## 2 Nanomaterials in Endodontics  
The human oral cavity is a habitat for over seven hundred species of microorganisms that create complex ecosystems. Most of these bacteria form biofilms attached to tooth surfaces and mucous membranes. This biofilm, also known as dental plaque, plays a key role in both oral health and the pathogenesis of various dental diseases--it is a major factor in the development of tooth decay and periodontal disease [25; 27]. Bacteria in the biofilm produce acids that demineralize tooth enamel, leading to tooth decay. In periodontal disease, bacterial toxins and enzymes degrade the supporting tissues of the tooth, leading to gingivitis and bone loss. Traditional antibacterial agents and fillers used in dentistry have numerous limitations that may affect their effectiveness in treating infections in this complex oral environment. Often, they do not effectively reach all areas of infection in the mouth and may not be effective enough against a wide range of bacteria, which can lead to the recurrence of infections, and what is more, many antibacterial agents (e.g., 2 or 5% sodium hypochlorite (NaOCI), 3% hydrogen peroxide, 40 or 50% citric acid (C\({}_{6}\)H\({}_{8}\)O\({}_{7}\)), chlorhexidine (C\({}_{22}\)H\({}_{30}\)Cl\({}_{2}\)N\({}_{10}\); CHX), 17% sodium edetate solution (C\({}_{10}\)H\({}_{14}\)N\({}_{2}\)Na\({}_{2}\)O\({}_{8}\)\(\times\) 2H\({}_{2}\)O; EDTA) degrade quickly or release active substances in a short time, which prevents a long-term antibacterial effect [31]. Nanoparticles, on the other hand, can significantly improve the effect of antibacterial agents and fillers in dentistry, eliminating many of the above-mentioned disadvantages, and they can penetrate deep into the bacterial biofilm and tissues, delivering drugs directly to the site of infection. Some nanoparticles also have strong antimicrobial properties and can fight a wide range of bacteria, even resistant strains. Nanoparticles can also be designed to control the gradual release of active substances,which ensures long-lasting antibacterial effects. This can reduce the risk of the recurrence of infection [32; 33; 34]. Moreover, nanocomposites can increase the mechanical strength of root sealants and fillers for pulp reconstruction (Table 1). To summarize, nanoparticles are intended to increase the strength of filling and sealing materials, which leads to more lasting treatment results; to act as carriers of drugs with extended biological activity, enabling their precise delivery to sites of infection and increasing the effectiveness of therapy; to reduce the need to use large doses of drugs, which may reduce the risk of side effects and toxicity; and above all, to improve the antimicrobial effect of substances used in endodontic treatment [23]. The oligodynamic effect of nanoparticles means that they can attack multiple target sites inside the cell, which distinguishes them from traditional antibiotics, which usually act selectively on specific structures or intracellular processes. However, due to this extraordinary ability to affect various cellular areas, it is necessary to thoroughly understand the mechanisms of action of nanoparticles in order to use them in an effective fight against bacteria resistant to conventional treatment [24]. Nanoparticles, especially those with a positive surface charge, can interact electrostatically with the negatively charged bacterial cell membrane, leading to destabilization and damage of the cell membrane, resulting in the leakage of cell contents. Some nanoparticles can also penetrate the cell membrane, creating pores or holes in it, leading to a loss of its integrity [26]. Many nanoparticles, especially metal oxides (e.g., zinc oxide, titanium oxide), can generate reactive oxygen species (ROS) through photocatalytic or redox activity. ROS are highly reactive and can cause the oxidation of lipids in the cell membrane, which leads to its peroxidation and damage.