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Carbonate Formation on Bioactive Glasses Marta Cerruti and Claudio Morterra* Department of Chemistry I.F.M., University of Turin, and Consortium INSTM, Research Unit of Turin University, via P. Giuria 7, 10125 Torino, Italy Received February 2, 2004. In Final Form: May 10, 2004 The system termed 58S is a sol-gel-synthesized bioactive glass composed of SiO2, CaO, and P2O5, used in medicine as bone prosthetic because, when immersed in a physiological fluid, a layer of hydroxycarbonate apatite is formed on its surface. The mechanism of bioactive glass 58S carbonation was studied in the vacuum by means of in-situ FTIR spectroscopy with the use of CO2, H2O, and CD3CN as probe molecules. The study in the vacuum was necessary to identify both the molecules specifically involved in the carbonation process and the type of carbonates formed. Bioactive glass 58S was compared to a Ca-doped silica and to CaO. On CaO, ionic carbonates could form by contact with CO2 alone, whereas on 58S and on Ca-doped silica carbonation occurred only if both CO2 and an excess of H2O were present on the sample. The function of H2O was not only to block surface cationic sites, so that CO2 could not manifest its Lewis base behavior, but also to form a liquid-like (mono)layer that allowed the formation of carbonate ions. The presence of H2O is also supposed to promote Ca2+ migration from the bulk to the surface. Carbonates formed at the surface of CaO and of Ca-bearing silicas (thus including bioactive glasses) are of the same type, but are produced through two different mechanisms. The finding that a water excess is necessary to start heavy carbonation on bioactive glasses seemed to imply that the mechanism leading to in-situ carbonation simulates, in a simplified and easy-to-reproduce system, what happens both in solution, when carbonates are incorporated in the apatite layer, and during sample shelf-aging.
Introduction The bioactive glass termed 58S, studied in this work, is part of the new class of sol-gel-synthesized bioactive glasses. Bioactive glasses are used in medicine, as bone prosthetics.1 They all descend from Bioglass 45S5, a meltderived glass made of a mixture of SiO2, CaO, Na2O, and P2O5 discovered by L. Hench in the 1970s.2 If bioactive glasses are immersed in a physiological fluid, a layer of hydroxy-carbonate apatite (HCA) is formed on their surface, analogous to the mineral phase present in bones. The sequence of reactions involved was at first hypothesized by Hench and Clark.3 The sequence would include the dissolution of cations from the glass to the solution, that causes a pH increase, the formation of surface silanols, the loss of soluble silica, and eventually the formation of a silica-rich layer surrounded by a Ca/P-rich amorphous layer. On the Ca/P-rich amorphous layer, hydroxyl ions and carbonate ions are included, so that at last HCA crystallizes. Once the crystalline HCA layer is formed, collagen is incorporated, and specific biological reactions involving osteoblasts can occur, so that new bone is formed. Sol-gel bioactive glasses represent a second generation of bioactive materials.4 No Na is included in their composition, and in fact pH does not increase as much as during Bioglass dissolution. The surface area is hundreds of times higher than for melt-derived glasses, and thus interfacial reactions run much faster. Moreover, a complete resorption of sol-gel bioactive glass can be achieved, if desired. Many authors studied the formation of the HCA layer. Most of the works, though, analyze the final layer of HCA, * Corresponding author. Phone: +39 011 6707589. Fax: +39 011 6707588. E-mail:
[email protected]. (1) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487-1510. (2) Hench, L. L.; Splinter, R. J.; Allen, W. C.; Greenlee, T. K. J. Biomed. Mater. Res. Symp. 1971, 2 (Part I), 117-141. (3) Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 1976, 10, 161174. (4) Hench, L. L. Bioactive materials: The potential for tissue regeneration. J. Biomed. Mater. Res. 1998, 41, 511-518.
to understand the bioactivity of samples synthesized in different ways.5-8 Recently, the very first stages of dissolution of small particles of Bioglass have been analyzed in some detail.9 Still, no previous works ever focused on the carbonation phase of bioactive glasses. This seems to be an important factor, because it is observed not only when bioactive glasses are immersed in physiological solutions, but also during simple shelf-aging of these materials. In the present work, we use some of the techniques typical of surface chemistry to understand the mechanism of carbonation of the sol-gel-derived bioactive glass 58S. These techniques already proved to be useful for the study of hydrophilicity and surface acidity of bioactive glasses 58S and 77S,10,11 because the high surface area possessed by sol-gel samples allows the analysis of the interactions with probe molecules. We decided to study bioactive glass 58S because a higher amount of Ca is present in its composition, and Ca is of paramount importance in the mechanism of carbonate formation, as it will be shown. In any event, the results presented in this Article for the 58S system were confirmed also for 77S or other compositions of bioactive glasses. The easy formation of carbonates in Ca-containing minerals is well-known; for example, Ca-containing silicates are often used as CO2 sequestering agents,12 and (5) Pereira, M. M.; Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 1994, 28, 693-698. (6) Nakamura, T.; Yamamuro, T.; Higashi, S.; Kokubo, T.; Ito, S. J. Biomed. Mater. Res. 1985, 19, 685-698. (7) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487-1510. (8) Andersson, O. H.; Kangasniemi, I. J. Biomed. Mater. Res. 1991, 25, 1019-1030. (9) Cerruti, M.; Greenspan, D.; Powers, K. Biomaterials, accepted. (10) Cerruti, M.; Magnacca, G.; Bolis, V.; Morterra, C. J. Mater. Chem. 2003, 13, 1279-1286. (11) Cerruti, M.; Bolis, V.; Magnacca, G.; Morterra, C. Phys. Chem. Chem. Phys. 2004, 6, 2468-2479. (12) Goldberg, P.; Chen, Z.-Y.; O’Connor, W.; Walters, R.; Ziock, H. Proceedings of Fifth International Conference on Greenhouse Gas Technology; Cairns convention center: Australia, 2000.
10.1021/la049723c CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004
Carbonate Formation on Bioactive Glasses Scheme 1. Linear Coordination of CO2 on Cationic Centers (Lewis Acid Sites)
Scheme 2. Possible Surface Carbonate-like Species18 a
a
Multiple bonds and charges have been omitted for simplicity.
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carbonates (type 2), ∆ν3 is usually equal to or slightly larger than 100 cm-1. For bidentate carbonates (type 3), ∆ν3 is ∼300 cm-1, whereas for bridged (or “organic”) carbonates (type 4), ∆ν3 is usually 400 cm-1 or higher. For bicarbonates (hydrogen-carbonates, type 5), ∆ν3 is ∼200 cm-1, and, in addition, there is a sharp δCOH mode at ∼1250 cm-1. Of course, these values are only indicative, because they depend on the polarizing power of the cation and on other factors, such as the presence of water molecules and/or of other cations external to the main coordination sphere. In the present Article, we will analyze the reactivity of CO2 and other selected probe molecules toward the bioactive glass 58S, to hypothesize which is the mechanism of surface carbonates formation. We believe that this study, using a simplified gas/solid interaction approach, could give some hints about the phenomena occurring both during HCA formation on bioactive glasses and upon bioactive glass shelf-aging. Materials and Methods
the carbonation of lime is widely studied for many important industrial processes.13 Carbonation has been described as “a liquid-phase reaction of carbon dioxide with aqueous hydroxide and cations to form carbonates.”14 Still, the mechanism of carbonation varies depending on the type of carbonates formed, and on the type of substrates. Surface carbonates, for example, can be formed just by contact with gaseous CO2, even on not strongly basic systems such as transition aluminas.15 The present Article will analyze carbonate formation by means of in-situ IR spectroscopy. The IR technique has been widely used for the analysis of carbonates both in silicates16 and in apatites.17 In fact, depending on IR absorption band positions, it is possible to distinguish different types of surface carbonate-like species. (A useful reference work on this subject was done by Busca and Lorenzelli.18) When CO2 interacts with metal oxides, two behaviors can be singled out: (1) Basic behavior. CO2 is a weak Lewis base and can coordinate on coordinatively unsaturated (CUS) surface cations, preserving its linear shape (see Scheme 1). The frequency of the asymmetric νCO stretching mode (symmetry species ∑u+) varies depending on the cation and is usually in the 2400-2300 cm-1 range.19 (2) Acidic behavior. If sufficiently basic surface species are present (like, for instance, basic O2- ions, OH- ions, or CUS cation-anion pairs), CO2 can coordinate on them, giving rise to many possible types of carbonate-like species (see Scheme 2). The identification of the carbonate-like species formed is usually made on the basis of the splitting of the two high-ν carbonate vibrations (i.e., the two components of the otherwise degenerate ν3 mode of species E′ in the D3h point group of the CO32- ion, bearing character of asymmetric OCO stretching vibration). For the “free” or purely ionic carbonate ion (type 1 in Scheme 2), which still has D3h symmetry, the vibrations are still degenerate (∆ν3 ) 0, ν3 ) 1420-1470 cm-1). For monodentate (13) Xu, B.-A.; Giles, D. E.; Ritchie, I. M. Hydrometallurgy 1998, 48, 205-224. (14) Gervais, C.; Garrabrants, A. C.; Sanchez, F.; Barna, R.; Moszkowicz, P.; Kosson, D. S. Cem. Concr. Res. 2004, 34, 119-131. (15) Morterra, C.; Magnacca, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1993, 89, 135-150. (16) Brooker, R. A.; Kohn, S. C.; Holloway, J. R.; McMillan, P. F. Chem. Geol. 2001, 174, 241-254. (17) Stoch, A.; Jastrzebski, W.; Brozek, A.; Trybalska, B.; Cichocinska, M.; Szarawar, E. J. Mol. Struct. 1999, 511-512, 287-294. (18) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89-126. (19) Morterra, C.; Cerrato, G.; Emanuel, C. Mater. Chem. Phys. 1991, 29, 447-456.
Bioactive glass 58S was synthesized via sol-gel by NovaMin Technology Inc. (Alachua, FL). The sol-gel procedure consisted in hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), and Ca(NO3)2‚4H2O, using HCl as catalyst. After aging and drying at low temperature, the glasses were stabilized at high temperature (873-973 K) for many hours.20 The sample, amorphous (as revealed by XRD and TEM analysis), was in a powder form. The final composition of Bioactive glass 58S is 60% SiO2, 36% CaO, and 4% P2O5 (expressed as mol %). Reference samples were CaO and a Ca-doped silica system. Nominally pure CaO powder was provided by Carlo Erba (starting purity 99.6%). As in CaO shelf-aging induces the formation of a thick layer of Ca hydroxide and/or Ca carbonates, the powder was vacuum calcined at 800 °C (5 h), to obtain a “clean” CaO surface, that is, free of carbonate and/or hydroxyl species. Ca-doped reference silica was the amorphous nonporous Aerosil A200, obtained by flame pyrolysis of SiCl4, Degussa (Frankfurt A.M., Germany), Ca-doped with the “incipient wetness” method. A titrated Ca(NO3)2 aqueous solution was used in amount just sufficient to wet, but not overwet, all of the powder. A thermal treatment in air at 873 K for 2 h was then carried out, to decompose and eliminate the nitrates. For IR measurements, the powders were compressed in the form of self-supporting pellets of some 10 mg cm-2. All spectra were obtained with an FTIR spectrometer (Bruker IFS 113v, equipped with a MCT criodetector). The homemade quartz infrared cell, equipped with KBr windows, was connected to a conventional vacuum line (residual pressure ≈ 10-5 Torr) and allowed to perform in strictly in-situ conditions both thermal treatments on the sample pellets and probe molecules adsorption/ desorption cycles on the activated samples. All IR spectra were recorded at beam-temperature (BT), that is, the temperature reached by (white) sample pellets in the IR beam. The BT is estimated to be some 20-30 °C higher than the actual room temperature (RT).
Results and Discussion IR spectra of bioactive glass 58S evacuated for 1 h at BT (nominally, RT) and at 400 °C are shown in Figure 1A. Main peak assignments are summarized in Table 1. (For a more detailed explanation, see refs 10 and 11.) The vacuum treatment at 400 °C induces a partial dehydration of the sample (as witnessed by the drastic decrease in the number of interacting hydroxyl groups, the increase in intensity of the peak of isolated silanols, and the appearance of a weak peak due to isolated CaOH species). Moreover, in the 1550-1400 cm-1 region, a decrease of (20) Vallet-Regi, M.; Romero, A. M.; Ragel, C. V.; LeGros, R. Z. J. Biomed. Mater. Res. 1999, 44, 416-421.
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Figure 1. (A) Absorbance background spectrum of bioactive glass 58S activated in-vacuo (residual pressure ≈ 10-5 Torr) for 1 h at room temperature (a) and at 400 °C (b). Inset: blown-up segments in the 1800-1200 cm-1 spectral range. (B) Absorbance background spectra of 58S, collected on the same sample of (A) after long shelf-aging (9 months). Table 1. IR Peak Assignment for 58S Evacuated for 1 h at Room Temperature and 400 °C (See Figure 1) peak position (cm-1) and description 3747 - sharp peak 3700-2500 - broad tail 3570 - small peak
presence (after activation at..)
assignment
both RT and 400 °C; more pronounced in the latter condition RT 400 °C
νOH of isolated SiOH (silanol groups)
1980, 1880, and, partially, 1630 weak and broad bands 1630 - intense peak
both RT and 400 °C
complex absorption at ∼1550-1400
both RT and 400 °C, with slightly different positions; less intense at 400 °C
RT
surface carbonates is observed. In particular, the intensity of a component at 1480 cm-1 decreases most. The very small ∆ν3 value observed indicates that carbonates on 58S can be thought of as either monodentate complexes (thus involving mostly surface O2- anions) or as still ionic [CO3]2- anions (involving mainly surface Ca2+ cations). Should the latter be the case, one could suppose that the location at the surface induced a slight asymmetry in the planar XY3 ion, so that the ν3 mode is split into two very close components. Something similar is known to occur in (the bulk of) crystalline calcium carbonate, when passing from the calcite (site group symmetry D3 and E′ f E) to the aragonite modification (site group symmetry Cs and E′ f 2A′).21 At the moment, there is no way to decide whether carbonates formed at the surface of 58S are of type 2 or of type 1, except for the very small ∆ν3 (
