Fig. 1. (a) TEM image of GQDs; (b) histogram of size distribution; (c) XRD pattern of GQDs compared with JCPDS 75-1621 standard; (d) TEM image of GQDs/EDC composites

Fig. 2. FTIR spectra of CA and GQDs

Fig. 3. (a) PL spectra of EDC, PBS, GQDs and GQDs/EDC and (b) its local magnification

Fig. 4. Fluorescence spectra of (a) GQDs and (c) GQDs/EDC at different excitation wavelengths, and excitation spectra of (b) GQDs and (d) GQDs/EDC at different detection wavelengths

Fig. 5. (a) Absorption spectra of EDC, GQDs, GQDs/EDC, and excitation spectrum of GQDs/EDC at 460 nm detection wavelength. (b) Absorption spectrum of GQDs and excitation spectrum at 460 nm detection wavelength. Transient PL decay curves of (c) GQDs and (d) GQDs/EDC at different detection wavelengths. The final volume of each sample was 2 mL and EDC content is 50 µL, 15.28 mg/mL

Fig. 6. (a) Fluorescence spectra of GQDs/EDC with different EDC contents (0−1700 µL), and (b) the trend of fluorescence integrated intensity with EDC content. (c) The integrated PL intensity varies with the reaction time of GQDs/ EDC with the pH value of 7.2 and 2.3, respectively

Fig. 7. (a) Fluorescence spectra (λ_ex=360 nm) and (c) absorption spectra of EDC and GQDs at different pH values after 10 min mixed reaction. (b) Integrated fluorescence and (d) absorption intensity as a function of pH value, after removing the baseline. (e) Bright-field and (f) dark-field images of GQDs/EDC under different pH environments with excitation by a 365 nm light source

Fig. 8. (a) Transient fluorescence attenuation curves of GQDs/ EDCs under different EDC contents and (b) different pH conditions

Fig. 9. (a) Selective tests, the fluorescence intensity response of GQDs/EDC to H⁺ and other commonly used ions (5 mM), the Blank group was only PBS (pH 7.2) solution. (b) The survival rate of MCF-7 cells was determined by MTT method after 24 h co-culture with different concentrations of GQDs/ EDC