Laccases Immobilization Onto Polypropylene Microporous Membrane Based On Dopamine Modification

Laccases are blue oxidoreductase, but also represent the largest category branch of multi-copper oxidase. They have been widespreadly concerned and applied in many fields from the 19th century, because they can oxidize a wide range of substrates. However, the free laccases perform badly under alkali and high temperature conditions, which limit the prospects for its industrial application. Enzyme immobilization technique acquires a lot of attention for it is a very effective way to improve the stability. Enzyme immobilization methods is abundant, including chemical, physical, and self-immobilization method. In addition to immobilization technology, we also need to consider the operating parameters and economic costs in practical applications. Thus, immobilizing enzymes onto the membranes shows good prospects, whose structure and properties are tunable. Product separation can be achieved during catalytic process, and energy consumption is pretty low. Moreover, operating parameters are easy to adjust, which make it promising for large-scale industrialization.Microporous polypropylene membranes have good chemical stability, high mechanical strength, low cost, and are widely used as polymer membranes. For enzyme immobilization via either chemical or physical methods, membrane surfaces are mostly required surface modification, such as UV/gamma radiation, plasma treatment and so on. These methods are complicated, sometimes undermine the intrinsic properties of the polymer, and even produce harmful contaminants. Therefore, in search of greener and more convenient method for modification is imminent. Messersmith and his colleagues found that dopamine-modified layer gives the membrane biocompatibility and functional groups for enzyme immobilization.Accordingly, we use dopamine-modified polypropylene microporous membranes to fixe laccases. The optimal deposition time is 8 h after studying the properties of modified membrane surface via SEM, Water Contact Angle, FT-IR/ATR and XPS. Moreover, the optimal time, laccase concentration and pH for immobilization are 8 h,1.0 mg/mL,4.0, respectively. Further, looking from amount and distribution of functional groups of laccase, pH 5.0 is the critical point that affects the immobilization efficiency, because there is a balance between phenolic and quinone-type structures of dopamine depending on the pH. The immobilized laccase stability has been greatly improved, including pH stability, temperature stability, and long-term storage stability. Finally, we use model surfaces (gold chips) to understand the immobilizing behavior of laccase on dopamine-modified MPPM is a single-layer adsorption behavior by using SPR (Surface Plasmon Resonance).In addition, in order to solve the low enzyme activity and enzyme loading, we introduced polyethyleneimine (PEI) to co-deposit with dopamine. The suitable deposition time is 4 h by analyzing the density and chemical structure of the modified layer. By modifying morphology and chemical composition of mineralization layer, immobilization of laccase activity optimum results mineralized time 8 h. Laccase loading increases by 25% with the increase of the original concentration of laccase solution compared with that of the chemical immobilization method, and laccase activity reaches the highest at 0.8 mg/mL concentration. In addition, free laccase still retains 90% activity after immersion in the silicon solution for 8 h, indicating mineralization condition is moderate, which is helpful to retain the activity of the immobilized laccase. Immobilized laccase’s pH and temperature stability, recovery capabilities after inactivation under high temperature are perfect than free laccase, and it still keep its 90% activity after 10 cycles, indicating that encapsulation effectively improves the stability of laccase. Finally, the dye degradation activity of laccase-immobilized polypropylene micro-membrane increases by 17% compared to chemical immobilization. This simple and efficient method of immobilization has good prospects for industrial applications.