Revisiting silicate substituted hydroxyapatite by solid-state NMR

Area of Science:

• Biomaterials Science
• Materials Chemistry
• Solid-State Chemistry

Background:

• Silicon-substituted hydroxyapatite (Si-HAp) exhibits enhanced bone remodeling properties, making it a promising material for biomedical implants.
• Previous structural characterization of Si-HAp primarily relied on indirect methods like X-ray and neutron diffraction.

Purpose of the Study:

• To precisely determine the structural location of silicon within Si-HAp synthesized via precipitation.
• To differentiate between silicon incorporated into the hydroxyapatite lattice and silicon present as separate silica phases.

Main Methods:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy was employed for detailed structural analysis.
• Specific NMR techniques included (29)Si Magic Angle Spinning (MAS), (1)H -->(29)Si Cross-Polarization MAS (CP MAS), and T(1)rho((1)H) edited (1)H -->(29)Si CP MAS experiments.

Main Results:

• Solid-state NMR demonstrated that in 4.6 wt% Si-HAp, only a fraction of silicon atoms are incorporated into the hydroxyapatite (HAp) lattice as Q(0) (SiO(4) (4-)) species.
• A significant portion of the silicate units were identified as silica-gel species located outside the HAp structure.
• The T(1)rho((1)H) edited sequence proved crucial for unambiguously distinguishing the location of SiO(4) (4-) moieties.

Conclusions:

• Precipitation methods yield Si-HAp where silicon incorporation into the HAp lattice is limited.
• The presence of external silica-gel units alongside lattice-incorporated silicon impacts the material's properties.
• Solid-state NMR, particularly with advanced editing sequences, provides definitive insights into the structural state of silicon in Si-HAp.