A label-free colorimetric detection of lead ions by controlling the ligand shells of gold nanoparticles

We have developed a simple, colorimetric and label-free gold nanoparticle (Au NP)-based probe for the detection of Pb2+ ions in aqueous solution, operating on the principle that Pb2+ ions change the ligand shell of thiosulfate (S2O32−)-passivated Au NPs. Au NPs reacted with S2O32− ions in solution to form Au+·S2O32− ligand shells on the Au NP surfaces, thereby inhibiting the access of 4-mercaptobutanol (4-MB). Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI–TOF MS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements revealed that PbAu alloys formed on the surfaces of the Au NPs in the presence of Pb2+ ions; these alloys weakened the stability of the Au+·S2O32− ligand shells, enhancing the access of 4-MB to the Au NP surfaces and, therefore, inducing their aggregation. As a result, the surface plasmon resonance (SPR) absorption of the Au NPs red-shifted and broadened, allowing quantitation of the Pb2+ ions in the aqueous solution. This 4-MB/S2O32−–Au NP probe is highly sensitive (linear detection range: 0.5–10 nM) and selective (by at least 100-fold over other metal ions) toward Pb2+ ions. This cost-effective sensing system allows the rapid and simple determination of the concentrations of Pb2+ ions in real samples (in this case, river water, Montana soil and urine samples).