Morphology dependent antibacterial activity of zinc oxide nanoparticles against clinically relevant bacteria

The present study focuses on the antibacterial activity of ZnO nanoparticles, evaluating variations based on morphology. Interest in ZnO nanoparticles arises from their high surface-to-volume ratio that enables effective interaction with bacterial cells. Their antibacterial properties depend on size and nanostructure, with enhanced activity attributed to increased surface area, promoting maximum contact with microbial membranes, cellular damage, and growth inhibition. Zinc oxide (ZnO) nanoparticles with two distinct shapes-ellipsoidal nanorods (Z1) and 3D microspheres (Z2)-were synthesized using a simple, template-free aqueous precipitation method. Zinc nitrate and hexamethylenetetramine (HMT) served as starting materials. The nanoparticles were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). The point of zero charge (PZC) was determined by the salt addition method. SEM images showed that particle shape changed with reaction time. XRD confirmed a hexagonal wurtzite crystal structure, with average crystallite sizes of 22.09 nm for Z1 and 27.18 nm for Z2. FT-IR spectra showed Zn-O bond vibrations between 540 and 411 cm^-1. Antibacterial activity was evaluated using the agar well diffusion method against Gram-positive (Streptococcus mutans, Staphylococcus aureus) and Gram-negative (Escherichia coli, Enterobacter cloacae) bacteria. The synthesized ZnO nanoparticles exhibited significantly higher antibacterial effects than commercial ZnO, which showed no activity at tested concentrations (0.25, 0.50, and 0.75 µg/µL). At 0.75 µg/µL, Z1 nanorods produced inhibition zones of 30 mm (E. coli), 28 mm (E. cloacae), 28 mm (S. mutans), and 30 mm (S. aureus). Z2 microspheres showed even stronger effects: 35 mm, 32 mm, 30 mm, and 31 mm, respectively. These findings demonstrate the superior antibacterial properties of the synthesized ZnO nanoparticles, particularly the 3D microspheres, highlighting their potential in antimicrobial coatings and biomedical applications.