Low-temperature synthesis of N,S-Co-doped fluorescent carbon nanoparticles for highly sensitive Fe³⁺ detection in water

• Synthesized at 60 °C for 6 h from single l -cysteine precursor, no purification. • Nanoparticle size: 31.4 ± 1.8 nm, interlayer spacing: 0.428 nm. • Broad excitation–emission gap: 135 nm (Ex 400 nm / Em 535 nm). • Fe³⁺ detection limit: 7 ppb, far below WHO guideline of 300 ppb. We report a facile, low-temperature, and environmentally friendly base-catalyzed hydrothermal method for synthesizing nitrogen and sulfur co-doped fluorescent carbon nanoparticles (N,S-FC NPs) from l -cysteine as a single precursor. The reaction proceeds at only 60 °C in aqueous medium, eliminating the need for high-energy input or post-synthesis purification steps. By adjusting the alkaline reaction conditions, the optical absorption, band gap, and surface chemistry of the nanoparticles were tuned, as confirmed by FT–IR, XRD, EDS, XPS, and HR-TEM analyses. The as-prepared N,S-FC NPs exhibited an average size of 31.4 ± 1.8 nm, stable green emission at 535 nm under 400 nm excitation, and a fluorescence quantum yield of 3.9 %. Among the synthesized variants, the C4 formulation displayed outstanding performance as a Fe³⁺ sensor in aqueous media, with high selectivity and a detection limit of 7 ppb, well below the WHO guideline for drinking water. Compared to conventional carbon-dot syntheses requiring higher temperatures, this low-temperature route offers significant energy savings, reduced environmental impact, and preservation of surface functionalities derived from the precursor. The method provides a scalable platform for producing heteroatom-doped carbon nanomaterials with tailored optical properties for environmental sensing and other advanced applications.