Environmental photochemistry of aqueous fullerene clusters

Because large scale application of buckminsterfullerene C 60 in commercial products has nearly come to realization, it is anticipated that significant quantities of C60 will reach the natural environment over the next decades. Therefore, it is imperative to understand its persistence, fate, and potential health impacts in the environment. To address part of this knowledge gap, this study has examined a potentially significant environmental fate process - photochemical transformation of aqueous C60 nanoparticles (nC60). Stable nC60 suspensions form upon mixing C 60 with water, and may be an important route for C60 to enter the environment. In this study, nC60 was observed to phototransform with a half-life of approximately 1 day under continuous fall sunlight (West Lafayette, IN, 86°55’ W, 40°26’ N). The loss of C 60 from the aqueous suspension occurred concurrent with the loss of the yellow-brownish color of parent nC60 and the decrease in the nanoparticle size. The lost C60 occurred with accumulation of soluble photoproducts in the aqueous phase. The phototransformation kinetics in sunlight was dependent on the cluster size, but was independent of pH (3-11), the presence of Suwannee River fulvic acid (10 mg/L), or preparation method (THF/nC 60 versus SON/nC60). In contrast to reports showing no 1O2 formation during short-term irradiations (\u3c 2 h) under similar conditions, the results of this study indicate that 1O2 forms upon irradiation of nC60 (\u3e 15 h), using furfuryl alcohol (FFA) as a reactive 1O2 scavenger. Further, 1O2 generation was more pronounced after soluble photoproducts had accumulated in solution. Removing O2 completely quenched the losses of FFA and C60, indicating nC 60 phototransformation requires 1O2 in accordance with the phototransformation mechanism reported for molecular C60 in organic solvents. Additional evidence for 1O2 as a reactive intermediate is that reactions in D2O or in the presence of azide ion accelerated or slowed 1O2 formation, respectively. Further studies in summer sunlight indicate 1O 2 occurred during photolysis of nC60 with a concentration 1 order of magnitude higher than the average value typically found in natural waters containing an equivalent amount of natural organic carbon. Light within the visible portion of the solar spectrum (λ ≥ 400nm) was shown to be sufficient for nC60 phototransformation and 1O 2 production. Under monochromatic light at either 366 or 435 nm, the apparent quantum yields were 1.48 × 10-5 and 2.95 × 10-5, respectively, again indicating the visible light is sufficient for transformation. Characterization of the photoproducts by XPS, FTIR and 13C-NMR methods collectively reveal the occurrence of multiple oxygen-containing functionalities on the remaining carbon molecules, with analysis by LDI-TOF mass spectrometry indicating that most of the material retains its original 60-carbon structure