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 λ(ex) 490 nm (2.53 eV) to emit excitation-independent photoluminescence at a maximum wavelength of λ(em) 550 nm. This is thus far the longest emission wavelength reported for GQDs. Stable photoluminescence is achieved at pH 4-10 with an ionic strength of 1.2 mol L(-1) KCl. These features make the p-GQDs excellent probes for bio-imaging and bio-labeling, which is demonstrated by imaging live HeLa cells.