1. The oxidative stress created by the reactive oxygen species can damage the
unsaturated fatty acids of the bacterial cell membrane leading to the permanent
damage of the bacterial activity (Kääriäinen et al. 2013).
2. Zn
2+ ions released from the ZnO can transport through the cell membrane and
reduce the intracellular ATP levels again leading to the improper functioning of
the bacteria (Tong et al. 2015).
3. The cell membrane can damage due to the adhesion of the aggregates of ZnO
nanoparticles (Li et al. 2008).
2.5.2.1 Nanostructured ZnO
Researchers have improved the photocatalytic antibacterial efficiency of ZnO by
fabricating nanostructures with different morphologies such as nanoflowers,
nanorods, thin films etc. For instance, 3-D flower like ZnO structures fabricated by
surfactant free co precipitation method were shown to have efficient photocatalytic
antibacterial property towards Enterococcus faecalis (E. faecalis) and Micrococcus
luteus (M. luteus) (Quek et al. 2018). The bacteria were eradicated completely in
130 min of visible light irradiation which was detected by bacterial morphological
change, K
+ ions leakage and protein leakage. The enhanced activity was attributed to
the flower like morphology which increase the number of surface hydroxyls groups
and photogenerated charge carriers on the ZnO surfaces leading to the formation of
reactive oxygen species mainly H 2 O 2 (Fig. 2.8a and b). Zhang and coworkers (2008)
employed ZnO nanofluids for the disinfection of E.coli. The improved activity of the
optimized ZnO sample stored for 120 days was attributed to the increased surface
area and the electrostatic binding of the particle and the bacteria. A morphology
dependent antibacterial activity was deduced for ZnO nanoparticle fabricated by
solvothermal method (Talebian et al. 2013). The trend in the antibacterial activity of
different nanostructures towards inactivation of gram-negative Escherichia coli and
gram-positive Staphylococcus aureus under UV light was found as nanoflower >
nanorods > spherical nanoparticle (Fig. 2.8c and d). The interstitial defect present in
the nanoflowers reduced the electron-hole recombination and hence improved the
light induced antibacterial activity. Raja et al. (2018) fabricated ZnO nanoparticles
through an ecofriendly green synthesis route, and studied their antibacterial activity
towards Salmonella paratyphi, Escherichia coli and Staphylococcus aureus. The
bacterial strains were investigated and their result showed a higher antibacterial
efficiency towards S.aurus and E.coli compared to S. paratyphi.
Another approach was the fabrication of ZnO nanoleaves reported by Gupta and
Srivastava (2018), where disperser assisted sonochemical approach was employed as
the synthesis route. The schematic diagrams of the synthesis route of the ZnO
nanoleaves are shown in Fig. 2.8e. The synthesis route was claimed to be scalable
in nature and the product can be employed for the disinfection of S. aureus. A green
synthesis approach was employed by Bhuyan et al. (2015) for the fabrication of ZnO
nanoparticles using neem leaf extract. The as prepared ZnO nanoparticles showed
efficient antibacterial activity towards gram positive and gram negative bacteria
(Staphylococcus aureus, Streptococcus pyogenes and Escherichia coli) by inhibiting
2 Photo-Assisted Antimicrobial Activity of Transition Metal Oxides
43
unsaturated fatty acids of the bacterial cell membrane leading to the permanent
damage of the bacterial activity (Kääriäinen et al. 2013).
2. Zn
2+ ions released from the ZnO can transport through the cell membrane and
reduce the intracellular ATP levels again leading to the improper functioning of
the bacteria (Tong et al. 2015).
3. The cell membrane can damage due to the adhesion of the aggregates of ZnO
nanoparticles (Li et al. 2008).
2.5.2.1 Nanostructured ZnO
Researchers have improved the photocatalytic antibacterial efficiency of ZnO by
fabricating nanostructures with different morphologies such as nanoflowers,
nanorods, thin films etc. For instance, 3-D flower like ZnO structures fabricated by
surfactant free co precipitation method were shown to have efficient photocatalytic
antibacterial property towards Enterococcus faecalis (E. faecalis) and Micrococcus
luteus (M. luteus) (Quek et al. 2018). The bacteria were eradicated completely in
130 min of visible light irradiation which was detected by bacterial morphological
change, K
+ ions leakage and protein leakage. The enhanced activity was attributed to
the flower like morphology which increase the number of surface hydroxyls groups
and photogenerated charge carriers on the ZnO surfaces leading to the formation of
reactive oxygen species mainly H 2 O 2 (Fig. 2.8a and b). Zhang and coworkers (2008)
employed ZnO nanofluids for the disinfection of E.coli. The improved activity of the
optimized ZnO sample stored for 120 days was attributed to the increased surface
area and the electrostatic binding of the particle and the bacteria. A morphology
dependent antibacterial activity was deduced for ZnO nanoparticle fabricated by
solvothermal method (Talebian et al. 2013). The trend in the antibacterial activity of
different nanostructures towards inactivation of gram-negative Escherichia coli and
gram-positive Staphylococcus aureus under UV light was found as nanoflower >
nanorods > spherical nanoparticle (Fig. 2.8c and d). The interstitial defect present in
the nanoflowers reduced the electron-hole recombination and hence improved the
light induced antibacterial activity. Raja et al. (2018) fabricated ZnO nanoparticles
through an ecofriendly green synthesis route, and studied their antibacterial activity
towards Salmonella paratyphi, Escherichia coli and Staphylococcus aureus. The
bacterial strains were investigated and their result showed a higher antibacterial
efficiency towards S.aurus and E.coli compared to S. paratyphi.
Another approach was the fabrication of ZnO nanoleaves reported by Gupta and
Srivastava (2018), where disperser assisted sonochemical approach was employed as
the synthesis route. The schematic diagrams of the synthesis route of the ZnO
nanoleaves are shown in Fig. 2.8e. The synthesis route was claimed to be scalable
in nature and the product can be employed for the disinfection of S. aureus. A green
synthesis approach was employed by Bhuyan et al. (2015) for the fabrication of ZnO
nanoparticles using neem leaf extract. The as prepared ZnO nanoparticles showed
efficient antibacterial activity towards gram positive and gram negative bacteria
(Staphylococcus aureus, Streptococcus pyogenes and Escherichia coli) by inhibiting
2 Photo-Assisted Antimicrobial Activity of Transition Metal Oxides
43