Hierarchically ordered truncated single crystal mesoporous ZIF-8 as a solid platform for lipase immobilization
Most pristine metal organic frameworks (MOFs) are inherently microporous. To introduce desired porosity for easy enzyme infiltration and immobilization, polystyrene spheres was used as sacrificial template to design hierarchical ordered micro-mesoporous truncated single-crystalline ZIF-8 (SOM-ZIF-8). The surface area and pore diameter of the SOM-ZIF-8 were 821 m 2 /g and 7.09 nm which changed to 669 m 2 /g and 4.98 nm respectively, after lipase immobilization, suggesting the pores and surface of the SOM-ZIF-8 served as binding sites for the enzyme, and could reach a loading capacity of 134 mg/g after 24 h. The optimal conditions for achieving maximum lipase activity for free LipaseELP, ZIF-8@LipaseELP and SOM-ZIF-8@LipaseELP were pH 7.8 at 45 °C, and maintained 33.67 %, 45.6 % and 57.78 % residual activity after incubation at 80 °C for 2 h. The specific activity towards tributyrin and p-nitrophenyl acetate were observed to be SOM-ZIF-8@LipaseELP (0.25 and 45.85 U/mg) compared to ZIF-8@LipaseELP (0.211 and 43.62 U/mg) and LipaseELP (0.179 and 41.09 U/mg) respectively. The SOM-ZIF-8@LipaseELP could further be recovered and reused for 9 rounds while maintaining 73.8 % of its original activity. This study demonstrates that introducing mesoporous structures into ZIF-8 could improve its binding enzyme property for enhanced hydrolytic function.
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