QUANTITATIVE ANALYSIS OF IMIDACLOPRID IN ENVIRONMENTAL WATER MATRICES BY SURFACE-ENHANCED RAMAN SPECTROSCOPY

Abstract

Neonicotinoids are synthetic pesticides commonly employed in agricultural fields to combat pests. Due to their high water solubility and persistence, they can be transported via percolating water or runoff into groundwater and surface water. Given the potential risks associated with acute and chronic human toxicity, regular monitoring of neonicotinoid concentrations in drinking water has become increasingly important. This study aims to develop a robust analytical method based on surface-enhanced Raman spectroscopy (SERS) was developed for the quantitative analysis of imidacloprid in various water matrices. This innovative analytical method is much faster and cheaper than the traditional methods based on liquid chromatography-tandem mass spectroscopy (LC-MS/MS).

The experimental approach primarily involved laboratory investigations, wherein varying concentrations of imidacloprid solutions were mixed with the benchmark citrate-coated gold nanoparticle (AuNP) colloid and electrolytes before Raman spectrum acquisition. Initially, deionized water devoid of inorganic and organic interferences was employed as the representative water sample. The fingerprinting SER spectra of imidacloprid were immediately obtained following vertexing, indicating the fast adsorption kinetics of imidacloprid onto AuNP surface. Furthermore, solution pH was adjusted to ensure that the molecular structure and fingerprinting spectra of imidacloprid were preserved under environmentally relevant pH values. These experimental assessments effectively confirmed the feasibility of the proposed method, consequently paving the way for subsequent investigations into its adaptability within complex water matrices.

Subsequently, environmental water matrices encompassing tap water and lake water were utilized as representative samples. The results exhibited the remarkable selectivity and sensitivity of SERS for imidacloprid detection in environmental water matrices. Notably, findings from various water samples collectively revealed that the competitive adsorption between citrate and imidacloprid on AuNP surfaces was the major interference with SERS detection of imidacloprid, especially at concentrations lower than 10 nM. In order to discern the imidacloprid-specific Raman bands concealed within the citrate bands, spectrum obtained from the samples were differentially analyzed against the citrate spectra. This comparative analysis demonstrated that the position of the characteristic bands associated with imidacloprid remained consistent across varying concentrations of imidacloprid in various water matrices.

Building upon these findings, we defined the relative intensities of the characteristic bands at fixed positions as the SERS pattern of imidacloprid. Each specific imidacloprid concentration in the samples exhibited a distinctive SERS pattern, thus substantiating that the SERS patterns can be used for imidacloprid quantification in water without using internal standards. To further ascertain the practical applicability of this method, principal component analysis (PCA) was conducted, resulting in successful classification of data collected from multiple parallel measurements encompassing different imidacloprid concentrations. In summary, this study exhibits immense potential to evolve into a cost-effective and high-throughput method for analyzing imidacloprid in environmental water matrices. Its implementation can significantly contribute to the development of a comprehensive map depicting imidacloprid pollution in natural and engineered water systems with an unprecedented level of spatiotemporal resolution.