Sunlight-Mediated Synthesis of Metallic Silver Nanoparticles with Coffee (<i>Robusta/Arabica</i>) Pulp Aqueous Extract for Hg²⁺ Ion Detection

Abstract

On-site and portable detection of heavy metals, especially in water, is critical for public health safety. Mercury is a toxic heavy metal and poses environmental and health hazard concerns causing neurological and behavioral disorders. Metallic nanoparticles possess unique optical properties, which can be used for heavy metal sensing applications. In this study, a colorimetric method for detecting mercury using silver nanoparticles was developed. The reduction of Ag+ to AgNP was initiated using sunlight with coffee pulp aqueous extract as a reducing agent. The UV-Vis spectrum of the as-prepared AgNP solution shows a maximum absorption peak at 433 nm due to the metal’s localized surface plasmon resonance (LSPR). The SEM analysis of the dried sample shows an aggregate of AgNPs with spherical morphology with diameters less than 100 nm. The dynamic light scattering distribution curve shows a bimodal peak with a mean hydrodynamic radius of 12.73 nm and 145 nm, respectively. The large hydrodynamic radius of more than 100 nm may be attributed to the presence of AgNP aggregates in the solution. The synthesized AgNP was shown to selectively detect Hg2+ ions in solution colorimetrically. A linear calibration curve was obtained for the Hg2+ solution between 0.04 mM and 0.2 mM with R2 equal to 0.9934. This proposed method can potentially be used in the analysis of actual water samples.