Composition Tunable Manganese Ferrite Nanoparticles for Optimized T₂ Contrast Ability

Manganese-doped magnetite nanoparticles as magnetic resonance imaging (MRI) contrast agents have been well developed in recent years due to their higher saturation magnetization and stronger transverse (T₂) contrast ability compared to parent magnetite. However, the underlying role that manganese doping plays in altering the contrast ability of magnetite is still not thoroughly understood. Herein, we investigate the effects of manganese doping on changes of ferrite crystal structures, magnetic properties, and contrast abilities. We developed a successful one-pot synthesis of uniform manganese-doped magnetite (Mn_xFe_3–xO₄) nanoparticles with different manganese contents (x = 0–1.06). The saturation magnetization and T₂ contrast ability of ferrite nanoparticles increase along with rising manganese proportion, peak when the doping level of Mn_xFe_3–xO₄ reaches x = 0.43, and decrease dramatically as the manganese percentage continues to augment. At high manganese doping level, the manganese ferrite nanoparticles may undergo lattice distortion according to analysis of XRD patterns and lattice distances, which may result in low saturation magnetization and eventually low T₂ contrast ability. The Mn_xFe_3–xO₄ nanoparticles (x = 0.43) with a diameter of ∼18.5 nm exhibit the highest T₂ relaxivity of 904.4 mM^–1 s^–1 at 7.0 T among all the samples and show a much stronger T₂ contrast effect for liver imaging than that of other iron oxide contrast agents. These results indicate that the optimized T₂ contrast ability of manganese ferrite nanoparticles could be achieved by tuning the manganese doping level. This work also opens a new field of vision for developing high-performance T₂ contrast agents by modulating the metal composition of nanoparticles.