Synthesis of hollow structural hydroxyapatite with different morphologies using calcium carbonate as hard template

Hydroxyapatite (HAp) with hollow structure was successfully synthesized by hydrothermal process of as-prepared calcium carbonate used as a hard template and calcium sources in a diammonium phosphate solution. Calcium carbonate was fabricated by precipitation, which possessed different morphologies such as balls, rods and blocks through regulating the amount of citric acid. The synthesized powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrograph (FT-IR), field-emission scanning electron microscope (FESEM) and high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption–desorption. Results indicated that different morphologies calcium carbonate could convert to hollow structural HAp with the higher BET surface area and the mesopores. Hydrothermal temperature and hydrothermal time play a slight role on transition percentage. As hydrothermal temperature and hydrothermal time increased, the conversion rate of calcium carbonate to hydroxyapatite increased. The possible formation mechanism of hydroxyapatite was preliminarily investigated. The resultants of HAp are interesting materials for drug delivery and sustained-release.     

Thermal decomposition of calcium carbonate polymorphs precipitated in the presence of ammonia and alkylamines

Precipitated calcium carbonate was obtained by CO₂ bubbling in CaCl₂ solution. Ammonia and alkylamines were used to enhance CO₂ absorption and control the polymorphic phases. Vaterite and calcite mixtures were obtained, mainly vaterite in ammonia and mono-methylamine and mostly calcite in tri-methylamine environment. The thermal decomposition of precipitated calcium carbonate leads to high purity CaO. During the thermal decomposition the vaterite to calcite transformation was noticed but the final spherical shape of vaterite was maintained. As the use of CaO as catalyst in biodiesel synthesis may recommend a spherical shape and high contact surface, a precipitated calcium carbonate, rich in vaterite phase, may be a good precursor for CaO catalyst preparation.     

Synthesis of Ti(IV)-substituted calcium hydroxyapatite microparticles by hydrolysis of phenyl phosphates

Ti(IV)-substituted calcium hydroxyapatite (Ti-CaHap) microparticles were synthesized by hydrolyzing the phenyl phosphate (C₆H₅PO₄H₂) in a mixture of aqueous Ca(OH)₂ and Ti(SO₄)₂ solutions at pH = 8 and 85 °C. Then, the atomic ratio Ti/(Ca + Ti) in the starting solution was ranged from 0 to 0.20. The XRD pattern of the product formed at Ti/(Ca + Ti) = 0 was identified as CaHap. On increasing Ti/(Ca + Ti) ratio, the diffraction intensity of the CaHap peaks weakened and the unit-cell dimension c of the material was reduced. The Ti content in the particles was increased by raising Ti/(Ca + Ti) ratio and the Ti(IV) was more easily incorporated in the particles than Ca(II). All the phenyl phosphates were hydrolyzed during preparation of the particles. The CaHap formed at Ti/(Ca + Ti) = 0 was needle-like particles with the size of ca. 3.2 μm in length and ca. 0.3 μm in width. The size of Ti-CaHap particles was essentially unchanged on increasing Ti/(Ca + Ti) ratio up to 0.05. At Ti/(Ca + Ti) ? 0.10, the amorphous TiO₂ particles with a size of ca. 90 nm were generated and the size of Ti-CaHap particles was decreased, leading to the steep increase of specific surface area. It can be, therefore, presumed that the Ti(IV) is substituted with Ca(II) in CaHap crystal to form Ti-CaHap microparticles at Ti/(Ca + Ti) = 0–0.05, though the incorporation of Ti(IV) in the materials impedes the crystallization and growth of Ti-CaHap particles and accelerates the formation of amorphous TiO₂ particles at Ti/(Ca + Ti) = 0.10–0.20. Diffuse reflectance UV measurements indicated that the Ti-CaHap microparticles possess a UV absorption property and the ability is enhanced on increasing Ti/(Ca + Ti) ratio. The Beer’s plot revealed that the UV absorption ability of the synthetic Ti-CaHap microparticles is higher than that of amorphous TiO₂ ones.     

Co-substitution of carbonate and fluoride in hydroxyapatite: Effect on substitution type and content

The nanosized hydroxyapatite substituted by fluoride and carbonate ions (CFHA) had been synthesized by aqueous precipitation method. CFHA had been considered as potential bone graft material for orthopedic and dental applications. The objective of this study was to determine the effects of simultaneously incorporated CO^2--₃ and F^-- on the substitution type and content. The morphologies of CFHAs were observed by TEM. The carbonate substitution type and content were characterized by FTIR. The fluoride contents were determined by F-selective electrode. The phase compositions and crystallinity of the samples were investigated by XRD. The fluoride and carbonate contents of CFHA increase with the dopant concentrations nonlinearly. The carbonate substitution has much more obvious effect on morphology compared with the fluoride substitution. The co-existence of CO^2--₃ and F^-- ions can influence the corresponding substitution fraction. The isomorphic substitution of sodium for calcium in the substitution process of CO^2--₃ can improve crystal degree and favor the B-type substitutions. Due to the closeness of the ion radii and equivalent substitution of F^-- and OH^--, F^-- will occupy the OH^-- sites of HA crystals more easily, compelling most of the CO^2--₃ to be located in the B sites.     

Control the polymorphism and morphology of calcium carbonate precipitation from a calcium acetate and urea solution

Two metastable calcium carbonate polymorphs, rod-like aragonite and spherical vaterite are selectively formed in this study. Aragonite rods were synthesized from a calcium acetate (Ca(AC)₂) and urea (CO(NH₂)₂) solution under a given condition. In contrast, the addition of polyacrylamide (PAM) and oleic acid results in the formation of spherical vaterite. The morphology, size and crystal structure were characterized by means of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and X-ray diffraction (XRD). The results show that PAM and oleic acid can be used as additives to select the polymorph from aragonite and vaterite. The contact angle of the modified products reached 112.49°. We have succeeded in surface modification of particles in situ at the same time.     

Synthesis of nanocrystalline fluorinated hydroxyapatite by microwave processing and its in vitro dissolution study

Synthetic hydroxyapatite, (Ca₁₀(PO₄)₆(OH)₂, HA), is an important material used for orthopedic and dental implant applications. The biological hydroxyapatite in the human bone and tooth is of nanosize and differs in composition from the stoichiometric HA by the presence of other ions such as carbonate, magnesium, fluoride, etc. Osseointegration is enhanced by using nanocrystalline HA. This stimulates the interest in synthesizing nanocrystalline HA by different routes and among the methods, microwave processing seems to form the fine grain size and uniform characteristic nanocrystalline materials. Fluorinated hydroxyapatite, (FHA, Ca₁₀(PO₄)₆(OH)_2−x F _x ), possesses higher corrosion resistance in biofluids than pure HA and reduces the risk of dental caries. The present work deals with the synthesis of nanocrystalline FHAs by microwave processing. The crystal size and morphology of the nanopowers were examined by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) methods. The functional groups present in FHA powders were ascertained by Fourier transform infrared spectroscopy (FT-IR) and laser Raman spectroscopy. Since the physiological stability is an important parameter while selecting the material for implantation, the in vitro dissolution studies of FHAs with different fluorine contents were carried out.     

Synthesizing a composite material of amorphous calcium carbonate and aspartic acid

A composite material of amorphous calcium carbonate and aspartic acid (Asp) was synthesized using a highly concentrated solution of calcium aspartate: a new approach. A transparent and amorphous solid with approximately 1 mm thickness was obtained. UV-vis transmittance spectrum of the composite shows no characteristic absorption in visible region. A Raman spectrum of the composite revealed a peak assigned to the symmetric stretching of carbonate ion. This study demonstrated that amorphous calcium carbonate could be stabilized using not only organic artificial macromolecules but also using Asp, a small biomolecule. This result is expected to engender development of new biomimetic materials.     

In situ preparation of calcium carbonate films

The in situ preparation of calcium carbonate films in an ultra high vacuum (UHV) is inhibited by the decomposition of CO₂ molecules at the surface and the absence of CO₂ bulk diffusion. Therefore, it is not possible to prepare such films simply by CO₂ exposure to a calcium layer.We investigated different approaches for the preparation of CaCO₃ films in an UHV. Among these, only the simultaneous evaporation of Ca atoms in a mixed O₂ and CO₂ atmosphere is able to produce well defined stoichiometric calcium carbonate films. Metastable Induced Electron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and X-ray Photoelectron Spectroscopy are employed to verify quality and purity of the films.     

Preparation of calcium carbonate microparticles containing organic fluorescent molecules from vaterite

The encapsulation of fluorescent organic molecules into crystalline calcium carbonate was examined using calcium carbonate microcapsule, whose crystalline phase is vaterite as a metastable phase of calcium carbonate. A calcium carbonate microcapsule with impregnated pyrene that is a water insoluble fluorescent molecule was soaked into suitable aqueous solutions to promote the phase transition of vaterite toward calcite as the stable phase of calcium carbonate. When 0.2 M calcium chloride solution was used, the largest amount of pyrene (approximately 0.06 wt%) was encapsulated into the calcite particle. Pyrene thus included was not eliminated even after thorough washing with THF. The calcite particle thus prepared produced the excimer emission of pyrene by UV irradiation. Rhodamine B was also introduced into calcium carbonate by the immersion of the microcapsule into the aqueous solutions of Rhodamine B. The fluorescence of rhodamine B was observed from the calcium carbonate particles by visible light irradiation. Acetaminophen, a common drug poorly soluble in water, was also included in the calcium carbonate particle by the same procedures as the pyrene encapsulation. As acetaminophen thus encapsulated was released by the dissolution of the calcium carbonate particle in acidic solution, the particle is expected to be applied for a dissolution-triggered drug delivery.     

独特形貌碳酸钙的微波水热合成与表征

以醋酸钙和尿素为原料,在无添加剂的条件下采用微波水热法合成技术,直接制得了高度对称的双头矛状、双头扁平铲形、拉长六边形、拉长四边形、侧表面为六边形的多面体和六瓣花状等奇异形貌的碳酸钙晶体,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、傅里叶红外光谱仪(FT-IR)等对样品进行了表征,并对碳酸钙晶体的形成过程进行了分析。结果表明醋酸钙用量、尿素用量和微波辐照能量对碳酸钙晶体的结构与形貌具有重要调控作用。     

Synthesis and thermal stability of selenium-doped hydroxyapatite with different substitutions

Selenium (Se) plays a specific role in human health, especially for its anti-tumor effect. Incorporation of selenium into biocompatible hydroxyapatite (HAP) may endow the materials with novel characteristics. In the current work, a series of selenium-doped hydroxyapatite (Se-HAP) nanoparticles with different Se/P ratios were synthesized by a modified chemical precipitation. It was revealed that the powders with/without heat-treatment were nano-sized needle-like HAP while the heat-treated samples have high crystallinity. The addition of selenium decreases the crystallinity of the synthesized apatite, and also takes a negative effect on the thermal stability of the as-prepared powders. The Se-HAP nanoparticles with Se/P molar ratio not more than 5% sintered at 900°C can achieve good crystallinity and thermal stability.     

Continuous microwave assisted flow synthesis and characterization of calcium deficient hydroxyapatite nanorods

Synthetic calcium deficient hydroxyapatite (CDHA) nanorods (<100?nm) were rapidly prepared with the help of a new continuous microwave assisted flow synthesis (CMFS) reactor in 5?min only from aqueous solution of calcium hydroxide and orthophosphoric acid at pH 8.5. The effect of various reaction parameters like, pH, concentration, temperature, residence time, degree of crystallinity and particle surface area were studied in detail. The phase purity, particle size and morphology of the powder samples were characterised by techniques such as X-ray diffraction analysis, transmission electron microscopy, scanning electron microscopy and FTIR and Raman spectroscopy. With the help of X-ray photoelectron spectroscopy, the chemical analysis was completed. Measurements were taken into account to estimate the particle size following the dynamic light scattering. The results showed that the employed synthesis procedure offered an efficient and economical route to achieve high quality nano-sized products with suitable size and low level of impurities.     

A two-fluid model for calcium carbonate precipitation in highly supersaturated solutions

Precipitation of calcium carbonate is a common phenomenon in nature, which has attracted attention from researchers due to its importance in biomineralization processes, climatic changes, and especially incrustations in pipelines. In this work, a two-fluid model is proposed for homogeneous crystallization of calcium carbonate in highly supersaturated solutions. By making an analogy between the dynamics of a granular gas and the dynamics of a solid dispersion, the proposed model not only accounts for the interaction forces between the solid particles and dispersion medium, but also for the mass transfer and changes in the particles size during the precipitation reaction. Moreover, by using a numerical scheme based on an iterative algorithm, 3D numerical simulations are performed for the homogeneous crystallization of calcium carbonate, and the results are compared with experimental particles size distributions and curves of pH versus time. The good agreement between theoretical and experimental results indicates that the two-fluid model can be successfully used to evaluate the growth kinetics of calcium carbonate nuclei.     

Properties of metakaolin based geopolymer incorporating calcium carbonate

An alkaline solution, thermally activated kaolinite clay and a mineral additive (calcium carbonate) were mixed with the aim to elaborate a geopolymer material with physical and mechanical properties comparable to those of classical construction materials.The starting reagents were characterized by quantitative chemical analyses (XRF), mineralogical analyses (XRD), thermal gravimetric analyses (TGA), and grain size distribution measurements. The setting of the mixture (polymerization) was implemented by measuring the evolution of the viscosity as a function of time at different temperatures.The geopolymers were synthesized at a temperature of 40 °C. The investigation of the mechanical behavior reveals that these materials display acceptable characteristics: the flexural and compression strength are around 4.6 and 26 MPa respectively, for an added calcium carbonate over dry matter ration up to 12% by weight.The promising results exposed in this paper show that the geopolymer formulations can be adapted for applications in construction and civil engineering structures as an alternative to conventional materials.     

超细碳酸钡粒子的合成

采用共沉淀法合成了碳酸钡粒子。通过添加合适的晶形控制剂,合成了球状和哑铃状的碳酸钡粉体,利用扫描电子显微镜(SEM)和X射线衍射仪(XRD)进行了表征,并对不同晶形碳酸钡的形成机理进行了初步的探讨。     

Trapping heavy metals by using calcium hydroxyapatite and dielectrophoresis

We propose a novel technique for the removal of heavy metal waste from contaminated water. Our method consists in using dielectrophoresis (DEP) to trap hydroxyapatite (HAP) particles of 1 microm size in water after they have adsorbed heavy metal (Pb, Zn, Cu, Co and Cr). Although HAP can adsorb heavy metals in water and as such offers great promise as a waste-cleaning tool , one of the current challenges is the efficient removal of the HAP particles once they have adsorbed the heavy metals. We show in this paper that DEP can be used to concentrate such particles in certain regions, thus rendering the rest of the solution volume nearly free of contaminated particles. We present here both experimental and numerical results for suspensions at low concentrations.     

Characterization and thermal behavior of calcium deficient hydroxyapatite whiskers with various Ca/P ratios

Calcium deficient hydroxyapatite (Ca-def HA) whiskers with various Ca/P ratios were synthesized by a homogeneous hydrothermal precipitation method using acetamide. They had high crystallinity for Ca/P > 1.54, with a mean length of 79–113 μm and an aspect ratio of 85–103. No whisker entanglement and aggregation were found in the products. Thermal analyses revealed that the Ca/P ratio and crystallinity of the products showed significant effects on the crystalline phase, constitution and morphological stability of the whiskers at an elevated temperature. The whiskers with Ca/P > 1.54 were morphologically stable at temperatures below 1200 °C, with minor TCP appearance after heat-treatment at 800 °C; no further decomposition of the whiskers was found with increasing temperature. Such whiskers are possibly one promising reinforcing materials for preparing highly reliable apatite ceramics and composites either in dense or porous block materials with high mechanical properties and good enhanced biological performance.     

3D spatial distribution of the calcium carbonate caused by carbonation of cement paste

Carbonation of cement-based materials is one area of concern for the durability of concrete structures. The calcium carbonate caused by carbonation is an important indicator of carbonation degrees. The present paper, using 3D tomography data, proposes a nondestructive method to characterize the 3D spatial distributions of calcium carbonate. It allows monitoring of 3D carbonation evolutions. The evolution of the calcium carbonate distributions in a specimen of cement paste with different carbonation degrees is given using the current method. The results are compared with the average quantity of calcium carbonate determined by thermal analysis. From the sharp edge of the calcium carbonate distribution, we conclude that the accelerated carbonation in this experimental condition is a diffusion controlling process.     

Growth of calcium carbonate on non-covalently modified carbon nanotubes

The crystallization of calcium carbonate was investigated on pristine and non-covalently modified carbon nanotubes (CNTs) using the vapor diffusion technique in a calcium chloride solution. Non-covalent modification was accomplished by treating the carbon nanostructures with the amphiphilic copolymer poly(isoprene-b-acrylic acid). Calcium carbonate crystals grown on the surface and in the interstitial channels of CNT buckypapers were observed in both cases. Scanning electron microscopy analysis of the untreated CNTs showed the characteristic rhombohedral morphology of calcite crystals, while in the case of modified material spherical and ellipsoidal crystals, consisted of nanocrystallites, were observed. X-ray diffraction analysis showed the presence of calcite crystals in both cases.     

室温固相反应合成碱式碳酸锌

以七水硫酸锌(ZnSO4.7H2O)和碳酸氢铵(NH4HCO3)为原料,以聚乙二醇-400(PEG-400)为模板,通过室温固相反应制备了碳酸锌(ZnCO3)和碱式碳酸锌(ZnCO3.3Zn(OH)2.H2O,basic zinc carbon-ate,BZC)。通过XRD测试及其半定量成分分析,研究了PEG-400剂量和NH4HCO3与ZnSO4.7H2O摩尔比x值对合成产物物相的影响。总结了PEG-400模板辅助合成ZnCO3和BZC的反应机制。结果表明,PEG-400包覆ZnSO4.7H2O颗粒形成模板,模板层的厚度影响固相反应的微观机制———薄层单向扩散与厚层互扩散,局部微环境的酸碱性决定着产物物相,酸性抑制ZnCO3水解,碱性促进ZnCO3水解生成BZC。由据此设计的较高x值(x=3.0)、较低PEG-400剂量(70μL)的合成工艺,制备了单相BZC粉体。