Green and simple synthesized graphene/MnO2 quantum dot nanocomposite: characterization and application as an efficient adsorbent for solid-phase extraction of heavy metals Maryam Babaei, Parviz Aberoomand Azar*, Mohammad Saber Tehrani, Manoochehr Farjaminezhad, Syed Waqif Hussain, 10.1007/s40097-021-00410-z XML