The Enhancement Mechanism Of Surface-Enhanced Raman Scattering Applied In Food Security

The harmful residues and illegal additives are always exist in food and do harm to our healthy,the detection of it in food by Surface-enhanced Raman scattering(SERS)make the safety of food be possible.The quantitative study of the surface-enhanced Raman mechanism has played an important role in the further development and application of surface-enhanced Raman scattering.Therefore,the SERS mechanisms of Ethyl carbamate and 2-Acetylpyridine in details by ab initio calculation combined with the discrete dipole approximation(DDA)method are presented based on the experiment.What’s more,the SERS mechanisms are expressed by chemical mechanism and electromagnetic mechanism.Ethyl carbamate is a potentially toxic carcinogen contained in fermented food and alcoholic beverages,which is a by-product of fermentation and storage of food and beverages.Many countries have set maximal levels of it.Therefore,the low levels of EC and the complex matrix require sensitive and selective methods to detect it.Danting Yang et al had done experimental research on the SERS of Ethyl carbamate interacting with Ag particles.In our thesis,the normal and surface-enhanced Raman scattering of EC with the M06-2X function are studied and consistent with experimental results.Different contributions to raman enhancements of EC absorbed on Ag20 cluster are presented in details to explore the enhanced mechanism.The EC absorbed on Ag20 cluster forming a new EC-Ag20 complex involves in the static chemical enhancement(factor of 10).The charge-transfer enhancement(factor of 104)is found when 352nm a wavelength corresponding to the absorption maximum of the EC-Ag20 complex is taken as an incident light.The electromagnetic enhancement(3.6×106)due to the localised surface plasnon resonance(LSPR)of Ag nanosphere(radius 15nm)at the same excitation wavelength are acquired by the discrete dipole approximation(DDA)method.The combined effect of the chemical and electromagnetic enhancement can result in the total relative enhancements factor up to 3.6×1010.Therefore,though low levels and toxic of the EC in food and beverages is,the surface-enhanced Raman scattering(SERS)technique can be a sensitive method to detect it.The ministry of Health of the People’s Republic of China forbids manufacturers adding flavouring agent in rice.However,2-Acetylpyridine(2AP)is used as the main flavour enhancer in rice additives.It is sensitive and selective to detect 2AP by using surface-enhanced Raman scattering.In our thesis,the wB97XD functional with 6-311G(d,p)(C,H,O,N)set is taken to calculate the normal raman spectrum,and the results are in good agreement with the experiment’s.Moreover,a systematic study of different contributions to SERS enhancements of the 2AP is presented by using ab initio calculation combined with the discrete dipole approximation method.A 2AP-Ag20 system without excitation wave involves in static chemical enhancement(factor of 10).Simulating SERRS spectrum with a wavelength of 350nm with respect to the absorption maximum,the charge-transfer enhancements is found and the enhancement factor is 104.Meanwhile,the same incident wavelength resonant with Ag sphere(R=15nm),where the electromagnetic enhancements of the order of 2.33 × 106 are predicted.Above all,the total relative enhancement factor can be 2.33 × 1010 while regarding it as a product of both chemical and electromagnetic effect which express as REtotal = REchem · REem(Where RE means relative enhancements).Such enhancements are in agreement with predictions of SERS enhancements.What’s more,the noraml raman spectra of isomers of Acetylpyridine are simulated by the same level and what we find is that the raman characteristic speaks of 2AP being different from 3,4-Acetylpyridine(3,4AP).The characteristic speaks are more obvious when the Surface-enhanced spectra are simulated,which make a easier distinction between 2AP and 3,4AP.Last but not least,surface-enhanced Raman scattering(SERS)technique can be a selective method to distinct substance.