Hierarchically Porous and Water-Tolerant Metal–Organic Frameworks for Enzyme Encapsulation

Metal–organic frameworks (MOFs) for in situ enzyme encapsulation commonly possess weak metal–ligand coordination bonds and rather small pores, which are instable in aqueous solution and present rather high diffusion resistance of reactants. Herein, we prepare a type of hierarchically porous and water-tolerant MOFs through a facile polyphenol treatment method for enzyme encapsulation. In brief, enzymes are first in situ encapsulated in a zeolitic imidazolate framework-8 (ZIF-8) through coprecipitation of enzymes, zinc ions (Zn²⁺), and 2-imidazole molecules (2-MI). Then, tannic acid (TA, a typical polyphenol) is introduced to functionalize the surface and etch the void of ZIF-8, acquiring the biocatalyst termed as E@ZIF-8@ZnTA. The hierarchically porous structure would accelerate the diffusion process of reactants, whereas the Zn-O bond in a TA-Zn nanocoating would improve the structural stability against water corrosion compared to ZIF-8. Taking glucose oxidase (GOD) as a model enzyme for the catalytic conversion of β-d-glucose, the resultant GOD@ZIF-8@ZnTA exhibits the equilibrium conversion of 77.4%, which is comparable to GOD@ZIF-8 but much higher than GOD@ZIF-8@ZnTA without void etching. More importantly, the GOD@ZIF-8@ZnTA shows significantly enhanced recycling and storage stabilities compared to GOD@ZIF-8. It is expected that our study provides a facile and generic method to encapsulate a broad range of enzymes in MOFs with enhanced activity and stabilities.