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.