Mihaela Georgeta Moisescu, Tudor Savopol, Liviu Dimitriu, Jaka Cemazar, Eugenia Kovacs, Mihai Radu
Cellular electrical properties are modulated by various physical and/or chemical stresses and detection of these changes is a challenging issue. Optical tweezers (OT) and dielectrophoresis (DEP) are frequently integrated to devices dedicated to the investigation of cells properties. Here we provide a technique to detect changes in cytosol conductivity of cells by using a combination of DEP and OT. The method was exemplified for the case of cells electroporation and is based on balancing the DEP force by a controlled OT force. We observed a decrease of the DEP force in the case of electroporated cells which was correlated to a decrease of cytosol conductivity by means of Clausius-Mossotti factor modeling. For highly stressing electroporation pulses, the cytosol conductivity drops to values close to those of the cells suspending medium. These results are consistent with those reported in the literature proving the robustness of our proposed sensing method.
DOI
Monday, May 28, 2018
New antineoplastic agent based on a dibenzoylmethane derivative: Cytotoxic effect and direct interaction with DNA
Fernanda R. Nascimento, Tiago A. Moura, Jefferson V.P.B. Baeta, Bruno C. Publio, Pollyanna M.F. Ferreira, Anésia A. Santos, Andressa A.P. França, Marcio S. Rocha, Gaspar Diaz-Muñoz, Marisa A.N. Diaz
Melanoma accounts for only 4% of all skin cancers but is among the most lethal cutaneous neoplasms. Dacarbazine is the drug of choice for the treatment of melanoma in Brazil through the public health system mainly because of its low cost. However, it is an alkylating agent of low specificity and elicits a therapeutic response in only 20% of cases. Other drugs available for the treatment of melanoma are expensive, and tumor cells commonly develop resistance to these drugs. The fight against melanoma demands novel, more specific drugs that are effective in killing drug-resistant tumor cells. Dibenzoylmethane (1,3-diphenylpropane-1,3-dione) derivatives are promising antitumor agents. In this study, we investigated the cytotoxic effect of 1,3-diphenyl-2-benzyl-1,3-propanedione (DPBP) on B16F10 melanoma cells as well as its direct interaction with the DNA molecule using optical tweezers. DPBP showed promising results against tumor cells and had a selectivity index of 41.94. Also, we demonstrated the ability of DPBP to interact directly with the DNA molecule. The fact that DPBP can interact with DNA in vitro allows us to hypothesize that such an interaction may also occur in vivo and, therefore, that DPBP may be an alternative to treat patients with drug-resistant melanomas. These findings can guide the development of new and more effective drugs.
DOI
Melanoma accounts for only 4% of all skin cancers but is among the most lethal cutaneous neoplasms. Dacarbazine is the drug of choice for the treatment of melanoma in Brazil through the public health system mainly because of its low cost. However, it is an alkylating agent of low specificity and elicits a therapeutic response in only 20% of cases. Other drugs available for the treatment of melanoma are expensive, and tumor cells commonly develop resistance to these drugs. The fight against melanoma demands novel, more specific drugs that are effective in killing drug-resistant tumor cells. Dibenzoylmethane (1,3-diphenylpropane-1,3-dione) derivatives are promising antitumor agents. In this study, we investigated the cytotoxic effect of 1,3-diphenyl-2-benzyl-1,3-propanedione (DPBP) on B16F10 melanoma cells as well as its direct interaction with the DNA molecule using optical tweezers. DPBP showed promising results against tumor cells and had a selectivity index of 41.94. Also, we demonstrated the ability of DPBP to interact directly with the DNA molecule. The fact that DPBP can interact with DNA in vitro allows us to hypothesize that such an interaction may also occur in vivo and, therefore, that DPBP may be an alternative to treat patients with drug-resistant melanomas. These findings can guide the development of new and more effective drugs.
DOI
Analytical description of lateral binding force exerted on bi-sphere induced by high-order Bessel beams
J.Bai, Z.S.Wu, C.X.Ge, Z.J.Li, T.Qu, Q.C.Shang
Based on the generalized multi-particle Mie equation (GMM) and Electromagnetic Momentum (EM) theory, the lateral binding force (BF) exerted on bi-sphere induced by an arbitrary polarized high-order Bessel beam (HOBB) is investigated with particular emphasis on the half-conical angle of the wave number components and the order (or topological charge) of the beam. The illuminating HOBB with arbitrary polarization angle is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz-Mie theories (GLMT). Utilizing the vector addition theorem of the spherical vector wave functions (SVWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the bi-sphere is considered. Numerical effects of various parameters such as beam polarization angles, incident wavelengths, particle sizes, material losses and the refractive index, including the cases of weak, moderate, and strong than the surrounding medium are numerically analyzed in detail. The observed dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction. Accurate investigation of BF induced by HOBB could provide an effective test for further research on BF between more complex particles, which plays an important role in using optical manipulation on particle self-assembly.
DOI
Based on the generalized multi-particle Mie equation (GMM) and Electromagnetic Momentum (EM) theory, the lateral binding force (BF) exerted on bi-sphere induced by an arbitrary polarized high-order Bessel beam (HOBB) is investigated with particular emphasis on the half-conical angle of the wave number components and the order (or topological charge) of the beam. The illuminating HOBB with arbitrary polarization angle is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz-Mie theories (GLMT). Utilizing the vector addition theorem of the spherical vector wave functions (SVWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the bi-sphere is considered. Numerical effects of various parameters such as beam polarization angles, incident wavelengths, particle sizes, material losses and the refractive index, including the cases of weak, moderate, and strong than the surrounding medium are numerically analyzed in detail. The observed dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction. Accurate investigation of BF induced by HOBB could provide an effective test for further research on BF between more complex particles, which plays an important role in using optical manipulation on particle self-assembly.
DOI
Analyzing the micromechanics of the cell division apparatus
Yuta Shimamoto, Tarun M. Kapoor
Cell division involves mechanical processes, such as chromosome transport and centrosome separation. Quantitative micromanipulation-based approaches have been central to dissecting the forces driving these processes. We highlight two biophysical assays that can be employed for such analyses. First, an in vitro “mini-spindle” assay is described that can be used to examine the collective mechanics of mitotic motor proteins cross-linking two microtubules. In the spindle, motor proteins (e.g., kinesin-5, kinesin-14, and dynein) can localize to overlapping microtubules that slide relative to each other, work as an ensemble, and equilibrate between cytoplasm and the microtubules. The “mini-spindle” assay can recapitulate these features and allows measurements of forces generated between adjacent microtubules and their dependence on filament orientation, sliding speed, overlap length, and motor protein density. Second, we describe a force-calibrated microneedle-based “whole-spindle” micromechanics assay. Microneedle-based micromanipulation can be a useful technique to examine cellular scale mechanics, but its use has been restricted by the difficulty in getting probes to penetrate the plasma membrane without disrupting cell physiology. As detailed here, the use of cell-free extracts prepared from metaphase-arrested Xenopus eggs can address this limitation. These micromanipulation studies also benefit from the use of frozen stocks of Xenopus egg extract. Together, these approaches can be used to decipher how micromechanics and biochemical activities ensure successful cell division.
DOI
Cell division involves mechanical processes, such as chromosome transport and centrosome separation. Quantitative micromanipulation-based approaches have been central to dissecting the forces driving these processes. We highlight two biophysical assays that can be employed for such analyses. First, an in vitro “mini-spindle” assay is described that can be used to examine the collective mechanics of mitotic motor proteins cross-linking two microtubules. In the spindle, motor proteins (e.g., kinesin-5, kinesin-14, and dynein) can localize to overlapping microtubules that slide relative to each other, work as an ensemble, and equilibrate between cytoplasm and the microtubules. The “mini-spindle” assay can recapitulate these features and allows measurements of forces generated between adjacent microtubules and their dependence on filament orientation, sliding speed, overlap length, and motor protein density. Second, we describe a force-calibrated microneedle-based “whole-spindle” micromechanics assay. Microneedle-based micromanipulation can be a useful technique to examine cellular scale mechanics, but its use has been restricted by the difficulty in getting probes to penetrate the plasma membrane without disrupting cell physiology. As detailed here, the use of cell-free extracts prepared from metaphase-arrested Xenopus eggs can address this limitation. These micromanipulation studies also benefit from the use of frozen stocks of Xenopus egg extract. Together, these approaches can be used to decipher how micromechanics and biochemical activities ensure successful cell division.
DOI
Particle trapping and conveying using an optical Archimedes’ screw
Barak Hadad, Sahar Froim, Harel Nagar, Tamir Admon, Yaniv Eliezer, Yael Roichman, and Alon Bahabad
Trapping and manipulation of particles using laser beams has become an important tool in diverse fields of research. In recent years, particular interest has been devoted to the problem of conveying optically trapped particles over extended distances either downstream or upstream of the direction of photon momentum flow. Here, we propose and experimentally demonstrate an optical analog of the famous Archimedes’ screw where the rotation of a helical-intensity beam is transferred to the axial motion of optically trapped micrometer-scale, airborne, carbon-based particles. With this optical screw, particles were easily conveyed with controlled velocity and direction, upstream or downstream of the optical flow, over a distance of half a centimeter. Our results offer a very simple optical conveyor that could be adapted to a wide range of optical trapping scenarios.
DOI
Trapping and manipulation of particles using laser beams has become an important tool in diverse fields of research. In recent years, particular interest has been devoted to the problem of conveying optically trapped particles over extended distances either downstream or upstream of the direction of photon momentum flow. Here, we propose and experimentally demonstrate an optical analog of the famous Archimedes’ screw where the rotation of a helical-intensity beam is transferred to the axial motion of optically trapped micrometer-scale, airborne, carbon-based particles. With this optical screw, particles were easily conveyed with controlled velocity and direction, upstream or downstream of the optical flow, over a distance of half a centimeter. Our results offer a very simple optical conveyor that could be adapted to a wide range of optical trapping scenarios.
DOI
Optomechanically Assisted Assembly of Surface‐Functionalized Zeolite‐L‐Based Hybrid Soft Matter
Álvaro Barroso, Tim Buscher, Katrin Ahlers, Armido Studer, Cornelia Denz
Nanoporous particles are particularly interesting for the assembly of functional nano‐ and microsystems because they provide hierarchical supramolecular organization of a large variety of guest molecules. In this work, arbitrary nanoarchitectures consisting of nanoporous zeolite‐L crystals are assembled by combining holographic optical tweezers (HOT) with polymer brush functionalized particles to overcome the limitations of 1D and restricted self‐assembly of zeolite‐L crystals. Readily prepared and functionalized polymer shells allow for controlled, instant, and highly efficient particle–particle and particle–surface adhesion without the need for an external trigger. In contrast to earlier studies, these assemblies remain permanently stable after release out of the HOT system. This novel strategy can be used to fabricate either motile units or locally grounded 1D, 2D, and 3D microconstructions, which can be further utilized as microtools in microfluidic and nanophotonic applications.
DOI
Nanoporous particles are particularly interesting for the assembly of functional nano‐ and microsystems because they provide hierarchical supramolecular organization of a large variety of guest molecules. In this work, arbitrary nanoarchitectures consisting of nanoporous zeolite‐L crystals are assembled by combining holographic optical tweezers (HOT) with polymer brush functionalized particles to overcome the limitations of 1D and restricted self‐assembly of zeolite‐L crystals. Readily prepared and functionalized polymer shells allow for controlled, instant, and highly efficient particle–particle and particle–surface adhesion without the need for an external trigger. In contrast to earlier studies, these assemblies remain permanently stable after release out of the HOT system. This novel strategy can be used to fabricate either motile units or locally grounded 1D, 2D, and 3D microconstructions, which can be further utilized as microtools in microfluidic and nanophotonic applications.
DOI
Friday, May 25, 2018
Quantifying Local Molecular Tension Using Intercalated DNA Fluorescence
Graeme A. King, Andreas S. Biebricher, Iddo Heller, Erwin J. G. Peterman, and Gijs J. L. Wuite
The ability to measure mechanics and forces in biological nanostructures, such as DNA, proteins and cells, is of great importance as a means to analyze biomolecular systems. However, current force detection methods often require specialized instrumentation. Here, we present a novel and versatile method to quantify tension in molecular systems locally and in real time, using intercalated DNA fluorescence. This approach can report forces over a range of at least ∼0.5–65 pN with a resolution of 1–3 pN, using commercially available intercalating dyes and a general-purpose fluorescence microscope. We demonstrate that the method can be easily implemented to report double-stranded (ds)DNA tension in any single-molecule assay that is compatible with fluorescence microscopy. This is particularly useful for multiplexed techniques, where measuring applied force in parallel is technically challenging. Moreover, tension measurements based on local dye binding offer the unique opportunity to determine how an applied force is distributed locally within biomolecular structures. Exploiting this, we apply our method to quantify the position-dependent force profile along the length of flow-stretched DNA and reveal that stretched and entwined DNA molecules—mimicking catenated DNA structures in vivo—display transient DNA–DNA interactions. The method reported here has obvious and broad applications for the study of DNA and DNA–protein interactions. Additionally, we propose that it could be employed to measure forces in any system to which dsDNA can be tethered, for applications including protein unfolding, chromosome mechanics, cell motility, and DNA nanomachines.
DOI
The ability to measure mechanics and forces in biological nanostructures, such as DNA, proteins and cells, is of great importance as a means to analyze biomolecular systems. However, current force detection methods often require specialized instrumentation. Here, we present a novel and versatile method to quantify tension in molecular systems locally and in real time, using intercalated DNA fluorescence. This approach can report forces over a range of at least ∼0.5–65 pN with a resolution of 1–3 pN, using commercially available intercalating dyes and a general-purpose fluorescence microscope. We demonstrate that the method can be easily implemented to report double-stranded (ds)DNA tension in any single-molecule assay that is compatible with fluorescence microscopy. This is particularly useful for multiplexed techniques, where measuring applied force in parallel is technically challenging. Moreover, tension measurements based on local dye binding offer the unique opportunity to determine how an applied force is distributed locally within biomolecular structures. Exploiting this, we apply our method to quantify the position-dependent force profile along the length of flow-stretched DNA and reveal that stretched and entwined DNA molecules—mimicking catenated DNA structures in vivo—display transient DNA–DNA interactions. The method reported here has obvious and broad applications for the study of DNA and DNA–protein interactions. Additionally, we propose that it could be employed to measure forces in any system to which dsDNA can be tethered, for applications including protein unfolding, chromosome mechanics, cell motility, and DNA nanomachines.
DOI
Probing Position-Dependent Diffusion in Folding Reactions Using Single-Molecule Force Spectroscopy
Daniel A.N. Foster, Rafayel Petrosyan, Andrew G.T. Pyo, Armin Hoffmann, Feng Wang, Michael T. Woodside
Folding of proteins and nucleic acids involves a diffusive search over a multidimensional conformational energy landscape for the minimal-energy structure. When examining the projection of conformational motions onto a one-dimensional reaction coordinate, as done in most experiments, the diffusion coefficient D is generally position dependent. However, it has proven challenging to measure such position-dependence experimentally. We investigated the position-dependence of D in the folding of DNA hairpins as a simple model system in two ways: first, by analyzing the round-trip time to return to a given extension in constant-force extension trajectories measured by force spectroscopy, and second, by analyzing the fall time required to reach a given extension in force jump measurements. These methods yielded conflicting results: the fall time implied a fairly constant D, but the round-trip time implied variations of over an order of magnitude. Comparison of experiments with computational simulations revealed that both methods were strongly affected by experimental artifacts inherent to force spectroscopy measurements, which obscured the intrinsic position-dependence of D. Lastly, we applied Kramers’s theory to the kinetics of hairpins with energy barriers located at different positions along the hairpin stem, as a crude probe of D at different stem positions, and we found that D did not vary much as the barrier was moved along the reaction coordinate. This work underlines the difficulties faced when trying to deduce position-dependent diffusion coefficients from experimental folding trajectories.
DOI
Folding of proteins and nucleic acids involves a diffusive search over a multidimensional conformational energy landscape for the minimal-energy structure. When examining the projection of conformational motions onto a one-dimensional reaction coordinate, as done in most experiments, the diffusion coefficient D is generally position dependent. However, it has proven challenging to measure such position-dependence experimentally. We investigated the position-dependence of D in the folding of DNA hairpins as a simple model system in two ways: first, by analyzing the round-trip time to return to a given extension in constant-force extension trajectories measured by force spectroscopy, and second, by analyzing the fall time required to reach a given extension in force jump measurements. These methods yielded conflicting results: the fall time implied a fairly constant D, but the round-trip time implied variations of over an order of magnitude. Comparison of experiments with computational simulations revealed that both methods were strongly affected by experimental artifacts inherent to force spectroscopy measurements, which obscured the intrinsic position-dependence of D. Lastly, we applied Kramers’s theory to the kinetics of hairpins with energy barriers located at different positions along the hairpin stem, as a crude probe of D at different stem positions, and we found that D did not vary much as the barrier was moved along the reaction coordinate. This work underlines the difficulties faced when trying to deduce position-dependent diffusion coefficients from experimental folding trajectories.
DOI
Active Mechanics Reveal Molecular-Scale Force Kinetics in Living Oocytes
Wylie W. Ahmed, Étienne Fodor, Maria Almonacid, Matthias Bussonnier, Marie-Hélène Verlhac, Nir Gov, Paolo Visco, Frédéric van Wijland, Timo Betz
Active diffusion of intracellular components is emerging as an important process in cell biology. This process is mediated by complex assemblies of molecular motors and cytoskeletal filaments that drive force generation in the cytoplasm and facilitate enhanced motion. The kinetics of molecular motors have been precisely characterized in vitro by single molecule approaches, but their in vivo behavior remains elusive. Here, we study the active diffusion of vesicles in mouse oocytes, where this process plays a key role in nuclear positioning during development, and combine an experimental and theoretical framework to extract molecular-scale force kinetics (force, power stroke, and velocity) of the in vivo active process. Assuming a single dominant process, we find that the nonequilibrium activity induces rapid kicks of duration τ ∼ 300 μs resulting in an average force of F ∼ 0.4 pN on vesicles in in vivo oocytes, remarkably similar to the kinetics of in vitro myosin-V. Our results reveal that measuring in vivo active fluctuations allows extraction of the molecular-scale activity in agreement with single-molecule studies and demonstrates a mesoscopic framework to access force kinetics.
DOI
Active diffusion of intracellular components is emerging as an important process in cell biology. This process is mediated by complex assemblies of molecular motors and cytoskeletal filaments that drive force generation in the cytoplasm and facilitate enhanced motion. The kinetics of molecular motors have been precisely characterized in vitro by single molecule approaches, but their in vivo behavior remains elusive. Here, we study the active diffusion of vesicles in mouse oocytes, where this process plays a key role in nuclear positioning during development, and combine an experimental and theoretical framework to extract molecular-scale force kinetics (force, power stroke, and velocity) of the in vivo active process. Assuming a single dominant process, we find that the nonequilibrium activity induces rapid kicks of duration τ ∼ 300 μs resulting in an average force of F ∼ 0.4 pN on vesicles in in vivo oocytes, remarkably similar to the kinetics of in vitro myosin-V. Our results reveal that measuring in vivo active fluctuations allows extraction of the molecular-scale activity in agreement with single-molecule studies and demonstrates a mesoscopic framework to access force kinetics.
DOI
Pure nanodiamonds for levitated optomechanics in vacuum
A C Frangeskou, A T M A Rahman, L Gines, S Mandal, O A Williams, P F Barker and G W Morley
Optical trapping at high vacuum of a nanodiamond containing a nitrogen vacancy centre would provide a test bed for several new phenomena in fundamental physics. However, the nanodiamonds used so far have absorbed too much of the trapping light, heating them to destruction (above 800 K) except at pressures above ~10 mbar where air molecules dissipate the excess heat. Here we show that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity is raised above 700 GW m−2 below 5 mbar of pressure.
DOI
Optical trapping at high vacuum of a nanodiamond containing a nitrogen vacancy centre would provide a test bed for several new phenomena in fundamental physics. However, the nanodiamonds used so far have absorbed too much of the trapping light, heating them to destruction (above 800 K) except at pressures above ~10 mbar where air molecules dissipate the excess heat. Here we show that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity is raised above 700 GW m−2 below 5 mbar of pressure.
DOI
Metalens optical 3D-trapping and manipulating of nanoparticles
Yurii E Geints and Alexander A. Zemlyanov
A principal design of a single-beam optical trap is proposed based on the planar metalens assembled from an ordered array of dielectric microspheres arranged in a closely packed micro-assembly with a hollow center. By means of FDTD numerical simulation, we study in the detail the spatial structure of the optical field within the trap active zone and present an example of metalens-trap operation demonstrating the capturing and propulsion of a glass nanoparticle by the optical field.
DOI
A principal design of a single-beam optical trap is proposed based on the planar metalens assembled from an ordered array of dielectric microspheres arranged in a closely packed micro-assembly with a hollow center. By means of FDTD numerical simulation, we study in the detail the spatial structure of the optical field within the trap active zone and present an example of metalens-trap operation demonstrating the capturing and propulsion of a glass nanoparticle by the optical field.
DOI
Cell contraction induces long-ranged stress stiffening in the extracellular matrix
Yu Long Han, Pierre Ronceray, Guoqiang Xu, Andrea Malandrino, Roger D. Kamm, Martin Lenz, Chase P. Broedersz, and Ming Guo
Animal cells in tissues are supported by biopolymer matrices, which typically exhibit highly nonlinear mechanical properties. While the linear elasticity of the matrix can significantly impact cell mechanics and functionality, it remains largely unknown how cells, in turn, affect the nonlinear mechanics of their surrounding matrix. Here, we show that living contractile cells are able to generate a massive stiffness gradient in three distinct 3D extracellular matrix model systems: collagen, fibrin, and Matrigel. We decipher this remarkable behavior by introducing nonlinear stress inference microscopy (NSIM), a technique to infer stress fields in a 3D matrix from nonlinear microrheology measurements with optical tweezers. Using NSIM and simulations, we reveal large long-ranged cell-generated stresses capable of buckling filaments in the matrix. These stresses give rise to the large spatial extent of the observed cell-induced matrix stiffness gradient, which can provide a mechanism for mechanical communication between cells.
DOI
Animal cells in tissues are supported by biopolymer matrices, which typically exhibit highly nonlinear mechanical properties. While the linear elasticity of the matrix can significantly impact cell mechanics and functionality, it remains largely unknown how cells, in turn, affect the nonlinear mechanics of their surrounding matrix. Here, we show that living contractile cells are able to generate a massive stiffness gradient in three distinct 3D extracellular matrix model systems: collagen, fibrin, and Matrigel. We decipher this remarkable behavior by introducing nonlinear stress inference microscopy (NSIM), a technique to infer stress fields in a 3D matrix from nonlinear microrheology measurements with optical tweezers. Using NSIM and simulations, we reveal large long-ranged cell-generated stresses capable of buckling filaments in the matrix. These stresses give rise to the large spatial extent of the observed cell-induced matrix stiffness gradient, which can provide a mechanism for mechanical communication between cells.
DOI
Thursday, May 24, 2018
High-precision joint amplitude and phase control of spatial light using a digital micromirror device
Lei Liu, Yesheng Gao, Xingzhao Liu
Control over both amplitude and phase of optical beams plays an important role in optics, especially in the field of optical trapping. Trapping particles needs optical beams with specified intensity distribution and desired phase distribution, so joint control over light beams with high precision is indispensable. In this paper, a novel joint amplitude and phase control method specially designed for high precision using a digital micromirror device (DMD) is proposed. An off-axis 4f-configuration and a pattern formation algorithm are required for the implementation of the proposed method. Getting DMD pattern by considering the correlations among all pixels guarantees the high precision but increases the computation complexity, which makes it difficult to realize. Thus we present a pattern formation algorithm based on mixed integer programming and its improved version with higher computing efficiency. Experimental tests have been conducted to verify the superior performance of the proposed method over the conventional methods, such as Lee holography and Superpixel method. Measured results and quantitative analyses show that the proposed method enables simultaneous and independent control over amplitude and phase of light beams with a higher precision and performs better when steepest phase gradients exist in the target field. What is more, vortex beam and line beam can be generated accurately by the proposed method.
Control over both amplitude and phase of optical beams plays an important role in optics, especially in the field of optical trapping. Trapping particles needs optical beams with specified intensity distribution and desired phase distribution, so joint control over light beams with high precision is indispensable. In this paper, a novel joint amplitude and phase control method specially designed for high precision using a digital micromirror device (DMD) is proposed. An off-axis 4f-configuration and a pattern formation algorithm are required for the implementation of the proposed method. Getting DMD pattern by considering the correlations among all pixels guarantees the high precision but increases the computation complexity, which makes it difficult to realize. Thus we present a pattern formation algorithm based on mixed integer programming and its improved version with higher computing efficiency. Experimental tests have been conducted to verify the superior performance of the proposed method over the conventional methods, such as Lee holography and Superpixel method. Measured results and quantitative analyses show that the proposed method enables simultaneous and independent control over amplitude and phase of light beams with a higher precision and performs better when steepest phase gradients exist in the target field. What is more, vortex beam and line beam can be generated accurately by the proposed method.
Optical guiding-based cell focusing for Raman flow cell cytometer
Ravi Shanker Verma, Sunita Ahlawat and Abha Uppal
We report the use of an optical guiding arrangement generated in a microfluidic channel to produce a stream of single cells in a line for single-cell Raman spectroscopic analysis. The optical guiding arrangement consisted of dual-line optical tweezers, generated using a 1064 nm laser, aligned in the shape of a ‘Image ID:c8an00037a-u1.gif’ symbol. By controlling the laser power in the tweezers and the flow rate in the microfluidic channel, a single line flow of cells could be produced in the tail of the guiding arrangement, where the 514.5 nm Raman excitation beam was also located. Furthermore, by resonantly exciting the Raman spectrum, a good-quality Raman spectrum could be recorded from the flowing single cells as they passed through the Raman excitation focal spot without the need to trap the cells. As a proof of concept, it was shown that red blood cells (RBCs) could be guided to the tail of the optical guide and the Raman spectra of the resonantly excited cells could be recorded in a continuous manner without trapping the cells at a cell flow rate of ∼500 cells per h. From the recorded spectra, we were able to distinguish between RBCs containing hemoglobin in the normal form (normal-RBCs) and the met form (met-RBCs) from a mixture of RBCs comprising met-RBCs and normal-RBCs in a ratio of 1 : 9.
DOI
We report the use of an optical guiding arrangement generated in a microfluidic channel to produce a stream of single cells in a line for single-cell Raman spectroscopic analysis. The optical guiding arrangement consisted of dual-line optical tweezers, generated using a 1064 nm laser, aligned in the shape of a ‘Image ID:c8an00037a-u1.gif’ symbol. By controlling the laser power in the tweezers and the flow rate in the microfluidic channel, a single line flow of cells could be produced in the tail of the guiding arrangement, where the 514.5 nm Raman excitation beam was also located. Furthermore, by resonantly exciting the Raman spectrum, a good-quality Raman spectrum could be recorded from the flowing single cells as they passed through the Raman excitation focal spot without the need to trap the cells. As a proof of concept, it was shown that red blood cells (RBCs) could be guided to the tail of the optical guide and the Raman spectra of the resonantly excited cells could be recorded in a continuous manner without trapping the cells at a cell flow rate of ∼500 cells per h. From the recorded spectra, we were able to distinguish between RBCs containing hemoglobin in the normal form (normal-RBCs) and the met form (met-RBCs) from a mixture of RBCs comprising met-RBCs and normal-RBCs in a ratio of 1 : 9.
DOI
Optical trapping and manipulation of single particles in air: Principles, technical details, and applications
Zhiyong Gong, Yong-Le Pan, Gorden Videen, Chuji Wang
Trapping a single aerosol particle allows detailed investigation of its fundamental properties over extended time periods without external interferences. Optical trapping has developed into a powerful tool to perform such single-particle studies. However, trapping and manipulating a single particle in air, especially an irregularly shaped, absorbing particle, is much more challenging than that of a particle in a liquid solution. Even though the underlying mechanisms are not fully understood, recent experimental developments advanced the technique for trapping single particles in air, making it possible to manipulate and characterize a wide range of single particles. In this paper, we review recently demonstrated optical configurations for trapping and manipulating single airborne particles. Based on different trapping principles, we tentatively categorize them into radiation-pressure traps, photophoretic traps, and universal optical traps (UOTs). Radiation-pressure traps are based on the radiation pressure force resulting from photon momentum transfer; they include the early optical levitation configurations and the well-known optical tweezers. Photophoretic traps are based on the complex photophoretic forces that occur in absorbing particles; they are classified by the optical arrangements and include single-beam, dual-beam, and confocal-beam traps. UOTs can trap a variety of different types of particles, including transparent or absorbing, spherical or irregularly shaped, and liquid or solid particles. In order to evaluate each optical trapping scheme, four key aspects, i.e., simplicity, robustness, flexibility, and efficiency, of an optical trapping configuration are discussed. In addition to the stable optical trapping, optical manipulations from one dimension to three dimensions allow studying various single particles with great flexibility. With the single particle stably trapped and flexibly manipulated in air, other analytical techniques can be used to characterize these particles. Recent updates on optical methods for characterizing and monitoring single particles in air are discussed, such as light scattering, Raman spectroscopy, and cavity ringdown spectroscopy (CRDS).
DOI
Trapping a single aerosol particle allows detailed investigation of its fundamental properties over extended time periods without external interferences. Optical trapping has developed into a powerful tool to perform such single-particle studies. However, trapping and manipulating a single particle in air, especially an irregularly shaped, absorbing particle, is much more challenging than that of a particle in a liquid solution. Even though the underlying mechanisms are not fully understood, recent experimental developments advanced the technique for trapping single particles in air, making it possible to manipulate and characterize a wide range of single particles. In this paper, we review recently demonstrated optical configurations for trapping and manipulating single airborne particles. Based on different trapping principles, we tentatively categorize them into radiation-pressure traps, photophoretic traps, and universal optical traps (UOTs). Radiation-pressure traps are based on the radiation pressure force resulting from photon momentum transfer; they include the early optical levitation configurations and the well-known optical tweezers. Photophoretic traps are based on the complex photophoretic forces that occur in absorbing particles; they are classified by the optical arrangements and include single-beam, dual-beam, and confocal-beam traps. UOTs can trap a variety of different types of particles, including transparent or absorbing, spherical or irregularly shaped, and liquid or solid particles. In order to evaluate each optical trapping scheme, four key aspects, i.e., simplicity, robustness, flexibility, and efficiency, of an optical trapping configuration are discussed. In addition to the stable optical trapping, optical manipulations from one dimension to three dimensions allow studying various single particles with great flexibility. With the single particle stably trapped and flexibly manipulated in air, other analytical techniques can be used to characterize these particles. Recent updates on optical methods for characterizing and monitoring single particles in air are discussed, such as light scattering, Raman spectroscopy, and cavity ringdown spectroscopy (CRDS).
DOI
Multiple optical trapping assisted bead-array based fluorescence assay of free and total prostate-specific antigen in serum
Di Cao, Cheng-Yu Li, Chu-Bo Qi, Hong-Lei Chen, Dai-Wen Pang, Hong-WuTang
Although suspension bead-based assay technology has been widely used owing to its advantages of high-throughput and microvolume detection, its sensitivity is greatly limited because it detects the fluorescence signal emitted by microbeads for a short time in the flowing fluid. In this work, we present the approach for prostate-specific antigen (PSA) detection of both free PSA (fPSA) and total PSA (tPSA) based on bead-array based fluorescence imaging by combining multiple optical trapping and bead-based bioassays. The polystyrene beads were employed to enrich the targets using the classic sandwich immuno-binding and tagged with fluorescent quantum dots (QDs), and the QDs-tagged beads in suspension were trapped array-by-array using multiple optical tweezers constructed with a diffraction optical element and excited with a 405 nm fiber laser for wide-field fluorescence imaging. The distinctive size information from the image of the trapped beads enabled the sorting of different targets. Moreover, the limits of detection for fPSA and tPSA are 3.8 pg/mL and 2.5 pg/mL respectively with good specificity. More importantly, this strategy was successfully used to detect fPSA and tPSA simultaneously in real serum samples. The high sensitivity, good selectivity, and tiny sample volume make this strategy a promising method for life sciences and clinical applications.
DOI
Although suspension bead-based assay technology has been widely used owing to its advantages of high-throughput and microvolume detection, its sensitivity is greatly limited because it detects the fluorescence signal emitted by microbeads for a short time in the flowing fluid. In this work, we present the approach for prostate-specific antigen (PSA) detection of both free PSA (fPSA) and total PSA (tPSA) based on bead-array based fluorescence imaging by combining multiple optical trapping and bead-based bioassays. The polystyrene beads were employed to enrich the targets using the classic sandwich immuno-binding and tagged with fluorescent quantum dots (QDs), and the QDs-tagged beads in suspension were trapped array-by-array using multiple optical tweezers constructed with a diffraction optical element and excited with a 405 nm fiber laser for wide-field fluorescence imaging. The distinctive size information from the image of the trapped beads enabled the sorting of different targets. Moreover, the limits of detection for fPSA and tPSA are 3.8 pg/mL and 2.5 pg/mL respectively with good specificity. More importantly, this strategy was successfully used to detect fPSA and tPSA simultaneously in real serum samples. The high sensitivity, good selectivity, and tiny sample volume make this strategy a promising method for life sciences and clinical applications.
DOI
Aberration correction in holographic optical tweezers using a high-order optical vortex
Yansheng Liang, Yanan Cai, Zhaojun Wang, Ming Lei, Zhiliang Cao, Yue Wang, Manman Li, Shaohui Yan, Piero R. Bianco, and Baoli Yao
Holographic optical tweezers are a powerful optical trapping and manipulation tool in numerous applications such as life science and colloidal physics. However, imperfections in the spatial light modulator and optical components of the system will introduce detrimental aberrations to the system, thereby degrading the trapping performance significantly. To address this issue, we develop an aberration correction technique by using a high-order vortex as the correction metric. The optimal Zernike polynomial coefficients for quantifying the system aberrations are determined by comparing the distorted vortex and the ideal one. Efficiency of the proposed method is demonstrated by comparing the optical trap intensity distribution, trap stiffness, and particle dynamics in a Gaussian trap and an optical vortex trap, before and after aberration corrections.
DOI
Holographic optical tweezers are a powerful optical trapping and manipulation tool in numerous applications such as life science and colloidal physics. However, imperfections in the spatial light modulator and optical components of the system will introduce detrimental aberrations to the system, thereby degrading the trapping performance significantly. To address this issue, we develop an aberration correction technique by using a high-order vortex as the correction metric. The optimal Zernike polynomial coefficients for quantifying the system aberrations are determined by comparing the distorted vortex and the ideal one. Efficiency of the proposed method is demonstrated by comparing the optical trap intensity distribution, trap stiffness, and particle dynamics in a Gaussian trap and an optical vortex trap, before and after aberration corrections.
DOI
Microrheology, advances in methods and insights
Qiuyang Xia, Huining Xiao, Yuanfeng Pan, Lidong Wang
Microrheology is an emerging technique that probes mechanical response of soft material at micro-scale. Generally, microrheology technique can be divided into active and passive versions. For active microrheology, a user-controlled force, e.g. magnetic force, electrostatic force, optical tweezers etc., is applied to embedded particle in medium of interest, and the particle motion under this force is tracked. For passive microrheology, the embedded particles only move due to thermal fluctuations in the medium, i.e. Brownian motion, and their trajectories are tracked and analyzed. After Mason's seminal paper that developed reliable theory to calculate the relation between viscoelastic property and Brownian motion of embedding particles, corresponding methods and equipments to track particle Brownian motion both in laboratory and commercially available, together with software for data analysis. During last two decades, extensive efforts have been paid to improve both the experiment techniques and data analysis methods, especially about how to link consequential particle positions into trajectories. Also, some insights have been revealed in soft matter system using this technique. In this review paper we attempt to go through the advances in experiment techniques and data analysis methods developed in last ten years along with some recent results obtained from these methods.
DOI
Microrheology is an emerging technique that probes mechanical response of soft material at micro-scale. Generally, microrheology technique can be divided into active and passive versions. For active microrheology, a user-controlled force, e.g. magnetic force, electrostatic force, optical tweezers etc., is applied to embedded particle in medium of interest, and the particle motion under this force is tracked. For passive microrheology, the embedded particles only move due to thermal fluctuations in the medium, i.e. Brownian motion, and their trajectories are tracked and analyzed. After Mason's seminal paper that developed reliable theory to calculate the relation between viscoelastic property and Brownian motion of embedding particles, corresponding methods and equipments to track particle Brownian motion both in laboratory and commercially available, together with software for data analysis. During last two decades, extensive efforts have been paid to improve both the experiment techniques and data analysis methods, especially about how to link consequential particle positions into trajectories. Also, some insights have been revealed in soft matter system using this technique. In this review paper we attempt to go through the advances in experiment techniques and data analysis methods developed in last ten years along with some recent results obtained from these methods.
DOI
Wednesday, May 23, 2018
Cell Mechanotransduction With Piconewton Forces Applied by Optical Tweezers
Fabio Falleroni, Vincent Torre and Dan Cojoc
Mechanical stresses are always present in the cellular environment and mechanotransduction occurs in all cells. Although many experimental approaches have been developed to investigate mechanotransduction, the physical properties of the mechanical stimulus have yet to be accurately characterized. Here, we propose a mechanical stimulation method employing an oscillatory optical trap to apply piconewton forces perpendicularly to the cell membrane, for short instants. We show that this stimulation produces membrane indentation and induces cellular calcium transients in mouse neuroblastoma NG108-15 cells dependent of the stimulus strength and the number of force pulses.
DOI
Mechanical stresses are always present in the cellular environment and mechanotransduction occurs in all cells. Although many experimental approaches have been developed to investigate mechanotransduction, the physical properties of the mechanical stimulus have yet to be accurately characterized. Here, we propose a mechanical stimulation method employing an oscillatory optical trap to apply piconewton forces perpendicularly to the cell membrane, for short instants. We show that this stimulation produces membrane indentation and induces cellular calcium transients in mouse neuroblastoma NG108-15 cells dependent of the stimulus strength and the number of force pulses.
DOI
Dynamics of a Protein Chain Motor Driving Helical Bacteria under Stress
Julian Roth, Matthias D. Koch, Alexander Rohrbach
The wall-less, helical bacterial genus Spiroplasma has a unique propulsion system; it is not driven by propeller-like flagella but by a membrane-bound, cytoplasmic, linear motor that consists of a contractile chain of identical proteins spanning the entire cell length. By a coordinated spread of conformational changes of the proteins, kinks propagate in pairs along the cell body. However, the mechanisms for the initiation or delay of kinks and their coordinated spread remain unclear. Here, we show how we manipulate the initiation of kinks, their propagation velocities, and the time between two kinks for a single cell trapped in an optical line potential. By interferometric three-dimensional shape tracking, we measured the cells’ deformations in response to various external stress situations. We observed a significant dependency of force generation on the cells’ local ligand concentrations (likely ATP) and ligand hydrolysis, which we altered in different ways. We developed a mechanistic, mathematical model based on Kramer’s rates, describing the subsequent cooperative and conformational switching of the chain’s proteins. The model reproduces our experimental observations and can explain deformation characteristics even when the motor is driven to its extreme. Nature has invented a set of minimalistic mechanical driving concepts. To understand or even rebuild them, it is essential to reveal the molecular mechanisms of such protein chain motors, which need only two components—coupled proteins and ligands—to function.
DOI
The wall-less, helical bacterial genus Spiroplasma has a unique propulsion system; it is not driven by propeller-like flagella but by a membrane-bound, cytoplasmic, linear motor that consists of a contractile chain of identical proteins spanning the entire cell length. By a coordinated spread of conformational changes of the proteins, kinks propagate in pairs along the cell body. However, the mechanisms for the initiation or delay of kinks and their coordinated spread remain unclear. Here, we show how we manipulate the initiation of kinks, their propagation velocities, and the time between two kinks for a single cell trapped in an optical line potential. By interferometric three-dimensional shape tracking, we measured the cells’ deformations in response to various external stress situations. We observed a significant dependency of force generation on the cells’ local ligand concentrations (likely ATP) and ligand hydrolysis, which we altered in different ways. We developed a mechanistic, mathematical model based on Kramer’s rates, describing the subsequent cooperative and conformational switching of the chain’s proteins. The model reproduces our experimental observations and can explain deformation characteristics even when the motor is driven to its extreme. Nature has invented a set of minimalistic mechanical driving concepts. To understand or even rebuild them, it is essential to reveal the molecular mechanisms of such protein chain motors, which need only two components—coupled proteins and ligands—to function.
DOI
Quantifying the Precision of Single-Molecule Torque and Twist Measurements Using Allan Variance
Maarten M. van Oene, Seungkyu Ha, Tessa Jager, Mina Lee, Francesco Pedaci, Jan Lipfert, Nynke H. Dekker
Single-molecule manipulation techniques have provided unprecedented insights into the structure, function, interactions, and mechanical properties of biological macromolecules. Recently, the single-molecule toolbox has been expanded by techniques that enable measurements of rotation and torque, such as the optical torque wrench (OTW) and several different implementations of magnetic (torque) tweezers. Although systematic analyses of the position and force precision of single-molecule techniques have attracted considerable attention, their angle and torque precision have been treated in much less detail. Here, we propose Allan deviation as a tool to systematically quantitate angle and torque precision in single-molecule measurements. We apply the Allan variance method to experimental data from our implementations of (electro)magnetic torque tweezers and an OTW and find that both approaches can achieve a torque precision better than 1 pN · nm. The OTW, capable of measuring torque on (sub)millisecond timescales, provides the best torque precision for measurement times Math Eq10 s, after which drift becomes a limiting factor. For longer measurement times, magnetic torque tweezers with their superior stability provide the best torque precision. Use of the Allan deviation enables critical assessments of the torque precision as a function of measurement time across different measurement modalities and provides a tool to optimize measurement protocols for a given instrument and application.
DOI
Single-molecule manipulation techniques have provided unprecedented insights into the structure, function, interactions, and mechanical properties of biological macromolecules. Recently, the single-molecule toolbox has been expanded by techniques that enable measurements of rotation and torque, such as the optical torque wrench (OTW) and several different implementations of magnetic (torque) tweezers. Although systematic analyses of the position and force precision of single-molecule techniques have attracted considerable attention, their angle and torque precision have been treated in much less detail. Here, we propose Allan deviation as a tool to systematically quantitate angle and torque precision in single-molecule measurements. We apply the Allan variance method to experimental data from our implementations of (electro)magnetic torque tweezers and an OTW and find that both approaches can achieve a torque precision better than 1 pN · nm. The OTW, capable of measuring torque on (sub)millisecond timescales, provides the best torque precision for measurement times Math Eq10 s, after which drift becomes a limiting factor. For longer measurement times, magnetic torque tweezers with their superior stability provide the best torque precision. Use of the Allan deviation enables critical assessments of the torque precision as a function of measurement time across different measurement modalities and provides a tool to optimize measurement protocols for a given instrument and application.
DOI
Single fiber dual-functionality optical tweezers based on graded-index multimode fiber
Zhihai Liu, Tong Wang, Yaxun Zhang, Xiaoyun Tang, Peikun Liu, Yu Zhang, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan
We propose and demonstrate single fiber dual-functionality optical tweezers based on a graded-index multimode fiber. By using the multi-angle fiber grinding and polishing technology, we fabricate the multimode fiber tip to be a special tapered shape, contributing to focus the outgoing beam with a large intensity gradient for the first functionality—three-dimensional contactless trapping of a microparticle. By adjusting the radial direction offset between the lead-in single mode fiber and the graded-index multimode fiber, we perform the second functionality—axial shift of the trapped microparticle with respect to the fiber tip without need of moving the fiber probe itself. It is convenient for practical applications. The theoretical and experimental results about the relationship between the radial offset and the equilibrium positions of the microparticle have the good consistency. Tailoring the trap and axial shift of the microparticle based on the graded-index multimode fiber provides convenient avenues for fiber optical tweezers applied in practical researches.
DOI
We propose and demonstrate single fiber dual-functionality optical tweezers based on a graded-index multimode fiber. By using the multi-angle fiber grinding and polishing technology, we fabricate the multimode fiber tip to be a special tapered shape, contributing to focus the outgoing beam with a large intensity gradient for the first functionality—three-dimensional contactless trapping of a microparticle. By adjusting the radial direction offset between the lead-in single mode fiber and the graded-index multimode fiber, we perform the second functionality—axial shift of the trapped microparticle with respect to the fiber tip without need of moving the fiber probe itself. It is convenient for practical applications. The theoretical and experimental results about the relationship between the radial offset and the equilibrium positions of the microparticle have the good consistency. Tailoring the trap and axial shift of the microparticle based on the graded-index multimode fiber provides convenient avenues for fiber optical tweezers applied in practical researches.
DOI
Out-of-plane Rotation Control of Biological Cells with a Robot-Tweezers Manipulation System for Orientation-based Cell Surgery
Mingyang Xie; Adnan Shakoor; Yajing Shen; James K. Mills; Dong Sun
In many cell surgery applications, cell must be oriented properly such that the micro-surgery tool can access the target components with minimum damage to the cell. In this paper, a scheme for out of image plane orientation control of suspended biological cells using robotic controlled optical tweezers is presented for orientation-based cell surgery. Based on our previous work on planar cell rotation using optical tweezers, the dynamic model of cell out-of-plane orientation control is formulated by using the T-matrix approach. Vision-based algorithms are developed to extract the cell out of image plane orientation angles, based on 2D image slices obtained under optical microscope. A robust feedback controller is then proposed to achieve cell out-of-plane rotation. Experiments of automated out of image plane rotational control for cell nucleus extraction surgery are performed to demonstrate the effectiveness of the proposed approach. This approach advances robot-aided single cell manipulation and produces impactful benefits to cell surgery applications such as nucleus transplantation and organelle biopsy in precision medicine.
DOI
In many cell surgery applications, cell must be oriented properly such that the micro-surgery tool can access the target components with minimum damage to the cell. In this paper, a scheme for out of image plane orientation control of suspended biological cells using robotic controlled optical tweezers is presented for orientation-based cell surgery. Based on our previous work on planar cell rotation using optical tweezers, the dynamic model of cell out-of-plane orientation control is formulated by using the T-matrix approach. Vision-based algorithms are developed to extract the cell out of image plane orientation angles, based on 2D image slices obtained under optical microscope. A robust feedback controller is then proposed to achieve cell out-of-plane rotation. Experiments of automated out of image plane rotational control for cell nucleus extraction surgery are performed to demonstrate the effectiveness of the proposed approach. This approach advances robot-aided single cell manipulation and produces impactful benefits to cell surgery applications such as nucleus transplantation and organelle biopsy in precision medicine.
DOI
Electromagnetic stress tensor for an amorphous metamaterial medium
Neng Wang, Shubo Wang, and Jack Ng
We analytically and numerically investigated the internal optical forces exerted by an electromagnetic wave inside an amorphous metamaterial medium. We derived, by using the principle of virtual work, the Helmholtz stress tensor, which takes into account the electrostriction effect. Several examples of amorphous media are considered, and different electromagnetic stress tensors, such as the Einstein-Laub tensor and Minkowski tensor, are also compared. It is concluded that the Helmholtz stress tensor is the appropriate tensor for such systems.
DOI
We analytically and numerically investigated the internal optical forces exerted by an electromagnetic wave inside an amorphous metamaterial medium. We derived, by using the principle of virtual work, the Helmholtz stress tensor, which takes into account the electrostriction effect. Several examples of amorphous media are considered, and different electromagnetic stress tensors, such as the Einstein-Laub tensor and Minkowski tensor, are also compared. It is concluded that the Helmholtz stress tensor is the appropriate tensor for such systems.
DOI
Tuesday, May 22, 2018
A polarized view on DNA under tension
Joost van Mameren, Karen Vermeulen, Gijs J. L. Wuite, and Erwin J. G. Peterman
In the past decades, sensitive fluorescence microscopy techniques have contributed significantly to our understanding of the dynamics of DNA. The specific labeling of DNA using intercalating dyes has allowed for quantitative measurement of the thermal fluctuations the polymers undergo. On the other hand, recent advances in single-molecule manipulation techniques have unraveled the mechanical and elastic properties of this intricate polymer. Here, we have combined these two approaches to study the conformational dynamics of DNA under a wide range of tensions. Using polarized fluorescence microscopy in conjunction with optical-tweezers-based manipulation of YOYO-intercalated DNA, we controllably align the YOYO dyes using DNA tension, enabling us to disentangle the rapid dynamics of the dyes from that of the DNA itself. With unprecedented control of the DNA alignment, we resolve an inconsistency in reports about the tilted orientation of intercalated dyes. We find that intercalated dyes are on average oriented perpendicular to the long axis of the DNA, yet undergo fast dynamics on the time scale of absorption and fluorescence emission. In the overstretching transition of double-stranded DNA, we do not observe changes in orientation or orientational dynamics of the dyes. Only beyond the overstretching transition, a considerable depolarization is observed, presumably caused by an average tilting of the DNA base pairs. Our combined approach thus contributes to the elucidation of unique features of the molecular dynamics of DNA.
DOI
In the past decades, sensitive fluorescence microscopy techniques have contributed significantly to our understanding of the dynamics of DNA. The specific labeling of DNA using intercalating dyes has allowed for quantitative measurement of the thermal fluctuations the polymers undergo. On the other hand, recent advances in single-molecule manipulation techniques have unraveled the mechanical and elastic properties of this intricate polymer. Here, we have combined these two approaches to study the conformational dynamics of DNA under a wide range of tensions. Using polarized fluorescence microscopy in conjunction with optical-tweezers-based manipulation of YOYO-intercalated DNA, we controllably align the YOYO dyes using DNA tension, enabling us to disentangle the rapid dynamics of the dyes from that of the DNA itself. With unprecedented control of the DNA alignment, we resolve an inconsistency in reports about the tilted orientation of intercalated dyes. We find that intercalated dyes are on average oriented perpendicular to the long axis of the DNA, yet undergo fast dynamics on the time scale of absorption and fluorescence emission. In the overstretching transition of double-stranded DNA, we do not observe changes in orientation or orientational dynamics of the dyes. Only beyond the overstretching transition, a considerable depolarization is observed, presumably caused by an average tilting of the DNA base pairs. Our combined approach thus contributes to the elucidation of unique features of the molecular dynamics of DNA.
DOI
A fluid membrane enhances the velocity of cargo transport by small teams of kinesin-1
Qiaochu Li, Kuo-Fu Tseng, Stephen J. King, Weihong Qiu, and Jing Xu
Kinesin-1 (hereafter referred to as kinesin) is a major microtubule-based motor protein for plus-end-directed intracellular transport in live cells. While the single-molecule functions of kinesin are well characterized, the physiologically relevant transport of membranous cargos by small teams of kinesins remains poorly understood. A key experimental challenge remains in the quantitative control of the number of motors driving transport. Here we utilized “motile fraction” to overcome this challenge and experimentally accessed transport by a single kinesin through the physiologically relevant transport by a small team of kinesins. We used a fluid lipid bilayer to model the cellular membrane in vitro and employed optical trapping to quantify the transport of membrane-enclosed cargos versus traditional membrane-free cargos under identical conditions. We found that coupling motors via a fluid membrane significantly enhances the velocity of cargo transport by small teams of kinesins. Importantly, enclosing a cargo in a fluid lipid membrane did not impact single-kinesin transport, indicating that membrane-dependent velocity enhancement for team-based transport arises from altered interactions between kinesins. Our study demonstrates that membrane-based coupling between motors is a key determinant of kinesin-based transport. Enhanced velocity may be critical for fast delivery of cargos in live cells.
DOI
Kinesin-1 (hereafter referred to as kinesin) is a major microtubule-based motor protein for plus-end-directed intracellular transport in live cells. While the single-molecule functions of kinesin are well characterized, the physiologically relevant transport of membranous cargos by small teams of kinesins remains poorly understood. A key experimental challenge remains in the quantitative control of the number of motors driving transport. Here we utilized “motile fraction” to overcome this challenge and experimentally accessed transport by a single kinesin through the physiologically relevant transport by a small team of kinesins. We used a fluid lipid bilayer to model the cellular membrane in vitro and employed optical trapping to quantify the transport of membrane-enclosed cargos versus traditional membrane-free cargos under identical conditions. We found that coupling motors via a fluid membrane significantly enhances the velocity of cargo transport by small teams of kinesins. Importantly, enclosing a cargo in a fluid lipid membrane did not impact single-kinesin transport, indicating that membrane-dependent velocity enhancement for team-based transport arises from altered interactions between kinesins. Our study demonstrates that membrane-based coupling between motors is a key determinant of kinesin-based transport. Enhanced velocity may be critical for fast delivery of cargos in live cells.
DOI
Single particle states of colloidal particles in 2D periodic potentials
E. Sarmiento-Gómez, J. A. Rivera-Morán and J. L. Arauz-Lara
Colloidal particles when subjected to a periodic array of potential wells are observed to adopt discrete stable configurations depending on the particle size/array wavelength ratio. Experimentally, the configuration states are determined for singlets, doublets and triplets of identical spheres in a periodic array of traps. The energy landscape of a single spherical particle is obtained by considering the refraction of the incident light as it passes throughout the particle. Then, the energy of a dumbbell is determined as the superposition of two singlets. The energy of a triplet is calculated as the superposition of a dumbbell and a single particle. As it is shown here, this direct method predicts accurately the stable particle configurations as observed in the experiments. The method can be generalized to obtain the potential energy of an n-particle aggregate, using as building blocks the energies of singlets and doublets.
DOI
Colloidal particles when subjected to a periodic array of potential wells are observed to adopt discrete stable configurations depending on the particle size/array wavelength ratio. Experimentally, the configuration states are determined for singlets, doublets and triplets of identical spheres in a periodic array of traps. The energy landscape of a single spherical particle is obtained by considering the refraction of the incident light as it passes throughout the particle. Then, the energy of a dumbbell is determined as the superposition of two singlets. The energy of a triplet is calculated as the superposition of a dumbbell and a single particle. As it is shown here, this direct method predicts accurately the stable particle configurations as observed in the experiments. The method can be generalized to obtain the potential energy of an n-particle aggregate, using as building blocks the energies of singlets and doublets.
DOI
Particle Manipulation by Optical Forces in Microfluidic Devices
Petra Paiè, Tommaso Zandrini, Rebeca Martínez Vázquez, Roberto Osellame and Francesca Bragheri
Since the pioneering work of Ashkin and coworkers, back in 1970, optical manipulation gained an increasing interest among the scientific community. Indeed, the advantages and the possibilities of this technique are unsubtle, allowing for the manipulation of small particles with a broad spectrum of dimensions (nanometers to micrometers size), with no physical contact and without affecting the sample viability. Thus, optical manipulation rapidly found a large set of applications in different fields, such as cell biology, biophysics, and genetics. Moreover, large benefits followed the combination of optical manipulation and microfluidic channels, adding to optical manipulation the advantages of microfluidics, such as a continuous sample replacement and therefore high throughput and automatic sample processing. In this work, we will discuss the state of the art of these optofluidic devices, where optical manipulation is used in combination with microfluidic devices. We will distinguish on the optical method implemented and three main categories will be presented and explored: (i) a single highly focused beam used to manipulate the sample, (ii) one or more diverging beams imping on the sample, or (iii) evanescent wave based manipulation.
Since the pioneering work of Ashkin and coworkers, back in 1970, optical manipulation gained an increasing interest among the scientific community. Indeed, the advantages and the possibilities of this technique are unsubtle, allowing for the manipulation of small particles with a broad spectrum of dimensions (nanometers to micrometers size), with no physical contact and without affecting the sample viability. Thus, optical manipulation rapidly found a large set of applications in different fields, such as cell biology, biophysics, and genetics. Moreover, large benefits followed the combination of optical manipulation and microfluidic channels, adding to optical manipulation the advantages of microfluidics, such as a continuous sample replacement and therefore high throughput and automatic sample processing. In this work, we will discuss the state of the art of these optofluidic devices, where optical manipulation is used in combination with microfluidic devices. We will distinguish on the optical method implemented and three main categories will be presented and explored: (i) a single highly focused beam used to manipulate the sample, (ii) one or more diverging beams imping on the sample, or (iii) evanescent wave based manipulation.
Focus shaping and optical manipulation using highly focused second-order full Poincaré beam
Yuxiong Xue, Yusong Wang, Sichao Zhou, Hongwei Chen, Guanghao Rui, Bing Gu, and Qiwen Zhan
Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in plenty of applications. In this work, we propose and experimentally demonstrate the generation of a second-order full Poincaré (FP) beam and its application in two-dimensional (2D) flattop beam shaping with spatially variant polarization under a high numerical aperture focusing condition. In addition, the force mechanism of the focal field with 2D flattop beam profile is numerically studied, demonstrating the feasibility to trap a dielectric Rayleigh particle in three-dimensional space. The results show that the additional degree of freedom provided by the FP beam allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force exerted on a dielectric Rayleigh particle. The findings reported in the work may find useful applications in laser micromachining, optical trapping, and optical assembly.
DOI
Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in plenty of applications. In this work, we propose and experimentally demonstrate the generation of a second-order full Poincaré (FP) beam and its application in two-dimensional (2D) flattop beam shaping with spatially variant polarization under a high numerical aperture focusing condition. In addition, the force mechanism of the focal field with 2D flattop beam profile is numerically studied, demonstrating the feasibility to trap a dielectric Rayleigh particle in three-dimensional space. The results show that the additional degree of freedom provided by the FP beam allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force exerted on a dielectric Rayleigh particle. The findings reported in the work may find useful applications in laser micromachining, optical trapping, and optical assembly.
DOI
Trapping two types of particles by using a tightly focused radially polarized power-exponent-phase vortex beam
Chengjin Fan, Yongxin Liu, Xiaoyan Wang, Ziyang Chen, and Jixiong Pu
We investigate the intensity of a radially polarized power-exponent-phase vortex (PEPV) beam focused by a high-numerical-aperture objective. A bright focal spot and a focal annulus with a dark core can be generated by changing the phase of the PEPV beam. The possibility of trapping a gold particle with the bright focal spot and trapping an air bubble with the focal annulus is discussed, and the force and trapping stability are calculated. It is shown that a tightly focused radially polarized PEPV beam is applicable to trapping two types of particles.
DOI
We investigate the intensity of a radially polarized power-exponent-phase vortex (PEPV) beam focused by a high-numerical-aperture objective. A bright focal spot and a focal annulus with a dark core can be generated by changing the phase of the PEPV beam. The possibility of trapping a gold particle with the bright focal spot and trapping an air bubble with the focal annulus is discussed, and the force and trapping stability are calculated. It is shown that a tightly focused radially polarized PEPV beam is applicable to trapping two types of particles.
DOI
Surface characterization of nanoparticles using near-field light scattering
Eunsoo Yoo, Yizhong Liu, Chukwuazam A. Nwasike, Sebastian R. Freeman, Brian C. DiPaolo, Bernardo Cordovez and Amber L. Doiron
The effect of nanoparticle surface coating characteristics on colloidal stability in solution is a critical parameter in understanding the potential applications of nanoparticles, especially in biomedicine. Here we explored the modification of the surface of poly(ethylene glycol)-coated superparamagnetic iron oxide nanoparticles (PEG-SPIOs) with the synthetic pseudotannin polygallol via interpolymer complexation (IPC). Changes in particle size and zeta potential were indirectly assessed via differences between PEG-SPIOs and IPC-SPIOs in particle velocity and scattering intensity using near-field light scattering. The local scattering intensity is correlated with the distance between the particle and waveguide, which is affected by the size of the particle (coating thickness) as well as the interactions between the particle and waveguide (related to the zeta potential of the coating). Therefore, we report here the use of near-field light scattering using nanophotonic force microscopy (using a NanoTweezerTM instrument, Halo Labs) to determine the changes that occurred in hydrated particle characteristics, which is accompanied by an analytical model. Furthermore, we found that altering the salt concentration of the suspension solution affected the velocity of particles due to the change of dielectric constant and viscosity of the solution. These findings suggest that this technique is suitable for studying particle surface changes and perhaps can be used to dynamically study reaction kinetics at the particle surface.
DOI
The effect of nanoparticle surface coating characteristics on colloidal stability in solution is a critical parameter in understanding the potential applications of nanoparticles, especially in biomedicine. Here we explored the modification of the surface of poly(ethylene glycol)-coated superparamagnetic iron oxide nanoparticles (PEG-SPIOs) with the synthetic pseudotannin polygallol via interpolymer complexation (IPC). Changes in particle size and zeta potential were indirectly assessed via differences between PEG-SPIOs and IPC-SPIOs in particle velocity and scattering intensity using near-field light scattering. The local scattering intensity is correlated with the distance between the particle and waveguide, which is affected by the size of the particle (coating thickness) as well as the interactions between the particle and waveguide (related to the zeta potential of the coating). Therefore, we report here the use of near-field light scattering using nanophotonic force microscopy (using a NanoTweezerTM instrument, Halo Labs) to determine the changes that occurred in hydrated particle characteristics, which is accompanied by an analytical model. Furthermore, we found that altering the salt concentration of the suspension solution affected the velocity of particles due to the change of dielectric constant and viscosity of the solution. These findings suggest that this technique is suitable for studying particle surface changes and perhaps can be used to dynamically study reaction kinetics at the particle surface.
DOI
Monday, May 21, 2018
Newtonian to non-newtonian fluid transition of a model transient network
Giovanni Nava, Tie Yang, Valerio Vitali, Paolo Minzioni, Ilaria Cristiani, Francesca Bragheri, Roberto Osellame, Lucas Bethge, Sven Klussmann, Elvezia Maria Paraboschi, Rosanna Asseltaf and Tommaso Bellini
The viscosity of gel-forming fluids is notoriously complex and its study can benefit from new model systems that enable a detailed control of the network features. Here we use a novel and simple microfluidic-based active microrheology approach to study the transition from Newtonian to non-Newtonian behavior in a DNA hydrogel whose structure, connectivity, density of bonds, bond energy and kinetics are strongly temperature dependent and well known. In a temperature range of 15 °C, the system reversibly and continuously transforms from a Newtonian dispersion of low-valence nanocolloids into a strongly shear-thinning fluid, passing through a set of intermediate states where it behaves as a power-law fluid. We demonstrate that the knowledge of network topology and bond free energy enables to quantitatively predict the observed behavior using established rheology models.
DOI
The viscosity of gel-forming fluids is notoriously complex and its study can benefit from new model systems that enable a detailed control of the network features. Here we use a novel and simple microfluidic-based active microrheology approach to study the transition from Newtonian to non-Newtonian behavior in a DNA hydrogel whose structure, connectivity, density of bonds, bond energy and kinetics are strongly temperature dependent and well known. In a temperature range of 15 °C, the system reversibly and continuously transforms from a Newtonian dispersion of low-valence nanocolloids into a strongly shear-thinning fluid, passing through a set of intermediate states where it behaves as a power-law fluid. We demonstrate that the knowledge of network topology and bond free energy enables to quantitatively predict the observed behavior using established rheology models.
DOI
Nonlinear optical tweezers for longitudinal control of dielectric particles
Quy Ho Quang, Thanh Thai Doan, Tuan Doan Quoc, Thang Nguyen Manh
A model of the optical tweezers using thin layer of organic dye as the addition nonlinear lens in configuration is proposed. The expressions of the focal length of nonlinear lens, intensity distribution of reshaped Gaussian beam, and total longitudinal optical force acting on dielectric particles are theoretically derived using paraxial approximation. The influence of the average power of incoming Gaussian laser beam, thickness of thin layer, and nonlinear coefficient of refractive index on properties of nonlinear optical tweezers are numerically observed. The results also are discussed to find out the conditions that the dielectric particle could be stable trapped and finely controlled by tuning of average power of incoming laser beam.
DOI
A model of the optical tweezers using thin layer of organic dye as the addition nonlinear lens in configuration is proposed. The expressions of the focal length of nonlinear lens, intensity distribution of reshaped Gaussian beam, and total longitudinal optical force acting on dielectric particles are theoretically derived using paraxial approximation. The influence of the average power of incoming Gaussian laser beam, thickness of thin layer, and nonlinear coefficient of refractive index on properties of nonlinear optical tweezers are numerically observed. The results also are discussed to find out the conditions that the dielectric particle could be stable trapped and finely controlled by tuning of average power of incoming laser beam.
DOI
Mechanical Protein Unfolding and Degradation
Adrian O. Olivares, Tania A. Baker, and Robert T. Sauer
AAA+ proteolytic machines use energy from ATP hydrolysis to degrade damaged, misfolded, or unneeded proteins. Protein degradation occurs within a barrel-shaped self-compartmentalized peptidase. Before protein substrates can enter this peptidase, they must be unfolded and then translocated through the axial pore of an AAA+ ring hexamer. An unstructured region of the protein substrate is initially engaged in the axial pore, and conformational changes in the ring, powered by ATP hydrolysis, generate a mechanical force that pulls on and denatures the substrate. The same conformational changes in the hexameric ring then mediate mechanical translocation of the unfolded polypeptide into the peptidase chamber. For the bacterial ClpXP and ClpAP AAA+ proteases, the mechanical activities of protein unfolding and translocation have been directly visualized by single-molecule optical trapping. These studies in combination with structural and biochemical experiments illuminate many principles that underlie this universal mechanism of ATP-fueled protein unfolding and subsequent destruction.
AAA+ proteolytic machines use energy from ATP hydrolysis to degrade damaged, misfolded, or unneeded proteins. Protein degradation occurs within a barrel-shaped self-compartmentalized peptidase. Before protein substrates can enter this peptidase, they must be unfolded and then translocated through the axial pore of an AAA+ ring hexamer. An unstructured region of the protein substrate is initially engaged in the axial pore, and conformational changes in the ring, powered by ATP hydrolysis, generate a mechanical force that pulls on and denatures the substrate. The same conformational changes in the hexameric ring then mediate mechanical translocation of the unfolded polypeptide into the peptidase chamber. For the bacterial ClpXP and ClpAP AAA+ proteases, the mechanical activities of protein unfolding and translocation have been directly visualized by single-molecule optical trapping. These studies in combination with structural and biochemical experiments illuminate many principles that underlie this universal mechanism of ATP-fueled protein unfolding and subsequent destruction.
Light‐Activated Upconverting Spinners
Paloma Rodriguez‐Sevilla Tianli Lee Liangliang Liang Patricia Haro‐González Ginés Lifante Xiaogang Liu Daniel Jaque
Rare earth doped upconverting particles (UCPs, capable of efficient infrared‐to‐visible light conversion) have played a fundamental role in the development of up‐to‐date photonics. Thanks to their unique combination of properties (high brightness, superior spectral and intensity stabilities, and high biocompatibility) old dreams have become possible such as obtaining intracellular dynamical images or remote measurement of temperature in the nanoscale. In this work, it is demonstrated how a rarely considered property of UCNPs, their intrinsic optical birefringence, expands their multifunctionality by converting them into fully controlled, optically activated luminescent spinners. Thanks to the luminescence‐based thermal sensing ability of upconverting spinners, it is possible, by comparison between experimental data and numerical modeling, to explain the supralinear behavior of spinning rate with optical power. The complete understanding of rotation dynamics allows the use of UCPs as mechanical microthermometers with thermal sensitivities larger than those traditionally achieved by luminescence‐based thermometry. Experimental demonstration of the potential use of UCPs as remote light‐activated microrotors is also provided. Results included in this work constitute the first step toward the overcoming of new challenges in photonics including those raising in modern biophotonics and colloidal science.
DOI
Rare earth doped upconverting particles (UCPs, capable of efficient infrared‐to‐visible light conversion) have played a fundamental role in the development of up‐to‐date photonics. Thanks to their unique combination of properties (high brightness, superior spectral and intensity stabilities, and high biocompatibility) old dreams have become possible such as obtaining intracellular dynamical images or remote measurement of temperature in the nanoscale. In this work, it is demonstrated how a rarely considered property of UCNPs, their intrinsic optical birefringence, expands their multifunctionality by converting them into fully controlled, optically activated luminescent spinners. Thanks to the luminescence‐based thermal sensing ability of upconverting spinners, it is possible, by comparison between experimental data and numerical modeling, to explain the supralinear behavior of spinning rate with optical power. The complete understanding of rotation dynamics allows the use of UCPs as mechanical microthermometers with thermal sensitivities larger than those traditionally achieved by luminescence‐based thermometry. Experimental demonstration of the potential use of UCPs as remote light‐activated microrotors is also provided. Results included in this work constitute the first step toward the overcoming of new challenges in photonics including those raising in modern biophotonics and colloidal science.
DOI
Nanoscale Optical Trapping: A Review
Carlo Bradac
Optical trapping is the craft of manipulating objects with light. Decades after its first inception in 1970, the technique has become a powerful tool for ultracold‐atom physics and manipulation of micron‐sized particles. Yet, optical trapping of objects at the intermediate—nanoscale—range is still beyond full grasp. This matters because the nanometric realm is where several promising advances, from mastering single‐molecule experiments in biology, to fabricating hybrid devices for nanoelectronics and photonics, as well as testing fundamental quantum phenomena in optomechanics, are anticipated to produce impactful breakthroughs. After a comprehensive, theoretical introduction to the phenomenon of optical trapping, this review delves into assessing the current state‐of‐the‐art for optical manipulation of objects at the nanoscale. Emphasis is put on presenting the challenges that coalesced into driving the field to its current development, as well as discussing the outstanding barriers, which might lead to future advancements in the field.
DOI
Optical trapping is the craft of manipulating objects with light. Decades after its first inception in 1970, the technique has become a powerful tool for ultracold‐atom physics and manipulation of micron‐sized particles. Yet, optical trapping of objects at the intermediate—nanoscale—range is still beyond full grasp. This matters because the nanometric realm is where several promising advances, from mastering single‐molecule experiments in biology, to fabricating hybrid devices for nanoelectronics and photonics, as well as testing fundamental quantum phenomena in optomechanics, are anticipated to produce impactful breakthroughs. After a comprehensive, theoretical introduction to the phenomenon of optical trapping, this review delves into assessing the current state‐of‐the‐art for optical manipulation of objects at the nanoscale. Emphasis is put on presenting the challenges that coalesced into driving the field to its current development, as well as discussing the outstanding barriers, which might lead to future advancements in the field.
DOI
Tuning and Switching a Plasmonic Quantum Dot “Sandwich” in a Nematic Line Defect
Haridas Mundoor, Ghadah H. Sheetah, Sungoh Park, Paul J. Ackerman, Ivan I. Smalyukh, and Jao van de Lagemaat
We study the quantum-mechanical effects arising in a single semiconductor core/shell quantum dot (QD) controllably sandwiched between two plasmonic nanorods. Control over the position and the “sandwich” confinement structure is achieved by the use of a linear-trap liquid crystal (LC) line defect and laser tweezers that “push” the sandwich together. This arrangement allows for the study of exciton–plasmon interactions in a single structure, unaltered by ensemble effects or the complexity of dielectric interfaces. We demonstrate the effect of plasmonic confinement on the photon antibunching behavior of the QD and its luminescence lifetime. The QD behaves as a single emitter when nanorods are far away from the QD but shows possible multiexciton emission and a significantly decreased lifetime when tightly confined in a plasmonic “sandwich”. These findings demonstrate that LC defects, combined with laser tweezers, enable a versatile platform to study plasmonic coupling phenomena in a nanoscale laboratory, where all elements can be arranged almost at will.
DOI
We study the quantum-mechanical effects arising in a single semiconductor core/shell quantum dot (QD) controllably sandwiched between two plasmonic nanorods. Control over the position and the “sandwich” confinement structure is achieved by the use of a linear-trap liquid crystal (LC) line defect and laser tweezers that “push” the sandwich together. This arrangement allows for the study of exciton–plasmon interactions in a single structure, unaltered by ensemble effects or the complexity of dielectric interfaces. We demonstrate the effect of plasmonic confinement on the photon antibunching behavior of the QD and its luminescence lifetime. The QD behaves as a single emitter when nanorods are far away from the QD but shows possible multiexciton emission and a significantly decreased lifetime when tightly confined in a plasmonic “sandwich”. These findings demonstrate that LC defects, combined with laser tweezers, enable a versatile platform to study plasmonic coupling phenomena in a nanoscale laboratory, where all elements can be arranged almost at will.
DOI
Wednesday, May 16, 2018
Stochastic Ratcheting on a Funneled Energy Landscape Is Necessary for Highly Efficient Contractility of Actomyosin Force Dipoles
James E. Komianos and Garegin A. Papoian
Current understanding of how contractility emerges in disordered actomyosin networks of nonmuscle cells is still largely based on the intuition derived from earlier works on muscle contractility. In addition, in disordered networks, passive cross-linkers have been hypothesized to percolate force chains in the network, hence, establishing large-scale connectivity between local contractile clusters. This view, however, largely overlooks the free energy of cross-linker binding at the microscale, which, even in the absence of active fluctuations, provides a thermodynamic drive towards highly overlapping filamentous states. In this work, we use stochastic simulations and mean-field theory to shed light on the dynamics of a single actomyosin force dipole—a pair of antiparallel actin filaments interacting with active myosin II motors and passive cross-linkers. We first show that while passive cross-linking without motor activity can produce significant contraction between a pair of actin filaments, driven by thermodynamic favorability of cross-linker binding, a sharp onset of kinetic arrest exists at large cross-link binding energies, greatly diminishing the effectiveness of this contractility mechanism. Then, when considering an active force dipole containing nonmuscle myosin II, we find that cross-linkers can also serve as a structural ratchet when the motor dissociates stochastically from the actin filaments, resulting in significant force amplification when both molecules are present. Our results provide predictions of how actomyosin force dipoles behave at the molecular level with respect to filament boundary conditions, passive cross-linking, and motor activity, which can explicitly be tested using an optical trapping experiment.
DOI
Current understanding of how contractility emerges in disordered actomyosin networks of nonmuscle cells is still largely based on the intuition derived from earlier works on muscle contractility. In addition, in disordered networks, passive cross-linkers have been hypothesized to percolate force chains in the network, hence, establishing large-scale connectivity between local contractile clusters. This view, however, largely overlooks the free energy of cross-linker binding at the microscale, which, even in the absence of active fluctuations, provides a thermodynamic drive towards highly overlapping filamentous states. In this work, we use stochastic simulations and mean-field theory to shed light on the dynamics of a single actomyosin force dipole—a pair of antiparallel actin filaments interacting with active myosin II motors and passive cross-linkers. We first show that while passive cross-linking without motor activity can produce significant contraction between a pair of actin filaments, driven by thermodynamic favorability of cross-linker binding, a sharp onset of kinetic arrest exists at large cross-link binding energies, greatly diminishing the effectiveness of this contractility mechanism. Then, when considering an active force dipole containing nonmuscle myosin II, we find that cross-linkers can also serve as a structural ratchet when the motor dissociates stochastically from the actin filaments, resulting in significant force amplification when both molecules are present. Our results provide predictions of how actomyosin force dipoles behave at the molecular level with respect to filament boundary conditions, passive cross-linking, and motor activity, which can explicitly be tested using an optical trapping experiment.
DOI
Detailed Analyses of Stall Force Generation in Mycoplasma mobile Gliding
Masaki Mizutani, Isil Tulum, Yoshiaki Kinosita, Takayuki Nishizaka, Makoto Miyata
Mycoplasma mobile is a bacterium that uses a unique mechanism to glide on solid surfaces at a velocity of up to 4.5 μm/s. Its gliding machinery comprises hundreds of units that generate the force for gliding based on the energy derived from ATP; the units catch and pull sialylated oligosaccharides fixed to solid surfaces. In this study, we measured the stall force of wild-type and mutant strains of M. mobile carrying a bead manipulated using optical tweezers. The strains that had been enhanced for binding exhibited weaker stall forces than the wild-type strain, indicating that stall force is related to force generation rather than to binding. The stall force of the wild-type strain decreased linearly from 113 to 19 picoNewtons after the addition of 0–0.5 mM free sialyllactose (a sialylated oligosaccharide), with a decrease in the number of working units. After the addition of 0.5 mM sialyllactose, the cells carrying a bead loaded using optical tweezers exhibited stepwise movements with force increments. The force increments ranged from 1 to 2 picoNewtons. Considering the 70-nm step size, this small-unit force may be explained by the large gear ratio involved in the M. mobile gliding machinery.
DOI
Mycoplasma mobile is a bacterium that uses a unique mechanism to glide on solid surfaces at a velocity of up to 4.5 μm/s. Its gliding machinery comprises hundreds of units that generate the force for gliding based on the energy derived from ATP; the units catch and pull sialylated oligosaccharides fixed to solid surfaces. In this study, we measured the stall force of wild-type and mutant strains of M. mobile carrying a bead manipulated using optical tweezers. The strains that had been enhanced for binding exhibited weaker stall forces than the wild-type strain, indicating that stall force is related to force generation rather than to binding. The stall force of the wild-type strain decreased linearly from 113 to 19 picoNewtons after the addition of 0–0.5 mM free sialyllactose (a sialylated oligosaccharide), with a decrease in the number of working units. After the addition of 0.5 mM sialyllactose, the cells carrying a bead loaded using optical tweezers exhibited stepwise movements with force increments. The force increments ranged from 1 to 2 picoNewtons. Considering the 70-nm step size, this small-unit force may be explained by the large gear ratio involved in the M. mobile gliding machinery.
DOI
Two distinct conformational states define the interaction of human RAD51‐ATP with single‐stranded DNA
Ineke Brouwer, Tommaso Moschetti, Andrea Candelli, Edwige B Garcin, Mauro Modesti, Luca Pellegrini, Gijs JL Wuite, Erwin JG Peterman
An essential mechanism for repairing DNA double‐strand breaks is homologous recombination (HR). One of its core catalysts is human RAD51 (hRAD51), which assembles as a helical nucleoprotein filament on single‐stranded DNA, promoting DNA‐strand exchange. Here, we study the interaction of hRAD51 with single‐stranded DNA using a single‐molecule approach. We show that ATP‐bound hRAD51 filaments can exist in two different states with different contour lengths and with a free‐energy difference of ~4 kBT per hRAD51 monomer. Upon ATP hydrolysis, the filaments convert into a disassembly‐competent ADP‐bound configuration. In agreement with the single‐molecule analysis, we demonstrate the presence of two distinct protomer interfaces in the crystal structure of a hRAD51‐ATP filament, providing a structural basis for the two conformational states of the filament. Together, our findings provide evidence that hRAD51‐ATP filaments can exist in two interconvertible conformational states, which might be functionally relevant for DNA homology recognition and strand exchange.
DOI
An essential mechanism for repairing DNA double‐strand breaks is homologous recombination (HR). One of its core catalysts is human RAD51 (hRAD51), which assembles as a helical nucleoprotein filament on single‐stranded DNA, promoting DNA‐strand exchange. Here, we study the interaction of hRAD51 with single‐stranded DNA using a single‐molecule approach. We show that ATP‐bound hRAD51 filaments can exist in two different states with different contour lengths and with a free‐energy difference of ~4 kBT per hRAD51 monomer. Upon ATP hydrolysis, the filaments convert into a disassembly‐competent ADP‐bound configuration. In agreement with the single‐molecule analysis, we demonstrate the presence of two distinct protomer interfaces in the crystal structure of a hRAD51‐ATP filament, providing a structural basis for the two conformational states of the filament. Together, our findings provide evidence that hRAD51‐ATP filaments can exist in two interconvertible conformational states, which might be functionally relevant for DNA homology recognition and strand exchange.
DOI
Determination of the refractive index of insoluble organic extracts from atmospheric aerosol over the visible wavelength range using optical tweezers
Rosalie H. Shepherd, Martin D. King, Amelia A. Marks, Neil Brough, and Andrew D. Ward
Optical trapping combined with Mie spectroscopy is a new technique used to record the refractive index of insoluble organic material extracted from atmospheric aerosol samples over a wide wavelength range. The refractive index of the insoluble organic extracts was shown to follow a Cauchy equation between 460 and 700 nm for organic aerosol extracts collected from urban (London) and remote (Antarctica) locations. Cauchy coefficients for the remote sample were for the Austral summer and gave the Cauchy coefficients of A = 1.467 and B = 1000 nm2 with a real refractive index of 1.489 at a wavelength of 589 nm. Cauchy coefficients for the urban samples varied with season, with extracts collected during summer having Cauchy coefficients of A = 1.465 ± 0.005 and B = 4625 ± 1200 nm2 with a representative real refractive index of 1.478 at a wavelength of 589 nm, whilst samples extracted during autumn had larger Cauchy coefficients of A = 1.505 and B = 600 nm2 with a representative real refractive index of 1.522 at a wavelength of 589 nm. The refractive index of absorbing aerosol was also recorded. The absorption Ångström exponent was determined for woodsmoke and humic acid aerosol extract. Typical values of the Cauchy coefficient for the woodsmoke aerosol extract were A = 1.541 ± 0.03 and B = 14 800 ± 2900 nm2, resulting in a real refractive index of 1.584 ± 0.007 at a wavelength of 589 nm and an absorption Ångström exponent of 8.0. The measured values of refractive index compare well with previous monochromatic or very small wavelength range measurements of refractive index. In general, the real component of the refractive index increases from remote to urban to woodsmoke. A one-dimensional radiative-transfer calculation of the top-of-the-atmosphere albedo was applied to model an atmosphere containing a 3 km thick layer of aerosol comprising pure water, pure insoluble organic aerosol, or an aerosol consisting of an aqueous core with an insoluble organic shell. The calculation demonstrated that the top-of-the-atmosphere albedo increases by 0.01 to 0.04 for pure organic particles relative to water particles of the same size and that the top-of-the-atmosphere albedo increases by 0.03 for aqueous core-shell particles as volume fraction of the shell material increases to 25 %.
DOI
Optical trapping combined with Mie spectroscopy is a new technique used to record the refractive index of insoluble organic material extracted from atmospheric aerosol samples over a wide wavelength range. The refractive index of the insoluble organic extracts was shown to follow a Cauchy equation between 460 and 700 nm for organic aerosol extracts collected from urban (London) and remote (Antarctica) locations. Cauchy coefficients for the remote sample were for the Austral summer and gave the Cauchy coefficients of A = 1.467 and B = 1000 nm2 with a real refractive index of 1.489 at a wavelength of 589 nm. Cauchy coefficients for the urban samples varied with season, with extracts collected during summer having Cauchy coefficients of A = 1.465 ± 0.005 and B = 4625 ± 1200 nm2 with a representative real refractive index of 1.478 at a wavelength of 589 nm, whilst samples extracted during autumn had larger Cauchy coefficients of A = 1.505 and B = 600 nm2 with a representative real refractive index of 1.522 at a wavelength of 589 nm. The refractive index of absorbing aerosol was also recorded. The absorption Ångström exponent was determined for woodsmoke and humic acid aerosol extract. Typical values of the Cauchy coefficient for the woodsmoke aerosol extract were A = 1.541 ± 0.03 and B = 14 800 ± 2900 nm2, resulting in a real refractive index of 1.584 ± 0.007 at a wavelength of 589 nm and an absorption Ångström exponent of 8.0. The measured values of refractive index compare well with previous monochromatic or very small wavelength range measurements of refractive index. In general, the real component of the refractive index increases from remote to urban to woodsmoke. A one-dimensional radiative-transfer calculation of the top-of-the-atmosphere albedo was applied to model an atmosphere containing a 3 km thick layer of aerosol comprising pure water, pure insoluble organic aerosol, or an aerosol consisting of an aqueous core with an insoluble organic shell. The calculation demonstrated that the top-of-the-atmosphere albedo increases by 0.01 to 0.04 for pure organic particles relative to water particles of the same size and that the top-of-the-atmosphere albedo increases by 0.03 for aqueous core-shell particles as volume fraction of the shell material increases to 25 %.
DOI
Surface enhanced Raman scattering of gold nanoparticles aggregated by a gold-nanofilm-coated nanofiber
Chang Cheng, Juan Li, Hongxiang Lei, and Baojun Li
Aggregation of metal nanoparticles plays an important role in surface enhanced Raman scattering (SERS). Here, a strategy of dynamically aggregating/releasing gold nanoparticles is demonstrated using a gold-nanofilm-coated nanofiber, with the assistance of enhanced optical force and plasmonic photothermal effect. Strong SERS signals of rhodamine 6G are achieved at the hotspots formed in the inter-particle and film-particle nanogaps. The proposed SERS substrate was demonstrated to have a sensitivity of 10−12 M, reliable reproducibility, and good stability.
DOI
Aggregation of metal nanoparticles plays an important role in surface enhanced Raman scattering (SERS). Here, a strategy of dynamically aggregating/releasing gold nanoparticles is demonstrated using a gold-nanofilm-coated nanofiber, with the assistance of enhanced optical force and plasmonic photothermal effect. Strong SERS signals of rhodamine 6G are achieved at the hotspots formed in the inter-particle and film-particle nanogaps. The proposed SERS substrate was demonstrated to have a sensitivity of 10−12 M, reliable reproducibility, and good stability.
DOI
Investigation of gel formation and volatilization of acetate acid in magnesium acetate droplets by the optical tweezers
Xi-Juan Lv, Yang Wang, Chen Cai, Shu-Feng Pang, Jia-Bi Ma, Yun-Hong Zhang
Hygroscopicity and volatility of single magnesium acetate (MgAc2) aerosol particles at various relative humidities (RHs) are studied by a single-beam optical tweezers, and refractive indices (RIs) and morphology are characterized by cavity enhanced Raman spectroscopy. Gel formation and volatilization of acetate acid (HAc) in MgAc2 droplets are observed. Due to the formation of amorphous gel structure, water transposition in droplets at RH < 50% is significantly impeded on a time scale of 140,000 s. Different phase transition at RH < 10% is proposed to explain the distinct water loss after the gel formation. To compare volatilization of HAc in different systems, MgAc2 and sodium acetate (NaAc) droplets are maintained at several different stable RHs during up to 86,000 s. At RH ≈ 74%, magnesium hydroxide (Mg(OH)2) inclusions are formed in MgAc2 droplets due to the volatilization of HAc, and whispering gallery modes (WGMs) of MgAc2 droplets in the Raman spectrum quench after 50,000 s. In sharp contrast, after 86,000 s at RH ≈ 70%, NaAc droplets are in well-mixed liquid states, containing soluble sodium hydroxide (NaOH). At this state, the RI of NaAc droplet is increased, and the quenching of WGMs is not observable.
DOI
Hygroscopicity and volatility of single magnesium acetate (MgAc2) aerosol particles at various relative humidities (RHs) are studied by a single-beam optical tweezers, and refractive indices (RIs) and morphology are characterized by cavity enhanced Raman spectroscopy. Gel formation and volatilization of acetate acid (HAc) in MgAc2 droplets are observed. Due to the formation of amorphous gel structure, water transposition in droplets at RH < 50% is significantly impeded on a time scale of 140,000 s. Different phase transition at RH < 10% is proposed to explain the distinct water loss after the gel formation. To compare volatilization of HAc in different systems, MgAc2 and sodium acetate (NaAc) droplets are maintained at several different stable RHs during up to 86,000 s. At RH ≈ 74%, magnesium hydroxide (Mg(OH)2) inclusions are formed in MgAc2 droplets due to the volatilization of HAc, and whispering gallery modes (WGMs) of MgAc2 droplets in the Raman spectrum quench after 50,000 s. In sharp contrast, after 86,000 s at RH ≈ 70%, NaAc droplets are in well-mixed liquid states, containing soluble sodium hydroxide (NaOH). At this state, the RI of NaAc droplet is increased, and the quenching of WGMs is not observable.
DOI
Integrated dual-tomography for refractive index analysis of free-floating single living cell with isotropic superresolution
Vinoth B., Xin-Ji Lai, Yu-Chih Lin, Han-Yen Tu & Chau-Jern Cheng
Digital holographic microtomography is a promising technique for three-dimensional (3D) measurement of the refractive index (RI) profiles of biological specimens. Measurement of the RI distribution of a free-floating single living cell with an isotropic superresolution had not previously been accomplished. To the best of our knowledge, this is the first study focusing on the development of an integrated dual-tomographic (IDT) imaging system for RI measurement of an unlabelled free-floating single living cell with an isotropic superresolution by combining the spatial frequencies of full-angle specimen rotation with those of beam rotation. A novel ‘UFO’ (unidentified flying object) like shaped coherent transfer function is obtained. The IDT imaging system does not require any complex image-processing algorithm for 3D reconstruction. The working principle was successfully demonstrated and a 3D RI profile of a single living cell, Candida rugosa, was obtained with an isotropic superresolution. This technology is expected to set a benchmark for free-floating single live sample measurements without labeling or any special sample preparations for the experiments.
DOI
Digital holographic microtomography is a promising technique for three-dimensional (3D) measurement of the refractive index (RI) profiles of biological specimens. Measurement of the RI distribution of a free-floating single living cell with an isotropic superresolution had not previously been accomplished. To the best of our knowledge, this is the first study focusing on the development of an integrated dual-tomographic (IDT) imaging system for RI measurement of an unlabelled free-floating single living cell with an isotropic superresolution by combining the spatial frequencies of full-angle specimen rotation with those of beam rotation. A novel ‘UFO’ (unidentified flying object) like shaped coherent transfer function is obtained. The IDT imaging system does not require any complex image-processing algorithm for 3D reconstruction. The working principle was successfully demonstrated and a 3D RI profile of a single living cell, Candida rugosa, was obtained with an isotropic superresolution. This technology is expected to set a benchmark for free-floating single live sample measurements without labeling or any special sample preparations for the experiments.
DOI
Tuesday, May 15, 2018
Particle trapping and manipulation using hollow beam with tunable size generated by thermal nonlinear optical effect
Bo He, Xuemei Cheng, Hui Zhang, Haowei Chen, Qian Zhang, Zhaoyu Ren, Shan Ding and Jintao Ba
We report micron-sized particle trapping and manipulation using a hollow beam of tunable size, which was generated by cross-phase modulation via the thermal nonlinear optical effect in an ethanol medium. The results demonstrated that the particle can be trapped stably in air for hours and manipulated in millimeter range with micrometer-level accuracy by modulating the size of the hollow beam. The merits of flexibility in tuning the beam size and simplicity in operation give this method great potential for the in situ study of individual particles in air.
DOI
We report micron-sized particle trapping and manipulation using a hollow beam of tunable size, which was generated by cross-phase modulation via the thermal nonlinear optical effect in an ethanol medium. The results demonstrated that the particle can be trapped stably in air for hours and manipulated in millimeter range with micrometer-level accuracy by modulating the size of the hollow beam. The merits of flexibility in tuning the beam size and simplicity in operation give this method great potential for the in situ study of individual particles in air.
DOI
Synchronization in pairs of rotating active biomotors
Neus Oliver, Christina Alpmann, Álvaro Barroso, Lena Dewenter, Mike Woerdemann and Cornelia Denz
Although synchronization is a well-known physical phenomenon, experimental studies of its emergence in living bacterial cells are still scarce. The difficulty in generating a controlled scenario to detect synchronization has limited the experimental outcomes so far. We present a realization based on holographic optical tweezers in which adhered pairs of self-propelled bacteria rotate in a plane. The separation distance between the bacteria determines the strength of the hydrodynamic coupling. Despite the noisy environment and autonomous dynamics of the living bacteria, we find evidence of phase locking and frequency entrainment in their rotation. The observation of higher order frequency synchronization is also discussed.
DOI
Although synchronization is a well-known physical phenomenon, experimental studies of its emergence in living bacterial cells are still scarce. The difficulty in generating a controlled scenario to detect synchronization has limited the experimental outcomes so far. We present a realization based on holographic optical tweezers in which adhered pairs of self-propelled bacteria rotate in a plane. The separation distance between the bacteria determines the strength of the hydrodynamic coupling. Despite the noisy environment and autonomous dynamics of the living bacteria, we find evidence of phase locking and frequency entrainment in their rotation. The observation of higher order frequency synchronization is also discussed.
DOI
A laser Raman tweezers study of eryptosis
Surekha Barkur, Deepak Mathur, Santhosh Chidangil
Eryptosis—the suicidal death of erythrocytes—is characterized by membrane blebbing and cell shrinkage. Eryptosis can be triggered by various xenobiotics such as carbon monoxide, lead, and amyloid, and by stressors such as oxidative stress, osmotic shock, and rapid alteration of ambient conditions. We have used Raman tweezers spectroscopy to study eryptosis in single, live cells and have attempted to explore the underlying mechanism, specifically to identify possible Raman signatures of eryptosis. Erythrocytes (red blood cells) were exposed to free radicals, silver nanoparticles, glucose, heat, and osmotic shock to induce eryptosis, and a comparison was made of their Raman spectra, which indicated that these conditions lead to a transition of haemoglobin from the R to the T state. Consequences of eryptosis include dehydration, cell shrinkage, and pH changes, which result in deoxygenation of haemoglobin. This, in turn, can be detected by monitoring the wavenumber shifts associated with Raman marker bands of R to T transitions. In addition, the principal component analysis results indicate differentiation among red blood cells undergone eryptosis due to different conditions.
DOI
Eryptosis—the suicidal death of erythrocytes—is characterized by membrane blebbing and cell shrinkage. Eryptosis can be triggered by various xenobiotics such as carbon monoxide, lead, and amyloid, and by stressors such as oxidative stress, osmotic shock, and rapid alteration of ambient conditions. We have used Raman tweezers spectroscopy to study eryptosis in single, live cells and have attempted to explore the underlying mechanism, specifically to identify possible Raman signatures of eryptosis. Erythrocytes (red blood cells) were exposed to free radicals, silver nanoparticles, glucose, heat, and osmotic shock to induce eryptosis, and a comparison was made of their Raman spectra, which indicated that these conditions lead to a transition of haemoglobin from the R to the T state. Consequences of eryptosis include dehydration, cell shrinkage, and pH changes, which result in deoxygenation of haemoglobin. This, in turn, can be detected by monitoring the wavenumber shifts associated with Raman marker bands of R to T transitions. In addition, the principal component analysis results indicate differentiation among red blood cells undergone eryptosis due to different conditions.
DOI
Nucleosomes of polyploid trophoblast giant cells mostly consist of histone variants and form a loose chromatin structure
Koji Hayakawa, Kanae Terada, Tomohiro Takahashi, Hidehiro Oana, Masao Washizu & Satoshi Tanaka
Trophoblast giant cells (TGCs) are one of the cell types that form the placenta and play multiple essential roles in maintaining pregnancy in rodents. TGCs have large, polyploid nuclei resulting from endoreduplication. While previous studies have shown distinct gene expression profiles of TGCs, their chromatin structure remains largely unknown. An appropriate combination of canonical and non-canonical histones, also known as histone variants, allows each cell to exert its cell type-specific functions. Here, we aimed to reveal the dynamics of histone usage and chromatin structure during the differentiation of trophoblast stem cells (TSCs) into TGCs. Although the expression of most genes encoding canonical histones was downregulated, the expression of a few genes encoding histone variants such as H2AX, H2AZ, and H3.3 was maintained at a relatively high level in TGCs. Both the micrococcal nuclease digestion assay and nucleosome stability assay using a microfluidic device indicated that chromatin became increasingly loose as TSCs differentiated. Combinatorial experiments involving H3.3-knockdown and -overexpression demonstrated that variant H3.3 resulted in the formation of loose nucleosomes in TGCs. In conclusion, our study revealed that TGCs possessed loose nucleosomes owing to alterations in their histone composition during differentiation.
DOI
Trophoblast giant cells (TGCs) are one of the cell types that form the placenta and play multiple essential roles in maintaining pregnancy in rodents. TGCs have large, polyploid nuclei resulting from endoreduplication. While previous studies have shown distinct gene expression profiles of TGCs, their chromatin structure remains largely unknown. An appropriate combination of canonical and non-canonical histones, also known as histone variants, allows each cell to exert its cell type-specific functions. Here, we aimed to reveal the dynamics of histone usage and chromatin structure during the differentiation of trophoblast stem cells (TSCs) into TGCs. Although the expression of most genes encoding canonical histones was downregulated, the expression of a few genes encoding histone variants such as H2AX, H2AZ, and H3.3 was maintained at a relatively high level in TGCs. Both the micrococcal nuclease digestion assay and nucleosome stability assay using a microfluidic device indicated that chromatin became increasingly loose as TSCs differentiated. Combinatorial experiments involving H3.3-knockdown and -overexpression demonstrated that variant H3.3 resulted in the formation of loose nucleosomes in TGCs. In conclusion, our study revealed that TGCs possessed loose nucleosomes owing to alterations in their histone composition during differentiation.
DOI
Force-detected nanoscale absorption spectroscopy in water at room temperature using an optical trap
Alexander Parobek, Jacob W. Black, Maria Kamenetska, and Ziad Ganim
Measuring absorption spectra of single molecules presents a fundamental challenge for standard transmission-based instruments because of the inherently low signal relative to the large background of the excitation source. Here we demonstrate a new approach for performing absorption spectroscopy in solution using a force measurement to read out optical excitation at the nanoscale. The photoinduced force between model chromophores and an optically trapped gold nanoshell has been measured in water at room temperature. This photoinduced force is characterized as a function of wavelength to yield the force spectrum, which is shown to be correlated to the absorption spectrum for four model systems. The instrument constructed for these measurements combines an optical tweezer with frequency domain absorption spectroscopy over the 400-800 nm range. These measurements provide proof-of-principle experiments for force-detected nanoscale spectroscopies that operate under ambient chemical conditions.
DOI
Measuring absorption spectra of single molecules presents a fundamental challenge for standard transmission-based instruments because of the inherently low signal relative to the large background of the excitation source. Here we demonstrate a new approach for performing absorption spectroscopy in solution using a force measurement to read out optical excitation at the nanoscale. The photoinduced force between model chromophores and an optically trapped gold nanoshell has been measured in water at room temperature. This photoinduced force is characterized as a function of wavelength to yield the force spectrum, which is shown to be correlated to the absorption spectrum for four model systems. The instrument constructed for these measurements combines an optical tweezer with frequency domain absorption spectroscopy over the 400-800 nm range. These measurements provide proof-of-principle experiments for force-detected nanoscale spectroscopies that operate under ambient chemical conditions.
DOI
Eigenmode orthogonality breaking and anomalous dynamics in multimode nano-optomechanical systems under non-reciprocal coupling
Laure Mercier de Lépinay, Benjamin Pigeau, Benjamin Besga & Olivier Arcizet
Thermal motion of nanomechanical probes directly impacts their sensitivities to external forces. Its proper understanding is therefore critical for ultimate force sensing. Here, we investigate a vectorial force field sensor: a singly-clamped nanowire oscillating along two quasi-frequency-degenerate transverse directions. Its insertion in a rotational optical force field couples its eigenmodes non-symmetrically, causing dramatic modifications of its mechanical properties. In particular, the eigenmodes lose their intrinsic orthogonality. We show that this circumstance is at the origin of an anomalous excess of noise and of a violation of the fluctuation dissipation relation. Our model, which quantitatively accounts for all observations, provides a novel modified version of the fluctuation dissipation theorem that remains valid in non-conservative rotational force fields, and that reveals the prominent role of non-axial mechanical susceptibilities. These findings help understand the intriguing properties of thermal fluctuations in non-reciprocally-coupled multimode systems.
Thermal motion of nanomechanical probes directly impacts their sensitivities to external forces. Its proper understanding is therefore critical for ultimate force sensing. Here, we investigate a vectorial force field sensor: a singly-clamped nanowire oscillating along two quasi-frequency-degenerate transverse directions. Its insertion in a rotational optical force field couples its eigenmodes non-symmetrically, causing dramatic modifications of its mechanical properties. In particular, the eigenmodes lose their intrinsic orthogonality. We show that this circumstance is at the origin of an anomalous excess of noise and of a violation of the fluctuation dissipation relation. Our model, which quantitatively accounts for all observations, provides a novel modified version of the fluctuation dissipation theorem that remains valid in non-conservative rotational force fields, and that reveals the prominent role of non-axial mechanical susceptibilities. These findings help understand the intriguing properties of thermal fluctuations in non-reciprocally-coupled multimode systems.
Wednesday, May 9, 2018
Levitating Micro-Actuators: A Review
Kirill V. Poletkin, Asa Asadollahbaik, Ronald Kampmann and Jan G. Korvink
Through remote forces, levitating micro-actuators completely eliminate mechanical attachment between the stationary and moving parts of a micro-actuator, thus providing a fundamental solution to overcoming the domination of friction over inertial forces at the micro-scale. Eliminating the usual mechanical constraints promises micro-actuators with increased operational capabilities and low dissipation energy. Further reduction of friction and hence dissipation by means of vacuum leads to dramatic increases of performance when compared to mechanically tethered counterparts. In order to efficiently employ the benefits provided by levitation, micro-actuators are classified according to their physical principles as well as by their combinations. Different operating principles, structures, materials and fabrication methods are considered. A detailed analysis of the significant achievements in the technology of micro-optics, micro-magnets and micro-coil fabrication, along with the development of new magnetic materials during recent decades, which has driven the creation of new application domains for levitating micro-actuators is performed.
DOI
Through remote forces, levitating micro-actuators completely eliminate mechanical attachment between the stationary and moving parts of a micro-actuator, thus providing a fundamental solution to overcoming the domination of friction over inertial forces at the micro-scale. Eliminating the usual mechanical constraints promises micro-actuators with increased operational capabilities and low dissipation energy. Further reduction of friction and hence dissipation by means of vacuum leads to dramatic increases of performance when compared to mechanically tethered counterparts. In order to efficiently employ the benefits provided by levitation, micro-actuators are classified according to their physical principles as well as by their combinations. Different operating principles, structures, materials and fabrication methods are considered. A detailed analysis of the significant achievements in the technology of micro-optics, micro-magnets and micro-coil fabrication, along with the development of new magnetic materials during recent decades, which has driven the creation of new application domains for levitating micro-actuators is performed.
DOI
Bioconjugated Core–Shell Microparticles for High‐Force Optical Trapping
Juan Carlos Cordova Dana N. Reinemann Daniel J. Laky William R. Hesse Sophie K. Tushak Zane L. Weltman Kelsea B. Best Rizia Bardhan Matthew J. Lang
Due to their high spatial resolution and precise application of force, optical traps are widely used to study the mechanics of biomolecules and biopolymers at the single‐molecule level. Recently, core–shell particles with optical properties that enhance their trapping ability represent promising candidates for high‐force experiments. To fully harness their properties, methods for functionalizing these particles with biocompatible handles are required. Here, a straightforward synthesis is provided for producing functional titania core–shell microparticles with proteins and nucleic acids by adding a silane–thiol chemical group to the shell surface. These particles display higher trap stiffness compared to conventional plastic beads featured in optical tweezers experiments. These core–shell microparticles are also utilized in biophysical assays such as amyloid fiber pulling and actin rupturing to demonstrate their high‐force applications. It is anticipated that the functionalized core–shells can be used to probe the mechanics of stable proteins structures that are inaccessible using current trapping techniques.
DOI
Due to their high spatial resolution and precise application of force, optical traps are widely used to study the mechanics of biomolecules and biopolymers at the single‐molecule level. Recently, core–shell particles with optical properties that enhance their trapping ability represent promising candidates for high‐force experiments. To fully harness their properties, methods for functionalizing these particles with biocompatible handles are required. Here, a straightforward synthesis is provided for producing functional titania core–shell microparticles with proteins and nucleic acids by adding a silane–thiol chemical group to the shell surface. These particles display higher trap stiffness compared to conventional plastic beads featured in optical tweezers experiments. These core–shell microparticles are also utilized in biophysical assays such as amyloid fiber pulling and actin rupturing to demonstrate their high‐force applications. It is anticipated that the functionalized core–shells can be used to probe the mechanics of stable proteins structures that are inaccessible using current trapping techniques.
DOI
Optical gradient forces in PT-symmetric coupled-waveguide structures
Xinbiao Xu, Lei Shi, Linhao Ren, and Xinliang Zhang
Optical gradient force in a parity-time (PT)-symmetric coupled-waveguide system is theoretically studied. We find that when the system evolves from PT-symmetric region to broken-PT-symmetric region, the normalized optical forces of the two eigenmodes decrease first and become the same when the exceptional point is reached. Besides, the optical force induced PT phase transition is demonstrated. It is worth noting that, when the system is in the broken-PT-symmetric region and the length of the waveguide is much longer than the propagation length of the lossy eigenmode, the total optical gradient force acting on the two waveguides will decrease with the decreasing of the gap. This work gives us a new understanding of integrated optomechanics by combining with PT symmetry.
DOI
Optical gradient force in a parity-time (PT)-symmetric coupled-waveguide system is theoretically studied. We find that when the system evolves from PT-symmetric region to broken-PT-symmetric region, the normalized optical forces of the two eigenmodes decrease first and become the same when the exceptional point is reached. Besides, the optical force induced PT phase transition is demonstrated. It is worth noting that, when the system is in the broken-PT-symmetric region and the length of the waveguide is much longer than the propagation length of the lossy eigenmode, the total optical gradient force acting on the two waveguides will decrease with the decreasing of the gap. This work gives us a new understanding of integrated optomechanics by combining with PT symmetry.
DOI
Building one molecule from a reservoir of two atoms
L. R. Liu, J. D. Hood, Y. Yu, J. T. Zhang, N. R. Hutzler, T. Rosenband
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combine exactly two atoms into a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms (one sodium and one cesium) in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photo-association forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.
DOI
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combine exactly two atoms into a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms (one sodium and one cesium) in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photo-association forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.
DOI
Generation of a ring-shaped focusing spot with precisely controllable position and diameter
Jiannong Chen, Chenglong Zhao, Dawei Zhang, Bo Dai, Linwei Zhu, and Qinfeng Xu
Ring-shaped focusing spots play a critical role in double beam super-resolution fluorescence imaging and can also be used in single beam optical trapping. The alignment and size matching of two beams is a significant issue in a double beam application and a transversely movable and diameter-controllable ring-shaped focusing spot is very useful in single beam trapping. Until now, however, the size and the position of the ring-shaped focusing spot in the focal plane could not be precisely controlled. Here, we demonstrate that a ring-shaped focusing, circularly symmetrical spot with a controllable position and an adjustable diameter can be generated in the focal plane of a high numerical aperture objective. The ring-shaped spot is composed of equally spaced multiple spots with small enough diameters arranged on the circumference of the ring. By updating the phase modulation, we also demonstrated the dynamic manipulation of the yeast cell in the focal plane with this controllable ring-shaped spot.
DOI
Ring-shaped focusing spots play a critical role in double beam super-resolution fluorescence imaging and can also be used in single beam optical trapping. The alignment and size matching of two beams is a significant issue in a double beam application and a transversely movable and diameter-controllable ring-shaped focusing spot is very useful in single beam trapping. Until now, however, the size and the position of the ring-shaped focusing spot in the focal plane could not be precisely controlled. Here, we demonstrate that a ring-shaped focusing, circularly symmetrical spot with a controllable position and an adjustable diameter can be generated in the focal plane of a high numerical aperture objective. The ring-shaped spot is composed of equally spaced multiple spots with small enough diameters arranged on the circumference of the ring. By updating the phase modulation, we also demonstrated the dynamic manipulation of the yeast cell in the focal plane with this controllable ring-shaped spot.
DOI
Observation of an optical spring with a beam splitter
Jonathan Cripe, Baylee Danz, Benjamin Lane, Mary Catherine Lorio, Julia Falcone, Garrett D. Cole, and Thomas Corbitt
We present the experimental observation of an optical spring without the use of an optical cavity. The optical spring is produced by interference at a beam splitter and, in principle, does not have the damping force associated with optical springs created in detuned cavities. The experiment consists of a Michelson–Sagnac interferometer (with no recycling cavities) with a partially reflective GaAs microresonator as the beam splitter that produces the optical spring. Our experimental measurements at input powers of up to 360 mW show the shift of the optical spring frequency as a function of power and are in excellent agreement with theoretical predictions. In addition, we show that the optical spring is able to keep the interferometer stable and locked without the use of external feedback.
DOI
We present the experimental observation of an optical spring without the use of an optical cavity. The optical spring is produced by interference at a beam splitter and, in principle, does not have the damping force associated with optical springs created in detuned cavities. The experiment consists of a Michelson–Sagnac interferometer (with no recycling cavities) with a partially reflective GaAs microresonator as the beam splitter that produces the optical spring. Our experimental measurements at input powers of up to 360 mW show the shift of the optical spring frequency as a function of power and are in excellent agreement with theoretical predictions. In addition, we show that the optical spring is able to keep the interferometer stable and locked without the use of external feedback.
DOI
Monday, May 7, 2018
Optical trapping two types of particles using a focused vortex beam
Hanghang Zhang, Jinhong Li, Miaojun Guo, Meiling Duan, Zhifang Feng, Wen Yang
Propagations of Gaussian Schell-model (GSM) vortex beams through a focusing optical system are formulated. The radiation force acting on Rayleigh dielectric sphere with different refractive indices produced by focused GSM vortex beams is investigated theoretically. Numerical results demonstrate that the focused GSM non-vortex beam can not trap the low index of refraction particles, but can capture the high index of refraction particles. The focused GSM vortex beam can be used to trap high index of refraction particles to a bright ring of the focal plane, and simultaneously capture low index of refraction particles to z-axis. The larger the topological charge m is, the larger the value of the spatial correlation length σ0 is, the easier it is to trap two types of particles is. Trapping stability is also analyzed.
DOI
Propagations of Gaussian Schell-model (GSM) vortex beams through a focusing optical system are formulated. The radiation force acting on Rayleigh dielectric sphere with different refractive indices produced by focused GSM vortex beams is investigated theoretically. Numerical results demonstrate that the focused GSM non-vortex beam can not trap the low index of refraction particles, but can capture the high index of refraction particles. The focused GSM vortex beam can be used to trap high index of refraction particles to a bright ring of the focal plane, and simultaneously capture low index of refraction particles to z-axis. The larger the topological charge m is, the larger the value of the spatial correlation length σ0 is, the easier it is to trap two types of particles is. Trapping stability is also analyzed.
DOI
Theory for controlling individual self-propelled micro-swimmers by photon nudging II: confinement
Markus Selmke, Utsab Khadka, Andreas P. Bregulla, Frank Cichos and Haw Yang
Photon nudging allows the manipulation and confinement of individual self-propelled micro-swimmers in 2D and 3D environments using feedback controls. Presented in this second part of a two-part contribution are theoretical models that afford the characterization for the positioning distribution associated with active localization. A derivation for the optimal nudging speed and acceptance angle is given for minimal placement uncertainty. The analytical solutions allow for a discussion on the physical underpinning that underlies controllability and optimality.
DOI
Photon nudging allows the manipulation and confinement of individual self-propelled micro-swimmers in 2D and 3D environments using feedback controls. Presented in this second part of a two-part contribution are theoretical models that afford the characterization for the positioning distribution associated with active localization. A derivation for the optimal nudging speed and acceptance angle is given for minimal placement uncertainty. The analytical solutions allow for a discussion on the physical underpinning that underlies controllability and optimality.
DOI
Theory for controlling individual self-propelled micro-swimmers by photon nudging I: directed transport
Markus Selmke, Utsab Khadka, Andreas P. Bregulla, Frank Cichos and Haw Yang
Photon nudging is a new experimental method which enables the force-free manipulation and localization of individual self-propelled artificial micro-swimmers in fluidic environments. It uses a weak laser to stochastically and adaptively turn on and off the swimmer's propulsion when the swimmer, through rotational diffusion, points towards or away from its target, respectively. This contribution presents a theoretical framework for the statistics of both 2D and 3D controls. The main results are: the on- and off-time distributions for the controlling laser, the arrival time statistics for the swimmer to reach a remote target, and how the experimentally accessible control parameters influence the control, e.g., the optimal acceptance angle for directed transport. The results are general in that they are independent of the propulsion or the actuation mechanisms. They provide a concrete physical picture for how a single artificial micro-swimmer could be navigated under thermal fluctuations—insights that could also be useful for understanding biological micro-swimmers.
DOI
Photon nudging is a new experimental method which enables the force-free manipulation and localization of individual self-propelled artificial micro-swimmers in fluidic environments. It uses a weak laser to stochastically and adaptively turn on and off the swimmer's propulsion when the swimmer, through rotational diffusion, points towards or away from its target, respectively. This contribution presents a theoretical framework for the statistics of both 2D and 3D controls. The main results are: the on- and off-time distributions for the controlling laser, the arrival time statistics for the swimmer to reach a remote target, and how the experimentally accessible control parameters influence the control, e.g., the optimal acceptance angle for directed transport. The results are general in that they are independent of the propulsion or the actuation mechanisms. They provide a concrete physical picture for how a single artificial micro-swimmer could be navigated under thermal fluctuations—insights that could also be useful for understanding biological micro-swimmers.
DOI
Light Manipulation in Inhomogeneous Liquid Flow and Its Application in Biochemical Sensing
Yunfeng Zuo, Xiaoqiang Zhu, Yang Shi, Li Liang and Yi Yang
Light manipulation has always been the fundamental subject in the field of optics since centuries ago. Traditional optical devices are usually designed using glasses and other materials, such as semiconductors and metals. Optofluidics is the combination of microfluidics and optics, which brings a host of new advantages to conventional solid systems. The capabilities of light manipulation and biochemical sensing are inherent alongside the emergence of optofluidics. This new research area promotes advancements in optics, biology, and chemistry. The development of fast, accurate, low-cost, and small-sized biochemical micro-sensors is an urgent demand for real-time monitoring. However, the fluid flow in the on-chip sensor is usually non-uniformed, which is a new and emerging challenge for the accuracy of optical detection. It is significant to reveal the principle of light propagation in an inhomogeneous liquid flow and the interaction between biochemical samples and light in flowing liquids. In this review, we summarize the current state of optofluidic lab-on-a-chip techniques from the perspective of light modulation by the unique dynamic properties of fluid in heterogeneous media, such as diffusion, heat transfer, and centrifugation etc. Furthermore, this review introduces several novel photonic phenomena in an inhomogeneous liquid flow and demonstrates their application in biochemical sensing.
DOI
Light manipulation has always been the fundamental subject in the field of optics since centuries ago. Traditional optical devices are usually designed using glasses and other materials, such as semiconductors and metals. Optofluidics is the combination of microfluidics and optics, which brings a host of new advantages to conventional solid systems. The capabilities of light manipulation and biochemical sensing are inherent alongside the emergence of optofluidics. This new research area promotes advancements in optics, biology, and chemistry. The development of fast, accurate, low-cost, and small-sized biochemical micro-sensors is an urgent demand for real-time monitoring. However, the fluid flow in the on-chip sensor is usually non-uniformed, which is a new and emerging challenge for the accuracy of optical detection. It is significant to reveal the principle of light propagation in an inhomogeneous liquid flow and the interaction between biochemical samples and light in flowing liquids. In this review, we summarize the current state of optofluidic lab-on-a-chip techniques from the perspective of light modulation by the unique dynamic properties of fluid in heterogeneous media, such as diffusion, heat transfer, and centrifugation etc. Furthermore, this review introduces several novel photonic phenomena in an inhomogeneous liquid flow and demonstrates their application in biochemical sensing.
DOI
Template-stripped nanoaperture tweezer integrated with optical fiber
Jamal M. Ehtaiba and Reuven Gordon
We demonstrate an optical trapping technique that integrates the light guiding of an optical fiber with the field localization of a nanoaperture in a gold film. A key innovation of our technique is to use template-stripping for easy planar fabrication without the need for nanofabrication on the tip itself. As a proof of principle, we demonstrate the trapping of 20 nm and 30 nm polystyrene nanoparticles in solution, as observed by a jump in the transmitted laser intensity through the aperture. We use the finite difference time domain technique to simulate this intensity jump with the addition of a nanoparticle in the aperture, showing reasonable agreement with the experimental data. This simple nano-aperture optical fiber tip eliminates the need for a microscope setup while allowing for trapping nanoparticles, so it is anticipated to have applications in biology (e.g. viruses), biophysics (e.g. protein interactions), physics (e.g. quantum emitters), and chemistry (e.g. colloidal particles).
DOI
We demonstrate an optical trapping technique that integrates the light guiding of an optical fiber with the field localization of a nanoaperture in a gold film. A key innovation of our technique is to use template-stripping for easy planar fabrication without the need for nanofabrication on the tip itself. As a proof of principle, we demonstrate the trapping of 20 nm and 30 nm polystyrene nanoparticles in solution, as observed by a jump in the transmitted laser intensity through the aperture. We use the finite difference time domain technique to simulate this intensity jump with the addition of a nanoparticle in the aperture, showing reasonable agreement with the experimental data. This simple nano-aperture optical fiber tip eliminates the need for a microscope setup while allowing for trapping nanoparticles, so it is anticipated to have applications in biology (e.g. viruses), biophysics (e.g. protein interactions), physics (e.g. quantum emitters), and chemistry (e.g. colloidal particles).
DOI
Noise induced aperiodic rotations of particles trapped by a non-conservative force
Ignacio Ortega-Piwonka, Christopher N. Angstmann, Bruce I. Henry, and Peter J. Reece
We describe a mechanism whereby random noise can play a constructive role in the manifestation of a pattern, aperiodic rotations, that would otherwise be damped by internal dynamics. The mechanism is described physically in a theoretical model of overdamped particle motion in two dimensions with symmetric damping and a non-conservative force field driven by noise. Cyclic motion only occurs as a result of stochastic noise in this system. However, the persistence of the cyclic motion is quantified by parameters associated with the non-conservative forcing. Unlike stochastic resonance or coherence resonance, where noise can play a constructive role in amplifying a signal that is otherwise below the threshold for detection, in the mechanism considered here, the signal that is detected does not exist without the noise. Moreover, the system described here is a linear system.
DOI
We describe a mechanism whereby random noise can play a constructive role in the manifestation of a pattern, aperiodic rotations, that would otherwise be damped by internal dynamics. The mechanism is described physically in a theoretical model of overdamped particle motion in two dimensions with symmetric damping and a non-conservative force field driven by noise. Cyclic motion only occurs as a result of stochastic noise in this system. However, the persistence of the cyclic motion is quantified by parameters associated with the non-conservative forcing. Unlike stochastic resonance or coherence resonance, where noise can play a constructive role in amplifying a signal that is otherwise below the threshold for detection, in the mechanism considered here, the signal that is detected does not exist without the noise. Moreover, the system described here is a linear system.
DOI
Friday, May 4, 2018
Optical Tweezers Bring Micromachines to Biology
I.A. Favre-Bulle, S. Zhang, A.V. Kashchuk, I.C.D. Lenton, L.J. Gibson, A.B. Stilgoe, T.A. Nieminen, and H. Rubinsztein-Dunlop
Optical tweezers, using focused laser beams to manipulate objects on the nano- and microscale, allow exploration of a world of curiosities at the borderline between physics and biology.
DOI
Optical tweezers, using focused laser beams to manipulate objects on the nano- and microscale, allow exploration of a world of curiosities at the borderline between physics and biology.
DOI
LabVIEW-Based Software for Optical Stiffness Determination Using Boltzmann Statistics, Equipartition Theorem and Power Spectral Density Methods
Hamid, Muhammad Yunus; Ayop, Shahrul Kadri
An optical tweezers applications are becoming vast over time due to its unique ability to optically trap micron sized objects. However, calibration in term of optical stiffness determination for force-related experiments is essential. The optical stiffness calculation may take a drag full amount of time. The aim of this study is to develop a user friendly and quick calculation LabVIEW based software for the determination of the optical stiffness of an optical tweezers. The determination employs three available methods: Equipartition Theorem (ET), Boltzmann Statistics (BS), and Power Spectral Density (PSD). The software requires only temporal trajectory of a trapped bead in an optical tweezers to calculate the optical stiffness and to evaluate the optical trap condition.
DOI
An optical tweezers applications are becoming vast over time due to its unique ability to optically trap micron sized objects. However, calibration in term of optical stiffness determination for force-related experiments is essential. The optical stiffness calculation may take a drag full amount of time. The aim of this study is to develop a user friendly and quick calculation LabVIEW based software for the determination of the optical stiffness of an optical tweezers. The determination employs three available methods: Equipartition Theorem (ET), Boltzmann Statistics (BS), and Power Spectral Density (PSD). The software requires only temporal trajectory of a trapped bead in an optical tweezers to calculate the optical stiffness and to evaluate the optical trap condition.
DOI
Two-point active microrheology in a viscous medium exploiting a motional resonance excited in dual-trap optical tweezers
Shuvojit Paul, Randhir Kumar, and Ayan Banerjee
Two-point microrheology measurements from widely separated colloidal particles approach the bulk viscosity of the host medium more reliably than corresponding single-point measurements. In addition, active microrheology offers the advantage of enhanced signal to noise over passive techniques. Recently, we reported the observation of a motional resonance induced in a probe particle in dual-trap optical tweezers when the control particle was driven externally [Paul et al., Phys. Rev. E 96, 050102(R) (2017)]. We now demonstrate that the amplitude and phase characteristics of the motional resonance can be used as a sensitive tool for active two-point microrheology to measure the viscosity of a viscous fluid. Thus, we measure the viscosity of viscous liquids from both the amplitude and phase response of the resonance, and demonstrate that the zero crossing of the phase response of the probe particle with respect to the external drive is superior compared to the amplitude response in measuring viscosity at large particle separations. We compare our viscosity measurements with those using a commercial rheometer and obtain an agreement ∼1%. The method can be extended to viscoelastic material where the frequency dependence of the resonance may provide further accuracy for active microrheological measurements.
DOI
Two-point microrheology measurements from widely separated colloidal particles approach the bulk viscosity of the host medium more reliably than corresponding single-point measurements. In addition, active microrheology offers the advantage of enhanced signal to noise over passive techniques. Recently, we reported the observation of a motional resonance induced in a probe particle in dual-trap optical tweezers when the control particle was driven externally [Paul et al., Phys. Rev. E 96, 050102(R) (2017)]. We now demonstrate that the amplitude and phase characteristics of the motional resonance can be used as a sensitive tool for active two-point microrheology to measure the viscosity of a viscous fluid. Thus, we measure the viscosity of viscous liquids from both the amplitude and phase response of the resonance, and demonstrate that the zero crossing of the phase response of the probe particle with respect to the external drive is superior compared to the amplitude response in measuring viscosity at large particle separations. We compare our viscosity measurements with those using a commercial rheometer and obtain an agreement ∼1%. The method can be extended to viscoelastic material where the frequency dependence of the resonance may provide further accuracy for active microrheological measurements.
DOI
Cooke–Triplet tweezers: more compact, robust, and efficient optical tweezers
Tim Stangner, Tobias Dahlberg, Pontus Svenmarker, Johan Zakrisson, Krister Wiklund, Lene B. Oddershede, and Magnus Andersson
We present a versatile three-lens optical design to improve the overall compactness, efficiency, and robustness for optical tweezers based applications. The design, inspired by the Cooke–Triplet configuration, allows for continuous beam magnifications of 2–10×, and axial as well as lateral focal shifts can be realized without switching lenses or introducing optical aberrations. We quantify the beam quality and trapping stiffness and compare the Cooke–Triplet design with the commonly used double Kepler design through simulations and direct experiments. Optical trapping of 1 and 2 μm beads shows that the Cooke–Triplet possesses an equally strong optical trap stiffness compared to the double Kepler lens design but reduces its lens system length by a factor of 2.6. Finally, we demonstrate how a Twyman–Green interferometer integrated in the Cooke–Triplet optical tweezers setup provides a fast and simple method to characterize the wavefront aberrations in the lens system and how it can help in aligning the optical components perfectly.
DOI
We present a versatile three-lens optical design to improve the overall compactness, efficiency, and robustness for optical tweezers based applications. The design, inspired by the Cooke–Triplet configuration, allows for continuous beam magnifications of 2–10×, and axial as well as lateral focal shifts can be realized without switching lenses or introducing optical aberrations. We quantify the beam quality and trapping stiffness and compare the Cooke–Triplet design with the commonly used double Kepler design through simulations and direct experiments. Optical trapping of 1 and 2 μm beads shows that the Cooke–Triplet possesses an equally strong optical trap stiffness compared to the double Kepler lens design but reduces its lens system length by a factor of 2.6. Finally, we demonstrate how a Twyman–Green interferometer integrated in the Cooke–Triplet optical tweezers setup provides a fast and simple method to characterize the wavefront aberrations in the lens system and how it can help in aligning the optical components perfectly.
DOI
Surfactant Copolymer Annealing of Chemically Permeabilized Cell Membranes
Hongfeng Chen, Colin McFaul, Igor Titushkin, Michael Cho, Raphael Lee
Structural breakdown of the cell membrane is a primary mediator in trauma-induced tissue necrosis. When membrane disruption exceeds intrinsic membrane sealing processes, biocompatible multi-block amphiphilic copolymer surfactants such as Poloxamer 188 (P188) have been found to be effective in catalyze or augment sealing. Although in living cells copolymer-induced sealing of membrane defects has been detected by changes in membrane transport properties, it has not been directly imaged. In this project, we used Atomic force microscopy (AFM) to directly image saponin permeabilized and poloxamer-sealed plasma membranes of monolayer-cultured MDCK and 3T3 fibroblasts. AFM image analysis resulted in the density and diameter ranges for membrane indentations per 5 × 5 μm area. For control, saponin lysed, and P188 treatment of saponin-lysed membranes, the supra-threshold indentation density was 3.6 ± 2.8, 13.8 ± 6.7, and 4.9 ± 3.3/cell, respectively. These results indicated that P188 catalyzed reduction in size of AFM indentations which correlated with increase cell survival. This evidence confirms that biocompatible surfactant P188 augments natural cell membrane sealing capability when intrinsic processes are incapable alone.
DOI
Structural breakdown of the cell membrane is a primary mediator in trauma-induced tissue necrosis. When membrane disruption exceeds intrinsic membrane sealing processes, biocompatible multi-block amphiphilic copolymer surfactants such as Poloxamer 188 (P188) have been found to be effective in catalyze or augment sealing. Although in living cells copolymer-induced sealing of membrane defects has been detected by changes in membrane transport properties, it has not been directly imaged. In this project, we used Atomic force microscopy (AFM) to directly image saponin permeabilized and poloxamer-sealed plasma membranes of monolayer-cultured MDCK and 3T3 fibroblasts. AFM image analysis resulted in the density and diameter ranges for membrane indentations per 5 × 5 μm area. For control, saponin lysed, and P188 treatment of saponin-lysed membranes, the supra-threshold indentation density was 3.6 ± 2.8, 13.8 ± 6.7, and 4.9 ± 3.3/cell, respectively. These results indicated that P188 catalyzed reduction in size of AFM indentations which correlated with increase cell survival. This evidence confirms that biocompatible surfactant P188 augments natural cell membrane sealing capability when intrinsic processes are incapable alone.
DOI
Two-dimensional arbitrary nano-manipulation on a plasmonic metasurface
Min Jiang, Guanghui Wang, Wenhao Xu, Wenbin Ji, Ningmu Zou, Ho-pui Ho, and Xuping Zhang
In this Letter, we report on a plasmonic nano-ellipse metasurface with the purpose of trapping and two-dimensional (2D) arbitrary transport of nanoparticles by means of rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off the adjacent hot spots and then convey the trapped target by rotating the incident polarization state. For the case of a metasurface with a unit cell of perpendicularly orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and counterclockwise rotation would guide the particle in a different direction, which makes it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by the three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.
DOI
In this Letter, we report on a plasmonic nano-ellipse metasurface with the purpose of trapping and two-dimensional (2D) arbitrary transport of nanoparticles by means of rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off the adjacent hot spots and then convey the trapped target by rotating the incident polarization state. For the case of a metasurface with a unit cell of perpendicularly orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and counterclockwise rotation would guide the particle in a different direction, which makes it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by the three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.
DOI
Calibration and energy measurement of optically levitated nanoparticle sensors
Erik Hebestreit, Martin Frimmer, René Reimann, Christoph Dellago, Francesco Ricci, and Lukas Novotny
Optically levitated nanoparticles offer enormous potential for precision sensing. However, as for any other metrology device, the absolute measurement performance of a levitated-particle sensor is limited by the accuracy of the calibration relating the measured signal to an absolute displacement of the particle. Here, we suggest and demonstrate calibration protocols for levitated-nanoparticle sensors. Our calibration procedures include the treatment of anharmonicities in the trapping potential, as well as a protocol using a harmonic driving force, which is applicable if the sensor is coupled to a heat bath of unknown temperature. Finally, using the calibration, we determine the center-of-mass temperature of an optically levitated particle in thermal equilibrium from its motion and discuss the optimal measurement time required to determine the said temperature.
DOI
Optically levitated nanoparticles offer enormous potential for precision sensing. However, as for any other metrology device, the absolute measurement performance of a levitated-particle sensor is limited by the accuracy of the calibration relating the measured signal to an absolute displacement of the particle. Here, we suggest and demonstrate calibration protocols for levitated-nanoparticle sensors. Our calibration procedures include the treatment of anharmonicities in the trapping potential, as well as a protocol using a harmonic driving force, which is applicable if the sensor is coupled to a heat bath of unknown temperature. Finally, using the calibration, we determine the center-of-mass temperature of an optically levitated particle in thermal equilibrium from its motion and discuss the optimal measurement time required to determine the said temperature.
DOI
Thursday, May 3, 2018
Opto-thermoelectric nanotweezers
Linhan Lin, Mingsong Wang, Xiaolei Peng, Emanuel N. Lissek, Zhangming Mao, Leonardo Scarabelli, Emily Adkins, Sahin Coskun, Husnu Emrah Unalan, Brian A. Korgel, Luis M. Liz-Marzán, Ernst-Ludwig Florin & Yuebing Zheng
Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers. By optically heating a thermoplasmonic substrate, a light-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles and tunable working wavelength, opto-thermoelectric nanotweezers will become a powerful tool in colloid science and nanotechnology.
DOI
Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers. By optically heating a thermoplasmonic substrate, a light-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles and tunable working wavelength, opto-thermoelectric nanotweezers will become a powerful tool in colloid science and nanotechnology.
DOI
Microstructures Fabricated by Two‐Photon Polymerization and Their Remote Manipulation Techniques: Toward 3D Printing of Micromachines
Yang Lin, Jie Xu
As a promising microfabrication method, two‐photon polymerization (TPP) underwent a rapid development for various applications in chemistry, biology, pharmaceuticals, microfluidics, and so forth. In recent years, the method has received particular attention because of its application for micromachines, including those requiring remote manipulation. Different manipulating techniques such as magnetic, optic, and acoustic manipulation are realized on TPP‐fabricated microstructures, demonstrating the great potential for further development of micromachines. Nonetheless, most of the work is still at early stages, only proofing conceptual ideas. For instance, magnetically driven microswimmers are only investigated in artificial experimental environments, and it is still challenging to tackle complex in vivo conditions. Although a long journey is still ahead for using these micromachines in daily life, it is predictable that the combination of two‐photon polymerization associated with remotely driven techniques will benefit various fields.
DOI
As a promising microfabrication method, two‐photon polymerization (TPP) underwent a rapid development for various applications in chemistry, biology, pharmaceuticals, microfluidics, and so forth. In recent years, the method has received particular attention because of its application for micromachines, including those requiring remote manipulation. Different manipulating techniques such as magnetic, optic, and acoustic manipulation are realized on TPP‐fabricated microstructures, demonstrating the great potential for further development of micromachines. Nonetheless, most of the work is still at early stages, only proofing conceptual ideas. For instance, magnetically driven microswimmers are only investigated in artificial experimental environments, and it is still challenging to tackle complex in vivo conditions. Although a long journey is still ahead for using these micromachines in daily life, it is predictable that the combination of two‐photon polymerization associated with remotely driven techniques will benefit various fields.
DOI
Lipid packing defects and membrane charge control RAB GTPase recruitment
Guillaume Kulakowski Hugo Bousquet Jean‐Baptiste Manneville Patricia Bassereau Bruno Goud Lena K. Oesterlin
Specific intracellular localization of RAB GTPases has been reported to be dependent on protein factors, but the contribution of the membrane physicochemical properties to this process has been poorly described. Here, we show that three RAB proteins (RAB1/RAB5/RAB6) preferentially bind in vitro to disordered and curved membranes, and that this feature is uniquely dependent on their prenyl group. Our results imply that the addition of a prenyl group confers to RAB proteins, and most probably also to other prenylated proteins, the ability to sense lipid packing defects induced by unsaturated conical‐shaped lipids and curvature. Consistently, RAB recruitment increases with the amount of lipid packing defects, further indicating that these defects drive RAB membrane targeting. Membrane binding of RAB35 is also modulated by lipid packing defects but primarily dependent on negatively charged lipids. Our results suggest that a balance between hydrophobic insertion of the prenyl group into lipid packing defects and electrostatic interactions of the RAB C‐terminal region with charged membranes tunes the specific intracellular localization of RAB proteins.
DOI
Specific intracellular localization of RAB GTPases has been reported to be dependent on protein factors, but the contribution of the membrane physicochemical properties to this process has been poorly described. Here, we show that three RAB proteins (RAB1/RAB5/RAB6) preferentially bind in vitro to disordered and curved membranes, and that this feature is uniquely dependent on their prenyl group. Our results imply that the addition of a prenyl group confers to RAB proteins, and most probably also to other prenylated proteins, the ability to sense lipid packing defects induced by unsaturated conical‐shaped lipids and curvature. Consistently, RAB recruitment increases with the amount of lipid packing defects, further indicating that these defects drive RAB membrane targeting. Membrane binding of RAB35 is also modulated by lipid packing defects but primarily dependent on negatively charged lipids. Our results suggest that a balance between hydrophobic insertion of the prenyl group into lipid packing defects and electrostatic interactions of the RAB C‐terminal region with charged membranes tunes the specific intracellular localization of RAB proteins.
DOI
Optical forces, torques, and force densities calculated at a microscopic level using a self-consistent hydrodynamics method
Kun Ding and C. T. Chan
The calculation of optical force density distribution inside a material is challenging at the nanoscale, where quantum and nonlocal effects emerge and macroscopic parameters such as permittivity become ill-defined. We demonstrate that the microscopic optical force density of nanoplasmonic systems can be defined and calculated using the microscopic fields generated using a self-consistent hydrodynamics model that includes quantum, nonlocal, and retardation effects. We demonstrate this technique by calculating the microscopic optical force density distributions and the optical binding force induced by external light on nanoplasmonic dimers. This approach works even in the limit when the nanoparticles are close enough to each other so that electron tunneling occurs, a regime in which classical electromagnetic approach fails completely. We discover that an uneven distribution of optical force density can lead to a light-induced spinning torque acting on individual particles. The hydrodynamics method offers us an accurate and efficient approach to study optomechanical behavior for plasmonic systems at the nanoscale.
DOI
The calculation of optical force density distribution inside a material is challenging at the nanoscale, where quantum and nonlocal effects emerge and macroscopic parameters such as permittivity become ill-defined. We demonstrate that the microscopic optical force density of nanoplasmonic systems can be defined and calculated using the microscopic fields generated using a self-consistent hydrodynamics model that includes quantum, nonlocal, and retardation effects. We demonstrate this technique by calculating the microscopic optical force density distributions and the optical binding force induced by external light on nanoplasmonic dimers. This approach works even in the limit when the nanoparticles are close enough to each other so that electron tunneling occurs, a regime in which classical electromagnetic approach fails completely. We discover that an uneven distribution of optical force density can lead to a light-induced spinning torque acting on individual particles. The hydrodynamics method offers us an accurate and efficient approach to study optomechanical behavior for plasmonic systems at the nanoscale.
DOI
Robotic Cell Rotation Based on Optimal Poking Direction
Chunlin Zhao, Yaowei Liu, Mingzhu Sun, and Xin Zhao
It is essential to have three-dimensional orientation of cells under a microscope for biological manipulation. Conventional manual cell manipulation is highly dependent on the operator’s experience. It has some problems of low repeatability, low efficiency, and contamination. The current popular robotic method uses an injection micropipette to rotate cells. However, the optimal poking direction of the injection micropipette has not been established. In this paper, a strategy of robotic cell rotation based on optimal poking direction is proposed to move the specific structure of the cell to the desired orientation. First, analysis of the force applied to the cell during rotation was done to find the optimal poking direction, where we had the biggest moment of force. Then, the moving trajectory of the injection micropipette was designed to exert rotation force based on optimal poking direction. Finally, the strategy was applied to oocyte rotation in nuclear transfer. Experimental results show that the average completion time was up to 23.6 s and the success rate was 93.3% when the moving speed of the injection micropipette was 100 μm/s, which demonstrates that our strategy could overcome slippage effectively and with high efficiency.
DOI
It is essential to have three-dimensional orientation of cells under a microscope for biological manipulation. Conventional manual cell manipulation is highly dependent on the operator’s experience. It has some problems of low repeatability, low efficiency, and contamination. The current popular robotic method uses an injection micropipette to rotate cells. However, the optimal poking direction of the injection micropipette has not been established. In this paper, a strategy of robotic cell rotation based on optimal poking direction is proposed to move the specific structure of the cell to the desired orientation. First, analysis of the force applied to the cell during rotation was done to find the optimal poking direction, where we had the biggest moment of force. Then, the moving trajectory of the injection micropipette was designed to exert rotation force based on optimal poking direction. Finally, the strategy was applied to oocyte rotation in nuclear transfer. Experimental results show that the average completion time was up to 23.6 s and the success rate was 93.3% when the moving speed of the injection micropipette was 100 μm/s, which demonstrates that our strategy could overcome slippage effectively and with high efficiency.
DOI
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