Layered Double Hydroxide for Long Term Delivery Of Fluoride

Abstract

Background

The prevention of early-stage carious lesions requires a continuous and controlled release of fluoride in the oral environment, which is widely recognised. However, dental caries continues to be a significant global health problem. Therefore, there is a requirement for the development of a fluoride delivery system that can consistently release fluoride at therapeutic levels. Layered double hydroxides (LDHs) offer a promising solution to this challenge. LDHs have the capability to absorb and release fluoride repeatedly, thus achieving the sustained therapeutic release that will potentially enhance remineralisation.

Objective

Therefore, the aim of this study was to investigate the structure and fluoride absorption and release capabilities of two types of LDH powders, namely 2:1 MgAl and ZnAl, after being charged with two different sodium fluoride concentrations (1500 ppm and 5000 ppm) over two cycles, as well as to observe the fluoride release over 7 days without charging.

Materials and Methods

The MgAl and ZnAl LDH powders with a divalent to trivalent ion ratio 2:1 were successfully synthesised. These pristine LDH powders were characterised using powder X-ray diffraction (pXRD), Fourier-transform infrared spectroscopy (FTIR), and particle size analysis. Both LDH samples were charged with fluoride solutions of different concentrations, 1500 ppm and 5000 ppm for 24 hours. The fluoride absorption and release in deionised water from the LDHs over two cycles 5 (absorption, release, re-absorption, and re-release) and the 7 days were measured using fluoride-ion selective electrode (ISE). Furthermore, the 2:1 MgAl and ZnAl LDH powders after the charging and following the release process over two cycles and 7 days were characterised using various techniques, including solid state 19F nuclear magnetic resonance NMR, FTIR, and pXRD.

Results

The LDH of both formulations 2:1 MgAl and ZnAl was largely maintain stable after charging with 1500 and 5000ppm sodium fluoride solution. The 2:1 MgAl LDH showed overall higher absorption and release of fluoride under all conditions. The release from all the formulations continued even without recharging for 7 days of study. The second cycle of fluoride release showed higher concentration of fluoride concentration than was released in the first cycle of release. The FTIR spectra of the charged LDH formulations and the samples following the release showed higher intensity of carbonate in the structure compared to the as-synthesised LDH. Solid-state 19F NMR showed diversity of the fluorine environment in all types of LDH formulations, with the spectra of the 5000ppm charged samples exhibiting quite different spectra.

Conclusions

A novel finding emerged from the release results, demonstrating a significant fluoride release from the LDHs over the 7 days, without any additional re-charge, particularly with the 5000 ppm fluoride concentration. These findings hold promise and open avenues for further research in the field of dental materials.