Evaluation of polyol‐made Gd 3+ ‐substituted Co 0.6 Zn 0.4 Fe 2 O 4 nanoparticles as high magnetization MRI negative contrast agents

The structural, microstructural, and magnetic properties of ~5‐nm‐sized Co 0.6 Zn 0.4 Fe 2 −  x Gd x O 4 nanoparticles were investigated in order to evaluate their capability to enhance the magnetic resonance imaging contrast as high magnetization agents. A focus was made on the solubility of Gd 3+ cations within the spinel lattice. By coupling X‐ray diffraction to X‐ray fluorescence spectroscopy, we demonstrated that only a limited fraction of Gd 3+ can substitute Fe 3+ ions into the whole crystal structure and does not exceed 6 at.‐%. At this concentration, the room temperature (27°C) saturation magnetizations of the prepared superparamagnetic nanocrystals were found to be close to 80 emu g −1 . Coating these nanoparticles with hydrophilic dopamine ligands leads to the formation of ~50‐nm‐sized clusters in water. As a consequence, relatively high r 2 / r 1 ratios of transverse to longitudinal proton relaxivities and high r 2 values were measured in the resulting colloids at physiological temperature (37°C) for an applied magnetic field of 1.41 T: 33 and 188 mM −1  sec −1 , respectively, for the richest system in gadolinium. Moreover, after incubation with healthy human model cells (fibroblasts) at doses as high as 10 μg mL −1 , they induce neither cellular death nor acute cellular damage making the engineered probes particularly valuable for negative magnetic resonance imaging contrasting.