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.