Green Synthesis of Zinc Oxide Nanoparticles Using Castor Plant Extract and Their Antibacterial Activity Against Gram-Positive and Gram-Negative Bacteria

Abstract Zinc oxide nanoparticles (ZnO NPs) were synthesized through an environmentally friendly green methodology, using Ricinus communis (castor) seed extract as a bioreducing/capping agent at three zinc acetate concentrations (C1–C3: 0.01, 0.05 and 0.07 M), with the objective of fabricating a readily available antimicrobial nanomaterial while avoiding harmful traditional reagents. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy, field emission scanning electron microscopy with energy dispersive X-ray analysis (FE-SEM–EDX) and transmission electron microscopy (TEM). XRD confirmed the formation of the crystalline hexagonal wurtzite structure of ZnO with dominant (100), (002) and (101) peaks. FTIR revealed the Zn–O vibration band together with surface hydroxyl/adsorbed species characteristic of oxide NPs. FE-SEM and TEM images showed agglomerated nanocrystallites, with mean TEM sizes increasing from about 28 to 54 nm with precursor concentration. UV–Vis spectra revealed a wide band gap of about 3.1 eV. The antibacterial activity showed clear, dose-dependent inhibition of Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus); inhibition zones were absent at the lowest doses and increased up to about 22 mm for E. coli and 20 mm for S. aureus at 3 mg/mL. This is consistent with the commonly reported antibacterial mechanisms of ZnO NPs (reactive oxygen species (ROS)-induced stress, membrane disintegration and Zn 2+ -mediated effects), indicating that castor-mediated ZnO NPs could be used as an antimicrobial material.