Underlying Mechanisms of Reactive Oxygen Species and Oxidative Stress photoinduced by Graphene and its Surface Functionalized Derivatives
Graphene can be modified by different functional groups through various transformation processes in the environment. Toxicological activity of graphene is closely relevant to its ability of reactive oxygen species (ROS) production, which can be altered by the surface modification. Herein, unfunctionalized graphene (u-G), carboxylated graphene (G-COOH) and aminated graphene (G-NH2) were selected to determine their ability of photogenerating ROS in aqueous phase. Oxidative stress (ROS concentration and superoxide dismutase activity) in Daphnia magna under the simulated sunlight radiation induced by the materials were also investigated. Based on density functional theory (DFT) calculation, the photochemical pathways of ROS production were identified. G-COOH and G-NH2 produced singlet oxygen in aqueous phase by mediating energy transfer. G-COOH, G-NH2 and u-G generated superoxide anion and further produced hydroxyl radicals by inducing electron transfer. By comparing the biological redox potential and the lowest occupied molecular orbitals values (ELUMO) of the substances, u-G and G-COOH were identified to have the potential of inducing oxidative stress. The predictive result was validated by the significant increase of oxidative stress biomarkers in Daphnia magna. By coupling experimental observations with the theoretically predictions, the present results provide mechanistic insight to understand the photochemical activity and toxicity of graphene and its surface functionalized derivatives.