Metal-induced aggregation of valine capped gold nanoparticles: An efficient and rapid approach for colorimetric detection of Pb ions.

In this study, we report a novel application of valine-capped gold nanoparticles for colorimetric and visual detection of lead ions. The -COO group of the hydrophobic valine molecules present efficient electrostatic repulsion resulting in generation of stable, well-dispersed and size-controlled GNPs. The GNPs were highly selective for Pb ions and showed visible colour change in the assay mixture on addition of solution containing lead ions. Interestingly, a decrease in the intensity of original SPR peak at 530 nm was observed, with concomitant appearance of a new peak at longer wavelength due to agglomerated GNPs. The free -COO groups on GNP surface interacted with Pb and ion-dependent chelation mechanism lead to cross-linking of particles and subsequent agglomeration. Binding of Pb ions and valine-capped GNPs occur in a stochiometric ratio of 1:2. The GNPs displayed colorimetric sensing in the range of 0 to 100 ppm concentration with a very high selectivity towards lead even in the presence of other metal ions. The minimum detection limit (MDL) for Pb was 30.5 µM. We anticipate that these valine-capped GNPs may be employed for lead detection in polluted water/wastewater through a cost-efficient, one-step assay protocol as it does not require additional functionalization with specific ligand molecules.