Wednesday, October 31, 2012

Measuring Nanoscale Forces with Living Probes

S. N. Olof, J. A. Grieve, D. B. Phillips, H. Rosenkranz, M. L. Yallop, M. J. Miles, A. J. Patil, S. Mann, and D. M. Carberry

Optical trapping techniques have been used to investigate fundamental biological processes ranging from the identification of the processive mechanisms of kinesin and myosin to understanding the mechanics of DNA. To date, these investigations have relied almost exclusively on the use of isotropic probes based on colloidal microspheres. However, there are many potential advantages in utilizing more complex probe morphologies: use of multiple trapping points enables control of the interaction volume; increasing the distance between the optical trap and the sample minimizes photodamage in sensitive biological materials; and geometric anisotropy introduces the potential for asymmetric surface chemistry and multifunctional probes. Here we demonstrate that living cells of the freshwater diatomNitzschia subacicularis Hustedt can be exploited as advanced probes for holographic optical tweezing applications. We characterize the optical and material properties associated with the high shape anisotropy of the silica frustule, examine the trapping behavior of the living algal cells, and demonstrate how the diatoms can be calibrated for use as force sensors and as force probes in the presence of rat B-cell hybridoma (11B11) cells.

Optical forces on Mie particles in an Airy evanescent field

Yang Yang, Wei-Ping Zang, Zhi-Yu Zhao, and Jian-Guo Tian
Using vector potential and spectrum representation, we derive the expressions of the Airy evanescent field existed at the interface. Utilizing these expressions and the Arbitrary Beam Theory, the optical forces exerted on a Mie dielectric particle in the Airy evanescent field were theoretically investigated in detail. Numerical results show that the optical forces exhibit strong oscillations which are corresponding to the distributions of the evanescent field. With the increasing the size of particle radius, Morphology Dependent Resonance occurs for the particle with specific refractive index.

Optical sorting in holographic trap arrays by tuning the inter-trap separation

S Ahlawat, R Dasgupta, R S Verma, V N Kumar and P K Gupta
Particle motion through a holographic trap array has been investigated theoretically and experimentally, and it is shown that a change in inter-trap separation can be used to selectively control the motion of particles of different sizes. By an appropriate choice of inter-trap separation in a holographically generated two-dimensional trap array, optical potential channels can be created in orthogonal directions such that, from a suspension having a mixture of two different particle sizes, the particles can be sorted in the two orthogonal channels. The use of the approach to sort 3 and 5 μm silica spherical particles in the two orthogonal channels, from a mixed suspension of these, has also been demonstrated.

Tuesday, October 30, 2012

Direct imaging of RecA nucleation and growth on single molecules of SSB-coated ssDNA

Jason C. Bell, Jody L. Plank, Christopher C. Dombrowski & Stephen C. Kowalczykowski

Escherichia coli RecA is the defining member of a ubiquitous class of DNA strand-exchange proteins that are essential for homologous recombination, a pathway that maintains genomic integrity by repairing broken DNA. To function, filaments of RecA must nucleate and grow on single-stranded DNA (ssDNA) in direct competition with ssDNA-binding protein (SSB), which rapidly binds and continuously sequesters ssDNA, kinetically blocking RecA assembly. This dynamic self-assembly on a DNA lattice, in competition with another protein, is unique for the RecA family compared to other filament-forming proteins such as actin and tubulin. The complexity of this process has hindered our understanding of RecA filament assembly because ensemble measurements cannot reliably distinguish between the nucleation and growth phases, despite extensive and diverse attempts. Previous single-molecule assays have measured the nucleation and growth of RecA—and its eukaryotic homologue RAD51—on naked double-stranded DNA and ssDNA; however, the template for RecA self-assembly in vivo is SSB-coated ssDNA. Using single-molecule microscopy, here we directly visualize RecA filament assembly on single molecules of SSB-coated ssDNA, simultaneously measuring nucleation and growth. We establish that a dimer of RecA is required for nucleation, followed by growth of the filament through monomer addition, consistent with the finding that nucleation, but not growth, is modulated by nucleotide and magnesium ion cofactors. Filament growth is bidirectional, albeit faster in the 5′3′ direction. Both nucleation and growth are repressed at physiological conditions, highlighting the essential role of recombination mediators in potentiating assembly in vivo. We define a two-step kinetic mechanism in which RecA nucleates on transiently exposed ssDNA during SSB sliding and/or partial dissociation (DNA unwrapping) and then the RecA filament grows. We further demonstrate that the recombination mediator protein pair, RecOR (RecO and RecR), accelerates both RecA nucleation and filament growth, and that the introduction of RecF further stimulates RecA nucleation.


Laser Trapping Chemistry: From Polymer Assembly to Amino Acid Crystallization

Teruki Sugiyama, Ken-ichi Yuyama, and Hiroshi Masuhara

Laser trapping has served as a useful tool in physics and biology, but, before our work, chemists had not paid much attention to this technique because molecules are too small to be trapped in solution at room temperature. In late 1980s, we demonstrated laser trapping of micrometer-sized particles, developed various methodologies for their manipulation, ablation, and patterning in solution, and elucidated their dynamics and mechanism. In the 1990s, we started laser trapping studies on polymers, micelles, dendrimers, and gold, as well as polymer nanoparticles. Many groups also reported laser trapping studies of nanoclusters, DNA, colloidal suspensions, etc. Following these research streams, we have explored new molecular phenomena induced by laser trapping. Gradient force leading to trapping, mass transfer by local heating, and molecular reorientation following laser polarization are intimately coupled with molecular cluster and aggregate formation due to their intermolecular interactions, which depend on whether the trapping position is at the interface/surface or in solution.
In this Account, we summarize our systematic studies on laser trapping chemistry and present some new advances and our future perspectives. We describe the laser trapping of nanoparticles, polymers, and amino acid clusters in solution by focusing a continuous wave 1064 nm laser beam on the molecules of interest and consider their dynamics and mechanism. In dilute solution, nanoparticles with weak mutual interactions are individually trapped at the focal point, while laser trapping of nanoparticles in concentrated solution assembles and confines numerous particles at the focal spot. The assembly of polymers during their laser trapping extends out from the focal point because of the interpolymer interactions, heat transfer, and solvent flow. When the trapping laser is focused at an interface between a thin heavy water solution film of glycine and a glass substrate, the assembled molecules nucleate and evolve to a liquid–liquid phase separation, or they will crystallize if the trapping laser is focused on the solution surface. Laser trapping can induce spatiotemporally the liquid and solid nucleation of glycine, and the dense liquid droplet or crystal formed can grow to a bulk scale. We can control the polymorph of the formed glycine crystal selectively by tuning trapping laser polarization and power. These results provide a new approach to elucidate dynamics and mechanism of crystallization and are the fundamental basis for studying not only enantioselective crystallization but also confined polymerization, trapping dynamics by ultrashort laser pulses, and resonance effect in laser trapping.


Force measurements on cargoes in living cells reveal collective dynamics of microtubule motors

Adam G. Hendricks, Erika L. F. Holzbaur, and Yale E. Goldman

Many cellular cargoes move bidirectionally along microtubules, driven by teams of plus- and minus-end–directed motor proteins. To probe the forces exerted on cargoes during intracellular transport, we examined latex beads phagocytosed into living mammalian macrophages. These latex bead compartments (LBCs) are encased in membrane and transported along the cytoskeleton by a complement of endogenous kinesin-1, kinesin-2, and dynein motors. The size and refractive index of LBCs makes them well-suited for manipulation with an optical trap. We developed methods that provide in situ calibration of the optical trap in the complex cellular environment, taking into account any variations among cargoes and local viscoelastic properties of the cytoplasm. We found that centrally and peripherally directed forces exerted on LBCs are of similar magnitude, with maximum forces of ∼20 pN. During force events greater than 10 pN, we often observe 8-nm steps in both directions, indicating that the stepping of multiple motors is correlated. These observations suggest bidirectional transport of LBCs is driven by opposing teams of stably bound motors that operate near force balance.

Simultaneous detection of biomolecular interactions and surface topography using photonic force microscopy

Seungjin Heo, Kipom Kim, Rodriguez Christophe, Tae-Young Yoon, Yong-Hoon Cho

We developed a photonic force microscope that can map multiple parameters simultaneously, including the surface topography and biomolecular interactions. To track the position of the probe bead and to determine contact position with the sample surface, we adopted a video analysis method using the diffraction pattern of monochromatic light passing through the probe bead. To demonstrate the capability of the microscope, we report the simultaneous measurement of the molecule distribution of DNA oligonucleotides on the surface, the binding strength of DNA hybridization between the bead and surface, and the topography of the smooth moulded surface.

Oxidative aging of mixed oleic acid/sodium chloride aerosol particles

Benjamin J. Dennis-Smither, Rachael E. H. Miles, Jonathan P. Reid

Studies of the oxidative aging of single mixed component aerosol particles formed from oleic acid (OL) and sodium chloride over a range of relative humidities (RH) and ozone concentrations by aerosol optical tweezers are reported. The rate of loss of OL and changes in the organic phase volume are directly measured, comparing particles with effloresced and deliquesced inorganic seeds. The kinetics of the OL loss are analyzed and the value of the reactive uptake coefficient of ozone by OL is compared to previous studies. The reaction of OL is accompanied by a decrease in the particle volume, consistent with the evaporation of semivolatile products over a time scale of tens of thousands of seconds. Measurements of the change in the organic phase volume allow the branching ratio to involatile components to be estimated; between 50 and 85% of the initial organic volume remains involatile, depending on ozone concentration. The refractive index (RI) of the organic phase increases during and after evaporation of volatile products, consistent with aging followed by a slow restructuring in particle morphology. The hygroscopicity of the particle and kinetics of the response of the organic phase to changes in RH are investigated. Both size and RI of unoxidized and oxidized particles respond promptly to RH changes with values of the RI consistent with linear mixing rules. Such studies of the simultaneous changes in composition and size of mixed component aerosol provide valuable data for benchmarking kinetic models of heterogeneous atmospheric aging.


Friday, October 26, 2012

A generalization of the dipolar force

Marian Apostol, Stelian Ilie, Aurel Petrut, Marcel Savu, and Stefan Toba
The static dipolar force is generalized to time-dependent classical distributions of dipoles and electromagnetic fields. This force may exhibit a remarkable resonance character for induced dipoles, related to the pole structure of the polarizabilities. The resonance phenomenon is illustrated for two macroscopic polarizable bodies, with mutually induced polarizations, using the well-known Lorentz-Drude model for the dielectric response with optical dispersion and a characteristic (resonance) frequency. Specifically, the calculations are performed for distances much longer than the dimension of the bodies (“point-like” bodies), but shorter than the characteristic wavelength (sub-wavelength, stationary, near-field regime). The polarizations are induced via a localized external field acting upon only one body. The force is practically vanishing for distinct substances and acquires a non-vanishing value for identical substances. It falls off as the 7-th power of the distance, being reminiscent of the van der Waals-London force. The conditions of validity of this resonance phenomenon are emphasized. Particular cases corresponding to independent external fields or two isolated, interacting bodies (closed system) are also analyzed, with similar conclusions regarding the resonance character of the force.

Size-Dependent Partitioning of Nano/Microparticles Mediated by Membrane Lateral Heterogeneity

Tsutomu Hamada , Masamune Morita, Makiyo Miyakawa, Ryoko Sugimoto, Ai Hatanaka, Mun’delanji C. Vestergaard , and Masahiro Takagi

It is important that we understand the physical, chemical, and biological mechanisms that govern the interaction between nanoparticles (NPs) and heterogeneous cellular surfaces because of the possible cytotoxicity of engineered nanomaterials. In this study, we investigated the lateral localization of nano/microparticles within a biomimetic heterogeneous membrane interface using cell-sized two-phase liposomes. We found that lateral heterogeneity in the membrane mediates the partitioning of nano/microparticles in a size-dependent manner: small particles with a diameter of ≤200 nm were localized in an ordered phase, whereas large particles preferred a fluidic disordered phase. This partitioning behavior was verified by temperature-controlled membrane miscibility transition and laser-trapping of associated particles. In terms of the membrane elastic energy, we present a physical model that explains this localization preference of nano/microparticles. The calculated threshold diameter of particles that separates the particle-partitioning phase was 260 nm, which is in close agreement with our observation (200 nm). These findings may lead to a better understanding of the basic mechanisms that underlie the association of nanomaterials within a cell surface.

Plasmon-Based Optical Trapping of Polymer Nano-Spheres as Explored by Confocal Fluorescence Microspectroscopy: A Possible Mechanism of a Resonant Excitation Effect

Tatsuya Shoji, Yoshihiko Mizumoto, Hajime Ishihara, Noboru Kitamura, Mai Takase, Kei Murakoshi, and Yasuyuki Tsuboi

In optical trapping using photon force much enhanced by localized surface plasmon (LSP) in solution, we found that a resonant excitation effect can further enhance photon force. In this LSP-based optical trapping under a resonant excitation condition, an incident laser beam excites both LSP and electronic resonant transition of a target object simultaneously. Fluorescence microspectroscopy clearly showed that nanospheres under the resonant condition were much more efficiently trapped as compared to that under a non-resonant condition. The resonant LSP-based trapping mechanism was further reinforced by theoretical calculations taking the resonant excitation effect into account. Such resonant LSP-based trapping methodology will provide a novel approach for efficient trapping of small molecules.

Studying Single Red Blood Cells Under a Tunable External Force by Combining Passive Microrheology with Raman Spectroscopy

Saurabh Raj, Michal Wojdyla and Dmitri PetrovThe dynamic micromechanical and structural properties of single human red blood cells are studied using a combination of dual trap optical tweezers and confocal Raman spectroscopy. Such a combination permits us to show a direct relationship between the rheological properties and chemical structure conformation. The frequency dependence of the complex stiffness of the cells was measured using both one and two probe response functions under identical experimental conditions. Both the microrheology and Raman measurements were performed at different stretching forces applied to the cell. A detailed analysis of the auto- and cross-correlated probe motions allows exploring the local and overall viscoelastic properties of the cells over a controlled range of the deformations. The observed growth of the cell viscoelasticity with stretching was associated with structural changes in the cell membrane monitored via the Raman spectroscopy.

Thursday, October 25, 2012

Radiation pressure on a moving body: beyond the Doppler effect

S. A. R. Horsley, M. Artoni, and G. C. La Rocca

The dependence of macroscopic radiation pressure on the velocity of the object being pushed is commonly attributed to the Doppler effect. This need not be the case, and here we highlight velocity-dependent radiation pressure terms that have their origins in the mixing of s and p polarizations brought about by the Lorentz transformation between the lab and the material rest frame, rather than in the corresponding transformation of frequency and wavevector. The theory we develop may be relevant to the nano-optomechanics of moving bodies.

Wednesday, October 24, 2012

Optical tweezers reveal relationship between microstructure and nanoparticle penetration of pulmonary mucus

Julian Kirch, Andreas Schneider, Bérengère Abou, Alexander Hopf, Ulrich F. Schaefer, Marc Schneider,Christian Schall, Christian Wagner, and Claus-Michael Lehr

In this study, the mobility of nanoparticles in mucus and similar hydrogels as model systems was assessed to elucidate the link between microscopic diffusion behavior and macroscopic penetration of such gels. Differences in particle adhesion to mucus components were strongly dependent on particle coating. Particles coated with 2 kDa PEG exhibited a decreased adhesion to mucus components, whereas chitosan strongly increased the adhesion. Despite such mucoinert properties of PEG, magnetic nanoparticles of both coatings did not penetrate through native respiratory mucus, resisting high magnetic forces (even for several hours). However, model hydrogels were, indeed, penetrated by both particles in dependency of particle coating, obeying the theory of particle mobility in an external force field. Comparison of penetration data with cryogenic scanning EM images of mucus and the applied model systems suggested particularly high rigidity of the mucin scaffold and a broad pore size distribution in mucus as reasons for the observed particle immobilization. Active probing of the rigidity of mucus and model gels with optical tweezers was used in this context to confirm such properties of mucus on the microscale, thus presenting the missing link between micro- and macroscopical observations. Because of high heterogeneity in the size of the voids and pores in mucus, on small scales, particle mobility will depend on adhesive or inert properties. However, particle translocation over distances larger than a few micrometers is restricted by highly rigid structures within the mucus mesh.


Tuesday, October 23, 2012

Force probing of individual molecules inside the living cell is now a reality

Lene B Oddershede

Biological systems can be quantitatively explored using single-molecule manipulation techniques such as optical or magnetic tweezers or atomic force microscopy. Though a plethora of discoveries have been accomplished using single-molecule manipulation techniques in vitro, such investigations constantly face the criticism that conditions are too far from being physiologically relevant. Technical achievements now allow scientists to take the next step: to use single-molecule manipulation techniques quantitatively in vivo. Considerable progress has been accomplished in this realm; for example, the interaction between a protein and the membrane of a living cell has been probed, the mechanical properties of individual proteins central for cellular adhesion have been measured and even the action of molecular motors in living cells has been quantified. Here, we review the progress of in vivo single-molecule manipulation with a focus on the special challenges posed by in vivo conditions and how these can be overcome.

Scattering forces and electromagnetic momentum density in crossed circularly polarized standing waves: erratum

Manuel I. Marqués and Juan José Saénz
In a previous paper [Opt. Lett. 37, 2787–2789 (2012)], several typos have been found. Results of the paper remain unchanged.


Saturday, October 20, 2012

Driving Potential and Noise Level Determine the Synchronization State of Hydrodynamically Coupled Oscillators

Nicolas Bruot, Jurij Kotar, Filippo de Lillo, Marco Cosentino Lagomarsino, and Pietro Cicuta

Motile cilia are highly conserved structures in the evolution of organisms, generating the transport of fluid by periodic beating, through remarkably organized behavior in space and time. It is not known how these spatiotemporal patterns emerge and what sets their properties. Individual cilia are nonequilibrium systems with many degrees of freedom. However, their description can be represented by simpler effective force laws that drive oscillations, and paralleled with nonlinear phase oscillators studied in physics. Here a synthetic model of two phase oscillators, where colloidal particles are driven by optical traps, proves the role of the average force profile in establishing the type and strength of synchronization. We find that highly curved potentials are required for synchronization in the presence of noise. The applicability of this approach to biological data is also illustrated by successfully mapping the behavior of cilia in the alga Chlamydomonas onto the coarse-grained model.


Metallic nanoparticles in a standing wave: optical force and heating

Martin Šiler, Lukáš Chvátal, Pavel Zemánek

We have investigated the absorbed power in a single gold or silver metallic nanoparticle together with the optical force acting upon it if the particle is illuminated by two counter-propagating plane waves forming a standing wave. We have used the Generalized Lorenz-Mie theory (GLMT) and considered the incident wavelengths and particles size parameter 0.1⩽d/λvac⩽4. Similarly as in the case of dielectric particle we have found that the optical force is equal to zero for all particle positions in the standing wave for certain wavelengths and particle sizes. However, in the case of a metallic object this phenomenon occurs for considerably smaller particles and the conditions change considerably with the illuminating wavelength especially near the localized surface plasmon resonances. Similarly, we have found that the absorbed heat does not change with the position of the particle in the standing wave for certain wavelengths and particle sizes. These sizes generally differ from those giving zero optical force and, therefore, the particle can be trapped at the intensity maximum or minimum and in both cases its heating is maximal or minimal depending on the particle size.


Friday, October 19, 2012

Optical Conveyors: A Class of Active Tractor Beams

David B. Ruffner and David G. Grier 

We experimentally demonstrate a class of tractor beams created by coherently superposing coaxial Bessel beams. These optical conveyors have periodic intensity variations along their axes that act as highly effective optical traps for micrometer-scale objects. Trapped objects can be moved selectively upstream or downstream along the conveyor by appropriately changing the Bessel beams’ relative phase. The same methods used to project a single optical conveyor can project arrays of independent optical conveyors, allowing bidirectional transport in three dimensions.


Thursday, October 18, 2012

Optical levitation and long-working-distance trapping: From spherical up to high aspect ratio ellipsoidal particles

Besira Mihiretie, Jean-Christophe Loudet, Bernard Pouligny

Radiation pressure forces from a moderately focused vertical laser beam are used to levitate transparent particles, a few micrometers in size. Having recalled basic results about levitation of spheres, and applications to long-working distance trapping, we turn to ellipsoid-shaped particles. Experiments are carried out with polystyrene particles, inside a glass chamber filled with water. The particles are lifted up to contact with the chamber top surface. We examine particle equilibrium in such conditions and show that the system “bifurcates” between static on-axis equilibrium with short ellipsoids, to sustained oscillations with longer ones. A similar Hopf bifurcation is found using a simple ray-optics model of the laser-ellipsoid interaction, providing a qualitative account of the observed oscillations.

Single aerosol trapping with an annular beam: Improved particle localisation

Richard Dear , Daniel Burnham , Michael Summers , David McGloin and Grant Ritchie
In this paper we explore the trapping of aerosol droplets using an annular beam, formed by blocking the central portion of a Gaussian beam, and quantify the improvements over conventional Gaussian beam traps. Recent work on the modelling of single aerosol dynamics within an optical tweezer trap [Burnham et al., Journal of the Optical Society of America B, 2011, 28, 2856] has indicated that the use of annular beams can allow smaller droplets to be trapped, which we experimentally verify. We also demonstrate that annular beams allow droplets to be trapped at higher powers, and with reduced axial displacement with increasing power, than Gaussian beams. We confirm these results, due to a reduction in the axial scattering forces, using this theoretical model. Finally back focal plane interferometry is used to determine the axial and lateral trap stiffnesses for a series of droplets, showing a significant increase in the axial:lateral trap stiffness ratio from 0.79 ± 0.04 to 1.15 ± 0.04 when an annular beam is used.

Micro-fabrication by laser radiation forces: A direct route to reversible free-standing three-dimensional structures

Loukas Athanasekos, Miltiadis Vasileiadis, Christos Mantzaridis, Vagelis C. Karoutsos, Ioannis Koutselas, Stergios Pispas, and Nikolaos A. Vainos

The origins and first demonstration of structurally stable solids formed by use of radiation forces are presented. By experimentally proving that radiation forces can indeed produce stable solid material forms, a novel method enabling two- and three-dimensional (2d and 3d) microfabrication is introduced: An optical, non-contact single-step physical operation, reversible with respect to materials nature, based on the sole use of radiation forces. The present innovation is elucidated by the formation of polyisoprene and polybutadiene micro-solids, as well as plasmonic and fluorescent hybrids, respectively comprising Au nanoparticles and CdS quantum dots, together with novel concepts of polymeric fiber-drawing by radiation forces.


Monday, October 15, 2012

Video-based and interference-free axial force detection and analysis for optical tweezers

Sebastian Knust, Andre Spiering, Henning Vieker, André Beyer, Armin Gölzhäuser, Katja Tönsing, Andy Sischka, and Dario Anselmetti

For measuring the minute forces exerted on single molecules during controlled translocation through nanopores with sub-piconewton precision, we have developed a video-based axial force detection and analysis system for optical tweezers. Since our detection system is equipped with a standard and versatile CCD video camera with a limited bandwidth offering operation at moderate light illumination with minimal sample heating, we integrated Allan variance analysis for trap stiffness calibration. Upon manipulating a microbead in the vicinity of a weakly reflecting surface with simultaneous axial force detection, interference effects have to be considered and minimized. We measured and analyzed the backscattering light properties of polystyrene and silica microbeads with different diameters and propose distinct and optimized experimental configurations (microbead material and diameter) for minimal light backscattering and virtually interference-free microbead position detection. As a proof of principle, we investigated the nanopore threading forces of a single dsDNA strand attached to a microbead with an overall force resolution of ±0.5 pN at a sample rate of 123 Hz.


Saturday, October 13, 2012

All-optically-controlled nanoparticle transporting and manipulating at SOI waveguide intersections

Hao Li, Xin Yu, Xiang Wu, Wei Shi, Mo Chen, Liying Liu, and Lei Xu

All-optically controlled nanoparticle manipulating units based on optical waveguide intersections are designed and their performance on nanoparticle trapping, redirecting, sorting and binding force measurement are theoretically analyzed. Our calculation shows that these simple units have trapping abilities comparable with most near field trapping tools and are capable of realizing multiple sorting and analyzing functions.


Friday, October 12, 2012

Optical Trapping in Air of an Individual Nanotube-Sphere Device

Gurpreet Singh, Andrew Slifka, Paul Rice, Damian Lauria, and Roop L. Mahajan

We demonstrated the optical manipulation of a polystyrene bead supported in air by an individual carbon nanotube. We have also utilized this technique to demonstrate the calibration of a nanotube-sphere force sensor in the ≈10-10 N range. A focused IR laser (at 1.064 µm, 100 mW power) was used to trap the bead. This simple device consisted of a tungsten probe with a long nanotube (length, ≥15 µm) attached to its tip, while the other end of the nanotube supported a polystyrene microsphere. Decreasing the nanotube length to 8 µm did not show any sphere motion in the trap.

Practical axial optical trapping

A. H. Mack, D. J. Schlingman, L. Regan, and S. G. J. Mochrie
We describe a new method for calibrating optical trapping measurements in which tension is applied in the direction of the laser beam to a molecule tethered between a surface and an optically trapped bead. Specifically, we present a generally-applicable procedure for converting from the measured scattering intensity and the measured stage displacement to applied tension and bead-coverslip separation, using measurements of the light intensity scattered from an untethered, trapped bead. Our calibration accounts for a number of effects, including aberrations and the interference of forward-reflected bead-scattered light with the trapping beam. To demonstrate the accuracy of our method, we show measurements of the DNA force-versus-extension relation using a range of laser intensities, and show that these measurements match the expected extensible wormlike-chain (WLC) behavior. Finally, we also demonstrate a force-clamp, in which the tension in a tether is held fixed while the extension varies as a result of molecular events.

Measuring the pressures across microfluidic droplets with an optical tweezer

Yuhang Jin, Antony Orth, Ethan Schonbrun, and Kenneth B. Crozier

We introduce a novel technique that enables pressure measurements to be made in microfluidic chips using optical trapping. Pressure differentials across droplets in a microfluidic channel are determined by monitoring the displacements of a bead in an optical trap. We provide physical interpretation of the results. Our experiments reveal that our device has high sensitivity and can be operated over a wide range of pressures from several Pascals to several thousand Pascals.


Wednesday, October 10, 2012

Optical forces induced behavior of a particle in a non-diffracting vortex beam

Martin Šiler, Petr Jákl, Oto Brzobohatý, and Pavel Zemánek

An interaction between a light field with complex field spatial distribution and a micro-particle leads to forces that drag the particle in space and may confine it in a stable position or a trajectory. The particle behavior is determined by its size with respect to the characteristic length of the spatially periodic or symmetric light field distribution. We study theoretically and experimentally the behavior of a microparticle near the center of an optical vortex beam in a plane perpendicular to the beam propagation. We show that such particle may be stably trapped either in a dark spot on the vortex beam axis, or in one of two points placed off the optical axis. It may also circulate along a trajectory having its radius smaller or equal to the radius of the first bright vortex ring.


Ultrastrong Optical Binding of Metallic Nanoparticles

Vassili Demergis and Ernst-Ludwig Florin
We demonstrate nanometer precision manipulation of multiple nanoparticles at room temperature. This is achieved using the optical binding force, which has been assumed to be weak compared to the optical gradient and scattering forces. We show that trapping by the optical binding force can be over 20 times stronger than by the gradient force and leads to ultrastable, rigid configurations of multiple nanoparticles free in solution – a realization of “optical matter.” In addition, we demonstrate a novel trapping scheme where even smaller nanoparticles are trapped between larger “anchor” particles. Optical binding opens the door for the observation of collective phenomena of nanoparticles and the design of new materials and devices made from optical matter.

Electron Beam Manipulation of Nanoparticles

Haimei Zheng , Utkur M Mirsaidov , Lin-Wang Wang , and Matsudaira Paul
We report on electron beam manipulation and simultaneous transmission electron microscopy imaging of gold nanoparticle movements in an environmental cell. Nanoparticles are trapped with the beam and move dynamically towards the location with higher electron density. Their global movements follow the beam positions. Analysis on the trajectories of nanoparticle movements inside the beam reveals a trapping force in the piconewton range at the electron density gradient of 10^3~4 e/(nm2·s)/nm. Multiple nanoparticles can also be trapped with the beam. By rapidly converging the beam, we further can collect nanoparticles on the membrane surface and assemble them into a cluster.


Tuesday, October 9, 2012

Evaluation of rare earth doped silica sub-micrometric spheres as optically controlled temperature sensors

P. Haro-González, L. Martínez Maestro, M. Trevisani, S. Polizzi, D. Jaque, J. García Sole, and M. Bettinelli

We report on the evaluation of rare earth (Er3+, Eu3+, and Tb3+ ions) SiO2 sub-micrometric spheres as potential optically controllable temperature sensors. Details about fabrication, optical manipulation and spectroscopic characterization of the sub-micrometric spheres are presented. The fluorescence properties of the micros-spheres in the biological range (25–60 °C) have been systematically investigated. From this systematic study, the thermal resolution potentially achieved in each case has been determined and compared to previous works.


Optical forces in lossless arbitrary refractive index optical trapping and micromanipulation

Leonardo A. Ambrosio, Hugo E. Hernández-Figueroa

This paper shows, using both a ray optics approach and in the framework of the generalized Lorenz–Mie theory (GLMT), what happens to the optical forces exerted on a lossless spherical particle with an arbitrary (positive or negative) relative refractive index, allowing the external medium also to be metamaterial. It is shown that the anti-parallelism between the linear momentum p of each photon and the Poynting vector Sassociated with the propagating wave, observed in negative refractive index media, leads to shifts in the direction of the optical force of a single ray and, consequently, to the total optical force exerted by an arbitrary-shaped laser beam. This extends the possible realizable traps and reveals how arbitrary-shaped laser beams can be used to trap particles with arbitrary refractive indices.


Plasmonic Nanostructures as Accelerators for Nanoparticles: Optical Nanocannon

Alexander S. Shalin and Sergey V. Sukhov

We suggest a model of an optical structure that allows to accelerate nanoparticles to velocities on the order of tens of centimeters per second using low-intensity external optical fields. The nano-accelerator system employs metallic V-grooves which concentrate the electric field in the vicinity of their bottoms and creates large optical gradient forces for the nanoparticles in that groove. The conditions are found when this optical force tends to eject particles away from the groove.


Monday, October 8, 2012

A light-driven turbine-like micro-rotor and study on its light-to-mechanical power conversion efficiency

Xiao-Feng Lin, Guo-Qing Hu, Qi-Dai Chen, Li-Gang Niu, Qi-Song Li, Andreas Ostendorf, and Hong-Bo Sun
A light driven micro-rotor is a useful telecontrolled device free of mechanical contact for power supply. However, low efficiency in converting light to mechanical power detracts from its advantages because it incurs a high power consumption that might result in unwanted effects. For a systematic study on conversion efficiency, we designed a turbine-like micro-rotor and made a quantitative analysis by computational fluid dynamics and semiclassical optics. Much larger in size than those ever reported, our rotor could rotate at over 500 r/min. Denoted by average angular momentum transfer, its conversion efficiency was experimentally determined as high as 34.55 ℏ/photon.

Limitations of Constant-Force-Feedback Experiments

Phillip J. Elms, John D. Chodera, Carlos J. Bustamante, Susan Marqusee

Single-molecule force spectroscopy has provided important insights into the properties and mechanisms of biological molecules and systems. A common experiment is to measure the force dependence of conformational changes at equilibrium. Here, we demonstrate that the commonly used technique of force feedback has severe limitations when used to evaluate rapid macromolecular conformational transitions. By comparing the force-dependent dynamics of three major classes of macromolecules (DNA, RNA, and protein) using both a constant-force-feedback and a constant-trap-position technique, we demonstrate a problem in force-feedback experiments. The finite response time of the instrument’s force feedback can modify the behavior of the molecule, leading to errors in the reported parameters, such as the rate constants and the distance to the transition state, for the conformational transitions. We elucidate the causes of this problem and provide a simple test to identify and evaluate the magnitude of the effect. We recommend avoiding the use of constant force feedback as a method to study rapid conformational changes in macromolecules.


Use of motion peculiarities of test particles for estimating degree of coherence of optical fields

Zenkova C. Yu., Gorsky M. P., Soltys I. V. and Angelsky P. O.
We discuss interconnections between the depth of spatial inhomogeneity 
of energy distribution of the optical fields and the velocity of motion of test particles
for the cases of different light scattering mechanisms during interaction with light.
We suggest an additional tool for determining the degree of coherence of
superposing waves that propagate along mutually orthogonal directions and have
orthogonal polarisations, being linearly polarised in the incidence plane. The use of
velocity of the test particles while estimating the degree of coherence of optical
fields is suggested for the first time.

Optical Forces in Metal/Dielectric/Metal Fishnet Metamaterials in the Visible Wavelength Regime

Cao, T. and Zhang, L.; Cryan, M. J.

The optical force is calculated on a nanoparticle in close proximity to the surface of a fishnet metamaterial based on a metal/dielectric/metal film when illuminated at visible wavelengths. We show that the optical force can be enhanced by the strong magnetic dipole in the fishnet metamaterial. In contrast to other plasmonic nanostructures, which exhibit an attractive force in all regions, our presented structure provides good flexibility in pushing and dragging particular size nanoscale particles at certain distances above the surface of the structure. Therefore, it is suitable for size selection and optical trapping of nanoscale particles at illumination intensities of $(1 hbox{mW}/muhbox{m}^{2})$. At this power level, calculation shows that the optical force can be up to four orders of magnitude larger than the gravitational force for a 100-nm radius nanoparticle.

Fractionalization of interstitials in curved colloidal crystals

William T. M. Irvine, Mark J. Bowick & Paul M. Chaikin

Understanding the effect of curvature and topological frustration in crystals yields insights into the fragility of the ordered state. For instance, a one-dimensional crystal of identical charged particles can accommodate an extra particle (interstitial) if all the particle positions are readjusted, yet in a planar hexagonal crystal interstitials remain trapped between lattice sites and diffuse by hopping. Using optical tweezers operated independently of three-dimensional imaging, we inserted interstitials in a lattice of similar colloidal particles sitting on flat or curved oil/glycerol interfaces, and imaged the ensuing dynamics. We find that, unlike in flat space, the curved crystals self-heal through a collective particle rearrangement that redistributes the increased density associated with the interstitial. This process can be interpreted in terms of the out-of-equilibrium interaction of topological defects with each other and with the underlying curvature. Our observations suggest the existence of particle fractionalization on curved surface crystals.

Saturday, October 6, 2012

Optical levitation of a non-spherical particle in a loosely focused Gaussian beam

Cheong Bong Chang, Wei-Xi Huang, Kyung Heon Lee, and Hyung Jin Sung

The optical force on a non-spherical particle subjected to a loosely focused laser beam was calculated using the dynamic ray tracing method. Ellipsoidal particles with different aspect ratios, inclination angles, and positions were modeled, and the effects of these parameters on the optical force were examined. The vertical component of the optical force parallel to the laser beam axis decreased as the aspect ratio decreased, whereas the ellipsoid with a small aspect ratio and a large inclination angle experienced a large vertical optical force. The ellipsoids were pulled toward or repelled away from the laser beam axis, depending on the inclination angle, and they experienced a torque near the focal point. The behavior of the ellipsoids in a viscous fluid was examined by analyzing a dynamic simulation based on the penalty immersed boundary method. As the ellipsoids levitated along the direction of the laser beam propagation, they moved horizontally with rotation. Except for the ellipsoid with a small aspect ratio and a zero inclination angle near the focal point, the ellipsoids rotated until the major axis aligned with the laser beam axis.


Toward efficient optical trapping of sub-10-nm particles with coaxial plasmonic apertures

Amr A. E. Saleh and Jennifer A. Dionne

Optical trapping using focused laser beams has emerged as a powerful tool in the biological and physical sciences. However, scaling this technique to nano-sized objects remains challenging due to the diffraction limit of light and the high power levels required for nanoscale trapping. In this paper, we propose plasmonic coaxial apertures as low-power optical traps for nano-sized specimens. Illumination of a coaxial aperture with a linearly polarized plane wave generates a dual optical trapping potential well. We theoretically show that this potential can stably trap dielectric particles smaller than 10 nm in diameter while keeping the trapping power level below 20 mW. By tapering the thickness of the coaxial dielectric channel, trapping can be extended to sub-2 nm particles. The proposed structures may enable optical trapping and manipulation of dielectric particles ranging from single proteins to small molecules with sizes previously inaccessible.

Friday, October 5, 2012

Mechanical Coupling between Myosin Molecules Causes Differences between Ensemble and Single-Molecule Measurements

Sam Walcott, David M. Warshaw, Edward P. Debold
In contracting muscle, individual myosin molecules function as part of a large ensemble, hydrolyzing ATP to power the relative sliding of actin filaments. The technological advances that have enabled direct observation and manipulation of single molecules, including recent experiments that have explored myosin’s force-dependent properties, provide detailed insight into the kinetics of myosin’s mechanochemical interaction with actin. However, it has been difficult to reconcile these single-molecule observations with the behavior of myosin in an ensemble. Here, using a combination of simulations and theory, we show that the kinetic mechanism derived from single-molecule experiments describes ensemble behavior; but the connection between single molecule and ensemble is complex. In particular, even in the absence of external force, internal forces generated between myosin molecules in a large ensemble accelerate ADP release and increase how far actin moves during a single myosin attachment. These myosin-induced changes in strong binding lifetime and attachment distance cause measurable properties, such as actin speed in the motility assay, to vary depending on the number of myosin molecules interacting with an actin filament. This ensemble-size effect challenges the simple detachment limited model of motility, because even when motility speed is limited by ADP release, increasing attachment rate can increase motility speed.

Nematic Colloids – Interaction between Particles in Anisotropic Liquids

Yasuyuki Kimura, Takahiro Kishita, Kosuke Kita, and Noboru Kondo

We discuss novel interparticle force between colloids in nematic liquid crystal. The dependence of the interparticle force on the interparticle distance was experimentally measured by optical tweezers for the same and different sized particles. The obtained force curves quantitatively make agreement with the theory based on electrostatic analogy and the numerical calculation. We also present the self-assembling and artificial ways for building colloidal assemblies in nematic liquid crystal.

Tension induces a base-paired overstretched DNA conformation

Niklas Bosaeus, Afaf H. El-Sagheer, Tom Brown, Steven B. Smith, Björn Åkerman, Carlos Bustamante, and Bengt Nordén

Mixed-sequence DNA molecules undergo mechanical overstretching by approximately 70% at 60–70 pN. Since its initial discovery 15 y ago, a debate has arisen as to whether the molecule adopts a new form [Cluzel P, et al. (1996)Science 271:792–794; Smith SB, Cui Y, Bustamante C (1996) Science 271:795–799], or simply denatures under tension [van Mameren J, et al. (2009) Proc Natl Acad Sci USA 106:18231–18236]. Here, we resolve this controversy by using optical tweezers to extend small 60–64 bp single DNA duplex molecules whose base content can be designed at will. We show that when AT content is high (70%), a force-induced denaturation of the DNA helix ensues at 62 pN that is accompanied by an extension of the molecule of approximately 70%. By contrast, GC-rich sequences (60% GC) are found to undergo a reversible overstretching transition into a distinct form that is characterized by a 51% extension and that remains base-paired. For the first time, results proving the existence of a stretched basepaired form of DNA can be presented. The extension observed in the reversible transition coincides with that produced on DNA by binding of bacterial RecA and human Rad51, pointing to its possible relevance in homologous recombination.


Thursday, October 4, 2012

Optical lift from dielectric semicylinders

Stephen H. Simpson, Simon Hanna, Timothy J. Peterson, and Grover A. Swartzlander
A wave optics numerical analysis of the force and torque on a semicylindrical optical wing is presented. Comparisons with a recently reported ray optics analysis indicate good agreement when the radius is large compared with the wavelength of light, as expected. Surprisingly, we find that the dominant rotationally stable angle of attack at α≈−15° is relatively invariant to changes in radius and refractive index. However, the torsional stiffness at the equilibrium point is found to increase, approximately, as the cubic power of the radius. Quasi-resonant internal modes of light produce complex size-dependent variations of the angle and magnitude of the optical lift force.

Magnetic interaction in all silicon waveguide spherical coupler device

Lei Shi and Francisco Meseguer

The magnetic field component of light in dielectric materials generally plays a negligible role at optical frequency values. However, it is a key component of metal based metamaterials. Here we report on the dominant role of the magnetic interaction in a dielectric spherical silicon nanocavity coupled to a silicon waveguide. The analytical method, as well as the finite difference time domain (FDTD) simulation, show a three dimensional (3D) magnetic trap effect when the magnetic like Mie resonances of the nanocavity are excited.


Auto- and cross-power spectral analysis of dual trap optical tweezer experiments using Bayesian inference

Yann von Hansen, Alexander Mehlich, Benjamin Pelz, Matthias Rief, and Roland R. Netz

The thermal fluctuations of micron-sized beads in dual trap optical tweezer experiments contain complete dynamic information about the viscoelastic properties of the embedding medium and—if present—macromolecular constructs connecting the two beads. To quantitatively interpret the spectral properties of the measured signals, a detailed understanding of the instrumental characteristics is required. To this end, we present a theoretical description of the signal processing in a typical dual trap optical tweezer experiment accounting for polarization crosstalk and instrumental noise and discuss the effect of finite statistics. To infer the unknown parameters from experimental data, a maximum likelihood method based on the statistical properties of the stochastic signals is derived. In a first step, the method can be used for calibration purposes: We propose a scheme involving three consecutive measurements (both traps empty, first one occupied and second empty, and vice versa), by which all instrumental and physical parameters of the setup are determined. We test our approach for a simple model system, namely a pair of unconnected, but hydrodynamically interacting spheres. The comparison to theoretical predictions based on instantaneous as well as retarded hydrodynamics emphasizes the importance of hydrodynamic retardation effects due to vorticity diffusion in the fluid. For more complex experimental scenarios, where macromolecular constructs are tethered between the two beads, the same maximum likelihood method in conjunction with dynamic deconvolution theory will in a second step allow one to determine the viscoelastic properties of the tethered element connecting the two beads.


Brownian Motion in a Designer Force Field: Dynamical Effects of Negative Refraction on Nanoparticles

A. Cuche, B. Stein, A. Canaguier-Durand, E. Devaux, C. Genet, and T. W. Ebbesen

Photonic crystals (PC) have demonstrated unique features that have renewed the fields of classical and quantum optics. Although holding great promises, associated mechanical effects have proven challenging to observe. We demonstrate for the first time that one of the most salient properties of PC, namely negative refraction, can induce specific forces on metal nanoparticles. By integrating a periodically patterned metal film in a fluidic cell, we show that near-field optical forces associated with negatively refracted surface plasmons are capable of controlling particle trajectories. Coupling particle motions to PC band structures draws new approaches and strategies for parallel and high resolution all-optical control of particle flows with applications for micro- and nanofluidic systems.


Tuesday, October 2, 2012

Validation and perspectives of a femtosecond laser fabricated monolithic optical stretcher

Nicola Bellini, Francesca Bragheri, Ilaria Cristiani, Jochen Guck, Roberto Osellame, and Graeme Whyte

The combination of high power laser beams with microfluidic delivery of cells is at the heart of high-throughput, single-cell analysis and disease diagnosis with an optical stretcher. So far, the challenges arising from this combination have been addressed by externally aligning optical fibres with microfluidic glass capillaries, which has a limited potential for integration into lab-on-a-chip environments. Here we demonstrate the successful production and use of a monolithic glass chip for optical stretching of white blood cells, featuring microfluidic channels and optical waveguides directly written into bulk glass by femtosecond laser pulses. The performance of this novel chip is compared to the standard capillary configuration. The robustness, durability and potential for intricate flow patterns provided by this monolithic optical stretcher chip suggest its use for future diagnostic and biotechnological applications.

Photophoretic trampoline—Interaction of single airborne absorbing droplets with light

Michael Esseling, Patrick Rose, Christina Alpmann, and Cornelia Denz
We present the light-induced manipulation of absorbing liquid droplets in air. Ink droplets from a printer cartridge are used to demonstrate that absorbing liquids—just like their solid counterparts—can interact with regions of high light intensity due to the photophoretic force. It is shown that droplets follow a quasi-ballistic trajectory after bouncing off a high intensity light sheet. We estimate the intensities necessary for this rebound of airborne droplets and change the droplet trajectories through a variation of the manipulating light field.

Structural responses of quasi-two-dimensional colloidal fluids to excitations elicited by nonequilibrium perturbations

Jelena Pesic, Joseph Zsolt Terdik, Xinliang Xu, Ye Tian, Alejandro Lopez, Stuart A. Rice, Aaron R. Dinner, and Norbert F. Scherer
We investigate the response of a dense monodisperse quasi-two-dimensional colloid suspension when a particle is dragged by a constant velocity optical trap. Consistent with microrheological studies of other geometries, the perturbation induces a leading density wave and trailing wake. We also use a hybrid version of Stokesian dynamics simulations to parse direct colloid-colloid and hydrodynamic interactions. We go on to analyze the underlying individual particle-particle collisions in the experimental images. The displacements of particles occur in chains reminiscent of stress propagation in sheared granular materials. From these data, we can reconstruct steady-state dipolar-like flow patterns that were predicted for dilute suspensions and previously observed in granular analogs to our system. The decay of this field differs, however, from point Stokeslet calculations, indicating that the nonzero size of the colloids is important. Moreover, there is a pronounced angular dependence that corresponds to the surrounding colloid structure, which develops in response to the perturbation. Put together, our results show that the response of the complex fluid is highly anisotropic owing to the fact that the effects of the perturbation propagate through the structured medium via chains of colloid-colloid collisions.