Monday, April 28, 2014

Observation of the dynamics of magnetic nanoparticles induced by a focused laser beam by using dark-field microscopy

Hai-Dong Deng, Guang-Can Li, Hai Li

The dynamics of Fe3O4 magnetic nanoparticles under the irradiation of a tightly focused laser beam was investigated by using a high-intensity dark-field microscopy. A depletion region of magnetic nanoparticles was found at the center of the laser beam where the dissipative force (absorption and scattering forces) dominated the dynamics of the magnetic nanoparticles. In contrast, the dynamics of magnetic nanoparticles was dominated by thermal and mass diffusions at the edge of the laser beam where the dissipative force was negligible. In addition, the transient variation in the concentration of magnetic nanoparticles was characterized by recording the transient scattering light intensity. The coefficients of thermal diffusion, mass diffusion and the Soret effect for this kind of magnetic nanoparticles were successfully extracted by using this technique.


Accurate measurement of force and displacement with optical tweezers using DNA molecules as metrology standards

Damian del Toro and Douglas E. Smith

Optical tweezers facilitate measurement of piconewton-level forces and nanometer-level displacements and have broad applications in biophysics and soft matter physics research. We have shown previously that DNA molecules can be used as metrology standards to define such measurements. Force-extension measurements on two DNA molecules of different lengths can be used to determine four necessary measurement parameters. Here, we show that the accuracy of determining these parameters can be improved by more than 7-fold by incorporating measurements of the DNA overstretching transition and using a multi-step data analysis procedure. This method results in very robust and precise fitting of DNA force-extension measurements to the worm-like chain model. We verify the accuracy through independent measurements of DNA stretching, DNA unzipping, and microsphere contact forces.


Friday, April 25, 2014

Optical tweezers: Theory and modelling

Timo A. Nieminen, Nathaniel du Preez-Wilkinson, Alexander B. Stilgoe, Vincent L.Y. Loke, Ann A.M. Bui, Halina Rubinsztein-Dunlop

Since their development in the 1980s, optical tweezers have become a widely used and versatile tool in many fields. Outstanding applications include the quantitative measurement of forces in cell biology and biophysics. Computational modelling of optical tweezers is a valuable tool in support of experimental work, especially quantitative applications. We discuss the theory, and the theoretical and computational modelling of optical tweezers.


Shape-induced force fields in optical trapping

D. B. Phillips, M. J. Padgett, S. Hanna, Y.-L. D. Ho, D. M. Carberry, M. J. Miles & S. H. Simpson
Advances in optical tweezers, coupled with the proliferation of two-photon polymerization systems, mean that it is now becoming routine to fabricate and trap non-spherical particles. The shaping of both light beams and particles allows fine control over the flow of momentum from the optical to mechanical regimes. However, understanding and predicting the behaviour of such systems is highly complex in comparison with the traditional optically trapped microsphere. In this Article, we present a conceptually new and simple approach based on the nature of the optical force density. We illustrate the method through the design and fabrication of a shaped particle capable of acting as a passive force clamp, and we demonstrate its use as an optically trapped probe for imaging surface topography. Further applications of the design rules highlighted here may lead to new sensors for probing biomolecule mechanics, as well as to the development of optically actuated micromachines.


Dynamic stereo microscopy for studying particle sedimentation

M. P. Lee, G. M. Gibson, D. Phillips, M. J. Padgett, and M. Tassieri

We demonstrate a new method for measuring the sedimentation of a single colloidal bead by using a combination of optical tweezers and a stereo microscope based on a spatial light modulator. We use optical tweezers to raise a micron-sized silica bead to a fixed height and then release it to observe its 3D motion while it sediments under gravity. This experimental procedure provides two independent measurements of bead diameter and a measure of Faxén’s correction, where the motion changes due to presence of the boundary.


Sunday, April 13, 2014

Plasmonic Optical Trapping in Biologically Relevant Media

Brian J. Roxworthy, Michael T. Johnston, Felipe T. Lee-Montiel, Randy H. Ewoldt, Princess I. Imoukhuede, Kimani C. Toussaint Jr

We present plasmonic optical trapping of micron-sized particles in biologically relevant buffer media with varying ionic strength. The media consist of 3 cell-growth solutions and 2 buffers and are specifically chosen due to their widespread use and applicability to breast-cancer and angiogenesis studies. High-precision rheological measurements on the buffer media reveal that, in all cases excluding the 8.0 pH Stain medium, the fluids exhibit Newtonian behavior, thereby enabling straightforward measurements of optical trap stiffness from power-spectral particle displacement data. Using stiffness as a trapping performance metric, we find that for all media under consideration the plasmonic nanotweezers generate optical forces 3–4x a conventional optical trap. Further, plasmonic trap stiffness values are comparable to those of an identical water-only system, indicating that the performance of a plasmonic nanotweezer is not degraded by the biological media. These results pave the way for future biological applications utilizing plasmonic optical traps.


Magnetic dipole super-resonances and their impact on mechanical forces at optical frequencies

Iñigo Liberal, Iñigo Ederra, Ramón Gonzalo, and Richard W. Ziolkowski

Artificial magnetism enables various transformative optical phenomena, including negative refraction, Fano resonances, and unconventional nanoantennas, beamshapers, polarization transformers and perfect absorbers, and enriches the collection of electromagnetic field control mechanisms at optical frequencies. We demonstrate that it is possible to excite a magnetic dipole super-resonance at optical frequencies by coating a silicon nanoparticle with a shell impregnated with active material. The resulting response is several orders of magnitude stronger than that generated by bare silicon nanoparticles and is comparable to electric dipole super-resonances excited in spaser-based nanolasers. Furthermore, this configuration enables an exceptional control over the optical forces exerted on the nanoparticle. It expedites huge pushing or pulling actions, as well as a total suppression of the force in both far-field and near-field scenarios. These effects empower advanced paradigms in electromagnetic manipulation and microscopy.


Direct evidence for three-dimensional off-axis trapping with single Laguerre-Gaussian beam

T. Otsu, T. Ando, Y. Takiguchi, Y. Ohtake, H. Toyoda, and H. Itoh

Optical tweezers are often applied to control the dynamics of objects by scanning light. However, there is a limitation that objects fail to track the scan when the drag exceeds the trapping force. In contrast, Laguerre-Gaussian (LG) beams can directly control the torque on objects and provide a typical model for nonequilibrium systems such as Brownian motion under external fields. Although stable “mid-water” trapping is essential for removing extrinsic hydrodynamic effects in such studies, three-dimensional trapping by LG beams has not yet been clearly established. Here we report the three-dimensional off-axis trapping of dielectric spheres using high-quality LG beams generated by a special holographic method. The trapping position was estimated as ~ half the wavelength behind the beam waist. These results establish the scientific groundwork of LG trapping and the technical basis of calibrating optical torque to provide powerful tools for studying energy-conversion mechanisms and the nonequilibrium nature of biological molecules under torque.


Optical tweezers: Dressed for success

Patrick C. Chaumet & Adel Rahmani

Controlled optical manipulation of a single dielectric nanoparticle is achieved with a bowtie nanoantenna placed at the end of the probe of a near-field scanning microscope.


Force-dependent isomerization kinetics of a highly conserved proline switch modulates the mechanosensing region of filamin

Lorenz Rognoni, Tobias Möst, Gabriel Žoldák, and Matthias Rief

Biological processes in the cell are highly dynamic and complex, and their correct interplay is ensured by a multitude of regulatory mechanisms. Among these, proline isomerization acts as a molecular switch that toggles two protein conformations, and thus functions, over time. In mechanosensing, mechanical stress is transduced into chemical signals. The molecular mechanisms underlying this regulation are crucial for understanding cell behavior and development. However, only a few experimental techniques are capable of studying force at the molecular level. In this paper, we use single-molecule mechanical experiments to investigate how the force-sensing region of the cytoskeletal cross-linker filamin is modulated by a proline switch.


Wednesday, April 9, 2014

Label-Free Biosensing over a Wide Concentration Range with Photonic Force Microscopy

Seungjin Heo, Prof. Kipom Kim and Prof. Yong-Hoon Cho

We present a label-free biosensor that measures molecular interactions between biomolecules on the surface of a probe bead and substrate over a wide concentration range. This system is capable of detecting target biomolecules with concentrations varying from 10 nm to 0.1 pm, with high selectivity and sensitivity.


Single-cell control of initial spatial structure in biofilm development using laser trapping

Jaime B. Hutchison , Christopher A Rodesney , Karishma Kaushik , Henry Le , Daniel Hurwitz , Yasuhiko Irie , and Vernita Gordon

Biofilms are sessile communities of microbes that are spatially structured by embedding matrix. Biofilm infections are notoriously intractable. This arises, in part, from changes in bacterial phenotype that result from spatial structure. Understanding these interactions requires methods to control the spatial structure of biofilms. We present a method for growing biofilms from initiating cells whose positions are controlled with single-cell precision using laser trapping. The native growth, motility, and surface adhesion of positioned microbes are preserved, as we show for model organisms Pseudomonas aeruginosa and Staphylococcus aureus. We demonstrate that laser-trapping and placing bacteria on surfaces can reveal the effects of spatial structure on bacterial growth in early biofilm development.


Theory for optical assembling of anisotropic nanoparticles by tailored light fields under thermal fluctuations

Mamoru Tamura, Syoji Ito, Shiho Tokonami, Takuya Iida
In order to evaluate the assembling processes of arbitrary-shaped nanoparticles (NPs) by the irradiation of a tailored laser beam under thermal fluctuations, we have developed a “Light-induced-force Nano Metropolis Method (LNMM)” as a new theoretical method based on the stochastic algorithm in the energy region and the general formula of light-induced force. By using LNMM, we have investigated the change of configurations of silver NPs with anisotropic shapes under the irradiation of laser beams with various polarizations and intensity distributions (Gaussian beam and axially-symmetric vector beams) in an aqueous solution at room temperature. As a result, it has been clarified that silver NPs can be selectively arranged into a characteristic spatial configuration reflecting the properties of an irradiated laser beam (wavelength, intensity distribution, and polarization distribution), and that the assembled structures possess broadband spectra and exhibit a strong optical response to the irradiated laser beam through the optimization with the help of fluctuations.


Dynamic relaxation of a levitated nanoparticle from a non-equilibrium steady state

Jan Gieseler, Romain Quidant, Christoph Dellago & Lukas Novotny

Fluctuation theorems are a generalization of thermodynamics on small scales and provide the tools to characterize the fluctuations of thermodynamic quantities in non-equilibrium nanoscale systems. They are particularly important for understanding irreversibility and the second law in fundamental chemical and biological processes that are actively driven, thus operating far from thermal equilibrium. Here, we apply the framework of fluctuation theorems to investigate the important case of a system relaxing from a non-equilibrium state towards equilibrium. Using a vacuum-trapped nanoparticle, we demonstrate experimentally the validity of a fluctuation theorem for the relative entropy change occurring during relaxation from a non-equilibrium steady state. The platform established here allows non-equilibrium fluctuation theorems to be studied experimentally for arbitrary steady states and can be extended to investigate quantum fluctuation theorems as well as systems that do not obey detailed balance.


Polarization-dependent optomechanics mediated by chiral microresonators

M. G. Donato, J. Hernandez, A. Mazzulla, C. Provenzano, R. Saija, R. Sayed, S. Vasi, A. Magazzù, P. Pagliusi, R. Bartolino, P. G. Gucciardi, O. M. Maragò & G. Cipparrone

Chirality is one of the most prominent and intriguing aspects of nature, from spiral galaxies down to aminoacids. Despite the wide range of living and non-living, natural and artificial chiral systems at different scales, the origin of chirality-induced phenomena is often puzzling. Here we assess the onset of chiral optomechanics, exploiting the control of the interaction between chiral entities. We perform an experimental and theoretical investigation of the simultaneous optical trapping and rotation of spherulite-like chiral microparticles. Due to their shell structure (Bragg dielectric resonator), the microparticles function as omnidirectional chiral mirrors yielding highly polarization-dependent optomechanical effects. The coupling of linear and angular momentum, mediated by the optical polarization and the microparticles chiral reflectance, allows for fine tuning of chirality-induced optical forces and torques. This offers tools for optomechanics, optical sorting and sensing and optofluidics.


Tuesday, April 8, 2014

Negative azimuthal force of nanofiber-guided light on a particle

Fam Le Kien and A. Rauschenbeutel

We calculate the force of a quasicircularly polarized guided light field of a nanofiber on a dielectric spherical particle. We show that the orbital parts of the axial and azimuthal components of the Poynting vector are always positive, while the spin parts can be either positive or negative. We find that, for appropriate values of the size parameter of the particle, the azimuthal component of the force is directed oppositely to the circulation direction of the energy flow around the nanofiber. The occurrence of such a negative azimuthal force indicates that the particle undergoes a negative torque.


Applied physics: Optical trapping for space mirrors

David McGloin

Might it be possible to create mirrors for space telescopes, using nothing but microscopic particles held in place by light? A study that exploits a technique called optical binding provides a step towards this goal.


Optimal estimation of diffusion coefficients from single-particle trajectories

Christian L. Vestergaard, Paul C. Blainey, and Henrik Flyvbjerg

How does one optimally determine the diffusion coefficient of a diffusing particle from a single-time-lapse recorded trajectory of the particle? We answer this question with an explicit, unbiased, and practically optimal covariance-based estimator (CVE). This estimator is regression-free and is far superior to commonly used methods based on measured mean squared displacements. In experimentally relevant parameter ranges, it also outperforms the analytically intractable and computationally more demanding maximum likelihood estimator (MLE). For the case of diffusion on a flexible and fluctuating substrate, the CVE is biased by substrate motion. However, given some long time series and a substrate under some tension, an extended MLE can separate particle diffusion on the substrate from substrate motion in the laboratory frame. This provides benchmarks that allow removal of bias caused by substrate fluctuations in CVE. The resulting unbiased CVE is optimal also for short time series on a fluctuating substrate. We have applied our estimators to human 8-oxoguanine DNA glycolase proteins diffusing on flow-stretched DNA, a fluctuating substrate, and found that diffusion coefficients are severely overestimated if substrate fluctuations are not accounted for.


Monday, April 7, 2014

Observation of Brownian Motion in Liquids at Short Times: Instantaneous Velocity and Memory Loss

Simon Kheifets, Akarsh Simha, Kevin Melin, Tongcang Li, Mark G. Raizen

Measurement of the instantaneous velocity of Brownian motion of suspended particles in liquid probes the microscopic foundations of statistical mechanics in soft condensed matter. However, instantaneous velocity has eluded experimental observation for more than a century since Einstein’s prediction of the small length and time scales involved. We report shot-noise–limited, high-bandwidth measurements of Brownian motion of micrometer-sized beads suspended in water and acetone by an optical tweezer. We observe the hydrodynamic instantaneous velocity of Brownian motion in a liquid, which follows a modified energy equipartition theorem that accounts for the kinetic energy of the fluid displaced by the moving bead. We also observe an anticorrelated thermal force, which is conventionally assumed to be uncorrelated.


Discovering the Power of Single Molecules

Robert A. Forties, Michelle D. Wang

Mechanical manipulations of single biological molecules have revealed highly dynamic and mechanical processes at the molecular level. Recent developments have permitted examination of the impact of torque on these processes and visualization of detailed molecular motions, enabling studies of increasingly complex systems. Here we highlight some recent important discoveries.


Optorheological thickening under the pulsed laser photocrosslinking of a polymer

Stephen Richard Okoniewski, Danielle Wisniewski, N. Laszlo Frazer, Weiqiang Mu, Andrew Arceo, Pranjali Rathi and J. B. Ketterson

Electro-, magneto-, and other rheological effects can be used to externally control fluid viscosity. However, they are largely reversible and in addition subject to colloidal settling, electrostatic breakdown, or high cost. In the experiments described here the dependence of the viscosity of a polymer solution under pulsed laser photocrosslinking as a function of radiation dose is determined using the Brownian motion of colloidal polystyrene tracers that were optically confined to a one dimensional channel. The system studied was a transparent aqueous solution of poly(ethylene glycol) dimethacrylate together with a 1-hydroxycyclohexyl phenyl ketone photoinitiator. An increase in the viscosity of the solution with the laser fluence was observed. The growth was exponential, stable between pulses, and spanned nearly three orders of magnitude.


Induced phagocytic particle uptake into a giant unilamellar vesicle

Andreas Meinel, Benjamin Tränkle, Winfried Römer and Alexander Rohrbach

Phagocytosis, the uptake and ingestion of solid particles into living cells, is a central mechanism of our immune system. Due to the complexity of the uptake mechanism, the different forces involved in this process are only partly understood. Therefore the usage of a giant unilamellar vesicle (GUV) as the simplest biomimetic model for a cell allows one to investigate the influence of the lipid membrane on the energetics of the uptake process. Here, a photonic force microscope (PFM) is used to approach an optically trapped 1 μm latex bead to an immobilized GUV to finally insert the particle into the GUV. By analysing the mean displacement and the position fluctuations of the trapped particle during the uptake process in 3D with nanometre precision, we are able to record force and energy profiles, as well as changes in the viscous drag and the stiffness. After observing a global followed by a local deformation of the GUV, we measured uptake energies of 2000 kT to 5500 kT and uptake forces of 4 pN to 16 pN for Egg-PC GUVs with sizes of 18–26 μm and varying membrane tension. The measured energy profiles, which are compared to a Helfrich energy model for local and global deformation, show good coincidence with the theoretical results. Our proof-of-principle study opens the door to a large number of similar experiments with GUVs containing more biochemical components and complexity. This bottom-up strategy should allow for a better understanding of the physics of phagocytosis.


Mechanical analysis of the optical tweezers in time-sharing regime

Lingyao Yu and Yunlong Sheng

Time-sharing optical tweezers is a versatile technique to realize multiple traps for manipulating biological cells and macromolecules. It has been based on an intuitive hypothesis that the trapped viscoelastic object does not “sense” blinking of the optical beam. We present a quantitative analysis using mechanical modeling and numerical simulation, showing that the local stress and strain are jumping all the time and at all locations with the jumping amplitude independent of the recovery time of the viscoelastic material and the jumping frequency. Effects of the stress and strain jumping on the object deformation and the internal energy dissipation are analyzed.


The Kinesin-12 Kif15 is a processive track-switching tetramer

Hauke Drechsler, Toni McHugh, Martin R Singleton, Nicholas J Carter, Andrew D McAinsh

Kinesin-12 motors are a little studied branch of the kinesin superfamily with the human protein (Kif15) implicated in spindle mechanics and chromosome movement. In this study, we reconstitute full-length hKif15 and its microtubule-targeting factor hTpx2 in vitro to gain insight into the motors mode of operation. We reveal that hKif15 is a plus-end-directed processive homotetramer that can step against loads of up to 3.5 pN. We further show that hKif15 is the first kinesin that effectively switches microtubule tracks at intersections, enabling it to navigate microtubule networks, such as the spindle. hKif15 tetramers are also capable of cross-linking microtubules, but unexpectedly, this does not depend on hTpx2. Instead, we find that hTpx2 inhibits hKif15 stepping when microtubule-bound. Our data reveal that hKif15 is a second tetrameric spindle motor in addition to the kinesin-5 Eg5 and provides insight into the mechanisms by which hKif15 and its inhibitor hTpx2 modulate spindle microtubule architecture.


Wednesday, April 2, 2014

Optical binding forces between plasmonic nanocubes: A numerical study based on discrete-dipole approximation

Zhang Xiao-Ming, Xiao Jun-Jun and Zhang Qiang

Plasmonic nanocubes are ideal candidates in realizing controllable reflectance surfaces, unidirectional nanoantennas and other plasmon-associated applications. In this work, we perform full-wave calculations of the optical forces in three-dimensional gold nanocube dimers. For a fixed center-to-center separation, the rotation of the plasmonic nanocube leads to a slight shift of the plasmonic resonance wavelength and a strong change in the optical binding forces. The effective gap and the near field distribution between the two nanocubes are shown to be crucial to this force variation. We further find that the optical binding force is dominated by the scattering process while the optical forces in the wavevector direction are affected by both scattering and absorption, making the former relatively more sensitive to the rotation of (an effective gap between) the nanocubes. Our results would be useful for building all-optically controllable meta-surfaces.


Particles Optical Trapping and Binding in Optofluidic Stable Fabry–Pérot Resonator with Single-Sided Injection

Noha Gaber, Maurine Malak, Frédéric Marty, Dan E. Angelescu, Elodie Richalot, Tarik Bourouina

In this article, micro particles are manipulated inside an optofluidic Fabry–Pérot cylindrical cavity embedding a fluidic capillary tube, taking advantage of field enhancement and multiple reflections within the optically-resonant cavity. This enables trapping of suspended particles with single-side injection of light and with low optical power. A Hermite-Gaussian standing wave is developed inside the cavity, forming trapping spots at the locations of the electromagnetic field maxima with strong intensity gradient. The particles get arranged in a pattern related to the mechanism affecting them: either optical trapping and/or optical binding. This is proven to eventually translate into either an axial one dimensional (1D) particle array or a cluster of particles, respectively. Numerical simulations are performed to model the field distributions inside the cavity allowing a behavioral understanding of the phenomena involved in each case.


Optical trapping and manipulation of plasmonic nanoparticles: fundamentals, applications, and perspectives

Alexander S. Urban, Sol Carretero-Palacios, Andrey A. Lutich, Theobald Lohmüller, Jochen Feldmanna and Frank Jäckel

This feature article discusses the optical trapping and manipulation of plasmonic nanoparticles, an area of current interest with potential applications in nanofabrication, sensing, analytics, biology and medicine. We give an overview over the basic theoretical concepts relating to optical forces, plasmon resonances and plasmonic heating. We discuss fundamental studies of plasmonic particles in optical traps and the temperature profiles around them. We place a particular emphasis on our own work employing optically trapped plasmonic nanoparticles towards nanofabrication, manipulation of biomimetic objects and sensing.