Stable nitrogen-doped carbon quantum dots with pH-controlled fluorescence response for Fe(3+) 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 Fe ions was critically examined under different pH conditions, revealing that Fe detection in alkaline media is severely hindered by Fe(OH) precipitation, which leads to misleading quenching effects. By conducting sensing experiments under strongly acidic conditions pH = 2, where Fe 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.