A Combined Normal Raman, Surface-Enhanced Raman Scattering (SERS), and Theoretical Calculation (DFT and PED) Study of Alpha-Tocopherol

Abstract

In this work, the band assignment for the vibration mode of the functional groups contained in α-tocopherol (aT) molecules was analyzed entirely for the first time using their normal Raman spectra (NRS), density functional theory (DFT) calculations, and potential energy distributions (PED). The bands obtained through the theory calculation method were in accordance with experimentally observed values. The surface-enhanced Raman scattering (SERS) spectra of aT adsorbed on nanoparticles were studied in gold colloidal solutions and on modified gold substrates. Some crucial differences were observed in the SERS spectra of the aT molecules adsorbed on these two substrates, indicating that the behavior of aT adsorbed on gold colloids was different from that of aT on the modified gold substrate. The aT molecules probably lie flat on the surfaces of the gold nanoparticles through the ring in the gold colloidal solution, whereas they stand with a certain angle on the gold surfaces of the modified substrate. Furthermore, the concentration dependence of SERS spectra of aT is characterized by variation of the enhanced intensity of the bands, primarily suggesting that the adsorption behavior of the aT molecules adsorbed on the gold nanoparticles surface is changed with different concentrations. With the concentration around 7.5 × 10−2 mol/L, the sub-monolayer mode, and more aT molecules allowed to lie on the gold surface, then Raman signals are greatly enhanced in the SERS spectrum.