Uniform and water stable magnetite nanoparticles with diameters around the monodomain-multidomain limit

A direct method for the preparation of uniform magnetite nanoparticles with sizes around 30 nm and stable in aqueous media at pH 7 has been developed. This method is based on the precipitation of an iron (II) salt (FeSO4) in the presence of a base (NaOH) and a mild oxidant (KNO3). Reaction rate seems to be controlled by the iron salt concentration and the presence of ethanol in the media. Thus lower iron concentration and a water/ethanol ratio equal to one lead to the formation of the smallest particles, 30 nm in diameter. Colloidal suspensions of these particles were directly obtained by simple ultrasonic treatment of the powders leading to very stable ferrofluids at pH 7. Sulphate anions present at the particle surface seem to be responsible for the colloidal stability, providing a biocompatible character to the suspensions. The structural, morphological and magnetic characterization of the nanoparticles is also described and suggests that the smallest particles have a diameter close to the limit between monodomain-multidomain magnetic structure, which could account for the high powder absorption of magnetic fields. According to this calorimetric experiments resulted in specific power absorption rates of ca 80-95 W g-1, which are among the highest values reported in the literature and make these nanoparticles very interesting for hyperthermia