Color-tunable carbon dots via control the degree of self-assembly in solution at different concentration

Using carbon spheres and sucrose as the carbon sources, two kinds of color-tunable carbon dots (Cdots) named as Cdots-C and Cdots-S were prepared via change the solution concentration. As the concentration of Cdots solution increased, varying degrees of self-assembly occur between monodisperse Cdots. Hence aggregates with different types formed, which can be observed. Appearance of new excitation bands at 465 and 520 nm caused by H- and J-type Cdots aggregates caused the emission at long wavelengths. At the same time, the excitation band at 300–400 nm induced blue-green emission and the competitions of three excitation bands resulted in the color-tunable PL behaviour which cover the region from green to red emission. The difference of carbon sources of Cdots-C, Cdots-S and Cdots-G (graphite) explained the reason why concentration-dependent PL behaviour were not observed in Cdots-G at a high solution concentration. The process of self-assembly were ascribed to hydrogen bonding interaction between abundant hydroxyl groups on the surface of Cdots-C and Cdots-S. This work provided a simple approach to modulate photoluminescence (PL) color of Cdots and will extend the application in bioimaging.