Concentration-tuned multicolor carbon dots: microwave-assisted synthesis, characterization, mechanism and applications

New multicolor carbon dots made from d-fructose and NH4HCO3, denoted as F-CDs, have been synthesized via a time-saving and energy-saving microwave-assisted method. By just adjusting the concentration, the optimal emission wavelength of the F-CDs could be tuned from 404 nm to 583 nm. Furthermore, the sp(2) carbon core and abundant functional groups provided the foundation for concentration-dependent photoluminescence properties. The relationship between the aggregation process and the concentration-induced photoluminescence mechanism is fully discussed. The maintenance of the F-CDs' PL intensity with increasing concentration occurred by a mechanism similar to the crosslinked enhanced emission (CEE) effect. Finally, the highly stable and biocompatible multicolor F-CDs were successfully applied in fluorescent ink and multicolor bioimaging.