An asymmetric Schiff base-functionalized gold nanoparticle-based colorimetric sensor for Hg ion determination: experimental and DFT studies.

We report a colorimetric sensor for the detection of Hg ions utilizing surface-modified gold nanoparticles. Gold nanoparticles (GNPs) were synthesized by direct reduction and were subsequently functionalized using Schiff base ligands. Schiff base ligands as electron transfer agents have been frequently used for the determination of heavy metal ions. From the spectroscopic analysis, it was found that the mechanism could be defined as coordination between azomethine nitrogen and the carbonyl oxygen of the ligand with Hg ions. The affinity of Hg ions towards the bidentate Schiff base on the GNPs result from their self-aggregation and investigated to be a powerful asset for the development of Hg ion-selective sensors, which is accompanied by a visible color change from pink to purple or can be detect by UV-Vis spectroscopy. The optimized structures and binding mechanisms were supported with a high correlation and agreement via spectroscopy and DFT calculations. These simple colorimetric tests can be extended for the rapid pre-screening of a wide variety of heavy metal ions for onsite detection and mitigation.