The effect of preparation and aging conditions on the internal environment of sol-gel derived materials as probed by 7-azaindole and pyranine fluorescence

Sol-gel derived silicate materials were prepared by a two-stage processing method similar that commonly used for entrapment of biomolecules. In the first step, tetraethylorthosilicate (TEOS) was hydrolyzed by sonicating in the presence of water (molar ratio of H2O:Si = 4:1) and 0.044 M HCl with no added alcohol. The hydrolyzed TEOS was then mixed with varying amounts of a buffer solution (1:1 up to 4:1 vol:vol of silane:buffer) to induce gelation, and the monoliths were aged at room temperature either in air or in buffer for several days, followed by prolonged drying in air over several months. The emission spectra of the entrapped probes 7-azaindole (7AI) and 8-hydroxy-1,3,6-trisulfonated pyrene (pyranine) were examined at various times during the aging process to monitor the effects of the H2O:Si ratio on the evolution of the internal environment of the silicate materials. The results showed that (i) the internal environment of the silicate materials continued to evolve long after the drying of the matrix (as determined by mass changes) was complete; (ii) the internal structure of the silicate materials, as monitored by both 7AI fluorescence and infrared spectroscopy, was affected by the proportion of water used for gelation of dry-aged materials, but was similar for all wet-aged samples, and (iii) both wet-aged and dry-aged monoliths had a distribution of final environments present both during and after drying. The implications of these findings for protein entrapment are discussed.