Study On The Detection Method For Pesticide Residues Using The Interfacial Self-assembled Metal Droplet SERS Array

Pesticide is a kind of chemical agents used to improve grain yield and control diseases and pests,which is widely used in modern agricultural production.However,the abuse of pesticides inevitably leads to the pollution of agricultural ecosystem and pesticide residues in food.Therefore,a variety of pesticide detection technologies have been developed to meet the needs of on-site detection.Compared with traditional detection methods,surface enhanced Raman spectroscopy（SERS）can provide fingerprint information of specific molecules,which has the advantages of simple operation,short time consuming,high sensitivity and strong selectivity.Therefore,it is widely used in the fields of food safety,environmental monitoring and biosensor.In this paper,gold nanoparticles（AuNPs）were used as SERS active materials to construct a self-assembled metal droplet SERS array（MDA）at the water/oil interface.Combined with theoretical calculation,the optimal assembled structure was obtained,and the quantitative analysis of thiram,acetamiprid and triazophos was realized.The main research contents and conclusions of this study are as follows:（1）Preparation and application of AuNPs with different particle sizes.On the basis of14.5 nm gold seeds,29,55,83,98 and 137 nm AuNPs were obtained by seed growth method.The prepared AuNPs were characterized by transmission electron microscopy（TEM）and ultraviolet visible（UV-Vis）absorption spectroscopy,and then rhodamine 6G（R6G）was used as a beacon molecule to evaluate the relationship between the particle size of AuNPs and the SERS enhancement effect.Finally,the optimal particle size was selected and used for the quantitative analysis of triazophos solution.The results showed that AuNPs with different particle sizes had good dispersion and homogeneity,and the SERS enhancement effect of 98nm AuNPs was the best.It was calculated that the concentration of 98 nm gold sol was 1.17×10¹⁰ nanoparticles/m L,the enhancement factor（EF）was 6.31×10³.In addition,the gold sol was used for quantitative analysis of triazophos standard solution.The characteristic peak intensity(1603 cm^-1,logarithm)of triazophos had a linear relationship with its concentration（logarithm）in the range of 0.1–33.3μg/m L,the linear relationship was Y=3.19+0.33X,and the coefficient of determination was R²=0.99,and the limit of detection（LOD）was as low as0.082μg/m L.（2）The fabrication of self-assembled MDA at water/oil interface.Using tetrathiafulvalene as beacon molecule,gold sol as water phase and chloroform as oil phase,different proportions of 98 nm gold sol and chloroform solution containing tetrathiafulvalene were assembled by mixing and shaking,and the prepared metal droplets were concentrated and then assembled.Finally,MDA with uniform distribution was prepared at the water/oil interface.The results showed that the MDA with the ratio of water phase to oil phase=3:1 had the best SERS enhancement effect.It was calculated that the density of AuNPs on the MDA surface was 60AuNPs/μm²,the distance between AuNPs was 31 nm,and the molecular density of tetrathiafulvalene adsorbed on the surface was 0.18 AuNPs/μm².On this basis,the MDA was concentrated and assembled by removing part of trichloromethane from the metal droplet.It was calculated that the density of AuNPs on the surface of MDA was 67 AuNPs/μm²,and the distance between AuNPs was 24 nm.It was confirmed that the characteristic peak intensity of tetrathiafulvalene was significantly improved.Meanwhile,it was also proved that the MDA had good reproducibility and uniformity of SERS signal by using R6G as the beacon molecule.（3）The MDA based on the water/oil interface realized the SERS detection of a variety of pesticides.The mentioned MDA was used to quantitatively analyze thiram,triazophos,and acetamiprid pesticide with different hydrophilicities.The results showed that the LOD of the three pesticide standard solutions using MDA as the SERS substrate were 0.21、0.97 and 0.087ng/m L,respectively.Combined with a simple extraction and soaking pretreatment process,the LOD of triazophos in apple peel was 0.005 mg/L with a recovery rate of 96%-110%,and the entire analysis process only took 30 minutes.In addition,when chloroform was used as the internal standard to correct the fluctuation of the SERS signal,the corresponding coefficient R²of this detection method was significantly improved,and was higher than 0.98.