Multicore Manganese Ferrite Nanoparticles for Theranostics

Collective behavior mediated by the assembly of nanoparticles (NPs) can result in theranostic applications. Herein, multicore Mn-ferrite-based magnetic nanostructures of various sizes were obtained by tuning the ionic force in the colloidal media, facilitated by a magnetophoresis phase-separation process. The hydrodynamic diameters ranged from 55 nm for the preseparation sample (0 h) to 35 nm at 48 h post separation. X-ray diffraction patterns confirmed the spinel structure. The saturation magnetization decreased from 262.5 kA m^–1 at 0 h to 111.9 kA m^–1 at 48 h, indicating a decrease in size. Transmission electron microscopy images corroborate these results but also reveal that lower cluster sizes contained smaller NPs, changing from 15 ± 4 nm at 0 h to 5 ± 2 nm at 48 h. Optical studies revealed a blue-shift phenomenon and a decrease in absorbance with a decrease in size that is consistent with the lower photothermal conversion coefficient, while the band gap energy varied from 1.72 to 2.06 eV due to confinement effects. The longitudinal and transverse (r₂) relaxivities at 20 °C decreased from 20 mM^–1 s^–1 to 8 mM^–1 s^–1 and from 750 mM^–1 s^–1 to 300 mM^–1 s^–1 from 0 to 48 h, respectively. While the r₂ of the NPs at 0 h decreased at 37 °C to 568 mM^–1 s^–1, confirming that the clusters are in the motional averaging regime. The results are explained by outer sphere theory and suggest that the magnetic cluster is a thermally sensitive contrast agent with potential for MRI thermometry.