Oxidative Stress Potential Of Graphene And Its Surface Functionalized Derivatives To Daphnia Magna

Some carbon nanomaterials can be photoinduced to generate reactive oxygen species (ROS) via energy and/or electron transfer pathways under irradiation, cause oxidative stress, and further initiate toxic effects on organisms. Toxicology studies involving graphene, a new type of carbon nanomaterial, have mainly focused on its oxidized form, i.e. graphene oxide. However, it needs further research that how different kinds of surface functionalization could change the electronic structure of graphene, and thus influence the generation of ROS which could further cause oxidative stress.In this study, Electron Paramagnetic Spectroscopy (EPR) and molecular probes were employed to determine the types and levels of ROS photoinduced by carboxylated graphene, amide graphene and unfunctionalized graphene. Based on density functional theory, the reaction pathways and mechanisms of ROS generation induced by graphene and corresponding functionalized derivatives were studied. Oxidative stress potential of graphene and its surface functionalized derivatives were predicted by using energy band structure, and were verified by measuring oxidative stress biomarkers with Daphnia magna as a model organism.(1) Based on EPR technique, carboxyl graphene and amide graphene were determined to generate 1O2 via energy transfer pathway, and produce ·OH via electron transfer pathway under UV irradiation. Based on molecular probe method, the ROS levels generated by graphene and its surface functionalized derivatives were quantitatively detected. The results showed the ability of 1O2 production as follows:carboxylated graphene> amide graphene; the intensity of O2·- production:carboxylated graphene> amide graphene> unfunctionalized graphene; and for ·OH generation:amide graphene?carboxylated graphene.(2) Based on density functional theory, the energy band structure of graphene and its surface functionalized derivatives were calculated. The energy gap (Eg) of carboxylated graphene and amide graphene were greater than the transition energy of molecular oxygen (0.97 eV), which indicated that the energy transfer pathway of 1O2 production could occur. Unfunctionalized graphene of which Eg was less than 0.97 eV was unable to produce 1O2. The lowest unoccupied molecular orbital energies (ELUMO) of graphene and its functionalized derivatives were greater than -4.30 eV, indicating that they have ability to generate O2·- via electron transfer pathway. However, the highest occupied molecular orbital energies (EHOMO) of graphene and its functionalized derivatives were greater than -6.70 eV, implying that the water cannot be oxidized to produce ·OH. Therefore, ·OH production may be generated through the disproportionation of O2·-(3) The calculated ?umo of carboxylated graphene and unfunctionalized graphene were within the range of the standard redox potentials (-4.12 eV?-4.84 eV) in biological media, indicating that they have a biological effect of oxidative stress. Through the determination of oxidative stress markers for Daphnia magna, it was found that unfunctionalized graphene and carboxylated graphene significantly enhanced the total ROS levels, for which the trend was: unfunctionalized graphene> carboxylated graphene. The total superoxide dismutase activity was significantly increased after exposure to unfunctionalized graphene and carboxylated graphene, while the total superoxide dismutase activity was not significantly changed after exposure to amide graphene. When vitamin C was added, the total ROS levels induced by unfunctionalized graphene and carboxylated graphene were reduced. The results showed that carboxylated graphene and unfunctionalized graphene can lead to oxidative stress, but amide graphene had no oxidative stress.