Over the last decade, nanoparticles used as protein carriers have opened new avenues for a variety of biomedical applications. The main concern for these applications is changes in biological activity of immobilized proteins due to conformational changes on the surface of the carrier. To evaluate this concern, the preparation and biocatalyst activity of α-chymotrypsin-Fe3O4 @ Au core/shell nanoparticles were investigated. First, Fe3O4 @ Au core/shell nanoparticles were synthesized by coprecipitation method and citrate reduction of HAuCl4. TEM imaging revealed a core size of 13 ± 3？nm and a shell thickness of 4 ± 1？nm for synthesized nanoparticles. X-ray diffraction (XRD) was used to study the crystalline structure of the nanoparticles. Next, the enzyme was immobilized on the surface of synthesized nanoparticles by covalent bonding of Au shell with thiol and amine groups present in the protein structure (e.g., cysteine and histidine residues). FTIR and fluorescence spectroscopy were utilized to study secondary and tertiary structures of the immobilized enzyme. Results show that the secondary and tertiary structures of the enzyme remain virtually unchanged after immobilization on the nanoparticles surface. However, the biocatalyst activity of the enzyme was reduced by thirty percent, indicating possible conformational changes or active site occlusion. 1. Introduction In the past decade, magnetic nanoparticles have been studied for their biomedical applications [1] such as cellular therapy [2, 3], drug delivery [4, 5], hyperthermia, and magnetic resonance imaging (MRI) [6, 7]. While one can find a comprehensive discussion on critical factors for interaction of nanoparticles with living cell and proteins in reviews by Rauch et al. [8] and Mahmoudi et al. [9], it could be summarized that nanoparticles must have biocompatibility and interactive functions at the surface to allow their use in biomedical applications [10, 11]. Both organic and inorganic coatings have been used for surface modification of magnetic nanoparticles [10, 12]. Polyethylene glycol (PEG) and polyvinyl alcohol (PVA) are mainly used for polymeric surface modification [13, 14]. Silica and gold are the most common inorganic molecules used to modify the surfaces of magnetic nanoparticles [10, 13]. A silica layer provides a rich surface of silanol groups that react easily with alcohols and silane coupling agents [15, 16]. However, of the available surface modifiers, gold shows great potentials for biomedical applications [10]. Gold not only confers stability to nanoparticles in oxidative
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