The Antimicrobial and Organic Substance Degrading Effects of Radicals Generated by Light-shielded Titanium Oxide After Irradiation

Abstract

Titanium oxide (TiO_2) is known to exhibit photocatalytic activity by producing reactive oxygen species (ROS) when irradiated with light within an excitation wavelength band, in addition to strong oxidation-reduction effects and superior hydrophilic effects. In particular, an anatase-type TiO_2 shows a strong tendency for photocatalysis. In dentistry, although TiO_2 has been used for tooth bleaching, its application to other dental treatments has not been comprehensively studied because it is difficult to provide sufficient light for exciting TiO_2 within the confined space of the oral cavity. The present study aimed to examine the generation of ROS in addition to the antimicrobial and organic substance degrading effects of light-shielded TiO_2 after activation by irradiation with light. An anatase-type titanium oxide was used in this study. ROS generation by the light-shielded TiO_2 was measured by electron spin resonance. The results revealed that generation of ROS continued for 100min after light-shielding the TiO_2. The antimicrobial effect was evaluated with Streptococcus mutans, Staphylococcus aureus, Enterococcus faecalis and Candida albicans. After light irradiation of TiO_2 for 5h, a solution of each microbe was mixed with TiO_2 powder, then at a predetermined period, each solution was measured to determine the number of viable microorganisms. Results revealed that the viability of S. mutans and S. aureus decreased significantly from 1min after being exposed to the irradiated TiO_2, whereas the viability of E. faecalis and C. albicans decreased significantly from 2h. The organic substance degrading effect was evaluated using albumin as an organic sample. TiO_2 was irradiated under the same conditions as that used for examining the antimicrobial effect and mixed with an albumin suspension, which was subsequently retrieved at specified times for evaluation. Degradation of albumin, evaluated by polyacrylamide gel electrophoresis, was found immediately after exposure to the TiO_2 in the polyacrylamide gel. There was a significant post-exposure decrease in albumin volume as determined by the Lowry method. The results of the present study indicate that an anatase-type TiO_2 maintains a ROS-generating effect after irradiation with light even when subsequently shielded. Moreover, it was shown that antimicrobial and organic degrading effects occurred as a result of ROS generation.