Nitrogen-Sulfur-Doped Graphene Quantum Dots with Metal Ion-Resistance for Bioimaging

The development of ultra-stable and highly fluorescent heteroatoms-doped graphene quantum dots (GQDs) for bioimaging remains a challenge due to the fluorescence quenching caused by binding between the heteroatoms-based functional groups of the GQDs and common metal ions in biological systems. Here, we developed a facile hydrothermal method to prepare nitrogen-sulfur doped GQDs (NS-GQDs). The fluorescence signals of the NS-GQDs are highly stable in the existence of different metal ions. Two natural products, aspartic acid and cysteine, were utilized as the carbon precursors and heteroatomic (nitrogen and sulfur) sources. The produced NS-GQDs showed a quantum yield up to 19.3  1.7 % with a maximum emission of 480 nm under the excitation of 400 nm. The elemental analysis, including X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS), and Fourier-transform infrared spectroscopy (FTIR), clearly demonstrated the presence of –NH2 and –SH groups on the surface of the NS-GQDs. Addition...