Preparation and characterization of self assembled monolayers of 2-mercaptonicotinic acid on Au(1Â 1Â 1)

In alkaline aqueous medium, 2-MNA adsorbs through a thiolate chemical bond on the gold surface and an additional interaction of the pyridinic nitrogen stabilizes the monolayer. One cathodic peak is obtained when reductive desorption is made in alkaline medium, while a splitting in two cathodic current contributions for pH values lower than ca. 7 indicates that two different molecular structures coexist on the gold surface in neutral or acidic media. This is assigned to the occurrence of an equilibrium between the molecular structure bonded through sulfur and nitrogen and the surface-bonded zwitterionic species by intramolecular proton transfer from the carboxylic acid group to the ring nitrogen, leading to the protonation of the pyridinic nitrogen and allowing the interaction of the carboxylate group with the gold surface.