Synthesis, characterization, and preliminary in vitro antibacterial evaluation of ZnO nanoparticles derived from soursop (Annona muricata L.) leaf extract as a green reducing agent

Synthesis of zinc oxide nanoparticles (ZnO NPs) was performed using microwave irradiation technique in the presence of Annona muricata L (soursop) leaf extract as a green reducing agent and Zn(NO3)2.6H2O. X-ray diffraction (XRD) characterization verified the structural identity of the ZnO NPs to be hexagonal wurtzite structure. Similarly, Raman spectrum revealed peaks at 109, 447 and 577 cm−1, verifying formation of pristine ZnO NPs. Based on field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results, the plant extract was found to assist the formation of quasi-spherical nanoparticles with an average size of 37 nm. UV–vis Absorption spectrum indicates that the green-synthesized ZnO NPs absorb at the wavelength centered at 365 nm, corresponding to band gap of 3.39 eV. The Fourier transform infrared (FTIR) spectroscopic findings demonstrate that phytochemicals in the soursop leaf extract are responsible for the formation of ZnO NPs with a sharp and intense characteristic peak at 466 cm−1. The ZnO NPs demonstrated antibacterial activity against two Gram-positive (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative (Klebsiella pneumoniae and Pseudomonas aeruginosa) bacteria with the inhibition zone between 8–15 mm. For all the bacterial strains, the optimal inhibitory concentration of ZnO NPs was determined to be 50 mg/mL. FESEM analysis of S. aureus depicted partial damage and shape distortion of the cells upon treatment with ZnO NPs at 10 mg/mL, thus proving successful antibacterial activity by the green synthesized ZnO NPs. The microwave irradiated, A. muricata L. extract-mediated green synthesis proposed in this work is a fast, economical and effective technique to synthesize ZnO NPs with promising antibacterial activity.