Subject modified electrodes

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. 

In the first part of the present review, new techniques of preparation of modified electrodes and their electrochemical properties are presented. The second part is devoted to applications based on electrochemical reactions of solute species at modified electrodes. Special focus is given to the general requirements for the use of modified electrodes in synthetic and analytical organic electrochemistry. The subject has been reviewed several times Besides the latest general review by Murray a number of more recent overview articles have specialized on certain aspects macro-molecular electronics theoretical aspects of electrocatalysis organic applicationssensor electrodes and applications in biological and medicinal chemistry. 

It is also of interest to note the age of these quotes. It has now been 20 years since the publication of the first papers on chemically modified electrodes. During these 20 years this concept has been the subject of intense research activity. Indeed, it is fair to say that chemically modified electrodes have been the most popular (important ) research area in electrochemistry during the previous two decades. There are now thousands of papers in the literature on this subject fortunately, a number of authoritative reviews of this voluminous literature are available [1-8]. Because of the importance of this field, it is essential that students of modern electrochemistry have a working knowledge of chemically modified electrodes. The objective of this chapter is to provide this knowledge. 

One of the goals in the chemically modified electrode research area has been to develop new types of electrochemical sensors. Several review articles have recently been published on this subject [86,87], Our intent is not to provide another review of this voluminous literature. Rather, we would like to introduce the reader to the concepts behind the use of chemically modified electrodes as electrochemical sensors. As we will see, the key to developing new sensors is... 

The asymmetric eiectroreduction of an imino group has also been the subject of several studies. The use of modified electrodes,or chiral additives that supposedly create a chiral environment close to the electrode surface, give only low optical yields. 49,so other strategies will have to be explored in order to obtain high optical purities useful for synthetic work. 

Instead of chemical oxidative polymerization, electropolymerization can also be considered. A recent study shows that slow but efficient electropolymerization is possible if anilinium-exchanged zeolite Y is subjected to oxidative treatment at the electrode-electrolyte interface. Cyclic voltammetric signatures of the polymerization suggest that it occurs mostly through one dimer (p-aminodiphenylamine) which imdergoes oxidative polymerization. Electrochemical polymerization of aniline in zeolite molecular sieves was studied. A zeolite-modified electrode showed shape-selectivity for 12-molybdophosphoric acid. 

As an alternative to working in non-aqueous solutions like DMSO and AN, considerable research has been conducted on the subject of electrodes modified by... 

Based on their regular pore structure, it should be possible to impart the molecular sieving capabilities of zeolites on the surface of electrodes in electrochemical reactions. Several research groups have addressed this issue by (i) developing ways to modify electrodes with thin zeolite films and (ii) by studying the resulting changes in electrode behaviour in electrochemical reactions. The subject has been reviewed.[91]... 

If the redox species is confined to the immediate vicinity of the electrode surface, either by adsorption forces or by covalent bonds (modified electrodes), there is no need for transport from the bulk of the electrolyte in order to induce the electrochemical reaction. No concentration profile develops. On the other hand, the number of molecules subject to electron transfer is limited to those attached to the surface. These facts lead to a characteristic voltammetric response, see e.g. Fig. 4. 

This approach has also been extended to other modified electrode reactions and this has been the subject of a comprehensive review [10] a good example is the reaction between an immobilized enzyme and its substrate, in which complexation between the immobilized enzyme (the redox species) and the substrate occurs [11,12]. We have also shown [13] that the electropolymerization and trimer film formation of a range of 5-substituted indoles at the RDE... 

We have described thiol derivatives with photo-functionalities and presented schematic illustrations of the photoelec-trochemicaUy active SAMs and multilayers constructed on the electrode surfaces as many as possible in this section. However, since there are a large number of studies on wide range of photoactive alkylthiol SAM-modified electrodes, we cannot review all the important subjects in this section such as photo-patterning [152-156] and nonlinear optical properties [157,158]. Instead we have listed many references for the interested readers. 

Chemically modified electrodes are widely used for analytical purposes. For instance, Nafionglassy carbon electrodes modified with diethyldithiocarbamic acid or 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) were used to promote the sensitivity in the stripping voltammetry determination of Pb(II), Cu(II), and Hg(II). The Kfetime of each electrode exceeds three weeks [147]. The subject [148] is covered in recent reviews, which also... 

As often happens for the electrode coatings that are the subject of this monograph, enzymatic biosensors constitute an area in which ICP-modified electrodes have found the highest number of applications and, for this reason, the number of review articles written in this specific field is particularly high [140-145]. Table 2.5 reports a selection made from the very high number of amperometric biosensors found in the literature, aiming to give an idea of the applications proposed. 

Although some attempt was made to select examples from a number of the research groups who have made the most extensive and significant contributions in the field, this is not a comprehensive review of the subject area, or of its contributors. A conceptual history of the body of knowledge about modified electrodes developed by electroanalytical chemists in the past decade is presented using illustrative examples. 

The development of modified electrodes is now intimately linked with practical and theoretical progress in the use of ultramicroelectrodes, electrodes with dimensions in the pm range. A good introduction to the subject of ultramicroelectrodes is available ( ), and several new papers in this field appear each month. 

Characterizing modified electrode surfaces. Various surface analysis techniques have been used to characterize modified electrodes. Efforts should be made to characterize the CME in conditions similar to those it will be subjected to as a sensor. For example, if the CME is designed for solution analysis (the most common application), the electrochemical characterization methods which can be performed in solution are most relevant. 

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