In this paper, a phase transfer method is reported which was used to prepare ultrasmall manganese(II) sulfide (MnS) nanocrystals (approximately 5 nm) in which prefabricated MnS aggregations are transferred from cyclohexane into an aqueous solution of sodium citrate. The MnS nanocrystals capped with citrate were extremely stable in the aqueous solution. Furthermore, the smart MnS nanocrystals showed super good behavior as MRI contrast agents. The relaxivity of the nanocrystals at a low magnetic field was 3.4 mM−1 s−1, which was higher than most reported values for manganese oxide. The MnS nanocrystal agent could produce highly efficient T1-weighted magnetic resonance images in vivo at a low dose, which is only 1/8–1/16 that of the commercial contrast agent gadolinium–diethylenetriamine penta-acetic acid. The MnS nanocrystals clearly enhanced the magnetic resonance imaging (MRI) contrast for use with the liver and kidney in vivo. The metabolic process of MnS nanocrystals was also studied. The in vivo results demonstrate that the simply prepared and smart MnS nanocrystals had a high performance and were promising candidates for MRI T1 contrast agents.
