Ultrasmall PEGylated Mn_xFe_3-xO₄ (x = 0-0.34) nanoparticles: Effects of Mn(ii) doping on T₁- and T ₂-weighted magnetic resonance imaging

We report a facile synthesis of water-soluble, ultrasmall, PEGylated Mn_xFe_3-xO₄ nanoparticles (MFNPs) (x = 0-0.34) and the Mn(ii) doping effects on T₁- and T₂-weighted magnetic resonance imaging (MRI). By adjusting the reaction conditions, the 'x' value can be continuously tuned from 0 to 0.34. The produced MFNPs are of high crystallinity and size uniformity with an average diameter of ∼6 nm, which show excellent colloidal stability in H₂O, PBS, and tolerate a high salt concentration (1 M NaCl) and a wide pH range from 7 to 11. The results of FTIR demonstrate that both HOOC-PEG-COOH and TEG were modified on the nanocrystal surfaces. The saturation magnetization of the MFNPs gradually increases with increasing Mn²⁺ concentration and reaches 75.5 emu g^-1 for x = 0.34. Careful investigation of the Mn(ii) doping effects on T₁- and T₂-weighted MRI reveals that T₂ contrast effects are enhanced while T₁ contrast effects are weakened with the increase of the 'x' value of the MFNPs. Furthermore, the T₁ contrast effects of the MFNPs are concentration dependant. A concentration which is lower than 0.500 mM is needed for the MFNPs to act as T₁ and T₂ dual contrast agents at 3 T.