Abstract A highly stable and magnetized citric acid (CA)-functionalized iron oxide aqueous colloidal solution (Fe 3 O 4 @CA) was synthesized by using a simple and rapid method of one-step co-participation via a chemical reaction between Fe 3+ and Fe 2+ in a NaOH solution at 65 °C, followed by CA addition to functionalize the Fe 3 O 4 surface in 25 min. The NPs were synthesized at lower temperatures and shortened time compared with conventional methods. Surface functionalization is highly suggested because bare Fe 3 O 4 nanoparticles (Fe 3 O 4 NPs) are frequently deficient due to their low stability and hydrophilicity. Hence, 19 nm-sized Fe 3 O 4 NPs coated with CA (Fe 3 O 4 @CA) were synthesized, and their microstructure, morphology, and magnetic properties were characterized using X-ray diffraction, transmission electron microscopy, Zeta potential, Fourier transform infrared spectroscopy, and vibrating sample magnetometer. CA successfully modified the Fe 3 O 4 surface to obtain a stabilized (homogeneous and well dispersed) aqueous colloidal solution. The Zeta potential value of the as-prepared Fe 3 O 4 @CA increases from − 31 to − 45 mV. These CA-functionalized NPs with high magnetic saturation (54.8 emu/g) show promising biomedical applications.