Effect of Electrolyte Ions on the Formation, Electroactivity, and Rectification Properties of Films Obtained by Electrografting

The oxidation of 4-(4-nitrophenyl)butyrate ions in acetonitrile was used to modify glassy carbon surfaces with films bearing 3-(4-nitrophenyl)propyl groups. The main features of the voltammetric reduction of the grafted nitrophenyl groups were studied in two different supporting electrolytes; n-Bu4NPF6 and Me4NPF6. The best response was obtained with the less bulky electrolyte, which was explained by the major capability of these ions to diffuse inside the channels of the grafted film, as well as the better stabilization of the radical anions generated during the reduction process. It was also shown that the films electrografted in presence of the bulky n-Bu4NPF6 are thicker but less compact than those prepared in Me4NPF6. The amount of nitrophenyl groups in the grafted film that can be reduced depends on the size of the electrolyte ions. The permeation of the electrolyte ions inside the film channels determines also the voltammetric behavior of ferrocene as redox probe. A current rectification phenomenon was observed in acetonitrile solutions of bulky and hydrophobic electrolytes like n-Bu4NPF6 and n-Hx(4)NPF(6), whose marginal inclusion inside the film channels promotes both the entrance of ferrocene and the expelling of ferrocenium ions from these ones. This phenomenon was emulated through a CEC mechanism in solution, where the diffusion inside the channels was described as single chemical equilibria.