The contribution of indirect photolysis to the degradation of graphene oxide in sunlight

This paper probes the role of hydroxyl radicals (·OH) generated by H2O2 photolysis on graphene oxide (GO) phototransformation, under simulated sunlight. It focuses on comparing the photoreaction of GO with (indirect) and without (direct) added H2O2 under simulated sunlight conditions. The biomarker responses of fish epithelial cells in in vitro assays of parent GO and GO photoreacted with H2O2 and their interaction with model biomembranes are also compared. GO was found to be far more extensively photodecomposed in the presence of H2O2, with ∼85% of the initial carbon content converted to CO2 during 48 h of irradiation. Direct and indirect photoreactions occurred concurrently in GO samples containing H2O2, and indirect photoreaction accounted for ∼70% of GO conversion to CO2. Reaction with ·OH causes increases in the concentrations of carboxylic acid groups of photoreacted GO and low-molecular-weight (LMW) species as part of the intermediate photoproducts. Compared to parent GO, intermediate photoproducts exhibited reduced interaction with model cell membranes and altered biomarker responses. Kinetic analysis extrapolating our data to conditions prevalent in sunlit surface waters predicts that initial GO photoreaction is dominated by direct photolysis, while indirect photoreactions involving ·OH determine subsequent conversion of intermediate GO photoproducts to CO2.