Tleugabulova Dina, Sui Jie, Ayers Paul W, Brennan John D
Department of Chemistry, McMaster University, Hamilton, ON, L8S 4M1, Canada.
J Phys Chem B. 2005 Apr 28;109(16):7850-8. doi: 10.1021/jp045569r.
Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.
阳离子探针罗丹明6G（R6G）与水中二氧化硅颗粒的阴离子表面之间的强离子结合提供了一种便捷的标记方法，可通过时间分辨荧光各向异性（TRFA）来研究颗粒生长动力学和表面改性。分散在稀释的Ludox硅溶胶中的R6G的衰减通常符合皮秒和纳秒衰减成分的总和，以及一个显著的残余各向异性成分。纳秒衰减成分（phi2）的起源尚未完全理解，有人将其归因于探针在二氧化硅表面的摆动、Ludox溶胶中存在的小纳米颗粒亚群，或游离R6G与结合R6G之间的快速交换。为了阐明phi2的物理意义，我们在各种基于二氧化硅的胶体系统中使用不同浓度的二氧化硅进行了测量。我们发现，phi2的比例在Ludox中通常高于硅酸钠水溶液，并且随着二氧化硅浓度的增加而降低；当硅酸钠在pH 7下凝胶化时，phi2消失，导致R6G去极化完全丧失（r(t) = 常数）。这些结果排除了离子结合到胶体二氧化硅上时局部R6G摆动的存在，并支持用刚性球体模型来描述R6G-Ludox的TRFA衰减。这一结论完全得到了稳态各向异性数据以及R6G分子和二氧化硅表面结构考虑因素的支持。
