Lipases (glycerol ester hydrolases E.C.184.108.40.206) are an important group of enzymes which have been widely used in the catalysis of different reactions [1, 2]. These enzymes have been applied in chemical and pharmaceutical industrial applications due to their catalytic activity in both hydrolytic and synthetic reactions. However, free lipases are easily inactivated and difficult to recover for reuse. Therefore, especially in large-scale applications, lipases are often immobilized on solid supports in order to facilitate recovery and improve operational stability under a wide variety of reaction conditions. Some lipase immobilization strategies involve the conjugation of lipases via covalent attachment, cross-linking, adsorption and entrapment onto hydrophobic or hydrophilic polymeric and inorganic matrixes [3–5].
In recent years, magnetic nanoparticles (MNPs) based on iron oxides, have attracted much interest thanks to their multifunctional properties, such as biocompatibility, superparamagnetism, small size and low toxicity . They have been applied in magnetic resonance imaging (MRI) , biosensors  and as anti-cancer drugs carriers . Due to their high specific surface area and easy separation from the reaction medium by the use of a magnet, they have been employed in enzymatic catalysis applications [10, 11]. Typical strategies for immobilizing lipase onto MNPs rely on surface grafting via low molecular weight linkers or polymers containing amino or epoxy functional groups to which lipases are reacted via covalent conjugation methods [1, 11]. Using such methods, the maximum reported loading capacity of lipase on nanoparticles is approximately 130 mg/g, using a complex methodology . One drawback of existing lipase immobilization technologies is that the activity of lipases decreases significantly upon immobilization due possibly to changes in enzyme secondary structure, or limited access of substrate to the active site of the surface bound enzyme . Thus, despite numerous reported approaches for immobilization of lipases on magnetic nanoparticles, there is still the need for simple, cost-effective and high loading capacity methods.
In this work, we present a facile, biomimetic approach to immobilize lipases onto iron oxide MNP surfaces modified with polydopamine, an in-situ formed coating inspired by the adhesive proteins secreted by marine mussels . The ortho-dihydroxyphenyl (catechol) functional group found in dopamine is also present in mussel adhesive proteins in the form of the amino acid DOPA, where it is highly adhesive to oxide surfaces [14, 15] and under alkaline conditions oxidizes to form quinone, a species that is reactive toward nucleophiles such as primary amines. Dopamine, containing both catechol and primary amine, was previously found to produce conformal coatings on surfaces by self-polymerization , which are further capable of immobilizing biomolecules . In the method described here, polydopamine serves as a conformal coating for the purposes of lipase immobilization onto MNPs. Our results demonstrate that PD-MNPs exhibit high efficiency for lipase immobilization under aqueous conditions, and the enzyme retains high activity after many cycles of magnetic separation and reuse.