Recycling Synthetic Route to Full-Color Fluorescent Carbon Nanodots

In the process of commercialization, it is critical to develop appropriate routes to decrease the pollution and the cost for preparing and purifying carbon nanomaterials in a large scale. Here, a low-pollution and recyclable approach has been developed to synthesize fluorescent carbon nanodots (CDs). In this route, citric acid and urea were used as precursors and N,N-diethylformamide (DMF) was used as the solvent and eluent for the solvent-thermal polymerization of CDs and the separation of different emissive CDs in chromatography. The unreacted precursors (citric acid and urea), effluent, and DMF reclaimed from the evaporated CD solution were separately reused as the reaction source and the eluent for the next cycle preparation process. In this way, CDs with their emission wavelength covering the full visible region have been synthesized. The CDs exhibit good biocompatibility and can be used for full-color fluorescent down-conversion films and high-contrast cellular bioimaging through single-photon or two-photon excitation. This work provides an environmental-friendly route to fluorescent CDs, which is essential for future industrial mass production, and thus may push forward their applications.