Effect of Coulombic Interactions on Rotational Mobility of Guests in Sol-Gel Silicate Thin Films

James W. Gilliland, Kazushige Yokoyama, Wai Tak Yip*

Conducted at: Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019

Recent Progress: We investigate the effect of coulombic interactions on the mobility of rhodamine 6G (R6G) and Oregon Green 514 (ORG) in sol-gel silicates by measuring their mobility distributions using single molecule polarization measurements. While R6G is a cationic dye, ORG (pKa = 3.69±0.08) is an anionic dye at neutral pH. The presence of more tumbling ORG molecules in sol-gel silicates indicates that R6G and ORG experience opposite coulombic interactions with a predominately anionic silica sol-gel surface. On the other hand, the fact that tumbling ORG only represents a minor portion of ORG investigated, even at pH 7, clearly illustrates that coulombic interaction alone does not control the mobility of an encapsulated guest molecule in sol-gel silicates.

 

 

BACKGROUND INFORMATION

Surface adsorption can alter the physical and chemical properties of an adsorbate. This has been advantageously exploited in silica sol-gel chemistry to synthesize new optical and biocomposite materials. For example, organic dyes encapsulated inside a sol-gel silicate were reported to exhibit a higher photostability.  It has also been demonstrated that a higher temperature is required to denature a silica sol-gel bioencapsulate.  In these materials a guest molecule entrapped inside a mesoporous silica sol-gel matrix is expected to experience a substantial amount of surface interaction, which has a direct impact on the mobility of the guest molecule. As a result, a wide range of molecular mobility originating from different extents of surface interaction has been observed.  Such a broad distribution of molecular mobility will inevitably lead to diverse physical and chemical properties of the entrapped guest molecules. Electrostatic interaction is an important driving force for molecular adsorption on a silica surface. Under physiological conditions, a negatively charged silica surface can facilitate the immobilization of small organic dye, DNA, RNA, and large protein molecules through adsorption. To facilitate the development of silica sol-gel composite materials and to gain control of molecular dynamics inside sol-gel silicates, we examine the significance of coulombic interaction on the mobility of a guest molecule entrapped inside a silica sol-gel matrix. Owing to the extensive structural and chemical heterogeneities found inside sol-gel silicates, we employ single molecule spectroscopy to examine how coulombic interaction is manifested at the molecular level.  The mobility of a molecule can be examined by monitoring the changes of its orientation in real time. This can be accomplished at levels ranging from simple two-dimensional projection to sophisticated three-dimensional orientation determinations.  We have successfully demonstrated how single molecules inside a silica sol-gel matrix can be separated into different mobility classes using fluorescence polarization measurement.  Now we employ molecular mobility to monitor the extent of guest-host interaction that is specifically contributed by coulombic interaction. Here we assume that guest-host interaction will hinder the rotational motion of a guest molecule, with weak guest-host interaction favoring the encapsulation of freely tumbling guest molecules whereas strong anisotropic guest-host interaction at a liquid/surface interface will lead to efficient immobilization of the guest molecules.

To examine the effect of coulombic interaction on guest-host interaction, we report the mobility measurement of rhodamine 6G (R6G) and Oregon Green 514 (ORG) encapsulated in silica sol-gel thin films. R6G contains an iminium ion and is regarded as a positively charged probe molecule. Since the positive charge in R6G delocalizes throughout the entire xanthene moiety, the iminium proton normally does not participate in acid-base equilibria. R6G fluorescence is therefore quite insensitive to external pH variations at physiological conditions. At high pH however, say pH 12, the ester group of R6G will undergo base-catalyzed hydrolysis (pK ~ 11) and turn into a zwitterion.  For the choice of negatively charged probe molecule, we chose ORG in favor of fluorescein for our investigation in view of its higher photostability. Similar to fluorescein, the fluorescence properties of ORG are also pH sensitive. Depending on its surrounding pH, an ORG molecule can be either neutral (weakly fluorescent) or negatively charged (strongly fluorescent). Since the carboxylic acid groups in ORG have low pKa values (via infra), its fluorescence gradually becomes insensitive to pH from 6.0 and higher. In this study, ORG is regarded as a negatively charged probe molecule because, by default, it is more difficult to observe the weakly fluorescent neutral ORG molecules at a single molecule level.

The effect of coulombic interaction was readily observed when more tumbling ORG than R6G molecules were found in sol-gel silicates. Presumably charge-charge repulsion between ORG and a silica surface promotes ORG mobility by preventing it from adhering strongly to the surface of the inorganic matrix. On the other hand, coulombic attraction between R6G and the silica matrix favors surface adsorption and substantially restricts R6G mobility. Surprisingly, we also found that tumbling R6G and ORG only account for a small portion of single molecules that we investigated. The majority of the molecules we examined were either completely fixed or loosely bound to a surface and capable of changing orientation occasionally. Our results therefore indicate that long-range coulombic interaction influence molecular mobility inside a sol-gel silicate only to a limited extent. The nanoscopic confinement of a highly polarizable guest molecule imposed by a silica sol-gel matrix may favor guest-host interactions contributed by shorter range electrostatic interactions such as van der Waals interactions and H-bonding. Alternatively, it is also quite possible that guest-host interaction is dominated by mere physical confinement as a result of molecular templating.