Piezo-catalysis for nondestructive tooth whitening

Tooth discoloration from habits and diet has driven demand for whitening, but current methods have drawbacks. Professional cleaning and coverings can irreversibly damage enamel and are time-consuming and costly; abrasive toothpastes rely on mechanical friction with limited efficacy and can scratch enamel; hydrogen peroxide bleaching is effective but may increase enamel micro-roughness, cause loss of organic matrix, gingival irritation, and cytotoxicity. Photocatalytic approaches (e.g., blue-light-activated TiO₂) can whiten without peroxide but require external light sources and pose risks of photo-toxicity and photo-allergy. The authors hypothesize that mechanically stimulated piezoelectric nanoparticles can generate reactive oxygen species via piezo-catalysis during routine toothbrushing, enabling effective, non-destructive, and convenient tooth whitening without added equipment or time. They propose replacing traditional toothpaste abrasives with piezoelectric particles (e.g., BaTiO₃) to exploit everyday mechanical vibration to drive catalysis.

Prior work documents adverse effects of peroxide-based bleaching on enamel microstructure and biocompatibility, and the limited efficacy and abrasive wear from traditional dentifrices. Blue-light-activated TiO₂ nanoparticles have been shown to whiten teeth effectively and non-destructively, but the need for blue light introduces practicality and safety issues (photo-toxic and photo-allergic reactions) and requires specialized equipment. Piezoelectric materials, which generate surface charges under mechanical stress, have been widely used in sensors, actuators, and energy harvesting, and several (ZnO, BaTiO₃, BiFeO₃) exhibit piezo-catalytic activity for degrading organic compounds. Mechanical vibration from brushing could therefore be harnessed to generate reactive oxygen species (·OH, ·O₂⁻) analogous to photocatalysis, providing a potential route to safe and convenient whitening.

• Materials synthesis: Ferroelectric tetragonal BaTiO₃ (BTO) nanoparticles were synthesized hydrothermally. Ti precursor was hydrolyzed, mixed with Ba(OH)₂·8H₂O at Ba:Ti = 3:1, adjusted to pH 12 (6 M KOH), and reacted at 180 °C for 48 h. Products were washed (acetic acid, ethanol, deionized water) and dried. Commercial cubic BTO nanoparticles were used as non-piezoelectric controls.
• Poling: As-prepared BTO nanoparticles were poled via a lab-made corona-poling setup (non-contact), aligning ferroelectric domains to enhance piezoelectric response. Unpoled tetragonal and cubic BTO served as controls. For comparison, [001]-oriented PMN-PT single crystals (high d₃₃ ~2930 pC N⁻¹) were ball-milled to powders and tested in poled and unpoled states.
• Structural and piezoelectric characterization: XRD confirmed perovskite tetragonal phase with doublet splitting near (002)c. SEM and TEM/HRTEM assessed morphology (~100–150 nm) and crystallinity; SAED indexed (110) planes (d ≈ 2.828 Å). PFM amplitude/phase imaging and hysteresis loops evidenced robust piezoelectricity and ferroelectric switching in nanoparticles.
• Piezo-catalysis dye degradation tests: Poled or unpoled BTO (50 mg) dispersed in 50 mL of 10 mg L⁻¹ Indigo Carmine or Rhodamine B (RhB); mixtures were stirred 30 min to reach adsorption equilibrium, then subjected to ultrasonic vibration (80 W, 40 kHz) to simulate brushing. Aliquots were centrifuged and dye concentration quantified by UV-Vis. Kinetics were fitted to a pseudo-first-order model to extract rate constants (k). PMN-PT powders were tested similarly.
• Reactive species detection: EPR with DMPO spin trapping was used to detect DMPO-·OH in water and DMPO-O₂⁻ in DMSO under vibration at 0, 1, 5 min. A Fenton reaction provided an ·OH reference with 3% H₂O₂.
• Tooth whitening experiments: Extracted, healthy human teeth were stained by immersion in black tea, blueberry juice, wine, or mixtures for 1 week, rinsed, and dried. Teeth were vibrated in 50 mL deionized water, poled BTO suspension (1 mg mL⁻¹), or unpoled BTO (1 mg mL⁻¹) for up to 10 h (ultrasonic). Color was quantified using the CIELab system (L, a, b) and ΔE = sqrt(ΔL²+Δa²+Δb²). Additional tests used separate stains (tea, wine, blueberry, vinegar), continuous vs discontinuous vibration, and PMN-PT powders. A lab-made electric toothbrush setup brushed teeth periodically (2 min intervals over 10 h) in water or BTO turbid liquid to simulate daily brushing.
• Nondestructive assessment: SEM imaged identical enamel regions before treatment, after 3 h and 10 h in BTO suspension, and after subsequent exposure to 30% or 3% H₂O₂ to compare enamel effects. Vickers microhardness (HV) was measured on teeth in original, stained, and whitened states.
• Biocompatibility and safety: A7r5 rat arterial smooth muscle cells were cultured with BTO suspension (1 mg mL⁻¹), 15% H₂O₂, or control. AO/EB staining and MTT assays (O.D. at 490 nm) assessed viability over 3 days. Potential Ba²⁺ ion release during vibration was measured (30 min exposure).

• Dye degradation efficacy: Under ultrasonic vibration, poled BTO degraded >90% Indigo Carmine within 35 min, while unpoled BTO showed negligible degradation. Pseudo-first-order rate constants: poled BTO k = 0.059 min⁻¹ vs unpoled BTO k = 0.002 min⁻¹ (~30× higher). For RhB, poled BTO k = 0.448 h⁻¹ vs unpoled k = 0.062 h⁻¹ (~8× higher). Poled PMN-PT exhibited ultrahigh activity: k ≈ 0.036 min⁻¹ (2.16 h⁻¹), completely degrading RhB in 105 min; unpoled PMN-PT k ≈ 0.003 min⁻¹ (0.18 h⁻¹).
• Reactive species generation: EPR detected DMPO-·OH and DMPO-O₂⁻ signals that increased with vibration time and scaled with piezoelectricity (poled PMN-PT > poled BTO > unpoled BTO > cubic BTO), confirming ·OH and ·O₂⁻ as active species.
• Tooth whitening performance: Stained teeth whitened visibly with poled BTO suspension; CIELab metrics showed increased L and decreased a and b over time. Color difference ΔE increased with time and was roughly three times larger for BTO suspension than deionized water under identical vibration; differences were significant (t-test, p < 0.01). Teeth showed notable whitening after ~3 h and were completely whitened by ~10 h continuous ultrasonic vibration across various stains. PMN-PT powders achieved more rapid whitening (~1.5 h pronounced effect). Continuous vibration provided stronger whitening than discontinuous protocols of equal total duration.
• Electric toothbrush simulation: Brushing with BTO turbid liquid induced whitening compared to water, though less pronounced than ultrasonic baths due to lower local nanoparticle concentration and weaker vibration amplitude; cumulative daily use is expected to build effect.
• Nondestructive to enamel: SEM of identical enamel regions showed no mechanical damage after 3–10 h in BTO suspension, whereas subsequent 30% and even 3% H₂O₂ treatments produced evident surface corrosion, pitting, and holes. Vickers microhardness remained ~300 HV and unchanged across original, stained, and whitened states.
• Biocompatibility and safety: A7r5 cells exhibited normal morphology and viability with BTO nanoparticles (no cytotoxicity by MTT), whereas 15% H₂O₂ was highly cytotoxic. No detectable Ba²⁺ release was observed after 30 min vibration, exceeding typical daily brushing exposure.

The results validate the central hypothesis that mechanically excited piezoelectric nanoparticles generate sufficient surface charges to produce reactive oxygen species (·OH, ·O₂⁻) that oxidize chromogenic compounds on teeth, enabling whitening without abrasive wear or peroxide-induced damage. The strong dependence of degradation and whitening rates on poling and on intrinsic piezoelectric coefficients (poled > unpoled; PMN-PT > BTO; cubic BTO ineffective) ties the effect directly to piezoelectricity-driven charge release. EPR corroborates the mechanistic pathway by detecting the required radicals under vibration. Quantitative CIELab analyses confirm significant color improvement (ΔE) with BTO relative to controls, and SEM/microhardness data demonstrate that the reactive species levels produced under these conditions are insufficient to damage enamel, contrasting sharply with peroxide treatments. The electric toothbrush tests, while less intense than ultrasonic excitation, indicate practical feasibility during routine brushing; cumulative daily vibration could provide gradual, safe whitening at home. Overall, piezo-catalysis offers a non-destructive, biocompatible alternative to conventional bleaching or abrasive methods, with potential for integration into consumer dentifrices.

This study introduces a nondestructive, biocompatible tooth whitening strategy based on piezo-catalysis using poled BaTiO₃ nanoparticles that generate reactive oxygen species under mechanical stimulation. BTO nanoparticles were synthesized and poled to enhance piezoelectricity, verified structurally and functionally. They efficiently degraded model dyes and whitened teeth stained by common agents, achieving notable whitening in ~3 h and complete whitening by ~10 h under ultrasonic vibration, with performance scaling with piezoelectric strength (PMN-PT superior). The approach preserved enamel microstructure and hardness and exhibited no cytotoxicity, in contrast to peroxide-based agents. Whitening was also demonstrated using an electric toothbrush, supporting real-world applicability. Future work could optimize nanoparticle formulations, concentrations, and poling methods for consumer products; assess long-term oral safety, microbiome impacts, and clinical efficacy in vivo; and explore higher-performance lead-free piezoelectric materials and toothbrush excitation parameters to accelerate outcomes.

• The whitening demonstrations were in vitro on extracted teeth; in vivo clinical performance, long-term safety, and user variability remain to be established.
• Ultrasonic baths provided strong excitation leading to faster whitening than achievable with typical consumer electric toothbrushes; practical at-home whitening rates may be slower and depend on brushing habits and formulation retention on enamel.
• Whitening of deeper root stains required longer durations (up to ~10 h cumulative vibration), indicating time dependence for heavily stained regions.
• While no Ba²⁺ release was detected within 30 min and BTO appeared biocompatible in cell assays, comprehensive assessments of long-term exposure, oral tissue responses, and nanoparticle clearance are needed.
• The study focused on BTO and PMN-PT; broader evaluation of alternative, lead-free high-piezoelectric materials and their stability in oral environments is warranted.