Nicotinamide and trigonelline studied with surface-enhanced FT-Raman spectroscopy

Surface-enhanced Raman scattering spectroscopy with a near-IR excitation (FT-SERS) and Ag nanoparticles and was used for the vibrational characterization of biologically active nicotinamide (NA) and trigonelline (TR). The adsorption of these two molecules is studied here on the basis of the results obtained by varying pH of the medium. An assignment of SERS bands of NA and TR was performed by the comparison of their Raman spectra in the solid state and the solutions, and supported by DFT modelling. We showed that nicotinamide adopts a tilted orientation of the ring in regard to the silver substrate and the amide group does not participate in the adsorption. Trigonelline, on the other hand, does not show SER activity unless the carboxylic group is deprotonated, and accordingly, the adsorption on the Ag nanoparticles occurs via the deprotonated site. The SERS profile of both molecules indicates that the enhancement of Raman signal appears mainly due to the long distance electromagnetic mechanism of the SERS phenomenon with some contribution of the charge-transfer effect.