Detection of pesticide residues on apples using surface-enhanced Raman spectroscopy

Abstract

Pesticides are an integral part of agriculture, while increasing use leads to residues in/on agricultural products. Federal monitoring and enforcement action is dependent on the technical capability to detect pesticides. However, current methods are elaborate, time-consuming and not cost-effective. In the first part of this work, a rapid and simple surface-enhanced Raman spectroscopy (SERS) method coupled with a surface swab method for recovery and quantitative detection of thiabendazole (TBZ) on apple surfaces was developed, optimized and validated. The whole apple surface was swabbed and the swab was vortexed to release the pesticides. After that, silver dendrites (AgD) were used to bind the pesticide for Raman measurement. The limit of detection of TBZ in methanol was 0.01µg/mL, (10ppb), while the Environmental Protection Agency (EPA) limit is 5µg/gram apple-weight (5 ppm). The concentration of the recovered TBZ was predicted using a partial least square model. The recovery from the surface swab method was calculated to be 59.7% to 76.6% for intentional contamination at 0.1, 0.3, 3 and 5 ppm (µg/g apple-weight) level, respectively. The final accuracy of the swab-SERS method was calculated to be between 90.0% and 115.4%, after corrected by the releasing factor (66.6%). In the second part, a new approach was proposed to detect acetamiprid using an aptamer-based SERS method. The acetamiprid aptamer was chosen from the literature, thoilated and conjugated onto AgD. To block the unbounded surface on the substrate surface after aptamer immobilization, bovine serum albumin (BSA), 2-mercaptoethanol (ME) 6-mercaptohexanol (MCH) and were investigated as blocking agents. MCH and ME cannot fully block the surface when encountered with interference. The typical peaks from acetamiprid did not show on the aptamer-blocking agent-acetampired spectra when using BSA as blocking agents. The aptamer and blocking agent immobility on AgD should be further investigated and the method should be further modified. Last, the swab method was further developed and validated using UV-visible spectroscopy as a reference method. A standard curve was established based on the absorbance at 245 nm at different concentrations from 0 ppm to 1000 ppm. The assay standard curve well fit the five-parameter logistic model (r2 =0.995). The concentration of acetamiprid in the extracts was determined using this standard curve. The recovery rate of refined surface swab method is 90.6 % ± 1.4 % (n=5). This assay has a low intra- and inter-assay coefficient of variation (CV < 5 %).