Topological and electron-transfer properties of the 2-thiobarbituric Acid adlayer on polycrystalline gold electrodes.

The voltammetric behavior of [Ru(NH(3))(6)](3+) on bare gold and that on 2-thiobarbituric acid (TBA)-modified gold surfaces are almost identical, with formal rate constants for the electron-transfer process of 0.25 and 0.21 cm s(-1), respectively. A detailed analysis of the modified surface allowed us to establish that this behavior is due to (i) a high surface coverage of 0.67, (ii) a low adsorption resistance that minimizes the potential drop across the TBA monolayer, (iii) the enhanced hydrophilic character of the modified surface compared with that of bare gold, and (iv) a low decay constant for the electronic coupling of the TBA adlayer that minimizes the tunneling barrier for the electron transfer. The electron-transfer process from Au and Au|TBA electrodes to the soluble [Ru(NH(3))(6)](3+/2+) redox couple can be explained according to the multistate model under the Landau-Zener formalism in the nonadiabatic regime that was recently proposed (Feldberg, S. W.; Sutin, N. Chem. Phys. 2006, 324, 216-225). The behavior of soluble [Ru(NH(3))(6)](3+) changes from semi-infinite linear diffusion on Au to finite-length bounded on Au|TBA, in agreement with a surface dimension of 2.17 for the TBA adlayer with a bidimensional underlying gold surface. This value for the surface dimension was determined by two essentially different electrochemical techniques with different sensing capabilities: cyclic voltammetry and electrochemical impedance spectroscopy. The estimated dielectric constant of the adlayer (around 37) and the low potential drop across the monolayer suggest the formation of a "mirror" pattern of water molecules in the diffusion layer, which explains this result.

Myoglobin modified electrodes as anchors for d metal cationic complexes.

The capability of adsorption of different electroactive cationic Re(V)-amine complexes onto myoglobin-containing electrodes has been investigated. The goal of this work was the development of an Au/thiol/myo electrode and, after incubation of such ensemble in the presence of three different Re(V)-amine complexes, the evaluation of the extent of surface coverage by the complexes (as a way to evaluate the interaction complex-protein) using electrochemical techniques. Our results showed that a protein-containing electrode could therefore be used for the detection of the interaction of small electroactive cationic complexes and the biomolecule. The extent of the coverage of the myoglobin electrode by the complex depends on the number of free tails from the ligands and the total charge of the complex.

Electrochemical behavior of aqueous acid perrhenate-containing solutions on noble metals: critical review and new experimental evidence.

The actual state of the art in the reduction of perrhenate ions on noble metals is reviewed and discussed. Also, with the aim of contributing to better knowledge of this process, results of several experiments are presented. For the first time, spectroscopic evidence on the nature of the deposited rhenium layer on Pt and Rh and the detection of an intermediate in the reduction pathway toward metallic rhenium is provided. The role of the substrate in the electroreduction of perrhenate ions in aqueous acid media is emphasized, because it is directly associated with the formation of different H-containing species as reducing agents. Thus, those metals capable of adsorbing H atoms are able to reduce ReO(4)(-) to ReO(2) by H(ad) at potentials more positive than that of the hydrogen evolution reaction. Moreover, H(ad) reacts with the ReO(2) layer previously deposited, resulting in the formation of Re(III)-soluble species, which subsequently undergo disproportionation to Re and ReO(2). For metals that are not capable of adsorbing H, i.e., Au, molecular hydrogen is the reducing agent, leading to the formation of metallic Re. In addition, ReO(4)(-) is chemically reduced to metallic Re by hydride.