Mn(<scp>ii</scp>) chelate-coated superparamagnetic iron oxide nanocrystals as high-efficiency magnetic resonance imaging contrast agents

In this communication, a paramagnetic bifunctional manganese(II) chelate ([Mn(Dopa-EDTA)]2−) containing a catechol group is designed and synthesized. The catechol can bind iron ions on the surface of superparamagnetic iron oxide (SPIO) nanocrystals to form core–shell nanoparticles. Both 4 and 7 nm SPIO@[Mn(Dopa-EDTA)]2− show good water solubility, single-crystal dispersion, and low cytotoxicity. The study of the interplay between the longitudinal and transverse relaxation revealed that 4 nm SPIO@[Mn(Dopa-EDTA)]2− with lower r2/r1 = 1.75 at 0.5 T tends to be a perfect T1 contrast agent while 7 nm SPIO@[Mn(Dopa-EDTA)]2− with a higher r2/r1 = 15.0 at 3.0 T tends to be a T2 contrast agent. Interestingly, 4 nm SPIO@[Mn(Dopa-EDTA)]2− with an intermediate value of r2/r1 = 5.26 at 3.0 T could act as T1–T2 dual-modal contrast agent. In vivo imaging with the 4 nm SPIO@[Mn(Dopa-EDTA)]2− nanoparticle shows unique imaging features: (1) long-acting vascular imaging and different signal intensity changes between the liver parenchyma and blood vessels with the CEMRA sequence; (2) the synergistic contrast enhancement of hepatic imaging with the T1WI and T2WI sequence. In summary, these Fe/Mn hybrid core–shell nanoparticles, with their ease of synthesis, good biocompatibility, and synergistic contrast enhancement ability, may provide a useful method for tissue and vascular MR imaging.