Effect Of Different Surface Functional Groups Of Graphene On Toxic Mechanism In Daphnia Magna

Graphene could be modified by functional groups through a series of transformation processes after being released into the environment.Meanwhile,the properties of graphene,such as solubility and biotoxicity,could be altered by surface modification.In order to illuminate the toxicity effect and mechanism of graphene and surface functionalized derivatives on aquatic organisms,we prepared a series of graphene nanomaterials including unfunctionalized unfunctionalized graphene?u-G?,carboxylated graphene?G-COOH?,aminated graphene?G-NH₂?,hydroxylated graphene?G-OH?and sulfydryl graphene?G-SH?,and determined the levels of reactive oxygen species?ROS?,antioxidant enzymes,antioxidant and lipid peroxidation in Daphnia magna induced by these graphene nanomaterials.In addition,based on transcriptomics techniques,the effect and molecular mechanisms of toxicity of graphene and surface functionalized derivatives in Daphnia magna were explored under chronic exposure conditions.?1?The results showed that malondialdehyde,under the same exposure conditions?24 h,2 mg/L?,was significantly increased by u-G,G-COOH and G-OH,respectively?p<0.05?.The increased level of malondialdehyde under u-G exposure?78%?was significantly higher than that under G-COOH?28%?or G-OH?30%?exposure.It was 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?p<0.05?,and activities of superoxide dismutase?SOD?and catalase?CAT?were decreased by u-G and G-OH?p<0.05?.However,G-SH and G-NH₂ 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-NH₂ did not affect the levels of ROS,antioxidant enzymes,antioxidant and lipid peroxidation.After depuration for 24 h,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-NH₂.?2?The result of 21-day reproductive and developmental toxicity test showed that u-G inhibited the growth and reproduction of Daphnia magna,and G-NH₂ and G-SH promoted the growth and reproduction of Daphnia magna.The reproduction of Daphnia magna was affected by G-OH or G-COOH,but the growth of Daphnia magna was not significantly affected by G-OH or G-COOH exposure.Gene ontology?GO?analysis showed that graphene and its surface functionalized derivatives promoted the gene expression related to chitin,glucose metabolism and cuticle structure components in Daphnia magna,indicating Daphnia magna might promote the detoxification of graphene and its surface functional derivatives by increasing the expression of the genes.Kyoto Encyclopedia of Genes and Genomes?KEGG?analysis showed that the toxic effects of u-G in Daphnia magna were related to the mechanisms of oxidative stress pathways?�Mineral absorption�,Corrected p-value<0.05?.The toxic effects of functionalized graphene in Daphnia magna were related to the mechanisms of several metabolic pathways,such as the�protein digestion and absorption�pathway and�carbohydrate digestion and absorption�pathway?Corrected p-value<0.05?.In addition,G-NH₂ and G-OH inhibited the genes expression in�DNA replication�and�mismatch repair�pathways,which may cause DNA damage.G-NH₂ and G-OH also inhibited the transcription and translation of genes in Daphnia magna,resulting in a decrease in the number of transcriptional proteins,affecting the function of proteins and normal life activities.These results will help us to understand the aquatic toxicity mechanism of graphene with different surface functional groups,and establish an important foundation for revealing the broad impacts of carbon nanomaterials on aquatic environments.