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We experimentally determined the characteristics and Langmuir parameters of the binding of calcium ions to different polycarboxylates. By using potentiometric titrations and isothermal titration calorimetry, the effects of side chain chemistry, pH value, and chain length were systematically investigated using the linear polymers poly(aspartic acid), poly(glutamic acid), and poly(acrylic acid). We demonstrate that for polymers with high polymerization degrees, the binding process is governed by higher-order effects, such as the change of apparent p

The determination of solubility limits of compounds in water is unprecise and relies on certain prerequisites such as UV–vis absorption activity. In this study, we designed an experimental approach...

Separation, analysis and recycling technologies are of high interest for our modern societies, where colloidal iron and aluminium (hydr)oxides have important applications. However, there are significant gaps in the fundamental understanding of how these phases form in real systems. Classical nucleation theory cannot account for many experimental observations, and there is a dichotomy between the chemistry of hydrolysing/condensating systems and the physical notion of supersaturation. Reviewing parts of the established and recent literature, we demonstrate that concepts of nonclassical nucleation pathways can overcome these issues. This broader, chemistry-based conceptual framework has a high potential for advancing current applications, and developing new strategies towards separation, analysis and recycling applications, which seem to be urgently required for the future.

In experimental studies, heavy water (D

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. published

Understanding the role of polymers rich in aspartic acid (Asp) and glutamic acid (Glu) is the key to gaining precise control over mineralization processes. Despite their chemical similarity, experiments revealed a surprisingly different influence of Asp and Glu sequences. We conducted molecular dynamics simulations of Asp and Glu peptides in the presence of calcium and chloride ions to elucidate the underlying phenomena. In line with experimental differences, in our simulations, we indeed find strong differences in the way the peptides interact with ions in solution. The investigated Asp pentapeptide tends to pull a lot of ions into its vicinity, and many structures with clusters of calcium and chloride ions on the surface of the peptide can be observed. Under the same conditions, comparatively fewer ions can be found in proximity of the investigated Glu pentapeptide, and the structures are characterized by single calcium ions bound to multiple carboxylate groups. Based on our simulation data, we identified three reasons contributing to these differences, leading to a new level of understanding additive-ion interactions. published

The work presented here investigates the early stages of barium sulfate crystallization from aqueous solutions. By monitoring free-barium activity during the crystallization, we identified a metastable BaSO4 phase that precedes barite formation. Ex situ investigations provide direct, unambiguous experimental evidence of an amorphous BaSO4 phase in an additive-free scenario.

Der Nukleationsmechanismus von Kristallen kleiner organischer Moleküle wurde durch Computersimulationen postuliert, doch bisher mangelt es an experimentellen Nachweisen. Hier wurde ein experimenteller Ansatz entwickelt, mit dem die frühen Stadien der Kristallisation von Ibuprofen als Modellsystem aufgezeigt werden. Ibuprofen vollzieht eine flüssig-flüssig Entmischung. Die binodalen und spinodalen Grenzen dieser Mischungslücke wurden analysiert und der flüssige Charakter der Phase wurde bestätigt. Ein Anstieg der Viskosität hält die kinetische Stabilität der dichten flüssigen Phase aufrecht, die intermolekularen Abstände entsprechen denen in den Kristallstrukturen. Die dichte flüssige Phase wurde als Vorläuferphase der Nukleation von Ibuprofen identifiziert, bei der zunächst eine Verdichtung stattfindet und sich erst später eine strukturelle Ordnung ausbildet. Diese Entdeckung verspricht die Möglichkeit, schwerlösliche Wirkstoffe in wässriger Lösung über klassische Löslichkeitsgrenzen hinaus zu befördern. published

Biomineralization is an active, biologically governed process of mineral formation, established early on in the history of life. The appearance of biomineralizing organisms heavily influenced the course of evolution, leading to the development of the large diversity of the extant taxa. Yet, we are still only beginning to grasp the intricate, genetically regulated mechanisms involved. Since prokaryotic organisms were the first to emerge from the primordial environments, we investigated bacteria-mineral interactions using titration and gas diffusion systems adapted to emulate conditions, which may have facilitated the development of biomineralization initially. By screening the minerals and bacteria from titration experiments with scanning electron microscopy, we discovered a broad spectrum of behavioral strategies employed by bacteria confronted with calcification, which fell into three main categories: (1) evasion of mineralization by the formation of the biofilm, (2) random embedding into the mineral, and (3) control over the mineral shape during its formation. The latter phenomenon we termed pseudo-biomineralization. Our experiments indicate that pseudo-biomineralization is an active process obligatorily reliant on the external calcifying conditions and allowing considerable degree of control over mineral shape, thus producing structures reminiscent of true biominerals. Here, we describe this notion for the first time, thus providing vital insight into the genesis of a transitional stage to calcium carbonate-based biomineralization systems.

Biomedical applications of calcium carbonate minerals have been receiving an increasing amount of attention. With a simple chemical composition, low production cost, and ease to produce in large quantities, this non-toxic (bio)mineral has been used preferentially in the pharmaceutical industry as a diluent, bulking or coating agent. Recently, novel products have been developed and amorphous calcium carbonate was successfully used as a hybrid carrier for drugs, proteins and genes. It is obvious that any impurities might have adverse effects in any of the aforementioned applications. However, the synthesis of impurity-free amorphous calcium carbonate often proves to be challenging due to its instability and rapid transformation to crystalline phases. Herein, we describe a novel, simple and scalable protocol for the synthesis of such impurity-free amorphous calcium carbonate nanoparticles that is potentially invaluable for critical health applications, improving the applicability of amorphous calcium carbonates in pharmaceutical and medical domains. published

Calcium orthophosphates (CaPs) are the hard constituents of bones and teeth, and thus of ultimate importance to humankind, while amorphous CaPs (ACPs) may play crucial roles in CaP biomineralization. Among the various ACPs with Ca/P atomic ratios between 1.0–1.5, an established structural model exists for basic ACP (Ca/P = 1.5), while those of other ACPs remain unclear. Herein, the structure of amorphous calcium hydrogen phosphate (ACHP; Ca/P = 1.0) obtained via aqueous routes at near-neutral pH values, without stabilizers, was studied by experiments (mainly, TEM with ED, XRD, IR, and NMR spectroscopies, as well as XAS) and computer simulation. Our results globally show that ACHP has a distinct short-range structure, and we propose calcium hydrogen phosphate clusters (CHPCs) as its basic unit. This model is consistent with both computer simulations and the experimental results, where CHPCs are arranged together with water molecules to build up ACHP. We demonstrate that Posner’s clusters, which are convent...

Cellular machineries guide the bottom-up pathways toward crystal superstructures based on the transport of inorganic precursors and their precise integration with organic frameworks. The biosynthesis of mesocrystalline spines entails concerted interactions between biomolecules and inorganic precursors; however, the bioinorganic interactions and interfaces that regulate material form and growth as well as the selective emergence of structural complexity in the form of nanostructured crystals are not clear. By investigating mineral nucleation under the regulation of recombinant proteins, we show that SpSM50, a matrix protein of the sea urchin spine, stabilizes mineral precursors via vesicle-confinement, a function conferred by a low-complexity, disordered region. Site-specific proteolysis of this domain by a collagenase initiates phase transformation of the confined mineral phase. The residual C-type lectin domain molds the fluidic mineral precursor into hierarchical mesocrystals identical to structural crystal modules constituting the biogenic mineral. Thus, the regulatory functions of proteolytic enzymes can guide biomacromolecular domain constitutions and interfaces, in turn determining inorganic phase transformations toward hybrid materials as well as integrating organic and inorganic components across hierarchical length scales. Bearing striking resemblance to biogenic mineralization, these hybrid materials recruit bioinorganic interactions which elegantly intertwine nucleation and crystallization phenomena with biomolecular structural dynamics, hence elucidating a long-sought key of how nature can orchestrate complex biomineralization processes., Chemical modifications guide bioinorganic interactions, intertwining the nucleation and crystallization of inorganic phases with biomolecular dynamics, hence elucidating a long-sought key of how nature orchestrates material genesis.

Amorphous calcium carbonate (ACC) 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 ACC in biomedicine. Here, we report the mutual effect between paramagnetic gadolinium ions and ACC, resulting in ultrafine paramagnetic amorphous carbonate nanoclusters (ACNC) in the presence of both gadolinium occluded highly hydrated ACC-like environment and poly(acrylic acid). Gadolinium is confirmed to enhance the water content in ACC, and the high water content of ACNC (23 molecules H2O per 1 Gd) contributes to the much enhanced magnetic resonance imaging (MRI) contrast efficiency compared with commercially available gadolinium-based contrast agents. Furthermore, the enhanced T1 weighted MRI performance and biocompatibility of ACNC are further evaluated in various animals including rat, rabbit and beagle dog, in combination with promising safety in vivo. Overall, exceptionally facile mass-productive ACNC exhibits superb imaging performance and impressive stability, which provides a promising strategy to design MR contrast agents.

Liquid-phase transmission electron microscopy is used to study a wide range of chemical processes, where its unique combination of spatial and temporal resolution provides countless insights into nanoscale reaction dynamics. However, achieving sub-nanometer resolution has proved difficult due to limitations in the current liquid cell designs. Here, a novel experimental platform for in situ mixing using a specially developed 2D heterostructure-based liquid cell is presented. The technique facilitates in situ atomic resolution imaging and elemental analysis, with mixing achieved within the immediate viewing area via controllable nanofracture of an atomically thin separation membrane. This novel technique is used to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment. The observations provide the first direct visual confirmation of the recently developed liquid-liquid phase separation theory, while the technological advancements open an avenue for many other studies of early stage solution-phase reactions of great interest for both the exploration of fundamental science and developing applications.

Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO3 biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO3 biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO3 mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca2+ and CO32– compared to the addition of only Ca2+. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization.

Nucleation of minerals in the presence of additives is critical for achieving control over the formation of solids in biomineralization processes or during syntheses of advanced hybrid materials. Herein, we investigated the early stages of Fe(III) (oxy)(hydr)oxide formation with/without polyglutamic acid (pGlu) at low driving force for phase separation (pH 2.0 to 3.0). We employed an advanced pH-constant titration assay, X-ray diffraction, thermal analysis with mass spectrometry, Fourier Transform infrared spectroscopy, and scanning electron microscopy. Three stages were observed: initial binding, stabilization of Fe(III) pre-nucleation clusters (PNCs), and phase separation, yielding Fe(III) (oxy)(hydr)oxide. The data suggest that organic–inorganic interactions occurred via binding of olation Fe(III) PNC species. Fourier Transform Infrared Spectroscopy (FTIR) analyses revealed a plausible interaction motif and a conformational adaptation of the polypeptide. The stabilization of the aqueous Fe(III) system against nucleation by pGlu contrasts with the previously reported influence of poly-aspartic acid (pAsp). While this is difficult to explain based on classical nucleation theory, alternative notions such as the so-called PNC pathway provide a possible rationale. Developing a nucleation theory that successfully explains and predicts distinct influences for chemically similar additives like pAsp and pGlu is the Holy Grail toward advancing the knowledge of nucleation, early growth, and structure formation., Institute of Technical Sciences of SASA - Ministry of Education, Science and Technological Development of the Republic of Serbia (451-03-9/2021-14/200175), IAESTE Belgrade organization and DAAD

Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO3 biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO3 biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO3 mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca2+ and CO32– compared to the addition of only Ca2+. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization. published

The ammonia diffusion method (ADM) is one of the most widely used strategies for the bioinspired synthesis of minerals. Herein, we present investigations of the mineralization mechanism using an advanced ADM to solve the limitations of the conventional ADM. This allows us to confirm the presence of liquid calcium carbonate precursor species in additive-free and polymer-stabilized gas diffusion systems, indicating that liquid–liquid phase separated species exhibit sufficient kinetic stability to be detected. Time-dependent experiments reveal the role of these precursor phases during the early stages of the crystallization process, demonstrating that liquid calcium carbonate mineral precursors play an important role in the precipitation pathway and must be generally considered for the interpretation of gas diffusion experiments, with and without additives. This discovery poses an important step in the understanding of how minerals are formed, highlighting that nonclassical mineralization processes must be considered for material synthesis. Last but not least, the advanced ADM may be useful for the exploration of the formation mechanism of other minerals than calcium carbonate, which are also of broad interest in the materials chemistry community.

Mechanistic pathways relevant to mineralization are not well‐understood fundamentally, let alone in the context of their biological and geological environments. Through quantitative analysis of ion association at near‐neutral pH, we identify the involvement of HCO3 − ions in CaCO3 nucleation. Incorporation of HCO3 − ions into the structure of amorphous intermediates is corroborated by solid‐state nuclear magnetic resonance spectroscopy, complemented by quantum mechanical calculations and molecular dynamics simulations. We identify the roles of HCO3 − ions as being through (i) competition for ion association during the formation of ion pairs and ion clusters prior to nucleation and (ii) incorporation as a significant structural component of amorphous mineral particles. The roles of HCO3 − ions as active soluble species and structural constituents in CaCO3 formation are of fundamental importance and provide a basis for a better understanding of physiological and geological mineralization., An important role of bicarbonate at near‐neutral pH during CaCO3 formation was found. Its structural incorporation in amorphous intermediates, revealed by a combination of potentiometric titrations, NMR, computer simulations, and other techniques, can be quantitatively explained by a model based on the binding of bicarbonate ions to pre‐nucleation clusters. The findings seem to be crucial for a better understanding of, e.g., CaCO3 biomineralization.

The determination of solubility limits of compounds in water is unprecise and relies on certain prerequisites such as UV-vis absorption activity. In this study, we designed an experimental approach based on potentiometric titrations to determine solubility limits of various organic compounds by exploiting their pH-active carboxylic acid groups. By applying the law of mass action, utilizing a double-dosing method ensuring a constant compound concentration, it is possible to determine the intrinsic solubility limits, which are independent of the pH value. The derived equations enable the precise and fast determination of intrinsic solubility limits of organic compounds in aqueous solutions within 2-4 h. Moreover, it is shown how the p

The effect of Mg2+ on the precipitation pathway of CaCO3 in absolute ethanol has been studied to investigate the role of ion solvation in the crystallization process. Our data reveal that high concentrations of Mg2+ promote the precipitation of an amorphous transient phase together with non-stoichometric hydrated phases of calcium carbonate. published

Flüssig‐flüssig‐Phasenseparation (liquid‐liquid phase separation, LLPS) ist eine Zwischenstufe während der Fällung von Calciumcarbonat und man vermutet, dass sie eine entscheidende Rolle in Biomineralisationsprozessen spielt. In diesem Artikel stellen wir ein Modell vor, in dem die Flüssig‐flüssig‐Mischungslücke im wässrigen Calciumcarbonatsystem durch die Thermodynamik der Ionenassoziation in der homogenen Phase bestimmt wird, was experimentell durch potentiometrische Titrationen und kinetische Studien mittels ATR‐FTIR‐Spektroskopie bestätigt wird. Der vorgeschlagene Mechanismus erklärt die variable Löslichkeit von amorphen Calciumcarbonaten und räumt Widersprüche in Literaturdaten aus. Da flüssig‐flüssig amorphe Polymorphie berücksichtigt wird, liefert das Modell Hinweise zum Mechanismus der Polymorphselektion. Es ist allgemein und sollte für andere Systeme als Calciumcarbonat überprüft werden. Ausgehend von stabilen Pränukleationsclustern anstelle von in‐ oder metastabilen Fluktuationen ergibt sich ein neuer Blickwinkel auf die physikalische Chemie der Flüssig‐flüssig‐Phasenseparationen in Biomineralisationsprozessen und darüber hinaus. published

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., A quantitative, general model for liquid–liquid phase separation is introduced based on “nonclassical” nucleation via prenucleation clusters. Potentiometric titrations and stopped‐flow ATR‐FTIR spectroscopy verify the predictive power of the model for the aqueous calcium carbonate system. This model reconciles the previously inconsistent literature values for the solubilities of amorphous calcium carbonates.

While dissolution-precipitation, plastic deformation and fracture have been proposed to explain the compaction of carbonates in geological formations, the role of these mechanisms on the densification process of calcium carbonate nanoparticles in synthetic systems remains poorly understood. Here, we systematically investigate the effect of pH of the aqueous phase (1 ≤ pH ≤ 7), temperature (10 ≤ T ≤ 90 °C), and pressure (10 ≤ P ≤ 800 MPa) on the cold compaction of nanovaterite powder with water to shed light on the mechanisms underlying this unique densification. Compaction experiments reveal that the applied pressure plays a major role on the densification of vaterite nanopowder with water. Our experimental data thus suggest that plastic deformation or subcritical crack growth might be important densification mechanisms for vaterite nanoparticles. These findings provide a new perspective into the cold compaction of nanopowders with water and may open promising routes for the manufacturing of CO2-based structural materials at mild processing conditions.

Classical nucleation theory (CNT) is based on the notion of critical nuclei serving as transition states between supersaturated solutions and growing particles. Their excess standard free energy depends on supersaturation, and determines the height of the barrier for phase separation. However, predictions of CNT nucleation rates can deviate from experimental observations by many orders of magnitude. We argue that this is due to oversimplifications within CNT, rendering the critical nucleus essentially a conceptual notion, rather than a truly existing physical entity. Still, given adequate parametrization, CNT is useful for predicting and explaining nucleation phenomena, since it is currently the only quantitative framework at hand. In the recent years, we have been introducing an alternative theory, the so-called pre-nucleation cluster (PNC) pathway. The truly “non-classical” aspect of the PNC pathway is the realization that critical nuclei, as defined within CNT, are not the key to nucleation, but that the transition state relevant for phase separation is based on a change in dynamics of PNCs rather than their size. We provide a summary of CNT and the PNC pathway, thereby highlighting this major difference. The discussion of recent works claiming to provide scientific evidence against the existence of PNCs reveals that such claims are indeed void. Moreover, we illustrate that an erroneous interpretation of the concentration dependence of the free energy has led to a postulated rationalization of the standard free energy of ion pairs and stable ion associates within CNT, which is not sustainable. In fact, stable ion associates are stuck in a free energy trap from the viewpoint of CNT and cannot be considered in a straightforward manner. On the other hand, the notions of the PNC pathway, by dismissing the idea of the CNT-type critical nucleus as a required transition state, overcome this issue. While a quantitative theory of the PNC pathway is eagerly anticipated, the rationalization of experimental observations that are inconsistent with CNT proves its qualitative explanatory power, underpinning great promise towards a better understanding of, for instance, polymorph selection and crystallization control by additives.

In the nacre layer of the Pinctada fucata oyster shell there exists a multimember proteome, known as the framework family, which regulates the formation of the aragonite mesoscale tablets and participates in the creation of an organic coating around each tablet. Several approaches have been developed to understand protein-associated mechanisms of nacre formation, yet we still lack insight into how protein ensembles or proteomes manage nucleation and crystal growth. To provide additional insights we have created a proportionally defined combinatorial model consisting of two recombinant framework proteins, r-Pif97 (containing a von Willebrand Factor Type A domain (vWA)) and r-n16.3 (containing an EGF-like domain), whose individual in vitro mineralization functionalities are distinct from one another. We find that at 1:1 molar ratios r-Pif97 and r-n16.3 exhibit little or no synergistic activity regarding modifying existing calcite crystals. However, during the early stages of nucleation in solution, we note synergistic effects on nucleation kinetics and ACC formation/stability (via dehydration) that are not observed for the individual proteins. This selective synergism is generated by Ca

Nature produces hierarchical, functional materials by shaping amorphous mineral precursors under physiological conditions. Although biominerals inspire the architectures of synthetic counterparts, the biogenic phase transformations yielding precise crystalline forms, polymorphs and structures are unclear. Elucidating the transformation and structuration of amorphous minerals, herein we show distinct crystallization and structuration schemes synergistically controlled by environmental water contents and the Mg/Ca atomic ratio within amorphous mixed metal carbonates. Control of phase transformation, as well as resultant crystalline micro- and nano-structures, reflects the significance of the amorphous precursors of biominerals as disordered by design. Thereby, we complement the literature-known, suggested (bio)polymer-mediated 'divide and protect' mechanism of amorphous mineral stabilization by a Mg2+-based 'unite and protect' strategy. Altogether, this allows delineating a novel mechanism for mesocrystal formation based on the interface-coupled dissolution-re-precipitation of mesoscale amorphous precursors, which appear important in biomineralization. In the latter case, the recruitment of environmentally abundant Mg2+ species can also supplement the functions of biomolecules.

Crystal formation is a highly debated problem. This report shows that the crystallization of l-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation; this indicates the recurrence of water molecules that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which can be assigned to a natural vibration of large hydrated aggregates, vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration. published

Gastroliths are highly calcified structures formed in the cardiac stomach wall of crustaceans for the temporary storage of amorphous CaCO3 (ACC). The gastrolithic ACC is stabilized by the presence of biomolecules, and represents a novel model for research into biomineralization. For the first time, an in vitro biomimetic retrosynthesis of scaffolds of gastrolithic matrices with CaCO3 is presented. With the help of synthetic polyacrylic (PAA) and phytic (PA) acids, amorphous precursor particles were stabilized in double (DD) and gas (GD) diffusion crystallization assays. The presence of these synthetic molecules as efficient inhibitors of nucleation and growth of CaCO3, and the use of biological gastrolith scaffolds as confined reaction environments determined the kinetics of crystallization, and controlled the morphogenesis of CaCO3. The formation of ACC particles was demonstrated and their crystallization was followed by light microscopy, scanning and transmission electron microscopy, and electron diffraction. published

Obwohl schon viele Studien auf dem Gebiet der Nukleation und Kristallisation von Eisen(oxyhydr)oxiden durchgeführt wurden, ist die Rolle der Spezies, die sich in den frühen Stadien entwickeln, besonders primäre Cluster und amorphe Übergangspartikel, immer noch schlecht verstanden. Hier wurden sowohl ligandenfreie als auch ligandengeschützte amorphe Eisenoxid-Cluster (“amorphous iron oxide”, AIO

Proteins controlling mineralization in vivo are diverse, suggesting that there are various ways by which mineralization can be directed in bio-inspired approaches. While well-defined three-dimensional (3D) structures occur in biomineralization proteins, the design of synthetic, soluble, bio-inspired macromolecules with specific, reproducible and predictable 3D arrangements of mineral-interacting functions poses an ultimate challenge. Thus, the question how certain arrangements of such functions on protein surfaces influence mineralization, and in which way, subsequently, specific alterations affect this process, remains elusive. Here, we used genetically engineered Ubiquitin (Ub) proteins in order to overcome the limitations of generic bio-inspired additive systems. By advancing existing protocols, we introduced an unnatural amino acid and, subsequently, mineral-interacting functions via selective pressure incorporation and click chemistry, respectively, without affecting the Ub secondary structure. Indeed, as-obtained Ub with three phosphate functions at defined positions shows unique effects, based on a yet unmatched capability towards the stabilization of a film of a dense liquid mineral phase visible even by naked eye, its transformation into amorphous nanoparticles, and afterwards crystals with complex shapes. We thereby demonstrate that Ub designer proteins pose a unique, new generation of crystallization additives where the 3D arrangement of mineral-interacting functions can be designed at will, promising a future use for programmable, target-oriented mineralization control. published

The mineral replacement of gypsum (CaSO4·2H2O) by baryte (BaSO4) is relevant to technological and industrial applications, including its use as a plaster or stone consolidant in cultural heritage conservation. In the present study, we provide experimental evidence suggesting that, during the interaction of gypsum cleavage surfaces with barium-bearing solutions, a pseudomorphic replacement takes place and results in the formation of a crystallographically oriented baryte layer. This mineral replacement process is favoured by the porosity generated, due to the differences in molar volume and solubility between parent and product sulfate phases, allowing the progress of the reaction. The homogeneous micrometre-sized layer of baryte occurs most likely via a fluid-mediated interface-coupled dissolution–precipitation mechanism. A certain degree of crystallographic control on the polycrystalline BaSO4 product layer by the structure of the parent substrate (gypsum) is confirmed by electron microscopy observations and X-ray diffraction analyses. The structural control exerted by the cleavage gypsum surface on the baryte layer can be defined by the epitactic relationship: Gyp (010) || Bar (010). The formation of baryte increases with reaction time until passivation occurs at the replacement interface, probably due to a decreased porosity and loss of connectivity that thereby prevents further reaction. The investigation of these processes occurring on freshly cleaved single crystals of gypsum were complemented by studying the replacement of polycrystalline gypsum cubes, showing a homogeneous baryte surface layer on the sample. The results of this study thus offer interesting insights into the application of the replacement of gypsum by baryte as a conservation method for gypsum sculptures and plasterwork, increasing their resistance against water and humidity while preserving the surface features of the original mineral substrate. published

Hematite (α-Fe2 O3 ) is thermodynamically stable under ambient conditions, of vast geological importance, and widely used in applications, for example, as corrosion protection and as a pigment. It forms at elevated temperatures, whereas room-temperature reactions typically yield metastable akaganeite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the prenucleation hydrolysis reaction categorically implies the presence of prenucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above approximately 50 °C, whereby the reaction limitation towards oxolation in phase-separated clusters is overcome. A model that rationalizes the occurrence of hematite, akaganeite, and ferrihydrite based on the chemistry of olation PNCs is proposed. Supersaturation and the temperature dependence of olation and oxolation rates from monomeric precursors are irrelevant in this nonclassical mechanism.

A general gas diffusion strategy using 10,12-pentacosadiynoic acid as a ligand and ethanol as a solvent was developed to fabricate stable amorphous mineral clusters with a core size of less than 2 nm., While nature exerts precise control over the size and chemical composition of minerals, this is still a challenging task for artificial syntheses. Despite its significance, until now, there are still no reports on colloidal mineral nanoparticles in the subnanometer range. Here we developed a general gas diffusion strategy using 10,12-pentacosadiynoic acid as a ligand and ethanol as a solvent to fabricate stable amorphous mineral clusters with a core size of less than 2 nm. First discovered for CaCO3, the method was successfully extended to produce monolayer protected clusters of MgCO3, SrCO3, Eu2(CO3)3, Tb2(CO3)3, Ce2(CO3)3, Cax(PO4)y, CaC2O4 and their hybrid minerals, CaxMgy(CO3)z and Cax(CO3)y(PO4)z. All the mineral clusters can be well dispersed in organic solvents like toluene, and are stable for a long period without further crystallization. Our work paves a way for the artificial synthesis of colloidal mineral clusters, which may have various uses in both fundamental research and industry.

Nacre continues to be an inspiration for the fabrication of strong and tough materials from renewable and earth-abundant raw materials. Herein, we showed how a nacre-like hybrid material based on nanocellulose (NC) and CaCO3 can be prepared via the sequential infiltration of polymer-stabilised CaCO3 liquid precursors into layers of pre-deposited NC films. Layer-by-layer assembly of the NC films followed by controlled spreading and infiltration with liquid CaCO3 precursors generated a lamellar material with an architecture and iridescent appearance similar to those of nacre. The wettability of the NC films towards the liquid CaCO3 precursors was controlled by hydroxyl and carboxyl functionalization of the NC fibrils and the addition of magnesium ions. The combination of a high stiffness and plasticity of the nacre-like NC/CaCO3 hybrid materials show that excellent mechanical properties can be obtained employing a fibrillar organic constituent that is relatively hard. The fabrication of a nacre-like hybrid material via an aqueous route of assembly and infiltration processing demonstrates how a sustainable composite material with outstanding properties can be produced using the most abundant biopolymer and biomineral on earth. published

Viele Naturphänomene hängen mit der Nukleation von CaCO3 zusammen, wobei die Rolle des Wassers bis jetzt weitgehend unklar war. Änderungen in der Absorption im THz-Frequenzbereich während der frühen Stadien der CaCO3-Nukleation belegen Veränderungen in den gekoppelten Bewegungen der hydratisierten Calcium- und Carbonat-Ionen. Der direkte Zusammenhang zwischen diesen Effekten und der kontinuierlichen Entwicklung des Ionenaktivitätsproduktes zeigt die Lage einer flüssig-flüssig-binodalen Grenze. Die Daten lassen stark darauf schließen, dass sich proto-strukturiertes, amorphes CaCO3 über die Verfestigung von anfänglich flüssigen Vorstufen bildet. Außerdem erhöhen Polycarboxylate, die flüssige Vorstufen von CaCO3 stabilisieren, die kinetische Stabilität des metastabilen flüssig-flüssigen Zustands signifikant, aber sie beeinflussen nicht die Lage der binodalen Grenze. Die mechanistische Bedeutung der Dynamik von Wassernetzwerken bei der Phasentrennung kann als alternativer Weg der Nukleation über Pränukleationscluster rationalisiert werden und ist wahrscheinlich allgemein für wässrige Systeme gültig. published

Many phenomena depend on CaCO3 nucleation where the role of water remains enigmatic. Changes in THz absorption during the early stages of CaCO3 nucleation evidence altered coupled motions of hydrated calcium and carbonate ions. The direct link between these changes and the continuous development of the ion activity product reveals the locus of a liquid–liquid binodal limit. The data strongly suggest that proto-structured amorphous CaCO3 forms through solidification of initially liquid precursors. Furthermore, polycarboxylates, which stabilize liquid precursors of CaCO3, significantly enhance the kinetic stability of the metastable liquid–liquid state, but they do not affect the locus of the binodal limit. The importance of water network dynamics in phase separation mechanisms can be understood based on the notions of the pre-nucleation cluster pathway, and is likely to be more general for aqueous systems. published