Effect of Ormosil and Polymer Doping on the Morphology of Separately and Co-hydrolyzed Silica Films Formed by a Two-Step Aqueous Processing Method

The entrapment of biomolecules within organic−inorganic nanocomposite materials derived by a sol−gel 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.