Synthesis of Nitrogen-Carbon Dots (N-CDs) from Waste Polystyrene for the Development of Heavy Metal (Cu²⁺) Detection Paper in Simulated Wastewater

Copper contamination in wastewater poses environmental and public-health risks, creating a need for simple and accessible screening methods. This study developed a fluorescence-based paper sensor using nitrogen-doped carbon dots (N-CDs) synthesized from waste polystyrene for detecting Cu²⁺ in simulated wastewater. The N-CDs were prepared through hydrothermal treatment at 200 °C for 5 h, followed by nitric-acid functionalization. Their optical, structural, surface, and elemental properties were evaluated using photoluminescence and ultraviolet–visible spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The synthesized N-CDs exhibited blue fluorescence with a prominent emission peak at approximately 370 nm. Characterization indicated the presence of graphitic and amorphous carbon domains and nitrogen- and oxygen-containing surface groups that may facilitate interactions with Cu²⁺. The N-CDs were deposited onto hydrogen peroxide-treated cellulose paper to produce a fluorescent sensing platform. A full-factorial experiment examined the effects of pH (1, 6, and 12), N-CD loading concentration (25%, 50%, and 100%), and paper-loading immersion time (20, 30, and 40 min). Fluorescence intensity decreased as Cu²⁺ concentration increased, demonstrating a concentration-dependent quenching response. Among the conditions tested, the most favorable response was obtained at pH 6 using a 50% N-CD concentration and a 20-minute immersion time, yielding a fluorescence-quenching difference of 51.67 arbitrary units. These findings demonstrate the feasibility of converting waste polystyrene into a functional fluorescent material for paper-based Cu²⁺ screening. Further validation using standard Cu²⁺ concentrations, selectivity testing, detection-limit determination, and real wastewater samples is required before quantitative or field application.