Cathodic protected Mn 2+ by Na x WO 3 nanorods for stable magnetic resonance imaging-guided tumor photothermal therapy

The stability and safety of magnetic resonance imaging (MRI) contrast agents (CAs) are crucial for accurate diagnosis and real-time monitor of tumor development. Paramagnetic Mn as nonlanthanide metal ion has been widely studied for use in T -MRI CAs, but unfortunately, Mn can be oxidized by H O in tumor to nonparamagnetic Mn via a Fenton-like reaction. The concurrent loss of paramagnetism and production of toxic hydroxyl radical (OH) go against the basic requirment of CAs, thus restricting the further development of Mn -based CAs. Based on the different standard potential of W /W (~0.26 V) and Mn /Mn (~1.2 V), a "cathodic protection" strategy was exploited in Mn -doped Na WO nanorods (Na MnWO ), with W as the sacrificial anode and Mn as the protected cathode, to protect Mn from oxidation in tumor for stable MR contrast performance, as well as repress its Fenton-like reaction activity for good biosafety. Moreover, the tungsten bronze crystal structure endows Na MnWO with excellent near-infrared (NIR)-photothermal properties for effective tumor hyperthermia, without effect from the changed oxidation state of W. This "cathodic protection" strategy offers a new method for the development of reliable and hypotoxic biomaterials for stable imaging and therapeutic applications in clinic.