Validation of SERS-Based Quantification of Neonicotinoids in Agriculture-Impacted Groundwater

Abstract

Conventional laboratory-based chromatographic analysis provides sensitive pesticide measurements, but its cost, labor, and analytical turnaround can constrain sampling frequency in routine field monitoring. Here, we report a surface-enhanced Raman spectroscopy (SERS) workflow for rapid, molecularly specific screening of neonicotinoids in agriculture-impacted groundwater. HCl-assisted ligand exchange on gold nanoparticle surfaces, multi-band analyte identification, and hot-spot normalization enabled reliable quantification of two representative neonicotinoids in environmental waters. Field samples were collected across Wisconsin’s Central Sands from five irrigation wells and nine groundwater-fed ditch sites. SERS detected imidacloprid (IMD) and clothianidin (CLO) at calibration-derived concentrations ranging from 0.25 to 4.02 nM, with detections below the SERS limits of detection (LOD) treated as semi-quantitative estimates. Sample processing involved filtration, acidification, and incubation with the SERS substrate, followed by rapid Raman spectra acquisition of approximately 5 min per map, with an estimated material cost of ∼$0.17 per substrate. Results were cross-validated against high-performance liquid chromatography (HPLC) measurements. For measurements above the LOD, SERS recoveries relative to HPLC ranged from 96% to 117%, demonstrating the applicability of the workflow to low-nanomolar pesticide screening in field-collected waters. This practical screen-then-confirm approach can help identify where and when definitive measurements are warranted in dynamic agriculture-impacted waters.