Anal. Chem. 1996, 68, 379R-444R
Dynamic Electrochemistry: Methodology and Application James L. Anderson
Department of Chemistry, University of Georgia, Athens, Georgia 30602 Edmond F. Bowden*
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695 Peter G. Pickup
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 Review Contents Books and Reviews Mass Transport Microelectrodes Stochastic and Chaotic Phenomena Hydrodynamic Methods Spatial Sampling Methods Analytical Voltammetry Methodologies Stripping Voltammetry Catalytic Methods Chemometric Approaches Heterogeneous/Homogeneous Kinetics Diffusionless Heterogeneous Kinetics Diffusional Heterogeneous Kinetics Theory of Heterogeneous Electron Transfer Homogeneous Kinetics Surface Electrochemistry Theoretical Aspects Underpotential Deposition Mercury Platinum Gold Silver Carbon Other Electrode Materials Modified Electrodes Theoretical Aspects Conducting Polymers and Organic Redox Polymers Ion-Exchange Polymers Zeolites and Clays Metal Complex Films New Modifying Materials Other Bulk Films Self-Assembled Monolayers (SAMs) Langmuir-Blodgett Monolayers Other Monolayers Self-Assembled Bilayers and Multilayers Bioelectrochemistry Small Molecules of Biological Importance Protein Electrochemistry Enzyme Electrodes (Amperometric Biosensors) Polynucleotides and Nucleic Acids Cellular and In Vivo Electrochemistry Immunological and Recognition-Based Electrochemistry S0003-2700(96)00015-7 CCC: $25.00
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© 1996 American Chemical Society
Characterization of Redox Reactions Spectroelectrochemistry UV/Visible Absorption or Reflection Spectroelectrochemical Methods Infrared Spectroelectrochemical Methods Raman Spectroelectrochemical Methods Luminescence and Fluorescence Electron Paramagnetic Resonance Electrochemical Mass Spectrometry X-ray Absorption, Scattering, or Diffraction Methods Instrumentation General Electrochemical Instrumentation Electrochemical Quartz Crystal Microbalance Scanning Electrochemical Microscopy Electrochemical Mass Spectrometry Electrochemical Cell Design Electrode Design Literature Cited
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This review article covers the literature on electroanalytical chemistry for the time period of November 1, 1993 through October 31, 1995. There will be a few exceptions to this time frame, particularly for the October-November transition period separating the preceding review (A1) from the present one. The format of this review is similar to the preceding one. The major focus is on fundamental advances in chemistry and electrochemistry that underlie and impact on the practice of electroanalytical chemistry. This is not a comprehensive review. Rather, we have attempted to critically identify and document the most significant advances and trends that have been reported during the targeted two-year period. It is intended that the citations provided are among the most significant and will serve to provide the interested reader with adequate entry into the primary and review literature. Because this is not a comprehensive review, we have by necessity had to exclude a large number of interesting and worthy publications from the cited literature. By examining the detailed Review Contents at the front of this article, the reader can immediately grasp the main topics and subject areas covered. Clearly, topics of direct electroanalytical significance such as voltammetric analysis, enzyme electrodes, microelectrodes, and instrumentation are covered. Additionally, many fundamental electrochemical topics, such as electrode kinetics, surface electrochemistry, self-assembled monolayers, and redox polymers, are addressed in detail. As in previous reviews, Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 379R
the goal is to include fundamental topics that will have direct or potentially direct impact on electroanalytical chemistry. Electrochemistry that is oriented toward corrosion, fuel cells, batteries, and other such subjects of technological relevance are not included. The present review does not include a separate section on photoelectrochemistry and only an abbreviated section on characterization of redox reactions, but the reader will find references to these topics throughout. Various means were employed by the authors to identify and choose the cited literature. In addition to personal perusal of journals, extensive use of literature searching was conducted using computer-based Chemical Abstracts searching and Current Contents on Disk. BOOKS AND REVIEWS A second edition of the Sawyer and Roberts’ classic has been published with the addition of a third author, Andrzej Sobkowiak, and a slightly altered title, now simply Electrochemistry for Chemists (A2). Although developed with the nonelectrochemist in mind, the first edition has proven to be an invaluable source for the electrochemists among us as well, and the new edition is destined to serve the same purpose. The new edition is fully updated and, as one might expect, contains a wealth of well-organized, pragmatic information about performing electrochemical experiments and interpreting electrochemical data. In order, the chapter topics are as follows: intro/fundamental, potentiometry, controlledpotential methods, titrations, indicator electrodes, cells and instrumentation, solvents and electrolytes, hydrogen species, oxygen species, metallic species, inorganic nonmetals, organic compounds, and organometallics. Among books published during the time period of interest, several edited volumes stand out for providing authoritative review chapters of current interest. Physical Electrochemistry, a new volume edited by Rubinstein (A3), is especially outstanding. This book gets off to a promising start with a 25-page chapter by Rubinstein in which the key fundamental principles of physical electrochemistry are collected and presented in a most concise and lucid fashion. The following 11 chapters are all high-quality contributions dealing with very timely subjects and authored by leading experts. In order they are as follows: Miller on electrontransfer kinetics at metal electrodes, Rudolph on digital simulations with the fast implicit finite difference algorithm, Amatore on microelectrodes, Bard et al. on scanning electrochemical microscopy, Gabrielli on electrochemical impedance spectroscopy, Ward on the electrochemical quartz crystal microbalance, McBreen on in situ synchrotron techniques, Gottesfeld et al. on ellipsometry and spectroellipsometry, Hubbard et al. on analysis of well-defined electrode surfaces, Hodes on electrodeposition of II-VI semiconductors, and Diaz et al. on electronically conducting polymers. The volume does, unfortunately, have one drawback for many who would be interested in adding it to their personal bookshelves: it is fairly high-priced. Lipkowski and Ross have edited a new volume focusing on novel electrochemical materials (A4). Although most of this volume is concerned with electrochemical technology applications, electroanalytically relevant chapters include those of Doblhofer on polymer film electrodes and Baker and Senaratne on clay and zeolite modified electrodes. Volume 18 in Bard’s Electroanalytical Chemistry series was published in 1994 (A5). Although also cited in the preceding 380R
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Dynamic Electrochemistry review (A1), it is worth mentioning again as it contains authoritative chapters by Rusling on electrochemistry in organized microheterogeneous solution media (micelles, microemulsions), by Inzelt on charge transport mechanisms in polymer film electrodes, and by Bard, Fan, and Mirkin on scanning electrochemical microscopy. The fourth volume of the Advances in Electrochemical Science and Engineering series (A6) contains six comprehensive chapters including Galus on nonaqueous and mixed solvent electrochemistry, Samec and Kakiuchi on charge transfer kinetics at the water/ organic interface, and Iwasita and Nart on in situ FT-IR of metal/ electrolyte interfaces, plus contributions on STM of semiconductor electrodes, surface chemistry of silicon, and electrolysis for pollution abatement. The long-running Modern Aspects of Electrochemistry series added a 26th volume (A7) with chapters by Benderskii et al. on interfacial phase transitions and reconstructed surfaces, Rusling again on electrochemistry in microemulsions, as well as several chapters focused on electrochemical technology, i.e., fuel cells, electrogalvanization, aluminum electrometallurgy, and cracking in metals due to corrosion and embrittlement. Two new single-authored texts of modest size became available. The 100 µm diameter) copper electrode to the CE capillary outlet in an easily aligned wall-jet configuration for the detection of sugars in mixtures (B148). Huang et al. have characterized a wall-jet electrode for flow injection analysis. Theory and experiment were in good agreement for ferricyanide and hydrogen peroxide analytes over a wide range of conditions, with the exception of the flow rate dependence of peak current when a packed-bed reactor was inserted between the injector and the detector (B149). (We might note that this behavior is not surprising if injected analytes are at all retained by the packing, due to changes in hydrodynamics and the consequent flow rate dependence in the packed column.) A wall-jet graphite electrode has been used to sense NADH produced by flow injection reaction of L- and D-lactate and 3-hydroxybutyrate with NAD+ at a packed-bed immobilized enzyme column reactor, enabling detection of these three species, although it was noted that the system needed further refinement (B150). Schindler et al. have developed immobilized-enzyme membrane electrodes for oxygen-sensitive continuous-flow analysis of β-D-glucose and L-lactate. The enzyme membranes are stable for periods of weeks to months (B151). Glutamine has been determined in mammalian cell cultures by means of flow injection analysis with a packed-bed enzyme reactor and wall-jet electrode detection of the resultant liberated hydrogen peroxide (B152). Dewald et al. have applied flow injection potentiometric and voltammetric stripping analysis at a cellulose triacetate dialysis membrane-covered mercury wall-jet electrode for determination of lead in environmental and clinical samples (B153). McCreedy and Fielden have reported a segmented ring/disk array of eight independent glassy carbon disk wall-jet electrodes located around a central disk electrode, as an amperometric detector for high-performance liquid chromatography of a series of phenolic compounds (B154). Numerous reports have appeared regarding detection of biologically important species. Ravant et al. have reported the determination of 8-oxoguanine in DNA by HPLC with electrochemical detection, coupled with gas chromatography/mass spectrometry (GC/MS). The electrochemical detection approach enables avoidance of a serious interference due to oxidation during derivatization for the GC/MS analysis (B155). Acetaminophen was determined in urine by direct injection micellar liquid chromatography with a wall-jet cell/carbon fiber microelectrode (B156). NADH was determined by flow injection analysis with detection at a poly(thionine)-modified electrode (B157). LAscorbic acid was determined by flow injection analysis with copper(II)-mediated electrochemical detection (B158). Serum alkaline phosphatase activity was assayed by electrochemical 390R
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detection with flow injection analysis, with minimal interferences (B159). Intercalation of DNA was detected by flow injection analysis coupled with photochemical reaction (B160). Sulfite was determined in wine by flow injection with indirect electrochemical detection based on electrogeneration of iodine and its reaction with sulfite (B161). A gold electrode modified with a self-assembled monolayer of thiols was used as an electrochemical detector for ionic surfactants (B162). An automated thin-layer electrochemical flow-through reactor was used to fabricate and characterize CdTe thin films formed by electrochemical atomic layer epitaxy (B163). Microelectrode Arrays Controlled at a Single Potential. Kounaves et al. have described the microlithographic fabrication of an iridium-based disk ultramicroelectrode array consisting of 19 electrically coupled disks of 10-µm diameter and applied it as a base for formation of a stable, reproducible array of mercury microsphere electrodes. The mercury microsphere array was successfully applied for the analysis of Cd(II), Pb(II), and Cu(II) in neat spring water samples (B164). Uhlig et al. have described an array of three independent noble metal microelectrode arrays microlithographically fabricated on a silicon substrate and plated with mercury amalgam. All microelectrodes in each array are held at the same potential. A multipotentiostat enables simultaneous differential pulse anodic stripping voltammetry with independent deposition potential optimized at each independent microelectrode array for a particular trace metal, minimizing intermetallic interferences (B165). Seddon et al. have described a disposable printed microelectrode array and amperometric sensor for environmental monitoring of trace metals by potentiometric stripping voltammetry. The sensor consists of a network of 1000-3000 carbon disk microelectrodes with recessed geometry on a polycarbonate film, fabricated using thick-film printing, vacuum polymerization of a dielectric, and laser ablation to define the electrode pattern (B166). Wang et al. have described disposable mercury-coated screenprinted microelectrode arrays and applied them for on-site stripping measurements of trace metals in drinking water and wine (B167). Anderson and Groeber have used a microelectrode array as a coarse diffraction grating for sensing electrode reaction products to focus light at a selected detector position, with intensity influenced by the production of an absorbing reaction product (B168). Fiedler et al. have described an intriguing approach for the generation of pH gradients on a micrometer or submicrometer distance scale over arrays of microelectrodes via pulsed potential initiation of electrode reactions that involve proton consumption or generation, with fluorescence microscopic visualization and quantitation. They envision possible applications, including implementation of titration techniques within microcompartments, and visualization of the working state of microelectrode arrays (B169). Composite Electrodes Functioning as Microelectrode Arrays. Fernandez et al. have reported on the use of graphite-poly(tetrafluoroethylene) electrodes as electrochemical detectors of herbicides, phenol, and Zn(II) in flowing systems (B170). These electrodes function as disordered microelectrode arrays. Bartroli et al. have discussed the use of a conductive epoxy-graphite composite as a solid internal reference electrode for a sodium
ion-selective electrode for flow injection analysis (B171). Oungpipat et al. have described the use of a nickel-modified glassy carbon electrode for flow injection detection of tetracyclines by electrocatalytic oxidation (B172). Tessema et al. have reported the flow injection amperometric determination of glucose and other saccharides by composite electrodes consisting of a mixture of enzyme with carbon paste, mediated by benzoquinone or related organic species (B173). Gun et al. have electrochemically and morphologically characterized composite porous gas electrodes consisting of palladium-modified carbon ceramic electrodes based on a mixture of graphite powder percolating through hydrophobically modified silica xerogels (B174). Pankratov and Lev have described a sol/gel-derived, polishable, amperometric biosensor based on enzymes immobilized in porous, organically modified silica-carbon matrices in the presence or absence of tetrathiafulvalene as a mediator. Application to glucose detection is demonstrated (B175). Montgomery and Anderson have described the fabrication of poly(chlorotrifluoroethylene) (i.e., Kel-F)/precious metal composite electrodes. Grinding proved unsuccessful for fabrication of platinum composite electrodes, due to physical and electrical isolation of the particles, but sputter coating of the Kel-F particles with gold prior to compression molding enabled successful fabrication of electrodes having less than 1% active area, which behaved as isolated electrodes on time scales less than 0.1 s (B176). Menon and Martin have described the fabrication and evaluation of nanoelectrode ensembles consisting of gold electrodes with diameters as small as 10 nm, electrolessly deposited in the pores of nanoporous membrane filters. Cyclic voltammetric detection limits are reported to be as much as 1000-fold lower than for large-diameter solid gold electrodes (B177). Wang and Armalis have applied a composite electrode consisting of an ensemble of carbon microdisk electrodes embedded in epoxy for stripping voltammetry. The nonplanar diffusion behavior enabled enhanced metal deposition and preconcentration from quiescent solutions and afforded good precision and detection limits for short deposition times (B178). Fernandez et al. have applied graphite-poly(tetrafluoroethylene) electrodes as electrochemical detectors in flowing systems for the flow injection determination of the herbicides thiram and disulfiram and concomitant species in water samples by both direct measurement and adsorptive stripping voltammetry (B179). Kunitake et al. have described a microelectrode array based on domains in a Langmuir-Blodgett monolayer formed from a binary mixture of an electroactive and an electroinactive component on gold (B180). Johnson et al. have synthesized and characterized a composite material consisting of 0.5% ruthenium in a Ti4O7 matrix which behaves like a well-behaved microelectrode array, showing no rotation rate dependence of the limiting current when applied as a rotating disk electrode for the oxidation of iodide to iodine (B181). Swain and co-workers have investigated properties of highly boron-doped polycrystalline diamond thin-film electrodes, either as grown or after surface treatments, by examining the cyclic voltammetric behavior of a series of analytes of varying charge and structure. The results are interpreted in terms of two models: conventional electron transfer at a p-type semiconductor/ electrolyte interface and electron transfer at a composite electrode composed of an array of non-diamond C impurities with much
higher electron-transfer rate constants than observed for the bulk diamond matrix (B182). Winkler has compared electron-transfer kinetics of the Fe(CN)64-/3- redox system on platinum standard size electrodes and microelectrodes, using fast cyclic voltammetry, steady-state voltammetry, and FFT-square wave voltammetry. He noted that reactant decomposition products partially block the macroelectrode, converting it into a pseudomicroelectrode array, and leading to smaller observed electron-transfer rate constants than observed at microelectrodes (B183). Bond and co-workers have theoretically and experimentally investigated the electrocatalytic reduction of hydrogen peroxide at a stationary pyrolytic graphite electrode surface in the presence of cytochrome c peroxidase. A microelectrode array model for adsorbed enzyme molecules gave a good fit of experimental results (B184). Hutton and Williams have applied photothermal deflection microscopy to visualize a random array of gold disk microelectrodes (B42). Dual Electrodes and Microelectrode Arrays Controlled at More Than One Potential: Interactive. Although the SECM functions in many instances as a dual-electrode assembly, this topic will be discussed elsewhere. Compton et al. have developed the theory for chronoamperometry at dual-channel electrodes in the generator/collector mode (B185) and verified the theory experimentally (B186). Oltra et al. have described an interesting use of a channelflow dual electrode to detect soluble chemical species released by a passivated iron electrode to a downstream-collecting electrode for detection following laser-induced depassivation of the iron (B187). Seddon et al. have investigated the steady-state generator and collector electrode currents at parallel platinum dual-cylinder electrodes as a function of varying gap width for narrow gap widths controlled by a micromanipulator (B188). Wang et al. have described a sensitive and selective determination of riboflavin by flow injection analysis using parallel dual-cylinder microelectrodes of 20-µm diameter, separated by a 2-µm gap, in the regenerative generator/collector mode (B189). Kirchhoff and co-workers have reported the fabrication and application of ring/disk microelectrodes with tip diameters as small as 25 µm. The electrodes exhibit high collection efficiency, due to radial diffusion, which is dominant at slow voltammetric scan rates (B190). Che and Dong have described the application of a generator/collector electrode array consisting of a generator microelectrode sandwiched between two collector macroelectrodes for investigation of homogeneous catalytic EC′ mechanisms. An advantage of the use of the macroelectrodes is the feasibility of studying slower catalytic reactions than the microelectrode alone would be capable of following due to rapid divergent diffusion of products (B23). A number of workers have reported interdigitated array (IDA) microelectrode array sensors based on electrochemical recycling of redox components to enhance sensitivity. Niwa has reviewed fabrication, properties, and electroanalytical applications of IDA microelectrodes (B191). Paeschke et al. have described an IDA with submicrometer dimensions (B192), characterized IDA properties as a function of geometric arrangement (B193), used the steady-state current of enzymatically generated 4-aminophenol at an IDA to determine the enzyme activity of alkaline phosphatase and β-galactosidase (B194), and incorporated four independent IDA devices controlled by a multichannel potentiostat into a single flow cell fabricated in an integrated device by micromachining (B195). Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Montelius et al. have fabricated and characterized IDAs with nanometer sized electrodes and gaps, using electron beam lithography and liftoff techniques, and applied them for admittance spectroscopy of biomolecules. They note that unusually high electric fields can be achieved with small voltage differences imposed across nanometer spacing, which should lead to interesting consequences for electrochemical and other investigations (B196). An example of the possibilities has been given by Wang et al., who have demonstrated the feasibility of trapping bioparticles and cells in the electric fields generated between adjacent electrodes of an IDA (B197). Iwasaki and Morita have reported the use of an IDA with 2-µm gaps as an electrochemical detector for HPLC of dopamine, with excellent (100 pM) detection limits (B198). Niwa and Tabei have investigated the highly sensitive voltammetric detection of reversible and quasi-reversible redox species and characterized the potential limits of utility in both the anodic and cathodic potential regions at a an amorphous carbon-based IDA microelectrode fabricated by pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride on a surface-oxidized Si wafer (B199). Niwa et al. have characterized electrochemically and by Raman spectroscopy carbon-based microband IDAs fabricated by electropolymerization and subsequent pyrolysis of polypyrrole on a platinum IDA substrate (B200). Niwa et al. have also demonstrated the suitability of a carbon IDA for microbore column HPLC, with detection limits lower than 10 fg (B201, B202). Niwa and coworkers have also reported application of an IDA for voltammetric detection of 4-aminophenol in an electrochemical enzyme immunoassay of mouse IgG antibody (B203). Paeschke et al. have applied an IDA to characterize the electrochemical behavior and determine the diffusion coefficient of the redox protein cytochrome c (B204). Nakamura et al. have applied an IDA to determine the diffusion coefficient of acid in a chemically amplified photoresist (B205). Niwa et al. have reported highly selective electrochemical detection of dopamine in the presence of up to 100-fold excesses of ascorbic acid at an IDA modified with layered films of Kodak AQ29D ionomer with Nafion outer layer, exploiting the anion exclusion properties of the Nafion layer to minimize permeation of ascorbate, in addition to the rapid and irreversible oxidation of ascorbate at the anode, minimizing its steady-state amperometric response while amplifying that of dopamine by means of its reversible cycling of dopamine to effect selectivity (B206). Niwa and co-workers have described a clever substitutional stripping voltammetric approach for sensitive detection of reversible redox species that do not readily form a deposit at an electrode, thus ruling out conventional stripping voltammetry. The approach uses a dual cell containing an IDA in contact with the analyte solution and a larger electrode in a second solution in contact with the first cell by means of a salt bridge and containing a species that can form a deposit at an electrode. The analyte is reduced at one set of electrodes of the IDA. When the reduced product molecules reach an interdigitated electrode shorted to the larger electrode in the second cell, they can be reoxidized, by transferring electrons to the larger electrode, with corresponding reduction and electrodeposition of a substitute species on the larger electrode which will be detected in the analytical step. After accumulation, stripping voltammetry is carried out at the larger electrode, enabling a very low detection limit (B207). 392R
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Niwa and co-workers have also effectively exploited to good advantage the geometry of an IDA consisting of alternating optically transparent (indium tin oxide, ITO) and nontransparent (Pt) band microelectrodes on a quartz substrate to increase the efficiency of the Ru(bpy)32+-Fe3+ photogalvanic system by backilluminating the system through the IDA, exploiting the shadow of the nontransparent electrode to physically separate photoinduced active species (B208). Bustin et al. reported the use of an IDA electrode assembly for trace determination of iron in ultrapure spectroscopic carbon and found the approach competitive with graphite furnace atomic absorption for iron concentrations in the low ppm range (B209). Murray et al. have used an IDA to investigate the anisotropy of diffusion in thin lyotropic liquid crystal films (B210) and to facilitate generation and freezing of the concentration gradient in a mixed valent viologen molten salt for investigation of the properties of the gradient-containing film (B211). Belmont and Girault have demonstrated the utility of microband IDAs for electrosynthesis. They have considered the benefits of minimized ohmic losses (B212) and presented results for a platinum microband IDA fabricated with platinum ink screen printed on alumina to facilitate the electrosynthetic epoxidation of propylene (B213), and the electrosynthetic methoxylation of furan (B214), by facilitating mass transfer between electrodes to enable reaction between products of reactions at anodes and cathodes with improved current efficiency, decreased energy consumption, and decreased dependence on bulk mass transport conditions in the reactor. Microelectrode Arrays Held at More Than One Potential: Independent Electrodes. Smyth and co-workers have investigated the performance of a four-channel electrode array for determination of mixtures of metals by flow injection analysis. Simultaneous determination and resolution of species that have overlapping voltammograms is feasible in favorable cases (B215). Mashige et al. have reported the simultaneous determination of catecholamines, their basic metabolites, serotonin, and 5-hydroxyindoleacetic acid in urine by high-performance liquid chromatography using a mixed-mode column and an eight-channel electrochemical detector (B216, B217). Fiaccabrino et al. have described a square array of 100 square, individually addressable platinum microelectrodes. An on-chip multiplexing approach was used to enable all 100 electrodes to be addressed with 22 connections. Voltammograms at individual microelectrodes compared favorably with those at bulk platinum electrodes (B218). Ross and Cammann have described a procedure for fabrication of a microbiosensor array of multiple independent enzymes immobilized in polypyrrole films on individual, independently addressable electrodes of a microfabricated microelectrode array (B219). Paeschke et al. have described an integrated flow cell containing an array of four independent IDA array devices controlled by a multichannel potentiostat (B195). Spatial Sampling Methods. Microelectrode Spatial Probes. Wightman and co-workers have used carbon fiber microelectrodes adjacent to single adrenal medullary cells to spatially sample the concentration profile of exocytotically released vesicular catecholamines from the cells after stimulation. They established the quantal nature of the secretions in packets and used the temporal distribution of the widths of the current pulses due to arrival of
packets to infer that kinetic limitations attributed to unraveling of the vesicle matrix during release were of significance (B220). Stamford et al. have reviewed theoretical and practical considerations constraining the use of fast cyclic voltammetry at microelectrodes to assess spatial and temporal measurements of neurotransmitters in the mammalian brain (B221). Nishidome et al. have used an oxygen microelectrode to map the concentration profile of dissolved oxygen inside and outside biofilms in a rotating biological contactor and thereby determine the oxygen-transfer rate to the contactor. The diffusion layer was found to have a thickness of 70 µm (B222). In a similar vein, de Beer et al. have used microelectrodes with tip diameters of ∼10 µm to measure concentration profiles of chlorine in the micromolar concentration range penetrating into biofilms during disinfection and have established that the concentration profile was the result of simultaneous effects of reaction kinetics and diffusion (B223). Flaetgen and Krischer have developed a general model for pattern formation in electrode reactions (B224). Scanning Electrochemical Microscopy. The level of activity in this area is clearly picking up, and the number of groups using the methodology is growing. Bard’s group has been very active in this area, and a number of the groups working in the area have had important contacts with the Bard laboratory. Bard and coworkers have reviewed the state of the field (B38, B39, B225, B226). Macpherson and Unwin have developed a theory for investigation of dissolution processes at a surface by means of scanning electrochemical microscopy, based on the alternating direction implicit finite difference method (B40). Fahidy and co-workers have modeled the effect of surface roughness on degradation of scanning electrochemical microscope images and introduced a method for restoring the images, suggesting the potential feasibility of resolving features smaller than the tip diameter by using sampling intervals small relative to tip size and using a deconvolution procedure (B41). Macpherson and Unwin have successfully applied a simple theoretical model for a perfect single-crystal surface, based on the assumption that dissolution sites are formed only when the interfacial saturation ratio is below the critical value to account for oscillations observed during the dissolution process induced by a scanning electrochemical microscope microelectrode probe. They have carried out studies at the (100) face of copper sulfate pentahydrate single crystals in regions of the surface where the average dislocation spacing is much greater than the size of the microelectrode probe (B227) and at the (010) surface of (monoclinic) potassium ferrocyanide trihydrate crystals in initially saturated aqueous solutions containing 3.5 mol dm-3 potassium chloride. Dissolution was induced and observed by potential step chronoamperometry as the scanning probe tip, positioned close to the crystal surface, was stepped from a value at which no reactions occurred to a value at which the oxidation of ferrocyanide to ferricyanide occurred at a diffusion-controlled rate. The reaction order was deduced to be second order in undersaturation. Both dissolution kinetics and topographic information could be obtained (B228). Dissolution of silver chloride was also investigated (B229). Toth et al. have investigated the proton concentration profile at the interface of a silver iodide-based ion-selective membrane during corrosion by cyanide solutions of low buffer capacity (B230).
(1) SECM Modulation. Several groups have been investigating hydrodynamic modulation schemes for SECM, which can exploit convective effects due to movement of the scanning microelectrode tip to distinguish between topographical features and localized redox reactivity of a surface, as well as to improve signal/ noise ratio. Heinze et al. have described a “picking mode”, with modulation perpendicular to the object being visualized (B231, B232). Ludwig et al. have described an SECM tip modulation technique suitable for topographic feedback in which the damping of the amplitude of lateral vibrations of the scanning microelectrode tip by hydrodynamic coupling with the sample is optically monitored. Microelectrode current response is slightly but linearly affected by the amplitude of hydrodynamic modulation (B233). Bard and Wipf were recently awarded a patent for another SECM tip modulation scheme with lock-in detection, which greatly increases resolution and signal/noise ratio (B234). (2) SECM Probes of Reaction Mechanisms. SECM has been used very effectively to probe electrochemical reaction mechanisms. In some cases, the SECM results have proven especially valuable for confirmation of mechanistic proposals obtained from other electrochemical methods. Bard and co-workers have investigated the mechanism of electrochemical reduction of a rhenium aryldiazenido complex (B235), the oxidative dimerization of 4-nitrophenolate in acetonitrile (B236), the detection of the electrohydrodimerization intermediate of the acrylonitrile radical anion in DMF (B237), and the nominally thermodynamically uphill electron transfer at the liquid/liquid interface, driven by interfacial transfer of ions determining the interfacial potential difference (B238). (3) SECM Probes of Polymer Films and Enzyme Electrode Membranes. Bard et al. have probed ion transport and electron transfer in polypyrrole films (B239) and assessed the effect of polymer resistance on the electrochemistry of poly(vinylferrocene) (B240). Frank and Denuault have probed the ingress and egress of protons from a polyaniline film (B241). SECM has also been applied to investigation of the phenomena occurring in the immobilized enzyme membrane of enzyme electrodes, including studies of local enzyme activity or concentration profiles in an enzyme layer of a biosensor by SECM in either the amperometric or the potentiometric mode (B242, B243), the local enzyme activity of GOx immobilized in 8-µm-diameter pores of a filtration membrane, or individual whole mitochondria with NADH cytochrome reductase outer membrane enzymes (B244). Conversely, an enzyme microelectrode biosensor can be made the scanning probe of the SECM. This approach has been used to map the distribution of hydrogen peroxide in the diffusion layer during the electrochemical reduction of oxygen at gold and carbon electrodes and to detect hydrogen peroxide produced by the action of another GOx-based enzyme electrode. The resolution and response time are rather poor relative to other scanning microelectrode probes, primarily due to the kinetic sluggishness imparted by the membrane coating of the scanning microelectrode probe (B245). SECM has also been used to probe biological tissues. Unwin et al. have probed fluid flow through porous solids, such as tooth dentinal tubules, with application to the measurement of convective rates through a single dentinal tubule in presence or absence of a blocking agent (B246, B247). White et al. have directly imaged molecular transport through skin (B248). Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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(4) SECM Microfabrication Applications. The fabrication of organic polymer films and semiconductors with spatially ordered chemical modifications has been reviewed, as well as a titanium dioxide/platinum microelectrode array as a prototype of an integrated chemical system (B249). Polypyrrole lines have been deposited on a gold substrate by controlled electropolymerization of pyrrole (B250). Gold patterns have been deposited on ITO by the controlled dissolution of the gold scanning microelectrode tip in the presence of bromide ion (B251). Nickel hydroxide structures have been deposited on platinum surfaces as a byproduct of local pH elevation on the electrode surface coupled with a redox process at the electrode (B252). Circular and linear micropatterns were created by scanning an SECM microelectrode probe across diaphorase-patterned glass surfaces. Oxidation of bromide or chloride ion at the SECM probe tip produced a reactive species that locally deactivated the immobilized diaphorase adjacent to the tip scan path. The catalytic current at the scanning probe tip due to oxidation of ferrocenylmethanol coupled to oxidation of NADH enabled quantitation of the immobilized enzyme concentration (B253). Fluorescent micropatterns were generated in a fluorescent dye ionic conducting polymer film system, by scanning a platinum or tungsten tip at negative potentials (-0.9 to -1.2 V vs Ag/AgCl). The reaction mechanism was elucidated on the basis of the tip electrode potential and fluorescence of the film (B254). (5) SECM Spatial Imaging. Finally, the SECM has been used for the property most evident from its title: the spatial mapping and visualization of surfaces. Titanium dioxide films on titanium in aqueous solutions were mapped to show that the oxidation of iodide, bromide, and ferrocyanide occur at randomly positioned and very sparse microscopic sites with diameters of ∼50 µm. Spatial localization at active sites with more metallike properties than bulk TiO2 was observed for reactions at potentials positive of the flat-band potential of TiO2, but uniform deposition was observed for sufficiently negative potentials (B255). Wittstock et al. have used SECM in conjunction with scanning electrochemical microscopy to characterize the image and topography of various gold electrodes modified by superficial carbon-spray layers. The two microscopic methods provided complementary information to help understand the electrochemical response of the electrode structure and its dependence on the conditions of carbon spray deposition particularly for carbon sprayed on gold minigrid substrates (B256). Wittstock et al. have imaged immobilized antibody layers by saturating the antigen-binding sites with an alkaline phosphataseantigen conjugate, which catalyzes hydolysis of 4-aminophenyl phosphate to form the electroactive product 4-aminophenol, which is detected by the scanning microelectrode tip in the collection mode. The imaging provided by SECM was valuable in optimizing the immobilization of antibodies on glass (B257). Solomon and Bard have imaged the interface between two immiscible electrolyte solutions with glass micropipet tips, achieving resolution comparable to that achieved with metallic electrode tips of the same size (B258). Bard et al. have also investigated the kinetics of electron transfer between ferrocene in nitrobenzene and other redox species in water and the coupled ion transfer at the interface between two immiscible electrolyte solutions (ITIES) and observed an apparently sharp, wave-free liquid/liquid interface between water and nitrobenzene on the submicrometer scale. The electron-transfer and ion-transfer components could be quantita394R
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tively resolved, and estimates of kinetic constants were obtained (B259). Bard and co-workers have also used ion-selective potentiometric microelectrodes as probes to image local concentration profiles of ions including ammonium, potassium, and zinc, in the diffusion layer of a microelectrode during an electrode reaction (B260). Several groups have investigated corrosion of metal surfaces. Tanabe et al. have imaged local distribution of reaction species H+ and Cl- adjacent to corrosion pits on austenitic stainless steels (B261). Wipf has used the SECM to initiate and study localized corrosion pits on stainless steel and aluminum by localized electrogeneration of chloride ions (B262). A number of modes of SECM imaging of metallic biomaterials have been reviewed, including oxygen reaction rate imaging, or metal dissolution product imaging, with deposition of the released species on the SECM scanning microelectrode probe. The SECM can selectively image Faradaic reactions while the surface is in the active, passive, or transpassive state (B263), as illustrated to good effect for a Co-Cr-Mo alloy, which was imaged as a function of applied potential in active, passive, and transpassive states. Scanning electron microscopy and atomic emission spectroscopy of metallic deposits on the scanning microelectrode tip and on the sample surface revealed that the solution composition of dissolved alloy was enriched in Co, while the surface was enriched in Cr and Mo (B264). SECM has also been used to monitor changes in film thicknesses and topography of thin films as a result of deposition processes, including deposition of AgBr on Ag or TiO2 on Ti by surface oxidation, or Cu metal deposition by reduction of copper ions, with resolution in the range of 10 nm to 1 µm, depending on tip diameter (B265). Smyrl and co-workers have described a novel approach to combine scanning electrochemical microscopy and scanning photoelectrochemical microscopy, using an optical fiber coated with gold as the scanning probe, with the fiber introducing light for photochemical excitation, and the gold serving as a ring electrode for the SECM, with both probes interacting with the same spatial zone (B266). Fahidy and co-workers have developed a model to assess and potentially overcome some of the blurring and degradation of scanning electrochemical microscope images due to surface roughness via a deconvolution procedure (B41). Finally, in a tour de force, Fan and Bard have succeeded in pushing SECM to the ultimate limit of single molecule detection, by trapping a single molecule physically between the substrate and the scanning microelectrode, which had a recessed tip allowing confinement of molecules within the volume of solution between the tip and the substrate. In this limit, amperometric response is quantized and stochastic, with current being either finite or zero as the molecule randomly wanders into and away from the sample probe microelectrode, requiring the collection of many events and assessment of their statistical distribution to enable quantitative interpretation of experimental observations (B47). ANALYTICAL VOLTAMMETRY Methodologies. An incredible volume of electroanalytical papers is published each year. Thousands of electrochemical papers were published during the period of this report. A small sampling will be give here. As usual, stripping methods continue to be heavily emphasized, although there are numerous papers published for every conceivable means of controlling and measur-
ing potential and current. In addition, many papers are published each year involving electrochemical detectors in flow systems, including flow injection, liquid chromatography, and capillary electrophoresis or variants thereon. It is likely that increasing automation will continue to become more important in the future. Flow-coupled systems lend themselves nicely to automation and to integration of multiple analytical steps. An important extension of this flow analysis theme is the development of micromachined analytical devices and analytical measurement systems, leading ultimately to a laboratory on a chip. Although this area is still in its infancy, and dominated by the “gee-whiz” phenomenon, it is likely to make a major impact down the road. In addition, there is likely to be major growth in the area of disposable sensors and measurement systems. Economies of scale will be required before such systems are dominant, although a number of researchers are already exploiting commercially available disposable electrode sensor strips. Despite these exciting possibilities on the horizon, many papers seem to repeat the same theme over and over again on the same analytes and the same general methodology, although there are certainly occasional real gems amid the tumult. Kauffmann and Guiberteau have reviewed trends in electroanalytical methods, with emphasis on potentiometry and voltammetry at classical and modified electrodes (C1). Tunon-Blanco and Costa-Garcia have reviewed new trends in the design and application of microelectrodes for electroanalysis (C2). Zhou et al. have reviewed electroanalytical methods for transition metal analysis in heavy metal fluoride melts (C3). Aguei et al. determined the antioxidant Irganox 1076 by differential pulse voltammetry and flow injection with pulsed amperometric detection at a glassy carbon electrode (C4). Stripping Voltammetry. Anodic Stripping Voltammetry (ASV). Feldman et al. have applied ASV at a disposable carbon microelectrode array to determine low blood lead concentrations (C5). Luque de Castro et al. have presented flow injection methods coupled with ASV for determination of Se(IV) (C6), Zn and Cd (C7), and Cu and Pb (C8), and for simultaneous determination of Cu, Pb, Cd, and Zn in lyophilized biological tissues (C9b). Matysik et al. have used a capillary flow injection system and ASV to determine a series of trace metals in tears (C9a). Dalangin and Gunasingham have reported a mercury(II) acetate-Nafion-modified electrode for anodic stripping voltammetry of lead and copper with flow injection analysis. The Hg(II) incorporated into the Nafion film facilitates the preparation of a mercury film for the analysis (C10). Komorsky-Lovric and Branica have studied the effects of surface-active substances in square wave voltammetry and potentiometric stripping analysis of amalgam-forming metal ions (C11). Caruso et al. have used an on-line ASV cell to preconcentrate Cr(VI) and V(V), for determination by inductively coupled plasma mass spectrometry (C12), and As(III) and Se(IV), for determination by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry (C13). Cathodic Stripping Voltammetry (CSV) and Adsorptive Stripping Voltammetry (AdSV). Dobney and Greenway have reported the on-line flow determination of chromium by adsorptive CSV (C14). Harbin and van den Berg have reported the determination of ammonia in seawater using catalytic cathodic stripping voltammetry based on the reaction of NH3 with formaldehyde to form methyleneimine, which adsorbs on a mercury drop electrode at pH 3.8 (C15). Turyan and Mandler have used self-assembled
monolayers of ω-mercaptocarboxylic acid for adsorptive accumulation and square wave CSV determination of ultralow levels of cadmium(II) (C16). Panneli and Voulgaropoulos have reported the simultaneous AdSV voltammetric determination of cobalt, nickel, and labile zinc, using 2-quinolinethiol at a hanging mercury drop electrode in the presence of surfactants without prior digestion (C17). Economou et al. reported batch and flow determination of uranium(VI) by adsorptive square wave stripping voltammetry on mercury film electrodes (C18). Wang et al. have determined remarkably low levels of RNA by coupling its adsorptive accumulation onto carbon paste electrodes with constantcurrent potentiometric stripping analysis (C19). Adeloju and Shaw have described the indirect determination of traces of the surfactants sodium dodecylbenzenesulfonate (C20), hexadecyltrimethylammonium bromide, and cetylpyridinium chloride in industrial and consumer products by adsorptive voltammetry (C21). Catalytic Methods. A wide variety of catalytic methods have been reported. Some examples of the variety of reported methods are presented below. Many of these methods are based on flow injection or chromatographic methodologies. Casella et al. have reported the electrocatalytic oxidation and liquid chromatographic detection of aliphatic alcohols at a nickel-based glassy carbonmodified electrode (C22). Oungpipat et al. have reported the flow injection detection of tetracyclines by electrocatalytic oxidation at a nickel-modified glassy carbon electrode (C23). Harbin and van den Berg have reported the determination of ammonia in seawater using catalytic adsorptive cathodic stripping voltammetry based on reaction of NH3 with formaldehyde to form methyleneimine, which adsorbs on a hanging mercury drop electrode (C15). Cai et al. have reported the flow injection electrocatalytic amperometric detection of hydroxylamine in river water with a palladiummodified carbon paste electrode (C24). Chen et al. have reported the flow injection catalytic oxidation of NADH at a methylene green chemically modified electrode (C25). Zen and Tang have reported the flow injection catalytic amperometric detection of hydrazine at a perfluorosulfonated ionomer/ruthenium oxide pyrochlore chemically modified electrode (C26). Cookeas and Efstathiou have reported the flow injection catalytic amperometric determination of thiocyanate and selenocyanate at a cobalt phthalocyanine-modified carbon paste electrode (C27). Mannino et al. have reported the flow injection catalytic amperometric detection of sugars, H2O2, and cysteine in media such as milk at a cobalt(II, III) oxide chemically modified electrode (C28). Wakabayashi et al. have reported the simultaneous determination of oxidized and reduced coenzyme Q and R-tocopherol in biological samples by high-performance liquid chromatography with platinum catalyst reduction and electrochemical detection (C29). Chemometric Approaches. Brown and Bear have critically reviewed the use of chemometric techniques in electrochemistry, with brief exposition of the relevant background information (C30). Data Analysis. Reviejo et al. have applied partial least-squares regression for polarographic analysis of emulsified mixtures in any combination of four organochlorine pesticides, using a calibration set of 35 samples, with current measurements at nine different potentials (C31). Expert Systems. Garcia-Armada et al. have developed a knowledge-based system for differential pulse polarography. A database of information about possible constituents of the system to be Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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studied (electrolytes, metal ions present, etc.) can be processed to facilitate the best approach for simultaneous multielement analysis with maximum efficiency, interpret the resulting data, and identify the constituents of the sample (C32). Esteban and co-workers have extended the development of an expert system for voltammetric determination of trace metals, which guides the user on choice of sample treatment, and the best choice of voltammetric procedure among the options of differential pulse voltammetry and anodic, cathodic, or adsorptive stripping voltammetry, with either hanging mercury drop or gold electrodes. Provision is made for identification and resolution of overlapping peaks, and quantification by means of the multiple standard addition method, with statistical validation tests. They have reported application of the system to the determination of mercury, selenium, tellurium, and vanadium (C33), the speciation of chromium and arsenic (as Cr(III), Cr(VI), As(III), As(V)) (C34), and the determination of total iron, manganese(II), aluminum, and titanium (C35). Palys et al. have further developed a knowledge-based system for the voltammetric elucidation of electrode reaction mechanisms and applied it to two metallomacrocyclic compounds, a nickel salen derivative, and a binaphthyl-uranyl salen crown ether at a mercury electrode in DMSO. Within one working day, the expert system enabled unambiguous assignment of a unique reaction mechanism (C36). Finally, Balasubramanian et al. have developed an expert system for the design of an electrochemical reactor, using both plug flow and stirred tank reactor models (C37). Experimental Design. Lan et al. have reported a five-level orthogonal array experimental design for optimization of the differential pulse polarographic determination of selenium. The significance of each factor was assessed by means of an analysis of variance, and a response surface was constructed (C38). HETEROGENEOUS/HOMOGENEOUS KINETICS Diffusionless Heterogeneous Kinetics. The fastest growing segment of the electrochemical kinetics literature is that dealing with surface-confined redox couples. Both simple electrontransfer and more complex mechanisms have been considered. The absence of mass-transfer contributions to the rate can serve to unveil the underlying electron-transfer details, which results in data amenable to interpretation by Marcus or other modern theories. This advantage was brought out in several studies in which self-assembled monolayers were used for immobilization of redox sites in more-or-less well-defined geometries and electrontransfer distances. Previous studies, primarily by Chidsey and Finklea, had used chronoamperometry to investigate such reactions. In the current time period, three papers described the use of linear potential sweep voltammetry in conjunction with Marcus electron-transfer theory for such investigations. Contributions from the groups of Murray (D1) and Creager (D2) discussed numerical solutions of the i-E responses in investigations of ferrocene-terminated alkylthiol layers on gold. Nahir et al. (D3) derived an analytic solution for the i-E response, as well as a simple method for determining k°(η) from voltammograms, and gave results for adsorbed cytochrome c adsorbed to COOHterminated self-assembled monolayers. These three studies provide the means for extracting electron-transfer reorganization energies from LSV and CV experiments. Ferrocene-terminated SAM/gold systems, typically diluted twodimensionally, were the target of several other illuminating studies 396R
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of fundamental electron-transfer kinetics. Hockett and Creager (D4) found that electron-transfer kinetics were nearly independent of film permeability and, therefore, presumably the defectiveness of the film. Richardson et al. (D5) examined ferrocene-substituted octanethiol films on gold and silver electrodes at cryogenic temperatures, 115-170 K, and measured the reorganization energy. Comparable experiments were also carried out for ferrocene-substituted dodecanethiol films at cryogenic temperatures (D6). The impact of kinetic dispersity, discussed in both of these papers, was the subject of a more in-depth analysis by Rowe et al. (D7). Smalley et al. (D8) gave a detailed report of the distance dependence of electron-transfer rate in ferrocene SAMs over a wide range of polymethylene spacer length. A laser-induced temperature jump method allowed fast rate measurements to be made for number of methylenes (n) in the 5 e n e 9 range. Carter et al. (D9) also examined the distance dependence of interfacial electron-transfer in ferrocene-terminated SAMs, in this case at cryogenic temperatures. Guo et al. (D10) examined electron-transfer as a function of SAM thickness in SAM/LB bilayers in which the ferrocene was confined to the interface between the two layers. Similar tunneling decay parameters were found in the preceding three studies. Guo et al. (D10) also found evidence for weaker electronic coupling in their system as a result of the noncovalent attachment of the ferrocene to the alkanethiol SAM. Cruanes et al. (D11) reported the first high-pressure investigation (up to 1000 atm) of ferrocene-SAMs, finding evidence for steric constraints on the electron-transfer process. Peck et al. (D12) measured the electron-transfer kinetics for ferroceneSAMs on ultrathin gold overlaid on a high-temperature superconducting material, Tl2Ba2CaCu2O8, at temperatures above and below the transition temperature, Tc. Although ferrocene was the kineticists’ choice during the last two years, it was not exclusively so. Ravenscroft and Finklea (D13) continued the research from this group on rutheniummodified SAMs by examining electron-transfer kinetics in several nonaqueous solvents. No correlation with solvent relaxation times was observed, apparently because of increased monolayer disorder and the presence of water at the interface. Also of significance are the pair of in-depth papers by Forster and Faulkner (D14) on spontaneously adsorbed monolayers of bis(bpy)chloro complexes of osmium. Chronoamperometry was used to determine the electron-transfer kinetics, which were interpreted using Marcus theory. One perplexing result was the observation that the electron-transfer kinetics are senstive to solvent dynamics while at the same time being nonadiabatic, which is a result not predicted by theory. Jaworski and Kebede (D15) also described their continuing work with surface-confined pentaaminecobalt(III) complexes attached to mercury electrodes. Electron-transfer rate constants measured as a function of temperature in water/acetone mixtures suggest solvent friction effects are important in the electron-transfer mechanism. The use of square wave voltammetry (SWV) for the measurement of quasi-reversible standard electron-transfer rate constants of surface-confined redox couples was the subject of three papers by Komorsky-Lovric and Lovric (D16-D18), who investigated the irreversibly adsorbed azobenzene/mercury system. The kinetics of more complex surface systems was examined in new papers from Laviron. CV and polarographic analysis of solution reactants involving strong adsorption of the reactant was
described for the case of simple electron transfer and a Langmuir isotherm (D19). Polarographic theory for chemically coupled redox reactions, in which all reactants and products are strongly adsorbed at submonolayer coverage according to Langmuir isotherms, was described for several cases involving followup chemical steps (D20). Xie and Anson (D21) discussed the extraction of kinetic parameters for surface-confined redox catalysts in the absence and presence of substrate. Diffusional Heterogeneous Kinetics. The papers in this subsection are roughly divided between those addressing particular chemical systems and mechanisms and those addressing techniques. First we consider the mechanistic papers. Miller’s group continued their elegant studies of electron transfer of solution redox species at insulating self-assembled monolayer electrodes with a systematic study of ligand substitution in ferricyanide (D22). Progressive replacement with bipyridine ligands decreased the reorganization energy, in agreement with Marcus theory, and revealed an interesting electronic coupling effect at these hydroxy-terminated SAMs. Methyl-terminated SAMs were employed by Xu et al. (D23) using the same methodological strategy. Distance-dependent electron-transfer rates for ferricyanide and ferric ion resulted in a consistent value for the tunneling decay constant, i.e., 1.0 per methylene unit. Several other papers addressed fundamental mechanistic features of interfacial electron-transfer kinetics of diffusing electroreactants. Activation barriers due to solvent reorganization effects was probed by Hecht and Fawcett (D24) for the reduction of CrIIIEDTA at mercury electrodes, by Urbanczyk et al. (D25) for the reduction of the tris(acetylacetonato)Cr(III) at mercury, and by Hupp and Zhang for several fast redox couples at HOPG electrodes (D26). This latter study makes use of low-defect HOPG to slow down the electron-transfer rates of normally fast reactants to readily measureable rates, a strategy related to that advocated by Miller for SAM/gold electrodes. Outer-sphere reorganization also played a dominant role in a CV study of decamethylferrocene at cryogenic temperatures by Richardson et al. (D27). This study represents the lowest temperatures ever employed in electrochemical kinetic studies of diffusional reactants in liquid solvents. Inner-sphere contributions to electron-transfer kinetic barriers were examined by Crawford and Schultz (D28) for an interesting series of metal complexes, M(tacn)23+/2+ (M ) Fe, Co, Ni, Ru; tacn ) 1,4,7-triazacyclononane), that exhibit similar outer-sphere activation energies but a range of redox-induced changes in metal-nitrogen bond distance. Structural effects were also the subject of Zhang et al.’s study (D29) of the cis and trans isomers of (O)2ReV(bpy)(py)2+ at HOPG electrodes, in which electrochemical rate measurements along with X-ray and Raman data were used to interpret the 3-fold higher rate of the trans species. Bond et al. (D30) investigated the dependence of electrontransfer rate on overpotential for the reduction of 2-methyl-2nitropropane in acetonitrile at the mercury electrode. A mild parabolic dependence of log(k°) on η was found and judged to be compatible with Marcus theory predictions. Such effects are, however, decidedly less pronounced and difficult to obtain on diffusional systems relative to surface-confined cases. McCreery’s group continued to advance our knowledge of interfacial kinetics at a variety of carbon-based electrode materials. Low-defect HOPG appears to be a viable alternative to alkanethiol SAMs for slowing down the rates of fast redox reactions, apparently due to a low density of electronic states and carriers
(D31). The rates of 18 redox systems showed a correlation with homogeneous self-exchange rate constants, consistent with Marcus theory. McCreery’s group also explored the role of specific surface chemistry at carbon electrodes in a study of several model transition metal complexes at modified glassy carbon surfaces (D32). Laser activation of carbon microdisk electrodes was also investigated with particular emphasis on temporal changes in k° following activation (D33). Ferri/ferrocyanide is certainly an intriguing beast. Although considered by many to be a “simple” redox couple, most electrochemists appreciate the complexity that can accompany heterogeneous electron transfer with this system. Winkler (D34) and Beriet and Pletcher (D35), reporting on its reaction at Pt microelectrodes (D34, D35) and macroelectrodes (D34), both find evidence in certain situations for the formation of surface decomposition products that impact on measured electron-transfer rates. The important role of counterion association and catalysis was also explored in the latter paper (D35) as well as in a related microelectrode paper (D36) addressing the ruthenium hexaamine and iridium chloride redox couples in addition to iron hexacyanide. In a similar vein, Campbell and Peter (D37) found a first-order kinetic dependence on potassium ion for ferri/ferrocyanide electron-transfer at gold electrodes, due to ionic association. Ion effects of a different sort were revealed in a study by Fawcett and Yee (D38) in which they found the rate of reduction of Pb(II) at mercury in DMF was controlled by the tetraalkylammonium present in solution. The kinetics of charge transfer between the conducting polymer electrode surface poly(N-methylpyrrole) and a selection of transition metal complexes were interpreted by Maksymiuk and Doblhofer in terms of a bimolecular kinetic model (D39). We now turn to those papers dealing with methodology and techniques for examining the heterogeneous electron-transfer kinetics of diffusing redox reactants. Much attention continues to be directed toward the measurement of very fast electrontransfer rates. First we consider microelectrode techniques, for which an informative review was previously cited (A10). Among the newer advances is the microjet electrode (MJE), a hydrodynamic electrode featuring a high-velocity jet of solution impinging on a disk microelectrode. Developed by Unwin’s group (D40, D41), this electrode offers well-defined and high rates of mass transfer for measuring fast kinetics as well as the unique ability to use the microelectrode for imaging of the impinging jet with regard to mass transfer. Compton’s group reported on the application of their microband high-velocity channel electrode to heterogeneous rate measurements finding a value of 0.30 cm/s for benzoquinone in acetonitrile (D42). Baars et al. (D43) have described the application of the dropping mercury microelectrode (DMµE) for polarographic determination of rate constants as large as 0.5 cm/s. Polarogram acquisition can be performed in the subsecond regime due to exceedingly short drop times. Results of electrochemical impedance spectroscopy (EIS) of the ferri/ ferrocyanide reaction at platinum microelectrodes were reported by Bruce et al. (D44). The measurement of electrochemical reaction rates using transient methods at macroelectrodes was examined by KirowaEisner et al. (D45) in a study of Cd(II) reduction at mercury. They critically compared three different coulostatic and galvanostatic techniques. New techniques of general applicability that were reported include harmonic impedance spectroscopy (D46), which Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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makes use of second and higher harmonics, and Fourier transform SWV (D47), which utilizes a repeated square wave cycle on each step of the staircase. It is claimed for the latter technique that data comparable to that from ac voltammetric experiments can be obtained in shorter times with less expensive instrumentation. On the LSV/CV front, Myland and Oldham (D48) derived an analytic solution to the problem of a linear potential sweep, quasireversible reaction, while Paul and Leddy (D49) showed that there is something useful that can be done with the isopoints obtained when diffusional CVs are obtained over a range of sweep rates, i.e., determining the transfer coefficient, R. Horrocks et al. (D50) used the SECM to measure the electrontransfer kinetics of outer-sphere couples at semiconductor interfaces in the dark. The SECM appears to hold some notable advantages with respect to conventional electrochemical methods for making these measurements. Very small rate constants in the 10-16 cm/s range were reported for ruthenium hexaamine at p-WSe2. Hutton and Williams (D51) employed photothermal imaging with a confocal scanning laser microscope in an investigation of localized reaction dynamics. Reaction hot spots and electrode/shield edge effects were observed for ferrocyanide oxidation at a gold electrode. Rusling and Nassar (D52) applied normal pulse voltammetry theory to the problem of measuring heterogeneous electrontransfer rate constants for myoglobin inside a liquid crystal film of didodecylidimethylammonium, for which the signal due to electrolysis of the diffusing reactant is small relative to the background current. Theory of Heterogeneous Electron Transfer. Kornyshev et al. (D53) investigated the possibility of the electronic overlap integral being overpotential dependent for nonadiabatic electrontransfer reactions at metal electrodes, i.e., a potential-dependent transmission coefficient. Low-density metals or long-range electrontransfer situations were predicted to be the most likely candidates for detecting this effect. Rose and Benjamin (D54) developed a molecular model of electron transfer at the solution/electrode interface for predicting both nonadiabatic and adiabatic reaction rates. The ferric/ferrous system was examined. Straus et al. (D55) also examined the ferric/ferrous reaction at Pt(111) for the adiabatic case using a molecular dynamics computer simulation method. The manner in which water is treated, i.e., classical vs quantized, was found to be a critical issue. MD computer simulations were also employed by Xia and Berkowitz (D56) in a study of changes in solvation and reaction energetics due to ion transfer from bulk to the interface. Benderskii and Grebenshchikov (D57) considered reorganization phenomena involved with electron/proton transfer and derived rate constants pertaining to hydrogen ion discharge reactions. Homogeneous Kinetics. General approaches to accurate, rapid simulation of electrochemical reactions with coupled homogeneous kinetics exhibiting wide dynamic range is a challenging goal that was the subject of several papers. Mocak and Feldberg (D58) described the Richtmyer modification of the fully implicit finite difference algorithm for simulating electrochemical processes covering a range of the dimensionless diffusion parameter from 10 to 1020. The already well-known DigiSim simulator for cyclic voltammetry of a diverse number of mechanisms was explained and discussed in an A-page article of this journal by Rudolph et al. (D59). Bieniasz previously introduced the adaptive moving grid strategy for simulating electrochemical processes, 398R
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and in three subsequent papers, this strategy has been further developed and applied (D60-D62). Electrochemical situations examined include extremely thin reaction layers at electrodes, e.g., fast EC mechanism involving homogeneous dimerization (D60), discontinuous boundary condition situations involved in potential step and potential sweep experiments (D61), and the case of extremely thin reaction layers removed from the electrode surface (e.g., electrogenerated chemiluminescence) (D62). A prepeak method for determining very rapid second-order homogeneous kinetics following electron transfer was described by Parker et al. (D63) for several mechanisms involving rate constants ranging from 106 M-1 s-1 upwards to the diffusion limit. Continued use of microelectrodes in investigations of fast homogeneous reactions was evident. From Bard’s group, the SECM was successfully employed in the detection of the acrylonitrile radical anion in dry N,N-DMF, which dimerized with a rate constant of 6 × 107 M-1 s-1 (D64). A similarly rapid rate, 1.2 × 108 M-1 s-1, was measured with the SECM for the dimerization of the phenoxy radical generated by 4-nitrophenolate oxidation at a gold microelectrode (D65). Collinson et al. (D66) measured ion annihilation rates in the 109-1010 M-1 s-1 regime for electrogenerated chemiluminescence reactions of substituted anthracenes and Ru(II)tris(bipy). High-frequency generation was made feasible by employing microelectrodes. Other examples of microelectrode use for exploring homogeneous reactions include investigations of the EC′ catalytic mechanism. Che and Dong (D67) described triple- and twin-electrode configurations for characterizing the EC′ catalytic mechanism under pseudo-first-order conditions and verified their kinetic model with test systems. Tutty gave a theoretical model for the secondorder steady-state kinetic case at a disk microelectrode (D68). Microband channel electrodes were developed and applied by Rees et al. (D69) for the study of ECE processes with homogeneous rate constants as large as 105 M-1 s-1. The reduction of o-bromonitrobenzene in DMF was examined with this hydrodynamic microelectrode and was shown to obey an ECE mechanism with a homogeneous rate constant of 250 s-1 (D70). Compton’s group has been very active in developing channel electrodes in macroscopic variations as well as the microversions. Reports during this time period included a photoelectrochemical investigation of the o-bromonitrobenzene species using channel electrode voltammetry (D71). The authors were able to distinguish a photoECE mechanism from other possible mechanisms such as photoECEE or photo-DISP. A theoretical treatment of chronoamperometry transients at double-channel electrodes was also described (D72). Laviron reported new studies of chemically coupled electrode reactions. In one study, part 16 of his series on equilibrium protonation-coupled electrochemistry, a 1e-, 2H+ electrochemical reaction was considered in which the reaction products undergo irreversible dimerizations (D73). In another theoretical study, of the EC mechanism involving an irreversible first-order chemical step, the possibility of reactant adsorption was considered and the benzidine rearrangement was examined (D74). There was also a theoretical study of the kinetics of dissociative reductions of methyl halides in polar solvents by German et al. (D75). A comparison of predictions with experimental results revealed good correlation with methyl chloride and methyl bromide but not for methyl iodide.
SURFACE ELECTROCHEMISTRY Although surface microscopy and surface spectroscopy have played an major role in the development of surface electrochemistry, they will not be covered in this review. The surface analysis aspects of surface electrochemistry have been recently reviewed (E1, E2) and special issues of Electrochimica Acta have been devoted to “Surface Structure and Electrochemical Reactivity” (E3) and “Nanotechniques in Electrochemistry” (E4). Trasatti (E5) has critically discussed the relationship between results from ultrahigh vacuum and electrochemical experiments. Conway (E6) has reviewed oxide film formation at noble metals. Theoretical Aspects. There has been continued development in the theory for the voltammetry of systems involving adsorbed species. Laviron (E7) has modelled polarography and linear-sweep voltammetry of a redox system when both O and R can be adsorbed, using the results of a previous rigorous theoretical treatment for rotating disk voltammetry. Although the new treatment is semiquantitative, it provides valuable insight into the mechanisms of reactions involving adsorption and allows the advantages of non-steady-state methods to be exploited. Bhugun and Saveant (E8) have developed a model for the selfinhibition of irreversible electrochemical reactions due to blocking of the electrode surface by adsorbing or electrode-derivatizing species. The decrease in the voltammetric current upon repetitive cycling and the peak characteristics during the first cycle are described as a function of a single dimensionless parameter that is a measure of the competition between the surface and solution reactions and of the matching between the diffusion flux of the substrate and the area occupied by the inhibiting species. Cyclic voltammograms for systems with kinetically controlled electrosorption have been numerical simulated (E9). The new treatment includes more parameters and a more rigorous solution of the kinetic equations arising from the Frumkin isotherm. A comprehensive and rigorous analysis of normal pulse polarography for labile metal-complex systems with ligand and complex adsorption has been reported (E10). Cases in which there is not excess ligand were treated, and corrections to the DeFord-Hume procedure are discussed. The adsorption and behavior of organic species on electrode surfaces has continued to attract much attention from theoreticians. Nikitas (E11) has presented a simple lattice statistics model based on the mean field approximation to treat the adsorption and coadsorption of neutral organic molecules and their reorientation. The simplified approach taken in this work makes it applicable to a wide range of situations, without losing an almost quantitative agreement with experimental data. This treatment was extended in a following paper (E12) to cover the aggregation of molecules on the electrode surface to form oligomers or surface micelles, and phase transformations within the adsorbed monolayer. The combined simplicity and effectiveness of these models makes them of value for both pedagogical and research purposes. Nikitas (E13) has also reported adsorption isotherms for neutral organic absorbates, derived from generalized expressions for chemical potential from a combination of classical thermodynamics and the electrostatic theory of dielectrics. The results are claimed to be more general and more reliable than results obtained from a particular model. Electron-transfer inhibition by adsorbed aliphatic alcohols on Hg has been studied using a semiempirical quantum chemical approach (E14). Theoretical calculations were made for a single
alcohol molecule interacting with an Hg28 cluster and compared with experimental data from the literature. The results indicated a vertical orientation of adsorbed alcohol molecules, even at incomplete coverage of the electrode surface. Preliminary results of a molecular dynamics study of the adsorption of aromatic hydrocarbons on metal electrodes have been reported (E15). A modified electrostatic model of water/dipole interactions at charged interfaces takes into account the internal dipole structure and both surface charge/dipole and dipole/dipole interactions (E16). The capacitance of the charged interface is shown to be sensitive to the dipole structural parameters, demonstrating the necessity of accounting for the charge distribution within the solvent molecule. The application of cyclic voltammetry to two-dimensional phase transitions has been analyzed (E17). At low scan speeds a general limiting behavior that is independent of whether nucleation under potentiostatic conditions is instantaneous or progressive is obtained. Analytical criteria for determining whether a phase transition is involved are given. The model has been used to analyze experimental data for two-dimensional phase formation by methyl viologen cation radicals on Hg (E18). Conway (E19) has discussed the role of changes in solvation energy in specific ion adsorption at electrodes. The importance of solvation effects is demonstrated by comparing anion adsorption at metal/water and air/water interfaces. It is concluded that they can be of comparable importance to the electronic interaction between the ion and the metal. Lamperski has used MPB5 (modified Poisson-Boltzmann equation) theory to assess the role of the diffuse layer in anion adsorption (E20). It is shown that at sufficiently strong negative charges of the diffuse layer, anion adsorption is enhanced by the diffuse layer while in the remaining range of charges anion adsorption is inhibited. There have been a number of theoretical reports on metal deposition. Blum et al. (E21) have extended their equilibrium statistical mechanics study of phase transitions at electrode interfaces to include reaction and diffusion kinetics. The case of Cu/Au(111) underpotential deposition (UPD) is treated as a sequence of three uncoupled phase transitions. Parsons (E22) has shown that application of the approach to monolayer adsorption developed by Frumkin to the adsorption of metal monolayers (UPD) is equivalent to the approach normally used for specific ionic adsorption. Explicit expressions for the deposition of Hg droplets on Pt have been derived and used to simulate the spacial distribution of nuclei (E23). The results were in qualitative agreement with experimental light-scattering results. The hydrogen waves on the low index planes of Pt singlecrystal electrodes have been qualitatively explained using Huckel calculations and a free-electron model (E24). Armstrong (E25) has discussed the impedance of a system where there is a discontinuous change with potential due to the formation of a two-dimensional condensed phase. A previous model developed by Wandlowski and Delevie is criticized for not taking into account the discontinuity of the system. Wandlowski and de Levie have responded to this criticism (E26). Underpotential Deposition. The effects of anions on UPD have attracted considerable attention. Lipkowski and co-workers (E27-E30) have published a series of papers on the coadsorption of Cu and various anions on Au(111). The compositions of the Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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deposited overlayers were determined from charge density measurements, allowing a comprehensive description of the effects of anions on Cu UPD to be developed (E28). For Cl-- and Br-containing solutions, a bilayer of Cu atoms and halide ions is formed, while sulfate forms a mixed monolayer with the Cu atoms. Abruna and co-workers (E31) have shown that the UPD of submonolayer amounts of Cu induces an enhanced adsorption of chloride and bromide on Pt(111). On the basis of further electrochemical work and in situ surface EXAFS and X-ray standing wave studies (E32) they have proposed a mechanism for the UPD of Cu on Pt(111) which involves the transient formation of a Cu-anion adlayer. This in turn gives rise to a Cu monolayer which itself is covered by an anion adlayer. A similar model has been proposed by Markovic et al. with support from a variety of techniques including a rotating ring/disk study (E33). Valuable corroboratory evidence has been obtained from EQCM studies (E34, E35). The effects of anions on the UPD of Tl on Pt(111) have also been investigated (E36). Solvent effects and the effects of organic adsorbates on UPD have also been investigated. Scherson and co-workers (E37) obtained considerable new insight into UPD phenomena by investigating four different UPD systems in a variety of solvents. For example, differences in the UPD of Cd on Ag(111) in propylene carbonate and water were ascribed to partial discharge of Cd adions. Large distortions of UPD voltammetric features in low dielectric constant solvents were ascribed to ion pairing. Abruna and co-workers (E38, E39) have investigated the effects of competing organic adsorbates on the UPD of Ag on Pt(111). Adsorbates bonding through a nitrogen heteroatom significantly hinder both the Ag UPD and bulk deposition processes, while sulfur-containing adsorbates completely inhibited Ag UPD. This behavior indicates a higher bond strength between the sulfur atom and the Pt surface than between ring nitrogens and the Pt surface, consistent with the expected strengths of adsorption. A Pt/Ag/adsorbate intermediary structure is proposed based on the voltammetric evidence. A multitechnique approach has been used to probe the effects of chloride, thiourea, acetonitrile, and poly(ethylene glycol) on the formation and properties of UPD Cu on Pt (E40). The observed surface coverages, electrosorption valencies, and optical properties of the UPD Cu adsorbates are interpreted in terms of their charge and size distribution and adsorbate/substrate interactions. A number of reports on electrocatalysis at UPD-modified electrodes have appeared. Some of these are reviewed here while other are included in the section on Pt electrodes. Papoutsis and Kokkinidis (E41) found that the reduction of nitroimidazoles and a nitropyridine N-oxide on Pb or Tl UPD-modified Au proceeded via different mechanisms depending on the UPD adatom coverage. With almost complete adatom coverage, the reduction proceeds through the dihydroxylamine with no adsorption, while on partially covered surfaces the reaction proceeds through chemisorption of the nitro group in a bridged Au-N-O-M structure (M ) Pb or Tl). Other reports of electrocatalysis by UPD-modified electrodes include the reduction of 2,4-dinitrosoresorcinol and related compounds at Pb-, Tl-, and Bi-modified Pt and Au electrodes (E42), the reduction of nitrate at Pd-modified Au (E43), and the selective oxidation of lactose to lactobionic acid at Pd-modified Pt (E44). 400R
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The inhibition of oxygen reduction at Pt(111) by UPD Cu has also been studied (E45). A thorough voltammetric and XPS study of the UPD of Sn on Pt by Lamy-Pitara et al. (E46) is timely given the conflicting reports of the electrocatalytic effects of this system. Some of the Sn adatoms were found to diffuse into the Pt bulk (to a depth of 200 Å) as Sn(II) and could not be desorbed. This unusual behavior may be responsible for the variable catalytic activity. The UPD of Ag on a polycrystalline Pt electrode can produce a Pt/Ag surface alloy by a place-exchange mechanism (E47). This process occurs at room temperature on a time scale of minutes, reaching the steady state in about 1 h. Partially charged metal ions have been shown to coexist with completely discharged metal atoms in UPD deposits of Hg, Tl, Bi, and Cu on Au (E48). It is concluded that the adsorption of partially charged metal ions in UPD is quite general. Results of a voltammetric and potential step investigation of the UPD of Cu onto Au(111) have been interpreted using a classical two-dimensional phase transition model (E49). However, it was found to be necessary to take surface imperfections into account by adding a term for adsorption of Cu atoms at surface defects (E50). Studies of Cu deposition on stepped Pt (E51) and Au (E52) have been used to further elucidate the role of surface defects in UPD. Mercury. The adsorption of organic species on Hg electrodes has continued to attract significant interest. Saba (E53) studied the coadsorption of 1-butanol and thiourea using a Zn(II) ion probe. Butanol inhibits Zn(II) reduction while thiourea accelerates it, and these effects were maintained in mixed layers. Additivity of thiourea and butanol adsorption was found. A differential capacitance study of the coadsorption of thiourea and thiocyanate ions on Hg has been used to probe the influence of organic species on anion adsorption (E54). Thiocyanate adsorption is enhanced by the thiourea at low thiocyanate concentrations but inhibited at high thiocyanate concentrations. A structural model for the coadsorbed layer is proposed based on a comprehensive data set from 72 solutions. The formation of anodic films of thiourea on Hg has also been investigated by chronoamperometry (E55). The transients can be well described theoretically with a transition from two-dimensional to three-dimensional nucleation. Adsorption isotherms and adsorption kinetics of the cis and trans isomers of the 2-alkyl-5-hydroxy-1,3-dioxane nonionic surfactants have been measured by ac polarography (E56). For the trans isomers, the experimental adsorption isotherms can be adequately described by the localized Langmuir model with negligible lateral interactions. On the other hand, in the case of the cis isomers, attractive lateral interactions play a considerable role, resulting in Frumkin-type isotherms. The kinetics and thermodynamics of thioglycol adsorption on Hg ultramicroelectrodes was investigated by fast sweep rate cyclic voltammetry (E57). Data were fitted to a theoretical model (E9) based on the Frumkin isotherm. The redox properties of ubiquinone adsorbed on Hg have been investigated at submonolayer coverages (E58). Differential capacitance measurements have been used to investigate the adsorption of cetylpyridinium (E59) and a homologous series of N-alkyl pyrimidinium salts on Hg (E60). At concentrations below the critical micelle concentration, cetylpyridinium cations form a monolayer film of surface micelles at positive potentials. At higher concentrations, a bilayer and then
a polylayer are formed. At -0.95 V the layers are reduced and collapse into a compact layer. Similar behavior was observed with the N-alkyl pyrimidinium cations. Pospisil and Svestka (E61) deduced from measurements of interfacial capacitance over long times that adsorption of β-cyclodextrin results in the formation of compact multilayers by an instantaneous nucleation and growth mechanism. Cyclodextrin was found to influence the electrochemistry of N,N ′-dialkyl-4,4′bipyridines (viologens) at Hg by forming inclusion complexes which adsorb on the electrode surface. The adsorption of nonanoic acid at Hg has been studied by means of capacitance and surface tension measurements (E62). The results suggest that the acid is adsorbed in a partially coiled conformation in a disordered network, excluding the occurrence of a two-dimensional phase transition involving the formation of a two-dimensional solidlike film. Adsorption of the two-tailed surfactant, bis(2-ethylhexyl) sodium sulfosuccinate at Hg has also been investigated (E63). Compilations of data for the adsorption of 38 aromatic compounds on Hg have been analyzed (E64). In addition to its aromaticity, the hardness of the compound must be taken into account to explain its adsorption behavior. A study of 23 solvents at Hg (E65) has revealed that capacitance minima depend mainly on the size and dipole moment of the solvent molecules. Interestingly, water behaves consistently as a zero carbon chain homologue of the simple aliphatic alcohols. Chen and Abruna (E66) were able to observe the potentialinduced reorientation of a phospholipid monolayer on Hg by rapidscan cyclic voltammetry. The tails-down structure is the more stable one at potentials of up to -0.80 V. Cycling of the potential to about -1.20 V for an electrode with the monolayer deposited in a heads-down fashion results in a complete reorientation of the monolayer to the tails-down configuration. Anson and co-workers have found that adsorption can plays a significant role in the electrochemistry of various inorganic complexes at Hg. The high electrocatalytic activity of Ni(cyclam)2+ toward the reduction of CO2 at Hg was found to be due to reductive adsorption of Ni(cyclam)+ (E67). CO, the primary product, was found to inhibit the catalytic activity. Formation of an insoluble Ni(cyclam)CO film was proposed. On the other hand, the poor electrochemistry of multiply charged heteropolytungstate anions on Hg electrodes is due to extraordinarily strong adsorption (E68). The adsorbed layer can interfere with the electrochemistry of complexes in the solution, resulting in self-inhibition. The adsorption of heteropoly and isopoly anions on Au has been studied in more detail using an EQCM (E69). Wandlowski and de Levie (E70) have continued their work on the effects of absorbed tetrabutylammonium ions at the Hg/water interface on electron-transfer kinetics. The reduction of hexamminecobalt(III) is strongly affected by the presence of a condensed tetrabutylammonium film. When the anions present are fluoride and/or tetrafluoroborate, electron transfer through the film proceeds via an inner-sphere mechanism; i.e., the film must be opened so that hexamminecobalt(III) can make direct contact with the metal. In the presence of bromide anions, an outer-sphere pathway is indicated, probably involving bridging by bromide ions. Previously derived equations accounting for the coupling between double-layer charging and reactant adsorption in chronocoulometry have been tested for the Cd(II), KBr/Hg system (E71). The method can be used to obtain the surface excess. An
isotherm allowing correlation between reactant and ligand adsorption is proposed. In a later paper (E72), a statistical thermodynamics approach is used to compare competitive adsorption and surface complexation models. Both models lead to the conclusion that the anionic tricoordinate metal complex CdBr3- and the tetracoordinate CdBr42- are the adsorbed species on the electrode surface. In other work, the adsorption of PbBr2 on Hg was found to occur via the surface complexation mechanism (E73). The maximum surface concentration of adsorbed PbBr2 is linearly correlated to the surface concentration of initially adsorbed bromide ions. The dissolution of anodic films of HgS has been investigated by analyzing current/time transients (E74). A model involving dissolution of adclusters at their rims into admolecules that diffuse across the surface and are reduced and desorbed was proposed. The dissolution of an HgS multilayer was found to proceed via two-dimensional progressive hole nucleation and growth. The anodic deposition of the first monolayer of calomel has been investigated (E75). A new theoretical model for twodimensional nucleation and growth has been developed that considers termination of nucleation due to a drop in monomer supersaturation. The surface structure of thin Hg films on glassy carbon, and changes associated with anodic stripping voltammetry, were studied by voltammetry and optical microscopy (E76). The initially smooth thin films, consisting of very small ( Au(111)-(px O ¨ 3) > Ag(100) > Ag(111) > Hg. Two-dimensional phase transitions have been demonstrated in thymine adlayers on Ag single-crystal electrodes (E138). Carbon. Recent work on carbon electrodes has been concerned primarily with the effects of various surface pretreatments. McCreery and co-workers (E139) found that the electrontransfer kinetics of aquated ions (e.g., Fe3+/2+) are catalyzed by surface CdO groups, while those of the other redox systems such as [Fe(CN6]3-/4- are not. They suggest that hydrogen bonding between a complexed water molecule and a surface CdO group may promote electron transfer. McCreery’s group and others have continued to investigate the effects of laser activation. Fast-scan rate voltammetry of Ru(NH3)63+/2+ at laser-activated carbon microdisk electrodes showed a slow decay in the electron-transfer rate constant with time (E140). This was attributed to formation of a neutral partial oxide layer following laser activation. Fast-scan rate voltammetry of catechols at laser-activated carbon microdisk electrodes showed marked changes in electron-transfer kinetics and the transfer coefficient following laser treatment (E141). The charge on the analyte, buffer type, and pH also influenced these parameters, suggesting a change in mechanism following laser activation. The effects of electrochemical oxidation of carbon fiber microelectrodes in solutions of various pH, on electron transfer to luminescent probes have been investigated in a series of elegant experiments (E142). The use of negatively and positively charged probe ions allowed the spacial distribution of carboxylate groups to be measured. Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Impedance spectroscopy and scanning tunneling microscopy have been used to investigate the effects of electrochemical oxidation on the surface structure of glassy carbon electrodes (E143). Oxidation leads to an increased distributed capacitance, which was attributed to edge-plane carboxyl groups. Corrosion of small-scale structures at the surface leads to local roughening, despite smoothening of the overall surface on a macroscopic scale. A voltammetric study of the effects of potential, pH, and time on the electrochemical pretreatment of carbon electrodes has shown that a graphitic oxide layer can be formed in basic solutions (E144). However, this oxide is unstable in 1 M NaOH, in which anodic pretreatment produces some other, unidentified surface species. The electrochemical characteristics of glassy carbon electrode anodized in water and 1-octanol have been compared (E145). Both treatments caused suppression of the voltammetry of Fe(CN)63+, and in both cases the voltammetric response could be restored by Ca2+. This was attributed to anionic sites at the electrode treated in water. However, an alternative and as yet unknown mechanism appears to be operating for the 1-octanol-treated electrode, which becomes modified with a covalently bound 1-octanol monolayer. There have been several studies of the early stages of the oxidative damaging of highly ordered pyrolytic graphite (E146, E147). Hathcock et al. (E146) found that the raised, bubblelike features observed in their earlier work were hollow with an inner roof layer of graphitic oxide. Blister formation correlates with the ease of intercalation of the electrolyte anion, supporting a model involving subsurface gas formation following electrolyte intercalation. Electron-transfer rates for 17 inorganic redox couples and methyl viologen have been compared at highly ordered pyrolytic graphite and glassy carbon electrodes (E148). The lower rates observed at graphite were attributed to its low density of states and carrier density. The adsorption/desorption dynamics of ferrocene-labeled alkylammonium surfactants on glassy carbon have been studied by flow voltammetry (E149). The method involves flowing an adsorbate-free blank electrolyte solution past the working electrode and switching the flow to a solution of adsorbate, or vice versa. Changes in surface concentration of adsorbate with time were monitored by fast cyclic voltammetry. Adsorption rates were found to be controlled by both intrinsic adsorption kinetics and diffusion. Electron-transfer rate constants for hemin and Nile Blue A adsorbed on a glassy carbon electrode have been determined by potential-modulated UV/visible reflectance spectroscopy (E150). A novel type of carbon electrode has been fabricated in an interdigitated microarray by pyrolysis of polypyrrole films on a Pt array (E151). Dopamine exhibited reversible cyclic voltammetry at the new electrode. Optically transparent graphite electrodes have been prepared on a Au mesh substrate by spay coating with colloidal graphite and by vapor deposition of pyrolytic graphite from acetone (E152). Diamond films have been attracting some interest as materials for electrochemistry. Several groups have characterized the basic electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes in aqueous electrolytes (E153, E154), and Zhu et al. (E155) have used them for anodic stripping analysis of lead. The electrochemical deposition of Pt, Pb, and Hg adlayers has 404R
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been demonstrated (E156). The doped diamond films appear to be suitable for use in a variety of electrochemical applications and are more corrosion resistant than other carbon electrode materials (E157). Other Electrode Materials. Cyclic voltammograms of LEED characterized Pt/Rh(100) bimetallic surfaces and Rh(100) have been compared (E158). Annealing of the Pt/Rh(100) electrode in O2 leads to chemical restructuring to produce a RhO/Pt/ Rh(100) trilayer with a Rh(100) surface. Multilayers of palladium and rhodium deposited on Pt(110) single-crystal electrodes, and the adsorption of CO on these surfaces, have been characterized by cyclic voltammetry, charge displacement experiments, and infrared spectroscopy (E159). Oxide/hydrous oxide formation/reduction in base was found to be more facile at a Pd electrode obtained by reduction of a PdO-coated Ti than at bulk Pd (E160). Differences in electrocatalytic activity were also observed for the higher oxides, with only the PdO-coated titanium electrode possessing electrocatalytic activity for ethanol oxidation. The mechanism of hydrous oxide growth at Pd has also been investigated (E161). The conversion of the anhydrous oxide to the hydrous oxide commences at certain sites and eventually produces columns of hydrous oxide. The voltammetric characteristics of vanadium electrodes have been investigated in aqueous media (E162). Repetitive cycling resulted in a gradual accumulation of an electroactive V(IV/III) oxide or hydrous oxide layer on the electrode surface. The conducting ceramics TiO and Ebonex (Ti4O7) have been considered as electrode materials (E163). Neither was found to be useful, because of slow electron-transfer kinetics. However, TiO proved to be an excellent support for electrochemically deposited Pt thin films. The adsorption of Cl- and Br- ions on the (001) plane of a bismuth single-crystal electrode from solutions in 2-propanol has been investigated using differential capacity measurements (E164). In situ atomic force microscopy has been used to enhance and control the location of Cu electrodeposition on Cu and Au singlecrystal electrodes (E165). On Cu, the AFM tip/sample force physically creates defects in a partially passivating oxygen adlayer, forming active sites for Cu adsorption. The general applicability of this scheme is demonstrated on Au(111) passivated with selfassembled octadecanethiol monolayers. MODIFIED ELECTRODES Inzelt has reviewed charge transport in polymer films (F1) and the use of the EQCM, radiotracer techniques, and impedance spectroscopy in the study of polymer-modified electrodes (F2). Doblhofer (F3) has reviewed the electrochemistry of polymermodified electrodes from a polymer science/physical chemistry perspective. Theoretical Aspects. The current interest in self-assembled monolayers has prompted several groups to develop treatments based on Marcus theory for linear sweep voltammetry of redoxactive groups immobilized in monolayers on electrodes (F4-F6). A local dielectric continuum model has been use to calculate the solvent reorganization energy (F7). Fawcett (F8) has extended the analysis of the double layer at an electrode modified by a selfassembled monolayer containing redox centers by including discreetness of charge effects. Models for mediation and catalysis at modified electrodes continue to be extended and refined. Xie and Anson have
extended the theoretical treatment of cyclic voltammetry at electrodes coated with catalyst monolayers by including interactions between catalyst sites and slow electron transfer between these sites and the electrode (F9). They describe procedures for the evaluation of the rate constant for the catalytic reaction. Leddy and co-workers (F10) have extended the theoretical treatment of mediated reactions at film-modified electrodes to include variation of the electrode potential. It is assumed that the concentrations of oxidized and reduced sites at the electrode surface are given by the Nernst equation. However, Sabatani and Anson (F11) have stressed that nonidealities in the electrochemistry of the polymer film should be taken into account. Experimental data for cationic complexes of Ru(II) confined within Nafion were analyzed according to a model in which the reactants in the Nafion exhibit an array of formal potentials with a Gaussian distribution. A new treatment of the cyclic voltammetry of polymer-modified electrodes rigorously includes the effects of short-range interactions between the charged components of the polymer phase and the changing polymer/solution interfacial potential (Donnan potential) (F12). Many of the unusual voltammetric wave shapes observed in experimental work have been simulated, including very narrow peaks and large peak separations that are almost independent of scan rate. Redepenning and co-workers (F13) have also incorporated the changing Donnan potential into a theoretical treatment of the cyclic voltammetry of modified electrodes. A comparison with experimental results for a viologenbased polymer provides good support for the model. The impedance of conducting polymers and other redox polymers has continued to attract much attention. Albery and Mount have reviewed their somewhat controversial approach (F14) and extended it to additional cases. They have shown their approach to be perfectly compatible with the fundamental equation describing the bimolecular hopping of electrons in the polymer and the Nernst-Planck equation describing the motion of each ion (F15). Models developed by Buck et al. (F16, F17) and Vorotyntsev et al. (F18) appear to give essentially the same results. A numerical model, which takes into account Butler-Volmer type heterogeneous redox kinetics, electron flux by hopping, diffusion and migration of the electrolyte, the Poisson equation, Donnan partition kinetics, an iR drop, and the double-layer capacitance including a constant phase element, has also been reported (F19). The effects of slow ion transport in solution and nonuniform current distributions have also been considered (F20), and the reliability of electrochemical parameters derived from impedance spectra of polymer-modified electrodes has been analyzed (F21). A theoretical analysis describing the transient current response of a conducting polymer-based amperometric chemical sensor to a concentration step has been reported (F22). Good agreement was obtained with experimental results for the amperometric detection of ascorbate at polypyrrole-coated electrodes. Conducting Polymers and Organic Redox Polymers. Conducting polymers continue to receive a tremendous amount of attention, with a huge number of new materials being described in the two years covered by this review. Since the focus here is on fundamental advances in electroanalytical chemistry, new materials will only be discussed where they present new insights into electrochemistry or new opportunities in electroanalysis. A recent review (F23) will serve as an entrance to the literature on the synthesis of conducting polymers. A special issue of Electro-
chemica Acta was devoted to conducting polymer-modified electrodes in 1994 (F24), and Lyons has reviewed the use of conducting polymers in electrocatalysis and amperometric sensors (F25). The ion-exchange and ion transport properties of conducting polymers are crucial to many electroanalytical applications and so have continued to be important areas for both fundamental and applied research. Bard and co-workers (F26) have used scanning electrochemical microscopy to follow the rates of anion ejection from bromide- and ferrocyanide-doped polypyrrole films during reduction. The nature of the cation was an important factor determining the kinetics of ion transport in the ferrocyanide-doped films. The physical localization and mechanisms of ferrocenium and Os(bpy)33+ (bpy ) 2,2′-bipyridine) reduction at polypyrrolecoated electrodes were also investigated. Chronoamperometric data for the oxidation of polypyrrole have been analyzed using a nucleation-type kinetic model (F27). It is assumed that entry of counterions occurs only at certain (nucleating) points on the film’s surface and that oxidized domains propagate from these points. The growing nuclei of oxidized polymer were observed visually. An EQCM study of ion transport in polypyrroles with and without N-alkyl substitution has shown that reduction involves reversible anion expulsion for the former but irreversible cation incorporation for the latter (F28). Hydrogen bonding between anions and NH groups in the unsubstituted polymer is postulated as the cause of the immobility of its counterions. The relative contributions of anions and protons during potential cycling of polyaniline-coated electrodes in 1 M aqueous acids was studied using an EQCM (F29). Anion transport dominates at high potentials and is more important for thicker films and in HClO4. For poly(5-amino-1-naphthol) thin films, in situ FT-IR and probe beam deflection studies revealed that protons were the dominant mobile species in acidic aqueous solution (F30). Anion transport is predominant in acidic or neutral nonaqueous solutions. Contrary to previous observations, the electrochemistry of polyaniline has been found to be stable at pH values as high as 9, as long as the pernigraniline form is avoided (F31). Loss of activity at high pH values is attributed to slow reduction of the pernigraniline form rather than deprotonation of the emeraldine form. The ion transport processes in films of poly(styrenesulfonate)/ poly(xylyl viologen) have been manipulated by varying the ratio of the two polymers (F32). Poly(styrenesulfonate)-rich films show predominant cation transport, while for poly(xylyl viologen)-rich films anion transport dominates. Surprisingly, comparisons in D2O and H2O revealed that cation insertion was accompanied by solvent expulsion. This was attributed to free-volume constraints. The apparent diffusion coefficients of BF4- ions during the undoping process of polyanilines prepared at various electropolymerization current densities were found to decrease with decreasing fiber diameter (F33). X-ray diffraction results suggest that this is due to a higher degree of crystallinity in thinner fibers. Bilayers involving conducting polymers have been attracting increasing interest, particularly for the insight that they can provide into the mechanisms of charge transport processes. Hillman and Glidle (F34) have continued their excellent work in this area with an EQCM study of poly(bithiophene)/poly(xylyl viologen) bilayers. On the relatively long time scales employed, Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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bilayer mass changes were dominated by perchlorate anion transfer with relatively little solvent transfer. The mass/charge characteristics of bilayer and single-layer films were quite similar, despite their very different mass/potential characteristics. On the other hand, experiments with polypyrrole poly(styrenesulfonate)/ poly(vinylferrocene) bilayers have shown that the outer film can act as a barrier to ion and solvent transport between the inner film and the electrolyte solution (F35). These bilayers allow controlled release or uptake of anions or cations, depending on the potential. Ion binding by electronically conducting ion-exchange polymers has also been receiving significant interest. In one study, the electrochemistry of anthraquinone-2-sulfonate in a series of cationic amphiphilic polypyrroles was found to depend on the length of the alkyl chain between the pyrrole and the alkyammonium head group (F36). Splitting of the anthraquinone redox wave was taken to indicate an organized film structure providing two distinct environments for the anion. The binding of large electroactive cations by poly[4-(pyrrol-1-ylmethyl)benzoic acid] films (F37) and electrochemically polymerized films of metal-free octacyanophthalocyanine (F38) have also been reported. Experimental data from a RDE study of the mediated electrochemistry of ionic redox couples in solution by anionic and cationic polypyrrole-based films have been analyzed using a redox polymer model (F39). The rate of electron transfer was found to be (i) proportional to the concentration of the oxidized or reduced polymeric sites, (ii) dependent to some extent on the Donnan potential prevailing at the interface, and (iii) correlated with the thermodynamic driving force for the reaction between the polymer and redox species. Kazarinov et al. (F40) have reviewed their experimental work on the mediation of solution redox reactions by conducting polymer and redox polymer films. The mediation of solution electrochemistry by the ladder redox polymer poly(o-phenylenediamine) has also been investigated (F41). The reproducibility and stability of conducting polymer films continue to be important issues. Reproducibility was addressed for poly(3-methylthiophene) in a collaboration among three separate laboratories (F42). Disappointingly, significant differences were observed in their results, and no recommendations for improving reproducibility were given. The deposition of conducting polymers on a suitable self-assembled monolayer base may prove to be a useful route to reproducibility. Polyaniline films deposited galvanostatically on gold electrodes precoated with selfassembled monolayers of p-aminothiophenol were found to be considerably denser than those grown on bare Au, and their electrochemical switching rate was enhanced (F43). Similarly, extremely adherent and smooth films of polypyrrole have been deposited on Au precoated with an ω-(N-pyrrolyl)alkanethiol selfassembled monolayer (F44). Ordered lamellar polypyrrole-based films have been prepared by electrochemical polymerization of a series of self-assembling potassium 3-(3-alkylpyrrol-1-yl)propanesulfonates (F45). The lamellar phase consists of head-to-head packed layers of the surfactant. The electrochemical stability and degradation of conducting polymers has been reviewed recently (F46). Overoxidation in the presence of nucleophilic species causes a loss of electroactivity but can also be used to deliberately modify polymers to advantage. For example, the permeability and selectivity of overoxidized polypyrrole films has been controlled by carrying out the overoxidation in different electrolyte solutions (F47). Ultrathin over406R
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oxidized polypyrrole films have been reported to exhibit permselectivity against anions and excellent selectivity toward cations (F48). The permeability of overoxidized polypyrrole films to various species has been correlated with structural information from XPS (F49). The existence of two domains consisting of hydrophilic oxygenated groups and hydrophobic imine groups was postulated to explain the observed dependence of probe permeability on its charge and hydrophobicity. Carboxylic acid groups, observed for the first time, may be responsible for the cation selectivity observed by Gao et al. (F47). Electrodes coated with overoxidized polypyrrole (F50) or polypyrrole(dodecyl sulfate) (F51) films are selective for dopamine oxidation in the presence of ascorbate. Dopamine can be preconcentrated in the film by ion exchange, while ascorbate is excluded. Preconcentration of Br3- has also been reported for electrodes coated with deactivated polypyrrole (F52), although in this case the mechanism is somewhat unusual. Br3- generated electrochemically at the polymer/electrode interface is trapped by the low permeability of the polymer film. An interesting alternative to overoxidized polypyrrole as a permselective barrier is underoxidized polypyrrole (F53). Electrochemical deposition of polypyrrole in a phosphate solution produces a nonconductive film that cannot mediate the oxidation of solution species. The selectivity of this novel modified electrode can be controlled by selection of the electrolyte counteranion and the pH. A variety of novel conducting polymer-modified electrodes with important potential applications in electroanalysis have been reported. Several polythiophene-based materials with molecular recognition properties observable via changes in resistance have been reported. A cyclophane was incorporated into the polymer for recognition of methyl viologen (F54), and a calix[4]arene was used as an ion receptor for detection of Li+, Na+, and K+ (F55). A polymerizable calix[4]arene containing N-substituted pyrrole moieties has also been reported (F56). Homopolymer films were insulating, but conducting films believed to contain the calix[4]arene were prepared by copolymerization with pyrrole. Several peptide-derivatized poly(pyrrole)-modified electrodes have been described (F57, F58). Cystine-derivatized copolymers were used to bind ferredoxin-type centers. The first synthesis of an optically active polyaniline has been achieved via the enantioselective electropolymerization of aniline in the presence of (1S)-(+)- or (1R)-(-)-10-camphorsulfonic acid (F59). The polymers exhibited strong circular dichroism spectra typical of polymers possessing helical chirality. A variety of anionic complexing ligands such as bathocuproine sulfonate have been incorporated into polypyrrole during electrochemical polymerization to produce films capable of preconcentrating Cu2+ and Cu+ ions (F60, F61). In similar work, diethyldithiocarbamate was incorporated into preformed films by ion exchange (F62). Electrochemically polymerized films of the ligand salicylaldoxime have also been used to bind Cu2+ ions (F63). The electrocatalytic activity of various polytungstate and polymolybdate anions has been exploited by their incorporation into polypyrrole films. The highly charged anions are readily incorporated into the film as counterions during anodic polymerization. The preparation and electrochemistry of polypyrrole films doped with isopolytungstate anions has been investigated
using an EQCM (F64). Films containing the iron-substituted heteropolytungstates [(H2O)FeIIIPW11O39]4- and [(H2O)FeIIISiW11O39]5- were found to catalyze the reduction of nitrite, providing a sensitive method for its electrochemical detection (F65). Polypyrrole films containing the dysprosium molybdosilicic heteropolyanion [Dy(SiMo11O39)2]13- have been reported to catalyze the reduction of ClO3- and BrO3- (F66). Catalyst materials prepared from polyaniline and 12-tungstophosphoric acid are effective for isopropyl alcohol oxidation (F67). The electrocatalytic reduction and cleavage of aromatic sulfones and disulfide at electrodes modified with polyfluorene or poly(dibenzofuran) films has been the subject of a comprehensive study (F68). Ion-Exchange Polymers. Nafion continues to be the most studied ion-exchange polymer for use in modified electrodes. To a large extent, fundamental studies of charge and mass transport have given way to studies involving the incorporation of novel ions, electrocatalysis, and development of electroanalytical methods for specific species. The latter topic will not be considered here. Deviations from the generally used membrane model have been observed at high rotation rates for RDV of ferricyanide and ferric ions at Nafion-coated Pt electrodes (F69). They were attributed to inhomogeneities or pinholes. The effectiveness of Nafion protective layers on electrodes has been investigated by comparing the effects of toluene and benzoic acid on the kinetics of Fe2+ oxidation at Pt and Nafion-coated Pt electrodes (F70). The Nafion coating effectively prevents inhibition by toluene but not by benzoic acid. However, electrochemical deposition of Pt on Nafion-coated glassy carbon produces an electrode immune to both organic species. It is suggested that a more intimate contact between the Nafion and the Pt is achieved in the latter case. Nafion-coated microelectrodes have been used to investigate the effects of fluid density on the voltammetry of ferrocene in both subcritical and supercritical CO2 containing water (F71). Exposure of the Nafion films to high-density fluids causes swelling of the film and decreases the transport rate of ferrocene to the microelectrode. Partitioning of ferrocene into the Nafion film is also decreased. The phenothiazine dyes thionine and methylene blue produce two redox waves under certain conditions when incorporated into Nafion films (F72). They were attributed to electroactive species in different domains of the Nafion film. The cyclic voltammetry of a range of cage complexes of Co, Fe, Ni, and Mn incorporated in high concentrations in Nafion films has been comprehensively characterized (F73). Mobility of the cage complexes rather than electron hopping was found to be the dominating charge transport mechanism. Nafion films containing the cis-dioxo Mn(VI) complex [MoO2(O2CC(S)C6H5)2]2- have been shown to catalyze the reduction of nitrite to ammonium ions (F74). This in an unusual case in which an anionic polymer is used to immobilize an anionic complex. An RDE study of nitrate reduction at electrodes modified with cobaltcyclam incorporated into Nafion showed mixed kinetic and nitrate diffusion control (F75). The activation parameters associated with charge propagation in a series of quaternized poly(4-vinylpyridine) polymers, which contained electrostatically bound or coordinated redox sites, have been obtained from the temperature dependence of chrono-
amperometrically measured diffusion coefficients (F76). Activation enthalpies were found to be virtually independent of the nature of the rate-limiting step, suggesting that entropic factors are primarily responsible for the observed differences in charge transport rates. Protonated polybenzimidazole films have been used to bind Fe(CN)63-/4- by ion exchange (F77). Their electrocatalytic effect on the oxidation of ascorbic acid was investigated. Zeolites and Clays. Several reviews have appeared on the electrochemistry of zeolite- (F78, F79) and clay- (F79) modified electrodes. One of the key fundamental issues in the electrochemistry of zeolites is the electrochemical accessibility of redox-active species within the zeolite framework. For zeolites containing Ag+ ions this has become a quite contentious issue. Baker and co-workers (F80, F81) have shown that electron transfer occurs at the support electrode surface following ion exchange with electrolyte cations (extrazeolite mechanism). The ability to reduce Ag+ depends on the properties of the electrolyte cation, the nature of the largechannel cations, and the framework charge of the zeolite. The current is controlled by the rate of ion exchange of Ag+ with electrolyte cations. A variable-temperature chronocoulometric study indicated that the rate-limiting step can be diffusion within the zeolite channels or ion exchange between the small and large cages, depending on the location of the silver ions (F81). On the other hand, Calzaferri and co-workers (F82, F83) appear to have provided evidence for intrazeolite electron transfer (i.e., electron transport within the zeolite particles) in Ag+-A zeolite-modified electrodes, although this has been hotly disputed (F84). The disagreement arises over the assignment of what could be as many as eight distinct reduction waves in Calzaferri’s voltammograms. Four peaks are tentatively assigned to the reduction of Ag+ at different sites in the zeolite framework and three to the reduction of silver clusters formed in the R-cages during the cathodic scan. Unfortunately, the speculative nature of these assignments has provided the critics with ammunition, and the real issue of whether the results support an intrazeolite electron transport mechanism has not been decided. Differences in the method of electrode preparation, elaborated in the reply (F83), does appear to be an important issue here. A voltammetric study of Fe(III)Salen+, Mn(III)Salen+, and Co(II)- and Fe(II)-hexadecafluorophthalocyanines physically encapsulated in the supercages of faujasite Y zeolite has demonstrated that intrazeolite electrochemistry is possible (F85). However, only a very small fraction (1-3%) of the encapsulated redox centers were electrochemically active at a scan speed of 10 mV s-1. The degree of electroactivity was increased when silver particles were deposited within the zeolite framework (F86). These results provide further support for an intrazeolite electron transport mechanism. A study of Cu2+-Y zeolite-modified electrodes has suggested that the electrochemistry of the copper ions involves their diffusion though the zeolite particle to the zeolite/electrode interface (intrazeolite ion transport mechanism) (F87). The inactivity of the outer layers of thick films, and differences from the voltammetric behavior of Cu2+ at an unmodified electrode, are cited as evidence that the mechanism does not involve ion exchange and reduction at the solution/electrode interface (extrazeolite mechanism). Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Cyclic voltammetry of synthetic faujasite-type zeolites modified with cobalt(II) and copper(II) hexadecafluorophthalocyanines provided more well-defined waves for the Co(II)/Co(I) and Cu(II)/ Cu(I) couples than observed in solution (F88). This supports a prior hypothesis that the poor electrochemistry observed in solution is due to aggregation. The electrochemistry of 5,10,15,20tetra(4-N-methylpyridinium)porphyrinatomanganese(III) adsorbed on ZSM-5 and EMC-2 zeolites and VPI-5 molecular sieves has also been investigated (F89). Positive potential shifts as large as 380 mV were observed for the Mn(III/II) couple. Interest in the electrochemistry of clay-modified electrodes has continued to grow. As with zeolites, one of the key issues in this area is the electrochemical accessibility of redox-active species in the clay film. Kaviratna and Pinnavaia (F90) addressed this by investigating the effects of pH and film thickness on the electrochemical activity of Ru(NH3)63+ gallery exchange ions, Fe(bpy)32+ An- surface ion pairs, and Fe(CN)63- anions. Qiu and Villemure (F91) have taken advantage of the electrochemical activity of nickel(II) sites in layered double hydroxide films (anionic clays) to facilitate electron transport. Stoichiometric oxidation of the Ni(II) sites up to Ni(IV) and mediation of Fe(CN)64- and Mo(CN)64- oxidation were demonstrated. Another approach to improving electron transport is to incorporate strands of a conducting polymer within the clay matrix. Thus, Fe3+ and Cu2+ have been used to chemically polymerize pyrrole within montmorillonite clays (F92). Vibrational spectroscopy, thermal analysis, and conductivity data all indicate that polypyrrole is present in the interlayer region of the clays utilized. The time dependence of the voltammetric waves of [Fe(bpy)3]2+ adsorbed in clay-modified electrodes has been found to be anomalous when compared with those of [Ru(bpy)3]2+ and [Os(bpy)3]2+ (F93). UV/visible spectroscopy showed that this unusual behavior is due to dissociation of the iron complex. Clay-modified electrodes containing the anionic and cationic dyes indigo carmine and methylene blue have been compared (F94). Methylene blue could be adsorbed into the interlamellar spaces, where it became electrochemically inactive, while indigo carmine did not interact strongly with the clay and remained electrochemically active. A study of m-nitrobenzenesulfonate and anthraquinone mono- and disulfonates in the layered double hydroxide Zn2Cr(OH)6Cl‚2H2O (an anion-exchanging clay) also showed low degrees of electroactivity (F95). The analytical applications of clay-modified electrodes have been explored with neutral and cationic ferrocenes and cobaltocenes, which were analyzed voltammetrically following preconcentration in clay films (F96). Metal Complex Films. Electrodes modified with redox polymers based on bipyridine complexes continue to be one of the most studied groups of chemically modified electrode. Vos and co-workers have extended their work in this area by investigating how stabilization of [Os(bpy)2(pvp)10Cl]Cl (pvp ) poly(4-vinylpyridine)) films by cross-linking can influence their selectivity (F97, F98). The redox and electron transport properties of the films were little affected, but the partitioning of Fe3+ was physically prevented by use of 1,5-dibromopentane and 1,10dibromodecane cross-linking agents, while reduced partitioning was evident for p-dibromobenzene cross-linked films. In contrast, NO2- was found to enter all the cross-linked polymer films. The possibility of using the different partitioning of these two analytes into the cross-linked polymers to simultaneously determine their 408R
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concentration is demonstrated. The electrocatalytic oxidation of ascorbic acid has also been investigated at [Os(bpy)2(pvp)10Cl]Cl-modified electrodes (F99). An EQCM study of overall ion and solvent transfers in [Os(bpy)2(pvp)10Cl]+ films exposed to aqueous p-toluenesulfonic acid solutions has revealed a surprisingly complicated concentration (pH) dependence (F100). As the acid concentration is increased, the mass of the film increases due to protonation of free pyridine sites at a pKa of 2.9. At higher concentrations, sharp apparent mass changes with changing concentration are attributed to changes in the viscoelasticity of the film rather than changes in ion/solvent populations. These results show that great care must be exercised in the interpretation of EQCM data for polymermodified electrodes. The electroactivity of thin-film polymers derived from bipyridyl and phenanthroline complexes of iron have been found to be modulated to a quite remarkable extent by organosulfonate anions (F101, F102). For example, decanesulfonate decrease the voltammetric charge by 65-80% relative to perchlorate. This would appear to be due to a significant reduction in the charge transport diffusion coefficient (F101), although surprisingly it has been argued that this is not the case (F102). The permeation of ferrocene ethylene oxide and propylene oxide oligomers into electropolymerized films of [Ru(vinyl-bpy)3](ClO4)2 has been investigated (F103). Permeability is controlled more by the partition coefficient than the diffusion coefficient within the polymer and is strongly dependent on the permeant molecular volume. Meyer and co-workers have reported a comprehensive study of the synthesis, electrochemistry, and electrocatalytic activity toward benzyl alcohol and chloride oxidation of electropolymerized films containing cis-[Ru(4-(2-pyrrol-1-ylethyl)-4′-methyl-2,2′-bipyridine)2(H2O)2]2+ (F104). Importantly, immobilization of the dioxoruthenium complex in a polymer film increases its stability with respect to ligand dissociation, while maintaining its electrocatalytic activity. The electrocatalytic activity for CO2 reduction of a series of mono(bipyridine)carbonylruthenium(II) complexes has been found to be due to the formation of a polymeric film containing Ru0Ru0 bonds (F105, F106). Electrodes deliberately modified with similar films by anodic polymerization and then reduction of the pyrrole-substituted analogue [Ru(4-(2-pyrrol-1-ylethyl)-4′-methyl2,2′-bipyridine)(CO)2Cl2] were also active, and were significantly more stable. The electrocatalytic reduction of CO2 and O2 with electropolymerized films of vinyl-terpyridine complexes of Fe, Ni, and Co has been investigated (F107). These complexes were found to be more active catalysts when immobilized in polymer films than in solution, indicating the occurrence of cooperative effects between metal centers. An interesting approach to novel modified electrodes is to used a redox polymer to electrostatically bind other redox-active species (F108). Thus, films of poly(vinylpyridine-co-styrene) with covalently bound [Ru(edta)(OH2)]- or [Ru(NH3)5(OH2)]2+ were used to bind methyl viologen and [Mo(CN)8]4-, respectively. A film containing all four species could be switched between six different redox states. This approach can provide useful information about charge transport in polymer films and can potentially be used to create useful new electroactive materials. It has been used to immobilize the poly(oxometalate) ions, PMo12O403-, PW12O403-,
and SiW12O404-, in poly(Ru(vinyl-bpy)32+) (F109). The bound poly(oxometalate) anions catalyze the electrochemical reduction of hydrogen ion. Poly(vinylferrocene)-modified electrodes continue to attract attention because of their value in both fundamental studies and practical applications. The unusual electrochemistry of poly(vinylferrocene) films in aqueous media has been reexamined with the aid of scanning electrochemical microscopy (F110). The microscope was used to probe changes in film resistance with potential and depth into the film. The results provide support for a theoretical model for chronoamperometry based on a potentialdependent charge transport diffusion coefficient. Galvanostatically controlled EQCM measurements on poly(vinylferrocene)-modified electrodes have demonstrated temporal separation of mobile species transfers (F111). It is proposed that the dynamic separation of the ion (fast), salt, and solvent (slow) transfer processes could provide selectivity in analytical applications. Uptake of iodide, thiocyanate, and cyanide by oxidized poly(vinylferrocenium)-coated electrodes, and subsequent analysis by cathodic stripping, have been investigated (F112). Cyanide uptake causes an irreversible change in the physical properties of the film, whereas thiocyanate and iodide are merely incorporated in the film structure as counterions. There has been continuing strong interest in electrodes modified with prussian blue and related materials. The stability of prussian blue films is dependent on both the cation and the anion present in solution (F113). In aqueous media, the sulfate ion clearly destabilizes the crystalline structure whereas the nitrate ion favors stabilization. An impedance study of prussian bluecoated electrodes has provided support for a bilayer structure with a compact inner layer (F114). The mechanical attachment of microcrystalline solid prussian blue particles to graphite and gold electrodes provides for useful comparisons with the traditional electrochemical deposition method (F115). The deposition and electrochemistry of nickel(II) hexacyanoferrate(III) films have been the subject of an EQCM study (F116). Charge compensation during redox cycling was concluded to be dominated by partially hydrated cations, although some co-anion transport, as ion pairs, was also suggested by the results. A mixed-valent ruthenium oxide-ruthenium cyanide film on glassy carbon was reported to exhibit excellent electrocatalytic activity toward oxidation of simple aliphatic alcohols and polyhydric compounds in acidic media (F117). Applications in flow injection analysis and liquid chromatography are demonstrated. A polishable carbon/epoxy-based bulk-modified electrode containing a similar mixed-valent ruthenium oxide-ruthenium cyanide catalyst has also been developed (F118). Electrocatalytic oxidation and flow injection analysis of various N-nitrosamines, As(III) glutathione, isotocin, and myoglobin are reported. Work on electrodes modified with porphyrins and phthalocyanines has been mainly concerned with electrocatalysis and the development of applications in electroanalysis. The immobilization of metalloporphyrins in polymer films on electrodes has recently been reviewed (F119). Lever and co-workers have investigated a series of phthalocyanine-modified electrodes for use in sulfide and thiol analysis. Electrochemically deposited N,N ′,N ′′,N ′′′-tetramethyltetra-3,4pyridinoporphyrazinocobalt(II) films gave linear potentiometric plots (E vs [Na2S]) in the presence of air, with slopes of between
-61 and -92 mV (F120). The mechanism for this response involves a mixed potential generated by simultaneous O2 reduction and HS- oxidation. Both processes were considered to be catalyzed by the immobilized phthalocyanine complex. Electrochemically polymerized tetraaminophthalocyanatocobalt(II) films were found to function similarly and provide better stability at high pH values (F121). Hexadecachlorophthalocyanatoiron(II) adsorbed on graphite was also found to be an effective catalyst for sulfide oxidation, although fouling of the surface by sulfur was a problem (F122). Graphite electrodes modified by an irreversibly adsorbed alizarin-Cu(II) complex were found to display electrocatalytic activity toward the oxidation of sulfide ion to sulfur (F123). Reduced films showed catalytic activity similar to submonolayer Cu deposits, indicating that the active catalyst in both cases was a submonolayer of copper(II) sulfide. The catalytic reduction of hydrogen peroxide has been studied at glassy carbon electrodes modified with an adsorbed σ-bonded pyrroleiron(III) octaethylporphyrin complex (F124). Despite the low stability of the modified electrode, good quality kinetic data were obtained by scanning the electrode rotation rate (at 1000 rpm s-1) at fixed potential. The oxidation of hydrogen peroxide has been studied at cobalt phthalocyanine-modified screen-printed carbon electrodes (F125). Iron tetrapyridylporphyrin intercalated into hydrated vanadium(V) oxide was found to exhibit pronounced electrocatalytic activity in the reduction of molecular dioxygen, reflecting an enhanced reactivity of the catalyst in a confined medium (F126). The mechanism of electron transfer from a scanning tunneling microscope tip to an electrode modified with a multilayer film of dry protoporphyrin(IX)FeIIICl has been investigated (F127). Mediation of the transfer allows estimation of the tip to film electron-transfer rate constant. A wide variety of novel modified electrodes based on metal complexes have been reported. Many of these exhibit useful electrocatalytic properties and have potential applications in electroanalysis. Anson and co-workers have investigated a number of new modified electrodes for O2 reduction. An apparently polymeric coating formed by prolonged exposure of graphite electrodes coated with cobalt tetrapyridylporphyrin to solutions of fac-Ru(NH3)5(OH2)2+ catalyzed the four-electron reduction of O2 at unusually positive potentials (F128). A series of Cu(II)-1,10phenanthroline complexes were also effective for the four-electron reduction of O2 when irreversibly adsorbed onto graphite electrodes (F129). The absorbed complexes are much more effective catalysts than when in solution. The ability of the electrode to provide multiple electrons rapidly appears to be a significant factor in the enhanced activity of the adsorbed species. Lever and coworkers (F130) have used the poisoning by trace SCN-, H2S, or HCN of the adsorbed [Cu(1,10-phenanthroline)]2+ complex as a sensitive new method for electroanalysis of these species. The mechanism is shown to involve coordination of the added ligand to the copper complex. Two new electrocatalytic modified electrodes derived from Ru(H2O)62+ have been described (F131). Anodic electrodeposition and ion exchange into Nafion both produce electrodes with pronounced electrocatalytic activity toward the oxidation of alcohols. An iron-alizarin ([[(3,4-dihydroxy-2-anthraquinolyl)methyl]imino]diacetic acid) complexone irreversibly adsorbed on graphite Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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is effective for the electrocatalytic reduction of nitrite and nitric oxide (F132). Conductive mixed-valent molybdenum(VI/V) oxide films have been electrochemically deposited on carbon fiber microelectrodes from Na2MoO4 solutions (F133). These films catalyze the electrochemical reduction of chlorate to chloride and bromate to bromide in H2SO4. Carbon electrodes modified with adsorbed or electrochemically deposited PMo12O403- have also been found to catalyze the reduction of bromate (F134). Electrochemical cycling of electrodes coated with electrochemically polymerized [NiII(5,7,12,14-tetramethyldibenzo[b,i]1,4,8,11-tetraaza[14]annulene)] films in base produces an electrocatalytic surface similar to nickel hydroxide (F135). This new modified electrode exhibits high electrocatalytic activity toward the oxidation of carbohydrates in alkaline solution. Cobalt-cyclam (trans-CoIII(1,4,8,11-tetraazacyclotetradecane)Cl2) adsorbed on a Au electrode has been investigated for the electrocatalytic reduction of nitrate (F136). The voltammetric peak potential for nitrate reduction at an electrode modified with the adsorbed catalyst was 80 mV more positive than at a Nafionmodified electrode containing the same catalyst. Electrodes modified with Ni-cyclam films were found to be effective catalysts for the oxidation of methanol and other simple alcohols (F137). Based on a RDE study, the catalytic reductions of iodoethane and 2-iodopropane at carbon electrodes coated with anodically polymerized films of nickel(II/I) salen have been analyzed according to Saveant’s classification of kinetic cases (F138). A conducting platinum cluster compound, K1.64Pt(C2O4)2, has been electrochemically synthesized on a glassy carbon electrode in the form of a fibrous deposit (F139). It is claimed that the modified electrode behaves as a microelectrode array, although a porous metal electrode would probably be a better analogy. New Modifying Materials. Layered poly(oxometalate) films have been deposited on electrodes by alternate immersion in solutions of the poly(oxometalate) anion and a large water-soluble cation (F140). The film is built up one layer at a time as each ion causes adsorption (incipient precipitation) of the other. The precipitate-forming cations include tetrabutylammonium, Fe(1,10phenanthroline)32+, and protonated poly(4-vinylpyridine). Composite films of heteropoly-12-tungstate anions with protonated poly(4-vinylpyridine) were the most robust. The method provides an excellent route to well-defined, electroactive 3-D molecular assemblies. A somewhat similar strategy has been used to deposit thin films of zinc and copper alkanebisphosphonates (F141). A gold surface was first modified with (4-mercaptobutyl)phosphonic acid and then immersed alternately in ethanolic solutions of the metal acetate or perchlorate salt and H2O3P(CH2)nPO3H2 (n ) 8, 10, 12, or 14). A novel ferrocene-containing modified electrode with the ferrocene units covalently bonded within a silica network has been reported (F142). The high stability of this material suggests applications in reference electrodes. Electrodes have been modified with films of silane-based dendrimers with ferrocene terminal groups (F143). The dendrimers, which contain four or eight independent ferrocene moieties, form stable films on the electrode when oxidized in CH2Cl2. The electrochemistry of highly ordered phthalocyaninatopolysiloxane thin films, produced using Langmuir-Blodgett thin-film deposition techniques, has been characterized (F144). The films 410R
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show facile electron and ion transport during electrochemical and chemical oxidation. Glassy carbon and Pt electrodes have been derivatized with TiCl4 and other early transition metal complexes (F145). The modified electrodes have a thin multilayer coating of metal oxide which supports a metal oxychloride overlayer. TiCl4-modified electrodes were further derivatized with molecules containing alcohol or thiol functionality (e.g., (hydroxymethyl)ferrocene and 6-ferrocenylhexanethiol). A new type of oligosiloxane with oligo(ethylene glycol) spacers appears to be very promising for use in modified electrodes, and especially for electrocatalytic applications (F146). Films of these materials exhibit high diffusivities and can be modified with catalytic species. Catalysis of the oxidation of 4-methoxybenzyl alcohol to anisaldehyde by films containing covalently bound 3-carboxy-2,2,5,5-tetramethyl-∆3-pyrroline-1-oxyl was demonstrated. Pt electrodes modified with an ultrathin (15-100 Å) silica film have been found to behave as ideally polarizable electrodes in aqueous media (F147). The silica films are stable under electrochemical conditions and are several orders of magnitude more conductive than bulk silica. The underpotential deposition of copper onto palladium particles on the silica layer and methanol oxidation on platinum particles have been demonstrated. Other Bulk Films. Bard and co-workers (F148) have reviewed the electrochemistry of fullerene-modified electrodes. Rusling and Howe (F149) have reviewed electron transfer in surfactant films on electrodes and present an electrochemical study of copper phthalocyaninetetrasulfonate didodecyldimethylammonium bromide films. Films were prepared by first casting the surfactant film onto the electrode and then introducing the phthalocyanine by ion exchange or by casting the films directly from the CuPCTS(didodecyldimethylammonium) salt. Charge transport and electron transfer were much faster in the ionexchanged films, indicating that they are more ordered than the cast salt films. Normal pulse voltammetry has been used to estimate standard heterogeneous rate constants and charge transport diffusion coefficients for films of didodecyldimethylammonium bromide containing the heme protein myoglobin (F150). The ability of thin films of 4-pentadecylpyridine on Au(111) to block the electrochemistry of Fe(CN)63- in solution has been investigated (F151). The modified electrode behaves as a potential dependent array of electrochemically active sites. A novel modified mercury electrode coated with a very thin hydrophilic dialysis polymer tube has been described (F152). Renewal of the mercury can conveniently be performed during use. Microelectrodes coated with poly(ethylene oxide) films containing electrolyte have been used to study the voltammetry of p-benzoquinone and anthracene in CO2-based fluids (F153). Conductive carbon cement has been tested as a matrix material for the preparation of electrodes bulk modified with Cu2O or cobalt phthalocyanine (F154). The reversibility of model couples was comparable with that of a freshly polished glassy carbon electrode and better than that of carbon paste electrodes. A Cu2O-modified electrode was effective for electrocatalysis of glucose oxidation in flow injection analysis. Self-Assembled Monolayers (SAMs). SAMs offer unique opportunities for the study of fundamental aspects of electro-
chemistry, including electron-transfer kinetics/mechanisms, adsorption processes, solvation effects, ion pairing, and the effects of intermolecular interactions. In addition, SAMs have significant potential for use in sensors and other analytical applications, molecular electronic devices, and possibly in electrocatalysis. Most studies to date have utilized reactions of thiols with a gold electrode to produce stable SAMs. The synthesis of a wide variety of substituted alkanethiols has made this approach very versatile, and the large body of work devoted to characterizing thiol SAMs has led to a clear understanding of their basic structure and properties. However, a number of basic issues regarding the formation, structure, and stability of SAMs have only recently been addressed. Several EQCM studies of SAM formation have appeared (F155, F156). From a study of the kinetics of the self-assembly of octadecanethiol from ethanolic electrolytes it was concluded that the adsorption reaction proceeds with formation of H2 (F155). A loosely packed, disordered monolayer is produced initially. Over a period of hours, it becomes denser and well-ordered. Deposition of submonolayer amounts of alkanethiols has been found to be much faster in DMF than in acetonitrile (F156). In acetonitrile the thiols initially form a multilayer which slowly converts to a monolayer. The kinetics of adsorption of redox-active self-assembling molecules of the type [Os(bpy)2LCl]+ (L ) 4,4′-bipyridine, trans1,2-bis(4-pyridyl)ethylene, 1,3-bis(4-pyridyl)propane, or 1,2-bis(4pyridyl)ethane) has been investigated as a function of concentration, applied potential, and solvent (F157). The adsorption process appears to be under kinetic rather than diffusion control. The electrode potential affects the equilibrium coverage but not the kinetics of adsorption. The rate of adsorption was not very sensitive to the nature of the solvent, but the equilibrium value of the surface coverage was. Rowe and Creager (F158, F159) have investigated the competitive self-assembly of ferrocenylalkanethiols and 1-alkanethiol onto gold. The affinity of the ferrocenylalkanethiols for the surface, relative to that of the corresponding alkanethiol, is a function of the polarity of the functional group linking the ferrocene to the alkanethiol chain. In general, ferrocenylalkanethiols with nonpolar linking groups show a stronger affinity for the surface than do polar groups and especially charged groups, suggesting that electrostatic effects are critically important during self-assembly. Redox potentials for the surface-confined molecules are consistently more positive than in solution (F160). Two physical models for the electrode/monolayer/solution interface are invoked to explain the influence of various experimental parameters on the magnitude of the positive shift. It is concluded that both interfacial solvation and ion spatial distribution effects must be considered to adequately explain the data. Potential-dependent wetting experiments and differential capacitance measurements on alkanethiol-modified gold electrodes have shown that potential-induced conformational changes within the thiol layer are insignificant (F161). The potential dependence of the wettability originates solely from the formation of an electrical double layer at the thiol/solution interface. The formation of SAMs by aromatic thiols has been reported (F162). Thiophenol forms poorly defined layers on gold but p-biphenyl mercaptan, and p-terphenyl mercaptan form monolayers with reproducible contact angles and ellipsometric thicknesses.
An EQCM study of 11-ferrocenyl-1-undecanethiol monolayers has revealed that there is strong ion pairing between perchlorate and oxidized ferrocenium sites in 1 M HClO4(aq) (F163). The rigidity of fully oxidized compact layers suggests formation of a two-dimensional ionic lattice. Incorporation of water molecules within the monolayer was observed if the electrodes were washed with pure water before immersion in the aqueous electrolyte solution. The stability of self-assembled monolayers has attracted some interest (F155, F164, F165). Everett et al. (F164) have measured the potential and time dependence of the loss of ferrocene-labeled alkanethiols from gold in CH2Cl2. In a second paper, the influence of solvent, water, oxygen, convection, and substrate type on the stability of self-assembled organothiols on gold was investigated (F165). The kinetics of electron transfer from electroactive moieties immobilized on SAMs has attracted considerable interest. Much of the key work in this area has been preformed using ferrocenelabeled alkanethiols, although a variety of other metal complexes have been used. Murray and co-workers have measured electrontransfer kinetics for mixed monolayers of (ferrocenylcarboxy)alkanethiols and n-alkanethiols in chloroethane/butyronitrile at low temperatures (F166, F167). The use of low temperatures improves the stability of the monolayers in a nonaqueous environment. The results of this work were complicated by an apparent kinetic dispersion, thought to originate from a distribution of formal potentials (F167). Murray’s group have also measured electron-transfer rates for self-assembled ferroceneoctanethiol monolayers on metal-coated high-temperature superconductor electrodes at sub-Tc temperatures (F168). The ferrocene monolayers displayed well-defined voltammetry down to 105 K. The voltammetry and rate measurements are the first of their kind for a molecular monolayer on an electrode in the superconducting state. Smalley et al. have used a laser-induced temperature jump method to measure electron-transfer kinetics for the same type of ferrocene-containing monolayer (F169). This method allows rates to be measured that are too fast for conventional chronoamperometry. For short-chain thiols, the decrease in the reorganization energy as the chain length was decreased was found to be larger than expected. Electron-transfer kinetics have also been investigated for a Langmuir-Blodgett monolayer of 16-ferrocenylhexadecanoic acid deposited on a self-assembled alkanethiol monolayer (F170). Voltammetry of the ferrocene groups, which reside at the hydrophobic monolayer/monolayer interface is suppressed when the outer layer is well packed and prevents ingress of ions. The factors influencing packing and ion transfer were investigated. Forster and Faulkner (F171, F172) have reported a comprehensive investigation of the effects of solvent, electrolyte, and temperature on the electrochemical response of spontaneously adsorbed monolayers of [Os(bpy)2Cl(pNp)]+, where pNp is 4,4′bipyridyl, 1,2-bis(4-pyridyl)ethane, or 4,4′-trimethylenedipyridine. Changes in the formal potential are used to probe the extent of ion pairing and the local environment within the monolayer, while changes in electron-transfer kinetics are used to test contemporary theories of electron transfer. Surprisingly, the electron-transfer kinetics depend strongly on solvent dynamics. Mixed monolayers of adsorbed Os and Ru bipyridine complexes, or two Os complexes with different anchoring ligands, have Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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also been investigated (F173). In the later case it was possible to distinguish between the two Os complexes, which had identical formal potentials, on the basis of their different heterogeneous electron-transfer rate constants. The same method was also used to distinguish between two anthraquinone-disulfonic acid isomers coadsorbed on mercury (F174). Solvent effects on the thermodynamics and kinetics of electron transfer have also been investigated for HS(CH2)nCONHCH2pyRu(NH3)5(PF6)2/HS(CH2)nCOOH (n ) 10 or 15) mixed monolayers (F175). Cyclic voltammetry indicated the presence of strong ion pairing and a relatively disordered structure of the monolayers. The dominant factors that control the kinetic parameters of the monolayers in nonaqueous solvents appear to be monolayer disorder and the local water concentration. The influence of SAMs on the electrochemistry of species in solution has attracted considerable attention, since it can provide valuable information about the quality of the SAM and can form the basis for selectivity in analytical applications. The permselectivity and sensitivity of 1,2-dithiolane-3-pentanoic acid/1-hexanethiol mixed monolayers to various probes has been controlled by changing the extent of dissociation of the monolayer COOH head groups and changing the hexanethiol content of the films (F176). The influence of carboxylic acid head groups on permeability has also been investigated using SAMs of unsymmetrical dialkyl sulfides (F177). Although the monolayer structures are disordered, no solution permeation occurs into purely hydrocarbon monolayers, while substantial permeation occurs with the acid-containing monolayers. Permeation increases as the acid group is placed closer to the monolayer/solution interface. The carboxylic acid group can buffer pH changes within the monolayer (F178). The intrinsic pKa values for phosphatidic acid in self-organized monolayers deposited on mercury have been determined from differential capacitance measurements (F179). The effects of OH, COOH, and NH2 terminal groups on the redox responses of Fe(CN)63-, Ru(NH3)63+ (F180), and several ferrocene derivatives (methanol, trimethylammonium, and sulfonate substituted) (F181) in aqueous solution have also been investigated. The results can be explained primarily in terms of electrostatic interactions between the terminal group of the alkanethiol and ionic redox species. The electrochemical activity of Fe(CN)63-/4- at thiol-based SAMs on gold has also been investigated by impedance spectroscopy (F182). The experimental results are fitted to a model which considers both throughfilm and at-defect electron-transfer paths. A variety of novel SAMs with electroactive head groups have been reported. Efforts have been largely concentrated on their synthesis and characterization, but some interesting results and applications are beginning to emerge. SAMs of a new series of redox-active transition metal complexes of the type [Os(bpy)2Cl(Py(CH2)nSH)](PF6), with n ) 4, 6, or 9, have been described (F183). The effect of the working electrode potential on the amount of material deposited was found to provide a facile way to determine Epzc for solid electrodes. SAMs of n-mercaptoalkyl tetrathiafulvalenecarboxylates react with tetracyanoquinodimethane to form a monolayer containing a charge-transfer complex (F184). An interesting two-step method for the preparation of SAMs with metal complex head groups has been described (F185). Bis(salicylaldehydo)copper(II) was reacted in situ with 2-amino412R
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ethanethiol or cystamine SAM-coated gold electrodes to form an immobilized Schiff base. Self-assembled monolayer films of several cobalt porphyrins have been used for electrocatalytic dioxygen reduction (F186, F187). The films retain catalytic activity for more than 105 turnovers and are more active and durable than nonspecifically adsorbed cobalt tetraphenylporphyrin (F188). A self-assembled monolayer of cystamine has been used to immobilize the quinonoid enzyme cofactor, pyrroloquinolino quinone (PQQ), on an electrode surface for the first time (F189). The electrochemistry of PQQ is completely irreversible on a bare Au electrode but reversible at the cystamine-modified electrode and when immobilized. The same method was used to immobilize a spiropyran monolayer for use in a device demonstrating thermal and photochemical control of electron transfer (F190). Electron transfer to a solution probe could be turned on by thermal isomerization of the monolayer and turned off by photochemical isomerization. SAMs of ω-(N-pyrrolyl)alkanethiols on gold have been electrochemically polymerized (F191). Dilution of the pyrrole sites with 75% n-hexanethiol in the assembly solution results in mixed monolayers which still display surface-confined pyrrole oxidation but no voltammetric evidence for polymer formation (F192). This suggests that the two thiols are homogeneously mixed in the monolayer. One of the most attractive potential applications of SAMs is in sensors with molecular recognition properties. A number of SAMs incorporating host or guest functionalities have therefore been developed and, in some cases, shown to demonstrate molecular recognition. The binding of monolayer-anchored ferrocenyl groups by an amphiphilic calixarene host in solution has been found to significantly change the formal potential of the immobilized ferrocene moieties (F193). Molecular recognition is demonstrated at concentrations as low as 0.05 µM. The parent nonamphiphilic calixarene produces no response. A self-assembled monolayer containing a bis(bipyridinium)-pphenylene head group was sensitive to π-donor aromatic molecules such as indole and catechol but insensitive to π-acceptors such as benzonitrile and nitrobenzene (F194). The host/guest interactions were detected by a shift in the bipyridinium voltammetric response. SAMs prepared from a variety of functionalized resorcin[4]arenes with four di-n-decyl sulfide chains have been prepared and thoroughly characterized (F195). SAMs of ω-mercaptocarboxylic acids on mercury film and gold electrodes have been used for the very sensitive and selective analysis of ultralow levels of cadmium(II) (F196). A detection limit of 4 × 10-12 M was achieved using a cathodic stripping procedure. The adsorption of tetraphenylborate and tetraphenylphosphonium ions in self-assembled monolayers of phosphatidylcholine and phosphatidylserine on mercury has been investigated by chronoamperometry and differential capacitance measurements (F197). Deviations from the Henry isotherm behavior were interpreted on the basis of an adsorption isotherm which accounts both for discreteness-of-charge effects and for the presence of two regions of different dielectric constant. Langmuir-Blodgett Monolayers. The electrochemical properties of thin films of electroactive materials formed on
electrode surfaces by the Langmuir-Blodgett (L-B) technique have been reviewed (F198). Ubiquinone has been incorporated into otherwise passivating octadecanethiol/octadecanol L-B monolayer films on gold to allow controlled access of Ru(NH3)63+ ions to the electrode surface (F199). At low concentrations, the ubiquinone “gate sites” act as an array of molecular size microelectrodes. The electrochemical properties of L-B films of Ru(P(C18H37)3)2(3,5-di-tert-butyl-o-benzosemiquinonate)2 have been characterized in situ in the L-B trough (F200). Interactions between the L-B monolayer and electroactive species in the subphase, including blocking, mediation, and ion pairing, have been probed. Rectified transmembrane electron transfer through mixed L-B monolayers of totally π-conjugated electroactive compounds 1-methyl-4-[2-[4-[2-(4-quinolyl)vinyl]phenyl]vinyl]quinolinium perchlorate and 1-methyl-4-[2-[4-[2-(4-pyridyl)vinyl]phenyl]vinyl]pyridinium perchlorate to hexacyanoferrate(II) in solution has been reported (F201, F202). Other Monolayers. The strong interest in SAM and L-B monolayer-modified electrodes has rekindled interest in other methods for immobilizing monolayers. Covalent binding of the monolayer to the electrode surface offers the potential for increased stability, but the generation of an ordered layer becomes a problem that has not yet been solved. Oxidized platinum electrode surfaces have been modified with chlorosilane monolayers by reaction with SiCl4 vapor (F203). The reaction of such surfaces with nucleophilic agents such as alcohols, amines, thiols, and Grignard reagents provides a versatile route to covalently bound monolayers. Although monolayer coverages of the nucleophilic agent can be achieved, and surfaces modified with long-chain alkyl groups efficiently block electrode reactions of redox probes in solution, the films are not highly ordered, self-assembled monolayers. Electrochemical methods for covalent attachment of 1-alcohols (F204) and amine-containing compounds (F205) to glassy carbon electrodes have been reported. These methods have significant potential as a general method for producing functionalized electrodes. The utility of the amine-based method was demonstrated by the oxidation of β-NADH at a dopamine-modified electrode. Electrodes of the superconductor YBa2Cu3O7-δ have been modified with adsorbed monolayers of a number of different ferrocene-labeled ligands (F206). An amine functionality was found to give the largest surface coverage and best stability. Self-Assembled Bilayers and Multilayers. Interactions of calcium ions with L-B bilayers of the synthetic phospholipid 1,3dihexadecylglycero-2-phosphoethanolamine on an electrode surface have been investigated using Fe(CN)63-/4- as a redox probe and an EQCM (F207). Ca2+ ions disrupt electrostatic intermolecular interactions and expand the monolayer packing, allowing increased access of Fe(CN)63-/4- to the electrode surface. The structure of bilayer assemblies consisting of octadecanethiol and L-R-dipalmitoyl phosphatidylcholine (DPPC) has been investigated using Ru(NH3)63+ as a solution redox probe and ubiquinone within the bilayer (F208). The ubiquinone is mobile in the DPPC layer and its electrochemistry is affected by the density of pinhole defects in the octadecanethiol monolayer. The mechanism of transient current generation at bilayer lipid membranes formed from a mixture of dipalmitoylphosphatidic acid and phosphatidylcholine has been investigated using a combina-
tion of electrochemical and fluorescence studies (F209). The transient current was found to be due to a combination of doublelayer reorganization and lipid head group reorganization. The ionic resistance of films of a bilayer lipid/polystyrenesulfonate complex immersed in KCl(aq) was found to change by a factor of 106 over a narrow temperature range encompassing a phase transition (F210). It is speculated that the phase transition causes Cl- ions in the film to become mobile. BIOELECTROCHEMISTRY Small Molecules of Biological Importance. Efforts continued on a number of small molecules including NADH, amino acids and peptides, carbohydrates, catecholamines, nitric oxide, flavins, and various others. Some articles of note on these various species are cited in this particular subsection. Additional research on catecholamines and neutrotransmitters can be found in the “Cellular and in Vivo Electrochemistry” subsection. Electrochemical studies directed toward hydrogen peroxide electrocatalysis have been included in the “Enzyme Electrodes” subsection. Intensive research effort continues to be directed toward the goal of facile electrochemical oxidation of reduced NADH. Although mediator-modified electrodes have received the most attention for NADH oxidation, three interesting reports of unmediated oxidation did appear. Kinoshita et al. (G1) found that “plastic formed carbon” electrodes, which are composed of a graphite/ carbon composite that can be easily fabricated into electrodes of various sizes and geometry, resulted in reproducible NADH oxidation at moderate overpotential without pretreatment. Another interesting study by Silber et al. (G2) found that bare screenprinted thick-film gold electrodes, which are porous, exhibited electrocatalytic activity toward NADH oxidation at low overpotential. Nowall and Kuhr used an electrochemical treatment to modify the surface of carbon fiber electrodes (G3). This electrode exhibited very low overpotential for oxidation of NADH, DOPAC, dopamine, uric acid, and ascorbate and also provided a discriminatory capability. New reports of mediator-modified electrodes for NADH oxidation included that of Grundig et al. (G4), who examined the mediation mechanism for cationic quinonoid dyes such as Meldola Blue when immobilized in graphite-epoxy electrodes. Other mediators employed in more-or-less monolayer configuration on electrodes included PQQ covalently attached to gold and platinum electrodes (G5), phenoxazine and phenothiazine derivatives covalently attached to gold electrodes (G6), toluidine blue covalent attached to gold electrodes (G7), and various organic dyes, e.g., methylene blue, toluidine blue, and brilliant cresyl blue, strong adsorbed to glassy carbon electrodes (G8). All of these mediatormodified electrodes oxidized NADH at low potentials, in the vicinity of 0 V. Several reports of NADH mediation using electroactive polymer-modified electrodes also provided evidence for low overpotential catalysis. Persson et al. (G9) incorporated Meldola Blue, perhaps the most successful NADH mediator, into a random block methylsiloxane polymer layer formed by dropcoating on graphite electrodes. Electropolymerized films exhibiting activity toward NADH included poly(3,4-dihydroxybenzaldehyde) on glassy carbon, which functions via quinone mediation (G10), and poly(phenosafranine), which is structurally similar to Meldola Blue, containing the phenazinium ring system (G11). Turning our attention to the electrochemical detection of amino acids (and proteins), a quite interesting and general strategy based Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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on a flow-through dual-electrode configuration was described (G12). In this method, bromide is oxidized at the upstream electrode to generate bromine, which readily reacts with most amino acids. Any reaction is then detected at the downstream electrode as a decrease in re-reducible bromine. Weber’s group continued their investigations of peptide analysis using the biuret reaction, which involves complexation with the Cu(II)/Cu(III) couple, in an RRDE study of several model glycine peptides (G13). Ye and Baldwin continued their work on the use of copper electrodes for amino acid and peptide analysis. Using copper walljet electrode detection with capillary electrophoresis, the detection limits for most nonhydrophobic amino acids were found to be 1-10 fmol (G14). Pulsed electrochemical detection has enjoyed widespread success for analysis of various biomolecules including sulfur-containing compounds. Johnson and co-workers added substantial insight into the relevant mechanism through voltammetric and EQCM studies of cysteine adsorption at pure gold electrodes following oxide reduction (G15) and of the role of cysteine in promoting the dissolution of gold during gold oxide formation/reduction (G16). Voltammetric determination of adenosine was reported using electrochemical treated carbon fiber electrodes by Chen et al. (G17). Following the lead of Johnson’s group for noble metals, a pulsed electrochemical waveform was developed that resulted in reproducible voltammetry at the carbon fibers. Carbohydrates have been successfully determined voltammetrically by pulsed electrochemical detection at noble metal electrodes and by electrocatalysis at modified electrodes. Three recent studies of successful modified electrodes are as follows: Kano et al. (G18), who electrocatalytically oxidized carbohydrates at copper oxide-modified electrodes presumably involving a surface Cu(III) species; Lyons et al. (G19), who concluded that surface-bound oxyruthenium groups are the catalytically active species in the electrooxidation of glucose at RuO2/carbon paste composite electrodes; and Cataldi et al. (G20), who described physical characterization of a previously reported electrode, the NiII-tetramethyldibenzotetraaza[14]annulene-modified electrode, which bears operational similarity to the nickel hydroxide electrode. Catecholamines were the subject of several investigations. Ciolkowski et al. (G21) characterized the possible impact that following chemical reactions, in particular disproportionation, could have on the Coulombic quantitation of catecholamine release from biological cells by electrooxidation at microelectrodes. It was concluded that, for most microelectrode applications, the impact would be negligible. Tabei et al. (G22) achieved a 0.5fmol detection limit for dopamine at an IDA carbon microelectrode detector coupled to a microbore-HPLC system. Low background noise and enhanced current due to redox cycling at the IDA were key factors in achieving this level of detection. Jaramillo et al. (G23) described how ionic strength and cation charge impacts the electrochemistry of dopamine at surfactant-modified graphite electrode surfaces. Nitric oxide has emerged during the last decade as a small biomolecule of exceeding importance in numerous physiological processes. A challenging molecule to detect quantitatively for a variety of reasons, NO is attracting increased attention from the electrochemical viewpoint by various workers. Bedioui and coworkers (G24) provided new insight into the metalloporphyrinicmodified electrode approach to NO detection previously intro414R
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duced by Malinski. Using nickel-containing porphyrin film electrodes with a nitrite-excluding Nafion overlayer, linear current/ concentration calibration was obtained from 1 nM to 40 µM. Yu and Su (G25) considered the mechanism of NO reduction in aqueous solution using manganese porphyrins and found it to proceed by an ECE pathway. A detailed mechanistic investigation of the NO reduction pathway at iron porphyrins, which is of relevance to the physiological process in nitrite reductases, was provided by Liu and Ryan (G26). Two fundamental investigations of flavin electrochemistry were noted. Time-resolved SERS was used to elucidate the detailed mechanism of photoinduced charge transfer from flavin mononucleotide (FMN) to the silver electrode. Two short-lived radical ion intermediates were identified in this study by Zhang et al. (G27). In situ FT-IR thin-layer spectroelectrochemistry with a flat mercury electrode was employed by Birss et al. (G28) to obtain, for the first time, the infrared spectrum of the unstable reduced form of flavin adenine dinucleotide (FAD) in aqueous solution. Various modifications of electrode surfaces with flavin moieties were also reported. These included an electropolymerized FAD film electrode that electrocatalytically reduces dioxygen (G29) as well as two examples of flavin monolayers on solid electrodes, namely, riboflavin covalently attached to glassy carbon (G30) and vitamin B12 adsorbed on graphite (G31). Voltammetric characterization of these electroactive monolayers in the absence and in the presence of both oxidizable and reducible substrates was described. Electrocatalytic oxidation of ascorbic acid was observed and characterized at polypyrrole/dodecyl sulfate-modified electrodes (G32) and at the bis(4-pyridyl) disulfide-modified gold electrode (G33). The results of a fundamental voltammetric/FT-IR mechanistic investigation of ascorbic acid adsorption and oxidation on Pt and bismuth-covered Pt electrodes were given by Climent et al. (G34). Finally we note a few isolated but quite significant reports on other small biomolecules of interest. Taniguchi et al. (G35) incorporated Meldola Blue into self-assembled decanethiol monolayer-modified electrodes and demonstrated that this interfacial system could function as a biomembrane model with molecular gating. Gordillo and Schiffrin gave an interesting mechanistic accounting of the redox properties of ubiquinone adsorbed to the mercury electrode (G36). Bacteriochlorophyll a, substituted with a variety of metals, was extensively characterized for the first time using electrochemical and visible/near-IR spectroelectrochemical methodology (G37). Pickett and co-workers have described some interesting work on the incorporation of biological structural motifs into polymer films on electrodes. They have successfully incorporated ironsulfur clusters into cationic polymer films on electrodes and characterized their voltammetric and spectroscopic behavior (G38). They have followed this up with a report of a polypyrrole film with amino acid and peptide incorporation achieved by polymerization of appropriate pyrrole derivatives (G39). Use of cysteinyl derivatives was found to result in subsequent binding of ferredoxin-like centers in the polymer. Protein Electrochemistry. (This section emphasizes interfacial electron-transfer and biochemical studies.) As in the previous review, this material has been divided among the following four subsections: simple electron-transfer proteins, myoglobin/hemoglobin, unmediated enzymes, and other studies.
Examples of both diffusional and diffusionless electrochemistry can be found under individual categories. Under the “other studies” subheading, several informative papers dealing specifically with protein adsorption at solid/aqueous interfaces can be found, as well as mediated enzyme electrochemistry, photobioelectrochemistry, and several articles on the electrochemistry of metallothioneins, which were not counted as simple electrontransfer proteins because they involve cysteinyl redox chemistry. “Simple” Electron-Transfer Proteins (Cytochromes, Ferredoxins, Blue Copper Proteins). In this subsection, we have roughly organized articles as follows: first, interfacial studies of basic proteins, mainly c-type cytochromes; second, interfacial studies of acidic proteins, mainly blue copper proteins and ferredoxins; and third, direct electrochemical studies concerned primarily with properties of the protein rather than the interface. The distinction drawn between these latter studies and the interfacial studies is, by necessity, somewhat artificial in certain cases. Cytochrome c, especially the equine species, has continued to be the object of many interfacial electrochemical studies, typically utilizing electrode surfaces exhibiting some negative charge. Cotton, Dong, and co-workers have nicely demonstrated the promoter effects of adsorbed halides on gold surfaces for cytochrome c electrochemistry (G40, G41). The ability of halides to promote cytochrome c electrochemistry, iodide > bromide > chloride > fluoride, was found to depend on halide adsorption energy, which in turn controlled surface concentration (G41). Some reports describing cytochrome c electrochemistry promoted by more conventional promoters by Safronov, Hill, and co-workers were overlooked in the last review and are included here (G42G44). In these studies dipeptide promoters, consisting of cysteine for anchoring to the gold and a second amino acid, were examined primarily with regard to the effect of the pH of the solution from which the promoter was adsorbed. Szucs and Novak reported some thought-provoking electrochemical studies of diffusing and adsorbed cytochrome c at bare gold and glassy carbon electrodes (G45, G46). They were able to obtain examples of both well-behaved and poor responses for nonchromatographically purified cytochrome c by manipulating experimental procedures and solution conditions. A model involving an adsorbed monolayer of cytochrome c was described in which the key element dictating diffusional electrochemical behavior was the structural state of the protein adsorbates, i.e., monomeric vs oligomeric. Cytochrome c was also the subject of two studies using lipidmodified electrodes. Salamon and Tollin, who previously published an interesting series of papers on these electrodes, described the cyclic voltammetry and chlorophyll-sensitized photoelectrochemistry of cytochrome c at phosphatidylcholine films (G47). Incorporation of dihexadecylphosphate in the film resulted in enhanced responses, presumably due to attractive electrostatic interactions. Bianco and Haladjian used mixed lipid layers, composed of phosphatidylcholine and cholesterol, in electrochemical studies of four different c-type cytochromes (G48). Electrostatic effects were also pronounced in their studies, as exemplified by the dramatically favorably effect on current that resulted when laurate anions were included in the film composition. A comparable study of cytochrome c3 electrochemistry at these same lipidmodified electrodes was also reported (G49). The immobilization of cytochrome c in (sub)monolayer configurations continued to receive notable attention. Niki’s group
continued their work in this area with an electroreflectance spectroscopic determination of the diffusionless standard electrontransfer rate constants of cytochrome c immobilized on COOHterminated self-assembled monolayers (G50). Hobara et al. (G51) discussed results obtained from SERS studies of cytochrome c on gold and silver electrodes modified with the bis(4-bipyridyl) disulfide and 4-mercaptopyridine promoters. Previously proposed models, in which cytochrome c is pictured as existing in three predominant interfacial conformations, depending upon the interfacial environment, were confirmed. Kuznetsov et al. (G52) used carbodiimide reagent to covalently attach cytochrome c to COOH self-assembled monolayers. Some interesting results were obtained in these experiments in which preferential orientations of cytochrome c are believed to have been obtained due to applied potential control prior to covalent binding. Cytochrome c bound under negative applied potential exhibited much better electrochemistry than that bound at positive potentials, which is consistent with the notion of facile electron transfer through the positively charged heme-edge face of the protein. Adsorbed cytochrome c3 was addressed in two articles. Zhang et al. (G53) conducted a detailed electrochemical examination of adsorbed cytochrome c3 at mercury, glassy carbon, and gold electrodes. The behavior of the protein was quite varied depending on the electrode material with the most complex results occurring as a result of strong adsorption on mercury. Sagara et al. (G54) utilized an electroactive cytochrome c3 layer on silver to demonstrate a new electroreflectance phase-shift technique that can separate overlapping currents arising from different adsorbed and diffusing species. We now turn to examples of simple proteins whose surface interaction region for facilitating electron transfer is believed to be anionic in nature. Interfacial electrochemical investigations of blue copper proteins have been reported by three groups in ongoing efforts. Ikeda and Sakurai (G55) described the direct electrochemistry of stellacyanin at bare glassy carbon electrodes under a variety of solution conditions. Results provided support for a primarily hydrophobic interfacial interaction between the electrode and the protein as well as evidence of radial diffusion at higher concentrations of protein. Kohzuma et al. (G56) characterized the electrochemistry of pseudoazurin at the bis(4pyridyl) disulfide-modified gold electrode at pH values corresponding to protonated and unprotonated promoter states. Differences in rate were interpreted in terms of molecular structure and the roles of the two possible electron-transfer pathways featured in this protein. Conrad et al. (G57) observed stable direct electrochemistry of parsley plastocyanin at bare edge-pyrolytic graphite (EPG) electrodes in contradistinction to previous results with plastocyanins from other species. This observation, unusual in that both EPG and plastocyanin are negatively charged, was interpreted in terms of more subtle structural features of this particular species. Armstrong’s group continued their investigations of adsorbed ferredoxin monolayers with an interesting study of proton-gated electron transfer in the 7Fe protein ferredoxin I from Azotobacter vinelandii (G58). Mutation of an aspartate believed to be involved in proton transfer to the 3Fe-4S cluster of this species resulted in dramatically different interfacial kinetics. In their work, aminocyclitol coadsorbates were used to promote favorable interactions of ferredoxin to the EPG electrode. Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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New strategies for coupling anionic proteins to electrode surfaces were also reported in several studies. Indium oxide electrodes, long known to be excellent substrates for cationic protein electrochemistry as well as being optically transparent, were successfully used for the first time with anionic proteins. Nishiyama et al. (G59) described an aminosilanization of indium oxide electrodes that yielded well-behaved direct electrochemistry of ferredoxin. In a second report from the Taniguchi group (G60), it was shown that adsorption of polylysine or polyarginine on indium oxide also gives rise to a modified surface suitable for ferredoxin electrochemistry. Bianco et al. (G61) also reported the use of polylysine adsorption to create modified electrodes that are well-suited for ferredoxin. In their study the substrate electrode was polished pyrolytic graphite. In a study from Walker’s group, Rivera et al. (G62) took a related but somewhat different approach to realize facile direct electrochemistry of an acidic electron-transfer protein. They examined the heme protein cytochrome b5 at gold electrodes modified with β-mercaptopropionic acid and found that addition of a short-chain polylysine (degree of polymerization 16) promoted the electrode reaction extremely well. Molecular models based on NMR measurements were proposed for the polylysine/cytochrome complex. The use of lipid-modified electrodes, discussed above for cytochrome c, was also employed by Bianco and Haladjian for ferredoxin electrochemistry (G63). Incorporation of long-chain species carrying positive charge, e.g., dodecylamine, into phophatidylcholine/cholesterol films was required to realize facile electrochemistry, once again supporting the notion of electrostatics, at least in large part, in controlling these reactions. To finish up this subsection of simple electron-transfer proteins, we cite a number of studies in which direct electrochemistry was used in important characterizations of the protein itself. Sun et al. (G64) utilized the bis(4-bipyridyl)-modified electrode in a highpressure electrochemical cell (0-200 MPa) to measure the reaction volume associated with electron transfer in cytochrome c. A small reaction volume, 5.0 ( 0.8 cm3/mol, was reported. Cai et al. (G65) utilized a gold microband electrode modified with a new promoter, 4,6-dimethyl-2-mercaptopyrimidine, to measure the redox potential of cytochrome c as a function of temperature, pH, and solution composition. A thermodynamic and kinetic study was performed by Verhagen et al. (G66) on the monohemic cytochrome c553 from Desulfovibrio vulgaris (Hildenborough) using bare glassy carbon electrodes. The formal potential of cytochrome c553 was determined as a function of temperature and pH, and the second-order kinetics of its reaction with hydrogenase were measured. Hagen’s group was also quite active with iron-sulfur proteins, employing direct electrochemistry to examine several of these. In one study, the effect of mutating the conserved Glu-92 residue in spinach ferredoxin was investigated at glassy carbon electrodes in the presence of neomycin as a promoter (G67). Spinach ferredoxin, along with Spirulina platensis ferredoxin and the watersoluble fragment of the Rieske protein, was the subject of an electrochemical characterization of 2Fe-2S proteins (G68). Evidence supporting “superreduction”, i.e., beyond the Fe(II), Fe(II) state, was found for the Reiske cluster at about -0.8 V vs SCE, the first observation of this phenomenon for solution species. The redox chemistry of 4Fe-4S clusters in seven high-potential ironsulfur proteins (HiPIPs) was described by Heering et al. (G69). Only one of these exhibited superreduction, which was confirmed 416R
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by EPR spectroscopy. Moura’s group also remained active in applying direct electrochemistry to the characterization of redox proteins. In a recent report Moreno et al. (G70) employed square wave voltammetry to characterize the redox chemistry of a series of metal (M)-substituted iron-sulfur clusters of MFe3S4 structure as well as two related ferredoxins from Desulfovibria gigas, the 3Fe-4S ferredoxin I and the 4Fe-4S ferredoxin II. In these experiments, Mg(II) was used to promote the electrochemistry at glassy carbon electrodes. Evidence was also found here for superreduction of the 3Fe-4S cluster. The use of thin-layer difference FT-IR spectroelectrochemistry in studies of redox proteins has been pioneered by Mantele’s group. In a recent report they have thoroughly characterized the electrochemistry and redox-induced vibrational structural changes for an archaebacterial blue copper protein, halocyanin, using a pyridine-3-carboxyaldehyde thiosemicarbazone-modified gold electrode to drive the reaction (G71). Mabrouk has demonstrated the feasibility of performing direct electrochemistry of biomolecules in nonaqueous media (G72). Microperoxidase-11, a cytochrome c fragment, exhibited quasireversible cyclic voltammetry at the bare glassy carbon electrode in dimethyl sulfoxide solutions. Finally, a simple but clever technical advance was reported by Haladjian et al. (G73) for performing protein electrochemical studies when amounts are limited. By simply entrapping microliter amounts of the protein solution between the working electrode and a dialysis membrane, one readily obtains wellbehaved thin-layer electrochemical behavior without the usual uncompensated resistance problems. The strategy was tested with cytochrome c and then used to characterize a bacterial cytochrome c552. Myoglobin and Hemoglobin. Rusling and co-workers published an impressive series of papers (G74-G77) describing the incorporation of myoglobin into liquid crystalline films prepared from didoceyldimethylammonium bromide (DDAB) as well as films formed from other surfactants. Film-bound myoglobin, which exhibits well-behaved quasi-reversible direct electrochemistry in aqueous media as well as in contact with microemulsions, is able to electrocatalytically reduce organohalide pollutants such as ethylene dibromide. In a recent paper, experimental evidence was presented supporting the view that these surfactants function in part by preventing deleterious adsorption of contaminating species present in protein preparations while allowing or facilitating myoglobin access to the underlying substrate electrode (G77). In other reports dealing with the electrochemistry of the globins, Torkornoo and Hawkridge (G78) examined the urea unfolding of the myoglobin-imidazole couples using CV and absorption spectroscopy. Ohno and Tsukuda (G79) continued their studies on PEO-modified myoglobin by examining the direct but kinetically irreversible electrochemistry of this species in an oligomeric PEO (MW ) 400) film. Chen and co-workers (G80) presented evidence for the direct electrochemistry of hemoglobin on bare silver electrodes. Zhang et al. (G81) described the STM of hemoglobin adsorbed on HOPG. When adsorbed on an anodized surface at open circuit, the hemoglobin image reflects a globular structure, but when adsorbed for 1-h periods at very negative potentials, STM indicates highly unfolded structures on the HOPG surface. Faulkner et al. (G82, G83) have reinvestigated in detail the thermodynamics of the hemoglobin redox system using thin-layer spectroelectrochemistry. Electronic and steric
contributions to hemoglobin functionality were highlighted in this work. Enzymes: Unmediated Electrochemistry. Two impressive reports of integral enzyme incorporation into electrode-supported bilayers were published. Cullison et al. (G84) immobilized cytochrome oxidase in a lipid bilayer membrane using gold/thiol self-assembly techniques in conjunction with deoxycholate dialysis. The immobilized enzyme exhibited direct electron transfer with the gold substrate as well as diffusing cytochrome c in solution. Naumann et al. (G85) also described incorporation of a large integral enzyme complex, ATPase, into a gold-supported lipid bilayer by fusing ATPase-containing liposomes with a preformed thiolipid layer. Square wave voltammetric evidence for reduction of protons that were translated enzymatically across the bilayer was presented. Lui and Cowan (G86) performed an extensive direct electrochemical study on two sulfite reductases from D. vulgaris (Hillsborough) using SWV at chromiumIIIhexamine-promoted edgepyrolytic graphite. The hexameric assimilatory reductase, containing 4Fe-4S and siroheme groups, weighs in at 200 000 Da, which apparently makes it the largest redox protein characterized to-date by direct electrochemistry. The monomeric assimilatory sulfite reductase species was also examined, and the effect of pH was carefully examined for both proteins. SWV was also used in a nice report by Tong and Feinberg (G87) of a direct electrochemical study at edge-pyrolytic graphite electrodes of another important iron-sulfur protein, aconitase, whose ironsulfur cluster can be interconverted between 3Fe-4S and 4Fe4S forms. Previously determined as well as new redox transformations involving the superoxidized 4Fe-4S and superreduced 3Fe-4S states of the protein were elucidated. Kohzuma and co-workers reported interesting new results on the direct electrochemistry of copper-containing nitrite reductases (NiR) from Achromobacter denitrifying bacteria. These molecules are trimers of ∼105 kDa. The NiR from Achromobacter xylosoxidans exhibited direct electrochemistry at the bis(4-pyridyl) disulfide-modified gold electrode (G88) whereas the NiR from Achromobacter cycloclastes required a promoter, apopseudoazurin (G89). Electrocatalytic reduction of nitrite was observed and kinetically characterized (G89). The first report of direct electrochemistry on hyperthermophilic enzymes was reported by Adams and co-workers (G90). These workers used differential pulse voltammetry and polished pyrolytic graphite electrodes to characterize the redox potentials of four enzymes and two ferredoxins from two different hyperthermophiles over the temperature range 25-95 °C. Other Studies of Proteins and Enzymes. Four papers dealing with mediated enzyme electrochemistry were chosen as particularly noteworthy. Yang and Murray (G91) examined the electrocatalytic behavior of poly(ethylene glycol)-modified horseradish peroxidase (PEG-HRP) in both organic and aqueous solvents. The organic solvents used, ethyl acetate and 1,2-dichlorobenzene, resulted in the same PEG-HRP-catalyzed reactions as in water for catechol and ferrocene reductants. Faulkner et al. (G92) achieved the electrocatalytic ω-hydroxylation of lauric acid by cytochrome P450 4A1 using cobalt(II)sepulchrate as the mediator reductant. A detailed mechanistic study of the hydroxybenzoyl-CoA reductase by mediated spectroelectrochemistry was described by El Kasmi et al. (G93). Farhangrazi et al. (G94) employed mediated thinlayer spectroelectrochemistry to determine the formal potentials
of the compound I/compound II and compound II/ferric redox couples for HRP. These quite high potentials, near +1 V, were determined using hexachloroiridate(IV) as the mediator. Photobioelectrochemistry was examined in a number of papers. Here we cite three of these, two involving orientation of photosynthetic systems on metal substrates with the third involving mediated electrocatalysis. Katz (G95) immobilized reaction centers in a specified orientation on carbon electrodes using covalent linkage via a cysteine residue. Photoinduced currents were much higher for these than for reaction centers anchored in random orientations via lysine residues. Picorel et al. (G96) were able to determine orientations of three different photosystem II pigment-protein complexes on silver electrodes and silver colloids using SERRS. Finally, Amako et al. (G97) used dimethylbenzoquinone as a mediator to drive the photoelectrochemical oxidation of water at a membrane-restrained photosystem II layer on carbon paste electrodes. The electrochemistry of metallothionein proteins, which is complex due to the sulfur chemistry of cysteines, were reported by two different groups. Rodriguez and co-workers examined a cysteinous peptide fragment, Lys-Cys-Thr-Cys-Cys-Ala (G98), and also Cd,Zn-metallothioneins (G99) as a function of pH at the dropping mercury electrode. Sestakova et al. (G100) examined Cd,Zn-metallothionein, Cd-metallothionein, and apometallothionein at both mercury and carbon electrodes, for which quite different behaviors were observed. We wrap up this protein electrochemistry section by pointing out four articles dealing primarily with the protein adsorption process rather than electrochemistry per se. Jordan et al. (101) used polarization-modulated FT-IR spectroscopy and surface plasmon resonance to characterize the adsorption of poly(L-lysine) on COOH-terminated self-assembled monolayers. They found that lysine interaction with the SAM surface gave rise to the formation of carboxylate-ammonium ion pairs that electrostatically bound the plypeptide to the substrate. Feng and Andrade (G102) examined the adsorption of several proteins on silicon-alloyed lowtemperature isotropic carbon (LTIC), which is used in heart valves, and on gold, as a function of applied potential and pH. Electrostatic effects were more important at gold whereas adsorption at the carbon surface appeared to be driven primarily by stronger hydrophobic interactions. Randriamahazaka and Nigretto (G103) reported interesting new results in their ongoing study of the interaction of blood coagulation proteins at carbon electrodes. They found that adsorbed human factor XII could be activated by applied potential on carbon paste electrodes. Adsorption of two proteins, immunoglobulin (IgG) and glucose oxidase (GOx), onto highly oriented pyrolytic graphite was characterized by Cullen and Lowe (G104) using in situ AFM. Different time courses of nucleation for the two proteins were observed. GOx adsorbed in a patchy fashion and also appeared to be in a denatured state. For those using ellipsometry to characterize protein layers on gold, Martensson and Arwin (G105) caution that use of the conventional three-phase model (ambient/protein/ substrate) is insufficient for accurate determinations. They advise using a four-phase model in which the fourth phase is believed to represent a thin electron depletion zone in the gold arising from its interaction with the protein. Enzyme Electrodes (Amperometric Biosensors). As was the case two years ago (A1), this topic generated a large number of publications for the current review. Unlike two years ago, the Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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interested reader will find this year’s version of this section to be shorter with fewer cited publications cited. General trends are highlighted along with some representative publications, and any articles that seem particularly innovative or of general interest were also included. As a result, numerous quality articles describing worthwhile biosensor developments and experimental results have, of necessity, not been cited. There is also less descriptive text introducing and elaborating the various subsections. The reader is referred to the prior review (A1) for a more detailed description if interested. Also, informative review articles on various topics relating to amperometric biosensors were highlighted above in section A. Finally, the reader is referred to the review on sensors in this same issue, as well as Wang’s review (A24) in the Clinical Chemistry chapter of the 1995 Application Reviews issue of this journal. Theory. Wang presented a generalized method of assessing the extent of interferences in amperometric sensors through the introduction of selectivity coefficients (G106, G107). Bacha et al. (G108) presented a numerical model for analyzing the transient response of multilayer amperometric biosensors and compared it to experimental results. Martens and Hall (G109) described a steady-state model for three-substrate-mediated oxidase electrodes where an artificial substrate-mediator competes with a natural cosubstrate. Rhodes et al. (G110) presented a detailed and generalized computer simulation for modeling and optimizing the GOx electrode under a variety of design parameters and solution mass transfer conditions. Redox Wired Polymer Film Electrodes. Considerable activity continued in this area with several groups reporting on the development of new polymers and previously unincorporated enzymes, all of which are not cited here. Some examples include reports on new osmium-based polyacrylamide redox hydrogels (G111) and osmium-based poly(1-vinylimidazole) hydrogels (G112) for glucose sensing by Heller and co-workers, on glucose sensors based on incorporation of GOx in poly(ether amine quinone)s by Kaku et al. (G113), and on a nitrate sensor based on nitrate reductase incorporation into a viologen polypyrrole film by Cosnier et al. (G114). In two comprehensive articles, Willner’s group addressed the electrochemical, photochemical, and photoelectrochemical reactions of glutathione reductase wired in redox polymers (G115, G116). Oyama et al. (G117, G118) described the application of a thermoshrinking ferrocenic polymer in amperometric enzyme applications. They demonstrated the capability of repeatedly loading and unloading enzyme into the film (G118). Elmgren et al. (G119) described an investigation of charge propagation rates through osmium-based redox polymer films with an emphasis on the effect of enzyme loading. Riklin et al. (G120) reported a novel approach for wiring GOx by modification of the cofactor with an electron-transfer relay. FAD was first removed from the enzyme, derivatized with a ferrocene moiety, and then reconstituted with the apoprotein, giving rise to an electroactive GOx species. Coury et al. (G121) developed an amperometric biosensor for acetominophen based on a multienzyme chemical amplication phenomenon. Polymer Film Electrodes (Except Redox Wired). One the hottest topics here was the issue of built-in selectivity exhibited by certain enzyme-entrapping electropolymerized films, especially for glucose sensors. Poly(phenylenediamine) films have shown promising selectivity in previous years and were further investigated in 418R
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reports during this time period by Zambonin and co-workers (G122, G123) and Lowry et al. (G124). Overoxidized polypyrrole is another material that exhibits promising selectivity properties that again were characterized by Zambonin’s group (G122, G125). Zhao and Luong (G126) have emphasized on the other hand neutral or underoxidized polypyrrole as a material whose selectivity can be controlled. Some promising new reports on Nafion, which is well-known for eliminating certain interferences, appeared. Rishpon et al. (G127) were able to incorporate GOx in Nafion to produce a stable glucose sensor. Moussy et al. (G128) have described a trilayer enzyme electrode consisting of platinum/ poly(phenylenediamine)/cross-linked GOx/Nafion that can be cured at 120 °C to improve the in vivo durability of the outer Nafion coating without loss of sensor sensitivity. Three other noteworthy articles addressing aspects of glucose sensors are cited here. Gros and Bergel (G129) performed a useful theoretical and experimental study of the behavior of polypyrrole/glucose oxide amperometric electrodes. Lowry’s group reported the interesting observation that glucose could be readily sensed by an electropolymerized poly(o-phenylenediamine)/GOx electrode in nitrogen-saturated solutions with sensitivity identical to that obtained in air-saturated solutions even though dioxygen is presumably the precursor for the hydrogen peroxide that is actually detected (G130). Alternatives to solution dioxygen reaction were proposed. One other glucose sensor, in this case designed to operate in an oxygen-independent manner, was described by Palmisano et al. (G131). The construction was based on the electropolymerization of a GOx-entrapping poly(phenyenediamine) layer on top of an organic conducting salt electrode. Coche-Guerente et al. (G132) provided a detailed characterization of their previously reported glucose sensor based on electropolymerization of a preadsorbed layer of GOx and an amphiphilic pyrrole derivative. Bartlett and Birkin (G133) have fully characterized a previously reported “enzyme switch”, also referred to as a microelectrochemical transistor. This device responds to glucose by a mechanism involving enzymatically based reduction of a polyaniline gate. Monolayer-Type Electrodes. Covalent attachment of enzymes and enzyme cofactors to metallic electrodes was exploited by several groups in new strategies for creating monolayer amperometric biosensors. PQQ serves as a redox cofactor for a number of different enzymes. Willner and Riklin (G134) linked PQQ to a gold electrode via chemisorbed cysteamine after which malic enzyme was covalently linked to the PQQ species. Bioelectrocatalyzed oxidation of malic acid proceeded by NADPH cycling between the enzyme and the PQQ moiety. Katz and co-workers followed a different approach by reconstituting an enzyme, glucose dehydrogenase, onto a gold electrode surface (G135). PQQ, which is the native cofactor of this enzyme, was covalently bound to a cysteamine/gold electrode surface through flexible spacer groups, which allowed the apoprotein to reconstitute with it. The interested reader should also consult their detailed voltammetric study of the PQQ monolayer in the absence of enzyme (G136). For site-specific covalent attachment of proteins to surfaces, the use of cysteine/gold chemistry appears highly promising as reported in the previous review. An example was reported recently by Vigmond et al. (G137), who bound dihydrofolate
reductase to gold via a single genetically engineered cysteine residue. A different, immunological, approach to preparing monolayer enzyme electrodes was advanced by Bourdillon et al. (G138). In their approach, an antigen, rabbit IgG, was first adsorbed on a carbon electrode, and this was followed by a binding reaction with a GOx/antibody conjugate which irreversibly attached the enzyme. In a subsequent paper, this approach was extended to create multilayer films in which the overlayers were initiated by binding of a monoclonal antibody to the conjugate GOx (G139). In a series of original papers, Willner and co-workers (G140G143) enunciated a new approach to monolayer enzyme electrodes in which the interaction of light assumed a central role. These devices demonstrate the possibility of amperometric transduction of optical signals as well as the use of light to control interfacial bioelectrode process, i.e., a photoelectrochemical switch. The key element in their systems is a reversible photoisomerizable molecule, such as spiropyran, which when covalently bound using thiol/gold chemistry to an electrode, can be interconverted by light between two isomer states. Several different schemes for coupling the structural isomerization to electron transfer between proteins and the underlying gold electrode were presented for cytochrome c (G140, G142), PQQ (G141), and several redox enzymes (G142, G143). Other examples of the increasing utilization of gold/thiol chemistry in the development of amperometric biosensor have been noted. Creager and Olsen (G144) used hydroxy-terminated alkanethiol self-assembled monolayers to dramatically diminish background currents in a glucose sensor. Kajiya et al. (G145) immobilized a mediator, 2-(aminoethyl)ferrocene, and GOx on a 4-(aminothio)phenol-modified electrode with the use of glutaraldehyde cross-linking to produce a glucose sensor. Irreversible adsorption of enzymes on carbon electrodes has long been a fruitful method for immobilizing active enzymes that exhibit electrical interfacial communication without the need for exogenous mediators. Peroxidases have continued to be a popular and worthwhile enzymatic system to investigate. Several papers dealing with kinetics and reaction mechanisms include those by Ruzgas et al. (G146) and by Bogdanovskaya et al. (G147) on adsorbed HRP, and by Scott and Bowden (G148) on adsorbed cytochrome c peroxidase (CCP). Dong and Guo (G149) described a method for preparing an adsorbed HRP graphitic electrode that operated well in nonaqueous media for the detection of hydrogen peroxide. Ikeda et al. (G150) discussed the bioelectrocatalytic reaction mechanism of adsorbed alcohol dehydrogenase, a quinohemoprotein, on the basis of electrochemical and electroreflectance methods at both carbon and metal electrodes. Evidence was found for a preferred orientation with the heme c site of the enzyme in close proximity to the electrode surface. Finally, an article by Broderick et al. (G151) is noted, not because it is concerned with bioelectrocatalysis, but rather because it advances our understanding of enzyme adsorption on metal surfaces. A non-heme enzyme, chlorocatechol dioxygenase, was found to retain its native properties when adsorbed on silver colloids, as shown by SERRS spectra of the enzyme complexed with substrate, product, and inhibitor. To conclude this monolayer section, we note three papers dealing with imaging of enzyme adsorption. Shiku et al. (G152) used the scanning electrochemical microscope to micropattern and then image diaphorase on glass. Controlled inactivation of
enzyme to create nonactive regions was performed by electrogenerating bromide or chloride at the microelectrode tip. Chi et al. (G153) reported STM images for folded and unfolded forms of GOx on HOPG. Hutton et al. (G154) used laser photoelectrochemical microscopy (PEM), which results in current variations due to localized heating, to image GOx adsorption on a platinized carbon paper. GOx was found to adsorb in a patchy arrangement with active regions of 10-30 µm in diameter. Carbon Paste and Other Bulk Composite Electrodes. The majority of important developments in renewable bulk materials continues to reside with composite carbon pastes, which offer some very attractive analytical attributes. The carbon paste materials are reviewed first, followed by four reports on other composites. Kulys and Hansen (G155) characterized the longterm response of GOx-containing glucose sensors constructed with a variety of enzyme stabilizers and pasting oils. They found enzyme stability to be the dominant factor in the long-term response. Investigations of additives for improving enzyme stability and biosensor performance were the focus of many publications. Lutz et al. (G156) investigated the effects of a large number of chemical and solid additives on the performance of an o-quinone-mediated tyrosinase carbon paste electrode material finding improved performance with most of the chemical additives. Gasparini et al. (G157) found that incorporation of Sepharose resulted in drastically reduced amounts of required enzyme needed for comparable sensor response. Wang and co-workers (G158, G159) reported on the improved sensitivity and stability of carbon paste sensors with incorporation of fumed silica, a high surface area powder. Wang and co-workers also report on another noteworthy continuing development, namely, dispersed metal carbon paste enzyme electrodes (G160, G161). By incorporating dispersed rhodium, which preferentially reduces the oxidase enzymatic product hydrogen peroxide in an electrocatalytic manner at low potentials, a glucose biosensor that exhibits impressive selectivity was demonstrated (G160). A brief review on metallized carbon paste biosensors was given (G161). Kulys et al. (G162) investigated the effects of protein and polycation incorporation on the electrode reaction of ferri/ferrocyanide. Finally, we note a paper by Kacaniklic et al. (G163) in which the emphasis was on the development of bienzymatic carbon paste composite sensors for detection of L- and D-amino acids. Incorporation of L-amino acid oxidase, which generates hydrogen peroxide product, along with HRP, which electrocatalytically reduces the hydrogen peroxide, resulted in a sensor that could readily detect all 20 common L-amino acids. Four other reports are noted that give descriptions of enzymatically active bulk electrode materials that can be renewed by mechanical polishing. The strategy of codeposition of enzymes and noble metals was applied by Sakslund et al. (G164) in a study of the palladium/GOx system. Korell and Spichiger gave a detailed accounting of their previously reported bulk composite biosensor material prepared by mixing the organic conducting salt (OCS) TTF-TCNQ with silicone oil binder and xanthine oxidase (G165) and reported the first example of coupling a reductase, in this case HRP, to an OCS electrode (G166). An interesting electrically conductive enzyme-encapsulated sol/gel biosensor was described by Pankratov and Lev (G167). Conductivity was imparted through the incorporation of enzymeimpregnated graphite powder into the silica sol solution prior to gelation. Gilmartin et al. (G168) gave a more detailed account of Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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their previously reported uric acid biosensor based on the incorporation of uricase and cobalt phthalocyanine (CoPC) catalyst into a graphite-epoxy resin, as well as a mechanistic voltammetric and XPS characterization of the CoPC-catalyzed hydrogen peroxide oxidation reaction (G169). Microenzyme Electrodes. Redox hydrogel-wired peroxidase microsensors were employed to detect hydrogen peroxide generated photoelectrochemically at ITO and titanium dioxide surfaces (G170) as well as hydrogen peroxide generated electrochemically in the diffusion layer of gold and carbon electrodes (G171). In the latter application the microelectrode was operating in an SECM. Adsorbed peroxidase carbon fiber electrodes were employed by Csoregi et al. (G172) in an investigation of microelectrode performance in a flow-through amperometric detection arrangement. In a paper of general importance to those researchers developing carbon fiber biosensors, Hopper and Kuhr (G173) continued their surface characterization studies with a combined voltammetric and fluorescence/ECL study of the effectiveness of various oxide-generating pretreatments with regard to electrontransfer properties. Two microelectrode array papers are noted. Meyer et al. (G174) described a 20 × 20 array of sequentially addressed platinum microelectrodes that were used to locally detect dioxygen and hydrogen peroxide and, after overcoating with a polypyrrole/ GOx film, glucose. It was proposed that the array could be considered as an alternative imaging device to the SECM. Ross and Cammann (G175) also fabricated a polypyrrole/enzymecoated microelectrode array device, which could detect glucose, choline, and lactate. Other Enzyme Electrode Studies. Some papers that did not end up in any of the preceding sections are roughly organized among the following topics: techniques, new substrates and materials, organic-phase biosensors, and mediated bioelectrocatalysis. Janata and co-workers (G176) described the first chronopotentiometric analysis of an enzyme electrode process utilizing the GOx system for this purpose. It is believed that this technique offers promise as an investigative tool for bioelectrode systems, in particular for the development of redox-enzyme-FET devices. Hall’s group utilized impedance techniques extensively in informative studies of an important organic conducting salt enzymic system, N-methylphenazinium (NMP)-TCNQ overcoated with GOx (G177, G178), and of several key conducting polymers in biosensor situations (G178). SECM studies have been cited in several other sections. It was also used to image, on the several micrometer-scale, localized enzymatic reactions due to GOx confined to the pores of a microfiltration membrane and of NADH cytochrome reductase signaling the presence of individual mitochondria (G179). Several other materials developments did not obviously fall into previous subsections. These include Wang et al.’s report (G180) of ultrathin spongelike porous carbon film transducers that allow large enzyme loadings in small geometric areas; Higson and Vadgama’s reports (G181, G182) of diamond-like microporous membranes, which have useful biocompatibility and selectivity properties; and two new immobilization matrices for sensor enzymes: a cubic liquid crystalline phase containing monoolein, as described by Razumas et al. (G183), and crystalline bacterial cell surface layer microparticles cross-linked with glutaraldehyde, which were used by Neubauer et al. (G184) to make sucrose sensors. 420R
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Two different approaches to organic phase biosensing were described. Iwuoha et al. (G185) examined the effect of solvent in 90/10 solvent/water mixtures on the catalytic performance of GOx sensors in an RDE kinetic study. Dong and Guo (G186) developed a peroxidase immobization strategy that allowed hydrogen peroxide to be amperometrically detected in water-free organic solvents. The immobilization matrix was a polyhydroxyl cellulose that gels upon refrigeration to form a suitable aqueous enzymatic microenvironment protected from the organic phase. Mediated electrocatalysis was utilized by Moore et al. (G187) in multienzymatic chemical amplification sensor for salicylate. Rivas and Solis (G188) described the solution enzyme kinetic characterization of tyrosinase by voltammetric detection of the enzyme product. Kuwabata et al. (G189) described the mediated electrochemical conversion of carbon dioxide to methanol using formate dehydrogenase and methanol dehydrogenase. Miki et al. (G190) reported that cytochrome c, hemoglobin, and myoglobin could be used to electrocatytically reduce nitric oxide. Polynucleotides and Nucleic Acids. The development of electrochemical DNA sensors, what could now be termed the conventional general strategy, involves immobilization of a singlestranded DNA sequence (the “probe”) on an electrode surface, which upon hybridization with its complementary strand (the “analyte”) gives rise to an amperometric or voltammetric signal. Surface immobilization of single-stranded DNA in a manner that allows it to readily hybridize is an issue of critical importance, as is the actual transduction event, which could, for example, involve electroactive intercalation. Mikkelsen and co-workers have adapted their prior strategy, based upon voltammetry of surface-accumulated cobalt(III)trisbipyridine, which associates with doublestranded DNA, to carbon paste electrodes (G191). Voltammetric detection of a base sequence indicative of cystic fibrosis was demonstrated for an 18-base oligodeoxynucleotide. Hashimoto and co-workers reported further development of their approach, also based on voltammetric detection of a hybridization indicator molecule (G192, G193). Single-stranded DNA was attached to a gold electrode through a mercaptohexyl linker, and the hybridization event was signaled by accumulation of Hoechst 33258, an electroactive dye that binds the minor groove of DNA. Using a 20-base probe, a targeted oncogene sequence was detected over a 10-7-10-13 g/mL range. Bard and co-workers also continue to be active in this sensor area, describing the immobilization of single-stranded DNA on aluminum(III) alkanebisphosphonate films through binding of the DNA phosphate groups to the exposed metal sites on the film (G194). Hybridization was also observed, and in a following study, Xu and Bard (G195) described ECL detection schemes based upon accumulation of intercalated Ru(II)tris(phenanthroline) as well as using indicator-labeled analyte strands. A potentially very attractive approach for immobilizing DNA was reported by Livache et al. (G196) who electrochemically copolymerized pyrrole and pyrrole-derivatized oligonucleotides into a conducting polypyrrole matrix. Hybridization was verified nonelectrochemically by using radioactively labeled complementary strands. A nonheterogeneous electrochemical approach to sequencedependent determination of polynucleotides utilizes solution hybridization of analyte sequences with redox probe-labeled DNA probes followed by HPLC separation and electrochemical detec-
tion. Takenaka et al. (G197) extended this approach to ferrocenelabeled probes and reported detection levels of 20-100 fmol. Homogeneous hybridization was also used by Pandey and Weetall in an investigation of photochemically active anthraquinone intercalators (G198). In previous years, Palecek and co-workers developed an adsorptive transfer stripping voltammetry (AdTSV) method for the detection of trace amounts of DNA at mercury electrodes. Palecek and Fojta (G199) have recently extended this strategy to the detection of RNA at the subnanogram level. They found that traces of RNA ( GG > GT > GC, were interpreted in terms of alterations in effective electron-transfer distance. Johnston et al. (G203) also reported on a promising new electrocatalytic cleavage reagent, a trans-dioxorhenium(VI/V) redox complex, which readily catalyzes the one-electron oxidation of guanine in doublestranded DNA. Welch et al. (G204) improved significantly upon existing methods for electrochemical determination of DNA diffusion coefficients by employing a trisbidentate osmium complex that exhibits simple noncleaving oxidative electron transfer and that binds quantitatively via intercalation and electrostatic attraction. Normal pulse voltammetry in conjunction with data analysis with the COOL algorithm were used to extract the diffusivities, which were in agreement with independent methods. A report that could have significant implications for STM studies of DNA is that of Jelen et al. (G205). They examined the two-dimensional condensation of benzalkonium chloride (BAC) at the mercury electrode as a model for adsorption on mica, a negatively charged surface used for STM imaging. It was found that BAC facilitated the anchoring of DNA on mica for imaging. Cellular and In Vivo Electrochemistry. Although not strictly falling in either one of these categories, the in vitro electrochemical mechanistic work from Dryhurst’s group has
direct and important ramifications for brain chemistry and disease. A previously cited introduction and review is a good place to start (A38). Detailed reaction product characterization and mechanistic interpretations have been reported in a series of recent papers addressing the oxidation chemistry of serotonin (G206, G207), dopamine (G208), 5-hydroxytryptophan (G209), and indole-3acetic acid (G210) and the possible implications of oxidative metabolites with regard to neurodegenerative diseases. Jackson et al. (G211) examined the transient oxidation of 5-hydroxytryptamine (5-HT) using fast-scan cyclic voltammetry at carbon microelectrodes and reported methodology for detecting 5-HT in the presence of other neuronal compounds. Both in vitro and in vivo (rat brain) experiments were described. Kawagoe and Wightman (G212) examined amperometry as an alternative technique to CV for in vivo measurement of dopamine in the rat brain. Although they found it to be a viable technique for this application, it had little in the way of advantages to recommend it over CV. Other significant in vivo experiments were those of Garguilo and Michael (G213, G214), who developed an enzymemodified microelectrode and employed it for the detection and quantitation of choline in brain tissue. The microsensor incorporated HRP and choline oxidase into a cross-linkable redox polymer with a Nafion overlayer to prevent interference from ascorbate. Heller’s group, utilizing several technological advances including redox hydrogel-wired enzyme layers, demonstrated the feasibility of fabricating a 0.29-mm flexible electrode for in vivo subcutaneous determination of glucose (G215). This device featured four layers including an interference-eliminating enzyme layer and allowed for one-point in vivo calibration to be performed. Another approach to getting at physiological fluids involves microdialysis with electrochemical detection. An example of this approach is Murphey and Galley’s account of glycerol detection in human plasma using a microdialysis electrode (G216). The amperometric electrode operated by oxidizing hydrogen peroxide liberated by reaction of glycerol in a glycerol kinase/L-glycerol3-phosphate oxidase bienzymatic reaction layer. Amperometric and voltammetric monitoring of exocytotic and other secretory release processes at isolated individual cells was once again a microelectrode subject that yielded some spectacular experimental results. Huang et al. (G217) employed a ruthenium oxide/cyanoruthenate microelectrode to detect exocytotic insulin release from individual pancreatic β-cells. In another contribution from the Kennedy group (G218), the oxidation of tyrosine and tryptophan-containing peptides was used as the basis for detection of R-melanocyte stimulating hormone (R-MSH) released from individual melanotrophs isolated from rat pituitary glands. Wightman’s group, in a series of papers (G219-G221), described amperometric and cyclic voltammetric measurements of the release of epinephrine, norepinephrine, catecholamines, and ascorbate from adrenal medullary cells. In these studies, it was possible to distinguish different cell compartments holding different molecules. Furthermore, for catecholamine secretion, evidence was presented for catecholamine dissociation from the vesicular matrix at the cell surface as being the rate-determining step. Chen et al. (G222) reported detection of zeptomole amounts of catecholamine release from stimulated rat pheochromocytoma (PC12) cells using carbon fiber electrodes. Shifting gears to reach our final topic in this subsection, we found several fascinating accounts that address the issue of whole Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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cell immobilization on electrode transducers. Wong et al. (G223) were able to culture aortic endothelial cells on fibronectin-coated polypyrrole grown on ITO electrodes. Potential control of the conducting state of the polymer could be used to control cell shape and growth rates. Martens and Hall (G224) immobilized viable photosynthetic bacteria in film-forming emulsion polymers of the methacrylate/acrylate family. Amperometry was used to assess the viability of the electron transport chain, and rotating disk experiments were used to characterize film diffusion properties. Korzeniewski and co-workers characterized the respiration of NB2a neuroblastoma cells by immobilizing them on aminated fluoropolymer gas-permeable membranes (G225). Dioxygen consumption was measured using a Clark-type oxygen sensor. Ikeda’s group reported new results from their continuing program on immobilized cell bioelectrocatalysis (G226). Bioelectrocatalytic hydroxylation of nicotinic acid to 6-hydroxynicotinic acid, a useful compound for pesticide synthesis, was achieved via mediated electrolysis of bacterial cells of Pseudomonas fluorescens constrained behind a dialysis membrane on a carbon electrode. Immunological and Recognition-Based Electrochemistry. Wittstock et al. (G227) imaged immobilized antibodies attached to glass surfaces using the SECM. This was achieved by first saturating the antibody binding sites with an antigen-enzyme conjugate and then using the immobilized enzyme to catalyze a solution reaction whose product can be detected amperometrically at the SECM tip. Using alkaline phosphatase (AP) as the enzyme, the conversion of 4-(aminophenyl)phosphate to the electroactive 4-aminophenol (PAP) served to signal the localized sites of the immobilized goat anti-digoxin antibodies. Another significant contribution from the Heineman group was the development of an electrochemical capillary immunoassay for assays of low molecular weight species such as therapeutic drugs (G228). This method involves sequential saturation of immobilized antibody by antigen-AP in a 0.53-mm-i.d. silica capillary followed by PAP generation and detection. Digoxin could be determined with short incubation times in 20-µL samples with an absolute detection limit of 260 amol. The same group also described the development of a simultaneous dual-analyte immunoassay method based upon releasable metal ion labels that are then detected by anodic stripping voltammetry (G229). The electrochemical detection of anti-digoxin antibody was investigated by Suzawa et al. (G230) in a homogeneous immunoassay involving an immobilized conjugate of digoxin and GOx. Multilabeling of GOx with ferrecene mediator sites resulted in improved sensitivity. A clever and promising strategy was developed by Duan and Meyerhoff (G231) toward the goal of a noncompetitive sandwich enzyme immunassay that requires no washing steps. All current noncompetitive sandwich assays require such manipulations before introducing the enzyme substrate in order to remove the large amount of antigen-enzyme conjugate remaining after the incubation step. In their new approach, tested with human chorionic gonadotropin (hCG), antibody was immobilized on a porous gold membrane after which the enzyme substrate was introduced from the membrane backside through the pores. By localizing the enzyme-substrate reaction and product detection near the surface, efficient discrimination was achieved against the unbound conjugate that remained in solution. This immunoassay method is also compatible for hCG detection in whole blood samples. A second example of a heterogeneous affinity assay that 422R
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requires no washing steps is based upon a redox-wired enzyme electrode and was demonstrated by Vreeke et al. (G232) for the biotin-avidin reaction. In this strategy, a layer of avidin-containing osmium redox polymer (poly(1-vinylimidazole)) was first prepared on a carbon disk electrode. After exposure to HRP-labeled biotin, which accumulates in the layer, a current due to the electrocatalytic reduction of exogenous hydrogen peroxide can be observed at +0.1 V vs Ag/AgCl that is dependent on biotin levels. Degrand and co-workers have continued to develop strategies for electrochemical immunoassay based upon voltammetric detection of accumulated cationic species in Nafion films. The key properties of Nafion that can be advantageously exploited include permselectivity of hydrophobic cations over hydrophilic cations, exclusion of proteins, and exclusion of anions. In one study (G233), a method for detecting AP, an important immunoassay enzyme, was developed. The method was based on enzymatic conversion of a ferrocene-labeled anionic substrate to a neutral electroactive product that then accumulated in the Nafion layer for subsequent detection by square wave voltammetry. Competitive enzyme immunoassays based on the Nafion-modified electrode were also developed for small antigens in both heterogeneous and homogeneous formats (G234, G235). Molecular imprinting has been used to develop artificial recognition sites for various applications. Kriz and Mosbach reported the development of a molecularly imprinted polymer that was used in a competitive binding scheme for assaying morphine concentration, which was detected amperometrically (G236). The concept of a reversible multicycle immunosensor based on an amperometric photoisomerizable monolayer-modified electrode was developed by Willner et al. (G237). This approach, which requires a photoisomerizable component of the antigen monolayer that binds antibody in one photoisomeric state but not in the other, thus releasing it, was demonstrated using a dinitrospyran moiety attached to gold via thiolate chemistry. CHARACTERIZATION OF REDOX REACTIONS Although a considerably abbreviated version of that found in past accounts is included here, it was felt best to maintain some continuity with previous reviews by inclusion of this section. Typically, many citations that would (have) be(en) found here would (will) also be found cited in other sections of the review. The papers cited below generally fall under the category of electrochemically driven homogeneous redox reactions. Citations are organized in the following order: metallic species, metal porphyrins, organic species, micellar and miscellaneous. Several investigations that caught the reviewer’s eye included the Freund and Lewis study (H1) of the irreversible electrocatalytic reduction of VO2+ to VO2+ in sulfuric acid, in which phosphomolybdic acid was found to catalyze the reaction by redoxstate-dependent differential binding. Richards and Geiger (H2) used CV to quantitatively characterize a square scheme reaction mechanism sequence for an equilibrium mixture of two isomers of CpCo(C8H8), finding a lower limit of 2 × 105 s-1 for the isomerization rate constant. Sabatini and Anson (H3) measured the cross-electron-transfer rate constants between Nafion-confined Ru(II) complexes and solution oxidants, concluding that distributed redox potentials of the film-bound reactant have negligible effect on Marcus theory predictions. Compton and co-workers (H4, H5) described hydrodynamic channel electrode mechanistic investigations of some electrodimerization reactions of cobalt
complexes and photochemically driven CE reactions of iron complexes. A number of other publications from this group can be found in other sections of this review. Some novel polyazinebridged Os(II)/Ru(II) trimetallic species exhibiting unique electrochemical behavior were described by Richter and Brewer (H6). Reduction of Pt(IV) and oxidation of Pt(II) dithiocarbamate species were examined by Bond et al. (H7) using electrochemistry, ESR, and MS. The choice of ligand dictated whether the reaction proceeded through a stable Pt(III) intermediate. Takashi et al. (H8) utilized UV/visible spectroelectrochemistry in the characterization of a series of trinuclear Rh(III) and Ir(III) complexes. The use of thin-layer FT-IR spectroelectrochemistry is becoming more widespread in the characterization of redox reactions of transition metal complexes. Among the investigations utilizing this powerful method were those of Chin et al. (H9) on boronic mixed-valent metallocarborane complexes; of van Outersterp et al. (H10, H11) on unusually stable radical anion reduction products of Mn-Re and Os3 metal-metal complexes; of Lyons et al. (H12) on a series of molybdenum hexacarbonyls with progressive CO substitution by 2,6-dimethylphenyl isocyanide ligands; and by Hill et al. (H13), who examined electroreductive metal-metal bond cleavage in tetranuclear complexes containing rhodium with either Mn or Fe. Metalloporphyrins continue to be an active area of electrochemical and spectroelectrochemical research. Kadish and coworkers reported on σ-bonded Fe(III) porphyrins with nonplanar porphyrin rings (H14), antimony and phosphorus tetra-p-tolylporphyrins (H15), and zirconium and hafnium double-decker complexes of porphyrin-metal-phthalocyanine structure (H16). Saveant and co-workers investigated the role of the CO ligand in electrochemical reduction of Fe(I) porphyrins and Zn(II) porphyrins to the phlorin anion complexes (H17). Arnold and Heath (H18) spectroelectrochemically characterized covalently bridged conjugated porphyrin dimers containing various metal centers. The covalent bridge was 1,3-dialkynyl. Thin-layer FT-IR spectroelectrochemistry was applied by Hinman and Olorunyolemi (H19) to the characterization of series of metal-substituted octaethylporphyrin complexes in dibromomethane. Turning to the characterization of organic redox reactions, we note the impressive work being continued by Saveant’s group on reductive dissociative electron-transfer reactions of halogenated species. These included studies of reduction of various dihaloalkanes that can result in ring closure or fragmentation (H20) and an investigation of cleavage rates of electrogenerated halogenated radical anions that focused attention on the role of charge localization (H21). An extensive study of the electrochemical reduction mechanism of nitrosobenzene in acetonitrile was reported by Steudel et al. (H22). Finally, Anne et al. (H23) investigated deprotonation rate constants and kinetic isotope effects of cation radicals of synthetic analogs of NADH; they found no evidence for significant involvement of proton or hydrogen atom tunneling processes. Lastly, a few papers are cited that deal with redox reactions in microheterogeneous media. Gounili et al. (H24) measured electron-transfer rate constants for oxidation of ferrocenic species in a bicontinuous microemulsion. Myers et al. (H25) measured redox potentials of various species in microemulsions and developed relations for predicting them. Ryabov et al. (H26) also measured ferrocenic formal potentials but in anionic, cationic, and nonionic micelles. They also demonstrated that micellar ferro-
cenium could be captured by GOx as an electron acceptor for glucose oxidation, but that n-dodecylferrocenium and decamethylferrocenium could not, presumably due to stronger hydrophobic interactions with the micelles. In a different vein, Kitamura et al. (H27) examined electron transfer and mass transfer at the microdroplet/water interface using Fe(III) oxidation of ferrocene contained in oil microdroplets. SPECTROELECTROCHEMISTRY Spectroelectrochemistry is a very active field, and mutiple hundreds of papers were produced during the period of this review, which is restricted to selections from this vast output. In addition to the great bulk of papers dealing with UV/visible or near-infrared absorption or reflection spectroscopy, there is an increasing number of papers using Fourier transform infrared spectrometry, as well as Raman spectroscopy (normal, resonance, and surface-enhanced Raman techniques), with a significantly smaller number using other techniques, including surface plasmon resonance, X-ray absorption or scattering, and magnetic resonance, with emphasis on electron paramagnetic resonance (EPR). Activity in the area of electrochemical mass spectrometry is currently building up a head of steam. Numerous groups are pursuing this line. UV/Visible Absorption or Reflection Spectroelectrochemical Methods. Holze has reviewed spectroelectrochemical methods, with the novice in mind. Examples are presented, including applications to conducting polymers and adsorption of aliphatic alcohols (I1). Several groups have investigated the effects of uncompensated ohmic iR drop on the performance of thin-layer spectroelectrochemical cells. Dong et al. derived a simple, linear relationship to relate iR drop to peak potential separation in thinlayer spectroelectrochemistry under static conditions (I2). Sundholm and Talonen used the backward implicit method to numerically solve the differential equations for convective diffusion for radial flow from the periphery to the center of a thin-layer cell under conditions used for infrared external reflection spectroelectrochemistry. They showed that the reaction was isolated to the outer periphery of the working electrode unless the cell was sufficiently thick (>7 µm) and that experimental deviations from theory resulted from nonidealities in electrode flatness and parallelism (I3). Dong and co-workers have used finite difference simulations to model potential step and cyclic voltammetric response in an OTTLE cell, and devised a method for estimating the rate constant for following chemical reactions based on the time required for exhaustive electrolysis (I4). Several groups have investigated the effect of temperature on electrochemical processes using spectroelectrochemistry. Segelbacher et al. have studied the temperature dependence of spectra of a homologous series of end-capped oligothiophenes (as models for polythiophene) in a thin-layer optical device, observing a relationship with chain length but an unusual temperature dependence (I5). Hoyer and Weller have investigated quantum size effects on the redox potentials of zinc oxide colloidal particles using an OTTLE cell. Measured size-dependent potentials are consistent with particle-in-a-box calculations (I6). Scherson et al. have described UV/visible spectroelectrochemical studies in a thin-layer channel under conditions of laminar flow (I7). Niu and Dong have investigated the mechanism Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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of electrooxidation of biliverdin by rapid-scanning spectrophotometry in an OTTLE cell (I8). Numerous UV/visible spectroelectrochemical studies of inorganic redox chemistry have been reported. For example, a number of papers cover metalloporphyrins, including solvent-related changes of axial ligation of Mn(II) and Mn(III) tetraphenylporphyrins (I9), characterization of a series of nonplanar s-bonded phenyl- and (perfluorophenyl)iron(III) porphyrins (I10), characterization of a tetraruthenated zinc porphyrin (I11), and photoinduced electron-transfer reactions of a Mg(II) porphyrin (I12). Several of these studies involve multiple spectroelectrochemical modes in addition to UV/visible absorption. Saveant and co-workers found OTTLE spectroelectrochemical measurements coupled with other electrochemical methods and Raman spectroscopy quite useful in elucidating the reduction pathways of iron(I) porphyrins. The presence of an axial carbon monoxide ligand promotes reduction of the ring instead of Fe(I) (I13). Park et al. have applied bifurcated fiber-optic UV/ visible illumination and a charge-coupled device with a spectrograph to afford full-spectrum kinetic studies on time scales as short as 25 ms over a range of 200-800 nm (I14). Potential Modulation. Potential modulation coupled with spectroelectrochemical measurements has been demonstrated to be a useful tool. Zhao and Scherson applied this approach, using near-normal incidence reflection and absorption UV/visible spectroelectrochemistry at a rotating disk electrode for investigation of electrode processes in which both reactant and product may absorb light, with application to both systems with soluble reactant and product (ferricyanide/ferrocyanide), and codeposition systems in which the product is insoluble (Ni-Fe codeposition) (I15). Niki and co-workers applied the approach to characterize hemin and Nile Blue A adsorbates and their electron-transfer kinetics on a stationary glassy carbon electrode (I16). Long Optical Path and Diffusion Layer Imaging Methods. Several groups have worked on spectroelectrochemical studies with long optical paths, involving either grazing incidence or illumination parallel to an electrode surface. Hudson and Riley have devised a procedure for determining the electrode position and source coherence, based on an intensity profile analysis of the diffraction pattern focused on a diode array (I17). Schindler et al. have applied diffusion layer imaging to investigate the effects of the solvent and the supporting electrolyte on reaction parameters for the benzophenone/benzophenone radical anion redox couple formed at a Hg-covered Au wire in aprotic solvents (I18) and to study intermediates and products in the electrochemical reduction of nitrosobenzene at a platinum mesh working electrode (I19). They have also reported results and compared experiment with finite difference simulations for kinetic investigations of a heterogeneous electron-transfer reaction with spatially resolved UV/visible absorption measurements in planar, cylindrical, and spherical diffusion layers (I20). Heering and Hagen have presented a mathematical treatment of the dependence of absorbance on concentration distribution in thin-layer electrochemical cells with a long optical path parallel to the electrode surface (I21). Wanzhei et al. have numerically simulated a potential step catalytic reaction mechanism in a spectroelectrochemical cell with long optical path length, using an expanding grid finite difference method, and applied the results to determine kinetic parameters and diffusion coefficient in the ferrocyanide/ascorbic acid system in glycine/nitric acid solution (I22). Lee et al. have determined the equilibrium constant for 424R
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dimerization of methyl viologen radical cation using a spectroelectrochemical cell with a long optical path under conditions of semiinfinite diffusion (I23). Xie et al. have coupled grazing incidence spectroelectrochemistry with quartz crystal microbalance methodology to characterize the reduction of Cu(II) and stripping of Cu(0) in presence of ammonia (I24). Imaging. Sawada and Harata have reported an interesting approach based on the ultrasonic generation of a transient reflecting grating for time-resolved subsurface characterization and imaging of a thin film at an electrochemical interface (I25). Flaetgen et al. have reported the two-dimensional imaging of potential waves on an electrode surface by surface plasmon microscopy (I26). Engstrom et al. have observed the spatial distribution of adsorbate coverage with electrogenerated chemiluminescence imaging (I27). Li and White have reported the interferometric imaging of the depletion layer structure adjacent to a disk-shaped platinum microelectrode by means of phasemeasurement interferometric microscopy based on gradients of refractive index. Optical voltammograms of apparent optical height vs potential tracked the voltammetric current response (I28). Anderson and Groeber have used a microelectrode array as a crude diffraction grating for sensing electrode reaction products to focus light at a selected detector position, with intensity influenced by the production of an absorbing reaction product (I29). Circular Dichroism. Several workers have used circular dichroism in spectroelectrochemical experiments. Dong and coworkers have extracted kinetic parameters for the electrochemical oxidation of tryptophan using circular dichroism in a long optical path length thin-layer cell (I30) and have investigated conformational changes occurring during the electrochemical oxidation of bilirubin and a complex of bilirubin with serum albumin (I31). Taniguchi et al. have investigated the circular dichroism spectra of spinach ferredoxin as a function of applied potential using thinlayer ellipsometry (I32). Infrared Spectroelectrochemical Methods. Infrared spectroelectrochemistry, particularly based on the use of FT-IR spectrophotometers, is becoming increasingly popular. A significant limitation remains due to the need to use very thin cells to overcome solvent absorption problems, particularly in aqueous media, leading to problems of uncompensated ohmic drop and slow response times. A number of cell designs and new experimental approaches have been developed to address such problems. Budevska and Griffiths have described the first application of step-scan FT-IR external reflection spectrometry to investigations of the electrode/electrolyte interface, based on potential modulation. The Stark shift of CO adsorbed on a polycrystalline Pt electrode was monitored. The approach is contrasted with traditional FT-IR spectroelectrochemical methods, including subtractively normalized FT-IR spectroscopy (SNIFTIRS); potential difference IR spectroscopy (PDIRS); and electrochemically modulated IR spectroscopy (EMIRS). The limitations in modulation frequency of conventional approaches are alleviated by the use of a step-scanning spectrometer, facilitating time-dependent studies, and the use of phase modulation and detection provides information on the time lag of effective electrode potential vs applied potential waveform, although the signal/noise ratio is slightly compromised (I33).
Scherson and co-workers have reported the use of Fourier transform PDIRS in several modes. They have compared internal (attenuated total reflection, ATR; PD-ATR-FTIRS) and external reflection (PD-FTIRS(external)) methods for characterization of 2,5-dihydroxybenzyl mercaptan (DHBM) irreversibly adsorbed on gold electrodes in quiescent solution, obtaining comparable results by both methods (I34); and also applied PD-ATR-FTIRS at a thin, optically transparent gold electrode on one wall of a thin-layer flow channel to study solution species, including reactant bisulfite and reduction product dithionite, as the reaction medium flows by the electrode under laminar flow conditions (I35). Sundholm and Talonen have theoretically modeled an external reflection IR spectroelectrochemical cell with radial liquid flow and tested the model for the oxidation of ferrocyanide to ferricyanide. Nonidealities of cell construction are readily detected (I3). Samant et al. have quantitatively evaluated the role of ion migration in an external reflection thin-layer electrochemical FTIR cell, using perchlorate ion as a probe of the potential dependence of ion concentration adjacent to the electrode (I36). Niu and Dong have used IR to study the electrochemical behavior of the ferricyanide/ferrocyanide system in aqueous media and the ferrocene/ferrocenium system in nonaqueous media, and to identify reaction products of bilirubin reduction, in a thin-layer optically transparent electrochemical cell constructed without any soluble adhesive materials (I37). Hartl et al. reported the investigation of several electrogenerated carbonyl complexes of Mn(I) and Ru(II) at variable low temperatures in a cryostated OTTLE cell suitable for both IR and UV/visible spectroelectrochemistry (I38). A number of papers have been reported in which FT-IR spectroelectrochemistry is used in tandem with membrane inlet mass spectrometry, to study the electrochemical reactivity and adsorption at metal electrodes of small molecules, including alcohols, formic acid, and their reaction products (I39-I43). FT-IR spectroelectrochemistry has been used to characterize the reaction pathways and products of inorganic and organometallic species, including as examples the oxidation or reduction of mononuclear and binuclear mixed-valent Fe and Co metallacarborane complexes (I44); reduction of edge-shared bioctahedral rhenium dimers, in which reversible structural rearrangement was found upon reduction, with evidence suggesting formation of a metal-metal-bonded product (I45); oxidation of Mo-carbonyl isocyanide complexes, with associated structural isomerism (I46); reduction of a series of rhenium cobalt diimine complexes and the dependence of product stability on the identity of the diimine ligand (I47); reduction and reoxidation of dimeric metal carbonyl complexes of Mn, W, and Mo, with cross-coupling and ligand substitution reactions of the electrochemically generated organometallic radicals (I48); reduction of metal-to-metal bonded complexes containing Re and Mn or Os to form unusually stable radical anionic complexes, studied over a wide range of temperatures (I49); oxidation and subsequent reduction of reaction products of tetranuclear complexes containing Rh and Mn or Re, with a number of metal-metal bond cleavage and subsequent bond re-formation steps (I50); and oxidation of a series of octaethylporphyrin metal complexes, in which reaction at the ring vs the metal site and dimer formation could be distinguished (I51). An important advantage of FT-IR spectroelectrochemical methodology in many cases is that the products are not stable enough to be isolated, but the use of FT-IR spectroelectrochemical
methods allows both generation and detection of the products on a short enough time scale that useful chemical information can be acquired, as evidenced for some trinuclear Ru complexes in acetonitrile. Even here, not all intermediates are sufficiently stable for spectral characterization (I52). A particularly useful application of FT-IR spectroelectrochemistry is to characterize biochemical or biological processes. For example, the binding and interaction of the primary and the secondary electron acceptor quinones in Rhodobacter sphaeroides bacterial photosynthetic reaction centers have been investigated in response to electrochemically or light-induced activation (I53). FT-IR spectroelectrochemical methods have been very fruitful in the characterization of reactions occurring on electrode surfaces. Considerable insight into processes at single-crystal electrodes has been obtained, both in situ and ex situ. For example, the adsorption and oxidation of glucose at Pt(111) and Pt(100) electrodes in sulfuric acid solution have been reported, together with an assessment of the merits and limitations of in situ reflection/absorption spectroscopy (I54). Adsorbed intermediates in the oxidation of methanol on Pt single crystals in alkaline media have been identified, and their bonding has been characterized (I55). Investigations of the IR spectroelectrochemical properties and potential dependence of high-nuclearity platinum carbonyl clusters in nonaqueous solvents in contrast with single-crystal platinum electrodes have been shown useful in distinguishing between the charge dependence of localized surface bonding and the potential dependence of surface properties (I56). Alternatively, studies of model electrochemical interfaces in ultrahigh vacuum have afforded insights into solvation of cations and chemisorbed carbon monoxide at Pt(111) electrode surfaces (I57). Work-function measurements and infrared Stark effects afford some clues about possible roles of surface solvation in electrochemical reaction systems for a number of different solvents (I58), including the solvation of K+ by methanol in high vacuum as a model of double-layer interactions (I59). A number of studies have characterized processes involving monolayers of species adsorbed on electrodes, including investigation with grazing incidence of adsorption of a hydroxyalkanethiol, HS(CH2)12OH, deposited onto a gold electrode of an EQCM and gold-coated quartz microscope slides with and without attachment of an immobilized antibody (I60); FTIRRAS of structural changes and mass transport accompanying the redox of a ferrocenylundecanethiol monolayer at an EQCM (I61); the adsorption and reoxidation of reduction products of CO2 on Pt (I62); the chemisorption of ethyl xanthate on gold (I63); thinlayer reflectance spectroscopy of flavin adenine dinucleotide in oxidized and reduced state adsorbed at a mercury/gold electrode (I64); and surface films formed on lithium and other, nonactive electrodes at low potentials in lithium salt solutions containing carbon dioxide (I65). Finally, FT-IR spectroelectrochemical studies have been popular for characterization of a number of polymer films deposited on electrodes and processes occurring in these films. Example studies have examined polyaniline (I66), phenazine, phenothiazine, and related polymer films, with assessment of molecular orientation based on polarized absorption (I67) or with characterization of intermediates and the electropolymerization mechanism (I68); and electropolymerization and characterization of poly(1,5-diaminonaphthalene) (I69, I70), polythiophene (I71), and polyphenylene (I72, I73). Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Raman Spectroelectrochemical Methods. The majority of Raman spectroelectrochemical studies have emphasized the investigation of species adsorbed on electrodes, although the approach has also been useful for characterization of species in solution as well. Important contributions to the understanding of reactions at carbon electrodes have been provided from the laboratory of McCreery. Non-resonance-enhanced, normal Raman spectra have been used to characterize the adsorption and electrochemical reaction products of monolayers of nitrophenyl groups covalently bonded to glassy carbon (GC) and HOPG in acetonitrile. The similarity of the results for both modified HOPG and GC surfaces suggests that the nitrophenyl radical attacks both edge plane and basal plane sites (I74). Particularly interesting resonance Raman measurements of the density of surface carbonyl groups on HOPG enabled classification of the reactivity of a series of aquated ions of differing charge as a function of the surface structure of the graphite surface, the ionic charge, and the competing adsorption of various neutral, cationic, and anionic nonspecifically adsorbing substances. It was possible to classify reaction systems into two groups, one of which was catalyzed by surface carbonyl groups, and one of which was not (I75). A theoretical model for the adsorbate/metal surface interaction and a charge-transfer mechanism for surface-enhanced Raman scattering (SERS) was developed to calculate the SERS intensity as a function of applied potential and exciting radiation, with good agreement with experiment for pyridine and for a dimeric iron bipyridine complex adsorbed on a silver electrode (I76). The significance of a charge-transfer mechanism of enhancement in the SERS of adsorbates on metal electrodes has been debated. Evidence of a charge-transfer mechanism with a ∼4-fold enhancement for pyridine adsorbed on a gold electrode was obtained from a study of the dependence of SERS intensity on applied potential as well as Raman excitation energy (I77). Evidence of a noncharge-transfer enhancement mechanism was presented for pyrazine and methyl viologen on Ag and Au electrodes with 1.06-µm excitation (I78). ATR-surface plasmon polariton SERS was explored as a means of improving the sensitivity of Raman scattering from self-assembled monolayers of p-(nitrothio)phenol and p-(aminothio)phenol adsorbed on Ag electrodes (I79). A number of SERS studies have examined adsorbed films on electrodes. Experimental SERS comparisons were made between the adsorption of benzonitrile and benzyl cyanide on a silver electrode (I80). Resonance Raman spectra of Prussian Bluemodified electrodes derived from a complex containing isonicotinamide exhibit prominent isonicotinamide vibrational peaks, whereas SERS spectra of the same system show only metal-CN vibrational peaks, suggesting that the cyanide ligands are involved in the adsorption mechanism (I81). Resonance Raman spectra of zinc phthalocyanine films revealed the inclusion of electrolyte anions upon oxidation of the film, with preferential axial coordination of the zinc phthalocyanine molecule and concomitant changes in both Raman and reflectance spectra (I82). SERS experiments have also been used to enhance analytical sensitivity of solution species by deposition on a suitable electrode. A spectroelectrochemical flow detector based on resonanceenhanced SERS (SERRS) detection at an electrochemically deposited silver film on a glassy carbon electrode was used for flow injection determination of femtomole quantities of Fe(II) (I83). A spectroelectrochemical flow detector based on a silver 426R
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electrode was used for flow injection determination of pyridine, adenosine, and adenosine 3′-monophosphate (I84). Several studies have illustrated the utility of Raman spectroelectrochemistry to characterize solution species. Time-resolved Raman and resonance Raman spectroelectrochemical studies been used to characterize the structure and reactivity of intermediates and products of reductive electrochemical reactions of a binuclear tungsten carbonyl complex ligand-bridged by 4,4′-bipyridyl (I85) as well as to characterize and identify the lowest energy chargetransfer excited states of Cu(I) complexes with several polypyridyl ligands (I86). Resonance Raman studies have enabled characterization of four oxidation states of a ruthenium complex and its sequential reduction products (I87). Resonance Raman studies have been used to characterize the one- and two-electron reduction products of two Fe(II) tetraphenylporphyrins as a function of the different solvent coordination ability of dimethyl sulfoxide and butyronitrile. The tendency of some complexes (especially Fe(I)) to photodecompose was partially addressed by using a rotating cell (I88). Raman spectroelectrochemical studies have also been useful in the characterization of the behavior of tetrachloro-pbenzoquinone in molten salts such as aluminum chloride/sodium chloride mixtures (I89). Luminescence and Fluorescence. Several workers have applied fluorescence or luminescence spectroelectrochemistry to characterize reactions or materials. Lee and Kirchhoff have investigated the electrogeneration and absorption and luminescence spectroelectrochemistry of a highly luminescent Re(II) complex. They assigned the luminescence to a relatively rare ligand-to-metal charge-transfer excited state (I90). Sugimura et al. generated fluorescent micropatterns in a methyl viologen conducting polymer film by scanning the platinum or tungsten tip of a scanning electrochemical microscope at an applied potential of -0.9 to -1.2 V vs Ag/AgCl (I91). Compton et al. have coupled spectrofluorometry with channel hydrodynamic voltammetry to study reaction products and homogeneous reactions in solution following electron transfer (I92) or heterogeneous reactions at films containing a fluorophore attached to either the cell walls or to an electrode on one wall of the cell (I93). Araki and Toma investigated the spectroelectrochemistry, luminescence, and photoelectrochemistry of a tetraruthenated zinc porphyrin and were able to characterize six distinct redox states. Fluorescence involving the porphyrin ring was the dominant form of luminescence at room temperature (I11). Szulbinski characterized the photoinduced electron-transfer reaction between a Mg(II) porphyrin and ethyl viologen, yielding a fluorescent porphyrin cation radical product which appeared to be oxidatively quenched by reaction with ethyl viologen, with generation of the ethyl viologen radical cation (I12). Electron Paramagnetic Resonance. A number of applications of EPR to spectroelectrochemical investigations have been reported. Stankovich et al. reviewed applications of EPR in spectroelectrochemical titrations of biological species (I94) and applied EPR spectroelectrochemical titrations to determine the formal potentials for the hydroxylase component of a methane monooxygenase hydroxylase enzyme (I95). Stange et al. examined the variable structure of a Cu2S2 core in doubly thiolatebridged dicopper(I) model complexes (I96). MacGregor et al. reported EPR and optical spectroelectrochemical studies of a series of coordination compounds involving binuclear mixed-valent ruthenium and osmium complexes and mononuclear platinum
species in the presence of phosphine ligands (I97). Jackowska et al. used EPR and optical spectroelectrochemical measurements to determine the number of electrons transferred in redox processes in polymer films including polypyrrole (I98). Wilgocki et al. characterized tungsten(0) organometallic complexes using EPR spectroelectrochemistry in conjunction with other electrochemical measurements in a variable-temperature spectroelectrochemical cell (I99). Lapkowski and Bidans investigated the reactions of three poly(pyrrole-viologen) polymers (I100). Lu et al. characterized the metal centers of the Ni/Fe-S component of a carbon monoxide dehydrogenase enzyme complex (I101). Finally, Wang and Wu have used the implicit finite difference method to model the expected ESR response for the EC2 electrode reaction mechanism in an electrochemical channel flow cell (I102). Electrochemical Mass Spectrometry. Most of the workers in this area are focusing on a technique somewhat misleadingly called differential electrochemical mass spectrometry (DEMS). The technique is best suited for detection of volatile reactants and products of electrode reactions and relies upon the transport of analyte species from the electrochemical cell across a thin membrane interface to the mass spectrometer. The membrane is typically made of either silicone rubber or microporous Teflon (I103). Membrane inlet electrochemical mass spectrometry might be a better name than “DEMS”, which gets the “differential” part of its name from the differential pumping commonly required to keep the pressure in the mass spectrometric source and analyzer regions manageable in the presence of the gas loads imposed by the transfer across the membrane into the vacuum phase of volatile species and solvent from the electrochemical cell. Other approaches include direct transport of solution species into the mass spectrometer interface, most prominently represented by electrospray ionization mass spectrometry, which has the advantage of sampling solution species directly, but the potential disadvantage of minimal fragmentation for structure identification, and off-line sampling of solids such as polymer membranes by means of secondary ion mass spectrometry. One example of direct electrochemistry in the ion source of the mass spectrometer is also presented. Membrane Inlet Electrochemical Mass Spectrometry (MIEMS). Wasmus et al. described an improved, more universal approach to MIEMS, which reduces the response time for species diffusing across the membrane into the mass spectrometer to ∼20-50 ms, enabling cyclic voltammetric scanning at rates as high as 1 V/s, while also allowing facile switching of the mass spectrometer between monitoring of electrochemical processes and other experiments (I104). A number of reactions have been studied by MIEMS. Oxidation and reduction of ammonia, hydroxylamine, nitrite, and nitrate were studied at Pt black electrodes (I105), and oxidation and reduction of azide were studied at porous Pt and Au electrodes (I106), with important insight into the reaction mechanisms, intermediates, and products afforded by the mass spectrometric measurements (I105). Oxidation and reduction of nitromethane and its sometimes catalytic and sometimes inhibiting effects on the oxidation of formic acid and methanol were treated (I107). Several groups have investigated the adsorption, hydrogenation, and electrochemical reactivity of alkenes by MIEMS, including adsorption and hydrogenation of ethene, ethanol, and cyclohexene at single-crystal and polycrystalline Pt and polycrystalline
Pd electrodes (I108, I109); hydrogenation of ethene at Pt single crystals (I110) or polycrystalline Au electrodes (I111); reduction of acetylene at Pt single crystals (I112); and adsorption and oxidation of acetylene at Au electrodes (I113). Others have studied the effects of inhibitors, including adsorbed benzene, aniline, and pyridine on single-crystal and polycrystalline Pt, Rh, and Pd electrodes (I114); and acetonitrile, dimethyl sulfoxide, and lead on electrochemical oxidation of small organic molecules, including formamide and formic acid at Pt (I115). The catalytic effect of lead on oxidation of formic acid at Pt was also studied (I40). Numerous investigations have focused on the oxidation of alcohols, particularly methanol, which is of interest as a fuel for fuel cells, at Au (I103); Pt and Pt-Ru alloys (I116-I119); Pt with and without submonolayers of Ru or Sn (I120); or Ru (I119); ethanol on Pt (I39, I40); propanol on Pt (I41); 2-propanol on Pt (I42); and allyl alcohol on Pt (I43). MIEMS has also proven useful for characterization of the chemical processes involved in the electroless deposition of copper by catalytic reduction of Cu(II) ions by formaldehyde (I121), characterization of the oxidation of formaldehyde at a copper electrode (I122), characterization of photoelectrocatalytic oxidation processes at TiO2 including observations of competitive oxidation of chloride ion vs water to form Cl2 in competition with O2 (I123), and competitive oxidation of HCOOH vs H2O to form CO2 in competition with O2 (I124). Numerous MIEMS studies have benefited greatly from isotopic labeling experiments, which afford means of assessing where key constituents of reaction products originated. Deuterium labeling (typically by using deuterated solvents or reagents) was used to investigate H/D exchange to test whether reactions proceeded through adsorbed intermediates, during propane formation from 2-propanol on Pt (I42), hydrogenation of alkenes at Pt single crystals (I108), or oxidation of ethene at Pt (I110, I111), and oxidation of formaldehyde at a Cu electrode (I122). Carbon-13 labeling of one carbon of ethanol was used to determine which carbon was involved in product formation during ethanol oxidation (I39). Oxygen-18 enrichment of TiO2 was used to assess the source of oxygen in photooxidation products of HCOOH and H2O (I124). Oxygen-18-labeled water was used to demonstrate that oxygen was incorporated into some adsorbed ethene molecules during ethene oxidation (I109). Electrospray Interface. Several papers appeared during the review period using this interface. It seems likely that we will hear more about this approach in years to come. An important observation made by several groups is that electrochemical reactions or their equivalent taking place at the electrospray interface tip are important factors in the generation of some of the ions detected with this interface, whether an electrochemical cell is intentionally coupled to the interface or not (I125-I129). The coupling of an electrochemical cell with the electrospray interface has also been reported. Van Berkel and Zhou have discussed key experimental considerations for successful formation of ions using an electrospray interface. They have demonstrated the use of a platinum capillary in the interface for detection of neutral metallocenes, metalloporphyrins, and polycyclic aromatic hydrocarbons (I126). They have also discussed several electrochemical flow cell designs and possible strategies for electrically floating or decoupling the electrochemical cell from the very high (kV) potentials imposed in the electrospray interface (I130) for detection of neutral molecules such as perylene, nickel Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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octaethylporphyrin, and β-carotene. The latter compound undergoes a follow-up chemical reaction that can be used as a yardstick for the transit time of analyte between the electrochemical cell and the mass spectrometer. They also have reported the coupling of anodic stripping voltammetry with the electrospray interface to preconcentrate samples and afford very low detection limits (I130). Dupont et al. have discussed the electrochemical oxidation or reduction of fullerenes to generate positive or negative ions for the electrospray interface. They note that the approach is particularly valuable for the electrospray ionization of species otherwise difficult to observe (I129). Other Interfaces. House and Anderson have reported a novel electrochemical cell that can be placed directly in the ion source of the mass spectrometer for generation of volatile molecules and ions. The electrodes constitute an interdigitated microelectrode array, with either poly(ethylene glycol) or hexamethylphosphoric triamide as solvent (I131). TOF-SIMS Characterization of Electropolymerized Films. In an alternative, off-line approach, a polypyrrole film was characterized by means of time-of-flight secondary ion mass spectrometry (TOFSIMS). The effects on the films of dopant anions, including chloride, bromide, sulfate, and tosylate, were investigated. Naturally abundant chlorine and bromine isotopes were used to verify the identity of polypyrrole and dopant species (I132). X-ray Absorption, Scattering, or Diffraction Methods. In situ X-ray absorption or scattering methods continue to be used to characterize processes at the electrode/solution interface during electrochemical reactions. Many of these methods are variations on extended X-ray absorption fine structure (EXAFS) techniques, with additional initials for specific variations (e.g., NEXAFS for near-edge, etc.). Ross et al. investigated copper UPD on Pt(111) by means of anomalous surface X-ray scattering in addition to a number of additional electrochemical and off-line surface analysis techniques (I133). Schultz et al. reported low-energy NEXAFS studies of a molybdenum dithiolate complex model for the nitrogenase enzyme cofactor at a reticulated vitreous carbon electrode in a cell which enabled complete electrolysis in 10 min (I134). Yoshitake et al. reported a spectroelectrochemical cell for in situ EXAFS spectroscopy on gas-generating platinum electrodes during hydrogen evolution (I135). Conway et al. applied X-ray absorption spectroscopy to investigate the effect of modifiers such as Bi(III) on the electrochemical behavior of MnO2 battery cathode material, concluding that Bi(III) influences Mn-Mn coordination but not O coordination (I136). Millet et al. used EXAFS spectroscopy in the dispersive mode to characterize the mass transport of Pt(NH3)42+ through a Nafion 117 membrane and its effect on electrochemical performance of the assembly during water electrolysis (I137). Schmickler et al. investigated the dependence of adsorbed water oxygenelectrode metal distance on applied potential at electrodes consisting of a monolayer of Pb deposited on Ag or a monolayer of Ag deposited on Au by means of SEXAFS spectroscopy (I138). Allen et al. applied EXAFS spectroscopy to follow surface oxide formation and reduction on platinum clusters as a function of time and potential at a platinum electrode surface, concluding that reactions at clusters may differ from behavior at bulk platinum and that platinum restructures during these reactions (I139). Choy et al. reported EXAFS and XANES studies indicating the formation of Cu(III) at sites in the superconductor La2CuO4.08 as a result of doping with excess O during electrochemical or chemical oxida428R
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tion (I140). Mukherjee et al. reported X-ray absorption studies of several metal hydride electrodes, showing that introduction of hydrogen caused large electronic and structural changes (I141). O’Grady et al. reported EXAFS studies of irreversible structural changes in the initial charging step of nickel oxide electrodes, followed by reversible charging cycles between two states (I142). You et al. reported an X-ray reflectivity study showing the lifting of surface Pt atoms in a place-exchange mechanism during the electrochemical oxidation of a Pt(111) surface. The process was reversible for charge transfers of e1.7 electrons/Pt atom but led to irreversible roughening for greater oxidation (I143). Herron et al. performed potential modulated difference X-ray diffraction studies of platinized platinum electrodes using synchrotron radiation and noted evidence for a change in lattice spacing between the hydrogen adsorption, double-layer, and oxide formation potential regions (I144). Several applications of spectroelectrochemical methods to the characterization of molten salt systems have been reported by Mamantov and co-workers. They have investigated the behavior of tetrachloro-p-benzoquinone (I89) and rhenium chloride species (I145, I146) in molten sodium chloroaluminates. INSTRUMENTATION General Electrochemical Instrumentation. Kissinger has briefly outlined future prospects for electroanalytical instrumentation (J1). Bond and Svestka have reviewed trends and commercial availability of instrumentation (hardware and software) for microcomputer-based voltammetry (J2). Thomsen et al. have described a flexible computer-controlled instrument for voltammetry, amperometry, and stripping potentiometry, which is capable of automating several other devices including pumps, valves, and a sample carousel (J3). Liao et al. have described a PC-based instrument for differential normal pulse voltammetry, which adaptively adjusts the pulse duration as a function of potential for a blank solution, based on decay of the charging current to an assigned percent of its initial value. A vector of decay times is then used for the test solution to obtain a backgroundcorrected voltammogram (J4). Williams and D’Silva have developed a hand-held, battery-powered instrument for environmental monitoring of trace metals, based on anodic stripping voltammetry at a mercury thin-film electrode (J5). Thomsen et al. have described a flexible instrument for voltammetry, amperometry, and stripping potentiometry and illustrated the use of command macros for building highly automated analytical procedures for the amperometric detection of carbohydrates at a Cu electrode in a flow injection setup (J3). Edwards and Durst have developed an automated flow injection liposome immunoanalytical system with a reusable immunoreactor column and a constant-potential amperometric electrochemical detector to measure the herbicide alachlor in an assay based on competition in the immunoreactor column between free alachlor and alachlor-tagged liposomes containing ferrocyanide as an electroactive tag. Quantitative signal amplification is achieved by lysis of bound liposomes with surfactant to release the tag for detection (J6). Creasey and Varney have described a rapid-response amperometric thick-film electrochemical sensor and computer-controlled analog instrumentation for determination of carbon dioxide partial pressure in the marine environment, based on diffusion of CO2 through a membrane to lower the pH and effect the dissociation
of a metal complex. The increase in uncomplexed metal ion is voltammetricallly monitored at a polymeric thick-film electrode (Au, Pt, or Ag/AgCl) printed on a porous substrate to provide a measure of pCO2 (J7). Fei et al. have described a low-noise contact for rotating electrodes based on a copper wire in contact with a small mercury pool which rotates with the electrode. The noise characteristics are superior to silver-carbon brushes and are especially valuable in studies of oscillations and chaotic behavior (J8). Electrochemical Quartz Crystal Microbalance. Wu et al. have developed a dynamically time-resolved EQCM based on measurement of a difference signal between the working quartz crystal resonator and a reference crystal. The period of this signal is measured as a function of time by using the signal to gate a high-frequency time-base generator into a 16-bit synchronous counter. The system was applied to investigations of the underpotential deposition of lead on a silver electrode (J9). Kadish and co-workers have described an improved cell and holder for EQCM electrochemical studies, which was well-suited for cyclic voltammetric studies (J10). Xie et al. have applied an EQCM piezoelectric quartz crystal as an optically transparent electrode for absorption spectroelectrochemistry (J11). Scanning Electrochemical Microscopy. Numerous authors have reported developments of instrumentation for SECM. Developments range from new designs for the basic static scanning mechanism and feedback control to designs for moving-tip modulation modes of operation. Bard et al. have reviewed instrumentation as well as theory and applications (J12). Mao et al. have described an SECM design based on piezoelectric bimorph X-Y scanners, with a battery-powered bipotentiostat, analog control circuits, and computer control (J13). Wittstock et al. have described the construction of an SECM from a commercially available open-loop micropositioning system and a digitally addressable bipotentiostat and assessed both the capabilities and limitations of the system. They introduced a new, experimentally simpler approach for quantifying the spatial resolution, using a newly derived expression based on the tip radius and the tip-substrate distance (J14). Smyrl and co-workers have coupled SECM and scanning photoelectrochemical microscopy to monitor electroassisted and/ or photoassisted reactions on semiconductor surfaces either concurrently or sequentially, using a scanning probe tip fabricated from an optical fiber coated with gold and insulated on the sides by a polymer film (J15). Heinze et al. have reported a hydrodynamic modulation technique for SECM which they call the picking mode, in which the scanning microelectrode tip is vertically modulated relative to the substrate. This convective modulation emphasizes the effects of mass transport phenomena over heterogeneous electrontransfer phenomena, enabling independent assessment of topography as opposed to surface reactivity (J16). Ludwig et al. have described an SECM tip modulation technique suitable for topographic feedback in which the damping of the amplitude of lateral vibrations of the scanning microelectrode tip by hydrodynamic coupling with the sample is optically monitored. Microelectrode current response is slightly but linearly affected by the amplitude of hydrodynamic modulation (J17). Bard and Wipf were recently awarded a patent for another SECM tip modulation scheme with lock-in detection, which greatly increases resolution and signal/ noise ratio (J18).
Electrochemical Mass Spectrometry. Several workers have reported instrumental developments in the coupling of electrochemical experimentation with mass spectrometric detection. One of the developments illustrates the ironically misleading nature of the name DEMS, since it involves minimization of the additional pumps usually required for DEMS. Ianniello and Schmidt have described a simplified interface for DEMS, based on use of only one turbomolecular pump and a modified gas inlet system. The system was set up for electrocatalytic studies of the electrooxidation of methanol in acid solution at porous platinumruthenium alloys (J19). Savinell and co-workers have also reported a multipurpose interface for electrochemical mass spectrometry, which allowed faster response time and switching between spectroelectrochemical mode and thermogravimetric analysis mode, by switching two valves (J20). Skou and Munk have described an electrochemical cell and interface based on a microporous, electronically conducting film electrode (e.g., gold) evaporated onto a silicone rubber membrane, with time constant and sensitivity for small molecules comparable to the more commonly used microporous Teflon membrane. The permeability of this membrane assembly is sufficiently low to allow elimination of an extra turbomolecular pump in the mass spectrometer. The apparatus was applied for mass spectrometric cyclic voltammetric investigation of the oxidation of methanol on platinum black (J21). Gao et al. have designed an electrochemical cell and interface suitable for investigations of the reduction of acetylene on a platinum single-crystal electrode. The single crystal and solution were brought into contact with a Teflon film which covered a pinhole (several millimeters in diameter) gas inlet to the mass spectrometer. They reported a high signal/noise ratio and were able to show that Pt(110) had significantly higher activity for acetylene reduction than Pt(111) and Pt(100) (J22). In contrast, House and Anderson placed the electrochemical cell directly inside the mass spectrometric ion source vacuum. They generated volatile molecules at a twin interdigitated electrode pair, with either PEG (average molecular weight 400) as the solvent, due to its extremely low volatility, or hexamethylphosphoric triamide as the solvent, due to its wider potential range. Applications were made to the generation of ferrocene by the reduction of a ferrocenium salt, the oxidation of carboxylic acids in PEG, and the reduction of dibromododecane in HMPA (J23). Electrochemical Cell Design. RDE. Bressers and Kelly have reported a new cell design for the inverted rotating disk electrode, which gives similar hydrodynamic conditions as a conventional RDE, but is much more immune to interference by gas bubble generation (J24). Wall-Jet Electrode Cell. Laevers et al. have described the design and construction of a wall-jet electrode reactor and its application to the study of electrode reaction mechanisms (J25). Unwin et al. have described the construction and hydrodynamic properties of a “microjet” electrode, consisting of a high-velocity jet directed with precise positioning at a microelectrode (J26). Medina et al. have described a “uniform injection” cell, which essentially involves a wall-jet configuration but a porous disk through which the solution enters in place of a jet nozzle (J27). Kissinger and co-workers have described a new miniaturized wall-jet detector well-suited for use with microbore liquid chromatography (J28). Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Barisci and Wallace have designed photoelectrochemical flow cells, including a wall-jet and a thin-layer configuration, with three possible configurations for the relative positions of the irradiation and detection points (J29). Thin-Layer Flow Cells. An automated thin-layer electrochemical flow-through reactor was used to fabricate and characterize CdTe thin films formed by electrochemical atomic layer epitaxy (J30). Spectroelectrochemical Cells. Christensen et al. have described a spectroelectrochemical cell for investigation of the photoelectrochemical detoxification of water, based on a TiO2 working electrode, for oxidation of organic species by means of photoelectrochemical generation of oxygen (J31). Designs for FT-IR spectroelectrochemical cells have also been reported, including a thin-layer reflection FT-IR microspectroelectrochemical cell (J32) and a thin-layer transmission FT-IR or UV/visible spectroelectrochemical cell with integrated CaF2 windows (J33). Other spectroelectrochemical cell designs include a channel-flow cell for UV/ visible spectroelectrochemistry (J34), a combination absorption/ luminescence cell based on reticulated vitreous carbon with drilled optical channels (J35), an anaerobic thin-layer cell for biological component titrations (J36), a nonaqueous thin-layer cell (J37), a variable-temperature cryostated thin-layer cell (J38), a variabletemperature thin-layer cell suitable for both UV/visible and EPR measurements (J39), and a cell suitable for in situ EXAFS spectroscopy (J40). Fiber Optics for Spectroelectrochemistry. Xie et al. have used a bifurcated fiber-optic probe to couple the incident light beam to a perpendicular reflectance spectroelectrochemical cell consisting of a pair of fibers coupled to an electrode on an EQCM quartz crystal for the study of reduction of Cu(II) in aqueous glycine and nitrate media (J41). Ichimura et al. have described a thinlayer spectroelectrochemical cell based on a platinum mesh OTE between two fiber optical light guides and suitable for use over a range of temperatures between -50 and +50 °C (J42). Multisensor Flow-Through Apparatus. A digitally controlled multisensor flow-through apparatus has been described, with numerous channels and individually controlled flow valving (J43). Electrode Design. RDE and RRDE. Both RDE (J44) and RRDE (J45) designs have been reported for use in hightemperature molten salt media. Spectroelectrochemical Electrodes. Kummer and Kirchhoff have reported procedures for production of graphite-coated metal mesh OTE electrodes based on either chemical vapor deposition from acetone pyrolysis or spraying of a colloidal graphite suspension (J46). Neudeck and Dunsch have reported the fabrication of microstructured “LIGA” spectroelectrochemical electrodes from metal foils (J47). Microelectrodes, Microelectrode Arrays, and Composite Electrodes. Whitesides et al. have described the fabrication of microelectrodes using a combination of micromachining and molecular self-assembly based on alkanethiol adsorption on gold (J48). Seddon et al. have reported the use of laser micromachining to fabricate arrays of carbon ink microelectrodes in a composite membrane or thin laminate. Holes between 7- and 38µm diameter generated in a film by laser pulses are filled by thickfilm printing with carbon ink (J49). Wang and Chen have reported screen-printed, disposable enzyme microelectrode array strips for glucose and lactate. The commercially available screen-printed carbon microdisk arrays were prepared by thick-film technology with a dielectric overlayer and laser micromachining to expose 430R
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the carbon microdisks below laser-drilled holes (J50). Johnston et al. have described fabrication of an array of gold/palladium thinfilm composite electrodes formed by evaporation through a mask onto a plastic substrate for the amperometric measurement of hydrogen peroxide (J51). Tsionsky et al. have described the fabrication of a new class of porous sol/gel-derived ceramic-carbon composite electrodes consisting of carbon dispersed in a silica matrix, whose properties can be tailored for a range of applications (J52). Montgomery and Anderson have described the fabrication of poly(chlorotrifluoroethylene) (i.e., Kel-F)/precious metal composite electrodes with less than 1% active area (J53). Menon and Martin have described the fabrication of gold nanoelectrode ensembles electrolessly deposited in the pores of nanoporous membrane filters (J54). Park and Shaw have described the fabrication of improved Ketjenblack carbon black/Kel-F composite electrodes with and without incorporation of cobalt phthalocyanine. The electrodes showed good electrochemical properties and worked well for electrochemical detection of L-cysteine in HPLC (J55). Kunugi et al. have prepared hydrophobic composite zinc and lead electrodes with high hydrogen overpotentials by coating the metals with fine particles of hydrophobic materialsspoly(tetrafluoroethylene) (PTFE), tetrafluoroethylene oligomer (TFEO), and fluorocarbon ((CF)n) (J56). Wang and Liu have described alcohol and lactate dehydrogenase-based carbon paste biosensors modified by addition of fumedsilica nanoparticles to enhance retention of dehydrogenase and NAD+ cofactor and to catalyze detection of the NADH produced by the reaction of the enzyme with analyte (J57). Kitamura et al. have discussed the microchemical fabrication and modification of a number of surfaces by coupling photocatalytic surface reactions and a scanning electrochemical microscope to prepare integrated chemical systems, e.g., a TiO2-Pt microelectrode array, with a series of microchemical reaction sites in a well-defined spatial arrangement (J58). Reimer et al. have described the fabrication of microelectrode arrays in the quarter micrometer regime for biotechnological applications, with electrode dimensions and spacing as small as 100-200 nm using electron beam lithography and either a platinum lift-off process or a gold electroplating process (J59). Cammann et al. have described the design and fabrication of a monolithic sensor array having as many as 400 individually addressable working microelectrodes and two reference electrodes, plus integrated read-out amplifiers, digital circuits for addressing the individual components and test circuits, in a chip of dimensions less than 1.4 cm × 1.4 cm (J60). They have also described a fabrication and packaging process for an amperometric microelectrode array with incorporation of reagents in an integrated sensor device which can be mass-produced (J61). Fiaccabrino et al. have described the design and fabrication of a multiplexed array of 100 square, individually addressable platinum microelectrodes (J62). James L. Anderson is Professor of Chemistry at the University of Georgia. He received his B.A. in chemistry from Kalamazoo College in 1967, and his Ph.D. in analytical chemistry at the University of Wisconsin in 1974, under the guidance of Irving Shain. After a postdoctoral appointment in the laboratory of Theodore Kuwana, he was an Assistant Professor at North Dakota State University from 1975 to 1979, when he moved to the University of Georgia. His research interests are in the general area of electroanalytical chemistry and include electrochemical investigations of environmental redox processes in the water and soil environment, electron-transfer kinetics from both experimental and theoretical perspectives, electrochemical flow detectors, and spectroelectrochemistry.
Edmond F. Bowden is currently an associate professor in the Department of Chemistry at North Carolina State University and a member of the Biotechnology Faculty. After earning a B.S. degree in aerospace engineering at Syracuse University in 1970, he spent several years working in the aerospace and chemical industries. He obtained his Ph.D. at Virginia Commonwealth University in 1982 under the guidance of Fred M. Hawkridge and then held a postdoctoral appointment at the University of Minnesota with John F. Evans before moving to NCSU. His research interests include interfacial bioelectrochemistry, biological electron transfer and bioenergetics, enzyme electrodes for bioanalysis, and electroactive monolayers. Peter G. Pickup is an Associate Professor of Chemistry at Memorial University of Newfoundland. He received his B.A. in chemistry (1979) and D.Phil. (1982) from Oxford University. He joined Memorial University in 1986 following postdoctoral appointments with Royce Murray (Chapel Hill), Robert Osteryoung (Buffalo), and Viola Birss (Calgary). His research interests include the electrochemistry of modified electrodes, the synthesis of novel conducting polymers and metallopolymers, and proton-exchange membrane fuel cells.
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