The antioxidant activity of graphene quantum dots (GQD) with different chemical composition and sp2-hybridized carbon content has been evaluated and compared to that of a standard antioxidant, ascorbic acid. GQD were prepared by top down and bottom up synthetic approaches from three different precursors (carbon black, glucose and pyrene) in order to vary significantly the chemical composition, electron density and sp2-hybridized carbon content. For a given radical, the three types of GQD exhibited very different radical scavenging activity (RSA). Moreover, the RSA varied with the type of free radical and reactive oxygen species (ROS) tested, indicating different radical inhibition mechanisms. Overall, GQD with strong hydrogen donor behavior and large content of sp2–hybridized carbon domains were the most effective radical scavengers. Thus, highly graphitic GQD with abundant edge functional groups produced from pyrene exhibited an extraordinary high antioxidant activity, with inhibition effective concentrations much lower than those of ascorbic acid. The great potential of highly antioxidant GQD as protective films against organic dye decoloration by ROS was demonstrated with two model target molecules, methylene blue and rhodamine B.
