Renny Mathew

Melt-derived bioactive phosphosilicate glasses are widely utilized as bone-grafting materials for various surgical applications. However, the insight into their structural features over a medium-range scale up to ∼1 nm remains limited. We present a comprehensive assessment of the spatial distribution of phosphate groups across the structures of 11 Na₂O–CaO–SiO₂–P₂O₅ glasses that encompass both bioactive and nonbioactive compositions, with the P contents and silicate network connectivities varied independently. Both parameters are known to strongly influence the bioactivity of the glass in vitro. The phosphate distribution was investigated by double-quantum ³¹P nuclear magnetic resonance (NMR) experiments under magic-angle spinning (MAS) conditions and by molecular dynamics (MD) simulations. The details of the phosphate-ion dispersion were probed by evaluating the MD-derived glass models against various scenarios of randomly distributed, as well as clustered, phosphate groups. From comparisons of the P–P interatomic-distance spreads and the statistics of small phosphate clusters assessed for variable cutoff radii, we conclude that the spatial arrangement of the P atoms in phosphosilicate glasses is well-approximated by a statistical distribution, particularly across a short-range scale of ≤450 pm. The primary distinction is reflected in slightly closer P–P interatomic contacts in the MD-derived structures over the distance span of 450–600 pm relative to that of randomly distributed phosphate groups. The nature of the phosphate-ion dispersion remains independent of the silicate network polymerization and nearly independent of the P content of the glass throughout our explored parameter space of 1–6 mol % P₂O₅ and silicate network connectivities up to 2.9.

The physiological responses of silicate-based bioactive glasses (BGs) are known to depend critically on both the P content (n_P) of the glass and its silicate network connectivity (N̅_BO^Si). However, while the bioactivity generally displays a nonmonotonic dependence on n_P itself, recent work suggest that it is merely the net orthophosphate content that directly links to the bioactivity. We exploit molecular dynamics (MD) simulations combined with ³¹P and ²⁹Si solid-state nuclear magnetic resonance (NMR) spectroscopy to explore the quantitative relationships between N̅_BO^Si, n_P, and the silicate and phosphate speciations in a series of Na₂O–CaO–SiO₂–P₂O₅ glasses spanning 2.1 ≤ N̅_BO^Si ≤ 2.9 and variable P₂O₅ contents up to 6.0 mol %. The fractional population of the orthophosphate groups remains independent of n_P at a fixed N̅_BO^Si-value, but is reduced slightly as N̅_BO^Si increases. Nevertheless, P remains predominantly as readily released orthophosphate ions, whose content may be altered essentially independently of the network connectivity, thereby offering a route to optimize the glass bioactivity. We discuss the observed composition-structure links in relation to known composition-bioactivity correlations, and define how Na₂O–CaO–SiO₂–P₂O₅ compositions exhibiting an optimal bioactivity can be designed by simultaneously altering three key parameters: the silicate network connectivity, the (ortho)phosphate content, and the n_Na/n_Ca molar ratio.

A new molecular adduct of MgCl₂ with isobutanol, namely MgCl₂·4((CH₃)₂CHCH₂OH) (MgiBOH), has been prepared as a precursor to the supporting material for an olefin polymerization catalyst. The MgiBOH adduct and final titanated Ziegler–Natta catalysts have been thoroughly characterized by powder XRD, thermal analysis, Raman spectroscopy and solid-state NMR for structural and spectroscopy aspects. A peak observed at 712 cm⁻¹ in the Raman spectra of MgiBOH indicates the characteristic Mg–O₆ breathing mode and the formation of the adduct. The diffraction feature at 2θ = 7.8° (d = 11.223 Å) in the XRD confirms the adduct formation and the layered structure. The aim of the present article is to study how the insertion of a bulky isobutanol moiety affects the structural and electronic properties of the MgCl₂·isobutanol molecular adduct. Indeed, the focus of the present study is to explore how the presence of isobutanol, in the initial molecular adduct, influences the final Z–N catalyst properties and its activity.

We review the benefits of using ²⁹ Si and ¹ H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy for probing the local structures of both bulk and surface portions of mesoporous bioactive glasses (MBGs) of the CaO–SiO ₂ −(P ₂ O ₅ ) system. These mesoporous materials exhibit an ordered pore arrangement, and are promising candidates for improved bone and tooth implants. We discuss experimental MAS NMR results from three MBGs displaying different Ca, Si and P contents: the ²⁹ Si NMR spectra were recorded either directly by employing radio-frequency pulses to ²⁹ Si, or by magnetization transfers from neighbouring protons using cross polarization, thereby providing quantitative information about the silicate speciation present in the pore wall and at the MBG surface, respectively. The surface modifications were monitored for the three MBGs during their immersion in a simulated body fluid (SBF) for intervals between 30 min and one week. The results were formulated as a reaction sequence describing the interconversions between the distinct silicate species. We generally observed a depletion of Ca ²⁺ ions at the MBG surface, and a minor condensation of the silicate-surface network over one week of SBF soaking.

By employing solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (PXRD), and scanning electron microscopy coupled with energy-dispersive X-ray (EDX) spectroscopy, we compare the biomimetic growth of calcium hydroxyapatite (HA) from an ordered mesoporous bioactive glass (MBG) in simulated body fluid (SBF) and buffered water solutions. For the latter medium, we also examine the effects of using two different MBG concentrations. We evaluate the predicting powers of PXRD and ³¹P NMR for directly quantifying the relative amounts of biomimetic amorphous calcium phosphate (ACP) and HA: we observe a very good agreement between the two analytical techniques. Thanks to their mesoporous channel system, fluids readily penetrate throughout sub-mm sized MBG grains, as evidenced by EDX. The latter revealed distinct element-mappings across the material after its exposure to SBF compared to water. Under our in vitro conditions involving relatively high MBG-loadings in the solutions, the HA formation reduces in SBF relative to buffered water, particularly for increasing MBG concentration. These features stem from a high [Ca²⁺]/[PO³⁻ ₄] ratio resulting in the fluid medium, which retards the HA crystallization by inducing a rapid ACP precipitation and an accompanying depletion of phosphate ions in the solution. This has bearings on the design of bioactivity comparisons of bioglasses exhibiting significantly different cation compositions.

By exploiting ¹H and ³¹P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, we explore the proton and orthophosphate environments in biomimetic amorphous calcium phosphate (ACP) and hydroxy-apatite (HA), as grown in vitro at the surface of a 10CaO–85SiO₂–5P₂O₅ mesoporous bioactive glass (MBG) in either a simulated body fluid or buffered water. Transmission electron microscopy confirmed the presence of a calcium phosphate layer comprising nanocrystalline HA. Two-dimensional ¹H–³¹P heteronuclear correlation NMR established predominantly ¹H₂O↔³¹PO₄^3– and O¹H↔³¹PO₄^3– contacts in the amorphous and crystalline component, respectively, of the MBG surface-layer; these two pairs exhibit distinctly different ¹H→³¹P cross-polarization dynamics, revealing a twice as large squared effective ¹H–³¹P dipolar coupling constant in ACP compared with HA. These respective observations are mirrored in synthetic (well-crystalline) HA, and the amorphous calcium orthophosphate (CaP) clusters that are present in the pristine MBG pore walls: besides highlighting very similar local ¹H and ³¹P environments in synthetic and biomimetic HA, our findings evidence closely related NMR characteristics, and thereby similar local structures, of the CaP clusters in the pristine MBG relative to biomimetic ACP.