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.
Méndez
Eduardo
E
Laboratorio de Biomateriales, Instituto de Química Biológica, Facultad de Ciencias, Universidad de la República, 11400 Montevideo, Uruguay. emendez@fcien.edu.uy
Wörner
Michael
M
Lages
Carol
C
Cerda
María F
MF
D003460:Crystallization; D004566:Electrodes; D004583:Electrons; D006046:Gold / Q000737:chemistry*; D015394:Molecular Structure; D013499:Surface Properties; D013858:Thiobarbiturates / Q000737:chemistry*
D013858:Thiobarbiturates; D006046:Gold; C029684:thiobarbituric acid