Solvent Effect on Structural Elucidation of Photoluminescent Graphitic Carbon Nanodots

Photoluminescence (PL) of carbon nanodots (CNDs) is proposed to originate from the polycyclic aromatic carbon-core and in situ synthesized molecular fluorophores. This work reports the CNDs prepared by direct pyrolysis of citric acid only at a prolonged time, 40 h, and their fluorescence emission parameters in a variety of solvents by steady-state and time-resolved emission spectroscopies. The response of fluorescence emission lifetime and emission quenching rate constants to changes in solvent parameters such as polarity and tumbling lifetime were essentially independent, unlike molecular fluorophores that display solvent-dependent emission parameters. Fluorescence emission was quenched in nitromethane additionally indicating to the polycyclic aromatic carbon-core as a predominant structural feature of the CNDs. The quenching of CND emission in the presence of benzophenone that has a strong triplet component in the excited state was observed. Quenching demonstrates the Stern-Volmer behavior and reveals the additional nonradiative decay pathways of CNDs. The main photophysical features of CNDs are discussed in terms of fluorescence emission originating from the excited state of the polycyclic aromatic carbon-core where contribution from the potential molecular fluorophores is considered minimal.