Wednesday, July 29, 2009

In-Situ Characterization of Three-Dimensional Optical Matters by Light Diffraction

Jiang Lai-Dong, Dai Qiao-Feng, Feng Tian-Hua, Liu Jin, Wu Li-Jun, Lan Sheng, A. V. Gopal and V. A. Trofimov

Three-dimensional optical matters are created by combining the single beam optical trapping with the conventional Z-scan technique. Dynamic light diffraction is employed to evaluate the structure and quality of the optical matter formed at the optimum trapping power. The lattice constant of the optical matter is extracted based on the Bragg and Snell laws, showing that polystyrene spheres are nearly close-packed in the optical matter, confirmed by comparing the diffraction pattern of the optical matter with that of a colloidal photonic crystal fabricated by the self-assembled technique. The relatively broad diffraction peaks observed in the optical matter indicate that the density of disorders in it is higher than that in the photonic crystal. It is suggested that the optical matter possesses a random close-packed structure rather than a face centered cubic one.


Control of cell membrane tension by myosin-I

Rajalakshmi Nambiar, Russell E. McConnell and Matthew J. Tyska

All cell functions that involve membrane deformation or a change in cell shape (e.g., endocytosis, exocytosis, cell motility, and cytokinesis) are regulated by membrane tension. While molecular contacts between the plasma membrane and the underlying actin cytoskeleton are known to make significant contributions to membrane tension, little is known about the molecules that mediate these interactions. We used an optical trap to directly probe the molecular determinants of membrane tension in isolated organelles and in living cells. Here, we show that class I myosins, a family of membrane-binding, actin-based motor proteins, mediate membrane/cytoskeleton adhesion and thus, make major contributions to membrane tension. These studies show that class I myosins directly control the mechanical properties of the cell membrane; they also position these motor proteins as master regulators of cellular events involving membrane deformation.

Tuesday, July 28, 2009

Investigation of inclined dual-fiber optical tweezers for 3D manipulation and force sensing

Yuxiang Liu and Miao Yu

Optical tweezers provide a versatile tool in biological and physical researches. Optical tweezers based on optical fibers are more flexible and ready to be integrated when compared with those based on microscope objectives. In this paper, the three-dimensional (3D) trapping ability of an inclined dual-fiber optical tweezers is demonstrated. The trapping efficiency with respect to displacement is experimentally calibrated along two dimensions. The system is studied numerically using a modified ray-optics model. The spring constants obtained in the experiment are predicted by simulations. It is found both experimentally and numerically that there is a critical value for the fiber inclination angle to retain the 3D trapping ability. The inclined dual-fiber optical tweezers are demonstrated to be more robust to z-axis misalignment than the counter-propagating fiber optical tweezers, which is a special case of th former when the fiber inclination angle is 90º. This inclined dual-fiber optical tweezers can serve as both a manipulator and a force sensor in integrated systems, such as microfluidic systems and lab-on-a-chip systems.

Surface Enhanced Raman Scattering from Pseudoisocyanine on Ag Nanoaggregates Produced by Optical Trapping with a Linearly Polarized Laser Beam

Yoshito Tanaka and Hiroyuki Yoshikawa

We have investigated surface enhanced Raman scattering (SERS) stemming from pseudoisocyanine (PIC) molecules adsorbed on Ag nanoparticles by using optical trapping techniques in aqueous solution. By focusing a near-infrared (NIR) laser beam with linear polarization in addition to a visible excitation laser beam, the PIC concentration necessary to detect SERS is dramatically reduced to 10−14 M, whereas no effect is confirmed with circular polarization. This finding suggests that optical trapping by a linearly polarized laser beam induces the formation of Ag nanoaggregates that incorporate PIC molecules at a specific nanosite where the localized electromagnetic (EM) field is strongly enhanced. The optical force exerted on these Ag nanoparticles and PIC molecules is discussed on the basis of experimental results for the laser-polarization dependence of the SERS and Rayleigh scattering spectra.

Single colloid electrophoresis

I. Semenov, O. Otto, G. Stober, P. Papadopoulos, U.F. Keyser and F. Kremer

Optical tweezers enable one to trap a single particle without any mechanical contact and to measure its position and the forces acting on it with high resolution (±4 nm, ±160 fN). Taking advantage of a specially designed microfluidic cell the electrophoretic response of the colloid under study and the electroosmotic effect on the surrounding medium are determined using the identical colloid. The former is found to be by more than one order of magnitude larger than the electroosmotic effect. It is shifted in phase with respect to the external field, hence giving rise to a complex electrophoretic mobility which can be theoretically described by a strongly damped driven harmonic oscillator model. By exchanging the medium surrounding the colloid it is possible to deduce the (KCl) concentration dependence of the single colloid electrophoretic response. The results are compared with conventional Zetasizer measurements.

Propulsion of Gold Nanoparticles with Surface Plasmon Polaritons: Evidence of Enhanced Optical Force from Near-Field Coupling between Gold Particle an

Kai Wang, Ethan Schonbrun and Kenneth B. Crozier

We experimentally demonstrate the enhanced propulsion of gold nanoparticles by surface plasmon polaritons (SPPs). Three dimensional finite difference time domain (FDTD) simulations indicate considerably enhanced optical forces due to the field enhancement provided by SPPs and the near-field coupling between the gold particles and the film. This coupling is an important part of the enhanced propulsion phenomenon. Finally, the measured optical force is compared with that predicted by FDTD simulations and proven to be reasonable.

Thursday, July 23, 2009

Interaction of Gaussian beam with near-spherical particle: an analytic-numerical approach for assessing scattering and stresses

Lars Boyde, Kevin J. Chalut, and Jochen Guck

We derive a straightforward theoretical method to determine the electromagnetic fields for the incidence of a monochromatic laser beam on a near-spherical dielectric particle. The beam-shape coefficients are obtained from the radial laser fields and expressed as a finite series in a form that has, to our knowledge, not been published before. Our perturbation approach to solve Maxwell's equations in spherical coordinates employs two alternative techniques to match the boundary conditions: an analytic approach for small particles with low eccentricity and an adapted point-matching method for larger spheroids with higher aspect ratios. We present results for the internal and external fields, scattering intensities, and stresses exerted on the particle. While similarly accurate as others, our approach is easily implemented numerically and thus particularly useful in praxis, e.g., for analyzing optical traps, such as the optical stretcher.

Optofluidic particle concentration by a long-range dual-beam trap

S. Kühn, E. J. Lunt, B. S. Phillips, A. R. Hawkins, and H. Schmidt

Ultrahigh sensitivity detection of particles in solution implies the ability to detect at very low concentrations. At the single-particle level, this is achieved through fluorescence detection, reaching down to single fluorophores. Sensitivity may also be improved by concentrating many particles into a compact cluster, thus “integrating” the signal of many particles. We show how both ways can be combined on an optofluidic chip in a fully planar geometry utilizing counterpropagating liquid-core waveguide modes to form a loss-based optical trap. This all-optical concentrator can increase the concentration of particles by more than 2 orders of magnitude and also provides a convenient, nondispersive means of transport for particle ensembles.

Lorentz force and radiation pressure on a spherical cloak

Hongsheng Chen, Baile Zhang, Yu Luo, Brandon A. Kemp, Jingjing Zhang, Lixin Ran, and Bae-Ian Wu

The mechanical behavior of a transformation based spherical cloak under wave illumination is derived. We show that the equatorial region of the cloak is subject to much higher stress than the polar regions, where the polar axis is defined along the wave propagation direction. These forces do not exist before transformation but stem from the squeezed electromagnetic space. The trajectory of the ray can be interpreted as a result of the recoil force that the cloak exerts upon the ray. The total radiation pressure on an ideal cloak is shown to be exactly zero, effecting a stationary cloak.

Detecting Sequential Bond Formation Using Three-Dimensional Thermal Fluctuation Analysis

Tobias F. Bartsch, Samo Fiinger, Martin D. Kochanczyk, Rongxin Huang, Alexandr Joná, Ernst-Ludwig Florin

We present a novel experimental method that solves two key problems in nondestructive mechanical studies of small biomolecules at the single-molecule level, namely the confirmation of single-molecule conditions and the discrimination against nonspecific binding. A biotin avidin ligand-receptor couple is spanned between a glass slide and a 1 mu m latex particles using short linker molecules. Optical tweezers are used to initiate bond formation and to follow the particles thermal position fluctuations with nanometer spatial and microsecond temporal resolution. Here we show that each step in the specific binding process leads to an abrupt change in the magnitude of the particle's thermal position fluctuations, allowing us to count the number of bonds formed one by one. Moreover, three-dimensional position histograms calculated from the particle's fluctuations can be separated into well-defined categories reflecting different binding conditions (single specific, multiple specific, nonspecific). Our method brings quantitative mechanical single-molecule studies to the majority of proteins, paving the way for the investigation of a wide range of phenomena at the single-molecule level.

Probing Dynein and Kinesin Stepping with Mechanical Manipulation in a Living Cell

Peter A. Sims, X. Sunney Xie

We report a label free assay for simultaneous optical manipulation and tracking of endogenous lipid droplets as actively transported cargoes in a living mammalian cell with sub milli-second time resoultion. Using an EM-Cd camera as a highly sensitive quadrant detector. we can detect steps of dynein- and kinesin-driven cargoes under known force loads. We can distinguish single and multiple motor-driven cargoes and show that the stall forces for inward and outward transported cargoes are similar By combining the stall force observable with the ability to detect individual steps, we can characterize kinesin and dynein-driven active transport in difference force regimes.

Monday, July 20, 2009

Intracellular nanomanipulation by a photonic-force microscope with real-time acquisition of a 3D stiffness matrix

E Bertseva, A S G Singh, J Lekki, P Thévenaz, M Lekka, S Jeney, G Gremaud, S Puttini,W Nowak, G Dietler, L Forró, M Unser and A J Kulik

A traditional photonic-force microscope (PFM) results in huge sets of data, which requires tedious numerical analysis. In this paper, we propose instead an analog signal processor to attain real-time capabilities while retaining the richness of the traditional PFM data. Our system is devoted to intracellular measurements and is fully interactive through the use of a haptic joystick. Using our specialized analog hardware along with a dedicated algorithm, we can extract the full 3D stiffness matrix of the optical trap in real time, including the off-diagonal cross-terms. Our system is also capable of simultaneously recording data for subsequent offline analysis. This allows us to check that a good correlation exists between the classical analysis of stiffness and our real-time measurements. We monitor the PFM beads using an optical microscope. The force-feedback mechanism of the haptic joystick helps us in interactively guiding the bead inside living cells and collecting information from its (possibly anisotropic) environment. The instantaneous stiffness measurements are also displayed in real time on a graphical user interface. The whole system has been built and is operational; here we present early results that confirm the consistency of the real-time measurements with offline computations.

Extreme axial optical force in a standing wave achieved by optimized object shape

J. Trojek, V. Karásek, and P. Zemanek

Standing wave optical trapping offers many useful advantages in comparison to single beam trapping, especially for submicrometer size particles. It provides axial force stronger by several orders of magnitude, much higher axial trap stiffness, and spatial confinement of particles with higher refractive index. Mainly spherical particles are nowadays considered theoretically and trapped experimentally. In this paper we consider prolate objects of cylindrical symmetry with radius periodically modulated along the axial direction and we present a theoretical study of optimized objects shapes resulting in up to tenfold enhancement of the axial optical force in comparison with the original unmodulated object shape. We obtain analytical formulas for the axial optical force acting on low refractive index objects where the light scattering by the object is negligible. Numerical results based on the coupled dipole method are presented for objects with higher refractive indices and they support the previous simplified analytical conclusions.

Quantifying Noise in Optical Tweezers by Allan Variance

Fabian Czerwinski, Andrew C. Richardson, and Lene B. Oddershede

Much effort is put into minimizing noise in optical tweezers experiments because noise and drift can mask fundamental behaviours of, e.g., single molecule assays. Various initiatives have been taken to reduce or eliminate noise but it has been difficult to quantify their effect. We propose to use Allan variance as a simple and efficient method to quantify noise in optical tweezers setups.We apply the method to determine the optimal measurement time, frequency, and detection scheme, and quantify the effect of acoustic noise in the lab. The method can also be used on-the-fly for determining optimal parameters of running experiments.

Microelectrophoresis in a laser trap: A platform for measuring electrokinetic interactions and flow properties within microstructures

V. Kahl, A. Gansen, R. Galneder, and J. O. Rädler

We describe a combination of microelectrophoresis and laser-trap methodology to accurately measure the electric force acting on a charged microsphere which is trapped in an optical tweezer. This field/trap apparatus allows measuring of the zeta potential with submillivolt accuracy and high temporal resolution. The combination with stop-flow techniques in principle provides a mean to observe adsorption or enzyme kinetics with single molecule sensitivity. We show that it is possible to accurately profile the position and frequency dependent hydrodynamic and electro-osmotic flow inside a microchannel structure of dimensions typically used in microfluidic applications without the need of fluorescent markers. We found good agreement to the theory of electrophoretic flow when retardation effects for rapidly alternating electric fields are included.

Monday, July 13, 2009

Brownian vortexes

Bo Sun, Jiayi Lin, Ellis Darby, Alexander Y. Grosberg, and David G. Grier

Mechanical equilibrium at zero temperature does not necessarily imply thermodynamic equilibrium at finite temperature for a particle confined by a static but nonconservative force field. Instead, the diffusing particle can enter into a steady state characterized by toroidal circulation in the probability flux, which we call a Brownian vortex. The circulatory bias in the particle's thermally driven trajectory is not simply a deterministic response to the solenoidal component of the force but rather reflects interplay between advection and diffusion in which thermal fluctuations extract work from the nonconservative force field. As an example of this previously unrecognized class of stochastic heat engines, we consider a colloidal sphere diffusing in a conventional optical tweezer. We demonstrate both theoretically and experimentally that nonconservative optical forces bias the particle's fluctuations into toroidal vortexes whose circulation can reverse direction with temperature or laser power.

Multipoint viscosity measurements in microfluidic channels using optical tweezers

Stephen Keen, Alison Yao, Jonathan Leach, Roberto Di Leonardo, Chris Saunter, Gordon Love, Jonathan Cooper and Miles Padgett

We demonstrate the technique of multipoint viscosity measurements incorporating the accurate calibration of micron sized particles. We describe the use of a high-speed camera to measure the residual motion of particles trapped in holographic optical tweezers, enabling us to calculate the fluid viscosity at multiple points across the field-of-view of the microscope within a microfluidic system.

Optical trapping studies of colloidal interactions in liquid films

R. Di Leonardo, F. Ianni, F. Saglimbeni, G. Ruocco, S. Keen, J. Leach and M. Padgett

A tightly focused light beam can stably trap small objects in three dimensions. Using spatial light modulators we can engineer the wavefront of a laser beam in such a way that, once focused by a microscope objective, it produces an almost arbitrary light intensity distribution. Arrays of optical traps can be thus generated in three-dimensional space and dynamically reconfigured. Optical traps allow direct manipulation and sensing on those length and energy scale that are most relevant in many colloidal processes. In the presence of long range interactions optical traps actually provide a unique tool of direct investigation allowing the precise relative positioning of particle pairs, far from boundaries or other particles. We have used optical trapping to directly measure two very long range interactions governing colloidal dynamics in two-dimensional fluid films: hydrodynamic interactions, which are found to decay logarithmically slow with distance, and capillary forces, whose intensity decreases as a power law with an exponent slightly smaller than one.

Wednesday, July 1, 2009

Published Papers

Published papers on Optical Tweezers added in this Blog for the months of 01-06 of 2009.
Here is the results for these first half year of the published papers on optical tweezers, micromanipulation and trapping.

The top 10 Journals are:
Optics Express 15.2%
Journal of Optics A 6.0%
Applied Optics 4.0%
Optics Letters 4.0%
Physical Review A 4.0%
Applied Physics Letters 3.3%
Lab on a Chip 3.3%
Physical Review Letters 3.3%
Nano Letters 2.6%
Physical Review E 2.6%

Below is also a cloud tag from the words found in the title and abstracts:

analysis, based, beam, cells, colloidal, complex, control, dielectric, different, DNA, energy, experimental, field, focal, force, holographic, intensity, interaction, laser, light, manipulation, measurements, mechanical, method, model, molecule, momentum, motion, observed, optical, particles, phase, position, potential, power, probe, properties, results, single, size, surface, technique, through, time, transition, trapping, tweezers