Electrode chemically modified

Polymer-modified electrodes are prepared either by casting polymer solution on the solid electrode surface and allowing the solvent to evaporate or by electropolymerization of a monomer [13], The polymer film is either an electronic insulator, but an ionic conductor, or a mixed electronic and ionic conductor. The first type of films is used for the preparation of permselective coatings that serve to prevent unwanted matrix constituents reaching the electrode surface. Electrochemical reactions in mixed conductor films depend on several physicochemical processes, such 

Parsons R (1985) The single electrode potential its significtince and ctilculation. In Bard AJ, Parsons R, Jordan J (eds) Standard potentials in aqueous solution. Mtircel Dekker, New York, pl3 

Bockris JO M, Khan SUM (1993) Surface electrochemistry. Plenum Press, New York Heyrovsky J, Kuta J (1966) Principles of polarography. Academic Press, New York Weppner W (1995) Electrode performance. In Bruce PG (ed) Solid state electrochemistry. Cambridge University Press, Cambridge, p 199 

Gains Z (1994) Fundamentals of electrochemical analysis. Ellis Horwood, New York and 

Cattral RW (1997) Chemical sensors. Oxford University Press, Oxford 

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

It is difficult to give a definition of a chemically modified electrode, as there are so many different experimental approaches to prepare electrodes with modified surfaces. Most widespread are methods based on attaching a certain compound. 

Bockris JO M, Khan SUM (1993) Surface electrochemistry. Plenum Press, New York 

Heyrovsky J, Kuta J (1966) Principles of polarography. Academic Press, New York 

Weppner W (1995) Electrode performance. In Bruce PG (ed) Solid state electrochemistry, Cambridge Univ Press, Cambridge, p 199 

The electrochemical reduction of silver ions and the understanding of limiting currents by Solomen (1) was perhaps the first indication that voltammetry could be analytically useful. However, it was not until the introduction of a practical, reproducible working electrode, the dropping mercury electrode (DME), that the analytical utility of the technique was realised. The DME alleviated problems associated with surface phenomena which complicated the electrochemistry of solid electrodes previously employed. These same surface phenomena are now much more clearly understood and the chemistry associated with them may be used in appropriate analyses to enhance analytical responses. 

Such is the area of chemically modified electrodes (CMEs). What may have been considered a contaminated or fouled electrode 20 years ago may now be considered a CME. The difference now is that the modification is purposefully carried out and carefully controlled to produce a desired result. 

Several excellent reviews on CMEs already exist (2-4). The purpose of this chapter is to highlight the considerations necessary when these devices are to be used as chemical sensors and to illustrate some useful examples. The design, preparation, characterization and application of CMEs as sensors will be discussed. Challenges which need to be overcome to expand the range of applications will be highlighted. 

Following breakthroughs in electrochemical theory in the 1960s and in instrumentation in the 1970s, the future of electroanalytical chemistry now 

The ideal CME for chemical sensing should have the following properties. 

Charles R. Martin Colorado State University, Fort Collins, Colorado Colby A. Foss, Jr. Georgetown University, Washington, D.C. 

when a substituent of interest is incorporated into an olefinic substance and the resulting compound allowed to react with the electrode surface, the substituent becomes connected to the surface.. .. By this means, ionic species have been tethered within the double layer region in order to probe the mechanisms of electrode reactions involving platinum complexes.. . . Alternatively, the electrochemical reactant itself can be connected to the electrode surface, allowing its reactivity to be observed as a function of charge, orientation, and structure, as described here. 

If a method for securely anchoring such molecules could be found, advantage could be taken of the molecular structure to build surfaces with unique and widely varying properties. Indeed, the attached molecules could be used in the sense of chemical reagents to perform reactions in tandem with the electron transfer processes characteristic of chemically inert electrodes. 

We see this line of research as eventually leading to a wide array of chemically modified electrode surfaces with unusual analytical, chemical, catalytic and optical properties. 

These quotes were chosen to introduce this chapter on chemically modified electrodes because they are from some of the earliest papers in the field and because they review the concepts and objectives of this research area. We learn that the field of chemically modified electrodes involves attaching specific molecules to the surfaces of conventional inert electrodes. We also discover the two major reasons for wanting to attach molecules to electrode surfaces. As explained by Lane and Hubbard, one objective is to obtain fundamental information about the mechanism of electron transfer at electrode surfaces. The second objective, as expressed by Watkins et al. and Elliott and Murray, is to impart to the electrode surface some chemical specificity not available at the unmodified electrode. For example, the modified electrode might catalyze a specific chemical reaction. Alternatively, the modified electrode might be able to recognize a specific molecule present in a contacting solution phase. 

Grant A. Edwards, Adam Johan Bergren, and Marc D. Porter 

Electrode surfaces are modified in a quest to render an electrochemical function either not possible or difficult to achieve using conventional electrodes. Targeted improvements include increased selectivity, sensitivity, chemical and electrochemical stability, as well as a larger usable potential window and improved resistance to fouling. Furthermore, electrodes with tailored surfaces enhance fundamental studies of interfacial processes. Therefore, the need for improved electrode performance and logically designed interfaces is rapidly growing in many areas of science. 

Ideally, the properties of the adlayer impart a predictable function to the electrode. Recent improvements in surface characterization techniques enable a molecular-level understanding of modified interfaces. These techniques, coupled with electrochemical characterizations, not only provide a means to verify the function, but also serve as a basis for refinements of the modification strategy to further enhance its performance. 

The extensive nature of electrode modification procedures prevents a comprehensive description of all types of modified electrodes, their uses, and the techniques used to characterize them. This chapter therefore focuses on providing the reader with concepts central to electrode modification through several examples of established systems. Specialized reviews are noted in each section, where appropriate these and topical texts (3-10) should be consulted for more detailed information. 

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Explain clearly how chemically modified electrodes can benefit electrochemical measurements ... 

Chemically modified electrodes, 39, 118 Chemometrics, 197 Chemoreceptor, 187 Chip, 194, 195 Chloramphenicol, 70 Chloride electrode, 159 Chlorpromazine, 34 Cholesterol, 182 Cholinesterase, 182 Chromium, 85, 86 Chronoahsorptometry, 42 Chronoamperometry, 21, 60, 130, 135, 132, 177... 

The electrochemistry of conducting polymers has been the subject of several reviews2-8 and has been included in articles on chemically modified electrodes.9-14 The primary purpose of this chapter is to review fundamental aspects of the electrochemistry of conducting polymer films. Applications, the diversity of materials available, and synthetic methods are not covered in any detail. No attempt has been made at a comprehensive coverage of the relevant literature and the materials that have been studied. Specific examples have been selected to illustrate general principles, and so it can often be assumed that other materials will behave similarly. 

The historical development of chemically electrodes is briefly outlined. Following recent trends, the manufacturing of modified electrodes is reviewed with emphasis on the more recent methods of electrochemical polymerization and on new ion exchanging materials. Surface derivatized electrodes are not treated in detail. The catalysis of electrochemical reactions is treated from the view of theory and of practical application. Promising experimental results are given in detail. Finally, recent advances of chemically modified electrodes in sensor techniques and in the construction of molecular electronics are given. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

Some porous ceramic structures of oxides on titanium (CT2O3, RuOj, MnOj, VOJ obtained by baking films of metal complexes like acetylacetonates on titanium surfaces can also be regarded as chemically modified electrodes Applications... 

The reductive cleavage of the alkylcobalamine is facilitated by light irradiation and can then proceed at a much more positive potential. A demonstration photoelec-trochemical reactor for the Bij-catalyzed photoelectrochemical synthesis of Michael adduct 17, the alarm pheromone of the ant atta texana (Scheme 9) has been constructed where the complete device is driven solely by solar energy . Hopefully, mediated photoelectrochemical reactions of this type will also be realized at chemically modified electrodes. 

The oxidation of N ADH has been mediated with chemically modified electrodes whose surface contains synthetic electron transfer mediators. The reduced form of the mediator is detected as it is recycled electrochemically. Systems based on quinones 173-175) dopamine chloranil 3-P-napthoyl-Nile Blue phenazine metho-sulphatemeldola blue and similar phenoxazineshave been described. Conducting salt electrodes consisting of the radical salt of 7,7,8,8-trtra-cyanoquinodimethane and the N-methylphenazium ion have been reported to show catalytic effects The main drawback to this approach is the limited stability... 

Murray, R. W. Chemically Modified Electrodes. Electroanalytical Chemistry 13 (ed.) Bard, A. J., p. 411, New York, Marcel Dekker 1984... 

The use of non-inert and chemically modified electrodes and other strategies for the detection of species that are difficult to analyze with the normal electrode materials have been reviewed.55 Photosensitization prior to amperometric detection is another tactic that has proved useful for the analysis of substances that are normally considered to be electrochemically inert.56 The use of pulsed amperometry has recently been reviewed.57... 

Clearly this approach is not suitable for preparing large quantities of products, its main purpose being to permit the greatest amount of information to be obtained concerning the reactivity of a new material. If the coreactant is expensive and/or difficult to prepare then this procedure is invaluable. However, it is important to consider that the quantities of derivatized polymer obtained in this approach [ 10-6 mole based on -N=C repeat unit] might well represent sufficient material if such a process were to be used for the direct preparation of chemically modified electrodes (12), or incorporated into a planar microfabrication process, with which it would appear to be compatible. 

The new edition of Principles of Electrochemistry has been considerably extended by a number of new sections, particularly dealing with electrochemical material science (ion and electron conducting polymers, chemically modified electrodes), photoelectrochemistry, stochastic processes, new aspects of ion transfer across biological membranes, biosensors, etc. In view of this extension of the book we asked Dr Ladislav Kavan (the author of the section on non-electrochemical methods in the first edition) to contribute as a co-author discussing many of these topics. On the other hand it has been necessary to become less concerned with some of the classical topics the details of which are of limited importance for the reader. 

Fig. 5.31 Cyclic voltammogram of a chemically modified electrode with a monolayer of a reversible mediator. The shaded area corresponds to the charge Q...

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

Murray, R. W., Chemically modified electrodes, in Electroanalytical Chemistry (Ed. 

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