Preparation Of Two-dimensional Substrates Based On Au@Ag Nanorods And Their Application In Surface Enhanced Raman Detection Of Thiram In Apple

In modern agricultural production,the problem of pesticide residues is becoming more and more serious with the widespead application of pesticides around the world,.Effective monitoring of pesticide residues in food is of great significance to food safety and environmental protection.In recent years,surface-enhanced Raman spectroscopy(SERS)has achieved rapid development in pesticide residues detection due to its high sensitivity and rapid non-destructive detection.However,the lack of stability of the SERS substrate hinders the practical application of the SERS technique to some extent.While retaining high sensitivity,how to improve the stability of the substrate has received more and more attention.The purpose of this paper is to detect pesticide residue in apple reliably and sensitively with fungicide thiram as a target.Gold-silver core-shell nanorods(Au@Ag NRs)were first synthesized,following the construction of two two-dimensional Raman substrates,including interface self-assembled substrate and cellulose based Au@Ag NRs substrate with great improvement in stability compared to Au@Ag NRs colloid.Finally,these substrates were successfully applied to the detection of thiram.The main research contents of the paper are as follows:(1)Preparation and characterization of SERS substrate Au@Ag NRs.Gold nanorods with high yield and uniform shape were successfully synthesized by seed-mediated growth method.Based on the gold nanorods,Au@Ag NRs with uniform silver shell thickness were prepared.Characterization of the synthesized nanorods were carried out using UV-visible spectrum,transmission electron microscopy,scanning electron microscopy and SERS,while the enhancement effect of Au@Ag NRs were optimized.Studies shown that increasing amount of silver nitrate could improve the yield and uniformity of gold nanorods.Besides,Au@Ag NRs obtained the best SERS effect with an enhancement factor of 1.95×10~7 with the addition of 200μL silver nitrate.The relative standard deviation of SERS intensity of Raman probe 4-mercaptobenzoic acid in different batches of substrates was 10.99%.After 7days of storage,the SERS intensity remained approximate to 86%.The prepared Au@Ag NRs showed good reproducibility and stability,meeting the requirements of subsequent applications.(2)Preparation and application of Au@Ag NRs interface self-assembled two-dimensional substrate.Using cyclohexane as the organic phase,Au@Ag NRs colloid as the water phase,and absolute ethanol as the inducer,the Au@Ag NRs interface self-assembled substrate was synthesized at the oil/water interface and transferred onto a silicon wafer to prepare a two-dimensional Au@Ag NRs self-assembled substrate.Using Au@Ag NRs concentrated sol and Au@Ag NRs self-assembled substrate,the rapid detection of thiram was successfully achieved with detection limits of 0.003 mg/L and 0.018 mg/L,respectively.In the detection using Au@Ag NRs concentrated sol,the concentration of thiram and the corresponding SERS intensity was proportional under low concentrations,the linear range was from 0.005 mg/L to 0.5 mg/L,the linear regression equation was y=68546.13 x+90.73,and R~2 was 0.9922.With regard to the self-assembled substrate,linear correlation was found between the logarithmic value of the thiram concentration and the SERS intensity,the linear range was from 0.01 mg/L to 10 mg/L,and the linear regression equation was y=15210.94 x+26717.74,and R~2 was 0.9816.The self-assembled substrate had a wider linear range and stability.The self-assembled substrate was also successfully used in the detection of thiram in apple,with a recovery rate of 93%–116%.(3)Preparation and application of Au@Ag NRs-carboxymethyl cellulose two-dimensional substrate.Au@Ag NRs-carboxymethyl cellulose two-dimensional substrate was prepared by mixing carboxymethyl cellulose with Au@Ag NRs sol and drying on a silicon wafer.Experiments revealed that cellulose could effectively slow down the silver shell oxidation process on the surface of Au@Ag NRs and effectively improved the stability of SERS performance.Raman intensity remained 86%after a 21-day storage period.The Au@Ag NRs-carboxymethyl cellulose two-dimensional substrate was successfully applied to the trace detection of thiram with a detection limit of 0.053 mg/L.The logarithmic value of thiram concentration was linearly correlated with the SERS signal intensity,with a linear range from 0.05 mg/L to 10 mg/L,the regression equation was y=15509.87 x+19893.12,and R~2 was 0.9923.In the thiram recovery test in apple,the recovery was 90%–115%.Au@Ag NRs-carboxymethyl cellulose two-dimensional substrate could be completely peeled from the silicon wafer to form a flexible substrate,which had the potential to be directly used for pesticide residues detection on the surface of apple.