Synthesis and characterization of different nano-hydroxyapatites and their impact on dental enamel following topical application for dental bleaching

Objective: This study aims to synthesize, characterize, and assess the penetration of hydrogen peroxide (HP), color change (CC), and surface morphology changes after the application of two distinct nano-hydroxyapatite (nano-HAp).

Methods: Two nano-HAp were previously synthesized by co-precipitation: one with rod-shaped particles (RS) and the other with spherical-shaped particles (SS). The surface charge of the nano-HAp particles was determined at varying pH levels and characterized by X-ray diffraction patterns and Fourier transform infrared spectroscopy. The morphology of the samples was assessed using scanning electron microscopy (SEM). The nano-HAp particles were applied before the dental bleaching procedure. Forty teeth were divided into four groups (n = 10) according to the bleaching treatment: no treatment, bleaching with 35 % HP only, RS application and bleaching with 35 % HP, and SS application and bleaching with 35 % HP. HP concentration (µg mL^-1) was measured using UV-Vis, while CC was evaluated with a digital spectrophotometer (ΔE_ab, ΔE₀₀ and WI_D). Additionally, four teeth from each group were selected for SEM analysis. Statistical analysis encompassed one-way ANOVA, Tukey's, and Dunnet's tests.

Results: RS and SS were successfully synthesized by coprecipitation, primarily differing in pH during synthesis. Both variations of nano-HAp morphology significantly reduced HP diffusion into the pulp chamber (p < 0.001). Regarding enamel morphology, groups analyzed post dental bleaching exhibited greater HAp deposition on the enamel surface. Notably, this deposition did not impede CC.

Significance: The utilization of different nano-HAp morphologies prior to dental bleaching appears to be a promising strategy for mitigating adverse effects associated with dental bleaching procedures.