Cyclic mediated electron transfer

This paper describes a process for activating polyimide surfaces for electroless metal plating. A thin surface region of a polyimide film can be electro-chemically reduced when contacted with certain reducing agent solutions. The electroactivity of polyimides is used to mediate electron transfer for depositing catalytic metal (e.g., Pd, Pt, Ni, Cu) seeds onto the polymer surface. The proposed metal deposition mechanism presented is based on results obtained from cyclic voltammetric, UV-visible, and Rutherford backscattering analysis of reduced and metallized polyimide films. This process allows blanket and full-additive metallization of polymeric materials for electronic device fabrication. 

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

In electrochemical redox reactions it is thought that the electron is transferred through the carbonyl ligand. Cyclic voltammetry experiments, in which the separation of the anodic and cathodic waves can be viewed as a measure of the ease of electron transfer, have been used to indirectly show that the carbonyl ligand can mediate electron transfer (265). For example, the electron transfer rate is inhibited by bulky ligands for R CCo3(CO)9 BLB (n = 1-3 and L = PR3) (266, 267). The interpretation is that steric bulk of the phosphines prevents the cluster carbonyls from approaching the platinum... 

Figure 14-7. Cyclic voltammogram demonstrating mediated electron transfer from glucose oxidase to a platinum electrode with ferrocene carboxylic acid, a) 2mM ferrocene carboxylic acid b) +10 mg glucose oxidase c) +30 mg glucose. —100 to +700 mV vs. SCE 5 mV s 0,1 M phosphate buffer (ph 7.4) with 0.5 M NaCl platinum disk electrode, 1 mm diameter.

The principle of "mediated" electron transfer, whereby electrons are passed from the reduced form of a relatively negative redox couple to the oxidized form of a relatively positive couple, has been demonstrated to occur between two polymer layers of slightly different Ru (bpy)3 complex polymers by Murray and coworkers (18), This kind of stepwise, unidirectional electron transfer may be very significant in future polymer coated PEC cells which seek to separate charge, and of additional Interest, Ru (bpy)3 complexes are frequently used as cyclic PEC catalysts in water splitting experiments. Some details of this experiment are thus Informative. 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

Other redox-active polyelectrolyte films were prepared from ferrocene-derivatized polly(allylamine) and poly(vinyl pyridine) as well as an osmium complex of poly(vinyl pyridine) [44-46]. These films were synthesized to mediate electron transfer between the electrode and a charged enzyme that was a constituent of the polyelectrolyte film. In the case of ferrocene-derivatized poly(allylamine) or polyfvinyl pyridine), cyclic voltammetry of the bound ferrocene moiety showed small peak splittings (<50 mV at a scan rate of 50 and 20 mV s respectively) [45, 46). The amount of electroactive material increased with the number of deposited layers, but the first layer contained significantly more electroactive ferrocene than the later layers in the poly(allylamine) system [46]. 

Fukuzumi reported a detailed mechanistic study of quinone reduction catalyzed by protonated amino acids [245]. Kinetic experiments, EPR spectroscopy and cyclic voltammetry were employed to illustrate the role protonated histidine plays in mediating electron transfer between NADH analog 9,10-dihydro-10-methy-lacridine (AcrH2) and l-(p-tolylsulfinyl)-2,5- benzoquinone (TolSQ). Cyclic voltammetry experiments demonstrate a 0.55-V positive shift in the one-electron reduction potential of TolSQ in the presence of 5.0 x 10 M of protonated... 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

The role of anthraquinones as mediators of one-electron transfer to molecular oxygen has been studied by cyclic voltammetry in DMSO and DMF solution.217 The reduction potentials of those anthraquinones containing OH groups were substantially shifted towards more positive values in the presence of 02, whereas those without OH groups... 

Cyclic voltammetry, kinetic studies, and DFT calculations using a BP functional and the TZVP basis set showed that the major pathway of the non-regiospeciflc zinc-reduced titanocene-mediated ring opening of epoxides was initiated by a titanium dimer-epoxide compound that reacted in a rate-determining electron transfer mechanism 25 The calculations showed that the transition state is early so the stereoselectivity is determined by steric effects rather than by the stability of intermediate radicals. This was confirmed by studies with more sterically crowded catalysts. 

Benzyl chloride and its substituted derivatives are electrochemically reduced indirectly through the mediator, l,4-dihydro-4-methoxycarbonyl-l-methylpyridine anion 35, (equation 27). The rates of the electron transfer between the mediators and benzyl halides, measured by cyclic voltammetry, were found to be about 961 80 M 1 s1. The ratedetermining step was proposed to involve a single electron transfer from the mediator to... 

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