Photocatalytic bactericidal effect of TiO2 thin films: dynamic view of the active oxygen species responsible for the effect

doi:10.1016/S1010-6030(97)00038-5

Journal of Photochemistry and Photobiology A: Chemistry

Volume 106, Issues 1–3, 1 June 1997, Pages 51-56

https://doi.org/10.1016/S1010-6030(97)00038-5 Get rights and content

Abstract

The role of active oxygen species in the photocatalytic bactericidal effect was investigated using a thin transparent titanium dioxide (TiO₂) film. The viable number of Escherichia coli (E. coli) significantly decreased on the illuminated TiO₂ film, and the bactericidal effect was observed even when E. coli was separated from the TiO₂ surface with a 50 μm porous membrane. Mannitol, a hydroxyl radical scavenger, inhibited the effect only in the absence of the membrane. In contrast, catalase inhibited the effect in all cases. On the basis of these results, the long-range bactericidal effect of hydrogen peroxide was proposed, together with a cooperative effect due to other oxygen species. © 1997 Elsevier Science S.A.

Introduction

Since the discovery of photocatalytic water cleavage on TiO₂ electrodes [1], research work on TiO₂ photocatalysts has been intensively pursued 2, 3. Recently, TiO₂ photocatalysts have been spotlighted as functional materials to aid in the cleaning of the environment [4]. Illuminated TiO₂ photocatalysts decompose organic compounds by oxidation, with holes (h⁺) generated in the valence band and with hydroxyl radicals (OH^·) produced by the oxidation of water. Such photocatalytic oxidation causes damage to microorganisms, which also consist of organic compounds. Thus photocatalysts are expected to find applications in materials possessing antibacterial functions.

Reports have appeared concerning the bactericidal effects of TiO₂ powder 5, 6, 7, 8, often referring to OH^· as the toxic agent. However, there remains some doubt about the actual bactericidal agents because several active oxygen species other than OH^· are generated by photocatalytic reactions, e.g. superoxide anion (O₂⁻), perhydroxyl radical (HOO^·) and hydrogen peroxide (H₂O₂). These species are better known for their role in biological reactions [9] than in the decomposition of ordinary organic molecules. In addition, bacteria can act as powerful probes to investigate these active oxygen species. Since bacteria are much larger than single molecules, the photocatalytic bactericidal effect necessarily involves long-range interactions between the reactants (bacteria) and the photocatalyst. Such interactions are usually neglected in photocatalytic reactions, which are typically surface reactions for ordinary molecules.

We have successfully produced TiO₂ thin films which are transparent in the visible region, and have demonstrated their high photocatalytic efficiency [10]. The use of a thin film localizes the reaction and also enables modifications to be made conveniently. In this paper, we report the bactericidal effect of TiO₂ thin films with and without modification in order to determine which active oxygen species are responsible for the bactericidal effect and the possible mechanisms.

Section snippets

Materials

Soda-lime glass plates, previously coated with silica thin film (approximately 100 nm), were dipped in titanium isopropoxide solution (Type NTi-500, Nippon Soda) and were slowly pulled from the solution at a fixed rate of 20 cm min⁻¹ in dry air. The plates were quickly placed in a furnace and calcined at 500 °C for 1 h. Four such coating steps produced a TiO₂ thin film of approximately 0.4 μm on both sides of the glass plate. The thickness was estimated from the interference oscillations in the

Bactericidal effects in the liquid film

The survival ratio for E. coli in the liquid film on the TiO₂ film under black light illumination decreases to a negligible level (i.e. essentially complete sterilization) within 1 h (Fig. 2). The efficiency is remarkably higher than that without a TiO₂ film, in which UV light (300–400 nm) causes only 50% sterilization within 4 h. The film in the dark does not affect the survival ratio, indicating that the film itself is not toxic for E. coli; this time dependence is almost the same as that on

Bactericidal agent

Hydroxyl radical (OH^·) reacts with various organic compounds at essentially a diffusion-limited rate [15] and is considered to be the primary agent in the photocatalytic bactericidal effect. A similar conclusion may be reached from the result that the photocatalytic bactericide in the liquid film is strongly inhibited by mannitol. In the light of the other results reported here, however, this scheme should be modified.

The contribution of hydrogen peroxide (H₂O₂) to the effect should be

Conclusions

In the photocatalytic bactericidal effect of E. coli on illuminated TiO₂ films, it was confirmed that both oxidation and reduction sites contribute, corresponding to OH^· and O₂⁻ production respectively. However, it is proposed that the actual lethal agent is H₂O₂, subsequently produced from OH^· and O₂⁻, particularly in the long-range bactericidal effect. The concentration of H₂O₂ produced photocatalytically is low, and thus a cooperative effect with other oxygen species is postulated.

Acknowledgements

The authors thank Dr D.A. Tryk for careful reading of the manuscript and useful discussions. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

References (27)

A. Fujishima, K. Honda, Nature 37 (1972)...
A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995)...
M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)...
D.F. Ollis, E. Pelizetti, N. Serpone, Environ. Sci. Technol. 25 (1991)...
T. Matsunaga, R. Tomoda, T. Nakajima, H. Wake, FEMS Microbiol. Lett. 22 (1985)...
J.C. Ireland, P. Klostermann, W. Rice, R.M. Clark, Appl. Environ. Microbiol. 59 (1993)...
C. Wei, W.-Y. Lin, Z. Zainal, N.E. Williams, K. Zhu, A.P. Kruzic, R.L. Smith, K. Rajeshwar, Environ. Sci. Technol. 28...
R.J. Watts, S. Kong, M.P. Orr, G.C. Miller, B.E. Henry, Water Res. 29 (1995)...
B. Halliwell, J.M.C. Gutteridge, Biochem. J. 219 (1984)...
N. Neghishi, T. Iyoda, K. Hashimoto, A. Fujishima, Chem. Lett. (1995)...

C. Matsubara, T. Iwamoto, Y. Nishikawa, K. Takamura, S. Yano, S. Yoshikawa, J. Chem. Soc., Dalton Trans. (1985)...

S. Marklund, J. Biol. Chem. 251 (1976)...

M. Abdullah, G.K.-C. Low, R.W. Matthews, J. Phys. Chem. 94 (1990)...

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