Ingredients used in Oxidative Whitening
Hydrogen peroxide: Hydrogen peroxide is the most common ingredient used for oxidative tooth whitening. A PubMed search of hydrogen peroxide and tooth whitening reveals over 2000 peer-reviewed citations. Hydrogen peroxide is used in numerous whitening applications including in-office based whitening systems, toothpastes and gels, strip delivery systems, and in some paint on tooth whitening systems. Its chemical structure is shown below (Figure 10).
Figure 10. Hydrogen Peroxide - H2O2
As described earlier, oxidation is primarily directed at the reactivity with organic stain components within the teeth. It is possible that this occurs due to the presence of catalytic levels of trace metals within the teeth (e.g., iron), which can promote formation of radical intermediates, a highly active form for oxidation.
Carbamide peroxide: Carbamide peroxide is a commonly used ingredient for oxidative tooth whitening. Its chemical structure is shown below (Figure 11) which is an adduct of urea and hydrogen peroxide. However, even with the presence of a small amount of water, the urea releases from hydrogen peroxide into solution.
Figure 11. Carbamide Peroxide - CH6N2O3
Figure 11. Carbamide Peroxide - CH6N2O3
The ratio of hydrogen peroxide created for a quantity of carbamide peroxide on a mass basis is roughly 3 to 1 – that is, 3% (by weight) carbamide peroxide will create 1% hydrogen peroxide. So, for example, the commercial in-office products that provide 35% carbamide peroxide provide a little over 10% as hydrogen peroxide. The hydrogen peroxide then carries out the whitening reactions in the tooth. Carbamide peroxide was the ingredient of choice in early dental practice-based mouthguard tray systems1, as it is relatively easy to formulate in anhydrous, hydrophilic gels which can activate when the patient places the mouthguard intra-orally. Water from the tooth and from saliva dissolve and dissociate the hydrogen peroxide. Our body has innate mechanisms to manage peroxide. This will be discussed in a later section regarding design tolerability.
Chlorite: Sodium chlorite has emerged in numerous topical mouthwashes and toothpastes as an ingredient to provide antimicrobial as well as tooth whitening properties.29,30
Sodium chlorite, which breaks down to chlorine dioxide in an acidic environment. Its chemical composition is shown below (Figure 12).
Figure 12. Sodium Chlorite - NaClO2
Chlorite participates in chlorine-based oxidation reactions (similar to bleach in laundry) through complex chemistries in aqueous solutions which are strongly pH dependent. Because of the dependency of pH, some chlorite products may contain a citric acid accelerator which can lead to demineralization of enamel causing safety concerns. The penetrability of chlorite-based whitening has not been assessed – it is unlikely to diffuse into teeth nearly as rapidly as hydrogen peroxide based on its molecular size (the molecular weight of chlorite is roughly twice that of hydrogen peroxide). The concentration of chlorine-based whiteners must be high for efficacy, however, this may also limit tolerability or safety for soft tissue since there is no enzymatic protection mechanism of tissues to protect against oxidative damage as there is with hydrogen peroxide.
PAP: 6-phthalimido peroxyhexanoic acid is a peroxy acid – structure shown below (Figure 13). The oxidation reactivity comes from the peroxide bound to the terminal carbonyl group on the molecule, shown in Figure 13 below, this structure is the configuration of the peroxy acid.
Figure 13. PAP: 6-phthalimido peroxyhexanoic acid – C14H15NO5
Peroxy acids contain a functional oxidizing group on a backbone resembling a peracid. They are widely used in laundry applications due to their strong oxidizing capabilities. Peroxy acids are often used in combination with other oxidizing agents in laundry bleaches as activators. Reports in the literature suggest the use of PAP gel in a tray application with efficacy for improving tooth color.29
As with other non H2O2 based oxidation-based whiteners, the breadth and extent of efficacy has less published evidence, however the strong oxidation activity of peroxyacids may make them effective in tooth whitening. The high molecular weight of the peroxy acid will limit its diffusion into dental enamel even more so than chlorite, and like chlorite the concentrations and doses used may be limited by safety considerations since oral enzymes would not protect against peroxy acid oxidative damage to tissues.