Year-long aqueous colloidal stability and enhanced AC magnetic heating of N,N-Dimethyl dodecyl amine N-Oxide (DDAO) functionalized magnetite ferrofluids

Combining durable colloidal stability with strong magnetic heating in a single aqueous ferrofluid (FF) is a long-standing obstacle in biomedical hyperthermia, because the coating that disperses the nanoparticles typically weakens their magnetic response. Aqueous magnetite FFs must therefore satisfy three requirements that are mediated by one nanoparticle surface and are consequently difficult to reconcile such as durable colloidal stability, retention of the magnetic response and efficient field-driven heating. Here, magnetite (Fe₃O₄) nanoparticles were synthesized by alkaline co-precipitation and cappedin situwith N,N-Dimethyl dodecyl amine N-Oxide (DDAO), an inexpensive zwitterionic amine-oxide surfactant. X-ray diffraction and HRTEM confirm a phase-pure, monocrystalline core of approximately 10.43 nm (particle size). FTIR and XPS verify the presence of DDAO and support coordination anchored through itshead-group oxygen to surface iron. The functionalized particles show markedly improved hydrophilicity and colloidal stabilization, with the water contact angle decreasing from 30.2° for the uncoated reference to 16.6° for DDAO-Fe₃O₄and the zeta potential becoming strongly negative at approximately -56.5 mV. The DDAO FF remains visually stable in water for one year, whereas the uncoated reference rapidly sediments. Although the non-magnetic organic shell and surface-spin perturbation moderately reduce the saturation magnetization from 63.62 to 53.81 emu g^-1, superparamagnetic behaviour is retained. DDAO-Fe₃O₄exhibits a higher SAR than the uncoated reference at every concentration, reaching 222.5 W g^-1at 0.5 mg ml^-1, about 40% above the reference at 443 Oe and 313 kHz. With closely matched magnetic cores, the heating enhancement is attributed primarily to improved dispersion, reduced aggregation-driven dipolar coupling and retention of active Néel and Brownian relaxation channels. MTT assays showed IC₅₀= 68.52µg ml^-1in HeLa cells, while L929 cells retained 79% viability at 100µg ml^-1, indicating comparatively lower normal-cell cytotoxicity. This work demonstrates that a single amine-oxide surfactant can integrate surface hydrophilicity, year-long aqueous stability and enhanced magnetic heating from one interfacial coordination in one practical FF formulation.