Title: Light-responsive nanomaterials with pro-oxidant and anti-oxidant activity. Authors: Soumik Podder 1,2, *, Chandan Kumar Ghosh 2 , Avijit Das 1 , John George Hardy 3,4, * Affiliations: 1. Department of Electronics and Communication Engineering, Guru Nanak Institute of Technology, Kolkata-700114, India 2. School of Materials Science and Nanotechnology, Jadavpur University, India 3. Department of Chemistry, Lancaster University, Lancaster, Lancashire LA1 4YB, U.K 4. Materials Science Institute, Lancaster University, Lancaster, Lancashire LA1 4YB, U.K *Corresponding authors: nanotechsoumik@gmail.com (S.P.); j.g.hardy@lancaster.ac.uk (J.G.H.) Abstract Nanomaterials are capable of generating reactive oxygen species (ROS) due to defect induced electronic interactions with oxygen and water stimulated by environmental and structural factors (e.g., photonic energy, band edge energy and morphology) resulting in excellent pro- oxidant activity of nanomaterials. The pro-oxidant activities are demonstrated by the antibacterial activity of nanomaterials under different environmental conditions (e.g., varying light levels). This review examines research related to the pro-oxidant activity of metallic, non-metallic, metal oxide nanoparticles (NPs) and their composites. Moreover, there is a scavenging phenomenon for nanomaterials that manifests itself as inhibition of ROS (i.e., anti-oxidant activity) which is also dependent on the electronic property of the nanomaterials, which is examined. These nanomaterials experience a crossover between pro-oxidant and anti-oxidant activities depending on concentration, morphology etc., which offers the nanomaterials potential for application in cancer therapy and inflammatory disease treatment. Keywords: Prooxidant activity, Antioxidant activity, Metal Oxide, ROS, Antibacterial activity Introduction Nanomaterials tend to have high surface/volume ratios and opportunities for enrichment with various defect related properties,[1] potentially enabling the generation of reactive oxygen species (ROS).[2] ROS are primarily generated from nanomaterials after light excitement whose energy level is higher than the band gap of the nanomaterial,[3] and can be generated in dark too due to presence of defect states [4,5] The photometric excitement generates electron-hole pairs that eventually recombine with each other resulting in the emission of photons 2 but defect states of nanomaterial can trap these charge carriers where their lifetimes are many times longer than triplet states of dyes[6] and thus lower the chance of recombination of photogenerated electron-hole pairs, thereby offering opportunities to react with adsorbed species (molecular oxygen, water molecules, etc.) on the surface of the nanomaterials producing ROS.[2] The ROS generation occurs either when electrons are donated to molecular oxygen or other acceptors(a reductive process) and when holes are