ZnO is found to possess high photocatalytic efficiency among all inorganic photocatalytic materials, and is more biocompatible than TiO2 [63]. ZnO can highly absorb UV light [64], and it has a better response to UV light, thus its conductivity dramatically enhances, and this feature significantly activates the interaction of ZnO with bacteria. Its photoconductivity persists long after turning off the UV light, and it has been attributed to surface electron depletion region strongly associated to negative oxygen species ( O−2,O2−2O2−,O22− ), adsorbed on the surface [65]. UV illumination rapidly causes desorption of this loosely bound oxygen from the surface. This results in reducing the surface electron depletion region and causing improved photoconductivity [66]. The photocatalysis is described as a photo-induced oxidation process that can damage and inactivate organisms [67]. ZnO-NPs in aqueous solution under UV radiation have phototoxic effect that can produce ROS such as hydrogen peroxide (H2O2) and superoxide ions (O2−). Such species are extremely essential for bio-applications [68]. The generated active species are capable to penetrate into the cells, thus inhibit or kill microorganisms. This process inspired the use of the photocatalytic activity of ZnO-NPs in bionanotechnology and in bionanomedicine for many antibacterial applications. Therefore, enhancement of ZnO bioactivity was considered as a result of the produced free radicals, as ZnO absorbs UV light [15]. A detailed reaction mechanism which explains this phenomenon was proposed by Seven et al. [69] and Padmavathy and Vijayaraghavan [12] as follows.