Carbon dots are small carbon nanoparticles with effective surface passivation by mostly organic species. For such a simple and well-defined nanoscale structure, the classical and most reliable synthesis is the use of pre-existing small carbon nanoparticles for surface chemical functionalization with organic molecules. However, a more popular synthetic approach in the literature has been the “one-pot” carbonization of organic precursors, which with appropriate processing conditions could in principle create local structures in the resulting dot-like entities that may be comparable to the structural configuration in carbon dots, though the carbonization can also easily produce colored organic materials crosslinked into the sample structures. An extreme example was the thermal processing of the specific organic precursor mixtures including only citric acid with formamide or urea to yield samples of red/near-IR absorption and emission features, which prompted the claims of “red/near-IR carbon dots”. In reality, these spectral features have nothing to do with nanoscale carbon, let alone carbon dots, but simply associated with molecular dyes/chromophores produced in chemical reactions of the specific precursor mixtures under the thermal processing conditions intended for carbonization. In this work, the isolation and identification of the responsible molecular dyes/chromophores were pursued. The results present further evidence for the conclusion that the red/near-IR absorption and emission features have nothing to do with carbon dots.

Carbon quantum dots (C-dots) are emerging semiconductor nanomaterials consisting of earth-abundant C, O, and N elements. C-dots have many advantages such as high quantum yield, good photo- and chemical-stability, low-cost, low-toxicity and easy synthesis with earth-abundant precursors. In this minireview, we summarized and updated the most recent research works for the synthesis of the fluorescent and phosphorescent C-dots. In the end, we also give our owner views for the challenges and research directions for C-dots.