Fig. 4. (a) The fluorescence responses of the PNCDs in PBS (pH: 7.4) after the addition of different concentrations of Cr(vi) (0, 0.1, 0.2, 0.3, 0.5, 0.7, 1, 2, 3, 5, 7, 10, 15, 20, 25, 40, 50, 70, 90, 100, 120, 140, 160 μmol L⁻¹). (b) Plot of the (F₀ − F)/F₀ with the concentration of Cr(vi). (c) and (d) (F₀ − F)/F₀ of the PNCDs in PBS (pH: 7.4) with 100 μM different metal ions. F and F₀ correspond to the fluorescence intensities of the PNCDs with and without 100 μM of different metal ions (the numbers 1 to 17 are corresponding to blank, Ba²⁺, Ca²⁺, Co²⁺, Al³⁺, Fe³⁺, Fe²⁺, K⁺, Cu²⁺, Na⁺, Pb²⁺, Zn²⁺, Mg²⁺, Ni²⁺, MnO₄⁻, ClO₃⁻, ClO₄⁻, respectively).