Fluorescence and NMR Characterization and Biomolecule Entrapment Studies of Sol−Gel-Derived Organic−Inorganic Composite Materials Formed by Sonication of Precursors

Optically clear sol−gel-derived organic−inorganic hybrid materials were prepared by a sonication method suitable for entrapment of biological compounds. Sonication at pH 2.5 hydrolyzed mixtures of tetraethyl orthosilicate (TEOS) and one of the organosilanes methyltriethoxysilane (MTES), propyltrimethoxysilane (PTMS), or dimethyldimethoxysilane (DMDMS). Buffer solutions containing the fluorescent probes 7-azaindole or prodan, or the proteins human serum albumin (HSA) or lipase, were then added to promote gelation and the resulting materials were aged at 4 °C over several months. The optical clarity, hardness, and degree of cracking were examined, in conjunction with solid-state 29Si and 13C NMR of the materials and fluorescence spectra of the entrapped probes. These studies revealed that MTES can be added to TEOS up to a level of 20% (v/v) with retention of good physical characteristics, thus allowing control over the hydrophobicity and cross-linking within the matrix. Materials with more than 20% MTES, or incorporating PTMS or DMDMS at levels above 5%, showed significantly poorer physical characteristics, indicating phase separation. Proteins entrapped into these hybrid materials could be examined by optical methods. Both entrapped HSA and lipase showed improvements in function with increased ormosil content, indicating that such materials are suitable for encapsulation of lipophilic proteins for optical sensor development.