Chemical synthesis and characterization of magnesium substituted amorphous calcium phosphate (MG-ACP)

Amorphous calcium phosphate (ACP) was synthesized by a simple aqueous precipitation using CaCl₂ and Na₃PO₄ in the presence of MgCl₂ to ensure the formation of the ACP phase at room temperature. Magnesium substituted ACP phases corresponding to two different compositions representing the two most prominent calcium phosphate phases (hydroxyapatite: Ca + Mg/P = 1.67 and tricalcium phosphate: Ca + Mg/P = 1.5) were synthesized by this simple approach. Both compositions of ACP phases resulted in their transformation into β-tricalcium phosphate upon heat treatment in air at 600 °C. X-ray diffraction (XRD), heat treatment, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Brunauer-Emmett-Teller (BET) analyses were used to characterize the phase, thermal stability, surface area, and morphology of the synthesized ACP powders corresponding to the two different nominal Ca/P compositions. Although it is known that α-TCP is the phase that appears upon heat treatment at 600 °C unsubstituted ACP, substitution of magnesium ion in ACP (both TCP and HA composition) stabilized the structure of β-TCMP phase at 600 °C. Moreover, FT-IR analysis revealed that the ACP phase regardless of the composition, exhibited characteristic bands corresponding to that of HA, with the exception of the ACP corresponding to HA composition which exhibited a prominent OH vibrational mode.     

Cement from magnesium substituted hydroxyapatite

Brushite cement may be used as a bone graft material and is more soluble than apatite in physiological conditions. Consequently it is considerably more resorbable in vivo than apatite forming cements. Brushite cement formation has previously been reported by our group following the mixture of nanocrystalline hydroxyapatite and phosphoric acid. In this study, brushite cement was formed from the reaction of nanocrystalline magnesium-substituted hydroxyapatite with phosphoric acid in an attempt to produce a magnesium substituted brushite cement. The presence of magnesium was shown to have a strong effect on cement composition and strength. Additionally the presence of magnesium in brushite cement was found to reduce the extent of brushite hydrolysis resulting in the formation of HA. By incorporating magnesium ions in the apatite reactant structure the concentration of magnesium ions in the liquid phase of the cement was controlled by the dissolution rate of the apatite. This approach may be used to supply other ions to cement systems during setting as a means to manipulate the clinical performance and characteristics of brushite cements.     

Monetite and brushite coated magnesium: in vivo and in vitro models for degradation analysis

The use of magnesium (Mg) as a biodegradable metallic replacement of permanent orthopaedic materials is a current topic of interest and investigation. The appropriate biocompatibility, elastic modulus and mechanical properties of Mg recommend its suitability for bone fracture fixation. However, the degradation rates of Mg can be rapid and unpredictable resulting in mass hydrogen production and potential loss of mechanical integrity. Thus the application of calcium phosphate coatings has been considered as a means of improving the degradation properties of Mg. Brushite and monetite are utilized and their degradation properties (alongside uncoated Mg controls) are assessed in an in vivo subcutaneous environment and the findings compared to their in vitro degradation behaviour in immersion tests. The current findings suggest monetite coatings have significant degradation protective effects compared to brushite coatings in vivo. Furthermore, it is postulated that an in vitro immersion test may be used as a tentative predictor of in vivo subcutaneous degradation behavior of calcium phosphate coated and uncoated Mg.     

Novel in-situ synthesis and characterization of nanostructured magnesium substituted β-tricalcium phosphate (β-TCMP)

Tricalcium phosphate (TCP, Ca₃(PO₄)₂) in its pure form cannot be synthesized under physiological conditions in normal aqueous solutions due to phase instability, resulting in its transformation to hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) in the presence of water. However, substituting magnesium in lieu of calcium is known to stabilize TCP, preventing its conversion to hydroxyapatite. There are several methods known for synthesizing magnesium substituted tricalcium phosphate (TCMP). In the present study, a novel in-situ method has been developed to synthesize β-TCMP using magnesium substituted brushite as a precursor.Substitution of 50% of calcium by magnesium results in the formation of semi-spherical nanocrystalline particles (～ 100 nm) of brushite. Boiling the nanocrystalline brushite powder in aqueous condition for only 30 min results in the generation of rosette shaped nanocrystals (～ 80 nm) of β-TCMP that emerge from the original brushite spheres. The β-TCMP particles exhibit a specific surface area of ～ 200 m²/g. Details about the synthesis procedure and the possible mechanisms involved in the formation of β-TCMP from Mg-substituted brushite is further discussed.     

Chemical stabilization of MSW incinerator fly ashes

In this work, the relationship between heavy metal content of fly ash and that of the solid wastes incinerated was correlated and compared. It is found that the former is a function of the latter. Hence, it is important to prevent heavy metal-rich wastes from being incinerated in order to reduce the content of toxic metals in the fly ash. The leachability of fly ash from incineration was usually beyond the scope of toxicity standard and must be properly treated before discharge. Secondly, chemical stabilization for the heavy metals in fly ash was explored. Among the chemicals used, it was found that sodium hydroxide was not suitable for the adequate extraction of the heavy metals from the fly ash. Ethylenediaminetetraacetic acid disodium salt (EDTA) was also tested and seems to be effective for the leaching of toxic metals from the fly ash. On the other hand, sodium sulfide and thiourea are one of excellent chemicals for the effective treatment of fly ashes, since they convert soluble and leachable toxic metals into non-leachable and insoluble forms such as lead and zinc sulfide or their similar forms of thiourea. These chemical species are supposed to be stable in nature. A comparison between chemical stabilization noted above and cement or asphalt solidification methods is made. Chemical stabilization processes, especially using sodium sulfide as the chemical agent, are strongly recommended for the practical uses, in terms of the volume expansion and environmental safety of the stabilized products and cost balances, in comparison with the traditional cement or asphalt solidification methods.     

Liquid phase epitaxial growth of indium substituted magnesium ferrite films

Single crystal films of Mg(Fe,In)₂O₄ have been grown by means of liquid-phase-epitaxy on (100)/1bMgO substrates using a PbO/1bB₂O₃/1bFe₂O₃ solvent. The final polishing of the substrates appeared to be very important in connection with the necessary large supersaturation of the flux. Electron microprobe analysis revealed Pt and high Pb contents in our films. The easy axis of magnetization is in-plane, probably due to the compressive strain of the films caused by the difference in thermal expansion of MgO and Mg(Fe,In)₂O₄.     

Mechanochemical synthesis of amorphous and crystalline magnesium diboride

We have studied the phase composition of materials obtained by mechanochemical processing and subsequent heat treatment of mixtures of magnesium and boron powders in the atomic ratio 1: 2. Differential dissolution, differential scanning calorimetry, and X-ray diffraction data indicate that, during mechanical processing, some of the magnesium reacts with boron to form amorphous magnesium diboride. During annealing of the activated powder mixture, X-ray amorphous magnesium diboride forms at 340°C and crystallizes at 480°C. As shown by high-resolution transmission electron microscopy, the unreacted crystalline magnesium is covered with an amorphous layer consisting of magnesium diboride and boron. The amorphous material obtained by milling contains nuclei of MgB₂ crystallites 3–5 nm in size. During subsequent heating of the activated mixture, magnesium and boron react further to form amorphous magnesium diboride and the amorphous phase crystallizes. Heating of mechanically activated mixtures to just below the crystallization temperature allow MgB₂ nanoparticles to be produced. The formation of nanocrystalline magnesium diboride nuclei along with the amorphous phase during mechanochemical processing facilitates mechanochemical synthesis compared to thermal synthesis.     

Borohydride synthesis and stabilization of flake-like tetragonal zirconia nanocrystallites

Flake-like particles of tetragonal zirconia (t-ZrO₂) were synthesized using ZrOCl₂·8H₂O as zirconium precursor, and sodium borohydride (NaBH₄) and cetyltrimethylammonium bromide (CTAB) as precipitating agent and surfactant, respectively. Small-sized nuclei, which formed during the borohydride synthesis, play an important role in the formation of small sized crystallites (5 nm) even at 600 °C and stabilization of t-ZrO₂ in both the samples synthesized with and without surfactant. In the sample synthesized without CTAB, a minor m-ZrO₂ phase (5 vol.%) along with the major t-ZrO₂ phase having a crystallite size of 20 nm, was observed at 700 °C. However, the use of surfactant leads to the formation of stabilized t-ZrO₂ nanoparticles with a smaller crystallite size of 15 nm. SEM micrographs of t-ZrO₂ show elliptical as well as elongated shaped flake-like morphology. The formation of small sized crystallites and slow growth of nucleation embryo play an important role in stabilizing the t-ZrO₂ up to as high temperature as 700 °C.     

Aqueous deposition of calcium phosphates and silicate substituted calcium phosphates on magnesium alloys

Attempts were made to deposit homogeneous films of calcium phosphates (CaPs) on two magnesium alloy systems, AZ31 and Mg-4Y, through an aqueous phosphating bath method. The deposition of silicate substituted CaPs by this aqueous method was also explored as silicate substitution is believed to increase the bioactivity of CaPs. The effect of doped and undoped coatings on the in vitro degradation and bioactivity of both alloy systems was studied. FTIR and EDX confirmed the deposition of Ca, P, and Si on both alloys and the coatings appeared to consist primarily biphasic mixtures of hydroxyapatite and β-TCP. These largely inhomogeneous coatings, as observed by SEM, were not shown to have any significant effect on maintaining the physiological pH of the culture medium in comparison to the uncoated samples, as the pH remained approximately in the 8.4-8.7 range. Interestingly, despite similar pH profiles between the coated and uncoated samples, CaP coatings affected the degradation of both alloys. These doped and undoped calcium phosphate coatings were observed to decrease the degradation of AZ31 whereas they increased the degradation of Mg-4Y. In vitro studies on cell attachment using MC3T3-E1 mouse osteoblasts showed that between the uncoated alloys, Mg-4Y appeared to be the more biocompatible of the two. Silicate substituted CaP coatings were observed to increase the cell attachment on AZ31 compared to bare and undoped CaPs coated samples, but did not have as great of an effect on increasing cell attachment on Mg-4Y.     

A study of brushite crystallization from calcium-phosphate solution in the presence of magnesium under the action of a low magnetic field

The paper discusses the crystallization of dicalcium phosphate dehydrate (DCPD) with subsequent transformation to nanocrystalline hydroxyapatite (НA) under the permanent magnetic field in the presence of magnesium. It was found that the presence of magnesium in the initial solution in concentrations of 0.01–0.03 g/l decreased the crystallinity of calcium-phosphates. The precipitation of DCPD under the magnetic field of 0.3 T was carried out in proximity of the north and south magnetic poles. The differences in the particle morphology and structure of precipitates with the same phase composition (DCPD) were observed in the neighborhood of the north and the south pole. Lattice parameters of DCPD precipitates obtained biomimetically near opposite magnet poles were calculated using XRD results. It was found that the increased crystallization time (more than 3 days) leads to a complete attenuation of DCPD peaks, whereas НА peaks are still present.     

Chemical denitrification of water by zero-valent magnesium powder

A laboratory-scale study was conducted in batch mode to investigate the feasibility of using zero-valent magnesium (Mg(0)), for removal of nitrate from aqueous solution. Reaction pH, dose of Mg(0), initial nitrate concentration and temperature were considered variable parameters during the study. Strong acidic condition enhanced nitrate reduction and in absence of external proton addition, reaction pH increased rapidly above ten and insignificant nitrate removal (7-16%) was achieved. At Mg(0):NO(3)(-)-N molar ratio of 5.8 and controlled reaction pH of 2, 84% denitrification efficiency was achieved (initial NO(3)(-)-N 50 mg/L) under ambient temperature and pressure and total nitrogen removal was 70% with 3.2% and 10% conversion of initial NO(3)(-)-N to NO(2)(-)-N and NH(4)(+)-N, respectively. The reaction was first order with respect to nitrate concentration. Nitrate removal rate decreased with solution pH and increased linearly with Mg(0) dose. Nitrate removal was coupled with 96-100% removal of dissolved oxygen and 85-90% generation of soluble Mg(2+) ion. An activation energy (E(a)) of nitrate reduction over the temperature range of 10-50 degrees C was observed as 17.7 kJ mol(-1).     

Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel

X-ray amorphous precursor phases for the synthesis of spinel (MgAl₂O₄) have been prepared by grinding mixtures of gibbsite (Al(OH)₃) with brucite (Mg(OH)₂) or hydromagnesite (4MgCO₃·Mg(OH)₂·4H₂O). The mechanochemical treatment does not remove any water or carbonate, but converts some of the gibbsite octahedral Al sites into tetrahedral sites and other sites with a ²⁷Al MAS NMR resonance at about 38 ppm. The brucite-derived precursor forms spinel on heating at 850°C, by contrast with unground mixtures which show little spinel formation even at 1250°C. The hydromagnesite-derived precursor transforms at about 850°C into a mixture of spinel and hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O), the latter decomposing to spinel and MgO by 1050°C. Spinel derived from the hydromagnesite-containing precursor shows superior pressureless sintering properties at 1400–1600°C, producing a body of 97% theoretical bulk density at 1600°C. Under the same conditions, the brucite-derived spinel sintered to 72% theoretical density and showed a morphology consisting of widely disparate grain sizes.     

Evaluation of the magnesium and silicon diffusion rates in forsterite synthesis using various magnesium compounds

In forsterite synthesis from highly disperse SiO₂ (white soot or silica sol) and magnesium chloride, nitrate, sulfate, acetate, or citrate through sol-gel processing, with polyvinyl alcohol as a gel former, the reaction products are forsterite, magnesium oxide, silica, and clinoenstatite. From analysis of the synthesis products, we assess the relationship between the diffusion rates of the magnesium and silicon cations. Phase-pure forsterite can be obtained by reacting white soot and magnesium acetate.     

Chemical synthesis of nanosized oxides

Fine powder of single and binary mixed oxides can be produced by decomposition of the respective metal nitrates and polyvinyl alcohol (PVA) or, a mixture of PVA and polyacrylic acids. These mixtures, after spray drying, yield a brown fluffy mass, which is spontaneously combustible and the heat liberated is sufficient for the crystallization of the desired oxide phase. The rate of combustion controls the growth of the particles. This can be manifested by combustion of the mixture in controlled atmosphere. The nanoparticles of the oxide system studied are: spinels [MFe₂O₄ where M = Ni(II), Co(II), Zn(II), Mg(II)]; orthoferrites [MFeO₃ where M = Gd(III), Sm(III)]; LaAlO₃, NdGaO₃, CaO/MgO/Y₂O₃ stabilized zirconia (ZrO₂); lead zirconate titanate (PZT), lanthanum modified lead zirconate titanate (PLZT) and BaTiO₃.     

Microwave assisted synthesis of amorphous magnesium phosphate nanospheres

Magnesium phosphate (MgP) materials have been investigated in recent years for tissue engineering applications, attributed to their biocompatibility and biodegradability. This paper describes a novel microwave assisted approach to produce amorphous magnesium phosphate (AMP) in a nanospherical form from an aqueous solution containing Mg²⁺ and HPO₄ ^2?/PO₄ ^3?. Some synthesis parameters such as pH, Mg/P ratio, solution composition were studied and the mechanism of AMP precursors was also demonstrated. The as-produced AMP nanospheres were characterized and tested in vitro. The results proved these AMP nanospheres can self-assemble into mature MgP materials and support cell proliferation. It is expected such AMP has potential in biomedical applications.     

Chemical stabilization of porous silicon for enhanced biofunctionalization with immunoglobulin

Abstract

Porous silicon (PSi) is widely used in biological experiments, owing to its biocompatibility and well-established fabrication methods that allow tailoring its surface. Nevertheless, there are some unresolved issues such as deciding whether the stabilization of PSi is necessary for its biological applications and evaluating the effects of PSi stabilization on the surface biofunctionalization with proteins. In this work we demonstrate that non-stabilized PSi is prone to detachment owing to the stress induced upon biomolecular adsorption. Biofunctionalized non-stabilized PSi loses the interference properties characteristic of a thin film, and groove-like structures resulting from a final layer collapse were observed by scanning electron microscopy. Likewise, direct PSi derivatization with 3-aminopropyl-triethoxysilane (APTS) does not stabilize PSi against immunoglobulin biofunctionalization. To overcome this problem, we developed a simple chemical process of stabilizing PSi (CoxPSi) for biological applications, which has several advantages over thermal stabilization (ToxPSi). The process consists of chemical oxidation in H₂O₂, surface derivatization with APTS and a curing step at 120 °C. This process offers integral homogeneous PSi morphology, hydrophilic surface termination (contact angle θ = 26°) and highly efficient derivatized and biofunctionalized PSi surfaces (six times more efficient than ToxPSi). All these features are highly desirable for biological applications, such as biosensing, where our results can be used for the design and optimization of the biomolecular immobilization cascade on PSi surfaces.     

Growth of epitaxial substituted garnet films by hydrothermal synthesis

A new hydrothermal technique is described in which the garnet synthesis is performed directly in the autoclave in low concentration basic solutions (NaOH or KOH). This method avoids a strong attack of the substrate and leads to uniform thickness and composition films. The best experimental conditions are given for the growth of Ga:YIG, Gd:Ga:YIG, Eu:Ga:YIG, Eu:Ga:ErIG and Gd:Ga:ErIG films on GdGaG substrates. Magnetic characterizations are presented and the properties of hydrothermal garnet films are discussed and compared with those of L.P.E. and C.V.D. garnet films.     

Chemical stabilization of porous silicon for enhanced biofunctionalization with immunoglobulin

Porous silicon (PSi) is widely used in biological experiments, owing to its biocompatibility and well-established fabrication methods that allow tailoring its surface. Nevertheless, there are some unresolved issues such as deciding whether the stabilization of PSi is necessary for its biological applications and evaluating the effects of PSi stabilization on the surface biofunctionalization with proteins. In this work we demonstrate that non-stabilized PSi is prone to detachment owing to the stress induced upon biomolecular adsorption. Biofunctionalized non-stabilized PSi loses the interference properties characteristic of a thin film, and groove-like structures resulting from a final layer collapse were observed by scanning electron microscopy. Likewise, direct PSi derivatization with 3-aminopropyl-triethoxysilane (APTS) does not stabilize PSi against immunoglobulin biofunctionalization. To overcome this problem, we developed a simple chemical process of stabilizing PSi (CoxPSi) for biological applications, which has several advantages over thermal stabilization (ToxPSi). The process consists of chemical oxidation in H₂O₂, surface derivatization with APTS and a curing step at 120 °C. This process offers integral homogeneous PSi morphology, hydrophilic surface termination (contact angle θ = 26°) and highly efficient derivatized and biofunctionalized PSi surfaces (six times more efficient than ToxPSi). All these features are highly desirable for biological applications, such as biosensing, where our results can be used for the design and optimization of the biomolecular immobilization cascade on PSi surfaces.