Potential concerns in fullerene application to water treatment related to transformation, cellular uptake and intracellular catalysis

Fullerene (C-60) exhibits versatile properties that shows great potential for improving water treatment technologies. However, the probable transformation of C-60 during water treatment, which consequently changes the physicochemical properties and toxicity of the parent compound, may introduce doubt concerning its application. Our results demonstrated that the C-60 aggregate (nC(60)) was transformed to a more oxidized form under common water disinfection processes (i.e., ultraviolet irradiation and photochlorination). The light-irradiated product (UV_nC(60)) exhibited lower cytotoxicity toward macrophage J774A.1 cells relative to nC(60), whereas the photochlorinated product (UV/Cl_nC(60)) increased the toxic effect. Particularly, the internalization of nanoparticles and the mimetic superoxide dismutase (SOD) activity resulted in the selective accumulation of intracellular hydrogen peroxide. Thus, sequential exposure to a nonlethal dose of nanoparticles followed by 5 mu M copper ions (which is a much lower level than the EPA-regulated level of 20 mu M in drinking water) led to the significant production of hydroxyl radicals inside cells. The uptake and SOD-like activity were highly structure-related, with the most noteworthy activity obtained for UV/Cl_nC(60). These results emphasize that environmental transformation-induced property changes should be given adequate consideration in the risk assessment of C-60.