Mechanistic insight into Pb²⁺ sensing by gallic acid-capped gold nanoparticles via SP-ICP-MS and complementary techniques

Background: Lead (Pb²⁺) contamination in water is a major environmental and public health concern, creating a demand for sensitive, simple, and field-deployable detection methods. Gold nanoparticles capped with gallic acid (GA-AuNPs) offer a promising colorimetric platform because their optical properties change upon interaction with metal ions. However, reliable sensor performance requires not only analytical sensitivity but also a clear understanding of the underlying sensing mechanism. Integrating complementary analytical techniques is therefore essential to elucidate metal–ligand interactions, nanoparticle aggregation behavior, and signal generation, enabling rational sensor design and improved confidence in real-world environmental monitoring applications. Results: A one-pot synthesis of gallic acid-capped gold nanoparticles (GA-AuNPs) was developed, allowing Pb²⁺ to interact directly with the nanoparticles during formation and produce a distinct color change from red to purple to blue. Key analytical findings supporting the proposed sensing mechanism and its selectivity toward Pb²⁺. A detection limit of 0.62 μg L⁻¹ was achieved by UV-Vis spectroscopy, accompanied by clear spectral shifts, along with a distinct naked-eye color change that enables simple onsite screening. SP-ICP-MS revealed increases in particle size and decreases in particle number concentration, consistent with Pb²⁺-induced aggregation. FlFFF-ICP-MS showed co-elution of Pb and Au signals, confirming their associations. XPS identified O–Pb coordination, verifying that Pb²⁺ binds to the hydroxyl groups of gallic acid, and TEM provided direct visual evidence of nanoparticle aggregation. Significance: Beyond a simple and cost-effective approach for Pb²⁺ detection, this study provides mechanistic insight into the GA-AuNP colorimetric response. Combined SP-ICP-MS, FlFFF-ICP-MS, XPS, and TEM analyses reveal a Pb²⁺-induced aggregation pathway driven by specific O–Pb coordination, advancing fundamental understanding of metal–ligand–nanoparticle interactions and supporting rational sensor design for environmental monitoring.