Enhancement Mechanism Study On Sers Of Aflatoxins-Silver Clusters System

Aflatoxins (AFs) are a group toxins of most powerful toxicity and carcinogenicity, as the most common mycotoxins are detected in human food and animal feed. Aflatoxins can cause gene mutation and leat to malignant tumors or hepatocellular carcinoma, thus we need detecting of Aflatoxins content. But the Raman signal of Aflatoxins molecules is very weak and difficult to detect by conventional Raman spectroscopy. In this thesis, the Surface-enhanced Raman scattering (SERS) and pre-resonance Raman spectra of Aflatoxin Bi, B2, G1and G2molecules adsorbed on sliver clusters were calculated using density functional theory (DFT) method with B3LYP/6-311G(d, p)(C, H, O)/LANL2DZ(Ag) basis set. The SERS enhancement factors and chemical and electric field enhancement mechanism were obtained. We investigated the structural properties and Raman spectra of AFB1-Agn(n=1,2,4,6,7) complexes under the external electric field.The two type of Aflatoxins adsorbed on sliver clusters through different adsorption site, resulting in Surface-enhanced Raman scattering effect. The Aflatoxins molecule prefer to a perpendicular orientation adsorbed on Ag clusters by C=O site, which is consistent with the experimental phenomena. Compared with the Raman spectra of AFs-Ag, AFs-Ag2, AFs-Ag4and AFs-Ag6, AFs-Ag7complexes and experimental data, it was obtained that the effect of adsorption substrate to Surface-enhanced Raman scattering of Aflatoxins molecule. The calculated results showed that the SERS spectra were strongly dependent on Ag clusters site and the configuration of new complexes. When the four Aflatoxins molecule adsorbed on diamond silver cluster (Ag4clusters) and Ag7clusters, the enhancement factors were strongest, and up to104, attributed to C=O stretching vibration mode. The enhancement mechanism was ascribed to the ground state static chemical enhancement from the static polarizability changes.The absorption spectra and electronic transitional energy of the two type Aflatoxins molecule were carried out based on Time-dependent DFT (TD-DFT) method. We found the pre-resonance Raman spectra were strongly dependent on the charge tranfer resonant state of new complexes. Wavelengths were nearly resonant with the charge transfer excitation states, which were adopted as incident light when simulating the pre-resonance Raman spectra for AFBi-Agn(n=2,4,6) and AFGi-Agn(n=2,4,6,7)complexes, respectively. The enhancement factors were obtained about102-104compared with the normal Raman spectra. The Raman intensities of charge transfer resonance of the AFB2-Ag2complex were significantly greater than the charge transfer excitation. The pre-resonance Raman spectra of AFB2-Ag2complex are explored at1144.1nm and544nm, which were charge transfer excitation energy, the enhancement factors were102. The charge transfer pre-resonant wavelength of432.5nm and410nm as the incident light, the pre-resonance enhancement factor of AFEB2-Ag2complex was up to104, mainly caused by the charge transfer excitation resonance. The charge transfer resonant energy of Aflatoxins-Ag clusters system were in430nm(AFB1molecule),540nm (AFB2) and400nm (AFG1molecule), this resules will provides corresponding reference to the experiment.With the increase of the external electric field, the probability of the electons from low enregy state to high energy level increases in AFB1-Agn(n=1,2,4,6,7) complexes, the stability of AFB1-Agn(n=l,2,4,6,7) complexes were decreased; the energy gap of the complexes decrease gradually with external electric increase, the electron that occupied orbits were induced to empty orbits easily, the probability of the electron transition spectral frequency decreases. With the effect of the different external field, electronic structure and Raman spectrum of the AFB1-Agn(n=1,2,4,6,7) complexes showed that structural parameter of the AFB1-Agn(n=1,2,4,6,7) complexes were changed. Therefore, Raman spectra of the complexes has changed. With the increase of the external electric field, the Raman spectra were occured blue shift significantly, the maximum blue shift was up to100cm-1, and the single vibration mode was difficult to be distinguished, the coupling of vibration modes were happen. The maximum vibrational mode of AFB1-Agn(n=l,2,4,6,7) complexes was along with the electric field direction, and it was the largest direction of the polarization change.The local electric field distribution of silver nanoparticles, which were different structures (spherical, cylindrical, prismatic columnar, three pyramid and cage-like) and different size, were simulated using three-dimensional finite difference time-domain (3D-FDTD). When UV or deep UV region wavelengths as the incident light which were corresponding to charge transfer resonance of AFB1-Agn(n=1,2,4,6,7) complexes, the local surface electric field of the silver nanoparticles were significantly stronger, the maximum enhancement factor was up to109, it is different with the conclusion of the reported in the literature that can not be obtained SERS enhancement in the ultraviolet region. The electric field enhancement was consistent with change trend of the pre-resonance Ranam spectra of corresponding complexes, which were only to considering the chemical enhancement effect. The local electric field distribution of Cage-like structural silver nanoparticles was showed that the electric field distribution was not significantly enhanced under different incident light of the ultraviolet or infrared. This is due to the the Caged silver nanoparticle structure having a plurality of vertices that produce higher-order multipole moments scattering, and internal electronic collective agitation behavior is difficult to numerical analysis.