The generation of ROS is the cause of cellular death for most bactericidal

antibiotics and ENMs. However, Cui et al. (2012) reported that bactericidal action

of Au NPs is independent of oxidative damage-related mechanisms such as ROS

generation. Au NPs have been shown to cause cell death specifically in two ways:

firstly, to alter the membrane potential and suppress the activity of ATP synthase to

decrease the amount of ATP, suggesting a general decrease in metabolism, and,

secondly, to inhibit the ribosome subunit from tRNA binding, indicating a failure of

the biological mechanism. In another study, Zhu et al. (2014) reported that the near

infrared laser irradiation of the Au nanorod arrays resulted in fast photo-heating with

noteworthy bactericidal properties which could be used for quick, efficient, and real-

time killing of pathogenic bacteria and thus producing microbe-free water.

27.4.3 Zinc (Zn)-Based ENMs

Among several transitions metal oxide, ZnO is one of the most promising inorganic

materials with a broad range of uses in the field of pharmaceutical, textile, cosmetic,

catalysis, photoelectronic, environmental remediation, electronics, and so on. ZnO is

registered as a safe substance by the US FDA, and owing to its distinctive electronic

configuration, low production costs, and appropriate properties, ZnO is considered

as one of the potential antibacterial materials (Joe et al. 2017; Abebe et al. 2020).

The key mechanism that has been identified for the antibacterial activity of ZnO

ENMs involves the production of ROS, antimicrobial ion release (Zn²⁺), electro-

static interaction, loss of cellular integrity, and internalization ENMs. Among

different mechanisms, the most widely described mechanism in the literature for

antimicrobial activity is the production of ROS, particularly, during the light absorp-

tion of characteristic wavelengths. ROS may include superoxide anions (^•O2

),

hydroxyl (^•OH) and perhydroxyl radicals (HOO^•), H2O2, and ¹O2 which can cause

the destruction of cellular components such as DNA, proteins, and lipids. For the

interaction of ENMs with the bacteria cell and production of ROS, the direct

production of ROS inside the bacterial cell and indirect production of ROS outside

the bacterial cell methods have been reported.

Semiconductor metal oxides such as TiO2, α-Fe2O3, MgO, CaO, etc. have a

specific band gap (such as 3.3 eV in case of ZnO NMs) that absorbs the characteristic

wavelength of light for the generation of electron (e^

CB^{) and hole (h+}

VB^{) pairs in the}

conduction and valence band, respectively. These electron-hole pairs have the

probability of recombining in picoseconds to produce thermal energy without any

chemical reactions or migrate/diffuse to the surface and initiate various reactions by

reacting with other species such as O2, H2O, or other moieties adsorbed on the

surface of the semiconductor. The ROS generated through different chain redox

reactions are extremely reactive and believed to degrade the bacterial cell into CO2,

H2O, and other nontoxic minerals; therefore, microorganisms in air and water can be

destroyed when they come into contact with the surfaces of a photocatalyst.

27

Antimicrobial Applications of Engineered Metal-Based Nanomaterials

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