Tuesday, June 29, 2010

Mechanical Characterization of Human Red Blood Cells Under Different Osmotic Conditions by Robotic Manipulation With Optical Tweezers

Tan, Y.; Sun, D.; Wang, J.; Huang, W.

The physiological functions of human red blood cells (RBCs) play a crucial role to human health and are greatly influenced by their mechanical properties. Any alteration of the cell mechanics may cause human diseases. The osmotic condition is an important factor to the physiological environment, but its effect on RBCs has been little studied. To investigate this effect, robotic manipulation technology with optical tweezers is utilized in this paper to characterize the mechanical properties of RBCs in different osmotic conditions. The effectiveness of this technology is demonstrated first in the manipulation of microbeads. Then the optical tweezers are used to stretch RBCs to acquire the force–deformation relationships. To extract cell properties from the experimental data, a mechanical model is developed for RBCs in hypotonic conditions by extending our previous work , and the finite element model is utilized for RBCs in isotonic and hypertonic conditions. Through comparing the modeling results to the experimental data, the shear moduli of RBCs in different osmotic solutions are characterized, which shows that the cell stiffness increases with elevated osmolality. Furthermore, the property variation and potential biomedical significance of this study are discussed. In conclusion, this study indicates that the osmotic stress has a significant effect on the cell properties of human RBCs, which may provide insight into the pathology analysis and therapy of some human diseases.


Measurement of the Instantaneous Velocity of a Brownian Particle

Tongcang Li, Simon Kheifets, David Medellin, Mark G. Raizen

Brownian motion of particles affects many branches of science. We report on the Brownian motion of micrometer-sized beads of glass held in air by an optical tweezer, over a wide range of pressures, and we measured the instantaneous velocity of a Brownian particle. Our results provide direct verification of the energy equipartition theorem for a Brownian particle. For short times, the ballistic regime of Brownian motion was observed, in contrast to the usual diffusive regime. We discuss the applications of these methods toward cooling the center-of-mass motion of a bead in vacuum to the quantum ground motional state.

Optical vortex beams for trapping and transport of particles in air

V. G. Shvedov, A. S. Desyatnikov, A. V. Rode, Y. V. Izdebskaya, W. Z. Krolikowski and Y. S. Kivshar

In this paper we show that laser beams containing phase singularity can be used for trapping and guiding light-absorbing particles in air. The experiments were performed with agglomerates of carbon nanoparticles with the size in the range 0.1–10 μm; the typical cw laser power was of a few mW. The stability of open-air three-dimensional trapping was within ±2 μm in both the transverse and the longitudinal directions. The particle position on the beams axis within the trap can be controlled by changing the relative intensity of two beams. The distinguishing feature of the trapping strategy is that particles are trapped at the intensity minimum of the beam, thus with minimum heating and intervention into the particle properties, which is important for direct studies of particle properties and for air-trapping of living cells.


The evaluation of interaction between red blood cells in blood coagulation by optical tweezers

Yang, Bor-Wen; Mu, Yu-Hong; Huang, Kui-Teng; Li, Zhe; Wu, Jie-Lung; Lin, Yu-An

To maintain the life of patients with hemophilia, apoplexy or hemorrhage, appropriate blood coagulation is crucial. To study the microscopic phenomena of blood coagulation and the therapeutic effects of blood medication, optical tweezers were applied to estimate the interaction between red blood cells in the coagulation process. By measuring minimum optical power required to trap the coagulating blood cells, the pN-scale interaction between them can be evaluated. In normal blood sample, the interaction rises in accordance with coagulation time. The addition of heparin attenuates the interaction and postpones the coagulation, whereas the addition of tranexamic acid starts the coagulation early at the beginning and allows the process completed in less time.


Friday, June 25, 2010

Protein-Mediated DNA Loop Formation and Breakdown in a Fluctuating Environment

Yih-Fan Chen, J. N. Milstein, and Jens-Christian Meiners

Living cells provide a fluctuating, out-of-equilibrium environment in which genes must coordinate cellular function. DNA looping, which is a common means of regulating transcription, is very much a stochastic process; the loops arise from the thermal motion of the DNA and other fluctuations of the cellular environment. We present single-molecule measurements of DNA loop formation and breakdown when an artificial fluctuating force, applied to mimic a fluctuating cellular environment, is imposed on the DNA. We show that loop formation is greatly enhanced in the presence of noise of only a fraction of kBT, yet find that hypothetical regulatory schemes that employ mechanical tension in the DNA—as a sensitive switch to control transcription—can be surprisingly robust due to a fortuitous cancellation of noise effects.


Radiation force of highly focused cosine-Gaussian beam on a particle in the Rayleigh scattering regime

Jiang Yun-Feng, Lu Xuan-Hui, Zhao Cheng-Liang

The radiation force of highly focused cosine-Gaussian beam acting on a particle in the Rayleigh scattering regime is theoretically investigated in this paper. The results show that it is feasible to utilize cosine-Gaussian beam in optical trapping system. Unlike the conventional optical beams, the cosine-Gaussian beam could simultaneously trap particles whose refractive index is lower or higher than the ambient.

Thursday, June 24, 2010

Stable trapping and manually controlled rotation of an asymmetric or birefringent microparticle using dual-mode split-beam optical tweezers

Fang-Wen Sheu, Tzu-Kai Lan, Yu-Chung Lin, Shiung Chen, and Chyung Ay

Inserting a coverslip into half of a Gaussian laser beam at a suitable tilting angle can make the single-mode laser beam become closely spaced dual light spots at the laser focus. In this way, we can reform the conventional single-beam optical tweezers easily and construct a set of dual-mode split-beam optical tweezers, which can be used to manually rotate a trapped and twisted red blood cell around the optical axis. Furthermore, we demonstrate that the split-beam optical tweezers can also stably trap and orient a birefringent polystyrene micro strip particle, which otherwise will self rotate at a varying speed along the structural principal axes, fast spin about the optical axis in a tilting pose, or precess like a gyroscope, in the original linearly polarized single-beam optical tweezers.


Wednesday, June 23, 2010

A force detection technique for single-beam optical traps based on direct measurement of light momentum changes

Arnau Farré and Mario Montes-Usategui

Despite the tremendous success of force-measuring optical traps in recent years, the calibration methods most commonly used in the field have been plagued with difficulties and limitations. Force sensing based on direct measurement of light momentum changes stands out among these as an exception. Especially significant is this method’s potential for working within living cells, with non-spherical particles or with non-Gaussian beams. However, so far, the technique has only been implemented in counter-propagating dual-beam traps, which are difficult to align and integrate with other microscopy techniques. Here, we show the feasibility of a single-beam gradient-trap system working with a force detection technique based on this same principle.


Tuesday, June 22, 2010

Regulation of a heterodimeric kinesin-2 through an unprocessive motor domain that is turned processive by its partner

Melanie Brunnbauer, Felix Mueller-Planitz, Süleyman Kösem, Thi Hieu Ho, Renate Dombi, J. Christof M. Gebhardt, Matthias Rief, and Zeynep Ökten

Cilia are microtubule-based protrusions of the plasma membrane found on most eukaryotic cells. Their assembly is mediated through the conserved intraflagellar transport mechanism. One class of motor proteins involved in intraflagellar transport, kinesin-2, is unique among kinesin motors in that some of its members are composed of two distinct polypeptides. However, the biological reason for heterodimerization has remained elusive. Here we provide several interdependent reasons for the heterodimerization of the kinesin-2 motor KLP11/KLP20 of Caenorhabditis elegans cilia. One motor domain is unprocessive as a homodimer, but heterodimerization with a processive partner generates processivity. The “unprocessive” subunit is kept in this partnership as it mediates an asymmetric autoregulation of the motor activity. Finally, heterodimerization is necessary to bind KAP1, the in vivo link between motor and cargo.


Sunday, June 20, 2010

Raman tweezers sorting of single microbial cells

Wei E. Huang, Andrew D. Ward and Andrew S. Whiteley

We have selectively isolated microbial cells by identifying and then manipulating cells using a combination of Raman microspectroscopy and optical trapping. The criterion for cell discrimination is based on spectral peak shifts within the Raman spectrum of individual cells. A specific shift in the phenylalanine peak position from 1001 rel. cm−1 to 965 rel. cm−1 is utilized to indicate the uptake of 13C within the cell that utilized 13C-substrate. Cells were captured and manipulated using an infrared (1064 nm) laser while Raman spectra were acquired over shorter timescales (30 s) using a co-aligned 514.5 nm laser beam. Selected cells were manoeuvred to a clean part of a capillary tube and the tubes were cleaved to physically separate the cells. The technique was tested for cell viability and cross-contamination effects using 70 single yeast cells (Saccharomyces cerevisia). Following these tests, 58 single bacterial cells (Escherichia coli DH5α, and Pseudomonas fluorescens SBW25::Km-RFP) that exhibited 13C uptake were sorted from bacterial populations. Among those isolated cells, 11 out of 18 yeast cells and 7 out of 18 single SBW25::Km-RFP cells were recovered by incubation; 2 out of 7 sorted yeast cells and 3 out of 8 sorted bacterial cells (single SBW25::Km-RFP) were genome amplified correctly. We show that the Raman tweezers approach has the potential to open a new frontier to study unculturable microorganisms, which account for more than 99% microbes in natural environment.


Intracellular Dielectric Tagging for Improved Optical Manipulation of Mammalian Cells

Mthunzi, P.; Lee, W. M.; Riches, A.; Brown, C. T. A.; Gunn-Moore, F. J.; Dholakia, K.

Optical micromanipulation of transparent microparticles such as cellular materials relies upon the application of optical forces that are crucially dependent on the refractive index contrast between the particle and the surrounding medium. We briefly review the application of optical forces for cell manipulation and sorting, highlighting some of the key experiments over the last twenty years. We then introduce a new technique for enhancing the dielectric contrast of mammalian cells, which is a result of cells naturally taking up microspheres from their environment. We explore how these intracellular dielectric tags can influence the scattering and gradient forces upon these cells from an externally applied optical field. We show that intracellular polymer microspheres can serve as highly directional optical scatterers and that scattering forces can enable sorting through axial guiding onto laminin-coated glass coverslips upon which the selected cells adhere. Such internal dielectric tagging presents a simple, inexpensive, sterile technique to enhance optical manipulation procedures for cellular material and may enable new sorting techniques within microfluidic systems.

Evaluation of radiation force acting on macromolecules by combination of Brownian dynamics simulation with fluorescence correlation spectroscopy

Syoji Ito, Naoki Toitani, Hiroaki Yamauchi, and Hiroshi Miyasaka

The effect of optical gradient force from a focused laser beam on the fluorescence correlation spectroscopy (FCS) was investigated by a computing method based on Brownian dynamics simulation. A series of calculations revealed that, in relatively shallow optical force potential up to 1.0kTR (TR=298.15 K), the conventional theoretical model of FCS without consideration of the optical gradient force could evaluate the increase in the average number of molecules and the diffusion time in the potential. On the other hand, large deviation between the simulated fluorescence correlation curve and the theoretical model was observed under the potential depth >1.0kTR. In addition, by integrating the optical force potential with the temperature elevation under optical trapping condition, it was deduced that the temperature rise does not seriously affect the average number of particles in the sampling area, but the average residence time is more sensitively affected by the temperature elevation. The present study using the simulation also provides a method to experimentally estimate molecular polarizabilities from FCS measurements.


Friday, June 18, 2010

Biaxial crystal-based optical tweezers

Angelsky Oleg, V., Maksimyak Andrew, P., Maksimyak Peter, P., Hanson Steen, G.

We suggest an optical tweezer setup based on an optically biaxial crystal. To control movements of opaque particles, we use shifts. The results of experimental studies are reported which are concerned with this laser tweezer setup. We demonstrate a movement of microparticles of toner using a singular-optical trap, rotation of particles due to orbital angular momentum of the field, and converging or diverging of two different traps when changing transmission plane of polariser at the input of our polarisation interferometer.


Bidirectional Transport by Molecular Motors: Enhanced Processivity and Response to External Forces

Melanie J.I. Müller, Stefan Klumpp and Reinhard Lipowsky

Intracellular transport along cytoskeletal filaments is often mediated by two teams of molecular motors that pull on the same cargo and move in opposite directions along the filaments. We have recently shown theoretically that this bidirectional transport can be understood as a stochastic tug-of-war between the two motor teams. Here, we further develop our theory to investigate the experimentally accessible dynamic behavior of cargos transported by strong motors such as kinesin-1 or cytoplasmic dynein. By studying the run and binding times of such a cargo, we show that the properties of biological motors, such as the large ratio of stall/detachment force and the small ratio of superstall backward/forward velocity, are favorable for bidirectional cargo transport, leading to fast motion and enhanced diffusion. In addition, cargo processivity is shown to be strongly enhanced by transport via several molecular motors even if these motors are engaged in a tug-of-war. Finally, we study the motility of a bidirectional cargo under force. Frictional forces arising, e.g., from the viscous cytoplasm, lead to peaks in the velocity distribution, while external forces as exerted, e.g., by an optical trap, lead to hysteresis effects. Our results, in particular our explicit expressions for the cargo binding time and the distance of the peaks in the velocity relation under friction, are directly accessible to in vitro as well as in vivo experiments.


Force spectroscopy and fluorescence microscopy of dsDNA–YOYO-1 complexes: implications for the structure of dsDNA in the overstretching region

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

When individual dsDNA molecules are stretched beyond their B-form contour length, they reveal a structural transition in which the molecule extends 1.7 times its contour length. The nature of this transition is still a subject of debate. In the first model, the DNA helix unwinds and combined with the tilting of the base pairs (which remain intact), results in a stretched form of DNA (also known as S-DNA). In the second model the base pairs break resulting effectively in two single-strands, which is referred to as force-induced melting. Here a combination of optical tweezers force spectroscopy with fluorescence microscopy was used to study the structure of dsDNA in the overstretching regime. When dsDNA was stretched in the presence of 10 nM YOYO-1 an initial increase in total fluorescence intensity of the dye–DNA complex was observed and at an extension where the dsDNA started to overstretch the fluorescence intensity leveled off and ultimately decreased when stretched further into the overstretching region. Simultaneous force spectroscopy and fluorescence polarization microscopy revealed that the orientation of dye molecules did not change significantly in the overstretching region (78.0°± 3.2°). These results presented here clearly suggest that, the structure of overstretched dsDNA can be explained accurately by force induced melting.


Longitudinal optical trapping and sizing of aerosol droplets

A. E. Carruthers, J. P. Reid, and A. J. Orr-Ewing

We present evidence that aerosol droplets, ~1-2μm in diameter, can be optically bound over a 4mm distance within a volume formed by the overlap of the central cores and rings of two counterpropagating Bessel beams. The sizes of the individual polydisperse aerosol particles can be estimated from the angular variation of the elastic light scattering. Scattered light from the two orthogonally polarized trapping beams and from a Gaussian probe beam of different wavelength can be used to provide independent estimations of size. The coalescence of two droplets was observed and characterized.


Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic-bandgap fiber: erratum

T. G. Euser, M. K. Garbos, J. S. Y. Chen, and P. St.J. Russell

In a previous Letter [Opt. Lett. 34, 3674 (2009)], the material of the spheres used in the experiment was incorrectly stated. That error is corrected here.


Wednesday, June 16, 2010

Optical binding of electrically small magnetodielectric particles

Kesava Jay, Patrick C. Chaumet, T. N. Langtry and Adel Rahmani
An ensemble of spherical particles with arbitrary dielectric permittivity and magneticpermeability was considered in the dipole approximation. Each particle was described by complex electric and magnetic polarizabilities. A computational approach based on the coupled dipole method, also called the discrete dipole approximation, was used to derive the optical force experienced by each particle due to an incident electromagnetic field andthe fields scattered by all other particles. This approach is general and can handle material dispersion and losses. In order to illustrate this approach, we studied the case of two spherical particles separated by a distance d, and illuminated by an incident plane wave whose wave vector is normal to the axis of the particles. We computed the optical force experienced by each particle in the direction of the beam (radiation pressure), and perpendicular to the beam (optical binding) for particles with positive and negative refractive indices. We also considered the effect of material losses.

Tuesday, June 15, 2010

Proportional enlargement of movement by using an optically driven multi-link system with an elastic joint

Yu Jin Jeong, Tae Woo Lim, Yong Son, Dong-Yol Yang, Hong-Jin Kong, and Kwang-Sup Lee

Diverse movements using optical manipulation have been introduced. These are generally performed in the focal region of the laser beam. To achieve a wider range of movements based on precise motion transformation, an effective method for optical manipulation that overcomes the important obstacles such as small optical trapping forces, friction, and the viscosity of fluids is required. A multi-link system with an elastic joint is introduced that provides precise motion transformation and amplification. By considering the physical properties of the structure and the optical trapping force, an elastic micron-scale joint with the simple shape of a thin plate was designed. As a further example of a multi-link system with an elastic joint, a double 4-link system for motion enlargement was designed and fabricated. By performing experimental evaluations of the fabricated structures, it was confirmed that multi-link systems with an elastic joint were effective tools for precise motion transformation through optical manipulation.


Friday, June 11, 2010

A few books on Optical Tweezers

Optical Tweezers: Methods and Applications (Series in Optics and Optoelectronics) Micromechanical Photonics Micromanipulation by Light in Biology and Medicine: the Laser Microbeam and Optical Tweezers (Methods in Bioengineering)

Two-body scattering in a trap and a special periodic phenomenon sensitive to the interaction

Y. Z. He and C. G. Bao

Two-body scattering of neutral particles in a trap is studied theoretically. The control of the initial state is realized by using optical traps. The collisions inside the trap occur repeatedly; thereby the effect of interaction can be accumulated. Two periodic phenomena with a shorter and a much longer period, respectively, are found. The latter is sensitive to the interaction. Instead of measuring the differential cross section as is usually done, the measurement of the longer period and the details of the periodic behavior might be a valid source of information on weak interactions among neutral particles.


Single DNA molecule detection in an optical trap using surface-enhanced Raman scattering

Satish Rao, Saurabh Raj, Stefan Balint, Carlota Bardina Fons, Susana Campoy, Montserrat Llagostera, and Dmitri Petrov

Raman spectra from single DNA molecules in their natural aqueous environment are presented. A DNA molecule that is anchored between two optically trapped dielectric beads is suspended in a solution with nanosized silver colloid particles. The nonspecific binding of the metal to the DNA enhances the Raman scattering that is excited by a near-infrared beam. A Raman spectrum is first recorded followed by a force-extension curve that verifies the presence of a single DNA molecule.


Optical bottles: A quantitative analysis of optically confined nanoparticle ensembles in suspension

Joseph Junio, Seongmin Park, Mahn-Won Kim and H. Daniel Ou-Yang

We present a novel method, an optical bottle, that uses a focused laser beam to trap and a second laser to analyze optically confined multiple nanoparticles. A theoretical framework based on the mechanical equilibrium of the optical radiation pressure produced by the focused laser beam and the osmotic pressure produced by the enriched particle concentration in the optical trap is developed for analyzing the ensemble behavior of the optically confined nanoparticles. Experiments were conducted for fluorescently labeled polystyrene nanospheres and unilamellar phospholipid vesicles to determine the optical trapping energy of individual particles as well as the osmotic compressibility of the colloids. The new method is not limited by the particle concentration and is relatively easy to implement.


Thursday, June 10, 2010

Elastic and Inelastic Light Scattering from Single Bacterial Spores in an Optical Trap Allows the Monitoring of Spore Germination Dynamics

Lixin Peng, De Chen, Peter Setlow and Yong-qing Li

Raman scattering spectroscopy and elastic light scattering intensity (ESLI) were used to simultaneously measure levels of Ca-dipicolinic acid (CaDPA) and changes in spore morphology and refractive index during germination of individual Bacillus subtilis spores with and without the two redundant enzymes (CLEs), CwlJ and SleB, that degrade spores’ peptidoglycan cortexes. Conclusions from these measurements include (1) CaDPA release from individual wild-type germinating spores was biphasic; in a first heterogeneous slow phase, Tlag, CaDPA levels decreased 15%, and in the second phase ending at Trelease, remaining CaDPA was released rapidly; (2) in l-alanine germination of wild-type spores and spores lacking SleB (a) the ESLI rose 2-fold shortly before Tlag at T1, (b) following Tlag, the ESLI again rose 2-fold at T2 when CaDPA levels had decreased 50%, and (c) the ESLI reached its maximum value at Trelease and then decreased; (3) in CaDPA germination of wild-type spores, (a) Tlag increased and the first increase in ESLI occurred well before Tlag, consistent with different pathways for CaDPA and l-alanine germination, (b) at Trelease, the ESLI again reached its maximum value; (4) in l-alanine germination of spores lacking both CLEs and unable to degrade their cortex, the time ΔTrelease (Trelease − Tlag) for excretion of ≥75% of CaDPA was 15-fold higher than that for wild-type or sleB spores; and (5) spores lacking only CwlJ exhibited a similar but not identical ESLI pattern during l-alanine germination to that seen with cwlJ sleB spores and the high value for ΔTrelease.


Monday, June 7, 2010

Assembly of Acircular SnO2 Rod Using Optical Tweezers and Laser Curing of Metal Nanoparticles

Chanhyuk Nam, Daehie Hong, Jaeik Chung, Jaewon Chung, Insung Hwang, Jongheun Lee, Seunghwan Ko, and Costas P. Grigoropoulos
Acicular tin dioxide (SnO2) rods (1–2 µm in diameter, 5–20 µm long) were assembled and fused on the patterned gold electrode by an optical tweezer. In addition, the electrical contact between the assembled SnO2 rod and the gold electrode was improved by laser curing of gold nanoparticles and the subsequent sintering in the oven. Here, the nanoparticles covered the entire area of the assembled SnO2 rod by evaporating a droplet of nanoparticle solution dripped on the assembled SnO2 rod. Subsequently, nanoparticles near the contact area between the rod and electrode were locally cured by direct heating with a focused infrared laser beam, which induced desorption of the surface monolayer. Therefore, the cured gold nanoparticles could be sintered after the non-laser irradiated nanoparticles were cleaned by the initial solvent application. Without sintering of the nanoparticles, the resistance of the assembled SnO2 rod was measured over several MΩ. After the nanoparticle sintering it could be reduced to a few hundred kΩ, which was in agreement with the resistance of the assembled SnO2 rod.


Optical forces near a nanoantenna

Martin Ploschner, Michael Mazilu, Thomas F. Krauss, and Kishan Dholakia

The Maxwell stress tensor method is used to calculate the optical forces acting upon a glass nanosphere in the proximity of optically excited gold nanoantenna structures. The dependence of optical forces over a full range of excitation angles is explored: the total internal reflection excitation does not bring any particular advantage to trapping efficiency when compared to the normal incidence excitation. Our calculations show multiple trapping sites with similar trapping properties for the normal and the total internal reflection cases, respectively; furthermore, the convective heating probably dominates over any optical forces in such systems.


Friday, June 4, 2010

Gripped by light: Optical binding

Kishan Dholakia and Pavel Zemánek

The light-matter interaction has been at the heart of major advances from the atomic scale right to the microscopic scale over the past four decades. Confinement by light, embodied by the area of optical trapping, has had a major influence across all of the natural sciences. However, an emergent and powerful topic within this field that has steadily merged but not gained much recognition is optical binding: the importance of exploring the optically mediated interaction between assembled objects that can cause attractive and repulsive forces and dramatically influence the way they assemble and organize themselves. This offers routes for colloidal self-assembly, crystallization, and organization of templates for biological and colloidal sciences. In this Colloquium, this emergent area is reviewed looking at the pioneering experiments in the field and the various theoretical approaches that aim to describe this behavior. The latest experimental studies in the field are reviewed and theoretical approaches are now beginning to converge to describe the binding behavior seen. Recent links between optical binding and nonlinearity are explored as well as future themes and challenges.

Multiple optical trapping and binding: new routes to self-assembly

T Čižmár, L C Dávila Romero, K Dholakia and D L Andrews

The impact of optical forces in the physical and biological sciences now enables the manipulation of objects ranging in size from a cell down to a single atom. The mechanical effects of optical fields have profound and far-reaching consequences, and attention is increasingly focused upon the opportunities for the non-contact assembly of particles into specific geometries. The present overview focuses on the two aspects of multi-particle trapping and optical binding. These can broadly be grouped as methods based on light-mediated inter-particle interactions, offering potential for the organization of large numbers of micro- or nano-particles using optical forces alone.


On-chip supercontinuum optical trapping and resonance excitation of microspheres

Arthur Nitkowski, Alexander Gondarenko, and Michal Lipson

We demonstrate the simultaneous optical manipulation and analysis of microscale particles in a microfluidic channel. Whispering gallery modes (WGMs) in dielectric microspheres are excited using the evanescent field from a silicon nitride waveguide. A supercontinuum source is used to both trap the microspheres to the surface of the waveguide and excite their resonant modes. All measurements are in plane, thus providing an integrated optofluidic platform for lab-on-a-chip biosensing applications.


Real-time detection of single-living pancreatic -cell by laser tweezers Raman spectroscopy: High glucose stimulation

Xi Rong, Shu-Shi Huang, Xiao-Cong Kuang, Hong Liu

Glucose acts as a -cell stimulus factor and leads to cellular responses that involve a large amount of biomolecule formation, relocation, and transformation. We hypothesize that information about these changes can be obtained in real-time by laser tweezers Raman spectroscopy. To test this hypothesis, repeated measurements designs in accordance with the application of Raman spectroscopy detection were used in the current experiment. Single rat -cells were measured by Raman spectroscopy in 2.8 mmol/l glucose culture medium as a basal condition. After stimulation with high glucose (20 mmol/l), the same cells were measured continuously. Each cell was monitored over a total time span of 25 min, in 5 min intervals. During this period of time, cells were maintained at an appropriate temperature controlled by an automatic heater, to provide near-physiological conditions. It was found that some significant spectral changes induced by glucose were taking place during the stimulation time course. The most noticeable changes were the increase of spectral intensity at the 1002, 1085, 1445, and 1655 cm-1 peaks, mainly corresponding to protein and lipid. We speculate that these changes might have to do with -cell protein and lipid synthesis. Using laser tweezers Raman spectroscopy in combination with glucose stimulation, optical spectral information from rat -cells was received and analyzed.


Thursday, June 3, 2010

The folding cooperativity of a protein is controlled by its chain topology

Elizabeth A. Shank, Ciro Cecconi, Jesse W. Dill, Susan Marqusee & Carlos Bustamante

The three-dimensional structures of proteins often show a modular architecture comprised of discrete structural regions or domains. Cooperative communication between these regions is important for catalysis, regulation and efficient folding; lack of coupling has been implicated in the formation of fibrils and other misfolding pathologies1. How different structural regions of a protein communicate and contribute to a protein’s overall energetics and folding, however, is still poorly understood. Here we use a single-molecule optical tweezers approach to induce the selective unfolding of particular regions of T4 lysozyme and monitor the effect on other regions not directly acted on by force. We investigate how the topological organization of a protein (the order of structural elements along the sequence) affects the coupling and folding cooperativity between its domains. To probe the status of the regions not directly subjected to force, we determine the free energy changes during mechanical unfolding using Crooks’ fluctuation theorem. We pull on topological variants (circular permutants) and find that the topological organization of the polypeptide chain critically determines the folding cooperativity between domains and thus what parts of the folding/unfolding landscape are explored. We speculate that proteins may have evolved to select certain topologies that increase coupling between regions to avoid areas of the landscape that lead to kinetic trapping and misfolding.


Observation of DNA pinning at laser focal point on Au surface and its application to single DNA nanowire and cross-wire formation

Sho Fujii, Katsuaki Kobayashi, Katsuhiko Kanaizuka, Tetsuaki Okamoto, Shoichi Toyabe, Eiro Muneyuki and Masa-aki Haga
We report a new technique for fabricating a single DNA nanowire at a desired position in a sequential manner using the micronanobubble generated by laser local heating at the Au/water interface. In our previous report, we found the reversible pull-in/shrinkage of one end immobilized DNA strands near a Nd:YAG laser focal point on an Au surface. In further experiments, the pinning of DNA strands in the stretched state was observed on the Au surface only when the bubble has touched the free end of DNA. This pinning phenomenon was observed even on the alkane thiol modified Au surface as self-assembled monolayers (SAMs) such as hexanethiol, mercaptohexanol, and hexadecanethiol. However, no pinning was observed on the bovine serum albumin (BSA) modified surface. Since optical tweezers can manipulate a DNA modified bead (radius = 1.87 μm), the bead was firstly fixed on a solid surface by being compressed with the optical tweezers, and the pulling and pinning of DNA on the bead were achieved. As a consequence, the laser local heating on the Au surface enables us to control the number and position of the one end immobilized DNA strands as DNA nanowires.


Monitoring ligand–receptor interactions by photonic force microscopy

Sylvia Jeney, Flavio Mor, Roland Koszali, László Forró and Vincent T Moy

We introduce a method for the acquisition of single molecule force measurements of ligand–receptor interactions using the photonic force microscope (PFM). Biotin-functionalized beads, manipulated with an optical trap, and a streptavidin-functionalized coverslip were used to measure the effect of different pulling forces on the lifetime of individual streptavidin–biotin complexes. By optimizing the design of the optical trap and selection of the appropriate bead size, pulling forces in excess of 50 pN were achieved. Based on the amplitude of three-dimensional (3D) thermal position fluctuations of the attached bead, we were able to select for a bead–coverslip interaction that was mediated by a single streptavidin–biotin complex. Moreover, the developed experimental system was greatly accelerated by automation of data acquisition and analysis. In force-dependent kinetic measurements carried out between streptavidin and biotin, we observed that the streptavidin–biotin complex exhibited properties of a catch bond, with the lifetime increasing tenfold when the pulling force increased from 10 to 20 pN. We also show that silica beads were more appropriate than polystyrene beads for the force measurements, as tethers, longer than 200 nm, could be extracted from polystyrene beads.


Tuesday, June 1, 2010

A simple control scheme for the manipulation of a particle by means of optical tweezers

Carlos Aguilar-Ibañez, Miguel S. Suarez-Castanon, Luis I. Rosas-Soriano

We present a simple control scheme for changing the position of a microscopic particle immersed in a viscous medium and trapped by optical tweezers. We derive a simple feedback controller under the consideration that the particle mass is so small that it can be discarded from the motion equations. This approximation is well justified in practice, since the inertial force produced by the motion of a micron-scaled trapped particle is completely dominated by the medium viscous drag force. Finally, we formally prove that the obtained controller is able to globally asymptotically stabilize the system when the particle mass is considered, if some suitable values of some control parameter are used. The stability analysis of the controlled system was carried out by using the standard Lyapunov stability theory. Also, by means of numerical simulations, we show that the obtained closed-loop system is robust when random thermal noise is presented.


Dynamic electrical response of colloidal micro-spheres in compliant micro-channels from optical tweezers velocimetry

Jan A. van Heiningen, Aliasghar Mohammadi and Reghan J. Hill

We report the dynamic response of colloidal silica in aqueous electrolytes to oscillatory electric fields at frequencies up to 50 kHz. Particles were optically trapped at various positions across the gap of straight and crossed parallel-plate micro-channels. Using back-focal-plane interferometry, we measured the apparent electrophoretic mobility in NaCl and CaCl2 electrolytes over a wide range of salt concentrations. The mobility has a strikingly complex dependence on channel position and forcing frequency that cannot be understood on the basis of standard electrokinetic theory for rigid micro-channels. We ascribe the anomalous dynamics to coupling of electro-osmotic flow and elastic modes of the micro-channel and auxiliary hardware. By integrating into the classical theory a complex-valued channel-compliance parameter—that modulates the phase and amplitude of the dynamic electro-osmotic flow—theoretical interpretation of the frequency-dependent mobility furnishes robust measurements of the intrinsic particle electrophoretic mobility and the upper and lower channel-wall -potentials. Together, the single-particle experiments and accompanying theoretical interpretation highlight—for the first time—how spatially and temporally resolved particle dynamics are exquisitely sensitive to channel compliance. Accordingly, specially designed compliant micro-fluidic channels and flexible tube connections might be tailored for dynamic electrical micro-fluidic diagnostic applications.