Stable nitrogen-doped carbon quantum dots with pH-controlled fluorescence response for Fe³⁺ detection

Nitrogen-doped carbon quantum dots (NCQDs) were synthesized via a facile and optimized hydrothermal method. The as-prepared CQDs were comprehensively characterized using AFM, HRTEM, DLS, ζ-potential, EDX, XRD, Raman, FTIR, UV-Vis, and fluorescence spectroscopy, confirming their nanoscale dimensions, graphitic structure, abundant nitrogen- and oxygen-containing surface functionalities, and excellent aqueous dispersibility. The CQDs exhibited strong blue fluorescence with excitation-independent emission and a relatively high quantum yield of 37.8%. Their fluorescence stability was systematically evaluated over a wide range of pH values, ionic strengths, and solvent environments, demonstrating robust optical performance. Importantly, the fluorescence sensing behavior toward Fe³⁺ ions was critically examined under different pH conditions, revealing that Fe³⁺ detection in alkaline media is severely hindered by Fe(OH)₃ precipitation, which leads to misleading quenching effects. By conducting sensing experiments under strongly acidic conditions (pH = 2), where Fe³⁺ remains fully soluble, a clear and reliable fluorescence quenching response was achieved over a wide linear range (20-1000 µM), with a detection limit of 10.25 µM. In addition, the practical applicability of the CQDs was demonstrated through fluorescent ink and CQDs/PVA composite films.