The sol-to-gel transition was monitored via the use of time-resolved recording of the fluorescence emission of viscosity-sensitive probes. Two dyes were chosen for the study, water-soluble DASPMI and a hydrophobic BODIPY, and steady-state, time-resolved and time-tagged fluorescence measurements were performed. These techniques, coupled with the probes different solubility, allowed complementary fluorescence lifetime and intensity data to be obtained from the dyes introduced into the matrix-forming mixture to produce sol-gel derived monoliths. Two different precursors were used as examples. A hydrogel was formed from a commercially available gellan gum (Gelrite), and a glass-like monolith was formed using tetraethyl orthosilicate. Changes in fluorescence lifetime could be related to those in the local viscosity sensed by the probe. The combination of this type of probe with time-resolved measurements is extremely useful in monitoring the microscopic changes that occur during the sol-to-gel transition within this important class of materials.

The fast and efficient collection of time-resolved fluorescence decay data is shown using a high repetition rate semiconductor laser excitation source optimally matched to low dead-time counting electronics. The 100 MHz repetition rate allows a measurement range of 10 ns without re-excitation of the sample. Rapid acquisition of time-resolved fluorescence data is essential for microscopy applications, such as fluorescence lifetime imaging. With this in mind we measured a representative dye (lifetime 376 ps) to determine the fastest collection time for a single exponential decay. Other dyes were also used with lifetimes ranging from ∼90 ps to 4 ns.

In this work we collate and review the usage that we have made of fluorescence techniques employed to follow the sol to gel transition and aging in different tetraethylorthosilicate based materials. The sol-gel method allows porous glasslike of good optical quality to be produced at relatively low (ambient) temperatures, which facilitates the incorporation of a range of molecules; from laser dyes to biomolecules. Here the use of “common” viscosity (DASPMI) and polarity (Nile red) sensitive fluorescence probes to monitor the host manufacture is made. Nile red was also used to label two catalytically active proteins (cytochrome c and subtilisin Carlsberg). These were incorporated into the different host media and the dye used to ascertain changes in protein conformation, both upon incorporation and at the end of an aging period. Complementary measurements of catalytic activity were performed. The probe emission was monitored via steady state and time-resolved fluorescence techniques and comparison made with the catalytic activity measurements to elucidate the amount of accessible and active protein. Overall it was found that the hosts became stable after an aging period approaching 20 days and that the major influence on the catalytic reaction rates was that of host mediated mass transport.