Effect of different surface functional groups of graphene on oxidative stress in Daphnia magna

Graphene could be modified by functional groups through a series of transformation processes after being released into environment. Meanwhile, the properties of graphene, such as solubility and biotoxicity, could be altered by surface modification. It is important to investigate the aquatic toxicity of graphene and its surface functionalized derivatives for assessing their ecological risks. Oxidative stress is one of the main toxicity mechanisms of graphene nanomaterials, but the effect mechanism of different surface functional groups (amino and thiol) on the oxidative stress induced by graphene is still unclear. In order to illuminate the toxicity mechanism and evaluate the extent of oxidative stress, a series of graphene nanomaterials including unfunctionalized grapheme (u-G), carboxylated grapheme (G-COOH), aminated grapheme (G-NH 2 ), hydroxylated grapheme (G-OH) and sulfydryl grapheme (G-SH) were selected to determine the levels of reactive oxygen species(ROS), antioxidant enzymes, antioxidant and lipid peroxidation in Daphnia magna induced by these graphene nanomaterials. Daphnia magna were exposed to graphene and its surface functionalized derivatives during 24 h period. Following treatment, the parameters reflecting oxidative stress such as ROS, superoxide dismutase (SOD), catalase (CAT), glutathione (GSH) and malondialdehyde in Daphnia magna were measured. The results showed that malondialdehyde, a marker of lipid peroxidation in Daphnia magna , was significantly increased by u-G, G-COOH and G-OH, respectively. The level of malondialdehyde was significantly higher under u-G exposure than that under G-COOH or G-OH exposure. It is indicated that the degree of oxidative damage induced by u-G in Daphnia magna was more serious than that of G-COOH and G-OH. In addition, the three nanomaterials also caused the changes in ROS and glutathione (GSH) levels, and activities of superoxide dismutase (SOD) and catalase (CAT) were decreased by u-G and G-OH. However, G-SH and G-NH 2 did not cause oxidative damage within 24 h exposure period. Marked inactivation of antioxidant enzymes in an ROS-independent manner was observed in response to G-SH, suggesting that G-SH may cause a structural change in enzymes, leading to functional inactivity. G-NH 2 did not affect the levels of ROS, antioxidant enzymes, antioxidant and lipid peroxidation. At 24 h after stopping exposure, Daphnia magna can alleviate the oxidative damage induced by u-G, G-COOH and G-OH to a certain extent by its own regulation. The ability of graphene nanomaterials to cause oxidative stress in Daphnia magna is as follows: u-G>G-COOH≈G-OH>G-SH>G-NH 2 . The main reasons of oxidative stress in Daphnia Magna induced by u-G, G-COOH and G-OH were the generation of ROS, which led to the imbalance of antioxidant defense system and lipid peroxidation. G-SH mainly inhibited the activities of SOD and CAT antioxidant enzymes and affected the antioxidant defense system. However, there were no significant changes in ROS levels and antioxidant enzymes in Daphnia magna under G-NH 2 exposure.