Surface chemical composition of chromatographically fractionated graphite nanofiber-derived carbon dots

Carbon dots (C-dots) were synthesized from graphite nanofiber starting material by oxidizing in strong acid (i.e., top-down synthesis). The resulting product was heterogeneous with regard to size and surface functionality. After separating the mixture into fractions by means of anion-exchange (AE) high-performance liquid chromatography (HPLC), fractions with unique photoluminescent properties were selected. We examined the surface of these unique C-dot fractions by means of X-ray photoelectron spectroscopy and infrared spectroscopy and identified the surface functional groups on the C-dot fractions that exhibited higher photoluminescence. Although some surface functionalities were common to all fractions (e.g., carboxylic groups), others were different (e.g., aliphatic C. H bonding). Among the luminescent nanoparticles examined, those with the higher luminescence showed less oxygen content. This strongly suggests that in the mixture of the as-synthesized C-dots, all nanoparticles did not have identical surface functionality, and they contributed differently to the observed photoluminescence.