Amorphous CaCO3 Research Articles

Tailoring Calcium Carbonate to Serve as Optical Functional Material: Examples from Biology and Materials Science

In nature and technology there are many examples of micron sized optical systems based on minerals. The thermodynamically most stable polymorph of calcium carbonate (CaCO3) –calcite– exhibits birefringence. However, nature uses biomineralization approaches to produce CaCO3 based optical functional materials, for example calcite mircrolens arrays as part of the photoreceptor system of brittlestars, amorphous CaCO3 (ACC) plant cystoliths as light scatterers and calcite trilobite eyes. In this short review, we illustrate basic strategies to produce optical materials from CaCO3 by manipulating the material structure. These strategies are driven by the aim to eliminate or reduce the birefringent properties of calcite, or even, to turn this property into an advantage. We report on the formation, structure, and functions of micron sized (hemi‐)spherical objects made from CaCO3 with biological origin but also in bio‐inspired synthetic examples. We highlight aspects, which pave the way to learn from nature and how to characterize natural and artificial systems. In conclusion, we identify three main possibilities to make optical materials based on CaCO3: (i) orienting the optical axis along the desired light propagation direction, (ii) stabilizing metastable phases, and (iii) producing nanocrystalline structures, which reduce birefringent properties.

Hydrogels from Amorphous Calcium Carbonate and Polyacrylic Acid: Bio-Inspired Materials for "Mineral Plastics".

Given increasing environmental issues due to the large usage of non-biodegradable plastics based on petroleum, new plastic materials, which are economic, environmentally friendly, and recyclable are in high demand. One feasible strategy is the bio-inspired synthesis of mineral-based hybrid materials. Herein we report a facile route for an amorphous CaCO3 (ACC)-based hydrogel consisting of very small ACC nanoparticles physically cross-linked by poly(acrylic acid). The hydrogel is shapeable, stretchable, and self-healable. Upon drying, the hydrogel forms free-standing, rigid, and transparent objects with remarkable mechanical performance. By swelling in water, the material can completely recover the initial hydrogel state. As a matrix, thermochromism can also be easily introduced. The present hybrid hydrogel may represent a new class of plastic materials, the "mineral plastics".

Bio-inspired in situ crosslinking and mineralization of electrospun collagen scaffolds for bone tissue engineering.

Bone disorders are the most common cause of severe long term pain and physical disability, and affect millions of people around the world. In the present study, we report bio-inspired preparation of bone-like composite structures by electrospinning of collagen containing catecholamines and Ca(2+). The presence of divalent cation induces simultaneous partial oxidative polymerization of catecholamines and crosslinking of collagen nanofibers, thus producing mats that are mechanically robust and confer photoluminescence properties. Subsequent mineralization of the mats by ammonium carbonate leads to complete oxidative polymerization of catecholamines and precipitation of amorphous CaCO3. The collagen composite scaffolds display outstanding mechanical properties with Young's modulus approaching the limits of cancellous bone. Biological studies demonstrate that human fetal osteoblasts seeded on to the composite scaffolds display enhanced cell adhesion, penetration, proliferation, differentiation and osteogenic expression of osteocalcin, osteopontin and bone matrix protein when compared to pristine collagen or tissue culture plates. Among the two catecholamines, mats containing norepinephrine displayed superior mechanical, photoluminescence and biological properties than mats loaded with dopamine. These smart multifunctional scaffolds could potentially be utilized to repair and regenerate bone defects and injuries.

Water as the Key to Proto-Aragonite Amorphous CaCO3.

Temperature and pH value can affect the short-range order of proto-structured and additive-free amorphous calcium carbonates (ACCs). Whereas a distinct change occurs in proto-vaterite (pv) ACC above 45 °C at pH 9.80, proto-calcite (pc) ACC (pH 8.75) is unaffected within the investigated range of temperatures (7-65 °C). IR and NMR spectroscopic studies together with EXAFS analysis showed that the temperature-induced change is related to the formation of proto-aragonite (pa) ACC. The data strongly suggest that the binding of water molecules induces dipole moments across the carbonate ions in pa-ACC as in aragonite, where the dipole moments are due to the symmetry of the crystal structure. Altogether, a (pseudo-)phase diagram of the CaCO3 polyamorphism in which water plays a key role can be formulated based on variables of state, such as the temperature, and solution parameters, such as the pH value.

Enhancing the mechanical properties of electrospun chitosan/poly(vinyl alcohol) fibers by mineralization with calcium carbonate

Mineralization of electrospun chitosan/poly(vinyl alcohol) (PVA) with CaCO3 was performed as a reinforcement strategy to produce a biocompatible material with good mechanical properties. Disk-like layered and cubic CaCO3 particles were formed when the composites were dried at low and high concentrations of CO2, respectively, followed by a transformation of the amorphous CaCO3 into calcite. The highest ultimate stress value was found to be 44 ± 3.2 MPa for the composite with 17 wt% of CaCO3, which was crosslinked by glutaraldehyde and dried at high concentrations of CO2. The highest value of the Young’s modulus was 2328.35 ± 204.9 MPa for the composite with 34 wt% of CaCO3 that was dried at high concentrations of CO2. The bioactivity of the obtained composites was tested by 6-day incubation in simulated body fluid (SBF), resulting in flake-like apatite structures grown on the composite surface. The composites retained around 50 % of their ultimate stress value after immersion in SBF. The mineralization of electrospun chitosan/PVA with CaCO3 produced composites with enhanced mechanical properties, which can be potentially used as scaffolds in tissue engineering applications.

Enzyme-assisted growth of nacreous CaCO3/polymer hybrid nanolaminates via the formation of mineral bridges

Laminated nanostructures in nacre have been adopted as models in the fabrication of strong, tough synthetic nanocomposites. However, the utilization of CaCO3 biominerals in these composites is limited by the complexity of the synthesis method for nanosized biominerals. In this study, we use the enzymatic reaction of urease to generate a nanoscale CaCO3 thin film to prepare CaCO3/polymer hybrid nanolaminates. Additional layers of CaCO3 thin film are consecutively grown over the base CaCO3 layer with the intercalation of organic layers. The morphology and crystallinity of the added CaCO3 layers depend strongly on the thickness of the organic layer coated on the underlying CaCO3 layer. When the organic layer is less than 20nm thick, the amorphous CaCO3 layer is spontaneously transformed into crystalline calcite layer during the growth process. We also observe crystalline continuity between adjacent CaCO3 layers through interconnecting mineral bridges. The formation of these mineral bridges is crucial to the epitaxial growth of CaCO3 layers, similar to the formation of natural nacre.

Aqueous Emulsion Droplets Stabilized by Lipid Vesicles as Microcompartments for Biomimetic Mineralization.

Mineral deposition within living cells relies on control over the distribution and availability of precursors as well as the location and rates of nucleation and growth. This control is provided in large part by biomolecular chelators, which bind precursors and regulate their availability, and compartmentalization within specialized mineralizing vesicles. Biomimetic mineralization in self-assembled lipid vesicles is an attractive means of studying the mineralization process, but has proven challenging due to vesicle heterogeneity in lamellarity, contents, and size across a population, difficulties encapsulating high and uniform precursor concentrations, and the need to transport reagents across an intact lipid bilayer membrane. Here, we report the use of liposome-stabilized all-aqueous emulsion droplets as simple artificial mineralizing vesicles (AMVs). These biomimetic microreactors allow the entry of precursors while retaining a protein catalyst by equilibrium partitioning between internal and external polymer-rich phases. Small molecule chelators with intermediate binding affinity were employed to control Ca(2+) availability during CaCO3 mineralization, providing protection against liposome aggregation while allowing CaCO3 formation. Mineral deposition was limited to the AMV interior, due to localized production of CO3(2-) by compartmentalized urease. Particle formation was uniform across the entire population of AMVs, with multiple submicrometer amorphous CaCO3 particles produced in each one. The all-aqueous emulsion-based approach to biomimetic giant mineral deposition vesicles introduced here should be adaptable for enzyme-catalyzed synthesis of a wide variety of materials, by varying the metal ion, enzyme, and/or chelator.

Chitin: Formation of Helically Structured Chitin/CaCO3 Hybrids through an Approach Inspired by the Biomineralization Processes of Crustacean Cuticles (Small 38/2015).

Biomineral-inspired hybrids forming helically ordered structures are developed by T. Kato and co-workers on page 5127. These helical hybrids consist of liquid-crystalline chitin and CaCO3 . They resemble the structures of crustacean cuticles such as the exoskeleton of a lobster or crayfish. These hybrids are formed through CaCO3 crystallization on the liquidcrystalline chitin templates. Polymer-stabilized amorphous CaCO3 is incorporated into the liquid-crystalline chitin templates. This approach is useful for the development of new hierarchical hybrid materials from abundant natural resources.

Application of Ca-doped mesoporous silica to well-grouting cement for enhancement of self-healing capacity

Carbonation has been considered as an alternative to prevent CO2 leakage. In this study, calcium-doped mesoporous silica (CDMS) was introduced into Portland cement as a promoter of carbonation. An admixture of CDMS and American Petroleum Institute (API) Class G Portland cement exposed to CO2-saturated water was analyzed under geologic sequestration conditions (40°C and 80MPa) to assess the carbonation properties and self-healing effect of CDMS for CO2 storage. The capacity of CDMS to synthesize CaCO3 from CO2via carbonation was identified in an in vitro crystallization test. Analysis of the cut surface of a cement core showed the rapid synthesis of CaCO3 including calcite and aragonite. Rietveld analysis was employed for quantitative phase analysis. The quantitative analysis of the cement carbonation showed that tricalcium silicate (C3S) and dicalcium silicate (C2S) play important roles in cement carbonation. Several crystal phases of CaCO3 were identified in this study including calcite, aragonite and amorphous CaCO3. X-ray diffractometry (XRD), a field emission-scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray spectrometer (EDS), 29Si MAS-NMR (magic angle spinning–nuclear magnetic resonance) spectrometry, thermogravimetry-differential thermal analysis (TG-DTA) and FT-IR (Fourier transform infrared spectroscopy) were applied to characterize the admixture.

Formation of Helically Structured Chitin/CaCO3 Hybrids through an Approach Inspired by the Biomineralization Processes of Crustacean Cuticles.

Chitin/CaCO3 hybrids with helical structures are formed through a biomineralization-inspired crystallization process under ambient conditions. Liquid-crystalline chitin whiskers are used as helically ordered templates. The liquid-crystalline structures are stabilized by acidic polymer networks which interact with the chitin templates. The crystallization of CaCO3 is conducted by soaking the templates in the colloidal suspension of amorphous CaCO3 (ACC) at room temperature. At the initial stage of crystallization, ACC particles are introduced inside the templates, and they crystallize to CaCO3 nanocrystals. The acidic polymer networks induce CaCO3 crystallization. The characterization of the resultant hybrids reveals that they possess helical order and homogeneous hybrid structures of chitin and CaCO3 , which resemble the structure and composition of the exoskeleton of crustaceans.

Transformation of amorphous calcium carbonate to rod-like single crystal calcite via “copying” collagen template

Collagen Langmuir films were prepared by spreading the solution of collagen over deionized water, CaCl2 solution and Ca(HCO3)2 solution. Resultant collagen Langmuir monolayers were then compressed to a lateral pressure of 10 mN/m and held there for different duration, allowing the crystallization of CaCO3. The effect of crystallization time on the phase composition and microstructure of CaCO3 was investigated. It was found that amorphous calcium carbonate (ACC) was obtained at a crystallization time of 6 h. The amorphous CaCO3 was transformed to rod-like single crystal calcite crystals at an extended crystallization time of 12 h and 24 h, via "copying" the symmetry and dimensionalities of collagen fibers. Resultant calcite crystallites were well oriented along the longitudinal axis of collagen fibers. The ordered surface structure of collagen fibers and electrostatic interactions played key roles in tuning the oriented nucleation and growth of the calcite crystallites. The mineralized collagen possessing both desired mechanical properties of collagen fiber and good biocompatibility of calcium carbonate may be assembled into an ideal biomaterial for bone implants.

A sea urchin Na+K+2Cl− cotransporter is involved in the maintenance of calcification-relevant cytoplasmic cords in Strongylocentrotus droebachiensis larvae

The cellular mechanisms of calcification in sea urchin larvae are still not well understood. Primary mesenchyme cells within the larval body cavity form a syncytium to secrete CaCO3 spicules from intracellular amorphous CaCO3 (ACC) stores. We studied the role of Na+K+2Cl− cotransporter (NKCC) in intracellular ACC accumulation and larval spicule formation of Strongylocentrotus droebachiensis. First, we incubated growing larvae with three different loop diuretics (azosemide, bumetanide, and furosemide) and established concentration-response curves. All loop diuretics were able to inhibit calcification already at concentrations that specifically inhibit NKCC. Calcification was most effectively inhibited by azosemide (IC50=6.5μM), while larval mortality and swimming ability were not negatively impacted by the treatment. The inhibition by bumetanide (IC50=26.4μM) and furosemide (IC50=315.4μM) resembled the pharmacological fingerprint of the mammalian NKCC1 isoform. We further examined the effect of azosemide on the maintenance of cytoplasmic cords and on the occurrence of calcification vesicles using fluorescent dyes (calcein, FM1-43). Fifty micromolars of azosemide inhibited the maintenance of cytoplasmic cords and resulted in increased calcein fluorescence within calcification vesicles. The expression of NKCC in S. droebachiensis was verified by PCR and Western blot with a specific NKCC antibody. In summary, the pharmacological profile of loop diuretics and their specific effects on calcification in sea urchin larvae suggest that they act by inhibition of NKCC via repression of cytoplasmic cord formation and maintenance.

Nonenzymatic Transformation of Amorphous CaCO3 into Calcium Phosphate Mineral after Exposure to Sodium Phosphate in Vitro: Implications for in Vivo Hydroxyapatite Bone Formation.

Studies indicate that mammalian bone formation is initiated at calcium carbonate bioseeds, a process that is driven enzymatically by carbonic anhydrase (CA). We show that amorphous calcium carbonate (ACC) and bicarbonate (HCO3 (-) ) cause induction of expression of the CA in human osteogenic SaOS-2 cells. The mineral deposits formed on the surface of the cells are rich in C, Ca and P. FTIR analysis revealed that ACC, vaterite, and aragonite, after exposure to phosphate, undergo transformation into calcium phosphate. This exchange was not seen for calcite. The changes to ACC, vaterite, and aragonite depended on the concentration of phosphate. The rate of incorporation of phosphate into ACC, vaterite, and aragonite, is significantly accelerated in the presence of a peptide rich in aspartic acid and glutamic acid. We propose that the initial CaCO3 bioseed formation is driven by CA, and that the subsequent conversion to calcium phosphate/calcium hydroxyapatite (exchange of carbonate by phosphate) is a non-enzymatic exchange process.

Tracking the transformations of mesoporous microspheres of calcium silicate hydrate at the nanoscale upon ibuprofen release: a XANES and STXM study

XANES and STXM imaging of mesoporous CSH microspheres indicate that amorphous silica provide aggregation site of HAp, and amorphous CaCO3 was formed during CSH biomineralization.

Liquid-crystalline calcium carbonate: biomimetic synthesis and alignment of nanorod calcite.

Liquid-crystalline CaCO3 has been prepared for the first time. The nanorods of CaCO3 calcite are obtained by bio-inspired crystallization through aqueous colloidal precursors of amorphous CaCO3 stabilized by poly(acrylic acid). The synthesized calcite nanocrystals have well-tuned morphologies that are preferable for formation of liquid-crystalline phases in concentrated aqueous colloidal solution. The one-dimensional alignment of calcite crystals is achieved by mechanical shearing of the aqueous colloidal solution showing liquid-crystalline phases. These CaCO3-based liquid crystals formed by a self-organization process in mild conditions may have great potential for use as environmentally friendly materials.

Multiphase calcification associated with the atmophytic cyanobacterium Scytonema julianum

Scytonema julianum, which is an atmophytic cyanobacterium that lives in small clusters in shaded vadose settings throughout the world, is prone to rapid calcification. Specimens found on cavity walls in an inactive spring tower located in Shiqiang (Stone Wall) in China and cavity walls in a breccia that fills sinkholes in dolostones of the Cayman Formation (Miocene) on Grand Cayman are morphologically identical. The microbes (4–11μm diameter) are encased with thick, well-developed calcified sheaths with external diameters of 11 to 25μm, which developed through the sequential precipitation of amorphous CaCO3 (ACC), acicular calcite crystals, triradiate calcite crystals, and dendrite calcite crystals. The paragenetic relationship of these precipitates to the skeletal rhombic crystals that cover some specimens is unknown. Precipitation probably took place in the extracellular polymeric substances (EPS) that covered the microbes when they were alive. Critically, the type of crystal evident on the surface of the sheath depends on the thickness of the calcified sheath, which is in accord with the sequential development of the different crystal forms. Available evidence indicates that precipitation was largely “microbially influenced” rather than “environment influenced” and also demonstrates that crystals commonly morphed from one crystal form into another as precipitation progressed. There is, for example, clear evidence that the dendrite crystals developed from the triradiate crystals. S. julianum can play a significant role in the development of microbialites in vadose settings by acting as nuclei for CaCO3 precipitation.

Entrapment and Removal of Carbon Nanotubes and Fullerenes by Coprecipitation with Calcium Carbonate Beads

Production, growth, and obvious health and environmental concerns about engineered nanomaterials (ENM) require development of methods for their entrapping/removal. Here, we propose a facile method for removal of carbon nanomaterials from water solutions based on coprecipitation with vaterite (CaCO3) beads. CaCO3 beads are formed by an aggregation of initially formed amorphous CaCO3 nanoparticles that efficiently incorporate nanomaterials in solution into the growing beads so that can be finally removed by settling. We show that by using this approach, a high percentage of fullerenes and carbon nanotubes (typically over 95%) can be removed in a broad range of ENM concentrations. We also discuss potential applications of the method for treatment of contaminated water.

Nanocrystalline calcitic lens arrays fabricated by self-assembly followed by amorphous-to-crystalline phase transformation.

Natural calcium carbonate-based nanocomposites often have superior physical properties and provide a comprehensive source for bioinspired synthetic materials. Here we present thermodynamically stable, transparent CaCO3 microlens arrays (MLA) produced by transforming an amorphous CaCO3 phase into nanocrystalline calcite. We analyze the structure and properties of crystallized MLA by X-ray scattering, transmitted and polarized light microscopy, and electron microscopy and find that MLA are crystallized in spherulite-like patterns without changing the shape of the microlens. The key finding is that nanocrystallinity of the calcite formed diminishes structural anisotropy on the wavelength scale and results in greatly reduced birefringent effects. The remnant preferred orientation of the optical axes of calcite crystals in the plane of the microlens arrays leads to some directionality of optical properties, which may be beneficial for technical applications.

Fabrication and Characterization of Sulfonate‐containing Polystyrene/CaCO3 Core‐shell Nanoparticles

AbstractPolystyrene (PSt) seed latex was first prepared via soap‐free emulsion polymerization in the presence of a small amount of methacrylic acid using ammonium persulfate as initiator, and then seeded emulsion polymerization of sodium 4‐styrenesulfonate (NaSS) and St was carried out to synthesize P(St‐NaSS) core latex using 2,2′‐azobisisobutyronitrile as initiator. After that, P(St‐NaSS)/CaCO3 core‐shell nanoparticles were fabricated by sequentially introducing Ca(OH)2 aqueous solution and CO2 gas into the core latex. The morphology of the core and core‐shell nanoparticles was characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM), and the state of CaCO3 shell was confirmed with high‐resolution scanning transmission electron microscope (HR‐STEM) and selected area electron diffraction (SAED). Results showed that PNaSS chains were successfully grafted onto the PSt seed surface, and length of the PNaSS "hairs" could be modulated by adjusting NaSS amount. Sulfonic groups of the PNaSS hairs served as additives in the formation and stabilization of amorphous CaCO3(ACC) and prevented ACC from sequent transformation into crystalline states. The amount of the anchored CaCO3 increased with the growth of PNaSS hair length, and reached 51 wt% (by thermalgravimetric analysis) under the optimal encapsulating temperature of 45°C. Moreover, the forming mechanism of P(St‐NaSS)/CaCO3 core‐shell nanoparticles was proposed.

Temperature Dependent Effects of Elevated CO2 on Shell Composition and Mechanical Properties of Hydroides elegans: Insights from a Multiple Stressor Experiment

The majority of marine benthic invertebrates protect themselves from predators by producing calcareous tubes or shells that have remarkable mechanical strength. An elevation of CO2 or a decrease in pH in the environment can reduce intracellular pH at the site of calcification and thus interfere with animal’s ability to accrete CaCO3. In nature, decreased pH in combination with stressors associated with climate change may result in the animal producing severely damaged and mechanically weak tubes. This study investigated how the interaction of environmental drivers affects production of calcareous tubes by the serpulid tubeworm, Hydroides elegans. In a factorial manipulative experiment, we analyzed the effects of pH (8.1 and 7.8), salinity (34 and 27‰), and temperature (23°C and 29°C) on the biomineral composition, ultrastructure and mechanical properties of the tubes. At an elevated temperature of 29°C, the tube calcite/aragonite ratio and Mg/Ca ratio were both increased, the Sr/Ca ratio was decreased, and the amorphous CaCO3 content was reduced. Notably, at elevated temperature with decreased pH and reduced salinity, the constructed tubes had a more compact ultrastructure with enhanced hardness and elasticity compared to decreased pH at ambient temperature. Thus, elevated temperature rescued the decreased pH-induced tube impairments. This indicates that tubeworms are likely to thrive in early subtropical summer climate. In the context of climate change, tubeworms could be resilient to the projected near-future decreased pH or salinity as long as surface seawater temperature rise at least by 4°C.