<p>This research provides insights about the synthesis and characterization of copper nanoparticles (CuNPs) using a simple, environmentally conscious approach, employing sodium dodecyl sulphate (SDS), starch, and polyethyleneimine (PEI). The synthesis aimed to produce stable CuNPs with controlled properties (particle size, shape, size distribution, crystal structure, surface chemistry, etc.) for heavy metal detection applications. Comprehensive characterization was performed utilizing a suite of analytical techniques, including UV–Vis, FTIR, XRD, DLS, and TEM. Atomic absorption spectroscopy (AAS) was used to quantify the copper content. Subsequently, the synthesized CuNPs were applied for the rapid screening of heavy metal ions in aqueous solutions, with a focus on mercury (Hg²⁺) detection. A colorimetric and spectrophotometric method was developed, utilizing the interaction between Hg²⁺ ions and the CuNPs, which resulted in a visually evident colour change and a corresponding shift in the UV–Visible absorption spectrum. Metal ions tested for detection were Fe³⁺, Pb²⁺, Zn²⁺, Cd²⁺, Ni²⁺, As³⁺, Al³⁺, Mn²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Co²⁺, Sn⁴⁺, and Mn²⁺. This method enabled rapid detection of Hg²⁺ ions within a mere 5&#xa0;min. The limit of detection (LOD) for Hg²⁺ ions by the visual colorimetric method was determined to be around 0.277&#xa0;ppm, demonstrating the high sensitivity of the synthesized CuNPs for heavy metal detection. This study underscores the potential of SDS, starch, and PEI-stabilized CuNPs as a promising material for rapid, sensitive, cost-effective, and extremely selective heavy metal detection in environmental monitoring and remediation applications, offering a viable alternative to traditional analytical methods.&#xa0;</p>