Facile synthesis of superparamagnetic nickel-doped iron oxide nanoparticles as high-performance contrast agents for magnetic resonance imaging.

Small-sized iron oxide nanoparticles (IONPs) are an excellent alternative to clinical gadolinium-based contrast agents (GBCAs) in -weighted magnetic resonance imaging (MRI) due to their biosafety. However, their relaxation efficiency and contrast enhancement are not significant compared with those of GBCAs, and the nanomaterials face stability problems and difficulties in large-scale synthesis. Here, we designed and synthesized a series of 7 nm monodispersed biocompatible and superparamagnetic nickel-doped iron oxide nanoparticles (NiIO NPs) as safe contrast agents (CAs) for ultra-sensitive -weighted MR imaging. The nickel doping strategy transforms the iron oxide nanoparticles from traditional CAs to CAs. The NiFeO NPs exhibited a high longitudinal relaxivity () of 9.1 mM s, which is much higher than that of IONPs and 2.4 times that of clinical CAs (, 3.8 mM s). NiIO NPs enable high-resolution -weighted and -weighted dual-mode MR imaging with long circulation time. These results, in hand with the excellent colloidal stability and superior biosafety, demonstrate the potential use of NiIO NPs for accurate diagnosis in deep-tissue - dual-mode MR imaging.