Photocatalytic Antibacterial Effects Are Maintained on Resin-Based TiO2 Nanocomposites after Cessation of UV Irradiation
Welch K (2013) Photocatalytic Antibacterial Effects Are Maintained on Resin-Based TiO2 Nanocomposites after Cessation of UV
Irradiation. PLoS ONE 8(10): e75929. doi:10.1371/journal.pone.0075929
Photocatalytic Antibacterial Effects Are Maintained on Resin-Based TiO2 Nanocomposites after Cessation of UV Irradiation
Yanling Cai 0
Maria Strmme 0
Ken Welch 0
Vipul Bansal, RMIT University, Australia
0 Division for Nanotechnology and Functional Materials, Department of Engineering Sciences, The A ngstro m Laboratory, Uppsala University , Uppsala , Sweden
Photocatalysis induced by TiO2 and UV light constitutes a decontamination and antibacterial strategy utilized in many applications including self-cleaning environmental surfaces, water and air treatment. The present work reveals that antibacterial effects induced by photocatalysis can be maintained even after the cessation of UV irradiation. We show that resin-based composites containing 20% TiO2 nanoparticles continue to provide a pronounced antibacterial effect against the pathogens Escherichia coli, Staphylococcus epidermidis, Streptococcus pyogenes, Streptococcus mutans and Enterococcus faecalis for up to two hours post UV. For biomaterials or implant coatings, where direct UV illumination is not feasible, a prolonged antibacterial effect after the cessation of the illumination would offer new unexplored treatment possibilities.
ihnycdlurodginegn NpOerHo; xidOe2N 2(H2aOnd2); HR2 Oea2c; tiavere ocxoidnasitdiveeredspeacsietshe(RmOaSin)
Titanium dioxide (TiO2) is a photocatalyst that has been
investigated for a wide variety of applications such as the
decontamination of water  and air  as well as for
selfcleaning surfaces . When irradiated with UV-A light
(l,385 nm for the anatase form of TiO2), crystalline TiO2 is
activated and electron (e2) hole (h+) pairs are formed. The
excited electron e2 can be trapped at surface TiIV sites of TiO2 to
form the reduced TiIII sites (Eq. 1), which in turn can react with
O2 to generate superoxide radicals (O2N2) (Eq. 2). Conversely, the
positive holes generate hydroxyl radicals (NOH) through the
reaction with water or hydroxyl ions on the hydrated metal oxide
(Eqs. 3 and 4). Further reactions can lead to the formation of
products of photocatalytic reaction .
TiO2 induced photocatalysis has been investigated for
applications in various antimicrobial materials . The antibacterial
mechanism is a result of the reaction between the ROS and
bacterial structural components, like the cell wall and cell
membrane [11,12]. Particularly, unsaturated phosphate lipids in
the cell membrane are the most sensitive targets for the ROS
attack and lipid peroxidation is considered to be the lethal mode of
action in the photocatalytic antibacterial process [13,14].
TiO2 nanoparticles are widely used as photocatalysts because of
their high photocatalytic activity, mainly due to their high specific
surface area . A number of polymer-TiO2 composites have
been developed to take advantage of their combined synergetic
properties. For example, TiO2 nanoparticles embedded in
polysulfone  and polyethersulfone  membranes improve
the water affinity, mechanical strength and anti-fouling ability of
the membranes. Bactericidal effect under UV irradiation has also
been shown in composite materials through the addition of TiO2
nanomaterials to polymers like polyurethane , polypropylene
[19,20], aromatic polyamide , cellulose  and polystyrene
. We have previously demonstrated that a resin-TiO2
nanocomposite prepared through the addition of TiO2
nanoparticles to a dental adhesive resin material endows the
nanocomposite with antibacterial and bioactive properties without affecting
the functional bonding strength of the adhesive .
The photocatalytic effect requires the application of UV light
which is both an advantage; since the effect can be induced on
demand, and a disadvantage; since UV illumination is not always
possible or practical. For certain biomaterials applications a
prolonged antibacterial effect after the cessation of UV irradiation
would offer benefits. In this work, we present a novel property of a
resin-TiO2 nanocomposite in which an antibacterial effect is
shown to be present for a significant time following UV
irradiation. This post-UV antibacterial property of the
resinTiO2 samples was tested on five bacterial strains including
Escherichia Coli, Staphylococcus epidermidis, Streptococcus pyogenes,
Streptococcus mutans and Enterococcus faecalis. Additionally, the tendency for
S. epidermidis adhesion on the UV-treated resin-TiO2 samples was
Materials and Methods
Escherichia coli (DH5a) and four pathogenic bacterial strains:
Staphylococcus epidermidis (CCUG18000A), Streptococcus pyogenes
(BM137), Streptococcus mutans (UA159), Enterococcus faecalis (JH2-2)
were activated in Brain Heart Infusion (BHI) broth and cultured at
37uC to late log phase. The bacteria were collected by
centrifugation (4000 rpm, 10 min, EBA 30 centrifuge, Hettich,
Tuttlingen, Germany) and then re-suspended in sterile PBS
(phosphate buffered saline, Sigma-Aldrich, Germany). Bacterial
density was adjusted to 109 CFU/mL for all strains using optical
The resin consists of two types of monomer, 2, 2-bis
[4-(2hydroxy-3- methacryloxypropoxy) phenyl-propane (BisGMA,
Polysciences Europe GmbH, Eppelheim, Germany) and
2hydroxyethyl methacrylate (HEMA, Sigma-Aldrich, Schnelldorf,
Germany) in a 55/45 wt/wt ratio. Photoinitiator and coinitiators
were added as follows: 0.5 mol% camphorquinone (CQ);
0.5 mol% 2-(dimethylamino) ethyl methacrylate (DMAEMA);
0.5 mol% ethyl-4-(dimethylamino) benzoate (EDMAB); and
1 wt% diphenyliodoniumhexafluorophosphate (DPIHP) (all from
Sigma-Aldrich, Steinheim, Germany).
The resin-TiO2 nanocomposite was made by mixing 20 wt% of
photocatalytic TiO2 nanoparticles (P25, Evonik Industries
(previously Degussa) AG, Germany) in the resin. P25 TiO2
nanoparticles are comprised of anatase and rutile crystalline phases in a
ratio of about 3:1 . It has previously been shown  that the
composition of 20 wt% P25 TiO2 nanoparticles in the resin matrix
provides optimal antibacterial effect without negatively affecting its
functional performance as a dental adhesive. The container with
the resin-TiO2 mixture was sonicated for 1 hour to minimize
nanoparticle aggregation. The resin-TiO2 mixture was then cast in
Teflon molds (diameter 8 mm, thickness 1 mm) and cured under
460 nm light (BlueLEX GT1200, Monitex, Taiwan) for 30 s
under N2 flow. Scanning Electron Microscopy (SEM, LEO 1550,
Zeiss) images of resin disks with and without TiO2 nanoparticles
were recorded after sputter coating with gold/palladium (Polaron
SC7640, Thermo VG Scientific). Additionally, the resin-TiO2
nanocomposite disks were characterized through X-ray diffraction
(XRD, D-5000 X-ray diffractometer, Siemens).
UV pre-treatment of resin-TiO2 disks
Three groups of resin-TiO2 sample disks were prepared:
resinTiO2 disks without UV irradiation treatment (resin-TiO2 control
disks); resin-TiO2 disks pre-treated with UV irradiation under
ambient conditions (post-UV dry disks) and resin-TiO2 disks
pretreated with UV irradiation under aqueous conditions (post-UV
To treat the resin-TiO2 disks with UV irradiation in ambient
conditions, disks were placed in a petri dish covered with its
transparent lid and irradiated with a UV-A diode (peak
wavelength at 365 nm, NSCU033B (T), Nichia, Japan) for 1 hour
at an intensity of 10 mW/cm2 (UV light meter, UV-340, Lutron).
To treat the resin-TiO2 disks with UV irradiation in aqueous
environment, 50 mL of deionized water was spread on the surface
of each disk prior to UV irradiation. The disks were subsequently
placed in a petri dish with its transparent lid and irradiated with
the UV-A diode for 1 hour (10 mW/cm2). After the UV
treatment, the water drops were collected from the surface of
the disks by pipetting and transferred to individual wells in a
Post-UV antibacterial tests
Immediately after the UV treatment, the antibacterial
properties of the resin-TiO2 control disks, post-UV dry disks, post-UV
wet disks and water droplets extracted from the surfaces of the
post-UV wet disks were investigated. All antibacterial tests were
performed in triplicate for each bacterial strain.
For antibacterial testing of the disks, 10 mL of bacterial
suspension were spread on each resin-TiO2 disk. Disks with
bacteria were placed in a petri dish, covered with its lid and
incubated for 15 minutes at room temperature. Individual disks
were subsequently transferred into wells in a 48-well plate with
300 mL sterile PBS in each well and orbital shaking at 500 rpm for
2 min was applied to re-suspend the bacteria from the resin-TiO2
disks. The disks were then removed from the wells.
For antibacterial testing of the water drops from the post-UV
wet disks, 10 mL of bacterial suspension were mixed into the water
drop by pipetting and incubated for 15 minutes at room
temperature. Following the incubation, 300 mL sterile PBS were
added to the wells containing water drops with bacteria and orbital
shaking at 500 rpm for 2 min was applied to re-suspend the
bacteria from the bottom of the wells.
Bacterial viability of the samples was quantified using a
metabolic assay incorporating the indicator resazurin. One
milliliter of BHI broth with resazurin (1.25 mg/mL) was added
to each well containing a sample of bacterial suspension.
Concurrently, a dilution series of bacteria suspension of the
corresponding bacterial strain with known bacteria concentration
was measured to produce a standard curve for quantification of
the test samples. The assays were incubated at 37uC for 3 hours
and the production of resorufin, indicating the bacterial metabolic
activity, was measured with a fluorescent multiplate reader
(excitation at 530 nm, emission at 590 nm, Tecan F200). The
number of viable bacteria in each test was determined with aid of
the standard curve. Bacteria from the resin-TiO2 control disks
were utilized to define 100% bacterial viability.
UV pre-treatment of pure resin disks and TiO2
In order to determine if the post-UV antibacterial effect was
only observed with the combination of TiO2 nanoparticles and
resin, antibacterial tests were also performed using TiO2
nanoparticles and disks comprised of the resin polymer without
TiO2 nanoparticles. Both the UV pre-treatment (1 hour, 10 mW/
cm2) in ambient and aqueous environments and the post-UV
antibacterial testing were performed as detailed above. Only S.
epidermidis was used in these control tests.
Bacterial adhesion tests
The post-UV antibacterial effect was also tested on the tendency
for S. epidermidis adhesion on the disk surfaces as well as the
viability of the adhered bacteria. Gram-positive S. epidermidis
(CCUG 18000A) is a skin flora species and a common cause for
infections associated with implant or biomedical devices .
Adhesion testing was performed on both post-UV dry disks and
post-UV wet disks following a delay period of 0, 15, 30, 60 or
120 minutes after the UV pre-treatment described above. Four
disks were used for each delay period and disk type. Additionally,
two control groups consisting of four resin-TiO2 disks without UV
pre-treatment were included in the adhesion testing. Control
Group 1 were tested in the same manner as the post-UV dry and
post-UV wet disks, while control Group 2 received a UV dose of
36 J/cm2 during the adhesion testing (10 mW/cm2 for 1 hour).
Adhesion testing was performed through the incubation of the
disks in a 10 mL bacterial suspension in a well of a 6-well plate
under shear forces generated with the help of an orbital shaker.
The bacterial suspension for adhesion testing was prepared by
suspending S. epidermidis in sterile PBS with a concentration of
Figure 3. Post-UV antibacterial effects against five bacterial strains. Standard deviations are derived from three measurements. 100%
viability corresponds to the bacterial viability of each strain measured on the resin-TiO2 disks without UV pre-treatment.
108 CFU/mL. The resin-TiO2 disks were secured on the bottom
of the well with the aid of a rubber mold such that all disks were
located at the same distance from the center of the well and
therefore experienced the same shear forces during the culturing.
The 6-well plate containing the disks was fixed on an orbital
shaking incubator (Talboys, Troemner, USA) set to 37uC. The
culturing process consisted of 30 min static incubation to allow for
initial adhesion followed by 30 min incubation with 100 rpm
orbital shaking. After the adhesion culturing, the resin-TiO2 disks
were removed from the bacterial suspension and rinsed gently with
sterile PBS to wash away non-adherent bacteria.
One disk from each of the control disks, the post-UV dry disks
and the post-UV wet disks were observed with SEM to check for
bacterial adhesion on the surface. Before SEM observation, disks
with adhered bacteria were dried in ambient environment and
sputter coated with gold/palladium. SEM images were recorded
with a LEO 1550 SEM (Zeiss, Oberkochen, Germany) using the
in-lens detector and 10 kV acceleration voltage.
Three disks from each of the control disks, the post-UV dry
disks and the post-UV wet disks were assessed for viability after the
adhesion culturing process. The amount of viable bacteria on each
disk was quantified using the metabolic assay incorporating
resazurin. Each disk was placed upside-down in a well of a
48well plate with 500 mL sterile PBS. The plate was placed in an
ultrasonic bath for 1 minute to detach the bacteria from the
surface. Afterwards, 100 mL of this bacterial suspension was
transferred to a well with 900 mL BHI broth with resazurin
(1.25 mg/mL) in a 48-well plate. A dilution series of S. epidermidis
with known bacterial concentrations was performed in parallel to
provide a standard curve. Both the test sample and calibration
assays were incubated at 37uC for 4 hours and the production of
resorufin was measured with fluorescent multiplate reader
(excitation at 530 nm, emission at 590 nm). The number of
bacteria in each test sample well was determined with aid of the
standard curve from the S. epidermidis calibration series.
Figure 1 shows SEM images of the surfaces of a pure resin disk
and a resin-TiO2 nanocomposite disk. The TiO2 nanoparticles
encased in the resin matrix can be observed in the image of the
resin-TiO2 nanocomposite. An XRD pattern of the resin-TiO2
disk (Figure 2) shows distinct diffraction peaks of anatase and rutile
phases of TiO2, which are attributed to the P25 nanoparticles
encased in the resin.
Post-UV antibacterial tests
Figure 3 shows the post-UV antibacterial effect of the
resinTiO2 nanocomposite disks after UV irradiation in ambient
condition (post-UV dry disks) and aqueous condition (post-UV
wet disks) against five bacterial strains. The prolonged bactericidal
effects of the water drops collected from the post-UV wet disks are
The post-UV dry disks exhibited a more efficient antibacterial
effect than the post-UV wet disks. In the tests with E. coli, S.
epidermidis, S. pyogenes, S. mutans and E. faecalis, the post-UV dry
disks achieved a 33%, 57%, 50%, 50% and 27% reduction of
bacterial viability, respectively, compared to the bacterial viability
measured on the resin-TiO2 control disks for each strain. The
corresponding reduction of bacterial viability caused by the
postUV wet disks was 27%, 52%, 27%, 25% and 16%. The water
drops extracted from the post-UV wet disks provided less
bactericidal effect than the post-UV disks. The water drops
reduced the viability of E. coli, S. epidermidis and E. faecalis by 17%,
17% and 11% respectively, but did not reduce the viability of S.
pyogenes and S. mutans.
These results show that susceptibilities of the five bacterial
strains to the post-UV antibacterial effect vary. Among the five
species, S. epidermidis is the most sensitive to both dry and wet
postUV disks. S. pyogenes and S. mutans are relatively more sensitive to
post-UV dry disks than to post-UV wet disks. Finally, E. coli and E.
faecalis are the least affected by the post-UV disks compared to
Neither the pure resin disks nor the TiO2 nanoparticles
exhibited a significant post-UV antibacterial effect as shown in
Figure 4. This indicates that the post-UV antibacterial effect
requires a combination of TiO2 nanoparticles and resin.
Bacterial adhesion tests
Figure 5 displays SEM images of the surfaces of four types of
disks after the adhesion culturing: a resin-TiO2 disk without UV
pre-treatment (Control Group 1, panel a), post-UV dry disk (panel
b), post-UV wet disk (panel c) and a resin-TiO2 disk without UV
pre-treatment but irradiated with UV during adhesion culturing
(Control Group 2, panel d). In all disks a similar pattern of S.
epidermidis adhesion was observed. Under a shear force during
culturing, bacteria are prone to attach to each other while
adhering to the resin-TiO2 surface rather than dispersing
separately on the surface. Qualitatively, no significant differences
could be observed in the adhesion tendencies of S. epidermidis on
the four types of disks.
The viability of adhered S. epidermidis bacteria on the four groups
of resin-TiO2 disks is shown in Figure 6. The viability of the
bacteria adhered to the resin-TiO2 disks was derived from the
metabolic activity assays based on measurements of resazurin
conversion. The viability of Control Group 2 shows that the
photocatalytic antibacterial effect resulting from the UV
irradiation during the culturing produced the greatest reduction in
viability in adhered bacteria (61%) compared to Control Group 1
Figure 5. SEM images of resin-TiO2 disks after S. epidermidis adhesion culturing. a) disk from Control Group 1 without UV pre-treatment; b)
post-UV dry disk; c) post-UV wet disk; d) disk from Control Group 2 without UV pre-treatment but irradiated with UV during adhesion culturing.
in which no antibacterial effect can be assumed. Note that the total
UV dose irradiated on Control Group 2 disks (10 mW/cm2 for
1 h) is equivalent to the UV dose used in the UV pre-treatment of
the post-UV dry and post-UV wet disks. The post-UV dry disks
and post-UV wet disks showed a 37% and 26% reduction,
respectively, of viable adhered bacteria compared to Control
From Fig. 6 we can also observe a diminishing post-UV
antibacterial effect over time. A similar degree of bactericidal effect
was observed for both the post-UV dry and post-UV wet disks that
were cultured within 30 min following the UV pre-treatment.
However, this effect diminished if the delay period following the
UV pre-treatment was increased. For example, the viability of
adhered bacteria on the post-UV wet disks that were cultured
120 min after the UV pre-treatment showed the same viability as
disks that did not receive a UV pre-treatment (i.e., Control Group
A visual indication of a diminishing post-UV effect can be
observed in Figure 7 where the appearance of resin-TiO2 disks
both before and following a UV treatment of 1 h at 10 mW/cm2
is displayed. During the UV irradiation, the color of the disks
changed from white to blue. When the UV light was removed, the
blue color of the disks faded away gradually and eventually
changed back to white.
Whereas existing literature indicates only an immediate
antibacterial effect associated with the photocatalytic process
[4,13], the results presented in this study clearly reveal a prolonged
antibacterial effect after the cessation of UV irradiation of the
resin-based TiO2 nanocomposites under study. With all bacterial
strains tested, the antibacterial effect of the post-UV dry disks was
larger than that of the post-UV wet disks. One possible
Figure 6. Viability of adhered bacteria on resin-TiO2 disks. Post-UV dry and post-UV wet disks were cultured following a delay period of 0, 15,
30, 60 or 120 minutes after the UV pre-treatment. Control Group 1 are resin-TiO2 disks without UV pre-treatment and Control Group 2 are resin-TiO2
disks without UV pre-treatment, but illuminated during the adhesion culturing. Standard deviations are derived from three measurements.
explanation for this is that the UV pre-treatment may have
resulted in different surface modifications on the two types of disks
and, thus, may have influenced how the bacterial suspensions
placed on the disks interacted with the surface. For example, a
more hydrophilic surface may have allowed the bacteria to come
closer to the surface and, hence, resulted in a greater antibacterial
effect than on a more hydrophobic surface. Another explanation
could be that the post-UV wet disks simply had a diminished
antibacterial capacity due to the release of ROS into the water
drop during the UV pre-treatment. The antibacterial effect
provided by the water drop, as shown in Fig. 3, can be attributed
to the presence of ROS generated during the UV pre-treatment.
Hydrogen peroxide is the most likely ROS due to the relatively
short lifetimes of the NOH and O2N radicals.
From Fig. 3 we can also observe that the degree of inactivation
varied depending on the bacterial strain. The ability of bacteria to
withstand attack from ROS depends on their innate
characteristics, such as, cell wall and cell membrane structure and thickness as
well as ROS-scavenging systems [4,27]. Bacteria, like many other
organisms, have ROS-scavenging systems to protect themselves
from oxidative stress, like intracellular ROS produced during
aerobic metabolism and extracellular ROS due to host defense
response or environmental stresses [28,29]. For example, E. coli
and Staphylococcus are catalase- and SOD-positive, which means
they have the capability of scavenging H2O2 and superoxide
radicals (O2N), respectively . Streptococcus and Enterococcus are
catalase-negative and SOD-positive [31,32]. Therefore, all strains
utilized in this work have a certain level of resistance to ROS.
However, in photocatalytic disinfection, large amounts of ROS are
produced that attack the bacteria from outside of the cell wall.
Such an excess of ROS would also likely overwhelm the
ROSscavenging systems. In this work, E. coli (the only Gram-negative
strain in the tests) and E. faecalis exhibited the greatest resistance to
the post-UV antibacterial effect of the disks while S. epidermidis was
the most susceptible. Both of the Streptococcus bacteria, S. pyogenes
and S. mutans showed very similar behavior in response to both
types of post-UV disks and the water drop extracted from the
postUV wet disk. S. mutans are known to be able to survive in
Figure 7. Appearance of the resin-TiO2 disks before and after one hour of UV irradiation under ambient conditions at 10 mW/cm2.
environments with relatively high concentrations of H2O2 since
they co-exist with Streptococcus sanguinis, a H2O2 producing bacteria
in dental biofilm . This helps explain the absence of
antibacterial effect of the water drop on S. mutans.
The bacterial adhesion testing showed that neither direct
photocatalysis on the Control Group 2 disks nor the post-UV
effect appeared to affect the adhesion tendencies of S. epidermidis.
This can be seen in Fig. 5 where all disk types show approximately
the same adhesion patterns and amounts of adherent bacteria. We
cannot expect that the antibacterial effects will remove the bacteria
as it is known that decomposition of bacteria due to photocatalysis
takes significantly longer time than inactivation of the bacteria. For
example, previous studies have shown that it can take as much as a
week of UV irradiation to completely remove E. coli from a TiO2
surface . On the other hand, the viability of the adhered
bacteria was significantly reduced on both the Control Group 2
disks and the post-UV disks. Again we observe a slightly greater
antibacterial effect from the post-UV dry disks compared to the
post-UV wet disks. The fact that direct UV irradiation of the
Control Group 2 disks caused an even higher degree of
inactivation is perhaps not surprising as the bacteria would be
affected by ROS generated both during and post UV irradiation
(note that the UV dose with the Control Group 2 disks was
equivalent to the UV pre-treatment dose of the post-UV disks).
Additionally, bacterial viability may have been further reduced by
the antibacterial effect of UV alone in the Control Group 2 tests.
An interesting observation in the viability measurements of the
adherent bacteria is the diminishing of the post-UV effect when
the S. epidermidis was cultured more than 30 min after the end of
the UV pre-treatment. Another indication of this diminishing
effect with time can be seen in Fig. 7 in which we can see a change
in color of the disks with time after a UV pre-treatment. In fact,
most of the blue color in the disks disappears after 30 min. This
blue color can be attributed to the presence of TiIII sites caused by
photo-produced electrons that are trapped at the surface of the
TiO2 nanoparticles [35,36]. It is likely that the resin material
encasing the nanoparticles slows the diffusion of oxygen to the
surface of the nanoparticles that then react with the TiIII sites to
create the radical. This could explain the prolonged
antibacterial effect of the post-UV disk.
The post-UV effect of the resin-TiO2 nanocomposite has
foreseeable benefits for biomaterial and biomedical device
applications. For example, if the nanocomposite is used as a
dental material, not only can UV irradiation be used to inactivate
infectious bacteria, the post-UV effect can help reduce
contamination of the surface after the treatment. Similarly, if such a
material is used as a coating for implants, the post-UV effect can
help prevent contamination of the surface after sterilization of the
surface with UV/photocatalysis.
In this work, we presented a novel property of a resin-TiO2
nanocomposite in which an antibacterial effect is shown to be
present for a significant time following UV irradiation. This
postUV effect of the resin-TiO2 disks was shown to be effective against
five bacterial strains, including E. Coli, S. epidermidis, S. pyogenes, S.
mutans and E. faecalis. The post-UV antibacterial effect was also
tested on the tendency for S. epidermidis adhesion on the disk
surface. Although the ability of the bacteria to adhere to the
surface was not affected by the post-UV disks, the viability of the
adhered bacteria was reduced by 37% compared to disks that did
not receive the UV pre-treatment. Additionally, the post-UV
antibacterial effect remained for at least 30 minutes after the UV
treatment. The prolonged antibacterial effect could help reduce
the chance of contamination by pathogenic microbes after the
cessation of UV irradiation and is therefore promising for
applications in biomaterials or implant coatings.
Conceived and designed the experiments: YC MS KW. Performed the
experiments: YC. Analyzed the data: YC MS KW. Contributed reagents/
materials/analysis tools: YC MS KW. Wrote the paper: YC MS KW.
12. Hirakawa K , Mori M , Yoshida M , Oikawa S , Kawanishi S ( 2004 ) Photoirradiated titanium dioxide catalyzes site specific DNA damage via generation of hydrogen peroxide . Free Radical Res 38 : 439 - 447 .
13. Maness PC , Smolinski S , Blake DM , Huang Z , Wolfrum EJ , et al. ( 1999 ) Bactericidal activity of photocatalytic TiO2 reaction: Toward an understanding of its killing mechanism . Appl Environ Microbiol 65 : 4094 - 4098 .
14. Kiwi J , Nadtochenko V ( 2004 ) New evidence for TiO2 photocatalysis during bilayer lipid peroxidation . J Phys Chem B 108 : 17675 - 17684 .
15. Beydoun D , Amal R , Low G , Mcevoy S ( 1999 ) Role of nanoparticles in photocatalysis . J Nanopart Res 1 : 439 - 458 .
16. Yang YN , Zhang HX , Wang P , Zheng QZ , Li J ( 2007 ) The influence of nanosized TiO2 fillers on the morphologies and properties of PSFUF membrane . J Membr Sci 288 : 231 - 238 .
17. Vatanpour V , Madaeni SS , Khataee AR , Salehi E , Zinadini S , et al. ( 2012 ) TiO2 embedded mixed matrix PES nanocomposite membranes: Influence of different sizes and types of nanoparticles on antifouling and performance . Desalination 292 : 19 - 29 .
18. Charpentier PA , Burgess K , Wang L , Chowdhury RR , Lotus AF , et al. ( 2012 ) Nano-TiO2/polyurethane composites for antibacterial and self-cleaning coatings . Nanotechnology 23.
19. Bahloul W , Melis F , Bounor-Legare V , Cassagnau P ( 2012 ) Structural characterisation and antibacterial activity of PP/TiO2 nanocomposites prepared by an in situ sol-gel method . Mater Chem Phys 134 : 399 - 406 .
20. Chiu CW , Lin CA , Hong PD ( 2011 ) Melt-spinning and thermal stability behavior of TiO2 nanoparticle/polypropylene nanocomposite fibers . J Polym Res 18 : 367 - 372 .
21. Kwak SY , Kim SH , Kim SS ( 2001 ) Hybrid Organic/inorganic reverse osmosis (RO) membrane for bactericidal anti-fouling. 1. preparation and characterization of TiO2 nanoparticle self-assembled aromatic polyamide thin-filmcomposite (TFC) membrane . Environ Sci Technol 35 : 2388 - 2394 .
22. Zhou JP , Liu SL , Qi JN , Zhang LN ( 2006 ) Structure and properties of composite films prepared from cellulose and nanocrystalline titanium dioxide particles . J Appl Polym Sci 101 : 3600 - 3608 .
23. Wang ZB , Li GC , Peng HR , Zhang ZK , Wang X ( 2005 ) Study on novel antibacterial high-impact polystyrene/TiO2 nanocomposites . J Mater Sci 40 : 6433 - 6438 .
24. Welch K , Cai YL , Engqvist H , Strmme M ( 2010 ) Dental adhesives with bioactive and on-demand bactericidal properties . Dent Mater 26 : 491 - 499 .
25. Jensen H , Pedersen JH , Jorgensen JE , Pedersen JS , Joensen KD , et al. ( 2006 ) Determination of size distributions in nanosized powders by TEM , XRD, and SAXS. J Exp Nanosci 1 : 355 - 373 .
26. Patel JD , Colton E , Ebert M , Anderson JM ( 2012 ) Gene expression during S. epidermidis biofilm formation on biomaterials . J Biomed Mater Res , Part A 100A: 2863 - 2869 .
27. Foster HA , Ditta IB , Varghese S , Steele A ( 2011 ) Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity . Appl Microbiol Biotechnol 90 : 1847 - 1868 .
28. Ziegelhoffer EC , Donohue TJ ( 2009 ) Bacterial responses to photo-oxidative stress . Nat Rev Microbiol 7 : 856 - 863 .
29. Imlay JA ( 2013 ) The molecular mechanisms and physiological consequences of oxidative stress: lessons from a model bacterium . Nat Rev Microbiol 11 : 443 - 454 .
30. Podbielska A , Galkowska H , Olszewski WL ( 2011 ) Staphylococcal and enterococcal virulence - a review . Central Eur J Immunol 36 : 56 - 64 .
31. Brioukhanov AL , Netrusov AI ( 2004 ) Catalase and superoxide dismutase: Distribution, properties, and physiological role in cells of strict anaerobes . Biochemistry-Moscow 69 : 949 - 962 .
32. Lynch M , Kuramitsu H ( 2000 ) Expression and role of superoxide dismutases (SOD) in pathogenic bacteria . Microbes Infect 2 : 1245 - 1255 .
33. Kreth J , Merritt J , Shi WY , Qi FX ( 2005 ) Competition and coexistence between Streptococcus mutans and Streptococcus sanguinis in the dental biofilm . J Bacteriol 187 : 7193 - 7203 .
34. Hashimoto K , Irie H , Fujishima A ( 2005 ) TiO2 photocatalysis: A historical overview and future prospects . Jpn J Appl Phys, Part 1 44 : 8269 - 8285 .
35. Fujishima A , Zhang X , Tryk DA ( 2008 ) TiO2 photocatalysis and related surface phenomena . Surf Sci Rep 63 : 515 - 582 .
36. Chae YK , Mori S , Suzuki M ( 2009 ) Visible-light photocatalytic activity of anatase TiO2 treated with argon plasma . Thin Solid Films 517 : 4260 - 4263 .