Biosynthesis protocols for colloidal metal oxide nanoparticles

Metal oxide nanoparticles (MO-NPs) have attracted great attention among nanomaterials in many fields including environmental remediation, catalysis, drug delivery, agriculture, molecular sensing and medicine. This has led to the development of different synthetic pathways for these nanostructures. Currently, the synthesis of MO-NPs largely depends on chemical methods where synthetic routes involve toxic chemicals and harsh reaction conditions that have been identified as a major contributor to environmental pollution. As an alternative, synthesis based on green chemistry principles has recently gained a large audience because they are eco-friendly, cost-effective and minimize waste. In this review, we focus on the green synthetic approaches to metal oxide nanoparticles such as ZnO, TiO2, Fe3O4. The synthesis based on microbes such as bacteria, fungi, algae and plant-mediated route are given major attention. In addition, the possible mechanisms for some of the syntheses and, the merits and demerits of green synthesis of metal oxide nanoparticles are also discussed.