Synthesis of N,S co-doped carbon dots for fluorescence detection of Fe3+ ions and intracellular pH imaging

Using citric acid and thiourea as precursors, sulfur-nitrogen co-doped dual emission carbon dots (DECDs) were prepared by a simple solvothermal method. At the excitation wavelength of 360 nm, DECDs have two emission peaks located at 465 nm (blue) and 592 nm (yellow-green) which are attributed by core and surface of DECDs. It is found that Fe3+ ions can coordinate with oxygen-containing groups on DECDs, and then the electrons transfer from DECDs to Fe3+ ions, which quench the fluorescence of DECDs. The fluorescence intensity is linear with the concentration of Fe3+ ions in the range of 0–100 μM and the detection limit is 0.77 μM. Moreover, the designed sensor can be applied to detect Fe3+ ions in tap water with high selectivity, which provides a basis for the quantitative detection of Fe3+ ions. At the same time, DECDs can be used as a pH fluorescence probe. In the pH 2.5–11.5 range, the fluorescence increases with pH whilst the solution color changes, providing a visualization platform for pH detection. Moreover, the pH fluorescence probe can be used for intracellular pH fluorescence imaging in living cells, and DECDs have an excellent biocompatibility (e.g., cell viability remains about 80% after treatment for 24 h at 200 μg·mL−1 of DECDs). Therefore, DECDs has a certain application prospect in food detection, biological detection and determination of intracellular pH change.