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.