Novel systems based on suspensions of colloidal magnetic nanoparticles have been investigated as perspective superparamagnetic contrast agents (CA) for magnetic resonance imaging (MRI). The nanostructures that we have studied contain surfactant-capped magnetite (Fe3O4) inorganic cores with different controlled sizes, ranging from 5.5 to 12 nm. The as-synthesized nanostructures are passivated by hydrophobic surfactants and thus are fully dispersible in nonpolar media. The magnetic nanocrystals have been transferred into aqueous media by a procedure based on the surface intercalation and coating with an amphiphilic polymer shell. The MRI efficiency in contrasting images, i.e., the NMR relaxivities r1 and r2, have been compared with Endorem and Sinerem, commercial superparamagnetic MRI contrast agents. We found that our nanostructures exhibit r1 and r2 relaxivities comparable to those of commercial CA over the whole frequency range. The MRI efficiency of our samples was related to their microstructural and magnetic properties. The transverse relaxivity r2, leading the contrast in “negative” superparamagnetic agents, was found to improve as the diameter of the inorganic core is increased. The NMR relaxometry profile confirmed the nature of the physical mechanisms inducing the increase of the nuclear relaxation rates at low (magnetic anisotropy) and high (Curie relaxation) fields.
The atomic level structure of a series of monodisperse single crystalline nanoparticles with a magnetic core of manganese ferrite was studied using X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) techniques at both the Fe and Mn K-edges, and conventional and high resolution transmission electron microscopy (TEM and HRTEM). In particular, insights on the non-stoichiometry and on the inversion degree of manganese ferrite nanocrystals of different size were obtained by the use of complementary structural and spectroscopic characterization techniques. The inversion degree of the ferrite nanocrystals, i.e. the cation distribution between the octahedral and tetrahedral sites in the spinel structure, was found to be much higher (around 0.6) than the literature values reported for bulk stoichiometric manganese ferrite (around 0.2). The high inversion degree of the nanoparticles is ascribed to the partial oxidation of Mn(2+) to Mn(3+) which was evidenced by XANES, leading to non-stoichiometric manganese ferrite.
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