Nitrogen and sulfur co-doped carbon dots with bright fluorescence for intracellular detection of iron ion and thiol

Carbon dots (CDs) have been widely used in recent years because of their excellent water solubility and abundant surface functional groups. However, compared with quantum dots or biological probes, the quantum yield of CDs is lower, and the fluorescence mainly concentrated in the blue-green range, which significantly limits the biological applications of CDs. Heteroatoms doping is the most common method to improve the luminescence of CDs. In this work, nitrogen and sulfur co-doped luminescent CDs were successfully synthesized by microwave assisted method using glutathione (GSH) and p-phenylenediamine (PPD) as raw materials. It can emit bright green fluorescence in ethanol solution, and the maximum emission wavelength is 535 nm when excited at 374 nm, and the absolute quantum yield is as high as 63%. Iron ion (Fe3+) can interact with the functional groups on the surface of the CDs to form CDs/Fe3+, which is a non-fluorescence complex, and Fe3+ can be reduced to ferrous ion (Fe2+). In other words, the reaction mechanism of CDs and Fe3+ is a combination of dynamic quenching and static quenching. The fluorescence of CDs quenched by Fe3+ can be restored by thiol, because there is a stronger binding force between sulfhydryl (-SH) on the surface of thiol and Fe3+, which enables CDs to be released. In addition, the CDs has good biocompatibility and stability, indicating that it has excellent potential in bioimaging. This discovery will expand the application of CDs in the fields of biosensing and imaging.