Fig. 1. —OH-activated small molecule^[32]. (a) Structure and mutual transformation of Hydro-1080 and Et-1080; (b) absorption and fluorescence spectra of Hydro-1080 and Et-1080; (c) in vivo NIR-Ⅱ FLI of mice

Fig. 2. H₂O₂-activated small molecule^[34]. (a) Structure and fluorescence activation of TC-H₂O₂; (b) linear fit of fluorescence and photoacoustic signals to H₂O₂ concentration; (c) in vivo NIR-Ⅱ FLI of mouse; (d) MOST of mouse liver

Fig. 3. NIR-Ⅱ fluorescent probes based on ClO^-/HClO-activated small molecules^[37-38]. (a) Fluorescence activation mechanism of SPNP25; (b) NIR-Ⅱ FLI of SPNP25 at normal and inflammation sites; (c) construction and activation mechanisms of DCNP@SeTT; (d) NIR-Ⅱ ratiometric FLI of mouse tumors, inflammation sites, and rabbit osteoarthritis

Fig. 4. NO-activated small molecules^[39-40]. (a) Construction of AOSNP-NO and in vivo NIR-Ⅱ FLI; (b) fluorescence activation mechanism of QY-N; (c) MOST to detect the recovery of liver injury in mice

Fig. 5. ONOO^--activated small molecules and probes^[44-45]. (a) Structural transformation of IRBTP-B; (b) molecular structures of CX-1, CX-2 and CX-3; (c) absorption and fluorescence emission spectra of CX-1, CX-2 and CX-3; (d) schematic diagram of the detection mechanism of PN1100; (e) ratiometric images of livers of mice

Fig. 6. H₂S-activated small molecules^[49-52]. (a) Construction of NIR-Ⅱ@Si; (b) design and fluorescence activation of SBOD-2; (c) synthesis of SSS and its application in photothermal therapy of CRC guided by NIR-Ⅱ FLI; (d) design strategy of WH-X(WX-1, WX-2, WX-3, and WX-4)and their activated fluorescence emission spectra; (e) tissue penetration depth test of NIR-Ⅱ FLI; (f) NIR-Ⅱ fluorescence images of tumors with different sizes

Fig. 7. NO and H₂S dual-activated small molecule^[53]. (a) Fluorescence activation and fluorescence conversion mechanism of BOD-NH-SC; (b) repeatedly cycled S-nitrosation and transnitrosation processes revealed by fluorescence emission spectra; (c) visualization of the dynamic and alternating presence of NO and H₂S in living cells

Fig. 8. Enzyme-activated small molecules^[55-58]. (a) Activation mechanism of IR1048-MZ; (b) IR1048-MZ and its activated fluorescence emission spectra; (c) NIR-Ⅱ FLI of the tumor site after 14 h injection; (d) penetration depth of PAI in the longitudinal section of tumor site after 14 h injection; (e) photothermal therapy after activation of IR1048-MZ; (f) schematic diagram of the reaction between Q-NO₂ and NTR; (g) activation mechanism of BOD-M-βGal; (h) construction of HISSPNPs and its fluorescence activation process

Fig. 9. pH-activated small molecules^[61-62]. (a) Molecular structures and protonation process of Lyso880, Lyso1005, Lyso855, and Lyso950; (b) mechanism of CEAF probe's fluorescence lighting and enhancement in tumor cell; (c) NIR-Ⅱ FLI-guided tumor resection; (d) design and molecular structures of NIRⅡ-RT1‒4; (e) design and synthesis of NIRⅡ-RT-pH and its pH activation mechanism; (f) molecular structures of NIRⅡ-RT-Hg and NIRⅡ-RT-ATP; (g) in vivo imaging monitoring of ATP fluctuations

Fig. 10. Viscosity-activated small molecule^[63]. (a) Response mechanism of WD-X to viscosity; (b) in vivo imaging detection of liver viscosity changes of mice

Table 1. Summary of activatable NIR-Ⅱ small molecules and probes