Photochemical production of reactive oxygen species by C₆₀ in the aqueous phase during UV irradiation

The objective of this study was to investigate photochemical production of singlet oxygen (¹O₂) and superoxide radical anion (O ₂^•-) by C₆₀ in water. It was demonstrated that photoexcited C₆₀ in the aqueous phase efficiently mediated transfer of absorbed energy to oxygen and produced singlet oxygen when associated with surfactant (Triton X100 and Brij 78) or polymer (polyvinylpyrrolidone), which is consistent with previously observed behavior in organic solvents. However, when C₆₀ was present as colloidal aggregate suspension, prepared through solvent exchange or sonication, this intrinsic character was lost. Similarly, C₆₀ associated with surfactant mediated electron transfer from electron donor (triethylamine) to oxygen producing superoxide radical, while C₆₀ aggregates and C ₆₀ associated with polymer did not. These results suggest that the ability of C₆₀ to mediate energy and electron transfer may be affected by the degree of C₆₀ aggregation in the aqueous phase as well as characteristics of associated stabilizing molecules. Dependence of photochemical reactivity of C₆₀ on its dispersion status in the aqueous phase is critical in assessing environmental impact and cytotoxicity of this material, as C₆₀ associated with model natural organic matter was found to exist in aggregate form and did not produce reactive oxygen species under UV irradiation.