Adverse effects of graphene incorporated in TiO2 photocatalyst on minuscule animals under solar light irradiation

The adverse effect of graphene-titanium oxide composite films (containing sheet-like surface morphology) on Caenorhabditis elegans nematodes (as a model for minuscule animals) was investigated in a solar light-induced photocatalytic process. X-ray photoelectron spectroscopy demonstrated photocatalytic reduction of the chemically exfoliated graphene oxide sheets included in the TiO2 film. Furthermore, formation of TiC and Ti-O-C bonds in the composite film (obtained through annealing at 450 degrees C in air) resulted in a substantial delay in the recombination rate of the photoexcited electron-hole pairs and more efficient photocatalytic processes. The composite film showed a type of concentration-dependent cytotoxic effect on the nematodes under irradiation. The rapid (4 h) photoinactivation of the nematodes by the composite films was assigned to a high generation of reactive oxygen species (ROS) under solar light irradiation (similar to 25x of the ROS level of a control sample in dark), rather than the strong photoinactivation of Escherichia coli bacteria, which was utilized as the food of the nematodes in a culture medium. The threshold ROS level for inactivation of the nematodes was found similar to 19x of the control level which was unreachable using TiO2. These results warn us of the destruction of minuscule animals of our environment through extensive application of highly efficient graphene-titanium oxide photocatalysts.