Nature often uses precursor phases for the controlled development of crystalline materials with welldefined morphologies and unusual properties. Mimicking such a strategy in in vitro model systems would potentially lead to the water-based, room-temperature synthesis of superior materials. In the case of magnetite (Fe 3 O 4 ), which in biology generally is formed through a ferrihydrite precursor, such approaches have remained largely unexplored. Here we report on a simple protocol that involves the slow coprecipitation of Fe III /Fe II salts through ammonia diffusion, during which ferrihydrite precipitates first at low pH values and is converted to magnetite at high pH values. Direct coprecipitation often leads to small crystals with superparamagnetic properties. Conversely, in this approach, the crystallization kineticsand thereby the resulting crystal sizescan be controlled through the NH 3 influx and the Fe concentration, which results in single crystals with sizes well in the ferrimagnetic domain. Moreover, this strategy provides a convenient platform for the screening of organic additives as nucleation and growth controllers, which we demonstrate for the biologically derived M6A peptide. ■ INTRODUCTIONLiving organisms exploit the properties of minerals by building a wide variety of specialized organic−inorganic hybrid materials for a variety of purposes, such as for protection and skeletal support and also for navigation and the detection of light. 1,2 The high level of control over the composition, structure, size, and morphology of such biominerals results in materials with amazing complexity and fascinating functionality that are in strong contrast with those of geological minerals and often surpass those of synthetic analogues. 3 Consequently, the processes involved in biomineralization have intrigued scientists for many decades since they could provide sustainable production routes for advanced materials with highly controllable and specialized properties. 4,5 With inspiration from biological mineralization processes, many studies have addressed the interaction between organic and inorganic components using in vitro experiments and in particular have addressed the biomimetic formation of calcium carbonate, 6 calcium phosphate, 7 and silica-based materials. 8 For these systems, a large variety of synthetic methods were developed, which allow for the controlled deposition of minerals in the presence of organic additives and templates. For the formation of crystalline biominerals, the use of precursor phases is of special interest since they allow the efficient transport of significant amounts of material and provide the organism with a feasible route to minerals with complex, nonequilibrium morphologies. 9 The development of precursor-based strategies for in vitro mineralization has boosted the field of bioinspired crystallization and provided a toolbox with which to mimic the effect of biological templates, additives, and confinement in a laboratory environment. This development has also allowed researc...

While biogenic calcites frequently contain appreciable levels of magnesium, the pathways leading to such high concentrations remain unclear. The production of high-magnesian calcites in vitro is highly challenging, because Mg-free aragonite, rather than calcite, is the favored product in the presence of strongly hydrated Mg(2+) ions. While nature may overcome this problem by forming a Mg-rich amorphous precursor, which directly transforms to calcite without dissolution, high Mg(2+)/Ca(2+) ratios are required synthetically to precipitate high-magnesian calcite from solution. Indeed, it is difficult to synthesize amorphous calcium carbonate (ACC) containing high levels of Mg, and the Mg is typically not preserved in the calcite product as the transformation occurs via a dissolution-reprecipitation route. We here present a novel synthetic method, which employs a strategy based on biogenic systems, to generate high-magnesian calcite. Mg-containing ACC is produced in a nonaqueous environment by reacting a mixture of Ca and Mg coordination complexes with CO(2). Control over the Mg incorporation is simply obtained by the ratio of the starting materials. Subsequent crystallization at reduced water activities in an organic solvent/water mixture precludes dissolution and reprecipitation and yields high-magnesian calcite mesocrystals with Mg contents as high as 53 mol %. This is in direct contrast with the polycrystalline materials generally observed when magnesian calcite is formed synthetically. Our findings give insight into the possible mechanisms of formation of biogenic high-magnesian calcites and indicate that precise control over the water activity may be a key element.

Biological systems show impressive control over the shape, size and organization of mineral structures, which often leads to advanced physical properties that are tuned to the function of these materials. Such control is also found in magnetotactic bacteria, which produce-in aqueous medium and at room temperature-magnetite nanoparticles with precisely controlled morphologies and sizes that are generally only accessible in synthetic systems with the use of organic solvents and/or the use of high-temperature methods. The synthesis of magnetite under biomimetic conditions, that is, in water and at room temperature and using polymeric additives as control agents, is of interest as a green production method for magnetic nanoparticles. Inspired by the process of magnetite biomineralization, a rational approach is taken by the use of a solid precursor for the synthesis of magnetite nanoparticles. The conversion of a ferrous hydroxide precursor, which we demonstrate with cryo-TEM and low-dose electron diffraction, is used to achieve control over the solution supersaturation such that crystal growth can be regulated through the interaction with poly-(α,β)-dl-aspartic acid, a soluble, negatively charged polymer. In this way, stable suspensions of nanocrystals are achieved that show remanence and coercivity at the size limit of superparamagnetism, and which are able to align their magnetic moments forming strings in solution as is demonstrated by cryo-electron tomography.

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publicationCitation for published version (APA): Lenders, J. J. M., Zope, H., Yamagishi, A., Bomans, P. H. H., Arakaki, A., Kros, A., ... Sommerdijk, N. A. J. M. (2015). Bioinspired magnetite crystallization directed by random copolypeptides. Advanced Functional Materials, 25(5), 711-719. DOI: 10.1002711-719. DOI: 10. /adfm.201403585, 10.1002 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. often single-domain crystals as, e.g., encountered in the magnetosomes of magnetotactic bacteria, [ 4 ] but also in the magnetoreceptive organs of migratory birds, [ 5 ] honeybees, [ 5a , 6 ] and certain fi sh. [ 5a , 7 ] In contrast, the most commonly used industrial process for magnetite production up to date, direct coprecipitation of Fe (II) and Fe (III) ions from solution, typically yields small (<20 nm) superparamagnetic particles with little control over size or shape, [ 8 ] while protocols allowing better control usually involve non-aqueous media and/or high temperatures. [ 8d,e ] Applying strategies from biomineralization in materials chemistry could open the door to the additive-directed synthesis of magnetite-based nanomaterials with control over the dimensions and organization of the particles and thereby their magnetic properties, using green, bioinspired production methods, i.e., using aqueous media and ambient temperatures. [ 3a , 9 ] However, compared to other biominerals (e.g., calcium carbonate, calcium phosphate, and silica), for which the biomimetic synthesis of materials with controlled morphology through the action of d...

Living organisms can produce inorganic materials with unique structure and properties. The biomineralization process is of great interest as it forms a source of inspiration for the development of methods for production of diverse inorganic materials under mild conditions. Nonetheless, regulation of biomineralization is still a challenging task. Magnetotactic bacteria produce chains of a prokaryotic organelle comprising a membrane-enveloped single-crystal magnetite with species-specific morphology. Here, we describe regulation of magnetite biomineralization through controlled expression of the mms7 gene, which plays key roles in the control of crystal growth and morphology of magnetite crystals in magnetotactic bacteria. Regulation of the expression level of Mms7 in bacterial cells enables switching of the crystal shape from dumbbell-like to spherical. The successful regulation of magnetite biomineralization opens the door to production of magnetite nanocrystals of desired size and morphology.