Determination of chemical species of fluoride during uptake mechanism of glass-ionomer cements with NMR spectroscopy

Abstract

Objective: The aim of the present study was to determine the chemical species formed inside glass-ionomer cements after fluoride uptake and to investigate the depth of penetration of fluoride ions within the cement matrix.

Methods: An experimental fluoride-free glass with composition 2SiO₂-AlO₃-CaO was produced. The glass powder was mixed with aqueous poly(acrylic acid) (PAA), and allowed to set. The resulting specimens were stored in 20ml KF solution with 1000ppm fluorine for 24h and then placed into the same amount of water as for 24h. A fluoride selective electrode was used to give the F concentration of the respective solutions. ¹⁹F MAS-NMR spectra were recorded on powdered cement specimens using a Bruker AVANCE-NEO 600 spectrometer. In addition, SEM observation and EDX chemical analysis were conducted on the cross-section of a carefully fractured specimen.

Results: Fluoride was shown to be mainly present in the surface layers of the specimen after placement in the KF solution, and only a small fraction was re-released into water. ¹⁹F NMR spectroscopy showed that AlF complexes were formed within the cement.

Significance: The fluoride taken up by a free-fluoride glass ionomer cement mostly occupies surface layers and is retained because it bonds to aluminum within the matrix. This finding explains why the majority of fluoride taken up by conventional glass ionomer cements is retained.

Keywords: Energy dispersive X-ray spectroscopy; Fluoride; Glass ionomer cements; NMR spectroscopy; Scanning electron microscopy.