Green synthesis, characterization and antimicrobial activity of ZnO and Co-doped ZnO nanoparticles obtained using aqueous extracts of Platycladus orientalis leaves

Abstract This study investigates the synthesis, structural characteristics, and antimicrobial applications of zinc oxide (ZnO) and cobalt-doped ZnO (Co-doped ZnO) nanoparticles synthesized via a green method utilizing Platycladus orientalis aqueous extract. Comprehensive characterization techniques, including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis spectroscopy, and Fourier transformed infra-red (FTIR) spectroscopy, were employed to evaluate the structural, morphological, and optical properties. XRD analysis confirmed the hexagonal wurtzite structure of ZnO with successful Co-doping, evidenced by lattice distortion and reduced crystallite size. The estimated crystallite size and lattice microstrain of the ZnO and Co-doped ZnO nanoparticles were 26.9 and 19.0 nm, and 0.16% and 0.2%, respectively. Furthermore, the lattice parameters showed a significant distortion of the lattice along the c-axis in the Co-doped ZnO nanoparticles. SEM and TEM revealed improved particle uniformity and decreased agglomeration in Co-doped ZnO nanoparticles. UV–Vis spectroscopy demonstrated a narrowing of the optical band gap, enhancing visible light absorption. Antimicrobial studies showed significant concentration-dependent activity, with Co-doped ZnO nanoparticles exhibiting superior antibacterial and antifungal properties compared to pristine ZnO. Co-doped ZnO demonstrated enhanced inhibition zones against Listeria monocytogenes (13.50 mm) , Escherichia coli (13.65 mm), and Enterococcus faecalis (14.05 mm), and also showed better minimum inhibitory concentrations (MICs) against fungal strains such as Mucor mucedo (0.05 mg/mL), Penicillium chrysogenum (0.05 mg/mL), and Aspergillus niger (0.03 mg/mL). The superior antimicrobial performance is attributed to modifications in particle size, morphology, and lattice defects induced by cobalt doping. These findings highlight the potential of Co-doped ZnO nanoparticles as advanced antimicrobial agents, suitable for applications in biomedical fields.