Selective and sensitive colorimetric determination of cobalt ions using Ag–Au bimetallic nanoparticles

Abstract

Plasmonic nanoparticle-based colorimetric sensing has gained wide attention due to its advantages including rapidity, simplicity and expensive instrument-free characters. In the present study, Ag–Au bimetallic nanoparticles were proposed as novel optical probes to determine cobalt ion (Co²⁺) concentration. The sensing mechanism is based on the fact that Co²⁺ can react with ethylenediamine (en) and S₂O₃²⁻ to form (en)₂CoS₂O₃⁺ on the nanoparticle surface, which can decrease the negative charge of the nanoparticle, reduce the repulsion between adjacent nanoparticles, and subsequently induce nanoparticle aggregation, was verified by dynamic light scattering. The effects of several parameters on the detection sensitivity, including the concentration of S₂O₃²⁻ and en, pH value and incubation time, were investigated. The assay demonstrated high selectivity toward Co²⁺ in the presence of K⁺, Cu²⁺, Pb²⁺, Ag⁺, Zn²⁺, Ba²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Ni²⁺ and Hg²⁺. Under optimal conditions, the absorbance ratio of A₆₀₀/A₄₃₄ showed a linear relationship versus Co²⁺ concentration from 0 to 0.6 μM with a correlation coefficient (R²) of 0.980, and the detection limit was calculated to be 0.02 μM. The practical application of the proposed nanoprobes was evaluated with river and tap water samples.