Yi-Yeoun Kim

Structure-property relationships of a biological mesocrystal in the adult sea urchin spine

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

Dehydration and crystallization of amorphous calcium carbonate in solution and in air

The mechanisms by which amorphous intermediates transform into crystalline materials are poorly understood. Currently, attracting enormous interest is the crystallization of amorphous calcium carbonate, a key intermediary in synthetic, biological and environmental systems. Here we attempt to unify many contrasting and apparently contradictory studies by investigating this process in detail. We show that amorphous calcium carbonate can dehydrate before crystallizing, both in solution and in air, while thermal analyses and solid-state nuclear magnetic resonance measurements reveal that its water is present in distinct environments. Loss of the final water fraction—comprising less than 15% of the total—then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid-state transformation. Such mechanisms are likely to be widespread in solid-state reactions and their characterization will facilitate greater control over these processes.

Bio‐Inspired Synthesis and Mechanical Properties of Calcite–Polymer Particle Composites

2010 WILEY-VCH Verlag Gmb Of themany roles performed by biominerals, skeletal support and protection are amongst the most common. The principal minerals of choice for the fabrication of structural biominerals are calcium phosphates in vertebrates and calcium carbonate in invertebrates, neither of which would be traditionally recognized for their mechanical properties. Despite this, biology employs such materials to manufacture biominerals whose mechanical properties can rival those of many engineering ceramics fabricated at high temperatures and pressures, a task it achieves through a range of design strategies. Characteristic features of these biominerals are their composite structures, and also their complex, hierarchical organizations which can be instrumental in effectively distributing stress over the structure and in controlling the propagation of cracks through the material. Considering the origin of their composite structures, the mineral phase of any biomineral is intimately associated with organic molecules, which can be located either between crystal units, or actually embedded within individual crystals. Thus, even single crystal biominerals can exhibit mechanical properties far superior to their synthetic counterparts, a feature which can be attributed to occlusion of often very low levels of macromolecules within the crystals. The mechanism of incorporation of organic macromolecules within single crystals – and in particular calcite – remains a subject of significant interest. It also offers many challenges due to problems associated with locating organic molecules within crystals using direct imaging methods. The traditional specificbinding model considers these molecules to locate on specific crystal faces, before being incorporated within the crystal in an overgrowth process. Many CaCO3 biominerals actually form via an amorphous calcium carbonate (ACC) precursor phase rather than by direct ion-by-ion growth, and the additives present within the ACC are believed to modulate its reactivity, possibly binding to specific lattice planes during crystallization and being retained within the product crystal. Finally, there is considerable current interest in crystallization via directed assembly of precursor units, where product particles can exhibit ‘‘mesocrystal’’ structures in which the constituent nanoparticle units are aligned such that the particle diffracts as a single crystal. Here it is envisaged that organic molecules may be located between the nanoparticle units. Historically, of course, the incorporation of additives within single crystals, from the dyeing of crystals, to entrapment of gels and particles within crystals, is well recognized. Recent work on the precipitation of calcite crystals in gels has demonstrated that gel incorporation can be achieved through control of the gel’s rigidity and the rate of crystal growth. Latex particle incorporation within zinc oxide and calcite crystals has been studied, with effective occlusion being achieved with the zinc oxide system. Limited success was achieved with the calcium carbonate system, however, with particles only being entrapped in the outer surfaces of the calcite crystals. Calcium carbonate has also been precipitated in the presence of surface-modified carbon nanotubes, leading to some incorporation within calcite crystals. In this work, the incorporation of functionalized polystyrene particles within single crystals of calcite is investigated as a straightforward approach to the synthesis of crystals with composite structures. It is emphasized that our goal is to occlude particles within single crystals rather than polycrystalline materials, which, being subject to far fewer constraints, can be achieved using a range of techniques such as crystallizing within colloidal crystal templates and in the presence of particulate additives. Here, we develop effective methods for achieving high levels of encapsulation, and then study the mechanical properties of the resultant crystals using nanoindentation. Further, using particles as growth additives provides the potential for probing the mechanism of additive incorporation within crystals; due to their sizes and well-defined morphologies, particles are easily located within a crystal. This strategy hasmuch in common with that active in biological systems, but rather than occluding organic molecules within the crystals, we here encapsulate particles. By combining two dissimilar materials, a methodology which provides the ethos for composite manufacture, it offers an intriguing and extremely flexible route for modifying the mechanical behavior of such brittle crystals, where systematic control of these properties can potentially be achieved through selection of the size, shape and composition of the particle additives. While crystallization within a rigid template such as a polymer matrix or a gel can result in growth around the template, and thus its encapsulation within the crystal, use of a simple one-pot method in which the particles are employed as growth additives offers a far more versatile and general synthetic approach.

Early stages of crystallization of calcium carbonate revealed in picoliter droplets.

In this work, we studied the heterogeneous nucleation and growth of CaCO(3) within regular arrays of picoliter droplets created on patterned self-assembled monolayers (SAMs). The SAMs provide well-defined substrates that offer control over CaCO(3) nucleation, and we used these impurity-free droplet arrays to study crystal growth in spatially and chemically controlled, finite-reservoir environments. The results demonstrate a number of remarkable features of precipitation within these confined volumes. CaCO(3) crystallization proceeds significantly more slowly in the droplets than in the bulk, allowing the mechanism of crystallization, which progresses via amorphous calcium carbonate, to be easily observed. In addition, the precipitation reaction terminates at an earlier stage than in the bulk solution, revealing intermediate growth forms. Confinement can therefore be used as a straightforward method for studying the mechanisms of crystallization on a substrate without the requirement for specialized analytical techniques. The results are also of significance to biomineralization processes, where crystallization typically occurs in confinement and in association with organic matrices, and it is envisaged that the method is applicable to many crystallizing systems.

A critical analysis of calcium carbonate mesocrystals.

The term mesocrystal has been widely used to describe crystals that form by oriented assembly, and that exhibit nanoparticle substructures. Using calcite crystals co-precipitated with polymers as a suitable test case, this article looks critically at the concept of mesocrystals. Here we demonstrate that the data commonly used to assign mesocrystal structure may be frequently misinterpreted, and that these calcite/polymer crystals do not have nanoparticle substructures. Although morphologies suggest the presence of nanoparticles, these are only present on the crystal surface. High surface areas are only recorded for crystals freshly removed from solution and are again attributed to a thin shell of nanoparticles on a solid calcite core. Line broadening in powder X-ray diffraction spectra is due to lattice strain only, precluding the existence of a nanoparticle sub-structure. Finally, study of the formation mechanism provides no evidence for crystalline precursor particles. A re-evaluation of existing literature on some mesocrystals may therefore be required.

Three-dimensional imaging of dislocation propagation during crystal growth and dissolution

Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials 1,2. Their effects range from increased chemical reactivity 3,4 to enhanced mechanical properties 5,6. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations 7 in two-dimensions (2D), while transmission electron microscopy (TEM) can now visualise strain fields in three-dimensions (3D) with near atomic resolution 8–10. However, these techniques cannot offer 3D imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) 11,12 can be used to visualize in 3D, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli.

Tuning hardness in calcite by incorporation of amino acids

Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unit-mineral single crystals containing embedded macromolecules-remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite. We analysed lattice distortions in these model crystals by using X-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.

One-pot synthesis of an inorganic heterostructure: uniform occlusion of magnetite nanoparticles within calcite single crystals

A facile one-pot method is described for the formation of novel heterostructures in which inorganic nanoparticles are homogeneously distributed throughout an inorganic single crystal matrix. Our strategy uses nanoparticles functionalised with a poly(sodium 4-styrenesulphonate)-poly(methacrylic acid) [PNaStS-PMAA] diblock copolymer as a soluble crystal growth additive. This copolymer plays a number of essential roles. The PMAA anchor block is physically adsorbed onto the inorganic nanoparticles, while the PNaStS block acts as an electrosteric stabiliser and ensures that the nanoparticles retain their colloidal stability in the crystal growth solution. In addition, this strong acid block promotes binding to both the nanoparticles and the host crystal, which controls nanoparticle incorporation within the host crystal lattice. We show that this approach can be used to achieve encapsulation loadings of at least 12 wt% copolymer-coated magnetite particles within calcite single crystals. Transmission electron microscopy shows that these nanoparticles are uniformly distributed throughout the calcite, and that the crystal lattice retains its continuity around the embedded magnetite particles. Characterisation of these calcite/magnetite nanocomposites confirmed their magnetic properties. This new experimental approach is expected to be quite general, such that a small family of block copolymers could be used to drive the incorporation of a wide range of pre-prepared nanoparticles into host crystals, giving intimate mixing of phases with contrasting properties, while limiting nanoparticle aggregation and migration.

Polymer-induced liquid precursor (PILP) phases of calcium carbonate formed in the presence of synthetic acidic polypeptides—relevance to biomineralization

Polymer-induced liquid precursor (PILP) phases of calcium carbonate have attracted significant interest due to possible applications in materials synthesis, and their resemblance to intermediates seen in biogenic mineralisation processes. Further, these PILP phases have been formed in vitro using polyelectrolytes such as poly(aspartic acid) which bears many structural parallels to the highly acidic biomacromolecules that are associated with biogenic calcium carbonate. This article describes experiments which investigate how the composition of acidic polypeptides determines their ability to form PILP phases of CaCO3, and therefore whether it is feasible that the acidic biomacromolecules extracted from CaCO3 biominerals could also function in this way. A series of random copoly(amino acid)s constructed from 80–20%, 50–50% and 20–80% aspartic acid and serine residues were synthesised and their effect on CaCO3 precipitation was determined. A strong correlation between the composition and function of the polypeptide was observed. Only the polypeptide containing 80% aspartic acid residues (Asp80%–Ser20%) induced the formation of continuous CaCO3 films, which provide a fingerprint of an intermediary PILP phase, while addition of Mg2+ also facilitated the formation of expanded film-like structures with the polypeptide Asp50%–Ser50%. In contrast, the weakly-acidic polypeptide Asp20%–Ser80% had only a minor effect on the crystal morphologies and also failed to aid infiltration of CaCO3 into small pores. These results therefore demonstrate that counter-ion induced phase separation of highly acidic biomacromolecules proteins appears to be entirely feasible based upon their composition, but that evidence for the operation of this mineralisation mechanism in vivo is still required.

Substrate-directed formation of calcium carbonate fibres

This article describes the formation of fibres of calcium carbonate on a range of substrates in the presence of a polyacid diblock copolymer, and it is demonstrated that the morphologies and structures of the fibres can be controlled by judicious selection of both the substrate and the reaction conditions. While fibres precipitated on single crystals of calcite and aragonite proved to be single crystals of calcite, those formed on glass and mica were amorphous and frequently displayed remarkable helical morphologies. Investigation of the fibre growth mechanism(s) suggests that all formed via the self-assembly of polymer-stabilised precursor units. Particles forming the calcite single-crystal fibres were elongated calcite crystallites, and fibre formation was interpreted in terms of an oriented assembly process, where anisotropy in the precursor unit morphology and surface distribution of copolymer chains promoted one-dimensional assembly. In contrast, the particles that formed amorphous fibres were spherical and amorphous, with fibre formation being attributed to polarisation of these copolymer-rich particles. This work therefore demonstrates that directional aggregation processes can be applied to both amorphous and crystalline units, opening up the possibility of using block copolymers to control the morphologies of a very wide range of materials.