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

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 235 cm−1 at −0.60 V electrode potential. Slight downshift (∼4 cm−1) without noticeable changes in intensity was observed for this mode at −0.90 V potential. Theoretical modeling of interaction of lipoic acid derivative with cluster of eight Ag atoms has confirmed the origin of 235 cm−1 band to ν(AgS) vibrational mode. Two SERS bands observed at 606 and 641 cm−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 (60 cm−1) because of involvement of 1,2-dithiolane ring sulfur atoms in covalent bonding with Ag surface. Relatively low wavenumber of amide-I band (1636 cm−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 2836 cm−1 in potential-difference spectrum indicating interaction of methylene groups with electrode surface at sufficiently negative electrode potentials.