Fig. 4. (A) Fluorescence response of CdSe QDs upon addition of Hg²⁺ ions. The arrow indicates the decrease in the fluorescence maximum with increasing concentration of Hg²⁺ ions. (B) Plot of change in fluorescence intensity at 550 nm versus Hg²⁺ ions. Linearity was observed in the concentration range of Hg²⁺ ions from 0–10 ppb. (C) Stern–Volmer plot display linearity between 0–30 ppb, suggesting the formation CdSe QDs–Hg²⁺ complex.