Electrochemical characterization of redox centers organized at Hg surfaces

The electrochemical characterization of a series of redox sites absorbed at Hg surface by different interactions is reported. The redox centers, based on Fe(II) and Ru(II), are incorporated, respectively, in the molecules Fe(C5H5)(C5H4)(CH2)(4)SH and [Ru(NH3)(5-)NC5H4CH2NHCO(CH2)(10)SH](PF6), and are anchored on the Hg surface in one component self-assembled monolayers. The electrochemical behaviour of these systems indicates that redox centers are located onto a uniform, homogeneous environment at the external surface of the monolayer. We also report the electrochemical behaviour of the positively charged redox species [Ru(NH3)(6)](3+) when the Hg electrode surface is functionalized with a negatively charged SAM. The SAM is formed by 11-mercaptoundecanoic acid that exposes carboxylic acid groups to solutions of different pH values. At a pH lower than 4, the cyclic voltammograms show negligible current, and pH from 5 to 9, the voltammograms are essentially identical and show a well-defined redox wave. From a study of the voltammetric responses of the Ru(NH3)(6)(3+/2+) couple as a function of the electrolyte composition and concentration at pH 9, we suggest that the redox reaction takes place at the defects of the SAMs created by the repulsion of the -COO- head groups and that the current is determined by a diffusion-controlled mechanism. (c) 2006 Elsevier B.V. All rights reserved.