Synthesis of Bifunctional Graphene Quantum Dots and Its Application in Fluorescence Detection of pH and Cell Imaging

Functional groups may change the electrical characteristics of graphene quantum dots, thus leading to the improvement of their properties and related applications. To improve the optical properties, we designed and synthesized pentaethylene hexamine and dodecylamine functionalized graphene quantum dots (PEHA-GQD-DA). Citric acid was mixed with pentaethylene hexamine and heated at 170 degrees C for 0-5 h. Then dodecylamine was added and the reaction was continued at 160 degrees C for 1-5 h to obtain PEHA-GQD-DA. The Yesults revealed that the PEHA-GQD-DA was composed of the graphene sheets from 1 nm to 3 nm, and their edges contained abundant functional groups. The introduction of pentaethylene hexamine greatly improved the fluorescence emission. The fluorescence quantum yield reached 72.7%, which was much higher than that of the GQD prepared by the thermal hydrolysis of single citric acid-The introduction of dodecylamine created a special amphiphilic structure that allows the quantum dots more likely to enter cell through the phospholipid bilayer of cell membrane. The PEHA-GQD-DA exhibited an excellent optical behavior for pH value of the medium-Within pH range of 1.0-6.5, the fluorescence intensity increased with the increase of pH value. The fluorescence spectrum sensitively changed with the change of pH value. Where was a good linear relationship between the maximum emission wavelength and the pH value. When the pH was in the range of 6.5-12.0, the fluorescence spectrum didn't change with the change of pH value-However, its fluorescence intensity linearly decreased with the increase of pH value. The existence of common inorganic ions and organic small molecules does not interfere with pH response of the quantum dots. The PEHA-GQD-DA has been successfully applied to fluorescent detection of pH value in water samples and Hela cell imaging.