Electrochemical Characterization of the Self-Assembled Monolayer of 8-Mercaptoquinoline on Polycrystalline Gold Electrode

Self-assembled monolayer of 8-mercaptoquinoline (MQ) on the surface of gold from MQ dilute ethanolic solutions is investigated by electrochemical methods. Some aqueous redox probes, such as ferrocene carboxylic acid and Fe(CN)₆ ^4–/3– can sufficiently diffuse into the monolayer because significant diffusion-limited current peaks are observed when the redox reactions take place, showing that the monolayer is very loosely packed or dominated by defects. However, the study on the electron transfer of other aqueous probes, such as Cu²⁺ and Ru(NH₃)₆ ^3+/2+, confirm that the monolayer can block the electron transfer on the gold electrode surface rather effectively for its low ratio of pinhole defects. These studies show that the MQ monolayer on the electrode can provide an excellent barrier for penetration of some probes but cannot resist the penetration of other probes effectively. The unusual properties of the self-assembled monolayers are attributed to the entity of the very large heterocyclic moiety.