Photochemistry of Aqueous C60 Clusters: Wavelength Dependency and Product Characterization

To construct accurate risk assessment models for engineered nanomaterials, there is urgent need for information on the reactivity (or conversely, persistence) and transformation pathways of these materials in the natural environment. As an important step toward addressing this issue, we have characterized the products formed when aqueous C-60 clusters (nC(60)) are exposed to natural sunlight and also have assessed the wavelengths primarily responsible for phototransformation. Long-wavelength light (lambda >= 400 nm) isolated from sunlight, was shown to be important in both the phototransformation of nC(60) and in the production of O-1(2). The significance of visible light in mediating the phototransformation of nC(60) was supported by additional experiments with monochromatic light in which the apparent quantum yield at 436 nm (Phi(436) (nm) = (2.08 +/- 0.08) x 10(-5)) was comparable to that at 366 nm (Phi(356) (nm) = (2.02 +/- 0.07) x 10(-5)). LDI-TOF mass spectrometry indicated that most of the photoproducts formed after 947 h of irradiation in natural sunlight retain a 60 atom carbon structure. A combination of C-13 NMR analysis of C-13-enriched nC(60), X-ray photoelectron spectroscopy and FTIR indicated that photoproducts have olefinic carbon atoms as well as a variety of oxygen-containing functional groups, including vinyl ether and carbonyl or carboxyl groups, whose presence destroys the native pi-electron system of C-60. Thus, the photoreactivity of nC(60) in sunlight leads to the formation of water-soluble C-60 derivatives.