## 4 ZnO-Based Nanomaterials for Antimicrobial Application in Water Treatment
### Mechanisms of ZnO-Based Antimicrobial Nanomaterials
#### 4.2.3 Influence of Physical Effects of ZnO-Based Nanomaterials on Antibacterial Performance  
In the process of exploring the antibacterial mechanism of ZnO-based nanomaterials, there are three main chemical antibacterial mechanisms: plasma membrane disruption through ZnO interactions [163], cellular internalization of ZnO-based nanoparticles [164], and mechanical damage of the cell envelope [165].  
At suitable pH, the bacterial surface is negatively charged due to the dissociation of carboxyl and other functional groups. Meanwhile, ZnO-based nanomaterials are positively charged with a zeta potential of +24 mV [161]. As shown in Figure 13, the opposite charges carried by bacterial cells and ZnO NPs are the reasons for the strong electrostatic attraction between them. Strong electrostatic interactions force particles larger than 10 nm in size to accumulate on the outer surface of the plasma membrane and neutralize the surface potential of the bacterial membrane, resulting in increased surface tension and membrane depolarization. In addition, strong electrostatic interactions can induce bacterial  
Figure 11: Schematic diagram of cell damage to G+ bacteria by Zn\({}^{2+}\) ions. Reprinted/adapted with permission from Ref. [27]. Copyright© 2022 Elsevier B.V.  
Figure 12: Schematic diagram of the damage to bacterial cells caused by ZnO nanomaterials producing H\({}_{2}\)O\({}_{2}\). Reprinted/adapted with permission from Ref. [27]. Copyright© 2022, Elsevier B.V.  
cell changes such as changes in cell membrane and membrane vesicle structure, rupture, morphological changes, and components, leading to bacterial cell death [166; 167]. Since interactions play an important role in the bactericidal effect, surface modifiers and templates of ZnO-based nanomaterials would enhance the interaction with bacterial cell walls.  
Another important mechanism is cellular internalization. Cellular internalization is simply summarized as those nanostructures with a size of less than 10 nm pass through the plasma membrane, accumulate in bacterial cells, and destroy intracellular components such as nucleic acids [168; 164; 169; 170; 164]. In addition, it has also been suggested that the cellular interaction of ZnO with bacteria can enhance cell permeability (Figure 14) [171]. In conclusion, cellular internalization is one of the physical methods that plays an important role in the antibacterial process of ZnO-based NPs.  
Figure 14: TEM images of _B. atrophaeus_ (**a**) control, (**b**) ZnO powders, (**c**) ZnO nanorods, and (**d**) ZnO nanoparticles. Reprinted/adapted with permission from Ref. [172]. Copyright © 2022, Elsevier B.V.  
Figure 13: Schematic diagram of the physical action of ZnO nanomaterials for sterilization. Reprinted/adapted with permission from Ref. [27]. Copyright © 2022, Elsevier B.V.  
The last physical mechanism is to use ZnO NPs to create cell membrane damage to destroy bacterial cells and achieve antibacterial effects. Compared with bulk ZnO materials, the presence of surface defects, uneven surface texture, and rough edges and corners on the surface of ZnO-based nanomaterials can lead to effective abrasiveness, resulting in excessive mechanical damage to bacterial cell membranes [163].