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Influence of the Chemical Composition on Nature and Activity of the Surface Layer of Zn-Substituted Sol-Gel (Bioactive) Glasses Valentina Aina,† Francesca Bonino,† Claudio Morterra,† Marta Miola,‡ Claudia L. Bianchi,§ Gianluca Malavasi,|| Marco Marchetti,^ and Vera Bolis*,# †

)

Dipartimento di Chimica IFM & Centro di Eccellenza Interdipartimentale “NIS”, Universita di Torino, INSTM (Italian National Consortium for Materials Science and Technology), UdR Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy ‡ Dipartimento di Scienza dei Materiali e Ingegneria Chimica - Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy § Dipartimento di Chimica Fisica ed Elettrochimica, INSTM (Italian National Consortium for Materials Science and Technology), UdR Universita di Milano, Via Golgi 19, 20133 Milano, Italy Dipartimento di Chimica, Universita di Modena e Reggio Emilia, Via Campi 183, 41125 Modena, Italy ^ Dipartimento di Chimica “G. Ciamician”, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy # Dipartimento DiSCAFF, Universita del Piemonte Orientale “Amedeo Avogadro”, INSTM (Italian National Consortium for Materials Science and Technology), UdR Piemonte Orientale, Largo G. Donegani 2, 28100 Novara, Italy

bS Supporting Information ABSTRACT: Two Zn-doped sol-gel glasses with the same ZnO content (5 wt %; 4% mol) but different overall composition have been synthesized and characterized, in comparison with a bioactive Zn-free reference glass. The role of ZnO in modifying the bioactivity of sol-gel glasses was investigated by soaking the glasses in a simple tris(hydroxymethyl)aminomethane-buffered solution (TRIS-BS), so as to maximize the solubility and to minimize back-precipitation phenomena, which will depend only on the nature and concentration of dissolved glass components. Glass dissolution/ions release in TRIS-BS was monitored by ion coupled plasma emission spectroscopy, whereas modifications of surface composition upon reaction were checked by X-ray photoelectron spectroscopy (XPS). The deposition of a Ca-P layer and the consequent crystallization to hydroxy-apatite (HA) and/or hydroxy-carbonate-apatite (HCA) at the glass surface were investigated by X-ray diffraction and Raman, Fourier transform infrared (FTIR), and XPS spectroscopies. Glass dissolution rate, back-precipitation of silica gel, and formation/crystallization of an apatite-like layer on Zn-containing glasses were found to be either inhibited or delayed, according to the overall glass composition, in that the presence of the network former ZnO component enhances glass reticulation, with the consequent formation of Si-O-Zn units. The presence of a ZnO component has no effect per se, but its influence depends on the overall composition of the glass and, in particular, on the CaO/SiO2 and ZnO/CaO ratios, which determine the nature/structure of Zn and Ca surface species. Glass surface features were investigated by the combined use of in situ FTIR spectroscopy and adsorption microcalorimetry. The role played by surface Ca species, thought to be the most hydrophilic sites, was found to be a decisive factor in both glass dissolution mechanism and formation of an apatite-like surface layer: (i) the scarce dissolution in aqueous media of a (non bioactive) low-Ca and high-silica glass is due to the high reticulation caused by the scarce population of Ca2þ cations in the role of network modifiers; and (ii) the amount of the latter species is, instead, much larger in the corresponding (moderately bioactive) high-Ca and low-silica glass, which dissolves more, although exhibiting a larger durability in aqueous solution than the Zn-free glass.

’ INTRODUCTION Glasses and glass-ceramic represent an important class of biomaterials, which are widely employed in medicine for bone repair and replacement in that they spontaneously bond and integrate with living tissues.1 The integration with the bone takes place through the formation of a thin layer of biologically active hydroxy-apatite (HA) and/or hydroxy-carbonate-apatite (HCA) at the implanted biomaterial/physiological fluids interface.2-5 This unique property is defined as bioactivity,6,7 and its development is closely dependent on both chemical composition and textural properties of the materials.8 r 2011 American Chemical Society

The first steps of the biological fixation are intrinsically inorganic in nature, as first proposed by Hench,9,10 and involve a rapid exchange of cations between the glass components and the physiological fluids, followed by a partial dissolution of the silica matrix and subsequent formation of a new silica-rich layer. The latter will provide favorable sites for the incorporation of ions from the solution. The precipitated Ca-P layer progressively Received: October 24, 2010 Revised: December 20, 2010 Published: January 18, 2011 2196

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The Journal of Physical Chemistry C crystallizes into biologically reactive HA/HCA, which is chemically and structurally equivalent to the mineral phase in bone and is responsible for the interfacial bonding.11,12 The first bioactive glass (Bioglass 45S5: 24.5% Na2O - 24.5% CaO - 45.0% SiO2 - 6% P2O5 by weight, with SiO2 and P2O5 as network formers and CaO and Na2O as network modifiers) was synthesized by Hench in the early 1970s,4,13,14 and since then a great variety of glasses and glass-ceramics have been extensively studied to design new materials with improved biological, chemical, and mechanical properties.4,5,15-18 Biocompatibility/bioactivity properties of the new materials have been carefully investigated both in vitro and in vivo. As for the bioactivity features, Kokubo et al.19,20 demonstrated that the HA/HCA layer is formed on bioactive glasses even in an acellular simulated body fluid (SBF),21 thus introducing a useful and now generally accepted method to assess the bioactivity of a glass.22 It was also observed that, by varying the chemical nature and concentration of inorganic dopants, novel important biological properties can be introduced, so that the glass can be tailored to specific clinical applications.23,24 In the early 1990s, a new way to obtain materials by sol-gel synthesis25,26 was successfully introduced also for bioactive glasses preparation.27-29 Several examples of bioactive powdery ternary (CaO-P2O5-SiO2) materials, possessing bioactivity considerably higher than melt-derived glasses, were so obtained.3,30-35 Sol-gel synthesis, typically conducted at low temperatures, yields very pure, homogeneous, and high surface area bioactive materials, which cover a large range of chemical compositions,27 including very simple ones as binary SiO2-CaO glasses.26,34,36 Further, glass porosity (a key factor for bioactivity) can be varied by controlling the sol-gel process conditions. In fact, in vitro studies proved that high specific surface area37 and porosity are responsible for the enhancement of bioactivity properties of solgel glasses with respect to the glasses obtained by the conventional melt process, as testified by the increased rate of HA/HCA formation. Unfortunately, high porosity and surface area are also responsible for the weakening of materials mechanical properties,38 so that, to overcome the disadvantage, intermediate or modifying oxides can be added to the basic glass composition. In this respect, a major role is played by ZnO, which is expected to enhance the glass reticulation and is known to promote bone formation around the implant, stimulating osteoblasts proliferation and differentiation.37,39-43 The possibility of incorporating Zn in bioactive glasses has received special interest in recent years,44 and several formulations of bioactive glasses doped with ZnO have been recently obtained, both by melting45-49 and sol-gel techniques.23,31,50,51 Interestingly, it has been observed that Zn-doped Bioglass 45S5 and, in general, Zn-doped alkali-silicate glasses exhibit, besides a promoting effect on tissue regeneration, also a reinforced structure with improved thermal and mechanical properties.47,49,52 Among other features reported in the literature, it has been observed that the presence of ZnO in (phosphate) glasses composition: (i) improves the chemical durability of melt-derived glassceramics in aqueous solutions like (simulated) body fluids,45,47,53 resulting in a decrease in the rate of apatite formation;46 and (ii) enhances SSA in sol-gel glasses, so increasing the number of sites suitable for the nucleation of Ca-phosphate precipitates.51 Because of the interest of Zn-modified glasses in clinical applications, several papers have been reported in the literature that deal with Zn influence on the bioactive process in sol-gel glasses.23,31,40,44,46,48,49,51,54-56 Nonetheless, there is still a lack of

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quantitative information on the physicochemical interactions occurring at the glasses interface and regarding the correlation of the physicochemical features with the expected biological effects. As far as biocompatibility is concerned, it has been reported by some of us that melt-quenched bioactive glasses with a ZnO content in the 5-20 mol % range exhibit significant cytotoxicity on human osteoblasts,57 whereas good biocompatibility toward murine osteoblasts (and high bioactivity in vitro) has been reported for a Zn-doped sol-gel glass (5 mol % ZnO).31 Still, in vitro and in vivo studies on Zn-glasses (based on Bioglass 45S5) recently evidenced that glass degradation processes are strictly dependent on ZnO content and that, with Zn being released very slowly in the biological fluids, osteogenic processes are favorably stimulated.56 It has also been reported that both heat treatment conditions and chemical composition of ternary SiO2-CaO-ZnO sol-gel glasses affect their bioactive response, in that they determine both textural properties and crystallinity of the material.40 Interesting chemical/mechanical features exhibited by Zncontaining glasses can be associated with the structural role played by Zn species in the glass network. A traditional view, based on IR58 and Raman59 spectroscopy, assumes that in Na-Zn-silicates Zn can exist in either tetrahedral (network former) or octahedral (network modifier) coordination depending on both Zn and Na concentration, the 6-fold coordination being preferred at low alkali concentration. X-rays absorption spectroscopy studies on multicomponent glasses60,61 claim that the network-forming role of tetrahedral Zn is responsible for the structure-reinforcing effect observed in Zn-containing glasses. By a combined experimental/computational (molecular dynamics simulations) approach,45,47,48 it was assessed that in phosphosilicate glasses Zn adopts a tetrahedral coordination (so acting as a weak tetrahedral network former), causing an overall increment of glass reticulation with respect to Bioglass 45S5. In fact, the presence of ZnO was found to delay the silicate network breakdown of a glass soaked in biological fluids, thus slowing glass dissolution and the consequent growth of a Ca-phosphate layer.45 Further, it has been observed that Zn acts as a chaperon for the insertion of P into the 3D-glass network, originating a highly branched network of interconnected Si-Zn-P tetrahedra (note that, in Zn-free glasses, P is present as isolated tetrahedra), whereas Naþ cations are preferentially associated with negatively charged [ZnO4]2- tetrahedra as charge compensators, so contributing to a drastic reduction of the overall leaching activity of the Zn-glass.47 With surface properties and microstructure being important in determining glasses bioactivity, both in vitro and in vivo,2,62 we started a research project aimed at better understanding the role played by Zn in modifying bulk and surface properties of sol-gel glasses and their reactivity in SBF. Particular attention will be paid to the relationship between Zn-glass physicochemical features, glass-dissolution/ion-release rates, and the rate of HA/ HCA layer formation, the latter taken as an indication of the potential bioactivity of the materials. In the present work, two Zn-doped sol-gel glasses with the same Zn content (5 wt %; 4% mol) but different overall composition have been synthesized and characterized, in comparison with the bioactive Zn-free reference glass termed 77S (77% SiO2 - 14% CaO - 9% P2O5 by weight).63,64 It is here recalled that, in the case of melt-derived glasses, such a low amount of Zn was demonstrated not to be cytotoxic for 2197

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Table 1. Chemical Composition of the As-Synthesized Sol-Gel Glasses, Reported as Either wt % or mol %a chemical composition (wt %)

chemical composition (mol %)

SiO2

CaO

P2O5

77S

77

14

9

77S5Zn

77

9

9

5

72S5Zn

72

14

9

5

samples

ZnO

ZnO

Scheme 1 (mol %)

Scheme 2 (mol %)

CaO network modifier

CaO charge compensator

SiO2

CaO

P2O5

80

16

4

82

10

4

4

6

4

76

16

4

4

12

4

16

a

The CaO mol % contribution as either network modifier or negative charge compensator is also quantified (1 CaO mol is required to compensate the negative charge of 1 [ZnO4]2- unit mol).

human osteoblast cells.57 With the CaO/ZnO ratio being sufficiently large in both synthesized glasses, the coordination of Zn is expected to be tetrahedral.65 As a consequence, Ca species are expected to play a dual role in that a fraction of them act as glass network modifiers and the other fraction as charge compensators for negatively charged network-forming [ZnO4]2- tetrahedra. The mobility of the network-modifiers Ca2þ cations in the glass network is expected to be higher than the mobility of the chargecompensators ones, and so the former are expected to be released faster than the latter in aqueous solution during durability tests. Our present purpose is 2-fold: (i) to investigate the role of ZnO in modifying the bioactivity of sol-gel glasses and (ii) to investigate the microstructure of the samples and their surface features as a function of the overall glass composition. As for the purpose of point (i), the study dealt with soaking the glasses in a simple tris(hydroxymethyl)aminomethane-buffered solution (TRIS-BS),63 instead of a more complex simulated body fluid (SBF) solution.66 This simplified procedure was adopted because the contact with a plain TRIS-BS is expected to maximize the solubility and to minimize the back-precipitation, so allowing one to investigate dissolution and reprecipitation processes as a function of the nature of the sole glass components. Glass dissolution/ions release in TRIS-BS was monitored by ion coupled plasma emission spectroscopy (ICP-ES), whereas the modification of the surface composition upon reaction was checked by X-ray photoelectron spectroscopy (XPS). The Ca-P layer deposition and the consequent crystallization to HA/HCA at the glass surface were investigated by a number of techniques: X-ray diffraction (XRD) and Raman, Fourier transform infrared (FTIR), and XPS spectroscopies. In particular, with Raman spectroscopy being virtually insensitive to the presence of water, the in vitro bioactivity was monitored in real in situ conditions.63 As for the purpose of point (ii), H2O was chosen as the probe molecule (to be adsorbed from the vapor phase) to assess and quantify the hydrophilic character of the glass surface, by means of the combined use of in situ infrared spectroscopy and adsorption microcalorimetry.67,68 The structure of the hydrated layer and the related surface affinity toward water69-71 is expected to play a major role in determining the behavior of biomaterials immersed in biological fluids.72,73

2. EXPERIMENTAL SECTION 2.1. Materials. Three sol-gel materials were prepared: (i) a ternary glass (77% SiO2 - 14% CaO - 9% P2O5 by weight), prepared according to refs 23,31,50,51, and (ii) two quaternary glasses, prepared by introducing in the reactant mixture 5 wt % of ZnO substituting either 5 wt % of CaO (77% SiO2 - 9% CaO 9% P2O5 - 5% ZnO by weight) or 5 wt. % of SiO2 (72% SiO2 14% CaO - 9% P2O5 - 5% ZnO by weight). The nominal

Scheme 1. CaO Network Modifier

Scheme 2. CaO Charge Compensator

chemical composition of the three glasses (termed 77S, 77S5Zn, and 72S5Zn, respectively) is reported in Table 1. The dual role of CaO, as either network modifier (scheme 1) or charge compensator of negative tetrahedral [ZnO4]2- units (scheme 2), is also evidenced and quantified. Three apatite-like nanosized materials were prepared to obtain suitable reference materials for the interpretation of FTIR-KBr and Raman spectroscopic data: (i) hydroxy-apatite (HA), (ii) a type-B hydroxy-carbonate-apatite (HCA), and (iii) a Znloaded type-B hydroxy-carbonate-apatite (Zn-HCA, 12 000 ppm Zn). All apatite-like materials were synthesized in aqueous medium by dropping a solution of H3PO4 in a Ca(OH)2 suspension for HA and HCA, and by adding also ZnCO3 for ZnHCA.74 The chemical composition of the apatite materials is Ca10(PO4)6(OH)2, Ca10(PO4)6-y(CO3)y(OH)2, and Ca10-x Znx(PO4)6-y(CO3)y(OH)2 for HA, HCA, and Zn-HCA, respectively. Glasses Preparation. The glasses were synthesized by hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) in distilled water, in the presence of triethyl phosphate (TEP), calcium nitrate [Ca(NO3)2 3 4H2O], and, for the Zn-loaded glasses, zinc nitrate [Zn(NO3)2 3 6H2O] as starting reagents, following a well-established protocol described in refs 12,63,75 All chemicals were purchased from Sigma Aldrich. TRIS Buffer Solution Preparation. The reactivity of sol-gel materials was investigated by dipping the glass powders in a TRIS buffer solution (TRIS-BS). Pure Sigma-Aldrich TRIS [tris(hydroxymethyl)aminomethane] was dissolved in distilled water to obtain a concentration of 6.1 g/L. The solution pH was lowered to 2198

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The Journal of Physical Chemistry C ∼8 by acidifying with HCl 1 M. To maximize samples dissolution rate and to inhibit reprecipitation, a sample/solution ratio of 1.5 mg/mL was used, as described in ref 63. Distilled Water. Distilled Milli-Q water was employed to prepare the TRIS-BS solution. Further, to prepare a H2O vapor reservoir to be employed in adsorption experiments, distilled water was purified in vacuo and rendered gas-free by several “freeze-pump-thaw” cycles.68,69 N2 Gas. Specpure N2 (gas purity 99,9995%) was employed for adsorption measurements at T = 77 K to: (i) determine the specific surface area (SSA) of the solid materials by BET (Brunauer, Emmet, Teller) model,76 both as-such and after soaking in TRISBS; and (ii) evaluate the possible porosity of the glass derived from TRIS-BS soaking, as previously observed in the case of fluoride-containing bioactive glasses, and described in ref 77. 2.2. Methods Reaction in TRIS. In all cases, ∼0.3 g of glass was dispersed in 200 mL of TRIS-BS and was kept under continuous stirring (to ensure optimal dispersion and homogeneity) at T ≈ 298 K in a parafilm-sealed beaker to prevent evaporation. After reaction (1 week or 1 month), the suspensions were filtered through Millipore filters (0.25/0.45 μm). The solid component of the suspensions was dried in air and then analyzed by X-ray diffraction (XRD), N2 adsorption, XPS, and FTIR spectroscopies, whereas the liquid supernatant was analyzed by ion coupled plasma emission spectroscopy (ICP-ES). X-ray Diffraction. The structural features of the glasses (both as-such and after reaction in TRIS) were analyzed by means of X-ray diffraction (XRD) by employing an X’Pert Philips (PW1830) diffractometer using a Co KR incident radiation, in the 10-70 2θ range. Pattern analysis was carried out by using X’Pert High Score software and the PCPDF data bank. Scanning Electron Microscopy. The influence of ZnO on glass samples morphology was investigated by scanning electron microscopy (SEM). Secondary electron images were obtained with a ZEISS EVO 50 XVP scanning electron microscope (SEM) equipped with a LaB6 source. An Oxford INCA X-ray energy-dispersive spectrometer (EDS) on the SEM was used for elemental analysis. Images and EDS spectra were acquired at 15 kV. Specific Surface Area and Porosity. These properties of the glasses (both as-such and after reaction in TRIS-BS) were evaluated by N2 adsorption at T = 77 K using a Micromeritics ASAP 2020 porosimeter. For SSA determination, data were processed by employing the BET model,76 whereas the BJH (Barrett, Joyner, Halenda) model78 was used to analyze mesopores, and the “t-plot” (statistical thickness method)79 was used to evaluate the presence of micropores. Before adsorption measurements, all samples were outgassed at T = 423 K for 24 h (residual pressure p ≈ 10-3 Torr, 1 Torr = 133.3 Pa), to get rid of all physisorbed contaminants. Fourier Transform Infrared (FTIR) Spectroscopy - KBr Pellets. To track the changes in vibrational modes of both bulk and surface species induced in the glasses by reaction in TRIS-BS, dried samples were mixed with KBr (glass/KBr = 1/5 weight ratio) under controlled conditions in glovebox. IR spectra of KBr pellets were recorded in the 4000-400 cm-1 spectral range, by means of a FTIR spectrometer Bruker IFS 28, using a DTGS detector to reach lower wavenumbers. In all cases, 128 scans per measurement were performed.77 Raman Spectroscopy. Raman spectra were recorded by using a Renishaw inVia Raman microscope spectrometer. A diode laser emitting at 785 nm with 65 mW output power at the sample was

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used. Photons scattered by the sample were dispersed by a 1200 lines/mm grating monochromator and simultaneously collected on a charged coupled device (CCD) camera; the collection optic was set at 20 Ultra Long Working Distance (ULWD) objective (5 μm diameter laser spot). A spectra collection setup of 10 acquisitions, each of them taking 10 s, was adopted. The samples were pressed in a thin self-supporting wafer and put in a holey gold envelope allowing laser focusing during the Raman spectrum acquisition. The pellets were immersed, by adopting a homemade manifold, into a continuously stirred TRIS-BS.63 X-ray Photoelectron Spectroscopy (XPS). XPS spectra were taken in an M-probe apparatus (Surface Science Instrument). The source was monochromatic Al KR radiation (1486 eV). A spot size of 400  1.000 μm and a pass energy of 150 eV with a resolution of 1.39 were used. The accuracy of the reported quantitative data (atomic percentage) can be estimated to be (1%.80 This technique was effective to reveal and quantify the presence at the surface of the elements composing the bulk materials, as well as to follow the evolution of the chemical species at the glass surface as a function of soaking in TRIS-BS.81 Ion Coupled Plasma Emission Spectroscopy (ICP-ES). ICPES82 was employed to analyze the chemical composition of the supernatant arising from the glass powder filtration from TRISBS, in which glasses were soaked during either 1 week (1 w) or 1 month (1 m). The supernatant solutions were analyzed by a Perkin-Elmer Optima 2000 DV spectrometer, to reveal and quantify the following elements: Ca (317.933 nm, radial torch), Si (251.611 nm, radial torch), P (213.617 nm, axial torch), and Zn (206.876 nm, radial torch).83 Adsorption Experiments. Parallel IR spectroscopy and microcalorimetry experiments of H2O vapor adsorption were performed to describe quantitatively the surface hydrophilic properties of the samples. Prior to the adsorption measurements, all samples were vacuum activated at T = 423 K for 2 h, to get rid of all physisorbed contaminants without inducing any irreversible modification of the surface structure and, in particular, to avoid any appreciable surface dehydroxylation.67,68 i. In Situ FT-IR Spectroscopy. A homemade quartz infrared cell connected to a conventional high-vacuum line (residual pressure p ≈ 10-5 Torr) was used, which allowed both in situ thermal treatments of the pure sample pellets and in situ H2O vapor adsorption experiments on the activated samples. All IR spectra were recorded, in the 4000-400 cm-1 spectral range, using a Bruker IFS 28 spectrometer equipped with a MCT cryodetector. The temperature reached by59 sample pellets under the IR beam (BT) was estimated to be some 30 C higher than the actual room temperature (rt). Absorbance IR spectra were normalized to both BET-SSA and sample weight, that is, to the total surface area exposed. ii. Adsorption Microcalorimetry. Heats of adsorption of H2O vapor were measured at T = 303 K by a heat-flow microcalorimeter (Calvet C80, by Setaram-France) connected to a high-vacuum gas-volumetric glass apparatus (residual pressure p e 10-5 Torr), following a well-established stepwise procedure.67,84 The equilibrium pressure was monitored by a transducer gauge (Ceramicell 0-100 Torr, Varian). Adsorbed amounts, normalized to the unit surface area, will be plotted in the form of volumetric isotherms (nads, μmol/m2 vs pH2O, Torr). The calorimetric outputs, routinely processed,69,85 will be reported in the form of differential heats of adsorption, as a function of the increasing water uptake (qdiff, kJ/mol vs nads, μmol/m2). In both IR-spectroscopic and volumetric-calorimetric experiments, the 2199

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reversibility/irreversibility of water adsorption was investigated through adsorption-desorption-adsorption measurements. After the first run of adsorption (ads. I), each sample was outgassed overnight (p e 10-5 Torr) at the adsorption temperature, and a subsequent second run of adsorption (ads. II) was performed. 2.3. Nomenclature. As-synthesized samples will be termed 77S, 77S5Zn, and 72S5Zn, respectively (see the nominal chemical composition in Table 1), whereas the samples reacted in TRIS-BS either 1 week (1 w) or 1 month (1 m) will be termed 77S-1w, 77S5Zn-1w, 72S5Zn-1w and 77S-1 m, 77S5Zn-1 m, 72S5Zn-1 m, respectively.

3. RESULTS AND DISCUSSION 3.1. Bioactivity. The possible bioactivity of the two Znglasses was checked by analyzing, in parallel, both the solid material and the supernatant TRIS buffer solution, and in comparison with the behavior of glass 77S, the bioactivity of which was already shown.64 The formation of an apatite-like layer at the surface of the glass (characterization of the solid material) and the changes of chemical composition in the buffer solution (characterization of the species released upon reaction in TRIS-BS) have been monitored. 3.1.1. Characterization of the Solid Material Structural and Morphological Characterization. The presence of Zn in glasses 77S5Zn and 72S5Zn was confirmed by EDS (spectra not reported for brevity). XRD patterns of the glasses, either as-synthesized (A), or after 1 week of soaking in TRIS-BS (B) are reported in Figure 1. Literature XRD patterns of HA86 and HCA87 species are also reported for comparison purposes. The glassy structure of all as-synthesized materials is witnessed, in the XRD traces, by the presence of a broad halo (in the 20 and 30 2θ interval), typical of amorphous materials,23 and by the absence of sharp peaks. The presence of ZnO as a fourth component in the glass does not influence the overall shape of the XRD traces, confirming that the amorphous nature of the parent Zn-free glass 77S is preserved. XRD patterns of the three glasses after 1 week dipping in TRIS-BS indicate that the precipitation/crystallization of apatite-like Ca-phosphate occurred only in the case of the Zn-free bioactive glass 77S (trace a0 ), as witnessed by peaks at 2θ ≈ 32 and 37 typical of apatite materials (see PCPDF reference code 00-001-1008 and refs 23,31,50,51). In the case of the glasses 77S5Zn (trace b0 ) and 72S5Zn (trace c0 ), no evidence of crystalline apatite peaks is observed, suggesting that the presence of ZnO tends to inhibit glass bioactivity. It is worth noting that HA XRD pattern is hardly discriminated from that of HCA. For this reason, it seems not possible from the present XRD data to determine which apatite phase is formed at the glass surface. In Figure 2, SEM micrographs of the reference glass 77S (A) and of the two Zn-containing glasses 77S5Zn (B) and 72S5Zn (C) are reported. From the inspection of the three images, it is rather evident that the microparticles making up the Zn-free glass 77S are sharp-edged, whereas the Zn-glasses microparticles are smoother and less well-defined. This effect is particularly evident for the glass 72S5Zn (C), which is characterized by a low silica content (72% wt instead of 77% wt). The micrographs obtained at higher magnification (not reported for the sake of brevity) do confirm this evidence. In all glasses, the distribution of the grain size is highly heterogeneous (from a few micrometers up to ∼40 μm), with a large proportion of small microparticles

Figure 1. (A) XRD patterns of the amorphous as-synthesized glasses 77S (a), 77S5Zn (b), and 72S5Zn (c). (B) XRD patterns of the reacted glasses 77S-1w (a), 77S5Zn-1w (b), and 72S5Zn-1w (c) after 1 week of soaking in TRIS buffer solution. XRD patterns of hydroxy-apatite (HA; red lines) and of type-B hydroxy-carbonate-apatite (HCA-B, blue lines) are also reported for comparison purposes.

(grain size even lower than 1 μm) in the case of the low-silica glass 72S5Zn. Textural features including SSA, micropores volume, and pore width for the materials as-synthesized and after reaction in TRISBS are listed in Table 2. BET-SSA of high-silica glasses 77S and 77S5Zn is quite similar (338 vs 312 m2/g), while that of the low-silica glass 72S5Zn is significantly lower (230 m2/g). This datum confirms the major role played by the silica matrix in determining the micromorphology of powdery silicate glasses. It is worth noting that the high-silica glasses 77S and 77S5Zn are microporous, whereas the low-silica glass 72S5Zn does not exhibit significant microporosity. The average pore width of the two as-such high-silica glasses (77S and 77S5Zn) is virtually not affected by the presence of the ZnO component (30 Å in both cases), whereas in the case of the low-silica glass (72S5Zn) the average pore width is some 25% larger (38 Å). Glasses morphology turns out to be significantly modified after 1 week reaction in TRIS-BS. Among the three glasses, the most affected one is, as expected, the high-silica Znfree bioactive glass 77S, the behavior of which has been already described in ref 64. BET-SSA is decreased by ∼25% (from 338 to 254 m2/g), microporosity is suppressed, and both effects are interpreted as caused by the fast precipitation of Ca-phosphate. 2200

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Meanwhile, both volume and average width of the mesopores increase (the latter, from 30 to 50 Å), and this effect is most likely ascribable to the process of glass dissolution (ions leaching). By contrast, neither BET-SSA nor porosity features of the high-silica Zn-glass (77S5Zn) was found to change after 1 week

of soaking in TRIS-BS, and the same behavior was observed also for the low-silica Zn-glass (72S5Zn). However, by dipping the two Zn-glasses in TRIS-BS for 1 month, the response of the two materials was different, in that: (i) still no significant changes of textural features were observed for the high-silica 77S5Zn; (ii) in the case of the low-silica 72S5Zn, an appreciable decrease of BET-SSA (∼25% reduction, similar to that found for 77S after 1 week of dipping) and an increase of both average pore width (from 42 to 59 Å) and pore volume were observed. These morphological results, concerning the different reactivity of the three glasses, can be taken as a first indication of possible different bioactivity features, to be specifically investigated in the following. X-ray Photoelectron Spectroscopy (XPS). The surface composition of the glasses as such and after 1 week of reaction in TRIS-BS was monitored by XPS, as described in Table 3. The elements Si, O, Ca, Zn, and P were investigated in that characteristic of the glass composition and possibly variably involved in glass dissolution and/or ions leaching. The amount of the element C was also monitored, in that present at the surface as a consequence of the formation of carbonate-like species and/or as surface contaminant. The Ca/Si and Ca/P ratios have been reported to monitor glass dissolution and the deposition of Ca phosphate, respectively. The Zn/Ca ratio is also reported to quantify the ratio between the two metal cations possibly exposed at the surface, taking into account that ZnO and CaO play a different role in the glass network, the former acting typically as network former and the latter as both network modifier and compensator of the negative charge of tetrahedral [ZnO4]2- units. Table 3. XPS Data: Elements Present at the Surface (Data Reported as at. %) and Ca/P, Ca/Si, and Zn/Ca Ratios samples

Figure 2. SEM micrographs of Zn-free glass 77S (A) and of Zncontaining glasses 77S5Zn (B) and 72S5Zn (C).

Si2p

O1s Ca2p Zn2p P2p

C1s

Ca/Si Ca/P Zn/Ca

77S

30.1 58.9

3.0

1.0

6.9

0.10

77S-1w

15.0 51.6

8.6

1.3 23.5

0.57

3.0 6.6

77SZn5

28.3 63.3

1.5

1.0

5.2

0.05

1.5

77SZn5-1w 28.5 59.6

1.0

0.6

2.0

8.2

0.04

0.75

0.60

72SZn5

25.2 59.8

2.7

0.8

2.0

9.5

0.11

1.35

0.30

72SZn5-1w 27.1 57.1

3.4

0.3

1.6 10.5

0.13

2.1

0.089

Table 2. BET-SSAa and Porosity Features of Glasses 77S, 77S5Zn, and 72S5Zn, As Synthesized and after Reaction in TRIS-BSb BET (m2/g)

t-plot micropore volume (cm3/g)

t-plot micropore area (m2/g)

77S

338

0.109

181

77S-1w

254

77S5Zn

312

0.088

77S5Zn-1w

296

0.085

77S5Zn-1m

280

72S5Zn 72S5Zn-1w

230 248

5Zn72S-1 m SiO2 sol-gel

samples

a

BJH adsorption mesopores volume (cm3/g)

BJH adsorption average pore width (Å)

0.170

30

0.315

50

147

0.161

30

130

0.185

42

0.080

120

0.190

40

0.001

8

0.203 0.238

38 42

170

0.311

59

492

0.185

42

It is recalled that the accuracy of surface areas obtained through the universally adopted BET method is relatively low ((5%), whereas the instrumental accuracy and reproducibility of data obtained with modern automatic gas-volumetric instrumentation are quite high. b Data for an all-silica specimen are also reported for further comparison purposes. The all-silica specimen was prepared by the sol-gel technique in the Laboratory of Prof. F. Pinna, University of Venezia, Italy. 2201

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The Journal of Physical Chemistry C Let us consider first the surface composition of the glasses as such (rows 1, 3, and 5 of the table). (i) As expected, Si content is lower in Zn-containing samples with respect to the parent 77S glass, and mostly in the case of the low-silica 72S5Zn (25.2 and 28.3 vs 30.1). (ii) O content does not vary significantly but for glass 77S5Zn, which seems slightly richer in O than the other two (63.3 vs 59.8 and 58.9). (iii) As expected, Ca surface content in 77S5Zn is significantly lower than in the parent Zn-free 77S (1.5 vs 3.0), and in 72S5Zn only slightly lower (2.7 vs 3.0).65 (iv) Zn surface content is negligible for the high-silica glass 77S5Zn, whereas it is evident for the lowsilica glass 72S5Zn, indicating that the presence/absence of surface Zn species strongly depends on the overall composition of the glass. (v) P surface amount is much larger in the glass 72S5Zn than in the two high-silica samples 77S and 77S5Zn (2.0 vs 1.0 and 1.0), indicating that a decreased amount of silica in the glass composition favors the surface exposition of P species (to be possibly interpreted in terms of competing surface anionic species). (vi) The surface amount of C is definitely larger for 77S than for 77S5Zn (6.9 vs 5.2), as expected. In fact, the surface of the latter system is less rich in Ca species, responsible for the formation of surface carbonates through the reaction with atmospheric CO2 (vide infra IR spectra, Figure 6, traces a and b, respectively). Conversely, far less expected is the dramatic increase of C content at the surface of the low-silica glass 72S5Zn with respect to both 77S and 77S5Zn (9.5 vs 6.9 and 5.2, respectively). This can be interpreted as an indication that a lower Si content allows surface Ca species to be more reactive toward carbonatation (possibly, another aspect of the competition between/among surface anionic species). (vii) The Ca/Si ratio is dramatically lower for the glass 77S5Zn with respect to the reference glass 77S (0.05 vs 0.10), as expected on the basis of the lower overall Ca content. Conversely, the Ca/Si ratio is virtually unchanged for the lowsilica 72S5Zn sample with respect to the glass 77S (0.11 vs 0.10). (viii) The Ca/P ratio is significantly lower for both 77S5Zn and 72S5Zn with respect to the reference glass 77S (1.5 and 1.35 vs 3.0, respectively). This was actually expected for the 77S5Zn sample, the overall composition of which is the poorest in Ca, but is somehow puzzling for the glass 72S5Zn, the overall Ca content of which is close to that of the reference 77S material. This is another indication that the Zn/Ca ratio (zero in 77S5Zn, 0.30 in 72S5Zn) does affect the structure of the surface as a consequence of the dual role played by calcium species in Zn-containing glasses. By inspection of rows 2, 4, and 6 in Table 3, relative to 1 week reacted glasses, it turns out that: (i) surface O and Si contents decrease significantly only in the case of the highly reactive glass 77S, giving clear indication of an abundant solubilization of the silica component; (ii) surface Ca content increases significantly for the reacted 77S (from 3.0 to 8.6) and slightly for the reacted low-silica glass 72S5Zn (from 2.7 to 3.4), indicating a considerable precipitation of Ca phosphate for the former system, and a little precipitation for the latter; however, surface Ca content decreases by some 30% for the reacted low-Ca glass 77S5Zn, indicating that the solubilization of Ca species is not followed by precipitation of Ca-phosphate; (iii) surface P content increases slightly in reacted 77S (from 1.0 to 1.3) and significantly in 77SZn5 (from 1.0 to 2.0), whereas it decreases slightly in the reacted glass 72S5Zn (from 2.0 to 1.6);65 (iv) surface Zn content increases in reacted 77S5Zn (it is recalled that no surface Zn species were detected on this glass in its starting form; vide supra), whereas it decreases in the reacted glass 72S5Zn

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(from 0.8 to 0.3); it is worth noting that the increase/decrease of surface Zn species in the two Zn-containing glasses can be correlated to the increase/decrease of surface P species in the materials; (v) the Ca/Si ratio increases, as expected, only in the reference material 77S (from 0.10 to 0.57), which is largely soluble in TRIS-BS; conversely, the Ca/Si ratio remains virtually unchanged in the case of the two Zn-glasses, and this result may be taken as an indication that the presence of Zn depresses the glass solubility; and (vi) as for the Ca/P ratio, which can be taken as an indicator of the potential bioactivity of the glasses, it was found that the ratio increases significantly in the case of the bioactive Zn-free material 77S (from 3.0 in the glass as such to 6.6 in the reacted glass). A Ca/P ratio of 6.6, surprisingly high by considering that in HA the Ca/P ratio is just 1.67, is likely to be due to the simultaneous precipitation of calcium carbonate, as witnessed by the large increase of C species for this glass (from 6.9 for the glass as-such to 23.5 for the reacted glass). In the lowsilica and high-Ca glass 72S5Zn, the Ca/P ratio increases to a lesser extent (from 1.35 in the glass as such to 2.1 in the reacted glass), whereas it decreases in the high-silica (and low-Ca) glass 77S5Zn (from 1.5 in the glass as such to 0.75 in the reacted glass). From the whole set of XPS data, it turns out that the surface species distribution in the three glasses (either as-such or after reaction in TRIS-BS) is strongly affected by the overall composition of the materials. Both Zn-containing glasses exhibit a lower propensity to dissolve in TRIS-BS than the Zn-free 77S, and also a lower propensity (if any) to allow an apatite-like layer to grow at the interface. However, the two Zn-glasses exhibit some remarkable differences in both pristine surface features and behavior toward TRIS-BS. In particular, the low-silica glass 72S5Zn, which exhibits Zn species at the surface, appears more reactive in both dissolution and apatite-like layer growth than does the high-silica glass 77S5Zn, which is poor in Ca and does not exhibit Zn species at the surface. As a matter of fact, the different features of the two Zn-containing glasses are bound to be due to the different Zn/Ca ratio and to the different role of Ca species in the two materials. The low-calcium glass 77S5Zn is characterized by a comparable proportion of charge compensators and network modifiers Ca2þ cations. The former species are tightly bound to the negative [ZnO4]2- units and migrate toward the surface far more slowly than the latter ones. As a consequence, charge-compensating cations are hardly released in TRIS-SB, opposite to what happens to the network modifiers Ca2þ cations, which are more abundant in the high-Ca glass 72S5Zn (vide supra Table 1) and are more easily released in aqueous solution. FTIR-KBr Spectroscopy. Figure 3 reports the IR spectra of glass/KBr pellets run in the 900-500 cm-1 interval. The features of the three as-synthesized glasses 77S, 77S5Zn, and 72S5Zn (traces a, b, c, respectively) are contrasted with the ones of the same glasses after 1 week of soaking in TRIS-BS (traces a0 , b0 , c0 , respectively). The glasses 77S5Zn and 72S5Zn were investigated also after 1 month reaction in TRIS-BS (traces b00 and c00 , respectively). Further, the spectral features of suitable reference materials, (i) a hydroxy-apatite (HA, trace 1), (ii) a type-B hydroxy-carbonate-apatite (HCA, trace 2), and (iii) a type-B Zn-loaded hydroxy-carbonate-apatite (Zn-HCA, trace 3), are also reported in the figure for comparison purposes. Let us start from the spectral pattern of the parent bioactive glass 77S (traces a).64 From the 550-650 cm-1 spectral interval, it is clearly evident that, after 1 week of soaking in TRIS-BS, the broad and ill-resolved phosphates band present in the pristine 77S sample (trace a) becomes well resolved into two discrete 2202

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Figure 3. FTIR spectra, in the 900-500 cm-1 range, of glass/KBr pellets. Traces a, b, c: as-synthesized glasses 77S, 77S5Zn, and 72S5Zn, respectively. Traces a0 , b0 , c0 : 77S-1w, 77S5Zn-1w, and 72S5Zn-1w glasses (soaked 1 week in TRIS-BS). Traces b00 , c00 : 77S5Zn-1 m and 72S5Zn-1 m glasses (soaked 1 month in TRIS-BS). Trace 1, hydroxy-apatite (HA); trace 2, type-B hydroxy-carbonate-apatite (HCAB); trace 3, type-B Zn-hydroxy-carbonate-apatite (Zn-HCA-B).

components centered at ∼566 and ∼600 cm-1, respectively (trace a0 ), which are typical of crystalline apatite P-O vibrational modes.77 In fact, by comparing the spectrum of reacted 77S (trace a0 ) with the spectra of the two apatitic reference materials HA (trace 1) and HCA (trace 2), the two spectral features are easily assigned to the crystallization of Ca-phosphate, partly present on the starting material and partly precipitated at the interface layer as a consequence of the reaction in TRIS-BS. In the spectrum of the reference HCA material (trace 2) is well evident a band located at 633 cm-1, assigned to the carbonate component of HCA,63 which is not visible in the reacted glass 77S. This datum and the close similarity of traces a0 and 1 suggest that, in the early stages of crystallization on 77S, Ca-phosphate is mainly in the form of HA rather than HCA. It is worth noting that by recording the IR spectrum of the reacted 77S glass after a couple of weeks exposure to the atmosphere, also the 633 cm-1 band becomes visible. It is worth noting that by FTIR-KBr, opposite to XRD (vide supra) and Raman (vide infra) methods, it is possible to discriminate HA and HCA materials. It appears quite clearly that in the early stage of the process HA and not HCA is formed at the glass/fluid interface. HA then tends to transform into HCA as a consequence of the contact with atmospheric CO2 in humid environment. By contrast, in the case of the high-silica glass 77S5Zn (trace b), no apatite-like peaks are observed, either after 1 week (trace b0 ) or after 1 month reaction in TRIS-BS (trace b00 ). This datum indicates that this low-Ca Zn-glass in not able to induce the backprecipitation of Ca-phosphate and subsequent crystallization of an apatite-like layer at its surface. As for the low-silica 72S5Zn glass, the behavior is somehow intermediate between those of the two high-silica materials 77S and 77S5Zn. The spectra shown in the figure indicate that, after 1 week of reaction (trace c0 ), the

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intensity of the broad band at ∼600 cm-1 (already present in the pristine glass, trace c) is simply increased, without showing yet any two-components resolution. Conversely, after 1 month of reaction, two discrete components centered at ∼566 and∼ 600 cm-1, typical of crystalline apatite-like phases of the HA type, are evidenced (trace c00 ). (Note that, as for the reacted glass 77S, the carbonate-component band located at 633 cm-1 is not visible in the just-reacted glass 72S5Zn.) The latter datum is important: it does confirm the precipitation and subsequent crystallization of Ca-phosphate at the surface of 72S5Zn glass, and thus indicates that, in this Ca-rich Zn-glass, the formation of an apatite-like layer is not inhibited but simply appreciably delayed. Also, this confirms what was already suggested by the analysis of morphological changes produced in the TRIS-reacted glasses and by XPS data. For further comparison, also the IR spectrum of a Zn-containing HCA was recorded (trace 3). No specific bands nor changes in the spectral position and/or relative intensity of the typical crystalline apatite P-O vibrational modes were observed. Should then Zn be contained in the crystalline apatitic phase slowly formed at the surface of 72S5Zn, it is most likely that the spectrum would result (as it does) undistinguishable from that of plain HA/HCA. Raman Spectroscopy. Figure 4 reports in situ Raman spectra (recorded in the spectral region 1200-800 cm-1) relative to the three glasses, either just-synthesized or soaked in TRIS-BS for different times. Note that, for the starting Raman spectra (traces a, b, and c, respectively), no plain “as-such” samples but specimens really “just-synthesized” and “kept-dry” were used, to reduce to a minimum the extent of carbonates surface contamination brought about by contact with the atmosphere. It is in fact known88 that, on silica-based glasses exposed to the atmosphere, the formation of (mainly-Ca) surface carbonates is a rather slow process and requires the simultaneous presence of CO2 and water vapor. Still, even traces of surface carbonates induce in the Raman spectrum strong fluorescence that, beyond a certain level, renders nonreadable the vibrational spectroscopic response looked for. In Figure 4A, the (known) Raman spectroscopic response of the reference Zn-free bioactive glass 77S is illustrated63 and is compared to the spectra of the reference apatite-like materials (HA, trace 1; HCA-B, trace 2; Zn-HCA-B, trace 3). Figure 4B and C shows the Raman spectra relative to the glasses 77S5Zn and 72S5Zn, respectively. The broad and complex band located in the 1080-1040 cm-1 interval in the glass 77S (trace a, A) is due to (a small amount of) carbonate-like species that, despite the precautions adopted, are formed at the surface of the glass upon short contact with the atmosphere. Conversely, the 950 cm-1 band is assigned to a ν(P-O) mode of the phosphate-like species present in the assynthesized glass of CaO-SiO2-P2O5 composition. By comparing the spectral features of the glass as-such with those of the same glass soaked 1 week in TRIS-BS (trace a0 ), it turns out that: (i) the phosphate-like ν(P-O) feature of the 1 week reacted glass (77S-1w) is located at higher frequency with respect to the glass as-such (∼960 vs ∼950 cm-1) and is definitely sharper; (ii) the ∼960 cm-1 peak of 77S-1w is quite similar, in both spectral position and bandwidth, to that observed at the same frequency in HA (trace 1), in HCA-B (trace 2), and in Zn-HCA-B (trace 3); and (iii) also in the higher-ν range (1080-1040 cm-1) the spectrum of 77S-1w (trace a0 ) became modified, in that the broad and unresolved spectral feature ascribed to surface carbonate contaminants evolved toward a partly resolved doublet band, reminiscent of two well-resolved peaks at ∼1072 and ∼1040 cm-1 2203

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Figure 4. Raman spectra (spectral region 1200-800 cm-1). (A) Trace a, glass 77S just-synthesized; trace a0 , 1 week soaked glass 77S-1w; trace 1, hydroxy-apatite (HA); trace 2, type B hydroxy-carbonate-apatite (HCA-B); trace 3, type B Zn-hydroxy-carbonate-apatite (Zn-HCA-B). (B) Trace b, 77S5Zn just-synthesized; trace b0 , 1 week soaked glass 77S5Zn-1w; trace b00 , 1 month soaked glass 77S5Zn-1 m. (C) Trace c, 72S5Zn justsynthesized; trace c0 , 1 week soaked glass 72S5Zn-1w; trace c00 , 1 month soaked glass 72S5Zn-1 m.

present, although with different relative intensities, in both reference HA and HCA-B systems (traces 1 and 2, respectively). All of these features confirm that a crystalline hydroxy-apatite-like structure has grown at the glass 77S/TRIS-BS interface, after only 1 week of contact. In Figure 4B, the spectra obtained for the high-silica glass 77S5Zn are illustrated. The features of the just-synthesized sample (trace b) are compared to those of the 1 week (trace b0 ) and the 1 month (trace b00 ) soaked glass. The overall spectrum of the sample as-such reproduces closely that of the glass 77S discussed above, but for the intensity of the bands (see the dramatic scale difference in sections A, B, and C of the figure). Conversely, the spectra of the sample soaked in TRIS-BS (either 1 week, trace b0 , or 1 month, trace b00 ) do not reveal any significant change in the structure of the interface layer. Thus, also from Raman spectroscopic data, it is inferred that the high-silica Zn-glass does not allow the precipitation/crystallization of a surface apatite-like layer. It is worth recalling that, for this system, the Ca/P ratio determined by XPS (vide supra) was significantly lower than that of the pristine glass, indicating that the Ca and P species released during the reaction in TRIS-BS (see next section) remain in the supernatant solution without precipitating onto the silica layer. Still, the Raman spectra of the low-silica 72S5Zn glass (Figure 4C) tend to confirm that the behavior of this glass is somehow intermediate between those of the two high-silica glasses 77S and 77S5Zn. In fact, also in this case, no crystalline apatite-like peaks are observed in the 1 week soaked glass 72S5Zn-1w (trace c0 ). However, by comparing the spectrum of the glass 72S5Zn assuch (trace c) with the spectrum of the 1 month soaked glass 72S5Zn-1 m (trace c00 ), it is rather evident that the ν(P-O) band of phosphate-like species is, in the latter case, located at higher frequency with respect to the pristine material (∼962 vs ∼955 cm-1) and is also somewhat sharper than in the case of the glass as-such. Also, the higher-ν spectral region (1100-1000 cm-1), still not resolved into two distinct components, presents an increased relative intensity. These observations are further indication, in addition to those deriving from XRD, FTIR-KBr, and XPS (vide supra), that 72S5Zn

glass is able to induce the precipitation/crystallization of an apatite-like layer at its surface, even if the process is significantly reduced and delayed with respect to the bioactive glass 77S. This feature allows one to classify, in principle, the glass 72S5Zn as potentially bioactive and, a positive aspect, possessing an improved durability in physiological solutions with respect to the parent Zn-free glass 77S. 3.1.2. Characterization of the Species Released upon Reaction in TRIS-BS: Ion Coupled Plasma Emission Spectroscopy (ICP-ES). The chemical composition of the supernatant TRISBS was monitored by ICP-ES spectroscopy as a function of soaking time. The ppm concentrations of Ca (A), Si (B), P (C), and Zn (D) species released in TRIS-BS after soaking the three glasses of interest are reported in Figure 5. Mean % dispersion per element, calculated for these ICP-ES data, was ∼5% for Ca, ∼5% for Zn, ∼6% for P, and ∼11% for Si. By inspection of Figure 5A, it is evident that (i) Ca species are initially released by Zn-containing glasses to a definitely lower extent than by the Zn-free glass 77S, and (ii) their amount in the supernatant solution keeps growing for at least 2 days, whereas the maximum Ca concentration in the supernatant solution is reached, for the Zn-free glass 77S, in less than one-half an hour (see Table 1SM in the Supporting Information). However, major differences have been observed between the two Zn-glasses. First, the low-silica glass 72S5Zn was found to release Ca to a much larger extent than the high-silica glass 77S5Zn, consistent with both the larger overall CaO amount and the larger proportion of network modifiers Ca2þ cations in the former system. Second, after 2 days of soaking, the Ca-release by glass 72S5Zn approaches very closely the amount detected for 77S glass. The same trend as for Ca was observed for Si species present in the supernatant solution, as illustrated in Figure 5B. The concentration of P species in solution (Figure 5C) increases very fast in the early stages of the reaction for the Zn-free glass 77S, and then (after less than one-half an hour) it starts to decrease, approaching zero in 1 day. This is clearly a consequence of the precipitation of a Ca-phosphate layer at the glass surface, as already evidenced by XRD, FTIR, and XPS data (vide supra). It is worth 2204

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Figure 5. Chemical composition (ppm concentration) of the supernatant after soaking the glasses 77S (9), 77S5Zn (b), and 72S5Zn (2) in TRIS-BS, as monitored by ICP-ES spectroscopy. (A) Calcium Ca; (B) silicon Si; (C) phosphorus P; (D) zinc Zn species released in TRIS-BS as a function of soaking time in days. Mean % dispersion per element, calculated for these ICP-ES data, was ∼5% for Ca, ∼5% for In Zn, ∼6% for P, and ∼11% for Si.

noting that the precipitation of Ca-phosphate was not so clearly evidenced by the Ca concentration profile discussed above (A) because, with the Ca amount being released quite large, only a slight decline in its concentration was observed. Conversely, in Zn-glasses the P species concentration increases quite slowly (as already observed for Ca-species) and is definitely larger for the low-silica glass 72S5Zn than for the high-silica glass 77S5Zn. Still, in the case of 72S5Zn, after ∼2 days of soaking, the P concentration starts to decrease, confirming that a Ca-phosphate layer is being formed at the 72S5Zn/TRIS-BS interface. In the case of 77S5Zn, the released amounts of Ca and P species seem not to reach the critical concentration that allows the precipitation of Ca-phosphate. Also, Zn release in TRIS-BS is larger, at any time of soaking, for the low-silica glass 72S5Zn than for the high-silica glass 77S5Zn, as illustrated in Figure 5D. In all cases, ICP data do agree with what was suggested above by XRD, FTIR, and XPS data. In fact, ICP results indicate that: (i) the presence of Zn in the glass composition delays the dissolution of the glass, so improving the chemical durability of the glass with an overall reduction of ion leaching; (ii) the formation of an apatite layer at the interface takes place only at the surface of the low-silica glass 72S5Zn, and for this latter material it requires a definitely longer contact time with TRIS-BS than for the Zn-free bioactive glass 77S; and (iii) the formation of a Ca-phosphate layer at the glass/TRIS-BS interface, which is in general taken as an indication of developing bioactivity, is not inhibited by the presence of ZnO per se, but depends on the overall glass composition and, in particular, on the ZnO/CaO ratio, which determines nature and structure of surface species. 3.2. Characterization of the Surface Structure by Water Vapor Adsorption. To better understand the role played by nature and structure of surface species on the behavior of the glasses in contact with physiological fluids, the interaction of H2O vapor with the “clean” surface of the glasses has been

investigated. It is well-known that the surface interaction with water molecules dominates the initial contact of whichever biomaterial with biological fluids.72,73 H2O molecules are rapidly adsorbed at the solid surface to form a H-bonding network, the structure of which is expected to be quite different from that of liquid water. In fact, the former is likely to be strongly affected by the nature/structure of the underlying surface. In Situ FTIR Spectroscopy. BT IR spectra obtained in the 3800-1350 cm-1 region will be considered first, to describe the surface features of as-synthesized and mild vacuum-activated glasses. The spectra relative to the glasses 77S, 77S5Zn, and 72S5Zn (traces a, b, c, respectively) are reported in Figure 6, together with the spectrum of an all-silica sol-gel specimen (see Table 2), inserted for comparison purposes (trace d). By comparing the spectra, it turns out that the sharp peak located at 3745 cm-1, typical of isolated (i.e., H-bond free) Si-OH species and ubiquitous in any silica-based material,89-91 is more or less evident, according to the chemical composition of the material. In particular, the band is well visible in the reference all-silica spectrum (trace d) and is still relatively sharp and rather evident in both Zn-containing glasses (traces b and c, corresponding to 77S5Zn and 72S5Zn, respectively), whereas it is hardly distinguished at all in the Zn-free bioactive glass 77S (trace a). Further, in all glasses, the isolated Si-OH band is slightly shifted toward lower frequencies with respect to the 3745 cm-1 band of the all-silica system, indicating that the acidic strength of isolated silanols is slightly enhanced, as expected, by the presence of surface Ca and/or Zn species. At lower frequencies, a broad band of H-bonding interacting Si-OH, typical of all the all-silica materials,89-91 is observed also in the case of the mixed oxides investigated. The apparent maximum of this broad band is shifted toward lower wavenumbers with respect to the all-silica band (trace d) in all cases examined, but for the low-silica glass 72S5Zn (trace c). 2205

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Figure 6. Beam-temperature (BT) IR spectra (3800-1350 cm-1 region) of glasses 77S, 77S5Zn, and 72S5Zn (traces a, b, and c, respectively), in comparison with an all-silica sol-gel reference material (trace d). All materials were outgassed at T = 423 K.

The band, still with the exception of 72S5Zn (trace c) and of the reference all-silica specimen (trace d), is quite broad and has an apparently structured aspect, which is typically observed with microporous systems and can be ascribed to extended perturbative effects within narrow cavities. Also, in fact, both the glass 72S5Zn and the pure SiO2 material used in the present work are not microporous (vide supra, Table 2). Further, due to the intensity and broadness of the low frequency band of H-bonded OH terminations, it was not possible to distinguish any component specifically ascribable to Ca-OH and/or Zn-OH species, possibly present at the surface of our mixed-oxide systems. Coming now to the lower-ν side of the IR spectra reported in Figure 6, the bands at 1994, 1870, and 1640 cm-1, which are typical of the structural overtone and combination bands of the silica network (trace d),89,90 are present, as expected, also in all our silica-based glasses (traces a-c). Conversely, major differences among the different samples are observed in the 1600-1400 cm-1 region, that is, the spectral region typical of the vibrations of surface carbonate-like species.88 In fact, (obviously) no bands in this region are observed for the all-silica specimen, whereas a sharp band doublet is well evident at 1503 and 1430 cm-1 for the parent glass 77S, as already described in ref 64. This doublet is typical of ionic carbonate surface species of the aragonite type92 (the antisymmetric and symmetric νOCO modes, degenerate in the D3h CO32- ion, are slightly resolved by the surface asymmetry), which are formed at cationic sites on the glass surface (Ca2þ species) upon reaction with atmospheric water and CO2.88 A similar reactivity is revealed also in the case of the low-silica glass 72S5Zn (trace c), the chemical composition of which includes a large CaO component, but not for the high-silica and low-Ca glass 77S5Zn (trace b; vide supra Table 1). (Note that the apparently different intensity of carbonate bands in spectra a and c has virtually no quantitative meaning, as in these spectra there are no fundamental solid bands of constant intensity to be used as internal reference standard, whereas

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Figure 7. Differential IR spectra (1750-1500 cm-1 region) of H2O molecularly adsorbed at BT on glasses 77S (traces a, a0 ), 77S5Zn (traces b, b0 ), and 72S5Zn (traces c, c0 ). Differential spectra were obtained by subtracting the bare-sample spectrum (i.e., the IR spectrum of the sample vacuum-activated at T = 423 K) from the IR spectrum of the sample either in equilibrium with ∼6 Torr of H2O (dashed-line traces a, b, c) or after evacuation (at BT, 1 h) of the reversible adsorbed phase (solid-line traces a0 , b0 , c0 ).

obviously meaningful is the presence/absence of carbonate bands observed in spectra a-c/b-d). The IR spectra of the activated samples in equilibrium at BT with ∼6 Torr of H2O vapor were then recorded. The whole spectra in the 3800-1350 cm-1 region will not be reported for the sake of brevity. It is just recalled that the band at 3745 cm-1, typical of isolated Si-OH groups, progressively disappears upon water uptake, whereas the broad band located in the 3500-3000 cm-1 region increases and broadens, as expected of an extended H-bonding network being created at the water-oxide interface. Conversely, changes of major interest are observed in the band located at ∼1630 cm-1, associated with the bending mode (δHOH) of molecularly adsorbed water. The relevant differential IR spectra, run in the 1750-1500 cm-1 region, are reported in Figure 7. Differential spectra were obtained by subtracting the bare-sample background spectrum (i.e., the IR spectrum of the sample vacuum-activated at T = 423 K) from the IR spectrum of the sample either in equilibrium with ∼6 Torr of H2O [77S (a), 77S5Zn (b), and 72S5Zn (c)], or after evacuation (1 h) of the H2O phase reversibly adsorbed at BT [77S (a0 ), 77S5Zn (b0 ), and 72S5Zn (c0 )]. The integrated areas (cm-1) of the ∼1630 cm-1 band are reported on the right side of the figure for the individual samples (the bands have been normalized to the exposed surface area, so that their intensities can be directly compared). In particular, the intensity of the ∼1630 cm-1 band relative to the samples in contact with H2O (dashed traces a, b, c) allows one to compare the total water uptakes, whereas the intensity of the same band after vacuum removal of the reversibly adsorbed phase (solid traces a0 , b0 , c0 ) allows one to compare the amounts of water irreversibly held at the surface. It turns out that (i) the intensity of the ∼1630 cm-1 band relative to the samples in 2206

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The Journal of Physical Chemistry C equilibrium with H2O increases in the order: 77S5Zn (15.5) < 77S (21.8) < 72S5Zn (27.7); (ii) in all cases the ∼1630 cm-1 band is still present, although reduced in intensity, after outgassing the samples at BT, indicating that in such conditions a significant fraction of molecular H2O remains tightly bound to the surface; and (iii) also the intensity of the ∼1630 cm-1 band not removed by BT evacuation increases in the series: 77S5Zn (8.4) < 77S (11.5) < 72S5Zn (13.2). The first indication of these IR spectral results is that: (i) the affinity toward water of all investigated glasses is quite high, as expected of still largely hydroxylated surfaces; and (ii) a large fraction of the overall water uptake (some 50% of the amount adsorbed at ∼6 Torr) is essentially due to a (molecular) coordinative adsorption, as indicated by the abundant partial irreversibility of the ∼1630 cm-1 band. Note that the possibility to reveal whether the irreversible fraction of water adsorption is also due, in part, to a dissociative chemisorption (often occurring at the surface of ionic oxides,23,31,50,51 although preliminarily dehydrated at medium-high temperatures) is missed in our case in that it has not been possible to detect, in the OH stretching region, any significant spectral evidence attributable in a straightforward way to the formation of new OH groups. In fact, after vacuum removal of reversibly adsorbed H2O from all of the examined samples, neither the sharp ∼3745 cm-1 band (free OH groups) nor the broad absorption below 3600 cm-1 (H-bonded OH species) did recover the shape/intensity they had before water adsorption. However, in this respect, we must consider that each one of the coordinated molecules constituting the abundant irreversible water phase has two possibilities of H-bond interacting with nearby OH species. A second observation deriving from the data of Figure 7 is that the presence of (the same amount of) Zn in the glass structure does not affect per se the hydrophilicity of the surface. In fact, the low-silica and high-Ca 72S5Zn surface does exhibit the highest capability of water uptake (both total and irreversible), whereas the high-silica and low-Ca 77S5Zn exhibits the lowest capability. In particular, with the irreversibly adsorbed component being the highest for 72S5Zn and the lowest for 77S5Zn, we can infer that is the lowsilica and high-Ca 72S5Zn surface that exhibits the strongest interaction with water. In summary, the present IR spectral results indicate that the hydrophilicity of the glass surface is far more strongly affected by the CaO/SiO2 and the ZnO/CaO ratios than by the simple presence and overall amount of ZnO. The CaO/SiO2 ratio of the investigated materials varies in the sequence 77S5Zn (0.12 mol %) < 77S (0.20 mol %) ≈ 72S5Zn (0.21 mol %), in rough agreement with the hydrophilicity sequence. However, in Zn-containing glasses, we have to consider that the amount of CaO acting as network modifier (net-mod) is reduced with respect to the Zn-free glass, because of the chargecompensator role of a fraction of the CaO component (vide supra Table 1). So, it seems more convenient to compare the CaOnet-mod/SiO2 ratio, which turns out to be quite low for 77S5Zn (0.073 mol %) with respect to both 77S (0.20 mol %) and 72S5Zn (0.16 mol %). It is also worth noticing that, in the Zn-free glass 77S, the H2O interaction with Ca sites can be somewhat inhibited by the large amount of carbonate-like species formed at the surface upon reaction with atmospheric CO2 and not removed by evacuation at T = 423 K (vide supra Figure 6). The same holds, although to a possibly different extent, for the high-Ca glass 72S5Zn at the surface of which the IR spectrum revealed the presence of carbonate-like species (vide supra Figure 6). Conversely, in the

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Figure 8. Volumetric isotherms of the first (A, solid symbols) and second (B, open symbols) run of H2O vapor adsorption at T = 303 K on glasses 77S (9, 0), 77S5Zn (b, O), and 72S5Zn (2, 4). All samples were preliminarily outgassed at T = 423 K. The experimental incertitude of the reported quantitative data is estimated to be lower than 5%.

low-Ca glass 77S5Zn, Ca sites turn out to be particularly little reactive (according to the low CaOnet-mod/SiO2 ratio) in that the surface is the least hydrophilic one, even though Ca ions are not engaged in carbonate-like species formation. In fact, the amount of Ca species free from the interaction with negatively charged tetrahedral [ZnO4]2- units is quite low. This datum confirms that is not just the presence of ZnO that affects the surface properties/ reactivity, but is also the nature/structure of the surface species, and the latter strongly depend on the CaO/SiO2 and the ZnO/ CaO ratios. Adsorption Microcalorimetry. In parallel with IR spectroscopic experiments, an adsorption-desorption-adsorption cycle of H2O vapor was performed at T = 303 K on the as-synthesized vacuum-activated glasses, ito examine, also from a quantitative/ energetic point of view, the affinity toward water of the glasses of interest. In Figure 8, the volumetric isotherms (nads vs pH2O) of the first (A) and second (B) adsorption run are reported. The total adsorption capacity of the three glasses, measured as μmol of H2O adsorbed per unit surface area during the first run, increases in the same sequence already suggested by the intensity (integrated absorbance) IR spectral data of Figure 7, that is, 77S5Zn < 77S < 72S5Zn. For instance, at pH2O = 6 Torr, the firstrun adsorbed amounts (μmol/m2) are 4.91 on 77S5Zn, 7.84 on 77S, and 10.61 on 72S5Zn. This datum quantitatively confirms that the surface of the low-silica Zn-glass 72S5Zn is, among the three specimens, the most hydrophilic one. By comparing the adsorption isotherms in Figure 8A and B, it is evident that the water amounts adsorbed during the second run (ads. II) are definitely lower than for the first run (ads. I), indicating that in all 2207

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Figure 9. Irreversible H2O uptake obtained for glasses 77S, 77S5Zn, and 72S5Zn at low and high H2O pressure (1 Torr, white; 6 Torr, gray). (A) (nads(I) - nads(II)), μmol/m2; (B) irreversible/total uptake ratio calculated as % irreversible uptake = [nads(I) - nads(II)/nads(I)]  100. See also Table 2SM in the Supporting Information. The experimental incertitude of the reported quantitative data is estimated to be lower than 5%.

cases part of the surface has been irreversibly modified by the interaction with water, as already evidenced by IR spectra (Figure 7). Also, the ads. II adsorbed amounts increase on the three glasses in the same sequence as for ads. I. For instance, at pH2O = 6 Torr, the adsorbed amounts (μmol/m2) are 3.96 on 77S5Zn, 6.06 on 77S, and 7.46 on 72S 5Zn. This datum confirms in quantitative terms that, in all the investigated materials, a significant fraction of H2O molecules resists overnight evacuation and remains irreversibly bound to specific surface sites. All volumetric and calorimetric data, not shown here for brevity, are reported in Table 2SM of the Supporting Information. To quantify the extent of the irreversible component (in the adopted conditions), the ads. II isotherms have been subtracted from the corresponding ads. I ones, and the (ads. I - ads. II) differences were evaluated for the three glasses. In Figure 9, the irreversible uptake is reported as μmol of H2O adsorbed per unit surface area at low and high water pressure (pH2O = 1 and 6 Torr, respectively). The percent irreversible/total uptake ratio was calculated as follows: % irreversible uptake = [nads(I) - nads(II)/ nads(I)]  100. The histograms reported in Figure 9 indicate that both irreversible uptake and the percent irreversible/total uptake ratio are largest for the low-silica glass 72S5Zn, which is confirmed to possess the most hydrophilic surface. This means that the sites exposed at the 72S5Zn surface exhibit the largest propensity to strongly interact with water molecules, which will remain tightly bound to the surface upon prolonged evacuation (overnight at T = 303 K). This feature is bound to have major implications in the behavior of the glass in contact with biological fluids and its propensity to dissolve in them.

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Figure 10. Differential heat of adsorption versus uptake for the first (A, solid symbols) and second (B, open symbols) runs of H2O vapor adsorption at T = 303 K on the glasses 77S (9, 0), 77S5Zn (b, O), and 72S5Zn (2, 4). All samples were preliminarily outgassed at T = 423 K.

The energy of the water/surface interaction, measured as differential heats of adsorption (which represent the enthalpy change associated with the process: qdiff = -ΔadsH), and reported as a function of surface coverage, reflects the behavior mentioned above, as illustrated in Figure 10. The heats of first (A) and second (B) adsorption runs are plotted versus water uptake for glasses 77S, 77S5Zn, and 72S5Zn. The three curves are all typical of heterogeneous surfaces, as expected in the case of mixed oxides that necessarily exhibit a variety of surface sites.69 All curves are also typical of hydrophilic surfaces,84,93 as witnessed by heat values, which lie above the latent heat of liquefaction of water (qL = 44 kJ/mol) in the whole interval of H2O pressure examined. This datum indicates that, even at high coverage, all surfaces (and mostly in the case of the two bioactive glasses 77S and 72S5Zn) are able to interact with H2O molecules more strongly than H2O molecules in the liquid phase. The heat of adsorption extrapolated to vanishing-coverage [q0 = -(ΔadsH)0] represents the enthalpy changes associated with the adsorption on the most energetic sites, active in the earliest stages of the process. The sequence of q0 values obtained for the ads. I curves in Figure 9A is 110 < 120 < 140 kJ/mol for 77S5Zn, 77S, and 72S5Zn, respectively. The reported sequence follows the same order as the adsorbed amounts (and the intensities of the ∼1630 cm-1 bands; see IR spectra in Figure 7), so confirming that the surface of the high-Ca Zn-glass 72S5Zn exhibits not only the most abundant population of hydrophilic sites, but also the most energetic ones. The same holds for the second run of adsorption (Figure 9B). It is worth noting that, for all glasses, the q0 values for ads. II are still quite high (only in the case of the nonbioactive glass 77S5Zn is the heat value slightly lower than ∼100 kJ/mol) and that the second run curves of the two bioactive glasses 77S and 72S5Zn are virtually coincident. 2208

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4. CONCLUSIONS From the whole set of data obtained by a multitechniques approach, it turns out that: (i) The presence of (a limited amount of) ZnO in the glass composition determines dramatic changes in the behavior of the glass in contact with TRIS-BS with respect to the Zn-free glass 77S. In fact, the glass dissolution rate, backprecipitation of silica gel, and formation/crystallization of the apatite-like layer in Zn-containing glasses were found to be either inhibited (a negative aspect) or delayed (a positive aspect). In fact, the presence of the network former ZnO component determines a global enhancement of the glass reticulation, with a consequent formation of Si-O-Zn units. (ii) The influence of ZnO component has no effect per se, but depends on the overall composition of the glass, and in particular on the CaO/SiO2 and ZnO/CaO ratios. In fact, both composition and structure of the surface and, consequently, surface reactivity strongly depend on these ratios: the two Zn-glasses, which contain the same amount of ZnO (5 wt %) and structurally similar tetrahedral Zn species, exhibit dramatically different behavior. (iii) The role played by Ca species, which are thought to be the most hydrophilic sites, is expected to be a decisive factor in the glass dissolution mechanism and in the formation of the apatite-like surface layer. In fact, the low propensity to dissolve in aqueous solution of the low-Ca and high-silica Zn-glass 77S5Zn is due to the high reticulation caused by the scarce population of Ca2þ cations in the role of network modifiers;65 the controversial data found in the literature on the bioactivity features of Zn-containing glasses are most likely due to the fact that, in general, the glasses dealt with in those works exhibit different Zn-loadings and/or different overall chemical composition. Our present case is, in this respect, emblematic: a constant Zn loading in different overall glass compositions leads to fairly different surface features. The presence of ZnO is expected to play a role also in the structure of P species, and work is presently in progress to model (by a molecular dynamics approach) the structure of Zn-containing sol-gel glasses, and to investigate on one hand the physicochemical features of the present work glasses in comparison with P-free model systems, and to investigate on the other hand the bioactivity in SBF of the various glasses as a function of their chemical composition/surface structure. ’ ASSOCIATED CONTENT

bS

Supporting Information. Table 1SM: Concentration (ppm) of Si, Ca, Zn, and P elements in supernatant TRIS-BS, at different soaking times of the investigated glasses. Table 2SM: Volumetric and calorimetric data at pH2O = 1 and 6 Torr, respectively, obtained for H2O adsorption at T = 303 K on glasses. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ39-0321-375-840. Fax: þ39-0321-375-621. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the Italian Ministry MUR (Project COFIN-2006, Prot. 2006032335_004: “Interface phenomena in silica-based nanostructured biocompatible materials

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contacted with biological systems”) and by Regione PiemonteItaly (Project CIPE-2004: “Nanotechnologies and Nanosciences. Nanostructured materials biocompatible for biomedical applications”), whose contribution is gratefully acknowledged. Prof. E. Verne (Dip. Scienza dei Materiali e Ingegneria Chimica Politecnico di Torino, Italy) is gratefully acknowledged for fruitful discussions.

’ REFERENCES (1) Wilson, J.; Yli-Urpo, A.; Risto-Pekka, H. Bioactive glasses: clinical applications. In An Introduction to Bioceramics; Hench, L. L., Wilson, J., Eds.; World Scientific: Singapore, 1993; p 63. (2) Hench, L. L. J. Am. Ceram. Soc. 1998, 81, 1705. (3) Hench, L. L. Biomaterials 1998, 19, 1419. (4) Hench, L. L.; Splinter, R. J.; Allen, W. C.; Greenlee, T. K., Jr. J. Biomed. Mater. Res. 1971, 36, 117. (5) Hench, L. L. J. Mater. Sci.: Mater. Med. 2006, 17, 967. (6) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487. (7) Hench, L. L. Am. Ceram. Soc. Bull. 1998, 77, 67. (8) Kokubo, T.; Kim, H. M.; Kawashita, M. Biomaterials 2003, 24, 2161. (9) Hench, L. L.; Wilson, J. Introduction to Bioceramics; World Scientific: River Edge, NJ, 1993. (10) Saravanapavan, P.; Jones, J. R.; Pryce, R. S.; Hench, L. L. J. Biomed. Mater. Res., Part A 2003, 66A, 110. (11) Ohura, K.; Nakamura, T.; Yamamuro, T.; Kokubo, T.; Ebisawa, Y.; Kotoura, Y.; Oka, M. J. Biomed. Mater. Res. 1991, 25, 357. (12) Zhong, J. P.; Greenspan, D. C. J. Biomed. Mater. Res. 2000, 53, 694. (13) Hench, L. L.; Splinter, R. J.; Allen, W. C.; Greenle, T. J. Biomed. Mater. Res. 1971, 5, 117. (14) Hench, L. L.; Paschall, H. A. J. Biomed. Mater. Res. 1973, 7, 25. (15) Gross, U.; Kinne, R.; Schmitz, H. J.; Strunz, V. Crit. Rev. Biocompat. 1988, 4, 155. (16) Hench, L. L. J. Biomed. Mater. Res. 1998, 41, 511. (17) Vitale-Brovarone, C.; Verne, E.; Robiglio, L.; Martinasso, G.; Canuto, R. A.; Muzio, G. J. Mater. Sci.: Mater. Med. 2008, 19, 471. (18) Vitale-Brovarone, C.; Baino, F.; Bretcanu, O.; Verne, E. J. Mater. Sci.: Mater. Med. 2009, 20, 2197. (19) Ohtsuki, C.; Kokubo, T.; Yamamuro, T. J. Non-Cryst. Solids 1992, 143, 84. (20) Filgueiras, M. R.; Latorre, G. R.; Hench, L. L. J. Biomed. Mater. Res. 1993, 27, 445. (21) Kokubo, T.; Kushitani, H.; Sakka, S.; Kitsugi, T.; Yamamuro, T. J. Biomed. Mater. Res. 1990, 24, 721. (22) Gamble, J. Chemical Anatomy, Physiology and Pathology of Extracellular Fluid: A Lecture Syllabus; Harvard University Press: Cambridge, MA, 1967. (23) Bini, M.; Grandi, S.; Capsoni, D.; Mustarelli, P.; Saino, E.; Visai, L. J. Phys. Chem. C 2009, 113, 8821. (24) Verne, E.; Ferraris, S.; Miola, M.; Fucale, G.; Maina, G.; Martinasso, G.; Canuto, R. A.; Di Nunzio, S.; Vitale-Brovarone, C. Adv. Appl. Ceram. 2008, 107, 234. (25) Dislich, H. J. Non-Cryst. Solids 1985, 73, 599. (26) Wright, J. D.; Sommerdijk, N. A. Sol-Gel Materials: Chemistry and Applications; The Netherlands Gordon and Breach Science Publishers: Amsterdam, 2001. (27) Li, R.; Clark, A. E.; Hench, L. L. J. Appl. Biomater. 1991, 2, 231. (28) Li, R.; Clark, A. E.; Hench, L. L. Effect of structure and surface area on bioactive powders by sol-gel process. In Chemical Processing of Advanced Materials; Hench, L. L., West, J. K., Eds.; Wiley: New York, 1992; p 627. (29) Pereira, M. M.; Clark, A. E.; Hench, L. L. J. Biomed. Mater. Res. 1994, 28, 693. (30) Gupta, R.; Kumar, A. Biomed. Mater. 2008, 3, 34005. (31) Balamurugan, A.; Balossier, G.; Kannan, S.; Michel, J.; Rebelo, A. H. S.; Ferreira, J. M. F. Acta Biomater. 2007, 3, 255. 2209

dx.doi.org/10.1021/jp1101708 |J. Phys. Chem. C 2011, 115, 2196–2210

The Journal of Physical Chemistry C (32) Vallet-Regi, M.; Salinas, A. J.; Arcos, D. J. Mater. Sci.: Mater. Med. 2006, 17, 1011. (33) Roman, J.; Padilla, S.; Vallet-Regi, M. Chem. Mater. 2003, 15, 798. (34) Vallet-Regi, M.; Ragel, C. V.; Salinas, A. J. Eur. J. Inorg. Chem. 2003, 1029. (35) Saravanapavan, P.; Hench, L. L. J. Biomed. Mater. Res. 2001, 54, 608. (36) Sepulveda, P.; Jones, J. R.; Hench, L. L. J. Biomed. Mater. Res. 2002, 59, 340. (37) Oki, A.; Parveen, B.; Hossain, S.; Adeniji, S.; Donahue, H. J. Biomed. Mater. Res., Part A 2004, 69A, 216. (38) Wheeler, D. L.; Hoellrich, R. G.; McLoughlin, S. W.; Chamberland, D. L.; Stokes, K. E. In Bioceramics; Sedel, L., Rey, C., Eds.; Elsevier: Oxford, 1997; p 349. (39) Day, R. M.; Boccaccini, A. R. J. Biomed. Mater. Res. 2005, 73A, 73. (40) Jaroch, D. B.; Clupper, D. C. J. Biomed. Mater. Res., Part A 2007, 82A, 575. (41) Kishi, S.; Yamaguchi, M. Biochem. Pharmacol. 1994, 48, 1225. (42) Ito, A.; Kawamura, H.; Otsuka, M.; Ikeuchi, M.; Ohgushi, H.; Ishikawa, K.; Onuma, K.; Kanzaki, N.; Sogo, Y.; Ichinose, N. Mater. Sci. Eng., C 2002, 22, 21. (43) Yamaguchi, M.; Igarashi, A.; Ychiyama, S. J. Health Sci. 2004, 50, 75. (44) Du, R. L.; Chang, J.; Ni, S. Y.; Zhai, W. Y.; Wang, J. Y. J. Biomater. Appl. 2006, 20, 341. (45) Linati, L.; Lusvardi, G.; Malavasi, G.; Menabue, L.; Menziani, M. C.; Mustarelli, P.; Pedone, A.; Segre, U. J. Non-Cryst. Solids 2008, 354, 84. (46) Kamitakahara, M.; Ohtsuki, C.; Inada, H.; Tanihara, M.; Miyazaki, T. Acta Biomater. 2006, 2, 467. (47) Linati, L.; Lusvardi, G.; Malavasi, G.; Menabue, L.; Menziani, M. C.; Mustarelli, P.; Segre, U. J. Phys. Chem. B 2005, 109, 4989. (48) Lusvardi, G.; Malavasi, G.; Menabue, L.; Menziani, M. C.; Segre, U.; Carnasciali, M. M.; Ubaldini, A. J. Non-Cryst. Solids 2004, 345-346, 710. (49) Lusvardi, G.; Malavasi, G.; Menabue, L.; Menziani, M. C. J. Phys. Chem. B 2002, 106, 9753. (50) Boyd, D.; Carroll, G.; Towler, M. R.; Freeman, C.; Farthing, P.; Brook, I. M. J. Mater. Sci.: Mater. Med. 2009, 20, 413. (51) Courtheoux, L.; Lao, J.; Nedelec, J. M.; Jallot, E. J. Phys. Chem. C 2008, 112, 13663. (52) Ennas, G.; Musinu, A.; Piccaluga, G.; Montenero, A.; Gnappi, G. J. Non-Cryst. Solids 1990, 125, 181. (53) Dellamea, G.; Gasparotto, A.; Bettinelli, M.; Montenero, A.; Scaglioni, R. J. Non-Cryst. Solids 1986, 84, 443. (54) Dietrich, E.; Oudadesse, H.; Lucas-Girot, A.; Le Gal, Y.; Jeanne, S.; Cathelineau, G. Appl. Surf. Sci. 2008, 255, 391. (55) Lusvardi, G.; Malavasi, G.; Menabue, L.; Menziani, M. C.; Pedone, A.; Segre, U.; Aina, V.; Perardi, A.; Morterra, C.; Boccafoschi, F.; Gatti, S.; Bosetti, M.; Cannas, M. J. Biomater. Appl. 2008, 22, 505. (56) Lusvardi, G.; Zaffe, D.; Menabue, L.; Bertoldi, C.; Malavasi, G.; Consolo, U. Acta Biomater. 2009, 5, 419. (57) Aina, V.; Perardi, A.; Bergandi, L.; Malavasi, G.; Menabue, L.; Morterra, C.; Ghigo, D. Chem.-Biol. Interact. 2007, 167, 207. (58) Hurt, J. C.; J, P. C. J. Am. Ceram. Soc. 1971, 53, 269. (59) Furukawa, T.; White, W. B. J. Non-Cryst. Solids 1980, 38, 87. (60) McKeown, D. A.; Muller, I. S.; Buechele, A. C.; Pegg, I. L. J. Non-Cryst. Solids 2000, 261, 155. (61) Le Grand, M.; Ramos, A. Y.; Calas, G.; Galoisy, L.; Ghaleb, D.; Pacaud, F. J. Mater. Res. 2000, 15, 2015. (62) Vallet-Regi, M. Dalton Trans. 2006, 5211. (63) Bonino, F.; Damin, A.; Aina, V.; Miola, M.; Verne, E.; Bretcanu, O.; Bordiga, S.; Zecchina, A.; Morterra, C. J. Raman Spectrosc. 2008, 39, 260. (64) Chen, X.; Meng, Y.; Li, Y.; Zhao, N. Appl. Surf. Sci. 2008, 255, 562.

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(65) Calas, G.; Cormier, L.; Galoisy, L.; P., J. C. R. Chemie 2002, 5, 831. (66) Kokubo, T.; Takadama, H. Biomaterials 2006, 27, 2907. (67) Bolis, V.; Busco, C.; Aina, V.; Morterra, C.; Ugliengo, P. J. Phys. Chem. C 2008, 112, 16879. (68) Cerruti, M.; Magnacca, G.; Bolis, V.; Morterra, C. J. Mater. Chem. 2003, 13, 1279. (69) Bolis, V.; Busco, C.; Ugliengo, P. J. Phys. Chem. B 2006, 110, 14849. (70) Corno, M.; Busco, C.; Bolis, V.; Tosoni, S.; Ugliengo, P. Langmuir 2009, 25, 2188. (71) Corno, M.; Rimola, A.; Bolis, V.; Ugliengo, P. Phys. Chem. Chem. Phys. 2010, 12, 6309. (72) Vogler, E. A. How Water Wets Biomaterials Surfaces. In Water in Biomaterials Surface Science; Morra, M., Ed.; John Wiley & Sons Ltd.: Chichester, 2001; p 269. (73) Vogler, E. A. Role of Water in Biomaterials. In Biomaterial Science: An Introduction to Materials in Medicine; Ratner, B. D., Schoen, F. J., Lemons, J. E., Eds.; Elsevier Inc.: San Diego, CA, 2004; p 59. (74) Sakhno, Y.; Bertinetti, L.; Iafisco, M.; Tampieri, A.; Roveri, N.; Martra, G. J. Phys. Chem. C 2010, 114, 16640. (75) Perez-Pariente, J.; Balas, F. J. Biomed. Mater. Res. 1999, 47, 170. (76) Brunauer, S.; Emmett, P. H.; Teller, E. J. J. Am. Chem. Soc. 1938, 60, 309. (77) Lusvardi, G.; Malavasi, G.; Menabue, L.; Aina, V.; Morterra, C. Acta Biomater. 2009, 5, 3548. (78) Barrett, E. P.; Joyner, L. S.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 375. (79) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319. (80) Cattania, M. G.; Ardizzone, S.; Bianchi, C. L.; Carella, S. Colloids Surf., A 1993, 76, 233. (81) Verne, E.; Bretcanu, O.; Balagna, C.; Bianchi, C. L.; Cannas, M.; Gatti, S.; Vitale-Brovarone, C. J. Mater. Sci.: Mater. Med. 2009, 20, 75. (82) Moore, L. Introduction to Inductively Coupled Plasma Atomic Emission Spectrometry; Elsevier: Amsterdam, 1989. (83) Aina, V.; Malavasi, G.; Pla, A. F.; Munaron, L.; Morterra, C. Acta Biomater. 2009, 5, 1211. (84) Fubini, B.; Bolis, V.; Bailes, M.; Stone, F. S. Solid State Ionics 1989, 32/33, 258. (85) Cerruti, M.; Bolis, V.; Magnacca, G.; Morterra, C. Phys. Chem. Chem. Phys. 2004, 6, 2468. (86) Wilson, R. M.; Elliot, J. C.; Dowker, S. E. P. Am. Mineral. 1999, 84, 1406. (87) Fleet, M. E.; Liu, X.; King, P. L. Am. Mineral. 2004, 89, 1422. (88) Cerruti, M.; Morterra, C. Langmuir 2004, 20, 6382. (89) Morrow, B. A.; Gay, I. D. Infrared and NMR Characterization of the Silica Surface. In Adsorption on Silica Surfaces; Papirer, E., Ed.; M. Dekker Inc.: New York, 2000; p 9. (90) Vansant, E. F.; Van der Voort, P.; Vrancken, K. C. The surface chemistry of silica. In Characterization and Chemical Modification of the Silica Surface; Delmon B., Y. J. T., Ed.; Elsevier Science B.V.: Amsterdam, 1995; Vol. 93. (91) Zhuralev, L. T. Colloids Surf., A 2000, 173, 1. (92) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: London, 1966. (93) Bolis, V.; Cavenago, A.; Fubini, B. Langmuir 1997, 13, 895.

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