Microstructure and transport properties of biocompatible silica hydrogels

Abstract

Silica matrices are suitable for encapsulation of biomolecules and microorganisms to build bioactive functional materials. For many applications of these host–guest systems, the performance highly depends on the tuning of transport properties. Here we analyze the microstructure of silica hydrogels from small-angle X-ray scattering (SAXS) experiments and its correlation with their transport properties evaluated from the fitting of diffusional profiles of the cationic dye crystal violet (CV). We found a clear correlation between the microstructure parameters and the transport of CV over a wide range of synthesis conditions (SiO2 total content from 3.6 to 9.0 % and pH of silica condensation from 4.5 to 7.5). At pH ~ 6, non-monotonic changes in transport properties can be attributed to the discontinuity observed in microscopic parameters, revealing the inherent complexity of the sol–gel transition. However, regardless of the pH of synthesis and for each set of samples with a fixed silica concentration, CV apparent diffusion coefficient (D app) is inversely proportional to the parameter S (related to the silica/aqueous-solution interfacial area) derived from SAXS. These results indicate that macroscopic properties cannot be easily predicted from the pH of synthesis, in particular around neutral pH that is relevant for biotechnological applications. Nonetheless, the close correlation between D app and the microstructure parameters of the studied systems allows proposing a predictive value of any of these approaches toward the other.