Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Materials Science
• Biomaterials Chemistry

Background:

• Carbonate substitution in hydroxyapatite (CHAp) significantly impacts its properties.
• Understanding the structural arrangement of carbonates (order/disorder and clustering) is crucial for CHAp applications.
• Traditional NMR methods face limitations in characterizing these structural features.

Purpose of the Study:

• To demonstrate the capability of NMR/DNP for detailed structural analysis of CHAp.
• To investigate the order/disorder and clustering of carbonates within the CHAp lattice.
• To explore the efficiency and applicability of DNP experiments for CHAp characterization.

Main Methods:

• Utilizing Nuclear Magnetic Resonance (NMR) with Dynamic Nuclear Polarization (DNP) enhancement.
• Employing distance-sensitive DNP experiments with ¹³C-¹³C recoupling.
• Performing numerical simulations to assess spin diffusion in nanoparticles.

Main Results:

• Achieved unprecedented description of CHAp structure using NMR/DNP.
• Successfully tackled key structural questions regarding carbonate order, disorder, and clustering.
• Demonstrated efficient DNP mediation via spin diffusion through hydroxyl columns in CHAp.
• Showed that DNP studies at 100 K yield comparable data to room temperature measurements.
• Interpreted 2D DNP experiments to identify domains with specific substitution types, favoring local order and carbonate clustering.

Conclusions:

• NMR/DNP offers a powerful, unprecedented approach for characterizing carbonate substituted hydroxyapatite.
• The study elucidates the structural details of carbonate arrangement, including local order and clustering.
• DNP-enhanced NMR is a viable technique for studying CHAp, even at reduced temperatures and for nanoparticles.