King Yan Fong, Hao-Kun Li, Rongkuo Zhao, Sui Yang, Yuan Wang & Xiang Zhang
Heat transfer in solids is typically conducted through either electrons or atomic vibrations known as phonons. In a vacuum, heat has long been thought to be transferred by radiation but not by phonons because of the lack of a medium1. Recent theory, however, has predicted that quantum fluctuations of electromagnetic fields could induce phonon coupling across a vacuum and thereby facilitate heat transfer2,3,4. Revealing this unique quantum effect experimentally would bring fundamental insights to quantum thermodynamics5 and practical implications to thermal management in nanometre-scale technologies6. Here we experimentally demonstrate heat transfer induced by quantum fluctuations between two objects separated by a vacuum gap. We use nanomechanical systems to realize strong phonon coupling through vacuum fluctuations, and observe the exchange of thermal energy between individual phonon modes. The experimental observation agrees well with our theoretical calculations and is unambiguously distinguished from other effects such as near-field radiation and electrostatic interaction. Our discovery of phonon transport through quantum fluctuations represents a previously unknown mechanism of heat transfer in addition to the conventional conduction, convection and radiation. It paves the way for the exploitation of quantum vacuum in energy transport at the nanoscale.
DOI
Friday, December 20, 2019
Direct observation and characterization of optical guiding of microparticles by tightly focused non-diffracting beams
Yansheng Liang, Shaohui Yan, Baoli Yao, and Ming Lei
Due to the propagation-invariant and self-healing properties, nondiffracting beams are highly attractive in optical trapping. However, little attention has been paid to investigating optical guiding of microparticles in nondiffracting beams generated by high-numerical-aperture (NA) optics with direct visualization. In this letter, we report a technique for direct observation and characterization of optical guiding of microparticles in a tight focusing system. With this technique, we observed a parabolic particle guiding trajectory with a longitudinal distance of more than 100µm and a maximal lateral deviation of 20 µm when using Airy beams. We also realized the tilted-path transport of microparticles with controllable guiding direction using tilted zeroth-order quasi-Bessel beams. For an NA of the focusing lens equal to 0.95, we achieved the optical guiding of microparticles along a straight path with a tilt angle of up to 18.8° with respect to the optical axis over a distance of 300 µm. Importantly, quantitative measurement of particle’s motion was readily accessed by measuring the particle’s position and velocity during the transport process. The reported technique for direct visualization and characterization of the guided particles will find its potential applications in optical trapping and guiding with novel nondiffracting beams or accelerating beams.
DOI
Due to the propagation-invariant and self-healing properties, nondiffracting beams are highly attractive in optical trapping. However, little attention has been paid to investigating optical guiding of microparticles in nondiffracting beams generated by high-numerical-aperture (NA) optics with direct visualization. In this letter, we report a technique for direct observation and characterization of optical guiding of microparticles in a tight focusing system. With this technique, we observed a parabolic particle guiding trajectory with a longitudinal distance of more than 100µm and a maximal lateral deviation of 20 µm when using Airy beams. We also realized the tilted-path transport of microparticles with controllable guiding direction using tilted zeroth-order quasi-Bessel beams. For an NA of the focusing lens equal to 0.95, we achieved the optical guiding of microparticles along a straight path with a tilt angle of up to 18.8° with respect to the optical axis over a distance of 300 µm. Importantly, quantitative measurement of particle’s motion was readily accessed by measuring the particle’s position and velocity during the transport process. The reported technique for direct visualization and characterization of the guided particles will find its potential applications in optical trapping and guiding with novel nondiffracting beams or accelerating beams.
DOI
Massive nanophotonic trapping and alignment of rod-shaped bacteria for parallel single-cell studies
Haitao Zhao, Lip Ket Chin, Yuzhi Shi, Kim Truc Nguyen, Patricia Yang Liu, Yi Zhang, Meng Zhang, Jingbo Zhang, Hong Cai, Eric Peng Huat Yap, Wee Ser, Ai-Qun Liu
The emerging single-cell technologies call for novel biological tools that can manipulate target cells in a massive and spatially-arranged manner. Here we report a nanophotonic platform, named WANTS (Waveguide-pair Array-based Nanophotonic Trapping System), for massive trapping and alignment of rod-shaped bacteria. This platform leverages silicon waveguide-pair arrays to engineer an optical lattice pattern and the accompanying optical force field. The rod-shaped bacteria inside the field are trapped and aligned by three motions: the out-of-plane rotation, the in-plane rotation, and the translational motion. Massive shigella are arranged into a closely-seated distribution at a trapping rate of ∼12 shigella/min. As a demonstration, we utilize the platform to investigate the bacterial biophysical property and find that the measured bacterial lengths are 23.65% more accurate than the results measured with free solutions. Subsequently, we study the bacterial viability in situ and find that shigella present high heterogeneity in response to chemical stimuli. The WANTS holds significant promise to integrate with lab-on-a-chip technologies and yield a compact and robust platform for practical biological studies at the single-cell level.
DOI
The emerging single-cell technologies call for novel biological tools that can manipulate target cells in a massive and spatially-arranged manner. Here we report a nanophotonic platform, named WANTS (Waveguide-pair Array-based Nanophotonic Trapping System), for massive trapping and alignment of rod-shaped bacteria. This platform leverages silicon waveguide-pair arrays to engineer an optical lattice pattern and the accompanying optical force field. The rod-shaped bacteria inside the field are trapped and aligned by three motions: the out-of-plane rotation, the in-plane rotation, and the translational motion. Massive shigella are arranged into a closely-seated distribution at a trapping rate of ∼12 shigella/min. As a demonstration, we utilize the platform to investigate the bacterial biophysical property and find that the measured bacterial lengths are 23.65% more accurate than the results measured with free solutions. Subsequently, we study the bacterial viability in situ and find that shigella present high heterogeneity in response to chemical stimuli. The WANTS holds significant promise to integrate with lab-on-a-chip technologies and yield a compact and robust platform for practical biological studies at the single-cell level.
DOI
Numerical and analytical models for calculating optical forces near auxiliary plasmonic substrates
Alexander S. Shalin, Aliaksandra Ivinskaya, Natalia Kostina, Mihail I. Petrov, Andrey A. Bogdanov, Sergei Sukhov, and Pavel Ginzburg
The optical force acting on a nanoparticle near a planar substrate is governed by incident light and excitation of surface and volume modes of the substrate. The realization of negative optical forces (“tractor beams”) via propagating plasmon-polaritones and volume modes will be shown and considered in detail on the basis of the described analytical and numerical models for certain types of anisotropic substrates. In addition, optical tweezers performance is investigated when the Gaussian beam is focused on the metal substrate with nanoparticle. When the beam is focused above the substrate optical force increases about an order of magnitude due to evanescent field of surface plasmon. Novel effect of repulsion from Gaussian beam (“anti-trapping”) is obtained when the beam waist is moved below the substrate which is confirmed by both the analytical approach and finite element simulation.
DOI
The optical force acting on a nanoparticle near a planar substrate is governed by incident light and excitation of surface and volume modes of the substrate. The realization of negative optical forces (“tractor beams”) via propagating plasmon-polaritones and volume modes will be shown and considered in detail on the basis of the described analytical and numerical models for certain types of anisotropic substrates. In addition, optical tweezers performance is investigated when the Gaussian beam is focused on the metal substrate with nanoparticle. When the beam is focused above the substrate optical force increases about an order of magnitude due to evanescent field of surface plasmon. Novel effect of repulsion from Gaussian beam (“anti-trapping”) is obtained when the beam waist is moved below the substrate which is confirmed by both the analytical approach and finite element simulation.
DOI
Nanoscale spectroscopic origins of photoinduced tip–sample force in the midinfrared
Junghoon Jahng, Eric O. Potma, and Eun Seong Lee
When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip–sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light–matter coupling in optomechanical force-based spectronanoscopy.
DOI
When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip–sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light–matter coupling in optomechanical force-based spectronanoscopy.
DOI
Thursday, December 19, 2019
Optical tweezers-based characterisation of gold core-satellite plasmonic nano-assemblies incorporating thermo-responsive polymers
Fei Han, Thomas Armstrong, Ana Andres-Arroyo, Danielle Bennett, Alexander Soeriyadi, Ali Alinezhad Chamazketi, Padmavathy Bakthavathsalam, Richard Tilley, John Justin Gooding and Peter Reece
We report on the characterisation of the optical properties and dynamic behaviour of optically trapped single stimuli-responsive plasmonic nanoscale assemblies. Nano-assemblies consist of a core-satellite arrangement where the constituent nanoparticles are connected by the thermoresponsive polymer, poly(DEGA-co-OEGA). The optical tweezers allow the particles to be held isolated in solution and interrogated using dark-field spectroscopy. Additionally, controlling the optical trapping power provides localised heating for probing the thermal response of the nanostructures. Our results identify a number of distinct core-satellite configurations that can be stably trapped, which are verified using finite element modelling. Laser heating of the nanostructures through the trapping laser yields irreversible modification of the arrangement, as observed through the scattering spectrum. We consider which factors may be responsible for the observed behaviour in the context of the core-satellite geometry, polymer-solvent interaction, and the bonding of the nanoparticles.
DOI
We report on the characterisation of the optical properties and dynamic behaviour of optically trapped single stimuli-responsive plasmonic nanoscale assemblies. Nano-assemblies consist of a core-satellite arrangement where the constituent nanoparticles are connected by the thermoresponsive polymer, poly(DEGA-co-OEGA). The optical tweezers allow the particles to be held isolated in solution and interrogated using dark-field spectroscopy. Additionally, controlling the optical trapping power provides localised heating for probing the thermal response of the nanostructures. Our results identify a number of distinct core-satellite configurations that can be stably trapped, which are verified using finite element modelling. Laser heating of the nanostructures through the trapping laser yields irreversible modification of the arrangement, as observed through the scattering spectrum. We consider which factors may be responsible for the observed behaviour in the context of the core-satellite geometry, polymer-solvent interaction, and the bonding of the nanoparticles.
DOI
Non-spherical particles in optical tweezers: A numerical solution
Joonas Herranen, Johannes Markkanen, Gorden Videen, Karri Muinonen
We present numerical methods for modeling the dynamics of arbitrarily shaped particles trapped within optical tweezers, which improve the predictive power of numerical simulations for practical use. We study the dependence of trapping on the shape and size of particles in a single continuous wave beam setup. We also consider the implications of different particle compositions, beam types and media. The major result of the study is that for different irregular particle shapes, a range of beam powers generally leads to trapping. The trapping power range depends on whether the particle can be characterized as elongated or flattened, and the range is also limited by Brownian forces.
DOI
We present numerical methods for modeling the dynamics of arbitrarily shaped particles trapped within optical tweezers, which improve the predictive power of numerical simulations for practical use. We study the dependence of trapping on the shape and size of particles in a single continuous wave beam setup. We also consider the implications of different particle compositions, beam types and media. The major result of the study is that for different irregular particle shapes, a range of beam powers generally leads to trapping. The trapping power range depends on whether the particle can be characterized as elongated or flattened, and the range is also limited by Brownian forces.
DOI
Microrheology with optical tweezers: peaks & troughs
ManlioTassieri
Since their first appearance in the 1970s, optical tweezers have been successfully exploited for a variety of applications throughout the natural sciences, revolutionising the field of microsensing. However, when adopted for microrheology studies, there exist some peaks and troughs on their modus operandi and data analysis that I wish to address and possibly iron out, providing a guide to future rheological studies from a microscopic perspective.
DOI
Since their first appearance in the 1970s, optical tweezers have been successfully exploited for a variety of applications throughout the natural sciences, revolutionising the field of microsensing. However, when adopted for microrheology studies, there exist some peaks and troughs on their modus operandi and data analysis that I wish to address and possibly iron out, providing a guide to future rheological studies from a microscopic perspective.
DOI
Role of a Kinesin Motor in Cancer Cell Mechanics
Kalpana Mandal, Katarzyna Pogoda, Satabdi Nandi, Samuel Mathieu, Amal Kasri, Eric Klein, François Radvanyi, Bruno Goud, Paul A. Janmey, Jean-Baptiste Manneville
Molecular motors play important roles in force generation, migration, and intracellular trafficking. Changes in specific motor activities are altered in numerous diseases. KIF20A, a motor protein of the kinesin-6 family, is overexpressed in bladder cancer, and KIF20A levels correlate negatively with clinical outcomes. We report here a new role for the KIF20A kinesin motor protein in intracellular mechanics. Using optical tweezers to probe intracellular mechanics and surface AFM to probe cortical mechanics, we first confirm that bladder urothelial cells soften with an increasing cancer grade. We then show that inhibiting KIF20A makes the intracellular environment softer for both high- and low-grade bladder cancer cells. Upon inhibition of KIF20A, cortical stiffness also decreases in lower grade cells, while it surprisingly increases in higher grade malignant cells. Changes in cortical stiffness correlate with the interaction of KIF20A with myosin IIA. Moreover, KIF20A inhibition negatively regulates bladder cancer cell motility irrespective of the underlying substrate stiffness. Our results reveal a central role for a microtubule motor in cell mechanics and migration in the context of bladder cancer.
Molecular motors play important roles in force generation, migration, and intracellular trafficking. Changes in specific motor activities are altered in numerous diseases. KIF20A, a motor protein of the kinesin-6 family, is overexpressed in bladder cancer, and KIF20A levels correlate negatively with clinical outcomes. We report here a new role for the KIF20A kinesin motor protein in intracellular mechanics. Using optical tweezers to probe intracellular mechanics and surface AFM to probe cortical mechanics, we first confirm that bladder urothelial cells soften with an increasing cancer grade. We then show that inhibiting KIF20A makes the intracellular environment softer for both high- and low-grade bladder cancer cells. Upon inhibition of KIF20A, cortical stiffness also decreases in lower grade cells, while it surprisingly increases in higher grade malignant cells. Changes in cortical stiffness correlate with the interaction of KIF20A with myosin IIA. Moreover, KIF20A inhibition negatively regulates bladder cancer cell motility irrespective of the underlying substrate stiffness. Our results reveal a central role for a microtubule motor in cell mechanics and migration in the context of bladder cancer.
A conserved ATP- and Scc2/4-dependent activity for cohesin in tethering DNA molecules
Pilar Gutierrez-Escribano, Matthew D. Newton, Aida Llauró, Jonas Huber, Loredana Tanasie, Joseph Davy, Isabel Aly, Ricardo Aramayo, Alex Montoya, Holger Kramer, Johannes Stigler, David S. Rueda, and Luis Aragon
Sister chromatid cohesion requires cohesin to act as a protein linker to hold chromatids together. How cohesin tethers chromatids remains poorly understood. We have used optical tweezers to visualize cohesin as it holds DNA molecules. We show that cohesin complexes tether DNAs in the presence of Scc2/Scc4 and ATP demonstrating a conserved activity from yeast to humans. Cohesin forms two classes of tethers: a “permanent bridge” resisting forces over 80 pN and a force-sensitive “reversible bridge.” The establishment of bridges requires physical proximity of dsDNA segments and occurs in a single step. “Permanent” cohesin bridges slide when they occur in trans, but cannot be removed when in cis. Therefore, DNAs occupy separate physical compartments in cohesin molecules. We finally demonstrate that cohesin tetramers can compact linear DNA molecules stretched by very low force (below 1 pN), consistent with the possibility that, like condensin, cohesin is also capable of loop extrusion.
DOI
Sister chromatid cohesion requires cohesin to act as a protein linker to hold chromatids together. How cohesin tethers chromatids remains poorly understood. We have used optical tweezers to visualize cohesin as it holds DNA molecules. We show that cohesin complexes tether DNAs in the presence of Scc2/Scc4 and ATP demonstrating a conserved activity from yeast to humans. Cohesin forms two classes of tethers: a “permanent bridge” resisting forces over 80 pN and a force-sensitive “reversible bridge.” The establishment of bridges requires physical proximity of dsDNA segments and occurs in a single step. “Permanent” cohesin bridges slide when they occur in trans, but cannot be removed when in cis. Therefore, DNAs occupy separate physical compartments in cohesin molecules. We finally demonstrate that cohesin tetramers can compact linear DNA molecules stretched by very low force (below 1 pN), consistent with the possibility that, like condensin, cohesin is also capable of loop extrusion.
DOI
Wednesday, December 18, 2019
Chaperone-Assisted Host–Guest Interactions Revealed by Single-Molecule Force Spectroscopy
Shankar Pandey, Dilanka V. D. Walpita Kankanamalage, Xiao Zhou, Changpeng Hu, Changpeng Hu, Lyle Isaacs, Janarthanan Jayawickramarajah, Hanbin Mao
The recent discovery of ultra-high binding affinities in cucurbit[7]uril (CB7)-based host–guest pairs in an aqueous environment has rendered CB7 a rather attractive material in analytical and biomedical applications. Due to the lack of a molecular platform that can follow the same host–guest complex during repetitive mechanical measurements, however, mechanical stabilities of the CB7 system have not been revealed, hindering its potential to rival widely used conjugation pairs, such as streptavidin–biotin. Here, we assembled a DNA template in which a flexible DNA linker was exploited to keep the host (CB7) and guest (adamantane) in proximity. This platform not only increased the efficiency of the single-molecule characterization in optical tweezers but also clearly revealed mechanical features of the same host–guest complex. We found that positively charged adamantane guest demonstrated higher mechanical stability (49 pN) than neutral adamantane (44 pN), a trend consistent with the chemical affinity between guest molecules and the CB7 host. Surprisingly, we found that a hexyl group adjacent to the adamantane served as a chaperone to assist the formation of the adamantane–CB7 pairs. The discovery of an unprecedented chaperone-assisted interaction mechanism provides new approaches to efficiently assemble host–guest-based supramolecules with increased mechanical stabilities, which can be exploited in various biomedical, biosensing, and materials fields.
DOI
The recent discovery of ultra-high binding affinities in cucurbit[7]uril (CB7)-based host–guest pairs in an aqueous environment has rendered CB7 a rather attractive material in analytical and biomedical applications. Due to the lack of a molecular platform that can follow the same host–guest complex during repetitive mechanical measurements, however, mechanical stabilities of the CB7 system have not been revealed, hindering its potential to rival widely used conjugation pairs, such as streptavidin–biotin. Here, we assembled a DNA template in which a flexible DNA linker was exploited to keep the host (CB7) and guest (adamantane) in proximity. This platform not only increased the efficiency of the single-molecule characterization in optical tweezers but also clearly revealed mechanical features of the same host–guest complex. We found that positively charged adamantane guest demonstrated higher mechanical stability (49 pN) than neutral adamantane (44 pN), a trend consistent with the chemical affinity between guest molecules and the CB7 host. Surprisingly, we found that a hexyl group adjacent to the adamantane served as a chaperone to assist the formation of the adamantane–CB7 pairs. The discovery of an unprecedented chaperone-assisted interaction mechanism provides new approaches to efficiently assemble host–guest-based supramolecules with increased mechanical stabilities, which can be exploited in various biomedical, biosensing, and materials fields.
DOI
Superchiral Surface Waves for All-Optical Enantiomer Separation
Giovanni Pellegrini, Marco Finazzi, Michele Celebrano, Lamberto Duò, Maria Antonia Iatì, Onofrio M. Maragò, Paolo Biagioni
We introduce the use of superchiral surface waves for the all-optical separation of chiral compounds. Using a combination of electrodynamics modeling and analytical techniques, we show that the proposed approach provides chiral optical forces 2 orders of magnitude larger than those obtained with circularly polarized plane waves. Superchiral surface waves allow for enantiomer separation on spatial, temporal, and size scales that would not be achievable with alternative techniques, thus representing a viable route toward all-optical enantiomer separation.
DOI
We introduce the use of superchiral surface waves for the all-optical separation of chiral compounds. Using a combination of electrodynamics modeling and analytical techniques, we show that the proposed approach provides chiral optical forces 2 orders of magnitude larger than those obtained with circularly polarized plane waves. Superchiral surface waves allow for enantiomer separation on spatial, temporal, and size scales that would not be achievable with alternative techniques, thus representing a viable route toward all-optical enantiomer separation.
DOI
Stiffness of Cargo–Motor Linkage Tunes Myosin VI Motility and Response to Load
Rachit Shrivastava, Ashim Rai, Murti Salapaka, Sivaraj Sivaramakrishnan
We examine the effect of cargo–motor linkage stiffness on the mechanobiological properties of the molecular motor myosin VI. We use the programmability of DNA nanostructures to modulate cargo–motor linkage stiffness and combine it with high-precision optical trapping measurements to measure the effect of linkage stiffness on the motile properties of myosin VI. Our results reveal that a stiff cargo–motor linkage leads to shorter step sizes and load-induced anchoring of myosin VI, while a flexible linkage results in longer steps with frequent detachments from the actin filament under load. Our findings suggest a novel regulatory mechanism for tuning the dual cellular roles of the anchor and transporter ascribed to myosin VI.
A Tour de Force on the Double Helix: Exploiting DNA Mechanics To Study DNA-Based Molecular Machines
Michael R. Wasserman and Shixin Liu
DNA is both a fundamental building block of life and a fascinating natural polymer. The advent of single-molecule manipulation tools made it possible to exert controlled force on individual DNA molecules and measure their mechanical response. Such investigations elucidated the elastic properties of DNA and revealed its distinctive structural configurations across force regimes. In the meantime, a detailed understanding of DNA mechanics laid the groundwork for single-molecule studies of DNA-binding proteins and DNA-processing enzymes that bend, stretch, and twist DNA. These studies shed new light on the metabolism and transactions of nucleic acids, which constitute a major part of the cell’s operating system. Furthermore, the marriage of single-molecule fluorescence visualization and force manipulation has enabled researchers to directly correlate the applied tension to changes in the DNA structure and the behavior of DNA-templated complexes. Overall, experimental exploitation of DNA mechanics has been and will continue to be a unique and powerful strategy for understanding how molecular machineries recognize and modify the physical state of DNA to accomplish their biological functions.
DOI
DNA is both a fundamental building block of life and a fascinating natural polymer. The advent of single-molecule manipulation tools made it possible to exert controlled force on individual DNA molecules and measure their mechanical response. Such investigations elucidated the elastic properties of DNA and revealed its distinctive structural configurations across force regimes. In the meantime, a detailed understanding of DNA mechanics laid the groundwork for single-molecule studies of DNA-binding proteins and DNA-processing enzymes that bend, stretch, and twist DNA. These studies shed new light on the metabolism and transactions of nucleic acids, which constitute a major part of the cell’s operating system. Furthermore, the marriage of single-molecule fluorescence visualization and force manipulation has enabled researchers to directly correlate the applied tension to changes in the DNA structure and the behavior of DNA-templated complexes. Overall, experimental exploitation of DNA mechanics has been and will continue to be a unique and powerful strategy for understanding how molecular machineries recognize and modify the physical state of DNA to accomplish their biological functions.
DOI
Newtonian orbits of nanoparticles interacting with structured light beams
Manuel F Ferrer-Garcia and Dorilian Lopez-Mago
We perform numerical analysis to study the orbits described by subwavelength-size particles interacting with complex structured light beams. Our solution to the particle dynamics considers: (i) the gradient force, (ii) the transverse radiation pressure, and (iii) polarisation-dependent curl force. The last two terms, (ii) and (iii), constitute the scattering forces. Multiples examples are provided to show the polarisation effects in the trajectories. For the single optical vortex case, the particle is always expelled due to the polarisation-dependent terms. Optical forces due to vector beams, such as cylindrical vector beams and full-Poincaré beams have been analysed finding closed and open orbits, respectively. Trapping control has been achieved by varying the separation distance in the off-axis superposition of optical vortices.
DOI
We perform numerical analysis to study the orbits described by subwavelength-size particles interacting with complex structured light beams. Our solution to the particle dynamics considers: (i) the gradient force, (ii) the transverse radiation pressure, and (iii) polarisation-dependent curl force. The last two terms, (ii) and (iii), constitute the scattering forces. Multiples examples are provided to show the polarisation effects in the trajectories. For the single optical vortex case, the particle is always expelled due to the polarisation-dependent terms. Optical forces due to vector beams, such as cylindrical vector beams and full-Poincaré beams have been analysed finding closed and open orbits, respectively. Trapping control has been achieved by varying the separation distance in the off-axis superposition of optical vortices.
DOI
Tuesday, December 17, 2019
Induction and measurement of the early stage of a host‐parasite interaction using a combined optical trapping and Raman microspectroscopy system
Faris Sinjab, Hany M. Elsheikha, Max Dooley, Ioan Notingher
Understanding and quantifying the temporal acquisition of host cell molecules by intracellular pathogens is fundamentally important in biology. In this study, a recently developed holographic optical trapping (HOT)‐based Raman microspectroscopy (RMS) instrument is applied to detect, characterize and monitor in real time the molecular trafficking of a specific molecular species (isotope‐labeled phenylalanine (L‐Phe(D8)) at the single cell level. This approach enables simultaneous measurement of the chemical composition of human cerebrovascular endothelial cells and the protozoan parasite Toxoplasma gondii in isolation at the very start of the infection process. Using a model to decouple measurement contributions from host and pathogen sampling in the excitation volume, the data indicate that manipulating parasites with HOT coupled with RMS chemical readout was an effective method for measurement of L‐Phe(D8) transfer from host cells to parasites in real‐time, from the moment the parasite enters the host cell.
DOI
Understanding and quantifying the temporal acquisition of host cell molecules by intracellular pathogens is fundamentally important in biology. In this study, a recently developed holographic optical trapping (HOT)‐based Raman microspectroscopy (RMS) instrument is applied to detect, characterize and monitor in real time the molecular trafficking of a specific molecular species (isotope‐labeled phenylalanine (L‐Phe(D8)) at the single cell level. This approach enables simultaneous measurement of the chemical composition of human cerebrovascular endothelial cells and the protozoan parasite Toxoplasma gondii in isolation at the very start of the infection process. Using a model to decouple measurement contributions from host and pathogen sampling in the excitation volume, the data indicate that manipulating parasites with HOT coupled with RMS chemical readout was an effective method for measurement of L‐Phe(D8) transfer from host cells to parasites in real‐time, from the moment the parasite enters the host cell.
DOI
Rotational dynamics of Bacillus subtilis in an optical trap
Ashwini V Bhat, Praveen Parthasarathi, Shruthi S Iyengar, Balaji Yendeti, D C Mohana, Ashok Vudayagiri and Sharath Ananthamurthy
The swimming of a bacterium in fluids occurs in a low Reynolds number regime. The ability to confine the swimming motion by trapping a bacterium in laser light, can give information on the propulsion coefficients, which are important in explaining the efficiency of swimming of these bacteria. In this work, we report the results of an optically trapped Bacillus subtilis in an optical tweezer and the studies on the rotatory motion of the bacterium. The data is gathered and analysed using video microscopy. The propulsion coefficients of such swimming bacterium are determined through a power spectral analysis of the rotatory motion of the bacterium in the trap.
DOI
The swimming of a bacterium in fluids occurs in a low Reynolds number regime. The ability to confine the swimming motion by trapping a bacterium in laser light, can give information on the propulsion coefficients, which are important in explaining the efficiency of swimming of these bacteria. In this work, we report the results of an optically trapped Bacillus subtilis in an optical tweezer and the studies on the rotatory motion of the bacterium. The data is gathered and analysed using video microscopy. The propulsion coefficients of such swimming bacterium are determined through a power spectral analysis of the rotatory motion of the bacterium in the trap.
DOI
An Atomic-Array Optical Clock with Single-Atom Readout
Ivaylo S. Madjarov, Alexandre Cooper, Adam L. Shaw, Jacob P. Covey, Vladimir Schkolnik, Tai Hyun Yoon, Jason R. Williams, and Manuel Endres
Currently, the most accurate and stable clocks use optical interrogation of either a single ion or an ensemble of neutral atoms confined in an optical lattice. Here, we demonstrate a new optical clock system based on an array of individually trapped neutral atoms with single-atom readout, merging many of the benefits of ion and lattice clocks as well as creating a bridge to recently developed techniques in quantum simulation and computing with neutral atoms. We evaluate single-site-resolved frequency shifts and short-term stability via self-comparison. Atom-by-atom feedback control enables direct experimental estimation of laser noise contributions. Results agree well with an ab initio Monte Carlo simulation that incorporates finite temperature, projective readout, laser noise, and feedback dynamics. Our approach, based on a tweezer array, also suppresses interaction shifts while retaining a short dead time, all in a comparatively simple experimental setup suited for transportable operation. These results establish the foundations for a third optical clock platform and provide a novel starting point for entanglement-enhanced metrology, quantum clock networks, and applications in quantum computing and communication with individual neutral atoms that require optical-clock-state control.
DOI
Currently, the most accurate and stable clocks use optical interrogation of either a single ion or an ensemble of neutral atoms confined in an optical lattice. Here, we demonstrate a new optical clock system based on an array of individually trapped neutral atoms with single-atom readout, merging many of the benefits of ion and lattice clocks as well as creating a bridge to recently developed techniques in quantum simulation and computing with neutral atoms. We evaluate single-site-resolved frequency shifts and short-term stability via self-comparison. Atom-by-atom feedback control enables direct experimental estimation of laser noise contributions. Results agree well with an ab initio Monte Carlo simulation that incorporates finite temperature, projective readout, laser noise, and feedback dynamics. Our approach, based on a tweezer array, also suppresses interaction shifts while retaining a short dead time, all in a comparatively simple experimental setup suited for transportable operation. These results establish the foundations for a third optical clock platform and provide a novel starting point for entanglement-enhanced metrology, quantum clock networks, and applications in quantum computing and communication with individual neutral atoms that require optical-clock-state control.
DOI
Optical nanomanipulation on solid substrates via optothermally-gated photon nudging
Jingang Li, Yaoran Liu, Linhan Lin, Mingsong Wang, Taizhi Jiang, Jianhe Guo, Hongru Ding, Pavana Siddhartha Kollipara, Yuji Inoue, Donglei Fan, Brian A. Korgel & Yuebing Zheng
Constructing colloidal particles into functional nanostructures, materials, and devices is a promising yet challenging direction. Many optical techniques have been developed to trap, manipulate, assemble, and print colloidal particles from aqueous solutions into desired configurations on solid substrates. However, these techniques operated in liquid environments generally suffer from pattern collapses, Brownian motion, and challenges that come with reconfigurable assembly. Here, we develop an all-optical technique, termed optothermally-gated photon nudging (OPN), for the versatile manipulation and dynamic patterning of a variety of colloidal particles on a solid substrate at nanoscale accuracy. OPN takes advantage of a thin surfactant layer to optothermally modulate the particle-substrate interaction, which enables the manipulation of colloidal particles on solid substrates with optical scattering force. Along with in situ optical spectroscopy, our non-invasive and contactless nanomanipulation technique will find various applications in nanofabrication, nanophotonics, nanoelectronics, and colloidal sciences.
DOI
Constructing colloidal particles into functional nanostructures, materials, and devices is a promising yet challenging direction. Many optical techniques have been developed to trap, manipulate, assemble, and print colloidal particles from aqueous solutions into desired configurations on solid substrates. However, these techniques operated in liquid environments generally suffer from pattern collapses, Brownian motion, and challenges that come with reconfigurable assembly. Here, we develop an all-optical technique, termed optothermally-gated photon nudging (OPN), for the versatile manipulation and dynamic patterning of a variety of colloidal particles on a solid substrate at nanoscale accuracy. OPN takes advantage of a thin surfactant layer to optothermally modulate the particle-substrate interaction, which enables the manipulation of colloidal particles on solid substrates with optical scattering force. Along with in situ optical spectroscopy, our non-invasive and contactless nanomanipulation technique will find various applications in nanofabrication, nanophotonics, nanoelectronics, and colloidal sciences.
DOI
Manipulating rod-shaped bacteria with optical tweezers
Zheng Zhang, Tom E. P. Kimkes & Matthias Heinemann
Optical tweezers have great potential in microbiology for holding and manipulating single cells under a microscope. However, the methodology to use optical tweezers for live cell studies is still at its infancy. In this work, we determined suitable parameters for stable trapping of single Escherichia coli bacteria, and identified the upper limits of IR-exposure that can be applied without affecting viability. We found that the maximum tolerable IR-exposure is 2.5-fold higher when employing oscillating instead of stationary optical trapping (20 J and 8 J, respectively). We found that good stability of cells in an oscillating trap is achieved when the effective trap length is 20% larger than the cell length, the oscillation frequency higher than 100 Hz and the trap oriented perpendicular to the medium flow direction. Further, we show, using an IR power just sufficient for stable holding, that bacteria remain viable during at least 30 min of holding in an oscillating trap. In this work, we established a method for long-term stable handling of single E. coli cells using optical tweezers. This work will pave the way for future use of optical tweezers in microbiology.
DOI
Optical tweezers have great potential in microbiology for holding and manipulating single cells under a microscope. However, the methodology to use optical tweezers for live cell studies is still at its infancy. In this work, we determined suitable parameters for stable trapping of single Escherichia coli bacteria, and identified the upper limits of IR-exposure that can be applied without affecting viability. We found that the maximum tolerable IR-exposure is 2.5-fold higher when employing oscillating instead of stationary optical trapping (20 J and 8 J, respectively). We found that good stability of cells in an oscillating trap is achieved when the effective trap length is 20% larger than the cell length, the oscillation frequency higher than 100 Hz and the trap oriented perpendicular to the medium flow direction. Further, we show, using an IR power just sufficient for stable holding, that bacteria remain viable during at least 30 min of holding in an oscillating trap. In this work, we established a method for long-term stable handling of single E. coli cells using optical tweezers. This work will pave the way for future use of optical tweezers in microbiology.
DOI
Monday, December 16, 2019
Mechanical regulation of formin-dependent actin polymerization
Shimin Le, Miao Yu, Alexander Bershadsky, Jie Yan
The actomyosin cytoskeleton network plays a key role in a variety of fundamental cellular processes such as cell division, migration, and cell adhesion. The functions of cytoskeleton rely on its capability to receive, generate, respond to and transmit mechanical signals throughout the cytoskeleton network within the cells and throughout the tissue via cell-extracellular matrix and cell-cell adhesions. Crucial to the cytoskeleton’s functions is actin polymerization that is regulated by many cellular factors. Among these factors, the formin family proteins, which bind the barbed end of an actin filament (F-actin), are known to be a major actin polymerization promoting factor. Mounting evidence from single-molecule mechanical manipulation experiments have suggested that formin-dependent actin polymerization is sensitively regulated by the force and torque applied to the F-actin, making the formin family an emerging mechanosensing factor that selectively promotes elongation of the F-actin under tensile forces. In this review, we will focus on the current understanding of the mechanical regulation of formin-mediated actin polymerization, the key technologies that have enabled quantification of formin-mediated actin polymerization under mechanical constraints, and future perspectives and studies on molecular mechanisms involved in the mechanosensing of actin dynamics.
DOI
The actomyosin cytoskeleton network plays a key role in a variety of fundamental cellular processes such as cell division, migration, and cell adhesion. The functions of cytoskeleton rely on its capability to receive, generate, respond to and transmit mechanical signals throughout the cytoskeleton network within the cells and throughout the tissue via cell-extracellular matrix and cell-cell adhesions. Crucial to the cytoskeleton’s functions is actin polymerization that is regulated by many cellular factors. Among these factors, the formin family proteins, which bind the barbed end of an actin filament (F-actin), are known to be a major actin polymerization promoting factor. Mounting evidence from single-molecule mechanical manipulation experiments have suggested that formin-dependent actin polymerization is sensitively regulated by the force and torque applied to the F-actin, making the formin family an emerging mechanosensing factor that selectively promotes elongation of the F-actin under tensile forces. In this review, we will focus on the current understanding of the mechanical regulation of formin-mediated actin polymerization, the key technologies that have enabled quantification of formin-mediated actin polymerization under mechanical constraints, and future perspectives and studies on molecular mechanisms involved in the mechanosensing of actin dynamics.
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All-optical targeted drug delivery and real-time detection of a single cancer cell
Xiaole Liu, Jie Yuan, Dong Wu, Xiaobin Zou, Qing Zheng, Weina Zhang, Hongxiang Lei
Targeted drug delivery and real-time detection both play an important role for studying the specificity of a single cancer cell and the development of anticancer drugs. However, a method that simultaneously enables safe and efficient targeted drug delivery and noninvasive, free-label cell detection is highly desirable but challenging. Here, we report an all-optical method that combines fiber optical tweezers with laser Raman microspectroscopy, which can achieve targeted drug delivery to a single cancer cell using optical manipulation in vitro quickly and accurately by a tapered fiber probe, and simultaneously record the corresponding active characteristics of the targeted cancer cell under the contact of delivered drug through a Raman spectrometer. Using the method, drug delivery and release can be flexibly controlled by turning on/off the trapping laser beam propagating in the fiber, which can avoid the complex systems and is highly autonomous and controllable. Moreover, the detection of cell activity does not require any dye calibration and processing, and it is noninvasive. In addition, for a single suspension cell, optical trapping of the cell using another fiber tip can overcome the low efficiency of targeted drug delivery and the poor stability of the Raman spectrum caused by Brownian motion of the cell. This all-optical method provides a promising approach to conduct pharmacologic studies with the reaction of cancer cell and drugs at the level of a single cell.
DOI
Targeted drug delivery and real-time detection both play an important role for studying the specificity of a single cancer cell and the development of anticancer drugs. However, a method that simultaneously enables safe and efficient targeted drug delivery and noninvasive, free-label cell detection is highly desirable but challenging. Here, we report an all-optical method that combines fiber optical tweezers with laser Raman microspectroscopy, which can achieve targeted drug delivery to a single cancer cell using optical manipulation in vitro quickly and accurately by a tapered fiber probe, and simultaneously record the corresponding active characteristics of the targeted cancer cell under the contact of delivered drug through a Raman spectrometer. Using the method, drug delivery and release can be flexibly controlled by turning on/off the trapping laser beam propagating in the fiber, which can avoid the complex systems and is highly autonomous and controllable. Moreover, the detection of cell activity does not require any dye calibration and processing, and it is noninvasive. In addition, for a single suspension cell, optical trapping of the cell using another fiber tip can overcome the low efficiency of targeted drug delivery and the poor stability of the Raman spectrum caused by Brownian motion of the cell. This all-optical method provides a promising approach to conduct pharmacologic studies with the reaction of cancer cell and drugs at the level of a single cell.
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Theoretical investigation of dielectrophoretic effect for carbon nanotubes in optoelectronic tweezers
Sheng Hu, Lv Cai, Jiangtao Lv, Xiaoxiao Jiang
Optoelectronic tweezers (OET) utilizes the optically induced dielectrophoresis (ODEP) force to manipulate and assemble carbon nanotube (CNT) particles in an aqueous solution. This work can help us to gain exciting and promising applications in electronic devices and sensing areas. In this paper, a numerical model based on the Maxwell stress tensor (MST) method has been presented to study a single CNT particle subjected to both the ODEP force and torque in a non-uniform electric field. In addition, a single-sided OET, which is unlike traditional OET chips and enables the assembly and alignment of CNT particles, has been introduced and studied. The calculated results on the CNT particles analogy to non-spherical shapes demonstrate that the MST method can provide more accurate predictions than the effective dipole moment. Furthermore, both the DEP force and torque exerted on the CNT particle, as well as shell thickness, spatial position, and distance between CNT particle and electrode, have been studied in detail. These results are in agreement with those obtained by other researchers. This work can help us to gain new insights into the analysis of motions of the CNT particles suspended in OET chips.
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Optoelectronic tweezers (OET) utilizes the optically induced dielectrophoresis (ODEP) force to manipulate and assemble carbon nanotube (CNT) particles in an aqueous solution. This work can help us to gain exciting and promising applications in electronic devices and sensing areas. In this paper, a numerical model based on the Maxwell stress tensor (MST) method has been presented to study a single CNT particle subjected to both the ODEP force and torque in a non-uniform electric field. In addition, a single-sided OET, which is unlike traditional OET chips and enables the assembly and alignment of CNT particles, has been introduced and studied. The calculated results on the CNT particles analogy to non-spherical shapes demonstrate that the MST method can provide more accurate predictions than the effective dipole moment. Furthermore, both the DEP force and torque exerted on the CNT particle, as well as shell thickness, spatial position, and distance between CNT particle and electrode, have been studied in detail. These results are in agreement with those obtained by other researchers. This work can help us to gain new insights into the analysis of motions of the CNT particles suspended in OET chips.
DOI
Wednesday, December 11, 2019
Influence of Pulsed He–Ne Laser Irradiation on the Red Blood Cell Interaction Studied by Optical Tweezers
Ruixue Zhu, Tatiana Avsievich, Alexander Bykov, Alexey Popov and Igor Meglinski
Optical Tweezers (OT), as a revolutionary innovation in laser physics, has been extremely useful in studying cell interaction dynamics at a single-cell level. The reversible aggregation process of red blood cells (RBCs) has an important influence on blood rheological properties, but the underlying mechanism has not been fully understood. The regulating effects of low-level laser irradiation on blood rheological properties have been reported. However, the influence of pulsed laser irradiation, and the origin of laser irradiation effects on the interaction between RBCs remain unclear. In this study, RBC interaction was assessed in detail with OT. The effects of both continuous and pulsed low-level He–Ne laser irradiation on RBC aggregation was investigated within a short irradiation period (up to 300 s). The results indicate stronger intercellular interaction between RBCs in the enforced disaggregation process, and both the cell contact time and the initial contact area between two RBCs showed an impact on the measured disaggregation force. Meanwhile, the RBC aggregation force that was independent to measurement conditions decreased after a short time of pulsed He–Ne laser irradiation. These results provide new insights into the understanding of the RBC interaction mechanism and laser irradiation effects on blood properties.
DOI
Optical Tweezers (OT), as a revolutionary innovation in laser physics, has been extremely useful in studying cell interaction dynamics at a single-cell level. The reversible aggregation process of red blood cells (RBCs) has an important influence on blood rheological properties, but the underlying mechanism has not been fully understood. The regulating effects of low-level laser irradiation on blood rheological properties have been reported. However, the influence of pulsed laser irradiation, and the origin of laser irradiation effects on the interaction between RBCs remain unclear. In this study, RBC interaction was assessed in detail with OT. The effects of both continuous and pulsed low-level He–Ne laser irradiation on RBC aggregation was investigated within a short irradiation period (up to 300 s). The results indicate stronger intercellular interaction between RBCs in the enforced disaggregation process, and both the cell contact time and the initial contact area between two RBCs showed an impact on the measured disaggregation force. Meanwhile, the RBC aggregation force that was independent to measurement conditions decreased after a short time of pulsed He–Ne laser irradiation. These results provide new insights into the understanding of the RBC interaction mechanism and laser irradiation effects on blood properties.
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Simultaneous, hybrid single-molecule method by optical tweezers and fluorescence
Guoteng Ma, Chunguang Hu, Shuai Li, Xiaoqin Gao, Hongbin Li, Xiaotang Hu
As studies on life sciences progress toward the single-molecule level, new experiments have put forward more requirements for simultaneously displaying the mechanical properties and conformational changes of biomolecules. Optical tweezers and fluorescence microscopy have been combined to solve this problem. The combination of instruments forms a new generation of hybrid single-molecule technology that breaks through the limitations of traditional biochemical analysis. Powerful manipulation and fluorescence visualization have been widely used, and these techniques provide new possibilities for studying complex biochemical reactions at the single-molecule level. This paper explains the features of this combined technique, including the application characteristics of single-trap and dual-traps, the anti-bleaching method, and optical tweezers combined with epi-fluorescence, confocal fluorescence, total internal reflection fluorescence, and other fluorescence methods. Using typical experiments, we analyze technical solutions and explain the factors and principles that instrument designers should consider. This review aims to give an introduction to this novel fusion technology process and describe important biological results.
DOI
As studies on life sciences progress toward the single-molecule level, new experiments have put forward more requirements for simultaneously displaying the mechanical properties and conformational changes of biomolecules. Optical tweezers and fluorescence microscopy have been combined to solve this problem. The combination of instruments forms a new generation of hybrid single-molecule technology that breaks through the limitations of traditional biochemical analysis. Powerful manipulation and fluorescence visualization have been widely used, and these techniques provide new possibilities for studying complex biochemical reactions at the single-molecule level. This paper explains the features of this combined technique, including the application characteristics of single-trap and dual-traps, the anti-bleaching method, and optical tweezers combined with epi-fluorescence, confocal fluorescence, total internal reflection fluorescence, and other fluorescence methods. Using typical experiments, we analyze technical solutions and explain the factors and principles that instrument designers should consider. This review aims to give an introduction to this novel fusion technology process and describe important biological results.
DOI
Binary square axicon with chiral focusing properties for optical trapping
Vinoth Balasubramani; Anand Vijayakumar; Mani Ratnam Rai; Joseph Rosen; Chau-Jern Cheng; Oleg V. Minin; Igor V. Minin
We introduce a novel phase-only diffractive optical element called chiral binary square axicon (CBSA). The CBSA is designed by linearly rotating the square half-period zones of the binary square axicon with respect to one another. A quadratic phase mask (QPM) is combined with the CBSA using modulo-2π phase addition technique to bring the far-field intensity pattern of CBSA at the focal plane of the QPM and to introduce quasiachromatic effects. The periodically rotated zones of CBSA produce a whirlpool phase profile and twisted intensity patterns at the focal plane of QPM. The degree of twisting seen in the intensity patterns is dependent upon the angular step size of rotation of the zones. The intensity pattern was found to rotate around the optical axis along the direction of propagation. The phase patterns of CBSA with different angles of zone rotation are displayed on a phase-only spatial light modulator, and the experimental results were found to match with the simulation results. To evaluate the optical trapping capabilities of CBSA, an optical trapping experiment was carried out and the optical fields generated by CBSA were used for trapping and rotating yeast cells.
DOI
We introduce a novel phase-only diffractive optical element called chiral binary square axicon (CBSA). The CBSA is designed by linearly rotating the square half-period zones of the binary square axicon with respect to one another. A quadratic phase mask (QPM) is combined with the CBSA using modulo-2π phase addition technique to bring the far-field intensity pattern of CBSA at the focal plane of the QPM and to introduce quasiachromatic effects. The periodically rotated zones of CBSA produce a whirlpool phase profile and twisted intensity patterns at the focal plane of QPM. The degree of twisting seen in the intensity patterns is dependent upon the angular step size of rotation of the zones. The intensity pattern was found to rotate around the optical axis along the direction of propagation. The phase patterns of CBSA with different angles of zone rotation are displayed on a phase-only spatial light modulator, and the experimental results were found to match with the simulation results. To evaluate the optical trapping capabilities of CBSA, an optical trapping experiment was carried out and the optical fields generated by CBSA were used for trapping and rotating yeast cells.
DOI
Probing gravity by holding atoms for 20 seconds
Victoria Xu, Matt Jaffe, Cristian D. Panda, Sofus L. Kristensen, Logan W. Clark, Holger Müller
Atom interferometers are powerful tools for both measurements in fundamental physics and inertial sensing applications. Their performance, however, has been limited by the available interrogation time of freely falling atoms in a gravitational field. By suspending the spatially separated atomic wave packets in a lattice formed by the mode of an optical cavity, we realize an interrogation time of 20 seconds. Our approach allows gravitational potentials to be measured by holding, rather than dropping, atoms. After seconds of hold time, gravitational potential energy differences from as little as micrometers of vertical separation generate megaradians of interferometer phase. This trapped geometry suppresses the phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters.
DOI
Atom interferometers are powerful tools for both measurements in fundamental physics and inertial sensing applications. Their performance, however, has been limited by the available interrogation time of freely falling atoms in a gravitational field. By suspending the spatially separated atomic wave packets in a lattice formed by the mode of an optical cavity, we realize an interrogation time of 20 seconds. Our approach allows gravitational potentials to be measured by holding, rather than dropping, atoms. After seconds of hold time, gravitational potential energy differences from as little as micrometers of vertical separation generate megaradians of interferometer phase. This trapped geometry suppresses the phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters.
DOI
Efficient methods for determining folding free energies in single-molecule pulling experiments
A Severino, A M Monge, P Rissone and F Ritort
The remarkable accuracy and versatility of single-molecule techniques make new measurements that are not feasible in bulk assays possible. Among these, the precise estimation of folding free energies using fluctuation theorems in nonequilibrium pulling experiments has become a benchmark in modern biophysics. In practice, the use of fluctuation relations to determine free energies requires a thorough evaluation of the usually large energetic contributions caused by the elastic deformation of the different elements of the experimental setup (such as the optical trap, the molecular linkers and the stretched-unfolded polymer). We review and describe how to optimally estimate such elastic energy contributions to extract folding free energies, using DNA and RNA hairpins as model systems pulled by laser optical tweezers. The methodology is generally applicable to other force-spectroscopy techniques and molecular systems.
DOI
The remarkable accuracy and versatility of single-molecule techniques make new measurements that are not feasible in bulk assays possible. Among these, the precise estimation of folding free energies using fluctuation theorems in nonequilibrium pulling experiments has become a benchmark in modern biophysics. In practice, the use of fluctuation relations to determine free energies requires a thorough evaluation of the usually large energetic contributions caused by the elastic deformation of the different elements of the experimental setup (such as the optical trap, the molecular linkers and the stretched-unfolded polymer). We review and describe how to optimally estimate such elastic energy contributions to extract folding free energies, using DNA and RNA hairpins as model systems pulled by laser optical tweezers. The methodology is generally applicable to other force-spectroscopy techniques and molecular systems.
DOI
Collective Force Generation by Molecular Motors Is Determined by Strain-Induced Unbinding
Mehmet Can Uçar, Reinhard Lipowsky
In the living cell, we encounter a large variety of motile processes such as organelle transport and cytoskeleton remodeling. These processes are driven by motor proteins that generate force by transducing chemical free energy into mechanical work. In many cases, the molecular motors work in teams to collectively generate larger forces. Recent optical trapping experiments on small teams of cytoskeletal motors indicated that the collectively generated force increases with the size of the motor team but that this increase depends on the motor type and on whether the motors are studied in vitro or in vivo. Here, we use the theory of stochastic processes to describe the motion of N motors in a stationary optical trap and to compute the N-dependence of the collectively generated forces. We consider six distinct motor types, two kinesins, two dyneins, and two myosins. We show that the force increases always linearly with N but with a prefactor that depends on the performance of the single motor. Surprisingly, this prefactor increases for weaker motors with a lower stall force. This counter-intuitive behavior reflects the increased probability with which stronger motors detach from the filament during strain generation. Our theoretical results are in quantitative agreement with experimental data on small teams of kinesin-1 motors.
DOI
In the living cell, we encounter a large variety of motile processes such as organelle transport and cytoskeleton remodeling. These processes are driven by motor proteins that generate force by transducing chemical free energy into mechanical work. In many cases, the molecular motors work in teams to collectively generate larger forces. Recent optical trapping experiments on small teams of cytoskeletal motors indicated that the collectively generated force increases with the size of the motor team but that this increase depends on the motor type and on whether the motors are studied in vitro or in vivo. Here, we use the theory of stochastic processes to describe the motion of N motors in a stationary optical trap and to compute the N-dependence of the collectively generated forces. We consider six distinct motor types, two kinesins, two dyneins, and two myosins. We show that the force increases always linearly with N but with a prefactor that depends on the performance of the single motor. Surprisingly, this prefactor increases for weaker motors with a lower stall force. This counter-intuitive behavior reflects the increased probability with which stronger motors detach from the filament during strain generation. Our theoretical results are in quantitative agreement with experimental data on small teams of kinesin-1 motors.
DOI
Monday, December 9, 2019
Single Molecule Force Spectroscopy Reveals the Mechanical Design Governing the Efficient Translocation of the Bacterial Toxin Protein RTX
Han Wang, Xiaoqing Gao, Hongbin Li
The efficient translocation of bacterial toxin adenylate cyclase toxin (CyaA) from the bacterial cytosol to the extracellular environment by the type 1 secretion system (T1SS) is essential for the toxin to function. To understand the molecular features that are responsible for the efficient translocation of CyaA, here we used optical tweezers to investigate the mechanical properties and conformational dynamics of the RTX domain of CyaA at the single molecule level. Our results revealed that apo-RTX behaves like an ideal random coil. This property allows the T1SS to translocate RTX without overcoming enthalpic resistance. In contrast, the folded holo-RTX is mechancially stable, and its folding occurs in a vectorial, co-translocational fashion starting from its C-terminus. Moreover, our results showed that the folding of holo-RTX generates a stretching force, which can further facilitate the translocation of RTX. Our results highlight the important role played by the Ca2+-triggered folding of RTX in the translocation of RTX and provide mechanistic insights into the mechanical design that governs the efficient translocation of RTX.
DOI
The efficient translocation of bacterial toxin adenylate cyclase toxin (CyaA) from the bacterial cytosol to the extracellular environment by the type 1 secretion system (T1SS) is essential for the toxin to function. To understand the molecular features that are responsible for the efficient translocation of CyaA, here we used optical tweezers to investigate the mechanical properties and conformational dynamics of the RTX domain of CyaA at the single molecule level. Our results revealed that apo-RTX behaves like an ideal random coil. This property allows the T1SS to translocate RTX without overcoming enthalpic resistance. In contrast, the folded holo-RTX is mechancially stable, and its folding occurs in a vectorial, co-translocational fashion starting from its C-terminus. Moreover, our results showed that the folding of holo-RTX generates a stretching force, which can further facilitate the translocation of RTX. Our results highlight the important role played by the Ca2+-triggered folding of RTX in the translocation of RTX and provide mechanistic insights into the mechanical design that governs the efficient translocation of RTX.
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A Proposal for Realization of MIR to NIR Up-conversion Process based on Nano-Optomechanical System
N. Khataeizadeh, G. Rostami, M. Dolatyari, A. Rostami, I. S. Amiri
In this work, a nanoscale optomechanical system driven by optical force resulted from light scattering is designed and demonstrated. The proposed system is able to control and manipulate the propagating light in single-mode waveguides in the structure. The optical part of the guiding wave structure consists of two coupled single-mode optical waveguides and a series of nanogold metallic spheres on a silica substrate. The structure is illuminated with a plane wave with a wavelength of 4.3 micrometers. The resonance of the nanogold spheres due to the incidence plane wave provides scattering and thus the optical force can be generated and causes the two waveguides to deform and start bending toward each other. The generated optical force has been calculated with a maximum value ofImage 1. The designed system could achieve a maximum waveguide displacement of 963 nm. The change in the power of the incident plane wave allows controlling the displacement of the waveguides hence the amount of the NIR (850 nm) light power coupled between the two waveguides can be controlled. Here, the electromagnetic wave with a wavelength of 850 nm has been utilized to propagate in the single-mode waveguides. The induced optical force changes the distance between the single-mode optical waveguides and this causes the coupling of the optical power (NIR) into another waveguide. By choosing the distance between the two waveguides it is possible to couple any desired fraction of the NIR input light from one waveguide to another. Therefore, any changes in the incident MIR light power will affect and change the distance between the two optical waveguides and thus the NIR coupled light changes accordingly. Finally, we have calculated a linear relationship between the coupled light in NIR band and the incident light power in MIR using the proposed structure and in fact this is an optomechanical up-conversion system.
DOI
In this work, a nanoscale optomechanical system driven by optical force resulted from light scattering is designed and demonstrated. The proposed system is able to control and manipulate the propagating light in single-mode waveguides in the structure. The optical part of the guiding wave structure consists of two coupled single-mode optical waveguides and a series of nanogold metallic spheres on a silica substrate. The structure is illuminated with a plane wave with a wavelength of 4.3 micrometers. The resonance of the nanogold spheres due to the incidence plane wave provides scattering and thus the optical force can be generated and causes the two waveguides to deform and start bending toward each other. The generated optical force has been calculated with a maximum value ofImage 1. The designed system could achieve a maximum waveguide displacement of 963 nm. The change in the power of the incident plane wave allows controlling the displacement of the waveguides hence the amount of the NIR (850 nm) light power coupled between the two waveguides can be controlled. Here, the electromagnetic wave with a wavelength of 850 nm has been utilized to propagate in the single-mode waveguides. The induced optical force changes the distance between the single-mode optical waveguides and this causes the coupling of the optical power (NIR) into another waveguide. By choosing the distance between the two waveguides it is possible to couple any desired fraction of the NIR input light from one waveguide to another. Therefore, any changes in the incident MIR light power will affect and change the distance between the two optical waveguides and thus the NIR coupled light changes accordingly. Finally, we have calculated a linear relationship between the coupled light in NIR band and the incident light power in MIR using the proposed structure and in fact this is an optomechanical up-conversion system.
DOI
Single-acquisition 2-D multifocal Raman spectroscopy using compressive sensing
Pengfei Zhang, Guiwen Wang, Xiujuan Zhang, Yong-Qing Li
Confocal Raman microscopy is a powerful method for nondestructive and noninvasive detection of chemicals with high spatial resolution, but its long acquisition time hinders its applications in large-scale monitoring of fast dynamics. Here, we report the development of a compressive sensing technique for single-acquisition multifocal Raman spectroscopy, which is capable of improving the speed of conventional confocal Raman spectroscopy by 2-3 orders of magnitude. A sample is excited with a 2-D multifocus pattern and the Raman scatterings from the multiple foci were projected onto the spectrometer’s entrance in a 2-D array. The superimposed spectra within each row of the array were processed with an algorithm such that the spectra from the individual foci were retrieved in a single acquisition and with reduced noise. The performances of the developed technique were demonstrated by parallel Raman spectroscopy of multiple individual particles as well as by single-acquisition confocal Raman imaging of a large scale with high spatial resolution when combined with spatially sparse sampling. The technique is expected to find wide applications in investigating fast dynamics in large-scale biological systems.
DOI
Confocal Raman microscopy is a powerful method for nondestructive and noninvasive detection of chemicals with high spatial resolution, but its long acquisition time hinders its applications in large-scale monitoring of fast dynamics. Here, we report the development of a compressive sensing technique for single-acquisition multifocal Raman spectroscopy, which is capable of improving the speed of conventional confocal Raman spectroscopy by 2-3 orders of magnitude. A sample is excited with a 2-D multifocus pattern and the Raman scatterings from the multiple foci were projected onto the spectrometer’s entrance in a 2-D array. The superimposed spectra within each row of the array were processed with an algorithm such that the spectra from the individual foci were retrieved in a single acquisition and with reduced noise. The performances of the developed technique were demonstrated by parallel Raman spectroscopy of multiple individual particles as well as by single-acquisition confocal Raman imaging of a large scale with high spatial resolution when combined with spatially sparse sampling. The technique is expected to find wide applications in investigating fast dynamics in large-scale biological systems.
DOI
Regular Assembly of Polymer Nanoparticles by Optical Trapping Enhanced with a Random Array of Si Needles for Reconfigurable Photonic Crystals in Liquid
Itsuo Hanasaki, Tatsuya Shoji, Yasuyuki Tsuboi
We report the optical trapping of many particles with feasible laser powers by the nanostructured semiconductor-assisted (NASSCA) trapping technique. Furthermore, we have found that the random array of silicon needles with spacings substantially smaller than the nanoparticle sizes is advantageous not only for the trapping force-field enhancement but also for the realization of close-packed assembly of nanoparticles. This counterintuitive approach is promising for the realizations of collective structural orders such as reconfigurable photonic crystals in liquid, which have been often regarded to require either top-down templates or full self-assembly beyond control.
DOI
We report the optical trapping of many particles with feasible laser powers by the nanostructured semiconductor-assisted (NASSCA) trapping technique. Furthermore, we have found that the random array of silicon needles with spacings substantially smaller than the nanoparticle sizes is advantageous not only for the trapping force-field enhancement but also for the realization of close-packed assembly of nanoparticles. This counterintuitive approach is promising for the realizations of collective structural orders such as reconfigurable photonic crystals in liquid, which have been often regarded to require either top-down templates or full self-assembly beyond control.
DOI
Friday, December 6, 2019
Analytically decomposing optical force on a spherical particle in Bessel beams into conservative and non-conservative parts
Guangji Ha, Hongxia Zheng, Xinning Yu, and Zhifang Lin
Based on the recently developed Cartesian multipole expansion theory, we analytically analyze the conservative and non-conservative nature of the optical force acting on a spherical particle of arbitrary size and isotropic composition immersed in the optical Bessel beams of arbitrary orders and polarizations. It is rigorously proved that the conservative force on the particle in Bessel beams aligns in the radial direction transverse to beam propagation, while the non-conservative force is completely non-radial, lying in the azimuthal and longitudinal directions. To the best of our knowledge, our work provides the first analytical partition between the conservative and non-conservative components of the optical force on a particle of arbitrary size and composition placed in a class of extensively employed optical beams in practical optical manipulation, beyond the small particle limit.
DOI
Based on the recently developed Cartesian multipole expansion theory, we analytically analyze the conservative and non-conservative nature of the optical force acting on a spherical particle of arbitrary size and isotropic composition immersed in the optical Bessel beams of arbitrary orders and polarizations. It is rigorously proved that the conservative force on the particle in Bessel beams aligns in the radial direction transverse to beam propagation, while the non-conservative force is completely non-radial, lying in the azimuthal and longitudinal directions. To the best of our knowledge, our work provides the first analytical partition between the conservative and non-conservative components of the optical force on a particle of arbitrary size and composition placed in a class of extensively employed optical beams in practical optical manipulation, beyond the small particle limit.
DOI
Non-monotonic dependence of stiffness on actin crosslinking in cytoskeleton composites
Madison L. Francis, Shea N. Ricketts, Leila Farhadi, Michael J. Rust, Moumita Das, Jennifer L. Ross and Rae M. Robertson-Anderson
The cytoskeleton is able to precisely tune its structure and mechanics through interactions between semiflexible actin filaments, rigid microtubules and a suite of crosslinker proteins. However, the role that each of these components, as well as the interactions between them, plays in the dynamics of the composite cytoskeleton remains an open question. Here, we use optical tweezers microrheology and fluorescence confocal microscopy to reveal the surprising ways in which actin crosslinking tunes the viscoelasticity and mobility of actin–microtubule composites from steady-state to the highly nonlinear regime. While previous studies have shown that increasing crosslinking in actin networks increases elasticity and stiffness, we instead find that composite stiffness displays a striking non-monotonic dependence on actin crosslinking – first increasing then decreasing to a response similar to or even lower than un-linked composites. We further show that actin crosslinking has an unexpectedly strong impact on the mobility of microtubules; and it is in fact the microtubule mobility – dictated by crosslinker-driven rearrangements of actin filaments – that controls composite stiffness. This result is at odds with conventional thought that actin mobility drives cytoskeleton mechanics. More generally, our results demonstrate that – when crosslinking composite materials to confer strength and resilience – more is not always better.
DOI
The cytoskeleton is able to precisely tune its structure and mechanics through interactions between semiflexible actin filaments, rigid microtubules and a suite of crosslinker proteins. However, the role that each of these components, as well as the interactions between them, plays in the dynamics of the composite cytoskeleton remains an open question. Here, we use optical tweezers microrheology and fluorescence confocal microscopy to reveal the surprising ways in which actin crosslinking tunes the viscoelasticity and mobility of actin–microtubule composites from steady-state to the highly nonlinear regime. While previous studies have shown that increasing crosslinking in actin networks increases elasticity and stiffness, we instead find that composite stiffness displays a striking non-monotonic dependence on actin crosslinking – first increasing then decreasing to a response similar to or even lower than un-linked composites. We further show that actin crosslinking has an unexpectedly strong impact on the mobility of microtubules; and it is in fact the microtubule mobility – dictated by crosslinker-driven rearrangements of actin filaments – that controls composite stiffness. This result is at odds with conventional thought that actin mobility drives cytoskeleton mechanics. More generally, our results demonstrate that – when crosslinking composite materials to confer strength and resilience – more is not always better.
DOI
Calculating optical forces with skew line ray model for Gaussian beam
Shuhe Zhang, Meng Shao, Xiao Yang, Jinhua Zhou
Ray optics models in optical tweezers are confined to two specific cases: ray pencil model for a highly focused beam and spherical wave front model for a weakly focused beam. In this manuscript, the skew lines of one sheet hyperboloid are introduced as rays’ trajectories to trace the propagation of a Gaussian beam for calculating optical forces. The simulations of trapping efficiencies demonstrate the skew-line ray model is valid in comparison with traditional ray-optics models including spherical wave front model and ray pencil model. Our results of transverse and axial trapping efficiencies show that the skew-line ray model has good performances in both highly and weakly focused beams. Furthermore, the influence of the spherical aberration is discussed, and our results are accordance with that from traditional ray-optics models. The SLR method unifies ray pencil model and spherical wave front model into one way, and can be used to calculate optical forces in either paraxial or nonparaxial conditions. Thus, this model is more appropriate in extensive simulations in ray optics regime.
DOI
Ray optics models in optical tweezers are confined to two specific cases: ray pencil model for a highly focused beam and spherical wave front model for a weakly focused beam. In this manuscript, the skew lines of one sheet hyperboloid are introduced as rays’ trajectories to trace the propagation of a Gaussian beam for calculating optical forces. The simulations of trapping efficiencies demonstrate the skew-line ray model is valid in comparison with traditional ray-optics models including spherical wave front model and ray pencil model. Our results of transverse and axial trapping efficiencies show that the skew-line ray model has good performances in both highly and weakly focused beams. Furthermore, the influence of the spherical aberration is discussed, and our results are accordance with that from traditional ray-optics models. The SLR method unifies ray pencil model and spherical wave front model into one way, and can be used to calculate optical forces in either paraxial or nonparaxial conditions. Thus, this model is more appropriate in extensive simulations in ray optics regime.
DOI
Wednesday, December 4, 2019
Coupling between axial and radial motions of microscopic particle trapped in the intracavity optical tweezers
Guangzong Xiao, Tengfang Kuang, Bin Luo, Wei Xiong, Xiang Han, Xinlin Chen, and Hui Luo
Intracavity optical tweezers have been proposed and demonstrated recently, which allows orders-of-magnitude higher optical confinement with lower-numerical-aperture lens and lower laser power in contrast to the standard optical tweezers. We further investigate its characteristics about the position stability of trapped particles. The dependence of the radial and axial position stability on the laser intensity acting on the particle of 10-µm diameter in intracavity optical tweezers and standard optical tweezers are compared experimentally. Result shows that higher laser intensity can make stronger optical confinement in intracavity optical tweezers under the condition of good trap operation, compared with standard optical tweezers. We demonstrate and analyze the coupling between the particle’s radial and axial motion, and then provide two approaches to reduce it. Our work will benefit the further enhancement of position stability for the trapped particle in intracavity optical tweezers.
DOI
Intracavity optical tweezers have been proposed and demonstrated recently, which allows orders-of-magnitude higher optical confinement with lower-numerical-aperture lens and lower laser power in contrast to the standard optical tweezers. We further investigate its characteristics about the position stability of trapped particles. The dependence of the radial and axial position stability on the laser intensity acting on the particle of 10-µm diameter in intracavity optical tweezers and standard optical tweezers are compared experimentally. Result shows that higher laser intensity can make stronger optical confinement in intracavity optical tweezers under the condition of good trap operation, compared with standard optical tweezers. We demonstrate and analyze the coupling between the particle’s radial and axial motion, and then provide two approaches to reduce it. Our work will benefit the further enhancement of position stability for the trapped particle in intracavity optical tweezers.
DOI
Impact of the evanescent waves on the backflow of power in the near field
V V Kotlyar, A A Kovalev and D S Kalinkina
For an elliptically polarized optical vortex with an arbitrary integer topological charge, using the expressions for all six components of the electric and magnetic field strength vectors, we obtain an expression for the longitudinal component of the Poynting vector in the initial plane. For the particular case of a narrow angular spectrum of plane waves (Bessel beam) and for the circular polarization, it is shown that in the presence of the inhomogeneous evanescent waves in the initial light field, a reverse flux of light energy can occur near the op-tical axis. It is shown that this reverse energy flux is due to toroidal vortices in the longitudinal plane.
DOI
For an elliptically polarized optical vortex with an arbitrary integer topological charge, using the expressions for all six components of the electric and magnetic field strength vectors, we obtain an expression for the longitudinal component of the Poynting vector in the initial plane. For the particular case of a narrow angular spectrum of plane waves (Bessel beam) and for the circular polarization, it is shown that in the presence of the inhomogeneous evanescent waves in the initial light field, a reverse flux of light energy can occur near the op-tical axis. It is shown that this reverse energy flux is due to toroidal vortices in the longitudinal plane.
DOI
Energetic dependencies dictate folding mechanism in a complex protein
Kaixian Liu, Xiuqi Chen, and Christian M. Kaiser
Large proteins with multiple domains are thought to fold cotranslationally to minimize interdomain misfolding. Once folded, domains interact with each other through the formation of extensive interfaces that are important for protein stability and function. However, multidomain protein folding and the energetics of domain interactions remain poorly understood. In elongation factor G (EF-G), a highly conserved protein composed of 5 domains, the 2 N-terminal domains form a stably structured unit cotranslationally. Using single-molecule optical tweezers, we have defined the steps leading to fully folded EF-G. We find that the central domain III of EF-G is highly dynamic and does not fold upon emerging from the ribosome. Surprisingly, a large interface with the N-terminal domains does not contribute to the stability of domain III. Instead, it requires interactions with its folded C-terminal neighbors to be stably structured. Because of the directionality of protein synthesis, this energetic dependency of domain III on its C-terminal neighbors disrupts cotranslational folding and imposes a posttranslational mechanism on the folding of the C-terminal part of EF-G. As a consequence, unfolded domains accumulate during synthesis, leading to the extensive population of misfolded species that interfere with productive folding. Domain III flexibility enables large-scale conformational transitions that are part of the EF-G functional cycle during ribosome translocation. Our results suggest that energetic tuning of domain stabilities, which is likely crucial for EF-G function, complicates the folding of this large multidomain protein.
DOI
Large proteins with multiple domains are thought to fold cotranslationally to minimize interdomain misfolding. Once folded, domains interact with each other through the formation of extensive interfaces that are important for protein stability and function. However, multidomain protein folding and the energetics of domain interactions remain poorly understood. In elongation factor G (EF-G), a highly conserved protein composed of 5 domains, the 2 N-terminal domains form a stably structured unit cotranslationally. Using single-molecule optical tweezers, we have defined the steps leading to fully folded EF-G. We find that the central domain III of EF-G is highly dynamic and does not fold upon emerging from the ribosome. Surprisingly, a large interface with the N-terminal domains does not contribute to the stability of domain III. Instead, it requires interactions with its folded C-terminal neighbors to be stably structured. Because of the directionality of protein synthesis, this energetic dependency of domain III on its C-terminal neighbors disrupts cotranslational folding and imposes a posttranslational mechanism on the folding of the C-terminal part of EF-G. As a consequence, unfolded domains accumulate during synthesis, leading to the extensive population of misfolded species that interfere with productive folding. Domain III flexibility enables large-scale conformational transitions that are part of the EF-G functional cycle during ribosome translocation. Our results suggest that energetic tuning of domain stabilities, which is likely crucial for EF-G function, complicates the folding of this large multidomain protein.
DOI
Quantitative characterization of mechano-biological interrelationships of single cells
Ying Li, Junyang Li, Zhijie Huan, Yuanchao Hu
This paper presented a quantitative investigation on the alteration of cell biological functions in relation to the change of mechanical properties of single cells. Leukemia NB4 cells were used as cell line. The dielectrophoresis (DEP) method was utilized to verify the mechanical properties variation of NB4 cells under the electric field treatment. A quantitative study of cell mechanical properties was then carried out using optical tweezers (OT), and cell biological properties using gene expression measurement. The result shows cell stiffness decreased after electric treatment. Biological properties related to cell motility, structure, apoptosis, migration, invasion, and metastases changed with cell mechanical properties variation.
DOI
This paper presented a quantitative investigation on the alteration of cell biological functions in relation to the change of mechanical properties of single cells. Leukemia NB4 cells were used as cell line. The dielectrophoresis (DEP) method was utilized to verify the mechanical properties variation of NB4 cells under the electric field treatment. A quantitative study of cell mechanical properties was then carried out using optical tweezers (OT), and cell biological properties using gene expression measurement. The result shows cell stiffness decreased after electric treatment. Biological properties related to cell motility, structure, apoptosis, migration, invasion, and metastases changed with cell mechanical properties variation.
DOI
In situ pH measurements of individual levitated microdroplets using aerosol optical tweezers
Hallie C. Boyer, Kyle Gorkowski, Ryan Christopher Sullivan
The pH of microscale reaction environments controls numerous physicochemical processes, requiring a real-time pH micro-probe. We present highly accurate real-time pH measurements of microdroplets using aerosol optical tweezers (AOT) and analysis of the Whispering Gallery Modes (WGMs) contained in the cavity-enhanced Raman spectra. Uncertainties ranging from ±0.03 to 0.06 in pH for picoliter droplets are obtained through averaging Raman frames acquired at 0.5 Hz over 3.3 minutes. The high accuracy in pH determination is achieved by combining two independent measurements uniquely provided by the AOT approach: the anion concentration ratio from the spontaneous Raman spectra, and the total solute concentration from the refractive index retrieved from WGM analysis of the stimulated cavity-enhanced Raman spectra. pH can be determined over a range of -0.36 to 0.76 using the aqueous sodium bisulfate system. This technique enables direct measurements of pH-dependent chemical and physical changes experienced by individual microparticles, and exploration of the role of pH in the chemical behavior of confined microenvironments.
DOI
The pH of microscale reaction environments controls numerous physicochemical processes, requiring a real-time pH micro-probe. We present highly accurate real-time pH measurements of microdroplets using aerosol optical tweezers (AOT) and analysis of the Whispering Gallery Modes (WGMs) contained in the cavity-enhanced Raman spectra. Uncertainties ranging from ±0.03 to 0.06 in pH for picoliter droplets are obtained through averaging Raman frames acquired at 0.5 Hz over 3.3 minutes. The high accuracy in pH determination is achieved by combining two independent measurements uniquely provided by the AOT approach: the anion concentration ratio from the spontaneous Raman spectra, and the total solute concentration from the refractive index retrieved from WGM analysis of the stimulated cavity-enhanced Raman spectra. pH can be determined over a range of -0.36 to 0.76 using the aqueous sodium bisulfate system. This technique enables direct measurements of pH-dependent chemical and physical changes experienced by individual microparticles, and exploration of the role of pH in the chemical behavior of confined microenvironments.
DOI
Monday, December 2, 2019
Nanobore fiber focus trap with enhanced tuning capabilities
Malte Plidschun, Stefan Weidlich, Martin Šiler, Karina Weber, Tomáš Čižmár, and Markus A. Schmidt
Confinement in fiber traps with two optical fibers facing one another relies on balancing the optical forces originating from the interaction of a scattering micro-object with the light beams delivered through the fibers. Here we demonstrate a novel type of dual fiber trap that involves the use of nanobore fibers, having a nano-channel located in the center of their fiber cores. This nano-element leads to a profound redistribution of the optical intensity and to considerably higher field gradients, yielding a trapping potential with greatly improved tuning properties compared to standard step-index fiber types. We evaluate the trap performance as a function of the fiber separation and find substantially higher stiffness for the nanobore fiber trap, especially in the range of short inter-fiber separations, while intermediate distances exhibit axial stiffness below that of the standard fiber. The results are in agreement with theoretical predictions and reveal that the exploitation of nanobore fibers allows for combinations of transverse and axial stiffness that cannot be accessed with common step-index fibers.
DOI
Confinement in fiber traps with two optical fibers facing one another relies on balancing the optical forces originating from the interaction of a scattering micro-object with the light beams delivered through the fibers. Here we demonstrate a novel type of dual fiber trap that involves the use of nanobore fibers, having a nano-channel located in the center of their fiber cores. This nano-element leads to a profound redistribution of the optical intensity and to considerably higher field gradients, yielding a trapping potential with greatly improved tuning properties compared to standard step-index fiber types. We evaluate the trap performance as a function of the fiber separation and find substantially higher stiffness for the nanobore fiber trap, especially in the range of short inter-fiber separations, while intermediate distances exhibit axial stiffness below that of the standard fiber. The results are in agreement with theoretical predictions and reveal that the exploitation of nanobore fibers allows for combinations of transverse and axial stiffness that cannot be accessed with common step-index fibers.
DOI
Chemo-treated 4T1 breast cancer cells radiation response measured by single and multiple cell ionization using infrared laser trap
Endris Muhammed, Li Chen, Ying Gao & Daniel Erenso
We present a study that uses a laser trapping technique for measurement of radiation sensitivity of untreated and chemo-treated cancer cells. We used a human mammary tumor cell line (4T1) treated by an antitumor compound, 2-Dodecyl-6-methoxycyclohexa-2, 5-diene-1,4-dione (DMDD), which was extracted from the root of Averrhoa carambola L. The untreated control group, and both 2-hour and 24-hour treated groups of 4T1 cells were used in this study. The absorbed threshold ionization energy (TIE) and the threshold radiation dose (TRD) were determined using a high-power infrared laser (at 1064 nm) trap by single and multiple cells trapping and ionization. The results were analyzed using descriptive and t-statistics. The relation of the TIE and TRD to the mass of the individual cells were also analyzed for different hours of treatment in comparison with the control group. Both TIE and TRD decrease with increasing treatment periods. However, the TRD decreases with mass regardless of the treatment. Analyses of the TRD for single vs multiple cells ionizations within each group have also consistently showed this same behavior regardless of the treatment. The underlying factors for these observed relations are explained in terms of radiation, hyperthermia, and chemo effects.
DOI
We present a study that uses a laser trapping technique for measurement of radiation sensitivity of untreated and chemo-treated cancer cells. We used a human mammary tumor cell line (4T1) treated by an antitumor compound, 2-Dodecyl-6-methoxycyclohexa-2, 5-diene-1,4-dione (DMDD), which was extracted from the root of Averrhoa carambola L. The untreated control group, and both 2-hour and 24-hour treated groups of 4T1 cells were used in this study. The absorbed threshold ionization energy (TIE) and the threshold radiation dose (TRD) were determined using a high-power infrared laser (at 1064 nm) trap by single and multiple cells trapping and ionization. The results were analyzed using descriptive and t-statistics. The relation of the TIE and TRD to the mass of the individual cells were also analyzed for different hours of treatment in comparison with the control group. Both TIE and TRD decrease with increasing treatment periods. However, the TRD decreases with mass regardless of the treatment. Analyses of the TRD for single vs multiple cells ionizations within each group have also consistently showed this same behavior regardless of the treatment. The underlying factors for these observed relations are explained in terms of radiation, hyperthermia, and chemo effects.
DOI
Heating in Nanophotonic Traps for Cold Atoms
Daniel Hümmer, Philipp Schneeweiss, Arno Rauschenbeutel, and Oriol Romero-Isart
Laser-cooled atoms that are trapped and optically interfaced with light in nanophotonic waveguides are a powerful platform for fundamental research in quantum optics as well as for applications in quantum communication and quantum-information processing. Ever since the first realization of such a hybrid quantum-nanophotonic system about a decade ago, heating rates of the atomic motion observed in various experimental settings have typically been exceeding those in comparable free-space optical microtraps by about 3 orders of magnitude. This excessive heating is a roadblock for the implementation of certain protocols and devices. Still, its origin has so far remained elusive and, at the typical atom-surface separations of less than an optical wavelength encountered in nanophotonic traps, numerous effects may potentially contribute to atom heating. Here, we theoretically describe the effect of mechanical vibrations of waveguides on guided light fields and provide a general theory of particle-phonon interaction in nanophotonic traps. We test our theory by applying it to the case of laser-cooled cesium atoms in nanofiber-based two-color optical traps. We find excellent quantitative agreement between the predicted heating rates and experimentally measured values. Our theory predicts that, in this setting, the dominant heating process stems from the optomechanical coupling of the optically trapped atoms to the continuum of thermally occupied flexural mechanical modes of the waveguide structure. Surprisingly, the effect of the high-Q mechanical resonances which have previously been observed in this system can be neglected, even if they coincide with the trap frequencies. Beyond unraveling the long-standing riddle of excessive heating in nanofiber-based atom traps, we also study the dependence of the heating rates on the relevant system parameters and find a strong R−5/2 scaling with the inverse waveguide radius. Our findings allow us to propose several strategies for minimizing the heating which also provide guidelines for the design of next-generation nanophotonic cold-atom systems. Finally, given that the predicted heating rate is proportional to the mass of the trapped particle, our findings are also highly relevant for optomechanics experiments with dielectric nanoparticles that are optically trapped close to nanophotonic waveguides.
DOI
Laser-cooled atoms that are trapped and optically interfaced with light in nanophotonic waveguides are a powerful platform for fundamental research in quantum optics as well as for applications in quantum communication and quantum-information processing. Ever since the first realization of such a hybrid quantum-nanophotonic system about a decade ago, heating rates of the atomic motion observed in various experimental settings have typically been exceeding those in comparable free-space optical microtraps by about 3 orders of magnitude. This excessive heating is a roadblock for the implementation of certain protocols and devices. Still, its origin has so far remained elusive and, at the typical atom-surface separations of less than an optical wavelength encountered in nanophotonic traps, numerous effects may potentially contribute to atom heating. Here, we theoretically describe the effect of mechanical vibrations of waveguides on guided light fields and provide a general theory of particle-phonon interaction in nanophotonic traps. We test our theory by applying it to the case of laser-cooled cesium atoms in nanofiber-based two-color optical traps. We find excellent quantitative agreement between the predicted heating rates and experimentally measured values. Our theory predicts that, in this setting, the dominant heating process stems from the optomechanical coupling of the optically trapped atoms to the continuum of thermally occupied flexural mechanical modes of the waveguide structure. Surprisingly, the effect of the high-Q mechanical resonances which have previously been observed in this system can be neglected, even if they coincide with the trap frequencies. Beyond unraveling the long-standing riddle of excessive heating in nanofiber-based atom traps, we also study the dependence of the heating rates on the relevant system parameters and find a strong R−5/2 scaling with the inverse waveguide radius. Our findings allow us to propose several strategies for minimizing the heating which also provide guidelines for the design of next-generation nanophotonic cold-atom systems. Finally, given that the predicted heating rate is proportional to the mass of the trapped particle, our findings are also highly relevant for optomechanics experiments with dielectric nanoparticles that are optically trapped close to nanophotonic waveguides.
DOI
Isolating live cells after high-throughput, long-term, time-lapse microscopy
Scott Luro, Laurent Potvin-Trottier, Burak Okumus & Johan Paulsson
Single-cell genetic screens can be incredibly powerful, but current high-throughput platforms do not track dynamic processes, and even for non-dynamic properties they struggle to separate mutants of interest from phenotypic outliers of the wild-type population. Here we introduce SIFT, single-cell isolation following time-lapse imaging, to address these limitations. After imaging and tracking individual bacteria for tens of consecutive generations under tightly controlled growth conditions, cells of interest are isolated and propagated for downstream analysis, free of contamination and without genetic or physiological perturbations. This platform can characterize tens of thousands of cell lineages per day, making it possible to accurately screen complex phenotypes without the need for barcoding or genetic modifications. We applied SIFT to identify a set of ultraprecise synthetic gene oscillators, with circuit variants spanning a 30-fold range of average periods. This revealed novel design principles in synthetic biology and demonstrated the power of SIFT to reliably screen diverse dynamic phenotypes.
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