Probing the fluorescence quenching mechanism of N-doped carbon quantum dots by inorganic ions

Fluorescence quenching is a common phenomenon of carbon quantum dots (CQDs) and has been utilized to detect various compounds. Since CQDs are prepared by varied synthesis routes and by diverse raw materials, a variety of fluorescence quenching mechanisms have been proposed. In this work, blue-emitted nitrogen-doped CQDs (N-CQDs) were prepared using citric acid and ethylenediamine as precursors, and its fluorescence quenching by inorganic ions was probed. It is found that only oxidizing inorganic ions (including Fe3+, MnO4−, Cr2O72−, CrO42−, ClO−, BrO3−, BrO3−+Br− and S2O82−) can remarkably quench the fluorescence, and a redox quenching mechanism was further proposed by analyzing the UV–vis absorption spectra, the fluorescence lifetime spectra, the electron paramagnetic resonance spectra, the zeta potential and the X-ray photoelectron spectra. In addition, when overlap between the absorption spectra of ions and the excitation or emission spectra of N-CQDs occurs, inner filter effect (IFE) can also quench the fluorescence as an auxiliary way, although it is less important compared with redox quenching. This work contributes to a better understanding of CQDs in sensing field.