Meisam Soleimani, Shahab Sahraee, Peter Wriggers
In this paper, a novel 3D numerical method has been developed to simulate red blood cells (RBCs) based on the interaction between a shell-like solid structure and a fluid. RBC is assumed to be a thin shell encapsulating an internal fluid (cytoplasm) which is submerged in an external fluid (blood plasma). The approach is entirely based on the smoothed particle hydrodynamics (SPH) method for both fluid and the shell structure. Both cytoplasm and plasma are taken to be incompressible Newtonian fluid. As the kinematic assumptions for the shell, Reissner–Mindlin theory has been introduced into the formulation. Adopting a total Lagrangian (TL) formulation for the shell in the realm of small strains and finite deflection, the presented computational tool is capable of handling large displacements and rotations. As an application, the deformation of a single RBC while passing a stenosed capillary has been modeled. If the rheological behavior of the RBC changes, for example, due to some infection, it is reflected in its deformability when it passes through the microvessels. It can severely affect its proper function which is providing the oxygen and nutrient to the living cells. Hence, such numerical tools are useful in understanding and predicting the mechanical behavior of RBCs. Furthermore, the numerical simulation of stretching an RBC in the optical tweezers system is presented and the results are verified. To the best of authors’ knowledge, a computational tool purely based on the SPH method in the framework of shell–fluid interaction for RBCs simulation is not available in the literature.
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Wednesday, November 21, 2018
Acidosis affects muscle contraction by slowing the rates myosin attaches to and detaches from actin
Katelyn Jarvis, Mike Woodward, Edward P. Debold, Sam Walcott
The loss of muscle force and power during fatigue from intense contractile activity is associated with, and likely caused by, elevated levels of phosphate ( [Math Processing Error]) and hydrogen ions (decreased pH). To understand how these deficits in muscle performance occur at the molecular level, we used direct measurements of mini-ensembles of myosin generating force in the laser trap assay at pH 7.4 and 6.5. The data are consistent with a mechanochemical model in which a decrease in pH reduces myosin’s detachment from actin (by slowing ADP release), increases non-productive myosin binding (by detached myosin rebinding without a powerstroke), and reduces myosin’s attachment to actin (by slowing the weak-to-strong binding transition). Additional support of this mechanism is found by incorporating it into a branched pathway model for the effects of [Math Processing Error] on myosin’s interaction with actin. Including pH-dependence in one additional parameter (acceleration of [Math Processing Error]-induced detachment), the model reproduces experimental measurements at high and low pH, and variable [Math Processing Error], from the single molecule to large ensemble levels. Furthermore, when scaled up, the model predicts force-velocity relationships that are consistent with muscle fiber measurements. The model suggests that reducing pH has two opposing effects, a decrease in attachment favoring a decrease in muscle force and a decrease in detachment favoring an increase in muscle force. Depending on experimental details, the addition of [Math Processing Error] can strengthen one or the other effect, resulting in either synergistic or antagonistic effects. This detailed molecular description suggests a molecular basis for contractile failure during muscle fatigue.
DOI
The loss of muscle force and power during fatigue from intense contractile activity is associated with, and likely caused by, elevated levels of phosphate ( [Math Processing Error]) and hydrogen ions (decreased pH). To understand how these deficits in muscle performance occur at the molecular level, we used direct measurements of mini-ensembles of myosin generating force in the laser trap assay at pH 7.4 and 6.5. The data are consistent with a mechanochemical model in which a decrease in pH reduces myosin’s detachment from actin (by slowing ADP release), increases non-productive myosin binding (by detached myosin rebinding without a powerstroke), and reduces myosin’s attachment to actin (by slowing the weak-to-strong binding transition). Additional support of this mechanism is found by incorporating it into a branched pathway model for the effects of [Math Processing Error] on myosin’s interaction with actin. Including pH-dependence in one additional parameter (acceleration of [Math Processing Error]-induced detachment), the model reproduces experimental measurements at high and low pH, and variable [Math Processing Error], from the single molecule to large ensemble levels. Furthermore, when scaled up, the model predicts force-velocity relationships that are consistent with muscle fiber measurements. The model suggests that reducing pH has two opposing effects, a decrease in attachment favoring a decrease in muscle force and a decrease in detachment favoring an increase in muscle force. Depending on experimental details, the addition of [Math Processing Error] can strengthen one or the other effect, resulting in either synergistic or antagonistic effects. This detailed molecular description suggests a molecular basis for contractile failure during muscle fatigue.
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A long overdue recognition
Arthur Ashkin has been awarded half of this year’s Nobel Prize in Physics for his invention of optical tweezers. On 2 October it was announced that the 2018 Nobel Prize in Physics was being awarded “for groundbreaking inventions in the field of laser physics”. One half of the award went to Arthur Ashkin “for the optical tweezers and their application to biological systems”, and the other half jointly to Gérard Mourou and Donna Strickland “for their method of generating high-intensity, ultra-short optical pulses”.
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Processive Nanostepping of Formin mDia1 Loosely Coupled with Actin Polymerization
Hiroaki Kubota, Makito Miyazaki, Taisaku Ogawa, Togo Shimozawa, Kazuhiko Kinosita, Jr., and Shin’ichi Ishiwata
Formins are actin-binding proteins that construct nanoscale machinery with the growing barbed end of actin filaments and serve as key regulators of actin polymerization and depolymerization. To maintain the regulation of actin dynamics, formins have been proposed to processively move at every association or dissociation of a single actin molecule toward newly formed barbed ends. However, the current models for the motile mechanisms were established without direct observation of the elementary processes of this movement. Here, using optical tweezers, we demonstrate that formin mDia1 moves stepwise, observed at a nanometer spatial resolution. The movement was composed of forward and backward steps with unitary step sizes of 2.8 and −2.4 nm, respectively, which nearly equaled the actin subunit length (∼2.7 nm), consistent with the generally accepted models. However, in addition to steps equivalent to the length of a single actin subunit, those equivalent to the length of two or three subunits were frequently observed. Our findings suggest that the coupling between mDia1 stepping and actin polymerization is not tight but loose, which may be achieved by the multiple binding states of mDia1, providing insights into the synergistic functions of biomolecules for the efficient construction and regulation of nanofilaments.
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Formins are actin-binding proteins that construct nanoscale machinery with the growing barbed end of actin filaments and serve as key regulators of actin polymerization and depolymerization. To maintain the regulation of actin dynamics, formins have been proposed to processively move at every association or dissociation of a single actin molecule toward newly formed barbed ends. However, the current models for the motile mechanisms were established without direct observation of the elementary processes of this movement. Here, using optical tweezers, we demonstrate that formin mDia1 moves stepwise, observed at a nanometer spatial resolution. The movement was composed of forward and backward steps with unitary step sizes of 2.8 and −2.4 nm, respectively, which nearly equaled the actin subunit length (∼2.7 nm), consistent with the generally accepted models. However, in addition to steps equivalent to the length of a single actin subunit, those equivalent to the length of two or three subunits were frequently observed. Our findings suggest that the coupling between mDia1 stepping and actin polymerization is not tight but loose, which may be achieved by the multiple binding states of mDia1, providing insights into the synergistic functions of biomolecules for the efficient construction and regulation of nanofilaments.
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A microflow velocity measurement system based on optical tweezers: A comparison using particle tracking velocimetry
Pedro Almendarez-Rangel, Beatriz Morales-Cruzado, Erick Sarmiento-Gómez, Ricardo Romero-Méndez, Francisco G.Pérez-Gutiérrez
Lab-on-a-chip devices have become useful to study substances not available in macrometric amounts. To manipulate the substances under study, ways to induce flow within such devices have been developed and, at the same time, methods to measure flow velocity have been required for the micro-scale. Several velocimetry techniques have been adapted from the macro to the micro scale, such as micro-particle image velocimetry (micro-PIV) and micro-particle tracking velocimetry (micro-PTV). However, the use of a system based on optical tweezers (OT) to measure flow velocity, specifically in microenvironments is possible. With the aim of highlighting the capabilities of an OT-based velocimetry system (OTV), we report the use of such system to measure flow velocity in a rectangular microchannel. Velocity measurements at different depths from the channel wall were carried out. As expected, an increment of the flow velocity with depth was observed. The results obtained with the OTV system were compared with measurements carried out by means of a time-averaged particle tracking velocimetry (TA-PTV). We found that both techniques provided similar results, therefore we demonstrate the capability of the OTV system to measure flow velocities in micrometric scale. The advantages of the OTV system are: (1) better space resolution, (2) minimization of the Brownian motion influence in the measurements and (3) the possibility to have submicrometric spatial resolutions without the employment of high particle concentrations, special data processing, or complex illumination systems like in other microvelocimetry techniques.
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Lab-on-a-chip devices have become useful to study substances not available in macrometric amounts. To manipulate the substances under study, ways to induce flow within such devices have been developed and, at the same time, methods to measure flow velocity have been required for the micro-scale. Several velocimetry techniques have been adapted from the macro to the micro scale, such as micro-particle image velocimetry (micro-PIV) and micro-particle tracking velocimetry (micro-PTV). However, the use of a system based on optical tweezers (OT) to measure flow velocity, specifically in microenvironments is possible. With the aim of highlighting the capabilities of an OT-based velocimetry system (OTV), we report the use of such system to measure flow velocity in a rectangular microchannel. Velocity measurements at different depths from the channel wall were carried out. As expected, an increment of the flow velocity with depth was observed. The results obtained with the OTV system were compared with measurements carried out by means of a time-averaged particle tracking velocimetry (TA-PTV). We found that both techniques provided similar results, therefore we demonstrate the capability of the OTV system to measure flow velocities in micrometric scale. The advantages of the OTV system are: (1) better space resolution, (2) minimization of the Brownian motion influence in the measurements and (3) the possibility to have submicrometric spatial resolutions without the employment of high particle concentrations, special data processing, or complex illumination systems like in other microvelocimetry techniques.
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Monday, November 19, 2018
Single-molecule force spectroscopy reveals folding steps associated with hormone binding and activation of the glucocorticoid receptor
Thomas Suren, Daniel Rutz, Patrick Mößmer, Ulrich Merkel, Johannes Buchner, and Matthias Rief
The glucocorticoid receptor (GR) is a prominent nuclear receptor linked to a variety of diseases and an important drug target. Binding of hormone to its ligand binding domain (GR-LBD) is the key activation step to induce signaling. This process is tightly regulated by the molecular chaperones Hsp70 and Hsp90 in vivo. Despite its importance, little is known about GR-LBD folding, the ligand binding pathway, or the requirement for chaperone regulation. In this study, we have used single-molecule force spectroscopy by optical tweezers to unravel the dynamics of the complete pathway of folding and hormone binding of GR-LBD. We identified a “lid” structure whose opening and closing is tightly coupled to hormone binding. This lid is located at the N terminus without direct contacts to the hormone. Under mechanical load, apo-GR-LBD folds stably and readily without the need of chaperones with a folding free energy of 41kBT(24kcal/mol)41 kBT (24 kcal/mol). The folding pathway is largely independent of the presence of hormone. Hormone binds only in the last step and lid closure adds an additional 12kBT12 kBT of free energy, drastically increasing the affinity. However, mechanical double-jump experiments reveal that, at zero force, GR-LBD folding is severely hampered by misfolding, slowing it to less than 1·s−1. From the force dependence of the folding rates, we conclude that the misfolding occurs late in the folding pathway. These features are important cornerstones for understanding GR activation and its tight regulation by chaperones.
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The glucocorticoid receptor (GR) is a prominent nuclear receptor linked to a variety of diseases and an important drug target. Binding of hormone to its ligand binding domain (GR-LBD) is the key activation step to induce signaling. This process is tightly regulated by the molecular chaperones Hsp70 and Hsp90 in vivo. Despite its importance, little is known about GR-LBD folding, the ligand binding pathway, or the requirement for chaperone regulation. In this study, we have used single-molecule force spectroscopy by optical tweezers to unravel the dynamics of the complete pathway of folding and hormone binding of GR-LBD. We identified a “lid” structure whose opening and closing is tightly coupled to hormone binding. This lid is located at the N terminus without direct contacts to the hormone. Under mechanical load, apo-GR-LBD folds stably and readily without the need of chaperones with a folding free energy of 41kBT(24kcal/mol)41 kBT (24 kcal/mol). The folding pathway is largely independent of the presence of hormone. Hormone binds only in the last step and lid closure adds an additional 12kBT12 kBT of free energy, drastically increasing the affinity. However, mechanical double-jump experiments reveal that, at zero force, GR-LBD folding is severely hampered by misfolding, slowing it to less than 1·s−1. From the force dependence of the folding rates, we conclude that the misfolding occurs late in the folding pathway. These features are important cornerstones for understanding GR activation and its tight regulation by chaperones.
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Multi-level sorting of nanoparticles on multi-step optical waveguide splitter
Xiaofu Xu, Guanghui Wang, Wenxiang Jiao, Wenbin Ji, Min Jiang, and Xuping Zhang
We propose an optofluidic sorting method for nanoparticles with different size by using optical waveguide splitter, and moreover, multiple cascaded splitters with different threshold could act as multi-level sorting unit. For a directional coupler (DC) with a specific wavelength excitation, the power splitting ratio is related to the coupling length and the gap between parallel waveguides. The power splitting ratio further determines the trapping force and potential wells distribution of both output ports. Most importantly, the potential well distribution is dependent on the particle size. For larger particles, the potential wells of both waveguides are inclined to merge, which makes it easier to be attracted and transfers to the adjacent waveguide with deeper potential well. The critical size of sorting is corresponding to the case when the barrier between wells just disappears, or the second derivative of the potential distribution is exactly zero. Moreover, since the sorting threshold of nanoparticles is related to coupling length and gap, multiple cascaded splitters with length or gap gradually varied could act as a multi-level sorting unit. A four-level sorting unit with a critical particle size of 600nm, 700nm, and 800nm are demonstrated. By considering the Brownian motion of particles and using particle-tracking method, the random distribution of nanoparticles on parallel waveguides in the sorting process is statistically presented, which agreed well with its corresponding potential wells distribution analysis. This sorting method based on multi-step optical waveguide splitter offers a number of advantages including single wavelength excitation, low loss, low power performance and ease of fabrication. This design can realize the high-throughput and large-scale nanoparticle automatic sorting in integrated photonic circuits, which have great potential for a large scale lab-on-a-chip system.
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We propose an optofluidic sorting method for nanoparticles with different size by using optical waveguide splitter, and moreover, multiple cascaded splitters with different threshold could act as multi-level sorting unit. For a directional coupler (DC) with a specific wavelength excitation, the power splitting ratio is related to the coupling length and the gap between parallel waveguides. The power splitting ratio further determines the trapping force and potential wells distribution of both output ports. Most importantly, the potential well distribution is dependent on the particle size. For larger particles, the potential wells of both waveguides are inclined to merge, which makes it easier to be attracted and transfers to the adjacent waveguide with deeper potential well. The critical size of sorting is corresponding to the case when the barrier between wells just disappears, or the second derivative of the potential distribution is exactly zero. Moreover, since the sorting threshold of nanoparticles is related to coupling length and gap, multiple cascaded splitters with length or gap gradually varied could act as a multi-level sorting unit. A four-level sorting unit with a critical particle size of 600nm, 700nm, and 800nm are demonstrated. By considering the Brownian motion of particles and using particle-tracking method, the random distribution of nanoparticles on parallel waveguides in the sorting process is statistically presented, which agreed well with its corresponding potential wells distribution analysis. This sorting method based on multi-step optical waveguide splitter offers a number of advantages including single wavelength excitation, low loss, low power performance and ease of fabrication. This design can realize the high-throughput and large-scale nanoparticle automatic sorting in integrated photonic circuits, which have great potential for a large scale lab-on-a-chip system.
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A review of sorting, separation and isolation of cells and microbeads for biomedical applications: microfluidic approaches
Arash Dalili, Ehsan Samiei and Mina Hoorfar
Several biomedical analyses are performed on particular types of cells present in body samples or using functionalized microparticles. Success in such analyses depends on the ability to separate or isolate the target cells or microparticles from the rest of the sample. In conventional procedures, multiple pieces of equipment, such as centrifuges, magnets, and macroscale filters, are used for such purposes, which are time-consuming, associated with human error, and require several operational steps. In the past two decades, there has been a tendency to develop microfluidic techniques, so-called lab-on-a-chip, to miniaturize and automate these procedures. The processes used for the separation and isolation of the cells and microparticles are scaled down into a small microfluidic chip, requiring very small amounts of sample. Differences in the physical and biological properties of the target cells from the other components present in the sample are the key to the development of such microfluidic techniques. These techniques are categorized as filtration-, hydrodynamic-, dielectrophoretic-, acoustic- and magnetic-based methods. Here we review the microfluidic techniques developed for sorting, separation, and isolation of cells and microparticles for biomedical applications. The mechanisms behind such techniques are thoroughly explained and the applications in which these techniques have been adopted are reviewed.
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Several biomedical analyses are performed on particular types of cells present in body samples or using functionalized microparticles. Success in such analyses depends on the ability to separate or isolate the target cells or microparticles from the rest of the sample. In conventional procedures, multiple pieces of equipment, such as centrifuges, magnets, and macroscale filters, are used for such purposes, which are time-consuming, associated with human error, and require several operational steps. In the past two decades, there has been a tendency to develop microfluidic techniques, so-called lab-on-a-chip, to miniaturize and automate these procedures. The processes used for the separation and isolation of the cells and microparticles are scaled down into a small microfluidic chip, requiring very small amounts of sample. Differences in the physical and biological properties of the target cells from the other components present in the sample are the key to the development of such microfluidic techniques. These techniques are categorized as filtration-, hydrodynamic-, dielectrophoretic-, acoustic- and magnetic-based methods. Here we review the microfluidic techniques developed for sorting, separation, and isolation of cells and microparticles for biomedical applications. The mechanisms behind such techniques are thoroughly explained and the applications in which these techniques have been adopted are reviewed.
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Radiation-Pressure-Antidamping Enhanced Optomechanical Spring Sensing
Fei Pan, Kaiyu Cui, Guoren Bai, Xue Feng, Fang Liu, Wei Zhang, and Yidong Huang
On-chip refractive index sensing plays an important role in many fields, ranging from chemical, biomedical, and medical to environmental applications. Recently, optomechanical cavities have emerged as promising tools for precision sensing. In view of the sensors based on optomechanical cavities, the Q factor of mechanical modes is a key parameter for achieving high sensitivity and resolution. Here we demonstrated an integrated optomechanical cavity based on a silicon nanobeam structure. Our cavity supports a fundamental mechanical mode with a frequency of 4.36 GHz and a record-high mechanical Q of 18300 in the ambient environment, facilitated by the radiation-pressure antidamping. The distinctive nature of the optomechanical spring sensing approach combined with our high mechanical Q silicon cavity allows for a sensing resolution of δλ/λ0 ∼ 10–7, which is at least 1 order of magnitude higher than that of conventional silicon-based approaches and paves the way for on-chip sensors with unprecedented sensitivity.
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On-chip refractive index sensing plays an important role in many fields, ranging from chemical, biomedical, and medical to environmental applications. Recently, optomechanical cavities have emerged as promising tools for precision sensing. In view of the sensors based on optomechanical cavities, the Q factor of mechanical modes is a key parameter for achieving high sensitivity and resolution. Here we demonstrated an integrated optomechanical cavity based on a silicon nanobeam structure. Our cavity supports a fundamental mechanical mode with a frequency of 4.36 GHz and a record-high mechanical Q of 18300 in the ambient environment, facilitated by the radiation-pressure antidamping. The distinctive nature of the optomechanical spring sensing approach combined with our high mechanical Q silicon cavity allows for a sensing resolution of δλ/λ0 ∼ 10–7, which is at least 1 order of magnitude higher than that of conventional silicon-based approaches and paves the way for on-chip sensors with unprecedented sensitivity.
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An optical reaction micro-turbine
Silvio Bianchi, Gaszton Vizsnyiczai, Stefano Ferretti, Claudio Maggi & Roberto Di Leonardo
To any energy flow there is an associated flow of momentum, so that recoil forces arise every time an object absorbs or deflects incoming energy. This same principle governs the operation of macroscopic turbines as well as that of microscopic turbines that use light as the working fluid. However, a controlled and precise redistribution of optical energy is not easy to achieve at the micron scale resulting in a low efficiency of power to torque conversion. Here we use direct laser writing to fabricate 3D light guiding structures, shaped as a garden sprinkler, that can precisely reroute input optical power into multiple output channels. The shape parameters are derived from a detailed theoretical analysis of losses in curved microfibers. These optical reaction micro-turbines can maximally exploit light’s momentum to generate a strong, uniform and controllable torque.
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To any energy flow there is an associated flow of momentum, so that recoil forces arise every time an object absorbs or deflects incoming energy. This same principle governs the operation of macroscopic turbines as well as that of microscopic turbines that use light as the working fluid. However, a controlled and precise redistribution of optical energy is not easy to achieve at the micron scale resulting in a low efficiency of power to torque conversion. Here we use direct laser writing to fabricate 3D light guiding structures, shaped as a garden sprinkler, that can precisely reroute input optical power into multiple output channels. The shape parameters are derived from a detailed theoretical analysis of losses in curved microfibers. These optical reaction micro-turbines can maximally exploit light’s momentum to generate a strong, uniform and controllable torque.
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Three-dimensional tracking of microbeads attached to the tip of single isolated tracheal cilia beating under external load
Takanobu A. Katoh, Koji Ikegami, Nariya Uchida, Toshihito Iwase, Daisuke Nakane, Tomoko Masaike, Mitsutoshi Setou & Takayuki Nishizaka
To study the properties of tracheal cilia beating under various conditions, we developed a method to monitor the movement of the ciliary tip. One end of a demembranated cilium was immobilized on the glass surface, while the other end was capped with a polystyrene bead and tracked in three dimensions. The cilium, when activated by ATP, stably repeated asymmetric beating as in vivo. The tip of a cilium in effective and recovery strokes moved in discrete trajectories that differed in height. The trajectory remained asymmetric in highly viscous solutions. Model calculation showed that cilia maintained a constant net flux during one beat cycle irrespective of the medium viscosity. When the bead attached to the end was trapped with optical tweezers, it came to display linear oscillation only in the longitudinal direction. Such a beating-mode transition may be an inherent nature of movement-restricted cilia.
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Friday, November 16, 2018
Repulsion of polarized particles from two-dimensional materials
Francisco J. Rodríguez-Fortuño, Michela F. Picardi, and Anatoly V. Zayats
Repulsion of nanoparticles, molecules, and atoms from surfaces can have important applications in nanomechanical devices, microfluidics, optical manipulation, and atom optics. Here, through the solution of a classical scattering problem, we show that a dipole source oscillating at a frequency ω can experience a robust and strong repulsive force when its near-field interacts with a two-dimensional material. As an example, the case of graphene is considered, showing that a broad bandwidth of repulsion can be obtained at frequencies for which propagation of plasmon modes is allowed 0<ℏω<(5/3)μc, where μc is the chemical potential tunable electrically or by chemical doping.
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Repulsion of nanoparticles, molecules, and atoms from surfaces can have important applications in nanomechanical devices, microfluidics, optical manipulation, and atom optics. Here, through the solution of a classical scattering problem, we show that a dipole source oscillating at a frequency ω can experience a robust and strong repulsive force when its near-field interacts with a two-dimensional material. As an example, the case of graphene is considered, showing that a broad bandwidth of repulsion can be obtained at frequencies for which propagation of plasmon modes is allowed 0<ℏω<(5/3)μc, where μc is the chemical potential tunable electrically or by chemical doping.
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Optical transportation of micro-particles by non-diffracting Weber beams
Weiwei Liu, Jie Gao and Xiaodong Yang
The optical transportation of solid polystyrene particles by the non-diffracting Weber beams is demonstrated with both forward and backward motion along the parabolic main lobes of Weber beams in three dimensions. The Weber beams are generated from the complex field modulation based on the spatial light modulator for realizing the optical transportation of micrometer polystyrene particles in an optical tweezers setup. The particle motion and velocity distribution along the main lobes of Weber beams in two different parabolic shapes are characterized and compared.
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The optical transportation of solid polystyrene particles by the non-diffracting Weber beams is demonstrated with both forward and backward motion along the parabolic main lobes of Weber beams in three dimensions. The Weber beams are generated from the complex field modulation based on the spatial light modulator for realizing the optical transportation of micrometer polystyrene particles in an optical tweezers setup. The particle motion and velocity distribution along the main lobes of Weber beams in two different parabolic shapes are characterized and compared.
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Optical manipulation of chiral nanoparticles in vector Airy beam
Wanli Lu, Xu Sun, Huajin Chen, Shiyang Liu and Zhifang Lin
The optical manipulation of chiral nanoparticles in a vector Airy beam with linear polarization is theoretically investigated, and to calculate the optical forces acting on a spherical chiral particle of an arbitrary size beyond the paraxial approximation, a rigorous numerical method based on the generalized Lorenz–Mie theory and Maxwell stress tensor method is presented. It is found that the chiral nanoparticle not only can be stably trapped within the main lobe due to the transverse optical gradient force but also can be transported faster than a conventional particle without chirality along curved trajectories because of the longitudinal scattering optical force. In addition, the particle chirality significantly enhances the longitudinal optical force while slightly affecting the transverse optical force. Our results may provide an additional handle for the optical manipulation of chiral nanoparticles.
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The optical manipulation of chiral nanoparticles in a vector Airy beam with linear polarization is theoretically investigated, and to calculate the optical forces acting on a spherical chiral particle of an arbitrary size beyond the paraxial approximation, a rigorous numerical method based on the generalized Lorenz–Mie theory and Maxwell stress tensor method is presented. It is found that the chiral nanoparticle not only can be stably trapped within the main lobe due to the transverse optical gradient force but also can be transported faster than a conventional particle without chirality along curved trajectories because of the longitudinal scattering optical force. In addition, the particle chirality significantly enhances the longitudinal optical force while slightly affecting the transverse optical force. Our results may provide an additional handle for the optical manipulation of chiral nanoparticles.
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Creating Multifunctional Optofluidic Potential Wells for Nanoparticle Manipulation
Fan Nan and Zijie Yan
Optical forces have enabled various nanomanipulation in microfluidics such as optical trapping, sorting, and transporting of nanoparticles (NPs), but the manipulation is usually specific with a certain optical field. Tightly focused Gaussian beams can trap NPs but not sort them; moderately focused Gaussian beams allow sorting microparticles in a flow but not NPs; quasi-Bessel beams can sort NPs in a flow but cannot control their positions due to low trapping stiffness. All these methods rely on the axial variation of laser intensity. Here we show that multifunctional and tunable optofluidic potential wells can be created for nanomanipulation by synchronizing optical phase gradient force with fluid drag force. We demonstrate controlled trapping and transporting of 150 nm Ag NPs over 10 μm and sorting of 80 and 100 nm Au NPs using optical line traps with tunable phase gradients in experiments. Our simulations further predict that simultaneous sorting and trapping of sub-50 nm Au NPs can be achieved with a sorting resolution of 1 nm using optimized optical fields. Our method provides great freedom and flexibility for nanomanipulation in optofluidics with potential applications in nanophotonics and biomedicine.
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Optical forces have enabled various nanomanipulation in microfluidics such as optical trapping, sorting, and transporting of nanoparticles (NPs), but the manipulation is usually specific with a certain optical field. Tightly focused Gaussian beams can trap NPs but not sort them; moderately focused Gaussian beams allow sorting microparticles in a flow but not NPs; quasi-Bessel beams can sort NPs in a flow but cannot control their positions due to low trapping stiffness. All these methods rely on the axial variation of laser intensity. Here we show that multifunctional and tunable optofluidic potential wells can be created for nanomanipulation by synchronizing optical phase gradient force with fluid drag force. We demonstrate controlled trapping and transporting of 150 nm Ag NPs over 10 μm and sorting of 80 and 100 nm Au NPs using optical line traps with tunable phase gradients in experiments. Our simulations further predict that simultaneous sorting and trapping of sub-50 nm Au NPs can be achieved with a sorting resolution of 1 nm using optimized optical fields. Our method provides great freedom and flexibility for nanomanipulation in optofluidics with potential applications in nanophotonics and biomedicine.
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Kinetics coming into focus: single-molecule microscopy of riboswitch dynamics
Sujay Ray, Adrien Chauvier and Nils G. Walter
Riboswitches are dynamic RNA motifs that are mostly embedded in the 5ʹ-untranslated regions of bacterial mRNAs, where they regulate gene expression transcriptionally or translationally by undergoing conformational changes upon binding of a small metabolite or ion. Due to the small size of typical ligands, relatively little free energy is available from ligand binding to overcome the often high energetic barrier of reshaping RNA structure. Instead, most riboswitches appear to take advantage of the directional and hierarchical folding of RNA by employing the ligand as a structural ‘linchpin’ to adjust the kinetic partitioning between alternate folds. In this model, even small, local structural and kinetic effects of ligand binding can cascade into global RNA conformational changes affecting gene expression. Single-molecule (SM) microscopy tools are uniquely suited to study such kinetically controlled RNA folding since they avoid the ensemble averaging of bulk techniques that loses sight of unsynchronized, transient, and/or multi-state kinetic behavior. This review summarizes how SM methods have begun to unravel riboswitch-mediated gene regulation.
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Riboswitches are dynamic RNA motifs that are mostly embedded in the 5ʹ-untranslated regions of bacterial mRNAs, where they regulate gene expression transcriptionally or translationally by undergoing conformational changes upon binding of a small metabolite or ion. Due to the small size of typical ligands, relatively little free energy is available from ligand binding to overcome the often high energetic barrier of reshaping RNA structure. Instead, most riboswitches appear to take advantage of the directional and hierarchical folding of RNA by employing the ligand as a structural ‘linchpin’ to adjust the kinetic partitioning between alternate folds. In this model, even small, local structural and kinetic effects of ligand binding can cascade into global RNA conformational changes affecting gene expression. Single-molecule (SM) microscopy tools are uniquely suited to study such kinetically controlled RNA folding since they avoid the ensemble averaging of bulk techniques that loses sight of unsynchronized, transient, and/or multi-state kinetic behavior. This review summarizes how SM methods have begun to unravel riboswitch-mediated gene regulation.
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Length dependence of viscoelasticity of entangled-DNA solution with and without external stress
Masaya Tanoguchi and Yoshihiro Murayama
We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution to investigate the effect of the viscoelasticity on the bead motion. In the absence of external stress (passive microrheology), subdiffusion appears in the timescale of 0.1–10 s, and the normal diffusion recovers in longer timescales. We evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium. We found that the absence of DNA-length dependence for the time-dependent diffusion is explained by the simple relaxation of the viscoelastic media rather than the reptation dynamics, including the disentanglement. On the other hand, in the presence of a small external stress in active microrheology, the bead motion showed clear length dependence owing to the viscoelasticity. These results suggest that the viscoelasticity of the entangled DNA is highly sensitive to the external stress, even in the linear response regime.
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We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution to investigate the effect of the viscoelasticity on the bead motion. In the absence of external stress (passive microrheology), subdiffusion appears in the timescale of 0.1–10 s, and the normal diffusion recovers in longer timescales. We evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium. We found that the absence of DNA-length dependence for the time-dependent diffusion is explained by the simple relaxation of the viscoelastic media rather than the reptation dynamics, including the disentanglement. On the other hand, in the presence of a small external stress in active microrheology, the bead motion showed clear length dependence owing to the viscoelasticity. These results suggest that the viscoelasticity of the entangled DNA is highly sensitive to the external stress, even in the linear response regime.
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An optical trapping system for particle probes in plasma diagnostics
Viktor Schneider and Holger Kersten
We present one of the first experiments for optically trapping of single microparticles as probes for low temperature plasma diagnostics. Based on the dual laser beam, counter-propagating technique, SiO2 microparticles are optically trapped at very large distances in low-temperature, low-pressure rf plasma. External forces on the particle are measured by means of the displacement of the probe particle in the trap. Measurements can be performed during plasma operation as well as without plasma. The paper focuses on the optical setup and the verification of the system and its principle. Three examples for the particle behavior in the trapping system are presented: First, we measured the neutral gas damping as a verification of the technique. Second, an experiment without a plasma studies the changing particle charge by UV light radiation, and third, by moving the probe particle in the vertical direction into the sheath or into the plasma bulk, respectively, the acting forces on the probe particle are measured.
DOI
We present one of the first experiments for optically trapping of single microparticles as probes for low temperature plasma diagnostics. Based on the dual laser beam, counter-propagating technique, SiO2 microparticles are optically trapped at very large distances in low-temperature, low-pressure rf plasma. External forces on the particle are measured by means of the displacement of the probe particle in the trap. Measurements can be performed during plasma operation as well as without plasma. The paper focuses on the optical setup and the verification of the system and its principle. Three examples for the particle behavior in the trapping system are presented: First, we measured the neutral gas damping as a verification of the technique. Second, an experiment without a plasma studies the changing particle charge by UV light radiation, and third, by moving the probe particle in the vertical direction into the sheath or into the plasma bulk, respectively, the acting forces on the probe particle are measured.
DOI
Wednesday, November 14, 2018
Experimental realization of Feynman's ratchet
Jaehoon Bang, Rui Pan, Thai M Hoang, Jonghoon Ahn, Christopher Jarzynski, H T Quan and Tongcang Li
Feynman's ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures T A and T B , that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium (T A = T B ), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference (${T}_{A}\ne {T}_{B}$), it can operate as a heat engine, rectifying thermal fluctuations to perform work. While it has attracted much interest, the operation of Feynman's ratchet as a heat engine has not been realized experimentally, due to technical challenges. In this work, we realize Feynman's ratchet with a colloidal particle in a one-dimensional optical trap in contact with two heat reservoirs: one is the surrounding water, while the effect of the other reservoir is generated by a novel feedback mechanism, using the Metropolis algorithm to impose detailed balance. We verify that the system does not produce work when T A = T B , and that it becomes a microscopic heat engine when ${T}_{A}\ne {T}_{B}$. We analyze work, heat and entropy production as functions of the temperature difference and external load. Our experimental realization of Feynman's ratchet and the Metropolis algorithm can also be used to study the thermodynamics of feedback control and information processing, the working mechanism of molecular motors, and controllable particle transportation.
DOI
Feynman's ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures T A and T B , that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium (T A = T B ), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference (${T}_{A}\ne {T}_{B}$), it can operate as a heat engine, rectifying thermal fluctuations to perform work. While it has attracted much interest, the operation of Feynman's ratchet as a heat engine has not been realized experimentally, due to technical challenges. In this work, we realize Feynman's ratchet with a colloidal particle in a one-dimensional optical trap in contact with two heat reservoirs: one is the surrounding water, while the effect of the other reservoir is generated by a novel feedback mechanism, using the Metropolis algorithm to impose detailed balance. We verify that the system does not produce work when T A = T B , and that it becomes a microscopic heat engine when ${T}_{A}\ne {T}_{B}$. We analyze work, heat and entropy production as functions of the temperature difference and external load. Our experimental realization of Feynman's ratchet and the Metropolis algorithm can also be used to study the thermodynamics of feedback control and information processing, the working mechanism of molecular motors, and controllable particle transportation.
DOI
Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams
Nils Odebo Länk, Peter Johansson, and Mikael Käll
Nanoparticles made of high index dielectric materials have seen a surge of interest and have been proposed for various applications, such as metalenses, light harvesting and directional scattering. With the advent of fabrication techniques enabling colloidal suspensions, the prospects of optical manipulation of such nanoparticles becomes paramount. High index nanoparticles support electric and magnetic multipolar responses in the visible regime and interference between such modes can give rise to highly directional scattering, in particular a cancellation of back-scattered radiation at the first Kerker condition. Here we present a study of the optical forces on silicon nanoparticles in the visible and near infrared calculated using the transfer matrix method. The zero-backscattering Kerker condition is investigated as an avenue to reduce radiation pressure in an optical trap. We find that while asymmetric scattering does reduce the radiation pressure, the main determining factor of trap stability is the increased particle response near the geometric resonances. The trap stability for non-spherical silicon nanoparticles is also investigated and we find that ellipsoidal deformation of spheres enables trapping of slightly larger particles.
DOI
Nanoparticles made of high index dielectric materials have seen a surge of interest and have been proposed for various applications, such as metalenses, light harvesting and directional scattering. With the advent of fabrication techniques enabling colloidal suspensions, the prospects of optical manipulation of such nanoparticles becomes paramount. High index nanoparticles support electric and magnetic multipolar responses in the visible regime and interference between such modes can give rise to highly directional scattering, in particular a cancellation of back-scattered radiation at the first Kerker condition. Here we present a study of the optical forces on silicon nanoparticles in the visible and near infrared calculated using the transfer matrix method. The zero-backscattering Kerker condition is investigated as an avenue to reduce radiation pressure in an optical trap. We find that while asymmetric scattering does reduce the radiation pressure, the main determining factor of trap stability is the increased particle response near the geometric resonances. The trap stability for non-spherical silicon nanoparticles is also investigated and we find that ellipsoidal deformation of spheres enables trapping of slightly larger particles.
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Plasmonic Nanogaps: From Fabrications to Optical Applications
Panpan Gu , Wei Zhang, Gang Zhang
Metallic nanostructures with nanogap features are proved to be highly effective building blocks for plasmonic systems, as they can provide ultrastrong electromagnetic (EM) fields and controllable optical properties. A wide range of fields, including surface enhanced spectroscopy, sensing, imaging, nonlinear optics, optical trapping, and metamaterials, are benefited from these enhanced EM fields. This review outlines the latest development of the fabrication methods for nanogap structures (metal nanoparticle assembly, nanosphere lithography, electron beam lithography (EBL), focused ion beam (FIB) lithography, oblique angle shadow evaporation, edge lithography, and so on), followed by a summary of their optical applications. The present review will inspire more ingenious designs and fabrications of plasmonic nanogap structures with lithography‐free fabrication techniques, and promote their applications in optics and electronics.
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Metallic nanostructures with nanogap features are proved to be highly effective building blocks for plasmonic systems, as they can provide ultrastrong electromagnetic (EM) fields and controllable optical properties. A wide range of fields, including surface enhanced spectroscopy, sensing, imaging, nonlinear optics, optical trapping, and metamaterials, are benefited from these enhanced EM fields. This review outlines the latest development of the fabrication methods for nanogap structures (metal nanoparticle assembly, nanosphere lithography, electron beam lithography (EBL), focused ion beam (FIB) lithography, oblique angle shadow evaporation, edge lithography, and so on), followed by a summary of their optical applications. The present review will inspire more ingenious designs and fabrications of plasmonic nanogap structures with lithography‐free fabrication techniques, and promote their applications in optics and electronics.
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Optical trapping and controllable targeted delivery of nanoparticles by a nanofiber ring
Ying Li, Yanjun Hu
The stable manipulation and position-designated delivery of nanoparticles has shown potential application in targeted drug delivery and enhanced detection of viruses. In this paper, we demonstrate optical trapping and controllable targeted delivery of 700 nm diameter particles using a nanofiber ring. Based on 3D FDTD simulations, the bending loss for bent nanofibers was calculated at different fiber diameters, bending radio, and laser wavelengths. Therefore, according to the theoretical analysis, the 500 nm diameter nanofiber and 808 nm wavelength laser light were chosen. The experimental results indicate that, by directing a laser beam with a wavelength of 808 nm into a nanofiber ring, nanoparticles were trapped and transported along the ring, and released into the water at a designated position because of bending loss. The release position of particles was controllable by the input optical power.
DOI
The stable manipulation and position-designated delivery of nanoparticles has shown potential application in targeted drug delivery and enhanced detection of viruses. In this paper, we demonstrate optical trapping and controllable targeted delivery of 700 nm diameter particles using a nanofiber ring. Based on 3D FDTD simulations, the bending loss for bent nanofibers was calculated at different fiber diameters, bending radio, and laser wavelengths. Therefore, according to the theoretical analysis, the 500 nm diameter nanofiber and 808 nm wavelength laser light were chosen. The experimental results indicate that, by directing a laser beam with a wavelength of 808 nm into a nanofiber ring, nanoparticles were trapped and transported along the ring, and released into the water at a designated position because of bending loss. The release position of particles was controllable by the input optical power.
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Nuclear recoil spectroscopy of levitated particles
Alexander Malyzhenkov, Vyacheslav Lebedev, and Alonso Castro
We propose a method for the detection and characterization of nuclear decay processes. Specifically, we describe how nuclear decay recoil can be observed within small particles levitated in an optical trap with high positional resolution. Precise measurements of the magnitude of each recoil as well as their rate of occurrence can provide accurate information about the isotopic composition of a radioactive sample. We expect that this technique for nuclear material characterization will be especially useful in the area of nuclear forensic analysis.
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We propose a method for the detection and characterization of nuclear decay processes. Specifically, we describe how nuclear decay recoil can be observed within small particles levitated in an optical trap with high positional resolution. Precise measurements of the magnitude of each recoil as well as their rate of occurrence can provide accurate information about the isotopic composition of a radioactive sample. We expect that this technique for nuclear material characterization will be especially useful in the area of nuclear forensic analysis.
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Simultaneous printing and deformation of microsystems via two-photon lithography and holographic optical tweezers
Samira Chizari, Lucas A. Shaw and Jonathan B. Hopkins
The purpose of this work is to enable the simultaneous printing and deformation of polymer microsystems using an integrated two-photon lithography (TPL) and holographic optical tweezers (HOT) approach. This approach is the first of its kind to enable the fabrication of advanced metamaterials, micromechanisms, soft microrobots, and sensors that require embedded strain energy in their constituent compliant elements to achieve their intended behaviors. We introduce a custom-developed photopolymer chemistry that is suitable for near-infrared (NIR) TPL fabrication but remains unreactive in the visible-light regime for HOT-based handling. We facilitated the optimal HOT-based actuation of TPL-fabricated microsystems by advancing a ray-optics-based optical-force simulation tool to work with microbodies of any arbitrary shape. We demonstrate the utility of this integrated system via fabrication of three unique case studies, which could not be achieved using any alternative technologies.
DOI
The purpose of this work is to enable the simultaneous printing and deformation of polymer microsystems using an integrated two-photon lithography (TPL) and holographic optical tweezers (HOT) approach. This approach is the first of its kind to enable the fabrication of advanced metamaterials, micromechanisms, soft microrobots, and sensors that require embedded strain energy in their constituent compliant elements to achieve their intended behaviors. We introduce a custom-developed photopolymer chemistry that is suitable for near-infrared (NIR) TPL fabrication but remains unreactive in the visible-light regime for HOT-based handling. We facilitated the optimal HOT-based actuation of TPL-fabricated microsystems by advancing a ray-optics-based optical-force simulation tool to work with microbodies of any arbitrary shape. We demonstrate the utility of this integrated system via fabrication of three unique case studies, which could not be achieved using any alternative technologies.
DOI
Monday, November 12, 2018
Theoretical analysis of optical force density distribution inside subwavelength-diameter optical fibers
Zhang Yun-Yuan, Yu Hua-Kang, Wang Xiang-Ke, Wu Wan-Ling, Gu Fu-Xing, Li Zhi-Yuan
We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter (SD) fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain (FDTD) method. Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.
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We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter (SD) fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain (FDTD) method. Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.
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Single Particle Automated Raman Trapping Analysis
Jelle Penders, Isaac J. Pence, Conor C. Horgan, Mads S. Bergholt, Christopher S. Wood, Adrian Najer, Ulrike Kauscher, Anika Nagelkerke & Molly M. Stevens
Enabling concurrent, high throughput analysis of single nanoparticles would greatly increase the capacity to study size, composition and inter and intra particle population variance with applications in a wide range of fields from polymer science to drug delivery. Here, we present a comprehensive platform for Single Particle Automated Raman Trapping Analysis (SPARTA) able to integrally analyse nanoparticles ranging from synthetic polymer particles to liposomes without any modification. With the developed highly controlled automated trapping process, single nanoparticles are analysed with high throughput and sensitivity to resolve particle mixtures, obtain detailed compositional spectra of complex particles, track sequential functionalisations, derive particle sizes and monitor the dynamics of click reactions occurring on the nanoparticle surface. The SPARTA platform opens up a wide range of new avenues for nanoparticle research through label-free integral high-throughput single particle analysis, overcoming key limitations in sensitivity and specificity of existing bulk analysis methods.
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Enabling concurrent, high throughput analysis of single nanoparticles would greatly increase the capacity to study size, composition and inter and intra particle population variance with applications in a wide range of fields from polymer science to drug delivery. Here, we present a comprehensive platform for Single Particle Automated Raman Trapping Analysis (SPARTA) able to integrally analyse nanoparticles ranging from synthetic polymer particles to liposomes without any modification. With the developed highly controlled automated trapping process, single nanoparticles are analysed with high throughput and sensitivity to resolve particle mixtures, obtain detailed compositional spectra of complex particles, track sequential functionalisations, derive particle sizes and monitor the dynamics of click reactions occurring on the nanoparticle surface. The SPARTA platform opens up a wide range of new avenues for nanoparticle research through label-free integral high-throughput single particle analysis, overcoming key limitations in sensitivity and specificity of existing bulk analysis methods.
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Optomechanical Manipulation with Hyperbolic Metasurfaces
Aliaksandra Ivinskaya, Natalia Kostina, Alexey Proskurin, Mihail I. Petrov , Andrey A. Bogdanov, Sergey Sukhov, Alexey V. Krasavin, Alina Karabchevsky, Alexander S. Shalin, and Pavel Ginzburg
Auxiliary nanostructures introduce additional flexibility into optomechanical manipulation schemes. Metamaterials and metasurfaces capable to control electromagnetic interactions at the near-field regions are especially beneficial for achieving improved spatial localization of particles, reducing laser powers required for trapping, and for tailoring directivity of optical forces. Here, optical forces acting on small particles situated next to anisotropic substrates, are investigated. A special class of hyperbolic metasurfaces is considered in details and is shown to be beneficial for achieving strong optical pulling forces in a broad spectral range. Spectral decomposition of Green’s functions enables identifying contributions of different interaction channels and underlines the importance of the hyperbolic dispersion regime, which plays the key role in optomechanical interactions. Homogenized model of the hyperbolic metasurface is compared to its metal-dielectric multilayer realizations and is shown to predict the optomechanical behavior under certain conditions related to composition of the top layer of the structure and its periodicity. Optomechanical metasurfaces open a venue for future fundamental investigations and a range of practical applications, where accurate control over mechanical motion of small objects is required.
DOI
Auxiliary nanostructures introduce additional flexibility into optomechanical manipulation schemes. Metamaterials and metasurfaces capable to control electromagnetic interactions at the near-field regions are especially beneficial for achieving improved spatial localization of particles, reducing laser powers required for trapping, and for tailoring directivity of optical forces. Here, optical forces acting on small particles situated next to anisotropic substrates, are investigated. A special class of hyperbolic metasurfaces is considered in details and is shown to be beneficial for achieving strong optical pulling forces in a broad spectral range. Spectral decomposition of Green’s functions enables identifying contributions of different interaction channels and underlines the importance of the hyperbolic dispersion regime, which plays the key role in optomechanical interactions. Homogenized model of the hyperbolic metasurface is compared to its metal-dielectric multilayer realizations and is shown to predict the optomechanical behavior under certain conditions related to composition of the top layer of the structure and its periodicity. Optomechanical metasurfaces open a venue for future fundamental investigations and a range of practical applications, where accurate control over mechanical motion of small objects is required.
DOI
Studying Glycolytic Oscillations in Individual Yeast Cells by Combining Fluorescence Microscopy with Microfluidics and Optical Tweezers
Anna‐Karin Gustavsson, Amin A. Banaeiyan, David D. van Niekerk, Jacky L. Snoep, Caroline B. Adiels, Mattias Goksör
In this unit, we provide a clear exposition of the methodology employed to study dynamic responses in individual cells, using microfluidics for controlling and adjusting the cell environment, optical tweezers for precise cell positioning, and fluorescence microscopy for detecting intracellular responses. This unit focuses on the induction and study of glycolytic oscillations in single yeast cells, but the methodology can easily be adjusted to examine other biological questions and cell types. We present a step‐by‐step guide for fabrication of the microfluidic device, for alignment of the optical tweezers, for cell preparation, and for time‐lapse imaging of glycolytic oscillations in single cells, including a discussion of common pitfalls. A user who follows the protocols should be able to detect clear metabolite time traces over the course of up to an hour that are indicative of dynamics on the second scale in individual cells during fast and reversible environmental adjustments.
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In this unit, we provide a clear exposition of the methodology employed to study dynamic responses in individual cells, using microfluidics for controlling and adjusting the cell environment, optical tweezers for precise cell positioning, and fluorescence microscopy for detecting intracellular responses. This unit focuses on the induction and study of glycolytic oscillations in single yeast cells, but the methodology can easily be adjusted to examine other biological questions and cell types. We present a step‐by‐step guide for fabrication of the microfluidic device, for alignment of the optical tweezers, for cell preparation, and for time‐lapse imaging of glycolytic oscillations in single cells, including a discussion of common pitfalls. A user who follows the protocols should be able to detect clear metabolite time traces over the course of up to an hour that are indicative of dynamics on the second scale in individual cells during fast and reversible environmental adjustments.
DOI
Friday, November 9, 2018
Numerical calculation and Cartesian multipole decomposition of optical pulling force acting on Si nanocube in visible region
E.A. Gurvitz and A. S. Shalin
The multipole decomposition of optical force and scattering cross-section is considered for the two plane waves incident on Si nanocube. The obtained results show the high impact of a toroidal moment and high order multipoles in optical force, while they aren't represented main resonances in scattering cross-section.
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The multipole decomposition of optical force and scattering cross-section is considered for the two plane waves incident on Si nanocube. The obtained results show the high impact of a toroidal moment and high order multipoles in optical force, while they aren't represented main resonances in scattering cross-section.
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Lateral radiative forces exerted by evanescent fields along a hyperbolic metamaterial slab
I S Nefedov, C A Valagiannopoulos and A S Shalin
We show and investigate the optical forces acting on a particle in the vicinity of a planar waveguide which is filled with hyperbolic material and supports propagation across its plane (two-dimensional). The anisotropy axis of its medium lies in plane of the waveguide. In contrast to commonly considered pushing or pulling forces, acting in one-dimensional guiding structures, in the case of two-dimensional wave propagation, the angles between the momentum and the total energy flow may take any value around the circle. Accordingly, evanescent fields out of the slab exert lateral radiative forces on a nanoparticle oriented parallel to momentum being controllably different from the total energy flow direction. This provides a flexibility in manipulation by nanoparticles by employing suitably engineered hyperbolic structures.
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We show and investigate the optical forces acting on a particle in the vicinity of a planar waveguide which is filled with hyperbolic material and supports propagation across its plane (two-dimensional). The anisotropy axis of its medium lies in plane of the waveguide. In contrast to commonly considered pushing or pulling forces, acting in one-dimensional guiding structures, in the case of two-dimensional wave propagation, the angles between the momentum and the total energy flow may take any value around the circle. Accordingly, evanescent fields out of the slab exert lateral radiative forces on a nanoparticle oriented parallel to momentum being controllably different from the total energy flow direction. This provides a flexibility in manipulation by nanoparticles by employing suitably engineered hyperbolic structures.
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Tractor beams at metamaterial substrates
A Ivinskaya, N Kostina, M I Petrov, A A Bogdanov, S Sukhov, P Ginzburg and A S Shalin
Optical forces acting on nanoobjects can be enhanced, inverted or cancelled out by the presence of carefully chosen environment. Here we consider metamaterial substrate which can modify optical forces through the excitation of surface waves and volumetric hyperbolic modes. Both types of interaction channels will be discussed in the present work where we show new effect of pulling forces on subwavelength dielectric particles above multilayer hyperbolic substrates.
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Optical forces acting on nanoobjects can be enhanced, inverted or cancelled out by the presence of carefully chosen environment. Here we consider metamaterial substrate which can modify optical forces through the excitation of surface waves and volumetric hyperbolic modes. Both types of interaction channels will be discussed in the present work where we show new effect of pulling forces on subwavelength dielectric particles above multilayer hyperbolic substrates.
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Experimental stochastic systems based on optical forces
Oto Brzobohaty, Stephen Simpson, Martin Siler, Petr Jakl, Jana Damkova, Vojtech Svak, Alejandro Arzola, Karen Volke-Sepulveda, Radim Filip and Pavel Zemanek
We will present our recent theoretical and experimental results related to the behaviour of micro- and nanoparticle placed into nonlinear optical potentials under overdamped or underdamped regime. The two-dimensional optical ratchet rectifies motion of Brownian particles in any direction in the plane, unstable cubic optical potential results in noise-induced particle motion and action of non-conservative optical spin-force leads to orbiting of a levitated particle.
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We will present our recent theoretical and experimental results related to the behaviour of micro- and nanoparticle placed into nonlinear optical potentials under overdamped or underdamped regime. The two-dimensional optical ratchet rectifies motion of Brownian particles in any direction in the plane, unstable cubic optical potential results in noise-induced particle motion and action of non-conservative optical spin-force leads to orbiting of a levitated particle.
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The zoo of nonconservative optical forces
S V Sukhov
Optical forces are usually described as conservative ones originating from intensity gradients in optical tweezers. However, the fundamental optical action on matter is nonconservative. In contrast to gradient forces, the spectrum of action of nonconservative forces is much wider: they can propel, pull, rotate objects or move objects along complicated trajectories. Different manifestations of nonconservative optical forces will be reviewed and their dependence on the specific spatial properties of optical fields will be discussed. New developments relevant to the nonconservative optical forces such as negative forces (tractor beams) and transversal forces will also be discussed.
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Optical forces are usually described as conservative ones originating from intensity gradients in optical tweezers. However, the fundamental optical action on matter is nonconservative. In contrast to gradient forces, the spectrum of action of nonconservative forces is much wider: they can propel, pull, rotate objects or move objects along complicated trajectories. Different manifestations of nonconservative optical forces will be reviewed and their dependence on the specific spatial properties of optical fields will be discussed. New developments relevant to the nonconservative optical forces such as negative forces (tractor beams) and transversal forces will also be discussed.
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Sensitivity of displacement detection for a particle levitated in the doughnut beam
Lei-Ming Zhou, Ke-Wen Xiao, Zhang-Qi Yin, Jun Chen, and Nan Zhao
Displacement detection of a spherical particle in focused laser beams with a quadrant photodetector provides a fast and high precision way to determine the particle location. In contrast to the traditional Gaussian beams, the sensitivity of displacement detection using various doughnut beams is investigated. The sensitivity improvement for large spherical particles along the longitudinal direction is reported. With appropriate vortex charge 𝑙 of the doughnut beams, they can outperform the Gaussian beam to get more than one order of magnitude higher sensitivity and, thus, have potential applications in various high-precision measurements. By using the levitating doughnut beam to detect the particle displacement, the result will also facilitate the recent proposal of levitating a particle in doughnut beams to suppress the light absorption.
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Displacement detection of a spherical particle in focused laser beams with a quadrant photodetector provides a fast and high precision way to determine the particle location. In contrast to the traditional Gaussian beams, the sensitivity of displacement detection using various doughnut beams is investigated. The sensitivity improvement for large spherical particles along the longitudinal direction is reported. With appropriate vortex charge 𝑙 of the doughnut beams, they can outperform the Gaussian beam to get more than one order of magnitude higher sensitivity and, thus, have potential applications in various high-precision measurements. By using the levitating doughnut beam to detect the particle displacement, the result will also facilitate the recent proposal of levitating a particle in doughnut beams to suppress the light absorption.
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Tuesday, November 6, 2018
Sequential trapping of single nanoparticles using a gold plasmonic nanohole array
Xue Han, Viet Giang Truong, Prince Sunil Thomas, and Síle Nic Chormaic
We have used a gold nanohole array to trap single polystyrene nanoparticles, with a mean diameter of 30 nm, into separated hot spots located at connecting nanoslot regions. A high trap stiffness of approximately [Math Processing Error] at a low-incident laser intensity of [Math Processing Error] at 980 nm was obtained. The experimental results were compared to the simulated trapping force, and a reasonable match was achieved. This plasmonic array is useful for lab-on-a-chip applications and has particular appeal for trapping multiple nanoparticles with predefined separations or arranged in patterns in order to study interactions between them.
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We have used a gold nanohole array to trap single polystyrene nanoparticles, with a mean diameter of 30 nm, into separated hot spots located at connecting nanoslot regions. A high trap stiffness of approximately [Math Processing Error] at a low-incident laser intensity of [Math Processing Error] at 980 nm was obtained. The experimental results were compared to the simulated trapping force, and a reasonable match was achieved. This plasmonic array is useful for lab-on-a-chip applications and has particular appeal for trapping multiple nanoparticles with predefined separations or arranged in patterns in order to study interactions between them.
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Probe chameleon dark energy with the coupling between a levitated nanodiamond and the single-spin
Jian Liu and Ka-Di Zhu
The chameleon scalar field is a dark energy candidate with the screening mechanism. In the present article we provide a spectral scheme to search the chameleon field between an optically levitated nanodiamond with a nitrogen-vacancy (NV) center and a silicon microsphere. The results show that a large optical nonlinear effect with ultranarrow bandwidth can be induced by the coupling of single spin and the levitated resonator. We find that the oscillator will present a distinct shift due to the chameleons, and the shift can be read out on the probe spectrum. Considering the noise limit, this constraint is about 2–3 orders of magnitude stronger than the ones from atom interferometry under the ultralow pressure. We expect this technique will become a valuable tool in optimizing future searches for chameleon fields and related theories.
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The chameleon scalar field is a dark energy candidate with the screening mechanism. In the present article we provide a spectral scheme to search the chameleon field between an optically levitated nanodiamond with a nitrogen-vacancy (NV) center and a silicon microsphere. The results show that a large optical nonlinear effect with ultranarrow bandwidth can be induced by the coupling of single spin and the levitated resonator. We find that the oscillator will present a distinct shift due to the chameleons, and the shift can be read out on the probe spectrum. Considering the noise limit, this constraint is about 2–3 orders of magnitude stronger than the ones from atom interferometry under the ultralow pressure. We expect this technique will become a valuable tool in optimizing future searches for chameleon fields and related theories.
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Fibrous polymer nanomaterials for biomedical applications and their transport by fluids: an overview
S. Pawłowska, T. A. Kowalewski and F. Pierini
Over the past few decades, there has been strong interest in the development of new micro- and nanomaterials for biomedical applications. Their use in the form of capsules, particles or filaments suspended in body fluids is associated with conformational changes and hydrodynamic interactions responsible for their transport. The dynamics of fibres or other long objects in Poiseuille flow is one of the fundamental problems in a variety of biomedical contexts, such as mobility of proteins, dynamics of DNA or other biological polymers, cell movement, tissue engineering, and drug delivery. In this review, we discuss several important applications of micro and nanoobjects in this field and try to understand the problems of their transport in flow resulting from material-environment interactions in typical, crowded, and complex biological fluids. Our aim is to elucidate the relationship between the nano- and microscopic structures of elongated polymer particles and their flow properties, thus opening the possibility to design nanoobjects that can be efficiently transported by body fluids for targeted drug release or local tissue regeneration.
DOI
Over the past few decades, there has been strong interest in the development of new micro- and nanomaterials for biomedical applications. Their use in the form of capsules, particles or filaments suspended in body fluids is associated with conformational changes and hydrodynamic interactions responsible for their transport. The dynamics of fibres or other long objects in Poiseuille flow is one of the fundamental problems in a variety of biomedical contexts, such as mobility of proteins, dynamics of DNA or other biological polymers, cell movement, tissue engineering, and drug delivery. In this review, we discuss several important applications of micro and nanoobjects in this field and try to understand the problems of their transport in flow resulting from material-environment interactions in typical, crowded, and complex biological fluids. Our aim is to elucidate the relationship between the nano- and microscopic structures of elongated polymer particles and their flow properties, thus opening the possibility to design nanoobjects that can be efficiently transported by body fluids for targeted drug release or local tissue regeneration.
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Single-particle analysis with 2D electro-optical trapping on an integrated optofluidic device
Mahmudur Rahman, Matthew A. Stott, Yucheng Li, Aaron R. Hawkins, and Holger Schmidt
Optical and optically assisted trapping has developed into an essential tool for studying and manipulating small objects with applications in numerous fields at the intersection of physics and the life sciences. Here, we report a waveguide-based optofluidic platform that restricts particle movement in two dimensions for full in-plane suppression of Brownian motion based on fluorescence tracking and application of an electrokinetic feedback force. Single-microbead trapping is demonstrated with confinement in two dimensions in a specially designed trapping volume. Tighter particle confinement and a 14× improvement in trap stiffness are achieved compared to 1D trapping along a fluidic channel. This paves the way for a chip-based high-throughput single-molecule analysis platform.
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Optical and optically assisted trapping has developed into an essential tool for studying and manipulating small objects with applications in numerous fields at the intersection of physics and the life sciences. Here, we report a waveguide-based optofluidic platform that restricts particle movement in two dimensions for full in-plane suppression of Brownian motion based on fluorescence tracking and application of an electrokinetic feedback force. Single-microbead trapping is demonstrated with confinement in two dimensions in a specially designed trapping volume. Tighter particle confinement and a 14× improvement in trap stiffness are achieved compared to 1D trapping along a fluidic channel. This paves the way for a chip-based high-throughput single-molecule analysis platform.
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Current Trends of Microfluidic Single-Cell Technologies
Pallavi Shinde, Loganathan Mohan, Amogh Kumar, Koyel Dey, Anjali Maddi, Alexander N. Patananan, Fan-Gang Tseng, Hwan-You Chang, Moeto Nagai and Tuhin Subhra Santra
The investigation of human disease mechanisms is difficult due to the heterogeneity in gene expression and the physiological state of cells in a given population. In comparison to bulk cell measurements, single-cell measurement technologies can provide a better understanding of the interactions among molecules, organelles, cells, and the microenvironment, which can aid in the development of therapeutics and diagnostic tools. In recent years, single-cell technologies have become increasingly robust and accessible, although limitations exist. In this review, we describe the recent advances in single-cell technologies and their applications in single-cell manipulation, diagnosis, and therapeutics development.
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Patterned Optoelectronic Tweezers: A New Scheme for Selecting, Moving, and Storing Dielectric Particles and Cells
Shuailong Zhang Nika Shakiba Yujie Chen Yanfeng Zhang Pengfei Tian Jastaranpreet Singh M. Dean Chamberlain Monika Satkauskas Andrew G. Flood Nazir P. Kherani Siyuan Yu Peter W. Zandstra Aaron R. Wheeler
Optical micromanipulation has become popular for a wide range of applications. In this work, a new type of optical micromanipulation platform, patterned optoelectronic tweezers (p‐OET), is introduced. In p‐OET devices, the photoconductive layer (that is continuous in a conventional OET device) is patterned, forming regions in which the electrode layer is locally exposed. It is demonstrated that micropatterns in the photoconductive layer are useful for repelling unwanted particles/cells, and also for keeping selected particles/cells in place after turning off the light source, minimizing light‐induced heating. To clarify the physical mechanism behind these effects, systematic simulations are carried out, which indicate the existence of strong nonuniform electric fields at the boundary of micropatterns. The simulations are consistent with experimental observations, which are explored for a wide variety of geometries and conditions. It is proposed that the new technique may be useful for myriad applications in the rapidly growing area of optical micromanipulation.
DOI
Optical micromanipulation has become popular for a wide range of applications. In this work, a new type of optical micromanipulation platform, patterned optoelectronic tweezers (p‐OET), is introduced. In p‐OET devices, the photoconductive layer (that is continuous in a conventional OET device) is patterned, forming regions in which the electrode layer is locally exposed. It is demonstrated that micropatterns in the photoconductive layer are useful for repelling unwanted particles/cells, and also for keeping selected particles/cells in place after turning off the light source, minimizing light‐induced heating. To clarify the physical mechanism behind these effects, systematic simulations are carried out, which indicate the existence of strong nonuniform electric fields at the boundary of micropatterns. The simulations are consistent with experimental observations, which are explored for a wide variety of geometries and conditions. It is proposed that the new technique may be useful for myriad applications in the rapidly growing area of optical micromanipulation.
DOI
Mechanochemical feedback control of dynamin independent endocytosis modulates membrane tension in adherent cells
Joseph Jose Thottacherry, Anita Joanna Kosmalska, Amit Kumar, Amit Singh Vishen, Alberto Elosegui-Artola, Susav Pradhan, Sumit Sharma, Parvinder P. Singh, Marta C. Guadamillas, Natasha Chaudhary, Ram Vishwakarma, Xavier Trepat, Miguel A. del Pozo, Robert G. Parton, Madan Rao, Pramod Pullarkat, Pere Roca-Cusachs & Satyajit Mayor
Plasma membrane tension regulates many key cellular processes. It is modulated by, and can modulate, membrane trafficking. However, the cellular pathway(s) involved in this interplay is poorly understood. Here we find that, among a number of endocytic processes operating simultaneously at the cell surface, a dynamin independent pathway, the CLIC/GEEC (CG) pathway, is rapidly and specifically upregulated upon a sudden reduction of tension. Moreover, inhibition (activation) of the CG pathway results in lower (higher) membrane tension. However, alteration in membrane tension does not directly modulate CG endocytosis. This requires vinculin, a mechano-transducer recruited to focal adhesion in adherent cells. Vinculin acts by controlling the levels of a key regulator of the CG pathway, GBF1, at the plasma membrane. Thus, the CG pathway directly regulates membrane tension and is in turn controlled via a mechano-chemical feedback inhibition, potentially leading to homeostatic regulation of membrane tension in adherent cells.
DOI
Plasma membrane tension regulates many key cellular processes. It is modulated by, and can modulate, membrane trafficking. However, the cellular pathway(s) involved in this interplay is poorly understood. Here we find that, among a number of endocytic processes operating simultaneously at the cell surface, a dynamin independent pathway, the CLIC/GEEC (CG) pathway, is rapidly and specifically upregulated upon a sudden reduction of tension. Moreover, inhibition (activation) of the CG pathway results in lower (higher) membrane tension. However, alteration in membrane tension does not directly modulate CG endocytosis. This requires vinculin, a mechano-transducer recruited to focal adhesion in adherent cells. Vinculin acts by controlling the levels of a key regulator of the CG pathway, GBF1, at the plasma membrane. Thus, the CG pathway directly regulates membrane tension and is in turn controlled via a mechano-chemical feedback inhibition, potentially leading to homeostatic regulation of membrane tension in adherent cells.
DOI
Probing Cell Mechanics with Bead-Free Optical Tweezers in the Drosophila Embryo
Claire Chardès, Raphael Clement, Olivier Blanc, Pierre-François Lenne
Morphogenesis requires coordination between genetic patterning and mechanical forces to robustly shape the cells and tissues. Hence, a challenge to understand morphogenetic processes is to directly measure cellular forces and mechanical properties in vivo during embryogenesis. Here, we present a setup of optical tweezers coupled to a light sheet microscope, which allows to directly apply forces on cell-cell contacts of the early Drosophila embryo, while imaging at a speed of several frames per second. This technique has the advantage that it does not require the injection of beads into the embryo, usually used as intermediate probes on which optical forces are exerted. We detail step by step the implementation of the setup, and propose tools to extract mechanical information from the experiments. By monitoring the displacements of cell-cell contacts in real time, one can perform tension measurements and investigate cell contacts' rheology.
DOI
Morphogenesis requires coordination between genetic patterning and mechanical forces to robustly shape the cells and tissues. Hence, a challenge to understand morphogenetic processes is to directly measure cellular forces and mechanical properties in vivo during embryogenesis. Here, we present a setup of optical tweezers coupled to a light sheet microscope, which allows to directly apply forces on cell-cell contacts of the early Drosophila embryo, while imaging at a speed of several frames per second. This technique has the advantage that it does not require the injection of beads into the embryo, usually used as intermediate probes on which optical forces are exerted. We detail step by step the implementation of the setup, and propose tools to extract mechanical information from the experiments. By monitoring the displacements of cell-cell contacts in real time, one can perform tension measurements and investigate cell contacts' rheology.
DOI
Thursday, November 1, 2018
Spin and Orbital Rotation of Plasmonic Dimer Driven by Circularly Polarized Light
Jiunn-Woei Liaw, Mao-Chang Huang, Hsueh-Yu Chao and Mao-Kuen Kuo
The plasmon-enhanced spin and orbital rotation of Au dimer, two optically bound nanoparticles (NPs), induced by a circularly polarized (CP) light (plane wave or Gaussian beam) were studied theoretically. Through the optomechanical performances of optical forces and torques, the longitudinal/transverse spin-orbit coupling (SOC) of twisted electromagnetic fields was investigated. The optical forces show that for the long-range interaction, there exist some stable-equilibrium orbits for rotation, where the stable-equilibrium interparticle distances are nearly the integer multiples of wavelength in medium. In addition, the optical spin torque drives each NP to spin individually. For a plane wave, the helicities of the longitudinal spin and orbital rotation of the coupled NPs are the same at the stable-equilibrium orbit, consistent with the handedness of plane wave. In contrast, for a focused Gaussian beam, the helicity of the orbital rotation of dimer could be opposite to the handedness of the incident light due to the negative optical orbital torque at the stable-equilibrium interparticle distance; additionally, the transverse spin of each NP becomes profound. These results demonstrate that the longitudinal/transverse SOC is significantly induced due to the twisted optical field. For the short-range interaction, the mutual attraction between two NPs is induced, associated with the spinning and spiral trajectory; eventually, the two NPs will collide. The borderline of the interparticle distance between the long-range and short-range interactions is approximately at a half-wavelength in medium.
DOI
The plasmon-enhanced spin and orbital rotation of Au dimer, two optically bound nanoparticles (NPs), induced by a circularly polarized (CP) light (plane wave or Gaussian beam) were studied theoretically. Through the optomechanical performances of optical forces and torques, the longitudinal/transverse spin-orbit coupling (SOC) of twisted electromagnetic fields was investigated. The optical forces show that for the long-range interaction, there exist some stable-equilibrium orbits for rotation, where the stable-equilibrium interparticle distances are nearly the integer multiples of wavelength in medium. In addition, the optical spin torque drives each NP to spin individually. For a plane wave, the helicities of the longitudinal spin and orbital rotation of the coupled NPs are the same at the stable-equilibrium orbit, consistent with the handedness of plane wave. In contrast, for a focused Gaussian beam, the helicity of the orbital rotation of dimer could be opposite to the handedness of the incident light due to the negative optical orbital torque at the stable-equilibrium interparticle distance; additionally, the transverse spin of each NP becomes profound. These results demonstrate that the longitudinal/transverse SOC is significantly induced due to the twisted optical field. For the short-range interaction, the mutual attraction between two NPs is induced, associated with the spinning and spiral trajectory; eventually, the two NPs will collide. The borderline of the interparticle distance between the long-range and short-range interactions is approximately at a half-wavelength in medium.
DOI
Optical fiber tip tweezers, a complementary approach for nanoparticle trapping
Godefroy Leménager; Khalid Lahlil; Thierry Gacoin; Gérard Colas des Francs; Jochen Fick
Nanoparticles of different compositions, sizes, and shapes are elaborated and trapped in a reproducible and stable manner using our optical fiber tip tweezers. Here we report trapping of spherical YAG : Ce3 + particles of 300 and 60 nm diameters and of NaYF4 : Er3 + , Yb3 + nanorods of lengths from 640 nm to 1.9 μm. The properties of the tweezers are analyzed by video observations using Boltzmann statistics and power spectra analysis. Efficient trapping is found for the spherical particles and the small nanorods, whereas the large nanorods are efficiently trapped with only one fiber tip. The optical emission of a trapped nanorod completes the experimental results. Force calculations using the exact Maxwell stress tensor formalism are conducted to explain the experimental observations.
DOI
Nanoparticles of different compositions, sizes, and shapes are elaborated and trapped in a reproducible and stable manner using our optical fiber tip tweezers. Here we report trapping of spherical YAG : Ce3 + particles of 300 and 60 nm diameters and of NaYF4 : Er3 + , Yb3 + nanorods of lengths from 640 nm to 1.9 μm. The properties of the tweezers are analyzed by video observations using Boltzmann statistics and power spectra analysis. Efficient trapping is found for the spherical particles and the small nanorods, whereas the large nanorods are efficiently trapped with only one fiber tip. The optical emission of a trapped nanorod completes the experimental results. Force calculations using the exact Maxwell stress tensor formalism are conducted to explain the experimental observations.
DOI
Laser interference‐based technique for dynamic measurement of single cell deformation manipulated by optical tweezers
Jiaqi Liu, Fan Zhang, Lianqing Zhu, Xinghua Qu, Daping Chu
A laser interference‐based method was proposed to measure the deformation response of cell manipulated by optical tweezers. This method was implemented experimentally by integrating a laser illuminating system and optical tweezers with an inverted microscope. Interference fringes generated by the transmitted and reflected lights were recorded by a complementary metal oxide semiconductor camera. From the acquired images, cell height was calculated and cell morphology was constructed. To further validate this method, the morphological analyses of HeLa cells were performed in static state and during detachment process. Subsequently, the dynamic deformation responses of red blood cells were measured during manipulation with optical tweezers. Collectively, this laser interference‐based method precludes the requirement of complex optical alignment, allows easy integration with optical tweezers, and enables dynamic measurement of cell deformation response by using a conventional inverted microscope.
A laser interference‐based method was proposed to measure the deformation response of cell manipulated by optical tweezers. This method was implemented experimentally by integrating a laser illuminating system and optical tweezers with an inverted microscope. Interference fringes generated by the transmitted and reflected lights were recorded by a complementary metal oxide semiconductor camera. From the acquired images, cell height was calculated and cell morphology was constructed. To further validate this method, the morphological analyses of HeLa cells were performed in static state and during detachment process. Subsequently, the dynamic deformation responses of red blood cells were measured during manipulation with optical tweezers. Collectively, this laser interference‐based method precludes the requirement of complex optical alignment, allows easy integration with optical tweezers, and enables dynamic measurement of cell deformation response by using a conventional inverted microscope.
Photothermal Convection Lithography for Rapid and Direct Assembly of Colloidal Plasmonic Nanoparticles on Generic Substrates
Chang Min Jin, Wooju Lee, Dongchoul Kim, Taewook Kang, Inhee Choi
Controlled assembly of colloidal nanoparticles onto solid substrates generally needs to overcome their thermal diffusion in water. For this purpose, several techniques that are based on chemical bonding, capillary interactions with substrate patterning, optical force, and optofluidic heating of light‐absorbing substrates are proposed. However, the direct assembly of colloidal nanoparticles on generic substrates without chemical linkers and substrate patterning still remains challenging. Here, photothermal convection lithography is proposed, which allows the rapid placement of colloidal nanoparticles onto the surface of diverse solid substrates. It is based on local photothermal heating of colloidal nanoparticles by resonant light focusing without substrate heating, which induces convective flow. The convective flow, then, forces the colloidal nanoparticles to assemble at the illumination point of light. The size of the assembly is increased by either increasing the light intensity or illumination time. It is shown that three types of colloidal gold nanoparticles with different shapes (rod, star, and sphere) can be uniformly assembled by the proposed method. Each assembly with a diameter of tens of micrometers can be completed within a minute and its patterned arrays can also be achieved rapidly.
DOI
Controlled assembly of colloidal nanoparticles onto solid substrates generally needs to overcome their thermal diffusion in water. For this purpose, several techniques that are based on chemical bonding, capillary interactions with substrate patterning, optical force, and optofluidic heating of light‐absorbing substrates are proposed. However, the direct assembly of colloidal nanoparticles on generic substrates without chemical linkers and substrate patterning still remains challenging. Here, photothermal convection lithography is proposed, which allows the rapid placement of colloidal nanoparticles onto the surface of diverse solid substrates. It is based on local photothermal heating of colloidal nanoparticles by resonant light focusing without substrate heating, which induces convective flow. The convective flow, then, forces the colloidal nanoparticles to assemble at the illumination point of light. The size of the assembly is increased by either increasing the light intensity or illumination time. It is shown that three types of colloidal gold nanoparticles with different shapes (rod, star, and sphere) can be uniformly assembled by the proposed method. Each assembly with a diameter of tens of micrometers can be completed within a minute and its patterned arrays can also be achieved rapidly.
DOI
Mechanical Force-Driven Adherens Junction Remodeling and Epithelial Dynamics
Diana Pinheiro, Yohanns Bellaïche
During epithelial tissue development, repair, and homeostasis, adherens junctions (AJs) ensure intercellular adhesion and tissue integrity while allowing for cell and tissue dynamics. Mechanical forces play critical roles in AJs’ composition and dynamics. Recent findings highlight that beyond a well-established role in reinforcing cell-cell adhesion, AJ mechanosensitivity promotes junctional remodeling and polarization, thereby regulating critical processes such as cell intercalation, division, and collective migration. Here, we provide an integrated view of mechanosensing mechanisms that regulate cell-cell contact composition, geometry, and integrity under tension and highlight pivotal roles for mechanosensitive AJ remodeling in preserving epithelial integrity and sustaining tissue dynamics.
DOI
During epithelial tissue development, repair, and homeostasis, adherens junctions (AJs) ensure intercellular adhesion and tissue integrity while allowing for cell and tissue dynamics. Mechanical forces play critical roles in AJs’ composition and dynamics. Recent findings highlight that beyond a well-established role in reinforcing cell-cell adhesion, AJ mechanosensitivity promotes junctional remodeling and polarization, thereby regulating critical processes such as cell intercalation, division, and collective migration. Here, we provide an integrated view of mechanosensing mechanisms that regulate cell-cell contact composition, geometry, and integrity under tension and highlight pivotal roles for mechanosensitive AJ remodeling in preserving epithelial integrity and sustaining tissue dynamics.
DOI
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