Green Synthesis of Zinc Oxide Nanoparticles Using Moringa oleifera Leaf Extract and Their Antibacterial Activity

Abstract

Zinc oxide nanoparticles made with plant extracts are widely reported to kill bacteria, and the leaf of Moringa oleifera, a fast-growing tree valued as food and medicine, is a popular source of the reducing and capping agents. This narrative review asks how much weight the published evidence can bear for the claim that zinc oxide nanoparticles synthesised with Moringa leaf extract are antibacterial. It draws on work on the chemistry of the plant, on the green synthesis of zinc oxide and other metal nanoparticles, on proposed antibacterial mechanisms, and on the design of susceptibility assays, and it reports no new data. The plausible picture is that particle size, shape, surface chemistry and dissolved zinc together determine activity, and that the extract adds a variable of its own because its composition shifts with cultivar, season and preparation. The common weaknesses are assays that rely on a single zone-of-inhibition reading, a small panel of bacterial strains, missing controls for the extract and the zinc salt, and thin characterisation, all of which make results hard to compare between laboratories. Questions of toxicity to non-target cells and of fate after release receive far less attention than activity. The review concludes that antibacterial activity is plausible and often reported, that its size and cause are not firmly established for the Moringa route, and it sets out what a convincing study and a fair comparison would contain.