Comparative electrochemical study of self-assembled monolayers of 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and 2-mercaptobenzimidazole formed on polycrystalline gold electrode

Comparative electrochemical behavior of self-assembled monolayers (SAMs) of three heteroaromatic thiols, 2-mercaptobenzoxazole (MBO), 2-mercaptobenzothiazole (MBT), and 2-mercaptobenzimidazole (MBI) are investigated by means of cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The electrochemical characteristics of the electrode/solution interface are considerably and differently affected by thiols constructing the SAMs. The consumed charges for reductive desorption of SAMs, which is criterion for the amount of chemically adsorbed thiol, are significantly different for these three SAMs, specially for MBT, implying that SAM of MBT is formed through both sulfur atoms; the thiol sulfur and skeleton sulfur of the thiazole ring. Desorption potentials of the SAMs have shown the following order for strength of gold-sulfur bond: MBT > MBO > MBI. Activity of the three SAMs as pH-sensitive interfaces was also investigated and their surface-pK_a values derived from the EIS measurements showed this order for acidic strength of SAMs: MBO > MBT > MBI. This is the same order expected due to the difference in electronegativity of the O, S, and N heteroatoms, and confirms that the most electron-rich ring imidazole is attached to the benzene ring of MBI. A comparison of the interfacial charge transfer resistance variation as a function of gold immersion time in thiols solution reveals that kinetics of Au-MBT assembly is different from those of two others and confirms formation of Au-MBT SAM via both sulfur atoms of MBT.