Detection Of Pesticide Residues By Surface-enhanced Raman Spectroscopy Based On Silver Colloid

The excessive use of pesticides poses a serious risk for food safety and human health,so it is urgent to build an efficient,stable and accurate pesticide residue detection system.Chromatography is widely used as the gold standard for pesticide residue detection.However,it has the shortcomings such as complicated pre-treatment steps,slow detection speed and high cost,which cannot meet the demand for rapid and accurate detection of pesticide residues.Surface-enhanced Raman spectroscopy(SERS)technique has a good application prospect for the advantages of high sensitivity,good selectivity,low water interference and in situ nondestructive detection of various organic pesticide molecules.Currently,the SERS technology has problems such as high cost and poor stability,so it is important to develop a cheap,stable and widely applied SERS pesticide residue detection technology.In this paper,we designed nano-silver colloid SERS substrate suitable for mass production and application by improving the preparation process of nano-silver colloid and achieved rapidly qualitative and quantitative analysis of thiram and ziram pesticides.The major research contents and conclusions of this thesis are as follows.(1)Nano-silver colloid was prepared by using the chemical reduction method and the particle size was controlled by the centrifugation process.The obtained nano-silver colloid SERS substrate has an obvious enhancement effect.The SERS signal reproducibility of the nano-silver colloid substrate was investigated using Rhodamine B as the probe molecule.The result showed that the SERS signals have good reproducibility(RSD<9%),which were much better than the traditional liquid substrates and even comparable to some complicated solid substrates.(2)The synthesized nano-silver colloid enhanced substrate can effectively enhance the SERS signals of thiram and ziram standard aqueous solutions with enhancement factor of 2.38×107 and 9.58×106,respectively.The RSD of the SERS signals of thiram and ziram standard aqueous solutions can be reduced to within 10%by optimizing the detection conditions such as collection time,type and dosage of activators.(3)It was found that the SERS signal intensity of thiram and ziram aqueous solutions presented a non-linear variation with their concentrations,which made it impossible to establish their quantitative analytical curves by the conventional method.It was concluded from the dynamic light scattering study that this nonlinear relationship is caused by the number change of "hot spots" induced by the different aggregation state of Ag nanoparticles in nano-silver colloid with the different concentrations of thiram and ziram.To achieve the quantitative analysis of thiram and ziram,a method,the exhaustive peak-seeking method,was proposed to find the internal standard peaks in the analyte's own groups.The method was successfully applied to the quantitative analysis of thiram and ziram pesticide residues in a wide concentration range,the determination of pesticide residues in apple juice.The range of detectable concentration meets the requirements of national standard.(4)An analytical method for thiram-ziram mixture with different concentrations in juice was established by using SERS technique combined with chemometric method.The qualitative discrimination of thiram,ziram and their mixture was achieved by the soft independent modeling of class analogy method.The quantitative determination of thiram-ziram mixture was established using the partial least square method with standard normal variate as the pretreatment method.