Separation and online optical characterization of fluorescent components of pyrogenic carbons for carbon dots identification

Optical features of fluorescent components of pyrogenic carbons as flame-formed carbon particulate matter (PM) were studied by setting up a facile and fast method (SEC-Abs/Fluo) based on size exclusion chromatography (SEC) coupled with UV–Visible absorption and fluorescence spectrometers as detectors. The SEC-Abs/Fluo method simultaneously allowed the molecular weight (MW) separation and identification of blue- and green-fluorescent PM components exhibiting size and spectroscopic features typical of carbon dots (CDs). The SEC-Abs/Fluo method allowed the online determination of quantum yield (QY) and optical band gap of fluorescent components also in strongly-scattering carbon suspensions so avoiding tedious pretreatment of carbon samples. From the detailed separation and analysis of blue- and green-fluorescent PM components along with two commercial pitches, aromatic species having MW > 1000 u and band gap <1–1.5 eV resulted to be scarcely or no fluorescent indicating these are the MW and band gap thresholds for fluorescence emission. Indeed, a sharp QY decrease was for the first time noticed in a narrower band gap (1–1.5 eV) range, where carriers can more easily tunnel to defects of the carbon network reducing fluorescence emission. The exponential increase of QY as the band gap rises was found with a more regular trend for the MW-segregated components in comparison to the bulk samples. The method can support and guide the choice of the optimal operating conditions to maximize CD formation and their purification from not-fluorescing components and can also be helpful in disentangling CD fluorescence attribution.