3.2. Bacteria growth behavior under various light sources
Fig. 5 (a) and (b) presents the volume absorption of the bacteria
liquid of Staphylococcus aureus and Escherichia coli tested in dark
condition or under various light sources. A parallel comparison of the
results for inoculation on bare stainless steel and TiO2-coated stainless
steel was also made. Independently of the exposure conditions, the
volume absorption of the bacteria liquid inoculated on bare stainless
steel corresponds to a typical growth pattern throughout the period of
exposure, indicating that the stainless steel has almost no antimicrobial
effect. This volume absorption value gradually reach saturation
after 15 h of testing and reveal a change from the exponential growth
phase of the bacteria to the stationary growth phase [23,24]. This
result also verifies that without the TiO2 photocatalysis, UV or
fluorescent lamp alone has almost no effect on bacterial growth.
For both Staphylococcus aureus and Escherichia coli on the other
hand, when exposed to fluorescent lamp and black light source after
24 h test, the volume absorption of bacteria liquid on TiO2-coated
stainless steel has a markedly lower value than that on bare stainless
steel, indicating that the photocatalysis associated with TiO2 occurs to
inhibit bacterial growth. However, according to our results, the
bacteria initially underwent 8–12 h of exponential growth, followed
by growth inhibition rather than normal stationary growth. Next,
during growth inhibition, the volume absorption decreases with
exposure time, indicating that bacterial growth was effectively
inactivated.
When in the dark or exposed to visible light, both Staphylococcus
aureus and Escherichia coli on the TiO2-coated stainless steel yield the
same bacteria growth pattern as on bare stainless steel. Clearly,
neither visible light nor the dark condition without an energy
threshold can activate photocatalysis on the TiO2 film, thus revealing
no features associated with inhibited bacterial growth.
Fig. 5 (a) presents the growth patterns of the particular Staphylococcus
aureus on TiO2-coated stainless steel under a fluorescent lamp and black
light. The onset of the growth inhibition phase under the former
condition is at 12 h, while that under the latter conditions is at only 8 h.
This finding also holds true for Escherichia coli. Good reasons exist to
believe that the black light source, which emits more strongly in the UV
region, as presented in Fig. 2, has the advantage of triggering the
inactivation of the bacteria sooner.
Above test results elucidate how the bacteria respond to the
synergistic action of TiO2 film and particular light sources. The anatase
TiO2 thin film has the photocatalysis effect only when the light source
emits with an energy that exceeds the band gap energy of anatase
TiO2, which is the threshold energy for triggering the inactivation
mechanism.