Electrochemical study of monolayers of heterocyclic thiols self-assembled on polycrystalline gold electrode: the effect of solution pH on redox kinetics

Monolayer assembly of 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzimidazole (MBI) onto gold surface from its dilute ethanol solutions has been investigated by electrochemistry. Aqueous redox probes, such as Fe(CN)(6)(4-/3-) and Ru(NH3)(6)(3+/2+) can sufficiently diffuse into the monolayer of MBT and MBI, because significant diffusion-limited current peaks are observed when the reduction/oxidation reactions take place in acidic solution, showing that the monolayer is very loosely packed or dominated by defects. However, the study on the electron transfer (ET) of Fe(CN)(6)(4-/3-) and Ru(NH3)(6)(3+/2+) in neutral solution confirms that the MBT and MBI monolayer, can passivate the gold electrode surface rather effectively for its low ratio of pinhole defects. These unusual properties of MBT and MBI monolayers are attributed to the protonation of the N atom, at the MBT and MBI molecules in acidic solution. It is proposed that two effects, which exist at the same time, decide the electron transfer kinetics in acidic solution. First, the attractive or repulsive interaction between the positively charged monolayer and negatively or positively charged probes. Second, the repulsion among the positively charged monolayer molecules that results in the structure reorganization or reorientation, and less compactness of the monolayer, even the formation of open channels in monolayer provided that the repulsion among monolayer molecules is strong enough. (C) 2003 Elsevier B.V. All rights reserved.