Monday, June 29, 2009

Using electrical and optical tweezers to facilitate studies of molecular motors

Mark E. Arsenault, Yujie Sun, Haim H. Bau and Yale E. Goldman

Dielectrophoresis was used to stretch and suspend actin filaments across a trench etched between two electrodes patterned on a glass slide. Optical tweezers were used to bring a motor protein-coated bead into close proximity to a pre-selected, suspended actin filament, facilitating the attachment of the myosin-coated bead to the filament. The clearance beneath the filament allowed the bead to move freely along and around its filamentous track, unhindered by solid surfaces. Using defocused images, the three-dimensional position of the bead was tracked as a function of time to obtain its trajectory. Experiments were carried out with myosin V and myosin X. Both motor proteins followed left-handed helical paths with the myosin X motor exhibiting a shorter pitch than the myosin V. The combined use of electrostatic and optical tweezers facilitates the preparation of motility assays with suspended tracks. Variants of this technique will enable higher complexity experiments in vitro to better understand the behavior of motors in cells.

Hysteresis in cross-bridge models of muscle

Sam Walcott and Sean X. Sun

A dynamical system is said to exhibit hysteresis if its current state depends on its history. Muscle shows hysteretic properties at constant length, such as residual force enhancement after stretch. There is no generally accepted explanation for residual force enhancement. Here we examine a very simple kinetic model for the interaction between actin and myosin, the two main proteins involved in muscle contraction. We demonstrate that this model shows hysteresis at constant force. Since muscle is not a continuum but rather a group of repeating elements, called sarcomeres, arranged in series, we perform simulations of three sarcomeres. These simulations show hysteresis at constant length. This result is the first time that residual force enhancement has been demonstrated using an experimentally motivated kinetic model and multi-sarcomere simulations without passive elastic elements, damping and/or force-length relationships. We conclude by suggesting some experiments to test the model's predictions. If these experiments support the model, it becomes important to understand multiple sarcomere systems, since their behavior may be very different from most current simulations that neglect the coupling between sarcomeres.

Twist–stretch coupling and phase transition during DNA supercoiling

Maxim Y. Sheinin and Michelle D. Wang

As a single DNA molecule is positively supercoiled under constant tension, its extension initially increases due to a negative twist–stretch coupling. The subsequent attainment of an extension maximum has previously been assumed to be indicative of the onset of a phase transition from B- to scP-DNA. Here we show that an extension maximum in fact does not coincide with the onset of a phase transition. This transition is evidenced by a direct observation of a torque plateau using an angular optical trap. Instead we find that the shape of the extension curve can be well explained with a theory that incorporates both DNA twist–stretch coupling and bending fluctuations. This theory also provides a more accurate method of determining the value of the twist–stretch coupling modulus, which has possibly been underestimated in previous studies that did not take into consideration the bending fluctuations. Our study demonstrates the importance of torque detection in the correct identification of phase transitions as well as the contribution of the twist–stretch coupling and bending fluctuations to DNA extension.

Direct experimental evidence for quadruplex-quadruplex interaction within the human ILPR

Joseph D. Schonhoft, Rabindra Bajracharya, Soma Dhakal, Zhongbo Yu, Hanbin Mao and Soumitra Basu

Here we report the analysis of dual G-quadruplexes formed in the four repeats of the consensus sequence from the insulin-linked polymorphic region (ACAGGGGTGTGGGG; ILPRn4). Mobilities of ILPRn4 in nondenaturing gel and circular dichroism (CD) studies confirmed the formation of two intramolecular G-quadruplexes in the sequence. Both CD and single molecule studies using optical tweezers showed that the two quadruplexes in the ILPRn4 most likely adopt a hybrid G-quadruplex structure that was entirely different from the mixture of parallel and antiparallel conformers previously observed in the single G-quadruplex forming sequence (ILPRn2). These results indicate that the structural knowledge of a single G-quadruplex cannot be automatically extrapolated to predict the conformation of multiple quadruplexes in tandem. Furthermore, mechanical pulling of the ILPRn4 at the single molecule level suggests that the two quadruplexes are unfolded cooperatively, perhaps due to a quadruplexquadruplex interaction (QQI) between them. Additional evidence for the QQI was provided by DMS footprinting on the ILPRn4 that identified specific guanines only protected in the presence of a neighboring G-quadruplex. There have been very few experimental reports on multiple G-quadruplex-forming sequences and this report provides direct experimental evidence for the existence of a QQI between two contiguous G-quadruplexes in the ILPR.

Radius measurements of optically trapped aerosols through Brownian motion

    D R Burnham and D McGloin

Optical trapping of liquid aerosols from polydisperse samples provides unique problems for measuring their radii. Perhaps the most precise method, cavity-enhanced Raman spectroscopy (CERS), is limited to relatively large aerosols (>2 mu m in radius). Determining particle perimeters in video microscopy lacks precision and, although simple, can be ambiguous. Here we demonstrate a simple and precise method based on studying the Brownian motion of droplets as they approach a nearby surface. We obtain results with greater precision and reliability than video microscopy, and with no size limitation conclude the technique could compete with CERS in terms of precision and accuracy.

Generalized Lorenz-Mie theory and applications

James A. Lock and Gérard Gouesbet

The basic formulas of generalized Lorenz–Mie theory are presented, and are applied to scattering of a focused Gaussian laser beam by a spherical particle. Various applications of focused beam scattering are also described, such as optimizing the rate at which morphology-dependent resonances are excited, laser trapping, particle manipulation, and the analysis of optical particle sizing instruments. Each of these applications requires either special positioning the beam with respect to the particle or illumination of only part of the particle by the beam.

Effects of magnesium salt concentrations on B-DNA overstretching transition

H. Fu, H. Chen, C. G. Koh and C. T. Lim

In this study, we use optical tweezers to investigate the ionic effects of magnesium salt solutions on the overstretching transition of single B-DNA molecules. The experimental data are compared with those in sodium salt solutions. The overstretching transition force increases when the NaCl or MgCl2 salt concentration increases. Magnesium cations have much stronger effects on the overstretching transition force than sodium cations. For both NaCl and MgCl2 salt solutions, the overstretching transition force is linear with the natural logarithm of salt concentration, which confirms the theory proposed in previous paper. The modified ZZO model is applied to study the electrostatic contribution of magnesium salt solutions to the overstretching transition of single B-DNA molecules. The consistency between the experimental data and analytical results shows that the modified ZZO model can simulate the transition behavior of single B-DNA molecules in different NaCl and MgCl2 salt solutions.

Thursday, June 25, 2009

Coalescence of levitated polystyrene microspheres

A.J. Trevitt, P.J. Wearne and E.J. Bieske

The slow, laser-induced coalescence of two conjoined, polystyrene spheres levitated in a quadrupole ion trap is investigated by monitoring optical morphology dependent resonances (MDRs) appearing in the fluorescence emission spectrum. The heated bisphere is driven by surface tension to become a single, larger sphere with a volume equal to the combined volumes of the two initial spheres. In the final stage of the structural transformation the particle is a prolate spheroid whose dimensions are ascertained by analyzing frequency shifts of the non-degenerate azimuthal MDRs. The relaxation time for the deformed viscous sphere is used to estimate the polystyrene viscosity and temperature. The study highlights the feasibility of using a temperature-controlled quadrupole ion trap to investigate the coalescence dynamics of viscous microstructures free from the perturbative effects of any solvent or substrate.

Touching the microworld with force-feedback optical tweezers

Cécile Pacoret, Richard Bowman, Graham Gibson, Sinan Haliyo, David Carberry,Arvid Bergander, Stéphane Régnier, and Miles Padgett

Optical tweezers are a powerful tool for micromanipulation and measurement of picoNewton sized forces. However, conventional interfaces present difficulties as the user cannot feel the forces involved. We present an interface to optical tweezers, based around a low-cost commercial force feedback device. The different dynamics of the micro-world make intuitive force feedback a challenge. We propose a coupling method using an existing optical tweezers system and discuss stability and transparency. Our system allows the user to perceive real Brownian motion and viscosity, as well as forces exerted during manipulation of objects by a trapped bead.

Focal shift and focal switch of flat-topped Mathieu–Gaussian beams passing through an apertured lens system

Bin Tang and Wei Wen

By introducing a hard aperture function into a finite sum of complex Gaussian functions, an approximate analytical expression predominating the distribution of axial intensity for the flat-topped Mathieu–Gauss (FTMG) beams passing through a system with the aperture and lens separated has been derived. The focal shift and the focal switch effect of FTMG beams passing through the system is studied in detail. Numerical calculations have shown that the position of real focal plane is not coincident with the geometrical focus but is somewhat shifted toward the lens. The focal shift and focal switch of FTMG beams take place when the relative separation s/f = 1 by a suitable choice of beam parameter and truncation parameter, for example, the beam parameter is smaller than its corresponding critical value or the truncation parameter is between its two corresponding critical values.

Sequence-specific physical properties of African green monkey alpha-satellite DNA contribute to centromeric heterochromatin formation

Malte Bussiek, Christian Hoischen, Stephan Diekmann and Martin L. Bennink

Satellite DNA, a major component of eukaryotic centromeric heterochromatin, is potentially associated with the processes ensuring the faithful segregation of the genetic material during cell division. Structural properties of alpha-satellite DNA (AS) from African green monkey (AGM) were studied. Atomic force microscopy imaging showed smaller end-to-end distances of AS fragments than would be expected for the persistence length of random sequence DNA. The apparent persistence length of the AS was determined as 35 nm. Gel-electrophoresis indicated only a weak contribution of intrinsic curvature to the DNA conformations suggesting an additional contribution of an elevated bending flexibility to the reduced end-to-end distances. Next, the force-extension behavior of the naked AS and in complex with nucleosomes was studied using optical tweezers. The naked AS showed a reduced overstretching transition force (−18% the value determined for random DNA) and higher forces required to straighten the DNA. Finally, reconstituted AS nucleosomes disrupted at significantly higher forces as compared with random DNA nucleosomes which is probably due to structural properties of the AS which stabilize the nucleosomes. The data support that the AS plays a role in the formation of centromeric heterochromatin due to specific structural properties and suggest that a relatively higher mechanical stability of nucleosomes is important in AGM–AS chromatin.

Studying taxis in real time using optical tweezers: Applications for Leishmania amazonensis parasites

L.Y. Pozzo, A. Fontes, A.A. de Thomaz, B.S. Santos, P.M.A. Farias, D.C. Ayres, S. Giorgio and C.L. Cesar

Beads trapped by an optical tweezers can be used as a force transducer for measuring forces of the same order of magnitude as typical forces induced by flagellar motion. We used an optical tweezers to study chemotaxis by observing the force response of a flagellated microorganism when placed in a gradient of attractive chemical substances. This report shows such observations for Leishmania amazonensis, responsible for leishmaniasis, a serious disease. We quantified the movement of this protozoan for different gradients of glucose. We were able to observe both the strength and the directionality of the force. The characterization of the chemotaxis of these parasites can help to understand the mechanics of infection and improve the treatments employed for this disease. This methodology can be used to quantitatively study the taxis of any kind of flagellated microorganisms under concentration gradients of different chemical substances, or even other types of variable gradients such as temperature and pressure.

Nanometer gold–silica composite particles manipulated by optical tweezers

Mariela Rodriguez-Otazo, Angel Augier-Calderin and Jean-Pierre Galaup

We have studied the behavior of nano or micro size composite particles submitted to optical trapping forces and a comparison was made with homogeneous particles of similar dimension. The forces were measured using the power spectrum signal analysis. Most of the results presented were obtained using a lateral effect position sensitive detector (PSD), which allowed the fluctuations of the particle position in the optical trap to be monitored. A 4-quadrant photodiode was also used for the same purpose. We bring clear experimental evidence that the trapping force was increased by a factor of about 2–3 for composite particles made of a colloidal gold core encapsulated in a silica shell, with respect to homogeneous silica or latex beads. These results were discussed in the frame of the various approaches currently used for modeling optical tweezing forces.

Raman spectroscopic analysis of apoptosis of single human gastric cancer cells

Huilu Yao, Zhanhua Tao, Min Ai, Lixin Peng, Guiwen Wang, Bijuan He and Yong-qing Li

A drug (5-FU) was employed to treat the gastric carcinoma cells and induce apoptosis of the cancer cells. Raman spectra obtained from single gastric carcinoma cells and the induced apoptotic cells through scan-excitation mode were used to analyze the effectiveness of the treatment. The major difference of the apoptotic cells from the cancer cells are the reduction in intensities of vibration bands generated by cellular lipids, proteins and nucleic acids. In particular, large intensity reduction in nucleic vibrations at 782, 1092, 1320, 1340, and 1578 cm−1 was observed upon apoptosis of the gastric carcinoma cells. Up to 45% reduction in the magnitude of the 782 cm−1 peak in Raman spectra of the apoptotic cells was observed, which suggests the breakdown of phosphodiester bonds and DNA bases. We showed that the principal components analysis (PCA), a multivariate statistical tool, can be used to distinguish single apoptotic cells and gastric carcinoma cells based on their Raman spectra.


Influence of time delay on trap stiffness in computer-controlled scanning optical tweezers

Sun-Uk Hwang and Yong-Gu Lee

In scanning optical tweezers, the positions of optically trapped particles are individually controlled by periodically updating the scan data, which make up the data set of time-shared positions of a single-beam optical trap. The scan data are usually generated from the computer and are then transferred to the controller that sends electrical signals to the scanner. During the data generation and transfer phases, a short but noticeable time delay, indicated by the updating of scan data, is incurred and the scanner momentarily stalls. In this paper, we investigate the influence of this time delay on the trapped beads in scanning optical tweezers having a low scanning frequency. To understand the influence of the time delay, we analyzed the motions of two beads, trapped by the scanning optical traps, having a finite distance between them. We found that the trapped beads undergo relatively large vibrations during scanning, and the trap stiffness for one of the two beads significantly decreases with the time delay. We anticipate that our observation is essential to design scanning optical traps that need to manipulate trapped beads in real-time.

Dynamic force spectroscopy of the Helicobacter pylori BabA–Lewis b Binding

Oscar Björnham, Jeanna Bugaytsova, Thomas Borén and Staffan Schedin

The binding strength of the Helicobacter pylori adhesin–receptor complex BabA-ABO/Lewis b has been analyzed by means of dynamic force spectroscopy. High-resolution measurements of rupture forces were performed in situ on single bacterial cells, expressing the high-affinity binding BabA adhesin, by the use of force measuring optical tweezers. The resulting force spectra revealed the mechanical properties of a single BabA–Leb bond. It was found that the bond is dominated by one single energy barrier and that it is a slip-bond. The bond length and thermal off-rate were assessed to be 0.86 ± 0.07 nm and 0.015 ± 0.006 s− 1, respectively.


Tuesday, June 23, 2009

Optical manipulation of a particle placed within a planar dielectric cavity

Pavlos G. Galiatsatos and Vassilios Yannopapas

We study theoretically the electromagnetic force exerted on a small dielectric particle lying within a planar, dielectric cavity, under plane-wave illumination. We find, in particular, that due to the confinement of an incident wavefield within the cavity, the force exerted on the particle is several orders of magnitude stronger than the corresponding force when the particle is placed next to a single planar slab. We also observed when the particle is placed in the middle of the cavity, it can be trapped at frequencies corresponding to the transmission maxima. We have also found that, for any given position within the cavity, there exist several frequencies for which the particle can be trapped. For the case of a Bragg-stack cavity, there exist more trapping frequencies in a given spectral region allowing for easier optical manipulation of the particle.

Friday, June 19, 2009

High rotation speed of single molecular microcrystals in an optical trap with elliptically polarized light

Mariela Rodriguez-Otazo, Angel Augier-Calderin, Jean-Pierre Galaup, Jean-François Lamère, and Suzanne Fery-Forgues

We build an experiment of optical tweezers based on the use of an inverted optical microscope for manipulating microsized single crystals, which are made of an organic dye and parallelepiped in shape. The microcrystals are directed so that their long axis is in the axial direction of the trapping beam. Their short axis follows the direction of the linear polarization of the beam. In circular or elliptic polarization, the crystals are spontaneously put in rotation with a high speed of up to 500 turns per second. It is the first time, to the best of our knowledge, that such a result is reported for particles of the size of our crystals. Another surprising result is that the rotation speed was first increased as expected by increasing the incident power;, but after passing by a maximum it decreased until the complete stop of rotation, whereas the power continued growing. This evolution was not reversible. Several hypotheses are discussed to explain such behavior.

Quantitative assessment of non-conservative radiation forces in an optical trap

Giuseppe Pesce, Giorgio Volpe, Anna Chiara De Luca, Giulia Rusciano and Giovanni Volpe

The forces acting on an optically trapped particle are usually assumed to be conservative. However, the presence of a non-conservative component has recently been demonstrated. Here, we propose a technique that permits one to quantify the contribution of such a non-conservative component. This is an extension of a standard calibration technique for optical tweezers and, therefore, can easily become a standard test to verify the conservative optical force assumption. Using this technique, we have analyzed optically trapped particles of different size under different trapping conditions. We conclude that the non-conservative effects are effectively negligible and do not affect the standard calibration procedure, unless for extremely low-power trapping, far away from the trapping regimes usually used in experiments.

Visible anisotropic deformation of chalcogenide glass by optical force

Keiji Tanaka, Nobuaki Terakado, Akira Saitoh

Semi-free As2S3 glass flakes undergo sub-mm scale anisotropic deformation upon illumination of linearly-polarized cw bandgap light. The mechanism has been ascribed to photo-induced birefringence, optical torque, and photo-induced fluidity. Quantitative measurements demonstrate that generated optical torque is 10-20 Nm under a light intensity of 1 mW. This value combined with measured deformation rates for As2S3 cantilevers suggests a related photo-induced fluidity of 10-1 P-1, which is much more fluidal (ten orders) than that reported in the conventional photo-induced fluidity. Reasons for this discrepancy are discussed.

Novel contact probing method using single fiber optical trapping probe

Sang In Eom, Yasuhiro Takaya and Terutake Hayashi

A novel contact probing method for microdevices with high aspect ratio or biological samples is proposed. In this technique, a dielectric microsphere is optically trapped by an optical fiber and used as a touch probe. In the simulations, the finite difference time domain (FDTD) method and Maxwell stress theory are applied to obtain a suitable shape for the tip of the optical fiber. The results show that it is possible to trap the microsphere by using a single optical fiber. In experiments, single fiber optical trapping is successfully demonstrated by considering the simulation results. In order to use the trapped microsphere in the touch probe, the intensity of the reentered beam that is reflected from the surface of the microsphere is monitored. When the probe is in contact with the surface of the object, the intensity of the beam changes and this change is used as the contact signal. Because the probe is trapped optically and the trapping force is very small, this system can be used in a low invasive method.

Stretching Submicron Biomolecules with Constant-Force Axial Optical Tweezers

Yih-Fan Chen, Gerhard A. Blab and Jens-Christian Meiners

Optical tweezers have become powerful tools to manipulate biomolecular systems, but are increasingly difficult to use when the size of the molecules is <1>

Laser Fabrication and Manipulation of an Optical Rotator Embedded inside a Transparent Solid Material

Kiyama, S, Tomita, T, Matsuo, S, Hashimoto, S

We report the fabrication and rotation of an optical rotator confined in a microcavity inside silica glass material, based on the techniques of femtosecond laser-assisted etching and laser manipulation. The rotator thus prepared may act as a micropump and micromixer. The material of this rotator is identical to that of the host substrate, because femtosecond laser-assisted etching is an internal removal process. As a result, the rotator as well as substrate has high chemical stability and good optical transparency. We also describe tests in which we demonstrated that this fabrication technique is versatile enough to allow one to parepare two types of rotators with different shapes.

Generating Simplified Trapping Probability Models From Simulation of Optical Tweezers System

Banerjee AG, Balijepalli A, Gupta SK, LeBrun TW

This paper presents a radial basis function based approach to generatesimplified models to estimate the trapping probability in optical trappingexperiments using offline simulations. The difference form of Langevin's equation is used to perform physically accurate simulations of a particleunder the influence of a trapping potential and is used to estimate trapping probabilities at discrete points in the parameter space. Gaussian radial basis functions combined with kd-tree based partitioning of the parameter space are then used to generate simplified models of trapping probability. We show that the proposed approach is computationally efficient in estimating the trapping probability and that the estimated probability using the simplified models is sufficiently close to the probability estimates from offline simulation data.

Tuesday, June 16, 2009

Manipulation of Nanoparticles Using Dark-Field-Illumination Optical Tweezers with Compensating Spherical Aberration

Zhou Jin-Hua, Tao Run-Zhe, Hu Zhi-Bin, Zhong Min-Cheng, Wang Zi-Qiang, Cai Junand Li Yin-Mei

Based on our previous investigation of optical tweezers with dark field illumination [Chin. Phys. Lett. 25(2008)329], nanoparticles at large trap depth are better viewed in wide field and real time for a long time, but with poor forces. Here we present the mismatched tube length to compensate for spherical aberration of an oil-immersion objective in a glass-water interface in an optical tweezers system for manipulating nanoparticles. In this way, the critical power of stable trapping particles is measured at different trap depths. It is found that trap depth is enlarged for trapping nanoparticles and trapping forces are enhanced at large trap depth. According to the measurement, 70-nm particles are manipulated in three dimensions and observed clearly at large appropriate depth. This will expand applications of optical tweezers in a nanometre-scale colloidal system.

Friday, June 5, 2009

Nanophotonics: Gradient force shows its potential

Mark Freeman & Wayne Hiebert

An all-optical chip-based method has been used to actuate and detect the motion of silicon nanocantilevers. Multiplexed read-out has also been demonstrated.


Thursday, June 4, 2009

Optical neuronal guidance in three-dimensional matrices

Catherine E. Graves, Ryan G. McAllister, William J. Rosoff and Jeffrey S. Urbach

We demonstrate effective guidance of neurites extending from PC12 cells in a three-dimensional collagen matrix using a focused infrared laser. Processes can be redirected in an arbitrarily chosen direction in the imaging plane in approximately 30 min with an 80% success rate. In addition, the application of the laser beam significantly increases the rate of neurite outgrowth. These results extend previous observations on 2D coated glass coverslips. We find that the morphology of growth cones is very different in 3D than in 2D, and that this difference suggests that the filopodia play a key role in optical guidance. This powerful, flexible, non-contact guidance technique has potentially broad applications in tissues and engineered environments.


High-Resolution Probing of Cellular Force Transmission

Daisuke Mizuno, Rommel Bacabac, Catherine Tardin, David Head, and Christoph F. Schmidt

Cells actively probe mechanical properties of their environment by exerting internally generated forces. The response they encounter profoundly affects their behavior.Here we measure in a simple geometry the forces a cell exerts suspended by two optical traps. Our assay quantifies both the overall force and the fraction of that force transmitted to the environment. Mimicking environments of varying stiffness by adjusting the strength of the traps, we found that the force transmission is highly dependent on external compliance. This suggests a calibration mechanism for cellular mechanosensing.

Wednesday, June 3, 2009

Optical stress on the surface of a particle: Homogeneous sphere

Feng Xu, James A. Lock, Gérard Gouesbet, and Cameron Tropea

A rigorous solution (generalized Lorenz-Mie theory) and an approximate solution (geometrical optics) are developed for the optical stress distribution on the surface of a homogeneous sphere. Demonstration calculations are made for plane-wave and Gaussian-beam illuminations. The influence of diffracted waves, surface waves, interference effects, caustic points, beam width, and refractive index on the stress is analyzed. The Debye series is used to evaluate the validity of geometrical optics and provides a quantitative analysis of the contribution of rays of different orders.


Plasmonic Interactions and Optical Forces between Au Bipyramidal Nanoparticle Dimers

Rene A. Nome, Mason J. Guffey, Norbert F. Scherer and Stephen K. Gray

Interparticle forces that can be driven by applied (optical) fields could lead to the formation of new particle arrangements when assembled in arrays. Furthermore, the potentially large interactions and large local fields associated with plasmon excitations in anisotropic nanoparticles can lead to enhanced nonlinear responses and applications for sensing. These and other applications would benefit from simulations of spectra and forces arising from plasmonic interactions. We present the results of rigorous three-dimensional, finite-difference, time-domain calculations of near- and far-field properties of pairs of Au bipyramidal nanoparticles in three different configurations: side-by-side, head-to-tail, and face-on. The absorption and scattering spectra depend strongly on the geometry as well as on the interparticle separation, as intuitively expected from a dipole coupling picture. Bipyramidal dimers in head-to-tail and face-on geometries exhibit an increasingly red-shifted (longitudinal) plasmon resonance with decreasing separation, whereas side-by-side dimers exhibit a blue shift. Large resonant field enhancements at the gap between particles in a head-to-tail configuration indicate the strong coupling of plasmonic modes. The Maxwell stress tensor formalism is employed to calculate the optical force one particle exerts on the other. Both significant attraction and weak repulsion can be obtained, depending on the relative arrangement of the particles. The force between bipyramids in the head-to-tail configuration can be greater than 10 times the force between pairs of Au nanospheres with the same volume. Experimental linear scattering spectra of particles trapped using the plasmon-resonance-based optical trapping method are found to be consistent with two particles trapped in the side-by-side configuration.

Monday, June 1, 2009

Cell separation by the combination of microfluidics and optical trapping force on a microchip

Masaya Murata, Yukihiro Okamoto, Yeon-Su Park, Noritada Kaji, Manabu Tokeshi, and Yoshinobu Baba

We investigated properties of cells affecting their optical trapping force and successfully established a novel cell separation method based on the combined use of optical trapping force and microfluidics on a microchip. Our investigations reveal that the morphology, size, light absorption, and refractive index of cells are important factors affecting their optical trapping force. A sheath flow of sample solutions created in a microchip made sample cells flow in a narrow linear stream and an optical trap created by a highly focused laser beam captured only target cells and altered their trajectory, resulting in high-efficiency cell separation. An optimum balance between optical trapping force and sample flow rate was essential to achieve high cell separation efficiency. Our investigations clearly indicate that the on-chip optical trapping method allows high-efficiency cell separation without cumbersome and time-consuming cell pretreatments. In addition, our on-chip optical trapping method requires small amounts of sample and may permit high-throughput cell separation and integration of other functions on microchips.


Bending and Twisting of Suspended Single-Walled Carbon Nanotubes in Solution

Ya-Qiong Xu, Arthur Barnard, and Paul L. McEuen

We combine suspended carbon nanotube transistors with optical trapping techniques and scanning photocurrent microscopy to investigate the motion of suspended single-walled carbon nanotubes in solution. We study the movement of nanotubes by monitoring their photocurrent images and measure their thermal fluctuations by imaging microbeads that are tightly attached to nanotubes by single-stranded DNA. By analyzing their thermal fluctuations, we are able to obtain the torsional and bending stiffness of nanotubes and then calculate their diameters. We can also measure, with subangstrom resolution, the effective attachment point of the microbead to the nanotube.


Universal transduction scheme for nanomechanical systems based on dielectric forces

Quirin P. Unterreithmeier, Eva M. Weig & Jörg P. Kotthaus

Any polarizable body placed in an inhomogeneous electric field experiences a dielectric force. This phenomenon is well known from the macroscopic world: a water jet is deflected when approached by a charged object. This fundamental mechanism is exploited in a variety of contexts—for example, trapping microscopic particles in an optical tweezer, where the trapping force is controlled via the intensity of a laser beam, or dielectrophoresis, where electric fields are used to manipulate particles in liquids. Here we extend the underlying concept to the rapidly evolving field of nanoelectromechanical systems (NEMS). A broad range of possible applications are anticipated for these systems, but drive and detection schemes for nanomechanical motion still need to be optimized. Our approach is based on the application of dielectric gradient forces for the controlled and local transduction of NEMS. Using a set of on-chip electrodes to create an electric field gradient, we polarize a dielectric resonator and subject it to an attractive force that can be modulated at high frequencies. This universal actuation scheme is efficient, broadband and scalable. It also separates the driving scheme from the driven mechanical element, allowing for arbitrary polarizable materials and thus potentially ultralow dissipation NEMS. In addition, it enables simple voltage tuning of the mechanical resonance over a wide frequency range, because the dielectric force depends strongly on the resonator–electrode separation. We use the modulation of the resonance frequency to demonstrate parametric actuation. Moreover, we reverse the actuation principle to realize dielectric detection, thus allowing universal transduction of NEMS. We expect this combination to be useful both in the study of fundamental principles and in applications such as signal processing and sensing.


Co- and cross-flow extensions in an elliptical optical trap

E. Schonbrun, J. Wong, and K. B. Crozier

The extension of a particle (translational deviation) in an isotropic harmonic potential well is linearly proportional and parallel to the applied force. In an anisotropic trapping potential, the extension is instead related to the applied force by a compliance tensor. Using the focal spot of a high numerical aperture zone plate to create an elliptical potential and microfluidics to apply a calibrated force, we measure the two-dimensional extension of a trapped spherical particle. As a function of the orientation of the elliptical potential, the extension sweeps out a circular trajectory, exhibiting extensions both parallel (coflow) and perpendicular (cross flow) to thedirection of the flow. The results fit well to a compliance tensor model.