23.11.2015 | Minerals article available online, "The Role of Chloride Ions during the Formation of Akaganéite Revisited"
Iron(III) hydrolysis in the presence of chloride ions yields akaganéite, an iron oxyhydroxide mineral with a tunnel structure stabilized by the inclusion of chloride. Yet, the interactions of this anion with the iron oxyhydroxide precursors occurring during the hydrolysis process, as well as its mechanistic role during the formation of a solid phase are debated. Using a potentiometric titration assay in combination with a chloride ion-selective electrode, we have monitored the binding of chloride ions to nascent iron oxyhydroxides. Our results are consistent with earlier studies reporting that chloride ions bind to early occurring iron complexes. In addition, the data suggests that they are displaced with the onset of oxolation. Chloride ions in the akaganéite structure must be considered as remnants from the early stages of precipitation, as they do not influence the basic mechanism, or the kinetics of the hydrolysis reactions. The structure-directing role of chloride is based upon the early stages of the reaction. The presence of chloride in the tunnel-structure of akagenéite is due to a relatively strong binding to the earliest iron oxyhydroxide precursors, whereas it plays a rather passive role during the later stages of precipitation.
[Minerals 5, 778 (2015)]
15.11.2015 | Peck-Yin Sow has joined our team to work on her Master Project, which is a collaboration with Prof. Karin Hauser. Welcome Yin!
01.09.2015 | The CrystEngComm themed issue entitled “Fundamentals of Nanocrystal Formation”, guest edited by Georg Garnweitner, Denis Gebauer and Markus Niederberger, has been published. Read our editorial here.
29.08.2015 | "Prenucleation clusters and non-classical nucleation" is among Nano Today's most cited articles.
10.06.2015 | CrystEngComm article available online, "Synergy of Mg2+ and poly(aspartic acid) in additive-controlled calcium carbonate precipitation"
Additive-controlled precipitation of calcium carbonate is central to various fields of research. Technically, scale formation is an important problem, where polycarboxylates are most commonly employed as inhibitors. Herein, we show that the combination of poly(aspartic acid) with magnesium ions leads to synergistic effects that bring about a dramatic increase in the efficiency towards inhibition of nucleation and growth of nanoscopic CaCO3 precursors. These effects can also be crucial in biomineralization processes, where polycarboxylates and magnesium ions are thought to play important roles.
copyright [CrystEngComm (2015); DOI: 10.1039/C5CE00452G]
01.06.2015 | Julian Gale visits our group as a Senior Fellow of the Zukunftskolleg, and will stay until July 3rd. Julian's research concerns the development and application of theoretical or computational methods to problems in chemistry, physics, geochemistry, mineralogy and materials science. Current foci include; polymorphism, crystallisation, mineral-water interfaces, fuel cells, lithium battery materials, solvent extraction, and desalination membranes. Within the framework of the theory colloquium of the Physics Department, Julian will give a talk in P603 on Monday June 8, 13:30.
01.06.2015 | CrystEngComm article available online "Disordered amorphous calcium carbonate from direct precipitation"
Amorphous calcium carbonate (ACC) is known to play a prominent role in biomineralization. Different studies on the structure of biogenic ACCs have illustrated that they can have distinct short-range orders. However, the origin of so-called proto-structures in synthetic and additive-free ACCs is not well understood. In the current work, ACC has been synthesised in iso-propanolic media by direct precipitation from ionic precursors, and analysed utilising a range of different techniques. The data suggest that this additive-free type of ACC does not resemble clear proto-structural motifs relating to any crystalline polymorph. This can be explained by the undefined pH value in isopropanolic media, and the virtually instantaneous precipitation. Altogether, this work suggests that aqueous systems and pathways involving pre-nucleation clusters are required for the generation of clear proto-structural features in ACC. Experiments on the ACC-to-crystalline transformation in solution with and without ethanol highlight that polymorph selection is under kinetic control, while the presence of ethanol can control dissolution re-crystallisation pathways.
copyright [CrystEngComm (2015), DOI: 10.1039/c5ce00720h]
12.01.2015 | Faraday Discussions article available online "High-resolution insights into the early stages of silver nucleation and growth"
Nucleation and growth of silver nanoparticles has already been investigated with various experimental and computational tools, however, owing to inherent problems associated with the analytical characterization of nucleation processes, there is a generic lack of experimental data regarding the earliest precursors and smallest Ag(0) clusters. Here, we address this problem by the application of Synthetic Boundary Crystallization Ultracentrifugation, utilizing a multiwavelength detector for the first time, complemented by a specialized Titration Assay. These techniques shed new light on silver nanoparticle precursors existing in the pre-nucleation regime and the initially nucleated ensemble of nanoclusters. For the first time, we present experimental data of UV-vis spectra for fractionated silver clusters. These allow for unsurpassed insights into the sequence of nucleation and early growth species as well as their optical properties.
[Faraday Discussions (2015), DOI: 10.1039/C4FD00269E]
01.11.2014 | Grazyna Durak has joined the team as a postdoctoral researcher. Welcome!
Grazyna's project is a collaboration with Thomas Böttcher.
09.10.2014 | Nanoscale article available online "New Insights into the Early Stages of Silica-Controlled Barium Carbonate Crystallisation"
Recent work has demonstrated that the dynamic interplay between silica and carbonate during co-precipitation can result in the self-assembly of unusual, highly complex crystal architectures with morphologies and textures resembling those typically displayed by biogenic minerals. These so-called biomorphs were shown to be composed of uniform elongated carbonate nanoparticles that are arranged according to a specific order over mesoscopic scales. In the present study, we have investigated the circumstances leading to the continuous formation and stabilisation of such well-defined nanometric building units in these inorganic systems. For this purpose, in-situ potentiometric titration measurements were carried out in order to monitor and quantify the influence of silica on both the nucleation and early growth stages of barium carbonate crystallisation in alkaline media at constant pH. Complementarily, the nature and composition of particles occurring at different times in samples under various conditions were characterised ex situ by means of high-resolution electron microscopy and elemental analysis. The collected data clearly evidence that added silica affects carbonate crystallisation from the very beginning on (i.e. already prior to, during, and shortly after nucleation), eventually arresting growth at the nanoscale by cementation of BaCO3 particles within a siliceous matrix. Our findings thus shed novel light on the fundamental processes driving bottom-up self-organisation in silica-carbonate materials and, for the first time, provide a direct experimental proof that silicate species are responsible for the miniaturisation of carbonate crystals during growth of biomorphs, hence confirming previously discussed theoretical models for their formation mechanism.
copyright [Nanoscale (2014), DOI: 10.1039/C4NR05436A]
01.10.2014 | Yu-Chieh Huang has joined the team as a PhD student. Welcome!
19.09.2014 |Bingqiang Lu has joined the team as a postdoctoral researcher. Welcome!
24.07.2014 | Denis Gebauer has been elected as a new member of the executive board of the Graduate School Chemistry.
04.02.2014 | Denis Gebauer has been promoted to a Research Fellow (5-year fellowship) of the Zukunftskolleg of the University of Konstanz.
23.01.2014 | Chemical Society Reviews article available online "Pre-Nucleation Clusters as Solute Precursors in Crystallisation" (OPEN ACCESS)
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.
[Chem. Soc. Rev. (2014), DOI: 10.1039/C3CS60451A]
05.11.2013 | Advanced Materials article available online "A Straightforward Treatment of Activity in Aqueous CaCO3 Solutions and the Consequences for Nucleation Theory"
The aqueous calcium carbonate system is rigorously investigated with respect to ionic activity. Ideal treatment is found to be a good approximation at relevant concentrations. The data further show that bound CaCO3 species cannot be regarded as "inactive" during nucleation but rather appear to play a key role in the phase-separation process, and that amorphous calcium carbonate (ACC) can be precipitated from much lower levels of supersaturation than previously believed.
copyright [Adv. Mater. (2013), DOI: 10.1002/adma.201303643]
11/2013 | new publications available online:
Nachrichten aus der Chemie 61, 1097, "Wie bilden sich Kristalle?"
"Research Methods in Biomineralization Science", in: Methods in Enzymology 532, 45, "Investigating the Early Stages of Mineral Precipitation by Potentiometric Titration and Analytical Ultracentrifugation".
09.08.2013 | JACS article available online "Mg2+ Tunes the Wettability of Liquid Precursors of CaCO3: Toward Controlling Mineralization Sites in Hybrid Materials"
Amorphous and liquid precursors of calcium carbonate are believed to be central species of biomineralization, which serves as an important inspiration for materials chemists in the quest for new and improved organic–inorganic hybrid materials. It has become increasingly clear that magnesium ions exhibit an important function through kinetic stabilization of the metastable precursors. We show that they additionally tune the wettability of liquid precursors of CaCO3, which is a crucial requirement for successful mineralization of proteinaceous organic matrices. Moreover, tunable wettability offers straightforward means to control mineralization sites in organic–inorganic hybrids.
copyright [J. Am. Chem. Soc. (2013), DOI: 10.1021/ja404979z]
21.06.2013 | Angewandte Chemie International Edition article available online "Bio-Inspired Materials Science at Its Best—Flexible Mesocrystals of Calcite"
(invited highlight of F. Natalio et al., Science 339, 1298 (2013))
Minerals are the benchmark of hard and brittle materials. Self-assembled calcitic spicules were recently obtained utilizing a protein from silica biomineralization, silicatein-α. The synthetic spicules show remarkable material properties including extreme flexibility. Breakthroughs in bio-inspired materials science are highlighted. Will it be possible to obtain similar composites with truly artificial organic constituents?
copyright [Angew. Chem. Int. Ed. (2013), DOI: 10.1002/anie.201303933]
Mineralien sind ein Bezugspunkt für harte und spröde Materialien. Vor kurzem wurden über Selbstorganisation mithilfe des Proteins Silicatein-α Nadeln aus Calcit synthetisiert, die eine bemerkenswerte Flexibilität aufweisen. Die erreichten Durchbrüche auf dem Gebiet der biologisch inspirierten Materialwissenschaften werden herausgestellt. Ist es möglich, ähnliche Kompositmaterialien mit wahrlich künstlichen, organischen Bestandteilen zu verwirklichen?
[Angew. Chem. (2013), DOI: 10.1002/ange.201303933]
05/2013 | Johanna Scheck and Stefan Wolf have joined the team as PhD students.
04.03.2013 | Masoud Farhadi Khouzani has been selected to participate in the 63rd Lindau Nobel Laureate Meeting, to be held from 30th June to 5th July 2013. Only the 550 most qualified young researchers can be given the opportunity to enrich and share the unique atmosphere of the Lindau Nobel Laureate Meetings. Congratulations, Masoud!
01.02.2013 | Denis Gebauer has been awarded a fellowship of the Zukunftskolleg of the University of Konstanz.
23.11.2012 | CrystEngComm article available online, "Porous tablets of crystalline calcium carbonate via sintering of amorphous nanoparticles"
Porous tablets of crystalline calcium carbonate were formed upon sintering of a precursor powder of amorphous calcium carbonate (ACC) under compressive stress (20 MPa) at relatively low temperatures (120-400ºC), induced by pulsed direct currents. Infrared spectroscopy ascertained the amorphous nature of the precursor powders. At temperatures of 120-350ºC and heating rates of of 20-100ºC/min, the nanoparticles of ACC transformed into crystallites of mainly aragonite, which is generally difficult to achieve using wet-chemicals under kinetic control. The amorphous precursor particles (~10 nm) transformed into crystallites (~30-50 nm) during sintering. Consistently, the specific surface areas of 140-160 m2/g for the precursor particles were reduced to 10-20 m2/g for the porous sintered tablets. The porous network within the tablets consisted of fused aragonite and vaterite particles in a ratio of ~80:20. The fraction of aragonite to vaterite was invariant to the temperature and heating rate used. The particle size increased only slightly on an increased heating rate. At temperatures above 400ºC, porous tablets of calcite formed under thermodynamic control together with a minor reduction of the specific surface area of the tablets. The size of the crystallites remained small, and the transformation to calcite appeared to be a solid-state transformation. Porous, template- and binder-free tablets of calcium carbonate could find applications in, for example, biology or water treatment.
copyright [CrystEngComm 15, 1257 (2013), DOI: 10.1039/C2CE26604K]
04.11.2012 | Angewandte Chemie International Edition Review article available online, "Calcium Carbonate Polyamorphism and Its Role in Biomineralization: How Many Amorphous Calcium Carbonates Are There?"
Although the polymorphism of calcium carbonate is well known, and its polymorphs—calcite, aragonite, and vaterite—have been highly studied in the context of biomineralization, polyamorphism is a much more recently discovered phenomenon, and the existence of more than one amorphous phase of calcium carbonate in biominerals has only very recently been understood. Here we summarize what is known about polyamorphism in calcium carbonate as well as what is understood about the role of amorphous calcium carbonate in biominerals. We show that consideration of the amorphous forms of calcium carbonate within the physical notion of polyamorphism leads to new insights when it comes to the mechanisms by which polymorphic structures can evolve in the first place. This not only has implications for our understanding of biomineralization, but also of the means by which crystallization may be controlled in medical, pharmaceutical, and industrial contexts.
copyright [Angewandte Chemie International Edition 51, 11960 (2012), DOI: 10.1002/anie.201203125]
Während die Polymorphie des Calciumcarbonats gut erforscht ist und seine kristallinen Modifikationen – Calcit, Aragonit und Vaterit – oft insbesondere im Zusammenhang mit der Biomineralisation untersucht wurden, ist das Phänomen der Polyamorphie noch nicht lange bekannt. Auch die Existenz von mehr als einer amorphen Calciumcarbonatphase ist erst vor Kurzem klar geworden. In diesem Aufsatz fassen wir zusammen, was über die Polyamorphie des Calciumcarbonats auf der einen Seite und über die Rolle des amorphen Calciumcarbonats in der Biomineralisation auf der anderen Seite bekannt ist. Unsere Betrachtungen ermöglichen uns aufzuzeigen, dass die Behandlung amorphen Calciumcarbonats im Rahmen des physikalischen Verständnisses der Polyamorphie zu neuartigen Einblicken führt, wie unterschiedliche Polymorphe überhaupt aus ihren Vorstufen entstehen können. Dies ist von großer Bedeutung für unser Verständnis der Prinzipien der Biomineralisation sowie der Mittel und Wege, wie Kristallisation in medizinischen, pharmazeutischen oder industriellen Zusammenhängen gesteuert werden kann.
[Angewandte Chemie 124, 12126 (2012), DOI: 10.1002/ange.201203125]
24.10.2012 | Zeitschrift für Kristallographie article available online, "The multiple effects of amino acids on the early stages of calcium carbonate crystallization"
Abstract. Proteins have found their way into many of Nature’s structures due to their structural stability, diversity in function and composition, and ability to be regulated as well as be regulators themselves. In this study, we investigate the constitutive amino acids that make up some of these proteins which are involved in CaCO3 mineralization – either in nucleation, crystal growth, or inhibition processes. By assaying all 20 amino acids with vapor diffusion and in situ potentiometric titration, we have found specific amino acids having multiple effects on the early stages of CaCO3 crystallization. These same amino acids have been independently implicated as constituents in liquid-like precursors that form mineralized tissues, processes believed to be key effects of biomineralization proteins in several biological model systems.
[Zeitschrift für Kristallographie 227, 744 (2012).
08.10.2012 | Casper Ibsen has joined the team as a visiting PhD student.
01.09.2012 | conference highlights from Faraday Discussions 159 available online
14.06.2012 | Advanced Functional Materials article available online, "Colloidal Stabilization of Calcium Carbonate Prenucleation Clusters with Silica"
Calcium carbonate precipitation proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which finally transform to crystals. Although the existence of stable clusters in solution prior to nucleation has been demonstrated, the molecular mechanisms by which they precipitate are still obscure. Here, direct insight into the processes that drive the transformation of individual clusters into amorphous nanoparticles is provided by progressive colloidal stabilization of different transient states in silica-containing environments. Nucleation of calcium carbonate in the presence of silica can only take place via cluster aggregation at low pH values. At higher pH, prenucleation clusters become colloidally stabilized and cannot aggregate. Nucleation through structural reorganization within the clusters is not observed under these conditions, indicating that this pathway is blocked by kinetic and/or thermodynamic means. The degree of stabilization against nucleation is found to be sufficient to allow for a dramatic enrichment of solutions with prenucleation clusters and enable their isolation into the dry state. This approach renders direct analyses of the clusters by conventional techniques possible and is thus likely to facilitate deeper insight into the chemistry and structure of these elusive species in the future.
[Advanced Functional Materials 22, 4301 (2012), DOI: 10.1002/adfm.201200953]
07.06.2012 | Faraday Discussions article available online, "A Meta-Stable Liquid Precursor Phase of Calcium Carbonate and its Interactions with Polyaspartate"
Invertebrate organisms that use calcium carbonate extensively in the formation of their hard tissues have the ability to deposit biominerals with control over crystal size, shape, orientation, phase, texture, and location. It has been proposed by our group that charged polyelectrolytes, like acidic proteins, may be employed by organisms to direct crystal growth through an intermediate liquid phase in a process called the polymer-induced liquid-precursor (PILP) process. Recently, it has been proposed that calcium carbonate crystallization, even in the absence of any additives, follows a non-classical, multi-step crystallization process by first associating into a liquid precursor phase before transition into solid amorphous calcium carbonate (ACC) and eventually crystalline calcium carbonates. In order to determine if the PILP process involves the promotion, or stabilization, of a naturally occurring liquid precursor to ACC, we have analyzed the formation of saturated and supersaturated calcium carbonate–bicarbonate solutions using Ca2+ ion selective electrodes, pH electrodes, isothermal titration calorimetry, nanoparticle tracking analysis, 13C T2 relaxation measurements, and 13C PFG-STE diffusion NMR measurements. These studies provide evidence that, in the absences of additives, and at near neutral pH (emulating the conditions of biomineralization and biomimetic model systems), a condensed phase of liquid-like droplets of calcium carbonate forms at a critical concentration, where it is stabilized intrinsically by bicarbonate ions. In experiments with polymer additive, the data suggests that the polymer is kinetically stabilizing this liquid condensed phase in a distinct and pronounced fashion during the so called PILP process. Verification of this precursor phase and the stabilization that polymer additives provide during the PILP process sheds new light on the mechanism through which biological organisms can exercise such control over deposited CaCO3 biominerals, and on the potential means to generate in vitro mineral products with features that resemble biominerals seen in nature.
[Faraday Discuss. (2012); DOI: 10.1039/C2FD20080E]
04.06.2012 | Faraday Discussions article available online, "Exploring the influence of organic species on pre- and post-nucleation calcium carbonate"
Organic additives are well known to influence the nucleation and growth of minerals. A combination of experimental and theoretical methods has been used to probe how three simple additives, containing varying numbers of carboxylate groups, influence the early stages of the growth of calcium carbonate. Computationally, the free energy landscape has been examined for each additive binding to Ca2+, the calcium carbonate ion pair, the surface of an amorphous calcium carbonate nanoparticle, and the basal plane of calcite. The different influence of the three organic ligands on the early stages of growth of calcium carbonate observed experimentally can be rationalised in terms of the degree of association of each anion with the species present prior to, and immediately after nucleation.
[Faraday Discuss. (2012); DOI: 10.1039/C2FD20052J]
01.06.2012 | Masoud Farhadi Khouzani has joined the team to work on his PhD project. Welcome!
Masoud will work on amorphous calcium carbonate, with a focus on polyamorphism on one hand, but also on its application as a functional material.
23.05.2012 | award ceremony of the Heinz Maier-Leibnitz Prizes 2012 in Berlin
DFG-President Matthias Kleiner, Stefan Roth, Katrin Paeschke, Yee Lee Shing, Federal Minister Annette Schavan, Pieter Samyn, Lisa Kaltenegger, Denis Gebauer, DFG-Vice-President Dorothea Wagner (from left to right)
copyright DFG / David Ausserhofer
12.04.2012 | career profile published on Science Careers: Going Against the Grain
01.04.2012 | John Berg has joined the team as a postdoctoral researcher. Welcome!
John is funded transitionally by the Young Scholar Fund of the University of Konstanz, and will work on fundamental aspects of calcium phosphate precipitation as well as advanced approaches to additive-controlled crystallization of calcium carbonates and phosphates.
15.03.2012 | Denis Gebauer has been awarded the Heinz Maier-Leibnitz Prize.
For more information see the DFG website or the press release of the University of Konstanz.
20.12.2011 | Nature Communications article available online, "Stable prenucleation mineral clusters are liquid-like ionic polymers" (open content)
Calcium carbonate is an abundant substance that can be created in several mineral forms by the reaction of dissolved carbon dioxide in water with calcium ions. Through biomineralization, organisms can harness and control this process to form various functional materials that can act as anything from shells through to lenses. The early stages of calcium carbonate formation have recently attracted attention as stable prenucleation clusters have been observed, contrary to classical models. Here we show, using computer simulations combined with the analysis of experimental data, that these mineral clusters are made of an ionic polymer, composed of alternating calcium and carbonate ions, with a dynamic topology consisting of chains, branches and rings. The existence of a disordered, flexible and strongly hydrated precursor provides a basis for explaining the formation of other liquid-like amorphous states of calcium carbonate, in addition to the non-classical behaviour during growth of amorphous calcium carbonate.
[Nature Communications 2, 590 (2011)]
25.11.2011 | Nano Today review article available online, "Prenucleation Clusters and Non-classical Nucleation"
In this contribution, we review the recent literature on an alternative crystallization pathway involving stable clusters prior to nucleation and show that the prenucleation cluster pathway is a truly non-classical concept of nucleation. Prenucleation clusters are solutes with “molecular” character in aqueous solution. It becomes evident that the stable clusters may have been concealed by the so-called ion pair concept and activity effects. We show that non-classical nucleation via stable prenucleation clusters is a highly valuable concept for a novel understanding of phenomena observed in bio- and biomimetic mineralization, which however may hardly be rationalized by means of classical nucleation theory. The importance to combine experimental and theoretical studies is emphasized.
copyright [Nano Today 6, 564 (2011)]