FIG. 1.

Schematic mechanisms of Pb²⁺ and Al³⁺ detection using AuNPs: (A) A GSH molecule binds with a label-free AuNP at its thiol group and simultaneously coordinates with Pb²⁺ at its carboxylic groups, leading to aggregation of AuNPs and shift of absorbance spectrum of the AuNP solution. (B) Citrate-capped AuNPs aggregate in the presence of Al³⁺, inducing shift of absorbance spectrum of the AuNP solution. (C) Molecular structures of the GSH and citrate cap.