Friday, August 28, 2015

Application of axial symmetric phase plate and circular diffraction waveplate in optical tweezers

V. K. Abrahamyan

In development of optical tweezers can be used diffractive optical elements, as laser beam steering systems, as well the phase plates, which allow modulating the intensity distribution of the laser beam for manipulating trapping forces. The system, consisting of an axial symmetric phase retarder–glass substrate coated by axially symmetric oriented liquid crystal (LC) polymer, and a circular diffraction wave plate–glass substrate, coated by LC polymer with polarization patterned orientation, is considered. Diffracted beams are obtained at the output of the system in ±1 order, the intensity distribution in which is determined by the state of light polarization at the system input. The possibility of use this system for trapping, scrolling and moving the particles of micro- and nanosizes by modification of the shape and intensity of the beams at the system output is considered.

Optically Trapped Surface-Enhanced Raman Probes Prepared by Silver Photoreduction to 3D Microstructures

Gaszton Vizsnyiczai, Tamás Lestyán, Jaroslava Joniova, Badri L. Aekbote, Alena Strejčková, Pál Ormos, Pavol Miskovsky, Lóránd Kelemen, and Gregor Bánó

3D microstructures partially covered by silver nanoparticles have been developed and tested for surface-enhanced Raman spectroscopy (SERS) in combination with optical tweezers. The microstructures made by two-photon polymerization of SU-8 photoresist were manipulated in a dual beam optical trap. The active area of the structures was covered by a SERS-active silver layer using chemically assisted photoreduction from silver nitrate solutions. Silver layers of different grain size distributions were created by changing the photoreduction parameters and characterized by scanning electron microscopy. The structures were tested by measuring the SERS spectra of emodin and hypericin.


Thursday, August 27, 2015

Tracking of colloids close to contact

Chi Zhang, Georges Brügger, and Frank Scheffold
The precise tracking of micron sized colloidal particles - held in the vicinity of each other using optical tweezers - is an elegant way to gain information about the particle-particle pair interaction potential. The accuracy of the method, however, relies strongly on the tracking precision. Particularly the elimination of systematic errors in the position detection due to overlapping particle diffraction patterns remains a great challenge. Here we propose a template based particle finding algorithm that circumvents these problems by tracking only a fraction of the particle image that is insignificantly affected by nearby colloids. Under realistic experimental conditions we show that our algorithm significantly reduces systematic errors compared to standard tracking methods. Moreover our approach should in principle be applicable to almost arbitrary shaped particles as the template can be adapted to any geometry.


Structural features of the αβTCR mechanotransduction apparatus that promote pMHC discrimination

Kristine N. Brazin, Robert J. Mallis, Dibyendu K. Das, Yinnian Feng, Wonmuk Hwang, Jia-huai Wang, Gerhard Wagner, Matthew J. Lang and Ellis L. Reinherz

The αβTCR was recently revealed to function as a mechanoreceptor. That is, it leverages mechanical energy generated during immune surveillance and at the immunological synapse to drive biochemical signaling following ligation by a specific foreign peptide-MHC complex (pMHC). Here we review the structural features that optimize this transmembrane receptor for mechanotransduction. Specialized adaptations include: 1) the CβFG loop region positioned between Vβ and Cβ domains that allosterically gates both dynamic TCR-pMHC bond formation and lifetime; 2) the rigid super β-sheet amalgams of heterodimeric CD3εγ as well as CD3εδ ectodomain components of the αβTCR complex; 3) the αβTCR subunit connecting peptides (CP) linking the extracellular and transmembrane (TM) segments, particularly the oxidized CxxC motif in each CD3 heterodimeric subunit that facilitates force transfer through the TM segments and surrounding lipid, impacting cytoplasmic tail conformation; and 4) quaternary changes in the αβTCR complex that accompany pMHC ligation under load. How bioforces foster specific αβTCR-based pMHC discrimination and why dynamic bond formation is a primary basis for kinetic proofreading are discussed. We suggest that the details of the molecular rearrangements of individual αβTCR subunit components can be analyzed utilizing a combination of structural biology, single molecule FRET, optical tweezers and nanobiology, guided by insightful atomistic molecular dynamic studies. Finally, we review very recent data showing that the preTCR complex employs a similar mechanobiology to that of the αβTCR to interact with self-pMHC ligands, impacting early thymic repertoire selection prior to the CD4+CD8+ double positive thymocyte stage of development.


Controlled modulation of laser beam and dynamic patterning of colloidal particles using optical tweezers

Brijesh Kumar Singh, Dalip Singh Mehta, Ranjeet Kumar & Paramasivam Senthilkumaran

We present controlled generation of complex-structured beam profiles using diffractive optical element and demonstrate multiple dynamic trapping of colloidal particles. The phase element is programmed to generate various tailored optical fields having structures, similar to that of number three, spiral, and circle but in a tractable manner. Thus, the generated spatially tailored optical fields are confined to focal volume in optical tweezers. This enabled real-time trapping of multiple microscopic objects whereby its transverse organization was controlled in a dynamic manner from one structure to another with the help of spatial light modulator. Such a controlled beam shaping finds potential applications in biophotonics, super resolution imaging, and measurement of biophysical parameters, cell sorting, and micro-manipulation of colloidal particles.


A Single-Strand Annealing Protein Clamps DNA to Detect and Secure Homology

Marcel Ander, Sivaraman Subramaniam, Karim Fahmy, A. Francis Stewart, Erik Schäffer

Repair of DNA breaks by single-strand annealing (SSA) is a major mechanism for the maintenance of genomic integrity. SSA is promoted by proteins (single-strand-annealing proteins [SSAPs]), such as eukaryotic RAD52 and λ phage Redβ. These proteins use a short single-stranded region to find sequence identity and initiate homologous recombination. However, it is unclear how SSAPs detect homology and catalyze annealing. Using single-molecule experiments, we provide evidence that homology is recognized by Redβ monomers that weakly hold single DNA strands together. Once annealing begins, dimerization of Redβ clamps the double-stranded region and nucleates nucleoprotein filament growth. In this manner, DNA clamping ensures and secures a successful detection for DNA sequence homology. The clamp is characterized by a structural change of Redβ and a remarkable stability against force up to 200 pN. Our findings not only present a detailed explanation for SSAP action but also identify the DNA clamp as a very stable, noncovalent, DNA–protein interaction.


Thursday, August 20, 2015

Rotation of large asymmetrical absorbing objects by Laguerre–Gauss beams

Catherine M. Herne, Kristina M. Capuzzi, Emily Sobel, and Ryan T. Kropas

In this Letter, we show the manipulation and rotation of opaque graphite through adhesion with optically trapped polystyrene spheres. The absorbing graphite is rotated by the orbital angular momentum transfer from a Laguerre–Gauss laser mode and is trapped due to the presence of refracting spheres. This technique is effective for trapping and rotating absorbing objects of all sizes, including those larger than the laser mode.


Evanescent wave optical binding forces on spherical microparticles

Xiang Han and Philip H. Jones

In this Letter, we demonstrate stable optical binding of spherical microparticles in counter-propagating evanescent optical fields formed by total reflection at a dielectric interface. The microspheres are observed to form one-dimensional chains oriented parallel to the direction of propagation of the beams. We characterize the strength of the optical binding interaction by measuring the extent of Brownian position fluctuations of the optically bound microspheres and relating this to a binding spring constant acting between adjacent particles. A stronger binding interaction is observed for particles near the middle of the chain, and the dependence of the binding strength on incident laser power and number of particles in the chain is determined.


Plasmonic random nanostructures on fiber tip for trapping live cells and colloidal particles

Jiajie Chen, Zhiwen Kang, Siu Kai Kong, and Ho-Pui Ho

We demonstrate optical trapping on a gold-coated single-mode fiber tip as excited by 980-nm laser radiation. The trapping force here is not due to common plasmonic localization, but dominated by the combined effect of thermophoresis and thermal convection. The reported scheme only requires simple thin-film deposition. More importantly, efficient broadband plasmonic absorption of the gold random nanostructures, aided by purely Gaussian excitation profile from the fiber core, has led to very low trapping-power threshold typically in hundreds of microwatts. This highly versatile fiber-based trapping scheme clearly offers many potential application possibilities in life sciences as well as engineering disciplines.


Magnetically self-assembled colloidal 3D structures as cell growth scaffold

Gašper Kokot , Špela Zemljič-Jokhadar , Urška Batista , and Dusan Babic

Understanding the chemical and physical conditions for cell growth is important from biological and medical aspects. Many tissues and cell types (e.g. epithelial cells, neurons) naturally grow on surfaces that span in three-dimensions and offer structural or mechanical support. The scaffold surface has to promote adhesion and cell proliferation as well as support their weight and retain its structural integrity. Here we present a flexible method which uses self-assembly of micrometer superparamagnetic particles to produce appropriate scaffold surfaces with controllable general appearance in three dimensions, such as oriented membranes, branched structure or void network. As a proof of principle Chinese hamster ovary epithelial cell line was successfully grown for several days on inclined membranes. Robustness of the oriented membrane architecture was probed with optical tweezers. We measured the magnetic force holding one particle in a self-assembled upright hexagonal sheet and modeled it as a sum of pair interaction forces between spatially arrested static dipoles.