pH value at which the surface charge is zero at the catalyst surface and is also
known as point of zero charge.
(c) Temperature: In photocatalytic disinfection process, temperature is not considered as an important parameter. However, in some cases, the disinfection process
is found to increase with increase in operating temperature. In sunlight based
disinfection process, thermal energy is contributed from the light source itself
and is presumed to play a minor role in controlling the disinfection process.
(d) Light wavelength, intensity and irradiation time: These two important parameters play a crucial role in the photocatalytic disinfection process. Generally, UV
assisted bactericidal process is an effective approach where the light energy is
quiet higher compared to the band gap of the material. Also, higher irradiation
intensity leads to higher reactive oxygen species generation leading to higher
efficiency. With increase in irradiation time, the efficiency of the disinfection
process increases, but it is desirable to obtain faster photocatalytic disinfection at
a commercial level. An important parameter to be considered in this case is the
irradiation surface, where the surface area of the semiconducting material is
preferred to be high on which the reactive oxygen species has to be generated.
More than these parameters, the structural complexity of the microorganism is an
important parameter. This decides the kinetics of the disinfection process, where
structural factors such as type of strains, cell wall structure, and type of organ
targeted for mineralization etc. play crucial role.
2.4 Different Models Proposed for Photocatalytic
Disinfection
The antimicrobial activity can be a cooperative action by all kinds of reactive oxygen
species. It was inferred initially that the photo-assisted destruction of coenzyme A in
the bacteria is the origin of death of bacteria due to the inhibition of respiratory
activity (Matsunaga et al. 1985). Later on, it was observed that the cell wall
destruction led to the leaking of potassium ions and other cellular components
leading to the destruction of cells (Saito et al. 1992). It was then confirmed by
various morphological and structural characterization that, the complete mineralization of the cell walls and components are occurring by photo catalytically generated
reactive oxygen species (Bagchi et al. 1993; Jacoby et al. 1998; Kiwi and
Nadtochenko 2005; Maness et al. 1999; Sökmen et al. 2001). A peroxidation
mechanism of the bacterial cell wall on TiO 2 surface was studied by Kiwi and
coworkers and was observed that, the first step of the complete disintegration of the
cell wall is the competition between the oxidation process by photo catalytically
generated holes and oxidation of lipid polysaccharide layer (Kiwi and Nadtochenko
2005). Nanoparticle interaction with the bacterial cell wall also played a major role
in cell damage where electrostatic interaction of the semiconducting surface and cell
wall, reactivity of reactive oxygen species on the particle surface and metal ions play
2 Photo-Assisted Antimicrobial Activity of Transition Metal Oxides
33
known as point of zero charge.
(c) Temperature: In photocatalytic disinfection process, temperature is not considered as an important parameter. However, in some cases, the disinfection process
is found to increase with increase in operating temperature. In sunlight based
disinfection process, thermal energy is contributed from the light source itself
and is presumed to play a minor role in controlling the disinfection process.
(d) Light wavelength, intensity and irradiation time: These two important parameters play a crucial role in the photocatalytic disinfection process. Generally, UV
assisted bactericidal process is an effective approach where the light energy is
quiet higher compared to the band gap of the material. Also, higher irradiation
intensity leads to higher reactive oxygen species generation leading to higher
efficiency. With increase in irradiation time, the efficiency of the disinfection
process increases, but it is desirable to obtain faster photocatalytic disinfection at
a commercial level. An important parameter to be considered in this case is the
irradiation surface, where the surface area of the semiconducting material is
preferred to be high on which the reactive oxygen species has to be generated.
More than these parameters, the structural complexity of the microorganism is an
important parameter. This decides the kinetics of the disinfection process, where
structural factors such as type of strains, cell wall structure, and type of organ
targeted for mineralization etc. play crucial role.
2.4 Different Models Proposed for Photocatalytic
Disinfection
The antimicrobial activity can be a cooperative action by all kinds of reactive oxygen
species. It was inferred initially that the photo-assisted destruction of coenzyme A in
the bacteria is the origin of death of bacteria due to the inhibition of respiratory
activity (Matsunaga et al. 1985). Later on, it was observed that the cell wall
destruction led to the leaking of potassium ions and other cellular components
leading to the destruction of cells (Saito et al. 1992). It was then confirmed by
various morphological and structural characterization that, the complete mineralization of the cell walls and components are occurring by photo catalytically generated
reactive oxygen species (Bagchi et al. 1993; Jacoby et al. 1998; Kiwi and
Nadtochenko 2005; Maness et al. 1999; Sökmen et al. 2001). A peroxidation
mechanism of the bacterial cell wall on TiO 2 surface was studied by Kiwi and
coworkers and was observed that, the first step of the complete disintegration of the
cell wall is the competition between the oxidation process by photo catalytically
generated holes and oxidation of lipid polysaccharide layer (Kiwi and Nadtochenko
2005). Nanoparticle interaction with the bacterial cell wall also played a major role
in cell damage where electrostatic interaction of the semiconducting surface and cell
wall, reactivity of reactive oxygen species on the particle surface and metal ions play
2 Photo-Assisted Antimicrobial Activity of Transition Metal Oxides
33