Friday, October 28, 2016

Plasmonic Optophoresis for Manipulating, In Situ Position Monitoring, Sensing, and 3-D Trapping of Micro/Nanoparticles

Mostafa Ghorbanzadeh; Mohammad Kazem Moravvej-Farshi; Sara Darbari

An optophoresis system based on surface plasmons in a simple microfluidic environment is proposed, which introduces new functionalities, including three -dimensional trapping of particles, bidirectional manipulation, controllable sorting, and in situ sensing. Its operating principle is based on plasmonic field, which is excited by the Kretschmann configuration on a gold stripe. In this system, trapping, manipulating, and sorting mechanisms are based on the balance of two oppositely exerted scattering forces induced by two counter-propagating surface plasmons. Moreover, for detecting particle's intrinsic properties and in situ position monitoring, we take advantage of the particle's lens-like behavior and stripe's edge effects. The proposed low-power functional system that can be simply and inexpensively integrated into lab-on-a-chip devices is a promising candidate for biological applications and developing integrated optical manipulation chips.


Dynamics of topological monopoles annihilation on a fibre in a thick and thin nematic layer

M. NikkhouM. ŠkarabotS. ČoparI. Muševič

We study topological defect annihilation on a glass fibre with homeotropic surface anchoring of nematic liquid crystal molecules. The fibre is set parallel to the nematic director of a planar cell with variable thickness and we create pairs of Saturn ring and Saturn anti-ring using the laser tweezers. In thick cells we observe in the whole region of defect separation a Coulomb-like pair attraction with no background force, F∝1/dαF∝1/dα with α≈2±0.3α≈2±0.3 . In cells with thickness comparable to glass fibre diameter, we observe the Coulomb-like attraction only at small separations of the defect pair. For separations larger than the fibre diameter, the pair interaction force is independent of separation. This string-like force is attributed to the formation of defect lines, connecting both monopoles and are indeed visible only on extremely confined fibre, where the fibre diameter is practically equal to the nematic layer thickness. Numerical simulations confirm the formation of defect lines connecting both rings.


Integrating Optical Tweezers, DNA Tightropes, and Single-Molecule Fluorescence Imaging: Pitfalls and Traps

J. Wang, J.T. Barnett, M.R. Pollard, N.M. Kad

Fluorescence imaging is one of the cornerstone techniques for understanding how single molecules search for their targets on DNA. By tagging individual proteins, it is possible to track their position with high accuracy. However, to understand how proteins search for targets, it is necessary to elongate the DNA to avoid protein localization ambiguities. Such structures known as “DNA tightropes” are tremendously powerful for imaging target location; however, they lack information about how force and load affect protein behavior. The use of optically trapped microstructures offers the means to apply and measure force effects. Here we describe a system that we recently developed to enable individual proteins to be directly manipulated on DNA tightropes. Proteins bound to DNA can be conjugated with Qdot fluorophores for visualization and also directly manipulated by an optically trapped, manufactured microstructure. Together this offers a new approach to understanding the physical environment of molecules, and the combination with DNA tightropes presents opportunities to study complex biological phenomena.


Self-assembly of colloidal particles in deformation landscapes of electrically driven layer undulations in cholesteric liquid crystals

Michael C. M. Varney, Qiaoxuan Zhang, Bohdan Senyuk, and Ivan I. Smalyukh

We study elastic interactions between colloidal particles and deformation landscapes of undulations in a cholesteric liquid crystal under an electric field applied normal to cholesteric layers. The onset of undulation instability is influenced by the presence of colloidal inclusions and, in turn, layers’ undulations mediate the spatial patterning of particle locations. We find that the bending of cholesteric layers around a colloidal particle surface prompts the local nucleation of an undulations lattice at electric fields below the well-defined threshold known for liquid crystals without inclusions, and that the onset of the resulting lattice is locally influenced, both dimensionally and orientationally, by the initial arrangements of colloids defined using laser tweezers. Spherical particles tend to spatially localize in the regions of strong distortions of the cholesteric layers, while colloidal nanowires exhibit an additional preference for multistable alignment offset along various vectors of the undulations lattice. Magnetic rotation of superparamagnetic colloidal particles couples with the locally distorted helical axis and undulating cholesteric layers in a manner that allows for a controlled three-dimensional translation of these particles. These interaction modes lend insight into the physics of liquid crystal structure-colloid elastic interactions, as well as point the way towards guided self-assembly of reconfigurable colloidal composites with potential applications in diffraction optics and photonics.


Thursday, October 27, 2016

Manipulating the Lorentz force via the chirality of nanoparticles

Maoyan Wang, Hailong Li, Yuliang Dong, Xiaochuan Zhang, Ming Du, Rui Wang, Tong Xu, Jian Wu

We demonstrate that a single plane wave pulls a chiral nanoparticle toward the light source. The nanoparticle exhibits optical gain in a particular wavelength region. The equivalence of the generalized and alternative expressions of the Lorentz force density relating to bound charges for chiral media is numerically validated. By considering the two-dimensional electromagnetic problem of incident plane waves normally impinged on active chiral cylinders, it is shown that the gradient force is mainly contributed by the bound electric and magnetic current densities of the cross-polarized waves. We also investigate how the medium parameters and impedance mismatch can be used to manipulate the pulling or pushing Lorentz forces between two chiral cylinders. This finding may provide a recipe to understand the light interaction with multiple chiral nanoparticles of arbitrary shapes (in general) with the aid of the numerical approach. It could be a promising avenue in controlling the optical micromanipulation for chiral nanoparticles with mirroring asymmetry.


Tapered optical fiber loops and helices for integrated photonic device characterization and microfluidic roller coasters

Yundong Ren, Rui Zhang, Chaoyang Ti, and Yuxiang Liu

Tapered optical fibers with special geometries are desired for probing monolithic in-plane nanophotonic devices, as well as for optical trapping and manipulation. In this work, we demonstrate two special geometries of tapered optical fibers, namely fiber loops and helices. The fiber loops in this work are distinct from previous ones in terms of their superior mechanical stability and high optical quality factors in air, thanks to a post-annealing process. We experimentally measured an intrinsic optical quality factor of 32,500 and a finesse of 137 for a fiber loop. A fiber helix was used to characterize a monolithic cavity optomechanical device. Moreover, a microfluidic “roller coaster” was demonstrated, where microscale particles in water were optically trapped and transported by a fiber helix. Tapered fiber loops and helices can find various applications ranging from on-the-fly characterization of integrated photonic devices to particle manipulation and sorting in microfluidics.


Yeast Nanobiotechnology

Ronnie Willaert, Sandor Kasas, Bart Devreese and Giovanni Dietler

Yeast nanobiotechnology is a recent field where nanotechniques are used to manipulate and analyse yeast cells and cell constituents at the nanoscale. The aim of this review is to give an overview and discuss nanobiotechnological analysis and manipulation techniques that have been particularly applied to yeast cells. These techniques have mostly been applied to the model yeasts Saccharomyces cerevisiae and Schizosaccaromyces pombe, and the pathogenic model yeast Candida albicans. Nanoscale imaging techniques, such as Atomic Force Microscopy (AFM), super-resolution fluorescence microscopy, and electron microscopy (scanning electron microscopy (SEM), transmission electron microscopy (TEM), including electron tomography) are reviewed and discussed. Other nano-analysis methods include single-molecule and single-cell force spectroscopy and the AFM-cantilever-based nanomotion analysis of living cells. Next, an overview is given on nano/microtechniques to pattern and manipulate yeast cells. Finally, direct contact cell manipulation methods, such as AFM-based single cell manipulation and micropipette manipulation of yeast cells, as well as non-contact cell manipulation techniques, such as optical, electrical, and magnetic cells manipulation methods are reviewed.


Characterization of shear stress preventing red blood cells aggregation at the individual cell level: The temperature dependence

Lee, K.; Priezzhev, A.; Shin, S.; Francois, Y.; Meglinski, I.

The novel measure of the red blood cells (RBC) aggregation (RBC-A) – the critical (minimum) shear stress (CSS) to prevent the cells from aggregation was found to be a promising clinically significant parameter. However, the absolute values of this parameter were found to change significantly depending on the shearing geometry (cup-and-bob, cone-plate or microchannel-flow) and have different temperature dependences along with it. The direct confirmation of these dependences aimed to find out the correct values is still pending. In this work, we aim to assess the absolute values of CSS at different temperatures. The single cell level measurements of CSS were performed using optical tweezers. The measurements were carried out in heavily diluted suspensions of RBCs in plasma. RESULTS: The temperature dependent changes in CSS were measured at the points (22 and 38°C), in which the cup-and-bob and cone-plate systems yielded about 1.5-fold different values, while the microchannel-flow system yielded a constant value. The single cell CSS were found to be 362±157 mPa (22°C) and 312±57 mPa (38°C). Our results prove that the microfluidic-flow approach is reflecting the RBC-A correctly. While the CSS values measured with other systems show the temperature dependent effect of the shearing geometry.


Tuesday, October 25, 2016

Kinetics of DNA uptake during transformation provide evidence for a translocation ratchet mechanism

Christof Hepp and Berenike Maier

Horizontal gene transfer can speed up adaptive evolution and support chromosomal DNA repair. A particularly widespread mechanism of gene transfer is transformation. The initial step to transformation, namely the uptake of DNA from the environment, is supported by the type IV pilus system in most species. However, the molecular mechanism of DNA uptake remains elusive. Here, we used single-molecule techniques for characterizing the force-dependent velocity of DNA uptake by Neisseria gonorrhoeae. We found that the DNA uptake velocity depends on the concentration of the periplasmic DNA-binding protein ComE, indicating that ComE is directly involved in the uptake process. The velocity–force relation of DNA uptake is in very good agreement with a translocation ratchet model where binding of chaperones in the periplasm biases DNA diffusion through a membrane pore in the direction of uptake. The model yields a speed of DNA uptake of 900 bp⋅s−1 and a reversal force of 17 pN. Moreover, by comparing the velocity–force relation of DNA uptake and type IV pilus retraction, we can exclude pilus retraction as a mechanism for DNA uptake. In conclusion, our data strongly support the model of a translocation ratchet with ComE acting as a ratcheting chaperone.


Light-driven micro-tool equipped with a syringe function

Mark Jayson Villangca, Darwin Palima, Andrew Rafael Bañas and Jesper Glückstad

Leveraging developments in microfabrication open new possibilities for optical manipulation. With the structural design freedom from three-dimensional printing capabilities of two-photon polymerization, we are starting to see the emergence of cleverly shaped ‘light robots’ or optically actuated micro-tools that closely resemble their macroscopic counterparts in function and sometimes even in form. In this work, we have fabricated a new type of light robot that is capable of loading and unloading cargo using photothermally induced convection currents within the body of the tool. We have demonstrated this using silica and polystyrene beads as cargo. The flow speeds of the cargo during loading and unloading are significantly larger than when using optical forces alone. This new type of light robot presents a mode of material transport that may have a significant impact on targeted drug delivery and nanofluidics injection.


Optical disassembly of cellular clusters by tunable ‘tug-of-war’ tweezers

Anna S Bezryadina, Daryl C Preece, Joseph C Chen and Zhigang Chen

Bacterial biofilms underlie many persistent infections, posing major hurdles in antibiotic treatment. Here we design and demonstrate ‘tug-of-war’ optical tweezers that can facilitate the assessment of cell–cell adhesion—a key contributing factor to biofilm development, thanks to the combined actions of optical scattering and gradient forces. With a customized optical landscape distinct from that of conventional tweezers, not only can such ‘tug-of-war’ tweezers stably trap and stretch a rod-shaped bacterium in the observing plane, but, more importantly, they can also impose a tunable lateral force that pulls apart cellular clusters without any tethering or mechanical movement. As a proof of principle, we examined a Sinorhizobium meliloti strain that forms robust biofilms and found that the strength of intercellular adhesion depends on the growth medium. This technique may herald new photonic tools for optical manipulation and biofilm study, as well as other biological applications.


Monday, October 24, 2016

Plasmonic trapping of sub-micro objects with metallic antennae

Eishi Sugawara, Jun-ichi Kato, Yutaka Yamagata, Miyu Ozaki and Ryoshu Furutani

Since optical trapping was first reported, its methods and targets have been broadened. In this paper, we propose 'plasmonic clipping', which traps objects on the plasmonic dot array. Localized surface plasmon polaritons (LSPPs), which localize optical energy in the nanometer-scale size and enhances the optical field, are excited in gaps between the dots. The objects are trapped by electric-field-gradient forces of LSPPs along the dot array. The dot arrays are arranged radially so that LSPPs are selectively excited in dot array corresponding to polarization direction of excitation light. The selective excitation results in directionally-selective 'plasmonic clipping'. The radial dot arrays made of silver are numerically designed and fabricated by means of a focused ion beam (FIB). The arrays are illuminated with laser beam through the half wavelength plate to rotate polarization direction. As a result, the plasmonic clipping is observed along the array corresponding to polarization of the excitation light. It is expected to be utilized to align functional components for manufacturing, measurement, and material technologies.


Dynamic enhancement of autofocusing property for symmetric Airy beam with exponential amplitude modulation

Weiwei Liu, Yao Lu, Lei Gong, Xiuxiang Chu, Guosheng Xue, Yuxuan Ren, Mincheng Zhong, Ziqiang Wang, Jinhua Zhou and Yinmei Li

A symmetric Airy beam (SAB) autofocuses during free space propagation. Such autofocusing SAB is useful in optical manipulation and biomedical imaging. However, its inherently limited autofocusing property may degrade the performance of the SAB in those applications. To enhance the autofocus, a symmetric apodization mask was proposed to regulate the SAB. In combination with the even cubic phase that shapes the SAB, this even exponential function mask with an adjustable parameter regulates the contribution of different frequency spectral components to the SAB. The propagation properties of this new amplitude modulated SAB (AMSAB) were investigated both theoretically and experimentally. Simulation shows that the energy distribution and autofocusing property of an AMSAB can be adjusted by the exponential amplitude modulation. Especially, the beam energy will be more concentrated in the central lobe once the even cubic phase is modulated by the mask with a higher proportion of high-frequency spectral components. Consequently, the autofocusing property and axial gradient force of AMSABs are efficiently enhanced. The experimental generation and characterization for AMSABs were implemented by modulating the collimated beam with a phase-only spatial light modulator. The experimental results well supported the theoretical predictions. With the ability to enhance the autofocus, the proposed exponential apodization modulation will make SAB more powerful in various applications, including optical trapping, fluorescence imaging and particle acceleration.


Optical trapping of nanoparticles by full solid-angle focusing

Vsevolod Salakhutdinov, Markus Sondermann, Luigi Carbone, Elisabeth Giacobino, Alberto Bramati, and Gerd Leuchs

Optical dipole traps are used for trapping and localizing particles in various scientific fields, including classical optics, quantum optics, and biophysics. Here, we propose and implement a dipole trap for nanoparticles that is based on focusing from the full solid angle with a deep parabolic mirror. The key aspect is the generation of a linear-dipole mode, which is predicted to provide a tight trapping potential. We demonstrate the trapping of rod-shaped nanoparticles and validate the trapping frequencies to be of the order of the expected ones. The described realization of an optical trap is applicable for various other kinds of solid-state targets. The obtained results demonstrate the feasibility of optical dipole traps that simultaneously provide high trap stiffness and allow for efficient interaction of light and matter in free space.


Optically bound colloidal lattices in evanescent optical fields

Xiang Han, Hui Luo, Guangzong Xiao, and Philip H. Jones

In this Letter, we demonstrate the formation of a stable two-dimensional lattice of colloidal particles in the interference pattern formed by four evanescent optical fields at a dielectric interface. The microspheres are observed to form a two-dimensional square lattice with lattice vectors inclined relative to the beam propagation directions. We use digital video microscopy and particle tracking to measure the Brownian motion of particles bound in the lattice, and use this to characterize fluctuations in the local ordering of particles using the bond orientational order parameter, the probability distribution of which is shown to be a chi-squared distribution. An explanation for the form of this distribution is presented in terms of fluctuations of the modes of a ring of particles connected by springs.


Friday, October 21, 2016

Detecting Swelling States of Red Blood Cells by “Cell–Fluid Coupling Spectroscopy”

Carla Zensen, Isis E. Fernandez, Oliver Eickelberg, Jochen Feldmann, Theobald Lohmüller

Red blood cells are “shaken” with a holographic optical tweezer array. The flow generated around cells due to the periodic optical forcing is measured with an optically trapped “detector” particle located in the cell vicinity. A signal-processing model that describes the cell's physical properties as an analog filter illustrates how cells can be distinguished from each other.


Polymorphic beams and Nature inspired circuits for optical current

José A. Rodrigo & Tatiana Alieva

Laser radiation pressure is a basis of numerous applications in science and technology such as atom cooling, particle manipulation, material processing, etc. This light force for the case of scalar beams is proportional to the intensity-weighted wavevector known as optical current. The ability to design the optical current according to the considered application brings new promising perspectives to exploit the radiation pressure. However, this is a challenging problem because it often requires confinement of the optical current within tight light curves (circuits) and adapting its local value for a particular task. Here, we present a formalism to handle this problem including its experimental demonstration. It consists of a Nature-inspired circuit shaping with independent control of the optical current provided by a new kind of beam referred to as polymorphic beam. This finding is highly relevant to diverse optical technologies and can be easily extended to electron and x-ray coherent beams.


Controllable optical trap arrays

N. V. Shostka, M. O. Ivanov,V. I. Shostka
A method of generating 3D optical trap arrays (OTAs) using a uniaxial crystal has been proposed and implemented. It is shown that the properties of obtained OTAs can be controlled by changing the parameters of the optical system and the state of radiation polarization past the crystal.


Tuesday, October 18, 2016

Single-Molecule Interactions of a Monoclonal Anti-DNA Antibody with DNA

Tatiana A. Nevzorova, Qingze Zhao, Yakov A. Lomakin, Anastasia A. Ponomareva, Alexander R. Mukhitov, Prashant K. Purohit, John W. Weisel, Rustem I. Litvinov

Interactions of DNA with proteins are essential for key biological processes and have both a fundamental and practical significance. In particular, DNA binding to anti-DNA antibodies is a pathogenic mechanism in autoimmune pathology, such as systemic lupus erythematosus. Here we measured at the single-molecule level binding and forced unbinding of surface-attached DNA and a monoclonal anti-DNA antibody MRL4 from a lupus erythematosus mouse. In optical trap-based force spectroscopy, a microscopic antibody-coated latex bead is trapped by a focused laser beam and repeatedly brought into contact with a DNA-coated surface. After careful discrimination of non-specific interactions, we showed that the DNA-antibody rupture force spectra had two regimes, reflecting formation of weaker (20–40 pN) and stronger (>40 pN) immune complexes that implies the existence of at least two bound states with different mechanical stability. The two-dimensional force-free off-rate for the DNA-antibody complexes was ∼2.2 × 10−3 s−1, the transition state distance was ∼0.94 nm, the apparent on-rate was ∼5.26 s−1, and the stiffness of the DNA-antibody complex was characterized by a spring constant of 0.0021 pN/nm, suggesting that the DNA-antibody complex is a relatively stable, but soft and deformable macromolecular structure. The stretching elasticity of the DNA molecules was characteristic of single-stranded DNA, suggesting preferential binding of the MRL4 antibody to one strand of DNA. Collectively, the results provide fundamental characteristics of formation and forced dissociation of DNA-antibody complexes that help to understand principles of DNA-protein interactions and shed light on the molecular basis of autoimmune diseases accompanied by formation of anti-DNA antibodies.


Hybrid microfluidics combined with active and passive approaches for continuous cell separation

Sheng Yan, Jun Zhang, Dan Yuan, Weihua Li

Microfluidics, which is classified as either active or passive, is capable of separating cells of interest from a complex and heterogeneous sample. Active methods utilise external fields such as electric, magnetic, acoustic, and optical to drive cells for separation, while passive methods utilise channel structures, intrinsic hydrodynamic forces, and steric hindrances to manipulate cells. However, when processing complex biological samples such as whole blood with rare cells, separation with a single module microfluidic device is difficult. Hybrid microfluidics is an emerging technique which utilises active and passive methods whilst fulfilling higher requirements for stable performance, versatility, and convenience, including (i) the ability to process multi-target cells, (ii) enhanced ability for multiplexed separation, (iii) higher sensitivity, and (iv) tunability for a wider operational range. This review introduces the fundamental physics and typical formats for subclasses of hybrid microfluidic devices based on their different physical fields; presents current examples of cell sorting to highlight the advantage and usefulness of hybrid microfluidics on biomedicine, and then discusses the challenges and perspective of future development and the promising direction of research in this field.


Marangoni effect visualized in two-dimensions Optical tweezers for gas bubbles

A. Miniewicz, S. Bartkiewicz, H. Orlikowska & K. Dradrach

In the report we demonstrate how, using laser light, effectively trap gas bubbles and transport them through a liquid phase to a desired destination by shifting the laser beam position. The physics underlying the effect is complex but quite general as it comes from the limited to two-dimension, well-known, Marangoni effect. The experimental microscope-based system consists of a thin layer of liquid placed between two glass plates containing a dye dissolved in a solvent and a laser light beam that is strongly absorbed by the dye. This point-like heat source locally changes surface tension of nearby liquid-air interface. Because of temperature gradients a photo-triggered Marangoni flows are induced leading to self-amplification of the effect and formation of large-scale whirls. The interface is bending toward beam position allowing formation of a gas bubble upon suitable beam steering. Using various techniques (employing luminescent particles or liquid crystals), we visualize liquid flows propelled by the tangential to interface forces. This helped us to understand the physics of the phenomenon and analyze accompanying effects leading to gas bubble trapping. The manipulation of sessile droplets moving on the glass surface induced via controlled with laser light interface bending (i.e. “droplet catapult”) is demonstrated as well.


Controlling and tracking of colloidal nanostructures through two-photon fluorescence

Dipankar Mondal and Debabrata Goswami

Multiphoton absorbing dye-coated trapped spherical bead at the focal plane of femtosecond optical tweezers shows nonlinear optical (NLO) phenomena. One such NLO process of two-photon fluorescence (TPF) has been used for the background-free imaging of a femtosecond laser-trapping event. Due to the high peak powers of femtosecond laser pulses with low average powers, it is possible to not only trap single nanospheres, but encourage optically directed self-assembly. The TPF signatures of trapped particles show evidence of such a directed self-assembly process which, in turn, can provide information about the structural dynamics during the process of cluster formation. We are able to trap and characterize structure and dynamics in 3D until pentamer formation from the decay characteristics of trapping at the focal plane.


Study of a Microfluidic Chip Integrating Single Cell Trap and 3D Stable Rotation Manipulation

Liang Huang, Long Tu, Xueyong Zeng, Lu Mi, Xuzhou Li and Wenhui Wang
Single cell manipulation technology has been widely applied in biological fields, such as cell injection/enucleation, cell physiological measurement, and cell imaging. Recently, a biochip platform with a novel configuration of electrodes for cell 3D rotation has been successfully developed by generating rotating electric fields. However, the rotation platform still has two major shortcomings that need to be improved. The primary problem is that there is no on-chip module to facilitate the placement of a single cell into the rotation chamber, which causes very low efficiency in experiment to manually pipette single 10-micron-scale cells into rotation position. Secondly, the cell in the chamber may suffer from unstable rotation, which includes gravity-induced sinking down to the chamber bottom or electric-force-induced on-plane movement. To solve the two problems, in this paper we propose a new microfluidic chip with manipulation capabilities of single cell trap and single cell 3D stable rotation, both on one chip. The new microfluidic chip consists of two parts. The top capture part is based on the least flow resistance principle and is used to capture a single cell and to transport it to the rotation chamber. The bottom rotation part is based on dielectrophoresis (DEP) and is used to 3D rotate the single cell in the rotation chamber with enhanced stability. The two parts are aligned and bonded together to form closed channels for microfluidic handling. Using COMSOL simulation and preliminary experiments, we have verified, in principle, the concept of on-chip single cell traps and 3D stable rotation, and identified key parameters for chip structures, microfluidic handling, and electrode configurations. The work has laid a solid foundation for on-going chip fabrication and experiment validation.


Monday, October 17, 2016

High-resolution, hybrid optical trapping methods, and their application to nucleic acid processing proteins

Yann R. Chemla

Optical tweezers have become a powerful tool to investigate nucleic-acid processing proteins at the single-molecule level. Recent advances in this technique have now enabled measurements resolving the smallest units of molecular motion, on the scale of a single base pair of DNA. In parallel, new instrumentation combining optical traps with other functionalities have been developed, incorporating mechanical manipulation along orthogonal directions or fluorescence imaging capabilities. Here, we review these technical advances, their capabilities, and limitations, focusing on benchmark studies of protein-nucleic acid interactions they have enabled. We highlight recent work that combines several of these advances together and its application to nucleic-acid processing enzymes. Finally, we discuss future prospects for these exciting developments.


Label-free optical sensor based on red blood cells laser tweezers Raman spectroscopy analysis for ABO blood typing

Duo Lin, Zuci Zheng, Qiwen Wang, Hao Huang, Zufang Huang, Yun Yu, Sufang Qiu, Cuncheng Wen, Min Cheng, and Shangyuan Feng

The clinical significance of ABO blood typing extends beyond transfusion medicine and is demonstrated to be associated with susceptibility to various diseases, even including cancer. In this study, a home-made laser tweezers Raman spectroscopy (LTRS) system was applied to detect red blood cells (RBCs) with the aim to develop a label-free, simple and objective blood typing method for the first time. High-quality Raman spectra of RBCs in the fingerprint region of 420-1700 cm−1 can be obtained, meanwhile exciting blood typing results can be achieved, especially with an accuracy of 100% for identifying Type AB from other blood types with the use of multivariate statistical analysis based on principal component analysis (PCA) combined with linear discriminant analysis (LDA). This primary work demonstrates that the label-free RBCs LTRS analysis in conjunction with PCA-LDA diagnostic algorithms has great potential as a biosensor for ABO blood typing.


Biophysical Tools for Cellular and Subcellular Mechanical Actuation of Cell Signaling

Allen P. Liu

The ability to spatially control cell signaling can help resolve fundamental biological questions. Optogenetic and chemical dimerization techniques along with fluorescent biosensors to report cell signaling activities have enabled researchers to both visualize and perturb biochemistry in living cells. A number of approaches based on mechanical actuation using force-field gradients have emerged as complementary technologies to manipulate cell signaling in real time. This review covers several technologies, including optical, magnetic, and acoustic control of cell signaling and behavior and highlights some studies that have led to novel insights. I will also discuss some future direction on repurposing mechanosensitive channel for mechanical actuation of spatial cell signaling.


DNA-doxorubicin interaction: new insights and peculiarities

E. F. Silva, R. F. Bazoni, E. B. Ramos, M. S. Rocha

We have investigated the interaction of the DNA molecule with the anticancer drug doxorubicin (doxo) by using three different experimental techniques: single molecule stretching, single molecule imaging and dynamic light scattering. Such techniques allowed us to get new insights on the mechanical behavior of the DNA-doxo complexes as well as on the physical chemistry of the interaction. Firstly, the contour length data obtained from single molecule stretching were used to extract the physicochemical parameters of the DNA-doxo interaction under different buffer conditions. This analysis has proven that the physical chemistry of such interaction can be modulated by changing the ionic strength of the surrounding buffer. In particular we have found that at low ionic strengths doxo interacts with DNA by simple intercalation (no aggregation) and/or by forming bound dimers. For high ionic strengths, otherwise, doxo-doxo self-association is enhanced, giving rise to the formation of bound doxo aggregates composed by 3 to 4 molecules along the double-helix. On the other hand, the results obtained for the persistence length of the DNA-doxo complexes is strongly force-dependent, presenting different behaviors when measured with stretching or non-stretching techniques. This article is protected by copyright. All rights reserved.


Friday, October 14, 2016

A Force-Induced Directional Switch of a Molecular Motor Enables Parallel Microtubule Bundle Formation

Maxim I. Molodtsov, Christine Mieck, Jeroen Dobbelaere, Alexander Dammermann, Stefan Westermann, Alipasha Vaziri

Microtubule-organizing centers (MTOCs) nucleate microtubules that can grow autonomously in any direction. To generate bundles of parallel microtubules originating from a single MTOC, the growth of multiple microtubules needs to coordinated, but the underlying mechanism is unknown. Here, we show that a conserved two-component system consisting of the plus-end tracker EB1 and the minus-end-directed molecular motor Kinesin-14 is sufficient to promote parallel microtubule growth. The underlying mechanism relies on the ability of Kinesin-14 to guide growing plus ends along existing microtubules. The generality of this finding is supported by yeast, Drosophila, and human EB1/Kinesin-14 pairs. We demonstrate that plus-end guiding involves a directional switch of the motor due to a force applied via a growing microtubule end. The described mechanism can account for the generation of parallel microtubule networks required for a broad range of cellular functions such as spindle assembly or cell polarization.


Optical configurations for photophoretic trap of single particles in air

Zhiyong Gong, Yong-Le Pan and Chuji Wang

Since Ashkin’s pioneering work in the 1970’s, optical trapping (OT) and manipulation have become an indispensable tool in diverse research fields. Today, there are multiple optical trapping schemes in use. In this article, we explore six different optical trapping schemes based on the photophoretic force (PPF). Within these schemes we explore 21 variants differing in such details as laser source, power, beam shape, and focusing optics. We evaluate and rate the trapping quality and performance of the six trapping schemes in terms of four key aspects: simplicity, robustness, flexibility, and efficiency. One of the schemes is novel: we introduce a simple, high quality scheme using a confocal design in which one trapping beam is effectively converted to two counter-propagating beams. The versatility of this new trapping scheme is demonstrated via application of the scheme to cavity ringdown spectroscopy. We hope this exploration of the diversity of PPF trapping schemes will extend applications of OT by providing researchers with information to assist in the selection of specific optical trapping schemes from the first-of-its-kind list of 21 configurations presented herein.


Temperature-driven volume phase transition of a single stimuli-responsive microgel particle using optical tweezers

Deepak K. Gupta, D. Karthickeyan, B. V. R. Tata, T. R. Ravindran

Poly(N-isopropylacrylamide) (PNIPAM)-based microgels respond to temperature and exhibits a transition from swollen to deswollen state upon variation of temperature, which is known as volume phase transition (VPT). Dynamic light scattering (DLS) is a popular technique to identify the volume phase transition temperature (VPTT) of microgel particles, which measures variation of particle size with temperature in a suspension having microgel particle concentration of 107–108 particles/cm3. Here, we employ optical tweezers to trap a single microgel particle and identify its VPTT by measuring the lateral trap stiffness, κ as a function temperature. It is shown that near the VPTT, κ increases gradually upon increasing temperature, which is due to a gradual decrease in particle size with simultaneous increase in its refractive index.


Cylindrical particle manipulation and negative spinning using a nonparaxial Hermite−Gaussian light-sheet beam

F G Mitri

Based on the angular spectrum decomposition method (ASDM), a nonparaxial solution for the Hermite−Gaussian (HG m ) light-sheet beam of any order m is derived. The beam-shape coefficients (BSCs) are expressed in a compact form and computed using the standard Simpson's rule for numerical integration. Subsequently, the analysis is extended to evaluate the longitudinal and transverse radiation forces as well as the spin torque on an absorptive dielectric cylindrical particle in 2D without any restriction to a specific range of frequencies. The dynamics of the cylindrical particle are also examined based on Newton's second law of motion. The numerical results show that a Rayleigh or Mie cylindrical particle can be trapped, pulled or propelled in the optical field depending on its initial position in the cross-sectional plane of the HG m light-sheet. Moreover, negative or positive axial spin torques can arise depending on the choice of the non-dimensional size parameter ka (where k is the wavenumber and a is the radius of the cylinder) and the location of the absorptive cylinder in the beam. This means that the HG m light-sheet beam can induce clockwise or anti-clockwise rotations depending on its shift from the center of the cylinder. In addition, individual vortex behavior can arise in the cross-sectional plane of wave propagation. The present analysis presents an analytical model to predict the optical radiation forces and torque induced by a HG m light-sheet beam on an absorptive cylinder for applications in optical light-sheet tweezers, optical micro-machines, particle manipulation and opto-fluidics to name a few areas of research.


Vortex-based line beam optical tweezers

Shubo Cheng and Shaohua Tao

A vortex-based line beam, which has a straight-line shape of intensity and possesses phase gradient along the line trajectory is developed and applied for optical manipulation in this paper. The intensity and phase distributions of the beam in the imaging plane of the Fourier transform are analytically studied. Simulation results show that the length of the line and phase gradient possessed by a vortex-based line beam are dependent on the topological charge and the azimuthal proportional constant. A superposition of multiple phase-only holograms with elliptical azimuthal phases can be used to generate an array of vortex-based line beams. Optical trapping with the vortex-based line beams has been implemented. Furthermore, the automatic transportation of microparticles along the line trajectory perpendicular to the optical axis is realized with an array of the beams. The generation method for the vortex-based line beam is simple. The beam would have potential applications in fields such as optical trapping, laser machining, and so on. 

Thursday, October 13, 2016

Single-cell bacterium identification with a SOI optical microcavity

M. Tardif, J.-B. Jager, P. R. Marcoux, K. Uchiyamada, E. Picard, E. Hadji and D. Peyrade

Photonic crystals and microcavities act as on-chip nano-optical tweezers for identification and manipulation of biological objects. Until now, optical trapping of virus and bacteria has been achieved and their presence in the vicinity of the optical resonator is deduced by the shift in the resonant wavelength. Here, we show trapping and identification of bacteria through a properly tuned silicon on insulator microcavity. Through the spatial and temporal observations of bacteria–cavity interaction, the optical identification of three different kinds of bacteria is demonstrated.


Optical tractor Bessel polarized beams

F.G. Mitri, R.X. Li, L.X. Guo, C.Y. Ding

Axial and transverse radiation force cross-sections of optical tractor Bessel polarized beams are theoretically investigated for a dielectric sphere with particular emphasis on the beam topological charge (or order), half-cone angle and polarization. The angular spectrum decomposition method (ASDM) is used to derive the non-paraxial electromagnetic (EM) field components of the Bessel beams. The multipole expansion method using vector spherical harmonics is utilized and appropriate beam-shape coefficients are derived in order to compute the radiation force cross-sections. The analysis has no limitation to a particular range of frequencies such that the Rayleigh, Mie or geometrical optics regimes can all be considered effectively using the present rigorous formalism. The focus of this investigation is to identify some of the tractor beam conditions so as to achieve retrograde motion of a dielectric sphere located arbitrarily in space. Numerical computations for the axial and transverse radiation force cross-sections are presented for linear, right-circular, radial, azimuthal and mixed polarizations of the individual plane waves forming the Bessel beams of zeroth- and first-order (with positive or negative helicity), respectively. As the sphere shifts off the beam׳s axis, the axial pulling (tractor) force is weakened. Moreover, the transverse radiation force cross-section field changes with the sphere׳s size factor ka (where k is the wavenumber and a is the sphere radius). Both stable and unstable equilibrium regions around the beam׳s axis are found, depending on the choice of ka and the half-cone angle α0. These results are particularly important in the development of emergent technologies for the photophoretic assembly of optically-engineered (meta)materials with designed properties using optical tractor (vortex) beams, particle manipulation, levitation and positioning, and other applications.


Optical tweezers with fractional fractal zone plate

Shubo Cheng ; Shaohua Tao ; Xinyu Zhang ; Wenzhuo Ma

Free-space propagations of the optical beams generated by the fractal zone plates with fractional structural parameters (i.e., fractional FZP) are analytically studied in this paper. The results demonstrate that the axial location of the main focus and axial distance between two neighboring foci of the fractional FZP beam can be precisely customized. Furthermore, we first demonstrate optical manipulation with the fractional FZP beam. The experimental results verified that such an FZP beam can simultaneously trap multiple particles positioned in different focal planes of the beam owing to the multiple foci and self-reconstruction property of the FZP beam. The customized locations of trapped particles can also be realized by the fractional FZP beam, which would be useful for constructing three-dimensional optical tweezers


Enhanced Transverse Optical Force between Paired Graphene Nanoribbons

Lu Sun ; Sinan Lai ; Chun Jiang

We present a study of transverse optical forces arising in a free-standing graphene nanoribbon pair. The force is evaluated using a numerical procedure based on finite difference time domain simulations. We find that optical forces on the order of nN·μm-1·mW-1 can be obtained in both vertically and horizontally aligned ribbon pair. The influence of graphene loss on the total optical force obtainable is also investigated in this paper. Numerical results show that we can get greater transverse optical force in a shorter actuation distance, considerably promising for nano-optomechanical applications.


Tuesday, October 11, 2016

Manipulation of resonant metallic nanoparticle using 4Pi focusing system

Xiaoyan Wang, Guanghao Rui, Liping Gong, Bing Gu, and Yiping Cui

Metallic nanoparticles have fascinated scientists for over a century and are now heavily utilized in biomedical sciences and engineering. Due to its noncontact and holding nature, optical trapping is suitable to be combined with various applications to manipulate metallic nanoparticles. However, stable trapping of resonant metallic nanoparticles remains challenging due to the strong axial scattering force and severe optical heating effect. In this work, we propose novel optical tweezers constructed around a 4Pi focusing system that is capable of manipulating metallic nanoparticles even under the resonant condition. By properly modulating the spatial distribution of the illumination and adjusting the focusing condition, specific numbers of spherical spots with controllable locations can be generated in the focal region, providing multiple probes to interrogate the sample properties. Besides, stable three-dimensional optical trapping can be formed since the axial scattering force is canceled by the counter-propagating light. The greatly enhanced optical force arising from the extremely high focusing efficiency of the 4Pi focusing system enables to avoid the overheating effect by reducing the input power without destroying the mechanical stability. Moreover, complex motion trajectory of the metallic nanoparticles can be realized via introducing specific phase modulation to the illumination sequentially. The technique demonstrated in this work may open up new avenues for optical manipulation and their applications in various scientific fields.


Mechanical perturbations trigger endothelial nitric oxide synthase activity in human red blood cells

Shunmugan Nagarajan, Rajendran Kadarkarai Raj, Venkatesan Saravanakumar, Uma Maheswari Balaguru, Jyotirmaya Behera, Vinoth Kumar Rajendran, Yogarajan Shathya, B. Mohammed Jaffar Ali, Venil Sumantran & Suvro Chatterjee

Nitric oxide (NO), a vascular signaling molecule, is primarily produced by endothelial NO synthase. Recently, a functional endothelial NO synthase (eNOS) was described in red blood cells (RBC). The RBC-eNOS contributes to the intravascular NO pool and regulates physiological functions. However the regulatory mechanisms and clinical implications of RBC-eNOS are unknown. The present study investigated regulation and functions of RBC-eNOS under mechanical stimulation. This study shows that mechanical stimuli perturb RBC membrane, which triggers a signaling cascade to activate the eNOS. Extracellular NO level, estimated by the 4-Amino-5-Methylamino-2′, 7′-Difluorofluorescein Diacetate probe, was significantly increased under mechanical stimuli. Immunostaining and western blot studies confirmed that the mechanical stimuli phosphorylate the serine 1177 moiety of RBC-eNOS, and activates the enzyme. The NO produced by activation of RBC-eNOS in vortexed RBCs promoted important endothelial functions such as migration and vascular sprouting. We also show that mechanical perturbation facilitates nitrosylation of RBC proteins via eNOS activation. The results of the study confirm that mechanical perturbations sensitize RBC-eNOS to produce NO, which ultimately defines physiological boundaries of RBC structure and functions. Therefore, we propose that mild physical perturbations before, after, or during storage can improve viability of RBCs in blood banks.


Dominant chiral optical forces in the vicinity of optical nanofibers

M. H. Alizadeh and B. M. Reinhard

Transverse spin angular momentum (SAM) of light and associated transverse chiral optical forces have received tremendous attention recently, as the latter may lead to an optical separation of chiral biomolecules. In this context, the relative magnitude of chiral and non-chiral forces is a challenge when implementing chiral separation schemes. In this work we have demonstrated that, by spatially separating the maxima of transverse spin density from the gradient of field intensity, it is possible to dominate chiral-specific components of the force over non-chiral ones. To that end, we studied optical nanofibers and nanowires as candidates for such a scheme and demonstrated that in their vicinity, chiral optical forces can emerge that are stronger than gradient and scattering forces. This finding may be of significance in the design of improved optical separation schemes for chiral biomolecules.


Fractal zone plate beam based optical tweezers

Shubo Cheng, Xinyu Zhang, Wenzhuo Ma & Shaohua Tao

We demonstrate optical manipulation with an optical beam generated by a fractral zone plate (FZP). The experimental results show that the FZP beam can simultaneously trap multiple particles positioned in different focal planes of the FZP beam, owing to the multiple foci and self-reconstruction property of the FZP beam. The FZP beam can also be used to construct three-dimensional optical tweezers for potential applications.


Monday, October 10, 2016

Phototaxis of synthetic microswimmers in optical landscapes

Celia Lozano, Borge ten Hagen, Hartmut Löwen & Clemens Bechinger

Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient. Here we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles because of diffusiophoretic torques, which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Since the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Because light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.


Theoretical Modeling of Average Force Acted on Nano Plasma Spheres in Presence of Radiation of Long Wavelength Point Source

Z. Hajijamali-AraniB. Jazi, S. Jahanbakht

Using the solutions of field equation, due to the electromagnetic wave scattering phenomena from a nano plasma sphere(NPS), the inserted force on nano plasma sphere is simulated. For this purpose, with using suitable Green’s function, the scattering phenomena of long wavelength electromagnetic waves from a nano plasma sphere will be investigated. A point electromagnetic source is considered in finite distance from the NPS. The computations will be generalized to two monopole point sources with the same strength but with opposite sign in two sides of sphere to simulating a NPS in presence of a plane electromagnetic wave. The resonance frequency and pattern scattering for these problems will be presented. The graphs of variations of inserted force with respect to wave frequency and geometrical dimension variations are presented.


Whispering gallery mode temperature sensor of liquid microresonastor

Zhihai Liu, Lu Liu, Zongda Zhu, Yu Zhang, Yong Wei, Xiaonan Zhang, Enming Zhao, Yaxun Zhang, Jun Yang, and Libo Yuan

We propose and demonstrate a whispering gallery mode (WGM) resonance-based temperature sensor, where the microresonator is made of a DCM (2-[2-[4-(dimethylamino)phenyl] ethenyl]-6-methyl-4H-pyran-4-ylidene)-doped oil droplet (a liquid material) immersed in the water solution. The oil droplet is trapped, controlled, and located by a dual-fiber optical tweezers, which prevents the deformation of the liquid droplet. We excite the fluorescence and lasing in the oil droplet and measure the shifts of the resonance wavelength at different temperatures. The results show that the resonance wavelength redshifts when the temperature increases. The testing sensitivity is 0.377 nm/°C in the temperature range 25°C–45°C. The results of the photobleaching testing of the dye indicate that measured errors can be reduced by reducing the measured time. As far as we know, this is the first time a WGM temperature sensor with a liquid state microcavity has been proposed. Compared with the solid microresonator, the utilization of the liquid microresonator improves the thermal sensitivity and provides the possibility of sensing in liquid samples or integrating into the chemical analyzers and microfluidic systems.


Biophysical approaches promote advances in the understanding of von Willebrand factor processing and function

Achim Löf, Jochen P. Müller, Martin Benoit, Maria A. Brehm

The large multimeric plasma glycoprotein von Willebrand factor (VWF) is essential for primary hemostasis by recruiting platelets to sites of vascular injury. VWF multimers respond to elevated hydrodynamic forces by elongation, thereby increasing their adhesiveness to platelets. Thus, the activation of VWF is force-induced, as is its inactivation. Due to these attributes, VWF is a highly interesting system from a biophysical point of view, and is well suited for investigation using biophysical approaches. Here, we give an overview on recent studies that predominantly employed biophysical methods to gain novel insights into multiple aspects of VWF: Electron microscopy was used to shed light on the domain structure of VWF and the mechanism of VWF secretion. High-resolution stochastic optical reconstruction microscopy, atomic force microscopy (AFM), microscale thermophoresis and fluorescence correlation spectroscopy allowed identification of protein disulfide isomerase isoform A1 as the VWF dimerizing enzyme and, together with molecular dynamics simulations, postulation of the dimerization mechanism. Advanced mass spectrometry led to detailed identification of the glycan structures carried by VWF. Microfluidics was used to illustrate the interplay of force and VWF function. Results from optical tweezers measurements explained mechanisms of the force-dependent functions of VWF's domains A1 and A2 and, together with thermodynamic approaches, increased our understanding of mutation-induced dysfunctions of platelet-binding. AFM-based force measurements and AFM imaging enabled exploration of intermonomer interactions and their dependence on pH and divalent cations. These advances would not have been possible by the use of biochemical methods alone and show the benefit of interdisciplinary research approaches.


Friday, October 7, 2016

Number Density Distribution of Small Particles around a Large Particle: Structural Analysis of a Colloidal Suspension

Ken-ichi Amano, Mitsuhiro Iwaki, Kota Hashimoto, Kazuhiro Fukami, Naoya Nishi, Ohgi Takahashi, and Tetsuo Sakka
Some colloidal suspensions contain two types of particles—small and large particles—to improve the lubricating ability, light absorptivity, etc. Structural and chemical analyses of such colloidal suspensions are often performed to understand their properties. In a structural analysis study, the observation of the number density distribution of small particles around a large particle (gLS) is difficult because these particles are randomly moving within the colloidal suspension by Brownian motion. We obtained gLS using the data from a line optical tweezer (LOT) which can measure the potential of mean force between two large colloidal particles (ΦLL). We propose a theory that transforms ΦLL into gLS. The transform theory is explained in detail and tested. We demonstrate for the first time that LOT can be used for the structural analysis of a colloidal suspension. LOT combined with the transform theory will facilitate structural analyses of the colloidal suspensions, which is important for both understanding colloidal properties and developing colloidal products.


Substrate-translocating loops regulate mechanochemical coupling and power production in AAA+ protease ClpXP

Piere Rodriguez-Aliaga, Luis Ramirez, Frank Kim, Carlos Bustamante & Andreas Martin

ATP-dependent proteases of the AAA+ family, including Escherichia coli ClpXP and the eukaryotic proteasome, contribute to maintenance of cellular proteostasis. ClpXP unfolds and translocates substrates into an internal degradation chamber, using cycles of alternating dwell and burst phases. The ClpX motor performs chemical transformations during the dwell and translocates the substrate in increments of 1–4 nm during the burst, but the processes occurring during these phases remain unknown. Here we characterized the complete mechanochemical cycle of ClpXP, showing that ADP release and ATP binding occur nonsequentially during the dwell, whereas ATP hydrolysis and phosphate release occur during the burst. The highly conserved translocating loops within the ClpX pore are optimized to maximize motor power generation, the coupling between chemical and mechanical tasks, and the efficiency of protein processing. Conformational resetting of these loops between consecutive bursts appears to determine ADP release from individual ATPase subunits and the overall duration of the motor's cycle.


Tunable potential well for plasmonic trapping of metallic particles by bowtie nano-apertures

Yu Lu, Guangqing Du, Feng Chen, Qing Yang, Hao Bian, Jiale Yong & Xun Hou

In this paper, the tunable optical trapping dependence on wavelength of incident beam is theoretically investigated based on numerical simulations. The Monte Carlo method is taken into account for exploring the trapping characteristics such as average deviation and number distribution histogram of nanoparticles. It is revealed that both the width and the depth of potential well for trapping particles can be flexibly adjusted by tuning the wavelength of the incident beam. In addition, incident wavelengths for the deepest potential well and for the strongest stiffness at bottom are separated. These phenomena are explained as the strong plasmon coupling between tweezers and metallic nanoparticles. In addition, required trapping fluence and particles’ distributions show distinctive properties through carefully modifying the incident wavelengths from 1280 nm to 1300 nm. Trapping with lowest laser fluence can be realized with 1280 nm laser and trapping with highest precision can be realized with 1300 nm laser. This work will provide theoretical support for advancing the manipulation of metallic particles and related applications such as single-molecule fluorescence and surface enhanced Raman spectroscopy.


Energy-based scheme for reconstruction of piecewise constant signals observed in the movement of molecular machines

Joachim Rosskopf, Korbinian Paul-Yuan, Martin B. Plenio, and Jens Michaelis

Analyzing the physical and chemical properties of single DNA-based molecular machines such as polymerases and helicases requires to track stepping motion on the length scale of base pairs. Although high-resolution instruments have been developed that are capable of reaching that limit, individual steps are oftentimes hidden by experimental noise which complicates data processing. Here we present an effective two-step algorithm which detects steps in a high-bandwidth signal by minimizing an energy-based model (energy-based step finder, EBS). First, an efficient convex denoising scheme is applied which allows compression to tuples of amplitudes and plateau lengths. Second, a combinatorial clustering algorithm formulated on a graph is used to assign steps to the tuple data while accounting for prior information. Performance of the algorithm was tested on Poissonian stepping data simulated based on published kinetics data of RNA polymerase II (pol II). Comparison to existing step-finding methods shows that EBS is superior in speed while providing competitive step-detection results, especially in challenging situations. Moreover, the capability to detect backtracked intervals in experimental data of pol II as well as to detect stepping behavior of the Phi29 DNA packaging motor is demonstrated.


Enhanced and selective optical trapping in a slot-graphite photonic crystal

Aravind Krishnan, Ningfeng Huang, Shao-Hua Wu, Luis Javier Martínez, and Michelle L. Povinelli

Applicability of optical trapping tools for nanomanipulation is limited by the available laser power and trap efficiency. We utilized the strong confinement of light in a slot-graphite photonic crystal to develop high-efficiency parallel trapping over a large area. The stiffness is 35 times higher than our previously demonstrated on-chip, near field traps. We demonstrate the ability to trap both dielectric and metallic particles of sub-micron size. We find that the growth kinetics of nanoparticle arrays on the slot-graphite template depends on particle size. This difference is exploited to selectively trap one type of particle out of a binary colloidal mixture, creating an efficient optical sieve. This technique has rich potential for analysis, diagnostics, and enrichment and sorting of microscopic entities.


Thursday, October 6, 2016

How curvature-generating proteins build scaffolds on membrane nanotubes

Mijo Simunovic, Emma Evergren, Ivan Golushko, Coline Prévost, Henri-François Renard, Ludger Johannes, Harvey T. McMahon, Vladimir Lorman, Gregory A. Voth, and Patricia Bassereau

Bin/Amphiphysin/Rvs (BAR) domain proteins control the curvature of lipid membranes in endocytosis, trafficking, cell motility, the formation of complex subcellular structures, and many other cellular phenomena. They form 3D assemblies that act as molecular scaffolds to reshape the membrane and alter its mechanical properties. It is unknown, however, how a protein scaffold forms and how BAR domains interact in these assemblies at protein densities relevant for a cell. In this work, we use various experimental, theoretical, and simulation approaches to explore how BAR proteins organize to form a scaffold on a membrane nanotube. By combining quantitative microscopy with analytical modeling, we demonstrate that a highly curving BAR protein endophilin nucleates its scaffolds at the ends of a membrane tube, contrary to a weaker curving protein centaurin, which binds evenly along the tube’s length. Our work implies that the nature of local protein–membrane interactions can affect the specific localization of proteins on membrane-remodeling sites. Furthermore, we show that amphipathic helices are dispensable in forming protein scaffolds. Finally, we explore a possible molecular structure of a BAR-domain scaffold using coarse-grained molecular dynamics simulations. Together with fluorescence microscopy, the simulations show that proteins need only to cover 30–40% of a tube’s surface to form a rigid assembly. Our work provides mechanical and structural insights into the way BAR proteins may sculpt the membrane as a high-order cooperative assembly in important biological processes.


Capture of 2D Microparticle Arrays via a UV-Triggered Thiol-yne “Click” Reaction

Debora Walker, Dhruv P. Singh, Peer Fischer

Immobilization of colloidal assemblies onto solid supports via a fast UV-triggered click-reaction is achieved. Transient assemblies of microparticles and colloidal materials can be captured and transferred to solid supports. The technique does not require complex reaction conditions, and is compatible with a variety of particle assembly methods.


Biocompatible and High Stiffness Nanophotonic Trap Array for Precise and Versatile Manipulation

Fan Ye, Ryan P. Badman, James T. Inman, Mohammad Soltani, Jessica L. Killian, and Michelle D. Wang

The advent of nanophotonic evanescent field trapping and transport platforms has permitted increasingly complex single molecule and single cell studies on-chip. Here, we present the next generation of nanophotonic Standing Wave Array Traps (nSWATs) representing a streamlined CMOS fabrication process and compact biocompatible design. These devices utilize silicon nitride (Si3N4) waveguides, operate with a biofriendly 1064 nm laser, allow for several watts of input power with minimal absorption and heating, and are protected by an anticorrosive layer for sustained on-chip microelectronics in aqueous salt buffers. In addition, due to Si3N4’s negligible nonlinear effects, these devices can generate high stiffness traps while resolving subnanometer displacements for each trapped particle. In contrast to traditional table-top counterparts, the stiffness of each trap in an nSWAT device scales linearly with input power and is independent of the number of trapping centers. Through a unique integration of microcircuitry and photonics, the nSWAT can robustly trap, and controllably position, a large number of nanoparticles along the waveguide surface, operating in an all-optical, constant-force mode without need for active feedback. By reducing device fabrication cost, minimizing trapping laser specimen heating, increasing trapping force, and implementing commonly used trapping techniques, this new generation of nSWATs significantly advances the development of a high performance, low cost optical tweezers array laboratory on-chip.


Platelet clearance via shear-induced unfolding of a membrane mechanoreceptor

Wei Deng, Yan Xu, Wenchun Chen, David S. Paul, Anum K. Syed, Matthew A. Dragovich, Xin Liang, Philip Zakas, Michael C. Berndt, Jorge Di Paola, Jerry Ware, Francois Lanza, Christopher B. Doering, Wolfgang Bergmeier, X. Frank Zhang & Renhao Li

Mechanisms by which blood cells sense shear stress are poorly characterized. In platelets, glycoprotein (GP)Ib–IX receptor complex has been long suggested to be a shear sensor and receptor. Recently, a relatively unstable and mechanosensitive domain in the GPIbα subunit of GPIb–IX was identified. Here we show that binding of its ligand, von Willebrand factor, under physiological shear stress induces unfolding of this mechanosensory domain (MSD) on the platelet surface. The unfolded MSD, particularly the juxtamembrane ‘Trigger’ sequence therein, leads to intracellular signalling and rapid platelet clearance. These results illustrate the initial molecular event underlying platelet shear sensing and provide a mechanism linking GPIb–IX to platelet clearance. Our results have implications on the mechanism of platelet activation, and on the pathophysiology of von Willebrand disease and related thrombocytopenic disorders. The mechanosensation via receptor unfolding may be applicable for many other cell adhesion receptors.


Wednesday, October 5, 2016

High frequency viscoelastic measurements using optical tweezers on wormlike micelles of nonionic and cationic surfactants in aqueous solutions

Ken Morishima and Tadashi Inoue

Linear viscoelasticity of two kinds of wormlike micelles in aqueous solutions, one is nonionic surfactants and the other one is cationic surfactant with organic salt, was measured over a wide frequency range with Brownian motion tracking microrheology (BMTR) using optical tweezers and a conventional rheometer. In BMTR measurements, the Brownian motion of a small particle embodied in the sample is traced and the complex modulus is calculated from the trajectory. The wideband linear viscoelastic spectra thus obtained for each of wormlike micelles were classified into the following three relaxation types as already known. Type A is similar to the spectrum of the nonentangle polymer solutions, type B similar to that of the entanglepolymer systems, and type C has a single Maxwell relaxation at low frequencies. In the high-frequency region, spectra of all types showed a common power-law relaxation, which reflects the reorientation of the viscoelastic segment of the wormlike micelles, indicating that the dynamics of wormlike micelles is identical with that of the ordinary polymeric systems. For type A solutions, the molar mass of wormlike micelles was estimated by fitting the beads-spring models to the viscoelastic spectra. For type B solutions, the molar mass was estimated by using the universality of entangled system. For the case of nonionic micelles, thus determined molar mass is in good agreement with the reported result with the light scattering measurement.


Digital selective laser methods for nanomaterials: From synthesis to processing

Sukjoon Hong, Habeom Lee, Junyeob Yeo, Seung Hwan Ko

Laser has long been used for material processing, and its applications to nanomaterials for their direct synthesis, positioning and processing are currently active fields of study. The main mechanism of typical laser processes is photothermal reaction by a focused laser that remotely generates confined temperature field at a desired position with high controllability. The laser-induced elevated temperature enables direct synthesis of nanomaterials in both gas and liquid environment as well as photophysical processing of nanomaterials through melting or vaporization, represented by laser sintering and ablation processes, in spatially selective manners. On the other hand, recent advances in laser process further incorporates not only different optical responses such as optical forces and photochemical reactions for more advanced manipulation of nanomaterials, but also the interaction between electromagnetic waves, nanostructures and underlying substrates to facilitate novel processing those cannot be achieved by any other means including laser nanowelding for percolation network and laser thinning for two dimensional nanomaterials. At the same time, the shortcomings of laser process in nanomaterial processing such as limited resolution and low throughput are tackled through introducing different optical schemes together with the integration with other systems. In this review, we summarize the development and current status of digital selective laser methods for nanomaterials in broad aspects that cover from nanomaterial synthesis to its processing.


Optical tweezer calibration by using a part of the intensity distribution of a trapped particle

Harun Yücel and Nazmi Turan Okumuşoğlu

In this paper, we demonstrated that an optical tweezer setup can be calibrated by using a part of the symmetric intensity distribution of the trapped particle in digital video microscopy. First, we modified the radial symmetry center method, which was a recently proposed position detection algorithm. This algorithm uses gradient vectors of the particle intensity distribution, which allows us to use a part of the symmetric intensity distribution in the calculation of the particle center. We applied the modified algorithm to different camera image configurations, which are obtained by cutting the same experimental video frames. We further calibrated the trap stiffness for each camera configuration. Then we compared the trap stiffness values and the position distributions. As a result, we can conclude that optical tweezer setups can be calibrated by using a part of the intensity distribution of the trapped particle.


Nanoparticle Trapping and Characterization Using Open Microcavities

A. A. P. Trichet, P. R. Dolan, D. James, G. M. Hughes, C. Vallance, and J. M. Smith

Characterization and trapping of nanoparticles in solution is of great importance for lab-on-a-chip applications in biomedical, environmental, and materials sciences. Devices are now starting to emerge allowing such manipulations and investigations in real-time. Better insights into the interaction between the nanoparticle and the optical trap is therefore necessary in order to move forward in this field. In this work, we present a new kind of nanotweezers based on open microcavities. We show that by monitoring the cavity mode wavelength shift as the particle diffuses through the cavity, it is possible to establish both the nanoparticle polarizability and its coefficient of friction. Additionally, our experiment provides a deep insight in the interaction between the nanoparticle and the cavity mode. The technique has built-in calibration of the trap strength and spring constant, making it attractive for practical applications. This work illustrates the potential of such optical microcavities for future developments in nanoparticle sensors and lab-on-a-chip devices.


Tuesday, October 4, 2016

Simultaneous Characterization of Nanoparticle Size and Particle-Surface Interactions with Three-Dimensional Nanophotonic Force Microscopy

Dakota O’Dell, Perry Schein, and David Erickson

The behavior of a nanoparticle in solution depends strongly on the particle’s physical and chemical characteristics, most notably the particle size and the surface properties. Accurately characterizing these properties is critical for quality control in a wide variety of industries. To understand a complex and polydisperse nanoparticle suspension, however, ensemble averaging is not sufficient, and there is a great need for direct measurements of size and surface properties at the individual nanoparticle level. In this work, we present an analysis technique for simultaneous characterization of particle-surface interactions and size using near-field light scattering and verify it using Brownian-dynamics simulations. Using a nanophotonic waveguide, single particles can be stably held near the waveguide’s surface by strongly localized optical forces. By tracking the dynamic 3D motion of the particle under the influence of these forces using an optical microscope, it is possible to extract the particle-surface interaction forces, as well as to estimate the size and refractive index of the nanoparticle. Because of the strong light-scattering signal, this method is viable for high-throughput characterization of particles as small as 100 nm in only a few seconds each.


Measurement of back-scattering patterns from single laser trapped aerosol particles in air

Yong-Le Pan, Chuji Wang, Leonid A. Beresnev, Alex J. Yuffa, Gorden Videen, David Ligon, and Joshua L. Santarpia

We demonstrate a method for measuring elastic back-scattering patterns from single laser trapped micron-sized particles, spanning the scattering angle range of 𝜽=167.7°–180°θ=167.7°–180° and 𝜑=0°–360°φ=0°–360° in spherical coordinates. We calibrated the apparatus by capturing light-scattering patterns of 10 μm diameter borosilicate glass microspheres and comparing their scattered intensities with Lorenz–Mie theory. Back-scattering patterns are also presented from a single trapped Johnson grass spore, two attached Johnson grass spores, and a cluster of Johnson grass spores. The method has potential use in characterizing airborne aerosol particles, and may be used to provide back-scattering data for lidar applications.


The Effect of Temperature on Microtubule-Based Transport by Cytoplasmic Dynein and Kinesin-1 Motors

Weili Hong, Anjneya Takshak, Olaolu Osunbayo, Ambarish Kunwar, Michael Vershinin

Cytoplasmic dynein and kinesin are both microtubule-based molecular motors but are structurally and evolutionarily unrelated. Under standard conditions, both move with comparable unloaded velocities toward either the microtubule minus (dynein) or plus (most kinesins) end. This similarity is important because it is often implicitly incorporated into models that examine the balance of cargo fluxes in cells and into models of the bidirectional motility of individual cargos. We examined whether this similarity is a robust feature, and specifically whether it persists across the biologically relevant temperature range. The velocity of mammalian cytoplasmic dynein, but not of mammalian kinesin-1, exhibited a break from simple Arrhenius behavior below 15°C—just above the restrictive temperature of mammalian fast axonal transport. In contrast, the velocity of yeast cytoplasmic dynein showed a break from Arrhenius behavior at a lower temperature (∼8°C). Our studies implicate cytoplasmic dynein as a more thermally tunable motor and therefore a potential thermal regulator of microtubule-based transport. Our theoretical analysis further suggests that motor velocity changes can lead to qualitative changes in individual cargo motion and hence net intracellular cargo fluxes. We propose that temperature can potentially be used as a noninvasive probe of intracellular transport.


Light-Induced Polarization-Directed Growth of Optically Printed Gold Nanoparticles

Ianina L. Violi, Julián Gargiulo, Catalina von Bilderling, Emiliano Cortés, and Fernando D. Stefani

Optical printing has been proved a versatile and simple method to fabricate arbitrary arrays of colloidal nanoparticles (NPs) on substrates. Here, we show that is also a powerful tool for studying chemical reactions at the single NP level. We demonstrate that 60 nm gold NPs immobilized by optical printing can be used as seeds to obtain larger NPs by plasmon-assisted reduction of aqueous HAuCl4. The final size of each NP is simply controlled by the irradiation time. Moreover, we show conditions for which the growth occurs preferentially in the direction of light polarization, enabling the in situ anisotropic reshaping of the NPs in predetermined orientations.


Light-driven transport of plasmonic nanoparticles on demand

José A. Rodrigo & Tatiana Alieva

Laser traps provide contactless manipulation of plasmonic nanoparticles (NPs) boosting the development of numerous applications in science and technology. The known trapping configurations allow immobilizing and moving single NPs or assembling them, but they are not suitable for massive optical transport of NPs along arbitrary trajectories. Here, we address this challenging problem and demonstrate that it can be handled by exploiting phase gradients forces to both confine and propel the NPs. The developed optical manipulation tool allows for programmable transport routing of NPs to around, surround or impact on objects in the host environment. An additional advantage is that the proposed confinement mechanism works for off-resonant but also resonant NPs paving the way for transport with simultaneous heating, which is of interest for targeted drug delivery and nanolithography. These findings are highly relevant to many technological applications including micro/nano-fabrication, micro-robotics and biomedicine.