Colorimetric detection of Cd2+ using gold nanoparticles cofunctionalized with 6-mercaptonicotinic acid and l-Cysteine

Abstract

We have developed a colorimetric assay for the highly sensitive and selective detection of Cd²⁺ using gold nanoparticles (AuNPs) cofunctionalized with 6-mercaptonicotinic acid (MNA) and L-Cysteine (L-Cys) through the formation of an Au–S bond. In the presence of Cd²⁺, the aggregation of functionalized AuNPs occurred by means of a metal–ligand interaction that led to visible color changes. Most importantly, cofunctionalized AuNPs had better responses for Cd²⁺ than that functionalized by either MNA or L-Cys. Cd²⁺ could be detected by the colorimetric response of AuNPs that could be detected by the naked eye or a UV-vis spectrophotometer. The absorbance ratio (A₆₂₀/A₅₂₃) was linear with the Cd²⁺ concentration in the range of 2.0 × 10⁻⁷ to 1.7 × 10⁻⁶ M. Under optimum conditions (2.0 × 10⁻⁵ M MNA, 2.0 × 10⁻⁶ M L-Cys and 0.020 M NaCl at pH 10.0), the detection limit (3σ) of Cd²⁺ could be as low as 1.0 × 10⁻⁷ M. Interference experiments showed that Pb²⁺ and Cu²⁺ caused a slight interference for Cd²⁺ determination while other metal ions caused no interference. The proposed method was successfully applied to determine the concentration of Cd²⁺ in environmental samples (lake water).