Polymer-coated electrodes selective

Since the incorporation of a certain anion would take place for the system to reach equilibrium at the polymer coated electrode, the potential at the membrane-electrolyte interface would indicate the amount of anions incorporated. Indeed, a few groups of investigators realized that this was the case [274-276]. The potential at the PPy film coated electrode was shown to vary depending on the concentration of the Fe /Fe + pair [274], hydroxide ion [275], and many other anions and cations [276]. Recently, Hutchins and Bachas have developed a nitrate ion selective sensor using a PPy film coated electrode [277]. The electrode showed a near Nemstial slope of —56 1 mV per decade, good dynamic linearity, and a detection limit of (2 1) X 10 M for nitrate. The selectivity coefficients over CIO4 and 1 were 5.7 x 10 and 5.1 X 10 respectively. 

Nation is a perfluorosulfonated ion-exchange polymer and has received a great deal of attention as a modifier for polymer-coated electrodes mainly because of its outstanding chemical stability and excellent ionic conductivity. Nafion-coated electrodes have been proved to be effective for the determination of electroactive compounds " . The attractive features of Nafion which are useful for electroanalytical purposes are its preconcentrating ability sof cations and its ionic selectivity. 

Polymer coated electrodes can be made selective to particular compounds by entrapping in the polymer salts, ion-exchangers or chelating ligands, but these active components tend to be leached from the matrix, limiting the sensitivity of the electrode. This inconvenience can be avoided by using components which are covalently attached to the polymer . However, physical entrapment is often easier to perform and renewal of the surface can be made very rapidly. 

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]. 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

Electrodes obtained by coating Nafion and cellulose acetate on electrode surface or the combination of Nafion/NO selective membranes and nonconducting polymer-modified electrode. 

Discovery of polymer coatings that maintain sensitivity, promote selectivity, protect the electrode from the biological fluid, and provide a biocompatible surface to the measured system... 

Various modified polymers and functional polymers have been used to coat electrode surfaces either to promote electrocatalysis or to act as permselective barriers [115, 116] and thus increase selectivity. The work in this area has been... 

Dubois et al. (21,22) have showed that the electrochemical oxidation of phenol euid its derivatives, on metal surfaces, produced hydrophobic, adherent, euid insulating polymer films of uniform thickness. Both Yacynych and Mark (17). and Helneman et al. (13) showed the oxidation of 1,2-diaminobenzene to be irreversible, and with successive cyclic voltammetric scans formed an insulating polymer film completely covering the electrode surface. Heineman et al. ( 22) further showed that 1,2-diaminobenzene forms a polymeric film over a pH range of 4 to 10, and that platinum electrodes coated with the poly(1,2-diaminobenzene) provided a nearly Nernstian response to pH. Cheek et al. (J[6) studied the pH response of platinum and vitreous carbon with polymer films of either 1,2-diaminobenzene or phenol. These polymer films are selective enough to allow the permeation of protons, while limiting access to larger molecules, which could be potential interferents. 

The modification of electrode surfaces with polymer films has received considerable attention because of many advantageous properties of polymers (2,3). Polymer films are chemically stable, provide diffusional barriers that can lead to selectivity based on size or charge exclusion properties, provide a means of preconcentrating analytes by ionic or other complexation interactions, and are a convenient matrix for the immobilization of other reagents, such as enzymes. Coating electrode surfaces with polymer films takes advantage of these properties. Complexation of a specific... 

Through the employment of polymer- or SAM-modified electrodes, it is possible to monitor electrochemical transformations in the film using AFM adhesion-based measurements [54,55). For example, Hudson and Abruna demonstrated the ability to directly control the adhesion force between a tip and substrate surface, both coated with an electroactive polymer, via the selective oxidation or reduction of the polymer film [54]. In these studies, both a gold-coated tip and gold foil were modified... 

When water is used as a medium for CO2 reduction, it is important to suppress proton reduction to produce H2, which is more favorable in water than CO2 reduction. A hydrophobic polymer environment can provide such conditions by suppressing proton reduction to selectively carry out CO2 reduction. In an electrocatalytic CO2 reduction by CoPc confined in a poly(vinylpyridine) membrane coated on an electrode, selective CO2 reduction takes place, producing CO [119]. It is of further interest that in the electrocatalytic system, a third electron is injected into the Co(I)Pc(-3) - CO2 intermediate to produce CO and Co(I)Pc(-2) complex after the starting Co(II)Pc(-2) complex is reduced by two electrons to Co(I)Pc(-3) to form Co(I)Pc(-3) - CO2 with CO2. Such a mechanistic scheme can be represented by Fig. 20 [119]. Path I is the previously accepted scheme, and path II is the proposal of a new two-electron reduction pathway. Note also that for the CO2 reduction, a proton is also involved in an equilibrium process. It was inferred that in the polyvinylpyridine matrix, protonation and deprotonation takes place easily with the help of the pendant pyridine groups in a concerted fashion, resulting in favorable CO production (see also Fig. 20) [119]. 

Conductive polymer films on electrodes have been prepared by electrochemical polymerization of electroactive monomers such as a pyrrole-substituted mediator, or by evaporating solutions containing preformed polymer. Examples of electrocatalyses reported include the oxidation of alcohols by pyrrole-substituted 2,2,5,5-tetramethyl-3-pyrroline-l-oxyl [26] and organohalide de-halogenation by pyrrole-substituted 4,4 -bipyridinium salt [27]. The preparation of mediator-modified electrode by evaporating solutions of preformed polymers was carried out by dip-coating polymers including mediators on electrode surface or by covalent attachment of mediators to dip-coated polymers on electrode surfaces. Examples of the former electrocatalyses are selected from the several reports on the oxidation of NADH by dopamine... 

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