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

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 (A(620)/A(523)) was linear with the Cd2+ concentration in the range of 2.0 x 10(-7) to 1.7 x 10(-6) M. Under optimum conditions (2.0 x 10(-5) M MNA, 2.0 x 10(-6) M L-Cys and 0.020 M NaCl at pH 10.0), the detection limit (3 sigma) of Cd2+ could be as low as 1.0 x 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).