SERS chemical enhancement by copper - nanostructures: Theoretical study of Thiram pesticide adsorbed on Cu₂₀ cluster

Abstract

Surface-enhanced Raman spectroscopy (SERS), a surface-sensitive technique, allows the practicability of detecting chemical compounds in ultra-low concentration. In this work, a chemical enhancement mechanism of SER process of Thirarn pesticide adsorbed on copper nanomaterial surface was proposed based on density functional theory (DFT) approaches. Structural and electronic properties of Thirarn and Thiram-Cu₂₀ complexes were optimized using PBE method with LanL2DZ basis set for copper atoms and cc-pVDZ basis set for the non-metal atoms. In the most stable adsorption configuration, Thirarn interacted with Cu₂₀ cluster via two S(sp²) atoms. The main peaks on normal Raman spectrum of Thirarn were characterized at 371, 576, 1414 and 1456 cm⁻¹ responsible for the stretching vibrations of C-S, C=S, S-C-S and C-N groups, respectively. Otherwise, the main peaks of Thiram-Cu2U SERS spectrum were found at 534, 874. 982, 1398 and 1526 cm 1 corresponding to the stretching vibrations of S-S, C-S, S C S, C-N and Clfi-N bonds, respectively. The SERS chemical enhancement of Thirarn by Cu₂₀ cluster was about 2 and 6 times stronger than those obtained from Ag₂₀ and Au₂₀ cluster, respectively. The chemical enhancement mechanism was also explained by analyzing HOMO and LUMO energies gap and density of states.