Green Synthesis of Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe

Carbon quantum dots (CQDs), as zero-dimensional carbon-based fluorescent materials, have garnered significant attention in environmental and biological monitoring due to their low toxicity, high stability, and tunable synthesis. This study pioneers a green synthesis of nitrogen-doped carbon quantum dots (N-CQDs) using medium-molecular-weight chitosan as a single precursor via one-step hydrothermal optimization (190 ℃, 24 h). The resulting N-CQDs exhibit uniform spherical morphology (typical diameter: 3 nm) with amorphous carbon cores and abundant surface moieties including amino, hydroxyl, and pyrrolic groups (C/N/O = 64:4:32). These structural attributes enable strong blue fluorescence (λ _ex /λ _em  = 330nm/402nm) and selective Fe ³⁺ detection through a dual-mechanism: (i) Coordination binding between Fe ³⁺ and surface functional groups, (ii) Photoinduced electron transfer (PET) facilitated by the high redox potential of Fe ³⁺ (E⁰ = +0.77 V vs. SHE). The N-CQDs achieve a 2.72 µM detection limit (S/N = 3) for Fe ³⁺ with excellent selectivity against 15 interfering ions (Al ³⁺ , Cu ²⁺ , etc.) and validated performance in tap/lake water samples (99.98-100.04% recoveries). Furthermore, the chitosan-derived N-CQDs were successfully applied as a fluorescent probe for cell imaging in onion epidermis, where they predominantly accumulated on the cell membrane with excellent color rendering performance. Notably, the fluorescence was effectively quenched upon the introduction of Fe ³⁺ ions, demonstrating their possibility as a responsive probe in biological systems. This work demonstrates the significant potential of biopolysaccharide-derived nanomaterials, highlighting their dual promise for both precise environmental sensing and responsive bioimaging applications.