Synergistic Effect of Thermal Energy on Bactericidal Action of Photolysis of H2O2 in Relation to Acceleration of Hydroxyl Radical Generation

The purpose of the present study is to evaluate the effect of thermal energy on the yield of and the bactericidal action of hydroxyl radical generated by photolysis of H2O2. Different concentrations of H2O2 (250, 500, 750, and 1,000 mM) were irradiated with light-emitting diodes (LEDs) at a wavelength of 400 +/- 20 nm at 25 degrees C to generate hydroxyl radical. The 500 mM H2O2 was irradiated with the LEDs at different temperatures (25, 35, 45, and 55 degrees C). Electron spin resonance spin trapping analysis showed that the yield of hydroxyl radicals increased with the temperature, as well as the concentration of H2O2. Streptococcus mutans and Enterococcus faecalis were used in the bactericidal assay. The LED-light irradiation of the bacterial suspensions in 500 mM H2O2 at 25 degrees C could hardly kill the bacteria within 3 min, while the bactericidal effect was markedly enhanced with the temperature rise. For instance, a temperature increase to 55 degrees C resulted in >99.999% reduction of viable counts of both bacterial species only within 1 min. The photolysis of 500 mM H2O2 at 55 degrees C could reduce the viable counts of bacteria more efficiently than did the photolysis of 1,000 mM H2O2 at 25 degrees C, although the yields of hydroxyl radical were almost the same under the both conditions. These findings suggest that the thermal energy accelerates the generation of hydroxyl radical by photolysis of H2O2, which in turn results in a synergistic bactericidal effect of hydroxyl radical and thermal energy.