Ultrasmall PEGylated MnxFe3-xO4 (x=0-0.34) nanoparticles: effects of Mn(II) doping on T1-and T2-weighted magnetic resonance imaging

We report a facile synthesis of water-soluble, ultrasmall, PEGylated MnxFe3-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 similar to 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) 1 for x 0.34. Careful investigation of the Mn(II) doping effects on T-1- and T-2-weighted MRI reveals that T-2 contrast effects are enhanced while T-1 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.