Nanoparticles (NPs) react with oxygen molecules to produce different types of ROS. For instance, titanium dioxide is a semiconductor that can be activated by photons with wavelengths below about 385 nm, or UVA, which are photocatalyzed to allow electrons to move freely within the conduction band (Bono et al., 2021). Oxygen and water can trap charges and produce reactive oxygen species, such as superoxide anions (O⁺ ₂ ⁻•) and hydroxyl radicals (•OH), which degrade cell walls and plasma membranes, leading to leakage of cell contents and bacterial death. In addition, the ROS produced can diffuse into bacteria, attack proteins and inhibit the activity of certain surrounding plasmids that maintain the normal morphology and physiological processes of bacterial cells.

The positively charged metal ions of NPs are released and bind to the negatively charged functional groups of the bacterial cell membrane, resulting in the chaotic dispersion of the originally ordered and tightly spaced cell membrane, resulting in the death of the bacteria. In addition, some metal ions can directly interact with functional groups of proteins and nucleic acids. For example, zinc ions have specific affinity for mercaptan groups and can be oxidized by mercaptans as inhibitors of glycolytic enzymes (Liao et al., 2020).

Under physiological conditions, some organic compounds, such as polyethylene glycol, can provide a hydrophilic environment on the substrate surface by hydrogen bonding with water molecules, forming a stable thin water layer, interfering with the interaction between the substrate surface and the biofilm, which can effectively prevent bacterial adhesion (Peng et al., 2017).

Materials can be endowed with antibacterial properties by adding functional groups such as quaternary ammonium cation (QAC) (Makvandi et al., 2018) and long alkyl chain (LAC) (Park et al., 2022). Although the detailed mechanism of the antimicrobial action of QACs has not been determined, its antibacterial effect is related to the strong affinity and destructive interaction between the positively charged QACs ions and the negatively charged acidic phospholipid head groups in the microbial membrane. Long lipophilic alkyl chains penetrate bacterial cell membranes by binding to cell wall components, leading to cytoplasmic material leakage, bacterial autolysis, and cell death.

A number of different metals and their compound materials have been coated onto fixed orthodontic appliances, including silver (Ag), titanium dioxide (TiO₂), zinc oxide (ZnO), zirconium oxide (ZrO₂), and titanium nitride (TN). Among these, TiO₂ is most widely studied and utilized.

Silver nanoparticles (Ag NPs), have become common materials in the fields of dentistry and orthodontics due to their high antibacterial activity. Studies have shown that without changing the physical and chemical properties of archwires, the silver nanoparticle coated Abzil^® archwires showed a significant reduction in the presence of S. aureus and S. mutans, while Orthometric^® archwires did not (Goncalves et al., 2020). The nano-silver coated bracket shows great antibacterial characteristic against S. mutans, and its contact inhibition feature is effective especially in the reduction of smooth surface caries around the brackets (Ghasemi et al., 2017; Metin-Gürsoy et al., 2017).

The nano-silver coatings for brackets can be obtained by physical vapor deposition (PVD), galvanic silver and plasma immersion ion implantation and deposition (PIIID). For the antibacterial effect, there’s no significant difference between the PIIID procedure with silver ions and a PVD or galvanic silver coating (Meyer-Kobbe et al., 2019).

The combination of TiO₂ and Ag NPs can compensate for the weakness in the decreased antibacterial property due to insufficient light in the mouth. A combination of Ag and TiO₂ for the coating onto SS brackets showed excellent anti-adherent properties for the major oral pathogens S. mutans and Porphyromonas gingivalis (P. gingivalis). Ag NPs can also be combined with ZnO NPs. The Ag/ZnO coated brackets exhibit the highest antibacterial effect in comparison with Ag and ZnO individually coated brackets on S. mutans and L. acidophilus, and the antibacterial activity of the bracket coatings can persist over time (Zeidan et al., 2022).

TiO₂ has attracted much attention because of its excellent antibacterial activity and biocompatibility (Mollabashi et al., 2020; Venkatesan et al., 2020). There are two phases of TiO₂, the anatase phase and the rutile phase, and they both exhibit significant antibacterial characteristics. The unwanted cytotoxicity is also seen in these phases—moderate to severe in the rutile phase, but only mild in the anatase phase (Baby et al., 2017). Thus, it is recommended that anatase phase TiO₂ be coated onto brackets, so that antibacterial property is obtained while being only slightly cytotoxic at the same time. A clinical trial showed that TiO₂ coated archwires can effectively reduce the adherence of S. mutans during the initial stage of orthodontic treatment (Mollabashi et al., 2020).

The antibacterial effect of TiO₂ is achieved through the process of photocatalysis in the ultra violet (UV) region (<380 nm) (Salehi et al., 2018). The way of doping and surface modification allows TiO₂ to exhibit catalytic activity within the visible light region, consequently improving the photocatalytic efficiency of TiO₂ (Salehi et al., 2018). For instance, Nitrogen-doped TiO₂ (N-doped TiO₂) can expand the absorption edge to the visible region and narrow the band gap (Amini et al., 2017), showing the characteristic of exceptional visible light and UV light activities (Asahi et al., 2001). Under the condition of ultraviolet filtration, the bactericidal rate of N-doped TiO₂ coated composite archwires (CAWs) and TiO₂ coated CAWs were 87.2% and 5.9% respectively (Liu J M et al., 2017). Brackets coated with N-doped TiO₂ show a significant antibacterial effect against S. mutans, L. acidophilus, A. viscous, and C. albicans (Salehi et al., 2018). Also, this longer lasting antibacterial effect for TiO₂ coated brackets and may not decrease over time, regardless of whether the illumination time was 24 h or 60 m (Salehi et al., 2018).

Another metallic oxide material coated onto orthodontic appliances is ZnO. ZnO nanoparticles (ZnO NPs) have photocatalytic bactericidal activity, wide antibacterial spectrum and low drug resistance (Gharpure and Ankamwar 2020; Pushpalatha et al., 2022). The effect of coating five kinds of ZnO NPs onto NiTi archwires on inhibiting S. mutans was studied (Gholami et al., 2021). The results indicated that chemical vapor deposition (CVD) method had the highest bacteriostatic rate of 98%. ZnO coated brackets could exhibit enhanced antibacterial effect against S. mutans and L. acidophilus in comparison to the uncoated (Zeidan et al., 2022). The carbon quantum dots (CQDs) is a kind of unique materials with the upconversion fluorescence property, which can convert visible light into UV or near-UV light. Combining the antibacterial ability of ZnO and the upconversion fluorescence property of CQD, the ZnO/CQD composite coating exhibits great antibacterial activity under natural visible light (Zhang et al., 2018).

ZrO₂ nanoparticles (ZrO₂ NPs) have excellent biocompatibility and good adhesion on metal surfaces. ZrO₂ NPs coated archwires showed good antibacterial activity against S. mutans and S. aureus. The antibacterial activity increased with the increase of corresponding concentration, but the inhibition on C. albicans was the same at different concentrations (Selvaraj et al., 2021).

Besides metallic oxide, TN is another metallic compound that has been used. The TN coating has been in use for implants since 2000 (Jabbari et al., 2012) while in orthodontic field, TN coatings onto the surface of SS brackets have no influence on the formation of the S. mutans biofilm and do not reduce the growth of S. mutans.

The SS archwires coated with polyethylene glycol (molecular weight 5,000) have good anti-bacterial adhesion performance, which can inhibit the growth of bacteria up to 10 h. (Peng et al., 2017). Lysozyme selectively breaks down the cell wall of microorganisms without destroying other tissues, and was used for antibacterial modification of archwires. The 40 g/L lysozyme coating on the CAWs had the strongest inhibitory effect on S. aureus (He et al., 2020).

Graphene oxide (GO) is an oxide of graphene, which has increased oxygen-containing functional groups, making its properties more active than graphene. Dai et al. (2022) found that with the increase of graphite oxide concentration, the bacteriostatic effect of GO coating was enhanced. In vitro and in vivo studies have been conducted to compare the antibacterial characteristics for different coatings onto orthodontic appliances (Table 1).