A colorimetric approach for measuring mercuric ions with high selectivity using label-free gold nanoparticles and thiourea

Abstract

We report a simple colorimetric assay for Hg²⁺ measurements using label-free gold nanoparticles (Au NPs, 13.0 ± 1.6 nm) with the aid of thiourea (TU). The addition of TU into Au NPs resulted in aggregation of Au NPs with a red-to-blue color change. While in the presence of Hg²⁺, it inhibited the aggregation of Au NPs induced by TU, resulting in a reverse color change from blue to purple and finally to wine red. These color variations are dependent on the Hg²⁺ concentration and can be utilized for the colorimetric sensing of Hg²⁺. This assay requires only 3 min to reach a naked-eye detection limit of 100 nM, which satisfies the guideline concentration of Hg²⁺ (250 nM) in industrial water regulated by the EPA. And the concentration of Hg²⁺ is quantitatively determined using a UV-vis spectrophotometer with a limit of detection (LOD) of 40 nM. The response of the assay caused by Hg²⁺ (2 μM) was 6.0 to 9.1 times higher than those induced by other metal ions (100 μM), indicating its high specificity towards Hg²⁺. The assay has been applied for the analysis of Hg²⁺ in drinking and lake water samples with recoveries in the range of 98.3–120.0%.