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Published online by Cambridge University Press: 01 February 2011
We report on high transversal relaxivity values of composite iron oxide-silica nanoparticles. To obtain the material, pre-formed maghemite nanoparticles were coated with silica by sol-gel chemistry, using supercritical fluids as the reaction media. The composite particles were monodisperse and consisted of a core of several maghemite nanoparticles, surrounded by a thick silica shell. The high pressure and high temperature process did not affect the iron oxide particle size but induced an increase on their saturation magnetization values, possibly due to an improvement of the particle crystallinity. These iron oxide-based materials present very high transversal relaxivity values which can be correlated to the magnetic moment and to the silica shell width of the composite particles. Moreover, composite particles are not cytotoxic and they are dispersable in polar solvents.