Chemical analysis and identification the fluorophores of photoluminescent carbon dots beyond infrared and X-ray photoelectron energy spectra

Carbon dots or Carbon quantum dots (herein after abbreviated as CDs) with bight luminescence, derived from carbohydrates such as citric acid, glucose or sucrose, have been widely reported in recent publications. However, as to which functional groups and structures in CDs are responsible for luminescence, most explanations in current references are normally based on assumptions, instead of being convinced by the accurate information resting on experimental facts. Herein, we conducted an experimental study targeting at finding the fluorophores of CDs prepared from citric acid and glycine by fluorescent quenching. The experimental results revealed that the essence of fluorescence was manipulated by fluorophores of conjugated C=C double bonds and aldehyde groups. The destruction of aldehyde groups or C=C double bonds in the CDs led to a complete decay of luminescence. In addition, the results of this experiment were similar to those of CDs prepared from the sole citric acid, glucose or sucrose, which predicted the analogy of the CDs in chemical structures. Such information about fluorophore of CDs was valuable for further exploring more extensive application of CDs in sense technologies.