Bleaching of stained resin using nitrogen doped-TiO₂ nanoparticles

Introduction

Due to the high expectation for clean and white teeth regardless of gender and generation, there have been many trials from Roman era using urine to recent using peroxide-based agents. Thus far, to satisfy diverse needs of the users, many tooth bleaching agents, mainly peroxide-based, have been introduced and the resultant bleaching was mainly affected by the strength and treated time of agent regardless of products (1-3). To satisfy diverse needs of the users, many different bleaching agents are available now from OTC (over-the-counter) to specialist products.

There are many complicated causes of teeth discoloration, and they can be categorized into two factors: intrinsic and extrinsic (4-6). Of the factors, aging, pigments or metal ions in colored foods or drinks, and smoking are the prevalent sources of teeth discoloration. Generally colored compounds from these sources are adsorbed onto the tooth surface or absorbed into the subsurface and then present stain. Compounds adsorbed onto the tooth surface can be readily removed by routine tooth brushing. Abrasives in tooth paste easily detach the adsorbed compounds. However, stains by the absorbed compounds are not readily removed by tooth brushing if tooth is not fully wear out.

Bleaching (whitening) agents are the substance that makes the stained (discolored) teeth to be white. Also, they affect the color (shade) of resin composites (7, 8). Among them, peroxide-based bleaching agents (carbamide peroxide [CH₆N₂O₃] or hydrogen peroxide [H₂O₂]) at the concentration of 3-3.5% or over to 30-35% are the common options. Bleaching by peroxide is achieved through the formation of reactive free radicals. Stable peroxide becomes radicals by external heat or ultraviolet light (9,10). Free radicals penetrate into the tooth subsurface and decompose stains of long molecular size into short size. During the bleaching process by radicals, mineral and water loss occurs and then dehydrated tooth with shortened stains gradually looks white (11, 12).

Despite excellent bleaching capability of peroxide-based agents, it usually takes some weeks to get acceptable whitening if used with lower concentration. Higher concentration is good for reducing treatment time (less than one hour with 2-3 treatments), however, higher concentration can make sharp poking pains on teeth and some loss of minerals and water from the treated teeth (13, 14). Both in user and practitioner sides, short treatment time and less teeth damage with acceptable whiteness of teeth would be highly welcomed.

Titanium dioxide (TiO₂) induces photocatalytic reaction if interact with ultraviolet light (UV) (15-17). UV makes electrons in valence band transit to conduction band by leaving holes. Formed holes and moved electrons induce redox process and produce hydroxyl radicals. Formed radicals can effectively bleach stains in materials as oxygen-rich peroxide does. However, since UV induces carcinogenic problems, using in oral cavity is not recommended. To handle the problem, doped-titanium dioxide with gas or metal ions was suggested. According to diverse studies, doped-titanium dioxide showed high absorption of visible light and effective treatment of diverse bacteria and pollutants (15-17). The purpose of the present study was to evaluate if nitrogen (N)-doped TiO₂ nanoparticles (NPs) can bleach stains under laser irradiation as H₂O₂ does. In this study, composite resin was used as a test material prior to teeth for a preliminary test.

Materials and Methods

Results

Figure 1 shows the tested powders for this study. TiN was used as a precursor for doping TiO₂. Unlike white TiO₂, N-TiO₂ has dark brown color similar to its precursor TiN.

Figure 1.

The tested powders for this study.

Figure 2 shows the absorbance spectrum of TiN, TiO₂, N-TiO₂. Unlike TiO₂, doped N-TiO₂ showed high absorbance after 400 nm.

Figure 2.

Absorbance spectrum of the tested powders.

Figure 3 shows the XRD patterns of the tested samples. N-doped TiO₂ had well-formed crystalline nature. Over the TiN peaks, the Bragg diffraction peaks indexed as (110), (101), (111), (211), (220), (002), (311), (301), and (112) are correspond to tetragonal rutile TiO₂ (JCPDS, no. 21-1276).

Figure 3.

XRD patterns of the tested powders.

Figure 4 and Table 1 show the color difference (ΔE*) of stained resin by laser irradiation for 3 h (180 min). Initial color values (L*, a*, b*) of specimen resins before stain were 60.75±0.21, -0.85±0.07, 6.87±0.21, respectively. After stain for 1 week, these values changed to 49.34±0.76, 1.63±0.26, 14.33±0.55, respectively. For bleaching test, color values of the stained resin became the initial values before treatments. During laser irradiation, stained resins showed a linear color difference (R>0.94) regardless of incident light wavelength. Among the conditions, N-TiO₂+3% H₂O₂+405 nm laser showed the greatest (up to 5.0) and significant (p<0.001) color difference (bleaching). Also, laser irradiated specimens showed higher color difference compared to that treated only with H₂O₂.

Table 1.

Different carbomer gel conditions used in the present study

Figure 4.

Color difference (ΔE*) of the stained resins treated with different treatment conditions for 3 h.

Discussion

Everybody wishes to have clean and white teeth. However, achieving and maintaining this wish is not easy owing to many daily obstacles and life styles. Tooth bleaching agents are materials that make this wish to be achieved easily and quickly with various effective dental treatments. However, peroxide (H₂O₂)-based bleaching agents usually confront two issues depending on the concentration of H₂O₂: long treatment time issue for low concentration agent; safety and oral damage issue for high concentration agent. Concerning teeth whitening, everybody wishes the bleaching to be achieved within short treatment time with minor soft tissue and teeth damage. The present study evaluated how N-doped TiO₂ NPs affects bleaching (whitening) if they are irradiated with visible light.

The basic mechanism that explains bleaching (whitening) of stained material is cleavage of long molecular chain that causes discoloration into short molecular chain by reactive oxygen radicals (4,6). Optically short molecular chain becomes colorless. Peroxide-based bleaching agents are the source of radicals. To break H₂O₂ for forming radicals, one of two different energies are needed: heat or light. Heat breaks molecular bond of H₂O₂ which has weak O-O bond. Heat can boost radical formations, but it simultaneously produces unpleasant feeling on teeth and can cause pulp damage if temperature rises by heat over 5℃ for long time. UV light can break H₂O₂ because photon energy of UV exceeds the molecular binding energy of H₂O₂. However, owing to high photon energy, it can generate carcinogenetic damage on tissue. Hydrogen peroxide can also be broken down via photocatalytic effect of TiO₂ with an aid of light. To initiate photocatalytic reaction from TiO₂, at least 3.2 eV or over photon energy is required to make electrons in valence band to transit to conduction band by leaving holes in valence band. To supply this amount photon energy, UV light is required and in that case biohazard issue is inevitable.

Doping (gas or metal) ions into TiO₂ as an impurity can lower photocatalytic activation energy from UV to visible light owing to narrowed energy band gap (18-20). One advantage using visible light as a light source is that it is free from biohazard issue if it is not operated at high power. In the present study, N-TiO₂ has much higher absorbance than TiO₂ after 400 nm (Figure 2). Compared to its precursor TiN, N-TiO₂ does not have decreasing absorbance after 400 nm and this means that any other visible light can be absorbed in N-TiO₂ NPs. For the present study, lasers of two different wavelengths have used: 405 and 660 nm with same light intensity.

Composite resins were immersed in coffee for 1 week to uniformly stain resin (21, 22). The stained resins have a coffee-adsorbed layer on their surface. To bleach the stained resins, carbomer gel which containing 1 wt% NPs without or with 3% H₂O₂ was pasted on the resin surface and then treated laser for 3 h (180 min). In the present study, as treated time increased, color difference by bleaching linearly increased. 405 and 660 nm laser treated specimens showed 1.36-1.87 and 1.12-1.15 times, respectively, higher color change (bleaching) compared to those treated only with 3 or 15% H₂O₂ after 3 h. In the laser treated specimens, addition of 3% H₂O₂ enhanced color change to 1.22-1.68 and 1.00-1.38 times, respectively, compared to those of no H₂O₂ specimens. The results here indicate that visible lasers (light) surely bleaching stain in conjunction with N-doped NPs, and in two lasers, shorter wavelength yields higher color difference (bleaching) than that by longer wavelength. Actually, from the relation E (photon energy)=hν=hc/λ, where h and λ are Planck’s constant and wavelength of the irradiated light, respectively, light of shorter wavelength is better than that of longer wavelength owing to higher photon energy of the light of shorter wavelength.

Conclusion

Within the limitations of the present study, the following conclusions could be reached:

1. N-TiO₂ NPs showed the greatest color difference (ΔE*: up to 5.0) if combined with 3% H₂O₂ and 405 nm laser irradiation for 3 h. However, 15% H₂O₂ case showed only 2.7.
2. Two lasers (405 and 660 nm) achieved higher color difference compared to that by 15% H₂O₂. Between two lasers, 405 nm laser achieved higher bleaching (color difference) than that by 660 nm laser.

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