Rapid Determination Of Three Pesticides In Tea By Magnetic Enrichment And Surface-Enhanced Raman Spectroscopy

The control of tea pesticides residue is the key problem restricting the healthy development of tea industry.The pretreatment process of commonly laboratory pesticide residues detection methods is complicated,the detection cycle is relatively long,and instruments and equipment are expensive.SERS detection technology has been widely studied in the field of rapid trace detection due to the high sensitivity,strong selectivity,short detection time and easy sample processing.In this paper,combining SERS magnetic nano-substrate preparation and tea solid phase extraction pre-processing technology,two kinds of composite magnetic nanomaterials,Fe₃O₄@Ag and GO/Fe₃O₄@Au were innovatively prepared.And a tea pre-processing method was proposed.The rapid detection study of paraquat pesticide in tea and SERS detection method exploration of bifenthrin pesticide and lambda-cyhalothrin pesticide were completed.The main results are as follows:1.Preparation of magnetic nanosubstrates and pretreatment of teaFe₃O₄ with a particle size of about 250 nm was used as the magnetic core and PEI as the surface amino substance.Using seed growth method,gold seeds were adsorbed on the magnetic core surface and silver nitrate and reducing agent were added to prepare Fe₃O₄@Ag magnetic nanoparticles.The structure and appearance of Fe₃O₄@Ag were characterized by XPS,EDS Mapping,SEM and TEM.The SERS effect of Fe₃O₄@Ag was tested using 4-ATP as probe molecule,and the detected concentration was as low as 10^-8 mol/L.With the characteristic peak at 1075 cm^-1 as reference,the R² reached97.36%.The 20 nm Au NPs were prepared by using PEI to adsorb the aminated Fe₃O₄magnetic core by sodium citrate reduction method.After coated with a small amount of GO lamellar structure,GO/Fe₃O₄@Au magnetic nanoparticles with benzene ring pesticide molecules were prepared.Magnetic changes were characterized by hysteresis regression curve,the displacement of absorption peaks in UV-Vis absorption spectrum and changes in electrical properties of sample surface were used to characterize the structure of magnetic beads,and SERS effect of GO/Fe₃O₄@Au was tested using CV as a probe molecule.The detected concentration was as low as 10^-8 mol/L and the characteristic peak at 1178 cm^-1 was taken as reference.The correlation coefficient of the fitting curve reached 95.96%.The simplified tea pretreatment scheme used for SERS detection was studied,the pretreatment effect of PAS,GCB and Florisil on paraquat in tea was screened and compared,and the ratio of pre-treatment fillers was optimized.The results showed that using 200 mg GCB filler to make SPE column filtration tea decoction,pigment and tea polyphenols in tea matrix could be removed.SERS detection of the target molecule was achieved.2.SERS spectroscopic study of paraquat in teaThe effects of colloidal gold volume,silver nitrate dosage and ultrasonic time on the morphology of nanoparticles were studied by SEM,TEM,and the SERS enhanced effect of silver shell magnetic beads under different growth conditions was evaluated.The optimum preparation parameters of 200 m L colloidal gold,3 mg silver nitrate and10 min ultrasonic time were obtained.The minimum detection concentrations of paraquat on this substrate were 10^-10 mol/L,and the correlation curve between peak intensity of 855 cm^-1 and paraquat concentration was y=-3.738x+16.465,and the correlation coefficient（R²）was 98.07%.The results showed that the pretreatment effect of the SPE column for TPT tea was better than that of the self-made GCB column for paraquat pretreatment in tea.Magnetic enrichment detection method was used to set the experiment of adding paraquat with different concentration gradient in tea soup matrix,and the results showed that the lowest concentration of paraquat detected reached 10^-7mol/L.3.SERS spectroscopic study of bifenthrin and lambda-cyhalothrin in teaThe amount of Au NPs and GO in the GO/Fe₃O₄@Au growth process was optimized.The related structures were characterized by SERS,UV-Vis and TEM,and8 m L Au NPs and 0.2 m L GO were determined as the best preparation parameters of GO/Fe₃O₄@Au substrate.Then the minimum detection concentrations of bifenthrin on the substrate were 10^-8 mol/L,and the correlation curve between the peak intensity of1357 cm^-1 and the concentration of bifenthrin was y=5.677+8.139x,and the determination coefficient（R²）was 96.90%.It was proved that GO/Fe₃O₄@Au MNPs could be used as an effective SERS adsorption enhanced substrate for bifenthrin.The detection method of lambda-cyhalothrin based on aptamer specific binding was preliminarily-explored.The minimum concentration of lambda-cyhalothrin detected by colloidal gold colorimetry was 5×10^-7 mol/L,and the concentration of lambda-cyhalothrin detected by wafer aptamer sensor and competitive adsorption aptamer sensor was 10^-6 mol/L.