Fig. 1. (a) The UV-Visible spectrum, excitation and emission spectrum of CDs; (b) The FT-IR spectrum of CDs

Fig. 2. XPS spectrum of the CDs

Fig. 3. (a) The change of CDs solution was mixed with Hg²⁺ aqueous solution with different concentration (0.0~75.0 μmol·L^-1); (b) Fluorescence intensities at 450 nm when the Hg²⁺@CDs was mixed with different concentrations of I^-

Fig. 4. (a) The fluorescence recovery ratios after they were mixed with other anions; (b) Interference experiments of aqueous Hg²⁺@CDs solution towards I^- (black bars) and other anions (striated bars), the concentration of other ion was detected by fluorescence spectrometer