BACKGROUND: Recently, numerous studies have utilized graphene/nanoparticle hybrid materials in bioapplications such as biosensing, bioimaging, photothermal therapies, delivery of drugs and genes, and stem cell and tissue engineering applications. In this study, we demonstrate a new, simple, low cost, environmentally safe process for the biosynthesis of graphene/silver (GN/Ag) and graphene/gold (GN/Au) nanocomposites using Xanthium strumarium plant extract. RESULTS: After synthesis of Ag or Au nanoparticles by treatment of an aqueous solution of AgNO3 or HAuCl4 with Xanthium strumarium plant extract, graphite was added to the synthesized nanoparticle solution and sonicated in the ultrasonication bath. The synthesized nanocomposites were characterized by UV-visible spectroscopy, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive X-ray spectroscopy. GN/Ag and GN/Au nanocomposites showed higher antimicrobial activity and higher peroxidase-like activity, respectively, than individual nanoparticles, graphene alone, or reaction product without plant extract. CONCLUSION: The present results show that GN/Ag and GN/Au nanocomposites can be ecofriendly synthesized using Xanthium strumarium leaf extract. The graphene/metal nanocomposites produced may be less-toxic, biocompatible, and useful for bioapplications. (C) 2016 Society of Chemical Industry