Friday, February 26, 2010

Controllable rotation of optical beams with bored helical phases

Stein Alec Baluyot and Nathaniel Hermosa, II

We achieve controllable noninterferometric rotation of a bored helical beam by introducing a phase shift exclusively to the annular helical region of the phase. We present a derivation based on the decomposition of the beams, which shows that a constant phase shift of ΔΦ between the bore and the surrounding helical phase with topological charge ℓ will rotate the intensity profile by −ΔΦ/ℓ about its center. The effect of the phase shifting is verified with experiments. This technique is simple, while it preserves the transverse intensity profiles of the beams. Our report may find applications in optical manipulation and trapping.

Observation of bistability of trapping position in aerosol optical tweezers

Kerry J. Knox, Daniel R. Burnham, Lowell I. McCann, Shawntel L. Murphy, David McGloin, and Jonathan P. Reid

Bistability in the axial trapping position of aqueous aerosol droplets has been observed for the first time, to our knowledge, in optical tweezers. The behavior has been observed for two distinct trapping configurations, with the trapping beam oriented either along the vertical or horizontal axis. This represents the first report of the optical tweezing of aerosol droplets with a horizontally propagating laser beam. Side imaging was used in conjunction with imaging in the plane of the optical trap. Droplet sizing was performed using cavity-enhanced Raman spectroscopy or by applying a circular regression routine to the acquired images. Predictions from a theoretical model of optical forces are shown to be in good agreement with the experimental observations of bistability in the trapping position. These studies have significance both for the rigorous interpretation of data obtained using aerosol optical tweezers and for the modeling of aerosol optical traps.

Optical manipulation of plasmonic nanoparticles, bubble formation and patterning of SERS aggregates

Zuwei Liu, Wei Hsuan Hung, Mehmet Aykol, David Valley and Stephen B Cronin

We present an optical method for patterning SERS (surface-enhanced Raman spectroscopy)—enhancing aggregates of gold nanoparticles, using a focused laser beam to optically trap the nanoparticles in suspension. At high laser powers, heat generated from the plasmonic excitation causes boiling of the aqueous suspension and the formation of gaseous bubbles of water vapor. By measuring the Raman peak of the hydroxyl bond of water, the temperature in the laser spot during the aggregation can be determined in situ. The hydrophilic nanoparticles are found to aggregate at the liquid–vapor interface. By allowing the suspension to dry, a ring of gold nanoparticles is deposited on the substrate, producing a highly SERS-active region. These aggregates are studied using optical microscopy, scanning electron microscopy and micro-Raman spectroscopy.

Thursday, February 25, 2010

Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament regulatory properties in vitro

Edward P. Debold, Walid Saber, Yaser Cheema, Carol S. Bookwalter, Kathleen M. Trybus, David M. Warshaw and Peter VanBuren

Two cardiomyopathic Mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system: E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly slower velocities than wild type (WT) at maximal Ca++. At submaximal Ca++, R312H thin filaments demonstrated significantly increased Ca++ sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca++ revealed that neither actin mutation significantly affected the myosin's unitary step size (d) OF duration Of Strong actin binding (t(on)) at 20 mu M ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.

Trapping two types of particles using a double-ring-shaped radially polarized beam

Yaoju Zhang, Biaofeng Ding, and Taikei Suyama

An optical-trap method based on the illumination of a double-ring-shaped radially polarized beam (R-TEM11*) is proposed. The numerical results based on the vector diffraction theory show that a highly focused R-TEM11* beam not only can produce a bright spot but also can form an optical cage in the focal region by changing the truncation parameter β, defined as the ratio of the radius of the aperture to the waist of the beam. The radiation forces acting on Rayleigh particles are calculated by using the Rayleigh scattering theory. The bright spot generated by the R-TEM11* beam with a β value close to 2 can three-dimensionally trap a particle with a refractive index larger than that of the ambient. An optical cage or three-dimensional dark spot generated by the R-TEM11* beam with a β value close to 1.3 can three-dimensionally trap a particle with refractive index smaller than that of the ambient. Because the adjustment of the truncation parameter can be actualized by simply changing the radius of a circular aperture inserted in the front of the lens, only one optical-trap system in the present method can be used to three-dimensionally trap two types of particles with different refractive indices.

Thursday, February 18, 2010

Scheme for Information Erasure in a Double-Well Potential

Wang Xin-Xin and Bao Jing-Dong

We design an experimental scheme to realize one-bit information erasure and restoring processes by considering an overdamped colloidal particle in a double-well optical trap, which is added by a controllable laser tweezer. Using the Monte Carlo method, we simulate numerically the Langevin equation to calculate the mean work spent during the entire process and validate the entropy production fluctuation theory. Our result shows that the distribution of entropy production becomes narrow with increasing temperature and becomes stationary, represents the diminishing extent of irreversibility.

Direct physical study of kinetochore–microtubule interactions by reconstitution and interrogation with an optical force clamp

Andrew D. Franck, Andrew F. Powers, Daniel R. Gestaut, Trisha N. Davis and Charles L. Asbury

We detail our use of computer-controlled optical traps to study interactions between kinetochore components and dynamic microtubules. Over the last two decades optical traps have helped uncover the working principles of conventional molecular motors, such as kinesin and dynein, but only recently have they been applied to study kinetochore function. The most useful traps combine sensitive position detectors and servo-control, allowing them to be operated as force clamps that maintain constant loads on objects as they move. Our instrument, which is among the simplest designs that permits force clamping, relies on a computer-controlled piezoelectric stage and a single laser for trapping and position detection. We apply it in motility assays where beads coated with pure microtubule-binding kinetochore components are attached to the tips of individual dynamic microtubules. Like kinetochores in vivo, the beads remain tip-attached, undergoing movements coupled to filament assembly and disassembly. The force clamp provides many benefits over instruments that lack feedback control. It allows tension to be applied continuously during both assembly- and disassembly-driven movement, providing a close match to the physiological situation. It also enables tracking with high resolution, and simplifies data interpretation by eliminating artifacts due to molecular compliance. The formation of persistent, load-bearing attachments to dynamic microtubule tips is fundamental to all kinetochore activities. Our direct, physical study of kinetochore–microtubule coupling may therefore furnish insights into many vital kinetochore functions, including correction of aberrant attachments and generation of the ‘wait-anaphase’ signals that delay mitosis until all kinetochores are properly attached.

Highly selective separation of DNA fragments using optically directed transport

Avital Braiman, Fedor Rudakov, and Thomas Thundat

We present a design that allows selective separation of biomolecules of a particular size without performing complete separation of the sample by size. By focusing a laser beam onto a photoelectrode in contact with an electrolyte medium, a highly localized andoptically controlled photoelectrophoretic trap is created. Moving the light beam along the photoelectrode consequently moves the trap. We demonstrate that by manipulating the speed of the photoelectrophoretic trap biomolecules of a particular size can be selectively separated from the mixture. We achieve a qualitative agreement between our experimental results and numerical simulations.

Resolution of the Abraham-Minkowski Dilemma

Stephen M. Barnett

The dilemma of identifying the correct form for the momentum of light in a medium has run for a century and has been informed by many distinguished contributions, both theoretical and experimental. We show that both the Abraham and Minkowski forms of the momentum density are correct, with the former being the kinetic momentum and the latter the canonical momentum. This identification allows us to explain why the experiments supporting each of the rival momenta gave the results that they did. The inclusion of dispersion and absorption provides an interesting subtlety, but does not change our conclusion.

Wednesday, February 17, 2010

Direct Force Measurements on Double-Stranded RNA in Solid-State Nanopores

Michiel van den Hout, Igor D. Vilfan, Susanne Hage and Nynke H. Dekker

Solid-state nanopores can be employed to detect and study local structure along single molecules by voltage driven translocation through the nanopore. Their sensitivity and versatility can be augmented by combining them with a direct force probe, for example, optical tweezers. Such a tool could potentially be used to directly probe RNA secondary structure through the sequential unfolding of duplex regions. Here, we demonstrate the first application of such a system to the study of RNA by directly measuring the net force on individual double-stranded RNA (dsRNA) molecules. We have probed the force on dsRNA over a large range of nanopore sizes from 35 nm down to 3.5 nm and find that it decreases as the pore size is increased, in accordance with numerical calculations. Furthermore, we find that the force is independent of the distance between the optical trap and the nanopore surface, permitting force measurement on quite short molecules. By comparison with dsDNA molecules trapped in the same nanopores, we find that the force on dsRNA is on the order of, but slightly lower than, that on dsDNA. With these measurements, we expand the possibilities of the nanopore-optical tweezers to the study of RNA molecules with potential applications to the detection of RNA-bound proteins, the determination of RNA secondary structure, and the processing of RNA by molecular motors.

Finite-difference time-domain analysis of refractive index grating on planar light waveguide circuit with optically trapped gold particles

Ryosuke Yotsutani and Hiroo Ukita

The super-resolution capability of scanning near-field optical microscopy (SNOM) with a gold particle is studied by the two-dimensional finite-difference time-domain (2D FDTD) method. We obtain SNOM signals by integrating the far field within the numerical aperture of an objective lens for a refractive index grating by scanning optically trapped gold particles with different diameters illuminated by focused laser light at the wavelength of 515 nm. The signal is strong at a high refractive index of the grating and exhibits similar behavior to that obtained in the experiment with the grating fabricated on a planar light waveguide circuit with a period of 1060 nm. Furthermore, the signal modulation increases as the gold particle diameter decreases and reaches 0.82 at a diameter of 50 nm.

Full distance-resolved folding energy landscape of one single protein molecule

J. Christof M. Gebhardt, Thomas Bornschlögl, and Matthias Rief

Kinetic bulk and single molecule folding experiments characterize barrier properties but the shape of folding landscapes between barrier top and native state is difficult to access. Here, we directly extract the full free energy landscape of a single molecule of the GCN4 leucine zipper using dual beam optical tweezers. To this end, we use deconvolution force spectroscopy to follow an individual molecule’s trajectory with high temporal and spatial resolution. We find a heterogeneous energy landscape of the GCN4 leucine zipper domain. The energy profile is divided into two stable C-terminal heptad repeats and two less stable repeats at the N-terminus. Energies and transition barrier positions were confirmed by single molecule kinetic analysis. We anticipate that deconvolution sampling is a powerful tool for the model-free investigation of protein energy landscapes.

Monday, February 15, 2010

Debye series analysis of radiation pressure force exerted on a multilayered sphere

Renxian Li, Xiang'e Han, and Kuan Fang Ren

On the basis of generalized Lorenz-Mie theory, the Debye series expansion (DSE) for radiation pressure forces (RPF) exerted on a multilayered sphere induced by focused beams is introduced. The DSE can isolate the contribution of each scattering process to RPF, and give a physical explanation of RPF. Typically, the RPF induced by a Gaussian beam is studied. The DSE is employed to the simulation of RPF corresponding to different scattering processes (diffraction, reflection, refraction, etc.) in detail, and gives the physical mechanism of RPF. The effects of various parameters, such as scattering mode p, beam position, and radius of core for coated spheres, to RPF is researched.

Optical forces in time domain on arbitrary objects

Patrick C. Chaumet, Kamal Belkebir, and Adel Rahmani

We develop a general theoretical and computational framework to describe, in time domain, the exchange of momentum between light and arbitrary three-dimensional objects. Our formulation can be used to study the time evolution of optical forces on any object with linear material response, including inhomogeneous, dispersive, and absorbing dielectrics and metals. We illustrate our approach by studying the behavior of the Abraham force on an object illuminated by a sequence of electromagnetic pulses.

Friday, February 12, 2010

Tracking Cell-Nanoparticle Interactions

Selhuber-Unkel, Christine

Nanoparticles are highly attractive tools for future biomedical applications. Essential steps towards using nanoparticles in living organisms include the development of strategies for a defined nanoparticle uptake by the cell, the availability of appropriate particle tracking methods and the design of nanoparticles with low cytotoxicity. Optical tweezers are versatile tools to study the interaction between particles and cells with nanometre, piconewton and microsecond resolution and they have proven capable of trapping and tracking individual nanoparticles in three dimensions. This mini-review presents recent achievements for the usage of nanoparticles in in vivo studies and discusses how optical tweezers can be employed to quantify cell-nanoparticle interactions at the single particle level.

Thursday, February 11, 2010

Arrangement dependence of interparticle force in nematic colloids

Takahiro Kishita, Kenji Takahashi, Masatoshi Ichikawa, Jun-ichi Fukuda, and Yasuyuki Kimura

We have experimentally and theoretically studied the interparticle force between two colloidal particles with different sizes accompanied by hyperbolic hedgehog defects in a nematic liquid crystal. The force f was directly measured using dual-beam optical tweezers and calculated theoretically from the equilibrium tensor field around the particles. The dependence of f on the center-to-center distance between particles of different sizes R is different from that for particles with the same size. The magnitude of f depends on the relative arrangement of the particles: f is larger when a defect between the particles belongs to the larger particle. From the theoretical calculation, the difference in f between the two arrangements, δf, monotonically increases with increasing size difference. The difference δf was experimentally and theoretically found to be proportional to R^−4.6 at large R. The obtained exponent is comparable to the exponent of −5 predicted by electrostatic analogy.

Microdisplacement sensor using an optically trapped microprobe based on the interference scale

Masaki Michihata, Terutake Hayashi, Daisuke Nakai, and Yasuhiro Takaya

Positioning technology is one of the most important technologies for developingmicrosystems. In particular, displacement sensors are necessary for positioning deviceswith nanoscale accuracy. In this study, we propose a new displacement sensor that uses an interference scale as a linear scale and a laser-trapped microsphere as a sensing probe. This sensor has a wide measuring range, high resolution, and accessibility for narrow target areas. A glass microsphere was optically trapped by means of the laser trapping technique. Between the target surface and the probe, an interference scale was generated along the optical axis. The scale origin was fixed on the target surface. The distance between the probe and the target surface could be measured in terms of the shift in the interference scale. This study investigated the fundamental performance of the sensor. The resolution and accuracy of the sensor were 10 and ±50 nm, respectively; these values could be improved by using trapping lasers having shorter wavelengths. The measurable range was 250 µm. This sensor can provide useful displacement information from a target area having dimensions smaller than 15 µm. In addition, the displacement sensor can measure the distance even for surfaces inclined at angles less than 15°; thus, a flexible arrangement can be used to carry out measurements. In addition, the direction of displacement can be identified.

Construction, figures of merit, and testing of a single-cell fluorescence excitation spectroscopy system

Laura S. Hill, Tammi L. Richardson, Luisa T. M. Profeta, Timothy J. Shaw, Christopher J. Hintz, Benjamin S. Twining, Evelyn Lawrenz, and Michael L. Myrick

Characterization of phytoplankton community composition is critical to understanding the ecology and biogeochemistry of the oceans. One approach to taxonomic characterizationtakes advantage of differing pigmentation between algal taxa and thus differences in fluorescence excitation spectra. Analyses of bulk water samples, however, may be confounded by interference from chromophoric dissolved organic matter or suspended particulate matter. Here, we describe an instrument that uses a laser trap based on a Nikon TE2000-U microscope to position individual phytoplankton cells for confocal fluorescence excitation spectroscopy, thus avoiding interference from the surrounding medium. Quantitative measurements of optical power give data in the form of photonsemitted per photon of exposure for an individual phytoplankton cell. Residence times for individual phytoplankton in the instrument can be as long as several minutes with no substantial change in their fluorescence excitation spectra. The laser trap was found togenerate two-photon fluorescence from the organisms so a modification was made to release the trap momentarily during data acquisition. Typical signal levels for an individual cell are in the range of 106 photons/s of fluorescence using a monochromated 75 W Xe arc lamp excitation source with a 2% transmission neutral density filter.

Monday, February 8, 2010

Stability of Novel Time-Sharing Dual Optical Tweezers Using a Rotating Tilt Glass Plate

Ren Yu-Xuan, Wu Jian-Guang, Chen Man, Li Huang and Li Yin-Mei

A novel realization of time-sharing optical tweezers (TSOT) is demonstrated using a tilt glass plate. Objects are trapped in the time-sharing dual traps; each of them acts like a single beam gradient trap with an effective stiffness. The effective stiffness of TSOT is experimentally measured through analysis of dynamical images. In comparison, it is numerically calculated by adopting the Monte Carlo technique. Both simulation and experimental results agree well with each other and show a good linear relationship between the effective stiffness and trap switching frequency in the range from 5 Hz to 70 Hz.

The Lever Arm Effects a Mechanical Asymmetry of the Myosin-V-Actin Bond

J. Christof M. Gebhardt, Zeynep Ökten and Matthias Rief

Myosin-V is a two-headed molecular motor taking multiple ATP-dependent steps toward the plus end (forward) of actin filaments. At high mechanical loads, the motor processively steps toward the minus end (backward) even in the absence of ATP, whereas analogous forward steps cannot be induced. The detailed mechanism underlying this mechanical asymmetry is not known. We investigate the effect of force on individual single headed myosin-V constructs bound to actin in the absence of ATP. If pulled forward, the myosin-V head dissociates at forces twice as high than if pulled backward. Moreover, backward but not forward distances to the unbinding barrier are dependent on the lever arm length. This asymmetry of unbinding force distributions in a single headed myosin forms the basis of the two-headed asymmetry. Under load, the lever arm functions as a true lever in a mechanical sense.

Measurements of the Equilibrium Size of Supersaturated Aqueous Sodium Chloride Droplets at Low Relative Humidity Using Aerosol Optical Tweezers and...

G. Hargreaves, N.-O. A. Kwamena, Y. H. Zhang, J. R. Butler, S. Rushworth, S. L. Clegg and J. P. Reid

An approach for examining the hygroscopicity of single aerosol particles over I broad range in relative humidity (RH) using aerosol optical tweezers is presented and compared with measurements made using an electrodynamic balance. In particular. benchmark measurements oil aqueous sodium chloride aerosol are presented over the RH range 45-75% (293 K), a RH range that had not previously been explored with aerosol optical tweezers. Measurements of the variation in equilibrium wet droplet size with RH are made using cavity-enhanced Raman scattering. with all accuracy of 1 nm in the determination of the wet particle radius. The Full range of optical tweezers experimental measurements (including previous dual trapping comparative studies approaching a saturation relative humidity of 100%) are compared with determinations using other experimental techniques and with a range of model treatments. An assessment of the models and all experimental data for estimating the equilibrium size of a sodium chloride droplet suggests that the size can be predicted with all accuracy of better than 0.1% over the RH range 48-100%. Discrepancies between different measurements lead to all increase ill uncertainty above 1% below all RH of 48% as efflorescence is approached. The optical tweezers' measurements of equilibrium size consistently agree with model predictions to within an error of 1% and mostly with all error of less than +/- 0.1%. These data demonstrate the highly accurate nature of measurements of thermodynamic equilibrium size by aerosol Optical tweezers and suggest that this approach may be used to investigate the competition between thermodynamic and kinetic factors ill governing aerosol particle size over the full RH range.


Superresolution imaging in optical tweezers using high-speed cameras

Juan Pablo Staforelli, Esteban Vera, José Manuel Brito, Pablo Solano, Sergio Torres, and Carlos Saavedra

High-speed cameras are reliable alternatives for the direct characterization of optical trap force and particle motion in optical tweezers setups, replacing indirect motion measurements often performed by quadrant detectors. In the present approach, subpixel motion data of the trapped particle is retrieved from a high-speed low-resolution video sequence. Due to the richness structure of motion diversity of microscopic trapped particles, which are subjected to a Brownian motion, we propose to also use the obtained motion information for tackling the inherent lack of resolution by applying superresolution algorithms on the low-resolution image sequence. The obtained results both for trapping calibration beads and for living bacteria show that the proposed approach allows the proper characterization of the optical tweezers by obtaining the real particle motion directly from the image domain, while still providing high resolution imaging.

Tuesday, February 2, 2010

Light distribution analysis of optical fibre probe-based near-field optical tweezers using FDTD

B H Liu, L J Yang and Y Wang

Optical fibre probe-based near-field optical tweezers overcomes the diffraction limit of conventional optical tweezers, utilizing strong mechanical forces and torque associated with highly enhanced electric fields to trap and manipulate nano-scale particles. Near-field evanescent wave generated at optical fibre probe decays rapidly with the distance that results a significant reduced trapping volume, thus it is necessary to analyze the near-field distribution of optical fibre probe. The finite difference time domain (FDTD) method is applied to characterize the near-field distribution of optical fibre probe. In terms of the distribution patterns, depolarization and polarization, the near-field distributions in longitudinal sections and cross-sections of tapered metal-coated optical fibre probe are calculated. The calculation results reveal that the incident polarized wave becomes depolarized after exiting from the nano-scale aperture of probe. The near-field distribution of the probe is unsymmetrical, and the near-field distribution in the cross-section vertical to the incident polarized wave is different from that in the cross-section parallel to the incident polarized wave. Moreover, the polarization of incident wave has a great impact on the light intensity distribution.

Multidimensional Optical Fractionation of Colloidal Particles with Holographic Verification

Ke Xiao and David G. Grier

The trajectories of colloidal particles driven through a periodic potential energy landscape can become kinetically locked-in to directions dictated by the landscape’s symmetries. When the landscape is realized with a structured light field, the path a given particle follows has been predicted to depend exquisitely sensitively on such properties as the particle’s size and refractive index. We confirm these predictions by measuring the transport of colloidal silica spheres through arrays of holographic optical traps, using holographic video microscopy to track individual spheres’ motions in three dimensions and simultaneously to measure each sphere’s radius and refractive index with part-per-thousand resolution. These measurements demonstrate optical fractionation’s ability to sort with part-per-thousand resolution on multiple characteristics simultaneously.

Transverse radiation force in a tailored optical fiber

Iver Brevik and Simen Å. Ellingsen

We show, by means of simple model calculations, how a weak laser beam sent though an optical fiber exerts a transverse radiation force if there is an azimuthal asymmetry present in the fiber such that one side has a slightly different refractive index than the other. The refractive index difference Δn needs only to be very low, of order 10-3, to produce an appreciable transverse displacement of order 10 μm. We argue that the effect has probably already been seen in a recent experiment by W. She et al. [Phys. Rev. Lett. 101, 243601 (2008)], and we discuss the correspondence between these observations and the theory presented. The effect could be used to bend optical fibers in a predictable and controlled manner and we propose that it could be useful for micron-scale devices.

Femtonewton Entropic Forces Can Control the Formation of Protein-Mediated DNA Loops

Yih-Fan Chen, J. N. Milstein, and Jens-Christian Meiners

We show that minuscule entropic forces, on the order of 100 fN, can prevent the formation of DNA loops—a ubiquitous means of regulating the expression of genes. We observe a tenfold decrease in the rate of LacI-mediated DNA loop formation when a tension of 200 fN is applied to the substrate DNA, biasing the thermal fluctuations that drive loop formation and breakdown events. Conversely, once looped, the DNA-protein complex is insensitive to applied force. Our measurements are in excellent agreement with a simple polymer model of loop formation in DNA, and show that an antiparallel topology is the preferred LacI-DNA loop conformation for a generic loop-forming construct.

Towards Holonomic Control of Janus Particles in Optomagnetic Traps

Randall M. Erb, Nathan J. Jenness, Robert L. Clark, Benjamin B. Yellen

A novel "dot" Janus particle is presented, which is compatible with optical traps and magnetic fields, allowing for direct control over five of the particle's degrees of freedom. With an additional constraint of the final sixth degree of freedom, this system represents the highest control ever achieved over freely suspended colloids, opening up the possibility for novel applications in intermolecular force measurement, microfluidics, and self-assembly.

Binding and Cleavage of E. coli HUβ by the E. coli Lon Protease

Jiahn-Haur Liao, Yu-Ching Lin, Jowey Hsu, Alan Yueh-Luen Lee, Tse-An Chen, Chun-Hua Hsu, Jiun-Ly Chir, Kuo-Feng Hua, Tzu-Hua Wu, Li-Jenn Hong, Pei-Wen Yen, Arthur Chiou and Shih-Hsiung Wu

The Escherichia coli Lon protease degrades the E. coli DNA-binding protein HUβ, but not the related protein HUα. Here we show that the Lon protease binds to both HUβ and HUα, but selectively degrades only HUβ in the presence of ATP. Mass spectrometry of HUβ peptide fragments revealed that region K18-G22 is the preferred cleavage site, followed in preference by L36-K37. The preferred cleavage site was further refined to A20-A21 by constructing and testing mutant proteins; Lon degraded HUβ-A20Q and HUβ-A20D more slowly than HUβ. We used optical tweezers to measure the rupture force between HU proteins and Lon; HUα, HUβ, and HUβ-A20D can bind to Lon, and in the presence of ATP, the rupture force between each of these proteins and Lon became weaker. Our results support a mechanism of Lon protease cleavage of HU proteins in at least three stages: binding of Lon with the HU protein (HUβ, HUα, or HUβ-A20D); hydrolysis of ATP by Lon to provide energy to loosen the binding to the HU protein and to allow an induced-fit conformational change; and specific cleavage of only HUβ.

Letter Nanomanipulation Using Silicon Photonic Crystal Resonators

Sudeep Mandal, Xavier Serey and David Erickson

Optical tweezers have enabled a number of microscale processes such as single cell handling, flow-cytometry, directed assembly, and optical chromatography. To extend this functionality to the nanoscale, a number of near-field approaches have been developed that yield much higher optical forces by confining light to subwavelength volumes. At present, these techniques are limited in both the complexity and precision with which handling can be performed. Here, we present a new class of nanoscale optical trap exploiting optical resonance in one-dimensional silicon photonic crystals. The trapping of 48 nm and 62 nm dielectric nanoparticles is demonstrated along with the ability to transport, trap, and manipulate larger nanoparticles by simultaneously exploiting the propagating nature of the light in a coupling waveguide and its stationary nature within the resonator. Field amplification within the resonator is shown to produce a trap several orders of magnitude stronger than conventional tweezers and an order of magnitude stiffer than other near-field techniques. Our approach lays the groundwork for a new class of optical trapping platforms that could eventually enable complex all-optical single molecule manipulation and directed assembly of nanoscale material.

Cell Palpation System Based on a Force Measurement by Optical Tweezers for Investigation of Local Mechanical Properties of a Cell Membrane

Hideaki Miyoshi, Tadao Sugiura, and Kotaro Minato

We have developed a cell palpation system, which enables to investigate mechanical properties of a cell in local with an optically manipulated particle. In this system, the particle attached on a cell surface is forced to move back-and-forth in sinusoidal manner by optical tweezers. Position of the particle position is recorded and analyzed to extract force exerted on the particle by using a physical model of the particle. We have demonstrated to measure cell membrane stiffness in local and temporal change of that with the detection limit of the force in pN order.

Mechanical Stress Analysis of Microfluidic Environments Designed for Isolated Biological Cell Investigations

Sean S. Kohles, Nathalie Nève, Jeremiah D. Zimmerman, and Derek C. Tretheway

Advancements in technologies for assessing biomechanics at the cellular level have led to discoveries in mechanotransduction and the investigation of cell mechanics as a biomarker for disease. With the recent development of an integrated optical tweezer with micron resolution particle imagevelocimetry, the opportunity to apply controlled multiaxial stresses to suspended single cells is available (Nève, N., Lingwood, J. K., Zimmerman,J., Kohles, S. S., and Tretheway, D. C., 2008, “The µPIVOT: An Integrated Particle Image Velocimetry and Optical Tweezers Instrument for Microenvironment Investigations,” Meas. Sci. Technol., 19(9), pp. 095403). A stress analysis was applied to experimental and theoretical flow velocitygradients of suspended cell-sized polystyrene microspheres demonstrating the relevant geometry of nonadhered spherical cells, as observed for osteoblasts, chondrocytes, and fibroblasts. Three flow conditions were assessed: a uniform flow field generated by moving the fluid sample with an automated translation stage, a gravity driven flow through a straight microchannel, and a gravity driven flow through a microchannel cross junction. The analysis showed that fluid-induced stresses on suspended cells (hydrodynamic shear, normal, and principal stresses in the range of 0.02–0.04 Pa) are generally at least an order of magnitude lower thanadhered single cell studies for uniform and straight microchannel flows (0.5–1.0 Pa). In addition, hydrostatic pressures dominate (1–100 Pa) overhydrodynamic stresses. However, in a cross junction configuration, orders ofmagnitude larger hydrodynamic stresses are possible without the influence of physical contact and with minimal laser trapping power.

ArfGAP1 generates an Arf1 gradient on continuous lipid membranes displaying flat and curved regions

Ernesto Ambroggio, Benoît Sorre, Patricia Bassereau, Bruno Goud, Jean-Baptiste Manneville and Bruno Antonny

ArfGAP1, which promotes GTP hydrolysis on the small G protein Arf1 on Golgi membranes, interacts preferentially with positively curved membranes through its amphipathic lipid packing sensor (ALPS) motifs. This should influence the distribution of Arf1-GTP when flat and curved regions coexist on a continuous membrane, notably during COPI vesicle budding. To test this, we pulled tubes from giant vesicles using molecular motors or optical tweezers. Arf1-GTP distributed on the giant vesicles and on the tubes, whereas ArfGAP1 bound exclusively to the tubes. Decreasing the tube radius revealed a threshold of R≈35 nm for the binding of ArfGAP1 ALPS motifs. Mixing catalytic amounts of ArfGAP1 with Arf1-GTP induced a smooth Arf1 gradient along the tube. This reflects that Arf1 molecules leaving the tube on GTP hydrolysis are replaced by new Arf1-GTP molecules diffusing from the giant vesicle. The characteristic length of the gradient is two orders of magnitude larger than a COPI bud, suggesting that Arf1-GTP diffusion can readily compensate for the localized loss of Arf1 during budding and contribute to the stability of the coat until fission.

Optical Torque Induced on a Geometrically Anisotropic Multi-Lamellar Vesicle with Form Birefringence

C. Ha, H.K. Pak, K. Kim

When a laser beam with an elliptical polarization passes through a material with an optical birefringence, the ordinary and the extraordinary components of the laser beam experience different phase shifts. In this case, an optical torque due to angular momentum conservation can be exerted on the material. In this work, a, shrunken multi-lamellar vesicle (SMLV), which has a geometrically anisotropic internal nano-layered structure, is used as a form birefringence material. By trapping the SMLV using optical tweezers with a polarized laser beam, we rotate the SMLV. This rotational motion is explained by using a simple model with optical torque. In the model, lipid bi-layers are treated as thin parallel plates. This optically-induced rotational motion can be used to control the motion of biological materials.

Single-molecule experiment with optical tweezers: improved analysis of the diffusion of the λ-receptor in E. coli's outer membrane

Lene Oddershede, Henrik Flyvbjerg and Kirstine Berg-Sørensen

The motion of a single protein, a λ-receptor in the outer membrane of E. coli, is studied with optical tweezers. We present improved measurements and an analysis that accounts for stochastic errors, filters in the detection system, aliasing and hydrodynamic effects. We test a simple model for the protein's motion and find it in agreement with the low-frequency part of the data. We thus determine physical parameter values for the system with improved precision. The diffusion coefficient describing the motion of the λ-receptor is now obtained with 2% relative error for individual receptors, and varies between bacteria with roughly 70% root-mean-square deviation about the mean. This improved analysis also reveals that the simple model must be revised in order to agree with the high-frequency part of the power spectrum.


Displacement and Force Measurements with Quadrant Photodetector in Optical Tweezers

Guo Hong-Lian, Liu Chun-Xiang, Li Zhao-Lin, Duan Jian-Fa, Han Xue-Hai, Cheng Bing-Ying and Zhang Dao-Zhong

A technique of displacement and force measurements with a photodiode quadrant detector in an optical tweezers system is presented. The stiffness of optical trap is calibrated and the leukemia cell membrane tension is measured. The results show that the optical tweezers combined with the quadrant detector is a very useful tool for detecting the displacement and force with a millisecond-order response.

Optimized multi-view imaging improves the observation of optically manipulated non-spherical particles

L Selvaggi, E Ferrari, A Moradi, S C Santucci, P Beuzer and D Cojoc

We report the optimization of two-side sample imaging through two orthogonal objectives and sample manipulation by single-beam optical trapping through one of the objectives. A high numerical aperture objective (NA = 1.0) is used to generate multiple laser traps configured by phase diffractive optical elements implemented on a phase programmable modulator. Supplementary lateral imaging through a NA = 0.55 objective represents a trade-off between the resolution and the field of view that provides additional information about the sample and its environment, impossible to obtain from conventional optical trapping setups where imaging is performed only along the trapping objective axis. The usefulness of two-side imaging combined with optical manipulation is illustrated by three different experiments: multiple beads trapped in 3D arrays, red blood cells rotated by multiple tweezers and manipulation of a glass grain with irregular shape.

Blinking Optical Tweezers for microrheology measurements of weak elasticity complex fluids

Giuseppe Pesce, Giulia Rusciano, and Antonio Sasso

Optical tweezers have become a powerful tool to explore the viscoelasticity of complex fluids at micrometric scale. In the experiments, the Brownian trajectories of optically confined microparticles are properly analysed to provide the viscous and elastic moduli G' and G'. Nevertheless, the elastic response of the medium is inherently superimposed on the trap stiffness itself. Usually, this drawback is removed by subtracting the elastic trap contribution from the measured medium response. However, it is clear that when trap and medium elasticity become comparable this procedure is no longer reliable. Still, there exists a wide class of complex fluids that exhibit a low elasticity (diluted biopolymers, Boger fluids, etc) for which alternative experimental approaches would be desirable. Herein we propose a new method based on blinking optical tweezers. It makes use of two independent laser beams: the first is used to trap a single bead while the second one, of very weak power, acts as probe to monitor its position with a quadrant photodiode. The trap laser intensity is modulated on-off: when the laser is off the bead follows a free diffusion trajectory that, hence, leads to an estimation of G' and G' free of the influence of the trap. We have successfully applied this technique to highly-diluted hyaluronic acid solutions (c < 0.1 mg/ml) reaching to measure very weak G' modulus (∼ 0.01 Pa) in a wide range of frequencies.


Selective trapping of multiple particles by volume speckle field

Vladlen G. Shvedov, Andrei V. Rode, Yana V. Izdebskaya, Anton S. Desyatnikov, Wieslaw Krolikowski, and Yuri S. Kivshar

We suggest a new approach for selective trapping of light absorbing particles in gases by multiple optical bottle-beam-like traps created by volume speckle field. We demonstrate stable simultaneous confinement of a few thousand micro-particles in air with a single lowpower laser beam. The size distribution of trapped particles exhibits a narrow peak near the average size of an optical speckle. Thus, the speckleformed traps act as a sieve with the holes selecting particles of a similar size.