Publikationen

Liquid-phase electron microscopy (LPEM) is capable of imaging native (unstained) protein structure in liquid, but the achievable spatial resolution is limited by radiation damage. This damaging effect is more pronounced when targeting small molecular features than for larger structures. The matter is even more complicated because the critical dose that a sample can endure before radiation damage not only varies between proteins but also critically depends on the experimental conditions. Here, we examined the effect of the electron beam on the observed protein structure for optimized conditions using a liquid sample enclosure assembled from graphene sheets. It has been shown that graphene can reduce the damaging effect of electrons on biological materials. We used radiation sensitive microtubule proteins and investigated the radiation damage on these structures as a function of the spatial frequencies of the observed features with transmission electron microscopy (TEM). Microtubule samples were also examined using cryo-electron microscopy (cryo-TEM) for comparison. We used an electron flux of 11 ± 1–16 ± 1 e–/Å2s and obtained a series of images from the same sample region. Our results show that graphene-encapsulated microtubules can maintain their structural features of spatial frequencies of up to 0.20 nm–1 (5 nm), reflecting protofilaments for electron densities of up to 7.2 ± 1.4 × 102 e–/Å2, an order of magnitude higher than measured for frozen microtubules in amorphous ice.

A graphene liquid cell for transmission electron microscopy (TEM) uses one or two graphene sheets to separate the liquid from the vacuum in the microscope. In principle, graphene is an excellent material for such an application because it allows the highest possible spatial resolution, provides a flexible covering foil, and effectively protects the liquid from evaporating. Examples in open literature have demonstrated atomic-resolution TEM using small liquid pockets and the coverage of whole biological cells with graphene sheets. A total of three different basic types of liquid cells are discerned: (i) one graphene sheet is used to cover a liquid sample supported by a thin membrane of another material (for example, silicon nitride, SiN), (ii) two graphene sheets pressed together leaving liquid pockets with graphene at both sides, and (iii) a spacer material with liquid pockets covered at both sides by graphene. A total of four different process flows are available for liquid cell assembly, but there is not yet a consensus on the best routes, and a number of variations exist. The key step is the transfer of graphene to a liquid sample, which is complicated by practical issues that arise from imperfections in the graphene sheets, such as cracks. This review provides an overview of these different approaches to assembling graphene liquid cells and discusses the main obstacles and ideas to overcome them with the prospect of developing the nanoscale technology needed for graphene liquid cells so that they become available on a routine basis for electron microscopy in liquid. It also provides guidance in selecting the appropriate type of graphene liquid cell and the best assembly method for a specific experiment.

The utilization of abundantly available elements in key technologies is a promising way to save precious and rare metals. Iron and titanium offer the highest abundance of all transition metals in the earth's crust and their application in catalytic processes is preferable regarding sustainable material development. The photocatalytic decontamination of wastewater using visible light-responsive materials is of high interest due to the demand for clean water and the increasing accumulation of harmful organic compounds resulting from medical or industrial waste. Herein, we report on a novel photocatalyst based on the generation of crystalline Fe2O3 and TiO2 on size-optimized colloidal metal-organic framework crystallites. The reusable photocatalyst permits the efficient oxidative degradation of pharmaceutical compounds and toxic dyes under visible light illumination and without the requirement of additives or noble metals. We observed a higher photocatalytic activity for our Fe2O3/TiO2@MIL-101 material than for commercially available Fe2O3, TiO2, and TiO2 (P25).

Two-dimensional atomically flat sheets with a high mechanical flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for non-indenting low-force spectroscopy on micrometer-sized freestanding graphene membranes. The method is based on applying quasi-static voltage ramps with active feedback at low tunneling currents and ultimately relies on the attractive electrostatic force between the tip and the membrane. As a result a bulge-test scenario can be established. The convenience and simplicity of the method relies on the fact that the loading force and the membrane deflection detection are both provided simultaneously by the STM. This permits the continuous measurement of the stress-strain relation. Electrostatic forces applied are typically below 1 nN and the membrane deflection is detected at sub-nanometer resolution. Experiments on single-layer graphene membranes with a strain of 0.1% reveal a two-dimensional elastic modulus E2D = 220 N m-1.

Molecular layering of liquids in nanometer-scale confinement is demonstrated for typical lubricant constituents such as polyalphaolefins (PAO) and an ester by means of atomic force microscopy. Layering is observed in force vs. distance curves for poly-(1-decene) tetramers (PAO6) and undecamers (PAO40) and for a 2-ethylhexyl monoester on graphite, mica, and polished steel surfaces and is compared to the layering of hexadecane and 1-hexadecene. On graphite surfaces, the confined molecules are oriented parallel to the surfaces for all liquids, resulting in layers with a thickness comparable to the diameter of the alkyl chains. On mica, confined hexadecane molecules also lie parallel to the surface, while the molecules in the first layer of 1-hexadecene and PAOs take a more upright orientation. Confinement on the oxidized polished steel surfaces results in a molecular layering which most often resembles the layering on graphite and differs significantly from layering on the ionic oxide mica.

Friction and wear of a commercially available polyetheretherketone were investigated by two different testing approaches, namely the standard pin-on-disc (POD) configuration and an unidirectional pin-on-flat (POF) scratch test, in a wide range of pv-products from 0.001 to 8 MPa m/s under dry sliding condition. It was found that the steady state friction coefficient gained from POD tests slightly decreases with increasing sliding velocity from 0.1 to 1 m/s, further increase in the velocity to 4 m/s results in an obvious raise of the friction coefficient. It is assumed that this increase can be attributed to the high interfacial temperature induced strong adhesion between PEEK surface and steel counterbody. No obvious difference of the friction coefficients between POD and POF tests is noted in the studied range. With respect to the wear rate, the wear rate measured from POD increases with monotonously increasing velocity. Possible reasons for these observations are discussed based on the analysis of the worn surfaces of polymer samples and transfer films formed on the steel counterface as well as the investigations on the thermal characteristics of different tribo-systems.

In this work we report the use of polarization active porous 1D Bragg microcavities (BM) prepared by physical vapor deposition at oblique angles for the optofluidic analysis of liquid solutions. These photonic structures consist of a series of stacked highly porous layers of two materials with different refractive indices and high birefringence. Their operational principle implies filling the pores with the analyzed liquid while monitoring with linearly polarized light the associated changes in optical response as a function of the solution refractive index. The response of both polarization active and inactive BMs as optofluidic sensors for the determination of glucose concentration in water solutions has been systematically compared. Different methods of detection, including monitoring the BM wave retarder behavior, are critically compared for both low and high glucose concentrations. Data are taken in transmission and reflection modes and different options explored to prove the incorporation of these nanostructured transducers into microfluidic systems and/or onto the tip of an optical fiber. This analysis has proven the advantages of the polarization active transducer sensors for the optofluidic analysis of liquids and their robustness even in the presence of light source instabilities or misalignments of the optical system used for detection.

The iminostannylene HNSn was successfully incorporated in a molecular cage of composition (Me2RSi–NSn)3(HNSn) with group R either being a methyl (1) or vinyl (2) substituent. An X-ray structure analysis reveals that 2 consists of a distorted Sn4N4 cube. The Sn–N(H) bond lengths [2.189(2) Å] are in the range for Sn4N4 hetero cubanes. When stored in a toluene solution the clusters 1 and 2 decompose slowly into the symmetric cubanes (Me2RSi–NSn)4 [R = Me (3), CHCH2 (4)] and an amorphous and insoluble powder of composition HNSn. The decomposition follows a first order rate law as established for 2 with a half life time t1/2 = 320 d at 20 °C. The compounds 1 and 2 can thus be regarded as a result of interaction between three entities {Me2RSi–NSn} and one entity {HNSn}. We also isolated the twistane-like Me2Si(NtBu)2Sn2NtBu (5) in a crystalline form. The central structure of this molecule, which has almost C2v symmetry, has a trigonal bipyramid Sn2N3 unit with the nitrogen atoms occupying the equatorial plane. Each nitrogen atom has a tert-butyl ligand and two of the N atoms are further connected by the dimethylsilyl group. There is one nitrogen atom in an almost planar environment (only bonding to tert-butyl and two tin atoms) with a remarkable short Sn–N bond length of 2.048(5) Å. Both tin atoms in cage 5 can bond to Cr(CO)5 to form [Me2Si(NtBu)2Sn2NtBu][Cr(CO)5]2 (6) with an almost linear Cr–Sn···Sn–Cr arrangement and Sn–Cr bond lengths of 2.581(1) Å (X-ray diffraction).