A facile ambient post modification method towards red fluorescent flame-formed carbon dots

In this work, a room-temperature HNO 3 wash is found to be effective in red-shifting the fluorescence of flame-formed carbon dots, especially for the green fluorescent carbon dots from premixed flames. Characterization techniques together with quantum chemistry calculations are carried out to investigate the mechanisms behind the red-shifted fluorescence. It was found that a room-temperature HNO 3 wash can significantly red-shift the fluorescence of flame-formed carbon dots from ∼535 nm to ∼620 nm and cause a shorter fluorescence lifetime. The particle diameter and in-plane correlation length of the aromatic lands were not significantly changed, but the N-containing species were detected after the HNO 3 wash. Further spectroscopic analysis showed that the responsible nitrogen species for the red-shifted fluorescence was –NO 2 , which possibly stemmed from the C=C reactive sites on the flame-formed carbon dots, as the C=C content decreased while the –NO 2 content increased after the HNO 3 wash. The excited state analysis shows that –NO 2 functional groups can lower the HOMO-LUMO gap of the flame-formed carbon dots and red-shift the 1π→π* transition. The hole-electron analysis shows that the excitation of the unmodified model is the local excitation within the π-conjugated domain, while that of the N-modified model is the charge transfer excitation from the π-conjugated domain to the peripheral –NO 2 functional groups, which can lead to the non-radiative decay and low quantum yield. This work also provides direct evidence for the presence of special reactive sites relevant to C=C on the flame-formed carbon dots that can readily react with HNO 3 at room temperature and load –NO 2 functional groups on them.