Hybrid Biomimetic Materials from Silica/Carbonate Biomorphs

The authors thank the Particle Analysis Center of the University of Konstanz (SFB 1214), the Nanostructure Laboratory and the Bioimaging center of the University of Konstanz for access to their instruments and Andra-Lisa Hoyt for corrections.The formation of a polymer protection layer around fragile mineral architectures ensures that structures stay intact even after treatments that would normally destroy them going along with a total loss of textural information. Here we present a strategy to preserve the shape of silica-carbonate biomorphs with polymers. This method converts non-hybrid inorganic-inorganic composite materials such a silica/carbonate biomorphs into hybrid organic/carbonate composite materials similar to biominerals.The authors thank the European Research Council under the European Union’s seventh Framework Program (FP7/2007-2013)/ERC grant agreement no. 340863

Structural Transition of Inorganic Silica–Carbonate Composites Towards Curved Lifelike Morphologies

The self-assembly of alkaline earth carbonates in the presence of silica at high pH leads to a unique class of composite materials displaying a broad variety of self-assembled superstructures with complex morphologies. A detailed understanding of the formation process of these purely inorganic architectures is crucial for their implications in the context of primitive life detection as well as for their use in the synthesis of advanced biomimetic materials. Recently, great efforts have been made to gain insight into the molecular mechanisms driving self-assembly in these systems, resulting in a consistent model for morphogenesis at ambient conditions. In the present work, we build on this knowledge and investigate the influence of temperature, supersaturation, and an added multivalent cation as parameters by which the shape of the forming superstructures can be controlled. In particular, we focus on trumpet- and coral-like structures which quantitatively replace the well-characterised sheets and worm-like braids at elevated temperature and in the presence of additional ions, respectively. The observed morphological changes are discussed in light of the recently proposed formation mechanism with the aim to ultimately understand and control the major physicochemical factors governing the self-assembly process.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 340863

Diffusion and Precipitation Processes in Iron-Based Silica Gardens

Silica gardens are tubular structures that form along the interface of multivalent metal salts and alk. solns. of sodium silicate, driven by a complex interplay of osmotic and buoyant forces together with chem. reaction. They display peculiar plant-​like morphologies and thus can be considered as one of the few examples for the spontaneous biomimetic self-​ordering of purely inorg. materials. Recently, we could show that silica gardens moreover are highly dynamic systems that remain far from equil. for considerable periods of time long after macroscopic growth is completed. Due to initial compartmentalization, drastic concn. gradients were found to exist across the tube walls, which give rise to noticeable electrochem. potential differences and decay only slowly in a series of coupled diffusion and pptn. processes. The effect of the nature of the metal cations on the dynamic behavior of the system has been studied. The authors have grown single macroscopic silica garden tubes by controlled addn. of sodium silicate sol to pellets of iron(II) and iron(III) chloride. In the following, the concns. of ionic species were measured as a function of time on both sides of the formed membranes, while electrochem. potentials and pH were monitored online by immersing the corresponding sensors into the two sepd. soln. reservoirs. At the end of the expts., the solid tube material was furthermore characterized with respect to compn. and microstructure by a combination of ex situ techniques. The collected data are compared to the previously reported case of cobalt-​based silica gardens and used to shed light on ion diffusion through the inorg. membranes as well as progressive mineralization at both surfaces of the tube walls. These results reveal important differences in the dynamics of the three studied systems, which can be explained based on the acidity of the metal cations and the porosity of the membranes, leading to substantially dissimilar time-​dependent soln. chem. as well as distinct final mineral structures. The insight gained in this work may help to better understand the diffusion properties and pptn. patterns in tubular iron (hydr)​oxide​/silicate structures obsd. in geol. environments and during steel corrosion

Growth of organic crystals via attachment and transformation of nanoscopic precursors

A key requirement for the understanding of crystal growth is to detect how new layers form and grow at the nanoscale. Multistage crystallization pathways involving liquid-like, amorphous or metastable crystalline precursors have been predicted by theoretical work and have been observed experimentally. Nevertheless, there is no clear evidence that any of these precursors can also be relevant for the growth of crystals of organic compounds. Herein, we present a new growth mode for crystals of DL-glutamic acid monohydrate that proceeds through the attachment of preformed nanoscopic species from solution, their subsequent decrease in height at the surface and final transformation into crystalline 2D nuclei that eventually build new molecular layers by further monomer incorporation. This alternative mechanism provides a direct proof for the existence of multistage pathways in the crystallization of molecular compounds and the relevance of precursor units larger than the monomeric constituents in the actual stage of growth.publishe

Precipitation and Crystallization Kinetics in Silica Gardens

Silica gardens are extraordinary plant-like structures resulting from the complex interplay of relatively simple inorganic components. Recent work has highlighted that macroscopic self-assembly is accompanied by the spontaneous formation of considerable chemical gradients, which induce a cascade of coupled dissolution, diffusion, and precipitation processes occurring over timescales as long as several days. In the present study, this dynamic behavior was investigated for silica gardens based on iron and cobalt chloride by means of two synchrotron- based techniques, which allow the determination of concentration profiles and time-resolved monitoring of diffraction patterns, thus giving direct insight into the progress of dissolution and crystallization phenomena in the system. On the basis of the collected data, a kinetic model is proposed to describe the relevant reactions on a fundamental physicochemical level. The results show that the choice of the metal cations (as well as their counterions) is crucial for the development of silica gardens in both the short and long term (i. e. during tube formation and upon subsequent slow equilibration), and provide important clues for understanding the properties of related structures in geochemical and industrial environments

Pre-nucleation clusters as solute precursors in crystallisation

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid–liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted

Co-mineralization of alkaline-earth carbonates and silica

This thesis is concerned with the manifold interactions that occur when alkaline-earth metal carbonates are crystallized in the presence of dissolved silica as an additive. The described work subdivides into two main lines of research. On the one hand, an understanding of the potential roles of silica during crystallization was sought on a fundamental level. That is, the mineral - in this case calcium carbonate - was directly precipitated from silica-containing solutions and the effect on growing particles and phase interconversions were characterized at different stages. On the other hand, experiments were performed in which the components were allowed to interact under conditions of low supersaturation, thus leading to gradual mineralization and enabling complex structuring. This resulted in the spontaneous self-assembly of hierarchical polycrystalline architectures, termed "silica biomorphs", which in the case of barium carbonate displayed delicate morphologies with sinuous shapes such as regular helicoids. Studies in the field of calcium carbonate precipitation showed that addition of silica is an effective means to arrest the crystallization process at virtually any stage simply by adjusting experimental parameters like species concentrations, pH, or temperature. In dilute systems, it was found that initially nucleated, metastable particles of amorphous calcium carbonate (ACC) become enveloped by a skin of silica due to local gradients in pH at growing surfaces and the intimate dependence of carbonate and silicate solubility on the pH. This spontaneous coating was investigated by a number of techniques and it was confirmed to increase the kinetic stability of the amorphous phase, causing decelerated conversion to stable calcite or fully preventing this energetically favored transformation. When stabilized temporarily, the inner ACC fraction of the as-formed core-shell particles served as a depot for CaCO3, slowly releasing growth units to the solution. Under these circumstances, a broad variety of unusual calcite morphologies was obtained, ranging from single crystals with uncommon habit to aggregates of nanocrystals with shapes beyond crystallographic restraints. In brines at higher supersaturation of CaCO3, a distinct silica-mediated stabilization could be verified also for the metastable crystalline CaCO3 modifications vaterite and aragonite, as evidenced by on-line diffraction analysis using synchrotron radiation. These findings suggest that the presence of silica, along with a sensible adjustment of conditions, does not only permit to study crystallization mechanisms in general by capturing transient intermediates, but also allows for a more or less concerted selection of polymorphs in the course of crystallization. The latter is clearly of interest for the synthesis of CaCO3 powders with tunable properties in view of industrial applications, while the former and in particular the achieved stabilization of ACC may serve as novel approaches to scale inhibition. Finally, systems at fairly low CaCO3 supersaturation and relatively high silica concentration were investigated. Here, the added silica was found to interfere with the crystallization process already before nucleation. In fact, so-called prenucleation clusters - recently discovered solute-like precursors of solid CaCO3 - were traced in the samples and found to be protected against otherwise rapidly occurring nucleation by binding of silica in their periphery. This concept was elaborated to develop an experimental procedure for the isolation these elusive species and render their analysis by conventional methods possible. Further, the degree of stabilization could be tuned by varying the pH, such that nucleation proceeded gradually to later stages. Eventually, this strategy enabled a direct observation of the processes underlying nucleation, and a non-classical model for homogeneous nucleation of calcium carbonate is proposed. Work addressing silica biomorphs was primarily focused on unraveling the morphogenetic mechanisms governing the evolution of complex and biomimetic form in simple inorganic environments. To that end, existing ideas on a pH-based chemical coupling of the speciations of carbonate and silicate in aqueous solution and an associated dynamic interplay during precipitation were assessed and efforts were made to advance and substantiate corresponding models. By a series of experiments conducted under conditions of forced convection, it was ascertained that growth of silica biomorphs is a local phenomenon taking place only at the front of developing aggregates, where the supersaturation and thus the driving force for precipitation is enhanced as compared to the bulk. In particular, the mineralization of one of the components (BaCO3 and silica) is thought to affect locally the chemistry of the other via the pH, ultimately triggering its precipitation. As a consequence, carbonate and silica are alternately deflected from equilibrium in time and therefore mineralized, depicting a synergetic scenario strongly reminiscent of the Belousov-Zhabotinsky reaction. This leads to a continuous production of silica-bearing BaCO3 nanocrystals, which constitute the emerging silica biomorph. On larger length scales, the assembly of crystallites grows free from constraints, usually forming flat sheets that later curl and give rise to twisted morphologies by mutual winding of different segments. In light of the performed experiments, it appears as if the morphological evolution of the crystal aggregates is widely determined by the beneficial impact of extrinsic and intrinsic surfaces on nucleation barriers. In this paradigm, smooth curvature in silica biomorphs results from the propensity of forming crystal aggregates to fold back on themselves and use their existing surface as a substrate. In turn, flat morphologies are favored when the assembly grows in direct contact with a foreign interface, such as a container wall. These particular characteristics, together with the observed structural hierarchy, delineate a fascinating analogy between the obtained abiotic precipitates and biologically produced mineral frameworks found in the tissues and exoskeletons of diverse living organisms. In this manner and thanks to the ease of their preparation and handling, silica biomorphs can be considered valuable laboratory model systems for the study of biomineralization concepts. With respect to these properties, further work was devoted to widen the range of morphologies and structures accessible with silica biomorphs. This was achieved for instance by introducing specific additives or varying distinct synthesis parameters, partially triggering remarkable alterations in the self-assembly behavior. The observed changes are interpreted in the realm of the envisaged morphogenetic mechanism and can mainly be ascribed to subtle shifts in the speciation of silica under the respective conditions. Eventually, the kinetics of the formation of silica biomorphs were studied. For this purpose, the concentrations of the involved reagents were monitored over prolonged periods of time and compared with growth rates determined for individual crystal aggregates by video microscopy. Results confirm reaction control for the process, thus supporting the autocatalytic character of growing fronts. The gathered data moreover shed novel light on the role of silica during crystallization as well as the mode and degree of its incorporation into forming aggregates. The drawn conclusions contribute to a deeper understanding of the phenomena leading to complex self-organization in these simple systems and to some extent devise routes to transfer the inherent concepts to other minerals and the design of novel promising materials

Beyond Biomineralization

A review. Crystn. in purely inorg. systems can yield smoothly curved forms that resemble those of biomaterials. Garcia-Ruiz et al. (2009) used video microscopy to provide insight into the formation of so-called "biomorphs" obtained by the pptn. from alk. media of nanocryst. BaCO3 (witherite) and amorphous SiO2 in a self-assembly process. Dynamic, pH-based coupling of equil. induces alternating crystn. of the BaCO3 and SiO2. The SiO2 acts, via pptn., as an inhibitor for continued carbonate growth. The coupling described by Garcia-Ruiz et al. (2009) explains the origin of nanocrystals and their uniformity; it also sheds light on the dynamics of their formation but details on mol.-scale interactions remain to be clarified