closely dependent on the oxidation state and biocidal activity of Cu2O against E. coli

was found to be high than CuO. However, instead of Cu ion toxicity, ROS genera-

tion and binding of proteins were argued as major contributing factors for the

antibacterial activity of CuO and Cu2O NPs, respectively.

Applerot et al. (2012) studied the size-dependent (from the microscale size

(800 nm) down to ultra-small nanoscale size (30 and 2 nm)) antibacterial activity

of CuO particles. The antibacterial properties of CuO particles were found to be

associated with their size, where smallest NPs were observed to have the highest

biocidal activity. The effective antibacterial activity of CuO NPs was attributed to

the increase of intracellular oxidative stress due to the generation of ROS by the NPs

attached to the bacterial cells. Electron microscopy study further indicated that the

ultra-small CuO NPs penetrated the bacterial cells. Similarly, Chauhan et al. (2019)

investigated the effect of size and morphology on the antimicrobial activity of CuO

NMs against the pathogenic bacteria (S. aureus) and reported that it varied as a

function of size and surface area. Karim and co-worker fabricated CuO-based

nanozymes which produce ROS in dark conditions, and upon irradiation of visible

light, there was 20% enhancement in the ROS production. This enhanced ROS

damages E. coli cell wall and leads to cell death (Karim et al. 2018).

27.4.5 Titanium (Ti)-Based ENMs

TiO2 is one of the most extensively studied metal oxide semiconductors due to its

great potential in the field of photocatalysis. Owing to various properties like

biologically inertness, non-toxicity, substantial stability and production of ROS

when illuminated with UV light makes TiO2 the most suitable component for

antibacterial applications. All of these radicals are known to be very reactive and

easily disrupt organic compounds.

The novel concept of photochemical sterilization was first demonstrated by

Matsunaga et al. (1985) using a powder of TiO2 semiconductor loaded with platinum

(TiOE/Pt). The authors reported the antimicrobial activity for TiOE/Pt powder under

the exposure of metal halide lamp irradiation against bacteria (E. coli and

L. acidophilus), yeast (S. cerevisiae), and algae (C. vulgaris). Inhibition of respira-

tory function due to oxidation of coenzyme A has been identified as a cause of cell

death. They further suggested that bactericidal effects of catalyst were not caused by

toxic substances such as H2O2 and free radicals released during electrolysis and

direct oxidation of the microbial cell was responsible for the loss of viability.

Following this initial study, research work on TiO2 photocatalytic killing has

been extensively conducted on a wide spectrum of organisms including viruses,

protozoa, bacteria, fungi, algae, and cancer cells. Tsuang et al. (2008) investigated

the photo-killing effects of TiO2 NPs against five different bacteria (i.e. E. coli,

P. aeruginosa, S. aureus, E. hirae, and B. fragilis) under UV light. At the end of the

study, it was revealed that UV light alone did not affect the viability of bacteria,

while TiO2 NPs, especially under UV light exposure, showed significant effects on

bacteria viability. Approximately, all the bacterial cells were killed within a 50 min

510

M. Chauhan et al.