Green synthesis of zinc oxide nanoparticles: A review of the synthesis methodology and mechanism of formation

Zinc oxide is of significant importance for many industries due to its versatile properties, which have been enhanced with the production of this material in the nanoscale. Nonetheless, the increase in concerns related to environmental impact has led to the development of eco-friendly processes for its production. Recent interest in obtaining metal and metal oxide nanoparticles using biological approaches has been reported in the literature. This method was termed 'green synthesis' as it is a less hazardous process than chemical and physical synthesis methods currently used in the industry to obtain these nanomaterials. Zinc oxide nanoparticles have been successfully obtained by green synthesis using different biological substrates. However, large scale production using green synthesis approaches remains a challenge due to the complexity of the biological extracts that poses a barrier onto the elucidation of the reactions and mechanism of formation that occur during the synthesis. Hence, the current review includes a summary of the different sources of biological substrates and methodologies applied to the green synthesis of zinc oxide nanoparticles and the impact on their properties. This work also describes the advances on the understanding of the mechanism routes reported in the literature.