Abstract
Objectives
The purpose of this study was to characterize commercial glass polyalkenoate cement (GPC) or glass ionomer cement (GIC), Glass Carbomer ® , which is designed to promote remineralization to fluorapatite (FAp) in the mouth. The setting reaction of the cement was followed using magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy.
Methods
Glass Carbomer ® initial glass powder and cements were subjected to 27 Al, 31 P, 19 F and 29 Si MAS-NMR analysis. X-ray powder diffraction (XRD) was employed to determine the presence of crystalline phases.
Results
27 Al MAS-NMR showed the Al to be predominantly four coordinate, Al(IV), and the presence of Al–O–P species in the glass. The proportion of Al(IV) was reduced with setting reaction of the cement and significant amount of six coordinate Al, Al(VI), was found in the cement. The 31 P MAS-NMR spectra showed clearly a decrease of the orthophosphate peak of apatite on initial setting. 19 F MAS-NMR showed only a small fraction of FAp. 29 Si MAS-NMR demonstrated the presence of largely Q 4 (2Al) in the glass which changed only little in the aged cement.
Significance
This study also demonstrated how the setting reaction in Glass Carbomer ® cement and other GICs can be followed by 27 Al MAS-NMR examining the conversion of Al(IV) to Al(VI). Our data revealed that the apatite in this cement was not FAp but largely hydroxyapatite, which was partially consumed during the cement formation.
1
Introduction
The perfect restorative filling material would be the one that can degrade in the mouth and be replaced by natural tooth enamel and dentin. Such an ideal material does not yet exist, however two previous studies indicate that some commercial glass polyalkenoate or ionomer cements (GPCs/GICs) made headway toward this goal and are capable of remineralizing in the mouth to apatite . It is worth noting that the two commercial cements, Fuji IX and Ketac Molar, that have been shown to promote remineralization in the mouth are based on glasses that can either have orthophosphate phases that are similar to hydroxyapatite (HAp) or crystallize to fluorapatite (FAp) . Previous studies showed that glasses containing phosphate crystallize to apatite . In the case of Fuji IX, it crystallizes to strontium FAp. The remineralization process in the mouth may be aided, not only by the presence of fluoride, but also by the presence of strontium in the glass. Strontium has a known synergistic effect in the conjunction with fluoride on the apatite mineralization . The Sr 2+ cation has a very similar ionic size to Ca 2+ and exhibits complete solid solution phase behavior with calcium in apatite crystal lattices .
Glass Carbomer ® is a relatively new commercially available GIC used as a restorative filling material and fissure sealant that is designed to deliberately remineralize in the mouth. Glass Carbomer ® is claimed to contain nanocrystals of calcium FAp, which can act as nuclei for the remineralization process and initiate the formation of FAp. The glass has a much finer particle size compared to conventional GICs which is thought to aid its dissolution and ultimate conversion to FAp.
Addition of HAp and FAp are also reported to enhance the mechanical properties of the GICs . However, the influence of FAp on the mechanical properties is not fully understood.
Magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy is a powerful technique that probes coordination states and gives information on chemical bonding and next nearest neighbors. It is particularly suitable for investigating the structure of the amorphous glasses and cements. One of the earliest study was done by Matsuya et al. where they used 29 Si, 31 P and 27 Al MAS-NMR to follow and characterize the setting reaction of GIC. They observed the conversion of Al with tetrahedral or four-fold coordination in the glass, Al(IV), to the Al ions with six-fold coordination, Al(VI), the species that crosslinks the cement matrix. Repolymerization of silicate network during glass degradation and cement formation was also observed by 29 Si MAS-NMR. More recent work by Pires et al. and Stamboulis et al. also used the same technique to investigate the setting chemistry of the GICs. Recently, we have carried out 27 Al MAS-NMR at much higher magnetic field (14.1 T) than used in previous studies, which has enabled quantitative measurements and the ability to study the kinetics of the setting reaction of the cements by following the ratio of Al(IV) to Al(VI).
In the present study, the Glass Carbomer ® glass is characterized and the setting reaction of Glass Carbomer ® cements aged from 5 min to 10 months is followed by using 29 Si, 31 P, 19 F and 27 Al MAS-NMR spectroscopy.
2
Materials and methods
2.1
Cement preparation
Commercial Glass Carbomer ® capsules for filling applications were provided by GCP Dental, Netherlands. The untreated glass in granular form and FAp in powder form were additionally provided by the First Scientific (GmbH Germany). The chemical composition of the commercial glass powder was analyzed at the Ceram Research Limited (Stoke-on-Trent, England) using the X-ray fluorescence spectroscopy. The cement capsules were activated and mixed in a mixer for 10–15 s. No heating via the light gun was applied to accelerate the setting. The cement was placed into a disc mold, clamped and allowed to set for 1 h at 37 °C. The termination of setting reaction was done by the method described by Matsuya et al. . The samples of the cement with aging time less than 1 h were quenched into liquid nitrogen and then dehydrated with ethanol. Cement specimens with aging time greater than 1 h were demoulded and placed into deionized water at 37 °C for the appropriate time prior to termination of the reaction by using the above method. This was done to prevent dehydration at longer setting times. The cements were then ground to a fine powder in an agate pestle and mortar prior to MAS-NMR spectroscopy.
2.2
X-ray diffraction
X-ray diffraction (XRD) was performed on Philips Powder Diffractometer with a copper (Cu K α ) X-ray source (Philips PW 1700 series diffractometer, Leiden, Netherlands). The powder samples (<45 μm particle size) were scanned between 2 = 10–80° with a step size of 2 = 0.04°.
2.3
MAS-NMR spectroscopy
The sample powders were packed in 4 mm zirconia rotor and sealed with Kel-F cap. MAS-NMR analyses were conducted on 29 Si, 31 P and 19 F nuclei at resonance frequencies of 39.8, 81.0 and 188.3 MHz, respectively, using FT-NMR spectrometer (AM-200, Bruker, Germany). Spinning rates of the samples at the magic angle were 5 kHz for 29 Si, 31 P and 10–12 kHz for 19 F MAS-NMR. Recycle times were 2–10 s for 29 Si, 31 P and 1–120 s for 19 F MAS-NMR depending on the nature of the sample. Background subtraction was performed on the 19 F MAS-NMR. Reference materials for chemical shift (in ppm) were polydimethylsilane for 29 Si, 85% H 3 PO 4 for 31 P and CaF 2 for 19 F.
27 Al MAS-NMR spectroscopy measurements were conducted at resonance frequency of 156.3 MHz using a higher magnetic field (14.1T) on a 600 MHz Bruker FT-NMR spectrometer. The recycle time was 1 second with spinning rate of 10–15 kHz. The short rf pulse of 0.3 μs corresponding to the π/12 magnetization tip angle has been used. The spectra were referenced to 0 ppm of 1 M AlCl 3 solution. In order to follow the setting reaction of the cements, the deconvolution of the 27 Al MAS-NMR spectra was conducted using dmfit software . This was carried out assuming a Gaussian curve to model the peak shape.
2
Materials and methods
2.1
Cement preparation
Commercial Glass Carbomer ® capsules for filling applications were provided by GCP Dental, Netherlands. The untreated glass in granular form and FAp in powder form were additionally provided by the First Scientific (GmbH Germany). The chemical composition of the commercial glass powder was analyzed at the Ceram Research Limited (Stoke-on-Trent, England) using the X-ray fluorescence spectroscopy. The cement capsules were activated and mixed in a mixer for 10–15 s. No heating via the light gun was applied to accelerate the setting. The cement was placed into a disc mold, clamped and allowed to set for 1 h at 37 °C. The termination of setting reaction was done by the method described by Matsuya et al. . The samples of the cement with aging time less than 1 h were quenched into liquid nitrogen and then dehydrated with ethanol. Cement specimens with aging time greater than 1 h were demoulded and placed into deionized water at 37 °C for the appropriate time prior to termination of the reaction by using the above method. This was done to prevent dehydration at longer setting times. The cements were then ground to a fine powder in an agate pestle and mortar prior to MAS-NMR spectroscopy.
2.2
X-ray diffraction
X-ray diffraction (XRD) was performed on Philips Powder Diffractometer with a copper (Cu K α ) X-ray source (Philips PW 1700 series diffractometer, Leiden, Netherlands). The powder samples (<45 μm particle size) were scanned between 2 = 10–80° with a step size of 2 = 0.04°.
2.3
MAS-NMR spectroscopy
The sample powders were packed in 4 mm zirconia rotor and sealed with Kel-F cap. MAS-NMR analyses were conducted on 29 Si, 31 P and 19 F nuclei at resonance frequencies of 39.8, 81.0 and 188.3 MHz, respectively, using FT-NMR spectrometer (AM-200, Bruker, Germany). Spinning rates of the samples at the magic angle were 5 kHz for 29 Si, 31 P and 10–12 kHz for 19 F MAS-NMR. Recycle times were 2–10 s for 29 Si, 31 P and 1–120 s for 19 F MAS-NMR depending on the nature of the sample. Background subtraction was performed on the 19 F MAS-NMR. Reference materials for chemical shift (in ppm) were polydimethylsilane for 29 Si, 85% H 3 PO 4 for 31 P and CaF 2 for 19 F.
27 Al MAS-NMR spectroscopy measurements were conducted at resonance frequency of 156.3 MHz using a higher magnetic field (14.1T) on a 600 MHz Bruker FT-NMR spectrometer. The recycle time was 1 second with spinning rate of 10–15 kHz. The short rf pulse of 0.3 μs corresponding to the π/12 magnetization tip angle has been used. The spectra were referenced to 0 ppm of 1 M AlCl 3 solution. In order to follow the setting reaction of the cements, the deconvolution of the 27 Al MAS-NMR spectra was conducted using dmfit software . This was carried out assuming a Gaussian curve to model the peak shape.
3
Results
The chemical composition of the initial Glass Carbomer ® glass was determined by X-ray fluorescence technique and is given in Table 1 . The analysis showed that the Glass Carbomer ® glass has higher amount of silica and fluorine and is lower in alkali oxides and phosphorus than many experimental ionomer glasses studied to date .
| Components | SiO 2 | Al 2 O 3 | P 2 O 5 | SrF 2 | CaF 2 | NaF | ZnO | BaO |
|---|---|---|---|---|---|---|---|---|
| Mol% | 54.6 | 20.0 | 3.2 | 11.9 | 4.1 | 5.4 | 0.7 | 0.2 |
Fig. 1 shows the XRD pattern for Glass Carbomer ® glass and FAp. The glass had a broad halo which is indicative of the amorphous nature of the glass phase and the FAp showed sharp lines characteristic of a crystalline phase. The diffraction pattern of the FAp sample matched both calcium HAp and calcium FAp. It is impossible to distinguish these two apatites by XRD because of their almost identical lattice parameters.
