Hidenobu Murata

Mechanism of incorporation of zinc into hydroxyapatite.

The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

Strontium Substitution in Bioactive Calcium Phosphates: A First-Principles Study

First-principles calculations are performed to investigate atomic and electronic structures of Sr(2+) ions substituting for Ca(2+) in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr(2+) and Ca(2+) determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the estimated equilibrium concentrations of Sr(2+) in OCP and HAp are in reasonable agreement with previous experimental results obtained in physiological conditions. It is also found that Sr(2+) ions can be more favorably substituted in OCP than in HAp. It is thought, therefore, that Sr(2+) plays its role to promote bone formation by being incorporated into the metastable OCP phase occurring during HAp nucleation.

Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics.

Silicon dioxide has eight stable crystalline phases at conditions of the Earths rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

First-principles calculations of Zn-K XANES in Ca-deficient hydroxyapatite.

The local environment of substitutional Zn(2+) in Ca-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses of the x-ray absorption near edge structure (XANES). For Zn-K XANES calculations, two situations of Zn(2+) were considered. One was Zn(2+) substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutional Zn(2+) associated with a Ca(2+) vacancy charge compensated by two protons. The model of Zn(2+) in perfect HAp did not reproduce the experimental Zn-K XANES spectrum. In contrast, the Ca-deficient HAp model agreed well with the experimental spectrum. This indicates that substitutional Zn(2+) in Ca-deficient HAp is associated with the Ca(2+) vacancy complex in HAp.

Tetravalent Dysprosium in a Perovskite‐Type Oxide

The existence of tetravalent dysprosium in perovskite-type oxide barium zirconate is confirmed in this work. This discovery will stimulate many researchers in diverse fields such as catalysts, solid state ionics, sensors, and fluorescent materials.

Pt nanoparticles supported on carbon nanowalls with different domain sizes for oxygen reduction reaction

Carbon nanowalls (CNWs) with different domain sizes were synthesized by a dc plasma-enhanced chemical vapor deposition. Platinum (Pt) loading on the CNWs (Pt/CNW) was carried out by a solution reduction method. As a result, Pt nanoparticles were preferentially deposited along the domain boundaries in CNWs as reported previously. It should be noted that the Pt particle size strongly depends on the domain size. Namely, the smaller the domain size is, the smaller the Pt particle size is. Moreover, the Pt/CNW with smaller Pt particles exhibit high catalytic activity. Especially, the mass activity related to cathodic oxygen reduction reaction (ORR) is high and reach about five times as much as that of commercial T-Pt/CB with good performance. The high ORR activity can be attributed to the domain boundaries in CNWs, which might lower not only the activation energy of O2 dissociation on Pt particles but also the energy barrier of the rate-limiting step in the ORR process.

Cytotoxicity of stoichiometric hydroxyapatites with different crystallite sizes

Abstract Hydroxyapatite (HAp) samples were synthesized by a solution reaction method followed by heat-treatments at three different temperatures. Special attention was given to optimizing the processing parameters to obtain the chemical composition near to the stoichiometry of HAp. No trace of secondary crystalline phase was found from powder X-ray diffraction in all samples. X-ray fluorescence measurements found that the Ca/P ratio was 1.68 ± 0.02, which is close to the stoichiometry of HAp, i.e., 1.67. Electron microscope observations revealed that the grain size was uniform within a sample, which was dependent on the heat treatment temperature. Dissolution rates in acid solution and cytotoxicity of the samples were measured. Tendency to decrease both the dissolution rates and the cytotoxicity with increasing crystallite size was observed. After heat-treatment at 1000 °C, the cytotoxicity of the sample was found to be minimal, which had uniaxial and faceted grains with a mean diameter of 200 nm.

Theoretical calculations of the thermodynamic stability of ionic substitutions in hydroxyapatite under an aqueous solution environment.

Defect formation energies in materials generally depend on chemical potentials determined by a chemical equilibrium condition. In particular, an aqueous solution environment is important for biomaterials such as hydroxyapatite studied here. Therefore, a methodology to obtain ionic chemical potentials under chemical equilibrium between solid and aqueous solution was introduced, and was applied to substitutional divalent cations formed via ion exchange with Ca(2+) in hydroxyapatite. The calculated ranking of the stability of substitutional cations in HAp was in good agreement with the experimentally observed trend. The present theoretical approach would be useful to explore the thermodynamic stability of defects in materials subjected to an aqueous solution environment.

Local environment of silicon in cubic boron nitride

Si-doped cubic boron nitride (c-BN) is synthesized at high pressure and high temperature, and the local environment of Si is investigated using X-ray absorption near edge structure (XANES) and first-principles calculations. Si-K XANES indicates that Si in c-BN is surrounded by four nitrogen atoms. According to first-principles calculations, the model for substitutional Si at the B site well reproduces experimental Si-K XANES, and it is energetically more favorable than substitutional Si at the N site. Both the present experimental and theoretical results indicate that Si in c-BN prefers the B site to the N site.

Controlled release of DNA from zinc and magnesium ion-doped hydroxyapatites

Hydroxyapatite (HA) is a biocompatible and porous material that is useful for gene delivery. In this study, various Zn- and Mg-doped HA samples were produced by adding the ions at different dopant concentrations to control the association and dissociation of DNA. Although the HA crystal structure was retained after doping, it became distorted with increasing doped metal concentration. In 20% Zn-doped HA, a secondary Zn(OH)2 phase was observed. Mg-doped HA did not show significant cytotoxicity against breast tumor cells and osteoblasts. But, highly Zn-doped HAs were toxic, suggesting that doped Zn ions affected the cells. DNA could effectively adsorb to HA, regardless of metal ion doping, but the dissociation behavior of DNA differed. DNA was gradually released from Mg-doped HA for over a week, independent of Mg dope concentration, but not from Zn-doped HA, except for 20% doping. Therefore, the release properties of HA can be controlled by doping with suitable metal ion species at an appropriate dosage.