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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., (© 2022 The Authors. Small published by Wiley-VCH GmbH.)

Designing bio-inspired, multifunctional organic-inorganic composite materials is one of the most popular current research objectives. Due to the high complexity of biocomposite structures found in nacre and bone, for example, a one-pot scalable and versatile synthesis approach addressing structural key features of biominerals and affording bio-inspired, multifunctional organic-inorganic composites with advanced physical properties is highly challenging. This article reviews recent progress in synthesizing organic-inorganic composite materials via various self-assembly techniques and in this context highlights a recently developed bio-inspired synthesis concept for the fabrication of hierarchically structured, organic-inorganic composite materials. This one-step self-organization concept based on simultaneous liquid crystal formation of anisotropic inorganic nanoparticles and a functional liquid crystalline polymer turned out to be simple, fast, scalable and versatile, leading to various (multi-)functional composite materials, which exhibit hierarchical structuring over several length scales. Consequently, this synthesis approach is relevant for further progress and scientific breakthrough in the research field of bio-inspired and biomimetic materials.

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as "non-classical crystallization", including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

In this article, we present a systematic, controlled synthesis of water-dispersed, polymer-stabilized Au nanoparticles with different sizes by either self-reduction or ultraviolet (UV) irradiation-enhanced reduction in the presence of double hydrophilic block copolymers with different functional patterns. The morphology of the particles was found to be spherical, triangular, truncated triangular, and hexagonal under various conditions. In addition, both the particle size and shape could be controlled by variation of the functional group pattern of the block copolymers. With a hexacyclen functional block, we achieved a perfect match to the Au (111) face, resulting in extremely thin free-standing triangular and hexagonal nanoplatelets that show interference patterns in the electron beam due to deformations in the sub-Angstrom range. UV-visible spectra show that the plasmon absorption band can be fine tuned, depending on the type of the polymer and the irradiation power. The influence of the functional polymer groups and the irradiation on the formation of gold nanoparticles was investigated in more detail.

Medical treatment options for bones and teeth can be significantly enhanced by taking control over the crystallization of biomaterials like hydroxyapatite in the healing process. Light‐induced techniques are particularly interesting for this approach as they offer tremendous accuracy in spatial resolution. However, in the field of calcium phosphates, light‐induced crystallization has not been investigated so far. Here, proof of principle is established to successfully induce carbonate‐hydroxyapatite precipitation by light irradiation. Phosphoric acid is released by a photolabile molecule exclusively after irradiation, combining with calcium ions to form a calcium phosphate in the crystallization medium. 4‐Nitrophenylphosphate (4NPP) is established as the photolabile molecule and the system is optimized and fully characterized. A calcium phosphate is crystallized exclusively by irradiation in aqueous solution and identified as carbonate apatite. Control over the localization and stabilization of the carbonate apatite is achieved by a pulsed laser, triggering precipitation in calcium and 4NPP‐containing gel matrices. The results of this communication open up a wide range of new opportunities, both in the field of chemistry for more sophisticated reaction control in localized crystallization processes and in the field of medicine for enhanced treatment of calcium phosphate containing biomaterials. [ABSTRACT FROM AUTHOR]

Photochemical activation is proposed as a general method for controlling the crystallization of sparingly soluble carbonates in space and time. The photogeneration of carbonate in an alkaline environment is achieved upon photo‐decarboxylation of an organic precursor by using a conventional 365 nm UV LED. Local irradiation was conducted focusing the LED light on a 300 μm radius spot on a closed glass crystallization cell. The precursor solution was optimized to avoid the precipitation of the photoreaction organic byproducts and prevent photo‐induced pH changes to achieve the formation of calcium carbonate only in the corresponding irradiated area. The crystallization was monitored in real‐time by time‐lapse imaging. The method is also shown to work in gels. Similarly, it was also shown to photo‐activate locally the formation of barium carbonate biomorphs. In the last case, the morphology of these biomimetic structures was tuned by changing the irradiation intensity. [ABSTRACT FROM AUTHOR]

The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to monitor the early stages of the crystallization of ibuprofen as a model system for small organic molecules. Ibuprofen undergoes liquid–liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid–liquid miscibility gap were analyzed and confirmed. An increase in viscosity sustains the kinetic stability of the dense liquid intermediate. Since the distances between ibuprofen molecules within the dense liquid phase are similar to those in the crystal forms, this dense liquid phase is identified as a precursor phase in the nucleation of ibuprofen, in which densification is followed by generation of structural order. This discovery may make it possible to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The role of the double-hydrophilic block copolymer poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) on the morphogenesis of calcium carbonate (CaCO3) was studied by applying the contrast variation small angle neutron scattering technique. The morphology and size of CaCO3 crystals is strongly affected by the addition of PEG-b-PMAA. In order to determine the partial scattering functions of the polymer and CaCO3 mineral, we developed both an experimental and theoretical approach with a sophisticated method of their determination from the scattering intensity. Partial scattering functions give detailed information for each component. In particular, the partial scattering function of the polymer, Spp, shows a monotonic slope with Q ² to ³ where the scattering vector Q is low (Q<0.01 Å ¹), which is a clear evidence that the polymer within the CaCO3 mineral has a mass fractal dimension. The other partial scattering functions reflected the geometry of the CaCO3 particles or the "interaction" of polymer and CaCO3 on a microscopic scale, which leads to a coherent view with Spp [ABSTRACT FROM AUTHOR]

Abstract: Biogenic nucleation and crystallization occur in confined spaces with defined interfacial properties. However, the regulatory functions of organic players in the stabilization and transport of inorganic precursors such as ion clusters, liquid‐condensed phases, and amorphous particles are unclear. Given the prevalence of unstructured proteins in biogenic materials, the present study investigates the effects of biomineral‐associated, intrinsically disordered protein domains with simple and repetitive amino acid compositions on mineral nucleation and their capability to form distinct supramolecular assemblies. The quantitative assessment and structural evaluation of the nucleation process reveal that disordered regions confine hydrated mineral precursors within vesicles, transiently suppressing mineral precipitation. Stabilization of the amorphous mineral is attributed to protein self‐association and restructuration toward β‐configurations, triggered by specific bioinorganic interactions. In consequence, the conditioned macromolecules localize at phase boundaries formed upon liquid–liquid demixing of mineral precursors and stabilize the fluidic mineral precursors against crystallization. Thus, the conformational plasticity and self‐association of intrinsically disordered sequences in response to crystallization environments mediates the selection of functional macromolecular subensembles dedicated to biomaterial growth. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The investigation of mineralization and demineralization processes is important for the understanding of many phenomena in daily life. Many crystalline materials are exposed to decay processes, resulting in lesions, cracks, and cavities. Historical artifacts, for example, often composed of calcium carbonate (CaCO3), are damaged by exposure to acid rain or temperature cycles. Another example for lesions in a crystalline material is dental caries, which lead to the loss of dental hard tissue, mainly composed of hydroxyapatite (HAp). The filling of such cavities and lesions, to avoid further mineral loss and enable or support the remineralization, is a major effort in both areas. Nevertheless, the investigation of the filling process of these materials into the cavities is difficult due to the non-transparency and crystallinity of the concerned materials. In order to address this problem, we present a transparent, inexpensive, and reusable test system for the investigation of infiltration and crystallization processes in situ, being able to deliver datasets that could potentially be used for quantitative evaluation of the infiltration process. This was achieved using a UV-lithography-based micro-comb test system (MCTS), combined with self-assembled monolayers (SAMs) to mimic the surface tension/wettability of different materials, like marble, sandstone, or human enamel. Moreover, the potential of this test system is illustrated by infiltration of a CaCO3 crystallization solution and a hydroxyapatite precursor (HApP) into the MCTS. [ABSTRACT FROM AUTHOR]

Water is a fundamental solvent sustaining life, key to the conformations and equilibria associated with solute species. Emerging studies on nucleation and crystallization phenomena reveal that the dynamics of hydration associated with mineral precursors are critical in determining material formation and growth. With certain small molecules affecting the hydration and conformational stability of co-solutes, this study systematically explores the effects of these chaotropes and kosmotropes as well as certain sugar enantiomers on the early stages of calcium carbonate formation. These small molecules appear to modulate mineral nucleation in a class-dependent manner. The observed effects are finite in comparison to the established, strong interactions between charged polymers and intermediate mineral forms. Thus, perturbations to hydration dynamics of ion clusters by co-solute species can affect nucleation phenomena in a discernable manner. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Synergistic relations between organic molecules and mineral precursors regulate biogenic mineralization. Given the remarkable material properties of the egg shell as a biogenic ceramic, it serves as an important model to elucidate biomineral growth. With established roles of complex anionic biopolymers and a heterogeneous organic scaffold in egg shell mineralization, the present study explores the regulation over mineralization attained by applying synthetic polymeric counterparts (polyethylene glycol, poly(acrylic acid), poly(aspartic acid) and poly(4-styrenesulfonic acid-co-maleic acid)) as additives during remineralization of decalcified eggshell membranes. By applying Mg2+ ions as a co-additive species, mineral retrosynthesis is achieved in a manner that modulates the polymorph and structure of mineral products. Notable features of the mineralization process include distinct local wettability of the biogenic organic scaffold by mineral precursors and mineralization-induced membrane actuation. Overall, the form, structure and polymorph of the mineralization products are synergistically affected by the additive and the content of Mg2+ ions. We also revisit the physicochemical nature of the biomineral scaffold and demonstrate the distinct spatial distribution of anionic biomolecules associated with the scaffold-mineral interface, as well as highlight the hydrogel-like properties of mammillae-associated macromolecules. [ABSTRACT FROM AUTHOR]

Proteins are known to play various key roles in the formation of complex inorganic solids during natural biomineralisation processes. However, in most cases our understanding of the actual underlying mechanisms is rather limited. One interesting example is perlucin, a protein involved in the formation of nacre, where it is believed to promote the crystallisation of calcium carbonate. In the present work, we have used potentiometric titration assays to systematically investigate the influence of recombinant GFP-labeled perlucin derivatives on the early stages of CaCO3 formation. Our results indicate that different parts of the protein can impact nucleation in distinct ways and act in either a retarding or promoting fashion. The most important finding is that full-length GFP-perlucin changes the nature of the initially precipitated phase and seems to favour the direct formation of crystalline polymorphs over nucleation of ACC and subsequent phase transformation, as observed in reference experiments without protein. This confirms the supposed role of perlucin in nacre biomineralisation and may rely on specific interactions between the protein and the crystal lattice of the emerging mineral phase. [ABSTRACT FROM AUTHOR]

Biogenic mineralization processes are generally regulated by soluble additives and insoluble matrices. This endows precise control over the different stages of mineralization such as the uptake, transport of mineral precursors as well as the subsequent deposition of the mineral phases with consistent compositions and morphologies. Programmed in the interactions of organic molecules with different precursor species and the fine modulation of the niche environments, a formative elegance is reflected in the biological means for crystal formation in comparison to the synthetic counterparts. In order to spotlight the role of prevalent biophysical environments in the emergence of fascinating materials, we revisit biologically modulated mineralization to describe nucleation and crystallization under physicochemical highly non-ideal conditions on account of macromolecular crowding and the gel-like nature of cellular matrices. [ABSTRACT FROM AUTHOR]

Amorphous intermediate phases are vital precursors in the crystallization of many biogenic minerals. While inherent short-range orders have been found in amorphous calcium carbonates (ACCs) relating to different crystalline forms, it has never been clarified experimentally whether such orders already exist in very small clusters less than 2 nm in size. Here, we studied the stability and structure of 10,12-pentacosadiynoic acid (PCDA) protected ACC clusters with a core size of ca. 1.4 nm consisting of only seven CaCO3 units. Ligand concentration and structure are shown to be key factors in stabilizing the ACC clusters. More importantly, even in such small CaCO3 entities, a proto-calcite short-range order can be identified but with a relatively high degree of disorder that arises from the very small size of the CaCO3 core. Our findings support the notion of a structural link between prenucleation clusters, amorphous intermediates, and final crystalline polymorphs, which appears central to the understanding of polymorph selection. [ABSTRACT FROM AUTHOR]

The identity of the crystallized protein in the article by Juneja et al. [(2014), Acta Cryst. F70, 260–262] is corrected. [ABSTRACT FROM AUTHOR]

The effect of amphoteric siderophore deferoxamine on the crystallization behavior of calcium carbonate was investigated under bioinspired conditions. Amphoteric siderophore deferoxamine possesses self-organization ability, surface activity and ion-chelating properties. It induces in the present case the formation of unusual, highly organized aragonite mesocrystals during the precipitation of calcium carbonate from liquid phase. A detailed investigation of the structure and growth of the particles provides an insight into the role of the deferoxamine and its function in the crystallization process. [ABSTRACT FROM AUTHOR]

Abstract: Mesocrystals are superstructures of nanoparticles with mutual order and can be intermediates in a non-classical particle-mediated crystallization reaction. Mesocrystals have various potential applications as functional materials due to their unique combination of nanoparticle properties and order combined with a microscopic or even macroscopic size. They can also show collective and emergent properties. In the past, most attention has been paid to the synthesis and the formation mechanism of mesocrystals and in some cases properties and potential applications were reported. Only recently, the number of studies on mesocrystal applications has increased with a clear focus on photocatalysis, Li-ion battery and electrode applications. In this opinion paper, the remarkable progress in the field of mesocrystal applications of the last three years is highlighted including examples, where mesocrystals already now can by far outperform traditional materials with their excellent material properties, which can even reach the theoretically maximally possible value. The future application potential of mesocrystals is also discussed. [Copyright &y& Elsevier]

It takes two different functional additives to produce the title structures. The proposed mechanism based on the nonclassical particle‐mediated crystallization of calcium carbonate demonstrates the individual and cooperative effects of the polymer poly(sodium 4‐styrenesulfonate) and small folic acid molecules on the formation of heterostructures at different reaction stages. [ABSTRACT FROM AUTHOR]

Biomineralization can afford crystal frameworks of great diversity and utmost complexity, frequently featuring hierarchical structures and morphologies beyond any crystallographic restrictions. The formation of such architectures is usually directed by organic molecules or matrices, which modify crystallization in a deliberate manner. Their influence often leads to sinuous forms, which, by intuition, suggest the presence of life and distinguish these minerals from their inanimate, mostly euhedral counterparts. However, such a strict distinction does not hold. In fact, smooth curvature and higher-order structuring can occur also in purely inorganic environments: simply by precipitating alkaline earth carbonates in silica-containing media, aggregates of highly oriented carbonate nanocrystals can be obtained that display striking noncrystallographic morphologies such as regular helicoids. Thereby, individual crystallites as well as the entire assembly are sheathed by amorphous silica, thus giving a composite material with various levels of hierarchy. These exceptional forms, called 'silica biomorphs', self-assemble through a bottom-up process, which relies on local variations in the conditions and is driven by a pH-based coupling of the carbonate and silicate. Here, we review recent progress in the field of silica biomorphs with particular focus on their mechanism of formation, provide insight into structural details at different length scales, and discuss implications of these biomimetic crystal aggregates for both primitive life detection and materials science. [ABSTRACT FROM AUTHOR]

Structuring over many length scales is a design strategy widely used in Nature to create materials with unique functional properties. We here present a comprehensive analysis of an adult sea urchin spine. and in revealing a complex, hierarchical structure, show how Nature fabricates a material which diffracts as a single crystal of calcite and yet fractures as a glassy material. Each spine comprises a highly oriented array of Mg-calcite nanocrystals in which amorphous regions and macromolecules are embedded. It is postulated that this meso-crystalline structure forms via the crystallization of a dense array of amorphous calcium carbonate (ACC) precursor particles. A residual surface layer of ACC and/or macromolecules remains around the nanoparticle units which creates the mesocrystal structure and contributes to the conchoidal fracture behavior. Nature's demonstration of how crystallization of an amorphous precursor phase can create a crystalline material with remarkable properties therefore provides inspiration for a novel approach to the design and synthesis of synthetic composite materials. [ABSTRACT FROM AUTHOR]

Summary: In this contribution, we review the recent literature on an alternative crystallization pathway involving stable clusters prior to nucleation and show that the prenucleation cluster pathway is a truly non-classical concept of nucleation. Prenucleation clusters are solutes with “molecular” character in aqueous solution. It becomes evident that the stable clusters may have been concealed by the so-called ion pair concept and activity effects. We show that non-classical nucleation via stable prenucleation clusters is a highly valuable concept for a novel understanding of phenomena observed in bio- and biomimetic mineralization, which however may hardly be rationalized by means of classical nucleation theory. The importance to combine experimental and theoretical studies is emphasized. [Copyright &y& Elsevier]

Stable monodispersed amorphous calcium carbonate (ACC) nanoparticles can be synthesized in ethanol media by a facile method, and crystallization of ACC is kinetically controlled, resulting in the formation of three polymorphs in a mixed solvent of ethanol–water at different pH values. [ABSTRACT FROM AUTHOR]

Wet washing is a widely used method for biodiesel purification. However, this technique generates a large amount of wastewater that needs to be treated afterward, which is costly and time-consuming. Thus, solvent-aided crystallization (SAC) with ultrasonic irradiation as solution movement assistance was introduced. This technique is based on the addition of 1-butanol to biodiesel to enhance purification via crystallization. During crystallization, two phases are formed, where glycerol solidifies (solid phase) and pure biodiesel remains (liquid phase). Technically, the implementation of ultrasonic technology can optimize laboratory work by saving time, as no cleaning or washing of the propeller is needed. Biodiesel purity was analyzed using gas chromatography-mass spectroscopy (GC-MS), where a purity of 99% was achieved. The optimum parameters in achieving higher purity fatty acid methyl ester (FAME) were a 1-butanol concentration of 1 wt.%, a coolant temperature of 9 °C, and a crystallization time of 40 min. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Crystal formation is a highly debated problem. By employing THz waves, it is shown that the crystallization of L‐(+)‐tartaric acid from water follows a non‐classical path involving intermediate hydrated states. The frontispiece image illustrates this crystallization process. The balloons on the top show structural schemes of the molecular arrangement in the different stages. The center of the figure represents a small section of the probed volume at a fixed time. The probed volume contains a mixture of the different stages. As time proceeds, the relative contributions of these phases to the total volume change. For more information see the Communication by H. Cölfen et al. on page 14128 ff. [ABSTRACT FROM AUTHOR]

Highlights • Several beam artifacts in the Liquid phase TEM (LP-TEM) are dicussed. • Methods to prevent the dissolution of oxides in the LP- TEM. • Modifying solution and/or window chemistry to control the mobility of particles. • Better controlled experiment by maintaining flow to LP-TEM and performing controlled heating. Abstract In situ liquid phase scanning/ transmission electron microscopy (LP-S/TEM), enables direct observation of particle formation and evolution in the hydrated liquid state. Though powerful, this technique has significant technical limitations, which need to be carefully addressed in order to obtain reliable quantitative data. In this paper, we highlight several common challenges seen in LP-TEM, including electron-beam induced particle dissolution and motion, particle-membrane adhesion, contamination, and triggering of reactions. We describe our efforts to address these challenges by modifying solution and interface chemistry, maintaining solution flow, performing systematic post in situ analyses on the samples and applying controlled heating. [ABSTRACT FROM AUTHOR]

This article focuses on mesocrystals and morphology change of CaCO3 crystals in the presence of a polyelectrolyte additive. Crystallization control is one of the most important techniques of preparation, purification, and application of solid substances, making it an intensely studied field. Besides conventional crystallization, there are also nonclassical pathways of crystallization via colloidal intermediates and mesoscale transformation, in which crystalline structures are constructed and/or transformed from larger units instead of single ions. The authors present a simple system of calcite paired with polystyrene sulfonate, which undergoes mesocrystal formation. Appropriate concentration adjustment of both partners allows broad, systematic variation of the mesocrystal morphology that demonstrates long-range interactions. Moreover, the authors reports that selective coding of nanocrystal surfaces by polymers can not only lead to mesocrystals exposing unusual crystal faces in their superstructure, but also can form self-assembled structures with changed symmetry.

Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. Huge amounts of carbon dioxide are retained as carbonate ions, and calcium ions represent a major contribution to water hardness. Despite its relevance, little is known about the precipitation mechanism of calcium carbonate, and specified complex crystal structures challenge the classical view on nucleation considering the formation of metastable ion clusters. We demonstrate that dissolved calcium carbonate in fact contains stable prenucleation ion clusters forming even in undersaturated solution. The cluster formation can be characterized by means of equilibrium thermodynamics, applying a multiple-binding model, which allows for structural preformation. Stable clusters are the relevant species in calcium carbonate nucleation. Such mechanisms may also be important for the crystallization of other minerals. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis and folding behavior of two different series of double hydrophilic block copolymers based on polyethyleneglycol and short acidic polypeptide blocks are described. The employment of those species in the crystallization of CaCO3 and BaSO4 shows that in contrast to the expectations from biomineral examples, a random conformation of the peptide is more advantageous in mineralization control than an α-helical arrangement of the same chain, which is contradictory to the epitaxial adsorption model in biomineralization. Otherwise, the peptide block copolymers support formation of superstructures similar to those already reported, e.g. well-defined spherical, prolate, and dumbbell morphologies are found. [Copyright &y& Elsevier]

The debate about crystallization processes is still ongoing and nonclassical crystallization mechanisms attract more and more attention. This work indicates that polymer induced liquid precursor (PILP) phases play a role for nonclassical calcium carbonate crystallization and growth processes. Here we report the observation of gel-like precursors for the crystal growth on a calcite surface by means of an in situ AFM study. These precursors spread out on the surface with time supporting their liquid character. This study will give new insights into biomineralization and crystallization processes in general. [ABSTRACT FROM AUTHOR]

Electrospun submicrometer-sized poly(ε-caprolactone) (PCL) meshes and nanosized multiwalled carbon nanotubes (MWCNTs) were used as a template for preparing porous and interconnected inorganic-organic hybrid materials composed of CaCO3. Herein, we describe the proportion and incorporation method of submicrometer-sized plasma-treated PCL meshes over areas >1 mm² with CaCO3 using three crystallization methods including the use of poly(acrylic acid) (PAA). We found that flexible and rigid acid-functionalized MWCNTs showed a clear capacity and effects to penetrate calcite particles. MWCNTs interacted differently with the individual growth planes of CaCO3, indicating that fibers can undergo changes depending on sulfonate or carboxylate groups, adopt different orientations in solution, and thereby elicit changes in CaCO3 morphology. In summary, the use of PCL and acidic MWCNT fibers as an additive for substrate templates and experimental crystallization provides a viable approach for studying various aspects of biomineralization, including the production of controlled particles, control of porosities, and defined morphologies at microscale and nanoscale levels. [ABSTRACT FROM AUTHOR]

The article discusses research on particle attachment crystallization (CPA) in synthetic, biogenic, and geologic environments. Topics include the nonclassical thermodynamic and dynamic mechanisms for crystallization pathways, the control of nanomaterials synthesis, and the role of free-energy landscapes and reaction dynamics in allowing for multiple pathways. The relation of ensemble behavior to synthetic systems, biominerals, and natural mineralization is addressed.

Abstract: In this study, split-and-mix peptide libraries from one to four amino acids bound to functionalized beads were used to identify active morphogenesis peptides for the systems CaCO3 and dl-alanine. Density gradient ultracentrifugation was used to remove all beads without crystals as well as crystals homogeneously nucleated in solution. From the remaining fractions, beads were selected, which account for differences in crystal morphologies from the default crystal morphology on a micrometer scale and their amino acid sequence was analyzed. Our results show that multiple and different peptide sequences are found to be active in the morphogenesis of CaCO3 and dl-alanine. It was not possible to find a correlation, which connects active single amino acids with the sequences of di-, tri- and tetrapeptides. However, peptide charge was found to be important for morphogenesis. Peptides active in CaCO3 morphogenesis were enriched in basic amino acids while those active for dl-alanine morphogenesis contained more acidic amino acids. This can be explained by charge–charge interactions of the crystallizing species with the countercharged peptide moieties. Tests for chiral separation for the dl-alanine system showed that with the applied oligopeptide libraries, no enantioselective crystallization was achieved to a significant extent. The presented combinatorial crystallization assay provides an easy tool for crystallization control, which can be used for the straightforward selection of interesting species under a light microscope. However, suitable staining techniques to identify individual beads with crystals, which are for example a pure enantiomer still need to be developed. [Copyright &y& Elsevier]