Biomass extract of green macroalga Halimeda opuntia assisted ZnO nanoparticles: preparation, physico-chemical characterization, and antibacterial activity against Vibrio harveyi

Zinc oxide (ZnO) nanoparticles are particularly interesting for antibacterial applications, unlike many other forms of metal and metal oxide nanoparticles. ZnO is an FDA-approved nanomaterial that is widely considered safe, efficient, and non-toxic at lower doses. In this study, biomass extract of Halimeda opuntia assisted preparation of ZnO nanoparticles and their antibacterial effect against aquaculture pathogenic Vibrio harveyi. X-ray diffraction (XRD) studies revealed that the ZnO nanoparticles are crystalline in nature with hexagonal wurtzite structure. The crystallite size (D) of ZnO nanoparticles was calculated from the full width at half-maximum of the most intense peak (101) using Scherrer’s formula and was around 33 nm. Furthermore, the various lattice parameters such as lattice constants (a and c), c/a ratio, unit cell volume (V), interplanar angle (φ), dislocation density (δ), and measure of atom displacement (μ) were calculated. Crystalline quality and binding energy were also investigated using X-ray photoelectron spectroscopy (XPS). From XPS studies, the chemical valence of Zn at the surface of ZnO nanoparticles was found to be + 2 oxidation state. The morphologies and elemental composition have been studied using SEM equipped with EDAX. The identification and composition of the major bioactive compounds present in the biomass extract of H. opuntia were characterized using GC–MS analysis. Furthermore, antibacterial test was performed by well diffusion assay and bacterial growth kinetics studies against shrimp pathogenic V. harveyi. The results revealed that ZnO nanoparticles at 1, 2.5, 5, 7.5, and 10 µg mL⁻¹ caused 5, 10, 13, 16, and 19 mm (diameter) growth inhibition zone against V. harveyi, respectively. Graphical abstract: [Figure not available: see fulltext.]