We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.amorphous calcium carbonate ͉ biomineralization ͉ high-resolution transmission EM ͉ solid-state NMR T he delicate mineral structures produced by organisms in the process of biomineralization are widely recognized as inspiration for future materials science and nanotechnology because of their unique materials properties and their hierarchical order often over several length scales (1). Therefore, recent multidisciplinary research has focused on understanding biomineralization processes and exploring ways to mimic them (1). Particularly well investigated is nacre, possessing a 3,000-fold enhanced fracture resistance compared with pure aragonite, with implications on building material design. Nacre is composed of aragonite platelets, a usually metastable CaCO 3 polymorph, with [001] orientation toward protein-covered ␤-chitin layers (2).The present paradigm discusses an epitaxial match of acidic proteins adsorbed on the insoluble matrix with the atomic structure of the aragonite (001) plane (3). Indeed, two independent studies reported aragonite formation in the presence of soluble proteins extracted from a nacreous aragonite biomineral layer (4, 5). However, because the extracted macromolecules are disordered species and mixtures, too (5), an epitaxial match seems questionable. We therefore revisited nacre aragonite single crystalline platelets from the abalone Haliotis laevigata (6) with high-resolution transmission electron microscopy (HRTEM) supplemented by solid-state 13 C and 1 H NMR to obtain information on the organic-inorganic interface. Materials and MethodsNacre was obtained from the shell of the abalone H. laevigata, which belongs to the gastropoda. The structure of the nacreous layer is described in refs. 6-8. Thin cuts from the nacreous part were made with a diamond knife in a Leica ultracut UCT and transferred onto an amorphous carbon-coated copper grid. By using this technique, artifacts in the form of amorphous regions in the sample as can be observed by the ion milling technique (9) can be avoided. A Philips CM200 FEG transmission electron microscope, operating at 200 kV, equipped with a field emission gun was used. Alternatively, a JEOL 4010 operated at 400 kV equipped with a LaB 6 cathode was applied. The NMR experiments have been carried out by using an AVANCE 600 spectrometer (Bruker, Billerica, MA) using a double-resonant 7-mm probe at sample rotation magic angle spinning (MAS) frequencies of 6.5 kHz.1 H MAS NMR spectr...

Crystalline hexagonal-shaped superstructures of calcium carbonate, synthesized in the presence of ammonia, are shown to be assembled by a three-dimensional oriented attachment of vaterite nanoparticles. This unusual crystallographic lock-in mechanism enables the formation of complicated rounded structures with a crystallographic orientation from nanosized building blocks, which has so far only been found for transition metal systems.

Synthetic nacre morphologically indistinguishable from the natural archetype was synthesized with amorphous calcium carbonate precursors by confinement in the scaffold of the original insoluble nacre matrix. The precursors were generated using a synthetic polyelectrolyte highlighting the physicochemical aspects of biomineralization.

The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self‐assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi‐core nanoparticles are determined. In addition, a self‐consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower‐shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)2, polyol‐mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long‐term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi‐core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower‐shaped nanoparticles.

Many organisms make use of calcium carbonate as a construction material, and for this purpose are able to selectively control the formation of the different polymorphs of this material. This is not the case for technical processes. Calcite is thermodynamically more stable at ambient pressure and temperature [1] than the other anhydrous CaCO 3 polymorphs (vaterite and aragonite), and thus is most easily obtained with long reaction times. There are some technical procedures which generate vaterite (usually the first polymorph formed as a result of the Ostwald rule of stages) by performing the precipitation along the kinetic pathway and yielding the kinetic metastable vaterite product or by trapping and stabilizing the very early crystals [2] with appropriate stabilizers. However, the mechanically very interesting aragonite (usually a high-pressure modification) is virtually inaccessible by chemical means, except by adding extreme amounts of Mg 2+ ions to the mother liquor. [3,4] It should be recalled that nacre, with its extraordinary mechanical performance, is based on pure aragonite platelets [5] and shows clear long-time stability even in the presence of water. In contrast, aragonite usually starts to transform to calcite within a day or faster depending on the pH value and temperature. [6,7] Thus, one of the main challenges in the crystallization of calcium carbonate remains the synthesis of pure aragonite of uniform size and morphology under ambient conditions. It has previously been described that aragonite is formed in a biomimetic pathway in the presence of several extracted

We review current synthetic routes to magnetic iron oxide nanoparticles for biomedical applications. We classify the different approaches used depending on their ability to generate magnetic particles that are either single-core (containing only one magnetic core, i.e. a single domain nanocrystal) or multi-core (containing several magnetic cores, i.e. single domain nanocrystals). The synthesis of single-core magnetic nanoparticles requires the use of surfactants during the particle generation, and careful control of the particle coating to prevent aggregation. Special attention has to be paid to avoid the presence of any toxic reagents after the synthesis if biomedical applications are intended. Several approaches exist to obtain multi-core particles based on the coating of particle aggregates; nevertheless, the production of multi-core particles with good control of the number of magnetic cores per particle, and of the degree of polydispersity of the core sizes, is still a difficult task. The control of the structure of the particles is of great relevance for biomedical applications as it has a major influence on the magnetic properties of the materials.

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