Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
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Symbiosis of Silica Biomorphs and Magnetite Mesocrystals
Silica biomorphs are extraordinary inorganic superstructures formed via autocatalytic co‐precipitation and bottom‐up self‐assembly of alkaline‐earth carbonates and silica. However, they show no inherent functionality except for their striking textural motifs and curved morphologies. This work presents strategies to magnetize silica biomorphs, thus creating thermally stable ceramic microswimmers with unique elaborate shapes. This is achieved by growing super paramagnetic magnetite mesocrystals on and around the complex curved surfaces of biomorphs, while keeping their morphology and maintaining mesocrystal integrity. Selective mesocrystal formation on certain parts of the substrates is induced by chemical modification of the biomorph surface, increasing the loading of magnetite on the silica–carbonate structures and, in suitable cases, rendering them able to respond to external magnetic fields and move as microswimmer entities. In this way, the complex ultrastructure of silica biomorphs is successfully used as a template for functional ceramics. Furthermore, selective dissolution of the carbonate core from the biomorphs leads to hollow magnetic structures that could be filled with actives, thus serving as microcarriers with considerable loading capacity.
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.
Probing local pH-based precipitation processes in self-assembled silica-carbonate hybrid materials
Crystallisation of barium carbonate in the presence of silica can lead to the spontaneous assembly of highly complex superstructures, consisting of uniform and largely co-oriented BaCO3 nanocrystals that are interspersed by a matrix of amorphous silica. The formation of these biomimetic architectures (so-called silica biomorphs) is thought to be driven by a dynamic interplay between the components, in which subtle changes of conditions trigger ordered mineralisation at the nanoscale. In particular, it has been proposed that local pH gradients at growing fronts play a crucial role in the process of morphogenesis. In the present work, we have used a special pH-sensitive fluorescent dye to directly trace these presumed local fluctuations by means of confocal laser scanning microscopy. Our data demonstrate the existence of an active region near the growth front, where the pH is locally decreased with respect to the alkaline bulk solution on a length scale of few microns. This observation provides fundamental and, for the first time, direct experimental support for the current picture of the mechanism underlying the formation of these peculiar materials. On the other hand, the absence of any temporal oscillations in the local pH - another key feature of the envisaged mechanism - challenges the notion of autocatalytic phenomena in such systems and raises new questions about the actual role of silica as an additive in the crystallisation process.
A simple strategy for the synthesis of well-defined bassanite nanorods
Bassanite is a metastable but industrially important form of calcium sulfate, which is commonly produced by heating of gypsum. Here we show that pure bassanite can also be obtained at ambient conditions by quenching aqueous CaSO4 solutions in ethanol. This highlights that organic solvents can actually induce the formation of metastable phases rather than freezing precipitation processes.
Hybrid Biomimetic Materials from Silica/Carbonate Biomorphs
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.
Functionalisation of silica–carbonate biomorphs
Biomorphs are a unique class of self-organised silica–carbonate mineral structures with elaborate shapes. Here we report first approaches to modify these complex inorganic architectures through silane chemistry, binding of nanoparticles, and organic polymerisation. This leads to functional nanostructures in which the complexity of the originally inorganic template is preserved, and offers new diagnostic tools to study the mechanisms underlying their formation.
Selektive Kontrolle der Bildung von Calciumsulfat-Polymorphen bei Raumtemperatur
Calciumsulfat ist ein in der Natur weit verbreitetes und technologisch wichtiges Mineral mit breitem Anwendungsspektrum. Allerdings stellte die Kontrolle der CaSO4-Polymorphie und damit der Materialeigenschaften nach wie vor eine beträchtliche Herausforderung dar, und bis jetzt gibt es keine universelle Methode für die selektive Herstellung der verschiedenen hydratisierten und wasserfreien Formen unter milden Bedingungen. Hier berichten wir über die erste erfolgreiche Synthese von reinem Anhydrit aus einer Lösung bei Raumtemperatur. Erreicht wurde dies durch Ausfällen von Calciumsulfat in alkoholischen Medien mit niedrigen Wassergehalten. Durch Variieren der Wassermenge gelang es uns außerdem, zwischen einzelnen Polymorphen zu schalten und das Ergebnis der Reaktion fein abzustimmen, um entweder eine gewünschte CaSO4-Phase in reiner Form oder binäre Mischungen mit festgelegten Zusammensetzungen herzustellen. Das Konzept bietet die volle Kontrolle über die Auswahl der Phasen in der CaSO4-Mineralisation und könnte die gezielte Herstellung entsprechender Materialien für diverse Anwendungsgebiete ermöglichen.
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.
Recombinant perlucin derivatives influence the nucleation of calcium carbonate
Proteins are known to play various key roles in the formation of complex inorganic solids during natural biomineralisation processes. However, in most cases our understanding of the actual underlying mechanisms is rather limited. One interesting example is perlucin, a protein involved in the formation of nacre, where it is believed to promote the crystallisation of calcium carbonate. In the present work, we have used potentiometric titration assays to systematically investigate the influence of recombinant GFP-labeled perlucin derivatives on the early stages of CaCO3 formation. Our results indicate that different parts of the protein can impact nucleation in distinct ways and act in either a retarding or promoting fashion. The most important finding is that full-length GFP-perlucin changes the nature of the initially precipitated phase and seems to favour the direct formation of crystalline polymorphs over nucleation of ACC and subsequent phase transformation, as observed in reference experiments without protein. This confirms the supposed role of perlucin in nacre biomineralisation and may rely on specific interactions between the protein and the crystal lattice of the emerging mineral phase.
Controlling the Selective Formation of Calcium Sulfate Polymorphs at Room Temperature
Calcium sulfate is a naturally abundant and technologically important mineral with a broad scope of applications. However, controlling CaSO4 polymorphism and, with it, its final material properties still represents a major challenge, and to date there is no universal method for the selective production of the different hydrated and anhydrous forms under mild conditions. Herein we report the first successful synthesis of pure anhydrite from solution at room temperature. We precipitated calcium sulfate in alcoholic media at low water contents. Moreover, by adjusting the amount of water in the syntheses, we can switch between the distinct polymorphs and fine-tune the outcome of the reaction, yielding either any desired CaSO4 phase in pure state or binary mixtures with predefined compositions. This concept provides full control over phase selection in CaSO4 mineralization and may allow for the targeted fabrication of corresponding materials for use in various areas.