Thursday, October 27, 2011

Trapping, manipulation and rapid rotation of NBD-C8 fluorescent single microcrystals in optical tweezers

J. P. –Galaup, M. Rodríguez-Otazo, A. Augier-Calderín, J. –F. Lamère y S. Fery-Forges

We have built an optical tweezers experiment based on an inverted microscope to trap and manipulate single crystals of micro or sub-micrometer size made from fluorescent molecules of 4-octylamino-7-nitro-benzoxadiazole (NBD-C8). These single crystals have parallelepiped shapes and exhibit birefringence properties evidenced through optical experiments between crossed polarizers in a polarizing microscope. The crystals are uniaxial with their optical axis oriented along their largest dimension. Trapped in the optical trap, the organic micro-crystals are oriented in such a way that their long axis is along the direction o f the beam propagation, and their short axis follows the direction of the linear polarization. Therefore, with linearly polarized light, simply rotating the light polarization can orient the crystal. When using circularly or only elliptically polarized light, the crystal can spontaneously rotate and reach rotation speed of several hundreds of turns per second. A surprising result has been observed: when the incident power is growing up, the rotation speed increases to reach a maximum value and then decreases even when the power is still growing up. Moreover, this evolution is irreversible. Different possible explanations can be considered. The development of a 3D control of the crystals by dynamical holography using liquid crystal spatial modulators will be presented and discussed on the basis of the most recent results obtained.


Nonlinear Optical Processes in Optically Trapped InP Nanowires

Fan Wang, Peter J. Reece, Suriati Paiman, Qiang Gao, Hark Hoe Tan, and Chennupati Jagadish

We report on the observation of nonlinear optical excitation and related photoluminescence from single InP semiconductor nanowires held in suspension using a gradient force optical tweezers. Photoexcitation of free carriers is achieved through absorption of infrared (1.17 eV) photons from the trapping source via a combination of two- and three-photon processes. This was confirmed by power-dependent photoluminescence measurements. Marked differences in spectral features are noted between nonlinear optical excitation and direct excitation and are related to band-filling effects. Direct observation of second harmonic generation in trapped InP nanowires confirms the presence of nonlinear optical processes.


Enhanced cell sorting and manipulation with combined optical tweezer and microfluidic chip technologies

Xiaolin Wang, Shuxun Chen, Marco Kong, Zuankai Wang, Kevin D. Costa, Ronald A. Li and Dong Sun

Sorting (or isolation) and manipulation of rare cells with high recovery rate and purity are of critical importance to a wide range of physiological applications. In the current paper, we report on a generic single cell manipulation tool that integrates optical tweezers and microfluidic chip technologies for handling small cell population sorting with high accuracy. The laminar flow nature of microfluidics enables the targeted cells to be focused on a desired area for cell isolation. To recognize the target cells, we develop an image processing methodology with a recognition capability of multiple features, e.g., cell size and fluorescence label. The target cells can be moved precisely by optical tweezers to the desired destination in a noninvasive manner. The unique advantages of this sorter are its high recovery rate and purity in small cell population sorting. The design is based on dynamic fluid and dynamic light pattern, in which single as well as multiple laser traps are employed for cell transportation, and a recognition capability of multiple cell features. Experiments of sorting yeast cells and human embryonic stem cells are performed to demonstrate the effectiveness of the proposed cell sorting approach.


Mechanical stability of low-humidity single DNA molecules

Silvia Hormeño, Borja Ibarra, José M. Valpuesta, José L. Carrascosa, J. Ricardo Arias-Gonzalez

DNA electrostatic character is mostly determined by both water and counterions activities in the phosphate backbone which, together with base sequence, further confer its higher-order structure. We overstretch individual double-stranded DNA molecules in water-ethanol solutions to investigate the modulation of its mechanical stability by hydration and polycations. We find that DNA denatures as ethanol concentration is increased and spermine concentration decreased. This is manifested by an increase in melting hysteresis between the stretch and release curves, with sharp transition at 10% ethanol and reentrant behavior at 60%, by a loss of cooperativity in the overstretching transition and by a dramatic decrease of both the persistence length and the flexural rigidity. Changes in base-stacking stability which are characteristic of the B-A transition between 70% and 80% ethanol concentration do not manifest in the mechanical properties of the double-helical molecule at low or high force or in the behavior of the overstretching and melting transitions within this ethanol concentration range. This is consistent with a mechanism in which A-type base-stacking is unstable in the presence of tension. Binding of motor proteins to DNA locally reduces the number of water molecules and therefore, our results may shed light on analogous reduced-water activity of DNA conditions caused by other molecules which interact with DNA in vivo.


Wednesday, October 26, 2011

Plasmon-Exciton Interactions on Single Thermoresponsive Platforms Demonstrated by Optical Tweezers

Silvia Hormeño, Neus G. Bastús, Andrea Pietsch, Horst Weller, J. R. Arias-Gonzalez, and Beatriz H. Juárez

Optical and hydrodynamic-size studies on single bare thermo-responsive microspheres, and microspheres covered either with Au nanoparticles, CdSe/CdS quantum dots, or a combination of both have been performed by optical tweezers. The photothermal heating of water in the focal region boosts the shrinkage of the microspheres, an effect that is intensified in the presence of Au nanoparticles. In contrast, bigger microspheres are measured when they are covered with quantum dots. Plasmon-exciton interactions are observable in the trap in the combined Au and quantum dots hybrid systems.


Planar silicon microrings as wavelength-multiplexed optical traps for storing and sensing particles

Shiyun Lin and Kenneth B. Crozier

We demonstrate the trapping of particles with silicon microring resonators integrated with waveguides. Multiple microrings with different resonant wavelengths are integrated with each waveguide. We demonstrate that tuning the laser wavelength to the resonance wavelengths of different rings enables trapped particles to be transferred back and forth between the rings. We demonstrate that the change in output power arising from particle-induced resonance shift enables the real-time monitoring of trapped particles, such as their number and velocities, without the need for an external imaging system. The techniques we describe here could form the basis for small footprint systems in which objects are moved between multiple locations on a chip, at each of which different operations are performed and the objects' properties sensed.


Towards controlling molecular motions in fluorescence microscopy and optical trapping: a spatiotemporal approach

Arijit Kumar De & Debabrata Goswami

This account reviews some recent studies pursued in our group on several control experiments with important applications in (one-photon) confocal and two-photon fluorescence laser-scanning microscopy and optical trapping with laser tweezers. We explore the simultaneous control of internal and external (i.e. centre-of-mass motion) degrees of freedom, which require the coupling of various control parameters to result in the spatiotemporal control. Of particular interest to us is the implementation of such control schemes in living systems. A live cell is a system of a large number of different molecules which combine and interact to generate complex structures and functions. These combinations and interactions of molecules need to be choreographed perfectly in time and space to achieve intended intra-cellular functions. Spatiotemporal control promises to be a versatile tool for dynamical control of spatially manipulated bio-molecules.


Tuesday, October 25, 2011

On optical forces in spherical whispering gallery mode resonators

J. T. Rubin and L. Deych

In this paper we discuss the force exerted by the field of an optical cavity on a polarizable dipole. We show that the modification of the cavity modes due to interaction with the dipole significantly alters the properties of the force. In particular, all components of the force are found to be non-conservative, and cannot, therefore, be derived from a potential energy. We also suggest a simple generalization of the standard formulas for the optical force on the dipole, which reproduces the results of calculations based on the Maxwell stress tensor.


Response theory of optical forces in two-port photonics systems: a simplified framework for examining conservative and non-conservative forces

Zheng Wang and Peter Rakich

We extend the response theory of optical forces to general electromagnetic systems which can be treated as multi-port systems with multiple mechanical degrees of freedom. We demonstrate a fundamental link between the scattering properties of an optical system to its ability to produce conservative or non-conservative optical forces. Through the exploration of two nontrivial two-port systems, including an analytical Fabry-Perot interferometer and a more complex particle-in-a-waveguide structure, we show perfect agreement between the response theory and numerical first-principle calculations. We show that new insights into the origins of optical forces from the response theory provide clear means of understanding conservative and non-conservative forces in a regime where traditional gradient force picture fails.


Long-range and long-term interferometric tracking by static and dynamic force-clamp optical tweezers

A. Guiggiani, B. Torre, A. Contestabile, F. Benfenati, M. Basso, M. Vassalli, and F. Difato

Optical tweezers are recognized single-molecule technique to resolve forces and motion on the molecular scale. Complex biological phenomena, such as cell differentiation and locomotion, require long range tracking capabilities with nanometer resolution over an extended period, to resolve molecular processes on the cellular scale. Here we introduce a real-time control of the microscope stage position to perform long-term tracking, with sub-millisecond resolution, of a bead attached to a neuron, preserving sub-nanometer sensitivity on a spatial range of centimeters, seven orders of magnitude larger. Moreover, the suitability of the system is tested by time- modulating the force-clamp condition to study the role of statically and dynamically applied forces in neuronal differentiation.


Monday, October 24, 2011

Nanochannel electroporation delivers precise amounts of biomolecules into living cells

Pouyan E. Boukany, Andrew Morss, Wei-ching Liao, Brian Henslee, HyunChul Jung, Xulang Zhang, Bo Yu, Xinmei Wang, Yun Wu, Lei Li, Keliang Gao, Xin Hu, Xi Zhao, O. Hemminger, Wu Lu, Gregory P. Lafyatis & L. James Lee

Many transfection techniques can deliver biomolecules into cells, but the dose cannot be controlled precisely. Delivering well-defined amounts of materials into cells is important for various biological studies and therapeutic applications. Here, we show that nanochannel electroporation can deliver precise amounts of a variety of transfection agents into living cells. The device consists of two microchannels connected by a nanochannel. The cell to be transfected is positioned in one microchannel using optical tweezers, and the transfection agent is located in the second microchannel. Delivering a voltage pulse between the microchannels produces an intense electric field over a very small area on the cell membrane, allowing a precise amount of transfection agent to be electrophoretically driven through the nanochannel, the cell membrane and into the cell cytoplasm, without affecting cell viability. Dose control is achieved by adjusting the duration and number of pulses. The nanochannel electroporation device is expected to have high-throughput delivery applications.


Healing of Defects at the Interface of Nematic Liquid Crystals and Structured Langmuir-Blodgett Monolayers

Núria Petit-Garrido, Rahul P. Trivedi, Jordi Ignés-Mullol, Josep Claret, Clayton Lapointe, Francesc Sagués, and Ivan I. Smalyukh

We use Langmuir-Blodgett molecular monolayers and nematic liquid crystals as model two- and three-dimensional orientationally ordered systems to study the stability and healing of topological defects at their contact interfaces. Integer-strength defects at the monolayer induce disclinations of similar strength in the nematic that, however, do not propagate deep into the bulk, but rather form single- or double-split arch-shaped loops pinned to the interface. This behavior is qualitatively independent of the far-field director orientation and involves either half-integer singular or twist-escaped unity-strength nonsingular nematic disclinations. These two defect configurations can be selected by varying sample preparation given their comparable free energy, consistently with direct probing by use of laser tweezers.


Friday, October 21, 2011

Fast parallel interferometric 3D tracking of numerous optically trapped particles and their hydrodynamic interaction

Dominic Ruh, Benjamin Tränkle, and Alexander Rohrbach

Multi-dimensional, correlated particle tracking is a key technology to reveal dynamic processes in living and synthetic soft matter systems. In this paper we present a new method for tracking micron-sized beads in parallel and in all three dimensions – faster and more precise than existing techniques. Using an acousto-optic deflector and two quadrant-photo-diodes, we can track numerous optically trapped beads at up to tens of kHz with a precision of a few nanometers by back-focal plane interferometry. By time-multiplexing the laser focus, we can calibrate individually all traps and all tracking signals in a few seconds and in 3D. We show 3D histograms and calibration constants for nine beads in a quadratic arrangement, although trapping and tracking is easily possible for more beads also in arbitrary 2D arrangements. As an application, we investigate the hydrodynamic coupling and diffusion anomalies of spheres trapped in a 3 × 3 arrangement.


Thursday, October 20, 2011

Mechanical Action of Inhomogeneously Polarized Optical Fields and Detection of the Internal Energy Flows

A. Ya. Bekshaev, O. V. Angelsky, S. V. Sviridova, and C. Yu. Zenkova

We analyze numerically correspondence between the mechanical action, experienced by a spherical microparticle, and the internal energy flows in the light field incident on the particle. The inhomogeneous incident field is modelled by superposition of two plane waves; the mechanical action is calculated via the Mie theory for dielectric and conducting particles of different sizes and optical properties. It is shown that both spin and orbital components of the field momentum can produce the mechanical action whose value and sign depend on many additional details of the field-particle interaction. Besides, forces that are not associated with any sort of the energy flow (e.g., the gradient force owing to the inhomogeneous intensity and the polarization-dependent dipole force emerging due to inhomogeneous polarization) can strongly modify the observed mechanical action. The polarization-dependent mechanical action on particles can be treated as a form of the spin-orbit interaction of light.


Stretching Short Sequences of DNA with Constant Force Axial Optical Tweezers

Krishnan Raghunathan, Joshua N. Milstein, Jens -Christian Meiners

Single-molecule techniques for stretching DNA of contour lengths less than a kilobase are fraught with experimental difficulties. However, many interesting biological events such as histone binding and protein-mediated looping of DNA, occur on this length scale. In recent years, the mechanical properties of DNA have been shown to play a significant role in fundamental cellular processes like the packaging of DNA into compact nucleosomes and chromatin fibers. Clearly, it is then important to understand the mechanical properties of short stretches of DNA. In this paper, we provide a practical guide to a single-molecule optical tweezing technique that we have developed to study the mechanical behavior of DNA with contour lengths as short as a few hundred basepairs.
The major hurdle in stretching short segments of DNA is that conventional optical tweezers are generally designed to apply force in a direction lateral to the stage (see Fig. 1). In this geometry, the angle between the bead and the coverslip, to which the DNA is tethered, becomes very steep for submicron length DNA. The axial position must now be accounted for, which can be a challenge, and, since the extension drags the microsphere closer to the coverslip, steric effects are enhanced. Furthermore, as a result of the asymmetry of the microspheres, lateral extensions will generate varying levels of torque due to rotation of the microsphere within the optical trap since the direction of the reactive force changes during the extension.
Alternate methods for stretching submicron DNA run up against their own unique hurdles. For instance, a dual-beam optical trap is limited to stretching DNA of around a wavelength, at which point interference effects between the two traps and from light scattering between the microspheres begin to pose a significant problem. Replacing one of the traps with a micropipette would most likely suffer from similar challenges. While one could directly use the axial potential to stretch the DNA, an active feedback scheme would be needed to apply a constant force and the bandwidth of this will be quite limited, especially at low forces.
We circumvent these fundamental problems by directly pulling the DNA away from the coverslip by using a constant force axial optical tweezers. This is achieved by trapping the bead in a linear region of the optical potential, where the optical force is constant-the strength of which can be tuned by adjusting the laser power. Trapping within the linear region also serves as an all optical force-clamp on the DNA that extends for nearly 350 nm in the axial direction. We simultaneously compensate for thermal and mechanical drift by finely adjusting the position of the stage so that a reference microsphere stuck to the coverslip remains at the same position and focus, allowing for a virtually limitless observation period.


Wednesday, October 19, 2011

Large-area manipulation of microdroplets by holographic optical tweezers based on a hybrid diffractive system

Yusuke Ogura, Yuki Kazayama, Takahiro Nishimura, and Jun Tanida

We report on large-area manipulation of microdroplets by holographic optical tweezers based on a hybrid diffractive system, in which a static computer-generated hologram and a spatial light modulator (SLM) are used. The hybrid diffractive system is useful to manipulate microdroplets on distant areas with the same manner. Experimental results demonstrated that microdroplets were transported successfully in parallel with approximately equivalent velocities over the entire manipulation area. Fusion of microdroplets was also achieved at a position where the optical pattern generated by the SLM alone did not reach.


Tuesday, October 18, 2011

Optical Force Stamping Lithography

Spas Nedev, Alexander S. Urban, Andrey A. Lutich, and Jochen Feldmann

Here we introduce a new paradigm of far-field optical lithography, optical force stamping lithography. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used.


Mapping electric fields generated by microelectrodes using optically trapped charged microspheres

Giuseppe Pesce, Biagio Mandracchia, Emanuele Orabona, Giulia Rusciano, Luca De Stefano and Antonio Sasso

In this work, we show a new technique to measure the direction and amplitude of the electric field generated by microelectrodes in a liquid environment, as often used in microfluidic devices. The method is based on the use of optical tweezers as a force transducer. A trapped, charged particle behaves as a probe. With this technique, it is possible to obtain a detailed map of the electric field, even for very complex electrode structures with a resolution below a micrometre and with a sensitivity as low as a few hundreds of V m−1.


Monday, October 17, 2011

Positional stability of holographic optical traps

Arnau Farré, Marjan Shayegan, Carol López-Quesada, Gerhard A. Blab, Mario Montes-Usategui, Nancy R. Forde, and Estela Martín-Badosa

The potential of digital holography for complex manipulation of micron-sized particles with optical tweezers has been clearly demonstrated. By contrast, its use in quantitative experiments has been rather limited, partly due to fluctuations introduced by the spatial light modulator (SLM) that displays the kinoforms. This is an important issue when high temporal or spatial stability is a concern. We have investigated the performance of both an analog-addressed and a digitally-addressed SLM, measuring the phase fluctuations of the modulated beam and evaluating the resulting positional stability of a holographic trap. We show that, despite imparting a more unstable modulation to the wavefront, our digitally-addressed SLM generates optical traps in the sample plane stable enough for most applications. We further show that traps produced by the analog-addressed SLM exhibit a superior pointing stability, better than 1 nm, which is comparable to that of non-holographic tweezers. These results suggest a means to implement precision force measurement experiments with holographic optical tweezers (HOTs).


Thursday, October 13, 2011

Single-Molecule DNA Force Spectroscopy to Probe Interactions with the Tri-Peptide Lys-Trp-Lys

Dr. Chandrashekhar U. Murade, Prof. Dr. Vinod Subramaniam, Dr. Cees Otto, Dr. Martin L. Bennink

May the force be with you: It is shown that single-molecule optical tweezers force spectroscopy can be used successfully to detect the effects of binding of low-affinity ligands to a single DNA molecule (see picture) and to determine various kinetic parameters of the reaction at single-molecular level.


Tuesday, October 11, 2011

Role of condenser iris in optical tweezer detection system

Akbar Samadi and S. Nader S. Reihani

Optical tweezers have proven to be very useful in various scientific fields, from biology to nanotechnology. In this Letter we show, both by theory and experiment, that the interference intensity pattern at the back focal plane of the condenser consists of two distinguishable areas with anticorrelated intensity changes when the bead is moved in the axial direction. We show that the space angle defining the border of two areas linearly depends on the NA of the objective. We also propose a new octant photodiode, which could significantly improve the axial resolution compared to the commonly used quadrant photodiode technique.


Numerical investigation and optimisation of hollow-core photonic crystal fibre for optical trapping of fluorescent microparticles

Shinoj, V.K.; Murukeshan, V.M.

This Letter proposes and demonstrates the optical trapping of fluorescence sample in a transversely-probed microstructured hollow-core photonic crystal fibres (HC-PCFs). The transmission intensity distribution at the central core of liquid-filled HC-PCFs with different central wavelengths is monitored for both the transverse electric and transverse magnetic modes, with illumination in the G??M direction. Forces acting on a sphere located inside the central core along the transverse direction of these liquid-filled fibres are calculated and compared using finite-difference time-domain method and Maxwell stress tensor-based method. The proposed concept is illustrated by trapping fluorescent microsphere particles to the core of liquid-filled HC-PCF and the presence of sample particles is confirmed by fluorescence signatures. The obtained results indicate that the proposed concepts have tangible potential for developing novel optical manipulation and trapping inside HC-PCFs and are expected to find potential biomedical diagnostic applications.


Monday, October 10, 2011

Calculation of Maxwell Stress Tensor Using 3D FDTD for Trapping Force in Near-Field Optical Tweezers

Bing Hui Liu, Li Jun Yang, Yang Wang

New forms of trapping force are proposed for the design of near-field optical tweezers. Without the limitation of dipole approximation, the trapping force acting on a nano-particle located in near-field region can be solved by direct calculation of Maxwell stress tensor using 3D FDTD method. The new forms are used to design near-field optical trapping with a metal-coated fiber probe. Calculations show that the fiber probe can trap a nano-particle with tens of nanometres diameter to different positions with different distance from the probe tip. In order to achieve higher trapping capability, the feasibility of near-field trapping near the optical fiber probe after adding the AFM metallic probe is shown by analyzing trapping forces along three axis directions. The correctness of new forms is demonstrated by numerical results.


Highly Anisotropic Stability and Folding Kinetics of a Single Coiled Coil Protein under Mechanical Tension

Ying Gao, George Sirinakis, and Yongli Zhang

Coiled coils are one of the most abundant protein structural motifs and widely mediate protein interactions and force transduction or sensation. They are thus model systems for protein engineering and folding studies, particularly the GCN4 coiled coil. Major single-molecule methods have also been applied to this protein and revealed its folding kinetics at various spatiotemporal scales. Nevertheless, the folding energy and the kinetics of a single GCN4 coiled coil domain have not been well determined at a single-molecule level. Here we used high-resolution optical tweezers to characterize the folding and unfolding reactions of a single GCN4 coiled coil domain and their dependence on the pulling direction. In one axial and two transverse pulling directions, we observed reversible, two-state transitions of the coiled coil in real time. The transitions equilibrate at pulling forces ranging from 6 to 12 pN, showing different stabilities of the coiled coil in regard to pulling direction. Furthermore, the transition rates vary with both the magnitude and the direction of the pulling force by greater than 1000 folds, indicating a highly anisotropic and topology-dependent energy landscape for protein transitions under mechanical tension. We developed a new analytical theory to extract energy and kinetics of the protein transition at zero force. The derived folding energy does not depend on the pulling direction and is consistent with the measurement in bulk, which further confirms the applicability of the single-molecule manipulation approach for energy measurement. The highly anisotropic thermodynamics of proteins under tension should play important roles in their biological functions.


Size-Scaling in Optical Trapping of Silicon Nanowires

Alessia Irrera, Pietro Artoni, Rosalba Saija, Pietro G. Gucciardi, Maria Antonia Iatì, Ferdinando Borghese, Paolo Denti, Fabio Iacona, Francesco Priolo, and Onofrio M. Maragò

We investigate size-scaling in optical trapping of ultrathin silicon nanowires showing how length regulates their Brownian dynamics, optical forces, and torques. Force and torque constants are measured on nanowires of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement.


Friday, October 7, 2011

Integration of Optical Manipulation and Electrophysiological Tools to Modulate and Record Activity in Neural Networks

F. Difato, L. Schibalsky, F. Benfenati & A. Blau

We present an optical system that combines IR (1064 nm) holographic optical tweezers with a sub-nanosecond-pulsed UV (355 nm) laser microdissector for the optical manipulation of single neurons and entire networks both on transparent and non-transparent substrates in vitro. The phase-modulated laser beam can illuminate the sample concurrently or independently from above or below assuring compatibility with different types of microelectrode array and patch-clamp electrophysiology. By combining electrophysiological and optical tools, neural activity in response to localized stimuli or injury can be studied and quantified at sub-cellular, cellular, and network level.


Towards total photonic control of complex-shaped colloids by vortex beams

Clayton P. Lapointe, Thomas G. Mason, and Ivan I. Smalyukh

We demonstrate optical trapping and orientational control over colloidal particles having complex shapes in an anisotropic host fluid using a dynamic holographic optical tweezers system. Interactions between a colloidal particle and the toroidal intensity distributions of focused Laguerre-Gaussian beams allow for stable optical tweezing and provide a tunable tilt of the particle out of the focal plane. Use of an aligned nematic liquid crystal as the host fluid suppresses rotations about the optical axis arising from angular momentum transfer from the beam and effectively defines a rotational axis for the colloid within the trap.


Custom-Built Optical Tweezers for Locally Probing the Viscoelastic Properties of Cancer Cells

Federica Tavano, Serena Bonin, Giulietta Pinato, Giorgio Stanta & Dan Cojoc

We report a home built optical tweezers setup to investigate the mechanism of the membrane tether formation from single cells in vitro. Using an optically trapped microbead as probe, we have determined the force-elongation curve during tether formation and extracted several parameters characterizing the viscoelastic behavior of the cell membrane: tether stiffness, force, and viscosity. Breast cancer MDA-MB-231 cells have been studied in two different conditions, at room and physiological temperatures, showing a strong temperature dependence of the visoelastic properties of the cell membrane. To get detailed inside information about the tether formation mechanism we have extended the analysis of the force-elongation curves fitting them with a Kelvin model. These preliminary results are part of a larger project of whose goal is to compare the viscoelastic properties of several types of cancer cell lines, characterized by different aggressiveness and metastatic potential.


Measuring Forces between Two Single Actin Filaments during Bundle Formation

Martin Streichfuss, Friedrich Erbs, Kai Uhrig, Rainer Kurre, Anabel E.-M. Clemen, Christian H. J. Bohm, Tamas Haraszti, and Joachim P. Spatz

Bundles of filamentous actin are dominant cytoskeletal structures, which play a crucial role in various cellular processes. As yet quantifying the fundamental interaction between two individual actin filaments forming the smallest possible bundle has not been realized. Applying holographic optical tweezers integrated with a microfluidic platform, we were able to measure the forces between two actin filaments during bundle formation. Quantitative analysis yields forces up to 0.2 pN depending on the concentration of bundling agents.


Nucleosome-remodelling machines and other molecular motors observed at the single-molecule level

Christophe Lavelle, Elise Praly, David Bensimon, Eric Le Cam, Vincent Croquette

Through its capability to transiently pack and unpack our genome, chromatin is a key player in the regulation of gene expression. Single-molecule approaches have recently complemented conventional biochemical and biophysical techniques to decipher the complex mechanisms ruling chromatin dynamics. Micromanipulations with tweezers (magnetic or optical) and imaging with molecular microscopy (electron or atomic force) have indeed provided opportunities to handle and visualize single molecules, and to measure the forces and torques produced by molecular motors, along with their effects on DNA or nucleosomal templates. By giving access to dynamic events that tend to be blurred in traditional biochemical bulk experiments, these techniques provide critical information regarding the mechanisms underlying the regulation of gene activation and deactivation by nucleosome and chromatin structural changes. This minireview describes some single-molecule approaches to the study of ATP-consuming molecular motors acting on DNA, with applications to the case of nucleosome-remodelling machines.


Accurate measurement of microscopic forces and torques using optical tweezers

Melanie McLaren, Elias Sidderas-Haddad, Andrew Forbes

It is now well known that matter may be trapped by optical fields with high intensity gradients. Once trapped, it is then possible to manipulate microscopic particles using such optical fields, in so-called optical tweezers. Such optical trapping and tweezing systems have found widespread application across diverse fields in science, from applied biology to fundamental physics. In this article we outline the design and construction of an optical trapping and tweezing system, and show how the resulting interaction of the laser light with microscopic particles may be understood in terms of the transfer of linear and angular momentum of light. We demonstrate experimentally the use of our optical tweezing configuration for the measurement of microscopic forces and torques. In particular, we make use of digital holography to create so-called vortex laser beams, capable of transferring orbital angular momentum to particles. The use of such novel laser beams in an optical trapping and tweezing set-up allows for the control of biological species at the single-cell level.


Single-base pair unwinding and asynchronous RNA release by the hepatitis C virus NS3 helicase

Wei Cheng, Srikesh G. Arunajadai, Jeffrey R. Moffitt, Ignacio Tinoco Jr., Carlos Bustamante

Nonhexameric helicases use adenosine triphosphate (ATP) to unzip base pairs in double-stranded nucleic acids (dsNAs). Studies have suggested that these helicases unzip dsNAs in single–base pair increments, consuming one ATP molecule per base pair, but direct evidence for this mechanism is lacking. We used optical tweezers to follow the unwinding of double-stranded RNA by the hepatitis C virus NS3 helicase. Single–base pair steps by NS3 were observed, along with nascent nucleotide release that was asynchronous with base pair opening. Asynchronous release of nascent nucleotides rationalizes various observations of its dsNA unwinding and may be used to coordinate the translocation speed of NS3 along the RNA during viral replication.


Hydrodynamic Pair Attractions between Driven Colloidal Particles

Yulia Sokolov, Derek Frydel, David G. Grier, Haim Diamant, and Yael Roichman

Colloidal spheres driven through water along a circular path by an optical ring trap display unexpected dynamical correlations. We use Stokesian dynamics simulations and a simple analytical model to demonstrate that the path’s curvature breaks the symmetry of the two-body hydrodynamic interaction, resulting in particle pairing. The influence of this effective nonequilibrium attraction diminishes as either the temperature or the stiffness of the radial confinement increases. We find a well-defined set of dynamically paired states whose stability relies on hydrodynamic coupling in curving trajectories.


Thursday, October 6, 2011

Resonances arising from hydrodynamic memory in Brownian motion

Thomas Franosch, Matthias Grimm, Maxim Belushkin, Flavio M. Mor, Giuseppe Foffi, László Forró & Sylvia Jeney

Observation of the Brownian motion of a small probe interacting with its environment provides one of the main strategies for characterizing soft matter. Essentially, two counteracting forces govern the motion of the Brownian particle. First, the particle is driven by rapid collisions with the surrounding solvent molecules, referred to as thermal noise. Second, the friction between the particle and the viscous solvent damps its motion. Conventionally, the thermal force is assumed to be random and characterized by a Gaussian white noise spectrum. The friction is assumed to be given by the Stokes drag, suggesting that motion is overdamped at long times in particle tracking experiments, when inertia becomes negligible. However, as the particle receives momentum from the fluctuating fluid molecules, it also displaces the fluid in its immediate vicinity. The entrained fluid acts back on the particle and gives rise to long-range correlations. This hydrodynamic ‘memory’ translates to thermal forces, which have a coloured, that is, non-white, noise spectrum. One hundred years after Perrin’s pioneering experiments on Brownian motion, direct experimental observation of this colour is still elusive. Here we measure the spectrum of thermal noise by confining the Brownian fluctuations of a microsphere in a strong optical trap. We show that hydrodynamic correlations result in a resonant peak in the power spectral density of the sphere’s positional fluctuations, in strong contrast to overdamped systems. Furthermore, we demonstrate different strategies to achieve peak amplification. By analogy with microcantilever-based sensors, our results reveal that the particle–fluid–trap system can be considered a nanomechanical resonator in which the intrinsic hydrodynamic backflow enhances resonance. Therefore, instead of being treated as a disturbance, details in thermal noise could be exploited for the development of new types of sensor and particle-based assay in lab-on-a-chip applications.


High precision and continuous optical transport using a standing wave optical line trap

Vassili Demergis and Ernst-Ludwig Florin

We introduce the Standing Wave Optical Line Trap (SWOLT) as a novel tool for precise optical manipulation and long-range transport of nano-scale objects at low laser power. We show that positioning and transport along the trap can be achieved by controlling the lateral component of the scattering force while the confinement of the particles by the gradient force remains unaffected. Multiple gold nanoparticles with a diameter of 100nm were trapped at a power density 3 times smaller than previously reported while their transverse fluctuations remained sufficiently small (±36nm) to maintain the order of the particles. The SWOLT opens new doors for sorting, mixing, and assembly of synthetic and biological nanoparticles.


Optically trapped microsensors for microfluidic temperature measurement by fluorescence lifetime imaging microscopy

Mathieu A. Bennet, Patricia R. Richardson, Jochen Arlt, Aongus McCarthy, Gerald S. Buller and Anita C. Jones
The novel combination of optical tweezers and fluorescence lifetime imaging microscopy (FLIM) has been used, in conjunction with specially developed temperature-sensitive fluorescent microprobes, for the non-invasive measurement of temperature in a microfluidic device. This approach retains the capability of FLIM to deliver quantitative mapping of microfluidic temperature without the disadvantageous need to introduce a fluorescent dye that pervades the entire micofluidic system. This is achieved by encapsulating the temperature-sensitive Rhodamine B fluorophore within a microdroplet which can be held and manipulated in the microfluidic flow using optical tweezers. The microdroplet is a double bubble in which an aqueous droplet of the fluorescent dye is surrounded by an oil shell which serves both to contain the fluorophore and to provide the refractive index differential required for optical trapping of the droplet in an external aqueous medium.


Poly(ethylene oxide) Adsorption onto and Desorption from Silica Microspheres: New Insights from Optical Tweezers Electrophoresis

Jan A. van Heiningen and Reghan J. Hill

By measuring the changing electrophoretic mobility of single optically trapped silica microspheres (radius a ≈ 0.4 μm) during poly(ethylene oxide) homopolymer adsorption and desorption, we study polymer-layer kinetics at various polymer solution flow rates, concentrations, molecular weights, and polydispersities. At polymer concentrations c 5 ppm (mg L–1), Péclet numbers Pe 20, and Reynolds numbers Re 1, the adsorbing layer growth is mass-transport-limited, with time scales 10 s that are resolved on the uniquely small, micrometer length scale of optical tweezers electrophoresis (OTE) experiments. However, during adsorption, layer growth becomes limited by surface diffusion, reconformation, and exchange processes. Two characteristic relaxation times are revealed by the OTE time series. The faster time scale increases with polymer concentration and plateaus to 3 s when c 10 ppm. This reflects layer development kinetics limited by surface diffusion and reconformation. The slower time scale is 100 s and reflects polymer exchange, which thermodynamically favors large adsorbed coils when solutions are polydisperse. Desorption is even slower but occurs faster than expected by local-equilibrium theory, possibly because of high shear rates 100 s–1. The dynamic states probed by OTE are often sufficiently far from equilibrium that they cannot be adequately described by theories for equilibrium polymer adsorption, mass-transport-limited kinetics, or kinetics based on local equilibrium.


Elliptical optical tweezers for trapping a red blood cell aggregate

Antti J. Kauppila, Matti Kinnunen, Artashes Karmenyan, Risto A. Myllylä

Red blood cells (RBCs) and their aggregates scatter light strongly. Optical tweezers facilitate the manipulation of cells and light scattering measurements at the single cell level. We show in this letter the successful trapping of an RBCaggregate with elliptical optical tweezers in a free suspension. Two cylindrical lenses are used to form the trap. The distance between the trapped objects and the bottom of the cuvette had a clear effect on image quality. Rotating the cylindrical lenses changes the orientation of the aggregate, which produces a visible change in the light scattering intensity images taken by a CCD camera.


Wednesday, October 5, 2011

Position clamping of optically trapped microscopic non-spherical probes

D. B. Phillips, S. H. Simpson, J. A. Grieve, G. M. Gibson, R. Bowman, M. J. Padgett, M. J. Miles, and D. M. Carberry

We investigate the degree of control that can be exercised over an optically trapped microscopic non-spherical force probe. By position clamping translational and rotational modes in different ways, we are able to dramatically improve the position resolution of our probe with no reduction in sensitivity. We also demonstrate control over rotational-translational coupling, and exhibit a mechanism whereby the average centre of rotation of the probe can be displaced away from its centre.


Multiplying optical tweezers force using a micro-lever

Chih-Lang Lin, Yi-Hsiung Lee, Chin-Te Lin, Yi-Jui Liu, Jiann-Lih Hwang, Tien-Tung Chung, and Patrice L. Baldeck

This study presents a photo-driven micro-lever fabricated to multiply optical forces using the two-photon polymerization 3D-microfabrication technique. The micro-lever is a second class lever comprising an optical trapping sphere, a beam, and a pivot. A micro-spring is placed between the short and long arms to characterize the induced force. This design enables precise manipulation of the micro-lever by optical tweezers at the micron scale. Under optical dragging, the sphere placed on the lever beam moves, resulting in torque that induces related force on the spring. The optical force applied at the sphere is approximately 100 to 300 pN, with a laser power of 100 to 300 mW. In this study, the optical tweezers drives the micro-lever successfully. The relationship between the optical force and the spring constant can be determined by using the principle of leverage. The arm ratio design developed in this study multiplies the applied optical force by 9. The experimental results are in good agreement with the simulation of spring property.


Laser-guidance-based cell deposition microscope for heterotypic single-cell micropatterning

Zhen Ma, Russell K Pirlo, Qin Wan, Julie X Yun, Xiaocong Yuan, Peng Xiang, Thomas K Borg and Bruce Z Gao

Cell patterning methods enable researchers to control specific homotypic and heterotypic contact-mediated cell–cell and cell–ECM interactions and to impose defined cell and tissue geometries. To micropattern individual cells to specific points on a substrate with high spatial resolution, we have developed a cell deposition microscope based on the laser-guidance technique. We discuss the theory of optical forces for generating laser guidance and the optimization of the optical configuration (NA ≈ 0.1) to manipulate cells with high speed in three dimensions. Our cell deposition microscope is capable of patterning different cell types onto and within standard cell research devices and providing on-stage incubation for long-term cell culturing. Using this cell deposition microscope, rat mesenchymal stem cells from bone marrow were micropatterned with cardiomyocytes into a substrate microfabricated with polydimethylsiloxane on a 22 mm × 22 mm coverglass to form a single-cell coculturing microenvironment, and their electrophysiological property changes were investigated during the coculturing days.


Trapping and two-photon fluorescence excitation of microscopic objects using ultrafast single-fiber optical tweezers

Yogeshwar N. Mishra, Ninad Ingle, and Samarendra K. Mohanty

Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest. We report on the development of single fiber ultrafast optical tweezers and its use in simultaneous two-photon fluorescence (TPF) excitation of trapped fluorescent microscopic objects. Using this method, trapping depth of a few centimeters was achieved inside a colloidal sample with TPF from the trapped particle being visible to the naked eye. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of the axicon was engineered so as to provide better trapping stability and high axial confinement of TPF. Trapping of the floating objects led to stable fluorescence emission intensity over a long period of time, suitable for spectroscopic measurements. Furthermore, the stability of the fiber optic trapping was confirmed by holding and maneuvering the fiber by hand so as to move the trapped fluorescent particle in three dimensions. Apart from miniaturization capability into lab-on-a-chip microfluidic devices, the proposed noninvasive microaxicon tipped optical fiber can be used in multifunctional mode for in-depth trapping, rotation, sorting, and ablation, as well as for two-photon fluorescence excitation of a motile sample.


FDTD simulation of trapping nanowires with linearly polarized and radially polarized optical tweezers

Jing Li and Xiaoping Wu
In this paper a model of the trapping force on nanowires is built by three dimensional finite-difference time-domain (FDTD) and Maxwell stress tensor methods, and the tightly focused laser beam is expressed by spherical vector wave functions (VSWFs). The trapping capacities on nanoscale-diameter nanowires are discussed in terms of a strongly focused linearly polarized beam and radially polarized beam. Simulation results demonstrate that the radially polarized beam has higher trapping efficiency on nanowires with higher refractive indices than linearly polarized beam.


Monday, October 3, 2011

Dynamic Force Sensing Using an Optically Trapped Probing System

Yanan Huang; Peng Cheng; Chia-Hsiang Menq

This paper presents the design of an adaptive observer that is implemented to enable real-time dynamic force sensing and parameter estimation in an optically trapped probing system. According to the principle of separation of estimation and control, the design of this observer is independent of that of the feedback controller when operating within the linear range of the optical trap. Dynamic force sensing, probe steering/clamping, and Brownian motion control can, therefore, be developed separately and activated simultaneously. The adaptive observer utilizes the measured motion of the trapped probe and input control effort to recursively estimate the probe-sample interaction force in real time, along with the estimation of the probing system's trapping bandwidth. This capability is very important to achieving accurate dynamic force sensing in a time-varying process, wherein the trapping dynamics is nonstationary due to local variations of the surrounding medium. The adaptive estimator utilizes the Kalman filter algorithm to compute the time-varying gain in real time and minimize the estimation error for force probing. A series of experiments are conducted to validate the design of and assess the performance of the adaptive observer.


Controlling molecular transport through nanopores

Ulrich F. Keyser

Nanopores are emerging as powerful tools for the detection and identification of macromolecules in aqueous solution. In this review, we discuss the recent development of active and passive controls over molecular transport through nanopores with emphasis on biosensing applications. We give an overview of the solutions developed to enhance the sensitivity and specificity of the resistive-pulse technique based on biological and solid-state nanopores.


The morphology of aerosol particles consisting of hydrophobic and hydrophilic phases: hydrocarbons, alcohols and fatty acids as the hydrophobic component

Jonathan P. Reid, Benjamin J. Dennis-Smither, Nana-Owusua A. Kwamena, Rachael E. H. Miles, Kate L. Hanford and Christopher J. Homer

The morphology of bi-phase aerosol particles containing phase separated hydrophobic and hydrophilic components is considered, comparing simulations based on surface and interfacial tensions with measurements made by aerosol optical tweezers. The competition between the liquid phases adopting core–shell and partially engulfed configurations is considered for a range of organic compounds including saturated and unsaturated hydrocarbons, aromatics, alcohols, ketones, carboxylic acids, esters and amines. When the solubility of the organic component and the salting-out of the organic component to the surface by the presence of concentrated inorganic solutes in the aqueous phase are considered, it is concluded that the adoption of a partially engulfed structure predominates, with the organic component forming a surface lens. The aqueous surface can be assumed to be stabilised by a surface enriched in the organic component. The existence of acid–base equilibria can lead to the dissociation of organic surfactants and to significant lowering of the surface tension of the aqueous phase, further supporting the predominance of partially engulfed structures. Trends in morphology from experimental measurements and simulations are compared for mixed phased droplets in which the organic component is decane, 1-octanol or oleic acid with varying relative humidity. The consequences of partially engulfed structures for aerosol properties are considered.


Anisotropic hydrodynamic mean-field theory for semiflexible polymers under tension

Hinczewski, M., Netz, R.R.

We introduce an anisotropic mean-field approach for the dynamics of semiflexible polymers under intermediate tension, the force range where a chain is partially extended but not in the asymptotic regime of a nearly straight contour. The theory is designed to exactly reproduce the lowest order equilibrium averages of a stretched polymer, and it treats the full complexity of the problem: the resulting dynamics include the coupled effects of long-range hydrodynamic interactions, backbone stiffness, and large-scale polymer contour fluctuations. Validated by Brownian hydrodynamics simulations and comparison to optical tweezer measurements on stretched DNA, the theory is highly accurate in the intermediate tension regime over a broad dynamical range, without the need for additional dynamic fitting parameters.


Unique single molecule binding of cardiac myosin binding protein-C to actin and phosphorylation-dependent inhibition of actomyosin motility requires 17 amino acids of the motif domain

Abbey Weith, Sakthivel Sadayappan, James Gulick, Michael J. Previs, Peter VanBuren, Jeffrey Robbins, David M. Warshaw

Cardiac myosin binding protein-C (cMyBP-C) has 11 immunoglobulin or fibronectin-like domains, C0 through C10, which bind sarcomeric proteins, including titin, myosin and actin. Using bacterial expressed mouse N-terminal fragments (C0 through C3) in an in vitro motility assay of myosin-generated actin movement and the laser trap assay to assess single molecule actin-binding capacity, we determined that the first N-terminal 17 amino acids of the cMyBP-C motif (the linker between C1 and C2) contain a strong, stereospecific actin-binding site that depends on positive charge due to a cluster of arginines. Phosphorylation of 4 serines within the motif decreases the fragments’ actin-binding capacity and actomyosin inhibition. Using the laser trap assay, we observed individual cMyBP-C fragments transiently binding to a single actin filament with both short (~ 20 ms) and long (~ 300 ms) attached lifetimes, similar to that of a known actin-binding protein, α-actinin. These experiments suggest that cMyBP-C N-terminal domains containing the cMyBP-C motif tether actin filaments and provide one mechanism by which cMyBP-C modulates actomyosin motion generation, i.e. by imposing an effective viscous load within the sarcomere.


Separability of electrostatic and hydrodynamic forces in particle electrophoresis

Brian A. Todd, Joel A. Cohen

By use of optical tweezers we explicitly measure the electrostatic and hydrodynamic forces that determine the electrophoretic mobility of a charged colloidal particle. We test the ansatz of O'Brien and White [ J. Chem. Soc. Faraday II 74 1607 (1978)] that the electrostatically and hydrodynamically coupled electrophoresis problem is separable into two simpler problems: (1) a particle held fixed in an applied electric field with no flow field and (2) a particle held fixed in a flow field with no applied electric field. For a system in the Helmholtz-Smoluchowski and Debye-Hückel regimes, we find that the electrostatic and hydrodynamic forces measured independently accurately predict the electrophoretic mobility within our measurement precision of 7%; the O'Brien and White ansatz holds under the conditions of our experiment.


Combined acoustic and optical trapping

G. Thalhammer, R. Steiger, M. Meinschad, M. Hill, S. Bernet, and M. Ritsch-Marte

Combining several methods for contact free micro-manipulation of small particles such as cells or micro-organisms provides the advantages of each method in a single setup. Optical tweezers, which employ focused laser beams, offer very precise and selective handling of single particles. On the other hand, acoustic trapping with wavelengths of about 1 mm allows the simultaneous trapping of many, comparatively large particles. With conventional approaches it is difficult to fully employ the strengths of each method due to the different experimental requirements. Here we present the combined optical and acoustic trapping of motile micro-organisms in a microfluidic environment, utilizing optical macro-tweezers, which offer a large field of view and working distance of several millimeters and therefore match the typical range of acoustic trapping. We characterize the acoustic trapping forces with the help of optically trapped particles and present several applications of the combined optical and acoustic trapping, such as manipulation of large (75 μm) particles and active particle sorting.