Easy to use and easy to produce biosensors would have a huge range of applications. To reach this goal many see the incorporation of a protein into a sol–gel network as one of the most viable options. The current most prevalent technique of predoping presents inherent limits on the concentration possible for the resulting thin film. In this study we demonstrate a new process utilizing the newly developed kinetic doping method to load silica sol–gel thin films with cytochrome C (CytC) and horseradish peroxidase (HRP). Both enzymes are shown to successfully load and have a concentration increase over their original loading solution by factors of 1300× and 2600×, respectively. Furthermore, each enzyme once loaded retained the ability to act as a catalyst for the detection of hydrogen peroxide. Ultimately the CytC- and HRP-loaded thin films were found to have enzyme concentrations of 11 ± 1 mM and 6.0 ± 0.4 mM, respectively, a considerable step up from any doping method reported in the past.

Efficient and benign doping of thin films is key for materials applications and sensor development. Herein, an alternative method of doping is presented where R6G is loaded to an evolving silica thin film. Dopant loading is markedly enhanced and doping can be carried out under relatively benign conditions. The film exhibits outstanding optical quality while its mechanical strength is well-preserved. Early results show diffusion and encapsulation can be tunable.

The mobility and photostability of single rhodamine 6G (R6G) molecules encapsulated in organosilane modified silica alcogel films were used to examine how postsynthesis grafting alters guest−host interactions. While physical confinement remains the major factor that controls mobility in modified alcogels, both R6G mobility and photostability register discernible changes after surface charges are respectively reversed and neutralized by aminopropyltrimethoxysilane (APTS) and methyltriethoxysilane (MTES) grafting to weaken R6G/silica attraction on pore surfaces. Among the two methods, the change in R6G photostability was found to be more sensitive to surface grafting, which is more capable of inducing localized dynamic motions than full scale molecular rotation under the stringent physical confinement inside alcogel films. In addition, silica films modified by 0.4% APTS is as efficient as that by pure MTES in lowering R6G photostability, suggesting that surface charge reversal is more effective than charge neutralization in disrupting R6G/silica attraction. Collectively, our results demonstrate that single-molecule mobility and photostability can be used to monitor the extent of grafting reaction underneath a film surface and complement other surface characterization techniques that are only sensitive to modifications made on a film surface.

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 × 10−6 cm2 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.

Efficient and benign doping of thin films is key for materials applications and sensor development. Herein, an alternative method of doping is presented where R6G is loaded to an evolving silica thin film. Dopant loading is markedly enhanced and doping can be carried out under relatively benign conditions. The film exhibits outstanding optical quality while its mechanical strength is well-preserved. Early results show diffusion and encapsulation can be tunable.