Superparamagnetic carbon-coated Fe_3O_4 nanoparticles with high magnetization(85 emu·g^(-1)) and high crystallinity were synthesized using polyethylene glycol-4000(PEG(4000)) as a carbon source.Fe_3O_4 water-based bilayer-surfactant-enveloped ferrofluids were subsequently prepared using sodium oleate and PEG(4000) as dispersants.Analyses using X-ray photoelectron spectroscopy,X-ray diffraction,and Fourier-transform infrared spectroscopy indicate that the Fe_3O_4 nanoparticles with a bilayer surfactant coating retain the inverse spinel-type structure and are successfully coated with sodium oleate and PEG(4000).Transmission electron microscopy,vibrating sample magnetometry,and particle-size analysis results indicate that the coated Fe_3O_4 nanoparticles also retain the good saturation magnetization of Fe_3O_4(79.6 emu·g^(-1)) and that the particle size of the bilayer-surfactant-enveloped Fe_3O_4 nanoparticles is 42.97 nm,which is substantially smaller than that of the unmodified Fe_3O_4 nanoparticles(486.2 nm).UV-vis and zeta-potential analyses reveal that the ferrofluids does not agglomerate for 120 h at a concentration of 4 g·L^(-1),which indicates that the ferrofluids are highly stable.
