Large-scale hydrophilic Fe3O4 nanoparticles (NPs) were prepared in the presence of citrate and sodium nitrate via a facile method. The Fe3O4 NPs are quite stable and can be freely dispersed in water. The as-prepared magnetic nanoparticle solution can be stable for more than 1 month. The mean diameter of the Fe3O4 NPs can be controlled in the range of ∼20 to ∼40 nm in mean diameter. The NPs show superparamagnetic properties with a relatively high saturation magnetization moment 58 emu/mg at room temperature. Furthermore, a possible formation mechanism is proposed to explain why the magnetic nanoparticles are very well soluble in water.
