Characterization of the Calcium-Fluoroaluminosilicate Glass Prepared by a Non-Hydrolytic Sol-Gel Route for Future Dental Application as Glass Ionomer Cement

Glass ionomer cements are widely employed in dentistry due to their physical, biological and mainly anti-caries properties. Glass ionomers consist of an aluminosilicate glass matrix modified with other elements, and they contain large quantities of fluorine. In this study, we report on the preparation of calcium-fluoroaluminosilicate glasses by a nonhydrolytic sol-gel route as an alternative approach to obtaining alumina-silica matrices. The glass powders were prepared via the non-hydrolytic sol-gel method, by mixing AlCl 3 , SiCl 4 , CaF 2 , AlF 3 , NaF, and AlPO 4 . The powders were studied by thermal analysis (TG/DTA/DSC), photoluminescence (PL), nuclear magnetic resonance (NMRAl-Si), and X ray diffraction (XRD). TG/DTA/DSC analyses revealed a constant mass loss due to structural changes during the heating process, which was confirmed by NMR and PL. A stable aluminosilicate matrix with potential future application as a glass ionomer base was obtained.


Introduction
Glass ionomer cements were firstly developed by Wilson and Kent in 1969.Due to their sustained fluoride release, they have found use as a restorative dental material with long-term adhesion to the dental structure, good biocompatibility, and cariostatic properties.These cements became commercially available in Europe in 1975 and since then they have been modified for use in specific clinical situations [1][2][3] .The glass ionomer consists in an aluminum and silicon matrix comprising an amorphous structure 2 , which renders glass ionomer cements their special mechanical strength properties (compressive, diametral tensile, and biaxial flexural strength) 4 .
Nowadays, cement formulations allow for their application as base or dentine substitute under a composite resin, luting cement in crown and bridgework, lining cement under a metal restoration, long-term sealant over an active carious lesion, and restoration material in its own right 5,6 .
The nonhydrolytic sol-gel route is an alternative process for the production of multicomponent oxide materials with chemical, physical and thermal properties that cannot be obtained by the hydrolytic sol-gel route and fusion methodologies 12,13 .This alternative method is based on the condensation between an alkoxide and a metal halide.The alkoxide can be added to the reaction or obtained in situ by reaction of a metal chloride with oxygen donors; i.e., ethers or alcohols, in the absence of water (Scheme 1) [14][15][16] .
The alkoxide is condensed with a metal chloride, to form a metal oxide (Scheme 2) 14 .
This route requires low temperatures, close to 110 °C, and it is advantageous because the products are more homogeneous, the method is easier to reproduce, no solvents are required, and the formation of residual Metal-OH groups is reduced 15,17,18 .
In this work, the calcium-fluoroaluminosilicate glass was prepared in oven-dried glassware.To this end, AlCl 3 , SiCl 4 , CaF 2 , AlF 3 , NaF, AlPO 4 and ethanol were reacted in reflux under argon atmosphere, and europium (III) chloride was added as a structural probe.The powder mixtures were dried at 50 °C and heat-treated at 1,000 °C for 4 hours.The synthesis and structure of the powders were studied by thermal analysis (TG/DTA/DSC), photoluminescence (PL), nuclear magnetic resonance (NMR 27 Al-29 Si), and X ray diffraction (XRD).

Experimental
The nonhydrolytic sol-gel route described in the literature 19 and modified by Ciuffi 20 was employed in the preparation of the calcium-fluoroaluminosilicate glass.The glasses were prepared in oven-dried glassware.
The reagents AlCl 3 , AlF 3 , CaF 2 , NaF, SiCl 4 , and AlPO 4 in weight percentages of 28.6, 1.6, 15.7, 9.3, 41.9, and 3.8, respectively, were added to 40 mL of anhydrous ethanol and 10 mL of diisopropyl ether, used as oxygen donors.One percent of EuCl 3 in weight was added as structural probe.The reaction was kept under reflux and magnetic stirring for 4 hours in inert argon atmosphere.The condenser was placed in a thermostatic bath at -5 °C.After the reflux, the mixture was cooled and aged overnight in the mother liquor at room temperature (RT), and precipitation continued through aging in the mother liquor 20 .The solvent was then removed under vacuum.The powder was dried for one week at 50 °C, resulting in a fine white powder.
Thermal Analysis (TG/DTA/DSC) was carried out in a thermal analyzer (TA -Instruments -SDT Q600 -Simultaneous DTA-TG) under nitrogen atmosphere, at a heating rate of 20 °C/min, from 25 to 1,500 °C.The luminescence data were obtained on a Spex Fluorolog II spectrofluorometer, at room temperature.The emission was collected at 22.5° (front face) from the excitation beam.The material was Materials Research submitted to 29 Si and 27 Al NMR analysis (59.5 MHz) in an INOVA 300 Varian spectrophotometer, using silicon nitride as reference.X ray diffractograms (XRD) were obtained using a Siemens ® (D 5005) X ray diffractometer under Cu K α radiation.

Results and Discussion
Figure 1 depicts the DTA and DSC curves of the material prepared in this work.These curves reveal the presence of an exothermic peak at 932 °C, which is characteristic of the glass crystallization temperature (T c ), and can be attributed to the transition from the amorphous to the crystalline system, as indicated by X ray analysis.
The TG curves obtained for the sample before it was submitted to thermal treatment reveal a considerable mass loss between 30 and 400 °C, which is attributed to ethanol and water molecules.
Figure 2 shows the XRD patterns of the calcium-fluoroaluminosilicate glass dried at 50 °C and heat-treated at 1,000 °C.The SiO 2 -Al 2 O 3 -CaO or SiO 2 -Al 2 O 3 -CaO-CaF 2 systems studied by Bertolini et al. 21presented the anorthite phase (CaAl 2 Si 2 O 8 ) when heat-treated at 1,020 °C.In the present work, an amorphous structure predominates in the system dried at 50 °C, confirming the glass structure.As for the material heat-treated at 1,000 °C, it displays defined peaks characteristic of the anorthite crystalline structure.
The 27 Al NMR spectrum indicates the aluminum coordination.Figure 3 presents the NMR spectrum of the calcium-fluoroaluminosilicate glass dried at 50 °C and of the same glass heat-treated at 1,000 °C.
The central transition (CT) frequency of the spectrum of a quadrupolar nucleus of half integer spin, such as 27 Al (I = 5/2), depends on the orientation of each crystalline in the static magnetic field to the second order in perturbation theory.The quadrupolar interaction between the nuclear electric quadrupole moment (eQ) and the electric field gradient of the nucleus (eq), arising from any lack of symmetry in the local electron distribution, is described by the quadrupolar coupling constant Cq (e 2 qQ/h) and the symmetry parameter η.It should be noted that disordered materials such as glasses have a wide range of interatomic distances, so CT line broadening occurs due to the δ iso distribution and quadrupolar interactions 22 .
After the material was heat-treated at 1,000 °C, a single peak corresponding to Al (VI) predominated at 0.0 ppm, indicating that a structural change in the coordination state of aluminum occurred.When Al atoms are in tetrahedral coordination Al (IV) , their chemical shifts vary from 55 to 80 ppm, as indicated in Figure 2. Chemical shifts in the range of -10 to 10 ppm correspond to coordinated octahedral Al (VI) [17][18][19] .The spectra of the two samples display three peaks, namely at 10.4, 59.4 and 140.1 ppm, which are characteristic of Al (VI) , Al (IV) , and spinning side bands 23 , respectively.Although some authors have reported the presence of Al (V) atoms with chemical shifts at 20 ppm 8 , we did not observe this peak.The dominant species in the sample dried at 50 °C corresponds to Al (IV) .
To be employed as a glass ionomer cement base, calciumfluoroaluminosilicate should display considerable basicity.To this end, the AlO 4 tetrahedron should bind to a SiO 4 tetrahedron via an oxygen atom.When Al 3+ ions are incorporated into an amorphous SiO 2 network, it acquires a negative charge that is counter-balanced by cations, such as Ca 2+ and P 5+ .Phosphorus is present as tetrahedral PO 4 adjacent to tetraedral AlO 4 (forming AlPO 7 ) in the network as a result of the charge balance between the Al 3+ and P 5+ ions.
The bond established between the aluminum and silicon tetrahedra via the same oxygen atom is protophylic, as in the case of non-bonding oxygens.This makes the vitreous network susceptible   to acid attack, which is desirable for materials based on calciumfluoroaluminosilicates if they are to be used as glass ionomer cement base.In fact, the material dried at 50 °C presents a chemical shift value typical of tetracoordinated aluminum, which is one of the requirements for dental cement bases.
The exact location of the chemical shifts due to Al (IV) between 50 and 60 ppm depend on the Al:P molar ratio.A chemical shift at 60 ppm for Al (IV) has been found for model glasses based on SiO 2- Al 2 O 3 CaOCaF 2 , and at about 50 ppm in glasses containing phosphate at an Al:P molar ratio of 2:1 23 .The Al:P molar ratio in our sample is approximately 10, which is higher than the one encountered in commercial glasses.In our case, this chemical shift is very difficult to observe because of the lower prevalence of Al-O-P bonds.
The 29 Si NMR analysis allows for investigation of the chemical environment of silicon atoms in silicates.Si-atoms are generally bound to four oxygen atoms in silicates, which can be represented by a tetrahedron whose corners link to other tetrahedra.The Q n notation serves to describe the substitution pattern around a specific silicon atom, with Q representing a silicon atom surrounded by four oxygen atoms and n indicating the connectivity 24 .Figure 4 depicts the 29 Si NMR spectrum of the sample dried at 50 °C.
The material displays a peak at -100 ppm and a shoulder at -110 ppm, which can be attributed to Si atoms Q 4 and Q 4 or Q 3 , respectively.Figure 5 illustrates the Q 3 and Q 4 structure.
The chemical shift indicates the environment around the Si atoms in the glass.The commercial calcium-fluoroaluminosilicate glass presents a broad peak between -90 and -99 ppm 8 , so our material exhibits a vitreous lattice.
The number of nearest neighboring aluminum atoms is given in brackets; thus, Q 4 (3/4 Al) and Q 4 (1/2) 22 are the structure represented in Figure 6.
The chemical shift ranges overlap, so the resonances in Fuji II cement (commercial glass) at -87 ppm may be due to Q 4 (3/4 Al), while the resonances at -92 ppm may be due to Q 4 (3 Al), at -99 ppm to Q 4 (1/2 Al), and at -109 ppm to Q 4 (0 Al) 22 .In this case, the chemical shifts at -110 and 100 ppm may be due to the Si atoms Q 4 (1/2 Al) and Q 4 (0 Al), because the Al:Si molar ration is less than 1.
Figure 7 shows the 29 Si NMR of the sample heat-treated at 1,000 °C for 4 hours.Only one peak at -88 ppm is observed, which is attributed to Q 4 (3/4 Al) site Si atoms due to the structural rearrangement of the aluminosilicate crystalline structures and is consistent with our X ray data.
Figure 8 presents the excitation spectrum of the Eu 3+ ion in the sample treated at 1,000 °C.The line observed in this spectrum can be assigned to the transition between the 7 F 0 and the 5 L 6 , 5 L 7 , 5 D 4 , 5 D 3 , 5 D 2 , and 5 D 1 levels Figure 9 depicts the emission spectra of the Eu 3+ ion recorded in the 550-720 nm range for the sample treated at 1,000 °C and excited at 394 and 462 nm.
Excitation and emission of the Eu 3+ ion in the material dried at 50 °C could not be observed because of the quenching caused by the vibrational mode of the solvent molecules in the material.This fact also occurred when the nonhydrolytic sol-gel route was employed for preparation of yttrium-alumina-garnet (YAG) at room temperature 25,26 and alumina oxide 27 .
The emission spectra obtained by excitation in the 5 D 0 → 5 L 6 and 5 D 0 → 5 D 2 transition of Eu 3+ ions at 394 and 462 nm, respectively, are composed by the 5 D 0 → 7 F J (J = 0, 1, 2, 3 and 4) emission lines   Figure 7. 29 Si NMR spectrum of the sample heat-treated at 1,000 °C. of Eu 3+ .The large magnitude of spin-orbit coupling in the lanthanides causes the individual J levels of the various electronic terms to be well separated from one another, except for the ground 7 F 0 and emissive 5 D 0 states of Eu 3+ , which are nondegenerated.The highly forbidden 7 F 0 → 5 D 0 transition of Eu 3+ is particularly important in that only a single transition is possible for a single Eu 3+ ion environment 28 .The 7 F 0 → 5 D 0 transition is clearly visible in Figure 8, indicating that the Eu 3+ ions occupy sites without an inversion center 29 .The presence of nonhomogeneous sites in the material was observed based on the band-width emission 30 .The various J levels are further split by ligand fields in the maximum 2J + 1; therefore, in the 7 F 0 → 5 D 1 and 7 F 0 → 5 D 2 transitions the number of bands may be 3 and 5, respectively.We observed more transitions than the ones allowed, a fact that confirmed that the Eu 3+ ions occupy sites with different symmetries such as C n ou C nv .This is an indication that the calcium-fluoroaluminosilicate glass has different symmetries, which can be observed by 27 Al NMR.

Conclusions
The physical and chemical characteristics of glass ionomer cements render them useful and advisable for application in a variety of clinical situations.However, the industrial preparation of glass ionomer powders based on calcium-fluoroaluminosilicate glass is very expensive because it requires high temperatures, between 1,200 and 1,550 °C, which makes it economically unreasonable for large-scale production.To offset this situation, the nonhydrolytic sol-gel method has proven to be efficient for the production of materials with glass properties, as shown in this work.This process allows for reaction control and the use of stoichiometric amounts of Al and Si reagents at low temperatures, around 110 °C, thus reducing production costs.
The powder obtained by this methology was tested and showed to be a rigid cement.It was also shown that the material exhibits the basicity required for its reaction with polyacrilic acid, to form the cement.

Figure 1 .
Figure 1.DTA and DSC curves for the calcium-fluoroaluminosilicate glass.

Figure 3 .
Figure3.27 Al NMR spectrum of the sample dried at 50 °C and treated at 1,000 °C.

Figure 4 .
Figure 4. 29 Si NMR spectrum of the sample dried at 50 °C.

Figure 5 .
Figure 5. Schematic representation of the Q 3 and Q 4 structures.

Figure 8 .
Figure 8. Excitation spectrum of the Eu 3+ ion in the sample heat-treated at 1,000 °C.