Abstract
Fe${}_{3}$O${}_{4}$ nanoparticles have been investigated as they are biocompatible and their surface can be functionalized. We synthesized iron oxide nanoparticles using a water-in-oil microemulsion method. Bare and silica-coated iron oxide nanoparticles of a core size of 6 nm dispersed in ethanol have been investigated by means of x-ray absorption spectroscopy (XAS). Due to a dedicated experimental setup the particles can be measured directly in dispersion. XAS allows us to disentangle the contributions of the Fe${}^{2+}$ and Fe${}^{3+}$ ions and therefore to estimate the amount of Fe${}_{3}$O${}_{4}$ in the particles. In case of the silica coated particles a high amount of magnetite was obtained. In contrast, the bare nanoparticles showed indications of a further oxidation into $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$ even in dispersion.
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