New Horizons of Nonclassical Crystallization

Complex Pathways Drive Pluripotent Fmoc-Leucine Self-Assemblies

Nature uses complex self-assembly pathways to access distinct functional non-equilibrium self-assemblies. This remarkable ability to steer same set of biomolecules into different self-assembly states is done by avoiding thermodynamic pit. In synthetic systems, on demand control over 'Pathway Complexity' to access self-assemblies different from equilibrium structures remains challenging. Here we show versatile non-equilibrium assemblies of the same monomer via alternate assembly pathways. The assemblies nucleate using non-classical or classical nucleation routes into distinct metastable (transient hydrogels), kinetic (stable hydrogels) and thermodynamic structures [(poly)-crystals and 2D sheets]. Initial chemical and thermal inputs force the monomers to follow different assembly pathways and form soft-materials with distinct molecular arrangements than at equilibrium. In many cases, equilibrium structures act as thermodynamic sink which consume monomers from metastable structures giving transiently formed materials. This dynamics can be tuned chemically or thermally to slow down the dissolution of transient hydrogel, or skip the intermediate hydrogel altogether to reach final equilibrium assemblies. If required this metastable state can be kinetically trapped to give strong hydrogel stable over days. This method to control different self-assembly states can find potential use in similar biomimetic systems to access new materials for various applications.

Nonclassical crystallization of goethite nanorods in limpet teeth by self-assembly of silica-rich nanoparticles reveals structure-mechanical property relations

The intricate organization of goethite nanorods within a silica-rich matrix makes limpet teeth the strongest known natural material. However, the mineralization pathway of goethite in organisms under ambient conditions remains elusive. Here, by investigating the multi-level structure of limpet teeth at different growth stages, it is revealed that the growth of goethite crystals proceeds by the attachment of amorphous nanoparticles, a nonclassical crystallization pathway widely observed during the formation of calcium-based biominerals. Importantly, these nanoparticles contain a high amount of silica, which is gradually expelled during the growth of goethite. Moreover, in mature teeth of limpet, the content of silica correlates with the size of goethite crystals, where smaller goethite crystals are densely packed in the leading part with higher content of silica. Correspondingly, the leading part exhibits higher hardness and elastic modulus. Thus, this study not only reveals the nonclassical crystallization pathway of goethite nanorods in limpet teeth, but also highlights the critical roles of silica in controlling the hierarchical structure and the mechanical properties of limpet teeth, thus providing inspirations for fabricating biomimetic materials with excellent properties.

Synthetic and analytical considerations for the preparation of amorphous metal-organic frameworks

Metal-organic frameworks (MOFs) are hybrid porous materials presenting several tuneable properties, allowing them to be utilised for a wide range of applications. To date, focus has been on the preparation of novel crystalline MOFs for specific applications. Recently, interest in amorphous MOFs (aMOFs), defined by their lack of correlated long-range order, is growing. This is due to their potential favourable properties compared to their crystalline equivalents, including increased defect concentration, improved processability and gas separation ability. Direct synthesis of these disordered materials presents an alternative method of preparation to post-synthetic amorphisation of a crystalline framework, potentially allowing for the preparation of aMOFs with varying compositions and structures, and very different properties to crystalline MOFs. This perspective summarises current literature on directly synthesised aMOFs, and proposes methods that could be utilised to modify existing syntheses for crystalline MOFs to form their amorphous counterparts. It outlines parameters that could discourage the ordering of crystalline MOFs, before examining the potential properties that could emerge. Methodologies of structural characterisation are discussed, in addition to the necessary analyses required to define a topologically amorphous structure.

Insight from Boric Acid into Bioskeleton Formation: Inscribed Circle Effect on the Edge-Base Plate Growth

Complex morphologies in nature often arise from the assembly of elemental building blocks, leading to diverse and intricate structures. Understanding the mechanisms that govern the formation of these complex morphologies remains a significant challenge. In particular, the edge-base plate growth of biogenic crystals plays a crucial role in directing the development of intricate bioskeleton morphologies. However, the factors and regulatory processes that govern edge-base plate growth remain insufficiently understood. Inspired by biological skeletons and based on the soluble property of boric acid (BA) in both water and alcohols, we obtained a series of novel BA morphologies, including coccolith, and anemone biological skeletons. Here, we unveil the "inscribed circle effect", a concise mathematical model that reveals the underlying causative factors and regulatory mechanisms driving edge-base plate growth. Our findings illuminate how variations in solvent environments can exert control over the edge-base plate growth pathways, thereby resulting in the formation of diverse and complex morphologies. This understanding holds significant potential for guiding the chemical synthesis of bioskeleton materials.

Liquid-Liquid Phase Separation Mediated Formation of Chiral 2D Crystalline Nanosheets of a Co-Assembled System

The role of macromolecule-macromolecule and macromolecule-H2O interactions and the resulting perturbation of the H-bonded network of H2O in the liquid-liquid phase separation (LLPS) process of biopolymers are well-known. However, the potential of the hydrated state of supramolecular structures (non-covalent analogs of macromolecules) of synthetic molecules is not widely recognized for playing a similar role in the LLPS process. Herein, LLPS occurred during the co-assembly of hydrated supramolecular vesicles (bolaamphiphile, BA1) with a net positive charge (zeta potential, ζ = +60 ± 2 mV) and a dianionic chiral molecule (disodium l-[+]-tartrate) is reported. As inferred from cryo-transmission electron microscopy (TEM), the LLPS-formed droplets serve as the nucleation precursors, dictating the structure and properties of the co-assembly. The co-assembled structure formed by LLPS effectively integrates the counter anion's asymmetry, resulting in the formation of ultrathin free-standing, chiral 2D crystalline sheets. The significance of the hydrated state of supramolecular structures in influencing LLPS is unraveled through studies extended to a less hydrated supramolecular structure of a comparable system (BA2). The role of LLPS in modulating the hydrophobic interaction in water paves the way for the creation of advanced functional materials in an aqueous environment.

Exploring the Potential of Nonclassical Crystallization Pathways to Advance Cementitious Materials

Understanding the crystallization of cement-binding phases, from basic units to macroscopic structures, can enhance cement performance, reduce clinker use, and lower CO2 emissions in the construction sector. This review examines the crystallization pathways of C-S-H (the main phase in PC cement) and other alternative binding phases, particularly as cement formulations evolve toward increasing SCMs and alternative binders as clinker replacements. We adopt a nonclassical crystallization perspective, which recognizes the existence of critical intermediate steps between ions in solution and the final crystalline phases, such as solute ion associates, dense liquid phases, amorphous intermediates, and nanoparticles. These multistep pathways uncover innovative strategies for controlling the crystallization of binding phases through additive use, potentially leading to highly optimized cement matrices. An outstanding example of additive-controlled crystallization in cementitious materials is the synthetically produced mesocrystalline C-S-H, renowned for its remarkable flexural strength. This highly ordered microstructure, which intercalates soft matter between inorganic and brittle C-S-H, was obtained by controlling the assembly of individual C-S-H subunits. While large-scale production of cementitious materials by a bottom-up self-assembly method is not yet feasible, the fundamental insights into the crystallization mechanism of cement binding phases presented here provide a foundation for developing advanced cement-based materials.

Biomimetic Remineralization of Artificial Caries Lesions with a Calcium Coacervate, Its Components and Self-Assembling Peptide P11-4 In Vitro

The application of calcium coacervates (CCs) may hold promise for dental hard tissue remineralization. The aim of this study was to evaluate the effect of the infiltration of artificial enamel lesions with a CC and its single components including polyacrylic acid (PAA) compared to that of the self-assembling peptide P11-4 in a pH-cycling (pHC) model. Enamel specimens were prepared from bovine incisors, partly varnished, and stored in demineralizing solution (DS; pH 4.95; 17 d) to create two enamel lesions per sample. The specimens were randomly allocated to six groups (n = 15). While one lesion per specimen served as the no-treatment control (NTC), another lesion (treatment, T) was etched (H3PO4, 5 s), air-dried and subsequently infiltrated for 10 min with either a CC (10 mg/mL PAA, 50 mM CaCl2 (Ca) and 1 M K2HPO4 (PO4)) (groups CC and CC + DS) or its components PAA, Ca or PO4. As a commercial control, the self-assembling peptide P11-4 (CurodontTM Repair, Credentis, Switzerland) was tested. The specimens were cut perpendicularly to the lesions, with half serving as the baseline (BL) while the other half was exposed to either a demineralization solution for 20 d (pH 4.95; group CC + DS) or pHC for 28 d (pH 4.95, 3 h; pH 7, 21 h; all five of the other groups). The difference in integrated mineral loss between the lesions at BL and after the DS or pHC, respectively, was analyzed using transversal microradiography (ΔΔZ = ΔZpHC - ΔZbaseline). Compared to the NTC, the mineral gain in the T group was significantly higher in the CC + DS, CC and PAA (p < 0.05, Wilcoxon). In all of the other groups, no significant differences between treated and untreated lesions were detected (p > 0.05). Infiltration with the CC and PAA resulted in a consistent mineral gain throughout the lesion body. The CC as well as its component PAA alone promoted the remineralization of artificial caries lesions in the tested pHC model. Infiltration with PAA further resulted in mineral gain in deeper areas of the lesion body.

Nucleation-Supersaturation Dual-Drive Crystallization Strategy Enables Efficient Protein Crystallization

A rational crystallization strategy is essential to obtain high-quality protein crystals, yet the established methods suffer from different limitations arising from the single regulation on either nucleation or supersaturation. Herein, a nucleation-supersaturation dual-driven crystallization (DDC) strategy that realizes synergistic regulation of heterogeneous nucleation sites and solution supersaturation based on dual surface and confinement effects for efficient protein crystallization is reported. This strategy relies on a p(PEGDA-co-DMAA) hydrogel template with pre-filled NaCl under designed concentrations. Once dropping hen egg white lysozyme (HEWL) protein solution on the hydrogel, the wrinkled surface provides numerous nucleation sites, while the internal structure regulates the solution supersaturation in the crystallization region through diffusion. Finally, DDC strategy can create high-quality HEWL crystals with large sizes (100-300 µm), well-defined morphologies (hexagon and tetragon), and a significantly accelerated nucleation time (9-12 times faster than that achieved using the conventional hanging drop method). It also performs well at wider protein concentrations (10-50 mg mL-1) and categories (e.g., achieving fast crystallization and large-size crystals of trypsin), therefore demonstrating clear advantages and great potential for efficiently fabricating protein crystals desirable for diverse applications.

Amorphous 1-D nanowires of calcium phosphate/pyrophosphate: A demonstration of oriented self-growth of amorphous minerals

Amorphous inorganic solids are traditionally isotropic, thus, it is believed that they only grow in a non-preferential way without the assistance of regulators, leading to the morphologies of nanospheres or irregular aggregates of nanoparticles. However, in the presence of (ortho)phosphate (Pi) and pyrophosphate ions (PPi) which have synergistic roles in biomineralization, the highly elongated amorphous nanowires (denoted ACPPNs) form in a regulator-free aqueous solution (without templates, additives, organics, etc). Based on thorough characterization and tracking of the formation process (e.g., Cryo-TEM, spherical aberration correction high resolution TEM, solid state NMR, high energy resolution monochromated STEM-EELS), the microstructure and its preferential growth behavior are elucidated. In ACPPNs, amorphous calcium orthophosphate and amorphous calcium pyrophosphate are distributed at separated but close sites. The ACPPNs grow via either the preferential attachment of ∼2 nm nanoclusters in a 1-dimension way, or the transformation of bigger nanoparticles, indicating an inherent driving force-governed process. We propose that the anisotropy of ACPPNs microstructure, which is corroborated experimentally, causes their oriented growth. This study proves that, unlike the conventional view, amorphous minerals can form via oriented growth without external regulation, demonstrating a novel insight into the structures and growth behaviors of amorphous minerals.

Effect of calcium-coacervate infiltration of artificial enamel caries lesions in de- and remineralizing conditions

OBJECTIVES

Calcium-coacervate emulsions (CC) might be considered as mineral precursors to foster remineralization of carious dental hard tissues. This study analyzed the instant effect of repeated infiltration of artificial caries lesions with a CC emulsion as well as the effects of subsequent exposure of CC-infiltrated lesions to demineralizing and remineralizing environments.

METHODS

Bovine enamel specimens were partly covered with varnish to leave three exposed windows. Artificial enamel caries lesions were created (pH 4.95, 17d). Baseline controls (BL) were obtained by preparing a thin section of each specimen. Specimens were allocated to five groups. In three groups lesions were etched with 37 % phosphoric acid gel, infiltrated with dipotassium hydrogen phosphate and subsequently with a calcium coacervate emulsion, prepared by mixing CaCl2 ⋅ 2H2O with polyacrylic acid sodium salt (PAA-Na). Subsequently, the infiltration effect was either analyzed immediately (Inf.) or after exposition to either de- (Inf.+DS) or remineralizing solution (Inf.+RS) for 10 or 20 days, respectively. In two control groups specimens were exposed to either DS or RS, respectively without prior CC infiltration. Integrated mineral loss [ΔZ(vol%×µm)] was analyzed using transverse microradiography (TMR).

RESULTS

Infiltration of enamel caries lesions with coacervate solution resulted in only subtle immediate mineral gain even if repeated. When exposed to demineralizing conditions, infiltrated lesions showed significantly less mineral loss compared to untreated controls (p < 0.05; Kruskal Wallis) and exhibited characteristic mineral depositions within the lesion body.

CONCLUSIONS

While immediate mineral gain by infiltration was only modest, the CC-emulsion might be able to prevent demineralization in acidic conditions.

CLINICAL SIGNIFICANCE

Calcium coacervates might act protective against further demineralization when infiltrated into enamel caries lesions.

Mesoscale self-organization of polydisperse magnetic nanoparticles at the water surface

In this study, we investigated the self-ordering process in Langmuir films of polydisperse iron oxide nanoparticles on a water surface, employing in situ x-ray scattering, surface pressure-area isotherm analysis, and Brewster angle microscopy. X-ray reflectometry confirmed the formation of a monolayer, while grazing incidence small-angle x-ray scattering revealed short-range lateral correlations with a characteristic length equal to the mean particle size. Remarkably, our findings indicated that at zero surface pressure, the particles organized into submicrometer clusters, merging upon compression to form a homogeneous layer. These layers were subsequently transferred to a solid substrate using the Langmuir-Schaefer technique and further characterized via scanning electron microscopy and polarized neutron reflectometry. Notably, our measurements revealed a second characteristic length in the lateral correlations, orders of magnitude longer than the mean particle diameter, with polydisperse particles forming circular clusters densely packed in a hexagonal lattice. Furthermore, our evidence suggests that the lattice constant of this mesocrystal depends on the characteristics of the particle size distribution, specifically the mean particle size and the width of the size distribution. In addition, we observed internal size separation within these clusters, where larger particles were positioned closer to the center of the cluster. Finally, polarized neutron reflectometry measurements provided valuable insights into the magnetization profile across the layer.

Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium

A mechanistic understanding of the principles governing the hierarchical organization of supramolecular polymers offers a paradigm for tailoring synthetic molecular architectures at the nano to micrometric scales. Herein, the unconventional crystal growth mechanism of a supramolecular polymer of superbenzene(coronene)-diphenylalanine conjugate (Cr-FFOEt ) is demonstrated. 3D electron diffraction (3D ED), a technique underexplored in supramolecular chemistry, is effectively utilized to gain a molecular-level understanding of the gradual growth of the initially formed poorly crystalline hairy, fibril-like supramolecular polymers into the ribbon-like crystallites. The further evolution of these nanosized flat ribbons into microcrystals by oriented attachment and lateral fusion is probed by time-resolved microscopy and electron diffraction. The gradual morphological and structural changes reveal the nonclassical crystal growth pathway, where the balance of strong and weak intermolecular interactions led to a structure beyond the nanoscale. The role of distinct π-stacking and H-bonding interactions that drive the nonclassical crystallization process of Cr-FFOEt supramolecular polymers is analyzed in comparison to analogous molecules, Py-FFOEt and Cr-FF forming helical and twisted fibers, respectively. Furthermore, the Cr-FFOEt crystals formed through nonclassical crystallization are found to improve the functional properties.

Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions

The formation of aragonite under ambient conditions is typically linked to Mg-rich aqueous environments. The grains that form in such environments show peculiar properties such as aggregate-like appearance and mesocrystalline character. We tested the effect of dissolved Mg2+ ions on the formation of aragonite mesocrystals by synthesizing aragonite with an automatic titrator at constant pH and at different dissolved Mg : Ca ratios, and by studying the properties of the precipitated material with various scanning transmission electron microscopy (STEM) techniques. At all studied Mg : Ca ratios the firstly condensed carbonate phase was Mg-bearing amorphous calcium carbonate (Mg-ACC) that transformed into aragonite during the synthesis experiments. The aragonite grains had typically aggregate-like appearance and spindle shapes, with the external morphologies of the spindles unaffected by variation in solution chemistry. The alignment of the nanocrystals within the aggregates was crystallographically highly coherent, the [001] directions of nanocrystals showing only a small misorientation with respect to one another; however, both parallel and twin assembly of neighbouring crystals occurred. An increase in the dissolved Mg concentration decreased the crystallographic coherence between the aragonite nanocrystals, suggesting an important role of Mg2+ ions in the assembly of aragonite spindles. Whereas the mesoscale-ordered arrangement of nanocrystals implies a particle-mediated assembly, the observed differences in particle size and composition between the amorphous precursor and the crystalline end-product suggest that the crystallization includes at least partial dissolution and re-precipitation. These findings provide insight into the formation of aragonite and could contribute to the understanding of important aspects of the formation of mesocrystals and hierarchically structured biogenic minerals.

Kinetics of Oriented Attachment of Mica Crystals

Oriented attachment (OA), that is, the coalescence of crystals through attachment on coaligned crystal faces, is a nonclassical crystal growth process. Before attachment, a mesocrystal consisting of coaligned parallel crystals but with liquid separating them was observed. Fundamental questions such as why OA is kinetically favored and whether a mesocrystal stage is a prerequisite for OA are raised. Through combining brute-force molecular dynamics simulations and path samplings based on extensive umbrella simulations, we address these questions with a case study on the OA of a mica nanocrystal onto a mica crystal substrate in water. Brute-force simulations show that if two mica crystals are attached but largely misaligned, coalignment hardly appears. Thus, if OA is possible, then coalignment must appear before the attachment between crystals. Electrophoresis of the nanocrystal toward the substrate surface is spontaneous, but mesocrystal formation is occasional, also shown by brute-force simulations. Free energies along different pathways show that OA is spontaneous and kinetically favored over non-OA, and a mesocrystal formation is just a bifurcation in the pathway. OA is through a pathway in which the nanocrystal is tilted with respect to the substrate. Part of the nanocrystal is attached to the substrate first, and then, OA is gradually completed. Once a mesocrystal is occasionally formed, then a jump event is needed for the nanocrystal to get back to the OA pathway. The sampling technique here can hopefully guide the design of nanostructured materials facilitated by OA.

Colloidal pathways of amorphous calcium carbonate formation lead to distinct water environments and conductivity

CaCO3 is the most abundant biomineral and a major constituent of incrustations arising from water hardness. Polycarboxylates play key roles in controlling mineralization. Herein, we present an analytical and spectroscopic study of polycarboxylate-stabilized amorphous CaCO3 (ACC) and its formation via a dense liquid precursor phase (DLP). Polycarboxylates facilitate pronounced, kinetic bicarbonate entrapment in the DLP. Since bicarbonate is destabilized in the solid state, DLP dehydration towards solid ACC necessitates the formation of locally calcium deficient sites, thereby inhibiting nucleation. Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy of poly-aspartate-stabilized ACC reveals the presence of two distinct environments. The first contains immobile calcium and carbonate ions and structural water molecules, undergoing restricted, anisotropic motion. In the second environment, water molecules undergo slow, but isotropic motion. Indeed, conductive atomic force microscopy (C-AFM) reveals that ACC conducts electrical current, strongly suggesting that the mobile environment pervades the bulk of ACC, with dissolved hydroxide ions constituting the charge carriers. We propose that the distinct environments arise from colloidally stabilized interfaces of DLP nanodroplets, consistent with the pre-nucleation cluster (PNC) pathway.

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

In Situ Liquid Cell Transmission Electron Microscopy Study of Studtite Particle Formation and Growth via Electron Beam Radiolysis

This study presents in situ observations of studtite (UO2O2(H2O)2·2H2O) crystal growth utilizing liquid phase transmission electron microscopy (LP-TEM). Studtite was precipitated from a uranyl nitrate hexahydrate solution using hydrogen peroxide formed by the radiolysis of water in the TEM electron beam. The hydrogen peroxide (H2O2) concentration, directly controlled by the electron beam current, was varied to create local environments of low and high concentrations to compare the impact of the supersaturation ratio on the nucleation and growth mechanisms of studtite particles. The subsequent growth mechanisms were observed in real time by TEM and scanning TEM imaging. After the initial precipitation reaction, a post-mortem TEM analysis was performed on the samples to obtain high-resolution TEM images and selected area electron diffraction patterns to investigate crystallinity as well as energy-dispersive X-ray spectroscopy spectra to ensure that studtite was produced. The results reveal that studtite particles form through various mechanisms based on the concentration ratio of uranyl to H2O2 and that studtite is initially produced through an amorphous intermediary prior to formation of the crystalline material commonly reported in the literature.

Nanoprecipitation through solvent-shifting using rapid mixing: Dispelling the Ouzo boundary to reach large solute concentrations

HYPOTHESIS

The addition of a non-solvent to a solute in good solvent solution leads to nanoprecipitation, which is the spontaneous formation of nanodomains. Yet, increasing solute concentration usually leads to the formation of macrodomains that quickly separate into a bulk phase, which is a severe process limitation. The corresponding concentration threshold, often termed as the Ouzo boundary, remains a mystery that could find its origin in the complex interplay between nanoprecipitation and mixing.

EXPERIMENTS

We performed a systematic investigation of nanoprecipitation thermodynamics and kinetics as well as its interplay with mixing hydrodynamics for the hexadecane-acetone-water system, in the presence of the non-ionic C16EO8 surfactant. The binodal curve and its underlying tie-lines were obtained using Raman spectroscopy, allowing the computation of the spinodal curve. Kinetics were probed using a continuous flow setup that combines two sequential rapid mixers. The impact of mixing efficiency was probed systematically by varying the oil concentration for respectively slow and rapid mixing, while the uncoupling from mixing and nanoprecipitation was quantified by modifying systematically the flow rate in a continuous flow approach.

FINDINGS

We elucidate the nature of the Ouzo boundary that marks the maximal solute concentration leading to nanoobjects. Rather than a thermodynamic boundary, as evidenced by its uncorrelation to the spinodal curve, it results from the coupling of nanoprecipitation and mixing when both processes occur within the same time range, leading to heterogeneous conditions and the escape of some objects to the macroscale. Increasing the solute concentration speeds up nanoprecipitation and thus requires increasingly faster mixing times to uncouple both processes. Accordingly, if the mixing efficiency is large enough, it is possible to dispel the Ouzo boundary and reach very large solute concentrations. Implementing rapid mixing strategies in continuous flow approaches is thus the solution to overcome the most stringent condition of nanoprecipitation and open the way to scale-up, while also providing efficient means to probe its fast mechanism. Overall, the simultaneous control of hydrodynamics and physical chemistry is thus key to boost up the Ouzo effect.

Insight on the Intracellular Supramolecular Assembly of DTTO: A Peculiar Example of Cell-Driven Polymorphism

The assembly of supramolecular structures within living systems is an innovative approach for introducing artificial constructs and developing biomaterials capable of influencing and/or regulating the biological responses of living organisms. By integrating chemical, photophysical, morphological, and structural characterizations, it is shown that the cell-driven assembly of 2,6-diphenyl-3,5-dimethyl-dithieno[3,2-b:2',3'-d]thiophene-4,4-dioxide (DTTO) molecules into fibers results in the formation of a "biologically assisted" polymorphic form, hence the term bio-polymorph. Indeed, X-ray diffraction reveals that cell-grown DTTO fibers present a unique molecular packing leading to specific morphological, optical, and electrical properties. Monitoring the process of fiber formation in cells with time-resolved photoluminescence, it is established that cellular machinery is necessary for fiber production and a non-classical nucleation mechanism for their growth is postulated. These biomaterials may have disruptive applications in the stimulation and sense of living cells, but more crucially, the study of their genesis and properties broadens the understanding of life beyond the native components of cells.

Non-classical crystallization of CeO2 by means of in situ electron microscopy

During in situ liquid-phase electron microscopy (LP-EM) observations, the application of different irradiation dose rates may considerably alter the chemistry of the studied solution and influence processes, in particular growth pathways. While many processes have been studied using LP-EM in the last decade, the extent of the influence of the electron beam is not always understood and comparisons with corresponding bulk experiments are lacking. Here, we employ the radiolytic oxidation of Ce3+ in aqueous solution as a model reaction for the in situ LP-EM study of the formation of CeO2 particles. We compare our findings to the results from our previous study where a larger volume of Ce3+ precursor solution was subjected to γ-irradiation. We systematically analyze the effects of the applied irradiation dose rates and the induced diffusion of Ce ions on the growth mechanisms and the morphology of ceria particles. Our results show that an eight orders of magnitude higher dose rate applied during homogeneous electron-radiation in LP-EM compared to the dose rate using gamma-radiation does not affect the CeO2 particle growth pathway despite the significant higher Ce3+ to Ce4+ oxidation rate. Moreover, in both cases highly ordered structures (mesocrystals) are formed. This finding is explained by the stepwise formation of ceria particles via an intermediate phase, a signature of non-classical crystallization. Furthermore, when irradiation is applied locally using LP scanning transmission electron microscopy (LP-STEM), the higher conversion rate induces Ce-ion concentration gradients affecting the CeO2 growth. The appearance of branched morphologies is associated with the change to diffusion limited growth.

Nonclassical Crystallization Causes Dendritic and Band-Like Microscale Patterns in Inorganic Precipitates

The self-organization of complex solids can create patterns extending hierarchically from the atomic to the macroscopic scale. A frequently studied model is the chemical garden system which consists of life-like precipitate shapes. In this study, we examine the thin walls of chemical gardens using microfluidic devices that yield linear Ni(OH)2 precipitate membranes. We observe distinct light-scattering patterns within the compositionally pure membranes, including disorganized spots, dendrites, and parallel bands. The bands are tilted with respect to the membrane axis and their spacing (20-100 μm) increases with increasing flow rates. Scanning electron microscopy reveals that the bands consist of submicron particles embedded in a denser material and these particles are also found in the reactant stream. We propose that dendrites and bands arise from the attachment of solution-borne nanoparticles. The bands are generated by particle-aggregation zones moving upstream along the slowly advancing membrane surface. The speed of the aggregation zones is proportional to the band distance and defines the system's dispersion relation. This speed-wavelength dependence and the flow-opposing motion of the aggregation zones are likely caused by low particle concentrations in the wake of the zones that only slowly recover due to Brownian motion and particle nucleation.

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value-added chemicals, and metal-air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost-effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

Embedding Hierarchical Pores by Mechanochemistry in Carbonates with Superior Chemoselective Catalysis and Stability

Hierarchical porosity of carbonates can facilitate their performance in massive applications as compared to their corresponding bulk samples. Traditional solution-based precipitation is typically utilized to fabricate porous carbonates. However, this tactic is generally employed under humid conditions, which demand soluble metal precursors, solvents, and extended dry periods. A salt-assisted mechanochemistry is exploited in contemporary work to settle the shortcomings. Enlighted by solid-state technology, this approach eliminates the utilization of solvents, and the process of ball milling can create pores in 5 min. A range of highly porous carbonates and their derivatives are acquired, with several materials surpassing recording surface areas (e.g., H-CaCO3: 108 m2/g, SrCO3: 125 m2/g, BaCO3: 172 m2/g, Pd/H-CaCO3 catalyst: 101 m2/g). The results display that Pd/H-CaCO3 shows superior catalytic efficiency in the synthesis of aniline (turnover frequency [TON] = 1.33 × 104/h-1, yield ≥ 99%, and recycle stability: 11 cycles) and dye degradation. Combining mechanochemistry and salt-assisted tactic provides a facile and efficient pathway for processing porous materials.

Morphology Transition with Temperature and its Effect on Optical Properties of Colloidal MoS2 Nanostructures

Morphology plays a crucial role in determining the chemical and optical properties of nanomaterials due to confinement effects. We report the morphology transition of colloidal molybdenum disulfide (MoS2) nanostructures, synthesized by a one-pot heat-up method, from a mix of quantum dots (QDs) and nanosheets to predominantly nanorods by varying the synthesis reaction temperature from 90 to 160 °C. The stoichiometry and composition of the synthesized QDs, nanosheets, and nanorods were quantified to be MoS2 using energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. A nanostructure morphology transition due to variation in the reaction temperature resulted in a photoluminescence quantum yield enhancement from 0 to 4.4% on increasing the temperature from 90 to 120 °C. On further increase in the temperature to 160 °C, a decrease in the quantum yield to 3.06% is observed. Red-shifts of ≈18 and ≈140 nm in the emission maxima and absorption edge, respectively, are observed for the synthesized nanostructures with an increase in the reaction temperature from 90 to 160 °C. The change in the quantum yield is attributed to the change in shape and hence confinement of charge carriers. To the best of our knowledge, microscopic analysis of variation in shape and optical properties of colloidal MoS2 nanostructures with temperature, explained by a nonclassical growth mechanism is presented here for the first time.

Moisture-Induced Degradation of Quantum-Sized Semiconductor Nanocrystals through Amorphous Intermediates

Elucidating the water-induced degradation mechanism of quantum-sized semiconductor nanocrystals is an important prerequisite for their practical application because they are vulnerable to moisture compared to their bulk counterparts. In-situ liquid-phase transmission electron microscopy is a desired method for studying nanocrystal degradation, and it has recently gained technical advancement. Herein, the moisture-induced degradation of semiconductor nanocrystals is investigated using graphene double-liquid-layer cells that can control the initiation of reactions. Crystalline and noncrystalline domains of quantum-sized CdS nanorods are clearly distinguished during their decomposition with atomic-scale imaging capability of the developed liquid cells. The results reveal that the decomposition process is mediated by the involvement of the amorphous-phase formation, which is different from conventional nanocrystal etching. The reaction can proceed without the electron beam, suggesting that the amorphous-phase-mediated decomposition is induced by water. Our study discloses unexplored aspects of moisture-induced deformation pathways of semiconductor nanocrystals, involving amorphous intermediates.

Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I-V-VI2 Nanocrystals with Tunable Composition and Interesting Properties

The growth mechanism and synthetic controls for colloidal multinary metal chalcogenide nanocrystals (NCs) involving alkali metals and the pnictogen metals Sb and Bi are unknown. Sb and Bi are prone to form metallic nanocrystals that stay as impurities in the final product. Herein, we synthesize colloidal NaBi_1-xSb_xSe_2-yS_y NCs using amine-thiol-Se chemistry. We find that ternary NaBiSe₂ NCs initiate with Bi⁰ nuclei and an amorphous intermediate nanoparticle formation that gradually transforms into NaBiSe₂ upon Se addition. Furthermore, we extend our methods to substitute Sb in place of Bi and S in place of Se. Our findings show the initial quasi-cubic morphology transforms into a spherical shape upon increased Sb substitution, and the S incorporation promotes elongation along the <111> direction. We further investigate the thermoelectric transport properties of the Sb-substituted material displaying very low thermal conductivity and n-type transport behavior. Notably, the NaBi_0.75Sb_0.25Se₂ material exhibits an ultralow thermal conductivity of 0.25 W·m^-1·K^-1 at 596 K with an average thermal conductivity of 0.35 W·m^-1·K^-1 between 358 and 596 K and a ZT_max of 0.24.

A Review of Laser-Induced Crystallization from Solution

Crystallization abounds in nature and industrial practice. A plethora of indispensable products ranging from agrochemicals and pharmaceuticals to battery materials are produced in crystalline form in industrial practice. Yet, our control over the crystallization process across scales, from molecular to macroscopic, is far from complete. This bottleneck not only hinders our ability to engineer the properties of crystalline products essential for maintaining our quality of life but also hampers progress toward a sustainable circular economy in resource recovery. In recent years, approaches leveraging light fields have emerged as promising alternatives to manipulate crystallization. In this review article, we classify laser-induced crystallization approaches where light-material interactions are utilized to influence crystallization phenomena according to proposed underlying mechanisms and experimental setups. We discuss nonphotochemical laser-induced nucleation, high-intensity laser-induced nucleation, laser trapping-induced crystallization, and indirect methods in detail. Throughout the review, we highlight connections among these separately evolving subfields to encourage the interdisciplinary exchange of ideas.

Enhanced and stabilized photoluminescence of perovskite cesium lead bromide nanocubes through ordered assemblies

This work clarified the effects of self-assembly of perovskite cesium lead bromide (CsPbBr₃) nanocubes (NCs) covered with didodecyldimethyl ammonium bromide (DDAB) on photoluminescence (PL) properties. Although the PL intensity of isolated NCs was weakened in the solid state even under inert conditions, the quantum yield of PL (PLQY) and the photostability of DDAB-covered NCs were drastically improved by the formation of two-dimensional (2D) ordered arrays on a substrate. The PLQY of the 2D arrays increased to ca. 60% by initial excitation illumination at 468 nm and was maintained for over 4000 h. The improved PL properties are attributable to the fixation of the surface ligand around the NCs in the specific ordered arrays.

Sequential Sol-Gel Self-Assembly and Nonclassical Gel-Crystal Transformation of the Metal-Organic Framework Gel

Metal-organic framework (MOF) gel, an emerging subtype of MOF structure, is unique in formation and function; however, its evolutionary process remains elusive. Here, the evolution of a model gel-based MOF, UiO-66(Zr) gel, is explored by demonstrating its sequential sol-gel self-assembly and nonclassical gel-crystal transformation. The control of the sol-gel process enables the observation and characterization of structures in each assembly stage (phase-separation, polycondensation, and hindered-crystallization) and facilitates the preparation of hierarchical materials with giant mesopores. The gelation mechanism is tentatively attributed to the formation of zirconium oligomers. By further utilizing the pre-synthesized gel, the nonclassical gel-crystal transformation is achieved by the modulation in an unconventional manner, which sheds light on crystal intermediates and distinct crystallization motions ("growth and splitting" and "aggregation and fusion"). The overall sol-gel and gel-crystal evolutions of UiO-66(Zr) enrich self-assembly and crystallization domains, inspire the design of functional structures, and demand more in-depth research on the intermediates in the future.

Multimodal imaging of cubic Cu2O@Au nanocage formation via galvanic replacement using X-ray ptychography and nano diffraction

Being able to observe the formation of multi-material nanostructures in situ, simultaneously from a morphological and crystallographic perspective, is a challenging task. Yet, this is essential for the fabrication of nanomaterials with well-controlled composition exposing the most active crystallographic surfaces, as required for highly active catalysts in energy applications. To demonstrate how X-ray ptychography can be combined with scanning nanoprobe diffraction to realize multimodal imaging, we study growing Cu₂O nanocubes and their transformation into Au nanocages. During the growth of nanocubes at a temperature of 138 °C, we measure the crystal structure of an individual nanoparticle and determine the presence of (100) crystallographic facets at its surface. We subsequently visualize the transformation of Cu₂O into Au nanocages by galvanic replacement. The nanocubes interior homogeneously dissolves while smaller Au particles grow on their surface and later coalesce to form porous nanocages. We finally determine the amount of radiation damage making use of the quantitative phase images. We find that both the total surface dose as well as the dose rate imparted by the X-ray beam trigger additional deposition of Au onto the nanocages. Our multimodal approach can benefit in-solution imaging of multi-material nanostructures in many related fields.

Nonclassical Crystallization and Core-Shell Structure Formation of Ibuprofen from Binary Solvent Solutions

Liquid-liquidphase separation (LLPS) or dense liquid intermediates during the crystallization of pharmaceutical molecules is common; however, their role in alternative nucleation mechanisms is less understood. Herein, we report the formation of a dense liquid intermediate followed by a core-shell structure of ibuprofen crystals via nonclassical crystallization. The Raman and SAXS results of the dense phase uncover the molecular structural ordering and its role in nucleation. In addition to the dimer formation of ibuprofen, which is commonly observed in the solution phase, methyl group vibrations in the Raman spectra show intermolecular interactions similar to those in the solid phase. The SAXS data validate the cluster size differences in the supersaturated solution and dense phase. The focused-ion beam cut image shows the attachment of nanoparticles, and we proposed a possible mechanism for the transformation from the dense phase into a core-shell structure. The unstable phase or polycrystalline core and its subsequent dissolution from inside to outside or recrystallization by reversed crystal growth produces the core-shell structure. The LLPS intermediate followed by the core-shell structure and its dissolution enhancement unfold a new perspective of ibuprofen crystallization.

Angular orientation between the cores of iron oxide nanoclusters controls their magneto-optical properties and magnetic heating functions

Oriented attachment of nanobricks into hierarchical multi-scale structures such as inorganic nanoclusters is one of the crystallization mechanisms that has revolutionized the field of nano and materials science. Herein, we show that the mosaicity, which measures the misalignment of crystal plane orientation between the nanobricks, governs their magneto-optical properties as well as the magnetic heating functions of iron oxide nanoclusters. Thanks to high-temperature and time-resolved millifluidic, we were able to isolate and characterize (structure, properties, function) the different intermediates involved in the diverse steps of the nanocluster's formation, to propose a detailed dynamical mechanism of their formation and establish a clear correlation between changes in mosaicity at the nanoscale and their resulting physical properties. Finally, we demonstrate that their magneto-optical properties can be described using simple molecular theories.

Crystallization of SrAl12O19 Nanocrystals from Amorphous Submicrometer Particles

Advanced instrumentation and modern analysis tools such as transmission electron microscopy (TEM) have led to phenomenal progress in understanding crystallization, in particular from solution, which is a prerequisite for the design-based preparation of a target crystal. Nevertheless, little has been understood about the crystallization pathway under high-temperature annealing (HTA) conditions. Metal oxide crystals are prominent materials that are usually obtained via HTA. Despite the widespread application of hydro-/solvothermal methods on the laboratory scale, HTA is the preferred method in many industries for the mass production of metal oxide crystals. However, poor control over the morphology and grain sizes of these crystals under extreme HTA conditions limits their applications. Here, applying ex-situ TEM, the transformation of a single amorphous spherical submicrometer precursor particle of SrAl12O19 (SA6) at 1150 °C toward a nanosized thermodynamically favored hexagonal crystal is explored. It is illustrated in real space, step by step, how both kinetic and thermodynamic factors contribute to this faceting and morphology evolution. These results demonstrate a nonclassical nucleation and growth process consisting of densification, crystallite domain formation, oriented attachment, surface nucleation, 2-dimensional (2D) growth, and surface diffusion of the atoms to eventually result in the formation of a hexagonal platelet crystal. The TEM images further delineate a parent crystal driving the crystal lattice and morphological orientation of a network of interconnected platelets.

Impact of a polymer modifier on directing the non-classical crystallization pathway of TS-1 zeolite: accelerating nucleation and enriching active sites

The crystallization process directly affects the physicochemical properties and active centers of zeolites; however, controllable tuning of the zeolite crystallization process remains a challenge. Herein, we utilized a polymer (polyacrylamide, PAM) to control the precursor structure evolution of TS-1 zeolite through a two-step crystallization process, so that the crystallization path was switched from a classical to a non-classical mechanism, which greatly accelerated nucleation and enriched active Ti sites. The TS-1 crystallization process was investigated by means of various advanced characterization techniques. It was found that specific interactions between PAM and Si/Ti species promoted the assembly of colloidal precursors containing ordered structural fragments and stabilized Ti species in the precursors, leading to a 1.5-fold shortened crystallization time and enriched Ti content in TS-1 (Si/Ti = 29). The PAM-regulated TS-1 zeolite exhibited enhanced catalytic performance in oxidative reactions compared to conventional samples.

Asymmetric Interfacet Adatom Migration as a Mode of Anisotropic Nanocrystal Growth

Crystals are known to grow nonclassically or via four classical modes (the layer-by-layer, dislocation-driven, dendritic, and normal modes, which generally involve minimal interfacet surface diffusion). The field of nanoscience considers this framework to interpret how nanocrystals grow; yet, the growth of many anisotropic nanocrystals remains enigmatic, suggesting that the framework may be incomplete. Here, we study the solution-phase growth of pentatwinned Au nanorods without Br, Ag, or surfactants. Lower supersaturation conditions favored anisotropic growth, which appeared at variance with the known modes. Temporal electron microscopy revealed kinetically limited adatom funneling, as adatoms diffused asymmetrically along the vicinal facets (situated inbetween the {100} side-facets and {111} end-facets) of our nanorods. These vicinal facets were perpetuated throughout the synthesis and, especially at lower supersaturation, facilitated {100}-to-vicinal-to-{111} adatom diffusion. We derived a growth model from classical theory in view of our findings, which showed that our experimental growth kinetics were consistent with nanorods growing via two modes simultaneously: radial growth occurred via the layer-by-layer mode on {100} side-facets, whereas the asymmetric interfacet diffusion of adatoms to {111} end-facets mediated longitudinal growth. Thus, shape anisotropy was not driven by modulating the relative rates of monomer deposition on different facets, as conventionally thought, but rather by modulating the relative rates of monomer integration via interfacet diffusion. This work shows how controlling supersaturation, a thermodynamic parameter, can uncover distinct kinetic phenomena on nanocrystals, such as asymmetric interfacet surface diffusion and a fundamental growth mode for which monomer deposition and integration occur on different facets.

Biomineralization-Inspired Intermediate Precursor for the Controllable Gelation of Polyphenol-Macromolecule Hydrogels

Hydrogels composed of polyphenols and various macromolecules have been widely reported to have the advantage of facile preparation, mainly through the formation of hydrogen bonds. However, the traditional preparation method involves the direct mixing of polyphenols and macromolecules, which generally occurs too quickly and uncontrollably, and results in poor homogeneity, injectability, and shape designability. Here, inspired by the intermediate precursor during biomineralization, to facilitate transformation in a controllable way, we propose a novel and universal internal gelation method that creates an intermediate precursor by controlling the pH value to manipulate the elimination and generation of hydrogen bonds between a polyphenol and macromolecules. The precursor strategy greatly improves the homogeneity, injectability, and shape designability of the hydrogel while also achieving a controllable gelation process, and the gelation time can be accurately adjusted. The hydrogels prepared with this method exhibited superior capability to seal leaks, provided complete wound coverage, and showed the potential to be a shape-designable wearable strain sensor. Our study opens up a new way to construct and apply polyphenol-macromolecule hydrogels in a more controllable manner.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Imaging Cu2O nanocube hollowing in solution by quantitative in situ X-ray ptychography

Understanding morphological changes of nanoparticles in solution is essential to tailor the functionality of devices used in energy generation and storage. However, we lack experimental methods that can visualize these processes in solution, or in electrolyte, and provide three-dimensional information. Here, we show how X-ray ptychography enables in situ nano-imaging of the formation and hollowing of nanoparticles in solution at 155 °C. We simultaneously image the growth of about 100 nanocubes with a spatial resolution of 66 nm. The quantitative phase images give access to the third dimension, allowing to additionally study particle thickness. We reveal that the substrate hinders their out-of-plane growth, thus the nanocubes are in fact nanocuboids. Moreover, we observe that the reduction of Cu2O to Cu triggers the hollowing of the nanocuboids. We critically assess the interaction of X-rays with the liquid sample. Our method enables detailed in-solution imaging for a wide range of reaction conditions.

Multistep Crystallization of Pharmaceutical Amorphous Nanoparticles via a Cognate Pathway of Oriented Attachment: Direct Evidence of Nonclassical Crystallization for Organic Molecules

Crystallization of organic molecules is important in a wide range of scientific disciplines. However, in contrast to maturely studied crystallization of inorganic materials, the crystallization mechanisms of organic molecules involving nucleation and crystal growth are still poorly understood. Here, we used time-resolved cryogenic transmission electron microscopy to directly map the morphological evolution of amorphous cyclosporin A (CyA) nanoparticles during CyA crystallization. We successfully observed its initial nucleation and found that the amorphous CyA nanoparticles crystallized via a pathway cognate with oriented attachment, which is the nonclassical crystallization mechanism usually reported for inorganic compounds. Crystalline mesostructured intermediates (mesocrystals) were formed during crystallization. This study revealed clear and direct evidence of mesocrystal formation and oriented attachment in organic pharmaceuticals, providing new insights into the crystallization of organic molecules and theories of nonclassical crystallization.

Calcium carbonate: controlled synthesis, surface functionalization, and nanostructured materials

Calcium carbonate (CaCO₃) is an important inorganic mineral in biological and geological systems. Traditionally, it is widely used in plastics, papermaking, ink, building materials, textiles, cosmetics, and food. Over the last decade, there has been rapid development in the controlled synthesis and surface modification of CaCO₃, the stabilization of amorphous CaCO₃ (ACC), and CaCO₃-based nanostructured materials. In this review, the controlled synthesis of CaCO₃ is first examined, including Ca²⁺-CO₃^2- systems, solid-liquid-gas carbonation, water-in-oil reverse emulsions, and biomineralization. Advancing insights into the nucleation and crystallization of CaCO₃ have led to the development of efficient routes towards the controlled synthesis of CaCO₃ with specific sizes, morphologies, and polymorphs. Recently-developed surface modification methods of CaCO₃ include organic and inorganic modifications, as well as intensified surface reactions. The resultant CaCO₃ can then be further engineered via template-induced biomineralization and layer-by-layer assembly into porous, hollow, or core-shell organic-inorganic nanocomposites. The introduction of CaCO₃ into nanostructured materials has led to a significant improvement in the mechanical, optical, magnetic, and catalytic properties of such materials, with the resultant CaCO₃-based nanostructured materials showing great potential for use in biomaterials and biomedicine, environmental remediation, and energy production and storage. The influences that the preparation conditions and additives have on ACC preparation and stabilization are also discussed. Studies indicate that ACC can be used to construct environmentally-friendly hybrid films, supramolecular hydrogels, and drug vehicles. Finally, the existing challenges and future directions of the controlled synthesis and functionalization of CaCO₃ and its expanding applications are highlighted.

The Non-Classical Crystallization Mechanism of a Composite Biogenic Guanine Crystal

Spectacular colors and visual phenomena in animals are produced by light interference from highly reflective guanine crystals. Little is known about how organisms regulate crystal morphology to tune the optics of these systems. By following guanine crystal formation in developing spiders, a crystallization mechanism is elucidated. Guanine crystallization is a "non-classical," multistep process involving a progressive ordering of states. Crystallization begins with nucleation of partially ordered nanogranules from a disordered precursor phase. Growth proceeds by orientated attachment of the nanogranules into platelets which coalesce into single crystals, via progressive relaxation of structural defects. Despite their prismatic morphology, the platelet texture is retained in the final crystals, which are composites of crystal lamellae and interlamellar sheets. Interactions between the macromolecular sheets and the planar face of guanine appear to direct nucleation, favoring platelet formation. These findings provide insights on how organisms control the morphology and optical properties of molecular crystals.

Biomineral-Inspired Colloidal Liquid Crystals: From Assembly of Hybrids Comprising Inorganic Nanocrystals and Organic Polymer Components to Their Functionalization

Bioinspired organic/inorganic synthetic composites have been studied as high-performance and functional materials. In nature, biominerals such as pearls, teeth, and bones are self-organized organic/inorganic composites. The inorganic components are composed of calcium carbonate (CaCO3) and hydroxyapatite (HAp), while the organic components consist of peptides and polysaccharides. These composites are used as structural materials in hard biological tissues. Biominerals do not show significantly higher performances than synthetic composites such as glass-fiber- or carbon-fiber-reinforced plastics. However, biominerals consist of environmentally friendly and biocompatible components that are prepared under mild conditions. Moreover, they form elaborate nanostructures and self-organized hierarchical structures. Much can be learned about material design from these biomineral-based hierarchical and nanostructured composites to assist in the preparation of functional materials.Inspired by these biological hard tissues, we developed nanostructured thin films and bulk hybrid crystals through the self-organization of organic polymers and inorganic crystals of CaCO3 or HAp. In biomineralization, the combination of insoluble components and soluble acidic macromolecules controls the crystallization process. We have shown that poly(acrylic acid) (PAA) or acidic peptides called polymer additives induce the formation of thin film crystals of CaCO3 or HAp by cooperation with insoluble organic templates such as chitin and synthetic polymers bearing the OH group. Moreover, we recently developed CaCO3- and HAp-based nanostructured particles with rod and disk shapes. These were obtained in aqueous media using a macromolecular acidic additive, PAA, without using insoluble polymer templates. At appropriate concentrations, the anisotropic particles self-assembled and formed colloidal liquid-crystalline (LC) phases.LC materials are generally composed of organic molecules. They show ordered and mobile states. The addition of stimuli-responsive properties to organic rod-like LC molecules led to the successful development of informational displays, which are now widely used. On the other hand, colloidal liquid crystals are colloidal self-assembled dispersions of anisotropic organic and inorganic nano- and micro-objects. For example, polysaccharide whiskers, clay nanosheets, gibbsite plate-shaped particles, and silica rod-shaped particles exhibit colloidal LC states.In this Account, we focused on the material design and hierarchical aspects of biomineral-based colloidal LC polymer/inorganic composites. We describe the design and preparation, nanostructures, and self-assembled behavior of these new bioinspired and biocompatible self-organized materials. The characterization results for these self-assembled nanostructured colloidal liquid crystals found using high-resolution transmission electron microscopy, small-angle X-ray scattering, and neutron scattering and rheological measurements are also reported. The functions of these biomineral-inspired liquid crystals are presented. Because these biomineral-based LC colloidal liquid crystals can be prepared under mild and aqueous conditions and they consist of environmentally friendly and biocompatible components, new functions are expected for these materials.

Pressure-induced evolution in the durability of nickel-metal hydride batteries under high-current charge

We found that an AAA-type battery (min. 750 mAh) pressurized with Ar or N₂ at pressures of up to 5 MPa exhibited a significant durability enhancement even under high-current conditions. As an example of a charge-discharge cycle test at 3 amperes, the residual ratio of capacity at atmospheric pressure decreased to approximately 90% of the standard capacity before 50 cycles. However, at a pressure of 3 MPa of N₂, the capacity remained at more than 90% until 180 cycles. With an increase in the pressure, the residual ratio of capacity was further improved. It has been considered that, at the positive electrode of the Ni-MH battery, the chemical reaction from nickel(II) hydroxide (Ni(OH)₂) crystals to nickel oxide hydroxide (NiOOH) crystals occurs during the charging process. However, X-ray diffraction (XRD) results in the present study do not support this solid-solid reaction between these two types of crystal. To address this contradiction, we propose a different reaction mechanism by introducing the concept of non-crystalline fine particles of compounds, which are undetected by XRD. This mechanism clearly explains how the pressure affects the durability improvement. Pressurized batteries, which are capable of fast charge-discharge operation under high-current conditions, can provide a new route for application fields of unconventional energy storage.

Anisotropic Crystal Growth of Layered Nickel Hydroxide along the Stacking Direction Using Amine Ligands

Edge surfaces of two-dimensional crystals play crucial roles in their properties, such as intercalation behavior and catalytic activities; however, reports on the preparation of crystals with a high aspect ratio of thickness to lateral size, typically a prism-like crystal morphology composed of stacked layers, are scarce. We report the anisotropic crystal growth of β-Ni(OH)₂ along the stacking direction using bidentate amine ligands, which act as both the base and the reservoir of Ni²⁺ through the formation of Ni-diamine complexes. Various characterization results of the crystal structure, composition, and crystal orientation indicate the formation of hexagonal prisms of β-Ni(OH)₂ with an unusually high aspect ratio of the thickness to the lateral size higher than 1. A systematic investigation focusing on the molar ratio of amine ligands to Ni²⁺, the concentration of Ni-diamine complexes, and stability constants of the complexes revealed that anisotropic growth was promoted when the supersaturation was relatively high and was maintained constant for a long time. We clarified the role of amine ligands in controlling supersaturation through the controlled release of metal ions from stable complexes. β-Co(OH)₂ with a hexagonal prism shape was prepared using this protocol. This study provides valuable indications for developing synthetic chemistry for various layered compounds to achieve a controlled aspect ratio.

Field-Assisted Formation of NH4CoF3 Mesocrystals toward Hierarchical Co3O4 Cuboids as Anode Materials for Lithium-Ion Batteries

Porous NH₄CoF₃ mesocrystalline cuboids with highly exposed {100} facets are grown by in situ reaction of products produced by high field anodization of cobalt metal foil, via a nonclassical crystallization process involving oriented particle aggregation. 3D nano-micro hierarchical Co₃O₄ cuboids are obtained by thermal annealing of NH₄CoF₃ mesocrystals. The microstructure and morphology of products are characterized by electron microscopy and X-ray diffraction. The combination of small nanoparticle subunits, micrometer-sized overall particles, and porous structure provides the obtained hierarchical Co₃O₄ cuboids with large electrolyte-electrode contact areas, channels for large lithium ion flux, pore accessibility, and structural stability, leading to excellent rate and cyclic performance as lithium-ion battery (LIB) anodes.

A Symmetry-Based Kinematic Theory for Nanocrystal Morphology Design

The growth of crystalline nanoparticles (NPs) generally involves three processes: nucleation, growth, and shape evolution. Among them, the shape evolution is less understood, despite the importance of morphology for NP properties. Here, we propose a symmetry-based kinematic theory (SBKT) based on classical growth theories to illustrate the process. Based on the crystal lattice, nucleus (or seed) symmetry, and the preferential growth directions under the experimental conditions, the SBKT can illustrate the growth trajectories. The theory accommodates the conventional criteria of the major existing theories for crystal growth and provides tools to better understand the symmetry-breaking process during the growth of anisotropic structures. Furthermore, complex dendritic growth is theoretically and experimentally demonstrated. Thus, it provides a framework to explain the shape evolution, and extends the morphogenesis prediction to cases, which cannot be treated by other theories.

Biomineralized Materials as Model Systems for Structural Composites: Intracrystalline Structural Features and Their Strengthening and Toughening Mechanisms

Biomineralized composites, which are usually composed of microscopic mineral building blocks organized in 3D intercrystalline organic matrices, have evolved unique structural designs to fulfill mechanical and other biological functionalities. While it has been well recognized that the intricate architectural designs of biomineralized composites contribute to their remarkable mechanical performance, the structural features within and corresponding mechanical properties of individual mineral building blocks are often less appreciated in the context of bio-inspired structural composites. The mineral building blocks in biomineralized composites exhibit a variety of salient intracrystalline structural features, such as, organic inclusions, inorganic impurities (or trace elements), crystalline features (e.g., amorphous phases, single crystals, splitting crystals, polycrystals, and nanograins), residual stress/strain, and twinning, which significantly modify the mechanical properties of biogenic minerals. In this review, recent progress in elucidating the intracrystalline structural features of three most common biomineral systems (calcite, aragonite, and hydroxyapatite) and their corresponding mechanical significance are discussed. Future research directions and corresponding challenges are proposed and discussed, such as the advanced structural characterizations and formation mechanisms of intracrystalline structures in biominerals, amorphous biominerals, and bio-inspired synthesis.

In situ optical spectroscopy of crystallization: One crystal nucleation at a time

While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement single crystal nucleation spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a nonsupervised spectral decomposition technique, uncovered the Raman spectrum of prenucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of prenucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of linear hydrogen-bonded networks. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.