New frontiers in the biosynthesis of metal oxide nanoparticles and their environmental applications: an overview

Nanotechnology has become a promising and emerging field of research in creating and modifying nanomaterials for different applications. Nanoparticles are considered to be the basic element of nanotechnology as they are the primary source of several nanostructured materials. During the last few decades, several metal oxide nanoparticles (NPs) were synthesized and their applications were investigated in various fields of science and technology, including biomedical, environmental, energy and agricultural practices. Moreover, metal oxide NPs have been synthesized by physical and chemical methods, while the chemical method used different chemicals as reducing and stabilizing agents. However, the wet chemical synthesis strategy become responsible for various biological and environmental risks due to the toxicity of used chemicals. Recently, biological synthesis of metal oxide nanoparticles using plants, algae, and microbes as a source of precursor material has emerged as a green and safe method. Additionally, the green synthesized metal oxide nanoparticles have shown a pivotal role in several applications such as nano-adsorbents, nano-membranes, photocatalysts and disinfection of wastewater from microbes. In this review, we present an overview of the biosynthesis of metal oxide nanoparticles such as; ZnO, CeO2, TiO2, CdO, CuO, Fe3O4, SnO2, NiO etc. Their method of characterization and properties is also discussed. Finally, their applications towards environmental protection is presented with particular attention to water treatment and remediation.