Magnetic Polymer Nanoparticles Immobilized with Enzymes for Stabilizing Pickering Emulsions: A Recyclable Interfacial Catalysis System.

The Pickering interfacial catalysis (PIC) system based on enzyme-immobilized carriers offers an efficient platform for the enzymatic reactions. In this study, we developed enzyme-immobilized magnetic polymer nanoparticles (MPNs) capable of stabilizing Pickering emulsions while enabling the rapid and stimuli-responsive recycling of catalytic products. Nano-Fe3O4 was prepared via coprecipitation, and MPNs with an average diameter of 252 nm were synthesized through hybrid emulsion polymerization of a mixture of styrene and methyl methacrylate containing a magnetic fluid. After hydrolysis in an alkaline solution to generate carboxyl groups on the MPN surface, Candida antarctica lipase B (CALB) was covalently immobilized, yielding MPNs-CALB with an enzyme loading. An oil-in-water Pickering emulsion (oil/water ratio of 0.6:1) was prepared by homogenizing n-heptane into phosphate-buffered saline containing the enzyme-loaded particles (MPNs-CALB), forming the PIC system. Microscopic observation revealed that increasing the enzyme loading is beneficial to the stability of the Pickering emulsion. The hydrolysis of 4-nitrophenyl butyrate (p-NPB) to p-nitrophenol (p-NP) was conducted in this system at room temperature to evaluate the catalytic efficiency. Compared with free CALB, the immobilized enzyme in the Pickering emulsion exhibited markedly enhanced activity, achieving 56.0% conversion within 1 h and approaching equilibrium. Moreover, as the enzyme loading increased, the hydrolysis conversion rate at equilibrium of the catalytic reaction rose accordingly. After five reaction cycles, immobilized CALB retained 95.2% of its initial activity. In each cycle, the aqueous phase was magnetically separated after reaction equilibrium, replenished with fresh buffer, and the reaction continued until substrate depletion. The oil phase was subsequently recovered by centrifugation. This strategy provides a smart, responsive Pickering interfacial biocatalyst system for sustainable biphasic enzymatic catalysis.