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Thin films (100-500 nm thick) of Class I, Class II and Class I/II organic-inorganic hybrid silicates were prepared by casting mixtures of tetraethyl orthosilicate, methyltriethoxysilane and/or dimethyldimethoxysilane that were first hydrolyzed via sonication using acid catalysis, followed by induction of gelation using added buffer solutions containing no polymer or 3% w/v of poly(ethylene glycol). In some cases, the fluorescent probe 6-propionyl-2-dimethylaminonaphthalene (PRODAN) or the protein human serum albumin (HSA) were also added to the buffer solution, which permitted the fluorescence probing of the internal environment and biological activity within the various silicate films, respectively. Parameters such as the type and level of organosilane precursor, polymer level, casting speed, and aging time were varied to determine how such factors affected the thickness, morphology, and internal environment of the resulting films that were formed by this two-step process. The results show that composite films evolve in two stages: (1) rapid drying during the initial 1-2 h after casting of the film, followed by (2) a slower aging process that continues for many months. The aging of these thin films is different to that of bulk materials and the final state of the films can be quite different from that obtained for bulk materials. Preliminary studies of HSA-doped films show that the entrapped protein adopts a partially expanded conformation, but in most cases is still able to bind to ligands.

The entrapment of biomolecules within organic-inorganic nanocomposite materials derived by a solgel method has proven to be a viable route for the development of biosensors and biocatalysts. However, the phase separation behavior within nanocomposite materials formed by a protein-compatible two-step aqueous processing method is not well-understood. In this study, a range of imaging methods was used to assess the degree of heterogeneity in a series of dipcast thin films formed with different types and levels of ormosils in the presence and absence of polyethylene glycol (PEG), using both separate and co-hydrolysis of precursors. Both microscopic (bright-field and fluorescence microscopy) and nanoscale (atomic force microscopy and scanning electron microscopy) imaging demonstrate that short chain monofunctional ormosils such as methyltrimethoxysiline do not lead to significant heterogeneity when mixed with TEOS, while disubstituted (dimethyldimethoxysilane) or longer chain (isobutyltrimethoxysilane) ormosils show significant heterogeneity at the microscopic and nanoscopic scale when prepared by a separate hydrolysis method. The addition of PEG can improve the homogeneity in some materials, likely due to the coating of silica sol particles, which reduces microscopic phase separation; however, cohydrolysis of the precursors provides a more general route to create homogeneous materials. Interestingly, the heterogeneity observed by bright-field microscopy, which reflects variations in the refractive index, did not fully correlate with the fluorescence microscopy images of entrapped fluorophores, suggesting that chemical heterogeneity exists even when samples appear to be homogeneous. The implications of these findings for biosensor development will be discussed.

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