【作者】 高思敏；

【导师】 洪时明； 王红艳；

【作者基本信息】 西南交通大学 ， 一般力学与力学基础， 2013， 博士

【摘要】 黄曲霉素(AFs)是一种强致癌、致畸的物质,普遍存在于人类食品和动物饲料中,可导致基因变异发生癌变,因此需要对其进行含量限度检测,但黄曲霉素拉曼信号非常弱,采用常规拉曼光谱(NRS)很难检测。因此本论文采用密度泛函理论(DFT)B3LYP方法,6-311G(d, p)(C, H, O)/LANL2DZ(Ag)基组,计算了黄曲霉素B1、B2、G1以及G2分子吸附在不同结构银团簇的表面增强拉曼散射光谱和预共振拉曼光谱,获得了表面增强拉曼散射的增强因子,分析了表面增强拉曼散射的化学增强和电场增强的机理,并且研究了外加电场作用对黄曲霉素B1分子表面增强拉曼散射的影响。两类黄曲霉素分子可以通过不同位点吸附于银团簇,产生表面增强拉曼散射效应。B和G类黄曲霉素分子通过C=O位垂直吸附于银团簇为最有利构型,与实验检测结果-致。计算得到两类黄曲霉素分子(AFs)的常规拉曼光谱和表面增强拉曼光谱与实验结果符合很好。通过比较复合物AFs-Ag、AFs-Ag2、AFs-Ag4以及AFs-Ag6、AFs-Ag7的表面增强拉曼光谱(SERS)和相关实验数据,获得了吸附基底对黄曲霉素表面增强拉曼散射的影响：四种黄曲霉素分子在菱形(Ag4)和五角双锥型十面体的银团簇(Ag7)表面吸附时,增强因子最大达到了104,对应pyrane环C=O伸缩振动,主要是由黄曲霉素分子周围化学环境改变而引起的基态静极化率改变导致的,增强机理为基态化学增强。通过含时密度泛函(TD-DFT)方法,计算了两类黄曲霉素-银团簇复合物的吸收光谱和跃迁能。两类黄曲霉素-银团簇体系的电荷转移共振激发分别发生在430nm (AFB1分子),420nm (AFB2分子)以及400nm左右(AFG1分子)。选择电荷转移预共振激发波长作为入射光,计算得到复合物AFB1-Agn(n=2,4,6)以及AFG1-Agn(n=2,4,6,7)的预共振拉曼光谱的增强因子约为102～104。复合物AFB2-Ag2中电荷转移共振拉曼信号增强因子明显大于电荷转移激发：电荷转移态激发波长1144.1nm和544nm可以使拉曼信号增强102,而选择电荷转移预共振波长432.5nm和410nm作为入射光,其拉曼信号增强了104。表面增强拉曼散射的增强机理为银团簇和黄曲霉素分子之间的电荷转移共振增强。随着外加电场的增加,复合物AFB1-Agn(n=1,2,4,6,7)中低能态电子向高能级跃迁的几率增大,体系稳定性降低；能隙逐渐减小,占据轨道的电子容易被激发到空轨道,可能的电子跃迁光谱频率减小。不同的外电场(0.0005a.u,0.005a.u,0.01a.u)作用下复合物的电子结构和拉曼光谱显示：外加电场使复合物AFB1-Agn(n=1,2,4,6,7)的结构参数发生明显改变,从而引起复合物拉曼光谱的变化：共振峰随着外电场的增大出现明显的蓝移,最大蓝移达到100cm-1,且单个振动模式很难被区分,同一频率出现几种振动模式的耦合。复合物AFB1-Agn(n=1,2,4,6,7)中增强因子最大的振动模,其振动方向均沿着外加电场方向,此方向也为极化率改变最大的方向。最后,采用三维时域差分法(3D-FDTD)模拟了不同形状(球状、柱状、棱形柱状、三棱锥以及笼状),不同结构和尺寸的银纳米粒子表面局域增强电场分布。同一种银纳米粒子采用位于紫外或深紫外区域且与AFs-Agn复合物相应的电荷转移预共振的入射光激发时,局域表面电场明显更强,最大增强因子达到了109,不同于文献报道中紫外区域无法获得SERS增强的结论。电场增强与仅考虑化学增强效应得到的对应复合物预共振拉曼光谱强度的变化趋势一致,这为研究单分子表面增强拉曼散射的实验提供了参考。笼状结构银纳米粒子表面的电场分布表明,笼状银纳米粒子表面局域电场分布在紫外或红外入射光激发下没有明显的增强,这是由于笼状银纳米粒子的结构具有多个顶点,产生高阶多极矩散射效应导致的,内部电子的集体激荡行为难以简单地数值分析也是产生此现象的原因。更多还原

【Abstract】 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.更多还原