Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface

Excitation-dependent luminescence (EDE) or excitation-independent luminescence (EIE) property of carbon dots (CDs) has attracted considerable attention. For the first time, we found that nitrogen doped CDs possess adjustable EDE and EIE properties by changing the corresponding environmental pH values for photoluminescence (PL) properties. Structural characterizations and property tests demonstrate that the unique photoluminescence properties of CDs can be attributed to the synergistic effect from the PL centers in the carbon core and on the surface. Doping nitrogen into the carbon core improves the emission efficiency of PL centers; these enhanced PL sites can even dominate fluorescence emission. Passivated surfaces of CDs modified with amino groups not only further boost the emission efficiency, but also make energy levels of PL centers more uniform. In addition, the mechanism of synergistic effect is investigated, which opens up the possibility of designing CDs with desirable PL characteristics. Such nitrogen-doped CDs with adjustable excitation-related luminescence properties have potential applications in detecting acidity or alkalinity of a certain physiological environment by intuitively observing the change in the wavelength.