巯基苯并咪唑在银电极表面的自组装吸附特性及表面增强拉曼光谱

采用循环伏安法处理Ag电极,得到活化的具有表面增强拉曼光谱(SERS)效应的粗糙Ag表面,进一步采用激光拉曼光谱探讨了2-巯基苯并咪唑(MBI)在其表面的自组装分子层的吸附特性。实验表明,在活性Ag表面的MBI自组装分子层能够产生理想的SERS效应,其强度随探针分子MBI浓度的增加先提高后减弱,达到一定浓度时因受其空间位阻等因素的影响,增强效应减弱。MBI在1×10^-6 mol·L^-1 浓度时增强效果最大。拉曼增强效应随着体系酸度的变化有着明显不同,在强酸性条件下的增强效应明显优于中性和碱性条件。MBI分子存在两种不同的异构体和在不同酸度下存在3种不同的存在形态,并形成动态平衡。pH <2时,MBI分子主要以硫酮式MBI⁺存在,并以巯基上的S:与活性Ag以配位方式吸附成键,其整个大π键平面垂直地吸附于Ag表面,产生相对较大的SERS信号。pH >2时,由于硫酮式和硫醇式与活性Ag的键合方式和能力不同,硫醇式上的S与Ag以S-Ag共价方式同时双键侧上的N以配位方式协同参与吸附成键,比硫酮式MBI⁺的单纯配位吸附要强,因而形成了竞争吸附,表现为SERS在pH=2~3.7之间的急剧下降。MBI硫醇式由于以S-Ag、N-Ag键的协同吸附,形成了倾斜侧卧式垂直吸附,而使拉曼增强效应相对减弱。

Self-assembly Behavior of 2-Mercaptobenzimidazole on Silver Electrode Investigated by Surface Enhancement Raman Scattering

The activated Ag electrode which was treated by electrochemical cyclic voltammetry to obtain surface enhancement Raman scattering (SERS) effect was used to investigate the self-assembled monolayers (SAMs) behavior of 2-mercaptobenzimidazole (MBI). The results show that SAMs of MBI are of ideal SERS effect on roughened Ag electrode and its SERS effect is related to concentration and pH. With the increasing of concentration, the SERS effect is strengthened firstly but weakened latterly because of its steric hindrance when it happens to the high concentration. As a result, the most effective enhancement is at 1×10-6 mol·L-1. On the other hand, the SERS effect of MBI in acid solution is better than in alkaline or neutral solution. It is further illustrated that MBI molecule has two different isomers and three kinds of existing forms, and all of them keep dynamic balance in solution. When the pH is lower than 2, MBI+, the main form of MBI molecule, is absorbed on roughened Ag electrode with coordinate bond between S: and Ag. MBI molecules stand vertically on Ag surface through π bond and produce relatively higher SERS effect. When the pH is higher than 2, thiol absorbed on Ag surface with S—Ag covalent bond and N—Ag coordinate bond is stronger than thione. This results in competitive absorption between thiol and thione of MBI, so SERS effect rapidly decreases from pH=2 to pH=3.7. As a result, thiol form of MBI lays aslant to Ag surface with S—Ag covalent bond and N—Ag coordinate bond, making the weaker SERS effect than thione.