Efficient Oxidative Polymerization of Hydroxytyrosol Using Horseradish Peroxidase Immobilized on a Magnetic Silica-Coated Nanocatalyst

Enzyme-based catalysis has gained considerable attention in recent years due to its eco-friendly and selective nature. However, the inability to recover enzymes after the reaction significantly increases operational costs. In this study, a reusable nanocatalyst Fe3O4@SiO2@APTES@GA@HRP is synthesized and applied in the oxidative polymerization of hydroxytyrosol. Fe3O4 nanoparticles are prepared using the solvothermal method, followed by silica coating via the Stöber process. Amino-functionalization is achieved with 3-aminopropyltriethoxysilane (APTES), and horseradish peroxidase (HRP) is immobilized through glutaraldehyde (GA)-mediated azomethine bonding. The synthesized catalyst is characterized using SEM, EDS, FTIR, Q-TOF, ¹H-NMR, and Zetasizer analyses, all confirming successful immobilization. Polymerization reactions are conducted in acetate buffer (pH 5, 25 °C), resulting in a 73% yield. The catalyst is reusable for up to ten cycles, and the molecular weight of the produced poly(hydroxytyrosol) is approximately 30,000 g mol−1. These findings demonstrate the promising application of recyclable enzyme nanocatalysts in green polymer chemistry.