A pH-controlled synthetic route to violet, green, and orange fluorescent carbon dots for multicolor light-emitting diodes

Carbon dots (CDs) with tunable photoluminescence (PL), especially with long wavelength PL, offer great potential for various applications including sensing, bioimaging, photomedicine, and optoelectronic devices. Here, we use 2,3-diaminopyridine as a single precursor to synthesize colorful CDs in different pH conditions. By simply regulating the reaction media from alkali to neutral to acid, bright CDs emitting violet, green, and orange fluorescence are prepared, respectively. Detailed characterizations reveal that the red-shift of PL is caused by the growing size of sp2 conjugated domain and increasing content of graphite nitrogen. We embed CDs in starch to suppress self-quenching and enhance the PL quantum yields. The CD/starch composites, possessing efficient solid-state fluorescence, can be coated on the ultraviolet chips (365 nm) to fabricate multicolor light-emitting diodes toward indoor illumination. This work presents a new strategy to adjust the PL wavelength of CDs in a wide range, and clarifies the emission mechanism.