A two-step ligand exchange reaction generates highly water-dispersed magnetic nanoparticles for biomedical applications

The high-temperature thermolysis of fatty acid-iron complexes generates magnetic nanoparticles (MNPs) of a precisely controlled size coated with fatty acids and dispersed in oil. Because they are water-immiscible, MNPs are unsuitable for water-based biomedical applications. Ligand exchange reactions that transform oil-into water-dispersed MNPs have attracted considerable attention, but are difficult to perform. In this paper, we report the successful preparation of size-controlled and highly water-dispersed MNPs, which have 4, 8 and 20 nm diameter by a unique two-step ligand exchange reaction. As temporary ligands, we selected thiomalic acid (TMA), which possesses moderate affinity toward MNPs and is soluble in both oil and water to remove fatty acids by XANES analyses. Next we selected the citric acids as secondary ligands for TMA-exchanged MNPs to be highly dispersed in water to remove TMA from the surface of MNPs. And the resulting highly water-dispersed MNPs are expected to be available as MRI contrast agents and hyperthermia carriers.