Entrapment of enzymes in silica aerogels

Aerogels, the world`s lightest solids, possess extraordinary traits. They have immense surface area, high porosity and ultra-low heat conductivity. These traits made aerogels favorable in various fields, from aerospace to building insulation. Nonetheless, the use of pure silica aerogels for the entrapment of enzymes is not known. While enzyme entrapment helps stabilize the enzymes and aids in catalyst separation from the reaction pot, the kinetics are usually impaired, as diffusional limitations of the substrate and product arise. When considering aerogels as potential enzyme carriers, their highly porous structure may help facilitate the diffusion of substrates to the embedded enzyme. And yet, reports of enzyme entrapment in aerogels are restricted to lipases only, and even those are in hydrophobically modified silica. This is probably because of the harsh synthesis procedure of aerogels that only robust enzymes as lipases can withstand. There is a need of a bio-friendly synthesis route for the entrapment of more sensitive enzymes in aerogel. In this work, we present a generalized procedure for the entrapment of enzymes in silica aerogel. All aerogel synthesis steps were modified and optimized, for reducing the risk of enzyme denaturation. The entrapment of three different types of enzymes was demonstrated: glucose oxidase, acid phosphatase and xylanase. All entrapped enzymes showed superior activity over the standard method of sol-gel entrapment in xerogel. Michaelis-Menten kinetics was observed for all three entrapped enzymes, indicating that they are highly accessible and diffusional limitations are negligible. A key observation is that the Michaelis-Menten constant, K_m, has remained at the same level upon entrapment, indicating that enzyme-substrate affinity was not negatively affected. Thermal stabilization was observed in the case of acid phosphatase with the entrapped enzyme reaching peak activity at 70^O_C. Aerogel enzymes entrapment might be especially attractive for health applications and for catalyzing biotechnological processes, even of large substrate molecules.