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

ABSTRACT

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.