Helmut Cölfen

Controlling Mineral Morphologies and Structures in Biological and Synthetic Systems

2.3. Amorphous Minerals 4354 3. Biological Routes to Controlling Morphology 4354 3.1. General Mechanisms 4356 3.1.1. Soluble and Insoluble Organic Molecules 4356 3.1.2. Control over Crystal Polymorph 4357 3.1.3. Control over Crystal Orientation 4359 3.2. Single-Crystal Biominerals 4359 3.2.1. Organic and Inorganic Soluble Additives 4360 3.2.2. Templating of Single-Crystal Morphologies 4361 3.3. Polycrystalline Biominerals 4366 3.3.1. Nacre Formation in Mollusks 4366 3.3.2. ForaminiferasA Biogenic Mesocrystal 4368 3.4. Amorphous Biominerals 4370 3.4.1. Silicification in Diatoms 4370 3.4.2. In Vitro Studies of Silicification in Diatoms 4371 4. Bioinspired Routes to Controlling Crystal Morphologies 4371

Stable Prenucleation Calcium Carbonate Clusters

Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. Huge amounts of carbon dioxide are retained as carbonate ions, and calcium ions represent a major contribution to water hardness. Despite its relevance, little is known about the precipitation mechanism of calcium carbonate, and specified complex crystal structures challenge the classical view on nucleation considering the formation of metastable ion clusters. We demonstrate that dissolved calcium carbonate in fact contains stable prenucleation ion clusters forming even in undersaturated solution. The cluster formation can be characterized by means of equilibrium thermodynamics, applying a multiple-binding model, which allows for structural preformation. Stable clusters are the relevant species in calcium carbonate nucleation. Such mechanisms may also be important for the crystallization of other minerals.

Mesocrystals-Ordered Nanoparticle Superstructures

Mesocrystals are 3D ordered nanoparticle superstructures, often with internal porosity, which receive much recent research interest. While more and more mesocrystal systems are found in biomineralization or synthesized, their potential as material still needs to be explored. It needs to be revealed, which new chemical and physical properties arise from the mesocrystal structure, or how they change by the ordered aggregation of nanoparticles to fully exploit the promising potential of mesocrystals. Also, the mechanisms for mesocrystal synthesis need to be explored to adapt it to a wide class of materials. The last three years have seen remarkable progress, which is summarized here. Also potential future directions of this reaserch field are discussed. This shows the importance of mesocrystals not only for the field of materials research and allows the appliction of mesocrystals in advanced materials synthesis or property improvement of existing materials. It also outlines attractive research directions in this field.

Double-Hydrophilic Block Copolymers: Synthesis and Application as Novel Surfactants and Crystal Growth Modifiers

Double-hydrophilic block copolymers are a new class of amphiphilic molecules of rapidly increasing importance with unique and fascinating properties poten tially connecting materials science, pharmacy, biochemistry and polymer science. Characteristic of these polymers is their application in aqueous environments and that amphiphilicity is just induced in the presence of a subtrate or by temperature and pH changes, respectively, Their chemical structure may be turned for a wide range of applications covering as different aspects as stabilization of colloids, crystal growth modification, indnced micelle formation, polyelectrolyte complexing towards novel drug carrier systems. As the potential of this novel polymer class is relatively unexplored vet it can be expected that more applications will arise due to the possibility to adapt the chemical structure to either the desired substrate in contact with water or the stimulus for the induction of structural changes. This review describes the synthetic strategies to wards these AB block copolymers, as well as their applications.

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

Growing crystals by attaching particles Crystals grow in a number a ways, including pathways involving the assembly of other particles and multi-ion complexes. De Yoreo et al. review the mounting evidence for these nonclassical pathways from new observational and computational techniques, and the thermodynamic basis for these growth mechanisms. Developing predictive models for these crystal growth and nucleation pathways will improve materials synthesis strategies. These approaches will also improve fundamental understanding of natural processes such as biomineralization and trace element cycling in aquatic ecosystems. Science, this issue 10.1126/science.aaa6760 Materials nucleate and grow by the assembly of small particles and multi-ion complexes. BACKGROUND Numerous lines of evidence challenge the traditional interpretations of how crystals nucleate and grow in synthetic and natural systems. In contrast to the monomer-by-monomer addition described in classical models, crystallization by addition of particles, ranging from multi-ion complexes to fully formed nanocrystals, is now recognized as a common phenomenon. This diverse set of pathways results from the complexity of both the free-energy landscapes and the reaction dynamics that govern particle formation and interaction. Whereas experimental observations clearly demonstrate crystallization by particle attachment (CPA), many fundamental aspects remain unknown—particularly the interplay of solution structure, interfacial forces, and particle motion. Thus, a predictive description that connects molecular details to ensemble behavior is lacking. As that description develops, long-standing interpretations of crystal formation patterns in synthetic systems and natural environments must be revisited. Here, we describe the current understanding of CPA, examine some of the nonclassical thermodynamic and dynamic mechanisms known to give rise to experimentally observed pathways, and highlight the challenges to our understanding of these mechanisms. We also explore the factors determining when particle-attachment pathways dominate growth and discuss their implications for interpreting natural crystallization and controlling nanomaterials synthesis. ADVANCES CPA has been observed or inferred in a wide range of synthetic systems—including oxide, metallic, and semiconductor nanoparticles; and zeolites, organic systems, macromolecules, and common biomineral phases formed biomimetically. CPA in natural environments also occurs in geologic and biological minerals. The species identified as being responsible for growth vary widely and include multi-ion complexes, oligomeric clusters, crystalline or amorphous nanoparticles, and monomer-rich liquid droplets. Particle-based pathways exceed the scope of classical theories, which assume that a new phase appears via monomer-by-monomer addition to an isolated cluster. Theoretical studies have attempted to identify the forces that drive CPA, as well as the thermodynamic basis for appearance of the constituent particles. However, neither a qualitative consensus nor a comprehensive theory has emerged. Nonetheless, concepts from phase transition theory and colloidal physics provide many of the basic features needed for a qualitative framework. There is a free-energy landscape across which assembly takes place and that determines the thermodynamic preference for particle structure, shape, and size distribution. Dynamic processes, including particle diffusion and relaxation, determine whether the growth process follows this preference or another, kinetically controlled pathway. OUTLOOK Although observations of CPA in synthetic systems are reported for diverse mineral compositions, efforts to establish the scope of CPA in natural environments have only recently begun. Particle-based mineral formation may have particular importance for biogeochemical cycling of nutrients and metals in aquatic systems, as well as for environmental remediation. CPA is poised to provide a better understanding of biomineral formation with a physical basis for the origins of some compositions, isotopic signatures, and morphologies. It may also explain enigmatic textures and patterns found in carbonate mineral deposits that record Earth’s transition from an inorganic to a biological world. A predictive understanding of CPA, which is believed to dominate solution-based growth of important semiconductor, oxide, and metallic nanomaterials, promises advances in nanomaterials design and synthesis for diverse applications. With a mechanism-based understanding, CPA processes can be exploited to produce hierarchical structures that retain the size-dependent attributes of their nanoscale building blocks and create materials with enhanced or novel physical and chemical properties. Major gaps in our understanding of CPA. Particle attachment is influenced by the structure of solvent and ions at solid-solution interfaces and in confined regions of solution between solid surfaces. The details of solution and solid structure create the forces that drive particle motion. However, as the particles move, the local structure and corresponding forces change, taking the particles from a regime of long-range to short-range interactions and eventually leading to particle-attachment events. Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

A Systematic Examination of the Morphogenesis of Calcium Carbonate in the Presence of a Double-Hydrophilic Block Copolymer

In this paper, a systematic study of the influence of various experimental parameters on the morphology and size of CaCO3 crystals after room-temperature crystallization from water in the presence of poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) is presented. The pH of the solution, the block copolymer concentration, and the ratio [polymer]/[CaCO3] turned out to be important parameters for the morphogenesis of CaCO3, whereas a moderate increase of the ionic strength (0.016 M) had no influence. Depending on the experimental conditions, the crystal morphologies can be tuned from calcite rhombohedra via rods, ellipsoids or dumbbells to spheres. A morphology map is presented which allows the prediction of the crystal morphology from a combination of pH, and CaCO3 and polymer concentration. Morphologies reported in literature for the same system but under different crystallization conditions agree well with the predictions from the morphology map. A closer examination of the growth of polycrystalline macroscopic CaCO3 spheres by TEM and time-resolved dynamic light scattering showed that CaCO3 macrocrystals are formed from strings of aggregated amorphous nanoparticles and then recrystallize as dumbbell-shaped or spherical calcite macrocrystal.

Bio-inspired crystal morphogenesis by hydrophilic polymers

The latest advances in hydrophilic polymer controlled morphosynthesis and bio-inspired mineralization of various technically important inorganic crystals are reviewed with a focus on how to generate inorganic crystals with unusual structural specialty and complexity by double hydrophilic block copolymers (DHBCs). The systematic morphogenesis of different inorganic minerals with controlled morphology and novel superstructures by using DHBCs with varying patterns of functional groups will be described and influence of parameters such as crystallization sites, temperature, concentrations of reactants and copolymers, as well as cosolvents or the introduction of foreign colloidal structures on the morphology, crystallization, and superstructure will be discussed. We will demonstrate the ability of the copolymer to interact with inorganic crystals as well as the fine-tuning of morphosynthesis of inorganic crystals. Several different morphogenesis mechanisms are identified including selective polymer adsorption, mesoscopic transformations and higher order assembly. Mesoscopic transformation and formation of novel organic–inorganic superstructures by DHBC-mediated crystallization, combination of DHBCs with normal surfactants for the formation of new superstructures and the DHBC–crystal interaction will be reviewed as well as recent advances in the analysis of these systems and their formation mechanisms. Current developments emphasize that probably all inorganic crystals will be amenable to morphosynthetic control by use of either flexible molecular templates or suitable self-assembly mechanisms. Further exploration in these areas should provide new possibilities for the rational design of various kinds of inorganic materials with ideal hierarchy and controllable length scales. These unique hierarchical materials of structural specialty and complexity with a size range spanning from nanometers to micrometers are expected to find potential applications in various fields.

Precipitation of carbonates: recent progress in controlled production of complex shapes

Abstract The recent progress in carbonate precipitation with complex shapes mainly concerns CaCO3 due to its importance for industry and biomineralization. Two general morphogenesis approaches were pursued: Either, soluble—predominately polymeric—additives were employed in the precipitation process leading to microparticles with complex shape which are partly even hybrids of two different crystal polymorphs, or macroscopic templates were used as confined reaction environments for precipitation. Despite a considerable amount of work and knowledge gain, the investigation of the structure formation processes with impact on understanding of biomineralization principles still remains a challenge. Nevertheless, some remarkable progress is reported in the areas of template and additive controlled CaCO3 crystallization as well as its morphogenesis based on mesoscopic transformation which lead to a deeper understanding of the underlying structuration principles.

Crystal Design of Barium Sulfate using Double-Hydrophilic Block Copolymers.

Peach, peanut, fiber, and flower (see picture) crystal morphologies are achieved from the precipitation of simple minerals in the presence of specifically adsorbing polymers. These crystal design effects are illustrated using BaSO(4) and double-hydrophilic block copolymers, the latter featuring carboxylate, sulfonate, phosphonate, and aspartic acid groups.

Biomineralization: A crystal-clear view

The mechanisms of biomineralization remain hotly debated. Now high-resolution microscopy yields unsurpassed insight into mechanisms relevant both to the biomineralization of bone and teeth and to pathological mineralization.