Humic acid - functionalized silver nanoparticles (HA-AgNPs) were successfully synthesized and used to detect Cu (II) ions in aqueous solutions. The HA-AgNPs was shown to have an average hydrodynamic diameter of 101.4 nm and a polydispersity index of 0.447. The absorbance spectra of HA-AgNPs showed the characteristic local surface plasmon resonance (LSPR) peak of AgNPs at 408.3 nm. Addition of Cu (II) in the HA-AgNPs led to their agglomeration as evidenced by the change in their surface morphology and their corresponding optical absorbance spectra. The synthesized HA-AgNPs showed a strong linear response for Cu (II) concentrations in the range of 0.00 – 1.25 mM with a limit of detection (LoD) of 4.4428 ± 0.1091 mg L-1, a limit of quantification (LoQ) of 14.8094 ± 0.3636 mg L-1, and a limit of blank (LoB) of 0.1214 ± 0.0065 mg L-1. Statistical analysis showed that this calibration curve could be used to quantify Cu (II) concentrations within a 95% confidence level. Furthermore, HA-AgNPs was found to be selective for Cu (II) detection based on the selectivity study against common metal ions found in drinking water. This shows that the synthesized HA-AgNPs can be used as an environment-friendly colorimetric nanosensor for rapid and point-of-need quantification of Cu (II) ions in aqueous media.

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.