MOF-templated rough, ultrathin inorganic microcapsules for enzyme immobilization

Ultrathin titania microcapsules with rough surfaces were prepared by using a metal-organic framework (ZIF-8) as one kind of hard template to mediate the structures of the microcapsule shell. Specifically, CaCO₃ particles were first coated with tannic acid (TA) followed by the deposition of hydrophobic ZIF-8 and another TA layer, the obtained particles were then assembled with protamine/TiO₂ bilayers through biomimetic mineralization. Finally, the microcapsules (Z-TiO₂) were obtained after simultaneously removing CaCO₃ and ZIF-8 templates using ethylenediaminetetraacetic acid (EDTA). The coordination interaction between TA and ZIF-8 ensured the robust templating which endowed the microcapsules with a rough surface and an ultrathin microcapsules shell (100 nm) with 4.4 nm pore size. Moreover, the surface roughness of microcapsules can be regulated by changing the size of ZIF-8 crystals. The microcapsules were then utilized to immobilize penicillin G acylase (PGA). And PGA@Z-TiO₂ retained 69% activity of equivalent free PGA with a loading capacity of 160 mg g^-1. The PGA@Z-TiO₂ microcapsules exhibited superior reusability: after recycling 8 times, the conversion of the enzymatic reaction remained 36.0%, which was twice higher than that of PGA@TiO₂ (14.7%). Moreover, compared with free PGA and PGA@TiO₂ microcapsules, PGA@Z-TiO₂ microcapsules exhibited higher thermal and storage stability. After storing for 60 days, the relative activity of PGA@Z-TiO₂ remained 89.6%, which was higher than that of free PGA (34.5%) and PGA@TiO₂ (73.6%). ZIF-8 can be envisioned to be a novel class of hard template for preparing a broad variety of microcapsules with different hierarchical structures.