Fluoride doped γ-FeO nanoparticles with increased MRI relaxivity.

Iron oxide nanoparticles (IONs) are being actively researched and experimented with as contrast agents for Magnetic Resonance Imaging (MRI), as well as image-directed delivery of therapeutics. The efficiency of an MRI contrast agent can be described by its longitudinal and transverse relaxivities, r and r. γ-FeO nanoparticles - doped with fluoride in a controlled manner and functionalised with citric acid - showed a 3-fold increase in r and a 17-fold increase in r in a magnetic field of 3 T and almost 6-fold increase in r and a 14-fold increase in r at 11 T. Following fluorination, PXRD shows that the crystal structure of γ-FeO is maintained, Mössbauer spectroscopy shows that the oxidation state of the Fe cation is unchanged and HREM shows that the particle size does not vary. However, magnetisation curves show a large increase in the coercive field, pointing towards a large increase in the magnetic anisotropy for the fluorinated nanoparticles compared to the un-doped γ-FeO nanoparticles. Therefore, a chemically induced increase in magnetic anisotropy appears to be the most relevant parameter responsible for the large increase in relaxivity for γ-FeO nanoparticles.