Tailoring color emissions from N-doped graphene quantum dots for bioimaging applications

Unlike inorganic quantum dots, fluorescent graphene quantum dots (GQDs) display excitation-dependent multiple color emission. In this study, we report N-doped GQDs (N-GQDs) with tailored single color emission by tuning pi-conjugation degree, which is comparable to the inorganic quantum dot. Starting from citric acid and diethylenetriamine, as prepared N-GQDs display blue, green, and yellow light emission by changing the reaction solvent from water, dimethylformamide (DMF), and solvent free. The X-ray photoelectron spectroscopy, ultraviolet-visible spectra results clearly show the N-GQDs with blue emission (N-GQDs-B) have relatively short effective conjugation length and more carboxyl group because H2O is a polar protic solvent, which tends to donate proton to the reagent to depress the H2O elimination reaction. On the other hand, the polar aprotic solvent (DMF) cannot donate hydrogen, the elimination of H2O is promoted and more nitrogen units enter GQD framework. With the increase of effective pi-conjugation length and N content, the emission band of N-GQDS red-shifts to green and yellow. We also demonstrate that N-GQDs could be a potential great biomarker for fluorescent bioimaging.