and/or chemically modified for a desired application. A large number of synthetic

techniques for the synthesis of ENMs with varied morphologies, sizes, and dimen-

sion have been explored by using different chemical and physical approaches.

Generally, to fabricate ENMs synthetic techniques can be broadly categorized

into top-down and bottom-up approach. As suggested by its name, the top-down

approach starts with a bulk material which is broken down into nanoscale dimension,

using techniques such as ball milling or attrition. Although this is a simple technique

for fabrication of ENMs, however, this technique results in a broad size distribution

and nonuniform particle geometries and contains increased impurities.

On the contrary, bottom-up approaches utilize diverse techniques to build ENMs

from raw chemicals and physical environments to a completed, finished product.

This approach can be time-consuming, however, permits for precise control over the

chemical output and produces consistent particle shapes, sizes, and geometries with

little defects. Examples of this fabrication technique include solution-based method,

sol-gel method, electrochemical method, colloidal methods, atomic layer deposition

method, solid state thermal conversion method, etc. (Table 27.1). Several compre-

hensive reviews are available, providing a thorough discussion of the basic under-

standing and properties in metal-based ENMs synthesis.

27.3

Mode of Action of ENMs Against Bacteria

ENMs have unique chemical, electrical, magnetic, thermal, physical, and biological

properties. Due to these interesting properties in ENMs, these were used as a

potential antibacterial agent. Effect of ENMs varies from exposer time of ENMs

against different bacterial culture with the environmental condition influences the

antibacterial activity. The mechanism of antibacterial property varies for different

ENMs. Yet, for all the ENMs, a mechanistic way is still not established for the

antibacterial property by which it kills the bacterial cell (Xie et al. 2018; Cai et al.

2019; Li et al. 2020; Huang et al. 2020).

27.3.1 Reactive Oxygen Species (ROS) Generation

The term ROS denoted the reactive radicals which includes, singlet oxygen (¹O2),

hydrogen peroxide (H2O2), superoxide (O^2), hydroxyl ions (OH^), and free

radicals (OH^•), which are produced as a side product of some biological processes.

ENMs generate ROS in presence of light, ultrasounds, or sometimes without light

give oxidative stress to bacterial cell. These ROS action modes are applied for a short

period which is also induced by catalase and superoxide. Normally, ROS plays an

important role in the different physiological processes in animals and plants, but the

excess level of ROS causes oxidation of different cellular components such as DNA,

lipids, and proteins which is a menace to cell survival. Out of all these

abovementioned ROS, ¹O2 is a highly toxic type of ROS which is responsible for

the killing of bacteria. Normally, the ¹O2 is generated by putting a suitable

27

Antimicrobial Applications of Engineered Metal-Based Nanomaterials

501