Atomic level observation and structural analysis of phosphoric-acid ester interaction at dentin

Area of Science:

• Biomaterials Science
• Dental Materials
• Nanotechnology

Background:

• The functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) is crucial in dental adhesives, known for self-assembling into nano-layers at adhesive-tooth interfaces.
• Previous studies suggested various states for the POH groups of 10-MDP calcium (Ca) salts, but their exact status remained unclear.

Purpose of the Study:

• To mechanistically investigate the nano-layering of 10-MDP calcium (Ca) salts at adhesive-dentin interfaces.
• To elucidate the precise interaction state of the POH groups of 10-MDP with Ca in these nano-layers.

Main Methods:

• Correlative investigation using scanning transmission electron microscopy with energy-dispersive X-ray spectrometry (STEM-EDS).
• X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (NMR) were employed to analyze the nano-layer structure and chemical bonding.

Main Results:

• STEM-EDS confirmed the presence of Calcium (Ca) and Phosphorus (P) within each nano-layer at the adhesive-dentin interface.
• XRD and NMR analyses revealed that both terminal POH groups of 10-MDP reacted with Ca, forming stable CaRPO4 structures.
• This twofold POH interaction with Ca was found to be stable in water.

Conclusions:

• The stable, water-resistant nano-layering of 10-MDP_Ca salts is attributed to the dual reaction of 10-MDP's POH groups with Ca.
• This robust nano-layer formation is expected to significantly contribute to the durability of the hybrid and adhesive layers.
• The findings support the clinical longevity of adhesively bonded tooth restorations utilizing 10-MDP-based dental adhesives.