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,
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.