A phosphorescent probe for in vivo imaging in the second near-infrared window

In the second near-infrared spectral window (NIR-II; with wavelengths of 1,000–1,700 nm), in vivo fluorescence imaging can take advantage of reduced tissue autofluorescence and lower light absorption and scattering by tissue. Here, we report the development and in vivo application of a NIR-II phosphorescent probe that has lifetimes of hundreds of microseconds and a Stokes shift of 430 nm. The probe is made of glutathione-capped copper–indium–selenium nanotubes, and in acidic environments (pH 5.5–6.5) switches from displaying fluorescence to phosphorescence. In xenograft models of osteosarcoma and breast cancer, intravenous or intratumoral injections of the probe enabled phosphorescence imaging at signal-to-background ratios, spatial resolutions and sensitivities higher than NIR-II fluorescence imaging with polymer-stabilized copper–indium–sulfide nanorods. Phosphorescence imaging may offer superior imaging performance for a range of biomedical uses.