Facile Synthesis of Matrix-Free Room-Temperature Phosphorescent Nitrogen-Doped Carbon Dots and Their Application as Security Inks

Room-temperature phosphorescent (RTP) nitrogen-doped carbon dots (N-CDs) have attracted great interest but their efficiency is usually dependent on a solid matrix. Developing efficient matrix-free RTP CDs is still a great challenge. Here, a simple strategy is reported to synthesize efficient matrix free RTP N-CDs from citric acid (CA) and basic fuchsine (BF) via a single step hydrothermal process. The synthesized N-CDs show strong fluorescence in aqueous solution and the solid state, and the RTP lifetime is long-lived (51.9 ms at 298 K). The phosphorescence of the N-CDs is attributed to the formation of an internal hydrogen bonded network by the organic polymers on the surface of the N-CDs. This reduces the nonradiative transitions of the triplet excitons and mitigates oxygen quenching. The application of N-CDs as security inks is demonstrated.