Friday, February 16, 2018

Bioconjugated Core–Shell Microparticles for High-Force Optical Trapping

Juan Carlos Cordova, Dana N. Reinemann, Daniel J. Laky, William R. Hesse, Sophie K. Tushak, Zane L. Weltman, Kelsea B. Best, Rizia Bardhan, Matthew J. Lang

Due to their high spatial resolution and precise application of force, optical traps are widely used to study the mechanics of biomolecules and biopolymers at the single-molecule level. Recently, core–shell particles with optical properties that enhance their trapping ability represent promising candidates for high-force experiments. To fully harness their properties, methods for functionalizing these particles with biocompatible handles are required. Here, a straightforward synthesis is provided for producing functional titania core–shell microparticles with proteins and nucleic acids by adding a silane–thiol chemical group to the shell surface. These particles display higher trap stiffness compared to conventional plastic beads featured in optical tweezers experiments. These core–shell microparticles are also utilized in biophysical assays such as amyloid fiber pulling and actin rupturing to demonstrate their high-force applications. It is anticipated that the functionalized core–shells can be used to probe the mechanics of stable proteins structures that are inaccessible using current trapping techniques.


Influence on the saturable absorption of the induced losses by photodeposition of zinc nanoparticles in an optical fiber

L. C. Gómez-Pavón, G. J. Lozano-Perera, A. Luis-Ramos, J. M. Muñoz-Pacheco, J. P. Padilla-Martínez, and P. Zaca-Morán

In this work, the influence of induced losses on the saturable absorption by zinc nanoparticles photodeposited onto the core of an optical fiber end is reported. Samples with different losses were obtained by the photodeposition technique using a continuous wave laser at 1550 nm. The nonlinear absorption of the saturable absorber was characterized by the P-scan technique using a high-gain pulsed erbium-doped fiber amplifier. The results have demonstrated that for optical fibers with variable induced losses by deposited nanoparticles, the modulation depth increases proportionally based on the nonlinear absorption coefficient. With induced losses fixed at 3 dB, it was demonstrated that the modulation depth increased as a function of the optical power used in the photodeposition process. The saturation intensity of the saturable absorber presents small shifts for higher intensities.


Self-Organization of Metal Nanoparticles in Light: Electrodynamics–Molecular Dynamics Simulations and Optical Binding Experiments

Patrick McCormack, Fei Han, and Zijie Yan

Light-driven self-organization of metal nanoparticles (NPs) can lead to unique optical matter systems, yet simulation of such self-organization (i.e., optical binding) is a complex computational problem that increases nonlinearly with system size. Here we show that a combined electrodynamics–molecular dynamics simulation technique can simulate the trajectories and predict stable configurations of silver NPs in optical fields. The simulated dynamic equilibrium of a two-NP system matches the probability density of oscillations for two optically bound NPs obtained experimentally. The predicted stable configurations for up to eight NPs are further compared to experimental observations of silver NP clusters formed by optical binding in a Bessel beam. All configurations are confirmed to form in real systems, including pentagonal clusters with five-fold symmetry. Our combined simulations and experiments have revealed a diverse optical matter system formed by anisotropic optical binding interactions, providing a new strategy to discover artificial materials.


Interactions of single nanoparticles in nematic liquid crystal

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

We studied interactions of single silica nanoparticles with homeotropic surface anchoring in a confined planar nematic liquid crystal cell. Nanoparticles form stable pairs down to a size of 35 nm with the pair binding energy proportional to the size of the particle. 22 nm particles do not form stable pairs, only metastable states were observed due to strong thermal motion and electrostatic repulsion. These nanoparticles strongly interact with the confining surfaces and the larger clusters, which prevents the formation of stable homogenous dispersions. In addition we have shown that nanoparticles are strongly attracted by topological defects induced by the microparticles. The origin of the attractive forces in nanoparticle dispersions is the minimization of free energy. These forces strongly depend on the strength and type of the liquid crystal anchoring at the surface of the nanoparticles.


Immersion and Contact Efflorescence Induced by Mineral Dust Particles

Shuichi B. Ushijima, Ryan D. Davis, and Margaret A. Tolbert

The phase state of inorganic salt aerosols impacts their properties, including the ability to undergo hygroscopic growth, catalyze heterogeneous reactions, and act as cloud condensation nuclei. Here, we report the first observation of contact efflorescence by mineral dust aerosol. The efflorescence of aqueous ammonium sulfate ((NH4)2SO4) and sodium chloride (NaCl) droplets by contact with three types of mineral dust particles (illite, montmorillonite, and NX illite), were examined using an optical levitation chamber. Immersion mode efflorescence was also studied for comparison. We find that in the presence of mineral dust particles, crystallization occurred at a higher relative humidity (RH) when compared to the homogeneous phase transition. Additionally, crystallization by contact mode efflorescence occurred at a higher RH than the corresponding immersion mode. Crystallization efficiencies in the contact mode exhibited an ion-specific trend consistent with the Hoffmeister series. Estimates for lifetimes of a salt droplet to collide with dust particles suggests that collisions between the two aerosol types are likely to occur before the salt aerosol is removed by other atmospheric processes. Such collisions could then lead to the crystallization of salt droplets that would otherwise have remained liquid, changing the overall impact that salt aerosols have on atmospheric chemistry and climate.


Statics and dynamics of DNA knotting

Enzo Orlandini

Knots and entanglement in polymers and biopolymers such as DNA and proteins constitute a timely topic that spans various scientific disciplines ranging from physics to chemistry, biology and mathematics. Although in the past many advancements have been made in understanding the equilibrium knotting probability and knot complexity of long polymer chains in solutions, many questions have been addressed in recent years by both experimental and theoretical means—for instance, how the knotting probability depends on the quality of the solvent, the elastic properties of the molecule and its degree of confinement. How knots form, evolve and eventually disappear in a fluctuating chain. Are the equilibrium and non-equilibrium properties of knotted molecules affected by the knot swelling/shrinking dynamics? Moreover, thanks to the great advance in nanotechnology and micromanipulation techniques, nowadays knots can be 'manually' tied in a single DNA molecule, followed during their motion along the chains, forced to pass through nanopores, or stretched by external forces or elongational flows. All these experimental approaches allow access to new information on the interplay of topology and polymer physics, and this has opened new perspectives in the field. Here, we provide an overview of the current knowledge of this topic, stressing the main results obtained, including the recent developments in experimental and computational approaches. Since almost all experiments on knotting involve DNA, the review will be mainly focused on the topological properties of this fascinating and biologically relevant molecule.


Thursday, February 15, 2018

Single charging events on colloidal particles in a nonpolar liquid with surfactant

Caspar Schreuer, Stijn Vandewiele, Toon Brans, Filip Strubbe, Kristiaan Neyts, and Filip Beunis
Electrical charging of colloidal particles in nonpolar liquids due to surfactant additives is investigated intensively, motivated by its importance in a variety of applications. Most methods rely on average electrophoretic mobility measurements of many particles, which provide only indirect information on the charging mechanism. In the present work, we present a method that allows us to obtain direct information on the charging mechanism, by measuring the charge fluctuations on individual particles with a precision higher than the elementary charge using optical trapping electrophoresis. We demonstrate the capabilities of the method by studying the influence of added surfactant OLOA 11000 on the charging of single colloidal PMMA particles in dodecane. The particle charge and the frequency of charging events are investigated both below and above the critical micelle concentration (CMC) and with or without applying a DC offset voltage. It is found that at least two separate charging mechanisms are present below the critical micelle concentration. One mechanism is a process where the particle is stripped from negatively charged ionic molecules. An increase in the charging frequency with increased surfactant concentration suggests a second mechanism that involves single surfactant molecules. Above the CMC, neutral inverse micelles can also be involved in the charging process.


The mechano-chemistry of a monomeric reverse transcriptase

Omri Malik, Hadeel Khamis, Sergei Rudnizky, Ariel Kaplan

Retroviral reverse transcriptase catalyses the synthesis of an integration-competent dsDNA molecule, using as a substrate the viral RNA. Using optical tweezers, we follow the Murine Leukemia Virus reverse transcriptase as it performs strand-displacement polymerization on a template under mechanical force. Our results indicate that reverse transcriptase functions as a Brownian ratchet, with dNTP binding as the rectifying reaction of the ratchet. We also found that reverse transcriptase is a relatively passive enzyme, able to polymerize on structured templates by exploiting their thermal breathing. Finally, our results indicate that the enzyme enters the recently characterized backtracking state from the pre-translocation complex.


An Accurate Perception Method for Low Contrast Bright Field Microscopy in Heterogeneous Microenvironments

Keshav Rajasekaran, Ekta Samani, Manasa Bollavaram, John Stewart and Ashis G. Banerjee

Automated optical tweezers-based robotic manipulation of microscale objects requires real-time visual perception for estimating the states, i.e., positions and orientations, of the objects. Such visual perception is particularly challenging in heterogeneous environments comprising mixtures of biological and colloidal objects, such as cells and microspheres, when the popular imaging modality of low contrast bright field microscopy is used. In this paper, we present an accurate method to address this challenge. Our method combines many well-established image processing techniques such as blob detection, histogram equalization, erosion, and dilation with a convolutional neural network in a novel manner. We demonstrate the effectiveness of our processing pipeline in perceiving objects of both regular and irregular shapes in heterogeneous microenvironments of varying compositions. The neural network, in particular, helps in distinguishing the individual microspheres present in dense clusters.


Lossless Brownian Information Engine

Govind Paneru, Dong Yun Lee, Tsvi Tlusty, and Hyuk Kyu Pak

We report on a lossless information engine that converts nearly all available information from an error-free feedback protocol into mechanical work. Combining high-precision detection at a resolution of 1 nm with ultrafast feedback control, the engine is tuned to extract the maximum work from information on the position of a Brownian particle. We show that the work produced by the engine achieves a bound set by a generalized second law of thermodynamics, demonstrating for the first time the sharpness of this bound. We validate a generalized Jarzynski equality for error-free feedback-controlled information engines.