Water as the Key to Proto-Aragonite Amorphous CaCO3

Studying crystallisation processes using electron microscopy: The importance of sample preparation

We present a comparison of common electron microscopy sample preparation methods for studying crystallisation processes from solution using both scanning and transmission electron microscopy (SEM and TEM). We focus on two widely studied inorganic systems: calcium sulphate, gypsum (CaSO4·2H2O) and calcium carbonate (CaCO3). We find significant differences in crystallisation kinetics and polymorph selection between the different sample preparation methods, which indicate that drying and chemical quenching can induce severe artefacts that are capable of masking the true native state of the crystallising solution. Overall, these results highlight the importance of cryogenic (cryo)-quenching crystallising solutions and the use of full cryo-TEM as the most reliable method for studying the early stages of crystallisation.

Uncovering the Dynamics of Urease and Carbonic Anhydrase Genes in Ureolysis, Carbon Dioxide Hydration, and Calcium Carbonate Precipitation

The hydration of CO2 suffers from kinetic inefficiencies that make its natural trapping impractically sluggish. However, CO2-fixing carbonic anhydrases (CAs) remarkably accelerate its equilibration by 6 orders of magnitude and are, therefore, "ideal" catalysts. Notably, CA has been detected in ureolytic bacteria, suggesting its potential involvement in microbially induced carbonate precipitation (MICP), yet the dynamics of the urease (Ur) and CA genes remain poorly understood. Here, through the use of the ureolytic bacteriumSporosarcina pasteurii, we investigate the differing role of Ur and CA in ureolysis, CO2 hydration, and CaCO3 precipitation with increasing CO2(g) concentrations. We show that Ur gene up-regulation coincides with an increase in [HCO3-] following the hydration of CO2 to HCO3- by CA. Hence, CA physiologically promotes buffering, which enhances solubility trapping and affects the phase of the CaCO3 mineral formed. Understanding the role of CO2 hydration on the performance of ureolysis and CaCO3 precipitation provides essential new insights, required for the development of next-generation biocatalyzed CO2 trapping technologies.

Highly hydrated paramagnetic amorphous calcium carbonate nanoclusters as an MRI contrast agent

Amorphous calcium carbonate plays a key role as transient precursor in the early stages of biogenic calcium carbonate formation in nature. However, due to its instability in aqueous solution, there is still rare success to utilize amorphous calcium carbonate in biomedicine. Here, we report the mutual effect between paramagnetic gadolinium ions and amorphous calcium carbonate, resulting in ultrafine paramagnetic amorphous carbonate nanoclusters in the presence of both gadolinium occluded highly hydrated carbonate-like environment and poly(acrylic acid). Gadolinium is confirmed to enhance the water content in amorphous calcium carbonate, and the high water content of amorphous carbonate nanoclusters contributes to the much enhanced magnetic resonance imaging contrast efficiency compared with commercially available gadolinium-based contrast agents. Furthermore, the enhanced T₁ weighted magnetic resonance imaging performance and biocompatibility of amorphous carbonate nanoclusters are further evaluated in various animals including rat, rabbit and beagle dog, in combination with promising safety in vivo. Overall, exceptionally facile mass-productive amorphous carbonate nanoclusters exhibit superb imaging performance and impressive stability, which provides a promising strategy to design magnetic resonance contrast agent.

Sensitive, biocompatible and stable contrast agents for MRI are in demand. Here, the authors combine gadolinium ions with amorphous calcium carbonate to make stable paramagnetic amorphous carbonate nanoclusters with high MRI contrast and significantly improved biocompatibility over commercial gadolinium-based agents.

Crystal Nucleation and Growth of Inorganic Ionic Materials from Aqueous Solution: Selected Recent Developments, and Implications

In this review article, selected, latest theoretical, and experimental developments in the field of nucleation and crystal growth of inorganic materials from aqueous solution are highlighted, with a focus on literature after 2015 and on non-classical pathways. A key point is to emphasize the so far underappreciated role of water and solvent entropy in crystallization at all stages from solution speciation through to the final crystal. While drawing on examples from current inorganic materials where non-classical behavior has been proposed, the potential of these approaches to be adapted to a wide-range of systems is also discussed, while considering the broader implications of the current re-assessment of pathways for crystallization. Various techniques that are suitable for the exploration of crystallization pathways in aqueous solution, from nucleation to crystal growth are summarized, and a flow chart for the assignment of specific theories based on experimental observations is proposed.

Solvent-mediated isotope effects strongly influence the early stages of calcium carbonate formation: exploring D2O vs. H2O in a combined computational and experimental approach

In experimental studies, heavy water (D₂O) is employed, e.g., so as to shift the spectroscopic solvent background, but any potential effects of this solvent exchange on reaction pathways are often neglected. While the important role of light water (H₂O) during the early stages of calcium carbonate formation has been realized, studies into the actual effects of aqueous solvent exchanges are scarce. Here, we present a combined computational and experimental approach to start to fill this gap. We extended a suitable force field for molecular dynamics (MD) simulations. Experimentally, we utilised advanced titration assays and time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. We find distinct effects in various mixtures of the two aqueous solvents, and in pure H₂O or D₂O. Disagreements between the computational results and experimental data regarding the stabilities of ion associates might be due to the unexplored role of HDO, or an unprobed complex phase behaviour of the solvent mixtures in the simulations. Altogether, however, our data suggest that calcium carbonate formation might proceed "more classically" in D₂O. Also, there are indications for the formation of new structures in amorphous and crystalline calcium carbonates. There is huge potential towards further improving the understanding of mineralization mechanisms by studying solvent-mediated isotope effects, also beyond calcium carbonate. Last, it must be appreciated that H₂O and D₂O have significant, distinct effects on mineralization mechanisms, and that care has to be taken when experimental data from D₂O studies are used, e.g., for the development of H₂O-based computer models.

Pyrophosphate-stabilised amorphous calcium carbonate for bone substitution: toward a doping-dependent cluster-based model

Multiscale and multitool advanced characterisation of pyrophosphate-stabilised amorphous calcium carbonates allowed building a cluster-based model paving the way for tunable biomaterials.

In vitro crystallization of calcium carbonate mediated by proteins extracted from P. placenta shells

The ASM extracted from the shells of P. placenta can stabilize ACC and inhibit secondary nucleation for 10 hours, and an explosive secondary nucleation and quick crystal growth from 50 nm to 10 μm can be finished on the shell surface in one hour.

Variation in Properties of Pre-Nucleation Calcium Carbonate Clusters Induced by Aggregation: A Molecular Dynamics Study

Numerous studies have speculated calcium carbonate (CaCO3) nucleation induced by pre-nucleation clusters (PNCs) aggregation. However, it is challenging for experiments to directly obtain the relationship between PNCs aggregation and nucleation. Herein, we employ molecular dynamics simulations to explore the variation during PNCs aggregation, which can describe the beginning stage of CaCO3 nucleation induced by PNCs aggregation in supersaturated solutions. The results reveal that the formation of CaCO3 nucleus consists of PNCs spontaneous growth, PNCs solubility equilibrium, and aggregation of PNCs inducing nucleation. The PNCs aggregation, accompanied by the variation in the configuration and stability of CaCO3 aggregate, breaks the solubility equilibrium of PNCs and creates conditions for the formation of the more stable nucleus. Besides, the CaCO3 nucleus with the higher coordination number and the lower hydration number form when decreasing the CaCO3 concentration or increasing the temperature. This work not only sheds light on the formation of the CaCO3 nucleus but also contributes to the explanation for CaCO3 polymorphism.

Polymorph control of 5-fluorouracil during a ball milling process

Polymorph control of 5-fluorouracil was achieved by ball milling. Forms II and III were prepared separately through ball milling with different amounts of ethanol as an additive at the same period without influencing each other.

Forming amorphous calcium carbonate within hydrogels by enzyme-induced mineralization in the presence of N-(phosphonomethyl)glycine

Amorphous inorganic materials have a great potential in material science. Amorphous calcium carbonate (ACC) is a widely useable system, however, its stabilization often turns out to be difficult and the synthesis is mostly limited to precipitation in solution as nanoparticles. Stable ACC in bulk phases would create new composite materials. Previous work described the enzyme-induced mineralization of hydrogels with crystalline calcium carbonate by entrapping urease into a hydrogel and treating this with an aqueous mineralization solution containing urea und calcium chloride. Here, this method was modified using a variety of crystallization inhibitors attached to the hydrogel matrix or added to the surrounding mineralization solution. It was found that only N-(phosphonomethyl)glycine (PMGly) in solution completely inhibits the crystallization of ACC in the hydrogel matrix. The stability of the homogeneously precipitated ACC could be accounted to the combination of stabilizing effects of the additive and stabilization through confinement. The crystallization could be accelerated at higher temperatures up to 60 °C. Here, a combination of Mg ions and PMGly was required to stabilize ACC in the hydrogel. Variation of these two compounds can be used to control a number of different calcium carbonate morphologies within the hydrogel. While the ACC nanoparticles within the hydrogel are stable over weeks even in water, a calcite layer grows on the surface of the hydrogel, which might be used as self-hardening mechanism of a surface.

The Crystallization Process of Vaterite Microdisc Mesocrystals via Proto-Vaterite Amorphous Calcium Carbonate Characterized by Cryo-X-ray Absorption Spectroscopy

Investigation on the formation mechanism of crystals via amorphous precursors has attracted a lot of interests in the last years. The formation mechanism of thermodynamically meta-stable vaterite in pure alcohols in the absence of any additive is less known. Herein, the crystallization process of vaterite microdisc mesocrystals via proto-vaterite amorphous calcium carbonate (ACC) in isopropanol was tracked by using Ca K-edge X-ray absorption spectroscopy (XAS) characterization under cryo-condition. Ca K-edge X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift to lower photoelectron energy while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate. Ca K-edge extended X-ray absorption fine structure (EXAFS) spectra exhibit that the coordination number of the nearest neighbor atom O around Ca increases slowly with the increase of crystallization time and tends to be stable as 4.3 (±1.4). Crystallization time dependent XANES and EXAFS analyses indicate that short-range ordered structure in proto-vaterite ACC gradually transform to long-range ordered structure in vaterite microdisc mesocrystals via a non-classical crystallization mechanism.

Characterisation of CaCO3 phases during strain-specific ureolytic precipitation

Numerous microbial species can selectively precipitate mineral carbonates with enhanced mechanical properties, however, understanding exactly how they achieve this control represents a major challenge in the field of biomineralisation. We have studied microbial induced calcium carbonate (CaCO₃) precipitation (MICP) in three ureolytic bacterial strains from the Sporosarcina family, including S. newyorkensis, a newly isolated microbe from the deep sea. We find that the interplay between structural water and strain-specific amino acid groups is fundamental to the stabilisation of vaterite and that, under the same conditions, different isolates yield distinctly different polymorphs. The latter is found to be associated with different urease activities and, consequently, precipitation kinetics, which change depending on pressure-temperature conditions. Further, CaCO₃ polymorph selection also depends on the coupled effect of chemical treatment and initial bacterial concentrations. Our findings provide new insights into strain-specific CaCO₃ polymorphic selection and stabilisation, and open up promising avenues for designing bio-reinforced geo-materials that capitalise on the different particle bond mechanical properties offered by different polymorphs.

Stable Prenucleation Calcium Carbonate Clusters Define Liquid-Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is an intermediate step during the precipitation of calcium carbonate, and is assumed to play a key role in biomineralization processes. Here, we have developed a model where ion association thermodynamics in homogeneous phases determine the liquid-liquid miscibility gap of the aqueous calcium carbonate system, verified experimentally using potentiometric titrations, and kinetic studies based on stopped-flow ATR-FTIR spectroscopy. The proposed mechanism explains the variable solubilities of solid amorphous calcium carbonates, reconciling previously inconsistent literature values. Accounting for liquid-liquid amorphous polymorphism, the model also provides clues to the mechanism of polymorph selection. It is general and should be tested for systems other than calcium carbonate to provide a new perspective on the physical chemistry of LLPS mechanisms based on stable prenucleation clusters rather than un-/metastable fluctuations in biomineralization, and beyond.

Formation of nano-magnesite in the calcareous spicules prepared under ambient conditions

Nanocrystallites of magnesite were found in calcareous spicules prepared under ambient conditions.

Water: How Does It Influence the CaCO3 Formation?

Nature produces biomineral-based materials with a fascinating set of properties using only a limited number of elements. This set of properties is obtained by closely controlling the structure and local composition of the biominerals. We are far from achieving the same degree of control over the properties of synthetic biomineral-based composites. One reason for this inferior control is our incomplete understanding of the influence of the synthesis conditions and additives on the structure and composition of the forming biominerals. In this Review, we provide an overview of the current understanding of the influence of synthesis conditions and additives during different formation stages of CaCO₃ , one of the most abundant biominerals, on the structure, composition, and properties of the resulting CaCO₃ crystals. In addition, we summarize currently known means to tune these parameters. Throughout the Review, we put special emphasis on the role of water in mediating the formation of CaCO₃ and thereby influencing its structure and properties, an often overlooked aspect that is of high relevance.

Aerosol-based synthesis of pure and stable amorphous calcium carbonate

A facile aerosol-based method for the synthesis of pure and stable amorphous calcium carbonate (ACC) is presented. The method relies on the instantaneous carbonation of calcium hydroxide aerosols with carbon dioxide followed by rapid drying of the freshly formed ACC. The ACC display extended stability against humidity induced crystallization.

Functionalized Multiwalled CNTs in Classical and Nonclassical CaCO3 Crystallization

Multiwalled carbon nanotubes (MWCNTs) are interesting high-tech nanomaterials. MWCNTs oxidized and functionalized with itaconic acid and monomethylitaconate were demonstrated to be efficient additives for controlling nucleation of calcium carbonate (CaCO₃) via gas diffusion (GD) in classical as well as nonclassical crystallization, yielding aragonite and truncated calcite. For the first time, all amorphous calcium carbonate (ACC) proto-structures, such as proto calcite-ACC, proto vaterite-ACC and proto aragonite-ACC, were synthesized via prenucleation cluster (PNC) intermediates and stabilized at room temperature. The MWCNTs also showed concentration-dependent nucleation promotion and inhibition similar to biomolecules in nature. Incorporation of fluorescein-5-thiosemicarbazide (5-FTSC) dye-labeled MWCNTs into the CaCO₃ lattice resulted in fluorescent hybrid nanosized CaCO₃. We demonstrate that functionalized MWCNTs offer a good alternative for controlled selective crystallization and for understanding an inorganic mineralization process.

Anhydrous amorphous calcium carbonate (ACC) is structurally different from the transient phase of biogenic ACC

An in situ ambient pressure soft X-ray spectroscopic study of the phase transformation of ACC exposed to water vapor in the mbar pressure range in conjunction with heat treatment.

Microdynamic changes of moisture-induced crystallization of amorphous calcium carbonate revealed via in situ FTIR spectroscopy

A microdynamic mechanism of moisture-induced ACC crystallization involving three consecutive conversion stages is elucidated via in situ FTIR spectroscopy and two-dimensional correlation analysis.

A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite

Despite its thermodynamical metastability at near-surface conditions, aragonite is widespread in marine and terrestrial sediments. It abundantly forms in living organisms, and its abiotic formation is favored in waters of a Mg²⁺/Ca²⁺ ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO₃ polymorph, which precipitates before aragonite in a cave. The new phase, which we term monoclinic aragonite (mAra), is crystallographically related to ordinary, orthorhombic aragonite. Electron diffraction tomography combined with structure determination demonstrates that mAra has a layered aragonite structure, in which some carbonates can be replaced by hydroxyls and up to 10 atomic % of Mg can be incorporated. The diagnostic electron diffraction features of mAra are diffuse scattering and satellite reflections along aragonite {110}. Similar features have previously been reported-although unrecognized-from biogenic aragonite formed in stromatolites, mollusks, and cyanobacteria as well as from synthetic material. We propose that mAra is a widespread crystalline CaCO₃ that plays a hitherto unrecognized key role in metastable aragonite formation.

Hydrogen Bonding in Amorphous Alkaline Earth Carbonates

Amorphous intermediates play a crucial role during the crystallization of alkaline earth carbonates. We synthesized amorphous carbonates of magnesium, calcium, strontium, and barium from methanolic solution. The local environment of water and the strength of hydrogen bonding in these hydrated modifications were probed with Fourier transform IR spectroscopy, ¹H NMR spectroscopy, and heteronuclear correlation experiments. Temperature-dependent spin-lattice (T1) relaxation experiments provided information about the water motion in the amorphous materials. The Pearson hardness of the respective divalent metal cation predominantly determines the strength of the internal hydrogen-bonding network. Amorphous magnesium carbonate deviates from the remaining carbonates, as it contains additional hydroxide ions, which act as strong hydrogen-bond acceptors. Amorphous calcium carbonate exhibits the weakest hydrogen bonds of all alkaline earth carbonates. Our study provides a coherent picture of the hydrogen bonding situation in these transient species and thereby contributes to a deeper understanding of the crystallization process of carbonates.

Amorphous Calcium Carbonate Constructed from Nanoparticle Aggregates with Unprecedented Surface Area and Mesoporosity

Amorphous calcium carbonate (ACC), with the highest reported specific surface area of all current forms of calcium carbonate (over 350 m² g^-1), was synthesized using a surfactant-free, one-pot method. Electron microscopy, helium pycnometry, and nitrogen sorption analysis revealed that this highly mesoporous ACC, with a pore volume of ∼0.86 cm³ g^-1 and a pore-size distribution centered at 8-9 nm, is constructed from aggregated ACC nanoparticles with an estimated average diameter of 7.3 nm. The porous ACC remained amorphous and retained its high porosity for over 3 weeks under semi-air-tight storage conditions. Powder X-ray diffraction, large-angle X-ray scattering, infrared spectroscopy, and electron diffraction exposed that the porous ACC did not resemble any of the known CaCO₃ structures. The atomic order of porous ACC diminished at interatomic distances over 8 Å. Porous ACC was evaluated as a potential drug carrier of poorly soluble substances in vitro. Itraconazole and celecoxib remained stable in their amorphous forms within the pores of the material. Drug release rates were significantly enhanced for both drugs (up to 65 times the dissolution rates for the crystalline forms), and supersaturation release of celecoxib was also demonstrated. Citric acid was used to enhance the stability of the ACC nanoparticles within the aggregates, which increased the surface area of the material to over 600 m² g^-1. This porous ACC has potential for use in various applications where surface area is important, including adsorption, catalysis, medication, and bone regeneration.

A New Methodology for Evaluating the Structural Similarity between Different Phases Using a Dimensionality Reduction Technique

A new methodology for definitively evaluating the structural similarity between different phases in an impartial manner is proposed. This methodology utilizes a dimensionality reduction (DR) technique that was developed in the fields of machine learning and statistics. The basis of the proposed methodology is that the structural similarity between different phases can be evaluated by the geometrical similarity of pair and/or angular distribution functions that reflect the atomic-scale structure of each phase. The DR technique is used for the analysis of this geometrical similarity. In this study, the proposed methodology is applied to evaluate the similarity in the atomic-scale structure, as obtained from molecular dynamics simulations, between amorphous CaCO₃ and CaCO₃ crystal phases in the presence or absence of additives, namely, Mg²⁺ ions, Sr²⁺ ions, and water molecules. The results indicate that in the absence of additives, the structure of the amorphous phase is closer to that of vaterite than to those of calcite or aragonite. However, the degree of structural similarity between the amorphous phase and vaterite decreases if Mg²⁺ ions are present. This tendency is also evident when Sr²⁺ ions are present, although these ions do not influence the structure of the amorphous phase as strongly as Mg²⁺ ions. In addition, the results indicate that at a high water concentration, the amorphous phase is separated into small particles by hydrogen-bonded networks of water molecules and the structure of the amorphous phase more closely approaches that of vaterite. The proposed methodology is widely applicable to the evaluation of the structural similarity between different phases for complex multicomponent systems.

Minerals in the pre-settled coral Stylophora pistillata crystallize via protein and ion changes

Aragonite skeletons in corals are key contributors to the storage of atmospheric CO₂ worldwide. Hence, understanding coral biomineralization/calcification processes is crucial for evaluating and predicting the effect of environmental factors on this process. While coral biomineralization studies have focused on adult corals, the exact stage at which corals initiate mineralization remains enigmatic. Here, we show that minerals are first precipitated as amorphous calcium carbonate and small aragonite crystallites, in the pre-settled larva, which then evolve into the more mature aragonitic fibers characteristic of the stony coral skeleton. The process is accompanied by modulation of proteins and ions within these minerals. These findings may indicate an underlying bimodal regulation tactic adopted by the animal, with important ramification to its resilience or vulnerability toward a changing environment.

Coral biomineralization is an important example of natural mineralization and understanding the process will aid biomineralization research. Here, the authors identify the precipitation of amorphous calcium carbonate and small aragonite crystals in pre-settled larva of Stylophora pistillata.

How similar are amorphous calcium carbonate and calcium phosphate? A comparative study of amorphous phase formation conditions

Precipitation domains of ACP and ACP increase with the complexity of the system, the ACP one being always larger.

On mechanisms of mesocrystal formation: magnesium ions and water environments regulate the crystallization of amorphous minerals

Elucidating the emergence of crystalline superstructures from amorphous precursors, hydration environments and ionic constituents can guide transformation and structuration reactions towards distinct micro- and nano-structures.

A classical view on nonclassical nucleation

Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO₃) is often used as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO₃ nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid-liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO₃ in dilute aqueous solutions. We propose that a dense liquid phase (containing 4-7 H₂O per CaCO₃ unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO₃ in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca²⁺ + z CO₃^2- → z CaCO₃ The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.

Amorphous calcium carbonate particles form coral skeletons

Do corals form their skeletons by precipitation from solution or by attachment of amorphous precursor particles as observed in other minerals and biominerals? The classical model assumes precipitation in contrast with observed "vital effects," that is, deviations from elemental and isotopic compositions at thermodynamic equilibrium. Here, we show direct spectromicroscopy evidence in Stylophora pistillata corals that two amorphous precursors exist, one hydrated and one anhydrous amorphous calcium carbonate (ACC); that these are formed in the tissue as 400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally, crystallize into aragonite (CaCO₃). We show in both coral and synthetic aragonite spherulites that crystal growth by attachment of ACC particles is more than 100 times faster than ion-by-ion growth from solution. Fast growth provides a distinct physiological advantage to corals in the rigors of the reef, a crowded and fiercely competitive ecosystem. Corals are affected by warming-induced bleaching and postmortem dissolution, but the finding here that ACC particles are formed inside tissue may make coral skeleton formation less susceptible to ocean acidification than previously assumed. If this is how other corals form their skeletons, perhaps this is how a few corals survived past CO₂ increases, such as the Paleocene-Eocene Thermal Maximum that occurred 56 Mya.

Controlling nucleation and growth of nano-CaCO3 via CO2 sequestration by a calcium alkoxide solution to produce nanocomposites for drug delivery applications

Calcium carbonate is an extremely attractive material in a plethora of biomedical applications. Intensive efforts have recently been made to achieve the control over its nucleation and subsequent aggregation, growth and crystallization; focusing on bringing insight into the role of precursors, solvents and templates. Having analyzed the recently acquired knowledge, we addressed this challenge using CO₂ sequestration synthesis, using an unusual reactant, a solution of calcium ethoxide, Ca(OC₂H₅)₂, as precursor. By tailoring the reaction conditions, it was possible to produce extremely small and rather size-uniform single-phase calcite CaCO₃ nanoparticles, forming sols and subsequently gels in the applied medium. According to DLS and nanoparticle tracking analysis the particles are only to a minor extent aggregated in the mother liquor and can form transparent gels on concentration in less polar media, but produce large aggregates 400-800nm in size when dried and subsequently transferred to aqueous media. Complete drying of solutions renders xerogel type materials with only moderate active surface area, as identified by nitrogen adsorption, due to aggregation with development of densified surface layers. Such behaviour is typical for the sol-gel synthesis of particles possessing enhanced surface reactivity. The aggregation on drying was used to produce hybrid nanocomposites, with the hydrophobic model component, β-carotene, introduced in solution in a non-polar co-solvent and model medicine - ibuprofen. The obtained nanocomposite particles, characterized by SEM, TEM, XRD, AFM and FTIR studies, are hierarchically structured spheroidal aggregates about 200nm in size with uniform distribution of the organic components present in the amorphous state. The composite particles are stable in neutral aqueous environments but are readily dissolved in acidic medium or even in PBS at pH = 7.40, releasing the hydrophobic organic component in the form of a relatively stable colloid solution. Efficient release of ibuprofen as model drug was achieved in both acidic and PBS medium and could be slowed down by the addition of β-carotene as hydrophobic component.

STATEMENT OF SIGNIFICANCE

The proposed sol-gel synthesis of CaCO₃ proved to create unprecedented size of CaCO₃ nanoparticles with striking size uniformity. The obtained results clearly demonstrate their ability to incorporate hydrophobic components in a nanocomposite matrix converting them into amorphous nano sized particles, building stable colloids via release in acidic medium. Transfer of a sol produced in organic medium into water in the presence of albumen surfactant results in relatively uniform micro particles about 1μm size. The obtained materials show characteristics attractive for use in drug delivery and potentially also a variety of other industrial applications.

Ammonium-Carbamate-Rich Organogels for the Preparation of Amorphous Calcium Carbonates

Amine-CO2 chemistry is important for a range of different chemical processes, including carbon dioxide capture. Here, we studied how aspects of this chemistry could be used to prepare calcium carbonates. Chemically crosslinked organogels were first prepared by reacting hyperbranched polyethylene imine (PEI) dissolved in DMSO with carbon dioxide. The crosslinks of the organogel consisted of ammonium-carbamate ion pairs as was shown by IR spectroscopy. These carbamate-rich organogels were subsequently subjected to aqueous solutions of calcium acetate, and amorphous calcium carbonate (ACC) precipitated. The ACC did not crystalize during the mixing for up to 20 h, as was shown by a combination of IR spectroscopy, X-ray diffraction, scanning electron microscopy, and thermal analysis. Some PEI had been included or adsorbed on the ACC particles. Traces of calcite were observed in one sample that had been subjected to water in a work-up procedure.

Alignment of Amorphous Iron Oxide Clusters: A Non-Classical Mechanism for Magnetite Formation

Despite numerous studies on the nucleation and crystallization of iron (oxyhydr)oxides, the roles of species developing during the early stages, especially primary clusters and intermediate amorphous particles, are still poorly understood. Herein, both ligand-free and ligand-protected amorphous iron oxide (AIO) clusters (<2 nm) were synthesized as precursors for magnetite formation. Thermal annealing can crystallize the clusters into magnetite particles, and AIO bulk phases with domains of pre-aligned clusters are found to be direct precursors to crystals, suggesting a non-classical aggregation-based pathway that differs from the reported oriented attachment or particle accretion mechanisms.