Entrapment of enzymes in silica aerogels

Aerogels, the world's lightest solids, possess extraordinary traits such as very low density, very high surface area, very high porosity and ultra-low heat conductivity. These traits made aerogels favorable in various applications, including high-performance thermal insulators, catalyst supports, electrode materials, random laser matrices, cosmic dust collectors and more. Of the many potential applications of aerogels, one of the most challenging has been the development of a general procedure for bioactive aerogels by the entrapment of enzymes within these air-light materials. The difficulty in reaching this “holy-grail” was dual: The special procedures for obtaining the unique structure of aerogel are destructive to enzymes; and the aerogels are extremely sensitive to any procedural modification. Thus, the use of pure silica aerogel for the entrapment of enzymes was not known. Here we present a generalized, bio-friendly procedure for the entrapment of enzymes in silica aerogel, retaining both the enzymatic activity and the air-light structure of the aerogel. All of the aerogel synthesis steps were modified and optimized for reducing the risk of enzyme denaturation, while preserving the aerogel characteristic structure of the composite. The entrapment of three enzymes of different types was demonstrated: glucose oxidase, acid phosphatase and xylanase. All aerogel-entrapped enzymes showed superior activity over the common method of sol–gel entrapment in xerogels, due to the much wider and open pore network of the former. Michaelis-Menten kinetics was observed for the entrapped enzymes, indicating that the enzymes are highly accessible and diffusional limitations are negligible. The Michaelis-Menten constant, Km, has remained at the same level, indicating that enzyme-substrate affinity was not affected. Thermal stabilization was observed for entrapped acid phosphatase reaching peak activity at 70 °C. Large molecular weight substrates such as xylan for xylanase, are no obstacle for the aerogel matrix, while completely inapplicable for the xerogel. All of these properties are highly relevant for biotechnological applications.