Label-free colorimetric sensing of cobalt(ii) based on inducing aggregation of thiosulfate stabilized gold nanoparticles in the presence of ethylenediamine

Abstract

As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co²⁺ in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S₂O₃²⁻) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co²⁺ first reacted with en and formed complexes of Co(en)₃²⁺ in aqueous solutions, which was followed by the oxidation of Co(en)₃²⁺ to Co(en)₃³⁺ by dissolved oxygen. Co(en)₃³⁺ then attacked S₂O₃²⁻ ligands adsorbed on the AuNPs' surfaces, forming positively charged (en)₂CoS₂O₃⁺ on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S₂O₃²⁻ and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD = 0.0 4 μM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu²⁺) toward Co²⁺ with a linear range from 0.1 to 0.7 μM. The cost-effective method allows rapid and simple determination of the concentrations of Co²⁺ ions in drinking water.