## 2 Nanomaterials in Endodontics
### Metallic Nanoparticles
#### 2.4.2 Metal Oxide Nanoparticles  
Research on nanoparticles of stable metal oxides, such as zinc oxide, titanium oxide or zirconium oxide, has indeed yielded many promising results in recent years. One of the main areas of their use is dental fillings. Metal oxide nanoparticles are mainly added to composite resins to improve their mechanical properties, durability and biocompatibility. Additionally, they have antibacterial properties, which is very important in the context of preventing infections and maintaining oral hygiene [140; 141]. The antibacterial properties of zinc oxide nanoparticles (ZnO NP) are significantly effective in eliminating microorganisms. There are several mechanisms that contribute to this effectiveness. Firstly, the presence of zinc ions leads to damage to the bacterial cell membrane, which causes the loss of integrity of the cell structure and, consequently, leads to its death. This mechanism is associated with the increased permeability of the cell membrane in the presence of zinc ions [142]. Secondly, zinc nanoparticles can also increase the production of reactive oxygen species (oxygen radicals), which have a bactericidal effect by damaging the cell membrane and other elements of the bacterial cell. Additionally, zinc ions (Zn\({}^{2\epsilon}\)) are toxic to bacteria, disturbing their metabolic processes and inhibiting their growth [143]. The total combination of these mechanisms, i.e., damage to the cell membrane, increased production of reactive oxygen species and the toxicity of zinc ions, causes zinc nanoparticles to have a strong antibacterial effect--desirable in endodontics [144]. The hydrolysis process of tetrabutyl titanate and its kinetics can be regulated, which allows for the modification of the titanium ion content in the +3 oxidation state and the number of oxygen defects (OV) in the TiO\({}_{2}\) structure--this in turn allows for the adjustment of visible radiation-induced photocatalytic efficiency and antibacterial properties [145]. One of the key features of TiO\({}_{2}\) is its ability to generate reactive oxygen species, such as hydroxyl radicals (OH) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), when exposed to UV radiation. These reactive oxygen species are strong oxidants and can damage bacterial cell membranes through the oxidation process. This damage leads to the lysis of bacterial cells, which ultimately eliminates the microorganisms. In addition, TiO\({}_{2}\) also exhibits other properties that may be beneficial in dentistry, such as a high mechanical strength and corrosion resistance, which makes it an ideal material for use in dental implants and surgical instruments or as a coating on dental instruments. Additionally, the excellent biocompatibility of TiO\({}_{2}\) makes it safe for body tissues, which is a key factor in dentistry, where contact with the soft and hard tissues of the oral cavity is inevitable [146; 147; 148]. In terms of stiffness, performance, fatigue resistance and wear resistance, zirconium oxide is indeed a material that may be superior to other substances used in dentistry. The use of laser vaporization techniques to produce zirconium oxide nanoparticles of very small sizes, such as 20-50 nm, opens up new possibilities for the use of this material [149; 150]. Zirconium oxide nanoparticles of such small sizes have increased surface activity, which affects their properties, including chemical reactivity, biocompatibility, osteoconductivity and the tendency to reduce the accumulation of dental plaque [151; 152]. [PERSON] et al. examined the use of zirconium oxides in dentistry by doping PMMA with ZrO\({}_{2}\) nanoparticles. In comparative tests, zirconium oxide ceramics showed a higher strength and bending resistance than aluminum oxide ceramics [153]. In turn, [PERSON] et al. showed that ZrO\({}_{2}\) nanoparticles combined with glass ionomer cement were characterized by a higher compressive strength and crack resistance [154].