Breaking pore size limit of metal—organic frameworks: Bio-etched ZIF-8 for lactase immobilization and delivery in vivo

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Abstract
Expanding pore size range of metal—organic frameworks (MOFs) promotes their versatility and feasibility for various biomedical applications. However, natural pore size greatly restricts large guest molecule accommodation. Customizing and tailoring pore apertures ranging from micropores to mesopores controllably is desired but still critically challenging. Herein, we developed a facile method with super mildness based on pH-sensitive zeolitic imidazolate framework (ZIF)-8 to increase porosity, providing pore size with maximum 20 nm, which is 8 times larger than average. Glucose oxidase (GOx) was introduced in ZIF-8 for bioetching, benefitted from the resultant acidic microe-nvironment during biocatalytic process. Different synthesis methods were assessed for obtaining different morphologies and size distributions. Reaction time, GOx encapsulation efficiency, and Zn2+ concentration was optimized to precisely control the mesopore size distribution of MOFs. It was found that bio-etching strategy was capable of producing stable mesopores which were large enough for loading lactase with good enzymatic activity retained, verified both in vitro and in vivo. This strategy breaks natural pore size limitation of MOFs and thereby facilitates biomolecule delivery, catalysis, and other biomedical applications with enhanced stability and performance.

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