为探讨胞嘧啶(Cytosine, Cy)在基底银表面的吸附特性和规律, 采用表面增强拉曼散射(SERS)光谱对其吸附行为进行分析, 并结合量子化学密度泛函理论(DFT)/B3LYP计算方法对Cy分子的常规拉曼光谱(NRS)及Cy与Ag团簇吸附的SERS光谱进行计算, 与测定结果进行比对且对其拉曼峰进行系统指认及归属, 理论计算结合实测值探讨了Cy在基底Ag上的增强效应和吸附行为。考察了Cy分子在Ag纳米粒子上的不同吸附时间、浓度、pH等条件对SERS光谱的影响及优化, 发现pH影响最大, 在中性和强碱性条件下的增强效应明显优于酸性。Cy分子存在2种不同的异构体和3种不同的存在形态, 并随酸度变化相互转化而达动态平衡。基于Cy在不同pH时的形态分布和相应的SERS变化规律, 结合DFT算得的Cy分子中的电荷分布及在银基底表面的吸附机制, 详细探讨了酸碱对Cy分子的SRES光谱影响的内因和吸附机理, 指出在中性和弱碱性时, 是Cy中的N3和O与Ag形成配位吸附; 在pH大于11时, N与Ag形成配位吸附, 而O与Ag形成共价吸附。

The adsorption characteristics and regularity of cytosine (Cy) on the silver substrate surface were analyzed by the means of surface enhanced Raman scattering (SERS) combing with quantum chemistry density functional theory (DFT). The normal Raman spectrum (NRS) of Cy and Cy—Agn complex (n=1, 3, 5) SERS spectra were calculated by DFT/B3LYP method, which were further compared with the measured results to identify and assign the spectral peaks. The SERS spectra of Cy molecules on silver colloid nanoparticles were recorded at different adsorption time, concentration and pH. The results show that pH value has the greatest influence on its SERS signal, especially at neutral and strong alkaline conditions. It is inferred that there are two different isomers of Cy molecules and three kinds of existing forms, and all of them keep dynamic balance in the solution. The adsorption mechanism of Cy on the surface of silver substrate was further discussed in detail based on the morphological distribution of Cy at different pH by their corresponding SERS changes combining with the DFT calculation of charge distribution in Cy molecules. In conclusion, the N3 and O in Cy are coordinated to Ag at the neutral and weakly alkaline conditions. N and Ag form coordinated adsorption and O and Ag form covalent adsorption, when the pH is greater than 11.