Effects of C<sub>60</sub> on the Photochemical Formation of Reactive Oxygen Species from Natural Organic Matter

Buckminsterfullerenes (C<sub>60</sub>) are widely used nanomaterials that are present in surface water. The combination of C<sub>60</sub> and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (<sup>1</sup>O<sub>2</sub>), hydroxyl radical (HO<sup>•</sup>)-like species, superoxide radicals (O<sub>2</sub><sup>•–</sup>), hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), and triplet excited states (<sup>3</sup>C<sub>60</sub>*/<sup>3</sup>HA*) in solutions containing both C<sub>60</sub> and HA. The quantum yield coefficients of excited triplet states (<i>f</i><sub>TMP</sub>) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C<sub>60</sub> proved to have only a slight impact on the <sup>1</sup>O<sub>2</sub> formation from HA, C<sub>60</sub> played a key role in the inhibition of O<sub>2</sub><sup>•–</sup>. The photochemical formation of H<sub>2</sub>O<sub>2</sub> followed the conservative mixing model due to the reaction of C<sub>60</sub><sup>•–</sup> with HO<sub>2</sub><sup>•</sup>/O<sub>2</sub><sup>•–</sup>, and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 10<sup>6</sup> M<sup>–1</sup> s<sup>–1</sup>. The apparent <i>f</i><sub>TMP</sub> was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of <sup>3</sup>C<sub>60</sub>* with the TMP probe. In contrast, C<sub>60</sub> did not have an effect on the photochemical formation of HO<sup>•</sup> from HA, suggesting that HO<sup>•</sup> is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C<sub>60</sub> on aqueous environments.