Monday, March 30, 2009

Published papers on Optical Tweezers in the months of(01-03)/2009)

Here is the results for these first three months of the published papers on optical tweezers, micromanipulation and trapping.

The top Journals (more than 5% hits) are: 
Optics Express             20.3%
Journal of Optics A       9.5%
Lab on a Chip                 6.8%
Optics Letters                6.8%

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

analysis, angular, array, beam, binding, cells, complex, control, different, DNAdynamics, energy, experimental, focused, forces, holographic, interaction, laser, light, manipulation, measurements, mechanical, method, microfluidic, model, motors, optical, particles, plane, potential, power, probe, properties, results, single, size, spatialsphericalstable, stiffnesssurface, technique, theoreticaltimetorquetransporttrappingtweezers

Friday, March 27, 2009

Tailoring the excitation of localized surface plasmon-polariton resonances by focusing radially-polarized beams

Nassiredin M. Mojarad and Mario Agio

We study the interaction of focused radially-polarized light with metal nanospheres. By expanding the electromagnetic field in terms of multipoles, we gain insight on the excitation of localized surface plasmon-polariton resonances in the nanoparticle. We show that focused radially-polarized beams offer more opportunities than a focused plane wave or a Gaussian beam for tuning the near- and far-field system response. These results find applications in nano-optics, optical tweezers, and optical data storage.


The effect of Mie resonances on trapping in optical tweezers: reply

Timo A. Nieminen, Alexander B. Stilgoe, Vincent L. Loke, Norman R. Heckenberg, and Halina Rubinsztein-Dunlop

We show that errors in the calculation of spherical Hankel functions for very small size parameters does not affect the calculation of optical trapping forces; predicted forces agree with the Rayleigh formula.


The effect of Mie resonances on trapping in optical tweezers: comment

Bo Sun and David G. Grier

Recently, Stilgoe, et al., [Opt. Express 16, 15039 (2008)] reported calculations of the force on an optically trapped sphere performed using the “optical tweezers toolbox”. This software suffers from numerical inaccuracies that lead to qualitative errors in the state diagram for stable trapping, particularly for spheres smaller than the wavelength of light.


Numerical simulation of an optical chromatographic separator

Alex Terray, H. D. Ladouceur, Mark Hammond, and Sean J. Hart

Optical chromatography achieves microscale optical manipulation through the balance of optical and hydrodynamic forces on micron sized particles entrained in microfluidic flow traveling counter to the propagation of a mildly focused laser beam. The optical pressure force on a particle is specific to each particle’s size, shape and refractive index. So far, these properties have been exploited in our lab to concentrate, purify and separate injected samples. But as this method advances into more complex optofluidic systems, a need to better predict behavior is necessary. Here, we present the development and experimental verification of a robust technique to simulate particle trajectories in our optical chromatographic device. We also show how this new tool can be used to gather better qualitative and quantitative understanding in a two component particle separation.


Theoretical investigation of the transverse optical force between a silicon nanowire waveguide and a substrate

W. H. P. Pernice, Mo Li, and H. X. Tang

We present a study of transverse optical forces arising in a free-standing silicon nanowire waveguide. A theoretical framework is provided for the calculation of the optical forces existing between a waveguide and a dielectric substrate. The force is evaluated using a numerical procedure based on finite-element simulations. In addition, an analytical formalism is developed which allows for a simple approximate analysis of the problem. We find that in this configuration optical forces on the order of pN can be obtained, sufficient to actuate nano-mechanical devices.


Radiation force of coherent and partially coherent flat-topped beams on a Rayleigh particle

Chengliang Zhao, Yangjian Cai, Xuanhui Lu, and Halil T. Eyyuboğlu

Propagations of coherent and partially coherent flat-topped beams through a focusing optical system are formulated. The radiation force on a Rayleigh dielectric sphere induced by focused coherent and partially coherent flat-topped beams is investigated theoretically. It is found that we can increase the transverse trapping range at the planes near the focal plane by increasing the flatness (i.e., beam order) of the flat-topped beam, and increase the transverse and longitudinal trapping ranges at the focal plane by decreasing the initial coherence of the flat-topped beam. Moreover the trapping stiffness of flat-topped beam becomes lower as the beam order increases or the initial coherence decreases. The trapping stability is also analyzed.


Nanonewton Force Generation and Detection Based on a Sensitive Torsion Pendulum

Chen, S.-J.; Pan, S.-S.

In this paper, we introduce an experiment based on a sensitive torsion pendulum for measuring and calibrating small forces at the nanonewton scale. The force standard for calibration is the universal gravitation between four masses separated by known distances. It is realized by two test masses suspended as part of the torsion pendulum and two source masses on a rotation table. Two force-generation mechanisms, namely, the optical force from the radiation pressure and the electrostatic force by the capacitive actuation unit, are designed and will be calibrated by the gravitational force. We present our recent results on radiation pressure measurements and describe the design of the capacitive displacement sensing/actuating unit.


Thursday, March 26, 2009

Optical angular momentum transfer by Laguerre-Gaussian beams

Stephen H. Simpson and Simon Hanna

It is well known that Laguerre-Gaussian beams carry angular momentum and that this angular momentum has a mechanical effect when such beams are incident on particles whose refractive indices differ from those of the background medium. Under conditions of tight focusing, intensity gradients arise that are sufficiently large to trap micrometer-sized particles, permitting these mechanical effects to be observed directly. In particular, when the particles are spherical and absorbing, they rotate steadily at a rate that is directly proportional to the theoretical angular momentum flux of the incident beam. We note that this behavior is peculiar to absorbing spheres. For arbitrary, axially placed particles the induced torque for rotation angle ζ is shown to be Γz=Asin(2ζ+δ)+B, where A, B, and δ are constants that are determined by the mechanisms coupling optical and mechanical angular momentum. The resulting behavior need not be directly related to the total angular momentum in the beam but can, nonetheless, be understood in terms of an appropriate torque density. This observation is illustrated by calculations of the torque induced in optically and geometrically anisotropic particles using a T-matrix approach.


Monopole woodpile photonic crystal modes for light-matter interaction and optical trapping

Lingling Tang and Tomoyuki Yoshie

Two types of ultra-high-Q monopole modes are designed in a woodpile three-dimensional photonic crystal. The unit cell size modulation is applied to a woodpile photonic crystal waveguide in a complete photonic band gap. A monopole mode overlapping with a dielectric rod is designed for solid-state sub-wavelength-scale light-matter interaction devices such as nanolasers, cavity-QED and optical switches, whereas another type of monopole mode overlapping with vacuum is designed for optical trapping experiments. For the mode overlapping with vacuum, the mode volume is as small as 0.4 cubic half-wavelengths.


Interferometric 3D tracking of several particles in a scanning laser focus

Michael Speidel, Lars Friedrich, and Alexander Rohrbach

High-Speed tracking of several particles allows measuring dynamic long-range interactions relevant to biotechnology and colloidal physics. In this paper we extend the successful technique of 3D back-focal plane interferometry to oscillating laser beams and show that two or more particles can be trapped and tracked with a precision of a few nanometers in all three dimensions. The tracking rate of several kHz is only limited by the scan speed of the beam steering device. Several tests proof the linearity and orthogonality of our detection scheme, which is of interest to optical tweezing applications and various metrologies. As an example we show the position cross-correlations of three diffusing particles in a scanning line optical trap.


Optically adjustable light filaments generated by a compact laser convertor

V. Kollárová, T. Medrik, R. Celechovský, V. Chlup, Z. Bouchal, A. Pochylý, M. Kalman, and T. Kubina

In the paper, the geometrical parameters and energetics of the extremely narrow pseudo-nondiffracting beams with the spot size of several micrometers are examined. The main attention is focused on design, realization and testing of the set-up enabling conversion of the laser diode beam or the fiber mode to the narrow Bessel-Gauss beam whose spot can be continuously relocated across the plane perpendicular to the beam propagation direction. Application of the laser convertor to the optical manipulation is demonstrated on experiments enabling transport of microparticles along a desired trajectory.


Focusing surface plasmon polariton trapping of colloidal particles

Zheyu Fang, Feng Lin, Shan Huang, Wentao Song, and Xing Zhu

We report focusing surface plasmon polariton (SPP) trapping of colloidal particles without optical interactions. Using a silver nanostructure, we demonstrate SPP launching and propagation under a p-polarized incident laser. In-plane Fresnel zone plate (FZP) is used to focus the SPP waves. Colloidal particles can be trapped at the silver nanostructure in the illumination region and the FZP focus area by focusing SPP force. The SPP tweezers open new perspectives in the subwavelength trapping and applications to lab-on-chip devices. 

Optical guiding of absorbing nanoclusters in air

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

We suggest a novel approach in all-optical trapping employinga photophoretic force for manipulation of absorbing particles in open air.We demonstrate experimentally the robust three-dimensional guiding, overthe distances of a few millimeters, of agglomerates of carbon nanoparticleswith the size spanned from 100 nm to 10μm, as well as their accelerationup to velocities of 1 cm/sec. We achieve stable positioning and guiding ofparticles as well as simultaneous trapping of a large number of particles in adual beam optical trap created by two counter-propagating and co-rotatingoptical vortex beams.

Jamming prokaryotic cell-to-cell communications in a model biofilm

Winston Timp, Utkur Mirsaidov, Paul Matsudaira and Gregory Timp

We report on the physical parameters governing prokaryotic cell-to-cell signaling in a model biofilm. The model biofilm is comprised of bacteria that are genetically engineered to transmit and receive quorum-sensing (QS) signals. The model is formed using arrays of time-shared, holographic optical traps in conjunction with microfluidics to precisely position bacteria, and then encapsulated within a hydrogel that mimics the extracellular matrix. Using fluorescent protein reporters functionally linked to QS genes, we assay the intercellular signaling. We find that there isn't a single cell density for which QS-regulated genes are induced or repressed. On the contrary, cell-to-cell signaling is largely governed by diffusion, and is acutely sensitive to mass-transfer to the surroundings and the cell location. These observations are consistent with the view that QS-signals act simply as a probe measuring mixing, flow, or diffusion in the microenvironment of the cell.


Wednesday, March 25, 2009

Optical force sensor array in a microfluidic device based on holographic optical tweezers

Kai Uhrig, Rainer Kurre, Christian Schmitz, Jennifer E. Curtis, Tamás Haraszti, Anabel E.-M. Clemen and Joachim P. Spatz

Holographic optical tweezers (HOT) are a versatile technology, with which complex arrays and movements of optical traps can be realized to manipulate multiple microparticles in parallel and to measure the forces affecting them in the piconewton range. We report on the combination of HOT with a fluorescence microscope and a stop-flow, multi-channel microfluidic device. The integration of a highspeed camera into the setup allows for the calibration of all the traps simultaneously both using Boltzmann statistics or the power spectrum density of the particle diffusion within the optical traps. This setup permits complete spatial, chemical and visual control of the microenvironment applicable to probing chemo-mechanical properties of cellular or subcellular structures. As an example we constructed a biomimetic, quasi-two-dimensional actin network on an array of trapped polystyrene microspheres inside the microfluidic chamber. During crosslinking of the actin filaments by Mg2+ ions, we observe the build up of mechanical tension throughout the actin network. Thus, we demonstrate how our integrated HOT-microfluidics platform can be used as a reconfigurable force sensor array with piconewton resolution to investigate chemo-mechanical processes.


Array-based optical nanolithography using optically trapped microlenses

Euan McLeod and Craig B. Arnold

Current demands on optical nanolithography require the ability to rapidly and cost-effectively write arbitrary patterns over large areas with sub-diffraction limit feature sizes. The challenge in accomplishing this with arrays of near-field probes is maintaining equal separations between the substrate and each probe, even over non-planar substrates. Here we demonstrate array-based laser nanolithography where each probe is a microsphere capable of fabricating 100 nm structures using 355 nm light when self-positioned near a surface by Bessel beam optical trapping. We achieve both a feature size uniformity and relative positioning accuracy better than 15 nm, which agrees well with our model. Further improvements are possible using higher power and/or narrower Bessel beam optical traps.


Force mapping of an extended light pattern in an inclined plane: Deterministic regime

Alejandro V. Arzola, Karen Volke-Sepúlveda, and José L. Mateos

We present a full quantitative mapping of the non-linear optical trapping force associated to an extended interference pattern of fringes as a function of the position. To map this force, we studied the dynamics of microscopic spherical beads of different sizes (8, 10 and 14.5 microns in diameter) moving through the light pattern. For this range of particle sizes, the system is overdamped due to the viscous drag and the effect of thermal noise is negligible. The novel experimental approach consists in tilting the sample cell a small angle with respect to the horizontal, thus we have a deterministic particle in an inclined plane. The combined action of the optical force and gravity gives rise to a washboard potential. We compared our experimental results with a ray optics model and found a good quantitative agreement. For each size of the microsphere we studied different spatial periods of the interference fringes.


Dynamics of the bacterial flagellar motor with multiple stators

Giovanni Meacci and Yuhai Tu

The bacterial flagellar motor drives the rotation of flagellar filaments and enables many species of bacteria to swim. Torque is generated by interaction of stator units, anchored to the peptidoglycan cell wall, with the rotor. Recent experiments [Yuan J, Berg HC (2008) Proc Natl Acad Sci USA 105:1182–1185] show that at near-zero load the speed of the motor is independent of the number of stators. Here, we introduce a mathematical model of the motor dynamics that explains this behavior based on a general assumption that the stepping rate of a stator depends on the torque exerted by the stator on the rotor. We find that the motor dynamics can be characterized by two timescales: the moving-time interval for the mechanical rotation of the rotor and the waiting-time interval determined by the chemical transitions of the stators. We show that these two timescales depend differently on the load, and that their cross-over provides the microscopic explanation for the existence of two regimes in the torque-speed curves observed experimentally. We also analyze the speed fluctuation for a single motor by using our model. We show that the motion is smoothed by having more stator units. However, the mechanism for such fluctuation reduction is different depending on the load. We predict that the speed fluctuation is determined by the number of steps per revolution only at low load and is controlled by external noise for high load. Our model can be generalized to study other molecular motor systems with multiple power-generating units.


Three-dimensional force measurements in optical tweezers formed with high-NA micromirrors

Fabrice Merenda, Mathieu Grossenbacher, Sylvia Jeney, László Forró, and René-Paul Salathé

The three-dimensional trap stiffness of optical tweezers formed with high-NA micromirrors is investigated by back-focal-plane interferometry and power spectrum analysis. Normalized stiffness values of κxy/Ptrap=1.2(μN/m)/mW and κz/Ptrap=0.52(μN/m)/mW in the transverse and axial directions, respectively, have been measured for polystyrene spheres with a radius of 1.03 μm. Compared with high-NA microscope objectives, micromirrors achieve much better trapping performances, particularly in the axial direction.


Functional gel-microbead manipulated by optical tweezers for local environment measurement in microchip

Hisataka Maruyama, Toshio Fukuda and Fumihito Arai

A novel on-chip environment measurement with functional gel-microtool was developed. Environment measurement gel-microtool was fabricated by connecting the gel-microbeads impregnated with indicators in a microchip. In this artcle, Bromothymol blue (BTB) and Bromocresol green (BCG) were employed as pH indicators. BTB and BCG have the different indicator range. Rhodamine B is temperature sensitive fluorescent dye and is used for temperature measurement. Gel-microbead is made by salting-out of hydrophilic photo-crosslinkable resin and is manipulated by optical tweezers. Moreover, gel-microbead is polymerized by UV illumination and connected to other gel-microbead under an electrolyte solution. The connection of gel-microbeads is performed by contact of gel-microbeads under UV illumination. Environment measurement gel-microtool with an arbitrary shape is fabricated by connection of the gel-microtool impregnated with arbitrary indicator. Multiple environments measurement gel-microtool included with several indicators is realized by assembly of the gel-microbeads impregnated with different indicators. Environment measurement is performed by detecting the color and the fluorescence intensity of each gel-microbead. We succeeded in the on-chip fabrication of the environment measurement gel-microtools such as circular pH measurement gel-microtool and wide range pH measurement gel-microtool in a microchip.


Automated focusing of nuclei for time lapse experiments on single cells using holographic optical tweezers

Emma O. Eriksson, David Engström, Jan Scrimgeour, and Mattias Goksör

Experiments on single cells are currently gaining more and more interest. Single cell studies often concerns the spatio-temporal distribution of fluorescent proteins inside living cells, visualized using fluorescence microscopy. In order to extract quantitative information from such experiments it is necessary to image the sample with high spatial and temporal resolution while keeping the photobleaching to a minimum. The analysis of the spatial distribution of proteins often requires stacks of images at each time point, which exposes the sample to unnecessary amounts of excitation light. In this paper we show how holographic optical tweezers combined with image analysis can be used to optimize the axial position of trapped cells in an array in order to bring the nuclei into a single imaging plane, thus eliminating the need for stacks of images and consequently reducing photobleaching. This allows more images to be collected, as well as increasing the time span and/or the time resolution in time lapse studies of single cells.


Forces and Transport Velocities for a Particle in a Slot Waveguide

Allen H. J. Yang, Tadsanapan Lerdsuchatawanich and David Erickson

Optofluidic transport seeks to exploit the high-intensity electromagnetic energy in waveguiding structures to manipulate nanoscopic matter using radiation pressure and optical trapping forces. In this paper, we present an analysis of optical trapping and transport of sub-100 nm polystyrene and gold nanoparticles in silicon slot waveguides. This study focuses on the effect of particle size, particle refractive index, and slot waveguide geometry on trapping stability and the resulting transport speed. Our results indicate that stable trapping and transport can be achieved for objects as small as 10 or 20 nm in diameter with as much as a 100 fold enhancement in trapping stiffness over the state of the art.


Tuesday, March 24, 2009

Mechanical property analysis of stored red blood cell using optical tweezers

Yanjie Li, Cheng Wen, Huimin Xie, Anpei Ye and Yajun Yin

The deformation of human red blood cells subjected to direct stretching by optical tweezers was analyzed. The maximum force exerted by optical tweezers on the cell via a polystyrene microbead 5 μm in diameter was 315 pN. Digital image correlation (DIC) method was introduced to calculate the force and the deformation of the cell for the first time. Force–extension relation curves of the biconcave cell were quantitatively assessed when erythrocytes were stored in Alsever's Solution for 2 days, 5 days, 7 days and 14 days respectively. Experiment results demonstrated that the deformability of red blood cells was impaired with the stored time.


Thursday, March 19, 2009

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets

Jason R. Butler, Jon B. Wills, Laura Mitchem, Daniel R. Burnham, David McGloin and Jonathan P. Reid

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis.


Motion control of low-index microspheres in liquid based on optical repulsive force of a focused beam array

Masaya Miyazaki and Yoshio Hayasaki

In the optical tweezers technique a microdielectric object with a higher refractive index than a surrounding liquid is trapped near the focal point of a laser beam, whereas an object with a lower refractive index is subjected to a repulsive force from the focused beam. We demonstrate that the optical repulsive forces are generated with a focused-beam array dynamically formed by a computer-generated hologram displayed on a liquid-crystal spatial light modulator. We also demonstrate that the optical repulsive forces perform a size selection and flow control of hollow glass microspheres.


Physical manipulation of single-molecule DNA using microbead and its application to analysis of DNA–protein interaction

Hirofumi Kurita, Hachiro Yasuda, Kazunori Takashima, Shinji Katsura and Akira Mizuno

We carried out an individual DNA manipulation using an optical trapping for a microbead. This manipulation system is based on a fluorescent microscopy equipped with an IR laser. Both ends of linear DNA molecule were labeled with a biotin and a thiol group, respectively. Then the biotinylated end was attached to a microbead, and the other was immobilized on a thiol-linkable glass surface. We controlled the form of an individual DNA molecule by moving the focal point of IR laser, which trapped the microbead. In addition, we applied single-molecule approach to analyze DNA hydrolysis. We also used microchannel for single-molecule observation of DNA hydrolysis. The shortening of DNA in length caused by enzymatic hydrolysis was observed in real-time. The single-molecule DNA manipulation should contribute to elucidate detailed mechanisms of DNA–protein interactions.


Influence of the state of light on the optically induced interparticle interaction

Justo Rodríguez and David L. Andrews

A general expression for the energy of interparticle interaction induced by an arbitrary mode of light is determined using quantum electrodynamics, and it is shown that the Casimir-Polder potential is included within this quantum result. Equations are also derived for the corresponding coupling induced by multimode number states of light, and the dependence of the pair energy on the Poynting vector and polarization state is determined. Attention is then focused on the interactions between particles trapped in counterpropagating coherent beams, both with and without interference, and it is shown that the results afford insights into the multiparticle structures that can be optically fabricated with counterpropagating input. Brief consideration is also given to the effect of squeezing the optical coherent state. Extending previous studies of optical binding in Laguerre-Gaussian beams, results are given for the case of particles trapped at radially different locations within the beam structure. Finally, consideration is given to interparticle interactions induced by broadband light, and it is shown how the length of optically fabricated particle chains can be controlled by the use of wavelength filters.


Trap profiles of projector based optoelectronic tweezers (OET) with HeLa cells

Steven L. Neale, Aaron T. Ohta, Hsan-Yin Hsu, Justin K. Valley, Arash Jamshidi, and Ming C. Wu

In this paper we present trap profile measurements for HeLa cells in Optoelectronic Tweezers (OET) based on a data projector. The data projector is used as a light source to illuminate amorphous Si creating virtual electrodes which are used to trap particles through dielectrophoresis. We show that although the trap stiffness is typically greater at the edges of the optical spot it is possible to create a trap with constant trap stiffness by reducing the trap’s size until it is similar to the object being trapped. We have successfully created a trap for HeLa cells with a constant trap stiffness of 3x10-6 Nm-1 (capable of moving the cell up to 50μms-1) with a 12μm diameter trap. We also calculate the depth of the potential well that the cell will experience due to the trap and find that it to be 1.6x10-16J (4x104 kBT).


Flagella-generated forces reveal gear-type motor in single cells of the green alga, Chlamydomonas reinhardtii

Jacinta S. D’Souza, Mohanram Gudipati, Jayashree A. Dharmadhikari, Aditya K. Dharmadhikari, Abhishek Kashyap, Manaswini Aiyer, Usha Rao, Deepak Mathur and Basuthkar J. Rao

Optically trapped single cells of the biflagellated, green alga, Chlamydomonas reinhardtii, rotate. The rotational dynamics of trapped wild-type and mutant cells show that functional flagella play a decisive role: the entire flagellar apparatus (central microtubules, radial spokes, and dynein arms) is involved. Any aberration in this apparatus leads to non-functionality, indicating a gear-type mechanism. The translational and rotational motions of the wild-type and mutant cells do not differ significantly. Optical forces alone do not play a vital role in the rotational dynamics of this cellular motor, making them useful as probes of the internal dynamics without external influence.


Friday, March 13, 2009

Recovery of Free Energy Branches in Single Molecule Experiments

Ivan Junier, Alessandro Mossa, Maria Manosas, and Felix Ritort

We present a method for determining the free energy of coexisting states from irreversible work measurements. Our approach is based on a fluctuation relation that is valid for dissipative transformations in partially equilibrated systems. To illustrate the validity and usefulness of the approach, we use optical tweezers to determine the free energy branches of the native and unfolded states of a two-state molecule as a function of the pulling control parameter. We determine, within 0.6kBT accuracy, the transition point where the free energies of the native and the unfolded states are equal.


Dynamic force spectroscopy of DNA hairpins: II. Irreversibility and dissipation

M Manosas, A Mossa, N Forns, J M Huguet and F Ritort

We investigate irreversibility and dissipation in single molecules that cooperatively fold/unfold in a two-state manner under the action of mechanical force. We apply path thermodynamics to derive analytical expressions for the average dissipated work and the average hopping number in two-state systems. It is shown how these quantities only depend on two parameters that characterize the folding/unfolding kinetics of the molecule: the fragility and the coexistence hopping rate. The latter has to be rescaled to take into account the appropriate experimental set-up. Finally we carry out pulling experiments with optical tweezers in a specifically designed DNA hairpin that shows two-state cooperative folding. We then use these experimental results to validate our theoretical predictions.


Dynamic force spectroscopy of DNA hairpins: I. Force kinetics and free energy landscapes

A Mossa, M Manosas, N Forns, J M Huguet and F Ritort

We investigate the thermodynamics and kinetics of DNA hairpins that fold/unfold under the action of applied mechanical force. We introduce the concept of the molecular free energy landscape and derive simplified expressions for the force dependent Kramers–Bell rates. To test the theory we have designed a specific DNA hairpin sequence that shows two-state cooperative folding under mechanical tension and carried out pulling experiments using optical tweezers. We show how we can determine the parameters that characterize the molecular free energy landscape of such sequences from rupture force kinetic studies. Finally we combine such kinetic studies with experimental investigations of the Crooks fluctuation relation to derive the free energy of formation of the hairpin at zero force.


Optical nanotrapping using cloaking metamaterial

Edward P. Furlani and Alexander Baev

We study the electromagnetic behavior of spherical semishell structures that have cloaking material properties proposed by Pendry, Schurig, and Smith [Science 312, 1780 (2006)]. We use three-dimensional full-wave time-harmonic field analysis to evaluate the field and dipolar force distribution produced by these structures in free-space under plane wave illumination. We show that the optical force in proximity to these structures is suitable for active and size-selective manipulation and trapping of neutral nanoscale particles.


Comparison of Faxén's correction for a microsphere translating or rotating near a surface

J. Leach, H. Mushfique, S. Keen, R. Di Leonardo, G. Ruocco, J. M. Cooper, and M. J. Padgett

Boundary walls in microfluidic devices have a strong influence on the fluid flow and drag forces on moving objects. The Stokes drag force acting on a sphere translating in the fluid is increased by the presence of a neighboring wall by a factor given by Faxén's correction. A similar increase in the rotational drag is expected when spinning close to a wall. We use optical tweezers to confirm the translational drag correction and report the hitherto unmeasured rotational equivalent. We findthat the corrections for the rotational motion is only required for particle-wall separations an order of magnitude shorter than that for the translational cases. These results are particularly significant in the use of optical tweezers for measuring viscosity on a picolitre scale.

Friday, March 6, 2009

Array illumination with minimal non-uniformity based on generalized phase contrast

Darwin Palima and Jesper Glückstad

The generalized phase contrast method (GPC) has been previously shown to be an efficient technique for generating array illumination and is thus highly suitable for such applications as dynamic multiple beam trapping and optical micromanipulation. However, projected arrays usually exhibit intensity roll-offs that may be undesirable for some applications. We show that the uniformity of GPC-generated array illuminations can be improved using intuitive corrections to the input spatial phase modulation, by increasing or decreasing it to respectively raise or lower the intensity of the corresponding output spots to improve uniformity. This is combined with matching corrections to the phase shift introduced by the phase contrast filter. Results from numerical experiments show that the array illumination uniformity error improves from over 40% to less than 1% while maintaining the efficiency prior to implementing corrections.

Simultaneous separation of polydisperse particles using an asymmetric nonperiodic optical stripe pattern

Yasuyuki Hayashi, Satoshi Ashihara, Tsutomu Shimura, and Kazuo Kuroda

We present simultaneous separation of polydisperse particles driven by an optical gradient force in the absence of microfluidic flow. The separation mechanism involves particle-size dependence of the potential landscape generated by a one-dimensional asymmetric optical stripe pattern. The outcome is that the particles align in different stacks according to their sizes. The dynamics of Brownian particles inside the optical potential landscapes are investigated theoretically and experimentally for various optical intensities and particle sizes. By introducing sequential changes in the optical profile, we also show that this technique allows semipassive arrangement of particles in arbitrary configurations.

Optical Tweezers Experiments Resolve Distinct Modes of DNA-Protein Binding

Micah J. McCauley, Mark C. Williams

Optical tweezers are ideally suited to perform force microscopy experiments that isolate a single biomolecule, which then provides multiple binding sites for ligands. The captured complex may be subjected to a spectrum of forces, inhibiting or facilitating ligand activity. In the following experiments, we utilize optical tweezers to characterize and quantify DNA binding of various ligands. High mobility group type B (HMGB) proteins, which bind to double-stranded DNA, are shown to serve the dual purpose of stabilizing and enhancing the flexibility of double stranded DNA. Unusual intercalating ligands are observed to thread into and lengthen the double-stranded structure. Proteins binding to both double- and single-stranded DNA, such as the alpha polymerase subunit of E. coli Pol III, are characterized, and the subdomains containing the distinct sites responsible for binding are isolated. Finally, DNA binding of bacteriophage T4 and T7 single-stranded DNA (ssDNA) binding proteins is measured for a range of salt concentrations, illustrating a binding model for proteins that slide along double-stranded DNA, ultimately binding tightly to ssDNA. These recently developed methods quantify both the binding activity of the ligand as well as the mode of binding.

Wednesday, March 4, 2009

Laser Microbeams and Optical Tweezers in Ageing Research

Paulius Grigaraviius, Karl Otto Greulich, Shamci Monajembashi

We show how a technique developed within the framework of physics and physical chemistry - in a true interdisciplinary approach - can answer questions in life sciences that are not solvable by using other techniques. Herein, we focus on blood-pressure regulation and DNA repair in ageing studies. Laser microbeams and optical tweezers are now established tools in many fields of science, particularly in the life sciences. A short glimpse is given on the wide field of non-age-research applications in life sciences. Then, optical tweezers are used to show that exerting a vertical pressure on cells representing the inner lining of blood vessels results in bursts of NO liberation concomitant with large changes in cell morphology. Repeated treatment of such human umbilical vein endothelial cells (HUVEC) results in stiffening, a hallmark of manifest high blood pressure, a disease primarily of the elderly. As a second application in ageing research, a laser microbeam is used to induce, with high spatial and temporal resolution, DNA damages in the nuclei of U2OS human osteosarcoma cells. A pairwise study of the recruitment kinetics of different DNA repair proteins reveals that DNA repair starts with non-homologous end joining (NHEJ), a repair pathway, and may only after several minutes switch to the error-free homologous recombination repair (HRR) pathway. Since DNA damages - when incorrectly repaired - accumulate with time, laser microbeams are becoming well-used tools in ageing research.

Optical manipulation of nanoparticles: a review

Maria Dienerowitz, Michael Mazilu, and Kishan Dholakia

Optical trapping is an established field for movement of micron-size objects and cells. However, trapping of metal nanoparticles, nanowires, nanorods and molecules has received little attention. Nanoparticles are more challenging to optically trap and they offer ample new phenomena to explore, for example the plasmon resonance. Resonance and size effects have an impact upon trapping forces that causes nanoparticle trapping to differ from micromanipulation of larger micron-sized objects. There are numerous theoretical approaches to calculate optical forces exerted on trapped nanoparticles. Their combination and comparison gives the reader deeper understanding of the physical processes in an optical trap. A close look into the key experiments to date demonstrates the feasibility of trapping and provides a grasp of the enormous possibilities that remain to be explored. When constructing a single-beam optical trap, particular emphasis has to be placed on the choice of imaging for the trapping and confinement of nanoparticles.

Surface-plasmon-resonance enhancement: effects on optical trapping and manipulation of nano-objects

Samarendra K. Mohanty, K. Divakar Rao, and Pradeep K. Gupta

The utilization of the enhanced local field near trapped metallic nanoparticles due to surface-plasmon resonance (SPR) for the optical trapping of dielectric fluorescent nano-objects is of considerable interest for single-molecule manipulation. Theoretical calculations as well as experimental measurements showed that even with moderate SPR based field enhancement factors, gradient force based trapping of fluorescent molecules would be rather difficult. While trapping of the fluorescent molecule at resonance wavelength showed decreased stiffness, at wavelengths far away from resonance, increase in stiffness was found which was attributed to interplay of SPR-enhanced absorption and gradient forces.

A basic swimmer at low Reynolds number

Marco Leoni, Jurij Kotar, Bruno Bassetti, Pietro Cicuta and Marco Cosentino Lagomarsino

Swimming and pumping at low Reynolds numbers are subject to the Scallop theorem, which states that there is no net fluid flow for time-reversible motions. Microscale organisms such as bacteria and cells are subject to this constraint, and so are existing and future artificial nano-bots or microfluidic pumps. We study a very simple mechanism to induce fluid pumping, based on the forced motion of three colloidal beads through a cycle that breaks time-reversal symmetry. Optical tweezers are used to vary the inter-bead distance. This model is inspired by a theoretical swimmer proposed by Najafi and Golestanian (A. Najafi and R. Golestanian,Phys. Rev. E, 2004, 69, 062901), but in this work the relative softness of the optical trapping potential introduces a new control parameter. We show that this system is able to generate flow in a controlled fashion, characterizing the model experimentally and numerically.

The reciprocal coordination and mechanics of molecular motors in living cells

Jeneva A. Laib, John A. Marin, Robert A. Bloodgood and William H. Guilford

Molecular motors in living cells are involved in whole-cell locomotion, contractility, developmental shape changes, and organelle movement and positioning. Whether motors of different directionality are functionally coordinated in cells or operate in a semirandom “tug of war” is unclear. We show here that anterograde and retrograde microtubule-based motors in the flagella ofChlamydomonas are regulated such that only motors of a common directionality are engaged at any single time. A laser trap was used to position microspheres on the plasma membrane of immobilized paralyzed Chlamydomonas flagella. The anterograde and retrograde movements of the microsphere were measured with nanometer resolution as microtubule-based motors engaged the transmembrane protein FMG-1. An average of 10 motors acted to move the microsphere in either direction. Reversal of direction during a transport event was uncommon, and quiescent periods separated every transport event, suggesting the coordinated and exclusive action of only a single motor type. After a jump to 32 °C, temperature-sensitive mutants of kinesin-2 (fla10) showed exclusively retrograde transport events, driven by 7 motors on average. These data suggest that molecular motors in living cells can be reciprocally coordinated to engage simultaneously in large numbers and for exclusive transport in a single direction, even when a mixed population of motors is present. This offers a unique model for studying the mechanics, regulation, and directional coordination of molecular motors in a living intracellular environment.

Assembly and force measurement with SPM-like probes in holographic optical tweezers

Ikin, L, Carberry, DM, Gibson, GM, Padgett, MJ, Miles, MJ

In this paper we demonstrate the optical assembly and control of scanning probe microscopy (SPM)-like probes, using holographic optical tweezers. The probes are formed from cadmium sulphide rods and silica microspheres, the latter providing explicit trapping handles. Calibration of the trap stiffness allows us to use a precise measure of probe displacement to calculate the applied forces. We demonstrate that the optically controlled probe can exert a force in excess of 60 pN, over an area of 1 x 10(-13) m(2), with a force sensitivity of 50 fN. We believe that probes similar to the ones presented here will have applications as nanotools in probing laser-sensitive cells/materials.

Measuring erythrocyte deformability with fluorescence, fluid forces, and optical trapping

Bambardekar, K, Dharmadhikari, AK, Dharmadhikari, JA , Mathur, D, Sharma, S

A laser-based method has been developed for experimentally probing single red blood cell (RBC) buckling and determining RBC membrane rigidity. Our method combines a liquid flow cell, fluorescence microscopy, and an optical-trap to facilitate simple measurements of the shear modulus and buckling properties of single RBCs, under physiological conditions. The efficacy of the method is illustrated by studying buckling behavior ofnormal and Plasmodium-infected RBCs, and the effect of Plasmodium falciparum–conditioned medium on normal, uninfected cells. Our simple method, which quantifiessingle-RBC deformability, may ease detection of RBC hematological disorders.

Spectral discrimination of live prostate and bladder cancer cell lines using Raman optical tweezers

Harvey, TJ, Faria, EC, Henderson, A, Gazi, E, Ward, AD,Clarke, NW , Brown, MD , Snook, RD, Gardner, P

An investigation into the use of Raman optical tweezers to study urological cell lines is reported, with the ultimate aim of determining the presence of malignant CaP cells in urine and peripheral fluids. To this end, we trapped and analyzed live CaP cells (PC-3) and bladder cells (MGH-U1), because both prostate and bladder cells are likely to be present in urine. The laser excitation wavelength of 514.5 nm was used, with Raman light collected both in back- and forward-scattering geometric configurations. For the backscattering configuration the same laser was used for trapping and excitation, while for forward scattering a 1064 nm laser provided the trapping beam. Analysis of cell-diameter distributions for cells analyzed suggested normal distribution of cell sizes, indicating an unbiased cell-selection criterion. Principal components analysis afforded discrimination of MGH-U1 and PC-3 spectra collected in either configuration, demonstrating that it is possible to trap, analyze, and differentiate PC-3 from MGH-U1 cells using a 514.5 nm laser. By loading plot analysis, possible biomolecules responsible for discrimination in both configurations were determined. Finally, the effect of cell size on discrimination was investigated, with results indicating that separation is based predominantly on cell type rather than cell size.

Dynamical density functional theory for colloidal dispersions including hydrodynamic interactions

M. Rex and H. Löwen

A dynamical density functional theory (DDFT) for translational Brownian dynamics is derived which includes hydrodynamic interactions. The theory reduces to the simple Brownian DDFT proposed by Marconi and Tarazona (U. Marini Bettolo Marconi and P. Tarazona, J. Chem. Phys. 110, 8032 (1999); J. Phys.: Condens. Matter 12, A413 (2000)) when hydrodynamic interactions are neglected. The derivation is based on Smoluchowski’s equation for the time evolution of the probability density with pairwise hydrodynamic interactions. The theory is applied to hard-sphere colloids in an oscillating spherical optical trap which switches periodically in time from a stable confining to an unstable potential. Rosenfeld’s fundamental measure theory for the equilibrium density functional is used and hydrodynamics are incorporated on the Rotne-Prager level. The results for the time-dependent density profiles are compared to extensive Brownian dynamics simulations which are performed on the same Rotne-Prager level and excellent agreement is obtained. It is further found that hydrodynamic interactions damp and slow the dynamics of the confined colloid cluster in comparison to the same situation with neglected hydrodynamic interactions.

Constant power optical tweezers with controllable torque

 Funk, M, Parkin, SJ, Stilgoe, AB, Nieminen, TA, Heckenberg, NR, Rubinsztein-Dunlop, H

We describe a means for controlling the spin angular-momentum flux of a laser beam at constant power, without introducing any elliptical or linear polarization, This allows a controllable torque, acting to spin the particle uniformly, to be exerted on a birefringent particle in optical tweezers. The constant power means that transverse and axial trapping, and heating due to absorption, are unaffected by changing the torque. The torque can be computer controlled and rapidly changed. In addition, the lateral trapping is kept constant. Very low torques can be obtained such that rotational Brownian motion of birefringent particles can be observed. This has the potential to greatly extend the quantitative applications of the rotation of birefringent objects in optical tweezers.

Tuesday, March 3, 2009

Correction of aberration in holographic optical tweezers using a Shack-Hartmann sensor

Carol López-Quesada, Jordi Andilla, and Estela Martín-Badosa

Optical aberration due to the nonflatness of spatial light modulators used in holographic optical tweezers significantly deteriorates the quality of the trap and may easily prevent stable trapping of particles. We use a Shack-Hartmann sensor to measure the distorted wavefront at the modulator plane; the conjugate of this wavefront is then added to the holograms written into the display to counteract its own curvature and thus compensate the optical aberration of the system. For a Holoeye LC-R 2500 reflective device, flatness is improved from 0.8λ to λ/16 (λ=532 nm), leading to a diffraction-limited spot at the focal plane of the microscope objective, which makes stable trapping possible. This process could be fully automated in a closed-loop configuration and would eventually allow other sources of aberration in the optical setup to be corrected for.

Hands-on with optical tweezers: a multitouch interface for holographic optical trapping

J. A. Grieve, A. Ulcinas, S. Subramanian, G. M. Gibson, M. J. Padgett, D. M. Carberry, and M. J. Miles

We report the implementation of a multitouch console for control of a holographic optical tweezers system. This innovative interface enables the independent but simultaneous interactive control of numerous optical traps by multiple users, overcoming the limitations of traditional interfaces and placing the full power of holographic optical tweezing into the operators’ hands.