Solid-state NMR Study on S-PRG Releasing Ions Reacted with Dentin

Objectives: Surface pre-reacted glass ionomer (S-PRG) filler is a bioactive glass that releases six ions, fluoride (F), strontium (Sr), sodium, aluminum (Al), boric acid (BO₃), and silicic acid (SiO₂). Sr, B and F ions are supposed to account for acid-neutralizing properties and inhibition of demineralization. To investigate those ions reacted with dentin, we demonstrated ³¹P, ¹⁹F and ¹¹B solid-state NMR spectroscopy.
Methods: S-PRG fillers were immersed in distilled water in a tumbler mixer at 23°C for 24 h to obtained S-PRG eluate. Pulverized bovine dentin was immersed in the diluted eluate for 3 hours, followed by DW rinsing. Biomimetic nucleation of calcium phosphate was prepared to obtain nanocrystalline-hydroxyapatites (nano-HAp) using S-PRG eluate. To examine Sr-incorporation in hydroxyapatite lattice, the biomimetic nucleation was made at the Ca/Sr ion mM ratio as (7.8/3.9), (5.9/5.9) and (3.9/7.8), in which Ca, Sr, P intake % was measured by ICP - Atomic Emission Spectrometry. Dehydrated pulverized dentin samples were examined by ³¹P CP MAS, ¹H/³¹P/¹H double CP MAS, 2D ³¹P-¹H HETCOR, ¹¹B and ¹⁹F MAS NMR were acquired with a JEOL (ECA II spectrometer 700MHz) with a MAS frequency of 15 kHz, while ¹⁹F MAS NMR was acquired with a Bruker (AVANCE NEO 600MHz) with a MAS frequency of 60 kHz.
Results: Eluate-treated dentin showed a trace of fluoride-substituted apatite (FAp), while biomimetic nucleation produced FAp and CaF₂ in nano-HAp. The ¹¹B peak near 0 ppm was detected for eluate-treated dentin and biomimetic nano-HAp, although two ¹¹B peaks were identified near 16 and 0 ppm for S-PRG filler. Hydroxyapatite formation was confirmed by ¹H/³¹P/¹H double CP for all Ca/Sr samples, but full width in half maximum increased with increase of Sr ion. The phosphate intake in nano-HAP was not different among tested Ca/Sr ion mM ratio.
Conclusions:
B and F ions are incorporated to dentin even 3-hour treatment. S-PRG releasing ions, Sr, B and F ions, were involved during calcium phosphate nucleation to form modified nano-HAp. Sr, B and F ions play a role in promoting remineralization and inhibition of demineralization.