Stable Prenucleation Calcium Carbonate Clusters Define Liquid-Liquid Phase Separation

Lower-Temperature Nucleation and Growth of Colloidal CdTe Quantum Dots Enabled by Prenucleation Clusters with Cd-Te Bond Conservation

The reason why heating is required remains elusive for the traditional synthesis of colloidal semiconductor quantum dots (QDs) of II-VI metal chalcogenide (ME). Using CdTe as a model system, we show that the formation of Cd-Te covalent bonds with individual Cd- and Te-containing compounds can be decoupled from the nucleation and growth of CdTe QDs. Prepared at an elevated temperature, a prenucleation-stage sample contains clusters that are the precursor compound (PC) of magic-size clusters (MSCs); the Cd-Te bond formation occurs at temperatures higher than 120 °C in the reaction. Afterward, the PC-to-QD transformation appears via monomers at lower temperatures in dispersion. Our findings suggest that the number of Cd-Te bonds broken in the PC reactant is similar to that of Cd-Te bonds formed in the QD product. For the traditional synthesis of ME QDs, heating is responsible for the M-E bond formation rather than for nucleation.

Kinetic and Thermodynamic Interplay of Polymer-Mediated Liquid-Liquid Phase Separation for Poorly Water-Soluble Drugs

Understanding the interplay between kinetics and thermodynamics of polymer-mediated liquid-liquid phase separation is crucial for designing and implementing an amorphous solid dispersion formulation strategy for poorly water-soluble drugs. This work investigates the phase behaviors of a poorly water-soluble model drug, celecoxib (CXB), in a supersaturated aqueous solution with and without polymeric additives (PVP, PVPVA, HPMCAS, and HPMCP). Drug-polymer-water ternary phase diagrams were also constructed to estimate the thermodynamic behaviors of the mixtures at room temperature. The liquid-liquid phase separation onset point for CXB was detected using an inline UV/vis spectrometer equipped with a fiber optic probe. Varying CXB concentrations were achieved using an accurate syringe pump throughout this study. The appearance of the transient nanodroplets was verified by cryo-EM and total internal reflection fluoresence microscopic techniques. The impacts of various factors, such as polymer composition, drug stock solution pumping rates, and the types of drug-polymer interactions, are tested against the onset points of the CXB liquid-liquid phase separation (LLPS). It was found that the types of drug-polymer interactions, i.e., hydrogen bonding and hydrophobic interactions, are vital to the position and shapes of LLPS in the supersaturation drug solution. A relation between the behaviors of LLPS and its location in the CXB-polymer-water ternary phase diagram was drawn from the findings.

Prenucleation Cluster Pathway is Inconsistent with CaCO3 Kinetics

It has been debated whether CaCO3 nucleates classically with the attainment of a critical cluster size or nonclassically with the restructuring of a prenucleation cluster (PNC). Here, we determine from the nucleation kinetics of CaCO3 that the transition state is composed of about 10 formula units, irrespective of the supersaturation. Crucially, the size of the transition state is considerably smaller than the average PNC size estimated from experimental characterization. This size discrepancy suggests the PNCs are uninvolved in nucleation, and the kinetics indicate that if CaCO3 nucleates classically, the transition state must be an abnormally unstable (antimagic) cluster.

Pathway of Room-Temperature Formation of CdSeS Magic-Size Clusters from Mixtures of CdSe and CdS Samples

The synthetic application of prenucleation-stage samples of colloidal semiconductor quantum dots (QDs) is in its infancy. It is shown that when two prenucleation-stage samples of binary CdSe and CdS are mixed, ternary CdSeS magic-size clusters (MSCs) grow at room temperature in dispersion. As the amount of the CdS sample increases, the optical absorption of the CdSeS MSCs blueshifts from ≈380 to ≈360 nm. It is proposed that the cluster in the CdSe sample reacts with the CdS monomer from the CdS sample. The monomer substitution reaction of CdSe by CdS can proceed continuously; thus, CdSeS MSCs with tunable compositions are obtained. The present study provides compelling evidence that clusters formed in the prenucleation stage of QDs. The clusters are precursor compounds (PCs) of MSCs, transforming at room temperature with the thermoneutrality principle of isodesmic reactions. The nucleation and growth of QDs follows a multi-step non-classical instead of one-step classical nucleation model.

Redefined ion association constants have consequences for calcium phosphate nucleation and biomineralization

Calcium orthophosphates (CaPs), as hydroxyapatite (HAP) in bones and teeth are the most important biomineral for humankind. While clusters in CaP nucleation have long been known, their speciation and mechanistic pathways to HAP remain debated. Evidently, mineral nucleation begins with two ions interacting in solution, fundamentally underlying solute clustering. Here, we explore CaP ion association using potentiometric methods and computer simulations. Our results agree with literature association constants for Ca2+ and H2PO4-, and Ca2+ and HPO42-, but not for Ca2+ and PO43- ions, which previously has been strongly overestimated by two orders of magnitude. Our data suggests that the discrepancy is due to a subtle, premature phase separation that can occur at low ion activity products, especially at higher pH. We provide an important revision of long used literature constants, where association of Ca2+ and PO43- actually becomes negligible below pH 9.0, in contrast to previous values. Instead, [CaHPO4]0 dominates the aqueous CaP speciation between pH ~6-10. Consequently, calcium hydrogen phosphate association is critical in cluster-based precipitation in the near-neutral pH regime, e.g., in biomineralization. The revised thermodynamics reveal significant and thus far unexplored multi-anion association in computer simulations, constituting a kinetic trap that further complicates aqueous calcium phosphate speciation.

Time evolution of moduli of a polymer-induced liquid precursor (PILP) of calcium carbonate

In situ AFM observations show that when PILP droplets contact a surface, their initial properties are either a liquid with a high interfacial tension (350 mJ m-2) or a soft gel-like material with a low modulus (less than 0.2 MPa). These findings suggest that PILP may initially be liquid-like to infiltrate collagen fibrils, enabling the production of interpenetrating composites, and/or become viscoelastic, to provide a means for moulding minerals.

Reversion of ACP Nanoparticles into Prenucleation Clusters via Surfactant for Promoting Biomimetic Mineralization: A Physicochemical Understanding of Biosurfactant Role in Biomineralization Process

Amphiphilic biomolecules are abundant in mineralization front of biological hard tissues, which play a vital role in osteogenesis and dental hard tissue formation. Amphiphilic biomolecules function as biosurfactants, however, their biosurfactant role in biomineralization process has never been investigated. This study, for the first time, demonstrates that aggregated amorphous calcium phosphate (ACP) nanoparticles can be reversed into dispersed ultrasmall prenucleation clusters (PNCs) via breakdown and dispersion of the ACP nanoparticles by a surfactant. The reduced surface energy of ACP@TPGS and the electrostatic interaction between calcium ions and the pair electrons on oxygen atoms of C-O-C of D-α-tocopheryl polyethylene glycol succinate (TPGS) provide driving force for breakdown and dispersion of ACP nanoparticles into ultrasmall PNCs which promote in vitro and in vivo biomimetic mineralization. The ACP@TPGS possesses excellent biocompatibility without any irritations to oral mucosa and dental pulp. This study not only introduces surfactant into biomimetic mineralization field, but also excites attention to the neglected biosurfactant role during biomineralization process.

Exploring the interference mechanisms of surface and aqueous complexes with groundwater arsenate and arsenite adsorption

Ca2+, Mg2+, and HCO3- are extremely common coexisting ions with arsenic (As) in geogenic As-polluted groundwaters. Although extensive research has improved our knowledge of groundwater As removal techniques and mechanisms, there is still a lack of a definite explanation of the distinct influences of Ca2+ and Mg2+ on As immobilization. Furthermore, the question of whether the occurrence of metal-As aqueous complexes has positive or detrimental effects on As adsorption is still open, which hinders our ability to predict the effectiveness of groundwater As removal. The goal of our present work was to investigate the molecular-level interference mechanisms of Ca2+, Mg2+, and HCO3- on arsenic adsorption with batch/column filtration experiments and spectroscopic techniques. The results showed that the co-presence of Ca2+ and As significantly increased As(V) and As(III) adsorption by 22.1 and 12.2% in batch studies and by 20.1 and 16.7% in column adsorptive filtrations, which could be explained by forming a ternary Ca-As-TiO2 complex. Without the surface complex, Mg2+ only had a slightly positive effect on As removal. Co-existence of Ca2+ and HCO3- prevented the generation this surface complex, which significantly decreased the elimination of As(III). Inversely, the As(V) ternary complex and adsorption were not interfered by HCO3-. Moreover, an aqueous Ca-As(V) complex was detected without surface, which facilitated generation of the surface complex and As(V) adsorption. The results of this work clarified the distinct effects of Ca2+ and Mg2+ and As(V) and As(III) adsorption, which was critical in predicting the As adsorption efficiency in column filtration processes.

Formation and Transformation of CdS Clusters during the Prenucleation Stage and in a Dilute Dispersion at Room Temperature

The formation and transformation of colloidal semiconductor clusters remain poorly understood. With CdS as a model system, we show that, in the reaction of cadmium myristate (Cd(MA)2) and S powder in 1-octadecene (ODE), clusters form in the prenucleation stage of quantum dots (QDs). Called precursor compounds (PCs), the clusters can transform to magic-size clusters (MSCs) in reaction at a relatively high temperature (MSC-322 displaying optical absorption peaking at 322 nm) or in a dispersion at room temperature (MSC-360). When the reaction temperature is increased, PC-360 forms at 140 °C, while PC-322 and MSC-322 form at 180 °C. In a dispersion of cyclohexane and octylamine, MSC-322 transforms to MSC-360 via MSC-345. The MSC-345 to MSC-360 transformation displays continuous and discontinuous shifts in the optical absorption. The PCs and MSCs are a group of isomers. The present findings bring insight into the cluster formation and isomerization in the prenucleation stage of QDs and in a dispersion.

Nucleation Domains in Biomineralization: Biomolecular Sequence and Conformational Features

Biomolecules play a vital role in the regulation of biomineralization. However, the characteristics of practical nucleation domains are still sketchy. Herein, the effects of the representative biomolecular sequence and conformations on calcium phosphate (Ca-P) nucleation and mineralization are investigated. The results of computer simulations and experiments prove that the line in the arrangement of dual acidic/essential amino acids with a single interval (Bc (Basic) -N (Neutral) -Bc-N-Ac (Acidic)- NN-Ac-N) is most conducive to the nucleation. 2α-helix conformation can best induce Ca-P ion cluster formation and nucleation. "Ac- × × × -Bc" sequences with α-helix are found to be the features of efficient nucleation domains, in which process, molecular recognition plays a non-negligible role. It further indicates that the sequence determines the potential of nucleation/mineralization of biomolecules, and conformation determines the ability of that during functional execution. The findings will guide the synthesis of biomimetic mineralized materials with improved performance for bone repair.

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.

Thermally-Induced Isomerization of Prenucleation Clusters During the Prenucleation Stage of CdTe Quantum Dots

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic-size clusters (MSCs). When a prenucleation-stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC-371, MSC-417, or MSC-448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC-371 forms at 25 °C, and isomerizes to PC-417 at 45 °C and to PC-448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC-371 and PC-448 in dispersion, respectively. The present findings suggest the existence of PC-to-PC isomerization in the QD prenucleation stage.

Evidence for liquid-liquid phase separation during the early stages of Mg-struvite formation

The precipitation of struvite, a magnesium ammonium phosphate hexahydrate (MgNH4PO4 · 6H2O) mineral, from wastewater is a promising method for recovering phosphorous. While this process is commonly used in engineered environments, our understanding of the underlying mechanisms responsible for the formation of struvite crystals remains limited. Specifically, indirect evidence suggests the involvement of an amorphous precursor and the occurrence of multi-step processes in struvite formation, which would indicate non-classical paths of nucleation and crystallization. In this study, we use synchrotron-based in situ x-ray scattering complemented by cryogenic transmission electron microscopy to obtain new insights from the earliest stages of struvite formation. The holistic scattering data captured the structure of an entire assembly in a time-resolved manner. The structural features comprise the aqueous medium, the growing struvite crystals, and any potential heterogeneities or complex entities. By analysing the scattering data, we found that the onset of crystallization causes a perturbation in the structure of the surrounding aqueous medium. This perturbation is characterized by the occurrence and evolution of Ornstein-Zernike fluctuations on a scale of about 1 nm, suggesting a non-classical nature of the system. We interpret this phenomenon as a liquid-liquid phase separation, which gives rise to the formation of the amorphous precursor phase preceding actual crystal growth of struvite. Our microscopy results confirm that the formation of Mg-struvite includes a short-lived amorphous phase, lasting >10 s.

The nucleation of C-S-H via prenucleation clusters

The nucleation and growth of calcium-silicate-hydrate (C-S-H) is of fundamental importance for the strength development and durability of the concrete. However, the nucleation process of C-S-H is still not fully understood. The present work investigates how C-S-H nucleates by analyzing the aqueous phase of hydrating tricalcium silicate (C₃S) by applying inductively coupled plasma-optical emission spectroscopy as well as analytical ultracentrifugation. The results show that the C-S-H formation follows non-classical nucleation pathways associated with the formation of prenucleation clusters (PNCs) of two types. Those PNCs are detected with high accuracy and reproducibility and are two species of the 10 in total, from which the ions (with associated water molecules) are the majority of the species. The evaluation of the density and molar mass of the species shows that the PNCs are much larger than ions, but the nucleation of C-S-H starts with the formation of liquid precursor C-S-H (droplets) with low density and high water content. The growth of these C-S-H droplets is associated with a release of water molecules and a reduction in size. The study gives experimental data on the size, density, molecular mass, and shape and outlines possible aggregation processes of the detected species.

Secondary Structure of DNA Aptamer Influences Biomimetic Mineralization of Calcium Carbonate

Calcium materials, such as calcium carbonate, are produced in natural and industrial settings that range from oceanic to biomedical. An array of biological and biomimetic template molecules have been employed in controlling and understanding the mineralization reaction but have largely focused on small molecule additives or disordered polyelectrolytes. DNA aptamers are synthetic and programmable biomolecules with polyelectrolyte characteristics but with predictable and controllable secondary structure akin to native extracellular moieties. This work demonstrates for the first time the influence of DNA aptamers with known G-quadruplex structures on calcium carbonate mineralization. Aptamers demonstrate kinetic inhibition of mineral formation, sequence and pH-dependent uptake into the mineral, and morphological control of the primarily calcite material in controlled solution conditions. In reactions initiated from the complex matrix of ocean water, DNA aptamers demonstrated enhancement of mineralization kinetics and resulting amorphous material. This work provides new biomimetic tools to employ in controlled mineralization and demonstrates the influence that template secondary structure can have in material formation.

In Situ Fluorescence Probing of the Formation of Calcium Phosphate Prenucleation Clusters

Initial-stage prenucleation clusters (PNCs) are critical in calcium phosphate (CaP) biomineralization and thus the formation mechanisms of human bones and teeth. However, several features of PNCs require further examination, e.g., structure, ionic stoichiometry, kinetics, thermodynamics, and nucleation mechanism. In this study, we used poly(acrylic acid) (PAA)-Ca(Eu) complexes with partial Eu³⁺ substitution as pre-PNCs and established a fluorescence method to study PNC formation in situ based on Eu-O charge-transfer transitions. The kinetics and thermodynamics of PNC formation were explored by probing the fluorescence changes of Eu-O charge-transfer transitions during bonding between the pre-PNCs and PO₄^3-. PNC formation was consistent with the pseudo-second-order kinetic and Langmuir isothermal adsorption models. The flexible structures of PNCs aided in regulating the subsequent nucleation and crystallization. This study provides an in situ fluorescence probing method with critical guiding significance in addressing the features of PNC formation, in addition to biomineralization.

Coexistence of Transient Liquid Droplets and Amorphous Solid Particles in Nonclassical Crystallization of Cerium Oxalate

Crystallization from solution often occurs via "nonclassical" routes; that is, it involves transient, non-crystalline states like reactant-rich liquid droplets and amorphous particles. However, in mineral crystals, the well-defined thermodynamic character of liquid droplets and whether they convert─or not─into amorphous phases have remained unassessed. Here, by combining cryo-transmission electron microscopy and X-ray scattering down to a 250 ms reaction time, we unveil that crystallization of cerium oxalate involves a metastable chemical equilibrium between transient liquid droplets and solid amorphous particles: contrary to the usual expectation, reactant-rich droplets do not evolve into amorphous solids. Instead, at concentrations above 2.5 to 10 mmol L^-1, both amorphous and reactant-rich liquid phases coexist for several tens of seconds and their molar fractions remain constant and follow the lever rule in a multicomponent phase diagram. Such a metastable chemical equilibrium between solid and liquid precursors has been so far overlooked in multistep nucleation theories and highlights the interest of rationalizing phase transformations using multicomponent phase diagrams not only when designing and recycling rare earths materials but also more generally when describing nonclassical crystallization.

The Role of Intrinsically Disordered Proteins in Liquid–Liquid Phase Separation during Calcium Carbonate Biomineralization

Some animal organs contain mineralized tissues. These so-called hard tissues are mostly deposits of calcium salts, usually in the form of calcium phosphate or calcium carbonate. Examples of this include fish otoliths and mammalian otoconia, which are found in the inner ear, and they are an essential part of the sensory system that maintains body balance. The composition of ear stones is quite well known, but the role of individual components in the nucleation and growth of these biominerals is enigmatic. It is sure that intrinsically disordered proteins (IDPs) play an important role in this aspect. They have an impact on the shape and size of otoliths. It seems probable that IDPs, with their inherent ability to phase separate, also play a role in nucleation processes. This review discusses the major theories on the mechanisms of biomineral nucleation with a focus on the importance of protein-driven liquid–liquid phase separation (LLPS). It also presents the current understanding of the role of IDPs in the formation of calcium carbonate biominerals and predicts their potential ability to drive LLPS.

Role of Mass Transport in the Deposition, Growth, and Transformation of Calcium Carbonate on Surfaces at High Supersaturation

We demonstrate how combined in-situ measurements and finite element method modeling can provide new insight into the relative contribution of mass transport to the growth of calcium carbonate on two model surfaces, glass and gold, under high-supersaturation conditions relevant to surface scaling. An impinging jet-radial flow system is used to create a high-supersaturated solution at the inlet of different cells: an optical microscope cell presenting a glass surface for deposition and quartz crystal microbalance (QCM) and in-situ IR spectroscopy cells, both presenting a gold surface. The approach described is quantitative due to the well-defined mass transport, and both time-lapse optical microscopy images and QCM data are analyzed to provide information on the growth kinetics of the calcite crystals. Initially, amorphous calcium carbonate (ACC), formed in solution, dominates the deposition process. At longer times, the growth of calcite is more significant and, on glass, is observed to consume ACC from the surface, leading to surface regions depleted of ACC developing around calcite microcrystals. On Au, the mass increase becomes linear with time in this region. Taken together, these microscopic and macroscopic measurements demonstrate that calcite growth has a significant component of mass transport control at high supersaturation. Finite element method (FEM) simulations of mass-transport-limited crystal growth support the strong mass transport contribution to the growth kinetics and further suggest that the observed growth must be sustained by more than just the Ca²⁺ and CO₃ ^2- in solution, with dissolution/direct attachment of ACC and/or ion pairs also contributing to the growth process.

Molecular Mechanism of Organic Crystal Nucleation: A Perspective of Solution Chemistry and Polymorphism

Crystal nucleation determining the formation and assembly pathway of first organic materials is the central science of various scientific disciplines such as chemical, geochemical, biological, and synthetic materials. However, our current understanding of the molecular mechanisms of nucleation remains limited. Over the past decades, the advancements of new experimental and computational techniques have renewed numerous interests in detailed molecular mechanisms of crystal nucleation, especially structure evolution and solution chemistry. These efforts bifurcate into two categories: (modified) classical nucleation theory (CNT) and non-classical nucleation mechanisms. In this review, we briefly introduce the two nucleation mechanisms and summarize current molecular understandings of crystal nucleation that are specifically applied in polymorphic crystallization systems of small organic molecules. Many important aspects of crystal nucleation including molecular association, solvation, aromatic interactions, and hierarchy in intermolecular interactions were examined and discussed for a series of organic molecular systems. The new understandings relating to molecular self-assembly in nucleating systems have suggested more complex multiple nucleation pathways that are associated with the formation and evolution of molecular aggregates in solution.

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

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

Probing the effects of polymers on the early stages of calcium carbonate formation by stoichiometric co-titration

Potentiometric titrations are a powerful tool to study the early stages of the precipitation of minerals such as calcium carbonate and were used among others for the discovery and characterisation of key precursors like prenucleation clusters. Here we present a modified procedure for conducting such titration experiments, in which the reactants (i.e. calcium and (bi)carbonate ions) are added simultaneously in stoichiometric amounts, while both the amount of free calcium and the optical transmission of the solution are monitored online. Complementarily, the species occurring at distinct stages of the crystallisation process were studied using cryogenic transmission electron microscopy. This novel routine was applied to investigate CaCO₃ nucleation in the absence and presence of polymeric additives with different chemical functionalities. The obtained results provide new insights into the critical steps underlying nucleation and subsequent ripening, such as the role of liquid mineral-rich phases and their transformation into solid particles. The studied polymers proved to interfere at multiple stages along the complex mineralisation pathway of calcium carbonate, with both the degree and mode of interaction depending on the chosen polymer chemistry. In this way, the methodology developed in this work allows the mechanisms of antiscalants - or crystallisation modifiers in general - to be elucidated at an advanced level of detail.

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

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

Liquid-Liquid Phase Separation in Nucleation Process of Biomineralization

Biomineralization is a typical interdisciplinary subject attracting biologists, chemists, and geologists to figure out its potential mechanism. A mounting number of studies have revealed that the classical nucleation theory is not suitable for all nucleation process of biominerals, and phase-separated structures such as polymer-induced liquid precursors (PILPs) play essential roles in the non-classical nucleation processes. These structures are able to play diverse roles biologically or pathologically, and could also give inspiring clues to bionic applications. However, a lot of confusion and dispute occurred due to the intricacy and interdisciplinary nature of liquid precursors. Researchers in different fields may have different opinions because the terminology and current state of understanding is not common knowledge. As a result, our team reviewed the most recent articles focusing on the nucleation processes of various biominerals to clarify the state-of-the-art understanding of some essential concepts and guide the newcomers to enter this intricate but charming field.

The influence of L-aspartic acid on calcium carbonate nucleation and growth revealed by in situ liquid phase TEM

In situ transmission electron microscopy has permitted the study of nanomaterials in liquid environments with high spatial and temporal resolutions, allowing chemical reaction visualization in real time. The aim of...

Possible embryo and precursor of crystalline nuclei of calcium carbonate observed by LC-TEM

Several different building blocks or precursors play an important role in the early stages of crystallization of calcium carbonate (CaCO3). Substantial number of studies have been conducted to understand the...

In Situ TEM Imaging of Solution-Phase Chemical Reactions Using 2D-Heterostructure Mixing Cells

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

Uncovering the Role of Bicarbonate in Calcium Carbonate Formation at Near-Neutral pH

Mechanistic pathways relevant to mineralization are not well‐understood fundamentally, let alone in the context of their biological and geological environments. Through quantitative analysis of ion association at near‐neutral pH, we identify the involvement of HCO₃⁻ ions in CaCO₃ nucleation. Incorporation of HCO₃⁻ ions into the structure of amorphous intermediates is corroborated by solid‐state nuclear magnetic resonance spectroscopy, complemented by quantum mechanical calculations and molecular dynamics simulations. We identify the roles of HCO₃⁻ ions as being through (i) competition for ion association during the formation of ion pairs and ion clusters prior to nucleation and (ii) incorporation as a significant structural component of amorphous mineral particles. The roles of HCO₃⁻ ions as active soluble species and structural constituents in CaCO₃ formation are of fundamental importance and provide a basis for a better understanding of physiological and geological mineralization.

Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization

Mineralization of hydroxylapatite (HAp), the main inorganic phase in bone, follows nonclassical crystallization routes involving metastable precursors and is strongly influenced by organic macromolecules. However, the effect of small organic molecules such as citrate on the formation of HAp is not well constrained. Using potentiometric titration experiments and titration calorimetry, in combination with a multianalytical approach, we show that citrate stabilizes prenucleation species as well as a liquid-like calcium phosphate precursor formed before any solid phase nucleates in the system. The stabilization of a liquid-like precursor phase could facilitate infiltration into the cavities of the collagen fibrils during bone mineralization, explaining the enhancement of collagen-mediated mineralization by citrate reported in previous studies. Hence, citrate can influence bone mineralization way before any solid phase (amorphous or crystalline) is formed. We also show that HAp formation after amorphous calcium phosphate (ACP) in the absence and presence of citrate results in nanoplates of about 5-12 nm thick, elongated along the c axis. Such nanoplates are made up of HAp nanocrystallites with a preferred c axis orientation and with interspersed ACP. The nanoplatelet morphology, size, and preferred crystallographic orientation, remarkably similar to those of bone HAp nanocrystals, appear to be an intrinsic feature of HAp formed from an amorphous precursor. Our results challenge current models for HAp mineralization in bone and the role of citrate, offering new clues to help answer the long-standing question as to why natural evolution favored HAp as the mineral phase in bone.

Controlling CaCO3 Particle Size with {Ca2+}:{CO3 2-} Ratios in Aqueous Environments

The effect of stoichiometry on the new formation and subsequent growth of CaCO₃ was investigated over a large range of solution stoichiometries (10^-4 < r _aq < 10⁴, where r _aq = {Ca²⁺}:{CO₃ ^2-}) at various, initially constant degrees of supersaturation (30 < Ω_cal < 200, where Ω_cal = {Ca²⁺}{CO₃ ^2-}/K _sp), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r _aq = 1 and Ω_cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1-10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r _aq ≠ 1 (Ω_cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r _aq. Complementary molecular dynamics (MD) simulations confirm that r _aq affects CaCO₃ nanoparticle formation substantially. At r _aq = 1 and Ω_cal ≫ 1000, the largest nanoparticle in the system had a 21-68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω_cal, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO₃ in geological, industrial, and geo-engineering settings.

Role of Water in CaCO3 Biomineralization

Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO₃ biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO₃ biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO₃ mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca²⁺ and CO₃^2– compared to the addition of only Ca²⁺. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization.

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

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

Multiple Pathways in NaCl Homogeneous Crystal Nucleation

NaCl crystal nucleation from metastable solutions has long been considered to occur according to a single-step mechanism where the growth in the size and crystalline order of the emerging nuclei...

Generality of liquid precursor phases in gas diffusion-based calcium carbonate synthesis

We confirm the presence of liquid calcium carbonate precursor species in absence of additives in gas diffusion systems.

Nanoscale kinetics of amorphous calcium carbonate precipitation in H2O and D2O

Calcium carbonate (CaCO₃) is one of the most well-studied and abundant natural materials on Earth. Crystallisation of CaCO₃ is often observed to proceed via an amorphous calcium carbonate (ACC) phase, as a precursor to more stable crystalline polymorphs such as vaterite and calcite. Despite its importance, the kinetics of ACC formation have proved difficult to study, in part due to rapid precipitation at moderate supersaturations, and the instability of ACC with respect to all other polymorphs. However, ACC can be stabilised under confinement conditions, such as those provided by a nanopipette. This paper demonstrates electrochemical mixing of a Ca²⁺ salt (CaCl₂) and a HCO₃^- salt (NaHCO₃) in a nanopipette to repeatedly and reversibly precipitate nanoparticles of ACC under confined conditions, as confirmed by scanning transmission electron microscopy (STEM). Measuring the current as a function of applied potential across the end of the nanopipette and time provides millisecond-resolved measurements of the induction time for ACC precipitation. We demonstrate that under conditions of electrochemical mixing, ACC precipitation is extremely fast, and highly pH sensitive with an apparent third order dependence on CO₃^2- concentration. Furthermore, the rate is very similar for the equivalent CO₃^2- concentrations in D₂O, suggesting that neither ion dehydration nor HCO₃^- deprotonation represent significant energetic barriers to the formation of ACC. Finite element method simulations of the electrochemical mixing process enable the supersaturation to be estimated for all conditions and accurately predict the location of precipitation.

Pulsed electric fields induce modulation of protein liquid-liquid phase separation

The time-resolved dynamic assembly and the structures of protein liquid dense clusters (LDCs) were analyzed under pulsed electric fields (EFs) applying complementary polarized and depolarized dynamic light scattering (DLS/DDLS), optical microscopy, and transmission electron microscopy (TEM). We discovered that pulsed EFs substantially affected overall morphologies and spatial distributions of protein LDCs and microcrystals, and affected the phase diagrams of LDC formation, including enabling protein solutions to overcome the diffusive flux energy barrier to phase separate. Data obtained from DLS/DDLS and TEM showed that LDCs appeared as precursors of protein crystal nuclei, followed by the formation of ordered structures within LDCs applying a pulsed EF. Experimental results of circular dichroism spectroscopy provided evidence that the protein secondary structure content is changing under EFs, which may consequently modulate protein-protein interactions, and the morphology, dimensions, and internal structure of LDCs. Data and results obtained unveil options to modulate the phase diagram of crystallization, and physical morphologies of protein LDCs and microcrystals by irradiating sample suspensions with pulsed EFs.

Identifying the existence and molecular structure of the dissolved HCO3-Ca-As(V) complex in water

Calcium (Ca²⁺) and bicarbonate (HCO₃^-) ions co-exist with arsenic (As) in natural water systems, while Ca-based materials such as lime and cement are widely used to immobilize As(V) in contaminated solids. In this paper, a new dissolved ternary complex, HCO₃-Ca-As(V), was discovered and its molecular structure was identified. The results from the batch experiments showed that adding As(V) to the solutions containing Ca²⁺ and HCO₃^- increased the dissolved Ca concentration from 4.8 to 73.2 mg/L at pH 11. Both infrared and X-ray absorption spectroscopy indicated the presence of dissolved HCO₃-Ca-As(V) complex. Based on the quantitative geometric information obtained from the spectroscopic results, the molecule of (OH)OC-O-(OH₂)₄Ca-O₂-As(OH)₂ was identified by the density functional theory (DFT) calculation. Although Ca²⁺ and As(V) can form complex without HCO₃^-, the presence of HCO₃^- further enhanced the stability of the dissolved Ca complex, as evidenced by the lower binding energy (BE) of HCO₃-Ca-As(V) (-329.1959 kJ/mol) than Ca-As(V) (4.7171 kJ/mol). The discovery of dissolved HCO₃-Ca-As(V) complex is important for understanding the mobility of As(V) in natural water, and the possible release of As(V) in contaminated solids treated with Ca-based materials.

Stable Prenucleation Calcium Carbonate Clusters Define Liquid-Liquid Phase Separation

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