Monday, January 28, 2013

Influence of Electrostatic Forces on the Particle Propulsion in the Evanescent Field of Silver Ion-Exchanged Waveguides

Dmytro Grebennikov and Silvia Mittler
The effect of electrostatic interaction between carboxylate- and amino-functionalized polystyrene particles and a charged waveguide surface on the propulsion speed in an optical tweezers is considered as a function of pH and ionic strength. It was shown that with the variation of the pH of the aqueous solution, the particles were immersed in, a systematic change of propulsion speed with a maximum speed could be achieved. The appearance of a maximum speed was ascribed to changes in the particle-waveguide separation as a result of the combination of two forces: Coulomb repulsion/attraction and induced dipole forces. The highest maximum speed at low ionic strength was around 12 µm/sec. Changes in the ionic strength of the solution influenced the gradient of the dielectric constant near the involved surfaces and also lead to a slightly reduced hydrodynamic radius of the particles. The combination of these effects subsequently increased the maximum speed to about 23 µm/sec.


Surface-Enhanced Raman Scattering with Ag Nanoparticles Optically Trapped by a Photonic Crystal Cavity

Shiyun Lin , Wenqi Zhu , Yuhang Jin , and Kenneth B. Crozier
We demonstrate a reusable and reconfigurable surface enhanced Raman scattering (SERS) platform by optically trapping Ag nanoparticles with a photonic crystal cavity integrated with a microfluidic chip. High-performance SERS is performed in a very reproducible manner, owing to the fact that Ag aggregates are produced by optical trapping in a controllable process that is monitored in real-time by the cavity resonance shift that occurs with the trapping of each additional nanoparticle.

Experimental demonstration of optical transport, sorting and self-arrangement using a ‘tractor beam’

O. Brzobohatý, V. Karásek, M. Šiler, L. Chvátal, T. Čižmár & P. Zemánek
Following the Keplerian idea of optical forces, one would intuitively expect that an object illuminated by sunlight radiation or a laser beam will be accelerated along the direction of photon flow. Recent theoretical studies have shown that small particles can be pulled by light beams against the photon stream, even in beams with uniform optical intensity along the propagation axis. Here, we present a geometry to generate such a ‘tractor beam’, and experimentally demonstrate its functionality using spherical microparticles of various sizes, as well as its enhancement with optically self-arranged structures of microparticles. In addition to the pulling of the particles, we also demonstrate that their two-dimensional motion and one-dimensional sorting may be controlled conveniently by rotation of the polarization of the linearly polarized incident beam. The relative simplicity of this geometry could serve to encourage its widespread application, and ongoing investigations will broaden the understanding of the light–matter interaction through studies combining more interacting micro-objects with various properties.

Optical Trapping of Nanoparticles

Jarrah Bergeron, Ana Zehtabi-Oskuie, Saeedeh Ghaffari, Yuanjie Pang, Reuven Gordon

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec, which has serious implications for biological matter.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

Saturday, January 26, 2013

Photo-induced reversible uniform to Janus shape change of vesicles composed of PNIPAM-b-PAzPy2

Guangyong Shen , Guosheng Xue , Jun Cai , Gang Zou , Yinmei Li and Qijin Zhang

In this work, three kinds of vesicles are fabricated by the self-assembly of amphiphilic block copolymers (BCPs), in which the hydrophobic chains are side chain azobenzene polymers with spacers of 0, 2 and 6 methylene units, respectively. It has been found that vesicles formed by BCPs with a spacer of 0 methylene units have no photo-responsive behavior and vesicles with a spacer of 6 have a photo-induced swelling behavior under the irradiation of light at 365 nm. Unexpectedly, the vesicle formed by BCPs with a spacer of 2 shows a photo-induced reversible uniform to Janus shape change under the same irradiation. This reversible process means that a bistable shape change of the vesicle can be controlled by the switching of UV light. A UV-visible absorption spectrum and a laser-trapped Raman spectrum (LTRS) are used to investigate differences in the morphology and photo-induced behavior of these vesicles. Results have confirmed that the photo-induced Janus shape of vesicles formed by BCPs with a spacer of 2 is a metastable shape, different from the stable Janus shape of vesicles formed by BCPs with a spacer of 0. This is also testified by a two-photon confocal laser scanning microscope (CLSM). From the results it is realized that the spacer length in the hydrophobic chains of BCPs can affect the photo-induced behavior of vesicles formed by BCPs, which will be a key point in designing functional vesicles with special morphologies.

Friday, January 25, 2013

Control over hygroscopic growth of saline aqueous aerosol using Pluronic polymer additives

Allen E. Haddrell, Graham Hargreaves, James F. Davies, Jonathan P. Reid
The hygroscopic properties of an aerosol originating from a nebulizer solution can affect the extent of peripheral deposition within the respiratory tract, which in turn affects drug efficacy of drugs delivered to the lungs. Thus, the ability to tailor the degree and rate of hygroscopic growth of an aerosol produced by a nebulizer through modification of the formulation would serve to improve drug efficacy through targeted lung deposition. In this study, the kinetic and thermodynamic hygroscopic properties of sodium chloride aerosol mixed with commercially available Pluronic polymers, specifically F77 and F127, are reported using three complimentary single aerosol analysis techniques, specifically aerosol optical tweezers, a double ring electrodynamic balance and a concentric cylinder electrodynamic balance. The F77 polymer is shown to have a predictable effect on the hygroscopic properties of the aerosol: the ability of the droplet to uptake water from the air depends on the solute weight percent of sodium chloride present in a linear dose dependant manner. Unlike the smaller F77, a non-linear relationship was observed for the larger molecular weight F127 polymer, with significant suppression of hygroscopic growth (>50% by mass) for solution aerosol containing even only 1 wt% of the polymer and 99 wt% sodium chloride. The suppression of growth is shown to be consistent with the formation of mixed phase aerosol particles containing hydrophilic inorganic rich domains and hydrophobic polymer rich domains that sequester some of the inorganic component, with the two phases responding to changes in relative humidity independently. This independence of coupling with the gas phase is apparent in both the equilibrium state and the kinetics of water evaporation/condensation. By starting with a saline nebulizer solution with a concentration of F127 ∼10−2 mM, a 12% reduction in the radius of all aerosol produced at a relative humidity (RH) of 84% is possible. The difference in diameter is RH dependent, and may be much greater at higher humidities. These findings suggest that the addition of μM concentrations of larger Pluronic polymers to nebulizer formulations may greatly reduce the size of aerosols produced.

Molecular Adaptations Allow Dynein to Generate Large Collective Forces inside Cells

Arpan K. Rai, Ashim Rai, Avin J. Ramaiya, Rupam Jha, Roop Mallik
Many cellular processes require large forces that are generated collectively by multiple cytoskeletal motor proteins. Understanding how motors generate force as a team is therefore fundamentally important but is poorly understood. Here, we demonstrate optical trapping at single-molecule resolution inside cells to quantify force generation by motor teams driving single phagosomes. In remarkable paradox, strong kinesins fail to work collectively, whereas weak and detachment-prone dyneins team up to generate large forces that tune linearly in strength and persistence with dynein number. Based on experimental evidence, we propose that leading dyneins in a load-carrying team take short steps, whereas trailing dyneins take larger steps. Dyneins in such a team bunch close together and therefore share load better to overcome low/intermediate loads. Up against higher load, dyneins “catch bond” tenaciously to the microtubule, but kinesins detach rapidly. Dynein therefore appears uniquely adapted to work in large teams, which may explain how this motor executes bewilderingly diverse cellular processes.

A Polypeptide-DNA Hybrid with Selective Linking Capability Applied to Single Molecule Nano-Mechanical Measurements Using Optical Tweezers

Fatemeh Moayed, Alireza Mashaghi, Sander J. Tans

Many applications in biosensing, biomaterial engineering and single molecule biophysics require multiple non-covalent linkages between DNA, protein molecules, and surfaces that are specific yet strong. Here, we present a novel method to join proteins and dsDNA molecule at their ends, in an efficient, rapid and specific manner, based on the recently developed linkage between the protein StrepTactin (STN) and the peptide StrepTag II (ST). We introduce a two-step approach, in which we first construct a hybrid between DNA and a tandem of two STs peptides (tST). In a second step, this hybrid is linked to polystyrene bead surfaces and Maltose Binding Protein (MBP) using STN. Furthermore, we show the STN-tST linkage is more stable against forces applied by optical tweezers than the commonly used biotin-Streptavidin (STV) linkage. It can be used in conjunction with Neutravidin (NTV)-biotin linkages to form DNA tethers that can sustain applied forces above 65 pN for tens of minutes in a quarter of the cases. The method is general and can be applied to construct other surface-DNA and protein-DNA hybrids. The reversibility, high mechanical stability and specificity provided by this linking procedure make it highly suitable for single molecule mechanical studies, as well as biosensing and lab on chip applications.

Monday, January 21, 2013

Pulsed laser manipulation of an optically trapped bead: Averaging thermal noise and measuring the pulsed force amplitude

Thue B. Lindballe, Martin V. G. Kristensen, Kirstine Berg-Sørensen, Søren R. Keiding, and Henrik Stapelfeldt
An experimental strategy for post-eliminating thermal noise on position measurements of optically trapped particles is presented. Using a nanosecond pulsed laser, synchronized to the detection system, to exert a periodic driving force on an optically trapped 10 μm polystyrene bead, the laser pulse-bead interaction is repeated hundreds of times. Traces with the bead position following the prompt displacement from equilibrium, induced by each laser pulse, are averaged and reveal the underlying deterministic motion of the bead, which is not visible in a single trace due to thermal noise. The motion of the bead is analyzed from the direct time-dependent position measurements and from the power spectrum. The results show that the bead is on average displaced 208 nm from the trap center and exposed to a force amplitude of 71 nanoNewton, more than five orders of magnitude larger than the trapping forces. Our experimental method may have implications for microrheology.

Mechanical and kinetic properties of β-cardiac/slow skeletal muscle myosin

Bernhard Brenner, Nils Hahn, Eva Hanke, Faramarz Matinmehr, Tim Scholz, Walter Steffen, Theresia KraftWe aimed to establish reference parameters to identify functional effects of familial hypertrophic cardiomyopathy-related point mutations in the β-cardiac/slow skeletal muscle myosin heavy chain (β-cardiac/MyHC-1). We determined mechanical and kinetic parameters of the β-cardiac/MyHC-1 using human soleus muscle fibers that express the same myosin heavy chain (MyHC-1) as ventricular myocardium (β-cardiac). The observed parameters are compared to previously reported data for rabbit psoas muscle fibers. We found all of the examined kinetic parameters to be slower in soleus fibers than in rabbit psoas muscle. Somewhat surprisingly, however, we also found that the stiffness of the β-cardiac/MyHC-1 head domain is more than 3-fold lower than the stiffness of the fast isoform of psoas fibers. Furthermore, and different from rabbit psoas muscle, in human soleus fibers both the occupancy of force-generating cross-bridge states as well as the elastic extension of force-generating heads increase with temperature. Thus, a myosin head in the force generating states makes an increasing contribution to force with temperature. We support some of our fiber data by data from in vitro motility and optical trapping assays. Initial findings with FHC-related point mutations in the converter imply that the differences in stiffness of the head domain between the slow and fast isoform may well be due to particular differences in the amino acid sequence of the converter. We show that the slower kinetics may be linked to a larger flexibility of the β-cardiac/MyHC-1 isoform compared to fast MyHC isoforms.


Speeding up liquid crystal SLMs using overdrive with phase change reduction

Gregor Thalhammer, Richard W. Bowman, Gordon D. Love, Miles J. Padgett, and Monika Ritsch-Marte
Nematic liquid crystal spatial light modulators (SLMs) with fast switching times and high diffraction efficiency are important to various applications ranging from optical beam steering and adaptive optics to optical tweezers. Here we demonstrate the great benefits that can be derived in terms of speed enhancement without loss of diffraction efficiency from two mutually compatible approaches. The first technique involves the idea of overdrive, that is the calculation of intermediate patterns to speed up the transition to the target phase pattern. The second concerns optimization of the target pattern to reduce the required phase change applied to each pixel, which in addition leads to a substantial reduction of variations in the intensity of the diffracted light during the transition. When these methods are applied together, we observe transition times for the diffracted light fields of about 1 ms, which represents up to a tenfold improvement over current approaches. We experimentally demonstrate the improvements of the approach for applications such as holographic image projection, beam steering and switching, and real-time control loops.

Measuring forces at the leading edge: a force assay for cell motility

Brenda Farrell , Feng Qian , Anatoly Kolomeisky , Bahman Anvariand William E. Brownell
Cancer cells become mobile by remodelling their cytoskeleton to form migratory structures. This transformation is dominated by actin assembly and disassembly (polymerisation and depolymerisation) in the cytoplasm. Synthesis of filamentous actin produces a force at the leading edge that pushes the plasma membrane forward. We describe an assay to measure the restoring force of the membrane in response to forces generated within the cytoplasm adjacent to the membrane. A laser trap is used to form a long membrane nanotube from a living cell and to measure the axial membrane force at the end of the tube. When the tube, resembling a filopodium, is formed and in a relaxed state the axial membrane force exhibits a positive stationary value. This value reflects the influence of the cytoskeleton that acts to pull the tube back to the cell. A dynamic sawtooth force that rides upon the stationary value is also observed. This force is sensitive to a toxin that affects actin assembly and disassembly, but not affected by agents that influence microtubules and myosin light chain kinase. We deduce from the magnitude and characteristics of dynamic force measurements that it originates from depolymerisation and polymerisation of F-actin. The on- and off-rates, the number of working filaments, and the force per filament (2.5 pN) are determined. We suggest the force-dependent transitions are thermodynamically uncoupled as both the on- and off-rates decrease exponentially with a compressive load. We propose kinetic schemes that require attachment of actin filaments to the membrane during depolymerisation. This demonstrates that actin kinetics can be monitored in a living cell by measuring force at the membrane, and used to probe the mobility of cells including cancer cells.

Helicase-mediated changes in RNA structure at the single-molecule level

Sebastian L.B. Koenig, Pramodha S. Linayage, Roland K.O. Sigel and David Rueda

RNA helicases are a diverse group of RNA-dependent ATPases known to play a large number of biological roles inside the cell, such as RNA unwinding, remodeling, export, as well as degradation. Understanding how helicases mediate changes in RNA structure is therefore of fundamental interest. The advent of single-molecule spectroscopic techniques has unveiled with unprecedented detail the interplay of RNA helicases with their substrates. In this review, we describe the characterization of helicase-RNA interactions by single-molecule approaches. State-of-the-art techniques are presented, followed by a discussion of recent advancements in this exciting field.


Sunday, January 20, 2013

Mechanisms By Which von Willebrand Disease Mutations Destabilize the A2 Domain

Amy J. Xu and Timothy A. Springer

von Willebrand Factor (VWF) is an ultralong, concatameric, adhesive glycoprotein. On short timescales, adhesiveness for platelets is activated by elongation of VWF by altered hydrodynamics at sites of hemostasis. Over longer timescales, length of VWF is regulated by ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13), cleavage by which in the VWF A2 domain is dependent on elongational force. Patients with von Willebrand Disease (VWD) type 2A present with increased bleeding due to mutations within the von Willebrand factor (VWF) A2 domain that enhance cleavage. We tested using temperature and force the hypothesis that VWD mutations disrupt A2 force sensing by destabilizing the folded state. Mutations R1597W, M1528V, and E1638K reduced A2 thermal stability by 10-18 oC. M1528V and E1638K showed a marked further decrease in stability upon calcium removal. In contrast, R1597W, which resides within the A2 calcium-binding loop, exhibited similar stability in the presence and absence of calcium. Using single molecule optical tweezers and R1597W, we measured the force-dependence of unfolding and refolding kinetics. In the presence of calcium, the R1597W mutation slowed the rate of refolding but had no effect on unfolding. The three mutations highlight the calcium-binding loop (R1597W), the hydrophobic core around the vicinal disulfide (M1528V), and hydrogen bonds to the α4-less loop (E1638K), as structural features critically important to the function of A2 as a force sensor in regulating thrombogenic activity in the vasculature.

Trapping-Assisted Sensing of Particles and Proteins using On-Chip Optical Microcavities

Shiyun Lin and Kenneth B. Crozier

An improved ability to sense particles and biological molecules is crucial for continued progress in applications ranging from medical diagnostics to environmental monitoring to basic research. Impressive electronic and photonic devices have been developed to this end. However, several drawbacks exist. The sensing of molecules is almost exclusively performed via their binding to a functionalized device surface. This means that the devices are seldom re-usable, that their functionalization needs to be decided before use, and that they face the diffusion bottleneck. The latter challenge also applies to particle detection using photonic devices. Here, we demonstrate particle sensing using optical forces to trap and align them on waveguide-coupled silicon microcavities. A second probe laser detects the trapped particles by measuring the microcavity resonance shift. We also apply this platform to quantitatively sense green fluorescent proteins by detecting the size distribution of clusters of antibody-coated particles bound by the proteins.

Silicon-on-insulator multimode-interference waveguide-based arrayed optical tweezers (SMART) for two-dimensional microparticle trapping and manipulation

Ting Lei and Andrew W. Poon

We demonstrate two-dimensional optical trapping and manipulation of 1 μm and 2.2 μm polystyrene particles in an 18 μm-thick fluidic cell at a wavelength of 1565 nm using the recently proposed Silicon-on-insulatorMultimode-interference (MMI) waveguide-basedARrayed opticalTweezers (SMART) technique. The key component is a 100 μm square-core silicon waveguide with mm length. By tuning the fiber-coupling position at the MMI waveguide input facet, we demonstrate various patterns of arrayed optical tweezers that enable optical trapping and manipulation of particles. We numerically simulate the physical mechanisms involved in the arrayed trap, including the optical force, the heat transfer and the thermal-induced microfluidic flow.

Particles sorting in micro-channel system utilizing magnetic tweezers and optical tweezers

Yung-Chiang Chung, Po-Wen Chen, Chao-Ming Fu, Jian-Min Wu

This study evaluates a method for separating magnetic microparticles in a micro channel by using embedded inverted-laser tweezers, a microflow pump, and a micro magnet. Various particles were separated using optical and/or magnetic tweezers, and were identified and counted to determine the dependence of the sorting rate on the channel flow velocity. The particle sorting experiment showed good separation results when the designed channel and magnetic tweezers were used. For magnetic particles, lower flow velocities corresponded to larger separating rates with a maximum separating rate of 81%. When the designed channel and optical tweezers were used, the polystyrene particle separating rate was as high as 94%. When both the optical tweezers and the magnetic tweezers were used, the optical tweezers were more effective in trapping polystyrene particles with flow velocities between 0.09 and 0.25 μm/s. For flow velocities between 0.09 and 0.17 μm/s, the separating rate for polystyrene particles reached 95% and the separating rate for magnetic particles reached 85%. This hybrid system can be applied to the separation of various particles in unknown mixtures.


Thursday, January 17, 2013

End-faced waveguide mediated optical propulsion of microspheres and single cells in a microfluidic device

Lothar Lilge , Duoaud Shah and Luc Charron
Single cell transport in microfluidic devices is a topic of interest as their utility is becoming appreciated by cell and molecular biologist. Cell transport should minimize mechanical stress due to friction or pressure gradients. Optical forces have the advantage of applying their forces across the cell volume and not only at the cell membrane and are thus preferable. Optical pushing by scattering force is a suitable candidate so highly dependent on the photon irradiance field inside the propagation capillary which in turn is determined by the waveguide properties delivering the radiation pressure. Here we present a numerical approach to predict the optical scattering force, speed and trajectory of cells as a function of waveguide and propagation capillary geometry. Experimental verification of the simulation approach is demonstrated using polystyrene microspheres and leukemia cells. Effects of optical fibre to waveguide alignment, capillary wall angle and temperature on the dynamic viscosity on speed and position of the microspheres and cells inside the propagation capillary are demonstrated.

Optical manipulation of self-aligned graphene flakes in liquid crystals

Christopher W. Twombly, Julian S. Evans, and Ivan I. Smalyukh
Graphene recently emerged as a new two-dimensional material platform with unique optical, thermal and electronic properties. Single- or few-atom-thick graphene flakes can potentially be utilized to form structured bulk composites that further enrich these properties and enable a broad range of new applications. Here we describe optical manipulation of self-aligned colloidal graphene flakes in thermotropic liquid crystals of nematic and cholesteric types. Three-dimensional rotational and translational manipulation of graphene flakes by means of holographic optical tweezers allows for non-contact spatial patterning of graphene, control of liquid crystal defects, and low-power optical realignment of the liquid crystal director using these flakes. Potential applications include optically- and electrically-controlled reconfigurable liquid crystalline dispersions of spontaneously aligning colloidal graphene flakes and new electro-optic devices with graphene-based interconnected transparent electrodes at surfaces and in the bulk of liquid crystals.


Direct Observation of Single DNA Structural Alterations at Low Forces with Surface-Enhanced Raman Scattering

Satish Rao, Saurabh Raj, Benjamin Cossins, Monica Marro, Victor Guallar, Dmitri Petrov

DNA experiences numerous mechanical events, necessitating single-molecule force spectroscopy techniques to provide insight into DNA mechanics as a whole system. Inherent Brownian motion limits current force spectroscopy methods from observing possible bond level structural changes. We combine optical trapping and surface-enhanced Raman scattering to establish a direct relationship between DNA’s extension and structure in the low force, entropic regime. A DNA molecule is trapped close to a surface-enhanced Raman scattering substrate to facilitate a detectable Raman signal. DNA Raman modes shift in response to applied force, indicating phosphodiester mechanical alterations. Molecular dynamic simulations confirm the local structural alterations and the Raman sensitive band identified experimentally. The combined Raman and force spectroscopy technique, to our knowledge, is a novel methodology that can be generalized to all single-molecule studies.

Optical forces on cylinders near subwavelength slits illuminated by a photonic nanojet

F.J. Valdivia-Valero, M. Nieto-Vesperinas

We discuss optical forces exerted on particles, either dielectric or metallic, near a subwavelength slit illuminated by a photonic nanojet. We compare those cases in which the Mie resonances are or are not excited. The configurations on study are 2D, hence those particles are infinite cylinders and, in order to obtain extraordinary transmission, the illuminating beam is p-polarized. We show the different effects of these particle resonances on the optical forces: while whispering gallery modes under those illumination conditions weaken the force strength, this latter is enhanced by localized plasmon excitation. Also, illuminating the slit with a nanojet enhances the optical forces on the particle at the exit of the aperture by a factor between 3 and 10 compared with illumination of the slit with a Gaussian beam. In addition, the pulling force that such a small resonant metallic particle suffers on direct illumination by a nanojet can change by the presence of the slit, so that it may become repulsive at certain lateral positions of the particle.


Optical trapping and binding

Richard W Bowman and Miles J Padgett
The phenomenon of light's momentum was first observed in the laboratory at the beginning of the twentieth century, and its potential for manipulating microscopic particles was demonstrated by Ashkin some 70 years later. Since that initial demonstration, and the seminal 1986 paper where a single-beam gradient-force trap was realized, optical trapping has been exploited as both a rich example of physical phenomena and a powerful tool for sensitive measurement. This review outlines the underlying theory of optical traps, and explores many of the physical observations that have been made in such systems. These phenomena include 'optical binding', where trapped objects interact with one another through the trapping light field. We also discuss a number of the applications of 'optical tweezers' across the physical and life sciences, as well as covering some of the issues involved in constructing and using such a tool.

Tuesday, January 15, 2013

Manipulation of Micro-particles through Optical Interference Patterns Generated by Integrated Photonic Devices

Li-Chung Hsu , Te-Chang Chen , Yao-Tsu Yang , Chieh-Yang Huang , Da-Wei Shen , Ya-Tsu Chen and Ming-Chang Lee
Micro-particle transport and switch governed by guided-wave optical interference are presented. The optical interference, occurring in a directional coupler and a multi-mode interferometer made by inverted rib waveguides, displays a wavelength-controlled optical field. Through a detailed theoretical analysis, the field of induced optical force shows a correlative pattern as the optical field. Experimental results demonstrate that 10-m polystyrene beads are propelled with a trajectory subjected to the interference pattern accordingly. By launching different wavelengths, the polystyrene beads can be delivered to different output ports. Massive micro-particle manipulation is applicable.


Advanced optical trapping by complex beam shaping

Mike Woerdemann, Christina Alpmann, Michael Esseling, Cornelia Denz

Optical tweezers, a simple and robust implementation of optical micromanipulation technologies, have become a standard tool in biological, medical and physics research laboratories. Recently, with the utilization of holographic beam shaping techniques, more sophisticated trapping configurations have been realized to overcome current challenges in applications. Holographically generated higher-order light modes, for example, can induce highly structured and ordered three-dimensional optical potential landscapes with promising applications in optically guided assembly, transfer of orbital angular momentum, or acceleration of particles along defined trajectories. The non-diffracting property of particular light modes enables the optical manipulation in multiple planes or the creation of axially extended particle structures. Alongside with these concepts which rely on direct interaction of the light field with particles, two promising adjacent approaches tackle fundamental limitations by utilizing non-optical forces which are, however, induced by optical light fields. Optoelectronic tweezers take advantage of dielectrophoretic forces for adaptive and flexible, massively parallel trapping. Photophoretic trapping makes use of thermal forces and by this means is perfectly suited for trapping absorbing particles. Hence the possibility to tailor light fields holographically, combined with the complementary dielectrophoretic and photophoretic trapping provides a holistic approach to the majority of optical micromanipulation scenarios.

Optical forces through guided light deflections

Darwin Palima, Andrew Rafael Bañas, Gaszton Vizsnyiczai,Lóránd Kelemen, Thomas Aabo, Pál Ormos, and Jesper Glückstad

Optical trapping and manipulation typically relies on shaping focused light to control the optical force, usually on spherical objects. However, one can also shape the object to control the light deflection arising from the light-matter interaction and, hence, achieve desired optomechanical effects. In this work we look into the object shaping aspect and its potential for controlled optical manipulation. Using a simple bent waveguide as example, our numerical simulations show that the guided deflection of light efficiently converts incident light momentum into optical force with one order-of-magnitude improvement in the efficiency factor relative to a microbead, which is comparable to the improvement expected from orthogonal deflection with a perfect mirror. This improvement is illustrated in proof-of-principle experiments demonstrating the optical manipulation of two-photon polymerized waveguides. Results show that the force on the waveguide exceeds the combined forces on spherical trapping handles. Furthermore, it shows that static illumination can exert a constant force on a moving structure, unlike the position-dependent forces from harmonic potentials in conventional trapping.


Gearing up for optical microrobotics: micromanipulation and actuation of synthetic microstructures by optical forces

Darwin Palima, Jesper Glückstad

Optics is usually integrated into robotics as part of intelligent vision systems. At the microscale, however, optical forces can cause significant acceleration and so optical trapping and optical manipulation can enable the noncontact actuation of microcomponents. Microbeads are ubiquitous optically actuated structures, from Ashkin's pioneering experiments with polystyrene beads to contemporary functionalized beads for biophotonics. However, micro- and nanofabrication technologies are yielding a host of novel synthetic structures that promise alternative functionalities and new exciting applications. Recent works on the actuation of synthetic microstructures using optical trapping and optical manipulation are examined in this review. Extending the optical actuation down to the nanoscale is also presented, which can involve either direct manipulation of nanostructures or structure-mediated approaches where the nanostructures form part of larger structures that are suitable for interfacing with diffraction-limited optical fields.

Friday, January 11, 2013

Microscopic heterogeneity in viscoelastic properties of molecular assembled systems

Atsuomi Shundo, David P. Penaloza Jr., Keiji Tanaka
An important step in understanding molecular assembled systems is to examine the structure and physical properties at various length scales and clarify the correlation between them. However, while the structures of these systems have been extensively studied from nanoscopic to macroscopic scales, their viscoelastic properties have been often limited to bulk rheological measurements. By using optical tweezers and particle tracking, we here show the local viscoelastic properties and their spatial distributions for the following systems: worm-like micelle solution, supramolecular hydrogel and lyotropic liquid crystal, which are formed by self-assembly of amphiphilic molecules in water. We found that all systems studied possessed a spatial heterogeneity in their viscoelastic properties and this was originated from the heterogeneous structures. It is interesting to note that there is the heterogeneity with the characteristic length scale of sub-micrometer or micrometer scale, thereby structures, although the systems are formed by molecules with nanometer size. The findings of these studies should lead to a better understanding of the dynamics of such systems.

To Study the Effect of Paclitaxel on the Cytoplasmic Viscosity of Murine Macrophage Immune Cell RAW 264.7 Using Self-Developed Optical Tweezers System

Ying-chun Chen and Chien-ming Wu

In recent years, optical tweezers have become one of the tools to measure the mechanical properties of living cells. In this study, we first constructed an optical tweezers to investigate the cytoplasmic viscosity of immune cells. In addition to measuring viscosity of cells in a normal condition, we also treated cells with anti-cancer drug, Paclitaxel, and in order to study its effect on the cytoplasmic viscosity. The results showed that the viscosity decreased dramatically during the first 3 h. After 3 h, the change started to slow down and it remained nearly flat by the end of the experiment. In addition, we used the confocal laser scanning microscope to observe the cytoskeleton of the cell after drug treatment for 3 and 5 h, respectively, and found that actin filaments were disrupted and that the nucleus had disintegrated in some drug-treated cells, similar to the process of apoptosis. This study presents a new way for measuring the changes in cytoplasmic viscosity, and to determine if a cell is going into apoptosis as a result of a drug treatment.

Fiber-optic rotation of micro-scale structures enabled microfluidic actuation and self-scanning two-photon excitation

Bryan J. Black, Dijun Luo, and Samarendra K. Mohanty
Here, we report non-restricted, controlled fiber optic rotation of micro-motor, in counter-propagating fiber-optic beams having transverse-offset, for actuation of microfluidic flow. Ray-optics based simulations of the torque (and angular velocity) were conducted for different fiber transverse-offsets in order to determine optimal geometry for effective actuation. Further, self-scanning two-photon excitation of the fiber-optically rotated microscopic object is achieved by use of an ultrafast laser beam in one of the fiber arm.


Thursday, January 10, 2013

Optimizing bead size reduces errors in force measurements in optical traps

Rebecca K. Montange, Matthew S. Bull, Elisabeth R. Shanblatt, and Thomas T. Perkins
Optical traps are used to measure force (F) over a wide range (0.01 to 1,000 pN). Variations in bead radius (r) hinder force precision since trap stiffness (ktrap) varies as r3 when r is small. Prior work has shown ktrap is maximized when r is approximately equal to the beam waist (w0), which on our instrument was ~400 nm when trapping with a 1064-nm laser. In this work, we show that by choosing r ≈w0, we improved the force precision by 2.8-fold as compared to a smaller bead (250 nm). This improvement in force precision was verified by pulling on a canonical DNA hairpin. Thus, by using an optimum bead size, one can simultaneously maximize ktrap while minimizing errors in F.

Simultaneous rotation, orientation and displacement control of birefringent microparticles in holographic optical tweezers

A. Arias, S. Etcheverry, P. Solano, J. P. Staforelli, M. J. Gallardo, H. Rubinsztein-Dunlop, and C. Saavedra
We report the experimental implementation of a new method for generating multiple dynamical optical tweezers, where each one of them is generated with an independent linear polarization state with arbitrary orientation. This also allows an independent simultaneous polarization-rotation control. The laser beam, both for generating multiple traps and polarization control, has been modulated using a single reflective nematic liquid crystal with parallel alignment. We present experimental results of controlled displacement, orientation and rotation of birefringent particles. In addition, a simple method for estimating and canceling out the primary astigmatism present in the system is presented.


Wednesday, January 9, 2013

Long-distance laser propulsion and deformation- monitoring of cells in optofluidic photonic crystal fiber

Sarah Unterkofler, Martin K. Garbos, Tijmen G. Euser, Philip St. J. Russell

We introduce a unique method for laser-propelling individual cells over distances of 10s of cm through stationary liquid in a microfluidic channel. This is achieved by using liquid-filled hollow-core photonic crystal fiber (HC-PCF). HC-PCF provides low-loss light guidance in a well-defined single mode, resulting in highly uniform optical trapping and propulsive forces in the core which at the same time acts as a microfluidic channel. Cells are trapped laterally at the center of the core, typically several microns away from the glass interface, which eliminates adherence effects and external perturbations. During propagation, the velocity of the cells is conveniently monitored using a non-imaging Doppler velocimetry technique. Dynamic changes in velocity at constant optical powers up to 350 mW indicate stress-induced changes in the shape of the cells, which is confirmed by bright-field microscopy. Our results suggest that HC-PCF will be useful as a new tool for the study of single-cell biomechanics.


Force as a single molecule probe of multidimensional protein energy landscapes

Gabriel Žoldák, Matthias Rief

Force spectroscopy has developed into an indispensable tool for studying folding and binding of proteins on a single molecule level in real time. Design of the pulling geometry allows tuning the reaction coordinate in a very precise manner. Many recent experiments have taken advantage of this possibility and have provided detailed insight the folding pathways on the complex high dimensional energy landscape. Beyond its potential to provide control over the reaction coordinate, force is also an important physiological parameter that affects protein conformation under in vivo conditions. Single molecule force spectroscopy studies have started to unravel the response and adaptation of force bearing protein structures to mechanical loads.

Alba shapes the archaeal genome using a delicate balance of bridging and stiffening the DNA

Niels Laurens, Rosalie P.C. Driessen, Iddo Heller, Daan Vorselen, Maarten C. Noom, Felix J.H. Hol, Malcolm F. White, Remus T. Dame & Gijs J.L. Wuite

Architectural proteins have an important role in shaping the genome and act as global regulators of gene expression. How these proteins jointly modulate genome plasticity is largely unknown. In archaea, one of the most abundant proteins, Alba, is considered to have a key role in organizing the genome. Here we characterize the multimodal architectural properties and interplay of the Alba1and Alba2 proteins using single-molecule imaging and manipulation techniques. We demonstrate that the two paralogues can bridge and rigidify DNA and that the interplay between the two proteins influences the balance between these effects. Our data yield a structural model that explains the multimodal behaviour of Alba proteins and its impact on genome folding.


Optically induced 'negative forces'

Aristide Dogariu, Sergey Sukhov & José Sáenz

The idea of using optical beams to attract objects has long been a dream of scientists and the public alike. Over the years, a number of proposals have attempted to bring this concept to life. Here we review the most recent progress in this emerging field, including new concepts for manipulating small objects using optically induced 'negative forces', achieved by tailoring the properties of the electromagnetic field, the environment or the particles themselves.

DNA target sequence identification mechanism for dimer-active protein complexes

Markita P. Landry, Xueqing Zou, Lei Wang, Wai Mun Huang, Klaus Schulten and Yann R. Chemla

Sequence-specific DNA-binding proteins must quickly and reliably localize specific target sites on DNA. This search process has been well characterized for monomeric proteins, but it remains poorly understood for systems that require assembly into dimers or oligomers at the target site. We present a single-molecule study of the target-search mechanism of protelomerase TelK, a recombinase-like protein that is only active as a dimer. We show that TelK undergoes 1D diffusion on non-target DNA as a monomer, and it immobilizes upon dimerization even in the absence of a DNA target site. We further show that dimeric TelK condenses non-target DNA, forming a tightly bound nucleoprotein complex. Together with theoretical calculations and molecular dynamics simulations, we present a novel target-search model for TelK, which may be generalizable to other dimer and oligomer-active proteins.

3D multiple optical tweezers based on time-shared scanning with a fast focus tunable lens

Yoshio Tanaka
Three-dimensional controlled manipulation of individual micro-objects requires multiple optical tweezers that can be independently controlled in a 3D working space with high spatiotemporal resolution. Here, the author presents 3D multiple optical tweezers based on a time-shared scanning technique with an electrically focus tunable lens for axial steering and a two-axis steering mirror for lateral steering. Four typical examples of 3D controlled manipulation, including the rotation of a single bead on its axis, are demonstrated in real time. The optical system design and the control method are also described.