Photoluminescence properties of N-doped carbon dots prepared in different solvents and applications in pH sensing

Two kinds of nitrogen-doped carbon dots (N-CDs) were synthesized by choosing citric acid, l-serine and monoethanolamine as the precursors, respectively, in organic solvent N,N-dimethylformamide (DMF) and deionized water. The FTIR, UV–Vis, HRTEM and XRD were used to characterize the surface molecular structures of these two N-CDs. It was found that different reaction systems made the as-prepared N-CDs possess varied surface states and thus exhibit distinctive photoluminescence features. For the N-CDs prepared in DMF, its fluorescent property showed pH dependent and color-switchable in visible light and could be reversibly changing over in both alkaline and acidic environments due to the protonation and de-protonation of the carboxyl groups on the surface of the N-CDs. Also, its fluorescence intensity exhibited a linear fashion over the pH range from 1.5 to 7.5, indicating its potential applications in pH quantitative detection under acidic condition.