SERS characterization of imidazole ring terminated self-assembled monolayer formed from lipoic acid histamide on silver electrode

Abstract

Structure of self-assembled monolayer (SAM) of lipoic acid histamide derivative (LAH), which contains a disulfide group, adsorbed at silver electrode was studied in situ in buffered (pH 8.0) aqueous solution by surface enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. Contrary to LAH in solution, the tautomer-II (N3H protonated) form of imidazole ring was found to be dominant in the adsorbed state. The low frequency AgS stretching band was clearly detected near 235cm−1 at −0.60V electrode potential. Slight downshift (∼4cm−1) without noticeable changes in intensity was observed for this mode at −0.90V potential. Theoretical modeling of interaction of lipoic acid derivative with cluster of eight Ag atoms has confirmed the origin of 235cm−1 band to ν(AgS) vibrational mode. Two SERS bands observed at 606 and 641cm−1 have been assigned to predicted by calculations CS in-phase and out-of-phase vibrational modes, respectively. The in-phase CS stretching vibration exhibited unusually large frequency downshift (60cm−1) because of involvement of 1,2-dithiolane ring sulfur atoms in covalent bonding with Ag surface. Relatively low wavenumber of amide-I band (1636cm−1) revealed strong hydrogen bonding interaction of amide group in the monolayer. Reorientation of adsorbate to more parallel configuration with respect to the electrode surface at more negative potentials was proposed based on changes in intensity of in-phase CS stretching band and appearance of soft CH stretching mode near 2836cm−1 in potential-difference spectrum indicating interaction of methylene groups with electrode surface at sufficiently negative electrode potentials.