"Naked" gold nanoparticles supported on HOPG: melanin functionalization and catalytic activity.

Reductive electrodesorption has been used to produce "naked" gold nanoparticles (AuNPs) 3 nm in size on HOPG from different thiolate-capped AuNPs. The clean AuNPs transform the electrocatalytic inert HOPG into an active surface for hydrogen peroxide electroreduction, causing a lowering of the cathodic overpotential of 0.25 V with respect to the Au(111) surface. Compared to the plain gold substrates, the nanostructures promote only a slight increase in the hydrogen evolution reaction. In a second modification step a ∼1 nm thick melanin-iron coating is electrochemically formed around the AuNPs. This ultrathin melanin-iron coating largely improves the catalytic activity of the bare AuNPs for both hydrogen peroxide electroreduction and hydrogen evolution reaction. This strategy, which integrates electrochemistry and nanotechnology, can be applied to the preparation of efficient "naked" AuNPs and organic-iron capped AuNPs catalysts.

Oxidation-reduction dynamics in layer-by-layer self-assembled redox polyelectrolyte multilayer modified electrodes.

The oxidation-reduction dynamics of layer-by-layer (LbL) self-assembled redox polyelectrolyte multilayer films on electrodes has been studied by cyclic voltammetry, chrono-amperometry, electrochemical quartz crystal microbalance (EQCM), ellipsometry, and Fourier transform reflection-absorption infrared spectroscopy (FT-IRRAS). Thin layer electrochemistry with fast electron transfer at the underlying metal-film interface and charge propagation by electron hopping between adjacent redox sites in the finite thin film has been observed. An almost ideal cyclic voltammetry for a fixed number of redox sites in the thin surface film suggests that the multilayer can be fully oxidized and reduced in the time scale of the experiment (RT/vF > or = 0.05 sec). The electron hopping diffusion coefficient 3 x 10(-10) cm2 s(-1) was obtained from the chronoamperometric current transient and the ellipsometric thickness. Both cyclic voltammetry and potential step yield a surface osmium bipyridyl redox concentration of gamma Os = 4 x 10(-10) mol x cm(-2) for (PAH-Os)5(PVS)4 film. Exchange of ions and solvent occur simultaneously to the charge injection as revealed by the EQCM mass change and the ellipsometric thickness change. From the end-to-end mass-to-charge linear relationship, the molar mass of the ionic and neutral species exchanged largely exceeds the molar mass of any ions or solvent which suggests an important flux of solvent during redox switching. An initial "break in" effect is observed for the first oxidation-reduction cycles when a newly self-assembled film equilibrates with the electrolyte as charge is injected during the electrochemical perturbation.