PVAm-Grafted N, S-Co-Doped Carbon Dots: A Robust Fluorescent Probe for Selective Detection of Ferric Ions

The development of fluorescent probes with high stability and selectivity for ferric ion (Fe³⁺) detection remains crucial for environmental monitoring. Herein, we report a one-pot hydrothermal synthesis of nitrogen and sulfur co-doped carbon dots grafted with polyvinylamine (CDs-PVAm) using citric acid and L-cysteine as sustainable precursors. The obtained CDs-PVAm exhibited strong blue fluorescence with a high quantum yield of 49% and an average fluorescence lifetime of 9.75 ns, attributed to the synergistic effect of N, S co-doping and PVAm passivation which effectively suppresses non-radiative recombination. The material demonstrates excellent environmental stability over a wide pH range (3–9), under thermal stress (25–45 °C), and prolonged irradiation. As a fluorescent probe, CDs-PVAm shows high selectivity toward Fe³⁺ among various metal ions, with a linear response range of 0–200 µM and a detection limit of 11.40 µM. Practical application in natural water samples achieved satisfactory recoveries (100.46–101.45%) with low relative standard deviations (0.71–1.43%), confirming its reliability for Fe³⁺ detection in complex aqueous matrices. This work presents a robust nanoplatform for environmental sensing with significant application potential.