Synthesis of amino acid-functionalized Cit-T20-GNPs and investigation of their metal ion selectivity for colorimetric sensing

The use of functionalized gold nanoparticles (GNPs) as a colorimetric sensor for heavy metal ions has been extensively studied due to their unique optical properties and ease of functionalization. Amino acid-functionalized GNPs have emerged as a promising platform for colorimetric sensing of various pollutants, including metal ions. However, the differential metal ion selectivity of the GNPs functionalized with different amino acids remains poorly understood. This study describes the synthesis and functionalization of Tween-20-stabilized citrated gold nanoparticles (Cit-T20-GNPs) using various amino acids and their interaction with metal ions. Tween-20 was found to be essential to provide stability to the nanoparticles for functionalization. The morphology, hydrodynamic diameter, and zeta potential of the GNPs were characterized using transmission electron microscopy and dynamic electrophoretic light scattering analysis. The GNPs were then functionalized with L-threonine, L-glutamine, L-alanine, L-tryptophan, L-cysteine, L-methionine, L-arginine, L-lysine, L-proline, and glycine. The metal-induced aggregation of the functionalized GNPs was studied using twelve different metal ions. At the experimental pH (5-6), the colloidal integrity of most amino acid-functionalized Cit-T20-GNPs remained intact upon treatment with the metal ions, except for Cit-T20-Arg-GNPs and Cit-T20-Lys-GNPs. These two GNPs showed interaction with most metal ions, which led to their aggregation. The study provides insights into the specific interaction and affinity of metal ions toward different amino acid ligands and highlights the potential of designing ion-specific colorimetric sensors. The possible mechanism behind these interactions has also been speculated.