Colorimetric and Label-Free Optical Detection of Pb²⁺ Ions via Colloidal Gold Nanoparticles

The detection of the lead heavy metal (Pb) in water is crucial in many chemical processes, as it is associated with serious health hazards. Here, we report the selective and precise colorimetric detection of Pb²⁺ ions in water, exploiting the aggregation and self-assembly mechanisms of glutathione (GSH)-functionalized gold nanoparticles (GNPs). The carboxyl functional groups are able to create coordination complexes with Pb²⁺, inducing aggregation amongst the GSH-GNPs in the presence of Pb²⁺ due to the chelation of the GSH ligands. The resulting aggregation amongst the GSH-GNPs in the presence of Pb²⁺ increases the aggregate size depending on the available Pb²⁺ ions, affecting the plasmonic coupling. This causes a substantial shift in the plasmon wavelength to a longer wavelength side with increasing Pb²⁺ concentration, resulting in a red-to-blue colorimetric or visual change, enabling the instant determination of lead content in water.