Highly fluorescent carbon dots synthesized with binary dopants for “turn off” and “turn off-on” sensing and cell imaging

In the present study, highly fluorescent carbon dots were prepared by using citric acid as the carbon source with the binary dopants, i.e. melamine and formaldehyde, named as MF-CDs. The co-dopants played a crucial role to enhance the fluorescence quantum yield of the MF-CDs, reaching 48%, in which melamine was the major nitrogen source, while formaldehyde was the co-dopant to regulate the state of nitrogen in the MF-CDs. The fluorescence of the MF-CDs was found to be quenched by Hg 2+ , and then recovered equally by individual halogen ions (eg. Cl − , Br − and I − ). Correspondingly, the “turn off” and “turn off-on” sensing systems were separately established to detect Hg 2+ and total halogen ions with good accuracy and reliability. In addition, the fluorescence of the MF-CDs was quenched by ClO − , and recovered by Fe 2+ ; the “turn off” sensor was established to detect ClO − in tap water with low limit of detection of 0.18 μM, and the “turn off-on” sensor was to determine the ferrous succinate in ferrous succinate tablets. Furthermore, owing to the low cytotoxicity, the MF-CDs were successful for cellular labeling on HepG2 cells. The developed binary dopants were effective to enhance the fluorescence quantum yield and endow the MF-CDs with some special properties, which would help to widen the applicability of the carbon dots.