Single-Particle Emission Microscopy of Green-Emitting Carbon Dots Made from Top-Down and Bottom-Up Precursors

Carbon dots have received considerable attention due to their tunable emission. Single-particle techniques revealed that individual top-down and bottom-up green carbon dots can support several chromophores. In particular, several studies demonstrated that bottom-up synthesized carbon dots are typically made of amorphous carbon and are multichromophoric but may also just be chemically impure, with free dye in solution or polymerized in a carbon matrix. Carbon dots made by top-down precursors, however, are highly graphitic and more often single-chromophoric, begging the question if carbon dots made from bottom-up precursors could have similar optical properties compared to their top-down counterparts, if properly purified. Here, we compare green-emitting carbon dots made by two methods: top-down by chemical oxidation and bottom-up from small-molecule precursors in a solvothermal synthesis followed by rigorous purification. Such dots have cores of different crystallinity, but both types have oxidized surfaces. Just as ensemble absorption and emission spectra show only subtle differences, we find based on single-particle emission imaging that both types of carbon dots contain similar weights of carbon dots with single and multiple chromophores. Surprisingly, the carbon dots are optically similar, despite coming from opposing synthetic approaches. Although the majority of all carbon dots are single-chromophoric, top-down carbon dots are found to more likely have only one emitting chromophore, whereas bottom-up carbon dots are comparatively more multichromophoric. In the multichromophoric case, bottom-up carbon dots have on average a greater number of chromophores than top-down carbon dots. Our results showing that very differently made carbon dots with different structural properties exhibit strikingly similar emission properties reveal the important insight that out of structural heterogeneity emerges spectroscopic homogeneity.