© 2007 Nature Publishing Group Intrinsic peroxidase-like activity of

Nanoparticles containing magnetic materials, such as magnetite (Fe3O4), are particularly useful for imaging and separation techniques. As these nanoparticles are generally considered to be biologically and chemically inert, they are typically coated with metal catalysts, antibodies or enzymes to increase their functionality as separation agents. Here, we report that magnetite nanoparticles in fact possess an intrinsic enzyme mimetic activity similar to that found in natural peroxidases, which are widely used to oxidize organic substrates in the treatment of wastewater or as detection tools. Based on this finding, we have developed a novel immunoassay in which antibody-modified magnetite nanoparticles provide three functions: capture, separation and detection. The stability, ease of production and versatility of these nanoparticles makes them a powerful tool for a wide range of potential applications in medicine, biotechnology and environmental chemistry. There is currently intense interest in the use of nanoparticles for a wide range of biomedical and technological applications. Among known nanomaterials, magnetic nanoparticles (MNPs) are of particular interest because of their power in biological imaging or separation techniques. For instance, MNPs have been used for separating proteins, DNA and cells from samples1,2, for drug and gene targeting3, for tissue engineering4, for magnetic resonanc