Mediated electrodeposition

Chen X, Matsumoto N, Hu Y, Wilson GS. Electrochemically-mediated electrodeposition/ electropolymerization to yield a glucose microbiosensor with improved characteristics. Analytical Chemistry 2002, 74, 368-372. 

X. Chen, N. Matsumoto, Y. Hu and G.S. Wilson, Electrochemically mediated electrodeposition/electropolsrmerization to 5deld a glucose microbiosensor with improved characteristics, A aZ. Chem., 74 (2002) 368-372. 

Figure 15.15 shows the chronopotentiogram for the constant current-(l mA/cm ) mediated electrodeposition of a PPy film on AA 2024-T3 using Tiron as both mediator and dopant ion. Compared to the nonmediated electrodeposition in the presence of p-toluene sulfonic acid sodium salt (Na-pTS), both the nucleation potential (maximum potential reached in the transient) and the growth (plateau) potential have been lowered by --- 700 and 500 mV, respectively. The film deposited by Tiron mediation was uniform and complete, whereas the film deposited with Na-pTS was patchy even after two times the deposition time [57]. From measurements of film thickness, doping level and polymer density, the current efficiency for polymer deposition is estimated to be nearly 100%. Electrochemical AFM studies revealed many more nucleation sites during initial stages of electrodeposition in the presence of Tiron than in control experiments where Tiron was replaced by Na-pTS [59]. 

Chen, G., D.E. Tallman, and G.P. Bierwagen. 2004. Unusual microstructures formed during the mediated electrodeposition of polypyrrole on Al 2024-T3 at low current densities. Solid State Electrochem 8 (7) 505. 

Deepa M., Srivastava A. K., Sharma S. N. et al. (2008) Microstmc-tural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition. Appl. Surf. Sci. 254 2342-2352... 

Jensen, M. B., J. M. Karels, A. F. Guerard, and D. E. Tallman, Video microscopy and scanning electrochemical microscopy investigations of the mediated electrodeposition of polypyrrole on aluminum alloy AA 2024-T3, Electrochimica Acta, 2011 (Unpublished results, manuscript in preparation). 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

It is interesting to conclude this section with an example that, in a sense, brings the chapter full circle. The metallization of plastic materials used as metal substitutes is a process with actual and future commercial potential. Usually, plastics are plated after appropriate sensitization by an electroless process which involves reduction of metal ions (e.g. Ni2+, Cu2+) by chemical rather than electrical means.19 There seems no reason why the reducing agent should not be incorporated in the polymer and Murray and his collaborators101 have demonstrated that copper, silver, cobalt and nickel may each be electrodeposited on to films of [poly-Ru(bipy)2(4-vinylpyridine)2]2+ coated on to platinum electrodes. The metal reductions are mediated by the Ru1 and Ru° states of the polymer. 

A vast amount of literature exists on enzyme-modified metal nanopartides. Crumbliss and co-workers pioneered the use of metal nanopartides for enzymatic sensors for various analytes such as H2O2, glucose, xanthine and hypoxanthine [156-158]. GCE or Pt electrodes are modified with enzyme-capped Au colloids, either by simple evaporation or electrodeposition. The nanopartides act as mediators, transferring electrons between the redox-active site on the immobilized biomolecule and the electrode and thus eliminating the need for external mediators. These sensors are classified as third generation biosensors . 

Schuhmann et al. introduced the use of electrodeposition paints (EDPs) as immobilization matrices for biosensors [17[. Following work enabled the incorporation of redox mediators into the polymer structure of EDPs [18. 146]. 

Hudak, N.S., Gallaway, J.W., and Barton, S.C. (2009) Formation of mediated biocatalytic cathodes by electrodeposition of a redox polymer and laccase. Journal of Eiectroanaiytical Chemistry, 629 (1-2), 57-62. 

A major improvement was realized with the use of indium, a metal with a very low first ionization potential (5.8 eV) which works without ultrasonic radiation even at room temperature [87]. As the zero-valent indium species is regenerated by either zinc, aluminum, or tin, a catalytic amount of indium trichloride together with zinc, aluminum [88], or tin [89] could be utilized in the allylation of carbonyl compounds in aqueous medium. The regeneration of indium after its use in an allylation process could be readily carried out by electrodeposition of the metal on an aluminum cathode [90], Compared with tin-mediated allylation in ethanol-water mixtures, the indium procedure is superior in terms of reactivity and selectivity. Indium-mediated allylation of pentoses and hexoses, which were however facilitated in dilute hydrochloric acid, produced fewer by-products and were more dia-stereoselective. The reactivity and the diastereoselectivity are compatible with a chelation-controlled reaction [84, 91]. Indeed, the methodology was used to prepare 3-deoxy-D-galacto-nonulosonic acid (KDN) [92, 93], N-acetylneuraminic acid [93, 94], and analogs [95],... 

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