Single-molecule spectroscopic investigation of the influence that charge-charge interactions, solvation, and confinement have on guest molecule rotational mobility and photostability within silica sol-gel host matrices

Silica sol-gel is an amorphous silicon oxide glass formed at room temperature using the sol-gel process. The sol-gel process consists of hydrolysis and condensation reactions, which bind the starting alkoxide monomers into a three dimensional polymer network that spans the sample volume (the solution becomes a gel, sol-gel). The random hydrolysis and condensation reactions leave behind solvent filled pores within the silica sol-gel matrix, within which guest molecules can be trapped. Silica sol-gel encapsulation has been found to stabilize both fluorescent dye molecules and enzymes; however the guest properties are strongly influenced by interactions with the heterogeneous host environment.; We have chosen to investigate guest-host interactions within silica sol-gel thin films using single molecule spectroscopy. Using single molecule fluorescence polarization, we identified the intermediate molecule mobility population, which expands our understanding beyond simple fixed and tumbling motions. Thus, intermediate molecules help reveal the guest's rich dynamic interactions with the local environment. Investigation into the influence that charge-charge interactions have on guest molecule mobility showed that anionic repulsion away from the negative silica surface has a greater influence on mobility than cationic attraction toward the surface. The limited influence of charge-charge attraction on mobility indicates that charge-charge attraction is only one among many interactions that foster dye immobilization. By controlling buffer pH, the anionic guest protonation state and charge could be altered, influencing guest mobility. Solvent equilibration was found to increase the observed fixed population, due to preferential leaching of highly mobile molecules. Solvation also revealed the importance of guest molecule dissociation from the counter ion to achieve optimal charge-charge interaction. Guest molecule hydrogen bonding with the surface was found to reduce the influence of solvent dynamics on the guest rotational mobility and photostability. In all samples studied there remains a 20%, or greater, population physically immobilized by molecular templating, pore collapse, or matrix incorporation.; To investigate physical immobilization further, guest behavior was monitored within the more highly branched silica nanoparticle matrix, when covalently bonded to silica nanoparticles, and within pores templated by gold nanoparticles. The silica nanoparticle environment was found to enhance mobility by increasing the intermediate population relative to the silica thin films, while covalent bonding to the silica nanoparticle surface did not significantly alter the mobility distribution relative to guests physically doped within silica nanoparticles. The guest survival lifetime increased with covalent bonding, but the photobleaching quantum yield did not change, indicating that covalent attachment does not improve photostability. In contrast, guest trapped in pores templated by gold nanoparticles showed increased survival lifetimes and decreased photobleaching quantum yields, indicating greater guest stability within the templated pores. Investigation will continue on the gold nanoparticle templated samples to reveal the role that nanoscopic confinement plays in determining guest mobility and photostability. By developing an accurate model of guest-host interactions, we hope to gain control over the interactions, guest behavior, and eventually material properties.