Adsorption of Methimazole on Silver Nanoparticles: FTIR, Raman, and Surface-Enhanced Raman Scattering Study Aided by Density Functional Theory

FTIR, Raman, and surface-enhanced Raman scattering (SERS) of methimazole (MMI) have been investigated. MMI is an important antithyroid drug that inhibits the formation of thyroid hormone. It is widely used clinically in the treatment of hyperthyroidism, and thus it is useful to study its surface adsorption characteristics. The experimental FTIR and Raman data are supported with DFT calculations using B3LYP functional with LANL2DZ basis set. This is the first report on the vibrational analysis of the thiol and thione forms of MMI and their various possible silver complexes. pH-dependent normal Raman spectra have been recorded, which show the abundance of the thione form of MMI in acidic, neutral, and alkaline media. From the SERS spectra as well as theoretical calculations, it has been inferred that in neutral and alkaline media, the thiol form of MMI is chemisorbed to the silver surface through the ring N atom of the imidazole ring with an edge-on orientation and the imidazole ring lying in the plane of the silver surface. In contrast, it has been concluded that in the acidic medium, the thione form of MMI gets adsorbed to the silver surface. Thus, the pH-dependent SERS spectra have shown the preferential existence of thione and thiol tautomeric forms on the silver surface in acidic, neutral, and alkaline media.