Qi-Zhi Yao

Formation of aragonite mesocrystals and implication for biomineralization

Abstract Highly oriented aragonite tablets have been found in the nacre layers of molluscan shell (or mother of pearl). In this article, we show that highly organized aragonite rods can be prepared over a broad range of pH values (1.5 to 6.9) and in the absence of any bio- or organic macromolecules. The organized rods were characterized by XRD, FTIR, FESEM, TEM, SAED, and EDX techniques. FESEM results reveal that the mesoscale aragonite rods are not only assembled with aragonite microrods end-to-end, and side-to-side, but are also partially fused to one another, forming flat, faceted surfaces, i.e., mesocrystal structure. TEM and SAED analyses confirm that the organized rods have the same crystallographic symmetry as single-crystal aragonite, and thus the self-assembly process is energetically favorable. Similar assembly processes also occur for the mineral strontianite of the aragonite group, revealing the occurrence of a general self-assembly process. The driving force controlling the self-assembly process may originate from the inherent anisotropic dipole-dipole interactions between the assembled units. Such dipole interaction may generally occur in biomineralization of nacre layers in molluscan shell, and orchestrate aragonite nanocrystals in an aragonite tablet to coherently orient and array. Furthermore, the dipole-dipole interactions may also contribute to the co-orientation of the aragonite tablets in the same nacreous column. Therefore, our experimental results may provide insight into biomineralization mechanisms. It appears that biological genetic and crystallochemical factors may synergistically operate in biomineralization.

Controlled crystallization of unstable vaterite with distinct morphologies and their polymorphic transition to stable calcite

Vaterite with different morphologies, a thermodynamically unstable polymorph of calcium carbonate, was successfully synthesized by a simple injection-precipitation method. The phase composition and morphology of the products were characterized by the XRD, FT-IR, SEM, and TEM techniques. Experiments were performed at 37 and 25 °C using pH values of 1.5, 3.0, and 6.9. At 37 °C, precipitated vaterite has spindle-like morphology at the low pH 1.5 of the initial CaCl 2 solution, and shows coexistence of the spindle-, spheroid-, and cauliflower-like morphologies at the intermediate pH 3.0, whereas it is spheroidal at pH 6.9. SAED analyses revealed that the spindle-like vaterite superstructures were self-assembled by the oriented aggregation of vaterite micro-crystals along the crystallographic c direction. At 25 °C, however, the low pH (1.5) led to coexistence of cauliflower- and spheroid-shaped vaterite, whereas spherulite-like vaterite was always obtained at either pH 3.0 or 6.9. We show that simple inorganic precipitation processes lead to complex and unusual morphologies with hierarchical structure. Therefore, care must be taken when morphological criteria are claimed as proof for biogenic origin of minerals. Moreover, time studies of the polymorphic transition revealed that, although solution-mediated dissolution of precursor vaterite and reprecipitation of secondary calcite always occur, Ostwald ripening finally contributes to the formation of rhombohedral calcite. Therefore, the polymorphic transition sequentially proceeds from vaterite through irregular calcite aggregates to stable calcite rhombohedra.

Microwave-assisted controlled synthesis of monodisperse pyrite microspherolites

A microwave-assisted polyol method was reported to synthesize uniform and monodisperse pyrite (FeS2) microspherolites. The reaction processes for the synthesis involved the reduction of sulfur and reaction of the intermediate sulphur species with Fe2+. Polyvinyl pyrrolidone (PVP) added to the reaction system exerts an important role in the control of the phase composition and morphology of the products. The sample was characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and selected area electron diffraction (SAED) techniques. A series of TEM/HRTEM and SAED results reveal that the formation of pyrite FeS2 microspherolites is via a nanocrystal aggregation-based mechanism. The time-dependence experiments further demonstrate that primary FeS2 nanocrystals are first formed, and then aggregate into large spherolites, finally Ostwald ripening leads to the uniform and monodisperse microspherolites. The influence of microwave power on the size and morphology of the products and effect of microwave heating in the synthesis were also investigated.

Biomimetic synthesis of struvite with biogenic morphology and implication for pathological biomineralization

Recent studies have found that certain urinary proteins can efficiently inhibit stone formation. These discoveries are significant for developing effective therapies for stone disease, but the inhibition mechanism of crystallization remains elusive. In the present study, polyaspartic acid (PASP) was employed as a model peptide to investigate the effect of urinary proteins on the crystallization and morphological evolution of struvite. The results demonstrate that selective adsorption/binding of PASP onto the {010} and {101} faces of struvite crystals results in arrowhead-shaped morphology, which further evolves into X-shaped and unusual tabular structures with time. Noticeably, these morphologies are reminiscent of biogenic struvite morphology. Concentration-dependent experiments show that PASP can inhibit struvite growth and the inhibitory capacity increases with increasing PASP concentration, whereas aspartic acid monomers do not show a significant effect. Considering that PASP is a structural and functional analogue of the subdomains of aspartic acid-rich proteins, our results reveal that aspartic acid-rich proteins play a key role in regulating biogenic struvite morphology, and aspartic acid residues contribute to the inhibitory capacity of urinary proteins. The potential implications of PASP for developing therapeutic agents for urinary stone disease is also discussed.

Microwave-assisted synthesis of flower-like β-FeSe microstructures

Flower-like tetragonal iron selenide (β-FeSe) microstructures assembled from nanoplates have been successfully prepared using Se powder and FeCl3·6H2O as elemental precursors in the basic 1,2-propylene glycol (1,2-PG) solvent system by microwave irradiation for 1 h. The product was characterized by means of X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX). The reaction mechanism and assembly process of the flower-like β-FeSe microstructures have been investigated and discussed, and a novel Se disproportionation-based reaction mechanism was proposed for the microwave-assisted synthesis of flower-like β-FeSe microstructures. In addition, ethylene glycol (EG) was also used as the reaction medium to investigate the influence of the different polyols on the size and structure of the product.

Preparation of Hollow Core/Shell Microspheres of Hematite and Its Adsorption Ability for Samarium

Hollow core/shell hematite microspheres with diameter of ca. 1-2 μm have been successfully achieved by calcining the precursor composite microspheres of pyrite and polyvinylpyrrolidone (PVP) in air. The synthesized products were characterized by a wide range of techniques including powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and Brunauer-Emmett-Teller (BET) gas sorptometry. Temperature- and time-dependent experiments unveil that the precursor pyrite-PVP composite microspheres finally transform into hollow core/shell hematite microspheres in air through a multistep process including the oxidation and sulfation of pyrite, combustion of PVP occluded in the precursor, desulfation, aggregation, and fusion of nanosized hematite as well as mass transportation from the interior to the exterior of the microspheres. The formation of the hollow core/shell microspheres dominantly depends on the calcination temperature under current experimental conditions, and the aggregation of hematite nanocrystals and the core shrinking during the oxidation of pyrite are responsible for the formation of the hollow structures. Moreover, the adsorption ability of the hematite for Sm(III) was also tested. The results exhibit that the hematite microspheres have good adsorption activity for trivalent samarium, and that its adsorption capacity strongly depends on the pH of the solution, and the maximum adsorption capacity for Sm(III) is 14.48 mg/g at neutral pH. As samarium is a typical member of the lanthanide series, our results suggest that the hollow hematite microspheres have potential application in removal of rare earth elements (REEs) entering the water environment.

Synthesis of siderite microspheres and their transformation to magnetite microspheres

Carbonates are among the most ubiquitous minerals in nature and are of significant interest in the geology and materials sciences communities, but studies concerning iron carbonate (i.e., siderite) mineralization processes and morphogenesis are less well developed. In this paper, we describe an environmentally benign biomolecule-assisted hydrothermal synthesis strategy to obtain siderite spherulites. Our results show that the formation of siderite microspheres is a successive multistep growth process involving a rod!peanut!dumbbell!sphere transition, which is driven by the intrinsic electric forces of siderite crystallites. Addition of the biomolecule ascorbic acid not only prevents Fe 2þ oxidation in the alkaline solution environment but also modifies the growth of siderite. Moreover, with increasing Fe 2þ concentrations, siderite nanoparticles or nano-sized triangular pyramids appear one after the other on the preformed spherical surfaces, resulting in different surface textures of the siderite microspheres. The surface morpho- logical modifications of the siderite microspheres can be attributed to the secondary nucleation and overgrowth of siderite nanocrystals. The siderite microspheres so obtained were then used as solid precursor minerals to form secondary magnetite through oxygen-limited thermolysis at 300 � C. The thermolysis experiments demonstrate the possibility that the size and morphology of secondary magnetite particles are influenced by the precursor siderite, suggesting that magnetite microcrystals with elongated or other uncommon morphologies in nature may potentially inherit the antecedent morphologies of Fe-carbonate-bearing minerals. Moreover, measurements of the magnetic properties of magnetite products after thermolysis show that the saturation magnetizations (Ms) of three kinds of magnetite microspheres are 52.5, 62.8 and 78.6 emu/g, which vary according to increasing Fe 2þ concentrations used in the synthesis of the precursor siderite.

Controlled morphogenesis of amorphous silica and its relevance to biosilicification

Abstract Biogenetic biosilica displays intricate patterns that are structured on a nanometer-to-micrometer scale. At the nanoscale, it involves the polymerization products of silica, apparently mediated by the interaction between different biomolecules with special functional groups. In this paper, using tetraethyl orthosilicate [TEOS, Si(OCH2CH3)4] as a silica source, phospholipid (PL) and dodecylamine (DA) were introduced as model organic additives to investigate their influence on the formation and morphology of silica in the mineralization process. Morphology, structure, and composition of the products were characterized using a range of techniques including FESEM, TEM, SAXRD, TG-DTA, solid-state 29Si NMR, FTIR, and nitrogen physisorption. The FESEM and TEM analyses demonstrate that increasing PL concentrations at constant DA content leads to the formation of siliceous elongated structures. Localized enlargement can also be observed during further growth of elongated structures, displaying some features of the earliest recognizable stage of valve development in diatoms. In addition, in the presence or absence of PL, a series of control experiments using ammonia instead of DA show that no elongated structures are obtained, suggesting that the formation of elongated silica structures results from the cooperative interactions between PL and DA molecules. Because both organic amines (e.g., long-chain polyamines, LCPA) and phospholipid membranes (e.g., silicalemma) are of special importance for biosilicification in diatoms and sponges, our results imply that phospholipids are involved in the formation of organic aggregates, and thus influence the amines-mediated silica deposition. As such, our results may provide a new insight into the mechanism of biosilicification

Formation of asymmetrical structured silica controlled by a phase separation process and implication for biosilicification.

Biogenetic silica displays intricate patterns assembling from nano- to microsize level and interesting non-spherical structures differentiating in specific directions. Several model systems have been proposed to explain the formation of biosilica nanostructures. Of them, phase separation based on the physicochemical properties of organic amines was considered to be responsible for the pattern formation of biosilica. In this paper, using tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4) as silica precursor, phospholipid (PL) and dodecylamine (DA) were introduced to initiate phase separation of organic components and influence silica precipitation. Morphology, structure and composition of the mineralized products were characterized using a range of techniques including field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), infrared spectra (IR), and nitrogen physisorption. The results demonstrate that the phase separation process of the organic components leads to the formation of asymmetrically non-spherical silica structures, and the aspect ratios of the asymmetrical structures can be well controlled by varying the concentration of PL and DA. On the basis of the time-dependent experiments, a tentative mechanism is also proposed to illustrate the asymmetrical morphogenesis. Therefore, our results imply that in addition to explaining the hierarchical porous nanopatterning of biosilica, the phase separation process may also be responsible for the growth differentiation of siliceous structures in specific directions. Because organic amine (e.g., long-chair polyamines), phospholipids (e.g., silicalemma) and the phase separation process are associated with the biosilicification of diatoms, our results may provide a new insight into the mechanism of biosilicification.

Bacterially mediated morphogenesis of struvite and its implication for phosphorus recovery

Abstract Bacterially mediated struvite usually crystallizes as unusual morphologies. To better understand the relationship between growth habit of struvite and bacterial activity in struvite biomineralization process, Shewanella oneidensis MR-1 was selected as a model microbe to induce struvite mineralizationin the synthetic sludge liquor. A combination of bacterial and biomimetic mineralization strategies was adopted. Different bacterial components were isolated from the cultures by a set of separation techniques, and used to influence struvite crystallization and growth. The identification and characterization of the mineralized products were done using XRD, FTIR, FESEM, TG-DTA, XPS, and elemental analysis. Bacterial mineralization experiments demonstrated that S. oneidensis MR-1 cannot only trigger mineralization and growth of struvite, but also mediate the specific morphogenesis of struvite. Biomimetic mineralization experiments revealed that different bacterial components had different effects on struvite morphology, and low molecular-weight peptides secreted by the bacteria played a dominant role. Current results can provide a deeper insight into bacterially mediated struvite morphogenesis, and be potentially applied to phosphorus and nitrogen recovery from various eutrophic wastewaters.