In Situ Chromophore Doping: A New Mechanism for the Long-Wavelength Emission of Carbon Dots

Doping of carbon dots (CDs) with heteroatoms (e.g., N, S, P, and B) is known as an efficient method to modulate the electronic and optical properties of these fluorescent carbonaceous nanomaterials. Here, we have demonstrated the possibility of in situ azo doping in CDs through hydrothermal treatment of glucose and 3-nitroaniline in aqueous sulfuric acid solution. Interestingly, because of the pH sensitivity of the absorption spectrum of the doped azo groups, CDs exhibited three completely excitation-independent blue, red, and yellow emission colors under acidic, neutral, and alkaline conditions, respectively. Time-dependent density functional theory calculations verified how chromophore doping in CDs could modulate the HOMO-LUMO energy levels and accordingly shift their absorption and emission spectra to longer wavelength regions.