¹⁹F-NMR Study of Fluoride Reaction of Various Formulations on Enamel

Objectives: Fluoride plays a role in strengthening of tooth structure and inhibition of tooth demineralization. To investigate the action of fluoride and its reaction with enamel, we demonstrated ¹⁹F nuclear magnetic resonance (NMR) and ¹⁹F solid-state (SS) magic-angle spinning (MAS) NMR. The fluoride release capacity of CaF₂, which is considered to be one of the byproducts of fluoride on enamel, was investigated by ¹⁹F NMR.
Methods: Following fluoride-based formulations were used:
i) 2% neutral sodium fluoride (N-NaF)
ii) 2% acidulated Sodium Fluoride (A-NaF)
iii) 38% Silver diammine Ffluoride (SDF)
iv) Surface pre-reacted glass ionomer (S-PRG) filler eluate: (S-PRG eluate).
The ion concentrations in this S-PRG eluate were 0.68 mM Al³⁺, 126.6 mM BO₃^3-, 20.8 mM Na⁺, 0.3 mM SiO₂^3-, 1.9 mM Sr²⁺, and 6.7 mM F^-(127 ppm). N-NaF, A-NaF, and SDF were diluted to 1900 ppm F, and S-PRG was used without dilution for ¹⁹F NMR measurement. Each formulation was measured by ¹⁹F NMR spectroscopy to examine the fluoride signal. Bovine enamel powder was treated by each fluoride formulation and then dehydrated for 19F SS-MAS NMR measurement. The fluoride release of CaF₂ in acidic, neutral, and alkaline conditions was evaluated by ¹⁹F NMR.
Results: ¹⁹F NMR spectra showed mainly a peak of aqueous F- from all formulations. In contrast to N-NaF, an additional peak was detected in A-NaF. SS-MAS NMR and spectral deconvolution revealed the presence of two main peaks, fluorapatite and CaF₂, from all treated enamels. When compared between N-NaF and A-NaF, a more pronounced signal intensity was detected for A-NaF than N-NaF. The ¹⁹F spectra were different among each condition, but there was no apparent difference in the integral ratios of the spectra that indicated the presence ratio (molar ratio) of fluorine.
Conclusions: Fluoride reactions formed fluorapatite and CaF₂. In particular, A-NaF is acidic and thus dissolves enamel surfaces and tends to produce byproducts. CaF₂ may release a trace amount of fluoride over a long period of time, under acidic, neutral, and alkaline conditions.