Lead (Pb2+) and hexavalent chromium (Cr6+) are highly toxic pollutants with no safe exposure levels, posing significant health risks globally, especially in developing countries. Current detection methods for these metals are often complex and inaccessible, highlighting the urgent need for innovative approaches. In this study, we present a rapid, cost-effective colorimetric assay utilizing ascorbic acid-capped gold nanoparticles (AuNPs) for the selective detection of Pb2+ and Cr3+/6+ ions at levels recommended by regulatory bodies such as the WHO and EPA. The synthesis of our AuNPs was achieved by reducing gold (III) chloride with ascorbic acid, resulting in stable, negatively charged nanoparticles, as characterized by dynamic light scattering, UV-vis spectroscopy and high-resolution transmission electron microscopy. Our method demonstrated high sensitivity, with limits of detection (LOD) of 5.4 ± 0.25 ppb for Pb2+, and 6.3 ± 0.23 ppb for Cr6+, confirming specificity towards these ions in various water samples. The assay's efficacy was validated in real-world applications, including testing drinking water from multiple sources and assessing the performance of filtration systems. This straightforward assay offers a promising tool for monitoring water quality, enhancing public health initiatives and accessibility to critical environmental testing.

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