Preparation and characterization of self assembled monolayers of 2-mercaptonicotinic acid on Au(111)

DIEGO E. PISSINIS; OMAR E. LINAREZ PÉREZ; FERNANDO P. COMETTO; MANUEL LÓPEZ TEIJELO

JOURNAL OF ELECTROANALYTICAL CHEMISTRY

ELSEVIER

Lugar: Amsterdam; Año: 2014 vol. 712 p. 167 - 177

0022-0728

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 -0.76 V (vs. ECS). From desorption experiments, the value of the surface concentration was estimated, resulting similar to that reported for related aromatic molecules. After 30 min of dipping time 2-MNA layers are closely packed on the Au(111) surface and the structure of the SAM is stabilized. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed for obtaining structural information of the monolayers by analysis of the change of the kinetics of the electron transfer reaction for the Fe2+/Fe3+ redox probe on the SAM. The results of CV and EIS show a rather imperfect blocking behaviour of the 2-MNA SAM and the Fe2+/Fe3+ redox process behaves as a more irreversible electron transfer reaction. This behaviour 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. From EIS experiments, the estimation of coverage can be made. In aqueous solution, 2-mercaptonicotinic acid behaves as a weak acid presenting a complex acid-base behaviour as well as equilibrium reactions between tautomeric species, including the formation of zwitterionic forms. The results can be explained taking into account that in strong alkaline aqueous solutions, 2-MNA stable species corresponds mainly to the doubly-charged negative species and a strong thiolate-gold chemical bond as well as a moderate interaction involving the non-bonding electrons of the non-protonated pyridinic nitrogen with the gold surface are expected. Under these conditions, no interaction of the gold surface with the carboxylate group is detected by SERS, which means that the carboxylate group is exposed to the solution. 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, allowing the formation of the zwitterionic species for of 2-MNA by intramolecular proton transfer between the nitrogen of the ring and the carboxylic acid group. 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. Reductive desorption of SAMs of thiols at different pH values was also employed to evaluate properties such as the pKa values of the different functional groups in the adsorbed molecules. As pH of the electrolyte is lowered, protonation of the thiolate group and/or any other ionisable groups in the molecule may take place after the desorption process, resulting in a pH-dependent value of peak potential for reductive desorption. 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. Thus, when reductive desorption is made in alkaline medium, only one cathodic peak is obtained. Furthermore, the splitting in two cathodic current contributions for pH values lower than ca. 7, allows to state that two different molecular configurations for 2-MNA 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 sulphur 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. pKa values of the adsorbed and/or dissolved species for 2-MNA adsorption onto Au(111) were estimated from reductive desorption experiments performed at different pH values.