# Antibacterial Ceramics for Dental Applications
## 3 Results and Discussion
### Antibacterial Activity of Cerium Oxide  
Antibacterial ceramics are a focus of research due to their functions, such as self-cleaning and bacterial killing. Adding antibacterial agents to glaze ceramics to generate metal ions or free radicals with antibacterial activity is one of the main methods of preparing antibacterial ceramics. Adding well-known photocatalytic materials such as TiO\({}_{2}\) and ZnO to ceramics can generate photogenerated electrons and holes, which can generate free radicals to inactivate bacteria. However, these ceramics often require ultraviolet light for their antibacterial activity, so their application is limited [34; 49; 78; 105; 106]. The mechanism of the well-known and described photoactive properties of TiO\({}_{2}\) can be explained by the following chemical equations and is graphically summarized in Figure 20.  
Ag, Au, and Cu exhibit antibacterial activity in ceramic glazes in the absence of light, but their activity decreases when their ions are released into the environment, which can be  
Figure 20: Schematic drawing of the photocatalytic antibacterial process of TiO\({}_{2}\). harmful to patients [62]. Ce ions have been shown to stimulate the proliferation, differentiation, and mineralization of osteoblasts; increase collagen production in human mesenchymal cells; and enhance the mechanical properties of bones [107; 108; 109]. They are known for their antibacterial activity. The reversible conversion of Ce\({}^{3+}\) to Ce\({}^{4+}\) is the basis of ROS (reactive oxygen species) generation when CeO\({}_{2}\) is used as an antibacterial material. The antibacterial activity of CeO\({}_{2}\) is not based on the penetration into the cell membrane of bacteria but on the induction of oxidative stress with the production of ROS, which leads to the degradation of DNA and RNA and their proteins [107; 108; 109; 110; 111]. CeO\({}_{2}\) is known for its low cytotoxicity to mammalian cells and its unique antibacterial mechanism on Gram-negative and Gram-positive bacteria [62]. Almost all bacterial cells are surrounded by a cell wall comprised of peptidoglycans, which protects the cell membrane from rupture due to high intracellular turgor pressure and maintains the cell shape. In contrast to the Gram-positive cell wall, which consists of a thick, porous layer of peptidoglycans with attached teichoic acids, the Gram-negative bacterial cell wall consists of a relatively thin layer of peptidoglycans surrounded by an asymmetric, highly impermeable bilayer with lipopolysaccharides.rides in the external leaflet and phospholipids in the internal leaflet [111]. Bacteria are negatively charged when they are suspended in physiological solutions (_pH_ = 7.4). Gram-negative bacteria display a greater negative potential than Gram-positive bacteria. Cerium ions, which are positively charged, are electrostatically adsorbed on the bacterial surfaces, resulting in high antibacterial toxicity. Furthermore, CeO\({}_{2}\) is known for its antibacterial activity as well as for stimulating angiogenesis, osteogenesis, and wound healing. [PERSON] et al. prepared glass ceramics that exhibit long-lasting antimicrobial activity in both the dark and the light [62]. The compositions of CeO\({}_{2}\) glass-ceramics (CGC) were 64 wt% Bi\({}_{2}\)O\({}_{3}\), 22 wt% B\({}_{2}\)O\({}_{3}\), and 14 wt% CeO\({}_{2}\). They treated this ceramic with 0.2 M HCl and developed a new ceramic: BiOCL@CeO\({}_{2}\) (BCGC) [62]. This BCGC ceramic was tested for antibacterial activity against _E. coli_ in the dark and under illumination. [PERSON] et al. found that BCGC ceramics exhibit antibacterial activity when exposed to darkness and visible light. Figure 21 shows the mechanism of the antibacterial activity of BCGC ceramics. When irradiated with visible light, BCGC ceramics can generate hydroxyl radicals (OH\({}^{-}\)) and superoxide radicals (O2\({}^{-}\)), which damage the cell membranes of bacteria and lead to their death.