Effects of Hydrogen Peroxide Concentration and Activation Time on Hydroxyl Radical Generation and the Bleaching Effect

Purpose: Tooth bleaching is widely performed in clinical practice. According to Goldstein, the mechanism behind tooth bleaching is the decomposition of a carbon double bond (chromatic) molecule in a discolored tooth to an achromatic substance via hydroxyl radicals generated from hydrogen peroxide. In this study, we sought to clarify the mechanism by investigating the efficacy of bleaching and time taken to activate the hydroxyl radicals generated from various concentrations of hydrogen peroxide. Methods: Hydrogen peroxide solutions containing 3%, 10%, 20%, and 30% hydrogen peroxide were mixed with a spin trapping agent. The hydroxyl radicals generated were measured by electron spin resonance. Activation was achieved by light irradiation at a wavelength of 440 nm for 0.5, 1, 2, 3, 5, and 10 min. Filter papers dyed with β-carotene were immersed in 3%, 10%, 20%, and 30% hydrogen peroxide solutions and irradiated for 0.5, 2, and 10 min. The color difference, ΔE*ab, was calculated from the CIELab values at baseline and after bleaching. Results: The generation of hydroxyl radicals increased at higher concentrations of hydrogen peroxide and longer irradiation times. The ΔE*ab values also showed the same tendency as the results showing the amounts of hydroxyl radicals generated. That is, the ΔE*ab value increased as the concentration and activation time increased. Conclusions: The concentration of hydrogen peroxide and the activation time were important for the generation of hydroxyl peroxide radicals. However, based on the amount of hydroxyl radicals and the ΔE*ab values, other factors such as the catalysts, temperature rise, and the physical properties of the bleaching agent appear to be related to the efficacy. To pursue effective and safe tooth bleaching materials, other factors in addition to the concentration of hydrogen peroxide and the application time should be considered.