Carbon dots from citric acid and its intermediates formed by thermal decomposition

Thermal decomposition of citric acid is one of the most common syntheses of fluorescent C-dots; the reaction pathway is, however, quite complex and the details are still far from being understood. For instance, several intermediates form during the process and they also give rise to fluorescent species. In the present work, the formation of fluorescent C-dots from citric acid has been studied as a function of reaction time by coupling infrared analysis, X-ray photoelectron spectroscopy, liquid chromatography/mass spectroscopy (LC/MS) with the change of the optical properties, absorption and emission. The reaction intermediates, which have been identified at different stages, produce two main emissive species, in the green and blue, as also indicated by the decay time analysis. C-dots from the intermediates have also been synthesised via thermal decomposition observing an emission maximum around 450 nm. The citric acid C-dots in water have shown short temporal stability, but their functionalization with 3-aminopropyltriethoxysilane has reduced the quenching. The understanding of the citric acid thermal decomposition reaction is expected to improve the control and reproducibility of C-dots synthesis.