Colorimetric assay for lead ions based on the leaching of gold nanoparticles

A colorimetric, label-free, and nonaggregation-based gold nanoparticles (Au NPs) probe has been developed for the detection of Pb²⁺ in aqueous solution, based on the fact that Pb²⁺ ions accelerate the leaching rate of Au NPs by thiosulfate (S₂O₃^2-) and 2-mercaptoethanol (2-ME). Au NPs reacted with S₂O₃ ^2- ions in solution to form Au(S₂O₃) ₂^3- complexes on the Au NP surfaces, leading to slight decreases in their surface plasmon resonance (SPR) absorption. Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) data reveals the formation of Pb-Au alloys on the surfaces of the Au NPs in the presence of Pb²⁺ ions and 2-ME. The formation of Pb-Au alloys accelerated the Au NPs rapidly dissolved into solution, leading to dramatic decreases in the SPR absorption. The 2-ME/S₂O ₃^2--Au NP probe is highly sensitive (LOD = 0.5 nM) and selective (by at least 1000-fold over other metal ions) toward Pb²⁺ ions, with a linear detection range (2.5 nM-10 μM) over nearly 4 orders of magnitude. The cost-effective probe allows rapid and simple determination of the concentrations of Pb²⁺ ions in environmental samples (Montana soil and river), with results showing its great practicality for the detection of lead in real samples.