Enzymes Immobilization Based On Metal-Organic Frameworks And Its Catalytic Performance

Metal Organic framework?MOFs?is a new type of nanocomposite materials.It has been used as a carrier to immobilize enzyme due to large specific surface area and porosity.However,most MOFs are easy to be degraded in acid solution,and it is difficult to recover and reuse them because of smaller size.Therefore,MOFs has limited their practical applications in some cases.To overcome these problems,we utilized catalase as a model enzyme for studying the morphological changes of zeolite imidazole frameworks?ZIFs?and synthesized the ZIFs/Catalase with cruciate flower-like morphology.Based on ZIF-8,immobilized catalases with core-shell structure were prepared by self-assembly,and their catalytic performances were investigated.?1?Preparation of the ZIFs and immobilization of catalase as well as it's catalytic performanceThe concentration of imidazole and zinc nitrate was optimized.The results showed that concentration of imidazole and zinc nitrate had significant effects on ZIF-8morphology.ZIFs with cruciate flower-like morphology were obtained when the concentration of imidazole and zinc nitrate reached to 0.8 mol/L,respectively.Performance measurement of immobilized catalase showed that,the activity recovery of enzyme can reach to 80%;Compared with free enzyme,the optimal catalytic temperature increased 10?;In terms of temperature tolerance,free catalase and immobilized catalase retained 25%and 60%,respectively.The result showed that immobilized catalase had higher temperature tolerance.Furthermore,immobilized catalase also exhibited superior reusability and storage stability compared to free enzymes.?2?Preparation of SiO₂@ZIF-8-enzyme and it's catalytic performance.Firstly,catalase was encapsulated in ZIF-8 for preparing ZIF/Catalase.Then SiO2@ZIF-8/Catalase was synthesized by sol-gel.The optimum conditions for preparing SiO2@ZIF-8/Catalase are as followed:ammonium hydroxide 500?L,TMOS 60?L and the stirring time was 16 h.The performance measurement of immobilized catalase showed that the optimal catalytic pH of SiO₂@ZIF/Catalase and ZIF/Catalase were at pH 6.0 and7.0,respectively.The optimal catalytic temperature was 40?;The SiO₂@ZIF/Catalase and ZIF/Catalase retained 80%and 40%of their original activity after they were treated at pH 3.0 buffer for 90 min,respectively.Furthermore,SiO2@ZIF/Catalase still maintained50%of original activity after 5 cycles whereas ZIF/Catalase almost inactivated.These result showed that the presence of silica gel improved the stability of immobilized enzymes.?3?Preparation of immobilized enzyme with core-shell structure and it's catalytic performanceFirstly,we prepared ZIF/Catalase by absorption,then mesoporous silica shell around ZIF/Catalase was formed by self-assemble of TMOS under induction of CTAB?CT-SiO₂@ZIF/Catalase?.In addition,the Fe³⁺-TA film was coated on the ZIF/Catalase surface by self-assemble technology?TA-Fe@ZIF/Catalase?.The optimum conditions for preparing CT-SiO₂@ZIF/Catalase are as followed:TMOS and TA were 80?L and 400?L,respectively.The performance measurement of immobilized catalase showed that the optimal catalytic temperature of all immobilized enzymes were 40?;The optimum catalytic pH of CT-SiO₂@ZIF-8/Catalase was at 6.0,while ZIF/Catalase and TA-Fe@ZIF-8/CatalasewereatpH7.0.CT-SiO2@ZIF-8/Catalaseand TA-Fe@ZIF-8/Catalase exhibited higher pH-stability than that of ZIF-8/Catalase.Furthermore,afterstoringfor20days,CT-SiO2@ZIF-8/Catalaseand TA-Fe@ZIF-8/Catalase retained 50%of original activity,while ZIF-8/Catalase only maintained 20%original activity.The results showed that the presence of mesoporous silica shell and Fe³⁺-TA film prevented the loss of enzyme molecules and improved the stability.