A novel near-infrared fluorescence switching sensor based on polyamide derived carbon quantum dots for sequential detections of perfluorobutane sulfonic acid and Fe3⁺ ions

In this work, we report a sustainable near-infrared (NIR) fluorescence switching sensor based on polyamide-derived carbon quantum dots (PACQDs) synthesized from waste, providing a green and scalable route to NIR-active nanomaterials. The ultrasmall PACQDs (1.53 ± 0.33 nm) exhibit a high quantum yield (29%), a large Stokes shift (350/702 nm), zeta potential (–29.82 ± 0.60 mV), and exceptional photostability with no spectral drift over 1056 h. FTIR surface analysis reveal abundant –OH, –NH₂, –COOH, and C = O functionalities that facilitate electrostatic interactions and hydrogen bonding with target analytes. The PACQDs display a distinct sequential fluorescence switching mechanism. First, Perfluorobutane Sulfonic acid (PFBS) induces pronounced NIR fluorescence quenching through ground-state complexation and static quenching, as evidenced by UV–Vis red shifts, Stern–Volmer behavior, DFT/DMol 3 calculations, and a temperature-dependent decrease in binding constants [0.82 (at 298 K) > 0.67 (at 308 K) > 0.55 (at 313 K)]. HOMO–LUMO analysis further confirms electronic perturbation of PACQDs upon PFBS binding. Subsequently, Fe 3 ⁺ addition restores fluorescence intensity via preferential Fe 3 ⁺–PFBS coordination that disrupts the PACQD + PFBS complex, enabling a reversible “off–on” sensing pathway. This dual-mode system demonstrated detection limits of 2.38 mg/L for PFBS and 5.73 mg/L for Fe 3 ⁺, with excellent recoveries (91–99%) in spiked tap water. Thermodynamic analysis (ΔH < 0, ΔS < 0, ΔG ̴ 0.5–1.6) indicates a spontaneous, enthalpy-driven association between PACQDs and PFBS, dominated by hydrogen bonding and electrostatic interactions. Overall, this study introduces a novel waste-derived NIR fluorophore and establishes its effectiveness as a sequential fluorescence-switching probe, offering a promising, resource-efficient platform for monitoring PFAS and metal ions in aqueous systems.