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

Abstract

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 °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 (∼25× 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 ∼19× 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.