Fig. 3. (A) Fluorescence spectra of S, N, O-CDs upon addition of various concentrations of Cu²⁺ from 0 to 140 μM. Insets represent green fluorescence changes of S, N, O-CDs in the presence of different concentrations of Cu²⁺. (B) Plots of (F₀ − F)/F₀ with different concentrations of Cu²⁺. Inset: linear calibration curve between (F₀ − F)/F₀ and various concentrations of Cu²⁺. F₀ and F represent the fluorescence of S, N, O-CDs in the absence and in the presence of Cu²⁺, respectively. (C) Fluorescence lifetime decay of S, N, O-CDs in the absence and in the presence of Cu²⁺. (D) Comparison of S, N, O-CDs after addition of various metal ions at a concentration of 100 μM.