Enhancing NIR-II Phosphorescence through Phosphorescence Resonance Energy Transfer for Tumor-Hypoxia Imaging

Phosphorescence probes have emerged as a promising hypoxia detector for their excellent characters of long luminescence lifetime, large Stokes shift, and oxygen sensitivity. However, the low quantum yields of organic phosphorescence probes in the second near-infrared wavelength window (NIR-II, 1000–1700 nm) hinder their further development of NIR-II hypoxia imaging. Herein, this study reports a new NIR-II phosphorescence probe (PRET NPs) containing a phosphorescent long-lived triplet energy donor (PD) and a NIR-II fluorescent energy acceptor (FA) by using phosphorescence resonance energy transfer (PRET) strategy. Because of the suitable spectral overlap of PD and FA, PRET NPs exhibit high NIR-II quantum yield in both normoxic and oxygen-free atmospheres. Based on that, in the in vivo tumor hypoxia imaging experiments, PRET NPs also exhibit high brightness and signal-to-background ratio (SBR) of NIR-II tumor-hypoxia imaging. Therefore, this study illustrates a general approach based on "PRET" strategy to develop bright NIR-II phosphorescent probes.