<p>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&#xa0;h). The resulting N-CQDs exhibit uniform spherical morphology (typical diameter: 3&#xa0;nm) with amorphous through a dual-mechanism: (i) Coordination binding between Fe³⁺ and surface functional groups, (ii) Photoinduced electron transfer (PET) facilitated by the high redox potential of Fe³⁺ (E⁰ = +0.77&#xa0;V vs. SHE). The N-CQDs achieve a 2.72 µM detection limit (S/<i>N</i> = 3) for Fe³⁺ with excellent selectivity against 15 interfering ions (Al³⁺, Cu²⁺, 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 Fe³⁺ 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.</p>