MD and DFT Calculations to Analyze Raman and SERS Spectra of Paraquat—From Computer Aided Spectra Interpretation to Pesticide Identification

The use of pesticides is an important practice in today's agricultural and nutritional supply chain worldwide. Their potentially harmful effects require rapid and reliable monitoring. As an emerging technology, surface-enhanced Raman spectroscopy (SERS) is paving its way through established methodologies. The Raman enhancing effect is based on the interaction of the analyte and nanoparticles prepared from noble metals. To better support and exploit analytical applications, the interaction between gold nanoparticles and the pesticide paraquat were studied. To this purpose, molecular dynamic calculations were performed with paraquat on single-crystal structures of gold at a distance of 3.4 Å and an intramolecular dihedral angle of 18.8° between the two paraquat ring systems. The molecular dynamic calculations showed that the two noble metal surface models exhibited only slight differences in their effects on paraquat. Experimental SERS spectra with gold nanoparticles were recorded and compared to the experimental Raman spectrum. The observed differences were further investigated using density functional theory calculations and reducing the gold cell to a gold cluster of 20 atoms. A co-planar orientation of paraquat to the gold cluster surface was thereby deduced. Based on an optimized paraquat geometry including a dihedral angle of 36.77° at a distance of 3.85 Å to the gold cluster, an excellent agreement between computed and experimental spectra was obtained. A head-on geometry was discarded due to spectral mismatch. This computational approach may help to analyze SERS spectra and make SERS further suitable for pesticide analysis.