Effect of annealing on antibacterial activity of ZnO nanocapsules prepared by green-synthesized method

The zinc oxide nanocapsules were synthesized via green-synthesis method using Eucalyptus globulus labill leaf extract under optimized condition. The ZnO samples obtained from green synthesis were separately annealed at 200 °C, 400 °C, and 600 °C and the three samples were subjected into crystal structure, morphological, optical and anti-bacterial activity analyses and the results were reported. Powder X-ray diffraction patterns confirmed that all synthesized samples are crystalline in nature and crystallized into a hexagonal wurtzite structure with minimum crystallite size of 12 nm for ZnO-200 °C sample, calculated using Debye–Scherrer for (101) peak. The additional peaks in the XRD pattern for the sample annealed at 600 °C due to the oxygen vacancies. Raman spectra also confirmed that the active modes matched with the hexagonal wurtzite crystal structure of ZnO. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images showed that the samples have capsule-like morphology with unique length and thickness. The optical properties of samples were studied using UV–visible diffuse reflectance spectrum which confirmed all the samples showed wide band gap. Photoluminescence spectroscopy is used to identify the defects present in the samples and types of recombination occur in the samples. The antibacterial properties of zinc oxide nanocapsules were assessed against Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae). ZnO-200 °C sample showed higher antibacterial activity for all gram negative and positive bacteria compared to all other samples except Gram-negative Pseudomonas bacteria. The ZnO-400 °C sample showed the higher inhibition zone (2.5 cm) against the Gram-negative bacteria Pseudomonas. Graphical abstract