Proton Environments in Biomimetic Calcium Phosphates Formed from Mesoporous Bioactive CaO-SiO₂-P₂O₅ Glasses in Vitro: Insights from Solid-State NMR

When exposed to body fluids, mesoporous bioactive glasses (MBGs) of the CaO-SiO₂-P₂O₅ system develop a bone-bonding surface layer that initially consists of amorphous calcium phosphate (ACP), which transforms into hydroxy-carbonate apatite (HCA) with a very similar composition as bone/dentin mineral. Information from various ¹H-based solid-state nuclear magnetic resonance (NMR) experiments was combined to elucidate the evolution of the proton speciations both at the MBG surface and within each ACP/HCA constituent of the biomimetic phosphate layer formed when each of three MBGs with distinct Ca, Si, and P contents was immersed in a simulated body fluid (SBF) for variable periods between 15 min and 30 days. Directly excited magic-angle-spinning (MAS) ¹H NMR spectra mainly reflect the MBG component, whose surface is rich in water and silanol (SiOH) moieties. Double-quantum-single-quantum correlation ¹H NMR experimentation at fast MAS revealed their interatomic proximities. The comparatively minor H species of each ACP and HCA component were probed selectively by heteronuclear ¹H-³¹P NMR experimentation. The initially prevailing ACP phase comprises H₂O and "nonapatitic" HPO₄^2-/PO₄^3- groups, whereas for prolonged MBG soaking over days, a well-progressed ACP → HCA transformation was evidenced by a dominating O¹H resonance from HCA. We show that ¹H-detected ¹H → ³¹P cross-polarization NMR is markedly more sensitive than utilizing powder X-ray diffraction or ³¹P NMR for detecting the onset of HCA formation, notably so for P-bearing (M)BGs. In relation to the long-standing controversy as to whether bone mineral comprises ACP and/or forms via an ACP precursor, we discuss a recently accepted structural core-shell picture of both synthetic and biological HCA, highlighting the close relationship between the disordered surface layer and ACP.