Facile synthesis of ratiometric fluorescent carbon dots for pH visual sensing and cellular imaging

A novel ratiometric emission N, S dual-doped carbon dots (N, S-CDs) were facilely developed for pH visual sensing via one-step hydrothermal method. The proposed N, S-CDs displayed intrinsic pH-sensitive behavior and exhibited ratiometric fluorescence emission (F_{563 nm}/F_{645 nm}) characteristic with the variation of pH values. Interestingly, a significant red shift of emission wavelength can be observed at 645 nm along with the emission at 563 nm decreased accordingly when the pH changed from 3.0 to 1.0. Simultaneously, the fluorescence of N, S-CDs aqueous solution was visually varied from yellow to red. The ratiometric pH linear response was in the region of 3.6 to 2.4 and the pK_a was 2.90. Moreover, the N, S-CDs hold unique optical properties, good reversibility, superior biocompatibility and low cytotoxicity, which was further employed to monitor the intracellular pH fluctuations through the visible fluorescence changes between yellow and red. All these findings demonstrated that N, S-CDs can be utilized as the visual biosensor platform for tracking pH variations in extremely acidic environments such as gastric juice, which provided novel insights for clinical medical disease diagnosis (e.g., stomach disease detection) and other biomedical fields.