Pyro-catalysis for tooth whitening via oral temperature fluctuation

Abstract

Tooth whitening has recently become one of the most popular aesthetic dentistry procedures. Beyond classic hydrogen peroxide-based whitening agents, photo-catalysts and piezo-catalysts have been demonstrated for non-destructive on-demand tooth whitening. However, their usage has been challenged due to the relatively limited physical stimuli of light irradiation and ultrasonic mechanical vibration. To address this challenge, we report here a non-destructive and convenient tooth whitening strategy based on the pyro-catalysis effect, realized via ubiquitous oral motion-induced temperature fluctuations. Degradation of organic dyes via pyro-catalysis is performed under cooling/heating cycling to simulate natural temperature fluctuations associated with intake and speech. Teeth stained by habitual beverages and flavorings can be whitened by the pyroelectric particles-embedded hydrogel under a small surrounding temperature fluctuation. Furthermore, the pyro-catalysis-based tooth whitening procedure exhibits a therapeutic biosafety and sustainability. In view of the exemplary demonstration, the most prevalent oral temperature fluctuation will enable the pyro-catalysis-based tooth whitening strategy to have tremendous potential for practical applications.

Fig. 1. Pyro-catalysis for tooth whitening.

a Consumption of different temperature beverages results in changes in oral temperature. b Infrared imaging of the change in oral temperature after drinking cold and hot water. c–e Schematic diagram of pyro-catalysis principle; heating or cooling leads to an alteration of the polarization strength in pyroelectric materials, which further causes the absorption and release of screening charges and the generation of reactive radicals in water. f The proposed pyro-catalysis effect-based tooth whitening method wherein pyroelectric particles are combined with light-curing hydrogel to generate reactive oxygen species via pyro-catalysis to bleach tooth stains. g-i Stained tooth can be whitened by wearing braces containing pyroelectric particles that use the pyro-catalytic properties activated by changes in oral temperature brought about by daily diet to degrade stains on the surface of the tooth.

Fig. 2. Microstructural and morphology characterization.

a X-ray diffraction pattern of the BTO nanowires. b Room-temperature Raman spectra of the hydrothermal BTO nanowires. c Scanning electron microscope image of BTO nanowires, d Transmission electron microscope, e high-resolution transmission electron microscope images and f selected area electron diffraction patterns of the BTO nanowires, and g–i corresponding EDX element mapping of Ba (red), Ti (blue), and O (yellow) in BTO nanowires. PFM results of BTO nanowires k topography image; l vertical amplitude image; m vertical phase image and n piezoelectric hysteresis loop. Scale bar: c is 10 μm, d is 1 μm, e is 5 nm, f is 5 1/nm, g, h, i and j are 1 μm, k, l and m are 500 nm. The experiments in c–n were repeated independently for three times with similar results. Source data are provided as a Source Data file.

Fig. 3. Degradation properties of pyro-catalysis.

UV-Vis absorption spectra of Indigo Carmine solutions with respect to temperature fluctuations a–f ΔT = −10, −5, +5, +10, +15, +20 °C. g the pseudo-first-order reaction kinetics of different temperature fluctuations. Electron paramagnetic resonance spectra (EPR) of radical h •OH and i •O₂^- created by pyro-catalysis over different temperature range. Source data are provided as a Source Data file.

Fig. 4. Demonstration of tooth whitening based on pyro-catalysis effect.

Photographs of teeth under treatment in turbid liquid of BTO nanowires with different temperature fluctuations a-c ΔT = −10, +10, +25 °C, respectively. d Photographs of teeth under treatment in pure water with a temperature fluctuation of +25 °C. These photographs are successive images of the same tooth. Comparison of different temperature fluctuation on the tooth whitening levels demonstrated by CIELab results e luminance L, f color value of red–green axis a, g color value of blue-yellow axis b and h color difference ΔE (+25* means without BTO nanowires). Scale bar is 1 cm. Data are presented as mean values ± SD (n = 5). Source data are provided as a Source Data file.

Fig. 5. Degradation properties of BTO-Gel.

a UV-Vis absorption spectra of Indigo Carmine solutions using BTO-Gel with a temperature fluctuation of +5 °C. b Cyclic stability of BTO-Gel degraded indigo solution. c–h Electron paramagnetic resonance spectra (EPR) of radical created by pyro-catalysis over different temperature range, different cycling times and the stability of radical creation. i The hydrogel shows excellent fluidity before curing even can be ejected from the syringe. Photographs of the stained tooth at original state, coated by BTO-gel and whited by BTO-gel at j slow and k fast heating rate. Slow rate designates heating-cooling time is 5 min, and fast designates heating-cooling time is 5 s. Scale bars are 1 cm. Source data are provided as a Source Data file.

Fig. 6. Non-destructive characterization.

Scanning electron micrographs of the same area of tooth enamel a before whitening, b after whitening with BTO gel, and c after further whitening with commercial peroxide gel. d The Vickers microhardness of five points on the enamel of the same tooth in different states (n = 3) and e the comparison of the average microhardness of the enamel in different states (n = 5). Scale bar: c are 100 μm (top) and 50 μm (bottom), d is 1 cm. The experiments in a–c were repeated independently for three times with similar results. Data are presented as mean values ± SD. Source data are provided as a Source Data file.

Fig. 7. Cytotoxicity characterization.

a–d The fluorescence microscope images of L-929 cells exposed to BTO nanowires with different concentrations for 1,2 and 3 days respectively. Dead cells appear red, while living cells appear green. e-g The viability of L-929 cells exposed to BTO nanowires with different concentrations for 1, 2 and 3 days measured by CCK-8 assay. Scale bar is 150 μm. The experiments in a-d were repeated independently for three times with similar results. Data are presented as mean values ± SD (n = 6). Source data are provided as a Source Data file.