Heiner Friedrich

The role of collagen in bone apatite formation in the presence of hydroxyapatite nucleation inhibitors

Bone is a composite material in which collagen fibrils form a scaffold for a highly organized arrangement of uniaxially oriented apatite crystals. In the periodic 67 nm cross-striated pattern of the collagen fibril, the less dense 40-nm-long gap zone has been implicated as the place where apatite crystals nucleate from an amorphous phase, and subsequently grow. This process is believed to be directed by highly acidic non-collagenous proteins; however, the role of the collagen matrix during bone apatite mineralization remains unknown. Here, combining nanometre-scale resolution cryogenic transmission electron microscopy and cryogenic electron tomography with molecular modelling, we show that collagen functions in synergy with inhibitors of hydroxyapatite nucleation to actively control mineralization. The positive net charge close to the C-terminal end of the collagen molecules promotes the infiltration of the fibrils with amorphous calcium phosphate (ACP). Furthermore, the clusters of charged amino acids, both in gap and overlap regions, form nucleation sites controlling the conversion of ACP into a parallel array of oriented apatite crystals. We developed a model describing the mechanisms through which the structure, supramolecular assembly and charge distribution of collagen can control mineralization in the presence of inhibitors of hydroxyapatite nucleation.

Towards stable catalysts by controlling collective properties of supported metal nanoparticles

Supported metal nanoparticles play a pivotal role in areas such as nanoelectronics, energy storage/conversion and as catalysts for the sustainable production of fuels and chemicals. However, the tendency of nanoparticles to grow into larger crystallites is an impediment for stable performance. Exemplarily, loss of active surface area by metal particle growth is a major cause of deactivation for supported catalysts. In specific cases particle growth might be mitigated by tuning the properties of individual nanoparticles, such as size, composition and interaction with the support. Here we present an alternative strategy based on control over collective properties, revealing the pronounced impact of the three-dimensional nanospatial distribution of metal particles on catalyst stability. We employ silica-supported copper nanoparticles as catalysts for methanol synthesis as a showcase. Achieving near-maximum interparticle spacings, as accessed quantitatively by electron tomography, slows down deactivation up to an order of magnitude compared with a catalyst with a non-uniform nanoparticle distribution, or a reference Cu/ZnO/Al(2)O(3) catalyst. Our approach paves the way towards the rational design of practically relevant catalysts and other nanomaterials with enhanced stability and functionality, for applications such as sensors, gas storage, batteries and solar fuel production.

Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate

Despite its importance in many industrial, geological and biological processes, the mechanism of crystallization from supersaturated solutions remains a matter of debate. Recent discoveries show that in many solution systems nanometre-sized structural units are already present before nucleation. Still little is known about the structure and role of these so-called pre-nucleation clusters. Here we present a combination of in situ investigations, which show that for the crystallization of calcium phosphate these nanometre-sized units are in fact calcium triphosphate complexes. Under conditions in which apatite forms from an amorphous calcium phosphate precursor, these complexes aggregate and take up an extra calcium ion to form amorphous calcium phosphate, which is a fractal of Ca(2)(HPO(4))(3)(2-) clusters. The calcium triphosphate complex also forms the basis of the crystal structure of octacalcium phosphate and apatite. Finally, we demonstrate how the existence of these complexes lowers the energy barrier to nucleation and unites classical and non-classical nucleation theories.

Zeolite Y Crystals with Trimodal Porosity as Ideal Hydrocracking Catalysts

Effektive Poren: Zeolith-Y-Kristalle mit Mikroporen (ca. 1 nm), kleinen (ca. 3 nm) und grosen Mesoporen (ca. 30 nm) wurden aus zuvor mit Dampf und Saure behandeltem Material durch Auslaugen mit Base erhalten. Die Zeolith-Y-Kristalle mit trimodaler Porositat (siehe elektronentomographische Aufnahme) zeigen beim Hydrocracking eine nahezu ideale Selektivitat fur und erhohte Ausbeuten an Kerosin und Diesel.

Electron Tomography for Heterogeneous Catalysts and Related Nanostructured Materials

The full potential in catalyst development will only be realized if characterization techniques are available that can probe materials with subnanometer resolution. One of the most employed techniques to image heterogeneous catalysts at the nanometer and subnanometer scale is transmission electron microscopy (TEM). As suggested by the name, TEM uses electrons transmitted through the object for imaging. Since the interaction between electrons and matter is very strong, only thin parts, commonly much less than a micron in thickness, are imaged. Since heterogeneous catalysts are, in most cases, structured on a much smaller length scale, the sample thickness can be reduced to TEM requirements by appropriate preparation techniques and is, therefore, no limitation.

Imaging of self-assembled structures: Interpretation of TEM and Cryo-TEM images

The investigation of solution-borne nanostructures by transmission electron microscopy (TEM) is a frequently used analytical method in materials chemistry. In many cases, the preparation of the TEM sample involves drying and staining steps, and the collection of images leads to the interaction of the specimen with the electron beam. Both aspects call for cautious interpretation of the resulting electron micrographs. Alternatively, a near-native solvated state can be preserved by cryogenic vitrification and subsequent imaging by low-dose cryogenic TEM. In this Minireview, we provide a critical analysis of sample preparation, and more importantly, of the acquisition and interpretation of electron micrographs. This overview should provide a framework for the application of (cryo)-TEM as a powerful and reliable tool for the analysis of colloidal and self-assembled structures with nanoscopic dimensions.

A chaotic self-oscillating sunlight-driven polymer actuator

Nature provides much inspiration for the design of materials capable of motion upon exposure to external stimuli, and many examples of such active systems have been created in the laboratory. However, to achieve continuous motion driven by an unchanging, constant stimulus has proven extremely challenging. Here we describe a liquid crystalline polymer film doped with a visible light responsive fluorinated azobenzene capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air. The presence of simultaneous illumination by blue and green light is necessary for the oscillating behaviour to occur, suggesting that the dynamics of continuous forward and backward switching are causing the observed effect. Our work constitutes an important step towards the realization of autonomous, persistently self-propelling machines and self-cleaning surfaces powered by sunlight.

Fractal parameters of individual soot particles determined using electron tomography: Implications for optical properties

[1] The morphologies of soot particles are both complex and important. They influence soot atmospheric lifetimes, global distributions, and climate impacts. Particles can have complex geometries with overlapping projecting parts and pores that are difficult to infer from the conventional techniques used to study them. We used electron tomography with a transmission electron microscope (TEM) to determine three-dimensional (3D) properties such as fractal dimension (D f ), radius of gyration (R g ), volume (V), surface area (As), and structural coefficient (k a ) for individual soot particles from the ambient air of an Asian dust (AD) episode and from a U.S. traffic source. The respective median values of D f are 2.4 and 2.2, of R g are 274 and 251 nm, ofA s /Vare 9.2 and 13.7 x 10 7 m -1 , and of k a are 0.67 and 0.71. The corresponding parameters, when calculated from 2D projections such as TEM images, are considerably less precise and commonly erroneous. Unlike other methods that have been used to derive fractal parameters, our method is applicable to particles of any D f . Using the 3D data, we estimate that mass-normalized scattering cross sections of our AD and traffic soot particles are respectively about 15 and 30 times greater than those of unaggregated spheres, which is the shape assumed in global models to estimate radiative forcing. Accurate 3D information can be used to compute more precise optical properties, which are important for estimating direct radiative forcing and improving our understanding of the climate impact of soot.

Observation of a Ternary Nanocrystal Superlattice and Its Structural Characterization by Electron Tomography

The first genuine ternary colloidal crystal (see picture) is composed of PbSe nanocrystals of two different diameters (blue and green) and of CdSe nanocrystals (red). Electron tomography shows that the superlattice is isostructural with the atomic lattice AlMgB4.

Nucleation and Growth of Monodisperse Silica Nanoparticles

Although monodisperse amorphous silica nanoparticles have been widely investigated, their formation mechanism is still a topic of debate. Here, we demonstrate the formation of monodisperse nanoparticles from colloidally stabilized primary particles, which at a critical concentration undergo a concerted association process, concomitant with a morphological and structural collapse. The formed assemblies grow further by addition of primary particles onto their surface. The presented mechanism, consistent with previously reported observations, reconciles the different theories proposed to date.