Preparation Of Au@Ag Nanocuboids For SERS Detection And Quantitative Analysis On Food Contaminations

Objective:In recent years,there have been frequent incidents of food contaminants such as food additives phthalate plasticizers（PAEs）to pesticide residues.Food contaminants can enter the human body through direct contact and indirect intake,posing a serious threat to human health.At present,the conventional detection methods of food contaminants have defects such as insufficient detection limit,time consuming,and large sample volume,which restricts the development of rapid detection of food contaminants on site.Surface-enhanced Raman scattering spectroscopy（SERS）is widely used in trace substances detection due to its high sensitivity,mild detection conditions,no need for complex sample preparation,no damage to samples,simple operation,and real-time in-situ on-site detection.In this experiment,four different sizes of Au@Ag nanocuboids were synthesized,and the optimal substrate for detecting food contaminants was screened to improve the sensitivity of the SERS substrate.Then,the SERS active substrate with internal standard is prepared by liquid-liquid interface self-assembly method,which solves the repetitive problem of SERS technology.Method:Firstly,four Au@Ag nanocuboids（Au@Ag NCs）with different thicknesses of silver were obtained by controlling the amount of gold nanorods（Au NRs）,and then the optimal substrate for detecting food contaminants was screened.The crystal violet was used as a Raman probe to investigate the sensitivity and repeatability of the substrate.The substrate was used to detect PAEs added to the wine by an external addition method.In order to improve the accuracy of the obtained SERS substrate signal,4-mercaptobenzoic acid（4-MBA）was modified as an internal standard molecule on the substrate by liquid-liquid interface self-assembly method,and the fluctuation of SERS signal due to various factors was calibrated.The crystal violet was also used as the Raman probe to investigate the sensitivity and repeatability of the Au@AgNCs arrays after adding the internal standard.Subsequently,the malachite green（MG）in the fish pond water and the thiabendazole（TBZ）on the pear skin were detected by using the base film modified with the internal standard,and the SERS signal was calibrated.Result:1.A two-step method for the synthesis of Au@Ag NCs with a thickness of 3,7,12,and 18 nm,and comparing the thickness of the four different silver shell of Au@Ag NCs to detect a CV of 10^-6 M.The SERS signal is strongest when the silver shell thickness is 12 nm.The BBP solution of 10^-5 M was also detected and consistent results were obtained.Therefore,Au@Ag NCs with the Agshell thickness of 12 nm is the optimal substrate in this experiment.The detection limit of the optimal substrate with crystal violet was 10^-8 M,and the relative standard deviation was 10.22%,indicating that the substrate has good sensitivity and repeatability,and satisfies the SERS detection of subsequent food contaminants.The detection limits of BBP and DEHP using the optimal substrate were all 10^-9 M,and the toxic dose of BBP（5.3 mg/kg）added to the two commercially available liquors was successfully detected.2.Au@Ag NCs solution was used as the water phase,cyclohexane was used as the oil phase,and ethanol containing the internal standard molecule 4-MBA was quickly added as the inducer.The optimal substrate is assembled into an internal standard modified arrays by a liquid-liquid interface self-assembly method.The characterization data shows that the nanoparticles are arranged neatly,evenly distributed and with small gaps,which can provide a large number of uniform hot spots for SERS detection.In the process of examining repeatability,we found that the relative standard deviation（RSD,8.50%）after the addition of the internal standard was much smaller than the RSD without the internal standard calibration(RSD_4-MBA=11.53%,RSD_CV=9.94%).The experimental results show that the accuracy and repetition rate of SERS quantitative detection can be improved by the internal standard method.The Au@Ag NCs arrays was then used to detect the MG in fish pond water and the TBZ on the pear skin.The detection limit of MG in fish pond water can reach 5×10^-9 M,and the correlation coefficient R² before and after added the internal standard are 0.92821 and 0.98457,respectively.In the experiment of detecting TBZ residues on pear skin,the detection limit was up to 10^-10 M,and R²-I_TBZ/I_4-MBA=0.96555>R²-I_TBZ=0.993913,which was consistent with the detection results of MG in fish ponds.It shows that Au@Ag NCs/4-MBA arrays has great potential in detecting pesticide residues.Conclusion:In this experiment,two single metals were combined into a single nanostructure.By controlling the amount of gold nanorods added,four unique core-shell bimetallic nanocuboid structures of different sizes were obtained,which changed the characteristics of single metal and made the application range.It is more extensive and screens the optimal SERS substrate for the detection of food contaminants within the experimental range.The internal standard modified Au@Ag NCs arrays was prepared by liquid-liquid interface self-assembly method.As a stable component,the internal standard（IS）can largely eliminate or reduce the influence of unstable factors on the experimental results and improve the reliability and accuracy of SERS detection.