Chemically modified solid electrodes

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

In this report we address cytochrome c, which is the most well-understood electron transfer protein. It has occupied a prominent role in interfacial electrochemical investigations due to its high degree of structural and reactivity characterization and its ready availability and purification. Cytochrome c has been found to react in a reproducible, quasi-reversible manner at a number of solid electrode surfaces. Electrode surfaces which have been most successful in this regard are metal oxides and chemically modified metal electrodes . Cytochrome c also has the potential to react readily at unmodified metal electrodes, as exemplified by the recent report of a stable, quasi-reversible reaction at bare silver . 

A typical adsorption process in electrocatalysis is chemisorption, characteristic primarily for solid metal electrodes. The chemisorbed substance is often chemically modified during the adsorption process. Then either the substance itself or some fragment of it is bonded chemically to the electrode. As electrodes mostly have physically heterogeneous surfaces (see Sections 4.3.3 and 5.5.5), the Temkin adsorption isotherm (Eq. 4.3.46) is suitable for characterizing the adsorption. 

FIGURE 6.10 CVs obtained at (a) Cu, Zn-SOD/MPA-modified, (b) Fe-SOD/MPA-modified, and (c) Mn-SOD/MPA-modified Au electrodes in 25 mM phosphate buffer (pH 7.5) in the absence (dotted lines) and presence (solid lines) of 1.8 pM min-1 02. Potential scan rate lOOmV s 1. (Reprinted from [138], with permission from the American Chemical Society.)... 

M.H. Pournaghi-Azar and R. Sabzi, Preparation of a cobalt hexacyanoferrate film-modified aluminum electrode by chemical and electrochemical methods enhanced stability of the electrode in the presence of phosphate and ruthenium(III). J. Solid State Electrochem. 6, 553—559 (2002). 

Figure 2.5 Schematic representation of the Au/MPS/PAH-Os/solution interface modeled in Refs. [118-120] using the molecular theory for modified polyelectrolyte electrodes described in Section 2.5. The red arrows indicate the chemical equilibria considered by the theory. The redox polymer, PAH-Os (see Figure 2.4), is divided into the poly(allyl-amine) backbone (depicted as blue and light blue solid lines) and the pyridine-bipyridine osmium complexes. Each osmium complex is in redox equilibrium with the gold substrate and, dependingon its potential, can be in an oxidized Os(lll) (red spheres) or in a reduced Os(ll) (blue sphere) state. The allyl-amine units can be in a positively charged protonated state (plus signs on the polymer...

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Some derivatives with mediating properties are suitable to form chemically modified electrodes (CMEs) with catalytic properties for NADH oxidation (55). Various attempts have been tried with different classes of mediators to immobilize the mediator onto solid electrodes or in carbon paste electrodes since the first deliberately made CME for electrocatalytic oxidation of NADH was described by Tse and Kuwana in 1978 (56), see Table I. They and others (67-72) based their CMEs on immobilized ortho-quinone derivatives. However, these CMEs were rapidly inactivated in the presence of NADH, probably because of side reactions in the catalytic process (72). For some other immobilized mediators one major reaction route could be proposed as the CME turned out to be quite stable in the presence of NADH. The catalytic reaction sequence comprizes two steps, one chemical between NADH and the immobilized mediator (reaction (6)) and one electrochemical between the mediator and the electrode (reaction (7)). The sequence is given below for the simplest case ... 

Figure 8. Cyclic voltammograms of (A) a naked solid graphite electrode (surface area 0.0731 cnr) and (B and C) two equivalent graphite electrodes also chemically modified with the structures included in the figure.

Surface complexation — is complexation of metal ions by ligands immobilized on the electrode surface (-> electrode surface area). The ligands may be incorporated in the structure of a -> carbon paste electrode, covalently bound to the surface of a chemically modified electrode (-> surface-modified electrodes), or adsorbed (-> adsorption) on the electrode surface etc. Surface complexation is not confined to electrodes. It can occur on many surfaces, e.g., minerals, when in contact with metal ion solutions or solutions containing complexing ions (in the first case, the surface provides the ligand and the solution the metal ion, whereas in the second case, the surface provides the metal ion and the solution the ligand). Surface complexation can be an important step in the dissolution of solid phases [ii]. 

Although one of the more complex electrochemical techniques [1], cyclic voltammetry is very frequently used because it offers a wealth of experimental information and insights into both the kinetic and thermodynamic details of many chemical systems [2], Excellent review articles [3] and textbooks partially [4] or entirely [2, 5] dedicated to the fundamental aspects and apphcations of cyclic voltammetry have appeared. Because of significant advances in the theoretical understanding of the technique today, even complex chemical systems such as electrodes modified with film or particulate deposits may be studied quantitatively by cyclic voltammetry. In early electrochemical work, measurements were usually undertaken under equilibrium conditions (potentiometry) [6] where extremely accurate measurements of thermodynamic properties are possible. However, it was soon realised that the time dependence of signals can provide useful kinetic data [7]. Many early voltammet-ric studies were conducted on solid electrodes made from metals such as gold or platinum. However, the complexity of the chemical processes at the interface between solid metals and aqueous electrolytes inhibited the rapid development of novel transient methods. 

Electrodes for voltammetry are usually made of solid or liquid metals [2, 3, 6], or from carbon [10]. Less frequently, metal oxides or polymers are used [11-15]. The primary metallic conductor may be covered with a thin film of a secondary conductor (e.g., mercury, or oxides and polymers) [9,13], or a monolayer of covalently bound foreign atoms or molecules such as thiols on gold substrate [16]. These are called chemically modified electrodes. The chemical preparation of the electrode... 

Figure 5.17 Cyclic voltammograms of the viologen-substituted polyvinylpyridine modified Au electrode with 25 mM nitrate. 0.1 M phosphate buffer (pH 7.5) in the presence (solid line) and absence (broken line) of NR (A) sweep rate 2 mV s and (B) sweep rate 100 mV s". Reprinted with permission from ref. 103. Copyright 2003 American Chemical Society.

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