Oxidative coupling of methane over transition-metal-substituted strontium hydroxyapatite

Lead-substituted strontium hydroxyapatite (Sr_10-xPb_x(OH)₂(PO₄)₆) showed remarkably enhanced catalytic performance for the oxidative coupling of methane (OCM) when compared with the unsubstituted strontium hydroxyapatite. Other substituted transition metals such as zinc, cobalt and nickel were not so effective for improving the catalytic performance for the OCM. The Ni-substituted catalyst exhibited quite different catalytic behavior: CO and hydrogen were the major products instead of the C₂ products. The catalyst with the extent of Pb substitution(x) of 0.2 showed the highest C₂ selectivity and yield (about 47% and 17% at 1,023 K, respectively) and also exhibited quite stable behavior.     

A multifunctional strontium/silver-co-substituted hydroxyapatite derived from biogenic source as antibacterial biomaterial

The bioactive properties of hydroxyapatite (HAp) facilitate bone regeneration, however, its physico-chemical and bioactive properties can be further enhanced by ionic substitutions within the crystalline lattice. In this work, substitution with Sr²⁺ and Ag⁺ ions was investigated for the improvement of osseointegration and antibacterial activity in potential treatment of traumatic bone injuries. A series of single substituted HAp with Sr²⁺ or Ag⁺ ions and Ag/Sr-co-substituted HAp with different degrees of substitution (0, 1, 2.5, and 5 mol%) were obtained by wet precipitation from cuttlefish bone. Rietveld refinement indicated successful Ca²⁺ substitution by increasing cell parameters due to the larger ionic radii of Sr²⁺ and Ag⁺ compared to Ca²⁺, which was confirmed by elemental mapping showing uniform distribution of substituent cations. Characterization of the zeta-potential of Ag/Sr-co-substituted HAp showed negatively charged populations, at potentials not lower than ?15 mV. Increasing the degree of substitution resulted in decreasing zeta-potential. The higher absorption capacity of bovine serum albumin was determined on Ag/Sr-co-substituted HAp powders as compared to non-substituted HAp. All Ag-substituted HAps have shown an antibacterial activity towards Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli, as determined by inhibition zone, viability analysis and scanning electron microscopy. In addition, non-cytotoxicity towards human cells was confirmed by in vitro tests with embryonic kidney 293 and mesenchymal stem cells.     

In-situ grown hydroxyapatite whiskers reinforced porous HA bioceramic

To improve the mechanical properties of a porous bioceramic without reducing its porosity, a new kind of porous hydroxyapatite (HA) bioceramic with in-situ grown HA whiskers was fabricated using a simple sintering method. CaSO₄·2H₂O was used as a pore-forming medium and also as a catalyst for the growth of in-situ HA whiskers. The bioceramic was analyzed by XRD, SEM and mechanical tests. In-situ grown HA whiskers were stratified on the cliffs of pores in the bioceramic. The compressive strength is as high as 21.7 MPa with the porosity of about 26%. The results show that porous HA bioceramic can be improved in both compressive strength and porosity by the addition of CaSO₄·2H₂O. This novel HA bioceramic has a higher compressive strength without reducing its porosity in a certain weight ratio of CaSO₄·2H₂O, which depends on its two-step fracture pattern. This novel structure provides a new and promising reinforced pattern for porous materials.     

Sintering and mechanical properties of MgO-doped nanocrystalline hydroxyapatite

Hydroxyapatite (HA) has been extensively studied for its exceptional ability in promoting osseointegration as in bone graft substitute and biomimetic coating of prosthetic implants. However poor mechanical properties of HA, in particular its low fracture toughness, has made its widespread adaption in a number of biomedical applications challenging. Here we employ an optimized wet precipitation method to synthesize nanocrystalline HA with significantly improved mechanical properties. In addition doping by MgO is found to effectively suppress grain growth and enhance fracture toughness by nearly 50% while good densification and phase stability in all samples regardless of concentration of dopant are fully maintained. Microstructural analysis further suggests that the exceptionally superior mechanical properties can be explained by migration of MgO to grain boundaries where they transform the more common transgranular fracture into an intergranular mode. Our biodegradation tests also confirm that MgO-doped HA is indeed a suitable candidate for load bearing implants.     

Strontium substituted hydroxyapatite promotes direct primary human osteoblast maturation

Strontium-substituted hydroxyapatite (SrHAp) materials are known to actively promote bone formation. However, the optimum level of Sr inclusion needed to elicit a physiologically relevant response from bone cells is unclear and can vary dependent on the fabrication process employed. In this work hydroxyapatite (HAp), SrHAp powders (2, 5 and 10 wt% (i.e., 1, 2 and 5 at%) with respect to [Sr/(Sr + Ca) ?100]), were synthesized with the purity and Sr-substitution was confined in the range of 1–8 wt% (1–4 at%). All SrHAp samples contained rod-like crystals (<106 nm in length), which decreased in length with increasing Sr content, and exhibited larger flatter crystals (>300 nm in length). TEM-EDX confirmed the presence of Sr and maintenance of the HAp lattice structure for both types of crystals. Qualitative in vitro evaluation using primary human ostesoblast cells (HOBC) cultured in contact with the SrHAp over 28 days showed that the presence of Sr (in particular with the highest Sr content) directly promotes the maturation of osteoblasts into osteocytes as compared to the response observed for HAp. As these materials contain no additives other than Sr, the effects observed here can only be attributed to the physiologically important levels of Sr in the samples.     

SHS of mineral-like ceramics for immobilization of radioactive nuclear wastes

Synthetic analogs of titanate ceramics (perovskite and zirconolite) designed for use as a matrix for immobilization of high-level nuclear fuel reprocessing wastes (HLW). Such ceramics have been prepared by SHS method from a mixture of titanate ceramics and non-radioactive model oxides. Synthesis conditions have been optimized. The synthesized low-porosity cylindrical compacts exhibited a high strength and low rate for leaching Cs, Sr, Y, Ce, and La in bidistilled water. The phase composition and microstructure of synthesized products have been characterized. The immobilization of Cs was found to be accompanied by a marked loss of this element. The text was submitted by the authors in English.     

Strontium incorporated hydroxyapatite/hydrothermally reduced graphene oxide nanocomposite as a cytocompatible material

Hydroxyapatite (HA)/reduced graphene oxide (rGO) composites with different mol% of strontium and 1?wt% of GO were fabricated through a green hydrothermal reduction method and this combination has been reported for the first time. All the synthesized composites had strontium incorporated onto the crystal structure of HA as can be substantiated from XRD and FTIR. This paper also discusses a possible role of surface and pore characteristics on the in vitro cytocompatibility and the contribution of graphene oxide in directing the nucleation points resulting in dispersed strontium incorporated hydroxyapatite (SHA) based on P-31 NMR and TEM studies. In addition, a reasonable speculation also has been made to correlate the cytocompatibility with the selective occupancy of strontium ions in the apatite lattice. The in vitro cytocompatibility of SHA/rGO composites (SHAG) has been evaluated using cell proliferation tests with MG-63 cells, under a wider range of concentrations (1000–7.8?µg/ml) and by varying Sr/(Ca+Sr) molar ratio. SHAG with strontium substitution of 10?mol% exhibited the maximum viability among the samples tested. These results suggest that the SHAG composites will be a promising material for biomedical application.     

Microbial biomineralization of hydroxyapatite nanocrystals using Bacillus tequilensis

In this research, biomimetic deposition of “bone-like” apatite by novel Gram-positive bacterium Bacillus tequilensis was investigated. Hydroxyapatite (HA) was produced by Bacillus tequilensis using defined biomineralization media and dried. Calcination was carried out at different temperatures (100 °C–900 °C) and HA nanopowder was analysed for its structural phase composition, crystallinity, crystallite size and functional groups. X-ray diffractometry (XRD) indicated that increasing temperatures increased the crystallinity of HA nanocrystals. The presence of carbonate groups was evidenced by Fourier transform infrared (FTIR) spectrum and the purity of synthesized apatite nanocrystals was validated by absence of secondary peaks in XRD studies. Scanning electron microscopy (SEM) images depicted those uniform spherical agglomerates of HA comprised of nanosized crystallites. Transmission electron microscope (TEM) results identified needle-like crystal morphologies with average dimensions of 30–60 nm length and 3–10 nm width. Rich trace ion deposition was illustrated by energy dispersive x-ray spectroscopy (EDS) and quantified using inductively coupled plasma optical emission spectrometer (ICP-OES). Overall, microbial biomineralization by Bacillus tequilensis produced nanocrystals of HA that mimicked “bone-like apatite” as evidenced by pure phase, B-type carbonated form, poor crystallinity and trace amounts of vital elements (Mg, Na, K, Zn, Sr, Cl). Moreover, in vitro cytotoxicity studies revealed more than 80% cell viability highlighting the biocompatible nature of synthesized nano HA. Thereby, Bacillus tequilensis biomineralized nano HA reflects as a suitable candidate for applications in biomedicine addressing bone injuries and aiding regeneration.     

Strontium and zinc co-substituted nanophase hydroxyapatite

It is well documented that biological hydroxyapatite (HA) differs from pure and synthetically produced HA, and contains of a mixture of calcium phosphate (CaP) phases in addition to a range of impurity ions, such as strontium (Sr²⁺), zinc (Zn²⁺), magnesium (Mg²⁺), fluoride (F^-) and carbonate(CO₃^2-), but to name a few. Further to this, biological apatite is generally in the form of rod (or needle-like) crystals in the nanometre (nm) size range, typically 60 nm in length by 5–20 nm wide. In this study, a range of nano-hydroxyapatite (nHA), substituted nHA materials and co-substituted nHA (based on Sr²⁺ and Zn²⁺) were manufactured using an aqueous precipitation method. Sr²⁺ and Zn²⁺ were chosen due to the significant performance enhancements that these substitutions can deliver. The materials were then characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) techniques. The TEM results show that all of the samples produced were nano-sized, with Zn-substituted nHA being the smallest crystals around 27 nm long and 8 nm wide. The FTIR, XRD and XPS results all confirm that the materials had undergone substitution with either Sr²⁺ and Zn²⁺, for Ca²⁺ within the HA lattice (or both in the case of the co-substituted materials). The FTIR results confirmed that all of the samples were carbonated, with a significant loss of hydroxylation as a consequence of the incorporation of Sr²⁺ and Zn²⁺ into the HA lattice. None of the materials synthesised here in this study contained any other impurity CaP phases. Therefore this study has shown that substituted and co-substituted nanoscale apatites can be prepared, and that the degree of substitution (and the substituting ion) can have a profound effect of the attendant materials’ properties.     

Effects of strontium - erbium co-doping on the structural properties of hydroxyapatite: An Experimental and theoretical study

Five different samples of Sr-based Er-doped hydroxyapatites (HAps) in the dissimilar quantities like 0, 0.35, 0.70, 1.05 and 1.40 at% were produced via a wet chemical process. The prepared samples were investigated experimentally by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR) spectroscopy, differential thermal analysis (DTA), and in vitro biocompatibility tests. In addition, the density of states (DOS) and band structures were investigated theoretically. It was found that the presence of Er as a dopant affected the lattice parameters, while the EDX measurements confirmed that the presence of Er at various concentrations caused a Ca-deficiency because the addition of Er decreased the Calcium/Phosphorus molar ratio from 1.67 to 1.61. For all samples, the single-phase distribution of HAp was observed. The crystallinity percentage of the samples was found to be 89% or more according to two different methods. The calculations with respect to Scherrer and Williamson-Hall methods showed that the crystallite sizes of the samples were found to be in the ranges of 29–34 nm and 25-42 nm, respectively. DTA investigations revealed that all samples exhibited thermal stability in the temperature range of 25 °C - 1000 °C. No remarkable morphological alterations were observed. Furthermore, the theoretical studies confirmed that the band structures narrowed with an increase in Er concentration.     

Rapid synthesis of citrate-zinc substituted hydroxyapatite using the ultrasonication-microwave method

Bio-inspired citrate-hydroxyapatite (HAP) nanoparticles doped with zinc ions were prepared from mussel shells via a rapid method of ultrasonication and microwave irradiation. It was found that zinc ions can interact with citrate ions in the crystallization process of HAP. The growth of nanocrystals in citrate solution was slower than that in HCl solution. The addition of zinc ions reduced the residue of citrate in the products and increased the crystallinity of HAP. Furthermore, mineral platelets with the length of 29 ± 7 nm and the thickness of 3–5 nm were obtained for Zn/(Zn+Ca) = 5 mol%, which displayed similar morphology with HAP platelets in bone. Moreover, the obtained HAP nanoparticles are promising for use as bone graft materials in biomedical applications.     

Rapid preparation of Nd-doped zirconia ceramics for high-level radioactive waste immobilization

Rapid preparation of ceramic waste form at low temperature is of great engineering significance for immobilization of high-level radioactive waste. Herein, we demonstrated that Nd-doped zirconia ceramics can be rapidly prepared at low sintering temperature with the assistance of hydrothermal assisted sol-gel method. The effects of sintering temperature and Nd content on the phase and microstructure evolutions of the obtained ceramics were studied. With the increasing content of Nd, the phase of ZrO₂ transformed from monoclinic to cubic at first, then the pyrochlore Nd₂Zr₂O₇ phase appeared, and finally hexagonal neodymia solid solution structure formed. Moreover, the grain size and compactness of the obtained ceramics increased with the increment in sintering temperature. The successful accommodation of Nd within the ZrO₂ lattice was revealed by the detected increase of cell parameters. This work demonstrated that the hydrothermal assisted sol-gel process can be employed to effectively synthesize the Nd-doped zirconia ceramics as potential hosts for HLW immobilization.     

Comparative hydrothermal synthesis of hydroxyapatite by using cetyltrimethylammonium bromide and hexamethylenetetramine as additives

Comparative synthesis ways for preparing HA (Ca₁₀(PO₄)₆(OH)₂) nanoparticles in presence of hexamethylenetetramine (HMTA) and cetyltrimethylammonium bromide (CTAB) were carried out. The reactions were performed in a Teflon-lined stainless-steel reactor at 120 °C during 12 h. The effects of the additive concentration and the cooling mode (fast and slow) were analyzed. The obtained powders were characterized by X-ray Diffraction (XRD), Raman Spectroscopy, Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The two hydrothermal ways carried out for preparing HA nano powders produced a pure crystalline phase of HA. When the fast cooling mode was used, the obtained particles exhibited smaller mean particle sizes. The highest concentrations of used additives (HTMA or CTAB) resulted in opposite effect on the obtained mean particle size of HA particles. These observations were associated to the different behavior of these additives in the HA formation processes.     

Synthesis of hydroxyapatite nanoparticles using surface carboxyl-functionalized carbon dots as template

Carbon dots (CDs), which are discrete, nearly spherical nanoparticles with sizes below 10?nm and large amounts of carboxylic acid moieties on the surface, have been proposed as an ideal template candidate for heterogeneous nucleators to regulate hydroxyapatite (HAp) nucleation and growth. In this paper, small HAp nanoparticles formed on carboxyl-functionalized CDs in situ were fabricated via the hydrothermal method. Investigation for the corresponding morphologies and detailed formation mechanisms of samples were conducted by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and Rietveld refinement. The optimum size and crystallinity of HAp particle had been obtained in the preparation when the additive content of CDs was 1.11?g/L. Moreover, results also suggested that CDs were served as nucleators in the HAp particles. Therefore, a novel synthetic strategy is presented for small HAp nanoparticles using CDs as template.     

Low‐temperature synthesis of nanocrystalline hydroxyapatite: Effect of Mg and Sr content

Nanocrystalline Mg‐ or Sr‐containing hydroxyapatite powders were synthesized through low‐temperature chemical precipitation. The most significant factor for reduction in particle sizes included adjusting the reaction temperature between 0°C and 50°C. Syntheses products were characterized using several analytical tools to determine purity and influence of added amount (up to 15 mol%) of Mg or Sr on the composition and structure. Qualitative analysis by Fourier transform infrared spectroscopy and low intensity, broad X‐ray diffraction peaks indicated the presence of nanocrystalline and/or amorphous hydroxyapatite in all the products. Moreover, a significant decrease in the crystallinity was observed with increasing Mg (up to 2.8 ± 0.3 wt%) and Sr (up to 9.6 ± 1.0 wt%) concentration. N₂ adsorption and scanning electron microscopy characterizations confirmed the nanocrystalline nature of the synthesized products. The synthesized products had nanosized spherical‐like particle morphology with the specific surface area ranging from 89 ± 7 to 150 ± 20 m²/g.     

Phase and thermal stability of hydroxyapatite whiskers precipitated using amine additives

Two kinds of hydroxyapatite (HA) whiskers, prepared using urea and acetamide, were investigated to determine their thermal stability in air at 800-1200 °C. The thermal decomposition behavior and the phase stability of the whiskers upon heating were found to depend on the synthesis method, crystallinity and constitution of the whiskers. The phase transformation of the whiskers prepared using urea took place at a temperature below 800 °C due to low crystallinity and more carbonate and HPO₄ ions substitutions. Long rod-like particles appeared in the products treated at 1000-1200 °C. However, the whiskers prepared using acetamide were morphologically and structurally stable at 1200 °C due to their high crystallinity and low ions substitution, but with minor TCP caused by the decomposition of HPO₄ in the structure. Such whiskers would be useful and promising reinforcement for fabricating highly reliable HA ceramics or composites either in dense or porous form.     

锶掺杂纳米羟基磷灰石的制备

骨缺损的治疗是临床研究中的一项重大挑战,在天然骨骼中发现的各种人体必需元素中,锶是一种非常特殊的元素。在人体中,锶具有很强的亲骨性。锶掺杂后的纳米羟基磷灰石,与未掺杂的羟基磷灰石相比表现出增强的促成骨细胞分化活性。     

Influence of strontium and niobium on the physical and biological performance of hydroxyapatite as a bioactive coating on implant materials

The coating of hydroxyapatite (HAP) on the surface of bio-inert metallic implants to augment their bioactivity is in use for the last two decades. Substitution of various materials in HAP further improves the functionality of these coatings. We demonstrate coating of Ti6Al4V alloy sheets with strontium and niobium reinforced HAP using microwave (MW) irradiation technique. Physical characterization revealed, uniform semicrystalline hydroxyapatite coating with enhanced surface roughness and microhardness. The increased surface roughness was accompanied by higher wettability and more protein adsorption. Electrochemical corrosion assessment showed a dramatic increase in corrosion potential and a noticeable decline in corrosion current density suggesting an enhanced anticorrosive behaviour. These implants also exhibited improved hemocompatibility and bacteriostatic properties. Cell viability and confocal microscopy studies of the coated samples showed enhanced cell attachment on the surface. We propose microwave irradiation as a fast and hassle-free alternative for one-pot synthesis and deposition of ionic substituted HAP on metallic implants.     

Gelcasting of through-pore hydroxyapatite ceramics

Although the near-net-shape forming ability of gelcasting has been well demonstrated, current attention mainly was focused on manufacturing compact ceramics. In this work we demonstrated that the combination of the gelcasting technique and the ceramic sintering process could prepare the near-net-shape through-pore hydroxyapatite (HAP) ceramics, in which the pore diameter at the range of < 2 μm could be adjusted by changing the content of HAP in green bodies and with the aid of alcohol immersion. Experiments demonstrated that the gels in green bodies and alcohol immersion played important roles in improving the strength of green bodies and decreasing the shrinkage and deformation of green bodies, and thus improving the through-pore and near-net-shape forming of HAP ceramics. Such method reported here possibly extends the gelcasting technique to manufacture other through-pore ceramics for precise filtration, protein separation, or high performance liquid chromatography.