with citrate and amino acids in case of MD and LB, respectively, were observed to

decrease the concentration of Zn²⁺ ions, resulting in the lower toxicity in these

media. These species drastically reduced the Zn²⁺ ion concentration, which resulted

in lower toxicity in these media. Additionally, the isotonic and rich nutrient

conditions improved the tolerance of E. coli to toxicants.

In another study, Jain et al. (2013) explored the physiological effects of the ZnO

nanorods on the GP (S. aureus and B. subtilis) and GN (E. coli and A. aerogenes)

bacterial cells. The findings showed that the antibacterial activity of ZnO nanorods

were higher against GP bacteria than GN bacteria, suggesting that the structure of the

cell wall plays a major role in the interaction with ENMs and displays a high

sensitivity to the concentration of the particles.

The cause of antibacterial action in the dark is primarily attributed to the leaching

of Zn ions from ZnO suspension into the cells, causing DNA damage and cell death.

However, studies have shown the effective production of ROS even in the dark

(Xu et al. 2013). For instance, Prasanna and Vijayaraghavan (2015) have reported

that the production of ROS such as ^•OH, H2O2, ^•O2

, and •HOO from the aqueous

suspension of ZnO NPs in the dark can cause oxidative stress resulting in

antibacterial activity. This work further confirmed that surface defects play a major

role in the production of ROS both in the presence and absence of light.

27.4.4 Copper (Cu)-Based ENMs

Cu and its complexes are popular for their biocidal properties since ancient times.

The earliest record of Cu being used for medicinal application can be found in Smith

Papyrus, an ancient Egyptian medical text composed during 2600 to 2200 B.C.; it

describes the use of Cu to sterilize chest wounds and drinking water (Borkow and

Gabbay 2009). The doses required for the treatment of bacterial infections, however,

are reasonably high enough to cause concomitant damage to healthy surrounding

cells as well. Therefore, the direct use of Cu complexes for the treatment of bacterial

infections is restricted in many cases. Recent studies have confirmed that Cu based

ENMs have high antibacterial potency within a considerably low dose range. It

implies that Cu based ENMs (metallic Cu, cupric oxide (CuO), and cuprous oxide

(Cu2O)) can be used as a potential antimicrobial agent. Kruk et al. (2015) reported

the high antibacterial activity of monodispersed metallic Cu NPs (50 nm) against

standard and clinical strains of GP bacteria (MRSA) and antifungal activity against

Candida sp.

Oxides of Cu typically occur in two different forms, i.e. CuO and Cu2O, which

are both p-type semiconductors that have a band gap that ranges approximately

between 1.21–1.55 and 2.2–2.5 eV, respectively. Comparatively, CuO is thermody-

namically more stable and exhibit a broad spectrum of antibacterial activity. Some

studies (Kumar et al. 2019; Gunawan et al. 2011; Hans et al. 2013) have observed

high antimicrobial efficacy for Cu2O than CuO, since it can generate cuprous ions

(Cu¹⁺) which has been shown to be more toxic to bacteria than cupric (Cu²⁺) ions.

Meghana et al. (2015) reported in their study that antibacterial activity of CuO NPs is

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