The Fabrication Of Interfacial Self-Assembled Nanoparticle Array Substrates For SERS Detection Of Pesticides

Pesticides are a class of chemical substances or biological agents widely used in agricultural production to increase crop yields and prevent the occurrence of pests and diseases.In recent years,the irrational use of pesticides has caused a great risk to the ecosystem and public health.Traditional methods for detection of pesticides such as high-performance liquid chromatography,liquid chromatography-mass spectrometry,and gas chromatography-mass spectrometry are time-consuming,laborious,costly,and environmentally unfriendly,making them difficult to achieve the purpose of rapid and on-site detection.Surface-enhanced Raman spectroscopy(SERS)combines the technical advantages of Raman spectroscopy and nano-optics,and has received extensive attention in the fields of food safety detection,environment monitoring,and biosensing.This study aims to improve the sensitivity,uniformity,and stability of the plasmonic nanoparticle substrate for SERS detection.Firstly,the shell-dependent bimetallic silver-coated-gold nanoparticles(Au@AgNPs)were fabricated based on the seed-growth method;secondly,the self-assembly behavior of gold nanoparticles(AuNPs)and Au@AgNPs at the oil/water planar interface was explored;thirdly,the SERS performance of tape and polymer film stabilized flexible Au@Ag nanoparticle array was investigated;finally,the electric field enhancement mechanism of self-assembled metal nanoparticle array was explored by the finite difference time domain(FDTD)method.As a result,four pesticides(difenoconazole,thiabendazole,thiram and fenpropathrin)commonly used in agricultural production were selected as model pesticides,and the detection performance of the constructed SERS substrates to pesticide residues in food matrices were studied.This study was expected to provides a basis for promoting the application of SERS technology in pesticide detection.The research content and conclusion of this study are listed as follows:(1)The fabrication of shell thickness-dependent Au@AgNPs and studying its SERS response to difenoconazole in grapes.The core-shell Au@AgNPs were prepared by the seed-growth method with the 32 nm AuNPs as the core,and the optical properties and SERS performance of Au@AgNPs were regulated by control of the thickness of Ag shell.Results showed that when the thickness of the Ag shell increased from 0 to 9.5 nm,the intensity of surface plasmon resonance(SPR)band from the Ag shell was increased and red-shifted,while the SPR from the Au core decayed rapidly.The Au@AgNPs with the Ag shell thickness of 5.2nm had a relatively uniform particle size distribution,and possessed excellent SERS enhancement effect.The synthesized Au@AgNPs can be used for quantitative analysis of difenoconazole solutions with a concentration of 0.2-10 ppm.The established calibration curve based on the Raman peaks at 698 cm^-1 and 808 cm^-1gave correlation coefficients R² of0.979 and 0.987,respectively.In combination with the Qu ECh ERS(Quick,Easy,Cheap,Effective,Rugged,Safe)sample pretreatment method,the quantitative SERS analysis of difenoconazole in grape samples was carried out,which showed the limit of detection(LOD)of as low as 64?g/kg.The entire analysis process of the proposed method can be completed within 25 minutes.(2)The fabrication of self-assembled two-dimensional(2D)Au@Ag nanodot array at the planar interface of the biphasic oil/water system,and studying its SERS detection performance for thiram and thiabendazole in juice.Firstly,a biphasic cyclohexane/water system was developed,and ethanol was injected to induce Au@AgNPs in the water phase to form a metal film at the oil/water interface.Subsequently,the metal film was transferred to the surface of the silicon wafer substrate to obtain a densely packed self-assembled two-dimensional Au@Ag nanodot array SERS substrate.Results showed that the gap of adjacent Au@AgNPs was less than 3 nm,and intensive SERS“hot spots”can be formed between adjacent nanoparticles.The obtained Au@Ag nanodot array with an enhancement factor of 1.2×10⁶ showed better SERS activity than Au@AgNPs colloid.In addition,the Au@Ag nanodot array substrate had good uniformity and signal reproducibility.The relative standard deviations(RSD)of the SERS intensity of R6G at 612,1183,and 1363 cm^-1 were 8.51%,9.21%and 9.68%,respectively,and the RSD from substrate-to-substate was 10.51%.The obtained 2D Au@Ag nanodot array substrate was used to detect thiram and thiabendazole in water,pear juice,apple juice,and orange juice.The LOD values were 0.0011,0.0052,0.0130,and 0.0590 ppm for thiram,and0.051,0.100,0.180,and 0.680 ppm for thiabendazole,respectively.(3)The fabrication of a flexible and stable Au@Ag nanoparticle array enabled by acrylic tape and polyethylene terephthalate(PET)film,and studying its non-destructive SERS detection performance of thiram on the surface of fruits and vegetables.The 2D Au@Ag nanodot array on the surface of the silicon wafer was transferred to the flexible and adhesive acrylic tape,and then a layer of PET film was attached on tape surface to prevent the Au@Ag nanodot array from being affected by the external environment.Results indicated that acrylic tape can effectively maintain the structure of the densely packed Au@Ag nanodot array,which showed excellent SERS sensitivity,uniformity,and signal reproducibility.The excellent flexibility of the tape allowed the substrate to maintain 85%SERS activity after undergoing a20%tensile deformation treatment for 3 times.In addition,the PET film can protect the Au@Ag nanodot array from the external environment(air,high temperature and ultrasound treatments),effectively holding the stability of the Au@Ag nanodot array.The resulting flexible SERS substrate can directly attach the surface of fruits and vegetables,allowing detection of thiram on the surface of apple,tomato,and cucumber with the detection limit of 5 ng/cm².(4)The fabrication of a sensitive,uniform,stable and flexible SERS chip enabled by multi-layer polymer films packaged Au@Ag nanoparticle array,and studying its SERS performance for sensing thiabendazole in fruit juice.The polymethyl methacrylate(PMMA)toluene solution was used as the organic phase and ethanol was used as the inducer.The Au@Ag nanoparticle array fixed on the PMMA film was formed by self-assembly of Au@AgNPs at the oil/water interface.The amount of PMMA added had a significant effect on the fixation of Au@AgNPs.When the content of PMMA was 2.6 mg/cm²,an Au@Ag/PMMA film substrate with densely nanoparticle arrangement can be obtained.In addition,the sealing film for fluorescent quantitative PCR(q PCR film)and PET film can maintain the mechanical stability and storage stability of the substrate.The resulting Au@Ag/PMMA/q PCR-PET film SERS chip gave an enhancement factor of 3.14×10⁶,and it retained more than 90%of SERS activity after 60 days of storage at room temperature.The obtained SERS chip showed excellent sensing performance for thiabendazole,and the detection ranges for thiabendazole in tap water and juice were 0.05-10 ppm and 0.1-10 ppm,respectively.The LOD values of thiabendazole in pear juice,orange juice and grape juice can be as low as 21,43,69 ppb,respectively.(5)The fabrication of 1-butanethiol(1-BT)self-assembled monolayer functionalized AuNPs arrays,and studying its SERS detection performance for fenpropathrin.Results showed that the self-assembled AuNPs array at the oil/water interface exhibited a strong SERS signal of sodium citrate under the 785 nm laser excitation.With the help of ligand exchange between1-BT(1 m M)and the citrate on the surface of the AuNPs array,the SERS background signal of citrate residues on the surface of AuNPs array can be significantly reduced.After normalizing the SERS spectrum with the Raman peak of 1-BT at 892 cm^-1,the 1-BT/AuNPs array showed good signal uniformity.RSD values of 1-BT Raman peak intensity at 799 cm^-1and 1096 cm^-1 were 4.28%and 1.16%,respectively.Most importantly,the unmodified AuNPs array was difficult to directly detect the Raman signal of fenpropathrin,while fenpropathrin can be enriched on the surface of the 1-BT/AuNPs array through the interaction between 1-BT and fenpropathrin molecules,allowing SERS detection of fenpropathrin with a quantitative range of 0.1 to 1000 ppm.(6)The construction of FDTD numerical simulation model of core-shell Au@Ag nanoparticle array for studying the electric field enhancement mechanism.Resulted suggested that the gap between adjacent nanoparticles significantly affected the near-field coupling effect of the electric field.For the Au@AgNPs array with the Au core nanoparticle size of 32 nm and the Ag shell thickness of 5.2 nm,the SERS electric field enhancement factor decreased by about4 orders of magnitude with increasing the nanoparticle gap from 1 nm to 10 nm.As the thickness of the Ag shell increased from 0 to 9.5 nm,the electromagnetic enhancement effect of the Au@AgNPs array increased gradually.Compared with the excitation at 532 nm and 785 nm,633 nm excitation can produce a stronger SERS electric field enhancement factor.In addition,the environment around nanoparticles also affected the electric field enhancement.When the“hot spots”were embedded by PMMA,the electric field coupling effect of adjacent nanoparticles was weakened,and thus the SERS electric field enhancement factor was reduced.Therefore,rational design of Au@Ag/PMMA film substrate with optimal Ag shell thickness and PMMA amount was very important to achieve the maximum SERS activity.