Direct Synthesis of Monodisperse Water-Soluble Iron Oxide Nanoparticles for Bioimaging

The direct synthesis of highly water-soluble nanoparticles has attracted intensive interest, but systematic size control has not been reported. Here, we developed a general method for synthesizing monodisperse water-soluble iron oxide nanoparticles with nanometer-scale size increments from 4 nm to 15 nm in a one reaction. Precisely 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 in the synthesis, and no need for additional capping agents and reducing agents. This study reveals the “living growth” character of iron oxide nanoparticles synthesis in an amphiphilic solvent. The synthetic method shows high reproducibility. The as-prepared iron oxide nanoparticles are extremely water soluble without any surface modification. Surprisingly, the synthesized 9 nm iron oxide nanoparticles exhibit extremely high transversal and longitudinal relaxivities in the literature for sub-10 nm spherical nanoparticles. In addition, 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 could also stimulate the synthesis and application of iron oxide nanoparticles.