SiO₂-Coated Fe₃O₄ Nanoparticle/Polyacrylonitrile Beads for One-Step Lipase Immobilization

Nanoscale particle-based enzyme immobilization carries a risk of dissolving or breaking the material when applied. One effective strategy to improve the stability and functionality of this type of nanomaterial is compounding with polymers. In this study, Fe3O4 nanoparticles were synthesized and coated with SiO2, after which magnetic hollow composite beads with nanoscale porous structures, named, lipase/Fe3O4/PAN (L/FP), were fabricated by compounding polyacrylonitrile and SiO2-coated Fe3O4 nanoparticles to immobilize Candida antarctica lipase B (CALB) in one step. Fourier transform infrared spectroscopy, scanning electron microscopy, the Brunauer–Emmett–Teller method, vibrating sample magnetometer measurements, and thermogravimetry analysis were used to analyze the structure of hollow beads. It was found that composite beads retained nanoscale channels and had a larger specific surface area (27.2 m2/g), a higher pore volume (0.09 cm3/g), and better saturation magnetization values (9.8 emu/g) than PAN hollow beads (6.2 m2/g, 0.03 cm3/g, and 0 emu/g, respectively). After optimizing fabrication and immobilization conditions, L/FP showed a high lipase capacity (50 mg protein/g support) at 2 mg/mL lipase, and the immobilized yield was up to 98.4% when the mass ratio of the starting lipase and support was 1.3 (mg lipase/g support). The immobilized lipase in L/FP showed greater thermal stability and resistance to alkali than free CALB. L/FP also had good stability in most organic solvents. In all tested cycles, it also had better reusability (78.4, 47.2, and 23.8% retained activity after 10, 20, and 30 cycles, respectively) in ester hydrolysis than CALB-immobilized Fe3O4 nanoparticles or PAN beads and had higher activity than the commercial immobilized lipase (Novo 435).