Zhou Yongyao, Yip Wai Tak
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
J Phys Chem B. 2009 Apr 30;113(17):5720-7. doi: 10.1021/jp8036473.
We examined the behavior of various entrapped guest molecules within silica hydrogel and evaluated the effect of Coulombic interactions and physical confinement on molecular mobility. Although rhodamine 6G (R6G) and fluorescein (FL) share similar size and molecular structure, their behavior in silica hydrogel was found to be dramatically different. A good majority of R6G was immobilized with little to no exchangeable molecules, whereas FL displayed a considerable amount of mobility in silica hydrogel. Moreover, silica hydrogel encapsulated R6G failed to gain mobility even under low pH or high ionic strength conditions to minimize Coulombic interactions, implying that encapsulated R6G molecules were inaccessible and likely trapped deep inside the silica matrix of a hydrogel. On the contrary, FL was relatively free to rotate and translate inside a silica hydrogel, implying that FL remained solvated in the solvent phase and was able to maintain its mobility throughout the hydrogel formation process. Fluorescence recovery after photobleaching measurements put the diffusion coefficient of FL in silica hydrogel at ca. 2.1 x 10(-6) cm(2) s(-1), about a factor of 3 slower than that in solution. The substantial difference in mobility between cationic R6G and anionic FL led us to conclude that the effect of Coulombic interactions on mobility is more dominating in hydrogel than in alcogel. Our results also suggest that Coulombic interactions are strong enough to influence the eventual placement of a guest molecule in a silica hydrogel, causing R6G and FL to reside in different microenvironments. This has a profound implication on the use of molecular probes to study silica hydrogel since a slight difference in physical attribute may result in very diverse observations even from identically prepared silica hydrogel samples. As demonstrated, the repulsion between FL and silica renders FL liquid-bound, making FL more suitable for monitoring the change in viscosity and physical confinement during hydrogel formation, whereas other researchers have shown that silica-bound R6G is more suitably used as a reliable probe for monitoring the growth of silica colloids because of its strong attraction toward silica.
我们研究了二氧化硅水凝胶中各种被包封客体分子的行为，并评估了库仑相互作用和物理限制对分子迁移率的影响。尽管罗丹明6G（R6G）和荧光素（FL）具有相似的尺寸和分子结构，但发现它们在二氧化硅水凝胶中的行为存在显著差异。绝大多数R6G被固定，几乎没有可交换分子，而FL在二氧化硅水凝胶中表现出相当大的迁移率。此外，即使在低pH或高离子强度条件下以最小化库仑相互作用，二氧化硅水凝胶包封的R6G也未能获得迁移率，这意味着包封的R6G分子无法进入且可能被困在水凝胶的二氧化硅基质深处。相反，FL在二氧化硅水凝胶中相对自由地旋转和平移，这意味着FL在溶剂相中保持溶剂化状态，并能够在整个水凝胶形成过程中保持其迁移率。光漂白后荧光恢复测量得出FL在二氧化硅水凝胶中的扩散系数约为2.1×10⁻⁶ cm² s⁻¹，比在溶液中慢约3倍。阳离子R6G和阴离子FL迁移率的显著差异使我们得出结论，库仑相互作用对迁移率的影响在水凝胶中比在醇凝胶中更占主导地位。我们的结果还表明，库仑相互作用足够强大，足以影响客体分子在二氧化硅水凝胶中的最终位置，导致R6G和FL处于不同的微环境中。这对于使用分子探针研究二氧化硅水凝胶具有深远意义，因为即使是相同制备的二氧化硅水凝胶样品，物理属性上的微小差异也可能导致非常不同的观察结果。如所示，FL与二氧化硅之间的排斥使FL与液体结合，使FL更适合监测水凝胶形成过程中粘度和物理限制的变化，而其他研究人员表明，与二氧化硅结合的R6G由于对二氧化硅有强烈吸引力，更适合用作监测二氧化硅胶体生长的可靠探针。
