High‐Magnesium Calcite Mesocrystals: Formation in Aqueous Solution under Ambient Conditions

Mesocrystals of high‐magnesian calcites are commonly found in biogenic calcites. Under ambient conditions, it remains challenging to prepare mesocrystals of high‐magnesian calcite in aqueous solution. We report that mesocrystals of calcite with magnesium content of about 20 mol % can be obtained from the phase transformation of magnesian amorphous calcium carbonate (Mg‐ACC) in lipid solution. The limited water content on the Mg‐ACC surface would reduce the extent of the dissolution–reprecipitation process and bias the phase transformation pathway toward solid‐state reaction. We infer from the selected area electron diffraction patterns and the dark‐field transmission electron microscopic images that the formation of Mg‐calcite mesocrystals occurs through solid‐state secondary nucleation, for which the phase transformation is initiated near the mineral surface and the crystalline phase propagates gradually toward the interior part of the microspheres of Mg‐ACC.     

Structural development of mercaptophenol self-assembled monolayers and the overlying mineral phase during templated CaCO3 crystallization from a transient amorphous film

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.     

Stabilization of amorphous calcium carbonate in inorganic silica-rich environments

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.     

生物矿化研究中的化学模型

本文介绍了近年来生物矿化领域的研究情况,特别是从早期的有机-无机界面分子识别模型发展到介观组装和无定形前驱相转化模型的建立。与传统基于水溶液晶体生长的理解不同,人们发现在生物矿化过程中,在溶液中首先沉积的是胶体状的无定型前驱相,有机基质通过对无机相转化动力学及组装过程的控制实现矿物纳米晶体的有序组装,这为材料的仿生制备及生物医学应用提供了全新的策略。     

Two modes of transformation of amorphous calcium carbonate films in air

Large-area amorphous calcium carbonate (ACC) films in air are shown to be transformed into crystalline calcium carbonate (CaCO(3)) films via two modes-dissolution-recrystallization and solid-solid phase transition-depending on the relative humidity of the air and the temperature. Moisture in the air promotes the transformation of ACC into crystalline forms via a dissolution-recrystallization process. Increasing the humidity increases the rate of ACC crystallization and gives rise to films with numerous large pores. As the temperature is increased, the effect of moisture in the air is reduced and solid-solid transition by thermal activation becomes the dominant transformation mechanism. At 100 and 120 degrees C, ACC films are transformed into predominantly (110) oriented crystalline films. Collectively, the results show that calcium carbonate films with different morphologies, crystal phases, and structures can be obtained by controlling the humidity and temperature. This ability to control the transformation of ACC should assist in clarifying the role of ACC in the biomineralization of CaCO(3) and should open new avenues for preparing CaCO(3) films with oriented and fine structure.     

蚕腺体内和再生丝素蛋白水溶液的性能研究

用偏光显微镜观察了蚕腺体内和再生丝素蛋白水溶液的流动状态和各向异性现象,用乌氏粘度计测试了其流出时间,并用HAKKE流变仪测试了其粘度。结果发现蚕腺体内和再生丝素蛋白水溶液的性质差别非常大,认为丝素蛋白水溶液在蚕腺体内存在一个逐步熟成的过程。随着丝素蛋白水溶液在腺体内的前移,丝素蛋白分子逐渐沿移动方向取向而呈有序态,最终成为粘度非常大的各向异性的凝胶体,而再生丝素蛋白水溶液是粘度非常低的各向同性溶液。这些差异表明,丝素蛋白水溶液在蚕腺体内的熟成过程是蚕能够吐出优良蚕丝的关键步骤,要想制备出高性能的纤维,在“仿生纺丝”之前,首先要“仿生制备纺丝液”。     

Patterning inorganic (CaCO3) thin films via a polymer-induced liquid-precursor process

The biomimetic synthesis of patterned mineral thin films, based on a combination of the microcontact printing technique and a novel crystallization process called the polymer-induced liquid-precursor (PILP) process, is demonstrated. The PILP process enables the deposition of smooth and continuous calcitic mineral films (up to 1500 nm in thickness) under low-temperature and aqueous-based processing conditions. The films are formed by deposition of colloidal droplets composed of a liquid-phase mineral precursor that is induced by a polymeric process-directing agent (polyaspartate or polyacrylate salts). The droplets can be preferentially deposited onto patterned substrates templated with self-assembled monolayers (SAMs) of alkanethiolate on gold. The droplets coalesce to form an amorphous mineral film, which then transforms (solidifies and crystallizes) while retaining the shape of the patterned template, providing a means for patterning the location and morphology of two-dimensional calcite crystals. A vertical substrate experiment supports the premise that the calcite films are created by adsorption of colloidal droplets from solution, rather than heterogeneous nucleation and growth of an amorphous phase on the SAMs. Large single-crystalline domains, on the order of 50-100 microm, can be "molded" into nonequilibrium morphologies by constraining the mineral precursor to a chemically defined "compartment". Biominerals are well recognized for their elaborate nonequilibrium molded crystal morphologies, and increasing evidence suggests that many biominerals are formed from an amorphous precursor that is stabilized by polyanionic proteins. The biomimetic system examined here, which consists of a polyanionic process-directing agent in combination with a functionalized organic template, offers a practical tool for generating complex inorganic structures such as those found in biominerals.     

Plant cystoliths: a complex functional biocomposite of four distinct silica and amorphous calcium carbonate phases

Plant cystoliths are mineralized objects that are formed by specialized cells in the leaves of certain plants. The main mineral component of cystoliths by volume is amorphous calcium carbonate (ACC) and the minor component is silica. We show that the silica stalk is formed first and is essential for ACC formation. Furthermore, the cystolith is shown to be composed of four distinct mineral phases with different chemical properties: an almost pure silica phase grades into a Mg-rich silica phase. This Mg-rich silica is overlaid by a relatively stable ACC phase. A bulky and less stable ACC phase encapsulates the first ACC phase. This architecture poses interesting questions about the role of Mg in the silica phase and suggests a strategy for ACC stabilization that takes advantage of a precise regulation of the mineral-growth microenvironment.     

Calcium carbonate phase transformations during the carbonation reaction of calcium heavy alkylbenzene sulfonate overbased nanodetergents preparation

The preparation and application of overbased nanodetergents with excess alkaline calcium carbonate is a good example of nanotechnology in practice. The phase transformation of calcium carbonate is of extensive concern since CaCO(3) serves both as an important industrial filling material and as the most abundant biomineral in nature. Industrially valuable overbased nanodetergents have been prepared based on calcium salts of heavy alkylbenzene sulfonate by a one-step process under ambient pressure, the carbonation reaction has been monitored by the instantaneous temperature changes and total base number (TBN). A number of analytical techniques such as TGA, DLS, SLS, TEM, FTIR, and XRD have been utilized to explore the carbonation reaction process and phase transformation mechanism of calcium carbonate. An enhanced understanding on the phase transformation of calcium carbonate involved in calcium sulfonate nanodetergents has been achieved and it has been unambiguously demonstrated that amorphous calcium carbonate (ACC) transforms into the vaterite polymorph rather than calcite, which would be of crucial importance for the preparation and quality control of lubricant additives and greases. Our results also show that a certain amount of residual Ca(OH)(2) prevents the phase transformation from ACC to crystalline polymorphs. Moreover, a vaterite nanodetergent has been prepared for the first time with low viscosity, high base number, and uniform particle size, nevertheless a notable improvement on its thermal stability is required for potential applications.     

Properties of amorphous calcium carbonate and the template action of vaterite spheres

The fast mixing of aqueous solutions of calcium chloride and sodium carbonate could immediately result in amorphous calcium carbonate (ACC). Under vigorous stirring, the formed ACC in the precipitation system will dissolve first and, then, transform within minutes to produce crystalline forms of vaterite and calcite. After that, the solution-mediated mechanism dominates the transformation of the thermodynamically unstable vaterite into the thermodynamically stable calcite. Although ACC is the least stable form of the six anhydrous phases of calcium carbonate (CaCO(3)), it could be, however, produced and stabilized by a variety of organisms. To better understand the formation-transformation mechanism of ACC and vaterite into calcite, ex-situ methods (i.e., scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectroscopy) were used to characterize the formation-transformation process of ACC and vaterite in aqueous systems without organic additives, showing that ACC sampled at different conditions has different properties (i.e., lifetime, morphology, and spectrum characterization). It is also very interesting to capture the obviously polycrystalline particles of CaCO(3) during the transformation process from vaterite to calcite, which suggests the formation mechanism for the calcite superstructure with multidimensional morphology.     

A quasi-time-resolved CryoTEM study of the nucleation of CaCO3 under langmuir monolayers

Calcium carbonate biomineralization uses complex assemblies of macromolecules that control the nucleation, growth, and positioning of the mineral with great detail. To investigate the mechanisms involved in these processes, for many years Langmuir monolayers have been used as model systems. Here, we descibe the use of cryogenic transmission electron microscopy in combination with selected area electron diffraction as a quasi-time-resolved technique to study the very early stages of this process. In this way, we assess the evolution of morphology, polymorphic type, and crystallographic orientation of the calcium carbonate formed. For this, we used a self-assembled Langmuir monolayer of a valine-based bisureido surfactant (1) spread on a CaCl2-containing subphase and deposited on a holey carbon TEM grid. In a controlled environment, the grid is exposed to an atmosphere containing NH3 and CO2 (the (NH4)2CO3 diffusion method) for precisely determined periods of time (reaction times 30-1800 s) before it was plunged into melting ethane. This procedure allows us to observe amorphous calcium carbonate (ACC) particles growing from a few tens of nanometers to hundreds of nanometers and then crystallizing to form [00.1] oriented vaterite. The vaterite in turn transforms to yield [10.0] oriented calcite. We also performed the reaction in the absence of monolayer or in the presence of a nondirective monolayer of surfactant containing an oligo(ethylene oxide) 2 head group. Both experiments also showed the formation of a transient amorphous phase followed by a direct conversion into randomly oriented calcite crystals. These results imply the specific though temporary stabilization of the (00.1) vaterite by the monolayer. However, experiments performed at higher CaCl2 concentrations show the direct conversion of ACC into [10.0] oriented calcite. Moreover, prolonged exposure to the electron beam shows that this transformation can take place as a topotactic process. The formation of the (100) calcite as final product under different conditions shows that the surfactant is very effective in directing the formation of this crystal plane. In addition, we present evidence that more than one type of ACC is involved in the processes described.     

Capillarity creates single-crystal calcite nanowires from amorphous calcium carbonate

Single-crystal calcite nanowires are formed by crystallization of morphologically equivalent amorphous calcium carbonate (ACC) particles within the pores of track etch membranes. The polyaspartic acid stabilized ACC is drawn into the membrane pores by capillary action, and the single-crystal nature of the nanowires is attributed to the limited contact of the intramembrane ACC particle with the bulk solution. The reaction environment then supports transformation to a single-crystal product.     

Photoluminescence in the characterization and early detection of biomimetic bone-like apatite formation on the surface of alkaline-treated titanium implant: state of the art

Photoluminescence (PL) property is particularly important in the characterization of materials that contain significant proportions of noncrystalline components, multiple phases, or low concentrations of mineral phases. In this research, the ability of biomimetic bone-like apatite deposition on the surface of titanium alloy (Ti6Al4V) substrates in simulated body fluid (SBF) right after alkaline-treatment and subsequent heat-treatment was studied by the inherent luminescence properties of apatite. For this purpose, the metallic substrates were treated in 5 M NaOH solution at 60 °C. Subsequently, the substrates were heat-treated at 600 °C for 1 h for consolidation of the sodium titanate hydrogel layer. Then, they were soaked in SBF for different periods of time. Finally, the possibility to use of PL monitoring as an effective method and early detection tool is discussed. According to the obtained results, it was concluded that the PL emission peak did not have any significant shift to the shorter or higher wavelengths, and the PL intensity increased as the exposure time increased. This research proved that the observed inherent PL of the newly formed apatite coatings might be of specific interest for histological probing and bone remodelling monitoring.     

Evolution Mechanism of Calcium Carbonate in Solution

Calcium carbonate was synthesized in a CaCl₂/NaCO₃ mixed solution by using ethylenedi-aminetetraacetic acid (EDTA) as an additive. The thermodynamics and kinetics analyses indicate that although the driving force of amorphous calcium carbonate (ACC) precipi-tation is always less than that of calcite and vaterite precipitation, the nucleation rate of ACC is greater than that of calcite and vaterite at the initial stage of the precipitation reaction. With the increasing incubation time, vaterite and calcite particles nucleate het-erogeneously by using the as-formed particles as active sites. Scanning electron microscopyimages indicate that the transformation mechanism of ACC and vaterite to calcite is the dissolution-recrystallisation reaction. The presence of EDTA not only improves the stabil-ities of ACC and vaterite, but also leads to forming enlongated, connected rhombohedralcalcite crystals after incubation 7 days in solutions. The ACC and vaterite are stabler in air than in solutions at room temperature, although the dissolution-recrystallisation reaction occurs on the surface.     

Hydrolysis and biomineralization of porous PLA microspheres and their influence on cell growth

Poly(lactic acid) (PLA) microspheres have great potential in bone tissue engineering. However, their applications have been limited by surface and bulk properties such as hydrophobicity, lack of cell recognition sites and acidic degradation products. Apatite is a mineral which can effectively promote the adhesion and growth of bone cells. In this study, the bonelike mineral, carbonate apatite, was successfully used to functionalize porous PLA microspheres by a biomimetic mineralization method. To improve apatite formation, porous PLA microspheres were first selectively hydrolyzed in NaOH solution to increase the density of polar anionic groups on the surface, and then immersed in simulated body fluid for biomineralization. The morphology, composition, and phase structure of bioactive mineral grown on the original and hydrolyzed PLA microspheres were analyzed and compared quantitatively. The results showed that the hydrolysis which took place on the PLA microspheres enhanced the nucleation and growth of apatite. MG-63 cells attached well and spread actively on the mineralized PLA microspheres, indicating their strong potential in bone tissue engineering.     

Bioinspired mineralization of inorganics from aqueous media controlled by synthetic polymers

The formation of inorganic structures in nature is commonly controlled by biogenic macromolecules. The understanding of mineralization phenomena and the nucleation and growth mechanisms involved is still a challenge in science but also of great industrial interest. This article focuses on the formation and mineralization of two archetypical inorganic materials: zinc oxide and amorphous calcium carbonate (ACC). Zinc oxide is selected as a model compound to investigate the role that polymers play in mineralization. Most of the effort has been devoted to the investigation of the effects of double-hydrophilic block and graft copolymers. Recent work has demonstrated that latex particles synthesized by miniemulsion polymerization, properly functionalized by various chemical groups, have similar effects to conventional block copolymers and are excellently suited for morphology control of ZnO crystals. Latex particles might serve as analogues of natural proteins in biomineralization. The second example presented, ACC, addresses the issue of whether this amorphous phase is an intermediate in the biomineralization of calcite, vaterite, or aragonite. Conditions under which amorphous calcium carbonate can be obtained as nanometer-sized spheres as a consequence of a liquid-liquid phase segregation are presented. Addition of specific block copolymers allows control of the particle size from the micrometer to the submicrometer length scale. The physical properties of novel materials synthesized from concentrated solution and their potential applications as a filler of polymers are also discussed.     

Effect of solutes on the viscosity of supercritical solutions

This study shows that solutes can impart significant structure to supercritical solutions, resulting in unexpectedly high solution viscosity at pressures close to the critical value. The viscosity passes through a minimum as the pressure is increased, and this is accounted for by a solvation of the solutes leading to a decrease in solute-solute interactions. At high pressure, the solution viscosity is similar to that of the pure solvent as solvent-solvent interactions dominate. The increase in relative viscosity is modeled using a modified Dole-Jones equation, and it is shown that the change in relative viscosity is related to the volume fraction occupied by the solute. A general model is presented for simple solutes whereby the viscosity of a supercritical solution can be calculated from the molecular volume of the solute and the viscosity of the pure fluid. The higher than expected viscosity observed at low pressures is used to explain the variation of reaction rate constant with pressure.     

Factors involved in the formation of amorphous and crystalline calcium carbonate: a study of an ascidian skeleton.

The majority of invertebrate skeletal tissues are composed of the most stable crystalline polymorphs of CaCO(3), calcite, and/or aragonite. Here we describe a composite skeletal tissue from an ascidian in which amorphous and crystalline calcium carbonate coexist in well-defined domains separated by an organic sheath. Each biogenic mineral phase has a characteristic Mg content (5.9 and 1.7 mol %, respectively) and concentration of intramineral proteins (0.05 and 0.01 wt %, respectively). Macromolecular extracts from various biogenic amorphous calcium carbonate (ACC) skeletons are typically glycoproteins, rich in glutamic acid and hydroxyamino acids. The proteins from the crystalline calcitic phases are aspartate-rich. Macromolecules extracted from biogenic ACC induced the formation of stabilized ACC and/or inhibited crystallization of calcite in vitro. The yield of the synthetic ACC was 15-20%. The presence of Mg facilitated the stabilization of ACC: the protein content in synthetic ACC was 0.12 wt % in the absence of Mg and 0.07 wt % in the presence of Mg (the Mg content in the precipitate was 8.5 mol %). In contrast, the macromolecules extracted from the calcitic layer induced the formation of calcite crystals with modified morphology similar to that expressed by the original biogenic calcite. We suggest that specialized macromolecules and magnesium ions may cooperate in the stabilization of intrinsically unstable amorphous calcium carbonate and in the formation of complex ACC/calcite tissues in vivo.     

A carbonate controlled-addition method for amorphous calcium carbonate spheres stabilized by poly(acrylic acid)s

Stable amorphous calcium carbonate (ACC) composite particle with a size-controlled monodispersed sphere was obtained by a new simple carbonate controlled-addition method by using poly(acrylic acid) (PAA) (Mw = 5000), in which an aqueous ammonium carbonate solution was added into an aqueous solution of PAA and CaCl2 with a different time period. The obtained ACC composite products consist of about 50 wt % of ACC, 30 wt % of PAA, and H2O. Average particle sizes of the ACC spheres increased from (1.8 +/- 0.4) x 102 to (5.5 +/- 1.2) x 102 nm with an increase of the complexation time of the PAA-CaCl2 solution from 3 min to 24 h, respectively. The ACC formed from the complexation time for 3 min was stable for 10 days with gentle stirring as well as 3 months under a quiescent condition in the aqueous solution. Moreover, the ACC was also stable at 400 degrees C. Stability of the amorphous phase decreased with an increase of the complexation time of the PAA-CaCl2 solution. No ACC was obtained when the lower molar mass PAAs (Mw = 1200 and 2100) were used. In the higher molar mass case (Mw = 25 000), a mixture of the amorphous phase and vaterite and calcite crystalline product was produced. The present results demonstrate that the interaction and the reaction kinetics of the PAA-Ca2+-H2O complex play an important role in the mineralization of CaCO3.     

The photophysical properties of a julolidene-based molecular rotor

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.