Highly Selective Colorimetric Assay of Cu²⁺ Ions by Silver Nanoparticles Conjugated with 1-(2-mercaptoethyl)-1,3,5-Triazinane-2,4,6-Trione

A simple and sensitive colorimetric assay method for the determination of Cu²⁺ ions in aqueous samples was developed using silver nanoparticles functionalized with 1-(2-mercaptoethyl)-1,3,5-triazinane-2,4,6-trione (MTT-AgNPs). The Cu²⁺ ion appears to play the role of the metal center of the coordination complex, which can form N–Cu²⁺–O coordination bonds with the MTT-AgNPs, shorten their interparticle distance, and induce aggregation of the MTT-AgNPs. The binding site and sensing mechanism of Cu²⁺ to MTT-AgNPs were characterized by XPS and FT-IR spectroscopy. This aggregation results in a dramatic color change from yellow to dark red. Therefore, the concentration of Cu²⁺ ions in environmental samples can be quantitatively detected by the naked eye or by UV-Vis spectrometry when using the MTT-AgNP sensor. The sensitivity of the detection is significantly improved by modulating the solution to pH 9, leading to a more rapid color change in the optimized AgNP system. The absorption ratios (A₅₅₅/A₃₉₅) of the modified AgNP solution exhibited a linear correlation with the Cu²⁺ ion concentrations, with a limit of detection of 15.2 nM. This cost-effective detection system allows for the rapid and facile determination of the concentration of Cu²⁺ ions in aqueous samples.