This article describes an experimentally versatile strategy for producing inorganic/organic nanocomposites, with control over the microstructure at the nano- and mesoscales. Taking inspiration from biominerals, CaCO₃ is coprecipitated with anionic diblock copolymer worms or vesicles to produce single crystals of calcite occluding a high density of the organic component. This approach can also be extended to generate complex structures in which the crystals are internally patterned with nano-objects of differing morphologies. Extensive characterization of the nanocomposite crystals using high resolution synchrotron powder X-ray diffraction and vibrational spectroscopy demonstrates how the occlusions affect the short and long-range order of the crystal lattice. By comparison with nanocomposite crystals containing latex particles and copolymer micelles, it is shown that the effect of these occlusions on the crystal lattice is dominated by the interface between the inorganic crystal and the organic nano-objects, rather than the occlusion size. This is supported by in situ atomic force microscopy studies of worm occlusion in calcite, which reveal flattening of the copolymer worms on the crystal surface, followed by burial and void formation. Finally, the mechanical properties of the nanocomposite crystals are determined using nanoindentation techniques, which reveal that they have hardnesses approaching those of biogenic calcites.

Amorphous calcium carbonate (ACC) is an important intermediate in the formation of crystalline CaCO₃ biominerals, where its crystallization is controlled using soluble additives. However, although this transformation often occurs in the solid state, experiments mainly focus on the effect of additives on ACC crystallization in solution. This paper addresses this issue and compares the crystallization, in solution and in the solid state, of ACC precipitated in the presence of a range of additives. Surprisingly, these results show that some additives exhibit a Janus behavior, where they retard crystallization in solution, yet accelerate it in the solid state. This is observed for all of the larger molecules examined, while the small molecules retard crystallization in both solution and the solid state.

Aggregation-based crystal growth often gives rise to crystals with complex morphologies which cannot be generated via classical growth processes. Despite this, understanding of the mechanism is rather poor, particularly when organic additives or amorphous precursor phases are present. In this work, advantage is taken of the observation that aggregation-based growth of calcium carbonate, and indeed many other minerals, is most often observed using diffusion-based synthetic methods. By fully characterizing the widely used ammonia diffusion method (ADM)–which is currently used as a “black box”–the solution and supersaturation conditions which accompany CaCO₃ precipitation using this method are identified and insight is gained into the nucleation and growth processes which generate calcite mesocrystals. This reveals that the distinguishing feature of the ADM is that the initial nucleation burst consumes only a small quantity of the available ions, and the supersaturation then remains relatively constant, and well above the solubility of amorphous calcium carbonate (ACC), until the reaction is almost complete. New material is thus generated over the entire course of the precipitation, a feature which appears to be fundamental to the formation of complex, aggregation-based morphologies. Finally, the importance of this understanding is demonstrated using the identified carbonate and supersaturation profiles to perfectly replicate CaCO₃ mesocrystals through slow addition of reagents to a bulk solution. This approach overcomes many of the inherent problems of the ADM by offering excellent reproducibility, enabling the synthesis of such CaCO₃ structures in large-scale and continuous-flow systems, and ultimately facilitating in situ studies of assembly-based crystallization mechanisms.

Although many processes of great biological, technological, and environmental importance, such as the formation of biominerals, the templating of nanostructures, and salt weathering, occur in confinement rather than in bulk solution, little is known about the influence of confinement over the precipitation of inorganic crystals. The effects of confinement on the precipitation of calcium sulfate are investigated using a crossed-cylinder apparatus which offers confinement that varies continuously from zero to tens of micrometers. While the thermodynamically stable form of calcium sulfate (gypsum) is always observed at large surface separations, a remarkable stabilization of the metastable phases amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate (bassanite, plaster of Paris) is observed even at micrometer-scale separations. For the first time, the approach is extended to study the combined effects of soluble additives and confinement, which are often present in natural or synthetic systems, and it is shown that this considerably extends the lifetimes of ACS and hemihydrate. These confinement effects are attributed to hindered aggregation of precursor particles at small surface separations, which limits polymorph conversion. While these results have immediate relevance to salt weathering and biomineralization processes, they are also important to the many crystallization and aggregation-driven processes occurring in small volumes.

A key feature of biomineralization processes is that they take place within confined volumes, in which the local environment can have significant effects on mineral formation. Herein, we investigate the influence of confinement on the formation mechanism and structure of calcium phosphate (CaP). This is of particular relevance to the formation of dentine and bone, structures of which are based on highly mineralized collagen fibrils. CaP was precipitated within 25–300 nm diameter, cylindrical pores of track etched and anodised alumina membranes under physiological conditions, in which this system enables systematic study of the effects of the pore size in the absence of a structural match between the matrix and the growing crystals. Our results show that the main products were polycrystalline hydroxapatite (HAP) rods, together with some single crystal octacalcium phosphate (OCP) rods. Notably, we demonstrate that these were generated though an intermediate amorphous calcium phosphate (ACP) phase, and that ACP is significantly stabilised in confinement. This effect may have significance to the mineralization of bone, which can occur through a transient ACP phase. We also show that orientation of the HAP comparable, or even superior to that seen in bone can be achieved through confinement effects alone. Although this simple experimental system cannot be considered, a direct mimic of the in vivo formation of ultrathin HAP platelets within collagen fibrils, our results show that the effects of physical confinement should not be neglected when considering the mechanisms of formation of structures, such as bones and teeth.

Soluble macromolecules are essential to Nature's control over biomineral formation. Following early studies where macromolecules rich in aspartic and glutamic acid were extracted from nacre, research has focused on the use of negatively charged additives to control calcium carbonate precipitation. It is demonstrated that the positively charged additive poly(allylamine hydrochloride) (PAH) can also cause dramatic changes in calcite morphologies, yielding thin films and fibers of CaCO₃ analogous to those produced with poly(aspartic acid) via a so-called PILP (polymer-induced liquid precursor) phase. The mechanism by which PAH induces these effects is investigated using a range of techniques including cryo transmission electron microscopy (TEM), Raman microscopy, and thermogravimetric analysis, and the data show that hydrated Ca²⁺/PAH/CO₃²⁻ droplets initially form in solution, before coalescing and ultimately crystallizing to give calcite, together with small quantities of vaterite. It is suggested that it is the initial formation of hydrated Ca²⁺/PAH/CO₃²⁻ droplets that is key to this process, rather than a specific polymer/mineral interaction. These results are discussed in terms of their relevance to biomineralization processes and highlight the opportunity for using counter-ion-induced phase separation of polyelectrolytes as a method for generating minerals with non-crystallographic morphologies.

This article investigates the formation of nanostructured single crystals of calcite using direct, ion-by-ion precipitation methods and shows that single crystals with complex morphologies and curved surfaces can readily be formed using this technique. Calcite crystals with inverse opal and direct opal structures are prepared using templates of colloidal crystals and polystyrene reverse opals, respectively, and excellent replication of the template structures are achieved, including the formation of 200-nm spheres of calcite in the direct opal structure. These highly porous crystals also display extremely regular, crystalline gross morphologies. The methodology is extremely versatile and challenges the preconception that nanostructured crystals cannot be prepared by simple diffusion of reagents into the template due to blocking of the channels. The results are also discussed in light of alternative templating methods using amorphous calcium carbonate (ACC) as a precursor phase and provide insight into the role of ACC in biological calcification processes.

Biominerals typically form within localized volumes, affording organisms great control over the mineralization process. The influence of such confinement on crystallization is studied here by precipitating CaCO₃ within the confines of an annular wedge, formed around the contact point of two crossed half-cylinders. The cylinders are functionalized with self-assembled monolayers of mercaptohexadecanoic acid on gold. This configuration enables a systematic study of the effects of confinement since the surface separation increases continuously from zero at the contact point to macroscopic (mm) separations. While oriented rhombohedral calcite crystals form at large (>10 µm) separations, particles with irregular morphologies and partial crystallinity are observed as the surface separation approaches the dimensions of the unconfined crystals (5–10 µm). Further increase in the confinement has a significant effect on the crystallization process with flattened amorphous CaCO₃ (ACC) particles being formed at micrometer separations. These ACC particles show remarkable stability when maintained within the wedge but rapidly crystallize on separation of the cylinders. A comparison of bulk and surface free-energy terms shows that ACC cannot be thermodynamically stable at these large separations, and the stability is attributed to kinetic factors. This study therefore shows that the environment in which minerals form can have a significant effect on their stability and demonstrates that ACC can be stabilized with respect to the crystalline polymorphs of CaCO₃ by confinement alone. That ACC was stabilized at such large (micrometer) separations is striking, and demonstrates the versatility of this strategy, and its potential value in biological systems.