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Monday, September 17, 2018

Optical trapping and orientation manipulation of 2D inorganic materials using a linearly polarized laser beam

Tominaga, Makoto; Higashi, Yuki; Kumamoto, Takuya; Nagashita, Takashi; Nakato, Teruyuki; Suzuki, Yasutaka; Kawamata, Jun

Because inorganic nanosheets, such as clay minerals, are anisotropic, the manipulation of nanosheet orientation is an important challenge in order to realize future functional materials. In the present study, a novel methodology for nanosheet manipulation using laser radiation pressure is proposed. When a linearly polarized laser beam was used to irradiate a niobate (Nb6O17 4-) nanosheet colloid, the nanosheet was trapped at the focal point so that the in-plane direction of the nanosheet was oriented parallel to the propagation direction of the incident laser beam so as to minimize the scattering force. In addition, the trapped nanosheet was aligned along the polarization direction of the linearly polarized laser beam. Thus, unidirectional alignment of a nanosheet can be achieved simply by irradiation using a laser beam.

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Miniature scanning light-sheet illumination implemented in a conventional microscope

Anjan Bhat Kashekodi, Tobias Meinert, Rebecca Michiels, and Alexander Rohrbach

Living cells are highly dynamic systems responding to a large variety of biochemical and mechanical stimuli over minutes, which are well controlled by e.g. optical tweezers. However, live cell investigation through fluorescence microscopy is usually limited not only by the spatial and temporal imaging resolution but also by fluorophore bleaching. Therefore, we designed a miniature light-sheet illumination system that is implemented in a conventional inverted microscope equipped with optical tweezers and interferometric tracking to capture 3D images of living macrophages at reduced bleaching. The horizontal light-sheet is generated with a 0.12 mm small cantilevered mirror placed at 45° to the detection axis. The objective launched illumination beam is reflected by the micro-mirror and illuminates the sample perpendicular to the detection axis. Lateral and axial scanning of both Gaussian and Bessel beams, together with an electrically tunable lens for fast focusing, enables rapid 3D image capture without moving the sample or the objective lens. Using scanned Bessel beams and line-confocal detection, an average axial resolution of 0.8 µm together with a 10-15 fold improved image contrast is achieved.

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Plasmonic Cube Tetramers over Substrates: Reversal of Binding Force as the Effect of Fano Resonance and Light Polarization

Sakin S. Satter, M. R. C. Mahdy, M. A. R. Ohi, Farhan Islam, and Hamim Mahmud Rivy

Near-field optical binding force is an emerging new topic in the field of optical manipulation and plasmonics. However, so far, all the studies of near-field binding force and its counterintuitive reversal are only restricted to dimer sets. In this work, we have studied extensively the idea of near-field optical binding force and associated Lorentz force dynamics for more than two objects, such as plasmonic tetramers over different substrates. We have demonstrated that if closely positioned plasmonic cube tetramers are placed only over the plasmonic substrate and the circularly polarized light is impinged over them, all-optical control between their mutual attraction and repulsion is possible because of strong Fano resonance. In addition, the polarization state of light controls the shifting of the extinction spectra and the binding force reversal wavelength, making such nanostructures ideal for the polarization-dependent optical switching device. The high magnitude of attractive and repulsive binding forces has been obtained at the dark and bright resonant modes, respectively, because of strong induced currents in the plasmonic substrate. Because of its simple arrangement, our proposed tetramer configuration may open a novel route for all-optical particle clustering, aggregation, and crystallization, which can be verified by the simple experimental setup.

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Robust increase of the optical forces in waveguide-based optical tweezers using V-groove structure

Mahdi Sahafi and Amir Habibzadeh-Sharif

Waveguide-based optical tweezers have significant advantages for particle trapping and transporting in optofluidic chips due to their simple fabrication process. However, for effective trapping of nanoparticles, a high-power input field should be applied, which limits their applications. Here, we numerically show that a silicon-on-insulator-based V-groove waveguide has a much higher capability to trap nanoparticles compared with the traditional waveguides. According to the calculations, the trapping force exerted by the V-groove waveguide to a 5 nm radius nanoparticle can be 14 times greater than that of the strip waveguide. The scattering force is five times larger in the same conditions. This structure would be useful to be integrated into a lab-on-a-chip system to form a particle delivery and analysis device.

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Fibrin network adaptation to cell-generated forces

Fransisca A. S. van Esterik, Arianne V. Vega, Kristian A. T. Pajanonot, Daniel R. Cuizon, Michelle E. Velayo, Jahazel Dejito, Stephen L. Flores, Jenneke Klein-Nulend, Rommel G. Bacabac

Fibrin promotes wound healing by serving as provisional extracellular matrix for fibroblasts that realign and degrade fibrin fibers, and sense and respond to surrounding substrate in a mechanical-feedback loop. We aimed to study mechanical adaptation of fibrin networks due to cell-generated forces at the micron-scale. Fibroblasts were elongated-shaped in networks with ≤ 2 mg/ml fibrinogen, or cobblestone-shaped with 3 mg/ml fibrinogen at 24 h. At frequencies f < 102 Hz, G′ of fibroblast-seeded fibrin networks with ≥ 1 mg/ml fibrinogen increased compared to that of fibrin networks. At frequencies f > 103 Hz, G″ of fibrin networks decreased with increasing concentration following the power-law in frequency with exponents ranging from 0.75 ± 0.03 to 0.43 ± 0.03 at 3 h, and of fibroblast-seeded fibrin networks with exponents ranging from 0.56 ± 0.08 to 0.28 ± 0.06. In conclusion, fibroblasts actively contributed to a change in viscoelastic properties of fibrin networks at the micron-scale, suggesting that the cells and fibrin network mechanically interact. This provides better understanding of, e.g., cellular migration in wound healing.

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Cancer cell ability to mechanically adjust to extracellular matrix stiffness correlates with their invasive potential

Lena Wullkopf, Ann-Katrine V. West, Natascha Leijnse, Thomas R. Cox, Chris D. Madsen, Lene B. Oddershede, and Janine T. Erler

Increased tissue stiffness is a classic characteristic of solid tumors. One of the major contributing factors is increased density of collagen fibers in the extracellular matrix (ECM). Here, we investigate how cancer cells biomechanically interact with and respond to the stiffness of the ECM. Probing the adaptability of cancer cells to altered ECM stiffness using optical tweezers based micro-rheology and deformability cytometry, we find that only malignant cancer cells have the ability to adjust to collagen matrices of different densities. Employing micro-rheology on the biologically relevant spheroid invasion assay, we can furthermore demonstrate that even within a cluster of cells of similar origin there are differences in the intracellular biomechanical properties dependent on the cells’ invasive behavior. We reveal a consistent increase of viscosity in cancer cells leading the invasion into the collagen matrices in comparison to cancer cells following in the stalk or remaining in the center of the spheroid. We hypothesize that this differential viscoelasticity might facilitate spheroid tip invasion through a dense matrix. These findings highlight the importance of the biomechanical interplay between cells and their microenvironment for tumor progression.

DOI

Friday, August 31, 2018

Nonlinearity-Induced Multiplexed Optical Trapping and Manipulation with Femtosecond Vector Beams

Yuquan Zhang, Junfeng Shen, Changjun Min, Yunfeng Jin, Yuqiang Jiang, Jun Liu, Siwei Zhu, Yunlong Sheng, Anatoly V. Zayats, and Xiaocong Yuan

Optical trapping and manipulation of atoms, nanoparticles, and biological entities are widely employed in quantum technology, biophysics, and sensing. Single traps are typically achieved with linearly polarized light, while vortex beams form rotationally unstable symmetric traps. Here we demonstrate multiplexed optical traps reconfigurable with intensity and polarization of the trapping beam using intensity-dependent polarizability of nanoparticles. Nonlinearity combined with a longitudinal field of focused femtosecond vortex beams results in a stable optical force potential with multiple traps, in striking contrast to a linear trapping regime. The number of traps and their orientation can be controlled by the cylindrical vector beam order, polarization, and intensity. The nonlinear trapping demonstrated here on the example of plasmonic nanoparticles opens up opportunities for deterministic trapping and polarization-controlled manipulation of multiple dielectric and semiconductor particles, atoms, and biological objects since most of them exhibit a required intensity-dependent refractive index.

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Response to Bianco et al.: Interaction Forces between F-actin and Titin PEVK Domain Measured with Optical Tweezers

Kenneth S. Campbell, Martin Lakie

A recent publication in Biophysical Journal by Bianco et al. (“Interaction forces between F-actin and titin PEVK domain measured with optical tweezers”) shows that the PEVK domain of titin molecules interacts with F-actin. This newly discovered behavior could influence the mechanical properties of striated muscles, and Bianco et al. suggest that the interactions between actin and titin could modulate thixotropic behavior. In this Comment to the Editor, we suggest that the thixotropic properties of striated muscles in vivo are more likely to reflect dynamic changes in the proportion of myosin cross-bridges bound between the myofilaments.

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Pause sequences facilitate entry into long-lived paused states by reducing RNA polymerase transcription rates

Ronen Gabizon, Antony Lee, Hanif Vahedian-Movahed, Richard H. Ebright & Carlos J. Bustamante

Transcription by RNA polymerase (RNAP) is interspersed with sequence-dependent pausing. The processes through which paused states are accessed and stabilized occur at spatiotemporal scales beyond the resolution of previous methods, and are poorly understood. Here, we combine high-resolution optical trapping with improved data analysis methods to investigate the formation of paused states at enhanced temporal resolution. We find that pause sites reduce the forward transcription rate of nearly all RNAP molecules, rather than just affecting the subset of molecules that enter long-lived pauses. We propose that the reduced rates at pause sites allow time for the elongation complex to undergo conformational changes required to enter long-lived pauses. We also find that backtracking occurs stepwise, with states backtracked by at most one base pair forming quickly, and further backtracking occurring slowly. Finally, we find that nascent RNA structures act as modulators that either enhance or attenuate pausing, depending on the sequence context.

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Liquid–liquid phase separation and evaporation of a laser-trapped organic–organic airborne droplet using temporal spatial-resolved Raman spectroscopy

Aimable Kalume, Chuji Wang, Joshua Santarpia and Yong-Le Pan

Chemical reactions in aerosol particles can occur between the reactive components of the particle or between the particle and its surrounding media. The fate of atmospheric aerosols depends on the environment, the composition and the distribution of components within a particle. It could be very interesting to see how a liquid aerosol particle behaves in ambient air if the particle is composed of mixed chemicals. Do the chemical components remain homogeneously mixed within a particle or separate as the mixed liquid is aerosolized? How do the chemicals within a droplet separate and interact with the air? In this paper, a single microdroplet formed from an organic–organic mixture of diethyl phthalate (DEPh) and glycerol was investigated using laser-trapped position-resolved temporal Raman spectroscopy. For the first time, we were able to directly observe the gradient distributions of the two chemicals at different positions within such an airborne droplet, their time-resolved processes of liquid–liquid phase-separation, and changes of the physical microstructure and chemical micro-composition in the droplet. The results revealed that DEPh migrated to the surface and formed an outer layer and glycerol was more concentrated in the interior of the droplet, DEPh evaporated faster than glycerol, and both organic chemicals within the mixed droplet evaporated faster than either of them within their pure droplets. This technique also provides a new method for studying the fine structure and chemical reactions of different molecules taking place inside a particle and at the interface of a particle with the surrounding microenvironment.

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