Research On The Preparation Of Solid SERS Substrate And Its Detection Of Pesticide Residues On Fruit Surface

Pesticide residue has become one of the hot issues in the field of food safety.Longterm consumption of food with excessive pesticide residues may cause chronic poisoning and endanger life and health.Surface enhanced Raman spectroscopy(SERS)technology was widely used in the field of physical chemistry because of its unique,simple,fast,nondestructive and spectral fingerprint characteristics.However,the application of SERS technology in pesticide residue detection was only at the initial development stage,and there are still many problems restricting its further promotion.Aiming at the requirements of pesticide residue detection on fruit surface,this paper promoted the practical process of SERS technology by researching and developing new types of solid substrate with high sensitivity and repeatability.Firstly,based on SERS electromagnetic enhancement mechanism,finite difference time domain(FDTD)method was used to simulate the local plasma resonance electric field with different morphologies and sizes of nanostructures.The variation rule of local electric field intensity with parameters such as morphology distance and incident electric field polarization of nanoparticles was obtained,and the development route of nanoparticles was designed,which was dominated by silver nanostructure and controlled the uniformity of overall morphology through gold nucleus growth method,laying a foundation for the development of new active substrates.Then,based on density functional theory(DFT),Raman spectral theoretical calculation was carried out on the residues of four commonly used pesticides of fruits,namely thiram,tricyclazole,imazalil and benzimidazole,at the level of B3 LYP.Through the matching of theoretical spectrum,solid spectrum and SERS spectrum,the attribution relationship between characteristic peak position of Raman spectrum and molecular vibration mode was determined,providing theoretical basis for subsequent detection.Secondly,the monodisperse silver nanoparticles were prepared by the wet chemical method of reducing silver nitrate with sodium citrate and ascorbic acid.Then,with the aid of the silicon wafer,the silver nanospheric self-assembled monolayer was transferred to the polydimethylsiloxane(PDMS)sheet by the Langmuir-Schaefer horizontal adhesion deposition method to develop a novel lamellar solid substrate.The soft and transparent properties of PDMS material were used to detect the fruit surface in situ without pretreatment.The enhancement factor(EF)of rhodamine 6G probes on the silver nanoparticles PDMS substrate was up to 6.06?105 and the detection limit was 10-8 mol/L.The in-situ detection and analysis process of thiram and ttricyclazole pesticides on apple surface without pretreatment could be realized within 10 min,which fully reflected the advantages of SERS technology on site.Thirdly,the silver nanocube with uniform appearance was prepared by double injection epitaxial growth of silver shell on the gold nanosphere In addition,the enhancement of the strength of the local electric field caused by the tip charge enrichment effect at the corner of the cube was utilized to improve the enhancement ability of PDMS substrate.The enhancement factor of the new substrate to the R6 G probe was 2.75?106,which was an order of magnitude higher than that of the silver nanoparticle PDMS substrate.The in-situ detection limit of imazalil pesticide on the surface of apple,citrus and tomato could reach 2 mg/L.Finally,a silver-coated gold nanorods thermal shrinkage polystyrene(TSP)substrate with high enhanced SERS activity was designed and prepared.The uniformity of the nanoparticles was controlled by the core of the gold nanorod,and the absorption peak position of the nanoparticles was adjusted by controlling the thickness of the silver nanoroshell,so as to match the resonance absorption between 785 nm wavelength of the excitation light source.Moreover,the density of nanostructure region was improved through the characteristics of three-dimensional folds generated by thermal shrinkage of TSP material.The enhancement factor of the substrate was raised to the order of 107 through a combination of above two aspects.After simple pretreatment,the actual detection limit of benzimidazole pesticide residues on apple surface was 0.071 mg/kg.After correction of extraction efficiency,the recovery rate ranged from 89.8% to 111.7%,and the detection result showed a good performance,fully demonstrating the application potential of substrate in the field of pesticide residue detection.