Ingolf Mönch

Construction of highly conductive nanowires on a DNA template

We present measurements of the electrical conductivity of metallic nanowires which have been fabricated by chemical deposition of a thin continuous palladium film onto single DNA molecules to install electrical functionality. The DNA molecules have been positioned between macroscopic Au electrodes and are metallized afterwards. Low-resistance electrical interfacing was obtained by pinning the nanowires at the electrodes with electron-beam-induced carbon lines. The investigated nanowires exhibit ohmic transport behavior at room temperature. Their specific conductivity is only one order of magnitude below that of bulk palladium, confirming that DNA is an ideal template for the production of electric wires, which can be utilized for the bottom-up construction of miniaturized electrical circuits.

Catalytic Janus motors on microfluidic chip: deterministic motion for targeted cargo delivery.

We fabricated self-powered colloidal Janus motors combining catalytic and magnetic cap structures, and demonstrated their performance for manipulation (uploading, transportation, delivery) and sorting of microobjects on microfluidic chips. The specific magnetic properties of the Janus motors are provided by ultrathin multilayer films that are designed to align the magnetic moment along the main symmetry axis of the cap. This unique property allows a deterministic motion of the Janus particles at a large scale when guided in an external magnetic field. The observed directional control of the motion combined with extensive functionality of the colloidal Janus motors conceptually opens a straightforward route for targeted delivery of species, which are relevant in the field of chemistry, biology, and medicine.

Synthesis and properties of filled carbon nanotubes

Abstract Single- and multi-walled carbon nanotubes are very interesting nanoscaled materials with many possible applications in nanoelectronics. Especially, nanotubes filled with ferromagnetic materials (Fe, Co, Ni) may have significant potential in data storage. Such structures may help to exceed the best available storage densities (>65 Gb/inch2) and show in the case of Fe-filled nanotubes higher coercivities compared to bulk Fe. In addition, metal-filled carbon nanotubes are promising nanowires with excellent oxidation protection. In this paper we describe the synthesis of Fe-, Ni- and Co-filled carbon nanotubes by using the chemical vapor deposition method. Varying the deposition conditions we have obtained filled nanotubes with relatively uniform core diameters and different thicknesses of the carbon walls. The core diameters vary between 15 and 30 nm and the thickness of the carbon shells between 2 and 60 nm. The length of the tubes amounts up to 30 μm. The filled carbon nanotubes are characterised by scanning and transmission electron microscopy and energy dispersive X-ray analysis. The magnetic behaviour of the aligned Fe-filled tubes is investigated using alternating gradient magnetometry measurements and electron holography. The hysteresis loops indicate a magnetic anisotropy. The coercivity depends on the direction of the applied magnetic field. The observed enhanced coercivities are significantly higher than in bulk Fe.

Magnetic properties of aligned Fe-filled carbon nanotubes

We report on the magnetic properties of Fe-filled multiwalled carbon nanotubes(MWNTs) grown by chemical vapor deposition(CVD) on Si substrates with ferrocene as precursor. The MWNTs are aligned perpendicularly to the substrate plane. X-ray diffraction analyses indicate the presence of both bcc and fcc iron with a relatively strong texture. Magnetometry measurements show a pronounced magnetic anisotropy with the easy axis perpendicular to the substrate plane and parallel to the axis of the aligned MWNTs, respectively. The low-temperature behavior suggests a negligible coupling between the two iron phases. We accessed the magnetic properties of individual Fe-filled MWNTs by electron holography using a transmission electron microscope(TEM).

Spin-coherent transport in ferromagnetically contacted carbon nanotubes

The spin-coherent quantum transport through multiwall carbon nanotubes contacted by ferromagnetic Co pads is investigated experimentally. At 4.2 K, the devices show a remarkable increase of the magnetoresistance (MR) ratio with decreasing junction bias, reaching a maximum MR ratio of 30% at a junction bias current of 1 nA. The experimental results suggest the transport to be dominated by spin-dependent tunneling processes at the Co/nanotube interfaces and governed by the local magnetization. We also observe an asymmetry of the magnetoresistance peak position and width which is attributed to a local exchange biasing in the electrode material.

Rolled-Up Magnetic Sensor: Nanomembrane Architecture for In-Flow Detection of Magnetic Objects

Detection and analysis of magnetic nanoobjects is a crucial task in modern diagnostic and therapeutic techniques applied to medicine and biology. Accomplishment of this task calls for the development and implementation of electronic elements directly in fluidic channels, which still remains an open and nontrivial issue. Here, we present a novel concept based on rolled-up nanotechnology for fabrication of multifunctional devices, which can be straightforwardly integrated into existing fluidic architectures. We apply strain engineering to roll-up a functional nanomembrane consisting of a magnetic sensor element based on [Py/Cu](30) multilayers, revealing giant magnetoresistance (GMR). The comparison of the sensors characteristics before and after the roll-up process is found to be similar, allowing for a reliable and predictable method to fabricate high-quality ultracompact GMR devices. The performance of the rolled-up magnetic sensor was optimized to achieve high sensitivity to weak magnetic fields. We demonstrate that the rolled-up tube itself can be efficiently used as a fluidic channel, while the integrated magnetic sensor provides an important functionality to detect and respond to a magnetic field. The performance of the rolled-up magnetic sensor for the in-flow detection of ferromagnetic CrO(2) nanoparticles embedded in a biocompatible polymeric hydrogel shell is highlighted.

Geometry sensing by self-organized protein patterns

In the living cell, proteins are able to organize space much larger than their dimensions. In return, changes of intracellular space can influence biochemical reactions, allowing cells to sense their size and shape. Despite the possibility to reconstitute protein self-organization with only a few purified components, we still lack knowledge of how geometrical boundaries affect spatiotemporal protein patterns. Following a minimal systems approach, we used purified proteins and photolithographically patterned membranes to study the influence of spatial confinement on the self-organization of the Min system, a spatial regulator of bacterial cytokinesis, in vitro. We found that the emerging protein pattern responds even to the lateral, two-dimensional geometry of the membrane such that, as in the three-dimensional cell, Min protein waves travel along the longest axis of the membrane patch. This shows that for spatial sensing the Min system does not need to be enclosed in a three-dimensional compartment. Using a computational model we quantitatively analyzed our experimental findings and identified persistent binding of MinE to the membrane as requirement for the Min system to sense geometry. Our results give insight into the interplay between geometrical confinement and biochemical patterns emerging from a nonlinear reaction–diffusion system.

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.

Directional roll-up of nanomembranes mediated by wrinkling.

We investigate the relaxation of rectangular wrinkled thin films intrinsically containing an initial strain gradient. A preferential rolling direction, depending on wrinkle geometry and strain gradient, is theoretically predicted and experimentally verified. In contrast to typical rolled-up nanomembranes, which bend perpendicular to the longer edge of rectangular patterns, we find a regime where rolling parallel to the longer edge of the wrinkled film is favorable. A nonuniform radius of the rolled-up film is well reproduced by elasticity theory and simulations of the film relaxation using a finite element method.

Surface Topology Engineering of Membranes for the Mechanical Investigation of the Tubulin Homologue FtsZ

In spite of their small size, bacteria display highly organized cytoskeletal structures like coils, helices, or rings. Extensive mechanical modeling has been done to explain the occurrence of such specific structures within the small volume of bacterial cells. As they are difficult to image within cells, in vitro reconstitution provides a valuable approach to quantitatively analyze their properties under defined conditions. A particularly interesting cytoskeletal feature is the Z-ring, which plays a key role in cell division for many bacteria. It is composed of FtsZ, a tubulin homologue, and other components and has been implicated in constriction force generation. Mechanisms localizing FtsZ to the center of the cell are known, but how it takes the form of a functional helical or ring-like structure remains unclear. 6] We hypothesized that intrinsically curved FtsZ filaments should initially respond to the native shape of bacteria and align using geometric cues. Thus, we devised a controlled biomimetic platform with membrane-coated glass substrates mimicking biologically relevant curvatures, to elucidate the mechanical properties of membrane-associated FtsZ. We found that E. coli FtsZ is assembled into inherently curved and twisted filaments supporting a helical geometry, which showed preferential orientations at the native bacterial cell-like curvatures. Strikingly, the FtsZ did not recognize smaller curvatures in the same way, but rather oriented themselves at an angle in higher curvatures, which does not support the idea that FtsZ alone is able to exert a constriction force. In recent studies involving high-resolution imaging and cryo-electron microscopy, the “Z-ring” has generally been described as a helical structure. Purified FtsZ has been studied extensively by electron microscopy and atomic force microscopy. Consistently, the EM and AFM images from these studies show curved filaments. Cryo-EM on reconstituted FtsZ filaments in vitro seems to contradict the presence of any local spontaneous curvature. However, in a recent study, Osawa et al. showed the ability of FtsZ filaments with an artificially introduced membrane targeting sequence (MTS) to bend membranes, with an influence of the MTS placement in FtsZ on the membrane bending direction. They used an MTS from MinD at the C-terminus of FtsZ to mimic the recruitment of FtsZ to the membrane by adaptor proteins ZipA or FtsA. Upon shifting the MTS to the Nterminus, they find that the filaments bend the membrane in opposite directions. They interpret this to be caused by a constriction force produced by partial Z-rings. A dividing bacterial cell initially has a curvature of about 2 mm , but proceeds towards much higher curvature values as the cell progresses through division. It is unknown whether a bacterial membrane, fortified with many structural proteins, osmotic pressure, and a cell wall, would be as easily deformed. The spontaneous structure of FtsZ filaments may enable them to organize into highly curved suprastructures by sensing the inner cell-membrane curvature, but they may have to recruit other mechanically active factors for cytokinesis. The distortions observed in previous studies 18] could simply be caused by a bundle of curved filaments bending the flexible membrane towards their own curvature. We first repeated the experiments with MTS-FtsZ on freestanding giant unilamellar vesicle (GUV) membranes, and quantitatively evaluated the induced radii of curvature. We found that the filaments did not bend the membranes when the unilamellar vesicles were isotonic. They aligned into filament networks similar to those on planar supported bilayers (Figure 1 b). Changing the osmotic gradient across the membranes by adding 10 mm glucose decreased intravesicular pressure and relaxed the membrane surface tension. This resulted in a curved topology of the membrane as well as the filaments (Figure 1a,b). Only when the membrane tension was low, under hypertonic conditions, could the filaments [*] Prof. Dr. P. Schwille Dept. Cellular and Molecular Biophysics, Max Planck Institute of Biochemistry Am Klopferspitz 18, 82152 Martinsried (Germany) E-mail: schwille@biochem.mpg.de S. Arumugam, G. Chwastek, C. Ehrig Max Planck Institute for Cell Biology and Genetics Pfotenhauerstrasse 108, 01307 Dresden (Germany) and Biotechnology Center of the TU Dresden Tatzberg 47/51, 01307 Dresden (Germany)