Fluorescent carbon dots with two absorption bands: luminescence mechanism and ion detection

Herein, we report the synthesis of carbon dots (CDs) with two characterized absorption bands but without excitation wavelength-dependent fluorescence via a one-step hydrothermal method. The structure of CDs was characterized using X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, Fourier transform infrared, and UV–Vis spectroscopy. The structure and photoluminescence of CDs vary significantly with different raw materials and preparation methods, and the mechanism of luminescence is not clear yet. Hence, we studied the luminescence mechanism behind two characterized absorption bands of CDs using fluorescence quenching method with ninhydrin and several ions as quenchers. The influence of the surface groups of CDs on its photoluminescence properties was also discussed. Ninhydrin and a variety of other ions exhibited different quenching effects on the fluorescence emissions which obtained at the two absorption bands of CDs. Combining with the structure characterization results, it can be concluded that the emission wavelength is mainly determined by the carbon core, while the excitation wavelength is determined by the surface nitrogen-containing groups. (The excitation at 234 nm might be due to the Schiff base structure, while the excitation at 345 nm was mainly due to the amide structure.) Furthermore, based on the interaction of NO₂⁻ with the surface nitrogen-containing groups of CDs, a quantitative detection method of NO₂⁻ using CDs was proposed in our study. CDs exhibited high selectivity for NO₂⁻ at pH 1.6 with good linearity to NO₂⁻ concentration in the range of 1–10 μM.