Fig. 1. Synthetic route of 7-hydroxy-4-methylcoumarin-8-formaldehyde

Fig. 2. Synthetic route of probe AFY

Fig. 3. Ultraviolet absorption spectra and solution coloration chart of AFY solution (10 μmol/L) upon addition of various cations (1‒16 represent AFY, Fe³⁺, Cd²⁺, Cu²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Al³⁺, Ni²⁺, Co²⁺, Hg²⁺, Ba²⁺, Cr³⁺, Ag⁺, Fe²⁺, and Pb²⁺) (60 μmol/L) in DMSO/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L). (a) Ultraviolet absorption spectra; (b) solution coloration chart

Fig. 4. Visible absorption spectra of AFY solution (10 μmol/L) in the presence of Fe³⁺ (0‒60 μmol/L) in DMF/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (lower left illustration shows solution coloration chart of AFY after adding Fe³⁺; upper right illustration shows absorbance curve of probe AFY with concentration of Fe³⁺ at 465 nm)

Fig. 5. Fluorescence stability of AFY solution (10 μmol/L) in DMF/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (excitation wavelength λ_ex=465 nm, emission wavelength λ_em=630 nm)

Fig. 6. Fluorescence spectra and fluorescence color of AFY solution (10 μmol/L) upon addition of various cations (1 represents AFY solution which is used as control group, and 2‒16 represent Fe³⁺, Cd²⁺, Cu²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Al³⁺, Ni²⁺, Co²⁺, Hg²⁺, Ba²⁺, Cr³⁺, Ag⁺, Fe²⁺, and Pb²⁺) (60 μmol/L)(65 μmol/L) in DMSO/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (excitation wavelength λ_ex=465 nm). (a) Fluorescence spectra; (b) fluorescence coloration chart

Fig. 7. Fluorescent response time of recognition of Fe³⁺ (65 μmol/L) by AFY solution (10 μmol/L) in DMF/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (λ_ex=465 nm, λ_em=610 nm)

Fig. 8. Fluorescence spectra of AFY solution (10 µmol/L) in presence of Fe³⁺ (0‒65 µmol/L) in DMSO/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (lower right illustration shows fluorescence color of probe AFY solution before and after adding Fe³⁺, and upper right illustration shows fluorescence intensity of probe AFY solution with Fe³⁺ concentration at 610 nm) (λ_ex=465 nm)

Fig. 9. Fluorescence intensity diagram of adding Fe³⁺ (65 μmol/L) to probe AFY solution (10 μmol/L) containing other competing ions (1‒15 represent blank, Cd²⁺, Cu²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Al³⁺, Ni²⁺, Co²⁺, Hg²⁺, Ba²⁺, Cr³⁺, Ag⁺, Fe²⁺, and Pb²⁺,respectively) (60 μmol/L) in DMSO/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (λ_ex=465 nm, λ_em=610 nm)

Fig. 10. Fluorescence intensity of probe AFY solution (10 μmol/L) as a function of Fe³⁺ concentration (5‒55 μmol/L) in DMF/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L) (λ_ex=465 nm, λ_em=610 nm)

Fig. 11. Job curve and Benesi-Hildebrand curve for probe AFY recognizing Fe³⁺. (a) Job plot of recognization of Fe³⁺ by probe AFY in DMF/HEPES (ratio of volume fractions is 2∶8, pH=7.4, 20 mmol/L); (b) Benesi-Hildebrand plot of recognization of Fe³⁺ by AFY (10 μmol/L) (λ_ex=465 nm, λ_em=610 nm)

Fig. 12. Effect of pH on fluorescence response of probe AFY (10 μmol/L) in DMF/HEPES (ratio of volume fractions is 2∶8, 20 mmol/L) (λ_ex=465 nm, λ_em=610 nm)

Fig. 13. HR-MS of probe AFY

Fig. 14. HR-MS of Fe³⁺ recognitized by probe AFY

Fig. 15. Response mechanism of probe AFY to Fe³⁺

Fig. 16. Adding Fe³⁺ with different concentrations to probe AFY under sunlight. (a1)‒(a5) Naked-eye colorimetry of Fe³⁺ solution prepared with pure water; (b1)‒(b5) naked-eye colorimetry of Fe³⁺ solution prepared with tap water

Fig. 17. Chromatographic silica gel plate loaded with probe AFY under sunlight and “open eye” colorimetric paper soaked in Fe³⁺ solutions of different concentrations. (a1)‒(a5) Fe³⁺ solution prepared with pure water; (b1)‒(b5) Fe³⁺ solution prepared with tap water

Fig. 18. Variations of （R+G）/B of test paper with Fe³⁺ concentration. (a) AFY test paper in Fig. 17(a); (b) AFY test paper in Fig. 17(b)