Theoretical Study Of Surface-enhanced Raman Spectroscopy Of Pesticide Molecule

Pesticide residues are harmful to people’s health.So it is crucial to conduct trace detection of pesticide residues on the surface of vegetables and fruit.However,traditional methods have complex process,long cycles and expensive price,while Surface-Enhanced Raman spectroscopy（SERS）technology has important application prospects for the trace detection of pesticide residues due to its advantages of simplicity,speed and sensitivity.At present,SERS detection is in its early stages and only relevant experimental research has been conducted in the laboratory,which lacks certain theoretical guidance.Therefore,the theoretical research on SERS detection of pesticide residues is very important.Based on the above analysis,this paper uses the quantum chemistry method to carry out the SERS spectral simulation calculation of several kinds of pesticide residues,and the progress is as follows:Firstly,we proposed a stable configuration of thiabendazole-gold clusters,calculated the SERS spectra of thiabendazole,and assigned the peak positions of the spectra.Using the quantum chemistry calculation method of density functional theory,the adsorption behavior of thiabendazole and gold clusters and SERS enhancement effect were systematically studied from a theoretical perspective,and the optimal adsorption position of gold clusters on thiabendazole molecules was obtained.A theoretical study was conducted on the interaction between thiabendazole and gold clusters using five adsorption models,namely thiabendazole Au₄（n=1～5）,and the results showed that thiabendazole-Au₄is the most stable.Using the visualization software Gaussview for observation,the Raman peaks of thiabendazole molecules were assigned,and the differences between Raman spectra and SERS spectra were compared.The conclusion was drawn that different sizes of gold clusters have different effects on Raman enhancement,and the Raman peaks corresponding to the vibration modes close to the gold clusters have better enhancement effects,and the gold clusters also have a certain impact on Raman shift.And the static polarizability of thiabendazole and its complex was obtained by simulation calculation,and the charge transfer between molecules and clusters was studied.The static polarizability of Au clusters of different sizes varies greatly.The larger the gold cluster,the greater the static polarizability and the stronger the Raman signal.Secondly,the optimal adsorption position of thiram on the surface of gold clusters was obtained,and the electrostatic potential and SERS spectra of thiram molecules were calculated.The density functional theory is applied to calculate the energy of thiram,and the electrostatic potential diagram of the molecule was obtained,which can judge the binding site of thiram and gold cluster when they are adsorbed.The energy of the thiram-gold cluster was calculated,and then the binding energy of each model was obtained.Through comparison,the thiram-Au₃model is the most stable.Five models of thiram-Au_n（n=1～5）were simulated to analyze the SERS enhanced effect of thiram-gold clusters,and the Raman peaks of thiram molecules were assigned.The conclusion was drawn that different models have different enhancement effects,with the larger the size of the gold cluster,the better the enhancement effect,and the gold cluster also has a certain impact on Raman shift.Finally,the theoretical study of p-aminophenol was carried out as an example,and the effect of cluster size on the SERS spectrum of p-aminophenol was systematically studied.The structure of p-aminophenol was optimized to obtain a stable configuration.The electrostatic potential diagram was obtained by energy calculation,and the adsorption site of molecules and gold clusters were analyzed.Then,a model was established to calculate the frequency of p-aminophenol-Au_n,and the SERS spectrum of the molecules was obtained.After analyzing the calculation results,it is concluded that:The larger the size of the gold cluster,the better the enhancement effect,and the Raman peak corresponding to the vibration mode close to the gold cluster has a better enhancement effect,and the gold cluster also has a certain influence on the Raman displacement,there is a certain redshift phenomenon,and with the increase of the gold cluster,the redshift phenomenon becomes more obvious and finally becomes stable.