Monodisperse mesoporous silica (mSiO₂) coated superparamagnetic iron oxide (Fe₃O₄@mSiO₂) nanoparticles (NPs) have been developed as a potential magnetic resonance imaging (MRI) T₂ contrast agent. To evaluate the effect of surface coating on MRI contrast efficiency, we examined the proton relaxivities of Fe₃O₄@mSiO₂ NPs with different coating thicknesses. It was found that the mSiO₂ coating has a significant impact on the efficiency of Fe₃O₄ NPs for MRI contrast enhancement. The efficiency increases with the thickness of mSiO₂ coating and is much higher than that of the commercial contrast agents. Nuclear magnetic resonance (NMR) relaxometry of Fe₃O₄@mSiO₂ further revealed that mSiO₂ coating is partially permeable to water molecules and therefore induces the decrease of longitudinal relaxivity, r₁. Biocompatibility evaluation of various sized (ca. 35–95 nm) Fe₃O₄@mSiO₂ NPs was tested on OC‐k3 cells and the result showed that these particles have no negative impact on cell viability. The enhanced MRI efficiency of Fe₃O₄@mSiO₂ highlights these core–shell particles as highly efficient T₂ contrast agents with high biocompatibility. Copyright © 2012 John Wiley & Sons, Ltd.

We review current synthetic routes to magnetic iron oxide nanoparticles for biomedical applications. We classify the different approaches used depending on their ability to generate magnetic particles that are either single-core (containing only one magnetic core, i.e. a single domain nanocrystal) or multi-core (containing several magnetic cores, i.e. single domain nanocrystals). The synthesis of single-core magnetic nanoparticles requires the use of surfactants during the particle generation, and careful control of the particle coating to prevent aggregation. Special attention has to be paid to avoid the presence of any toxic reagents after the synthesis if biomedical applications are intended. Several approaches exist to obtain multi-core particles based on the coating of particle aggregates; nevertheless, the production of multi-core particles with good control of the number of magnetic cores per particle, and of the degree of polydispersity of the core sizes, is still a difficult task. The control of the structure of the particles is of great relevance for biomedical applications as it has a major influence on the magnetic properties of the materials.