Fig. 1. Synthesis route of probe H2

Fig. 2. UV‒vis absorption spectra of probe H2. (a) UV‒vis absorption spectra of H2 (10 µmol/L) in coexistence with Na₂S₂O₄ (0‒280 µmol/L) in Tris buffer; (b) UV‒vis absorption spectra of H2 (10 µmol/L) in coexistence with diverse analytical substances and Na₂S₂O₄ (280 µmol/L)

Fig. 3. Fluorescence stability and fluorescence titration of probe H2. (a) Changes of fluorescence intensities of H2 (10 µmol/L) with time in Tris-HCl buffer solution; (b) emission spectra of H2 (10 µmol/L) in coexistence with Na₂S₂O₄ (0‒280 µmol/L) in Tris-HCl buffer with λ_ex=380 nm and λ_em=455 nm, the inset shows color changes of H2 (10 µmol/L) (Ⅰ) without and (Ⅱ) with Na₂S₂O₄ (280 µmol/L) under 365 nm UV lamp irradiation at the wavelength of 455 nm

Fig. 4. Fluorescence selectivity and competitive test of probe H2. (a) Emission spectra of H2 (10 µmol/L) in coexistence with various analytes and Na₂S₂O₄ (280 µmol/L); (b) emission responses of H2 (10 µmol/L) towards Na₂S₂O₄ (280 µmol/L) in coexistence with the aforesaid analytical substances (280 µmol/L) with λ_ex=380 nm and λ_em=455 nm (1—ONOO^-, 2—HS^-, 3—Br^-, 4—Cl^-, 5—F^-, 6—HSO3-, 7—SO42-, 8—S2-, 9—NO2-, 10—NO3-, 11—P₂O74-, 12—PO43-, 13—SO32-, 14—GSH, 15—Hcy, 16—Cys, 17—H₂O₂, 18—HClO)

Fig. 5. Detection limit and fluorescence kinetic curves of probe H2. (a) Linear curve between emission intensity of H2 (10 µmol/L) at 455 nm and Na₂S₂O₄ concentrations (0‒250 µmol/L); (b) time-depending fluorescence spectra of probe H2 (10 µmol/L) under different concentrations of Na₂S₂O₄ [(1)—0 µmol/L, (2)—70 µmol/L, (3)—140 µmol/L, (4)—210 µmol/L, (5)—280 µmol/L]

Fig. 6. Job’s plot and Benesi‒Hildebranddate fitting of probe H2. (a) Job’s plot gradually adding CoCl₂·6H₂O (10 µmol/L) to L₂ (10 µmol/L) to the emission intensity at 455 nm; (b) Benesi‒Hildebrand data fitted in a 2∶1 coordination mode with the emission intensity at 455 nm in the above solution

Fig. 7. Magnetic resonance properties of probe H2. (a) Changes in the relaxivity (r₂-r)/r of H2 (10 µmol/L) with Na₂S₂O₄ concentration (0‒280 µmol/L), and the insert shows T₂‒MRI images of H2 under different concentrations of Na₂S₂O₄ (0, 200, 280 µmol/L); (b) relaxivity (r₂-r)/r₂ responses of H2 (10 µmol/L) to various analytes (280 µmol/L), including 1—S₂O42-, 2—HS^-, 3—Br^-, 4—Cl^-, 5—F^-, 6—HSO3-, 7—S2-, 8—NO2-, 9—NO3-, 10—P₂O74-, 11—PO43-, 12—SO32-, 13—SO42-, 14—GSH, 15—Hcy, 16—Cys, 17—ONOO^-, 18—HClO, and 19—H₂O₂

Fig. 8. Cytotoxicity of A549 cells after incubation with different concentrations of H2 by MTT assay

Fig. 9. Effect of pH on probe H2 identification of Na₂S₂O₄. (a) Fluorescence emission intensity of H2 (10 µmol/L) and in coexistence with Na₂S₂O₄ (280 µmol/L) at 455 nm in the pH range of 3.0‒11.5; (b) relaxivity of H2 (10 µmol/L) and in coexistence with Na₂S₂O₄ (280 µmol/L) in the pH range of 3.0‒11.5

Fig. 10. Fluorescence imaging of H2 (50 µmol/L, 100 µL) to exogenous Na₂S₂O₄ (20 mmol/L, 15 µL) in nude miceover time and average fluorescence intensity

Fig. 11. Sagittal and axial graphs of T₂-weighted imaging and corresponding pseudocolor images in vivo with time gradient of probe H2 (50 µmol/L,100 µL) to exogenous Na₂S₂O₄ (20 mmol/L, 15 µL)

Fig. 12. Sensing mechanism of probe H2 for the recognition of Na₂S₂O₄

Fig. 13. Cyclic V-I curves before and after H2 recognition of Na₂S₂O₄