Fig. 1. Scheme of the reaction between FPEI/EY and SHMP

Fig. 2. UV-Vis absorption and Fluorescence emission spectra of FPEI under different excitation wavelengths

Fig. 3. Fluorescence spectra of the reaction between FPEI/EY (a) and SHMP (b)
Concentration: EY: 4.0 μmol·L^-1; FPEI, 1.0 mL; pH 3.93; λ_ex=340 nm

Fig. 4. FTIR spectra of FPEI (a) and absorption spectra of the reaction between FPEI/EY and SHMP (b)

Fig. 5. Effect of excitation wavelength on the fluorescence intensity
FPEI: 1.0 mL; EY: 4 μmol·L^-1; pH 3.93

Fig. 6. Effect of pH on the reaction between FPEI/EY and SHMP
FPEI: 1.0 mL; EY: 4 μmol·L^-1; SHMP: 4 μmol·L^-1; λ_ex=340 nm

Fig. 7. Effect of formaldehyde on the fluorescence increasing of EY by SHMP
FPEI: 1.0 mL; EY: 4 μmol·L^-1; SHMP: 2.4 μmol·L^-1; pH 3.93

Fig. 8. Effect of EY on the reaction between FPEI/EY and SHMP
FPEI: 1.0 mL; EY: 4 μmol·L^-1; SHMP: 2.4 μmol·L^-1; λ_ex=340 nm; pH 3.93

Fig. 9. Plots of fluorescence intensity at 540 nm and F₅₄₀/F₄₇₀via the concentrations of SHMP
FPEI: 1.0 mL; pH 3.93, λ_ex=340 nm