Synthesis of highly stable red-emissive carbon polymer dots by modulated polymerization: from the mechanism to application in intracellular pH imaging

Great efforts have been made to develop facile and efficient methods to prepare carbonaceous nanostructures with long-wavelength emission. Herein, we report a low-temperature aqueous strategy to synthesize red-emissive carbon polymer dots (R-CPDs) through the regulation of oxidative polymerization of p-phenylenediamine at 80 degrees C. The morphology, chemical composition and photophysical properties of the R-CPDs are characterized and analyzed in detail, thereby elucidating their photoluminescence origins from the surface state and crosslink enhanced emission effect. The resulting R-CPDs possess unique features including high pH-sensitivity within pH 4-6 and a wide-range tunable solvent-color effect ((em) 528-600 nm). Moreover, the R-CPDs show high stability in physiological media with high salinity, and good resistance to photobleaching. In addition to their favorable biocompatibility, the R-CPDs are applied for monitoring the pH fluctuation in HeLa cells. This study not only provides a unique red emissive carbonaceous nanomaterial for cellular imaging and multicolor applications, but also presents a novel perspective for the construction of long-wavelength emission carbon-based nanomaterials by simple and controllable strategies.