Control of biomineralization dynamics by interfacial energies.
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Calcification, Physiologic↗
Biomedical subjects
Christine A Orme
Publications and source records attributed to Christine A Orme.
A new model for nanoscale enamel dissolution.
The dissolution kinetics of human tooth enamel surfaces was investigated using nanomolar-sensitive constant composition (CC) and in situ atomic force microscopy (AFM) under simulated caries formation conditions (relative undersaturation with respect to hydroxyapatite = 0.902, pH = 4.5). Scanning electron microscopic (SEM) examination of the resulting etched enamel surfaces showed that deminerzalization, initiated at core/wall interfaces of rods, developed anisotropically along the c-axes. After an initial rapid removal of surface polishing artifacts, the dissolution rate decreased as the reaction proceeded in accordance with our recently proposed crystal dissolution model, resulting in hollow enamel cores and nanosized remaining crystallites, resistant to further dissolution. Generally, dissolution of minerals is regarded as a spontaneous reaction in which all the solid phase can be dissolved in undersaturated solutions. However, the dissolution of some biominerals may be suppressed when the crystallites approach nanometer size. This study shows that CC demineralization of enamel in acidic medium follows this new model that can be used to mimic carious lesion formation. In dissolution studies, nanosized enamel crystallites exhibit a remarkable degree of self-preservation in the fluctuating physiological milieu.
Dental Caries↗
Dissolution of crystallites: surface energetic control and size effects.
Traditional understanding of dissolution assumes that the reaction is spontaneous and continues until equilibrium is reached. This paper presents theoretical and experimental data to support a dissolution mechanism that involves the existence of critical conditions for dissolution, in which the reaction is accompanied by the formation of pits and the subsequent displacement of pit steps. The accompanying increase in surface roughness leads to changes in surface energy with losses of crystal mass that are positive rather than negative and the existence of critical dissolution conditions. Critical pits and dissolution steps are verified experimentally and a relationship between the size and rate of displacement of steps is also demonstrated, in which the rate decreases with size and approaches zero at a critical size, r*. These microscopic step dynamics are consistent with the observed size-effects in bulk dissolution, which cannot be explained using traditional dissolution theories. The observed size effects include self-inhibition, in which the dissolution rate decreases with extent of reaction, dissolution suppression, and periodic resumption. These interesting dissolution phenomena are only readily displayed when the sizes of dissolving crystallites fall in the same range as the critical size (i.e., within 50r*). It is interesting to note that natural biominerals and many nanoparticles fall into this category, so that their suspensions can be dynamically stabilized without dissolution in undersaturated supporting media. The current research implies that dissolution kinetics cannot be understood well without appealing to fundamental physical concepts about the energetic control of dissolution steps on a molecular level. A new dissolution model for crystallites is introduced systemically.
Chemistry, Physical↗
Effect of hydrogen peroxide on titanium surfaces: in situ imaging and step-polarization impedance spectroscopy of commercially pure titanium and titanium, 6-aluminum, 4-vanadium.
To analyze titanium's response to representative surgical wound environments, a study was conducted on commercially pure titanium (CPTi) and titanium, 6-aluminum, 4-vanadium (Ti-6Al-4V) exposed to phosphate-buffered saline (PBS) with 30 mM of hydrogen peroxide (H(2)O(2)) added. The study was characterized by simultaneous electrochemical atomic force microscopy (EC AFM) and step-polarization impedance spectroscopy (SPIS). Surfaces were covered with protective oxide domes that indicated topography changes with potential and time of immersion. Less oxide dome coarsening was noted on surfaces treated with PBS containing H(2)O(2) than on surfaces exposed to pure PBS. Electrical data deduced from current transients collected while stepping voltage between 0 V and 1 V indicated that charge transfer in hydrogen peroxide solutions was an order of magnitude larger than it was in pure PBS. Oxide (early) resistances of CPTi samples were higher than were Ti-6Al-4V oxide resistances in both types of solutions, but CPTi oxide resistance was lower in the hydrogen peroxide solution compared to pure PBS. Capacitance data suggest that CPTi oxide films thicken in hydrogen peroxide solution more than they do in pure PBS. Differences in electrical properties between CPTi and Ti-6Al-4V surfaces suggest that CPTi, but not Ti-6Al-4V, has catalytic activity on H(2)O(2) and that the catalytic activity of CPTi oxide affects its ability to grow TiO(2). Differences in electrical properties are related to catalytic and oxidative mechanisms that take place directly on the titanium oxide surface and in wound environments. The study provides a foundation and theoretic basis for the porous oxide model on commercially pure titanium exposed to hydrogen peroxide.
Aluminum↗
Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy.
Intermolecular interactions drive the vast majority of condensed phase phenomena from molecular recognition to protein folding to particle adhesion. Complex energy barriers encountered in these interactions include contributions from van der Waals forces, hydrogen bonding, and solvent medium. With the spectacular exception of hydrophobic interactions, contributions from the medium are usually considered secondary. We report a variable temperature force microscopy study of the interactions between several hydrogen bonds in different solvents that challenges this point of view. Surprisingly, we observed an increase in the strength of the interaction between carboxylic acid groups in ethanol as the temperature increased. Moreover, when we switched to a nonpolar solvent we observed the opposite behavior: The binding force decreased as the temperature increased. Kinetic model of bond dissociation provided quantitative interpretation of our measurements. We attributed the observed phenomena to a large entropic contribution from the ordered solvent layers that are forming on the probe and sample surfaces upon detachment. The observed reversal in the force vs temperature trend is a manifestation of a transition between thermodynamic and kinetic regimes of unbinding predicted by the model. Our results indicate that entropic barriers dominated by the interactions of solvent molecules with the surface exist in a much wider variety of systems than previously thought.
Carboxylic Acids↗
In situ imaging and impedance measurements of titanium surfaces using AFM and SPIS.
Surfaces of commercially pure titanium and titanium, 6-aluminum, 4-vanadium were subjected to simultaneous polarization/impedance testing and in situ electrochemical atomic force microscopy imaging to evaluate how the structure and properties of the passive oxide film is affected by varying potential and hydration. Current transients were acquired via a step polarization impedance spectroscopy technique: the voltage was stepped between -1 and 1 V in 50 mV increments, while current transients and surface morphology were digitally recorded. Numerical Laplace transformation applied to the current transient data provided frequency-dependent admittance (impedance(-1)). Simultaneous AFM imaging of dry surfaces, initially hydrated surfaces, and surfaces immersed and changing with potential revealed that all sample surfaces were covered with protective titanium oxide domes that grew in area and coalesced due to hydration and as a function of increasing applied voltage and time. Reversal of dome growth did not occur upon voltage reduction, while impedance behavior was quasi-reversible, suggesting independence between structural and electrical properties. Oxide growth appeared to occur in part by lateral spreading and overgrowth of domes at the oxide-solution interface. Interfacial impedance data reflect oxide passivity and n-type semiconductor behavior. Non-linear Mott-Schottky fits specified multi-layer donor concentrations between 10(18) and 10(19)cm(-3), depending on the surface.
Alloys↗