Fig. 4. The influence of other metal ions on fluorescent intensity of Q-CD-Cu(ii) system ([Q] = 1.0 × 10⁻⁵ mol L⁻¹, [Cu²⁺] = 2.5 × 10⁻⁶ mol L⁻¹, the concentration of other transition metal cations and rare earth cations was 1.0 × 10⁻⁵ mol L⁻¹, the concentration of alkali metal cations and alkali earth metal cations was 5.0 × 10⁻⁵ mol L⁻¹).