Striking luminescence phenomena of carbon dots and their applications as a double ratiometric fluorescence probes for H₂S detection

Here, we report some new observations on the luminescence of carbon dots (CDs) that are passivated with polyethylene glycol, and explored them, for the first time, as highly sensitive H₂S detection probe. The as-prepared CDs have an ultraviolet (UV) emission at 350 nm and a green emission at 540 nm when excited at 270 nm, but they have only a green emission at 523 nm when excited at 365 nm. As Na₂S is added to the CDs, the UV emission decreases gradually, while the green emission increases slightly, and interestingly a new emission appears at 455 nm is increased linearly in intensity with the increase of Na₂S concentration. The blue emission at 455 nm is from the intrinsic core of CDs, and the green emissions at 540 or 523 nm are from their surface states, while the UV emission at 350 nm is from the anchored PEG. Both the emission intensity ratios of F₄₅₀/F₃₅₀ and F₄₅₅/F₅₂₃ show an excellent linear relationship with the Na₂S concentration in the range of 0–800 μM, and the detection limit estimated from both of them is approximately 7.0 nM, indicating our method is the most sensitive H₂S CDs based detection method reported thus far. Furthermore, the detection was tested successfully for H₂S imaging in live cells. We propose that the interactions of H₂S with the CDs and the anchored ligands, as well as the energy transfer between the ligands and the CDs, are responsible for the luminescence responses for H₂S detection. Our studies enrich the world of carbon dots with plenty of potential applications.