| Summary: | Heavy metal contamination in groundwater has become more prevalent due to the leaching of toxic wastes from various anthropogenic sources. When ingested, this can cause serious ill effects detrimental to human health. Hence, there is a need to monitor the levels of heavy metals in various water sources to ensure that they are fit for human consumption. Standard detection methods such as AAS and ICP-MS are typically used for quantifying the concentration of heavy metals. However, these require expensive equipment, not to mention the need for a trained and highly-skilled technician to operate the equipment. Nanosensors offer a low-cost alternative to these methods. By utilizing the localized surface plasmon resonance (LSPR) and the properties of noble metal nanoparticles such as AgNPs, the colorimetric detection of heavy metals is made possible. Herein, we report the synthesis of humic-acid-functionalized silver nanoparticles (HA-AgNPs) using a borohydride reduction approach as a colorimetric nanosensor for Ni (II) detection in aqueous solutions. Humic acid acts as a capping agent that stabilizes the AgNPs in the colloidal mixture while providing functional groups for the detection of heavy metals. The synthesized HA-AgNPs had an average hydrodynamic diameter of 42.9 nm, a polydispersity index of 0.438, and an LSPR peak of 400.6 nm. The nanosensor could be used for the colorimetric detection of Ni (II) ions within the linear range of 0.15–0.40 mM Ni (II) with a limit of detection (LoD) of 2.35 mg L<sup>−1</sup>. The HA-AgNPs were shown to be selective when detecting Ni (II) ions; common metals in water such as Ca (II), Mg (II), Al (III), Zn (II), Na (I), and K (I) did not interfere with Ni (II) detection. As such, HA-AgNPs can be used as reliable and environmentally friendly colorimetric nanosensors for the rapid and point-of-need detection of Ni (II) ions in aqueous solutions.
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