Study on luminescence mechanism of nitrogen-doped carbon quantum dots with different fluorescence properties and application in Fe 3+ detection

Carbon quantum dots surface doped with nitrogen (N-CQDs) were synthesized with quantum yield (24.9–66.4%) via an easy hydrothermal route while using citric acid as a precursor for carbon and varying concentrations of DL-Alaninol as the dopant. The as-prepared N-CQDs with adjustable fluorescence emission transformed from weak excitation-dependent to excitation-dependent and then to excitation-independent. After a detailed characterization and investigation, the N-CQDs were found to have a uniform particle size (1.76–3.07 nm) and good monodispersion. They were bearing similar functional groups on their surfaces but with different contents. The N/C ratio of N-CQDs increased gradually with the increasing contents of dopant, while the O/C ratio gradually decreased. The photoluminescence (PL) of (N-CQDs) has been explained by a proposed PL mechanism that involved quantum size effect and the energy levels by the surface state. When further investigated as a fluorescent probe for Fe3+ ion detection, the synthesized N-CQDs were found to be selectively and sensitively detecting Fe3+ ions. The N-CQD-based fluorescent probe was capable of detecting in a wider linear detection range and lower limits of detection (LODs).