Vibrant blue N-doped carbon quantum dots from plastic bottles as a highly sensitive fluorescent sensor of Cr6+ ion contaminant in water

Polyethylene terephthalate (PET) plastic waste represents a major environmental burden, yet it offers a valuable carbon source for functional nanomaterials. Here, we report the synthesis of carbon quantum dots (CQDs) from PET waste obtained from mineral water bottles, doped with urea, via carbonization at 300 °C, followed by 10 min of microwave irradiation. The obtained N-CQDs have an average size of 1.025 nm, exhibit strong optical properties with a quantum yield (29.3%), maximally absorb ultraviolet at 238 nm, and emit a blue light at 375 nm. The interaction between Cr 6+ and N-CQDs was evaluated using fluorescence (FL) quenching. The presence of Cr 6+ ions at the concentration range 0.03–4 mg L −1 progressively quenched FL intensity of N-CQDs, significantly reduced UV-light absorption capacity with a limit of detection (LOD) 0.0138 mg L⁻¹ . The N-CQDs probe was found effective in detecting Cr 6+ in the refilled drinking water, tap water, and river water with recovery rates ranging from 89.2% to 112.3% and RSD (3.33–9.31%). This study demonstrates an effective upcycling strategy for plastic waste into high-performance fluorescent nanomaterials and a sustainable platform for potential environmental monitoring, particularly for the detection of trace levels of Cr 6+ ions in aqueous systems.