Elsevier

Biomaterials

Volume 24, Issue 1, January 2003, Pages 107-119
Biomaterials

Fluoride uptake by glass ionomer cements:: a surface analysis approach

https://doi.org/10.1016/S0142-9612(02)00268-5Get rights and content

Abstract

Despite extensive research, the mechanism by which glass ionomer cements take up fluoride ions from solution remains unclear. To date, the majority of studies have concentrated on measuring the removal of ions from solution. In this study, we demonstrate the application of X-ray photoemission spectroscopy and secondary ion mass spectrometry to the surface analysis of the cements, after the introduction of fluoride either by doping or by immersion. Fluoride ion uptake from potassium fluoride solution is correlated with the formation of a surface layer which is rich in calcium as well as fluoride.

Introduction

The ability to take up and re-release ions from solution is an important property of glass ionomer (glass polyalkenoate) cements, which may allow their application as ‘rechargeable reservoirs’ for the distribution of ions including fluoride [1], [2], [3], [4]. This would have clear benefits in dentistry, where glass ionomer cements (GICs) may be used as filling materials combating caries whilst restoring form and function [5]. A GIC is comprised of a blend of glass, polymeric acid and water, typically achieved in the UK by mixing a blend of glass and polyacid powder with water [6]. The ensuing acid–base setting reaction involves the release of multivalent cations from the glass component of the cement, which form ionic crosslinks between the polymer chains [7]. The inorganic component of a GIC is generally based on an aluminosilicate glass, which may contain a range of other constituents such as CaO, Na2O or P2O5. Fluoride may be an intrinsic component of the glass, or may be introduced into the cement from solution during the mixing stage of preparation. Both fluoride-free and fluoride-containing cements have been shown to take up fluoride ions on immersion in aqueous fluoride solution and subsequent re-release into solution has been demonstrated [8]. Despite the significant amount of research aimed at establishing the mode of uptake and re-release [8], [9], [10], however, the underlying mechanism remains unclear.

The majority of the research into fluoride ion uptake has utilised analysis of ion concentrations in solution before and after GIC immersion. By following ionic concentration as a function of time, useful information has been obtained about how the initial GIC composition affects the removal of ions from solution and their subsequent re-release pattern. Such an approach, however, provides little information about the changes that occur to the GIC itself during immersion; the location of the incorporated ions, for example, remains unknown. Some analysis of ion distribution within GICs has been carried out. For example, X-ray microanalysis has been used to determine elemental distributions through GIC sections [11], although the distribution of fluoride was not considered. Stray-field nuclear magnetic resonance spectroscopy (STRAFI-NMR) has been used to examine the distribution of fluoride through a GIC [12]. However, only intrinsic F was considered and not ions introduced by doping or immersion. Most recently, Hadley et al. [13] used X-ray photoelectron spectroscopy (XPS) to assess GIC surface composition, as well as dynamic secondary ion mass spectrometry (SIMS) to assess ionic distribution as a function of depth into the sample. Fluoride uptake from KF(aq) was demonstrated and, using depth profiling, it was shown that the concentration of fluoride maximised at the surface of the samples. Although the uptake of potassium was also considered, no attempt was made to correlate the variation in fluoride signal with that of any other ion in the near surface region and the spectra were not explored in any great depth.

XPS [14], [15] is a powerful technique with which the elemental composition of a surface can be comprehensively examined. Semi-quantitative information can be obtained about the elemental composition of the surface. Furthermore, the binding energy of the photoelectrons is related to the chemical environment of the element from which they originate; by measuring binding energies, a degree of structural and bonding information may be inferred. For example, C1s photoelectrons originating from carbon atoms bound to different functional groups (C–OH, C=O, etc.) in an organic polymer have significantly different binding energies and the overall C1s lineshape reflects the relative contribution of each component. SIMS [16], [17], particularly in the surface-specific ‘static’ mode, has other advantages; namely a high elemental detection sensitivity and the capacity for surface chemical mapping.

The current work constitutes a preliminary but relatively in-depth study using these techniques to examine the surface chemistry of an undoped GIC and that of GICs which have been modified by the inclusion of fluoride via either mixing (‘doping’) with, or immersion in, fluoride solutions. The C1s XPS lineshape of the undoped GIC is compared in some detail with that of poly(acrylic acid) (PAA), the major organic constituent of the GICs, and the semi-quantitative information obtained from XPS and SIMS measurements is compared. The results highlight the complementary nature of the two techniques and their power for surface analysis. The data show significant correlation with results from previous studies and begin to shed some light on the surface processes occurring during the uptake of ionic species from aqueous solution by GICs.

Section snippets

Experimental

Discs (10 mm diameter×1 mm) were prepared using LG30 anhydrous restorative. The LG30 glass is a calcium aluminosilicate glass, containing neither fluoride nor a monovalent cation. The cements were mixed at a ratio of 7:1 with either deionised water, or KF(aq) at an Fconcentration of 90 ppm (4.74 μmol g−1), 900 ppm (47.4 μmol g−1), or 9000 ppm (474 μmol g−1). The cements mixed using fluoride solution are referred to throughout as ‘doped’ samples and contain KF levels of 0.593, 5.93 and 59.3 μmol g−1,

Results

The widescan (survey) X-ray photoelectron spectrum from the fluoride-free, water-mixed sample is shown in Fig. 1. The spectrum shows core-level photoelectron peaks due to all the expected elements, with major contributions from oxygen and carbon and minor peaks attributable to aluminium, silicon, calcium and phosphorus. Contaminants that were occasionally observed included nitrogen, sulphur and zinc, although these were present only in very small amounts (<3 at%).

Narrow region spectra for the

Bonding within the GIC matrix

The carbon 1s core-level peak shape provides several insights into the reaction occurring on mixing the GIC.

Firstly, the peak contains more ‘hydrocarbon-like’ carbon than expected. The ratio of C1:C2-type carbon is 1.8, compared with published values of 1.4 [23] and 1.5 [25] for PAA and an expected ratio of 1 on the basis of the PAA molecular structure. Possible explanations include hydrocarbon surface contamination, loss of carboxyl groups or alignment at the surface of the chains of the

Conclusions

XPS and SIMS are powerful techniques for investigating the chemistry of GICs. Together, they have provided a wealth of information about the changes in GIC surface composition that occur on the inclusion of fluoride by doping and by immersion. Using these techniques, it has been demonstrated that a GIC based on poly(acrylic acid) and a calcium aluminosilicate glass takes up fluoride on immersion in KF(aq) solution primarily by the formation of a calcium and fluoride rich surface layer, possibly

Acknowledgements

Many thanks to David Williams (UCL Chemistry) for access to the ESCALAB facility. Thanks also to Tim Carney (VG) for general advice on spectrometer operation and quantification and Neal Fairley for curve fitting using CasaXPS.

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