Thursday, December 24, 2009

Statistical Properties of Metastable Intermediates in DNA Unzipping

J. M. Huguet, N. Forns, and F. Ritort

We unzip DNA molecules using optical tweezers and determine the sizes of the cooperatively unzipping and zipping regions separating consecutive metastableintermediates along the unzipping pathway. Sizes are found to be distributed following a power law, ranging from one base pair up to more than a hundred base pairs. We find that a large fraction of unzipping regions smaller than 10 bp are seldom detected because of the high compliance of the released single stranded DNA. We show how the compliance of a single nucleotide sets a limit value around 0.1 N/m for the stiffness of any local force probe aiming to discriminate one base pair at a time in DNA unzipping experiments.

Wednesday, December 23, 2009

Laser Manipulation by Using Liquid Crystal Devices with Variable-Focusing and Beam-Steering Functions

Marenori Kawamura; Junji Onishi; Susumu Sato

A laser manipulation system for trapping and controlling the positions of microscopic transparent particles by using liquid crystal (LC) devices is developed. The LC device has functions of variable-focusing and beam-steering by controlling the applied voltages to the LC device without mechanical movements. The trapped particles suspended in deionized water can easily be shifted along the position of the focused laser spot. The microscopic rod-like particles can also be shifted and rotated in the clockwise or anticlockwise along the direction of the major axis of the elliptically distributed beam intensity. In addition, the multiple microscopic particles at the bright region of the linear interference fringe patterns of the LC device with comb-shaped electrodes can be trapped and shifted along the fringe patterns.

Monday, December 21, 2009

Trapping of low-refractive-index particles with azimuthally polarized beam

Fei Peng, Baoli Yao, Shaohui Yan, Wei Zhao, and Ming Lei

Azimuthally polarized beams, focused by a high-numerical-aperture (NA) objective lens, form a hollow intensity distribution near the focus, which is appropriate for trapping low-refractive-index particles, in contrast to common linearly polarized or radially polarized beams. In this paper, the field distribution of the azimuthally polarized beam focused by a high-NA objective is described by the vectorial diffraction integral, and then the radiation forces on spherical particles with different parameters such as radius and refractive index are calculated by the T-matrix method. Numerical results show that the azimuthally polarized beam not only can steadily trap low-refractive-index particles at the focus center but also can trap multiple high-refractive-index particles around the focus center by virtue of the hollow-ring configuration. The range of the sizes of low-refractive-index particles that can be trapped steadily are presented, corresponding to different parameters such as the NA of the objective and the relative refractive index, based on which the NA of the objective can be selected to trap the appropriate size of particles.

Multi-mode mitigation in an optofluidic chip for particle manipulation and sensing

Philip Measor, Sergei Kühn, Evan J. Lunt, Brian S. Phillips, Aaron R. Hawkins, andHolger Schmidt

A new waveguide design for an optofluidic chip is presented. It mitigates multi-mode behavior in solid and liquid-core waveguides by increasing fundamental mode coupling to 82% and 95%, respectively. Additionally, we demonstrate a six-fold improvement in lateral confinement of optically guided dielectric microparticles and double the detection efficiency of fluorescent particles.

Friday, December 18, 2009

An Optical Conveyor for Molecules

Franz M. Weinert and Dieter Braun

Trapping single ions under vacuum allows for precise spectroscopy in atomic physics. The confinement of biological molecules in bulk water is hindered by the lack of comparably strong forces. Molecules have been immobilized to surfaces, however often with detrimental effects on their function. Here, we optically trap molecules by creating the microscale analogue of a conveyor belt: a bidirectional flow is combined with a perpendicular thermophoretic molecule drift. Arranged in a toroidal geometry, the conveyor accumulates a hundredfold excess of 5-base DNA within seconds. The concentrations of the trapped DNA scale exponentially with length, reaching trapping potential depths of 14 kT for 50 bases. The mechanism does not require microfluidics, electrodes, or surface modifications. As a result, the trap can be dynamically relocated. The optical conveyor can be used to enhance diffusion-limited surface reactions, redirect cellular signaling, observe individual biomolecules over a prolonged time, or approach single-molecule chemistry in bulk water.

Thursday, December 17, 2009

Dynamic micro-bead arrays 
using optical tweezers combined with
intelligent control techniques

Yoshio Tanaka, Hiroyuki Kawada, Shogo Tsutsui, Mitsuru Ishikawa, and Hiroyuki Kitajima

Dynamic micro-bead arrays offer great flexibility and potential as sensing tools in various scientific fields. Here we present a software-oriented approach for fully automated assembly of versatile dynamic micro-bead arrays using multi-beam optical tweezers combined with intelligent control techniques. Four typical examples, including the collision-free sorting of array elements by bead features, are demonstrated in real time. Control algorithms and experimental apparatus for these demonstrations are also described.

Spatial Stability of Particles Trapped by Time-Division Optical Tweezers

Johtaro Yamamoto; Toshiaki Iwai

We developed an on-demand multiple-spot holographic optical tweezers (HOT) system based on quasi-simultaneous generation of two intensity-spot patterns by alternately sending the two corresponding hologram patterns to a spatial light modulator. This switching operation reduces the spatial stability of a Brownian particle trapped inside the generated intensity spot. In this study, numerical analysis of the conditions for stable particle trapping in the time-division HOT is conducted using the Smoluchowski equation under the Rayleigh scattering approximation. The relationship between the particle size, the switching rate, and the focused laser beam power is obtained. Experiments confirm the validity of the numerical analysis.

Passive and active microrheology for cross-linked F-actin networks in vitro

Hyungsuk Lee, Jorge M. Ferrer, Fumihiko Nakamura, Matthew J. Lang and Roger D. Kamm

Actin filament (F-actin) is one of the dominant structural constituents in the cytoskeleton. Orchestrated by various actin-binding proteins (ABPs), F-actin is assembled into higher-order structures such as bundles and networks that provide mechanical support for the cell and play important roles in numerous cellular processes. Although mechanical properties of F-actin networks have been extensively studied, the underlying mechanisms for network elasticity are not fully understood, in part because different measurements probe different length and force scales. Here, we developed both passive and active microrheology techniques using optical tweezers to estimate the mechanical properties of F-actin networks at a length scale comparable to cells. For the passive approach we tracked the motion of a thermally fluctuating colloidal sphere to estimate the frequency-dependent complex shear modulus of the network. In the active approach, we used an optical trap to oscillate an embedded microsphere and monitored the response in order to obtain network viscoelasticity over a physiologically relevant force range. While both active and passive measurements exhibit similar results at low strain, the F-actin network subject to high strain exhibits non-linear behavior which is analogous to the strain-hardening observed in macroscale measurements. Using confocal and total internal reflection fluorescent microscopy, we also characterize the microstructure of reconstituted F-actin networks in terms of filament length, mesh size and degree of bundling. Finally, we propose a model of network connectivity by investigating the effect of filament length on the mechanical properties and structure.

Direct Measurement of Shear-Induced Cross-Correlations of Brownian Motion

A. Ziehl, J. Bammert, L. Holzer, C. Wagner, and W. Zimmermann

Shear-induced cross-correlations of particle fluctuations perpendicular and along streamlines are investigated experimentally and theoretically. Direct measurements of the Brownian motion of micron-sized beads, held by optical tweezers in a shear-flow cell, show a strong time asymmetry in the cross-correlation, which is caused by the non-normal amplification of fluctuations. Complementary measurements on the single particle probability distribution substantiate this behavior and both results are consistent with a Langevin model. In addition, a shear-induced anticorrelation between orthogonal random displacements of two trapped and hydrodynamically interacting particles is detected, having one or two extrema in time, depending on the positions of the particles.

Wednesday, December 16, 2009

Optical tweezer for probing erythrocyte membrane deformability

Manas Khan, Harsh Soni, and A. K. Sood

We report that the average rotation speed of optically trapped crenated erythrocytes is direct signature of their membrane deformability. When placed in hypertonic buffer, discocytic erythrocytes are subjected to crenation. The deformation of cells brings in chirality and asymmetry in shape that makes them rotate under the scattering force of alinearly polarized optical trap. A change in the deformability of the erythrocytes, due to any internal or environmental factor, affects the rotation speed of the trapped crenated cells. Here we show how the increment in erythrocyte membrane rigidity with adsorption of Ca++ions can be exhibited through this approach.

Electrophoretic Force on a Protein-Coated DNA Molecule in a Solid-State Nanopore

Adam R. Hall, Stijn van Dorp, Serge G. Lemay and Cees Dekker

Using solid-state nanopores with optical tweezers, we perform force spectroscopy on DNA molecules that are coated with RecA proteins. We observe that the electrophoretic force is 2−4 times larger for RecA-DNA filaments than for uncoated DNA molecules and that this force increases at lower salt concentrations. The data demonstrate the efficacy of solid-state nanopores for locally probing the forces on DNA-bound proteins. Our results are described quantitatively by a model that treats the electrophoretic and hydrodynamic forces. The conductance steps that occur when RecA-DNA enters the nanopore change from conductance decreases at high salt to conductance increases at low salt, which allows the apparent charge of the RecA-DNA filament to be extracted. The combination of conductance measurements with local force spectroscopy increases the potential for future solid-state nanopore screening devices.

Monday, December 14, 2009

Wavelength dependence of optical tweezer trapping forces on dye-doped polystyrene microspheres

M. J. Kendrick, D. H. McIntyre, and O. Ostroverkhova

We present an experimental and numerical study of the wavelength dependence, near resonance, of the optical tweezer trap stiffness on three different dye-doped 1 μm polystyrene spheres with peak absorptions at λ=625, 775, and 840 nm. Experimentally, an increase in the trap stiffness of ~35% on the red side of resonance was observed for the dye-doped spheres relative to polystyrene spheres without dye. Numerical simulations for spheres of different sizes, between 20 nm and 1 μm, and for absorption strengths corresponding to peak extinction coefficient values between 0.0027 and 0.081 were also conducted. Numerical results showed a maximum increase in the trap stiffness of ~35%, which is consistent with experimental results.

Wednesday, December 9, 2009

Design considerations for micro- and nanopositioning: leveraging the latest for biophysical applications

Jordan SC, Anthony PC.

Biophysical applications ranging from fluorescence microassays to single-molecule microscopy are increasingly dependent on automated nanoscale positional control and stability. A whirlwind of motion-industry innovation has resulted in an array of new motion options offering significant improvements in application performance, reproducibility and throughput. The challenge to leverage these developments depends on researchers, engineers and motion vendors acquiring a common language of specifications and a shared understanding of the challenges posed by application needs. To assist in building this shared understanding, this article reviews today's motion technologies, beginning with a concise review of key principles of motion control focusing on applications. It progresses through illustrations of sensor/encoder technologies and servo techniques. A spectrum of classical and recent motion technologies is explored, from stepper and servo actuation of conventional microscopy stages, to advanced piezo stack nanopositioners capable of picometer precision, to novel ultrasonic resonant piezomotors and piezo-ceramic-based mechanisms capable of high-force positioning over many millimeters while providing resolutions down into the sub-nanometer range. A special emphasis is placed on the effects of integrating multiple motion technologies into an application, such as stacking a fine nanopositioner atop a long-travel stage. Examples and data are presented to clarify these issues, including important and insightful new stability measurements taken directly from an advanced optical trapping application. The important topics of software and interfacing are also explored from an applications perspective, since design-and-debugging time, synchronization capabilities and overall throughput are heavily dependent on these often-overlooked aspects of motion system design. The discussion is designed to illuminate specifications-related topics that become increasingly important as precision requirements tighten. Throughout, both traditional and novel techniques and approaches are explored so that readers are left with a solid overview of the state of the art, and an actionable perspective that readies them to discuss and evaluate specifications and vendor capabilities against practical application requirements.

Effect of antibiotics and antimicrobial peptides on single protein motilit

Winther T, Oddershede LB.

Following the movement of individual molecules of a bacterial surface protein in vivo we investigated the effects of antibiotics and antimicrobial peptides on protein motility and membrane structure. In previous work we engineered the lambda-receptor of Escherichia coli such that less than one receptor per cell is in vivo biotinylated and can bind to a streptavidin coated bead. Such a bead served as a handle for the optical tweezers to follow the motion of an individual receptor. In an un-perturbed living cell the lambda-receptor performs a confined diffusive motion. The lambda-receptor links to the peptidoglycan layer, and indeed, a perturbation of the peptidoglycan layer had a pronounced effect on the motility of the receptor: The motility significantly decreases upon treatment with vancomycin or ampicillin, to study the effect of vancomycin we used strains with increased membrane permeability. As the motility of an individual receptor was monitored over an extended amount of time we were able to observe a temporal evolution of the action of vancomycin. Antimicrobial peptides (AMPs) are alternatives to conventional antibiotics in the treatment of bacterial infections. Therefore, we also investigated the effect of the toxic AMP polymyxin B (PMB) which targets both the outer and inner membranes and kills the organism. PMB significantly decreased the motility of the lambda-receptor. On the basis of these findings we confirm that the lambda-receptor is firmly attached to the peptidoglycan layer, and that an antibiotic or AMP mediated destruction of the dynamic peptidoglycan synthesis decreases the receptor motion.

10 Years of Tension on Chromatin: Results from Single Molecule Force Spectroscopy

Chien, Fan-Tso; van Noort, John

The compact, yet dynamic organization of chromatin plays an essential role in regulating gene expression. Although the static structure of chromatin fibers has been studied extensively, the controversy about the higher order folding remains. In the past ten years a number of studies have addressed chromatin folding with single molecule force spectroscopy. By manipulating chromatin fibers individually, the mechanical properties of the fibers were quantified with piconewton and nanometer accuracy. Here, we review the results of force induced chromatin unfolding and compare the differences between experimental conditions and single molecule manipulation techniques like force and position clamps. From these studies, five major features appeared upon forced extension of chromatin fibers: the elastic stretching of chromatin's higher order structure, the breaking of internucleosomal contacts, unwrapping of the first turn of DNA, unwrapping of the second turn of DNA, and the dissociation of histone octamers. These events occur sequentially at the increasing force. Resolving force induced structural changes of chromatin fibers at the single molecule level will help to provide a physical understanding of processes involving chromatin that occur in vivo and will reveal the mechanical constraints that are relevant for processing and maintenance of DNA in eukaryotes.

Single-Molecule Force Spectroscopy Using the NanoTracker™ Optical Tweezers Platform: from Design to Application

Wozniak, A.; van Mameren, J.; Ragona, S.

Since the development of detection and analysis techniques for optical tweezers setups, there has been an everincreasing interest in optical tweezers as a quantitative method, shifting its applications from a pure manipulation tool towards the investigation of motions and forces. With the capability of manipulation and detection of forces of a few hundred picoNewtons down to a fraction of a picoNewton, optical tweezers are perfectly suitable for the investigation of single molecules. Accordingly, the technique has been extensively used for the biophysical characterization of biomolecules, ranging from the mechanical and elastic properties of biological polymers to the dynamics associated with enzymatic activity and protein motility. Here, the use of state-of-the-art optical tweezers on the elasticity of single DNA molecules is presented, highlighting the possibilities this technique offers for the investigation of protein-DNA interaction, but also for other single molecule applications. Technical in nature, design aspects of the NanoTracker™ optical tweezers setup are addressed, presenting the recent advances in the development of optical tweezers, ranging from noise reduction to detection and calibration methodology.


Transportin Mediates Nuclear Entry of DNA in Vertebrate Systems

Aurelie Lachish-Zalait, Corine K. Lau, Boris Fichtman, Ella Zimmerman, Amnon Harel, Michelle R. Gaylord , Douglass J. Forbes and Michael Elbaum

Delivery of DNA to the cell nucleus is an essential step in many types of viral infection, transfection, gene transfer by the plant pathogen Agrobacteriumtumefaciens and in strategies for gene therapy. Thus, the mechanism by which DNA crosses the nuclear pore complex (NPC) is of great interest. Using nuclei reconstituted in vitro inXenopus egg extracts, we previously studied DNA passage through the nuclear pores using a single-molecule approach based on optical tweezers. Fluorescently labeled DNA molecules were also seen to accumulate within nuclei. Here we find that this import of DNA relies on a soluble protein receptor of the importin family. To identify this receptor, we used different pathway-specific cargoes in competition studies as well as pathway-specific dominant negative inhibitors derived from the nucleoporin Nup153. We found that inhibition of the receptor transportin suppresses DNA import. In contrast, inhibition of importin β has little effect on the nuclear accumulation of DNA. The dependence on transportin was fully confirmed in assays using permeabilized HeLa cells and a mammalian cell extract. We conclude that the nuclear import of DNA observed in these different vertebrate systems is largely mediated by the receptor transportin. We further report that histones, a known cargo of transportin, can act as an adaptor for the binding of transportin to DNA.

Axially chiral facial amphiphiles with a dihydronaphthopentaphene structure as molecular tweezers for SWNTs

Renaud Marquis, Krystyna Kulikiewicz, Sergei Lebedkin, Manfred M. Kappes, Charles Mioskowski, Stéphane Meunier, Alain Wagner

Syntheses of chiral 6,15-dihydronaphtho[2,3-c]pentaphene derivatives of opposite configurations are reported. Starting from anthracene, the strategy involves two key steps: a Diels-Alder reaction on a prochiral dianthraquinone, and an enantiomeric resolution using (-)-menthol. The final molecules exhibit very strong optical activity, as shown by their circular dichroism spectra, and are examples of chiral facial amphiphiles. Their adsorption at the surface of single-walled carbon nanotubes (SWNTs) has also been studied, and has been found to occur preferentially on 0.8-1.0 nm diameter nanotubes among the population of a high-pressure CO conversion (HiPco) SWNT sample (0.8-1.2 nm). The synthesised facial amphiphiles act as nano-tweezers for the diameter-selective solubilisation of SWNTs in water. The expected optical activities of the SWNT samples solubilised by each of the chiral amphiphiles have been studied by circular dichroism spectroscopy, but the results are not yet conclusive.

Structural and micromechanical characterization of type I collagen gels

Olga Latinovic, Lawrence A. Hough and H. Daniel Ou-Yang

In this paper we report a study where we use a novel optical tweezers technique to measure the local viscoelastic properties of type I collagen solutions spanning the sol-to-gel transition. We use phase contrast optical microscopy to reveal dense and sparse regions of the rigid fibril networks, and find that the spatial variations in the mechanical properties of the collagen gels closely follow the structural properties. Within the dense phase of the connected network in the gel samples, there are regions that exhibit drastically different viscoelastic properties. Within the sparse regions of the gel samples, no evidence of elasticity is found. In type I collagen gels, we find a high degree of structural inhomogeneity. The inhomogeneity in the structural properties of collagen gels and the corresponding viscoelastic properties provide benchmark measurements for the behavior of desirable biological materials, or tissue equivalents.

Unfolding and refolding properties of S pili on extraintestinal pathogenic Escherichia coli

Mickaël Castelain, Annika E. Sjöström, Erik Fällman, Bernt Eric Uhlin and Magnus Andersson

S pili are members of the chaperone-usher-pathway-assembled pili family that are predominantly associated with neonatal meningitis (SII) and believed to play a role in ascending urinary tract infections (SI). We used force-measuring optical tweezers to characterize the intrinsic biomechanical properties and kinetics of SII and SIpili. Under steady-state conditions, a sequential unfolding of the layers in the helix-like rod occurred at somewhat different forces, 26 pN for SII pili and 21 pN for SI pili, and there was an apparent difference in the kinetics, 1.3 and 8.8 Hz. Tests with bacteria defective in a newly recognized sfa gene (sfaX II) indicated that absence of the sfaX IIgene weakens the interactions of the fimbrium slightly and decreases the kinetics. Data of SI are compared with those of previously assessed pili primary associated with urinary tract infections, the P and type 1 pili. S pili have weaker layer-to-layer bonds than both P and type 1 pili, 21, 28 and 30 pN, respectively. In addition, the S pili kinetics are ~10 times faster than the kinetics of P pili and ~550 times faster than the kinetics of type 1 pili. Our results also show that the biomechanical properties of pili expressed ectopically from a plasmid in a laboratory strain (HB101) and pili expressed from the chromosome of a clinical isolate (IHE3034) are identical. Moreover, we demonstrate that it is possible to distinguish, by analyzing force-extension data, the different types of pili expressed by an individual cell of a clinical bacterial isolate.

Impaired red cell deformability in iron deficient subjects

Marcelo M. Brandão, Maria de Lourdes R.B. Castro, Adriana Fontes, Carlos L. Cesar, Fernando F. Costa, Sara T.O. Saad

Iron deficiency is a systemic disorder, which affects a variety of different cell types and is one of the most frequent diseases throughout the world. The influence of iron deficiency upon erythrocyte deformability is controversial and could be a consequence of membrane peroxidation damage or cross linking of membrane proteins. The aim of this study was to determine the overall elasticity (the deformability of the entire cell is evaluated) of iron deficient red blood cells (RBC) using laser optical tweezers. In this study, the laser trapped the cell and the elasticity was then analyzed measuring cell deformation at six different drag velocities. Twenty-five RBCs from 11 healthy blood donors (controls) and 7 patients with iron deficiency anemia were analyzed. Iron deficiency subjects were classified into 3 groups based on Hb concentration for statistical analysis (group I: Hb = 7.0–7.9; group II: 8.0–10.2 and group III: 7.0–10.2 g/dl). The results showed an increased rigidity in the iron deficiency of deficient red blood cells when compared to normal control blood cells, and, this impaired deformability seems to be correlated to the hemoglobin concentration. In conclusion, the results obtained by optical tweezers showed that iron deficiency affects the elasticity of whole RBC.

Hydrodynamic Mobility of an Optically Trapped Colloidal Particle near Fluid-Fluid Interfaces

G. M. Wang, R. Prabhakar, and E. M. Sevick

Using optical tweezers, we measure the anisotropic hydrodynamic mobility of a colloidal particle by tracking its thermal motion in an optical trap located near fluid-fluid interfaces, namely, liquid-vapor and liquid-liquid interfaces. The method requires no controlled fluid flow, is independent of conservative interactions between particle and interface, and resolves distance dependent friction to within a fraction of the particle radius. Near the liquid-vapor interface, the friction decreases below that in the bulk, corresponding to predictions of a “perfect-slip” surface.

Combining Single-Molecule Optical Trapping and Small-Angle X-Ray Scattering Measurements to Compute the Persistence Length of a Protein ER/K alpha-Hel

S. Sivaramakrishnan, J. Sung, M. Ali, S. Doniach, H. Flyvbjerg and J.A. Spudich

A relatively unknown protein structure motif forms stable isolated single alpha-helices, termed ER/K alpha-helices, in a wide variety of proteins and has been shown to be essential for the function of some molecular motors. The flexibility of the ER/K alpha-helix determines whether it behaves as a force transducer, rigid spacer, or flexible linker in proteins. In this study, we quantity this flexibility in terms of persistence length, namely the length scale over which it is rigid. We use single-molecule optical trapping and small-angle x-ray scattering, combined with Monte Carlo simulations to demonstrate that the Kelch ER/K alpha-helix behaves as a wormlike chain with a persistence length of similar to 15 nm or similar to 28 turns of alpha-helix. The ER/K alpha-helix length in proteins varies from 3 to 60 nm, with a median length of similar to 5 nm. Knowledge of its persistence length enables us to define its function as a rigid spacer in a translation initiation factor, as a force transducer in the mechanoenzyme myosin VI, and as a flexible spacer in the Kelch-motif-containing protein.

Multi-particle three-dimensional coordinate estimation in real-time optical manipulation

J. S. Dam, I. Perch-Nielsen, D. Palima, J. Glückstad

We have previously shown how stereoscopic images can be obtained in our three-dimensional optical micromanipulation system [J. S. Dam et al, Opt. Express 16, 7244 (2008)]. Here, we present an extension and application of this principle to automatically gather the three-dimensional coordinates for all trapped particles with high tracking range and high reliability without requiring user calibration. Through deconvolving of the red, green, and blue colour planes to correct for bleeding between colour planes, we show that we can extend the system to also utilize green illumination, in addition to the blue and red. Applying the green colour as on-axis illumination yields redundant information for enhanced error correction, which is used to verify the gathered data, resulting in reliable coordinates as well as producing visually attractive images.

Radiation force of scalar and electromagnetic twisted Gaussian Schell-model beams

Chengliang Zhao, Yangjian Cai, and Olga Korotkova

Radiation force of a focused scalar twisted Gaussian Schell-model (TGSM) beam on a Rayleigh dielectric sphere is investigated. It is found that the twist phase affects the radiation force and by raising the absolute value of the twist factor it is possible to increase both transverse and longitudinal trapping ranges at the real focus where the maximum on-axis intensity is located. Numerical calculations of radiation forces induced by a focused electromagnetic TGSM beam on a Rayleigh dielectric sphere are carried out. It is found that radiation force is closely related to the twist phase, degree of polarization and correlation factors of the initial beam. The trapping stability is also discussed.

Monday, December 7, 2009

Optical forces on a Mie spheroidal particle arbitrarily oriented in a counterpropagating trap

Héctor Sosa-Martínez and Julio C. Gutiérrez-Vega

The axial and transverse optical forces exerted by Gaussian beams on an arbitrarily oriented and homogeneous spheroid are calculated and studied within the framework of the Mie theory. The results are applied to study the behavior of the forces in a counterpropagating optical trap. We calculate the trapping efficiencies for a wide range of physical parameters, including the beam waist separation distance, the equivalent spheroid radius, the spheroid eccentricity, and the refractive index ratio between the particle and the surrounding medium.

Manipulation and characterisation of accumulation and coarse mode aerosol particles using a Bessel beam trap

Helena Meresman, Jon B. Wills, Michael Summers, David McGloin and Jonathan P. Reid

Micron and sub-micron sized aerosol particles are captured, manipulated and characterised in a Bessel beam optical trap. Bright field microscopy and elastic light scattering measurements are used in combination to interrogate trapped particles and explore the optical landscape of the trap. We conclude that the Bessel trap has a number of advantages over optical tweezers in terms of characterization of accumulation mode particles, manipulation of particles over macroscopic length scales and effective control of the gas phase. As such, the Bessel trap is a valuable addition to the aerosol optical toolkit.

Direct observation of the transfer of orbital angular momentum to metal particles from a focused circularly polarized Gaussian beam

Yiqiong Zhao, David Shapiro, David Mcgloin, Daniel T. Chiu, and Stefano Marchesini

It is well known that a circularly polarized Gaussian beam carries spin angular momentum, but not orbital angular momentum. This paper demonstrates that focusing a beam carrying spin angular momentum can induce an orbital angular momentum which we used to drive the orbital motion of a micron-sized metal particle that is trapped off the beam axis. The direction of the orbital motion is controlled by the handedness of the circular polarization. The orbiting dynamics of the trapped particle, which acted as an optical micro-detector, were quantitatively measured and found to be in excellent agreement with the theoretical predictions.

Friday, December 4, 2009

Determining the Zero-Force Binding Energetics of an Intercalated DNA Complex by a Single-Molecule Approach

Tzu-Sen Yang, Yujia Cui, Chien-Ming Wu, Jem-Mau Lo, Chi-Shiun Chiang, Wun-Yi Shu, Wei-Ju Chung, Chung-Shan Yu, Kuo-Ning Chiang, Ian C. Hsu

Binding behavior of DNA: A wormlike chain model is applied to determine the zero-force binding energetics of an intercalated DNA complex. A mono-intercalating agent is synthesized which can be very easily inserted into the DNA bases (see figure). This approach is particularly important in nuclear medicine and new radiation therapies.

Thursday, December 3, 2009

Steady electrical and micro-rheological response functions for uncharged colloidal inclusions in polyelectrolyte hydrogels

Aliasghar Mohammadi and Reghan J. Hill

The electric-field-induced response of an uncharged colloidal sphere embedded in a quenched polyelectrolyte hydrogel is calculated from a model where the polymer network is treated as an elastic, porous skeleton saturated with an aqueous electrolyte. We present exact analytical solutions for the steady response to a uniform electric field, as well as the steady susceptibility, defined as the ratio of the particle displacement to the strength of an optical or magnetic force. Even though the particle is uncharged, it attains a finite electric-field-induced displacement owing to hydrodynamic coupling with electroosmotic flow. The steady susceptibility decreases with increasing charge and decreasing electrolyte concentration; in general, charge imparts a small correction to the classical theory for an uncharged linearly elastic continuum.

Tuesday, December 1, 2009

Work distribution for a particle moving in an optical trap and non-Markovian bath

Alok Samanta, K. Srinivasu and Swapan K. Ghosh

We propose a simple approach to derive an exact analytical expression of work distribution for a system consisting of a colloidal particle trapped in an optical harmonic potential well, which is being pulled at a constant velocity through a solution represented by a non-Markovian bath. The thermal environment is represented by a bath composed of an infinite set of harmonic oscillators, and a model Hamiltonian for the trapped colloidal particle is constructed by representing the interaction with the bath via linear dissipative mechanism. We have studied the effects of pulling time, pulling speed, and the adiabatic limit. It is also observed that only at long time the total work is completely converted into dissipative work.