matrixes, and nonconducting polymers. The search of Chemical Abstracts yielded 2964 references on modified electrodes, of which
