Redox-polymer modified electrodes

Theory of EHD Impedances for a Mediated Reaction on a Redox Polymer Modified Electrode [85]... 

Nieto, R, Jr., and Tucceri, R.L 1996. The effect of pH on the charge transport at redox polymer-modified electrodes An a.c. impedance smdy apphed to poly(o-aminophenol) film electrodes. Journal of Electroanalytical Chemistry 416, 1-24. 

There are many similarities (e.g., in charge storage and electro-chromic behavior, and in transport mechanisms) between inorganic hydrous oxide films and the largely organic redox polymer-modified electrodes which are also of much interest at the present time. Of special interest is the appearance in certain instances240 with the... 

All the ion and solvation changes discussed so far in this section relate to chemically reversible processes. Anecdotally, it is widely accepted in the redox polymer modified electrode literature that the first redox cycle of a newly deposited film is atypical. Although the reasons and processes are chemically rather different, this is reminiscent of the first cycle effect discussed in Sect. 2.7.3.6 for Prussian Blue films. PVF provides a typical example of this first cycle, or break-in , effect. The initial EQCM response to PVF oxidation in water (after its deposition from an organic solvent, typically dichloromethane) is quite different from that of a previously cycled film [132]. The result is most clearly demonstrated by considering the mass flux - or, better stiU, the difference between the total mass flux and the elec-tron/ion flux-as a function of apphed potential or film charge. Such plots show a once-only pulse of solvent into a new film this solvent (typically ca. five solvent molecules per ferrocene redox site) is retained ( trapped ) within the film thereafter and provides the baseline upon which subsequent redox-driven solvation... 

Inzelt, G., Role of polymeric properties in the electrochemical behaviour of redox polymer-modified electrodes, Electwchim. Acta, 34, 83-91 (1989). 

The main drawback of redox polymer electrodes, both in fundamental studies and in chemical analysis, stems from the difficulty in controlhng the concentration and spatial organization of redox sites within the film (4). The main use of redox polymer modified electrodes has been in fundamental studies of electron transfer mechanisms. These electrodes are also used as electrochemical sensor devices, but on a much more limited scale (117). Many of these limitations are overcome by the use of ion-exchange polymers, as outlined below. 

Forster RJ, Vos JG, Lyons MEG. Controlling processes in the rate of charge transport through [Os(bipy)2(PVP) Cl]Cl redox polymer-modified electrodes. J Chem Soc, Faraday Trans 1991 87 3761-3767. 

There has therefore been much interest in the mediation of redox reactions in solution by conducting polymer-modified electrodes. 

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

A horseradish peroxidase-osmium redox polymer-modified glassy carbon electrode (HRP-GCE) has also been applied to this analysis to improve sensitivity and reduce problems with faradic interference. Kato and colleagues (1996) employed this electrode in measurement of basal ACh in microdialysates using a precolumn enzyme reactor. This system was three to five times more sensitive than a conventional Pt electrode. ACh in rat hippocampus dialysate was quantitated at 9 5 fmol/15 pi (n = 8). ACh was analyzed in PC12 cells in a similar assay by Kim and colleagues (2004). No precolumn enzyme reactor was employed. 

Manipulation of the Donnan potential in random polymer-modified electrodes can also be achieved. In the case of cast redox polyelectrolyte-modified electrodes one can control ion permselectivity by mixing the redox polymer with an oppositely charged polyelectrolyte in an appropriate ratio before film casting [123]. The same strategy can be followed in electropolymerized films by mixing the electroactive monomer with one of opposite charge [124]. 

During continuous redox cycling, the first cycle usually differs from the following ones. This effect is referred as break-in. In poly(vinylferrocene), PVF, films this has been related to the incorporation of solvent and ions into the film, decreasing its resistivity [132]. This effect has been observed for several polyelectrolyte and polymer-modified electrodes, for example, polyaniline [155]. 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit-... 

Scanning tunneling (STM) was invented a decade ago by Binnig and Rohrer [72], and was first applied to the solid-liquid interface by Sonnenfeld and Hansma in 1986 [73]. Since then, there have been numerous applications of STM to in situ electrochemical experiments [74-76]. Because the STM method is based on tunneling currents between the surface and an extremely small probe tip, the sample must be reasonably conductive. Hence, STM is particularly suited to investigations of redox and conducting polymer-modified electrodes [76,77],... 

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. 

Polymer-modified electrodes have shown considerable utility as redox catalysts. In many cases, modified electrode surfaces show an improved electrochemical behavior towards redox species in solution, thus allowing them to be oxidized or reduced at less extreme potentials. In this manner, overpotentials can be eliminated and more selective determination of target molecules can be achieved. In this discussion, a mediated reduction process will be considered, although similar considerations can be used to discuss mediated oxidation processes. This mediation process between a surface-bound redox couple A/B and a solution-based species Y can be describes by the following ... 

A BASJ ion-exchange microbore column (45 cm x 1 mm i.d.) 0.05 Sodium phosphate buffer of pH 8.5 containing 0.1 mM EDTA [60 pL/min]. Electrochemical at horseradish peroxidase osmium redox polymer-modified vitreous C electrodes at 0 mV versus Ag/AgCl. Rat frontal cortex dialysate samples [188]... 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

Since the appearance of the redox [ii, iii] and conducting [iv] polymer-modified electrodes much effort has been made concerning the development and characterization of electrodes modified with electroactive polymeric materials, as well as their application in various fields such as -> sensors, actuators, ion exchangers, -> batteries, -> supercapacitors, -> photovoltaic devices, -> corrosion protection, -> electrocatalysis, -> elec-trochromic devices, electroluminescent devices (- electroluminescence) [i, v-viii]. See also -> electrochemically stimulated conformational relaxation (ESCR) model, and -> surface-modified electrodes. 

Our approach to the color-variable LEDs presented here has a number of important advantages (1) The two redox polymers modify the charge-inj ection properties of the polymer/metal interfaces, allowing the use of high-workfunction metals as electrodes. This potentially reduces the aging problems associated with conventional polymer LEDs, which must use reactive low-workfunction metals to... 

The diffusion step becomes important for polymer-modified electrodes. Thus, the apparent diffusion coefficient depends on the concentration of redox groups because the acceleration of the electron exchange decreases with the ion distance. These conclusions were drawn from a series of polyvalent ions anchored electrochemically to poly(4-vinyl pyridine) on graphite [47]. 

H.S. White, J. Leddy and A. Bard, Polymer films on electrodes. 8. Investigation of charge-transfer mechanism in Nation polymer modified electrodes, J. Am. Chem. Soc., 1982, 104, 4811-4817 F.C. Anson, D.N. Blauch, J.-M. Saveant and C.-F. Shu, Ion association and electric field effects on electron hopping in redox polymers. Application to the Os(bpy)33+/2+ couple in Nafion, J. Am. Chem. Soc., 1991, 113, 1922-1932. 

A last important aspect of these functionalized conducting polymers concerns their interest for the theoretical modelling of the mode of operation of modified electrodes. Previous work based on redox polymer coated electrodes raised a fundamental question concerning the transport of charge in these polymeric materials. 

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