Mechanism of tooth bleaching

Bleaching is a decolourisation or whitening process that can occur in solution or on a  surface. The colour producing materials in solution or on a surface are typically organic compounds that possess extended conjugated chains of alternating single or double bonds and often include heteroatoms, carbonyl, and phenyl rings in the conjugated system and are often referred to as a chromophore.


Bleaching and decolourisation of the chromophore can occur by destroying one or more of the double bonds in the conjugated chain, by cleaving the conjugated chain, or by oxidation of other chemical moieties in the conjugated chain. Hydrogen peroxide oxidises a wide variety of organic and inorganic compounds. The mechanisms of these reactions are varied and dependent on the substrate, the reaction environment, and catalysis. In general, the mechanism of bleaching by hydrogen peroxide is not well understood and it can form a number of different active oxygen species depending on reaction conditions, including temperature, pH, light and presence of transition metals. Under alkaline conditions, hydrogen peroxide bleaching generally proceeds via the perhydroxyl anion (HO2* ). Other conditions can give rise to free radical formation, for example, by homolytic cleavage of either an O–H bond or the O–O bond in hydrogen peroxide to give H* + *OOH and 2*OH (hydroxyl radical), respectively. Under photochemically initiated reactions using light or lasers, the formation of hydroxyl radicals from hydrogen peroxide has been shown to increase.

The mechanism by which teeth are whitened by oxidising materials such as hydrogen peroxide and carbamide peroxide are currently not fully understood. Considering the available literature, evidence points towards the initial diffusion of peroxide into and through the enamel to reach the enamel dentine junction and dentine regions. Indeed, in vitro experiments by a number of authors have demonstrated the penetration of low levels of peroxide into the pulp chambers of extracted teeth after exposure times of 15– 30 min from a range of peroxide products and solutions. The levels of peroxide measured in these experiments is considerably much lower than that needed to produce pulpal enzyme inactivation. As peroxide diffuses into the tooth, it can react with organic coloured materials found within the tooth structures leading to a reduction in colour.

This is particularly evident within dentine as demonstrated by McCaslin et al. who showed, using hemi-sectioned human teeth mounted on glass slides, that following external bleaching with carbamide peroxide, colour changes occurred throughout the dentine. Indeed, the treatment of dentine specimens with 10% carbamide peroxide, 5.3% and 6% hydrogen peroxide has been shown to give a significant reduction in yellowness and an increase in whiteness. In addition, Sulieman et al. showed using sectioned extracted teeth stained internally with black tea chromophores that significant bleaching occurred within the
dentine, particularly on the buccal surface where a 35% hydrogen peroxide gel had been applied. For tetracycline stained teeth, the colour is derived from photo-oxidation of tetracycline molecules bound within the tooth structures.

In some cases, it is possible to bleach these teeth to give significant and long lasting tooth whitening. The mechanism by which peroxide affects the tetracycline stain is considered to be by chemical degradation of the unsaturated quinone type structures found in tetracycline leading to less coloured molecules.However, in contrast there appears to be a paucity of information available in the literature regarding the nature and chemical composition of the coloured materials naturally found within the dental hard tissues and the mechanistic effects of peroxide on these structures. Thus, this is clearly an area that requires further research if the chemical mechanistic aspects of tooth bleaching are to be significantly resolved.