Protamine-templated biomimetic hybrid capsules: Efficient and stable carrier for enzyme encapsulation

Biosilica with a highly complex and intricate structure can be formed by a diatom under physiological conditions in the presence of silaffins. Herein, the biosilicification process in vivo is mimicked. A natural cationic polypeptide, prolamine, was for the first time utilized in vitro to inspire and template silica formation at ambient temperature and neutral pH conditions. The silica-precipitating and templating effect of protamine was first tentatively elucidated. Then, this biomimetic silicification process was performed on the outer surface of liquid-core alginate (Alg) capsules in which beta-glucuronidase (GUS) was pre-encapsulated. An alginate/protamine/silica (APSi) hybrid capsule with a distinct liquid core-solid shell structure was thus fabricated. The rigid, mesoporous silica shell dramatically inhibited the swelling of the capsule and effectively enhanced the mass transfer of substrates and products. Meanwhile, the biocompatible polysaccharide liquid core created a benign microenvironment and well preserved the three-dimensional structure of GUS. The stability of encapsulated GUS was significantly enhanced after silicification, and no loss in activity was found after 10 reaction cycles. Moreover, the relative activity of GUS encapsulated in APSi capsules reached 125%, not only exceeding that encapsulated in Alg capsules but also being higher than that of the free enzyme.