## 3 Results and Discussion
### Antibacterial and Antioxidant Properties
OH radicals are negatively charged, and thus cannot penetrate the outer cell membrane of _E. coli_, which is also negatively charged, and remain in direct contact with the outer surface of the bacterium [66; 67; 68]. The entry of ZnO NPs into the cell leads to integrity loss of phospholipid layer and blebbing of intracellular elements resulting in cell death. Nanoparticles react with membrane proteins and inactivate them decreasing membrane permeability and causing cell  
Figure 7: (**a**) Antibacterial activity of the biosynthesized ZnO NPs, _S. ebulus_ extract, and ciprofloxacin against _B. cereus_, _S. aureus_, and _E. coli_, (**b**) H\({}_{2}\)O\({}_{2}\) free radical scavenging activity of the biosynthesized ZnO NPs, _S. ebulus_ extract, and ascorbic acid, and (**c**) antibacterial activity against various pathogens of green synthesized ZnO NPs when compared to chemical ZnO NPs [83]. death. The antibacterial activities of ZnO NPs strongly depend on shape, size, powder concentration, specific surface area, zeta potential, etc. For example, it is reported that ZnO NPs with positive surface potential showed higher antimicrobial properties compared with ZnO NPs of the same size but with negative surface potential [80]. Furthermore, it is reported that small-sized nanoparticles can penetrate the bacterial membrane easily and spherically shaped ZnO NPs release Zn\({}^{2+}\) ions more effectively than rod-shaped ions [81; 82]. As observed above, biosynthesized ZnO NPs depict better antibacterial activity against various microorganisms when compared to pure extract. Previous studies imply that green ZnO nanoparticles showed more enhanced biocidal activity against various pathogens when compared to chemical ZnO nanoparticles [83]; furthermore, [PERSON] et al. reported that green synthesized nanoparticles show more effective antimicrobial activity than those of plant extracts [84]. Surface modifying reagent molecules result in differences in the release of Zn\({}^{2+}\) ions and the production of reactive oxygen species (ROS). _S. ebulus_ leaf extract contains enormous numbers of carbonyl, carboxyl, and hydroxyl groups in polyphenols and flavonoids [35; 69; 70; 71]. These compounds can have a positive effect on human health because they can act as antioxidants and anti-inflammatory and anticancer agents. Indeed, these compounds present a wide range of biological activities, and the presence of abundant hydroxyl and carbonyl groups is responsible for antibacterial applications [69; 70; 71]. These compounds can affect the release of Zn\({}^{2+}\) and the production of ROS. The existence of these compounds in the extract and the surface helps ZnO NPs to adhere to the bacterial cell membranes. The chemical interaction between the extract molecules and ZnO NPs enhance the reactivity of NPs and improves penetration via bonding to the surface of the bacteria. Plant extract compounds attach to the surface of the bacteria and create new pathways near the surface for better transportation of or penetration by ions. For example, hydrogen in these compounds acts as both an oxidizing and a reducing agent (due to its two distinct oxidation states) which can easily form many bonds with other molecules [72; 73]. In a recent study, it was found that _Albizia lebbeck_ stem bark increased the antibacterial effect of biosynthesized ZnO NPs [74]. In another investigation, ZnO NPs (100-190 nm) were synthesized using _Trifolium pratens_ flower extract and the high zone of their growth inhibition (29-31 mm) against _E. coli_, and _S. aureus_ at \(\sim\)1200 ug/mL was obtained [20]. In the work performed by [PERSON] et al. [75],  
Figure 8: A schematic diagram of the antibacterial activity mechanism. the biosynthesized ZnO NPs using _Aloe vera_ extract showed enhanced antibacterial properties compared with chemically prepared ZnO NPs [75]. On the other hand, [PERSON] et al. [76] synthesized ZnO NPs with _Silybum marianum_ L. seed extract and reported effective growth inhibition of Gram-negative _E. coli_.