The term dental bleaching is chemically summarized in a reaction where there is a breakdown of chromophore molecules present in organic and inorganic clusters of the tooth structure. During bleaching, the pigments, made up of unsaturated carbon molecules, are saturated by the local action of free radicals. Thus, the double bonds between the carbons are undone by modifying the absorption spectrum of those molecules, thus enabling a greater reflection of light and, consequently, a clearer dental structure. The generation of free radicals can be performed with hydrogen peroxide or with carbamide which, because they are unstable, produce the free radicals that will provide the tooth whitening reaction.
In order to trigger the acceleration of the chemical reaction different methods are used in the dental whitening procedure. Among these we can mention:; pH modification; temperature increase through molecular vibration (heat); and chemical catalysts. In this module we will illustrate the different actions of the photocatalysis in the acceleration of the reaction to obtain the tooth whitening.
In order to understand how light is capable of producing photocatalysis, we need to briefly understand what light is. Light is a form of electromagnetic energy whose wavelength extends from gamma radiation to infrared radiation, invisible to the human eye. The visible part of the spectrum is comprised in a very small interval between violet color and red color (Figure 1).
Figure 1 shows the different irradiations with their respective physical characteristics. Thus, because it has limited energy, visible light requires a molecule capable of absorbing it to generate catalysis, and consequently an increase in temperature occurs. It is like exposing a black or white material to the energy of the sun. Of course we will have a rise in temperature on the dark surface that absorbs light, while the light surface, reflecting it, will not heat up. Because of this, the most effective bleaching agents to be catalyzed by light are those that have complementary colors. The color of the light emitting source in most instruments is emitted by “Leds – Light Emission Diodes” blue, that is, if we are doing a lightening with blue light source we must use colors in the whitening agent that absorb blue, for example , yellow, red, orange, violet. The heating on the surface of the bleaching gel provides, by the laws of the chemical kinetics of Arrhenius, an increase of the speed of the catalysis reaction, thus releasing more quickly the free radicals that will be responsible for the dental whitening. This reaction is known as phototermocatalytic. On the other hand, the infrared radiation emitted by low power lasers is able to provide greater vibration in different molecules. This increase in vibration generates heat on the radiated surface.
Thus, in dental bleaching we have the possibility of providing two types of photocatalysis: one dependent on the absorption of light energy by aggregate dyes in bleaching agents and another by direct catalysis using infrared radiation.
Different studies show similar results in bleaching effectiveness, but they differ statistically in relation to the time of the procedure in which the light source provides a faster reaction by photocatalysis. In figure 2 it is observed that the final bleaching rate is the same when the substance is subjected to light source, or not. However, it is clear that with the light source the result occurs more rapidly thus benefiting the performance of the dental whitening procedure.
The new concepts of tooth whitening with light source not only provide a reduction in the time of the procedure, but also the possibility of using lower concentrations of the bleaching agent, obtaining less possibility of sensitivity, and greater safety for the patient, as shown by the figure 3.
With the advent of nanotechnology new concepts of photocatalysis were developed, among which, the incorporation of photosensitive nanomollectors into bleaching agents made possible direct catalytic reactions using visible light, without the need for temperature increase. As we have seen, all this technology led to a decrease in the concentration of bleaching agents due to the greater efficiency of the free radical release reaction. Example of this concept is the incorporation of titanium dioxide in bleaching agents.
Finally, we have seen that photocatalysis provides both the possibility of increasing the speed of the dental bleaching reaction, as well as facilitating the reduction of bleaching agents concentration, thus significantly reducing the patients’ sensitivity index (Figure 2).
The search for a minimally invasive dentistry makes us think of a change in aesthetic approaches in order to provide greater patient safety. Dental whitening should be understood as a conservative aesthetic restorative procedure seeking the natural coloration of dental structures that are unique to each individual. And for this we should increasingly be concerned with the indication of customized procedures for each patient, reducing the procedure time, or the concentration of the bleaching agent using, for example, the technology of LASERs and LEDs as light sources.
Hermes Pretel *, CD, Msc., Dr.
* Master’s and Doctoral Degree – Dental Sciences – Faculty of Dentistry of Araraquara – UNESP.