Synthesis of Fe3O4 magnetic fluid used for magnetic resonance imaging and hyperthermia

► Fe3O4 magnetic fluid (MF) was prepared by co-precipitation without protection gas and using NaOH as co-precipitation agent. ► Tween 80 was employed as the second layer surfactant. ► Hydrophilic ends of Tween 80 were exposed outside making the Fe3O4 MNPs suspending in water stably. ► Prepared MF can be used both in magnetic resonance imaging (MRI) and magnetic fluid hyperthermia (MFH).