## 4 ZnO-Based Nanomaterials for Antimicrobial Application in Water Treatment
### Strategies for Enhancing ZnO-Based Nanomaterials Antibacterial Activity
#### 4.4.4 Rare Earth Metal Doping into ZnO  
Doping rare earth metals into ZnO can improve the ability of the composite in trapping photogenerated carriers and reducing electron-hole recombination, which can enhance the photocatalytic antibacterial ability. In the rare earth doping process, f-orbital doping is the most common and efficient way, which can improve the photocatalytic activity by enhancing the adsorption of pollutants on the catalyst surface, while reducing the band gap energy to the visible light range [196]. Lanthanide ion doping is considered a versatile strategy to tune the optical response and improve the photocatalytic performance of ZnO. Lanthanides are composed of 17 elements in the periodic table, including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Lanthanides have attracted much attention due to their multifunctional properties resulting from their unique f-orbital structures, and due to the f-f or f-d intra-electron transitions, lanthanides are considered candidates for luminescent centers in doped materials, which is beneficial to prolonging the effective response time [197; 198; 199].  
Doping ZnO with Ln\({}^{3+}\) and Ce\({}^{4+}\) ions can convert the magnetism from diamagnetism to ferromagnetism, improve the n-type conductivity, enhance the photo response, increase the concentration of free electrons in the CB, and increase the electron mobility [200; 201; 202]. A novel Z-type ZnO-CeO\({}_{2}\)-Yb\({}_{2}\)O\({}_{3}\) heterojunction photocatalyst was prepared for the first time by [PERSON] et al. [203], and its physical, photocatalytic, and antibacterial properties were investigated. Growth samples were tested for antimicrobial properties against _E. coli_ and _S. aureus_. The effects of operating parameters such as catalyst dosage, dye concentration, and solution pH on the photocatalytic performance of the nanocomposites were investigated. The ZOIs of _S. aureus_ and _E. coli_ along with the standard antibiotic ciprofloxacin are shown in Figure 20a,b. The synthesized nanocomposites exhibited good activity against both bacteria with a ZOI > 6 mm, but higher activity against _E. coli_ with a ZOI of 14 mm shown in Figure 21a,b. Positively charged heavy metal ions such as Zn\({}^{2+}\), Ce\({}^{4+}\), and Yb\({}^{3+}\) can be released from the surface of the nanocomposite to interact with negatively charged microbial cell membranes. The entry of these metal ions into the cell membrane reduces the capacity and permeability of proteins, which in turn leads to the death of microorganisms such as bacteria and viruses. The above antibacterial action mechanism can be visualized in Figure 22C. In conclusion, the nanocomposites doped with rare earth ions are effective materials for preventing diseases caused by _S. aureus_ and _E. coli_.  
Figure 19: Photographs of antibacterial test results of ZnO, Ag2 MoO4/Ag, and ZAA samples against Escherichia coli (**a–c**) and Staphylococcus aureus (**d–f**). Reprinted/adapted with permission from Ref. [184]. Copyright © 2022, Elsevier B.V.  
Figure 21: Comparison of the zone of inhibition (ZOI) against different species of bacteria (**a**,**b**), mechanism of antibacterial activity of ZnO–CeO\({}_{2}\)-Yb\({}_{2}\)O\({}_{3}\) nanocomposite (**c**). Reprinted/adapted with permission from Ref. [203]. Copyright © 2022, Elsevier Masson SAS.  
Figure 20: Antibacterial effect of ZnO-CeO\({}_{2}\)-Yb\({}_{2}\)O\({}_{3}\) nanocomposites: (**a**) gram-positive _Staphylococcus aureus_ bacteria, (**b**) gram-negative _Escherichia coli_ bacteria at different concentrations, and (**c**,**d**) Standard antibiotics Ciprofloxacinacin against _S. aureus_ and _E. coli_. Reprinted/adapted with permission from Ref. [203]. Copyright © 2022, Elsevier Masson SAS.