Abstract: The superparamagnetic CoFe₂O₄ nanoparticles with core size of 510nm were prepared by high temperature polyol process using FeCl₃·6H₂O, CoCl₂·6H₂O and HOOC-PEG-COOH as reactants. The samples had good monodispersibility in aqueous solution. Moreover, the influences of reaction conditions such as varieties of modification agents and their amounts, reaction temperature and time on the size, dispersibility and magnetic properties of CoFe₂O₄ nanoparticles were investigated. The results indicate that choosing the modification agent with strong polar group, increasing the amount of modification agent, elevating reaction temperature and extending reaction time are beneficial to obtain samples with larger particle size, better waterdispersibility and narrower size distribution. The optimum reaction condition is determined as follows, the weight ratio of metal source to HOOC-PEG-COOH is 1∶10, reaction temperature is 210-220℃ and reaction time is 2h. Magnetic analysis shows that all the samples manifest a well-defined superparamagnetic behavior at room temperature. The larger the particle sizeis is, the higher saturation magnetizationis is.

Key words: high temperature polyol process, superparamagnetic CoFe₂O₄ nanoparticles, magnetic resonance imaging contrast agents