Precise air oxidation for continuous production of superparamagnetic Fe3O4 nanoparticles at room temperature through a microfilm reactor

Superparamagnetic iron oxide nanoparticles (SPIONs) find vast applications in biomedicine such as drug delivery, magnetic resonance imaging (MRI) contrast agents, cell separation, tissue repair, etc. The synthesis of pure and uniform SPIONs involves tedious processes, complex precursors, the assistance of organic solvents, capping agents and/or high temperature. Herein, we report a continuous mass production of pure and uniform SPIONs in aqueous microfilm at room temperature without using any surfactant. The air oxidation rate and nanocrystal formation kinetics were precisely controlled by process intensification in the continuously flowing microfilm. Precise oxidation of Fe(II) is a synergy of disc spinning speed and feeding concentration. The production process was continuous and conducted at room temperature in an aqueous medium with a high yield of 55.4 g h⁻¹. The obtained products were proved to be highly pure through XRD and a colorimetric method. The resulting SPIONs exhibited a narrow size distribution, ranging from 6.5 to 12.6 nm. All sizes of the produced nanoparticles showed characteristic superparamagnetism, with the 12.6 nm SPIONs exhibiting the highest magnetization saturation (M_s) of 87 emu g⁻¹.