Bioactive sol-gel glasses at the atomic scale: the complementary use of advanced probe and computer modelling methods

Sol-gel synthesised bioactive glasses may be formed via a hydrolysis condensation reaction, silica being introduced in the form of tetraethyl orthosilicate (TEOS) and calcium is typically added in the form of calcium nitrate. The synthesis reaction proceeds in an aqueous environment; the resultant gel is dried, before stabilisation by heat treatment. These materials, being amorphous, are complex at the level of their atomic-scale structure, but their bulk properties may only be properly understood on the basis of that structural insight. Thus, a full understanding of their structure : property relationship may only be achieved through the application of a coherent suite of leading-edge experimental probes, coupled with the cogent use of advanced computer simulation methods. Using as an exemplar a calcia-silica sol-gel glass of the kind developed by Larry Hench, to whose memory this paper is dedicated, we illustrate the successful use of high-energy x-ray and neutron scattering (diffraction) methods, magic-angle spinning solid state NMR, and molecular dynamics simulation as components to a powerful methodology for the study of amorphous materials

³¹P MAS Refocused INADEQUATE Spin−Echo (REINE) NMR Spectroscopy: Revealing <i>J</i> Coupling and Chemical Shift Two-Dimensional Correlations in Disordered Solids

Two-dimensional (2D) variations in 2JP1,P1, 2JP1,P2, and 2JP2,P2 are obtainedusing the REINE (REfocused INADEQUATE spin−Echo) pulse sequence presented by Cadars et al. (Phys. Chem. Chem. Phys. 2007, 9, 92−103)from pixel-by-pixel fittings of the spin−echo modulation for the 2D correlation peaks due to linked phosphate tetrahedra (P1−P1, P1−P2, P2−P1, and P2−P2) in a 31P refocused INADEQUATE solid-state MAS NMR spectrum of a cadmium phosphate glass, 0.575CdO−0.425P2O5. In particular, separate variations for each 2D 31P REINE peak are obtained which reveal correlations between the J couplings and the 31P chemical shifts of the coupled nuclei that are much clearer than those evident in previously presented 2D z-filtered 31P spin−echo spectra. Notably, such correlations between the J couplings and the 31P chemical shifts are observed even though the conditional probability distributions extracted using the protocol of Cadars et al. (J. Am. Chem. Soc. 2005, 127, 4466−4476) indicate that there is no marked correlation between the 31P chemical shifts of neighboring phosphate tetrahedra. For 2D peaks at the P2 31P chemical shift in the direct dimension, there can be contributions from chains of three units (P1−P2−P1), chains of four units (P1−P2−P2−P1), or longer chains or rings (−P2−P2−P2−): for the representative glass considered here, best fits are obtained assuming a glass comprised predominantly of chains of four units. The following variations are found: 2JP1,P1 = 13.4 ± 0.3 to 14.8 ± 0.5 Hz, 2JP1,P2 = 15.0 ± 0.3 to 18.2 ± 0.3 Hz, and 2JP2,P2 = 5.9 ± 0.6 to 9.1 ± 0.9 Hz from the fits to the P1−P1, P1−P2, and P2−P2 peaks, respectively. The correlation of a particular J coupling with the 31P chemical shifts of the considered nucleus and the coupled nucleus is quantified by the coefficients CF2 and CF1 that correspond to the average pixel-by-pixel change in the J coupling with respect to the chemical shift of the observed (F2) and neighboring (F1) 31P nuclei, respectively

Effects of Five Metals on the Evolution of Hydrogen Sulfide, Methanethiol, and Dimethyl Sulfide during Anaerobic Storage of Chardonnay and Shiraz Wines

The synergistic effects of Cu, Fe, Mn, Zn, and Al on the evolution of different volatile sulfur compounds (VSCs) in a Chardonnay and a Shiraz wine have been investigated. The evolution of H₂S, MeSH, and DMS were influenced by metal addition, and in some instances, a combination of metals was responsible for the largest variation in the concentration of VSCs. The metals and metal combinations associated with significant changes in VSC concentrations in both Chardonnay and Shiraz samples after anaerobic storage were Cu, Fe, Zn, Al, Cu*Fe, Cu*Mn*Al, and Cu*Zn*Al for H₂S; Cu, Zn, Fe*Mn, and Cu*Fe*Mn for MeSH; and Al and Zn*Al for DMS. The effect of Cu addition on the evolution of VSCs has previously been shown; however, this investigation has demonstrated that metals other than Cu could also be involved in the catalytic release of VSCs and that the interactions and combinations of metals are important. In some instances, the metal effect was reversed, associated with significant decreases during high oxygen conditions and with significant increases during low oxygen conditions

A High-Resolution ⁴³Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments

Synthesis and Structure of a Calcium Polyphosphate with a Unique Criss-Cross Arrangement of Helical Phosphate Chains

We report here the facile synthesis and structural characterization of γ-Ca(PO3)2, a novel polymorph of calcium polyphosphate. The single-step synthesis of this potential biomaterial was achieved using flux methods at 250 °C, yielding a single-phase product stable at room temperature. Single-crystal X-ray diffraction determined the structure to be of monoclinic symmetry (a = 9.5669(2) Å, b = 9.5023(2) Å, c = 10.3717(3) Å, and β = 93.474(2)°, space group Cc), with layers of helical polyphosphate chains directed alternately along the [110] and [−110] unit cell directions to produce a unique arrangement of criss-cross polyphosphate layers separated by calcium ions. Further structural characterization by MAS-NMR and FT-IR was found to be consistent with this structure. Thermal studies indicated that γ-Ca(PO3)2 is stable up to 690 °C, whereupon it irreversibly converts to the β-Ca(PO3)2 polymorph

Synthesis and Structure of a Calcium Polyphosphate with a Unique Criss-Cross Arrangement of Helical Phosphate Chains

We report here the facile synthesis and structural characterization of γ-Ca(PO3)2, a novel polymorph of calcium polyphosphate. The single-step synthesis of this potential biomaterial was achieved using flux methods at 250 °C, yielding a single-phase product stable at room temperature. Single-crystal X-ray diffraction determined the structure to be of monoclinic symmetry (a = 9.5669(2) Å, b = 9.5023(2) Å, c = 10.3717(3) Å, and β = 93.474(2)°, space group Cc), with layers of helical polyphosphate chains directed alternately along the [110] and [−110] unit cell directions to produce a unique arrangement of criss-cross polyphosphate layers separated by calcium ions. Further structural characterization by MAS-NMR and FT-IR was found to be consistent with this structure. Thermal studies indicated that γ-Ca(PO3)2 is stable up to 690 °C, whereupon it irreversibly converts to the β-Ca(PO3)2 polymorph

Synthesis and Structure of a Calcium Polyphosphate with a Unique Criss-Cross Arrangement of Helical Phosphate Chains

We report here the facile synthesis and structural characterization of γ-Ca(PO3)2, a novel polymorph of calcium polyphosphate. The single-step synthesis of this potential biomaterial was achieved using flux methods at 250 °C, yielding a single-phase product stable at room temperature. Single-crystal X-ray diffraction determined the structure to be of monoclinic symmetry (a = 9.5669(2) Å, b = 9.5023(2) Å, c = 10.3717(3) Å, and β = 93.474(2)°, space group Cc), with layers of helical polyphosphate chains directed alternately along the [110] and [−110] unit cell directions to produce a unique arrangement of criss-cross polyphosphate layers separated by calcium ions. Further structural characterization by MAS-NMR and FT-IR was found to be consistent with this structure. Thermal studies indicated that γ-Ca(PO3)2 is stable up to 690 °C, whereupon it irreversibly converts to the β-Ca(PO3)2 polymorph

¹⁷O and ¹⁵N Solid State NMR Studies on Ligand-Assisted Templating and Oxygen Coordination in the Walls of Mesoporous Nb, Ta and Ti Oxides

A multinuclear solid state NMR approach is applied to four templated mesoporous oxides (silica, titania, niobia and tantala) to include 15N and 17O magic angle spinning (MAS) NMR and double resonance 15N−93Nb, 17O Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR). The templated samples were ramped in steps of 20 °C for 2 days up to typically 110 °C where the samples were left for 2−4 days. 15N MAS NMR shows that amines are the only species present in the TiO2, Nb2O5, and Ta2O5. In SiO2, amines are only present as a minor coordination (10 ± 2%), but there are several strong ammonium 15N resonances. The REAPDOR experiments show that the nitrogen interacts with niobium, confirming a ligand interaction between the Nb and N, as previously believed. In the case of silica, the amine is quaternized and there is apparently no interaction with the Si, suggesting a RNH3+ −O−Si- hydrogen-bonding interaction with the walls. 17O MAS NMR provides the clearest indication of the local wall structure. In the aged, templated samples in all cases only OM2 coordinations are present which is very different from the pure bulk oxides (apart from SiO2) and must be due to the effects of amine coordination at the metal centers. On removal of the template, these oxides behave differently, with Ta2O5 showing a mixture of OTa2 (85 ± 5%) and OTa3 (15 ± 5%) which is similar to the types of coordination found in the bulk oxide. The previously reported 17O MAS NMR data from heat-treated mesoporous niobia shows only ONb2, which is very highly ordered. In contrast for titania, the OTi2 coordination is immediately lost on removal of the template to be replaced by a mixture of OTi3 (60 ± 5%) and OTi4 (40 ± 5%), with the OTi4 becoming dominant above 250 °C, very different behavior from the corresponding bulk oxide. In summary, this NMR study shows that the local oxygen coordination in amine-templated mesoporous transition metal oxides is present as OM2 which is relatively rare in bulk oxides. The data indicates that the template interaction is largely controlled by the N−M dative bond to the wall, suppressing higher oxygen coordination numbers. Qualitatively it appears that the strength of this interaction varies greatly in the different mesoporous oxides

A Solid-State NMR Study of Lead and Vanadium Substitution into Hydroxyapatite

A systematic study on cationic and anionic substitution in hydroxyapatite structures was carried out, with the aim of understanding the impact of ion exchange on the crystalline structure and properties of these materials. Lead and vanadium were chosen for the exchange, due to their known effects on the redox and catalytic properties of hydroxypatites. Hydroxyapatites with variable Pb and V contents, PbxCa10-x(VO4)y(PO4)6-y(OH)2 (x = 0, 2, 4, 6, 8 and 10 for y = 1; y = 0, 0.5, 1, 2, 3 and 6 for x = 10) were synthesized and characterized by NMR spectroscopy. Solid-state NMR allowed an analysis of the chemical environment of every ion after substitution into the hydroxyapatite network. 43Ca and 207Pb NMR spectra at different lead concentrations provided clear evidence of the preferential substitution of lead into the Ca(II) site, the replacement of the Ca(I) site starting at x = 4 for y = 1. Two NMR distinguishable Pb(I) sites were observed in Pb10(PO4)6(OH)2, which is compatible with the absence of a local mirror plane perpendicular to the c direction. In contrast with 31P NMR, for which only small variations related to the incorporation of Pb are observed, the strong change in the 51V NMR spectrum indicates that lead perturbs the vanadium environment more than the phosphorus one. The existence of a wide variety of environments for OH in substituted apatites is revealed by 1H NMR, and the mobility of the water molecules appears to vary upon introduction of lead into the structure

Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na8Si46 and Na2Ba6Si46. Although their structure and properties are well described, there is little understanding of the formation mechanism. Na8Si46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si44– polyanions. In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25–500 °C at 8 °C/min) under vacuum conditions (10–4 bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state 23Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na8Si46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum