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 C₆₀ clusters (nC ₆₀) are exposed to natural sunlight and also have assessed the wavelengths primarily responsible for phototransformation. Longwavelength light (λ ≥ 400 nm) isolated from sunlight, was shown to be important in both the phototransformation of nC₆₀ and in the production of ¹O₂. The significance of visible light in mediating the phototransformation of nC₆₀ was supported by additional experiments with monochromatic light in which the apparent quantum yield at 436 nm (Ψ_{436 nm}) (2.08 ± 0.08) × 10^-5) was comparable to that at 366 nm (Ψ_{366 nm}) (2.02 (0.07) × 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 of¹³C NMR analysis of ¹³C-enriched nC₆₀, 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 π-electron system of C₆₀. Thus, the photoreactivity of nC₆₀ in sunlight leads to the formation of water-soluble C₆₀ derivatives.