Insight into SERS Chemical Enhancement Mechanism of Fungicide Thiram Adsorbed on Silver Nanoparticles

Surface-enhanced Raman spectroscopy (SERS) is an efficient analytical method to detect the chemical compound at ultra-low concentration. In this work, chemical enhancement phenomenon during the interaction of fungicide thiram (THR) on silver nanoparticles (AgNPs) is studied by combining experimental and density functional theory approaches. Limit of detection value for THR being 1 mu M is achieved. Structural and electronic properties of THR and its complexes with pyramidal Ag-20 clusters as a surface model are investigated at the PBE/cc-pVDZ-PP//cc-pVDZ level of theory. The calculated normal Raman and SERS spectra are in good agreement with the experimental ones. Analysis of the most stable complex indicates that THR interacts with the silver cluster via two S(sp(2)) atoms at the top of the cluster. By increasing the adsorbed THR molecules from 1 to 3, the SERS peak intensities increase from 4 to 10 times, respectively. The spectral fingerprints of THR are also analyzed via the adsorption of thiram-chlorpyrifos mixture. Consequently, the peaks at 1496, 1350 and 956 cm(-1) are recommended as the three most characteristic SERS peaks for quantifying THR in a real mixture. Finally, natural population analysis (NPA) charges, NBO analysis, charge density difference (CDD) maps allow providing more insight into charge transfer process THR and AgNPs.