Alpha-Lipoic Acid Functionalized Silver Nanoparticles for Colorimetric Detection of Copper Ion

A facile, low cost, rapid and easy-to-use colorimetric sensor for the detection of Cu²⁺ ion is described using alpha-lipoic acid (ALA) functionalized AgNPs. The prepared ALA-AgNPs are yellow, and spherical with an average particle size of 9.23 ± 3.36 nm. The addition of Cu²⁺ ion resulted in the aggregation of ALA-AgNP solution and caused a visible color change from yellow to orange-brown, which could be detected by the naked eye and UV-vis spectrophotometer. Parameters affecting the sensitivity of the sensor were investigated, including ALA concentrations, reaction times, reaction volumes and pH. Under optimal conditions, the colorimetric sensor could detect Cu²⁺ ion in the range of 0.625 × 10^-5 to 10 × 10^-5 M with a linear coefficient (r) of 0.9978. The limit of detection (LOD) and limit of quantification (LOQ) were 0.43 × 10^-5 M (4.3 μM) and 1.45 × 10^-5 M (14.5 μM), respectively, which is lower than the limit set by the U.S. Environmental Protection Agency (EPA). This proposed method showed good selectivity and could detect Cu²⁺ within 5 min. Finally, the proposed method could be successfully applied for the analysis of Cu²⁺ ion in tap water samples.