size-controlled synthesis of cofe2o4 nanoparticles potential contrast agent for mri and investigation on their size-dependent magnetic properties

Cobalt ferrite nanoparticles (CoFe2O4 NPs) were synthesized by coprecipitation followed by treatments with diluted nitric acid and sodium citrate. Transmission electron microscope (TEM) and photon correlation spectroscopy (PCS) characterization showed that the size distributions of these nanoparticles were monodisperse and that no aggregation occurred. This colloid showed a long-term stability. Through adjustment of the concentrations of reactants and reaction temperature, the size of the NPs can be tuned from 6 to 80 nm. The size-control mechanism is explained by a nucleation-growth model, where the local concentration of monomers is assumed to decide the size of nuclei, and reaction temperatures influence the growth of nuclei. Magnetization and relaxivity r1,2 measurements showed that the NPs revealed size-dependent magnetization and relaxivity properties, which are explained via a “dead magnetic layer” theory where reductions of saturation magnetization (Ms) and r1,2 are assumed to be caused by the demagnetization of surface spins.