Wednesday, February 29, 2012

Measurement of probe displacement to the thermal resolution limit in photonic force microscopy using a miniature quadrant photodetector

Sambit Bikas Pal, Arijit Haldar, Basudev Roy, and Ayan Banerjee

A photonic force microscope comprises of an optically trapped micro-probe and a position detection system to track the motion of the probe. Signal collection for motion detection is often carried out using the backscattered light off the probe–however, this mode has problems of low S/N due to the small backscattering cross sections of the micro-probes typically used. The position sensors often used in these cases are quadrant photodetectors. To ensure maximum sensitivity of such detectors, it would help if the detector size matched with the detection beam radius after the condenser lens (which for backscattered detection would be the trapping objective itself). To suit this condition, we have used a miniature displacement sensor whose dimensions makes it ideal to work with 1:1 images of micrometer-sized trapped probes in the backscattering detection mode. The detector is based on the quadrant photo-integrated chip in the optical pick-up head of a compact disc player. Using this detector, we measured absolute displacements of an optically trapped 1.1 μm probe with a resolution of ∼10 nm for a bandwidth of 10 Hz at 95% significance without any sample or laser stabilization. We characterized our optical trap for different sized probes by measuring the power spectrum for each probe to 1% accuracy, and found that for 1.1 μm diameter probes, the noise in our position measurement matched the thermal resolution limit for averaging times up to 10 ms. We also achieved a linear response range of around 385 nm with cross talk between axes ≃4% for 1.1 μm diameter probes. The detector has extremely high bandwidth (few MHz) and low optical power threshold–other factors that can lead to its widespread use in photonic force microscopy.


Imaging the cellular response to transient shear stress using stroboscopic digital holography

Maciej Antkowiak, Yoshihiko Arita, Kishan Dholakia, Frank Gunn-Moore

We use stroboscopic quantitative phase microscopy to study cell deformation and the response to cavitation bubbles and transient shear stress resulting from laser-induced breakdown of an optically trapped nanoparticle. A bi-directional transient displacement of cytoplasm is observed during expansion and collapse of the cavitation bubble. In some cases, cell deformation is only observable at the microsecond time scale without any permanent change in cell shape or optical thickness. On a time scale of seconds, the cellular response to shear stress and cytoplasm deformation typically leads to retraction of the cellular edge most exposed to the flow, rounding of the cell body and, in some cases, loss of cellular dry mass. These results give a new insight into the cellular response to cavitation induced shear stress and related plasma membrane permeabilization. This study also demonstrates that laser-induced breakdown of a nanoparticle offers localized cavitation, which interacts with a single cell but without causing cell lysis.


Trapping metallic Rayleigh particles with radial polarization: reply to comment

Qiwen Zhan
This is a reply to the comment by Iglesias and Sáenz directed to a previous paper “Trapping metallic Rayleigh particles with radial polarization,” by Q. Zhan, Opt. Express12, 3377 (2004).


Tuesday, February 28, 2012

Optical Tweezers as a New Biomedical Tool to Measure Zeta Potential of Stored Red Blood Cells

Diego C. N. Silva, Cauêh N. Jovino, Carlos A. L. Silva, Heloise P. Fernandes, Milton M. Filho, Sheyla C. Lucena,Ana Maria D. N. Costa, Carlos L. Cesar, Maria L. Barjas-Castro, Beate S. Santos, Adriana Fontes

During storage, red blood cells (RBCs) for transfusion purposes suffer progressive deterioration. Sialylated glycoproteins of the RBC membrane are responsible for a negatively charged surface which creates a repulsive electrical zeta potential. These charges help prevent the interaction between RBCs and other cells, and especially among each RBCs. Reports in the literature have stated that RBCs sialylated glycoproteins can be sensitive to enzymes released by leukocyte degranulation. Thus, the aim of this study was, by using an optical tweezers as a biomedical tool, to measure the zeta potential in standard RBCs units and in leukocyte reduced RBC units (collected in CPD-SAGM) during storage. Optical tweezers is a sensitive tool that uses light for measuring cell biophysical properties which are important for clinical and research purposes. This is the first study to analyze RBCs membrane charges during storage. In addition, we herein also measured the elasticity of RBCs also collected in CPD-SAGM. In conclusion, the zeta potential decreased 42% and cells were 134% less deformable at the end of storage. The zeta potential from leukodepleted units had a similar profile when compared to units stored without leukoreduction, indicating that leukocyte lyses were not responsible for the zeta potential decay. Flow cytometry measurements of reactive oxygen species suggested that this decay is due to membrane oxidative damages. These results show that measurements of zeta potentials provide new insights about RBCs storage lesion for transfusion purposes.


Normal and system lupus erythematosus red blood cell interactions studied by double trap optical tweezers: direct measurements of aggregation forces

Maria D. Khokhlova, Eugeny V. Lyubin, Alexander G. Zhdanov, Andrey A. Fedyanin, Sophia Yu. Rykova, Irina A. Sokolova

Direct measurements of aggregation forces in piconewton range between two red blood cells in pair rouleau are performed under physiological conditions using double trap optical tweezers. Aggregation and disaggregation properties of healthy and pathologic (system lupus erythematosis) blood samples are analyzed. Strong difference in aggregation speed and behavior is revealed using the offered method which is proposed to be a promising tool for SLE monitoring at single cell level.


Calcium stabilizes the von Willebrand factor A2 domain by promoting refolding

Amy J. Xu and Timothy A. Springer

Von Willebrand factor (VWF) is a large, multimeric plasma glycoprotein that critically mediates hemostasis at sites of vascular injury. Very large VWF multimers have the greatest thrombogenic activity, which is attenuated by cleavage in the A2 domain by the metalloproteinase ADAMTS13. ADAMTS13 proteolysis requires mechanical force to expose the scissile bond and is regulated by a calcium-binding site within A2. In this study, we characterized the interaction between VWF A2 and calcium by examining the effect of calcium on VWF A2 stability and mechanical unfolding and refolding. Isothermal calorimetry yielded a calcium binding Kd = 3.8 ± 1.0 μM and reversible thermal denaturation showed that 5 mM calcium stabilized the unfolding transition from 56.7 ± 0.1 to 69.1 ± 0.1 °C. Using optical tweezers to apply tensile force to single domains, we found that calcium did not affect VWF A2 unfolding, but rather enhanced refolding kinetics fivefold, resulting in a 0.9 kcal/mol stabilization in the folding activation energy in the presence of calcium. Taken together, our data demonstrate that VWF binds calcium at physiologic calcium concentrations and that calcium stabilizes VWF A2 by accelerating refolding. 

Analysis of fluid flow around a beating artificial cilium

Mojca Vilfan, Gašper Kokot, Andrej Vilfan, Natan Osterman, Blaž Kavčič, Igor Poberaj and Dušan Babič 

Biological cilia are found on surfaces of some microorganisms and on surfaces of many eukaryotic cells where they interact with the surrounding fluid. The periodic beating of the cilia is asymmetric, resulting in directed swimming of unicellular organisms or in generation of a fluid flow above a ciliated surface in multicellular ones. Following the biological example, externally driven artificial cilia have recently been successfully implemented as micropumps and mixers. However, biomimetic systems are useful not only in microfluidic applications, but can also serve as model systems for the study of fundamental hydrodynamic phenomena in biological samples. To gain insight into the basic principles governing propulsion and fluid pumping on a micron level, we investigated hydrodynamics around one beating artificial cilium. The cilium was composed of superparamagnetic particles and driven along a tilted cone by a varying external magnetic field. Nonmagnetic tracer particles were used for monitoring the fluid flow generated by the cilium. The average flow velocity in the pumping direction was obtained as a function of different parameters, such as the rotation frequency, the asymmetry of the beat pattern, and the cilium length. We also calculated the velocity field around the beating cilium by using the analytical far-field expansion. The measured average flow velocity and the theoretical prediction show an excellent agreement.


Monday, February 27, 2012

DNA-cisplatin binding mechanism peculiarities studied with single molecule stretching experiments

F. A. P. Crisafuli, E. C. Cesconetto, E. B. Ramos, and M. S. Rocha

We propose a method to determine the DNA-cisplatin binding mechanism peculiarities by monitoring the mechanical properties of these complexes. To accomplish this task, we have performed single molecule stretching experiments by using optical tweezers, from which the persistence and contour lengths of the complexes can be promptly measured. The persistence length of the complexes as a function of the drug total concentration in the sample was used to deduce the binding data, from which we show that cisplatin binds cooperatively to the DNA molecule, a point which so far has not been stressed in binding equilibrium studies of this ligand.


The molten globule state is unusually deformable under mechanical force

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

Recently, the role of force in cellular processes has become more evident, and now with advances in force spectroscopy, the response of proteins to force can be directly studied. Such studies have found that native proteins are brittle, and thus not very deformable. Here, we examine the mechanical properties of a class of intermediates referred to as the molten globule state. Using optical trap force spectroscopy, we investigated the response to force of the native and molten globule states of apomyoglobin along different pulling axes. Unlike natively folded proteins, the molten globule state of apomyoglobin is compliant (large distance to the transition state); this large compliance means that the molten globule is more deformable and the unfolding rate is more sensitive to force (the application of force or tension will have a more dramatic effect on the unfolding rate). Our studies suggest that these are general properties of molten globules and could have important implications for mechanical processes in the cell.


S. pombe Kinesins-8 Promote Both Nucleation and Catastrophe of Microtubules

Muriel Erent, Douglas R. Drummond, Robert A. Cross

The kinesins-8 were originally thought to be microtubule depolymerases, but are now emerging as more versatile catalysts of microtubule dynamics. We show here that S. pombeKlp5-436 and Klp6-440 are non-processive plus-end-directed motors whose in vitro velocities on S. pombe microtubules at 7 and 23 nm s−1 are too slow to keep pace with the growing tips of dynamic interphase microtubules in living S. pombe. In vitro, Klp5 and 6 dimers exhibit a hitherto-undescribed combination of strong enhancement of microtubule nucleation with no effect on growth rate or catastrophe frequency. By contrast in vivo, both Klp5 and Klp6 promote microtubule catastrophe at cell ends whilst Klp6 also increases the number of interphase microtubule arrays (IMAs). Our data support a model in which Klp5/6 bind tightly to free tubulin heterodimers, strongly promoting the nucleation of new microtubules, and then continue to land as a tubulin-motor complex on the tips of growing microtubules, with the motors then dissociating after a few seconds residence on the lattice. In vivo, we predict that only at cell ends, when growing microtubule tips become lodged and their growth slows down, will Klp5/6 motor activity succeed in tracking growing microtubule tips. This mechanism would allow Klp5/6 to detect the arrival of microtubule tips at cells ends and to amplify the intrinsic tendency for microtubules to catastrophise in compression at cell ends. Our evidence identifies Klp5 and 6 as spatial regulators of microtubule dynamics that enhance both microtubule nucleation at the cell centre and microtubule catastrophe at the cell ends.


Friday, February 24, 2012

Light and Surface Plasma Wave Induced Force on Nanoparticles and Nanotubes

Santosh Jain, J. Parashar and Rajnish Kurchania

Expressions for force on spherical nanoparticles and cylindrical nanotubes due to laser and surface plasma wave are obtained. It shows Expressions for force on spherical nanoparticles and cylindrical nanotubes due to laser and surface plasma wave are obtained. It shows resonant enhancement at ω=ω p/√3 for the nanoparticle and ω=ω p/√2 for the nanotube. At frequencies lower than this resonance the ponderomotive force is along the intensity gradient of the laser while antiparallel for frequencies higher than the resonant frequency.


Observation of microsphere movement driven by optical pulse: comment

Tomaž Požar and Janez Možina

We show that fiber-delivered, pulsed laser propulsion of glass microspheres, as observed in a recent Letter [Opt. Lett.36, 1996 (2011).], due to only radiation pressure cannot explain the measured maximum velocity of microspheres. Our considerations, based on the reported results, indicate that the main momentum transfer mechanism is due to mass recoil that very likely follows a dielectric breakdown near or on the surface of the microsphere.


Characterizing the rotation of non symmetric objects in an optical tweezer

Yogesha Sarbari Bhattacharya, Sharath Ananthamurthy

We present an optical tweezer based study of the rotation of microscopic objects with shape asymmetry. Thermal fluctuations and rotations are simultaneously monitored through laser back scattering. The rotation causes a modulation in intensity of the back scattered light incident on a quadrant photo detector. The resulting power spectrum is a modified Lorentzian with additional peaks located at the fundamental rotational frequency of the object and at the integer harmonics. The manifestation of these peaks reveals that the rotations are periodic but with varying angular velocity. We model our experimental results to illustrate the hydrodynamic interplay between the rotor and the surrounding medium that results in the time dependence of the angular speed of the former. Further, we demonstrate the use of video microscopy for characterization of low reflectivity rotors, such as biological cells. We propose through these studies that an analysis of these rotations can provide insights into the role of hydrodynamics at micrometer levels.


Wednesday, February 22, 2012

Physical ageing of the contact line on colloidal particles at liquid interfaces

David M. Kaz, Ryan McGorty, Madhav Mani, Michael P. Brenner & Vinothan N. Manoharan

Young’s law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface.


Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces

Thomas Bohlein, Jules Mikhael & Clemens Bechinger

Friction between solids is responsible for many phenomena such as earthquakes, wear or crack propagation. Unlike macroscopic objects, which only touch locally owing to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel–Kontorova model, which considers a monolayer of interacting particles on a periodic substrate potential. In addition to the well-known stick–slip motion, such models also predict the formation of kinks and antikinks, which greatly reduce the friction between the monolayer and the substrate. Here, we report the direct observation of kinks and antikinks in a two-dimensional colloidal crystal that is driven across different types of ordered substrate. We show that the frictional properties only depend on the number and density of such excitations, which propagate through the monolayer along the direction of the applied force. In addition, we also observe kinks on quasicrystalline surfaces, which demonstrates that they are not limited to periodic substrates but occur under more general conditions.


Colloidal friction: Kinks in motion

Andrea Vanossi & Erio Tosatti

The ability of laser interference potentials to trap and control colloidal particles opens up a new potential area of 'toy systems' displaying real physics. A beautiful example is the study of friction between colloidal crystals and a variety of artificially created surface potentials.


Intramolecular folding in three tandem guanine repeats of human telomeric DNA

Deepak Koirala , Tomoko Mashimo , Yuta Sannohe , Zhongbo Yu , Hanbin Mao and Hiroshi Sugiyama

Intramolecular folding in three tandem guanine repeats of human telomeric DNA has been investigated using optical-tweezers, MD simulation and circular dichroism. A mechanically and thermodynamically stable species in this sequence shows a structure consistent with a triplex conformation. A similar species has also been observed to coexist with a G-quadruplex in a DNA sequence with four tandem guanine repeats.


Tuesday, February 21, 2012

Out-of-equilibrium forces between colloids

Indira Sriram and Eric M. Furst

Two colloidal probe particles are held with optical traps orthogonal to a uniformly flowing suspension of colloidal bath particles. Using confocal microscopy, the local bath suspension microstructure is characterized as a function of the probe separation and flow velocity. At sufficiently close separations, bath particles are excluded from passing between the probes, resulting in an asymmetric, non-equilibrium microstructure in which the major features are a depleted region between the probes and dense boundary layers along the surfaces that face away from the neighboring probe. As a consequence, the drag force acting on the probes is lower than that acting on a single probe and a net force pushes the probes together along their line of centers. The strength of the latter mutual force increases with increasing flow velocity. These experiments demonstrate that depletion-like forces can be induced between two particles by a non-equilibrium microstructure in a strongly driven suspension.


Photophoretic trapping of absorbing particles in air and measurement of their single-particle Raman spectra

Yong-Le Pan, Steven C. Hill, and Mark Coleman

A new method is demonstrated for optically trapping micron-sized absorbing particles in air and obtaining their single-particle Raman spectra. A 488-nm Gaussian beam from an Argon ion laser is transformed by conical lenses (axicons) and other optics into two counter-propagating hollow beams, which are then focused tightly to form hollow conical beams near the trapping region. The combination of the two coaxial conical beams, with focal points shifted relative to each other along the axis of the beams, generates a low-light-intensity biconical region totally enclosed by the high-intensity light at the surface of the bicone, which is a type of bottle beam. Particles within this region are trapped by the photophoretic forces that push particles toward the low-intensity center of this region. Raman spectra from individual trapped particles made from carbon nanotubes are measured. This trapping technique could lead to the development of an on-line real-time single-particle Raman spectrometer for characterization of absorbing aerosol particles.


Optical trap kits: issues to be aware of

I Alexeev, U Quentin, K-H Leitz and M Schmidt

An inexpensive and robust optical trap system can be built from off-the-shelf optical and opto-mechanical components or acquired as a kit to be assembled in a laboratory. The primary advantages of such a trap, besides being significantly more affordable, are its flexibility, and ease of modification and upgrade. In this paper, we consider several important issues to be addressed during development and application of a kit system. We numerically examine the performance of trapping systems equipped with oil and water immersion focusing objectives. We also investigate the effect of trapping laser beam quality on optical tweezing.


Tuesday, February 14, 2012

Improved direct binary search-based algorithm for generating holograms for the application of holographic optical tweezers

XuDong Zhao, Jing Li, Tao Tao, Qian Long, and Xiaoping Wu

This paper presents an improved direct binary search (DBS)-based algorithm for generating holograms to holographic optical tweezers. The simulations show that the improved algorithm greatly enhances computation speed while maintaining high hologram efficiency and high-intensity homogeneous target spots. The improved algorithm was applied to generate holographic optical tweezers in several experiments. The experiments demonstrate that real-time trap and manipulation can be realized with the improved algorithm if the number of trapped microparticles is small.


Force spectroscopy reveals the DNA structural dynamics that govern the slow binding of Actinomycin D

Thayaparan Paramanathan, Ioana Vladescu, Micah J. McCauley, Ioulia Rouzina and Mark C. Williams

Actinomycin D (ActD) is a small molecule with strong antibiotic and anticancer activity. However, its biologically relevant DNA-binding mechanism has never been resolved, with some studies suggesting that the primary binding mode is intercalation, and others suggesting that single-stranded DNA binding is most important. To resolve this controversy, we develop a method to quantify ActD’s equilibrium and kinetic DNA-binding properties as a function of stretching force applied to a single DNA molecule. We find that destabilization of double stranded DNA (dsDNA) by force exponentially facilitates the extremely slow ActD-dsDNA on and off rates, with a much stronger effect on association, resulting in overall enhancement of equilibrium ActD binding. While we find the preferred ActD–DNA-binding mode to be to two DNA strands, major duplex deformations appear to be a pre-requisite for ActD binding. These results provide quantitative support for a model in which the biologically active mode of ActD binding is to pre-melted dsDNA, as found in transcription bubbles. DNA in transcriptionally hyperactive cancer cells will therefore likely efficiently and rapidly bind low ActD concentrations (∼10 nM), essentially locking ActD within dsDNA due to its slow dissociation, blocking RNA synthesis and leading to cell death.


Saturday, February 11, 2012

Optical trapping and polarization-controlled scattering of dielectric spherical nanoparticles by femtosecond laser pulses

Anwar Usman, Wei-Yi Chiang, Hiroshi Masuhara

We present optical trapping behavior of 50-nm-sized polystyrene beads, suspended in water medium, by femtosecond pulsed laser beam. In addition to a higher number of nanoparticles trapped at the focal spot by the ultrashort laser pulses compared with that by continuous-wave laser, the nanoparticles are scattered out of the focal spot by the laser pulses to the surrounding area. The scattered particles form a partially opened folding fan-shaped bright locus in two opposite directions, in an alternating manner, perpendicular to the laser polarization. To understand those phenomena, we analyzed radiation (gradient and scattering) force of femtosecond laser pulses and their temporal force exerted on the dielectric spherical nanoparticles by taking into account the impulsive peak power and the axial component of electric light field produced by high numerical aperture of objective lens. We show that the axial electric field is responsible for lateral components of the scattering and temporal forces, and hence, controls the scattering directions of the Rayleigh particles. These findings provide important information about the dynamic optical trapping of the Rayleigh particles by highly focused ultrashort laser pulses.


Friday, February 10, 2012

Fourier optics along a hybrid optical fiber for Bessel-like beam generation and its applications in multiple-particle trapping

Jongki Kim, Yoonseob Jeong, Sejin Lee, Woosung Ha, Jeon-Soo Shin, and Kyunghwan Oh

Highly efficient Bessel-like beam generation was achieved based on a new all-fiber method that implements Fourier transformation of a micro annular aperture along a concatenated composite optical fiber. The beam showed unique characteristics of tilted washboard optical potential in the transverse plane and sustained a nondiffracting length over 400 μm along the axial direction. Optical trapping of multiple dielectric particles and living Jurkat cells were successfully demonstrated along the axial direction of the beam in the water.


Optical trapping in an absorbing medium: from optical tweezing to thermal tweezing

Poonam Kumari, J. A. Dharmadhikari, A. K. Dharmadhikari, H. Basu, S. Sharma, and D. Mathur

We report on optical trapping in a weakly absorbing medium, hemin, an iron-containing porphyrin that is an important component of hemoglobin. By altering the hemin concentration we are able to control the amount of optical energy that is absorbed; changing the hemin concentration from <12 mg/ml to >45 mg/ml enables the onset of thermal trapping to be observed. By estimating the trap strength using two different methods we are readily able to differentiate between the optical trapping and thermal trapping regimes. We also deduce the rise in temperature that occurs within the laser focal volume: temperature changes of 5-24 K are observed for laser power values of 10-90 mW for hemin concentrations of 0-50 mg/ml.

Thursday, February 9, 2012

Harmonic oscillator in heat bath: Exact simulation of time-lapse-recorded data and exact analytical benchmark statistics

Simon F. Nørrelykke, Henrik Flyvbjerg

The stochastic dynamics of the damped harmonic oscillator in a heat bath is simulated with an algorithm that is exact for time steps of arbitrary size. Exact analytical results are given for correlation functions and power spectra in the form they acquire when computed from experimental time-lapse recordings. Three applications are discussed: (i) The effects of finite sampling rate and time, described exactly here, are similar for other stochastic dynamical systems—e.g., motile microorganisms and their time-lapse-recorded trajectories. (ii) The same statistics is satisfied by any experimental system to the extent that it is interpreted as a damped harmonic oscillator at finite temperature—such as an AFM cantilever. (iii) Three other models of fundamental interest are limiting cases of the damped harmonic oscillator at finite temperature; it consequently bridges their differences and describes the effects of finite sampling rate and sampling time for these models as well.


Impairment of the biomechanical compliance of P pili: a novel means of inhibiting uropathogenic bacterial infections?

Jeanna E. Klinth, Jerome S. Pinkner, Scott J. Hultgren, Fredrik Almqvist, Bernt Eric Uhlin and Ove Axner

Gram-negative bacteria often initiate their colonization by use of extended attachment organelles, so called pili. When exposed to force, the rod of helix-like pili has been found to be highly extendable, mainly attributed to uncoiling and recoiling of its quaternary structure. This provides the bacteria with the ability to redistribute an external force among a multitude of pili, which enables them to withstand strong rinsing flows, which, in turn, facilitates adherence and colonization processes critical to virulence. Thus, pili fibers are possible targets for novel antibacterial agents. By use of a substance that compromises compliance of the pili, the ability of bacteria to redistribute external forces can be impaired, so they will no longer be able to resist strong urine flow and thus be removed from the host. It is possible such a substance can serve as an alternative to existing antibiotics in the future or be a part of a multi-drug. In this work we investigated whether it is possible to achieve this by targeting the recoiling process. The test substance was purified PapD. The effect of PapD on the compliance of P pili was assessed at the single organelle level by use of force-measuring optical tweezers. We showed that the recoiling process, and thus the biomechanical compliance, in particular the recoiling process, can be impaired by the presence of PapD. This leads to a new concept in the search for novel drug candidates combating uropathogenic bacterial infections—“coilicides”, targeting the subunits of which the pilus rod is composed.


Image-based algorithm for analysis of transient trapping in single-particle trajectories

Daphne Weihs, Dror Gilad, Moti Seon and Itai Cohen

Particle tracking has become an increasingly useful tool in microfluidics and biophysics, allowing measurement of microrheology, local structure, and flow. We introduce a novel, automated approach to analyze single-particle trajectories with transient elements, based on image-processing approaches and physical analysis of probe motion. In many physical and active biological systems, such as living cells, probe particles experience thermally mediated Brownian motion combined with active transport processes that can lead to transient-trajectories of local diffusion and trapping, punctuated by segments of active transport. Analyzing such a trajectory as a single unit masks the intermittent nature of the motion. Moreover, directly applying the generalized Stokes–Einstein relation in out-of-equilibrium systems is incorrect and returns inaccurate rheological parameters. We present an automated image-processing-based method to identify and segment transient trap-escape trajectories, allowing quantitative analysis of each segment. We define and discuss effects of controlling parameters, such as particle size and camera frame rate. Our algorithm provides a general and automated method to segment and analyze transient elements in trajectories of single particles, which can be applied to many different experiments. Our image-based approach allows identification of trapping segments, unbiased by specific step sizes within those traps or the mechanism driving those steps. As an example, we successfully apply this method to experiments of laser tweezers trapped particles and show that trajectory segmentation allows us to calculate both trap and fluid parameters. We accurately identify a round trap, calculate the trap stiffness at 3.1 pN/μm, and find that significant local heating reduces fluid viscosity.


Tuesday, February 7, 2012

Precise switching control of liquid crystalline microgears driven by circularly polarized light

Kiminori Ito, Hiroshi Frusawa, and Masahiro Kimura

Liquid crystalline molecules carrying photopolymerizable end groups absorb photon energy via a two-photon process, enabling the photofabrication of 3D structures. In this work, we prepared microgears with different heights and tooth lengths. These birefringent microgears can be induced to rotate by circularly polarized light. Here, we demonstrate that the use of phase plate for switching between left- and right-handed polarization reverses the optically induced rotation while maintaining the same rotational frequency. Due to the precise switching control, these birefringent microgears have advantages over previous microrotors that are fabricated from non-birefringent light-curing resins.


Saturday, February 4, 2012

Shape-Dependent Oriented Trapping and Scaffolding of Plasmonic Nanoparticles by Topological Defects for Self-Assembly of Colloidal Dimers in Liquid Crystals

Bohdan Senyuk, Julian S. Evans, Paul J. Ackerman, Taewoo Lee, Pramit Manna§, Leonid Vigderman, Eugene R. Zubarev, Jao van de Lagemaa, and Ivan I. Smalyukh
We demonstrate scaffolding of plasmonic nanoparticles by topological defects induced by colloidal microspheres to match their surface boundary conditions with a uniform far-field alignment in a liquid crystal host. Displacing energetically costly liquid crystal regions of reduced order, anisotropic nanoparticles with concave or convex shapes not only stably localize in defects but also self-orient with respect to the microsphere surface. Using laser tweezers, we manipulate the ensuing nanoparticle-microsphere colloidal dimers, probing the strength of elastic binding and demonstrating self-assembly of hierarchical colloidal superstructures such as chains and arrays.


Oriented imaging of 3D subcellular structures in bacterial cells using optical tweezers

G. Carmon, I. Fishov, and M. Feingold
Using oscillating optical tweezers, we show that controlled alignment of rod-shaped bacterial cells allows imaging fluorescently labeled three-dimensional (3D) subcellular structures from different, optimized viewpoints. To illustrate our method, we analyze the Z ring of E. coli. We obtain that the radial width of the Z ring in unconstricted cells is about 120 nm. This result suggests that the Z ring consists of an extremely sparse network of FtsZ filaments.


Controlled Photonic Manipulation of Proteins and Other Nanomaterials

Yih-Fan Chen, Xavier Serey, Rupa Sarkar, Peng Chen, and David Erickson
The ability to controllably handle the smallest materials is a fundamental enabling technology for nanoscience. Conventional optical tweezers have proven useful for manipulating microscale objects but cannot exert enough force to manipulate dielectric materials smaller than about 100 nm. Recently, several near-field optical trapping techniques have been developed that can provide higher trapping stiffness, but they tend to be limited in their ability to reversibly trap and release smaller materials due to a combination of the extremely high electromagnetic fields and the resulting local temperature rise. Here, we have developed a new form of photonic crystal “nanotweezer” that can trap and release on-command Wilson disease proteins, quantum dots, and 22 nm polymer particles with a temperature rise less than 0.3 K, which is below the point where unwanted fluid mechanical effects will prevent trapping or damage biological targets.


Friday, February 3, 2012

Speed enhancement of multi-particle chain in a traveling standing wave

Martin Šiler, Tomáš Čižmár, and Pavel Zemánek

A moving array of optical traps created by interference of two counter-propagating evanescent waves has been used for delivery of particle chains up to 18 micro-particles long immersed in water. The particles were optically self-arranged into a linear chain with well-separated distances between them. We observed a significant increase in the delivery speed of the whole structure as the number of particles in the chain increased. This could provide faster sample delivery in microfluidic systems. We quantified the contributions to the speed enhancement caused by the optical and hydrodynamic interactions between the particles.


Multiscale Modeling of Red Blood Cell Mechanics and Blood Flow in Malaria

Dmitry A. Fedosov, Huan Lei, Bruce Caswell, Subra Suresh, George E. Karniadakis

Red blood cells (RBCs) infected by a Plasmodium parasite in malaria may lose their membrane deformability with a relative membrane stiffening more than ten-fold in comparison with healthy RBCs leading to potential capillary occlusions. Moreover, infected RBCs are able to adhere to other healthy and parasitized cells and to the vascular endothelium resulting in a substantial disruption of normal blood circulation. In the present work, we simulate infected RBCs in malaria using a multiscale RBC model based on the dissipative particle dynamics method, coupling scales at the sub-cellular level with scales at the vessel size. Our objective is to conduct a full validation of the RBC model with a diverse set of experimental data, including temperature dependence, and to identify the limitations of this purely mechanistic model. The simulated elastic deformations of parasitized RBCs match those obtained in optical-tweezers experiments for different stages of intra-erythrocytic parasite development. The rheological properties of RBCs in malaria are compared with those obtained by optical magnetic twisting cytometry and by monitoring membrane fluctuations at room, physiological, and febrile temperatures. We also study the dynamics of infected RBCs in Poiseuille flow in comparison with healthy cells and present validated bulk viscosity predictions of malaria-infected blood for a wide range of parasitemia levels (percentage of infected RBCs with respect to the total number of cells in a unit volume).


Theory for collective dynamics and optical response of metallic nanoparticles under light-induced force and fluctuations

Takuya Iida

Due to the strong optical response of localized surface plasmon (LSP) in metallic nanoparticles (NPs), the light-induced force (LIF) is also strong and can be used for the control of their dynamics even at room temperature. However, properties of LIF are still unclear under the collective effects of LSP in multiple NPs. In this article, I discuss the fundamental properties of LIF exerted on metallic NPs taking into account photomediated interaction between NPs, and light-induced dynamics of NPs in fluid medium (for example, water) in the presence of the thermal fluctuations. Remarkably, it has been clarified that the collective optical response of LSP can be greatly modulated through the dynamical pattern formation process of NPs by LIF.


Probing of Ehrlich ascites carcinoma cell using in situ aggregates of Au-NPs as SERS label created by plasmon exciting hybrid-TEM*11 laser mode

R. Kumar, D. S. Mehta, S. Saraswati and C. Shakher

Apart from commonly employed target-specific labeling/adsorption of antibodies over Au-NPs surface for the creation of localized aggregates, an alternative approach using optical tweezers (OT) driven byhybrid-TEM*11 mode has been devised and exploited for in vitro detection of Ehrlich ascites carcinoma cells (EAC) relying on enhanced scattering. Intra-cavity generated spatially featured asymmetric (SFA) laser beam (λ = 532 nm) has effected simultaneous trapping of mice-EAC cells and in-situ crowd/assembly of incubated Au-NPs/small gold nano-aggregates (created from two or more individual Au-NPs). Relatively larger focus spot created by tightly focused SFA beam than frequently employed Gaussian-mode in OT has offered an extended working area and hence dilute heating has taken care of EAC cells. GNA improves significantly the sensitivity of diagnostics relying on scattered light and the safety and efficacy of therapeutic nanotechnologies for the diseases of cancer and vascular system in medicine.


Thursday, February 2, 2012

Orthonormal basis for nonparaxial focused fields in two dimensions, and its application to modeling scattering and optical manipulation of objects

Krista Lombardo and Miguel A. Alonso
A new family of rigorous solutions to the two-dimensional Helmholtz equation is discussed. These solutions form a complete orthonormal basis in terms of which any focused field can be expressed. The directionality of each element of this basis is regulated by a parameter which is related to an imaginary displacement of the focus. The forces and torques that these fields and any linear combination of them exert on a homogeneous circular scatterer can be calculated analytically, regardless of the relative position between the scatterer and the focus.


Linear Discriminant Analysis of Single-Cell Fluorescence Excitation Spectra of Five Phytoplankton Species

Bruckman, Laura S.; Richardson, Tammi L.; Swanstrom, Joseph A.; Donaldson, Kathleen A.; Allora, Michael, Jr.; Shaw, Timothy J.; Myrick, Michael L.

Linear discriminant analysis (LDA) of single-cell fluorescence excitation spectra (lambda(em) = 680 nm) for five species of marine phytoplankton was used to determine whether intra-species variation among single cells precluded discrimination among species. Single-cell spectra were recorded in an optical trap with a custom-built spectral fluorometer. For nitrogen (N)-replete cells, separation of all five species (Emiliania huxleyi, a coccolithophore, Thalassiosira pseudonana, a diatom, Dunaliella tertiolecta, a chlorophyte, Amphidinium carterae, a dinoflagellate, and Rhodomonas sauna, a cryptophyte) was possible using only a portion of the excitation spectra (570-610 nm). This wavelength region gave perfect classification of species with a minimum Fisher ratio of 62. For four species (E. huxleyi, T. pseudonana, D. tertiolecta, and A. carterae), variations in fluorescence excitation spectra as cells were starved of N did not impact the classification process adversely within the chosen spectral window. R. sauna cells grown with and without N showed significant differences in their fluorescence excitation spectra but could still be classified if a different spectral window (490-570 nm) was used. Overall, we conclude that intra-species variation among single-cell fluorescence excitation spectra does not preclude discrimination among species.


Calibration of the optical torque wrench

Francesco Pedaci, Zhuangxiong Huang, Maarten van Oene, and Nynke H. Dekker

The optical torque wrench is a laser trapping technique that expands the capability of standard optical tweezers to torque manipulation and measurement, using the laser linear polarization to orient tailored microscopic birefringent particles. The ability to measure torque of the order of kBT (∼4 pN nm) is especially important in the study of biophysical systems at the molecular and cellular level. Quantitative torque measurements rely on an accurate calibration of the instrument. Here we describe and implement a set of calibration approaches for the optical torque wrench, including methods that have direct analogs in linear optical tweezers as well as introducing others that are specifically developed for the angular variables. We compare the different methods, analyze their differences, and make recommendations regarding their implementations.


Wednesday, February 1, 2012

An integrated hollow-core photonic crystal fiber transverse optical trapping system for optical manipulation and detection

V. K. Shinoj and V. M. Murukeshan

Optical manipulation, separation, and detection of biological cells have immense potential biomedical applications, for example, in disease detection. In this paper, we present optical manipulation and detection of micron sized fluorescent particles inside hollow-core photonic crystal fiber (HC-PCF) by transverse optical trapping. An optical trapping system is designed where a near-infrared laser light is focused using a microscope objective to create an optical trap across a liquid-filled HC-PCF. The fluorescent microsphere particles trapped in the core of HC-PCF using the laser induced optical force further undergo imaging and fluorescence spectroscopic analysis. It is illustrated that the proposed method can track the particle into a different medium using the optical trap as well. The obtained results indicate that this proposed method has tangible potential for developing HC-PCF based lab-on-a-chip bio/chemical sensors capable of detecting reagents in ultra low sample volumes.


Orbital rotation without orbital angular momentum: mechanical action of the spin part of the internal energy flow in light beams

O. V. Angelsky, A. Ya. Bekshaev, P. P. Maksimyak, A. P. Maksimyak, S. G. Hanson, and C. Yu. Zenkova

The internal energy flow in a light beam can be divided into the “orbital” and “spin” parts, associated with the spatial and polarization degrees of freedom of light. In contrast to the orbital one, experimental observation of the spin flow seems problematic because it is converted into an orbital flow upon tight focusing of the beam, usually applied for energy flow detection by means of the mechanical action upon probe particles. We propose a two-beam interference technique that results in an appreciable level of spin flow in moderately focused beams and detection of the orbital motion of probe particles within a field where the transverse energy circulation is associated exclusively with the spin flow. This result can be treated as the first demonstration of mechanical action of the spin flow of a light field.


Multi-beam bilateral teleoperation of holographic optical tweezers

Kazuhisa Onda and Fumihito Arai

A multi-beam bilateral teleoperation system of holographic optical tweezers accelerated by a graphics processing unit is proposed and evaluated. This double-arm teleoperation system is composed of two haptic devices and two laser-trapped micro-beads. Each micro-bead is trapped and moved following the trajectory of each haptic device, and the forces to which the micro-beads are subjected, which are generated by Stokes drag, are measured and fed back to an operator via the haptic devices. This real-time telexistence was quantitatively evaluated based on the time response of the trapped beads and the fed-back forces. And the demonstration of touching red blood cells shows the effectiveness of this system for biomedical application.


An integrated microparticle sorting system based on near-field optical forces and a structural perturbation

Shiyun Lin and Kenneth B. Crozier

We demonstrate an integrated microparticle passive sorting system based on the near-field optical forces exerted by a 3-dB optical splitter that consists of a slot waveguide and a conventional channel waveguide. We show that 320 nm and 2 µm polystyrene particles brought into the splitter are sorted so that they exit along the slot waveguide and channel waveguide, respectively. Electromagnetic simulations and precise position tracking experiments are carried out to investigate the sorting mechanism. As the waveguides are separated by 200 nm, they provide two potential wells for the smaller particles, but only one broad potential well for the larger particles, since their diameters exceed the distance between the two field maxima. A structural perturbation consisting of a stuck bead transfers the smaller particles to the second well associated with the slot waveguide, while the larger particles are brought to the region between the two waveguides and eventually follow the channel waveguide, as it is associated with a deeper potential well. This label-free passive particle sorting system requires low guided power (20 mW in these experiments), and provides a new technique for sorting sub-micron particles.