Magnetic nanoparticles (MNp) have attracted great interest in the biomedical field due to the new perspectives that are forecasted in biomedicine, especially in magnetic targeted drug delivery and in magnetic fluid hyperthermia (MFH). Oxidebased spinel ferrites are very promising for hyperthermic treatment, and iron oxides are good candidates due to their wellknown biocompatibility.

However, the use of different materials with larger magnetic anisotropy and larger magnetic moments is envisioned, since it could allow a significant improvement of the material efficiency forMFH.The spinel cobalt ferrite CoFe2O4 has already been proposed for biomedical applications, and it is known to have large anisotropy compared to other oxide ferrites. As a consequence, the magnetic moment of cobalt ferrite relaxes much slower than that in magnetite nanoparticles of similar size. This means that, in principle, smaller cobalt ferrite particles can be used instead of iron oxide, allowing the assembly of smaller biocompatible devices that are known to promote cellular uptake and to better avoid the reticuloendothelial system.

Cobalt ferrite nanoparticle (NP) synthesis has been reported on a laboratory scale, but their use in medicine has not been possible because of numerous problems such as poor accessibility of the surface due to the presence of surfactant, aggregation in solution, and the remarkable amount of cobalt release in aqueoussolutions.

Cobalt ferrite nanoparticles (CoFe-1) have been produced via the following polyol method on a scale up to 1 kg cobalt acetate (2.88 mol) and iron acetate (2 equiv) were solubilized in DEG (20 kg) at 110 °C for 1 h, and then the solution was heated to 180 °C. After 3 h, the product was air cooled to room temperature and then stored (Scheme 1). Transmission electron microscopy (TEM, 100 keV) of CoFe-1 showed a uniform  dispersion of nanoparticles with a log-normal distribution(number) and with an average diameter of 5.4 nm (Figure 1).

The dispersion remained stable for more than 1 year, as proved by dynamic light scattering (DLS). DLS also showed a uniform size distribution with an average (volume) diameter of 13.2 nm. For comparison with the TEM observations, we have used the calculations suggested by Thomas to transform a dynamic light scattering average diameter into the number mean diameter that would result from TEM observations. Using the transformation with the polydispersity index (PDI ) 0.19), we obtained 5.56 nm, which is in very good agreement with the TEM value. The X-ray diffraction spectra indicated the formation of a cobalt ferrite phase, and the Sherrer analysis of the (311) peak gave an average diameter of 5.5 nm for the crystallites, which is again in good agreement with the TEM observations (the Sherrer analysis assumes the particles as spherical, and this could explain the small differences).

© 2007 American Chemical Society

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