Bone cell–material interactions on metal-ion doped polarized hydroxyapatite

The objective of this work is to study the influence of Mg²⁺ and Sr²⁺ dopants on in vitro bone cell–material interactions of electrically polarized hydroxyapatite [HAp, Ca₁₀(PO₄)₆(OH)₂] ceramics with an aim to achieve additional advantage of matching bone chemistry along with the original benefits of electrical polarization treatment relevant to biomedical applications. To achieve our research objective, commercial phase pure HAp has been doped with MgO, and SrO in single, and binary compositions. All samples have been sintered at 1200 °C for 2 h and subsequently polarized using an external d.c. field (2.0 kV/cm) at 400 °C for 1 h. Combined addition of 1 wt.% MgO/1 wt.% SrO in HAp has been most beneficial in enhancing the polarizability in which stored charge was 4.19 μC/cm² compared to pure HAp of 2.23 μC/cm². Bone cell–material interaction has been studied by culturing with human fetal osteoblast cells (hFOB) for a maximum of 7 days. Scanning electron microscope (SEM) images of cell morphology reveal that favorable surface properties and dopant chemistry lead to good cellular adherence and spreading on negatively charged surfaces of both Sr²⁺ and Mg²⁺ doped HAp samples over undoped HAp. MTT assay results at 7 days show the highest viable cell densities on the negatively charged surfaces of binary doped HAp samples, while positive charged doped HAp surfaces exhibit limited cellular growth in comparison to neutral surfaces.     

Abnormal blue-shift of Eu2+-activated calcium zirconium phosphates CaZr4(PO4)6

The electronic structure of CaZr₄(PO₄)₆ was calculated using the CASTEP code and the band gap for CaZr₄(PO₄)₆ can reach up to 4.30 eV. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) samples were prepared by a high temperature solid-state reaction method. XRD analysis shows that Eu²⁺ ion can be totally incorporated into CaZr₄(PO₄)₆ forming complete solid solutions with trigonal lattice. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) shows typical broad band emission in wavelength range from 400 to 650 nm for both under ultraviolet (UV) light and X-ray excitation, originating from the 4f⁶5d¹ → 4f⁷5d⁰ transition of Eu²⁺ ions. With increasing Eu²⁺ concentration, there is abnormal blue-shift of the emission peaks for Ca_1−xEu_xZr₄(PO₄)₆ due to the decreasing crystal field strength and Stokes shift. With increasing temperature in CaZr₄(PO₄)₆: Eu²⁺, its emission bands show the anomalous blue-shift with decreasing intensity. The overall scintillation efficiency of Ca_0.9Eu_0.1Zr₄(PO₄)₆ is 1.7 times of that of Bi₄Ge₃O₁₂ (BGO) powder under the same conditions. In addition, its predominant decay time is about 50 ns at room temperature. The potential application of Eu²⁺-doped CaZr₄(PO₄)₆ has been pointed out.     

In vitro biocompatibility and antimicrobial activity of wet chemically prepared Ca10−xAgx(PO4)6(OH)2 (0.0 ≤ x ≤ 0.5) hydroxyapatites

Herein, we report the effect of silver ions on the physical, antimicrobial and cytocompatibility properties of wet chemically synthesized silver doped Ca_10?xAg_x(PO₄)₆(OH)₂ (0.0 ≤ x ≤ 0.5) hydroxyapatites (HAp). Silver ions containing HAp exhibit the comparable density, hardness and enhanced antimicrobial properties, in comparison to parent HAp. The optical absorption measurements confirm the presence of silver ions in the doped compositions, which are responsible for as increased antimicrobial property of doped HAp materials for x > 0.3. The cytotoxicity behavior of the doped HAp was evaluated using mouse fibroblast (L929) cell line. The important result has been that doped HAp (x > 0.3) exhibit statistically (significant) lower cell viability in comparison to undoped HAp. However, no difference in cellular functionality on doped HAp surfaces, in terms of cell adhesion and proliferation could be qualitatively observed in reference to undoped HAp. In order to explain the observed antimicrobial and cell viability properties, the in vitro release of Ag⁺ ions has been quantified using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and solubility was measured by weight loss in acetate buffer solution.     

Multicolor-emitting Ca3 -x-ySry(PO4)2:xEu2 + (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors for light-emitting diodes

In this paper, a series of Ca_3 -x-ySr_y(PO₄)₂:xEu^2 +, (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors were prepared by flux assisted solid-state reaction method, and their photoluminescence properties were investigated. The β- to β′-phase transition of Ca_3 -ySr_y(PO₄)₂ for high Sr^2 + content was observed from the XRD patterns, and the corresponding optical bandgaps were obtained experimentally. Various Eu^2 + emission centers were found, which generate tunable emission depending on the Sr^2 + concentration. Broad and intense excitation bands exist in Eu^2 + activated Ca₃(PO₄)₂, and the introduction of Sr^2 + further extends and enhances the excitation bands beyond 350 nm, which is beneficial to the applications on near ultraviolet LEDs. The morphology measurement reveals that the average size of particles with smooth surface is about 11.2 μm, which is suitable for the practical applications. These results indicate that the Ca_3 -x-ySr_y(PO₄)₂:xEu^2 + phosphors could be promising candidates for LEDs.     

Biomimetic synthesis of hierarchical crystalline hydroxyapatite fibers in large-scale

Crystalline hierarchical hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, HAp)] fibers were successfully synthesized via a biomimetic route by using cotton cloth as a natural bio-template. The effects of pH value, aging time, ultrasonic cleaning time, and calcination temperature on the purity and morphology of the resulting hydroxyapatite (HAp) were monitored by scanning election microscope (SEM), X-ray diffraction (XRD), and infrared spectrophotometer (IR) to obtain an optimized reaction condition, namely, pH 9, ultrasonic cleaning for 1 min, aging for 24 h, and calcination at 600 °C for 4 h. We found that the natural cellulose could not only control the morphology of HAp but also lower its phase transformation temperature. The impact of this method lies in its low cost and successful production of large-scale patterning of three-dimensional hierarchical HAp fibers.     

Synthesis and photoluminescence properties of a cyan-emitting phosphor Ca3(PO4)2:Eu2+ for white light-emitting diodes

In this paper, a cyan-emitting phosphor Ca₃(PO₄)₂:Eu²⁺ (TCP:Eu²⁺) was synthesized and evaluated as a candidate for white light emitting diodes (WLEDs). This phosphor shows strong and broad absorption in 250–450 nm region, but the emission spectrum is prominent at around 480 nm. The emission intensity of the TCP:Eu²⁺ was found to be 60% and 82% of that of the commercial BaMgAl₁₀O₁₇:Eu²⁺ (BAM) under excitation at 340 nm and 370 nm, respectively. Upon excitation at 370 nm, the absolute internal and external quantum efficiencies of the Ca₃(PO₄)₂:1.5%Eu²⁺ are 60% and 42%, respectively. Moreover, a white LED lamp was fabricated by coating TCP:Eu²⁺ with a blue-emitting BAM and a red-emitting CaAlSiN₃:Eu²⁺ on a near-ultraviolet (375 nm) LED chip, driven by a 350 mA forward bias current, and it produces an intense white light with a color rendering index of 75.     

Chemical synthesis and stabilization of magnesium substituted brushite

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is the most ubiquitous calcium phosphate phase used in implant coatings and more recently in gene/drug delivery applications due to its chemical stability under normal physiological conditions (37 °C, pH ～ 7.5, 1 atm.). However, different calcium phosphate phases, such as brushite (CaH(PO₄)?2(H₂O)) and tricalcium phosphate (Ca₃(PO₄)₂) which are thermodynamically unstable under physiological conditions are also being explored for biomedical applications. One way of stabilizing these phases under physiological conditions is to introduce magnesium to substitute for calcium in the brushite lattice. The role of magnesium as a stabilizing agent for synthesizing brushite under physiological conditions at room temperature has been studied. Chemical analysis, Fourier transform infrared spectroscopy and X-ray diffraction have also been conducted to validate the formation of magnesium substituted brushite under physiological conditions.     

Luminescence properties of Eu2+-activated Sr5(PO4)2(SiO4) for green-emitting phosphor

A green-emitting phosphor of Eu²⁺-activated Sr₅(PO₄)₂(SiO₄) was synthesized by the conventional solid-state reaction. It was characterized by photoluminescence excitation and emission spectra, and lifetimes. In Sr₅(PO₄)₂(SiO₄):Eu²⁺, there are at least two distinguishable Eu²⁺ sites, which result in one broad emission situating at about 495 nm and 560 nm. The phosphor can be efficiently excited in the wavelength range of 250–440 nm where the near UV (～ 395 nm) Ga(In)N LED is well matched. The dependence of luminescence intensities on temperature was investigated. With the increasing of temperature, the luminescence of the phosphor shows good thermal stability and stable color chromaticity. The luminescence characteristics indicate that this phosphor has a potential application as a white light emitting diode phosphor.     

Accelerated transformation of brushite to octacalcium phosphate in new biomineralization media between 36.5 °C and 80 °C

This study investigated the hydrothermal transformation of brushite (dicalcium phosphate dihydrate, DCPD, CaHPO₄·2H₂O) into octacalcium phosphate (OCP, Ca₈(HPO₄)₂(PO₄)₄·5H₂O) in seven different newly developed biomineralization media, all inspired from the commercial DMEM solutions, over the temperature range of 36.5 °C to 90 °C with aging times varying between 1 h and 6 days. DCPD powders used in this study were synthesized in our laboratory by using a wet-chemical technique. DCPD was found to transform into OCP in the Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^?, Cl^? and H₂PO₄^? containing aqueous biomineralization media in less than 72 h at 36.5 °C, without stirring. The same medium was able to convert DCPD into OCP in about 2 h at 75–80 °C, again without a need for stirring. Samples were characterized by using powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM).     

Synthesis and luminescence properties of Ca8NaGd(PO4)6F2: Eu2+, Mn2+ for white LEDs

A series of luminescent emission-tunable phosphors Ca₈NaGd(PO₄)₆F₂: Eu²⁺, Mn²⁺ have been prepared by a combustion-assisted synthesis method. The X-ray diffraction measurement results indicate that the crystal structure of the phosphor is a single phase of Ca₈NaGd(PO₄)₆F₂. The photoluminescence (PL) properties of Eu²⁺ and Mn²⁺-codoped Ca₈NaGd(PO₄)₆F₂ phosphors were also investigated. The phosphors can be efficiently excited by ultraviolet (UV) light and show a blue emission band at about 450 nm and a yellow emission band at about 574 nm, which originated from the Eu²⁺ ions and the Mn²⁺ ions, respectively. The efficient energy transfer from the Eu²⁺ ions to the Mn²⁺ ions was observed and its mechanism should be a resonant type via a nonradiative dipole–quadrupole interaction. A color-tunable emission in Ca₈NaGd(PO₄)₆F₂ phosphors can be realized by Eu²⁺ → Mn²⁺ energy transfer. Our results indicate that the developed phosphor may be used as a potential white emitting phosphor for UV based white LEDs.     

Photocatalytic decolorization of methylene blue dye by Ag3PO4–AgX (X = Cl−, Br− and I−) under visible light

Three kinds of Ag₃PO₄–AgX (X = Cl⁻, Br⁻ and I⁻) were prepared by the co-precipitation method between AgNO₃ and Na₂HPO₄ + KX (X = Cl⁻, Br⁻ and I⁻) solutions. Their photocatalytic activities were also investigated by decolorization of a methylene blue solution. Under visible light irradiation, photocatalytic decolorization of the dye using Ag₃PO₄–AgX as photocatalyst could occur by interaction between a hole in the valence band of the photocatalyst and the methylene blue dye molecules as it cannot detect the emission of 7-hydroxylcoumarin. AgCl and AgBr induce the Ag₃PO₄ to increase its photocatalytic decolorization efficiency. On the other hand, the compound Ag₃PO₄–AgI also showed a high adsorption for methylene blue in the dark.     

Bioactivity and mineralization of hydroxyapatite with bioglass as sintering aid and bioceramics with Na3Ca6(PO4)5 and Ca5(PO4)2SiO4 in a silicate matrix

Hydroxyapatite and Bioglass^®-45S5 were sintered together creating new ceramic compositions that yielded increased apatite deposition and osteoblast differentiation and proliferation in vitro compared to hydroxyapatite. The sintered products characterized by X-ray diffraction, revealed hydroxyapatite as the main phase when small quantities (1, 2.5 and 5 wt.%) of bioglass was added. Bioglass behaved as a sintering aid with β-TCP (Ca₃(PO₄)₂) being the minor phase. The amount of β-TCP increased with the amount of bioglass added. In compositions with larger additions of bioglass (10 and 25 wt.%), new phases with compositions of calcium phosphate silicate (Ca₅(PO₄)₂SiO₄) and sodium calcium phosphate (Na₃Ca₆(PO₄)₅) were formed respectively within amorphous silicate matrices. In vitro cell culture studies of the ceramic compositions were examined using bone marrow stromal cell (BMSC). Cell proliferation and differentiation of bone marrow stromal cells into osteoblasts were determined by Pico Green DNA assays and alkaline phosphatase (ALP) activity, respectively. All hydroxyapatite–bioglass co-sintered ceramics exhibited larger cell proliferation compared to pure hydroxyapatite samples. After 6 days in cell culture, the ceramic with Ca₅(PO₄)₃SiO₄ in a silicate matrix formed by reacting hydroxyapatite with 10 wt.% bioglass exhibited the maximum proliferation of the BMSC's. The ALP activity was found to be largest in the ceramic with Na₃Ca₆(PO₄)₅ embedded in a silicate matrix synthesized by reacting hydroxyapatite with 25 wt.% bioglass.     

Synthesis and tunable luminescence of RbCaGd(PO4)2:Ce3+, Mn2+ phosphors

RbCaGd(PO₄)₂ doped with Ce³⁺, Mn²⁺ was synthesized by the sol-gel method. The crystal structure and crystallographic location of Ce³⁺ in RbCaGd(PO₄)₂ were identified by Rietveld refinement. Powder X-ray diffraction (XRD) revealed that the structure of RbCaGd(PO₄)₂:Ce³⁺ compounds is hexagonal structure which is similar to that of hexagonal LnPO₄ with the lattice constant of a = b = 7.005(57) Å, c = 6.352(05) Å, and V (cell volume) = 269.980 Å³. The photoluminescence behavior and emission mechanism were studied systematically by doping activators in the RbCaGd(PO₄)₂ host. The Mn²⁺ incorporated RbCaGd(PO₄)₂:Ce³⁺, Mn²⁺ compounds exhibited blue emission from the parity- and spin-allowed f-d transition of Ce³⁺ and orange-to-red emission from the forbidden ⁴T₁ → ⁶A₁ transition of Mn²⁺. The emission chromaticity coordinates of RbCaGd(PO₄)₂:0.10Ce³⁺, xMn²⁺ (x = 0.16, 0.25) are close to the white region due to an energy transfer process and the energy transfer mechanism from Ce³⁺ to Mn²⁺ in the RbCaGd(PO₄)₂ host was dominated by dipole-dipole interactions.     

In vitro behavior of fluoridated hydroxyapatite coatings in organic-containing simulated body fluid

A dense and pure hydroxyapatite [HA, Ca₁₀(PO₄)₆(OH)₂] coating and a fluoridated HA [Ca₁₀(PO₄)₆(OH)_0.67F_1.33] are deposited on Ti6Al4V substrates by sol-gel dip coating method. Glucose and bovine serum albumin have been added in standard simulated body fluid (SBF) to form organic-containing SBF in simulation of the physiological blood plasma. The HA and the fluoridated HA coatings are immersed in the standard and modified SBF for time periods of 2, 4, 7, 14 and 28 days at 37 ± 0.1°C. After soaking, the coating surface is examined for nucleation and growth of apatite using SEM morphological observation. The post-soaking SBF solutions are analyzed via Inductively Coupled Plasma spectroscopy for calcium ion concentration. The results show that at concentration of 40 g/L, bovine serum albumin has significant retardation effect on apatite precipitation from SBF onto pure or fluoridated HA coatings; Fluorine-incorporation in HA has positive bio-activation effect in both standard SBF and organic-containing SBF. However, glucose addition in SBF does not generate significant influence on the bioactivity of HA and fluoridated HA.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

An inorganic electroluminescent device using calcium phosphate doped with Eu3+ as the luminescent layer

In this work, the availability of calcium phosphates for the light emitting layer of a thin-film electroluminescent (TFEL) device was investigated. The goal of this work was to develop an electronic device with ordinary materials such as a calcium phosphate, the principal ingredient of the skeleton of the vertebrate. Compositions of 2CaO·P₂O₅ (Ca₂P₂O₇), 3CaO·P₂O₅ (Ca₃(PO₄)₂) and 4CaO·P₂O₅ (Ca₄O(PO₄)₂) were examined as the candidates for the light emitting layer. Before composing the TFEL device, the photoluminescence (PL) properties of the three compositions were investigated in the powder form to evaluate the performance as the light emitting layer. Among the examined calcium phosphates, Eu-doped β-Ca₃(PO₄)₂ showed the best PL properties. It showed typical red-emission from Eu³⁺. The PL intensity was enhanced with the heat-treatment temperature and the optimal temperature was 1250 °C. Then, a TFEL device was prepared by a spray pyrolysis method with the β-Ca₃(PO₄)₂:Eu³⁺ phosphor layer on a BaTiO₃ disk. The TFEL device exhibited the red emission originating in Eu³⁺ at 610 nm under applying alternating voltage. Different from the power sample, the intensity of EL decreased with the heat-treatment temperature from 1000 to 1250 °C. The deterioration of EL at the higher temperatures was attributed to chemical interaction between the phosphor layer and the BaTiO₃ disk.     

Biomimetic mineralization of calcium phosphate crystals in polyacrylamide hydrogel: Effect of concentrations of calcium and phosphate ions on crystalline phases and morphology

Calcium phosphate crystals were synthesized in polyacrylamide (PAAm) hydrogel, and the effects of the concentrations of calcium and phosphate ions on the crystalline phases and morphology were investigated. PAAm hydrogels containing diammonium hydrogen phosphate ((NH₄)₂HPO₄) were transformed into calcified materials by diffusion of calcium ions from calcium nitrate (Ca(NO₃)₂) aqueous solution into the gels. Several kinds of calcium phosphate crystals were precipitated at various Ca(NO₃)₂ concentrations (0.5–4.0 mol·dm^? 3), or (NH₄)₂HPO₄ contents (3.6–21.6 mmol) in the gels. The crystalline phases were mainly determined by the (NH₄)₂HPO₄ content in the gels. When the (NH₄)₂HPO₄ content was ≥ 10.8 mmol, hydroxyapatite (HAp) formed near the interfaces between Ca(NO₃)₂ solution and the gels, whereas octacalcium phosphate (OCP) formed in gels with ≤ 10.8 mmol (NH₄)₂HPO₄. HAp crystals were granular in form and about 200 nm in diameter, and OCP crystals were spherulitic with diameter 10–70 μm.     

Influence of temperature, [Ca2+], Ca/P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method

Nanocrystalline hydroxyapatite was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO₃)₂ and NH₄H₂PO₄ as source material and carbamide (NH₂CONH₂) as precipitator. The influence of Ca/P molar ratio, precipitation temperature, concentration of Ca²⁺ ([Ca²⁺]) and ultrasonic power on the crystallinity of the nanopowder were systematically investigated by XRD analysis. The size of the as-prepared particles was analyzed using TEM and XRD methods. The results revealed that the monophase hydroxyapatite could be obtained at the following technological conditions: [Ca²⁺] = 0.01–0.1 mol/L, ultrasonic power = 300 W, Ca/P (mol) = 1.2–2.5 and T = 313–353 K. In addition, the acicular and spherical particles could be prepared at different ultrasonic powers of 300 and 200 W, respectively.     

High-rate performance of xLiFePO4·yLi3V2(PO4)3/C composite cathode materials synthesized via polyol process

xLiFePO₄·yLi₃V₂(PO₄)₃/C composite cathode materials were synthesized via a polyol process, using LiOH·H₂O, Fe₃(PO₄)₂·8H₂O, V₂O₅ and H₃PO₄ as raw materials, citric acid and PEG as carbon sources, and TEG as both a solvent and a reductant. Structural and morphological characterizations of as-prepared materials were carried out by X-ray diffraction (XRD) as well as scanning electron microscopy (SEM), respectively. Furthermore, electrochemical properties of as-prepared materials were analyzed by charge–discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). XRD results indicated that the composites consisting of an olivine phase of LiFePO₄ and a monoclinic phase of Li₃V₂(PO₄)₃ are well-crystallized. It is found that the LF_0.6P·LV_0.4P/C composite exhibited better electrochemical performance than pristine LFP/C and LVP/C at 5 C and 10 C rate and delivered 126 mAh g^?1 and 110 mAh g^?1, respectively. The favorable particles morphology with less than 100 nm size and low extent agglomeration is believed as a factor. In addition, the co-existence of V³⁺-doped LiFePO₄/C and Fe²⁺-doped Li₃V₂(PO₄)₃/C was supposed as another reason.     

Investigation of energy absorption and transfer process of Tb3+ or Eu3+ excited Na3Gd(PO4)2 in the VUV region

Na₃Gd(PO₄)₂, Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ and Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ are prepared by solid-state reaction and their photoluminescence (PL) properties are investigated in the ultraviolet (UV) and vacuum ultraviolet (VUV) region. The obtained results show that Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ has an efficient emission under 147 nm excitation, but the emission efficiency of Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ is low under 147 nm excitation. We discuss the energy absorption and transfer process in the VUV region to solve the special phenomenon.