Ke Ma, Shoubao Han, Long Zhang, Yaocheng Shi, and Daoxin Dai
A theoretical analysis is given for the optical forces induced by the Bloch mode propagating along a silicon subwavelength-grating (SWG) waveguide for the first time. As a periodical structure, an SWG waveguide supports periodical light field distribution along the waveguide. This makes it possible to trap many nano-particles stably periodically, which is very different from the case with a conventional optical waveguide. The separation of the trapped nano-particles can be designed easily by modifying the grating period of an SWG waveguide. Furthermore, an SWG waveguide has larger working distance in the lateral direction to trap nano-particles around the waveguide than a conventional optical waveguide.
DOI
Wednesday, December 20, 2017
Single-molecule force spectroscopy of protein-membrane interactions
Lu Ma, Yiying Cai, Yanghui Li, Junyi Jiao, Zhenyong Wu, Ben O'Shaughnessy, Pietro De Camilli, Erdem Karatekin, Yongli Zhang
Many biological processes rely on protein–membrane interactions in the presence of mechanical forces, yet high resolution methods to quantify such interactions are lacking. Here, we describe a single-molecule force spectroscopy approach to quantify membrane binding of C2 domains in Synaptotagmin-1 (Syt1) and Extended Synaptotagmin-2 (E-Syt2). Syts and E-Syts bind the plasma membrane via multiple C2 domains, bridging the plasma membrane with synaptic vesicles or endoplasmic reticulum to regulate membrane fusion or lipid exchange, respectively. In our approach, single proteins attached to membranes supported on silica beads are pulled by optical tweezers, allowing membrane binding and unbinding transitions to be measured with unprecedented spatiotemporal resolution. C2 domains from either protein resisted unbinding forces of 2–7 pN and had binding energies of 4–14 kBT per C2 domain. Regulation by bilayer composition or Ca2+ recapitulated known properties of both proteins. The method can be widely applied to study protein–membrane interactions.
DOI
Many biological processes rely on protein–membrane interactions in the presence of mechanical forces, yet high resolution methods to quantify such interactions are lacking. Here, we describe a single-molecule force spectroscopy approach to quantify membrane binding of C2 domains in Synaptotagmin-1 (Syt1) and Extended Synaptotagmin-2 (E-Syt2). Syts and E-Syts bind the plasma membrane via multiple C2 domains, bridging the plasma membrane with synaptic vesicles or endoplasmic reticulum to regulate membrane fusion or lipid exchange, respectively. In our approach, single proteins attached to membranes supported on silica beads are pulled by optical tweezers, allowing membrane binding and unbinding transitions to be measured with unprecedented spatiotemporal resolution. C2 domains from either protein resisted unbinding forces of 2–7 pN and had binding energies of 4–14 kBT per C2 domain. Regulation by bilayer composition or Ca2+ recapitulated known properties of both proteins. The method can be widely applied to study protein–membrane interactions.
DOI
Knotty Problems during Mitosis: Mechanistic Insight into the Processing of Ultrafine DNA Bridges in Anaphase
Kata Sarlós, Andreas Biebricher, Erwin J.G. Petermann, Gijs J.L. Wuite and Ian D. Hickson
To survive and proliferate, cells have to faithfully segregate their newly replicated genomic DNA to the two daughter cells. However, the sister chromatids of mitotic chromosomes are frequently interlinked by so-called ultrafine DNA bridges (UFBs) that are visible in the anaphase of mitosis. UFBs can only be detected by the proteins bound to them and not by staining with conventional DNA dyes. These DNA bridges are presumed to represent entangled sister chromatids and hence pose a threat to faithful segregation. A failure to accurately unlink UFB DNA results in chromosome segregation errors and binucleation. This, in turn, compromises genome integrity, which is a hallmark of cancer. UFBs are actively removed during anaphase, and most known UFB-associated proteins are enzymes involved in DNA repair in interphase. However, little is known about the mitotic activities of these enzymes or the exact DNA structures present on UFBs. We focus on the biology of UFBs, with special emphasis on their underlying DNA structure and the decatenation machineries that process UFBs.
DOI
To survive and proliferate, cells have to faithfully segregate their newly replicated genomic DNA to the two daughter cells. However, the sister chromatids of mitotic chromosomes are frequently interlinked by so-called ultrafine DNA bridges (UFBs) that are visible in the anaphase of mitosis. UFBs can only be detected by the proteins bound to them and not by staining with conventional DNA dyes. These DNA bridges are presumed to represent entangled sister chromatids and hence pose a threat to faithful segregation. A failure to accurately unlink UFB DNA results in chromosome segregation errors and binucleation. This, in turn, compromises genome integrity, which is a hallmark of cancer. UFBs are actively removed during anaphase, and most known UFB-associated proteins are enzymes involved in DNA repair in interphase. However, little is known about the mitotic activities of these enzymes or the exact DNA structures present on UFBs. We focus on the biology of UFBs, with special emphasis on their underlying DNA structure and the decatenation machineries that process UFBs.
DOI
Influence of experimental parameters on the laser heating of an optical trap
Frederic Català, Ferran Marsà, Mario Montes-Usategui, Arnau Farré & Estela Martín-Badosa
In optical tweezers, heating of the sample due to absorption of the laser light is a major concern as temperature plays an important role at microscopic scale. A popular rule of thumb is to consider that, at the typical wavelength of 1064 nm, the focused laser induces a heating rate of B = 1 °C/100 mW. We analysed this effect under different routine experimental conditions and found a remarkable variability in the temperature increase. Importantly, we determined that temperature can easily rise by as much as 4 °C at a relatively low power of 100 mW, for dielectric, non-absorbing particles with certain sets of specific, but common, parameters. Heating was determined from measurements of light momentum changes under drag forces at different powers, which proved to provide precise and robust results in watery buffers. We contrasted the experiments with computer simulations and obtained good agreement. These results suggest that this remarkable heating could be responsible for changes in the sample under study and could lead to serious damage of live specimens. It is therefore advisable to determine the temperature increase in each specific experiment and avoid the use of a universal rule that could inadvertently lead to critical changes in the sample.
In optical tweezers, heating of the sample due to absorption of the laser light is a major concern as temperature plays an important role at microscopic scale. A popular rule of thumb is to consider that, at the typical wavelength of 1064 nm, the focused laser induces a heating rate of B = 1 °C/100 mW. We analysed this effect under different routine experimental conditions and found a remarkable variability in the temperature increase. Importantly, we determined that temperature can easily rise by as much as 4 °C at a relatively low power of 100 mW, for dielectric, non-absorbing particles with certain sets of specific, but common, parameters. Heating was determined from measurements of light momentum changes under drag forces at different powers, which proved to provide precise and robust results in watery buffers. We contrasted the experiments with computer simulations and obtained good agreement. These results suggest that this remarkable heating could be responsible for changes in the sample under study and could lead to serious damage of live specimens. It is therefore advisable to determine the temperature increase in each specific experiment and avoid the use of a universal rule that could inadvertently lead to critical changes in the sample.
Kinesin-dependent mechanism for controlling triglyceride secretion from the liver
Priyanka Rai, Mukesh Kumar, Geetika Sharma, Pradeep Barak, Saumitra Das, Siddhesh S. Kamat, and Roop Mallik
Despite massive fluctuations in its internal triglyceride content, the liver secretes triglyceride under tight homeostatic control. This buffering function is most visible after fasting, when liver triglyceride increases manyfold but circulating serum triglyceride barely fluctuates. How the liver controls triglyceride secretion is unknown, but is fundamentally important for lipid and energy homeostasis in animals. Here we find an unexpected cellular and molecular mechanism behind such control. We show that kinesin motors are recruited to triglyceride-rich lipid droplets (LDs) in the liver by the GTPase ARF1, which is a key activator of lipolysis. This recruitment is activated by an insulin-dependent pathway and therefore responds to fed/fasted states of the animal. In fed state, ARF1 and kinesin appear on LDs, consequently transporting LDs to the periphery of hepatocytes where the smooth endoplasmic reticulum (sER) is present. Because the lipases that catabolize LDs in hepatocytes reside on the sER, LDs can now be catabolized efficiently to provide triglyceride for lipoprotein assembly and secretion from the sER. Upon fasting, insulin is lowered to remove ARF1 and kinesin from LDs, thus down-regulating LD transport and sER–LD contacts. This tempers triglyceride availabiity for very low density lipoprotein assembly and allows homeostatic control of serum triglyceride in a fasted state. We further show that kinesin knockdown inhibits hepatitis-C virus replication in hepatocytes, likely because translated viral proteins are unable to transfer from the ER to LDs.
DOI
Despite massive fluctuations in its internal triglyceride content, the liver secretes triglyceride under tight homeostatic control. This buffering function is most visible after fasting, when liver triglyceride increases manyfold but circulating serum triglyceride barely fluctuates. How the liver controls triglyceride secretion is unknown, but is fundamentally important for lipid and energy homeostasis in animals. Here we find an unexpected cellular and molecular mechanism behind such control. We show that kinesin motors are recruited to triglyceride-rich lipid droplets (LDs) in the liver by the GTPase ARF1, which is a key activator of lipolysis. This recruitment is activated by an insulin-dependent pathway and therefore responds to fed/fasted states of the animal. In fed state, ARF1 and kinesin appear on LDs, consequently transporting LDs to the periphery of hepatocytes where the smooth endoplasmic reticulum (sER) is present. Because the lipases that catabolize LDs in hepatocytes reside on the sER, LDs can now be catabolized efficiently to provide triglyceride for lipoprotein assembly and secretion from the sER. Upon fasting, insulin is lowered to remove ARF1 and kinesin from LDs, thus down-regulating LD transport and sER–LD contacts. This tempers triglyceride availabiity for very low density lipoprotein assembly and allows homeostatic control of serum triglyceride in a fasted state. We further show that kinesin knockdown inhibits hepatitis-C virus replication in hepatocytes, likely because translated viral proteins are unable to transfer from the ER to LDs.
DOI
The motion of nanoparticles under the non-conservative forces mediated by surface plasmon polaritons
I D Toftul, A A Bogdanov and M I Petrov
We have developed the theoretical and numerical modelling of nanoparticle dynamics near planar metallic interface under light radiation. Using our model, we employed the Green's function formalism to simulate the dynamics of nanoparticles under the action of surface plasmon polariton mediated optical forces. By varying the illumination conditions, we determined the different regimes of motion, such as surface attraction and repulsion, and optical pulling regime. We showed that the topology of the trajectories dramatically change, when the surface plasmon polaritons are excited.
DOI
We have developed the theoretical and numerical modelling of nanoparticle dynamics near planar metallic interface under light radiation. Using our model, we employed the Green's function formalism to simulate the dynamics of nanoparticles under the action of surface plasmon polariton mediated optical forces. By varying the illumination conditions, we determined the different regimes of motion, such as surface attraction and repulsion, and optical pulling regime. We showed that the topology of the trajectories dramatically change, when the surface plasmon polaritons are excited.
DOI
Monday, December 18, 2017
Cell membrane biophysics with optical tweezers
H. Moysés Nussenzveig
Membrane elastic properties play important roles in regulating cell shape, motility, division and differentiation. Here I review optical tweezer (OT) investigations of membrane surface tension and bending modulus, emphasizing didactic aspects and insights provided for cell biology. OT measurements employ membrane-attached microspheres to extract long cylindrical nanotubes named tethers. The Helfrich–Canham theory yields elastic parameters in terms of tether radius and equilibrium extraction force. It assumes initial point-like microsphere attachment and no cytoskeleton content within tethers. Experimental force–displacement curves reveal violations of those assumptions, and I discuss proposed explanations of such discrepancies, as well as recommended OT protocols. Measurements of elastic parameters for predominant cell types in the central nervous system yield correlations between their values and cell function. Micro-rheology OT experiments extend these correlations to viscoelastic parameters. The results agree with a quasi-universal phenomenological scaling law and are interpreted in terms of the soft glass rheology model. Spontaneously-generated cell nanotube protrusions are also briefly reviewed, emphasizing common features with tethers. Filopodia as well as tunneling nanotubes (TNT), which connect distant cells and allow transfers between their cytoplasms, are discussed, including OT tether pulling from TNTs which mediate communication among bacteria, even of different species. Pathogens, including bacteria, viruses and prions, opportunistically exploit TNTs for cell-to-cell transmission of infection, indicating that TNTs have an ancient evolutionary origin.
DOI
Membrane elastic properties play important roles in regulating cell shape, motility, division and differentiation. Here I review optical tweezer (OT) investigations of membrane surface tension and bending modulus, emphasizing didactic aspects and insights provided for cell biology. OT measurements employ membrane-attached microspheres to extract long cylindrical nanotubes named tethers. The Helfrich–Canham theory yields elastic parameters in terms of tether radius and equilibrium extraction force. It assumes initial point-like microsphere attachment and no cytoskeleton content within tethers. Experimental force–displacement curves reveal violations of those assumptions, and I discuss proposed explanations of such discrepancies, as well as recommended OT protocols. Measurements of elastic parameters for predominant cell types in the central nervous system yield correlations between their values and cell function. Micro-rheology OT experiments extend these correlations to viscoelastic parameters. The results agree with a quasi-universal phenomenological scaling law and are interpreted in terms of the soft glass rheology model. Spontaneously-generated cell nanotube protrusions are also briefly reviewed, emphasizing common features with tethers. Filopodia as well as tunneling nanotubes (TNT), which connect distant cells and allow transfers between their cytoplasms, are discussed, including OT tether pulling from TNTs which mediate communication among bacteria, even of different species. Pathogens, including bacteria, viruses and prions, opportunistically exploit TNTs for cell-to-cell transmission of infection, indicating that TNTs have an ancient evolutionary origin.
DOI
Radiation pressure on a two-level atom: an exact analytical approach
L. Podlecki, R. D. Glover, J. Martin, and T. Bastin
The mechanical action of light on atoms is currently a tool used ubiquitously in cold atom physics. In the semiclassical regime, where atomic motion is treated classically, the computation of the mean force acting on a two-level atom requires numerical approaches in the most general case. Here we show that this problem can be tackled in a purely analytical way. We provide an analytical yet simple expression of the mean force that holds in the most general case, where the atom is simultaneously exposed to an arbitrary number of lasers with arbitrary intensities, wave vectors, and phases. This yields a novel tool for engineering the mechanical action of light on single atoms.
DOI
The mechanical action of light on atoms is currently a tool used ubiquitously in cold atom physics. In the semiclassical regime, where atomic motion is treated classically, the computation of the mean force acting on a two-level atom requires numerical approaches in the most general case. Here we show that this problem can be tackled in a purely analytical way. We provide an analytical yet simple expression of the mean force that holds in the most general case, where the atom is simultaneously exposed to an arbitrary number of lasers with arbitrary intensities, wave vectors, and phases. This yields a novel tool for engineering the mechanical action of light on single atoms.
DOI
Epigenetic Transitions and Knotted Solitons in Stretched Chromatin
D. Michieletto, E. Orlandini & D. Marenduzzo
The spreading and regulation of epigenetic marks on chromosomes is crucial to establish and maintain cellular identity. Nonetheless, the dynamic mechanism leading to the establishment and maintenance of tissue-specific, epigenetic pattern is still poorly understood. In this work we propose, and investigate in silico, a possible experimental strategy to illuminate the interplay between 3D chromatin structure and epigenetic dynamics. We consider a set-up where a reconstituted chromatin fibre is stretched at its two ends (e.g., by laser tweezers), while epigenetic enzymes (writers) and chromatin-binding proteins (readers) are flooded into the system. We show that, by tuning the stretching force and the binding affinity of the readers for chromatin, the fibre undergoes a sharp transition between a stretched, epigenetically disordered, state and a crumpled, epigenetically coherent, one. We further investigate the case in which a knot is tied along the chromatin fibre, and find that the knotted segment enhances local epigenetic order, giving rise to “epigenetic solitons” which travel and diffuse along chromatin. Our results point to an intriguing coupling between 3D chromatin topology and epigenetic dynamics, which may be investigated via single molecule experiments.
DOI
The spreading and regulation of epigenetic marks on chromosomes is crucial to establish and maintain cellular identity. Nonetheless, the dynamic mechanism leading to the establishment and maintenance of tissue-specific, epigenetic pattern is still poorly understood. In this work we propose, and investigate in silico, a possible experimental strategy to illuminate the interplay between 3D chromatin structure and epigenetic dynamics. We consider a set-up where a reconstituted chromatin fibre is stretched at its two ends (e.g., by laser tweezers), while epigenetic enzymes (writers) and chromatin-binding proteins (readers) are flooded into the system. We show that, by tuning the stretching force and the binding affinity of the readers for chromatin, the fibre undergoes a sharp transition between a stretched, epigenetically disordered, state and a crumpled, epigenetically coherent, one. We further investigate the case in which a knot is tied along the chromatin fibre, and find that the knotted segment enhances local epigenetic order, giving rise to “epigenetic solitons” which travel and diffuse along chromatin. Our results point to an intriguing coupling between 3D chromatin topology and epigenetic dynamics, which may be investigated via single molecule experiments.
DOI
Single floating cell irradiation technique with an X-ray microbeam
Fuminobu Sato, Kikuo Shimizu, Isao Murata
A single floating cell irradiation technique with X-ray microbeam has been developed for the microscopic research of radiation effects on floating cells in a liquid medium. This technique is a combination of X-ray microbeam and laser technologies. A preliminary experiment on the survival of budding cells of the yeast Saccharomyces cerevisiae (RAD52) was performed with the X-ray microbeam system. Micro-pits were fabricated on the bottom of a culture dish in a liquid medium, using green laser beams. The yeast cells were put into the micro-pits one by one by using focused infrared laser beams. A 50-kV X-ray microbeam 12 μm (FWHM) in diameter was propagated into the targeted yeast cell. The maximum dose rate on the targeted cell was estimated to be 0.09 Gy/s from the results of beam profile measurements and photon-electron transport calculations. The X-ray irradiation effect on the cell lethality was clearly observed for the cell exposed to X-rays of 100 Gy.
DOI
A single floating cell irradiation technique with X-ray microbeam has been developed for the microscopic research of radiation effects on floating cells in a liquid medium. This technique is a combination of X-ray microbeam and laser technologies. A preliminary experiment on the survival of budding cells of the yeast Saccharomyces cerevisiae (RAD52) was performed with the X-ray microbeam system. Micro-pits were fabricated on the bottom of a culture dish in a liquid medium, using green laser beams. The yeast cells were put into the micro-pits one by one by using focused infrared laser beams. A 50-kV X-ray microbeam 12 μm (FWHM) in diameter was propagated into the targeted yeast cell. The maximum dose rate on the targeted cell was estimated to be 0.09 Gy/s from the results of beam profile measurements and photon-electron transport calculations. The X-ray irradiation effect on the cell lethality was clearly observed for the cell exposed to X-rays of 100 Gy.
DOI
Raman Spectroscopy of Single Light-Absorbing Carbonaceous Particles Levitated in Air Using an Annular Laser Beam
Masaru Uraoka, Keisuke Maegawa, and Shoji Ishizaka
A laser trapping technique is a powerful means to investigate the physical and chemical properties of single aerosol particles in a noncontact manner. However, optical trapping of strongly light-absorbing particles such as black carbon or soot is quite difficult because the repulsive force caused by heat is orders of magnitude larger than the attractive force of radiation pressure. In this study, a laser trapping and Raman microspectroscopy system using an annular laser beam was constructed to achieve noncontact levitation of single light-absorbing particles in air. Single acetylene carbon black or candle soot particles were arbitrarily selected with a glass capillary connected to a three-axis oil hydraulic micromanipulator and introduced into a minute space surrounded by a repulsive force at the focal point of an objective lens. Using the developed system, we achieved optical levitation of micrometer-sized carbonaceous particles and observation of their Raman spectra in air. Furthermore, we demonstrated in situ observations of changes in the morphology and chemical composition of optically trapped carbonaceous particles in air, which were induced by heterogeneous oxidation reactions with ozone and hydroxyl radicals.
DOI
A laser trapping technique is a powerful means to investigate the physical and chemical properties of single aerosol particles in a noncontact manner. However, optical trapping of strongly light-absorbing particles such as black carbon or soot is quite difficult because the repulsive force caused by heat is orders of magnitude larger than the attractive force of radiation pressure. In this study, a laser trapping and Raman microspectroscopy system using an annular laser beam was constructed to achieve noncontact levitation of single light-absorbing particles in air. Single acetylene carbon black or candle soot particles were arbitrarily selected with a glass capillary connected to a three-axis oil hydraulic micromanipulator and introduced into a minute space surrounded by a repulsive force at the focal point of an objective lens. Using the developed system, we achieved optical levitation of micrometer-sized carbonaceous particles and observation of their Raman spectra in air. Furthermore, we demonstrated in situ observations of changes in the morphology and chemical composition of optically trapped carbonaceous particles in air, which were induced by heterogeneous oxidation reactions with ozone and hydroxyl radicals.
DOI
Alterations of biomechanics in cancer and normal cells induced by doxorubicin
Kaja Fraczkowska, Marcin Bacia, Magda Przybyło, Dominik Drabik, Aleksandra Kaczorowska, Justyna Rybka, Ewa Stefanko, Slawomir Drobczynski, Jan Masajada, Halina Podbielska, Tomasz Wrobel, Marta Kopaczynska
Mechanical properties of biological structures play an important role in regulating cellular activities and are critical for understanding metabolic processes in cancerous cells and the effects of drugs. For some cancers, such as acute myeloid leukaemia, chemotherapy is one of preferential methods. However, due to the lack of selectivity to cancer cells, cytostatic agents cause toxicity to normal tissues. Here, we study the effect of doxorubicin (DOX) on the mechanical properties of DNA molecules, leukemic blast cells and erythrocytes, using optical tweezers. In addition, we controlled the subcellular distribution of the drug by confocal microscopy. Our results indicated that doxorubicin affects mechanical properties of cellular structures. In all cases the drug reduced mechanical strength of examined objects. For the leukemic cells the drug subcellular distribution was predominantly nuclear with some particulate cytoplasmic fluorescence. In erythrocytes, doxorubicin showed fluorescence mainly in cytoplasm and plasma membrane. The lowering of blast cells stiffness may be due to the interaction of doxorubicin with nuclear structures, especially with nucleic acids, as our studies with DNA confirmed. In addition, it is known that DOX inhibits the polymerization of actin and thus cytoskeletal modification may also be important in reducing of cell mechanical strength. In the case of erythrocytes – the non-nucleated cells, a significant effect on the decrease of cell stiffness, besides the cytoskeleton, may have the interaction of the drug with the cell membrane. Experiments with model phospholipid membranes confirmed that observed increase in cell elasticity originates, among other things, from the drug incorporation in the lipid membrane itself. The lowering of mechanical strength of leukemic cells may have an significant impact on the effectiveness of chemotherapy. However, the fact that doxorubicin interacts not only with proliferating cancer cells, but also with the health ones may explains the high toxicity of the drug at the therapeutic concentrations. Our observations also suggest that chemotherapy with doxorubicin may decrease the risk of vascular complications in acute leukemia, due to increasing the cell elasticity.
DOI
Mechanical properties of biological structures play an important role in regulating cellular activities and are critical for understanding metabolic processes in cancerous cells and the effects of drugs. For some cancers, such as acute myeloid leukaemia, chemotherapy is one of preferential methods. However, due to the lack of selectivity to cancer cells, cytostatic agents cause toxicity to normal tissues. Here, we study the effect of doxorubicin (DOX) on the mechanical properties of DNA molecules, leukemic blast cells and erythrocytes, using optical tweezers. In addition, we controlled the subcellular distribution of the drug by confocal microscopy. Our results indicated that doxorubicin affects mechanical properties of cellular structures. In all cases the drug reduced mechanical strength of examined objects. For the leukemic cells the drug subcellular distribution was predominantly nuclear with some particulate cytoplasmic fluorescence. In erythrocytes, doxorubicin showed fluorescence mainly in cytoplasm and plasma membrane. The lowering of blast cells stiffness may be due to the interaction of doxorubicin with nuclear structures, especially with nucleic acids, as our studies with DNA confirmed. In addition, it is known that DOX inhibits the polymerization of actin and thus cytoskeletal modification may also be important in reducing of cell mechanical strength. In the case of erythrocytes – the non-nucleated cells, a significant effect on the decrease of cell stiffness, besides the cytoskeleton, may have the interaction of the drug with the cell membrane. Experiments with model phospholipid membranes confirmed that observed increase in cell elasticity originates, among other things, from the drug incorporation in the lipid membrane itself. The lowering of mechanical strength of leukemic cells may have an significant impact on the effectiveness of chemotherapy. However, the fact that doxorubicin interacts not only with proliferating cancer cells, but also with the health ones may explains the high toxicity of the drug at the therapeutic concentrations. Our observations also suggest that chemotherapy with doxorubicin may decrease the risk of vascular complications in acute leukemia, due to increasing the cell elasticity.
DOI
Wednesday, December 13, 2017
Super-Resolution Trapping: A Nanoparticle Manipulation Using Nonlinear Optical Response
Masayuki Hoshina, Nobuhiko Yokoshi, Hiromi Okamoto, and Hajime Ishihara
Optical manipulation of nanoparticles (NPs) with nanoscale precision has been a goal of several research fields. One of the promising ways to realize this is the usage of localized surface plasmon (LSP). The electric field at hotspots near metallic structures is highly localized, which generates a sufficient force to trap NPs, and at the same time, the optical nonlinearity of NPs appears. In this Letter, we propose a scheme to super-resolutionally trap the NP into a particular hotspot of the metallic nanostructure array. The scheme is based on the optical nonlinearity of NPs, and utilizes two kinds of structured light: Gaussian and Laguerre-Gaussian beams. The results show the significant role of the optical nonlinearity in LSP trappings, and they are expected to open up new degrees of freedom to manipulate NPs.
DOI
Optical manipulation of nanoparticles (NPs) with nanoscale precision has been a goal of several research fields. One of the promising ways to realize this is the usage of localized surface plasmon (LSP). The electric field at hotspots near metallic structures is highly localized, which generates a sufficient force to trap NPs, and at the same time, the optical nonlinearity of NPs appears. In this Letter, we propose a scheme to super-resolutionally trap the NP into a particular hotspot of the metallic nanostructure array. The scheme is based on the optical nonlinearity of NPs, and utilizes two kinds of structured light: Gaussian and Laguerre-Gaussian beams. The results show the significant role of the optical nonlinearity in LSP trappings, and they are expected to open up new degrees of freedom to manipulate NPs.
DOI
Optical torques and forces in birefringent microplate
C.Yu. Zenkova, D.I. Ivanskyi, T.V. Kiyashchuk
The paper presents a theoretical model for calculating the optical forces caused by the density of spin and orbital momentum and optical torque, caused by the angular momentum typical for a circularly polarized beam or a circular component of an elliptically polarized beam. Total internal reflection at the plate–air interface implemented by the birefringent plate spattered with nanoparticles of gold creates the conditions for the allocation of the predominant action of the vertical spin of the evanescent wave, which has recently been predicted theoretically.
DOI
The paper presents a theoretical model for calculating the optical forces caused by the density of spin and orbital momentum and optical torque, caused by the angular momentum typical for a circularly polarized beam or a circular component of an elliptically polarized beam. Total internal reflection at the plate–air interface implemented by the birefringent plate spattered with nanoparticles of gold creates the conditions for the allocation of the predominant action of the vertical spin of the evanescent wave, which has recently been predicted theoretically.
DOI
Holographic Plasmonic Nanotweezers for Dynamic Trapping and Manipulation
Preston R. Huft, Joshua D. Kolbow, Jonathan T. Thweatt, and Nathan C. Lindquist
We demonstrate dynamic trapping and manipulation of nanoparticles with plasmonic holograms. By tailoring the illumination pattern of an incident light beam with a computer-controlled spatial light modulator, constructive and destructive interference of plasmon waves create a focused hotspot that can be moved across a surface. Specifically, a computer-generated hologram illuminating the perimeter of a silver Bull’s Eye nanostructure generates surface plasmons that propagate toward the center. Shifting the phase of the plasmon waves as a function of space gives complete control over the location of the focus. We show that 200 nm diameter nanoparticles trapped in this focus can be moved in arbitrary patterns. This allows, for example, circular motion with linearly polarized light. These results show the versatility of holographically generated surface plasmon waves for advanced trapping and manipulation of nanoparticles.
DOI
We demonstrate dynamic trapping and manipulation of nanoparticles with plasmonic holograms. By tailoring the illumination pattern of an incident light beam with a computer-controlled spatial light modulator, constructive and destructive interference of plasmon waves create a focused hotspot that can be moved across a surface. Specifically, a computer-generated hologram illuminating the perimeter of a silver Bull’s Eye nanostructure generates surface plasmons that propagate toward the center. Shifting the phase of the plasmon waves as a function of space gives complete control over the location of the focus. We show that 200 nm diameter nanoparticles trapped in this focus can be moved in arbitrary patterns. This allows, for example, circular motion with linearly polarized light. These results show the versatility of holographically generated surface plasmon waves for advanced trapping and manipulation of nanoparticles.
DOI
Characterising the evaporation kinetics of water and semi-volatile organic compounds from viscous multicomponent organic aerosol particles
Stephen Ingram, Chen Cai, Young-Chul Song, David R. Glowacki, David O. Topping, Simon O’Meara and Jonathan P. Reid
The physicochemical changes experienced by organic aerosol particles undergoing dehydration into the surrounding gas phase can be drastic, forcing rapid vitrification of the particle and suppressing internal diffusion. Until recently, experimental studies have concentrated on quantifying diffusional mixing of either water or non-volatile components, while relatively little attention has been paid to the role of semivolatile organic component (SVOC) diffusion and volatilisation in maintaining the equilibrium between the gas and particle phases. Here we present methods to simultaneously investigate diffusivities and volatilities in studies of evolving single ternary aerosol particle size and composition. Analysing particles of ternary composition must account for the multiple chemical species that volatilise in response to a step change in gas phase water activity. In addition, treatments of diffusion in multicomponent mixtures are necessary to represent evolving heterogeneities in particle composition. We find that the contributions to observed size behaviour from volatilisation of water and a SVOC can be decoupled and treated separately. Employing Fickian diffusion modelling, we extract the compositional dependence of the diffusion constant of water and compare the results to recently published parametrisations in binary aerosol particles. The treatment of ideality and activity in each case is discussed, with reference to use in multicomponent core shell models. Meanwhile, the evaporation of an SVOC into an unsaturated gas flow may be treated by Maxwell's equation, with slow diffusional transport manifesting as a suppression in the extracted vapour pressure.
DOI
The physicochemical changes experienced by organic aerosol particles undergoing dehydration into the surrounding gas phase can be drastic, forcing rapid vitrification of the particle and suppressing internal diffusion. Until recently, experimental studies have concentrated on quantifying diffusional mixing of either water or non-volatile components, while relatively little attention has been paid to the role of semivolatile organic component (SVOC) diffusion and volatilisation in maintaining the equilibrium between the gas and particle phases. Here we present methods to simultaneously investigate diffusivities and volatilities in studies of evolving single ternary aerosol particle size and composition. Analysing particles of ternary composition must account for the multiple chemical species that volatilise in response to a step change in gas phase water activity. In addition, treatments of diffusion in multicomponent mixtures are necessary to represent evolving heterogeneities in particle composition. We find that the contributions to observed size behaviour from volatilisation of water and a SVOC can be decoupled and treated separately. Employing Fickian diffusion modelling, we extract the compositional dependence of the diffusion constant of water and compare the results to recently published parametrisations in binary aerosol particles. The treatment of ideality and activity in each case is discussed, with reference to use in multicomponent core shell models. Meanwhile, the evaporation of an SVOC into an unsaturated gas flow may be treated by Maxwell's equation, with slow diffusional transport manifesting as a suppression in the extracted vapour pressure.
DOI
Optical manipulation of individual strongly absorbing platinum nanoparticles
Akbar Samadi, Poul Martin Bendix and Lene B. Oddershede
Nanostructures with exceptional absorption in the near infrared (NIR) regime are receiving significant attention due to their ability to promote controlled local heating in biological material upon irradiation. Also, such nano-structures have numerous applications in nano-electronics and for bio-exploration. Therefore, significant effort is being put into controlling and understanding plasmonic nanostructures. However, essentially all focus has been on NIR resonant gold nanoparticles and remarkably little attention has been given to nanoparticles of other materials that may have superior properties. Here, we demonstrate optical control and manipulation of individual strongly absorbing platinum nanoparticles in three dimensions using a single focused continuous wave NIR laser beam. Also, we quantify how the platinum nanoparticles interact with light and compare to similarly sized absorbing gold nanoparticles, both massive gold and gold nanoshells. By finite element modeling, we find the scattering and absorption cross sections and the polarizability of all particles. The trapping experiments allow for direct measurements of the interaction between the nanoparticles and NIR light which compares well to the theoretical predictions. In the NIR, platinum nanoparticles are stronger absorbers than similarly sized massive gold nanoparticles and scatter similarly. Compared to NIR resonant gold nanoshells, platinum nanoparticles absorb less, however, they also scatter significantly less, thus leading to more stable optical trapping. These results pave the way for nano-manipulation and positioning of platinum nanoparticles and for using these for to enhance spectroscopic signals, for localized heating, and for manipulation of biological systems.
DOI
Nanostructures with exceptional absorption in the near infrared (NIR) regime are receiving significant attention due to their ability to promote controlled local heating in biological material upon irradiation. Also, such nano-structures have numerous applications in nano-electronics and for bio-exploration. Therefore, significant effort is being put into controlling and understanding plasmonic nanostructures. However, essentially all focus has been on NIR resonant gold nanoparticles and remarkably little attention has been given to nanoparticles of other materials that may have superior properties. Here, we demonstrate optical control and manipulation of individual strongly absorbing platinum nanoparticles in three dimensions using a single focused continuous wave NIR laser beam. Also, we quantify how the platinum nanoparticles interact with light and compare to similarly sized absorbing gold nanoparticles, both massive gold and gold nanoshells. By finite element modeling, we find the scattering and absorption cross sections and the polarizability of all particles. The trapping experiments allow for direct measurements of the interaction between the nanoparticles and NIR light which compares well to the theoretical predictions. In the NIR, platinum nanoparticles are stronger absorbers than similarly sized massive gold nanoparticles and scatter similarly. Compared to NIR resonant gold nanoshells, platinum nanoparticles absorb less, however, they also scatter significantly less, thus leading to more stable optical trapping. These results pave the way for nano-manipulation and positioning of platinum nanoparticles and for using these for to enhance spectroscopic signals, for localized heating, and for manipulation of biological systems.
DOI
Speed of the bacterial flagellar motor near zero load depends on the number of stator units
Ashley L. Nord, Yoshiyuki Sowa, Bradley C. Steel, Chien-Jung Lo, and Richard M. Berry
The bacterial flagellar motor (BFM) rotates hundreds of times per second to propel bacteria driven by an electrochemical ion gradient. The motor consists of a rotor 50 nm in diameter surrounded by up to 11 ion-conducting stator units, which exchange between motors and a membrane-bound pool. Measurements of the torque–speed relationship guide the development of models of the motor mechanism. In contrast to previous reports that speed near zero torque is independent of the number of stator units, we observe multiple speeds that we attribute to different numbers of units near zero torque in both Na+- and H+-driven motors. We measure the full torque–speed relationship of one and two H+ units in Escherichia coli by selecting the number of H+ units and controlling the number of Na+ units in hybrid motors. These experiments confirm that speed near zero torque in H+-driven motors increases with the stator number. We also measured 75 torque–speed curves for Na+-driven chimeric motors at different ion-motive force and stator number. Torque and speed were proportional to ion-motive force and number of stator units at all loads, allowing all 77 measured torque–speed curves to be collapsed onto a single curve by simple rescaling.
DOI
The bacterial flagellar motor (BFM) rotates hundreds of times per second to propel bacteria driven by an electrochemical ion gradient. The motor consists of a rotor 50 nm in diameter surrounded by up to 11 ion-conducting stator units, which exchange between motors and a membrane-bound pool. Measurements of the torque–speed relationship guide the development of models of the motor mechanism. In contrast to previous reports that speed near zero torque is independent of the number of stator units, we observe multiple speeds that we attribute to different numbers of units near zero torque in both Na+- and H+-driven motors. We measure the full torque–speed relationship of one and two H+ units in Escherichia coli by selecting the number of H+ units and controlling the number of Na+ units in hybrid motors. These experiments confirm that speed near zero torque in H+-driven motors increases with the stator number. We also measured 75 torque–speed curves for Na+-driven chimeric motors at different ion-motive force and stator number. Torque and speed were proportional to ion-motive force and number of stator units at all loads, allowing all 77 measured torque–speed curves to be collapsed onto a single curve by simple rescaling.
DOI
Monday, December 11, 2017
Image-based model of the spectrin cytoskeleton for red blood cell simulation
Thomas G. Fai , Alejandra Leo-Macias, David L. Stokes, Charles S. Peskin
We simulate deformable red blood cells in the microcirculation using the immersed boundary method with a cytoskeletal model that incorporates structural details revealed by tomographic images. The elasticity of red blood cells is known to be supplied by both their lipid bilayer membranes, which resist bending and local changes in area, and their cytoskeletons, which resist in-plane shear. The cytoskeleton consists of spectrin tetramers that are tethered to the lipid bilayer by ankyrin and by actin-based junctional complexes. We model the cytoskeleton as a random geometric graph, with nodes corresponding to junctional complexes and with edges corresponding to spectrin tetramers such that the edge lengths are given by the end-to-end distances between nodes. The statistical properties of this graph are based on distributions gathered from three-dimensional tomographic images of the cytoskeleton by a segmentation algorithm. We show that the elastic response of our model cytoskeleton, in which the spectrin polymers are treated as entropic springs, is in good agreement with the experimentally measured shear modulus. By simulating red blood cells in flow with the immersed boundary method, we compare this discrete cytoskeletal model to an existing continuum model and predict the extent to which dynamic spectrin network connectivity can protect against failure in the case of a red cell subjected to an applied strain. The methods presented here could form the basis of disease- and patient-specific computational studies of hereditary diseases affecting the red cell cytoskeleton.
DOI
We simulate deformable red blood cells in the microcirculation using the immersed boundary method with a cytoskeletal model that incorporates structural details revealed by tomographic images. The elasticity of red blood cells is known to be supplied by both their lipid bilayer membranes, which resist bending and local changes in area, and their cytoskeletons, which resist in-plane shear. The cytoskeleton consists of spectrin tetramers that are tethered to the lipid bilayer by ankyrin and by actin-based junctional complexes. We model the cytoskeleton as a random geometric graph, with nodes corresponding to junctional complexes and with edges corresponding to spectrin tetramers such that the edge lengths are given by the end-to-end distances between nodes. The statistical properties of this graph are based on distributions gathered from three-dimensional tomographic images of the cytoskeleton by a segmentation algorithm. We show that the elastic response of our model cytoskeleton, in which the spectrin polymers are treated as entropic springs, is in good agreement with the experimentally measured shear modulus. By simulating red blood cells in flow with the immersed boundary method, we compare this discrete cytoskeletal model to an existing continuum model and predict the extent to which dynamic spectrin network connectivity can protect against failure in the case of a red cell subjected to an applied strain. The methods presented here could form the basis of disease- and patient-specific computational studies of hereditary diseases affecting the red cell cytoskeleton.
DOI
Real-time observation of polymerase-promoter contact remodeling during transcription initiation
Cong A. Meng, Furqan M. Fazal & Steven M. Block
Critical contacts made between the RNA polymerase (RNAP) holoenzyme and promoter DNA modulate not only the strength of promoter binding, but also the frequency and timing of promoter escape during transcription. Here, we describe a single-molecule optical-trapping assay to study transcription initiation in real time, and use it to map contacts formed between σ70 RNAP holoenzyme from E. coli and the T7A1 promoter, as well as to observe the remodeling of those contacts during the transition to the elongation phase. The strong binding contacts identified in certain well-known promoter regions, such as the −35 and −10 elements, do not necessarily coincide with the most highly conserved portions of these sequences. Strong contacts formed within the spacer region (−10 to −35) and with the −10 element are essential for initiation and promoter escape, respectively, and the holoenzyme releases contacts with promoter elements in a non-sequential fashion during escape.
DOI
Critical contacts made between the RNA polymerase (RNAP) holoenzyme and promoter DNA modulate not only the strength of promoter binding, but also the frequency and timing of promoter escape during transcription. Here, we describe a single-molecule optical-trapping assay to study transcription initiation in real time, and use it to map contacts formed between σ70 RNAP holoenzyme from E. coli and the T7A1 promoter, as well as to observe the remodeling of those contacts during the transition to the elongation phase. The strong binding contacts identified in certain well-known promoter regions, such as the −35 and −10 elements, do not necessarily coincide with the most highly conserved portions of these sequences. Strong contacts formed within the spacer region (−10 to −35) and with the −10 element are essential for initiation and promoter escape, respectively, and the holoenzyme releases contacts with promoter elements in a non-sequential fashion during escape.
DOI
Multiplexing molecular tension sensors reveals piconewton force gradient across talin-1
Pia Ringer, Andreas Weißl, Anna-Lena Cost, Andrea Freikamp, Benedikt Sabass, Alexander Mehlich, Marc Tramier, Matthias Rief & Carsten Grashoff
Förster resonance energy transfer (FRET)-based tension sensor modules (TSMs) are available for investigating how distinct proteins bear mechanical forces in cells. Yet, forces in the single piconewton (pN) regime remain difficult to resolve, and tools for multiplexed tension sensing are lacking. Here, we report the generation and calibration of a genetically encoded, FRET-based biosensor called FL-TSM, which is characterized by a near-digital force response and increased sensitivity at 3–5 pN. In addition, we present a method allowing the simultaneous evaluation of coexpressed tension sensor constructs using two-color fluorescence lifetime microscopy. Finally, we introduce a procedure to calculate the fraction of mechanically engaged molecules within cells. Application of these techniques to new talin biosensors reveals an intramolecular tension gradient across talin-1 that is established upon integrin-mediated cell adhesion. The tension gradient is actomyosin- and vinculin-dependent and sensitive to the rigidity of the extracellular environment.
DOI
Förster resonance energy transfer (FRET)-based tension sensor modules (TSMs) are available for investigating how distinct proteins bear mechanical forces in cells. Yet, forces in the single piconewton (pN) regime remain difficult to resolve, and tools for multiplexed tension sensing are lacking. Here, we report the generation and calibration of a genetically encoded, FRET-based biosensor called FL-TSM, which is characterized by a near-digital force response and increased sensitivity at 3–5 pN. In addition, we present a method allowing the simultaneous evaluation of coexpressed tension sensor constructs using two-color fluorescence lifetime microscopy. Finally, we introduce a procedure to calculate the fraction of mechanically engaged molecules within cells. Application of these techniques to new talin biosensors reveals an intramolecular tension gradient across talin-1 that is established upon integrin-mediated cell adhesion. The tension gradient is actomyosin- and vinculin-dependent and sensitive to the rigidity of the extracellular environment.
DOI
Dual-mode optical fiber-based tweezers for robust trapping and manipulation of absorbing particles in air
Souvik Sil, Tushar Kanti Saha, Avinash Kumar, Sudipta K Bera and Ayan Banerjee
We develop an optical tweezers system using a single dual-mode optical fiber where mesoscopic absorbing particles can be trapped in three dimensions and manipulated employing photophoretic forces. We generate a superposition of fundamental and first order Hermite–Gaussian beam modes by the simple innovation of coupling a laser into a commercial optical fiber designed to be single mode for a wavelength higher than that of the laser. We achieve robust trapping of the absorbing particles for hours using both the pure fundamental and superposition mode beams and attain large manipulation velocities of ~5 mm s−1 in the axial direction and ~0.75 mm s−1 in the radial direction. We then demonstrate that the superposition mode is more effective in trapping and manipulation compared to the fundamental mode by around 80%, which may be increased several times by the use of a pure first order Hermite–Gaussian mode. The work has promising implications for trapping and spectroscopy of aerosols in air using simple optical fiber-based traps.
DOI
We develop an optical tweezers system using a single dual-mode optical fiber where mesoscopic absorbing particles can be trapped in three dimensions and manipulated employing photophoretic forces. We generate a superposition of fundamental and first order Hermite–Gaussian beam modes by the simple innovation of coupling a laser into a commercial optical fiber designed to be single mode for a wavelength higher than that of the laser. We achieve robust trapping of the absorbing particles for hours using both the pure fundamental and superposition mode beams and attain large manipulation velocities of ~5 mm s−1 in the axial direction and ~0.75 mm s−1 in the radial direction. We then demonstrate that the superposition mode is more effective in trapping and manipulation compared to the fundamental mode by around 80%, which may be increased several times by the use of a pure first order Hermite–Gaussian mode. The work has promising implications for trapping and spectroscopy of aerosols in air using simple optical fiber-based traps.
DOI
Direct measurement of Kramers turnover with a levitated nanoparticle
Loïc Rondin, Jan Gieseler, Francesco Ricci, Romain Quidant, Christoph Dellago & Lukas Novotny
Understanding the thermally activated escape from a metastable state is at the heart of important phenomena such as the folding dynamics of proteins1,2, the kinetics of chemical reactions3 or the stability of mechanical systems4. In 1940, Kramers calculated escape rates both in the high damping and low damping regimes, and suggested that the rate must have a maximum for intermediate damping5. This phenomenon, today known as the Kramers turnover, has triggered important theoretical and numerical studies6. However, as yet, there is no direct and quantitative experimental verification of this turnover. Using a nanoparticle trapped in a bistable optical potential, we experimentally measure the nanoparticle's transition rates for variable damping and directly resolve the Kramers turnover. Our measurements are in agreement with an analytical model that is free of adjustable parameters. The levitated nanoparticle presented here is a versatile experimental platform for studying and simulating a wide range of stochastic processes and testing theoretical models and predictions.
DOI
Understanding the thermally activated escape from a metastable state is at the heart of important phenomena such as the folding dynamics of proteins1,2, the kinetics of chemical reactions3 or the stability of mechanical systems4. In 1940, Kramers calculated escape rates both in the high damping and low damping regimes, and suggested that the rate must have a maximum for intermediate damping5. This phenomenon, today known as the Kramers turnover, has triggered important theoretical and numerical studies6. However, as yet, there is no direct and quantitative experimental verification of this turnover. Using a nanoparticle trapped in a bistable optical potential, we experimentally measure the nanoparticle's transition rates for variable damping and directly resolve the Kramers turnover. Our measurements are in agreement with an analytical model that is free of adjustable parameters. The levitated nanoparticle presented here is a versatile experimental platform for studying and simulating a wide range of stochastic processes and testing theoretical models and predictions.
DOI
Simultaneous fluorescence and surface charge measurements on organic semiconductor-coated silica microspheres in (non)polar liquids
R. Grollman, G. Founds, R. Wallace, and O. Ostroverkhova
We present an experimental platform which combines spectroscopic capabilities with time-resolved measurements of effective surface charge at solid-liquid interfaces. Silica microspheres, pristine and coated with various organic semiconductor molecules, were optically trapped either in water or in toluene. Adsorption of organic semiconductor molecules on the microspheres was observed via appearance of fluorescence and dramatic reduction in the effective surface charge, measured concurrently on individual spheres, with elementary charge resolution. The versatile platform accommodates possibilities to study a variety of photoinduced processes simultaneously with measurements of surface charge and can be incorporated in devices such as microreactors and microfluidics.
DOI
We present an experimental platform which combines spectroscopic capabilities with time-resolved measurements of effective surface charge at solid-liquid interfaces. Silica microspheres, pristine and coated with various organic semiconductor molecules, were optically trapped either in water or in toluene. Adsorption of organic semiconductor molecules on the microspheres was observed via appearance of fluorescence and dramatic reduction in the effective surface charge, measured concurrently on individual spheres, with elementary charge resolution. The versatile platform accommodates possibilities to study a variety of photoinduced processes simultaneously with measurements of surface charge and can be incorporated in devices such as microreactors and microfluidics.
DOI
Friday, December 8, 2017
Fast label-free microscopy technique for 3D dynamic quantitative imaging of living cells
José A. Rodrigo, Juan M. Soto, and Tatiana Alieva
The refractive index (RI) is an important optical characteristic that is often exploited in label-free microscopy for analysis of biological objects. A technique for 3D RI reconstruction of living cells has to be fast enough to capture the cell dynamics and preferably needs to be compatible with standard wide-field microscopes. To solve this challenging problem, we present a technique that provides fast measurement and processing of data required for real-time 3D visualization of the object RI. Specifically, the 3D RI is reconstructed from the measurement of bright-field intensity images, axially scanned by a high-speed focus tunable lens mounted in front of a sCMOS camera, by using a direct deconvolution approach designed for partially coherent light microscopy in the non-paraxial regime. Both the measurement system and the partially coherent illumination, that provides optical sectioning and speckle-noise suppression, enable compatibility with wide-field microscopes resulting in a competitive and affordable alternative to the current holographic laser microscopes. Our experimental demonstrations show video-rate 3D RI visualization of living bacteria both freely swimming and optically manipulated by using freestyle laser traps allowing for their trapping and transport along 3D trajectories. These results prove that is possible to conduct simultaneous 4D label-free quantitative imaging and optical manipulation of living cells, which is promising for the study of the cell biophysics and biology.
DOI
The refractive index (RI) is an important optical characteristic that is often exploited in label-free microscopy for analysis of biological objects. A technique for 3D RI reconstruction of living cells has to be fast enough to capture the cell dynamics and preferably needs to be compatible with standard wide-field microscopes. To solve this challenging problem, we present a technique that provides fast measurement and processing of data required for real-time 3D visualization of the object RI. Specifically, the 3D RI is reconstructed from the measurement of bright-field intensity images, axially scanned by a high-speed focus tunable lens mounted in front of a sCMOS camera, by using a direct deconvolution approach designed for partially coherent light microscopy in the non-paraxial regime. Both the measurement system and the partially coherent illumination, that provides optical sectioning and speckle-noise suppression, enable compatibility with wide-field microscopes resulting in a competitive and affordable alternative to the current holographic laser microscopes. Our experimental demonstrations show video-rate 3D RI visualization of living bacteria both freely swimming and optically manipulated by using freestyle laser traps allowing for their trapping and transport along 3D trajectories. These results prove that is possible to conduct simultaneous 4D label-free quantitative imaging and optical manipulation of living cells, which is promising for the study of the cell biophysics and biology.
DOI
Universal glass-forming behavior of in vitro and living cytoplasm
Kenji Nishizawa, Kei Fujiwara, Masahiro Ikenaga, Nobushige Nakajo, Miho Yanagisawa & Daisuke Mizuno
Physiological processes in cells are performed efficiently without getting jammed although cytoplasm is highly crowded with various macromolecules. Elucidating the physical machinery is challenging because the interior of a cell is so complex and driven far from equilibrium by metabolic activities. Here, we studied the mechanics of in vitro and living cytoplasm using the particle-tracking and manipulation technique. The molecular crowding effect on cytoplasmic mechanics was selectively studied by preparing simple in vitro models of cytoplasm from which both the metabolism and cytoskeletons were removed. We obtained direct evidence of the cytoplasmic glass transition; a dramatic increase in viscosity upon crowding quantitatively conformed to the super-Arrhenius formula, which is typical for fragile colloidal suspensions close to jamming. Furthermore, the glass-forming behaviors were found to be universally conserved in all the cytoplasm samples that originated from different species and developmental stages; they showed the same tendency for diverging at the macromolecule concentrations relevant for living cells. Notably, such fragile behavior disappeared in metabolically active living cells whose viscosity showed a genuine Arrhenius increase as in typical strong glass formers. Being actively driven by metabolism, the living cytoplasm forms glass that is fundamentally different from that of its non-living counterpart.
DOI
Physiological processes in cells are performed efficiently without getting jammed although cytoplasm is highly crowded with various macromolecules. Elucidating the physical machinery is challenging because the interior of a cell is so complex and driven far from equilibrium by metabolic activities. Here, we studied the mechanics of in vitro and living cytoplasm using the particle-tracking and manipulation technique. The molecular crowding effect on cytoplasmic mechanics was selectively studied by preparing simple in vitro models of cytoplasm from which both the metabolism and cytoskeletons were removed. We obtained direct evidence of the cytoplasmic glass transition; a dramatic increase in viscosity upon crowding quantitatively conformed to the super-Arrhenius formula, which is typical for fragile colloidal suspensions close to jamming. Furthermore, the glass-forming behaviors were found to be universally conserved in all the cytoplasm samples that originated from different species and developmental stages; they showed the same tendency for diverging at the macromolecule concentrations relevant for living cells. Notably, such fragile behavior disappeared in metabolically active living cells whose viscosity showed a genuine Arrhenius increase as in typical strong glass formers. Being actively driven by metabolism, the living cytoplasm forms glass that is fundamentally different from that of its non-living counterpart.
DOI
Numerical Investigation of Tunable Plasmonic Tweezers based on Graphene Stripes
Mohsen Samadi, Sara Darbari & Mohammad Kazem Moravvej-Farshi
We are proposing tunable plasmonic tweezers, consisting two parallel graphene stripes, which can be utilized to effectively trap and sort nanoparticles. We show that by electrostatically tuning the chemical potential of a graphene stripe by about 100 meV (equivalent to ΔVG ≈ 4.4 V), the plasmonic force can be switched efficiently, without a need to switch the laser intensity. This enables high speed and low power switching with a large number of switching cycles. By applying two independent and appropriate gate bias voltages to the stripes, the direction of the plasmonic force can be reversed, which leads to separation of nanoparticles that satisfy the trapping conditions. Numerical simulations show that the potential depths obtained for polystyrene nanoparticles of refractive index n = 1.5717 and radii r ≥ 50 nm is deeper than −10 kBT , confirming the ability of the proposed system to effectively separate such nanoparticles. This capability holds for smaller nanoparticles with larger refractive indices. Finally, performing thermal simulations, we have demonstrated that the heat induced by the illumination increases the fluid temperature by at most 9 °C, having negligible effect on the trapping mechanism. The proposed system opens up new possibilities in developing tunable on-chip manipulation devices, suitable for biological applications.
DOI
We are proposing tunable plasmonic tweezers, consisting two parallel graphene stripes, which can be utilized to effectively trap and sort nanoparticles. We show that by electrostatically tuning the chemical potential of a graphene stripe by about 100 meV (equivalent to ΔVG ≈ 4.4 V), the plasmonic force can be switched efficiently, without a need to switch the laser intensity. This enables high speed and low power switching with a large number of switching cycles. By applying two independent and appropriate gate bias voltages to the stripes, the direction of the plasmonic force can be reversed, which leads to separation of nanoparticles that satisfy the trapping conditions. Numerical simulations show that the potential depths obtained for polystyrene nanoparticles of refractive index n = 1.5717 and radii r ≥ 50 nm is deeper than −10 kBT , confirming the ability of the proposed system to effectively separate such nanoparticles. This capability holds for smaller nanoparticles with larger refractive indices. Finally, performing thermal simulations, we have demonstrated that the heat induced by the illumination increases the fluid temperature by at most 9 °C, having negligible effect on the trapping mechanism. The proposed system opens up new possibilities in developing tunable on-chip manipulation devices, suitable for biological applications.
DOI
Pushing, pulling and electromagnetic radiation force cloaking by a pair of conducting cylindrical particles
F.G. Mitri
The present analysis shows that two conducting cylindrical particles illuminated by an axially-polarized electric field of plane progressive waves at arbitrary incidence will attract, repel or become totally cloaked (i.e., invisible to the transfer of linear momentum carried by the incident waves), depending on their sizes, the interparticle distance as well as the angle of incidence of the incident field. Based on the rigorous multipole expansion method and the translational addition theorem of cylindrical wave functions, the electromagnetic (EM) radiation forces arising from multiple scattering effects between a pair of perfectly conducting cylindrical particles of circular cross-sections are derived and computed. An effective incident field on a particular particle is determined first, and used subsequently with its corresponding scattered field to derive the closed-form analytical expressions for the radiation force vector components. The mathematical expressions for the EM radiation force components (i.e. longitudinal and transverse) are exact, and have been formulated in partial-wave series expansions in cylindrical coordinates involving the angle of incidence, the interparticle distance and the expansion coefficients. Numerical examples illustrate the analysis for two perfectly conducting circular cylinders in a homogeneous nonmagnetic medium of wave propagation. The computations for the dimensionless radiation force functions are performed with particular emphasis on varying the angle of incidence, the interparticle distance, and the sizes of the particles. Depending on the interparticle distance and angle of incidence, the cylinders yield total neutrality (or invisibility); they experience no force and become unresponsive to the transfer of the EM linear momentum due to multiple scattering cancellation effects. Moreover, pushing or pulling EM forces between the two cylinders arise depending on the interparticle distance, the angle of incidence and their size parameters. This study provides a complete analytical method and computations for the longitudinal and transverse radiation force components in the multiple scattering of EM plane progressive waves with potential applications in particle manipulation, optically-engineered metamaterials with reconfigurable periodicities and cloaking devices to name a few examples.
DOI
The present analysis shows that two conducting cylindrical particles illuminated by an axially-polarized electric field of plane progressive waves at arbitrary incidence will attract, repel or become totally cloaked (i.e., invisible to the transfer of linear momentum carried by the incident waves), depending on their sizes, the interparticle distance as well as the angle of incidence of the incident field. Based on the rigorous multipole expansion method and the translational addition theorem of cylindrical wave functions, the electromagnetic (EM) radiation forces arising from multiple scattering effects between a pair of perfectly conducting cylindrical particles of circular cross-sections are derived and computed. An effective incident field on a particular particle is determined first, and used subsequently with its corresponding scattered field to derive the closed-form analytical expressions for the radiation force vector components. The mathematical expressions for the EM radiation force components (i.e. longitudinal and transverse) are exact, and have been formulated in partial-wave series expansions in cylindrical coordinates involving the angle of incidence, the interparticle distance and the expansion coefficients. Numerical examples illustrate the analysis for two perfectly conducting circular cylinders in a homogeneous nonmagnetic medium of wave propagation. The computations for the dimensionless radiation force functions are performed with particular emphasis on varying the angle of incidence, the interparticle distance, and the sizes of the particles. Depending on the interparticle distance and angle of incidence, the cylinders yield total neutrality (or invisibility); they experience no force and become unresponsive to the transfer of the EM linear momentum due to multiple scattering cancellation effects. Moreover, pushing or pulling EM forces between the two cylinders arise depending on the interparticle distance, the angle of incidence and their size parameters. This study provides a complete analytical method and computations for the longitudinal and transverse radiation force components in the multiple scattering of EM plane progressive waves with potential applications in particle manipulation, optically-engineered metamaterials with reconfigurable periodicities and cloaking devices to name a few examples.
DOI
A robust single-beam optical trap for a gram-scale mechanical oscillator
P. A. Altin, T. T.-H. Nguyen, B. J. J. Slagmolen, R. L. Ward, D. A. Shaddock & D. E. McClelland
Precise optical control of microscopic particles has been mastered over the past three decades, with atoms, molecules and nano-particles now routinely trapped and cooled with extraordinary precision, enabling rapid progress in the study of quantum phenomena. Achieving the same level of control over macroscopic objects is expected to bring further advances in precision measurement, quantum information processing and fundamental tests of quantum mechanics. However, cavity optomechanical systems dominated by radiation pressure – so-called ‘optical springs’ – are inherently unstable due to the delayed dynamical response of the cavity. Here we demonstrate a fully stable, single-beam optical trap for a gram-scale mechanical oscillator. The interaction of radiation pressure with thermo-optic feedback generates damping that exceeds the mechanical loss by four orders of magnitude. The stability of the resultant spring is robust to changes in laser power and detuning, and allows purely passive self-locking of the cavity. Our results open up a new way of trapping and cooling macroscopic objects for optomechanical experiments.
DOI
Precise optical control of microscopic particles has been mastered over the past three decades, with atoms, molecules and nano-particles now routinely trapped and cooled with extraordinary precision, enabling rapid progress in the study of quantum phenomena. Achieving the same level of control over macroscopic objects is expected to bring further advances in precision measurement, quantum information processing and fundamental tests of quantum mechanics. However, cavity optomechanical systems dominated by radiation pressure – so-called ‘optical springs’ – are inherently unstable due to the delayed dynamical response of the cavity. Here we demonstrate a fully stable, single-beam optical trap for a gram-scale mechanical oscillator. The interaction of radiation pressure with thermo-optic feedback generates damping that exceeds the mechanical loss by four orders of magnitude. The stability of the resultant spring is robust to changes in laser power and detuning, and allows purely passive self-locking of the cavity. Our results open up a new way of trapping and cooling macroscopic objects for optomechanical experiments.
DOI
Limiting (zero-load) speed of the rotary motor of Escherichia coli is independent of the number of torque-generating units
Bin Wang, Rongjing Zhang, and Junhua Yuan
Rotation of the bacterial flagellar motor is driven by multiple torque-generating units (stator elements). The torque-generating dynamics can be understood in terms of the “duty ratio” of the stator elements, that is, the fraction of time a stator element engages with the rotor during its mechanochemical cycle. The dependence of the limiting speed (zero-load speed) of the motor on the number of stator elements is the determining test of the duty ratio, which has been controversial experimentally and theoretically over the past decade. Here, we developed a method combining laser dark-field microscopy and optical trapping to resolve this controversy. We found that the zero-load speed is independent of the number of stator elements for the bacterial flagellar motor in Escherichia coli, demonstrating that these elements have a duty ratio close to 1.
DOI
Rotation of the bacterial flagellar motor is driven by multiple torque-generating units (stator elements). The torque-generating dynamics can be understood in terms of the “duty ratio” of the stator elements, that is, the fraction of time a stator element engages with the rotor during its mechanochemical cycle. The dependence of the limiting speed (zero-load speed) of the motor on the number of stator elements is the determining test of the duty ratio, which has been controversial experimentally and theoretically over the past decade. Here, we developed a method combining laser dark-field microscopy and optical trapping to resolve this controversy. We found that the zero-load speed is independent of the number of stator elements for the bacterial flagellar motor in Escherichia coli, demonstrating that these elements have a duty ratio close to 1.
DOI
Thursday, December 7, 2017
Ensembles of Bidirectional Kinesin Cin8 Produce Additive Forces in Both Directions of Movement
Todd Fallesen, Johanna Roostalu, Christian Duellberg, Gunnar Pruessner, Thomas Surrey
Most kinesin motors move in only one direction along microtubules. Members of the kinesin-5 subfamily were initially described as unidirectional plus-end-directed motors and shown to produce piconewton forces. However, some fungal kinesin-5 motors are bidirectional. The force production of a bidirectional kinesin-5 has not yet been measured. Therefore, it remains unknown whether the mechanism of the unconventional minus-end-directed motility differs fundamentally from that of plus-end-directed stepping. Using force spectroscopy, we have measured here the forces that ensembles of purified budding yeast kinesin-5 Cin8 produce in microtubule gliding assays in both plus- and minus-end direction. Correlation analysis of pause forces demonstrated that individual Cin8 molecules produce additive forces in both directions of movement. In ensembles, Cin8 motors were able to produce single-motor forces up to a magnitude of ∼1.5 pN. Hence, these properties appear to be conserved within the kinesin-5 subfamily. Force production was largely independent of the directionality of movement, indicating similarities between the motility mechanisms for both directions. These results provide constraints for the development of models for the bidirectional motility mechanism of fission yeast kinesin-5 and provide insight into the function of this mitotic motor.
DOI
Most kinesin motors move in only one direction along microtubules. Members of the kinesin-5 subfamily were initially described as unidirectional plus-end-directed motors and shown to produce piconewton forces. However, some fungal kinesin-5 motors are bidirectional. The force production of a bidirectional kinesin-5 has not yet been measured. Therefore, it remains unknown whether the mechanism of the unconventional minus-end-directed motility differs fundamentally from that of plus-end-directed stepping. Using force spectroscopy, we have measured here the forces that ensembles of purified budding yeast kinesin-5 Cin8 produce in microtubule gliding assays in both plus- and minus-end direction. Correlation analysis of pause forces demonstrated that individual Cin8 molecules produce additive forces in both directions of movement. In ensembles, Cin8 motors were able to produce single-motor forces up to a magnitude of ∼1.5 pN. Hence, these properties appear to be conserved within the kinesin-5 subfamily. Force production was largely independent of the directionality of movement, indicating similarities between the motility mechanisms for both directions. These results provide constraints for the development of models for the bidirectional motility mechanism of fission yeast kinesin-5 and provide insight into the function of this mitotic motor.
DOI
Optical assembly of microsnap-fits fabricated by two-photon polymerization
Jannis Köhler; Yunus Kutlu; Gordon Zyla; Sarah I. Ksouri; Cemal Esen; Evgeny L. Gurevich; Andreas Ostendorf
To respond to current demands of nano- and microtechnologies, e.g., miniaturization and integration, different bottom-up strategies have been developed. These strategies are based on picking, placing, and assembly of multiple components to produce microsystems with desired features. This paper covers the fabrication of arbitrary-shaped microcomponents by two-photon polymerization and the trapping, moving, and aligning of these structures by the use of a holographic optical tweezer. The main focus is on the assembly technique based on a cantilever microsnap-fit. More precisely, mechanical properties are characterized by optical forces and a suitable geometry of the snap-fit is designed. As a result of these investigations, a fast and simple assembly technique is developed. Furthermore, disassembly is provided by an optimized design. These findings suggest that the microsnap-fit is suitable for the assembly of miniaturized systems and could broaden the application opportunities of bottom-up strategies.
DOI
To respond to current demands of nano- and microtechnologies, e.g., miniaturization and integration, different bottom-up strategies have been developed. These strategies are based on picking, placing, and assembly of multiple components to produce microsystems with desired features. This paper covers the fabrication of arbitrary-shaped microcomponents by two-photon polymerization and the trapping, moving, and aligning of these structures by the use of a holographic optical tweezer. The main focus is on the assembly technique based on a cantilever microsnap-fit. More precisely, mechanical properties are characterized by optical forces and a suitable geometry of the snap-fit is designed. As a result of these investigations, a fast and simple assembly technique is developed. Furthermore, disassembly is provided by an optimized design. These findings suggest that the microsnap-fit is suitable for the assembly of miniaturized systems and could broaden the application opportunities of bottom-up strategies.
DOI
Trapping and manipulation of nanoparticles using multifocal optical vortex metalens
Yanbao Ma, Guanghao Rui, Bing Gu & Yiping Cui
Optical trapping and manipulation have emerged as a powerful tool in the biological and physical sciences. In this work, we present a miniature optical tweezers device based on multifocal optical vortex metalens (MOVM). The MOVM is capable of generating multiple focal fields with specific orbital angular momentum at arbitrary position. The optical force of the vortex field exerted on both high-refractive-index particle and low-refractive-index particle are analyzed. The simulation results show that the two kinds of dielectric particles can be trapped simultaneously. Besides, it is also feasible to manipulate plasmonic nanoparticles even under the resonant condition, which is realized by constructing a 4Pi focusing system with metalenses. Moreover, the metalens can be made into an array format that is suitable for trapping and manipulating various nanoparticles with diverse motion behaviors. The work illustrates the potential of such optical tweezers for further development in lab-on-a-chip devices, and may open up new avenues for optical manipulation and their applications in extensive scientific fields.
DOI
Optical trapping and manipulation have emerged as a powerful tool in the biological and physical sciences. In this work, we present a miniature optical tweezers device based on multifocal optical vortex metalens (MOVM). The MOVM is capable of generating multiple focal fields with specific orbital angular momentum at arbitrary position. The optical force of the vortex field exerted on both high-refractive-index particle and low-refractive-index particle are analyzed. The simulation results show that the two kinds of dielectric particles can be trapped simultaneously. Besides, it is also feasible to manipulate plasmonic nanoparticles even under the resonant condition, which is realized by constructing a 4Pi focusing system with metalenses. Moreover, the metalens can be made into an array format that is suitable for trapping and manipulating various nanoparticles with diverse motion behaviors. The work illustrates the potential of such optical tweezers for further development in lab-on-a-chip devices, and may open up new avenues for optical manipulation and their applications in extensive scientific fields.
DOI
Optofluidic trapping and delivery of massive mesoscopic matters using mobile vortex array
Jianxin Yang, Zongbao Li, Haiyan Wang, Debin Zhu, Xiang Cai, Yupeng Cheng, Mingyu Chen, Xiaowen Hu, and Xiaobo Xing
The realization of directional and controllable delivery of massive mesoscopic matters is of great significance in the field of microfluidics. Here, the mobile thermocapillary vortex array has achieved the enrichment and transport of massive mesoscopic matters in free or limited space. The ability of the vortex array to confine objects in the center ensures the controllability of particle trajectory. We also simulated the delivery process to reveal the stability of the mobile vortex. Owing to the distance between the vortex center and the heat source, the method provides the ability to protect trapped matters, including organisms and living cells. The mobile vortex array has opened the exciting possibilities of realizing that bridges the gap between remote optofluidics and lab on a chip.
DOI
The realization of directional and controllable delivery of massive mesoscopic matters is of great significance in the field of microfluidics. Here, the mobile thermocapillary vortex array has achieved the enrichment and transport of massive mesoscopic matters in free or limited space. The ability of the vortex array to confine objects in the center ensures the controllability of particle trajectory. We also simulated the delivery process to reveal the stability of the mobile vortex. Owing to the distance between the vortex center and the heat source, the method provides the ability to protect trapped matters, including organisms and living cells. The mobile vortex array has opened the exciting possibilities of realizing that bridges the gap between remote optofluidics and lab on a chip.
DOI
Viscoelastic Dissipation Stabilizes Cell Shape Changes during Tissue Morphogenesis
Raphaël Clément, Benoît Dehapiot, Claudio Collinet, Thomas Lecuit, Pierre-François Lenne
Tissue morphogenesis relies on the production of active cellular forces. Understanding how such forces are mechanically converted into cell shape changes is essential to our understanding of morphogenesis. Here, we use myosin II pulsatile activity during Drosophila embryogenesis to study how transient forces generate irreversible cell shape changes. Analyzing the dynamics of junction shortening and elongation resulting from myosin II pulses, we find that long pulses yield less reversible deformations, typically a signature of dissipative mechanics. This is consistent with a simple viscoelastic description, which we use to model individual shortening and elongation events. The model predicts that dissipation typically occurs on the minute timescale, a timescale commensurate with that of force generation by myosin II pulses. We test this estimate by applying time-controlled forces on junctions with optical tweezers. Finally, we show that actin turnover participates in dissipation, as reducing it pharmacologically increases the reversibility of contractile events. Our results argue that active junctional deformation is stabilized by actin-dependent dissipation. Hence, tissue morphogenesis requires coordination between force generation and dissipation.
DOI
Tissue morphogenesis relies on the production of active cellular forces. Understanding how such forces are mechanically converted into cell shape changes is essential to our understanding of morphogenesis. Here, we use myosin II pulsatile activity during Drosophila embryogenesis to study how transient forces generate irreversible cell shape changes. Analyzing the dynamics of junction shortening and elongation resulting from myosin II pulses, we find that long pulses yield less reversible deformations, typically a signature of dissipative mechanics. This is consistent with a simple viscoelastic description, which we use to model individual shortening and elongation events. The model predicts that dissipation typically occurs on the minute timescale, a timescale commensurate with that of force generation by myosin II pulses. We test this estimate by applying time-controlled forces on junctions with optical tweezers. Finally, we show that actin turnover participates in dissipation, as reducing it pharmacologically increases the reversibility of contractile events. Our results argue that active junctional deformation is stabilized by actin-dependent dissipation. Hence, tissue morphogenesis requires coordination between force generation and dissipation.
DOI
Hemodynamic forces can be accurately measured in vivo with optical tweezers
Sébastien Harlepp, Fabrice Thalmann, Gautier Follain, and Jacky G. Goetz
Force sensing and generation at the tissue and cellular scale is central to many biological events. There is a growing interest in modern cell biology for methods enabling force measurements in vivo. Optical trapping allows noninvasive probing of piconewton forces and thus emerged as a promising mean for assessing biomechanics in vivo. Nevertheless, the main obstacles lie in the accurate determination of the trap stiffness in heterogeneous living organisms, at any position where the trap is used. A proper calibration of the trap stiffness is thus required for performing accurate and reliable force measurements in vivo. Here we introduce a method that overcomes these difficulties by accurately measuring hemodynamic profiles in order to calibrate the trap stiffness. Doing so, and using numerical methods to assess the accuracy of the experimental data, we measured flow profiles and drag forces imposed to trapped red blood cells of living zebrafish embryos. Using treatments enabling blood flow tuning, we demonstrated that such a method is powerful in measuring hemodynamic forces in vivo with accuracy and confidence. Altogether this study demonstrates the power of optical tweezing in measuring low range hemodynamic forces in vivo and offers an unprecedented tool in both cell and developmental biology.
DOI
Force sensing and generation at the tissue and cellular scale is central to many biological events. There is a growing interest in modern cell biology for methods enabling force measurements in vivo. Optical trapping allows noninvasive probing of piconewton forces and thus emerged as a promising mean for assessing biomechanics in vivo. Nevertheless, the main obstacles lie in the accurate determination of the trap stiffness in heterogeneous living organisms, at any position where the trap is used. A proper calibration of the trap stiffness is thus required for performing accurate and reliable force measurements in vivo. Here we introduce a method that overcomes these difficulties by accurately measuring hemodynamic profiles in order to calibrate the trap stiffness. Doing so, and using numerical methods to assess the accuracy of the experimental data, we measured flow profiles and drag forces imposed to trapped red blood cells of living zebrafish embryos. Using treatments enabling blood flow tuning, we demonstrated that such a method is powerful in measuring hemodynamic forces in vivo with accuracy and confidence. Altogether this study demonstrates the power of optical tweezing in measuring low range hemodynamic forces in vivo and offers an unprecedented tool in both cell and developmental biology.
DOI
Friday, December 1, 2017
Arbitrary spin-to-orbital angular momentum conversion of light
Robert C. Devlin, Antonio Ambrosio, Noah A. Rubin, J. P. Balthasar Mueller, Federico Capasso
Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elements, SAM to orbital angular momentum (OAM) converters, are based on the geometric phase and only permit the conversion of left- and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We design a metasurface that converts left- and right-circular polarizations into states with independent values of OAM, and another device that performs this operation for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.
DOI
Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elements, SAM to orbital angular momentum (OAM) converters, are based on the geometric phase and only permit the conversion of left- and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We design a metasurface that converts left- and right-circular polarizations into states with independent values of OAM, and another device that performs this operation for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.
DOI
Filamentous phages as building blocks for reconfigurable and hierarchical self-assembly
Thomas Gibaud
Filamentous bacteriophages such as fd-like viruses are monodisperse rod-like colloids that have well defined properties of diameter, length, rigidity, charge and chirality. Engineering these viruses leads to a library of colloidal rods, which can be used as building blocks for reconfigurable and hierarchical self-assembly. Their condensation in an aqueous solution with additive polymers, which act as depletants to induce attraction between the rods, leads to a myriad of fluid-like micronic structures ranging from isotropic/nematic droplets, colloid membranes, achiral membrane seeds, twisted ribbons, π-wall, pores, colloidal skyrmions, Möbius anchors, scallop membranes to membrane rafts. These structures, and the way that they shape-shift, not only shed light on the role of entropy, chiral frustration and topology in soft matter, but also mimic many structures encountered in different fields of science. On the one hand, filamentous phages being an experimental realization of colloidal hard rods, their condensation mediated by depletion interactions constitutes a blueprint for the self-assembly of rod-like particles and provides a fundamental foundation for bio- or material-oriented applications. On the other hand, the chiral properties of the viruses restrict the generalities of some results but vastly broaden the self-assembly possibilities.
DOI
Filamentous bacteriophages such as fd-like viruses are monodisperse rod-like colloids that have well defined properties of diameter, length, rigidity, charge and chirality. Engineering these viruses leads to a library of colloidal rods, which can be used as building blocks for reconfigurable and hierarchical self-assembly. Their condensation in an aqueous solution with additive polymers, which act as depletants to induce attraction between the rods, leads to a myriad of fluid-like micronic structures ranging from isotropic/nematic droplets, colloid membranes, achiral membrane seeds, twisted ribbons, π-wall, pores, colloidal skyrmions, Möbius anchors, scallop membranes to membrane rafts. These structures, and the way that they shape-shift, not only shed light on the role of entropy, chiral frustration and topology in soft matter, but also mimic many structures encountered in different fields of science. On the one hand, filamentous phages being an experimental realization of colloidal hard rods, their condensation mediated by depletion interactions constitutes a blueprint for the self-assembly of rod-like particles and provides a fundamental foundation for bio- or material-oriented applications. On the other hand, the chiral properties of the viruses restrict the generalities of some results but vastly broaden the self-assembly possibilities.
DOI
Tailoring Optical Forces Behavior in Nano-optomechanical Devices Immersed in Fluid Media
Janderson R. Rodrigues & Vilson R. Almeida
Emerging nano-optofluidic devices have allowed a synergetic relation between photonic integrated circuits and microfluidics, allowing manipulation and transport at the realm of nanoscale science. Simultaneously, optical gradient forces have allowed highly precise control of mechanical motion in nano-optomechanical devices. In this report, we show that the repulsive optical forces of the antisymmetric eigenmodes in an optomechanical device, based on a slot-waveguide structure, increases as the refraction index of the fluid medium increases. This effect provides a feasible way to tailor the repulsive optical forces when these nano-optomechanical devices are immersed in dielectric liquids. Furthermore, the total control of the attractive and repulsive optical forces inside liquids may be applied to design novel nanophotonic devices, containing both microfluidic and nanomechanical functionalities, which may find useful applications in several areas, such as biomedical sensors, manipulators and sorters, amongst others.
DOI
Emerging nano-optofluidic devices have allowed a synergetic relation between photonic integrated circuits and microfluidics, allowing manipulation and transport at the realm of nanoscale science. Simultaneously, optical gradient forces have allowed highly precise control of mechanical motion in nano-optomechanical devices. In this report, we show that the repulsive optical forces of the antisymmetric eigenmodes in an optomechanical device, based on a slot-waveguide structure, increases as the refraction index of the fluid medium increases. This effect provides a feasible way to tailor the repulsive optical forces when these nano-optomechanical devices are immersed in dielectric liquids. Furthermore, the total control of the attractive and repulsive optical forces inside liquids may be applied to design novel nanophotonic devices, containing both microfluidic and nanomechanical functionalities, which may find useful applications in several areas, such as biomedical sensors, manipulators and sorters, amongst others.
DOI
Steering chemical reactions with force
Sergi Garcia-Manyes & Amy E. M. Beedle
Chemical reactivity underlies our fundamental understanding of many physical and biological phenomena. Chemical reactions are typically initiated by heat, electric current or light. Albeit far less studied, mechanical force is yet another way to orthogonally catalyse chemical reactions. An applied force can substantially reduce the reaction energy barrier, thus enabling reaction pathways that are too slow (or even forbidden) according to the laws of thermodynamics. Single-molecule nanomechanical techniques, including optical and magnetic tweezers and atomic force microscopy, offer the possibility to apply a directional force on an individual chemical bond. In non-covalent (or soft) mechanochemistry, low, sub-nN forces trigger bond rotation or hydrogen-bond rupture. By contrast, in covalent mechanochemistry, higher forces typically result in the breaking and re-forming of individual bonds. This Review focuses on the advances in our mechanistic understanding of single-bond mechanochemistry resulting from single-molecule measurements, as well as on the exciting new perspectives that we envision for this burgeoning field in the near future.
DOI
Chemical reactivity underlies our fundamental understanding of many physical and biological phenomena. Chemical reactions are typically initiated by heat, electric current or light. Albeit far less studied, mechanical force is yet another way to orthogonally catalyse chemical reactions. An applied force can substantially reduce the reaction energy barrier, thus enabling reaction pathways that are too slow (or even forbidden) according to the laws of thermodynamics. Single-molecule nanomechanical techniques, including optical and magnetic tweezers and atomic force microscopy, offer the possibility to apply a directional force on an individual chemical bond. In non-covalent (or soft) mechanochemistry, low, sub-nN forces trigger bond rotation or hydrogen-bond rupture. By contrast, in covalent mechanochemistry, higher forces typically result in the breaking and re-forming of individual bonds. This Review focuses on the advances in our mechanistic understanding of single-bond mechanochemistry resulting from single-molecule measurements, as well as on the exciting new perspectives that we envision for this burgeoning field in the near future.
DOI
Microparticles controllable accumulation, arrangement, and spatial shaping performed by tapered-fiber-based laser-induced convection flow
Yu Zhang, Jiaojie Lei, Yaxun Zhang, Zhihai Liu, Jianzhong Zhang, Xinghua Yang, Jun Yang & Libo Yuan
The ability to arrange cells and/or microparticles into the desired pattern is critical in biological, chemical, and metamaterial studies and other applications. Researchers have developed a variety of patterning techniques, which either have a limited capacity to simultaneously trap massive particles or lack the spatial resolution necessary to manipulate individual particle. Several approaches have been proposed that combine both high spatial selectivity and high throughput simultaneously. However, those methods are complex and difficult to fabricate. In this article, we propose and demonstrate a simple method that combines the laser-induced convection flow and fiber-based optical trapping methods to perform both regular and special spatial shaping arrangement. Essentially, we combine a light field with a large optical intensity gradient distribution and a thermal field with a large temperature gradient distribution to perform the microparticles shaping arrangement. The tapered-fiber-based laser-induced convection flow provides not only the batch manipulation of massive particles, but also the finer manipulation of special one or several particles, which break out the limit of single-fiber-based massive/individual particles photothermal manipulation. The combination technique allows for microparticles quick accumulation, single-layer and multilayer arrangement; special spatial shaping arrangement/adjustment, and microparticles sorting.
DOI
The ability to arrange cells and/or microparticles into the desired pattern is critical in biological, chemical, and metamaterial studies and other applications. Researchers have developed a variety of patterning techniques, which either have a limited capacity to simultaneously trap massive particles or lack the spatial resolution necessary to manipulate individual particle. Several approaches have been proposed that combine both high spatial selectivity and high throughput simultaneously. However, those methods are complex and difficult to fabricate. In this article, we propose and demonstrate a simple method that combines the laser-induced convection flow and fiber-based optical trapping methods to perform both regular and special spatial shaping arrangement. Essentially, we combine a light field with a large optical intensity gradient distribution and a thermal field with a large temperature gradient distribution to perform the microparticles shaping arrangement. The tapered-fiber-based laser-induced convection flow provides not only the batch manipulation of massive particles, but also the finer manipulation of special one or several particles, which break out the limit of single-fiber-based massive/individual particles photothermal manipulation. The combination technique allows for microparticles quick accumulation, single-layer and multilayer arrangement; special spatial shaping arrangement/adjustment, and microparticles sorting.
DOI
Frequency dependence of the electrophoretic mobility for single colloids as measured using optical tweezers
Jun Ma, Tim Stangner, and Friedrich Kremer
Optical tweezers accomplished with fast (100 µs) particle tracking are employed to determine the frequency (1–1000 Hz) dependence of the electrophoretic mobility µ of a single colloid in an external oscillating electric (ac) field. The colloid under study is held in a microfluidic channel filled with monovalent or divalent salt (KCl and CaCl2) solutions of varying concentrations (10−4 to 10−1 molar), and its response to an external ac field is measured. A pronounced steplike frequency dependence of the single colloid electrophoretic mobility (SCE-µ) is observed with a plateau on the low-frequency side (1–100 Hz) and a corner frequency, fc , which shifts with increasing concentration of the salt solution. The results are explained by considering the characteristic time needed for polarizing the electric double layer surrounding the colloidal particle under study.
DOI
Optical tweezers accomplished with fast (100 µs) particle tracking are employed to determine the frequency (1–1000 Hz) dependence of the electrophoretic mobility µ of a single colloid in an external oscillating electric (ac) field. The colloid under study is held in a microfluidic channel filled with monovalent or divalent salt (KCl and CaCl2) solutions of varying concentrations (10−4 to 10−1 molar), and its response to an external ac field is measured. A pronounced steplike frequency dependence of the single colloid electrophoretic mobility (SCE-µ) is observed with a plateau on the low-frequency side (1–100 Hz) and a corner frequency, fc , which shifts with increasing concentration of the salt solution. The results are explained by considering the characteristic time needed for polarizing the electric double layer surrounding the colloidal particle under study.
DOI
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