Fig. 7. (a) Quenching curves of CDs upon exposure to different concentration of Fe³⁺; (b) the linear region of the CDs-Fe³⁺ sensing response ([Fe³⁺]: 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 10, 15, 20, 25, 30, 35, 40, 45, 50, 100, 150, 200, 250 and 300 μmol L⁻¹, respectively); (c) images of the CDs before (left) and after (right) treatment with Fe³⁺ under sunlight and UV light, respectively; (d) fluorescence images of aqueous CDs after adding Fe²⁺ and Fe³⁺ ions; the on/off fluorescence was achieved by oxidation.