Fig. 3. The influence of different cations on fluorescent intensity of Q-CD system in CH₃OH–PBS buffer solution ([Q] = 1.0 × 10⁻⁵ mol L⁻¹, [Cu²⁺] = 1.0 × 10⁻⁵ mol L⁻¹, [CD] = 2.72 g L⁻¹, the concentration of other transition metal cations and rare earth cations was 2.0 × 10⁻⁵ mol L⁻¹, the concentration of alkali metal cations and alkali earth metal cations was 5.0 × 10⁻⁵ mol L⁻¹).