Synthesis, Optical Tuning, and Fluorescent Sensing of Fe³⁺ with N-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) have emerged as highly versatile nanomaterials due to their tunable optical properties, excellent water dispersibility, and chemical stability. In this work, undoped, N-doped, and N,S-doped CQDs were synthesized via a simple, low-temperature bottom-up method using different molecular additives to tailor their emission behavior. Systematic characterization by UV–Vis spectroscopy, photoluminescence (PL), and quantum yield measurements (QY) confirmed that heteroatom doping effectively modulates the electronic structure of CQDs, enabling controllable emission spanning the UV to visible region. Undoped CQDs exhibited excitation-dependent blue emission arising from surface states, whereas nitrogen doping introduced mid-gap states responsible for a pronounced red-shift and dual-band emission. Co-doping with nitrogen and sulfur further intensified defect-related blue emission, leading to a noticeable enhancement in photoluminescence intensity. In addition, the nitrogen-doped CQDs were successfully employed as a sensitive fluorescent probe for Fe³⁺ ion detection. The CQDs exhibited high selectivity toward Fe³⁺ with significant fluorescence quenching, attributed to strong coordination between Fe³⁺ ions and surface functional groups. The sensing performance was optimized by studying the effects of pH, reaction time, and Fe³⁺ concentration, revealing a rapid response within 5 minutes and effective operation over a wide pH range (3–11) with maximum quenching at pH 7. A linear Stern–Volmer relationship was observed between quenching efficiency and Fe³⁺ concentration (1–200 μM), demonstrating the suitability of N-CQDs for quantitative detection. Overall, this study highlights the ability to tune CQD emission through controlled heteroatom doping and demonstrates the practical potential of N-doped CQDs as a simple, sensitive, and selective fluorescent sensor for Fe³⁺ ions in aqueous environments.