period indicating that TiO2 NPs are very effective as antimicrobial agents. Liu et al.

(2010) studied the effect of TiO2, UV irradiation, and their combined exposure on

the E. coli cells. After treatment with TiO2 or UV light alone, cells sustained damage

to the outer membrane due to lipopolysaccharide rupture to some degree, but the

cells were still able to retain the initial rod morphology. However, the outer

membrane of E. coli was severely compromised and totally removed in the presence

of TiO2 under the exposure of UV light.

Conventional TiO2 ENMs are activated only under UV illumination, which is less

than 5% of the solar spectrum compared to 45% of visible light (Ikram et al. 2020).

In addition, overexposure to UV radiation has been known to cause serious genetic

damage to human cells and tissues, which restricts the practical application of TiO2

ENMs. To date, numerous studies have been carried out to design TiO2 ENMs with

an aim to draw its photoresponse into visible light region (Ismael et al. 2020). These

include doping TiO2 ENMs with metallic or non-metallic ion and coupling with

narrow band-gap semiconductors. For instance, Yadav et al. (2014) reported the

photocatalytic antibacterial activity of Ni-doped TiO2 NPs under fluorescent visible

light against GP (S. aureus and B. subtilis) and GN (E. coli and S. abony) bacteria.

Ananpattarachai et al. (2016) studied the effects of cation (Ni) and anion (N) doping

on the structure, visible light-absorbing capacity, and antimicrobial activity of the

TiO2 ENMs. N-doped TiO2 ENMs were observed to show higher antibacterial

activity than un-doped and Ni-doped TiO2, which was attributed to the band-gap

narrowing that leads to more visible light absorption and the superb antibacterial

properties.

In another study, Hamal et al. (2010) reported the fabrication of Ag, S, and C

co-doped TiO2 composite NPs as an effective biocide/sporicide in dark and

photocatalyst in visible light. It was reported that sporicidal efficacy of co-doped

NPs increased with increasing Ag doping, while photocatalytic efficacy of co-doped

NPs was higher at lower Ag concentration. According to this result, it is estimated

that Ag/(C, S)-TiO2 composite NPs can act as a potential biocide at higher loadings

and a photocatalyst under visible light at lower loadings. Liu et al. (2017) reported

the remarkable photocatalytic and antibacterial capability under visible light irradia-

tion for TiO2-Ag2O heterostructure composite, which was mainly attributed to the

synergistic effect between Ag2O NPs and TiO2 microsphere. Highly dispersed

smaller Ag2O NPs (5–30 nm) were suggested to enhance the visible light absorption

and efficient separation of photo-induced charge carriers. To enhance the light

absorption region of TiO2 ENMs, Wang and co-worker (2010) fabricated the

tri-doped TiO2 NPs by doping with Er³⁺, Yb³⁺, and Fe³⁺ ions. By this method,

they successfully broadened the light absorption region to the near infrared region

which leads to more penetration effect and enhancement in antibacterial activity in

this region.

In addition to extending the photoresponse to the visible or infrared region,

doping has also been reported to improve the antibacterial properties of the parent

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