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 beta-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.