Nitrogen and sulfur dual-doped yellow fluorescent carbon dots: strongly pH/polarity response and cancer cell...

Polarity, pH, and metal ions are closely related to normal cellular function, and their abnormal changes are linked to pathophysiological processes, making their real-time monitoring scientifically significant and practically valuable. In this research, nitrogen-sulfur dual-doped carbon dots (SN-CDs) sensitive to strongly acidic environments and polarity were synthesized via one-step solvothermal method. Under pH = 1-4 conditions, the amino and carboxyl groups on SN-CDs' surface underwent protonation, resulting in SN-CDs' aggregation and the subsequent quenching of fluorescence. The combination of Fe³⁺ and SN-CDs also resulted in fluorescence quenching. Meanwhile, as solvent polarity increased, the enhanced hydrogen bonding interactions exacerbated SN-CDs' aggregation and thus induced aggregation fluorescence quenching. In addition, due to the presence of pyridinic N and pyrrolic N structures in SN-CDs, the strengthened charge effect in polar solvents caused the fluorescence emission peak to redshift with increasing polarity. In cell imaging applications, effective differentiation between normal cells and cancer cells was achieved by leveraging the polarity difference between them. Moreover, the excellent photostability performance of SN-CDs enabled them to continuously present clear images of the cytoplasm and cell morphology within 20 min. The successful preparation of SN-CDs not only provides a new strategy for distinguishing normal cells from cancer cells but also points out the direction for the development of highly sensitive to pH/polarity and long-term continuous bioimaging probes in the future.