Deactivation of Singlet Oxygen by Titanium Dioxide in Aqueous

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Deactivation of Singlet Oxygen by Titanium Dioxide in Aqueous Solution Studied by Phosphorescence Quenching with Porphyrin Photosensitizers Hironobu Saito, and Yoshio Nosaka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508573s • Publication Date (Web): 29 Sep 2014 Downloaded from http://pubs.acs.org on October 2, 2014

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The Journal of Physical Chemistry

Deactivation of Singlet Oxygen by Titanium Dioxide in Aqueous Solution Studied by Phosphorescence Quenching with Porphyrin Photosensitizers

Hironobu Saito and Yoshio Nosaka*

Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, 940-2188 Japan

Corresponding author; *[email protected]； Tel/Fax: +81-258-47-9315

1

ACS Paragon Plus Environment


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Abstract Deactivation of singlet oxygen (1O2) by TiO2 nanoparticles was investigated for the first time. 1O2 was produced by pulse laser induced photosensitization with either cationic or anionic porphyrin dye. Generation and decay of 1O2 were measured by its phosphorescence at 1270 nm in the presence of seven commercially available TiO2 powders of various sizes and crystallites. To clarify the quenching mechanisms, the deactivation of triplet excited state of porphyrin dyes (3D*) with TiO2 powders was also measured by the transient T-T absorption. Stern-Volmer plot suggests that the quenching should take place dynamically by second order reaction. The obtained rate constants for the deactivation of 1O2 and 3D* with TiO2 were increased with the size of the particles, which is attributable to the increase of the local concentration of 1O2 and 3D* around the TiO2 particles. The intrinsic rate constant for the deactivation of 1O2 by TiO2 were elucidated to be 1.0x108 M-1s-1.

Key words; T-T absorption, Quenching rate constant, Triplet state, TPPS, TMPyP

2


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Introduction TiO2 has been used as pigments and engineering materials owing to its stable, unpoisonous, and low-cost characteristics. Recently, TiO2 is gathering much attention by its photocatalytic property, which could be utilized for environmental cleaning, defogging, dustless coating, and water splitting.1-4 Although many researchers are investigating the properties of TiO2, the photophysical and photochemical properties have not been fully clarified yet. One of the properties in the current arguments is the photogeneration of 1O2 on light irradiation over TiO2,5-13 modified TiO2,

14,15

doped

TiO2,16,17 and metal deposited TiO2.18,19 1

O2 is the primary agent of photooxidative stress in microorganisms,20 and it

mediates protein oxidation21 and genetic signaling of mutation.22 Furthermore, it inactivates cells such as Saccharomyces cerevisiae.23 Biological effects of TiO2 photocatalysts on bacteria such as Escherichia coli, keratinocytes,

25

17,26-28

and other biological systems

12,13,16,24

DNA damage in human

have been considered to be exerted

by the produced 1O2 over TiO2 . As for the mechanism of the 1O2 generation in photocalytic systems, Munuera et al. insisted that only in the presence of chloride ions 1O2 could be generated.6 Gao and co-workers29 reported the 1O2 generation on bare-TiO2 under 532-nm light irradiation, and suggested two-photon excitation across the band-gap of TiO2. Macyk and co-workers15 suggested an energy transfer mechanism for a modified-TiO2 system under visible-light irradiation. On the other hand, we suggested an electron transfer mechanism, in which reduced O2 is successively oxidized to form 1O2.19,30,31

The

conventional ESR detection method of 1O2 utilizes the formation of stable nitroxide radicals with sterically hindered cyclic amines. However, this method could not be used in photocatalysis due to the direct oxidation of the amine probe.32 Formation of 1O2 by the sensitization with porphyrins attached on TiO2 powder has been reported.33-35 Usually, 1O2 is generated by photosensitization using many kinds of dyes (D) .36

The excited singlet state of dye 1D* (eq 1) converts to the triplet state

(3D*) by the intersystem crossing (eq 2), or by the reaction with the ground state of O2 to form 1O2 (eq 3). In addition, the produced 3D* also reacts with the ground state of O2 to form 1O2 (eq 4).36

3



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1

D + hν → 1D*

(1)

1

D* → 3D*

(2)

1

D* + 3O2 → 3D* + 1O2

3

1

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3

1

(3)

1

D* + O2 → D + O2

(1/τr)

(4)

O2 emits the phosphorescence at the wavelength of 1270 nm and the

measurement of the 1270-nm emission is the most reliable method to detect 1O2.36 Since the phosphorescence intensity is proportional to the concentration of 1O2, we can obtain the rate constant kq for the deactivation (quenching) of 1O2 by TiO2 (eq 5) by measuring the time-profiles of the intensity in the presence of various amount of TiO2. 1

O2 + TiO2 → 3O2 + TiO2

kq

(5)

Because 1O2 is formed from 3D* (eq 4), to elucidate the quenching mechanisms of 1O2 in the presence of TiO2 powders, we should also take account of the deactivation of 3D* by TiO2 (eq 6). The amount of 3D* can be measured by the transient triplet-triplet (T-T) absorption of 3D* in the absence of 3O2. 3

D* + TiO2 → 1D + TiO2

k q’

(6)

Thus 1O2 can be generated by the sensitization with porphyrins attached on TiO2 powder.

34-36

However, the deactivation process of 1O2 in the TiO2 photocatalytic

systems has not been well understood. We found recently that the generated 1O2 could deactivate to the ground state with the interaction of TiO2 surface.30 To our best knowledge, there has been no report so far on the deactivation of 1O2 by TiO2. In the present report, to assure the deactivation effect in the TiO2 photocatalytic systems we tried to measure intrinsic rate constants for the deactivation of 1O2 by TiO2.

Experimental Section Materials

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Seven kinds of commercially available TiO2 powders with different particle size and crystalline structure were employed, and shown in Table 1 together with the properties. To convert the weight concentration of TiO2 powders to molar concentration for the analysis of experimental results, the number of particles per gram was calculated from the primary particle size with the density of 4.25 g cm-3 and 3.89 g cm-3 for rutile and anatase crystallite, respectively, and listed in Table 1. Table 1. Properties of TiO2 particles used40

TiO2

primary

Anatase

number

particle

fraction

particles

size / nm

of manufacturer

/1016 g-1

/%

ST-01

7

100

17.9

Ishihara Sangyo

UV100

10

100

6.13

Sachtleben Chemie

MT-150A

15

0

1.81

TAYCA

F-6

18

90

1.04

Showa Titanium

MT-600B

27

0

0.276

TAYCA

P25

32

80

0.184

Nippon Aerosil

ST-41

50

100

0.049

Ishihara Sangyo

Two types of porphyrin dye, TPPS (meso-tetra(4-sulfonatophenyl)porphine, purchased from Frontier Scientific) and TMPyP

(5,10,15,20-tetrakis(1-methyl-4

-pyridyl)-21H,23H-porphine, from Wako Chemicals) were used as a sensitizer for 1O2 generation. The quantum yield of 3D* and its intrinsic lifetime τ’o, are reported to be 0.78 and 420 µs for TPPS, and 0.92 and 156 µs for TMPyP, respectively.37 Since the counter ion of TMPyP, p-toluene sulfonate, has an anionic charge, some amount might be adsorbed on the TiO2 surface similarly to the case of TPPS. However, the size of p-toluenesulfonate is small and shows no optical absorption in visible light range. Therefore, the counter ion would not affect the deactivation process of 1O2. Even if it contributed to the deactivation, only τo was affected and no effect on the kq which is calculated from the change of the TiO2 concentration. To determine the wavelengths for excitation and the T-T absorption, absorption spectra of the dyes were measured with a 5



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UV-Vis spectrophotometer (UV-3150, Shimadzu). In Figure 1 the absorption spectra of 0.01 M TPPS and TMPyP aqueous solutions were shown, where pH was adjusted to 10 by KOH (Nacalai Tesque). Although the absorption wavelength of the Q-band for the porphyrins is known to vary

with pH of the solution,38

both porphyrin samples have

absorption band at 532 nm but no absorption appears around 700 nm under the alkaline conditions (pH 10). Therefore, in the present study to excite the dyes by a pulsed Nd:YAG laser at 532 nm, the pH of porphyrin sample solutions were adjusted to about 10.

Fig.1 Absorption spectra of 0.01 M TPPS and TMPyP in aqueous solution of pH 10. Insert is the expansion spectra from 500 nm to 800 nm. 1

O2 phosphorescence For the measurements of 1O2 phosphorescence, sample solutions were prepared as

follows. In 3 ml of D2O solution of 0.2 mM dye, various amounts ( 0, 10, 30, 50, 70, and 100 mg) of TiO2 powders were suspended. D2O was used instead of H2O, because the lifetime of 1O2 becomes longer by about 20 times.36 The solution was stirred in a 1 x 1 cm quartz cell with a magnetic stirrer. The front of the cell was irradiated with a pulse Nd:YAG laser (Minilite-II, 532 nm, 4 mJ/pulse, Continuum) at the repetition rate of 10 s-1. The emission light was gathered with a convex lens and focused on the incident slit of a monochromator (675 Grooves/mm, Shimadzu-BoschLomb) to select 1270-nm light. 6


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The output light from the monochromator was introduced into a photomultiplier (R5509-41, Hamamatsu) cooled at 200 K with liquid nitrogen. At the front of the convex lens, a cold mirror filter (CLDM-50S, Sigma Koki) was placed to decrease the obstruction of scattered and/or luminescent light. The signal from the photomultiplier was amplified with a pre-amplifier (SR445, Stanford Research Systems) and then accumulated for 10000 pulse excitations with a scalar/averager (SR430, Stanford Research Systems) for 1024 data of 320-ns bin-width. Thus, the time profiles of 1O2 phosphorescence after the pulse laser irradiation were obtained. T-T absorption The lifetimes of the triplet excited state of dyes 3D* were measured by T-T absorption at room temperature by the following procedure. Various amounts of TiO2 powder (10-50 mg) was added in 25 ml aqueous solution of 1 mM dyes. The solution was vigorously stirred, bubbled with N2 gas, and then flowed into a quartz cell of 1 mm x 10 mm in cross section at the rate of l00 ml/min by using a Master Flex pump (Cole-Parmer). The sample solution was irradiated with the same pulse Nd:YAG laser used in the 1O2 measurements and analyzed by the transmission intensity of a 690 nm diode laser (35 mW, LDP2-6935B, NEOARK). The 690 nm light passed through the quartz cell was introduced to a light guide. To eliminate the intense scattered light of the YAG laser, a 532 nm Notch filter (NF-532.0-E-25.0M, CVI Melles Griot) and a red sharp cut filter (R-60, HOYA) were placed in front of the light guide. The output light of the light guide was detected by a photomultiplier tube (R928, Hamamatsu) through an interference filter of 690 nm (Koshin Kougaku). The signal from the photomultiplier tube was averaged with a digital oscilloscope (54522A, Hewlett Packard) for 1024 times.

The triplet state of the porphyrin dyes (3D*) absorbs the light of 690 nm37, but

the ground state (1D) does not absorb this light as shown in Fig. 1. Since the analyzing light scatters by TiO2 powder, the absorbance of 3D* in solution could not be measured correctly at the high concentration of the TiO2 powder. But the lifetime of 3D* would not be affected by the light scattering because the degree of scattering did not change in the time scale of milli-second.

Results and discussion

7



Deactivation of 1O2 The time profile of phosphorescence intensity for TMPyP photosensitizer in the presence of various amount of TiO2 (ST-01) powders was shown in Figure 2 in logarithmic scale. The decay slopes were increased with the amount of TiO2 while the peak intensity decreased. The decrease of peak intensity may be caused by the increase of the scattering of excitation and phosphorescence light, and/or possible decrease of 1

O2 production in the presence of TiO2 powders. The fact that the decay slopes were

increased with the amount of TiO2 indicates a dynamic quenching of 1O2 by TiO2 powders. This time profile can be described approximately by the following three-term equation (eq 7)

1E+05

number of photons / -

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0mg 10mg 30mg 50mg 70mg 100mg

1E+04

1E+03

1E+02 0

50

100

150

time / µs

Fig. 2 The time profiles of phosphorescence intensities for

1

O2 generated by

photosensitization with 0.2 mM TMPyP in 3 ml D2O solution, for different amount of TiO2 powders (ST-01)

= exp − − 0.5 − 0.5 ∗ exp −

(7)

, where L(t) is the phosphorescence intensity, L0 is a pre-exponential factor, and τd and τr are the time-constants of the decay and rise phases, respectively. The second

term 0.5 and the third term 0.5 ∗ exp − in the equation (7) correspond to the

8


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fast and the slow generation of 1O2, or eqs (3) and (4), respectively. By fitting eq (7) to the observed time profile of phosphorescence, time constants of rise (τr) and decay (τd) for 1O2 were obtained.

τr could be estimated to be ca. 6 x10-5s for all measurements. When pseudo-first order reaction is assumed with the dissolved oxygen of 1.2 mM, the rate constant of the reaction (4) , i.e.,1O2 generation by 3D*, can be calculated to be 1.3 x 107 M-1s-1. This value corresponds to the rate constant for the 1O2 formation by the energy transfer from 3

D* with a higher excess energy.36 The values of L0 obtained by the data fitting were significantly dispersed and the

reproducibility was low most probably due to the light scattering caused by the TiO2 powders. Then, the value of L0 was not involved to discuss the amount of produced 1O2. For the decay of the phosphorescence intensity, the lifetime τd can be described with the concentration of TiO2 quenchers [Q] as expressed by eq 8 on the basis of dynamic quenching.

= + !Q#

(8)

Here, τ0 is the lifetime of 1O2 in the absence of TiO2 powders, kq is the rate constant for the quenching (deactivation) of 1O2 by TiO2, and the concentration [Q] of TiO2 particles is in the unit of molar concentration M (= mole dm-3). The molar concentration [Q] could be calculated from the weight concentration with the number of particle per weight of each TiO2 listed in Table 1. Figure 3 shows the Stern-Volmer plot (eq 8) for TiO2 (ST-01) with TMPyP sensitizer. The linear relationship indicates that a dynamic quenching mechanism is valid under the present experimental condition. The slope of the linear line corresponds to the quenching rate constant of kq for the reaction (5).

9



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Fig.3 Relationship between the lifetime τd of 1O2 and the concentration of quencher [Q] of TiO2 (ST-01) powder for TMPyP sensitizer. The same procedure was applied to the other six kinds of TiO2 powders and another dye TPPS. The kq values obtained for TMPyP and TPPS sensitizer were plotted as a function of the particle size of employed TiO2 in Fig. 4 in logarithmic scale. It is noticed that kq values significantly increase with the particle size of TiO2. This might be explained by the increase in the collisional cross section of TiO2 particles for 1O2. It is noticed that kq values significantly increase with the particle size of TiO2. This might be explained by the increase in the collisional cross section of TiO2 particles for 1O2. Though the cross section may be one fourth of the surface area and proportional to square of the particle size, we could not formulate the relation with kq. Therefore, the particle size was used for the horizontal scale in Fig. 4.

10


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Fig.4 Relationships between kq and particle size of TiO2 for TPPS(■) and TMPyP(◆) When a bimolecular reaction in solution is assumed, the rate constant should not exceed the diffusion limit rate constants, kdiff, which are calculated to be 6.4x109 M-1s-1 and 5.2x109 M-1s-1 in H2O and D2O solutions, respectively, based on the Stokes-Einstein model (kdiff = 8RT/3η)36. However, the actual observed value of kq exceeded the diffusion limit. This experimental result indicates that the local concentration of 1O2 to collide TiO2 particles was increased due to the increase of the interaction of dyes with TiO2 particles. The degree of the interaction is stronger for TPPS, because it carries sulfonate group. Therefore, kq for TPPS became larger than that for TMPyP in Fig. 4. The intrinsic value for kq , kq0, was estimated to be (1.0-1.1)x108M-1s-1 by extrapolating each trend curve to zero. If the deactivation takes place on every collision of 1O2 and TiO2 particle, the rate constant should become the diffusion controlled value. Then, it could be estimated that only one 1O2 deactivates in the every 52 collisions to TiO2 surface. In our previous study30 we suggested that 1O2 formed at TiO2 surface should be most probably deactivated by TiO2. From the study on the reaction of 1O2 in H2O with four kinds of organic molecules, methionine, pyrrole, collagen, and folic acid, we proposed that reactant molecules should be adsorbed on TiO2 surface for efficient reactions with 1O2. 39 However, the present study suggested that the deactivation of 1O2 should not be caused mainly by the surface of TiO2 but caused by the vibrational level of the surrounding solvent molecules, H2O. Deactivation of 3D* 11



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Figure 5 shows the time profile of the transient T-T absorption for TPPS with ST-01 TiO2 quencher, which was calculated by taking logarithm of the transmission intensity of 690 nm light. At 0.4 ms the porphyrin dye was excited by 532-nm pulse (eq 1) to produce 3D* (eqs 2 and 3). 3D* deactivated to 1D in 0.5 ms. The deactivation became faster with the amounts of TiO2 added, suggesting the dynamic quenching of 3

D* by TiO2 particles. As shown in Fig. 5, the absorbance for 40 (e) and 50 mg (f) of

TiO2 was particularly larger than the others. The large amount of TiO2 can scatter the 690 nm probe light to reduce the intensity of transmitted light. This would lead to the inaccuracy of the absorbance and a large noise in the time profiles.

Fig. 5. Time profiles of absorbance at 690 nm for TPPS on the pulsed 532-nm excitation at 0.4 ms in the presence of various amounts (a;0, b;10, c;20, d;30, e;40, f;50 mg) of TiO2 (ST-01) powder in 25 ml deaerated aqueous solution. The lifetimes of 3D* of porphyrins (τd’) were obtained by fitting the decay curve as an exponential function. On the assumption of the dynamic quenching with TiO2 particles (eq 6), τd’ can be expressed with Stern-Volmer relationship (eq 9) 12


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similarly to the case of 1O2.

%$

=

%

+ & !Q#

(9)

τd’ (the decay rate of 3D* (TPPS)) was plotted against [Q] (the concentration of TiO2 (ST-01) particles) in Fig. 6. From the slope, the deactivation rate constant kq’ could be calculated.

Fig.6 Relationship between the decay rate of 3D* (TPPS) and the concentration [Q] of TiO2 (ST-01) powders. The deactivation rate constants kq’ were obtained similarly for various TiO2 powders and for TMPyP photosensitizers. The obtained kq’ values were plotted against the particle size of employed TiO2 in Fig. 7. kq’ increases with the size of TiO2 particles. This could be explained by the increase in the local concentration of 3D* near the TiO2 particles as was the case for 1O2. kq’ s of

3

D* for

TPPS and TMPyP show similar

1

vales , while kq of O2 for TPPS is slightly larger than that for TMPyP as shown in Fig. 4. The difference may become too small to be recognized for 3D* quenching due to the fact that the intrinsic lifetime of 3D* is longer than 1O2 by about 10 times and that the intrinsic quenching rate is larger by 15 times.

13



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Fig.7 Relationship between the 3D* deactivation rate constant kq’ of TPPS and TMPyP and the particle size of each TiO2. The intrinsic deactivation rate constant kq0’ for the triplet excited state of porphyrins (3D* ) was obtained to be 1.5x109 M-1s-1 by extrapolating the size to zero. Since kq0’ is the same order of the diffusion controlled rate constant, deactivation of 3D* with TiO2 (eq 6) may affect the formation of 1O2 (eq 4 ; 1.3 x 107 M-1s-1) through the reaction of 3

D* with 3O2. As stated above the concentration of TiO2 particles was less than 10-3

times of that of 3O2 under the present experimental condition. Therefore, comparing the rate of reaction (4) 1.3 x 107 M-1s-1 with that of (6) 1.5x109 M-1s-1, reaction (4) would not affect the formation rate of 1O2. This is also supported experimentally, because the rise time τr of 1O2 observed was almost the same value of 6 x10-5s (Fig. 2) as stated above.

Concluding Remarks 1

O2 generated on TiO2 plays important roles in the biological systems in

diminishing hazardous materials. However, the formation and in particular the deactivation mechanisms have not been well understood yet. In the present study, the deactivation of

1

O2 by TiO2 nano-particles was investigated by measuring

1

O2

phosphorescence intensity in the presence of two kinds of porphyrin dye sensitizer, TMPyP and TPPS. To clarify the effect of TiO2 on the 1O2 formation and deactivation, 14


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the deactivation of the triplet state of porphyrin dyes 3D* by TiO2 particles was also measured. The plausible reaction processes are schematically presented in Fig. 8. In the absence of both TiO2 and O2, the singlet state 1D of porphyrin dyes (1Dye) becomes 1D* (1Dye*) by the absorption of 532 nm laser light (1), mostly to yield the triplet state 3D* (3Dye*) by intersystem crossing (2). In the absence of TiO2 and in the presence of O2 (3O2), 1D* becomes 3D* with the formation of 1O2 (3). 3D* deactivates to the ground state 1D by energy transfer to 3

O2 with producing 1O2 (4). 1O2 deactivates in 49 µs ( = τo) mainly by the coupling with

vibrational mode of D2O solvent.36 The lifetimes of 3D* (τo’) were 162 µs and 409 µs for TMPyP and TPPS, respectively.

Fig.8 Deactivation processes of 1O2 and 3Dye* (triplet sate of dye, 3D*) with TiO2 particle surface in porphyrin sensitization systems. The numbers at arrows indicate the number of the equation in the text. In the presence of both TiO2 particles and O2 (3O2), the lifetimes of both 1O2 and 3D* were decreased. The deactivation (quenching) rate constant depended on the particle size but not on the crystalline structure, anatase-rutile. The size dependence 15



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Page 16 of 21

could be explained by the increase of the local concentration of 1O2 and 3D* near the particles. As a summarized in Table 2, for 1O2 the intrinsic rate constant for the deactivation by TiO2 (kq0) was 1.0x108 M-1s-1 in D2O solution. Since diffusion controlled rate constant (kdiff) was 5.2x109 M-1s-1, only 2% of the collision was revealed to actually deactivate 1O2. On the other hand, the intrinsic rate constant (kq0’) for the deactivation of

3

D* was estimated to be about 1.5x109 M-1s-1. Therefore, the

deactivation of 3D* over the TiO2 surface may not be negligible in the 1O2 formation by the reaction (4), though it was not evidenced in the present experimental condition. Table 2. Intrinsic quenching rate constants, kq0 and kq0’ for 1O2 and 1D* with TiO2. Sensitizers

kq0 / 108 M-1 s-1

kq0’ / 109 M-1 s-1

TMPyP

1.0

1.5

TPPS

1.1

1.5

In conclusion, the present study revealed that the intrinsic rate constant for deactivation of 1O2 by TiO2 is considerably small. Instead, deactivation by energy transfer to the surrounding molecules such as H2O is dominant.36 The quenching of 1O2 by TiO2 occurs only in a limited case. To optimize the effect of the 1O2 it should be produced near the reactant molecule. Since H2O deactivate 1O2 by energy transfer, more hydrophobic environment would promote the effect.

On the other hand, to reduce the

1

biological hazardous effects of O2 reported so far,12,13,16,17,24-28 more hydrophilic environment would be desirable. In actual applications of TiO2 various kinds of active oxygen species, such as OH radical and H2O2, are formed under photoirradiation. Present results suggest that we can regulate the effects of the individual active species by changing the surrounding conditions such as hydrophobic and hydrophilic conditions.

ACKNOWLEDGMENT We thank Dr. Atsuko Y. Nosaka for the valuable comments on the manuscript preparation. 16


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References

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