Friday, October 30, 2009

Design of nanoslotted photonic crystal waveguide cavities for single nanoparticle trapping and detection

Shiyun Lin, Juejun Hu, Lionel Kimerling, and Kenneth Crozier

We design and numerically simulate an on-chip photonic device that integrates both optical manipulation and detection functionalities for a single nanoparticle or macromolecule. A unique combination of a photonic crystal waveguide cavity and a nanoslot structure leads to a ~1300 times enhancement of the optical gradient trapping force compared with a conventional waveguide trapping device. Numerical simulations indicate that the designed device is capable of stably trapping a single nanoparticle inside the nanoslot cavity, and thus provides an ideal platform for single particle detection and analysis using cavity-enhanced spectroscopic technologies.

Wednesday, October 28, 2009

Trapping and Rotation of Nanowires Assisted by Surface Plasmons

Miao , X. Wilson, B. K. Cao , G. Pun , S. H. Lin, L. Y.

We report long-range trapping of vanadium dioxide (VO2) and vanadium oxyhydroxide (H2V3O8) nanowires at a distance as large as 50 mu m outside the laser spot using plasmonic tweezers and controlled rotation of the nanowires by combining trapping with microfluidic drag force. The plasmonic tweezers are built upon a self-assembled gold nanoparticle array platform. In addition to the long-range trapping and rotation capability, the required optical intensity for the plasmonic tweezers to initiate trapping is much lower than that required by conventional optical tweezers for similar nanowires. We also investigate possible mechanisms for the unique long-range trapping of nanowires through performing control experiments.

Tuesday, October 27, 2009

Characterization of Photoactivated Singlet Oxygen Damage in Single-Molecule Optical Trap Experiments

Markita P. Landry, Patrick M. McCall, Zhi Qi and Yann R. Chemla

Optical traps or “tweezers” use high-power, near-infrared laser beams to manipulate and apply forces to biological systems, ranging from individual molecules to cells. Although previous studies have established that optical tweezers induce photodamage in live cells, the effects of trap irradiation have yet to be examined in vitro, at the single-molecule level. In this study, we investigate trap-induced damage in a simple system consisting of DNA molecules tethered between optically trapped polystyrene microspheres. We show that exposure to the trapping light affects the lifetime of the tethers, the efficiency with which they can be formed, and their structure. Moreover, we establish that these irreversible effects are caused by oxidative damage from singlet oxygen. This reactive state of molecular oxygen is generated locally by the optical traps in the presence of a sensitizer, which we identify as the trapped polystyrene microspheres. Trap-induced oxidative damage can be reduced greatly by working under anaerobic conditions, using additives that quench singlet oxygen, or trapping microspheres lacking the sensitizers necessary for singlet state photoexcitation. Our findings are relevant to a broad range of trap-based single-molecule experiments—the most common biological application of optical tweezers—and may guide the development of more robust experimental protocols.

Monday, October 26, 2009

Probing the dynamic differential stiffness of dsDNA interacting with RecA in the enthalpic regime

Chia-Hui Lien, Ming-Tzo Wei, Te- Yu Tseng, Chien-Der Lee, Chung Wang, Ting-Fang Wang, H. Daniel Ou-Yang, and Arthur Chiou

RecA plays a central role in homologous recombination of DNA. When RecA combines with dsDNA to form RecA-dsDNA nucleofilament, it unwinds dsDNA and changes its structure. The unwinding length extension of a DNA segment interacting with RecA has been studied by various techniques, but the dynamic differential stiffness of dsDNA conjugating with RecA has not been well characterized. We applied oscillatory optical tweezers to measure the differential stiffness of dsDNA molecules, interacting with RecA, as a function of time at a constant stretching force of 33.6pN. The values of the differential stiffness of DNA (for stretching force in the range of 20.0pN to 33.6pN) measured by oscillatory optical tweezers, both before and after its interaction with RecA, are consistent with those measured by stationary optical tweezers. In the dynamic measurement, we have shown that the association (or binding) rate increases with higher concentration of RecA; besides, we have also monitored in real-time the dissociation of RecA from the stretched RecA-dsDNA filament as ATPγS was washed off from the sample chamber. Finally, we verified that RecA (I26C), a form of RecA mutant, does not affect the differential stiffness of the stretched DNA sample. It implies that mutant RecA (I26C) does not bind to the DNA, which is consistent with the result obtained by conventional biochemical approach.

Friday, October 23, 2009

Unraveling the structure of DNA during overstretching by using multicolor, single-molecule fluorescence imaging

Joost van Mameren, Peter Gross, Geraldine Farge, Pleuni Hooijman, Mauro Modesti, Maria Falkenberg, Gijs J. L. Wuite and Erwin J. G. Peterman

Single-molecule manipulation studies have revealed that double-stranded DNA undergoes a structural transition when subjected to tension. At forces that depend on the attachment geometry of the DNA (65 pN or 110 pN), it elongates ≈1.7-fold and its elastic properties change dramatically. The nature of this overstretched DNA has been under debate. In one model, the DNA cooperatively unwinds, while base pairing remains intact. In a competing model, the hydrogen bonds between base pairs break and two single DNA strands are formed, comparable to thermal DNA melting. Here, we resolve the structural basis of DNA overstretching using a combination of fluorescence microscopy, optical tweezers, and microfluidics. In DNA molecules undergoing the transition, we visualize double- and single-stranded segments using specific fluorescent labels. Our data directly demonstrate that overstretching comprises a gradual conversion from double-stranded to single-stranded DNA, irrespective of the attachment geometry. We found that these conversions favorably initiate from nicks or free DNA ends. These discontinuities in the phosphodiester backbone serve as energetically favorable nucleation points for melting. When both DNA strands are intact and no nicks or free ends are present, the overstretching force increases from 65 to 110 pN and melting initiates throughout the molecule, comparable to thermal melting. These results provide unique insights in the thermodynamics of DNA and DNA-protein interactions.

Optically driven micropump with a twin spiral microrotor

Shoji Maruo, Akira Takaura, and Yohei Saito

An optically driven micropump that employs viscous drag exerted on a spinning microrotor with left- and right-handed spiral blades on its rotational axis has been developed using two-photon microfabrication. It was demonstrated that the twin spiral microrotor provides a higher rotation speed than a single spiral microrotor. The rotation speed reached 560 rpm at a laser power of 500 mW. The twin spiral microrotor was also applied to a viscous micropump with a U-shaped microchannel. To pump fluid, the twin spiral microrotor located at the corner of the U-shaped microchannel was rotated by focusing a laser beam. The flow field inside the U-shaped microchannel was analyzed using the finite element method (FEM) based on the Navier-Stokes equation to optimize the shape of the microchannel. It was confirmed that the rotation of the twin spiral microrotor generated a unidirectional laminar flow. Finally, a tandem micropump using two twin spiral microrotors was driven by a dual optical trapping system using a spatial light modulation technique.

Laser-induced self-assembly of silver nanoparticles via plasmonic interactions

Yoshito Tanaka, Hiroyuki Yoshikawa, Tamitake Itoh, and Mitsuru Ishikawa

We report laser induced self-assembly of silver nanoparticles via plasmonic interactions. By focusing a near-infrared laser in silver nanoparticle suspension, nanoparticle assembly is formed as a result of optical trapping. The shape of Rayleigh scattering spectra of the nanoassembly strongly depends on the polarization of the laser beam. Particularly, a linearly polarized laser induces the formation of arrayed structure along the laser polarization, that shows a sharp plasmon resonance band and harnesses excellent plasmonic properties applicable for nonlinear surface enhanced spectroscopy.

Real-time molecular assessment on oxidative injury of single cells using Raman spectroscopy

Wei-Tien Chang, Hung-Lung Lin, Hung-Che Chen, Yao-Ming Wu, Wen-Jone Chen, Yuan-Teh Lee, Ian Liau

Oxidative stress is encountered in many biological systems; the resultant oxidative injury plays a significant role in the pathogenesis of diverse diseases. Conventional measurements on oxidative injury are employed almost exclusively on a large population of cells either by counting the fraction of cell death or by observing the fluorometric change resulting from exogenous reagents, thereby lacking in molecular detail and temporal specificity. In this work we combine laser tweezers and Raman spectroscopy to observe the response of single cells to oxidative stress. By measuring the temporal changes of vibrational spectra of single optically trapped cells, we demonstrate a molecular-level assessment of cellular oxidative injury in real time, both qualitatively and quantitatively, without the introduction of exogenous reagents. The main experimental findings are supported by the observation of Raman spectra of intermediates and downstream products. The abrogation of the above changes by ascorbic acid further illustrates the therapeutic effect of antioxidants against cellular oxidative injury. This approach is extensible to studies exploring the biochemical transformation of single cells or intracellular organelles in response to various chemical or physical stimuli. With the aid of molecular fingerprints, single-cell Raman spectroscopy exhibits a great potential for accessing the chemical aspects of cellular bioactivity, yielding insight into pathophysiological processes and assisting the development of novel therapeutic interventions against diseases.

Emergence of exotic spatio-temporal structure under photon flux

K Yoshikawa

The operation of laser focusing on an object implies the creation of dielectric dielectric field under a thermodynamically open condition. Thus, we can expect the appearance of the effect of thermal irreversibility, such as breakdown of detailed balance, occurrence of circular state-flux in the phase-space, and limit-cycle oscillation. In the present article, we describe our recent experimental results on various kinds of exotic time-dependent phenomena induced by the continuous irradiation of laser. 1) Generation/annihilation of droplets from binary homogeneous liquid induced by laser: It will be shown that focused laser induces micro-phase separation on an oil/water isotropic solution. By choosing the proper experimental conditions, rhythmic change of generation, growth, and disappearance of a droplet at the focus is generated. This rhythmic phenomenon is a kind of limit-cycle oscillation. 2) Positive/negative photophoresis on a droplet: We show that a droplet is driven by a laser beam, either toward and backward along the direction of photon flux, through the change of the position of irradiation. Such photophoretic motion is induced by interfacial instability owe to the laser irradiation. 3) Rhythmic growth and bursting of a cluster with micro-beads: It is shown that negatively charged micro-beads are collected toward the focus of IR laser, i.e., optical tweezers. When the focusing angle is decreased from usual conditions, rhythmic change of the formation-growth-bursting of the beads cluster is generated.

Conservative and Nonconservative Torques in Optical Binding

D. Haefner, S. Sukhov, and A. Dogariu

We show that in the canonical case of two lossless spheres that are electromagnetically coupled there is interplay between conservative and nonconservative forces, which is controlled by the polarization of the bounding field. We demonstrate that this phenomenon leads to new mechanisms to induce torques on spherically symmetric, optically isotropic, and lossless objects. The electromagnetic interaction can be exploited to apply orbital torque about the mutual center of mass of thebounded spheres as well as spin around the individual axes. When the incident field is linearly polarized, the torques are mostly conservative and affect only transient behaviors while for circularly polarized fields, the torques are entirely nonconservative, resulting in steady rotations. Means to control the magnitudes of orbital and spin torques are presented and applications to nanorotator machines are discussed.

Wednesday, October 21, 2009

Lithium niobate nanowires synthesis, optical properties, and manipulation

Rachel Grange, Jae-Woo Choi, Chia-Lung Hsieh, Ye Pu, Arnaud Magrez, Rita Smajda, László Forró, and Demetri Psaltis

Free-standing lithium niobate nanowires (LiNbO3) are synthesized by the hydrothermal route. The polarization response of the second harmonic generation (SHG) signal is measured in a single nanowire and used to identify the crystal orientation by matching with bulk LiNbO3 nonlinear optical susceptibility. The electrical manipulation of a LiNbO3 nanowire and its monitoring through the SHG signal in a fluidic setup are demonstrated.

Polarization Raman study of protein ordering by controllable RBC deformation

Satish Rao, Stefan Bálint, Luisa del Carmen Frias, Dmitri Petrov

Polarized Raman spectroscopy is used to provide evidence of hemoglobin protein ordering as the red blood cell (RBC) is stretched with optical tweezers. The stretching of the cell is intended to mimic the deformation that it experiences as it travels through vessels and capillaries. The depolarization ratios for a number of heme Raman bands change as the cell is stretched, confirming the semi-ordered nature of the hemoglobin ensemble in the cytoplasm. Furthermore, trends observed in the ratio shifts point to increased packing and ordering of the Hb after cell stretching. This evidence should shed more light on to the role of deformation in the RBC function.

Giant Enhanced Diffusion of Gold Nanoparticles in Optical Vortex Fields

Silvia Albaladejo, Manuel I. Marqués, Frank Scheffold and Juan Jose Saenz

We study the diffusion of a metal nanoparticle in the nonconservative force field of an optical vertex lattice. Radiation in the vortex array is shown to induce a giant enhancement over the free thermal diffusion. Langevin dynamics simulations show that the diffusion coefficient of (50 nm radius) gold particles at room temperature is enhanced by 2 orders of magnitude at power densities of the order or smaller than those used to trap nanoparticles with optical tweezers.

Spontaneous Oscillations of a Minimal Actomyosin System under Elastic Loading

P.-Y. Plaçais, M. Balland, T. Guérin, J.-F. Joanny, and P. Martin

Spontaneous mechanical oscillations occur in various types of biological systems where groups of motor molecules are elastically coupled to their environment. By using an optical trap to oppose the gliding motion of a single bead-tailed actin filament over a substrate densely coated with myosin motors, we mimicked this condition in vitro. We show that this minimal actomyosin system can oscillate spontaneously. Our finding accords quantitatively with a general theoretical framework where oscillatory instabilities emerge generically from the collective dynamics of molecular motors under load.

Monday, October 19, 2009

History force on coated microbubbles propelled by ultrasound

Valeria Garbin, Benjamin Dollet, Marlies Overvelde, Dan Cojoc, Enzo Di Fabrizio, Leen van Wijngaarden, Andrea Prosperetti, Nico de Jong, Detlef Lohse, and Michel Versluis

In this paper the unsteady translation of coated microbubbles propelled by acoustic radiation force is studied experimentally. A system of two pulsating microbubbles of the type used as contrast agent in ultrasound medical imaging is considered, which attract each other as a result of the secondary Bjerknes force. Optical tweezers are used to isolate the bubble pair from neighboring boundaries so that it can be regarded as if in an unbounded fluid and the hydrodynamic forces acting on the system can be identified unambiguously. The radial and translational dynamics, excited by a 2.25 MHz ultrasound wave, is recorded with an ultrahigh speed camera at 15×106 frames/s. The time-resolved measurements reveal a quasisteady component of the translational velocity, at an average translational Reynolds number 0.5, and an oscillatory component at the same frequency as the radial pulsations, as predicted by existing models. Since the coating enforces a no-slip boundary condition, an increased viscous dissipation is expected due to the oscillatory component, similar to the case of an oscillating rigid sphere that was firstdescribed by Stokes [“On the effect of the internal friction of fluids on the motion of pendulums,” Trans. Cambridge Philos. Soc. 9, 8 (1851)]. A history force term is thereforeincluded in the force balance, in the form originally proposed by Basset and extended to the case of time-dependent radius by Takemura and Magnaudet [“The history force on a rapidly shrinking bubble rising at finite Reynolds number,” Phys. Fluids 16, 3247 (2004)]. The instantaneous values of the hydrodynamic forces extracted from the experimental data confirm that the history force accounts for the largest part of the viscous force. The trajectories of the bubbles predicted by numerically solving the equations of motion are in very good agreement with the experiment.


Optical manipulation of proteins in aqueous solution

Yasuyuki Tsuboi, Tatsuya Shoji, Masayuki Nishino, Seiji Masuda, Koichiro Ishimori and Noboru Kitamura

Optical trapping of lysozyme, cytochrome c, or myoglobin based on photon pressure generated by focusing 1064 nm laser beam in an aqueous solution was explored. For all the proteins, microparticle formation was observed at the focal point under an optical microscope. Furthermore, the microparticles were identified to the molecular assemblies of the corresponding protein by means of confocal Raman microspectroscopy. For lysozyme, molecular clusters in solution were optically trapped to form the microparticle and it took more than 1 h to produce the microparticle. By contrast, molecular assembling proceeded within 1 min for cytochrome c and myoglobin. Since heme in cytochrome c or myoglobin would have a high polarizability, that would contribute to rapid assembling of the protein. Thus we demonstrated that a focused laser beam was a powerful tool to manipulate protein molecules in solution.

Friday, October 16, 2009

Simple Route for Preparing Optically Trappable Probes for Surface-Enhanced Raman Scattering

Stefan Balint, Mark P. Kreuzer, Satish Rao, Gonal Badenes, Pavol Mikovsk and Dmitri Petrov

Surface-enhanced Raman scattering (SERS) has proven to be a powerful technique for a wide range of topics such as chemical analysis, materials research, and biological imaging. In this work, we report in detail on the development and characterization of micrometer-sized dielectric beads with metal colloids attached to their surface. The metalized beads were sufficiently transparent enabling optical trapping while the presence of metal islands provided the SERS. This method is fast and simple and is void of complications when compared to the Tollen’s test used in previous publications. This highly efficient probe can be placed and scanned with nanometric accuracy near living cells. The process to create such probes is described including discussion of various parameters that are critical in achieving the desired results. Additionally, their use is demonstrated by detecting low quantities of a drug in aqueous solution and in a cell membrane.

Size- and orientation-selective optical manipulation of single-walled carbon nanotubes: A theoretical study

Hiroshi Ajiki, Takuya Iida, Takahiro Ishikawa, Seiji Uryu, and Hajime Ishihara

We have theoretically studied the resonant radiation force exerted on single-walled carbon nanotubes (SWCNs) by taking into account the excitonic effect under the effective-mass approximation. When a light frequency is close to an exciton level, the radiation force becomes significantly large even at room temperature for conventional laser intensities in optical manipulation. The peak positions in radiation force spectra are sensitive to the tube diameter and light polarization. Furthermore, the chirality dependence on exciton for similar diameter is relatively large. Therefore, the selective sorting and trapping of SWCNs with a desired specific structure is possible by tuning the applied field frequency.

Minimum-variance Brownian motion control of an optically trapped probe

Yanan Huang, Zhipeng Zhang, and Chia-Hsiang Menq

This paper presents a theoretical and experimental investigation of the Brownian motion control of an optically trapped probe. The Langevin equation is employed to describe the motion of the probe experiencing random thermal force and optical trapping force. Since active feedback control is applied to suppress the probe's Brownian motion, actuator dynamics and measurement delay are included in the equation. The equation of motion is simplified to a first-order linear differential equation and transformed to a discrete model for the purpose of controller design and data analysis. The derived model is experimentally verified by comparing the model prediction to the measured response of a 1.87 μm trapped probe subject to proportional control. It is then employed to design the optimal controller that minimizes the variance of the probe's Brownian motion. Theoretical analysis is derived to evaluate the control performance of a specific optical trap. Both experiment and simulation are used to validate the design as well as theoretical analysis, and to illustrate the performance envelope of the active control. Moreover, adaptive minimum variance control is implemented to maintain the optimal performance in the case in which the system is time varying when operating the actively controlled optical trap in a complex environment.

Optical manipulation of microtubules for directed biomolecule assembly

Cerasela Zoica Dinu, Tania Chakrabarty, Elaine Lunsford, Christopher Mauer, Joseph Plewa, Jonathan S. Dordick and Douglas B. Chrisey

Optical trapping provides the ability to directly manipulate nano-objects in synthetic environment and hold the potential to produce the next generation of nanodevices. We report a computer-controlled strategy based on dynamic holographic optical trapping to efficiently capture and optically manipulate individual microtubules (25 nm in diameter and several µm in length) as well as hybrid complexes formed from microtubules and quantum dots, with nanometer spatial resolution (15 nm), in three dimensions (over distances exceeding 50 µm in thex–y plane and 10 µm in the z direction), in stationary flow and on engineered surfaces. We also show that individual hybrid complexes can be captured and manipulated for the assembly of user-directed architectures. This strategy can be used for the automated nanofabrication of complex macromolecular architectures and development of novel hybrid materials.

Tuesday, October 13, 2009

Generation and Mixing of Subfemtoliter Aqueous Droplets On Demand

Jianyong Tang, Ana M. Jofre, Rani B. Kishore, Joseph E. Reiner, Mark E. Greene, Geoffrey M. Lowman, John S. Denker, Christina C. C. Willis, Kristian Helmerson and Lori S. Goldner

We describe a novel method of generating monodisperse subfemtoliter aqueous droplets on demand by means of piezoelectric injection. Droplets with volumes down to 200 aL are generated by this technique. The droplets are injected into a low refractive index perfluorocarbon so that they can be optically trapped. We demonstrate the use of optical tweezers to manipulate and mix droplets. For example, using optical tweezers we bring two droplets, one containing a calcium sensitive dye and the other calcium chloride, into contact. The droplets coalesce with a resulting reaction time of about 1 ms. The monodispersity, manipulability, repeatability, small size, and fast mixing afforded by this system offer many opportunities for nanochemistry and observation of chemical reactions on a molecule-by-molecule basis.

Optical mirror trap with a large field of view

Maximilian Pitzek, Ruth Steiger, Gregor Thalhammer, Stefan Bernet, and Monika Ritsch-Marte

Holographic optical tweezers typically require microscope objectives with high numerical aperture and thus usually suffer from the disadvantage of a small field of view and a small working distance. We experimentally investigate an optical mirror trap that is created after reflection of two holographically shaped collinear beams on a mirror. This approach combines a large field of view and a large working distance with the possibility to manipulate particles in a large size range, since it allows to use a microscope objective with a numerical aperture as low as 0.2. In this work we demonstrate robust optical three-dimensional trapping in a range of 1mm x 1mm x 2mm with particle sizes ranging from 1.4 μm up to 45 μm. The use of spatial light modulator based holographic methods to create the trapping beams allows to simultaneously trap many beads in complex, dynamic configurations. We present measurements that characterize the mirror traps in terms of trap stiffness, maximum trapping force and capture range.

Force and Premature Binding of ADP Can Regulate the Processivity of Individual Eg5 Dimers

Megan T. Valentine and Steven M. Block

Using a high-resolution optical trapping instrument, we directly observed the processive motions of individual Eg5 dimers over a range of external loads and ATP, ADP, and phosphate concentrations. To constrain possible models for dissociation from the microtubule, we measured Eg5 run lengths and also compared the duration of the last step of a processive run to all previous step durations. We found that the application of large longitudinal forces in either hindering or assisting directions could induce Eg5-microtubule dissociation. At a constant moderate force, maintained with a force clamp, the premature binding of ADP strongly promoted microtubule release by Eg5, whereas the addition of ATP or phosphate had little effect on dissociation. These results imply that run length is determined not only by the load, but also by the concentration and type of nucleotides present, and therefore that the biochemical cycles of the two motor domains of the Eg5 dimer are coordinated to promote processive stepping.

On the Origin of Kinesin Limping

Adrian N. Fehr, Braulio Gutiérrez-Medina, Charles L. Asbury and Steven M. Block

Kinesin is a dimeric motor with twin catalytic heads joined to a common stalk. Kinesin molecules move processively along microtubules in a hand-over-hand walk, with the two heads advancing alternately. Recombinant kinesin constructs with short stalks have been found to “limp”, i.e., exhibit alternation in the dwell times of successive steps. Limping behavior implies that the molecular rearrangements underlying even- and odd-numbered steps must differ, but the mechanism by which such rearrangements lead to limping remains unsolved. Here, we used an optical force clamp to measure individual, recombinant dimers and test candidate explanations for limping. Introducing a covalent cross-link into the stalk region near the heads had no effect on limping, ruling out possible stalk misregistration during coiled-coil formation as a cause. Limping was equally unaffected by mutations that produced 50-fold changes in stalk stiffness, ruling out models where limping arises from an asymmetry in torsional strain. However, limping was enhanced by perturbations that increased the vertical component of load on the motor, including increases in bead size or net load, and decreases in the stalk length. These results suggest that kinesin heads take different vertical trajectories during alternate steps, and that the rates for these motions are differentially sensitive to load.

Flickering Analysis of Erythrocyte Mechanical Properties: Dependence on Oxygenation Level, Cell Shape, and Hydration Level

Young-Zoon Yoon, Ha Hong, Aidan Brown, Dong Chung Kim, Dae Joon Kang, Virgilio L. Lew and Pietro Cicuta

Erythrocytes (red blood cells) play an essential role in the respiratory functions of vertebrates, carrying oxygen from lungs to tissues and CO2 from tissues to lungs. They are mechanically very soft, enabling circulation through small capillaries. The small thermally induced displacements of the membrane provide an important tool in the investigation of the mechanics of the cell membrane. However, despite numerous studies, uncertainties in the interpretation of the data, and in the values derived for the main parameters of cell mechanics, have rendered past conclusions from the fluctuation approach somewhat controversial. Here we revisit the experimental method and theoretical analysis of fluctuations, to adapt them to the case of cell contour fluctuations, which are readily observable experimentally. This enables direct measurements of membrane tension, of bending modulus, and of the viscosity of the cell cytoplasm. Of the various factors that influence the mechanical properties of the cell, we focus here on: 1), the level of oxygenation, as monitored by Raman spectrometry; 2), cell shape; and 3), the concentration of hemoglobin. The results show that, contrary to previous reports, there is no significant difference in cell tension and bending modulus between oxygenated and deoxygenated states, in line with the softness requirement for optimal circulatory flow in both states. On the other hand, tension and bending moduli of discocyte- and spherocyte-shaped cells differ markedly, in both the oxygenated and deoxygenated states. The tension in spherocytes is much higher, consistent with recent theoretical models that describe the transitions between red blood cell shapes as a function of membrane tension. Cell cytoplasmic viscosity is strongly influenced by the hydration state. The implications of these results to circulatory flow dynamics in physiological and pathological conditions are discussed.

Removal of the cardiac myosin regulatory light chain increases isometric force production

Kiran Pant, James Watt, Michael Greenberg, Michelle Jones, Danuta Szczesna-Cordary, and Jeffrey R. Moore

The myosin neck, which is supported by the interactions between light chains and the underlying alpha-helical heavy chain, is thought to act as a lever arm to amplify movements originating in the globular motor domain. Here, we studied the role of the cardiac myosin regulatory light chains (RLCs) in the capacity of myosin to produce force using a novel optical-trap-based isometric force in vitro motility assay. We measured the isometric force and actin filament velocity for native porcine cardiac (PC) myosin, RLC-depleted PC (PCdepl) myosin, and PC myosin reconstituted with recombinant bacterially expressed human cardiac RLC (PCrecon). RLC depletion reduced unloaded actin filament velocity by 58% and enhanced the myosin-based isometric force similar to 2-fold. No significant change between PC and PCdepl preparations was observed in the maximal rate of actin-activated myosin ATPase activity. Reconstitution of PCdepl myosin with human RLC partially restored the velocity and force levels to near untreated values. The reduction in unloaded velocity after RLC extraction is consistent with the myosin neck acting as a lever, while the enhancement in isometric force can be directly related to enhancement of unitary force. The force data are consistent with a model in which the neck region behaves as a cantilevered beam.

Radiation forces on dielectric and absorbing particles studied via the finite-difference time-domain method

Lin Jia and Edwin L. Thomas

Using the three dimensional finite-difference time-domain (FDTD) method, we calculate the radiation force from an incident plane wave on both dielectric and absorbing particles in the Lorentz-Mie regime via the Maxwell stress tensor approach. We find that the radiation force changes with particle permittivity, and we categorize the force into three regions: increasing, fluctuating, and constant. We discuss how particle size, shape, orientation and absorption affect the radiation force. A nanoscale solar sail is proposed based on our calculation. A detailed understanding of the optical force of a plane wave on particles in the Lorentz-Mie regime is fundamental for designing nanoscale solar sail systems and optical traps from a set of interfering plane waves.

Friday, October 9, 2009

Many-Body Electrostatic Forces between Colloidal Particles at Vanishing Ionic Strength

Jason W. Merrill, Sunil K. Sainis, and Eric R. Dufresne

Electrostatic forces between small groups of colloidal particles are measured using blinking optical tweezers. When the electrostatic screening length is longer than the interparticle separation, forces are found to be non-pairwise-additive. Both pair and multiparticle forces are well described by the linearized Poisson-Boltzmann equation with constant potential boundary conditions. These findings may play an important role in understanding the structure and stability of a wide variety of systems, from micron-sized particles in oil to aqueous nanocolloids.


Deterministic Ratchet from Stationary Light Fields

I. Zapata, S. Albaladejo, J. M. R. Parrondo, J. J. Sáenz, and F. Sols

Ratchets are dynamic systems where particle transport is induced by zero-average forces due to the interplay between nonlinearity and asymmetry. Generally, they rely on the effect of a strong external driving. We show that stationary optical lattices can be designed to generate particle flow in one direction while requiring neither noise nor driving. Such optical fields must be arranged to yield a combination of conservative (dipole) and nonconservative (radiation pressure) forces. Under strong friction all paths converge to a discrete set of limit periodic trajectories flowing in the same direction.

Radiation force of a sphere with an eccentric inclusion illuminated by a laser beam

Han GX, Han YP

Based on the theory of the scattering of an eccentric sphere arbitrarily illuminated by a shaped beam which is recently presented by us, and according to generalized Lorenz - Mie theory (GLMT), we derived the series expressions of radiation forces exerted by the incident beam on an eccentric sphere by using the conservation law of electromagnetic momentum and the Mexwell - Tensor. As an example, numerical results of radiation force of absorbing eccentric spheres for off - axis incidence of Gaussian beam are given, along with a discussion about the influence of the beam - waist radius, relative complex refractive index, relative size and location of the inclusion on the forces.


Validation, In-Depth Analysis, and Modification of the Micropipette Aspiration Technique

Yong Chen, Baoyu Liu, Gang Xu and Jin-Yu Shao

The micropipette aspiration technique (MAT) has been successfully applied to many studies in cell adhesion such as leukocyte–endothelium interactions. However, this technique has never been validated experimentally and it has been only employed to impose constant forces. In this study, we validated the force measurement of the MAT with the optical trap and analyzed two technical issues of the MAT, force-transducer offset and cell-micropipette gap, with finite element simulation. We also modified the MAT so that increasing or decreasing forces can be applied. With the modified MAT, we studied tether extraction from endothelial cells by pulling single tethers at increasing velocities and constant force loading rates. Before the onset of tether extraction, an apparently linear surface protrusion of a few hundred nanometers was observed, which is likely related to membrane receptors pulling on the underlying cytoskeleton. The strength of the modified MAT lies in its capability and consistency to apply a wide range of force loading rates from several piconewtons per second up to thousands of piconewtons per second. With this modification, the MAT becomes more versatile in the study of single molecule and single cell biophysics.


On the interaction of nano-sized organic carbon particles with model lipid membranes

G. Rusciano, A.C. De Luca, G. Pesce and A. Sasso

The cytotoxic character of atmospheric ultra-fine-particles (UFPs) has been reported in numerous, mainly epidemiological, investigations. However, the detailed mechanism, at the molecular level, leading to UFP cytotoxicity is not yet fully understood. To address this question, a better characterization of UFP toxicity in relation to chemical composition, surface area and particle number is needed. To follow this bottom-up approach, we have investigated the interaction of nano-sized organic carbon particles (NOCs), produced in ordinary combustion processes, with cell-sized unilamellar vesicles. These particles have a size in the 1–5 nm range and constitute an abundant fraction of the total carbon emission to the environment due to human activity. The investigation was performed by spectroscopically analyzing the chemical modifications of membrane lipid tails in optically trapped vesicles after exposure to NOCs. Our experimental outcomes demonstrate that they are able to induce oxidative damage to liposome membranes. This modification, in turn, leads to a change in membrane permeability, as observed by surface enhanced Raman scattering. It should be noticed that the method reported herein paves the way for a deeper comprehension of the interaction of nanomaterials with lipid membranes, in relation to both nanoparticles characteristics and membrane lipid composition.

Combined holographic-mechanical optical tweezers: Construction, optimization, and calibration

Richard D. L. Hanes, Matthew C. Jenkins, and Stefan U. Egelhaaf

A spatial light modulator (SLM) and a pair of galvanometer-mounted mirrors (GMM) were combined into an optical tweezers setup. This provides great flexibility as the SLM creates an array of traps, which can be moved smoothly and quickly with the GMM. To optimize performance, the effect of the incidence angle on the SLM with respect to phase and intensity response was investigated. Although it is common to use the SLM at an incidence angle of 45°, smaller angles give a full 2pi phase shift and an output intensity which is less dependent on the magnitude of the phase shift. The traps were calibrated using an active oscillatory technique and a passive probability distribution method.

Thursday, October 8, 2009

Line optical tweezers: A tool to induce transformations in stained liposomes and to estimate shear modulus

E. Spyratou, E.A. Mourelatou, A. Georgopoulos, C. Demetzos, M. Makropoulou and A.A. Serafetinides

Liposomes have been actively studied as models of cell membranes and are currently used as drug delivery systems of bioactive molecules. Liposome transformations that mimic cellular processes and are associated with their physicochemical properties have become field of interest the last decade. However, there has been little experimental work on controlled vesicle transformations by non-contact, optical handling methods.In this paper we present the use of line optical tweezers to observe liposome state transitions and transformations. Dynamic shape deformations were induced by line optical tweezers in giant stained liposomes leading to budding transition, fission and pearling creation. Under the controlled effect of line optical tweezers reversible liposome deformations were observed. The shear modulus μ of the membrane was inferred by measuring deformation of stained liposomes induced by the applied optical force. Further laser radiation caused irreversible shape deformations of liposomes, which were transformed from spherical to tubular vesicles. The ability of the selective manipulation of liposomes brings us closer to study their physicochemical properties which play a key role in cellular–liposome interactions, drug encapsulation and delivery efficiency.

Tuesday, October 6, 2009

Frequency dependence of the optical force between two coupled waveguides

Lin Zhu

We investigate the frequency dependence of the optical force between two coupled waveguides using the supermode theory. We show that the frequency dependence of the optical force in a coupled waveguide system is determined by the dispersion relations of uncoupled individual waveguides. We further numerically calculate the optical force of three representative coupled systems at different frequencies and show that these results agree well with the supermode analysis.

Monday, October 5, 2009

Enhanced particle transport in an oscillating sinusoidal optical potential

W Mu, Z Liu, L Luan, G Wang, G C Spalding and J B Ketterson

We have studied the delivery of a colloidal particle in the presence of an oscillating, spatially periodic, optical potential. The average particle velocity relative to the fluid velocity in this potential depends greatly on the oscillation amplitude and frequency. The results of both our simulations and experiments show that for some combinations of these parameters, the average particle transportation velocity can be enhanced due to the synchronization of the particle's movement with the oscillating potential.

Thursday, October 1, 2009

Published Papers

Published papers on Optical Tweezers added in this Blog for the months of 01-09 of 2009.
Here is the results for these first half year of the published papers on optical tweezers, micromanipulation and trapping.

The top 10 Journals are:
Optics Express 13.1%
Physical Review Letters 5.1%
Journal of Optics A 4.2%
Physical Review E 4.2%
Biophysical Journal 3.8%
Lab on a Chip 3.8%
Applied Optics 3.4%
Applied Physics Letters 3.0%
Nano Letters 3.0%
Optics Letters 3.0%
Physical Review A 3.0%

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

analysis, approach, beam, beams, behavior, binding, cell, cells, colloidal, complex, dielectric, DNA, dynamics, energy, experimental, field, focal, focused, force, forces, found, holographic, individual, interaction, laser, light, manipulation, measurements, mechanical, membrane, method, model, molecular, molecule, momentum, motion, motor, observed, Optical, optically, particle, particles, phase, position, potential, power, probe, properties, Raman, results, scattering, single, spherical, stiffness, studies, surface, technique, time, trap, trapped, trapping, tweezers