Chemically modified electrodes preparation

FIGURE 7. Cyclic voltammograms of chemically modified electrodes prepared by electropolymerization of fac-Re(vbpy)(00) 01 in CH CN/O.IM TBAH with a Pt... 

Nitrobenzene Ni(protoporphyrin IX) CH2CI2 tetrabutylammonium perchlorate Chemically modified electrode prepared by electropolymerization [127]... 

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. 

J. Pei and X. Li, Amperometric glucose enzyme sensor prepared by immobilizing glucose oxidase on CuPtC16 chemically modified electrode. Electroanalysis 11, 1266-1272 (1999). 

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

Miller and his co-workers60) reported surprisingly high optical yields, close to 50 %, in the reduction of 2-acetylpyridine in the presence of strychnine. They also prepared chemically modified electrodes with optically active amino acids and attempted asymmetric induction in both reduction and oxidation61 . The best optical yield, only 14.5 %, seemed to be obtained in the reduction of 4-acetyl-pyridine on a graphite cathode modified with (S)-phenylalanine methyl ester. 

There are now over a half dozen demonstrated methods for preparing chemically modified electrodes. These range from simple chemisorption to electro-... 

In 1978, Miller s group and Bard s group independently showed that chemically modified electrodes could be prepared by coating electrode surfaces with polymer films [20,21]. This has since proven to be the most versatile approach for preparing chemically modified electrodes. Indeed, until the recent rebirth of chemisorption and new covalent-attachment schemes (see earlier discussion), the polymer-film method had essentially supplanted all other methods for preparing chemically modified electrodes. 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

The third class of polymers used to prepare chemically modified electrodes is the electronically conductive polymers [25]. The polymer chains in this family of materials are themselves electroactive. For example, the polymer redox reaction for polypyrrole (Table 13.2) can be written as follows ... 

Recently, considerable interest has been shown for electrodes on which the surface has been modified chemically (CME, chemically modified electrodes). The emphasis in the investigations has mainly been on studying the preparation, properties, and mechanism of the electron transfer, but gradually the emphasis in the field is shifting to applications the use of CME for induction of chirality is treated in Chapter 26. 

K.-I. Machida, A. Fukuoka, M. Ichikawa, M. Enyo, Preparation of chemically modified electrodes attachement of platinum carbonyl clusters, and their efficient electrocatalytic action in anodic oxidation of methanol. J. Chem. Soc. Chem. Commun. 1987, 1486-1487. 

Attention in what follows, however, will be focused on systems in which the adsorbate is irreversibly confined to the electrode surface (at least within a time scale longer than that required for electrocatalytic studies), even in the absence of solution phase material. Various aspects of the preparation and characterization of such chemically modified electrodes will be presented next. 

The adsorption of polymers, poly(vinyl pyridine) or poly(acrylonitrile) either to coordinate metal atoms or to adsorb biopolymers has been used to prepare chemically modified electrodes for immobihzation of enzymes either by physical or by chemical adsorption (carrier binding), cross-linking, and entrapping at lattice sites or in microcapsules [43]. A wide application of these types of electrodes has been made for electrochemical reactions of biological interest [44]. 

Finally, chemically modified electrodes have been prepared which allow the transposition of solution electrocatalytic chemistry to electrode surfaces. Although these studies are in their infancy it appears that new products (e.g., oxalate), and therefore new mechanistic pathways, have been found for some of the surface immoblized electrocatalsyts. 

High temperatures are required to melt the crystalline domains in the high-EW samples and promote dissolution. Martin et have recently found that Nafions with EWs of 1100 and 1200 dissolve in both 50 50 propanol-water and 50 50 ethanol-water, at 250°C and elevated pressure, because the crystallites of the materials are eliminated. McCain and Covitch have also reported a similar dissolution technique. The ionic membrane was chemically converted into the nonionic precursor (sulfonyl fluoride) form prior to the dissolution process. Due to the nonionic nature of the precursor, it dissolves under relatively mild conditions. These dissolution techniques for Nafion polymers provide an important means for preparation of chemically modified electrodes and membranes of any desired geometry. ... 

Recently, several interesting studies of the electrochemical properties of electrodes coated with thin films of Nafion have been reported. These chemically modified electrodes are prepared using low-EW polymers which are alcohol soluble, or using a solution of a 1100-EW polymer which has been dissolved at high pressure and temperature. Electrochemical studies for cations such as the Ru(bpy)3 couple yielded estimates of ionic diffusion coefficients in the polymer films. However, results also indicate that these films are far more porous than conventional Nafion membranes, so it is not possible to compare values directly with those discussed above. 

Therefore the concept of chemically modified electrodes has been developed, in which the mediator is integrated with the amperometric electrode (Fig. 19). The following methods for preparing mediator-chemically modified electrodes (MCME) have been described (Murray, 1984) ... 

Chapter 13 was largely concerned with adsorbed species that are not electroactive. In this chapter we consider electroactive monolayers and thicker films on conductive substrates these are frequently called chemically modified electrodes. This area of electrochemistry has been a very active one in recent years, and a number of reviews discussing the preparation, characterization, and electrochemical behavior of chemically modified electrodes are available (1-14). These electrodes are often prepared by the modification of a conductive substrate to produce an electrode suited to a particular function, whose properties are different from those of the unmodified substrate. Modified electrodes can be prepared in several different ways, as discussed in vSection 14.2, including irreversible adsorption, covalent attachment of a monolayer, and coating the electrode with films of polymers or other materials. 

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite eleetrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(0H)Ru 0Ru 0(bipy)2] , which was reduced to the corresponding [Ru -Ru ] dimer during the reaction. 

Enzyme-like behavior was observed for electrocatalyt-ic oxidation on Nafion/lead-ruthenium oxide pyrochlore chemically modified electrodes.Nafion is loaded with lead(II) and ruthenium(III) cations by ion exchange, and then their oxides are prepared by in situ precipitation in such a way that the catalytically active sites remain... 

Analysis in flowing solutions, as performed in particular with high performance liquid chromatography (HPLC) and flow injection analysis, (FIA) has developed rapidly over the last decade and now plays an important function in most analytical laboratories throughout the world. There is little doubt, however, that even HPLC lacks the resolving power required to solve analytical problems in complex matrices with minimal sample preparation. Often, the resolving power of the detection method is called upon to assist in the solution of these problems. This is particularly true with electrochemical detection (ED) systems which offer a certain degree of selectivity based on differences in oxidation or reduction potentials of the species to be determined. In recent years, the advent of chemically modified electrodes (CMEs) has provided a stimulus to further improve both the sensitivity and selectivity of ED systems used in HPLC and FIA. 

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