Functional graphene quantum dots (IL-GQDs) are synthesized by electrochemical exfoliation in a one-pot process. Ionic liquid in the mixed solution (water/acetonitrile) is used as the electrolyte, which can be bonded to the surface of GQDs. The method just needs a very low concentration of ionic liquid. Fluorescent spectra indicate that the maximum emission intensity is achieved at 465 nm. Further, the obtained IL-GQDs exhibit selective fluorescence quenching towards Cu(II) ion, with a detection limit of ppb level.