We have developed a novel eco-friendly approach involving hydrazine hydrate-assisted hydrothermal cutting followed by functionalization with poly (ethylene imine) (PEI) for fabricating highly fluorescent graphene quantum dots from coffee grounds. The PEI-functionalized graphene quantum dots exhibit enhanced band-edge photoluminescence with single exponential decay. Their preliminary applications in bioimaging and sensing of heavy metals have been demonstrated. (C) 2016 Elsevier B.V. All rights reserved.