Diego Romero R. Abujetas, José A. Sánchez-Gil
Spin and orbital angular momenta of light have been a subject of fundamental interest since long ago, classically associated to circular polarization and wave vector. In recent years, extraordinary spin angular momenta in structured electromagnetic waves have been investigated, mostly in sub-wavelength evanescent fields at the nanoscale. Here we present an in-depth theoretical analysis of the transverse spin density and related momentum induced by mode confinement inside waveguides, with alternating spin layers governed by guided mode spatial symmetry, different from and indeed richer than that in the evanescent region outside. Furthermore, hybrid guided modes with intrinsic helicity exhibit in addition longitudinal spin density. Such fundamental features are manifested through fascinating phenomenology relevant to spin-orbit coupling in nanophotonic waveguides. Thus guided light intrinsically carrying a wealth of spin momenta hold promise of superb devices to control spin-orbit interaction within confined geometries throughout the electromagnetic spectra.
DOI
Friday, January 31, 2020
Optomechanical resonating probe for very high frequency sensing of atomic forces
Pierre Etienne Allain, Lucien Schwab, Colin Mismer, Marc Gely, Estelle Mairiaux, Maxime Hermouet, Benjamin Walter, Giuseppe Leo, Sébastien Hentz, Marc Faucher, Guillaume Jourdan, Bernard Legrand and Ivan Favero
Atomic force spectroscopy and microscopy are invaluable tools to characterize nanostructures and biological systems. State-of-the-art experiments use resonant driving of mechanical probes, whose frequency reaches MHz in the fastest commercial instruments where cantilevers are driven at nanometer amplitude. Stiffer probes oscillating at tens of picometers provide a better access to short-range interactions, yielding images of molecular bonds, but they are little amenable to high-speed operation. Next-generation investigations demand combining very high frequency (>100 MHz) with deep sub-nanometer oscillation amplitude, in order to access faster (below microsecond) phenomena with molecular resolution. Here we introduce a resonating optomechanical atomic force probe operated fully optically at a frequency of 117 MHz, two decades above cantilevers, with a Brownian motion amplitude four orders below. Based on Silicon-On-Insulator technology, the very high frequency probe demonstrates single-pixel sensing of contact and non-contact interactions with sub-picometer amplitude, breaking open current limitations for faster and finer force spectroscopy.
DOI
Atomic force spectroscopy and microscopy are invaluable tools to characterize nanostructures and biological systems. State-of-the-art experiments use resonant driving of mechanical probes, whose frequency reaches MHz in the fastest commercial instruments where cantilevers are driven at nanometer amplitude. Stiffer probes oscillating at tens of picometers provide a better access to short-range interactions, yielding images of molecular bonds, but they are little amenable to high-speed operation. Next-generation investigations demand combining very high frequency (>100 MHz) with deep sub-nanometer oscillation amplitude, in order to access faster (below microsecond) phenomena with molecular resolution. Here we introduce a resonating optomechanical atomic force probe operated fully optically at a frequency of 117 MHz, two decades above cantilevers, with a Brownian motion amplitude four orders below. Based on Silicon-On-Insulator technology, the very high frequency probe demonstrates single-pixel sensing of contact and non-contact interactions with sub-picometer amplitude, breaking open current limitations for faster and finer force spectroscopy.
DOI
Reversible optical binding force in a plasmonic heterodimer under radially polarized beam illumination
Fajun Xiao, Jiachen Zhang, Weixing Yu, Weiren Zhu, Ting Mei, Malin Premaratne, and Jianlin Zhao
We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.
DOI
We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.
DOI
Mechanical Characterization of Vesicles and Cells: A Review
Adnan Morshed Buddini Iroshika Karawdeniya Y.M. Nuwan D.Y. Bandara Min Jun Kim Prashanta Dutta
Vesicles perform many essential functions in all living organisms. They respond like a transducer to mechanical stress in converting the applied force into mechanical and biological responses. At the same time, both biochemical and biophysical signals influence the vesicular response in bearing mechanical loads. In recent years, liposomes, artificial lipid vesicles, have gained substantial attention from the pharmaceutical industry as a prospective drug carrier which can also serve as an artificial cell‐mimetic system. The ability of these vesicles to enter through pores of even smaller size makes them ideal candidates for therapeutic agents to reach the infected sites effectively. Engineering of vesicles with desired mechanical properties that can encapsulate drugs and release as required is the prime challenge in this field. This requirement has led to the modifications of the composition of the bilayer membrane by adding cholesterol, sphingomyelin, etc. In this paper, we review the manufacturing and characterization techniques of various artificial/synthetic vesicles. We particularly focus on the electric field‐driven characterization techniques to determine different properties of vesicle and its membranes such as bending rigidity, viscosity, capacitance, conductance, etc., which are indicators of their content and mobility. Similarities and differences between artificial vesicles, natural vesicles, and cells are highlighted throughout the manuscript since most of these artificial vesicles are intended for cell mimetic functions.
DOI
Vesicles perform many essential functions in all living organisms. They respond like a transducer to mechanical stress in converting the applied force into mechanical and biological responses. At the same time, both biochemical and biophysical signals influence the vesicular response in bearing mechanical loads. In recent years, liposomes, artificial lipid vesicles, have gained substantial attention from the pharmaceutical industry as a prospective drug carrier which can also serve as an artificial cell‐mimetic system. The ability of these vesicles to enter through pores of even smaller size makes them ideal candidates for therapeutic agents to reach the infected sites effectively. Engineering of vesicles with desired mechanical properties that can encapsulate drugs and release as required is the prime challenge in this field. This requirement has led to the modifications of the composition of the bilayer membrane by adding cholesterol, sphingomyelin, etc. In this paper, we review the manufacturing and characterization techniques of various artificial/synthetic vesicles. We particularly focus on the electric field‐driven characterization techniques to determine different properties of vesicle and its membranes such as bending rigidity, viscosity, capacitance, conductance, etc., which are indicators of their content and mobility. Similarities and differences between artificial vesicles, natural vesicles, and cells are highlighted throughout the manuscript since most of these artificial vesicles are intended for cell mimetic functions.
DOI
Optical radiation force circular dichroism spectroscopy
F.G. Mitri
This work introduces the method of circular dichroism spectroscopy in the framework of the electromagnetic/optical radiation force theory. This analytical tool is defined here as the difference in radiation force of left-handed and right-handed circularly polarized electromagnetic waves illuminating an object exhibiting rotary polarization. The example of a lossless material, such as the perfect electromagnetic conductor (PEMC) cylinder having a circular geometric cross-section, is considered. The modal expansion method in cylindrical coordinates is used to obtain exact mathematical series expansions for the longitudinal radiation force per-length (i.e. acting along the direction of wave propagation) considering left-handed and right-handed circularly polarized cylindrically diverging waves emanating from a line source. The case of plane progressive waves is recovered when the source is located far from the cylinder. Numerical illustrative results for the dimensionless radiation force functions as well as the scattering, extinction and absorption energy efficiencies and their co-polarized and cross-polarized components are performed with particular emphasis on the size parameter of the cylinder, the dimensionless distance parameter from the line source, and the admittance parameter of the cylinder. The results reveal that the individual radiation force functions for left-handed and right-handed circularly polarized waves can be negative, zero, or positive depending on the cylinder distance from the source. Moreover, the optical radiation force circular dichroism (ORFCD) and the extinction energy efficiency circular dichroism (EEECD) are positive for a negative admittance of the cylinder, while they reverse sign for a positive admittance. While the EEECD shows some form of symmetry versus admittance sign change, the ORFCD does not. The possibility of achieving invisibility cloaking for a small PEMC cylinder is also investigated. The present ORFCD spectroscopy method is applicable to any cylinder material exhibiting rotary polarization such as chiral, topological insulator, plasma, liquid crystal etc.
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Dual optical trap created by tightly focused circularly polarized ring Airy beam
Zaili Chen, Yunfeng Jiang
The dual focus property of focused circularly polarized ring Airy beam (RAB) under the action of tightly focused lens is demonstrated in this paper. The radiation forces at two foci of tightly focused RAB are calculated, the numerical results show that the particle could be longitudinally and transversely trapped at the two foci. By varying corresponding parameters, we could control the property of two traps. The trapping force increases with NA and the scaling parameter w; and an appropriate initial radius r0 is necessary for the enhancement of either trap. The two traps could move closer as w increases or r0 decreases. To realize the dual optical trap, we should choose a smaller decaying parameter a and a larger NA, or the dual optical trap would degenerate into a single optical trap. Moreover, because of the influence of the Brownian motion and the scattering force, the size of the particle should be in a special range.
DOI
The dual focus property of focused circularly polarized ring Airy beam (RAB) under the action of tightly focused lens is demonstrated in this paper. The radiation forces at two foci of tightly focused RAB are calculated, the numerical results show that the particle could be longitudinally and transversely trapped at the two foci. By varying corresponding parameters, we could control the property of two traps. The trapping force increases with NA and the scaling parameter w; and an appropriate initial radius r0 is necessary for the enhancement of either trap. The two traps could move closer as w increases or r0 decreases. To realize the dual optical trap, we should choose a smaller decaying parameter a and a larger NA, or the dual optical trap would degenerate into a single optical trap. Moreover, because of the influence of the Brownian motion and the scattering force, the size of the particle should be in a special range.
DOI
Thursday, January 30, 2020
Optical trapping reveals differences in dielectric and optical properties of copper nanoparticles compared to their oxides and ferrites
Pablo Purohit, Akbar Samadi, Poul Martin Bendix, J. Javier Laserna & Lene B. Oddershede
In a nanoplasmonic context, copper (Cu) is a potential and interesting surrogate to less accessible metals such as gold, silver or platinum. We demonstrate optical trapping of individual Cu nanoparticles with diameters between 25 and 70 nm and of two ionic Cu nanoparticle species, CuFe2O4 and CuZnFe2O4, with diameters of 90 nm using a near infrared laser and quantify their interaction with the electromagnetic field experimentally and theoretically. We find that, despite the similarity in size, the trapping stiffness and polarizability of the ferrites are significantly lower than those of Cu nanoparticles, thus inferring a different light-particle interaction. One challenge with using Cu nanoparticles in practice is that upon exposure to the normal atmosphere, Cu is spontaneously passivated by an oxide layer, thus altering its physicochemical properties. We theoretically investigate how the presence of an oxide layer influences the optical properties of Cu nanoparticles. Comparisons to experimental observations infer that oxidation of CuNPs is minimal during optical trapping. By finite element modelling we map out the expected temperature increase of the plasmonic Cu nanoparticles during optical trapping and retrieve temperature increases high enough to change the catalytic properties of the particles.
DOI
In a nanoplasmonic context, copper (Cu) is a potential and interesting surrogate to less accessible metals such as gold, silver or platinum. We demonstrate optical trapping of individual Cu nanoparticles with diameters between 25 and 70 nm and of two ionic Cu nanoparticle species, CuFe2O4 and CuZnFe2O4, with diameters of 90 nm using a near infrared laser and quantify their interaction with the electromagnetic field experimentally and theoretically. We find that, despite the similarity in size, the trapping stiffness and polarizability of the ferrites are significantly lower than those of Cu nanoparticles, thus inferring a different light-particle interaction. One challenge with using Cu nanoparticles in practice is that upon exposure to the normal atmosphere, Cu is spontaneously passivated by an oxide layer, thus altering its physicochemical properties. We theoretically investigate how the presence of an oxide layer influences the optical properties of Cu nanoparticles. Comparisons to experimental observations infer that oxidation of CuNPs is minimal during optical trapping. By finite element modelling we map out the expected temperature increase of the plasmonic Cu nanoparticles during optical trapping and retrieve temperature increases high enough to change the catalytic properties of the particles.
DOI
Supercoiling DNA optically
Graeme A. King, Federica Burla, Erwin J. G. Peterman, and Gijs J. L. Wuite
Cellular DNA is regularly subject to torsional stress during genomic processes, such as transcription and replication, resulting in a range of supercoiled DNA structures. For this reason, methods to prepare and study supercoiled DNA at the single-molecule level are widely used, including magnetic, angular-optical, micropipette, and magneto-optical tweezers. However, it is currently challenging to combine DNA supercoiling control with spatial manipulation and fluorescence microscopy. This limits the ability to study complex and dynamic interactions of supercoiled DNA. Here we present a single-molecule assay that can rapidly and controllably generate negatively supercoiled DNA using a standard dual-trap optical tweezers instrument. This method, termed Optical DNA Supercoiling (ODS), uniquely combines the ability to study supercoiled DNA using force spectroscopy, fluorescence imaging of the whole DNA, and rapid buffer exchange. The technique can be used to generate a wide range of supercoiled states, with between <5 and 70% lower helical twist than nonsupercoiled DNA. Highlighting the versatility of ODS, we reveal previously unobserved effects of ionic strength and sequence on the structural state of underwound DNA. Next, we demonstrate that ODS can be used to directly visualize and quantify protein dynamics on supercoiled DNA. We show that the diffusion of the mitochondrial transcription factor TFAM can be significantly hindered by local regions of underwound DNA. This finding suggests a mechanism by which supercoiling could regulate mitochondrial transcription in vivo. Taken together, we propose that ODS represents a powerful method to study both the biophysical properties and biological interactions of negatively supercoiled DNA.
DOI
Cellular DNA is regularly subject to torsional stress during genomic processes, such as transcription and replication, resulting in a range of supercoiled DNA structures. For this reason, methods to prepare and study supercoiled DNA at the single-molecule level are widely used, including magnetic, angular-optical, micropipette, and magneto-optical tweezers. However, it is currently challenging to combine DNA supercoiling control with spatial manipulation and fluorescence microscopy. This limits the ability to study complex and dynamic interactions of supercoiled DNA. Here we present a single-molecule assay that can rapidly and controllably generate negatively supercoiled DNA using a standard dual-trap optical tweezers instrument. This method, termed Optical DNA Supercoiling (ODS), uniquely combines the ability to study supercoiled DNA using force spectroscopy, fluorescence imaging of the whole DNA, and rapid buffer exchange. The technique can be used to generate a wide range of supercoiled states, with between <5 and 70% lower helical twist than nonsupercoiled DNA. Highlighting the versatility of ODS, we reveal previously unobserved effects of ionic strength and sequence on the structural state of underwound DNA. Next, we demonstrate that ODS can be used to directly visualize and quantify protein dynamics on supercoiled DNA. We show that the diffusion of the mitochondrial transcription factor TFAM can be significantly hindered by local regions of underwound DNA. This finding suggests a mechanism by which supercoiling could regulate mitochondrial transcription in vivo. Taken together, we propose that ODS represents a powerful method to study both the biophysical properties and biological interactions of negatively supercoiled DNA.
DOI
Modulation of Kinesin’s Load-Bearing Capacity by Force Geometry and the Microtubule Track
Serapion Pyrpassopoulos, Henry Shuman, E. Michael Ostap
Kinesin motors and their associated microtubule tracks are essential for long-distance transport of cellular cargos. Intracellular activity and proper recruitment of kinesins is regulated by biochemical signaling, cargo adaptors, microtubule-associated proteins, and mechanical forces. In this study, we found that the effect of opposing forces on the kinesin-microtubule attachment duration depends strongly on experimental assay geometry. Using optical tweezers and the conventional single-bead assay, we show that detachment of kinesin from the microtubule is likely accelerated by forces vertical to the long axis of the microtubule due to contact of the single bead with the underlying microtubule. We used the three-bead assay to minimize the vertical force component and found that when the opposing forces are mainly parallel to the microtubule, the median value of attachment durations between kinesin and microtubules can be up to 10-fold longer than observed using the single-bead assay. Using the three-bead assay, we also found that not all microtubule protofilaments are equivalent interacting substrates for kinesin and that the median value of attachment durations of kinesin varies by more than 10-fold, depending on the relative angular position of the forces along the circumference of the microtubule. Thus, depending on the geometry of forces across the microtubule, kinesin can switch from a fast detaching motor (median attachment duration <0.2 s) to a persistent motor that sustains attachment (median attachment duration >3 s) at high forces (5 pN). Our data show that the load-bearing capacity of the kinesin motor is highly variable and can be dramatically affected by off-axis forces and forces across the microtubule lattice, which has implications for a range of cellular activities, including cell division and organelle transport.
DOI
Kinesin motors and their associated microtubule tracks are essential for long-distance transport of cellular cargos. Intracellular activity and proper recruitment of kinesins is regulated by biochemical signaling, cargo adaptors, microtubule-associated proteins, and mechanical forces. In this study, we found that the effect of opposing forces on the kinesin-microtubule attachment duration depends strongly on experimental assay geometry. Using optical tweezers and the conventional single-bead assay, we show that detachment of kinesin from the microtubule is likely accelerated by forces vertical to the long axis of the microtubule due to contact of the single bead with the underlying microtubule. We used the three-bead assay to minimize the vertical force component and found that when the opposing forces are mainly parallel to the microtubule, the median value of attachment durations between kinesin and microtubules can be up to 10-fold longer than observed using the single-bead assay. Using the three-bead assay, we also found that not all microtubule protofilaments are equivalent interacting substrates for kinesin and that the median value of attachment durations of kinesin varies by more than 10-fold, depending on the relative angular position of the forces along the circumference of the microtubule. Thus, depending on the geometry of forces across the microtubule, kinesin can switch from a fast detaching motor (median attachment duration <0.2 s) to a persistent motor that sustains attachment (median attachment duration >3 s) at high forces (5 pN). Our data show that the load-bearing capacity of the kinesin motor is highly variable and can be dramatically affected by off-axis forces and forces across the microtubule lattice, which has implications for a range of cellular activities, including cell division and organelle transport.
DOI
Single Photons Emitted by Nanocrystals Optically Trapped in a Deep Parabolic Mirror
Vsevolod Salakhutdinov, Markus Sondermann, Luigi Carbone, Elisabeth Giacobino, Alberto Bramati, and Gerd Leuchs
We investigate the emission of single photons from CdSe/CdS dots-in-rod which are optically trapped in the focus of a deep parabolic mirror. Thanks to this mirror, we are able to image almost the full 4π emission pattern of nanometer-sized elementary dipoles and verify the alignment of the rods within the optical trap. From the motional dynamics of the emitters in the trap, we infer that the single-photon emission occurs from clusters comprising several emitters. We demonstrate the optical trapping of rod-shaped quantum emitters in a configuration suitable for efficiently coupling an ensemble of linear dipoles with the electromagnetic field in free space.
DOI
We investigate the emission of single photons from CdSe/CdS dots-in-rod which are optically trapped in the focus of a deep parabolic mirror. Thanks to this mirror, we are able to image almost the full 4π emission pattern of nanometer-sized elementary dipoles and verify the alignment of the rods within the optical trap. From the motional dynamics of the emitters in the trap, we infer that the single-photon emission occurs from clusters comprising several emitters. We demonstrate the optical trapping of rod-shaped quantum emitters in a configuration suitable for efficiently coupling an ensemble of linear dipoles with the electromagnetic field in free space.
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A Model for DNA Interactions with Proteins and Other Large Ligands: Extracting Physical Chemistry from Pure Mechanical Measurements
Patricia S. Alves, Oscar N. Mesquita, Marcio Santos Rocha
We present a new model to describe DNA interactions with large ligands such as proteins, based on a quenched-disorder equation for ligand binding along the double-helix and on Manning's description for the local changes of the persistence length at the binding sites. Such model allows one to extract the physical chemistry of the interactions from pure mechanical measurements, such as those typically performed with the DNA-protein complexes in force spectroscopy assays. We have tested the proposed methodology here to investigate the DNA interaction with the protein lysozyme, determining binding parameters such as the equilibrium association constant, the cooperativity degree of the binding reaction, and the fraction of neutralized charges on the phosphate backbone. The model also allows one to get information on the size and positional conformation of the bound proteins.
DOI
We present a new model to describe DNA interactions with large ligands such as proteins, based on a quenched-disorder equation for ligand binding along the double-helix and on Manning's description for the local changes of the persistence length at the binding sites. Such model allows one to extract the physical chemistry of the interactions from pure mechanical measurements, such as those typically performed with the DNA-protein complexes in force spectroscopy assays. We have tested the proposed methodology here to investigate the DNA interaction with the protein lysozyme, determining binding parameters such as the equilibrium association constant, the cooperativity degree of the binding reaction, and the fraction of neutralized charges on the phosphate backbone. The model also allows one to get information on the size and positional conformation of the bound proteins.
DOI
Tuesday, January 28, 2020
Red blood cells under varying extracellular tonicity conditions: an optical tweezers combined with micro-Raman study
Jijo Lukose, Shamee Shastry, Mithun Nelliat, Ganesh Mohan, Akheel Ahmed and C Santhosh
Extracellular tonicity has a significant influence on human red blood cell deformation capability. Advancements in the area of laser physics and optical trapping have opened up a plethora of applications for understanding cell structure and dynamics. Here, Raman Tweezers technique was employed to investigate the impact of extracellular tonicity by exposing human red blood cells to both hypertonic and hypotonic intravenous fluids. Heme aggregation was observed in hypertonic saline solution, accompanied with damage in membrane protein. Loss of intracellular hemoglobin in hypotonic solution was evident from the decrease in porphyrin breathing mode present at 752 cm−1. Oxygen binding to the central iron in the red blood cell heme was also affected under both hyper/hypo tonicity conditions. Morphological deviation of discocytes to echinocytes/spherocytes were also evident from quantitative phase imaging. Principal component analysis have showed clear differentiation of samples in order to classify the control erythrocytes and the tonicity stressed erythrocytes. Present study has also demonstrated the application of Raman Tweezers spectroscopy as a potential tool for probing red blood cell under different stress conditions.
DOI
Extracellular tonicity has a significant influence on human red blood cell deformation capability. Advancements in the area of laser physics and optical trapping have opened up a plethora of applications for understanding cell structure and dynamics. Here, Raman Tweezers technique was employed to investigate the impact of extracellular tonicity by exposing human red blood cells to both hypertonic and hypotonic intravenous fluids. Heme aggregation was observed in hypertonic saline solution, accompanied with damage in membrane protein. Loss of intracellular hemoglobin in hypotonic solution was evident from the decrease in porphyrin breathing mode present at 752 cm−1. Oxygen binding to the central iron in the red blood cell heme was also affected under both hyper/hypo tonicity conditions. Morphological deviation of discocytes to echinocytes/spherocytes were also evident from quantitative phase imaging. Principal component analysis have showed clear differentiation of samples in order to classify the control erythrocytes and the tonicity stressed erythrocytes. Present study has also demonstrated the application of Raman Tweezers spectroscopy as a potential tool for probing red blood cell under different stress conditions.
DOI
Active diffusion in oocytes nonspecifically centers large objects during prophase I and meiosis I
Alexandra Colin, Gaëlle Letort, Nitzan Razin, Maria Almonacid, Wylie Ahmed, Timo Betz, Marie-Emilie Terret, Nir S. Gov, Raphaël Voituriez, Zoher Gueroui, Marie-Hélène Verlhac
Nucleus centering in mouse oocytes results from a gradient of actin-positive vesicle activity and is essential for developmental success. Here, we analyze 3D model simulations to demonstrate how a gradient in the persistence of actin-positive vesicles can center objects of different sizes. We test model predictions by tracking the transport of exogenous passive tracers. The gradient of activity induces a centering force, akin to an effective pressure gradient, leading to the centering of oil droplets with velocities comparable to nuclear ones. Simulations and experimental measurements show that passive particles subjected to the gradient exhibit biased diffusion toward the center. Strikingly, we observe that the centering mechanism is maintained in meiosis I despite chromosome movement in the opposite direction; thus, it can counteract a process that specifically off-centers the spindle. In conclusion, our findings reconcile how common molecular players can participate in the two opposing functions of chromosome centering versus off-centering.
DOI
Nucleus centering in mouse oocytes results from a gradient of actin-positive vesicle activity and is essential for developmental success. Here, we analyze 3D model simulations to demonstrate how a gradient in the persistence of actin-positive vesicles can center objects of different sizes. We test model predictions by tracking the transport of exogenous passive tracers. The gradient of activity induces a centering force, akin to an effective pressure gradient, leading to the centering of oil droplets with velocities comparable to nuclear ones. Simulations and experimental measurements show that passive particles subjected to the gradient exhibit biased diffusion toward the center. Strikingly, we observe that the centering mechanism is maintained in meiosis I despite chromosome movement in the opposite direction; thus, it can counteract a process that specifically off-centers the spindle. In conclusion, our findings reconcile how common molecular players can participate in the two opposing functions of chromosome centering versus off-centering.
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Rotational motion and rheotaxis of human sperm do not require functional CatSper channels and transmembrane Ca2+ signaling
Christian Schiffer, Steffen Rieger, Christoph Brenker, Samuel Young, Hussein Hamzeh, Dagmar Wachten, Frank Tüttelmann, Albrecht Röpke, U Benjamin Kaupp, Tao Wang, Alice Wagner, Claudia Krallmann, Sabine Kliesch, Carsten Fallnich, Timo Strünker
Navigation of sperm in fluid flow, called rheotaxis, provides long‐range guidance in the mammalian oviduct. The rotation of sperm around their longitudinal axis (rolling) promotes rheotaxis. Whether sperm rolling and rheotaxis require calcium (Ca2+) influx via the sperm‐specific Ca2+ channel CatSper, or rather represent passive biomechanical and hydrodynamic processes, has remained controversial. Here, we study the swimming behavior of sperm from healthy donors and from infertile patients that lack functional CatSper channels, using dark‐field microscopy, optical tweezers, and microfluidics. We demonstrate that rolling and rheotaxis persist in CatSper‐deficient human sperm. Furthermore, human sperm undergo rolling and rheotaxis even when Ca2+ influx is prevented. Finally, we show that rolling and rheotaxis also persist in mouse sperm deficient in both CatSper and flagellar Ca2+‐signaling domains. Our results strongly support the concept that passive biomechanical and hydrodynamic processes enable sperm rolling and rheotaxis, rather than calcium signaling mediated by CatSper or other mechanisms controlling transmembrane Ca2+ flux.
DOI
Navigation of sperm in fluid flow, called rheotaxis, provides long‐range guidance in the mammalian oviduct. The rotation of sperm around their longitudinal axis (rolling) promotes rheotaxis. Whether sperm rolling and rheotaxis require calcium (Ca2+) influx via the sperm‐specific Ca2+ channel CatSper, or rather represent passive biomechanical and hydrodynamic processes, has remained controversial. Here, we study the swimming behavior of sperm from healthy donors and from infertile patients that lack functional CatSper channels, using dark‐field microscopy, optical tweezers, and microfluidics. We demonstrate that rolling and rheotaxis persist in CatSper‐deficient human sperm. Furthermore, human sperm undergo rolling and rheotaxis even when Ca2+ influx is prevented. Finally, we show that rolling and rheotaxis also persist in mouse sperm deficient in both CatSper and flagellar Ca2+‐signaling domains. Our results strongly support the concept that passive biomechanical and hydrodynamic processes enable sperm rolling and rheotaxis, rather than calcium signaling mediated by CatSper or other mechanisms controlling transmembrane Ca2+ flux.
DOI
Dynein harnesses active fluctuations of microtubules for faster movement
Yasin Ezber, Vladislav Belyy, Sinan Can & Ahmet Yildiz
The cytoskeleton forms a dynamic network that generates fluctuations larger than thermal agitation of the cytoplasm1. Here, we tested whether dynein, a minus-end-directed microtubule (MT) motor2, can harness energy from these fluctuations using optical trapping in vitro. We show that dynein forms an asymmetric slip bond with MTs, where its detachment rate increases more slowly under hindering forces than assisting forces. This asymmetry enables dynein to generate unidirectional motility towards the minus-end from force fluctuations. Consistent with our model, oscillatory forces exerted by the trap drive dynein stepping without net force and ATP. Dynein is capable of ratcheting towards the minus-end, even when the net force is in the plus-end direction. With ATP, force oscillations increase the velocity and stall force of dynein as it transports cargos and glides MTs. Therefore, dynein is a mechanical ratchet that rectifies cytoskeletal fluctuations to move faster and resists higher forces along MTs.
DOI
The cytoskeleton forms a dynamic network that generates fluctuations larger than thermal agitation of the cytoplasm1. Here, we tested whether dynein, a minus-end-directed microtubule (MT) motor2, can harness energy from these fluctuations using optical trapping in vitro. We show that dynein forms an asymmetric slip bond with MTs, where its detachment rate increases more slowly under hindering forces than assisting forces. This asymmetry enables dynein to generate unidirectional motility towards the minus-end from force fluctuations. Consistent with our model, oscillatory forces exerted by the trap drive dynein stepping without net force and ATP. Dynein is capable of ratcheting towards the minus-end, even when the net force is in the plus-end direction. With ATP, force oscillations increase the velocity and stall force of dynein as it transports cargos and glides MTs. Therefore, dynein is a mechanical ratchet that rectifies cytoskeletal fluctuations to move faster and resists higher forces along MTs.
DOI
High precision detection technology of particle positions in optical trap
Miao Lijun, Yan Jingtao, Hu Huizhu, Ying Guangyao, Huang Tengchao, Che Shuangliang, Shu Xiaowu
Precision sensing and measuring based on optical trap technology is innovation and deepening for application of optical force effect from micro-precise manipulation to precise measurement of physical quantities. Precision measurement of position information of the particle in optical trap is key technology of precise sensing and measuring. The method was put forward, which used digital image processing and curve fitting algorithm to detect particle positions in optical trap. The normalized self-correlation function of particle positions was obtained by digital image correlation, and the quadratic curve fitting was used to the normalized self-correlation function curve by least-square method to realize particle position detection of sub-pixel accuracy. Experiments show that the method described in this paper can effectively suppress the quantization effect of hardware and realize fast and high-precision detection of particle position in optical traps. Compared with the direct correlation method, the detection accuracy can be improved by at least one order of magnitude to 0.03 pixel.
DOI
Precision sensing and measuring based on optical trap technology is innovation and deepening for application of optical force effect from micro-precise manipulation to precise measurement of physical quantities. Precision measurement of position information of the particle in optical trap is key technology of precise sensing and measuring. The method was put forward, which used digital image processing and curve fitting algorithm to detect particle positions in optical trap. The normalized self-correlation function of particle positions was obtained by digital image correlation, and the quadratic curve fitting was used to the normalized self-correlation function curve by least-square method to realize particle position detection of sub-pixel accuracy. Experiments show that the method described in this paper can effectively suppress the quantization effect of hardware and realize fast and high-precision detection of particle position in optical traps. Compared with the direct correlation method, the detection accuracy can be improved by at least one order of magnitude to 0.03 pixel.
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Dual-Balance Electrodynamic Trap as a Microanalytical Tool for Identifying Gel Transitions and Viscous Properties of Levitated Aerosol Particles
David S. Richards, Kristin L. Trobaugh, Josefina Hajek-Herrera, Ryan D. Davis
The formation of gelatinous networks within an aerosol particle significantly alters the physicochemical properties of the aerosol material. Existing techniques for studying gel transitions rely on bulk rheometry, which is limited by contact with the sample, or microrheological techniques such as holographic optical tweezers, which rely on expensive equipment and high-powered lasers that can degrade light-absorbing aerosol. Here, we present a new technique to probe the microrheological characteristics of aerosol particles and explore gel formation under atmospheric conditions in a contactless environment without the need for high-power light sources. In a dual-balance quadrupole electrodynamic balance, levitated droplets of opposite polarity are trapped and equilibrated at fixed relative humidity (RH) and then subsequently merged, and the physical characteristics of the merged droplets are monitored as a function of time and RH using imaging techniques. By comparing the RH-dependent characteristics of MgSO4 (known to undergo a gel transition) to glucose and sucrose (known to remain as viscous Newtonian fluids) under fixed equilibration time scales, we demonstrate that gel phase transitions can be identified in aerosol particles, with MgSO4 abruptly transitioning to a rigid microgel at 30% RH. Further, we demonstrate this technique can be used to also measure aerosol viscosity and identify non-Newtonian fluid dynamics in model sea spray aerosol composed of NaCl, CaCl2, and sorbitol. Thus, using this experimental technique, it is possible to distinguish between aerosol compositions that form viscous Newtonian fluids and those that undergo a gel transition or form non-Newtonian fluids. This technique offers a simple and cost-effective analytical tool for probing gel transitions outside of bulk solubility limits, with relevant applications ranging from atmospheric science to microengineering of soft matter materials.
DOI
The formation of gelatinous networks within an aerosol particle significantly alters the physicochemical properties of the aerosol material. Existing techniques for studying gel transitions rely on bulk rheometry, which is limited by contact with the sample, or microrheological techniques such as holographic optical tweezers, which rely on expensive equipment and high-powered lasers that can degrade light-absorbing aerosol. Here, we present a new technique to probe the microrheological characteristics of aerosol particles and explore gel formation under atmospheric conditions in a contactless environment without the need for high-power light sources. In a dual-balance quadrupole electrodynamic balance, levitated droplets of opposite polarity are trapped and equilibrated at fixed relative humidity (RH) and then subsequently merged, and the physical characteristics of the merged droplets are monitored as a function of time and RH using imaging techniques. By comparing the RH-dependent characteristics of MgSO4 (known to undergo a gel transition) to glucose and sucrose (known to remain as viscous Newtonian fluids) under fixed equilibration time scales, we demonstrate that gel phase transitions can be identified in aerosol particles, with MgSO4 abruptly transitioning to a rigid microgel at 30% RH. Further, we demonstrate this technique can be used to also measure aerosol viscosity and identify non-Newtonian fluid dynamics in model sea spray aerosol composed of NaCl, CaCl2, and sorbitol. Thus, using this experimental technique, it is possible to distinguish between aerosol compositions that form viscous Newtonian fluids and those that undergo a gel transition or form non-Newtonian fluids. This technique offers a simple and cost-effective analytical tool for probing gel transitions outside of bulk solubility limits, with relevant applications ranging from atmospheric science to microengineering of soft matter materials.
DOI
Monday, January 27, 2020
Atom femto trap: experimental realization
Anton E. Afanasiev, Anna A. Meysterson, Anastasiia M. Mashko, Pavel N. Melentiev, Victor I. Balykin
In this work, we demonstrate the trapping of rubidium (Rb) atoms in a pulsed optical dipole trap formed by femtosecond laser radiation with a pulse duration as small as 70 fs. The atom localization in such trap strongly depends on the heating of the atoms caused by the momentum diffusion due to the dipole force fluctuations. The atom femto traps can be used for localization of atoms others than alkaline and alkaline earth atomic elements by conversation of pulsed laser radiation of visible or near infrared to UV spectral.
DOI
In this work, we demonstrate the trapping of rubidium (Rb) atoms in a pulsed optical dipole trap formed by femtosecond laser radiation with a pulse duration as small as 70 fs. The atom localization in such trap strongly depends on the heating of the atoms caused by the momentum diffusion due to the dipole force fluctuations. The atom femto traps can be used for localization of atoms others than alkaline and alkaline earth atomic elements by conversation of pulsed laser radiation of visible or near infrared to UV spectral.
DOI
Influence of Laser Intensity Fluctuation on Single-Cesium Atom Trapping Lifetime in a 1064-nm Microscopic Optical Tweezer
Rui Sun, Xin Wang, Kong Zhang, Jun He and Junmin Wang
An optical tweezer composed of a strongly focused single-spatial-mode Gaussian beam of a red-detuned 1064-nm laser can confine a single-cesium (Cs) atom at the strongest point of the light intensity. We can use this for coherent manipulation of single-quantum bits and single-photon sources. The trapping lifetime of the atoms in the optical tweezers is very short due to the impact of the background atoms, the parametric heating of the optical tweezer and the residual thermal motion of the atoms. In this paper, we analyzed the influence of the background pressure, the trap frequency of optical tweezers and the laser intensity fluctuation of optical tweezers on the atomic trapping lifetime. Combined with the external feedback loop based on an acousto-optical modulator (AOM), the intensity fluctuation of the 1064-nm laser in the time domain was suppressed from ±3.360% to ±0.064%, and the suppression bandwidth in the frequency domain reached approximately 33 kHz. The trapping lifetime of a single-Cs atom in the microscopic optical tweezers was extended from 4.04 s to 6.34 s.
DOI
An optical tweezer composed of a strongly focused single-spatial-mode Gaussian beam of a red-detuned 1064-nm laser can confine a single-cesium (Cs) atom at the strongest point of the light intensity. We can use this for coherent manipulation of single-quantum bits and single-photon sources. The trapping lifetime of the atoms in the optical tweezers is very short due to the impact of the background atoms, the parametric heating of the optical tweezer and the residual thermal motion of the atoms. In this paper, we analyzed the influence of the background pressure, the trap frequency of optical tweezers and the laser intensity fluctuation of optical tweezers on the atomic trapping lifetime. Combined with the external feedback loop based on an acousto-optical modulator (AOM), the intensity fluctuation of the 1064-nm laser in the time domain was suppressed from ±3.360% to ±0.064%, and the suppression bandwidth in the frequency domain reached approximately 33 kHz. The trapping lifetime of a single-Cs atom in the microscopic optical tweezers was extended from 4.04 s to 6.34 s.
DOI
Optical pulling force on nonlinear nanoparticles with gain
Hongli Chen, Lei Gao, Chonggui Zhong, Guoqiu Yuan, Yanyan Huang, Zhongwei Yu, Min Cao, and Meng Wang
We investigate the optical force on the nonlinear nanoparticles with gain based on nonlinear Mie theory and Maxwell’s stress tensor method. For the nonlinear susceptibility χ(3) = 0 (i.e., the linear nanoparticle), the threshold gain to obtain the optical pulling force increases when the permittivity of the surrounding medium εm deviates from the real part of the permittivity of the nanoparticles εcr. For χ(3) > 0, one or two threshold fields exist for the switch of optical pulling and pushing force. However, for χ(3) < 0, only one threshold field is found. Moreover, the optical pulling force may be enhanced by tuning the incident field intensity. Our results for the optical force on the nonlinear nanoparticle will have potential applications in nonlinear optical manipulations and optical transportation.
DOI
We investigate the optical force on the nonlinear nanoparticles with gain based on nonlinear Mie theory and Maxwell’s stress tensor method. For the nonlinear susceptibility χ(3) = 0 (i.e., the linear nanoparticle), the threshold gain to obtain the optical pulling force increases when the permittivity of the surrounding medium εm deviates from the real part of the permittivity of the nanoparticles εcr. For χ(3) > 0, one or two threshold fields exist for the switch of optical pulling and pushing force. However, for χ(3) < 0, only one threshold field is found. Moreover, the optical pulling force may be enhanced by tuning the incident field intensity. Our results for the optical force on the nonlinear nanoparticle will have potential applications in nonlinear optical manipulations and optical transportation.
DOI
Plasmon-hybridization-induced optical torque between twisted metal nanorods
An’an Wu, Yoshito Y. Tanaka, and Tsutomu Shimura
We present a numerical study of optical torque between two twisted metal nanorods due to the angular momentum of the electromagnetic field emerging from their plasmonic coupling. Our results indicate that the interaction optical torque on the nanorods can be strongly enhanced by their plasmon coupling, which highly depends on not only the gap size but also the twisted angle between the nanorods. The behaviors of the optical torque are different between two plasmon coupling modes: hybridized bonding and anti-bonding modes with different resonances. The rotations of the twisted nanorods with the bonding and anti-bonding mode excitations lead to mutually parallel and perpendicular alignments, respectively. At an incident intensity of 10 mW/μm2, the rotational potential depths are more than 30 times as large as the Brownian motion energy, enabling the optical alignments with angle fluctuations less than ∼±10°. Thus, this optical alignment of the nanoparticles with the plasmon coupling allows dynamic control of the plasmonic characteristics and functions.
DOI
We present a numerical study of optical torque between two twisted metal nanorods due to the angular momentum of the electromagnetic field emerging from their plasmonic coupling. Our results indicate that the interaction optical torque on the nanorods can be strongly enhanced by their plasmon coupling, which highly depends on not only the gap size but also the twisted angle between the nanorods. The behaviors of the optical torque are different between two plasmon coupling modes: hybridized bonding and anti-bonding modes with different resonances. The rotations of the twisted nanorods with the bonding and anti-bonding mode excitations lead to mutually parallel and perpendicular alignments, respectively. At an incident intensity of 10 mW/μm2, the rotational potential depths are more than 30 times as large as the Brownian motion energy, enabling the optical alignments with angle fluctuations less than ∼±10°. Thus, this optical alignment of the nanoparticles with the plasmon coupling allows dynamic control of the plasmonic characteristics and functions.
DOI
Study of laser induced dynamics on macroscopic mirror
Yukun Yuan, Chunyang Gu, Siyu Huang, Le Song, Guangpeng Yan, Zexiao Li and FengZhou Fang
Light carries energy and momentum, which can be transferred to the irradiated target during the process of reflection, refraction or transmission. The energy transfer can be seen from heat generation and thermal expansion of the target, while the momentum can exert a microscopic force, called an optical force, that is extremely difficult to detect because it is so small. A theoretical model for describing the force distribution on a macroscopic spherical mirror irradiated by a single laser pulse is established in this study. Finite element analysis has been performed to predict the dynamics of the mirror and it shows good consistency with the experimental results. It indicates the differences in the variation of mechanical response due to the optical force and thermal deformation due to photon absorption. A parametric study was also performed to analyze the relationship between mirror geometry and laser dynamics. These studies verify the thermal-mechanical coupling in light-material interactions for a pulsed-laser irradiation model, thereby being applicable in precision measurement of optical force at a microscopic level.
DOI
Light carries energy and momentum, which can be transferred to the irradiated target during the process of reflection, refraction or transmission. The energy transfer can be seen from heat generation and thermal expansion of the target, while the momentum can exert a microscopic force, called an optical force, that is extremely difficult to detect because it is so small. A theoretical model for describing the force distribution on a macroscopic spherical mirror irradiated by a single laser pulse is established in this study. Finite element analysis has been performed to predict the dynamics of the mirror and it shows good consistency with the experimental results. It indicates the differences in the variation of mechanical response due to the optical force and thermal deformation due to photon absorption. A parametric study was also performed to analyze the relationship between mirror geometry and laser dynamics. These studies verify the thermal-mechanical coupling in light-material interactions for a pulsed-laser irradiation model, thereby being applicable in precision measurement of optical force at a microscopic level.
DOI
Single atom movement with dynamic holographic optical tweezers
S R Samoylenko, A V Lisitsin, D Schepanovich, I B Bobrov, S S Straupe and S P Kulik
We report an experimental implementation of dynamical holographic tweezers for single trapped atoms. The tweezers are realized with dynamical phase holograms displayed on the liquid crystal spatial light modulator. We experimentally demonstrate the possibility to trap and move single rubidium atoms with such dynamic potentials, and study its limitations. Our results suggest that high probability transfer of single atoms in the tweezers may be performed in large steps, much larger then the trap waist. We discuss intensity-flicker in holographic traps and techniques for its suppression. Loss and heating rates in dynamic tweezers are measured and no excess loss or heating is observed in comparison with static traps.
DOI
We report an experimental implementation of dynamical holographic tweezers for single trapped atoms. The tweezers are realized with dynamical phase holograms displayed on the liquid crystal spatial light modulator. We experimentally demonstrate the possibility to trap and move single rubidium atoms with such dynamic potentials, and study its limitations. Our results suggest that high probability transfer of single atoms in the tweezers may be performed in large steps, much larger then the trap waist. We discuss intensity-flicker in holographic traps and techniques for its suppression. Loss and heating rates in dynamic tweezers are measured and no excess loss or heating is observed in comparison with static traps.
DOI
Friday, January 24, 2020
Optical Fiber Tweezers: A Versatile Tool for Optical Trapping and Manipulation
Xiaoting Zhao, Nan Zhao, Yang Shi, Hongbao Xin and Baojun Li
Optical trapping is widely used in different areas, ranging from biomedical applications, to physics and material sciences. In recent years, optical fiber tweezers have attracted significant attention in the field of optical trapping due to their flexible manipulation, compact structure, and easy fabrication. As a versatile tool for optical trapping and manipulation, optical fiber tweezers can be used to trap, manipulate, arrange, and assemble tiny objects. Here, we review the optical fiber tweezers-based trapping and manipulation, including dual fiber tweezers for trapping and manipulation, single fiber tweezers for trapping and single cell analysis, optical fiber tweezers for cell assembly, structured optical fiber for enhanced trapping and manipulation, subwavelength optical fiber wire for evanescent fields-based trapping and delivery, and photothermal trapping, assembly, and manipulation.
DOI
Optical trapping is widely used in different areas, ranging from biomedical applications, to physics and material sciences. In recent years, optical fiber tweezers have attracted significant attention in the field of optical trapping due to their flexible manipulation, compact structure, and easy fabrication. As a versatile tool for optical trapping and manipulation, optical fiber tweezers can be used to trap, manipulate, arrange, and assemble tiny objects. Here, we review the optical fiber tweezers-based trapping and manipulation, including dual fiber tweezers for trapping and manipulation, single fiber tweezers for trapping and single cell analysis, optical fiber tweezers for cell assembly, structured optical fiber for enhanced trapping and manipulation, subwavelength optical fiber wire for evanescent fields-based trapping and delivery, and photothermal trapping, assembly, and manipulation.
DOI
Multiview microscopy of single cells through microstructure-based indirect optical manipulation
Gaszton Vizsnyiczai, András Búzás, Badri Lakshmanrao Aekbote, Tamás Fekete, István Grexa, Pál Ormos, and Lóránd Kelemen
Fluorescent observation of cells generally suffers from the limited axial resolution due to the elongated point spread function of the microscope optics. Consequently, three-dimensional imaging results in axial resolution that is several times worse than the transversal. The optical solutions to this problem usually require complicated optics and extreme spatial stability. A straightforward way to eliminate anisotropic resolution is to fuse images recorded from multiple viewing directions achieved mostly by the mechanical rotation of the entire sample. In the presented approach, multiview imaging of single cells is implemented by rotating them around an axis perpendicular to the optical axis by means of holographic optical tweezers. For this, the cells are indirectly trapped and manipulated with special microtools made with two-photon polymerization. The cell is firmly attached to the microtool and is precisely manipulated with 6 degrees of freedom. The total control over the cells' position allows for its multiview fluorescence imaging from arbitrarily selected directions. The image stacks obtained this way are combined into one 3D image array with a multiview image processing pipeline resulting in isotropic optical resolution that approaches the lateral diffraction limit. The presented tool and manipulation scheme can be readily applied in various microscope platforms.
DOI
Fluorescent observation of cells generally suffers from the limited axial resolution due to the elongated point spread function of the microscope optics. Consequently, three-dimensional imaging results in axial resolution that is several times worse than the transversal. The optical solutions to this problem usually require complicated optics and extreme spatial stability. A straightforward way to eliminate anisotropic resolution is to fuse images recorded from multiple viewing directions achieved mostly by the mechanical rotation of the entire sample. In the presented approach, multiview imaging of single cells is implemented by rotating them around an axis perpendicular to the optical axis by means of holographic optical tweezers. For this, the cells are indirectly trapped and manipulated with special microtools made with two-photon polymerization. The cell is firmly attached to the microtool and is precisely manipulated with 6 degrees of freedom. The total control over the cells' position allows for its multiview fluorescence imaging from arbitrarily selected directions. The image stacks obtained this way are combined into one 3D image array with a multiview image processing pipeline resulting in isotropic optical resolution that approaches the lateral diffraction limit. The presented tool and manipulation scheme can be readily applied in various microscope platforms.
DOI
Spatial vibrations suppressing resonant tunneling
Gilad Zangwill and Er'el Granot
The dynamics of resonant tunneling via a spatially oscillating narrow well is investigated. The well generates a quasiresonance state, which can trap the incoming particles. Four spectral regimes are found: (1) the adiabatic regime, when the vibrations’ frequency is lower than the spectral width of the resonance. In this regime, the mean current is independent of the vibration's frequency, and the current decreases as a function of the vibration's amplitude. (2) When the frequency of the vibration is higher than the spectral width of the resonance, the particle is partially trapped to the moving well and the dependence of the current on the vibrations’ amplitude is more moderate. (3) However, and this is the main result of this paper, beyond a certain frequency the kinetic energy of the trapped particle exceeds the spectral width of the resonance, in which case particles cannot be trapped in the moving well and the current is abruptly suppressed. (4) When the energy quanta of the vibrations are higher than the energy gap between the resonance energy and the barrier's potential height, a single phonon can be absorbed by the particle only when the final energy agrees with the resonances of the barrier, in which case current suppression is selective and occurs only for specifics frequencies. The model is solved exactly numerically, and analytical approximations are presented for the different regimes. The analytical solutions show high agreement with the numerical ones. This effect can be implemented in extremely sensitive accelerometers. Moreover, it may explain the odor receptor's sensitivity to molecular vibrations.
DOI
The dynamics of resonant tunneling via a spatially oscillating narrow well is investigated. The well generates a quasiresonance state, which can trap the incoming particles. Four spectral regimes are found: (1) the adiabatic regime, when the vibrations’ frequency is lower than the spectral width of the resonance. In this regime, the mean current is independent of the vibration's frequency, and the current decreases as a function of the vibration's amplitude. (2) When the frequency of the vibration is higher than the spectral width of the resonance, the particle is partially trapped to the moving well and the dependence of the current on the vibrations’ amplitude is more moderate. (3) However, and this is the main result of this paper, beyond a certain frequency the kinetic energy of the trapped particle exceeds the spectral width of the resonance, in which case particles cannot be trapped in the moving well and the current is abruptly suppressed. (4) When the energy quanta of the vibrations are higher than the energy gap between the resonance energy and the barrier's potential height, a single phonon can be absorbed by the particle only when the final energy agrees with the resonances of the barrier, in which case current suppression is selective and occurs only for specifics frequencies. The model is solved exactly numerically, and analytical approximations are presented for the different regimes. The analytical solutions show high agreement with the numerical ones. This effect can be implemented in extremely sensitive accelerometers. Moreover, it may explain the odor receptor's sensitivity to molecular vibrations.
DOI
Direct observation of the formation of a CRISPR–Cas12a R-loop complex at the single-molecule level
Yang Cui, Yangchao Tang, Meng Liang, Qinghua Ji, Yan Zeng, Hui Chen, Jie Lan, Peng Jin, Lei Wang, Guangtao Song and Jizhong Lou
Here, we develop an optical tweezers-based single-molecule manipulation assay to detect the formation of an R-loop complex in the Cas12a system and characterize its thermodynamic stability. We found that the formation of the R-loop complex induces a two-step unfolding of a DNA hairpin containing the target sequence, the non-target sequence binds loosely to Cas12a and can be easily released from the complex, and the Nuc domain of Cas12a plays key roles in target binding and R-loop formation.
DOI
Here, we develop an optical tweezers-based single-molecule manipulation assay to detect the formation of an R-loop complex in the Cas12a system and characterize its thermodynamic stability. We found that the formation of the R-loop complex induces a two-step unfolding of a DNA hairpin containing the target sequence, the non-target sequence binds loosely to Cas12a and can be easily released from the complex, and the Nuc domain of Cas12a plays key roles in target binding and R-loop formation.
DOI
Auxiliary Optomechanical Tools for 3D Cell Manipulation
Ivan Shishkin, Hen Markovich, Yael Roichman and Pavel Ginzburg
Advances in laser and optoelectronic technologies have brought the general concept of optomechanical manipulation to the level of standard biophysical tools, paving the way towards controlled experiments and measurements of tiny mechanical forces. Recent developments in direct laser writing (DLW) have enabled the realization of new types of micron-scale optomechanical tools, capable of performing designated functions. Here we further develop the concept of DLW-fabricated optomechanically-driven tools and demonstrate full-3D manipulation capabilities over biological objects. In particular, we resolved the long-standing problem of out-of-plane rotation in a pure liquid, which was demonstrated on a living cell, clamped between a pair of forks, designed for efficient manipulation with holographic optical tweezers. The demonstrated concept paves the way for the realization of flexible tools for performing on-demand functions over biological objects, such as cell tomography and surgery to name just few.
DOI
Advances in laser and optoelectronic technologies have brought the general concept of optomechanical manipulation to the level of standard biophysical tools, paving the way towards controlled experiments and measurements of tiny mechanical forces. Recent developments in direct laser writing (DLW) have enabled the realization of new types of micron-scale optomechanical tools, capable of performing designated functions. Here we further develop the concept of DLW-fabricated optomechanically-driven tools and demonstrate full-3D manipulation capabilities over biological objects. In particular, we resolved the long-standing problem of out-of-plane rotation in a pure liquid, which was demonstrated on a living cell, clamped between a pair of forks, designed for efficient manipulation with holographic optical tweezers. The demonstrated concept paves the way for the realization of flexible tools for performing on-demand functions over biological objects, such as cell tomography and surgery to name just few.
DOI
Thursday, January 23, 2020
Cell Death in Cells Overlying Lateral Root Primordia Facilitates Organ Growth in Arabidopsis
Sacha Escamez, Domenique André, Bernadette Sztojka, Benjamin Bollhöner, Hardy Hall, Béatrice Berthet, Ute Voß, Amnon Lers, Alexis Maizel, Magnus Andersson, Malcolm Bennett, Hannele Tuominen
Plant organ growth is widely accepted to be determined by cell division and cell expansion, but, unlike that in animals, the contribution of cell elimination has rarely been recognized. We investigated this paradigm during Arabidopsis lateral root formation, when the lateral root primordia (LRP) must traverse three overlying cell layers within the parent root. A subset of LRP-overlying cells displayed the induction of marker genes for cell types undergoing developmental cell death, and their cell death was detected by electron, confocal, and light sheet microscopy techniques. LRP growth was delayed in cell-death-deficient mutants lacking the positive cell death regulator ORESARA1/ANAC092 (ORE1). LRP growth was restored in ore1-2 knockout plants by genetically inducing cell elimination in cells overlying the LRP or by physically killing LRP-overlying cells by ablation with optical tweezers. Our results support that, in addition to previously discovered mechanisms, cell elimination contributes to regulating lateral root emergence.
DOI
Plant organ growth is widely accepted to be determined by cell division and cell expansion, but, unlike that in animals, the contribution of cell elimination has rarely been recognized. We investigated this paradigm during Arabidopsis lateral root formation, when the lateral root primordia (LRP) must traverse three overlying cell layers within the parent root. A subset of LRP-overlying cells displayed the induction of marker genes for cell types undergoing developmental cell death, and their cell death was detected by electron, confocal, and light sheet microscopy techniques. LRP growth was delayed in cell-death-deficient mutants lacking the positive cell death regulator ORESARA1/ANAC092 (ORE1). LRP growth was restored in ore1-2 knockout plants by genetically inducing cell elimination in cells overlying the LRP or by physically killing LRP-overlying cells by ablation with optical tweezers. Our results support that, in addition to previously discovered mechanisms, cell elimination contributes to regulating lateral root emergence.
DOI
Nanoparticle trapping and routing on plasmonic nanorails in a microfluidic channel
Shengqi Yin, Fei He, Nicolas Green, and Xu Fang
Plasmonic nanostructures hold great promise for enabling advanced optical manipulation of nanoparticles in microfluidic channels, resulting from the generation of strong and controllable light focal points at the nanoscale. A primary remaining challenge in the current integration of plasmonics and microfluidics is to transport trapped nanoparticles along designated routes. Here we demonstrate through numerical simulation a plasmonic nanoparticle router that can trap and route a nanoparticle in a microfluidic channel with a continuous fluidic flow. The nanoparticle router contains a series of gold nanostrips on top of a continuous gold film. The nanostrips support both localised and propagating surface plasmons under light illumination, which underpin the trapping and routing functionalities. The nanoparticle guiding at a Y-branch junction is enabled by a small change of 50 nm in the wavelength of incident light.
DOI
Plasmonic nanostructures hold great promise for enabling advanced optical manipulation of nanoparticles in microfluidic channels, resulting from the generation of strong and controllable light focal points at the nanoscale. A primary remaining challenge in the current integration of plasmonics and microfluidics is to transport trapped nanoparticles along designated routes. Here we demonstrate through numerical simulation a plasmonic nanoparticle router that can trap and route a nanoparticle in a microfluidic channel with a continuous fluidic flow. The nanoparticle router contains a series of gold nanostrips on top of a continuous gold film. The nanostrips support both localised and propagating surface plasmons under light illumination, which underpin the trapping and routing functionalities. The nanoparticle guiding at a Y-branch junction is enabled by a small change of 50 nm in the wavelength of incident light.
DOI
Probing Nanoparticle–Cell Interaction Using Micro-Raman Spectroscopy: Silver and Gold Nanoparticle-Induced Stress Effects on Optically Trapped Live Red Blood Cells
Surekha Barkur, Jijo Lukose, Santhosh Chidangil
Advancements in the field of nanotechnology have resulted in the emergence of a large variety of engineered nanomaterials for innumerable applications. Despite the ubiquitous use of nanomaterials in daily life, concerns regarding the potential toxicity and safety of these materials have also been raised. There is a high demand for assessing the unwanted effects of both gold and silver nanoparticles, which is increasingly being used in biomedical applications. This paper deals with the study of stress due to silver and gold nanoparticles of varying size on red blood cells (RBCs) using Raman tweezers spectroscopy. RBCs were incubated with nanoparticles of size in the 10–100 nm range with the same concentrations, and micro-Raman spectra were recorded by optically trapping the nanoparticle-treated live RBCs. Spectral modifications implicating hemoglobin deoxygenation were observed in all nanoparticle-treated RBCs. One of the probable reason for the deoxygenation trend can be the adhesion of nanoparticles onto the cell surface causing imbalance in cell functioning. Moreover, the higher spectral variations observed for silver nanoparticles indicate that oxidative stress is involved in cell damage. These mechanisms lead to the modification in the hemoglobin structure because of changes in the pH of cytoplasm, which can be detected using Raman spectroscopy.
DOI
Advancements in the field of nanotechnology have resulted in the emergence of a large variety of engineered nanomaterials for innumerable applications. Despite the ubiquitous use of nanomaterials in daily life, concerns regarding the potential toxicity and safety of these materials have also been raised. There is a high demand for assessing the unwanted effects of both gold and silver nanoparticles, which is increasingly being used in biomedical applications. This paper deals with the study of stress due to silver and gold nanoparticles of varying size on red blood cells (RBCs) using Raman tweezers spectroscopy. RBCs were incubated with nanoparticles of size in the 10–100 nm range with the same concentrations, and micro-Raman spectra were recorded by optically trapping the nanoparticle-treated live RBCs. Spectral modifications implicating hemoglobin deoxygenation were observed in all nanoparticle-treated RBCs. One of the probable reason for the deoxygenation trend can be the adhesion of nanoparticles onto the cell surface causing imbalance in cell functioning. Moreover, the higher spectral variations observed for silver nanoparticles indicate that oxidative stress is involved in cell damage. These mechanisms lead to the modification in the hemoglobin structure because of changes in the pH of cytoplasm, which can be detected using Raman spectroscopy.
DOI
A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
Wenfeng Liang, Lianqing Liu, Junhai Wang, Xieliu Yang, Yuechao Wang, Wen Jung Li and Wenguang Yang
Optoelectrokinetics (OEK), a fusion of optics, electrokinetics, and microfluidics, has been demonstrated to offer a series of extraordinary advantages in the manipulation and fabrication of micro/nanomaterials, such as requiring no mask, programmability, flexibility, and rapidness. In this paper, we summarize a variety of differently structured OEK chips, followed by a discussion on how they are fabricated and the ways in which they work. We also review how three differently sized polystyrene beads can be separated simultaneously, how a variety of nanoparticles can be assembled, and how micro/nanomaterials can be fabricated into functional devices. Another focus of our paper is on mask-free fabrication and assembly of hydrogel-based micro/nanostructures and its possible applications in biological fields. We provide a summary of the current challenges facing the OEK technique and its future prospects at the end of this paper.
DOI
Optoelectrokinetics (OEK), a fusion of optics, electrokinetics, and microfluidics, has been demonstrated to offer a series of extraordinary advantages in the manipulation and fabrication of micro/nanomaterials, such as requiring no mask, programmability, flexibility, and rapidness. In this paper, we summarize a variety of differently structured OEK chips, followed by a discussion on how they are fabricated and the ways in which they work. We also review how three differently sized polystyrene beads can be separated simultaneously, how a variety of nanoparticles can be assembled, and how micro/nanomaterials can be fabricated into functional devices. Another focus of our paper is on mask-free fabrication and assembly of hydrogel-based micro/nanostructures and its possible applications in biological fields. We provide a summary of the current challenges facing the OEK technique and its future prospects at the end of this paper.
DOI
Photonic Crystal Nanobeam Cavities for Nanoscale Optical Sensing: A Review
Da-Quan Yang, Bing Duan, Xiao Liu, Ai-Qiang Wang, Xiao-Gang Li and Yue-Feng Ji
The ability to detect nanoscale objects is particular crucial for a wide range of applications, such as environmental protection, early-stage disease diagnosis and drug discovery. Photonic crystal nanobeam cavity (PCNC) sensors have attracted great attention due to high-quality factors and small-mode volumes (Q/V) and good on-chip integrability with optical waveguides/circuits. In this review, we focus on nanoscale optical sensing based on PCNC sensors, including ultrahigh figure of merit (FOM) sensing, single nanoparticle trapping, label-free molecule detection and an integrated sensor array for multiplexed sensing. We believe that the PCNC sensors featuring ultracompact footprint, high monolithic integration capability, fast response and ultrahigh sensitivity sensing ability, etc., will provide a promising platform for further developing lab-on-a-chip devices for biosensing and other functionalities.
DOI
The ability to detect nanoscale objects is particular crucial for a wide range of applications, such as environmental protection, early-stage disease diagnosis and drug discovery. Photonic crystal nanobeam cavity (PCNC) sensors have attracted great attention due to high-quality factors and small-mode volumes (Q/V) and good on-chip integrability with optical waveguides/circuits. In this review, we focus on nanoscale optical sensing based on PCNC sensors, including ultrahigh figure of merit (FOM) sensing, single nanoparticle trapping, label-free molecule detection and an integrated sensor array for multiplexed sensing. We believe that the PCNC sensors featuring ultracompact footprint, high monolithic integration capability, fast response and ultrahigh sensitivity sensing ability, etc., will provide a promising platform for further developing lab-on-a-chip devices for biosensing and other functionalities.
DOI
Wednesday, January 22, 2020
Three-dimensional chirped Airy Complex-variable-function Gaussian vortex wave packets in a strongly nonlocal nonlinear medium
Xi Peng, Yingji He, and Dongmei Deng
Three-dimensional chirped Airy Complex-variable-function Gaussian vortex (CACGV) wave packets in a strongly nonlocal nonlinear medium (SNNM) are studied. By varying the distribution parameter, CACGV wave packets can rotate stably in a SNNM in different forms, including dipoles, elliptic vortices, and doughnuts. Numerical simulation results for the CACGV wave packets agree well with theoretical analysis results under zero perturbation. The Poynting vector related to the physics of the rotation phenomenon and the angular momentum as a torque corresponding to the force are also presented. Finally, the radiation forces of CACGV wave packets acting on a nanoparticle in a SNNM are discussed.
DOI
Three-dimensional chirped Airy Complex-variable-function Gaussian vortex (CACGV) wave packets in a strongly nonlocal nonlinear medium (SNNM) are studied. By varying the distribution parameter, CACGV wave packets can rotate stably in a SNNM in different forms, including dipoles, elliptic vortices, and doughnuts. Numerical simulation results for the CACGV wave packets agree well with theoretical analysis results under zero perturbation. The Poynting vector related to the physics of the rotation phenomenon and the angular momentum as a torque corresponding to the force are also presented. Finally, the radiation forces of CACGV wave packets acting on a nanoparticle in a SNNM are discussed.
DOI
3D control stretched length of lambda-phage WLC DNA molecule by nonlinear optical tweezers
Thang Nguyen Manh, Quy Ho Quang, Thanh Thai Doan, Tuan Doan Quoc, Viet Do Thanh & Khoa Doan Quoc
In this paper, the general Langevin equations of motion for the polystyrene bead linked to the lambda-phage worm-like chain DNA molecule embedded in the fluid under the nonlinear optical tweezers is derived in 3D space. Using the finite difference method, the dynamical properties of the bead trapped by the nonlinear optical tweezers using a thin layer of Acid Blue 29 are numerically studied. Results in, the stretched length of the lambda-phage worm-like chain DNA molecule can be controlled in 3D space by finely tuning of the laser power.
DOI
In this paper, the general Langevin equations of motion for the polystyrene bead linked to the lambda-phage worm-like chain DNA molecule embedded in the fluid under the nonlinear optical tweezers is derived in 3D space. Using the finite difference method, the dynamical properties of the bead trapped by the nonlinear optical tweezers using a thin layer of Acid Blue 29 are numerically studied. Results in, the stretched length of the lambda-phage worm-like chain DNA molecule can be controlled in 3D space by finely tuning of the laser power.
DOI
Vangl2 acts at the interface between actin and N-cadherin to modulate mammalian neuronal outgrowth
Steve Dos-Santos Carvalho, Maite M Moreau, Yeri Esther Hien, Michael Garcia, Nathalie Aubailly, Deborah J Henderson, Vincent Studer, Nathalie Sans, Olivier Thoumine, Mireille Montcouquiol
Dynamic mechanical interactions between adhesion complexes and the cytoskeleton are essential for axon outgrowth and guidance. Whether planar cell polarity (PCP) proteins, which regulate cytoskeleton dynamics and appear necessary for some axon guidance, also mediate interactions with membrane adhesion is still unclear. Here we show that Vangl2 controls growth cone velocity by regulating the internal retrograde actin flow in an N-cadherin-dependent fashion. Single molecule tracking experiments show that the loss of Vangl2 decreased fast-diffusing N-cadherin membrane molecules and increased confined N-cadherin trajectories. Using optically manipulated N-cadherin-coated microspheres, we correlated this behavior to a stronger mechanical coupling of N-cadherin with the actin cytoskeleton. Lastly, we show that the spatial distribution of Vangl2 within the growth cone is selectively affected by an N-cadherin-coated substrate. Altogether, our data show that Vangl2 acts as a negative regulator of axonal outgrowth by regulating the strength of the molecular clutch between N-cadherin and the actin cytoskeleton.
DOI
Dynamic mechanical interactions between adhesion complexes and the cytoskeleton are essential for axon outgrowth and guidance. Whether planar cell polarity (PCP) proteins, which regulate cytoskeleton dynamics and appear necessary for some axon guidance, also mediate interactions with membrane adhesion is still unclear. Here we show that Vangl2 controls growth cone velocity by regulating the internal retrograde actin flow in an N-cadherin-dependent fashion. Single molecule tracking experiments show that the loss of Vangl2 decreased fast-diffusing N-cadherin membrane molecules and increased confined N-cadherin trajectories. Using optically manipulated N-cadherin-coated microspheres, we correlated this behavior to a stronger mechanical coupling of N-cadherin with the actin cytoskeleton. Lastly, we show that the spatial distribution of Vangl2 within the growth cone is selectively affected by an N-cadherin-coated substrate. Altogether, our data show that Vangl2 acts as a negative regulator of axonal outgrowth by regulating the strength of the molecular clutch between N-cadherin and the actin cytoskeleton.
DOI
Nanophotonic Platforms for Chiral Sensing and Separation
Michelle L. Solomon, Amr A. E. Saleh, Lisa V. Poulikakos, John M. Abendroth, Loza F. Tadesse, Jennifer A. Dionne
Recent advances in nanophotonics lay the foundation toward highly sensitive and efficient chiral detection and separation methods. In this Account, we highlight our group’s effort to leverage nanoscale chiral light–matter interactions to detect, characterize, and separate enantiomers, potentially down to the single molecule level. Notably, certain resonant nanostructures can significantly enhance circular dichroism for improved chiral sensing and spectroscopy as well as high-yield enantioselective photochemistry. We first describe how achiral metallic and dielectric nanostructures can be utilized to increase the local optical chirality density by engineering the coupling between electric and magnetic optical resonances. While plasmonic nanoparticles locally enhance the optical chirality density, high-index dielectric nanoparticles can enable large-volume and uniform-sign enhancements in the optical chirality density. By overlapping these electric and magnetic resonances, local chiral fields can be enhanced by several orders of magnitude. We show how these design rules can enable high-yield enantioselective photochemistry and project a 2000-fold improvement in the yield of a photoionization reaction. Next, we discuss how optical forces can enable selective manipulation and separation of enantiomers. We describe the design of low-power enantioselective optical tweezers with the ability to trap sub-10 nm dielectric particles. We also characterize their chiral-optical forces with high spatial and force resolution using combined optical and atomic force microscopy. These optical tweezers exhibit an enantioselective optical force contrast exceeding 10 pN, enabling selective attraction or repulsion of enantiomers based on the illumination polarization. Finally, we discuss future challenges and opportunities spanning fundamental research to technology translation. Disease detection in the clinic as well as pharmaceutical and agrochemical industrial applications requiring large-scale, high-throughput production will gain particular benefit from the simplicity and relative low cost that nanophotonic platforms promise.
DOI
Recent advances in nanophotonics lay the foundation toward highly sensitive and efficient chiral detection and separation methods. In this Account, we highlight our group’s effort to leverage nanoscale chiral light–matter interactions to detect, characterize, and separate enantiomers, potentially down to the single molecule level. Notably, certain resonant nanostructures can significantly enhance circular dichroism for improved chiral sensing and spectroscopy as well as high-yield enantioselective photochemistry. We first describe how achiral metallic and dielectric nanostructures can be utilized to increase the local optical chirality density by engineering the coupling between electric and magnetic optical resonances. While plasmonic nanoparticles locally enhance the optical chirality density, high-index dielectric nanoparticles can enable large-volume and uniform-sign enhancements in the optical chirality density. By overlapping these electric and magnetic resonances, local chiral fields can be enhanced by several orders of magnitude. We show how these design rules can enable high-yield enantioselective photochemistry and project a 2000-fold improvement in the yield of a photoionization reaction. Next, we discuss how optical forces can enable selective manipulation and separation of enantiomers. We describe the design of low-power enantioselective optical tweezers with the ability to trap sub-10 nm dielectric particles. We also characterize their chiral-optical forces with high spatial and force resolution using combined optical and atomic force microscopy. These optical tweezers exhibit an enantioselective optical force contrast exceeding 10 pN, enabling selective attraction or repulsion of enantiomers based on the illumination polarization. Finally, we discuss future challenges and opportunities spanning fundamental research to technology translation. Disease detection in the clinic as well as pharmaceutical and agrochemical industrial applications requiring large-scale, high-throughput production will gain particular benefit from the simplicity and relative low cost that nanophotonic platforms promise.
DOI
Motion Reconstruction for Optical Tomography of Trapped Objects
Peter Elbau, Monika Ritsch-Marte, Otmar Scherzer and Denise Schmutz
Optical and acoustical trapping has been established as a tool for holding and moving microscopic particles suspended in a liquid in a contact-free and non-invasive manner. Opposed to standard microscopic imaging where the probe is fixated, this technique allows imaging in a more natural environment. This paper provides a method for estimating the movement of a transparent particle which is maneuvered by tweezers (assuming that the inner structure of the probe is not subject to local movements) by making use of the assumption of a smooth movement in time. The mathematical formulation of the motion estimation leads to an infinitesimal version of the common line technique used in cryogenic electron microscopy single particle imaging to estimate the orientations of the particles in the probe.
DOI
Optical and acoustical trapping has been established as a tool for holding and moving microscopic particles suspended in a liquid in a contact-free and non-invasive manner. Opposed to standard microscopic imaging where the probe is fixated, this technique allows imaging in a more natural environment. This paper provides a method for estimating the movement of a transparent particle which is maneuvered by tweezers (assuming that the inner structure of the probe is not subject to local movements) by making use of the assumption of a smooth movement in time. The mathematical formulation of the motion estimation leads to an infinitesimal version of the common line technique used in cryogenic electron microscopy single particle imaging to estimate the orientations of the particles in the probe.
DOI
Selective Manipulation of Biomolecules with Insulator-Based Dielectrophoretic Tweezers
Myungkeun Oh, Vidura Jayasooriya, Sung Oh Woo, Dharmakeerthi Nawarathna, Yongki Choi
Insulator-based dielectrophoretic (iDEP) trapping, separating, and concentrating nanoscale objects is carried out using a nonmetal, unbiased, mobile tip acting as a tweezers. The spatial control and manipulation of fluorescently labeled polystyrene particles and DNA were performed to demonstrate the feasibility of the iDEP tweezers. Frequency-dependent iDEP tweezers’ strength and polarity were quantitatively determined using two theoretical approaches to DNA, which resulted in a factor of 2–40 differences between them. In either approach, the strength of iDEP was at least 4 orders of magnitude stronger than the thermal force, indicating iDEP was a dominant force for trapping, holding, and separating DNA. The trapping strength and volume of the iDEP tweezers were also determined, which further supports direct capture and manipulation of DNA at the tip end.
DOI
Insulator-based dielectrophoretic (iDEP) trapping, separating, and concentrating nanoscale objects is carried out using a nonmetal, unbiased, mobile tip acting as a tweezers. The spatial control and manipulation of fluorescently labeled polystyrene particles and DNA were performed to demonstrate the feasibility of the iDEP tweezers. Frequency-dependent iDEP tweezers’ strength and polarity were quantitatively determined using two theoretical approaches to DNA, which resulted in a factor of 2–40 differences between them. In either approach, the strength of iDEP was at least 4 orders of magnitude stronger than the thermal force, indicating iDEP was a dominant force for trapping, holding, and separating DNA. The trapping strength and volume of the iDEP tweezers were also determined, which further supports direct capture and manipulation of DNA at the tip end.
DOI
Monday, January 20, 2020
Cell engineering: Biophysical regulation of the nucleus
Yang Song, Jennifer Soto, Binru Chen, Li Yang, Song Li
Cells live in a complex and dynamic microenvironment, and a variety of microenvironmental cues can regulate cell behavior. In addition to biochemical signals, biophysical cues can induce not only immediate intracellular responses, but also long-term effects on phenotypic changes such as stem cell differentiation, immune cell activation and somatic cell reprogramming. Cells respond to mechanical stimuli via an outside-in and inside-out feedback loop, and the cell nucleus plays an important role in this process. The mechanical properties of the nucleus can directly or indirectly modulate mechanotransduction, and the physical coupling of the cell nucleus with the cytoskeleton can affect chromatin structure and regulate the epigenetic state, gene expression and cell function. In this review, we will highlight the recent progress in nuclear biomechanics and mechanobiology in the context of cell engineering, tissue remodeling and disease development.
DOI
Cells live in a complex and dynamic microenvironment, and a variety of microenvironmental cues can regulate cell behavior. In addition to biochemical signals, biophysical cues can induce not only immediate intracellular responses, but also long-term effects on phenotypic changes such as stem cell differentiation, immune cell activation and somatic cell reprogramming. Cells respond to mechanical stimuli via an outside-in and inside-out feedback loop, and the cell nucleus plays an important role in this process. The mechanical properties of the nucleus can directly or indirectly modulate mechanotransduction, and the physical coupling of the cell nucleus with the cytoskeleton can affect chromatin structure and regulate the epigenetic state, gene expression and cell function. In this review, we will highlight the recent progress in nuclear biomechanics and mechanobiology in the context of cell engineering, tissue remodeling and disease development.
DOI
A 3D computational model of perfusion seeding for investigating cell transport and adhesion within a porous scaffold
Ziying Zhang, Jun Du, Zhengying Wei, Zhen Wang, Minghui Li, Jingda Ni
The process of cell seeding within a porous scaffold is an essential first step in the development of tissue-engineered bone grafts. Understanding the underlying mechanisms of cell distribution and adhesion is fundamental for the design and optimization of the seeding process. To that end, we present a numerical model to investigate the perfusion cell seeding process that incorporates cell mechanics, cell–fluid interaction, and cell–scaffold adhesion. The individual cells are modeled as deformable spherical capsules capable of adhering to the scaffold surface as well as to other cells with probabilistic bond formation and rupture. The mechanical deformation of the cell is calibrated with the stretching of mice mesenchymal stem cells induced by optical tweezers, while the predicted adhesive forces are consistent with the experimental data reported in the literature. A sub-domain is numerically reconstructed as the region of interest (ROI) which is representative of an actual scaffold. Through the simulations, the perfusion seeding kinetics within the ROI involving detailed transport and adhesion of cells over time is analyzed. The effects of the perfusion pressure and initial cell concentration on the seeding kinetics are studied in terms of adhesion rates, cell cluster formation, seeding uniformity, and efficiency, as well as scaffold permeability. The results highlight the importance of cell–fluid interaction and adhesion dynamics in modeling the dynamic seeding process. This bottom-up model provides a way to bridge detailed behaviors of individual cells to the seeding outcomes at the macroscopic scale, allowing for finding the best configuration to enhance cell seeding.
DOI
The process of cell seeding within a porous scaffold is an essential first step in the development of tissue-engineered bone grafts. Understanding the underlying mechanisms of cell distribution and adhesion is fundamental for the design and optimization of the seeding process. To that end, we present a numerical model to investigate the perfusion cell seeding process that incorporates cell mechanics, cell–fluid interaction, and cell–scaffold adhesion. The individual cells are modeled as deformable spherical capsules capable of adhering to the scaffold surface as well as to other cells with probabilistic bond formation and rupture. The mechanical deformation of the cell is calibrated with the stretching of mice mesenchymal stem cells induced by optical tweezers, while the predicted adhesive forces are consistent with the experimental data reported in the literature. A sub-domain is numerically reconstructed as the region of interest (ROI) which is representative of an actual scaffold. Through the simulations, the perfusion seeding kinetics within the ROI involving detailed transport and adhesion of cells over time is analyzed. The effects of the perfusion pressure and initial cell concentration on the seeding kinetics are studied in terms of adhesion rates, cell cluster formation, seeding uniformity, and efficiency, as well as scaffold permeability. The results highlight the importance of cell–fluid interaction and adhesion dynamics in modeling the dynamic seeding process. This bottom-up model provides a way to bridge detailed behaviors of individual cells to the seeding outcomes at the macroscopic scale, allowing for finding the best configuration to enhance cell seeding.
DOI
Isolating Nanocrystals with an Individual Erbium Emitter: A Route to a Stable Single-Photon Source at 1550 nm Wavelength
Amirhossein Alizadehkhaledi, Adriaan L. Frencken, Frank C. J. M. van Veggel, and Reuven Gordon
Single-photon emitters based on individual atoms or individual atomic-like defects are highly sought-after components for future quantum technologies. A key challenge in this field is how to isolate just one such emitter; the best approaches still have an active emitter yield of only 50% so that deterministic integration of single active emitters is not yet possible. Here, we demonstrate the ability to isolate individual erbium emitters embedded in 20 nm nanocrystals of NaYF4 using plasmonic aperture optical tweezers. The optical tweezers capture the nanocrystal, whereas the plasmonic aperture enhances the emission of the Er and allows the measurement of discrete emission rate values corresponding to different numbers of erbium ions. Three separate synthesis runs show near-Poissonian distribution in the discrete levels of emission yield that correspond to the expected ion concentrations, indicating that the yield of active emitters is approximately 80%. Fortunately, the trap allows for selecting the nanocrystals with only a single emitter, and so this gives a route to isolating and integrating single emitters in a deterministic way. This demonstration is a promising step toward single-photon quantum information technologies that utilize single ions in a solid-state medium, particularly because Er emits in the low-loss fiber-optic 1550 nm telecom band.
DOI
Single-photon emitters based on individual atoms or individual atomic-like defects are highly sought-after components for future quantum technologies. A key challenge in this field is how to isolate just one such emitter; the best approaches still have an active emitter yield of only 50% so that deterministic integration of single active emitters is not yet possible. Here, we demonstrate the ability to isolate individual erbium emitters embedded in 20 nm nanocrystals of NaYF4 using plasmonic aperture optical tweezers. The optical tweezers capture the nanocrystal, whereas the plasmonic aperture enhances the emission of the Er and allows the measurement of discrete emission rate values corresponding to different numbers of erbium ions. Three separate synthesis runs show near-Poissonian distribution in the discrete levels of emission yield that correspond to the expected ion concentrations, indicating that the yield of active emitters is approximately 80%. Fortunately, the trap allows for selecting the nanocrystals with only a single emitter, and so this gives a route to isolating and integrating single emitters in a deterministic way. This demonstration is a promising step toward single-photon quantum information technologies that utilize single ions in a solid-state medium, particularly because Er emits in the low-loss fiber-optic 1550 nm telecom band.
DOI
Microfluidic control over topological states in channel-confined nematic flows
Simon Čopar, Žiga Kos, Tadej Emeršič & Uroš Tkalec
Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate.
DOI
Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate.
DOI
Characterization of optofluidic devices for the sorting of sub-micrometer particles
James White, Cyril Laplane, Reece P. Roberts, Louise J. Brown, Thomas Volz, and David W. Inglis
In this work, we investigate methods of fabricating a device for the optical actuation of nanoparticles. To create the microfluidic channel, we pursued three fabrication methods: SU-8 to molded polydimethylsiloxane soft lithography, laser etching of glass, and deep reactive ion etching of fused silica. We measured the surface roughness of the etched sidewalls, and the laser power transmission through each device. We then measured the radiation pressure on 0.5-µm particles in the best-performing fabricated device (etched fused silica) and in a square glass capillary.
DOI
In this work, we investigate methods of fabricating a device for the optical actuation of nanoparticles. To create the microfluidic channel, we pursued three fabrication methods: SU-8 to molded polydimethylsiloxane soft lithography, laser etching of glass, and deep reactive ion etching of fused silica. We measured the surface roughness of the etched sidewalls, and the laser power transmission through each device. We then measured the radiation pressure on 0.5-µm particles in the best-performing fabricated device (etched fused silica) and in a square glass capillary.
DOI
Advanced apparatus for the integration of nanophotonics and cold atoms
J.-B. Béguin, A. P. Burgers, X. Luan, Z. Qin, S. P. Yu, and H. J. Kimble
We combine nanophotonics and cold atom research in a new apparatus enabling the delivery of single-atom tweezer arrays in the vicinity of photonic crystal waveguides.
DOI
We combine nanophotonics and cold atom research in a new apparatus enabling the delivery of single-atom tweezer arrays in the vicinity of photonic crystal waveguides.
DOI
Thursday, January 16, 2020
Stability and dynamics of chiral nanoparticles in lateral optical binding induced by high-order Bessel beams
Jing Bai, Cheng-xian Ge, Zhen-sen Wu
The generalized multi-particle Mie equation (GMM) and electromagnetic momentum (EM) theory are applied to investigate the stability and dynamics of chiral nanoparticles in lateral optical binding induced by a high-order Bessel beam (HOBB). Such non-diffracting light suppressed the influence of the axial intensity profile of the illuminating beams on the self-organization process which then depended critically upon the inter-particles interactions. The illuminating HOBB is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz–Mie theories (GLMT). Utilizing the addition theorem of the vector spherical wave functions (VSWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the chiral nanoparticles is considered. The observed lateral binding force (BF) dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction when the chiral spheres degenerate into isotropic spheres. We discuss the influence of the different parameters of the incident Bessel beam and of the chiral body on lateral BF in detail. Linearly and circularly polarized incident Bessel beams are considered, and the corresponding lateral BFs are compared and analyzed. The polarizations of incident HOBB considerably influence the lateral BF of chiral nanoparticles. In binding chiral nanoparticles, the polarization of incident beams should be chosen in accordance with the chirality. This finding may provide a recipe to understand the light interaction with multiple chiral particles of arbitrary shapes with the aid of the analytical approach. It could be a promising avenue in controlling the optical micromanipulation on chiral structures self-arrangement.
DOI
The generalized multi-particle Mie equation (GMM) and electromagnetic momentum (EM) theory are applied to investigate the stability and dynamics of chiral nanoparticles in lateral optical binding induced by a high-order Bessel beam (HOBB). Such non-diffracting light suppressed the influence of the axial intensity profile of the illuminating beams on the self-organization process which then depended critically upon the inter-particles interactions. The illuminating HOBB is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz–Mie theories (GLMT). Utilizing the addition theorem of the vector spherical wave functions (VSWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the chiral nanoparticles is considered. The observed lateral binding force (BF) dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction when the chiral spheres degenerate into isotropic spheres. We discuss the influence of the different parameters of the incident Bessel beam and of the chiral body on lateral BF in detail. Linearly and circularly polarized incident Bessel beams are considered, and the corresponding lateral BFs are compared and analyzed. The polarizations of incident HOBB considerably influence the lateral BF of chiral nanoparticles. In binding chiral nanoparticles, the polarization of incident beams should be chosen in accordance with the chirality. This finding may provide a recipe to understand the light interaction with multiple chiral particles of arbitrary shapes with the aid of the analytical approach. It could be a promising avenue in controlling the optical micromanipulation on chiral structures self-arrangement.
DOI
Optical Trapping, Sizing, and Probing Acoustic Modes of a Small Virus
Jeffrey Burkhartsmeyer, Yanhong Wang, Kam Sing Wong and Reuven Gordon
Prior opto-mechanical techniques to measure vibrational frequencies of viruses work on large ensembles of particles, whereas, in this work, individually trapped viral particles were studied. Double nanohole (DNH) apertures in a gold film were used to achieve optical trapping of one of the smallest virus particles yet reported, PhiX174, which has a diameter of 25 nm. When a laser was focused onto these DNH apertures, it created high local fields due to plasmonic enhancement, which allowed stable trapping of small particles for prolonged periods at low powers. Two techniques were performed to characterize the virus particles. The particles were sized via an established autocorrelation analysis technique, and the acoustic modes were probed using the extraordinary acoustic Raman (EAR) method. The size of the trapped particle was determined to be 25 ± 3.8 nm, which is in good agreement with the established diameter of PhiX174. A peak in the EAR signal was observed at 32 GHz, which fits well with the predicted value from elastic theory.
DOI
Prior opto-mechanical techniques to measure vibrational frequencies of viruses work on large ensembles of particles, whereas, in this work, individually trapped viral particles were studied. Double nanohole (DNH) apertures in a gold film were used to achieve optical trapping of one of the smallest virus particles yet reported, PhiX174, which has a diameter of 25 nm. When a laser was focused onto these DNH apertures, it created high local fields due to plasmonic enhancement, which allowed stable trapping of small particles for prolonged periods at low powers. Two techniques were performed to characterize the virus particles. The particles were sized via an established autocorrelation analysis technique, and the acoustic modes were probed using the extraordinary acoustic Raman (EAR) method. The size of the trapped particle was determined to be 25 ± 3.8 nm, which is in good agreement with the established diameter of PhiX174. A peak in the EAR signal was observed at 32 GHz, which fits well with the predicted value from elastic theory.
DOI
Munc13-1 MUN domain and Munc18-1 cooperatively chaperone SNARE assembly through a tetrameric complex
Tong Shu, Huaizhou Jin, James E. Rothman, and Yongli Zhang
Munc13-1 is a large multifunctional protein essential for synaptic vesicle fusion and neurotransmitter release. Its dysfunction has been linked to many neurological disorders. Evidence suggests that the MUN domain of Munc13-1 collaborates with Munc18-1 to initiate SNARE assembly, thereby priming vesicles for fast calcium-triggered vesicle fusion. The underlying molecular mechanism, however, is poorly understood. Recently, it was found that Munc18-1 catalyzes neuronal SNARE assembly through an obligate template complex intermediate containing Munc18-1 and 2 SNARE proteins—syntaxin 1 and VAMP2. Here, using single-molecule force spectroscopy, we discovered that the MUN domain of Munc13-1 stabilizes the template complex by ∼2.1 kBT. The MUN-bound template complex enhances SNAP-25 binding to the templated SNAREs and subsequent full SNARE assembly. Mutational studies suggest that the MUN-bound template complex is functionally important for SNARE assembly and neurotransmitter release. Taken together, our observations provide a potential molecular mechanism by which Munc13-1 and Munc18-1 cooperatively chaperone SNARE folding and assembly, thereby regulating synaptic vesicle fusion.
DOI
Munc13-1 is a large multifunctional protein essential for synaptic vesicle fusion and neurotransmitter release. Its dysfunction has been linked to many neurological disorders. Evidence suggests that the MUN domain of Munc13-1 collaborates with Munc18-1 to initiate SNARE assembly, thereby priming vesicles for fast calcium-triggered vesicle fusion. The underlying molecular mechanism, however, is poorly understood. Recently, it was found that Munc18-1 catalyzes neuronal SNARE assembly through an obligate template complex intermediate containing Munc18-1 and 2 SNARE proteins—syntaxin 1 and VAMP2. Here, using single-molecule force spectroscopy, we discovered that the MUN domain of Munc13-1 stabilizes the template complex by ∼2.1 kBT. The MUN-bound template complex enhances SNAP-25 binding to the templated SNAREs and subsequent full SNARE assembly. Mutational studies suggest that the MUN-bound template complex is functionally important for SNARE assembly and neurotransmitter release. Taken together, our observations provide a potential molecular mechanism by which Munc13-1 and Munc18-1 cooperatively chaperone SNARE folding and assembly, thereby regulating synaptic vesicle fusion.
DOI
Specific Nucleic Acid Chaperone Activity of HIV-1 Nucleocapsid Protein Deduced from Hairpin Unfolding
Micah J. McCauley, Ioulia Rouzina, Mark C. Williams
RNA and DNA hairpin formation and disruption play key regulatory roles in a variety of cellular processes. The 59-nucleotide transactivation response (TAR) RNA hairpin facilitates the production of full-length transcripts of the HIV-1 genome. Yet the stability of this long, irregular hairpin becomes a liability during reverse transcription as 24 base pairs must be disrupted for strand transfer. Retroviral nucleocapsid (NC) proteins serve as nucleic acid chaperones that have been shown to both destabilize the TAR hairpin and facilitate strand annealing with its complementary DNA sequence. Yet it has remained difficult to elucidate the way NC targets and dramatically destabilizes this hairpin while only weakly affecting the annealed product. In this work, we used optical tweezers to measure the stability of TAR and found that adding NC destabilized the hairpin and simultaneously caused a distinct change in both the height and location of the energy barrier. This data was matched to an energy landscape predicted from a simple theory of definite base pair destabilization. Comparisons revealed the specific binding sites found by NC along the irregular TAR hairpin. Furthermore, specific binding explained both the unusual shift in the transition state and the much weaker effect on the annealed product. These experiments illustrate a general method of energy landscape transformation that exposes important physical insights.
DOI
RNA and DNA hairpin formation and disruption play key regulatory roles in a variety of cellular processes. The 59-nucleotide transactivation response (TAR) RNA hairpin facilitates the production of full-length transcripts of the HIV-1 genome. Yet the stability of this long, irregular hairpin becomes a liability during reverse transcription as 24 base pairs must be disrupted for strand transfer. Retroviral nucleocapsid (NC) proteins serve as nucleic acid chaperones that have been shown to both destabilize the TAR hairpin and facilitate strand annealing with its complementary DNA sequence. Yet it has remained difficult to elucidate the way NC targets and dramatically destabilizes this hairpin while only weakly affecting the annealed product. In this work, we used optical tweezers to measure the stability of TAR and found that adding NC destabilized the hairpin and simultaneously caused a distinct change in both the height and location of the energy barrier. This data was matched to an energy landscape predicted from a simple theory of definite base pair destabilization. Comparisons revealed the specific binding sites found by NC along the irregular TAR hairpin. Furthermore, specific binding explained both the unusual shift in the transition state and the much weaker effect on the annealed product. These experiments illustrate a general method of energy landscape transformation that exposes important physical insights.
DOI
Mass Accommodation Coefficients of Water on Organics from Complementary Photoacoustic and Light Scattering Measurements on Laser-Trapped Droplets
Sandra Roy, Matus E. Diveky, Ruth Signorell
The mass accommodation coefficient, αM, describes evaporation and condensation kinetics at the liquid–vapor interface. In spite of numerous experimental efforts, reliable values of αM are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296 K and to decrease by more than an order of magnitude when the temperature increases to 309 K. The experimentally observed temperature dependence of αM shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on αM. No concentration dependence of αM is observed at low water concentrations, while at elevated water concentrations, we observe a 5-fold increase in αM. The technique presented in this work has the potential to become a reliable method for the retrieval of αM values at liquid–vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.
DOI
The mass accommodation coefficient, αM, describes evaporation and condensation kinetics at the liquid–vapor interface. In spite of numerous experimental efforts, reliable values of αM are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296 K and to decrease by more than an order of magnitude when the temperature increases to 309 K. The experimentally observed temperature dependence of αM shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on αM. No concentration dependence of αM is observed at low water concentrations, while at elevated water concentrations, we observe a 5-fold increase in αM. The technique presented in this work has the potential to become a reliable method for the retrieval of αM values at liquid–vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.
DOI
Homologous Recombination under the Single-Molecule Fluorescence Microscope
Dalton R. Gibbs and Soma Dhakal
Homologous recombination (HR) is a complex biological process and is central to meiosis and for repair of DNA double-strand breaks. Although the HR process has been the subject of intensive study for more than three decades, the complex protein–protein and protein–DNA interactions during HR present a significant challenge for determining the molecular mechanism(s) of the process. This knowledge gap is largely because of the dynamic interactions between HR proteins and DNA which is difficult to capture by routine biochemical or structural biology methods. In recent years, single-molecule fluorescence microscopy has been a popular method in the field of HR to visualize these complex and dynamic interactions at high spatiotemporal resolution, revealing mechanistic insights of the process. In this review, we describe recent efforts that employ single-molecule fluorescence microscopy to investigate protein–protein and protein–DNA interactions operating on three key DNA-substrates: single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), and four-way DNA called Holliday junction (HJ). We also outline the technological advances and several key insights revealed by these studies in terms of protein assembly on these DNA substrates and highlight the foreseeable promise of single-molecule fluorescence microscopy in advancing our understanding of homologous recombination.
DOI
Homologous recombination (HR) is a complex biological process and is central to meiosis and for repair of DNA double-strand breaks. Although the HR process has been the subject of intensive study for more than three decades, the complex protein–protein and protein–DNA interactions during HR present a significant challenge for determining the molecular mechanism(s) of the process. This knowledge gap is largely because of the dynamic interactions between HR proteins and DNA which is difficult to capture by routine biochemical or structural biology methods. In recent years, single-molecule fluorescence microscopy has been a popular method in the field of HR to visualize these complex and dynamic interactions at high spatiotemporal resolution, revealing mechanistic insights of the process. In this review, we describe recent efforts that employ single-molecule fluorescence microscopy to investigate protein–protein and protein–DNA interactions operating on three key DNA-substrates: single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), and four-way DNA called Holliday junction (HJ). We also outline the technological advances and several key insights revealed by these studies in terms of protein assembly on these DNA substrates and highlight the foreseeable promise of single-molecule fluorescence microscopy in advancing our understanding of homologous recombination.
DOI
Tuesday, January 14, 2020
Optimizing optical trap stiffness for Rayleigh particles with an Airy array beam
Rafael A. B. Suarez, Antonio A. R. Neves, and Marcos R. R. Gesualdi
Airy array beams are attractive for optical manipulation of particles owing to their non-diffraction and auto-focusing properties. An Airy array beam is composed of 𝑁 Airy beams that accelerate mutually and symmetrically in opposite directions, for different ballistics trajectories, i.e., with different initial launch angles. Based on this, we investigate the optical force distribution acting on Rayleigh particles. Results show that it is possible to obtain greater stability for optical trapping by increasing the number of beams in the array. Also, the intensity focal point and gradient and scattering force of the array on Rayleigh particles can be controlled through a launch angle parameter.
DOI
Airy array beams are attractive for optical manipulation of particles owing to their non-diffraction and auto-focusing properties. An Airy array beam is composed of 𝑁 Airy beams that accelerate mutually and symmetrically in opposite directions, for different ballistics trajectories, i.e., with different initial launch angles. Based on this, we investigate the optical force distribution acting on Rayleigh particles. Results show that it is possible to obtain greater stability for optical trapping by increasing the number of beams in the array. Also, the intensity focal point and gradient and scattering force of the array on Rayleigh particles can be controlled through a launch angle parameter.
DOI
A novel approach to study the interactions between polymeric stabilized micron-sized oil droplets by optical tweezers
An Chen, Shao-Wei Li, Jian-Hong Xu
The well understanding of interaction forces between single dispersed droplets is crucial to the understanding of emulsion stabilization mechanism. Recently, many studies have reported the direct quantitative measurements of interaction forces between 20-200 μm single droplets coated polymers by atomic force microscope (AFM). These studies have revealed many important results about the relationship of the interaction forces and the droplet deformation. However, these studies of the quantitative relationship between the measured interaction forces and the separation distance of the front end of the droplet have rarely been reported. Optical tweezers instrument can make it possible to establish the quantitative relationship between the measured force and the separation distance of the front end of the droplet, which will make better understanding of the interaction mechanisms between droplets. Due to the differences of the measuring mechanism between atomic force microscopy (AFM) and optical tweezers, the theory model of AFM measurements cannot be fitted with the force measurement by optical tweezers. We have made an exhaustive comparison of the measuring differences between AFM and optical tweezers instrument in this work. Moreover, we built a numerical model to derive the repulsive pressure through the measured force curve in order to quantify the measured force of two micron-sized oil droplets coated polymers by optical tweezers. Furthermore, the novel method can be extended to other micron-sized emulsion systems, and these findings will be a vital progress on quantitative force measurements between micron-sized droplets.
DOI
The well understanding of interaction forces between single dispersed droplets is crucial to the understanding of emulsion stabilization mechanism. Recently, many studies have reported the direct quantitative measurements of interaction forces between 20-200 μm single droplets coated polymers by atomic force microscope (AFM). These studies have revealed many important results about the relationship of the interaction forces and the droplet deformation. However, these studies of the quantitative relationship between the measured interaction forces and the separation distance of the front end of the droplet have rarely been reported. Optical tweezers instrument can make it possible to establish the quantitative relationship between the measured force and the separation distance of the front end of the droplet, which will make better understanding of the interaction mechanisms between droplets. Due to the differences of the measuring mechanism between atomic force microscopy (AFM) and optical tweezers, the theory model of AFM measurements cannot be fitted with the force measurement by optical tweezers. We have made an exhaustive comparison of the measuring differences between AFM and optical tweezers instrument in this work. Moreover, we built a numerical model to derive the repulsive pressure through the measured force curve in order to quantify the measured force of two micron-sized oil droplets coated polymers by optical tweezers. Furthermore, the novel method can be extended to other micron-sized emulsion systems, and these findings will be a vital progress on quantitative force measurements between micron-sized droplets.
DOI
Single-Molecule Nanotechnologies: An Evolution in Biological Dynamics Detection
Yu Li, Lihua Zhao, Yuan Yao, Xuefeng Guo
Single-molecule detection is a rapidly developing area within the analytical chemistry field that requires ultrasensitive technologies to detect a range of molecules. Over the past few decades, various optically-, mechanically-, and electrically-based strategies have been employed for single-molecule detection to uncover information in biological processes. These strategies enable real-time monitoring with single-molecule/single-event sensitivity. In addition, their high temporal resolution enables investigation of the underlying mechanisms of biological functions from static to dynamic, from qualitative to quantitative, and from one to multiple disciplines. In this review, we provide a brief overview of the prominent, real-time single-molecule detection nanotechnologies and their potential applications within the life science fields.
DOI
Single-molecule detection is a rapidly developing area within the analytical chemistry field that requires ultrasensitive technologies to detect a range of molecules. Over the past few decades, various optically-, mechanically-, and electrically-based strategies have been employed for single-molecule detection to uncover information in biological processes. These strategies enable real-time monitoring with single-molecule/single-event sensitivity. In addition, their high temporal resolution enables investigation of the underlying mechanisms of biological functions from static to dynamic, from qualitative to quantitative, and from one to multiple disciplines. In this review, we provide a brief overview of the prominent, real-time single-molecule detection nanotechnologies and their potential applications within the life science fields.
DOI
Monday, January 13, 2020
Nanoscale Inorganic Motors Driven by Light: Principles, Realizations, and Opportunities
Hana Šípová-Jungová, Daniel Andrén, Steven Jones, Mikael Käll
The prospect of self-propelled artificial machines small enough to navigate within biological matter has fascinated and inspired researchers and the public alike since the dawn of nanotechnology. Despite many obstacles toward the realization of such devices, impressive progress on the development of its basic building block, the nanomotor, has been made over the past decade. Here, we review this emerging area with a focus on inorganic nanomotors driven or activated by light. We outline the distinct challenges and opportunities that differentiate nanomotors from micromotors based on a discussion of how stochastic forces influence the active motion of small particles. We introduce the relevant light–matter interactions and discuss how these can be utilized to classify nanomotors into three broad classes: nanomotors driven by optical momentum transfer, photothermal heating, and photocatalysis, respectively. On the basis of this classification, we then summarize and discuss the diverse body of nanomotor literature. We finally give a brief outlook on future challenges and possibilities in this rapidly evolving research area.
DOI
The prospect of self-propelled artificial machines small enough to navigate within biological matter has fascinated and inspired researchers and the public alike since the dawn of nanotechnology. Despite many obstacles toward the realization of such devices, impressive progress on the development of its basic building block, the nanomotor, has been made over the past decade. Here, we review this emerging area with a focus on inorganic nanomotors driven or activated by light. We outline the distinct challenges and opportunities that differentiate nanomotors from micromotors based on a discussion of how stochastic forces influence the active motion of small particles. We introduce the relevant light–matter interactions and discuss how these can be utilized to classify nanomotors into three broad classes: nanomotors driven by optical momentum transfer, photothermal heating, and photocatalysis, respectively. On the basis of this classification, we then summarize and discuss the diverse body of nanomotor literature. We finally give a brief outlook on future challenges and possibilities in this rapidly evolving research area.
DOI
Brownian Disks Lab: Simulating time-lapse microscopy experiments for exploring microrheology techniques and colloidal interactions
Pablo Domínguez-García
Brownian Disks Lab (BDL) is a Java-based application for the real-time generation and visualization of the motion of two-dimensional Brownian disks using Brownian Dynamics (BD) simulations. This software is designed to emulate time-lapse microscopy experiments of colloidal fluids in quasi-2D situations, such as sedimented layers of particles, optical trap confinement, or fluid interfaces. Microrheology of bio-inspired fluids through optical-based techniques such as videomicroscopy is a classic tool for obtaining the mechanical properties and molecular behavior of these materials. The results obtained by microrheology notably depend of the time-lapse value of the videomicroscopy setup, therefore, a tool to test the influence of the lack of statistics by simulating Brownian objects in experimental-like situations is needed. We simulate a colloidal fluid by using Brownian Dynamics (BD) simulations, where the particles are subjected to different external applied forces and inter-particle interactions. This software has been tested for the analysis of the microrheological consequences of attractive forces between particles [1], the influence of image analysis on experimental microrheological results [2], and to explore experimental diffusion with optical tweezers [3]. The output results of BDL are directly compatible with the format used by standard microrheological algorithms [4]. In a context of microrheology of complex bio-inspired fluids, we use this tool here to study if the lack of statistics may influence the observed potential of a bead trapped by optical tweezers.
DOI
Brownian Disks Lab (BDL) is a Java-based application for the real-time generation and visualization of the motion of two-dimensional Brownian disks using Brownian Dynamics (BD) simulations. This software is designed to emulate time-lapse microscopy experiments of colloidal fluids in quasi-2D situations, such as sedimented layers of particles, optical trap confinement, or fluid interfaces. Microrheology of bio-inspired fluids through optical-based techniques such as videomicroscopy is a classic tool for obtaining the mechanical properties and molecular behavior of these materials. The results obtained by microrheology notably depend of the time-lapse value of the videomicroscopy setup, therefore, a tool to test the influence of the lack of statistics by simulating Brownian objects in experimental-like situations is needed. We simulate a colloidal fluid by using Brownian Dynamics (BD) simulations, where the particles are subjected to different external applied forces and inter-particle interactions. This software has been tested for the analysis of the microrheological consequences of attractive forces between particles [1], the influence of image analysis on experimental microrheological results [2], and to explore experimental diffusion with optical tweezers [3]. The output results of BDL are directly compatible with the format used by standard microrheological algorithms [4]. In a context of microrheology of complex bio-inspired fluids, we use this tool here to study if the lack of statistics may influence the observed potential of a bead trapped by optical tweezers.
DOI
Light Pressure on an Inhomogeneous Spherical Particle in the Field of Laser Tweezers
R. Artser, Yu. V. Rozhdestvenskii
The light pressure on an inhomogeneous spherical dielectric particle consisting of a shell and a core with different refractive indices is considered. The forces acting on this particle in a laser beam with a Gaussian intensity profile have been found in the geometrical optics approximation. The radiation propagation is analyzed by taking into account the different refractive indices of the core, the shell, and the medium surrounding the particle. The light pressure is shown to depend significantly on the core radius, which allows the core size to be estimated from the spatial dynamics of the particle in the field of optical tweezers.
DOI
The light pressure on an inhomogeneous spherical dielectric particle consisting of a shell and a core with different refractive indices is considered. The forces acting on this particle in a laser beam with a Gaussian intensity profile have been found in the geometrical optics approximation. The radiation propagation is analyzed by taking into account the different refractive indices of the core, the shell, and the medium surrounding the particle. The light pressure is shown to depend significantly on the core radius, which allows the core size to be estimated from the spatial dynamics of the particle in the field of optical tweezers.
DOI
Real-Time Monitoring of Temperature Variations around a Gold Nanobipyramid Targeted Cancer Cell under Photothermal Heating by Actively Manipulating an Optically Trapped Luminescent Upconversion Microparticle
Ya-Feng Kang, Bei Zheng, Cheng-Yu Li, Zhi-Ling Zhang, Hong-Wu Tang, Qiong-Shui Wu, Dai-Wen Pang
We demonstrate an effective approach to realize active and real-time temperature monitoring around the gold nanobipyramids (AuNBPs)-labeled cancer cell under 808 nm laser irradiation by combining optical tweezers and temperature-sensitive upconversion microparticles (UCMPs). On the one hand, the aptamer-modified AuNBPs that absorb laser at 808 nm not only act as an excellent photothermal reagent but also accurately and specifically bind the target cancer cells. On the other hand, the single optically trapped NaYF4:Yb3+, Er3+ UCMPs with a 980 nm laser exhibit temperature-dependent luminescence properties, where the intensity ratio of emission 525 and 547 nm varies with the ambient temperature. Therefore, real-time temperature variation monitoring is performed by 3D manipulation of the trapped single UCMP to control its distance from the AuNBPs-labeled cancer cell while being photothermally killed. The results show distance-related thermal propagation because the temperature increase reaches as high as 10 °C at a distance of 5 μm from the cell, whereas the temperature difference drops rapidly to 5 °C when this distance increases to 15 μm. This approach shows that the photothermal conversion from AuNBPs is sufficient to kill the cancer cells, and the temperature increase can be controlled within the micrometer level at a certain period of time. Overall, we present a micrometer-size thermometer platform and provide an innovative strategy to measure temperature at the micrometer level during photothermal killing of cancer cells.
DOI
We demonstrate an effective approach to realize active and real-time temperature monitoring around the gold nanobipyramids (AuNBPs)-labeled cancer cell under 808 nm laser irradiation by combining optical tweezers and temperature-sensitive upconversion microparticles (UCMPs). On the one hand, the aptamer-modified AuNBPs that absorb laser at 808 nm not only act as an excellent photothermal reagent but also accurately and specifically bind the target cancer cells. On the other hand, the single optically trapped NaYF4:Yb3+, Er3+ UCMPs with a 980 nm laser exhibit temperature-dependent luminescence properties, where the intensity ratio of emission 525 and 547 nm varies with the ambient temperature. Therefore, real-time temperature variation monitoring is performed by 3D manipulation of the trapped single UCMP to control its distance from the AuNBPs-labeled cancer cell while being photothermally killed. The results show distance-related thermal propagation because the temperature increase reaches as high as 10 °C at a distance of 5 μm from the cell, whereas the temperature difference drops rapidly to 5 °C when this distance increases to 15 μm. This approach shows that the photothermal conversion from AuNBPs is sufficient to kill the cancer cells, and the temperature increase can be controlled within the micrometer level at a certain period of time. Overall, we present a micrometer-size thermometer platform and provide an innovative strategy to measure temperature at the micrometer level during photothermal killing of cancer cells.
DOI
Switchable Assembly and Guidance of Colloidal Particles on an All-Dielectric One-Dimensional Photonic Crystal
Fengya Lu, Yan Kuai, Junxue Chen, Xi Tang, Yifeng Xiang, Yang Liu, Pei Wang, Joseph. R. Lakowicz, and Douguo Zhang
Dielectric multilayer photonic-band-gap structures, called one-dimensional photonic crystals (1DPCs), have drawn considerable attention in the fields of physics, chemistry, and biophotonics. Here, experimental results verify the feasibility of a 1DPC working as a substrate for switchable manipulations of colloidal microparticles. The optically induced thermal convective force on a 1DPC can assemble colloidal particles that are dispersed in a water solution, while the photonic scattering force on the same 1DPC caused by propagating evanescent waves can guide these particles. Additionally, in the 1DPC, one internal mode can be excited that has seldom been noticed previously. This mode shows an ability to assemble particles over large areas even when the incident power is low. The assembly and guidance of colloidal particles on the 1DPC are switchable just through tuning the polarization and angle of the incident laser beam. Numerical simulations are carried out, which are consistent with these experimental observations.
DOI
Dielectric multilayer photonic-band-gap structures, called one-dimensional photonic crystals (1DPCs), have drawn considerable attention in the fields of physics, chemistry, and biophotonics. Here, experimental results verify the feasibility of a 1DPC working as a substrate for switchable manipulations of colloidal microparticles. The optically induced thermal convective force on a 1DPC can assemble colloidal particles that are dispersed in a water solution, while the photonic scattering force on the same 1DPC caused by propagating evanescent waves can guide these particles. Additionally, in the 1DPC, one internal mode can be excited that has seldom been noticed previously. This mode shows an ability to assemble particles over large areas even when the incident power is low. The assembly and guidance of colloidal particles on the 1DPC are switchable just through tuning the polarization and angle of the incident laser beam. Numerical simulations are carried out, which are consistent with these experimental observations.
DOI
Optical trapping and arrangement with reconfigurable "bottle" beam for digital holographic microscopy
N V Shostka, B N Sokolenko, O S Karakcheva, D A Poletaev, A O Titova, A V Prisyazhniuk and I A Ismailov
The design and construction of optical tweezers based on uniaxial crystal anisotropy for generation of adjustable "bottle" beam trap carrying optical vortex with orbital angular momentum is considered. In coupling with digital holographic microscopy, optical trapping becomes a high precision instrument for visualization, shape definition and refractivity measurements of isolated micro structures and biological objects in-situ. The non-destructive and sterile non-contact tweezing of specimens or their parts in localized intensity minima of coherent vortex beam was performed with using of 200 mW semiconductor 532 nm trapping laser and LiNbO3 crystal. Visualization and position control of trapped marine centric diatoms was performed by a lens-free axial digital holographic microscopy in liquid medium.
DOI
The design and construction of optical tweezers based on uniaxial crystal anisotropy for generation of adjustable "bottle" beam trap carrying optical vortex with orbital angular momentum is considered. In coupling with digital holographic microscopy, optical trapping becomes a high precision instrument for visualization, shape definition and refractivity measurements of isolated micro structures and biological objects in-situ. The non-destructive and sterile non-contact tweezing of specimens or their parts in localized intensity minima of coherent vortex beam was performed with using of 200 mW semiconductor 532 nm trapping laser and LiNbO3 crystal. Visualization and position control of trapped marine centric diatoms was performed by a lens-free axial digital holographic microscopy in liquid medium.
DOI
Thursday, January 9, 2020
Impact of Nanocapsules on Red Blood Cells Interplay Jointly Assessed by Optical Tweezers and Microscopy
Tatiana Avsievich,Yana Tarakanchikova, Ruixue Zhu, Alexey Popov, Alexander Bykov, Ilya Skovorodkin, Seppo Vainio and Igor Meglinski
In the framework of novel medical paradigm the red blood cells (RBCs) have a great potential to be used as drug delivery carriers. This approach requires an ultimate understanding of the peculiarities of mutual interaction of RBC influenced by nano-materials composed the drugs. Optical tweezers (OT) is widely used to explore mechanisms of cells’ interaction with the ability to trap non-invasively, manipulate and displace living cells with a notably high accuracy. In the current study, the mutual interaction of RBC with polymeric nano-capsules (NCs) is investigated utilizing a two-channel OT system. The obtained results suggest that, in the presence of NCs, the RBC aggregation in plasma satisfies the ‘cross-bridges’ model. Complementarily, the allocation of NCs on the RBC membrane was observed by scanning electron microscopy (SEM), while for assessment of NCs-induced morphological changes the tests with the human mesenchymal stem cells (hMSC) was performed. The combined application of OT and advanced microscopy approaches brings new insights into the conception of direct observation of cells interaction influenced by NCs for the estimation of possible cytotoxic effects.
DOI
In the framework of novel medical paradigm the red blood cells (RBCs) have a great potential to be used as drug delivery carriers. This approach requires an ultimate understanding of the peculiarities of mutual interaction of RBC influenced by nano-materials composed the drugs. Optical tweezers (OT) is widely used to explore mechanisms of cells’ interaction with the ability to trap non-invasively, manipulate and displace living cells with a notably high accuracy. In the current study, the mutual interaction of RBC with polymeric nano-capsules (NCs) is investigated utilizing a two-channel OT system. The obtained results suggest that, in the presence of NCs, the RBC aggregation in plasma satisfies the ‘cross-bridges’ model. Complementarily, the allocation of NCs on the RBC membrane was observed by scanning electron microscopy (SEM), while for assessment of NCs-induced morphological changes the tests with the human mesenchymal stem cells (hMSC) was performed. The combined application of OT and advanced microscopy approaches brings new insights into the conception of direct observation of cells interaction influenced by NCs for the estimation of possible cytotoxic effects.
DOI
Studying the different coupling regimes for a plasmonic particle in a plasmonic trap
Jeonghyeon Kim and Olivier J. F. Martin
Plasmonic antennas improve the stiffness and resolution of optical tweezers by producing a strong near-field. When the antenna traps metallic objects, the optically-resonant object affects the near-field trap, and this interaction should be examined to estimate the optical force accurately. We study this effect in detail by evaluating the force using both Maxwell’s stress tensor and the dipole approximation. In spite of the strong optical interaction between the particle and the antenna, the results show that the dipole approximation remains accurate for calculating forces on Rayleigh particles. For particles whose sizes exceed the dipole limit, we observe different coupling regimes where the force becomes either attractive or repulsive. The distributions of field amplitudes and polarization charges explain such a behavior.
DOI
Plasmonic antennas improve the stiffness and resolution of optical tweezers by producing a strong near-field. When the antenna traps metallic objects, the optically-resonant object affects the near-field trap, and this interaction should be examined to estimate the optical force accurately. We study this effect in detail by evaluating the force using both Maxwell’s stress tensor and the dipole approximation. In spite of the strong optical interaction between the particle and the antenna, the results show that the dipole approximation remains accurate for calculating forces on Rayleigh particles. For particles whose sizes exceed the dipole limit, we observe different coupling regimes where the force becomes either attractive or repulsive. The distributions of field amplitudes and polarization charges explain such a behavior.
DOI
HIV restriction factor APOBEC3G binds in multiple steps and conformations to search and deaminate single-stranded DNA
Michael Morse, M Nabuan Naufer, Yuqing Feng, Linda Chelico, Ioulia Rouzina, Mark C Williams
APOBEC3G (A3G), an enzyme expressed in primates with the potential to inhibit human immunodeficiency virus type 1 (HIV-1) infectivity, is a single-stranded DNA (ssDNA) deoxycytidine deaminase with two domains, a catalytically active, weakly ssDNA binding C-terminal domain (CTD) and a catalytically inactive, strongly ssDNA binding N-terminal domain (NTD). Using optical tweezers, we measure A3G binding a single, long ssDNA substrate under various applied forces to characterize the binding interaction. A3G binds ssDNA in multiple steps and in two distinct conformations, distinguished by degree of ssDNA contraction. A3G stabilizes formation of ssDNA loops, an ability inhibited by A3G oligomerization. Our data suggests A3G securely binds ssDNA through the NTD, while the CTD samples and potentially deaminates the substrate. Oligomerization of A3G stabilizes ssDNA binding but inhibits the CTD’s search function. These processes explain A3G’s ability to efficiently deaminate numerous sites across a 10,000 base viral genome during the reverse transcription process.
DOI
APOBEC3G (A3G), an enzyme expressed in primates with the potential to inhibit human immunodeficiency virus type 1 (HIV-1) infectivity, is a single-stranded DNA (ssDNA) deoxycytidine deaminase with two domains, a catalytically active, weakly ssDNA binding C-terminal domain (CTD) and a catalytically inactive, strongly ssDNA binding N-terminal domain (NTD). Using optical tweezers, we measure A3G binding a single, long ssDNA substrate under various applied forces to characterize the binding interaction. A3G binds ssDNA in multiple steps and in two distinct conformations, distinguished by degree of ssDNA contraction. A3G stabilizes formation of ssDNA loops, an ability inhibited by A3G oligomerization. Our data suggests A3G securely binds ssDNA through the NTD, while the CTD samples and potentially deaminates the substrate. Oligomerization of A3G stabilizes ssDNA binding but inhibits the CTD’s search function. These processes explain A3G’s ability to efficiently deaminate numerous sites across a 10,000 base viral genome during the reverse transcription process.
DOI
Statistical physics and mesoscopic modeling to interpret tethered particle motion experiments
Manoel Manghi, Nicolas Destainville, Annaël Brunet
Tethered particle motion experiments are versatile single-molecule techniques enabling one to address in vitro the molecular properties of DNA and its interactions with various partners involved in genetic regulations. These techniques provide raw data such as the tracked particle amplitude of movement, from which relevant information about DNA conformations or states must be recovered. Solving this inverse problem appeals to specific theoretical tools that have been designed in the two last decades, together with the data pre-processing procedures that ought to be implemented to avoid biases inherent to these experimental techniques. These statistical tools and models are reviewed in this paper.
DOI
Tethered particle motion experiments are versatile single-molecule techniques enabling one to address in vitro the molecular properties of DNA and its interactions with various partners involved in genetic regulations. These techniques provide raw data such as the tracked particle amplitude of movement, from which relevant information about DNA conformations or states must be recovered. Solving this inverse problem appeals to specific theoretical tools that have been designed in the two last decades, together with the data pre-processing procedures that ought to be implemented to avoid biases inherent to these experimental techniques. These statistical tools and models are reviewed in this paper.
DOI
From Strong Dichroic Nanomotor to Polarotactic Microswimmer
Xiaojun Zhan, Jing Zheng, Yang Zhao, Bairen Zhu, Rui Cheng, Jizhuang Wang, Jun Liu, Jiang Tang, Jinyao Tang
Light‐driven micro/nanomotors are promising candidates for long‐envisioned next‐generation nanorobotics for targeted drug delivery, noninvasive surgery, nanofabrication, and beyond. To achieve these fantastic applications, effective control of the micro/nanomotor is essential. Light has been proved as the most versatile method for microswimmer manipulation, while the light propagation direction, intensity, and wavelength have been explored as controlling signals for light‐responsive nanomotors. Here, the controlling method is expanded to the polarization state of the light, and a nanomotor with a significant dichroic ratio is demonstrated. Due to the anisotropic crystal structure, light polarized parallel to the Sb2Se3 nanowires is preferentially absorbed. The core–shell Sb2Se3/ZnO nanomotor exhibits strong dichroic swimming behavior: the swimming speed is ≈3 times faster when illuminated with parallel polarized light than perpendicular polarized light. Furthermore, by incorporating two cross‐aligned dichroic nanomotors, a polarotactic artificial microswimmer is achieved, which can be navigated by controlling the polarization direction of the incident light. Compared to the well‐studied light‐driven rotary motors based on optical tweezers, this dichroic microswimmer offers eight orders of magnitude light‐intensity reduction, which may enable large‐scale nanomanipulation as well as other heat‐sensitive applications.
DOI
Light‐driven micro/nanomotors are promising candidates for long‐envisioned next‐generation nanorobotics for targeted drug delivery, noninvasive surgery, nanofabrication, and beyond. To achieve these fantastic applications, effective control of the micro/nanomotor is essential. Light has been proved as the most versatile method for microswimmer manipulation, while the light propagation direction, intensity, and wavelength have been explored as controlling signals for light‐responsive nanomotors. Here, the controlling method is expanded to the polarization state of the light, and a nanomotor with a significant dichroic ratio is demonstrated. Due to the anisotropic crystal structure, light polarized parallel to the Sb2Se3 nanowires is preferentially absorbed. The core–shell Sb2Se3/ZnO nanomotor exhibits strong dichroic swimming behavior: the swimming speed is ≈3 times faster when illuminated with parallel polarized light than perpendicular polarized light. Furthermore, by incorporating two cross‐aligned dichroic nanomotors, a polarotactic artificial microswimmer is achieved, which can be navigated by controlling the polarization direction of the incident light. Compared to the well‐studied light‐driven rotary motors based on optical tweezers, this dichroic microswimmer offers eight orders of magnitude light‐intensity reduction, which may enable large‐scale nanomanipulation as well as other heat‐sensitive applications.
DOI
Opto-thermophoretic separation and trapping of plasmonic nanoparticles
Kenji Setoura, Tetsuro Tsuji, Syoji Ito, Satoyuki Kawanoa and Hiroshi Miyasaka
Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 μm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.
DOI
Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 μm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.
DOI
Wednesday, January 8, 2020
Streamlining effects of extra telomeric repeat on telomeric DNA folding revealed by fluorescence-force spectroscopy
Jaba Mitra, Taekjip Ha
A human telomere ends in a single-stranded 3′ tail, composed of repeats of T2AG3. G-quadruplexes (GQs) formed from four consecutive repeats have been shown to possess high-structural and mechanical diversity. In principle, a GQ can form from any four repeats that are not necessarily consecutive. To understand the dynamics of GQs with positional multiplicity, we studied five and six repeats human telomeric sequence using a combination of single molecule FRET and optical tweezers. Our results suggest preferential formation of GQs at the 3′ end both in K+ and Na+ solutions, with minor populations of 5′-GQ or long-loop GQs. A vectorial folding assay which mimics the directional nature of telomere extension showed that the 3′ preference holds even when folding is allowed to begin from the 5′ side. In 100 mM K+, the unassociated T2AG3 segment has a streamlining effect in that one or two mechanically distinct species was observed at a single position instead of six or more observed without an unassociated repeat. We did not observe such streamlining effect in 100 mM Na+. Location of GQ and reduction in conformational diversity in the presence of extra repeats have implications in telomerase inhibition, T-loop formation and telomere end protection.
DOI
A human telomere ends in a single-stranded 3′ tail, composed of repeats of T2AG3. G-quadruplexes (GQs) formed from four consecutive repeats have been shown to possess high-structural and mechanical diversity. In principle, a GQ can form from any four repeats that are not necessarily consecutive. To understand the dynamics of GQs with positional multiplicity, we studied five and six repeats human telomeric sequence using a combination of single molecule FRET and optical tweezers. Our results suggest preferential formation of GQs at the 3′ end both in K+ and Na+ solutions, with minor populations of 5′-GQ or long-loop GQs. A vectorial folding assay which mimics the directional nature of telomere extension showed that the 3′ preference holds even when folding is allowed to begin from the 5′ side. In 100 mM K+, the unassociated T2AG3 segment has a streamlining effect in that one or two mechanically distinct species was observed at a single position instead of six or more observed without an unassociated repeat. We did not observe such streamlining effect in 100 mM Na+. Location of GQ and reduction in conformational diversity in the presence of extra repeats have implications in telomerase inhibition, T-loop formation and telomere end protection.
DOI
Subfemtonewton force fields measured with ergodic Brownian ensembles
Minghao Li, Oussama Sentissi, Stefano Azzini, Gabriel Schnoering, Antoine Canaguier-Durand, and Cyriaque Genet
We demonstrate that radiation pressure force fields can be measured and reconstructed with a resolution of 0.3 fN (at a 99.7% confidence level) using an ergodic ensemble of overdamped colloidal particles. The outstanding force resolution level is provided by the large size of the statistical ensemble built by recording all displacements from all diffusing particles, regardless of trajectory and time. This is only possible because the noise driving the particles is thermal, white, and stationary, so that the colloidal system is ergodic, as we carefully verify. Using an ergodic colloidal dispersion for performing ultrasensitive measurements of external forces is not limited to nonconservative optical force fields. Our experiments therefore give way to interesting opportunities in the context of weak force measurements in fluids.
DOI
We demonstrate that radiation pressure force fields can be measured and reconstructed with a resolution of 0.3 fN (at a 99.7% confidence level) using an ergodic ensemble of overdamped colloidal particles. The outstanding force resolution level is provided by the large size of the statistical ensemble built by recording all displacements from all diffusing particles, regardless of trajectory and time. This is only possible because the noise driving the particles is thermal, white, and stationary, so that the colloidal system is ergodic, as we carefully verify. Using an ergodic colloidal dispersion for performing ultrasensitive measurements of external forces is not limited to nonconservative optical force fields. Our experiments therefore give way to interesting opportunities in the context of weak force measurements in fluids.
DOI
Optimizing Brownian escape rates by potential shaping
Marie Chupeau, Jannes Gladrow, Alexei Chepelianskii, Ulrich F. Keyser, and Emmanuel Trizac
Brownian escape is key to a wealth of physico-chemical processes, including polymer folding and information storage. The frequency of thermally activated energy barrier crossings is assumed to generally decrease exponentially with increasing barrier height. Here, we show experimentally that higher, fine-tuned barrier profiles result in significantly enhanced escape rates, in breach of the intuition relying on the above scaling law, and address in theory the corresponding conditions for maximum speed-up. Importantly, our barriers end on the same energy on which they start. For overdamped dynamics, the achievable boost of escape rates is, in principle, unbounded so that the barrier optimization has to be regularized. We derive optimal profiles under 2 different regularizations and uncover the efficiency of N-shaped barriers. We then demonstrate the viability of such a potential in automated microfluidic Brownian dynamics experiments using holographic optical tweezers and achieve a doubling of escape rates compared to unhindered Brownian motion. Finally, we show that this escape rate boost extends into the low-friction inertial regime.
DOI
Brownian escape is key to a wealth of physico-chemical processes, including polymer folding and information storage. The frequency of thermally activated energy barrier crossings is assumed to generally decrease exponentially with increasing barrier height. Here, we show experimentally that higher, fine-tuned barrier profiles result in significantly enhanced escape rates, in breach of the intuition relying on the above scaling law, and address in theory the corresponding conditions for maximum speed-up. Importantly, our barriers end on the same energy on which they start. For overdamped dynamics, the achievable boost of escape rates is, in principle, unbounded so that the barrier optimization has to be regularized. We derive optimal profiles under 2 different regularizations and uncover the efficiency of N-shaped barriers. We then demonstrate the viability of such a potential in automated microfluidic Brownian dynamics experiments using holographic optical tweezers and achieve a doubling of escape rates compared to unhindered Brownian motion. Finally, we show that this escape rate boost extends into the low-friction inertial regime.
DOI
Validity of cylindrical approximation for spherical birefringent microparticles in rotational optical tweezers
Rahul Vaippully, Venkata Siva Gummaluri, C Vijayan and Basudev Roy
Rotational manipulation of microscopic birefringent particles has conventionally been done by manoeuvring the polarization of the trapping light in optical tweezers. The torque on the particle is a sum of contributions from the linear polarization and the circular polarization, while assuming that the difference in optical path lengths between the extraordinary and the ordinary components of polarization depends upon the wavelength of light, the thickness of the particle and the birefringence. Generally, the thickness of spherical microparticles is assumed to be the diameter which renders the particle appear cylindrical. We test this hypothesis for sizes relevant towards optical tweezers manipulation. We find that for a range of particles from the Rayleigh regime to the early Mie regime, the approximation holds good.
DOI
Rotational manipulation of microscopic birefringent particles has conventionally been done by manoeuvring the polarization of the trapping light in optical tweezers. The torque on the particle is a sum of contributions from the linear polarization and the circular polarization, while assuming that the difference in optical path lengths between the extraordinary and the ordinary components of polarization depends upon the wavelength of light, the thickness of the particle and the birefringence. Generally, the thickness of spherical microparticles is assumed to be the diameter which renders the particle appear cylindrical. We test this hypothesis for sizes relevant towards optical tweezers manipulation. We find that for a range of particles from the Rayleigh regime to the early Mie regime, the approximation holds good.
DOI
Azimuthal Imaginary Poynting Momentum Density
Xiaohao Xu and Manuel Nieto-Vesperinas
The momentum of light beams can possess azimuthal densities, circulating around the beam axis and inducing intriguing mechanical effects in local light-matter interaction. Belinfante’s spin momentum loops in circularly polarized beams, while the canonical momentum spirals in helically phased beams. However, a similar behavior of their imaginary counterpart, the so-called imaginary Poynting momentum (IPM), has not yet emerged. The foremost purpose of the present work is to put forward the discovery of this IPM vortex. We show that a simple superposition of radially and azimuthally polarized beams can form an IPM of completely azimuthal density. Additionally, the azimuthal IPM density can exist with a donut beam-intensity distribution and even with a vanishing azimuthal component of all other momenta. This uncovers the existence of a new mechanical effect which broadens the area of optical micromanipulation by achieving optical rotation of isotropic spheres, in the absence of both spin and orbital angular momenta. Our findings enrich the local dynamic properties of electromagnetic fields, highlighting the rotational action of their IPM, and thus its mechanical effect on microparticles and nanoparticles.
DOI
The momentum of light beams can possess azimuthal densities, circulating around the beam axis and inducing intriguing mechanical effects in local light-matter interaction. Belinfante’s spin momentum loops in circularly polarized beams, while the canonical momentum spirals in helically phased beams. However, a similar behavior of their imaginary counterpart, the so-called imaginary Poynting momentum (IPM), has not yet emerged. The foremost purpose of the present work is to put forward the discovery of this IPM vortex. We show that a simple superposition of radially and azimuthally polarized beams can form an IPM of completely azimuthal density. Additionally, the azimuthal IPM density can exist with a donut beam-intensity distribution and even with a vanishing azimuthal component of all other momenta. This uncovers the existence of a new mechanical effect which broadens the area of optical micromanipulation by achieving optical rotation of isotropic spheres, in the absence of both spin and orbital angular momenta. Our findings enrich the local dynamic properties of electromagnetic fields, highlighting the rotational action of their IPM, and thus its mechanical effect on microparticles and nanoparticles.
DOI
Monday, January 6, 2020
Optical tweezers-based characterisation of gold core–satellite plasmonic nano-assemblies incorporating thermo-responsive polymers
Fei Han, Thomas Armstrong, Ana Andres-Arroyo, Danielle Bennett, Alex Soeriyadi, Ali Alinezhad Chamazketi, Padmavathy Bakthavathsalam, Richard D. Tilley, J. Justin Gooding and Peter J. 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
Ultracold polar molecules as qudits
Rahul Sawant, Jacob A Blackmore, Philip David Gregory, Jordi Mur-Petit, Dieter Jaksch, Jesus Aldegunde, Jeremy M Hutson, Mike R Tarbutt and S L Cornish
We discuss how the internal structure of ultracold molecules, trapped in the motional ground state of optical tweezers, can be used to implement qudits. We explore the rotational, fine and hyperfine structure of $^{40}$Ca$^{19}$F and $^{87}$Rb$^{133}$Cs, which are examples of molecules with $^2\Sigma$ and $^1\Sigma$ electronic ground states, respectively. In each case we identify a subset of levels within a single rotational manifold suitable to implement a 4-level qudit. Quantum gates can be implemented using two-photon microwave transitions via levels in a neighboring rotational manifold. We discuss limitations to the usefulness of molecular qudits, arising from off-resonant excitation and decoherence. As an example, we present a protocol for using a molecular qudit of dimension $d=4$ to perform the Deutsch algorithm.
DOI
We discuss how the internal structure of ultracold molecules, trapped in the motional ground state of optical tweezers, can be used to implement qudits. We explore the rotational, fine and hyperfine structure of $^{40}$Ca$^{19}$F and $^{87}$Rb$^{133}$Cs, which are examples of molecules with $^2\Sigma$ and $^1\Sigma$ electronic ground states, respectively. In each case we identify a subset of levels within a single rotational manifold suitable to implement a 4-level qudit. Quantum gates can be implemented using two-photon microwave transitions via levels in a neighboring rotational manifold. We discuss limitations to the usefulness of molecular qudits, arising from off-resonant excitation and decoherence. As an example, we present a protocol for using a molecular qudit of dimension $d=4$ to perform the Deutsch algorithm.
DOI
Liquid-Liquid Interface Can Promote Micro-Scale Thermal Marangoni Convection in Liquid Binary Mixtures
Issei Aibara, Tatsuki Katoh, Chihiro Minamoto, Takayuki Uwada, Shuichi Hashimoto
Liquid-liquid phase separation, a physical transition in which a homogeneous solution spontaneously demixes into two coexisting liquid phases, has been a key topic in the thermodynamics of two-component systems and may find applications in separation, drug delivery, and protein crystallization. Here we applied a microscale temperature gradient using optothermal heating of a gold nanoparticle to overcome the experimental difficulties inherent in homogeneous heating: we aimed at highlighting precise structural development by avoiding randomly nucleating/growing microdomains. In response to laser illumination, a single gold nanoparticle immersed in a binary mixture of aqueous 2,6-dimethylpiridine (lutidine) and N-isopropylpropionamide (NiPPA) was clearly sensitive to the phase transition of the surrounding liquid as demonstrated by light scattering signals, spectral red-shifts and bright-spot images. The local phase separation encapsulating the gold nanoparticle resulted in immediate formation and growth of an organic-rich droplet which was confirmed by Raman spectroscopy. Remarkably, the droplet was stable under a non-equilibrium steady-state heating condition because of strong thermal confinement. Microdroplet growth was ascribed to thermocapillary flow induced by a newly formed liquid-liquid interface around the hot gold nanoparticle. Based upon a tracer experiment and numerical simulation, it is deduced that the transport of solute to the high temperature area is driven by this thermocapillary flow. This study enhances our understanding of phase separation in binary mixtures induced by microscale temperature confinement.
DOI
Liquid-liquid phase separation, a physical transition in which a homogeneous solution spontaneously demixes into two coexisting liquid phases, has been a key topic in the thermodynamics of two-component systems and may find applications in separation, drug delivery, and protein crystallization. Here we applied a microscale temperature gradient using optothermal heating of a gold nanoparticle to overcome the experimental difficulties inherent in homogeneous heating: we aimed at highlighting precise structural development by avoiding randomly nucleating/growing microdomains. In response to laser illumination, a single gold nanoparticle immersed in a binary mixture of aqueous 2,6-dimethylpiridine (lutidine) and N-isopropylpropionamide (NiPPA) was clearly sensitive to the phase transition of the surrounding liquid as demonstrated by light scattering signals, spectral red-shifts and bright-spot images. The local phase separation encapsulating the gold nanoparticle resulted in immediate formation and growth of an organic-rich droplet which was confirmed by Raman spectroscopy. Remarkably, the droplet was stable under a non-equilibrium steady-state heating condition because of strong thermal confinement. Microdroplet growth was ascribed to thermocapillary flow induced by a newly formed liquid-liquid interface around the hot gold nanoparticle. Based upon a tracer experiment and numerical simulation, it is deduced that the transport of solute to the high temperature area is driven by this thermocapillary flow. This study enhances our understanding of phase separation in binary mixtures induced by microscale temperature confinement.
DOI
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