Monday, August 31, 2009

Single molecule transcription elongation

Eric A. Galburt, Stephan W. Grill and Carlos Bustamante

Single molecule optical trapping assays have now been applied to a great number of macromolecular systems including DNA, RNA, cargo motors, restriction enzymes, DNA helicases, chromosome remodelers, DNA polymerases and both viral and bacterial RNA polymerases. The advantages of the technique are the ability to observe dynamic, unsynchronized molecular processes, to determine the distributions of experimental quantities and to apply force to the system while monitoring the response over time. Here, we describe the application of these powerful techniques to study the dynamics of transcription elongation by RNA polymerase II from Saccharomyces cerevisiae.

Protein Friction Limits Diffusive and Directed Movements of Kinesin Motors on Microtubules

Volker Bormuth, Vladimir Varga, Jonathon Howard, Erik Schäffer

Friction limits the operation of macroscopic engines and is critical to the performance of micromechanical devices. We report measurements of friction in a biological nanomachine. Using optical tweezers, we characterized the frictional drag force of individual kinesin-8 motor proteins interacting with their microtubule tracks. At low speeds and with no energy source,the frictional drag was related to the diffusion coefficient by the Einstein relation. At higher speeds, the frictional drag force increased nonlinearly, consistent with the motor jumping 8 nanometers between adjacent tubulin dimers along the microtubule, and was asymmetric, reflecting the structural polarity of the microtubule. We argue that these frictional forces arise from breaking bonds between the motor domains and the microtubule, and they limit the speed and efficiency of kinesin.

Friday, August 28, 2009

Variety in intracellular diffusion during the cell cycle

Christine Selhuber-Unkel, Pernille Yde, Kirstine Berg-Sørensen and Lene B Oddershede

During the cell cycle, the organization of the cytoskeletal network undergoes dramatic changes. In order to reveal possible changes of the viscoelastic properties in the intracellular space during the cell cycle we investigated the diffusion of endogenous lipid granules within the fission yeast Schizosaccharomyces Pombeusing optical tweezers. The cell cycle was divided into interphase and mitotic cell division, and the mitotic cell division was further subdivided in its stages. During all stages of the cell cycle, the granules predominantly underwent subdiffusive motion, characterized by an exponent α that is also linked to the viscoelastic moduli of the cytoplasm. The exponent α was significantly smaller during interphase than during any stage of the mitotic cell division, signifying that the cytoplasm was more elastic during interphase than during division. We found no significant differences in the subdiffusive exponents from granules measured in different stages of cell division. Also, our results for the exponent displayed no significant dependence on the position of the granule within the cell. The observation that the cytoplasm is more elastic during interphase than during mitotic cell division is consistent with the fact that elastic cytoskeletal elements such as microtubules are less abundantly present during cell division than during interphase.

Simulation and analysis of single-ribosome translation

Ignacio Tinoco Jr and Jin-Der Wen

In the cell, proteins are synthesized by ribosomes in a multi-step process called translation. The ribosome translocates along the messenger RNA to read the codons that encode the amino acid sequence of a protein. Elongation factors, including EF-G and EF-Tu, are used to catalyze the process. Recently, we have shown that translation can be followed at the single-molecule level using optical tweezers; this technique allows us to study the kinetics of translation by measuring the lifetime the ribosome spends at each codon. Here, we analyze the data from single-molecule experiments and fit the data with simple kinetic models. We also simulate the translation kinetics based on a multi-step mechanism from ensemble kinetic measurements. The mean lifetimes from the simulation were consistent with our experimental single-molecule measurements. We found that the calculated lifetime distributions were fit in general by equations with up to five rate-determining steps. Two rate-determining steps were only obtained at low concentrations of elongation factors. These analyses can be used to design new single-molecule experiments to better understand the kinetics and mechanism of translation.

Analytical calculation of axial optical force on a Rayleigh particle illuminated by Gaussian beams beyond the paraxial approximation

Jun Chen, Jack Ng, Shiyang Liu, and Zhifang Lin

We investigate the optical trapping of a Rayleigh particle by a linearly or radially polarized Gaussian beam. The Mie theory is applied to obtain a full solution, with the incident beam being described by the mixed dipole model, which is beyond the paraxial approximation. We then obtain approximate analytical expressions for the optical force, equilibrium position, and trap stiffness for a Rayleigh particle. At equilibrium, the displacement for the particle from the focus scales like a3 (where a is the radius) for a transparent particle, owing to scattering, whereas for absorptive particles it scales like C+Da2, owing to absorption. The trap stiffness is found to be proportional to a3, in agreement with the recent experiment. The radially polarized beam is found to be superior to the linearly polarized beam in the Rayleigh regime in terms of its ability to trap. It is found that the larger the ratio of epsilonr/epsiloni, the closer the equilibrium to the focus, and thus higher stability.

NanoPen: Dynamic, Low-Power, and Light-Actuated Patterning of Nanoparticles

Arash Jamshidi, Steven L. Neale, Kyoungsik Yu, Peter J. Pauzauskie, Peter James Schuck, Justin K. Valley, Hsan-Yin Hsu, Aaron T. Ohta and Ming C. Wu

We introduce NanoPen, a novel technique for low optical power intensity, flexible, real-time reconfigurable, and large-scale light-actuated patterning of single or multiple nanoparticles, such as metallic spherical nanocrystals, and one-dimensional nanostructures, such as carbon nanotubes. NanoPen is capable of dynamically patterning nanoparticles over an area of thousands of square micrometers with light intensities <10 style="vertical-align: 0.4em; font-size: 0.8em; line-height: 0; ">2 (using a commercial projector) within seconds. Various arbitrary nanoparticle patterns and arrays (including a 10 × 10 array covering a 0.025 mm2 area) are demonstrated using this capability. One application of NanoPen is presented through the creation of surface-enhanced Raman spectroscopy hot-spots by patterning gold nanoparticles of 90 nm diameter with enhancement factors exceeding 107 and picomolar concentration sensitivities.

Thursday, August 27, 2009

Single Centrosome Manipulation Reveals Its Electric Charge and Associated Dynamic Structure

S. Hormeño, B. Ibarra, F.J. Chichón, K. Habermann, B.M.H. Lange, J.M. Valpuesta, J.L. Carrascosa and J.R. Arias-Gonzalez

The centrosome is the major microtubule-organizing center in animal cells and consists of a pair of centrioles surrounded by a pericentriolar material. We demonstrate laser manipulation of individual early Drosophila embryo centrosomes in between two microelectrodes to reveal that it is a net negatively charged organelle with a very low isoelectric region (3.1 ± 0.1). From this single-organelle electrophoresis, we infer an effective charge smaller than or on the order of 103 electrons, which corresponds to a surface-charge density significantly smaller than that of microtubules. We show, however, that the charge of the centrosome has a remarkable influence over its own structure. Specifically, we investigate the hydrodynamic behavior of the centrosome by measuring its size by both Stokes law and thermal-fluctuation spectral analysis of force. We find, on the one hand, that the hydrodynamic size of the centrosome is 60% larger than its electron microscopy diameter, and on the other hand, that this physiological expansion is produced by the electric field that drains to the centrosome, a self-effect that modulates its structural behavior via environmental pH. This methodology further proves useful for studying the action of different environmental conditions, such as the presence of Ca2+, over the thermally induced dynamic structure of the centrosome.

Wednesday, August 26, 2009

Experimental studies of the transient fluctuation theorem using liquid crystals

Soma Datta and Arun Roy

In a thermodynamical process, the dissipation or production of entropy can only be positive or zero, according to the second law of thermodynamics. However, the laws of thermodynamics are applicable to large systems in the thermodynamic limit. Recently a fluctuation theorem, known as the transient fluctuation theorem (TFT), which generalizes the second law of thermodynamics to small systems has been proposed. This theorem has been tested in small systems such as a colloidal particle in an optical trap. We report for the first time an analogous experimental study of TFT in a spatially extended system using liquid crystals.

Monday, August 24, 2009

Drug Effect Unveils Inter-head Cooperativity and Strain-dependent ADP Release in Fast Skeletal Actomyosin

Nuria Albet-Torres, Marieke J. Bloemink, Tom Barman, Robin Candau, Kerstin Frölander, Michael A. Geeves, Kerstin Golker, Christian Herrmann, Corinne Lionne, Claudia Piperio, Stephan Schmitz, Claudia Veigel, and Alf Månsson

Amrinone is a bipyridine compound with characteristic effects on the force-velocity relationship of fast skeletal muscle,including a reduction in the maximum shortening velocity and increased maximum isometric force. Here we performed experiments to elucidate the molecular mechanisms for these effects, with the additional aim to gain insight into the molecular mechanisms underlying the force-velocity relationship. In vitro motility assays established that amrinone reduces the sliding velocity of heavy meromyosin-propelled actin filaments by 30% at different ionic strengths of the assay solution. Stopped-flow studies of myofibrils, heavy meromyosin and myosin subfragment 1, showed that the effects on sliding speed were not because of a reduced rate of ATP-induced actomyosin dissociation because the rateof this process was increased by amrinone. Moreover, optical tweezers studies could not detect any amrinone-induced changes in the working stroke length. In contrast, the ADP affinity of acto-heavy meromyosin was increased about 2-fold by 1 mM amrinone. Similar effects were not observed for acto-subfragment 1. Together with the other findings, this suggests that the amrinone-induced reduction in sliding velocity is attributed to inhibition of a strain-dependent ADP release step. Modeling results show that such an effect may account for the amrinone-induced changes of the force-velocity relationship. The data emphasize the importance of the rate of a strain-dependent ADP release step in influencing the maximum sliding velocity in fast skeletal muscle. The data also lead us to discuss the possible importance of cooperative interactions between the two myosin heads in muscle contraction.

The influence of resonant absorption and heating on the equilibrium size of aqueous-solute aerosol droplets

Rachael E. H. Miles, Marc Guillon, Laura Mitchem, David McGloin and Jonathan P. Reid

The time-dependent evolution in the equilibrium size of an optically trapped aqueous sodium chloride droplet (>2 m radius) within an environment of varying relative humidity (RH) is shown to depend on both the depression in vapour pressure due to the presence of the solute and the elevation in temperature due to optical absorption. In particular, the level of optical absorption is highly dependent on the size of the droplet relative to the wavelength of the absorbed light. Thus, as the droplet size tunes into a Mie resonance at the trapping laser wavelength, the increased level of optical absorption leads to an elevation in droplet temperature. This increase in resonant heating can balance a continual increase in RH, leading to only marginal growth in droplet size and change in solute concentration. Once the RH is sufficiently high that the resonance condition can be surpassed, the droplet cools instantaneously and the solute concentration again dominates in determining the vapour pressure, with a rapid increase in size and a decrease in solute concentration returning the droplet to equilibrium with the gas phase RH. Thus, a growing droplet is observed to pass through periods of apparent size stability followed by instantaneous growth, consistent with the variation in absorption efficiency with droplet size. This provides a clear example of the coupling between the optical and physical properties of an aerosol and their influence on the equilibrium state.

Friday, August 21, 2009

Interaction of oxazole yellow dyes with DNA studied with hybrid optical tweezers and fluorescence microscopy

C.U. Murade, V. Subramaniam, C. Otto and Martin L. Bennink

We have integrated single molecule fluorescence microscopy imaging into an optical tweezers set-up and studied the force extension behavior of individual DNA molecules in the presence of various YOYO-1 and YO-PRO-1 concentrations. The fluorescence modality was used to record fluorescent images during the stretching and relaxation cycle. Force extension curves recorded in the presence of either dye did not show the overstretching transition that is characteristic for bare DNA. Using the modified wormlike chain model to curve-fit the force extension data revealed a contour length increase of 6% and 30%, respectively, in the presence of YO-PRO-1 and YOYO-1 at 100 nM. The fluorescence images recorded simultaneously showed that the number of bound dye molecules increased as the DNA molecule was stretched and decreased again as the force on the complex was lowered. The binding constants and binding site sizes for YO-PRO-1 and YOYO-1 were determined as a function of the force. The rate of YO-PRO-1 binding and unbinding was found to be 2 orders of magnitude larger than that for YOYO-1. A kinetic model is proposed to explain this observation.

Microrheology with optical tweezers

Alison Yao, Manlio Tassieri, Miles Padgett and Jonathan Cooper

Microrheology is the study of the flow of materials over small scales. It is of particular interest to those involved with investigations of fluid properties within Lab-on-a-Chip structures or within other micron-scale environments. The article briefly reviews existing active and passive methods used in the study of fluids. It then explores in greater detail the use of optical tweezers as an emerging method to investigate rheological phenomena, including, for example, viscosity and viscoelasticity, as well as the related topic of flow. The article also describes, briefly, potential future applications of this topic, in the fields of biological measurement, in general, and Lab-on-a-Chip, in particular.

IOFF Generally Extends Fluorophore Longevity in the Presence of an Optical Trap

Ferrer, J. M.; Fangyuan, D.; Brau, R. R.; Tarsa, P. B.; Lang, M. J.

The combination of optical tweezers force microscopy and single molecule fluorescence has previously been complicated by trap-induced photobleaching. Recent studies have suggested that this effect is caused by a sequential absorption of photons, leading to ionization of the fluorescent singlet state. In this work, we show the range of effects of optical trapping radiation on common fluorescent dyes. Using the interlaced optical force fluorescence (IOFF) laser modulation technique, we show that the removal of simultaneous near infrared radiation dramatically reduces photobleaching effects. However, these studies show that the sequential addition of near infrared radiation in some cases extends photobleaching longevity beyond the natural intrinsic decay. We suggest a refined photoelectronic mechanism that accounts for the possibility of reverse intersystem crossing from a reactive triplet state and explains the nature of trap-induced photobleaching.

Tuesday, August 18, 2009

Dynamics of microbubble generation and trapping by self-focused femtosecond laser pulses

Kun Yang, Yun Zhou, Qiushi Ren, Jing Yong Ye, and Cheri X. Deng

Different from conventional optical tweezers used for trapping high refractive index micron-sized particles, bubble generation and trapping by femtosecond laser offer a unique strategy to manipulate microbubbles. Using high frequency ultrasound imaging and fast-frame optical video microscopy, we obtained results revealing the spatiotemporal characteristics of bubble generation and trapping by self-focused femtosecond laser pulses at multiple locations along the laser beam. We detected distinct acoustic signals associated with the laser focus and measured the trapping force by using acoustic radiation force to detrap the bubble from the laser beam.

Integrating a High-Force Optical Trap with Gold Nanoposts and a Robust Gold-DNA Bond

D. Hern Paik, Yeonee Seol, Wayne A. Halsey and Thomas T. Perkins

Gold-thiol chemistry is widely used in nanotechnology but has not been exploited in optical-trapping experiments due to laser-induced ablation of gold. We circumvented this problem by using an array of gold nanoposts (r = 50−250 nm, h ≈ 20 nm) that allowed for quantitative optical-trapping assays without direct irradiation of the gold. DNA was covalently attached to the gold via dithiol phosphoramidite (DTPA). By using three DTPAs, the gold−DNA bond was not cleaved in the presence of excess thiolated compounds. This chemical robustness allowed us to reduce nonspecific sticking by passivating the unreacted gold with methoxy-(polyethylene glycol)-thiol. We routinely achieved single beads anchored to the nanoposts by single DNA molecules. We measured DNA’s elasticity and its overstretching transition, demonstrating moderate- and high-force optical-trapping assays using gold-thiol chemistry. Force spectroscopy measurements were consistent with the rupture of the strepavidin−biotin bond between the bead and the DNA. This implied that the DNA remained anchored to the surface due to the strong gold−thiol bond. Consistent with this conclusion, we repeatedly reattached the trapped bead to the same individual DNA molecule. Thus, surface conjugation of biomolecules onto an array of gold nanostructures by chemically and mechanically robust bonds provides a unique way to carry out spatially controlled, repeatable measurements of single molecules.

Thursday, August 13, 2009

Study of the Line Optical Tweezers Characteristics Using a Novel Method and Establishing a Model for Cell Sorting

Ho-Chien Lin and Long Hsu

Optical tweezers have become a powerful tool in cellular and molecule biology. Line optical tweezers enhanced its function in cell sorting. This study presents the line trap model, based on the ray-optics model, and demonstrates its accuracy for the line optical tweezers. The line optical tweezers system is established to produce the optical intensity distribution of a line pattern and to trap the micro-sized beads. The main parameter, optical intensity distribution, is used to calculate the trapping force distribution in the model. The two forces, trapping force and water dragging force, and the equation of motion is used to simulate the trajectory of micro-sized beads as they pass through the line pattern in flowing water in the microchannel. The trajectory is analyzed to determine the effective separation distance between the micro-sized beads or cells. The method will be applied in biological and medical detection.

Triplex structures in an RNA pseudoknot enhance mechanical stability and increase efficiency of –1 ribosomal frameshifting

Gang Chen, Kung-Yao Chang, Ming-Yuan Chou,Carlos Bustamante,c and Ignacio Tinoco, Jr

Many viruses use programmed –1 ribosomal frameshifting to express defined ratios of structural and enzymatic proteins. Pseudoknot structures in messenger RNAs stimulate frameshifting in upstream slippery sequences. The detailed molecular determinants of pseudoknot mechanical stability and frameshifting efficiency are not well understood. Here we use single-molecule unfolding studies by optical tweezers, and frameshifting assays to elucidate how mechanical stability of a pseudoknot and its frameshifting efficiency are regulated by tertiary stem-loop interactions. Mechanical unfolding of a model pseudoknot and mutants designed to dissect specific interactions reveals that mechanical stability depends strongly on triplex structures formed by stem-loop interactions. Combining single-molecule and mutational studies facilitates the identification of pseudoknot folding intermediates. Average unfolding forces of the pseudoknot and mutants ranging from 50 to 22 picoNewtons correlated with frameshifting efficiencies ranging from 53% to 0%. Formation of major-groove and minor-groove triplex structures enhances pseudoknot stem stability and torsional resistance, and may thereby stimulate frameshifting. Better understanding of the molecular determinants of frameshifting efficiency may facilitate the development of anti-virus therapeutics targeting frameshifting.

Tuesday, August 11, 2009

Trajectory Approach to Two-State Kinetics of Single Particles on Sculpted Energy Landscapes

David Wu, Kingshuk Ghosh, Mandar Inamdar, Heun Jin Lee, Scott Fraser, Ken Dill, and Rob Phillips

We study the trajectories of a single colloidal particle as it hops between two energy wells which are sculpted using optical traps. Whereas the dynamical behaviors of such systems are often treated by master-equation methods that focus on particles as actors, we analyze them instead using a trajectory-based variational method called maximum caliber (MaxCal). We show that the MaxCal strategy accurately predicts the full dynamics that we observe in the experiments: From the observed averages, it predicts second and third moments and covariances, with no free parameters. The covariances are the dynamical equivalents of Maxwell-like equilibrium reciprocal relations and Onsager-like dynamical relations.

Phase Measurements of Barrier Crossings in a Periodically Modulated Double-Well Potential

Yeonee Seol, D. L. Stein, and Koen Visscher

We report on the experimental observation of the phase angle of a particle escaping over a periodically modulated potential barrier. Optical tweezers and back-focal plane position detection were used to record particle trajectories in the entire double-well potential. These measurements provide a sensitive test of theories proposed in the past decade of escape driven by random thermal noise from a periodically modulated potential. The observed phase shifts as a function ofmodulation frequency are consistent with those calculated using existing theories.

Brownian particles in stationary and moving traps: The mean and variance of the heat distribution function

Debarati Chatterjee and Binny J. Cherayil

A recent theoretical model developed by Imparato et al. [Phys. Rev. E 76, 050101(R) (2007)] of the experimentally measured heat and work effects produced by the thermal fluctuations of single micron-sized polystyrene beads in stationary and moving optical traps has proved to be quite successful in rationalizing the observed experimental data. The model, based on the overdamped Brownian dynamics of a particle in a harmonic potential that moves at a constant speed under a time-dependent force, is used to obtain an approximate expression for the distribution of the heat dissipated by the particle at long times. In this paper, we generalize theabove model to consider particle dynamics in the presence of colored noise, without passing to the overdamped limit, as a way of modeling experimental situations in which the fluctuations of the medium exhibit long-lived temporal correlations, of the kind characteristic of polymeric solutions, for instance, or of similar viscoelastic fluids. Although we have not been able to find an expression for the heat distribution itself, we do obtain exact expressions for its mean and variance, both for the static and for the moving trap cases. These moments are valid for arbitrary times and they also hold in the inertial regime, but they reduce exactly to the results of Imparato et al. in appropriate limits.

Optical tweezers for undergraduates: Theoretical analysis and experiments

M. S. Rocha

A theoretical treatment of optical tweezers is presented at a level suitable for undergraduates. We explore the Rayleigh and the geometrical optics regimes with an emphasis on the latter. We discuss a model for the geometrical optics regime, including spherical aberration effects, and show that the model can easily be implemented numerically. A comparison of the model with experimental data yields excellent agreement between theory and experiment. We also briefly discuss a theory of optical tweezers valid for microspheres of any size.

Friday, August 7, 2009

Nucleosomal Fluctuations Govern the Transcription Dynamics of RNA Polymerase II

Courtney Hodges, Lacramioara Bintu, Lucyna Lubkowska, Mikhail Kashlev, Carlos Bustamante

RNA polymerase II (Pol II) must overcome the barriers imposed by nucleosomes during transcription elongation. We have developed an optical tweezers assay to follow individual Pol II complexes as they transcribe nucleosomal DNA. Our results indicate that the nucleosome behaves as a fluctuating barrier that locally increases pause density, slows pause recovery, and reduces the apparent pause-free velocity of Pol II. The polymerase, rather than actively separating DNA from histones, functions instead as a ratchet that rectifies nucleosomal fluctuations. We also obtained direct evidence that transcription through a nucleosomeinvolves transfer of the core histones behind the transcribing polymerase via a transient DNA loop. The interplay between polymerase dynamics and nucleosome fluctuations provides a physical basis for the regulation of eukaryotic transcription.

Thursday, August 6, 2009

Manipulation of live mouse embryonic stem cells using holographic optical tweezers

Jonathan Leach, Daniel Howard, Scott Roberts, Graham Gibson, David Gothard, Jon Cooper, Kevin Shakesheff, Miles Padgett, Lee Buttery

We report the ability to move and arrange patterns of live embryonic stem cells using holographic optical tweezers. Single cell suspensions of mouse embryonic stem cells were manipulated with holographic optical tweezers into a variety of patterns including lines, curves and circles. Individual cells were also lifted out of the sample plane highlighting the potential for 3D positional control. Trypan blue dye exclusion and Live/Dead™ staining (CMFDA-1, EthHD-1) showed that the cells were still viable after manipulation with the optical tweezers. The ability to move individual stem cells into specific, pre-defined patterns provides a method to study how arrangement and associated small-scale interactions occur between neighbouring cells.

Application and Progress of Raman Tweezers in Single Cells

Min AI, Jun-Xian LIU, Shu-Shi HUANG, Gui-Wen WANG, Xiu-Li CHEN, Zhi-Cheng CHEN and Hui-Lu YAO

Raman tweezers is a new optical technique that combines laser tweezers with Raman spectroscopy, which has the capability of studying single biological cells or organelles in nearly natured solution. Owing to its characteristics of noncontact, noninvasion, fast identification, and real-time detection, it has found application in improving the Signal Noise Ratio of Raman spectroscopy, optimizing the detection and sorting single cells or organelles, practicing the real-time detection of biochemistry dynamics process so as to make the activity and mechanisms of biomolecules understood deeply. The source, principle of Raman tweezers, and its optical application in biology are depicted and reviewed particularly in this article.

A molecular brake, not a clutch, stops the Rhodobacter sphaeroides flagellar motor

Teuta Pilizota, Mostyn T. Brown, Mark C. Leake,Richard W. Branch, Richard M. Berry and Judith P. Armitage

Many bacterial species swim by employing ion-driven molecular motors that power the rotation of helical filaments. Signals are transmitted to the motor from the external environment via the chemotaxis pathway. In bidirectional motors, the binding of phosphorylated CheY (CheY-P) to the motor is presumed to instigate conformational changes that result in a different rotor-stator interface, resulting in rotation in the alternative direction. Controlling when this switch occurs enables bacteria to accumulate in areas favorable for their survival. Unlike most species that swim with bidirectional motors, Rhodobacter sphaeroides employs a single stop-start flagellar motor. Here, we asked, how does the binding of CheY-P stop the motor in R. sphaeroides—using a clutch or a brake? By applying external force with viscous flow or optical tweezers, we show that the R. sphaeroides motor is stopped using a brake. The motor stops at 27–28 discrete angles, locked in place by a relatively high torque, approximately 2–3 times its stall torque.


E. Brasselet and S. Juodkazis

The high sensitivity of liquid crystals to external fields, especially electromagnetic fields, confer to them fascinating properties. In the case of light fields, their large optical nonlinearities over a broad spectrum have great application potential for all-optical devices. The linear optical properties of liquid crystals, such as their high refractive index, birefringence and transparency, are also of great practical interest in optofluidics, which combines the use of optical tools in microfluidic environments. A representative example is the laser micromanipulation of liquid crystalline systems using optical tweezing techniques. Liquid crystal droplets represent a class of systems that can be easily prepared and manipulated by light, with or without a nonlinear light-matter coupling. Here we review different aspects of quasi-statics and dynamical optical angular manipulation of liquid crystal droplets trapped in laser tweezers. In particular, we discuss to the influence of the phase (nematic, cholesteric or smectic), the bulk ordering symmetry, the droplet size, the polarization state and power of the trapping light, together with the prominent role of light–matter angular momentum exchanges and optical orientational nonlinearities.

Theoretical determination of the radiation force for a spherical particle illuminated by a focused laser beam

R.R. Dorizzi and Z. Ulanowski

Trapping forces on dielectric spheres in single beam laser tweezers are computed. A focused beam description based on an exact solution of Maxwell's equations is compared to the 5th order Gaussian beam approximation due to Barton and Alexander. Forces on water droplets suspended in air and on polystyrene spheres suspended in water, exerted by beams focused to varying degree, are calculated. It is demonstrated that the 5th order approximation is accurate for almost paraxial beams (numerical aperture NA <not, vert, similar0.25), as compared to the exact treatment. However, for strongly focused beams the 5th order approximation breaks down. Thus it is established that an accurate beam description is vital for modeling optical traps, since, in order to hold a particle effectively in a single beam trap, a strongly focused beam is required.

Symmetry and the generation and measurement of optical torque

Timo A. Nieminen, Theodor Asavei, Vincent L.Y. Loke, Norman R. Heckenberg and Halina Rubinsztein-Dunlop

A key element in the generation of optical torque in optical traps, which occurs when electromagnetic angular momentum is transferred from the trapping beam to the trapped particle by scattering, is the symmetries of the scattering particle and the trapping beam. We discuss the effect of such symmetries on the generation and measurement of optical torque in optical tweezers, and some consequent general principles for the design of optically driven micromachines.

Particle levitation and laboratory scattering

Jonathan P. Reid

Measurements of light scattering from aerosol particles can provide a non-intrusive in situ method for characterising particle size distributions, composition, refractive index, phase and morphology. When coupled with techniques for isolating single particles, considerable information on the evolution of the properties of a single particle can be gained during changes in environmental conditions or chemical processing. Electrostatic, acoustic and optical techniques have been developed over many decades for capturing and levitating single particles. In this review, we will focus on studies of particles in the Mie size regime and consider the complimentarity of electrostatic and optical techniques for levitating particles and elastic and inelastic light scattering methods for characterising particles. In particular, we will review the specific advantages of establishing a single-beam gradient force optical trap (optical tweezers) for manipulating single particles or arrays of particles. Recent developments in characterising the nature of the optical trap, in applying elastic and inelastic light scattering measurements for characterising trapped particles, and in manipulating particles will be considered.

Generalized Lorenz-Mie theories, the third decade: A perspective

G. Gouesbet

During the year 2008, we have been commemorating, in several places, the hundredth anniversary of the famous 1908-paper by Mie describing the interaction between an electromagnetic plane wave and a homogeneous sphere defined by its diameter d and its complex refractive index m. Due to the existence of a prior version by Lorenz, Mie's theory may also be named as Lorenz-Mie theory (LMT). The generalized Lorenz-Mie theory (GLMT) stricto sensu deals with the more general case when the illuminating wave is an arbitrary shaped beam (say: a laser beam) still interacting with a homogeneous sphere defined by its diameter d and its complex refractive index m. The name "GLMTs" is generically used to designate various variants for other particle shapes when the method of separation of variables is used. The present paper provides a review of the work accomplished in this generalized field during the last decade (the third decade). As a convenient selection criterion, only papers citing the work of the group of Rouen have been essentially used, with ISIweb of knowledge providing a database.

Experimental Verification of a Modified Fluctuation-Dissipation Relation for a Micron-Sized Particle in a Nonequilibrium Steady State

J. R. Gomez-Solano, A. Petrosyan, S. Ciliberto, R. Chetrite, and K. Gawdzki

A modified fluctuation-dissipation theorem for a nonequilibrium steady state is experimentally checked by studying the position fluctuations of a colloidal particlewhose motion is confined in a toroidal optical trap. The nonequilibrium steady state is generated by means of a rotating laser beam which exerts on the particle a sinusoidal conservative force plus a constant nonconservative one. The modified fluctuation-dissipation theorem is perfectly verified by the experimental data. It can be interpreted as an equilibriumlike fluctuation-dissipation relation in the Lagrangian frame of the mean local velocity of the particle.

Monday, August 3, 2009

Configurable three-dimensional optical cage generated from cylindrical vector beams

Xi-Lin Wang, Jianping Ding, Jian-Qi Qin, Jing Chen, Ya-Xian Fan and Hui-Tian Wang

We propose a new approach to generate a controllable three-dimensional (3D) optical cage by double-mode cylindrical vector beam in the vicinity of focus, by controlling the polarization state of beam. The simulation results show that the 3D optical cage of spheroid surface shape with a sharp dark region surrounded by a uniform optical barrier with a maximum deviation of 2% is achieved. The finite-difference time-domain calculation of optical force validates that such a kind of 3D optical cage has the trapping capability of the low-refractive-index particles with the size being much smaller than the light wavelength.

Optical redistribution of microparticles and cells between microwells

Jörg Baumgartl, Gregor M. Hannappel, David J. Stevenson, Daniel Day, Min Gu and Kishan Dholakia

The shaping of laser beams has developed into a powerful tool for optical micromanipulation. In this context, Airy and parabolic laser beams which follow curved trajectories have drawn considerable attention. These beams may allow clearing of microparticles through particle transport along curved paths, a concept termed optically mediated particle clearing (OMPC). In this communication we apply this concept to microparticles and cells within specially designed microwells. Our results open novel perspectives for the redistribution of cells between different media within a microfluidic environment.

Loss-based optical trap for on-chip particle analysis

S. Kühn, P. Measor, E. J. Lunt, B. S. Phillips, D. W. Deamer, A. R. Hawkins and H. Schmidt

Optical traps have become widespread tools for studying biological objects on the micro and nanoscale. However, conventional laser tweezers and traps rely on bulk optics and are not compatible with current trends in optofluidic miniaturization. Here, we report a new type of particle trap that relies on propagation loss in confined modes in liquid-core optical waveguides to trap particles. Using silica beads and E. coli bacteria, we demonstrate unique key capabilities of this trap. These include single particle trapping with micron-scale accuracy at arbitrary positions over waveguide lengths of several millimeters, definition of multiple independent particle traps in a single waveguide, and combination of optical trapping with single particle fluorescence analysis. The exclusive use of a two-dimensional network of planar waveguides strongly reduces experimental complexity and defines a new paradigm for on-chip particle control and analysis.