(a) Schematic illustration of the construction of several H2S-activatable NIR-II fluorescence probes WH-X (WH-1, WH-2, WH-3, and WH-4). (b) Theoretical calculations used to verify the design strategy. The HOMO and LUMO energy levels of WH-X-HS were computed. (c) Fluorescence spectra of WH-4 in the presence of different concentrations of NaHS under the irradiation of 980 nm laser. (d) Intracellular fluorescence imaging of HCT-116 cells incubated with WH-3, WH-3 + NaHS, and WH-3 + AOAA. (e) NIR-II fluorescence imaging of tumor-bearing mice at various time points after treatment with WH-3 and WH-3 + AOAA. Reproduced with permission [121]. Copyright 2021, American Chemical Society.