Underlying mechanisms of reactive oxygen species and oxidative stress photoinduced by graphene and its surface-functionalized derivatives

Abstract

Graphene can be modified by different functional groups through various transformation processes in the environment. The toxicological activity of graphene is closely related to its ability to produce reactive oxygen species (ROS), which can be altered by surface modification. Herein, unfunctionalized graphene (u-G), carboxylated graphene (G-COOH) and aminated graphene (G-NH₂) were selected to determine their ability to photogenerate ROS in the aqueous phase. Oxidative stress (ROS concentration and superoxide dismutase activity) in Daphnia magna under simulated sunlight radiation induced by the materials was also investigated. Based on density functional theory (DFT) calculations, the photochemical pathways of ROS production were identified. G-COOH and G-NH₂ produced singlet oxygen in the aqueous phase by mediating energy transfer. G-COOH, G-NH₂ and u-G generated superoxide anions and further produced hydroxyl radicals by inducing electron transfer. By comparing the biological redox potential and the lowest occupied molecular orbital values (E_LUMO) of the substances, u-G and G-COOH were identified to have the potential to induce oxidative stress. The predictive results were validated by the significant increase of oxidative stress biomarkers in Daphnia magna. By coupling experimental observations with the theoretical predictions, the results provide mechanistic insight into understanding the photochemical activity and toxicity of graphene and its surface-functionalized derivatives.