Wednesday, December 22, 2010

Optical trapping with focused Airy beams

Zhu Zheng, Bai-Fu Zhang, Hao Chen, Jianping Ding, and Hui-Tian Wang

Airy beams are attractive owing to their two intriguing properties—self-bending and nondiffraction—that are particularly helpful for optical manipulation of particles. We perform theoretical and experimental investigations into the focusing property of Airy beams and provide insight into the trapping ability of tightly focused Airy beams. Experiment on optical tweezers demonstrates that the focused Airy beams can create multiple traps for two-dimensional confining particles, and the stable traps exist in the vicinity of the main intensity lobes in the focused beams. The trapping pattern can be varied with changes in the cross section of the focused beam. The focused Airy beam offers a novel way of optically manipulating microparticles.


Dual-trap Raman tweezers for probing dynamics and heterogeneity of interacting microbial cells

Yan Li, Guiwen Wang, Hui-lu Yao, Junxian Liu, Yong-qing Li

We report on development of dual-trap Raman tweezers for monitoring cellular dynamics and heterogeneity of interacting living cells suspended in a liquid medium. Dual-beam optical tweezers were combined with Raman spectroscopy, which allows capturing two cells that are in direct contact or closely separated by a few micrometers and simultaneously acquiring their Raman spectra with an imaging CCD spectrograph. As ademonstration, we recorded time-lapse Raman spectra of budding yeast cells held in dual traps for over 40 min to monitor the dynamic growth in a nutrient medium. We also monitored two germinating Bacillus spores after the initiation with L-alanine and observed their heterogeneity in the release of CaDPA under identical microenvironment.


Stable optical lift

Grover A. Swartzlander Jr, Timothy J. Peterson, Alexandra B. Artusio-Glimpse & Alan D. Raisanen

We have predicted and observed an optical analogue of aerodynamic lift, in which a cambered refractive object with differently shaped top and bottom surfaces experiences a transverse lift force when placed in a uniform stream of light. A semi-cylindrical rod is found to automatically torque into a stable angle of attack, and then exhibit uniform motion. We have experimentally verified this using a micrometer-scale ‘lightfoil’ which was fabricated using photolithographic techniques, immersed in water and illuminated with milliwatt-scale laser light. Unlike optical tweezers, an intensity gradient is not required to achieve a transverse force. Many rods may therefore be lifted simultaneously in a single quasi-uniform beam of light. We propose using optical lift to power micromachines, transport microscopic particles in a liquid, or to improve the design of solar sails for interstellar space travel.


Tuesday, December 21, 2010

Fast uncoiling kinetics of F1C pili expressed by uropathogenic Escherichia coli are revealed on a single pilus level using force-measuring optical tweezers

Mickaël Castelain, Sarah Ehlers, Jeanna Klinth, Stina Lindberg, Magnus Andersson, Bernt Eric Uhlin and Ove Axner
Uropathogenic Escherichia coli (UPEC) express various kinds of organelles, so-called pili or fimbriae, that mediate adhesion to host tissue in the urinary tract through specific receptor-adhesin interactions. The biomechanical properties of these pili have been considered important for the ability of bacteria to withstand shear forces from rinsing urine flows. Force-measuring optical tweezers have been used to characterize individual organelles of F1C type expressed by UPEC bacteria with respect to such properties. Qualitatively, the force-versus-elongation response was found to be similar to that of other types of helix-like pili expressed by UPEC, i.e., type 1, P, and S, with force-induced elongation in three regions, one of which represents the important uncoiling mechanism of the helix-like quaternary structure. Quantitatively, the steady-state uncoiling force was assessed as 26.4 ±1.4 pN, which is similar to those of other pili (which range from 21 pN for SI to 30 pN for type 1). The corner velocity for dynamic response (1,400 nm/s) was found to be larger than those of the other pili (400–700 nm/s for S and P pili, and 6 nm/s for type 1). The kinetics were found to be faster, with a thermal opening rate of 17 Hz, a few times higher than S and P pili, and three orders of magnitude higher than type 1. These data suggest that F1C pili are, like P and S pili, evolutionarily selected to primarily withstand the conditions expressed in the upper urinary tract.


Monday, December 20, 2010

Ionic concentration- and pH-dependent electrophoretic mobility as studied by single colloid electrophoresis

I Semenov, P Papadopoulos, G Stober and F Kremer

Optical tweezers are employed to measure separately the complex electrophoretic mobility of a single colloid and the complex electroosmotic response of the surrounding medium in a specially designed microfluidic cell. Using the very same colloid both quantities are determined in dependence on the concentration of the aqueous salt solution (10 − 5–10 − 1 mol l − 1), the valence of the ions (K + , Ca2 + ) and the pH (2.5–8.5). A pronounced effect is observed for all these examined parameters. The dependence on ion concentration agrees qualitatively—for the monovalent case—with the predictions of the standard electrokinetic model.


Optical trapping with high forces reveals unexpected behaviors of prion fibrils

Jijun Dong, Carlos E Castro, Mary C Boyce, Matthew J Lang & Susan Lindquist

Amyloid fibrils are important in diverse cellular functions, feature in many human diseases and have potential applications in nanotechnology. Here we describe methods that combine optical trapping and fluorescent imaging to characterize the forces that govern the integrity of amyloid fibrils formed by a yeast prion protein. A crucial advance was to use the self-templating properties of amyloidogenic proteins to tether prion fibrils, enabling their manipulation in the optical trap. At normal pulling forces the fibrils were impervious to disruption. At much higher forces (up to 250 pN), discontinuities occurred in force-extension traces before fibril rupture. Experiments with selective amyloid-disrupting agents and mutations demonstrated that such discontinuities were caused by the unfolding of individual subdomains. Thus, our results reveal unusually strong noncovalent intermolecular contacts that maintain fibril integrity even when individual monomers partially unfold and extend fibril length.


Hop2-Mnd1 Condenses DNA to Stimulate the Synapsis Phase of DNA Strand Exchange

Roberto J. Pezza, R. Daniel Camerini-Otero and Piero R. Bianco

Hop2-Mnd1 is a meiotic recombination mediator that stimulates DNA strand invasion by both Dmc1 and Rad51. To understand the biochemical mechanism of this stimulation, we directly visualized the heterodimer acting on single molecules of duplex DNA using optical tweezers and video fluorescence microscopy. The results show that the Hop2-Mnd1 heterodimer efficiently condenses double-stranded DNA via formation of a bright spot or DNA condensate. The condensation of DNA is Hop2-Mnd1 concentration-dependent, reversible, and specific to the heterodimer, as neither Hop2 nor Mnd1 acting alone can facilitate this reaction. The results also show that the rate-limiting nucleation step of DNA condensation is overcome in the presence of divalent metal ions, with the following order of preference: Mn2+>Mg2+>Ca2+. Hop2-Mnd1/Dmc1/single-stranded DNA nucleoprotein filaments also condense double-stranded DNA in a heterodimer concentration-dependent manner. Of importance, the concentration dependence parallels that seen in DNA strand exchange. We propose that rapid DNA condensation is a key factor in stimulating synapsis, whereas decondensation may facilitate the invasion step and/or the ensuing branch migration process.


Improvement of the axial trapping effect using azimuthally polarised trapping beam

Li Xue-Cong and Sun Xiu-Dong

A dual optical tweezers system, which consists of a doughnut mode optical tweezer (DMOT) with the azimuthally polarised trapping beam and a solid mode optical tweezer (SMOT) with the Gauss trapping beam was constructed to compare the axial trapping effect of DMOT and SMOT The long-distance axial trapping of ST68 microbubbles (MBs) achieved by DMOT was more stable than that of SMOT Moreover the axial trapping force measured using the viscous drag method, was depended on the diameter of the particle, the laser power, and the numerical aperture (NA) of the objective lens The measurement of the axial trapping force and the acquisition of CCD images of trapping effect confirmed that the DMOT showed excellent axial trapping ability than SMOT A simple and effective method is developed to improve axial trapping effect using the azimuthally polarized beam as trapping beam This is helpful for the long-distance manipulating of particles especially polarised biological objects in axial direction.


Friday, December 17, 2010

Local x-ray structure analysis of optically manipulated biological micro-objects

Dan Cojoc, Heinz Amenitsch, Enrico Ferrari, Silvia C. Santucci, Barbara Sartori, Michael Rappolt, Benedetta Marmiroli, Manfred Burghammer, and Christian Riekel
X-ray diffraction using micro- and nanofocused beams is well suited for nanostructure analysis at different sites of a biological micro-object. To conduct in vitro studies without mechanical contact, we developed object manipulation by optical tweezers in a microfluidic cell. Here we report x-ray microdiffraction analysis of a micro-object optically trapped in three dimensions. We revealed the nanostructure of a single starch granule at different points and investigated local radiation damage induced by repeated x-ray exposures at the same position, demonstrating high stability and full control of the granule orientation by multiple optical traps.


Electromagnetic forces on plasmonic nanoparticles induced by fast electron beams

Alejandro Reyes-Coronado, Rubén G. Barrera, Philip E. Batson, Pedro M. Echenique, Alberto Rivacoba, Javier Aizpurua
The total momentum transfer from fast electron beams, like those employed in scanning transmission electron microscopy (STEM), to plasmonic nanoparticles is calculated. The momentum transfer is obtained by integrating the electromagnetic forces acting on the particles over time. Numerical results for single and dimer metallic nanoparticles are presented, for sizes ranging between 2 and 80 nm. We analyze the momentum transfer in the case of metallic dimers where the different relevant parameters such as particle size, interparticle distance, and electron beam impact parameter are modified. It is shown that depending on the specific values of the parameters, the total momentum transfer yields a force that can be either attractive or repulsive. The time-average forces calculated for electron beams commonly employed in STEM are on the order of piconewtons, comparable in magnitude to optical forces and are thus capable of producing movement in the nanoparticles. This effect can be exploited in mechanical control of nanoparticle induced motion.


Thursday, December 16, 2010

Three dimensional optical manipulation and structural imaging of soft materials by use of laser tweezers and multimodal nonlinear microscopy

Rahul P. Trivedi, Taewoo Lee, Kris A. Bertness, and Ivan I. Smalyukh

We develop an integrated system of holographic optical trapping and multimodal nonlinear microscopy and perform simultaneous three-dimensional optical manipulation and non-invasive structural imaging of composite soft-matter systems. We combine different nonlinear microscopy techniques such as coherent anti-Stokes Raman scattering, multi-photon excitation fluorescence and multi-harmonic generation, and use them for visualization of long-range molecular order in soft materials by means of their polarized excitation and detection. The combined system enables us to accomplish manipulation in composite soft materials such as colloidal inclusions in liquid crystals as well as imaging of each separate constituents of the composite material in different nonlinear optical modalities. We also demonstrate optical generation and control of topological defects and simultaneous reconstruction of their three-dimensional long-range molecular orientational patterns from the nonlinear optical images.


Three dimensional nanoparticle trapping enhanced by surface plasmon resonance

Jingzhi Wu and Xiaosong Gan

We demonstrate a three dimensional nanoparticle trapping approach aided by the surface plasmon resonance of metallic nanostructures. The localized surface plasmon resonance effect provides strong electromagnetic field enhancement, which enables confinement of nanoparticles in the three-dimensional space. Numerical simulations indicate that the plasmonic structure provides approximately two orders of magnitude stronger optical forces for trapping nanoparticles, compared with that without nanostructures. The study of thermal effect of the plasmonic structure shows that the impact of the thermal force is significant, which may determine the outcome of the nanoparticle trapping.


Wednesday, December 15, 2010

Trapping of a microsphere pendulum through cavity-enhanced optical forces

Yuqiang Wu, Jonathan M Ward, Vladimir G Minogin and Síle Nic Chormaic

Optical forces resulting from evanescently coupled microcavities can produce remarkable mechanical effects on micro- and nanoscale systems. Excitation of the symmetric and antisymmetric modes of the interacting whispering gallery modes (WGM) leads to significant attractive and repulsive forces. Here, we propose a method to spatially trap a microspherical resonator pendulum via the optical forces produced by two simultaneously excited WGMs of a photonic molecule, comprising two microspherical cavities. We discuss how the cavity-enhanced optical force generated in the photonic molecule can create an optomechanical potential of about 5 eV deep and 10 pm wide, which can be used to trap the pendulum at any given equilibrium position by a simple choice of laser frequencies. This result presents opportunities for very precise all-optical self-alignment of microsystems. Frequency splitting of a co-resonant mode from two similar-sized microspheres was observed experimentally and the mechanical characteristics of a microsphere pendulum were also studied.


Tuesday, December 14, 2010

Conical diffraction of linearly polarised light controls the angular position of a microscopic object

D. P. O’Dwyer, C. F. Phelan, K. E. Ballantine, Y. P. Rakovich, J. G. Lunney, and J. F. Donegan

Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity profile. Rotation of the plane of polarisation produces the unique effect of spatially moving the crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them at any angular position on the ring. Continuous motion around the circle is also demonstrated. This crescent beam does not require an interferometeric arrangement to form it, nor does it carry optical angular momentum. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of light suggests that this optical process should find application in the manipulation and actuation of micro- and nano-scale physical and biological objects.


Control of micro-cantilever using passive optical feedback for force microscopy

Hao Fu, Yong Liu, Jian Chen and Gengyu Cao

We demonstrate the use of bolometric force backaction in lever-based miniature Fabry–Pérot (FP) optical cavity for micro-cantilever control. In our experiment, low finesse FP microcavity is formed by polished fiber end and mass loaded micro-cantilever. We show that the dynamics of micro-cantilever can be deeply modified when photon intensity stored in the FP microcavity is sufficiently large. For blue microcavity detuning, we confirm this control mechanism can optimize the dynamics of micro-cantilever and dramatically reduce its Brownian motion amplitude without deteriorating force resolution. This effective and low-cost control method can be simply realized in most optical detection force microscopes.


Optical orbital angular momentum from the curl of polarization

Xi-Lin Wang, Jing Chen, Yongnan Li, Jianping Ding, Cheng-Shan Guo, and Hui-Tian Wang

We predict a new category of optical orbital angular momentum that is associated with the curl of polarization and a kind of vector field with radial-variant hybrid states of polarization that can carry such novel optical orbital angular momentum. We present a scheme for creating the desired vector fields. Optical trapping experiments validate that the vector fields, which have no additional phase vortex, exert torques to drive the orbital motion of the trapped isotropic microspheres.


A microfluidic system with optical laser tweezers to study mechanotransduction and focal adhesion recruitment

Peyman Honarmandi, Hyungsuk Lee, Matthew J. Lang and Roger D. Kamm

We present a new method to locally apply mechanical tensile and compressive force on single cells based on integration of a microfluidic device with an optical laser tweezers. This system can locate a single cell within customized wells exposing a square-like membrane segment to a functionalized bead. Beads are coated with extracellular matrix (ECM) proteins of interest (e.g. fibronectin) to activate specific membrane receptors (e.g.integrins). The functionalized beads are trapped and manipulated by optical tweezers to apply mechanical load on the ECM-integrin-cytoskeleton linkage. Activation of the receptor is visualized by accumulation of expressed fluorescent proteins. This platform facilitates isolation of single cells and excitation by tensile/compressive forces applied directly to the focal adhesion via specific membrane receptors. Protein assembly or recruitment in a focal adhesion can then be monitored and identified using fluorescent imaging. This platform is used to study the recruitment of vinculin upon the application of external tensile force to single endothelial cells. Vinculin appears to be recruited above the forced bead as an elliptical cloud, centered 2.1 ± 0.5 μm from the 2 μm bead center. The mechanical stiffness of the membrane patch inferred from this measurement is 42.9 ± 6.4 pN μm−1 for a 5 μm × 5 μm membrane segment. This method provides a foundation for further studies of mechanotransduction and tensile stiffness of single cells.


Monday, December 13, 2010

Resolving interparticle position and optical forces along the axial direction using optical coherence gating

T. H. Chow, W. M. Lee, K. M. Tan, B. K. Ng, and C. J. R. Sheppard

In this paper, we demonstrate the use of coherence gating to resolve particle positions and forces in the axial direction. Through coherence gating, particle displacements and interparticle separations can be resolved with a high signal-to-noise ratio. We achieved both high depth resolvability (10−6 m) and weak optical force (10−15 N) measurements in an optical trapping system using a low coherence interferometry system. Trap stiffness as low as 1.46 fN μm−1 was measured. This technique is well-suited for the direct visualization of interparticle optical-mechanical interactions.


Determining the binding mode and binding affinity constant of tyrosine kinase inhibitor PD153035 to DNA using optical tweezers

Chih-Ming Cheng, Wei-Ting Wang, Chien-Ting Hsu, Jing-Shin Tsai, Chien-Ming Wu and Tzu-Sen Yang
Accurately predicting binding affinity constant (KA) is highly required to determine the binding energetics of the driving forces in drug–DNA interactions. Recently, PD153035, brominated anilinoquinazoline, has been reported to be not only a highly selective inhibitor of epidermal growth factor receptor but also a DNA intercalator. Here, we use a dual-trap optical tweezers to determining KA for PD153035, where KA is determined from the changes in B-form contour length (L) of PD153035–DNA complex. Here, L is fitted using a modified wormlike chain model. We found that a noticeable increment in L in 1 mM sodium cacodylate was exhibited. Furthermore, our results showed that KA = 1.18(±0.09) × 104 (1/M) at 23 ± 0.5 °C and the minimum distance between adjacent bound PD153035 ≈ 11 bp. We anticipate that by using this approach we can determine the complete thermodynamic profiles due to the presence of DNA intercalators.


Friday, December 10, 2010

Plasmon Hybridization Reveals the Interaction between Individual Colloidal Gold Nanoparticles Confined in an Optical Potential Well

Lianming Tong, Vladimir D. Miljković, Peter Johansson, and Mikael Käll

The understanding of interaction forces between nanoparticles in colloidal suspension is central to a wide range of novel applications and processes in science and industry. However, few methods are available for actual characterization of such forces at the single particle level. Here we demonstrate the first measurements of colloidal interactions between two individual diffusing nanoparticles using a colorimetric assay based on plasmon hybridization, that is, strong near-field coupling between localized surface plasmon resonances. The measurements are possible because individual gold nanoparticle pairs can be loosely confined in an optical potential well created by a laser tweezers. We quantify the degree of plasmon hybridization for a large number of individual particle pairs as a function of increasing salt concentration. The data reveal a considerable heterogeneity at the single particle level but the estimated average surface separations are in excellent agreements with predictions based on the classical theory of Derjaguin, Landau, Verwey, and Overbeek.

Tuesday, December 7, 2010

On-the-fly cross flow laser guided separation of aerosol particles based on size, refractive index and density–theoretical analysis

A. A. Lall, A. Terray, and S. J. Hart

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. In this paper, we present a theoretical analysis on laser separation of non-absorbing aerosol particles moving at speeds (1-10 cm/sec) which are several orders of magnitude greater than typical particle speeds used in previous studies in liquid medium. The calculations are presented for particle deflection by a loosely focused Gaussian 1064 nm laser, which simultaneously holds and deflects particles entrained in flow perpendicular to their direction of travel. The gradient force holds the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles, perpendicular to the flow, during this period. Our calculations show particle deflections of over 2500 µm for 15 µm aerosol particles, and a separation of over 1500 µm between 5 µm and 10 µm particles when the laser is operated at 10W. We show that a separation of about 421 µm can be achieved between two particles of the same size (10 µm) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 µm particles with a density difference of 600 kg/m3 can be separated by 193 µm. Examples are shown for separation distances between polystyrene, poly(methylmethacrylate), silica and water particles. These large laser guided deflections represent a novel achievement for optical separation in the gas phase.


Monday, December 6, 2010

Nanomanipulation of single influenza virus using dielectrophoretic concentration and optical tweezers for single virus infection to a specific cell on a microfluidic chip

Hisataka Maruyama, Kyosuke Kotani, Taisuke Masuda, Ayae Honda, Tatsuro Takahata and Fumihito Arai
A major problem when analyzing bionanoparticles such as influenza viruses (approximately 100 nm in size) is the low sample concentrations. We developed a method for manipulating a single virus that employs optical tweezers in conjunction with dielectrophoretic (DEP) concentration of viruses on a microfluidic chip. A polydimethylsiloxane microfluidic chip can be used to stably manipulate a virus. The chip has separate sample and analysis chambers to enable quantitative analysis of the virus functions before and after it has infected a target cell. The DEP force in the sample chamber concentrates the virus and prevents it from adhering to the glass substrate. The concentrated virus is transported to the sample selection section where it is trapped by optical tweezers. The trapped virus is transported to the analysis chamber and it is brought into contact with the target cell to infect it. This paper describes the DEP virus concentration for single virus infection of a specific cell. We concentrated the influenza virus using the DEP force, transported a single virus, and made it contact a specific H292 cell.


Friday, December 3, 2010

Out-of-equilibrium microrheology using optical tweezers to probe directional viscoelastic properties under shear

Manas Khan and A. K. Sood

Many wormlike micellar systems exhibit appreciable shear thinning due to shear-induced alignment. As the micelles get aligned introducing directionality in the system, the viscoelastic properties are no longer expected to be isotropic. An optical-tweezers–based active microrheology technique enables us to probe the out-of-equilibrium rheological properties of a wormlike micellar system simultaneously along two orthogonal directions —parallel to the applied shear, as well as perpendicular to it. While the displacements of a trapped bead in response to active drag force carry signature of conventional shear thinning, its spontaneous position fluctuations along the perpendicular direction manifest an orthogonal shear thickening, an effect hitherto unobserved.


Supercontinuum trap stiffness measurement using a confocal approach

Zhe Zhang, Haifeng Li, Peng Li, Kebin Shi, Perry Edwards,Fiorenzo Omenetto, Mark Cronin-Golomb, Guizhong Zhang, and Zhiwen Liu

We report a novel method for characterizing the stiffness of white light supercontinuum tweezers, in which the nonlinear photonic crystal fiber used for supercontinuum generation is also utilized as an effective confocal pinhole to track the motion of a trapped bead and as a scan head to realize rapid scanning of the optical trap. By measuring the phase of the bead’s motion in following the trap, a lateral stiffness value of about 7.9 μN/m was obtained with supercontinumm power of about 75mW. Our technique can potentially allow for trap stiffness calibration along an arbitrary direction in three dimensions.


Simultaneous calibration of optical tweezers spring constant and position detector response

Nathalie Westbrook, Antoine Le Gall, David Dulin, André Villing,Philippe Bouyer, Karen Perronet, and Koen Visscher

We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection. The method takes direct advantage of back-focal-plane interferometry measuring not an absolute but a differential position, i.e. the position of the trapped particle relative to the center of the optical tweezers. Therefore, a fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle. Calibration parameters such as the detector’s spatial and temporal response and the spring constant of the optical tweezers then follow readily from fitting the measured impulse response.


Thursday, December 2, 2010

All-Optical Optomechanics: An Optical Spring Mirror

S. Singh, G. A. Phelps, D. S. Goldbaum, E. M. Wright, and P. Meystre

The dominant hurdle to the operation of optomechanical systems in the quantum regime is the coupling of the vibrating element to a thermal reservoir via mechanical supports. Here we propose a scheme that uses an optical spring to replace the mechanical support. We show that the resolved-sideband regime of cooling can be reached in a configuration using a high-reflectivity disk mirror held by an optical tweezer as one of the end mirrors of a Fabry-Perot cavity. We find a final phonon occupation number of the trapped mirror n̅ =0.56 for reasonable parameters, the limit being set by our approximations, and not any fundamental physics. This demonstrates the promise of dielectric disks attached to optical springs for the observation of quantum effects in macroscopic objects.


Robust Mechanosensing and Tension Generation by Myosin VI

Peiying Chuan, James A. Spudich and Alexander R. Dunn

Myosin VI is a molecular motor that is thought to function both as a transporter and as a cytoskeletal anchor in vivo. Here we use optical tweezers to examine force generation by single molecules of myosin VI under physiological nucleotide concentrations. We find that myosin VI is an efficient transporter at loads of up to 2 pN but acts as a cytoskeletal anchor at higher loads. Our data and the resulting model are consistent with an indirect coupling of global structural motions to nucleotide binding and release. The model provides a mechanism by which load may regulate the dual functions of myosin VIin vivo. Our results suggest that myosin VI kinetics are tuned such that the motor maintains a consistent level of mechanical tension within the cell, a property potentially shared by other mechanosensitive proteins.


Optical manipulation of light-absorbing particles takes to the air

Ashley G. Smart

The idea that light can move matter is not new—Johannes Keplersuspected as much some 400 years ago when he noticed that the tails of comets always point away from the Sun. That suspicion was formalized in 1871 with James Clerk Maxwell's prediction of radiation pressure—the force imparted on a body by refracted, reflected, or absorbed light—and confirmed in 1900, when Pyotr Lebedev observed the effect in experiments.


Towards detection and identification of circulating tumour cells using Raman spectroscopy

U. Neugebauer, T. Bocklitz, J. H. Clement, C. Krafft and J. Popp

Body fluids are easily accessible and contain valuable indices for medical diagnosis. Fascinating tools are tumour cells circulating in the peripheral blood of cancer patients. As these cells are extremely rare, they constitute a challenge for clinical diagnostics. In this contribution we present the Raman spectroscopic-based identification of different single cells in suspension that are found in peripheral blood of cancer patients including healthy cells like leukocytes and erythrocytes, and tumour cells like leukaemic cells and cells originating from solid tumours. Leukocytes and erythrocytes were isolated from the peripheral blood of healthy donors while myeloid leukaemia cells (OCI-AML3) and breast carcinoma derived cells (MCF-7, BT-20) were obtained from cell cultures. A laser emitting 785 nm light was used for optical trapping the single cells in the laser focus and to excite the Raman spectrum. Support vector machines were applied to develop a supervised classification model with spectra of 1210 cells originating from three different donors and three independent cultivation batches. Distinguishing tumour cells from healthy cells was achieved with a sensitivity of >99.7% and a specificity of >99.5%. In addition, the correct cell types were predicted with an accuracy of approximately 92%.


Angular and position stability 
of a nanorod trapped in an optical tweezers

Paul B. Bareil and Yunlong Sheng

We analyze the trap stiffness and trapping force potential for a nano-cylinder trapped in the optical tweezers against its axial and lateral shift and tilt associated to the natural Brownian motion. We explain the physical properties of the optical trapping by computing and integrating the radiation stress distribution on the nano-cylinder surfaces using the T-matrix approach. Our computation shows that the force stiffness to the lateral shift is several times higher than that to the axial shift of the nano-cylinder, and lateral torque due to the stress on the side-face is 1-2 orders of magnitude higher than that on the end-faces of a nano-cylinder with the aspect ratio of 2 – 20. The torque due to the stress on the nano-cylinder surface is 2-3 orders of magnitude higher than the spin torque. We explain why a nano-cylinder of low aspect ratio is trapped and aligned normal to the trapping beam axis.


Wednesday, December 1, 2010

Optical forces from an evanescent wave on a magnetodielectric small particle

M. Nieto-Vesperinas and J. J. Saenz

We report the first study on the optical force exerted by an evanescent wave on a small sphere with both electric and magnetic responses to the incident field, immersed in an arbitrary nondissipative medium. New expressions and effects from their gradient, radiation pressure, and curl components are obtained owing to the particle induced electric and magnetic dipoles, as well as to their mutual interaction. We predict possible dramatic changes in the force depending on the host medium, the polarization, and the nature of the surface wave.


Mathieu beams as versatile light moulds for 3D micro particle assemblies

C. Alpmann, R. Bowman, M. Woerdemann, M. Padgett, and C. Denz

We present tailoring of three dimensional light fields which act as light moulds for elaborate particle micro structures of variable shapes. Stereo microscopy is used for visualization of the 3D particle assemblies. The powerful method is demonstrated for the class of propagation invariant beams, where we introduce the use of Mathieu beams as light moulds with non-rotationally-symmetric structure. They offer multifarious field distributions and facilitate the creation of versatile particle structures. This eneral technique may find its application in micro fluidics, chemistry, biology, and medicine, to create highly efficient mixing tools, for hierarchical supramolecular organization or in 3D tissue engineering.


Sorting colloidal particles into multiple channels with optical forces: Prismatic optical fractionation

Ke Xiao and David G. Grier 

Brownian particles drifting through a periodically structured force landscape can become entrained by the landscape’s symmetries. What direction a particular particle takes can depend strongly on subtle variations in its physical properties. Consequently, a homogeneously structured force field can sort a mixture of particles into spatially separated fractions, much as an optical prism refracts light into its component wavelengths. When the force landscape is implemented with structured light fields, such continuous multichannel sorting may be termed prismatic optical fractionation. We describe experimental and numerical studies of colloidal spheres’ transport through periodic arrays of optical tweezers, which reveal an important role for three-dimensional motion in determining a drifting particle’s fate. These studies also demonstrate sorting on the basis of statistically locked-in transport, in which Brownian fluctuations contribute to direction selection.


Tuesday, November 30, 2010

Optically trapped probes with nanometer-scale tips for femto-Newton force measurement

M R Pollard, S W Botchway, B Chichkov, E Freeman, R N J Halsall, D W K Jenkins, I Loader, A Ovsianikov, A W Parker, R Stevens, R Turchetta, A D Ward and M Towrie

We describe the development of a novel force probe, controlled by multiple optical traps, with a nanometer-scale tip that protrudes outside the direct laser radiation field. We have measured forces to an accuracy of 240 fN, which enables future experiments that probe photo-sensitive components (such as biological cells) and non-transparent objects. The probes were produced using two methods, electron beam lithography and two-photon polymerization, with the latter providing approximately twice as much trapping stiffness.


Crystallization in Unsaturated Glycine/D2O Solution Achieved by Irradiating a Focused Continuous Wave Near Infrared Laser

Thitiporn Rungsimanon, Ken-ichi Yuyama, Teruki Sugiyama, and Hiroshi Masuhara

The crystallization of glycine in unsaturated solution is made possible by laser trapping of its molecular clusters due to photon pressure of a focused continuous wave near-infrared laser beam. Always one single crystal is spatiotemporally formed at a focal spot, and then it undergoes dissolution, eventually leading to repetitive crystallization and dissolution. The polymorph characterization of the crystal formed in unsaturated solution confirmed the γ-form, which is not obtainable by conventional crystallization methods. The preparation probability of the γ-form compared to the α-form is much higher than that in the supersaturated solution.


All-Optical Patterning of Au Nanoparticles on Surfaces Using Optical Traps

Mason J. Guffey and Norbert F. Scherer

The fabrication of nanoscale devices would be greatly enhanced by “nanomanipulators” that can position single and few objects rapidly with nanometer precision and without mechanical damage. Here, we demonstrate the feasibility and precision of an optical laser tweezer, or optical trap, approach to place single gold (Au) nanoparticles on surfaces with high precision (approximately 100 nm standard deviation). The error in the deposition process is rather small but is determined to be larger than the thermal fluctuations of single nanoparticles within the optical trap. Furthermore, areas of tens of square micrometers could be patterned in a matter of minutes. Since the method does not rely on lithography, scanning probes or a specialized surface, it is versatile and compatible with a variety of systems. We discuss active feedback methods to improve positioning accuracy and the potential for multiplexing and automation.


Peptide Nucleic Acids as Tools for Single-Molecule Sequence Detection and Manipulation

Hagar Zohar, Craig L. Hetherington, Carlos Bustamante, and Susan J. Muller

The ability to strongly and sequence-specifically attach modifications such as fluorophores and haptens to individual double-stranded (ds) DNA molecules is critical to a variety of single-molecule experiments. We propose using modified peptide nucleic acids (PNAs) for this purpose and implement them in two model single-molecule experiments where individual DNA molecules are manipulated via microfluidic flow and optical tweezers, respectively. We demonstrate that PNAs are versatile and robust sequence-specific tethers.


Thursday, November 25, 2010

Mechanical force characterization in manipulating live cells with optical tweezers

Yanhua Wu, Dong Sun and Wenhao Huang

Laser trapping with optical tweezers is a noninvasive manipulation technique and has received increasing attentions in biological applications. Understanding forces exerted on live cells is essential to cell biomechanical characterizations. Traditional numerical or experimental force measurement assumes live cells as ideal objects, ignoring their complicated inner structures and rough membranes. In this paper, we propose a new experimental method to calibrate the trapping and drag forces acted on live cells. Binding a micro polystyrene sphere to a live cell and moving the mixture with optical tweezers, we can obtain the drag force on the cell by subtracting the drag force on the sphere from the total drag force on the mixture, under the condition of extremely low Reynolds number. The trapping force on the cell is then obtained from the drag force when the cell is in force equilibrium state. Experiments on numerous live cells demonstrate the effectiveness of the proposed force calibration approach.


A microfluidic device with integrated optics for microparticle switching

Siew-Kit Hoi, Zhi-Bin Hu, Yuanjun Yan, Chorng-Haur Sow, and Andrew A. Bettiol

We report a high efficiency and noninvasive microfluidic particle switching device with integrated optical microstructures. Microfluidic channels are combined with a cylindrical microlens and an optical fiber to achieve on-chip optical switching of colloidal particles without the need for an optical microscope. A laser beam is coupled into an optical fiber and redirected by the microlens. The angle of incidence of the optical force can be changed by varying the position of the optical fiber relative to the microlens. Under certain circumstances, a switching efficiency approaching 100% was achieved with a relatively fast response time for a solution containing 10 μm polystyrene spheres.


Tuesday, November 23, 2010

Optical tweezers for the micromanipulation of plant cytoplasm and organelles

Chris Hawes, Anne Osterrieder, Imogen A Sparkes and Tijs Ketelaar

Laser trapping of micron-sized particles can be achieved utilizing the radiation pressure generated by a focused infrared laser beam. Thus, it is theoretically possible to trap and manipulate organelles within the cytoplasm and remodel the architecture of the cytoplasm and membrane systems. Here we describe recent progress, using this under utilized technology, in the manipulation of cytoplasmic strands and organelles in plant cells.


Monday, November 22, 2010

Electrophoretic mobility of a growing cell studied by photonic force microscope

Mario Tonin, Stefan Bálint, Pau Mestres, Ignacio A. Martìnez, and Dmitri Petrov

Living cells have spatially localized charged groups such as nucleus, cell walls, and others that can move in an external electric field providing the cell electrophoretic mobility (EPM). We suggest to monitor the EPM of a single living cell during its growth using optical tweezers combined with a position detector. As an example, we studied the EPM during the yeast growth, and we observed a nonmonotonic behavior of the EPM during the cell cycle, such as that the maximal EPM was observed at the initial stage of the growth, strongly reducing when the cell cycle is near its final stage.


Experimental and theoretical determination of optical binding forces

O. Brzobohatý, T. Čižmár, V. Karásek, M. Šiler, K. Dholakia, and P. Zemánek

We present an experimental and theoretical study of long distance optical binding effects acting upon micro-particles placed in a standing wave optical field. In particular we present for the first time quantitatively the binding forces between individual particles for varying inter-particle separations, polarizations and incident angles of the binding beam. Our quantitative experimental data and numerical simulations show that these effects are essentially enhanced due to the presence of a reflective surface in a sample chamber. They also reveal conditions to form stable optically bound clusters of two and three particles in this geometry. We also show that the inter-particle separation in the formed clusters can be controlled by altering the angle of the beam incident upon the sample plane. This demonstrates new perspectives for the generation and control of optically bound soft matter and may be useful to understand various inter-particle effects in the presence of reflective surfaces.


All-silica fiber Bessel-like beam generator and its applications in longitudinal optical trapping and transport of multiple dielectric particles

Sung Rae Lee, Jongki Kim, Sejin Lee, Yongmin Jung, Jun Ki Kim, and K. Oh
A Bessel-like beam was generated in a novel all-fiber integrated structure. A concentric ring intensity pattern was achieved by the multimode interference along the coreless silica fiber, which was then focused by the integrated micro-lens to result in a Bessel-like beam. The average beam diameter of 7.5 μm maintained over 500 μm axial length for a continuous wave Yb-doped fiber laser input oscillating at the wavelength of 1.08 μm. The generated beam was successfully applied to two-dimension optical trapping and longitudinal transport of multiple dielectric particles confirming its unique non-diffracting and self-reconstructing nature. Physical principle of operation, fabrication, and experimental results are discussed.


Parallel Analysis of Individual Biological Cells Using Multifocal Laser Tweezers Raman Spectroscopy

Rui Liu, Douglas S. Taylor, Dennis L. Matthews, and James W. Chan

We report on the development and characterization of a multifocal laser tweezers Raman spectroscopy (M-LTRS) technique for parallel Raman spectral acquisition of individual biological cells. Using a 785-nm diode laser and a time-sharing laser trapping scheme, multiple laser foci are generated to optically trap single polystyrene beads and suspension cells in a linear pattern. Raman signals from the trapped objects are simultaneously projected through the slit of a spectrometer and spatially resolved on a charge-coupled device (CCD) detector with minimal signal crosstalk between neighboring cells. By improving the rate of single-cell analysis, M-LTRS is expected to be a valuable method for studying single-cell dynamics of cell populations and for the development of high-throughput Raman based cytometers.


Friday, November 19, 2010

Optical manipulation: Trapping ions

Timo A. Nieminen
The unexpected demonstration of all-optical trapping of ions offers new possibilities in the simulation of quantum spin systems, ultracold chemistry with ions and more.


Thursday, November 18, 2010

Parameter exploration of optically trapped liquid aerosols

D. R. Burnham, P. J. Reece, and D. McGloin

When studying the motion of optically trapped particles on the microsecond time scale, in low-viscosity media such as air, inertia cannot be neglected. Resolution of unusual and interesting behavior not seen in colloidal trapping experiments is possible. In an attempt to explain the phenomena we use power-spectral methods to perform a parameter study of the Brownian motion of optically trapped liquid aerosol droplets concentrated around the critically damped regime. We present evidence that the system is suitably described by a simple harmonic oscillator model which must include a description of Faxén’s correction, but not necessarily frequency dependent hydrodynamic corrections to Stokes’ law. We also provide results describing how the system behaves under several variables and discuss the difficulty in decoupling the parameters responsible for the observed behavior. We show that due to the relatively low dynamic viscosity and high trap stiffness, it is easy to transfer between over- and underdamped motion by experimentally altering either trap stiffness or damping. Our results suggest stable aerosol trapping may be achieved in underdamped conditions, but the onset of deleterious optical forces at high trapping powers prevents the probing of the upper stability limits due to Brownian motion.


Optical trapping of dielectric nanoparticles in resonant cavities

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

We theoretically investigate the opto-mechanical interactions between a dielectric nanoparticle and the resonantly enhanced optical field inside a high Q, small-mode-volume optical cavity. We develop an analytical method based on open system analysis to account for the resonant perturbation due to particle introduction and predict trapping potential in good agreement with three-dimensional (3D) finite-difference time-domain (FDTD) numerical simulations. Strong size-dependent trapping dynamics distinctly different from free-space optical tweezers arise as a consequence of the finite cavity perturbation. We illustrate single nanoparticle trapping from an ensemble of monodispersed particles based on size-dependent trapping dynamics. We further discover that the failure of the conventional dipole approximation in the case of resonant cavity trapping originates from a new perturbation interaction mechanism between trapped particles and spatially localized photons.


TimeSeriesStreaming.vi: LabVIEW program for reliable data streaming of large analog time series

Czerwinski, F., Oddershede, L.B. 

With modern data acquisition devices that work fast and very precise, scientists often face the task of dealing with huge amounts of data. These need to be rapidly processed and stored onto a hard disk. We present a LabVIEW program which reliably streams analog time series of MHz sampling. Its run time has virtually no limitation. We explicitly show how to use the program to extract time series from two experiments: For a photodiode detection system that tracks the position of an optically trapped particle and for a measurement of ionic current through a glass capillary. The program is easy to use and versatile as the input can be any type of analog signal. Also, the data streaming software is simple, highly reliable, and can be easily customized to include, e.g., real-time power spectral analysis and Allan variance noise quantification. Program summary: Program title: TimeSeriesStreaming.VI. Catalogue identifier: AEHT_v1_0. Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEHT_v1_0.html. Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland. Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html. No. of lines in distributed program, including test data, etc.: 250. No. of bytes in distributed program, including test data, etc.: 63 259. Distribution format: tar.gz. Programming language: LabVIEW (http://www.ni.com/labview/). Computer: Any machine running LabVIEW 8.6 or higher. Operating system: Windows XP and Windows 7. RAM: 60-360 Mbyte. Classification: 3. Nature of problem: For numerous scientific and engineering applications, it is highly desirable to have an efficient, reliable, and flexible program to perform data streaming of time series sampled with high frequencies and possibly for long time intervals. This type of data acquisition often produces very large amounts of data not easily streamed onto a computer hard disk using standard methods. Solution method: This LabVIEW program is developed to directly stream any kind of time series onto a hard disk. Due to optimized timing and usage of computational resources, such as multicores and protocols for memory usage, this program provides extremely reliable data acquisition. In particular, the program is optimized to deal with large amounts of data, e.g., taken with high sampling frequencies and over long time intervals. The program can be easily customized for time series analyses. Restrictions: Only tested in Windows-operating LabVIEW environments, must use TDMS format, acquisition cards must be LabVIEW compatible, driver DAQmx installed. Running time: As desirable: microseconds to hours.


Tuesday, November 16, 2010

An SLM-based Shack–Hartmann wavefront sensor for aberration correction in optical tweezers

Richard W Bowman, Amanda J Wright and Miles J Padgett
Holographic optical tweezers allow the creation of multiple optical traps in 3D configurations through the use of dynamic diffractive optical elements called spatial light modulators (SLMs). We show that, in addition to controlling traps, the SLM in a holographic tweezers system can be both the principal element of a wavefront sensor and the corrective element in a closed-loop adaptive optics system. This means that aberrations in such systems can be estimated and corrected without altering the experimental setup. Aberrations are estimated using the Shack–Hartmann method, where an array of spots is projected into the sample plane and the distortion of this array is used to recover the aberration. The system can recover aberrations of up to ten wavelengths peak–peak, and is sensitive to aberrations much smaller than a wavelength. The spot pattern could also be analysed by eye, as a tool for aligning the system.


Monday, November 15, 2010

Forces within Single Pairs of Charged Colloids in Aqueous Solutions of Ionic Liquids as Studied by Optical Tweezers

Mahdy M. Elmahdy, Christof Gutsche, and Friedrich Kreme

Forces of interaction within single pairs of negatively charged microsized blank colloids in aqueous solutions of water miscible room temperature ionic liquids (RTILs) have been measured at varying concentrations and pH by using optical tweezers (OT). Three different water miscible RTILs (1-butyl-3-methylimidazolium tetrafluoroborate [BMIM-BF4], 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM-TfO], and 1-butyl-3-methylimidazolium chloride [BMIM-Cl]) having the same organic cation [BMIM]+ and different inorganic anions ([BF4]−, [TfO]−, and Cl−) are used and compared with the high temperature molten salt (KCl). The experimental data are well described by a size-corrected screened Coulomb interaction approach which originates from the linearized Poisson−Boltzmann (PB) equation. The effective surface charge density σ derived from the fitted force-separation data is found to be concentration and pH dependent.


On the concept of “tractor beams”

S. Sukhov and A. Dogariu

We demonstrate the existence of a class of optical beams where the nonconservative forces can be locally oriented in a direction opposite to the propagation wave vector. Objects placed in the vicinity of these locations will move toward the source of light. The behavior of these negative forces is discussed for the particular case of nondiffracting rotating scale-invariant vector electromagnetic waves.


ion of a Gas Mixture by Means of Optical Trapping of the Gas

A. A. Shevyrin and M. S. Ivanov

Contributions of barodiffusion and thermodiffusion to separation of a methane–helium mixture are calculated with the use of a laser-induced interferential lattice with a non-resonant frequency. The process of separation is studied by the direct simulation Monte Carlo method of rarefied gas flows, which can be considered as a numerical method of the stochastic solution of the Boltzmann equation. An analysis of modeling results shows that barodiffusion arising under optical radiation owing to the influence of ponderomotive forces on the species of the gas mixture exerts a significant effect during separation of the gas mixture by means of optical trapping, in addition to the selective action of the lattice. The effect of thermodiffusion caused by heating of the mixture by the optical lattice is found to be significant only in peripheral areas of the lattice.


Friday, November 12, 2010

Laser speckle field as a multiple particle trap

V G Shvedov, A V Rode, Ya V Izdebskaya, D Leykam, A S Desyatnikov, W Krolikowski and Yu S Kivshar

We demonstrate that a speckle pattern in the spatially coherent laser field transmitted by a diffuser forms a multitude of three-dimensional intensity micro-pockets acting as particle traps for air-borne light absorbing particles. Confinement of up to a few thousand particles in air with a unidirectional single beam has been achieved. Theoretical analysis of the speckle defined trapping volume is in a good agreement with experimental results on capturing of aggregates of carbon nanoparticles in air.


Thursday, November 11, 2010

Optical Manipulation of Microparticles in an SU-8/PDMS Hybrid Microfluidic Chip Incorporating a Monolithically Integrated On-Chip Lens Set

Honglei Guo; Ping Zhao; Gaozhi Xiao; Zhiyi Zhang; Jianping Yao

An SU-8/PDMS microfluidic chip incorporating a monolithically integrated on-chip lens set for transport and manipulation of microparticles is developed. The components, including the on-chip lens set, the microfluidic channel, and the fiber grooves, are defined in a single layer of SU-8 by one-step photolithography. The design of the on-chip lens set and the fabrication of the microfluidic chip are fully described. The influence of the beam-waist radius on the manipulation performance is theoretically analyzed and experimentally verified for the first time. In the cross-type optofluidic architecture, the evaluation is performed by measuring the particle displacement with different beam-waist radii under different fluid-flow rates. The on-chip lens set is designed to have a specific dimension to achieve the required beam-waist radius. It is revealed that the particle displacement is counter-proportional to the beam-waist radius. An experiment is performed. The results show that the particle displacement is increased by reducing the beam-waist radius. The optical manipulation of microparticles is also demonstrated by using two counter-propagating light beams that are perpendicular to the fluid-flow direction with the beam-waist radius determined by two on-chip lens sets placed on the two sides of the microfluidic channel. The proposed architecture could be used to enhance the performance in particle transport, separation, and concentration.


Monday, November 8, 2010

Fundamentals of negative refractive index optical trapping: forces and radiation pressures exerted by focused Gaussian beams using the generalized Lorenz-Mie theory

Leonardo A. Ambrosio and Hugo E. Hernández-Figueroa

Based on the generalized Lorenz-Mie theory (GLMT), this paper reveals, for the first time in the literature, the principal characteristics of the optical forces and radiation pressure cross-sections exerted on homogeneous, linear, isotropic and spherical hypothetical negative refractive index (NRI) particles under the influence of focused Gaussian beams in the Mie regime. Starting with ray optics considerations, the analysis is then extended through calculating the Mie coefficients and the beam-shape coefficients for incident focused Gaussian beams. Results reveal new and interesting trapping properties which are not observed for commonly positive refractive index particles and, in this way, new potential applications in biomedical optics can be devised.


Heterogeneity of the electrostatic repulsion between colloids at the oil–water interface

Bum Jun Park, Jan Vermant and Eric M. Furst

The pairwise and multi-body interaction forces between polystyrene particles at an oil–water interface are measured. The electrostatic repulsive force has the expected dependence on particle separation for a dipole–dipole interaction, Frepr−4, but exhibits a distribution of magnitudes in which the force depends on the particle pairs tested and sample preparation method. A gamma distribution accurately models this variation in the repulsion between pairs of particles. Despite this heterogeneity, the multibody interactions measured in small ensembles are pairwise additive. Good agreement is found for the two-dimensional equilibrium suspension structure between experiments and Monte Carlo simulations when a heterogeneous interaction potential is implemented in the latter. The heterogeneity and long-range of the repulsive interaction accounts for the lower apparent pair interaction potential derived from the suspension radial distribution function at dilute, but finite, surface concentrations when compared to the direct pair interaction measurements made with laser tweezers at nearly infinite dilution.


Gradient forces on double-negative particles in optical tweezers using Bessel beams in the ray optics regime

Leonardo A. Ambrosio and Hugo E. Hernández-Figueroa

Gradient forces on double negative (DNG) spherical dielectric particles are theoretically evaluated for v-th Bessel beams supposing geometrical optics approximations based on momentum transfer. For the first time in the literature, comparisons between these forces for double positive (DPS) and DNG particles are reported. We conclude that, contrary to the conventional case of positive refractive index, the gradient forces acting on a DNG particle may not reverse sign when the relative refractive index n goes from |n| > 1 to |n| < 1, thus revealing new and interesting trapping properties.


Thursday, November 4, 2010

Geometry-induced electrostatic trapping of nanometric objects in a fluid

Madhavi Krishnan, Nassiredin Mojarad, Philipp Kukura & Vahid Sandoghdar

The ability to trap an object—whether a single atom or a macroscopic entity—affects fields as diverse as quantum optics, soft condensed-matter physics, biophysics and clinical medicine. Many sophisticated methodologies have been developed to counter the randomizing effect of Brownian motion in solution, but stable trapping of nanometre-sized objects remains challenging. Optical tweezers are widely used traps, but require sufficiently polarizable objects and thus are unable to manipulate small macromolecules. Confinement of single molecules has been achieved using electrokinetic feedback guided by tracking of a fluorescent label, but photophysical constraints limit the trap stiffness and lifetime. Here we show that a fluidic slit with appropriately tailored topography has a spatially modulated electrostatic potential that can trap and levitate charged objects in solution for up to several hours. We illustrate this principle with gold particles, polymer beads and lipid vesicles with diameters of tens of nanometres, which are all trapped without external intervention and independently of their mass and dielectric function. The stiffness and stability of our electrostatic trap is easily tuned by adjusting the system geometry and the ionic strength of the solution, and it lends itself to integration with other manipulation mechanisms. We anticipate that these features will allow its use for contact-free confinement of single proteins and macromolecules, and the sorting and fractionation of nanometre-sized objects or their assembly into high-density arrays.


Tuesday, November 2, 2010

Nanoscale ablation through optically trapped microspheres

Romain Fardel, Euan McLeod, Yu-Cheng Tsai and Craig B. Arnold

The ability to directly create patterns with size scales below 100 nm is important for many applications where the production or repair of high resolution and density features is needed. Laser-based direct-write methods have the benefit of being able to quickly and easily modify and create structures on existing devices, but ablation can negatively impact the overall technique. In this paper we show that self-positioning of near-field objectives through the optical trap assisted nanopatterning (OTAN) method allows for ablation without harming the objective elements. Small microbeads are positioned in close proximity to a substrate where ablation is initiated. Upon ablation, these beads are temporarily displaced from the trap but rapidly return to the initial position. We analyze the range of fluence values for which this process occurs and find that there exists a critical threshold beyond which the beads are permanently ejected.


Probing DNA with micro- and nanocapillaries and optical tweezers

L J Steinbock, O Otto, D R Skarstam, S Jahn, C Chimerel, J L Gornall and U F Keyser

We combine for the first time optical tweezer experiments with the resistive pulse technique based on capillaries. Quartz glass capillaries are pulled into a conical shape with tip diameters as small as 27 nm. Here, we discuss the translocation of λ-phage DNA which is driven by an electrophoretic force through the nanocapillary. The resulting change in ionic current indicates the folding state of single λ-phage DNA molecules. Our flow cell design allows for the straightforward incorporation of optical tweezers. We show that a DNA molecule attached to an optically trapped colloid is pulled into a capillary by electrophoretic forces. The detected electrophoretic force is in good agreement with measurements in solid-state nanopores.


Dynamic translocation of ligand-complexed DNA through solid-state nanopores with optical tweezers

Andy Sischka, Andre Spiering, Maryam Khaksar, Miriam Laxa, Janine König, Karl-Josef Dietz and Dario Anselmetti

We investigated the threading and controlled translocation of individual lambda-DNA (λ-DNA) molecules through solid-state nanopores with piconewton force sensitivity, millisecond time resolution and picoampere ionic current sensitivity with a set-up combining quantitative 3D optical tweezers (OT) with electrophysiology. With our virtually interference-free OT set-up the binding of RecA and single peroxiredoxin protein molecules to λ-DNA was quantitatively investigated during dynamic translocation experiments where effective forces and respective ionic currents of the threaded DNA molecule through the nanopore were measured during inward and outward sliding. Membrane voltage-dependent experiments of reversible single protein/DNA translocation scans yield hysteresis-free, asymmetric single-molecule fingerprints in the measured force and conductance signals that can be attributed to the interplay of optical trap and electrostatic nanopore potentials. These experiments allow an exact localization of the bound protein along the DNA strand and open fascinating applications for label-free detection of DNA-binding ligands, where structural and positional binding phenomena can be investigated at a single-molecule level.


Friday, October 29, 2010

Nonlinear optical effects in trapping nanoparticles with femtosecond pulses

Yuqiang Jiang, Tetsuya Narushima & Hiromi Okamoto

The optical trapping technique has been widely used in various areas to manipulate particles, cells, and so forth. The principle of trapping is based on the interaction between optical electric fields and induced linear polarizations. Here we show a novel phenomenon of trapping arising from nonlinear polarization when we trap gold nanoparticles by ultrashort near-infrared laser pulses. That is, the stable trap site is split into two equivalent positions (we call this ‘trap split’). The trap positions are aligned along the direction of the incident laser polarization. The dependencies of trap split on the trapping-laser power and wavelength were investigated. The results were successfully interpreted in terms of the nonlinear polarization caused by the femtosecond pulses. This method may give novel applications to micromachining, nanofabrication, and biological samples as well as atomic and molecular trapping at low temperatures.


Optical forces on arbitrary shaped particles in optical tweezers

Lin Ling, Fei Zhou, Lu Huang, and Zhi-Yuan Li

Discrete dipole approximation (DDA) method is an efficient method for computing electromagnetic (EM) field of nanometer/micrometer-sized dielectric particles with arbitrary geometric shape and topology. In this work we employ the DDA method to calculate the optical force of dielectric shaped particles embedded in optical tweezers made from focused Gaussian laser beams. The EM force is calculated based on the self-consistent solution of EM field distribution and discrete dipole moment distribution within the particles. The DDA method agrees well with the Mie theory for spherical dielectric particles and this supports the effectiveness of the DDA method in handling optical forces in optical tweezers. The optical force for shaped particles such as cubes, rectangles, cylinders, and core-shell composite particles shows many interesting features. The force strongly depends on the orientation of the particle with respect to the laser beam propagation and polarization direction and the aspect ratio of the anisotropic particle. For a core-shell composite particle the zero-force balance point shifts from the particle center to its two sides. When an additional particle comes close a trapped particle, the perturbation effect strongly depends on the relative location of the center of the focused laser beam with respect to the two particles. Furthermore, the geometry of shaped particles not only affects the magnitude of the optical force but also influences the optical trap stiffness.


DNA condensation by TmHU studied by optical tweezers, AFM and molecular dynamics simulations

Carolin Wagner, Carsten Olbrich, Hergen Brutzer, Mathias Salomo, Ulrich Kleinekathöfer, Ulrich F. Keyser and Friedrich Kremer

The compaction of DNA by the HU protein from Thermotoga maritima (TmHU) is analysed on a single-molecule level by the usage of an optical tweezers-assisted force clamp. The condensation reaction is investigated at forces between 2 and 40 pN applied to the ends of the DNA as well as in dependence on the TmHU concentration. At 2 and 5 pN, the DNA compaction down to 30% of the initial end-to-end distance takes place in two regimes. Increasing the force changes the progression of the reaction until almost nothing is observed at 40 pN. Based on the results of steered molecular dynamics simulations, the first regime of the length reduction is assigned to a primary level of DNA compaction by TmHU. The second one is supposed to correspond to the formation of higher levels of structural organisation. These findings are supported by results obtained by atomic force microscopy.


Wednesday, October 27, 2010

Focusing properties of Gaussian beam with mixed screw and conical phase fronts

Jinsong Li, Xiumin Gao, Shuqin Zhang and Songlin Zhuang

Vector diffraction theory is employed to investigate the focusing properties of the Gaussian beams with mixed screw and conical phase fronts. Numerical simulations show that the Gaussian beams with screw–conical phase fronts are different from both the ordinary Laguerre–Gaussian beams and the higher-order Bessel beams. Rather than forming the ring-shaped intensity distributions characteristic of optical vortices, focusing the Gaussian beams with screw–conical phase fronts produce non-symmetric spiral intensity distributions at the focal plane. The intensity distribution forms a counter-clockwise non-symmetric screw path around the focus. The rotation of intensity distributions was observed in the focal plane. The gradient force patterns of these beams focused with high NA are also investigated. The results show that the gradient force pattern shape depends principally on parameter topological charge n of the phase distribution. The gradient force pattern expands with increase in the parameter m of the phase distribution. Therefore, one can change the topological charge n or the parameter m of the phase mask to construct the tunable optical trap to meet different requirements. Its potential application might include rotational positioning of particles and accumulation of smaller non-symmetric particles towards the focus.


Research on the propagation properties of hollow laser beams with three-dimensional trap optical distribution

Yuan Dong, Xi-He Zhang, Feng-dong Zhang, Guo-Bin Ning, Guang-yong Jin, Wei Liang, Yan-Fei Lü and Kai Zhang

A new kind of hollow beams – hollow laser beams with three-dimension trap optical distribution – was put forward. With the help of the Collins formula in paraxial optical system, the analytical equation of propagation and transformation of the hollow laser beams was deduced. According to the analytical equation, the propagation properties of the kind of hollow beams that transform in free space were simulated. In the experiment, we obtained the hollow laser beams by means of the combinational optical system of reflecting positive-axis and negative-axis pyramids. The intensity of the vertical loop in different distances was tested, which shows that the analytical equation of propagation and transformation is in agreement with the result.


Concentric piecewise azimuthally polarized beam by a pure phase plate

Xiumin Gao, Jinsong Li, Jian Wang and Songlin Zhuang

Focusing properties of the azimuthally polarized beam induced by a pure phase plate are investigated theoretically. The pure phase plate consists of two concentric portions, one center circle portion and one outer annular portion, through which the azimuthally polarized beam passed evolves into concentric piecewise azimuthally polarized beam. When the phase shift of the center portion is π, one ring focus may evolve into novel focal patterns with increasing radius of the center circle portion, such as cylindrical crust focus, two-ring focus, and three-ring focus. And if the geometrical parameters are unchanged, focal patterns also changes considerably with tunable phase of the center portion. Ring focus shifts along the optical axis on the increasing phase. Some optical gradient force distributions and dependence of focal shift on phase shift are also illustrated. This kind of concentric piecewise azimuthally polarized beam can be used in optical manipulation technology.


Near-field-induced optical force on a metal particle and C60: Real-time and real-space electron dynamics simulation

Takeshi Iwasa and Katsuyuki Nobusada

Optical forces induced by a near field are calculated for a 1-mm-sized metal particle mimicked by a jellium model and for C60 in the framework of real-time and real-space time-dependent density-functional theory combined with a nonuniform light-matter interaction formalism, fully taking account of multipole interaction. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in C60. We also found that the optical force depends linearly on the intensity of the near field.


Tuesday, October 26, 2010

Rapid and Direct Cell-to-Cell Adherence Using Avidin-Biotin Binding System: Large Aggregate Formation in Suspension Culture and Small Tissue Element Formation Having a Precise Microstructure Using Optical Tweezers

Nobuhiko Kojima, Ken Miura, Tomoki Matsuo, Hidenari Nakayama, Kikuo Komori, Shoji Takeuchi, and Yasuyuki Sakai

Effectively organizing isolated cells to tissue elements having an appropriate microstructure is a fundamental issue in future tissue engineering, but biological cell-to-cell adhesion is too weak to assemble single cells directly. In order to overcome the difficulty, we applied an Avidin-Biotin Binding System (ABBS) to cell surfaces, and avidinylated and biotinylated cells could mutually bind in the short time they were mixed together. Unlike conventional intact cells, ABBS helped make larger spheroids. Interestingly, avidinylated and biotinylated cell adherence occurred within 1 sec using laser trapping, enabling single cell manipulation. We showed precise, direct single-cell-based tissue assembly using ABBS and optical tweezers, followed by damage-free tissue culture. The combination of ABBS and single cell manipulation has considerable potential for use in application such as tissue engineering, regenerative medicine, and drug screening system.


Polymer adsorption onto a micro-sphere from optical tweezers electrophoresis

Jan A. van Heiningen and Reghan J. Hill
We explore the design and operation of an optical-tweezers electrophoresis apparatus to resolve polymer adsorption dynamics onto a single micro-sphere in a micro-fluidic environment. Our model system represents a broader class of micro-fluidic electrophoresis experiments for biosensing and fundamental colloid and surface science diagnostics. We track the adsorption of 100 kDa poly(ethylene oxide) homopolymer onto a colloidal silica sphere that is optically trapped in a crossed parallel-plate micro-channel. The adsorption dynamics are probed on the similarμm particle length scale with similar1 s temporal resolution. Because the particle electrophoretic mobility and channel electro-osmotic flow are exquisitely sensitive to the polymer layer hydrodynamic thickness, particle dynamics can be complicated by polymer adsorption onto the micro-channel walls. Nevertheless, using experiments and a theoretical model of electro-osmotic flow in channels with non-uniform wall ζ-potentials, we show that such influences can be mitigated by adopting a symmetrical flow configuration. The equilibrium hydrodynamic layer thickness of 100 kDa poly(ethylene oxide) on colloidal silica is similar10 nm at polymer concentrations gtrsim R: greater, similar10 ppm (weight percent), with the dynamics reflecting polymer solution concentration, flow rate, and polydispersity.


Measuring micro-interactions between coagulating red blood cells using optical tweezers

Bor-Wen Yang and Zhe Li

Agents that alter the dynamics of hemostasis form an important part in management of conditions such as atherosclerosis, cerebrovascular disease, and bleeding diatheses. In this study, we explored the effects of heparin and tranexamic acid on the efficiency of blood coagulation. Using optical tweezers, we evaluated the pN-range micro-interaction between coagulating red blood cells (RBCs) by measuring the minimum power required to trap them. By observing the mobility of RBCs and the intensity of cellular interactions, we found that the coagulation process can be separated into three phases. The effects of heparin and tranexamic acid were examined by observing variations in cellular interaction during the coagulation phases. Heparin attenuated the interaction between RBCs and prolonged the first phase whereas the samples containing tranexamic acid bypassed the first two phases and immediately proceeded to the final one.


Tuesday, October 12, 2010

Real-time particle tracking at 10,000 fps using optical fiber illumination

Oliver Otto, Fabian Czerwinski, Joanne L. Gornall, Gunter Stober, Lene B. Oddershede, Ralf Seidel, and Ulrich F. Keyser

We introduce optical fiber illumination for real-time tracking of optically trapped micrometer-sized particles with microsecond time resolution. Our light source is a high-radiance mercury arc lamp and a 600μm optical fiber for short-distance illumination of the sample cell. Particle tracking is carried out with a software implemented cross-correlation algorithm following image acquisition from a CMOS camera. Our image data reveals that fiber illumination results in a signal-to-noise ratio usually one order of magnitude higher compared to standard Köhler illumination. We demonstrate position determination of a single optically trapped colloid with up to 10,000 frames per second over hours. We calibrate our optical tweezers and compare the results with quadrant photo diode measurements. Finally, we determine the positional accuracy of our setup to 2 nm by calculating the Allan variance. Our results show that neither illumination nor software algorithms limit the speed of real-time particle tracking with CMOS technology.


Detection of doxorubicin-induced apoptosis of leukemic T-lymphocytes by laser tweezers Raman spectroscopy

Tobias J. Moritz, Douglas S. Taylor, Denise M. Krol, John Fritch, and James W. Chan

Laser tweezers Raman spectroscopy (LTRS) was used to acquire the Raman spectra of leukemic T lymphocytes exposed to the chemotherapy drug doxorubicin at different time points over 72 hours. Changes observed in the Raman spectra were dependent on drug exposure time and concentration. The sequence of spectral changes includes an intensity increase in lipid Raman peaks, followed by an intensity increase in DNA Raman peaks, and finally changes in DNA and protein (phenylalanine) Raman vibrations. These Raman signatures are consistent with vesicle formation, cell membrane blebbing, chromatin condensation, and the cytoplasm of dead cells during the different stages of drug-induced apoptosis. These results suggest the potential of LTRS as a real-time single cell tool for monitoring apoptosis, evaluating the efficacy of chemotherapeutic treatments, or pharmaceutical testing.


Sunday, October 10, 2010

Negative radiation pressure on gain medium structures

Amit Mizrahi and Yeshaiahu Fainman

We demonstrate negative radiation pressure on gain medium structures, such that light amplification may cause a nanoscale body to be pulled toward a light source. Optically large gain medium structures, such as slabs and spheres, as well as deep subwavelength bodies, may experience this phenomenon. The threshold gain for radiation pressure reversal is obtained analytically for Rayleigh spheres, thin cylinders, and thin slabs. This threshold vanishes when the gain medium structure is surrounded by a medium with a matched refractive index, thus eliminating the positive scattering forces.


Friday, October 8, 2010

Massive Parallel Assembly of Microbeads for Fabrication of Microtools Having Spherical Structure and Powerful Manipulation by Optical Tweezers

Hisataka Maruyama, Ryo Iitsuka, Kazuhisa Onda, and Fumihito Arai

Production of functional microtools having an arbitrary shape by self-assembly of microparticles and heat treatment above the glass transition temperature of the microparticles was developed. Polystyrene microbeads were used as a material of the microtool. A solution including microparticles was dispersed onto the silicon substrate having microtool patterns fabricated by photolithography and etching. Dispersed particles were introduced to the pattern by gravity force. Microparticles in the pattern aggregate autonomously by surface tension through evaporation of the solution. Aggregated microparticles were fused by heating above the glass transition temperature (100°C). Fused microparticles were detached from the pattern by ultrasonic treatment and used as microtools. Produced microtool has spherical part since the microtool is made of microparticles. Spherical part is suitable for trapping point of optical tweezers. We demonstrated production of microtools using self-assembly and manipulation of the fabricated microtool on a chip.


Determinants of Plasma Membrane Wounding by Deforming Stress

Richard A Oeckler, Won-Yeon Lee, Mun-Gi Park, Othmar Kofler, Deborah L. Rasmussen, Heung-Bum Lee, Hewan Belete, Bruce J. Walters, Randolph W. Stroetz, and Rolf Dieter Hubmayr

Once excess liquid gains access to airspaces of an injured lung, the act of breathing creates and destroys foam and thereby contributes to the wounding of epithelial cells by interfacial stress. Since cells are not elastic continua, but rather complex network structures composed of solid as well as liquid elements, we hypothesize that plasma membrane (PM) wounding is preceded by a phase separation, which results in blebbing. We postulate that inteventions, such as a hypertonic treatment, increase adhesive PM/cytoskeletal (CSK) interactions, thereby preventing blebbing as well as PM wounds. We formed PM tethers in alveolar epithelial cells andfibroblasts and measured their retractive force as read-out of PM/CSK adhesive interactions using optical tweezers. A 50mOsm increase in media tonicity consistently increased the tether retractive force in epithelial cells, but lowered it in fibroblasts. The osmo-response was abolished by pretreatment with Latrunculin, Cytochalasin D and calcium chelation. Epithelial cells and fibroblasts were exposed to interfacial stress in a microchannel and the fraction of wounded cells measured. Interventions which increased PM/CSK adhesive interactions prevented blebbing and were cytoprotective regardless of cell type. Finally, we exposed ex-vivo perfused rat lungs to injurious mechanical ventilation and showed that hypertonic conditioning reduced the number of wounded subpleuralalveolus resident cells to baseline levels. Our observations support the hypothesis that PM/CSK adhesive interactions are important determinants of the cells response to deforming stress and pave the way for preclinical efficacy trials of hypertonic treatment in experimental models of acute lung injury.


Dynamic multiple-beam counter-propagating optical traps using optical phase-conjugation

Mike Woerdemann, Konrad Berghoff, and Cornelia Denz

Counter-propagating optical traps are widely used where long working distances, axially symmetric trapping potentials, or standing light waves are required. We demonstrate that optical phase-conjugation can automatically provide a counter-propagating replica of a wide range of incident light fields in an optical trapping configuration. The resulting counter-propagating traps are self-adjusting and adapt dynamically to changes of the input light field. It is shown that not only single counter-propagating traps can be implemented by phase-conjugation, but also structured light fields can be used. This step towards more complex traps enables advanced state-of-the-art applications where multiple traps or other elaborated trapping scenarios are required. The resulting traps cannot only be used statically, but they can be rearranged in real-time and allow for interactive dynamic manipulation.


Thursday, October 7, 2010

Multiple-trap laser tweezers Raman spectroscopy for simultaneous monitoring of the biological dynamics of multiple individual cells

Pengfei Zhang, Lingbo Kong, Peter Setlow, and Yong-qing Li

We report the development of a multiple-trap laser tweezers Raman spectroscopy (LTRS) array for simultaneously acquiring Raman spectra of individual cells in physiological environments. This LTRS-array technique was also combined with phase contrast and fluorescence microscopy, allowing measurement of Raman spectra, refractility, and fluorescence images of individual cells with a temporal resolution of ~5 s. As a demonstration, we used this technique to monitor multiple Bacillus cereus spores germinating in a nutrient medium for up to 90min and observed the kinetics of dipicolinic acid release and uptake of nucleic acid-binding stain molecules during spore germination.


A Prototype Optical Tweezer System Employing Adaptive Optics Technology

R. Nash; S. Bowman; C. Bradley; R. Conan

This article describes the design, implementation and characterization of a novel optical tweezer system. The system utilizes a deformable mirror, wavefront sensor and controller to manipulate an optically trapped micro-particle within a small chamber. This method for optical trapping employs technology adopted from astronomical instrumentation; in particular, adaptive optics. A deformable mirror is employed to control the wavefront phase of a laser beam before it is imaged into a chamber by a high numerical aperture microscope objective lens. The wavefront phase is measured by a Shack-Hartman wavefront sensor and the particle's position monitored by a video camera. The goals of the work presented here are to trap particles ranging in size from 1 μm to 10 μm; create a suitable controller for moving trapped particles in three dimensions; image the trapped particle; determine the prototype system's performance specifications; and determine the trap stiffness.


First-order nonconservative motion of optically trapped nonspherical particles

Stephen H. Simpson and Simon Hanna

It is well known that optical force fields are not conservative. This has important consequences for the thermal motion of optically trapped dielectric spheres. In particular, the spheres do not reach thermodynamic equilibrium. Instead, a steady state is achieved in which the stochastic trajectory contains an underlying deterministic bias toward cyclic motion, and the energy of the sphere deviates from that implied by the equipartition theorem. Such effects are second order and only observed at low trap powers when the sphere is able to explore regions of the trap beyond the linear regime. Analogous effects may be expected for particles of less than spherical symmetry. However, in this case the effects are first order and depend on the linear term in the optical force field. As such they are not suppressed by increases in beam power, although the frequency and amplitude of the cyclic motion will be affected by it. In this paper, we present an analysis of the first-order nonconservative behavior of nonspherical particles in optical traps. The analysis is supported by optical force calculations and Brownian dynamics simulations of dielectric microrods held vertically in Gaussian optical traps.


Hemoglobin degradation in human erythrocytes with long-duration near-infrared laser exposure in Raman optical tweezers

Raktim Dasgupta, Sunita Ahlawat, Ravi Shanker Verma, Abha Uppal, and Pradeep Kumar Gupta

Near-infrared laser (785-nm)-excited Raman spectra from a red blood cell, optically trapped using the same laser beam, show significant changes as a function of trapping duration even at trapping power level of a few milliwatts. These changes in the Raman spectra and the bright-field images of the trapped cell, which show a gradual accumulation of the cell mass at the trap focus, suggest photoinduced aggregation of intracellular heme. The possible role of photoinduced protein denaturation and hemichrome formation in the observed aggregation of heme is discussed.


Modulation of the mechano-chemical properties of myosin V by drebrin-E

Hiroaki Kubota, Ryoki Ishikawa, Takashi Ohki, Junji Ishizuka, Sergey V. Mikhailenko and Shin’ichi Ishiwata

The regulation of actin filament networks by various proteins has essential roles in the growth cone dynamics. In this study we focused on the actin–myosininteraction which has been suggested to be an important player in the neurite extension. We examined in vitro how the decoration of actin filaments with a side-binding protein, drebrin-E, affects the motile properties of an intracellular transporter myosin V. Single myosin V molecules landed on the drebrin-E-decorated actin filaments with a lower frequency and ran over shorter distances; however, their velocities were normal. Furthermore, the analysis of the movement of myosin V molecules in the optical trap revealed that the decoration of actin filaments with drebrin-E markedly increased the load-sensitivity of the myosin V stepping. These results are attributable to the delay in the attachment of the motor’s leading head (ADP·Pi state) to actin, induced by the competitive binding of drebrin-E to actin, whereas the rate of ADP release from the trailing head (the rate-limiting step in the ATPase cycle of myosin V) is unaffected. Our study indicates that, in addition to the regulation of binding affinity of myosin V, drebrin-E also modulates the chemo-mechanical coupling in the motile myosin V molecules, presumably affecting the movement of the growth cone.


Detection of Calcium-Induced Morphological Changes of Living Cells Using Optical Traps

Moradi, A. R.; Ali, M. K.; Daneshpanah, M.; Anand, A.; Javidi, B.

In this paper, we investigate an optical-trap-based method for the detection of structural changes of the red blood cell (RBC) membrane affected by$hbox{Ca}^{2+}$ ions. Individual cells are immobilized by the use of optical tweezers and are monitored live, while the concentration of $hbox{Ca}^{2+}$ions in the buffer is changed simultaneously. $hbox{Ca}^{2+}$ ions are known to affect the cells' membrane morphology. These changes are attributed to the formation of calcium-induced hydrophobic aggregates of phospholipid molecules in the RBC membrane, resulting in a net change in membrane rigidity. Membrane deformation results in the change of effective radius and the drag coefficient of the cell, both of which affect the Brownian motion of the cell in solution. This motion is indirectly measurable by monitoring the forward scattering light and its dependence on the size and drag coefficient of the cell. We show the relationship between the $hbox{Ca}^{2+}$ ion concentration and the optical trap specifications. The results are in agreement with previous biological studies and the phase contrast observations of living RBCs under investigation.