Highly selective colorimetric assay for nickel ion using N-acetyl-l-cysteine-functionalized silver nanoparticles

A colorimetric assay based on silver nanoparticles (NAC-Ag NPs) capped with N-acetyl-l-cysteine (C₅H₉NO₃S, NAC) has been developed which exhibits highly selectivity towards Ni²⁺ over other cations such as Al³⁺, Cr³⁺, Fe³⁺, Co²⁺, Pb²⁺, Ba²⁺, Cd²⁺, Fe²⁺, Cu²⁺, Hg²⁺, Zn²⁺, Mn²⁺, Ag⁺, Mg²⁺, Ca²⁺, Na⁺, and K⁺ under specified conditions. Silver nanoparticles were prepared by reducing AgNO₃ with sodium borohydride (NaBH₄) in the presence of NAC. The infrared spectra suggested that NAC was successfully capped on the surface of the silver nanoparticles. In the presence of Ni²⁺, rapid aggregation of NAC-Ag NPs was induced along with color change from yellow to deep orange. The absorbance ratio (A ₅₅₀/A ₃₉₀) was linear with the concentration of Ni²⁺ in the wide range from 2 to 48 μM with a detection limit of 0.23 μM. The proposed method was applied successfully to the determination of Ni²⁺ in tap water samples, and the recoveries were from 92 to 106 %.