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Friday, April 24, 2015

Focus issue introduction: optical cooling and trapping

Antonio A. R. Neves, Philip H. Jones, Le Luo, and Onofrio M. Maragò

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This Focus Issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint Focus Issue features 32 papers, including both experimental and theoretical works, which span this wide range of activities.

DOI

Optical cooling and trapping: introduction

Antonio A. R. Neves, Philip H. Jones, Le Luo, and Onofrio M. Maragò

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This focus issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint focus issue features 33 papers, including both experimental and theoretical works, which span this wide range of activities.

DOI

Cancellation of non-conservative scattering forces in optical traps by counter-propagating beams

Shawn Divitt, Loïc Rondin, and Lukas Novotny

Non-conservative forces in optical tweezers generate undesirable behavior, such as particle loss due to radiation pressure and the preclusion of the thermodynamic equilibrium. Here, we rigorously derive criteria for the elimination of non-conservative forces, and describe how these criteria can be met by a large class of counter-propagating, focused optical beams.

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Liquid-Liquid Phase Separation in Mixed Organic/Inorganic Single Aqueous Aerosol Droplets

David Stewart , Chen Cai , James Nayler , Thomas C Preston , Jonathan Philip Reid , Ulrich Krieger , Claudia Marcolli , and Yun-Hong Zhang

Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient-force optical trap and the evolving size, refractive index (RI) and morphology characterised by cavity enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase separated structure inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-di-acids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at marginally higher RH (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02).

DOI

Thursday, April 23, 2015

Single-molecule chemo-mechanical unfolding reveals multiple transition state barriers in a small single-domain protein

Emily J. Guinn, Bharat Jagannathan & Susan Marqusee

A fundamental question in protein folding is whether proteins fold through one or multiple trajectories. While most experiments indicate a single pathway, simulations suggest proteins can fold through many parallel pathways. Here, we use a combination of chemical denaturant, mechanical force and site-directed mutations to demonstrate the presence of multiple unfolding pathways in a simple, two-state folding protein. We show that these multiple pathways have structurally different transition states, and that seemingly small changes in protein sequence and environment can strongly modulate the flux between the pathways. These results suggest that in vivo, the crowded cellular environment could strongly influence the mechanisms of protein folding and unfolding. Our study resolves the apparent dichotomy between experimental and theoretical studies, and highlights the advantage of using a multipronged approach to reveal the complexities of a protein’s free-energy landscape.

DOI

Engineering of a superhelicase through conformational control

Sinan Arslan, Rustem Khafizov, Christopher D. Thomas, Yann R. Chemla, Taekjip Ha

Conformational control of biomolecular activities can reveal functional insights and enable the engineering of novel activities. Here we show that conformational control through intramolecular cross-linking of a helicase monomer with undetectable unwinding activity converts it into a superhelicase that can unwind thousands of base pairs processively, even against a large opposing force. A natural partner that enhances the helicase activity is shown to achieve its stimulating role also by selectively stabilizing the active conformation. Our work provides insight into the regulation of nucleic acid unwinding activity and introduces a monomeric superhelicase without nuclease activities, which may be useful for biotechnological applications.

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Step-by-step guide to the realization of advanced optical tweezers

Giuseppe Pesce, Giorgio Volpe, Onofrio M. Maragó, Philip H. Jones, Sylvain Gigan, Antonio Sasso, and Giovanni Volpe

Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have become an indispensable tool for contactless manipulation of micro- and nanoparticles. Nowadays OT are employed in a myriad of applications demonstrating their importance. While the basic principle of OT is the use of a strongly focused laser beam to trap and manipulate particles, more complex experimental setups are required to perform novel and challenging experiments. With this article, we provide a detailed step-by-step guide for the construction of advanced optical manipulation systems. First, we explain how to build a single-beam OT on a homemade microscope and how to calibrate it. Improving on this design, we realize a holographic OT, which can manipulate independently multiple particles and generate more sophisticated wavefronts such as Laguerre–Gaussian beams. Finally, we explain how to implement a speckle OT, which permits one to employ random speckle light fields for deterministic optical manipulation.

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Isolation and identification of bacteria by means of Raman spectroscopy

Susanne Pahlow, Susann Meisel, Dana Cialla-May, Karina Weber, Petra Rösch, Jürgen Popp

Bacterial detection is a highly topical research area, because various fields of application will benefit from the progress being made. Consequently, new and innovative strategies which enable the investigation of complex samples, like body fluids or food stuff, and improvements regarding the limit of detection are of general interest. Within this review the prospects of Raman spectroscopy as a reliable tool for identifying bacteria in complex samples are discussed.
The main emphasis of this work is on important aspects of applying Raman spectroscopy for the detection of bacteria like sample preparation and the identification process. Several approaches for a Raman compatible isolation of bacterial cells have been developed and applied to different matrices. Here, an overview of the limitations and possibilities of these methods is provided. Furthermore, the utilization of Raman spectroscopy for diagnostic purposes, food safety and environmental issues is discussed under a critical view.

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Principal-component analysis of particle motion

H. Y. Chen, Raphaël Liégeois, John R. de Bruyn, and Andrea Soddu

We demonstrate the application of principal-component analysis (PCA) to the analysis of particle motion data in the form of a time series of images. PCA has the ability to resolve and isolate spatiotemporal patterns in the data. Using simulated data, we show that this translates into the ability to separate individual frequency components of the particle motion. We also show that PCA can be used to extract the fluid viscosity from images of particles undergoing Brownian motion. PCA thus provides an efficient alternative to more traditional particle-tracking methods for the analysis of microrheological data.

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Wednesday, April 22, 2015

Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal

A. V. Ryzhkova, M. Škarabot, and I. Muševič

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is ∼4.5×10−3C/m2 for 20-nm nanocolloids, and two orders of magnitude lower, i.e., ∼2.3×10−5C/m2, for 1−μm colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.

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