Fig. 5. Fluorescence titration of Q-CD system and standard curve of fluorescence titration spectra with varying concentrations of Cu²⁺ in CH₃OH–PBS buffer solution ([Q] = 1 × 10⁻⁵ mol L⁻¹, from 1 to 38, [Cu²⁺] = (0, 0.5, 1.0, 3.0, 5.0, 7.0, 10.0, 13.0, 15.0, 17.0, 20.0, 23.0, 25.0, 27.0, 30.0, 33.0, 35.0, 37.0, 40.0, 43.0, 45.0, 47.0, 50.0, 53.0, 55.0, 57.0, 60.0, 63.0, 65.0, 67.0, 70.0, 73.0, 75.0, 77.0, 80.0, 81.0, 82.0, 83.0)× 10⁻⁷ mol L⁻¹, respectively.).