Antibacterial property of graphene oxide: the role of phototransformation

Graphene-related materials are emerging with a wide range of potential applications. Their possible ecological effect must be properly evaluated before their widespread use. This study examined the antibacterial activity of graphene oxide (GO) before and after solar transformation under two reaction scenarios. GO was directly phototransformed under simulated sunlight or indirectly photolyzed by photochemically generated hydroxyl radical (center dot OH). The results indicate that compared to parent GO, directly phototransformed GO showed increased toxicity to bacteria, while the indirectly phototransformed GO became less toxic. The reduced bacterial toxicity of indirectly photolyzed GO correlated with its large decrease in total organic carbon (TOC) concentration after the indirect photoreaction. In contrast, the directly phototransformed GO caused greater membrane disruption and oxidative stress, which is consistent with its increased antibacterial activity. The oxidative stress was likely associated with the intrinsic oxidation capability of the directly photolyzed GO. Given that direct photolysis of GO is expected to occur more rapidly in sunlit surface waters, as predicted in previous research, enhanced antibacterial activity of phototransformed GO will likely dominate the initial solar transformation of GO, while microbial toxicity will decrease in long term photolysis where GO is extensively decomposed by center dot OH-mediated photoreaction.