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The toxicity effect due to chronic exposure of ZnO nanoparticles (NPs) was systematically studied by repeatedly treating different lower concentrations of ZnO nanoparticles with culture media of E. coli strain. The chronic exposure of ZnO NPs of concentrations below minimum inhibitory concentration (MIC) exhibited higher toxicity than the single exposure of higher concentrations. Most striking result was 57% inhibition of growth corresponding to chronic exposure of 0.06 mg/mL of ZnO NPs which was two folds more than that exhibited by single exposure of 0.30 mg/mL ZnO NPs. The toxicity of ZnO NPs in E. coli was studied in the light of formation of reactive oxygen species (ROS), measured as malondialdehyde (MDA) equivalent by thiobarbituric acid-ROS (TBARS) assay, and effect of Zn dissolution from ZnO NPs. Higher inhibition of growth for the chronic exposure batches were correlated with higher ROS generation, which subsequently contributed to cause membrane lipid peroxidation, confirmed from observation of cell wall deformation by scanning electron microscopy study and energy dispersive X-ray analysis showed adherence of ZnO NPs on cell wall. The possibility of membrane lipid peroxidation was addressed by revealing in vitro oxidation of oleic acid, which is a monounsaturated fatty acid. Further in this study we have shown that the dissolution of ZnO NPs at pH 7.4 was not significant to cause Zn-induced toxicity.

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Photocatalysts suffer from a lack of separation of photogenerated excitons due to the fast recombination of charge carriers, so a strong synergistic effect exhibited by photocatalysts is promising for effective photocatalysis. Herein, we have synthesized efficient visible light functionalized graphene oxide nanosheet (FGS)/ZnO nanocomposite photocatalysts via a simple and economical approach with large scale production for practical applications. A series of nanocomposites (FGS/ZnO NCs) with different amounts by weight of graphene oxide (GO) have been synthesized via a facile solution route followed by calcination under environmental conditions. The phase, purity and morphological studies of the synthesized FGS/ZnO NCs were carried out using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The optical properties were studied using UV-visible diffuse reflectance

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