Читать книгу Microbial Interactions at Nanobiotechnology Interfaces - Группа авторов - Страница 42

Similar to metal NMs, oxides of metal also exhibit antimicrobial activity through the metal ions released from the system. The mechanism of action is the ROS production induced by metal ions or by the accumulation of NPs inside the cells. Most metal oxides are semiconductors in nature with larger bandgap energy. Among the metal oxides, ZnO (3.2 eV) has the highest bandgap energy similar to TiO_2, which gets activated at a wavelength of about 390 nm to induce ROS production. CdO has a bandgap energy of about 2.1 eV with activation wavelength at 590 nm (Table 1.2).

Among the metal oxides, the most widely used are TiO₂, ZnO, CuO, and MgO for antibacterial activity. A vastly studied metal oxide is TiO_2, which is used in various industrial and environmental applications such as in sunscreen preparation, implant coatings, and removal of water and air contaminants. The antimicrobial efficiency of a system depends on the wavelength of light used for activation, the intensity of light, concentration, interaction time, temperature, and target microbes (Markowska‐Szczupak, Ulfig, & Morawski, 2011). Efficacy of the TiO₂ system depends on the photoinduced generation of ROS, which happens effectively at the oxide anatase phase. When light of energy equal to or higher than the bandgap energy (3.22 eV, anatase phase) is exposed, photocatalytic activation produces an energy‐rich electron–hole pair. The electron produced is transferred to reducible species such oxygen to generate free radicals like superoxides O₂⁻. In a similar way, it induces the production of hydroxyl radicals and hydrogen peroxide in acidic conditions (Kühn et al., 2003). A schematic representation of photoactivated ROS generation and antimicrobial property of NMs is given in Figure 1.2 as described by Gardini et al. (2018)

Table 1.2 Bandgap energy and activation wavelength for various metal oxide NMs (Gardini et al., 2018).

S. No	Material	Bandgap energy (eV)	Activation wavelength (nm)
1	CdO	2.1	590
2	Fe2O3	2.2	565
3	WO3	2.8	443
4	TiO2	3.2	387
5	ZnO	3.2	390

Figure 1.2 Schematic representation of photoactivated ROS generation and antimicrobial property of NMs.

In a similar way, ZnO, a semiconductor with larger bandgap energy, is applied in coatings, paints, and sunscreens. Upon exposure of UV light, photocatalysis induces the ROS production, which is responsible for its antimicrobial effect. Here, the roughness of the system depends on the surface defects. Efficacy of the ZnO NP system increases with decrease in size where the roughness of the particle along with its high surface area causes the disruption of the microbial cell wall (Padmavathy & Vijayaraghavan, 2008). ZnO NMs have also been reported to interact with some disease target proteins. Chatterjee et al. (2010) studied the effect of ZnO NPs over periplasmic domain structure of ToxR protein of Vibrio cholerae. ToxR protein plays a critical role in the regulation of expression of many virulence factors of the bacteria. It was observed that the binding of the protein ToxR to ZnO NPs' surface reduced the stability of protein where it was more susceptible to denaturation. Further, significant change in the structure of the protein was also observed (Chatterjee et al., 2010).