The escalating demand for aesthetic dentistry, particularly tooth whitening, has spurred the development of various techniques. Current methods, such as hydrogen peroxide bleaching, while effective in removing stains, often lead to undesirable side effects including enamel demineralization, gingival irritation, and cytotoxicity. Furthermore, these methods can be time-consuming and expensive. Professional cleaning and procedures like veneers involve irreversible enamel damage and high costs. Abrasive toothpastes provide a less aggressive but less effective option, relying solely on mechanical friction for stain removal. Photocatalysis, using blue-light-activated TiO₂ nanoparticles, offers a non-destructive alternative, but its reliance on external light sources and potential for phototoxicity limits its practicality. This study introduces a novel approach leveraging piezo-catalysis, a process where mechanical stress on piezoelectric materials generates reactive oxygen species, offering a potentially safer, more convenient, and effective tooth whitening method. The piezoelectric effect, discovered by the Curie brothers, is the generation of an electric charge in response to mechanical stress. Piezoelectric materials, even when subjected to subtle vibrations, can produce electrical charges that, in an electrolytic environment, facilitate the generation of reactive oxygen species like ·OH and ·O₂⁻. These reactive species can oxidize organic molecules, including those responsible for tooth staining, resulting in their degradation and consequent tooth whitening. This research proposes a nondestructive tooth whitening strategy by incorporating piezoelectric nanoparticles into toothpaste, harnessing the mechanical vibrations generated during regular brushing to initiate the piezo-catalytic process. This method eliminates the need for specialized equipment or lengthy procedures, making it a potentially transformative advance in dental aesthetics.

Existing tooth whitening methods have limitations. Hydrogen peroxide-based bleaching, while effective, is associated with enamel demineralization, increased surface roughness, and potential cytotoxicity (Tredwin et al., 2006; Markovic et al., 2007; Abouassi et al., 2011). Abrasive toothpastes, though safer, lack efficacy due to their reliance on mechanical action (Lippert et al., 2017; Soeteman et al., 2018). Professional procedures like cleaning and veneers are expensive and cause irreversible enamel damage (Kimyai et al., 2017). Recent research demonstrated the efficacy of photocatalysis using blue light-activated TiO₂ nanoparticles (Zhang et al., 2018), but this approach suffers from potential phototoxicity and requires specialized equipment (Maran et al., 2018; Yoshino & Yoshida, 2018). The piezo-catalytic approach, converting mechanical energy into electrical energy for the generation of reactive oxygen species, offers an attractive alternative that addresses the shortcomings of existing methods. Previous studies have shown that certain piezoelectric materials, such as ZnO, BaTiO₃, and BiFeO₃, exhibit promising piezo-catalytic properties (Hong et al., 2016; Lin et al., 2018; Lan et al., 2019; Mushtaq et al., 2018). This study builds upon this foundation, focusing on the application of piezo-catalysis for a safe and effective tooth whitening process.

The study employed a hydrothermal method to synthesize BaTiO₃ (BTO) nanoparticles, a classical ferroelectric material. The synthesized nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and piezoresponse force microscopy (PFM) to confirm their tetragonal perovskite structure, morphology, and piezoelectric properties. Corona poling was used to enhance the piezoelectric response of the BTO nanoparticles. The piezo-catalytic activity of the poled and unpoled BTO nanoparticles was evaluated using Indigo Carmine and Rhodamine B (RhB) as model organic dyes. Degradation experiments were conducted under ultrasonic vibration to simulate the mechanical stress of tooth brushing. UV-Vis spectroscopy was used to monitor the degradation of the dyes over time. Electron paramagnetic resonance (EPR) spectroscopy, using DMPO as a spin trap, was employed to confirm the generation of reactive oxygen species (·OH and ·O₂⁻) during the piezo-catalytic process. In vitro tooth whitening experiments were performed using teeth stained with black tea, blueberry juice, wine, and vinegar. The teeth were exposed to poled BTO nanoparticle suspensions under ultrasonic vibration. Color changes were assessed using the Commission Internationale De L'Eclairage (CIELab) system, quantitatively evaluating the whitening effect. Scanning electron microscopy (SEM) was used to examine the enamel surface before and after treatment to assess the potential for damage. Vickers microhardness testing was conducted to evaluate the effect of the whitening process on enamel hardness. The biocompatibility of the BTO nanoparticles was assessed using a cytotoxicity assay (MTT) with rat arterial smooth muscle cells (A7r5) to ensure the safety of the proposed method. The potential release of Ba²⁺ ions during the process was also examined. Comparative tests were conducted using unpoled BTO, cubic BTO (lacking piezoelectric properties), and high-performance PMN-PT single crystal particles to confirm the role of piezoelectricity in the whitening process. Finally, an electric toothbrush setup was used to simulate a more realistic tooth brushing scenario, comparing the whitening efficacy to that achieved with an ultrasonic bath.

The synthesized BTO nanoparticles exhibited a tetragonal perovskite structure with an average size of ~130 nm. Corona poling significantly enhanced their piezoelectric properties. Ultrasonic vibration of poled BTO nanoparticles in a dye solution resulted in a substantial degradation of both Indigo Carmine and RhB, while unpoled BTO showed negligible effect, confirming the piezo-catalytic nature of the process. EPR spectroscopy confirmed the generation of reactive oxygen species (·OH and ·O₂⁻) during the vibration of poled BTO nanoparticles. In vitro tooth whitening experiments demonstrated significant whitening of teeth stained with various agents after 3 hours of ultrasonic vibration in a poled BTO suspension, with complete whitening achieved within 10 hours. SEM images showed no significant enamel damage after the piezo-catalytic treatment, in contrast to the damage observed after treatment with hydrogen peroxide. Vickers microhardness testing revealed no change in enamel hardness during the staining and whitening process. Cytotoxicity assays showed that the BTO nanoparticles were biocompatible and non-cytotoxic to A7r5 cells, unlike hydrogen peroxide. The experiment using an electric toothbrush also showed a whitening effect although less pronounced than with ultrasonic vibration. Cubic BTO nanoparticles, lacking piezoelectric properties, did not show any significant whitening effect, further confirming the role of piezoelectricity in the observed effects. High-performance PMN-PT single crystal particles also demonstrated a strong piezo-catalytic effect with more efficient tooth whitening.

The findings strongly support the efficacy and safety of the proposed piezo-catalytic tooth whitening strategy. The significant degradation of organic dyes in the presence of poled BTO nanoparticles under ultrasonic vibration, along with the absence of enamel damage and cytotoxicity, highlights the advantages of this approach compared to traditional methods. The generation of reactive oxygen species, confirmed by EPR, explains the whitening mechanism. The superior performance of poled BTO compared to unpoled BTO and cubic BTO demonstrates the critical role of piezoelectricity in the process. The successful demonstration of whitening using an electric toothbrush indicates the potential for practical application in daily oral hygiene. This research opens a new avenue for developing safe and effective tooth whitening agents, potentially revolutionizing the field of aesthetic dentistry. Future research can focus on optimizing nanoparticle size and composition, exploring other piezoelectric materials, and conducting in vivo studies to validate the findings.

This study successfully demonstrates a novel tooth whitening strategy utilizing the piezo-catalytic effect of BTO nanoparticles. The method is shown to be effective, safe, and convenient, offering a promising alternative to traditional hydrogen peroxide-based approaches. Future research should focus on long-term in vivo studies, exploring different piezoelectric materials and optimizing the formulation for commercial applications.

The study primarily utilizes in vitro experiments. Further in vivo studies are needed to confirm the long-term efficacy and safety of the piezo-catalytic tooth whitening method. The current study focuses on a specific type of piezoelectric material (BTO); further investigation into other piezoelectric materials is warranted to explore potential improvements in efficacy and biocompatibility. While the electric toothbrush experiment provides a more realistic setting, the whitening effect was less pronounced compared to the ultrasonic bath, suggesting further optimization might be needed for complete integration into electric toothbrushes.