# Antibacterial Ceramics for Dental Applications
## 3 Results and Discussion
### Antibacterial Activity of Cerium Oxide
In the dark, the release of cerium ions from the BCGC contributed to the antibacterial activity of the BCGC ceramics (Figure 21) [62]. The results of this study show that it is possible to develop durable and effective antibacterial ceramics that work in the dark and under illumination. [PERSON] et al. [64] investigated the effect of cerium oxide on the bioactivity and antibacterial activity of Bioglass 4555. The concentrations of CeO\({}_{2}\) were 0.25 (Ce0.25), 0.5 (Ce0.5), 1 (Ce1), and 2 mol% (Ce2). The antibacterial activity of cerium oxide-doped biogas was tested against _E. coli_, _S. aureus_, and _S. mutans_. The bioactivity was also inves  
Figure 21: Schematic drawing of the photocatalytic antibacterial process of BiOCL@CeO\({}_{2}\) under light irradiation and darkness [62]. tigated. All samples showed antibacterial activity against all bacterial strains, but samples Ce1 and Ce2 were more effective against _E. coli._ Cerium oxide-doped bioglasses showed less activity against _S. aureus._ Ce1 showed the highest activity against _S. aureus_, while Ce2 samples showed no significant effect. The antibacterial activity generally increased with increasing cerium oxide content. The different antibacterial activities of cerium oxide against Gram-positive and Gram-negative bacteria can be explained by the different structures of the cell walls of these bacteria [111]. The results showed an increase in the bioactivity of the BG for the Ce2 samples, which supports the use of Ce2 BG as a material for bone regeneration, the filling of bone defects, or as a coating material for implants [64]. The development of ceramics with multifunctional properties and stable antimicrobial activity is still a major challenge for researchers. An effective method for preventing enamel demineralization and the appearance of cavitated lesions is the use of dental materials that are resistant to bacterial accumulation. Ideally, the material should have bioactive properties for the mineralization of enamel. [PERSON] et al. investigated the antibacterial activity of a dental resin with CeO\({}_{2}\) nanoparticles (CeO\({}_{2}\)NP) as an additive [112]. The resin was doped with 3 wt% CeO\({}_{2}\). They tested the antibacterial activity of their experimental dental resin against _S. mutans, S. mitis, S. aureus,_ and _Lactobacillus_ spp. It was found that such a dental composite was effective against all tested bacterial strains [112]. The antibacterial activity can be explained by two mechanisms: (a) adsorption on the bacterial surface and (b) oxidative stress. Nanoparticles possess large contact surfaces, which allow positively charged CeO\({}_{2}\)NPs to adsorb onto the negatively charged bacterial membrane. CeO\({}_{2}\)NPs pass through the membrane, and the special ion pumps are hindered by the viscosity increase of the membrane. This has a significant impact on the transport exchange between bacterial cells and the fluid, leading to bacterial death. The released cerium ions can disrupt the electron flow and the respiration of the bacteria by reacting with thiol groups (-SH) or by adsorbing onto transporters, disrupting nutrient transport inside the bacteria [112]. Oxidative stress is a significant factor for the antibacterial activity of CeO\({}_{2}\) and is caused by the production of reactive oxygen species (ROS) in vivo, whereas ROS produced on bacterial membranes is caused by the reversible conversion of Ce\({}^{3+}\) to Ce\({}^{4+}\). ROS can damage nucleic acids, proteins, polysaccharides, lipids, and other biological components and even destroy them. Such reactions occur on CeO\({}_{2}\)NPs when they are irradiated with ultraviolet light (h\(v\)) [111]:  
\[\ce{CeO}_{2}+\ce{h}v\rightarrow\ce{CeO}_{2}\left(\ce{e}^{-}+\ce{h}^{\star}\right)\]  
\[\ce{e}^{-}+\ce{O}_{2}\rightarrow\ce{O}_{2}{}^{\star-}\]  
\[\ce{e}^{-}+\ce{H}_{2}\ce{O}_{2}\rightarrow\ce{OH}^{-}+{}^{\star}\ce{OH}\]  
\[\ce{h}^{+}+\ce{OH}^{-}\rightarrow{}^{\star}\ce{OH}\]  
Figure 22 schematically illustrates the antibacterial activity of CeO\({}_{2}\)NPs.