## 3 Results and Discussion
### Microstructural Characterisation  
The uniform distribution and almost spherically shaped ZnO NPs can be observed in the FE-SEM image in Figure 5a. Some NPs are present with a hexagonal shape and the average particle size is around 40-45 nm. The NPs were dispersed in ethanol and sonicated in an ultrasonic bath for 20 min for TEM analysis. The TEM image in Figure 5b clearly shows that the NPs are mostly spherical with a size ranging from 25 to 30 nm. The selected area electron diffraction pattern, depicted in Figure 5b, confirms the crystalline nature of the biosynthesized ZnO NPs.  
According to the dynamic light scattering (DLS) technique (Figure 5c), the average size of the NPs is around \(65\pm 4\) nm.  
It is known that the physicochemical properties of nanomaterials, including size, shape, surface area, zeta potential, and composition greatly effect on their toxicological effects. In this regard, ZnO NPs induce a significant cytotoxicity in a size-dependent trend; particularly soluble and smaller NPs provide more useful toxicological information on nanomaterials. ZnO-NP antimicrobial activity is significantly affected by particles' morphology and size, as well as concentration. The smaller the NP size, the higher their toxic effects on microorganisms, which have relatively large interfacial area and can easily penetrate bacterial membranes, thereby increasing their antibacterial effectiveness. The biosynthesized ZnO NPs used in the present work present an average diameter of 55-75 nm. It is reported that spherical shape nanoparticles (with average 21 nm size) were obtained using plant extract of _Ocimum americanum_, which showed significant antimicrobial activity against Gram-positive, Gram-negative, and fungal pathogens [24]. ZnO NPs prepared using aqueous _Tabernaemontana divaricata_ green leaf extract showed a spherical shape with particle size ranging from 20 to 50 nm [25]. A higher antibacterial activity against _S. aureus_ and _E. coli_ and lesser antibacterial activity against _Salmonella Paratyphi_ (_S. paratyphi_) compared to the standard pharmaceutical formulation, was also observed. ZnO NPs prepared using pomelo fruit juice with an average particle size of about 10-20 nm showed significant antibacterial  
Figure 5: (**a**) Field emission scanning electron microscopy and (**b**) transmission electron microscopy images, and (**c**) size distribution by dynamic light scattering of the biosynthesized ZnO NPs.  
activity against pathogenic strains such as _S. aureus_ and _K. aerogenes_ and a less significant role in _E. coli_ pathogenic bacterial strains [26]. ZnO NPs green synthesized from _Aristolochia indica_ of size 22.5 nm exhibited remarkable bactericidal activity [27].  
A zeta-potential study was carried out to detect the surface charge and stability of the prepared NPs and indicated the presence of a distinct peak at \(-\)38 mV, which suggests that the synthesized NPs have good stability. This means that the capping molecules present on the surface of biosynthesized ZnO NPs are mainly comprised of negatively charged groups and also responsible for moderate stability of the nanoparticles. The rich source of proteins and flavonoids in the extract may be responsible for the reduction of metal ions and efficient stabilization of ZnO NPs.