Reduced graphene oxide (rGO) generates reactive oxygen species (ROS) under visible light in air via a singlet oxygen–superoxide anion radical pathway which readily kills Enterobacter sp. The rGO⁺ intermediate reacts with a hydroxyl ion to produce graphene oxide (GO) as a coating on the surface of rGO resulting in enhanced fluorescence and a slow down in photo-induced ROS formation. GO is not toxic but on ageing it gets a surface coating of rGO and shows toxicity.