Active rheology of phospholipid vesicles

Aidan T. Brown, Jurij Kotar, and Pietro Cicuta

Optical tweezers are used to manipulate the shape of artificial dioleoyl-phosphatidylcholine (DOPC) phospholipid vesicles of around 30 μm diameter. Using a time-shared trapping system, a complex of traps drives oscillations of the vesicle equator, with a sinusoidal time dependence and over a range of spatial and temporal frequencies. The mechanical response of the vesicle membrane as a function of the frequency and wavelength of the driving oscillation is monitored. A simple model of the vesicles as spherical elastic membranes immersed in a Newtonian fluid, driven by a harmonic trapping potential, describes the experimental data. The bending modulus of the membrane is recovered. The method has potential for future investigation of nonthermally driven systems, where comparison of active and passive rheology can help to distinguish nonthermal forces from equilibrium fluctuations.

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Biophysical characterization of hematopoietic cells from normal and leukemic sources with distinct primitiveness

Youhua Tan, Tsz-Kan Fung, Haixia Wan, Kaiqun Wang, Anskar Y. H. Leung, and Dong Sun

This letter reported the biophysical characterization of immunophenotypically distinct hematopoietic cells from normal and leukemic sources, through manipulation with optical tweezers at single cell level. The results show that the percentage of cells that are stretchable and their deformability are significantly higher in the more primitive cell populations. This study provides the evidence that normal and leukemic hematopoietic cell populations with distinct primitiveness exhibit differential biophysical properties. These findings raise a hypothesis that the high deformability may be related to the unique functions and activities of primitive hematopoietic cells.

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Translation and manipulation of silicon nanomembranes using holographic optical tweezers

Stefan M Oehrlein, Jose R Sanchez-Perez, R B Jacobson, Frank S Flack, Ryan J Kershner and Max G Lagally

We demonstrate the use of holographic optical tweezers for trapping and manipulating silicon nanomembranes. These macroscopic free-standing sheets of single-crystalline silicon are attractive for use in next-generation flexible electronics. We achieve three-dimensional control by attaching a functionalized silica bead to the silicon surface, enabling non-contact trapping and manipulation of planar structures with high aspect ratios (high lateral size to thickness). Using as few as one trap and trapping powers as low as several hundred milliwatts, silicon nanomembranes can be rotated and translated in a solution over large distances.

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Lysophospatidic acid induced red blood cell aggregation in vitro

Lars Kaestner, Patrick Steffen, Duc Bach Nguyen, Jue Wang, Lisa Wagner-Britz, Achim Jung, Christian Wagner and Ingolf Bernhardt

Under physiological conditions healthy RBCs do not adhere to each other. There are indications that RBC display an intercellular adhesion under certain (pathophysioligical) conditions. Therefore we investigated signaling steps starting with transmembrane calcium transport by means of calcium imaging. We found a lysophosphatidic acid (LPA) concentration dependent calcium influx with an EC50 of 5 μM LPA. Downstream signaling was investigated by flow cytometry as well as by video-imaging comparing LPA induced with „pure“ calcium mediated phosphatidylserine exposure and concluded the coexistence of two branches of the signaling pathway. Finally we performed force measurements with holographic optical tweezers (HOT): The intercellular adhesion of RBCs (aggregation) exceeds a force of 25 pN. These results support (i) earlier data of a RBC associated component in thrombotic events under certain pathophysiological conditions and (ii) the concept to use RBCs in studies of cellular adhesion behavior, especially in combination with HOT. The latter paves the way to use RBCs as model cells to investigate molecular regulation of cellular adhesion processes.

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Rapid Microtubule Self-Assembly Kinetics

Melissa K. Gardner, Blake D. Charlebois, Imre M. Jánosi, Jonathon Howard, Alan J. Hunt and David J. Odde

Microtubule assembly is vital for many fundamental cellular processes. Current models for microtubule assembly kinetics assume that the subunit dissociation rate from a microtubule tip is independent of free subunit concentration. Total-Internal-Reflection-Fluorescence (TIRF) microscopy experiments and data from a laser tweezers assay that measures in vitro microtubule assembly with nanometer resolution, provides evidence that the subunit dissociation rate from a microtubule tip increases as the free subunit concentration increases. These data are consistent with a two-dimensional model for microtubule assembly, and are explained by a shift in microtubule tip structure from a relatively blunt shape at low free concentrations to relatively tapered at high free concentrations. We find that because both the association and the dissociation rates increase at higher free subunit concentrations, the kinetics of microtubule assembly are an order-of-magnitude higher than currently estimated in the literature.

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PicoNewton-Millisecond Force Steps Reveal the Transition Kinetics and Mechanism of the Double-Stranded DNA Elongation

Pasquale Bianco, Lorenzo Bongini, Luca Melli, Mario Dolfi and Vincenzo Lombardi

We study the kinetics of the overstretching transition in λ-phage double-stranded (ds) DNA from the basic conformation (B state) to the 1.7-times longer and partially unwound conformation (S state), using the dual-laser optical tweezers under force-clamp conditions at 25°C. The unprecedented resolution of our piezo servo-system, which can impose millisecond force steps of 0.5–2 pN, reveals the exponential character of the elongation kinetics and allows us to test the two-state nature of the B-S transition mechanism. By analyzing the load-dependence of the rate constant of the elongation, we find that the elementary elongation step is 5.85 nm, indicating a cooperativity of 25 basepairs. This mechanism increases the free energy for the elementary reaction to 94 kBT, accounting for the stability of the basic conformation of DNA, and explains why ds-DNA can remain in equilibrium as it overstretches.

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Optical accelerator of nanoparticles

A. S. Shalin
A model of the optical nanoaccelerator that allows the acceleration of nanoparticles to velocities of several meters per second with the aid of low-intensity external fields is proposed. The system is based on the application of the plasmon resonator that exhibits a relatively high gain of local field, which leads to a significant increase in the gradient optical force that is exerted on the nanoobject. The conditions under which the nanoparticle is pushed out from the resonator by the gradient force, so that a shot effect is observed, are determined. The effect of the perturbation that is introduced by the nanoparticle in the field distribution in the resonator on the electromagnetic forces and the acquired energy is analyzed.

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A universal pathway for kinesin stepping

Bason E Clancy, William M Behnke-Parks, Johan O L Andreasson, Steven S Rosenfeld & Steven M Block

Kinesin-1 is an ATP-driven, processive motor that transports cargo along microtubules in a tightly regulated stepping cycle. Efficient gating mechanisms ensure that the sequence of kinetic events proceeds in the proper order, generating a large number of successive reaction cycles. To study gating, we created two mutant constructs with extended neck-linkers and measured their properties using single-molecule optical trapping and ensemble fluorescence techniques. Owing to a reduction in the inter-head tension, the constructs access an otherwise rarely populated conformational state in which both motor heads remain bound to the microtubule. ATP-dependent, processive backstepping and futile hydrolysis were observed under moderate hindering loads. On the basis of measurements, we formulated a comprehensive model for kinesin motion that incorporates reaction pathways for both forward and backward stepping. In addition to inter-head tension, we found that neck-linker orientation is also responsible for ensuring gating in kinesin.

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Optical Trapping of 12 nm Dielectric Spheres Using Double-Nanoholes in a Gold Film

Yuanjie Pang and Reuven Gordon

Optical tweezers have found many applications in biology, but for reasonable intensities, conventional traps are limited to particles >100 nm in size. We use a double-nanohole in a gold film to experimentally trap individual nanospheres, including 20 nm polystyrene spheres and 12 nm silica spheres, at a well-defined trapping point. We present statistical studies on the trapping time, showing an exponential dependence on the optical power. Trapping experiments are repeated for different particles and several nanoholes with different gap dimensions. Unusually, smaller particles can be more easily trapped than larger ones with the double-nanohole. The 12 nm silica sphere has a size and a refractive index comparable to the smallest virus particles and has a spherical shape which is the worst case scenario for trapping.

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Ultimate capabilities of sharp metal tips for plasmon nanofocusing, near-field trapping and sensing

Dmitri K. Gramotnev and Michael W. Vogel

This Letter considers an ‘ultimate’ nanofocusing system with arguably the largest field enhancements achievable in plasmonic nanofocusing structures. These enhancements appear far beyond those required for single molecule detection. The proposed structure is demonstrated to be 4 orders of magnitude more efficient for trapping of small nanoparticles and single molecules, than the tip-assisted trapping. It does not require direct irradiation of the tip, and this will enable highly targeted delivery of the optical energy to nanoscale regions as small as a few nanometers (e.g., inside living cells) for trapping, imaging, localized heat discharge, and superior sensing in a new generation of nano-optical detectors.

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Cytoskeletal actin networks in motile cells are critically self-organized systems synchronized by mechanical interactions

Luca Cardamone, Alessandro Laio, Vincent Torre, Rajesh Shahapure, and Antonio DeSimone

Growing networks of actin fibers are able to organize into compact, stiff two-dimensional structures inside lamellipodia of crawling cells. We put forward the hypothesis that the growing actin network is a critically self-organized system, in which long-range mechanical stresses arising from the interaction with the plasma membrane provide the selective pressure leading to organization. We show that a simple model based only on this principle reproduces the stochastic nature of lamellipodia protrusion (growth periods alternating with fast retractions) and several of the features observed in experiments: a growth velocity initially insensitive to the external force; the capability of the network to organize its orientation; a load-history-dependent growth velocity. Our model predicts that the spectrum of the time series of the height of a growing lamellipodium decays with the inverse of the frequency. This behavior is a well-known signature of self-organized criticality and is confirmed by unique optical tweezer measurements performed in vivo on neuronal growth cones.

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Phase boundaries as agents of structural change in macromolecules

Ritwik Raj and Prashant K. Purohit

We model long rod-like molecules, such as DNA and coiled-coil proteins, as one-dimensional continua with a multi-well stored energy function. These molecules suffer a structural change in response to large forces, characterized by highly typical force-extension behavior. We assume that the structural change proceeds via a moving folded/unfolded interface, or phase boundary, that represents a jump in strain and is governed by the Abeyaratne–Knowles theory of phase transitions. We solve the governing equations using a finite difference method with moving nodes to represent phase boundaries. Our model can reproduce the experimental observations on the overstretching transition in DNA and coiled-coils and makes predictions for the speed at which the interface moves. We employ different types of kinetic relations to describe the mobility of the interface and show that this leads to different classes of experimentally observed force-extension curves. We make connections with several existing theories, experiments and simulation studies, thus demonstrating the effectiveness of the phase transitions-based approach in a biological setting.

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Myosin Va and myosin VI coordinate their steps while engaged in an in vitro tug of war during cargo transport

M. Yusuf Ali, Guy G. Kennedy, Daniel Safer, Kathleen M. Trybus, H. Lee Sweeney, and David M. Warshaw

Myosin Va (myoV) and myosin VI (myoVI) are processive molecular motors that transport cargo in opposite directions on actin tracks. Because these motors may bind to the same cargo in vivo, we developed an in vitro “tug of war” to characterize the stepping dynamics of single quantum-dot-labeled myoV and myoVI motors linked to a common cargo. MyoV dominates its myoVI partner 79% of the time. Regardless of which motor wins, its stepping rate slows due to the resistive load of the losing motor (myoV, 2.1 pN; myoVI, 1.4 pN). Interestingly, the losing motor steps backward in synchrony with the winning motor. With ADP present, myoVI acts as an anchor to prevent myoV from stepping forward. This model system emphasizes the physical communication between opposing motors bound to a common cargo and highlights the potential for modulating this interaction by changes in the cell’s ionic milieu.

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Nucleosome remodeling machines and other molecular motors observed at the single molecule level

Christophe Lavelle, Elise Praly, David Bensimon, Eric Le Cam, Vincent Croquette

Through its capability to transiently pack and unpack our genome, chromatin is a key player in the regulation of gene expression. Single-molecule approaches have recently complemented conventional biochemical and biophysical techniques to decipher the complex mechanisms ruling chromatin dynamics. Micromanipulations with tweezers (magnetic or optical) and imaging with molecular microscopies (electron or atomic force) have indeed opened opportunities to handle and visualize single molecules, and to measure the forces and torques produced by molecular motors, along with their effects on DNA or nucleosomal templates. By giving access to dynamical events that tend to be blurred in traditional biochemical bulk experiments, these techniques provide critical information regarding the mechanisms underlying the regulation of gene activation and desactivation by nucleosome and chromatin structural changes. This minireview describes some single molecule approaches to the study of ATP-consuming molecular motors acting on DNA, with applications to the case of nucleosomes remodeling machines.

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Laser-guidance based cell detection for identifying malignant cancerous cells without any fluorescent markers

Zhen Ma and Bruce Z. Gao

Laser guidance technique employs the optical forces generated from a focused Gaussian laser beam incident on a biological cell to trap and guide the cell along the laser propagation direction. The optical force, which determines the guidance speed, is dependent on the cellular characteristics of the cell being guided, such as size, shape, composition and morphology. Different cell populations or subpopulations can be detected without any fluorescent markers by measuring their guidance speeds. We found that cell guidance speeds were sensitive enough to monitor the subtle changes during the progression of mouse fibroblast cells from normal to cancerous phenotype. The results also demonstrated that this technique can effectively distinguish mouse mammary cancerous cells with different metastatic competence. Laser guidance technique can be used as a label-free cell detection method for basic cell biological investigation and cancer diagnosis.

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Optical trapping through the localized surface-plasmon resonance of engineered gold nanoblock pair

Yoshito Tanaka and Keiji Sasaki

We have investigated the plasmonic trapping of dielectric nanoparticles by using engineered gold nanoblock pairs with ~5-nm gaps. Pairs with surface-plasmon resonance peaks at the incident wavelength allow the trapping of 350-nm-diameter nanoparticles with 200 W/cm2 laser intensities, and their plasmon resonance properties and trapping performance are drastically modified by varying the nanoblock size of ~20%. In addition, plasmon resonance properties of nanoblock pairs strongly depend on the direction of the linear polarization of the incident laser, which determines the trapping performance.

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Wide-field Rayleigh scattering imaging and spectroscopy of gold nanoparticles in heavy water under laser trapping

Takayuki Uwada, Teruki Sugiyama and Hiroshi Masuhara

We demonstrated wide-field Rayleigh scattering spectroscopy and imaging of gold nanoparticle trapping upon a focused near-infrared laser irradiation under dark-field illumination. The migration, trapping, and assembling behavior of gold nanoparticles were examined at single nanoparticle level at the focus spot and its surrounding area in heavy water. It is clarified that particle migration to the laser spot within the focal plane is not appreciable while scattering force drives the migration vertically along the direction of light propagation. The gradient force at the focus point and the scattering force are theoretically estimated to be 3.1 and 0.17 pN, which are consistent with the direct observation. Analysis of light scattering intensity fluctuation at the focus shows that three nanoparticles can be trapped at most, which is well supported by light scattering spectral measurement. The particle existence probability at the focus is estimated as a function of lateral distance from the focus, and is compared with numerically obtained photon pressure potential.

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Optical forces: Laser tractor beams

Juan José Sáenz

Scientists have theoretically proposed that it is possible to pull objects from a far distance towards a light source in the absence of axial optical gradient forces.

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Optical pulling force

Jun Chen, Jack Ng, Zhifang Lin & C. T. Chan

A photon carries k of momentum, so it may be anticipated that light will ‘push’ on any object standing in its path by means of the scattering force. In the absence of an intensity gradient, using a light beam to pull a particle backwards is counter-intuitive. Here, we show that it is possible to realize a backward scattering force that pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle, and if the projection of the total photon momentum along the propagation direction is small, an attractive optical force is possible. This possibility adds ‘pulling’ as an additional degree of freedom to optical micromanipulation.

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Note: Direct force and ionic-current measurements on DNA in a nanocapillary

O. Otto, L. J. Steinbock, D. W. Wong, J. L. Gornall, and U. F. Keyser

We have developed optical tweezers, with force measurements based on fast video tracking, for analysis and control of DNA translocation through nanocapillaries. Nanocapillaries are single-molecule biosensors with very similar characteristics to solid-state nanopores. Our novel experimental setup allows for ionic-current measurements in which the nanocapillary is oriented perpendicular to the trapping laser. Using video-based particle tracking, we are able to measure the position of DNA coated colloids at sub-millisecond resolution and in real-time. We present the first electrophoretic force and simultaneous ionic-current measurements of a single DNA molecule inside the orifice of a nanocapillary.

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BioPhotonics workstation: A versatile setup for simultaneous optical manipulation, heat stress, and intracellular pH measurements of a live yeast cell

Thomas Aabo, Andrew Raphael Banás, Jesper Glückstad, Henrik Siegumfeldt, and Nils Arneborg

In this study we have modified the BioPhotonics workstation (BWS), which allows for using long working distance objective for optical trapping, to include traditional epi-fluorescence microscopy, using the trapping objectives. We have also added temperature regulation of sample stage, allowing for fast temperature variations while trapping. Using this modified BWS setup, we investigated the internal pH (pHi) response and membrane integrity of an optically trapped Saccharomyces cerevisiae cell at 5 mW subject to increasing temperatures. The pHi of the cell is obtained from the emission of 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester, at 435 and 485 nm wavelengths, while the permeability is indicated by the fluorescence of propidium iodide. We present images mapping the pHi and permeability of the cell at different temperatures and with enough spatial resolution to localize these attributes within the cell. The combined capability of optical trapping, fluorescence microscopy and temperature regulation offers a versatile tool for biological research.

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Waveguide trapping of hollow glass spheres

Balpreet Singh Ahluwalia, Pål Løvhaugen, and Olav Gaute Hellesø

Microparticles can be trapped and propelled by the evanescent field of optical waveguides. As the evanescent field only stretches 100–200 nm from the surface of the waveguide, only the lower caps of the microparticles interact directly with the field. This is taken advantage of by trapping hollow glass spheres on waveguides in the same way as solid glass spheres. For the chosen waveguide, numerical simulations show that hollow microspheres with a shell thickness above 60 nm can be stably trapped, while spheres with thinner shells are repelled. The average refractive index of the sphere–field intersection volume is used to explain the result in a qualitative way.

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On-a-chip surface plasmon tweezers

H. M. K. Wong, M. Righini, J. C. Gates, P. G. R. Smith, V. Pruneri, and R. Quidant

We report on an integrated optical trapping platform operated by simple fiber coupling. The system consists of a dielectric channel optical waveguide decorated with an array of gold micro-pads. Through a suitable engineering of the waveguide mode, we achieve light coupling to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. We demonstrate parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip with only 20 mW total incident laser power.

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Robust trapping and manipulation of airborne particles with a bottle beam

Vladlen G. Shvedov, Cyril Hnatovsky, Andrei V. Rode, and Wieslaw Krolikowski

We demonstrate that micron-sized light-absorbing particles can be trapped and transported photophoretically in air using an optical bottle formed inside the focal volume of a lens with a controlled amount of spherical aberration. This optical fiber-based single beam trap can be used in numerous applications where true 3D manipulation and delivery of airborne micro-objects is required.

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Calculation of radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam

Zheng-Jun Li, Zhen-Sen Wu, and Qing-Chao Shang
Using the theory of electromagnetic scattering of a uniaxial anisotropic sphere, we derive the analytical expressions of the radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam. The beam’s propagation direction is parallel to the primary optical axis of the anisotropic sphere. The effects of the permittivity tensor elements εt and εz on the axial radiation forces are numerically analyzed in detail. The two transverse components of radiation forces exerted on a uniaxial anisotropic sphere, which is distinct from that exerted on an isotropic sphere due to the two eigen waves in the uniaxial anisotropic sphere, are numerically studied as well. The characteristics of the axial and transverse radiation forces are discussed for different radii of the sphere, beam waist width, and distances from the sphere center to the beam center of an off-axis Gaussian beam. The theoretical predictions of radiation forces exerted on a uniaxial anisotropic sphere are hoped to provide effective ways to achieve the improvement of optical tweezers as well as the capture, suspension, and high-precision delivery of anisotropic particles.

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Potential mapping of optical tweezers

Tahmineh Godazgar, Rouzbeh Shokri, and S. Nader S. Reihani
Optical tweezers are very often used for measurement of piconewton range forces. Depending on the displacement of the trapped bead, the trap may become stiffer which causes considerable underestimation of the measured force. We have shown, both by theory and experiment, that such a stiffening occurs for beads larger than 0.5 μm in radius. For the first time, we have shown that the displacement at which the stiffening starts is size dependent and that the stiffening starts at higher forces for larger beads. We have shown that for the applications, which simultaneous force measurement and position sensing are on demand (such as biopolymer stretching), mid-range sized (∼1.5 μm in radius) beads could be the best choice.

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Acceleration control of Airy beams with optically induced refractive-index gradient

Zhuoyi Ye, Sheng Liu, Cibo Lou, Peng Zhang, Yi Hu, Daohong Song, Jianlin Zhao, and Zhigang Chen

We demonstrate both experimentally and theoretically controlled acceleration of one- and two-dimensional Airy beams in optically induced refractive-index potentials. Enhancement as well as reduction of beam acceleration are realized by changing the index gradient, while the beam shape is maintained during propagation through the linear optical potential. Our results of active acceleration manipulation in graded media are pertinent to Airy-type beam propagation in various environments.

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Application of the discrete dipole approximation to optical trapping calculations of inhomogeneous and anisotropic particles

Stephen H. Simpson and Simon Hanna

The accuracy of the discrete dipole approximation (DDA) for computing forces and torques in optical trapping experiments is discussed in the context of dielectric spheres and a range of low symmetry particles, including particles with geometric anisotropy (spheroids), optical anisotropy (birefringent spheres) and structural inhomogeneity (core-shell spheres). DDA calculations are compared with the results of exact T-matrix theory. In each case excellent agreement is found between the two methods for predictions of optical forces, torques, trap stiffnesses and trapping positions. Since the DDA lends itself to calculations on particles of arbitrary shape, the study is augmented by considering more general systems which have received recent experimental interest. In particular, optical forces and torques on low symmetry letter-shaped colloidal particles, birefringent quartz cylinders and biphasic Janus particles are computed and the trapping behaviour of the particles is discussed. Very good agreement is found with the available experimental data. The efficiency of the DDA algorithm and methods of accelerating the calculations are also discussed.

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Individual particle handling in a microfluidic system based on parallel laser trapping

Philippe Hamel, Bastien Rachet, Michael Werner, Mathieu Grossenbacher, Horst Vogel, Martin Forrer, Peter Ryser, and René P. Salathé
We present an optical trapping system combining individually addressable multiple laser traps with fluorescence spectroscopy. An in-line set of 64 near-IR laser diodes is used to create a line of individually addressable traps inside a microfluidic chip. This system is completed by an excitation/detection line for spectrally resolved fluorescence imaging of trapped particles. Highly parallel trapping in a constant flow (up to a few millimeters per second), fast particle handling rates (up to a few particles per second), and the possibility of recording fluorescence spectra of trapped objects lead to a performing bioanalytical platform, e.g., for highly parallel analysis and sorting.

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Driving and analysis of micro-objects by digital holographic microscope in microfluidics

F. Merola, L. Miccio, M. Paturzo, A. Finizio, S. Grilli, and P. Ferraro

We propose an optical configuration in which floating particles in a microfluidic chamber can be characterized by an interference microscopy configuration to obtain quantitative phase-contrast maps. The configuration is simply made by two laser beams from the same laser source. One beam provides the optical forces for driving the particle along appropriate paths, but at same time works as the object illumination beam in the holographic microscope. The second beam plays the role of the reference beam, allowing recording of an interference fringe pattern (i.e., the digital hologram) in an out-of-focus image plane. The system and method are illustrated and experimental results are offered for polymeric particles as well as for in vitro cells with the aim to demonstrate the approach.

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Reverse optical forces in negative index dielectric waveguide arrays

Alessandro Salandrino and Demetrios N. Christodoulides

Nonconservative optical forces acting on dipolar particles are considered in longitudinally invariant optical fields. We demonstrate that the orientation of these forces is strictly dictated by the propagation vector associated with such field configurations. As a direct consequence of this, it is impossible to achieve a reversal of optical forces in homogeneous media. We show instead that translation invariant optical tractor fields can in fact be generated in the negative index environment produced in a special class of fully dielectric waveguide arrays.

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Optical trapping induced by reorientational nonlocal effects in nematic liquid crystals

L. Lucchetti, L. Criante, F. Bracalente, F. Aieta, and F. Simoni

We report a detailed analysis of optical trapping of low index particles in liquid crystals under experimental conditions that prevent the effect of conventional trapping originated by optical gradient forces. The observation of stable, long-range trapping shows that this phenomenon in liquid crystals is regulated by a completely different mechanism than in isotropic media. In particular, the role of the nonlocality of optical reorientation is highlighted by showing the dependence of the trapping force on the size of the reoriented area. A model based on the actual form of the Gaussian focused beam impinging on the liquid-crystalline medium in the trapping experiment is also reported, with good agreement with experimental data.

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Dual-trap optical tweezers with real-time force clamp control

Anders E. Wallin, Heikki Ojala, Gabija Ziedaite, and Edward Hæggström

Single molecule force clamp experiments are widely used to investigate how enzymes, molecular motors, and other molecular mechanisms work. We developed a dual-trap optical tweezers instrument with real-time (200 kHz update rate) force clamp control that can exert 0–100 pN forces on trapped beads. A model for force clamp experiments in the dumbbell-geometry is presented. We observe good agreement between predicted and observed power spectra of bead position and force fluctuations. The model can be used to predict and optimize the dynamics of real-time force clamp optical tweezers instruments. The results from a proof-of-principle experiment in which lambda exonuclease converts a double-stranded DNA tether, held at constant tension, into its single-stranded form, show that the developed instrument is suitable for experiments in single molecule biology.

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Use of an Optical Trap for Study of Host-Pathogen Interactions for Dynamic Live Cell Imaging

J.M. Tam, C. E. Castro, R. J. W. Heath, M. K. Mansour, M. L. Cardenas, R. J. Xavier, M. J. Lang, J. M. Vyas
Dynamic live cell imaging allows direct visualization of real-time interactions between cells of the immune system; however, the lack of spatial and temporal control between the phagocytic cell and microbe has rendered focused observations into the initial interactions of host response to pathogens difficult. Historically, intercellular contact events such as phagocytosis have been imaged by mixing two cell types, and then continuously scanning the field-of-view to find serendipitous intercellular contacts at the appropriate stage of interaction. The stochastic nature of these events renders this process tedious, and it is difficult to observe early or fleeting events in cell-cell contact by this approach. This method requires finding cell pairs that are on the verge of contact, and observing them until they consummate their contact, or do not. To address these limitations, we use optical trapping as a non-invasive, non-destructive, but fast and effective method to position cells in culture.

Optical traps, or optical tweezers, are increasingly utilized in biological research to capture and physically manipulate cells and other micron-sized particles in three dimensions. Radiation pressure was first observed and applied to optical tweezer systems in 1970, and was first used to control biological specimens in 1987. Since then, optical tweezers have matured into a technology to probe a variety of biological phenomena.

We describe a method that advances live cell imaging by integrating an optical trap with spinning disk confocal microscopy with temperature and humidity control to provide exquisite spatial and temporal control of pathogenic organisms in a physiological environment to facilitate interactions with host cells, as determined by the operator. Live, pathogenic organisms like Candida albicans and Aspergillus fumigatus, which can cause potentially lethal, invasive infections in immunocompromised individuals (e.g. AIDS, chemotherapy, and organ transplantation patients), were optically trapped using non-destructive laser intensities and moved adjacent to macrophages, which can phagocytose the pathogen. High resolution, transmitted light and fluorescence-based movies established the ability to observe early events of phagocytosis in living cells. To demonstrate the broad applicability in immunology, primary T-cells were also trapped and manipulated to form synapses with anti-CD3 coated microspheres in vivo, and time-lapse imaging of synapse formation was also obtained. By providing a method to exert fine spatial control of live pathogens with respect to immune cells, cellular interactions can be captured by fluorescence microscopy with minimal perturbation to cells and can yield powerful insight into early responses of innate and adaptive immunity.

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Optically induced forces in a nanoparticle-on-substrate system

A. S. Shalin

Optical forces acting onto a nanoparticle in an electromagnetic field near the interface of two media have been investigated. Conditions have been determined under which the gradient optical force increases by an order of magnitude without an increase in the strength of the external field. It has been shown that, depending on the material of the particle and the radiation wavelength, the self field of a nanocluster can both enhance and weaken the total force acting on the cluster. The use of optically induced forces for constructing a nanoengine has been suggested.

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Power dependent oxygenation state transition of red blood cells in a single beam optical trap

Rui Liu, Lena Zheng, Dennis L. Matthews, Noriko Satake, and James W. Chan

Laser tweezers Raman spectroscopy (LTRS) was used to demonstrate that a red blood cell (RBC) in a single beam optical trap transitions from an oxygenated to a partially deoxygenated state with increasing trapping power. Continuous switching between the two states is possible by repeatedly cycling between low and high trapping powers. Alterations in the hemoglobin conformation and interactions due to cell folding in the trap are proposed to be responsible for the transition. This study demonstrates that mechanically induced biochemical changes by optical forces need to be considered when applying single beam optical tweezers for cell analysis. LTRS holds promise as a functional assay to characterize normal and diseased RBCs based on their biochemical response to the forces of a single beam optical trap.

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Drag reduction by DNA-grafting for single microspheres in a dilute λ-DNA solution

Olaf Ueberschär, Carolin Wagner, Tim Stangner, Konstanze Kühne, Christof Gutsche and Friedrich Kremer

The fluid resistance of single micrometre-sized blank and DNA-grafted polystyrene microspheres under shear flow is compared in purified water and dilute λ-DNA solutions by means of optical tweezers experiments with a high spatial (±4 nm) and temporal (±0.2 ms) resolution. The measurement results show that the drag experienced by a colloid in a dilute λ-DNA solution (molecular weight of 48,502 bp per molecule, radius of gyration of 0.5 μm) is significantly decreased if the microsphere bears a grafted DNA brush. This newly discovered drag reduction effect is studied for different parameters, comprising the molecular weight of the grafted DNA molecules (250 bp, 1000 bp and 4000 bp), the concentration of the λ-DNA solution (11, 17 and 23 μg ml−1, all being significantly smaller than the critical entanglement concentration c*), the microsphere core diameter (2 μm, 3 μm and 6 μm) as well as the flow speed of the medium (10–50 μm s−1). The maximum extent of the drag reduction is found to amount to (60 ± 20)% compared to the λ-DNA-induced contribution on the drag acting on blank colloids. We propose a theoretical explanation of this effect based on the combination of the dynamic density functional theory of Rauscher and co-workers [Rauscher M. J. Phys.: Condens. Matter 2010;22:364109] and the stagnation length theory of polymer brushes, as it was established by Kim, Lobaskin et al. [Kim et al. Macromolecules 2008;42(10):3650–3655]. In particular, the solution of the Stokes equation (i.e., the Navier–Stokes equation for creeping flow) for the studied system yields a numerical prediction that is found to be in full accord with our experimental results within measurement uncertainty.

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