Multimodal Light Microscopy Approaches to Reveal Structural and Functional Properties of Promyelocytic Leukemia Nuclear Bodies

Christian Hoischen, Shamci Monajembashi, Klaus Weisshart and Peter Hemmerich

The promyelocytic leukemia (pml) gene product PML is a tumor suppressor localized mainly in the nucleus of mammalian cells. In the cell nucleus, PML seeds the formation of macromolecular multiprotein complexes, known as PML nuclear bodies (PML NBs). While PML NBs have been implicated in many cellular functions including cell cycle regulation, survival and apoptosis their role as signaling hubs along major genome maintenance pathways emerged more clearly. However, despite extensive research over the past decades, the precise biochemical function of PML in these pathways is still elusive. It remains a big challenge to unify all the different previously suggested cellular functions of PML NBs into one mechanistic model. With the advent of genetically encoded fluorescent proteins it became possible to trace protein function in living specimens. In parallel, a variety of fluorescence fluctuation microscopy (FFM) approaches have been developed which allow precise determination of the biophysical and interaction properties of cellular factors at the single molecule level in living cells. In this report, we summarize the current knowledge on PML nuclear bodies and describe several fluorescence imaging, manipulation, FFM, and super-resolution techniques suitable to analyze PML body assembly and function. These include fluorescence redistribution after photobleaching, fluorescence resonance energy transfer, fluorescence correlation spectroscopy, raster image correlation spectroscopy, ultraviolet laser microbeam-induced DNA damage, erythrocyte-mediated force application, and super-resolution microscopy approaches. Since most if not all of the microscopic equipment to perform these techniques may be available in an institutional or nearby facility, we hope to encourage more researches to exploit sophisticated imaging tools for their research in cancer biology.

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A comparison of methods to assess cell mechanical properties

Pei-Hsun Wu, Dikla Raz-Ben Aroush, Atef Asnacios, Wei-Chiang Chen, Maxim E. Dokukin, Bryant L. Doss, Pauline Durand-Smet, Andrew Ekpenyong, Jochen Guck, Nataliia V. Guz, Paul A. Janmey, Jerry S. H. Lee, Nicole M. Moore, Albrecht Ott, Yeh-Chuin Poh, Robert Ros, Mathias Sander, Igor Sokolov, Jack R. Staunton, Ning Wang, Graeme Whyte & Denis Wirtz

The mechanical properties of cells influence their cellular and subcellular functions, including cell adhesion, migration, polarization, and differentiation, as well as organelle organization and trafficking inside the cytoplasm. Yet reported values of cell stiffness and viscosity vary substantially, which suggests differences in how the results of different methods are obtained or analyzed by different groups. To address this issue and illustrate the complementarity of certain approaches, here we present, analyze, and critically compare measurements obtained by means of some of the most widely used methods for cell mechanics: atomic force microscopy, magnetic twisting cytometry, particle-tracking microrheology, parallel-plate rheometry, cell monolayer rheology, and optical stretching. These measurements highlight how elastic and viscous moduli of MCF-7 breast cancer cells can vary 1,000-fold and 100-fold, respectively. We discuss the sources of these variations, including the level of applied mechanical stress, the rate of deformation, the geometry of the probe, the location probed in the cell, and the extracellular microenvironment.

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Dislocation-controlled formation and kinetics of grain boundary loops in two-dimensional crystals

François A. Lavergne, Arran Curran, Dirk G. A. L. Aarts, and Roel P. A. Dullens

The formation and kinetics of grain boundaries are closely related to the topological constraints imposed on their complex dislocation structure. Loop-shaped grain boundaries are unique structures to establish such a link because their overall topological “charge” is zero due to their null net Burgers vector. Here, we observe that a local rotational deformation of a 2D colloidal crystal with an optical vortex results in a grain boundary loop only if the product of its radius and misorientation exceeds a critical value. Above this value, the deformation is plastic and the grain boundary loop spontaneously shrinks at a rate that solely depends on this product, while otherwise, the deformation is elastically restored. We show that this elastic-to-plastic crossover is a direct consequence of the unique dislocation structure of grain boundary loops. At the critical value, the loop is structurally equivalent to the so-called “flower defect” and the shrinkage rate diverges. Our results thus reveal a general limit on the formation of grain boundary loops in 2D crystals and elucidate the central role of defects in both the onset of plasticity and the kinetics of grain boundaries.

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Mechanics, Structure and Function of Biopolymer Condensates

Louis-Philippe Bergeron-Sandoval, Stephen W. Michnick

The spontaneous nature of biopolymer phase separation in cells entails that the resulting condensates can be thermodynamic machines, which, in the process of condensing, can take on distinct forms themselves and deform neighboring cellular structures. We introduce here general notions of material and mechanical properties of protein condensates with an emphasis on how molecular arrangements and intermolecular interaction within condensates determine their ability to do work on their surroundings. We further propose functional implications of these concepts to cellular and subcellular morphology and biogenesis.

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Low-temperature laser-stimulated controllable generation of micro-bubbles in a water suspension of absorptive colloid particles

O. V. Angelsky, A. Ya. Bekshaev, P. P. Maksimyak, A. P. Maksimyak, and S. G. Hanson

A method is described for the generation of micrometer-sized vapor-gas bubbles in a water suspension containing absorptive pigment nanoparticles. The diluted suspension (mean interparticle distance 20 μm) absorbs the continuous laser radiation (wavelength 808 nm), and each particle in the best illuminated volume (~10 × 10 × 200 μm3) serves as a bubble-nucleation center. The suspension heating is inessential (several degrees above the room temperature) and the bubbles are formed mainly of the air gases dissolved in water. The bubbles can stably exist within or near the illuminated area where their location is governed by the competition between thermal and optical forces and can be controlled via the laser beam parameters. The method enables controllable creation, support, prescribed transportation, and destruction of the bubbles. This can be useful in applications aimed at precise sorting, transportation, and delivery of species in nano- and micro-engineering as well as for biomedical studies.

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Determining the size and refractive index of homogeneous spherical aerosol particles using Mie resonance spectroscopy

L. J. Nugent Lew, Michelle V. Ting, and Thomas C. Preston
Methods for determining the size and refractive index of single, homogeneous, micrometer-sized aerosol particles using Mie resonance spectroscopy are studied using measurements from optically trapped particles and light-scattering calculations based on Mie theory. We consider both single-particle broadband light scattering and cavity-enhanced Raman scattering (CERS) and demonstrate that, when resonances observed in either type of spectroscopy are fitted using Mie theory, the accuracy of the best fits are similar. However, broadband measurements can yield more resonances than CERS, thus reducing the uncertainty in the retrieved parameters of best fit and increasing the range of particles that can be characterized. Resonance fitting methods are also compared to methods that fit the entire Mie scattering spectrum. Through calculations, it is shown that measured scattering spectra are sensitive to small changes in how light is collected, while Mie resonance positions are much less sensitive. This means that additional parameters are required to accurately fit entire light-scattering spectra using Mie theory, but these parameters are not needed to accurately determine Mie resonance positions.

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Electrodynamic multiple-scattering method for the simulation of optical trapping atop periodic metamaterials

Vassilios Yannopapas & Emmanuel Paspalakis

We present a new theoretical tool for simulating optical trapping of nanoparticles in the presence of an arbitrary metamaterial design. The method is based on rigorously solving Maxwell’s equations for the metamaterial via a hybrid discrete-dipole approximation/multiple-scattering technique and direct calculation of the optical force exerted on the nanoparticle by means of the Maxwell stress tensor. We apply the method to the case of a spherical polystyrene probe trapped within the optical landscape created by illuminating of a plasmonic metamaterial consisting of periodically arranged tapered metallic nanopyramids. The developed technique is ideally suited for general optomechanical calculations involving metamaterial designs and can compete with purely numerical methods such as finite-difference or finite-element schemes.

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Viscoelastic properties of vimentin originate from nonequilibrium conformational changes

Johanna Block, Hannes Witt, Andrea Candelli, Jordi Cabanas Danes, Erwin J. G. Peterman, Gijs J. L. Wuite, Andreas Janshoff and Sarah Köster

Structure and dynamics of living matter rely on design principles fundamentally different from concepts of traditional material science. Specialized intracellular filaments in the cytoskeleton permit living systems to divide, migrate, and grow with a high degree of variability and durability. Among the three filament systems, microfilaments, microtubules, and intermediate filaments (IFs), the physical properties of IFs and their role in cellular mechanics are the least well understood. We use optical trapping of individual vimentin filaments to investigate energy dissipation, strain history dependence, and creep behavior of stretched filaments. By stochastic and numerical modeling, we link our experimental observations to the peculiar molecular architecture of IFs. We find that individual vimentin filaments display tensile memory and are able to dissipate more than 70% of the input energy. We attribute these phenomena to distinct nonequilibrium folding and unfolding of α helices in the vimentin monomers constituting the filaments.

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Study of single airborne particle using laser-trapped submicron position-resolved temporal Raman spectroscopy

Aimable Kalumea, Chuji Wangb, Joshua Santarpiac, Yong-Le Pana

Study of various molecules located in different positions and of their temporal reactions within a microsized particle in its natural phase is essential to deeper understanding the functions of these molecules. Here, we measured the temporal Raman spectra in different submicron positions of a laser-trapped droplet composed of diethyl phthalate and glycerol. Results demonstrated the micro-cavity enhanced effects, the morphology change, dynamics of phase-separation, and evaporation of the mixed droplet. This method offers a powerful tool to monitor various chemical reactions in different positions within a single cell, spore, or particle in life science and atmospheric science.

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The optical stretcher as a tool for single-particle X-ray imaging and diffraction

J.-D. Nicolas, J. Hagemann, M. Sprung and T. Salditt

For almost half a century, optical tweezers have successfully been used to micromanipulate micrometre and sub-micrometre-sized particles. However, in recent years it has been shown experimentally that, compared with single-beam traps, the use of two opposing and divergent laser beams can be more suitable in studying the elastic properties of biological cells and vesicles. Such a configuration is termed an optical stretcher due to its capability of applying high deforming forces on biological objects such as cells. In this article the experimental capabilities of an optical stretcher as a potential sample delivery system for X-ray diffraction and imaging studies at synchrotrons and X-ray free-electron laser (FEL) facilites are demonstrated. To highlight the potential of the optical stretcher its micromanipulation capabilities have been used to image polymer beads and label biological cells. Even in a non-optimized configuration based on a commercially available optical stretcher system, X-ray holograms could be recorded from different views on a biological cell and the three-dimensional phase of the cell could be reconstructed. The capability of the setup to deform cells at higher laser intensities in combination with, for example, X-ray diffraction studies could furthermore lead to interesting studies that couple structural parameters to elastic properties. By means of high-throughput screening, the optical stretcher could become a useful tool in X-ray studies employing synchrotron radiation, and, at a later stage, femtosecond X-ray pulses delivered by X-ray free-electron lasers.

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Measuring the size and complex refractive index of an aqueous aerosol particle using electromagnetic heating and cavity-enhanced Raman scattering

Aidan Rafferty and Thomas C. Preston

A quantitative understanding of light scattering by small homogeneous particles requires accurate knowledge of particle geometry and complex refractive index, m = n + ik. In weakly absorbing particles, k can be on the order of 10−9, which is well below the detection limit of almost all light scattering based instruments. Here, we describe a dual-beam optical trap that can simultaneously determine n, k, and the radius, s, of weakly absorbing aerosol particles. We utilize cavity-enhanced Raman scattering to determine n and s and electromagnetic heating from the trapping laser itself to determine k. The relationship between particle size, the trapping cell conditions, the parameters of the trapping laser, and electromagnetic heating is thoroughly discussed and it is shown that the proper choice of a light scattering model is necessary to retrieve accurate values of k when fitting measurements. The phenomenon of optical multistability and its connection to thermal locking and thermal jumping is investigated through both modeling and measurements as understanding this behavior is essential when interpreting results from electromagnetic heating experiments. Measurements are made on three different atmospheric aerosol model systems and k as low as 5.91 × 10−9 are found.

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Trapping of low-refractive-index nanoparticles in a hollow dark spherical spot

Guanghao Rui, Yusong Wang, Xiaoyan Wang, Bing Gu and Yiping Cui

Optical trapping techniques have been of great interest and advantages that enable the direct handling of nanoparticles. However, stable trapping of low-refractive-index nanoparticle remains challenging because the conventional two-dimensional hollow beams is only capable of trapping nanoparticle in the transverse plane. In this work, we propose a novel strategy to optically trap low-refractive-index nanoparticle in three-dimensional space with a hollow dark spherical focal spot, which is generated by 4Pi focusing of radially polarized first-order Laguerre-Gaussian beam. With the assumption that the laser power is 100 mW, the nanoparticles can be stably trapped with the maximal optical force of 0.3 pN, potential depth of 10 KBT and stiffness of 80 pN/μm. Moreover, both the number and the position of the focal spot can be controlled by modulating the focusing condition and the gradient phase of the illumination respectively, enabling the simultaneously trapping of multiple nanoparticles with complex motion trajectory. The technique demonstrated in this work may open up new avenues for optical manipulation and their applications in various scientific fields.

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Measurement of the average shear rate around a microparticle in the shear thinning medium with laser tweezers

Piotr Domagalski, Marek Dziubinski, Ryszard Pawlak & Mariusz Tomczyk
A novel method for the determination of the average shear rate around a microparticle moving in the fluid is presented. Although the shear rate around a particle moving in the fluid is a parameter of paramount importance in sedimentation studies, its determination is time-consuming, thus model-based solutions are preferred. However, the current literature models require still rigorous validation. The presented technique relies on optical tracking of a laser-tweezers trapped particle suspended in the liquid medium. The obtained experimental results for spherical particles of 1 micron diameter show a good agreement with the classical rheology and literature correlation models. The new method is a valuable tool for determination of sedimentation parameters as it reduces the time of experiments and the sample volume by order(s) of magnitude when compared to classical methods.

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Diffusive tail anchorage determines velocity and force produced by kinesin-14 between crosslinked microtubules

Annemarie Lüdecke, Anja-Maria Seidel, Marcus Braun, Zdenek Lansky & Stefan Diez

Form and function of the mitotic spindle depend on motor proteins that crosslink microtubules and move them relative to each other. Among these are kinesin-14s, such as Ncd, which interact with one microtubule via their non-processive motor domains and with another via their diffusive tail domains, the latter allowing the protein to slip along the microtubule surface. Little is known about the influence of the tail domains on the protein’s performance. Here, we show that diffusive anchorage of Ncd’s tail domains impacts velocity and force considerably. Tail domain slippage reduced velocities from 270 nm s−1 to 60 nm s−1 and forces from several piconewtons to the sub-piconewton range. These findings challenge the notion that kinesin-14 may act as an antagonizer of other crosslinking motors, such as kinesin-5, during mitosis. It rather suggests a role of kinesin-14 as a flexible element, pliantly sliding and crosslinking microtubules to facilitate remodeling of the mitotic spindle.

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Non-fluorescent nanoscopic monitoring of a single trapped nanoparticle via nonlinear point sources

Seung Ju Yoon, Jungmin Lee, Sangyoon Han, Chang-Kyu Kim, Chi Won Ahn, Myung-Ki Kim & Yong-Hee Lee

Detection of single nanoparticles or molecules has often relied on fluorescent schemes. However, fluorescence detection approaches limit the range of investigable nanoparticles or molecules. Here, we propose and demonstrate a non-fluorescent nanoscopic trapping and monitoring platform that can trap a single sub-5-nm particle and monitor it with a pair of floating nonlinear point sources. The resonant photon funnelling into an extremely small volume of ~5 × 5 × 7 nm3 through the three-dimensionally tapered 5-nm-gap plasmonic nanoantenna enables the trapping of a 4-nm CdSe/ZnS quantum dot with low intensity of a 1560-nm continuous-wave laser, and the pumping of 1560-nm femtosecond laser pulses creates strong background-free second-harmonic point illumination sources at the two vertices of the nanoantenna. Under the stable trapping conditions, intermittent but intense nonlinear optical spikes are observed on top of the second-harmonic signal plateau, which is identified as the 3.0-Hz Kramers hopping of the quantum dot trapped in the 5-nm gap.

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Emerging investigator series: determination of biphasic core–shell droplet properties using aerosol optical tweezers

Kyle Gorkowski, Neil M. Donahue and Ryan C. Sullivan

We present a new algorithm for the analysis of whispering gallery modes (WGMs) found in the cavity enhanced Raman spectra retrieved from optically tweezed droplets. Our algorithm improves the computational scaling when analyzing core–shell droplets (i.e. phase-separated or biphasic droplets) in the aerosol optical tweezers (AOT), making it computationally practical to analyze spectra collected at a few Hz over hours-long experiments. This enables the determination of the size and refractive index of both the core and shell phases with high accuracy, at 0.5 Hz time resolution. Phase-separated core–shell droplets are common morphologies in a wide variety of biophysical, colloidal, and aerosolized chemical systems, and have recently become a major focus in understanding the atmospheric chemistry of particulate matter. Our new approach reduces the number of parameters directly searched for, decreasing computational demands. We assess the accuracy of the diameters and refractive indices retrieved from a homogeneous or core–shell droplet. We demonstrate the performance of the new algorithm using experimental data from a droplet of aqueous glycerol coated by squalane. We demonstrate that a shell formation causes adjacent WGMs to split from each other in their wavenumber position through the addition of a secondary organic aerosol shell around a NaCl(aq) droplet. Our new algorithm paves the way for more in-depth physiochemical experiments into liquid–liquid phase separation and their consequences for interfacial chemistry—a topic with growing experimental needs for understanding the dynamics and chemistry of atmospheric aerosol particles, and in biochemical systems.

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Radiation force acting on a Rayleigh dielectric sphere produced by Whittaker-Gaussian beams

Bin Tang, Kai Chen, Li Huang, Xin Zhou, Xianzhong Lang

Optical trapping and manipulating of micro particles have attracted extensive interests due to the advantages of being noncontact and noninvasive. In this work, the field distribution of Whittaker-Gaussian (WG) beams propagating through a lens and the radiation force acting on the Rayleigh particle are investigated numerically and theoretically. The results show that the WG beams can trap the particles with both high and low index of refractive near the focus. The influences of optical parameters on the radiation forces are analyzed in detail. Furthermore, the conditions for trapping stability are also discussed.

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Probing Mitotic CENP-E Kinesin with the Tethered Cargo Motion Assay and Laser Tweezers

Nikita Gudimchuk, Ekaterina V. Tarasovetc, Vadim Mustyatsa, Alexei L. Drobyshev, Benjamin Vitre, Don W. Cleveland, Fazly I. Ataullakhanov, Ekaterina L. Grishchuk

Coiled-coil stalks of various kinesins differ significantly in predicted length and structure; this is an adaption that helps these motors carry out their specialized functions. However, little is known about the dynamic stalk configuration in moving motors. To gain insight into the conformational properties of the transporting motors, we developed a theoretical model to predict Brownian motion of a microbead tethered to the tail of a single, freely walking molecule. This approach, which we call the tethered cargo motion (TCM) assay, provides an accurate measure of the mechanical properties of motor-cargo tethering, verified using kinesin-1 conjugated to a microbead via DNA links in vitro. Applying the TCM assay to the mitotic kinesin CENP-E unexpectedly revealed that when walking along a microtubule track, this highly elongated molecule with a contour length of 230 nm formed a 20-nm-long tether. The stalk of a walking CENP-E could not be extended fully by application of sideways force with optical tweezers (up to 4 pN), implying that CENP-E carries its cargo in a compact configuration. Assisting force applied along the microtubule track accelerates CENP-E walking, but this increase does not depend on the presence of the CENP-E stalk. Our results suggest that the unusually large stalk of CENP-E has little role in regulating its function as a transporter. The adjustable stalk configuration may represent a regulatory mechanism for controlling the physical reach between kinetochore-bound CENP-E and spindle microtubules, or it may assist localizing various kinetochore regulators in the immediate vicinity of the kinetochore-embedded microtubule ends. The TCM assay and underlying theoretical framework will provide

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Multimode ultrasound viscoelastography for three-dimensional interrogation of microscale mechanical properties in heterogeneous biomaterials

Xiaowei Hong, Ramkumar T. Annamalai, Tyler S. Kemerer, Cheri X. Deng, Jan P. Stegemann

Both static and time-dependent mechanical factors can have a profound impact on cell and tissue function, but it is challenging to measure the mechanical properties of soft materials at the scale which cells sense. Multimode ultrasound viscoelastography (MUVE) uses focused ultrasound pulses to both generate and image deformations within soft hydrogels non-invasively, at sub-millimeter resolution, and in 3D. The deformation and strain over time data are used to extract quantitative parameters that describe both the elastic and viscoelastic properties of the material. MUVE was used in creep mode to characterize the viscoelastic properties of 3D agarose, collagen, and fibrin hydrogels. Quantitative comparisons were made by extracting characteristic viscoelastic parameters using Burger's lumped parameter constitutive model. Spatial resolution of the MUVE technique was found to be approximately 200 μm, while detection sensitivity, defined as the capability to differentiate between materials based on mechanical property differences, was approximately 0.2 kPa using agarose hydrogels. MUVE was superior to nanoindentation and shear rheometry in generating consistent microscale measurements of viscoelastic behavior in soft materials. These results demonstrate that MUVE is a rapid, quantitative, and accurate method to measure the viscoelastic mechanical properties of soft 3D hydrogels at the microscale, and is a promising technique to study the development of native and engineered tissues over time.

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Photonic force optical coherence elastography for three-dimensional mechanical microscopy

Nichaluk Leartprapun, Rishyashring R. Iyer, Gavrielle R. Untracht, Jeffrey A. Mulligan & Steven G. Adie

Optical tweezers are an invaluable tool for non-contact trapping and micro-manipulation, but their ability to facilitate high-throughput volumetric microrheology of biological samples for mechanobiology research is limited by the precise alignment associated with the excitation and detection of individual bead oscillations. In contrast, radiation pressure from a low-numerical aperture optical beam can apply transversely localized force over an extended depth range. Here we present photonic force optical coherence elastography (PF-OCE), leveraging phase-sensitive interferometric detection to track sub-nanometer oscillations of beads, embedded in viscoelastic hydrogels, induced by modulated radiation pressure. Since the displacements caused by ultra-low radiation-pressure force are typically obscured by absorption-mediated thermal effects, mechanical responses of the beads were isolated after independent measurement and decoupling of the photothermal response of the hydrogels. Volumetric imaging of bead mechanical responses in hydrogels with different agarose concentrations by PF-OCE was consistent with bulk mechanical characterization of the hydrogels by shear rheometry.

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Optical Trapping of Ion Coulomb Crystals

Julian Schmidt, Alexander Lambrecht, Pascal Weckesser, Markus Debatin, Leon Karpa, and Tobias Schaetz

The electronic and motional degrees of freedom of trapped ions can be controlled and coherently coupled on the level of individual quanta. Assembling complex quantum systems ion by ion while keeping this unique level of control remains a challenging task. For many applications, linear chains of ions in conventional traps are ideally suited to address this problem. However, driven motion due to the magnetic or radio-frequency electric trapping fields sometimes limits the performance in one dimension and severely affects the extension to higher-dimensional systems. Here, we report on the trapping of multiple barium ions in a single-beam optical dipole trap without radio-frequency or additional magnetic fields. We study the persistence of order in ensembles of up to six ions within the optical trap, measure their temperature, and conclude that the ions form a linear chain, commonly called a one-dimensional Coulomb crystal. As a proof-of-concept demonstration, we access the collective motion and perform spectrometry of the normal modes in the optical trap. Our system provides a platform that is free of driven motion and combines advantages of optical trapping, such as state-dependent confinement and nanoscale potentials, with the desirable properties of crystals of trapped ions, such as long-range interactions featuring collective motion. Starting with small numbers of ions, it has been proposed that these properties would allow the experimental study of many-body physics and the onset of structural quantum phase transitions between one- and two-dimensional crystals.

DOI

Quantifying the stability of oxidatively damaged DNA by single-molecule DNA stretching

Micah J McCauley Leah Furman Catherine A Dietrich Ioulia Rouzina Megan E Núñez Mark C Williams

One of the most common DNA lesions is created when reactive oxygen alters guanine. 8-oxo-guanine may bind in the anti-conformation with an opposing cytosine or in the syn-conformation with an opposing adenine paired by transversion, and both conformations may alter DNA stability. Here we use optical tweezers to measure the stability of DNA hairpins containing 8-oxoguanine (8oxoG) lesions, comparing the results to predictive models of base-pair energies in the absence of the lesion. Contrasted with either a canonical guanine-cytosine or adenine-thymine pair, an 8oxoG-cytosine base pair shows significant destabilization of several kBT. The magnitude of destabilization is comparable to guanine-thymine ‘wobble’ and cytosine-thymine mismatches. Furthermore, the measured energy of 8oxoG-adenine corresponds to theoretical predictions for guanine-adenine pairs, indicating that oxidative damage does not further destabilize this mismatch in our experiments, in contrast to some previous observations. These results support the hypothesis that oxidative damage to guanine subtly alters the direction of the guanine dipole, base stacking interactions, the local backbone conformation, and the hydration of the modified base. This localized destabilization under stress provides additional support for proposed mechanisms of enzyme repair.

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PLANT: A Method for Detecting Changes of Slope in Noisy Trajectories

Alberto Sosa-Costa, Izabela K. Piechocka, Lucia Gardini, Francesco S. Pavone, Marco Capitanio, Maria F. Garcia-Parajo, Carlo Manzo

Time traces obtained from a variety of biophysical experiments contain valuable information on underlying processes occurring at the molecular level. Accurate quantification of these data can help explain the details of the complex dynamics of biological systems. Here, we describe PLANT (Piecewise Linear Approximation of Noisy Trajectories), a segmentation algorithm that allows the reconstruction of time-trace data with constant noise as consecutive straight lines, from which changes of slopes and their respective durations can be extracted. We present a general description of the algorithm and perform extensive simulations to characterize its strengths and limitations, providing a rationale for the performance of the algorithm in the different conditions tested. We further apply the algorithm to experimental data obtained from tracking the centroid position of lymphocytes migrating under the effect of a laminar flow and from single myosin molecules interacting with actin in a dual-trap force-clamp configuration.

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Optoelectronic forces with quantum wells for cavity optomechanics in GaAs/AlAs semiconductor microcavities

V. Villafañe, P. Sesin, P. Soubelet, S. Anguiano, A. E. Bruchhausen, G. Rozas, C. Gomez Carbonell, A. Lemaître, and A. Fainstein

Radiation pressure, electrostriction, and photothermal forces have been investigated to evidence backaction, nonlinearities, and quantum phenomena in cavity optomechanics. We show here through a detailed study of the relative intensity of the cavity mechanical modes observed when exciting with pulsed lasers close to the GaAs optical gap that optoelectronic forces involving real carrier excitation and deformation potential interaction are the strongest mechanism of light-to-sound transduction in semiconductor GaAs/AlAs distributed Bragg reflector optomechanical resonators. We demonstrate that the ultrafast spatial redistribution of the photoexcited carriers in microcavities with massive GaAs spacers leads to an enhanced coupling to the fundamental 20-GHz vertically polarized mechanical breathing mode. The carrier diffusion along the growth axis of the device can be enhanced by increasing the laser power, or limited by embedding GaAs quantum wells in the cavity spacer, a strategy used here to prove and engineer the optoelectronic forces in phonon generation with real carriers. The wavelength dependence of the observed phenomena provide further proof of the role of optoelectronic forces. The optical forces associated with the different intervening mechanisms and their relevance for dynamical backaction in optomechanics are evaluated using finite-element methods. The results presented open the path to the study of hitherto seldom investigated dynamical backaction in optomechanical solid-state resonators in the presence of optoelectronic forces.

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Single-Stranded Condensation Stochastically Blocks G-Quadruplex Assembly in Human Telomeric RNA

Irene Gutiérrez, Miguel Garavís, Sara de Lorenzo, Alfredo Villasante, Carlos González, and J. Ricardo Arias-Gonzalez

TERRA is an RNA molecule transcribed from human subtelomeric regions toward chromosome ends potentially involved in regulation of heterochromatin stability, semiconservative replication, and telomerase inhibition, among others. TERRA contains tandem repeats of the sequence GGGUUA, with a strong tendency to fold into a four-stranded arrangement known as a parallel G-quadruplex. Here, we demonstrate by using single-molecule force spectroscopy that this potential is limited by the inherent capacity of RNA to self-associate randomly and further condense into entropically more favorable structures. We stretched RNA constructions with more than four and less than eight hexanucleotide repeats, thus unable to form several G-quadruplexes in tandem, flanked by non-G-rich overhangs of random sequence by optical tweezers on a one by one basis. We found that condensed RNA stochastically blocks G-quadruplex folding pathways with a near 20% probability, a behavior that is not found in DNA analogous molecules.

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KIF15 nanomechanics and kinesin inhibitors, with implications for cancer chemotherapeutics

Bojan Milic, Anirban Chakraborty, Kyuho Han, Michael C. Bassik, and Steven M. Block

Eg5, a mitotic kinesin, has been a target for anticancer drug development. Clinical trials of small-molecule inhibitors of Eg5 have been stymied by the development of resistance, attributable to mitotic rescue by a different endogenous kinesin, KIF15. Compared with Eg5, relatively little is known about the properties of the KIF15 motor. Here, we employed single-molecule optical-trapping techniques to define the KIF15 mechanochemical cycle. We also studied the inhibitory effects of KIF15-IN-1, an uncharacterized, commercially available, small-molecule inhibitor, on KIF15 motility. To explore the complementary behaviors of KIF15 and Eg5, we also scored the effects of small-molecule inhibitors on admixtures of both motors, using both a microtubule (MT)-gliding assay and an assay for cancer cell viability. We found that (i) KIF15 motility differs significantly from Eg5; (ii) KIF15-IN-1 is a potent inhibitor of KIF15 motility; (iii) MT gliding powered by KIF15 and Eg5 only ceases when both motors are inhibited; and (iv) pairing KIF15-IN-1 with Eg5 inhibitors synergistically reduces cancer cell growth. Taken together, our results lend support to the notion that a combination drug therapy employing both inhibitors may be a viable strategy for overcoming chemotherapeutic resistance.

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Machine-learning techniques for fast and accurate feature localization in holograms of colloidal particles

Mark D. Hannel, Aidan Abdulali, Michael O’Brien, and David G. Grier

Holograms of colloidal particles can be analyzed with the Lorenz-Mie theory of light scattering to measure individual particles’ three-dimensional positions with nanometer precision while simultaneously estimating their sizes and refractive indexes. Extracting this wealth of information begins by detecting and localizing features of interest within individual holograms. Conventionally approached with heuristic algorithms, this image analysis problem can be solved faster and more generally with machine-learning techniques. We demonstrate that two popular machine-learning algorithms, cascade classifiers and deep convolutional neural networks (CNN), can solve the feature-localization problem orders of magnitude faster than current state-of-the-art techniques. Our CNN implementation localizes holographic features precisely enough to bootstrap more detailed analyses based on the Lorenz-Mie theory of light scattering. The wavelet-based Haar cascade proves to be less precise, but is so computationally efficient that it creates new opportunities for applications that emphasize speed and low cost. We demonstrate its use as a real-time targeting system for holographic optical trapping.

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Microtubules soften due to cross-sectional flattening

Edvin Memet, Feodor Hilitsk, Margaret A Morris, Walter J Schwenger, Zvonimir Dogic, L Mahadevan

We use optical trapping to continuously bend an isolated microtubule while simultaneously measuring the applied force and the resulting filament strain, thus allowing us to determine its elastic properties over a wide range of applied strains. We find that, while in the low-strain regime, microtubules may be quantitatively described in terms of the classical Euler-Bernoulli elastic filament, above a critical strain they deviate from this simple elastic model, showing a softening response with increasing deformations. A three-dimensional thin-shell model, in which the increased mechanical compliance is caused by flattening and eventual buckling of the filament cross-section, captures this softening effect in the high strain regime and yields quantitative values of the effective mechanical properties of microtubules. Our results demonstrate that properties of microtubules are highly dependent on the magnitude of the applied strain and offer a new interpretation for the large variety in microtubule mechanical data measured by different methods.

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High-Resolution Large-Ensemble Nanoparticle Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface

Justus C. Ndukaife, Yi Xuan, Agbai George Agwu Nnanna, Alexander V. Kildishev, Vladimir M. Shalaev, Steven T. Wereley, and Alexandra Boltasseva

The intrinsic loss in a plasmonic metasurface is usually considered to be detrimental for device applications. Using plasmonic loss to our advantage, we introduce a thermoplasmonic metasurface that enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. In our work, an array of subwavelength nanoholes in a metal film is used as a plasmonic metasurface that supports the excitation of localized surface plasmon and Bloch surface plasmon polariton waves upon optical illumination and provides a platform for molding both optical and thermal landscapes to achieve a tunable many-particle assembling process. The demonstrated many-particle trapping occurs against gravity in an inverted configuration where the light beam first passes through the nanoparticle suspension before illuminating the thermoplasmonic metasurface, a feat previously thought to be impossible. We also report an extraordinarily enhanced electrothermoplasmonic flow in the region of the thermoplasmonic nanohole metasurface, with comparatively larger transport velocities in comparison to the unpatterned region. This thermoplasmonic metasurface could enable possibilities for myriad applications in molecular analysis, quantum photonics, and self-assembly and creates a versatile platform for exploring nonequilibrium physics.

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Direct Particle Tracking Observation and Brownian Dynamics Simulations of a Single Nanoparticle Optically Trapped by a Plasmonic Nanoaperture

Zhe Xu, Wuzhou Song, and Kenneth B. Crozier

Optical trapping using plasmonic nanoapertures has proven to be an effective means for the contactless manipulation of nanometer-sized particles under low optical intensities. These particles have included polystyrene and silica nanospheres, proteins, coated quantum dots and magnetic nanoparticles. Here we employ fluorescence microscopy to directly observe the optical trapping process, tracking the position of a polystyrene nanosphere (20 nm diameter) trapped in water by a double nanohole (DNH) aperture in a gold film. We show that position distribution in the plane of the film has an elliptical shape. Comprehensive simulations are performed to gain insight into the trapping process, including of the distributions of the electric field, temperature, fluid velocity, optical force, and potential energy. These simulations are combined with stochastic Brownian diffusion to directly model the dynamics of the trapping process, that is, particle trajectories. We anticipate that the combination of direct particle tracking experiments with Brownian motion simulations will be valuable tool for the better understanding of fundamental mechanisms underlying nanostructure-based trapping. It could thus be helpful in the development of the future novel optical trapping devices.

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Laser trap ionization for identification of human erythrocytes with variable hemoglobin quantitation

Michele Kelley; James Cooper; Daniel Devito; Robert Mushi; Maria del Pilar Aguinaga; Daniel B. Erenso

An approach to an established technique that is potentially applicable for a more comprehensive understanding of the electrical properties of red blood cells (RBCs) is presented. Using a high-intensity gradient laser trap, RBCs can be singly trapped and consequentially ionized. The subsequent dynamics of the ionized cell allows one to calculate the charge developed and the ionization energy (IE) through a Newtonian-based analysis. RBCs with two different hemoglobin (Hb) types were ionized. The first sample was identified as carrying Hb HbAA (normal Hb) and the second one was identified as carrying HbAC (HbC trait). By analyzing the charge developed on each cell and several other related factors, we were able to discern a difference between the main Hb types contained within the individual RBC, independent of cell size. A relationship between the charge developed and the IE of the cell was also established based on the electrical properties of RBCs. Thus, we present this laser trapping technique as a study of the electrical properties of RBCs and as possible biomedical tool to be used for the differentiation of Hb types.

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Optothermophoretic Manipulation of Colloidal Particles in Nonionic Liquids

Xiaolei Peng, Linhan Lin, Eric H. Hill, Pranaw Kunal, Simon M. Humphrey, and Yuebing Zheng

The response of colloidal particles to a light-controlled external temperature field can be harnessed for optothermophoretic manipulation of the particles. The thermoelectric effect is regarded as the driving force for thermophoretic trapping of particles at the light-irradiated hot region, which is thus limited to ionic liquids. Herein, we achieve optothermophoretic manipulation of colloidal particles in various nonionic liquids, including water, ethanol, isopropyl alcohol, and 1-butanol, and establish the physical mechanism of the manipulation at the molecular level. We reveal that the nonionic driving force originates from a layered structure of solvent molecules at the particle–solvent interface, which is supported by molecular dynamics simulations. Furthermore, the effects of hydrophilicity, solvent type, and ionic strength on the layered interfacial structures and thus the trapping stability of particles are investigated, providing molecular-level insight into thermophoresis and guidance on interfacial engineering for optothermal manipulation.

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Influence of particle viscosity on mass transfer and heterogeneous ozonolysis kinetics in aqueous–sucrose–maleic acid aerosol

Frances H. Marshall, Thomas Berkemeier, Manabu Shiraiwa, Lucy Nandy, Peter B. Ohm, Cari S. Dutcher and Jonathan P. Reid

Mass transfer between the gas and condensed phases in aerosols can be limited by slow bulk diffusion within viscous particles. During the heterogeneous and multiphase reactions of viscous organic aerosol particles, it is necessary to consider the interplay of numerous mass transfer processes and how they are impacted by viscosity, including the partitioning kinetics of semi-volatile organic reactants, water and oxidants. To constrain kinetic models of the heterogeneous chemistry, measurements must provide information on as many observables as possible. Here, the ozonolysis of maleic acid (MA) in ternary aerosol particles containing water and sucrose is used as a model system. By varying the mass ratio of sucrose to MA and by performing reactions over a wide range of relative humidity, direct measurements show that the viscosity of the particle can be varied over 7 orders of magnitude. Measurements of the volatilisation kinetics of MA show that this range in viscosity leads to a suppression in the effective vapour pressure of MA of 3–4 orders of magnitude. The inferred values of the diffusion coefficient of MA in the particle phase closely mirror the expected change in diffusion coefficient from the Stokes–Einstein equation and the change in viscosity. The kinetics of ozonolysis show a similar dependence on particle viscosity that can be further investigated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). Two scenarios, one constraining the diffusion coefficients for MA to those expected based on the Stokes–Einstein equation and the other including the diffusion coefficients as a fit parameter, yield similarly adequate representations of the ozonolysis kinetics, as inferred from the experimental decay in the signature of the vinylic C–H stretching vibration of MA. However, these two scenarios provide very different parameterisations of the compositional dependence of the diffusion coefficients of ozone within the condensed phase, yielding qualitatively different time-dependent internal concentration profiles. We suggest that this highlights the importance of providing additional experimental observables (e.g. particle size, heterogeneity in composition) if measurements and models are to be universally reconciled.

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Force-Mediating Magnetic Nanoparticles to Engineer Neuronal Cell Function

Trevor J. Gahl and Anja Kunze

Cellular processes like membrane deformation, cell migration, and transport of organelles are sensitive to mechanical forces. Technically, these cellular processes can be manipulated through operating forces at a spatial precision in the range of nanometers up to a few micrometers through chaperoning force-mediating nanoparticles in electrical, magnetic, or optical field gradients. But which force-mediating tool is more suitable to manipulate cell migration, and which, to manipulate cell signaling? We review here the differences in forces sensation to control and engineer cellular processes inside and outside the cell, with a special focus on neuronal cells. In addition, we discuss technical details and limitations of different force-mediating approaches and highlight recent advancements of nanomagnetics in cell organization, communication, signaling, and intracellular trafficking. Finally, we give suggestions about how force-mediating nanoparticles can be used to our advantage in next-generation neurotherapeutic devices.

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Sensing and Exploiting Static Femto-Newton Optical Forces by a Nanofiber with White-Light Interferometry

Jianhui Yu, Liheng Chen, Huazhuo Dong, Xingyu Liu, Hankai Huang, Weiqia Qiu, Shiqing Huang, Wenguo Zhu, Huihui Lu, Jieyuan Tang, Yi Xiao, Yongchun Zhong, Yunhan Luo, Jun Zhang, and Zhe Chen

Optical force determines the fundamental process of momentum exchange between light and matter. However, owing to the weak mechanical effect of the optical force and relatively large stiffness of optomechanical devices, pico-Newton (10–12 N) optical force is required to manipulate micro/nanoparticles and the optical response of optical devices. It is still extremely challenging to sense static femto-Newton (fN) optical forces and exploit such forces to actuate micro-optical devices. Here, using a tapered nanofiber (TNF) with a high mechanical efficiency of 2.13 nm/fN, a sensitive and cost-effective scheme is demonstrated to generate, sense, and exploit fN optical force. Strong light coupling from the TNF to a glass substrate can result in a fN repulsive optical force, which can induce a TNF deformation of up to 425.6 nm. Such a large deformation allows white-light interferometry to detect a fN static optical force (5.2 fN). Moreover, the high optomechanical efficiency (15.6 nm/μW) allows us to all-optically control the signal power at values ranging from 0.09 to 17.1 μW with only microwatt pump power, which paves the way toward microwatt and fN-optical-force optomechanical devices.

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Amorphous phase state diagrams and viscosity of ternary aqueous organic/organic and inorganic/organic mixtures

Aleksandra Marsh, Sarah Suda Petters, Nicholas Ernest Rothfuss, Grazia Rovelli, Young Chul Song, Jonathan Philip Reid and Markus Dirk Petters

A Dimer Coagulation, Isolation and Coalescence (DCIC) technique is used to probe the phase behaviour and glass transition temperatures of ternary aerosol mixtures. The DCIC technique is used to perform temperature and relative humidity dependent viscosity measurements at viscosities near 5 × 106 Pa s. Measurements include organic–organic and organic–inorganic mixtures composed of sucrose–citric acid and sucrose–sodium nitrate. The data reported here add additional insight into the wide discrepancies in glass transition temperatures reported for pure sodium nitrate. The phase diagram model used in the work of Rothfuss and Petters (Phys. Chem. Chem. Phys., 2017, 19, 6532–6545) is expanded to include multiple solute components. Data and model predictions of the mixtures are in good agreement with the modified model. These measurements are compared with values from Holographic Optical Tweezer (HOT) measurements taken at room temperature. Overall, the viscosities determined from the DCIC and HOT techniques are in good agreement.

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Photonic crystal resonances for sensing and imaging

Giampaolo Pitruzzello and Thomas F Krauss

This review provides an insight into the recent developments of photonic crystal (PhC)-based devices for sensing and imaging, with a particular emphasis on biosensors. We focus on two main classes of devices, namely sensors based on PhC cavities and those on guided mode resonances (GMRs). This distinction is able to capture the richness of possibilities that PhCs are able to offer in this space. We present recent examples highlighting applications where PhCs can offer new capabilities, open up new applications or enable improved performance, with a clear emphasis on the different types of structures and photonic functions. We provide a critical comparison between cavity-based devices and GMR devices by highlighting strengths and weaknesses. We also compare PhC technologies and their sensing mechanism to surface plasmon resonance, microring resonators and integrated interferometric sensors.

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Cell surface flip-flop of phosphatidylserine is critical for PIEZO1-mediated myotube formation

Masaki Tsuchiya, Yuji Hara, Masaki Okuda, Karin Itoh, Ryotaro Nishioka, Akifumi Shiomi, Kohjiro Nagao, Masayuki Mori, Yasuo Mori, Junichi Ikenouchi, Ryo Suzuki, Motomu Tanaka, Tomohiko Ohwada, Junken Aoki, Motoi Kanagawa, Tatsushi Toda, Yosuke Nagata, Ryoichi Matsuda, Yasunori Takayama, Makoto Tominaga & Masato Umeda

Myotube formation by fusion of myoblasts and subsequent elongation of the syncytia is essential for skeletal muscle formation. However, molecules that regulate myotube formation remain elusive. Here we identify PIEZO1, a mechanosensitive Ca2+ channel, as a key regulator of myotube formation. During myotube formation, phosphatidylserine, a phospholipid that resides in the inner leaflet of the plasma membrane, is transiently exposed to cell surface and promotes myoblast fusion. We show that cell surface phosphatidylserine inhibits PIEZO1 and that the inward translocation of phosphatidylserine, which is driven by the phospholipid flippase complex of ATP11A and CDC50A, is required for PIEZO1 activation. PIEZO1-mediated Ca2+ influx promotes RhoA/ROCK-mediated actomyosin assemblies at the lateral cortex of myotubes, thus preventing uncontrolled fusion of myotubes and leading to polarized elongation during myotube formation. These results suggest that cell surface flip-flop of phosphatidylserine acts as a molecular switch for PIEZO1 activation that governs proper morphogenesis during myotube formation.

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Optothermal Manipulations of Colloidal Particles and Living Cells

Linhan Lin , Eric H. Hill, Xiaolei Peng, and Yuebing Zheng

This Account covers the working principles, design concepts, and applications of a series of newly developed optothermal manipulation techniques, including bubble-pen lithography, opto-thermophoretic tweezers, opto-thermoelectric tweezers, optothermal assembly, and opto-thermoelectric printing. In bubble-pen lithography, optical heating of a plasmonic substrate generates microbubbles at the solid–liquid interface to print diverse colloidal particles on the substrates. Programmable bubble printing of semiconductor quantum dots on different substrates and haptic control of printing have also been achieved. The key to optothermal tweezers is the ability to deliver colloidal particles from cold to hot regions of a temperature gradient or a negative Soret effect. We explore different driving forces for the two types of optothermal tweezers. Opto-thermophoretic tweezers rely on an abnormal permittivity gradient built by structured solvent molecules in the electric double layer of colloidal particles and living cells in response to heat-induced entropy, and opto-thermoelectric tweezers exploit a thermophoresis-induced thermoelectric field for the low-power manipulation of small nanoparticles with minimum diameter around 20 nm. Furthermore, by incorporating depletion attraction into the optothermal tweezers system as particle–particle or particle–substrate binding force, we have achieved bottom-up assembly and reconfigurable optical printing of artificial colloidal matter. Beyond optothermal manipulation techniques in liquid environments, we also review recent progress of gas-phase optothermal manipulation based on photophoresis. Photophoretic trapping and transport of light-absorbing materials have been achieved through optical engineering to tune particle–molecule interactions during optical heating, and a novel optical trap display has been demonstrated.

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Photothermal Heating of Plasmonic Nanoantennas: Influence on Trapped Particle Dynamics and Colloid Distribution

Steven Jones, Daniel Andrén, Pawel Karpinski, and Mikael Käll

Plasmonic antennas are well-known and extremely powerful platforms for optical spectroscopy, sensing, and manipulation of molecules and nanoparticles. However, resistive antenna losses, resulting in highly localized photothermal heat generation, may significantly compromise their applicability. Here we investigate how the interplay between plasmon-enhanced optical and thermal forces affects the dynamics of nanocolloids diffusing in close proximity to gold bowtie nanoantennas. The study is based on an anti-Stokes thermometry technique that can measure the internal antenna temperature with an accuracy of ∼5 K over an extended temperature range. We argue that Kapitza resistances have a significant impact on the local thermal landscape, causing an interface temperature discontinuity of up to ∼20% of the total photothermal temperature increase of the antenna studied. We then use the bowties as plasmonic optical tweezers and quantify how the antenna temperature influences the motion and distribution of nearby fluorescent colloids. We find that colloidal particle motion within the plasmonic trap is primarily dictated by a competition between enhanced optical forces and enhanced heating, resulting in a surprising insensitivity to the specific resonance properties of the antenna. Furthermore, we find that thermophoretic forces inhibit diffusion of particles toward the antenna and drive the formation of a thermal depletion shell that extends several microns. The study highlights the importance of thermal management at the nanoscale and points to both neglected problems and new opportunities associated with plasmonic photothermal effects in the context of nanoscale manipulation and analysis.

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Optical manipulation of microparticles with a fiber tip containing a hollow cavity

Xiaoqi Ni, Ming Wang, Ri Wang, Yan Huang, Yiping Wang, Dongmei Guo

Optical manipulation is a non-contact and non-destructive method to capture and manipulate micro/nano particles, biological macromolecules and cells with application of optical forces. With the rapid development of optical manipulation, various kinds of optical fiber tips have been presented and demonstrated in recent years. In this letter, a novel structure of optical fiber tip with an imbedded hollow cavity is presented. The electric field distribution of the fiber tip and the influence of cavity geometry are simulated with finite-difference time-domain method. The unique advantage is that the electric field intensity will be significantly enhanced when the fiber tip is filled with high refractive index medium in the cavity, which brings the potential to generate a three-dimensional optical trap for particles. Through experiments, we also demonstrate that the velocity of the particles driven by the optical force has a quadratic relationship with the laser power. The relationship can be used in microfluidic technology to measure the velocity of micro fluid. And the balance between the scattering force and the fluid force can be used to study non-invasive migration of particles.

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Dextran adsorption onto red blood cells revisited: single cell quantification by laser tweezers combined with microfluidics

Kisung Lee, Evgeny Shirshin, Nataliya Rovnyagina, Francois Yaya, Zakaria Boujja, Alexander Priezzhev, and Christian Wagner

The aggregation of red blood cells (RBC) is of importance for hemorheology, while its mechanism remains debatable. The key question is the role of the adsorption of macromolecules on RBC membranes, which may act as “bridges” between cells. It is especially important that dextran is considered to induce “bridge”-less aggregation due to the depletion forces. We revisit the dextran-RBC interaction on the single cell level using the laser tweezers combined with microfluidic technology and fluorescence microscopy. An immediate sorption of ~104 molecules of 70 kDa dextran per cell was observed. During the incubation of RBC with dextran, a gradual tenfold increase of adsorption was found, accompanied by a moderate change in the RBC deformability. The obtained data demonstrate that dextran sorption and incubation-induced changes of the membrane properties must be considered when studying RBC aggregation in vitro.

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Topological Insulator Particles As Optically Induced Oscillators: Toward Dynamical Force Measurements and Optical Rheology

W. H. Campos, J. M. Fonseca, V. E. de Carvalho, J. B. S. Mendes, M. S. Rocha, and W. A. Moura-Melo

We report the first experimental study upon the optical trapping and manipulation of topological insulator (TI) particles. By virtue of the unique TI properties, which have a conducting surface and an insulating bulk, the particles present a peculiar behavior in the presence of a single laser beam optical tweezers: they oscillate in a plane perpendicular to the direction of the laser propagation. In other words, TI particles behave as optically induced oscillators, allowing dynamical measurements with unprecedented simplicity and purely optical control. Actually, optical rheology of soft matter interfaces and biological membranes, as well as dynamical force measurements in macromolecules and biopolymers, may be quoted as feasible possibilities for the near future.

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LED-based interference-reflection microscopy combined with optical tweezers for quantitative three-dimensional microtubule imaging

Steve Simmert, Mohammad Kazem Abdosamadi, Gero Hermsdorf, and Erik Schäffer

Optical tweezers combined with various microscopy techniques are a versatile tool for single-molecule force spectroscopy. However, some combinations may compromise measurements. Here, we combined optical tweezers with total-internal-reflection-fluorescence (TIRF) and interference-reflection microscopy (IRM). Using a light-emitting diode (LED) for IRM illumination, we show that single microtubules can be imaged with high contrast. Furthermore, we converted the IRM interference pattern of an upward bent microtubule to its three-dimensional (3D) profile calibrated against the optical tweezers and evanescent TIRF field. In general, LED-based IRM is a powerful method for high-contrast 3D microscopy.

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Sorting and manipulation of biological cells and the prospects for using optical forces

Arslan Atajanov, Alexander Zhbanov and Sung Yang

Contemporary biomedical research requires development of novel techniques for sorting and manipulation of cells within the framework of a microfluidic chip. The desired functions of a microfluidic chip are achieved by combining and integrating passive methods that utilize the channel geometry and structure, as well as active methods that include magnetic, electrical, acoustic and optical forces. Application of magnetic, electric and acoustics-based methods for sorting and manipulation have been and are under continuous scrutiny. Optics-based methods, in contrast, have not been explored to the same extent as other methods, since they attracted insufficient attention. This is due to the complicated, expensive and bulky setup required for carrying out such studies. However, advances in optical beam shaping and computer hardware, and software have opened up new opportunities for application of light to development of advanced sorting and manipulation techniques. This review outlines contemporary techniques for cell sorting and manipulation, and provides an in-depth view into the existing and prospective uses of light for cell sorting and manipulation.

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Multivalency of NDC80 in the outer kinetochore is essential to track shortening microtubules and generate forces

Vladimir A Volkov, Pim J Huis in 't Veld, Marileen Dogterom, Andrea Musacchio

Presence of multiple copies of the microtubule-binding NDC80 complex is an evolutionary conserved feature of kinetochores, points of attachment of chromosomes to spindle microtubules. This may enable multivalent attachments to microtubules, with implications that remain unexplored. Using recombinant human kinetochore components, we show that while single NDC80 complexes do not track depolymerizing microtubules, reconstituted particles containing the NDC80 receptor CENP-T bound to three or more NDC80 complexes do so effectively, as expected for a kinetochore force coupler. To study multivalency systematically, we engineered modules allowing incremental addition of NDC80 complexes. The modules’ residence time on microtubules increased exponentially with the number of NDC80 complexes. Modules with two or more complexes tracked depolymerizing microtubules with increasing efficiencies, and stalled and rescued microtubule depolymerization in a force-dependent manner when conjugated to cargo. Our observations indicate that NDC80, rather than through biased diffusion, tracks depolymerizing microtubules by harnessing force generated during microtubule disassembly.

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Manipulation of Biological Cells Using a Robot-Aided Optical Tweezers System

Mingyang Xie, Adnan Shakoor and Changcheng Wu

This article reviews the autonomous manipulation strategies of biological cells utilizing optical tweezers, mainly including optical direct and indirect manipulation strategies. The typical and latest achievements in the optical manipulation of cells are presented, and the existing challenges for autonomous optical manipulation of biological cells are also introduced. Moreover, the integrations of optical tweezers with other manipulation tools are presented, which broadens the applications of optical tweezers in the biomedical manipulation areas and will also foster new developments in cell-based physiology and pathology studies, such as cell migration, single cell surgery, and preimplantation genetic diagnosis (PGD).

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Colloidal transport through trap arrays controlled by active microswimmers

Wen Yang, Vyacheslav R Misko, Fabio Marchesoni and Franco Nori

We investigate the dynamics of a binary mixture consisting of active and passive colloidal particles diffusing in a 2D array of truncated harmonic wells, or traps. We explore the possibility of using a small fraction of active particles to manipulate a much larger fraction of passive particles, for instance, to confine them in or extract them from the traps. The results of our study have potential application in biology and medical sciences, for example, to remove dead cells or undesired contaminants from biological systems by means of self-propelled nano-robots.

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Coherent Optomechanical Switch for Motion Transduction Based on Dynamically Localized Mechanical Modes

Hao Fu, Zhi-cheng Gong, Li-ping Yang, Tian-hua Mao, Chang-pu Sun, Su Yi, Yong Li, and Geng-yu Cao

We present a coherent switch for motion transduction based on dynamically localized mechanical modes in an optomechanical system consisting of two coupled cantilevers. By placing one of the cantilevers inside a harmonically oscillating optical trap, the effective coupling strength between the degenerate cantilevers can be tuned experimentally. In particular, when the coupling is turned off, we show that mechanical motion becomes tightly bounded to the isolated cantilevers rather than propagating away as a result of destructive Landau-Zener-Stückelberg–like interference. The effect of dynamical localization is adopted to implement a coherent switch, through which the tunneling oscillation is turned on and off with well-preserved phase coherence. We provide a simple yet efficient approach for full control of the coupling between mechanical resonators, which is highly desirable for coherent control of transport phenomena in a coupled-mechanical-resonator array.

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Motion Control and Optical Interrogation of a Levitating Single Nitrogen Vacancy in Vacuum

Gerard P. Conangla, Andreas W. Schell, Raúl A. Rica, and Romain Quidant

Levitation optomechanics exploits the unique mechanical properties of trapped nano-objects in vacuum to address some of the limitations of clamped nanomechanical resonators. In particular, its performance is foreseen to contribute to a better understanding of quantum decoherence at the mesoscopic scale as well as to lead to novel ultrasensitive sensing schemes. While most efforts have focused so far on the optical trapping of low-absorption silica particles, further opportunities arise from levitating objects with internal degrees of freedom, such as color centers. Nevertheless, inefficient heat dissipation at low pressures poses a challenge because most nano-objects, even with low-absorption materials, experience photodamage in an optical trap. Here, by using a Paul trap, we demonstrate levitation in vacuum and center-of-mass feedback cooling of a nanodiamond hosting a single nitrogen-vacancy center. The achieved level of motion control enables us to optically interrogate and characterize the emitter response. The developed platform is applicable to a wide range of other nano-objects and represents a promising step toward coupling internal and external degrees of freedom.

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Single-Cell Force Spectroscopy of Interaction of Lipopolysaccharides from Yersinia pseudotuberculosis and Yersinia pestis with J774 Macrophage Membrane Using Optical Tweezers

A. A. Byvalov, V. L. Kononenko

In order to investigate quantitatively the role of lipopolysaccharides (LPS) from outer bacterial membrane at the initial state of bacterium adhesion to a host cell membrane, a model system for single cell force spectroscopy was developed and used. The system comprised of an LPS-coated microsphere placed into optical trap and a J774 macrophage being approached the microsphere to initiate their binding and then moved back to rupture the bond. An “object shadow” phenomenon was discovered, manifested as large-scale variations of the signal of photodetector registering the trapped microsphere displacement, such variations emerging long before the actual interaction between the macrophage and microsphere. The theory and the measurements technique were developed for registration of the force required for detachment of bounded microsphere from the object investigated by means of optical tweezers under the “object shadow” conditions. Characteristic spectra of binding force between J774 macrophage and microspheres functionalized with various LPS, as well as LPS plus complementary antibodies preparations were obtained at the rate of detachment force application of 3–6 pN/s. Force spectrum characteristic of Yersinia pseudotuberculosis LPS possessing O-antigen had a maximum at ~14 pN with half-width of ~23 pN. The treatment of O-antigen with complementary antibodies resulted in transformation of this spectrum into a spectrum with maximum at ~10 pN and half-width of ~14 pN, being almost identical to the spectrum of Y. pestis LPS devoid of O-antigen, with a maximum at ~9 pN and half-width of ~13 pN. A possible mechanism of force spectra formation has been proposed under assumptions of nonspecific binding of O-antigen and probable receptor-type binding of LPS core region to the macrophage surface. The elastic modulus of macrophage envelope, as estimated using analysis of displacement of the contacting microsphere as an indenter, was ≈0.17 pN/nm.

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Propagation characteristics of radially shifted Gaussian beams and their radiation forces on Rayleigh particles

Yunfeng Jiang, Shuofeng Zhao, Wenlei Yu, Xiuwei Zhu, Xin Zhang

A new kind of strictly Gaussian-shaped dark hollow beam (DHB), called radially shifted Gaussian beam (RSGB), is proposed in this paper. Compared with previous approximately Gaussian-shaped DHB [i.e., Hollow Gaussian beam (HGB) and hollow sinh-Gaussian beam (HsGB)], it is much more convenient to design any special sized DHB by using RSGB. The propagation characteristics of RSGB, especially its autofocusing property, are investigated in detail and compared with that of HGB and HsGB under the same condition. It is found that the hollow region of RSGB can remain for a longer distance; and its autofocusing property is much stronger. An analytical expression for the focal position is also obtained for Gaussian-shaped DHB for the first time. Radiation forces of these beams exerted on two kinds of Rayleigh particle are analyzed. Numerical results show that RSGB also has advantages over previous Gaussian-shaped DHB in two-dimensional optical micromanipulation. For the low-index particle, RSGB can trap it for a longer distance; for the high-index particle, RSGB can produce a larger trapping force at the focal point.

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Plasmonic Manipulation of Targeted Metallic Particles by Polarization-Sensitive Metalens

Xianyou Wang, Yanmeng Dai, Yuquan Zhang, Changjun Min, and Xiaocong Yuan 

As a tool in the manipulation of micro- and nano-objects, optical tweezers are found in applications in many areas. However, selective trapping still poses challenges. Recently, a meta-surface technique offers an approach to improve optical trapping and manipulation capabilities. Here, we demonstrate the selective trapping of metallic nanoparticles with tailored plasmonic fields using a polarization sensitive metalens. We show, both by theory and experiments, modulated trapping and antitrapping forces when beam polarizations are tuned. Combining the effects of two orthogonal circular polarizations, single target particles were stably trapped in the center, while all other particles were repelled. This particle isolation points toward targeted manipulations that may find applications in single-particle assistant molecular Raman detection and assembly of plasmonic structures.

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Controlled Mechanical Motions of Microparticles in Optical Tweezers

Jing Liu and Zhiyuan Li

Optical tweezers, formed by a highly focused laser beam, have intriguing applications in biology and physics. Inspired by molecular rotors, numerous optical beams and artificial particles have been proposed to build optical tweezers trapping microparticles, and extensive experiences have been learned towards constructing precise, stable, flexible and controllable micromachines. The mechanism of interaction between particles and localized light fields is quite different for different types of particles, such as metal particles, dielectric particles and Janus particles. In this article, we present a comprehensive overview of the latest development on the fundamental and application of optical trapping. The emphasis is placed on controllable mechanical motions of particles, including rotation, translation and their mutual coupling under the optical forces and torques created by a wide variety of optical tweezers operating on different particles. Finally, we conclude by proposing promising directions for future research.

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The glycolipid GM1 reshapes asymmetric biomembranes and giant vesicles by curvature generation

Raktim Dasgupta, Markus S. Miettinen, Nico Fricke, Reinhard Lipowsky, and Rumiana Dimova

The ganglioside GM1 is present in neuronal membranes at elevated concentrations with an asymmetric spatial distribution. It is known to generate curvature and can be expected to strongly influence the neuron morphology. To elucidate these effects, we prepared giant vesicles with GM1 predominantly present in one leaflet of the membrane, mimicking the asymmetric GM1 distribution in neuronal membranes. Based on pulling inward and outward tubes, we developed a technique that allowed the direct measurement of the membrane spontaneous curvature. Using vesicle electroporation and fluorescence intensity analysis, we were able to quantify the GM1 asymmetry across the membrane and to subsequently estimate the local curvature generated by the molecule in the bilayer. Molecular-dynamics simulations confirm the experimentally determined dependence of the membrane spontaneous curvature as a function of GM1 asymmetry. GM1 plays a crucial role in connection with receptor proteins. Our results on curvature generation of GM1 point to an additional important role of this ganglioside, namely in shaping neuronal membranes.

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Optofluidic Single-Cell Genome Amplification of Sub-micron Bacteria in the Ocean Subsurface

Zachary C. Landry, Kevin Vergin, Christopher Mannenbach, Stephen Block, Qiao Yang, Paul Blainey, Craig Carlson and Stephen Giovannoni

Optofluidic single-cell genome amplification was used to obtain genome sequences from sub-micron cells collected from the euphotic and mesopelagic zones of the northwestern Sargasso Sea. Plankton cells were visually selected and manually sorted with an optical trap, yielding 20 partial genome sequences representing seven bacterial phyla. Two organisms, E01-9C-26 (Gammaproteobacteria), represented by four single cell genomes, and Opi.OSU.00C, an uncharacterized Verrucomicrobia, were the first of their types retrieved by single cell genome sequencing and were studied in detail. Metagenomic data showed that E01-9C-26 is found throughout the dark ocean, while Opi.OSU.00C was observed to bloom transiently in the nutrient-depleted euphotic zone of the late spring and early summer. The E01-9C-26 genomes had an estimated size of 4.76–5.05 Mbps, and contained “O” and “W”-type monooxygenase genes related to methane and ammonium monooxygenases that were previously reported from ocean metagenomes. Metabolic reconstruction indicated E01-9C-26 are likely versatile methylotrophs capable of scavenging C1 compounds, methylated compounds, reduced sulfur compounds, and a wide range of amines, including D-amino acids. The genome sequences identified E01-9C-26 as a source of “O” and “W”-type monooxygenase genes related to methane and ammonium monooxygenases that were previously reported from ocean metagenomes, but are of unknown function. In contrast, Opi.OSU.00C genomes encode genes for catabolizing carbohydrate compounds normally associated with eukaryotic phytoplankton. This exploration of optofluidics showed that it was effective for retrieving diverse single-cell bacterioplankton genomes and has potential advantages in microbiology applications that require working with small sample volumes or targeting cells by their morphology.

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Optical tweezing using tunable optical lattices along a few-mode silicon waveguide

C. Pin, J.-B. Jager, M. Tardif, E. Picard, E. Hadji, F. de Fornel and B. Cluzel

Fourteen years ago, optical lattices and holographic tweezers were considered as a revolution, allowing for trapping and manipulating multiple particles at the same time using laser light. Since then, near-field optical forces have aroused tremendous interest as they enable efficient trapping of a wide range of objects, from living cells to atoms, in integrated devices. Yet, handling at will multiple objects using a guided light beam remains a challenging task for current on-chip optical trapping techniques. We demonstrate here on-chip optical trapping of dielectric microbeads and bacteria using one-dimensional optical lattices created by near-field mode beating along a few-mode silicon nanophotonic waveguide. This approach allows not only for trapping large numbers of particles in periodic trap arrays with various geometries, but also for manipulating them via diverse transport and repositioning techniques. Near-field mode-beating optical lattices may be readily implemented in lab-on-a-chip devices, addressing numerous scientific fields ranging from bio-analysis to nanoparticle processing.

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Microfluidic Cultivation and Laser Tweezers Raman Spectroscopy of E. coli under Antibiotic Stress

Zdeněk Pilát, Silvie Bernatová, Jan Ježek, Johanna Kirchhoff, Astrid Tannert, Ute Neugebauer, Ota Samek and Pavel Zemánek

Analyzing the cells in various body fluids can greatly deepen the understanding of the mechanisms governing the cellular physiology. Due to the variability of physiological and metabolic states, it is important to be able to perform such studies on individual cells. Therefore, we developed an optofluidic system in which we precisely manipulated and monitored individual cells of Escherichia coli. We tested optical micromanipulation in a microfluidic chamber chip by transferring individual bacteria into the chambers. We then subjected the cells in the chambers to antibiotic cefotaxime and we observed the changes by using time-lapse microscopy. Separately, we used laser tweezers Raman spectroscopy (LTRS) in a different micro-chamber chip to manipulate and analyze individual cefotaxime-treated E. coli cells. Additionally, we performed conventional Raman micro-spectroscopic measurements of E. coli cells in a micro-chamber. We found observable changes in the cellular morphology (cell elongation) and in Raman spectra, which were consistent with other recently published observations. The principal component analysis (PCA) of Raman data distinguished between the cefotaxime treated cells and control. We tested the capabilities of the optofluidic system and found it to be a reliable and versatile solution for this class of microbiological experiments.

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Direct observation of the external force mediated conformational dynamics of an IHF bound Holliday junction

Subhas C. Bera, Tapas Paul, A. N. Sekar Iyengar and Padmaja P. Mishra

We have investigated the isomerization dynamics and plausible energy landscape of 4-way Holliday junctions (4WHJs) bound to integration host factor (IHF, a DNA binding protein), considering the effect of applied external force, by single-molecule FRET methods. A slowing down of the forward as well as the backward rates of the isomerization process of the protein bound 4WHJ has been observed under the influence of an external force, which indicates an imposed restriction on the conformational switching. This has also been reflected by an increase in rigidity, as observed from the increase in the single-molecule FRET (smFRET)-anisotropy values (0.270 ± 0.012 to 0.360 ± 0.008). The application of an external force has assisted the conformational transitions to share the unstacked open structure intermediate, with different rate-limiting steps and a huge induced variation in the energy landscape. Furthermore, the associated landscape of the 4WHJ is visualized in terms of rarely interconverting states embedded into the two isoforms by using nonlinear dynamics analysis, which shows that the chaoticity of the system increases at intermediate force (0.4 to 1.6 pN). The identification of chaos in our investigation provides useful information for a comprehensive explanation of the origin of the complex behavior of the system, which effectively helps us to perceive the dynamics of IHF bound 4WHJs under the influence of external force, and also demonstrates the applicability of nonlinear dynamics analysis in the field of biology.

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A folding nucleus and minimal ATP binding domain of Hsp70 identified by single-molecule force spectroscopy

Daniela Bauer, Sarah Meinhold, Roman P. Jakob, Johannes Stigler, Ulrich Merkel, Timm Maier, Matthias Rief, and Gabriel Žoldák

The folding pathways of large proteins are complex, with many of them requiring the aid of chaperones and others folding spontaneously. Along the folding pathways, partially folded intermediates are frequently populated; their role in the driving of the folding process is unclear. The structures of these intermediates are generally not amenable to high-resolution structural techniques because of their transient nature. Here we employed single-molecule force measurements to scrutinize the hierarchy of intermediate structures along the folding pathway of the nucleotide binding domain (NBD) of Escherichia coli Hsp70 DnaK. DnaK-NBD is a member of the sugar kinase superfamily that includes Hsp70s and the cytoskeletal protein actin. Using optical tweezers, a stable nucleotide-binding competent en route folding intermediate comprising lobe II residues (183–383) was identified as a critical checkpoint for productive folding. We obtained a structural snapshot of this folding intermediate that shows native-like conformation. To assess the fundamental role of folded lobe II for efficient folding, we turned our attention to yeast mitochondrial NBD, which does not fold without a dedicated chaperone. After replacing the yeast lobe II residues with stable E. coli lobe II, the obtained chimeric protein showed native-like ATPase activity and robust folding into the native state, even in the absence of chaperone. In summary, lobe II is a stable nucleotide-binding competent folding nucleus that is the key to time-efficient folding and possibly resembles a common ancestor domain. Our findings provide a conceptual framework for the folding pathways of other members of this protein superfamily.

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Selectively transporting small chiral particles with circularly polarized Airy beams

Wanli Lu, Huajin Chen, Sandong Guo, Shiyang Liu, and Zhifang Lin

Based on the full wave simulation, we demonstrate that a circularly polarized vector Airy beam can selectively transport small chiral particles along a curved trajectory via the chirality-tailored optical forces. The transverse optical forces can draw the chiral particles with different particle chirality towards or away from the intensity maxima of the beam, leading to the selective trapping in the transverse plane. The transversely trapped chiral particles are then accelerated along a curved trajectory of the Airy beam by the chirality-tailored longitudinal scattering force, rendering an alternative way to sort and/or transport chiral particles with specified helicity. Finally, the underlying physics of the chirality induced transverse trap and de-trap phenomena are examined by the analytical theory within the dipole approximation.

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3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam

Yu Zhang, Xiaoyun Tang, Yaxun Zhang, Wenjie Su, Zhihai Liu, Xinghua Yang, Jianzhong Zhang, Jun Yang, Kyunghwan Oh, and Libo Yuan

We proposed and experimentally demonstrated 3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam. The Bessel beam was produced by concatenating single-mode fiber and a step index multimode fiber, which was then focused by a high refractive index glass microsphere integrated on the fiber end facet. The focused Bessel beam provided two dark fields along the axial direction, where stable trapping of low refractive index bio-cells was realized in a high refractive index liquid bath. The all-fiber and seamlessly integrated structure of the proposed scheme can find ample potential as a micro-optical probe in in situ characterization and manipulation of multiple bio-cells with refractive indices lower than that of the liquid bath.

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Nanofiber quantum photonics

Kali P Nayak, Mark Sadgrove, Ramachandrarao Yalla, Fam Le Kien and Kohzo Hakuta

Recent advances in the coherent control of single quanta of light, photons, is a topic of prime interest, and is discussed under the banner of quantum photonics. In the last decade, the subwavelength diameter waist of a tapered optical fiber, referred to as an optical nanofiber, has opened promising new avenues in the field of quantum optics, paving the way toward a versatile platform for quantum photonics applications. The key feature of the technique is that the optical field can be tightly confined in the transverse direction while propagating over long distances as a guided mode and enabling strong interaction with the surrounding medium in the evanescent region. This feature has led to surprising possibilities to manipulate single atoms and fiber-guided photons, e.g. the efficient channeling of emission from single atoms and solid-state quantum emitters into the fiber-guided modes, high optical depth with a few atoms around the nanofiber, trapping atoms around a nanofiber, and atomic memories for fiber-guided photons. Furthermore, implementing a moderate longitudinal confinement in nanofiber cavities has enabled the strong coupling regime of cavity quantum electrodynamics to be reached, and the long-range dipole–dipole interaction between quantum emitters mediated by the nanofiber offers a platform for quantum nonlinear optics with an ensemble of atoms. In addition, the presence of a longitudinal component of the guided field has led to unique capabilities for chiral light–matter interactions on nanofibers. In this article, we review the key developments of the nanofiber technology toward a vision for quantum photonics on an all-fiber interface.

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Engineering thermoresponsive phase separated vesicles formed via emulsion phase transfer as a content-release platform

Kaiser Karamdad, James W. Hindley, Guido Bolognesi, Mark S. Friddin, Robert V. Law, Nicholas J. Brooks, Oscar Ces and Yuval Elani

Giant unilamellar vesicles (GUVs) are a well-established tool for the study of membrane biophysics and are increasingly used as artificial cell models and functional units in biotechnology. This trend is driven by the development of emulsion-based generation methods such as Emulsion Phase Transfer (EPT), which facilitates the encapsulation of almost any water-soluble compounds (including biomolecules) regardless of size or charge, is compatible with droplet microfluidics, and allows GUVs with asymmetric bilayers to be assembled. However, the ability to control the composition of membranes formed via EPT remains an open question; this is key as composition gives rise to an array of biophysical phenomena which can be used to add functionality to membranes. Here, we evaluate the use of GUVs constructed via this method as a platform for phase behaviour studies and take advantage of composition-dependent features to engineer thermally-responsive GUVs. For the first time, we generate ternary GUVs (DOPC/DPPC/cholesterol) using EPT, and by compensating for the lower cholesterol incorporation efficiencies, show that these possess the full range of phase behaviour displayed by electroformed GUVs. As a demonstration of the fine control afforded by this approach, we demonstrate release of dye and peptide cargo when ternary GUVs are heated through the immiscibility transition temperature, and show that release temperature can be tuned by changing vesicle composition. We show that GUVs can be individually addressed and release triggered using a laser beam. Our findings validate EPT as a suitable method for generating phase separated vesicles and provide a valuable proof-of-concept for engineering content release functionality into individually addressable vesicles, which could have a host of applications in the development of smart synthetic biosystems.

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A technique to calibrate spatial light modulator for varying phase response over its spatial regions

Deepak K. Gupta, B. V. R. Tata, and T. R. Ravindran
Holographic Optical Tweezers (HOTs) employ the technique of beam shaping and holography in an optical manipulation system to create a multitude of focal spots for simultaneous trapping and manipulation of sub-microscopic particles. The beam shaping is accomplished by the use of a phase only liquid crystal spatial light modulator (SLM). The efficiency and the uniformity in the generated traps greatly depend on the phase response behavior of SLMs. In addition the SLMs are found to show different phase response over its different spatial regions, due to non-flat structure of SLMs. Also the phase responses are found to vary over different spatial regions due to non-uniform illumination (Gaussian profile of incident laser). There are various techniques to calibrate for the varying phase response by characterizing the phase modulation at various sub-sections. We present a simple and fast technique to calibrate the SLM suffering with spatially varying phase response. We divide the SLM into many sub-sections and optimize the brightness and gamma of each sub-section for maximum diffraction efficiency. This correction is incorporated in the Weighted Gerchberg Saxton (WGS) algorithm for generation of holograms.

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Response to “Comment on ‘Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers’”

Yang Zhao and Jennifer Dionne

Comment on “Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers”
ACS Photonics

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Comment on “Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers”

Martin Schäferling and Harald Giessen

Response to “Comment on ‘Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers’”
ACS Photonics

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The Role of Glycans in Bacterial Adhesion to Mucosal Surfaces: How Can Single-Molecule Techniques Advance Our Understanding?

Cécile Formosa-Dague, Mickaël Castelain, Hélène Martin-Yken, Karen Dunker, Etienne Dague and Marit Sletmoen

Bacterial adhesion is currently the subject of increased interest from the research community, leading to fast progress in our understanding of this complex phenomenon. Resent research within this field has documented the important roles played by glycans for bacterial surface adhesion, either through interaction with lectins or with other glycans. In parallel with this increased interest for and understanding of bacterial adhesion, there has been a growth in the sophistication and use of sensitive force probes for single-molecule and single cell studies. In this review, we highlight how the sensitive force probes atomic force microscopy (AFM) and optical tweezers (OT) have contributed to clarifying the mechanisms underlying bacterial adhesion to glycosylated surfaces in general and mucosal surfaces in particular. We also describe research areas where these techniques have not yet been applied, but where their capabilities appear appropriate to advance our understanding.

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Analysis of Egg White Protein Composition with Double Nanohole Optical Tweezers

Noa Hacohen, Candice J. X. Ip, and Reuven Gordon

We use a double nanohole optical tweezer to analyze the protein composition of egg white through analysis of many individual protein trapping events. The proteins are grouped by mass based on two metrics: standard deviation of the trapping laser intensity fluctuations from the protein diffusion and the time constant of these fluctuations coming from the autocorrelation. Quantitative analysis is demonstrated for artificial samples, and then, the approach is applied to real egg white. The composition found from real egg white corresponds well to past reports using gel electrophoresis. This approach differs from past works by allowing for individual protein analysis in heterogeneous solutions without the need for denaturing, labeling, or tethering.

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Micro-Dumbbells—A Versatile Tool for Optical Tweezers

Weronika Lamperska, Sławomir Drobczyński, Michał Nawrot, Piotr Wasylczyk and Jan Masajada

Manipulation of micro- and nano-sized objects with optical tweezers is a well-established, albeit still evolving technique. While many objects can be trapped directly with focused laser beam(s), for some applications indirect manipulation with tweezers-operated tools is preferred. We introduce a simple, versatile micro-tool operated with holographic optical tweezers. The 40 µm long dumbbell-shaped tool, fabricated with two-photon laser 3D photolithography has two beads for efficient optical trapping and a probing spike on one end. We demonstrate fluids viscosity measurements and vibration detection as examples of possible applications.

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Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture Using 10 nm Gap

Daehan Yoo, Kargal L. Gurunatha, Han-Kyu Choi, Daniel A. Mohr, Christopher T. Ertsgaard, Reuven Gordon, and Sang-Hyun Oh

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.

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