<![CDATA[ABSTRACT. Silver nanoparticles (AgNPs) have attracted significant attention from researchers and are widely used in the development of chemical sensors due to their unique surface plasmon resonance properties. This study focuses on a colorimetric method for determining Cu(II) using AgNPs capped with p-hydroxybenzoic acid. The AgNPs were synthesized through a chemical reduction process, using p-hydroxybenzoic acid as both a reducing and capping agent. The resulting AgNPs exhibited a yellow color and displayed a characteristic absorption peak at 420 nm, with an average particle size of about 60 nm. These nanoparticles were then used for the colorimetric detection of Cu(II) in the presence of ethylenediaminetetraacetic acid (EDTA), which serves as a ligand. The addition of Cu(II) to the mixture of AgNPs and EDTA induced nanoparticle aggregation, leading to a decrease in absorption intensity at 420 nm and the emergence of a new peak at 680 nm. A linear correlation was established between the concentration of Cu(II) and the absorbance ratio at 680 nm to 420 nm. The limits of detection and quantitation for determining Cu(II) were found to be 0.4 × 10⁻³ M and 1.4 × 10⁻³ M, respectively, with a recovery rate of 104%. These results demonstrate that AgNPs capped with p-hydroxybenzoic acid, combined with EDTA, can provide a simple colorimetric determination of Cu(II) and offer insight into ligand-mediated aggregation behavior in AgNPs-based sensing systems. Keywords: colorimetry; copper; hydroxybenzoic acid; nanoparticles; silver; water]]>
