This project evaluates how the toxicity of a graphene-based material changes over time during sunlight exposure. Dose-response curves were established and half maximal effective concentration (EC50) of GO without sunlight exposure were identified. These Ec50 values will be compared with the toxicity of GO after sunlight exposure. After sunlight was provided by a solar simulator at 1 Sun intensity, the toxicity of GO at different sunlight exposure time was measured. Finally, characterization of the change in physicochemical properties and structures were studied. This project revealed the relationship between the structure changes of GO induced by sunlight and its toxicity.