Fig. 3. (a) UV-vis absorption (blue line), PL excitation (red line), and emission (green line) spectra of PNCDs in aqueous solution (0.30 mg mL⁻¹). (b) Excitation-dependent PL behavior of PNCDs. (c) 3D wall spectra of the PNCDs with different excitation wavelength. (d) Fluorescence emission spectra of PNCDs at different conditions: PNCDs (black line), PNCDs + Cr₂O₇²⁻ (red line), PNCDs + Cr₂O₇²⁻ + DA (blue line).