Facile synthesis of ultrasmall MnFe2O4 nanoparticles with high saturation magnetization for magnetic resonance imaging

The optimal particle size and surface states are important to superparamagnetic nanoparticles for magnetic resonance imaging (MRI) contrast agents. Here, a simple one-pot approach is proposed to prepare ultrasmall hydrophobic MnFe 2 O 4 superparamagnetic nanoparticles with a uniform diameter of about 4.7 nm. To satisfy the requirements for MRI contrast agents, the MnFe 2 O 4 nanoparticles are reduced by chitosan to form MnFe 2 O 4 @chitosan nanoparticles (MnFe 2 O 4 @CS NPs), which dramatically increases the saturation magnetization (M s ) of the nanoparticles by 21%. The valence state analysis indicates that the amino group (NH 2 ) of chitosan shares lone pair electrons with Mn 4+ , resulting in high-spin state Mn 3+ to enhance the M s . The MnFe 2 O 4 @CS NPs show significant signal reduction and high r 2 value in the T 2 -weighted imaging. Therefore, due to their large specific surface area and high responsiveness to the external magnetic field, MnFe 2 O 4 @CS NPs are expected to be applied to nano platform in magnetic targeting therapy with the function of MRI contrast agents in vivo.