Role of Acids in Producing Ultrabright, Dual-Emissive Carbon Dots and their Urea/Biuret Composites with Ultralong Afterglow

Since 2015, m-phenylenediamines (mPD) have become a popular carbon source for the synthesis of carbonized polymer dots (CPDs). However, their exact fluorescence mechanism is still obscure. To elucidate this, inorganic acids that are carbon-free are chosen as additives for a comparative study. It is found that the green fluorescence quantum yield (nearly 80%), photostability, and reaction yield (over 90%) can be enhanced by introduction of most of inorganic acids with moderate amount. Besides, green-blue dual emission is observed in acid-assisted groups. UV-vis absorption, Fourier-transform infrared spectroscopy, and surface-enhanced Raman scattering results indicate that the green fluorescence center is composed of quinoid rings, whereas the blue fluorophore contains benzenoid rings. Moreover, room-temperature afterglow with lifetime up to 1.25 s is observed exclusively in acid-assisted CPDs composites with urea/biuret. The blue chromophore is proposed to be the origin of the triplet level that induces the long afterglow. This work provides an in-depth understanding on the macromolecular structures of CPDs derived from phenylenediamines, and contributes a new line of thought to the origin of phosphorescence in N-doped carbon dots.