Enhanced magnetic properties and MRI performance of bi-magnetic core-shell nanoparticles

Two sets of bi-magnetic Zn_0.5Mn_0.5Fe₂O₄@Fe₃O₄ core-shell nanoparticles were prepared by a seedmediated modified co-precipitation method. While the first set was obtained by fast addition of the alkaline solution to grow Fe₃O₄ shells over the ferrite seeds, a slow drop-wise addition of stoichiometric Fe²⁺/Fe³⁺ ion solution to the alkaline ferrite seeds solution was adopted to synthesize the second set. Samples were characterized by electron microscopy (STEM, TEM, UHRTEM) and magnetometry measurements. Viability MTT assay of the nanoparticles on L929 murine fibroblasts were performed, indicating that they are biocompatible. The coating of the Zn_0.5Mn_0.5FeO₄ nanoparticle by a magnetite or maghemite shell minimizes the effect of the magnetic dead layer at the core surface, improving the magnetic properties and offering thus outstanding values for biological application. Relaxometry values r₂ higher than 300 mM^-1 s^-1 at H 1.5 T, and cell viability at concentrations as high as 0.5 mg mL^-1 means these bi-magnetic nanoparticles have a vast potential as MRI contrast agents.