Effect of Ligand Concentration on the Stability and Copper (II) Sensing Performance of Humic Acid-Functionalized Silver Nanoparticles

Abstract

Detecting heavy metals in water is necessary to ensure its safety. However, current detection methods require costly equipment, making heavy metal monitoring challenging. Colorimetric detection of heavy metals using silver nanoparticles (AgNPs) relies on the optical spectra changes when it detects an analyte. We have previously shown that a colorimetric assay comprised of humic acid - functionalized AgNPs (HA-AgNPs) can selectively detect copper ions in water. Here, we investigated the effect of humic acid concentration on the stability of HA-AgNPs and their ability to detect copper. HA acts as a capping agent around the AgNPs, making them stable even for up to 48 days of storage in both ambient and cold storage environments. At critical HA concentrations of 5 mg L-1 and beyond, the changes in the optical properties of the HA-AgNPs are linearly dependent on Cu (II) concentration. Below this critical HA concentration, Cu (II) sensing is futile. The most stable HA-AgNPs is at HA = 25 mg L-1 based on zeta potential measurements, while the best assay for colorimetric copper (II) detection is at HA = 50 mg L-1, giving the lowest detection limit of 4.35 mg L-1 and R2 = 0.999 within a dynamic linear range of 0.00 to 1.25 mM Cu (II). We have shown that the ligand concentration is critical for achieving stable AgNP assays for heavy metal detection.