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Designing luminescence lifetime sensors in the second near‐infrared (NIR‐II) window is a great challenge due to the difficult structural construction. Here, we report a tumor redox responsive and easily synthesized material, amorphous manganese oxide (MnOx) with indirect band gap of 1.02 eV, as an energy acceptor to build a luminescence resonance energy transfer (LRET) toolbox for universally regulating NIR‐I to NIR‐II luminescence lifetimes of lanthanide nanoparticles, in which energy transfer is based on matched energy gap instead of conventional overlapped spectra. We further utilize ytterbium (Yb3+)‐doped YbNP@MnOx as an NIR‐II luminescence lifetime sensor to realize in vitro quantitative redox visualization with relative errors under 5 % in samples covered with mouse skin. Furthermore, HepG2 cells and tumors with high redox state have been accurately distinguished by NIR‐II luminescence lifetime imaging. The quantified intracellular and intratumor glutathione (GSH) levels are highly consistent with the commercial kit results, illustrating the reliable redox visualization ability in biological tissue. [ABSTRACT FROM AUTHOR]

To improve the bioimaging signal‐to‐noise ratio (SNR), long‐term imaging capability, and decrease the potential biotoxicity, an in vivo cross‐linking strategy was developed by using sub‐10 nm, glutathione‐modified, lanthanide nanoprobes. After administration, the nanoprobes cross‐link in response to reactive oxygen species (ROS) at the inflamed area and enable the quick imaging of ROS in the second near‐infrared (NIR‐II) window. These nanoprobes could be rapidly excreted due to their ultra‐small size. This strategy may also be applied to other ultra‐small contrast agents for the precise bioimaging by in situ lesion cross‐linking. Ray of light: An in vivo, reactive oxygen species (ROS)‐responsive, cross‐linking strategy was developed utilizing glutathione‐modified, sub‐10 nm, ultra‐small nanoprobes to improve NIR‐II bioimaging and excretion properties. [ABSTRACT FROM AUTHOR]