wavelength light on photosensitizer, and this phenomenon is known ‘photodynamic

therapy.’ There were many ENMs like ZnO, TiO2, MoO2, MoO3, WO3, metal-

organic framework (MOF), etc. that act as good metal-based photosensitizers which

are used in photodynamic therapy (Sun et al. 2018). When ROS is generated in the

presence of ultrasound, then the process is called son dynamic therapy (You et al.

2016).

The oxidative stress caused by ROS to the bacterial membrane causes the

variation in the membrane permeability which leads to cell membrane damage.

There are many examples in the literature related to the ENMs which generate

ROS and produce oxidative stress to the cell and causes death.

27.3.2 Self-Antibacterial Property of ENMs

During the study of the antibacterial property of metal based ENMs, the reorganiza-

tion of metal is very important because some metals have inherited antibacterial

property in their bulk form (e.g. Cu, Ag, Zn, etc.), while some become antibacterial

in its nanoparticles (NPs) form. Interestingly the self-antibacterial property of

ENMs successfully applied against both Gram-positive (GP) and Gram-negative

(GN) bacteria. Antibacterial properties are affected by the NPs surface to volume

ratio. As the ENMs size decreases, then their more surface area is available for the

dose for the surrounding environment, so different ENMs with an increase in surface

to volume thus enhance the antibacterial effect. There are relevant variables like

shape, chemistry, size, and surface charge which affect the antibacterial properties of

ENMs. For example, the antibacterial property of Au, Ag, Cu based NPs have shown

interesting antibacterial nature without creating any resistance so far (Yasuyuli et al.

2010).

A study by Yoon et al. report 90% killing of E. coli and B. subtilis by Ag NPs

with the dose of 58.4 and 32.1 μg/mL NPs, respectively, while for Cu based NPs, it

required 33.49 and 28.2 mg μg/mL, respectively (Yoon et al. 2007). This study has

shown that Cu based NPs are more effective as compare to Ag based NPs. Due to its

cheaper availability, greener synthesis, and the inherited antibacterial property of Cu

against wide range of microorganism, it has been recognized as the first metallic

antibacterial agent in 2008 by the American Environmental Protection Agency

(Vincent et al. 2016). Other than bacteria, metallic Cu coating is also effective

against yeast and virus killing. In this way, it was reported that Cu followed all the

mechanisms of bacteria killing, i.e. ROS generation, plasmid and genomic DNA

damage, lipid peroxidation occurrence, cell membrane rupturing and generating

dissolve ions (Grass et al. 2011). Zinc oxide (ZnO) is also a well-known antibacterial

NPs which shows antibacterial property against both GP and GN bacteria. Tiwari

and co-workers fabricated the ZnO ENMs and applied it against A. baumannii which

is a multidrug-resistant (MDR) pathogen. This also shown the ROS leads to bacterial

cell damage and caused the killing of the cell (Tiwari et al. 2018; Alves et al. 2017).

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