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New Insight Into Mixing Fluoride and Chloride in Bioactive Silicate Glasses

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New Insight Into Mixing Fluoride and Chloride in Bioactive Silicate Glasses www.nature.com/scientificreports OPEN New Insight into Mixing Fluoride and Chloride in Bioactive Silicate Glasses Received: 8 August 2017 Xiaojing Chen 1,2, Xiaohui Chen 3, Alfonso Pedone4, David Apperley5, Robert G. Hill2 & Accepted: 21 December 2017 Natalia Karpukhina2 Published: xx xx xxxx Adding fuoride into bioactive glasses leads to fuorapatite formation and a decrease in glass transition temperature. Recently, chloride has been introduced into glasses as an alternative to fuoride. The presence of the large chloride ion lowers glass crystallisation tendency and increases glass molar volume, which efectively facilitates glass degradation and bone-bonding apatite-like layer formation. However, there is no information regarding the efect of mixing fuoride and chloride on the glass structure and properties. This study aims to synthesize mixed fuoride and chloride containing bioactive glasses; investigate the structural role of fuoride and chloride and their efects on glass properties. The chloride content measurements reveal that 77–90% of chloride was retained in these Q2 type glasses. Glass transition temperature reduced markedly with an increase in CaX2 (X = F + Cl) content, while the glass molar volume increased. 29Si MAS-NMR results show that the incorporation of mixed fuoride and chloride did not cause signifcant change in the polymerization of the silicate network and no detectable concentration of Si-F/Cl bands were present. This agrees with 19F NMR spectra showing that F existed as F-Ca(n) species. Bioactive glasses (BGs) are well known for their bone regenerative properties1,2. When immersed in physiological solutions, they degrade, release ions and form an integrated bond with living bone via the formation of an apatite layer on the surface. Tey are of interest for applications in sof tissue repair due to their ability of promoting angiogenesis (formation of blood vessels)3–5; bone grafing6, toothpastes7 and as dental air abrasives8. Certain glass properties are desired for specifc applications. For example, it is thought that highly degradable glasses are favourable for resorbable bone grafing materials9, while BGs that release fuoride and form fuorapatite (FAP) are preferred for re-mineralizing toothpastes and the BGs with controllable hardness are particularly attractive for selective cutting of dental tissues. It is believed that glass composition and especially its structure defnes its properties10. Terefore, the knowledge of structure-property relationship is required in order to tailor the glass properties to the preferred applications. In recent years, there is an increase amount of interest in halogen-containing oxide glass systems, due to their unique mechanical, optical, electrical and medical properties. Fluoride containing BGs were frst investigated by 11 Hench and Spillman who substituted CaF2 for CaO or Na2O. It is noted that this approach results in an increase in the polymerization of the silicate tetrahedra12,13 and the network connectivity, which substantially reduces 14 the glass bioactivity . In contrast to replacing oxides with CaF2, adding calcium fuoride into BGs leads to no change in the glass Q silicate network structure15,16. Te incorporation of fuoride into the BGs reduces glass fring 17,18 temperature, glass transition temperature (Tg) and probably glass hardness . More recently, Sriranganathan et al.18 demonstrated that the presence of fuoride promotes and speeds up FAP formation upon immersion in bufer solution, this efect is more pronounced in Sr containing glasses. Fluorapatite is well-documented to be less soluble in an acidic condition than hydroxyapatite19–22. However, high fuoride content (≥9.3 mol%) results in an 1Xiangya Stomatological Hospital & School of Stomatology, Central South University, Changsha, Hunan, 410078, P.R. China. 2Dental Physical Sciences, Institute of Dentistry, Queen Mary University of London, Mile End Road, London, E1 4NS, United Kingdom. 3Division of Dentistry, School of Medical Sciences, University of Manchester, Manchester, M13 9PL, United Kingdom. 4Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Via G. Campi 103, 41125, Modena, Italy. 5Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom. Correspondence and requests for materials should be addressed to N.K. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:1316 | DOI:10.1038/s41598-018-19544-2 1 www.nature.com/scientificreports/ 17 uncontrollable fuorite (CaF2) crystallisation . Te insoluble CaF2 will inhibit the delivery of fuoride ions from glass and also reduce glass bioactivity23. Chloride volatilization is a severe problem in silicate glasses and results in very few studies on oxychloride silicate glasses and very rare practical applications, though chloride has been used as a refning aid in glass melt- ing24. Recently, chloride has been introduced to the sodium-free bioactive silicate glasses as an alternative to fuoride by Chen et al.9,25,26. A considerable amount of chloride (up to 16.7 mol%) has been retained in the BGs. It is found that adding chloride leads to a reduced Tg in a similar manner to fuoride and a signifcant expansion of glass molar volume, which is benefcial to fast glass degradation and rapid apatite-like phase formation in vitro. Moreover, the crystallisation tendency of the chloride containing glasses was much lower than the equivalent fuoride containing glasses. All the studied chloride containing glasses were largely amorphous, this shows a clear contrast to the equivalent fuoride glasses, which crystallised to fuorapatite, cuspidine and fuorite when the fu- oride content were higher than 9.3 mol%17. It is known that the chloride ion is larger than the hydroxyl ion, which is larger than the fuoride ion (Rion − − − (Cl ):Rion (OH ):Rion (F ) = 1.67:1.26:1.19 (Å)). As a result, chlorapatite is more soluble than hydroxyapatite, which is more soluble than fuorapatite25. Additionally, unlike the formation of fuorapatite in fuoride containing glasses, a hydroxyapatite-like phase was the phase formed in the chloride containing glasses upon immersion9. Tis is favorable for resorbable bone substitutes but less attractive for the toothpastes for re-mineralization and caries protection. BGs with the presence of fuoride and chloride are of special interest, since they are potentially able to achieve the benefts from both. However, to the best of our knowledge, mixed fuoride and chloride con- taining BGs have not been yet synthesized and investigated in the literature and the structure of fuorine and, especially chlorine in silicate glasses remains unclear. Brauer et al.27 has investigated the structural role of fuoride by using 19F MAS-NMR, which showed that fuoride complexes the modifer cations (Ca2+ and Na+) rather than forming Si-F bonds. Chloride is expected to have similar efects on the glass structure and properties to fuoride. 35Cl MAS-NMR is potentially a promising technique to provide a direct view of the chlorine environment on an atomic-scale. However, 35Cl MAS-NMR is not as straightforward as 19F MAS-NMR. 35Cl is a spin 3/2 low γ nuclide with a large quadrupole moment and a relatively low resonance frequency28. In addition chloride ofen exhibits a low solubility in silicate melts29. As a result of these problems, 35Cl MAS-NMR signals can be severely broadened and indiscernible from background, and require very high magnetic felds and the use of fast spinning frequencies. Terefore, to date, only a few direct studies on the atomic environment of chloride have been reported28,30. In general, these studies indicate that Cl is coordinated primarily by alkali or alkaline earth cations. In this paper, the possibility of incorporating both fuoride and chloride into glass GPFCl0.0 (38.1% SiO2, 6.3% P2O5, 55.5% CaO, in mol%) and their efects on the glass structure and properties were investigated and compared with only fuoride or chloride containing BGs (GPF and GPCl series) studied previously9,23. Various techniques, including Diferential Scanning Calorimetry, X-ray Difraction, Magic Angle Spinning-Nuclear Magnetic Resonance (29Si, 31P and 19F MAS-NMR) and Helium Pycnometer were employed to understand the glass structure and properties. To the best of our knowledge, this is the frst study on mixed chloride/fuoride containing BGs. Materials and Methods Glass synthesis. Novel mixed fuoride and chloride containing BGs in the system of SiO2-P2O5-CaO-CaF2/ CaCl2 were prepared by the melt-quench route. Instead of replacing CaO by CaX2 (X = F + Cl), a varied amount of CaX2, which is consist of 50% CaF2 and 50% CaCl2·2H2O, since CaCl2 picks up water easily, was added to 9 a calcium halide free composition (GPFCl0.0) , whilst other components (SiO2, P2O5 and CaO) were reduced proportionally. All the glasses were designed to have a constant network connectivity (NC) value of 2.08. Glass compositions are reported in Table 1. A 200 g batch size was made. Glass reagents including analytical grade SiO2 (Prince Minerals Ltd., Stoke-on-Trent, UK), CaCO3, P2O5, CaF2 and CaCl2.2H2O (all Sigma-Aldrich) were melted at high temperatures in a Pt/10Rh crucible for 1 hour in an electrical furnace (EHF 17/3 Lenton, Hope Valley, UK). Te melted glass was rapidly water quenched. Te collected glass frits were dried, Gy-Ro milled (Glen Creston, London, UK) and sieved through a 45 μm mesh analytical sieve (Endecotts Ltd, London, England). Te compositions of the individual chloride and fuoride series, which have been reported previously9,23, are presented in ESI (Table S1) for comparison purpose. Compositional analysis. Te chloride content in the initial mixed glasses was quantifed using a chloride ion selective electrode (ELIT Cl- 2844, NICO 2000 UK), according to the method described by Chen et al.9. Te actual glass compositions were re-calculated based on the chloride component analysis and summarized in Table 1. Glass characterisation. Glass thermal properties were evaluated by using a Stanton Redcrof DSC 1500 (Rheometric Scientifc, Epsom, UK). A 50 mg of glass frit was heated under Nitrogen (60 ml/min−1) from 25 °С to 1100 °С at a rate of 20 °С/min against an alumina reference in a Pt crucible.
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