Continuous Growth of Monodisperse Water-Dispersible Iron Oxide Nanoparticles for Potential Biomedical Applications

A direct synthesis of highly water-dispersible magnetic nanoparticles has been of great interest for biomedicine, but systematic control over size has not been achieved. Here, we have developed a general method to synthesize monodisperse water-dispersible iron oxide nanoparticles with nanometer-scale size increments, ranging from 4 nm to 15 nm, in a single reaction. Precise size control was achieved by continuous growth in an amphiphilic solvent, diethylene glycol (DEG), where growth step was separated from nucleation step by sequential addition of reactant. There was only one reactant used in the synthesis, and no additional capping agents and reducing agents were required. This approach suggests the “living growth” character of the synthesis of iron oxide nanoparticles in an amphiphilic solvent. The synthetic method demonstrates high reproducibility. The as-prepared iron oxide nanoparticles are highly water dispersible without the need for any surface modification. Furthermore, the synthesized 9 nm iron oxide nanoparticles exhibit extremely high transversal and longitudinal relaxivities, as reported in the literature for sub-10 nm spherical nanoparticles. Additionally, the iron oxide nanoparticles were studied for magnetic particles imaging (MPI). The performance of MPI increases with the increase of nanoparticles size. This study will not only shed light on the continuous growth phenomenon of iron oxide nanoparticles in amphiphilic solvent but also stimulate the synthesis and application of iron oxide nanoparticles.