The effect of irradiation wavelengths and the crystal structures of titanium dioxide on the formation of singlet oxygen for bacterial killing

Abstract

Safe and effective methods for oral bacterial disinfection have been desired, since bacteria cause many infectious diseases such as dental caries, periodontal disease, and endodontic infections. Singlet oxygen ((1)O(2)) is attractive, because it is toxic to prokaryotic cells, but not to eukaryotic cells. We selected irradiation of titanium dioxide (TiO(2)) as a source of (1)O(2), because it has been used in sunscreens and cosmetic products without complications. In order to establish the optimal oral photodynamic therapy conditions, we measured the rate of (1)O(2) formation from the irradiated anatase or rutile forms of TiO(2) using 365 or 405 nm lamps. The rate of (1)O(2) formation decreased in the following order: anatase, 365 nm > rutile, 405 nm > rutile, 365 nm > anatase, 405 nm. Therefore, we concluded that irradiation of the rutile form of TiO(2) by a 405 nm lamp is the most favorable photodynamic therapy condition, because visible light is more desirable than UV light from the viewpoint of patient safety. We also confirmed that there was no direct HO(•) formation from the irradiated TiO(2).

Keywords: 405 nm; electron spin resonance spectroscopy; photodynamic therapy; singlet oxygen; titanium dioxide.

Fig. 1

The EPR spectra for the production of 4-oxo-TEMPO following irradiation of the ¹O₂-specific reagent, 4-oxo-TMP (40 mM) with TiO₂ (4.0 mg/ml) for 5 min.

Fig. 2

The formation of 4-oxo-TEMPO upon irradiation of 4-oxo-TMP (40 mM) with TiO₂ (4.0 mg/ml). The data points are the mean values (n = 3) with standard deviation bars.

Fig. 3

The photooxidation of uric acid (100 mM) in the presence of TiO₂ (4.0 mg/ml). The values are the means of two independent and reproducible analyses.

Fig. 4

The formation of OH adducts and their disappearance following the addition of 10 U/ml SOD after the photooxidation of 100 mM CYPMPO in the presence of 0.4 mg/ml TiO₂.

Fig. 5

The oxidation of 2,6-di-tert-butyl-4-methylphenol (BHT, 100 µM) in the presence of TiO₂ (4.0 mg/ml) in aqueous 80% methanol. The values are the means of two independent and reproducible analyses.