Study On The Detection Of Catechins In Tea Soup Based On SERS Spectra

The catechin is the main functional component in tea,which is closely related to the quality of tea.The development of a simple,highly sensitive and low-cost detection methods is of great significance for tea research,tea processing and quality analysis.Surface enhanced Raman spectroscopy can identify the structural information of various chemical molecules and enables real-time in-situ detection with high sensitivity,simple data processing.In this paper,the preparation methods and SERS effects of flower-shaped silver and gold nanoparticles have been studied,and the infrared and Raman spectra of catechins are calculated theoretically based on the first principle.In addition,the two SERS substrates synthesized above were used for SERS detection of epicatechin and a quantitative analysis model was proposed.The main results are as follows:1.The controllable preparation of flower-shaped silver nanoparticles have been studied by using silver nitrate as precursor,ascorbic acid as reducing agent and polyethylene pyrrolidone as surfactant.The flower-shaped silver nanoparticles with different morphology were prepared by changing the concentration of silver nitrate.After the comprehensive analysis of SEM characterization and SERS characterization of R6G probe molecules,the preparation method of flower-shaped silver nanoparticles with good morphology and the best SERS enhancement effect was determined,that is,the amount of silver nitrate added to 0.3 mL,PVP added to 0.5 mL,and the amount of ascorbic acid added was 1 mL.The size of flower-shaped silver nanoparticles is around 700 nm,and the uniformity is high.Under this experimental scheme,the SERS performance of flower-shaped silver nanoparticles was characterized,and it is found that the minimum detection limit of R6G probe molecules can reach 10^-8 mol/L.2.The controllable preparation of gold nanoparticles was studied via chemical reduction method by using chloroacetic acid as precursor and sodium citrate as reducing agent.The gold nanoparticles of different particle sizes were prepared by changing the number of additions of chloroacetic acid and sodium citrate.The preparation method of gold nanoparticles with good morphology and the best SERS enhancement effect was determined by SEM characterization and SERS characterization of R6G probe molecules,that is,the 9th generation nanoparticles.The prepared gold nanoparticles have a particle size of about 50 nm,which is spherical and uniform.And it is found that the minimum detection limit of R6G probe molecules can reach 10^-9 mol/L.3.The first principle was used to study the structure of 13 main components in tea.After determining the lowest energy point of the studied molecule,the FTIR and Raman spectra were obtained according to the DFT calculation at the theoretical level of B3LYP/6-311+G（d,p）and compared with the existing experimental and theoretical researches.The normal Raman spectra of these tea components were then measured.There is a good correlation between the experimental and the theoretical data,which will provide effective theoretical support for the SERS trace detection of the main components of tea.4.The SERS detection of epicatechin solution was carried out by using the optimized flower-shaped silver nanoparticles and gold nanoparticles as substrates.The normal Raman and SERS spectra of epicatechin were compared to analyze the effect of flower-shaped silver nanoparticles and gold nanoparticles on the properties of epicatechin Raman scattering.The results showed that the flower-shaped silver nanoparticles had almost no SERS enhancement effect on epicatechin,which is because the chemisorption of epicatechin molecules was weak,and the contribution of electromagnetic field enhancement in the system was small.SERS detection of different concentration of epicatechin by gold nanoparticles showed that when the concentration of epicatechin solution was as low as 10^-7 mol/L,the SERS signals at 717 cm^-1,1384 cm^-1,1440 cm^-1,1540 cm^-1 are still identifiable,indicating that the SERS detection technique based on gold nanoparticles is extremely sensitive to epicatechin.And the detection limit can reach10^-7 mol/L;Based on the correspondence between the concentration of catechin solution and its SERS spectral intensity,a quantitative analysis model is established by principal component analysis.The fitting function is y=-0.81908x+5.36909,y means–lg C（C is the concentration of epicatechins,in mol/L）,x represents lg Z（Z is the main component analysis total score）,and the fitting coefficient R² is as high as 0.97492.In summary,the prepared gold nanoparticle SERS substrate has a good application prospect in the detection of catechin in aqueous solution.The research results of this subject provide an experimental basis and theoretical basis for the application of SERS technology in the qualitative and quantitative analysis of catechins in tea soup.