Characterization and Detection of Neonicotinoid Pesticides using Surface Enhanced Raman Spectroscopy and Nano-electrochemistry

Orthogonal sensing of nitroguanidine neonicotinoid insecticides is explored by combining both surface enhanced Raman Spectroscopy (SERS) and nanoelectrochemical analysis. Silver nanostructured surfaces were fabricated for qualitative SERS-based detection of clothianidin and imidacloprid, to provide characteristic molecular spectra of the molecules, Density functional theory (DFT) studies were undertaken to assign the Raman active vibrational modes for clothianidin and imidacloprid. The Raman spectrum of clothianidin is assigned for the first time. Discrete arrays of on-chip fully integrated gold nanowire electrodes were developed to provide quantitative detection of the insecticides. Square-wave voltammetry permits highly sensitive and rapid determination of the neonicotinoids. Two electrochemical reduction peaks for clothianidin and imidacloprid were identified, and detection limits of 0.22 ng/mL and 2.15 ng/mL were subsequently achieved, respectively. These detection limits are significantly lower than previously reported electrochemical techniques and are comparable with significantly more complex and expensive chromatographic methods. By employing both techniques in combination, SERS characterisation provides the unique molecular fingerprint of each pesticide, while the nanoelectrochemical measurement provides a quantitative determination to ultra-low limits of detection. This combined spectro-electrochemical approach has the potential to significantly reduce false positives, that arise in remote monitoring, greatly increasing the robustness and credibility of these measurements.