Reactions of graphene oxide and buckminsterfullerene in the aquatic environment
Due to unique physical and chemical properties, carbon-based nanomaterials, including C60 and graphene oxide, now being used in an increasing number of applications. Considering their widespread use, nanoparticles will inevitably find their way to the natural environment. However, their environmental fate and transport have not been intensively explored, resulting in a general lack of knowledge regarding their risk assessment and life cycle exposure concentrations. To this end, this study has investigated: (i) the photo mineralization of aqu/nC60 clusters under photo irradiation, and (ii) environmental transformation of graphene oxide in the aquatic environment. This study shows that CO2 was produced from aqu/nC60 when exposed to lamp light within the solar spectrum (300-410 nm), suggesting mineralization was indeed occurring to some extent. In addition, the ultraviolet-visible (UV-vis) spectrum and liquid chromatographic separation of photo-irradiated samples indicated that decomposition of C60 occurred. Aqueous graphene oxide suspensions produced reactive oxygen species (ROS), including superoxide anion (O2˙–)and hydrogen peroxide (H 2O2) when exposed to the same lamp light, under oxic conditions. The color of the GO suspension progressed from pale to dark brown during the photoreaction process, consistent with changes in the UV-vis spectrum. Raman spectra showed that the ratio of the ID/IG bands increased as irradiation proceeded, suggesting an increased number of defects (e.g., functional groups and vacancies) on the graphene oxide sheets. These defects may be the sites for ROS production. In dark environments, GO was able to accept electrons from a well know reducing agent and electron donor (NADH), oxidizing NADH to NAD+, and transferring these electrons to molecular oxygen in water, producing the reactive oxygen species (ROS) superoxide anion (O2˙–)and hydrogen peroxide (H 2O2). DNA cleavage was observed in air-satured GO suspensions that contained NADH, suggesting that ROS production could be a mechanism for DNA damage by GO within biological cells. Indirect photochemical reactions involving GO were shown to occur in an experiment in which hydroxyl radicals were generated by the photodecay of hydrogen peroxide. GO was oxidized by ˙OH within a 48 hr irradiation period, as measured by changes in the UV-Vis absorbance spectra over the wavelengths from 300 to 800 nm. Spectral changes were consistent with visible color changes (i.e., fading) from 0 to 48 hours. Hence, both direct and indirect photochemical reactions might greatly affect the lifetime and stability of GO in surface waters.
Jafvert, Purdue University.
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