Transformation Of Amorphous Calcium Carbonate Research Articles

The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate

The effects of pH and Mg on the crystallization of amorphous calcium carbonate (ACC) to vaterite and/or calcite were studied using a combination of in situ time resolved synchrotron-based techniques and electron microscopy. The experiments showed that Mg increased the stability of ACC and favoured the formation of calcite over vaterite. A neutral (∼7) starting pH during mixing promoted the transformation of ACC into calcite via a dissolution/reprecipitation mechanism. Conversely, when ACC formed in a solution that started with a high initial pH (∼11.5), the transformation to calcite occurred via metastable vaterite, which formed via a spherulitic growth mechanism. In a second stage this vaterite transformed to calcite via a surface-controlled dissolution and recrystallization mechanism. These crystallization pathways can be explained as a consequence of the pH-dependent composition, local structure, stability and dissolution rates of ACC.

Electrochemically assisted deposition on TiO2scaffold for Tissue Engineering: an apatite bio-inspired crystallization pathway

The interactions among inorganic materials and biological environments occurring at the nanoscale level are crucial to understand the mechanisms behind the bioactivity of biomaterials and for their better design. In the present study, we prepare TiO2 and TiO2/hydroxyapatite (HA) scaffolds for Tissue Engineering (TE) via a sol–gel/polymeric sponge method. The obtained architectures have a biomimetic morphology (trabecular bone) and an average pore dimension of 100 µm. Moreover, we introduce a versatile electrochemically assisted deposition method for non-conductive substrates that allows the formation of a stable coating on the pore walls of the scaffolds with the morphology and phase composition determined by SEM and DRIFTS. Both the materials properties are strictly correlated to the scaffold composition: pure TiO2 scaffolds are coated with amorphous calcium carbonate while onto TiO2/HA scaffolds we observe the growth of octacalcium phosphate nanocrystals. The bioactivity of the TiO2-based scaffolds is then predicted examining the apatite formation on their surface in simulated body fluid. The presence of apatite crystals is observed only for pure TiO2 scaffolds due to the transformation of amorphous calcium carbonate into crystalline biomimetic HA: a novel bio-inspired crystallization pathway toward the bioactivity of scaffold for TE.

Influence of viscosity on the phase transformation of amorphous calcium carbonate in fluids: An understanding of the medium effect in biomimetic mineralization

Biological mineral generation via an amorphous precursor is a topic of great current interest. Various factors such as the temperature, solution composition and presence of organic molecules can influence this important inorganic process. Here we demonstrate that this mineral transformation can actually readily be regulated by solution viscosity, a fundamental but often overlooked property. In our experiment, amorphous calcium carbonate (ACC), a key model compound in biomimetic mineralization studies, is synthesized and dispersed into inert dispersants with different viscosities and the crystallization process is examined by using FT-IR spectroscopy and XRD. It is found that the inhibition of the transformation of ACC becomes more significant with increasing fluid viscosity. This phenomenon can be explained by the differences in ion diffusion in different media. Furthermore, the resulting crystals always have different morphologies and size distributions although they all have the calcite structure. This study implies that the importance of the fluid medium cannot be ignored in building a complete understanding of biological control of biomimetic crystallizations.

The roles of water and polyelectrolytes in the phase transformation of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase of calcium carbonates and it is a precursor during the biomineralization of calcium carbonate in nature. Different ACC in the absence and presence of the additives of poly(sodium 4-styrene sulfonate) and poly(acrylic acid) were prepared and their transformation processes under controls of water were investigated. It was found that ACC powders generally contained about 15% water. Two different transformations via thermal and solution pathways were studied in the present work. We found that water was released from the ACC samples when the temperature was above 100 °C and the calcium carbonate began to crystallize at around 270–400 °C, which resulted the calcite crystals. The involvement of the polymer additives could inhibit this crystallization process and the crystallization temperature shifted to the higher values. During the solution transformation, the evolution steps of the ACC in different water–ethanol solution were monitored by FT-IR. In this case, water could accelerate the transformation and crystallization of ACC. An increasing of the water amount in the mixed solvents always led to the promotion of the transformation kinetics. Interestingly, the other metastable crystal phases of calcium carbonates, vaterite and aragonite, were readily to be induced during the solution transformation of ACC when the low water amounts were applied. However, water could still make them turned into the stable calcite phase with the experimental time prolonged. Since the additives influenced the transformation of ACC and inhibited the transformation of metastable crystal phase, they could be used to stabilize different phases of calcium carbonates in the solutions. The current experimental results confirmed that the controls of water amount and additives played important roles in the transformation of ACC.

Formation of Multilayered Vaterite via Phase Separation, Crystalline Transformation, and Self-Assembly of Nanoparticles at the Air/Water Interface

Flowerlike CaCO3 particles with multilayered superstructure were obtained at the air/water interface without any additives by the so-called vapor diffusion method. The X-ray and Fourier transform infrared analyses reveal that the flakes composing the flowerlike particles are vaterite crystals. The time-dependent shape evolution process at the air/water interface was followed by careful examination of the intermediate with scanning electron microscopy and transmission electron microscopy. The analysis results suggest that this process involves the liquid−liquid phase separation, transformation of amorphous calcium carbonate to vaterite and self-assembly of vaterite nanopatricles.

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.

Polymorphic Change of Calcium Carbonate during Reaction Crystallization in a Batch Reactor

In the reaction crystallization of calcium carbonate using a batch reactor system with aqueous CaCl2 and Na2CO3 at room temperature, the selective polymorphic control of crystalline calcium carbonate into calcite was achieved by adjusting the operating conditions. The polymorphic ratio of calcite and vaterite nucleated from amorphous calcium carbonate (ACC) was significantly influenced by the initial supersaturation and solution pH, thereby resulting in a different phase transformation rate for the product powder from unstable vaterite to stable calcite. With a high supersaturation and low solution pH, the phase fraction of calcite decreased owing to the favorable transformation of ACC into vaterite during the early stage of the crystallization. Thereafter, the phase fraction of calcite increased as a result of the transformation of vaterite into calcite as the aging progressed. However, with a low supersaturation and high solution pH, calcite was exclusively observed throughout the reaction time without ...

Formation of Amorphous Calcium Carbonate Thin Films and Their Role in Biomineralization

A new and simple method for preparing large-area and continuous calcium carbonate films under mild conditions is described. Amorphous calcium carbonate (ACC) films have been formed both in the presence and absence of a poly(acrylic acid) inhibitor. The transformation from ACC to crystalline vaterite/calcite has been observed by optical microscopy and confirmed by external reflection infrared spectroscopy. We have shown that the inhibiting effects of substrates and inhibitors on the transformation of ACC result in the formation of good CaCO3 films. From our results, we suggested that ACC precipitates are initially formed from highly supersaturated solutions, which then deposit as films through the cooperation between an insoluble matrix and a soluble inhibitor. The matrix and inhibitor were also found to affect the growth, morphology, and structure of CaCO3 crystal by influencing the phase transformation of ACC into crystalline forms. It has been shown that ACC plays an important role in the biomineralization and crystallization of calcium carbonate.

Taking Advantage of Disorder: Amorphous Calcium Carbonate and Its Roles in Biomineralization

AbstractAmorphous calcium carbonate (ACC) in its pure form is highly unstable, yet some organisms produce stable ACC, and cases are known in which ACC functions as a transient precursor of more stable crystalline aragonite or calcite. Studies of biogenic ACC show that there are significant structural differences, including the observation that the stable forms are hydrated whereas the transient forms are not. The many different ways in which ACC can be formed in vitro shed light on the possible mechanisms involved in stabilization, destabilization, and transformation of ACC into crystalline forms of calcium carbonate. We show here that ACC is a fascinating form of calcium carbonate that may well be of much interest to materials science and biomineralization.

The role of magnesium in stabilising amorphous calcium carbonate and controlling calcite morphologies

Calcium carbonate has been precipitated from aqueous solutions containing magnesium under conditions of high supersaturation. The precipitates were analysed over time using X-ray diffraction (XRD) and Infrared Spectroscopy (IR), and it was demonstrated that in all cases amorphous calcium carbonate (ACC) was the first phase formed. The magnesium content of the ACC was defined by the Mg:Ca ratio in the solution, and increased systematically with increasing Mg:Ca. In turn, ACC occluding higher magnesium concentrations was significantly more stable than low magnesium ACC. The crystalline phase produced on transformation of ACC depended on the Mg:Ca ratio, and ACC precipitated from solutions containing higher Mg concentrations crystallised to yield high-Mg calcites (occluding up to 47 mol% Mg), together with CaCO 3.H 2O and/or CaCO 3.6H 2O. The magnesium content of the ACC phase also directly affected the final morphology of the precipitates, such that continuous sheets of CaCO 3 were produced at the air/solution interface at solution ratios of 3:1 Mg:Ca and above. Previously, this effect has only been observed for ACC stabilised with organic additives. These results have relevance to the mechanism of formation of biological magnesian calcites, and suggest that the stabilisation of ACC by magnesium may provide organisms with a mechanism for controlling crystal morphologies.

An X-ray absorption spectroscopy study of the structure and transformation of amorphous calcium carbonate from plant cystoliths

Cystoliths are intracellular calcified bodies which are found in great numbers in the leaves of many higher plants such as Ficus retusa. The mineral part of these deposits is amorphous calcium carbonate, which transforms to calcite only when moistened. We have followed this transformation by using X-ray spectroscopy by measuring the local atomic structure around the calcium of the isolated dry cystoliths and after moistening them with water. The production and maintenance of the amorphous phase is clearly under biological control. The cystoliths may act as a pH-stat which neutralizes hydroxide ions. Potentially cytotoxic cations also accumulate in the cystoliths. Rapid precipitation of calcium carbonate into the organic matrix could favour the amorphous phase, which may be maintained by low concentrations of magnesium and phosphate, which are inhibitors of the nucleation of crystalline phases.