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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Photoexcited TiO2 has been found to generate reactive oxygen species, yet the precise mechanism and chemical nature of the generated oxy species especially regarding the different crystal phases remain to be elucidated. Visible light-induced reactions of a suspension of titanium dioxide (TiO2) in water were investigated using electron paramagnetic resonance (EPR) coupled with the spin-trapping technique. Increased levels of both hydroxyl (˙OH) and superoxide anion (˙O2) radicals were detected in TiO2 rutile and anatase nanoparticles (50 nm). The intensity of signals assigned to the ˙OH and ˙O2 radicals was larger for the anatase phase than that originating from rutile. Moreover, illumination with visible (nonUV) light enhanced ˙O2 formation in the rutile phase. Singlet oxygen was not detected in water suspension of TiO2 neither in rutile nor in anatase nanoparticles, but irradiation of the rutile phase with visible light revealed a signal, which could be attributed to singlet oxygen formation. The blue part of visible spectrum (400–500 nm) was found to be responsible for the light-induced ROS in TiO2 nanoparticles. The characterization of the mechanism of visible light-induced oxy radicals formation by TiO2 nanoparticles could contribute to its use as a sterilization agent.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Titanium dioxide (TiO2), especially in its nanoscale form, has emerged as an excellent photocatalyst material (in the UVA range) applied for environmental purification, including bacteria eradication (1,2). For example, compounds that are present even on the order of 10 parts per million by volume (ppmv), can be decomposed by TiO2 when exposed to UV light (1).

The photocatalytic mechanisms of the photoirradiated TiO2 surface have been studied intensively and a variety of reaction mechanisms have been proposed so far. In these mechanistic studies (3–6), the formation of reactive oxygen species (ROS) in water suspensions of TiO2 (particle size of 0.3 mm–0.45 μm) was demonstrated under exposure to UV light (320–365 nm), (3–6). In most of these studies, the formation of oxy radicals (˙OH, ˙O2 and 1O2) was monitored by EPR spectroscopy (3–5,7,8).

In general, TiO2 photocatalytic reactions proceed mainly from the hydroxyl radicals (˙OH) by the oxidation of water and superoxide radicals (˙O2) produced by the reduction of oxygen in air.

Recently, Nosaka et al. (9–13) directly detected singlet oxygen photogenerated at the surface of TiO2 nanoparticles using the gated photon-counting method. This direct method is based on the emission properties of singlet oxygen (14,15). They found that the production mechanism of 1O2 is most probably carried out by an electron transfer process. A two-step mechanism is proposed for the 1O2 formation. The first step is a reduction of aerial O2 to ˙O2 using photogenerated conduction band electrons. The second being the oxidation of ˙O2 to 1O2 using photogenerated valence band holes or trapped holes (8). As expected the amount of 1O2 is increased with a decrease in particle size. Yet, no clear dependence of ROS formation on the crystal phase (anatase or rutile) has been described.

In the present study, using electron paramagnetic resonance (EPR) coupled with the spin-trapping technique, we demonstrate the formation of ROS in nonilluminated aqueous suspension of nanoparticles TiO2 rutile and anatase. Moreover, illumination with blue light resulted in enhanced ROS formation. The rutile and anatase phases generate different oxy species in aqueous suspension and also respond differently to visible-light irradiation. The characterization of the mechanism of light-induced oxy radical formation by the different crystal phases of TiO2 nanoparticles is reported for the first time and could contribute to their use as sterilization agents.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Nanoparticles and chemical reagents.  TiO2 (anatase and rutile, particle size of 50 nm) were obtained from M.K. Impex (Mississauga, Canada). Catalase, super oxide dismutase (SOD), nitroxide probe TEMP and spin-trapping reagent DMPO, were purchased from Sigma (Milwaukee, WI). Sodium azide, DMSO, ethanol (analytical grade) were purchased from Frutarom LTD (Haifa, Israel) and used without further purification.

Spin-trapping measurements coupled with EPR spectroscopy Detection of ˙OH (hydroxyl), ˙O2 (superoxide anion) and 1O2 (singlet oxygen) radicals in a water suspension of TiO2 nanoparticles.  To detect ˙OH, ˙O2 and 1O2, we used the EPR-spin trapping technique coupled with the spin traps 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 0.02 m), and 2,2,6,6-tetramethyl-piperidine (TEMP, 0.1 m) respectively. Aqueous suspensions (10 mg mL−1) of TiO2 (anatase or rutile) and the appropriate spin trap were drawn by a syringe into a gas-permeable Teflon capillary (Zeus Industries, Raritan, NJ) and inserted into a narrow quartz tube that was kept open at both ends. The tube was then placed in the EPR cavity and the spectra were recorded before and after illumination through the EPR cavity, on a Bruker EPR 100d X-band spectrometer (Billerica, MA, USA). The EPR measurement conditions were as follows (unless otherwise stated): frequency: 9.74 GHz; microwave power: 20 mW; scan width: 65 G; resolution: 1024; receiver gain: 2 × 105; conversion time: 82 ms; time constant: 655 ms; sweep time: 84 s; scans: 2; modulation frequency 100 KHz; modulation amplitude: 2 G.

The EPR measurements for each suspension sample were performed at least three times, and the averaged value is plotted in the figures.

In all experiments, the illumination parameters were the same for both anatase and rutile nanoparticles. After acquisition, the spectra were processed using the Bruker WIN-EPR software version 2.11 for baseline correction. The peak intensity was calculated by double integration of the peak signals, and the intensity was expressed in arbitrary units.

Simulation of the recorded spectra was performed using an algorithm provided in the WINSIM program, which is available from NIEHS (National Institutes of Health), available at (http://epr.niehs.nih.gov/pest_mans/winsim.html).

Hffs data for DMPO adducts were taken from Buettner. (16).

Purification and characterization of the spin traps. 5,5-dimethyl-1-pyrroline N-oxide. DMPO was purified in the dark in ddH2O, with activated charcoal. After about 30–60 min, the solution was filtered and its concentration was determined spectrophotometrically using ε227 nm = 8.0 mm−1 × cm−1. The solution was stored at −20°C for no longer than 2 weeks.

2,2,6,6-tetramethyl-4-piperidone. (TEMP) reacts with singlet oxygen (1O2). The reaction of 1O2 with TEMP leads to formation of the free radical 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (TEMPONE) with a characteristic EPR spectrum comprised of three lines of equal intensity.

Illumination Light sources.  The following light sources were used: (1) A homemade broadband tungsten-halogen lamp, 400–800 nm, 40 mW cm−2 with appropriate filters was used in this study. In some experiments, color dichroic filters (Edmund Optics) were used; (2) A 415 nm, 100 mW cm−2 LED (light emitting diode) was employed in several settings for TiO2 irradiation; (3) UV, 294 nm, 0.1 mW cm−2.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

Hydroxyl and superoxide radical generation in a water suspension of TiO2 nanoparticles

To characterize the nature of the ROS formed upon the irradiation of TiO2 (anatase and rutile) nanoparticles, hydroxyl and superoxide-anion radicals were first measured in nonilluminated samples using EPR-spin trapping coupled with the DMPO spin trap. Water suspension of TiO2 (50 nm, 10 mg mL−1, anatase or rutile) was introduced to the EPR cavity, as described in the experimental section, and the EPR spectrum was recorded. The results are presented in Fig. 1.

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Figure 1.  ROS formation in water suspensions of nonilluminated nano-TiO2 (50 nm, 10 mg mL−1), (a) rutile, (b) anatase.

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In Fig. 1, a characteristic DMPO–OH spin adduct with four resolved peaks (aN = aH = 14.9 Gauss), attributed to the formation of the hydroxyl radical, was observed for anatase and rutile DMPO/TiO2 suspensions, the intensity of the anatase signal being significantly higher. As DMPO can also trap O2˙ to produce the spin adduct DMPO–OOH, which is unstable and decomposes to DMPO–OH adduct, the formation of the quartet cannot be attributed solely to the formation of the hydroxyl radical. To determine whether the spectrum presented in Fig. 1 arises from the production of superoxide and/or ˙OH, we used DMSO to scavenge ˙OH. Fig. 2 illustrates that visible light induces the generation of superoxide anion radicals as well as ˙OH. When the TiO2 is not illuminated the amount of the superoxide anion radicals formed is very small.

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Figure 2.  ROS formation in water suspensions of TiO2 (50 nm, 10 mg mL−1) nanoparticles with DMSO (15%): (a) rutile-TiO2/DMPO suspension before illumination, (b) rutile-TiO2/DMPO after illumination (400–800 nm, 40 mW cm−2), (c) anatase-TiO2/DMPO before irradiation, (d) anatase-TiO2/DMPO following illumination.

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As shown in Fig. 2, the addition of DMSO enabled the distinction between the DMPO spin adducts: DMPO–OH, DMPO–CH3 and DMPO–OOH obtained following the trapping of OH˙, ˙CH3 and O2˙ using DMPO, respectively (the different spin adducts formed are marked in the figure). For identification of the different DMPO adducts formed in the suspension, simulation (Fig. 3) of the recorded spectra was performed using an algorithm provided in the WINSIM program (see Experimental section for details).

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Figure 3.  Simulation spectra for (a) DMPO–CH3, (b) DMPO–OOH and (c) DMPO–OH.

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From simulated spectra, we calculated that the recorded spectra of both anatase and rutile nanoparticles, before illumination, represents a combination of: DMPO–CH3 (30%), DMPO–OOH (20%), and DMPO–OH (50%) obtained following the trapping of ˙CH3, ˙O2˙ and OH˙ using DMPO, respectively.

The ˙CH3 is formed according to the equations below (Eqs. 1 and 2).

  • image(1)
  • image(2)

Following illumination (400–800 nm, 40 mW cm−2, 5 min), the DMPO–OOH signal intensity was markedly increased in the rutile crystal phase (2b compared with 2a), whereas in contrast, a very moderate change was observed for the anatase phase (2d compared with 2c). It also can be noticed that the peak marking OH formation (in anatase) decreased following the illumination. This decrease is due to trapping of the OH by DMSO to form methyl radical as described by Eqs. 1 and 2. To verify the superoxide anion generation by rutile TiO2, we measured its formation in ethanol suspension.

Detection of superoxide radical in ethanol suspension of TiO2 nanoparticles.

The nature of the radicals produced by the different TiO2 crystals was further examined by measuring their production in ethanol. Ethanol suspension of TiO2 (50 nm, 10 mg mL−1) was introduced in the EPR cavity and the spectra were obtained before and after illumination using broadband visible light (400–800, 40 mW cm−2, 5 min) (Fig. 4) or a LED at 415 nm (5 min) as described in the experimental section (Fig. 5).

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Figure 4.  ROS formation in ethanol suspensions of TiO2 nanoparticles: (a) rutile: (b) anatase (black: before illumination, grey: after illumination).

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Figure 5.  ROS formation from TiO2/DMPO/ethanol suspensions following irradiation with 415 nm (100 mW cm−2, 5 min: (a) rutile (b) anatase (black: before irradiation, grey: after irradiation).

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Figure 4 shows the EPR spectra recorded after spin trapping with DMPO of TiO2 suspended in ethanol. Figure 4b reveals that there is no increase in the peak intensity after illumination of the anatase ethanolic suspension with white light. On the other hand, when the rutile TiO2 phase was illuminated (4a) an increase in the peak intensity became apparent. Due to the fact that only the rutile crystal phase was reactive with visible light, detailed examination of the nature of the radicals observed was performed only on this crystal phase. The observed spectrum of rutile before irradiation (a-black), showed a broad quartet pattern, suggesting the generation of the superoxide adduct, having hfsc values of AN = 13.1 AH = 10.3 G as was simulated using an algorithm provided in the WINSIM program.

Simulation of rutile illuminated suspensions represents a combination of DMPO–OOH (aN = 13.10 G, aH = 10.30 G; relative concentration of 80%) obtained following the trapping of O2˙ by DMPO and DMPO–C2H4OH (aN = 15.8 G, aH = 22.8 G, relative concentration of 20%), which is, in turn, obtained following the trapping of the hydroxyethyl radical by DMPO (Eqs. 3 and 4).

  • image(3)
  • image(4)

The previous experiments were repeated using 415 nm LED for the irradiation of the nanoparticle suspensions. The results are shown in Fig. 5.

Figure 5 shows that radical production from the rutile phase of TiO2 nanoparticles was markedly higher than in the anatase form following illumination with 415 nm light.

As described above DMPO can trap both the superoxide and hydroxyl radicals forming the DMPO–OH adduct producing the quartet EPR signal. As the hydroxyl radical cannot be formed in ethanol suspensions the quartet is attributed to the superoxide anion. In the absence of TiO2, irradiation of the DMPO/ethanol solution did not show any signal.

Detection of singlet oxygen in water suspension of TiO2 nanoparticles.

Singlet oxygen (1O2) generation was examined using the spin trap TEMP. TEMP reacts with 1O2 to yield the adduct TEMPONE, which shows a triplet EPR signal. Water suspensions of TiO2 (50 nm, 10 mg mL−1, rutile or anatase) were introduced to the EPR cavity and illuminated using visible light, 40 mW cm−2 for 5 min as described in the experimental section. The recorded spectra are presented in Fig. 6.

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Figure 6.  Singlet oxygen production in rutile -TiO2 water suspension: (a) TiO2 before illumination, (b) TiO2 after illumination (400–800 nm, 40 mW cm−2, 5 min), (c) TiO2 after illumination in the presence of sodium azide (10%).

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Figure 6 illustrates EPR spectra obtained before (a) and after (b) irradiation of rutile-TiO2 suspension using TEMP as the spin trap. Following illumination (Fig. 6b), the EPR signal displayed a 1:1:1 triplet signal, obtained by reaction of TEMP with singlet oxygen to yield TEMPONE possessing a hyperfine splitting constant (hfsc, aN 15.04 G), which is identical to that of commercial TEMPONE in water. In the absence of TiO2 or without irradiation (Fig. 6a), EPR spectra did not exhibit the TEMPONE signal. The signal was also not observed when the irradiation was carried out in the presence of sodium azide (Fig. 6c) (a singlet oxygen quencher). No singlet oxygen formation was observed in the anatase phase of TiO2 nanoparticles (results not shown).

To clarify the mechanism by which singlet oxygen is generated in an irradiated rutile TiO2 suspension, we measured its production in the presence of super oxide dismutase (SOD) and catalase respectively. A suspension of rutile TiO2 was illuminated (400–800 mW, 40 mW cm−2, 5 min) in the presence of different concentration of SOD or catalase, as shown in Figs 7 and 8.

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Figure 7.  EPR spectra of illuminated TiO2 in the presence of TEMP and SOD: (a) TEMP (alone) following irradiation; (b) TiO2-rutile: no irradiation; (c) TiO2 after irradiation using visible light; (d–g) illumination of TiO2 in the presence of different concentration of SOD: (d) 10 units (U) mL−1, (e) 30 U mL−1, (f) 50 U mL−1, (g) 100 U mL−1.

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Figure 8.  TEMPONE peak intensity (% relative to TEMPONE signal in the absence of catalase) of illuminated TiO2 in the presence of different concentration of catalase: 20, 40, 60, 150 and 300 U mL−1.

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Figure 7 illustrates the concentration-dependent changes in the EPR spectra upon illumination of the phosphate buffered saline (PBS) suspension of TiO2 with TEMP in the presence of SOD (10–100 U mL−1, d–g). In the absence of TiO2 (a) or without irradiation (b), EPR spectra did not show the characteristic TEMPONE signal. Figure 7 shows that the gradual increase in the SOD concentration leads to the elimination of the TEMPONE EPR signals suggesting that superoxide anion is involved in the formation of the singlet oxygen.

In Fig. 8, a concentration-dependent change of the TEMPONE signal (monitoring singlet oxygen formation) in PBS suspension of TiO2 in the presence of catalase (20–300 U mL−1, d–h) is demonstrated. In the absence of TiO2 or without irradiation, no signal for TEMPONE (Fig. 7a, b) was obtained. Similar to the results obtained with SOD, addition of catalase reduced singlet oxygen production proportional to the added enzyme. This reveals the involvement of hydrogen peroxide in the production of the singlet oxygen.

Effect of UV light illumination on radical production of TiO2 nanoparticles.

Illumination was carried out using 294 nm UV light having an intenisty of 0.1 mW cm−2. From Fig. 9 it can be seen that 294 UV light illumination of both anatase and rutile TiO2 nanoparticles increased the ROS production by the nanoparticles the increase in the rutile signal being significantly higher than that of the anatase.

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Figure 9.  ROS formation in water suspensions of TiO2 (50 nm, 5 mg mL−1) nanoparticles following illumination with 294 nm UV light: (a) rutile-TiO2/DMPO after illumination (294 nm, 0.1 mW cm−2), (b) rutile-TiO2/DMPO suspension before illumination, (c) anatase-TiO2/DMPO following illumination, (d) anatase-TiO2/DMPO before irradiation.

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Wavelength dependence of light-induced ROS by TiO2 nanoparticles.

To examine wavelength dependence of visible light-induced ROS by TiO2 nanoparticles, the light source was coupled with dichroic color filters (Edmond optics), transmitting light at wavelengths (wl) of 400–525 nm (blue), ≥600 (red) and 500 ≤ wl ≤600 (green).

The results presented in Fig. 10 suggest that the blue part of the visible light is responsible for the increase of the singlet oxygen formed by TiO2 nanoparticles: the relative enhancement of the peak intensity following illumination with white light was similar to that caused by illumination with blue light although the power density of the blue light is only a third of the white light. Similarly, the formation of hydroxyl radicals in TiO2 suspensions of both anatase and rutile was mainly due to the blue part of the visible spectrum. The illumination of the rutile suspension with red light had a slight effect on the formation of hydroxyl radicals but not of singlet oxygen. Green light did not change the production of either hydroxyl radicals or singlet oxygen in TiO2 suspensions.

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Figure 10.  Spectra of rutile TiO2 nanoparticles (with a TEMP spin trap) following irradiation with white light (black curve), and following irradiation with blue light (grey curve).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

The present study describes the ability of an aqueous suspension of TiO2, rutile or anatase nanoparticles (50 nm) to produce ROS in the presence or absence of visible light illumination. The formation and amount of ROS was monitored using the EPR-spin trapping technique, employing the spin traps, DMPO and TEMP. Using the DMPO spin trap, we showed that both hydroxyl (˙OH) and superoxide anion (˙O2) radicals are present in water suspension of TiO2 rutile and anatase nanoparticles (50 nm). We further showed (Fig. 1) that the quartet of DMPO–OH that monitors ˙OH and ˙O2 radicals is higher in the anatase phase 1(b) as compared with 1(a). As can be observed from Fig. 1, both anatase and rutile crystal phases form ROS prior to irradiation, thus supporting the study of Gurr et al. who found anatase (10 and 20 nm) TiO2 particles, to induced oxidative damage in human bronchial epithelial cell line in the absence of photoactivation (17). Similar results were also observed by Christie M. Sayes et al. who also reported that anatase produces more ROS than rutile before irradiation (18). The production of ROS by TiO2 nanoparticles before irradiation may be explained by surface properties of nanomaterials. The negatively charged surface hydroxyl groups, which terminate the TiO2 lattice, cause upward bending of the electron bands. There is, therefore, a tendency for the electrons to be repelled and the positive holes to be attracted to the surface (19). Consequently, the electrons and holes tend to move apart, forming electron hole pair, which reacts with the surrounding water or oxygen molecules in a similar way as following photoexitation.

As DMPO can trap both the hydroxyl and superoxide anion radicals, DMSO was added to the sample. In the presence of DMSO, superoxide and hydroxyl radicals have resolved hffs values, making the separation between them possible. We found that in the absence of illumination hydroxyl and superoxide anion radicals were present in both rutile and anatase TiO2 nanoparticles. Illumination of the nanoparticle suspensions caused an elevation in the production of ROS (Fig. 2b,d). A much higher elevation was observed when the rutile crystal was illuminated (Fig. 2b). To verify superoxide anion generation using rutile TiO2, we measured its formation in ethanol suspension. Similar to the results obtained in the presence of DMSO, illumination of the rutile TiO2 phase with visible light (Fig. 4) and with a LED emitting at 415 nm (Fig. 5) resulted in an elevation of superoxide anion peak intensity.

In a previous study (20), we have found that ZnO nanoparticles also responded to visible light, but in contrast to TiO2 nanoparticles no superoxide anion formation was observed in water suspension of ZnO nanoparticles or was formed after illumination.

To study singlet oxygen formation, we used the TEMP spin trap. Figure 6 demonstrates that only the rutile phase produces singlet oxygen, and only upon illumination with light in the visible range. No singlet oxygen signal was detected in the anatase phase (data not shown).

Singlet oxygen can be generated by several pathways,

Path 1: Direct production by energy transfer from TiO2.

Path 2: Dismutation of a superoxide anion to form hydrogen peroxide and singlet oxygen (Eq. 5):

  • image(5)

Path 3: Fenton-type reaction (Eq. 6):

  • image(6)

Path 4: Oxidation of ˙O2 by the hole (h+) (Eq. 7):

  • image(7)

To identify the mechanism operative here, the involvement of superoxide anion or hydrogen peroxide in singlet oxygen production by illuminated TiO2 rutile was examined using superoxide dismutase (SOD) and catalase respectively. Catalase and SOD are two antioxidant enzymes. While the SOD belongs to a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide, the catalase enzyme is responsible for the decomposition of hydrogen peroxide to water and oxygen. As both hydrogen peroxide and superoxide anion may be involved in production of singlet oxygen by TiO2, the different effect of the antioxidative enzymes may suggest the most probable pathway for their production

As seen in Fig. 7, SOD which reacts with ·O2 totally abolished the triplet signal of TEMPONE. The same phenomenon was observed following the addition of catalase (Fig. 8), which scavenges hydrogen peroxide, suggesting that singlet oxygen is formed through a Fenton-type reaction (Eq. 6). To verify the involvement of superoxide anion in singlet oxygen production in illuminated TiO2 rutile nanoparticles, we also added DMPO to the suspension of TiO2 and TEMP. No triplet signal of TEMPONE was detected (data not shown), as DMPO reacts with ·O2 to yield DMPO–OOH, demonstrating that in the absence of ·O2 no singlet oxygen is generated. This is in contrast to Konaka et al. (5) who observed an increase in TEMPONE signal following UV irradiation of a suspension of 0.3 mm rutile in the presence of DMPO and TEMP.

As studies in the literature suggest that a variety of active species such as trapped holes (13), OH (22), O2 (23), can form the nitroxide radicals (TEMPONE) and not only 1O2, we added sodium-azide, which is a specific quencher of singlet oxygen (24) to prove singlet oxygen formation. In the presence of azide, no TEMPONE signal was observed following illumination of TiO2 suspensions (Fig. 6). Surprisingly, in Konaka’s study (4), no reduction of the triplet signal of TEMPONE in the presence of azide was observed.

Finally, to study wavelength dependence of ROS production in TiO2 nanoparticles, filtered visible light of limited wavelengths was used (blue, green and red). Singlet oxygen generation as well as OH and superoxide were enhanced to almost the same extent when white or blue filtered light from the same light source was used (Fig. 10), although the energy dose of the blue light was three times smaller.

These results showed that only the blue part of the visible spectrum is responsible for ROS generation in TiO2 nanoparticles. The ability of nano-TiO2 to generate ROS by blue light illumination is explained by its light absorption in the UVA with a residual absorption in the blue range. The difference between anatase and rutile in visible photocatalysis is probably due to a difference in their band-gap energies (Eg). Eg (anatase) = 3.2 eV = 387 nm, whereas Eg (rutile) = 3 eV = 415 nm.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References

From the results of the present study, it is clear that the photocatalytic behavior of TiO2 rutile versus that of anatase nanoparticles (50 nm) is different. While they both generate hydroxyl (˙OH) and superoxide anion (O2˙) radicals in water suspension, the amount of ˙OH and O2˙ radicals is higher in the anatase phase. Illumination with blue light mainly enhances O2˙ formation in the rutile phase. Singlet oxygen was not detected in water suspension of TiO2 rutile or anatase nanoparticles, but irradiation of the rutile phase with blue light revealed a signal, that could be attributed to singlet oxygen formation. Thus, the TiO2 anatase phase is expected to have a better sterilization effect than that of TiO2 rutile, as the ROS content in water suspension of TiO2 anatase is higher. However, when nano-TiO2 are used in combination with blue light illumination, the rutile phase is expected be more efficient.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. References
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