Single quantum dot-micelles coated with gemini surfactant for selective recognition of a cation and an anion in aqueous solutions

The synthesis of quantum dot coated with cetyltrimethyl ammonium bromide (CTAB) and gemini surfactant [C₁₂H₂₅N⁺(CH₃)₂(CH₂)₄(CH₃)₂N⁺C₁₂H₂₅]·2Br⁻ (C_12-4-12) in aqueous solution have been described. It is characterized by photoluminescent spectroscopy, UV–vis spectroscopy and transmission electron microscopy (TEM), etc. In comparison with CTAB-coated QDs, the QDs coated with C_12-4-12 respond selectively to both transition metal ion copper and fluoride ion in aqueous media. When Cu²⁺ is bound to C_12-4-12-coated QD micelles, the fluorescence intensity is quenched. Linear relationships are found between the relative fluorescence intensity and the concentration of Cu²⁺ in the range 0–500 μM, which is best described by a Stern–Volmer-type equation. Meanwhile, it is found that F⁻ enhanced the luminescence of the C_12-4-12-coated QD micelles in a concentration dependence that is described by a Langmuir binding isotherm equation in the range 0–300 μM. The limits of detection of Cu²⁺ and F⁻ are 1.1 and 0.68 μM, respectively. The possible mechanism is discussed.