Anal. Chem. 1992. 64. 7913-116R (434) Kostennlkova, 2. P.; Chickova, I . V. Farmatslya 1888, 38, 35-39; Chem. Abstr. 1888, 7 7 7 , 219361n. (435) Singh, D. K.; Mishra, A. Himalayan Chem. Pharm. Bull. 1880, 7 , 11-13. (436) Salinas, F.; &res-Nevado, J. J.; Espinosa-Mansiila. A. An. Ouim. 1880, 86, 302-304; Chem. Absfr. 1881, 774, 220457s. (437) Lee, A. W. M.; Chan, W. H.; Ho, M. F. Anal. Chim. Acta 1981, 248, 443-445. (438) MedinaHernandez, M. J.; VlllanuevaCamanas, R. M.; Garcia-AlvarezCoqw, M. C. Mlcrochem. J. 1880, 42, 286-293. (439) Conflno, M.; Bontchev, P. Mlkrochim. Acta 1880, 3 , 305-309. (440) Zakhari, N. A. Anel. Left. 1990, 23,1843-1856. (441) htyanarayana, T.; Mushlni, R.; Anipindi, N. R. Indian J. Chem., Sect. A 1980, 29A, 936-937. (442) &to, H.; Ito, H.; Kobi, M. Jpn. Kokal Tokkyo Koho JP 63,305,000 [86,305,000] (Cl. Cl2Ql/40), 13 Dec 1986, Appl. 67/139,352, 03 Jun 1987; 7 pp; Chem. Abstr. 1880, 772, 512671. (443) Santegati, N. A,; Vlilarl, A.; Spadaro, A.; Puglisl, G. Int. J. Pharm. 1880, 65, 137-140. (444) Suh. J. H.; Kam, S. K. Yakhak Hoechi 1880. 3 4 , 22-23; Chem. Abstr. 1880, 773, 1981106. (445) Yars. U. Mermera Univ. Eczacilik L k g . 1888. 4 , 89-96. (446) hstry. C. S. P.; Suryanarayana, M. V.; Tiplrneni, A. S. R. P.; Sailaja, A. fast. Pharm. 1888, 32, 129-130. (447) U,S.; Berger, J.; Hartland, S. Anal. Chim. Acta 1880,232.409-412. (446) hpeda, A.; Paseiro, P.; Simal, J.; Rodriguez, J. L. Alimentarla 1880, 277, 23-27; Chem. Abstr. 1880, 773, 130804~. (449) NaMu, D. V.; NaMu, P. R. Talanta 1880. 37, 629-631. (450) Abdaila, M. A. Anal. Left.1981. 24, 55-67. (451) krnai, J. L.; Nozal, M. J.; Mega, E.; Vazques, D.; Beban, L. Rev. Awoauim. Tecnol. Aliment. 1888, 2 9 , 556-559: Chem. Abstr. 1890. 772, '153585a. (452) Bhowal, S. K.; Des, T. K. Anal. Left. 1891. 24, 25-37. (453) ksada, A.; Tadrox, N. 6.; Gawargious, Y. A. Mlkrochim. Acta 1888, 8. 143-148. (454) E l - k " s , M. E.; Emara, K. M. Bull. Pharm. Sci., Assiot Univ. 1888, 7 7 , 141-153. (455) Hussein, S. A.; Mohamed, A. M. I.; Hassan, H. Y. Talanta 1889, 36, 1147-1 149. (456) Latawkc, A. P. Analyst 1881, 776, 749-750. (457) Sastry, C. s. P.; Tlpirnenl, A. S. R. P.; Suryanarayana. M. V. Indian J. PhafITl. SCi. 1888, 57, 146-148. (456) Pal'chevskaya. T. A.; Bogutskaya, L. V. Zh. Anal. Khim. 1880, 45, 1218-1221; Chem. Abstr. 1880, 773, 204231th (459) Stuzka, V. Cesk. Farm. 1888, 38, 446-449; Chem. Abstr. 1880, 772, 1859406. (460) Karkiina, V.; Birska, I.; Veveris, A.; Duievska. I . Zh. Anal. Khim. 1880. 45. 2460-2464; Chem. Ab&. 1881, 775, 213179. (461) Herzog, W. Polimry 1888, 34, 311-312; Chem. Abstr. 1891, 774, 351891.
(462) Eross-Kiss, K.; Hanak-Feher, G.; Baiogh, S. Acta Chim. Hung. 1880, 727, 519-524. (463) Servat, F.; Montet, D.; Pina, M.; Grailie, J.; Marcou, L. Rev. Fr. Corps &as 1888, 36, 217-220; Chem. Abstr. 1880, 772, 97129r. (464) Chughtai, A. 6.; Zaka, S.; Saeed, M. A. Pak. J. Sci. Ind. Res. 1891, 34, 42-44. (465) Chen. X. Zhongguo Yaoxue Zazhi 1880. 25, 543-544; Chem. Abstr. 1881, 774, 129267~. (466) Figenschou, D. L.; Marais, J. P. Anal. Biochem. 1881, 795, 308-312. (467) Cacho, J.; Ferreira, V. Analyst 1880, 775, 857-661. (466) Emara, K. M.; ECKommos, M. E. Alexandria J . Pharm. Sci. 1880, 4 , 69-72. (469) Piiipenko, A. T.; Kulichenko. N. G. Izu. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnoi. 1881, 3 4 , 52-56; Chem. Abstr. 1881, 775, 166739r. (470) Onur, F.; Acar, N. Gad Univ. Eczacllik Fak. Derg. 1889, 6 , 23-30. (471) Morgii. F. I.; Kestek. A. J. Fac. Pharm. Gad Univ. 1880, 7 , 33-42. (472) Zhang, W. Shanghai Huanjing Kexue 1880, 9 , 30-31; Chem. Abstr. 1881, 774, 192095b. (473) Mikheeva. Y. R.; Bernshtein, L. A.; Neiman, E. Y. Zh. Anal. Khim. 1981, 46, 351-354; Chem. Abstr. 1881, 774, 214031g. (474) ECDin, M. S.; ECBrashy, A. Spectrosc. Lett. 1880, 23, 899-909. (475) Yang, Q.; Wei, H.; Yu, R. Zhongguo Ya&e Daxue Xuebao 1880, 21, 234-235; Chem. Abstr. 1881, 774, 109061~. (476) Guzev, K. S.; Gretskii, V. M. Farmatslya 1990, 3 9 , 66-67; Chem. Abstr. 1880, 773, 158777~. (477) Petrusevska-Tozi, L.; Stanimirovic, D.; Stankovic, 8.; Ristev, T. Hrana Ishrana 1888, 2 9 , 165-169; Chem. Abstr. 1880, 713, 130833m. (478) Liu, N.; Lln, G.; Zhang, Q. Zhongguo Yiyuan Yaoxue Zazbi 1891, 7 7 , 82-83; Chem. Abstr. 1881. 775, 994432. (479) Salinas, F.; Espinosa-Mansiila. A.; Berzas-Nevado, J. J. Anal. Chim. Acta 1880, 233, 289-294. (480) Ai-Abachi, M. Q.; Ahmad, A. K.; Flayeh, K. A. Iraqi J. Scl. 1880, 37, 265-272. (481) Shukia, I. C. Nafl. Aced. Sci. Lett. 1888, 72, 287-288. (482) Bonllla-Simon, M. M.; De Ehrlra-Cozar, A. A.; Polo-Diez, L. M. Analyst 1880, 775,337-339. (483) Zhebentyaev, A. L.; Taiut, I. E. Gig. Sanit. 1889, ( 5 ) ,56-57; Chem. Abstr. 1880, 772, 30085e. (464) Saha, U.; Sen, A. K.; Das, T. K.; Bhowal, S. K. Talanta 1880, 3 7 , 1193-1196. (485) Bedalr. M. M.; Alexandria J . Pharm. Sci. 1881, 5 , 64-67. (466) Artacho-Martin-Lagos, R.; Olea-Serrano, M. F. Ars fharm. 1888, 2 9 , 123-127; Chem. Abstr. 1880, 773, 243980a. (487) Katayama, A.; Kamidate, T.; Morita, M.; Watanabe, H. Anal. Sd. 1881, 7, 633-636. (486) Dahot, M. U.; Memon, M. A.; Memon, M. A. Pak. J. Pharm. Sci. 1880, 3, 53-59. (489) Xia, Z. Lab. Del0 1880, (2), 44-46; Chem. Abstr. 1880, 773, 168455~.
Dynamic Electrochemistry: Methodology and Application Michael D. Ryan Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
James Q.Chambers* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
INTRODUCTION
inor anic, organic, and organometallic couples, spectroelectrociemistry, and photoelectrochemistry. The subject is of course quite broad, the divisions overlap. It is perhaps easier to indicate topics not covered i s detail. Applications where there is no net current flow, e.g. potentiometric sensors, have traditionally been covered elsewhere in this review issue. Ion transport across liquidlliquid interfaces has not been covered explicit1 in this review although this is an im ortant and rapidly &vel0 ing area of electrochemistry. For tge most part articles that &al with exotic electrode materials and media where the principal subject is not electroanalytical in nature were excluded from this review. Industrial electrochemistry and fuel cell or battery applications of electrochemistry were also omitted from the coverage. The literature cited below was selected by scanning the Citation Index, CA Selects: Electrochemical Reactions, CA
This article reviews the literature of electroanalytical chemistry between the period of January 1990 and the end of November 1991. Althou h there has been a change in the authorshi of this review, &e format and topics covered remain sim& to the previous review (AO). It can be noted that one of the authors (M.D.R.) has provided some stability to the coverage of literature in this review for the entire decade of the 80s and this marks his sixth coauthorship of the ic Electrochemistry" review in Analytical Chemistry. he focus of this review is on fundamental advances and practical applications of electrochemistry that pertain to electroanalytical chemistry. Topics covered include microelectrodes, analytical voltammetry, heterogeneous and homogeneous electrode kinetics, surface electrode phenomena, modified electrodes, bioelectrochemistry, characterization of 0003-2700/92/0364-79R$10.00/0
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Selects: Analytical Electrochemistry, and our personal reading of the literature. The coverage is not exhaustive but is intended to highlight important developments and activity.
A. BOOKS AND REVIEWS The 200th anniversary of the birth of Michael Faraday has brought forth several brief biographies and accounts of his monumental contributions to science and philosophy. Am0 these, the brief account by Thomas is high1 recommend3 ( A I ) . Also in celebration of Faraday’s bird, the Beckman Center for the History of Chemistry distributed a wall chart that lists breakthrough developments in electrochemistry from Galvani’s experiments in 1791 on “animal electricity”to work of Bednorz and Muller on high T , superconductors reported in 1986. Heyrovsky’s original experiments on the droppingmercury electrode and their motivation have been described in some detail b Koryta (A2).Along these lines, a personal account of the geginnings of the International Society for Electrochemistry and Electrochimica Acta has appeared (A3). Pletcher has written an introductory text on electrochemistry suitable for the advanced undergraduate or beginnin graduate student level (A4). He has also coauthored a second edition of Industrial Electrochemistry (A5) which contains chapters on the chlor-alkali industry, electrowinning and electroplating, industrial organic electrosynthesis (mainly adiponitrile production), corrosion, batteries, and fuel cells, among others. Applied electrosynthesis is covered further in a comDendium of chaDters written by authorities with backgrounas in industry (A6). A readable tutorial review on Dulsed LCEC detection at Pt and Au electrodes has appearedA(A7). The versatility of this detection method, which was developed in large part by Johnson and co-workers a t Iowa State, is truly impressive. Wang summarized and divided the use of chemically modified electrodes in LCEC detectors into four areas: permselective coatings, electrocatalytic modifiers, bioactive molecules, and detectors for nonelectroactive ions (A8). The technological raison d’etra for the recent activity in the development of electrochemical sensors has been presented by Buck (A9). The beautiful review of chemical sensors and biosensors by Cammann et al. is highly recommended (A10). These authors follow the definitions of an IUPAC Commission in their presentation, namely that “chemical sensors are miniaturized transducers that selectively and reversibly respond to chemical compounds and ions and yield electrical signals which depend on concentration. Biosensors are then a sub-group of chemical sensors where biological recognition mechanisms or principles are used as a means of recognizing species”. In these contexts, the authors describe the principles and applications of the usual electrochemical sensors as well as fiber-optic sensors, mass-sensitive transducers, enzyme electrodes, biochips, immunosensors, and others. Historical aspects and recent advances in piezoelectric immunosensors were reviewed recently ( A l l ) ,and 25 papers have appeared from an American Chemical Societ symposium on chemical sensors and microinstrumentation &12). This subject is also covered in a brief review of miniaturized electrochemical biosensors (A13). Several exceptional reviews of the area of conducting olymers have a peared. One of the most lucid is that of heinze (A14),wfo has also written a brief overview of the electrochemistry of conducting polymers with emphasis on their electrosynthesis and char e storage properties (A15). Two authoritative reviews of pofyaniline have also appeared (A16, A17). The former review by Heinze is in the fourth volume of the Topics in Current Chemistry series devoted to electrochemistry. Also in this volume, Men does modified electrodes with applications stressed (AM). This too is a rather critical review in a field that has some age on it. The third chapter in this volume describes recent contributions of the Kolbe electrolysis to organic synthesis. This review is mechanism oriented with lots of examples of Kolbe, “non-Kolbe”, and “pseudo-Kolbe”, “photo-Kolbe”, and related electrode reactions. The mechanism of the Kolbe reaction has also been reviewed by Vassiliev and Grinberg (A19). Conducting polymers are also thoroughly convered in a monograph by Przyluski (A20) and a chapter by Evans in Advances in Electrochemical Science and Engineering (MI). Electrochemical aspects of conducting polymers are dealt with 80R
ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
in a large number of papers presented at conferences on synthetic metals held in September 1989 in Poland and in September 1990 in Tubingen and published in Synthetic Metals (A22, A23). As is usually the case with publications of this sort, you had to be there to get the most out of the papers. The synthesis and properties of organic superconductors based on or anic electron-donor molecules and electron-acceptor me$ or anic complexes (A24)and conducting oxide electrodes (A257 were covered in two reviews. In addition to the above chapter on conducting polymers, the latest volume of the Advances series (A21) contains chapters on photoelectron spectroscopy of practical electrode materials by Kotz, methanol oxidation b Iwasita-Vielstich, and rotating hemispherical electrode y Der-Tau Chin, electrochemical separation of gases by Winnick, and electrochemical aspects of thin-film storage media by Brusic, Horkans, and Barclay. This volume continues the fine tradition of the earlier series, Advances in Electrochemistry and Electrochemical Engineering. An entire issue of Chemical Reviews was devoted to electrochemistry in 1990 (A26-AX). In thia issue Janata reviewed potentiometric microsensors, Coetzee et al. nonaqueous applications of potentiometric ion sensors, and Sharpe et al. X-ray absorption spectroscopy from a spectroelectrochemical viewpoint. Andrieux et al. described methods to determine rate constants of fast chemical reactions coupled to electron-transfer steps, and Evans gave an analysis of solution electron-transfer reactions in organic and organometallic chemistry: the EE, ECE, ECEC schemes, etc. Peter reviewed the dynamic aspects of semiconductor photoelectrochemistry. The surface coordination chemistry of ligands on solid electrode surfaces was elegantly treated by Soriaga. Dryhurst illustrated the bioelectrochemistry electrochemlstry interface with an account of the oxidation c emistry of central nervous system indoles. A monograph related to the latter topic has appeared (A37). Parsons described recent developments in our understanding of the electrical double layer. Couper et al. surveyed electrode materials for electrosynthetic applications. This article, which has an applied bent, is useful becaw the topic is not often reviewed in the mainstream literature. Finally, Rolison reviewed zeolite-modified electrodes. This topic was also covered in a 1991 review (A38). Several chapters on s ectroscopic techniques for the characterization of electro8elsolution interfaces can be found in two recent volumes (A39, A40). Topics covered in the f i i t include infrared methods, photoacoustic spectroscopy, Raman spectroscopy, UV-visible reflectance spectroscopy, secondharmonic in situ methods, X-ray methods, photoelectron spectroscopy, Rutherford back-scattering,and ex situ methods involving transfer of the electrode into a vacuum system. In the second volume, which has more of a North American set of authors, there are chapters on X-ray methods, surface X-ray scattering, the use of X-ray standing waves, measurement of surface forces, SERS, nonlinear optical methods, infrared spectroelectrochemistry,Mossbauer spectroscopy, radioactive labelin methods, and the QCM technique. As documented below &ere is currently considerable activity in several of these a r m . One of these topics, o tical second-harmonic generation, especially as applied to soEd electrode interfaces, is nicely reviewed by Corn (A41). This technique can monitor chemisorption at interfaces and provide information on molecular orientation and aver e surface symmetry of adsorbates. This topic is also coveredyy Richmond in a chapter in the latest volume of the Bard series,Electroanalytical Chemistry (A42). 0 tical methods for the study of electrode reactions are em iasized in an up-to-date collection of chapters that is tecgnique oriented (A43). Topics covered include in situ infrared and Raman methods, ex situ electrode-transfer techniques, ESR-electrochemical methods, photocurrent spectroscopy, electroreflectance, and ellipsometry. Volume 20 of the Modern Aspects of Electrochemistry series (A44) also contains timely reviewe on theoretical aspecta of the metal solution interface, the theory of charge transfer, faradaic rectification, spectroelectrochemical X-ray methods, C02 reduction, and the electrochemistry of aluminum in aqueous solutions. The use of X-rays for the study of electrode surfacea has been reviewed in thorough fashion (A45). Horanyi has authored or coauthored two general descriptions of his radiotracer method for the study of sorption phenomena at
g
h
DYNAMIC ELECTROCHEMISTRY Mkhael D. Ryan is Associate Professor of Chemistry at Marquette University. In 1969 he received his B.S. degree from the Universlty 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 indkect reduction of nttrite, nitric oxide, and sulfite reductases and the kinetics of electron transfer reactions of biological compounds.
James Q. Chambers ea& 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 biologicallyimportant compounds.
I
electrode interfaces (A46, A47). Wieckowski has covered this topic from a somewhat different angle (A48). Several collections of papers presented at various meetings can be mentioned. Subjects include microelectrodes (A49), the structure of the electrified interface (A50),fuel cells (A51), and electrochemical impedance spectroscopy (A52). The latest volume of the Bard series (A42) also contains authoritative chapters on the quartz crystal microbalance by Buttry, electrochemical mass spectroscopy by Bittins-Cattaneo, and structural effects on electron-transfer kinetics by McCreery. Several other reviews of the electrochemical applications of the quartz crystal microbalance have appeared in the last two years (A53, A54). The review by Ward and Buttry describes the principles of several modes of motion of quartz crystal detectors including the surface acoustic wave (SAW), the flexural (or Lamb) wave, and shear horizontal acoustic plate mode. Biological and electrochemical applications are reviewed in detail. A very useful summary of eleven in situ and four ex situ techniques for the measurement of real surface area has appeared (A55). For each method the principle of the method is outlined in simple terms, its limitations are discussed, and a critical evaluation is given. A review of electrochemical applications of scanning tunneling microscopy (STM), an emerging technology, has appeared (A56). The scanning electrochemical microscopy (SECM) technique invented and developed by Bard and co-workers has been recently summarized (A57, A58). Pajkossy has briefly reviewed electrochemistry at fractal surfaces (A59). The review by Shannon et al. on electrode reactions of adsorbed molecules is especially noteworthy (A60). Much of the detailed experimental evidence presented here comes from ex situ surface spectroscopy on electrodes transferred into vacuum systems. The authors outline the justification and background for this methodology. Several examples of angular distribution Auger microscopy (ADAM) where adsorbates shadow electrons emitted from well-defined single-crystal electrodes are given. The various factors that interplay with surface electrode processes (surface structure, molecular orientation, solution conditions, electrode potential) are nicely illustrated in this essay. This review and the above mentioned review of Soriaga (A31) represent state of the art accounts of the characterization of electrode reactions. Hubbard covered surface electrochemistry in his Kendall award address, which also contains early ADAM images (A61). Lewis has given an important unified discussion of interfacial charge-transfer kinetics for donor/acceptor systems, heterogeneous semiconductor/metal systems, and semiconductor/liquid junctions (A62). The theoretical framework
presented allows heterogeneous electron-transfer rates a t semiconductor electrodes, dye sensitization rate constants,and steady-state current-voltage data to be correlated. Lewis has also reviewed the fundamentals of semiconductor photoelectrochemistry from a mechanistic point of view (A63). Fundamental concepts of semiconductor electrochemistry have been reviewed in an account that ties together the basics of this field in chemistry, solid-state physics, and photochemistry (A64). The material science aspects of photoelectrochemical solar cells have been emphasized in a discussion of these devices by Sharon et al. (A&). Several reviews related to these subjects on the topic of photcinduced electron-transfer processes have appeared recently in the Topics in Current Chemistry series. Among these, the article by Fox on electron transfer in arranged media caught this reviewer's eye (A66). Electrochemistry at the interface between two immiscible electrolyte solutions has been reviewed (A67). An attempt a t a comprehensive coverage of a mainstay technique of modem electrochemistry, cyclic voltammetry, has been made (A68). Rusling has reviewed his research on electrocatalysis in micellar solutions (A69),in vivo voltammetry was surveyed (A70), Beden et al. have briefly discussed the extensive literature on the electrooxidation of CO (A71), Ross has emphasized surface spectral methods in a review of alloy electrocatalysta for the direct oxidation of methanol (A72),and the kinetics of electrode processes for metal complexes at the DME have been outlined in classical fashion (A73).
B. MASS TRANSPORT B.l. Microelectrodes. There has continued to be intense activity in the area of microelectrodes. Advances have been made in theory, in fabrication methods, and in applications to classical problems and exotic media where the advantages inherent to microelectrodes have led to new data and understanding. On the theoretical front, the mass transport problem has been tackled for various electrode shapes, for the complexity of coupled chemical reactions, and in the absence of deliberately added supporting electrolytewhere electric field assisted migration is important. Oldham has continued to be a major contributor in this area. It was shown that steady-state diffusion currents at various hemis heroidal microelectrodes approximate (within &lo%) nFDCR( 2 X~ in magnitude (BI). Whisker electrodes, on the other hand, show a pronounced shape effect. The square root dependence on electrode area was also noted in Liu et al. (B2).Leddy considered the convolution integral of the thin-layer response and demonstrated that the sigmoidal responses of (i) radial diffusion to a point electrode, (ii) convective diffusion to a RDE, (iii) the integral of the ultrathin-layer voltammetric current, and (iv) the convolution of the voltammetric current for linear diffusion to a plane are mathematically equivalent (B3). In one of the more general treatments of microelectrode theory, Oldham has considered the short-time chronoamperometric response for a diffusion-controlled redox process occurring at an electrode of arbitrary shape (B4). In general, the current expression contains an "evanescent" component that decreases with time, a "prompt" component that is time independent, and "argumentative" components that increase with time. For shrouded surface regions of a given area, the generalized response is I = nFCbD[(7rDt)-1/2 +H + (terms of order t1/2,t , t3I2,..)I (1) where H is the mean curvature of the surface. Phillips and Jansons have also treated steady-state diffusion to surfaces of various curvatures (E%). This general approach was applied to several examples including bands, disks, inlays, mercury drops, parabolic dendrites, rings, spheres, strips, tetrahedra, and fmite and infiiite cylinders (R4).An earlier paper treated the steady-state diffusion to and short-time response at a hemitoroidal microelectrode (e.g. the surface one would obtain by deposition of mercury on a microring) (B6). Other have presented theoretical expression for the reversible current for an Ox/R couple under conditions of diffusion mass transport, arbitrary electrode geometry and arbitrary potential modulation (B7). This paper employed the integral method of Cope, Scott, and Tallman for the calculation of transient currents at microelectrodes (B8).The ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15. 1992
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versatility of this theory, which was verified experimentally at a Pt-microring electrode (B9),was demonstrated by comparison of caculated transient currents at ring, band, and disk electrodes (B10).It was found the ring electrode, with the largest perimeter-to-area ratio, has the largest current density at steady state and the entlest approach to steady state. Theory has also been devekoped for microband electrodes (B11, B12) and recessed microbands (B13). The approach to steady state for a microelectrode response has been addressed in several papers. Zoski has surveyed theory for the voltammetric approach to steady state for four microelectrode geometries (shrouded hemisphere, inlaid disk, inlaid ring, and shrouded hemitoroid) (B14). Denuault et al. (B15) observed that the ratio of the chronoamperometric "Cottrell" current at a microdisk divided by the steady-state current is a simple function of t-'lz (i/iB*)= [(r'/2a)/(4")''2)] +1 (2) where a is the disk radius. Thus for a microdisk electrode where the transition from "Cottrell" to steady-state behavior can be observed, diffusion coefficients can be determined without knowledge of the n value or the concentration. This method was applied to the oxidation of ferroc anide and borohydride ions. The latter is a stringent test ecause the n value is 8. Zoski et al. (B16)found both theoretically and experimentally that the time re uired to reach within E% of for a reversible the steady state is given by 109d2/(?r3E2D) couple for both hemispherical and dlsk electrodes of superficial diameter d. This equation is valid for both potentiostatic and galvanostatic modes. For an irreversible couple, the time to reach steady state is dependent on the potential current being faster on the ascending segment of the wave. he transient chronoamperometric response of a SECM (Scanning Electrochemical Microscope) was considered both in theoretical and experimental terms (BI7). When the diffusion field of the probing tip electrode is erturbed by the substrate, the response is approximately &at of a thin-layer cell. A procedure was developed on the basis of the theory that, provided the tip radius is known, allows the determination of the diffusion coefficient of an electroactive species without knowledge of the concentration and the n value of the electrode reaction. Several authors have calculated steady-state microelectrode current-voltage curves for conditions of quasireversible or irreversible heterogeneous electron transfer (B18-B22). The treatment of Oldham and Zoski appears to be quite useful. They compare three literature treatments of the roblem (B23-B25) and show that they are algebraically anlnumerically equivalent. They ive empirical formulas, with or without the assumption o f Butler-Volmer kinetics, that reproduce the I-E curves to within 0.3%. Experimental rate constants measured using microelectrodes are usually larger than those measured with electrodes of conventional size. Zhang et al. (B19),for example, found electron-transfer rates of several metalloporphyrins approximately 10 times larger at Pt microdisks with diameters of 1-25 pm than a t larger electrodes. Advances have been made in the theory and practice of microelectrodes for the study of coupled chemical reactions. The steady-state current is augmented in the classical case of an electrocatalytic scheme in which the reactant is regenerated by the follow-up reaction. This process has been studied at microelectrodes by several workers ( B 2 6 4 3 0 ) . Oldham (B26)in a very general treatment considers the CE, EC, ECE, and the EC' mechanisms at a hemispherical microelectrode. In the latter case, the variation of the steadystate limiting current with electrode radius is given by the simple expression ilim= nFCb((D/a)+ (Dk-,)l/z) (3)
i
I,
where k-l is the rate constant. As an important prelude to the development, Oldham treats the problem of codiffusion of two species (P and Q) in chemical equilibrium and derives expressions for the surface flux of each species as a function of the surface concentrations, the diffusion coefficients, and the rate constants for the interconversion of P and Q. Related to this is the chronoamperometric theory for the important situation where the electroactive species exists in two forms in solution with different diffusion coefficients. A note has 2
been published that calls attention to the role of the crossreaction involving electron exchange between the oxidized and reduced states in this case (B31). Several other applications of microelectrodes to the study of coupled chemical reactions can be cited (B32-B34). A good example is the reexamination of the oxidation mechanism of diphenylamine in acetonitrile at a 10-pm-diameter microdiik electrode (B32). At a microelectrode fast sweep rates are readily accessible, which allowed the detection of the radical cation intermediate. Digital simulations were performed to distinguish between the classical EC, product coupling, and parent-product coupling reactions. The variation of the peak width with sweep rate was found to be a sensitive diagnostic criterion. Wipf and Wightman found intermediate-sized electrodes (e.g. 100-pm disks) to be useful for the study of coupled chemical reactions when there is a trade-off between the desire to minimize ohmic drop in nonaqueous solvents and the advantage of achieving linear diffusion conditions at fast sweep rates (B35). Rates of coupled chemical reactions can be measured using the SECM methodology (B36, B37). The principle of the method involves variation of the tip-to-substrate distance such that the lifetime of the reacting species and its transient time are of the same magnitude. For an irreversible follow-u reaction, rate constants up to lo4s-l can be determined witf the present state of the art of the SECM technique. Feedback diffusion currenta have also been calculated at a dual-cylinder microelectrode as a function of the electrode radius and interelectrode gap (B38). The behavior of microelectrodes in the absence of deliberately added supporting electrolyte has been studied. For species that undergo successive electron-transfer steps (EE mechanism) unequal limiting currents have been explained in an important paper (B39). At low concentrations of the supporting electrolyte, the decreased limiting current for the second wave was attributed to the electric field induced miration of the charged species into the bulk of the solution. fn situations where the coproportionation reaction, i.e. A2+ -+ A = 2A+, occurs, this results in decreased flux of net A to the electrode surface. Simulation of the migration problem allowed estimates of the homogeneous electron transfer rates to be made. The effect of the electric double layer can be subtle upon extrapolation to infinitely dilute supporting electrolyte (B40). Under certain conditions, peak-shaped voltammograms can be seen under steady-state mass-transport conditions. The dependence of E,, for steady-state I-E curves on the supporting electrolyte kas been analyzed by Pendley et al. (&I). For a 1:l supporting electrolyte they found that the equation mi/z *(RT/F) ((C*ADA)/(~C*XYDAZ)) (4) was obeyed for the oxidation of ferrocene (Az = Fc+) and the reduction of TCNQ (Az = TCNQ-) in acetonitrile a t Pt 2.4pm-radius electrodes. (The asterisk denotes bulk concentrations.) The data analysis indicated an impurity level of 80 nM electrolyte and a diffusion coefficient of neutral ferrocene twice that of the ferrocenium ion. Several other careful emrimental studies emdovina microelectrodes in the absence ofsup orting electrolfie ihn Gganic solvents have appeared (B42-144). Pritzker (B45) and Baker et al. (B46) have solved the transport equations governing steady-state transport by diffusion and migration. The resulta apply to situations where a single char ed species reacts at the electrode and the Nernst-Planc! equation holds for all ionic species. Specific calculations were given in the latter paper for the disk and hemisphere electrodes; and the behavior in dilute supporting electrolyte solution and the extraction of kinetic data from the voltammograms were discussed. The experimental advantages and limitations of microcylinder wire electrodes in various pulse voltammetric techniques were thoroughly delineated recently (B47). The test system was ferrocene in acetonitrile using 25-pm Pt- or PtIr-wire electrodes. For normal-pulse voltammetry (NPV) the increased mass transport due to cylinder diffusion decreases the waiting time required to reatore the diffusion layer between pulaes. However, for NPV and for reverse-pulse voltammetry, natural convection, which becomes severe for thin-wire electrodes, is significant at short times. Reverse-pulse voltammetry in a "triple pulse mode" and square-wave (SWV) and
DYNAMIC ELECTROCHEMISTRY
staircase voltammetry showed excellent agreement with theory. It is noteworthy that the SWV net current response retained the shape and Ellz value of the planar electrode response even when the current was significantly enhanced by lindrical diffusion. Karpinski and Osteryoung have also s t u e d the effect of waiting times on the renewal of the diffusion layer in pulse voltammetry (B48).Pulse techniques have also been ap lied with succeas to the anodization of silver microelectrodes b49). Stri pin voltammetry has been performed with microelectroles (85H352). In the absence of added su orting were electrolyte, concentrations of Pbz+as low as 3.2 X detected by ASV at mercury films on carbon-ring electrodes (B50). Electroanalytical chemists have continued to robe exotic environments with microelectrodes. These lnclug solid-state polymer matrices where diffusion coefficients of ferrocene redox solutes as low as 1W12an2/& have been measured (B53), mixtures of butyronitrile and etb.. 1 chloride at -160 OC (E54), frozen HC104,and HzSO solutions (B55), and supercritical fluids (B56).The latter detectors were carbon fibers coated with an organic film. Analyte species, which partitioned into the film, were detected when water was added to the SCF to provide sufficient conductivity for useful voltammetry. Several exciting bioanalytical applications of microelectrodes have a peared. Carbon-fiber microelectrodes modified with demedated porphyrin layers were used for the determination of nickel in individual biological cells of dimensions approximately 1order of magnitude greater than the electrode tips (B57).The problem of tip sharpness and shape for unobtrusive implantation through a cell membrane was addressed in this paper. The carbon-ring microelectrode design of Ewin ’s group was the basis of an intracellular amperometric gfucose enzyme electrode that was used to monitor the glucose levels in large neurons of the pond snail (Bss). These electrodes had a diameter of 2 pm and a subsecond response time. Dopamine has been detected by voltammetry in the cytoplasm of a single biological cell (B59).Ndion-coated carbon-fiber electrodes have been used by Adams’ group for the detection of catecholamine neurotransmitters in brain tissue (B60).A enzyme microelectrode for the detection of catechol has been reported that was made by immobilizing horseradish eroxidase on a carbon-fiber microelectrode via a covalently ound biotin/avidine/biotin complex (B61).A more conventional 10-pm-diameter microelectrode has been used to detect catecholamines secreted from a single-cell vesicle (B62). Microelectrodes also have advantages for the precise measurement of routine electrochemical diffusion coefficients (B63). A dual microelectrode voltammetric pH sensor has been described that used a microelectrode array modified with both quinone and ferrocene moieties (B64).The surface wave for the quinone thiol self-assembledlayer exhibited the usual 60 mV pH-’ dependence, while the ferrocene wave was pH independent. Since both functionalities were confined to the same Au microelectrode, a separate reference was not required, only a counter electrode was necessary (which can be incorporated into the same assembly). There have been si ificant advances in microelectrode fabrication and relateEethodology. The most widely publicized of these was the fabrication of nanometer-size electrodes in Lewis’ laboratory (B65).‘Nanodes” and PF and Pt-Ir tip radii as small as 10-20 8, were used to obtain steady-state voltammograms for several couples. A rate constant of 220 f 120 cm s-l was obtained for the ferrocene+/Ocouple in acetonitrile and rate constants greater than this were indicated for the methylviologen2+/+and the (Cp# Co)+/O couples in acetonitrile. (cp#is a substituted c p ligand)The significance of this breakthrou h has been briefly critiqued by Heinze (B66).Micrqdisk efxtrodes with 5-nmdlmensions have been made b C a s k et al. using a STM (B69.Repeated cycling of the SGM bias voltage removes a thin TiOzsurface to expose a Pt underlayer. The SEM micrograph of one of these revealed an exceptionally defect-free microdisk. Possible sources of systematic error associated with the measurement of electron-transfer rates at ultramicroelectrodes and nanodes have been discussed (B65,B68). Cracks and cavities between the electrode edge and the insulating shroud can cause an apparent increase in the observed reversibility.
lWRb
\
The double-layer and ion-exchange properties of the glass insulation may influence the response of a very small electrode. The problem of background charging currents at ultramicroelectrodes has been addressed by Nomura et al. (B69). Procedures were ‘ven for construction of microelectrodesthat are cylindrically gelded by aluminum foil to reduce the stray capacitance contribution to the chargin current. Variation of the electrode size, in con‘unction wit% the shielding, permitted the contributions o/ both the stra capacitance and the double-layer capacity to be separated: Two rocedures for the fabrication of submicron electrodes for SE8M have been described that produce electrodes with tips which protrude slightly from an insulating plane (B70). In situ laser activation increased the Nernstian behavior of carbon-fiber microdisk electrodes (B71). Electrochemical preconditioning of Pt microelectrodes has been described (B72).Experiment and theory has been presented for voltammetry at a micro liquid/liquid interface supported on the tip of a micropipet (B73). Gold microtubules have been prepared by electrodeposition of eold into the Dores of silane-treated microDorous membr&es (B74). B.2. Flow-Through Electrochemical Detectors. Several novel flow cell detectors have been described. some of which take advantage of advances in the areas of m d i e d electrodes and microelectrodes. With a view to improved selectivity, a flow cell has been constructed in which each of four glassycarbon workin electrodes was coated with a different permselective film (j75).The coatings were based on size (cellulose acetate), charge (Nafion, polyvinylpyridine (PVP) or poly(ester of sulfonic acid)), and polarity ( hospholipid). An electrochemical GC detector was basexon a 25- or 50-pm Pt-disk working electrode coated with a thin f i i of polyethylene oxide (PEO)/LiCF3SOBcontaining ferrocene carboxy1.k acid (B76). The diffusion coefficient of the ferrocene derivative was a sensitive function of the partitioning of gas-phase eluant species into the PEO phase. The response time of this device is suitable for a GC detector if the thickness of the PEO film is comparable to that of the chromatographic partitioning phase in the GC column. A continuous gas-phase monitor for HzOz (or chlorine with slight modification) employed a P t working electrode downstream behind a 200-cm length of microporous polypropylene (B77). A polyaniline/Nafon composite flow cell electrode has been used for the detection of alkali-metaland ahheearth-metal cations (B78).An LCEC detector for carbohydrates based on a Nafion film on glassy carbon containing CuO/Cu particles had micromolar sensitivit (B79),and cationic drugs were detected at Nafion-coatedl glassy-carbon electrodes (B80). Details have been given for the simple fabrication of multiple (B81)or single (B82)carbon-fiber electrode flow cells in PVC tubes. The two-electrode flow cell of Bartels has an effective volume of approximately 1 pL (B83).A well-jet, carbon-fiber array ring/disk electrode assembly has been tested (B84)and a microelectrode, computer-interfaced flow cell described (B85). Nagy and Anderson have given details of a 1.0-cm-long optical path thin-layer spectroelectrochemical cell for simultaneous monitoring of absorbance/transmittance and faradaic current (B86). The cell consists of a tubular Pt working electrode that is pres-fit into a Z-type UV-vis HPLC detector cell. Photolytic LCEC detection of sulfur-containing amino acids, peptides, and proteins has been achieved (B87). A packed-bed flow cell containing 200-pm carbon particles was used for UV-vis and for XANES and EXAFS X-ray absorption spectroelectrochemistry (B88). The spectral compartment used in conjunction with this cell employed 0.1-mm-thick Kapton sheets as windows for the X-ray measurements. FIA and/or LCEC methods have been re orted for amino acids and peptides at Cu electrodes (B897,thiol- and disulfide-containing peptides (B90), carbohydrates at nickelflavins using dual-electrode activated lassy carbon (B91), detection (%92),and ethanol in beer using an enzyme flow cell assembly em loying membrane-bound alcohol dehydrogenase (B93).Ano8c prior electrolysis of an analyte solution at a carbon-felt electrode allowed glucose to be detected at a downstream electrode (B94).Pt, Au, Cu, Ni, Ag, and Co electrodes were evaluated and compared as am erometric flow cell detectors for carbohydrates, amino aci s, and related
B
ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
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compounds (B95).The Cu electrode provided su erior performance in terms of response range, detection imits, and stabilit The responses of glassy-carbon and TTF-TCNQpaste erectrodes were compared for the oxidation of hydroquinone, dopamine, and ascorbic acid (B96).The review of Baldwin and Thomaen is again recommended in this area (A8). The amperometric response of an electrode coated with a permselective membrane in a flow stream has been modeled for either a Gaussian or a Poisaon concentration profile (B97). The theory allowa diffusion coefficients of the analyte through the membrane to be estimated and the detector response to be o timized. Ou and Anderson have addressed the effect of eictrode geometry on the response of a flow channel electrode under conditions of negligible diffusion in the plane of flow (B98).The response of a circular solid electrode, which is commonly used in LCEC detectors, was found theoretically and ex erimentally to be 84.13% that of a solid square electrole of equivalent area (B98).The current at a microelectrode in a rectangular flow channel was computed numerically (B99).Mass transfer to a carbon-felt electrode BIOI), and the mass trans rt of has been considered (B100, Cu(I1) in flow cells with RVC cathodes was studied &02). Target factor analysis has been a plied to the analysis of electrochemical FIA data generated \y application of a large amplitude sine wave to a carbon-microfiber electrode (B103). Compton has continued to analyze the effect of coupled chemical reactions on electrochemicalmeasurements. Theoretical treatments have appeared for collection efficiency measurements for a dual-electrode configuration (B104)and for double-potential step chronoamperometry at a flow channel electrode (B85). In the latter case, the current transients permit the ECE and DISPl mechanisms to be distinguished. BJ. Array Electrodes. Magee and Osteryo have dealt with several aspects of flow cell detectors emp o ing conventional and microelectrode arrays (B105).The R C arrays employed,while giving a Cfold greater response than a circular Pt electode of equivalent area, were more susceptible to pump noise and more sensitive to dispersion of the concentration plug under FIA conditions. Several studies of composite electrode arra have appeared including that of Anderson et al. on Kel-$graphite poly(chlorotnfluoroethylene)electrodes (B106)and that of eund and Brajter-Toth on TTF-TCNQ/PVC electrodes (B107). A multichannel am erometric detector consistin of a circular array of 16 work& electrodes, which can be dikerent materials, and a multichannel potentiostat was described (B108).A detection limit of 25 pM was achieved for epiteelectrodes. Arrays nephrine in a FIA mode us' been evaluated for of 1-30 Pt microdisks s e a l 3 i ; k E e fast-scan voltammetry. These electrodes performed as N independent and noninteracting disks at both steady-state and fast sweep rate conditions (B109). Theory has been presented for the chronoamperometric current at a random ensemble of microdisk electrodes (B110). Several procedures have been refiied or developed for the fabrication of microelectrode arrays. These include laser activation of glassy-carbon or HOPG surfaces covered with the electrode osition of Pt into a thin organic film (BIII), the pores of a Nucleopore membrane (&12), the vacuum de ition of epoxy into the pores of carbonized microcellular the use of microporous Al poFacrylonitrile) foams (B113), as a template for an array of gold microdisks of 10-100-nm photolithographic techni ues for the condiameter (B114), struction of an arra of up to 40 OOO micro8isks that are recessed and insulateJfrom a surface electrode (B115), and the electrodeposition of Th-lead oxide arrays (B116). Photolithographic methods have also been used to fabricated microstep band electrodes (B117),multielement microelectrode arrays on silicon nitride/silicon substrates (B118), and arrays for the recor of neural electrical signals (B119). Pt, Au, V, Ir, and C electr e elementswere incorporated into the arrays of Glass et al. (B118)and used to obtain voltammetric data on explosives and related compounds in DMSO. An eight-line gold microarray on silicon nitride was functionalized with ferrocene-substituted thiols (B120). The increased current that arises from redox c cling was demonstrated at an interdigitated microarray ($A) in a flowing stream (B121). Here the current at the IDA was compared to that at a single potential microarray and that
P
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
at series rectangular dual electrodes. Harrington and Anderson ointed out that for IDAs operated in a collection mode, t1e approach to steady state is 4 times faster for the difference current than for either the anode or cathode current by itself (B122).The sensitivity and response time for an IDA to reach steady state has been determined experimentally ( B I B )and Licht et al. have used transient time measurements at IDAs for the direct determination of diffusion coefficients for redox species (B124).Aoki (B125)has evaluated various models of IDA electrodes operated in the collection mode as a function of the wgap/(wgap+ we,)ratio, where the w terms are the widths of the ga and the electrode fin era. He has also presented theory r ! i quasireversible an irreversible electrode reactions at IDAs (B126). The voltammetric response of a double-band and a trile-band electrode operated in the generator/collection mode !as been simulated using conformal maps to recast the diffusion problem (B127,B I B ) . Hi her collection efficiencies were realized at the triple-band t i a n at a double-band electrode with an identical gap width. The analysis was applied to the oxidation of 9,10-diphenylanthracene in the presence of pyridine in CH&N. B.4. Rotated Electrodes. Compton and co-workers have treated the problem of coupled chemical reactions for the RDE. These studies have included the EC' catalytic reaction (B129)and a comparison of the abilities of the wall-jet electrode (WJE) and the RDE to distin ish between reaction mechanisms such as the ECE, DI&, and EC2 schemes (B130-BI32).The WJE was shown to afford better resolution for these schemes and could also be applied to very slow processes. Nolan and Plambeck gave an excellent discussion of the evaluation of experimental limitin current data when an EC-catalytic scheme is operative at a 5 D E (B133).They investi ated the Fenton reaction in acid, i.e. the well-studied F e ( I 9 j k2O system, and the reaction of bi yridinium cation radicals wit! oxygen (B134).A Pt-ring&k study has appeared on the Cu(1)-catalyzedoxidation of cyanide (B135). The effect of homogeneous coupled chemical reactions at the RDE has been treated with and without electrical migration terms (B136).Texter examined the implications of unequal diffusion coefficients for the CE mechanism at a RDE (B137).Nyikos et al. have shown that the limiting current at a RDE is proportional to was where w is the rotation rate and a is related to the fractal dimension of the active surface area ( B I B ) . The RDE voltammetric behavior of several electroactive counterions in the resence of soluble polymers (e.g. rf(styrenesulfonate) a n i a polysiloxane containing attac e 4,4'-bipyridine groups) that bind the ions has been B140). The extent of association in these described (B139, polyelectrolyte systems is strongly dependent on the ionic strength. Furthermore, aa the degree of counterion binding increases, the structure of the complex will adopt a less extended, more coiled structure. Thus diffusion coefficients will not be constant as the concentration changes. The nonuniform current density at a RDE was observed b the variation of the ECL emission intensity for the dipienylanthracene/dihydropyridine system (B141).Other studies include the measurement of the current distribution at a recessed RDE (B142),a treatment of the convective diffusion maas trans rt at a vibrating plate electrode (B143), and an e x p e r i m e n s t u d y of the hangin meniscus RDE at latter electrode, rotation rates up to 104 rpm ( ~ 1 4 4 ) . which obeys the Levich equation, minimizes solution turbulence. B.5. Transport in Polymer Films. Charge and mass transport in lymer f i i is a topic that has been extensively studied by ecctrochemists in the last decade or so. Papers addressing aspects of this topic tend to fall in more than one category makin them somewhat difficult to organize in a survey rewew. hectron-transfer kinetics in redox polymers has been covered in a Faraday Discussions article from the Chapel Hill group (B145).Charge transport in polymer f i i is reviewed in more of a survey fashion below in section F. Hillman, Bruckenstein, and co-workers have analyzed the electrochemical quartz crystal microbalance experiment in elegant style (B146, B147).They derived a powerful f o r " for data treatment and analysis of the charge and mass transients for cyclic voltammetry of conductin or redox polymer-modified electrodes. The analysis is buft around a new function (a) that is a combination of the mass change
d
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DYNAMIC ELECTROCHEMISTRY
and charge injected into a film (Q) weighted by the char e/mass ratio for a particular ion, i.e. counter- or coion for d e redox ‘doping” process. (5) @ = Amass + Q(mio,/zi,,,.F) A diagnostic flow chart was given that allows the experimentalist to distinguish when global equilibrium holds and to determine the rate-limiting process in the event of kinetic control of the film doping associated with the motion of electrons, ions, or neutral species. The treatment was used to analyze the redox do ing of pol (bithio hene) films in contact with CH&N N&t)4BF4sorution. this case, the rate-limiting step invo ved transfer of onl net neutral species, solvent and salt, in o p p i t e directions. &e power of the data treatment was underscored in this example by the net mass change which superficially appeared to indicate solely countenon (BFc) transfer. Hillman et al. have also used t h QCM analysis for polyvinylferrocene, lythiophene, and poly(bithiophene) films (B147-BI49). Eansfer of a neutral species, solvent water in the latter two cases,was identifed as the slow step in the redox switc of these films. The authors noted that it seems likely that w en neutral species are re uired to move in theae and related systems, their transport be rate limiting. Fast scan cyclic voltammetry at microelectrodes indicates that charge transport for conducting polymer electrodes is very fast (B150, B151). CVs of 1 aniline in 1M H d 0 4 at 364000 V and polypyrrole in C E zN/NEt4C104at 50000 V s-l were not distorted from the slow sweep rate patterns (B150). Facci et al. have reported diffusion coefficients and hole mobilities (uo)for a charge-trans ort pol er possessing tetra hen lbenzidine moiehes (B154. The E t e i n equation, u = &/RT),was used to correlate the data within a fador of 2-4.%e mobility activation energy, however, was hi her than predicted due to differences in the physical state opthe polymer during the two types of measurement employed. A clear analysis of charge transport in a viologen redox polymer was iven by Dalton and Murray (B153). Self-exchange rates for the viol0 en+/ocouple were 20 times faster than the viologen2+/+coude and much slower than exchange between equivalent solution species. The exchange rates seemed to be dominated by polymer properties since they were not strongly influenced by the presence of a bathing acetonitrile atmosphere or the nature of the counterions. Electron hopping in a film containing immobile redox sites was described by a general equation that accounted for both diffusion and migration effects (B154). The model is only valid, however, for examples where explicit consideration of counterion migration is unnecessary. Gabrielli et al. have analyzed the ac impedance of redox polymer f i i s in some detail (B155). Theory is developed for two models: electron hopping and finite film conductivity. Experimental results were obtained for a Ru(bpy),Cl[PVP] film. (PVP is polyvin lpyridine and bpy is 2,2’-bipyridine). $he permeabilities of ultrathin (5-7-nm) films of poly(phenylene oxide) produced by the electrochemicaloxidation of henoxide anion have been studied in contact with water a n i acetonitrile solutions ( ~ 1 % ) mese . filmswere dieordered and relatively free of pinholes. The permeability could be controlled by the electropolymerization conditions; e.g. at potentials greater than the peak potential of the phenoxide anion, cross-linking of the polymer occurred. Manipulation of the ermeability of redox polymer films was also accomplishet by a clever photoelectrochemical demetalation of copolymer films of vinylpyridine and vinylbipyridine comlexea (B157). Photolysis was said to increase the permeability Ey decreasin the cross-linking and creatin molecular-size structure. The voids could%e electrochemvoids in the ically 1 ged by reduction of an Os(PR2Ph)3C1complex in CH C l .&tion. The same research group used pioto ancfmaskin techniques to roduce microstructure in 1 ms of oly(Ru(%py)a(v-pj)~2+ 63158). laveant cons1 ere t e uestion of membrane diffusion through a homo eneous film vs diffusion through small channels and pin!oles at the fii/solution interface (B159). Relationshi s were given between the parameters derived assuming t i e two models, which lead to the same formal kinetics. Solvent effects on the electron-transfer cross-reaction between a metal complex in solution and a redox polymer film
lYl
b
fi
T
electrode have been reported (B160). In addition to the usual monomeric solvents, rate constants were determined for a polymeric ether solvent where reorientational relaxation of the polymer chain segmental dipole was found to influence the frequen of the exchange reaction. Molecular rectification was observ3’for charge transport between layers of a hematoporphyrin(1X)-Ru(I1) an a flavin ilm (B161). Electron hopping was more than 100 times faster from the flavin to the porphyrin centers than in the reverse direction. B.6. Mercury Electrodes. Electrocapillar and capacitance vs potential curves for mercury electroies have continued to give information concerning adsorbed states of , vs E curves, various species. Depressions or “pits” in the C when combined with molecular surface areas, can give information on orientation of adsorbates. Accordin ly, capacitance pits have been reported for adenine (81627, several adenine derivatives (B163), adenosine (B164), adenosine dinucleotides (B165), uracil (B166, B167), 2-thiouracil (B168, B169), uridine (BI70), thymine and thymidine (B171), and camphor (B172). Nikitas has treated the thermodynamics of order-disorder transitions at electrosorbed layers (BI 73). Criteria for the occurrence of hase transitions were propoeed in the context of a mercuryJlectrolyte interface and capacitance “pita” in the Cd w E curves. Thermodynamic methods for analysis of adsorption data of or anic compounds at electrode surfaces have been reviewed (J174). A new method is proposed in which the only extrathermodynamicassumption is that the partial molecular areas of the surface components are constant during the adsorption process. Other studies of adsorption of organic compounds on mercury have included n-aliphatic alcohols (B175), several simple lycols (B176),acetamide (B177), and quinine (B178). The a&orption of thiourea, 1-butanol, and 1- entanol on dropping-mercury and drop ing-gallium electrdes has been compared (BI 79, B180). Ze&cand Lovric have reported the bromide-induced adsorption of Pb(I1) from pH 2 perchlorate solution (B181). Polarographic theory has been derived for the EE mechanism at an expanding mercury drop when the intermediate is adsorbed following a Lan uir isotherm (B182).The reduction of strongly adsorbegpecies via a totally irreversible reaction coupled to a diffusion-controlled adsorption step was nicely treated by Ribes and Osteryoung (B183). The normal-pulse polarographic technique was employed and graphical data analysis methods were given to analyze the pronounced maxima that appeared in the polarograms. The same ex erimental system was also analyzed via a model that assumefreversible adsorption of the reactant (B184). In this case, reduction of either adsorbed or diffusing material will have the same time dependence. Kinetic details of metal ion reduction at mercury have been discussed in several papers (B185, B186), including an interesting study of the pressure dependence of the Cd(II)/ Cd(Hg) couple over the range from 0.001 to 10 kbar (B187). Chronopotentiometry of 1-hydroxyanthraquinone at a HDE was found to be in good agreement with theory for a reversible surface confined redox couple (B188). Articles describing stripping voltammetric methods, which are commonly performed at thin-fib mercury electrodes,will not be cited here. The pa er of Coetzee and Ecoff on potentiometric stripping a n g s i s at both macro- and microelectrodes is worth noting however (B189). They employed thin mercury films on glass -carbon, carbon-fiber, and gold substrates and com a r e i otentiometric and voltammetric methods. For leafat mgn-fiber microelectrodes, the LODs for both methods were ca. 2 X 10-loM, while potentiometric stripping was superior for metals with sluggish electrode reactions. Two groups have described mercury microelectrodes made by electroplating 1-20-rm iridium disks with mercury (B190,B191), and another group used silver as a substrate for a Hg microelectrode (8192). The anodic dissolution of mercury in the presence of crown ligands was reported to proceed via complex formation (B193), and Hg(1) h dro en citrate was formed in the presence of citrate ion (i1947. ASV of mercury from dithiocarbamate surface has been performed (B195). Oxidation of Cu, Sn, Bi, and Zn amalgam electrodes in CH+212.generated weakly solvated and very reactive metal dicabons, mformation on the formation of BF4-and Clod-complexes was obtained (B196). ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
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Colyer et al. presented the thermodynamics of the growth of mercury droplets on an inlaid disk assumin that the Gibbs energy is minimized for individual droplets fl3197). Several growth patterns are possible, all of which are dependent on the various interfacial energies. Earlier the same group had described the growth of mercury de osits onto inlaid Pt microdisks with radii less than 2.5 pm (1198).The nucleation of Hg on glassy-carbon surfaces has also been described (Bl99). A polymer-dispersed mercury electrode was prepared by coating polypyrrole over a thin mercury film. Dispersed Hg droplets ca. 1pm in diameter were evident in the SEM photos (B200). A classical method for measurement of surface tension, the Wihelmy plate technique, has been applied to the Hg aqueous electrolyte interface dear to the hearts of old electrochemists (B201). This involved measurement of the force on an insulated Pt plate that was partially immersed in a mercury pool in contact with the solution. A method for determining the E of Hg was based on detection of changes in the curvature o g mercury drop using laser beam reflection from the surface (B202). A microelectrode DME was constructed that produces drops with radii of approximately 50 pm and drop times of 2.3 ms (B203) and a renewable Hg microelectrode with similar dimensions announced (B204). A pressurized DME was claimed to improve the chronocoulometric measurement of capacitive charge (B205). B.7. Other Electrode Systems. The procedure of determining n values and D values from the ratio of transient and steady-state measurements has been addressed by Amatore et al. (B206). They recommended the use of the chronoamperometric current, with a nD1I2dependence and the steady-state current with a nD dependence. They examined five systems and achieved a precision of f0.15 for the number of electrons per molecule. LSV at porous electrodes has been modeled assuming cylindrical pores (B207). Correlation e uations were given for the peak current and potential un%r conditions of negligible axial diffusion and/or ohmic resistance. Methodology was established for determination of the reversibility of an electrode process, the n value, the rate constant, and the cy value. Fourier transformation of voltammetric peaks has been considered (B208, B209) and theory resented for the convolution of staircase voltammograms (i210). Electrochemical nucleation and three-dimensionalgrowth has been treated by several authors in some detail (B211-B213). Zoski et al. (B214) presented a lobal kinetic analysis of cyclic voltammograms at a spheric3 electrode. The method, which does not assume a particular model of the electrode kinetics such as the Butler-Volmer equation, uses spherical convolution integral data at all potential values to obtain kinetic parameters, diffusion coefficients of reacting species, and the half-wave potential. Notable papers on numerical simulation of electrode rocesses include Feldber ’s descri tion of a fast quasiexpricit f i t e difference methof for calcPting cyclic voltammograms that seems well suited to computations on PC-level computers and for schemes with lar e homogeneous rate constants (B215). Britz and Nielsen fB216) have examined the choice of stability factors, e.g. the dimensionless diffusion coefficient in ex licit simulations, with regard to accuracy, and Kavanaug! et al. (B217) have extended an eigenvector-eigenvalue matrix algebra formulation of digital simulation to cyclic voltammetry and quasireversible heterogeneous kinetics. A fast im licit finite difference simulation procedure has been touted gy Rudolph (B218) and Speiser has given theoretical details of an orthogonal collocation simulation with a heterogeneous equivalent formulation (8219). Several papers have dealt with surface roughness as modeled by a fractal surface and its effect on electrodes processes (B220-B229). The interested reader should consult the lucid exposition of de Levie (B221, B222), who has given expressions for the effeds of diffusion and coupled homogeneous reactions on the electrode impedance at rough electrodes. The apparent transfer coefficient is simply xi times that on an equivalent flat surface where xi, which can vary between 0.5 for a porous electrode and 1.0 for a smooth electrode, is a constant that can be related to the fractal nature of the interface. A random number generation of voltammograms via Monte Carlo simulation of the diffusion problem is of interest (B224). On the 86R
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experimental side, remarkable SEM micrograph of Pt electrodes have been published which show the rough topography that developed upon application of repetitive square-wave potential waveforms (B230). Several interesting papers have appeared on electrochemistry at superconducting electrodes. Electrodeposition procedures have been given (B231-B233), and voltammetrically well-behaved su rconductor microband electrodes have been Analysis of Cu(I1) and Cu(1) fluxes at characterized a ring electrode gave the com osition and potential of a NdzCuOl superconducting disk epectrode (B235). Y-Ba-Cu-0 superconductorelectrodes dis layed a wide potential window in dry acetonitrile (B236) maexhibited photocurrents upon illumination (B237,B238). Lithium ion intercalation into this high-T, superconductor has been demonstrated (B239), and it was used as the cathode in a lithium battery (B240). The results of impedance and transient current measurements on the interface between a Y-Ba-Cu-0 superconductor and a silver ion conductor (RbAg41S)are most interesting (B241, B242). Both techniques indicated an increase in the apparent exchange current density around T N T,. The effect was interpreted to be a quantum-electrochemicalphenomenon caused by the contribution of Cooper pairs acting as correlated charge carriers crossing the electrochemical double layer. Proton conductivity has been measured in various media (B243,B244). For the Au/HC1O4.5.5Hz0 interface, possible tunneling processes were indicated at T < 80 K by an activation energy that tended toward zero. A proton-conducting membrane was used in a device for the partial gas-phase oxidation of ethylene (B245). Several diverse grou s have described the use of screenprinted electrodes, whicg lend themselves to inexpensive maas production, for voltammetric measurements (B246-B249). Gold, platinum, graphite, and TCNQ-coated graphite electrode materials were employed.
(EM).
C. ANALYTICAL VOLTAMMETRY As in the previous review, a significant fraction of recent research in analytical methods has focused on the use of adsorptive strippin methods. This method has expanded to encomgass several ferent approaches. The simplest method is the etermination of compounds that adsorbed strongly to the electrode surface at one potential and then are stripped off at a different potential. This method is used most often for organic or biological molecules. Besides this approach, adsorptive stripping also includes coprecipitation of (usually) a metal with a ligand or the use of modified electrodes that preferentially adsorb a given ion. These latter two approaches are commonly used for metal determinations. To enhance the adsorption process and to improve selectivity, many adsorption methods have used li id-modified glassy-carbon and carbon-paste electrodes (hy rophobic accumulation). Wang et al. (121)showed that hydrophobic voltammetry offered a highly useful method for imparting selectivity toward tricyclic antidepressants based on analyte polarity. &cos et al. (C2) obtained a limit of detection of 0.2 nM for the antitumor agent celiptium at carbon- aste and lipid-modified carbon-pasteelectrodes. Wang and &am used a lipid layer on a glassy-carbon electrode to enhance the sensitivity of antihypertensive drugs (C3) and ergot alkaloids (C4). Adsorptive voltammetric methods were used to investigate the binding of the proteins avidin and stre tavidin at a stationary mercury-dro electrode (C5).Chasteyet al. (C6) developed a lipid-modifief7carbon-pasteelectrode, which used phospholi ids in a carbon-paste matrix. The resulting eleca 8 lar head-grou s, which allowed interactions with trodes h positively c&ged +ugs. $he accumulation of the antitumor drug, marcellomycin, was enhanced 8-fold by this matrix. Folic acid was accumulated in a flow system at a mercury surface for 10 min and after a medium exchange, ac adsorptive stri ing voltammetry gave a limit of detection of 2 X M ~odikiedcarbon-paste electrodes were also used succeasf~lly for direct voltammetric determinations. Balha and Jung (CB) reported on the use of stearate-modified graphite-paste electrodes for the selective detective of dopamine. Lyne and ONeill (C9) found that the selectivity did not maintain itself after exposure to lipids and other surface-active agents. They emphasized the need to characterize electrochemical sensors in an environment similar to that of their intended applica-
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tions. A related study of the oxidation of ascorbic acid at lipid-treated carbon-paste electrodes showed that brain tissue removed pasting oil, which increased the electron-transferrate and shifted the ascorbic acid wave (ClO). Another approach to rovide selectivity,especially for metal ions, is to use a modifiecQelectrodethat selectively concentrates a given analyte. Bailey et al. ((211)used a carbon-fiber microelectrode modified with a conductive polymeric nickelorphyrin. The por hyrin was then demetalated, leavin a fole in the matrix, wkch exhibited selectivit for nickel. &a0 et al. used a com osite carbon-paste electrde of Nafion with 1,lO henanthro\ne to concentrate iron(I1) (C12) or 2,2'-bipyr&l to concentrate cobalt(I1) (C13). Nafion-coated thin mercury-film electrodes were also used to preconcentrate europium and ytterbium (C14). Preconcentration and voltammetric determination of silver was accomplished at TCNQ polymer film electrodes (CIS). The reduced polymer film reduced silver(I), which was held in the matrix for the strippin step. Co per and lead were determined at modified car on-paste erectrodes that contained natural ionic polysaccharides for the accumulation of the metals (C16). Modif in the carbon-paste electrode with. 2,2'-dithiodipyrid+e (817fprovided selectivity for silver, whde a modifcation w t h bis(8-quinol 1) disulfide provided a sensitive method for copper (C187. For the copper anal sis, the deposition ste involved reduction of the hulfide, w h h then complexed wi& copper. Copper was then reduced to copper metal in the stripping step. Reduction of Se(1V) in the presence of 0phenylenediamine produced an absorbable complex, which, m the stri ping step, was removed in two waves (C19). The latter (re&ction of Se(0)) was used analytically, and a limit of detection of 0.4 nM was obtained. Copper was accumulated and determined at an electropolymerized ruthenium-bipyridine film that incorporated the chromotrope 2B azo dye chelator (C20). An adsorbed layer of copper(I1)-poly-L-lysine was used to preconcentrate ferri anide at a hangin -mercury-drop electrode ( ~ 2 while 1 ~ a ry-L-histidine adsor%ed la er evels of copper (C22). ?' he was used to determine trace p" simultaneous analysis of zinc, lead, and copper in humam hair using UV irradiation for digestion and adsorptive strippin voltammetry (with catechol violet) was reported by Liu and Jiao (C23). Additional improvements in the limit of detection for adsorptive stripping voltammetry can be obtained by the addition of material which reads with the analyte catalytically. For example, a highly sensitive cathodic stripping method for titanium in seawater was developed that involyd the addition of chlorate to the solution durin the stri ping cycle (C24). As titanium was stripped off as d(III), chporate reacted mth the analyte to regenerate Ti(IV), which was reduced again. A 1order of magnitude decrease in the limit of detection was achieved. A similar method was used to the determination of cobalt using a nioxime/nitrite/ammonia buffer catalytic system (C25).A catalytic reaction was also used by Zhao et al. (C26)to develop a method for the trace determination of cobalt. In this case, the ligand was reduced in the stripping step in a four-electron transfer and oxidized in the catalytic step. In addition, the cobalt-ligand complex remained affmed to the electrode in both oxidation steps. Therefore, there was no loss of the anal e by diffusion. Coupling of the amino acids histidine an tyrosine with diazotized sulfanilic acid provided a sensitive differential pulse adsorptive stripping methods for these analytes. Fanelli et al. (C27) studied the adsorption-desorption processes related to adsorptive strip ing voltammetry usin Monte Carlo simulations. Wang ancfGrabaric (C28)appliei adsorptive stripping voltammetr for the indirect measurement of a nonelectroactive ion, Juoride, using com etitive com lex formation reactions. The method was basefon the dispracement of an electroactive ligand from zirconium by fluoride, which then adsorbed on the electrode surface. Ribes and Osteryoung (C29) determined benzodiazepine using adsorptive stripping with pulse voltammetry. In this work, the variation in the maximum pulse current as a function of surface coverage and pulse time was derived. Jin et al. used 1.5- and 2.5-order derivative adsor tion voltammetry to develop sensitive methods for cobalt (830)and vanadium (C31). While most of the focus of stripping methods have been on adsorptive methods, interesting approaches have also been explored for anodic and cathodic stripping voltammetry.
2
Wan and Taha (C32) examined the antifouling properties of pofy(ester of sulfonic acid) coatings for stripping voltammetric measurements of trace metals in the presence of organic surfactants such as gelatin, Triton X-100, albumin, or humic acid. Hepel (C33) studied the effect of the adsorption of foreign organic compounds on the stripping voltammetry of metal adatoms at solid electrodes. For irreversible metal deposition processes, the adatom deposition current or charge is sensitive to the presence of the adsorbed forei n s ecies. Trace amounts of lead and cadmium in silver tC347 were determined by anodic disaolution of milligram amounts of the sample. Most of the dissolved silver was then removed by electrol sis, using the sam le as the anode and a platinum electroie as the cathode. gurther electrolysis removed the remaining silver, and nanogram amounts of lead and cadmium were then determined by anodic stripping voltammetry. Anodic strip ing voltammetry was also used to analyze commercial tinpY,te by anodic dissolution of the sample (C35). Kopanica and Stara (C36) developed a method using a mercury film, deposited on the surface of a silica gellcarbon composite electrode, to determine amalgam-forming metals by anodic stripping voltammetry in the presence of oxygen. Trace iodine in food and biological samples was determined by cathodic stripping voltammetry, after combustion of the sample in an oxygen flask (C37). The iodine formed in the deposition step was complexed with Zephiramine, which was then strip ed off in the voltammetric scan. Determination of trace idine in table salt, laver, and eggs was demonstrated. Several reports on the theory and application of potentiostatic stri ing voltammetry have appeared recently. Jin and Wang if!%) developed the theory for derivative adsor tion potentiometry for a reversible reaction. Equations of d e derivative of time with respect to potential were developed and experimentally verified. Xie and Huber (C39)developed a method for the determination of hypochlorous acid and monochloramine using a film of copper(1). Theoretical aspecta of this problem were also derived. Labar et al. (C40)studied the effect of electrochemicalparameters on film potentiostatic stri ping analysis, with lead(I1) as the test ion and ferric ion as t\e oxidizing agent. Direct determinations of analytes using voltammetric methods paralleled the approaches used in adsorptive stri ing voltammetry. The u8e of modified electrodes has alreazy Eeen discussed. The enhanced eak current in differential pulse polarography due to an ecctrocatal ic reaction was exploited to determine vanadium (with ch orate) (C41) and molybdenum (with nitrate) (C42). Leech et al. (C43) fabricated a ruthenium dioxide modified graphiteepoxy composite electrode for the electrocatalytic detection of streptomycin and related antibiotics. The effect of adsorption in pulse voltammetric methods was examined by Osteryoung and co-workers (C44, C45). Significant differences were observed between linear scan and staircase voltammetry when adsorption was involved. For staircase voltammetry, variation of the current sampling time had a marked effect on the peak height of the adsor tion prewaves (C44). The kinetics of the totally irreversibfe reduction of adsorbed materials was also examined (C45),and applied to midamlam on mercury. The effects of mercury film morphology on square-wave voltammetric peak current res onse were ex lored by Kouvanes and co-workers ((746). &ous metho& for the deconvolution of overlapped peaks and separation of the analytical signal from the background discharge were studied. Engblom (C47) used Fourier transform methods for resolution enhancement of overla ping peaks. Pizeta et al. (C48)studied the detection and resoktion enhancement of two closely spaced electrochemical processes. Using differential pulse polarographic data, the signal was Fourier transformed and deconvoluted with a peak-shaped function. Inverse transformation gave a clear distinction between single and double processes. Palys and co-workers (C49)transformed linear scan voltammograms by the use of semidifferentiation. The undesired peaks were removed from the semiderivative curve and replaced with calculated baselines. The resulting semiderivative curve was semiintegrated back to a linear scan voltammopam that did not contain the peak that was removed. Individual electrochemical parameters from a dc polarographic wave overlapping with the discharge of the supportin electrolyte were obtained using a Gauss-Newton method
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Several interesting analytical applications of voltammetric methods have been reported over the past 2 years. Daniele et al. (C51) used a platinum mjcroelectrode as a voltammetric probe for o en directly in mdk. A microelectrode, modified with a thin?& of conducting poly(ethy1ene oxide), was used by Price and Clarke (C52) for the chemical sensing of amine antioxidants in turbine lubricants. Luong et al. (C53) BBBeeeed f i h freshness with an enzymic method using a polarographic probe to monitor the degradation of inosine monophosphate, inosine, and hypoxanthine. Voltammetric methods were applied b several groups to determine the speciation of different me in natural waters. Apte et al. (C54) used catechol as a competing ligand to determine the range of copper complexing ligands in natural waters. The analysis was carried out using adsorptive stripping voltammetry and computer simulations. Organotin s ecies in natural waters were analyzed by a combination of PLC and cathodic stripping voltammetry (C55). UV irradiation was used to destroy the organic species that would interfere. A multitechnique approach was also used by Muller and Kester (C56) to determine the lability of zinc and cadmium complexes in Narragansett Bay (RI) waters. Anodic stripp’ voltammetry at a rotating-disk electrode, with column an batch ion exchange, was used to vary the measurement time from 10 ms to 106s. A submersible robe with a flow-thro h cell was developed b Tercier anzco-workers (C57) t o r e termine man anese(I5 in anoxic lake water and trace metals (copper, lea ,cadmium, and zinc) in oxygen-saturated seawater. Various strategies for reducin the limit of detection with clic voltammetry were examine8 by Wiedemann et al. (C58). %ith a potentiostat, the measured noise was comparable to that expected for Johnson noise from the feedback resistor of the current transducer. Additional noise arose from the waveform generator and line noise. Line noise can be reduced by starting each cyclic voltammogram either in phase or 180° out of phase with the line frequency. For in vivo studies, additional noise will arise from the physiolo ical activity of the animal. Fast scan voltammetry was used%y Zimmerman and Wightman (C59) to determine simultaneouslydopamine and oxygen in vivo using a Nafion-coated 5-pm carbon-fiber electrode. Fast cyclic voltammetry was used by HafEi et al. (CSO) to measure do amine concentrations in the extracellular fluid of the animalirain. Physiological levels of dopamine precursors and metabolites did not seriously affect the voltammetric detection of dopamine in vivo.
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D. HETEROGENEOUS/HOMOGENEOUS KINETICS Many interesting reports on the fundamental processes involved in electron transfer have appeared over the part 2 years. The mechanism of the reduction of metal ions to an amalgam was investigated by Fawcett (01,021. The first report studied the differences between a two-electron transfer and an IA (ion-transfer, adsorption) mechanism. A careful estimate of double-layer effects shows that there was no clear way to distin ish these mechanisms. The second paper compared a C g a n d an IE (ion transfer to a location closer to the electrode precedes electron transfer) mechanism. These two m e d ” s can be distinguished by experiments in which the size of the electrolyte cation and, therefore, the location of the OHP are varied. An examination of solvent effects on the kinetics of amalgam formation led Fawcett and Foss (03) to conclude that the rate-determiningstep is ion transfer, not electron transfer. Furthermore, they found that this ratelimitin step is often located in the inner part of the double layer. ey also concluded that reactants are solvated mainly by solvent monomers. As a consequence,the dielectric constant near the reactant is reduced considerabl in protic solvents (04). Abbott and Rusling ( 0 5 ) also s t d e d solvent effects on electron-transfer kinetics. The solvent-dependent dielectric constant term in the Marcus expression for reor anizational energy was replaced by a measure of the so vent-solute interactions from the Taft theory. An improved correlation between predicted and theoretical rate constants for the Taft model over the dielectric continuum model was observed because specific solvent-solute interactions were
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recx:t&onship between hetero eneous and homogeneous electron-transfer rates was studied t y Weaver and co-workers. 88R
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An altemate approach to the intercomparison between these rates was described, where the heterogeneous electron-transfer rate was transformed into an “equivalent second-order”rate constant ( 0 6 ) . The rates of gas- hase metallocene electronexchange reactions were m e a s d a n d were on the same order itude as the solution values (07)The . combined role krpenetration into the metal electrode and solvent s atial correlations ( n o n l d electrostatics) upon the outer-she[ free energy of activation were explored (08). Baranski et al. (09) studied the eledrochemical oxidation of ferrocene on platinum in different solvents. The data were examined to decide whether the imaging term was important in estimating the inner-s here Gibbs activation energy. The a parent potential depenince of the entro y of activation in e L r o d e reactions was examined by Gileazi (DIU).The results show that any anomalous tem rature dependence of the Tafel plot will show u in a linear Gpendence of the intercept of In i versus 1/T p ot, as 1/T goes to zero. Unusual double-layer effects on voltammetric data at ultramicroelectrodes were examined by several workers. Norton et al. (011) studied the case where the thickness of the double layer was comparable to the radius of the electrode. Anomalous behavior may occur under some circumstances, with peaks being observed in the voltammograms rather than the expected sigmoidal shape. Amatore and Lefrou (012) found that the diffusion layer and the electrical double layer cannot be separated theoretically when the scan rate in cyclic voltammetry was in the m avolt per eecond range. As the result, the voltammograms wi be extremely dependent on their location with respect to the point of zero charge. The coupled effects of solution resistance, double-layer ca acitance, and finite eledrochemical kinetics were examined i y Safford and Weaver (013). Systematic errors in the determination of charge-transfer kinetics at microelectrodes were examined by Baranski (014). Possible sources of error may be the formation of a microcavity around the electrode, the ion-exchange behavior of the glass, and the double layer caused by ions adsorbed on the glass. An explicit Hopscotch algorithm and a conformally mapped space grid was used in the simulation to assess the error involved if resistance/capacitance effects were neglected. Baranski examined the optimization of ac voltammetric measurements at ultramicroelectrodes (015). With these optimizations, it is possible to measure rate constants up to 100 cm s-l. While fast voltammetric data can be obtained at microelectrodes due to the small time constant of the system, the small electrode size leads to very low currents. Strohben et al. (016) demonstrated the use of an array of 12.5-pm disks in cyclic voltammetry. The RC time constant for the array increased by onl a fador of 2 in going from 1to 26 disks, and the increase in o L c resistance was barely detectable at lo00 V s-l for a ten-electrode array. Several different approaches were reported for simulating slow electron transfer and homogeneous kinetics. The main focus of these studies was the development of fast, efficient, and stable algorithms. Kavana h et al. (017) described an eigenvalue-eigenvector approax to the solution of the diffusion equation and applied this ap roach to cyclic voltammetry w t h slow electron-transfer finetics. Britz and coworkers used the Saul‘yev method of digital simulation and applied this method to chronopotentiometry and quasi-reversible chronoamperometry (018). Rudolph found that a numerical integration algorithm coped very well with electrochemical rocewes involving fmt-order chemical reactions (019). In a t t e r article, a fast implicit finite difference method, which arose from the combination of the exponentially expanding space model with an implicit simulation algorithm, was described (020). Because of its unconditional stability, the algorithm can handle homogeneous and heterogeneous rate constants of any order of magnitude with potential steps of several millivolts. Feldberg (021) applied a fast +-explicit finite difference method to cyclic voltammetry. approach was extended by b r k e et al. (022) to the square scheme and used to examine the oxidation of a cyclopentadienyl tungsten com lex. Britz and Nielsen used accuracy contours in electrociemical digital simulations (023). Progress has continued on the development of the theory for a wide number of electrochemicalreaction mechanisms. Unwin and Bard ( 0 2 4 ) extended the theory for scanning electrochemical microscopy so that it could be applied to
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voltammet even for small potential ste s in the latter technique, x e n adsorbed speciea are involvefin the electrode reaction (E5).Jin et al. have derived theory for semiintegral and semiderivative LSV for the case of a reversible couple that ObeysLanTuir adsorption isotherms (E6). This group also extended t eir treatment to a totally irreversible adsorption reaction where both Ox and R are strongly adsorbed and obey Langmuir +therms (En.Kinetic t h ~ r that y does not entail the assumption of an equivalent circwt has been given for the ac i m d c e of a two-ste~faradaic reaction with an adsorbed interihediate (E8). E.1. Carbon Electrodes. The surface properties of glassy-carbon electrodes have been correlated with electrontransfer kinetics for ferri/ferrocyanide,dopamine, and ascorbic acid oxidation in a ueous media (E9). Raman spectra provided information %out the surface microstructure (Le. sp2 character of the surface carbons); capacitance and phenanthroquinone adsorption data measured true surface area. Surfaces were treated by fracturing, polishing, and laser activation. In a previous study, Rice and McCreery found a weak dependence of the surface activation on the wavelength and suggested that the laser activation mechanism was thermally driven (EIO). For glassy carbon, McCreery concluded that the active electron-transfer sites were at graphtie edges inherent in the structure and that laser activation principally o erated by removal of adsorbed impurities. The nature of gLy-carbon and HOPG electrode surfaces were also probed with SERS techniques after deposition of silver particles on the surface ( E l l ) . SEM micrographs of Cu de oaition atterns on graphite electrodes have also been pubrished ( d 2 ) . A related paper of interest is that of Jiang and Anson who (037)mechanisms. First- and second-order decomposition modeled an electrocatalytic surface by assuming a potential reactions were studied at double-channel electrodes (038). dependent Gaussian distribution of reactivities for a cataThese problem were treated numerically using the backward lyst-substrate complex on the electrode surface (E13). Eximplicit finite difference method. With the same type of perimental I-E curves for the reduction of oxygen at edgeelectrode, Unwin (039)derived a eneral resolution of the plane gra hite electrodes coated with cobalt tetraphenylECE-DISP1 problem. Nolan and %lambeck (040)used the porphine xisplayed inclined plateau currents, which were in rotatin -disk electrode for the EC' catalytic mechanism and good agreement with their calculations. ap lied the theory to the reaction of bipyridinium cations Fluorescence microscopy was used to map the carboxyl raficals with dioxygen. 'Split waves" at a rotating-disk groups on a carbon-fiber electrode. Surface images of fluorelectrode in homogeneous catalysis were studied by CompEn escein moieties that were covalently attached to the surface and reaction and S ackman (041).Two-component &ion were obtained with a CCD camera (E14).In a related method was &o studied b Texter for rotating-disk voltammetry Pharr et al. used electrochemiluminescence (ECL) from the (042). A photo-Cl$ mechanism at a channel electrode was surface to image the spatial variation of luminol oxidation used to study the oxidation of tri(p-toly1)amine in acetonitrile kinetics on various carbon electrodes (E15). (043). Several more conventional electrode pretreatment proceThe develo ment of elaborate mechanistic schemes and dures have been further researched (E16-El8). Apparent pK, their related Jeory r uires more sophisticated methodology values determined from El data can be shifted by as much in order to anal ze ezciently the ex erimental data. Palys as two K units by eledrockemical pretreatment (E17).The et al. developelan automated know edge-based system for methot! of Swain and Kuwana induced extensive fracturin the analysis of sampled dc polar0 aphic data for the eluciof 10-pm carbon-fiber microelectrodes as indicated by 10-fol! dation of electrode mechanisms 544). Speiser (045) deincrease in Cd (E16). scribed an approach to the determination of the rate and A low-temperature (600 "C) route has been described to equilibrium constants of a CE mechanism from cyclic voltdoped lassy carbon, which functioned as a stable working were able ammetric data Papanastaaiou and -workers (046) electroie material and which contained noble metals, e.g. 1 to determine the two rate constants from an ECECE mechatom % Pt (E19).Ion implantation of Li+ and K+ into glassy anism using a new iterative method. carbon decreased the electrode reversibility (E20). E.2. Scanning Tunneling Microscopy. In the last few E. SURFACE EFFECTS years several groups have employed in situ STM to characterize electrode surfaces. While some of the published images This rather eneral heading covers much of modern elechave been only partially convincing, there have been spectrochemist $he emphasis of the papers surveyed here is tacular images published that have contributed to the unmostly on %e characterization of solid electrodes and the derstanding of specific electrode reactions. It seems clear that dynamics of electrode reactions that take place on these STM will have a role to play along with other techniques such surfaces. There has been great activity in the last two years as in situ SERS and F"l'IR, and ex situ electron spectroscopies in this area, ea ially in studies detailing the role of the crystal in the characterization of electrode reactions when transforstructure of trelectrode surface and in the development of mations in the surface order occur. Since this is a developing new techniques for their study. Notable among the latter is methodolo a few papers which emphasized STM experiscanning tunneling microscopy (STM). ments will cited here in an effort to organize this general On the theoretical front, exlsting models of electrochemical section on surface effects. phase formation involvin both adsorption and nucleation/ The atomic level resolution of STM images has revealed growth have been modiked (El), a general statistical medetails of the surface electrochemistryof Pt(100) and Pt(ll1) chanical procedure that accounts for the influence of shortelectrodes for the well-studied iodine system (E21, E22). Large range interactions on the standard Gibba energy of adsorption substrate adlayers were evident in the images after treatment was roposed (E2), and two-dimensional adsor tion theory of the flame-annealed crystals with iodine vapor. Adsorption deveyoped (E3).The sweep rate dependence ofthe surface of CO in HClO,(aq) yielded smaller, potential dependent voltammetry of an adsorbed species has been anal ed by domains. Gonzalez-Velasco (E4),who showed that for y h LSrsweep The electrochemicalbehavior of CO at Rh(ll1) surfaces was rates, the peak current can be inde ndent o f t e sweep rate. revealed in detail using spectroscopy (E23). Two distinctly Stojek and Osteryoung observeEhat there are significant different types of STM images were obtained in potential differences between the responses for LSV and staircase measure chemical reaction kinetics (EC mechanism). Blauch examined the effects of interconversion and and Anson (025) electron transfer for two-component systems with differing diffusion coefficients. The theory for first-order coupled reactions in c clic staircase voltammet was developed by In this work, the gC, CE, and catalytic Murphy et (0%). mechanisms were solved, and the results were compared with cyclic voltammetry. The the0 for an ECE mechanism in square-wave voltammetry (02Vwas developed by ODea et al. Kant and Rangarajan (028used ) the Pade approach to tential transients for the EC and CE mechanisms. Difg e n t i a l pulse olarographic theory was develo ed to difClear ferentiate severapdifferent dimerization schemes (829). diagnostic experimental criteria were presented to distinguish these schemes. Canas et al. (030)used digital simulation to predict variations in the impedence foy the CE, EC, and CEC mechanisms. The theory and experimental illustration of preparative electrochemistry using redox catalysis ( related mechanisms) was studied by Alam et al. (031 Anand drieux and co-workers (032)applied homo eneous redox catalpis to bimolecular reactions and were ab6 to determine readily rate constants approaching the diffusion limit. The steady-state currents for fiist- and second-order catalytic reactions at a microelectrode were determined by Denuault et al. (033). The variation in the features of shady-state ams on electrode size was used by Oldham (034) volt" to study%, EC, ECE, and EC' catalytic mechanisms. The characterization of electrode reaction mechanisms was
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regions where CO was electroinactive, which were interpreted in terms of the CO-Rh(ll1) adlattice structure. At intermediate potentials, areas were evident in the images where both surface structures could be seen in adjacent regions. The FTIR spectra and geometrical considerations permitted binding sites for CO on the Rh(ll1) surface to be assigned. Honbo et al. (E24)have published beautiful STM images of Au(ll1) facet surfaces obtained in HC104(aq). Pits, a single atomic la er deep, were formed upon electrochemical reduction of go& oxide layers. Nichols et al. (EW)saw island growth of oxide layers under similar conditions at Au(100) surfaces. A picture album of STM images has been published that documents the reversible electrochemical roughening of Au(111)surfaces under potential control in a ueous salt solutions (2326). Monolayer films of ethanethiaate and n-octadecanethiolate have also been successfully imaged with atomic resolution on this substrate (E27). The cop er deposition process lends itself to characterization by SfTM techniques (E28). Exceptionally clear STM images of cop er deposits on Au(ll1) electrodes showed that electrocrysdzation initiated at step e es, dislocations, and defects in the substrate surface (E29). issolution occurred at positive potentials, but the last remnant of the deposit was removed only slowly, perhaps indicative of alloy formation. Copper deposition on Au(111)was also followed with atomic force microscopy (AFM) with atomic resolution (E30). The complete cycle from bare gold, through a UPD monolayer, to bulk copper was observed. The UPD monolayer had distinctly different structure in different electrolytes; e.g. in sulfate the lattice was more open than in perchlorate solution with 0.49 f 0.02 nm vs 0.29 f 0.02 nm spacings. Hachiya et al. also studied Cu on Au(ll1) and reported a ( 4 3 X 4/3)R3Oo structure for the first UPD layer in a two-step process (E31). STM images have revealed details of the dealloyin of C u d u surfaces in contact with pH 2.8 NazSo4(aq) (E327. STM and AFM images of glassy-carbon electrodes, which were obtained before and after electrochemical surface oxidation in 0.1 M KNOB,revealed the presence of carbon oxide layers (E33),guanine condensed on HOPG under potential control has been ima ed (E34),molecular detail was seen in and even STM images of Re(Ch)&v-b y)C1 on HOPG (E35), STM images of ordinary carton-paste electrodes have made it into print (E36). E.3. Studies at SingleCrystal Electrodes. The number of papers appearing weekly on electrode reactions at singlecrystal electrodes has increased considerably in the last 2 ears. A principal theme of man of these studies is the correration of electrode kinetics, mecianisms, adsorption ener ies with LGJnow the structure on the electrode side of the interface. well documented, the voltammetric si nature of many outwardly simple electrode processes is Aamatically different for different crystal faces of the same electrode material. Many recent spectroelectrochemical studies have revealed specific substrate electrode orientations at these well-defined surfaces. Studies at platinum single crystals have included oxidative adsorption of 12 aliphatic alcohols at Pt(ll1)(E37),oxidation of several terminal alkenols at Pt(ll1) where reaction patterns were a function of chain len h (E38),oxidation of simple alkenes at Pt(ll1) where oxi ation occurred at the adsorbed double bond and adjacent carbon atoms (E39),the oxidation of ethylene glycol at 12 sin le-crystal orientations of Pt (the ak was found for bt(ll1)in alkaline solution (E40), !%i%l% surface showed lower activity for ethylene glycol oxidation than Pt(100) and Pt(ll0) in HzS04(aq)),formic acid oxidation at Pt(ll1) in the presence of arsenic (E41), the dissociative adsorption of acet laldehyde (E42),the oxidation of oxalic acid on three ste pdsurface of platinum (E43),the electrooxidation of D-sorktol in acid where Pt(100) was the most active surface (E44),and the oxidation of glucose in NaOH(aq1 where a correlation between PtOH formation and the onset of oxidation was noted (E45). The oxidation of lucose was also found to be structure sensitive in HC104(aq) %y Popovic et al. (E461 who reported that hydrogen adatoms inhibited the reaction on all but Pt(100) out of 11orientations. The surface electrochemistry of pyrrole, furan, and thiophene on Pt(ll1) was explored by Gui et al. (E47). The oxidation of CO at old (E48), Pt(ll1) (E44-E5I), Ir(ll1) (E52),and Rh(100) (&3) electrodes has been studied
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by mainly voltammetric and FTIR spectroscopic methods. In the study on gold single-crystal electrodes, a marked dependence of the rate constants on the crystallographic orientation was found. The authors s eculated that the electrooxidation reaction between CO ancfHBOcoadsorbates was favored at step sites or on r o w of low-coordination surface atoms (E48). Structure dependence was found for the reduction of COz to CO on Pt singlecrystal electrodes in HC104(aq)(B4)where Pt(ll1)exhibited the lowest activity. Anion adsorption effects were invoked to explain the variations found for the reduction of oxygen at Pt single crystals in aqueous acidic electrolytes (E55). The characterization of the voltammetric pattern in the hydrogen and oxide formationlreduction regions for platinum and gold single-crystal electrodes continued to be a o ular area of research (E56E60). The minimum in Cd for i!ui!hl), -(loo), - ( l l O ) , -(210), and -(311) single-crystal electrodes in HC104(aq)was used to estimate the temperature coefficient of Epzc and to deduce the strength of the surface-water dipole interaction (43361). The a value for the proton reduction at Au(ll1) and Au(210) surfaces was found to be virtually independent of temperature, although markedly curved Tafel plots were obtained (E62). If you favor vo1ta”ogrms of the multiwave variety, check out the CV of Au(100) in 0.01 M HC104which was adorned with 13 features that are dependent on rotation rate and the presence of trace impurities (E63). Clavilier et al. have published protocols for electrochemical, thermal, and atmospheric treatments claimed to produce clean platinum single-crystal surfaces (E64), and surface reconstruction phenomena have been studied by LEED on gold(lll), 43111, -(llO), and 4210) surfaces (E&). The oxidation of gl cero1 at Au(100) and polycrystalline gold surfaces in NaOkaq) has been reported (2366). Two studies on Pt(ll1) surfaces modified with alladium have appeared (E67,EM). Adsorption of sulfate anfhydrogen sulfate anions on Rh(ll1) was studied by voltammetry and radiolabeling methods (E69)and the voltammetric reduction of perchlorate on Rh(100) reported (E70). LEED spectra nicely demonstrated the reordering of a disordered Pd(ll1) surface, which was prepared by anodization followed by reduction, upon immersion in NaI solutions (E71). Soriaga and co-workers have also given procedures for the electrochemical regeneration of a clean and well-ordered Pd(ll1) surface (E72). The key step was oxidative adsorption of iodine from an iodide solution followed by cathodic desorption, or displacement of iodine b CO and anodic desorption of CO. The adsorbed zerodent iodine promoted the anodic dissolution of Pd in HzSO (as) by a factor of at least 100 (E73). Oxidative adsorption of zerovalent sulfur took place on Pd(ll1) from pH 10 HS-(aq) solution (E74). The anodic dissolution of Pd(100) has also been studied (E75). Several studies of UPD on single-crystal electrodes have been published. These include a study of lead UPD on Pt( l l l ) , Pt(100), and Pt(ll0) (E76),Cu UPD on Pt(ll1) in the presence of various adsorbates (E77),Cu UPD on Au(ll1) (E78);As on Pt(ll1) (E79),and Pb on A (111) (E80).The anodic UPD of iodine atoms and the cat!odic strip ing of iodide ions have been studied at olycrystalline and A)u(lll) electrodes (E81). Oxidatively adPsorbed iodine was reduced at negative potentials and oxidized to iodate at considerably more positive potentials. Chloride and bromide adlayers on Ag(ll1) surfaces were ordered at positive potentials with electrosorption valencies of 1 and 0.6 for the C1- and Br- anions, respectively (E82). Partial charge transfer was also invoked to describe the adsorption of acetate ions on Ag(ll1) (E83). Potential dependent surface structure was found for two isomeric pyridinethiols adsorbed on Ag(111) from aqueous fluoride solutions (E84). dine adsorption has been studied on several silver (W, E z k d gold (E87,E88)single-crystal electrodes. Vertical adsorbate orientations around the E,, were proposed (E88). E.4. Studiee at Polycrystalline Noble Metal Electrodes. Carbon monoxide has been a popular substrate for a variety of reasons. It is a postulated (poisoning)intermediate in many anodic oxidation reactions, an environmental pollutant, and a possible feedstock for electrosyntheses, and its strong vibrational absorption bands are easily studied by in situ spectroelectrochemical methods. Several studies of the latter nature at a variety of solid electrodes have appeared
DYNAMIC ELECTROCHEMISTRY
in the last 2 years (E89-E94).The position of the CO infrared band is sensitive to the nature of the interface, i.e. the surface coverage and the dielectric constant. UV-vis potential modulated reflectance spectra of CO adsorbed on a variety of metals feature bands that have been assigned to chargetransfer transitions between the metal and adsorbed CO (Em. This phenomenon has been observed at several electrode surfaces ( E W E 1011. Several groups have probed the details of methanol oxidation at various electrodes, usually in acidic media. Electrochemically modulated infrared reflectance spectra allowed identification of bands due to methoxy radicals, formate, and formyl groups adsorbed on Pt (E102),and conditions were elucidated for formaldehyde formation in this electrode process (E103).Several groups have detected adsorbed CO and branded it as a poison in the methanol oxidation process (E104-EI08). Long-term poisoning of Pt and Pt-Ru alloy electrodes during methanol oxidation may be due to other causes, however, e.g. oxide formation for the former surface (BIOS,E110).Catalytic effects were not observed for methanol howoxidation at a PkSn alloy and a Pt(Sn) surface (E111); ever currenta up to ca. 5 mA cm-2 at 0.3 V vs SCE were obtained for the oxidation of methanol at Pt(Sn) particles in a polyaniline matrix at 298 K (E112). Other studies on simple alcohols include the oxidation of simple alcohols three butanediols on gold at H 13 (E113), and ethylene ggcol at gold surfaces modified on Pt (E114), with lead and bismuth adatoms (E115). Correlations between the catal ic activity of iridium electrodes modified with submono ayers of T1, Pb, Hg, Cd, Ag, Bi, and Cu and the atomic radii, work functions, and reversible potentials of the adsorbate metals were reported for the electrooxidation of formic acid (E116).On-line mass spectroscopy electrochemistrywas used to study the oxidation of formic acil4 and formaldehyde at Pd (E117).Several oups have observed fascinating oscillating electrocatalytic tJLvior for formate and formaldehyde anodic reactions. Systems studied included formaldehyde oxidation at Rh in alkaline media (EllB),formic acid oxidation at a rotating Pt electrode (E119),and the oxidation of formate at Pt(100) in pH 1 2 solution (E120). Bae et al. have detected CO intermediatea in the oxidation of g l u m both in NaOH(aq) @121) and in HC104(aq)(E122) Carboxylic acid and C02were among the reaction products: Anodic oxidation and hydrodesulfurization of thiophenols and related compounds on iridium and A@tl alloy electrodes have been reported by Sori a and his students (E123, E124). The oxidative adsorption %enzohydroquinone from HClO,(aq) was uantified using radioactive labelin techniques (El%) an! found to be in agreement with the c L i c a l studies of Hubbard, Soriaga, and co-workers. One-electron oxidation of the thiol head group took place upon the adsorption of monolayers of n-alkanethiols on Au and Ag electrodes (E126). On the cathodic side, the reaction of CO with adsorbed en atoms (E123and the reduction of 80, in HC104(aq) have been studied at Pt electrodes. The product distributions for the reduction of COP in KHCO,(aq) were analyzed as a function of the working electrode material (E129).In, Sn, Hg, and Pb electrodes were selective for formation of formate, Ag and Au were selective for formation of CO, and Cu had the highest electrocatalytic activity for formation of hydrocarbons, aldehydes, and alcohols. Low current efficiencies were general1 obtained in these electrolyses, although a 80% yield of C 6 has been reported for the reduction of C02 at Ni(Cd) surfaces (E130).A galvanostatic double-pulse study of the hydrogenation of allyl alcohol at Pt has appeared (E131). Several groups have published further detailed studies of the formation of hydrous oxide f i s on Pt and Au electrodes (El32-E136). Spectroscopicand related methods have been used to follow the adsorption of simple ions on solid eledrodes. Orientations of cyanide adsorbed on Pd and bisulfate on Pt were deduced from in situ FTIR spectra (E137,E138);the otential deendence of the adsorption of HS04- on Pt Zduced from h I R s ectra was explained b back-bonding adsorbateinteractions (E139). &e temperature dependence electr of the double-layer parameters for old in HC104(af s y e s t e d that perchlorate is adsorbed at (E140).T e in uced
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adsorption of Ca2+ions on Pt was documented using radiolabeled species (E141). A key role was assigned to a chlorine atom intermediate in an ac impedance study of anodic Clz formation at partially oxidized P t surfaces in NaCl/HCl solutions (E142).A useful appendix in this paper outlines the procedure used for extraction of the kinetic and capacitive parameters from the im edancedata. 8everal reports involving the effect of metal deposition on noble electrodes caught your reviewer’s eye. Adsorbed Sn(I1) chloride on Pt microelectrodes inhibits the oxidation of Sn(I1) in the bulk of the solution resultin in I-E curves that exhibit hysteresis (El#). UPD of Cd and!Te was realized on Au, Pt, and Cu substrates (E144).The stri ping of CdTe was also studied. The thin-layer deposition of 8dTe on a gold electrode was claimed as the first example of “electrochemical atomic layer epitaxy” (E145).Bartels and Salie studied the UPD of Hg(I),.Tl(I), Bi(II), and Cu(I1) on gold Using a rotatingrmg-disk electrode (E146).The growth of silver deposits on Pt under voltammetric conditions was found to be remarkabl dependent on surface roughness and sweep rate (E147).KO& and Michaelis reported that the XPS binding energy of UPD copper on Pt was d i r d y dependent on the electrode potential used for the deposition (E148).They concluded that the electronic state of the copper adsorbate was therefore shifted by as much as 1eV per volt applied for the deposition. Several investi ations at less conventional electrodes can be cited here. Vohammetry at RuO, (E149)and RuTi alloy electrodes (E150)has been performed in H&304(aq). Tin oxide doped with Sb has been used aa a high overvoltage anode for the oxidation of phenol and Ce(II1) (E151). Bismuth- and iron-doped Pb02 were shown to function as oxygen atom E153). transfer electrodes in electrooxidative rocesses (E152, QCM experiments on these electroLs have also been described (E154). The action of nanosecond laser pulses on Ni, Pb, Ti, Ta, Al, Zn, and W electrodes has been investigated for various solution conditions (E155). Other studies include the chemisorption of benzotriazole on chemically etched Cu the adsor tion of thiourea at silver (E157),and foils (E156), the oxidation of form8dehyde at silver in NaOH(aq) (E158).
F. CHEMICALLY MODIFIED ELECTRODES F.l. Ion-ExchangeFilms. Many of the studies on this topic have employed Nafion, a cation-exchange membrane, or polyvinylp ‘dine (PVP),an anion-exchange membrane in aqueous a c i c solutions, films on metal substrates: two modified electrode systems that have been around since the pioneering work in this area. While the nature of charge transport through these films when they are loaded with redox active ions continues to be of interest, many of the more recent apers describe more intricate interfaces designed for anaytical or various electrocatalytic applications. Physical diffusion and not electron hopping appeared to be the main charge-transport mechanism in a spectroelectrochemical study of viologen cations incorporated into a Ndion membrane (FI). The concentration dependence of the radical cation/dimer equilibrium was used in a novel manner to determine the diffusion coefficients of the various viologen s ecies in this study. Triply and quadruply charged viologen &rivatives substituted with quaternary amine groups tethered by pentamethylene arms have been studied at Nafion-modified and bare electrodes (272). In solution these species underwent a 0.5-electron reduction to a dimeric species, while in the f i the monomeric highly charged ions were stabilized and oneelectron processes were observed. A dual-mode charge transport for a heterobinuclear cation was operative in Nation coatings (273).Others have reported diffusion coefficients for methylene blue type dyes (F4)and amphiphilic ferrocene derivatives (275) in Nafion films. The behavior of Co(I1) ammine complexes as electrocatalysta for oxygen reduction has been examined in a solid-state, Nafion-coated cell and at a glassy-carbon Nafion-coated electrode in solution (F6). Anson et al. have made an important contribution in a paper that considered the role of ion association of redox sites in charge transport through redox polymers (F7). The variation of the apparent diffusion coefficient describin charge transport with the concentration of redox sites, whch exhibited a marked increase as the mole fraction of the redox species approached unity, was successfully modeled by incorporation of ion association equilibria for the electron donor
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into the treatment. Data for O ~ ( b p y ) ~ ~in+Ndion / ~ + films in contact with H S04(a ) were in accord with the theory, as were the data of Skmp ($8) where the electrolyte was N%S04. Nadjo and co-workers have continued their work on polytungstate anions entrapped in PVP films (F9,FIO). Shiu and Anson employed various strategies to immobilize heteropolytungstate ions on electrode surface including anion exchange into PVP, incorporation into oxidized pol yrrole, and others, with modest success (FII).Quaternized P% was used to immobilize the Fe(CN)63-/4-cou le for hotogeneration of voltammetric a mixed-valent catalytic surface ($12). analysis of ions after preconcentration in ion exchange films on electrode surfaces continued to be a popular subject (FI3-FI5). Ultrasmall particles of CdSe and CdS were formed in Nafion films by an ion-dilution method (FI6)and the band ap energies of FezOd microc stallites in Nafion were founf to vary with the extent of loading (F17). Various electrocatalytic applications of ion-exch films have been explored. Kost et al. e l e c t r o d e p o s ~ I @ ~ Ru, and Pt in PVP films on lassy-carbon electrodes with The films were stabilized electrocatalysison the mind on the surface by cross-linking them in place with 1,6-dibromohexane. Several Japanese groups have incorporated Pt particles into Ndion fiims for the electrocatalytic oxidation of methanol (FI9,F20). The com Mite membrane of Ogumi et al., which also contained a MnKV/II) couple, consisted of a 3-pm Pt film on a 200-pm Nafion support that separated the organic phase containing the substrate for the electrooxidation from the aqueous cathode compartment (F21).The oxidation of cinnamyl alcohol was carried out in this cell. hamata and Masuda found that a Pt-Ru-Sn alloy entrapped in Nafion, among several other Pt alloys, gave the greatest catalytic activity for the oxidation of methanol (F22). The four-electron reduction of 0,has been achieved at glassy-carbon electrodes modified with electrode osited Pt microparticles and a cobalt porphyrin containing 8afion film (F23).Ion transport in Nafion/GORETEX membranes has been investigated (F24)and mass transport parameters for O2and electrode kinetics of 0 reduction at the Pt/Nafion interface have been reported (525).The latter paper gives informative purification procedures for the Nafion membrane and details for construction of a cell which featured at 175pm-thick Nafion layer without a contacting li uid interface. Gels, which were prepared from Nafion a n 8 hydrophobic solvents like tributyl phosphate, and which contained 9phenylacridinium salts, exhibited catalytic action for the reduction of oxy en (F26).The electrocatalytic oxidation of cysteine took dace at Nafion-film electrodes containing the Os(bpy)t+/2+couple (F27).Shu and Anson reported that the electrochemistry of a cationic cobalt com lex, which generated an anionic fr ment upon reduction, fecame considerably more irreverszle when performed in a Nafion film (F28). Physical properties and various analytical applications of Nafion films have been reported. Layered films of Prussian blue and Nafion containing methylviologen exhibited a fivecolor electrochromic effect (F29).The transference number of H+ in Ndion in contact with HzS04(aq)was found to be the effect of curing humidity on recast close to unity (F30), Nafion films was noted (F3I), and temperature profies acrass a Ndion membrane separating the electrodes during the electrolysis of water were measured (F32).Huang and Dasgupta constructed a moisture sensor from a two-electrode sandwich cell containing a Nafion electrolyte (F33).In this device, the current due to the electrolysis of water, which partitions into the film,was a function of the relative humidity of the contacting atmosphere. Electronically conducting lymer films can be manipulated a t the molecular level in orgr to impart specific properties and functions to a modified electrode interface, including ion-exchange properties. Interesting exam les are the polypyrrole-based films of Zhong et al. (F34,$3.9, which functioned as anion exchangers a t positive potentials and cation e x c y r s at negative potentials. These films were prepared by oxi izing pyrrole with a N-sulfo ropylpyrrole comonomer or in the presence of dodecyl sulkte ions. Li and Albery documented an anion effect on the charge-transport process for related films prepared by the electrosynthesis of polyp ole in the presence of poly(styrenesulfonate)(F36).These f g were cation exchangers, and in the presence of favorable
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anions (NO - Br-, Cl-), relatively fast charge-transport rates were seen. klliott et al. have published a thorou h stud of this interface usin voltammetric, coulometric, Ef’R, anB ac impedance metho% (F37).Yano has observed that electrochemically repared poly(NJv-dibutylaniline)was an insulator except ine!t presence of iodide ions where the 12/1-couple was uasireversible (F38). This phenomenon was the basis of a %ilayer potentiometric sensor for dissolved iodide ion (F39).Polypyrrole films substituted with positively charged quaternary amine groups incorporated approximately 5 times more anionic species than lypyrrole films and had excellent optical pro erties (F40). amphiphilic substituted pyrrole has been erectropolymerized to produce ordered films with ion-exchange properties (F41). The ac im edance of a polypyrrole-based anion-exchange polymer has !een reported (F42).Films of electropolymerized [Rub-b . Y ) ~ ]have ~ + been used as an ion-exchange matrix for several igands that selectively complex copper ions (F43). Copper ions were preconcentrated into these films prior to voltammetric analysis. Electrodes coated with ion-exchange films other than Nafion or PVP have promising applications. Electrodes modified with fluid coatings of double-tailed cationic surfactants such as didodecyldimethylammonium bromide incorporate phthalocyanine complexes and display electrocatalytic properties (F44).The temperature dependence of the voltammetzic peak currents of highly charged anions in these films correlated with the gel-to-liquid transition temperatures of aqueous sus ensions of the surfactants (F45). The resistance of a mof& bilayer containii a synthetic lipid (Cl&137)2NMez+ also exhibited a marked change due to a phase transition of the bilayer (F46).Han and Kaifer have used similar surfactants to modify phospholipid films containing surfactant viol0 en derivatives (F47).Graphite electrodes coated with poly(k4nyl yrrolidone) were resistant to deactivation by the products of &e electrooxidation of several phenolic compounds
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Charge-transport diffusion coefficients and heterogeneous rate constants were measured for the Os(III/II) couple in PVP films (F49).Okajima et al. have reported the kinetic parameters for the Fe(CN) *I4- couple confined in films of a cationic couple at perfluoro polymer $50) and for the Ru(NH3)2+IZ+ clay-modified electrodes F51). Fitch reported the cyclic voltammet of Cr(bpy)t+ 2+ and R u ( N H ~ ) ~ +couples / ~ + also a t clay-mA f i e d electrodes (8’52, F53). Photocatalytic activities of ‘EO2microcrystaUites prepared in Nafion films were found to be low compared to those in clay-modified electrodes for the photodecomposition of acetic acid (F54). Clay-modified electrodes were made catalytic for the reduction of oxygen by incorporation of stearyltrimethylammonium oxometalate species into the clay layer (F55).A study of clay-modified electrodes indicated that most of the adsorbed and intercalated cations are electroinactive (F56). Two electrochemically distinct silver species were voltammetrically detected in Y-type zeolite electrodes (F57). F.2. Ionophore Films. The papers cited in this brief section emphasized voltammetry or electrode reactions in ion0 hore films or media Polymeric ion0 hores such as PEO and b E P (a methoxy ether-substitutetpolyphosphazene) have been used in various solid-state devices, often containing lithium ion salts and in conjunction with a lithium negative electrode. Papers dealing with ionic charge transport in these media or with the performance of solid-state batteries, for example, were not included in this survey. Also a few articles concerned with ion transport through li ophilic membranes are mentioned here; the interested reaier should also note section F.5 below on this topic. The solid-state voltammetry of the TCNQ- anion in poly(ethylene oxide)/LiC104 revealed an interesting electron-exchan e phenomenon. The self-exchan e rate for the TCN cou ye was larger than that for the #CNQ-l2- couple in t is megum, and as a result, the second reduction wave was considerably smaller than the first in the cyclic voltammoams (F58).Diffusion coefficients of silver ions in PEO and EEP were measured b a transient method usin an interdigitated arra electrodre (F59).Charge transport fas also been studied in tilayer films in PEO/LiC104 and electropolymerized metallocenes (F60).Armand published a voltammetric study of charge transfer at polymer electrolyte metal interfaces (F61).
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The electrochemical response of lip0 hilic quinones incorporated into lip0 hile exchange memtranes was studied as a function of memtrane composition of solution p~ ( ~ 6 2 ) . C clic voltamm ams of iron rotoporphyrin M (hemin) and a $EO-m&ied?femin were 01tainedin a PEO matrix (F63). Nelson has reported very interesting results on mercury electrodes modified with phospholipid monolayers. The transport of T1+ through gramicidin A channels, which is selective for the transport of K+ in biological systems, has been studied (RM)and a lyether antibiotic rendered phospholipid monola ers permeace to Cd(I1) and Cu(I1) ions (F65). The permeagility was voltage dependent which was attributed to a conformational change of the monolayer/iono hore metal complex. Voltammetric waves were seen for Earium and lithium ions in acetonitrile at membrane electrodes that consisted of Dolvacrvlamide covalentlv couded to a PEO derivative (F66): ” Ion channel sensors have also been based on LangmuirBlodgett film assemblies containing protonated derivatives of macrocyclic amines or cyclodextrin polyamines (F67).A sensitive glutamate sensor was based on the glutamate ion channel protein entrapped in a bilayer membrane (F68). F.3. Redox Films. In addition to the papers cited in section B.5 above, there have been several other significant i b electrodes. Murray studies of charge transport in lymer f and co-workers have publisEd a clear account of charge transport in redox polymers driven by concentration polarization and by electrical potential gradienta (F69). This paper deals with the separation of electron from ion mobility and dilution effects for redox polymers with fiied sites. Mao and Pickup interpreted electronic conductivity data obtained on polypyrrole and substituted polyp ole f i i s in contact with acetonitrile solution in terms of 0th electronic and redox conduction models (F70). It was concluded that electron transport occurred by a hopping mechanism and that the two models were equivalent for pyrrole-based polymers. The Nernst-Einstein equation was used to relate the electronic conductivity ( u ) and the electron diffusion coefficient (De): u = gFD,C,,/RT, where g * 0.5 and Coxis the concentration of redox sites in the film. Electron transport was also measured in osmium polypyridine films in which the Os centers were diluted by a copolymer with Ru or Zn centers (F71)and in poly(bipyridine) films containing several different metal centers (F72). Electron transport through co olymer films of pol (3methylthiophene) and metallopofypyrrole has been stuc$ed (F73). Char e-transport diffusion coefficients have been reported for refated films (F74), and an ac im edance study of TCN polymers has been published (F757. The effect of variab e film thickness on the low-frequency ac impedance of polymer-modified electrodes has been considered (F76). Ellipsometry measurements indicated that the redox switching of a thionine film proceeded from the “insideout”,i.e. from the electrode/film interface toward the solution (F77). Several quartz crystal microbalance studies of redox polyi ihave ap ared. (See the above discussion in section mer f B.5.) These inclugstudies of poly(vin lferrocene) (F78, F79), TCN polymer films (F80-F82), a n l a Os(bpy)2(PVP)loC1 film ( 83). Electron transfer and charge trans ort through various bilayer assemblies has been reported. fhese include a study of electron-transfer kinetics at polymer/polymer and polymer/Pt/polymer interfaces (F84), in which redox metal bipyridyl complexes, i.e. the Os(III/II) and Ru(III/II) complexes of Murray and co-workers, were employed. Others have studied electroactive bilayers of poly(bithi0phene) and poly(xylylviolo en) (F85, F86), bilayers of polypyrrole and solid-state diode bilayer-like films contain’ thiophene and bilayers cobaltocenium and ferrocene centers (F88), copolymers with Fe(I1) and Ru(I1) complexes in a poly(biyridine) network synthesized by electropolymerization of Eromometh$-substituted derivatives of this popular ligand (F89). Osmium- and ruthenium-containin metallopolymers have also been studied by Forster and Vos $FN), and a cyclic voltammetry and XPS study of copolymer films of Ru polypyridine and several polymers of the polypyrrole genre has appeared (F91). The electropolymerization of N-substituted pyrrole derivatives is a versatile method of incorporating various functionalities and properties into an electrode interface. Papers
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em loying this strate have continued to a pear. These in&de the synthesis ot%ma with a pol yrrole%ackbone that display the voltammetric waves of botppolypyrrole and Rubpy complexes (F92), homo- and copolymers containing anthraquinone, phenothiazine, and anthracene (F93),polymers substituted with both viologen and electron donors for hotoelectrochemical applications (F94, F95), nickel-cycramsubstituted polymers (FW), cobalt-porphyrin-substituted polymers that displayed the Co(II I) couple (F97); terpyridine metal redox polymers (F98), others (F99). Several electrocatalytic a plications of redox polymer film can be noted. These includPe the oxidation of benzyl alcohol at quite positive potentials (ca.1.6 V vs Ag/AgNO,) in CH3CN i ielectrode containing covalently attached at a polypyrrole f triruthenium clusters (F100).The polymeric porph in f i i electrodes of Malinski have been used for the oxi&ion of hydrazine, methanol, and water (F101,F102). The electrocatalytic oxidation of NADH was performed at a graphite electrode modified with a thionine mediator cross-linked in a polymer network (F103). Electrocatalyticreductive coupling of bromotoluene was reported at a polymer nickel phosphine interface (F1041, and hydrogenation of conjugated enones occurred at carbon felt electrodes modified with poly(pyrroleviologen) films containing metal micro articles (F105). Further studies on the catalysis of the hy&ogen evolution reaction at the oxometalate electrodes of Keita and Nadjo were described (F106).The mediated reduction of Fe3+at osmium containing PVP f i was analyzed in terms of the kinetic zone di ams of Saveant (F107). A polyviologen film electrode wirexcellent redox and ion-exchan e pro erties was described (F108). Finally, a polypyrroye moiified electrode containing a Rh(III/I) couple was linked to an enzyme catalyzed reduction of pyruvate (F109). The clever analytical application of demetalated porphyrin films for the concentration and determination of nickel ions can be cited here also (F110).These films could be treated with acid to restore the demetalated condition. Modified electrodes prepared by polymerization of a vitamin B1 derivative have been used to determine RX compounds. h e concentration occurs by alkylation to form a surface-confined alkylcobalamin which is subsequently quantified (F111). Voltammograms of silver deposited into a redox polymer have been used to determine trace levels of Ag(1) in solution (F112, F113). The “redox conducti epoxy cement” of Gregg and Heller is amon the more novzredox films of recent years (F114). These fiis are formed from a two-component mixture containing an osmium-bp complex and a commercially available poly(ethy1ene glycol) Jglycidyl ether. The resulting f i i are strongly attached to electrode surfaces and highly permeable to solution species. They support currents up to 5 mA cm-2 for the catalytic oxidation of hydroquinone. Ferrocene-functionalizedsurfaces have been prepared via reaction of a carbocation ferrocene derivative formed in situ (F115).Procedures have been iven for modifying tin oxide surfaces with a dicarboxylato ku-bpy species (F116). Reduction of hexachlorobutadiene produced an electroactive black film (F117)and electropolymerization of 5amino-1-naphthol produced a redox polymer film with ood electrochemical properties (F118). Arai et al. detectedi pH changes in a polymeric quinone redox film by impregnating the film with a series of pH indicators and monitoring the absorbance during the redox cycling (F119). Elliott et al. have studied the formation of poly(viny1bi yridine) metal complexes from a mechanistic viewpoint ($120). There continues to be interest in Prussian blue redox film electrodes (F121-FI23). Papers on related films include a study of the indium-ferricyanide analo ue (F124), several studies on cobalt cyanometalate films (#125-F127), and on nickel cyanometalate films (F128, F129), and a radiotracer sorption study of 4sCa2+flux at copper cyanometalate films (F130). The cobalt cyanometalate films behaved as reversible redox pol ers in the presence of aqueous solutions of Li+, Na+, or salts, but were inactive in the presence of T1+, NH,+, Cs+, or Rb+ cations (F127). The nickel-ferrocyanide surface of Lin and Bocarsly oxidized hydrazine via an oneelectron, one-proton rds forming N2H3which was rapidly oxidized further to N2 (F128). F.4. Electronically Conducting Films. Here is an area that is exceedingly difficult to keep abreast of since not only
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electrochemists,but also polymer and material scientists, and others have their fingers and electrodes in the pot. The following survey is purposely focused on articles that address electrochemical aspecta of electrodes modified with conducting polymers of the polypyrrole (PP), polythiophene (PT),and polyaniline (PA) type. There is necessarily overlap with the previous section since for a conducting film sandwiched between an electrolyte solution and a metal substrate, there is often little phenomenological distinction between redox and electronically conducting polymers. A recent Faraday Dkcussions contained several interesting papers of a theoretical nature on charge transfer in conducting polymers (F131-Fl33). Since the ori inal work of Dim and coworkers, conductin polymers have%een synthesized by simple electrochemica procedures. Refinements on these methods and details of the electropolymerization processes continue to ap ear in the literature. Christensen and Hamnett describe f a detailed ellipsometric and FTIR spectroscopic study of the electrosynthesis of PP in 1 M NaC10, (F134). The process was said to proceed by deposition of oligomers with a mean len h of 34 monomer units. Delocalized polaron states existe over 12 monomer units,and at her potentials, bipolarons of three monomer conju ation lengt were formed accompanied by a decrease in the film thickness. This seems to be in agreement with John and Wallace who indicated that PP growth took place via continual precipitation of oligomers and not via addition of pyrrole monomers to the ends of a living chain (F135). Other studies of the electrochemical synthesis of PP have appeared (F136,F137) including two rotating-ring-disk investigations (F138, F139). Lamellar hi hly-ordered conducting polymer films have been prepared %yelectropolymerization of a pyrrole surfactant (F140). Conditions have been iven for the electropolymerization of pyrrole on iron (F144 and aluminum (F142) electrodes. The overoxidation of PP, which was minimized b limiting the water content to 10 ppm, was followed by F T d spectra (F143),and PP was electropolymerized directly in an insulating state by Osaka et al. (F144). The molar mass of several substituted PTs, e.g. poly(thiophene-3-aceticacid), was estimated by gel permeation chromatography. Chain lengths greater than 2000 monomer units were found for the extracted and completely soluble polymers (F145). In situ neutron reflectivity of a poly(bithio hene) (PBT) film gave information on its solvent content, its tkickness, and combined with the electrochemical charge, the charge associated with each thiophene unit in the oxidized film (F146). A value of 0.31 f 0.05 was found for the latter value for a PBT film oxidized in the presence of NEt4BF4. Several other elli sometry studies of conducting polymer films have appeare! (F147-Fl49). The combination of the QCM and simultaneous ellipsometry allowed Rishpon et al. (F149)to determine the potential dependence of the density of a PA film. Poly(p- henylene) has been prepared by the electrooxidation of goth bi henyl in CH&l (F150) and benzene in NBu4BF4-oleum-8HzClzsolutions tF151). An electronically conducting polymer was synthesized by the oxidation of 1naphthol (F152),and conditions for the copolymerization of pyrrole and thio hene in LiClO,/PC solutions have been worked out (F1537. More interesting was the report of Iyoda et al. who prepared alternate layered and graded structures with 10-nm resolution by “potential-programmed” electropolymerization of pyrrole and methylthiophene (F154). Additional papers on electrosynthetic aspects include the preparation of highly conducting olymer from tetrathienylsilane (F155),reduction of hexackorobutadiene to form a conducting film (F156),the formation of conducting PA from protonated aniline in acetonitrile (F157), the oxidation of 2,5-dimethylaniline in molten NH4.2.35HF (F158),the synthesis of poly(o-hexylaniline),a soluble conducting polymer (F159),the synthesis of water-soluble conducting poly(ani1ine propanesulfonic acid) (F160),and electropolymerization of two methox anilines (F161). A redox-active, soluble, and gold-colorec? cyanide-functionalized polyaniline was synthesized by the electropolymerization of o-aminobenzonitrile (F162),and poly(butoxythiophenes) were shown to have good conductivity,excellent optical properties, and a low oxidation potential (F163).
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Several QCM studies of conducting polymer films have appeared (F149, F164-Fl69). In addition to the previously discussed work of Hillman et al. (see section B.5 above), the study of Borjas and Buttry has iven considerable detail on the nature of the formation anfdoping of PT films in acetonitrile. Gabrielli et al. have calibrated the QCM response by coulometric de osition of silver and noted the effects of nonuniform mass &stribution and electrode size on the sensitivity (Fl70). The voltammetric behavior of polyphenylene was compared to the behavior of oligomeric p-phenylenes in nonaqueous solution as a solid-state film on a Pt electrode. The reversible EE electrochemistry of the oligomers in solution colla sed to an irreversible two-electron system in the solid state b 1 7 1 ) . Zotti et al. (Fl72) suggested that dimeric associations in the polymer chain were responsible for n values equal to 0.5 that they measured for PT. In situ measurement of the relative conductivities of PT, PA, and PP as a function of potential revealed that finite windows of high conductivity exist in each case (F173). Polyacetylene films in contact with S02/electrolyte solutions behaved in a similar fashion (FI74). The problem of curve fitting nonlinear Nernst plots obtained from spectral data on conducting polymer films has been addressed specifically by two groups (F175, F176). Marque and Roncali attributed nonlinearity to variations in interactions between charged sites and mechanical strain associated with insertion of counterions into PT polymer matrices. Others have reported on the spectroelectrocheniistry of various PT films (F177). Analysis of the periodic& electrical and optical responses of PA films allowed the faradaic and non-faradaic charging responses to be separated (F178). The PA films were optically responsive over the entire positive-goin potential sweep, which was taken as an indication that re&, processes occur in the plateau region of the CVs. There was poor correlation, however, between capacitance values derived from impedance data and values obtained from large potential step perturbations. This problem has also been addressed by others (F179) and was discussed by Heinze in his review (A14). Genies et al. have reported differential capacities for PA as a function of counterion and solution pH (F180). In part because conducting polymers synthesized from the same monomer can have different chemical, morphological, and physical properties depending on the solution and electrochemical conditions, a wide range of treatments have been used to analyze electrochemical data obtained on conducting polymer films. Recent papers on PP include a transmission line analysis of ac im edance data (F181) and an analysis of the RC response of f P that was based on porous electrode theory (F182). Aoki has proposed a model for conducting polymers that assumed microdomains of conducting and insulating re ions were randomly distributed on either onedimensiondor cubic arrays (F183,F184). The latter situation gives rise to nonlinear Nernst plots. Other studies on PP include the use of a photodiode array detector to measure the prop ation speed of the conductive front during e1ectrochemicale;kopingof the film (F185). The ation speed was constant, typically 0.1 cm s-l at 0.8 V The permeability of functionalized PP films to ferrocene solutes was determined for films synthesized using different electrolyte anions in the polymerization step (F186). Details of the PA-switc processes continue to be actively researched. ESR spectra of A films exhibit line widths that narrow as the potential becomes more positive. Lapkowski and Genies suggested that a transition from a Curie to a Pauli spin state was at play (F187). Other recent spectroelectrochemical studies of PA included a Raman study (F188),a reflectance study (F189),and absorption measurements on a poly(2-methylaniline) film ( F I N ) . CVs of PA have been obtained at temperatures as low as -42 OC (F191). Variation of the pH of PA films with potential was measured from difference spectra of methyl violet indicator in both aqueous and acetonitrile solution (F192). A “small” insertion of protons was seen for the first oxidation step to the PA polaron state and ne ligible pH change was obtained upon oxidation to the bipofaron state. Kalaji et al. observed fast proton transport for PA using microelectrode techniques (F193). Anion effeds on the PA response continue to be noted (F194), includin an interesting study of the potential dependence of the fkfusion coefficient of BF4- ions in cylindrical
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PA fibrils (F195).Photoelectrochemical effects on the PA and response have been studied by two groups (F196,197) attributed to photothermally enhanced convection by the latter. The oxidation of formic acid at pol aniline electrodes has been studied in several laboratories ($198, F199). In accord with the methanol oxidation mentioned above (E112),Gholamian and Contractor observed that the rate of formic acid oxidation was increased further by deposition of Pt microparticles into the PA film (F199).Ascorbate oxidation gave a ak-shaped voltammo am at PP electrodes (F200)and a &erocyclic quinone exfibited reversible behavior at P P film electrodes (27201). F.S. Langmuir-Blodgett Films and Self-Assembly Monolayers. The L-B film technique and the use of selfassembled monolayers, often based on the adsorption of monolayers of thiol-terminated long-chain alkane molecules, allows the experimenter considerable control over the architecture of modified electrode interfaces. In combination with molecular design of the film components, many truly novel electrochemical applications are possible. It is likely that the activity seen in recent years merely represents the foot of the growth curve in this area. Among the ap lications of the L-B trough a paratus, the simple trace anapytical method of Miller and %ard for surfactant (and nonsurfactant) species is noted first (F2O2).They achieved detection at the picomole level using a lanar Pt electrode that was brought into contact with a sur ace monolayer in an orientation parallel to the air water interface. A more elaborate study of two-dimension diffusion of ferrocene surfactants at a cleverly des‘ ed microelectrode with a line geometry also stands out (803). Two-dimensional diffusion (not electron hop ing) in the surfactant layer produced CVs that displayed t1e classical Randles-Sevcik wave shape. Lateral diffusion of octadecylviologen species in bilayer assemblies was also investi ated by Majda’s group (F204, F205). Octadecylmeth lviofogen was also a key component of an aluminum oxide h m photoelectrode for conversion of solar energy to electricity (F206).Spectroelectrochemistry was performed on L-B films in a related manner using a trans arent IT0 electrode that horizontally contacted a free stan& monola er of a silver tetraneopentoxyphthalocyanine complex (~207, The electrochromic behavior of several rare-earth-metal diphthalocyanineL-B f h was reported by Liu et al. (F209). Ferrocene-substituted surfactants have often been used as probe molecules to characterize L-B and self-assembled monolayers on electrode surfaces (B121,F210-F216). The study of Nordyke and Buttry (F211)detailed especially interesting phenomena relating to the hydrophobic and ionic nature of gold surfaces modified by treatment with several short-chain disulfides. A novel method of preparing thin film organic surfaces is based on ferrocene surfactants. Oxidation to the ferrocenium cation state destroys the surfactant character such that solubilized organic species are released from the micelles and dispersed particles (F217). Electron transfer through highl uniform layers of n-alkanethiols has been of considera&e interest. Miller and Gratzel measured heterogeneous outer-sphere electron-transfer rates at lar e overvoltages for several simple transition-metal couples at &eae interfaces (F218).Electron tunneling between a gold underla er and cytochrome c adsorbed on a self-assembled monorayer of a carboxylate-substituted thioalkane was invoked to explain the kinetic data (F219).Electron tunneling through a dielectric surface film of cetyl alcohol was also proposed to be the key step in related systems (F220). Kunitake et al. (F221)as well argued for transmembrane electron transfer throu h mercaptan monolayers containing a small mole fraction of a dihydroxybenzenederivative that imparted redox character to the solution/ film interface. The role of the electric field in assisting ion binding to a self-assembled monolayer has been examined by Steinberg et al. (F222). In these well-designed experiments, a bis(ethylacetoacetate) sulfide film was found to complex cations such as Cu2+and Pb2+in a markedly potential dependent fashion increasing at potentials both positive and negative of An interesting pH-de endent ion gate behavior was the d e s c h for monola ers of a &ol-containing hosphate lipid on gold electrodes ($223). Charge- and pH-&pendent elec-
d
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trostatic binding to self-assembled monolayers of 4-aminothiophenol was studied by cyclic voltammetry (F224). A careful surface Raman spectra stud of self-assembled monola ers of 1-alkanethiols on Au andrAg electrodes has reveaddetails of the orientations of C-S bonds at the surface. Different m e t a l 4 bonding interactions and different C-S bond orientations are seen at the two metals (F225).Defects in self-assembled alkanethiol monolayers were imaged by de osition of co per metal at the defect sites followed by suLequent STdsurface analysis (F226).The defect surface density was found to decrease with the time allotted for assembly of the monolayer. Reflectance infrared techniques have also been applied to the study of these interfaces (F227). Several papers have addressed various aspects of monolayers of viologen surfactants on electrode surfaces (F228F235). Of interest is the report of Bae et al. (F228)that their films became infrared silent upon reduction. Mediated electron transfer to solution redox couples was observed in several instances (F230,F231). Bilewicz and Majda presented evidence that the isoprenoid side chain of ubiquinone (Q ), molecules incorporated into a L-B monolayer functionea as an ion channel former for transport of the R u ( N H ~ ) ~ +couple / ~ + to the electrode surface (F236).Redox mediation of the electron transfer was ruled out by the Q El/ value which was 200 mV more negative than that of the redox couple. Takehara et al. found that the reversibility of the ubi uinone and vitamin K species was dependent on the aUryl c%am length of the n-alkanethiol layer at self-assembled film electrodes (F237).The Fe(CN)63-/4couple was silent at these surfaces. Amphiphiles from quinone-functionalized liposomes form electroactive monolayers on glassy-carbon electrodes (F238). Cytochrome c eledrochemistryhas been performed at lauric acid and laurylamine-modified electrodes (F239)and at a L-B film of a flavin surfactant (F240).Reversible behavior was seen in the former casel while the flavin film reacted with the oxidized, but not the reduced, form of cytochrome c. The latter system is an approach to molecular rectification. Ueama has also observed “molecular rectification” at L-B birayers of hematoporphyrin IX Ru(I1)/flavin films (I3161 F241). Rectifying characteristics were reported for a L-B f i d consisting of a zwitterion acceptor with a hydrophobic tail sandwiched between Mg and Pt contacts (F242).Two simple e methods for the preparation of ordered monoion-exc% of b‘ yer-forming amphiphiles on gold electrodes were layers based on the adsorption of a 3-mercaptopropionate derivative of dioctadecyldimethylammonium cation on the electrode surface (F243). Films of thiol-terminated oligoimides exhibit reversible EE surface electrode reactions (F244,F245). These molecules accepted up to eight electrons per molecule without desorbing from the electrode. Selectivity to the reduction of whydroxyaldehyde was imparted by a L-B film of a dimethyldioctadecylammonium salt (F246). Several interesting photoelectrochemical applications of L-B films were noted. Liu et al. observed that a L-B monolayer of an azobenzene derivative participated in a square scheme where a low-energy reduction of the photogenerated cis state occurred at a potential ca. 0.4 V more positive than the trans form (F2474’249).They noted that a photoncounting device could be based on this phenomenon. Photocurrents were obtained for bacteriorhodo sin (F250)and for xanthene dves (F251,F2.52)incorDoratedPinto L-B films on SnOz electrodes. F.6. Other Modified Electrode Studies. Studies of the electrocatalysis of H202reduction at iron protoporphyrin (F253)and the H202 oxidation at cobalt protoporphyrin electrodes (F254)have been published. Tin oxide surfaces have also been modified bv covalent attachment of a lowmolar-mass heme eptide t o make them responsive to hydro en peroxide $255). Ox en reduction has also been studiied at iron porphyrin m o 18led pyrolytic graphite (F256) and at a surface prepared by evaporation of solution containing a cationic porphyrin/anionic porphyrin complex (F257).Rotating-ring-disk evidence was obtained on the latter surface for competing two- and four-electron O2 reduction pathways. Electroreflectance spectroscopy of dicobalt cofacial po hyrin adsorbed on gold and graphite electrodes provided eviTence that solution and surface oxidized species were not the same (F258).On the surface, the two-electron reduction ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
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ste of the dimeric species do not involve identical redox sites. -carbon electrodes coated with complex anions containin boFe3S4cores were shown to be electrocatalytic for the rekction of nitrate and nitrite (F259): Reduction of both of these ions to ammonia was achieved in pH = 10 buffers, while at less negative potentials, nitrite was reduced to N2. Ultrathin, highly insulating polymer films were formed by the anodic polymerization of phenoxide ion in acetonitrile (B156, F260). STM images of glass carbon taken during phenolic oxidation indicated that dif erent surface polymerization mechanisms were operative under constant potential and potential scanning rocedures (F261). Glass -carbon electrdes coated with permselective phospholipidjcholesterol membranes exhibited some selectivity for redox-active hydrophobic compounds (F263, F264). A gel volume change of up to 30% was realized by redox cyclin of a pol yrrole film in contact with pH-sensitive methykethacrycte microgel (F265). Others have examined redox solutes at electrodes coated with yirradiated immobilized hydrogels (F266) and have achieved the electrically controlled release of dru s from polyelectrolyte gels (F267). Tiemey and Martin neatfy performed electrically controlled release of solution species by the electrochemical dissolution/disru tion of membranes on a microporous substrate electrode ($268). Biomolecules such as insulin can be delivered in this fashion (F269). Finally, an electrolytic procedure for deposition of CaHP04.2H20(brushite) was reported (F270).
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G. BIOELECTROCHEMISTRY The following survey of the recent literature is more selective than in previous reviews in this series. The emphasis here will be on the electrochemistry of proteins in section G.2 and enz e electrodes in section G.3. G.1. g a c t i o n s of Low-Molar-Mass Biologically Important Molecules. Several studies have appeared on the determination of binding constants for the interaction of small ionic species with DNA and RNA molecules. These include the work of Rodriguez and Bard who estimated bindiyf site size for the interacbon of O ~ ( b p y ) ~ constantsand bin with calf thymus D A using peak potential and ECL data (GI), a study of nickel ion binding equilibria with tRNA using differential ulse polar0 aph (G2),a voltammetric study of Pb2+ biniing with DRA ( 8 3 ) , a report of one-electron reduction otentials of electron acceptors intercalated into the DNA Auble helix structure (G4),and the cyclic voltammetry of a bis intercalator for DNA with a viologen spacer group (G5). Palecek et al. have described the electrochemicalbehavior of a B-form decamer of DNA, d(CCAGGCCTGG) (G6). Sagara et al. examined the interaction of several 4-pyridyl derivatives with horse heart cytochrome c (G7). Variable binding strengths were found, and the number of protein binding sites was small. The 4,4'-bipyridyl molecule, which has been used to modify electrode surfaces for protein electrochemistry, did not bind with cytochrome c in neutral solution. The electrocatalytic oxidation of NADH has been further studied at aphite electrodes coated with henothiazinium salta and grivatives (G8,G9). The C~(p;en)~~+-mediated oxidation of NADH has also been studied (C10). NADH exhibited persistent oxidation waves at a poly(3-methylthiophene) film electrode in the absence of mediators (G11). A coulometric analytical procedure for both ascorbic acid and dehydroascorbic acid was described that was based on the reduction of the electroinactive dehydroascorbate form with excess dithiothreitol (G12). Ascorbic acid was determined before and after reaction by coulometric electrolysis at a carbon-felt anode. An im lantable needle-type amperometric glucose sensor employelthe direct oxidation of glucose at platinized Pt (C13). Speciation of various Cu(I1)-peptide complexes was achieved in a cyclic voltammetric and potentiometric titration study ((214). Bond et al. reported the polarography and voltammetry of oxindolylalanine and several other small pe tide molecules at mercury electrodes (G15). Voltammetric relox potentials were reported for tyrosine phenoxy and tr tophan indolyl radicals generated in peptides by pulse ra%olysis techniques (G16).
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Other articles of interest concerned the one-electron reductions of several alkylcobalamins in DMF/H20 solutions ((713,the voltammetry and spectroscopy of pyrrolcquinoline quinone coenzyme (G18),the cyclic voltammetry of isoalloxazine and a flavin cyclophane (G19), and the entrapment of flavinmononucleotide in a polypyrrole film (G20). 6.2. Protein Electrochemistry. Much work has been done on the electrochemistry of redox proteins in the last few years as researchers continue their struggle to understand the factors that govern electron transfer between metal electrodes and redox centers embedded in a protein matrix. This topic has been reviewed recently by Heller, who has made major contributions in this area (G21). Other major themes include the effect of structure modificationson protein electrochemical behavior and the differences between the redox reactions of adsorbed and solution species. Bowden and Willit found that the surface formal otential of horse heart cytochrome c adsorbed on fluoride-Aped tin oxide electrodes was 20-25 mV more negative than the solution value (G22). The reaction entropy was also 25 J K-' mol-' lower than the corres onding solution value. Various strategies have been use: to increase the hetero eneous electron-transfer rates of cytochrome species inclufing the modification of P t and Au electrodes with adatoms (G23),and in more elaborate fashion, with a lipid bilayer containing immobilized vinylferrocene as a mediator ( G H ) . Related to this subject is the argument of Biichi and Bond that the electron-transfer reaction of horse heart cytochrome c at carbon electrodes is markedly inhibited by adsorption of the protein itself. Thus electron transfer occurs, in their view, at micrometer-size uncovered active sites with an exchange rate much larger than previously reported (GW). Hill and co-workers, who did the seminal work in this area, have continued their studies. They found that adsorption of lauric acid or lauryl acid or laurylamine on basal plane pryrolytic graphite rendered the surface reversible for electron transfer to cytochrome c and cytochrome b5 (F239). Cytochrome c551, cytochrome b5, and azurin absorbed in Nafion films on gra hite electrodes gave quasireversible cyclic voltammetry (826). Others have reported that the presence of DNA and RNA (G27) or a hydrogenase from a sulfate-reducing bacteria (G28) increased the apparent reversibility of the cytochrome c couple at a glassy-carbon electrode. The previously cited study of electron transfer to cytochrome c thro h a thioalkanecarboxylate monolayer is also of interest hereF219). Other pa ers include a CV study of cytochrome c3 at graphite electroles (G29) and a galvanostatic doublepulse measurement of the heterogeneous rate constant and a value for cytochrome c3 at Hg (G30). The experimental values in the latter study were 0.48 for a and a fast 1.2 cm s-' for the electron-transfer rate. Formal potentials and exchange rates were meausred voltammetrically at cysteine-modified gold electrodes for several cytochrome b5 variants that were produced by mutaenesis so as to vary the surface charge on the native protein (331). The shift of the E"' values was modeled by theory that assumed a continuum dielectric within the protein. A good fit was obtained for a dielectric constant of 3 f 1. Raphael and Gray also studied several modified cytochrome c molecules using differential ulse polarography and spectroelectrochemical methods. bplacement of an axial methionine ligand by cysteine resulted in a substantial 600-mV negative shift of the Eo' value (G32). Reversible electrochemistry of hemoglobin has been observed at Janus green and methylene green dye modified electrodes (G33, G34). Armstrong et al. have published several interestin studies of redox protein electrochemistry. They compared t i e cyclic voltammetry of three enzymes (plastocyanin, cytochrome cS1, and a high-potential iron-sulfur protein) in the presence of a cationic re ent to the behavior obtained by attachment of the Ru(NH3?+12+ couple to specific histidine residues in the proteins. Attachment of a single complex resulted in significant changes in the electron-ex&any rates with edje plane pyrolytic gra hite electrodes (C35) n another stu et al. obtain3 CVs of surface f i i of ferredoxins coazzi% with aminoc clitol, again on ed e plane pyrolytic graphite electrodes ($36). Three reversifle waves were seen for two different iron-sulfur centers, and when the adsorbed proteins were treated with solutions of Fez+,Zn2+,and Cd2+ions, new
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voltammetric patterns were obtained indicative of ligation transformations of the iron-sulfur clusters. Others have obtained quasireversible cyclic voltammetry for spinach plastoc min at old electrodes modified by adsorption of [Cr(N&3)SNCSf+(G37) Denaturization of ‘xanthine oxidase allowed the FAD FADHz couple to be seen at lassy-carbon electrodes (G38L Greater electroactivity was ogserved at Hg electrodes where mercuric cysteinate, cysteine, and cystine couples were also evident in the voltammograms. Electron transfer between several viologen-accepting pyridine nucleotide oxidoreductases and modified glassy-carbon electrodes was demonstrated (C39).The electrode surfaces in this study were modified by covalent attachment of a viologen dication and in a second case a cobalt macrocyclic ammine com lex. Direct electrontransfer reactions to electrode surfaces iave been realized for several copper redox proteins ((340,G41). Redox and spectral properties of a flavodoxin have been reported (G42). Coury et al. studied electron-transfer enzyme kinetics in solution using voltammetric methods. EC’ electrocatalytic processes were involved between the molybdoheme protein sulfite oxidase and either transition-metal complexes or cyG44). tochrome c mediators ((343, Several miscellaneous studies have a peared on hi h-molar-mass proteins that are not normdy considerecf to be electroactive. However viol en groups have been covalently attached to bovine serum al umin to make it electroactive (C45),and anti-human serum albumin was incor rated into a polypyrrole mat& by carrying out the el&opo&erization in a solution of suitable antibody concentration (G46).Inhibition of the electrode reactions of sim le redox couples by the adsorption of bovine immunoglobuh G was described (G47). Adsorptive strippin voltammetry of an immunoglobulin at a static Hg-drop ekectrode gave a limit of detection of 1.2 X 10-loM (G48),the voltammetry of immunoglobulin G at a mercury drop electrode was reported (G49), and S E W spectra of an antibody adsorbed on a Ag electrode were obtained (G50). G.3. Enzyme Electrodes. The construction of modified electrodes with biological enzymes has proven to be a formidable challenge that is well suited to the skills of electrochemists. The unique selectivity afforded by the biolo ical catalysts is a primary motivation for development of ties, electrodes. The interface of these electrochemical detectors, which typically must contain the enzyme catal st and an electron mediator, also must be compatible w i d the environmental requirements for enzyme activity and stability. These are subjects that have been addressed by many research oups, and as a result, the literature on enzyme electrodes as mushroomed in the last 2 years. The well-studied glucose mediator/glucose oxidase (GOD) system has often been use as a test system. Enzyme electrodes have become smaller in conjunction with the development of microelectrode methodology. Microenzyme glucose electrodes for example have been made using 7-pm carbon fibers sealed in glass ((351). The small size of these sensors produces 10 times greater current density and less de endence on oxygen than more conventional-size electrJes. Glucose oxidase has also been immobilized on the 2-pm-diameter carbon-ring electrodes of Ewing’s group to produce a microenzyme electrode with subsecond response times (E58).A 26 auge syringe needle (G52)and printed screen electrodes ($53) have been used to fabricate enzyme electrodes. The latter electrodes rmitted the determination of acetoaminophen on a 20-pL g o p of whole blood using a disposable dry-strip electrode. A fiber-optic/electrochemical glucose sensor has been described (G54)and Coulet has compared two sensors, one electrochemical and one based on fiber optics, for the detection of HzOzgenerated in enzyme reactions (G55). Many different strategies and variations of strategies have been used to immobilize enzymes and to communicate elect r i d y with enzymes on electrode surfaces. One of the most ingenious is the back-to-nature approach pioneered by Rechnitz, in which plant tissues high in enzyme content and containing the necessar cofactors are directly incorporated into a working electroL. Recent entrees to the modifiedelectrode menu in this category are carbon-paste pea seedling eg lant sensors for plant owth regulating amines (G56), , electrodes for pEnols electrodes for catego1 ( ~ 5 7 jmushroom
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(C58), a zucchini carbon-paste electrode that eliminated the ascorbic acid si al by the presence of ascorbic acid oxidase (G59), and R V G l a n t tissue electrodes made from potatoes, mushrooms, and horseradish roots ((360). Nine different surface modification rocedures for gra hite were evaluated in a study of NADI!production b Jrect electron transfer between immobilized mediator a n 8 an immobilized viologen-accepting oxidoreductase (G61).Procedures for GOD immobilization by sim le adsor tion on pyaphite and by covalent b i n i n to gf)assy-carbon were compared (G62)and the fong-term stability of carbon-paste GOD enzyme electrodes was investigated (C63).GOD was immobilized directly on a platinized RVC in the detector of Heider et al. (G64),and Hu et al. attached GOD to a graphite electrode coated with a quinone redox couple using the gluteraldehyde method (G65). A ferrocene-mediated carbon-paste glucose electrode with excellent sensitivity was prepared by sealing the surface with cellulose acetate and covering the resulting electrode with a GODmodified Nylon net (1366).GOD electrodes have been described using a bis(benzophenoxaziny1) mediatbr on graphite (G67),various vi010 en mediators in a carbon-paste formuF as a mediator species (G69).Enzyme lation (G68),and electrodes have also been fabricated with GOD immobilized in the head group re ‘on of a bilayer assembly on microporous aluminum oxide (&O), in Nafion-coated polyester cationand on the outer surface of clay exchange membranes (C71), film containing a [RU(NH~),$+/~+ mediator species (G72). Sass0 et al. have noted that electropol erized 1,a-aminobenzene films minimized fouling of GGelectrodes (G73). An idea that has been around for a while is that of entra ping redox enzymes in redox polymers, which can function as L t h the mediator and immobilization matrix. Gre g and Heller have entrapped GOD in cross-linked redox g e t with this view in mind (G74),and their “redox conducting e oxy cement” (17114)Beems to be well suited for this purpose (875). Hale et al. employed ferrocene-modified siloxane ethylene oxide copolymers to construct amperometric enzyme electrodes for glutamate containing L-glutamate oxidase FAD redox centers (G76)and for glucose containing GOD in the same matrices (G77).Descriptions of several enzyme electrodes based on electropolymerized polypyrrole films conincluding taining various mediators have ap ared (G784380), and a horseradish pera cholesterol oxidase electrode oxidase (HRP) electrode that functions as a HzOzdetector (G82).Also NADH dehydrogenase entrapped in a polyp ole matrix was shown to catalyze the electrooxidation of &H (G83).A microarray ‘enzyme switch” for NADH was fabricated by polymerizing pyrrole and N-methylpyrrole in the presence of diaphorase, a flavin enzyme that catalyzes the reduction of oxidized species by NADH (G84).y irradiation was shown to be an effective wa to immobilize lactate oxidase and Xang and Varughese have dein a polymer film (G85), scribed a polishable enzyme electrode constructed using a commercial epoxy-bonded graphite ((386). An enzyme electrode that gave catalytic currents for the oxidation of simple alcohols was based on a silicone oil paste containing dispersed TTF-TCNQ particles and a methanol dehydrogenase (G87).Hydrogen eroxide sensing electrodes for the selective detection of L-gktamate were constructed by immobilization of L-glutamate oxidase on an Immobilon-AV afffity membrane (C88)and directly on a graphite electrode surface (G89). A novel bilayer enzyme electrode was made by covalent attachment of one enzyme to a tin oxide electrode followed by further modification by a second enzyme (G90).Sensors for lactate, yruvate, cholesterol, and uric acid based on this design h a v e k n developed us’ HRP as the surface-attached enzyme (G91).A dual-enzymzectrode approach using HRP was used to eliminate urate, ascorbate, and p-acetomidophenol interferences to the detection of glucose using a chemically modified GOD in a conducting gel (G92).HRP was also used in a carbon-fiber microelectrode for the detection of catechol ((793). A major advance in the last few years has been in the chemical synthesis and use of modified or “wired” enzymes (G21).Attachment of mediator electron shuttles to native enzymes can o n paths of electrical communication between metal electrogand the enzyme redox centers. Schuhmann et al., for example, synthesized several ferrocene-modified
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GODS that had the mediators attached to the periphery of the enzyme by flexible chains (G94).The electrocatalytic activity of the enzyme was dramatically turned on when the chain was greater than approximately five bond lengths long. Chemical modification of glutathione reductase by covalent attachment of a viologen species rendered it redox active (G95, 13%).This "wired enz e" was em loyed in a photochemical system consisting of a b p y ) 2+, gktathione, and sacrificial EDTA. Bartlett suggested t i a t the ability of TTF-TCNQ to mediate electron transfer with GOD was due to a hydrophobic TTF-enzyme interaction and that the modified enzyme exchanged electrons at the conduct' salt surface (G97). Previously it was demonstrated that 8F-modified GOD underwent direct electron transfer at metal electrodes and could sense glucose (G98). Application of enzymes modified with electron relays to practical devices, however, may be limited by the poor chemical stability of the covalently attached mediators (G99). A related strate is that of Mizutami et al. who used a pol ethylene glyco!?nodified enzyme for the construction of . modified enzyme carion-paste glucose electrodes ( ~ 1 0 0 )The exhibited almost a 10-fold greater activity on the hydrophobic paste surface than the native enzyme. Turner and co-workers have discussed the use of organicphase enzyme electrodes (G101) and described an enzymeoxy en electrode application for the detection of cholesterol in choroform/hexane mixtures (G102). Several carbon-paste xanthine oxidase electrodes have been described (G103-C105),and a HzOzsensing amino oxidase electrode has been used to detect polyamines such as putrescine and cadaverine (G106). A L-lactate sensor was constructed by entrapping lactate oxidase in a poly(viny1 alcohol) matrix on platinized graphite (G107).Platinized graphite was also used to immobillze a dehydrogenase for the detection of 3-hydroxybutyrate (G108).Lactate was determined with an electrode ensemble consisting of flavoc ochrome bz coated aphite particles confined to a glassy-car n electrode surface a Nylon mesh and a dialysis membrane ((2109). An enzyme gel was described by Willner et al. that was photoactive for the reduction of nitrate ion in the presence of Ru(bpy)3z+(C110). The gel was a cross-linked polyacryl=de containing viologen groups. A ferrocene-medmted HRp electrode for the detection of cyanide with sub-ppb sensitivity was reported (GI11),and a sulfite oxidase carbon- aste electrode for aseous SO was based on an agarose ge containing immo%ilizedsuldte oxidase (G112). Several papers describing enzyme reactors caught your reviewer's eye. These included an electrochemical fixation of CO that proceeded with 100% current efficiency (G113), FIA of phosphate at the 2 X M level in a cell containing a three-stage a phosphorylase and xanthine oxidase (G114), enzyme reactor for the determination of starch (G115), a reactor for the detection of ATP that used a modified aphite electrode to sense the formation of glucose (G116rand a reactor for the efficient oxidation of p-cresol (G117).
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H. CHARACTERIZATION OF REDOX REACTIONS H.1. Electron-Transfer Mechanisms. The advances in methodology, instrumentation, and theory have made it ossible to apply electrochemistry to a wide variety of probLms in all areas of chemistry. One of the issues of great importance is the electron-transfer event itself. Outer-sphere versus inner-sphere electron-transfer mechanisms for organic compounds were examined in a series of articles by Saveant et al. (H1-H4).The reaction of trifluoromethyl bromide with electrochemically generated aromatic anion radicals and s@ur dioxide anion radicals was used to probe whether the reactions were inner sphere or outer sphere (HI). These same issues were examined in the reductive elimination reaction of vicinal dibromoalkanes (H2). The rates of the debromination processes are controlled by the prim reduction of the dibromo com ounds. The direct and i x r e c t electrochemistry of pedoroalkyl bromides and iodides was examined to determine if the outer-sphere dissociative electron transfer led to radicals or carbanions (H3). The reaction mechanism of perfluoroalkyl halides with bromides is not a classical SRNl mechanism because the reduction potentials of the substrate and of the perfluoroalkyl radical are too close together (H4). The problem can be overcome by using an electrochemically 88R
ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
generated outer-sphere electron donor as an inductor. Voltammetry and coulometry were used to examine the nucleophilic substitution of molecules such as CF3Br and C6F J by using electron-transfer catalysis. The photocatalyzed and electrocatalyzed reduction of vicinal dibromides and activated ketones was carried out using a ruthenium tris(bipyridine) mediator (H5).Homogeneous redox catalysis was used by Ruhl et al. (H6) to study the fast cleavage of 1,l-dinitrocyclohexane in DMF. The results were compared with pulse radiolysis. Factors, that affect the rate of electron transfer such as solvent, electrolyte, and molecular rearrangement, were exstudied amined by several workers. Fawcett and Lasia (H7) the reduction of zinc(I1) in DMF and DMSO, and in the presence of various tetraalkylammonium salts. The rate of reduction was always slower in DMSO than in DMF due to the stronger solvating ability. The standard heterogeneous rate constant, k,, increased as the size of the electrolyte cation increased. This effect was discussed with respect to electroinstatic effects in the inner layer. Brisard and Lasia (H8) vestigated the reduction of cadmium ion to the amalgam in six solvents, and found that the corrected k, value decreased as one shifted to more coordinating solvents. Weaver et al. (H9)studied solvent dynamic effects in electron transfer. Rate constants for electrochemical exchange for tris(hexafluoroacety1acetonato)rutheniumat annealed gold were compared with the self-exchan e values. The rate constants were markedly smaller ant! insensitive to solvent dynamics than or anometallic systems which have adiabatic athways. This befmvior is consistent with markedly nonadiagatic athways, where the preexponential factor is solvent indepenzent. The electron-transfer event is controlled b the electron-tunne probability rather than by nuclear Jynamics. Fawcett an Yee (H10)demonstrated that, in the electroreduction of lead(I1) in DMF, the double layer has a large effect on the enthal y of activation. A significant dependence was shown on bot\ the electrode potential and nature of the cation of the supporting electrolyte. Focusing on the surface rather than the electroactive material, Rice et al. (H11) found a quantitative correlation between the k, value and surface properties of glassy-carbon electrodes. Mu and Schultz (H12)correlated the rate of electron transfer with the site of the electron transfer on metalloporphyrins. If the electron transfer is porphyrin-centered, the rate is uniformly fast, while the rates are much slower for metal-centered electron transfers. Spectroelectrochemistry was used to identify redox-induced conformational changes In when cytochrome c was reduced electrochemically (H13). contrast to the absolute infrared spectra, the difference spectra between the oxidized and reduced species allow for a more complete interpretation of the spectra. They were able to assign all but one of the major bands between 1500 and 1800 cm- Yuan et al. (H14)were able to determine the rate of conformational chan e in cytochrome c during electron transfer by using doutle-potential step chronoelliptometry. The rate constant for the relaxation of the cyt(II)* intermediate to cyt(I1) was greater than 50 s-l, while the analogous reaction for cyt(II1) was 20 s-l. Several reports have appeared on correlating structural changes with two-electron transfers. Fernandes et al. (HI51 examined the two-electron oxidation of a thiolate-bridged tungsten(0) dimer. The reaction exhibited facile heterogeneous kinetics despite large changes in structure and formation of a metal-metal bond within the WzSzcore. Digital simulation of the cyclic voltammograms showed that the electron transfer occurred in two closely-spaced one-electron steps. Therefore, smaller structural changes were associated with each step, enabling the overall rate to be fast. Hill and Mann (H16) used UV-visible and infrared spectroelectrochemistry to examine rhodium dimers. A single two-electron wave was observed for the Rhz(dimen)3(dppm)2+and Rhz(dimenId2+ (dimen = 1,8-diisocyanomenthane, dppm = bis(dipheny1phosphin0)methane) complexes. The disproportionation constants, as well as the AH and AS values for the disproportionation reaction, were determined. Geiger et al. (H17) examined the two-electron oxidation of the pseudo-tri ledecker complex Cp Ruz(y-COT) and found a reversigle, electrochemically incfuced insertion of ruthenium atoms into the cyclooctatetraene ligand. A similar rhodium pseudotriple-decker complex also undergoes a very quasi-reversible
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DYNAMIC ELECTROCHEMISTRY
two electron oxidation (H18). The twist to tub isomerization was measured using c clic voltammetr and chronoamperometry, and was foun to be about 2 s-! This isomerization was slower than the rate for the analogous cobalt complex (HI@,and was consistent with the larger rearrangement energies for Rh-olefm bonds as compared with Co-olefm bonds. The two-electron oxidation wave of the rhodium complex separated into two one-electron waves at low temperatures. The reaction under these conditions was an EEC mechanism, with the C step being an isomerization reaction. Structural effects on the dispro rtionation equilibrium of tethered tetra hen lethylene rad% anions were examined by Shultz andsox h19). In this study, the electrochemistry of four bis[n.l]metacyclo hanylidenes were studied by cyclic and differential pulse v o L m e t r i c methods. The potential differences between the first and second reduction steps for these cyclophanes depended on the length of the hydrocarbon compared the electrochemical tethers. Ram and Hupp (H20) and homogeneous redox reactivity for species involved in multi-electron transfers. In the homogeneous case, it was shown that the slope of a plot of the observed rate constant versus the thermodynamic driving force can be used to identify the cross-reaction rate-determining step. Lintvedt and Zehetmair (H21)used chronoamperometr to verify that bis[ 1,3-bis(2-methyl-4,6,8-trioxooctan~8-yl~benzenato]tetrakis(pyridine)tetracopper(II) underwent a single four-electron quasireversible wave. The U02-Zn complex was a tweelectron process. The heterogeneous electron-transfer rates for the four-electron reduction of
d
method. The resulting rate constant (1.2 cm with the homogeneous self-exchange rate spectroscopy. H.2. Micelles and Surfactants. Micellar media continues to provide a fertile area for electrochemical research. Martre studied the dimerization of acetophenone in the et al. (H23) resence of cyclodextrins and observed asymmetric induction. bavidovic (H24)examined the reduction of p-nitrosodiphenylamine in cationic micellar systems. As was observed in aqueous media, the reduction in CTAB was an ECE mechanism. The reaction rate in CTAB was somewhat slower due to solubilization of the substrate in the CTAB micelles, adsorption of monomeric surfactant at the electrode surface, and a lower rate of the base-catal ed dehydration reaction ex(C-ste ) in the micellar system. Euszalska et al. (H25) amn idthe dischar e of the lead citrate comples on a mercury electrode covered y! monolayers of aliphatic alcohols. The relationship between the bulk surfactant concentration and the rate of the electrode reaction was examined. An increase of the reactant dimensions can be related to the hindrance in the transport of the bulky readant ion thro h the surface layer of inhibitor molecules and to the greateryecrease of ita concentration in the surface layer. The effect of surfactants on the reduction of azobenzene in methanol in the presence of saturated phos hine oxide monolayers was also studied polar0 a hically ( 26). The determination of the size factors showeftiat the activated complex does not incorporate the surfactant molecules, even at full coverage. Mackay et al. (H27) examined changes in the microstructure of the microemulsions by usin ferrocene derivatives, methylviologen, and ferricyanide. hicrodroplets, as well as the bicontinuous microstructure, were detected. Electrochemical reversibility of the probes was affected by the structure of the microemulsions, and appeared to reflect the ease of mobility across interfaces, and the Ellz values depended on the composition of the microemulsion. Dayalan et al. (H28)also investigated the reduction of methylviologen in cationic, anionic, and nonionic microemulsions. The adsorption of the neutral form of methylviolo en was eliminated by microemulsions, and the Ellz for %oth reduction steps and the diffusion currents depended on the type and com osition of the microemulsion. Rusling and Couture (H29pobserved sharp peaks in the capacitance current versus potential curves at mercury and old electrodes in aqueous micellar systems containin CTAS and normal alcohols. These peaks were attributedi to rapid structural reorganization of a mixed alcohol/CTAB layer on the electrode surface. Organic anion radicals produced in these systems were more stable in the CTAB/alcohol layer than in the aqueous CTAB micelles.
a
Anion radicals stabilized in the CTAB/alcohol layer were used for the catalytic reduction of 2,3,5-trichlorophenol. Reviejo Garcia et al. (H30)used micellar solutions to develop an analytical method for the determination of several organochlorine pesticides (dieldrin, endosulfan, and endosulfan developed a methodology for the sulfate). Texter et al. (H31) voltammetric detection of micelle formation. Voltammetry at a rotating-disk electrode of a water- and oil-soluble electroactive probe was used to detect the onset of micellization in aqueous SDS, DTINS, and CTAB systems. H.3. Solvents and Electrolytes. Novel oxidation reactions continue to be reported in liquid sulfur dioxide because of ita high positive solvent limit, which is up to +6 V vs SCE! In this solvent, straight-chain hydrocarbons can be oxidized at platinum microelectrodes with CsAsFBas the supporting electrolyte (H32).In the same solvent, it was possible to oxidize cesium ion, as well as other alkali metals (H33). Fluoromethanesulfonic acid (triflic acid) was used for cyclic voltammetry up to +3.3 V (H34).Because triflic acid is a strong acid, deprotonation is su pressed when coordination compounds of high metal oxi&tion states are generated. Liquid- h e electrochemistry at ultralow temperatures was report81 using mixtures of propionitrile and dichloroethane (H35). Temperatures as low as -180 "C were obtained using tetrabutylammonium hexafluorophosphate or perchlorate. examined the use of n-butyronitrile over Bond et al. (H36) a wide range of temperatures and scan rates with conventional found and microsized electrodes. Chandrasekaran et al. (H37) that tetrafluoroborate formed a f i i on a carbon electrode at positive potentials, probably due to the fluoridation of the developed a technique for the carbons. Maran et al. (H38) determination of the pK, of weak acids in DMF using voltammetric methods. H.4.Organic Electrochemistry. Electrochemistry continues to be valuable in robing the reactions of organic radical used fasbscan cyclic anions and cations. AnLeux et al. (H39) voltammetry to observe cation radicals of pyrroles and to measure their lifetimes. This study was later extended using fast double-potential step techniques (H40).Systematic analysis of the reaction kinetics for three different pyrroles showed that the cation radicals, rather than the neutral radical (formed by deprotonation), were involved in the carboncarbon bond-forming process and that they couple between themselves rather than with the starting monomer. Yang and used rapid-scan cyclic voltammetry and digital Bard (H41) simulation to study the mechanism of diphenylamine oxidation and polymerization. The initial step in the polymerization was a second-order radical cation-radical cation coupling to form diphenylbenzidine. Digital simulation suggests that, after the dimerization, deprotonation occurs, followed by further oxidation. Cyclic voltammetry was also used to probe the electrolytically induced isomerization of 1-phenyl-1-hexyne to 1phenyl-1,Bhexadiene (an allene) (H42). The rate-determining step in the rearrangement is deprotonation of the alkyne by electrogenerated bases. It was also found that protonation of the conjugate base of the alkyne by the parent alkyne is a rapid process. The reduction of maleate dianion was examined at a mercury electrode using rotating-disk voltamIt was found that the reduction proceeded by metry (H43). a CEEC mechanism rather than a CECE mechanism. Pulse voltammetry at cylindrical electrodes was used to study the The data were not consistent oxidation of anthracene (H44). with a simple ECE mechanism but indicated that a disproportionation mechanism (probably DISP1) was occurring. The use of relatively small diameter electrodes is advantageous in pulse voltammetry since restoration of the diffusion layer between ulses is enhanced by the nonlinear diffusion process. The oxilation of formaldehyde on rhodium was used by H46)to study oscillating electrocatalytic Hachkar et al. (H45, systems. As with other areas of chemistry there has been a reat interest in the electrochemistry of fullerenes, which are farge clusters of carbon atoms. The first reduction potentials of two different fullerenes, Cm and CT0,were found to be esThe compounds sentially the same in several solvents (H47). were also able to form di-, tri-, and tetraanions, which were all EPR active (H48).The redox processes were examined both electrochemically and spectroelectrochemically (H48). In addition to the four reduction steps, an irreversible fourANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
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electron oxidation was also observed in benzonitrile and benzene, as well as a fifth reduction wave (H48). Seger et al. (H49)examined the rospects of using solid films of Cm and C,o fullerenes in lit ium batteries. Cyclic voltammetry, chrono otentiometry, and an effusion test showed that reduced fullerenes are soluble in the electrolyte tested. Unfortunate1 the poor electrochemical reversibility and stability of the higtly reduced material would seriously compromise the performance of batteries based on these materials. The SRNlmechanism was used by Degrand to synthesize selenium and tellurium compounds. For exam le, phenylseleno and phenyltelluro compounds were formed gy the direct and mediated reduction of unactivated bromoarenes in the reeence of an equivalent amount of PhSe- or PhTe- (H50). begrand and Prest (H51) determinedatheabsolute and relative reactivities of hen 1chalco enide mons toward aryl radicals. Reitstoen an Par er (H52 examined the reaction.between anthracene or 9-substituted anthracenes with pyridine nucleo hiles using voltammetric methods. Simonet et al. (H53) usefthe solvent itself as the electrode carrier for the indirect reduction of aromatic halides. Chlorobenzene, bromobenzene, and 2-chloropyridine were reduced catalytidy by the radical anion of the solvent, benzonitrile. The radical anion of anthracene and the dianion of thianthrene 9,9,10,10-tetroxide were both used to mediate the reduction of aromatic halides (H54). While the half-wave potentials of both mediators were about the same, the electron-transfer step was slower for the latter case than the former owin to a reorganization process involving ion pair formation. he competition between a catalytic process and a fmt-order deactivation of the mediator was examined for cyclic voltammetr by Medebielle et al. (H55).This mechanism was studied goth theoretically and experimentally for the reduction of C6FI3Iby 4-nitropyridine N-oxide anion radical in the presence of 2-methyl-5-nitroimidazolate. Sacrificial cathodes were used by Tholie-Gauter and Degto generate directly diary1 dichalchogenides by an rand (H56) Sml reaction. For this purpose, a tea-bag type electrode was used in which a graphite cloth bag was fabricated with selenium pearls or tellurium pellets. Cathodic olarization of selenium and tellurium electrodes generated e2- and Te2-, which reacted with 4-bromobenzophenone to form aromatic diselenides and ditellurides. Electrosynthesis was also used by Liddell et al. (H57)to synthesize porphyrins from a,?biladienes. Evidence was found for a common mechanistic pathway in the electrochemical and chemical routes. Carbon dioxide inserted itself into 1,3-di es by a nickel-catalyzed F w a s possible to generate electrochemical reaction (H58). 2-vinyliden-3-yne carboxylic acids from carbon dioxide and substituted 1,3-diynes. The use of redox mediators to catal ze a wide variety of examined reactions continues to expand. Do and 6 o u (H59) the kinetics of the oxidation of benzyl alcohol with a redox and cobalt mediator and a phase-transfer catalyst. Iron (H60) (H61) porphyrin modified electrodes were used by Jiang and Dong to mediate the reduction of dioxygen or hydrogen peroxide. The reduction of dioxy en with cobalt protorphyrin used a rotat' ring-disk ekctrode (H60), while the Using K t e r study used a rapTiotation-scan method (H61). this method, the rate constant for the catalytic process as a also function of pH was determined. Shi and Anson (H62) examined the electroreduction of dioxygen at iron orphyrin modified electrodes. Iron tetrakis(4-N-methyppyridy1)porphyrin absorbed strongly to edge plane pyrolytic graphite electrodes. The adsorbed species catalyzed the electroreduction of dioxygen at potentials more positive than can be obtained with electrodes on which the porphyrin does not adsorb. Saw chi et al. (H63) utilized an iron porphyrin ion com lex m& electrode to catalyze the same reaction. ?e cadytic material in this case consisted of an electrostatic aggregate of a cationic and anionic iron porphyrin. H.6. Organometallic Electrochemistry. The oxidative cleav e of the Ru-C bond in cyclopentadienylruthenium methy complexes was examined using electrochemical and spectroscopic techniques (H64). Differences in the decomposition mechanism were observed in comparing the phenyl with the cyclohexyl derivatives. The cyclohexyl complex decomposed b a single first-order process, while the henyl derivative exhigited com titive fmborder pathways. Jolvent effects indicated that cyclohexyl derivative underwent
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Ru-C bond cleava e via a 19-electron intermediate. The electrochemical rerfuction of a cobaltacycle was studied in THF by cyclic voltammetry, bulk coulometry, and rotatingring-disk voltammetry (H65).The electrogenerated 19electron species greatly enhanced the alkyl to acyl migratory insertion reaction, leading to the fluorenone anion. This reaction occurs only with great difficulty for the 18-electron species. The electrochemistry of cis- and trans-tetraazacyclotetradecane complexes of cobalt(II1)at gold electrodes was investigated by cyclic voltammetry and chronoamperometry (H66). The cis and trans cobalt(I1) isomers have roughly the , but the trans cobalt(II1) complex was 21.3 kJ than the cis complex. As a result, cis/trans isomerization must occur during the redox process. Amatore and Pfluger (H67) investigated the mechanism and rates of oxidative addition of substituted iodobenzenes to palladium(0) complexes in toluene by means of electrochemistry at ultramicroelectrodes. The Hammett slope suggested that the transition state for the addition of aryl halides to the coordinately unsaturated palladium complex had no significant ionic character. Alkyl halides were found to react rapidly with electrochemically generated iron(1) and iron(0) porOnce the u-alkyl porphyrins were formed, CO phyrins (Ha). was able to insert into the Fe-C bond. No insertion was observed for a-aryl iron porphyrins due to the increased strength of the Fe-C bond. The mechanism and rate constants were determined by cyclic voltammetry, thin-layer s ectroelectrochemistry, and reparative-scale electrolysis. $he oxidation of metalloporp yrins bearing secondary amide-linked superstructures were found to undergo a unique two-electron one-proton reversible ring-centered oxidation (H69). The reaction, studied by UV-visible-near IR and FTIR spectroelectrochemistry,involved the formation of an oxazine ring formed upon condensation of a meso-carbon of the porphyrin dication with the oxygen of the amide group. The transfer of the nitrosyl ligand to and from iron and cobalt porphyrin complexes was investi ated by Mu and Kadish (H70) by electrochemistry and F j I R and EPR spectroelectrochemistry. The driving force in these reactions is related to the nature and oxidation state of the central metal. The electronic structure of reduced and oxidized organometallic complexes can be readily invest' ated in combination with spectroelectrochemical methods. 'gV-visible spectroelectrochemistry has become quite standard, but other s ectroscopic techniques are becoming much more common. bhe spectral characteristics of one- and two-electron oxidation products of cobalt octaethylporphyrin was investigated b absorption and magnetic circular dichroism to distinguis metal-centered from ring-centered oxidations (H71).The MCD s ectral data showed that the wring oxidation 4wa s led to t i e appearance of characteristic spectral features in t e visible region that can be used to identify ring versus metal-based oxidation. The oxidation state of the metal for the reduced species for nickel(I1) PO hyrins was investigated by Kadish et al. 873)also studied the factors Nahor et al. (H72). that determined whether a nickel(1) porphyrin, a nickel(I1) porphyrin radical anion, or some combination was formed using spectral characterization. Similarly, the effect of structure and medium on the formation of nickel(II1) orphyrin or nickel(I1) porphyrin *-radical cation was stuiied by Nahor et al. (H74).Choi et al. (H75)used resonance Raman spectroelectrochemistry to determine that the reduction of Fe(TPP)(NO) was centered on the Fe-NO moiety. H.6. Inorganic Electrochemistry. The nitric oxide/ nitrosonium couple gave a reversible cyclic voltammetric wave at +1.28 V vs SCE in acetonitrile and nitromethane (H76). The electrochemistry of this couple was examined in several nonbonded complexes involving nitrosonium with crown ethers, hexamethylbenzene, durene, and pentamethylbenzene. The reversible redox potential for the N02+/N02couple was determined by cyclic voltammetry (H77).Chronoam erometry was used to determine the kinetics of the N2O$NO2 equilibrium. Mirkin and Bard examined the voltammetry of the borohydride anion in order to study the electrosynthesis of sodium borohydride from borate. The borohydride anion gave a well-defined irreversible voltammetric oxidation wave (EPabout -0.5 V) at a old electrode (H78), and the peak current was proportionaf to borohydride concentration. The reaction of ferric ion with DOPA led to the initial formation of a ferric-DOPA complex, which decomposed by an internal
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electron-transfer reaction (H79). The reaction was studied by chronoamperometric and spectroscopic methods, and it was found that the ultimate products were ferrous ion and dopaquinone. The electrochemistry of the iron-molybdenum cofactor, isolated from nitrogenase, was examined by Schultz et al. (H80). Electrochemical measurements were carried out to explore the surface reactivity of the oxide superconductorYBa Cu 0, (H81). The electrochemical response was found to b e h h y dependent on both the method of surface treatment an on the amount of water in the electrolytic fluid. Between 1.3 and -1.4 V vs SCE, the electrode behaved similar to a noble metal electrode like platinum. Outside this ran e degradation of the electrode surface occurred. Slezek and kieckowski (H82) studied the coprecipitation of Y,Ba, and Cu (as the hydroxides) for the electrochemical synthesis of yttrium barium cuprate superconductors. After high-temperature treatment, the films showed superconductive characteristics, but with low and broad transition temperatures. A superconductor (Bi&3r2CaCu20 microband electrode was fabricated from the &ea of thin of the superconductor sandwched between two insulating layers (H83). At room temperature, these microbands gave well-defined voltammograms. H.7. Activation of Small Molecules. Azuma et al (H84) examined the electrochemical reduction of carbon dioxide on 32 electrodes. The current efficiency of carbon dioxide reduction on nickel, silver, lead, and alladium increased significantly as the temperature was ecreased. Formation of methane and ethylene was observed on almost all electrodes, although the efficiency was very low except for copper. At a palladium electrode in 0.05 M bicarbonate buffer, though, carbon dioxide can be converted to hydrocarbons (H85). Reduction of carbon dioxide at a metal solid polymer electrolyte interface was also found to yield ydrocarbons (H86). A solid-state electrochemical cell was examined for the catalytic conversion of carbon dioxide and hydrogen to methane The electrolyte, made up of ytria-stabilized and oxygen (H87). zirconia, served as a sup rt material for the platinum catalyst, and an oxygen ion-con ucting membrane for the removal of surface oxygen formed during the reaction. A new type of cell for the detection of volatile-products by masa spectrometry was used to observe the reducbon of carbon The electrode consisted dioxide to methane and ethene (Ha). of bulk copper, electrodeposited copper on glassy carbon, and in situ electrodeposited copper on lassy carbon. The mass signal was observed as a function of potential and time, and a poasible reaction mechanism was discussed. A new catal ic system, using nickel(I1) associated with pentamethy diethylenetriarmne, was used to reduce carbon &oxide to alkynes (H89). The reduction mechanism of carbon dioxide by nickel(I1) cyclam complexes was studied b cyclic voltamOgura et met ,polar0 raphy and electroca illarity al. 8 9 1 ) use8 a hydrogen fuel c e i to catalytically reduce carbon dioxide to methanol. This process involved both hom eneous and heterogeneous catalysis. Iron("0") porphyrins in%e presence of magnesium were used by Hammouche et al. (H92)to catalyze the reduction of carbon dioxide. Molybdenum and tungsten com lexes with dinitrogen were studied by Becker and Avraham (fi93) by cyclic voltammetry in THF. Controlled-potential electrolysis yielded ammonia selective1 in chemical yields which ranged from 1.3% to 35.8%. daseous ethylene was reduced electrochemicallyon The ethylene rea platinized Ndion 117 membrane (H94). action kinetics were found to be similar to those reported for the reduction of ethylene dissolved in aqueous solution. Ethane and dihydrogen were detected. H.8. Miscellaneous. The kinetics of the de osition of colloidal graphite particles onto an indium tin oxi& electrode The rate of deposition was studied at a wall jet cell (H95). was followed by measuring the intensity of light from the evanescent wave scattered by the de osited particles. The effect of potential on the deposition finetics was examined. Thin-la er electrochemistrywas used to study the under otential dkpition of cadmium and tellurium on polycrys&e The strippin gold, platinum, and copper electrodes (H96). of galenite, which had been fabricated as a disk electrode wit a gold ring, was studied by Hoskova et al. (H97). Lead was reduced from the galenite at potentials more negative than -1 V and remained on the surface of the electrode. The reduced lead was then stripped from the electrode at potentials
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more positive than -0.5 V. At more positive potentials, PbS was oxidized to Pb(1V). Ultrasound hydrodynamic modulation voltammetry was described by Dewald and Peterson (H98).Ultrasound was used to increase the rate of mass transfer (similar to electrode rotation). Rather than use constant irradiation, the ultrasound irradiation was modulated to give an ac current res onse. Sonoelectrochemistry was reported by Mason et al. pH99). The examined the application of ultrasound to the Kolbe synthesis of several carboxylic acid salts, where it was used to improve mass transport, clean the surface, and degas the electrode. Golasz et al. (HI001prepared platinum cylindrical ultramicroelectrodes for high-pressure measurements. These electrodes were used between 1and 8OOO bar, and quantitative data on the pressure dependence of the diffusion coefficients of ferric anide and dioxygen as a function of pressure were obtainel Krasinski et al. (H101)studied the cadmium(11) cadmium amal am system up to 10 kbar. The partial mo ar volumes for Affusion activation of Cd(I1) in solution and cadmium in the amalgam and the volume of the electroreduction reaction of cadmium(I1) were determined. studied the photoelectrochemical Muzyka and Fox (H102) oxidation of vicinal cyclohexanedicarboxylic acids on irradiated T i 0 suspensions. Under these conditions, the one-electron oxdative decarboxylation occurred rather than the twoelectron bis decarboxylation, which was seen on poised metal electrodes. Smith et al. (H103) examined the voltammetric roperties of photochemically generated free radicals. 8teady-state currents for oxidation and reduction of photochemically generated radicals were detected using latinum microeledrodes. Radicals such as diphenylmethyl radlical were produced by photolysis (sym-tetraphenylacetone in this case).
I. SPECTROELECTROCHEMISTRY The following survey is organized principally by techni ue. Most of the articles cited involve applications of establis ed spectroelectrochemical techniques. New developments are indicated where appropriate, and details of specific reactions are given when they illustrate particularly well the versatility of a given method. On-line mass spectroscopy is one of the least ambi uous methods to study an electrode process. Hartung et a! (11) have published the design of a 1-pL thin-la er cell for electrochemical maas spectroscopy (ECMS) in a &ferential mode in which a porous Teflon membrane serves as the mass spectrometer inlet. They have studied the desorption and hydrogenation of benzene at Pt(ll1) single-c stal electrodes Other recent with a sensitivity of ca. 1% of a monolayer ECMS applications include a stud of electrode reactions at a porous Ni/Teflon membrane (137,the reduction of nitrate and nitrite to NO NzO/N2 gaseous mixtures (141,the oxidation of PC/LiCl solutions (1.9,the electrocatalysis of COz reduction by a nickef-cyclem complex (161,and the oxidation of formic acid at Pd (17). In recent years access to synchrotron radiation sources has become possible, and as a result, specific groups have been able to perform in situ electrochemical X-ray absorption and scattering experiments. Techniques such as EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) have provided a way to determine bond lengths and structures of electrode and solution heavy atom species in the first few molecular la ers of the Faraday electrochemical interface. A recent Discwswns of Society was devoted to the study of surfaces using synchrotron radiation (18).A good example of the information available from these experiments is the UPD of Bi on Ag(ll1) studied by in situ X-ray scattering techniques (19). Detailed strvctures of the bismuth layer were revealed including the observation that the bismuth monolayer compressed as the electrode potential decreased. In situ X-ray and the ex situ LEED results were in agreement for this system. EXAFS and XANES spectra have been obtained at several electrode surfacea for the electrodeposition of copper (11GZ13). On Au(100) the first gold layer undergoes a rearrangement with a decrease of the gold-gold spacing upon UPD of Cu and the oxidation state of the copper is close to + 1 (111).In situ X-ray and EXAFS techniques have also been ap lied to the oxide Ni/Ni(OH) /KOH(aq) interface (114),the CU(I)/~U(II) couple in a %oratebuffer solution (115),and the complexation of copper and iron incorporated into a conducting polymer
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film (116).In situ XANES spectra indicated that a (p-oxo)iron porphyrin dimer in an adsorbed oxidized state on carbon underwent a tweelectron reduction to a predominantly axially uncoordinated Fe(I1) species (117). EXAFS spectroelectrochemistry was carried out on a Cu(II/I) phenanthroline couple both in an aqueous and in a solid-state cationic polyelectrolyte. In the latter medium, coordination chan es were not seen upon oxidation of the Cu(1) com lex (1187, Details of the Elder and Heineman flow cell have Been published (B88).Nagy et al. have discussed and tested three cell designs for X-ray scattering measurements at electrode interfaces based on reflection and transmission geometries (119). Pyridine adsorption on Au has been studied using synchrotron radiation coupled with electrode potential modulation techniques (120).Bommarito et al. have studied the electrosorption of iodine and determined the distribution of iodide ions in the diffuse double layer a t an electrode by means of an X-ray standing wave method (121). In the less esoteric visible region of the EM s ectnun, there have been several interestin pa ers that can e noted. The principles of an e1ectromoduLJelectroreflectionabsorption spectroelectrochemistry technique (which cries for an acronym) have been outlined (122).In this method, the real and imaginary components of the modulated apparent reflectivity due to adsorbed species are measured when the electrode otential is subject to a small amplitude sine wave perturgation. Electron-transfer rates in the 60-120-s-' range were measured for the Nile blue couple on pyrolytic graphite. The wavelength sensitivity makes this a powerful method. As noted above in section E.3, Caram and Gutierrez have used this technique in less quantitative fashion to study the adsorption of CO on solid electrodes (E88,123). Potential modulated reflectance spectrosco y was also used to study thin anodic oxide films on Ti a n i was shown to be a ood method to obtain the flat band potential of the oxide (724). Several exemplary spectroelectrochemical studies in the UV-vis region have a eared recently (125128).Takeshita et al. used in situ vis& and Raman spectroelectrochemical methods to study thin films of octacyanophthalocyanine (129, 130),and Elzing et al. obtained transmission and reflectance spectra of tetrasulfonatophthalocyanine adsorbed on the basal plane of pyrolytic graphite electrodes (131).Dong and Zhu reported on an electric dichroism phenomenon for adsorbed cetylpyridinium cations in a long optical path length, thinlayer cell, which was attributed to reorientation of the adsorbed ions along the direction of the electric field in the double layer (132).Mu and Kadish employed FTIR, UV-vis, and ESR thin-layer spectroelectrochemistry to characterize Ru-porphyrin redox reactions (133). Theory was developed for the construction of Nernst plots from spectroelectrochemical absorbance data when there was overlap of two or more independent redox couples (134).In this situation, assumption of a simple Ox/R couple can result in nonlinear plots. Detailed information on the nature of electrode processes can be obtained when a probing light beam is aligned parallel to the electrode surface. McCreery has continued his work in this area with a study of the homogeneous second-order redox catalysis mechanism that employed the phenothiazine cation radical as a mediator species (135)and a study of the oxidation of do amine by the chlorpromazine cation radical (136). Xie an8Dong have presented theory for the time dependence of the absorbance for a potential step spectroelectrochemical experiment for several common situations (137-139). Two groups have observed that a laser beam aligned parallel to an electrode surface in the diffusion layer will deflect as a result of refractive index variations, the so-called mira e effect. Accordingly, the ejection of protons from Ir and &y oxide films upon oxidation (140), roton release and anion insertion at polyaniline films (1417,ion gradients a t redox ion flux at Prussian blue elecpolymer electrodes (142,143), trodes (144),and counterion flux at lutetium diphthalocyanine f h (145)are all processes that were studied by this method. The release of pyrenesulfonate and naphthalenesulfonate from polypyrrole films was monitored by the fluorescence spectra of these species in solution (146),and the release of ethidium ions from a carbon-paste com osite matrix was followed by the fluorescence of DNA-etkdium complexes
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formed in the diffusion layer (147).Trace amounts of DNA in solution could be detected by the latter procedure. Adsor tion of pyrene on silver was followed by a fluorescence proie technique (148). Infrared reflection-absorption spectroelectrochemistry has been used extensively in surface studies at solid electrodes and can be regarded as a mature technique. Many of the articles cited in sections E.2 and E.3 above employed FTIR spectroscopy especially when carbonyl or other strongly IRactive oups were involved in the electrode reaction. A good exampg is the study of Chang and Weaver on CO adsorption on Pt and Rh single-crystal electrodes in which the in situ electrochemical and the ex situ UHV data were compared
acetic acid as products of acetaldehyde oxidation at Pt (150) and determination of the orientation of SCN- adsorbed on silver electrodes (151).Polarization modulation ATR spectra were used to study adsorbed tetracyanoethylene radical anions on transparent gold films (152). The technique of optical second harmonic generation (SGH) can monitor surface reactions and provide information on orientation and average surface symmetry of adsorbates (A41, 153). SHG has been used to study a variety of surface electrode rocesses including deposition of Cd on Au(ll1) (154), TI U#D on Au(ll1) (155,156), Cu(ll1) electrodes in HC104(aq) (157),and others (158-161).Anion chemisorption at polycrystallinePt was nicely studied with SHG techni ues where the irreversible adsorption of zerovalent iodine an! its replacement by atomic hydrogen in basic solutions was confirmed (162).The adsorption of CN- and SCN- on Pt was also investigated using sum frequency generation infrared spectroscopy (163). SERS is another mature spectroelectrochemical technique. The recent work of Pemberton and her students (F225,164166)on the adsorption of simple alkanols and alkanethiols on Ag and Au surfaces illustrates well how careful interpretation of the spectra can give detailed information on the adsorbate structure and orientation at an electrode surface. Surface roughening effects were studied, and the comparison of emmersion and in situ spectra indicated that the interface was preserved upon emmersion a t atmospheric pressure. Other studies include SERS of buckminsterfullerene f i i (167,168), in situ Raman s ectra of the transformation of amorphous titanium oxide f i i to the anatase, brookite, and rutile forms (169), and several applicationsto reactions of adsorbed organic species, mostly on silver electrodes (170-179). In the SERS study of Byahut and Furtak (180), the full emission cone of emitted radiation was collected using a hemispherical glass prism in the "Kretchmann" configuration. Several papers have addressed aspects of the surface-adsorbate interactions in SERS and methods to enhance intensity on surfaces where the SERS effect is not operative. In situ Raman scatterin s ectra for C10 - at smooth Pt electrodes were reported (!81y Byahut and #urtak presented evidence that single-crystalAg(111), grown on mica surfaces, was devoid of active sites for enhancement of the Raman process (182). Controlled and gradual ro hening of the surface produced adatoms that gave the S E 8 effect. SERS of glassy-carbon and HOPG electrodes were obtained after deposition of silver particles (Ell).This rocedure gave a better sensitivity to the nature of the cargo, surface than normal Raman spectroscopy to that of the untreated surface. Several flow electrolysis cells for spectroelectrochemical measurements have been described (183,184). The cell of Oyama et al., which utilized a carbon-wool column, permitted s cies with lifetimes greater than 50 ms to be quantitatively etected. It was used to follow cation-radical intermediates in several electrode reactions (185,186).The flow cell of Roth and Weaver allowed potential difference infrared spectra to be obtained and thus distinguish between solution and surface components (E89,187). The principles and experimental demonstration of Hubbard's ADAM technique (Angular Distribution Auger Microscopy) were presented in an important pa er (188). In this UHV method the complete angular distrigution of Au er electrons from a single-crystal surface is collected. T is
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distribution consists rimarily of the silhouettes of nearsurface atoms that are fack-lit by emissions from atoms deeper in the crystal structure. Thus the multicolor, computer-enhanced (and somewhat surrealistic) ADAM images show the interstices in the outer atomic layers of the surface. Adsorbate s cies can be imaged on the surface. It should be noted that gbbard's description of the ADAM technique was controversial and may still be (189-191). Simultaneous electrochemical electron spin resonance (SEESR) continued to be a routine technique for the characterization of radical-ion intermediates in electrode reactions (192-197). Compton et al. developed ESR and RDE methodology to study EE mechanisms and applied it to the reduction of chloranil in CH&N (198).Two nice applications of the use of spin traps to detect radical intermediates in electrode reactions have appeared (199,1100). ESR methods are often used to detect spin density in "doped" conducting and redox olymers (1101). The line width study of Genies on polyaniyine can be cited again here (F187). A useful design for an ESR cell probe for the simultaneous determination of in situ con uctivity and ESR spectra has been published (1102). Spatial resolution of electrochemically generated radical ions in an ESR cavity was achieved by recordin spectra at a series of magnetic field gradients (1103,1104).&r example, the spectra of a quinone radical anion generated at a cathode and a 9,10-diphenylanthracene radical cation generated at the anode were observed simultaneously, separated in space by approximately 2 mm. Line width variations due to electron exchan e and indirect generation of radicals at the auxiliary electro e were seen in this in enious techni ue. Two reports of rarely usecfh henated e ectrochemical techniques have been published.%incey et al. described an in situ electrochemical NMR experiment that used a 5-mm NMR tube coated with a Sb-SnOz semiconductor film as a working electrode (1105).The performance of the cell was demonstrated for the reduction of 0.1 M benzoquinone to the hydroquinone in DzO. A reliminary report has appeared of in situ electrochemicalrezction adsorption m etic circular dichroism, in which the working electrode was ocated in the gap of a 80000-G superconducting magnet (1106). Finally a photoacoustic study of surface roughening phenomena of silver electrodes in aqueous electrolyte solutions will be mentioned (1107).
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J. INSTRUMENTATION Several potentiostat and related instruments have been described. These include a four-electrode potentiostat that uses the current interruption technique for IR drop elimination (Jl),a wide-bandwidth computer-based otentiostat that operates at 2 MHz for 1 pA full-scale (J2f a batterypowered, pro rammable function generator for stripping voltammetry f53),a device that controls the ratios of partial anodic currents at separate sacrificial counter electrodes for electrodeposition of ternary compound semiconductors (541, and a two-electrode current measurement circuit, based on a current feedback mode of operation for correction of ohmic drop, which does microelectrode CV at swee rates up to 50 000 V s-l (J5). Instrumentation and procefures have been described that allowed cyclic voltammetry to be carried out at sweep rates greater than lo6 V s-l using microelectrodes (J6).Battery-powered two-electrode microvoltammetry can be performed, however, with the simplest of circuits (J7). Details of an interface between a IBM PC XT and a PAR Model 273 potentiostat have been given ( 8). A variety of electrochemical cells with speclfic performance features have been described. Voltammetry was performed on a single drop of solution 1mm in diameter in the minielectrochemical cell of Howard and Koval (J9).A stainles steel needle assembly with a graphite-microfiber working electrode was used for a 10-pL volume cell (JlO). Pressures up to 125 psi above ambient can be employed in the cell of Anderson and Maher that was constructed from commercially available glass joints (511).Static and circulating devices, including a thin gap electrolysis cell, have been described for synthetic reductions in liquid ammonia (512).If you want to deep-six your gear, look up the submersible ASV cell of Tercier et al., which operates in oxy en-saturated sea water at depths of 20 m below the surface b13). Hansen and O'Grady have given the design of a cell for ex
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situ X-ray absorption studies of emmersed electrodes (J14). The measurement of ECL quantum yields was performed in a cell that was designed to control the influence of doublelayer geometry, and the efficiency of the electrode process ; J T f conventional sandwich O P L E flow cell employing a gold minigrid electrode for FTIR spectroscopy (J16),a s a m le-switching interface for FTIR of electrode surfaces (JI a sample holder for reproducible substrate positioning and an extremely simple in IR reflectance spectroscopy (Jl8), thin-layer cell for spectroelectrochemistry in a Spectronic 20 cell (J19)have been re orted. A circuit diagram anacell design for a two-contact in situ measurement of conductivities of electrochemically synthesized conducting polymers was published (J20). Commercial indium oxide electrodes, which remained colorless in both reduced and neutral forms after lo00 cycles, were used as counter electrode in spectroelectrochemical cells (J21). Details have been given for construction of a Pt/ SiOz/TiOz hotoelectrode (J22)and for a molecular glue for attaching t k n old layers to glass substrates (J23).Rooney et al. describef a laser-directed chemical vapor method for the depaition of thick and optically thin platinum electrodes (J24). The use of the ferrocene+/Ocouple as a reference potential standard in nonaqueous solvents was discussed by Hupp (JW). The Cp*,Fe molecule (where Cp* is pentamethylcyclopentadienyl) was suggested as a substitute for ferrocene for this purpose since the C e cation is more stable than the ferrocenium ion (J26).IP*F t was re orted that insertion of a Pt wire into a Luggin capillary proie mitigated the deleterious effects of stray capacitance and resistance in the reference electrode (J27). The SECM technique of Bard has been developed and refiied in the last 2 years as detailed in several papers. The sign and magnitude of the feedback current allows chemically different sites to be distin uished and resolution is now approaching the 100-A level fA58,B70,J28-J31). The related vibrating probe electrode technique for analyzing current density distributions was treated theoretically (J32). Another novel method of imaging electrode surfaces is the high-resolution phase-measurement interferometric microscopy technique reported by White et al. (J33).This method, which only requires that the surface to be imaged is partially reflecting, rovided 0.6-nm-vertical and 0.5-pm-horizontal HOPG, Pt, and corroding Fe electrodes. resolution Details have been given for digital sampling strate 'es and anal0 methods that reduce noise levels in electroc emical sign& (534,JS). An expert system for the determination of electrode mechanisms via artificial intelligence software was described (J36).It had 10 simple mechanisms in its memory and used the results of four electroanalytical methods: cyclic staircase voltammetry, chronoamperometry/chronocoulometry,and convolution voltammetry.
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K. PHOTOELECTROCHEMISTRY Gerischer has presented theory for electron-transferkinetics at the semiconductor/electrolyte solution interface (K1).Here a comparison was made with the electron-exchange rate at a semiconductor metal Schottky barrier and conditions were given that shod favor reaction via surface states. The effect of surface states on photocurrents and an analysis of photocurrents at semiconductor electrodes were also analyzed by Ramakrishna and Rangarajan (K2,K3). The review of Lewis (A62)can be cited in this context as well. Open-circuitphotovoltage transients at n-TiOzsinglecrystal electrodes (K4)and transient photocurrents at GaAs electrodes (K5)have been obtained and analyzed. The model used in the former study included terms for recombination and diffusion in the space charge region, drift in the electric field, and charge-transfer reactions at the surface. Research continued on photoelectrochemical cells for the conversion of light into electrical energy. Textured TiOz f i b s modified with cyanide-bridged trinuclear Ru complexes exhibited improved characteristics for this purpose: a fii factor of 75% and a power conversion efficiency of 11.3% at 520 nm (K6). Others have re orted the effects of postheating treatments (K7)and niotium doping (K8)on the properties of TiOz films. Absorption spectra of 4-pm-thick TiOz membranes (obtained between 350 and 800 nm and over a 2.5-V
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range) indicated formation of trapped conduction band electrons at negative potentials (K9).The directly obtained flatband otential obeyed the relation, Ecb = -0.34 - 0.06pH (V vs SCE). Matsumoto et al. have grown @PbO epitaxial layers on Ti02 by photoelectrochemical means (k10). Large photocurrents (25 mA cm-9 and o en-cell voltages (0.6 V) were obtained in photoelectrochemidcells with a bare Si(100) electrode immersed in acetone/methanol solutions containing ferrocene derivatives under Am1 100 mW cm-2 illumination (K11).The open-circuit voltage for the n-Si/ methanol junction was measured over the temperature range from -75 to +20 OC (K12).The use of a two-solvent cell in order to minimize hotocorrosion of semiconductor anodes has been suggeste8(K13). Peter and co-workers have ublished several papers on the photocorrosion of n-Si in N8,F (K14-KI7).The photodissolution of n-GaP was followed using labeled 32Pand 72Ga (KIB),and STM images of n-GaAs undergoing anodic photocorrosion showed increasin surface roughness with time (K19).The porous arsenic-ricg layer that forms on the latter surfaces in aqueous acidic solutions was also characterized by surface analysis and ellipsometry (K20).Papers on the nAl,Ga,,As (K21)and the p-InP (K22) hotoelectrodes, the photooxidation of formic acid at n-Cd sin le-crystal electrodes ( K B ) ,and the photaoxidation of ri-met!oxymbenzene at Ti02film electrodes (K24)have appeared. Electrochemiluminescence in the NIR region due to band transitions in n-InP single-crystal electrodes was reported (K25). Photoreduction of O2has been observed at IT0 electrodes coated with thin films of porphyrins and several phthalocyanines (K26).Photocurrents that switched direction upon reversal of the direction of illumination were seen in symmetrical photocells consisting of a liquid cr stal porphyrin, zinc octakis(4-oxtyloxyethyl)porphyrin,san wiched between I T 0 electrodes (K27). The detailed analysis of the kinetics of the pho-tooxidation of organic molecules on semiconductor particles in the presence of oxygen by Gerischer and Heller is pertinent to photoelectrochemicalstudies (K28). They concluded that catalysis of the oxygen reduction process will be mandatory for the realization of high quantum yields in these processes. Photoelectrochemistry of irradiated suspensions of 10-pm FeS2 particles was researched by Chen et al. (K29).Addition of the R u ( N H ~ ) mediator ~~+ to the system containing tartrate as a hole-acceptor greatly increased the measured photocurrents. Several articles on electrochemiluminescence(ECL) caught your reviewer’s eye. A microring optical fiber combination was used to detect ECL in the u-bpy/persulfate system (K30). The probe consisted of a 200-pm core diameter optical fiber surrounded by a 15pm gold ring that was in e p w and polished. A modified electrode prepare by adsor ing a Ru-bpy complex substituted with an alkanethiol on the surface was used for the electrogeneration of light in the oxalate system (K31).Other ECL studies include the report of ECL from bimetallic iridium complexes in nonaqueous solution (K32)and from several ar 1 derivatives of N,N-dimethylaniline (K33-K35).For the ratter systems, the intramolecular ECL from a twisted charge-transfer state was found to be dependent on the su porti electrolyte and the extent of proton production in t i e a n 3 i c step (K36). Long-terma ueous solution ECL was reported for the direct current electro ysis of the 2,2’-bi yrazine complex of Ru(I1) in the presence of S202-ions (&7). The same group used ECL in aqueous [ R ~ ( b p y ) ~ ] ~02+ / Ssolutions as the basis for the determination of peroxycfisulfate in the 10-9-10-3 M range (K38). Char e-transfer reaction inverse photoemission was observed from a gold electrode in acetonitrile solution (K39). Surface states were invoked to explain the dependence of the emission efficiency on potential and the nature of the redox couple. Several studies that involved electrode reactions of reactive photogenerated species can be noted. Smith et al. used a microelectrode to obtain voltammo ams of diphen lmeth 1 radicals and reported the p o t e n t i E f the Ph CH+Yocoupre to be 0.00 f 0.1 V vs ferrocene in acetonitrile (840). Nagaoka et al. have digitally simulated photomodulation voltammograms of reactive carbanions and carbocations (K41)and photoeffects on the electrogeneration of the viologen cation
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in the presence of oxygen have been reported (K42). Compton and his students have presented mechanistic analyses of several rocesses that involve chemical reactions coupled to a phohEctrochemical process. These include the reduction of tert-butyl bromide via the excited state of tetrachlorobenzoquinone radical anion, a photo-EC’ mechanism (K43);hotoex ulsion of iodide from 1-iodoanthraquinone, a mixed)photo-#CE/DISPl mechanism (K44);photooxidation of tris(p-tolylbine in acetonitrile, a photo-EC mechanism (K45); and photoreduction of pyrene at a mercury channel electrode via photoexcitation of the pyrene radical anion (K46). LITERATURE CITED A. BOOKS AND REVIEWS
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Scanning Tunneling Microscopy, Atomic Force Microscopy, and Related Techniques Shelly R. Snyder and Henry 5.White* Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue SE,Minneapolis, Minnesota 55455
INTRODUCTION The last decade has seen the invention and development of scanning tunnelin microscopy (STM) and atomic force microscopy (AFM). n 1986, the inventors of STM, Binnig received the Nobel Prize in Physics for their and Rohrer (AI), accom lishment. A nearly exponential increase in the number of pub$cations describing applicationsof STM and AFM has occurred during the past few years. This remarkable growth is the best indication that these and related scanned-probe
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techniques are being widely accepted as analytical tools for addressing fundamental issues in physics, chemistry,biology, and engineering. This manuscript reviews the literature concernin STM, AFM, and other scanned-probemicroscopies from fnuary 1 through December 15, 1991. Recent reviews on Surface Characterization (A21 and Chemical Microscopy (A3),published in this journal, report on developmenta and ap lications of STM and AFM through the end of 1990. The {terature 0 1992 American Chemical Society
