Anal. Chem. 1994, 66. 360R-427R
Dynamic Electrochemistry: Methodology and Application Michael D. Ryan
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233 Edmond F. Bowden
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695 James Q. Chambers*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
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Review Contents
Books and Reviews
360R
Mass Transport
363R
Microelectrodes Hydrodynamic Methods 368R
Analytical Voltammetry Methodologies
Organic Electrochemistry Organometallic Electrochemistry Inorganic Electrochemistry Activation of Small Molecules
407R 410R
Spectroelectrochemistry Instrumentation
Metal/Ligand Complexation Studies Chemometric Approaches Heterogeneous/Homogeneous Kinetics Electron-Transfer Theories Heterogeneous Kinetics Homogeneous Kinetics Double-Layer Studies Adsorption Studies
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Surface Electrochemistry Theoretical Aspects Mercury Electrodes
Carbon Electrodes Single Crystal Surfaces Surface Imaging Techniques Polycrystalline Electrodes Miscellaneous Electrodes Modified Electrodes Charge Transport in Polymer Films Electrocatalysis at Modified Electrodes Ion-Exchange Polymer Film Electrodes Ionophore Films Redox Polymer Films Electrochromism and Pattern Formation in Polymer Electrodes Conducting Polymer Electrodes Self-Assembled Monolayers Other Modified Electrodes
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Bioelectrochemistry Books and Reviews
Small Molecules of Biological Importance Protein ElectrochemistryEnzyme Electrodes Polynucleotides and Nucleic Acids In Vivo and Cellular Electrochemistry
A. BOOKS AND REVIEWS Three accounts of a historical nature on square-wave and pulse voltammetry have appeared, in part commemorating
Electrochemistry 12,
This article reviews the literature of electroanalytical chemistry in the period between December 1991 and the end of November 1993. An attempt was made to minimize the gap in the coverage between this and the previous Dynamic Electrochemistry review in Analytical Chemistry (Al). The focus of this review is on fundamental advances and practical applications of electrochemistry that pertain to electroanalytical chemistry. Topics covered include ultramicroelectrodes, analytical voltammetry, electrode kinetics, surface electrode phenomena, modified electrodes, bioelectrochemistry, characterization of inorganic, organic, and organometallic redox couples, spectroelectrochemistry, and instrumentation. The subject is of course quite broad and the divisions overlap. It is perhaps easier to indicate topics not covered in detail. Applications where there is no net current flow, e.g., potentiometric sensors, have traditionally been covered elsewhere in this review issue. There is not a separate section on photoelectrochemistry in the present review, although citations to articles relating to this topic can be found throughout the review. For the most part, articles were excluded that dealt with exotic electrode materials or media where the emphasis was not electroanalytical in nature. Industrial electrochemistry, fuel cells, and battery applications were also omitted from the coverage. The literature cited below was selected by scanning Citation Index, CA Selects: Electrochemical Reactions, CA Selects: Analytical Electrochemistry, and our personal reading of the literature. The coverage is not exhaustive, but is intended to
highlight important developments and activity.
Immunological and Recognition-Based
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Electrosynthesis Micelles and Surfactants
Stripping Voltammetry Catalytic Methods Derivatization Methods Analytical Use of Micelles Pulse and Sweep Methods
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Miscellaneous Bioelectrochemical Studies Characterization of Redox Reactions Electron-Transfer Mechanisms
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0003-2700/94/0366-0360$ 14.00/0 1994 American Chemical Society
©
Michael D. Ryan is Associate Professor of Chemistry at Marquette University. In 1969 he received his B.S. degree from the University of Notre Dame and in 1973 he was awarded a Ph.D. from the University of Wisconsin, Madison, Before joining the faculty of Marquette University in 1974, he served as Lecturer at the University of Arizona. His current interests include the study of indirect reduction of nitrite, nitric oxide, and sulfite reductases and the kinetics of electron-transfer reactions of biological compounds. 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 before returning to school. He obtained his Ph.D. at Virginia Commonwealth University in 1962 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.
James Q. Chambers earned his A.B. degree in chemistry from Princeton University in 1959. His graduate work under the direction of Ralph N. Adams was conducted at the University of Kansas, where he received the Ph.D. degree in 1964. The research interests of Prof. Chambers are in the general area of electroanalytical chemistry and are focused primarily on understanding and characterizing electrode reactions involving organic, polymeric, and biologically important compounds.
the original publication of Barker 40 years ago (A2-A4). Osteryoung also makes a passionate case for the advantages and virtues of pulse voltammetry in an Accounts of Chemical Research article (A5), Of practical value is the IUPAC commission on electroanalytical chemistry review on the effects that arise in pulse voltammetry when adsorption of the reactant is
significant (A6).
Several monographs or reviews have appeared recently that would make suitable reading for beginning students at various levels. Koryta has written a short introduction to ionic solutions, electrochemistry, and membrane phenomena that emphasizes concepts and is nonmathematical in nature (AT). A practical treatment of classical polarography also avoids mathematical detail (/IS). Runo and Peters have written an undergraduate level introduction to concepts involving electrode potentials (A 9). At the graduate level, the monograph by Gileadi on electrode kinetics is especially noteworthy (A 10). This text does a remarkable job of covering the fundamental concepts of electrode kinetics as well as presenting clear introductory descriptions of modern techniques, experimental details, and applications to batteries, fuel cells, corrosion, and electroplating. A number of impressive edited compilations of chapters on topics related to some aspect of electrochemical science have been published in the last two years. Before detailing these,
we will note the extensive, multiauthor review on the current state of understanding and research on the electrode/ electrolyte interface by Bard et al. (^4/7). This report focuses on new experimental capabilities and outstanding issues in three areas: structural characterization, dynamics, and materials aspects of the electrode/electrolyte interface.
Elsewhere, Bard has speculated on future directions of electrochemistry in a provocative article (AI2). Areas mentioned included UMEs and unusual media, scanning probe microscopies, and molecular biology. Three volumes of Modern Aspects of Electrochemistry have appeared (A13-AI5). Volume 25 has chapters on hydrogen ingress in metals, charge transfer across liquid/ liquid interfaces, dc techniques for measurement of corrosion rates, ellipsometry, and electrical breakdown in liquids. Volume 23 contains chapters on ion and electron transfer
monolayers of organic surfactants, determination of distributions by Laplace transformation, cathodic protection engineering, semiconductor/metal cluster surfaces, and electrical breakdown in anodic oxide films. Continuing with the eclectic nature of this series, Volume 24 treats nerve excitation, membrane energy transduction, the chlor-alkali process, bioelectrochemical field effects, electronic factors in charge-transfer reactions, and electrodeposition of metal powders with controlled particle grain size and morphology. In similar manner, a compilation with a high-sounding title (A16) contains chapters on the double layer, in situ spectroscopic examination of electrodes, electrode kinetics, organic electrochemistry, high-temperature electrochemistry, corrosion, and others. The most recent volume of Advances in Electrochemical Science and Engineering (AI7) contains four chapters: Trasatti on electrocatalysis of the HER, Hammou on solid oxide fuel cells, Richmond on second harmonic generation at single crystal electrodes, and Deslouis and Tribollet on flow modulation techniques. Lipkowski and Ross have edited two volumes of generally high caliber reviews relating to fundamental surface science at electrode interfaces (A 18, AI9). The first, which deals with adsorption of molecules, contains several chapters on radiochemical and spectroscopic characterization of adsorbed layers. The second, which is a collection of major reviews on the structure of the metal/ electrolyte interface, includes chapters by vacuum surface experimentalists, theorists, and electrochemists. Among the latter are reports by Ross on surface crystallography, Kolb on surface reconstruction, and Soriaga on molecular adsorption at single crystal electrodes. Omitted from the previous Analytical Chemistry review (Al) was mention of a compilation that contained chapters on the structure of halides and small organic molecules on metal surfaces by Hubbard, on heterogeneous catalysis of substitution reactions by Spiro, on the kinetics of crystallization of solids from aqueous solution by House, and a general introduction to corrosion of metals by Hammond (A20). In yet another volume one can learn about the semiconductor/electrolyte interface, electrode potentials, and energy scales, the application of STM to electrochemistry, adsorption and electron transfer at interfaces, and various electrochemical aspects of biomembranes (A 21). Two reviews of solvent effects on electron-transfer reactions have appeared (A22, A23). Weaver’s article is mostly across
current
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nonmathematical and emphasizes concepts. Galus summarizes theoretical treatments of the reaction rates for both soluble couples and amalgam-forming reactions such as Zn2+/ Zn(Hg). In a review by Saveant of his contributions to our understanding of dissociative electron-transfer reactions, he outlines criteria for distinguishing between stepwise and concerted mechanisms in electrode reactions (A24). Trasatti has reviewed recent theory on the adsorption of organics on electrode surfaces (A2S), and Parsons and Ritzoulis critically compare experimental results for adsorption on stepped surfaces of Pt and Au single crystal electrodes (A26). In the latter article, the evidence for the assignment of voltammetric peaks in the hydrogen region to hydrogen atom adsorption on steps of surface unit cells was clearly summarized. A thorough review of UHV techniques as applied to obtain atomic level information about the electrode interface at single crystal electrodes has been provided by Soriaga (A27). This treatment is suitable for a graduate student level introduction to the area. A substantial analysis of the kinetics of oxygen reduction at solid electrodes in aqueous solution has been written by Appleby (A28), and a book has appeared on the electrochemistry of surfaces from the critical perspective of Professor J. O’M. Bockris (A29). Catalysis of the hydrogen ion reduction by metal surfaces has been briefly reviewed (A30). An IUPAC Commission report has appeared that compiles kinetic parameters on the C12/C1- electrode reaction (A31). Also the IUPAC Commission report dealing with the measurement of real surface areas has been published a second time (A32). This report describes 15 methods, 11 in situ and 4 ex situ, in detail. A variety of ancillary techniques for the study of electrode processes are treated in a recent volume; included are chapters on ellipsometry, inferometric methods, SERS, Mossbauer spectroscopy, photothermal deflection spectroscopy, X-ray
electrodes by Inzelt, and SECM by Bard et al. (A40). Inzelt has also reviewed polymer film electrodes elsewhere (A41). Microelectrodes have been covered thoroughly in a publication of the proceedings of a NATO advanced study institute that contains articles from most of the laboratories which have contributed to the development of this area (A42). The authoritative review by Heinze on this subject is also recommended (A43). The latest in the Techniques in Chemistry series deals with the molecular design of electrode surfaces. Nine individual chapters written by active mod squad researchers cover subjects such as adsorption on single crystal electrodes, various aspects of redox polymer electrodes, and self-assembled monolayers. The volume, which includes more than 1400 references, is tightly edited by Royce Murray, who contributed a highly recommended overview chapter (A44). Reviews in this area have appeared elsewhere (A45-A49). The articles by Forster and Vos on the theory and applications of modified electrodes (A45) and by Zagal on electrocatalytic processes at metallophthalocyanine surfaces (A46) are nicely done. Several reviews treat subjects that strongly overlap electrochemistry and the amorphous, but fashionable, area of materials science. The chapter on processable conducting polymers in a compilation containing five chapters on nonlinear optics and conducting polymers is especially noteworthy (A50). Mirkin and Ratner have produced a provocative treatise on molecular electronics (A51) and electrochemistry at high-Tc superconducting working electrodes, an area where electrochemistry can contribute to both fabrication and characterization, is the subject of a report (AS2). Other reviews treated the structure and physical properties of PEO-type polymer electrolytes (AS3) and the electrochemical synthesis and properties of conducting polymers (AS4). The article by Curran et al. narrowly focused on the various methods by which polypyrrole can be employed as a support for electro-
absorption and neutron scattering, impedance spectroscopy, and others (A33). An introduction to STM and AFM with emphasis on basic theory and practicalities has appeared that discusses the application of these techniques to in situ
catalytic materials or substituent groups (/155). Recent work toward the development of practical biosensors has been reviewed from several different viewpoints (A56A62). These devices are generally based around a redox enzyme coupled with a molecular mediating species entrapped in an interfacial matrix of some kind. Both amperometric and potentiometric detection can be employed. The review of Alvarez-Icaza and Bilitewski gave a good summary of design parameters and their optimization (AS6). The article of Wring and Hart (A61) concentrated on the chemistry of the modification of carbon-based substrates for these devices, while that of Hilditch and Green had a practical bent describing
electrochemistry (A34). Scanning tunneling spectroscopy, which can map the surface electronic structure with atomic resolution in the best scenario, was also addressed. Buttry and Ward have written perhaps the most authoritative of several recent reviews on electrochemical quartz crystal microscopy (EQCM) (A35). Hillman et al. also reviewed the QCM technique with emphasis on the detection of mobile species transferred during the redox switching of polymer films (A36). A review of electrochemical mass spectroscopy (ECMS) contains some excellent examples of the use of a thermospray LC/MS interface with an electrochemical cell (A37). Solution IR spectroelectrochemistry has also been briefly reviewed (A38). Recent applications of spectroelectrochemistry have been described in a report that focuses on the redox chemistry of thin-film interfaces, e.g., inorganic semiconductors, oxide and chalcogenide films on native metals, dye-modified electrodes, conducting polymer films, and others (A39). Volume 18 of the Bard series, Electroanalytical Chemistry, contains reviews on electrochemistry in microheterogeneous fluids by Rusling, charge transport in polymer-modified 362R
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disposable electrochemical biosensors in or near to commercial
production (A62). Ewing et al. have reviewed progress on the difficult problem of analyzing the contents of single nerve cells with emphasis on in vivo electrochemistry and EC detection for capillary electrophoresis where the Penn State group has made major contributions (A63). Several works have appeared that concern more classical analytical voltammetry. These include a little monograph by Smyth on the voltammetry of biologically important molecules (A64) and reviews of analytical voltammetry theory by Cassidy (A6S), adsorptive cathodic stripping voltammetry by van den Berg (A66), the reduction of metal complexes on Hg by
Tur’yan (A67), and instrumentation for voltammetry by Barisci et al. (A68). In addition, a book on cyclic voltammetry has been recently advertized (A69). In the realm of organic electrochemistry, several important books or reviews have been published in recent years. A revised and expanded third edition of the Manual Baizer opus has appeared (A70). To complement this work, a book based on a symposium in honor of Manny Baizer contains 48 chapters organized under the following headings: Mechanism; Reduction; Oxidation; Mediated Reactions; Biochemical, Biomass and Natural Products; Modified, Sacrificial/Consumable Electrodes; Electrogenerated Bases; Film-Forming Electropolymerization; and Ion-Exchange (A71). Professor Shono has written a brief text that features experimental details for 150 electrochemical transformations of specific compounds (A72). Niyazymbetov and Evans have summarized the use of carbanions and heteroatom anions in electroorganic synthesis (A73). Recent examples of in situ generation of anions and anodic oxidation of anions are given. Commercial applications are emphasized in a brief review of the use of sacrificial anodes in synthetic electrochemical processes involving CO2 (A74). The problem of CO2 reduction has also been treated from several different angles in a collection of chapters by different authors (A75). Other reviews have appeared on the electrosynthesis of polymers with emphasis on intermediates (A76) and on the electrochemistry of chlorophyll (A77). Two excellent treatments of important topics have been produced by authorities in their respective fields. Wayner and Parker have provided an Accounts of Chemical Research article on the thermodynamic relationships between bond dissociation energies and redox potentials of the derived radicals and their corresponding ions (A78). A clear description is given here of the way to incorporate voltammetric peak potential data into the thermodynamic cycles. Koval and Howard have presented a thorough review of electron transfer at semiconductor electrode/liquid electrolyte interfaces (A79). While the emphasis of this review is on research advances since 1985, it also serves well as a lucid introduction to the terms and fundamental concepts of a complex subject. Gratzel has given an account of his research on photoelectrochemical energy conversion using a “molecular machine” based on thin films of colloidal TiC>2 particles that are sintered together to allow for charge carrier transport (A80, A81). The phenomenon of room-temperature photoluminescence from porous Si was reviewed in comprehensive fashion (A82), and a review of photoemission at metal/electrolyte interfaces includes a discussion of the cathodic generation of solvated electrons (A83). An issue of Electrochimica Acta was devoted to new trends in photoelectrochemistry (A84). Finally, two issues of the Journal of Electroanalytical Chemistry have accounts of the scientific careers and useful complete lists of their publications for two stalwarts of physical electrochemistry: Professors Brian Conway and John O’Mara Bockris in Vols. 355 and 357, respectively (A85, A86). B. MASS TRANSPORT Microelectrodes. Theory. The intense activity in the area of microelectrode theory has lessened in the last few years. Applications of UMEs have mushroomed, however, and there
have been some important and useful papers published that deserve mention here.
In the latter category, Mirkin and Bard have presented a theoretical analysis of quasi-reversible steady-state voltammograms that allows extraction of the kinetic parameters (k° and a) from a single i-E curve without independent determination of the E0' value (Bl). They gave extensive tables for the wave shape parameters, £1/4 £1/2 and £1/2 £3/4, that correlate with given sets of kinetic parameters. Correlation tables were given both for the case of uniformly accessible electrodes, such as a RDE and an UME hemisphere, and for a UME disk. Another procedure that appears to be easy to implement for the determination of heterogeneous rate constants from CV peak separation data is that of Lavagnini et al., which only requires intermediate diffusion control such that peak currents are evident in the CVs (B2). Exact formalism for the ac impedance of spherical, cylindrical disk, and ring UMEs was developed (B3). Real and imaginary components of the impedance, assuming uniform flux at the electrode surface, were tabulated as a function a2w/Z), a dimensionless quantity where a is the characteristic dimension of the UME, 01 is the frequency, and D is the diffusion coefficient of the Ox/R couple. Linear sweep voltammograms obtained at ring electrodes were calculated over 9 orders of magnitude sweep rate and compared to experimental results (B4). Calculation of the theoretical voltammograms required the value of a dimensionless parameter, 7 = (R2 + R\)/2(R2 R\), where R2 and R\ are the outer and inner ring radii, respectively. Oldham has continued his penetrating theoretical examination of microelectrode behavior in a very general treatment -
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of steady-state voltammetry at UMEs of arbitrary shape (B5). He showed that the steady-state current depends on three factors: the electrode area, an accessibility factor, and a heterogeneity function. A universal equation is given for the
i-E relationship. A sophisticated integral equation approach
was used by Bender and Stone to treat steady-state mass transport to microelectrodes (B6). They gave a numerical solution procedure for calculation of the surface flux that is applicable to the general case of an arbitrarily shaped planar electrode, including both surface and bulk catalytic regeneration reactions. Multidimensional integral equations were used in another mathematically sophisticated approach to UME diffusion problems (B7, B8). The approach was stated to have considerable advantage over the conceptually simpler finite difference and finite element digital simulations in terms of computer requirements and execution time. UME configurations considered were microdisks embedded in an insulating plane of infinite or finite extent, microbands, the SECM problem, and an array of inlaid planar electrodes of
arbitrary shape. Brodsky et al. presented closed-form solutions of the diffusion kinetic equation for individual or arrays of UMEs that were based on a “zero range approximation” (B9). They achieved excellent agreement between calculated and experimental collection efficiencies using literature data. Conformal mapping procedures for the digital simulation of diffusion at a microdisk have been optimized and improved (BIO, Bll).
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Oldham has also generalized his treatment of steady-state voltammetry in the absence of supporting electrolyte (SE) in two important papers (B12, B13). In the first, theory was developed for three classes of voltammograms: “sign-retention voltammograms”, where Ox and R have the same sign; “charge neutralization voltammograms”, where a neutral product is generated; and “sign reversal voltammograms”, where Ox and R have opposite signs. In the first two situations, plateaus will exist in the limiting current region, while in the latter the current was predicted to increase in a linear fashion with potential. For the charge reversal CVs, there is a marked dependence on trace amounts of supporting electrolyte (B12). The second paper gave steady-state voltammetry theory at a hemisphere UME for any degree of supporting electrolyte excess relative to the concentration of the reactant. Reversible, quasi-reversible, and irreversible i-E curves can be calculated from the equations given and representative examples of different cases are worked out. The criteria presented by Myland and Oldham should give the experimentalist new tools for characterizing electrode reactions by variation of the SE concentration. The theory, however, is developed by assuming that there is no adsorption of Ox or R on the electrode surface.
Myland and Oldham also derived the support ratio, [SE] / [reactant], needed to ensure that the limiting current is within 2% of (lim for infinite excess SE, and to ensure that E1/2 is likewise displaced by less than 1 mV. Convective mass transport at macroelectrodes, i.e., RDEs, may be treated in a way that exactly parallels the general theory of this paper with simple algebraic replacements (B13). Two important papers address problems that will arise for so-called nanodes, electrodes with characteristic dimensions on the order of nanometers. Smith and White calculated i-E curves for very small electrodes based on a numerical solution of the Nernst-Planck and Poisson equations (B14). They showed that the double-layer electrical field can markedly affect the currents, even in the presence of a large excess of SE and even for neutral reactants. Under conditions where the double layer and the depletion layer have similar dimensions, and where charge separation in the depletion region occurs due to ionic flux, the assumption of electroneutrality is not generally valid. Another possible artifact for very small electrodes is the problem of incomplete adhesion or cracks between the electrode surface and the insulating sheath. This can create a “lagoon” of electrolyte solution behind a pinhole. Oldham modeled this situation with a geometrically well-defined lagoon and solved Fick’s equation in elegant style (B15). The limiting currents, the time to reach steady state, and the kinetic parameters extracted from the CVs all are significantly altered. These lagooned electrodes, however, do behave as UMEs and can have some analytical virtues, e.g., more reversible behavior phenomenologically. Several articles have considered coupled homogeneous chemical reactions at UMEs (B16-B21). The EC, EC', ECE, and DISP1 reaction schemes were incorporated into simple theory using a steady-state reaction/diffusion layer concept (B17). Bond et al. have applied UMEs for the determination of E°' values and kinetic parameters for a Cr carbonyl complex that participates in a square scheme (522). This paper also described a neat procedure for correction of the iRv drop 364R
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involving the simultaneous measurement of the difference current between electrolyte solution in the presence and absence of analyte. Other miscellaneous applications include the chronoamperometric determination of the absolute concentration of a well-behaved electroactive species at a microwire UME (525). The procedure rested on the Aoki equation, whereby the concentration is a function of the intercept and slope of the it1/2/area vs t1/2 curve (and the electrode radius and n value). A procedure was given by Wikiel et al. for measurement of the stability constants of metal complexes by analysis of the anodic dissolution reaction of a metal UME operating in a normal pulse voltammetry mode (B24). Pulse techniques applied to dissolving metal UMEs such as Cu render the entire voltammetric wave accessible due to the reduced iR drop and double-layer capacity of the UME (B25). A Monte Carlo method of modeling the random motion of particles was employed to simulate diffusion noise at a UME (B26). Experimental Aspects. Procedures for the fabrication of UMEs have been pretty well worked out in recent years. Nonetheless, useful tidbits can be gleaned by perusal of experimental sections of the myriad of articles on UME applications. The reader is to be warned that surveying the literature in this manner is hit and miss. A careful study was carried out on methods to maximize the S/N ratio for the detection of dopamine using Nafioncoated carbon fiber UMEs (527). Sources of noise considered included Johnson noise from the feedback resistor in the current follower, waveform generator noise, line noise, and physiological noise in in vivo measurements. In this study dopamine was readily detected at a 100 nM concentration with a S/N ratio of 25 using fast-sweep voltammetry. Reasonable CVs of dopamine were obtained using very small carbon fiber UMEs (overall dimensions of 400 nm) that had been insulated with a phenol-allylphenol copolymer (B28). A simple procedure of sealing UME carbon fibers or wires in polypropylene has been published (529). On-line iRu compensation was employed to perform CV at sweep rates up to 11 kV/s using 7-^m carbon fibers (B30). Peng et al. have described fabrication and electrochemical activation of carbon fiber UMEs for the in vivo determination of neurotransmitters CB31).
Photolithographic methods were used to make carbon ID As with 3-fim-wide fingers separated by 2 pm of Si3N4 insulation (552). The carbon, which was vapor deposited by pyrolysis of a perylenetetracarboxylic dianhydride, exhibited an electrochemical behavior similar to that of glassy carbon. Wrighton’s group, which has pioneered the use of photolithography to make UME arrays, has used surface spectroscopy to characterize an array consisting of six or eight individually addressable Au or Pt UMEs on a Si3N4 substrate (555). The experimental details for carbon-based enzyme electrodes often involve state-of-the-art fabrication techniques. For example, amperometric enzyme electrodes were prepared using extremely thin (35-50 nm thick) carbon films prepared by the pyrolysis of spin-cast polyacrylonitrile (B34). The were entrapped on the nanobands by electropolymerization of 1,2-diaminobenzene in the presence of the enzyme. Mention is also made of the elegant surface
enzymes
modifications of carbon fiber UMEs by Kuhr and co-workers (B35, B36). Enzyme electrodes were made using TTF-TCNQ salt deposited in the recessed tips of 7-fim carbon-fiber UMEs. The conducting salt mediated the redox chemistry of flavoenzymes attached to the surface via the glutaraldehyde method (B37).
Platinum and gold UMEs were prepared by the direct electroreduction of Au(III) and Pt(IV/II) onto the tips of carbon fiber electrodes that had been coated with an insulating polymer (B38). Also, Ewing’s group has used Au ring UMEs prepared by electrodeposition of Au onto carbon rings (B39). A little paper on the measurement of k° values for the Fe(CN)63~'/4- couple has some interesting details (B40). The presence of millimolar amounts of cyanide in 1 M KC1 solution was found to stabilize the response. Pretreatment by either polishing with an alumina slurry containing KCN or by laser activation yielded k° values in the range of 0.5 cm/s. Particulars were given for construction of UME arrays using Buckbee-Mears minigrids (B41). The epoxy-potted electrodes were polishable and had relatively regular spacings (which were the cross sections of the minigrid wires). The morphology of micropit arrays formed by electrochemical etching of carbon fiber/epoxy electrodes was characterized by bullet-shaped tips (B42). A proof-of-principle submicrometer galvanic cell consisting of STM-deposited Cu and Ag pillars on a HOPG surface was demonstrated (B43). Atomic microscopy showed that the 70-nm cell discharged when immersed in a plating solution. Martin and his troops have been busy making and characterizing arrays of metal cylinders deposited in the pores of alumina microporous template membranes. They have prepared recessed gold disk array electrodes, for example, with very deep 200-nm-diameter microholes (B44). In another study, they showed that the color of arrays of Au cylinders with nanometer dimensions could be changed by variation of the aspect ratio of the nanocylinders (B45). Properly prepared arrays were transparent in the infrared region (2000-4000 cm-1) (B46). Also, arrays of CdSe and CdTe microfibrils were fabricated in this manner (B47). Iridium is known to be a good substrate for mercury electrodes. A nice application of anodic stripping square wave voltammetry employed an iridium substrate-based Hg UME where the Ir was etched to a radius of 5-10 /urn prior to Hg deposition (B48). Metal ions were determined without deoxygenation, without added SE, and without controlled stirring during the deposition step. The diffusion coefficient of Tl° in T1 amalgams was determined by UME chronoamperometric methods (B49). Applications. Even a cursory survey of the current literature reveals that UME methodology and theory have widened considerably the playing field of electroanalytical chemistry. Several accounts have described UME voltammetry in the absence, or at low ratios, of the SE to analyte concentration where agreement was sought with Oldham’s theory (B50). Drew et al. reported agreement for ratios greater than 0.1, but found that natural convection and the tendency of the generated ions to scavenge ions into the diffusion layer vitiated the theory (B51). For inorganic redox couples of varying charge and E\/2 values, Cooper et al. also reported anomalous behavior in several instances (B52). Lee and Anson
found that the electroreduction of Fe(CN)63~ at carbon and Pt UMEs was markedly suppressed in the complete absence of SE (B53). Reduction of this species, however, could be efficiently mediated by the positively charged M V2+/+ couple. Comproportionation kinetics of the latter system were studied by steady-state voltammetry in solutions of low SE concentration (B54). In an interesting study, Cooper and Bond found adsorption of neutral cobaltocene and passivation of the electrode surface for the (Cp)2Co+/°/_ system in CH3CN (B55). At UMEs this gave rise to stochastic processes at negative potentials where cathodic dissolution of the film, as (Cp)2_, is possible.
Homogeneous electron-transfer kinetics in monomeric organic liquids such as nitrobenzene were studied by Norten et al. (B56). At Pt UMEs, the second wave for the formation of the NB2~ dianion was depressed relative the first wave due to the combined effects of the comproportionation reaction and electrostatic repulsion of the NB'~ away from the negatively charged depletion layer surrounding the electrode. Others have looked at the UME voltammetry of nitrobenzenes in low ionic strength aprotic media (B57), and Ciszkowska and Stojek obtained well-defined anodic waves, with a 0.5 n value, for the oxidation of solvent in neat alcohol solution with LiC104 SE (B58). Several studies have employed UMEs at low or wide temperature ranges. Attention is drawn to the low-temperature CVs of the (Cp)2M2+/+/°/-/2- cobaltocene and nickelocene systems in liquid S02 (B59), the careful kinetic study of the ferrocene couple over the 200-300 K temperature range (B60), and the temperature-dependent phase transitions and diffusive electron and solute transport seen in polyether “solid-state” solvents (B61). CVs of the Fe3+/2+ couple in ice featured surface or thin-layer behavior, indicating the existence of liquid water microphases in contact with the UME surface at temperatures below the freezing point (B62). Modestly fast CVs and chronoamperometry of polyaniline films in contact with HC104‘5.5H20 at temperatures down to 220 K suggested that the oxidation process involved electrocrystallization phenomena (B63). Voltammetry and chronoamperometry has been performed at pressures up to 8000 bar using Pt microcylinder wire electrodes (B64, B65). A two-electrode cell with a UME coated with redox polymer/enzyme functioned remarkably well in a C02-based fluid near its critical point (B66). Several more strictly analytical applications of UMEs included studies of the reduction of acids in the presence and absence of SE (B67, B68), ASV of heavy metals in natural waters at Hg UMEs (B69), and a method for the determination of the total acidity of various wines (B70). Good results were obtained for analysis of Hg in the absence of SE by ex situ plating of a UME Pt disk followed by transfer of the electrode to an electrolyte solution for ASV (B71). Very nice CVs of solid-state vanadyl sulfate hydrate were obtained at a carbon disk UME (B72). UME voltammetry was carried out in a single drop of solution by Unwin and Bard, who measured the adsorption and ion exchange of H+ on a silicate mineral surface and of methylene blue on HOPG and polycrystalline graphite surfaces {B73). Likewise, Bowyer et al. did electrochemistry in volumes as small as 0.05 ^L using a three-band array consisting of a
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4-gm Pt working electrode, a 100-gm Ag reference electrode, and a 100-gm Pt counter electrode separated by heat-sealing film (B74). In continuing publications, mostly from the Texas laboratory of Bard and co-workers, the versatility of the scanning electrochemical microscopy (SECM) technique has been demonstrated. Wipf and Bard made significant improvement in the technique by employing small-amplitude tip-position modulation in combination with lock-in detection of the signal (B75). This gave unambiguous indication of whether a surface was conducting or insulating and markedly improved the contrast between those surfaces. In another study, it was shown that information on the tip shape could be obtained from the SECM response at a well-defined flat surface (B76). This is important for very small UMEs where conical-shaped electrodes are much more easily fabricated than insulated UME disks. SECM theory was developed for the determination of fast heterogeneous kinetics from steady-state currents (B77, B78). The method, which involves determining the current as a function of potential for fixed values of d/a, where d is the separation between the scanning tip of radius a and the conductive surface, should permit rate constants up to 10-20 cm/s to be measured. The technique was applied to the measurement of the rate constants for the ferrocene*/0 and C6o°/“ couples (B78, B79). Also SECM, operating in the feedback and generation/collection modes, was applied to the measurement of rates of chemical reactions coupled to electrontransfer steps for the electrodimerization of activated olefins (B80). Simulation of the redox kinetics in the tip-substrate gap was performed in conjunction with SECM to image the redox enzyme glucose oxidase immobilized on nonconducting substrates such as nylon, hydrogel membranes, or a L-B film (B81). An antimony UME was used with the SECM apparatus as a potentiometric sensor for hydrogen ion activity (B82). General theory was developed and applied to give pH images around a Pt electrode during the reduction of water, a corroding Agl disk in cyanide solution, a disk of immobilized urease hydrolyzing urea, and other systems. The SECM technique was used to characterize solid films of AgBr {B83). The diffusion coefficient of bromide ion in the AgBr matrix was deduced from the tip current transients produced when the tip/substrate pair was pulsed in a thin-layer electrochemical mode. SECM has also been employed to map ionic fluxes of electroactive species at various porous membranes including mica and mouse skins (B84), to detect proton motion at polyaniline film electrodes (B85), to monitor ion release from protonated poly(vinylpyridine) films (B86), and to characterize a 200-nm-thick polymer film (B87). Surface diffusion and desorption processes are readily handled by SECM. In this application, the probing tip, biased at a potential where the adsorbate is electroactive, yields a current that is a function of the rate of diffusion through solution, the adsorption/ desorption kinetics, and the rate of surface diffusion (B88). The approach was illustrated by detection of H*adS at rutile(001) and aluminosilicate surfaces. In vivo electrochemistry under physiological conditions, which was the original motivation for UME voltammetry, has provided some of the most impressive applications of UMEs in analytical chemistry. The detection of the release of 366R
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catecholamines at the femtomole level from individual adrenal medullary chromaffin cells continues this tradition (B89). Interestingly, both cells that released only epinephrine and cells that released a mixture of epinephrine and norepinephrine
identified. Wightman’s group have used UMEs to monitor the flux of catecholamines from other biological cells during exocytosis (B90). The process was analyzed in terms of diffusive mass transport from a point source. Ewing’s group has used platinized carbon microring electrodes to monitor O2 concentrations and to perform multiple pulse voltammetry in single neuron cells of their favorite pond snails (B91,B92). Oxidation currents have also been measured due to generation of superoxide anions at a carbon UME in contact with a single biological cell (B93), and a Pt on graphite UME was used to monitor oxygen efflux from illuminated algae protoplasts (B94). The release of NO from a single cell was detected with a carbon fiber UME coated with a polymeric porphyrin/ Nafion composite film (B95). Adams and co-workers used Nafion-coated carbon fibers to detect norepinephrine release and to profile glutamate-evoked ascorbic acid release in the brains of freely moving rats (B96, B97). Impedance analysis of 100-gm Pt electrodes covered with biological cell cultures indicated the formation of pores in the cell membranes upon application of small applied voltages (B98). UME arrays have been employed in innovative fashion. Electrochemical luminescence, generated by a radical cation plus radical anion annihilation reaction, was examined at double-band UME arrays both experimentally and theoretically using conformal mapping transformations (B99). For reversible systems the depletion effect in differential pulse voltammetry is minimized for an interdigitated array (IDA) working electrode operating in the feedback mode (B100). An IDA electrode, also operating in the collection mode, was shown to be a sensitive detector under conditions compatible with an enzyme immunoassay (B101). Volumes as small as 800 nL were successfully handled in this study. A clever application of an IDA incorporated an electrochemical coulometer in series with an IDA in a solution of a reversible redox couple. The coulometric process employed was electrodeposition of a metal, which was followed by AS V analysis allowing determination of the redox couple at the 10-8 M level (B102). IDAs have been used to measure apparent electron diffusion coefficients in polymeric matrices: good examples are the study by Nishihara et al. of poly(ethylene oxide) (B103) and the redox switching of poly (pyrrole) films (B104). Several papers have addressed theoretical aspects of various array geometries. These include a treatment of the overlap of diffusion layers at microband arrays (B105), cylinder arrays closely aligned parallel to a planar electrode (Bl 06), and dualcylinder UMEs in parallel operating in a biamperometric mode with a small imposed voltage difference (B107). The latter theory was applied to the titration of ascorbic acid with ferricyanide. Frequencies as high as 20-30 kHz were used for the generation of electrochemical luminescence by square-wave generation of the radical ions at UMEs (B108). A lower limit on the ion-annihilation rate constant for diphenylanthracene of 2 X 109 M~’ s-1 was determined. Double-band UME arrays have also been used to generate ECL in a steady-state mode of operation (B109). were
Hydrodynamic Methods. Rotated Electrodes. Verbrugge has given a theoretical analysis of the RDE convective diffusion problem for an Ox/R couple that is valid for a wide range of
Schmidt numbers and Reynolds numbers {B110). The current response was found to be bounded by the Levich equation for large Schmidt and Reynolds numbers and by the stationary disk response for zero Schmidt or Reynolds numbers. Vieil developed a general mathematical treatment of mass transfer that quantifies the transition between stationary convective mass transport and time-dependent accumulation at an electrode surface {Bill). A mass-transfer rate expression is given that can accommodate a variety of experimental conditions such as fractal geometries and transient behavior at RDEs. Simulation of transient diffusion and migration to a RDE during deposition of a metal film gave information on the current distribution, the effect of inert electrolyte, and the role of disk size (5772). Bartlett and Eastwick-Field have written a particularly cogent analysis of limiting currents at a RDE for a generalized ECE reaction scheme (5775). The effect of rotation rate on UME array composite RDEs was studied using composite electrodes of gold and graphite particles embedded in Kel-F (5714). As expected, significant enhancement of true current density was seen at the array electrode in comparison to solid electrodes; i.e., [(i/area)array/ (i/area)soiid] > 1, where the active area is used to calculate the current density. Several researchers have used rotation rate modulation techniques in various studies. The rotation rate step method of Blauch and Anson (57 75) was applied to a silver electrode to determine the diffusion coefficients of electroinactive ligands (5775). Anodic O-transfer reactions at several electrodes (Pt, Au, Pd, Ir, and glassy carbon) were studied using a current difference RDE method that diminished the contribution of O2 evolution to the observed response (57 77). A key role for adsorbed hydroxyl radicals was deduced in this study.
Engelhardt et al. have presented theory for hydrodynamic square-wave modulation of a rotating ring disk electrode (57 73-5720). The method is useful when there are parallel reactions and the ring current of interest is masked by large background currents. In the experiments of Schwartz (5727), the rotation rate of a commercial RDE was modulated by a sinusoidal or by a square root waveform. Fourier transformation then gave the frequency response of the system in a single experiment. Hydrodynamic impedance has also been employed by Deslouis and Tribollet (5722). Papers continue to be published that exploit the power of rotating electrodes in mechanistic studies. Rotating ring disk techniques were used to study the MV2+/+/° system (MV is methyl viologen) in aqueous solution, where surface blocking by adsorbed neutral species had to be taken into account {B123). Likewise, Kokkinidis et al. found that electrodeposition of neutral benzyl viologen onto Pt or Hg proceeded by direct nucleation and 3-D growth under mass transport control (5724). Other studies include chronocoulometric measurement of adsorbed intermediates in the oxidation of iodide at a Pt RDE (5725), voltammetry of adsorbed intermediates in the HER (5725), the electrocatalytic oxidation of CN~ at a glassy carbon RDE (5727), the mediated reduction of an alkyl halide (57 23), the detection of the anaesthetic halothane
via oxidation of released Br~ at the ring electrode {B129), and measurement of enhanced D values due to homogeneous electron exchange in the RDE voltammetry of Ru-EDTA-
poly(vinylpyridine) complexes {B130). On the more applied side, the photographic fixation process at a AgCl emulsion disk electrode was followed by monitoring the flux at a ring electrode (5737), and a novel RRDE method was described for the detection of CO2 expired in breath from a human subject {B132). Compton and Brown have proposed a RDE method to monitor particle size in solution that is based on the enhancement of mass transport in the presence of suspended particles (5733). Others have studied mass-transfer enhancement in a dilute suspension of rotating particles under the influence of shear flow {B134), and Gabrielli et al. treated the ac impedance of fluidized-bed electrodes, both theoretically and experimentally for gold beads in NaOH solution (5735). In the miscellaneous category are papers on the rotating ring cone electrode (5735), mass transfer at the entire surface of a vertical cylinder electrode (5737), the use of an inverted RDE for the study of gas-evolving reactions (5733), and mass transfer at a RDE with external forced convection {B139). Wall Jet and Channel Electrodes. R. G. Compton and his colleagues have continued their sophisticated analysis of wall jet and channel electrodes. As will be noted below in the section on electrochemical detectors for FIA or chromatographic columns, these configurations have real practical
significance. The Compton group has presented theory for the transient current response for a simple E step at a WJE (5740) and at the ring in a wall jet ring disk electrode (5747). The theory was experimentally verified in a later study {B142). They also developed theory for CV at a WJE where the electrode is substantially larger than the jet. The theory predicts transitions from steady-state CVs at slow sweep rates to peakshaped response at fast sweep rates {B143). The role of radial diffusion in the WJE response has been considered in a more recent paper (5744). Compton et al. have presented a general computational approach to calculation of i-E curves at channel electrodes (5745) and, in a related paper, calculated limiting currents for UME band electrodes in a rectangular flow channel under conditions where convection is the predominant mode of mass transport (5745). Tait et al. have also tackled the difficult problem of a UME disk electrode in a flow channel using finite difference simulation {B147). The effect of electrode size, solution velocity, and channel thickness on the magnitude of the near-steady-state currents and the time required to reach this condition were determined in the latter paper. A treatment of the catalytic EC' reaction at a flow channel electrode is representative of several articles on coupled chemical reactions under these conditions {B148). A four-element carbon paste array detector, which consisted of different graphite/metal oxide composite surfaces in a wall jet configuration, displayed different electrocatalytic sensitivities toward carbohydrates and amino acids {B149). In another study, the performance of an array WJE assembly was
optimized (5750). Flow- Through Electrochemical Detectors. Electrochemical detectors for chromatographic or FIA columns (LCEC) Analytical Chemistry, Vol. 66, No.
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routinely employed. Only a fraction of the recent papers that present possible advancements in the methodology will be cited here. The WJE configuration is well suited to the LCEC problem. Stojanovic et al. evaluated cylindrical wire, thin-layer, and WJE detectors with constant and pulsed amperometric (PAD) modes of operation for the determination of inorganic arsenic (5757). In their hands, the WJE had the best LOD. Other recent examples of WJE LCEC detection can be mentioned (B152, B153). It can be noted, however, that the impressive sensitivity of carbon-fiber detectors, which has been exploited in the determination of catecholamines in single bovine adrenomedullary cells (5754), is difficult to match. Electrochemical detection has been applied to capillary electrophoresis with much success. Sloss and Ewing, for example, have improved their methods by enlarging the end of the capillary to accommodate a larger UME. Problems with variation of the capillary/electrode alignment were also minimized with their new design, which gave detection limits as low as 11 amol for catechol (5755). Lunte and co-workers have given details on the construction of a complete capillary electrophoresis system with electrochemical detection (5756). They found that a 50-Mm-diameter Au(Hg) amalgam UME functioned well as a CE detector for free thiols (5757). Lu and Cassidy evaluated several UMEs in a WJE configuration as detectors for capillary electrophoresis columns. Not unexpectedly, mercury amalgam electrodes gave the best performance for 14 test metal ions (5755) and PAD at a Au UME worked well on the anodic side (5759). Mahoney et al. modified a commercial stationary mercury drop electrode apparatus so that square-wave voltammograms could be obtained under stopped-flow conditions (B160). Rapid-scan voltammetry at a UME detector was shown to give theoretical steady-state CVs under chromatographic conditions (6767). Trade-offs in sweep rate, UME diameter, and flow rate were evaluated. Two groups have put 16-element LCEC amperometric detectors to good use. In the study of Sparks and Geng, detection over a potential window of 0.75 V greatly increased the information content of a single chromatogram (67 62). In the impressive work of Aoki et al., 80-channel detection was achieved by application of a five-step £-staircase waveform with 10-mV step heights to the 16 elements of the array (6765). IDAs operated in the dual-electrode mode were shown to have good sensitivities as flow detectors: 100 pM dopamine was detected under HPLC conditions (B164). An advantage of this mode of detection is that the component of the current due to redox cycling is flow rate independent. Descriptions of several novel spectroelectrochemical flow detectors have appeared. Brown et al. used anodic photocurrents at TiC>2 to detect species with redox potentials more negative than the valence band edge of the semiconductor at the 100-pmol concentration level (6765). Another flow cell was based on indirect detection of eluents by the decrease in the intensity of electrogenerated luminol chemiluminescence (57 66). Also, a pulsed flash photolysis amperometric detector was described that appears to have some promise as a general purpose LCEC detector (B167). Several papers have proposed various surface modifications to improve LCEC response. The dual-electrode sensor of are
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Doherty et al., which is based on redox polymer-modified electrodes, nicely performs speciation analysis of Fe(II) and Fe(III) (5765). One electrode was coated with a Ru-bpy polymer (£1/2 = 0.75 V vs SCE), which mediated the oxidation of Fe(II), and the other was coated with an Os-bpy polymer = (£1/2 0.25 V vs SCE), which mediated the Fe(III) reduction. The latter electrode was also used for the FIA of nitrite ion with excellent sensitivity and stability of response (5769). Wang et al. employed self-assembled monolayers (SAMs) of n-alkanethiols on Au electrodes to vary the response of LCEC detectors to analytes such as chlorpromazine in urine samples (B170). The SAM-modified electrodes discriminated against small ionic electroactive species and were stable under the hydrodynamic conditions of the flow cells. Mark and colleagues (5777), and others (B172), have found that conducting polymer film electrodes show improved performance in terms of sensitivity and antifouling properties for the detection of ionic species. Numerous examples of chromatographic analyses employing electrochemical detection can be found in the forests of chromatography literature. Examples of pulse amperometric detection include a study of the sulfur compounds cysteine, cystine, methionine, and glutathione, all of which were detected in human blood samples using simple LC PAD procedures (5775) PAD and HPLC/MS were employed in complementary fashion for the analysis of aminoglyoside antibiotics (5774); SO2 has been analyzed in beer by IC with PAD detection (B175); and HPLC of amino acids using PAD had excellent sensitivity and required little to no sample preparation (5776) Johnson and co-workers have published two papers refining their PAD methodology. In one, pulsed voltammetry at a RDE was used to optimize the potential and time parameters for the PAD waveforms (5777), and the second gives construction details for a simple low-cost LCEC detector employing a gold working electrode (5775). Other examples of LCEC that caught your reviewer’s eye were a fast-scan voltammetric detection of fullerenes (67 79), detection of underivatized polypeptides using constant potential amperometry (5750), and the coulometric detection of neurotransmitters and respiratory metabolites in brain tissue (5757). Zhu and Curran considered porous flow-through amperometric detectors under conditions of low conversion efficiencies (5752). RVC electrodes were used to test the theory which predicted that the limiting currents were proportional to the 2/i power of the pore diameter. A UME biamperometric GC detector was evaluated (5755). .
.
C. ANALYTICAL VOLTAMMETRY Methodologies. The year 1992 marked the 40th anniversary of the publication of the seminal article by Barker and Jenkins on square-wave polarography. This paper was reprinted by the Analyst in honor of the event (C7), along with a retrospective by Barker and Gardner (C2). The development of square-wave voltammetry (SWV), as well as other electroanalytical techniques, has continued over the past two years. For example, Chin et al. (C5) reported on the mathematical enhancement of SWV. Lovricetal. (C4) treated theoretically the use of SWV in cathodic stripping, while Komorsky-Lovric et al. (C5) looked at peak current/frequency
relationships in adsorptive stripping. The theory and experimental verification of pseudopolarography at the mercury hemisphere ultramicroelectrode was examined for use in anodic stripping voltammetry and metal speciation studies (C6). A reference element (internal standard) method was examined for the analysis of natural waters by stripping voltammetry
(C7). As in the past review, adsorptive stripping voltammetry has been the most dominant approach that has been reported. Jin et al. (C8) studied the amount adsorbed in the adsorptive accumulation at a symmetric spherical electrode in a stirred solution. Li, James, and Magee(C9) examined the effect of the accumulation potential in the adsorptive stripping voltammetry of organochlorine compounds. As will be seen below, the analysis of metals by an adsorbed metal complex has been quite productive. The induced reactant adsorption in pulse polarography was examined by Puy et al. in a series of articles (CI0-C12). Jin et al. (Cl 3) compared conventional and derivative measuring techniques for linear potential sweep adsorption stripping voltammetry. Stripping Voltammetry. The scope and selectivity of adsorptive stripping voltammetry has been greatly extended by the use of complexing agents either in solution or attached to the electrode. This has enabled adsorptive stripping voltammetry to be extended to a wide range of metals. Carbon paste electrodes are excellent candidates in that a wide variety of reagents can be incorporated into the paste. For example, the incorporation of salicylideneamino-2-thiophenol allowed for the accumulation and adsorptive stripping of copper (Cl 4). A functionalized silica gel was incorporated into carbon paste for the adsorptive stripping of mercury(II) (Cl 5). Mercury (Cl6) and thallium (Cl7) were concentrated with anion exchangers which were present in the carbon paste. A diphenylcarbazide-modified carbon paste electrode was used for the determination of chromium(VI) and chromium(III) (C18). Gold was selectively extracted with triisooctylaminemodified carbon paste (Cl9) or with thiobenzanilide (C20). A long alkyl chain amine was used for the selective determination of pyridoxal in nonfat dry milk (C21). A mossmodified carbon paste electrode was found to efficiently bioaccumulate lead (C22). Organic cations such as paraquat were determined by adsorptive stripping voltammetry with Amberlite resin in carbon paste electrodes (C23) or an ion pair at a hanging mercury drop electrode (C24). Silicamodified carbon paste electrodes were used for the determination of todralazine in biological fluids (C25). OV-17 silicone-modified carbon paste electrodes selectively concentrated organic compounds such as benomyl prior to analysis (C26). A diphenyl ether graphite paste electrode was used in the analysis of vanillin (C27). While carbon paste has been the most popular electrode material, other electrodes have been derivatized or modified in some manner so as to provide for preconcentration prior to the stripping analysis. Nafion mercury film-modified electrodes were used for the anodic stripping voltammetry of bismuth (C28). A glassy carbon electrode coated with a Nafion film was used to preconcentrate a nitrosoamine (C29). Gold(III) was determined by anodic stripping voltammetry using a glassy carbon electrode with an aza crown ether (C30).
While significant sensitivity and selectivity enhancement be obtained by covalently bonding or mechanically entrapping an agent near the electrode, similar results can be can
obtained by forming strongly adsorbable complexes in solution. Some examples are the use of Beryllon III to form complexes
with beryllium (C31) or copper (C32) prior to adsorptive stripping analysis. The same technique was used to determine aluminum (C33) or uranium (C34) with cupferron. Vanadium (C35) was determined by cathodic stripping voltammetry after deposition as the Solochrome Violet RS complex, while this same ligand can be used to determine aluminum by adsorptive stripping voltammetry (C36). Cathodic stripping voltammetry was used to determine tripeptides by the formation of the copper complex at a mercury electrode (C37, C38). Fulvic acid enhanced the adsorption of the Mo(VI)-phenanthroline complex in cathodic stripping analysis (C39). A wide variety of organic compounds can be adsorbed on electrode surfaces and are ideal candidates for adsorptive stripping voltammetry. This is especially true of pharmaceutical compounds. Some examples of the drugs, electroanalytical techniques, and sample matrices that have been examined in the past two years are summarized below in order to give the reader a flavor for the scope of the technique. Villar et al. (C40) determined mitoxantrone using phaseselective ac adsorptive stripping voltammetry in a flow system. Phase-selective ac adsorptive stripping voltammetry was also used in the analysis of folic acid on a mercury thin-film electrode (C41) and aminopterin on a mercury thin-film carbon fiber microelectrode (C42). Mercury-coated carbon fiber microelectrodes were also used in the adsorptive stripping voltammetry of folic acid and mitoxantrone (C43). Flunitrazepam (a psychotropic drug) (C44) and lormetazepam (Cll) were determined in urine by adsorptive stripping. Ranitidine in stomach tissue was determined by the same technique (C45), as was metronidazole in human serum (C46). Cholesterol (C47) in blood serum and testosterone propionate in pharmaceutical preparations (C48) was determined following adsorptive preconcentration. Multispecies analysis was obtained for the determination of riboflavin and folic acid in multivitamin preparations (C49) and nickel(II) and cobalt(II) on a rotating disk mercury film electrode (C50). Economou and Fielden (C51), though, investigated the effect of surfactants on adsorptive stripping voltammetry and found that interferences can occur on the milligram per liter level. They examined the use of fumed silica gel and Nafion films to alleviate these problems.
A more forceful way of applying the sample to the surface utilized in abrasive stripping voltammetry, where the
was
sample is physically deposited unto the surface. KomorskyLovric et al. compared the use of electrochemical and abrasive deposition onto a paraffin-impregnated graphite electrode for the analysis of lead and mercury (C52). Scholz et al. (C53) examined the anodic dissolution of dental amalgams by abrasive stripping voltammetry. This same stripping technique was also used to study the thermodynamics of solid-phase
transitions (C54). There were several reports over the past two years on the use of new electrodes for anodic stripping voltammetry. Mercury films on conducting poly(3-methylthiophene) (C55) or poly-A-ethyltyramine (C56) on carbon surfaces were Analytical Chemistry, Vol. 66, No.
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reported. Wang et al. investigated the use of mercury-coated carbon foam composite electrodes (C57), vitreous carbon aerogel electrodes (C5S), and screen-printed stripping electrodes (C59). Frenzel (C60) discussed the attributes and problems of using mercury films on a glassy carbon support. Ultramicroelectrodes have also been applied to anodic stripping analysis. A mercury ultramicroelectrode electrode was used by Peng and Jin (C61) to determine lead in a sample-limited analysis (e.g., 1 mg of hair). Gold fiber microelectrodes were used to determine mercury in high-purity gallium arsenide (C62) or waters and fertilizers (C63), mushrooms (C64), and selenium(IV) in blood serum (C65). Copper (C66) and lead/ cadmium (C67) were determined in the absence or presence of low concentrations of supporting electrolyte. Kouvanes and Deng (C68) examined the use of an iridium-based mercury ultramicroelectrode with square-wave anodic stripping vol-
tammetry. The effectiveness of various methods to remove interferences in anodic stripping voltammetry was examined. The influence of complexing agents on the effectiveness of electrochemical masking with anionic surfactants was examined by Opydo (C69). The detection of Ga-Zn intermetallic compounds and its removal with antimony was reported by Cofre and Brinck (C70). Surfactants were used to suppress the indium peak in the determination of lead in samples that contained large concentrations of indium (C71). A photochemical process was reported by Barisci and Wallace (C72) for the removal of oxygen in flowing solutions. Photochemistry was also used in sample preparation of heavy metals in peat (C73), while a digestion method for soil samples was reported by Fernando and Plambeck (C74). Systematic errors due to adsorption of metal complexes onto cell components were investigated (C75). Sodium and other impurities in alkoxysilanes were determined by anodic stripping square-wave voltammetry (C76). Metals are generally the most likely candidates for determination by anodic stripping voltammetry, but there are several reports on the determination of organic compounds, either directly or indirectly. Cholesterol in blood serum was found to be amenable to anodic stripping voltammetry (C77), as were ionic alkyllead compounds in natural waters (C78). An indirect method for the analysis of NT A and EDTA in natural water by means of a bismuth complex was reported (C79). The bulk of the reports on cathodic stripping voltammetry has involved the determination of halides and chalcogens. For example, total inorganic iodine in seawater (C80) or a variety of sulfur species such as thiols, sulfides, cysteine, and cystine (C81) were also determined. Wang and Lu (C82) reported on the ultratrace measurement of selenium in the presence of rhodium. A mercury-coated carbon-fiber electrode was used for the determination of selenium(IV) in blood serum (C83). Smyth et al. (C84) reported on the determination of organic and inorganic selenium compounds, while Kotoucek et al. (C£5) determined arsenic. Organic compounds such as cytosine 3'-phosphate (C86), thiamine (C87), pentamidine isethionate (C88), and glutathione in natural waters (C89) were amenable to cathodic stripping voltammetry. The adsorption of “reduced CO2” on platinum was the basis of a new technique for the determination of carbon dioxide (C90). 370R
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Wang and Tian (C91) developed a mercury-free disposal lead sensor based on potentiometric stripping voltammetry using gold-coated screen-printed electrodes. Xie and Huber (C92) used constant-current enhanced potentiometric stripping voltammetry for the analysis of cadmium. Potentiometric stripping voltammetric techniques were also developed for the determination of cadmium and lead in whole blood (C95), copper and lead in tap water (C94), and manganese (C95). Komorsky-Lovric and Branica (C9im at the base. Other in situ STM and AFM imaging of carbon electrodes included the study of Hendricks et al. on lead deposition on HOPG that was previously decorated with monolayer deep pits. Lead deposition and stripping at the rims of the pits can be seen in the published images (£39). Bard’s group also achieved monolayer etching of the basal plane of HOPG using a STM tip under positive bias (£90). Lines and widths as small as
10 nm and squares 25 X 25 nm were
formed. Others have
STM to follow the nucleation and 3-D growth of Pt deposition at HOPG surface defects (E91), the formation of condensed layers of adenine (E92), and the deposition of oxometalates (E93). used
Glassy carbon surfaces have been most intensely studied, doubt due to their favorable characteristics as working electrodes. An important account of the modification of glassy carbon electrodes (E94) also gives a very thorough review of GC as a solid electrode material. no
The McCreery in situ laser activation technique for GC surfaces at power densities below 30 mW/cm2 produced only slight changes in the SERS spectra, the Tt value for phenanthrenequinone, Cdi, and the SEM appearance of the polished surfaces (E95). However at fractured GC, or at fractured GC activated with three 70 mW/cm2 laser pulses, a k° of 0.4 cm/s was measured for the Fe(CN)63~/*~ couple in 1 M KC1. STM images of GC surfaces that had been subjected to several pretreatment procedures showed varying degrees of roughness (E96). However, ET rates as measured by the k° for Fe(CN)63~/4~ did not correlate with the surface roughness. This observation was said to be consistent with the previously widely held view that electrode activity is determined, to a large extent, as a result of active site exposure by means of whatever activation method is employed.
Transient currents, which had components on the millisecond time scale, were produced at GC electrodes by intense laser pulses (9 ns at 1064 nm). They were attributed to perturbation and restoration of the diffuse double layer and adsorbed ions (E97). Zhang and Coury have made the useful observation that sonication of GC electrodes in dioxane leads to increased ET rates for aqueous redox couples (E98). Electrodes treated in this manner are more prone to adsorb redox-active compounds than more conventionally treated electrodes and remain active in aqueous solutions for days. Firouzi et al. imaged GC surfaces using phase detection interferometric microscopy. In NaOH(aq), application of 1.5-2.0 V vs SCE for several seconds generated mesas with heights up to 250 nm and diameters on the order of 30-70 jum (E99). An in situ ellipsometric study of the electrochemical activation of GC indicated formation of a highly porous, hydrated surface layer, which increased monotonically with activation time (El 00). GC electrodes activated in air at 400-800 °C or in steam at 790-980 0 C gave C Vs that exhibited a quinone/hydroquinonelike couple (El 01). An extensive account has appeared concerning the modification of GC, both surface and homogeneous modification, by low-temperature thermolysis of poly (phenylene diacetylene) precursors (E94). Homogeneous incorporation of Pt, for example, into GC produced solid electrodes with electrocatalytic response for the reduction of O2 and H+. These novel materials were prepared by thermolysis of either mixtures of platinum oxide microcrystallites in a carrier polymeric precursor to GC or an organometallic polymer containing covalent Pt(0). TEM of the electrodes indicated a narrow size distribution of Pt clusters in the doped GC, with an average diameterof ca. 1.6 nm (E102). Chlorine- and fluorine-doped GC was synthesized by this method using perhalogenated oligomeric materials (El 03). Electrodes of these materials
exhibited reasonably fast kinetics for the FefCN)^/4- couple, and interestingly, the fluoro-GC had very low double-layer capacities, on the order of 8 juF/cm2. Several more conventional modifications of GC surfaces have been pursued in the past two years. Tateishi et al. found that ultrafine gold particles, 1-12 nm in diameter deposited on GC, produced an active surface for the oxidation of ethanol and acetaldehyde in alkaline solution (E104). Similarly, silver-
modified GC was an efficient substrate for the oxidation of small organics (E105), and GC electrodes modified with the NiO/NiOOH couple worked well for the amperometric detection of aliphatic alcohols (El 06). Kulesza et al. reported that the use of a Pt counter electrode in acidic media can lead to electrodeposition of Pt particles with diameters of 20-40 nm on graphite cathodes (El 07). The electrocatalytic oxidation of As(III) was used as a sensitive indicator for the presence of the Pt particles. Several Russian groups have reported the modification of GC with fluorosulfonic groups (El08, E109) or with CF„ groups (El 10). Prewaves in the CVs of aromatic carbonyl compounds at GC electrodes were attributed to acidic surface functionalities interacting with the C=0 group of the carbonyl compound (El 11, El 12). GC electrode surfaces have been modified with covalently attached groups by the reduction of aromatic diazonium salts (El 13, El 14). For example, attachment of phenyl groups at coverages corresponding to a close-packed monolayer was demonstrated. The surfaces, which could be modified further by chemical reactions, were stable to ultrasonic cleaning and persistent over months.
Several interesting surface electrochemical investigations have been carried out on carbon fiber electrodes in conjunction with their use as UMEs. Pantano and Kuhr performed sophisticated imaging of 10-/mi fiber UMEs by two methods: (i) fluorescence from fluorophores attached to surface carboxyl groups via a linker arm containing a biotin-avidin complex and (ii) luminol ECL generated at ET sites on the surface. Surface heterogeneity was evident at the submicrometer level in the polished and electrochemically treated surfaces (El 15). Kawagoe et al., who analyzed the pH dependence of both quinone reduction and dopamine oxidation at carbon fiber electrodes, concluded that there were mechanistic differences between the reactions on carbon fiber and on conventional electrodes (El 16). Several papers have described the effects of electrochemical pretreatments of carbon fiber electrodes (El 17,El 18). Swain and Kuwana found that Dupont pitch-based carbon fibers, which had been subjected to a sequence of high-current anodizations, underwent a surface-reforming process when followed by a vacuum heat treatment (El 19). The ionexchange properties of oxidized carbon fiber bundles have been exploited in two studies of Jannakondakis and co-workers (E120, El 21). The ion exchange capacity of the fiber bundles was estimated at ca. 1 mequiv/g. Carbon fiber UMEs with cation selectivity were prepared by electropolymerization of
phenolic compounds bearing ion-exchanging carboxylic or sulfonic groups (E122). Anodic oxidation of pyrolytic graphite in alkaline solution did not destroy the surface structure while introducing hydroxyl groups (El 23). In acid, oxidation occurred to depths as great as 40 nm from the edge surface. Analytical Chemistry, Vol. 66, No.
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Several articles with innovative aspects have appeared describing carbon-based working electrodes. DNA-modified glassy carbon, immobilized via covalent bonds between deoxyguanosine residues and surface carboxylate groups, functioned as an electrochemical probe for the complementary oligonucleotide strand (El 24). The electrochemical signal was that of Co(phen)33+/2+ preconcentrated by the doublestrand hybrid structure. McFadden et al. prepared carbon electrodes by pyrolysis of natural gas onto Macor, a machinable ceramic substrate (El 25). ET rates on these surfaces for the Fe(CN)63_/,4_ couple were comparable to those obtained on conventionally polished GC. A highly active electrocatalytic porous carbon surface for 02 reduction was prepared by adsorbing anionic Co complexes into oxidized poly(pyrrole) followed by heat treatment at 820 °C under nitrogen (El 26). A TEMPO-modified graphite felt electrode was used for the
efficient electrocatalytic coupling of methylquinolines (El 27). Wang et al. (E128) evaluated epoxy-composite pellets as voltammetric working electrodes that were fabricated from carbon aerogel foams with high surface area and ultrafine pore sizes (80% at Pt(100) surfaces (£755). For hydrazine oxidation in acid, stable irreversibly adsorbed species were found at Pt(110), while reversible adsorption, without charge transfer, occurred on Pt(100) and Pt(lll) (El59). Nishihara et al. found competitive adsorption with H+ on both terrace and step sites for the oxidation of hydrazine at Pt(332) and Pt(l 11) in H2-
HN02/N0 were
S04(aq)
(El 60).
The adsorption of HS04_ (£767, El62) and phosphate ions (El63) on Pt was sensitive to the surface geometries. The hydrogen region electrode process in NaOH(aq) was found to be strongly dependent on the exposed crystal planes at Pt low-index and stepped surfaces (£764). Adsorbed Pd atoms
Pt(100), -(111), and -(110) exhibited different hydrogen adsorption behavior {El 65), and the growth of Pt oxide films was faster on Pt( 110) than on -(100), -(111) or polycrystalline surfaces {E166). Even the electropolymerization of 3-methylthiophene was dependent on the crystal structure of the Pt anode {El 67). Likewise, the optical properties of Prussian-blue films, electrochemically grown on Pt(l 11) and Au(l 11) substrates, were similar to those on glassy carbon, while on Pt( 110) little film formation was seen {E168). Surface reconstruction phenomena have been found to be structure sensitive as well. Rodes and Clavilier, for example, found that, for stepped-terrace Pt surfaces, the reconstruction phenomena, which could be rationalized in part by a hardsphere model of the surface, were distinctly dependent on the width of the terraces {El 69). Restructuring was indicated for Pt(110) undergoing the hydrogen adsorption process in carbonate and bicarbonate solutions {El 70) and for Pt(100), -(110), and -(111) surfaces in neutral phosphate buffers {E171). However, specific adsorption of anions on Pt(100) in acid did not induce irreversible surface reconstruction {El 7 2). on
Sumino and Shibata have reported the surface transformations of electrodeposited films of Pt on polycrystalline substrates, which can have (100) or (110) orientation, depending on the experimental conditions {E173, E174). Clavilier and Rodes investigated the effect of the quenching temperature on the C V response of Pt (331),-(553),and- (443) surfaces {El75).
Two major groups have addressed difficulties in the evaluation of absolute surface coverage of adsorbed CO on single crystal electrodes {E176, E177). Related to this is the difficulty of determining the Epzc for single crystal electrodes when reconstruction occurs {E178). Orts et al. reported that higher CO coverages were reached in solution than in the gas phase for Pt(l 11) in H2S04(aq) {El 77). Oxidation of CO and CO adlayer formation continue to be popular subjects for study at single crystal electrodes {E179-E181). Minimal COads poisoning was reported for the oxidative dissociation of methanol at Pt(100) in Na2C03(aq) {E182) and for the oxidation of glycolic acid at Pt(lll) and Pt(110) surfaces
{El 8 3). Detailed mechanistic studies on the important methanol oxidation process included a study in CD3OH and CH3OH at Pt single crystal surfaces which reached the conclusion that a C-H bond was broken in the initial step {El 84). This is in contrast to the UHV decomposition where 0-H undergoes
initial scission at Pt. The orientations of nitrogen heterocycles such as substituted pyridines at Pt(lll) surfaces have been deduced by a combination of surface spectroscopies and electrochemistry {El 85, El 86). In several instances, surface layers, which were stable under vacuum, displayed the same electrochemical behavior before and after evacuation. Gomez and Clavilier studied Pt(110) with a view to the role of surface domains and their size on the hydrogen desorption process {El 87). Mixed adlattices of CO and iodine produced immiscible domains on Pt(lll) surfaces {E188). Optical second harmonic generation methods applied to the
Pt( 111)—iodine surface revealed symmetry changes of the monolayer structure {E189). The electrocatalytic role of bismuth adatoms has been addressed by several authors. Chang et al. attributed the 30-40-fold enhancement of formic acid oxidation rates at Pt( 100) in HCIO4 to the attenuation of COadS coverage {El 90). Formic acid oxidation was also catalyzed on Pt( 111), although the major poison was not COads- In contrast, in the presence of predosed Bi, methanol oxidation was diminished on both Pt(l 11) and -(100). Campbell and Parsons also found that the oxidation of methanol was inhibited by submonolayer and monolayer coverages of both Sn and Bi on single crystal, polycrystalline, and dispersed Pt electrodes and that Bi submonolayers on Pt(l 11) enhanced the oxidation of formic acid {E191). Weaver’s group has also studied the influence of Bi adatoms on the oxidation of ethylene glycol at Pt( 111) {El 92). The redox behavior of Bi on Pt(l 11) indicated that the adatom sites were dependent on the extent of surface coverage
{El 93).
Copper deposition onto Pt single crystal electrodes has been the topic of several detailed studies. Reports have noted the dramatic effect of adsorbates on the UPD of Cu on Pt{n,n,n) electrodes. Adsorbates studied include anions such as Cl" and HSO4" {El 94, El 95) and organic molecules such as hydroquinone {E196, E197). Cu, Pb and CO adsorbates on Pt( 111) in acid inhibited hydrogen adsorption and decreased the voltammetric peak presumably due to HSO4" adsorption {El 98). Cadmium submonolayers also effect the adsorption of HSO4" on Pt(lll) {E199). Along this line, Varga et al. reported that Cu deposition on Pt( 111) produced active and inactive adlayers toward bisulfate adsorption {E200). In one of the more detailed studies, Michaelis and Kolb correlated the voltammetric waves in H2S04(aq) with copper deposition initially into every other trough in the Pt(l 10)-(1 X l)surface, followed by complete monolayer coverage in every trough in the second process {E201). In situ STM of Cu UPD on Pt( 111) had previously indicated a two-step process {E202). Leung et al. explained apparent discrepancies between stripping coulometric charge and theory for complete monolayer coverage of Cu on Pt(lll) surfaces by partial charge transfer to the substrate {E203, E204). In the UPD of silver on iodine-covered Pt(lll), the Ag deposited underneath the iodine layer to form a Pt( 111)Agl surface {E205). At a thickness of two monolayers, the adsorption behavior of bisulfate on Ag deposited on Pt( 111) was similar to that on bulk silver {E206). Other studies include the concentration dependence of the UPD of Ag on Pt(l 11) {E207), the epitaxial growth of Pd and Rh monolayers on Pt(l 11) and Pt( 100) {E208), and the adsorption of Ge adatoms on Pt single crystal electrodes {E209). Gold(n,n,n). Reconstruction phenomena have been prominent in studies using gold single crystal electrodes. Several groups have investigated the anion-induced transformations of (5 X 20) to (1 X 1) structures for Au( 100) electrodes (E210, E211). In a STM study, Gao and Weaver found that, in the presence of iodide, the conversion of the square planar (1 X 1) surface to a hexagonal reconstructed phase was remarkably rapid ( Ag(100) > > chemically polished Ag > mechanically polished polycrystalline Ag in KI solution (E257). STM of Pb UPD on Ag(100) and Ag(lll) electrodes revealed formation of
Ag(l 10)
well-ordered monolayers (E258). Electrochemical reordering of a disordered palladium oxide surface was demonstrated by McBride et al. (E259). They found that treatment of a disordered oxidized surface with dilute Nal solution at a potential where oxide was reduced resulted in the appearance of the LEED pattern of a Pd(100) surface. The same laboratory reported reorganization of surface bonding structures upon oxidation of CO adsorbed on Pd( 111) (E260) and the dissolution of Pd in a layer-by-layer process without loss of an iodine monolayer on a Pd( 111 )— iodine surface (E261). Two-stage adlattice formation was suggested for the UPD deposition of Cu on Pd(100) (E262), and thin Pd overlayers electrodeposited on Au(lll) and Au( 100) electrodes were shown to behave in a manner similar to Pd(lll) and Pd(100), respectively (E263). Differences were found in the measurement of CO surface coverage values on Rh(100) electrodes by coulometric and FT-IR spectroscopic techniques (E264). Adsorption of bisulfate on Rh( 111) featured adsorption plateaus over a wide potential range, in contrast to the behavior at polycrystalline Rh electrodes (E265). Perchlorate anions were reported to decompose on polycrystalline Rh and Ir( 111) electrodes to
produce a surface species, probably adsorbed chloride, that inhibited hydrogen adsorption (E266). Only negligible electrocatalysis was seen for the oxidation of methane when surface oxides and/or silver were deposited on Ru(001) electrodes (E267). Wang et al. have described a neat procedure for the preparation of Ni(l 11) surfaces for electrochemical study. The surfaces, which were formed under UH V conditions, were protected with a layer of adsorbed CO prior to transfer to solution and electrochemical stripping of the CO (E268). The UPD of T1 and Pb on Cu(l 11) films evaporated on mica surfaces was reported (E269), and an
AFM study of potential-controlled oxygen adsorption on Cu(100) has appeared (E270). STM images were obtained of the siliconfl 11) hydride phase that was revealed when an oxide layer was removed under potential control in HF(aq) solution (E271). Surface Imaging Techniques. As is evident in the previous section, surface electrochemists have been applying the various surface microscopies to the study of electrode interfaces since the initial introduction of these techniques. The student wishing a more complete compilation of references on this topic should scan citations from the above sections on UMEs and single crystal electrodes. The articles mentioned here are perhaps more technique oriented, although the distinctions are often arbitrary. Vogel et al. published beautiful STM images of Pt single crystal electrodes subjected to the “iodine procedure” both in air and in electrolyte solutions (E272). The images, which were in accord with previous LEED ex situ results, were obtained using a noncommercial STM apparatus, details of which were given. A prospectus for STM/electrochemistry has been published that contains many examples and impressive STM images (E273). Schmickler and Widrig presented some theoretical considerations of the STM/Echem experiment (E274). The Poisson-Boltzmann equation was solved for a sphere-plane configuration as a model for the tipsubstrate geometry (E275). Techniques for STM tip sharpening and related applications were extensively reviewed (E276). A combination of normal sharpening procedure under ac voltage with the tip oriented downward, followed by further sharpening with the tip oriented upward, was found to be effective for tungsten tips. Oxide layers on tungsten tips are easily removed in concentrated HF (£277). Details were given for the preparation of STM tips with reduced capacitive currents for use in a differential conductance mode of operation (E278) and for electrocoating STM tips with polyacrylic carboxylic acid (E279). Surface microscopes have been used in novel ways to characterize and/or to spatially modify film electrode interfaces. For example, Murray’s group has reported a procedure to produce spatially patterned, laterally heterogeneous polymermodified electrodes using in situ AFM (E280). They used an AFM tip to “nanodose” a defect in a thin film of insulating poly(phenylene oxide) (PEO). The defects were then filled by electropolymerized conducting polymers. Yang et al. proposed a STM technique to measure the thickness of polymer films on conducting substrates. The plot of tip current vs tip displacement exhibited linear regions due to (i) approach to the surface, (ii) penetration through the film, and (iii) contact with the substrate (E281). In situ AFM was successfully
used to follow the electrochemical formation of PEO (E282),
while STM images of poly(iV-methylpyrrole) were noisy, probably because the STM tip typically was buried in the poorly conducting polymer film (E283). Sugimoto et al. used the Bard SECM technique in a directscanning mode to obtain images of a Prussian blue film electrode that showed cracks and grain boundaries at the submicrometer level (E284). Engstrom et al. employed SECM to map the local electron-transfer kinetics of reactions occurring at kinetically heterogeneous Pt disks or epoxy impregnated RVC electrodes (E285). Several other applications of the SECM technique have been cited above in the section on UMEs. The improvement in the technique involving small-amplitude modulation of the tip position seems especially important (B75, E286). This permits automatic determination of whether a surface is insulating or conducting. Among the many applications of STM to electrochemistry reports of STM images of dissolving electrodes (E287, E288), of metal particles deposited in the pores of anodic aluminum oxide films (E289), and of electrochemically grown organic semiconductors (E290). In situ STM/Echem are several
of silver electrodes revealed time-dependent smoothing of the surface during redox cycles (E291), and the fractal dimensionality of Au and Pt electrodeposits was determined (E292). STM images of DNA molecules have been obtained on
Au(lll)
surfaces under potential control (E293). This particular application, and related methodology (E294), promises to become popular in molecular biology fields. Fluorescence imaging of electrode surfaces was achieved by generation of OH- in weakly buffered solutions of fluorescein. Thus, reduction of H2O or O2, the latter at cathodic corrosion sites, converted the dye into a strong fluorescing species and produced images of the electrode surface (E295). Miller et al. imaged L-B monolayer films of a Ru-bpy surfactant by observing ECL with a sensitive CCD camera (E296). Finally, local ac impedances were obtained by measuring the potential difference between two microelectrodes in a probe assembly (E297). Polycrystalline Electrodes. An interesting comparison of chronocoulometry, radiochemistry, and Raman spectroscopy applied to the measurement of pyridine adsorption on gold electrodes has appeared (E298). Agreement was found between surface concentrations determined by the first two techniques, but chronocoulometry, where double-layer, not faradaic, charge densities were measured, gave the better precision. Adsorption of HSO4-, Cl-, and I- on Pt was measured by three in situ methods: radiotracers, FT-IR, and ellipsometry (E299). Bell-shaped adsorption isotherms were reported for 20 organic compounds on Pt (E300), and the £pzc values and capacitance minima for Au were measured in NaF(aq) using a piezoelectric technique (E301). An extensive set of double-layer data at various metal electrodes in DMSO, DMF, PC, AN, MeOH, and H2O was collected and used to analyze metal/solvent interactions and the interfacial solvent structure (E302). Among the papers on the oxidation of methanol at solid electrodes are a study at mixed oxides of Pt and Sn (E303), a study of the effect of Ru deposition where RuOHadS intermediates were proposed (E304), a study of the effect of adsorbed Sn atoms (E305), a detailed examination of the
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a report of the use of electrodes coated with fine particles of hydrophobic nickel FT-IR In situ spectra revealed tetrafluoroethylene (E307). in the at several intermediates Pt, including three process a CH-containing a COH and forms of adsorbed CO, species, Raman spectra also have species (E308). Surface-enhanced been obtained for this system (E309).
process at
Pt-Ru alloys (E306), and
The surface electrochemical behavior of three isomeric pyridyl hydroquinones at polycrystalline Pt (and at Pt(l 11)) was dependent on the orientation of the adsorbed monolayers (E310). Oxidation of ethylene glycol gave different product distributions on Au, Pt, and Ni electrodes (E311). FT-IR spectra indicate C2 solution intermediates in route to oxalate and carbonate ions on gold electrodes, while formate was formed to the greatest extent on Ni. The electrocatalytic oxidation of toluene was seen at coatings of deposited hydrated platinum oxides on Pt, Ni, Ti, Fe, and glassy carbon supports (E312). Two pathways were described for the oxidation of phenol at Pt: one occurring at the inner Helmholtz plane, where ring cleavage took place, and one occurring at the outer layer, where a mixture of products was formed (E3I3). Squaric acid oxidation on Pt gave extensive formation of COads and CO2 products over a wide potential range (E314). Oxidation of surface mercaptoethanol films at Au proceeded by multiple pathways (E315). Evidence was presented to indicate that the first monolayer of adsorbed thionine is electroinactive at Pt. On sulfurmodified Pt, however, the first layer of adsorbed material is electroactive (E316). In similar fashion, sulfur adlayers on Pt changed the irreversible phenothiazine oxidation into reversible CV behavior (E317).
RDE and QCM measurement gave new insights on the well-studied iodine/iodide system at Pt in H2S04(aq) (E318). Michelhaugh et al. found that even submonolayer coverages of adsorbed iodine gave fast kinetics for the quinone/ hydroquinone couple, which they took to indicated selective ET at iodine surface sites (E319). Anodic O-atom transfer electrode reactions were proposed for the oxidation of I~ to 103“ at Pt, Au, Pd, Ir, and glassy carbon (57/7), for anodic reactions at PbC>2 electrode doped with acetate (E320), for the oxidation of oxysulfur anions at Pb02 (E321), and for the determination of As(III) at Pt where a key role was assigned to PtOH (E322). Anodic CI2 evolution at Pt was reported to take place on an oxide-free surface in anhydrous trifluoroacetic acid (E323). Oscillating phenomena continue to stimulate electrochemists, who have usually fingered COadS as a key intermediate in their mechanistic speculations (E324-E327). Wolf et al. modeled the oscillating electrochemical reduction of peroxodisulfate by a system of nonlinear differential equations based on a Nernst diffusion layer treatment for a diffusion current term and a Butler-Volmer expression with a Frumkin correction for the charge-transfer term (E328). Good agreement between theory and experiment was obtained. The potential oscillations seen in the galvanostatic oxidation of formic acid on Pt were directly coupled to frequency oscillations in the EQCM experiment (E329). Finally, a true ac battery was based on an ingenious concentration cell that consisted of two mass-coupled oscillating half-cells. Typical specifica382R
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tions
were
period, 58
s;
current, ±2.5 /aA; and emf, ±50 mV
(E330). The formation and growth of metal oxide films have been studied by a variety of methods including voltammetry, ac impedance, and EQCM. These include investigations of Pt electrodes (E331-E336), gold electrodes (E337-E340), and Pd electrodes (E341, E336). EQCM data indicated that the gold dissolution rate upon E cycling in H2S04(aq) was 550
ng/h (E337). In the miscellaneous category, the relationship of area to volume of dendritic Ag deposits on polycrystalline Pt was found to exhibit fractal behavior with area = k(vol)fl/3, where D = 2.50 ± 0.03 (E342). This value is consistent with a self-similar fractal surface. Finally, a description of a guillotine electrode, which was tested on Al electrodes in aqueous solution, was noted (E343). Miscellaneous Electrodes. Several reports have appeared
that featured superconducting working electrodes. Plots of Cdi vs T for two Tl-based high-To superconductors immersed in fluid electrolyte solutions displayed abrupt changes in the region of Tc (E344). An increase in faradaic current was observed for high-7^ superconducting electrodes in contact with Ag+ ion conductors at low temperatures (E345). A role was suggested for Cooper pairs crossing the double layer and participating in the electrode reaction. A quasi-reversible, almost irreversible, CV for the ferrocene+7° couple was obtained at a Bi-Pb-2223 superconducting UME at 102 K (E346). Kuznetsov developed theory explaining the increase in current in the Tc region for superconducting electrodes {E347). Conditions were given for the anodic electrosynthesis of millimeter-sized crystals of Bao.6Ko.4Bi03 with Tc values of 30.5 K (E348). Superconducting thin films of Y-Ba-Cu-0
Tl-Ba-Ca-Cu-0
were electrodeposited at negative in DMSO solution Electrochemical Li(E349). potentials in an increase of films resulted Tc doping high- superconducting in a lattice constant and/or the Tc value (E350). Electropolymerization of aniline on the surface of YBa2Cu307_j
and
film with protective properties (E351). More interestingly, redox cycling of poly(pyrrole) coated on thin superconducting films reversibly changed the Tc value by almost 15 K (E352). The Cu(III/II/I) system was examined at the latter surface (E353), and Ma et al. successfully produced
a
electrodeposited Cu contacts onto
a
superconducting substrate
(E354). Several interesting working electrode materials containing titanium have been studied in the last two years. Titanium diboride, an electroconductive ceramic, exhibited a wide potential window and was used for the reduction of CO2 (E355). Ebonex, a conducting ceramic mainly composed of the Magneli phase of titanium oxides Ti4C>7 and TisO^ coated with PbC>2, was found to be a suitable anode for ozone generation (E356). A previous report from Pletcher’s group had described conducting titanium oxide ceramic electrodes (E357). Polycrystalline thin films of cubic BaTiC>3 were prepared on Ti metal substrates by several methods, including an electrochemical anodization in Ba(OH)2 solution (E358).
F.
MODIFIED ELECTRODES
Charge Transport in Polymer Films. Several important papers on this topic have appeared in the last two years. Attention is also called to the review of Inzelt, who has surveyed theory and experiment up to ca. 1992 (A40). Fritsch-Faules and Faulkner simulated lateral charge transport in a thin film of redox centers electrostatically held in a polymer matrix (FI). Their model allowed for partitioning between the film and solution, which opened two diffusional paths for the ions in the charge transport process. In a nice experimental study, they determined the concentration profiles in methylquaternized poly(vinylpyridine) (PVP) films containing the Fe(CN)63~/*~ couple by means of potentiometric measurement at arrays of 4-^m-wide Au electrodes in contact with the film (F2). The concentration profiles under steady-state current flow between flanking electrodes were linear. The calibration curve relating potential to concentration was established by chronocoulometry in a companion paper (F3). The behavior was found to be Nernstian in spite of (i) different extent of partitioning of ferri- and ferrocyanide, (ii) oxidation-statedependent mass transport, and (iii) nonideal CV behavior. The charge transport was dominated by diffusion of the redox species through solution since the diffusion coefficients were 2-3 orders of magnitude greater in solution than in the film. The experiments of Larsson et al. (F4) on PVP films containing Fe(III/II) redox sites either directly bound to pyridine groups on the polymer or electrostatically bound to quaternary pyridinium sites relate to this model. In the former situation, the apparent charge-transfer diffusion constants (Dct) were almost 100 times smaller than in the more typical ion-exchange
polymer. The dynamics of electron hopping in assemblies of redox centers has been treated in a major contribution that is pertinent to charge transport in fixed-site redox polymers (F5). The authors found that when physical motion of the redox centers was either nonexistent or much slower than electron hopping, charge propagation was fundamentally a percolation process, in which electron hops occur between a random distribution of redox center clusters. In another paper, Blauch and Saveant modeled the charge transport by random walk of electrons through redox molecules in square and cubic lattices (F6). Below a critical concentration, finite cluster size makes charge transport impossible. Further, in their treatment, the mean-field physical diffusion model of Dahms and Ruff was shown to be inapplicable to systems in which the contribution of physical diffusion to charge transport is small compared to that of electron hopping. Rapid bounded diffusion in systems where the redox centers are irreversibly attached to the supramolecular structure, on the other hand, gives rise to mean-field behavior when it exceeds the rate of electron hopping.
In another approach, Mohan and Sangaranarayanan incorporated an exponential dependence of electron hopping rates on distance into a generalized diffusion/migration equation for redox film charge transport (F7). Also Deiss et al. published a quite general digital simulation of redox polymer CVs (F8). Their calculation accounted for mass transport by diffusion and migration, electron hopping by a Saveant mechanism, homogeneous reactions in the film, heterogeneous reactions and Cdi at the membrane/electrode interface, and
Donnan partitioning at the membrane/diffusion layer interface.
Impedance techniques have been refined for the analysis of charge transport in polymer film electrodes and successfully applied, notably by Pickup and Albery and their respective co-workers. Ren and Pickup have published several studies where they used a transmission line equivalent circuit to analyze charge transport in ion exchange polymers based on
poly (pyrrole) (PPy) (F9-F11). They employed a porous electrode model and generally found that ion mobility limited the charge transport. A good example is their study of polymer films of 3-methylpyrrole-4-carboxylic acid, where ion mobility was 103 faster than electronic conductivity (F9). The results were interpreted using a two-phase model in which ion transport was due to counterions in the polymer phase and excess electrolyte in the pores. Fletcher also used a porous electrode model to interpret impedance data for conducting polymer electrodes (FI 2, FI 3). In a similar fashion the transmission line model of Albery and Mount was based on a porous electrode with organic polymer and aqueous pore phases (FI 4). Resistances were obtained due to bimolecular electron exchange and anion buildup in the film. In another treatment of polymer film ac impedance, they proposed a transmission line model in which there were separate resistive rails for the cation and for the anion (FI 5). This would apply to the common situation when electron motion along the polymer backbone is faster than ion conduction in the pores. Mathias and Haas have developed theory for ac impedance of redox polymer films under conditions where either electron hopping or ion migration is slow relative to the other (FI 6). They assumed Donnan exclusion permitting only one mobile ion in the film. These authors have studied PVP films containing Os(bpy)2 centers under conditions of four bathing electrolytes where the charge transport was via electron and anion motion only (FI 7). (This paper gives a nice summary of the procedures for extracting parameters from raw impedance data.) In contrast to the above situations, ion transport in these films was found to be much faster than electron hopping, even for large anions such as toluenesulfonate. The impedance response of poly(pyrrole) bilayers, with perchlorate and poly(styrenesulfonate) counterions, indicated that the redox reaction was outside-inside, i.e., began at the polymer/solution interface (FI 8). This was consistent with a porous electrode model and with the redox polymer model of Albery et al. (FI 9). On the other hand, a chronopotentiometric study concluded that an inside-outside mechanism was operative for lightly doped PPy (F20). Sharp et al. gave a clear account of the interpretation of impedance data for Nafion film electrodes containing Os(bpy)33+/2+ and substituted ferrocene couples (F21). The dependence of the redox conductivity on the overall oxidation state of the film, in their view, was in agreement with an ion-pairing model in which electron hopping took place between, for example, a neutral Os(II) site and a positive Os(III)+ site. Forster and Vos have reported correlations between Dct and the heterogeneous electron-exchange rate(k°) for two redox polymers containing Os(III/II)-bpy sites (F22, F23). The effect of the extent of loading of Os(bpy)33+/2+ in Nafion was significant when the amount of
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Os was greater than one-third of the available anionic groups necessary for charge neutrality in the oxidized state. In this case, the CVs exhibited two waves: a reversible surface wave
Os(III/II) couple and an irreversible wave at greater potentials that involved ejection of the complex from the film (F24). The ejection was elegantly verified by SECM using a UME probe positioned above the film. for the
Mao and Pickup used RDE voltammetry of ferrocene to potential profile across a substituted poly(pyrrole) film. The gradient was nonlinear, which they took to indicate nonmetallic conductivity where the charge transport process is driven by a concentration gradient of oxidized sites in the polymer matrix (F25). Aoki and Heller measured apparent electron diffusion coefficients in a cross-linked redox polymer that contained Os(bpy)2 redox sites. In their interpretation of the data, they invoked hydration effects that were induced by counterions, ionic strength changes, or protonation of basic groups on the polymer backbone (F26). Water transport was noted in EQCM studies of PVP films containing Os(III/II)—bpy centers (F27) and poly(vinylferrocene) films (F28, F29). Slow structural changes for related polymer films were seen when the electrodes were transferred between perchloric and toluenesulfonic acid solutions (F30). For poly(l-hydroxyphenazine) films, the typical featureless CV was shown to involve two ion-exchange coupled steps: one with proton transport and the other with anion uptake and solvent loss (F31). Hydration effects were also noted for the solid-state charge transport in hexacyanoferrate films with fixed Fe(III/II) sites (F32). An equivalent circuit proposed for the interpretation of impedance data at redox polymer electrodes contained two capacity terms: one for the substrate/polymer interface and one for the polymer/solution interface (F33). Also the effect of surface roughness of the substrate on the impedance of polymer films has been considered (F34). The combination of ac impedance spectroscopy and “electromodulated optical transmittance spectroscopic impedance” was used by Amemiya et al. to study charge transport in polymer film electrodes (.F35, F36). Hillman and Bruckenstein have pointed out the important role of slow solvent transport in several studies of the redox kinetics of permselective polymer films. Electron transfer, solvent uptake, and polymer reconfiguration in a cube scheme were incorporated into a general model (F37). Often observed measure the
as “break-in” processes, charge and mass structural changes with redox cycling, and variation trapping, of charge transport rate and E°' values with time were encompassed by their theory. For a polythionine redox film, the kinetics were described in terms of a scheme of squares involving electron, proton, and solvent transfer (F38). The EQCM data showed that the coupled motion of electrons and protons preceded the rate-limiting solvent transfer in both anodic and cathodic steps. One of the later papers in the general Bruckenstein and Hillman treatment of the EQCM experiment has appeared in the last two years (F39). Proton transfer was also shown to be involved in the charge transport process operative in thin ubiquinone-Qio films (F40). A two-phase model was employed to explain the surprisingly large electrochemical diffusion coefficients in polyacrylate
phenomena such
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gels {F41). In spite of high gel viscosities, high diffusivity in the continuous aqueous phase was invoked to explain the data.
Electrocatalysis at Modified Electrodes. This subject, a raison d’etre of modified electrode research, has seen relatively little theoretical activity in the last two years. The general treatment of electrocatalysis at polymer-modified electrodes containing microparticles stands out however (F42). Equations were derived for the flux as a function of the number of particles per unit volume, the film thickness, the substrate and mediator concentrations, and the particle radii. Eight cases were described, along with the respective flux equations, that differed in the reaction orders with respect to the above experimental variables. In a second paper on metal oxide/ Nafion composite amperometric sensors, the kinetics were cast in the context of the Michaelis-Menten formalism (F43). Anson and Xie have addressed several important aspects of data analysis for the estimation of rates of cross-reactions that occur during electrochemical catalysis at polymermodified electrodes using the Koutecky-Levich equation (F44, F45). In a later paper, a modified kinetic model, which assumed an array of film mediator couples with a Gaussian distribution of E°' values, was developed (F46). When the parameters of the distribution were selected to fit the i-E curve for the non-Nernstian surface wave of the mediator couple, significant improvement in the agreement between
experimental and calculated currents was obtained for several involving redox couples in Nafion coatings. Numerous articles continue to be published on various polymer-modified electrodes designed to be catalytic for specific processes. The ones cited here will be organized in terms of the electrode reaction catalyzed and not by the nature of the polymer matrix. Electrocatalysis of O2 reduction was achieved at a porphyrin ligand coordinated by four Ru(NH3)5 groups in Nafion (F47), and by metal phthalocyanines in various matrices (F48-F50). Cobalt(II) complexes in Nafion (F51) and Prussian Blue/poly(aniline) (F52) films catalyzed the reduction of C02. Electrocatalytic reduction of nitrite took place at quite positive potentials at PVP films containing Os-bpy complexes (F53, F54); a mixture of N2O, N2, NH2OH, and NH3 was obtained at thin polymeric films of an iron(III) protoporphyrin (F55). Oxyanions such as chlorate and bromate were reduced at conducting polymer electrodes doped with molybdate species (F56,F57). Poly(pyrrole) films were robust enough to mediate the reduction of dichromate in acid (F58). Electrocatalytic films for the reduction of the disulfide bond in cystine (F59) and for the Cu(II/I)-mediated reduction of cytochrome c and tyrosinase (F60) have been described. Substituted poly(pyrrole) films with Pd(II) and Rh(III)—bpy centers were used, respectively, for the hydrogenation of organic compounds (F61) and the catalysis of hydrogen evolution (F62). Catalytic activity was imparted to insoluble liquid crystal films of a cationic surfactant on graphite electrodes by vitamin B12 hexacarboxylate (F63). On the oxidation side, several different polymer film electrodes have been used for the electrocatalytic oxidation of NADH (F64-F66). Pyrrole-substituted Mn tetraphenylporphryrins were precursors to catalytic polymer films for the epoxidation of alkenes and the oxidation of thioacetamide (F67). Ru(V/IV)-oxygen complexes in Nafion and poly(pyrrole) films mediated the oxidation of alcohols (F68, F69). cases
Electropolymerization of several free-base and metalated porphyrins produced conductive redox polymers with electrocatalytic activity for a variety of reactions (F70, F71), and a porous TiC>2 ceramic coated with Nafion containing R.UO2/ Ir02 catalyst was an efficient electrode for oxygen evolution (.F72).
Polymer film matrices have been employed in enzyme electrodes since the initial work on things such as the urease electrode of Updike and Hicks. The use of redox polymers and conducting polymers in conjunction with mediating species continues to be an active research area. Many different variations, and some not so different, have been published in the last two years on this topic, especially with glucose oxidase as the enzyme system. Only a few of these will be mentioned here. Ye et al. described a glucose electrode, which was made with Heller’s epoxy redox polymer and a quinoprotein glucose dehydrogenase, that exhibited exceptionally high current densities, 1.8 mA/cm2 (F73). An improved glucose sensor was fabricated via the substituted pyrrole route using glucose oxidase that had been covalently modified with pyrrole (F74). Your reviewer also liked the description of a glucose sensor
“switch” that was based
on a
poly(l,2-diaminobenzene) film
containing the enzyme, which was polymerized on top of a poly(aniline) electrode (F75). Another crafty approach was that of Anzai et al. who electrodeposited avidin on Pt and then complexed the surface with biotinylated glucose oxidase (F76). The mediated redox enzyme concept has been applied to enzyme systems other than glucose oxidase to develop sensors for other species including amino acids (F77), fructose (F78), lactate (F79), NADH (F80), and others (F81). Papers also continue to appear on electrocatalytic applications of surface-modified electrodes without a (often resistive) polymer film component. Some of these have been cited under Carbon Electrodes. Shi and Anson have studied their cobalt porphyrin substituted with Ru(NH3)s groups via pyridyl ligands when it is adsorbed on graphite (F82). The currents for oxygen reduction were greater at these surfaces than at Nafion film surfaces containing the same complexes, but the stability was not as good. In another study it was found that the number of Ru(NH3)5 groups on the complex determined whether a two-electron or a four-electron pathway was followed (F83), with the trisubstituted pomplex giving the latter behavior. For protoporphyrin IX-modified glassy carbon electrodes a two-step reduction of O2 was observed for pH >12, and a four-electron reduction for pH 2, film (FI 90, FI 91). A poly(aniline) image of a human subject is shown in the publications from Yoneyama’s laboratory. Illumination of the film was performed in pH 7 phosphate buffer containing methanol as a hole scavenger under conditions of low PAn conductivity to prevent image spreading. The yellow images of reduced PAn were easily erased by polarizing the films at 0.5 V vs SCE. Images in metal-bpy polymer films were also produced by a combination of photochemistry and electrochemistry (FI 92). In this case, a spatially controlled image of the original mask was produced. Poly(aniline)-methylene blue-Nafion composite film electrodes, which had been further modified with Ru(bpy)32+, developed images upon illumination (FI 93). In another approach, procedures were given for the photoproduction of very small, patterned tungsten nuclei in a microlithographic resist consisting of phosphotungstic acid and poly (vinyl alcohol). The nuclei served as nucleation sites for the subsequent electroless deposition of Ni. Features with dimensions on the order of 0.3 jtm were formed (FI 94). Another method for creating microstructure and patterned images in polymer films involved photochemical ligand loss from metal redox polymer films containing tetramethyl-bpy complexes of ruthenium. The photochemical reaction created “voids” that could be filled via reaction with osmium phosphine complexes (F195). Brumfield et al. also created patterns in polymer films by deposition of conducting polymers into defects that had been formed in insulating films with an AFM tip
(E280).
A novel display device was based on the pH change, induced by the oxidation of a PPy film in NaCl(aq), that caused a reversible flocculation of a poly(L-lysine) microgel (FI 96). Analytical Chemistry, Vol. 66, No.
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Instantaneous detection of light intensity changes and image production were achieved with a 64-element array contacting a bacteriorhodopsin film on a SnC>2-transparent electrode {FI 97). SEM was used to detect images formed in irradiated self-assembled monolayers containing photoactive aryl azide groups {FI 98). Micropatterning of PPy on insulating surfaces was performed by growing the conducting polymer film across surfaces that were hydrophobic {F199-F201). A maskless local deposition procedure was based on the electrodeposition
of poly(aniline) at laser-irradiated sites {F202). Papers describing modified electrodes with good electrochromic properties continue to appear. Recent examples include L-B films of several rare earth bis(phthalocyanine)s (F203), Cu and Ni phthalocyanine films {F204), poly(viologen) films {F205), poly(vinyloligothiophene) {F206), poly(quinolinium) salts {F207), and poly(aniline)/W03 composite films {F208, F209). Related to this application is the report of a procedure for growing a poly(thiophene) film that was transparent in the visible region for both the oxidized and reduced states {F210). Siekierski et al. also described a poly(thiophene) that was highly transparent in the conductive state {F211). The electrochromic specifications of a PPy/ Prussian Blue/K^SO^aq), poly(benzylviologen) system were especially impressive {F212). Light emission from conjugated polymers of the polyfpphenylenevinylene) type, which has been reported by several laboratories {F213-F215), represents an intriguing approach to large area displays. The flexible LEDs of Gustafsson et al. were fabricated from free-standing films of polyfethylene terephthalate) as a substrate, PAn as the hole injecting contact, an electroluminescent layer, and a calcium metal negative electrode {F215). Greenham et al. achieved quite high efficiencies with poly(cyanoterephthalylidiene) LEDs—up to 4% photon out per electron injected {F216). For the sandwich cell, Al/poly(thiophene)/ITO, light emission was seen at applied voltages of 10 V {F217). Photoluminescence intensity and wavelength from poly(3-hexylthiophene) was dependent on the regioregularity of the polymer chain {F218).
Conducting Polymer Electrodes. Recent work on electrochemical aspects of electronically conducting polymers, e.g., poly(pyrrole), poly(aniline), poly(thiophene) (PT), etc., will be reviewed here. The equally vast literature on the solidstate physics of these and related systems will not be covered. Synthetic Aspects. Radical cation coupling reactions have been suggested as the carbon-carbon bond formation steps for several pyrroles {F219) and for 3-MePT {F220) based on double-potential step chronoamperometric and spectroelectrochemical data, respectively. RRDE and EQCM techniques have also been employed to good effect to distinguish between coupling mechanisms and to identify intermediates in the electrosynthesis of conducting polymers {F221, F222). Electropolymerization of pyrrole in the presence of a A-methylphenothiazine mediator proceeded via a catalytic scheme {F223). STM images and in situ video recording of the nucleation and growth of 3-MePT were reported {F224, F225). The electropolymerization and deposition process of PT-3-acetic acid was concluded to proceed via a two-dimensional layerby-layer nucleation mechanism {F226, F227). The fast CV study of Vuki et al. {B63) indicated that, at room temperature, 388R
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the growth of PAn nucleation centers, and not slow electron or counterion motion, limited the current. At low temperature, ion transport controlled the current at long times. Perhaps related to this is the observation of spatial variation of the PAn conductivity, on the order of 20-30 nm, corresponding to granular metallic regions {F228). Several papers have addressed the question of the molecular
weights of electrosynthesized polymers. Wei and Tian found that the applied potential influenced the molecular weight of electrosynthesized 3-alkyl-PT {F229). The molecular weight exhibited a maximum of greater than 50K at an intermediate potential (1.6 V vs SCE) and was lower in the presence of additives such as 2,2'-bithiophene. The molecular weight of electrosynthesized PAn, which was less cross-linked than the chemically synthesized material, was estimated to be greater than 50K {F230). Also low-temperature (0 °C) was found to give increased doping levels and conjugation lengths for PPy electrosynthesis {F231). Photocurrent spectroscopy at ITO electrodes provided evidence that the first traces of electrosynthesized 3-MePT had long conjugation lengths
{F232). Two reports have described the effect of sweep rate on the morphology of PAn films: fast sweep rates (20 V/s) gave the more uniform dense films (F233, F234). Ellipsometry and EQCM data indicated that a periodic cathodic bias during anodic growth of PAn resulted in morphological changes characteristic of increased long-range order {F235). pPhenylenediamine increased the rate of PAn electropolymerization and altered the morphology of the resulting film {F236). Improved yields were obtained for the electropolymerization of thiophene in the presence of ultrasonic waves {F237). Electropolymerization of pyrrole from aqueous carbonate solutions produced pinhole-free insulating films, ca. 100-300 nm thick {F238). Two groups have studied the overoxidation and degradation of PPy {F239, F240), and the deactivation of 3-MePT in the presence of Cl- was partially restored by oxidation in CH3CN {F241). In the latter case, the reactivated film was chlorinated. Conditions were given for the electrosynthesis of brominated PPy {F242), and PPy films have been grown in the presence of zwitterionic buffers {F243). 3-MePT conducting fibers as long as 10 cm were grown in a capillary flow cell where the fluid flow pattern, in part, governed the shape and diameter of the fiber {F244). Especially interesting was the fabrication of flexible, conductive composite fibers by electrolysis along the surface of polyester and Kelvar strings attached to the electrodes {F245). PAn has been grown on nylon and glass cloth via chemical oxidation of aniline-permeated substrates {F246), and PPy electrosynthesized in a nematic liquid crystal medium exhibited only slightly anisotropic conductivity and no evidence of crystallinity {F247). Several papers have described the synthesis of PPy either on Ta or within the pores of sintered Ta electrodes (.F248-F250). A wide variety of different conducting polymer films have been electrosynthesized. Recent interesting examples include highly anisotropically conducting films produced by the electroreduction of a soluble naphthalenedicarboximide in the presence of a polycation {F251), films of electropolymerized thiophene, bithiophene, and terthiophene in the presence of Keggin heteropolyanions {F252), poly(o-anisidine) (F253),
and poly(o-toluidine) (F254). Let it be noted that 1992 was a good year for the electrosynthesis of poly(paraphenylene)
(.F255-F262). Several studies have appeared on the electrosynthesis of buckey ball films (F263-F265). Jehoulet et al. found that films fully reduced to a C62 particles have been given (F298, F299). Two interesting applications of conducting polymers as membranes stand out. Martin and co-workers reported that oxidized PPy membranes, supported in the pores of a Analytical Chemistry, Vol. 66, No.
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polycarbonate membrane, could transport electrons across the membrane between a donor/acceptor pair, while simultaneously transporting ions to maintain charge neutrality (F300). More interestingly, they also demonstrated that glucose oxidase could exchange electrons with the membrane and drive the oxidation of glucose in a transmembrane manner. Nafion/PAn and free-standing PAn membranes have been used as porous gas diffusion membrane electrodes for the oxidation of SO2 and N2H4, and for the reduction of NO2 and
02 {F301). The volume change that takes place when polymer films are redox cycled is the basis for possible servomechanical devices, in which a conducting polymer film is firmly attached to an electroinactive substrate, e.g., commercial adhesive tape in one example (F302). The curvature or bending of such strips has been studied by several groups (F303-F305). The dimensional changes, driven by proton or redox doping, were largely reversible for PAn films; values of percent elongation up to ca. 10% were reported for oriented films in the perpendicular direction to the draw axis (F305). A velocity of 5 X 10~5 m/s was reported for the propagation of the oxidized zone on a PAn strip (F306). Conducting polymer diodes and transistors are other curiosities that continue to attract attention. McCoy and
Wrighton configured two conducting polymer “gates” as a push-pull amplifier such that there was no crossover distortion when the output current went through zero (F307). Fox discussed ways to control directional charge transport in electroactive polymer arrays, including some voltammetry on PT/PPy and PPy/PT bilayers (F308). Buck et al. have discussed the analogies and differences between liquid/solid polyelectrolyte diodes and conventional solid-state semiconductor diodes (F309). They gave conditions for the chemical mimicking of p-n junctions involving ion-exchange and redox polymers. The experimental study of Han et al. on Nafion bilayers supported their ideas (F310). These workers reported diode behavior for a Nafion bilayer prepared by sandwiching two films together at 150 °C in a mechanical press—one film was loaded with Fe(phen)32+, and the other with Fe(phen)33+/2+. Others have reported diode-like behavior for junctions such as poly(bithiophene)/Si and PPy/poly(p-phenylenevinylene) (F311). Chemically deposited PPy has been used as a precoat for the metalization of printed circuit boards (F312). Electrodeposited PPy on carbon fibers improved the bonding between the fibers and an epoxy resin matrix (F313). Self-Assembled Monolayers. The surface chemistry of self-assembled monolayers, especially of alkanethiols bonded to well-defined gold surfaces, is a field to itself which has blossomed in the last decade. The ability to manipulate interfacial structure via the chemistry of SAMs is unprecedented. There are now a significant number of modified electrode studies in which the interface exhibits novel phenomena due to its microstructure in either the lateral or perpendicular direction. Several research groups have addressed the question of how the potential gradient across a SAM affects charge transport and the voltammetric response at a SAM-modified electrode. Smith and White showed that non-Nernstian wave shapes were a function of the thickness and dielectric constant 390R
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of the film, the surface concentration of the adsorbate, the supporting electrolyte, the solvent dielectric constant, and the EPzc (F314). Correction procedures that take into account the potential at the “plane of electron transfer” were described that are similar to Frumkin 4>2 corrections for irreversible electron-transfer reactions. The electric field strength in a SAM at a roughened Ag electrode was measured by observation of the Stark effect on the fluorescence spectra of a dye molecule in the film (E315). A value of ca. 4 X 104 V/cm was found at a point in the diffuse region of the double layer. The nature of a SAM interface was also probed by measurement of redox potentials of viologen groups positioned at different locations with respect to the interface. One conclusion of this study was that hydration of the viologen dication was the principal factor in determining the E\/2 values (F316). Kitamura reported spike-like voltammetric peaks, indicative of attractive interactions in the adsorbed state, were dependent on the alkyl chain length for AW'-dialkylviologen cations at Hg electrodes (F317). Creager and Weber calculated the effect of ions on the potential distribution, and the resulting effect on electron-transfer rates, across a monolayer (F318). Electron tunneling through thiol SAMs at gold electrodes has been a popular topic. Becka and Miller reported that the tunneling coefficient, 1.08 ± 0.20 per CH2 group, was almost independent of the potential and the redox couple in solution (F319). On the other hand, for redox molecules as different as ubiquinone and Fe(CN)63~, Takehara et al. found a marked difference in the effect of n-alkanethiol SAMs on the electrontransfer kinetics (F320). A careful and detailed study of electron-transfer kinetics of pentaammine(pyridine)Ru complexes tethered to gold via an alkanethiol linkage provided evidence for the existence of a small fraction of fast electronexchange centers that dominated the currents at low overpotential (F321). Analysis of the data for molecules with (CHijio-is tethers suggested that through-bond tunneling, as opposed to through-space tunneling, was the mechanism of electron transfer. Curtin et al. also investigated the effect of the length of the tethering arm on the low-temperature CVs obtained on ferrocene thiol SAMs (F322). The influence of film thickness on electron-transfer rates was very clear for an electrode covered with multilayer films of a metal phosphonate structure (F323). Obeng et al. reported “blocking” properties of rigid rod thiols on Au (F324). In other studies Finklea and co-workers obtained nearly ideal CV surface waves for electroactive thiols containing pendant pyRu(NH3)53+/2+ redox centers on gold electrodes (F325, F326). Electrodes modified with self-assembling molecules with terminal acid/base or ionic groups can enhance or suppress selected electrode reactions, depending on the state of the end group. For example, rates of cationic couples such as Ru(NH3)63+/2+ and anionic couples such as Fe(CN)63%4_ at SAMs with terminal ionic groups were affected in markedly different ways by solvent effects (F327). The effect of pH on the response of SAMs has been reported for the latter couples at thioctic acid-coated electrodes (F328), for Fe(CN)63~/4~ at 4-pyridyl sulfide electrodes (E329), and for carboxylic thiol SAMs on a QCM surface (F330). SAMs of HS(CH2)„C02H on Au discriminated against ascorbate anions in the voltammetric analysis of dopamine, with n = 5 giving the optimum differentiation (F331). Related to these studies
that maxima and minima will appear in i-E of molecular films containing acid/base groups that are due to variations of the differential capacity (F332). Activity and ion-pairing effects have been noted at SAM is the prediction
curves
modified electrodes. For the (NH3)sRu3+/2+ couple attached to Au in an organized monolayer, activity coefficient effects were consistent with a model in which one anion was transferred between solution and the metal center in the monolayer for each electron transferred {F333). (This paper also contains a good example of the influence of liquid junction potentials on measured electrode potentials.) Ion-pairing effects were invoked to explain the very rapid electron-transfer kinetics of a SAM of a redox-active Os(III/II)-bpy complex (F334) and for the (ferrocene)+/° couple in an n-alkanethiol SAM (F335). The lateral diffusion of octadecylferrocene in a L-B bilayer assembly was found to be dependent on the fluidity of the monolayer film (F336). In a previous paper Majda and coworkers documented the transition between two-dimensional diffusion and steady-state mass transport for the microband voltammetry of Ci8-ferrocene surfactant films (F337). Katz et al. reported that treatment of disordered viologen films, in which ester links were in the tethering arms, with C^SH produced distinctly ordered behavior (F338). Mixed monolayers of CisSH and CisOH have also been characterized electrochemically using ubiquinone probe molecules (F339). Reductive desorption of alkanethiols on evaporated Au surfaces was the basis of a method reported for the measurement of thiol surface coverage (F340). Porter and co-workers have also observed the potential controlled electrodeposition and stripping of alkanethiols in ethanolic KOH (F341). Nanoporous SAMs on Au were prepared by spontaneous adsorption of two thiols from solution. The structure and electrochemical response of the interface could be manipulated by variation of the ratio of the thiols and their nature (F342). Cyclic voltammograms were obtained at a L-B interface containing /3-cyclodextrin channels using the “horizontal touch method” (F343). Inhibition of the electron-transfer process for permeable species was seen when uncharged electroinactive guest molecules such as cyclohexanol were present in solution. A laser desorption procedure was described for the preparation of mixed thiol monolayers that functioned as “ion-gate” interfaces (F344). Kim and Bard were able to form pits and aggregates in alkanethiol SAMs by control of the bias voltage, the tunneling current and the position of a STM tip (F345). Gold electrodes treated with cystamine were further functionalized by treatment with tranj-stilbene diisocyanate. This produced a reactive surface for attachment of redox proteins and other electroactive groups (F346). Lu et al. published C V evidence for one of the more intricate of modified electrodes with a covalently attached donor/acceptor catenane complex (F347). The procedure involved attachment of both terminal thiols of substituted aromatic donor “needle” molecules to the electrode surface. Other interesting modified electrodes include the L-B films of rigid rod oligoimides and thiol-terminated SAMs of oligoimides of Miller and colleagues (F348, F349). The cis/trans isomerization of a single L-B monolayer of a polymeric azobenzene was followed after irradiation using a “displacement current” technique (F350). (In this method, the cell consists of two parallel electrodes where one is in air
1 mm above the surface of the L-B film.) A CV study of this isomerization over the temperature range 3-41 °C has also appeared (F351). This system is the basis of coulometric actinometer since the more easily reduced cis form of the azobenzene can be quantified using Faraday’s law (F352-
ca.
F354). Several groups have studied electron transfer to biologically important molecules such as cytochrome c at SAM-modified electrodes (F355-F358). These and related interfaces have been touted as ideal physiological membrane-mimetic systems for the study of redox proteins (F355, F359).
A hydrophobic hydrogenase enzyme was immobilized in bilayer assembly containing Qs-viologen as an electron mediator (F360). Efficient coupling of the enzymatic activity
a
to the electrode surface was realized using both potentiometric and steady-state voltammetry. Mediated electron transfer to the redox enzyme glutathione reductase covalently attached to a cysteic acid ester monolayer was achieved by reaction of the SAM/enzyme surface with a viologen mediator (F361). Reports of molecular diode-like behavior have appeared. SAMs of a u-substituted ferrocenyl alkanethiol on gold were shown to exhibit unidirectional electron transfer in the presence of the Fe3+/2+ couple in water (F362). The monolayer surface mediated reduction of Fe3+, but inhibited the oxidation of Fe2+. Asymmetric i-E curves indicative of “molecular rectification” were also obtained for L-B sandwich cells containing a donor/acceptor surfactant molecule (F363) and for a flavolipid/cytochrome c heterolayer prepared by L-B
techniques (F364). Several miscellaneous studies were noted. The voltammetry
of L-B films of phospholipids functionalized with
an-
thraquinone groups revealed anion effects related to the supramolecular structure of the monolayers (F365). Reductive dechlorination of an aryl chloride was carried out in a cationic surfactant film (F366). The CV behavior of simple cations at phosphatidylserine-coated Hg electrodes was markedly pH dependent. These films were relatively impermeable to cations (e.g., Tl+ or Pb2+) at low pH or in the presence of strongly bound ions such as La3+ (F367). Electron-transfer rates of amphiphilic ferrocene-substituted surfactants in cationic micellar media indicated that the molecules were oriented with cationic head groups toward the electrode surface (F368). Disordering effects were deduced from a CV study on SAMs of a cholesteryl viologen (F369). Salt formation was indicated for redox cycling of bis(phthalocyaninato)Yb(III)-stericacid L-B films (F370). Formation of a porous deposit was indicated when methylviologen was reduced on glassy carbon electrodes in the presence of sodium alkyl sulfate surfactants (F371). A two-capacitor model was advanced to explain double-layer capacity data obtained at thiol-coated Au electrodes (F372). Other Modified Electrodes. Electrochemistry in gel matrices has been the subject of several interesting studies. Rillema et al. performed photochemistry on the Ru(bpy)32+/ viologen system in a hydrogel matrix where the diffusion coefficients were only 1 order of magnitude smaller than in aqueous environments (F373). A sol/gel Si02 film doped with Ru(bpy)32+ was ceramic in nature and porous enough to allow electron transfer to the electrode substrate (F374). Electropolymerization of aqueous solutions of acrylamide gave Analytical Chemistry, Vol. 66, No.
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thin poly(acrylamide) films on carbon fiber electrodes with number-average molecular weights up to 430 000 (F375). The reversible, cooperative complexation of surfactants in a polymer gel was the basis of a device that converted electrochemical energy into mechanical energy (F376). Modified electrodes with permselective and electrocatalytic properties were prepared by casting cellulose acetate films, which had been hyrolyzed in base to provide porosity, over electrodeposited Pt and Pd surfaces (F377). Christie et al. found that poly(vinyl chloride) gave better selectivity than cellulose acetate as a barrier membrane in amperometric sensors for H2O2 and phenolics (F379). G. BIOELECTROCHEMISTRY Books and Reviews. Smyth wrote a new book that surveys the voltammetric analysis of a large number of small organic and inorganic molecules of biological importance (Gl). Schultz and Taniguchi edited the proceedings volume for the Fifth International Symposium on Redox Mechanisms and Interfacial Properties of Molecules of Biological Importance, held in May 1993 (G2). This is an excellent collection of richly diverse, high-quality papers that give a clear sense of the state of bioelectrochemistry as of 1993. The electrochemistry and spectroelectrochemistry of proteins, enzymes, small molecules, membranes, and cells, as well as new concepts and techniques, are covered. Volume 227 of the Methods in
Enzymology series will be of great interest to metalloprotein chemists as it covers physical and spectroscopic methods for probing metal ion bioenvironments. Of special interest to electrochemists are the three chapters by Stankovich and coworkers on EPR spectroelectrochemical titrations of redox enzymes (G3), Armstrong and co-workers on the voltammetry of adsorbed metalloproteins (G4), and Hill and Hunt on direct and indirect enzyme electrochemistry (G5). A brief yet succinct overview of the electrochemistry of biopolymers (proteins, polynucleotides) was provided by Cox and Przyjazny (G6). Ewing et al. discussed aspects of performing analytical chemistry (electrochemistry, separations, identification) in microenvironments, specifically single nerve cells (G7). The role of voltammetric methods in research and development of pharmaceuticals was reviewed by Kauffmann and Vire (G8). Volk et al. discussed the application of electrochemistry/mass spectrometry for the elucidation of biological redox mechanisms (G9). Many reviews and some books relating to amperometric biosensors were published over the past two years. In several excellent reviews, the crucial issue of coupling enzymatic reactions to current-carrying electrodes was addressed authoritatively. Encompassing reviews regarding enzyme/ electrode coupling were provided by Ikeda (G10) and Bourdillon (Gl 1). Gorton et al. reviewed the topic of biosensors based on apparent direct electron-transfer reactions of peroxidases (Gl 2). In another very informative review, Heller reviewed his and others’ work on redox “wiring” of enzymes to electrodes (G13). Redox “wiring” was also covered by Boguslavsky et al. in their article, which was more narrowly focused on ferrocene-derivatized siloxane and ethylene oxide polymers (G14). A very timely review article on glucose oxidase, with an emphasis on properties important to biosensor development, 392R
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published by Wilson and Turner (G15). A general biosensor review was given by Ivnitskii et al. (G16), which included reference to numerous Russian language papers. Hilditch and Green reviewed the topic of disposable electrochemical biosensors and discussed requirements that are needed for commercial viability (G17). was
Several books
on
biosensors that include chapters on
were recently published, but your amperometric reviewer has, regrettably, yet to examine them except for Table of Contents listings. Among these is the second volume in the Advances in Biosensors series edited by Turner (G18). This volume appears to contain a number of chapters of interest to amperometric biosensor scientists. Another book, edited by Nakamura et al., is concerned with immunochemical assays and biosensors (G19) and includes two chapters on amperometric biosensors and electrochemical immunoassay. The use of chemically modified carbon-type electrodes was reviewed by Wring and Hart (G20) with an emphasis on mediation reactions in biosensors. Bilayer lipid membranes were reviewed for electrochemical sensing applications by Nikolelis and Krull (G21) and for bioelectronic devices in a comprehensive review of BLMs by Ottova-Leitmannova and Tien (G22). Small Molecules of Biological Importance. Our ability to efficiently oxidize and reduce small biological molecules at electrode surfaces carries important ramifications. From a biochemical perspective, this ability opens the way for powerful sensors
investigations of the redox energetics and dynamics of these molecules in the context of biological function. From an analytical perspective, efficient redox conversion obviously provides a basis for sensitive voltammetric or amperometric detection of the molecules per se. Furthermore, for those particular molecules serving as enzyme cofactors, their electrodic behavior can assume paramount importance with
regard to designing amperometric biosensors. Because many small molecules of biological importance are organic species, they typically do not undergo clean facile electron-transfer reactions at metal electrodes. Large redox overpotentials and electrode fouling tend to be standard fare, and thus, numerous investigations over the years have sought to minimize these factors through mediation and electrocatalytic strategies. This trend continued during the past two years. In the coverage that follows, we have not included studies of pharmaceuticals or investigations of complex chemical reactions coupled to electrolysis. Our emphasis is decidedly interfacial. Porphyrinmodified electrode studies directed toward catalysis of O2 are also not included. Substantial interest continued with regard to the electrochemistry of nicotinamide adenine dinucleotide (NAD/ NADH). The oxidation of NADH is a pivotal component in the design of dehydrogenase-based enzyme electrodes. Typically, however, unmediated NADH oxidation proceeds at large overpotentials with comcomitant electrode fouling. Kuhr et al. described a most interesting and potentially useful CV study of NADH oxidation at carbon fiber microelectrodes
(G23). By using judicious electrochemical pretreatment (to minimize NADH adsorption) in combination with fast 100 V/s scan rates (to minimize electrode fouling), reproducible electrooxidation of NADH was achieved at bare carbon fiber surfaces. The application of this approach to the development
of hydrogenase-based microelectrodes was also considered. In past years, some very successful mediators have been developed by Gorton and others specifically for NADH oxidation. Perhaps the best one, Meldola blue, was incorporated successfully by Hale et al. into a siloxane polymer structure that is easily deposited on electrodes by evaporation (G24). A more complicated approach to the mediated oxidation of NADH involves the incorporation of a second enzyme, e.g., diaphorase, specifically to oxidize NADH, with the mediator then oxidizing the diaphorase (G25). Yet another potential solution to the problem at hand is direct electrocatalytic oxidation of NADH on electrochemically grown conducting polymers, although few have been found to date that function in this capacity. Two promising candidates were reported on, namely, poly(indole-5-carboxylic acid) (G26) and poly(thionine) (G27). Reduction of NAD+ to NADH, also a difficult reaction, is important synthetically in bioreactors but less so from an analytical standpoint. The incorporation of rhodium complexes in polymer film electrodes resulted in the catalysis of this reaction with good selectivity for 1,4-NADH and without the formation of NAD dimers (G28,G29). Catalytic reduction of NAD+ by hydrogenase at platinum electrodes was also described (G30). Basic electrochemical investigations of catechols and catecholamines on carbon-type electrodes emphasized voltammetric discrimination capabilities. Such studies are of direct relevance to in vivo neurochemical studies, which are reviewed under In Vivo and Cellular Electrochemistry. Tokuda and co-workers investigated the effects of electrochemical pretreatment on the oxidation of dopamine (DA) (G31). Discrimination against DOPA and DOPAC, but not ascorbic acid, can be achieved by employing a “mild” pretreatment. Caution against the indiscriminate use of “strong” pretreatments for DA detection was given. Over the last few years, McCreery’s group has provided much valuable insight into carbon electrode voltammetry. Of relevance to this section is a detailed investigation of theirs concerned with the adsorption of catechols on glassy carbon surfaces (G32). Whereas DA, 4-methylcatechol, and DOPAC all adsorb strongly on fresh-fractured GC, the interaction is weak on polished surfaces apparently due to impurity adsorption. Electrochemical pretreatment leads to adsorptive preference for DA, a cation, due to oxidation of the GC surface. Reverse differential-pulse voltammetry was used by Matysik et al. in a study aimed at discriminating among a serious of catechols (G33). Nafion coatings are frequently used with carbon electrode studies of neurotransmitters to electrostatically discriminate against ascorbate and other anions. A new electrostatic approach was described by Malem and Mandler in which COOH-terminated alkanethiolate self-assembled monolayers on gold were effective in discriminating against ascorbate during the detection of DA (G34).
The direct oxidation of carbohydrates, alcohols, and amino acids is now performed routinely at noble metal electrodes due to the efforts of Dennis Johnson and co-workers. In the latest chapter, Vandeberg and Johnson report on the pulsed electrochemical detection of the sulfurous compounds cysteine, cystine, methionine, and glutathione, at picomolar detection limits (G35). Lacourse and Johnson have described an
automated optimization of all waveform parameters for the pulsed amperometric detection of several representative carbohydrates (G36). Wang’s group has been developing chemically modified electrodes for HPLC and FIA determination of various biological compounds. Accounts were given of the design and characterization of a Prussian Blue electrode for glucose detection (G37) and a polymer film electrode containing nickel (oxy)hydroxide catalyst for carbohydrate and amino acid oxidations (G38). The oxidation of glucose can also occur readily at organic conducting salt electrodes as shown by Zhao and Lennox for TTF/TCNQ (G39). Electrodes based on HMTTeF-TCNQ (where HMTTeF represents hexamethylenetetratellurafulvalene) (G40) exhibited especially good versatility for a number of other important oxidations including glutathione, cysteine, dopamine, and ascorbate. This latter electrode is considerably more resistant to dissolution than TTF/TCNQ. These and other papers from this group shed considerable light on the biological oxidations that occur at organic conducting salt electrodes. The interaction of amino acids with copper has also been examined electrochemically. Weber’s group, in efforts to develop detection schemes for nonelectroactive peptides, have exploited the biuret reaction, which produces electroactive Cu(II)-peptide complexes. The influence of tyrosine, an electroactive amino acid, on the electrochemical response was addressed in a recent report (G41).
An amperometric sensor was described by Malinski and Taha for the detection of nitric oxide (G42), a molecule whose biological importance has only begun to be appreciated in the past few years. This sensor, which is constructed by electropolymerization of a Ni-porphyrin catalytic film on a carbon fiber electrode, had a detection limit of 10 nM. Furthermore, it was sufficiently miniaturized, 0.5-m fiber diameter, to monitor NO release from a single cell. Several other important molecules were the subject of significant investigations. The electrochemistry of the coenzyme pyrroloquinolinequinone (PQQ) was found to be reversible under acidic conditions at bis(4-pyridyl)disulfidemodified gold electrodes (G43). The oxidation of biliverdin, a key intermediate derived from bile metabolism, was examined by thin-layer spectroelectrochemistry and its formal potential was reported (G44). Hemin adsorbed on pyrolytic graphite electrodes was examined in-depth by UV/visible electroreflectance (G45). The adsorption and voltammetry of ubiquinones was examined at the mercury electrode (G46). Flavins and related compounds were explored electrochemically in both solution and monolayer formats. Verhagen and Hagen described some very nice electrochemistry of flavin adenine dinucleotide (FAD) at glassy carbon electrodes (G47). High electron-transfer rates were measured for adsorbed FAD, which appeared to act as a surface mediator for the subsequent reaction of diffusing FAD. The electrochemical behavior of solution FAD (and also a ubiquinone) was examined by Takehara et al. at gold electrodes covered by n-alkanethiolate self-assembled monolayers (G48). FAD was able to partition into the monolayer, but its electron-transfer rate decreased as film thickness increased, apparently due to increased ET distance. Nakashima et al. incorporated flavin molecules into synthetic lipid bilayers on gold electrodes (G49). In this study, Analytical Chemistry, Vol. 66, No.
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they demonstrated the regulation of flavin ET through control of the thermal phase transition of the bilayer. Mallik and Gani immobilized an isoalloxazine species on a gold electrode and conducted a detailed study of the effect of pH on its surface redox potential and electron-transfer rate constant (G50). pH-dependent conformational changes play a key role in the observed electrochemical properties.
Protein Electrochemistry (emphasizing interfacial electron transfer and biochemical studies). Exciting advances continue to emerge from the protein electrochemistry field, which has experienced a very active two-year period. Whereas simply obtaining a reproducible voltammetric response for small redox proteins remained a challenge some ten to fifteen years ago, that situation has since changed quite decisively. Obtaining voltammograms for cytochromes, ferredoxins, and small blue copper proteins is now a commonplace endeavor. Developing a clear, molecular-level understanding of such “simple” protein/electrode reactions, however, remains a challenge, and indeed, many publications and much controversy have ensued. Other major trends during this time period include the following: direct electronic communication between electrodes and larger enzymes; voltammetry of protein and enzyme monolayers; and the increasing success of protein voltammetry in solving significant biological problems. “Simple” Electron-Transfer Proteins (Cytochromes, Ferredoxins, Blue Copper Proteins). Gold electrodes modified with adsorbed “promoters” are widely used in electrochemical investigations of small ET proteins. Several recent papers addressed the mechanism by which such promoters work. Niki’s group described a spectroelectrochemical study of the cytochrome c reaction at a gold electrode in the presence of the original Eddowes/Hill promoter 4,4,-bipyridyl (G51). They concluded that the interaction of this promoter with cytochrome c is relatively weak and that it acts in the adsorbed state to inhibit the unfolding of coadsorbed cytochrome c as well as to provide a suitable interface. An electrochemical QCM study was reported that provided evidence for weak adsorption of 4,4'-bipyridine on gold electrodes (G52). Cotton and co-workers reported the interesting result that even more weakly adsorbed molecules, namely, 2,2'-bipyridine and pyrazine, could exhibit promoter activity for cytochrome c electrochemistry when appropriate adsorption protocols are followed (G53). These molecules had previously been viewed as nonpromoters. The most successful gold electrode promoter for cytochrome c electrochemistry is the strongly adsorbing bis(4-pyridyl) disulfide, which was first described by Taniguchi. Ellipsometric evidence was described supporting the idea that bis(4-pyridyl) disulfide prevents unfolding of irreversibly adsorbed cytochrome c at the gold surface (G54), and it was furthermore suggested that electron transfer occurs in some manner through an adsorbed monolayer of cytochrome c. In situ STM images of the bis(4-pyridyl) disulfide/gold interface in water suggested ordering of the organic monolayer (G55). In an interesting study from Taniguchi’s group, the effect of chemical modification of surface lysines, i.e., conversion to negative charge, was examined (G56). Promoter-modified electrodes turned out to be significantly less adept at recognizing modified cytochrome c molecules than cytochrome oxidase. Haladjian et al. used modified gold electrodes in the 394R
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first reported electrochemical investigation of rusticyanin (Thiobacillus ferrooxidans), a blue copper protein with a molecular weight of ca. 16.5 kDa (G57). An especially intriguing observation was that the direct electrochemistry of this basic protein (IEP = 9.1) was promoted well by bis(4pyridyl) disulfide but not by several other known promoters of cytochrome c (IEP = 10) electrochemistry. Finally, an examination of the influence of promoter surface coverage on cytochrome c electrochemistry was described by Bond et al. (G58). At suimonolayer coverage of bis(4-pyridyl) disulfide, a sigmoidal voltammetric wave shape resulted, which was attributed to radial diffusion at microscopic reactive sites. A different approach for promoting the electrochemistry of small proteins involves the use of lipid-modified electrodes. Nakashima et al. reported that cytochrome c undergoes direct electron-transfer reactions at gold electrodes modified by a
Langmuir-Blodgett mercaptophosphatidylcholine monolayer (G59). Tollin and co-workers described a self-assembled lecithin bilayer-modified gold electrode that successfully promoted the direct electrochemistry of thioredoxins (ca. 12 kDa molecular mass) via their disulfide/dithiol redox activity (G60). In previous studies, this particular modified electrode had shown electroactivity for metalloproteins. Conductive metal oxide electrodes, particularly indium oxide, also continue to be useful for studies with small ET proteins. It was shown definitively that the presence of deamidated or oligomeric forms of cytochrome c interfere with the reaction of this protein at indium oxide, but much less so at promoter-modified gold (G61). Daido and Akaike reported a detailed ionic strength and pH study of the reaction of cytochrome c at indium oxide (G62); their results supported Coulombic interfacial attraction as the dominating factor. Indium oxide electrodes also were shown to give good responses for negatively charged ferredoxins in the presence of polylysine (G63). Coulombic attraction is also known to be the dominating force in the interaction of cytochrome c with tin oxide electrodes. Using chronoabsorptometry, Collinson and Bowden determined adsorption isotherms for this system as a function of ionic strength, solution composition, and oxidation state (G64). A report of a “solid-state” promoter of cytochrome c, namely, a porous layer of 7-alumina on glassy carbon, was presented (G65). These results may have ramifications for those who use alumina polishing media. Edge-plane pyrolytic graphite (EPG) has been widely used in studies of small negatively charged proteins, typically in the presence of inorganic cationic promoters. Datta et al. proposed that, for ferredoxin reactions at EPG, the promoter induces a weak adsorption of the protein with Frumkin isotherm behavior subsequently resulting from lateral repulsive interactions (G66). EPG direct electrochemistry was used to characterize the thermodynamic and ET kinetic behavior of four plastocyanins (G4, the nitro group was reduced to form a nitrosamine. Mirallesroch et al. (H26) examined the electrochemical conversion of a-nitrobenzylic compounds into the corresponding oximes.
Anne et al. (H27, H28) examined the electrochemistry of synthetic analogues of NADH and NAD dimer analogues. Medebielle et al. (H29) investigated the perfluoroalkylation
of pyrine and pyrimidine bases by electrochemically induced Srni substitution. Combellas et al. (H30) carried out selective substitutions of 1,4-dichlorobenzene with 2,6-di-terf-butylphenoxide using the electrochemically induced Srni mechanism. Mortensen et al. (H31) studied the voltammetry of highly reduced oligoanthrylene systems and were able to generate the tetraanion of all the species studied. Cleghorn and Pletcher reported on the mechanism of the electrocatalytic hydrogenation of organic molecules at palladium black (H32) and palladium on nickel cathodes (H33). Mahdavi et al. (H34) examined the electrocatalytic hydrogenation of phenanthrene at Raney nickel electrodes. The electrochemical fluorination of benzene was carried out at +2.5 V in acetonitrile using tetraalkylammonium fluoride salts (H35).
Delgado et al. (H36) examined the electrochemistry of an alkali metal complex of quinone crown ethers and showed that the formation constants with the alkali metal with the attached crown ether varied with the redox state of the quinone. The binding of the alkali metal was qualitatively and quantitatively different from simple ion pairing. Urove and Peters (H37) examined the electrochemical reduction of cyclohexanecarbonyl chloride at mercury cathodes. Pritts et al. (H38) reported on a method to quantitatively determine volatile products formed in the electrolysis of organic compounds. Wandlowski et al. (H39) studied the electrochemical oxidation of 2,6-dichloro-l,4-phenylenediamine. Potential step and digital simulation of the voltammetric data was used to determine the kinetic parameters. Organometallic Electrochemistry. Redox-induced changes
in the conformation, bonding, or solvation of the metal atom of a complex can be readily probed by the use of electrochemical techniques. Electron-transfer-induced isomerization of cobalt, nickel, and palladium cyclooctatetraene complexes was examined by Geiger et al. (H40). The Ni and Pd complexes retained their 1,5-conformation upon reduction, while the Co complex underwent rapid isomerization to the 1,3-isomer in the 19e~ species. The differences were explained by the role played by the ligand vs metal composition of the redox orbital. Osella et al. reinvestigated the electrochemical behavior of the Co2(CO)6(ethynylstradiol) complex and found evidence of efficient recombination of the electrogenerated fragments (H41). They also found electrochemical evidence for the reorientation of alkynes on trimetallic clusters during a two-electron reduction (H42). Karpinski and Kochi (H43) used electron-transfer chain (ETC) catalysis in the electrochemical deligation of bis(arene)iron(II) dications. Mechanistic studies were carried out using normal and reverse-pulse voltammetry. Sanaullah et al. (H44) used chemical and electrochemical methods to examine the redox-associated conformation changes in the bis(l,4,7-trithiacyclononane)copper(II/I) system. Electrochemical studies on nioboceneketene complexes yielded redox-induced ketene fragmentation reactions (H45). Solvent effects on the redox behavior of organometallic complexes were examined by several groups. Boudon et al. (H46) studied the effects of axial anions and solvent on the redox behavior of nickel complexes with C-functionalized tetraazamacrocycles. McDevitt and Addison (H47) examined medium effects on the redox properties of tris(2,2'-bipyridyl)ruthenium complexes. The medium was also found to Analytical Chemistry, Vol. 66, No.
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observed, and the initial reduction led to the loss of the metal fragment. Koefod et al. (H73) studied the electrochemistry of an iridium buckminsterfullerene complex and found evidence for a C60 localized reduction.
modulate the two-electron activity of ferrocene metallocyclam conjugates (H48). Mu and Schultz (H49) studied the effect of methanol binding on the redox potential and electrontransfer reactivity of chloro(tetraphenylporphinato)manganese-
were
(III).
Sudha et al. (H74) reported on electrochemical evidence for a two-electron-reduction process in a ^-oxobis(ju-acetato)diruthenium(III) complex with a terminal 1 -methylimidazole ligand. The electrochemistry of the incomplete cubane-type clusters, M3S4 (M Mo, W), was examined (H75), as well as some molybdenum mononitrosyl complexes containing oxobiphenyl ligands (H76). The influence of pyridine substituents on binuclear rhenium(V) clusters was studied as a redox tuning procedure (H77). Low-temperature voltammetry was used to study the reduction and oxidation of [Re2(NSC)8]2“ (H78). The electrochemical reduction mechanism of a Ru3(CO)i2 was investigated in considerable detail by voltammetric techniques (H79). Cyr et al. (H80) studied the electrochemistry of boron-capped "Tc-dioxime complexes. Choi et al. (H81) studied the electrochemical reduction of thionyl chloride by cyclic voltammetry, chronocoulometry, and chronoamperometry. Opekar and Langmaier (H82) reported a procedure for electrochemically controlled generation of carbon monoxide.
The use of inert solvents such as liquid sulfur dioxide and/ or microelectrodes has enabled the voltage range to be extended, and highly reduced or oxidized species were observed. Liquid sulfur dioxide was used to study the oxidation of M(bpy)32+ complexes where M = Ni, Zn,and Cd (H50,H51).
Very negative and very positive potentials were used to generate Cp2Co2+, Cp2Co2~, and Cp2Ni2~ (H52). Ruthenium complexes are quite interesting in that they can undergo a large number of redox processes. Four one-electron-transfer steps were observed in the voltammetry of tra«5-[Ru(tpy)(0)2-
(H20)]2+ (H53). Ruthenium(II) complexes of 2,2'-bipyridine and 2-pyridylpyrazine were examined up to -3.1 V at -54 °C in DMF using cyclic voltammetry (H54). By the use of convolution techniques and digital simulation, it was possible to determine between 8 and 12 redox steps, depending upon ligation. Krejcik and Vlcek (H55) found that [(Ru(bpy)2)2bpm]4+ yielded 14 one-electron waves, 2 of which metal based and 12 ligand based. Debias et al. (H56) used pyridines with appended metallocyclam subunits as versatile building blocks to supramolecular multielectron redox systems. Reversible electrogenerated triply oxidized nickel porphyrins and porphycenes were reported by Kadish et al. (H57), where they were able to generate a stable nickel(III) were
x-dication. The oxidation state of the metal atom in the electrogenerated complex was the focus of several studies. Guldi et al. (H58) investigated whether chromium(III) porphyrins were reduced to Cr(II) porphyrins or Cr(III) porphyrin x-cation
radicals. Kadish et al. (H59) examined the site of electroreduction of rhodium porphyrins. A very complex redox scheme was elucidated for the reduction of cr-bonded iron(III) porphyrins in noncoordinating solvents (H60). Strojanovic and Bond (H61) examined the conditions under which the reduction of cobaltocenium cation could be used as a standard voltammetric reference process in organic and aqueous solvents. Kaminsky et al. (H62) reported on a reference electrode for organic solvents based on modified polyethylenimine loaded with ferrocyanide/ferricyanide.
Inorganic Electrochemistry. The electrochemistry of buckminsterfullerene and related complexes attracted the attention of many researchers. Xie et al. (H63) detected the The hexaanions, C6o6- and C706', using electrochemistry in of was also studied ammonia C60 liquid electrochemistry (H64). The kinetics and thermodynamics (H65) and the role of solvation (H66) in the electroreduction of C60 in aprotic solvents was investigated. Fast-scan cyclic voltammetry and scanning electrochemical microscopy (H67) were used to determine the kinetic parameters for the electroreduction of Cgo- An electrochemically reversible oxidation of C60 and C70 was reported (H68), as was the electrochemistry of C6oH2 (H69). Penicaud et al. (H70) electrocrystallized C60 for the synthesis and characterization of (Ph4P)2C6oK. Li et al. (H71) reported on unusual electrochemical properties of the chiral C76. Lerke et al. (H72) studied platinum, palladium, and nickel derivatives of buckminsterfullerene. Three to four waves .
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Activation of Small Molecules. The direct electrolysis or electrocatalytic activation of small molecules has been an active area of research. The largest area is probably the activation of carbon dioxide. These studies have involved both the direct reduction of carbon dioxide and the coupling of C02 to a substrate. Reports on the electrocatalyzed reduction of carbon dioxide have included the catalysis by molybdenum-ironsulfur clusters (H83), nickel phosphine clusters (H84), iron, cobalt, and nickel terdentate complexes (H85), osmium bipyridyl complexes (H86), and rhenium bipyridyl complexes incorporated into a coated Nafion membrane (H87). Electrocatalytic surfaces have been reported for the direct reduction of carbon dioxide, such as ruthenium-titanium oxide (H88), Cu 4- Au electrodes (H89), Perovskite-type electrocatalysts (H90), palladium (H91) or copper-modified palladium (H92) electrodes, and nickel electrodes at high pressure (H93). Carbon dioxide may also be electrochemically activated by the reductive addition of C02 to quinones in acetonitrile (H94). Carbon dioxide can be coupled electrochemically by a nickel catalyst to 1,3-enynes (H95), diynes (//96),oralkenes (H97). p- Anisic acid was formed from the reduction of p-iodoanisole at mercury in DMF, saturated with carbon dioxide (H98). The electrocatalytic generation of C2 and C3 compounds was reported for the reduction of C02 on a cobalt compleximmobilized dual-film electrode (H99). Kyriacou et al. (HI 00) examined the influence of C02 partial pressure and the supporting electrolyte cation on the product distribution. Naitoh et al. (HI01) studied the electrochemical reduction of carbon dioxide in methanol at low temperature. The electroactivation of other small molecules have also been reported. Formaldehyde was oxidized on ultrafine gold particles, supported on glassy carbon substrates (HI02). Methanol was electrooxidized on rhenium-tin oxide, platinumtin oxide, and iridium-tin oxide, and the results were compared with the oxidation on platinum (H103). The electrocatalytic oxidation of methanol at PTFE-bonded electrodes was studied for a direct methanol/air fuel cell (H104). Wong et al.
reported on the electrocatalytic oxidation of methanol (HI 05) and benzyl alcohol (HI 06) with a monooxoruthenium(V) complex. Cavalca et al. (HI 07) examined electrochemical modification of methanol oxidation selectivity and activity on a platinum single-pellet catalytic reactor. Gasteiger et al. (HI 08) studied methanol electrooxidation on well-characterized Pt-RN alloys. A quadruply aza bridged closely interspaced cofacial porphyrin was used to catalytically reduce dioxygen (HI 09). A rotating disk electrode was used to study the catalytic alkaline cyanide oxidation (HI 10). A cobalt(Ill)-mediated electrochemical oxidation was used to destroy
chlorinated organics (HI 11). F430-Model compounds, which contain nickel isobacteriochlorins, will dehydrohalogenate alkyl halides (H112). Lojou et al. (H113) examined the electroreduction of aryl halides in DMF on a cadmiummodified gold electrode. Che and Dong (HI 14) applied ultramicroelectrodes to the electrocatalytic reduction of organohalides by metalloporphyrins. The electrocatalytic reduction of nitrate was studied with foreign lead adatoms (HI 15). Gur and Huggins (HI 16) studied the direct electrochemical conversion of carbon to electrical energy in a high-temperature fuel cell. Electrosynthesis. Electrosynthetic procedures have often been the impetus for detailed mechanistic studies by electrochemical techniques. The interplay between electrosynthesis and electroanalytical studies has been quite synergistic over the years. One area of active research is the direct electrosynthesis of solid material. Matsumoto et al. (HI 17)
reported a new preparation method of LaCo03 Perovskite using electrochemical oxidation. Wade et al. (HI 18) electrosynthesized ceramic materials and precursors, while Singh and Tanveer electrosynthesized (CdHg)Se (HI 19, HI 20) and (ZnCd)Se (HI 21). Dennison (HI 22) studied the cathodic deposition of CdS from aqueous solution. Roberts et al. (HI 23) investigated the mechanism and electrosynthesis of the superconductor Bai_^KxBi03. The direct dissolution of solid electrodes has also been used electrosynthetically. Halo and mixed-halo complexes of palladium(II and IV) were synthesized by the dissolution of a sacrificial palladium anode (HI 24). Cathodic dissolution of an AuTe2 electrode led to the formation of Au2Te43~ (H125).
Niyazymbetov and Evans (HI 26, HI 27) reported
on
the
utility of carbanions and heteroatom anions in electroorganic synthesis. Biaryls and aromatic carboxylic acids were synthesized by palladium-catalyzed electrosynthesis using triflates (HI 28). Freshly metal coated electrodes were used to electrosynthesize 1,2-diketones by reduction of aromatic esters (HI29). Gard et al. (HI30) reported an efficient electrochemical method for the synthesis of nitrosobenzene from nitrobenzene. Momota et al. (HI 31) reported the electrochemical fluoridation of aromatic compounds in liquid R4NF-mHF. Wendt et al. (H132, H133) studied the anodic synthesis of benzaldehydes from the anodic oxidation of toluene. Amino acids were synthesized from a molybdenum nitride via nitrogen-carbon and carbon-carbon bond formation reactions involving imides and nitrogen ylides (HI 34). aNitrobenzylic acids were converted into oximes using macroscale electrolysis (HI 35). .The hydrodimerization of dimethyl maleate in methanol using an undivided cell was reported by Casanova et al. (HI36). Franklin et al. (HI37)
reported a method for the destruction of halogenated hydrocarbons accompanied by the generation of electricity. Kunai et al. (HI 38) synthesized poly(disilanylene)ethylenes by the electrolysis of bis(chlorosilyl)ethanes. Chakravorti et al. (HI 39) reported the first electrosynthesis of transition metal peroxofluoro complexes (HI 39). The selective electrosynthesis of (CH3)2C60 provided a novel method for the controlled functionalization of fullerenes (HI 40). Ferrate(VI) was prepared using an alternating current superimposed on the direct current (HI 41).
Micellar media can provide very interesting electrochemistry because of their ability to solubilize material in aqueous solutions. Nikitas (HI 42) reported a simple model for micellization and micelle transformations on electrode surfaces. Myers et al. (H143) studied solution microstructure and electrochemical reactivity. They examined the effect of probe partitioning on electrochemical formal potentials in microheterogeneous solutions. Abbott et al. (HI 44) studied electron transfer between amphiphilic ferrocenes and electrodes in cationic micellar solution, and the correlations between solvent polarity scales and electron-transfer kinetics, as applied to micellar media (HI 45). Gounili et al. (HI 46) studied the influence of micelles and microemulsions on the one-electron reduction of 1-alkyl4-carbomethoxypyridinium ions. The rate enhancement and control in electrochemical catalysis using a bicontinuous microemulsion was examined (HI 47), and this method was used to debrominate alkyl vicinal dibromides with neutral metal phthalocyanines (HI 48). An adsorbed film of cationic surfactant was used to dechlorinate 9-chloroanthracene (H149). Takisawa et al. (H150) reported on ultrasonic relaxation and electrochemical studies of the micellization of sodium decyl sulfate and decyltrimethylammonium bromide in glycerol/water mixtures. Phani et al. (H151) developed a microemulsion-based electrosynthesis of polyparaphenylene. Micelles and Surfactants.
for
some
I. SPECTROELECTROCHEMISTRY The following survey is organized principally by technique. While most spectroelectrochemical methods are well established, the cited articles either feature some experimental aspect of general interest or illustrate particularly well the versatility of a given technique. On-line electrochemical mass spectroscopy, which is a powerful technique for the study of complex electrode reactions of small molecules, has been applied to a variety of problems. Included among these are studies of redox reactions of alcohols on Pt and Au (11-13) and on carbon-based electrodes (14). The working electrode of the latter study was made from PTFE-bonded carbon supporting Pt and Pt-Ru catalysts on Norit BRX. The working electrode of Munk and Skou was microporous gold film on a commercial silicone rubber membrane (15). EC/MS has provided detailed mechanistic insight into the role of surface structure at single crystal electrodes during electrode reactions of unsaturated coma
pounds
(El 52, El 53,16).
EC/MS studies have appeared on the oxidation of formaldehyde (17-110), acetonitrile (111), DMSO and sulfolane (112), and propylene carbonate (113,114). The latter article provides a good example of the use of isotopically labeled Analytical Chemistry, Vol. 66, No.
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solvents (D20 and H2180) to trace the origin of intermediates and products of the electrode reactions (114).
An on-line EC/MS study of 02 reduction under conditions of a methanol fuel cell allowed parallel reactions to be sorted out (115). Information on the reductive pathways for CH3CCI3 at the Pt/H2SC>4 interface (116) and for nitrite at graphite-supported CuO electrodes (117) was obtained. A porous Ni-plated Teflon membrane allowed the electroless Ni-P deposition to be followed by mass spectroscopy (118). The oscillatory reaction involving bromate, malonic acid, and the Ce4+/3+ couple was followed by potentiometry and mass spectroscopy. The production of C02(g) tracked the potential oscillations in this system (119). Several simplified designs of differential EC/MS interfaces have appeared (120, 121). Articles continue to appear in which X-ray methods have been used to probe the electrode/solution interface. These techniques can give detailed specific information, i.e., bond lengths, in the best of circumstances, but require access to a synchrotron radiation source. Two new descriptions of cell designs for in situ X-ray spectroelectrochemistry were noted (122, 123). The latter employed transmission geometry through a drop of solution maintained on the electrode surface by capillary action. Adlayer formation via UPD of metals are well-suited to study by X-ray methods. Recent systems examined include the UPD of Pb and Th on Au( 111) (124), Cu UPD on Pt( 111) (125) and Pt(100) (126), Cu deposition on carbon-supported Pt (127), Ag UPD on Au(l 11) (E234), and iodine adsorption on Pt single crystal electrodes (128). In the UPD study at Au(l 11), it was found that the Au-Pb distance was potential dependent, while the Au-Th distance was not (124). In the surface EXAFS study of Cu UPD, chloride ion was shown to play an important role in the ordering of the adlayer (125). In situ X-ray methods have been used to follow intercalation reactions of M0O3 (129), Li*Co02 (130), and V6Ol3 (131) electrodes. In situ X-ray spectra demonstrated the conversion of «-Pb02 to the /3-form on Pt substrates (132) and the formation of Cu20 layers by the reduction of Cu022' in concentrated KOH (133). In situ XANES of Fe-26Cr stainless revealed peaks for Cr(VI) that could be correlated with the transpassive voltammetric wave (134). Near-edge EXAFS spectra demonstrated that disulfide bond scission occurred upon electroreduction of a sulfur polymer (135). In situ X-ray methods have monitored surface roughness of Pt( 111) and Au( 100) electrodes (136,137) and the formation of oxides on dispersed Pt/C fuel cell electrodes (138).
Spectroelectrochemistry in the UV/visible region of the spectrum is routinely practiced in the characterization of inorganic, organic, and biological redox couples. On the theoretical side, Wei et al. have published several papers on spectroelectrochemistry under “long-path-length” conditions, i.e., with the light beam parallel to the working electrode (139-143). One of the papers contains theoretical expressions for derivative linear sweep and derivative cyclic voltabsorptometry, e.g., expressions for d(ABS)/d^]Pk, under thin-layer conditions (140). The case of semiinfinite linear diffusion was also addressed. The catalytic EC' mechanism has also been treated under these conditions (144,145). Zamponi et al. have presented derivative linear sweep voltabsorptometry 408R
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theory for surface waves and OTTLE cells (146). The relationship between the d(ABS)/dt vs E curve and the corresponding voltammetric parameter is one of equivalence in most situations. A spectroelectrochemical sensor for Cl2 based on a planar optical waveguide was described (147). This novel device employed a thin Lu-biphthalocyanine film on ITO that could be electrochemically reset to the reduced state. Oxidation of the film by dissolved chlorine, which was monitored at 950 nm, resulted in an integral signal that was linear in the 0-30 ppm range. The transmittance changes were detected using a transverse magnetically polarized evanescent wave. Wavelength modulation spectroscopy was used to obtain spectra of methylene blue and Co tetrasulfonated phthalocyanine couples on graphite electrodes (148). The instrument described had a resolution of ca. 0.002 absorbance unit. Several papers on experimental aspects were noted. A simple procedure was given for the synthesis of Sn02 and the preparation of Sn02-coated ITO glass electrodes (149). An optically transparent carbon film electrode, with electrochemical properties similar to glassy carbon, was prepared by the pyrolysis of an aromatic anhydride on a quartz substrate (150). The general method of modifying ITO electrodes of Chen et al. deserves mention again (F88). Salbeck has published two standard designs for thin-layer cells, in one of which only Teflon components contact the solution (151,152). Shimazu et al. performed simultaneous UV/visible spectroelectrochemistry and QCM (153). Optically transparent contacts to the quartz crystal were used in a transmission mode configuration. Fluorescence spectroelectrochemistry was shown to be a sensitive method for the detection of intermediates and products of electrode reactions (154,155). These authors gave details of their flow cell, which was used with a commercial luminescence spectrometer. Littig and Nieman also obtained excellent sensitivity with an electrochemical FIA chemiluminescence method (156). Electrochemical reduction of 02 to H202 triggered the chemiluminescence of acridinium esters in a flow cell giving a LOD in the 10-fmol range. The sophisticated fluorescence imaging of electrode surfaces cited above can also be mentioned here (E115, E295). Two simple cell designs for luminescence spectroelectrochemistry have appeared (157, 158). Articles on ECL that were noted included a report that ultrasonic radiation markedly enhanced the ECL intensity in the Ru(bpy)32+/oxalate system (159) and the observation of the ECL of perylene in a room-temperature molten salt (160). A weak photoemission seen during the evolution of 02 at Pt in water was assigned to the recombination of singlet oxygen molecules (161). FT-IR spectroelectrochemical studies on adsorbed CO continue to give detailed information on the interfacial structure and electrode reactions. The spectra of Roth and Weaver indicated a terminal coordination of CO over a wide potential range at Pt/nonaqueous interfaces (162). Bands in the ATR-IR spectra of CO on Pt were assigned to a linearly bonded CO and possibly a multiply bonded species (163). FT-IR spectra of CO on Ni electrodes in KOH(aq) indicated oxidation via bridge-bonded CO to generate carbonate ions (164). Cation effects on the IR spectra of CO adsorbed on
Pt were interpreted in terms of an electrochemical Stark effect in which the cation altered the position of the outer Helmholtz plane (165). Quantum mechanical Xa calculations, which assumed a Pt4 cluster as a model for the electrode surface, were used to calculate the potential dependence of uco at Pt electrodes (166).
CO coverages on Pt were obtained from FT-IR absorbance values after oxidation of CO to CO2 in a thin-layer cell (E264). Differences
were
noted between the in situ and the ex situ
IR reflection absorption spectra of HSO4- adsorbed
on
Pt(lll) (167). The in situ spectra indicated adsorption at positive potentials and a potential dependence of Xmax. Adsorption of CH3CN on gold was followed by subtractively normalized FT-IR spectroscopy which gave a picture of the double layer containing two types of CH3CN and H2O in the interfacial region (168). In situ FT-IR spectroelectrochemistry was performed on cobalt electrodes in NaOH(aq) (169), Si single crystal wafer electrodes (170), and Ru electrodes in aqueous acid and alkaline solutions (171). In several instances, in situ infrared spectroscopy has given information on the orientations of molecules at electrode surfaces. For example, anthraquinonedisulfonates adopted a flat orientation initially and then a more perpendicular configuration as the adsorption proceeded (172). Polarization modulation FT-IR spectra of thick phenazine and phenothiazine films indicated that most of the molecules were oriented either perpendicular (in one case) or parallel (in two cases) to the electrode surface (173). In situ reflectance IR spectra indicated that the very narrow CV wave seen for heptylviologen on Hg was due to a faradaic reaction (174). While solution spectroelectrochemical studies have been generally omitted from this survey, attention will be called to the extensive set of data, including IR band assignments, for nine p-quinone molecules in five solvents (175). On the experimental side, a three-electrode IR optically transparent thin-layer electrochemical cell was detailed that allowed minimal diffusion of O2 into the cell (176). Electrodes used in ATR spectroelectrochemical cells included a gold minigrid placed on the surface of a ZnSe element (177) and a BaF2 crystal coated with a 30-nm Au layer (178). A detailed description of the problems that arise in the use of Ge or GaAs crystals for ATR spectroelectrochemistry has appeared (179).
FT-IR external reflection spectroelectrochemistry has been carried out using a step-scanning, phase-modulated spectrometer and controlled-potential electrochemical modulation of the signal (180). Since Fourier frequencies due to movement of the interferometer mirror are reduced to zero in stepscanning spectrometers, cross-talk between the Fourier frequencies and modulation of the electrode potential is minimized. The feasibility of the technique was established for the surface oxidation of CO on Pt. For spectra obtained with more conventional instruments, simple trapezoidal integration of the EMIRS spectra led to improved spectra and more convincing peak assignments (181,182). In situ spectroelectrochemistry was performed using synchrotron radiation in the far-IR region, which is 1001000 times brighter than conventional black body radiation (183,184). The decomposition of CICV in an acid electrolyte
indicated by the appearance of bands due to adsorbed chloride. Real-time surface-enhanced Raman spectroscopy (SERS) of the electrooxidation of Pt, Rh, Ru, and Au surfaces was performed using a charge-coupled device detector. Raman bands in the 250-850-cmr1 region were assigned to metaloxygen vibrations; M-0 and M-OH vibrations were distinguished by the use of D2O solvent (185). SERS spectra acquired during the oxidation of CO at Au, Pt, and Rh films on gold substrates detailed the interrelations between CO and metal surface oxidation processes (186). In situ SERS of adsorbed oxygen on Ag was performed under a wide range of conditions on various supports, including Y203-stabilized Zr02 (187). SERS spectra were reported for oxide films at Ti and copper electrodes (188,189). Pemberton and co-workers have continued their studies of the orientation of adsorbed alcohol molecules at silver and gold electrodes (190-194). They deduced the orientations, which were generally potential dependent, from the relative intensities of the symmetric and the asymmetric C-H vibrations of the methyl and methylene groups in the adsorbates. Interestingly, their spectra indicated that the solvent structure and orientation were maintained upon emersion from butanol solvents (192). SERS of adsorbed pyridine and related molecules continues to be a popular topic. Often perpendicular, or nearly perpendicular, orientations are reported (195-197), although SERS spectra of indole on roughened Ag were interpreted in terms of a parallel orientation (198). Articles appeared on SERS of pyridine adsorption on Cu and Ag (199), the effect of Pb UPD on pyridine adsorption on Cu electrodes (1100), and 4-mercaptopyridine adsorption at mechanically polished polycrystalline Pt (1101). A SERS study of coadsorbed nicotinic acid and 3-acetylpyridine on Ag featured detailed band assignments (1102). The intensity of SERS spectra of pyridine on Ag, as activated in the usual fashion by redox cycling, was found to be related to the magnitude of the cathodic charge applied in the activation and was roughly independent of the anion (1103). In situ SERS has been used to good effect to identify intermediates and products of electrode reactions. Systems studied recently include the oxidation of diphenylamine in CH3CN (II04), the oxidation of o-aminophenol (1105,1106), the surface redox chemistry of p-mercaptoaniline (1107), the oxidation of adsorbed sulfur on gold (II08), and the oxidation of pyrite in neutral solutions (1109). SERS spectra of the 3N,+ cation radical in CH3CN were obtained in a flow cell without interference from dimeric products (1110). SERS of organic sulfides was carried out at a rotating silver electrode in order to eliminate experimental artifacts due to photoreactions (1111). Other SERS electrochemical studies included electrodeposition of Ag from a cyanide bath (1112), the Cu/CuSCN electrode (1113), and Ni electrodes in the presence of electrodeposited Ag (1114). Time-resolved SERS was impressively performed in a pulse mode at low power in order to detect intermediates of electrode reactions on the nanosecond time scale (II15). Time-resolved SERS spectra for the reduction of heptylviologen on Ag indicated the existence of nucleation phenomena at short times was
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In situ Raman spectra of Zn-phthalocyanine films on Au and glassy carbon electrodes were obtained with a confocal spectrometer. A significant aspect of this study was that the irradiated spot on the electrode surface had a diameter of less than 1 /urn (1117). Simonet and co-workers have used spin traps to detect radical intermediates in the electrochemical reduction of several species (1118,1119). For the reduction of (C6H5)4P+ in nonaqueous solvents, it appeared that the CeHs" radical was not an intermediate. ESR was also used to follow the intercalation of lithium into V2O5 cathodes (II20). The time frame accessible in the ESR-electrochemistry experiment was ca. s in the study of Dunsch and Petr (1121). 13C NMR spectra were obtained for l3C-enriched CO on Pt-black surfaces under potential control (1122). Line narrowing and chemical shifts were seen associated with changes in the CO bonding at the surface. Another original spectroelectrochemical study was the in situ determination of atomic magnetic susceptibility using a nonspinning cell that operated in the bore of a 400-MHz NMR spectrometer (II23). The test system for this study was a 0.1 M FefCN)^3-/4- couple in D2O solution. Several applications of ellipsometry to electrochemistry have been described by Hamnett (1124). Ellipsometric transients were followed during adsorption of thiols on gold, growth of metal oxide films, and growth and switching of polymer films. The time scale was relatively slow, on the order of seconds, but the author predicted advances in instrumentation that would allow measurements in the millisecond range, as well as spatial resolution of ca. 10-4 cm2. Chao et al. measured effective dielectric constants for the electrode/solution interface at single crystal electrodes using an ellipsometry method (1125). Electrochemical quartz microbalance methodology has proved to be useful for the study of a variety of interfacial processes as evidenced by the many applications to surface electrochemistry and polymer film electrodes cited above. Several recent papers have addressed experimental artifacts that can arise with this technique. The problem of nonuniform mass sensitivity across a QCM electrode surface was treated authoritatively by Hillier and Ward (1126). Bacskai et al. have also considered the QCM response for uneven coatings of polymer films (II27). The effect of surface microstructure on the QCM response was analyzed, and the analysis applied to roughened Ag/AgCl surfaces (1128). Frequency shifts on the order of a few hertz (
