Directly aqueous synthesis of well-dispersed superparamagnetic Fe3O4 nanoparticles using ionic liquid-assisted co-precipitation method

•Using ionic liquid as templates to prepare the Fe3O4 nanoparticles.•Ionic liquid can be washed away completely by deionized water and absolute ethyl alcohol.•The synthesized Fe3O4 nanoparticles are well-dispersed and superparamagnetic.•The sizes of Fe3O4 nanoparticles can be easily tailored by varying the amount of ionic liquid.