Study On Rapid Detection Of Caffeine And Theanine In Tea Based On SERS Technology

Recently, many modern technologies have been applied to the detection of quality component in tea. However, these technologies are generally expensive, complex and time consuming. Herein, a simple, rapid, sensitive, and low cost analytical technology for detection of the quality component in tea is very essential. Surface Enhanced Raman Spectroscopy(SERS) technology is widely used in many chemical and biological detection fields due to its excellent performance, such as high sensitivity, great flexibility, reproducibility, and reliability. SERS signal depends largely on the shape, size of substrate and adsorption characteristics between the probe molecules and the substrate. In this paper, four SERS-active nanostructures with high SERS activity and large uniformity have been successfully synthesized, and the SERS enhancement mechanism of those nanostructures in SERS spectroscopy have been further discussed. In addition, the prepared noble metal nanomaterials can be served as an effective platform for SERS detection of caffeine and L-Theanine.The main results are as follows:1. Gold nanoparticles have been successfully synthesized by using chlor auric acid as predecessor and trisodium citrate as a reductant. It is found that the concentration of the chlor auric acid solution have great influence on the size, agglomeration and distribution of gold nanoparticles, and the SERS signal depends largely on the size and agglomeration of gold nanoparticles.2. Flower-like silver nanoparticles, with diameters of 450~500 nm and protrusions on the surface up to 10~15 nm, have been successfully synthesized via liquid phase reduction method, by using ascorbic acid as a reductant and polyvinyl pyrrolidone(PVP) as a surfactant. It is found that the proper coating degree of PVP on silver nuclei leads to anisotropic growth of the silver nuclei resulting to the formation of flower-like structure with more protrusions. Furthermore, when the obtained flower-like silver nanostructure is used as SERS substrates, SERS signal is still clearly even if the concentration of R6 G solution is reduced down to 10-8 mol/L.3. A kind of graphene oxide/silver(GO/Ag) nanocomposites with high SERS activity were fabricated via a facile and green liquid phase reduction method. The synthesized materials were characterized in detail using various microscopic and spectroscopic techniques. In this method, the GO sheets worked as a holder which makes silver nanoparticles(Ag NPs) aggregate to a particular morphology, and under a suitable dosage of silver ions, well-dispersed Ag NPs on the surface of GO were obtained, which could generate more “hot spots” of SERS.4. The Raman shift, relative intensity, and assignment of characteristic peaks at the normal Raman spectra and SERS spectra of caffeine were summarized, and the effect of silver nanoparticles on the Raman scattering of caffeine molecules were discussed. The prepared flower-like silver nanoparticles can be served as an effective platform for SERS detection of caffeine. The detection limit was estimated to be as low as 10-9 mol/L, and a multivariate linear regression model for the concentration of caffeine was established,which is Y =10041.0-0.9X807-3.1X1327+1.4X1269+1.3X958. In this model, Y represents the –log10C(C is the concentration of caffeine, and its unit is mmol/L), and Xn represents the SERS intensities at n cm-1. The coefficient of determination(R2) is as high as 0.9997.5. The Raman shifts and relative intensities of the characteristic peaks at the normal Raman spectra and SERS spectra of L-Theanine were summarized, and the possible vibrational assignments of these peaks were identified. The prepared GO/Ag nanocomposites can be served as an effective platform for SERS detection of L-Theanine. The detection limit is estimated to be around 10-7 mol/L, and the optimal fitting equation is Y=5676.5+30.7X1458-26.7X1251. The coefficient of determination(R2) is as high as 0.9899.In summary, the prepared flower-like silver nanoparticles and GO/Ag nanocomposites have high SERS activity, which can serve as an effective SERS detection platform for the quality component in tea. This work provides an experimental and theoretical basis for the SERS application in qualitative and quantitative detection of caffeine and L-Theanine.