Core-shell Magnetic Nanocomposites:Fabrication And Enzyme Immobilization

Immobilized enzymes have found numerous application for industrial production as they exhibit many benefits,such as high stability and easy recyclability.Usually,enzymes were immobilized on suitable carrier materials.Fe₃O₄ magnetic nanocarriers have been widely used in the field of enzyme immobilization due to their advantages of large specific surface area,small mass-transfer resistance,good biocompatibility and superparamagnetism.However,Fe₃O₄ magnetic nanoparticles（MNPs）are facile to be oxidized,corroded,aggregated or precipitated.More surface modification on Fe₃O₄ magnetic nanocomposites had been employed to overcome this problem.In this study,the core-shell structured Fe₃O₄ magnetic nanocomposites with good dispersibility and high stability were successfully prepared by introducing dopamine and zein as precusors,and then were used for the immobilization of enzymes.The results are as follows:Firstly,spherical Fe₃O₄ magnetic nanocomposites with uniform size of 10 nm were prepared by co-precipitation method,which were covered with polydopamine（PDA）layer to obtain spherical Fe₃O₄@polydopamine magnetic nanoparticles with size of 17 nm.Fe₃O₄@polydopamine/mercapto polyethylene glycol carboxyl magnetic nanoparticles were prepared by surface modification of Fe₃O₄@polydopamine magnetic nanoparticles with polymer mercapto polyethylene glycol carboxyl（HS-PEG-COOH）.The prepared magnetic nanoparticles had good superparamagnetism and was maintained the natural crystal structure of Fe₃O₄.Three kinds of immobilized glucose oxidase（GOx）were prepared by using Fe₃O₄,Fe₃O₄@Polydopamine and Fe₃O₄@Polydopamine/mercapto polyethylene glycol carboxyl magnetic nanoparticles as carrier materials.The enzyme activity of Fe₃O₄@Polydopamine/mercapto polyethylene glycol carboxyl-GOx was 78.45%,which displayed the highest retention rate.Compared with Fe₃O₄-GOx,Fe₃O₄@Polydopamine/mercapto polyethylene glycol carboxyl-GOx showed improved acid and reagent resistance,as well as thermal,operation and storage stability.Therefore,Fe₃O₄ magnetic nanocomposites were improved stability upon surface coating with polydopamine and further modification of mercapto polyethylene glycol carboxyl,which were more favorable for the retention of enzyme activity and the improvement of enzyme stabilitySecondly,in order to prepare magnetic nanocomposite suitable for multienzyme co-immobilization,the self-assembly property of zein was used to synthesise spherical Fe₃O₄@Zein magnetic nanoparticles of 100¹50 nm with smooth surface.The size of Fe₃O₄@Zein magnetic nanoparticles increased by 15±5 nm after the coating of polydopamine.Fe₃O₄@Zein@Polydopamine magnetic nanoparticles exhibited excellent superparamagnetism,stability and resolubility and maintained the crystal structure of Fe₃O₄.Fe₃O₄@Zein@polydopamine magnetic nanoparticles were modified by mercapto polyethylene glycol carboxyl to obtain Fe₃O₄@Zein@Polydopamine/mercapto polyethylene glycol carboxyl magnetic nanoparticles,which were used to co-immobilize GOx and horseradish peroxidase（HRP）.Under the optimal immobilization conditions,the activity of the immobilized bienzyme retained more than 70%.The stability of the immobilized bienzyme was much higher than that of the free bienzyme.Therefore,Fe₃O₄@Zein@Polydopamine/mercapto polyethylene glycol carboxyl magnetic nanoparticles can be used as good carrier material for enzyme immobilization,especially for co-immobilization of multienzymes.