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Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Number of Authors: 32017 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 41, p. 9737-9752Article in journal (Refereed) Published
Abstract [en]

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO2 by B2O3 in the respective phosphosilicate base compositions 24.1Na(2)O-23.3CaO-48.6SiO(2)-4.0P(2)O(5) (S49) and 24.6Na(2)O-26.7CaO-46.1SiO(2)-2.6P(2)O(5) (S46), the latter constituting the 45S5 Bioglass utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO4, BO4, and BO3 moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P-O-Si and P-O-B-[4] motifs, whose populations were estimated by heteronuclear P-31{B-11} NMR experimentation. The high Na+/Ca2+ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO4 < SiO4 < BO3 << PO4. The BO4/BO3 intermixing was examined by double-quantum-single-quantum correlation B-11 NMR experiments, which evidenced the presence of all three BO3-BO3, BO3-BO4, and BO4-BO4 connectivities, with B-[3]-O-B-[4] bridges dominating. Notwithstanding that B-[4]-O-B-[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B-[4]-NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si-O-Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO4 tetrahedra tend to self-associate into small chain/ring motifs embedded in BO3/BO4-dominated domains.

Place, publisher, year, edition, pages
2017. Vol. 121, no 41, p. 9737-9752
National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:su:diva-148999DOI: 10.1021/acs.jpcb.7b06654ISI: 000413617800028PubMedID: 28876931OAI: oai:DiVA.org:su-148999DiVA, id: diva2:1160386
Available from: 2017-11-27 Created: 2017-11-27 Last updated: 2018-10-31Bibliographically approved
In thesis
1. Understanding Composition–Structure–Bioactivity Correlations in Bioactive Glasses
Open this publication in new window or tab >>Understanding Composition–Structure–Bioactivity Correlations in Bioactive Glasses
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bioactive glasses integrate with bone/tooth tissues by forming a layer of hydroxy-carbonate apatite (HCA), which mimics the composition of bone mineral. In the current thesis, we investigated composition–structure–bioactivity correlations of phosphosilicate and borophosphosilicate (BPS) glasses. Bioactive phosphosilicate glasses extend the compositional space of the ”45S5 Bioglass®”, which has been in clinical use for decades. Recently developed bioactive BPS glasses with SiO2→B2O3 substitutions transform more completely into HCA and their glass dissolution behaviors can be tuned by varying the relative contents of B and Si. 

It is known that the average silicate network connectivity NSi and the phosphate content (x(P2O5)) affect the apatite formation (in vitro bioactivity) of phosphosilicate glasses, but the details remain poorly explored. Three series of phosphosilicate glasses were designed by independently varying NSi and x(P2O5). After immersion of the glasses in a simulated body fluid (SBF) for 24 hours, different degrees of their apatite formation were quantified by Fourier-transform infrared (FTIR) spectroscopy. The results revealed that a high P content widened the NSi range that generated optimum amounts of apatite and also mitigated the detrimental effects associated with using glass particles with < 50 μm. The amounts of apatite derived from FTIR agreed with those from 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The growth of apatite at bioactive glass surfaces was found to follow a sigmoidal growth model, in which the precursor phase, amorphous calcium phosphate (ACP), formed in the induction period and then crystallized into HCA in the following proliferation period, with an improvement in the structural ordering of HCA in the maturation period. This formation process closely resembles the apatite precipitated spontaneously from supersaturated Ca/P-containing solutions. The simultaneous growth of ACP and HCA is discussed in conjunction with a previously proposed mechanism for explaining in vitro bioactivity and apatite growth from bioactive glasses. 

The short- and medium- range structures of bioactive borophosphosilicate (BPS) glasses were investigated by solid-state MAS NMR. Two series of BPS glasses were designed by gradually replacing SiO2 with B2O3 in the 45S5 glass, as well as another base glass featuring a more condensed glass network. As the B2O3 content is increased, the glass networks become more polymerized, together with decreased fractions of the dominating BO3 and orthophosphate units. Borate groups are homogeneously mixed with the isolated orthophosphate groups, while the remaining phosphate groups exhibit a slight preference for bonding to BO4 over SiO4 units. Linkages among borate groups are dominated by B[3]–O–B[4] linkages at the expenses of B[3]–O–B[3] and B[4]–O–B[4] linkages, with the latter B[4]–O–B[4] motifs disfavored yet abundant. A similar fashion of borate mixing was observed in P-free Na/Ca-based borosilicate glasses that span a large compositional space. The content of B[4]–O–B[4] linkages was found to be controlled by the relative fractions of BO4 groups and non-bridging oxygen ions.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University, 2018. p. 76
Keywords
Bioactive glasses, Phosphosilicate glasses, Borophosphosilicate glasses, Solid-state NMR spectroscopy, Glass structure, Fourier-transform infrared spectroscopy, Hydroxyapatite, Amorphous calcium phosphate, Apatite formation, In vitro bioactivity testing
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-161477 (URN)978-91-7797-458-1 (ISBN)978-91-7797-459-8 (ISBN)
Public defence
2018-12-12, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
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Supervisors
Available from: 2018-11-19 Created: 2018-10-29 Last updated: 2019-02-04Bibliographically approved

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