Polymer-coated electrodes poly

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Other reference electrodes have been proposed for use in the nonaqueous solvents that are widely used in coordination chemistry. Their main advantage is that they allow one to work with a single solvent. Among these electrodes, the Ag+/Ag electrode is reversible in many solvents.4 Ag+ ions are introduced as salts, such as AgCl or AgBF4. However, the inner solution has to be refreshed due to the reactivity of Ag+. Another class consists of redox electrodes in which the two components are in solution, such as ferrocenium ion/ferrocene Fc+/Fc.5 Since the potential is dependent on the concentration ratio of the redox couple, this ratio must be kept constant. An attractive solution to prevent the use of a junction lies in the preparation of a functionalized-polymer coated electrode such as poly(vinylferrocene).6 The polymer is deposited by electrooxidation in its oxidized form, polyFc+, and then partially reduced to yield poly Fc+/Fc. Their use is limited by their relative stability in the different solvents. 

Much more effectual and very often applied are polymer-coated electrodes. Especially electrochemical polymerization is an attractive method for the immobilization of redox enzymes at electrode surfaces, and/or accumulation of electroactive reactants. An approximative analytical treatment of the response of an amperometric enzymatic electrode leading to plots of fluxes and concentration profiles has been made in [14]. The electron transport through poly-4-vinylpyridine and polystyrene-sulfonate films (widely used for immobilization of redox centers on electrodes) has been studied in [15]. 

TH" attached to poly(V-methylolacrylamide-co-acrylic acid), etc., was coated on a Pt electrode. Its cyclic voltammogram showed the formation of a complex between the coated dye and ferric/ferrous cyanide present in the solution. When the electrode is illuminated in a Fe aqueous solution, cathodic polarization at the coated electrode is observed, in contrast to the bare electrode dipped in a mixture of TH and Fe ", which gave an anodic response at the electrode" . It was proposed that, for the polymer-coated electrode, the excited states of TH and Fe " form a complex, which is stabilized by the polymer network and accepts an electron from the electrode. A flash photolysis study showed the formation of such a complex... 

When the polymer coated electrode was treated for 12 h with a ternary solvent of water, methanol, and 2-propanol (1 2 2, by volume) containing 0.5 wt-% KOH, the CV peaks have appeared at almost the same potential as, but of much smaller (oa. 30 %) current values than, those on the bare one. The alkaline treatment was made in order to hydrolyze the methyl ester group of the polypeptide to the carboxylate group. Thus, with the hydrolysis, the polypeptide segment of the block copolymer would be converted to hycfrophilic poly(L-glutamate) (PLG), which should allow the indicator ions to penetrate into the polymer layer. 

For nonaqueous electrochemistry, lUPAC recommended the use of a redox couple such as ferrocene/ferrocenium ion (Fc/Fc" ) as an internal standard [26]. An alternative to the liquid junction electrode is one based on an entirely solid-state design. Peerce and Bard [27] fabricated such an electrode by coating poly (vinylferrocene) (PVFc) on platinum. The polymer-coated electrode was brought to a 1 1 ratio of ferrocene to ferrocenium by poising the electrode at the PVTc/Fc" half-wave potential (0.39 V vs. SCE). Although this electrode maintained a constant, reproducible potential in deaerated acetonitrile over 21 h, it was unstable in other... 

Consider the polymer-coated electrode in contact with a redox electrolyte (Fig. 20. lb). When the system is undisturbed, electrons will exchange across both the Me/ poly and poly/S interfaces until the state of equilibrium is attained. This state is characterized by the condition that the partial molar free energy of electrons, i.e., the electrochemical potential of electrons /He in the three phases, is the same [165-167] ... 

Fig. 20.46 Nonequilibrium states of a redox polymer coated electrode (Me/poly) that mediates the oxidation of a solution (S) species (e.g., R— O), The chemical charge transfer reaction takes place at the poly/S interface. ]Lc is the electrochemical potential of electrons in the considered phase, (a) The system at equilibrium at electrode potential 2 [cf. Eq. (19)]. (b) The (nonequilibrium) situation shortly after changing from 2 to 3. (c) Possible further developments, depending on the system parameters and time.

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

Figure 2. Cyclic voltammograms of a poly(2,2 -bithiophene)-coated electrode in acetonitrile containing 0.1 M Bu4NC 04.34 (Reprinted from G. Zotti, C. Schiavon, and S. Zecchin, Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena, Synth. Met. 72 (3) 275-281, 1995, with kind permission from Elsevier Sciences S.A.)...

The bridging polymer is a conducting poly(3-methyIthiophene) or polyaniline and the solid state redox conduction between all electrodes is accomplished by a common coating with poly(ethyleneoxide)/Li" CF3S03- or poly(vinyl alcohol)/ The polyaniline based molecular transistor proved as a very sensitive moisture detector it works well in a dry argon atmosphere but in water saturated argon the device cuts out... 

One other design developed by Wang s group uses the same base sensor (GCE), which is coated with a layer of poly(4-vinylpyridine) (PVP). This cationic polyelectrolyte was one of the first polymers used to modify electrode surfaces [27]. Much research effort in this context has been directed to the characterization of the transport and electrostatic binding of multi-charged anions at PVP-coated electrodes. The ability of this polymer... 

The surface of a carbon electrode was at first coated with a thin film of an anionic polymer such as sodium poly(styrene-sulfonate) 95) or nafion 96) (thickness thousand A) then the cationic Ru(bpy)2+ was adsorbed in the anionic layer electrostatically. The modification was also made by coating water insoluble polymer pendant Ru(bpy)2 + ( ) from its DMF solution 97). These Ru(bpy) +/polymer modified electrode gave a photoresponse in the MV2+ solution with the Pt counter electrode 95-97) The time-current behaviours induced by irradiation and cutoff of the light under argon are shown in Fig. 28. It is interesting to see that the direction of the photocurrent reversed at the electrode potential of ca. 0.4 V (vs. Ag—AgCl) under... 

Assume that a disk-shaped electrode (gold, platinum, carbon, etc.) has been coated with a Film of poly (vinyl ferrocene) (Table 13.2). This can be accomplished by dissolving the polymer in chloroform, applying a drop of the solution to the electrode surface, and allowing the solvent to evaporate. The electrochemistry of the resulting polymer film-coated electrode can be investigated using the same electrochemical cell and equipment as described in the previous example. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

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