Towards pesticide identification in open fields by Surface-enhanced Raman spectroscopy: A preparative, spectroscopic, and quantum chemical route

Pesticides are used in agriculture worldwide. The use of pesticides is monitored due to their hazardous potential. One method that is used as an analytical method is Surface-Enhanced Raman Spectroscopy (SERS). The SERS technique is based on the enhancement of the Raman signal by electric fields induced by nanoparticles. The resulting SERS signal is several orders of magnitude more intense than a Raman signal, which also enables detection in the trace range. Different SERS methods have been developed to identify the pesticides imidacloprid, thiram and paraquat, which are used with portable Raman spectrometers. Firstly, a method was developed in which imidacopride can be detected directly in the field using a Raman handheld spectrometer. Various agglomeration agents were analysed, including hydrochloric acid and various salts, in order to increase the SERS signal through agglomeration. The investigations showed that hydrochloric acid is significantly more suitable for agglomeration than the salts and also produces a better amplification of the SERS signal. It was also investigated whether filter paper and polylactide fabric are suitable as SERS substrates. It was found that polylactide is better suited as a SERS substrate base due to its hydrophobic effect. Furthermore, a model calculation was carried out which shows that potential pesticide residues of around 300 mg/kg can occur in agricultural soils. In a further study, the aspect of the SERS substrate basis was taken up again and polylactide and polyethylene terephthalate/polyamide textiles were investigated as carrier materials for the SERS substrates, with polyethylene terephthalate/polyamide proving to be the most suitable. As part of this study, the method was extended to the pesticides thiram and paraquat. Detection limits in the nanogram to picogram range were determined. The comparison with the model calculation from the first study showed that the detection limits are therefore in a relevant range, namely below the quantity applied, which means that pesticide residues can also be detected. In a further study, the developed method was extended to silver nanoparticles as active SERS substrates and analysed with regard to their different amplification compared to gold nanoparticles. It was shown that thiram is equally amplified by gold and silver nanoparticles and produces similar SERS spectra. Paraquat, on the other hand, produces different SERS spectra with gold and silver nanoparticles. This suggests a different orientation of the paraquat molecule on the nanoparticle surfaces. In a further study, which was carried out using computer simulations with DFT calculations, further orientations were investigated. A gold cluster consisting of 20 goldatoms and a gold single crystal consisting of 256 gold atoms were used. The comparison of the different calculations with experimentally recorded spectra showed that gold nanoparticles and paraquat probably have a planar orientation of the paraquat molecule to the gold nanoparticle surface. In future studies, further geometries could be investigated by calculations, as well as a supplementary investigation of the SERS substrate fundamentals.