Feature 23-2 Modified Electrodes

An active area of research in electrochemistry is the development of electrodes that are produced by chemical modification of vai ious conductive substrates. Such electrodes can, in principle, be tailored to accomplish various functions. Modifications include the presence of irreversibly adsorbing substances with 

Modifed electrodes have many potential applications. A primaiy interest has been in the area of electrocatalysis. Here, electrodes capable of reducing oxygen to water have been sought for u.se in fuel cells and batteries. Another potential application is in the production of electrochromic devices that change color on oxidation and reduction. Such devices could be used in displays or smtin win- 

To the right, the cyclic voltammograms are shown. In (a), a Pt electrode is shown with a ferrocene attached. (Reprinted with permission from J. R. Lenhard and R, W. Murray, 

Copyright 1978 American Chemical Society.) In (b), a graphite electrode is shown with attached py-Ru(NH3). (Reprinted with permission from C. A. Koval and F. C. Anson, Anal. Chem.. 1978, 50, 223. Copyright 1978 American Chemical Society.) 

Stripping methods encompass a variety of electrochemical procedures that include a bulk electrolysis preconcentration step followed by a voltammetric step. In all these procedures, the analyte is first deposited into a small volume of mercury, usually from a stirred solution. A hanging mercury drop or a thin mercury film is most often used. After an accurately measured deposition time, the electrolysis is dis- 

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As discussed before, very high turnover numbers of the catalytic site and a large active electrode area are the most important features for effective catalysis. In the following sections three relatively successful approaches are illustrated in detail, all of which make use of one or both of these parameters. A further section will deal with non-redox modified electrodes for selectivity enhancement of follow-up reactions. 

The existence of charge in these polyelectrolyte-modified electrodes is responsible for some interesting features of the electrochemical response ... 

In general, the electrochemical performance of carbon materials is basically determined by the electronic properties, and given its interfacial character, by the surface structure and surface chemistry (i.e. surface terminal functional groups or adsorption processes) [1,2]. Such features will affect the electrode kinetics, potential limits, background currents and the interaction with molecules in solution [2]. From the point of view of electroanalysis, the remarkable benefits of CNT-modified electrodes have been widely praised, including low detection limits, increased sensitivity, decreased overpotentials and resistance to surface fouling [5, 9, 11, 17]. 

Another common feature of metal UPD on SAM-modified electrodes is the pronounced suppression of nucleation that for bare Au occurs at steps unless the step density is very low [218]. Even though nucleation still occurs mostly at steps for... 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

The generic approach to modifying electrodes with peptides for the detection of metal ions is depicted in Fig. 10.1 and outlined in Procedure 13 (in CD accompanying this book). The essential feature is an electrode modified with an SAM, which contains a carboxylic acid moiety at the distal end. The carboxylic acid moiety is activated using carbodiimides, typically l-ethyl-3-(3-dimethylaminopropyl)... 

Impedance spectroscopy is an effective technique for probing the features of chemically-modified electrodes and for understanding electrochemical reaction rates (87,88). Impedance is the totally complex resistance encountered... 

Some similar features were observed concerning the adsorption and electrochemical oxidation of DNA on glassy carbon and tin oxide electrodes [68]. Differential pulse voltammograms were recorded in buffer solution without DNA after adsorption of DNA onto the electrode surface during a predetermined time at a fixed potential suggesting the possibility of using adsorption to preconcentrate DNA on solid electrode surfaces and use this DNA-modified electrode for analytical purposes. 

Chemically modified electrodes (CME) — According to IUPAC, the distinguishing feature of a CME is that a generally quite thin film (from monomolecular to perhaps a few micrometers-thick multilayer) of a selected chemical is bonded to or coated on the electrode surface to endow the electrode with the chemical, electrochemical, optical, electrical, transport, and other desirable properties of the film in a rational, chemically designed manner. ... 

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