Abstract
Zeolitic imidazole framework-8 (ZIF-8) with tunable pore sizes and high surface areas have recently emerged as a promising support for immobilizing enzymes. However, the instability in the aqueous acidic environment and difficulty of recovery has limited their practical applications in some cases. In this study, catalase/ZIF-8 composites with a protective nanocoating were prepared by the controlled self-assembly of silanes or coordination complexes (tannic acid (TA) and Fe3+). The properties of the catalase (CAT)/ZIF-8 composites with a protective nanocoating were also determined. The recovered activity of CAT/ZIF-8 and CAT/ZIF-8 with protective nanocoating was 70% and 65%, respectively. Compared with the conventional CAT/ZIF-8 composites, CAT/ZIF-8 with protective nanocoating exhibited excellent acid resistance. For example, after treatment for 60 min in phosphate buffer solution (pH 3.0), CAT/ZIF-8 composites only maintained 20% of their initial activity (about 12 U/mg). However, CAT/ZIF-8 with a protective nanocoating could still retain about 50% of its initial activity (about 10 U/mg). Meanwhile, the thermostability and storage stability of the CAT/ZIF-8 composites was enhanced significantly due to the presence of nanocoating compared with conventional CAT/ZIF-8. More importantly, the CAT/ZIF-8 with a protective nanocoating retained 40% of its initial activity after 7 cycles, whereas CAT/ZIF-8 only retained 8% of the initial activity. The approach in this study could be an efficient strategy to prepare enzyme/ZIF-8 composites with both high acid resistance and excellent recyclability.
Highlights
Enzymes, as natural catalysts, have long been of abundant interest for the fine chemicals, pharmaceutical, cosmetic, and food industries [1,2]
For the Fe3+ -TA@CAT/Zeolitic imidazole framework-8 (ZIF-8) composites, the obtained CAT/Zeolitic imidazole framework (ZIF)-8 particles were mixed with TA and FeCl3 ·6H2 O water solution to enable the growth of an Fe3+ -TA nanocoating at the surface of the enzyme by the crystallization and deposition of Fe3+ -TA crystals, resulting in the formation of a mesoporous Fe3+ -TA nanocoating on the surface of the immobilized CAT
For SiO2 @CAT/ZIF-8 composites, cetyltrimethylammonium bromide (CTAB) was utilized to direct the overgrowth of mesostructured silica on the external surface of CAT/ZIF-8
Summary
As natural catalysts, have long been of abundant interest for the fine chemicals, pharmaceutical, cosmetic, and food industries [1,2] Their low stability, and the difficulty in the recovery and reusability of enzymes hampers their implementation in industries [3,4]. The solid supports for enzyme immobilization include microparticles [9], silica gel [10], hydrogels [11], and nanoporous materials [12] These conventional supports for enzymes are not finely tunable and crystalline, exhibiting non-uniformity and long-range ordering from the atomic to microscale regime, which leads to Polymers 2019, 11, 27; doi:10.3390/polym11010027 www.mdpi.com/journal/polymers
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