To understand ROS generation mechanism and its relationship with the

antibacterial potency of NPs, Li et al. (2012) studied the ROS generation kinetics

of different metal oxide NPs (i.e. TiO2, CeO2, ZnO, CuO, SiO2, Al2O3, and Fe2O3)

and their bulk counterparts under the exposure of UV radiation (365 nm).

The findings showed that different metal oxides had distinct photogenerated

ROS kinetics, where TiO2 and ZnO NPs were observed to generate all the three

kinds of ROS (¹O2, ^•OH, and ^•O2), while the rest of metal oxides produced either

one or two or did not produce any kind of ROS. NPs generated more ROS than

their bulk counterparts presumably by having more UV radiation absorption

sites due to the larger surface area. The average concentration of total ROS

(the sum of the concentrations of three types of ROS) followed the order:

TiO2 > ZnO > Al2O3 > SiO2 > Fe2O3 > CeO2 > CuO > ZnO (bulk) > TiO2

(bulk). The ROS generation process was interpreted by comparing the electronic

structures of metal oxides with the redox potentials of different ROS generation.

Furthermore, a linear correlation was found between the average concentration of

total ROS and the antibacterial activity of the NPs on E. coli cells as the model

bacterium. However, in the dark, none of these metal oxides were reported to

produce detectable ROS within the experimental period. Likewise, Lipovsky et al.

(2009) related the toxic effect of ZnO to the elevated levels of ROS, namely, ¹O2 and

•OH radicals, when the aqueous suspension ZnO was irradiated with blue light

(400–500 nm).

Padmavathy and co-workers studied the effect of size (ranging from micron to

nm) on the antibacterial property of ZnO against E. coli. The results demonstrated

that nanosized ZnO (10–50 nm) is more effective antimicrobial agent than bulk ZnO

(2 μm). Comparatively high antibacterial effect of ZnO NPs was attributed to its

abrasive surface texture due to rough edges and corners that contributed to the

mechanical damage of the cell membrane. Moreover, with a decrease in size of

ZnO particles, there is an increase in the generation of ROS, which killed bacteria

more effectively (Padmavathy and Vijayaraghavan 2008). Jones and co-workers

studied the antibacterial property of the purchased ENMs (ZnO, TiO2, MgO, CeO2,

and CuO) against both GP (B. subtilis, E. faecalis, S. pyogenes, S. epidermis, and

S. aureus) and GN bacteria (E. coli) and evaluated that among all, ZnO proved to be

an excellent material with maximum antibacterial property (Jones et al. 2008).

Many studies have ascribed the antimicrobial activity of ZnO ENMs to the release

of Zn²⁺ ions in a medium (Blinova et al. 2010; Wong et al. 2010; Heinlaan et al.

2008). When ZnO NMs are in solution, partial dissolution results in the release of

Zn²⁺ ions, which have antimicrobial activity. Li et al. (2011) investigated the effect

of dispersion medium on the toxicity of ZnO NMs and reported that the toxicity can

be related with the concentration of the free hydrated Zn²⁺ ions or labile Zn

complexes. In a typical study, five different aqueous medium, i.e. ultrapure water,

NaCl (0.85%), phosphate-buffered saline (PBS), minimal Davis (MD), and Luria-

Bertani (LB) were chosen to investigate the potential effect of water chemistry on the

toxicity of ZnO NMs to E. coli. The results showed that the toxic effect of ZnO NMs

in different media was in the order of ultrapure water > NaCl > MD > LB > PBS.

The formation of precipitates like Zn3(PO4)2 in case of PBS and complexes of Zn

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M. Chauhan et al.