Mark S. Friddin, Guido Bolognesi, Ali Salehi-Reyhani, Oscar Ces & Yuval Elani
Multicomponent lipid bilayers can give rise to coexisting liquid domains that are thought to influence a host of cellular activities. There currently exists no method to directly manipulate such domains, hampering our understanding of their significance. Here we report a system that allows individual liquid ordered domains that exist in a liquid disordered matrix to be directly manipulated using optical tweezers. This allows us to drag domains across the membrane surface of giant vesicles that are adhered to a glass surface, enabling domain location to be defined with spatiotemporal control. We can also use the laser to select individual vesicles in a population to undergo mixing/demixing by locally heating the membrane through the miscibility transition, demonstrating a further layer of control. This technology has potential as a tool to shed light on domain biophysics, on their role in biology, and in sculpting membrane assemblies with user-defined membrane patterning.
DOI
Wednesday, January 30, 2019
Propagation properties of Airy Ince–Gaussian wave packets in gradient-index media
Xi Peng, Yingji He, Dongmei Deng, Yunli Qiu, Xing Zhu and Liping Zhang
We study localized Airy Ince–Gaussian (AiIG) wave packets in gradient-index (GRIN) media by solving the spatiotemporal evolution equation analytically. The generation and manipulation of AiIG wave packets in GRIN media are affected by the direction of the self-accelerating, elliptical, refractive index distribution parameter, initial velocity and mode number. In addition, energy flow, angular momentum and radiation force, which change periodically, are also thoroughly discussed. The Airy helical Ince–Gaussian wave packets exhibit multiple vortices and carry orbital angular momentum, which have potential applications in optical tweezers and optical communications.
DOI
We study localized Airy Ince–Gaussian (AiIG) wave packets in gradient-index (GRIN) media by solving the spatiotemporal evolution equation analytically. The generation and manipulation of AiIG wave packets in GRIN media are affected by the direction of the self-accelerating, elliptical, refractive index distribution parameter, initial velocity and mode number. In addition, energy flow, angular momentum and radiation force, which change periodically, are also thoroughly discussed. The Airy helical Ince–Gaussian wave packets exhibit multiple vortices and carry orbital angular momentum, which have potential applications in optical tweezers and optical communications.
DOI
Synergy between RecBCD subunits is essential for efficient DNA unwinding
Rani Zananiri, Omri Malik, Sergei Rudnizky, Vera Gaydar, Roman Kreiserman, Arnon Henn, Ariel Kaplan
The subunits of the bacterial RecBCD act in coordination, rapidly and processively unwinding DNA at the site of a double strand break. RecBCD is able to displace DNA-binding proteins, suggesting that it generates high forces, but the specific role of each subunit in the force generation is unclear. Here, we present a novel optical tweezers assay that allows monitoring the activity of RecBCD’s individual subunits, when they are part of an intact full complex. We show that RecBCD and its subunits are able to generate forces up to 25–40 pN without a significant effect on their velocity. Moreover, the isolated RecD translocates fast but is a weak helicase with limited processivity. Experiments at a broad range of [ATP] and forces suggest that RecD unwinds DNA as a Brownian ratchet, rectified by ATP binding, and that the presence of the other subunits shifts the ratchet equilibrium towards the post-translocation state.
DOI
The subunits of the bacterial RecBCD act in coordination, rapidly and processively unwinding DNA at the site of a double strand break. RecBCD is able to displace DNA-binding proteins, suggesting that it generates high forces, but the specific role of each subunit in the force generation is unclear. Here, we present a novel optical tweezers assay that allows monitoring the activity of RecBCD’s individual subunits, when they are part of an intact full complex. We show that RecBCD and its subunits are able to generate forces up to 25–40 pN without a significant effect on their velocity. Moreover, the isolated RecD translocates fast but is a weak helicase with limited processivity. Experiments at a broad range of [ATP] and forces suggest that RecD unwinds DNA as a Brownian ratchet, rectified by ATP binding, and that the presence of the other subunits shifts the ratchet equilibrium towards the post-translocation state.
DOI
Near-field optical trapping in a non-conservative force field
Mohammad Asif Zaman, Punnag Padhy & Lambertus Hesselink
The force-field generated by a near-field optical trap is analyzed. A C-shaped engraving on a gold film is considered as the trap. By separating out the conservative component and the solenoidal component of the force-field using Helmholtz-Hodge decomposition, it was found that the force is non-conservative. Conventional method of calculating the optical potential from the force-field is shown to be inaccurate when the trapping force is not purely conservative. An alternative method is presented to accurately estimate the potential. The positional statistics of a trapped nanoparticle in this non-conservative field is calculated. A model is proposed that relates the position distribution to the conservative component of the force. The model is found to be consistent with numerical and experimental results. In order to show the generality of the approach, the same analysis is repeated for a plasmonic trap consisting of a gold nanopillar. Similar consistency is observed for this structure as well.
The force-field generated by a near-field optical trap is analyzed. A C-shaped engraving on a gold film is considered as the trap. By separating out the conservative component and the solenoidal component of the force-field using Helmholtz-Hodge decomposition, it was found that the force is non-conservative. Conventional method of calculating the optical potential from the force-field is shown to be inaccurate when the trapping force is not purely conservative. An alternative method is presented to accurately estimate the potential. The positional statistics of a trapped nanoparticle in this non-conservative field is calculated. A model is proposed that relates the position distribution to the conservative component of the force. The model is found to be consistent with numerical and experimental results. In order to show the generality of the approach, the same analysis is repeated for a plasmonic trap consisting of a gold nanopillar. Similar consistency is observed for this structure as well.
Polarization induced control of optical trap potentials in binary liquids
Dipankar Mondal, Sirshendu Dinda, Soumendra Nath Bandyopadhyay & Debabrata Goswami
We illustrate control of a polarized laser optical trapping potential landscape through the nonideal mixing of binary liquids. The inherent trapping potential asymmetry (ITPA) present in the trapping region results from the asymmetric intensity distribution in focal volume due to the high numerical aperture objective lens. Experimentally, we show that this ITPA effect can be modified and/or removed by the use of binary liquid mixtures. From our femtosecond optical tweezers experiments, we determine the topograph of the trapping potential base on the fluctuation-dissipation theorem. Additionally, the Brownian motion of the trapped bead is sensitive to the frictional force (FF) of the surroundings that is exerted by clusters of water and alcohol binary mixture through extended hydrogen bonding. Thus, using these two effects, ITPA and FF of the medium, we have shown that one can indeed modify the effective trapping potential landscape. Water-alcohol binary mixtures display a nonlinear dependence on the microrheological properties of the solvent composition as a result of rigid cluster formation. Volumetrically, at about 30% methanol in water binary mixture, the trapping asymmetry is minimal. In this particular binary mixture composition, the hydrophobic part of the methanol molecule is surrounded by ‘cages’ of water molecules. Enhanced H-bonding network of water molecules results in higher viscosity, which contributes to the higher frictional force. Increased viscosity decreases the degree of anisotropy due to hindered dipolar rotation. However, at higher methanol concentrations, the methanol molecules are no longer contained within the water cages and are free to move, which decrease their overall bulk viscosity. Thus, for pure solvents, experimentally measured anisotropy matches quite well with the theoretical prediction, but this fails in case of the binary mixtures due to the increased frictional force exerted by binary mixtures that result from the formation of cage-like structures.
DOI
We illustrate control of a polarized laser optical trapping potential landscape through the nonideal mixing of binary liquids. The inherent trapping potential asymmetry (ITPA) present in the trapping region results from the asymmetric intensity distribution in focal volume due to the high numerical aperture objective lens. Experimentally, we show that this ITPA effect can be modified and/or removed by the use of binary liquid mixtures. From our femtosecond optical tweezers experiments, we determine the topograph of the trapping potential base on the fluctuation-dissipation theorem. Additionally, the Brownian motion of the trapped bead is sensitive to the frictional force (FF) of the surroundings that is exerted by clusters of water and alcohol binary mixture through extended hydrogen bonding. Thus, using these two effects, ITPA and FF of the medium, we have shown that one can indeed modify the effective trapping potential landscape. Water-alcohol binary mixtures display a nonlinear dependence on the microrheological properties of the solvent composition as a result of rigid cluster formation. Volumetrically, at about 30% methanol in water binary mixture, the trapping asymmetry is minimal. In this particular binary mixture composition, the hydrophobic part of the methanol molecule is surrounded by ‘cages’ of water molecules. Enhanced H-bonding network of water molecules results in higher viscosity, which contributes to the higher frictional force. Increased viscosity decreases the degree of anisotropy due to hindered dipolar rotation. However, at higher methanol concentrations, the methanol molecules are no longer contained within the water cages and are free to move, which decrease their overall bulk viscosity. Thus, for pure solvents, experimentally measured anisotropy matches quite well with the theoretical prediction, but this fails in case of the binary mixtures due to the increased frictional force exerted by binary mixtures that result from the formation of cage-like structures.
DOI
Optical radiation force (per–length) on an electrically conducting elliptical cylinder having a smooth or ribbed surface
F. G. Mitri
The aim of this work is to develop a formal semi-analytical model using the modal expansion method in cylindrical coordinates to calculate the optical/electromagnetic (EM) radiation force-per-length experienced by an infinitely long electrically-conducting elliptical cylinder having a smooth or wavy/corrugated surface in EM plane progressive waves with different polarizations. In this analysis, one of the semi-axes of the elliptical cylinder coincides with the direction of the incident field. Initially, the modal matching method is used to determine the scattering coefficients by imposing appropriate boundary conditions and solving numerically a linear system of equations by matrix inversion. In this method, standard cylindrical (Bessel and Hankel) wave functions are used. Subsequently, simplified expressions leading to exact series expansions for the optical/EM radiation forces assuming either electric (TM) or magnetic (TE) plane wave incidences are provided without any approximations, in addition to integral equations demonstrating the direct relationship of the radiation force with the square of the scattered field magnitude. An important application of these integral equations concerns the accurate determination of the radiation force from the measurement of the scattered field by any 2D non-absorptive object of arbitrary shape in plane waves. Numerical computations for the non-dimensional radiation force function are performed for electrically conducting elliptic and circular cylinders having a smooth or ribbed/corrugated surface. Adequate convergence plots confirm the validity and correctness of the method to evaluate the radiation force with no limitation to a particular frequency range (i.e. Rayleigh, Mie, or geometrical optics regimes). Particular emphases are given on the aspect ratio, the non-dimensional size of the cylinder, the corrugation characteristic of its surface, and the polarization of the incident field. The results are particularly relevant in optical tweezers and other related applications in fluid dynamics, where the shape and stability of a cylindrical drop stressed by a uniform external electric/magnetic field are altered. Furthermore, a direct analogy with the acoustical counterpart is noted and discussed.
DOI
The aim of this work is to develop a formal semi-analytical model using the modal expansion method in cylindrical coordinates to calculate the optical/electromagnetic (EM) radiation force-per-length experienced by an infinitely long electrically-conducting elliptical cylinder having a smooth or wavy/corrugated surface in EM plane progressive waves with different polarizations. In this analysis, one of the semi-axes of the elliptical cylinder coincides with the direction of the incident field. Initially, the modal matching method is used to determine the scattering coefficients by imposing appropriate boundary conditions and solving numerically a linear system of equations by matrix inversion. In this method, standard cylindrical (Bessel and Hankel) wave functions are used. Subsequently, simplified expressions leading to exact series expansions for the optical/EM radiation forces assuming either electric (TM) or magnetic (TE) plane wave incidences are provided without any approximations, in addition to integral equations demonstrating the direct relationship of the radiation force with the square of the scattered field magnitude. An important application of these integral equations concerns the accurate determination of the radiation force from the measurement of the scattered field by any 2D non-absorptive object of arbitrary shape in plane waves. Numerical computations for the non-dimensional radiation force function are performed for electrically conducting elliptic and circular cylinders having a smooth or ribbed/corrugated surface. Adequate convergence plots confirm the validity and correctness of the method to evaluate the radiation force with no limitation to a particular frequency range (i.e. Rayleigh, Mie, or geometrical optics regimes). Particular emphases are given on the aspect ratio, the non-dimensional size of the cylinder, the corrugation characteristic of its surface, and the polarization of the incident field. The results are particularly relevant in optical tweezers and other related applications in fluid dynamics, where the shape and stability of a cylindrical drop stressed by a uniform external electric/magnetic field are altered. Furthermore, a direct analogy with the acoustical counterpart is noted and discussed.
DOI
Tuesday, January 29, 2019
Gerchberg-Saxton algorithm for fast and efficient atom rearrangement in optical tweezer traps
Hyosub Kim, Minhyuk Kim, Woojun Lee, and Jaewook Ahn
We demonstrate fast and efficient neutral atom rearrangements in an optical tweezer-trap array, using an enhanced hologram generation algorithm. The conventional Gerchberg-Saxton (GS) algorithm is modified to include zero-padding hologram expansion for optical tweezer sharpness, weighted iteration feedback for reduced crosstalk, and phase induction for successive phase continuity. With the new GS algorithm, we experimentally demonstrate defect-free formation of 2D atom arrays with various geometries, achieving a high loading probability of 0.98 for up to N ∼ 30 atoms. Furthermore, the hologram movie calculation speed is enhanced to cover a computational scalability up to 𝒪(103).
DOI
We demonstrate fast and efficient neutral atom rearrangements in an optical tweezer-trap array, using an enhanced hologram generation algorithm. The conventional Gerchberg-Saxton (GS) algorithm is modified to include zero-padding hologram expansion for optical tweezer sharpness, weighted iteration feedback for reduced crosstalk, and phase induction for successive phase continuity. With the new GS algorithm, we experimentally demonstrate defect-free formation of 2D atom arrays with various geometries, achieving a high loading probability of 0.98 for up to N ∼ 30 atoms. Furthermore, the hologram movie calculation speed is enhanced to cover a computational scalability up to 𝒪(103).
DOI
Regulation of Rep helicase unwinding by an auto-inhibitory subdomain
Monika A Makurath, Kevin D Whitley, Binh Nguyen, Timothy M Lohman, Yann R Chemla
Helicases are biomolecular motors that unwind nucleic acids, and their regulation is essential for proper maintenance of genomic integrity. Escherichia coli Rep helicase, whose primary role is to help restart stalled replication, serves as a model for Superfamily I helicases. The activity of Rep-like helicases is regulated by two factors: their oligomeric state, and the conformation of the flexible subdomain 2B. However, the mechanism of control is not well understood. To understand the factors that regulate the active state of Rep, here we investigate the behavior of a 2B-deficient variant (RepΔ2B) in relation to wild-type Rep (wtRep). Using a single-molecule optical tweezers assay, we explore the effects of oligomeric state, DNA geometry, and duplex stability on wtRep and RepΔ2B unwinding activity. We find that monomeric RepΔ2B unwinds more processively and at a higher speed than the activated, dimeric form of wtRep. The unwinding processivity of RepΔ2B and wtRep is primarily limited by ‘strand-switching’—during which the helicases alternate between strands of the duplex—which does not require the 2B subdomain, contrary to a previous proposal. We provide a quantitative model of the factors that enhance unwinding processivity. Our work sheds light on the mechanisms of regulation of unwinding by Rep-like helicases.
DOI
Helicases are biomolecular motors that unwind nucleic acids, and their regulation is essential for proper maintenance of genomic integrity. Escherichia coli Rep helicase, whose primary role is to help restart stalled replication, serves as a model for Superfamily I helicases. The activity of Rep-like helicases is regulated by two factors: their oligomeric state, and the conformation of the flexible subdomain 2B. However, the mechanism of control is not well understood. To understand the factors that regulate the active state of Rep, here we investigate the behavior of a 2B-deficient variant (RepΔ2B) in relation to wild-type Rep (wtRep). Using a single-molecule optical tweezers assay, we explore the effects of oligomeric state, DNA geometry, and duplex stability on wtRep and RepΔ2B unwinding activity. We find that monomeric RepΔ2B unwinds more processively and at a higher speed than the activated, dimeric form of wtRep. The unwinding processivity of RepΔ2B and wtRep is primarily limited by ‘strand-switching’—during which the helicases alternate between strands of the duplex—which does not require the 2B subdomain, contrary to a previous proposal. We provide a quantitative model of the factors that enhance unwinding processivity. Our work sheds light on the mechanisms of regulation of unwinding by Rep-like helicases.
DOI
High-Bandwidth 3-D Multitrap Actuation Technique for 6-DoF Real-Time Control of Optical Robots
Edison Gerena; Stéphane Régnier; Sinan Haliyo
Optical robots are microscale structures actuated using laser trapping techniques. However, the lack of robust and real-time three-dimensional (3-D) actuation techniques reduces most applications to planar space. We present here a new approach to generate and control several optical traps synchronously in 3-D with low latency and high bandwidth (up to 200 Hz). This time-shared technique uses only mirrors, hence, is aberration free. Simultaneous traps are used to actuate optical robots and provide 6-DoF telemanipulation. Experiments demonstrate the flexibility and dexterity of the implemented user control, paving the way to novel applications in microrobotics and biology.
DOI
Optical robots are microscale structures actuated using laser trapping techniques. However, the lack of robust and real-time three-dimensional (3-D) actuation techniques reduces most applications to planar space. We present here a new approach to generate and control several optical traps synchronously in 3-D with low latency and high bandwidth (up to 200 Hz). This time-shared technique uses only mirrors, hence, is aberration free. Simultaneous traps are used to actuate optical robots and provide 6-DoF telemanipulation. Experiments demonstrate the flexibility and dexterity of the implemented user control, paving the way to novel applications in microrobotics and biology.
DOI
Trapping and Optomechanical Sensing of Particles with a Nanobeam Photonic Crystal Cavity
Lin Ren, Yunpeng Li, Na Li and Chao Chen
Particle trapping and sensing serve as important tools for non-invasive studies of individual molecule or cell in bio-photonics. For such applications, it is required that the optical power to trap and detect particles is as low as possible, since large optical power would have side effects on biological particles. In this work, we proposed to deploy a nanobeam photonic crystal cavity for particle trapping and opto-mechanical sensing. For particles captured at 300 K, the input optical power was predicted to be as low as 48.8 μW by calculating the optical force and potential of a polystyrene particle with a radius of 150 nm when the trapping cavity was set in an aqueous environment. Moreover, both the optical and mechanical frequency shifts for particles with different sizes were calculated, which can be detected and distinguished by the optomechanical coupling between the particle and the designed cavity. The relative variation of the mechanical frequency achieved approximately 400%, which indicated better particle sensing compared with the variation of the optical frequency (±0.06%). Therefore, our proposed cavity shows promising potential as functional components in future particle trapping and manipulating applications in lab-on-chip.
DOI
Particle trapping and sensing serve as important tools for non-invasive studies of individual molecule or cell in bio-photonics. For such applications, it is required that the optical power to trap and detect particles is as low as possible, since large optical power would have side effects on biological particles. In this work, we proposed to deploy a nanobeam photonic crystal cavity for particle trapping and opto-mechanical sensing. For particles captured at 300 K, the input optical power was predicted to be as low as 48.8 μW by calculating the optical force and potential of a polystyrene particle with a radius of 150 nm when the trapping cavity was set in an aqueous environment. Moreover, both the optical and mechanical frequency shifts for particles with different sizes were calculated, which can be detected and distinguished by the optomechanical coupling between the particle and the designed cavity. The relative variation of the mechanical frequency achieved approximately 400%, which indicated better particle sensing compared with the variation of the optical frequency (±0.06%). Therefore, our proposed cavity shows promising potential as functional components in future particle trapping and manipulating applications in lab-on-chip.
DOI
Nano‐Optical Tweezing of Single Proteins in Plasmonic Nanopores
Daniel Verschueren, Xin Shi, Cees Dekker
Single‐molecule sensing technologies aim to detect and characterize single biomolecules, but generally need labeling while the measurement times and throughput are severely restricted by a lack of positional control over the molecule. Here, a plasmonic nanopore biosensor is reported where single molecules can be electrophoretically delivered into a nanopore sensor with a plasmonic nanoantenna that is used to optically trap single molecules for extended measurement times. Using the light transmission through the antenna as read‐out, optical trapping of 20 nm diameter polystyrene nanoparticles and individual beta‐amylase proteins, a 200 kDa enzyme, in the plasmonic nanoantenna are demonstrated. Application of an electrical bias voltage allows the increase of the event rate over an order of magnitude as well as shorten the residence time of the proteins in the plasmonic nanopore as they can controllably be drawn out of the trap by electrical forces. Trapping is found to be assisted by protein–surface interactions and trapped proteins can denature on the nanopore surface. The integration of two single‐molecule sensors, a plasmonic nanoantenna and solid‐state nanopore, creates independent control handles at the single‐molecule level—the optical trapping force and electrophoretic force—which provides augmented control over single molecules.
DOI
Single‐molecule sensing technologies aim to detect and characterize single biomolecules, but generally need labeling while the measurement times and throughput are severely restricted by a lack of positional control over the molecule. Here, a plasmonic nanopore biosensor is reported where single molecules can be electrophoretically delivered into a nanopore sensor with a plasmonic nanoantenna that is used to optically trap single molecules for extended measurement times. Using the light transmission through the antenna as read‐out, optical trapping of 20 nm diameter polystyrene nanoparticles and individual beta‐amylase proteins, a 200 kDa enzyme, in the plasmonic nanoantenna are demonstrated. Application of an electrical bias voltage allows the increase of the event rate over an order of magnitude as well as shorten the residence time of the proteins in the plasmonic nanopore as they can controllably be drawn out of the trap by electrical forces. Trapping is found to be assisted by protein–surface interactions and trapped proteins can denature on the nanopore surface. The integration of two single‐molecule sensors, a plasmonic nanoantenna and solid‐state nanopore, creates independent control handles at the single‐molecule level—the optical trapping force and electrophoretic force—which provides augmented control over single molecules.
DOI
Monday, January 28, 2019
Feynman's ratchet built from optical tweezers
Edwin Cartlidge
A thought experiment proposed more than 50 years ago by Richard Feynman has finally been realized in the lab by physicists in the US and China.
DOI
A thought experiment proposed more than 50 years ago by Richard Feynman has finally been realized in the lab by physicists in the US and China.
DOI
Online Characterization of Single Airborne Carbon Nanotube Particles Using Optical Trapping Raman Spectroscopy
Zhiyong Gong, Yong-le Pan, Gorden Videen, Chuji Wang
Carbon nanotubes (CNTs) have become recognized as a potential environmental and health hazard, as their applications are broadening and manufacturing costs are reducing. Fundamental information of CNTs in air is of significant importance to our understanding of their environmental fate as well as to the further applications. Extensive efforts have been made over decades on characterizing CNTs; however, a majority of the studies are of bulk or CNTs dispersed on substrates. In the present work, we characterize single CNT particles in air using optical trapping Raman spectroscopy (OT-RS). Different types of CNT particles, as well as glassy carbon spheres, were optically trapped in air. Their physical properties were viewed by microscopic bright field images and scattering images; their chemical properties and structural information can be inferred from characteristic Raman bands. The system also can spatially resolve the morphology and chemical distribution of optically trapped CNT particles in air. The OT-RS technique combines single-particle morphological and chemical information and offers an online method to characterize the physicochemical properties of single CNT particles at their native states in air.
DOI
Carbon nanotubes (CNTs) have become recognized as a potential environmental and health hazard, as their applications are broadening and manufacturing costs are reducing. Fundamental information of CNTs in air is of significant importance to our understanding of their environmental fate as well as to the further applications. Extensive efforts have been made over decades on characterizing CNTs; however, a majority of the studies are of bulk or CNTs dispersed on substrates. In the present work, we characterize single CNT particles in air using optical trapping Raman spectroscopy (OT-RS). Different types of CNT particles, as well as glassy carbon spheres, were optically trapped in air. Their physical properties were viewed by microscopic bright field images and scattering images; their chemical properties and structural information can be inferred from characteristic Raman bands. The system also can spatially resolve the morphology and chemical distribution of optically trapped CNT particles in air. The OT-RS technique combines single-particle morphological and chemical information and offers an online method to characterize the physicochemical properties of single CNT particles at their native states in air.
DOI
Selective Stiffening of Fibrin Hydrogels with Micron Resolution Via Photocrosslinking
Mark Keating, Micah Lim, Qingda Hu, Elliot Botvinick
Fibrin hydrogels are used as a model system for studying cell-ECM biophysical interactions. Bulk mechanical stiffness of these hydrogels has been correlated to mechanotransduction and downstream signaling. However, stiffness values proximal to cells can vary by orders of magnitude at the length scale of microns. Patterning of matrix stiffness at this spatial scale can be useful in studying such interactions. Here we present and evaluate a technique to selectively stiffen defined regions with in a fibrin hydrogel. Laser scanning illumination activates ruthenium-catalyzed crosslinking of fibrin tyrosine residues, resulting in tunable stiffness changes spanning distances as small as a few microns and a localized compaction of the material. As probed by active microrheology, stiffness increases by as much as 25X, similar to previously observed stiffness changes around single cells in 3D culture. In summary, our method allows for selective modification of fibrin stiffness at the micron scale with the potential to create complex patterns, which could be valuable for the investigation of mechanotransduction in a biologically meaningful way.
DOI
Fibrin hydrogels are used as a model system for studying cell-ECM biophysical interactions. Bulk mechanical stiffness of these hydrogels has been correlated to mechanotransduction and downstream signaling. However, stiffness values proximal to cells can vary by orders of magnitude at the length scale of microns. Patterning of matrix stiffness at this spatial scale can be useful in studying such interactions. Here we present and evaluate a technique to selectively stiffen defined regions with in a fibrin hydrogel. Laser scanning illumination activates ruthenium-catalyzed crosslinking of fibrin tyrosine residues, resulting in tunable stiffness changes spanning distances as small as a few microns and a localized compaction of the material. As probed by active microrheology, stiffness increases by as much as 25X, similar to previously observed stiffness changes around single cells in 3D culture. In summary, our method allows for selective modification of fibrin stiffness at the micron scale with the potential to create complex patterns, which could be valuable for the investigation of mechanotransduction in a biologically meaningful way.
DOI
Transport of solid bodies along tubular membrane tethers
D. R. Daniels
We study the crucial role of membrane fluctuations in maintaining a narrow gap between a fluid membrane tube and an enclosed solid particle. Solvent flows can occur in this gap, hence giving rise to a finite particle mobility along the tube. While our study has relevance for how cells are able to transport large organelles or other cargo along connecting membrane tubes, known as tunneling nanotubes, our calculations are also framed so that they can be tested by a specific in vitro experiment: A tubular membrane tether can be pulled from a membrane reservoir, such as an aspirated Giant Unilamellar Vesicle (GUV), e.g. using a conjugated bead that binds to the membrane and is held in a laser trap. We compute the subsequent mobility of colloidal particles trapped in the tube, focusing on the case when the particle is large compared to the equilibrium tube radius. We predict that the particle mobility should scale as ∼ σ−2/3, with σ the membrane tension.
DOI
We study the crucial role of membrane fluctuations in maintaining a narrow gap between a fluid membrane tube and an enclosed solid particle. Solvent flows can occur in this gap, hence giving rise to a finite particle mobility along the tube. While our study has relevance for how cells are able to transport large organelles or other cargo along connecting membrane tubes, known as tunneling nanotubes, our calculations are also framed so that they can be tested by a specific in vitro experiment: A tubular membrane tether can be pulled from a membrane reservoir, such as an aspirated Giant Unilamellar Vesicle (GUV), e.g. using a conjugated bead that binds to the membrane and is held in a laser trap. We compute the subsequent mobility of colloidal particles trapped in the tube, focusing on the case when the particle is large compared to the equilibrium tube radius. We predict that the particle mobility should scale as ∼ σ−2/3, with σ the membrane tension.
DOI
Chiral Optical Stern-Gerlach Newtonian Experiment
Nina Kravets, Artur Aleksanyan, and Etienne Brasselet
We report on a chiral optical Stern-Gerlach experiment where chiral liquid crystal microspheres are selectively displaced by means of optical forces arising from optical helicity gradients. The present Newtonian experimental demonstration of an effect predicted at molecular scale [New J. Phys. 16, 013020 (2014)] is a first instrumental step in an area restricted so far to theoretical discussions. Extending the Stern-Gerlach experiment legacy to chiral light-matter interactions should foster further studies, for instance towards the elaboration of chirality-enabled quantum technologies or spin-based optoelectronics.
DOI
We report on a chiral optical Stern-Gerlach experiment where chiral liquid crystal microspheres are selectively displaced by means of optical forces arising from optical helicity gradients. The present Newtonian experimental demonstration of an effect predicted at molecular scale [New J. Phys. 16, 013020 (2014)] is a first instrumental step in an area restricted so far to theoretical discussions. Extending the Stern-Gerlach experiment legacy to chiral light-matter interactions should foster further studies, for instance towards the elaboration of chirality-enabled quantum technologies or spin-based optoelectronics.
DOI
Friday, January 25, 2019
Toward characterizing extracellular vesicles at a single-particle level
Chun-yi Chiang and Chihchen Chen
Extracellular vesicles (EVs) are cell-derived membrane-bound vesicles that serve a means of cell-cell communication. Studying EVs at a single-particle level is important because EVs are inherently heterogeneous. Novel micro- and nanotechnological tools have open opportunities for realizing single-EV measurements exploiting their biochemical, electrical, mechanical, and/or optical properties. This review summarizes the recent development of technologies toward sorting and analyzing single EVs. Sorting EVs into a more homogeneous subset relaxes the sensitivity and throughput required on the EV detection, and hence related techniques are also included in this review. These exciting technologies are on the rise and will expand our understanding of EVs and their applications in the near future.
DOI
Extracellular vesicles (EVs) are cell-derived membrane-bound vesicles that serve a means of cell-cell communication. Studying EVs at a single-particle level is important because EVs are inherently heterogeneous. Novel micro- and nanotechnological tools have open opportunities for realizing single-EV measurements exploiting their biochemical, electrical, mechanical, and/or optical properties. This review summarizes the recent development of technologies toward sorting and analyzing single EVs. Sorting EVs into a more homogeneous subset relaxes the sensitivity and throughput required on the EV detection, and hence related techniques are also included in this review. These exciting technologies are on the rise and will expand our understanding of EVs and their applications in the near future.
DOI
Quadrupole interaction of non-diffracting beams with two-level atoms
Saud Al-Awfi, Smail Bougouffa
Recently it has been shown that the quadrupole interactions can be improved significantly as the atom interacts at near resonance with the Laguerre-Gaussian (LG) mode. In this paper, we illustrate that other kinds of optical vortex can be also led to a considerable enhancement of quadrupole interaction when the atom interacts with optical modes at near resonance. The calculations are performed on an interesting situation with Cs atom, where the process is concerned with dipole-forbidden and quadrupole-allowed transitions with a convenable choice of atomic and optical mode parameters. In this direction, we show that the quadrupole transitions can be significantly enhanced and therefore they can play an interesting role and lead to new features of atom-light interaction, which can have some constructive implications in experiments.
DOI
Recently it has been shown that the quadrupole interactions can be improved significantly as the atom interacts at near resonance with the Laguerre-Gaussian (LG) mode. In this paper, we illustrate that other kinds of optical vortex can be also led to a considerable enhancement of quadrupole interaction when the atom interacts with optical modes at near resonance. The calculations are performed on an interesting situation with Cs atom, where the process is concerned with dipole-forbidden and quadrupole-allowed transitions with a convenable choice of atomic and optical mode parameters. In this direction, we show that the quadrupole transitions can be significantly enhanced and therefore they can play an interesting role and lead to new features of atom-light interaction, which can have some constructive implications in experiments.
DOI
High-Performance Image-Based Measurements of Biological Forces and Interactions in a Dual Optical Trap
Jessica L. Killian, James T. Inman, and Michelle D. Wang
Optical traps enable the nanoscale manipulation of individual biomolecules while measuring molecular forces and lengths. This ability relies on the sensitive detection of optically trapped particles, typically accomplished using laser-based interferometric methods. Recently, image-based particle tracking techniques have garnered increased interest as a potential alternative to laser-based detection; however, successful integration of image-based methods into optical trapping instruments for biophysical applications and force measurements has remained elusive. Here, we develop a camera-based detection platform that enables accurate and precise measurements of biological forces and interactions in a dual optical trap. In demonstration, we stretch and unzip DNA molecules while measuring the relative distances of trapped particles from their trapping centers with sub-nanometer accuracy and precision. We then use the DNA unzipping technique to localize bound proteins with sub-base-pair precision, revealing how thermal DNA “breathing” fluctuations allow an unzipping fork to detect and respond to the presence of a protein bound downstream. This work advances the capabilities of image tracking in optical traps, providing a state-of-the-art detection method that is accessible, highly flexible, and broadly compatible with diverse experimental substrates and other nanometric techniques.
DOI
DOI
Microscopic Control and Detection of Ultracold Strontium in Optical-Tweezer Arrays
M. A. Norcia, A. W. Young, and A. M. Kaufman
Optical tweezers provide a versatile platform for the manipulation and detection of single atoms. Here, we use optical tweezers to demonstrate a set of tools for the microscopic control of atomic strontium, which has two valence electrons. Compared to the single-valence-electron atoms typically used with tweezers, strontium has a more complex internal state structure with a variety of transition wavelengths and linewidths. We report single-atom loading into an array of subwavelength scale optical tweezers and light-shift-free control of a narrow-linewidth optical transition. We use this transition to perform three-dimensional ground-state cooling and to enable high-fidelity nondestructive imaging of single atoms on subwavelength spatial scales. These capabilities, combined with the rich internal structure of strontium, open new possibilities including tweezer-based metrology, new quantum computing architectures, and new paths to low-entropy many-body physics.
Optical tweezers provide a versatile platform for the manipulation and detection of single atoms. Here, we use optical tweezers to demonstrate a set of tools for the microscopic control of atomic strontium, which has two valence electrons. Compared to the single-valence-electron atoms typically used with tweezers, strontium has a more complex internal state structure with a variety of transition wavelengths and linewidths. We report single-atom loading into an array of subwavelength scale optical tweezers and light-shift-free control of a narrow-linewidth optical transition. We use this transition to perform three-dimensional ground-state cooling and to enable high-fidelity nondestructive imaging of single atoms on subwavelength spatial scales. These capabilities, combined with the rich internal structure of strontium, open new possibilities including tweezer-based metrology, new quantum computing architectures, and new paths to low-entropy many-body physics.
Transcription factor regulation of RNA polymerase’s torque generation capacity
Jie Ma, Chuang Tan, Xiang Gao, Robert M. Fulbright Jr., Jeffrey W. Roberts, and Michelle D. Wang
During transcription, RNA polymerase (RNAP) supercoils DNA as it translocates. The resulting torsional stress in DNA can accumulate and, in the absence of regulatory mechanisms, becomes a barrier to RNAP elongation, causing RNAP stalling, backtracking, and transcriptional arrest. Here we investigate whether and how a transcription factor may regulate both torque-induced Escherichia coli RNAP stalling and the torque generation capacity of RNAP. Using a unique real-time angular optical trapping assay, we found that RNAP working against a resisting torque was highly prone to extensive backtracking. We then investigated transcription in the presence of GreB, a transcription factor known to rescue RNAP from the backtracked state. We found that GreB greatly suppressed RNAP backtracking and remarkably increased the torque that RNAP was able to generate by 65%, from 11.2 pN⋅nm to 18.5 pN·nm. Variance analysis of the real-time positional trajectories of RNAP after a stall revealed the kinetic parameters of backtracking and GreB rescue. These results demonstrate that backtracking is the primary mechanism by which torsional stress limits transcription and that the transcription factor GreB effectively enhances the torsional capacity of RNAP. These findings suggest a broader role for transcription factors in regulating RNAP functionality and elongation.
DOI
During transcription, RNA polymerase (RNAP) supercoils DNA as it translocates. The resulting torsional stress in DNA can accumulate and, in the absence of regulatory mechanisms, becomes a barrier to RNAP elongation, causing RNAP stalling, backtracking, and transcriptional arrest. Here we investigate whether and how a transcription factor may regulate both torque-induced Escherichia coli RNAP stalling and the torque generation capacity of RNAP. Using a unique real-time angular optical trapping assay, we found that RNAP working against a resisting torque was highly prone to extensive backtracking. We then investigated transcription in the presence of GreB, a transcription factor known to rescue RNAP from the backtracked state. We found that GreB greatly suppressed RNAP backtracking and remarkably increased the torque that RNAP was able to generate by 65%, from 11.2 pN⋅nm to 18.5 pN·nm. Variance analysis of the real-time positional trajectories of RNAP after a stall revealed the kinetic parameters of backtracking and GreB rescue. These results demonstrate that backtracking is the primary mechanism by which torsional stress limits transcription and that the transcription factor GreB effectively enhances the torsional capacity of RNAP. These findings suggest a broader role for transcription factors in regulating RNAP functionality and elongation.
DOI
Feeling the force: formin’s role in mechanotransduction
Dennis Zimmermann, David R Kovar
Fundamental cellular processes such as division, polarization, and motility require the tightly regulated spatial and temporal assembly and disassembly of the underlying actin cytoskeleton. The actin cytoskeleton has been long viewed as a central player facilitating diverse mechanotransduction pathways due to the notion that it is capable of receiving, processing, transmitting, and generating mechanical stresses. Recent work has begun to uncover the roles of mechanical stresses in modulating the activity of key regulatory actin-binding proteins and their interactions with actin filaments, thereby controlling the assembly (formin and Arp2/3 complex) and disassembly (ADF/Cofilin) of actin filament networks. In this review, we will focus on discussing the current molecular understanding of how members of the formin protein family sense and respond to forces and the potential implications for formin-mediated mechanotransduction in cells.
DOI
Fundamental cellular processes such as division, polarization, and motility require the tightly regulated spatial and temporal assembly and disassembly of the underlying actin cytoskeleton. The actin cytoskeleton has been long viewed as a central player facilitating diverse mechanotransduction pathways due to the notion that it is capable of receiving, processing, transmitting, and generating mechanical stresses. Recent work has begun to uncover the roles of mechanical stresses in modulating the activity of key regulatory actin-binding proteins and their interactions with actin filaments, thereby controlling the assembly (formin and Arp2/3 complex) and disassembly (ADF/Cofilin) of actin filament networks. In this review, we will focus on discussing the current molecular understanding of how members of the formin protein family sense and respond to forces and the potential implications for formin-mediated mechanotransduction in cells.
DOI
Thursday, January 24, 2019
Controlling the Dynamics and Optical Binding of Nanoparticle Homodimers with Transverse Phase Gradients
Curtis W. Peterson, John Parker, Stuart A. Rice, and Norbert F. Scherer
While transverse phase gradients enable studies of driven nonequilibrium phenomena in optical trapping, the behavior of electrodynamically interacting particles in a transverse phase gradient has not been explored in detail. In this Letter we study electrodynamically interacting pairs of identical nanoparticles (homodimers) in transverse phase gradients. We establish that the net driving force on homodimers is modulated by a separation-dependent interference effect for small phase gradients. By contrast, large phase gradients break the symmetry of the interaction between particles and profoundly change the electrodynamic interparticle energy landscape. Our findings are particularly important for understanding multiparticle dynamics during the self-assembly and rearrangement of optical matter.
DOI
While transverse phase gradients enable studies of driven nonequilibrium phenomena in optical trapping, the behavior of electrodynamically interacting particles in a transverse phase gradient has not been explored in detail. In this Letter we study electrodynamically interacting pairs of identical nanoparticles (homodimers) in transverse phase gradients. We establish that the net driving force on homodimers is modulated by a separation-dependent interference effect for small phase gradients. By contrast, large phase gradients break the symmetry of the interaction between particles and profoundly change the electrodynamic interparticle energy landscape. Our findings are particularly important for understanding multiparticle dynamics during the self-assembly and rearrangement of optical matter.
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Label-free detection of hydrogen peroxide-induced oxidative stress in human retinal pigment epithelium cells via laser tweezers Raman spectroscopy
Yang Chen, ZhiQiang Wang, Yan Huang, ShangYuan Feng, ZuCi Zheng, XiuJie Liu, and MengMeng Liu
Human retinal pigment epithelium cells under hydrogen peroxide-induced oxidative stress and a ligustrazine-based protective effect were investigated using laser tweezers Raman spectroscopy. Protein and lipid were significantly affected by oxidative damage, along with increased reactive oxygen species (ROS) level within cells. The effects of ligustrazine against the reaction of ROS with protein seemed to be able to inhibit such damages but were limited during the desamidization of amides, along with additional effect on nucleic acid base and DNA phosphoric acid skeleton. This work laid the basis for both understanding the molecular mechanisms of oxidative stress-induced injury and highlighting possible biomarkers in retinal diseases.
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Human retinal pigment epithelium cells under hydrogen peroxide-induced oxidative stress and a ligustrazine-based protective effect were investigated using laser tweezers Raman spectroscopy. Protein and lipid were significantly affected by oxidative damage, along with increased reactive oxygen species (ROS) level within cells. The effects of ligustrazine against the reaction of ROS with protein seemed to be able to inhibit such damages but were limited during the desamidization of amides, along with additional effect on nucleic acid base and DNA phosphoric acid skeleton. This work laid the basis for both understanding the molecular mechanisms of oxidative stress-induced injury and highlighting possible biomarkers in retinal diseases.
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Abruptly autofocusing property and optical manipulation of circular Airy beams
Wanli Lu, Xu Sun, Huajin Chen, Shiyang Liu, and Zhifang Lin
By employing the angular spectrum representation, we successfully derive the partial-wave expansion coefficients of the circular Airy beams (CABs) with different polarizations, based on which the scattering of a spherical particle in the CABs is solved exactly. Special attention is focused on exploring the potential applications of the CABs in the abruptly autofocusing (AAF) property and optical manipulation of microparticle by optical force. It is found that both the linear and circular polarizations are the best candidates to implement the AAF property robustly against strong disturbance by a large-sized particle. In addition, although the CABs can be intensively focused to a small region, resulting in an abrupt increase of light intensity by two orders of magnitude at the focal point and enabling three-dimensional stable trapping of a Rayleigh particle near the focal point, the usual CABs fail to generate an optical force outweighing the Brownian force to achieve a stable transverse trapping in the region before the focal point. On the other hand, a Mie particle can be confined transversely in the primary ring and accelerated along the curved trajectory of CABs to the focal point, then pushed further all the way through the focal point rather than being trapped therein, and eventually accelerated further along a straight trajectory in the direction of light propagation over 100 wavelengths, due to the weak diffraction characteristic of a morphed Bessel-like beam of CABs. The exotic curved-straight trajectory transport of particles by the CABs may find applications in the case where the transport path is partly blocked.
DOI
By employing the angular spectrum representation, we successfully derive the partial-wave expansion coefficients of the circular Airy beams (CABs) with different polarizations, based on which the scattering of a spherical particle in the CABs is solved exactly. Special attention is focused on exploring the potential applications of the CABs in the abruptly autofocusing (AAF) property and optical manipulation of microparticle by optical force. It is found that both the linear and circular polarizations are the best candidates to implement the AAF property robustly against strong disturbance by a large-sized particle. In addition, although the CABs can be intensively focused to a small region, resulting in an abrupt increase of light intensity by two orders of magnitude at the focal point and enabling three-dimensional stable trapping of a Rayleigh particle near the focal point, the usual CABs fail to generate an optical force outweighing the Brownian force to achieve a stable transverse trapping in the region before the focal point. On the other hand, a Mie particle can be confined transversely in the primary ring and accelerated along the curved trajectory of CABs to the focal point, then pushed further all the way through the focal point rather than being trapped therein, and eventually accelerated further along a straight trajectory in the direction of light propagation over 100 wavelengths, due to the weak diffraction characteristic of a morphed Bessel-like beam of CABs. The exotic curved-straight trajectory transport of particles by the CABs may find applications in the case where the transport path is partly blocked.
DOI
Chiral optical tweezers for optically active particles in the T-matrix formalism
Francesco Patti, Rosalba Saija, Paolo Denti, Giovanni Pellegrini, Paolo Biagioni, Maria Antonia Iatì & Onofrio M. Maragò
Modeling optical tweezers in the T-matrix formalism has been of key importance for accurate and efficient calculations of optical forces and their comparison with experiments. Here we extend this formalism to the modeling of chiral optomechanics and optical tweezers where chiral light is used for optical manipulation and trapping of optically active particles. We first use the Bohren decomposition to deal with the light scattering of chiral light on optically active particles. Thus, we show analytically that all the observables (cross sections, asymmetry parameters) are split into a helicity dependent and independent part and study a practical example of a complex resin particle with inner copper-coated stainless steel helices. Then, we apply this chiral T-matrix framework to optical tweezers where a tightly focused chiral field is used to trap an optically active spherical particle, calculate the chiral behaviour of optical trapping stiffnesses and their size scaling, and extend calculations to chiral nanowires and clusters of astrophysical interest. Such general light scattering framework opens perspectives for modeling optical forces on biological materials where optically active amino acids and carbohydrates are present.
DOI
Modeling optical tweezers in the T-matrix formalism has been of key importance for accurate and efficient calculations of optical forces and their comparison with experiments. Here we extend this formalism to the modeling of chiral optomechanics and optical tweezers where chiral light is used for optical manipulation and trapping of optically active particles. We first use the Bohren decomposition to deal with the light scattering of chiral light on optically active particles. Thus, we show analytically that all the observables (cross sections, asymmetry parameters) are split into a helicity dependent and independent part and study a practical example of a complex resin particle with inner copper-coated stainless steel helices. Then, we apply this chiral T-matrix framework to optical tweezers where a tightly focused chiral field is used to trap an optically active spherical particle, calculate the chiral behaviour of optical trapping stiffnesses and their size scaling, and extend calculations to chiral nanowires and clusters of astrophysical interest. Such general light scattering framework opens perspectives for modeling optical forces on biological materials where optically active amino acids and carbohydrates are present.
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Search for kilogram-scale dark matter with precision displacement sensors
Akio Kawasaki
The search for dark matter has been performed mainly for weakly interacting massive particles and massive compact halo objects, and the intermediate mass region has not been investigated experimentally. A method to search dark matter with precision displacement sensors is suggested for this mass range. The search is performed by detecting a characteristic motion of a test mass when it is attracted by a dark matter particle through gravity. Two different types of displacement sensors are examined: optically levitated microspheres and laser interferometers for gravitational wave detection. The state-of-the-art detectors’ sensitivity is several orders of magnitude lower to put constraints on dark matter particles. Among the two types of detectors, gravitational wave detectors have higher sensitivities, and a sensitivity 10 times more than the next generation detector can potentially address the existence of dark matter particles of a few kilograms.
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The search for dark matter has been performed mainly for weakly interacting massive particles and massive compact halo objects, and the intermediate mass region has not been investigated experimentally. A method to search dark matter with precision displacement sensors is suggested for this mass range. The search is performed by detecting a characteristic motion of a test mass when it is attracted by a dark matter particle through gravity. Two different types of displacement sensors are examined: optically levitated microspheres and laser interferometers for gravitational wave detection. The state-of-the-art detectors’ sensitivity is several orders of magnitude lower to put constraints on dark matter particles. Among the two types of detectors, gravitational wave detectors have higher sensitivities, and a sensitivity 10 times more than the next generation detector can potentially address the existence of dark matter particles of a few kilograms.
DOI
Tuesday, January 22, 2019
Spectral-optical-tweezer-assisted fluorescence multiplexing system for QDs-encoded bead-array bioassay
Qinghua He, Xuejing Chen, Yonghong He, Tian Guan, Guangxia Feng, Bangrong Lu, Bei Wang, Xuesi Zhou, Liangshan Hu, Donglin Cao
As an efficient tool in the multiplexed detection of biomolecules, bead-array could achieve separation-free detection to multiple targets, making it suitable to analyze valuable and scarce samples like antigen and antibody from living organism. Herein, we propose a spectral-optical-tweezer-assisted fluorescence multiplexing system to analyze biomolecule-conjugated bead-array. Using optical tweezer, we trapped and locked beads at the focus to accept stimulation, offering a stable and optimized analysis condition. Moving the system focus and scanning the sample slide, we achieved emissions collection to QDs-encoded bead-array after the multiplexed detection. The emission spectra of fluorescence were collected and recorded by the spectrometer. By recognizing locations of decoding peaks and counting the intensities of label signals of emission spectra, we achieved qualitative and quantitative detection to targets. As proof-of-concept studies, we use this system to carry out multiplexed detection to various types of anti-IgG in the single sample and the detection limit reaches 1.52 pM with a linear range from 0.31 to 10 nM. Through further optimization of experimental conditions, we achieved specific detection to target IgG with sandwich method in human serum and the detection limit reaches as low as 0.23 pM with a linear range from 0.88 to 28 pM, validating the practical application of this method in real samples.
DOI
As an efficient tool in the multiplexed detection of biomolecules, bead-array could achieve separation-free detection to multiple targets, making it suitable to analyze valuable and scarce samples like antigen and antibody from living organism. Herein, we propose a spectral-optical-tweezer-assisted fluorescence multiplexing system to analyze biomolecule-conjugated bead-array. Using optical tweezer, we trapped and locked beads at the focus to accept stimulation, offering a stable and optimized analysis condition. Moving the system focus and scanning the sample slide, we achieved emissions collection to QDs-encoded bead-array after the multiplexed detection. The emission spectra of fluorescence were collected and recorded by the spectrometer. By recognizing locations of decoding peaks and counting the intensities of label signals of emission spectra, we achieved qualitative and quantitative detection to targets. As proof-of-concept studies, we use this system to carry out multiplexed detection to various types of anti-IgG in the single sample and the detection limit reaches 1.52 pM with a linear range from 0.31 to 10 nM. Through further optimization of experimental conditions, we achieved specific detection to target IgG with sandwich method in human serum and the detection limit reaches as low as 0.23 pM with a linear range from 0.88 to 28 pM, validating the practical application of this method in real samples.
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Bio-Molecular Applications of Recent Developments in Optical Tweezers
Dhawal Choudhary, Alessandro Mossa, Milind Jadhav and Ciro Cecconi
In the past three decades, the ability to optically manipulate biomolecules has spurred a new era of medical and biophysical research. Optical tweezers (OT) have enabled experimenters to trap, sort, and probe cells, as well as discern the structural dynamics of proteins and nucleic acids at single molecule level. The steady improvement in OT’s resolving power has progressively pushed the envelope of their applications; there are, however, some inherent limitations that are prompting researchers to look for alternatives to the conventional techniques. To begin with, OT are restricted by their one-dimensional approach, which makes it difficult to conjure an exhaustive three-dimensional picture of biological systems. The high-intensity trapping laser can damage biological samples, a fact that restricts the feasibility of in vivo applications. Finally, direct manipulation of biological matter at nanometer scale remains a significant challenge for conventional OT. A significant amount of literature has been dedicated in the last 10 years to address the aforementioned shortcomings. Innovations in laser technology and advances in various other spheres of applied physics have been capitalized upon to evolve the next generation OT systems. In this review, we elucidate a few of these developments, with particular focus on their biological applications. The manipulation of nanoscopic objects has been achieved by means of plasmonic optical tweezers (POT), which utilize localized surface plasmons to generate optical traps with enhanced trapping potential, and photonic crystal optical tweezers (PhC OT), which attain the same goal by employing different photonic crystal geometries. Femtosecond optical tweezers (fs OT), constructed by replacing the continuous wave (cw) laser source with a femtosecond laser, promise to greatly reduce the damage to living samples. Finally, one way to transcend the one-dimensional nature of the data gained by OT is to couple them to the other large family of single molecule tools, i.e., fluorescence-based imaging techniques. We discuss the distinct advantages of the aforementioned techniques as well as the alternative experimental perspective they provide in comparison to conventional OT.
In the past three decades, the ability to optically manipulate biomolecules has spurred a new era of medical and biophysical research. Optical tweezers (OT) have enabled experimenters to trap, sort, and probe cells, as well as discern the structural dynamics of proteins and nucleic acids at single molecule level. The steady improvement in OT’s resolving power has progressively pushed the envelope of their applications; there are, however, some inherent limitations that are prompting researchers to look for alternatives to the conventional techniques. To begin with, OT are restricted by their one-dimensional approach, which makes it difficult to conjure an exhaustive three-dimensional picture of biological systems. The high-intensity trapping laser can damage biological samples, a fact that restricts the feasibility of in vivo applications. Finally, direct manipulation of biological matter at nanometer scale remains a significant challenge for conventional OT. A significant amount of literature has been dedicated in the last 10 years to address the aforementioned shortcomings. Innovations in laser technology and advances in various other spheres of applied physics have been capitalized upon to evolve the next generation OT systems. In this review, we elucidate a few of these developments, with particular focus on their biological applications. The manipulation of nanoscopic objects has been achieved by means of plasmonic optical tweezers (POT), which utilize localized surface plasmons to generate optical traps with enhanced trapping potential, and photonic crystal optical tweezers (PhC OT), which attain the same goal by employing different photonic crystal geometries. Femtosecond optical tweezers (fs OT), constructed by replacing the continuous wave (cw) laser source with a femtosecond laser, promise to greatly reduce the damage to living samples. Finally, one way to transcend the one-dimensional nature of the data gained by OT is to couple them to the other large family of single molecule tools, i.e., fluorescence-based imaging techniques. We discuss the distinct advantages of the aforementioned techniques as well as the alternative experimental perspective they provide in comparison to conventional OT.
Rotating Ag-Fe3O4-Au Nanograin by Optical Torque with a Monochromatic Light Beam
Xiaoqin Mao, Yan Li, Weiyan Jiao, Xinshun Wang, Benyang Wang
Optical torques of asymmetrical Ag–Fe3O4–Au nanograins were investigated by the method of discrete dipole approximation (DDA). The results show that surface plasmon resonance (SPR) causes the optical torques which can keep the nanograins rotating clockwise or counterclockwise. When the power density of optical radiation is I = 109 W/m2, the angular velocities of the hybrid sphere and cube heterotrimers can reach to about 104 rad/s in the ranges of 360–374 nm and 403–426 nm, which is ten times larger than that of Brownian rotation. The peak widths at half height of angular velocity curves for two kinds of grains are in the ranges of 31–47 nm and 54–70 nm, respectively. When light radiation force offers a regular driving force, such grains can serve as potential nanoscale optical wrench or microscopic mixers. In addition, the influences of Brownian rotation and photophoresis were discussed.
DOI
Optical torques of asymmetrical Ag–Fe3O4–Au nanograins were investigated by the method of discrete dipole approximation (DDA). The results show that surface plasmon resonance (SPR) causes the optical torques which can keep the nanograins rotating clockwise or counterclockwise. When the power density of optical radiation is I = 109 W/m2, the angular velocities of the hybrid sphere and cube heterotrimers can reach to about 104 rad/s in the ranges of 360–374 nm and 403–426 nm, which is ten times larger than that of Brownian rotation. The peak widths at half height of angular velocity curves for two kinds of grains are in the ranges of 31–47 nm and 54–70 nm, respectively. When light radiation force offers a regular driving force, such grains can serve as potential nanoscale optical wrench or microscopic mixers. In addition, the influences of Brownian rotation and photophoresis were discussed.
DOI
Observation of Ultrastrong Spin-Motion Coupling for Cold Atoms in Optical Microtraps
A. Dareau, Y. Meng, P. Schneeweiss, and A. Rauschenbeutel
We realize a mechanical analogue of the Dicke model, achieved by coupling the spin of individual neutral atoms to their quantized motion in an optical trapping potential. The atomic spin states play the role of the electronic states of the atomic ensemble considered in the Dicke model, and the in-trap motional states of the atoms correspond to the states of the electromagnetic field mode. The coupling between spin and motion is induced by an inherent polarization gradient of the trapping light fields, which leads to a spatially varying vector light shift. We experimentally show that our system reaches the ultrastrong coupling regime; i.e., we obtain a coupling strength that is a significant fraction of the trap frequency. Moreover, with the help of an additional light field, we demonstrate the in situ tuning of the coupling strength. Beyond its fundamental interest, the demonstrated one-to-one mapping between the physics of optically trapped cold atoms and the Dicke model paves the way for implementing protocols and applications that exploit extreme coupling strengths.
DOI
We realize a mechanical analogue of the Dicke model, achieved by coupling the spin of individual neutral atoms to their quantized motion in an optical trapping potential. The atomic spin states play the role of the electronic states of the atomic ensemble considered in the Dicke model, and the in-trap motional states of the atoms correspond to the states of the electromagnetic field mode. The coupling between spin and motion is induced by an inherent polarization gradient of the trapping light fields, which leads to a spatially varying vector light shift. We experimentally show that our system reaches the ultrastrong coupling regime; i.e., we obtain a coupling strength that is a significant fraction of the trap frequency. Moreover, with the help of an additional light field, we demonstrate the in situ tuning of the coupling strength. Beyond its fundamental interest, the demonstrated one-to-one mapping between the physics of optically trapped cold atoms and the Dicke model paves the way for implementing protocols and applications that exploit extreme coupling strengths.
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Experimental evidence of symmetry breaking of transition-path times
J. Gladrow, M. Ribezzi-Crivellari, F. Ritort & U. F. Keyser
While thermal rates of state transitions in classical systems have been studied for almost a century, associated transition-path times have only recently received attention. Uphill and downhill transition paths between states at different free energies should be statistically indistinguishable. Here, we systematically investigate transition-path-time symmetry and report evidence of its breakdown on the molecular- and meso-scale out of equilibrium. In automated Brownian dynamics experiments, we establish first-passage-time symmetries of colloids driven by femtoNewton forces in holographically-created optical landscapes confined within microchannels. Conversely, we show that transitions which couple in a path-dependent manner to fluctuating forces exhibit asymmetry. We reproduce this asymmetry in folding transitions of DNA-hairpins driven out of equilibrium and suggest a topological mechanism of symmetry breakdown. Our results are relevant to measurements that capture a single coordinate in a multidimensional free energy landscape, as encountered in electrophysiology and single-molecule fluorescence experiments.
DOI
While thermal rates of state transitions in classical systems have been studied for almost a century, associated transition-path times have only recently received attention. Uphill and downhill transition paths between states at different free energies should be statistically indistinguishable. Here, we systematically investigate transition-path-time symmetry and report evidence of its breakdown on the molecular- and meso-scale out of equilibrium. In automated Brownian dynamics experiments, we establish first-passage-time symmetries of colloids driven by femtoNewton forces in holographically-created optical landscapes confined within microchannels. Conversely, we show that transitions which couple in a path-dependent manner to fluctuating forces exhibit asymmetry. We reproduce this asymmetry in folding transitions of DNA-hairpins driven out of equilibrium and suggest a topological mechanism of symmetry breakdown. Our results are relevant to measurements that capture a single coordinate in a multidimensional free energy landscape, as encountered in electrophysiology and single-molecule fluorescence experiments.
DOI
Effects of the multi-order and off-axis vortex on quadratically chirped Airy beams in the right-handed and left-handed materials slabs
Shihan Hong, Jintao Xie, Xinyu Yang, Feng Ye, Guanghui Wang, Hongzhan Liu, Xiangbo Yang, Dongmei Deng
Based on the ABCD matrix and Huygens diffraction integral, the expressions for the quadratically chirped Airy vortex (QCAiV) beams with vortex’s arbitrary position and vortex’s topological charge propagating in the right-handed (RHMs) and left-handed materials (LHMs) slabs are deduced. The effects of the position and the topological charge of the vortex on the intensity distribution, the intensity concentration, the peak intensity, diffraction range and radiation forces are deeply investigated in the paper. It is shown that the acceleration of the optical vortex is always bigger than that of the corresponding QCAiV beams in both RHMs and LHMs slabs. Therefore, with suitable position of the vortex, plurality of intersections of the vortex and the QCAiV beams can be achieved and the intensity concentration can be controlled during propagation. The range of the diffraction can be controlled by the off-axis vortex and it depends on the relative distance from vortex to the origin. At last, we demonstrate the effect of the vortex on the maximum scattering force and gradient force distribution on the Rayleigh dielectric particles respectively.
DOI
Based on the ABCD matrix and Huygens diffraction integral, the expressions for the quadratically chirped Airy vortex (QCAiV) beams with vortex’s arbitrary position and vortex’s topological charge propagating in the right-handed (RHMs) and left-handed materials (LHMs) slabs are deduced. The effects of the position and the topological charge of the vortex on the intensity distribution, the intensity concentration, the peak intensity, diffraction range and radiation forces are deeply investigated in the paper. It is shown that the acceleration of the optical vortex is always bigger than that of the corresponding QCAiV beams in both RHMs and LHMs slabs. Therefore, with suitable position of the vortex, plurality of intersections of the vortex and the QCAiV beams can be achieved and the intensity concentration can be controlled during propagation. The range of the diffraction can be controlled by the off-axis vortex and it depends on the relative distance from vortex to the origin. At last, we demonstrate the effect of the vortex on the maximum scattering force and gradient force distribution on the Rayleigh dielectric particles respectively.
DOI
Monday, January 21, 2019
Optical manipulation of microparticles with the momentum flux transverse to the optical axis
Shubo Cheng, Tian Xia, Mengsi Liu, Shan Xu, Shufang Gao, Geng Zhang, Shaohua Tao
The beams with phase gradients along the line trajectory, i.e., with the momentum flux transverse to the optical axis were generated with the beam shaping method and applied for optical manipulations in this paper. The phase distributions of the reconstructed beam in the focal plane of the objective and the moving velocity of the trapped microparticles were experimentally investigated. The experimental results showed that the moving velocity of the trapped microparticles is linearly dependent on the phase gradient possessed by the reconstructed beam. Furthermore, a generated curve beam was used to transport microparticles quickly and automatically along the curved route to avoid an obstacle. The generation method of the beams with phase gradients is simple. The beams would have potential applications in the fields such as optical trapping and optical sorting.
DOI
The beams with phase gradients along the line trajectory, i.e., with the momentum flux transverse to the optical axis were generated with the beam shaping method and applied for optical manipulations in this paper. The phase distributions of the reconstructed beam in the focal plane of the objective and the moving velocity of the trapped microparticles were experimentally investigated. The experimental results showed that the moving velocity of the trapped microparticles is linearly dependent on the phase gradient possessed by the reconstructed beam. Furthermore, a generated curve beam was used to transport microparticles quickly and automatically along the curved route to avoid an obstacle. The generation method of the beams with phase gradients is simple. The beams would have potential applications in the fields such as optical trapping and optical sorting.
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Generalized Lorenz-Mie theories and mechanical effects of laser light, on the occasion of Arthur Ashkin’s receipt of the 2018 Nobel prize in physics for his pioneering work in optical levitation and manipulation: A review
Gérard Gouesbet
Among the many works of Arthur Ashkin, many have been devoted to optical tweezers, optical levitation and optical manipulation of macroscopic particles (“macroscopic” being here to beunderstood as opposed to atoms or molecules). From a theoretical point of view, these experiments have been studied in the framework of two limiting regimes, namely Rayleigh regime for small size parameter and ray optics for large size parameter. The generalized Lorenz-Mie theory (GLMT, and more generally GLMTs) bridges the gap between these two regimes. The present paper therefore reviews GLMTs and mechanical effects of laser light, in Rouen where the GLMT had originally been built, but also worldwide. A story in the review concerns the first experimental validations of GLMT using optical levitation experiments.
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Among the many works of Arthur Ashkin, many have been devoted to optical tweezers, optical levitation and optical manipulation of macroscopic particles (“macroscopic” being here to beunderstood as opposed to atoms or molecules). From a theoretical point of view, these experiments have been studied in the framework of two limiting regimes, namely Rayleigh regime for small size parameter and ray optics for large size parameter. The generalized Lorenz-Mie theory (GLMT, and more generally GLMTs) bridges the gap between these two regimes. The present paper therefore reviews GLMTs and mechanical effects of laser light, in Rouen where the GLMT had originally been built, but also worldwide. A story in the review concerns the first experimental validations of GLMT using optical levitation experiments.
DOI
Plasmonic resonant nonlinearity and synthetic optical properties in gold nanorod suspensions
Huizhong Xu, Pepito Alvaro, Yinxiao Xiang, Trevor S. Kelly, Yu-Xuan Ren, Chensong Zhang, and Zhigang Chen
We experimentally demonstrate self-trapping of light, as a result of plasmonic resonant optical nonlinearity, in both aqueous and organic (toluene) suspensions of gold nanorods. The threshold power for soliton formation is greatly reduced in toluene as opposed to aqueous suspensions. It is well known that the optical gradient forces are optimized at off-resonance wavelengths at which suspended particles typically exhibit a strong positive (or negative) polarizability. However, surprisingly, as we tune the wavelength of the optical beam from a continuous-wave (CW) laser, we find that the threshold power is reduced by more than threefold at the plasmonic resonance frequency. By analyzing the optical forces and torque acting on the nanorods, we show theoretically that it is possible to align the nanorods inside a soliton waveguide channel into orthogonal orientations by using merely two different laser wavelengths. We perform a series of experiments to examine the transmission of the soliton-forming beam itself, as well as the polarization transmission spectrum of a low-power probe beam guided along the soliton channel. It is found that the expected synthetic anisotropic properties are too subtle to be clearly observed, in large part due to Brownian motion of the solvent molecules and a limited ordering region where the optical field from the self-trapped beam is strong enough to overcome thermodynamic fluctuations. The ability to achieve tunable nonlinearity and nanorod orientations in colloidal nanosuspensions with low-power CW laser beams may lead to interesting applications in all-optical switching and transparent display technologies.
DOI
We experimentally demonstrate self-trapping of light, as a result of plasmonic resonant optical nonlinearity, in both aqueous and organic (toluene) suspensions of gold nanorods. The threshold power for soliton formation is greatly reduced in toluene as opposed to aqueous suspensions. It is well known that the optical gradient forces are optimized at off-resonance wavelengths at which suspended particles typically exhibit a strong positive (or negative) polarizability. However, surprisingly, as we tune the wavelength of the optical beam from a continuous-wave (CW) laser, we find that the threshold power is reduced by more than threefold at the plasmonic resonance frequency. By analyzing the optical forces and torque acting on the nanorods, we show theoretically that it is possible to align the nanorods inside a soliton waveguide channel into orthogonal orientations by using merely two different laser wavelengths. We perform a series of experiments to examine the transmission of the soliton-forming beam itself, as well as the polarization transmission spectrum of a low-power probe beam guided along the soliton channel. It is found that the expected synthetic anisotropic properties are too subtle to be clearly observed, in large part due to Brownian motion of the solvent molecules and a limited ordering region where the optical field from the self-trapped beam is strong enough to overcome thermodynamic fluctuations. The ability to achieve tunable nonlinearity and nanorod orientations in colloidal nanosuspensions with low-power CW laser beams may lead to interesting applications in all-optical switching and transparent display technologies.
DOI
Nonlinear dynamic analysis of a photonic crystal nanocavity resonator
Fengrui Liu, Han Yan, Wenming Zhang
A nonlinear dynamic model of a one-dimensional photonic crystal nanocavity resonator is presented. It considers the internal tensile stress and the geometric characteristics of a photonic crystal with rectangular (and circular) holes. The solution of the dynamic model shows that the internal tensile stress can suppress the hardening and softening behaviors of the resonator. However, the stress can reduce the amplitude, which is not conducive to an improvement of the sensitivity of the sensor. It is demonstrated that with an optimized beam length, the normalized frequency drift of the beam can be stabilized within 1% when the optical power increases from 2 mW to 6 mW. When the hole size of the resonator beam is close to the beam width, its increase can lead to a sharp rise of the resonant frequency and the promotion of hardening behavior. Moreover, the increase in the optical power initially leads to the softening behavior of the resonator followed by an intensification of the hardening behavior. These theoretical and numerical results are helpful in understanding the intrinsic mechanism of the nonlinear response of an optomechanical resonator, with the objective of avoiding the nonlinear phenomena by optimizing key parameters.
DOI
A nonlinear dynamic model of a one-dimensional photonic crystal nanocavity resonator is presented. It considers the internal tensile stress and the geometric characteristics of a photonic crystal with rectangular (and circular) holes. The solution of the dynamic model shows that the internal tensile stress can suppress the hardening and softening behaviors of the resonator. However, the stress can reduce the amplitude, which is not conducive to an improvement of the sensitivity of the sensor. It is demonstrated that with an optimized beam length, the normalized frequency drift of the beam can be stabilized within 1% when the optical power increases from 2 mW to 6 mW. When the hole size of the resonator beam is close to the beam width, its increase can lead to a sharp rise of the resonant frequency and the promotion of hardening behavior. Moreover, the increase in the optical power initially leads to the softening behavior of the resonator followed by an intensification of the hardening behavior. These theoretical and numerical results are helpful in understanding the intrinsic mechanism of the nonlinear response of an optomechanical resonator, with the objective of avoiding the nonlinear phenomena by optimizing key parameters.
DOI
Optically induced rotation of Rayleigh particles by arbitrary photonic spin
Guanghao Rui, Ying Li, Sichao Zhou, Yusong Wang, Bing Gu, Yiping Cui, and Qiwen Zhan
Optical trapping techniques hold great interest for their advantages that enable direct handling of nanoparticles. In this work, we study the optical trapping effects of a diffraction-limited focal field possessing an arbitrary photonic spin and propose a convenient method to manipulate the movement behavior of the trapped nanoparticles. In order to achieve controllable spin axis orientation and ellipticity of the tightly focused beam in three dimensions, an efficient method to analytically calculate and experimentally generate complex optical fields at the pupil plane of a high numerical aperture lens is developed. By numerically calculating the optical forces and torques of Rayleigh particles with spherical/ellipsoidal shape, we demonstrate that the interactions between the tunable photonic spin and nanoparticles lead to not only 3D trapping but also precise control of the nanoparticles’ movements in terms of stable orientation, rotational orientation, and rotation frequency. This versatile trapping method may open up new avenues for optical trapping and their applications in various scientific fields.
DOI
Optical trapping techniques hold great interest for their advantages that enable direct handling of nanoparticles. In this work, we study the optical trapping effects of a diffraction-limited focal field possessing an arbitrary photonic spin and propose a convenient method to manipulate the movement behavior of the trapped nanoparticles. In order to achieve controllable spin axis orientation and ellipticity of the tightly focused beam in three dimensions, an efficient method to analytically calculate and experimentally generate complex optical fields at the pupil plane of a high numerical aperture lens is developed. By numerically calculating the optical forces and torques of Rayleigh particles with spherical/ellipsoidal shape, we demonstrate that the interactions between the tunable photonic spin and nanoparticles lead to not only 3D trapping but also precise control of the nanoparticles’ movements in terms of stable orientation, rotational orientation, and rotation frequency. This versatile trapping method may open up new avenues for optical trapping and their applications in various scientific fields.
DOI
Emergence of collective dynamics of gold nanoparticles in an optical vortex lattice
R. Delgado-Buscalioni, M. Meléndez, J. Luis-Hita, M. I. Marqués, and J. J. Sáenz
Gold nanoparticles moving in an aqueous solution under an optical vortex lattice are shown to present a complex collective optofluidic dynamics. Above a critical field intensity and concentration, the system presents a spontaneous transition toward synchronized motion, driven by the interplay between nonconservative optical forces, thermal fluctuations, and hydrodynamic pairing. Above criticality, the system exhibits swarm behavior with strong unidirectional currents of nanoparticles reaching speeds of centimeters per second. This relatively simple optofluidic setup offers an alternative way to control nanoparticle transport in aqueous environments.
Gold nanoparticles moving in an aqueous solution under an optical vortex lattice are shown to present a complex collective optofluidic dynamics. Above a critical field intensity and concentration, the system presents a spontaneous transition toward synchronized motion, driven by the interplay between nonconservative optical forces, thermal fluctuations, and hydrodynamic pairing. Above criticality, the system exhibits swarm behavior with strong unidirectional currents of nanoparticles reaching speeds of centimeters per second. This relatively simple optofluidic setup offers an alternative way to control nanoparticle transport in aqueous environments.
Friday, January 18, 2019
Atoms in complex twisted light
Mohamed Babiker, David L Andrews and Vassilis E Lembessis
The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant inter-disciplinary connections. Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means. The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements. The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact. The main physical effects involve atoms in motion due to application of optical forces. The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose–Einstein condensates. The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields. Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation. These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light. Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces. From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom. Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below. A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed. Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date.
DOI
The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant inter-disciplinary connections. Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means. The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements. The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact. The main physical effects involve atoms in motion due to application of optical forces. The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose–Einstein condensates. The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields. Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation. These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light. Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces. From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom. Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below. A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed. Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date.
DOI
TEFM Enhances Transcription Elongation by Modifying mtRNAP Pausing Dynamics
Hongwu Yu, Cheng Xue, Mengping Long, Huiqiang Jia, Guosheng Xue, Shengwang Du, Yves Coello, Toyotaka Ishibashi
Regulation of transcription elongation is one of the key mechanisms employed to control gene expression. The single-subunit mitochondrial RNA polymerase (mtRNAP) transcribes mitochondrial genes, such as those related to ATP synthesis. We investigated how mitochondrial transcription elongation factor (TEFM) enhances mtRNAP transcription elongation using a single-molecule optical-tweezers transcription assay, which follows transcription dynamics in real time and allows the separation of pause-free elongation from transcriptional pauses. We found that TEFM enhances the stall force of mtRNAP. Although TEFM does not change the pause-free elongation rate, it enhances mtRNAP transcription elongation by reducing the frequency of long-lived pauses and shortening their durations. Furthermore, we demonstrate how mtRNAP passes through the conserved sequence block II, which is the key sequence for the switch between DNA replication and transcription in mitochondria. Our findings elucidate how both TEFM and mitochondrial genomic DNA sequences directly control the transcription elongation dynamics of mtRNAP.
DOI
Regulation of transcription elongation is one of the key mechanisms employed to control gene expression. The single-subunit mitochondrial RNA polymerase (mtRNAP) transcribes mitochondrial genes, such as those related to ATP synthesis. We investigated how mitochondrial transcription elongation factor (TEFM) enhances mtRNAP transcription elongation using a single-molecule optical-tweezers transcription assay, which follows transcription dynamics in real time and allows the separation of pause-free elongation from transcriptional pauses. We found that TEFM enhances the stall force of mtRNAP. Although TEFM does not change the pause-free elongation rate, it enhances mtRNAP transcription elongation by reducing the frequency of long-lived pauses and shortening their durations. Furthermore, we demonstrate how mtRNAP passes through the conserved sequence block II, which is the key sequence for the switch between DNA replication and transcription in mitochondria. Our findings elucidate how both TEFM and mitochondrial genomic DNA sequences directly control the transcription elongation dynamics of mtRNAP.
DOI
Combined Force Ramp and Equilibrium High-Resolution Investigations Reveal Multipath Heterogeneous Unfolding of Protein G
Dena Izadi, Yujie Chen, Miles L. Whitmore, Joseph D. Slivka, Kevin Ching, Lisa J. Lapidus, and Matthew J. Comstock
Over the past two decades, one of the standard models of protein folding has been the “two-state” model, in which a protein only resides in the folded or fully unfolded states with a single pathway between them. Recent advances in spatial and temporal resolution of biophysical measurements have revealed “beyond-two-state” complexity in protein folding, even for small, single-domain proteins. In this work, we used high-resolution optical tweezers to investigate the folding/unfolding kinetics of the B1 domain of immunoglobulin-binding protein G (GB1), a well-studied model system. Experiments were performed for GB1 both in and out of equilibrium using force spectroscopy. When the force was gradually ramped, simple single-peak folding force distributions were observed, while multiple rupture peaks were seen in the unfolding force distributions, consistent with multiple force-dependent parallel unfolding pathways. Force-dependent folding and unfolding rate constants were directly determined by both force-jump and fixed-trap measurements. Monte Carlo modeling using these rate constants was in good agreement with the force ramp data. The unfolding rate constants exhibited two different behaviors at low vs high force. At high force, the unfolding rate constant increased with increasing force, as previously reported by high force, high pulling speed force ramp measurements. However, at low force, the situation reversed and the unfolding rate constant decreased with increasing force. Taken together, these data indicate that this small protein has multiple distinct pathways to the native state on the free energy landscape.
DOI
Over the past two decades, one of the standard models of protein folding has been the “two-state” model, in which a protein only resides in the folded or fully unfolded states with a single pathway between them. Recent advances in spatial and temporal resolution of biophysical measurements have revealed “beyond-two-state” complexity in protein folding, even for small, single-domain proteins. In this work, we used high-resolution optical tweezers to investigate the folding/unfolding kinetics of the B1 domain of immunoglobulin-binding protein G (GB1), a well-studied model system. Experiments were performed for GB1 both in and out of equilibrium using force spectroscopy. When the force was gradually ramped, simple single-peak folding force distributions were observed, while multiple rupture peaks were seen in the unfolding force distributions, consistent with multiple force-dependent parallel unfolding pathways. Force-dependent folding and unfolding rate constants were directly determined by both force-jump and fixed-trap measurements. Monte Carlo modeling using these rate constants was in good agreement with the force ramp data. The unfolding rate constants exhibited two different behaviors at low vs high force. At high force, the unfolding rate constant increased with increasing force, as previously reported by high force, high pulling speed force ramp measurements. However, at low force, the situation reversed and the unfolding rate constant decreased with increasing force. Taken together, these data indicate that this small protein has multiple distinct pathways to the native state on the free energy landscape.
DOI
Direct Measurement of π Coupling at the Single-Molecule Level using a Carbon Nanotube Force Sensor
Tu Hong, Tianjiao Wang, and Ya-Qiong Xu
We report a carbon nanotube (CNT) force sensor that combines a suspended CNT transistor with dual-trap optical tweezers to explore the interactions between two individual molecules in the near-equilibrium regime with sub-piconewton resolution. The directly measured equilibrium force (1.2 ± 0.5 pN) is likely related to the binding force between a CNT and a single DNA base, where two aromatic rings spontaneously attract to each other due to the noncovalent forces between them. On the basis of our force measurements, the binding free energy per base is calculated (∼0.34 eV), which is in good agreement with theoretical simulations. Moreover, three-dimensional scanning photocurrent microscopy enables us to simultaneously monitor the morphology changes of the CNT, leading to a comprehensive reconstruction of the CNT-DNA binding dynamics. These experimental results shed light on the fundamental understanding of the mechanical coupling between CNTs and DNA molecules and, more importantly, provide a new platform for direct observation of intermolecular interfaces at the single-molecule level.
DOI
We report a carbon nanotube (CNT) force sensor that combines a suspended CNT transistor with dual-trap optical tweezers to explore the interactions between two individual molecules in the near-equilibrium regime with sub-piconewton resolution. The directly measured equilibrium force (1.2 ± 0.5 pN) is likely related to the binding force between a CNT and a single DNA base, where two aromatic rings spontaneously attract to each other due to the noncovalent forces between them. On the basis of our force measurements, the binding free energy per base is calculated (∼0.34 eV), which is in good agreement with theoretical simulations. Moreover, three-dimensional scanning photocurrent microscopy enables us to simultaneously monitor the morphology changes of the CNT, leading to a comprehensive reconstruction of the CNT-DNA binding dynamics. These experimental results shed light on the fundamental understanding of the mechanical coupling between CNTs and DNA molecules and, more importantly, provide a new platform for direct observation of intermolecular interfaces at the single-molecule level.
DOI
Nucleotide-dependent DNA gripping and an end-clamp mechanism regulate the bacteriophage T4 viral packaging motor
Mariam Ordyan, Istiaq Alam, Marthandan Mahalingam, Venigalla B. Rao, and Douglas E. Smith
ATP-powered viral packaging motors are among the most powerful biomotors known. Motor subunits arranged in a ring repeatedly grip and translocate the DNA to package viral genomes into capsids. Here, we use single DNA manipulation and rapid solution exchange to quantify how nucleotide binding regulates interactions between the bacteriophage T4 motor and DNA substrate. With no nucleotides, there is virtually no gripping and rapid slipping occurs with only minimal friction resisting. In contrast, binding of an ATP analog engages nearly continuous gripping. Occasional slips occur due to dissociation of the analog from a gripping motor subunit, or force-induced rupture of grip, but multiple other analog-bound subunits exert high friction that limits slipping. ADP induces comparably infrequent gripping and variable friction. Independent of nucleotides, slipping arrests when the end of the DNA is about to exit the capsid. This end-clamp mechanism increases the efficiency of packaging by making it essentially irreversible.
DOI
ATP-powered viral packaging motors are among the most powerful biomotors known. Motor subunits arranged in a ring repeatedly grip and translocate the DNA to package viral genomes into capsids. Here, we use single DNA manipulation and rapid solution exchange to quantify how nucleotide binding regulates interactions between the bacteriophage T4 motor and DNA substrate. With no nucleotides, there is virtually no gripping and rapid slipping occurs with only minimal friction resisting. In contrast, binding of an ATP analog engages nearly continuous gripping. Occasional slips occur due to dissociation of the analog from a gripping motor subunit, or force-induced rupture of grip, but multiple other analog-bound subunits exert high friction that limits slipping. ADP induces comparably infrequent gripping and variable friction. Independent of nucleotides, slipping arrests when the end of the DNA is about to exit the capsid. This end-clamp mechanism increases the efficiency of packaging by making it essentially irreversible.
DOI
Thursday, January 17, 2019
Enhanced Plasmonic Particle Trapping Using a Hybrid Structure of Nanoparticles and Nanorods
So Yun Lee, Hyung Min Kim, Jinho Park, Seong Keun Kim , Jae Ryoun Youn, and Young Seok Song
Plasmon-enhanced particle trapping was demonstrated using a hybrid structure of nanoparticles and nanorods. In order to intensify localized surface plasmon resonance (LSPR), gold nanoparticles (AuNPs) were deposited on zinc oxide nanorods (ZnONRs). The synergistic effect caused by the hybrid structure was identified experimentally. Numerical analysis revealed that the LSPR-induced photophysical processes such as plasmonic heating and near-field enhancement were improved by the existence of ZnONRs. The role of the ZnONR in enhancing the particle-trapping velocity was explored by examining the scattered electric field, Poynting vector, and temperature gradient over the nanostructures calculated from the simulation. It was found that polystyrene microparticles and Escherichia coli cells were successfully trapped by using the ZnONR/AuNP plasmonic structure. A relatively high dielectric constant and nanorod geometry of ZnO enabled the hybrid substrate to enhance trapping performance, compared with a control case fabricated using only gold nanoislands.
DOI
Plasmon-enhanced particle trapping was demonstrated using a hybrid structure of nanoparticles and nanorods. In order to intensify localized surface plasmon resonance (LSPR), gold nanoparticles (AuNPs) were deposited on zinc oxide nanorods (ZnONRs). The synergistic effect caused by the hybrid structure was identified experimentally. Numerical analysis revealed that the LSPR-induced photophysical processes such as plasmonic heating and near-field enhancement were improved by the existence of ZnONRs. The role of the ZnONR in enhancing the particle-trapping velocity was explored by examining the scattered electric field, Poynting vector, and temperature gradient over the nanostructures calculated from the simulation. It was found that polystyrene microparticles and Escherichia coli cells were successfully trapped by using the ZnONR/AuNP plasmonic structure. A relatively high dielectric constant and nanorod geometry of ZnO enabled the hybrid substrate to enhance trapping performance, compared with a control case fabricated using only gold nanoislands.
DOI
Optimal Nanoparticle Forces, Torques, and Illumination Fields
Yuxiang Liu, Lingling Fan, Yoonkyung E. Lee, Nicholas X. Fang, Steven G. Johnson, and Owen D. Miller
A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, λ2, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ2. In this article, we identify λ2/c and λ3/c as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal “holographic” incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.
DOI
A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, λ2, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ2. In this article, we identify λ2/c and λ3/c as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal “holographic” incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.
DOI
Absorbing particle 3D trap based on annular core fiber tweezers
Zhihai Liu, Zhenyu Zhang, Yu Zhang, Yaxun Zhang, Xiaoyun Tang, Keqiang Liu, Xinghua Yang, Jianzhong Zhang, Jun Yang, Libo Yuan
We proposed and demonstrated a new method to trap and manipulate a strongly absorbing particle by harnessing strong -type photophoretic forces in pure liquid glycerol with a focused annular beam, which was introduced by an annular core fiber optical tweezers. We performed the focused annular light field by integrating a high refractive index silica microsphere on the annular core fiber end facet. The silica microsphere, which acted as a lens, focused laser beam for trapping. We tested and calculated -type photophoretic forces tendency by using the fluorescence dye method, and the simulated results were in agreement with the experimental results. The proposed optical fiber tweezers were small in size, large in capture range, simple to manipulate, and inexpensive in cost. It provided a new approach for absorbing particles trapping. Tailoring the trapping and manipulation of absorbing particles in liquid environments will open avenues for studying the special, yet-unexplored characteristics of absorbing particles.
DOI
We proposed and demonstrated a new method to trap and manipulate a strongly absorbing particle by harnessing strong -type photophoretic forces in pure liquid glycerol with a focused annular beam, which was introduced by an annular core fiber optical tweezers. We performed the focused annular light field by integrating a high refractive index silica microsphere on the annular core fiber end facet. The silica microsphere, which acted as a lens, focused laser beam for trapping. We tested and calculated -type photophoretic forces tendency by using the fluorescence dye method, and the simulated results were in agreement with the experimental results. The proposed optical fiber tweezers were small in size, large in capture range, simple to manipulate, and inexpensive in cost. It provided a new approach for absorbing particles trapping. Tailoring the trapping and manipulation of absorbing particles in liquid environments will open avenues for studying the special, yet-unexplored characteristics of absorbing particles.
DOI
Transverse spin forces and non-equilibrium particle dynamics in a circularly polarized vacuum optical trap
V. Svak, O. Brzobohatý, M. Šiler, P. Jákl, J. Kaňka, P. Zemánek & S. H. Simpson
We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. As well as corroborating existing measurements of spin momentum, our results reveal its dynamic effect. We show how the under-damped motion of probe particles in structured light fields can expose the nature and morphology of optical momentum flows, and provide a testbed for elementary non-equilibrium statistical mechanics.
DOI
We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. As well as corroborating existing measurements of spin momentum, our results reveal its dynamic effect. We show how the under-damped motion of probe particles in structured light fields can expose the nature and morphology of optical momentum flows, and provide a testbed for elementary non-equilibrium statistical mechanics.
DOI
Optical lateral forces and torques induced by chiral surface-plasmon-polaritons and their potential applications in recognition and separation of chiral enantiomers
Qiang Zhang, Junqing Li and Xingguang Liu
Surface plasmon polaritons carry an intrinsic transverse spin angular momentum which is locked to their propagation direction due to the quantum spin Hall effect of light. We study the chirality-sorting lateral optical forces arising from this phenomenon in a Kretschmann configuration. We show that the characteristics of surface plasmon polaritons are affected by small changes of the environment's chirality. This property can be utilized to detect the medium's chirality in the macro-world. Furthermore, we explain how the the lateral forces and optical torques interact with the chirality of nano-particles located or adsorbed in the vicinity of the surface in the micro-world. Finally, we demonstrate that introducing one handedness of chirality to the dielectric medium in the Kretschmann configuration will benefit the discrimination of chiral enantiomers compared with the nonchiral case. Our work presents physical insights into chirality-selective lateral forces using surface plasmon polaritons and provides a new approach to discriminating and separating chiral enantiomers in the chemical and pharmaceutical industries.
DOI
Surface plasmon polaritons carry an intrinsic transverse spin angular momentum which is locked to their propagation direction due to the quantum spin Hall effect of light. We study the chirality-sorting lateral optical forces arising from this phenomenon in a Kretschmann configuration. We show that the characteristics of surface plasmon polaritons are affected by small changes of the environment's chirality. This property can be utilized to detect the medium's chirality in the macro-world. Furthermore, we explain how the the lateral forces and optical torques interact with the chirality of nano-particles located or adsorbed in the vicinity of the surface in the micro-world. Finally, we demonstrate that introducing one handedness of chirality to the dielectric medium in the Kretschmann configuration will benefit the discrimination of chiral enantiomers compared with the nonchiral case. Our work presents physical insights into chirality-selective lateral forces using surface plasmon polaritons and provides a new approach to discriminating and separating chiral enantiomers in the chemical and pharmaceutical industries.
DOI
Translational dynamics of individual microbubbles with millisecond scale ultrasound pulses
Christopher N. Acconcia
It is established that radiation forces can be used to transport ultrasound contrast agents, particularly for molecular imaging applications. However, the ability to model and control this process in the context of therapeutic ultrasound is limited by a paucity of data on the translational dynamics of encapsulated microbubbles under the influence of longer pulses. In this work, the translation of individual microbubbles, isolated with optical tweezers, was experimentally investigated over a range of diameters (1.8–8.8 μm, n = 187) and pressures (25, 50, 100, 150, and 200 kPa) with millisecond pulses. Data were compared with theoretical predictions of the translational dynamics, assessing the role of shell and history force effects. A pronounced feature of the displacement curves was an effective threshold size, below which there was only minimal translation. At higher pressures (≥150 kPa) a noticeable structure emerged where multiple local maxima occurred as a function of bubble size. The ability to accurately capture these salient features depended on the encapsulation model employed. In low Reynolds number conditions (i.e., low pressures, or high pressures, off-resonance) the inclusion of history force more accurately fit the data. After pulse cessation, bubbles exhibited substantial displacements consistent with the influence of history effects.
DOI
It is established that radiation forces can be used to transport ultrasound contrast agents, particularly for molecular imaging applications. However, the ability to model and control this process in the context of therapeutic ultrasound is limited by a paucity of data on the translational dynamics of encapsulated microbubbles under the influence of longer pulses. In this work, the translation of individual microbubbles, isolated with optical tweezers, was experimentally investigated over a range of diameters (1.8–8.8 μm, n = 187) and pressures (25, 50, 100, 150, and 200 kPa) with millisecond pulses. Data were compared with theoretical predictions of the translational dynamics, assessing the role of shell and history force effects. A pronounced feature of the displacement curves was an effective threshold size, below which there was only minimal translation. At higher pressures (≥150 kPa) a noticeable structure emerged where multiple local maxima occurred as a function of bubble size. The ability to accurately capture these salient features depended on the encapsulation model employed. In low Reynolds number conditions (i.e., low pressures, or high pressures, off-resonance) the inclusion of history force more accurately fit the data. After pulse cessation, bubbles exhibited substantial displacements consistent with the influence of history effects.
DOI
Monday, January 14, 2019
Structural heterogeneity of attC integron recombination sites revealed by optical tweezers
Ann Mukhortava, Matthias Pöge, Maj Svea Grieb, Aleksandra Nivina, Celine Loot, Didier Mazel, Michael Schlierf
A predominant tool for adaptation in Gram-negative bacteria is the functional genetic platform called integron. Integrons capture and rearrange promoterless gene cassettes in a unique recombination process involving the recognition of folded single-stranded DNA hairpins—so-called attC sites—with a strong preference for the attC bottom strand. While structural elements have been identified to promote this preference, their mechanistic action remains incomplete. Here, we used high-resolution single-molecule optical tweezers (OT) to characterize secondary structures formed by the attC bottom (attCbs) and top (attCts) strands of the paradigmatic attCaadA7 site. We found for both sequences two structures—a straight, canonical hairpin and a kinked hairpin. Remarkably, the recombination-preferred attCbs predominantly formed the straight hairpin, while the attCts preferentially adopted the kinked structure, which exposes only one complete recombinase binding box. By a mutational analysis, we identified three bases in the unpaired central spacer, which could invert the preferred conformations and increase the recombination frequency of the attCtsin vivo. A bioinformatics screen revealed structural bias toward a straight, canonical hairpin conformation in the bottom strand of many antibiotic resistance cassettes attC sites. Thus, we anticipate that structural fine tuning could be a mechanism in many biologically active DNA hairpins.
DOI
A predominant tool for adaptation in Gram-negative bacteria is the functional genetic platform called integron. Integrons capture and rearrange promoterless gene cassettes in a unique recombination process involving the recognition of folded single-stranded DNA hairpins—so-called attC sites—with a strong preference for the attC bottom strand. While structural elements have been identified to promote this preference, their mechanistic action remains incomplete. Here, we used high-resolution single-molecule optical tweezers (OT) to characterize secondary structures formed by the attC bottom (attCbs) and top (attCts) strands of the paradigmatic attCaadA7 site. We found for both sequences two structures—a straight, canonical hairpin and a kinked hairpin. Remarkably, the recombination-preferred attCbs predominantly formed the straight hairpin, while the attCts preferentially adopted the kinked structure, which exposes only one complete recombinase binding box. By a mutational analysis, we identified three bases in the unpaired central spacer, which could invert the preferred conformations and increase the recombination frequency of the attCtsin vivo. A bioinformatics screen revealed structural bias toward a straight, canonical hairpin conformation in the bottom strand of many antibiotic resistance cassettes attC sites. Thus, we anticipate that structural fine tuning could be a mechanism in many biologically active DNA hairpins.
DOI
Silver‐Nanowire‐Based Interferometric Optical Tweezers for Enhanced Optical Trapping and Binding of Nanoparticles
Fan Nan, Zijie Yan
Light‐induced self‐assembly offers a new route to build mesoscale optical matter arrays from nanoparticles (NPs), yet the low stability of optical matter systems limits the assembly of large‐scale NP arrays. Here it is shown that the interferometric optical fields created by illuminating a single Ag nanowire deposited on a coverslip can enhance the electrodynamic interactions among NPs. The Ag nanowire serves as a plasmonic antenna to shape the incident laser beam and guide the optical assembly of colloidal metal (Ag and Au) and dielectric (polystyrene) NPs in solution. By controlling the laser polarization direction, both the mesoscale interactions among multiple NPs and the near‐field coupling between the NPs and nanowire can be tuned, leading to large‐scale and stable optical matter arrays consisting of up to 60 NPs. These results demonstrate that single Ag nanowires can serve as multifunctional antennas to guide the optical trapping and binding of multiple NPs and provide a new strategy to control electrodynamic interactions using hybrid nanostructures.
DOI
Light‐induced self‐assembly offers a new route to build mesoscale optical matter arrays from nanoparticles (NPs), yet the low stability of optical matter systems limits the assembly of large‐scale NP arrays. Here it is shown that the interferometric optical fields created by illuminating a single Ag nanowire deposited on a coverslip can enhance the electrodynamic interactions among NPs. The Ag nanowire serves as a plasmonic antenna to shape the incident laser beam and guide the optical assembly of colloidal metal (Ag and Au) and dielectric (polystyrene) NPs in solution. By controlling the laser polarization direction, both the mesoscale interactions among multiple NPs and the near‐field coupling between the NPs and nanowire can be tuned, leading to large‐scale and stable optical matter arrays consisting of up to 60 NPs. These results demonstrate that single Ag nanowires can serve as multifunctional antennas to guide the optical trapping and binding of multiple NPs and provide a new strategy to control electrodynamic interactions using hybrid nanostructures.
DOI
An optical fibre tip with double tapers etched by the interfacial layer
Zilong Liu, Nao Zhang, Yu Tang, Yaxin Liu & Bo Zhang
We fabricate double-tapered optical fibre tips by the interfacial layer etching method. The optical fibre is first etched by means of meniscus-based etching to obtain one-tapered fibre tip, which is etched again by the interfacial layer between the etchant and the overlay to obtain a double-tapered tip. The formation mechanism of the second taper is analysed. The impacts of the interfacial layer etching time, HF concentration and the overlay on the second cone angle are discussed. The second cone angle can be controlled by the interfacial layer etching time and the overlay. But the HF concentration has few impacts on the second cone angle. The double-tapered fibre tips can be applied for the contactless trapping of particles.
DOI
We fabricate double-tapered optical fibre tips by the interfacial layer etching method. The optical fibre is first etched by means of meniscus-based etching to obtain one-tapered fibre tip, which is etched again by the interfacial layer between the etchant and the overlay to obtain a double-tapered tip. The formation mechanism of the second taper is analysed. The impacts of the interfacial layer etching time, HF concentration and the overlay on the second cone angle are discussed. The second cone angle can be controlled by the interfacial layer etching time and the overlay. But the HF concentration has few impacts on the second cone angle. The double-tapered fibre tips can be applied for the contactless trapping of particles.
DOI
Sorting of micron-sized particles using holographic optical Raman tweezers in aqueous medium
Uğur Parlatan, Gönül Başar & Günay Başar
We have constructed a holographic optical tweezers system combined with Raman spectroscopy to sort trapped particles. Our software automatically moves the trapped objects to the measurement positions to obtain individual Raman signals from multiple trapped particles. We performed the sorting by comparing their spectra with the previously measured training dataset using the correlation coefficients. We used yeast cells and polystyrene beads as test particles. This study aims to show that biological particles can be separated using single cell analysis with combined holographic optical tweezers and Raman spectroscopy system.
DOI
We have constructed a holographic optical tweezers system combined with Raman spectroscopy to sort trapped particles. Our software automatically moves the trapped objects to the measurement positions to obtain individual Raman signals from multiple trapped particles. We performed the sorting by comparing their spectra with the previously measured training dataset using the correlation coefficients. We used yeast cells and polystyrene beads as test particles. This study aims to show that biological particles can be separated using single cell analysis with combined holographic optical tweezers and Raman spectroscopy system.
DOI
Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain
Itia A.Favre-Bulle, Gilles Vanwalleghem, Michael A.Taylor, Halina Rubinsztein-Dunlop, Ethan K.Scott
The vestibular system, which reports on motion and gravity, is essential to postural control, balance, and egocentric representations of movement and space. The motion needed to stimulate the vestibular system complicates studying its circuitry, so we previously developed a method for fictive vestibular stimulation in zebrafish, using optical trapping to apply physical forces to the otoliths. Here, we combine this approach with whole-brain calcium imaging at cellular resolution, delivering a comprehensive map of the brain regions and cellular responses involved in basic vestibular processing. We find responses broadly distributed across the brain, with unique profiles of cellular responses and topography in each region. The most widespread and abundant responses involve excitation that is graded to the stimulus strength. Other responses, localized to the telencephalon and habenulae, show excitation that is only weakly correlated to stimulus strength and that is sensitive to weak stimuli. Finally, numerous brain regions contain neurons that are inhibited by vestibular stimuli, and these neurons are often tightly localized spatially within their regions. By exerting separate control over the left and right otoliths, we explore the laterality of brain-wide vestibular processing, distinguishing between neurons with unilateral and bilateral vestibular sensitivity and revealing patterns whereby conflicting signals from the ears mutually cancel. Our results confirm previously identified vestibular responses in specific regions of the larval zebrafish brain while revealing a broader and more extensive network of vestibular responsive neurons than has previously been described. This provides a departure point for more targeted studies of the underlying functional circuits.
DOI
The vestibular system, which reports on motion and gravity, is essential to postural control, balance, and egocentric representations of movement and space. The motion needed to stimulate the vestibular system complicates studying its circuitry, so we previously developed a method for fictive vestibular stimulation in zebrafish, using optical trapping to apply physical forces to the otoliths. Here, we combine this approach with whole-brain calcium imaging at cellular resolution, delivering a comprehensive map of the brain regions and cellular responses involved in basic vestibular processing. We find responses broadly distributed across the brain, with unique profiles of cellular responses and topography in each region. The most widespread and abundant responses involve excitation that is graded to the stimulus strength. Other responses, localized to the telencephalon and habenulae, show excitation that is only weakly correlated to stimulus strength and that is sensitive to weak stimuli. Finally, numerous brain regions contain neurons that are inhibited by vestibular stimuli, and these neurons are often tightly localized spatially within their regions. By exerting separate control over the left and right otoliths, we explore the laterality of brain-wide vestibular processing, distinguishing between neurons with unilateral and bilateral vestibular sensitivity and revealing patterns whereby conflicting signals from the ears mutually cancel. Our results confirm previously identified vestibular responses in specific regions of the larval zebrafish brain while revealing a broader and more extensive network of vestibular responsive neurons than has previously been described. This provides a departure point for more targeted studies of the underlying functional circuits.
DOI
Friday, January 11, 2019
Near-field coupling of a levitated nanoparticle to a photonic crystal cavity
Lorenzo Magrini, Richard A. Norte, Ralf Riedinger, Igor Marinković, David Grass, Uroš Delić, Simon Gröblacher, Sungkun Hong, and Markus Aspelmeyer
Quantum control of levitated dielectric particles is an emerging subject in quantum optomechanics. A major challenge is to efficiently measure and manipulate the particle’s motion at the Heisenberg uncertainty limit. Here we present a nanophotonic interface suited to address this problem. By optically trapping a 150 nm silica particle and placing it in the near field of a photonic crystal cavity, we achieve tunable single-photon optomechanical coupling of up to [Math Processing Error], three orders of magnitude larger than previously reported for levitated cavity optomechanical systems. Efficient collection and guiding of light through the nanophotonic structure results in a per-photon displacement sensitivity that is increased by two orders of magnitude compared to conventional far-field detection. The demonstrated performance shows a promising route for room temperature quantum optomechanics.
DOI
Quantum control of levitated dielectric particles is an emerging subject in quantum optomechanics. A major challenge is to efficiently measure and manipulate the particle’s motion at the Heisenberg uncertainty limit. Here we present a nanophotonic interface suited to address this problem. By optically trapping a 150 nm silica particle and placing it in the near field of a photonic crystal cavity, we achieve tunable single-photon optomechanical coupling of up to [Math Processing Error], three orders of magnitude larger than previously reported for levitated cavity optomechanical systems. Efficient collection and guiding of light through the nanophotonic structure results in a per-photon displacement sensitivity that is increased by two orders of magnitude compared to conventional far-field detection. The demonstrated performance shows a promising route for room temperature quantum optomechanics.
DOI
Integrated Photonic and Plasmonic Resonant Devices for Label‐Free Biosensing and Trapping at the Nanoscale
Caterina Ciminelli, Francesco Dell'Olio, Donato Conteduca, Mario Nicola Armenise
In the last few years, integrated photonic and plasmonic devices based on resonant cavities have become key building blocks in new microsystems, instruments, and diagnostic tools for a wide range of biomedical applications including point‐of‐care (POC) diagnostics, new drug development, and proteomics. Several resonant label‐free photonic and plasmonic biosensors for early diagnosis and monitoring of a wide range of pathologies have attracted a remarkable research interest due to their characteristic features such as high resolution, small size, immunity to electromagnetic interferences, compatibility with the CMOS technology, and strong light–matter interaction. Moreover, recently, photonic, plasmonic, and hybrid photonic/plasmonic micro and nano‐cavities have experimentally demonstrated a great potential also for trapping at the nanoscale and the interest toward these devices in the field of healthcare is quickly rising. Here, the recent advances in the field of integrated photonic and plasmonic devices based on resonant cavities for label‐free biosensing and trapping at the nanoscale are critically reviewed, with a special emphasis on the specific applications of these devices such as diseases diagnostics and new drugs development.
DOI
In the last few years, integrated photonic and plasmonic devices based on resonant cavities have become key building blocks in new microsystems, instruments, and diagnostic tools for a wide range of biomedical applications including point‐of‐care (POC) diagnostics, new drug development, and proteomics. Several resonant label‐free photonic and plasmonic biosensors for early diagnosis and monitoring of a wide range of pathologies have attracted a remarkable research interest due to their characteristic features such as high resolution, small size, immunity to electromagnetic interferences, compatibility with the CMOS technology, and strong light–matter interaction. Moreover, recently, photonic, plasmonic, and hybrid photonic/plasmonic micro and nano‐cavities have experimentally demonstrated a great potential also for trapping at the nanoscale and the interest toward these devices in the field of healthcare is quickly rising. Here, the recent advances in the field of integrated photonic and plasmonic devices based on resonant cavities for label‐free biosensing and trapping at the nanoscale are critically reviewed, with a special emphasis on the specific applications of these devices such as diseases diagnostics and new drugs development.
DOI
Dual-mode subwavelength trapping by plasmonic tweezers based on V-type nanoantennas
Ren-Chao Jin, Jia-Qi Li, Lin Li, Zheng-Gao Dong, and Yongmin Liu
We propose novel plasmonic tweezers based on silver V-type nanoantennas placed on a conducting ground layer, which can effectively mitigate the plasmonic heating effect and thus enable subwavelength plasmonic trapping in the near-infrared region. Using the centroid algorithm to analyze the motion of trapped spheres, we can experimentally extract the value of optical trapping potential. The result confirms that the plasmonic tweezers have a dual-mode subwavelength trapping capability when the incident laser beam is linearly polarized along two orthogonal directions. We have also performed full-wave simulations, which agree with the experimental data very well in terms of spectral response and trapping potential. It is expected that the dual-mode subwavelength trapping can be used in non-contact manipulations of a single nanoscale object, such as a biomolecule or quantum dot, and find important applications in biology, life science, and applied physics.
DOI
We propose novel plasmonic tweezers based on silver V-type nanoantennas placed on a conducting ground layer, which can effectively mitigate the plasmonic heating effect and thus enable subwavelength plasmonic trapping in the near-infrared region. Using the centroid algorithm to analyze the motion of trapped spheres, we can experimentally extract the value of optical trapping potential. The result confirms that the plasmonic tweezers have a dual-mode subwavelength trapping capability when the incident laser beam is linearly polarized along two orthogonal directions. We have also performed full-wave simulations, which agree with the experimental data very well in terms of spectral response and trapping potential. It is expected that the dual-mode subwavelength trapping can be used in non-contact manipulations of a single nanoscale object, such as a biomolecule or quantum dot, and find important applications in biology, life science, and applied physics.
DOI
Axisymmetric scalable magneto-gravitational trap for diamagnetic particle levitation
J. P. Houlton, M. L. Chen, M. D. Brubaker, K. A. Bertness, and C. T. Rogers
We report on the design, construction, and use of axisymmetric magnetic traps for levitating diamagnetic particles. The magnetic traps each consist of two pole pieces passively driven by a neodymium iron boron (NdFeB) permanent magnet. The magnetic field configuration between the pole pieces combined with the earth’s gravitational field forms a 3D confining potential capable of levitating a range of diamagnetic substances, e.g., graphite powder, silica microspheres, and gallium nitride (GaN) powder and nanowires. Particles trap stably at atmosphere and in high-vacuum for periods up to weeks with lifetimes largely determined by choices made to actively destabilize the trap. We describe the principles of operation, finite element design, approximate closed-form results for design rules, and examples of operation of such traps.
DOI
We report on the design, construction, and use of axisymmetric magnetic traps for levitating diamagnetic particles. The magnetic traps each consist of two pole pieces passively driven by a neodymium iron boron (NdFeB) permanent magnet. The magnetic field configuration between the pole pieces combined with the earth’s gravitational field forms a 3D confining potential capable of levitating a range of diamagnetic substances, e.g., graphite powder, silica microspheres, and gallium nitride (GaN) powder and nanowires. Particles trap stably at atmosphere and in high-vacuum for periods up to weeks with lifetimes largely determined by choices made to actively destabilize the trap. We describe the principles of operation, finite element design, approximate closed-form results for design rules, and examples of operation of such traps.
DOI
Robotic Micromanipulation: Fundamentals and Applications
Zhuoran Zhang, Xian Wang, Jun Liu, Changsheng Dai, and Yu Sun
Robotic micromanipulation is a relatively young field. However, after three decades of development and evolution, the fundamental physics; techniques for sensing, actuation, and control; tool sets and systems; and, more importantly, a research community are now in place. This article reviews the fundamentals of robotic micromanipulation, including how micromanipulators and end effectors are actuated and controlled, how remote physical fields are utilized for micromanipulation, how visual servoing is implemented under an optical microscope, how force is sensed and controlled at the micro- and nanonewton levels, and the similarities and differences between robotic manipulation at the micro- and macroscales. We also review representative milestones over the past three decades and discuss potential future trends of this field.
DOI
Robotic micromanipulation is a relatively young field. However, after three decades of development and evolution, the fundamental physics; techniques for sensing, actuation, and control; tool sets and systems; and, more importantly, a research community are now in place. This article reviews the fundamentals of robotic micromanipulation, including how micromanipulators and end effectors are actuated and controlled, how remote physical fields are utilized for micromanipulation, how visual servoing is implemented under an optical microscope, how force is sensed and controlled at the micro- and nanonewton levels, and the similarities and differences between robotic manipulation at the micro- and macroscales. We also review representative milestones over the past three decades and discuss potential future trends of this field.
DOI
A LASER BEAM INDUCED OPTICAL MANIPULATION OF A SMECTITE
Yuki Higashi, Takashi Nagashita, Teruyuki Nakato, Yasutaka Suzuki, Jun Kawamata
On-demand and local manipulation of exfoliated clay nanosheets in colloidal dispersions enables a variety of novel applications of clay-based materials. In this study, manipulation of a single fluorohectorite nanosheet by utilizing the radiation pressure of a tightly focused laser beam was demonstrated. When a linearly polarized continuous laser beam was irradiated to a fluorohectorite nanosheet, the nanosheet was oriented with its in-plane direction parallel to the propagation direction of the laser beam. Furthermore, the nanosheet edge was directed in parallel to the polarization direction of the laser beam. When the polarization direction of the laser beam was rotated, the nanosheet was rotated following the rotation of the polarization direction.
DOI
On-demand and local manipulation of exfoliated clay nanosheets in colloidal dispersions enables a variety of novel applications of clay-based materials. In this study, manipulation of a single fluorohectorite nanosheet by utilizing the radiation pressure of a tightly focused laser beam was demonstrated. When a linearly polarized continuous laser beam was irradiated to a fluorohectorite nanosheet, the nanosheet was oriented with its in-plane direction parallel to the propagation direction of the laser beam. Furthermore, the nanosheet edge was directed in parallel to the polarization direction of the laser beam. When the polarization direction of the laser beam was rotated, the nanosheet was rotated following the rotation of the polarization direction.
DOI
Performance metrics and enabling technologies for nanoplasmonic biosensors
Sang-Hyun Oh & Hatice Altug
Nanoplasmonic structures can tightly confine light onto a material’s surface to probe biomolecular interactions not easily accessed by other sensing techniques. New and exciting developments in nanofabrication processes, nano-optical trapping, graphene devices, mid-infrared spectroscopy, and metasurfaces will greatly empower the performance and functionalities of nanoplasmonic sensors.
DOI
Nanoplasmonic structures can tightly confine light onto a material’s surface to probe biomolecular interactions not easily accessed by other sensing techniques. New and exciting developments in nanofabrication processes, nano-optical trapping, graphene devices, mid-infrared spectroscopy, and metasurfaces will greatly empower the performance and functionalities of nanoplasmonic sensors.
DOI
Thursday, January 10, 2019
Biophysical control of the cell rearrangements and cell shape changes that build epithelial tissues
R Marisol Herrera-Perez, Karen E Kasza
Epithelial cell rearrangements and cell shape changes are fundamental mechanisms by which cells build and shape elaborate and diverse tissue architectures from simple tissue sheets. These cell behaviors are regulated by a complex interplay between physical and biochemical mechanisms, many of which have been uncovered in recent studies in Drosophila. While the regulation of these cell behaviors is still under investigation, emerging technologies are being used to gain experimental control over these behaviors, opening new possibilities for designing and engineering tissue structures. Analysis of the biophysical mechanisms governing cell shape and movement will be crucial for understanding morphogenesis and for harnessing this knowledge to build tissues of precise shapes and structures for basic science and engineering applications.
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DOI
Upconverting materials for boosting the development of advanced optical microrheometric techniques
P. Rodríguez-Sevilla, L. Labrador-Páez, P. Haro-González
The confluence of technological advances in optics and the need of a deeper understanding of cellular processes have boosted the development of diverse optical microrheometric techniques. These experimental methods have allowed a better understanding of not only the viscoelastic behaviour of cells, but also different intracellular processes. In this work, the fundamentals of the most commonly used optical microrheometric techniques and their more relevant applications in the biological field are expounded for a non-specialist public. In particular, the promising and emerging use of upconverting luminescent particles as probes in this field is highlighted, as they could be the cornerstone of the development of new methods or the improvement of already existing microrheometric techniques.
DOI
The confluence of technological advances in optics and the need of a deeper understanding of cellular processes have boosted the development of diverse optical microrheometric techniques. These experimental methods have allowed a better understanding of not only the viscoelastic behaviour of cells, but also different intracellular processes. In this work, the fundamentals of the most commonly used optical microrheometric techniques and their more relevant applications in the biological field are expounded for a non-specialist public. In particular, the promising and emerging use of upconverting luminescent particles as probes in this field is highlighted, as they could be the cornerstone of the development of new methods or the improvement of already existing microrheometric techniques.
DOI
Alkaline-Earth Atoms in Optical Tweezers
Alexandre Cooper, Jacob P. Covey, Ivaylo S. Madjarov, Sergey G. Porsev, Marianna S. Safronova, and Manuel Endres
We demonstrate single-shot imaging and narrow-line cooling of individual alkaline-earth atoms in optical tweezers; specifically, strontium trapped in 515.2−nm light. Our approach enables high-fidelity detection of single atoms by imaging photons from the broad singlet transition while cooling on the narrow intercombination line, and we extend this technique to highly uniform two-dimensional tweezer arrays with 121 sites. Cooling during imaging is based on a previously unobserved narrow-line Sisyphus mechanism, which we predict to be applicable in a wide variety of experimental situations. Further, we demonstrate optically resolved sideband cooling of a single atom to near the motional ground state of a tweezer, which is tuned to a magic-trapping configuration achieved by elliptical polarization. Finally, we present calculations, in agreement with our experimental results, that predict a linear-polarization and polarization-independent magic crossing at 520(2) nm and 500.65(50) nm, respectively. Our results pave the way for a wide range of novel experimental avenues based on individually controlled alkaline-earth atoms in tweezers—from fundamental experiments in atomic physics to quantum computing, simulation, and metrology.
DOI
We demonstrate single-shot imaging and narrow-line cooling of individual alkaline-earth atoms in optical tweezers; specifically, strontium trapped in 515.2−nm light. Our approach enables high-fidelity detection of single atoms by imaging photons from the broad singlet transition while cooling on the narrow intercombination line, and we extend this technique to highly uniform two-dimensional tweezer arrays with 121 sites. Cooling during imaging is based on a previously unobserved narrow-line Sisyphus mechanism, which we predict to be applicable in a wide variety of experimental situations. Further, we demonstrate optically resolved sideband cooling of a single atom to near the motional ground state of a tweezer, which is tuned to a magic-trapping configuration achieved by elliptical polarization. Finally, we present calculations, in agreement with our experimental results, that predict a linear-polarization and polarization-independent magic crossing at 520(2) nm and 500.65(50) nm, respectively. Our results pave the way for a wide range of novel experimental avenues based on individually controlled alkaline-earth atoms in tweezers—from fundamental experiments in atomic physics to quantum computing, simulation, and metrology.
DOI
Unscrambling Structured Chirality with Structured Light at the Nanoscale Using Photoinduced Force
Mohammad Kamandi, Mohammad Albooyeh, Mehdi Veysi, Mohsen Rajaei, Jinwei Zeng, H. Kumar Wickramasinghe, and Filippo Capolino
We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip–sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample’s magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally polarized beams as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively. The proposed technique may benefit branches of science such as stereochemistry, biomedicine, physical and material science, and pharmaceutics.
DOI
We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip–sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample’s magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally polarized beams as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively. The proposed technique may benefit branches of science such as stereochemistry, biomedicine, physical and material science, and pharmaceutics.
DOI
A small single-domain protein folds through the same pathway on and off the ribosome
Emily J. Guinn, Pengfei Tian, Mia Shin, Robert B. Best, and Susan Marqusee
In vivo, proteins fold and function in a complex environment subject to many stresses that can modulate a protein’s energy landscape. One aspect of the environment pertinent to protein folding is the ribosome, since proteins have the opportunity to fold while still bound to the ribosome during translation. We use a combination of force and chemical denaturant (chemomechanical unfolding), as well as point mutations, to characterize the folding mechanism of the src SH3 domain both as a stalled ribosome nascent chain and free in solution. Our results indicate that src SH3 folds through the same pathway on and off the ribosome. Molecular simulations also indicate that the ribosome does not affect the folding pathway for this small protein. Taken together, we conclude that the ribosome does not alter the folding mechanism of this small protein. These results, if general, suggest the ribosome may exert a bigger influence on the folding of multidomain proteins or protein domains that can partially fold before the entire domain sequence is outside the ribosome exit tunnel.
DOI
In vivo, proteins fold and function in a complex environment subject to many stresses that can modulate a protein’s energy landscape. One aspect of the environment pertinent to protein folding is the ribosome, since proteins have the opportunity to fold while still bound to the ribosome during translation. We use a combination of force and chemical denaturant (chemomechanical unfolding), as well as point mutations, to characterize the folding mechanism of the src SH3 domain both as a stalled ribosome nascent chain and free in solution. Our results indicate that src SH3 folds through the same pathway on and off the ribosome. Molecular simulations also indicate that the ribosome does not affect the folding pathway for this small protein. Taken together, we conclude that the ribosome does not alter the folding mechanism of this small protein. These results, if general, suggest the ribosome may exert a bigger influence on the folding of multidomain proteins or protein domains that can partially fold before the entire domain sequence is outside the ribosome exit tunnel.
DOI
Extraordinary Focusing Effect of Surface Nanolenses in Total Internal Reflection Mode
Brendan Dyett, Qiming Zhang, Qiwei Xu, Xihua Wang, and Xuehua Zhang
Microscopic lenses are paramount in solar energy harvesting, optical devices, and imaging technologies. This work reports an extraordinary focusing effect exhibited by a surface nanolens (i.e., with at least one dimension of subwavelength) that is situated in an evanescent field from the total internal reflection (TIR) of light illuminated to the supporting substrate above the critical angle. Our measurements show that the position, shape, and size of the surface area with enhanced light intensity are determined by the geometry of the nanolens and the incident angle, in good agreement with simulation results. This strong focusing effect of the surface nanolens is shown to significantly promote the plasmonic effect of deposited gold nanoparticles on the lens surface inlight conversion and to vaporize surrounding water to microbubbles by using low laser power. This work further demonstrates that the light redistribution by the surface nanolens in TIR enables a range of novel applications in selectively local visualization of specimens in fluorescence imaging, optical trapping of colloids from an external flow, and selective materials deposition from photoreactions.
DOI
Microscopic lenses are paramount in solar energy harvesting, optical devices, and imaging technologies. This work reports an extraordinary focusing effect exhibited by a surface nanolens (i.e., with at least one dimension of subwavelength) that is situated in an evanescent field from the total internal reflection (TIR) of light illuminated to the supporting substrate above the critical angle. Our measurements show that the position, shape, and size of the surface area with enhanced light intensity are determined by the geometry of the nanolens and the incident angle, in good agreement with simulation results. This strong focusing effect of the surface nanolens is shown to significantly promote the plasmonic effect of deposited gold nanoparticles on the lens surface inlight conversion and to vaporize surrounding water to microbubbles by using low laser power. This work further demonstrates that the light redistribution by the surface nanolens in TIR enables a range of novel applications in selectively local visualization of specimens in fluorescence imaging, optical trapping of colloids from an external flow, and selective materials deposition from photoreactions.
DOI
Father of optical trapping awarded a share of the Nobel Prize in Physics
Rachel Berkowitz
Optical tweezers have endured as an invaluable laboratory tool for manipulating molecules and other small particles.
DOI
Optical tweezers have endured as an invaluable laboratory tool for manipulating molecules and other small particles.
DOI
High throughput trapping and arrangement of biological cells using self-assembled optical tweezer
Zongbao Li, Jianxin Yang, Shaojing Liu, Xiaofang Jiang, Haiyan Wang, Xiaowen Hu, Sheng Xue, Sailing He, and Xiaobo Xing
Lately, a fiber-based optical tweezer that traps and arranges the micro/nano-particles is crucial in practical applications, because such a device can trap the biological samples and drive them to the designated position in a microfluidic system or vessel without harming them. Here, we report a new type of fiber optical tweezer, which can trap and arrange erythrocytes. It is prepared by coating graphene on the cross section of a microfiber. Our results demonstrate that thermal-gradient-induced natural convection flow and thermophoresis can trap the erythrocytes under low incident power, and the optical scattering force can arrange them precisely under higher incident power. The proposed optical tweezer has high flexibility, easy fabrication, and high integration with lab-on-a-chip, and shows considerable potential for application in various fields, such as biophysics, biochemistry, and life sciences.
DOI
Lately, a fiber-based optical tweezer that traps and arranges the micro/nano-particles is crucial in practical applications, because such a device can trap the biological samples and drive them to the designated position in a microfluidic system or vessel without harming them. Here, we report a new type of fiber optical tweezer, which can trap and arrange erythrocytes. It is prepared by coating graphene on the cross section of a microfiber. Our results demonstrate that thermal-gradient-induced natural convection flow and thermophoresis can trap the erythrocytes under low incident power, and the optical scattering force can arrange them precisely under higher incident power. The proposed optical tweezer has high flexibility, easy fabrication, and high integration with lab-on-a-chip, and shows considerable potential for application in various fields, such as biophysics, biochemistry, and life sciences.
DOI
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