Proton assisted dissolution of the dental hard tissue enamel as a non-bacterial process

The overall aim of this thesis was to examine the kinetics of proton-promoted dissolution of the dental hard tissue enamel as a non-bacterial process and the evaluation of inhibitors with the intent of minimising the dissolution process and effectively protecting the surface. A novel approach was taken, utilising scanning electrochemical microscopy (SECM) to galvanostatically generate controllable and well defined proton fluxes in defined areas of the surface. The resulting etch pits formed on the surface were characterised by optical microscopy and white light interferometry (WLI), which quantitatively determined etch pit dimensions. A theoretical finite element model (FEM) was used to elucidate the kinetics of dissolution based upon the analysis of the shape and dimensions of etch pits produced. A heterogeneous rate constant of dissolution of 0.08 ± 0.04 cm s-1 was attributed to untreated enamel, whereas 2 min treatment with 1000 ppm sodium fluoride (NaF) decreased this rate constant slightly to 0.05 ± 0.03 cm s-1. The impact of fluoride on the rate of proton attack was evident from the formation of shallower broader etch pits. In relation to both acid erosion and caries, the two most relevant acids pertinent to enamel dissolution are citric acid and lactic acid. These acids were investigated by protonating their respective sodium salts in-situ to produce localised weak acid directly under the probe tip. This permitted the surrounding enamel sample to remain largely unaltered giving a true surface for comparison, whilst allowing evaluation of the kinetics in the presence of each weak acid. Etching in the presence of lactic acid, showed a surface controlled process with a rate constant of 0.1 ± 0.03 cm s-1. Etching in the presence of the triprotic citric acid, also yielded a surface controlled process with a rate constant of 0.35 ± 2.6 cm s-1. Calcite was also investigated using SECM, WLI and FEM to validate the use of these techniques. The kinetic data extrapolated was comparable to rate constants found in literature, confirming the validity of these methods. In this case, a novel approach was the use of experimental data to parameterise the finite element model directly. Confocal laser scanning microscopy (CLSM) coupled with SECM was used to visualise proton fluxes from the tip of the UME. This allowed, not only, correlation of the current applied to the probe tip with the pH, but also quantitative data on the spread of protons across a particular surface. Rate constants found for untreated and fluoride-treated enamel were comparable to those found in SECM etching, however, zinc ion treatment proved to result in much greater inhibition of dissolution than fluoride

Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101̅4) cleavage surface (from natural “Iceland Spar”) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved “mirror” surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 μm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) × 10–4 m s–1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites

Scanning electrochemical microscopy as a quantitative probe of acid-induced dissolution: theory and application to dental enamel

This Article reports the use of scanning electrochemical microscopy (SECM) for the quantitative study of acid-induced dissolution. An ultramicroelectrode (UME) is used to generate a flux of protons galvanostatically just above a sample surface, creating controlled acid challenges relevant to acid erosion. The electrochemical technique produces etch features in the sample, which are characterized by white light interferometry (WLI). The technique has been applied to bovine enamel where understanding the kinetics of dissolution is important in the context of acid erosion. Dissolution has been observed as a fast process, but the high rates of mass transport in SECM allow the surface kinetics of dissolution to be evaluated. Key attributes of SECM for these studies are the ability to deliver high, controllable, and local acid challenges in a defined way and that multiple dissolution measurements can be performed on one sample, eliminating intersample variability effects. A novel moving boundary finite element model has been designed to describe the etching process, which allows the etch kinetics to be evaluated quantitatively, simply by measuring the size and shape of etch features over time