Photoenhanced oxidation of nC₆₀ in water: Exploring H₂O₂ and hydroxyl radical based reactions

The transformations of water-stable C₆₀ clusters (nC₆₀) to oxidized C₆₀ derivatives via photoreactions and ground-state reactions have been described as critical processes in understanding the ultimate environmental fate of fullerene-based materials. However, (photo)oxidation of aqueous-based C₆₀ (as water stable, nanoscale aggregates termed nC₆₀) with hydrogen peroxide (H₂O₂) and/or hydroxyl radical ([rad]OH), common environmental oxidants, has not been fully explored. To address this, the aqueous physicochemical transformations of C₆₀ (as nC₆₀ aggregates) in the presence of H₂O₂ and/or [rad]OH in both photoexcited-state and ground-state under environmentally relevant conditions are quantitatively described. Results show that nC₆₀ undergoes facile oxidation in the presence of H₂O₂ under UVA irradiation but not under dark conditions, and the oxidation reaction rates increase with effective [rad]OH concentration (via photodecomposition of H₂O₂), while being inversely related to solution pH. Product characterization via dynamic light scattering, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, total organic carbon, high-performance liquid chromatography, and octanol-water partition experiments collectively describe resulting C₆₀ derivatives with new covalent oxygen functionality which are also relatively more hydrophilic. For all cases, photoirradiation was observed to significantly enhanced the rates and extent of C₆₀ oxidation.