Nonclassical crystallization in vivo et in vitro (II): Nanogranular features in biomimetic minerals disclose a general colloid-mediated crystal growth mechanism

Microbial-induced calcium precipitation: Bibliometric analysis, reaction mechanisms, mineralization types, and perspectives

Microbial-induced calcium precipitation (MICP) refers to the formation of calcium precipitates induced by mineralization during microbial metabolism. MICP has been widely used as an ecologically sustainable method in environmental, geotechnical, and construction fields. This article reviews the removal mechanisms of MICP for different contaminants in the field of water treatment. The nucleation pathway is explained at both extracellular and intracellular levels, with a focus on evaluating the contribution of extracellular polymers to MICP. The types of mineralization and the regulatory role of enzyme genes in the MICP process are innovatively summarized. Based on this, the environmental significance of MICP is illustrated, and the application prospects of calcium precipitation products are discussed. The research hotspots and development trends of MICP are analyzed by bibliometric methods, and the challenges and future directions of MICP technology are identified. This review aims to provide a theoretical basis for further understanding of the MICP phenomenon in water treatment and the effective removal of multiple pollutants, which will help researchers to find the breakthroughs and innovations in the existing technologies, with a view to making significant progress in MICP technology.

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.

Unraveling the pathological biomineralization of monosodium urate crystals in gout patients

Crystallization of monosodium urate monohydrate (MSU) leads to painful gouty arthritis. Despite extensive research it is still unknown how this pathological biomineralization occurs, which hampers its prevention. Here we show how inflammatory MSU crystals form after a non-inflammatory amorphous precursor (AMSU) that nucleates heterogeneously on collagen fibrils from damaged articular cartilage of gout patients. This non-classical crystallization route imprints a nanogranular structure to biogenic acicular MSU crystals, which have smaller unit cell volume, lower microstrain, and higher crystallinity than synthetic MSU. These distinctive biosignatures are consistent with the template-promoted crystallization of biotic MSU crystals after AMSU at low supersaturation, and their slow growth over long periods of time (possibly years) in hyperuricemic gout patients. Our results help to better understand gout pathophysiology, underline the role of cartilage damage in promoting MSU crystallization, and suggest that there is a time-window to treat potential gouty patients before a critical amount of MSU has slowly formed as to trigger a gout flare.

In situ mapping of biomineral skeletal proteins by molecular recognition imaging with antibody-functionalized AFM tips

Spatial localizing of skeletal proteins in biogenic minerals remains a challenge in biomineralization research. To address this goal, we developed a novel in situ mapping technique based on molecular recognition measurements via atomic force microscopy (AFM), which requires three steps: (1) the development and purification of a polyclonal antibody elicited against the target protein, (2) its covalent coupling to a silicon nitride AFM tip ('functionalization'), and (3) scanning of an appropriately prepared biomineral surface. We applied this approach to a soluble shell protein - accripin11 - recently identified as a major component of the calcitic prisms of the fan mussel Pinna nobilis [1]. Multiple tests reveal that accripin11 is evenly distributed at the surface of the prisms and also present in the organic sheaths surrounding the calcitic prisms, indicating that this protein is both intra- and inter-crystalline. We observed that the adhesion force in transverse sections is about twice higher than in longitudinal sections, suggesting that accripin11 may exhibit preferred orientation in the biomineral. To our knowledge, this is the first time that a protein is localized by molecular recognition atomic force microscopy with antibody-functionalized tips in a biogenic mineral. The 'pros' and 'cons' of this methodology are discussed in comparison with more 'classical' approaches like immunogold. This technique, which leaves the surface to analyze clean, might prove useful for clinical tests on non-pathological (bone, teeth) or pathological (kidney stone) biomineralizations. Studies using implants with protein-doped calcium phosphate coating can also benefit from this technology. STATEMENT OF SIGNIFICANCE: Our paper deals with an unconventional technical approach for localizing proteins that are occluded in biominerals. This technique relies on the use of molecular recognition atomic force microscopy with antibody-functionalized tips. Although such approach has been employed in other system, this is the very first time that it is developed for biominerals. In comparison to more classical approaches (such as immunogold), AFM microscopy with antibody-functionalized tips allows higher magnification and keeps the scanned surface clean for other biophysical characterizations. Our method has a general scope as it can be applied in human health, for non-pathological (bone, teeth) and pathological (kidney stone) biomineralizations as well as for bone implants coated with protein-doped calcium phosphate.

Biologically driven isotopic fractionations in bivalves: from palaeoenvironmental problem to palaeophysiological proxy

Traditional bulk stable isotope (δ18 O and δ13 C) and clumped isotope (Δ47 ) records from bivalve shells provide invaluable histories of Earth's local and global climate change. However, biologically driven isotopic fractionations (BioDIFs) can overprint primary environmental signals in the shell. Here, we explore how conventional measurements of δ18 O, δ13 C, and Δ47 in bivalve shells can be re-interpreted to investigate these physiological processes deliberately. Using intrashell Δ47 and δ18 O alignment as a proxy for equilibrium state, we separately examine fractionations and/or disequilibrium occurring in the two major stages of the biomineralisation process: the secretion of the extrapallial fluid (EPF) and the precipitation of shell material from the EPF. We measured δ18 O, δ13 C, and Δ47 in fossil shells representing five genera (Lahillia, Dozyia, Eselaevitrigonia, Nordenskjoldia, and Cucullaea) from the Maastrichtian age [66-69 million years ago (Ma)] López de Bertodano Formation on Seymour Island, Antarctica. Material was sampled from both the outer and inner shell layers (OSL and ISL, respectively), which precipitate from separate EPF reservoirs. We find consistent δ18 O values across the five taxa, indicating that the composition of the OSL can be a reliable palaeoclimate proxy. However, relative to the OSL baseline, ISLs of all taxa show BioDIFs in one or more isotopic parameters. We discuss/hypothesise potential origins of these BioDIFs by synthesising isotope systematics with the physiological processes underlying shell biomineralisation. We propose a generalised analytical and interpretive framework that maximises the amount of palaeoenvironmental and palaeobiological information that can be derived from the isotopic composition of fossil shell material, even in the presence of previously confounding 'vital effects'. Applying this framework in deep time can expand the utility of δ18 O, δ13 C, and Δ47 measurements from proxies of past environments to proxies for certain biomineralisation strategies across space, time, and phylogeny among Bivalvia and other calcifying organisms.

Silica-Functionalized Nanolimes for the Conservation of Stone Heritage

The relatively recent development of nanolimes (i.e., alcoholic dispersions of Ca(OH)2 nanoparticles) has paved the way for new approaches to the conservation of important art works. Despite their many benefits, nanolimes have shown limited reactivity, back-migration, poor penetration, and lack of proper bonding to silicate substrates. In this work a novel solvothermal synthesis process is presented by which extremely reactive nanostructured Ca(OH)2 particles are obtained using calcium ethoxide as the main precursor species. Moreover, it is demonstrated that this material can be easily functionalized with silica-gel derivatives under mild synthesis conditions, thereby preventing particle growth, increasing total specific surface area, enhancing reactivity, modifying colloidal behavior, and functioning as self-integrated coupling agents. Additionally, the formation of calcium silicate hydrate (CSH) nanocement is promoted by the presence of water, resulting in optimal bonding when applied to silicate substrates, as evidenced by the higher reinforcement effect produced on treated Prague sandstone specimens as compared to those consolidated with nonfunctionalized commercial nanolime. The functionalization of nanolimes is not only a promising strategy for the design of optimized consolidation treatments for the cultural heritage, but may also have important implications for the development of advanced nanomaterials for building, environmental, or biomedical applications.

Bioprocess inspired formation of calcite mesocrystals by cation-mediated particle attachment mechanism

Calcite mesocrystals were proposed, and have been widely reported, to form in the presence of polymer additives via oriented assembly of nanoparticles. However, the formation mechanism and the role of polymer additives remain elusive. Here, inspired by the biomineralization process of sea urchin spine comprising magnesium calcite mesocrystals, we show that calcite mesocrystals could also be obtained via attachment of amorphous calcium carbonate (ACC) nanoparticles in the presence of inorganic zinc ions. Moreover, we demonstrate that zinc ions can induce the formation of temporarily stabilized amorphous nanoparticles of less than 20 nm at a significantly lower calcium carbonate concentration as compared to pure solution, which is energetically beneficial for the attachment and occlusion during calcite growth. The cation-mediated particle attachment crystallization significantly improves our understanding of mesocrystal formation mechanisms in biomineralization and offers new opportunities to bioprocess inspired inorganic ions regulated materials fabrication.

Microbial-induced calcium carbonate precipitation: Influencing factors, nucleation pathways, and application in waste water remediation

Microbial-induced calcium carbonate precipitation (MICP) is a technique that uses the metabolic action of microorganisms to produce CO₃^2- which combines with free Ca²⁺ to form CaCO₃ precipitation. It has gained widespread attention in water treatment, aimed with the advantages of simultaneous removal of multiple pollutants, environmental protection, and ecological sustainability. This article reviewed the mechanism of MICP at both intra- and extra-cellular levels. It summarized the parameters affecting the MICP process in terms of bacterial concentration, ambient temperature, etc. The current status of MICP application in practical engineering is discussed. Based on this, the current technical difficulties faced in the use of MICP technology were outlined, and future research directions for MICP technology were highlighted. This review helps to improve the design of existing water treatment facilities for the simultaneous removal of multiple pollutants using the MICP and provides theoretical reference and innovative thinking for related research.

Nanoscale Analysis of the Structure and Composition of Biogenic Calcite Reveals the Biomineral Growth Pattern

The vast majority of calcium carbonate biocrystals differ from inorganic crystals in that they display a patent nanoroughness consisting of lumps of crystalline material (calcite/aragonite) surrounded by amorphous pellicles. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) was used to map the calcite secreted by a barnacle chemically and structurally with ultrahigh resolution (down to 1 nm). The material is composed of irregular lumps of calcite (up to two hundred nm in diameter) surrounded by relatively continuous cortexes (up to 20 nm thick) of amorphous calcium carbonate (ACC) and/or nanocalcite plus biomolecules, with a surplus of calcium relative to carbonate. We develop a model by which the separation of the crystalline and amorphous phases takes place upon crystallization of the calcite from a precursor ACC. The organic biomolecules are expelled from the crystal lattice and concentrate in the form of pellicles, where they stabilize minor amounts of ACC/nanocalcite. In this way, we change the previously established conception of biomineral structure and growth.

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.

Confinement induces stable calcium carbonate formation in silica nanopores

Scalable efforts to remove anthropogenic CO₂via the formation of durable carbonates require us to harness siliceous nanoporous geologic materials for carbon storage. While calcium carbonate formation has been extensively reported in bulk fluids, there is a limited understanding of the influence of nanoconfined fluids on the formation of specific stable and metastable polymorphs of calcium carbonates in siliceous materials that are abundant in subsurface environments. To address this challenge, silica nanochannels with diameters of 3.7 nm are architected and the formation of specific calcium carbonate phases is investigated using X-ray diffraction (XRD), and molecular dynamics (MD) simulations. The formation of stable calcium carbonate (or calcite) is noted in silica nanochannels. The presence of fewer water molecules in the first hydration shell of calcium ions in confinement compared to in bulk fluids contributes to stable calcium carbonate formation. These studies show that nanoporous siliceous environments favor the formation of stable calcium carbonate formation.

Crystallographic control of the fabrication of an extremely sophisticated shell surface microornament in the glass scallop Catillopecten

The external surface microornament of the glass scallops Catillopecten natalyae and malyutinae is made by calcitic spiny projections consisting of a stem that later divides into three equally spaced and inclined branches (here called aerials). C. natalyae contains larger and smaller aerials, whereas C. malyutinae only secreted aerials of the second type. A remarkable feature is that aerials within each type are fairly similar in size and shape and highly co-oriented, thus constituting a most sophisticated microornament. We demonstrate that aerials are single crystals whose morphology is strongly controlled by the crystallography, with the stem being parallel to the c-axis of calcite, and the branches extending along the edges of the {104} calcite rhombohedron. They grow epitaxially onto the foliated prisms of the outer shell layer. The co-orientation of the prisms explains that of the aerials. We have developed a model in which every aerial grows within a periostracal pouch. When this pouch reaches the growth margin, the mantle initiates the production of the aerial. Nevertheless, later growth of the aerial is remote, i.e. far from the contact with the mantle. We show how such an extremely sophisticated microornament has a morphology and co-orientation which are determined by crystal growth.

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.

Particle-based hematite crystallization is invariant to initial particle morphology

Many crystallization processes occurring in nature produce highly ordered hierarchical architectures. Their formation cannot be explained using classical models of monomer-by-monomer growth. One of the possible pathways involves crystallization through the attachment of oriented nanocrystals. Thus, it requires detailed understanding of the mechanism of particle dynamics that leads to their precise crystallographic alignment along specific faces. In this study, we discover a particle-morphology–independent oriented attachment mechanism for hematite nanocrystals. Independent of crystal morphology, particles always align along the [001] direction driven by aligning interactions between (001) faces and repulsive interactions between other pairs of hematite faces. These results highlight that strong face specificity along one crystallographic direction can render oriented attachment to be independent of initial particle morphology.

Understanding the mechanism of particle-based crystallization is a formidable problem due to the complexity of macroscopic and interfacial forces driving particle dynamics. The oriented attachment (OA) pathway presents a particularly challenging phenomenon because it occurs only under select conditions and involves a precise crystallographic alignment of particle faces often from distances of several nanometers. Despite the progress made in recent years in understanding the driving forces for particle face selectivity and alignment, questions about the competition between ion-by-ion crystallization, near-surface nucleation, and OA remain. This study examines hydrothermal conditions leading to apparent OA for hematite using three initial particle morphologies with various exposed faces. All three particle types formed single-crystal or twinned one-dimensional (1D) chain-like structures along the [001] direction driven by the attractive interactions between (001) faces and repulsive interactions between other pairs of hematite faces. Moreover, simulations of the potential of mean force for iron species and scanning transmission electron microscopy (S/TEM) imaging confirm that the formation of 1D chains is a result of the attachment of independently nucleated particles and does not follow the near-surface nucleation or ion-by-ion crystallization pathways. These results highlight that strong face specificity along one crystallographic direction can render OA to be independent of initial particle morphology.

Nonclassical Pathway to Biomimetic Strained SrSO4 Crystals

Biomimetic strained SrSO4 crystals were produced through a nonclassical pathway involving fiber-by-fiber accumulation with poly(acrylic acid) in an aqueous solution system. Polymer-stabilized fibrous hydrate crystals 20–40 nm wide and several...

Stabilization of Calcium Oxalate Precursors during the Pre- and Post-Nucleation Stages with Poly(acrylic acid)

In this work, calcium oxalate (CaOx) precursors were stabilized by poly(acrylic acid) (PAA) as an additive under in vitro crystallization assays involving the formation of pre-nucleation clusters of CaOx via a non-classical crystallization (NCC) pathway. The in vitro crystallization of CaOx was carried out in the presence of 10, 50 and 100 mg/L PAA by using automatic calcium potentiometric titration experiments at a constant pH of 6.7 at 20 °C. The results confirmed the successful stabilization of amorphous calcium oxalate II and III (ACOII and ACO III) nanoparticles formed after PNC in the presence of PAA and suggest the participation and stabilization of polymer-induced liquid-precursor (PILP) in the presence of PAA. We demonstrated that PAA stabilizes CaOx precursors with size in the range of 20–400 nm. PAA additive plays a key role in the in vitro crystallization of CaOx stabilizing multi-ion complexes in the pre-nucleation stage, thereby delaying the nucleation of ACO nanoparticles. Indeed, PAA additive favors the formation of more hydrated and soluble phase of ACO nanoparticles that are bound by electrostatic interactions to carboxylic acid groups of PAA during the post-nucleation stage. These findings may help to a better understanding of the pathological mineralization resulting in urolithiasis in mammals.

DNA nanostructures as templates for biomineralization

Nature uses extracellular matrix scaffolds to organize biominerals into hierarchical structures over various length scales. This has inspired the design of biomimetic mineralization scaffolds, with DNA nanostructures being among the most promising. DNA nanotechnology makes use of molecular recognition to controllably give 1D, 2D and 3D nanostructures. The control we have over these structures makes them attractive templates for the synthesis of mineralized tissues, such as bones and teeth. In this Review, we first summarize recent work on the crystallization processes and structural features of biominerals on the nanoscale. We then describe self-assembled DNA nanostructures and come to the intersection of these two themes: recent applications of DNA templates in nanoscale biomineralization, a crucial process to regenerate mineralized tissues.

Origin of the biphase nature and surface roughness of biogenic calcite secreted by the giant barnacle Austromegabalanus psittacus

The calcite grains forming the wall plates of the giant barnacle Austramegabalanus psittacus have a distinctive surface roughness made of variously sized crystalline nanoprotrusions covered by extremely thin amorphous pellicles. This biphase (crystalline-amorphous) structure also penetrates through the crystal’s interiors, forming a web-like structure. Nanoprotrusions very frequently elongate following directions related to the crystallographic structure of calcite, in particular, the <− 441> directions, which are the strongest periodic bond chains (PBCs) in calcite. We propose that the formation of elongated nanoprotrusions happens during the crystallization of calcite from a precursor amorphous calcium carbonate (ACC). This is because biomolecules integrated within the ACC are expelled from such PBCs due to the force of crystallization, with the consequent formation of uninterrupted crystalline nanorods. Expelled biomolecules accumulate in adjacent regions, thereby stabilizing small pellicle-like volumes of ACC. With growth, such pellicles become occluded within the crystal. In summary, the surface roughness of the biomineral surface reflects the complex shape of the crystallization front, and the biphase structure provides evidence for crystallization from an amorphous precursor. The surface roughness is generally explained as resulting from the attachment of ACC particles to the crystal surface, which later crystallised in concordance with the crystal lattice. If this was the case, the nanoprotrusions do not reflect the size and shape of any precursor particle. Accordingly, the particle attachment model for biomineral formation should seek new evidence.

Microbe-Mediated Extracellular and Intracellular Mineralization: Environmental, Industrial, and Biotechnological Applications

Microbe-mediated mineralization is ubiquitous in nature, involving bacteria, fungi, viruses, and algae. These mineralization processes comprise calcification, silicification, and iron mineralization. The mechanisms for mineral formation include extracellular and intracellular biomineralization. The mineral precipitating capability of microbes is often harnessed for green synthesis of metal nanoparticles, which are relatively less toxic compared with those synthesized through physical or chemical methods. Microbe-mediated mineralization has important applications ranging from pollutant removal and nonreactive carriers, to other industrial and biomedical applications. Herein, the different types of microbe-mediated biomineralization that occur in nature, their mechanisms, as well as their applications are elucidated to create a backdrop for future research.

Reorderability of node-filtered order complexes

Growing graphs describe a multitude of developing processes from maturing brains to expanding vocabularies to burgeoning public transit systems. Each of these growing processes likely adheres to proliferation rules that establish an effective order of node and connection emergence. When followed, such proliferation rules allow the system to properly develop along a predetermined trajectory. But rules are rarely followed. Here we ask what topological changes in the growing graph trajectories might occur after the specific but basic perturbation of permuting the node emergence order. Specifically, we harness applied topological methods to determine which of six growing graph models exhibit topology that is robust to randomizing node order, termed global reorderability, and robust to temporally local node swaps, termed local reorderability. We find that the six graph models fall upon a spectrum of both local and global reorderability, and furthermore we provide theoretical connections between robustness to node pair ordering and robustness to arbitrary node orderings. Finally, we discuss real-world applications of reorderability analyses and suggest possibilities for designing reorderable networks.

Nanostructure of mouse otoconia

Mammalian otoconia of the inner ear vestibular apparatus are calcium carbonate-containing mineralized structures critical for maintaining balance and detecting linear acceleration. The mineral phase of otoconia is calcite, which coherently diffracts X-rays much like a single-crystal. Otoconia contain osteopontin (OPN), a mineral-binding protein influencing mineralization processes in bones, teeth and avian eggshells, for example, and in pathologic mineral deposits. Here we describe mineral nanostructure and the distribution of OPN in mouse otoconia. Scanning electron microscopy and atomic force microscopy of intact and cleaved mouse otoconia revealed an internal nanostructure (~50 nm). Transmission electron microscopy and electron tomography of focused ion beam-prepared sections of otoconia confirmed this mineral nanostructure, and identified even smaller (~10 nm) nanograin dimensions. X-ray diffraction of mature otoconia (8-day-old mice) showed crystallite size in a similar range (73 nm and smaller). Raman and X-ray absorption spectroscopy - both methods being sensitive to the detection of crystalline and amorphous forms in the sample - showed no evidence of amorphous calcium carbonate in these mature otoconia. Scanning and transmission electron microscopy combined with colloidal-gold immunolabeling for OPN revealed that this protein was located at the surface of the otoconia, correlating with a site where surface nanostructure was observed. OPN addition to calcite growing in vitro produced similar surface nanostructure. These findings provide details on the composition and nanostructure of mammalian otoconia, and suggest that while OPN may influence surface rounding and surface nanostructure in otoconia, other incorporated proteins (also possibly including OPN) likely participate in creating internal nanostructure.

Bioinspired Materials: From Living Systems to New Concepts in Materials Chemistry

Nature successfully employs inorganic solid-state materials (i.e., biominerals) and hierarchical composites as sensing elements, weapons, tools, and shelters. Optimized over hundreds of millions of years under evolutionary pressure, these materials are exceptionally well adapted to the specifications of the functions that they perform. As such, they serve today as an extensive library of engineering solutions. Key to their design is the interplay between components across length scales. This hierarchical design-a hallmark of biogenic materials-creates emergent functionality not present in the individual constituents and, moreover, confers a distinctly increased functional density, i.e., less material is needed to provide the same performance. The latter aspect is of special importance today, as climate change drives the need for the sustainable and energy-efficient production of materials. Made from mundane materials, these bioceramics act as blueprints for new concepts in the synthesis and morphosynthesis of multifunctional hierarchical materials under mild conditions. In this review, which also may serve as an introductory guide for those entering this field, we demonstrate how the pursuit of studying biomineralization transforms and enlarges our view on solid-state material design and synthesis, and how bioinspiration may allow us to overcome both conceptual and technical boundaries.

Polymer-Functionalised Nanograins of Mg-Doped Amorphous Calcium Carbonate via a Flow-Chemistry Approach

Calcareous biominerals typically feature a hybrid nanogranular structure consisting of calcium carbonate nanograins coated with organic matrices. This nanogranular organisation has a beneficial effect on the functionality of these bioceramics. In this feasibility study, we successfully employed a flow-chemistry approach to precipitate Mg-doped amorphous calcium carbonate particles functionalized by negatively charged polyelectrolytes-either polyacrylates (PAA) or polystyrene sulfonate (PSS). We demonstrate that the rate of Mg incorporation and, thus, the ratio of the Mg dopant to calcium in the precipitated amorphous calcium carbonate (ACC), is flow rate dependent. In the case of the PAA-functionalized Mg-doped ACC, we further observed a weak flow rate dependence concerning the hydration state of the precipitate, which we attribute to incorporated PAA acting as a water sorbent; a behaviour which is not present in experiments with PSS and without a polymer. Thus, polymer-dependent phenomena can affect flow-chemistry approaches, that is, in syntheses of functionally graded materials by layer-deposition processes.

The multiple roles of carbonic anhydrase in calcium carbonate mineralization

Carbonic anhydrase (CA) accelerates, templates and arrests calcium carbonate mineralization by playing both enzymatic and structural protein roles.

Development of a two-stage in vitro model system to investigate the mineralization mechanisms involved in idiopathic stone formation: stage 1-biomimetic Randall's plaque using decellularized porcine kidneys

Idiopathic calcium oxalate (CaOx) stone formers form stones that are commonly attached to calcium phosphate (CaP) deposits in the renal tissue, known as Randall's plaques (RP). Plaques are suggested to originate in the renal tubular basement membrane, where they exhibit a morphology of concentrically laminated apatitic spherules, while in the interstitial regions, the collagen fibrils and vesicles become mineralized. We hypothesize that these minerals might form by non-classical crystallization mechanisms, such as via amorphous precursors, some of which might originate from a polymer-induced liquid-precursor (PILP) process. Thus, our goal is to identify mineralogical 'signatures' of various stone formation mechanisms. To do this for idiopathic CaOx stones, we are developing a two-stage model system of CaP-CaOx composite stones, consisting of stage (1) CaP mineralized plaque, followed by stage (2) CaOx overgrowth into a stone. For the studies presented here, decellularized porcine kidneys were mineralized with CaP using polyaspartic acid or the protein osteopontin (OPN) to induce the PILP process and create biomimetic RP. Analysis of the PILP-mineralized tissues shows features that resemble the native plaques, including mineral spherules and collagen with intrafibrillar mineral. In contrast, the classical crystallization produced large apatitic spherulites, which is a very different morphology, but one which is also found in some stones. An alternative hypothesis regarding Randall's plaque, and if or when it becomes pathological, is discussed.

The Carbonation of Wollastonite: A Model Reaction to Test Natural and Biomimetic Catalysts for Enhanced CO2 Sequestration

One of the most promising strategies for the safe and permanent disposal of anthropogenic CO2 is its conversion into carbonate minerals via the carbonation of calcium and magnesium silicates. However, the mechanism of such a reaction is not well constrained, and its slow kinetics is a handicap for the implementation of silicate mineral carbonation as an effective method for CO2 capture and storage (CCS). Here, we studied the different steps of wollastonite (CaSiO3) carbonation (silicate dissolution → carbonate precipitation) as a model CCS system for the screening of natural and biomimetic catalysts for this reaction. Tested catalysts included carbonic anhydrase (CA), a natural enzyme that catalyzes the reversible hydration of CO2(aq), and biomimetic metal-organic frameworks (MOFs). Our results show that dissolution is the rate-limiting step for wollastonite carbonation. The overall reaction progresses anisotropically along different [hkl] directions via a pseudomorphic interface-coupled dissolution–precipitation mechanism, leading to partial passivation via secondary surface precipitation of amorphous silica and calcite, which in both cases is anisotropic (i.e., (hkl)-specific). CA accelerates the final carbonate precipitation step but hinders the overall carbonation of wollastonite. Remarkably, one of the tested Zr-based MOFs accelerates the dissolution of the silicate. The use of MOFs for enhanced silicate dissolution alone or in combination with other natural or biomimetic catalysts for accelerated carbonation could represent a potentially effective strategy for enhanced mineral CCS.

Nanocrystalline Precursors for the Co-Assembly of Crack-Free Metal Oxide Inverse Opals

Inorganic microstructured materials are ubiquitous in nature. However, their formation in artificial self-assembly systems is challenging as it involves a complex interplay of competing forces during and after assembly. For example, colloidal assembly requires fine-tuning of factors such as the size and surface charge of the particles and electrolyte strength of the solvent to enable successful self-assembly and minimize crack formation. Co-assembly of templating colloidal particles together with a sol-gel matrix precursor material helps to release stresses that accumulate during drying and solidification, as previously shown for the formation of high-quality inverse opal (IO) films out of amorphous silica. Expanding this methodology to crystalline materials would result in microscale architectures with enhanced photonic, electronic, and catalytic properties. This work describes tailoring the crystallinity of metal oxide precursors that enable the formation of highly ordered, large-area (mm² ) crack-free titania, zirconia, and alumina IO films. The same bioinspired approach can be applied to other crystalline materials as well as structures beyond IOs.

Calcium carbonate with nanogranular microstructure yields enhanced toughness

The presence of nanogranular microstructures is a widely reported feature of biominerals that form by classical and non-classical mineralization pathways. Inspired by nature, we have synthesized amorphous calcium carbonate nanoparticles with nanogranular microstructures, whose grain size is tuned by varying the polymer concentration. The response to indentation of single calcium carbonate nanoparticles proceeds via an intermittent stick-slip that reflects the characteristics of the nanogranular microstructure. A two-fold mechanism is thus proposed to enhance the toughness of the nanoparticles, namely nanogranular rearrangement and intergranular bridging by an organic phase and/or hydration. This work not only provides a synthesis route to design biologically inspired mineral nanoparticles with nanogranular structure, but also helps in understanding toughening mechanisms of biominerals arising from their nanoscale heterogeneity.

Crystallization and Colloidal Stabilization of Ca(OH)2 in the Presence of Nopal Juice (Opuntia ficus indica): Implications in Architectural Heritage Conservation

Hydrated lime (Ca(OH)₂) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca(OH)₂ crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001)_Ca(OH)2 faces. Adsorption of polysaccharides on Ca(OH)₂ crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties.

Transformation of ACC into aragonite and the origin of the nanogranular structure of nacre

Currently a basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase. The globular nanostructure of most biominerals is taken as evidence of this. Nevertheless, little is known as to how the amorphous-to-crystalline transformation takes place. To gain insight into this process, we have made a high-resolution study (by means of transmission electron microscopy and other associated techniques) of immature tablets of nacre of the gastropod Phorcus turbinatus, where the proportion of amorphous calcium carbonate is high. Tablets displayed a characteristic nanoglobular structure, with the nanoglobules consisting of an aragonite core surrounded by amorphous calcium carbonate together with organic macromolecules. The changes in composition from the amorphous to the crystalline phase indicate that there was a higher content of organic molecules within the former phase. Within single tablets, the crystalline cores were largely co-oriented. According to their outlines, the internal transformation front of the tablets took on a complex digitiform shape, with the individual fingers constituting the crystalline cores of nanogranules. We propose that the final nanogranular structure observed is produced during the transformation of ACC into aragonite.

Nanolimes: from synthesis to application

Cultural heritage objects and structures are subjected to a range of weathering processes that result in their decay and destruction. To slow weathering rates and/or mitigate their effects, several protective and consolidant materials have been used during conservation interventions. Treatments based on organic polymers and alkoxysilanes, as well as some traditional inorganic treatments such as lime water, are in many cases either incompatible and/or show limited efficacy. In recent years nanolimes, that is, dispersions of Ca(OH)2 nanoparticles in alcohol (as well as alcohol dispersions of other alkaline-earth metal hydroxide nanoparticles), have emerged as an effective and compatible conservation material. Here we review recent advances in the synthesis and application of nanolimes in the field of heritage conservation. First, we present an overview of lime-based conservation materials, with an emphasis on the earliest reports on the use of nanolimes. Subsequently, we present the different methods used to synthesize nanolimes. Afterwards, we describe their carbonation and its consolidation effects. Practical application of nanolimes in heritage conservation are summarized, including consolidation of stone, ceramics, lime mortars and mural painting, as well as deacidification of paper, canvas, and wood. The advantages and limitations of this novel nanotechnology for cultural heritage conservation are outlined. Finally, some conclusions and areas for future research are presented.

Solid-State Transformation of Amorphous Calcium Carbonate to Aragonite Captured by CryoTEM

Early-stage reaction mechanisms for aragonite-promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol-water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid-state transformation of amorphous calcium carbonate to aragonite, demonstrating the co-existence of both amorphous and crystalline material within the same aragonite needle. This supports non-classical formation of aragonite within both a synthetic and biological context.

Universal structure motifs in biominerals: a lesson from nature for the efficient design of bioinspired functional materials

Biominerals are typically indispensable structures for their host organism in which they serve varying functions, such as mechanical support and protection, mineral storage, detoxification site, or as a sensor or optical guide. In this perspective article, we highlight the occurrence of both structural diversity and uniformity within these biogenic ceramics. For the first time, we demonstrate that the universality-diversity paradigm, which was initially introduced for proteins by Buehler et al. (Cranford & Buehler 2012 Biomateriomics; Cranford et al. 2013 Adv. Mater.25, 802-824 (doi:10.1002/adma.201202553); Ackbarow & Buehler 2008 J. Comput. Theor. Nanosci.5, 1193-1204 (doi:10.1166/jctn.2008.001); Buehler & Yung 2009 Nat. Mater.8, 175-188 (doi:10.1038/nmat2387)), is also valid in the realm of biomineralization. A nanogranular composite structure is shared by most biominerals which rests on a common, non-classical crystal growth mechanism. The nanogranular composite structure affects various properties of the macroscale biogenic ceramic, a phenomenon we attribute to emergence. Emergence, in turn, is typical for hierarchically organized materials. This is a clear call to renew comparative studies of even distantly related biomineralizing organisms to identify further universal design motifs and their associated emergent properties. Such universal motifs with emergent macro-scale properties may represent an unparalleled toolbox for the efficient design of bioinspired functional materials.

Universal structure motifs in biominerals: a lesson from nature for the efficient design of bioinspired functional materials

Biominerals are typically indispensable structures for their host organism in which they serve varying functions, such as mechanical support and protection, mineral storage, detoxification site, or as a sensor or optical guide. In this perspective article, we highlight the occurrence of both structural diversity and uniformity within these biogenic ceramics. For the first time, we demonstrate that the universality-diversity paradigm, which was initially introduced for proteins by Buehler et al. (Cranford & Buehler 2012 Biomateriomics; Cranford et al. 2013 Adv. Mater.25, 802-824 (doi:10.1002/adma.201202553); Ackbarow & Buehler 2008 J. Comput. Theor. Nanosci.5, 1193-1204 (doi:10.1166/jctn.2008.001); Buehler & Yung 2009 Nat. Mater.8, 175-188 (doi:10.1038/nmat2387)), is also valid in the realm of biomineralization. A nanogranular composite structure is shared by most biominerals which rests on a common, non-classical crystal growth mechanism. The nanogranular composite structure affects various properties of the macroscale biogenic ceramic, a phenomenon we attribute to emergence. Emergence, in turn, is typical for hierarchically organized materials. This is a clear call to renew comparative studies of even distantly related biomineralizing organisms to identify further universal design motifs and their associated emergent properties. Such universal motifs with emergent macro-scale properties may represent an unparalleled toolbox for the efficient design of bioinspired functional materials.

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

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

STATEMENT OF SIGNIFICANCE

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

CaCO3 thin-film formation mediated by a synthetic protein-lysozyme coacervate

A thin film was formed through in vitro CaCO₃ crystallization in the presence of complex coacervates, which was expected to be planar and poorly crystalline CaCO₃ guided at the interface of two immiscible liquid phases upon complex coacervation.