Towards MRI T2 contrast agents of increased efficiency

Magnetic nanoparticles can be efficient contrast agents for T2 weighted magnetic resonance imaging (MRI) after tuning of some key parameters such as size, surface state, colloidal stability and magnetization, thus motivating the development of new synthetic pathways. In this paper we report the effects of surface coating on the efficiency of two different types of iron based nanoparticles (NPs) as MRI contrast agents. Starting from well-defined hydrophobic iron oxide nanospheres and iron nanocubes of 13 nm size, we have used three methods to increase their hydrophilicity and transfer them into water: surface ligand modification, ligand exchange or encapsulation. The NPs obtained have been characterized by dynamic light scattering and transmission electron microscopy, and the relaxivities of their stable colloidal solutions in water have been determined. Among all samples prepared, iron nanocubes coated by silica display the highest relaxivity (r2) value: 628 s-1 mM-1.