Preparation of Excitation-Independent Photoluminescent Graphene Quantum Dots with Visible-Light Excitation/Emission for Cell Imaging

We report the first pyrrole-ring surface-functionalized graphene quantum dots (p-GQDs) prepared by a two-step hydrothermal approach under microwave irradiation in an ammonia medium. The most distinct feature of the functionalized GQDs is that both the excitation and emission wavelengths fall into the visible-light region. The p-GQDs are excited by visible light at (ex) 490nm (2.53eV) to emit excitation-independent photoluminescence at a maximum wavelength of (em) 550nm. This is thus far the longest emission wavelength reported for GQDs. Stable photoluminescence is achieved at pH4-10 with an ionic strength of 1.2molL(-1) KCl. These features make the p-GQDs excellent probes for bio-imaging and bio-labeling, which is demonstrated by imaging live HeLa cells.