(color online) (a) Emission spectra of WS₂ QDs in the presence of Cu²⁺ with concentrations ranging from 0 μM to 200 μM. (b) Plot of (F₀ − F)/F₀ of WS₂ QDs at 365 nm versus the concentrations of Cu²⁺ (inset: The calibration curve between concentration of Cu²⁺ and (F₀ − F)/F₀). (c) PL emission spectra of the WS₂ QDs at a pH range from 2 to 12. (d) Fluorescence intensity change of WS₂ QDs in the presence and absence of 100-μMCu²⁺ at different pH values.