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

Abstract

The characterization by cyclic voltammetry, impedance spectroscopy and SERS measurements of 2-mercaptonicotinic acid (2-MNA) monolayers formed under different conditions on Au(111) surfaces, is reported.Self-assembled 2-MNA monolayers on Au(111) in alkaline solutions desorbs reductively from the gold surface at −0.76V (vs. ECS). From desorption experiments, the value of the surface concentration was estimated, resulting similar to that reported for related aromatic molecules.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show a rather imperfect blocking behavior of the 2-MNA SAMS. This behavior corresponds to that obtained for microelectrode arrays, which is attributed to the access of ions through pinholes, defects and/or pores present in the SAM.The characteristics of ionisable groups exposed to the solution in the 2-MNA monolayers prepared by dipping in alkaline solutions were obtained by impedance measurements at different pH using the [Fe(CN)6]3−/[Fe(CN)6]4− redox probe and the value of pKa(SAM) for the carboxylate group in the surface 2-MNA monolayer was estimated. For pH>pKa, the carboxylic acid group is deprotonated and the monolayer acquires a negative surface charge, meanwhile it remains neutral or positively charged at lower pH values due to the beginning of pyridinic nitrogen protonation, which takes place overlapped with the protonation of the carboxylate group. The protonation of the pyridinic nitrogen is accompanied by desorption of this group from the surface. This leads to a change in the orientation of 2-MNA species allowing the interaction between the carboxylate group and the gold surface, which was further corroborated from SERS measurements.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.