A naked eye aggregation assay for Pb²⁺ detection based on glutathione-coated gold nanostars

Gold nanostars (AuNS) with a mean hydrodynamic size of 40 nm, obtained with a seed-growth approach using a zwitterionic surfactant (laurylsulfobetaine, LSB), were successfully coated with glutathione (GSH), obtaining a stable and purified solid product which can be easily stored and re-dissolved on need in 0.1 M aqueous solution of Hepes buffered at pH 7. Upon exposure to micromolar concentrations of Pb²⁺ cation, the GSH-coated nano-objects undergo a fast aggregation followed by sedimentation leading to complete precipitation in about an hour. The subsequent disappearing of the intense LSPR extinction can of course be followed spectrophotometrically but, most importantly, can be easily detected with the naked eye. No signs of this event are noticed when other divalent cations are added to the colloidal suspension in the same condition. A careful investigation was performed to study this selectivity and the behaviour of aggregation as a function of time and Pb²⁺ cation concentration. We demonstrate that an easy, rapid, instrument-free, visual detection of micromolar levels of Pb²⁺ is thus possible with this assay, showing a good selectivity towards other investigated metal cations.