Effect of surfactant functionalization on Fe₃O₄aqueous ferrofluid stability and magnetic hyperthermia performance

Developing stable water-based ferrofluids with efficient magnetic heating remains a key challenge for biomedical hyperthermia. In this work, we functionalize Fe₃O₄nanoparticles with surfactants of different chemical nature namely ethylenediaminetetraacetic acid (EDTA), ethanolamine and betaine and examine the role of surface chemistry on colloidal stability and heating performance in aqueous media. High-resolution transmission electron microscopy study on these samples reveals particle sizes of ∼10-12 nm, indicating monocrystalline nanoparticles with a surface coating. We confirm the successful functionalization with different functional groups such as carboxylate, amine and zwitterionic groups by Fourier transform infrared spectroscopy, thermogravimetric analysis and x-ray photoelectron spectroscopy analysis. Zeta-potential values ranging from -18.56 to -45.2 mV indicate negatively charged nanoparticle surfaces, mainly arising from Fe-O^-groups and surfactant-derived functionalities that promote electrostatic stabilization in water. Vibrating sample magnetometer measurements confirm superparamagnetic behaviour with saturation magnetization of 55-63 emu g^-1indicating that the magnetic core structure is preserved. Among the studied systems, EDTA-functionalized Fe₃O₄exhibits exceptional long-term stability, retaining its ferrofluid state without sedimentation for over one year. This stable dispersion correlates with superior magnetic heating performance, achieving the highest specific absorption rate (∼245 W g^-1at 0.5 mg ml^-1). These results demonstrate that surface functionalization plays a critical role in stabilizing aqueous ferrofluids, enhance hyperthermia efficiency and provide a practical basis for designing biocompatible magnetic nanofluids.