Toxic chemical monitoring of agricultural bioproducts using nanomaterials-based sensors

Raman and SERS spectra of thiamethoxam and the Ag3-thiamethoxam complex: an experimental and theoretical investigation

The insecticide thiamethoxam (TMX) is one of the most important neonicotinoid pesticides. The chromatographic methods currently employed to detect TMX require multiple operational steps. This study proposes a simple method that detects TMX via surface-enhanced Raman scattering (SERS) spectroscopy with Ag nanoparticles (NPs) as the SERS active substrate. Density functional theory (DFT) was used to calculate the structures and vibrational modes of the Ag- and Ag₃-TMX complexes at the B3LYP/6-311++G(d,p)(C,H,N)/LanL2DZ(Ag) level of theory. The results reveal that the atoms in the thiazole ring all lie in the same plane, while the six-membered ring is perpendicular to the thiazole ring. Data from both Ultraviolet-visible and Raman spectroscopy indicated that TMX bonds to Ag through its nitro group, vertically. A weak intramolecular (N22-O23…H26) hydrogen bonding and Ag-O bands shift N-O symmetrical vibration to down to lower wavenumber. This was supported by the appearance of a strong 866 cm^-1 band in the SERS spectrum assigned to the N-O symmetrical vibration coupled with the N-N stretching vibrational mode of different excitation wavelength. Notably, a good linear relationship was observed in the TMX concentration range 1.0 × 10^-6-1.0 × 10^-4mol·L^-1 (R² = 0.9667). SERS is an extremely simple and rapid technique that requires little sample for analysis.