Green synthesis strategy of CeO2 nanowires, WO3 nanoparticles and CeO2@WO3 nanocomposites by using (Azadirachta Indica) leaf extract: its surface defects, Ce3+/W5+ ratio for their enhanced antibacterial activity

Abstract

This work presented the green synthesis of cerium oxide (CeO₂), tungsten oxide (WO₃) nanoparticles (NPs) and CeO₂/WO₃ nanocomposites (NCs) via simple precipitation method by using the leaf extract of the Azadirachta Indica (A. indica) plant. The morphological, structural, chemical composition, Ce³⁺/W⁵⁺ ratio and optical properties of all the synthesized samples were inspected by various techniques i.e. SEM, EDX, FT-IR, XRD, UV-vis, contact angle, Raman spectroscopy, XPS and zeta potential (ζ). The formations of single phase of CeO₂ and WO₃ NPs were confirmed through structural examination and no impurity signals were detected. The estimated crystallite’s size for CeO₂ nanowires (NW) is ~2 nm with SD ± 0.1–0.2, for oval stone like (OS) WO₃ NPs, the size is ~9.8 nm with SD ± 0.2–0.5, while for the CeO₂/WO₃ NCs it is about ~5.5 nm with SD is ±0.1–0.3 respectively was studied by XRD. The samples were tested as an antibacterial agent against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), and it was confirmed that they had greater antibacterial effects as compared to the negative (-ve) control group (water) which showed no response (0%). The antibacterial effect of pure CeO₂ and WO₃ NPs was also compared with the CeO₂/WO₃ NCs and predicted that these NCs showed excellent antibacterial activity (100%) verses pure NPs (70–80%) at the same 0.2 mg/L concentration for E. coli. While against S. aureus strain the CeO₂/WO₃ NCs also showed (90%) antibacterial action at the lowest concentration (0.2 mg/L) but 100% as the concentration increases i.e. 0.5 mg/L due to difference in the bacterial strain as compared to the pure NPs which showed good response at 5 mg/L. Furthermore, the detailed mechanism and the characteristics properties of the pure green synthesized CeO₂, WO₃ NPs as well as CeO₂/WO₃ NCs behind their antibacterial effects specifically at lowest concentrations are rarely studied, therefore, in this paper we elaborated the mechanisms of these NPs and NCs distinctly which is important for the development of more effective and unique antibacterial materials.

Graphical Abstract