Fluorescent Sensing And Nitrogen Doping Of Graphene Quantum Dots

Graphene quantum dots (GQDs) have been developed recently as a new kind of fluorescent carbon nano-materials with sizes usually below 10 nm. GQDs not only show good fluorescence property and small size features like conventional semiconductor quantum dots (QDs), but also have advantages of good water-solubility and low biotoxicity. Therefore, GQDs are ideal substitute for conventional semiconductor quantum dots in bioimaging, and have attracted great interests from scientists. To date, great effort has been devoted to the large-scale preparation of GQDs. The bright photoluminescence and established low cytotoxicity render GQDs applicable in fluorescence sensors and bioimaging. Chemical doping is an important and effective approach to study and tune the material property, and have been widely applied in the preparation of new type of carbon nano-materials.Based on the current developments of graphene and QDs in our group, we explored a new GQDs-based fluorescence sensor and the synthesis of doped-GQDs.1. Graphene quantum dots as fluorescence probes for sensing of melamine in the presence of Hg2+By making use of the fluorescence of GQDs, we have developed a novel type of rapid and sensitive GQDs-based sensor for the detection of melamine. The synthesized GQDs were strongly luminescent with predominantly aromatic sp2 domains. Melamine could coordinate with mercury and bring more Hg2+ to the surface of GQDs through π-π stacking, thus leading to quenching of the GQDs’fluorescence. The melamine demonstrated a linear range 0.15-20 μM and a detection limit of 0.12 μM, which was far below the regulatory level. This sensing system possessed high sensitivity and selectivity for melamine, suggesting the promising practical usage of this sensing system.2. Preparation and optical properties of nitrogen doped GQDsIn this work, we provided a two-step approach for the synthesis of nitrogen doped GQDs (N-GQDs). Melamine and graphene oxide (GO) powder, used as the nitrogen and carbon precursors respectively, were annealed in a nitrogen atmosphere to get the nitrogen doped graphene. Then the doped graphene was cleaved into N-GQDs under acid conditions. The nitrogen content of N-GQDs was around 3.11%-4.27% and affected by the mass ratio between GO and melamine. The synthesized N-GQDs exhibited good downconversion and upconversion fluorescence properties. We found that the emission peak exhibited a blue shift when the nitrogen doping content increased.