Magnetic Fe3O4@Au nanocomposite, nanostructured particles were prepared by depositing gold onto seeds of superparamagnetic iron oxide nanoparticles (SPION). The particles were characterized by SEM, TEM, XRD, visible and near infrared spectroscopy, and magnetic measurements. They were also functionalized and employed as enzymatic carriers for electrochemical biosensing. The nanostructured particles showed an urchin-like, roughly spherical morphology whose surface was covered with nanometric protuberances. The average diameter of the particles could be controlled in the 100-500 nm range by changing the concentration of surfactants (oleylamine and oleic acid) employed in the synthesis. XRD showed a preferential growth of {1 1 1} facets. 3-Mercaptopropionic acid was used to chemically modify the surface of the nanostructured particles and horseradish peroxidase was immobilized onto the gold surface. Attached enzymes showed good activity even after several cycles of magnetic collection, drying, and redispersion. (c) 2013 Elsevier Ltd. All rights reserved.