Fig. 7. Selectivity study of the CS/AuNPs@TDA for As³⁺ detection. The absorbance ratio (A₆₄₅/A₅₂₅) of the nanoprobe was plotted in the presence of a single analyte (As⁵⁺, Cu²⁺, Fe³⁺, Fe²⁺, Hg²⁺, Al³⁺, Cr³⁺, Cd²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Br⁻, Cl⁻, F⁻, SO₄²⁻, NO₃⁻ and PO₄²⁻) and a mixture (Mix) of As³⁺ and the other ions. All signals were compared with the signal for CS/AuNPs@TDA without cations and anions (blank) and As³⁺. The inset shows the corresponding colorimetric responses for ion detection. The concentrations of As³⁺, Cu²⁺ and Hg²⁺ were 0.25 mg L⁻¹ whereas the other cations were at 1.0 mg L⁻¹. The concentrations of the anions were 2.5 mg L⁻¹.