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.