Reaction Layer Analysis of In Situ Reinforced Titanium Composites: Influence of the Starting Material Composition on the Mechanical Properties

materials Article A Structural Comparison of Ordered and Non-Ordered Ion Doped Silicate Bioactive Glasses Seray Schmitz 1, Ana M. Beltrán2, Mark Cresswell 3and Aldo R. Boccaccini 1,* 1Institute of Biomaterials, University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany; seray[email protected] 2Department of Materials and Transport Science Engineering, University of Seville, 41011 Seville, Spain; [email protected] 3Lucideon Ltd., Queens Road, Stoke-on-Trent ST4 7LQ, UK; Mark.Cr[email protected] *Correspondence: [email protected]; Tel.: +49- (0)9131-85-28601 Received: 21 January 2020; Accepted: 20 February 2020; Published: 22 February 2020   Abstract: One of the key benefits of sol-gel-derived glasses is the presence of a mesoporous structure and the resulting increase in surface area. This enhancement in textural properties has a significant effect on the physicochemical properties of the materials. In this context the aim of this study was to investigate how sol-gel synthesis parameters can influence the textural and structural properties of mesoporous silicate glasses. We report the synthesis and characterization of metal ion doped sol-gel derived glasses with different dopants in the presence or absence of a surfactant (Pluronic P123) used as structure-directing templating agent. Characterization was done by several methods. Using a structure directing agent led to larger surface areas and highly ordered mesoporous structures. The chemical structure of the non-ordered glasses was modified to a larger extent than the one of the ordered glasses due to increased incorporation of dopant ions into the glass network. The results will help to further understand how the properties of sol-gel glasses can be controlled by incorporation of metal dopants, in conjunction with control over the textural properties, and will be important to optimize the properties of sol-gel glasses for specific applications, e.g., drug delivery, bone regeneration, wound healing, and antibacterial materials. Keywords: mesoporous; ordered; non-ordered; bioactive glass; sol-gel method 1. Introduction Mesoporous materials (MMs) are defined as materials having pores between 2 to 50 nm according to IUPAC (International Union of Pure and Applied Chemistry) [ 1 ]. They are used commonly in industry as catalysts, adsorbents, and for chromatographic applications [ 2 , 3 ]. Recently, the potential of mesoporous materials to be used for biomedical applications such as drug delivery systems and bone regeneration [ 4 , 5 ] has become more widely investigated. Further enhancement of the properties of mesoporous materials can be achieved by controlling the structural nature of their inherent porous network. A common method for doing this is to introduce structure-directing templating agents during the synthesis of mesoporous materials in order to generate products with highly ordered internal porous structures as opposed to porous structures that are non-ordered. Ordered MMs have important features such as long-range ordered porosity, narrow pore size distributions, and high surface areas. In addition, variable final mesostructures (e.g., rods, 3D structures, or sheets) can be obtained by modification of the type or amount of the surfactant acting as the structure directing mesophase template [ 6 ]. Surfactants can be divided into three groups; cationic, anionic, and non-ionic [ 7 , 8 ]. As a cationic surfactant, cetyltrimethylammonium bromide (CTAB) is mostly used. Also, Gemini surfactants, multihead group surfactants, and cationic fluorinated surfactants can be Materials 2020,13, 992; doi:10.3390/ma13040992 www.mdpi.com/journal/materials Materials 2020,13, 992 2 of 16 alternative templates to produce different mesoporous structures. These surfactants have specific properties such as excellent solubility, high critical micelle temperature, and they can be used in both acidic and basic media. On the other hand, they are toxic and relatively expensive [ 8 ]. Examples of anionic salt surfactants are carboxylates, sulfates, sulfonates, phosphates. In comparison to cationic surfactants, the repulsive interactions between anionic surfactants and silicate species do not lead to ordered mesostructures [ 8 ]. The mostly used non-ionic surfactants are oligomeric alkyl Poly(ethylene oxide) (PEO) surfactants, amphiphilic triblock–copolymers, sorbitan esters, etc. Non-ionic surfactants are non-toxic, biodegradable, and have relative low costs as compared with other surfactants [ 8 ]. Widely used non-ionic surfactants in mesoporous bioactive glass synthesis are P123 and F127. The first mesoporous bioactive glass was synthesized in 2004 by Yan et al. using the combination of P123, F127 and B50–6600 as non-ionic surfactants [ 9 ]. Due to its preferable properties in comparison to other surfactants, in this study a non-ionic surfactant, P123, was used. The benefits of mesoporous structures include i) the possibility of drug loading and controlled release from the mesopores, and ii) the ability to achieve consistent and controlled degradation due to the ordered and uniform structures obtained using structure-directing agents. These properties are important for biomedical applications (e.g., drug delivery, bone regeneration, wound healing, and antibacterial materials) of the mesoporous bioactive glasses. Ideally due to their ordered pore structure, ordered mesoporous glasses could be more effective in their bioactive behavior and drug delivery capability compared to non-ordered glasses. However, if doped with suitable therapeutic elements in the right amount, the non-ordered glasses can also become attractive for antibacterial applications, wound healing, and bone regeneration. Indeed mesoporous bioactive glasses doped with metal elements can degrade homogeneously, thus the release of the therapeutic metal ions can be controlled. Therefore, control of the mesoporosity characteristics plays an important role regarding the mentioned medical applications of mesoporous bioactive glasses. The preparation of mesoporous silica and mesoporous silicate glasses by sol-gel method has been extensively studied [ 4 – 7 , 9 – 11 ]. The impact of the variation of different sol-gel process parameters on the structure of mesoporous silica is well known. In particular, the use of structure-directing surfactants to generate mesoporous silica with highly ordered porous structures expanded the potential application of these materials significantly. The use of the sol-gel methodology to produce mesoporous silicate glasses represented a natural progression from pure silica and it enabled sol-gel derived glasses with compositional functionality to be produced, dependent on the nature of the additional oxides present. Amongst these, mesoporous silicate glasses with enhanced bioactivity [ 9 ], antibacterial [ 11 ], and angiogenesis [ 12 ] properties have been produced. In addition to the beneficial properties which result from the modification of the glass composition, the textural properties of mesoporous silicate glasses have also been found to be critical in enhancing their performance. Mesoporous silicate glasses (MSGs) synthesized with surfactants have enhanced surface area and higher pore volume and in the context of biomedical applications have an increased ability to induce in vitro apatite mineralization in simulated body fluid (SBF), whilst also having excellent cytocompatibility [13–16]. The purpose of this study was to synthesize MSGs with and without the use of structure directing templates (P123) in combination with different dopants (biologically active ions) and to evaluate the potential impact of such parameters on the textural and structural properties of the resultant glasses. The dopants chosen were Ag 2 O, CuO, ZnO, and CeO 2 . These oxides were chosen for their proven antibacterial properties [ 17 – 19 ]. They also exhibit osteogenesis and angiogenesis [ 4 , 12 ] promoting properties, thus being very useful biologically active dopants for MSGs. Undoped and Ag 2 O, CuO, ZnO, and CeO 2 -doped MSGs with compositions of (76-x)SiO 2− 13CaO − 11P 2 O 5 –x(Ag 2 O, CuO, ZnO, and CeO 2 ) (x =5 wt%) were synthesized by sol-gel (non-ordered MSGs) and surfactant-assisted sol-gel methods (ordered MSGs). A non-ionic surfactant was applied in the sol-gel process to synthesize the ordered MSGs. Materials 2020,13, 992 3 of 16 2. Materials and Methods 2.1. Materials Tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich, Steinheim, Germany), triethyl phosphate (TEP, 99.8%, Sigma-Aldrich, Steinheim, Germany), calcium nitrate tetrahydrate (Ca(NO 3 ) 2· 4H 2 O, 99%, Sigma-Aldrich, Steinheim, Germany), silver nitrate (AgNO 3 , 99.5%, Sigma-Aldrich, Steinheim, Germany), copper(II) nitrate hemi (pentahydrate) (Cu(NO 3 ) 2· 2.5H 2 O, 99.99%, Sigma-Aldrich, Steinheim, Germany), zinc nitrate hexahydrate (Zn(NO 3 ) 2· 6H 2 O, 98%, Sigma-Aldrich, Steinheim, Germany), cerium(III) nitrate hexahydrate (Ce(NO 3 ) 3· 6H 2 O, 99.999%, Sigma-Aldrich, Steinheim, Germany), nitric acid (HNO 3 , 69%, Sigma-Aldrich, Steinheim, Germany), hydrochloric acid (HCl, 37%, Sigma-Aldrich, Steinheim, Germany), ethanol (C 2 H 5 OH, 96%, Sigma-Aldrich, Steinheim, Germany), Pluronic P123 (M n 5800, Sigma-Aldrich, Steinheim, Germany), and distilled water were used without further purification for the preparation of non-ordered and ordered MSGs. 2.2. Method 2.2.1. Sol–gel Synthesis of Non-ordered mesoporous silicate glasses (MSGs) Silica-based undoped and Ag 2 O, CuO, ZnO, and CeO 2 -doped MSG compositions [76SiO 2− 13CaO − 11P 2 O 5 –5(Ag 2 O, CuO, ZnO, and CeO 2 )] were prepared by the sol–gel method. In the first step of undoped-MSG preparation, 50 mL TEOS was mixed with 33.12 mL of distilled water. As a catalyst 6.80 mL 0.5 M HCl was also added and mixed for 30 min. The preparation of doped-MSGs required 46.80 mL of TEOS. The catalyst used for Ag, Zn, and Ce-MSGs was 24.04 mL of 0.5 M HNO 3 and for Cu-MSGs 17.50 mL of 0.5 M HCl. In the next step, 4.76 mL TEP was added to the solution and 30 min mixing was done. Afterwards, 9.87 g of Ca(NO 3 ) 2· 4H 2 O was added to the solution and stirred for 60 min. For the doped-MSGs, the nitrates of the dopants were added to the sol and stirred for a further of 60 min. The resultant sols were aged for 3 days under a fume hood without stirring, and the gels were dried at 40 ◦ C for 48 h. In the last step, to obtain undoped, Cu, Zn, and Ce-MSGs, heat treatment at 700 ◦ C with 233.33 ◦ C/h heating rate for 3 h was applied. For the Ag-MSGs, the heat treatment was carried out at 500 ◦ C with a 166.66 ◦ C/h heating rate for 5 h. Afterwards, the glasses were left for slow cooling in the furnace. 2.2.2. Surfactant-Assisted Sol-gel Synthesis of Ordered MSGs MSGs with ordered mesoporosity were synthesized by evaporation induced self-assembly (EISA) [ 20 ] process with a non-ionic surfactant Pluronic P123. 4 g of P123 was stirred into 76 mL of 96 vol% ethanol and 1.00 mL of 0.5 M HCl (for undoped and Cu-MSGs) for at least 1 h until it became a clear solution. 1 mL of 0.5 M HNO 3 was used as the catalyst for Ag, Zn, and Ce-MSGs. After P123 dissolution, 7.13 mL TEOS was added to the solution and stirred for 30 min for undoped-MSGs. Doped-MSGs required 6.65 mL of TEOS. In the next step, 0.67 mL of TEP was added and stirred for a further 30 min. A total of 1.40 g of Ca(NO 3 ) 2· 4H 2 O was added and stirred for 1 day for undoped-MSGs. For the doped-MSGs, after stirring 1.40 g of Ca(NO 3 ) 2· 4H 2 O for 1 h, the nitrates of the dopants were added to the sol and stirred for 1 day. In the next step, these sols went through evaporation induced self-assembly (EISA) process, by leaving the sols in petri dishes under a fume hood. Finally, heat treatment was done at 700 ◦ C at a 233.33 ◦ C/h heating rate for 3 h in the case of undoped, Cu-MSG, Zn-MSG, and Ce-MSG synthesis. Ag-MSGs were obtained at 500 ◦ C at a 166.66 ◦ C/h heating rate for 5 h. Afterwards, the glasses were left for slow cooling in the furnace. Materials 2020,13, 992 4 of 16 2.3. Characterization Methods 2.3.1. High Resolution Transmission Electron Microscopy (HRTEM) High resolution transmission electron microscopy (HRTEM) studies were performed with a FEI Talos F200S microscope (ThermoFisher Scientific, Eindhoven, Netherlands) equipped with Super-X energy dispersive X-ray spectrometry (EDX) system, including two silicon drift detectors. The microscope operates at an accelerating voltage of 200 kV. Prior to the HRTEM analyses, MSG powders were deposited on a Holey carbon film on a copper grid. The pore profiles were obtained by using ImageJ analysis to observe the distance between the pore channels. Twenty-five to 150 pore size measurements were done on the MSG samples. The standard deviations of these measurements are also given. 2.3.2. Nitrogen Adsorption Analysis (Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)) The mesoporous properties (surface area, pore size diameter, and pore volume) of the undoped and doped-MSGs were investigated by nitrogen adsorption porosimetry. The machine used was TriStar II Plus 2.03 (Micromeritics, Norcross, GA, USA). Before performing the adsorption analyses, MSGs were degassed under vacuum for 5 h at 423 K. The analyses were performed at 77 K. Brunauer-Emmett-Teller (BET) method was used to determine the surface area of the MSGs. Barrett-Joyner-Halenda (BJH) method was applied to determine the pore size diameter of the MSGs from the desorption branch of the isotherm. 2.3.3. Fourier Transform Infrared Spectroscopy (FTIR) Undoped and doped-MSGs were placed directly onto the diamond crystal (spectral range: 4000–400 cm −1 ) of a GladiATR Vision (PIKE Technologies Ltd., Madison, Wisconsin, USA), attenuated total reflectance accessory that was installed in the sample compartment of an Agilent 660-IR FTIR spectrometer (Varian Australia Pty Ltd., Melbourne, Victoria, Australia). A press was used to secure the samples in place and to ensure that a sufficient homogeneous contact between the sample and attenuated total reflectance (ATR) crystal was achieved. The samples were measured in the region 4000–400 cm−1wavenumbers and 32 scans were collected at a resolution of 4 cm−1. 2.3.4. Semi-quantitative X-ray Fluorescence (XRF) To determine the elemental wt% compositions of the MSGs, semi-quantitative X-ray Fluorescence (XRF) method was applied. Axios wavelength dispersive (WD) XRF spectrometer (Panalytical, Almelo, Netherlands) was used. 2.3.5. Powder X-ray Diffraction (XRD) The X-ray diffraction (XRD) analyses of the MSGs were done by D8 Advance (Bruker, Karlsruhe, Germany), with Copper K α radiation at 40 kV and 30 mA. The diffraction range of the analyses was 5 to 70 ◦ 2 θ . The step size and the dwell time used to collect the data using Bragg-Brentano geometry was 0.02 ◦ 2 θ and 0.5 s per step, respectively. X-rays were detected simultaneously by a Lynxeye solid state detector over a 3◦2θwindow. 3. Results 3.1. High Resolution Transmission Electron Microscopy (HRTEM) Figure 1shows HRTEM images of the undoped and 5 wt% metal-doped non-ordered mesoporous glasses. It can be seen that the MSG particle surfaces have a globular and textured nature which is typical for materials synthesized by sol-gel method. These MSGs did not show any ordered mesoporous network. Materials 2020,13, 992 5 of 16 Materials 2020, 13, x FOR PEER REVIEW 5 of 17 Figure 1. High resolution transmission electron microscopy (HRTEM) images of non-ordered mesoporous silicate glasses (MSGs): (a) Undoped-MSG; (b) 5 wt% Ag-MSG; (c) 5 wt% Cu-MSG; (d) 5 wt% Zn-MSG; (e) 5 wt% Ce-MSG. Scale bar = 100 nm. In contrast, Figure 2 shows the highly ordered porous networks associated with the ordered undoped and 5 wt% metal-doped mesoporous glasses. Except 5 wt% Ce-MSG (Figure 2e), in the images of other ordered MSGs, just the lateral view (along the length of mesopores) of the mesoporous structures were seen. In the case of 5 wt% Ce-MSG (Figure 2e), in addition to the lateral view of pore channels, a face-on view of a honeycomb structure was seen. ImageJ was used to measure the average pore size diameters of the ordered MSGs. Figure 1. High resolution transmission electron microscopy (HRTEM) images of non-ordered mesoporous silicate glasses (MSGs): ( a ) Undoped-MSG; ( b ) 5 wt% Ag-MSG; ( c ) 5 wt% Cu-MSG; (d) 5 wt% Zn-MSG; (e) 5 wt% Ce-MSG. Scale bar =100 nm. In contrast, Figure 2shows the highly ordered porous networks associated with the ordered undoped and 5 wt% metal-doped mesoporous glasses. Except 5 wt% Ce-MSG (Figure 2e), in the images of other ordered MSGs, just the lateral view (along the length of mesopores) of the mesoporous structures were seen. In the case of 5 wt% Ce-MSG (Figure 2e), in addition to the lateral view of pore channels, a face-on view of a honeycomb structure was seen. ImageJ was used to measure the average pore size diameters of the ordered MSGs. Materials 2020, 13, x FOR PEER REVIEW 5 of 17 Figure 1. High resolution transmission electron microscopy (HRTEM) images of non-ordered mesoporous silicate glasses (MSGs): (a) Undoped-MSG; (b) 5 wt% Ag-MSG; (c) 5 wt% Cu-MSG; (d) 5 wt% Zn-MSG; (e) 5 wt% Ce-MSG. Scale bar = 100 nm. In contrast, Figure 2 shows the highly ordered porous networks associated with the ordered undoped and 5 wt% metal-doped mesoporous glasses. Except 5 wt% Ce-MSG (Figure 2e), in the images of other ordered MSGs, just the lateral view (along the length of mesopores) of the mesoporous structures were seen. In the case of 5 wt% Ce-MSG (Figure 2e), in addition to the lateral view of pore channels, a face-on view of a honeycomb structure was seen. ImageJ was used to measure the average pore size diameters of the ordered MSGs. Figure 2. HRTEM images of ordered MSGs: ( a ) Undoped-MSG; ( b ) 5 wt% Ag-MSG; ( c ) 5 wt% Cu-MSG; (d) 5 wt% Zn-MSG; (e) 5 wt% Ce-MSG. Scale bar =50 nm. Materials 2020,13, 992 6 of 16 Figure 3shows an example of how the average pore size was typically determined; the arrow indicates the direction along which the profile has been plotted. In order to increase the intensity, the profile has been integrated over the area. The measurements were taken from the well-ordered lateral areas of the MSGs. Ordered undoped-MSGs have a pore size diameter of 6.67 ± 0.33 nm. Ordered 5 wt% Ag-MSGs have pore sizes of 6.66 ± 0.20 nm. The pore sizes of ordered 5 wt% Cu-MSGs, 5 wt% Zn-MSGs, and 5 wt% Ce-MSGs are 6.22 ±0.63 nm, 6.58 ±0.92 nm, and 6.34 ±0.15 nm, respectively. Materials 2020, 13, x FOR PEER REVIEW 6 of 17 Figure 2. HRTEM images of ordered MSGs: (a) Undoped-MSG; (b) 5 wt% Ag-MSG; (c) 5 wt% Cu- MSG; (d) 5 wt% Zn-MSG; (e) 5 wt% Ce-MSG. Scale bar = 50 nm. Figure 3 shows an example of how the average pore size was typically determined; the arrow indicates the direction along which the profile has been plotted. In order to increase the intensity, the profile has been integrated over the area. The measurements were taken from the well-ordered lateral areas of the MSGs. Ordered undoped-MSGs have a pore size diameter of 6.67 ± 0.33 nm. Ordered 5 wt% Ag-MSGs have pore sizes of 6.66 ± 0.20 nm. The pore sizes of ordered 5 wt% Cu-MSGs, 5 wt% Zn-MSGs, and 5 wt% Ce-MSGs are 6.22 ± 0.63 nm, 6.58 ± 0.92 nm, and 6.34 ± 0.15 nm, respectively. Figure 3. Pore size determination of an ordered MSG. The average pore diameters of the ordered MSGs are shown in Table 1. Table 1. Average pore diameters of ordered MSGs. Dopant Pore Diameter (nm) ± Standard Deviation — 6.67 ± 0.33 5 wt% Ag 6.66 ± 0.20 5 wt% Cu 6.22 ± 0.63 5 wt% Zn 6.58 ± 0.92 5 wt% Ce 6.34 ± 0.15 3.2. Nitrogen Adsorption—BET and BJH 3.2.1. Non-ordered and Ordered Undoped Mesoporous Glasses According to the N2 adsorption-desorption isotherms of non-ordered and ordered, undoped- MSGs, both the non-ordered and ordered undoped-MSGs show type IV isotherms (Figure 4), which is characteristic of mesoporous materials [12,22]. The hysteresis type of non-ordered undoped-MSG is H2 (Figure 4), which is associated with ink-bottle shape pores, whilst the hysteresis type of ordered undoped-MSG is H1 (Figure 4), which is characteristic of cylindrical pores. H1 hysteresis loop of the ordered undoped-MSG has parallel adsorption and desorption branches that are almost vertical at the capillary condensation region. In comparison H2 hysteresis loop of non-ordered undoped-MSG has a steeper desorption branch than its adsorption branch. The pore properties of these MSGs are given in Table 2. Figure 3. Pore size determination of an ordered MSG. The average pore diameters of the ordered MSGs are shown in Table 1. Table 1. Average pore diameters of ordered MSGs. Dopant Pore Diameter (nm) ±Standard Deviation — 6.67 ±0.33 5 wt% Ag 6.66 ±0.20 5 wt% Cu 6.22 ±0.63 5 wt% Zn 6.58 ±0.92 5 wt% Ce 6.34 ±0.15 3.2. Nitrogen Adsorption—BET and BJH 3.2.1. Non-ordered and Ordered Undoped Mesoporous Glasses According to the N 2 adsorption-desorption isotherms of non-ordered and ordered, undoped-MSGs, both the non-ordered and ordered undoped-MSGs show type IV isotherms (Figure 4), which is characteristic of mesoporous materials [ 11 , 21 ]. The hysteresis type of non-ordered undoped-MSG is H2 (Figure 4), which is associated with ink-bottle shape pores, whilst the hysteresis type of ordered undoped-MSG is H1 (Figure 4), which is characteristic of cylindrical pores. H1 hysteresis loop of the ordered undoped-MSG has parallel adsorption and desorption branches that are almost vertical at the capillary condensation region. In comparison H2 hysteresis loop of non-ordered undoped-MSG has a steeper desorption branch than its adsorption branch. The pore properties of these MSGs are given in Table 2. Materials 2020,13, 992 7 of 16 Materials 2020, 13, x FOR PEER REVIEW 7 of 17 3.2.2. Non-ordered and Ordered 5 wt% Ag-, Cu-, Zn, and Ce-doped Mesoporous Glasses Figure 4. N2 adsorption-desorption isotherms of non-ordered and ordered, undoped-MSGs, 5 wt% Ag-MSGs, 5 wt% Cu-MSGs, 5 wt% Zn-MSGs, and 5 wt% Ce-MSGs. Crosses (+) and open circles (O) represent the adsorption and desorption branches, respectively. Figure 4. N 2 adsorption-desorption isotherms of non-ordered and ordered, undoped-MSGs, 5 wt% Ag-MSGs, 5 wt% Cu-MSGs, 5 wt% Zn-MSGs, and 5 wt% Ce-MSGs. Crosses (+) and open circles (O) represent the adsorption and desorption branches, respectively. Materials 2020,13, 992 8 of 16 Table 2. Pore properties of MSGs. (BET: Brunauer-Emmett-Teller; BJH: Barrett-Joyner-Halenda) Mesopore Structure Dopant BET Surface Area (m2/g) BJH Desorption Cumulative Pore Volume (cm3/g) BJH Desorption Average Pore Size Diameter (nm) Non-ordered — 246.7 0.1686 2.8019 Non-ordered 5 wt% Ag 307.8 0.2659 3.1422 Non-ordered 5 wt% Cu 167.1 0.1546 3.3767 Non-ordered 5 wt% Zn 188.3 0.1460 2.8718 Non-ordered 5 wt% Ce 250.5 0.1990 3.2015 Ordered — 411.9 0.6119 4.7230 Ordered 5 wt% Ag 528.6 0.8676 5.2439 Ordered 5 wt% Cu 328.3 0.4232 4.0138 Ordered 5 wt% Zn 380.9 0.5015 4.1182 Ordered 5 wt% Ce 438.1 0.6747 4.8182 3.2.2. Non-ordered and Ordered 5 wt% Ag-, Cu-, Zn, and Ce-doped Mesoporous Glasses The hysteresis types of non-ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs are H2 (Figure 4), while the hysteresis types of ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs are H1 (Figure 4). The hysteresis curves of ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs have parallel adsorption and desorption branches that are almost vertical at the capillary condensation region, which is characteristic of cylindrical pores. In contrast, the H2 hysteresis of non-ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs have steeper desorption branches than their adsorption branches. The values of the porosity-related properties (surface area, pore volume, and pore size diameter) of ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs are higher than those of the non-ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs. The relevant properties of these MSGs are given in Table 2. 3.3. Fourier Transform Infrared Spectroscopy (FTIR) The portion of the vibrational spectra of most interest (~1400–400 cm −1 ) for the non-ordered mesoporous glasses can be seen in Figure 5a and for the ordered mesoporous glasses in Figure 5b. This region is of primary importance since it is dominated by the vibrational bands associated with Si-O. All samples show strong and broad bands in the range 1300–1000 cm −1 , which can be assigned to the asymmetric Si-O-Si stretching mode [ 22 ]. Absorption bands centered at ~800 cm −1 in all samples result from symmetric Si-O-Si stretching modes [ 23 ], whilst bands at ~450 cm −1 can be attributed to rocking vibration modes of Si-O-Si [ 22 , 24 ]. Small shoulders of the main Si-O-Si stretching band at ~950 cm−1 are only visible for the non-ordered mesoporous Ag and Cu- MSGs and can be assigned as Si-O non-bonding oxygens (NBO) [ 22 , 23 ]. Absorption bands at ~560 cm −1 can be assigned as bending P-O vibrational mode [ 22 , 25 ]. In addition, all materials show a broad band (shown in the supplementary section (Figures S1 and S2) as an example for ordered 5 wt% Cu-MSG) at 3700–3000 cm −1 attributable to various vibrational modes of hydroxyl groups; hydrogen-bonded and non-hydrogen-bonded silanols and physisorbed water [22]. Materials 2020,13, 992 9 of 16 Materials 2020, 13, x FOR PEER REVIEW 9 of 17 Figure 5. Fourier transform infrared spectroscopy (FTIR) spectra of: (a) Non-ordered mesoporous glasses; (b) ordered mesoporous glasses. (r) Shows rocking, (as) is asymmetric and (s) is symmetric. The main vibrational bands and associated approximate peak wavenumbers can be seen in Table 3. Table 3. Vibrational band assignments and approximate peak band wavenumbers for non-ordered and ordered mesoporous undoped and doped glasses, where ns = not seen. Mesopore Structure Dopant νas (Si-O-Si) cm−1 ν (Si-O NBO) cm−1 νs (Si-O-Si) cm−1 B (P-O) cm−1 r (Si-O-Si) cm−1 Non-ordered — 1047 ns 796 556 442 Non-ordered 5 wt% Ag 1045 959 794 551 443 Non-ordered 5 wt% Cu 1077 936 799 604/566 456 Non-ordered 5 wt% Zn 1071 ns 801 559 445 Non-ordered 5 wt% Ce 1050 ns 797 555 448 Ordered — 1032 ns 804 568 441 Ordered 5 wt% Ag 1035 ns 797 559 440 Ordered 5 wt% Cu 1039 ns 801 557 443 Ordered 5 wt% Zn 1034 ns 800 556 440 Ordered 5 wt% Ce 1035 ns 800 558 441 3.4. Semi-quantitative X-ray Fluorescence The non-ordered and ordered, undoped, and 5 wt% doped mesoporous glasses were examined for their experimental wt% compositions by semi-quantitative (SQ) X-ray fluorescence technique. Results are shown in Tables 4 and 5. Even if the XRF results could be prone to variations, the data showed that the reproducibility of the compositions of the non-ordered MSGs was more consistent and their experimental compositions were closer to their theoretical values compared to ordered MSGs. Among the different dopant (Ag, Cu, Zn, and Ce) incorporated ordered MSGs, there were large compositional variabilities. Based on their theoretical 5 wt% dopant values, non-ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs had higher dopant incorporation than the same composition ordered MSGs as shown in Table 4. Table 4. Theoretical and experimental wt% compositions of mesoporous silicate glasses. Figure 5. Fourier transform infrared spectroscopy (FTIR) spectra of: ( a ) Non-ordered mesoporous glasses; (b) ordered mesoporous glasses. (r) Shows rocking, (as) is asymmetric and (s) is symmetric. The main vibrational bands and associated approximate peak wavenumbers can be seen in Table 3. Table 3. Vibrational band assignments and approximate peak band wavenumbers for non-ordered and ordered mesoporous undoped and doped glasses, where ns =not seen. Mesopore Structure Dopant νas (Si-O-Si) cm−1 ν(Si-O NBO) cm−1 νs(Si-O-Si) cm−1 B (P-O) cm−1 r (Si-O-Si) cm−1 Non-ordered —1047 ns 796 556 442 Non-ordered 5 wt% Ag 1045 959 794 551 443 Non-ordered 5 wt% Cu 1077 936 799 604/566 456 Non-ordered 5 wt% Zn 1071 ns 801 559 445 Non-ordered 5 wt% Ce 1050 ns 797 555 448 Ordered —1032 ns 804 568 441 Ordered 5 wt% Ag 1035 ns 797 559 440 Ordered 5 wt% Cu 1039 ns 801 557 443 Ordered 5 wt% Zn 1034 ns 800 556 440 Ordered 5 wt% Ce 1035 ns 800 558 441 3.4. Semi-quantitative X-ray Fluorescence The non-ordered and ordered, undoped, and 5 wt% doped mesoporous glasses were examined for their experimental wt% compositions by semi-quantitative (SQ) X-ray fluorescence technique. Results are shown in Tables 4and 5. Even if the XRF results could be prone to variations, the data showed that the reproducibility of the compositions of the non-ordered MSGs was more consistent and their experimental compositions were closer to their theoretical values compared to ordered MSGs. Among the different dopant (Ag, Cu, Zn, and Ce) incorporated ordered MSGs, there were large compositional variabilities. Based on their theoretical 5 wt% dopant values, non-ordered 5 wt% Ag-, Cu-, Zn-, and Ce-MSGs had higher dopant incorporation than the same composition ordered MSGs as shown in Table 4. Materials 2020,13, 992 16 of 16 19. Goh, Y.F.; Alshemary, A.Z.; Akram, M.; Abdul Kadir, M.R.; Hussain, R. In-vitro characterization of antibacterial bioactive glass containing ceria. Ceram. Int. 2014,40, 729–737. [CrossRef] 20. Brinker, C.J.; Lu, Y.; Sellinger, A.; Fan, H. Evaporation-induced self-assembly: Nanostructures made easy. Adv. Mater. 1999,11, 579–585. [CrossRef] 21. Zhao, D.; Wan, Y.; Zhou, W. Chapter 4- Structural Characterization Methods. In Ordered Mesoporous Materials; Wiley-VCH Verlag & Co. KGaA: Weinheim, Germany, 2013; pp. 117–151. ISBN 978-3-527-64789-7. 22. Aguiar, H.; Serra, J.; Gonz á lez, P.; Le ó n, B. Structural study of sol-gel silicate glasses by IR and Raman spectroscopies. J. Non-Cryst. Solids 2009,355, 475–480. [CrossRef] 23. Mariappan, C.R.; Ranga, N. Influence of silver on the structure, dielectric and antibacterial effect of silver doped bioglass-ceramic nanoparticles. Ceram. Int. 2017,43, 2196–2201. [CrossRef] 24. Kalampounias, A.G. IR and Raman spectroscopic studies of sol–gel derived alkaline-earth silicate glasses. Bull. Mater. Sci. 2011,34, 299–303. [CrossRef] 25. Notingher, I.; Jones, J.R.; Verrier, S.; Bisson, I.; Embanga, P.; Edwards, P.; Polak, J.M.; Hench, L.L. Application of FTIR and Raman spectroscopy to characterisation of bioactive materials and living cells. Spectroscopy 2003 , 17, 275–288. [CrossRef] 26. Gupta, N.; Santhiya, D.; Murugavel, S.; Kumar, A.; Aditya, A.; Ganguli, M.; Gupta, S. Effects of transition metal ion dopants (Ag, Cu and Fe) on the structural, mechanical and antibacterial properties of bioactive glass. Colloids Surf. A Physicochem. Eng. Asp. 2018,538, 393–403. [CrossRef] 27. Li, X.; Wang, X.; He, D.; Shi, J. Synthesis and characterization of mesoporous CaO–MO–SiO2–P2O5 (M =Mg, Zn, Cu) bioactive glasses/composites. J. Mater. Chem. 2008,18, 4103–4109. [CrossRef] 28. Yan, X.X.; Deng, H.X.; Huang, X.H.; Lu, G.Q.; Qiao, S.Z.; Zhao, D.Y.; Yu, C.Z. Mesoporous bioactive glasses. I. Synthesis and structural characterization. J. Non-Cryst. Solids 2005,351, 3209–3217. [CrossRef] 29. Thommes, M.; Kaneko, K.; Neimark, A.V.; Olivier, J.P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K.S.W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015,87, 1051–1069. [CrossRef] 30. Sing, K.S.W.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl. Chem. 1985,57, 603–619. [CrossRef] 31. Grosman, A.; Ortega, C. Nature of Capillary Condensation and Evaporation Processes in Ordered Porous Materials. Am. Chem. Soc. 2005,21, 10515–10521. [CrossRef] 32. Gonz á lez, P.; Serra, J.; Liste, S.; Chiussi, S.; Le ó n, B.; P é rez-Amor, M. Raman spectroscopic study of bioactive silica based glasses. J. Non-Cryst. Solids 2003,320, 92–99. 33. El-Kady, A.M.; Ali, A.F.; Rizk, R.A.; Ahmed, M.M. Synthesis, characterization and microbiological response of silver doped bioactive glass nanoparticles. Ceram. Int. 2012,38, 177–188. [CrossRef] 34. Atkinson, I.; Anghel, E.M.; Petrescu, S.; Seciu, A.M.; Stefan, L.M.; Mocioiu, O.C.; Predoana, L.; Voicescu, M.; Somacescu, S.; Culita, D.; et al. Cerium-containing mesoporous bioactive glasses: Material characterization, in vitro bioactivity, biocompatibility and cytotoxicity evaluation. Microporous Mesoporous Mater. 2019 ,276, 76–88. [CrossRef] 35. Salinas, A.J.; Shruti, S.; Malavasi, G.; Menabue, L.; Vallet-Reg í , M. Substitutions of cerium, gallium and zinc in ordered mesoporous bioactive glasses. Acta Biomater. 2011,7, 3452–3458. [CrossRef] [PubMed] 36. Wers, E.; Oudadesse, H.; Lefeuvre, B.; Lucas-Girot, A.; Rocherull é , J.; Lebullenger, R. Excess entropy and thermal behavior of Cu- and Ti-doped bioactive glasses. J. Therm. Anal. Calorim. 2014 ,117, 579–588. [CrossRef] 37. Arcos, D.; L ó pez-Noriega, A.; Ruiz-Hern á ndez, E.; Terasaki, O.; Vallet-Reg í , M. Ordered mesoporous microspheres for bone grafting and drug delivery. Chem. Mater. 2009,21, 1000–1009. [CrossRef] 38. Rabiee, S.M.; Nazparvar, N.; Azizian, M.; Vashaee, D.; Tayebi, L. Effect of ion substitution on properties of bioactive glasses: A review. Ceram. Int. 2015,41, 7241–7251. [CrossRef] 39. Sayah, E.; Brouri, D.; Wu, Y.; Musi, A.; Da Costa, P.; Massiani, P. A TEM and UV-visible study of silver reduction by ethanol in Ag-alumina catalysts. Appl. Catal. A Gen. 2011,406, 94–101. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).