Mesocrystals in Biominerals and Colloidal Arrays Bergström, Lennart; Sturm (née Rosseeva), Elena V; Salazar-Alvarez, German ...
Accounts of chemical research,
05/2015, Letnik:
48, Številka:
5
Journal Article
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Mesocrystals, which originally was a term to designate superstructures of nanocrystals with a common crystallographic orientation, have now evolved to a materials concept. The discovery that many ...biominerals are mesocrystals generated a large research interest, and it was suggested that mesocrystals result in better mechanical performance and optical properties compared to single crystalline structures. Mesocrystalline biominerals are mainly found in spines or shells, which have to be mechanically optimized for protection or as a load-bearing skeleton. Important examples include red coral and sea urchin spine as well as bones. Mesocrystals can also be formed from purely synthetic components. Biomimetic mineralization and assembly have been used to produce mesocrystals, sometimes with complex hierarchical structures. Important examples include the fluorapatite mesocrystals with gelatin as the structural matrix, and mesocrystalline calcite spicules with impressive strength and flexibility that could be synthesized using silicatein protein fibers as template for calcium carbonate deposition. Self-assembly of nanocrystals can also result in mesocrystals if the nanocrystals have a well-defined size and shape and the assembly conditions are tuned to allow the nanoparticles to align crystallographically. Mesocrystals formed by assembly of monodisperse metallic, semiconducting, and magnetic nanocrystals are a type of colloidal crystal with a well-defined structure on both the atomic and mesoscopic length scale. Mesocrystals typically are hybrid materials between crystalline nanoparticles and interspacing amorphous organic or inorganic layers. This structure allows to combine disparate materials like hard but brittle nanocrystals with a soft and ductile amorphous material, enabling a mechanically optimized structural design as realized in the sea urchin spicule. Furthermore, mesocrystals can combine the properties of individual nanocrystals like the optical quantum size effect, surface plasmon resonance, and size dependent magnetic properties with a mesostructure and morphology tailored for specific applications. Indeed, mesocrystals composed of crystallographically aligned polyhedral or rodlike nanocrystals with anisotropic properties can be materials with strongly directional properties and novel collective emergent properties. An additional advantage of mesocrystals is that they can combine the properties of nanoparticles with a structure on the micro- or macroscale allowing for much easier handling. In this Account, we propose that mesocrystals are defined as “a nanostructured material with a defined long-range order on the atomic scale, which can be inferred from the existence of an essentially sharp wide-angle diffraction pattern (with sharp Bragg peaks) together with clear evidence that the material consists of individual nanoparticle building units”. We will give several examples of mesocrystals and discuss the structural characteristics for biominerals, biomimetic materials, and colloidal arrays of nanocrystals. The potential of the mesocrystal materials concept in other areas will be discussed and future developments envisioned.
In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of ...mesocrystals and their relations with other classes of solid state materials in terms of their structure. Building up on this, we comment on the problems in mesocrystal research both fundamental and methodological. Additionally, we make the short overview of the mesocrystal formation principles and synthetic routes used for their fabrications. As an outlook into the future, we highlight the most notable trends in mesocrystal research and developments.
Cholinergic neurons of the medial forebrain are considered important contributors to brain plasticity and neuromodulation. A reduction of cholinergic innervation can lead to pathophysiological ...changes of neurotransmission and is observed in Alzheimer's disease. Here we report on six patients with mild to moderate Alzheimer's disease (AD) treated with bilateral low-frequency deep brain stimulation (DBS) of the nucleus basalis of Meynert (NBM). During a four-week double-blind sham-controlled phase and a subsequent 11-month follow-up open label period, clinical outcome was assessed by neuropsychological examination using the Alzheimer's Disease Assessment Scale-cognitive subscale as the primary outcome measure. Electroencephalography and (18)F-fluoro-desoxyglucose positron emission tomography were, besides others, secondary endpoints. On the basis of stable or improved primary outcome parameters twelve months after surgery, four of the six patients were considered responders. No severe or non-transitional side effects related to the stimulation were observed. Taking into account all limitations of a pilot study, we conclude that DBS of the NBM is both technically feasible and well tolerated.
Amorphous iron‐calcium phosphate (Fe‐ACP) plays a vital role in the mechanical properties of teeth of some rodents, which are very hard, but its formation process and synthetic route remain unknown. ...Here, the synthesis and characterization of an iron‐bearing amorphous calcium phosphate in the presence of ammonium iron citrate (AIC) are reported. The iron is distributed homogeneously on the nanometer scale in the resulting particles. The prepared Fe‐ACP particles can be highly stable in aqueous media, including water, simulated body fluid, and acetate buffer solution (pH 4). In vitro study demonstrates that these particles have good biocompatibility and osteogenic properties. Subsequently, Spark Plasma Sintering (SPS) is utilized to consolidate the initial Fe‐ACP powders. The results show that the hardness of the ceramics increases with the increase of iron content, but an excess of iron leads to a rapid decline in hardness. Calcium iron phosphate ceramics with a hardness of 4 GPa can be achieved, which is higher than that of human enamel. Furthermore, the ceramics composed of iron‐calcium phosphates show enhanced acid resistance. This study provides a novel route to prepare Fe‐ACP, and presents the potential role of Fe‐ACP in biomineralization and as starting material to fabricate acid‐resistant high‐performance bioceramics.
An iron‐bearing amorphous calcium phosphate (Fe‐ACP) is prepared in the presence of ammonium iron citrate. The Fe‐ACP particles can be highly stable in water and simulated body fluid. The particles show good biocompatibility and osteogenic properties. By applying the Fe‐ACP particles as initial powders, calcium iron phosphate ceramics with superior mechanical strength and acid resistance are prepared.
Monetite (CaHPO4, DCPA) and hydroxyapatite (Ca10(PO4)6(OH)2, HAp) are promising biomaterials with wide application in the biomedical field. In this study, we propose a novel method to prepare ...fiber-like DCPA without organic additives for the first time and show that nanostructured HAp can be synthesized without changing the macroscopic morphology of DCPA. Our results show that the temperature, pH, calcium to phosphate ratio (Ca/P), and concentration of NH4 + are essential factors in the formation of fiber-like DCPA. The synthesized materials were characterized by powder X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy, and the DCPA → HAp phase transformation process was examined. The growth of HAp on the DCPA crystal can be regulated by the composition of the hydrolyzing solution, forming HAp with its c-axis perpendicular HAp (001)//DCPA (100) or parallel HAp (001)//DCPA (010) to the elongation direction of the DCPA crystal. At a high concentration of Ca2+ in NH4OH solution, octacalcium phosphate was observed as an intermediate phase with its c-axis parallel to elongation of the initial DCPA crystal. The HAp crystals formed after complete transformation of the precursor phases have a preferred crystallographic orientation with c-axis coaxial to the elongation direction of DCPA. In the concentrated NH4OH solution, the direct transformation of DCPA to HAp was observed. At initial stages of the hydrolysis, the c-axis of newly formed HAp is perpendicular to the elongation direction of DCPA. This study provides a new insight into regulation of the crystal growth of DCPA without organic additives and a facile route to form HAp with hierarchical structures.
Inspired by chains of ferrimagnetic nanocrystals (NCs) in magnetotactic bacteria (MTB), the synthesis and detailed characterization of ferrimagnetic magnetite NC chain‐like assemblies is reported. An ...easy green synthesis route in a thermoreversible gelatin hydrogel matrix is used. The structure of these magnetite chains prepared with and without gelatin is characterized by means of transmission electron microscopy, including electron tomography (ET). These structures indeed bear resemblance to the magnetite assemblies found in MTB, known for their mechanical flexibility and outstanding magnetic properties and known to crystallographically align their magnetite NCs along the strongest magnetization easy axis. Using electron holography (EH) and angular dependent magnetic measurements, the magnetic interaction between the NCs and the generation of a magnetically anisotropic material can be shown. The electro‐ and magnetostatic modeling demonstrates that in order to precisely determine the magnetization (by means of EH) inside chain‐like NCs assemblies, their exact shape, arrangement and stray‐fields have to be considered (ideally obtained using ET).
The magnetite chain‐like structures synthesized in a gelatin hydrogel matrix provide unique similarities to the magnetite assemblies found in magnetotactic bacteria. Using electron holography and angular dependent magnetic measurements, the magnetic interaction between the crystallites as well as the generation of a magnetically anisotropic material is demonstrated.
The crystallization of calcium oxalates (weddellite and whewellite) by interaction of calcite marble with fungus Aspergillus niger, one of the most active stone destructors, was studied under in ...vitro conditions. The temporal development of acid production of fungus as well as the sequence of formation and morphogenesis of the growing oxalate hydrates crystals were investigated in detail. Furthermore, the relationships between morphology and growth conditions of crystals within the biofilms on the surface of carbonate rocks are discussed.
This letter describes the formation and detailed characterization of iron oxide mesocrystals produced by the directed assembly of superparamagnetic iron oxide-truncated nanocubes using the slow ...evaporation of the solvent within an externally applied homogeneous magnetic field. Anisotropic mesocrystals with an elongation along the direction of the magnetic field can be produced. The structure of the directed mesocrystals is compared to self-assembled mesocrystalline films, which are formed without the influence of a magnetic field. The remarkable structural difference of mesocrystals produced within the external magnetic field from those self-assembled without field indicates that the specific nanoparticle ordering within the superstructure is driven by competing of two types of anisotropic interactions caused by particle shape (i.e., faceting) and orientation of the magnetic moment (i.e., easy axes:
). Hence, these findings provide a fundamental understanding of formation mechanisms and structuring of mesocrystals built up from superparamagnetic nanoparticles and how a magnetic field can be used to design anisotropic mesocrystals with different structures.
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