Monday, November 30, 2015

Influence of arbitray mode in cluster formation in optical tweezers

R. Kumar

Three dimension multiparticle optical trapping and dynamic manipulation in a pre-defined fashion is a routine affair often realized by spatio-temporal modulation of fundamental Gaussian beam either using lens or diffractive optical elements (DOE). We present multi-particle trapping which is achieved due to cumulative effects of both the weak focusing of arbitrary beam and constructive interference arising from scrambling outwardly from trapped polystyrene spheres. The arbitrary mode is generated from diode pumped solid state source due to cavity imperfections per-se. The prime motivation of this article is to establish the usability of arbitrary mode in arranging as external optics in OTs for achieving colloidal clusters of microspheres from which forwardly scattered light can be utilized as feedback in shaping the wavefronts to be focused in highly turbid media. In general, it offers overall cost-cutting of experimental set-up and particularly suitable for fundamental demonstration of trapping to undergraduates only.


Transient dynamics of a colloidal particle driven through a viscoelastic fluid

Juan Ruben Gomez-Solano and Clemens Bechinger
We study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.


Femtosecond Nanostructuring of Glass with Optically Trapped Microspheres and Chemical Etching

Aleksander Shakhov, Artyom Astafiev, Alexander Gulin, and Victor A. Nadtochenko

Laser processing with optically trapped microspheres is a promising tool for nanopatterning at sub-diffraction limited resolution in a wide range of technological and biomedical applications. In this paper, we investigate sub-diffraction limited structuring of borosilicate glass with femtosecond pulses in the near-field of optically trapped microspheres combined with chemical post-processing. Glass surface was processed by single laser pulses at 780 nm focused by silica microspheres and then subjected to selective etching in KOH, which produced pits in the laser affected zones (LAZs). Chemical post-processing allowed obtaining structures with better resolution and reproducibility. We demonstrate production of reproducible pits with diameter as small as 70 nm (λ/11). Complex 2-Dimensional structures with 100 nm (λ/8) resolution were written on the glass surface point by point with microspheres manipulated by optical tweezers. Furthermore, the mechanism of laser modification underlying selective etching was investigated with mass-spectrum analysis. We propose that increased etching rate of laser-treated glass result from change in its chemical composition and oxygen deficiency.


Versatile microsphere attachment of GFP-labeled motors and other tagged proteins with preserved functionality

Michael Bugiel, Horatiu Fantana, Volker Bormuth, Anastasiya Trushko, Frederic Schiemann, Jonathon Howard, Erik Schäffer, Anita Jannasch

Microspheres are often used as handles for protein purification or force spectroscopy. For example, optical tweezers apply forces on trapped particles to which motor proteins are attached. However, even though many attachment strategies exist, procedures are often limited to a particular biomolecule and prone to non-specific protein or surface attachment. Such interactions may lead to loss of protein functionality or microsphere clustering. Here, we describe a versatile coupling procedure for GFP-tagged proteins via a polyethylene glycol linker preserving the functionality of the coupled proteins. The procedure combines well-established protocols, is highly reproducible, reliable, and can be used for a large variety of proteins. The coupling is efficient and can be tuned to the desired microsphere-to-protein ratio. Moreover, microspheres hardly cluster or adhere to surfaces. Furthermore, the procedure can be adapted to different tags providing flexibility and a promising attachment strategy for any tagged protein.


Thursday, November 26, 2015

Holographic Raman tweezers controlled by multi-modal natural user interface

Zoltán Tomori, Peter Keša, Matej Nikorovič, Jan Kaňka, Petr Jákl, Mojmír Šerý, Silvie Bernatová, Eva Valušová, Marián Antalík and Pavel Zemánek

Holographic optical tweezers provide a contactless way to trap and manipulate several microobjects independently in space using focused laser beams. Although the methods of fast and efficient generation of optical traps are well developed, their user friendly control still lags behind. Even though several attempts have appeared recently to exploit touch tablets, 2D cameras, or Kinect game consoles, they have not yet reached the level of natural human interface. Here we demonstrate a multi-modal 'natural user interface' approach that combines finger and gaze tracking with gesture and speech recognition. This allows us to select objects with an operator's gaze and voice, to trap the objects and control their positions via tracking of finger movement in space and to run semi-automatic procedures such as acquisition of Raman spectra from preselected objects. This approach takes advantage of the power of human processing of images together with smooth control of human fingertips and downscales these skills to control remotely the motion of microobjects at microscale in a natural way for the human operator.


Optical trapping and manipulation of nanoparticles using a meta plasmonic structure

Rehab Kotb, Mahmoud El Maklizi, Yehea Ismail and Mohamed A Swillam
In this paper, a novel structure of nano optical tweezers using triple-slit plasmonic grating structure is introduced and analyzed. The tweezers have deep potential wells that can trap sub-10 nm dielectric particle stably and efficiently. The resultant 50 KT potential well provides tight 2D trapping to the particle. The plasmonic device allows for steering the particle by simply changing the angle of the incident plane. This simple control allows efficient manipulation of the trapped particle over wide range of angles.


Probing the Red Blood Cells Aggregating Force With Optical Tweezers

Kisung Lee, Danilina, A.V. ; Kinnunen, M. ; Priezzhev, A.V. ; Meglinski, I.

The red blood cells (RBC) aggregation is of current basic science and clinical interest, as a determinant of blood microcirculation. Thus, the measurement and assessment of the RBC aggregation property (aggregability) and aggregation state at different physiologic conditions of a human individual or laboratory animal are an important issue. In this paper, in order to assess the dynamics of RBC interaction, optical tweezers were used to probe the forces during the RBC doublet formation or disruption. We show that in autologous plasma, RBC aggregating and disaggregating forces have different absolute values, ca 2-4 pN and dozens of piconewton, correspondingly. We speculate that in plasma, RBC aggregation and disaggregation processes have different driving forces.


Probing the Casimir force with optical tweezers

D. S. Ether jr., L. B. Pires, S. Umrath, D. Martinez, Y. Ayala, B. Pontes, G. R. de S. Araújo, S. Frases, G.-L. Ingold, F. S. S. Rosa

We propose to use optical tweezers to probe the Casimir interaction between microspheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double-layer forces between polystyrene microspheres at distances above 400 nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology.


Wednesday, November 25, 2015

Dimensionality constraints of light-induced rotation

László Oroszi, András Búzás, Péter Galajda, Lóránd Kelemen, Anna Mathesz, Tamás Vicsek, Gaszton Vizsnyiczai and Pál Ormos

We have studied the conditions of rotation induced by collimated light carrying no angular momentum. Objects of different shapes and optical properties were examined in the nontrivial case where the rotation axis is perpendicular to the direction of light propagation. This geometry offers important advantages for application as it fundamentally broadens the possible practical arrangements to be realised. We found that collimated light cannot drive permanent rotation of 2D or prism-like 3D objects (i.e., fixed cross-sectional profile along the rotation axis) in the case of fully reflective or fully transparent materials. Based on both geometrical optics simulations and theoretical analysis, we derived a general condition for rotation induced by collimated light carrying no angular momentum valid for any arrangement: Permanent rotation is not possible if the scattering interaction is two-dimensional and lossless. In contrast, light induced rotation can be sustained if partial absorption is present or the object has specific true 3D geometry. We designed, simulated, fabricated, and experimentally tested a microscopic rotor capable of rotation around an axis perpendicular to the illuminating light.


Direct observation of processive exoribonuclease motion using optical tweezers

Furqan M. Fazal, Daniel J. Koslover, Ben F. Luisi, and Steven M. Block

Bacterial RNases catalyze the turnover of RNA and are essential for gene expression and quality surveillance of transcripts. In Escherichia coli, the exoribonucleases RNase R and polynucleotide phosphorylase (PNPase) play critical roles in degrading RNA. Here, we developed an optical-trapping assay to monitor the translocation of individual enzymes along RNA-based substrates. Single-molecule records of motion reveal RNase R to be highly processive: one molecule can unwind over 500 bp of a structured substrate. However, enzyme progress is interrupted by pausing and stalling events that can slow degradation in a sequence-dependent fashion. We found that the distance traveled by PNPase through structured RNA is dependent on the A+U content of the substrate and that removal of its KH and S1 RNA-binding domains can reduce enzyme processivity without affecting the velocity. By a periodogram analysis of single-molecule records, we establish that PNPase takes discrete steps of six or seven nucleotides. These findings, in combination with previous structural and biochemical data, support an asymmetric inchworm mechanism for PNPase motion. The assay developed here for RNase R and PNPase is well suited to studies of other exonucleases and helicases.