Synthesis of Fluorinated Urchin-like Serried Hydroxyapatite with Improved Water Sorption-Solubility and Bioactivity for Dental Composites

Fluorine is present in the hydroxyapatite mineral in natural tooth enamel, which plays a key role in the prevention of dental caries. The aim of this study is to synthetize the fluorinated urchin-like serried hydroxyapatite(FnUHA) particles with different degrees of fluorine substitution and explore the effect of the fluorine element on the water absorption-solubility, mechanical strength, and biological activity of dental composites. The obtained FnUHA particles were further modified with 3-methacryloxypropyl trimethoxysilane(γ-MPS) to get the silanized FnUHA(SFnUHA) particles, which were then used as the reinforcement for dental composites. The morphology, compositional elements, and structure of the FnUHA were characterized by field-emission scanning electron microscopy(FE-SEM), transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS), X-ray diffractometer(XRD), and Fourier transform infrared spectrometer (FTIR), respectively. The mechanical properties of the SFnUHA reinforced composites with different filler loadings were measured with a universal testing machine. The results demonstrated that the 50%(mass fraction) SF5UHA filled composite exhibited the acceptable flexural strength and compressive strength, giving the respective improvements of 56.3% and 30.8% compared with those of the 50% SUHA filled composite. In addition, this composite also presented lower water absorption-solubility, better in vitro bioactivity, and excellent cell viability. Therefore, fluorinated hydroxyapatite is a promising filler to improve the mechanical properties and functionality of dental composites.     

A comparative study of the metastable equilibrium solubility behavior of high-crystallinity and low-crystallinity carbonated apatites using pH and solution strontium as independent variables

Using solution strontium and pH as independent variables, the metastable equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 degrees C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 degrees C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca(10-n)Sr(n)(PO4)6(OH)2 (n = 0-10)) and the calcium/hydroxide deficient hydroxyapatite template (Ca(9-n)Sr(n)(HPO4)(PO4)5OH (n = 0-9)). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.     

Self-assembly of synthetic hydroxyapatite nanorods into an enamel prism-like structure

The application of surfactants as reverse micelles or microemulsions for the synthesis and self-assembly of nanoscale structures is one of the most widely adopted methods in nanotechnology. These synthesized nanostructure assemblies sometimes have an ordered arrangement. The aim of this research was to take advantage of these latest developments in the area of nanotechnology to mimic the natural biomineralization process to create the hardest tissue in the human body, dental enamel. This is the outermost layer of the teeth and consists of enamel prisms, highly organized micro-architectural units of nanorod-like calcium hydroxyapatite (HA) crystals arranged roughly parallel to each other. In particular, we have synthesized and modified the hydroxyapatite nanorods surface with monolayers of surfactants to create specific surface characteristics which will allow the nanorods to self-assemble into an enamel prism-like structure at a water/air interface. The size of the synthetic hydroxyapatite nanorods can be controlled and we have synthesized nanorods similar in size to both human and rat enamel. The prepared nanorod assemblies were examined using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The specific Langmuir-Blodgett films were shown to be comprised of enamel prism-like nanorod assemblies with a Ca/P ratio between 1.6 and 1.7.     

The adsorption of fluoride containing polyelectrolytes on hydroxyapatite

In this study the preparation of three polyvinylpyridinium fluorides with different hexadecyl group contents is described. Their adsorption on hydroxyapatite (the main constituent of dental enamel) was studied as a function of time and concentration. It was found that increasing hexadecyl group content resulted in lower amounts of adsorbed polymer. The adsorption of the polyelectrolytes is accompanied with the uptake of fluoride by hydroxyapatite at a higher rate and with greater amounts. Phosphate ions present in aqueous suspensions of hydroxyapatite were responsible for this phenomenon. Desorption experiments showed that the adsorption of these macromolecules was irreversible in water. Excess calcium resulted to some extent in desorption.     

Adsorption of citric acid from dilute aqueous solutions by hydroxyapatite

The role of citric acid in the demineralization of dental enamel, which is mainly constituted by hydroxyapatite, is important for periodontal regeneration and in the conditioning of enamel or dentin for bonding restorative resins. The adsorption of citric acid from aqueous solutions onto synthetic hydroxyapatite at 278, 288, 298, and 308 K and pH 4.8 has been studied by means of UV spectroscopy. The adsorption reaction, which takes place by an interaction between phosphate groups and citrate anions at the solid-solution interface, yields an adsorbate-adsorbent complex of high stability. The adsorption isotherms fit the Langmuirian shape. The proposed adsorption model, where citrate species interact in a bidentate manner (one citrate ion links two phosphate sites), is coherent with the experimental data. The activation standard heat and activation standard entropy were calculated. All the thermodynamic and kinetic parameters were in concordance with the adsorption reaction proposed in this work.     

Linking microstructure and nanochemistry in human dental tissues

Mineralized dental tissues and dental pulp were characterized using advanced analytical transmission electron microscopy (TEM) methods. Quantitative X-ray energy dispersive spectroscopy was employed to determine the Ca/P and Mg/P concentration ratios. Significantly lower Ca/P concentration ratios were measured in peritubular dentine compared to intertubular dentine, which is accompanied by higher and variable Mg/P concentration ratios. There is strong evidence that magnesium is partially substituting calcium in the hydroxyapatite structure. Electron energy-loss near-edge structures (ELNES) of C-K and O-K from enamel and dentine are noticeably different. We observe a strong influence of beam damage on mineralized dental tissues and dental pulp, causing changes of the composition and consequently also differences in the ELNES. In this article, the importance of TEM sample preparation and specimen damage through electron irradiation is demonstrated.     

Microwave synthesis of hydroxyapatite and physicochemical study of its properties

Method for microwave liquid-phase synthesis of hydroxyapatite samples was developed. The composition of the synthesis products obtained and natural hydroxyapatite was studied by X-ray phase and X-ray fluorescence microanalysis. IR spectra were measured and the dispersity of hydroxyapatite powders obtained, their solubility in water at 20°C, and structural parameters were determined in comparison with natural hydroxyapatite and that produced by the common (“classical”) liquid-phase technique. The microwave-synthesized hydroxyapatite compares well in its properties with the biological and “classical” hydroxyapatites.     

Effects of Cerium and Combination of Fluoride and Cerium on the Hydrolysis of Hydroxyapatite and Calcium Hydrogen Phosphate

Researches have indicated that fluoride aided in decreasing the solubility of enamel of teeth and preventing caries through converting hydroxyapatite (HAP) to less dissoluble fluoroapatite (FAP). However, The toxicity of fluoride is not negligible. Recent studies^[1] suggested that rare-earth elements (for instance,lanthanum, cerium, etc.) might play an important role in enamel demineralization reduction. However, the effect of these rare-earth elements on the solubility of HAP and calcium hydrogen phosphate (CaHPO₄·2H₂O, DCPD),which are the major inorganic compositions in enamel, remains unclear. The purpose of this paper was to investigate the characteristics of the hydrolysis of DCPD and HAP under 37℃ when hydrolyzed (a) in solutions containing cerium alone, and (b) in solutions containing fluoride followed by solutions containing cerium, respectively.     

Multiple solubility maxima of oxolinic acid in mixed solvents and a new extension of Hildebrand solubility approach

A new extension of the Hildebrand solubility approach which describes drug solubility in solvent mixtures showing multiple solubility peaks, the chameleonic effect, is proposed. The experimental solubilities of oxolinic acid were measured at 25 degrees C in solvent mixtures of ethanol-water and ethanol-ethyl acetate. A plot of the mole fraction of the drug against the solubility parameter (delta) of the solvent mixtures displays two peaks at delta = 30.78 MPa1/2 (80% v/v of ethanol in water) and at delta = 20.90 MPa1/2 (30% v/v of ethanol in ethyl acetate). The new extension proposed reproduces two solubility peaks. The thermograms of the solid phase before and after equilibration with the solvent mixtures did not show significant changes. The new extension was also tested with experimental data previously reported for drugs showing two solubility peaks of different height. The accuracy of other published models for describing two solubility maxima is also compared.     

Solubility of Iodine in the System Water-Nitric Acid

The solubility of iodine in 30-55% nitric acid at 25°C was measured. Particular attention was given to the control of equilibration in the system. An equation was suggested for estimating the iodine solubility in nitric acid in the concentration range 0-70 wt % HNO₃.     

Proton microprobe determination of fluorine depth distributions and surface multielement characterization in dental enamel

Particle induced X-ray emission analysis (PIXE) and nuclear reaction analysis have been applied to multielemental analysis of human dental enamel in a proton microprobe. The PIXE technique was excercised for the characterization of multielemental lateral distributions on and just below the sample surface. For the assessment of depth distributions of fluorine the resonant nuclear reaction¹⁹F(p,aλ)¹⁶O was used. The combination of two analytical techniques comprising both atomic and nuclear interactions in a microprobe was applied to enamel samples of different origins. One sample constituted a healthy enamel and the other one enamel from a restored tooth. The microprobe in the combined mode was demonstrated to permit the establishment of lateral, concentration gradients of elements heavier than phosphorus with a resolution of 15 μm in enamel and simultaneously of depth distributions of fluorine better than 0.7 μm. The detection limits approached, 10–25 ppm for most of the elements considered. Supported by the Swedish Natural Science Research Council     

A MO-SCF study of hydroxyl binding and hydroxyl diffusion in pure and fluorine-containing hydroxyapatite

Hydroxyl diffusion and interhydroxyl binding in hydroxyapatite has been studied. LCGO -MO -SCF calculations have been carried out on groups of hydroxyl ions in a perfect and a vacancy-containing crystal. Electrostatic crystal effects were accounted for by including the crystal potential field in the SCF calculations. Nearest-neighbour interactions were calculated to first order. Effects exerted by fluorine impurities were considered for the vacancy-containing crystal. The calculations indicate that narrow bandwidths obtained in nmr experiments on biological hydroxyapatite may be caused by a hydroxyl displacement mechanism and that no interhydroxyl hydrogen bonds exist in hydroxyapatite. The suggestion that the caries-inhibiting properties of fluorine impurities in human dental enamel is caused by a diffusion-binding mechanism is supported by the results.     

Interactions of hydroxyapatite surfaces: conditioning films of human whole saliva

Hydroxyapatite is a very interesting material given that it is the main component in tooth enamel and because of its uses in bone implant applications. Therefore, not only the characterization of its surface is of high relevance but also designing reliable methods to study the interfacial properties of films adsorbed onto it. In this paper we apply the colloidal probe atomic force microscopy method to investigate the surface properties of commercially available hydroxyapatite surfaces (both microscopic particles and macroscopic discs) in terms of interfacial and frictional forces. In this way, we find that hydroxyapatite surfaces at physiological relevant conditions are slightly negatively charged. The surfaces were then exposed to human whole saliva, and the surface properties were re-evaluated. A thick film was formed that was very resistant to mechanical stress. The frictional measurements demonstrated that the film was indeed highly lubricating, supporting the argument that this system may prove to be a relevant model for evaluating dental and implant systems.     

Interaction of DNA with hydroxyapatite. Studies on the effect of the phosphate concentration of the column equilibration and washing buffer

The ability of hydroxyapatite to bind DNA effectively in phosphate solutions used for column equilibration, sample loading and column washing has been examined. It was demonstrated that substantial amounts of DNA (up to 40%) were eluted in the washing buffer when the phosphate concentration in the lysing solution or urea-phosphate used for column equilibration, sample loading and column washing was 0.24 M. A reduction in the phosphate concentration from 0.24 to 0.15 M in urea-phosphate solution led to almost 100% binding, whereas a similar reduction in the lysing solution did not. A modified method for loading and eluting DNA from hydroxyapatite columns is presented.     

Raman spectroscopic studies of CO2 laser-irradiated human dental enamel.

While the effects of carbon dioxide (CO2) laser radiation on the physical properties of human dental enamel are well characterized, little is known regarding laser-induced chemical changes. In this study, enamel was exposed to CO2 laser radiation to induce fusion and recrystallization, and the Raman spectra recorded using both dispersive and Fourier-transformed (FT) Raman spectroscopy. Spectra were compared to a heart-treated specimen of hydroxyapatite (HAP) and enamel. Laser irradiation induced chemical changes which differed from those induced by heat treatment. Comparing the Raman spectra of lased enamel to HAP and tricalcium phosphate (TCP), it is evident that CO2 laser irradiation of enamel causes the partial conversion of HAP to TCP. The effect of laser irradiation is not merely a simple local heating effect as previously thought, since simple heating of enamel leads to the formation of both TCP and Ca(OH)2, while laser treatment of enamel results in the formation of TCP but not Ca(OH)2.     

Synchrotron X-ray diffraction characterization of healthy and fluorotic human dental enamel

With the introduction of fluoride as the main anticaries agent used in preventive dentistry, and perhaps an increase in fluoride in our food chain, dental fluorosis has become an increasing world-wide problem. Visible signs of fluorosis begin to become obvious on the enamel surface as opacities, implying some porosity in the tissue. The mechanisms that conduct the formation of fluorotic enamel are unknown, but should involve modifications in the basic physical-chemistry reactions of demineralization and remineralisation of the enamel of the teeth, which is the same reaction of formation of the enamel's hydroxyapatite (HAp) in the maturation phase. The increase of the amount of fluoride inside of the apatite will result in gradual increase of the lattice parameters. The aim of this work is to characterize the healthy and fluorotic enamel in human tooth using Synchrotron X-ray diffraction. All the scattering profile measurements were carried out at the X-ray diffraction beamline (XRD1) at the Brazilian Synchrotron Light Laboratory—LNLS, Campinas, Brazil. X-ray diffraction experiments were performed both in powder samples and polished surfaces. The powder samples were analyzed to obtain the characterization of a typical healthy enamel pattern. The polished surfaces were analyzed in specific areas that have been identified as fluorotic ones. X-ray diffraction data were obtained for all samples and these data were compared with the control samples and also with the literature data.     

Temperature dependence of oxygen isotope acid fractionation for modern and fossil tooth enamels

The oxygen isotope ratio of CO(2) liberated from structural carbonate in tooth enamel apatite was measured at phosphoric acid reaction temperatures of 25 degrees C, 60 degrees C and 90 degrees C, and it was found that apparent acid fractionation factors for pristine enamel, fossilized enamel, and calcite follow different temperature relationships. Using sealed vessel reactions normalized to alpha(25) = 1.01025 (the fractionation factor for calcite at 25 degrees C), the apparent fractionation factor at 90 degrees C (alpha*(90)) for pristine enamel ranged between 1.00771 and 1.00820, and between 1.00695 and 1.00772 for fossilized enamel. Apparent fractionation factors for common acid bath reactions are similar to those for sealed vessel reactions. A significant correlation exists between alpha*(90) and F(-) content, suggesting that change in the acid fractionation factor may be related to the replacement of OH(-) with F(-) during fossilization of bioapatite. These results have important implications for making accurate comparisons between modern and fossil tooth enamel delta(18)O values, and for the uniformity of isotope data produced in different laboratories using different acid reaction temperatures.     

Determination of Zinc Sulfide Solubility to High Temperatures

A new experimental set-up and methodology for the measurement of ZnS solubility in aqueous solutions at 40, 60 and 80 °C (atmospheric pressure) is presented. The methodology implemented includes the preparation of the samples in a reduced oxygen atmosphere, particle size analysis of ZnS, quality control of the analytical technique and evaluation of equilibration time. ZnS solubility analyses were run for prolonged times (up to 11 days) to ensure that equilibrium conditions were met. The equilibration time was explored at three temperatures (40, 60 and 80 °C) observing small variations in the time required to reach the solid–liquid equilibrium at each temperature. Equilibrium was reached within 72 h. The concentration of zinc and of total sulfur were determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The experimental solubility data show an exponential dependency of the solubility with respect to temperature. An increase of 40 °C results in an increase of roughly 12 times for the solubility of ZnS.     

EPR spectroscopy of tooth enamel: the tooth radicals and the microcrystal alignment

The paramagnetic radicals induced by radiation in dental enamel are very important because they can be related to the crystalline structure of hydroxyapatite. The R-value, that is, the ratio of the amplitude of the lines of the EPR signal due to radiation, is a measure of the degree of microcrystal alignment in human tooth enamel. The aim of this study is to underline the importance of a correct evaluation of the R-value, by using the current method to increase reproducibility in EPR spectroscopy of tooth enamel. Data with and without correction show a significant difference and, consequently, they give rise to a different valuation of microcrystal alignment.     

Ideal gas solubilities and solubility selectivities in a binary mixture of room-temperature ionic liquids

This study focuses on the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazolium-based room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) at 40 degrees C and low pressures (approximately 1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % [C2mim][BF4] in [C2mim][Tf2N]. Results show that regular solution theory (RST) can be used to describe the gas solubility and selectivity behaviors in RTIL mixtures using an average mixture solubility parameter or an average measured mixture molar volume. Interestingly, the solubility selectivity, defined as the ratio of gas mole fractions in the RTIL mixture, of CO2 with N2 or CH4 in pure [C2mim][BF4] can be enhanced by adding 5 mol % [C2mim][Tf2N].