shows a comparison of bacterial inactivation process with time. Multifunctional thin
film polyamide membranes embedded with rGO@TiO 2 @Ag nanocomposites was
developed by Abadikhah and coworkers (2019). A microwave irradiation assisted
synthesis process was adopted for the fabrication of nanocomposites which
improved the hydrophilicity, water permeation, salt rejection and antifouling properties of the membrane (Fig. 2.7B). A 100% reduction in the live E.coli population
was observed when in contact for 3 hrs with the nanocomposite modified polyamide
membrane. Recently Shimizu and coworkers (2019) studied the antimicrobial properties of carbon nanotubes -TiO 2 nanocomposite for the disinfection of E. coli. The
photogenerated hydroxyl radical induced reactive oxygen species caused a physical
rupture of the bacterial cell wall leading to the destruction of the microbes on the
semiconductor surface. A fructose modifies titania was employed as a catalyst for
photocatalytic disinfection of waste water by Rokicka-Konieczna et al. (2019).
Fructose was incorporated as an inexpensive and nontoxic source of carbon on
hydrothermally modified carbon. A two-step bacterial destruction by OH radials is
proposed and the disinfection process began from cell wall towards intra cellular
components.
2.5.2 Antimicrobial Behaviour of Zinc Oxide
ZnO is another class of wide band gap semiconductor materials having a band gap of
3.3 eV and exist in three different polymorphs say, wurtzite, rock salt and zinc blend
structure. The thermodynamic stable structure is wurtzite which is widely used in
applications such as gas sensors (Ahn et al. 2009), biosensors (Wei et al. 2006),
piezoelectric materials (Gullapalli et al. 2010), photocatalysts for organic decomposition and photocatalytic antibacterial coating materials (Hatamie et al. 2015). The
progressive research of antibacterial properties of ZnO started in 1950s.
Similar to the case of TiO 2 , upon light irradiation, the photogenerated charge
carriers move to the conduction band and valence band and participate in redox
reactions. The conduction band of ZnO is situated at À0.5 V vs NHE which is more
negative compared to the O 2 /O
-.2 level (À0.33 V vs NHE. Whereas, the valence
band positioning is 2.7 V vs NHE which is more positive compared to OH
. /H 2 O
redox position (2.53 V vs NHE). Thus, the photogenerated electrons and holes can
create O 2 /O
-.
2 and OH
. radical for disinfection reaction. The disinfection process by
ZnO is also attributed to the reactive oxygen species assisted oxidation process. The
radicals (O 2 /O
-.
2 and OH
. ) generated by photo irradiation is oxidative enough to
react and damage the cellular constituents such as DNA, lipids, proteins, carbohydrates. Three main mechanisms are proposed for the antibacterial activity of ZnO in
aqueous environment.
42
R. P. Antony et al.
film polyamide membranes embedded with rGO@TiO 2 @Ag nanocomposites was
developed by Abadikhah and coworkers (2019). A microwave irradiation assisted
synthesis process was adopted for the fabrication of nanocomposites which
improved the hydrophilicity, water permeation, salt rejection and antifouling properties of the membrane (Fig. 2.7B). A 100% reduction in the live E.coli population
was observed when in contact for 3 hrs with the nanocomposite modified polyamide
membrane. Recently Shimizu and coworkers (2019) studied the antimicrobial properties of carbon nanotubes -TiO 2 nanocomposite for the disinfection of E. coli. The
photogenerated hydroxyl radical induced reactive oxygen species caused a physical
rupture of the bacterial cell wall leading to the destruction of the microbes on the
semiconductor surface. A fructose modifies titania was employed as a catalyst for
photocatalytic disinfection of waste water by Rokicka-Konieczna et al. (2019).
Fructose was incorporated as an inexpensive and nontoxic source of carbon on
hydrothermally modified carbon. A two-step bacterial destruction by OH radials is
proposed and the disinfection process began from cell wall towards intra cellular
components.
2.5.2 Antimicrobial Behaviour of Zinc Oxide
ZnO is another class of wide band gap semiconductor materials having a band gap of
3.3 eV and exist in three different polymorphs say, wurtzite, rock salt and zinc blend
structure. The thermodynamic stable structure is wurtzite which is widely used in
applications such as gas sensors (Ahn et al. 2009), biosensors (Wei et al. 2006),
piezoelectric materials (Gullapalli et al. 2010), photocatalysts for organic decomposition and photocatalytic antibacterial coating materials (Hatamie et al. 2015). The
progressive research of antibacterial properties of ZnO started in 1950s.
Similar to the case of TiO 2 , upon light irradiation, the photogenerated charge
carriers move to the conduction band and valence band and participate in redox
reactions. The conduction band of ZnO is situated at À0.5 V vs NHE which is more
negative compared to the O 2 /O
-.2 level (À0.33 V vs NHE. Whereas, the valence
band positioning is 2.7 V vs NHE which is more positive compared to OH
. /H 2 O
redox position (2.53 V vs NHE). Thus, the photogenerated electrons and holes can
create O 2 /O
-.
2 and OH
. radical for disinfection reaction. The disinfection process by
ZnO is also attributed to the reactive oxygen species assisted oxidation process. The
radicals (O 2 /O
-.
2 and OH
. ) generated by photo irradiation is oxidative enough to
react and damage the cellular constituents such as DNA, lipids, proteins, carbohydrates. Three main mechanisms are proposed for the antibacterial activity of ZnO in
aqueous environment.
42
R. P. Antony et al.