Friday, February 26, 2016

Repulsion of polarised particles from anisotropic materials with a near-zero permittivity component

Francisco J Rodríguez-Fortuño and Anatoly V Zayats
Reduction of adhesion and stiction is crucial for robust operation on nanomechanical and optofluidic devices as well as atom and molecule behaviour near surfaces. It can be achieved using electric charging, magnetic materials or light pressure and optical trapping. Here we show that a particle scattering or emitting in close proximity to an anisotropic substrate can experience a repulsive force if one of the diagonal components of the permittivity tensor is close to zero. We derive an analytic condition for the existence of such repulsive force depending on the optical properties of the substrate. We also demonstrate the effect using realistic anisotropic metamaterial implementations of a substrate. The anisotropic metamaterial approach using metal–dielectric and graphene–dielectric multilayers provides a tuneable spectral range and a very broad bandwidth of electromagnetic repulsion forces, in contrast to isotropic substrates.


Optically trapping tumor cells to assess differentiation and prognosis of cancers

M. Pradhan, S. Pathak, D. Mathur, and U. Ladiwala

We report an optical trapping method that may enable assessment of the differentiation status of cancerous cells by determining the minimum time required for cell-cell adhesion to occur. A single, live cell is trapped and brought into close proximity of another; the minimum contact time required for cell-cell adhesion to occur is measured using transformed cells from neural tumor cell lines: the human neuroblastoma SK-N-SH and rat C6 glioma cells. Earlier work on live adult rat hippocampal neural progenitors/stem cells had shown that a contact minimum of ~5 s was required for cells to adhere to each other. We now find the average minimum time for adhesion of cells from both tumor cell lines to substantially increase to ~20-25 s, in some cases up to 45 s. Upon in vitro differentiation of these cells with all-trans retinoic acid the average minimum time reverts to ~5-7 s. This proof-of-concept study indicates that optical trapping may be a quick, sensitive, and specific method for determining differentiation status and, thereby, the prognosis of cancer cells.


Thursday, February 25, 2016

Light manipulation of nanoparticles in arrays of topological defects

D. Kasyanyuk, P. Pagliusi, A. Mazzulla, V. Reshetnyak, Yu. Reznikov, C. Provenzano, M. Giocondo, M. Vasnetsov, O. Yaroshchuk & G. Cipparrone

We report a strategy to assemble and manipulate nanoparticles arrays. The approach is based on the use of topological defects, namely disclination lines, created in chiral liquid crystals. The control of nanoparticle-loaded topological defects by low power light is demonstrated. Large-scale rotation, translation and deformation of quantum dots light-emitting chains is achieved by homogeneous LED illumination. Full reconfigurability and time stability make this approach attractive for future developments and applications.


The Influence of Lateral Forces on the Cell Stiffness Measurement by Optical Tweezers Vertical Indentation

Fatou Ndoye, Muhammad Sulaiman Yousafzai, Giovanna Coceano, Serena Bonin, Giacinto Scoles, Oumar Ka, Joseph Niemela & Dan Cojoc

We studied the lateral forces arising during the vertical indentation of the cell membrane by an optically trapped microbead, using back focal plane interferometry to determine force components in all directions. We analyzed the cell-microbead interaction and showed that indeed the force had also lateral components. Using the Hertz model, we calculated and compared the elastic moduli resulting from the total and vertical forces, showing that the differences are important and the total force should be considered. To confirm our results we analyzed cells from two breast cancer cell lines: MDA-MB-231 and HBL-100, known to have different cancer aggressiveness and hence stiffness. 

Wednesday, February 24, 2016

Detection of Brownian Torque in a Magnetically-Driven Rotating Microsystem

Maria N. Romodina, Evgeny V. Lyubin & Andrey A. Fedyanin

Thermal fluctuations significantly affect the behavior of microscale systems rotating in shear flow, such as microvortexes, microbubbles, rotating micromotors, microactuators and other elements of lab-on-a-chip devices. The influence of Brownian torque on the motion of individual magnetic microparticles in a rotating magnetic field is experimentally determined using optical tweezers. Rotational Brownian motion induces the flattening of the breakdown transition between the synchronous and asynchronous modes of microparticle rotation. The experimental findings regarding microparticle rotation in the presence of Brownian torque are compared with the results of numerical Brownian dynamics simulations.


Enantioselective optical trapping of chiral nanoparticles with plasmonic tweezers

Yang Zhao, Amr A. E. Saleh, and Jennifer A. Dionne

Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength silica channel embedded in silver and can stably trap sub-20-nm dielectric nanoparticles. Using both full-field simulations and analytic calculations, we show that selective trapping of enantiomers can be achieved with circularly polarized illumination. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer-aperture separations. Our theory indicates that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures, providing a possible route toward all-optical enantiopure syntheses.


Direct measurement of the temperature profile close to an optically trapped absorbing particle

Martin Šiler, Jan Ježek, Petr Jákl, Zdeněk Pilát, and Pavel Zemánek

The surface temperature of an absorbing particle trapped in optical tweezers (OTs) is measured using a mixture of two fluorescent dyes. We analyze the dependence of temperature on both laser power and the radial distance from its surface, and we verify the 1/𝑟 decrease of temperature with increasing distance from the particle surface. We detect the variations of spectral profiles as the medium temperature changes. The temperature dependent signal, i.e., the ratio of summed intensities from two distinct spectral regions, is affected by the convolution of temperature profile with transfer function of the spectroscopic system. We analyze this effect and determine the temperature increase on the surface of a core-shell particle trapped by OTs.


Tuesday, February 23, 2016

Characterization at the individual cell level and in whole blood samples of shear stress preventing red blood cells aggregation

K. Lee, M. Kinnunen, A.V. Danilina, V.D. Ustinov, S. Shin, I. Meglinski, A.V. Priezzhev

The aggregation of red blood cells (RBC) is an intrinsic feature of blood that has a strong impact on its microcirculation. For a number of years it has been attracting a great attention in basic research and clinical studies. Here, we study a relationship between the RBC aggregation parameters measured at the individual cell level and in a whole blood sample. The home made optical tweezers were used to measure the aggregating and disaggregating forces for a pair of interacting RBCs, at the individual cell level, in order to evaluate the corresponding shear stresses. The RheoScan aggregometer was used for the measurements of critical shear stress (CSS) in whole blood samples. The correlation between CSS and the shear stress required to stop an RBC pair from aggregating was found. The shear stress required to disaggregate a pair of RBCs using the double channel optical tweezers appeared to be about 10 times higher than CSS. The correlation between shear stresses required to prevent RBCs from aggregation at the individual cell level and in whole blood samples was estimated and assessed quantitatively. The experimental approach developed has a high potential for advancing hemorheological studies.


Rational Design of Dinuclear Complexes Binding at Two Neighboring Phosphate Esters of DNA

Thorsten Glaser, Gabriele Fischer von Mollard, Dario Anselmetti

This microreview summarizes a study aiming at the development of a novel family of metal complexes that binds to the phosphate esters of the DNA backbone inspired by the cytotoxicity of the anticancer drug cisplatin and by the phosphate ester cleaving reactivity of metalloenzymes and their biomimetic model complexes. A rational design is presented that is based on the requirements to establish a molecular recognition for the phosphate esters of the DNA backbone and to suppress binding to the less exposed nucleobases. Two phosphate binding sites should be preoriented and fixed by a rigid backbone to the distance of two neighboring phosphate ester in the DNA backbone of 6-7 Å. Sterical demand close to the phosphate binding sites should inhibit coordination to the nucleobases. This was molecularly translated into an unprecedented family of dinuclear complexes based on 1,8-naphthalenediol ligands with sterically demanding chelate arms in 2,7-position. The synthesis of the ligand and its first dinuclear CuII2 complex is described. The binding of this complex to DNA has been studied by biochemical ensemble methods and biophysical single-molecule methods. The incubation of DNA with the CuII2 complex results in interstrand interactions forming aggregates that prevent the DNA to enter the gel in the electrophoresis experiments. AFM experiments show an increase of the DNA diameter and local entanglements of the DNA by intrastrand interactions. The stretching of a single DNA molecule by optical tweezers exhibits distinct force peaks upon treatment with the complex necessary to break the intrastrand interactions. Torsional measurements of a single DNA molecule by magnetic tweezers showed a shortening of the effective DNA length due to the intrastrand interactions. Incubation with the CuII2 complex suggests inhibition of DNA synthesis in polymerase chain reaction experiments and strong cytotoxicity to human HeLa cancer cells, both at lower concentration than with cisplatin. A coherent model is provided that explains all experimental observations by the intended binding of the CuII2 complex to two neighboring phosphates of the DNA backbone and the formation of intra- or interstrand interactions by π-π interactions of the outward oriented and freely exposed naphthalene rings.


Calculation of the radiation forces on a microsphere in the evanescent field of an optical nanofiber by ray tracing

B. Zhang, J. X. Luo, Z. L. Liu, and F. P. Wu

We use ray optics to calculate the radiation forces on a dielectric microsphere in the evanescent field of an optical nanofiber. We theoretically demonstrate that the gradient force may attract the microsphere onto the fiber surface. The scattering force may transport the microsphere along the fiber and in the light propagating direction. The impact of the sphere-fiber distance, sphere radius, and fiber radius on the scattering and gradient forces are investigated. The radius of nanofiber can be optimized for particle transportation.


An inverted dielectrophoretic device for analysis of attached single cell mechanics

Rebecca Lownes Urbano and Alisa Morss Clyne

Dielectrophoresis (DEP), the force induced on a polarizable body by a non-uniform electric field, has been widely used to manipulate single cells in suspension and analyze their stiffness. However, most cell types do not naturally exist in suspension but instead require attachment to the tissue extracellular matrix in vivo. Cells alter their cytoskeletal structure when they attach to a substrate, which impacts cell stiffness. It is therefore critical to be able to measure mechanical properties of cells attached to a substrate. We present a novel inverted quadrupole dielectrophoretic device capable of measuring changes in the mechanics of single cells attached to a micropatterned polyacrylamide gel. The device is positioned over a cell of defined size, a directed DEP pushing force is applied, and cell centroid displacement is dynamically measured by optical microscopy. Using this device, single endothelial cells showed greater centroid displacement in response to applied DEP pushing force following actin cytoskeleton disruption by cytochalasin D. In addition, transformed mammary epithelial cell (MCF10A-NeuT) showed greater centroid displacement in response to applied DEP pushing force compared to untransformed cells (MCF10A). DEP device measurements were confirmed by showing that the cells with greater centroid displacement also had a lower elastic modulus by atomic force microscopy. The current study demonstrates that an inverted DEP device can determine changes in single attached cell mechanics on varied substrates.


Single-molecule mechanics of protein-labelled DNA handles

Vivek S. Jadhav, Dorothea Brüggemann, Florian Wruck and Martin Hegner

DNA handles are often used as spacers and linkers in single-molecule experiments to isolate and tether RNAs, proteins, enzymes and ribozymes, amongst other biomolecules, between surface-modified beads for nanomechanical investigations. Custom DNA handles with varying lengths and chemical end-modifications are readily and reliably synthesized en masse, enabling force spectroscopic measurements with well-defined and long-lasting mechanical characteristics under physiological conditions over a large range of applied forces. Although these chemically tagged DNA handles are widely used, their further individual modification with protein receptors is less common and would allow for additional flexibility in grabbing biomolecules for mechanical measurements. In-depth information on reliable protocols for the synthesis of these DNA–protein hybrids and on their mechanical characteristics under varying physiological conditions are lacking in literature. Here, optical tweezers are used to investigate different protein-labelled DNA handles in a microfluidic environment under different physiological conditions. Digoxigenin (DIG)-dsDNA-biotin handles of varying sizes (1000, 3034 and 4056 bp) were conjugated with streptavidin or neutravidin proteins. The DIG-modified ends of these hybrids were bound to surface-modified polystyrene (anti-DIG) beads. Using different physiological buffers, optical force measurements showed consistent mechanical characteristics with long dissociation times. These protein-modified DNA hybrids were also interconnected in situ with other tethered biotinylated DNA molecules. Electron-multiplying CCD (EMCCD) imaging control experiments revealed that quantum dot–streptavidin conjugates at the end of DNA handles remain freely accessible. The experiments presented here demonstrate that handles produced with our protein–DNA labelling procedure are excellent candidates for grasping single molecules exposing tags suitable for molecular recognition in time-critical molecular motor studies.


Friday, February 19, 2016

Characterization of the Stiffness of Multiple Particles Trapped by Dielectrophoretic Tweezers in a Microfluidic Device

Myeonggu Son, Seungyeop Choi, Kwan Hwi Ko, Min Hyung Kim, Sei-Young Lee, Jaehong Key, Young-Ro Yoon, In Soo Park, and Sang Woo Lee

Characterization of the stiffness of multiple particles trapped by tweezers-based force spectroscopy is a key step in building simple, high-throughput, and robust systems that can investigate the molecular interactions in a biological process, but the technology to characterize it in a given environment simultaneously is still lacking. We first characterized the stiffness of multiple particles trapped by dielectrophoretic (DEP) tweezers inside a microfluidic device. In this characterization, we developed a method to measure the thermal fluctuations of the trapped multiple particles with DEP tweezers by varying the heights of the particles in the given environment at the same time. Using the data measured in this controlled environment, we extracted the stiffness of the trapped particles and calculated their force. This study not only provides a simple and high-throughput method to measure the trap stiffness of multiple particles inside a microfluidic device using DEP tweezers but also inspires the application of the trapped multiple particles to investigate the dynamics in molecular interactions.


Uptake and levels of the antibiotic berberine in individual dormant and germinating Clostridium difficile and Bacillus cereus spores as measured by laser tweezers Raman spectroscopy

Shiwei Wang, Barbara Setlow, Peter Setlow and Yong-qing Li

Spores of Clostridium difficile and Bacillus cereus are major causes of nosocomial diarrhoea and foodborne disease. Our aim was to measure the dynamics of the uptake of the antibiotic berberine by individual germinating spores and the levels of berberine accumulated in germinated spores. Laser tweezers Raman spectroscopy (LTRS) and differential interference contrast microscopy were used to measure levels of berberine accumulated in single germinating spores and to monitor berberine uptake and germination of individual C. difficile and B. cereus spores. MIC values of berberine for C. difficile and B. cereus spores were 640 and 256 mg/L, respectively. Levels of berberine accumulated at the berberine MICs in individual germinated spores were heterogeneous, with values of 17.1 ± 5.4 and 12.7 ± 5.5 g/L for C. difficile and B. cereus spores, respectively. These values were 25–50-fold higher than the MIC values. However, berberine did not affect the germination of C. difficile and B. cereus spores, but did block germinated spores' outgrowth. Berberine uptake kinetics were similar for these two kinds of spores. After the addition of germinants, berberine began to enter germinating spores at the time (Tlag) when rapid release of the spore core's large depot of the 1:1 chelate of Ca2+ with dipicolinic acid began, and the level of berberine taken up was maximal shortly after spore cortex lysis was completed (Tlysis). LTRS can be used to measure uptake and levels of berberine in single cells. High levels of berberine can enter spores of C. difficile and B. cereus soon after germination is initiated, thus inhibiting spore outgrowth and minimizing hazards posed by germinated spores.


Trapping and viability of swimming bacteria in an optoelectric trap

A. Mishra, T. R. Maltais, T. M. Walter, A. Wei, S. J. Williams and S. T. Wereley

Non-contact manipulation methods capable of trapping and transporting swimming bacteria can significantly aid in chemotaxis studies. However, high swimming speed makes the trapping of these organisms an inherently challenging task. We demonstrate that an optoelectric technique, rapid electrokinetic patterning (REP), can effectively trap and manipulate Enterobacter aerogenes bacteria swimming at velocities greater than 20 μm s−1. REP uses electro-orientation, laser-induced AC electrothermal flow, and particle–electrode interactions for capturing these cells. In contrast to trapping non-swimming bacteria and inert microspheres, we observe that electro-orientation is critical to the trapping of the swimming cells, since unaligned bacteria can swim faster than the radially inward electrothermal flow and escape the trap. By assessing the cell membrane integrity, we study the effect of REP trapping conditions, including optical radiation, laser-induced heating, and the electric field on cell viability. When applied individually, the optical radiation and laser-induced heating have negligible effect on cells. At the standard REP trapping conditions fewer than 2% of cells have a compromised membrane after four minutes. To our knowledge this is the first study detailing the effect of REP trapping on cell viability. The presented results provide a clear guideline on selecting suitable REP parameters for trapping living bacteria.


Hexadecapolar colloids

Bohdan Senyuk, Owen Puls, Oleh M. Tovkach, Stanislav B. Chernyshuk & Ivan I. Smalyukh

Outermost occupied electron shells of chemical elements can have symmetries resembling that of monopoles, dipoles, quadrupoles and octupoles corresponding to filled s-, p-, d- and f-orbitals. Theoretically, elements with hexadecapolar outer shells could also exist, but none of the known elements have filled g-orbitals. On the other hand, the research paradigm of ‘colloidal atoms’ displays complexity of particle behaviour exceeding that of atomic counterparts, which is driven by DNA functionalization, geometric shape and topology and weak external stimuli. Here we describe elastic hexadecapoles formed by polymer microspheres dispersed in a liquid crystal, a nematic fluid of orientationally ordered molecular rods. Because of conically degenerate boundary conditions, the solid microspheres locally perturb the alignment of the nematic host, inducing hexadecapolar distortions that drive anisotropic colloidal interactions. We uncover physical underpinnings of formation of colloidal elastic hexadecapoles and describe the ensuing bonding inaccessible to elastic dipoles, quadrupoles and other nematic colloids studied previously.


Continuous-wave laser-assisted injection of single magnetic nanobeads into living cells

Jing Zhong, Hengjun Liu, Hisataka Maruyama, Taisuke Masuda, Fumihito Arai

Fast and effective transportation of beads/molecules into living cells with high cell viability is essential for drug development and cell biology. This study presents a new approach of injecting single magnetic nanobeads with low surface temperature into living cells using a continuous-wave laser. Experimental results demonstrate the successful injection of magnetic nanobeads into living cells with high injection rates reaching 100%, short injection times of about 1 sec, and maximal cell survival rates of 100%.


Wednesday, February 17, 2016

Spin-controlled orbital motion in tightly focused high-order Laguerre-Gaussian beams

Yongyin Cao, Tongtong Zhu, Haiyi Lv, and Weiqiang Ding

Spin angular momentum can contribute to both optical force and torque exerted on spheres. Orbit rate of spheres located in tightly focused LG beams with the same azimuthal mode index l is spin-controlled due to spin-orbit coupling. Laguerre-Gaussian beams with high-order azimuthal mode are used here to study the orbit rate of dielectric spheres. Orbit rates of spheres with varying sizes and refravtive indices are investigated as well as optical forces acting on spheres in LG beams with different azimuthal modes. These results would be much helpful to investigation on optical rotation and transfer of spin and orbital angular momentum.

Rotation of millimeter-sized objects using ordinary light

Olivier Emile and Janine Emile

The ability to optically rotate bodies offers new degrees of control of micro-objects with applications in various domains, including microelectromechanical systems (MEMS), biomanipulations, or optofluidics. Here we demonstrate the optically-induced rotation of simple asymmetric two-dimensional objects using plane waves originating either from ordinary laser sources or from black body radiation. The objects are floating on an air/water interface. We observe a steady-state rotation depending on the light intensity and on the asymmetry of the object. We interpret this rotation in terms of light diffraction by the edges of the object. Such systems could be easily implemented in optofluidic devices to induce liquid flow without the need for special light sources.


Dynamics of a small particle in a fluctuating random light field

Manuel I. Marqués

The dynamics of an electric dipole in a light field consisting of electromagnetic plane waves with polarizations randomly distributed and fluctuating phases is theoretically analyzed. The expression for the optical random-force fluctuations is derived and found to be proportional to the scattering cross section and to the square of the intensity divided by the frequency of the electromagnetic field. Under these fluctuations, and in the absence of damping, the dipole behaves like a super-diffusive particle with a kinetic energy growing linearly with time. The analytic predictions are tested against numerical simulations for the particular case of a resonant dipole.


Picosecond Motional Relaxation of Nanoparticles in Femtosecond Laser Trapping

Masayasu Muramatsu, Tse-Fu Shen, Wei-Yi Chiang, Anwar Usman, and Hiroshi Masuhara

Repetitive drag and release dynamics by impulsive force is characteristic of optical trapping by femtosecond laser pulses. We studied the dynamics utilizing double pulse train, and found that trapped polystyrene particles are ejected repetitively from the focal spot and its frequencies become less for longer interval of the pulse trains. The ejection changes drastically in a few-ps interval region, although particles cannot move appreciable distance in such a short time. It means that displacement of particles by a conventional diffusive motion is not dominant and another fast process has an important role in femtosecond pulse trapping. We also revealed that the silica nanoparticles shows a decay at few-ps, indicating that the picosecond decay is not due to a material property but considered to be a general dynamics. We propose that a picosecond relaxation process of inertia force of particles is important for understanding laser trapping dynamics by femtosecond laser pulses. picosecond relaxation process of inertia force of particles is important for understanding laser trapping dynamics by femtosecond pulses.


Monday, February 15, 2016

Polygonal micro-whirlpools induced in ferrofluids

Marcin Bacia, Weronika Lamperska, Jan Masajada, Slawomir Drobczynski, Maciej Marc

We report on the observation of the polygonal whirlpools in the thin layer of ferrofluid under illumination with a laser beam carrying optical vortex and in the presence of a vertical magnetic field. This kind of structures have attracted attention after discovering a hexagonal storm in Saturn’s atmosphere. Our polygonal whirlpools were created in a closed system (no free surfaces) in micro-scale (whirlpool diameter <20 μm) by the use of holographic optical tweezers. The polygonal shape was changed by varying the magnetic field strength or value of the optical vortex topological charge.


Constructive spin-orbital angular momentum coupling can twist materials to create spiral structures in optical vortex illumination

Daisuke Barada, Guzhaliayi Juman, Itsuki Yoshida, Katsuhiko Miyamoto, Shigeo Kawata, Seigo Ohno and Takashige Omatsu

It was discovered that optical vortices twist isotropic and homogenous materials, e.g., azo-polymer films to form spiral structures on a nano- or micro-scale. However, the formation mechanism has not yet been established theoretically. To understand the mechanism of the spiral surface relief formation in the azo-polymer film, we theoretically investigate the optical radiation force induced in an isotropic and homogeneous material under irradiation using a continuous-wave optical vortex with arbitrary topological charge and polarization. It is revealed that the spiral surface relief formation in azo-polymer films requires the irradiation of optical vortices with a positive (negative) spin angular momentum and a positive (negative) orbital angular momentum (constructive spin-orbital angular momentum coupling), i.e., the degeneracy among the optical vortices with the same total angular momentum is resolved.


Luminescent nanoparticle trapping with far-field optical fiber-tip tweezers

Jean-Baptiste Decombe, Francisco Valdivia-Valero, Géraldine Dantelle, Godefroy Leménager, Thierry Gacoin, Gérard Colas des Francs, Serge Huant and Jochen Fick

We report stable and reproducible trapping of luminescent dielectric YAG:Ce3+ nanoparticles with sizes down to 60 nm using far-field dual fiber tip optical tweezers. The particles are synthesized by a specific glycothermal route followed by an original protected annealing step, resulting in significantly enhanced photostability. The tweezers properties are analyzed by studying the trapped particles residual Brownian motion using video or reflected signal records. The trapping potential is harmonic in the transverse direction to the fiber axis, but reveals interference fringes in the axial direction. Large trapping stiffnesses of 35 and 2 pN µm-1 W-1 are measured for a fiber tip-to-tip distance of 3 µm and 300-nm and 60-nm particles, respectively. The forces acting on nanoparticles are discussed within the dipolar approximation (gradient and scattering force contributions) or exact calculations using the Maxwell Stress Tensor formalism. Prospects for trapping even smaller particles are discussed.


Friday, February 12, 2016

Fano resonant Ge2Sb2Te5 nanoparticles realize switchable lateral optical force

Tun Cao, Libang Mao, Dongliang Gao, Weiqiang Ding and Cheng Wei Qiu

Sophisticated optical micromanipulation of small biomolecules usually relies on complex light, e.g., structured light, highly non-paraxial light, or chiral light. One emerging manipulation is to employ chiral light to drive the chiral nanoparticle along the direction perpendicular to the light propagation, i.e., the lateral optical force. Here, we theoretically study the lateral optical force exerted by a merely Gaussian beam. We for the very first time demonstrate that the Fano resonances(FRs) of the Ge2Sb2Te5 (GST) phase-change nanoparticle encapsulated with Au shell could enable a conventional Gaussian laser to exert lateral force on such a dielectric GST nanoparticle, attributed to the strongly asymmetric energy flow around the spheres in the dipole-quadrupole FRs. More interestingly, the direction of this lateral force could be reversible during the state transition (i.e., from amorphous to crystalline). By bonding small biomolecule to the outer surface of the phase-change nanoparticle, the particle behaves as a direction-selective vehicle to transport biomolecule along opposite directions, at pre-assessed states of the Ge2Sb2Te5 core correspondingly. Importantly, the origin of reversal of lateral optical forceis further unveiled by the optical singularity of Poynting vector. Our mechanism by tailoring FRs of phase-change nanoparticles, not just limited to GST, may bring a new twist to optical micromanipulation and biomedical application.


Optical Manipulation and Spectroscopy Of Silicon Nanoparticles Exhibiting Dielectric Resonances

Ana Andres-Arroyo, Bakul Gupta, Fan Wang, J. Justin Gooding, and Peter J. Reece

We demonstrate that silicon (Si) nanoparticles with scattering properties exhibiting strong dielectric resonances can be successfully manipulated using optical tweezers. The large dielectric constant of Si has a distinct advantage over conventional colloidal nanoparticles in that it leads to enhanced trapping forces without the heating associated with metallic nanoparticles. Further, the spectral features of the trapped nanoparticles provide a unique marker for probing size, shape, orientation and local dielectric environment. We exploit these properties to investigate the trapping dynamics of Si nanoparticles with different dimensions ranging from 50 to 200 nm and aspect ratios between 0.4 and 2. The unique combination of spectral and trapping properties make Si nanoparticles an ideal system for delivering directed nanoscale sensing in a range of potential applications.


Capture and sorting of multiple cells by polarization-controlled three-beam interference

Yu Hou, Zuobin Wang, Yaowei Hu, Dayou Li and Renxi Qiu

For the capture and sorting of multiple cells, a sensitive and highly efficient polarization-controlled three-beam interference set-up has been developed. With the theory of superposition of three beams, simulations on the influence of polarization angle upon the intensity distribution and the laser gradient force change with different polarization angles have been carried out. By controlling the polarization angle of the beams, various intensity distributions and different sizes of dots are obtained. We have experimentally observed multiple optical tweezers and the sorting of cells with different polarization angles, which are in accordance with the theoretical analysis. The experimental results have shown that the polarization angle affects the shapes and feature sizes of the interference patterns and the trapping force.


Plasmon-assisted trapping of nanoparticles using a silver-nanowire-embedded PMMA nanofiber

Chang Cheng, Xiaohao Xu, Hongxiang Lei & Baojun Li

The integration of surface plasmon with waveguide is a strategy for lab-on-a-chip compatible optical trapping. Here, we report a method for trapping of nanoparticles using a silver nanowire (AgNW) embedded poly(methyl methacrylate) (PMMA) nanofiber with the assistance of surface plasmon polaritons (SPPs). The nanoparticles (polystyrene, 700 nm diameter) are transported along the nanofiber and ultimately trapped at the AgNW embedded region because of the enhanced optical gradient force towards the nanofiber exerted on the nanoparticles and optical potential well generated by the excitation of SPPs. The low optical power requirement and the easy fabrication of the AgNW-embedded nanofiber with broad range of wavelength for SPPs are advantageous to the applications in optofluidics and plasmofluidics.


Novel Method for Neuronal Nanosurgical Connection

Nir Katchinskiy, Helly R. Goez, Indrani Dutta, Roseline Godbout & Abdulhakem Y. Elezzabi

Neuronal injury may cause an irreversible damage to cellular, organ and organism function. While preventing neural injury is ideal, it is not always possible. There are multiple etiologies for neuronal injury including trauma, infection, inflammation, immune mediated disorders, toxins and hereditary conditions. We describe a novel laser application, utilizing femtosecond laser pulses, in order to connect neuronal axon to neuronal soma. We were able to maintain cellular viability, and demonstrate that this technique is universal as it is applicable to multiple cell types and media.


Thursday, February 11, 2016

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air

Jeremy Moore, Leopoldo L. Martin, Shai Maayani, Kyu Hyun Kim, Hengky Chandrahalim, Matt Eichenfield, Inocencio R. Martin, and Tal Carmon

We experimentally reporton optical binding of many glass particles in air that levitate in a single optical beam. A diversity of particle sizes and shapes interact at long range in a single Gaussian beam. Our system dynamics span from oscillatory to random and dimensionality ranges from 1 to 3D. The low loss for the center of mass motion of the beads could allow this system to serve as a standard many body testbed, similar to what is done today with atoms, but at the mesoscopic scale.


Dynein Clusters into Lipid Microdomains on Phagosomes to Drive Rapid Transport toward Lysosomes

Ashim Rai, Divya Pathak, Shreyasi Thakur, Shampa Singh, Alok Kumar Dubey, Roop Mallik

Diverse cellular processes are driven by motor proteins that are recruited to and generate force on lipid membranes. Surprisingly little is known about how membranes control the force from motors and how this may impact specific cellular functions. Here, we show that dynein motors physically cluster into microdomains on the membrane of a phagosome as it matures inside cells. Such geometrical reorganization allows many dyneins within a cluster to generate cooperative force on a single microtubule. This results in rapid directed transport of the phagosome toward microtubule minus ends, likely promoting phagolysosome fusion and pathogen degradation. We show that lipophosphoglycan, the major molecule implicated in immune evasion of Leishmania donovani, inhibits phagosome motion by disrupting the clustering and therefore the cooperative force generation of dynein. These findings appear relevant to several pathogens that prevent phagosome-lysosome fusion by targeting lipid microdomains on phagosomes.


Label-free detection and manipulation of single biological nanoparticles

Michael C. DeSantis and Wei Cheng

In the past several years, there have been significant advances in the field of nanoparticle detection for various biological applications. Of considerable interest are synthetic nanoparticles being designed as potential drug delivery systems as well as naturally occurring or biological nanoparticles, including viruses and extracellular vesicles. Many infectious diseases and several human cancers are attributed to individual virions. Because these particles likely display different degrees of heterogeneity under normal physiological conditions, characterization of these natural nanoparticles with single-particle sensitivity is necessary for elucidating information on their basic structure and function as well as revealing novel targets for therapeutic intervention. Additionally, biodefense and point-of-care clinical testing demand ultrasensitive detection of viral pathogens particularly with high specificity. Consequently, the ability to perform label-free virus sensing has motivated the development of multiple electrical-, mechanical-, and optical-based detection techniques, some of which may even have the potential for nanoparticle sorting and multi-parametric analysis. For each technique, the challenges associated with label-free detection and measurement sensitivity are discussed as are their potential contributions for future real-world applications.


Load-dependent modulation of non-muscle myosin-2A function by tropomyosin 4.2

Nikolas Hundt, Walter Steffen, Salma Pathan-Chhatbar, Manuel H. Taft & Dietmar J. Manstein

Tropomyosin isoforms play an important role in the organisation of cytoplasmic actomyosin complexes in regard to function and cellular localisation. In particular, Tpm4.2 is upregulated in rapidly migrating cells and responsible for the specific recruitment of the cytoplasmic class-2 myosin NM-2A to actin filaments during the formation of stress fibres. Here, we investigate how the decoration of F-actin with Tpm4.2 affects the motor properties of NM-2A under conditions of low and high load. In the absence of external forces, decoration of actin filaments with Tpm4.2 does not affect the gated release of ADP from NM-2A and the transition from strong to weak actin-binding states. In the presence of resisting loads, our results reveal a marked increase in the mechanosensitive gating between the leading and trailing myosin head. Thereby, the processive behaviour of NM-2A is enhanced in the presence of resisting loads. The load- and Tpm4.2-induced changes in the functional behaviour of NM-2A are in good agreement with the role of this myosin in the context of stress fibres and the maintenance of cellular tension.


Tuesday, February 9, 2016

Large-scale dynamic assembly of metal nanostructures in plasmofluidic field

Partha Pratim Patra, Rohit Chikkaraddy, Sreeja Thampi, Ravi P. N. Tripathi and G. V. Pavan Kumar

We discuss two aspects of the plasmofluidic assembly of plasmonic nanostructures at the metal–fluid interface. First, we experimentally show how three and four spot evanescent-wave excitation can lead to unconventional assembly of plasmonic nanoparticles at the metal–fluid interface. We observed that the pattern of assembly was mainly governed by the plasmon interference pattern at the metal–fluid interface, and further led to interesting dynamic effects within the assembly. The interference patterns were corroborated by 3D finite-difference time-domain simulations. Secondly, we show how anisotropic geometry, such as Ag nanowires, can be assembled and aligned in unstructured and structured plasmofluidic fields. We found that by structuring the metal-film, Ag nanowires can be aligned at the metal–fluid interface with a single evanescent-wave excitation, thus highlighting the prospect of assembling plasmonic circuits in a fluid. An interesting aspect of our method is that we obtain the assembly at locations away from the excitation points, thus leading to remote assembly of nanostructures. The results discussed herein may have implications in realizing a platform for reconfigurable plasmonic metamaterials, and a test-bed to understand the effect of plasmon interference on assembly of nanostructures in fluids.


Separation and sorting of cells in microsystems using physical principles

Gi-Hun Lee, Sung-Hwan Kim, Kihoon Ahn, Sang-Hoon Lee and Joong Yull Park
In the last decade, microfabrication techniques have been combined with microfluidics and applied to cell biology. Utilizing such new techniques, various cell studies have been performed for the research of stem cells, immune cells, cancer, neurons, etc. Among the various biological applications of microtechnology-based platforms, cell separation technology has been highly regarded in biological and clinical fields for sorting different types of cells, finding circulating tumor cells (CTCs), and blood cell separation, amongst other things. Many cell separation methods have been created using various physical principles. Representatively, these include hydrodynamic, acoustic, dielectrophoretic, magnetic, optical, and filtering methods. In this review, each of these methods will be introduced, and their physical principles and sample applications described. Each physical principle has its own advantages and disadvantages. The engineers who design the systems and the biologists who use them should understand the pros and cons of each method or principle, to broaden the use of microsystems for cell separation. Continuous development of microsystems for cell separation will lead to new opportunities for diagnosing CTCs and cancer metastasis, as well as other elements in the bloodstream.


−1 Programmed Ribosomal Frameshifting as a Force-Dependent Process

Koen Visscher

−1 Programmed ribosomal frameshifting is a translational recoding event in which ribosomes slip backward along messenger RNA presumably due to increased tension disrupting the codon–anticodon interaction at the ribosome's coding site. Single-molecule physical methods and recent experiments characterizing the physical properties of mRNA's slippery sequence as well as the mechanical stability of downstream mRNA structure motifs that give rise to frameshifting are discussed. Progress in technology, experimental assays, and data analysis methods hold promise for accurate physical modeling and quantitative understanding of −1 programmed ribosomal frameshifting.


Monday, February 8, 2016

Spinning gold nanoparticles driven by circularly polarized light

Jiunn-Woei Liaw, Ying-Syuan Chen, Mao-Kuen Kuo

This study theoretically examines a spinning gold nanoparticle (GNP) driven by circularly polarized (CP) plane waves. The wavelength-dependent optical torques which were exerted on three different shapes of GNPs (spherical, prolate and oblate spheroidal) were analyzed by utilizing Mie theory for the former and the multiple multipole method for the latter two, respectively. Numerical results show that both the absorbed and scattered photons contribute to optical torques in most cases. For the case that the CP wave is incident along the long axis of an oblate spheroid or the short axis of a prolate one, the scattering effect in optical torque is more pronounced than the absorption one. This phenomenon is significant especially when the wavelength of the CP wave is close to the longitudinal surface plasmon resonance band of the GNP. In contrast, when the CP wave is incident along the axes of revolution of these shapes of GNPs, the ratio of optical torque to absorption power is directly proportional to the wavelength. Moreover, this ratio is independent of the size and even the aspect ratio of GNPs. This result suggests that only the absorbed photons contribute to optical torques, but not the scattered ones, due to the conservation of angular momentum for cases of rotational symmetry.


Engineering of frustration in colloidal artificial ices realized on microfeatured grooved lattices

Antonio Ortiz-Ambriz & Pietro Tierno
Artificial spin ice systems, namely lattices of interacting single domain ferromagnetic islands, have been used to date as microscopic models of frustration induced by lattice topology, allowing for the direct visualization of spin arrangements and textures. However, the engineering of frustrated ice states in which individual spins can be manipulated in situ and the real-time observation of their collective dynamics remain both challenging tasks. Inspired by recent theoretical advances, here we realize a colloidal version of an artificial spin ice system using interacting polarizable particles confined to lattices of bistable gravitational traps. We show quantitatively that ice-selection rules emerge in this frustrated soft matter system by tuning the strength of the pair interactions between the microscopic units. Via independent control of particle positioning and dipolar coupling, we introduce monopole-like defects and strings and use loops with defined chirality as an elementary unit to store binary information.


Insights into the interaction of the N-terminal amyloidogenic polypeptide of ApoA-I with model cellular membranes

Giulia Rusciano, Giuseppe Pesce, Gianluigi Zito, Antonio Sasso, Rosa Gaglione, Rita Del Giudice, Renata Piccoli, Daria Maria Monti, Angela Arciello

About twenty variants of apolipoprotein A-I (ApoA-I) are associated to hereditary systemic amyloidoses. Although the molecular bases of this disease are still largely unknown, it has been hypothesized that ApoA-I proteolysis is a key event in pathogenesis, since it triggers the release of an N-terminal fragment (80–100 residue long) that misfolds to form amyloid deposits in peripheral organs and tissues. It is also known that cell membrane lipids play a key role in the fibrillogenic pathway. In the case of ApoA-I related amyloidosis caused by L174S mutation, the 93-residue N-terminal fragment of ApoA-I ([1-93]ApoA-I) was found to be the major constituent of ex vivo fibrils. With the main goal to investigate the interaction of either [1-93]ApoA-I and ApoA-I with biomimetic membranes, we set-up an experimental system based on the Raman Tweezers methodology. We tested GUVs composed by two types of zwitterionic lipids with a different fluidity degree, i.e. dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC). We found that [1-93]ApoA-I induces conformational disorder in an ordered lipid bilayer. When interacting with fluid phases, instead, the fragment was found to be able to penetrate the membrane bilayer inducing an alignment of lipid chains. The interaction features of [1-93]ApoA-I with biomimetic membranes strongly depend on the lipid phase. Full-length ApoA-I was found to have similar effects, even if significantly less pronounced. Our observations shed light on still largely unknown molecular bases of ApoA-I fibrillogenic domain interaction with membranes.


Measurement of Elastic Modulus of Collagen Type I Single Fiber

Pavel Dutov, Olga Antipova, Sameer Varma, Joseph P. R. O. Orgel, Jay D. Schieber

Collagen fibers are the main components of the extra cellular matrix and the primary contributors to the mechanical properties of tissues. Here we report a novel approach to measure the longitudinal component of the elastic moduli of biological fibers under conditions close to those found in vivo and apply it to type I collagen from rat tail tendon. This approach combines optical tweezers, atomic force microscopy, and exploits Euler-Bernoulli elasticity theory for data analysis. This approach also avoids drying for measurements or visualization, since samples are freshly extracted. Importantly, strains are kept below 0.5%, which appear consistent with the linear elastic regime. We find, surprisingly, that the longitudinal elastic modulus of type I collagen cannot be represented by a single quantity but rather is a distribution that is broader than the uncertainty of our experimental technique. The longitudinal component of the single-fiber elastic modulus is between 100 MPa and 360 MPa for samples extracted from different rats and/or different parts of a single tail. Variations are also observed in the fibril-bundle / fibril diameter with an average of 325±40 nm. Since bending forces depend on the diameter to the fourth power, this variation in diameter is important for estimating the range of elastic moduli. The remaining variations in the modulus may be due to differences in composition of the fibril-bundles, or the extent of the proteoglycans constituting fibril-bundles, or that some single fibrils may be of fibril-bundle size.


Friday, February 5, 2016

Nano-optical conveyor belt with waveguide-coupled excitation

Guanghui Wang, Zhoufeng Ying, Ho-pui Ho, Ying Huang, Ningmu Zou, and Xuping Zhang

We propose a plasmonic nano-optical conveyor belt for peristaltic transport of nano-particles. Instead of illumination from the top, waveguide-coupled excitation is used for trapping particles with a higher degree of precision and flexibility. Graded nano-rods with individual dimensions coded to have resonance at specific wavelengths are incorporated along the waveguide in order to produce spatially addressable hot spots. Consequently, by switching the excitation wavelength sequentially, particles can be transported to adjacent optical traps along the waveguide. The feasibility of this design is analyzed using three-dimensional finite-difference time-domain and Maxwell stress tensor methods. Simulation results show that this system is capable of exciting addressable traps and moving particles in a peristaltic fashion with tens of nanometers resolution. It is the first, to the best of our knowledge, report about a nano-optical conveyor belt with waveguide-coupled excitation, which is very important for scalability and on-chip integration. The proposed approach offers a new design direction for integrated waveguide-based optical manipulation devices and its application in large scale lab-on-a-chip integration.


Optical Manipulation of Multiple Groups of Microobjects Using Robotic Tweezers

Haghighi, R.; Cheah, C.C.

Micromanipulation has received increasing attention from robotics researchers due to its wide applications in the manipulation of microobjects like biological cells and Bio-MEMS components. The demand for accurate and precise manipulation of microobjects opens up new challenges in automation of micromanipulation tasks. In this paper, we present a concurrent framework for optical manipulation of multiple groups of microobjects using robotic tweezers. The proposed framework is based on laser-stage coordination control and consists of two concurrent subschemes: 1) local coordination achieved by asynchronous manipulation of multiple groups of microobjects using laser beams and 2) global coordination achieved by manipulation of whole groups using a motorized stage. Unlike existing methods that are limited to the manipulation of a single microobject or a single group of microobjects, the proposed method considers concurrent laser-stage coordination of multiple groups of microobjects, which enhances the capability and flexibility in micromanipulation tasks. In addition, we introduce a unified social interaction function to achieve various cellular behaviors. A mathematical formulation is provided and stability analysis is presented. Using the proposed method, we are able to manipulate multiple groups of microobjects to construct time-varying microformations. Experimental results are presented to illustrate the performance of the proposed method.


Microtubule detyrosination guides chromosomes during mitosis

Marin Barisic, Ricardo Silva e Sousa, Suvranta K. Tripathy, Maria M. Magiera, Anatoly V. Zaytsev, Ana L. Pereira, Carsten Janke, Ekaterina L. Grishchuk, Helder Maiato

Before chromosomes segregate into daughter cells, they align at the mitotic spindle equator, a process known as chromosome congression. Centromere-associated protein E (CENP-E)/Kinesin-7 is a microtubule plus-end–directed kinetochore motor required for congression of pole-proximal chromosomes. Because the plus-ends of many astral microtubules in the spindle point to the cell cortex, it remains unknown how CENP-E guides pole-proximal chromosomes specifically toward the equator. We found that congression of pole-proximal chromosomes depended on specific posttranslational detyrosination of spindle microtubules that point to the equator. In vitro reconstitution experiments demonstrated that CENP-E–dependent transport was strongly enhanced on detyrosinated microtubules. Blocking tubulin tyrosination in cells caused ubiquitous detyrosination of spindle microtubules, and CENP-E transported chromosomes away from spindle poles in random directions. Thus, CENP-E–driven chromosome congression is guided by microtubule detyrosination.


Formation of single-mode laser in transverse plane of perovskite microwire via micromanipulation

Kaiyang Wang, Zhiyuan Gu, Shuai Liu, Jiankai Li, Shumin Xiao, and Qinghai Song
The synthesized perovskites are randomly distributed and their optical properties are fixed after synthesis. Here we demonstrate the tailoring of lasing properties of perovskite microwire via micromanipulation. One microwire has been lifted by a tungsten probe and repositioned on a nearby perovskite microplate with one end suspended in air. Consequently, the conventional Fabry–Perot lasers are completely suppressed and a single laser peak has been observed. The numerical calculations reveal that the single-mode laser is formed by the whispering-gallery mode in the transverse plane of microwire. Our research provides a simple way to tailor the properties of microwire postsynthesis.


Thursday, February 4, 2016

3D micromanipulation at low numerical aperture with a single light beam: the focused-Bessel trap

Yareni A. Ayala, Alejandro V. Arzola, and Karen Volke-Sepúlveda

Full-three-dimensional (3D) manipulation of individual glass beads with radii in the range of 2–8 μm is experimentally demonstrated by using a single Bessel light beam focused through a low-numerical-aperture lens (NA=0.40). Although we have a weight-assisted trap with the beam propagating upward, we obtain a stable equilibrium position well away from the walls of the sample cell, and we are able to move the particle across the entire cell in three dimensions. A theoretical analysis for the optical field and trapping forces along the lateral and axial directions is presented for the focused-Bessel trap. This trap offers advantages for 3D manipulation, such as an extended working distance, a large field of view, and reduced aberrations.


Transport of a spherical transparent nanoparticle by radiation forces in the field of a Gaussian laser beam

A. A. Afanas’ev, L. S. Gaida , D. V. Guzatov, D. V. Novitski, E. V. Matuk

The motion of a spherical transparent nanoparticle under the influence of radiation forces in the field of a Gaussian laser beam is investigated based on solution of Langevin equation. Expressions governing transverse and longitudinal velocities of the nanoparticle under the action of gradient and scattering forces are derived and analyzed. The possibility of spatial separation of nanoparticles having different sizes and optical properties is discussed.


Modeling Bessel Beams and Their Discrete Superpositions from the Generalized Lorenz-Mie Theory to Calculate Optical Forces over Spherical Dielectric Particles

Leonardo A. Ambrosio, Carlos. H. Silva Santos, Ivan E. L. Rodrigues, Ayumi K. de Campos, Leandro A. Machado

In this work, we propose an algorithm developed under Python language for the modeling of ordinary scalar Bessel beams and their discrete superpositions and subsequent calculation of optical forces exerted over dielectric spherical particles. The mathematical formalism, based on the generalized Lorenz-Mie theory, is implemented in Python for its large number of free mathematical (as SciPy and NumPy), data visualization (Matplotlib and PyJamas) and multiprocessing libraries. We also propose an approach, provided by a synchronized Software as Service (SaaS) in cloud computing, to develop a user interface embedded on a mobile application, thus providing users with the necessary means to easily introduce desired unknowns and parameters and see the graphical outcomes of the simulations right at their mobile devices. Initially proposed as a free Android-based application, such an App enables data post-processing in cloud-based architectures and visualization of results, figures and numerical tables.


Optical Trapping and Two-Photon Excitation of Colloidal Quantum Dots using Bowtie Apertures

Russell A. Jensen, I-Chun Huang, Ou Chen, Jennifer T Choy, Thomas S. Bischof, Marko Loncar, and Moungi G. Bawendi

We demonstrate bowtie apertures that were designed and fabricated by a lift-off process to optically trap individual, 30 nm silica coated quantum dots (scQD). Simulations and experiments confirm the trapping capability of the system with a relatively low continuous wave trapping flux of 1.56 MW/cm2 at 1064 nm. Additionally, the scQD emits upon trapping via two-photon excitation from the trapping laser due to strong field enhancement inside the aperture. This system is an exciting platform for studying light-matter interactions and mulitphoton processes in single emitters.


Wednesday, February 3, 2016

Thermal Scanning at the Cellular Level by an Optically Trapped Upconverting Fluorescent Particle

Paloma Rodríguez-Sevilla, Yuhai Zhang, Patricia Haro-González, Francisco Sanz-Rodríguez, Francisco Jaque, José García Solé, Xiaogang Liu, Daniel Jaque

3D optical manipulation of a thermal-sensing upconverting particle allows for the determination of the extension of the thermal gradient created in the surroundings of a plasmonic-mediated photothermal-treated HeLa cancer cell.


Tuesday, February 2, 2016

Measuring the interaction between a pair of emulsion droplets using dual-trap optical tweezers

Bill Williams, Marjorie Griffiths, Allan Raudsepp and Kate McGrath

Optical tweezers have been used to investigate the dependance of electrostatic inter-particle forces on separation, in systems consisting of pairs of either model silica beads or emulsion droplets. Measurements were carried out as a function of ionic strength and, at salt concentrations where the Debye length was larger than the standard deviation of Brownian fluctuations of the particles in the traps, results were found to agree reasonably well with the predictions of DLVO theory. Experiments were also carried out where the salt concentration of the environment was changed in real-time while interactions were continuously measured. Specifically, single pairs of particles or emulsion droplets were held in a microfluidic channel in close proximity to an interface created between milliQ water and a 5 mM NaCl solution. Changes in the force-separation curves were measured as a function of time and used to monitor changes in the Debye length, and thus the local salt concentration, as ions diffused away from the interface. The results were shown to be consistent with expectations based on a relevant diffusion equation.


Optical pulling force on a particle near the surface of a dielectric slab waveguide

Nayan Kumar Paul ; Brandon A. Kemp

Optical forces on a Rayleigh particle near the surface of a dielectric slab waveguide are considered. A light wave of the lowest-order TE0TE0 mode is used to excite the particle. The transverse and longitudinal forces acting on the particle are studied. The particle is always trapped near the surface of the slab, where the electric field intensity is high. The particle can be pushed away from or pulled toward the light source along the surface of the slab by tuning the frequency around a switching frequency. This phenomenon switches between scattering and gradient forces near the switching frequency of the dielectric slab waveguide.


Photonic Crystal Optical Tweezers with High Efficiency for Live Biological Samples and Viability Characterization

Peifeng Jing, Jingda Wu, Gary W. Liu, Ethan G. Keeler, Suzie H. Pun & Lih Y. Lin

We propose and demonstrate a new optical trapping method for single cells that utilizes modulated light fields to trap a wide array of cell types, including mammalian, yeast, and Escherichia coli cells, on the surface of a two-dimensional photonic crystal. This method is capable of reducing the required light intensity, and thus minimizing the photothermal damage to living cells, thereby extending cell viability in optical trapping and cell manipulation applications. To this end, a thorough characterization of cell viability in optical trapping environments was performed. This study also demonstrates the technique using spatial light modulation in patterned manipulation of live cell arrays over a broad area.


Coupling of Retrograde Flow to Force Production During Malaria Parasite Migration

Katharina A. Quadt, Martin Streichfuss, Catherine A. Moreau, Joachim P. Spatz, and Friedrich Frischknecht

Migration of malaria parasites is powered by a myosin motor that moves actin filaments, which in turn link to adhesive proteins spanning the plasma membrane. The retrograde flow of these adhesins appears to be coupled to forward locomotion. However, the contact dynamics between the parasite and the substrate as well as the generation of forces are complex and their relation to retrograde flow is unclear. Using optical tweezers we found retrograde flow rates up to 15 μm/s contrasting with parasite average speeds of 1–2 μm/s. We found that a surface protein, TLP, functions in reducing retrograde flow for the buildup of adhesive force and that actin dynamics appear optimized for the generation of force but not for maximizing the speed of retrograde flow. These data uncover that TLP acts by modulating actin dynamics or actin filament organization and couples retrograde flow to force production in malaria parasites.


Monday, February 1, 2016

Interaction dynamics of two diffusing particles: contact times and influence of nearby surfaces

B. Tränkle,a D. Ruha and A. Rohrbach

Interactions of diffusing particles are governed by hydrodynamics on different length and timescales. The local hydrodynamics can be influenced substantially by simple interfaces. Here, we investigate the interaction dynamics of two micron-sized spheres close to plane interfaces to mimic more complex biological systems or microfluidic environments. Using scanned line optical tweezers and fast 3D interferometric particle tracking, we are able to track the motion of each bead with precisions of a few nanometers and at a rate of 10 kilohertz. From the recorded trajectories, all spatial and temporal information is accessible. This way, we measure diffusion coefficients for two coupling particles at varying distances h to one or two glass interfaces. We analyze their coupling strength and length by cross-correlation analysis relative to h and find a significant decrease in the coupling length when a second particle diffuses nearby. By analysing the times the particles are in close contact, we find that the influence of nearby surfaces and interaction potentials reduce the diffusivity strongly, although we found that the diffusivity hardly affects the contact times and the binding probability between the particles. All experimental results are compared to a theoretical model, which is based on the number of possible diffusion paths following the Catalan numbers and a diffusion probability, which is biased by the spheres' surface potential. The theoretical and experimental results agree very well and therefore enable a better understanding of hydrodynamically coupled interaction processes.


Measuring forces and stresses in situ in living tissues

Kaoru Sugimura, Pierre-François Lenne, François Graner

Development, homeostasis and regeneration of tissues result from a complex combination of genetics and mechanics, and progresses in the former have been quicker than in the latter. Measurements of in situ forces and stresses appear to be increasingly important to delineate the role of mechanics in development. We review here several emerging techniques: contact manipulation, manipulation using light, visual sensors, and non-mechanical observation techniques. We compare their fields of applications, their advantages and limitations, and their validations. These techniques complement measurements of deformations and of mechanical properties. We argue that such approaches could have a significant impact on our understanding of the development of living tissues in the near future.


Folding and assembly of the large molecular machine Hsp90 studied in single-molecule experiments

Markus Jahn, Johannes Buchner, Thorsten Hugel, and Matthias Rief

Folding of small proteins often occurs in a two-state manner and is well understood both experimentally and theoretically. However, many proteins are much larger and often populate misfolded states, complicating their folding process significantly. Here we study the complete folding and assembly process of the 1,418 amino acid, dimeric chaperone Hsp90 using single-molecule optical tweezers. Although the isolated C-terminal domain shows two-state folding, we find that the isolated N-terminal as well as the middle domain populate ensembles of fast-forming, misfolded states. These intradomain misfolds slow down folding by an order of magnitude. Modeling folding as a competition between productive and misfolding pathways allows us to fully describe the folding kinetics. Beyond intradomain misfolding, folding of the full-length protein is further slowed by the formation of interdomain misfolds, suggesting that with growing chain lengths, such misfolds will dominate folding kinetics. Interestingly, we find that small stretching forces applied to the chain can accelerate folding by preventing the formation of cross-domain misfolding intermediates by leading the protein along productive pathways to the native state. The same effect is achieved by cotranslational folding at the ribosome in vivo.


Orbital Rotation of Trapped Particle in a Transversely Misaligned Dual-Fiber Optical Trap

Xiao, G.; Yang, K. ; Luo, H. ; Chen, X. ; Xiong, W.

We propose and demonstrate a novel optical orbital rotation technique for trapped particle in a transversely misaligned dual-fiber optical trap. The orbital rotation frequency can be controlled by varying the power of two counterpropagating beams. We theoretically analyze an optical force field exerted on a trapped 10- \mumbox{m} -diameter polystyrene particle and simulate its dynamic trajectory within a geometric optics regime framework. Results show that orbital rotation is realized by an optical force field with a vortex distribution that inherently stems from a transversely misaligned dual-fiber optical trap. The orbital rotation trajectory of the particle depends on the configuration of the transversely misaligned dual-beam optical trap rather than its initial position.


Strong optical force acting on a dipolar particle over a multilayer substrate

Shubo Wang and C. T. Chan

Optical forces acting on nano-sized particles are typically too small to be useful for particle manipulation. We theoretically and numerically demonstrate a mechanism that can significantly enhance the optical force acting on a small particle through a special type of resonant particle-substrate coupling. The resonance arises from the singular behavior of the particle’s effective polarizablity in the presence of a metal-dielectric-metal multilayer substrate. We show that this phenomenon is closely related to the existence of a flat-band plasmon mode supported by the multilayer substrate.