Anal. Chem. 1982, 54, 20 R-27 R
Analytical Electrochemistry: Methodology and Application of Dynamic Techniques Mlchael D. Ryan Department of Chemistry, Marquette Univers/ty, Milwaukee, Wisconsin 53233
George S. Wllson” Department of Chemistry, University of Arizona, Tucson, Arizona 85721
Following the tradition of our predecessors, we have attempted to report on recent developmentsin the area of finite current electrochemistry. References have been selected which, for the most part, have appeared in print from January 1980 through December 1981. The intent has been to choose from an extensive number of publications work reporting the most novel developments or indicating important trends. Time and space limitations preclude citing many excellent papers; however, we have tried to include review articles which should provide a “gateway- to more detailed aspects of an area.
CHEMICALLY MODIFIED ELECTRODES Since the demonstration of functionalized electrode surfaces in the mid 1970s by Murray’s group at North Carolina, there has been explosive growth in interest and possibilities for altering and controlling electrochemical reactions. The methodology for covalent attachment of functional groups to electrodes has recently been reviewed (1A). Hubbards group at Santa Barbara has evaluated the stability of organic molecules immobilized by chemisorbtion on platinum (2A). More recently attention has been directed toward the incorporation of the redox active component in a polymer matrix, which, in turn, adheres to an electrode. Among the redox polymers employed are poly(viny1ferrocene) (3A, 4A), polypyrroles @A),functionalized polystyrenes (6A, 7A),polyaniline @A),phenoxytetrathiafulvalene (9A),polyviologen (IOA),azobenzene ( I I A ) ,and poly(N-9,1O-anthraquinone-2carbonylethyleneimine) (12A). It is also possible to deposit an electrochemically inert film possessing functional groups capable of holding redox centers within the polymer matrix. Examples include transition metal complexes bound electrostatically to polyelectrolyte films (13A,14A) or covalently attached (15A, 16A). The polymer films are produced by plasma discharge, electrochemically induced deposition, covalent bonding, and dip or spin coating. A polymer film can contain the equivalent of 10-1000 monolayers of electroactive centers. There is some advantage in electrocatalytic efficiency to increasing the thickness of the film provided that suitable rates of electron transfer between the active centers and the electrode and substrates, respectively, can be maintained. Thus there is considerable current interest in developing experimental and theoretical techniques to evaluate the individual steps which contribute to the overall electrocatalytic process. It has been proposed (17A, 18A) that “hopping” of electrons between the electrode and the distributed electroactive centers can be regarded formally as a diffusion process, described by a diffusion coefficient. With such a model, it can be predicted that the catalytic efficiency will increase with film thickness to an optimal value. Diffusion of substrate through the polymer film is also shown to be important in defining the process (19A). This behavior has been verified experimentally (6A, 20A). Murray and coworkers (.HA,22A) have examined charge transfer diffusion rates by cyclic voltammetry and potential step measurements. Diffusion of substrates both in bulk solution and in the polymer film can affect the overall electron transfer rate and Anson’s group has used charged-polymer coated rotating disk electrodes to measure the electron transfer cross exchange reactions (between active center and substrate) (23A)and the 20 R
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self-exchangereaction (24A) for the active center. Differences are noted between films charged prior to and following electrode coating (25A). Kaufman and co-workers have also demonstrated an important role for counter ion flow within the polymer (9A). A complete theoretical treatment of electrochemical reactions mediated by redox polymer films will require simultaneous consideration of (a) substrate diffusion in bulk solution, (b) substrate diffusion within the film, (c) charge transfer diffusion within the film, and (d) the kinetics of the cross exchange reaction. Such a theory has recently been proposed by Sav6ant and co-workers (26A)and includes consideration of the partitioning of the substrate between the bulk solution and polymer film layers. Enhanced stability and reusability have often been cited as justification for immobilization of catalytic species. Less well understood are the effects of the active site matrix on the cross-exchange reaction which may provide catalysts with unique properties. Polyelectrolytefilms have been suggested to significantly alter self-exchangerates (24A)through ionic strength effects which can also substantially alter substrate partition coefficients (27A). The question of alteration of chemical reactivity has barely been considered (28A). Spatial arrangement can also be im ortant and an interesting discovery is the orientation of cfifferent active sites in discrete adjacent polymer layers resulting in “charge state” trapping which creates a rectifyin interface (29A, 30A, 31A). A number of indepenfent techniques have been used to study polymer film electrode morphology (32A)and characteristics of immobilized species including electronic (33A,%A, 35A) and photoelectron (35A,36A) spectroscopy, fluorescence (37A), chemiluminescence (38A), and infrared and EPR spectroscopy (39A). It was possible to demonstrate that a significant portion of immobilized redox centers are inactive even under conditions of slow voltammetric scan (33A). Redox catalysis by chemically modified electrodes of a number of important systems has been studied. The reduction of dioxygen to peroxide and water is facilitated by iron and cobalt phthalocyanines(40A,41A) and by covalently attached iron porphyrins (42A). In the latter case the distribution of products is dependent on surface coverage. Homogeneous electrocatalysis has been accomplished by using cobalt macrocycles (43A, 44A). The oxidation of NADH has been facilitated by modified electrodes with dopamine (20A),catechol functionalities (45A),and chloranil (46A). Vinylferrocenes have been used for the oxidation of ascorbic acid (47A) and metallotetraphenylporphins for the reduction of dibromoalkyl compounds (48A). Developments in photoassisted electrolysis have been reviewed by Heller (49A) as has the derivatization of n-type semiconductor electrodes to prevent corrosion (50A). Photoassisted reductions of chloromethanes (51A),cytochrome c (52A),and hydrogen ion (53A,54A) have been accomplished. The latter system, a viologen PtCle2-mediator has been immobilized on the inside of a yrex test tube. This will be a big seller among biochemists because shining light on the tube results in reduction of cytochrome c , stellacyanin, and myoglobin (55A). Perhaps one of the most interesting chemically modified electrodes utilizes an adsorbed modifier (4,4’-dipyridyl)(56A) which is not electroactive in the potential region of electron
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0 1982 American Chemlcal Society
METHOWLOGY AND APPLICATION OF DYNAMIC TECHNIWES
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