Transformation of aggregated C60 in the aqueous phase by UV irradiation

This study demonstrates that water-stable C60 cluster (nC60) undergoes a photochemical transformation(s) when irradiated with monochromatic UV light at 254 nm. Upon UV exposure, characteristic absorption of nC60 in the visible (ca. 450-550 nm, indicative of a cluster structure) and UV regions (indicative of underivatized molecular C60) gradually disappeared. Concurrently, a new product with absorption centered at 210 nm formed. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) analyses confirmed a complete reduction in aggregation and formation of a soluble product. Negligible loss of total organic carbon (TOC) and drastic retardation in degradation kinetics in the absence of oxygen collectively implied that photochemical transformation was accomplished via oxidative pathway without carbon mineralization. MS (LDI), FTIR, and XPS analyses indicated a 60 carbon cage product, containing various oxygen functional groups such as epoxides and ethers. In addition, this product demonstrated significantly less antibacterial effects on Escherichia coli when compared to the parent nC60. The results of this study suggest that accurate assessment of C60 in environmental life cycles and impact should consider the light-mediated transformation of C60 in the aqueous phase and resulting water-soluble products.