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Molecular Origin of the Elastic State of Aqueous Hyaluronic Acid
Get PDFThe macroscopic mechanical properties of biological hydrogels are broadly studied and successfully mimicked in synthetic materials, but little is known about the molecular interactions that mediate these properties. Here we use two-dimensional infrared spectroscopy (2D-IR) to study the pH-induced gelation of hyaluronic acid, a ubiquitous biopolymer, which undergoes a transition from a viscous to an elastic state in a narrow pH range around 2.5. We find that the gelation originates from the enhanced formation of strong inter-chain connections, consisting of a double amide-COOH hydrogen-bond and an N-D-COO- hydrogen-bond on the adjacent sugars of the hyaluronan disaccharide unit. We confirm the enhanced inter-chain connectivity in the elastic state by AFM imaging
Energy-Momentum Cathodoluminescence Imaging of Anisotropic Directionality in Elliptical Aluminium Plasmonic Bullseye Antennas
Get PDFWe show that geometric eccentricity can be used to strongly tailor the angular radiation profiles of aluminium plasmonic bullseye antennas. High-resolution energy/momentum maps are recorded using a novel cathodoluminescence Fourier imaging technique. The angular profiles for elliptical bullseyes (ellipticity e = 0, 0.6, 0.8) are well described by a 2D dipole scattering model in which the phase and amplitude of the scattering from the bullseye grooves dictates the angular profile at a given energy. We show that geometric eccentricity is an important parameter to control the radiation profile of bullseye antennas. The new energy-momentum cathodoluminescence imaging technique can be used to map the optical properties of a wide range of dispersive and anisotropic systems, paving the way for a broad range of studies on complex nanophotonic systems
Automated Tracking of Biopolymer Growth and Network Deformation with TSOAX
Get PDFStudies of how individual semi-flexible biopolymers and their network assemblies change over time reveal dynamical and mechanical properties important to the understanding of their function in tissues and living cells. Automatic tracking of biopolymer networks from fluorescence microscopy time-lapse sequences facilitates such quantitative studies. We present an open source software tool that combines a global and local correspondence algorithm to track biopolymer networks in 2D and 3D, using stretching open active contours. We demonstrate its application in fully automated tracking of elongating and intersecting actin filaments, detection of loop formation and constriction of tilted contractile rings in live cells, and tracking of network deformation under shear deformation
Supercoiling DNA optically
Get PDFCellular DNA is regularly subject to torsional stress during genomicprocesses, such as transcription and replication, resulting in arange of supercoiled DNA structures. For this reason, methods toprepare and study supercoiled DNA at the single-molecule levelare widely used, including magnetic, angular-optical, micropipette,and magneto-optical tweezers. However, it is currently challeng-ing to combine DNA supercoiling control with spatial manipulationand fluorescence microscopy. This limits the ability to studycomplex and dynamic interactions of supercoiled DNA. Here wepresent a single-molecule assay that can rapidly and controllablygenerate negatively supercoiled DNA using a standard dual-trapoptical tweezers instrument. This method, termed Optical DNASupercoiling (ODS), uniquely combines the ability to study super-coiled DNA using force spectroscopy, fluorescence imaging of thewhole DNA, and rapid buffer exchange. The technique can be usedto generate a wide range of supercoiled states, with between<5and 70% lower helical twist than nonsupercoiled DNA. Highlight-ing the versatility of ODS, we reveal previously unobserved effectsof ionic strength and sequence on the structural state of under-wound DNA. Next, we demonstrate that ODS can be used to di-rectly visualize and quantify protein dynamics on supercoiledDNA. We show that the diffusion of the mitochondrial transcrip-tion factor TFAM can be significantly hindered by local regions ofunderwound DNA. This finding suggests a mechanism by whichsupercoiling could regulate mitochondrial transcription in vivo.Taken together, we propose that ODS represents a powerfulmethod to study both the biophysical properties and biologicalinteractions of negatively supercoiled DNA
Perfect Absorption and Phase Singularities in Plasmon Antenna Array Etalons
Get PDFWe present an interferometrically resolved study of the amplitude and phase response of plasmon array etalons composed of a reflective surface with in front of it a metasurface of resonant plasmonic dipole antennas. We find that above a minimum antenna oscillator strength (set by antenna size and density), such structures show conditions of perfect absorption. Contrary to earlier findings on perfect absorption, we find that these singular points unavoidably come in pairs, and are associated with a phase singularity in the parameter space spanned by frequency and etalon spacing. The topologically oppositely charged point pairs occur around the geometric Fabry Perot condition. We elucidates the origin of these singularities, and their continuous evolution with oscillator strength in the 2D plane spanned by optical frequency and mirror-antenna spacing. Our findings extend the understanding of Salisbury screens and of ‘pixels’ in reflective metasurfaces for full control of amplitude and phase. Finally, our data demonstrates the limits of transfer-matrix approaches to predicting the response of arbitray stacks of metasurfaces and dielectric layers
Plasmon Nanocavity Array Lasers: Cooperating over Losses and Competing for Gain
Get PDFPlasmon nanocavity array lasers leverage the combination of locally enhanced electromagnetic fields at localized particle plasmons with collective diffractive effects in periodic lattice geometries for low-threshold lasing with excellent coherence, line width, and directivity. This combination is enabled by the collective reduction of ohmic and radiative loss of plasmon antennas that hybridize to form surface lattice resonances. At the same time, candidate lasing modes compete for gain in the tight confines of the unit cell, where electromagnetic fields and population inversion are strongly structured in space, time, and polarization. This Perspective reviews the state of the art in understanding and manipulating this balance to combat losses and to optimize gain
Development of photovoltaic technologies for global impact
Get PDFPhotovoltaic solar energy (PV) is expected to play a key role in the future global sustainable energy system. It has demonstrated impressive developments in terms of the scale of deployment, cost reduction and performance enhancement, most visibly over the past decade. PV conversion is and can be done with a wide range of materials, device architectures and technologies, at very different levels of technical and economic maturity. In this context it is customary to distinguish between first, second, third, and sometimes even fourth generation PV. This has initially been very useful to clarify the complex and, for many, confusing landscape of PV. In this paper it is argued, however, that in view of actual developments in PV over the past few decades there are good reasons to adopt another approach, that does more justice to the role and potential of existing and new PV concepts and technologies
Combined metagratings for efficient broad-angle scattering metasurface
Get PDFSpectrally controlled diffusion and reflection of light are key operations for light management in many optical devices. Integration of this operation in complex nanophotonic devices requires a 2D interface that provides tailored spectrum and directivity control. Here, we present a metagrating superstructure that realizes a resonant light reflector with tailored angular scattering profile. Millimeter-sized metasurfaces are built from arrays of combined supercells of 20-50 μm, composed of 5-7 differently pitched metagratings that tailor at will and with large efficiency the angular response. Each supercell is composed of one or more Si Mie resonators, arranged in a periodic array above a Ag back plane and tailored to resonantly scatter light at 650 nm into only the ±1 diffraction orders with very high efficiency. By varying the pitch and supercell design, we can tailor the overall metasurface reflection profile with large flexibility, realizing a broad-angle Lambertian-type scattering metasurface, as well as a large-angle (35-75°) scattering metasurface, both with resonant optical scattering efficiencies above 70%. These ultra-thin structures, fabricated using thin-film deposition, electron beam lithography and reactive ion etching, can find applications for light trapping and spectrum splitting in solar cells and other devices
CLASP stabilizes microtubule plus ends created by serving to drive cortical array reorientation
Get PDFCentral to building and reorganizing cytoskeletal arrays is the creation of new polymers. While nucleation has been the major focus of study for new microtubule generation, severing has been proposed as an alternative mechanism to create new polymers, a mechanism recently shown to drive the reorientation of cortical arrays of higher plants in response to blue light perception. As severing produces new plus ends behind the stabilizing GTP-cap, an important and unanswered question is how these are stabilized in vivo to promote net microtubule generation. Here we identify the conserved protein CLASP as a potent stabilizer of new plus ends created by katanin severing and find that CLASP is required for rapid cortical array reorientation. In clasp mutants both rescue of shrinking plus ends and the regrowth of plus ends immediately after severing are reduced, computational modeling reveals that it is the specific stabilization of severed ends that explains CLASP`s function in promoting microtubule amplification by severing and cortical array reorientation