WO2009133525A2

WO2009133525A2 - Composition for dental bleaching

Info

Publication number: WO2009133525A2
Application number: PCT/IB2009/051746
Authority: WO
Inventors: Jeroen Verheyen; Adriana Barylyak; Roeland De Moor; Peter Verheyen; Yaroslav Bobytskyy
Original assignee: High Tech Laser
Priority date: 2008-04-29
Filing date: 2009-04-29
Publication date: 2009-11-05
Also published as: WO2009133525A3

Abstract

This invention relates to a bleaching composition for the bleaching of discoloured teeth, whereby the bleaching composition contains an activatable peroxide compound and a TiO₂ photocatalyst with an average particle size of maximum 300 nm for photocatalytic decomposition of the peroxide and the release of free radicals capable of reacting with the discolouring substances present in the teeth, characterised in that the TiO₂ present in the composition is doped with S and the TiO₂particles have an average particle size of < 100 nm.

Description

Composition for dental bleaching

This invention relates to a bleaching composition for the bleaching of discoloured teeth, whereby the bleaching composition contains an activatable peroxide compound and a TiO₂ photocatalyst with an average particle size of maximum 300 nm for the photocatalytic decomposition of the peroxide and the release of free radicals capable of reacting with discolouring substances present in the teeth, according to the preamble of the first claim.

Background of the invention

In recent years there have been increasing efforts to find solutions for the bleaching of discoloured teeth. Discolouration of the teeth is a frequently occurring phenomenon that may have many causes. For example, discolouration may be restricted to the exterior of the teeth, but may also be accompanied by intrinsic discolouration due to the presence of discolourations in the crystalline structure of the tooth enamel and dentine.

Exterior discolouration is frequently caused by consumption of e.g. coffee, tobacco, fruit, etc., and can usually be removed by polishing. However, the existing methods frequently fail if the discolouration extends into the innermost part of the dentine. Intrinsic discolouration of the teeth may be endogenous or exogenous and can originate before or during odontogenisis. One of the primary causes of intrinsic tooth discolouration is tetracycline discolouration due to the incorporation of tetracyclines in the hydroxyapatite crystals that form the mineral part of the tooth, during the mineralisation of the tooth in the dental development phase. Discolouration may vary from yellow to yellow/brown, brown, grey or blue and may be accompanied by striping in the teeth. The degree of discolouration varies due to a number of factors, including age, dosage and type of tetracycline administered. Another important cause of tooth discolouration is fluorosis, with frequently involved hypoplasia of the enamel matrix and white stains. The hypoplasia may be pronounced to such an extent that rough spots and real defects appear in the enamel, which is then further seriously discoloured by exogenous factors (coffee, tea, tobacco, wine, herbs). Fluorosis is occasionally accompanied by pronounced staining on the enamel. Discolouration of the teeth can also be caused by other factors such as chromogenic bacteria on the teeth, components present in the saliva that can cause brown discolouration of the teeth, components originating in the blood, phenylketonuria, erythroblastosis fetalis, sickle-cell anaemia, amelogenisis and dentinogenic imperfecta.

In one of the methods available for the bleaching of teeth, a concentrated bleaching composition that contains to 30-35 weight % of hydrogen peroxide, is applied to the teeth to be bleached. However, the thermal activation of hydrogen peroxide makes teeth highly sensitive subsequent to bleaching. Up to now the intensity of the morphological changes of the biochemical properties of the tooth enamel following bleaching, is still not really known. Tooth enamel and dentine are highly permeable for hydrogen peroxide, although the process of molecular permeation has not been completely identified. Two-dimensional analysis of the surface using scanning electron microscopy reveals an increased roughness of the enamel after bleaching with hydrogen peroxide. Other side-effects include an increased permeability of the tooth enamel, which is probably caused by a loss of organic matrix material, calcium and phosphate, after exposure to hydrogen peroxide, a detectable reduction in hardness at the micro scale and reduced resistance to breakage of the enamel and dentine.

WO0222097 in the name of High Tech Laser describes a method for the photochemical bleaching of teeth. According to this method the teeth to be bleached are covered with a bleaching composition that contains an amount of a peroxide or a peroxide forming compound that generates free radicals under the influence of 532 nm electromagnetic radiation capable of decomposing the compounds responsible for the discolouration present in the tooth. Because the radicals are generated photochemically and not by thermal activation, heating of the teeth is minimised as well as the extreme sensitivity of the teeth after the bleaching associated therewith. Moreover, the bleaching of the teeth is not restricted to the tooth surface, but extends into the innermost part of the tooth material.

However, this method has the disadvantage that the bleaching agent contains a relatively large amount of H₂O₂, frequently 30-35%, despite the presence of rhodamine in order to obtain an increased radical yield.

WO2007036290 discloses a bleaching composition for the bleaching of teeth that contains an amount of a photocatalyst capable of accelerating the decomposition of hydrogen peroxide at room temperature. The photocatalyst contains 10-90 weight % of a material consisting of semiconductor particles such as ZnO₂, Si, α-Sn or TiO₂. TiO₂ is preferably used in the form of tetragonal anatase, whereby the particles have an average diameter smaller than 100 nm. The bleaching also contains a amount of hydrogen peroxide or a hydrogen peroxide generating compound as a radical producer, a thickening agent and a alkaline pH buffer. The bleaching composition is irradiated with light with a wavelength between 380 - 500 nm and an energy in the milliwatt - Watt range in order to prevent damage to the pulpa.

However, the method described in WO2007036290 has several disadvantages. Absorption by the TiO₂ nano particles is negligible in the wavelength range used, and the energy density of the light source is too low, as a result of which hardly any photocatalytic activation of the hydrogen peroxide takes place and hydrogen peroxide decomposition is mainly due to auto-decomposition. As a consequence the bleaching effect is minimal, tetracycline discolouration is not resolved. If increased radical production for an improved bleaching effect is envisaged, longer radiation times are required, which involves a considerable risk of increasing the sensitivity of the teeth during bleaching. Because light sources with a wavelength < 400 nm are oncogenic, light sources with a wavelength of 450 - 480 nm are usually chosen. However, these wavelengths are unable to induce photocatalytic activity. EP1457200 describes an agent for the bleaching of teeth that contains a first component that adheres temporarily to the surface of the teeth. A second component is then applied to the first component. The first component contains 1-30 weight % of titanium dioxide, titanium dioxide possibly doped with N and/or a titanium oxinitride with an average particle size of 1-500 nm, water, a solvent for improving adhesion to the teeth and has a pH of between 5 and 10. On the titanium dioxide surface islands of a ceramic material and of a charge separating compound are present, for the purpose of increasing the radical production. The second component contains 1-40 weight % of a hydrogen peroxide producing compound and a polyol acting as a carrier. The coated teeth are irradiated with visible or UV light. However, the use of an agent with a lower pH causes tooth erosion. On the other hand, the agent is not transparent for UV light due to the high titanium dioxide concentration , which means that insufficient radicals are produced. EP1393711 solves the problem of the limited transparency by reducing the amount of porous titanium dioxide powder doped with N to 0.01-5 weight %. The solution also contains alcohol, 1-20 weight % hydrogen peroxide and 2-45 weight % urea peroxide in order to increase the bleaching effect. Although these peroxide concentrations may cause serious post bleaching sensitivity, their presence is required in view of the fact that radical production upon irradiation with UV or visible light is insufficient. However, tetracycline discolouration cannot be treated with these wavelengths.

WO2007/118689 describes a method for the bleaching of teeth whereby a composition is applied to the teeth that contains an apatite- like compound with a particle size of < 500 nm, and preferably smaller than

100 nm. Nano particles of fluorapatite are preferred because of their low solubility in acidic environments. The composition also contains 5-75 weight % peroxide, one or more activators for the decomposition of the peroxide and a polyol. The composition is thermally activated, for example by heating with light, laser, etc., which in combination with the high hydrogen peroxide concentration, causes the occurrence of post bleaching sensitivity. It is assumed that bleaching involves oxidative cleaning of the tooth surface and that re-mineralising of the teeth occurs simultaneously. Analysis of the state of the art reveals that a vast number of techniques have been developed for the bleaching of teeth, whereby photocatalytic activation of peroxide is obtained by irradiation with UV light. Because UV can be oncogenic, use is made of visible light, which however is unable to induce significant photocatalytic activity. The low radical yield associated with the use of visible light involves that high peroxide concentrations are required in order to achieve a sufficient bleaching effect. However, a technique which permits to treat simple discolouration by using mild conditions, as well as a complex tetracycline discolouration by using appropriatel conditions while using relatively low amounts of hydrogen peroxide is still unavailable.

The object of this invention is to provide a bleaching composition in which photocatalytic decomposition of the peroxide is provided under the influence of visible light, and whereby even a minimum peroxide concentration provides optimum radical yield.

According to this invention, this is achieved with a bleaching composition showing the technical features of the charactering portion of the first claim.

To this end, the bleaching composition of this invention is characterised in that the TiO₂ present is doped with S and the TiO₂ particles have an average particle size of < 100 nm.

Doping of this type of TiO₂ particles with S involves that the absorption peak of TiO₂, with is a semi-conductor, for the exciting of an electron from the 3p condition to the valence band is shifted to a lower energy level. In particular, for TiO₂ doped with S an overlap of the S 3p state with the valence band has been observed. The inventors have now observed that this shift depends on the nature of the semi-conductor and is such for S doped TiO₂ that an absorption maximum is present in the wavelength range around 525-545 nm. This offers the advantage that TiO₂ doped with S can be activated for the photocatalytic decomposition of peroxide for the generation of reactive radicals by irradiation with visible light in a wavelength range where besides free oxygen radicals also fee hydroxyl radicals and the extremely reactive perhydroxyl HO₂ radicals are generated. Light sources in the wavelength range 525-545 are commercially available in varying energy densities. The result is that an increased radical yield can be obtained, even with a significantly lower peroxide content. It has namely been found that the peroxide content in the bleaching composition can be reduced to 0.5-20 weight % at minimum risk to adversely affecting the result, and also with a considerably reduced risk of post-bleaching sensitivity and tooth erosion. The fact that dentine is highly permeable for O, OH and HO₂ radicals means that the bleaching effect is not restricted to the tooth surface, but extends to at least part of the dentine. Nano particles of pure TiO₂ show photocatalytic activity when irradiated with UV light with a wavelength below 387 nm, which can be oncogenic. This is why the prior art prompts the use of light sources emitting visible light, particularly blue and violet light with a wavelength of 440-480 nm. The associated low level of radical production was compensated by an increase in the H₂O₂ concentration. However, almost no perhydroxyl radicals are generated at this wavelength.

Moreover, the bleaching composition of this invention enables use of the fact that light with a wavelength between 525 and 545 nm is able to induce immediate decomposition of discolourations including those associated with tetracycline, even without the intervention of the TiO₂ photocatalyst doped with S. In that case, bleaching is not limited to the surface layer, but extends into the dentine. Irradiation with 525-545 nm light breaks the connection between the tetracycline discolouration and the dentine, after which the perhydroxyl radicals are able to decompose the tetracycline discolouration and stabilise the decomposition products. The bleaching composition of this invention thus offers the possibility to not only bleach relatively superficial light yellow to brown discolouration, but also intrinsic grey discolouration, which uo to now could barely be removed or could only be resolved with the use of aggressive and painful treatments. Because the peroxide concentration can be kept low in the bleaching composition of this invention, the penetration depth of the peroxide into the teeth can be restricted to the outermost layers, which are bleached, regardless of the good permeability of dentine for hydrogen peroxide. This limits the risk to the occurrence of post-bleaching sensitivity and tooth erosion. However, some penetration of the peroxide into the dentine is required, especially in the case of intrinsic discolouration.

The significantly increased radical yield offers the possibility to control the irradiation time and therefore the contact time with the peroxide compound, taking into account the energy density supplied by the light source and the envisaged effect.

Optimum photocatalytic activity is achieved with TiO₂ particles doped with S that have an average particle size smaller than 100 nm, preferably smaller than 50 nm. Particles with a diameter smaller than 50 nm display photocatalytic activity. With larger diameters above 100 nm, the photocatalytic activity decreases rapidly and peroxide decomposition is mainly due to auto-reaction.

The TiO₂ can be doped with atomic or with cationic or anionic S, but are preferably doped with anionic S. Here, S can have a valence of both 4 and 6. The S atoms occupy O sites in the TiO₂, so that Ti-S compounds are formed.

In the TiO₂ preferably a maximum of 50% of the O atoms are replaced by S, with a greater preference for a maximum of 25% and a minimum of 5%.

The concentration of the TiO₂ doped with S in the composition of this invention is preferably not higher than 5 weight %, preferably lower than 2.5, more preferably lower than 1 weight %, so that the composition remains sufficiently transparent for visible light. The concentration of the TiO₂ doped with S in the composition of this invention is preferably not lower than 0.001 weight %, more preferably not lower than 0.01 weight % in order to guarantee a sufficiently high levels of photcatalytic activity.

The TiO₂ present in the composition of this invention can be crystalline or amorphous, but is preferably at least partially crystalline. The TiO₂ can occur as rutile or anatase, but occurs preferably as anatase in view of the fact that the tetragonal crystal lattice displays optimum photocatalytic activity. The TiO₂ present in the composition of this invention can be a mix of anatase and amorphous TiO₂. The shape of the TiO₂ nano particles is not relevant for this invention and can be any shape considered suitable by the skilled person. The shape of the particles is preferably chosen in such a way that the maximum contact area with the bleaching composition is obtained. For example, the particles may be tube-shaped, round, cylindrical or elongated or any other shape regarded as suitable by the expert. The nano particles may also have the shape of vesicles.

The TiO2 used in the composition of this invention can be further doped with F in order to promote the reinforcement and recovery of the teeth.

The composition preferably also contains 0.01-5 weight % of an apatite with an average particle size ≤ 100 nm, preferably ≤ 50 nm, with preference between 30 and 40 nm, whereby the apatite corresponds to the formula: Ca₅(PO₄)₃B_yAz(OH)_1-z,

In which

B is an anion that differs from PO₄ ^3", A is selected from the group of O²- , CO₃ ^2', F and Cl^", - and whereby 0 < y < 5 and 0 < z < 1 , whereby the sum of the cation charges compensates the sum of the anion charges. The apatite is preferably fluorapatite, whereby y = 0, z = land A = F.

The inventors have namely observed that tooth erosion takes place upon contact with the activatable peroxide compound, and particularly when using hydrogen peroxide. It may occur that the enamel and the material from which the tooth is built is damaged to a greater or lesser degree. Apatite that is offered to the enamel and dentine by the composition is apparently suitable for build-in and for ion exchange of F ions to the enamel and dentine, thereby providing re-mineralisation of the enamel and dentine and better resistance to dental caries. It is also possible to at least partially repair cracks and defects in the enamel by administering fluorapatite in particular. The presence of apatite also makes it possible to bleach teeth that had not previously been suitable for bleaching due to the fact that the apatite, and in particular the fluorapatite, is compatible with the tooth enamel and is able to repair defects.

The activatable peroxide compound may be any compound regarded as suitable by the expert. For example they may be chosen from the group of hydrogen peroxide, percarbamide, sodium perborate, potassium peroxomonosulphate, potassium chlorate, potassium carbonate, calcium peroxide, magnesium peroxide, perphosphate, persilicate, perborate, benzoyl peroxide, glycol peroxide, calcium hydrogen carbonate peroxide, sodium hydrogen carbonate peroxide, hydrogen peroxide and potassium peroxomonosulphate. Preferably, the bleaching composition of this invention will contain an adequate amount of hydrogen peroxide in view of the fact that it is able to release the reactive perhydroxyl radical. Peroxide forming oxidantia are preferred due to the fact that they permeate enamel and dentine easily. The concentration of the activatable peroxide compound may vary within wide limits, taking into account the intensity of the discolouration and the envisaged bleaching, and the activity and selectivity of the catalyst for the generation of free radicals containing oxygen. A bleaching composition that contains 0.1-20 weight % of activatable peroxide is usually employed for the bleaching of significantly discoloured teeth. If the discolouration is less intensive this concentration can be reduced to 0.1-10 weight %, even to 0.1-5 weight %. The Increased photocatalytic activity generates more radicals with a minimal risk of peroxide auto-decomposition.

In order to minimise the risk of damaging the enamel during bleaching and bleaching composition will normally be used that is alkaline, with a pH preferably between 8 and 11 , and more preferably 9-10.5, with the most preference for 9-9.5. A base or a mixture of two or more bases is normally used in the composition to control the pH. Examples of suitable bases are sodium carbonate, sodium hydroxide, sodium polysilicate, but other bases known to the skilled artisan can be used. The use of an alkaline pH reduces the risk of demineralisation of the tooth enamel. With an alkaline pH, the conversion of peroxide into hydroxyl and perhydroxyl free radicals and superoxides is promoted. The radical yield can be furthered increased by adding a amount of chromphore in order to improve the absorption of the radiated energy and the distribution of the electromagnetic energy over a larger area. This is especially important when using lasers that produce a high energy density on a small area. Examples of suitable chromophores are Rhodamine

(g), acid Fuchsin, Alixarin red, basic Fuchsine, Carmine, Congo red, Darrow red, methyl orange, natural red, orange, methyl red, chlorophenol red, phenol red, All these compounds are commercially available from Aldrich Chemie. However, the use of Rhodamine B(1) is preferred in view of the fact that this has an absorption leak around 532 nm. Rhodamine is preferably present in a concentration of 0.25-1 weight % in relation to the total weight of the bleaching compound, more preferably 0.5-0.6 weight %. The use of rhodamine offers the advantage that this material discolours from deep red to pink and even completely colourless upon photochemical reaction, which indicates that a photochemical reaction is taking place. When irradiated with light with a wavelength around 532 nm, rhodamine is converted to a triplet activated state, which activates free radical formation from H₂O₂.

The bleaching composition of this invention can contain all usual additives, including a thickener in order to make application as a gel or paste possible.

The composition of this invention may take various physical forms, for example the form of a powder, a gel a suspension or an emulsion or any form regarded as suitable by the expert. The nano particles can be present as individual particles or in the form of clusters. The thickener is preferably present in a amount of 80-

98 weight % in relation to the total weight of the composition, preferably in a amount of 90-98 weight %, more preferably 94-95 weight %. Suitable thickeners include silica, aluminium oxide, mixed silica aluminium oxides, sodium stearate, polymer thickeners, for example Carbopol®, Trolamine® and Polyox®.

This invention also relates to a method for the bleaching of discoloured teeth, whereby the teeth are at least partially covered with a bleaching composition that contains an activatable peroxide compound and a TiO₂ photocatalyst with an average particle size of a maximum of 300 nm for the photocatalytic activation of the decomposition of the peroxide and the release of free radicals that are able to react with the discolouring substances present in the teeth. This method is characterised in that the TiO₂ in the composition is doped with S₁ in that the TiO₂ particles have an average particle size < 100 nm and in that the composition is irradiated with electromagnetic radiation with a wavelength of 532 nm.

The generation of this energy can be carried out using the radiation sources known to the skilled person, such as a KTP laser. Other suitable sources are LED, which offers the advantage that it can irradiate a larger area, whereas lasers deliver concentrated energy to an extremely small area. However, in the context of this invention, the use of a source that delivers energy over a larger area offers advantages because several teeth can be irradiated simultaneously, the total treatment time can be shortened and that the risks to heating the teeth and post-bleaching sensitivity are minimised. An LED source is preferably used in combination with a spreader, so that a large part or the complete mouth can be irradiated.

An example of a suitable LED source is the green 525- 540 nm LED, which produces to an average of 5 Watts output capacity and has an average energy density of just 470 mW per cm2. The Led irradiates a surface area of approximately 10 cm2, whereby every tooth is irradiated simultaneously for 10 minutes. In three treatment sessions lasting ten minutes, which is required to achieve sufficient bleaching, this produces a total flux of 846,000 mJ per tooth. In contrast, using the KTP laser involves irradiating each tooth for 30 seconds. Therefore, for 20 teeth this corresponds with an interaction period with the bleaching composition of 10 minutes. The laser has an average output capacity of 1 Watt, a spot size of approximately 0.5 cm2, which therefore produces an energy density of approximately 2000 mW per cm2. In view of the fact that each tooth is irradiated for 30 seconds three times, this corresponds with a total flux of 180,000 mJ per tooth.

A further preferred form of the method of this invention uses a clamp that can be fastened to the teeth to be treated, whereby the side of the clamp facing the teeth is fitted with one or more LEDs or other sources of electromagnetic radiation. However, all other sources of electromagnetic radiation regarded as suitable by the expert can be used, whereby the aim must preferably be to deliver energy to the teeth in as uniform a manner as possible. Prior to the application of the composition of this invention the teeth are cleaned, debris is removed and the teeth are lightly dried in order to maximise the penetratability of the teeth for the peroxide and the radicals that are generated from it.

Nano particles of titanium dioxide doped with S can be produced in various ways. According to a first possible preparation the nano particles are produced by means of oxidative ageing of titanium sulphide. At a temperature of 600 ⁰C TiS₂ is converted into anatase, whereby a part of the Ti- O sites are occupied by Ti-S. According to another suitable method the titanium dioxide doped with S is by ion implantation followed by ageing. The invention is further explained on the basis of the following examples.

Comparative example A

A powder mix was prepared by mixing 400 mg of colloidal silica with an average particle size of approximately 200 ψn (Federa,

Belgium) with 2.4 mg Rhodamine B and 20 mg sodium carbonate, so that the pH of the mix was approximately 9.5. Approximately 5 ml of a 55 volume % alkaline H₂O₂ was added to the powder mix, after which a gel was obtained of which the H₂O₂ content had decreased by 25% in comparison with the added amount. The mix was stored in a closed container for approximately 5 minutes.

The specified quantities were apparently sufficient for the cleaning of an entire set of teeth.

The teeth to be bleached were thoroughly cleaned and cleared of any debris by means of treatment with a pressurised jet of air, sodium bicarbonate and water. In a different application this was done with a mixture of pumice stone and water.

A jaw retractor was inserted into the mouth of the patient in order to spread the lips and jaws open. The teeth were subjected to an intensive prophylaxis in order to remove all plaque and debris and to optimise the bleaching. In order to avoid the bleaching gel coming into contact with the gums, the gingiva edges and sulcus were covered with a blocking gel based on dimethylacrylate. The teeth were covered partially, 1 mm cervical, with the blocking gel, and also the exposed dentine and those teeth and parts of the teeth that did not require bleaching.

A amount of the bleaching composition was then applied to the teeth with a small brush in the following manner: 11 - 21 , 12 - 22, 13 - 23, 14 - 24, 15 - 25, followed by 41 - 31, 42 - 32, 43 - 33, 44 - 34, 45 - 35. Each tooth was irradiated with 0.6 Watts from a KTP laser at 532 nm for 30 seconds according to the sequence specified above.

The laser energy was reduced if the patient experienced pain. After the gel had been in contact with the teeth for 10 minutes it was sucked away, the teeth were rinsed with water and then dried. The bleaching gel was applied a second time in the following sequence: 21 - 11 , 22 - 12, 23 - 13, 24 - 14, 25 - 15, followed by 31 - 41 , 32- 42, 33 - 43, 34 - 44, 35 - 45, and irradiated with the laser light. The desired bleaching effect was achieved.

After the completion of the treatment the bleaching gel was removed and the teeth were rinsed with a large amount of water. The teeth that had been exposed to the bleaching composition were covered with a fluoride gel in order to strengthen them. The fluoride gel contained 3 weight % sodium fluoride, 0.7 weight % citric acid, 1 weight % sodium monophosphate, 3 weight % hydroxy ethyl cellulose and water. In another example, the gel contained fluorapatite, which is able to facilitate bleaching re-mineralisation.

In view of the fact that the teeth were significantly discoloured, the bleaching effect achieved after the first and second treatment was unsatisfactory, which meant that the treatment had to be repeated a third and a fourth time. During the treatment it was ensured that the total irradiation period of 30 minutes was not exceeded in order to avoid damage to the tooth structure. When the treatment was completed the bleaching gel was removed with water. The teeth that had been exposed to the bleaching comosition were covered with a fluoride gel in order to strengthen them.

Example 1 : Application of the bleaching composition according to the current invention. A powder mix was prepared by mixing 2 mg TiO2 with an average particle size of approximately 50 nm, doped with 10 atom % S in relation to O, and 400 mg colloidal silica with an average particle size of approximately 200 4m (Federa, Belgium) with 2.4 mg of Rhodamine B, and 20 mg sodium carbonate in order to achieve a pH of approximately 9.5.. Approximately 2 ml of a basic H₂O₂ 55 vol. % solution was added to the powder mix.

The teeth that had to be bleached were thoroughly cleaned, any debris was cleared and the bleaching composition was applied as described above. Satisfactory bleaching was achieved after two treatments.

Claims

1. Bleaching composition for the bleaching of discoloured teeth, whereby the bleaching composition comprises an activatable peroxide compound and a TiO₂ photocatalyst with an average particle size of maximum 300 nm for the photocatalytic decomposition of the peroxide and the release of free radicals capable of reacting with the discolouring substances present in the teeth, characterised in that the TiO₂ present in the composition is doped with S and the TiO₂ particles have an average particles size < 100 nm.

2. Bleaching composition according to claim 1 characterised in that the TiO₂ is doped with anionic S.

3. Bleaching composition according to claim 1 or 2 characterised in that a maximum of 50% of the O atoms in the TiO₂ is replaced by S, preferably a maximum of 25%.

4. Bleaching composition according to claim 1 or 2, characterised in that the composition contains 0.01 wt. % of TiO₂ doped with S.

5. Bleaching composition according to any one of claims 1 to 4, characterised in that the average particle size of the particles of TiO₂ doped with S is smaller than 50 nm.

6. Bleaching composition according to any one of claims 1 to 5, characterised in that at least part of the TiO₂ present in the bleaching compound is crystalline anatase TiO₂.

7. Bleaching composition according to any one of claims 1 to 6, characterised in that the composition also contains 0.01-5 weight

% of an apatite with an average particle size ≤ 250 nm, which corresponds to the formula

In which - B is an anion different from PO₄ ^3", A is selected from the group of O^2',

CO₃ ^2', F and Cl^",

- and whereby O < y < 5 and O < z < 1 , whereby the sum of the cationic charges compensates the sum of the anionic charges.

8. Bleaching composition according to claim 7 is characterised in that the apatite is fluorapatite, whereby y = 0, z = 1 and A = F.

9. Bleaching composition according to claim 7 or 8, characterised in that the apatite has an average particle size < 100 nm, in particular smaller than 50 nm, in particular between 30 and 40 nm.

10. Bleaching composition according to any one of claims 1 to 9, characterised in that the activatable peroxide compound is selected from the group of hydrogen peroxide, percarbamide, sodium perborate, potassium peroxomonosulphate, potassium chlorate, potassium percarbonate, sodium percarbonate, calcium peroxide, magnesium peroxide, perphosphate, persilicate, perborate, benzoyl peroxide, sodium hydrogen carbonate peroxide, hydrogen peroxide, percarbamide, sodium perborate and potassium peroxomonosulphate.

11. Bleaching composition according to any one of claims 1 to 11 characterised in that the composition contains 0.1-20 weight %, preferably 0.1-10 weight %, more preferably 0.1-5 weight % of the activatable peroxide compound.

12. Bleaching composition according to any one of claims 1 to 11 , characterised in that the bleaching composition has a pH between 8.5 and 10.5.

13. Method for the bleaching of discoloured teeth, whereby the teeth are at least partially covered with a bleaching composition that contains an activatable peroxide compound and a TiO₂ photocatalyst with an average particle size of maximum 300 nm for the photocatalytic decomposition of the peroxide and the release of free radicals capable of reacting with the discolouring substances present in the teeth, characterised in that the TiO₂ present in the compound is doped with S, in that the TiO₂ particles have an average particle size < 100 nm and that the composition is irradiated with electromagnetic radiation with a wavelength of 525-540 nm.

14. Method according to claim 13 characterised in that use is made of electromagnetic radiation with a wavelength of 532 nm.