Redox films

The electrocatalytic activity of novel redox films in regeneration of NAD/NADH has been investigated by means of chronoamperometry, hydrodynamic and potentiodynamic methods. In order to achieve the most efficient electrocatalytic properties indicated as both the highest heterogeneous rate constant and maximum sensitivity, the further optimization of electropolymerisation conditions has been made. 

If substrate diffusion becomes rate determining, only a small fraction of the film at the film/solution interface will be used. On the other hand, if charge diffusion becomes rate determining, the catalytic reaction can take place only in a film fraction close to the electrode surface. Each of these effects will render parts of the film superfluous, and it is obvious that there is no sense in designing very thick redox films, rather there is an optimal layer thickness to be expected depending on the individual system. 

What benefits and drawbacks to these problems can one expect from the use of cyclic voltammetry instead of RDEV They are related. In a general case, the application of cyclic voltammetry will be more complicated, because playing with the scan rate, one can make the diffusion layer penetrate the film or remain outside, as is the case with RDEV. We have already seen a fruitful application of the first of these possibilities in the use of cyclic voltammetry to the characterization of electron hopping transport within the redox films (Section 4.3.4). In the second situation, cyclic voltammetry may replace RDEV in a manner similar to what has been seen in Section 4.3.2 Each time a term (1 — ///a) is encountered in the analysis, it suffices to replace it by... 

In multiscan cyclic voltammetry measurements, e.g., of a redox film-coated electrode, the isopotential point is the potential of the same current value for voltam-mograms of different cycles. That is, by analogy to, e.g., the isosbestic point in absorption spectroscopy of two species remaining in equilibrium, this is the potential at which voltammograms of two redox species being in equilibrium in the film cross each other. A pair of such redox species can have several isopotential points in their multicyclic voltammograms. 

Scanning Probe Microscopy Studies of Molecular Redox Films... 

For STM of molecular redox films, theoretical models predict enhanced tuimeling through redox-active centers via resonant tuimeling modes specifically, reorganizations of the inner-sphere vibrational modes should significantly contribute to... 

The ability to remotely control the surface chemistry of an AFM tip, as we have done, represents a convenient method for studying potential-dependent chemical properties of molecular redox films and evidences the utility of redox molecules in nanotechnology. 

Investigations of molecular redox films are of fundamental importance to numerous technologies. Scanning probe microscopy, in combination with electrochemistry, is uniquely suited to provide invaluable information concerning the potential-dependent structural, chemical, and electronic properties on these systems. 

Prussian Blue and related inorganic redox films have proved very popular for spectroelectrochemical studies and elec-trochromic applications. Early investigations used rapid scan techniques to collect spectra as a function of potential [51], Prussian Blue grows by a three-dimensional nucleation and growth mechanism, which includes surface diffusion of Prussian Blue particles to kinks at growing nuclei [250]. DCVA traces were better defined than the CVs [251], and allowed determination of the molar absorptivity and the amount of film on the electrode. A recent study used a waveguide to study the formation of Prussian Blue [30]. It showed that the technique could detect submonolayer deposition of Prussian Blue film. The technique is typically 10 times more sensitive than rival techniques. 

The maximum redox capacities of several redox films have been measured and are listed in Table 2.6. 

The model used by Ho et al [103] proposed that the capacitive behaviour observed at low frequencies for electroactive redox films is caused by insertion of a guest species into a host of finite thickness. A similar model was used to describe the impedance of redox couples that have been sorbed into polymer flms [100, 107-108]. In both cases, the more complete expression for the faradaic impedance of an intercalating redox film is given by (except for the high frequency double layer capacitance)... 

Boland, S., Jenkins, P., Kavanagh, P., Leech, D. Biocatalytic fuel cells A comparison of surface pre-treatments for anchoring biocatalytic redox films on electrode surfaces. J. Electroanal. Chem. 626(1-2), 111-115 (2009). doi 10.1016/j.jelechem.2008.11.010... 

Since suitable conducting polymers with anionic backbones are not available, we have resorted to a more complex approach for cation binding. Following Martin s workio on terpolymer redox films of polyvinylferrocene, we produced a new type of composite polymer, of cationic poly-(N-methylpyrrole) (PMP" ) with anionic poly-styrenesulfonate (PSS ). Upon reduction of a film of this material, cations are taken up. Thus, the entangled polymeric anion is not flushed out by reduction of the pyrrole units. Instead, cations are incorporated to balance the sulfonate charges. This has been shown for a variety of cations including protonated dopamine. The scheme below shows how this polymer works to bind protonated dimethyldopamine (DH ) cathodically and to release it anodically. 

Redox films ean be produeed by electropolymerization of small redox molecules, whieh retain redox ehemistry in the polymerized state. The suggested structure of a polymerie version of poly (methylene blue) is shown in panel H of Table 9.1 [60]. In 1996, Zhou et al. demonstrated that methylene green could be electropolymerized on an electrode surface, resulting in a film with a half-wave potential in the vicinity of 0.21 V versus the standard hydrogen electrode (SHE) [100]. Furthermore, their work showed that this film eould mediate to NADH, reducing the required oxidation overpotential by 400 mV. Similar polymers have been demonstrated to mediate to NAD/NADH, sueh as poly(neutral red) [101]. 

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