Microscopic study of hydroxyapatite dissolution as affected by fluoride ions.

Area of Science:

• Materials Science
• Surface Chemistry
• Biomineralization

Background:

• Tooth decay is a prevalent global health issue.
• Fluoride ions are known for their cariostatic (tooth decay-preventing) properties.
• The precise mechanism of fluoride's protective effect at the molecular level on tooth enamel (hydroxyapatite) requires further elucidation.

Purpose of the Study:

• To investigate the microscopic effects of fluoride ions on hydroxyapatite (100) surface dissolution.
• To understand how fluoride concentration influences the dissolution kinetics and morphology of hydroxyapatite surfaces.
• To correlate observed morphological changes with the cariostatic mechanisms of fluoride.

Main Methods:

• In situ atomic force microscopy (AFM) was employed to observe hydroxyapatite (100) surface dissolution in real-time.
• Experiments were conducted under varying sodium fluoride (NaF) concentrations, including NaF-free, 10 mM, 50 mM, and tap water concentration (10 μM).
• Surface step retraction velocities and etch pit morphologies were analyzed.

Main Results:

• Fluoride ions significantly decreased surface step retraction velocities by approximately a factor of 5.
• Elongated hexagonal etch pits, typical of hydroxyapatite (100) dissolution, were absent in the presence of NaF.
• Altered etch pit shapes (triangular) were observed at higher NaF concentrations (50 mM), and step roughening leading to scalloped morphologies occurred at fluoride concentrations typical of tap water (10 μM).

Conclusions:

• Fluoride ions strongly interact with molecular steps on the hydroxyapatite surface.
• These interactions inhibit dissolution and alter surface morphology, providing a microscopic basis for fluoride's cariostatic effects.
• The study highlights the importance of fluoride's interaction with step edges in preventing tooth decay.