Cathodic protected Mn²⁺ by Na_xWO₃ nanorods for stable magnetic resonance imaging-guided tumor photothermal therapy.

Abstract

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 Mn²⁺ as nonlanthanide metal ion has been widely studied for use in T₁-MRI CAs, but unfortunately, Mn²⁺ can be oxidized by H₂O₂ in tumor to nonparamagnetic Mn⁴⁺ 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 Mn²⁺-based CAs. Based on the different standard potential of W⁶⁺/W⁵⁺ (~0.26 V) and Mn⁴⁺/Mn²⁺ (~1.2 V), a "cathodic protection" strategy was exploited in Mn²⁺-doped Na_xWO₃ nanorods (Na_xMnWO₃), with W⁵⁺ as the sacrificial anode and Mn²⁺ as the protected cathode, to protect Mn²⁺ 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_xMnWO₃ 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.