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    ABSTRACT: Nanopores have emerged over the past two decades to become an important technique in single molecule experimental physics and biomolecule sensing. Recently DNA nanotechnology, in particular DNA origami, has been used for the formation of nanopores in insulating materials. DNA origami is a very attractive technique for the formation of nanopores since it enables the construction of 3D shapes with precise control over geometry and surface functionality. DNA origami has been applied to nanopore research by forming hybrid architectures with solid state nanopores and by direct insertion into lipid bilayers. This review discusses recent experimental work in this area and provides an outlook for future avenues and challenges.
    FEBS Letters 10/2014; 588(19). DOI:10.1016/j.febslet.2014.06.013
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    ABSTRACT: In this opinions piece we give an introduction to the topic of spin-orbit torques, reviewing the most important experiments and providing an outlook for the future.
    Nano Today 04/2014; 9(2). DOI:10.1016/j.nantod.2014.02.011
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    ABSTRACT: A general semiconductor-independent two-dimensional character of the carrier distribution in top-gate polymer field-effect transistors is revealed by analysing temperature-dependent transfer characteristics and the sub-bandgap absorption tails of the polymer semiconductors. A correlation between the extracted width of the density of states and the Urbach energy is presented, corroborating the 2D accumulation layer and demonstrating an intricate connection between optical measurements concerning disorder and charge transport in transistors.
    Advanced Materials 02/2014; 26(5). DOI:10.1002/adma.201303060
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    ABSTRACT: Understanding mechanosensitivity (i.e., how cells sense the stiffness of their environment) is very important, yet there is a fundamental difficulty in understanding its mechanism: to measure an elastic modulus one requires two points of application of force-a measuring and a reference point. The cell in contact with substrate has only one (adhesion) point to work with, and thus a new method of measurement needs to be invented. The aim of this theoretical work is to develop a self-consistent physical model for mechanosensitivity, a process by which a cell detects the mechanical stiffness of its environment (e.g., a substrate it is attached to via adhesion points) and generates an appropriate chemical signaling to remodel itself in response to this environment. The model uses the molecular mechanosensing complex of latent TGF-β attached to the adhesion point as the biomarker. We show that the underlying Brownian motion in the substrate is the reference element in the measuring process. The model produces a closed expression for the rate of release of active TGF-β, which depends on the substrate stiffness and the pulling force coming from the cell in a subtle and nontrivial way. It is consistent with basic experimental data showing an increase in signal for stiffer substrates and higher pulling forces. In addition, we find that for each cell there is a range of stiffness where a homeostatic configuration of the cell can be achieved, outside of which the cell either relaxes its cytoskeletal forces and detaches from the very weak substrate, or generates an increasingly strong pulling force through stress fibers with a positive feedback loop on very stiff substrates. In this way, the theory offers the underlying mechanism for the myofibroblast conversion in wound healing and smooth muscle cell dysfunction in cardiac disease.
    Biophysical Journal 01/2014; 106(1):124-33. DOI:10.1016/j.bpj.2013.10.042
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    ABSTRACT: Recent biophysical approaches have provided key insights into the enthalpic and entropic forces that compact the nucleoid in the cell. Our biophysical approach combines two complementary, non-invasive and label-free techniques: a precisely timed steerable optical trap and a high throughput microcapillary Coulter counter. We demonstrate the ability of the latter technique to probe the physical properties and size of many purified nucleoids, at the individual nucleoid level. The DNA-binding protein H-NS is central to the organization of the bacterial genome. Our results show that nucleoids purified from the Δhns strain in the stationary phase expand approximately five fold more than the form observed in WT bacteria. This compaction is consistent with the role played by H-NS in regulating the nucleoid structure and the significant organizational changes that occur as the cell adapts to the stationary phase. We also study the permeability to the flow of ions and find that in the experiment nucleoids behave as solid colloids.
    Integrative Biology 12/2013; 6(8). DOI:10.1039/c3ib40147b
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    ABSTRACT: We show that the equilibrium Poisson ratio of electrically neutral gels depends on their shear modulus. When immersed in a good solvent, gels increase their volume on imposed external deformation, but stiffer gels swell less and exhibit a larger Poisson ratio, closer to 0.5, while the gels with a higher solvent content (and correspondingly lower shear modulus) approach a Poisson ratio of 0.25. We monitor the full process of stress and shape relaxation after an instantaneous deformation by using the technique of digital image correlation (DIC), and show that the amount of stress relaxation in uniaxially strained gels is proportional to the shear modulus of the free swollen state and a change in effective strain. Experiments were conducted on polyacrylamide (PAAm) gels in a custom built setup to give the Poisson ratio to high accuracy and time resolution, as well as verification of homogeneous deformation in equilibrium. In addition to water, hydrophilic gels were stretched in three poor solvents: silicone oil, mineral oil, and in air. All three exhibited water loss on imposed deformation and a resulting increase in stress, with mineral oil presenting the smallest change due to its lower permeability to water. Mineral oil and silicone oil are of particular interest as they are often used in mechanical testing to prevent solvent loss.
    Polymer 12/2013; 54(26):6954–6960. DOI:10.1016/j.polymer.2013.11.006
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    ABSTRACT: Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
    Nature Chemistry 12/2013; 5(12):1019-1024. DOI:10.1038/nchem.1801
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    ABSTRACT: A fast dark-field scattering technique capturing full broadband spectra with millisecond time-resolution enables us to monitor the growth or assembly of single nano-objects in situ and in real time. Applying this technique to study the growth of single gold nanorods, together with scanning electron microscopy and finite-difference time-domain simulations, reveals precise quantitative information about gold nanorod growth kinetics.
    Small 11/2013; 9(22). DOI:10.1002/smll.201300958
  • Nature Reviews Molecular Cell Biology 11/2013; 14(12):754. DOI:10.1038/nrm3706
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    ABSTRACT: The problem of transport through nanochannels is one of the major questions in cell biology, with a wide range of applications. In this paper we discuss the process of spontaneous translocation of molecules (Brownian particles) by ratcheted diffusion: a problem relevant for protein translocation along bacterial flagella or injectosome complex, or DNA translocation by bacteriophages. We use molecular dynamics simulations and statistical theory to identify two regimes of transport: at low rate of particle injection into the channel the process is controlled by the individual diffusion towards the open end (the first passage problem), while at a higher rate of injection the crowded regime sets in. In this regime the particle density in the channel reaches a constant saturation level and the resistance force increases substantially, due to the osmotic pressure build-up. To achieve a steady-state transport, the apparatus that injects new particles into a crowded channel has to operate with an increasing power consumption, proportional to the length of the channel and the required rate of transport. The analysis of resistance force, and accordingly - the power required to inject the particles into a crowded channel to overcome its clogging, is also relevant for many microfluidics applications.
    Scientific Reports 10/2013; 3:3103. DOI:10.1038/srep03103
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