Optimization of gold nanoparticles conjugated with four thiol-based ligands for the colorimetric detection of cadmium ions in water samples

Abstract

This study optimized colorimetric gold nanoparticle (AuNP) probes conjugated with four thiol-based ligands, namely, 1,3,4-thiadiazole-2,5-dithiol (TDDT), 2-amino-5-(methylthio)-1,3,4-thiadiazole (AMTTD), mercaptopropionic acid with l-alanine (MNA–Ala), and N-acetyl-l-cysteine (NAC), for cadmium (Cd2+) detection by tuning ligand loading, pH, salt, and incubation. UV-vis spectroscopy, FTIR spectroscopy, and HR-TEM data supported probe formation and response trends. Among the four ligands, MNA–Ala–AuNPs exhibited the most reliable performance under the optimized conditions of 10 mM MNA, 1 mM Ala, pH 10, and 100 mM NaCl with a 60-min pre-incubation, yielding a linear A640/A520vs. log[Cd2+] fit (R2 = 0.9933). TDDT, AMTTD, and NAC showed only minor spectral/color changes. The enhanced Cd2+ sensitivity of MNA/Ala compared to TDDT, AMTTD, and NAC is rationalized by its flexible donor architecture, enabling effective Cd2+ chelation, unlike the rigid, sterically hindered, or partially masked thiol donors of the other ligands. MNA–Ala–AuNPs were used to identify the presence of Cd2+ in a household water sample collected from Kelapa Dua, Depok, and the analytical results indicated that the sample did not contain Cd2+.