Abstract: Hydrophilic magnetic nanoparticles have been widely applied in biological sciences. In this work, we propose a convenient method to synthesize surface carboxylic magnetic nanoparticles. Firstly, hydrophobic magnetic nanoparticles coated with oleic acid were prepared from ferric chloride and ferrous chloride by co-precipitation method, with oleic acid as surfactant. Then the C＝C bond of oleic acid coated on the surface of particles was oxidized into -COOH by KMnO4 solution in situ, thereby gaining hydrophobic magnetic nanoparticles with monolayer carboxylic groups functioned. The magnetic nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), Zeta potential analyzer and so on. The results showed that magnetic nanoparticles surface was successfully carboxyl-functioned. The average diameter and the saturation magnetization of the nanoparticles were about 9nm and 64.5emu/g, respectively. The remnant magnetization and coercivity of the nanoparticles were nearly to be 0, which demonstrated it was classic superparamagnetic. The carboxyl-functioned magnetic nanoparticles can be dispersed into water to form ferrofluid, which are stable for more than 6 weeks without precipitation in the pH range of 7-10.

Key words: Magnetic nanoparticles, Hydrophilic, Carboxyl group, Ferrofluid