咖啡酸分子表面增强拉曼光谱的理论与实验研究

咖啡酸（CA）是一种具有很高的医学价值的药物成分，在抗菌抗病毒方面应用广泛，尤其是咖啡酸及其衍生物在抗肿瘤方面有着巨大作用，现在对咖啡酸的相关研究越来越多，但大部分都是关于咖啡酸医学性质的研究，所以对咖啡酸分子的微观结构研究是非常有必要的。目前关于CA在Ag表面上的表面增强拉曼散射（SERS）光谱的理论与实验结合的研究尚未见报道，而对其振动光谱及表面增强机理的研究可以为咖啡酸的各种药学机理的研究提供一种科学的物理解释，所以有必要将密度泛函理论（DFT）方法与表面增强拉曼散射技术相结合，对咖啡酸在Ag纳米颗粒上的吸附性质及表面增强机理进行全面的研究，这对推进它们在医药学等领域的相关研究有着重要的参考价值。采用SERS与DFT技术对CA分子在Ag纳米颗粒表面上的表面增强拉曼光谱进行了研究。在实验方面，利用热还原反应原理，使用柠檬酸三钠和硝酸银在加热搅拌情况下制备Ag纳米颗粒，并使用激光共聚焦显微拉曼光谱仪测量了CA分子的常规拉曼散射（NRS）光谱及其表面增强拉曼散射（SERS）光谱。在理论计算方面，采用DFT的B3LYP方法，以6-31+G**和LANL2DZ分别作为C，H，O和Ag的计算基组来优化咖啡酸的分子构型，羟基与Ag₄的吸附构型，羧基与Ag₄的吸附构型，羟基与羧基共同与Ag₄吸附的构型，并以此为基础分析计算了CA分子的NRS光谱以及三种可能吸附模型的SERS光谱，并结合实验结果进行比较。同时对CA分子的振动模式进行了详细指认。根据实验数据和理论结果分析，在452 cm-1处的谱峰归属为环面外弯曲振动和O-H面外弯曲振动的耦合，这说明CA分子上的酚羟基是与Ag纳米颗粒表面作用的，不过相互作用较弱，推测CA分子平面可能与Ag基底表面不垂直；出现在1 338 cm-1处的谱峰归属于COO-伸缩振动，则可以说明CA分子上的羧基可能与Ag纳米颗粒垂直吸附。结果表明，CA分子是以羧基和酚羟基为吸附位吸附在Ag纳米颗粒表面上的。同时对CA分子的振动模式进行了详细指认。该工作对推进咖啡酸在生物医药等领域进一步的应用将起到重要作用。

Theoretical and Experimental Study of Surface Enhanced Raman Spectroscopy of Caffeic Acid Molecules

Caffeic acid (CA) is a medicinal component with high medical value. It is widely applied in antibacterial and antiviral applications. In particular, caffeic acid and its derivatives have a enormous function in antitumor. Nowadays, there are many researches about caffeic acid. However, most of them are about the medicinal properties of caffeic acid, as a result the investigation of the microstructure of caffeic acid molecules is necessary. So far, there are no theoretical and experimental studies about the surface-enhanced Raman scattering spectroscopy (SERS) of CA on Ag surface. It is worth noting that the research on the vibrational spectrum and surface enhancement mechanism of caffeic acid can be a variety of pharmaceutical mechanisms of caffeic acid. Therefore, a combine surface-enhanced Raman scattering (SERS) and density functional theory (DFT) techniques are applied to conduct a comprehensive study of the adsorption properties and surface enhancement mechanism of caffeic acid on Ag nanoparticles, which can provide a scientific explanation to the medicinal properties of caffeic acid. This has important reference for advancing their related research in medicine and other fields. In this paper, surface-enhanced Raman spectroscopy of CA molecules on the surface of Ag nanoparticles was studied using combined SERS and DFT techniques. Ag nanoparticles were prepared using trisodium citrate and silver nitrate under heating and stirring using the principle of thermal reduction reaction and conventional Raman scattering (NRS) spectra and SERS spectra of CA molecules were measured usingthe laser confocal micro-Raman spectrometer. In terms of theoretical calculations, we applied B3LYP method to optimize the molecular configuration of caffeic acid, the adsorption configuration of Ag₄, the adsorption configuration of carboxyl group and Ag₄, and the configuration of adsorption of Ag₄ by both hydroxyl and carboxyl groups, using 6-31+G** and LANL2DZ as the basis set for C, H, O, and Ag, respectively. Then, the NRS spectra of CA molecules and the SERS spectra of three possible adsorption models were calculated and compared with experimental results. At the same time, the vibration mode of CA molecules was confirmed. According to the experimental data and theoretical results, the peak at 452 cm-1 was attributed to the coupling of the torsional bending vibration and the -OH out-of-plane bending vibration, which indicated that the phenolic hydroxyl group on the CA molecule have a weak interaction with the Ag nanoparticle. We speculated that the CA molecular plane may not be perpendicular to the surface of the Ag substrate. The peak appearing at 1 338 cm-1 was attributed to COO- stretching vibration, which indicated that the carboxyl group on the CA molecular is vertically adsorbed with the Ag nanoparticle. The results showed that CA molecules adsorbed on the surface of Ag nanoparticles with carboxyl groups and phenolic hydroxyl groups as adsorption sites. At the same time, we have identified the vibrational modes of CA molecules in detail. This work has an important effect on the further applications of caffeic acid in biomedicine and other fields.