Thursday, November 30, 2017

Sizing of single evaporating droplet with Near-Forward Elastic Scattering Spectroscopy

M. Woźniak, D. Jakubczyk, G. Derkachov, J. Archer

We have developed an optical setup and related numerical models to study evolution of single evaporating micro-droplets by analysis of their spectral properties. Our approach combines the advantages of the electrodynamic trapping with the broadband spectral analysis with the supercontinuum laser illumination. The elastically scattered light within the spectral range of 500–900  nm is observed by a spectrometer placed at the near-forward scattering angles between 4.3 ° and 16.2 ° and compared with the numerically generated lookup table of the broadband Mie scattering. Our solution has been successfully applied to infer the size evolution of the evaporating droplets of pure liquids (diethylene and ethylene glycol) and suspensions of nanoparticles (silica and gold nanoparticles in diethylene glycol), with maximal accuracy of  ± 25 nm. The obtained results have been compared with the previously developed sizing techniques: (i) based on the analysis of the Mie scattering images - the Mie Scattering Lookup Table Method and (ii) the droplet weighting. Our approach provides possibility to handle levitating objects with much larger size range (radius from 0.5  µm to 30  µm) than with the use of optical tweezers (typically radius below 8  µm) and analyse them with much wider spectral range than with commonly used LED sources.


Optical trapping for biosensing: materials and applications

P. Rodríguez-Sevilla, L. Labrador-Páez, D. Jaque and P. Haro-González

Since the 70's, when Arthur Ashkin and coworkers demonstrated that optical forces could displace and levitate microsized particles, optical trapping has seen a steady stream of developments and applications, particularly in the biological field. Since that demonstration, optical trapping has been especially exploited as a powerful tool for non-invasive sensitive measurements. The recent development of synthesis routes has further expanded the possibilities of optical trapping in the area of biosensing where new multifunctional particles are used as a single probe. The synergy between the development of new materials and experimental techniques has led to the appearance of numerous studies in which novel biosensing applications are demonstrated. The design of new materials and optical systems to face new challenges makes it necessary to have a clear idea about the latest developments achieved in the field. In this work, we summarize recent experimental advances in biosensing achieved by optical manipulation of micro- and nanoparticles providing a critical review on the state of the art and future prospects.


Recent advances in the use of microfluidic technologies for single cell analysis

Travis W. Murphy, Qiang Zhang, Lynette B. Naler, Sai Ma and Chang Lu

The inherent heterogeneity in cell populations has become of great interest and importance as analytical techniques have improved over the past decades. With the advent of personalized medicine, understanding the impact of this heterogeneity has become an important challenge for the research community. Many different microfluidic approaches with varying levels of throughput and resolution exist to study single cell activity. In this review, we take a broad view of the recent microfluidic developments in single cell analysis based on microwell, microchamber, and droplet platforms. We cover physical, chemical, and molecular biology approaches for cellular and molecular analysis including newly emerging genome-wide analysis.


Nanoscale imaging and force probing of biomolecular systems using atomic force microscopy: from single molecules to living cells

Mi Li, Dan Dang, Ning Xi, Yuechao Wanga and Lianqing Liu

Due to the lack of adequate tools for observation, native molecular behaviors at the nanoscale have been poorly understood. The advent of atomic force microscopy (AFM) provides an exciting instrument for investigating physiological processes on individual living cells with molecular resolution, which attracts the attention of worldwide researchers. In the past few decades, AFM has been widely utilized to investigate molecular activities on diverse biological interfaces, and the performances and functions of AFM have also been continuously improved, greatly improving our understanding of the behaviors of single molecules in action and demonstrating the important role of AFM in addressing biological issues with unprecedented spatiotemporal resolution. In this article, we review the related techniques and recent progress about applying AFM to characterize biomolecular systems in situ from single molecules to living cells. The challenges and future directions are also discussed.


Characterization of optomechanical modes in multilayer stack of graphene sheets

Mohammad Mahdi Salary, Sandeep Inampudi and Hossein Mosallaei

Graphene, a two-dimensional (2D) crystalline material exhibits unique electronic, optical, and mechanical properties which makes it a promising candidate for optomechanical and optoelectronic devices. The giant plasmonic activity of graphene sheets enables low-dimensional confinement of light and enhanced light–matter interaction leading to significant enhancement of optical forces which may give rise to large mechanical deformations on account of ultralow mass density and flexibility of graphene. The multilayer stack and heterostructures of 2D materials provide access to a spectrum of guided modes which can be used to tailor the optical forces and mechanical states of graphene sheets. Here, we study the optical forces arising from the coupling of guided modes in layered structures of graphene sheets. We obtain the mechanical deformation states corresponding to each guided mode and demonstrate that the optical forces can be adjusted by changing the interlayer spacing as well as the chemical potential of graphene layers. Our results can be used for various designs of graphene-based optomechanical devices.


Quantitative optical trapping and optical manipulation of micro-sized objects

Rania Sayed

An optical tweezers technique is used for ultraprecise micromanipulation to measure positions of micrometer scale objects with a precision down to the nanometer scale. It consists of a high performance research microscope with motorized scanning stage and sensitive position detection system. Up to 10 traps can be used quasi-simultaneously. Non photodamage optical trapping of Escherichia coli (E. coli) bacteria cells of 2 µm in length, as an example of motile bacteria, has been shown in this paper. Also, efficient optical trapping and rotation of polystyrene latex particles of 3 µm in diameter have been studied, as an optical handle for the pick and place of other tiny objects. A fast galvoscanner is used to produce multiple optical traps for manipulation of micro-sized objects and optical forces of these trapped objects quantified and measured according to explanation of ray optics regime. The diameter of trapped particle is bigger than the wavelength of the trapping laser light. The force constant (k) has been determined in real time from the positional time series recorded from the trapped object that is monitored by a CCD camera through a personal computer.


Monday, November 27, 2017

Doxorubicin hinders DNA condensation promoted by the protein bovine serum albumin (BSA)

C. H. M. Lima, H. M. C. de Paula, L. H. M. da Silva, M. S. Rocha

In this work, we have studied the interaction between the anticancer drug doxorubicin (doxo) and condensed DNA, using optical tweezers. To perform this task, we use the protein bovine serum albumin (BSA) in the working buffer to mimic two key conditions present in the real intracellular environment: the condensed state of the DNA and the abundant presence of charged macromolecules in the surrounding medium. In particular, we have found that, when doxo is previously intercalated in disperse DNA, the drug hinders the DNA condensation process upon the addition of BSA in the buffer. On the other hand, when bare DNA is firstly condensed by BSA, doxo is capable to intercalate and to unfold the DNA condensates at relatively high concentrations. In addition, a specific interaction between BSA and doxo was verified, which significantly changes the chemical equilibrium of the DNA–doxo interaction. Finally, the presence of BSA in the buffer stabilizes the double-helix structure of the DNA–doxo complexes, preventing partial DNA denaturation induced by the stretching forces.


Advances in optoplasmonic sensors – combining optical nano/microcavities and photonic crystals with plasmonic nanostructures and nanoparticles

Jolly Xavier, Serge Vincent, Fabian Meder, Frank Vollmer

Nanophotonic device building blocks, such as optical nano/microcavities and plasmonic nanostructures, lie at the forefront of sensing and spectrometry of trace biological and chemical substances. A new class of nanophotonic architecture has emerged by combining optically resonant dielectric nano/microcavities with plasmonically resonant metal nanostructures to enable detection at the nanoscale with extraordinary sensitivity. Initial demonstrations include single-molecule detection and even single-ion sensing. The coupled photonic-plasmonic resonator system promises a leap forward in the nanoscale analysis of physical, chemical, and biological entities. These optoplasmonic sensor structures could be the centrepiece of miniaturised analytical laboratories, on a chip, with detection capabilities that are beyond the current state of the art. In this paper, we review this burgeoning field of optoplasmonic biosensors. We first focus on the state of the art in nanoplasmonic sensor structures, high quality factor optical microcavities, and photonic crystals separately before proceeding to an outline of the most recent advances in hybrid sensor systems. We discuss the physics of this modality in brief and each of its underlying parts, then the prospects as well as challenges when integrating dielectric nano/microcavities with metal nanostructures. In Section 5, we hint to possible future applications of optoplasmonic sensing platforms which offer many degrees of freedom towards biomedical diagnostics at the level of single molecules.


On the validity of integral localized approximation for on-axis zeroth-order Mathieu beams

A. Chafiq, L. A. Ambrosio, G. Gouesbet, A. Belafhal

In a recent paper on radiation pressure forces exerted on a homogenous spherical particle by zeroth-order Mathieu beams (zMBs), the integral localized approximation(ILA) was used to calculate the beam shape coefficients (BSCs) encoding the shape of the beams. Unfortunately, this method is valid only for beams with a propagating factor exp ( ± ikz). In the case of non-diffracting beams the propagation factor is exp (± ikcos α z) which involves an extra-cosine term, with α being the axicon angle. Due to this term it has been demonstrated that localized approximations, including ILA, provide a satisfactory description of the intended beam only if the axicon angle is small enough. Zeroth-order Mathieu beams pertain to this type of beams. The present paper is therefore devoted to a comparison between BSCs calculated with an exact procedure and those calculated using ILA, in order to determine a range of validity of the approximate procedure. As a side result, we also establish exact closed-form expressions to the evaluation of BSCs of zMBs.


Nonlinear Actin Deformations Lead to Network Stiffening, Yielding, and Nonuniform Stress Propagation

Bekele Gurmessa, Shea Ricketts, Rae M. Robertson-Anderson

We use optical tweezers microrheology and fluorescence microscopy to apply nonlinear microscale strains to entangled and cross-linked actin networks, and measure the resulting stress and actin filament deformations. We couple nonlinear stress response and relaxation to the velocities and displacements of individual fluorescent-labeled actin segments, at varying times throughout the strain and varying distances from the strain path, to determine the underlying molecular dynamics that give rise to the debated nonlinear response and stress propagation of cross-linked and entangled actin networks at the microscale. We show that initial stress stiffening arises from acceleration of strained filaments due to molecular extension along the strain, while softening and yielding is coupled to filament deceleration, halting, and recoil. We also demonstrate a surprising nonmonotonic dependence of filament deformation on cross-linker concentration. Namely, networks with no cross-links or substantial cross-links both exhibit fast initial filament velocities and reduced molecular recoil while intermediate cross-linker concentrations display reduced velocities and increased recoil. We show that these collective results are due to a balance of network elasticity and force-induced cross-linker unbinding and rebinding. We further show that cross-links dominate entanglement dynamics when the length between cross-linkers becomes smaller than the length between entanglements. In accord with recent simulations, we demonstrate that post-strain stress can be long-lived in cross-linked networks by distributing stress to a small fraction of highly strained connected filaments that span the network and sustain the load, thereby allowing the rest of the network to recoil and relax.

Fano-enhanced pulling and pushing optical force on active plasmonic nanoparticles

Dongliang Gao, Ran Shi, Yang Huang, and Lei Gao

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures at plasmon singularity and Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. When the size of the nanostructure is beyond the quasistatic limit, one can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of an optical pump compensates the dissipative loss from the metal shell, thus enabling strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces originate from the reversal of the electric field at Fano resonance, which leads to pulling forces on bound currents and charges. Meanwhile, the separation of the Lorentz force helps to reveal the nature of the pulling forces in the gain system. We have shown that by applying the Lorentz force density formula, it is possible to obtain the correct value of the force inside our complex inhomogeneous structure made up of dispersive and lossy metamaterial irrespective of the electromagnetic momentum density. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.


Thursday, November 23, 2017

Objective-lens-free Fiber-based Position Detection with Nanometer Resolution in a Fiber Optical Trapping System

Chaoyang Ti, Minh-Tri Ho-Thanh, Qi Wen & Yuxiang Liu

Position detection with high accuracy is crucial for force calibration of optical trapping systems. Most existing position detection methods require high-numerical-aperture objective lenses, which are bulky, expensive, and difficult to miniaturize. Here, we report an affordable objective-lens-free, fiber-based position detection scheme with 2 nm spatial resolution and 150 MHz bandwidth. This fiber based detection mechanism enables simultaneous trapping and force measurements in a compact fiber optical tweezers system. In addition, we achieved more reliable signal acquisition with less distortion compared with objective based position detection methods, thanks to the light guiding in optical fibers and small distance between the fiber tips and trapped particle. As a demonstration of the fiber based detection, we used the fiber optical tweezers to apply a force on a cell membrane and simultaneously measure the cellular response.


Two Disease-Causing SNAP-25B Mutations Selectively Impair SNARE C-terminal Assembly

Aleksander A. Rebane, Bigeng Wang, Lu Ma, Hong Qu, Jeff Coleman, Shyam Krishnakumar, James E. Rothman, Yongli Zhang

Synaptic exocytosis relies on assembly of three soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins into a parallel four-helix bundle to drive membrane fusion. SNARE assembly occurs by stepwise zippering of the vesicle-associated SNARE (v-SNARE) onto a binary SNARE complex on the target plasma membrane (t-SNARE). Zippering begins with slow N-terminal association followed by rapid C-terminal zippering, which serves as a power stroke to drive membrane fusion. SNARE mutations have been associated with numerous diseases, especially neurological disorders. It remains unclear how these mutations affect SNARE zippering, partly due to difficulties to quantify the energetics and kinetics of SNARE assembly. Here, we used single-molecule optical tweezers to measure the assembly energy and kinetics of SNARE complexes containing single mutations I67T/N in neuronal SNARE synaptosomal-associated protein of 25 kDa (SNAP-25B), which disrupt neurotransmitter release and have been implicated in neurological disorders. We found that both mutations significantly reduced the energy of C-terminal zippering by ~ 10 kBT, but did not affect N-terminal assembly. In addition, we observed that both mutations lead to unfolding of the C-terminal region in the t-SNARE complex. Our findings suggest that both SNAP-25B mutations impair synaptic exocytosis by destabilizing SNARE assembly, rather than stabilizing SNARE assembly as previously proposed. Therefore, our measurements provide insights into the molecular mechanism of the disease caused by SNARE mutations.


Contrast and imaging performance in photo induced force microscopy

Mohammad Almajhadi and H. Kumar Wickramasinghe

We numerically analyze PiFM’s lateral and vertical (subsurface) imaging performance in the visible and IR regimes. The lateral spatial resolution and subsurface imaging capabilities are limited by the field spatial confinement near the tip apex, which is directly proportional to the excitation wavelength. In addition, we show that near-field optical force exerted on the tip due to sample molecular resonance is indeed in the detectable range. Moreover, driving sample on (off) resonance reveals high (low) contrast. The strength of the optical forces is assessed for metal (gold), polymers (Polystyrene and Polymethylmethacrylate), and solid (SiC). By increasing tip-coating thickness from 5 nm to 35 nm, the gap-field enhancement decreases to about 40%. In IR, force spectrum over an absorption band is predominantly following the real part of the polarizability, as predicted by dipole-dipole approximation.


Changes in hemoglobin–oxygen affinity with shape variations of red blood cells

Aniket Chowdhury; Raktim Dasgupta; Shovan K. Majumder

Shape variations of red blood cells (RBCs) are known to occur upon exposure to various drugs or under diseased conditions. The commonly observed discocytic RBCs can be transformed to echinocytic or stomatocytic shape under such conditions. Raman spectra of the three major shape variations, namely discocyte, echinocyte, and stomatocyte, of RBCs were studied while subjecting the cells to oxygenated and deoxygenated conditions. Analysis of the recorded spectra suggests an increased level of hemoglobin (Hb)–oxygen affinity for the echinocytes. Also, some level of Hb degradation could be noticed for the deoxygenated echinocytes. The effects may arise from a reduced level of intracellular adenosine triphosphate in echinocytic cells and an increased fraction of submembrane Hb.


Torque Induced on Lipid Microtubules with Optical Tweezers

T Na wichean, S Charrunchon, A Pattanaporkratana, J Limtrakul and N Chattham

Chiral Phospholipids are found self-assembled into cylindrical tubules of 500 nm in diameter by helical winding of bilayer stripes under cooling in ethanol and water solution. Theoretical prediction and experimental evidence reported so far confirmed the modulated tilt direction in a helical striped pattern of the tubules. This molecular orientation morphology results in optically birefringent tubules. We investigate an individual lipid microtubule under a single optical trap of 532 nm linearly polarized laser. Spontaneous rotation of a lipid tubule induced by radiation torque was observed with only one sense of rotation caused by chirality of a lipid tubule. Rotation discontinued once the high refractive index axis of a lipid tubule aligned with a polarization axis of the laser. We further explored a lipid tubule under circularly polarized optical trap. It was found that a lipid tubule was continuously rotated confirming the tubule birefringent property. We modified the shape of optical trap by cylindrical lens obtaining an elliptical profile optical trap. A lipid tubule can be aligned along the elongated length of optical trap. We reported an investigation of competition between polarized light torque on a birefringent lipid tubule versus torque from intensity gradient of an elongated optical trap.


Femtosecond Laser Trapping Dynamics of Nanoparticles: A Single Transient Assembly Formation Leading to Their Directional Ejection

Wei-Yi Chiang, Anwar Usman, Teruki Sugiyama, Johan Hofkens, and Hiroshi Masuhara

We investigated femtosecond laser trapping dynamics of silica nanoparticles with different hydrophobic surface properties. We demonstrated that the hydrophobic surface on the silica nanoparticles facilitates mutual association of the nanoparticles in the optical trapping site. Such association of optically trapped nanoparticles is a prerequisite to induce their directional ejection away from the trapping site. The directional ejection of the optically trapped nanoparticles is most probably due to asymmetric three-dimensional ejecting forces generated by the electromagnetic interaction between transient assembly in the focal spot and the incident pulses. These findings provide important insights into the directional ejection of nanoparticles from the trapping site in the femtosecond laser trapping, and this physicochemical phenomenon is controlled by both the trapping laser and material properties.


Wednesday, November 22, 2017

Effects of Infrared Optical Trapping on Saccharomyces cerevisiae in a Microfluidic System

Zdeněk Pilát, Alexandr Jonáš, Jan Ježek and Pavel Zemánek

Baker’s yeast (Saccharomyces cerevisiae) represents a very popular single-celled eukaryotic model organism which has been studied extensively by various methods and whose genome has been completely sequenced. It was also among the first living organisms that were manipulated by optical tweezers and it is currently a frequent subject of optical micromanipulation experiments. We built a microfluidic system for optical trapping experiments with individual cells and used it for the assessment of cell tolerance to phototoxic stress. Using optical tweezers with the wavelength of 1064 nm, we trapped individual Saccharomyces cerevisiae cells for 15 min and, subsequently, observed their stress response in specially designed microfluidic chambers over time periods of several hours by time-lapse video-microscopy. We determined the time between successive bud formations after the exposure to the trapping light, took account of damaged cells, and calculated the population doubling period and cell areas for increasing trapping power at a constant trapping time. Our approach represents an attractive, versatile microfluidic platform for quantitative optical trapping experiments with living cells. We demonstrate its application potential by assessing the limits for safe, non-invasive optical trapping of Saccharomyces cerevisiae with infrared laser light.


Interactive optical trapping shows that confinement is a determinant of growth in a mixed yeast culture

Nils Arneborg Henrik Siegumfeldt Grith H. Andersen Peter Nissen Vincent R. Daria Peter John Rodrigo Jesper Glückstad
Applying a newly developed user-interactive optical trapping system, we controllably surrounded individual cells of one yeast species, Hanseniaspora uvarum, with viable cells of another yeast species, Saccharomyces cerevisiae, thus creating a confinement of the former. Growth of surrounded and non-surrounded H. uvarum cells was followed under a microscope by determining their generation time. The average generation time of surrounded H. uvarum cells was 15% higher than that of non-surrounded cells, thereby showing that the confinement imposed by viable S. cerevisiae cells on H. uvarum inhibits growth of the latter. This study is the first to demonstrate that confinement is a determinant of growth in a microbial ecosystem.


Radiative Pulling Forces, Exerted by Evanescent Fields Along a Hyperbolic Metamaterial Slab

Igor Nefedov, Alexander S. Shalin

We show and investigate in detail the optical pulling forces acting on a particle in the vicinity of a waveguiding structure made of a hyperbolic metamaterial. We have shown analytically that if the waveguide dispersion is negative, the longitudinal radiative force outside of hyperbolic metamaterial is always a pulling force, that is directed toward the source of electromagnetic energy flow in contrast to the common situation when a strong propulsion force emerges near the surface of an open waveguide.


Guanidinium-based “molecular glues” for modulation of biomolecular functions

Rina Mogaki, P. K. Hashim, Kou Okuro and Takuzo Aida

Molecular adhesion based on multivalent interactions plays essential roles in various biological processes. Hence, “molecular glues” that can adhere to biomolecules may modulate biomolecular functions and therefore can be applied to therapeutics. This tutorial review describes design strategies for developing adhesive motifs for biomolecules based on multivalent interactions. We highlight a guanidinium ion-based salt-bridge as a key interaction for adhesion to biomolecules and discuss the application of molecular glues for manipulation of biomolecular assemblies, drug delivery systems, and modulation of biomolecular functions.


Rheological Study of Soft Matters: A Review of Microrheology and Microrheometers

Wei Liu, Chi Wu

Rheological properties of soft matter like polymer solutions/gels, colloidal dispersions, and biological materials have been extensively studied by macroscopic methods. Recently, a set of microrheometers has emerged as powerful tools to investigate the dynamics and structures of homogeneous or heterogeneous soft matter at the micro- or nanoscale. In this review, these microrheometers, including some novel hybrid microrheometers are summarized and compared.


Fluid photonic crystal from colloidal quantum dots

V. N. Mantsevich and S. A. Tarasenko

We study optical forces acting upon semiconductor quantum dots and the force-driven motion of the dots in a colloid. In the spectral range of exciton transitions in quantum dots, when the photon energy is close to the exciton energy, the polarizability of the dots is drastically increased. It leads to a resonant increase of both the gradient and the scattering contributions to the optical force, which enables the efficient manipulation with the dots. We reveal that the optical grating of the colloid leads to the formation of a fluid photonic crystal with spatially periodic circulating fluxes and density of the dots. Pronounced resonant dielectric response of semiconductor quantum dots enables a separation of the quantum dots with different exciton frequencies.


Tuesday, November 21, 2017

Spin-Momentum Locking in the Near Field of Metal Nanoparticles

Claudia Triolo, Adriano Cacciola, Salvatore Patanè, Rosalba Saija, Salvatore Savasta, and Franco Nori

Light carries both spin and momentum. Spin–orbit interactions of light come into play at the subwavelength scale of nano-optics and nanophotonics, where they determine the behavior of light. These phenomena, in which the spin affects and controls the spatial degrees of freedom of light, are attracting rapidly growing interest. Here we present results on the spin-momentum locking in the near field of metal nanostructures supporting localized surface resonances. These systems can confine light to very small dimensions below the diffraction limit, leading to a striking near-field enhancement. In contrast to the propagating evanescent waves of surface plasmon-polariton modes, the electromagnetic near-field of localized surface resonances does not exhibit a definite position-independent momentum or polarization. Close to the particle, the canonical momentum is almost tangential to the particle surface and rotates when moving along the surface. The direction of this rotation can be controlled by the spin of the incident light.


Laser refrigeration, alignment and rotation of levitated Yb3+:YLF nanocrystals

A. T. M. Anishur Rahman & P. F. Barker

The ability to cool and manipulate levitated nanoparticles in vacuum is a promising tool for exploring macroscopic quantum mechanics1,2, precision measurements of forces3 and non-equilibrium thermodynamics4,5. The extreme isolation afforded by optical levitation offers a low-noise, undamped environment that has been used to measure zeptonewton forces3 and radiation pressure shot noise6, and to demonstrate centre-of-mass motion cooling7,8. Ground-state cooling and the creation of macroscopic quantum superpositions are now within reach, but control of both the centre of mass and internal temperature is required. While cooling the centre-of-mass motion to micro-kelvin temperatures has now been achieved, the internal temperature has remained at or above room temperature. Here, we realize a nanocryostat by refrigerating levitated Yb3+:YLF nanocrystals to 130 K using anti-Stokes fluorescence cooling, while simultaneously using the optical trapping field to align the crystal to maximize cooling.


Reverse orbiting and spinning of a Rayleigh dielectric spheroid in a J0 Bessel optical beam

F. G. Mitri

Based on the electric dipole approximation, numerical computations for the optical orbital and spin radiation torques induced by a zeroth-order Bessel beam on a lossless dielectric subwavelength spheroid with arbitrary orientation in space are performed. The transverse optical force components are determined first, and then used to compute the longitudinal orbital torque component. Moreover, the Cartesian components of the spin radiation torque vector are evaluated. Numerical calculations illustrate the analysis with particular emphasis on the beam parameters, polarization of the magnetic vector potential forming the beam, and aspect ratio of the spheroid. The results demonstrate that the lossless dielectric subwavelength spheroid becomes irresponsive to the transfer of angular momentum, where the longitudinal orbital and spin radiation torque components vanish along singularity lines. Moreover, depending on the beam parameters and the spheroid location in space, the longitudinal orbital and spin torque components reverse sign, indicating a direction of rotation around the central axis of the beam and center of mass of the spheroid, respectively, in either the counterclockwise or clockwise directions. The results are important in particle dynamics and trapping applications, optical tweezers and spanners, and other related fields.


Optical manipulation from the microscale to the nanoscale: fundamentals, advances and prospects

Dongliang Gao, Weiqiang Ding, Manuel Nieto-Vesperinas, Xumin Ding, Mahdy Rahman, Tianhang Zhang, ChweeTeck Lim & Cheng-Wei Qiu

Since the invention of optical tweezers, optical manipulation has advanced significantly in scientific areas such as atomic physics, optics and biological science. Especially in the past decade, numerous optical beams and nanoscale devices have been proposed to mechanically act on nanoparticles in increasingly precise, stable and flexible ways. Both the linear and angular momenta of light can be exploited to produce optical tractor beams, tweezers and optical torque from the microscale to the nanoscale. Research on optical forces helps to reveal the nature of light–matter interactions and to resolve the fundamental aspects, which require an appropriate description of momenta and the forces on objects in matter. In this review, starting from basic theories and computational approaches, we highlight the latest optical trapping configurations and their applications in bioscience, as well as recent advances down to the nanoscale. Finally, we discuss the future prospects of nanomanipulation, which has considerable potential applications in a variety of scientific fields and everyday life.


Insights into Microalga and Bacteria Interactions of Selected Phycosphere Biofilms Using Metagenomic, Transcriptomic, and Proteomic Approaches

Ines Krohn-Molt, Malik Alawi, Konrad U. Förstner, Alena Wiegandt, Lia Burkhardt, Daniela Indenbirken, Melanie Thieß, Adam Grundhoff, Julia Kehr, Andreas Tholey and Wolfgang R. Streit

Microalga are of high relevance for the global carbon cycling and it is well-known that they are associated with a microbiota. However, it remains unclear, if the associated microbiota, often found in phycosphere biofilms, is specific for the microalga strains and which role individual bacterial taxa play. Here we provide experimental evidence that Chlorella saccharophila, Scenedesmus quadricauda, and Micrasterias crux-melitensis, maintained in strain collections, are associated with unique and specific microbial populations. Deep metagenome sequencing, binning approaches, secretome analyses in combination with RNA-Seq data implied fundamental differences in the gene expression profiles of the microbiota associated with the different microalga. Our metatranscriptome analyses indicates that the transcriptionally most active bacteria with respect to key genes commonly involved in plant–microbe interactions in the Chlorella (Trebouxiophyceae) and Scenedesmus (Chlorophyceae) strains belong to the phylum of the α-Proteobacteria. In contrast, in the Micrasterias (Zygnematophyceae) phycosphere biofilm bacteria affiliated with the phylum of the Bacteroidetes showed the highest gene expression rates. We furthermore show that effector molecules known from plant–microbe interactions as inducers for the innate immunity are already of relevance at this evolutionary early plant-microbiome level.


Optical Manipulation of nanoparticles by simultaneous electric and magnetic field enhancement within diabolo nanoantenna

Nyha Hameed, Ali Nouho Ali & Fadi I. Baida

In this paper, we propose and numerically simulate a novel optical trapping process based on the enhancement and the confinement of both magnetic and electric near-fields by using gold Diabolo Antenna (DA). The later was recently proposed to generate huge magnetic near-field when illuminated by linearly polarized wave along its axis. Numerical 3D – FDTD simulation results demonstrate the high confinement of the electromagnetic field in the vicinity of the DA. This enhancement is then exploited for the trapping of nano-particles (NP) as small as 30 nm radius. Results show that the trapping process greatly depends on the particle dimensions and that three different regimes of, trapping at contact, trapping without contact, or pushing can be achieved within the same DA. This doubly resonant structure opens the way to the design of a novel generation of efficient optical nano-tweezers that allow manipulation of nano-particles by simply changing the operation wavelength.


Monday, November 20, 2017

Strong Binding of Platelet Integrin αIIbβ3 to Fibrin Clots: Potential Target to Destabilize Thrombi

Peter Höök, Rustem I. Litvinov, Oleg V. Kim, Shixin Xu, Zhiliang Xu, Joel S. Bennett, Mark S. Alber & John W. Weisel
The formation of platelet thrombi is determined by the integrin αIIbβ3-mediated interactions of platelets with fibrinogen and fibrin. Blood clotting in vivo is catalyzed by thrombin, which simultaneously induces fibrinogen binding to αIIbβ3 and converts fibrinogen to fibrin. Thus, after a short time, thrombus formation is governed by αIIbβ3 binding to fibrin fibers. Surprisingly, there is little understanding of αIIbβ3 interaction with fibrin polymers. Here we used an optical trap-based system to measure the binding of single αIIbβ3 molecules to polymeric fibrin and compare it to αIIbβ3 binding to monomeric fibrin and fibrinogen. Like αIIbβ3 binding to fibrinogen and monomeric fibrin, we found that αIIbβ3 binding to polymeric fibrin can be segregated into two binding regimes, one with weaker rupture forces of 30–60 pN and a second with stronger rupture forces >60 pN that peaked at 70–80 pN. However, we found that the mechanical stability of the bimolecular αIIbβ3-ligand complexes had the following order: fibrin polymer > fibrin monomer > fibrinogen. These quantitative differences reflect the distinct specificity and underlying molecular mechanisms of αIIbβ3-mediated reactions, implying that targeting platelet interactions with fibrin could increase the therapeutic indices of antithrombotic agents by focusing on the destabilization of thrombi rather than the prevention of platelet aggregation.


Sharply Focused Azimuthally Polarized Beams with Magnetic Dominance: Near-Field Characterization at Nanoscale by Photoinduced Force Microscopy

Jinwei Zeng , Fei Huang, Caner Guclu, Mehdi Veysi, Mohammad Albooyeh, H. Kumar Wickramasinghe, and Filippo Capolino

Azimuthally polarized beams are gaining fundamental importance for near-field force microscopy systems to inspect photoinduced magnetism in special molecules or nanostructures, due to their strong axial magnetic field and vanishing electric field. The magnetic dominant region represents a unique trait of such a beam as a potentially ideal structured light to probe photoinduced magnetism at the nanoscale. Therefore, we present a near-field characterization of an optical, sharply focused azimuthally polarized beam using photoinduced force microscopy, a technique with simultaneous near-field excitation and detection, achieving nanoscale resolution well beyond the diffraction limit. Such a method exploits the photoinduced gradient force on a nanotip, mechanically detected as forced oscillations of the cantilever in an atomic force microscopy system upon external light illumination. The photoinduced force is strongly localized, which that depends only on the near-field signal free from background scattering photons, granting photoinduced force microscopy a superior performance over its precedent near-field scanning optical microscopy. We develop an analytical model to correct the tip-induced measurement anisotropy, suppress the background noise, and reveal the local electric field distribution of the azimuthally polarized beam. These measurements are used to retrieve its strong longitudinal axial magnetic field at the center of the polarization vortex where the electric field vanishes. This study can lead to a plethora of possibilities in optomechanical, chemical, or biomedical applications. We also propose and discuss how to use such beams with polarization azimuthal symmetry as a way to calibrate microscope nanotips.


Programming the mechanics of cohesive fiber networks by compression

Bart E. Vos, Luka C. Liebrand, Mahsa Vahabi, Andreas Biebricher, Gijs J. L. Wuite, Erwin J. G. Peterman, Nicholas A. Kurniawan, Fred C. MacKintosh and Gijsje H. Koenderink

Fibrous networks are ideal functional materials since they provide mechanical rigidity at low weight. Here, we demonstrate that fibrous networks of the blood clotting protein fibrin undergo a strong and irreversible increase in their mechanical rigidity in response to uniaxial compression. This rigidification can be precisely controlled by the level of applied compressive strain, providing a means to program the network rigidity without having to change its composition. To identify the underlying mechanism we measure single fiber–fiber interactions using optical tweezers. We further develop a minimal computational model of cohesive fiber networks that shows that stiffening arises due to the formation of new bonds in the compressed state, which develop tensile stress when the network is re-expanded. The model predicts that the network stiffness after a compression cycle obeys a power-law dependence on tensile stress, which we confirm experimentally. This finding provides new insights into how biological tissues can adapt themselves independently of any cellular processes, offering new perspectives to inspire the design of reprogrammable materials.


Optical trapping of nanoparticles with tunable inter-distance using a multimode slot cavity

Lin Wang, Yongyin Cao, Tongtong Zhu, Rei Feng, Fangkui Sun, and Weiqiang Ding

Optical trapping of nano-objects (i.e., the nano-tweezers) has been investigated intensively. Most of those nano-tweezers, however, were focused on the trapping of a single nanoparticle, while the interactions between them were seldom considered. In this work, we propose a nano-tweezers in a slot photonic crystal cavity supporting multiple modes, where the relative positions of two trapped nanoparticles can be tuned by selective excitation of different resonant mode. Results show that both the nanoparticles are trapped at the center of the cavity when the first order mode is excited. When the incident source is tuned to the second order mode, however, these two nanoparticles push each other and are trapped stably at two separated positions. Also, the inter-distance between them can be tuned precisely by changing the relative power of the two modes. This provides a potential method to control the interactions between two nano-objects via optically tuning the separation between them, and may have applications in various related disciplinary.


The temporal evolution process from fluorescence bleaching to clean Raman spectra of single solid particles optically trapped in air

Zhiyong Gong, Yong-Le Pan, Gorden Videen, Chuji Wang

We observe the entire temporal evolution process of fluorescence and Raman spectra of single solid particles optically trapped in air. The spectra initially contain strong fluorescence with weak Raman peaks, then the fluorescence was bleached within seconds, and finally only the clean Raman peaks remain. We construct an optical trap using two counter-propagating hollow beams, which is able to stably trap both absorbing and non-absorbing particles in air, for observing such temporal processes. This technique offers a new method to study dynamic changes in the fluorescence and Raman spectra from a single optically trapped particle in air.


Engineering Cell Surface Function with DNA Origami

Ehsan Akbari, Molly Y. Mollica, Christopher R. Lucas, Sarah M. Bushman, Randy A. Patton, Melika Shahhosseini, Jonathan W. Song, Carlos E. Castro

A specific and reversible method is reported to engineer cell-membrane function by embedding DNA-origami nanodevices onto the cell surface. Robust membrane functionalization across epithelial, mesenchymal, and nonadherent immune cells is achieved with DNA nanoplatforms that enable functions including the construction of higher-order DNA assemblies at the cell surface and programed cell–cell adhesion between homotypic and heterotypic cells via sequence-specific DNA hybridization. It is anticipated that integration of DNA-origami nanodevices can transform the cell membrane into an engineered material that can mimic, manipulate, and measure biophysical and biochemical function within the plasma membrane of living cells.


Wednesday, November 15, 2017

Influence of DNA Lesions on Polymerase-Mediated DNA Replication at Single-Molecule Resolution

Hailey L. Gahlon, Louis J. Romano, and David Rueda

Faithful replication of DNA is a critical aspect in maintaining genome integrity. DNA polymerases are responsible for replicating DNA, and high-fidelity polymerases do this rapidly and at low error rates. Upon exposure to exogenous or endogenous substances, DNA can become damaged and this can alter the speed and fidelity of a DNA polymerase. In this instance, DNA polymerases are confronted with an obstacle that can result in genomic instability during replication, for example, by nucleotide misinsertion or replication fork collapse. It is important to know how DNA polymerases respond to damaged DNA substrates to understand the mechanism of mutagenesis and chemical carcinogenesis. Single-molecule techniques have helped to improve our current understanding of DNA polymerase-mediated DNA replication, as they enable the dissection of mechanistic details that can otherwise be lost in ensemble-averaged experiments. These techniques have also been used to gain a deeper understanding of how single DNA polymerases behave at the site of the damage in a DNA substrate. In this review, we evaluate single-molecule studies that have examined the interaction between DNA polymerases and damaged sites on a DNA template.


A review on optical actuators for microfluidic systems

Tie Yang, Yue Chen and Paolo Minzioni

During the last few decades microfluidic systems have become more and more popular and their relevance in different fields is continually growing. In fact, the use of microchannels allows a significant reduction of the required sample-volume and opens the way to a completely new set of possible investigations, including the study of the properties of cells, the development of new cells' separation techniques and the analysis of single-cell proteins. One of the main differences between microscopic and macroscopic systems is obviously dictated by the need for suitable actuation mechanisms, which should allow precise control of microscopic fluid volumes and of micro-samples inside the fluid. Even if both syringe-pump and pneumatic-pump technologies significantly evolved and they currently enable sub-μL samples control, completely new approaches were recently developed for the manipulation of samples inside the microchannel. This review is dedicated to describing different kinds of optical actuators that can be applied in microfluidic systems for sample manipulation as well as for pumping. The basic principles underlying the optical actuation mechanisms will be described first, and then several experimental demonstrations will be reviewed and compared.


Stochastic Optical Trapping and Manipulation of Micro Object with Neural-Network Adaptation

Xiang Li ; Chien Chern Cheah

Optical tweezers are capable of manipulating micro/nano objects without any physical contact, and therefore widely used in biomedical engineering and biological science. While much progress has been achieved in automated optical manipulation of micro objects, Brownian motion is commonly ignored in the stability analysis in order to simplify the control problem. However, random Brownian perturbations exist in micromanipulation problem and therefore may result in failure of optical trapping due to the escape of micro object from the trap. In addition, it is usually assumed in the development of controller that the model of trapping stiffness is known, but the model is difficult to obtain because of its spatially varying feature around the centre of laser beam and variations with laser power and dimensions of objects. In this paper, a neural-network control method is proposed for optical trapping and manipulation of micro object, in the presence of stochastic perturbations and unknown trapping stiffness. The unknown trapping stiffness and dynamic parameters of micro objects, which vary with different laser power settings and sizes of the objects, are approximated by using adaptive neural networks. The stability analysis is carried out from stochastic perspectives, by considering the effect of Brownian motion in the dynamic model. Both experimental results and simulation results are presented.


Radiation forces of beams generated by Gaussian mirror resonator on a Rayleigh dielectric sphere

Bin Tang, Kai Chen, Lirong Bian, Xin Zhou, Li Huang & Yi Jin

Optical trapping and manipulating of micron-sized particles have attracted enormous interests due to the potential applications in biotechnology and nanoscience. In this work, we investigate numerically and theoretically the radiation forces acting on a Rayleigh dielectric particle produced by beams generated by Gaussian mirror resonator (GMR) in the Rayleigh scattering regime. The results show that the focused beams generated by GMR can be used to trap and manipulate the particles with both high and low index of refractive near the focus point. The influences of optical parameters of the beams generated by GMR on the radiation forces are analyzed in detail. Furthermore, the conditions for trapping stability are also discussed in this paper.


Ferdinando Borghese (26 May 1940–19 January 2017)

M.A. Iatì, R. Saija, O.M. Maragò, P. Denti

Here we summarize the life and scientific legacy of Ferdinando Borghese (1940–2017). He has been a pioneer in the theory and modeling of light scattering by nonspherical particles and clusters in the framework of the transition matrix approach. His work has found applications in many research fields ranging from interstellar dust to aerosol science, plasmonics, and optical trapping.


Dimerization regulates both deaminase-dependent and deaminase-independent HIV-1 restriction by APOBEC3G

Michael Morse, Ran Huo, Yuqing Feng, Ioulia Rouzina, Linda Chelico & Mark C. Williams

APOBEC3G (A3G) is a human enzyme that inhibits human immunodeficiency virus type 1 (HIV-1) infectivity, in the absence of the viral infectivity factor Vif, through deoxycytidine deamination and a deamination-independent mechanism. A3G converts from a fast to a slow binding state through oligomerization, which suggests that large A3G oligomers could block HIV-1 reverse transcriptase-mediated DNA synthesis, thereby inhibiting HIV-1 replication. However, it is unclear how the small number of A3G molecules found in the virus could form large oligomers. Here we measure the single-stranded DNA binding and oligomerization kinetics of wild-type and oligomerization-deficient A3G, and find that A3G first transiently binds DNA as a monomer. Subsequently, A3G forms N-terminal domain-mediated dimers, whose dissociation from DNA is reduced and their deaminase activity inhibited. Overall, our results suggest that the A3G molecules packaged in the virion first deaminate viral DNA as monomers before dimerizing to form multiple enzymatically deficient roadblocks that may inhibit reverse transcription.

Wednesday, November 8, 2017

Emulsified and Liquid–Liquid Phase-Separated States of α-Pinene Secondary Organic Aerosol Determined Using Aerosol Optical Tweezers

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

We demonstrate the first capture and analysis of secondary organic aerosol (SOA) on a droplet suspended in an aerosol optical tweezers (AOT). We examine three initial chemical systems of aqueous NaCl, aqueous glycerol, and squalane at ∼75% relative humidity. For each system we added α-pinene SOA—generated directly in the AOT chamber—to the trapped droplet. The resulting morphology was always observed to be a core of the original droplet phase surrounded by a shell of the added SOA. We also observed a stable emulsion of SOA particles when added to an aqueous NaCl core phase, in addition to the shell of SOA. The persistence of the emulsified SOA particles suspended in the aqueous core suggests that this metastable state may persist for a significant fraction of the aerosol lifecycle for mixed SOA/aqueous particle systems. We conclude that the α-pinene SOA shell creates no major diffusion limitations for water, glycerol, and squalane core phases under humid conditions. These experimental results support the current prompt-partitioning framework used to describe organic aerosol in most atmospheric chemical transport models and highlight the prominence of core–shell morphologies for SOA on a range of core chemical phases.


Elliptical orbits of microspheres in an evanescent field

Lulu Liu, Simon Kheifets, Vincent Ginis, Andrea Di Donato, and Federico Capasso

We examine the motion of periodically driven and optically tweezed microspheres in fluid and find a rich variety of dynamic regimes. We demonstrate, in experiment and in theory, that mean particle motion in 2D is rarely parallel to the direction of the applied force and can even exhibit elliptical orbits with nonzero orbital angular momentum. The behavior is unique in that it depends neither on the nature of the microparticles nor that of the excitation; rather, angular momentum is introduced by the particle’s interaction with the anisotropic fluid and optical trap environment. Overall, we find this motion to be highly tunable and predictable.


Mechanically switching single-molecule fluorescence of GFP by unfolding and refolding

Ziad Ganim and Matthias Rief

Green fluorescent protein (GFP) variants are widely used as genetically encoded fluorescent fusion tags, and there is an increasing interest in engineering their structure to develop in vivo optical sensors, such as for optogenetics and force transduction. Ensemble experiments have shown that the fluorescence of GFP is quenched upon denaturation. Here we study the dependence of fluorescence on protein structure by driving single molecules of GFP into different conformational states with optical tweezers and simultaneously probing the chromophore with fluorescence. Our results show that fluorescence is lost during the earliest events in unfolding, 3.5 ms before secondary structure is disrupted. No fluorescence is observed from the unfolding intermediates or the ensemble of compact and extended states populated during refolding. We further demonstrate that GFP can be mechanically switched between emissive and dark states. These data definitively establish that complete structural integrity is necessary to observe single-molecule fluorescence of GFP.


Optical tweezing and binding at high irradiation powers on black-Si

Tatsuya Shoji, Ayaka Mototsuji, Armandas Balčytis, Denver Linklater, Saulius Juodkazis & Yasuyuki Tsuboi

Nowadays, optical tweezers have undergone explosive developments in accordance with a great progress of lasers. In the last decade, a breakthrough brought optical tweezers into the nano-world, overcoming the diffraction limit. This is called plasmonic optical tweezers (POT). POT are powerful tools used to manipulate nanomaterials. However, POT has several practical issues that need to be overcome. First, it is rather difficult to fabricate plasmonic nanogap structures regularly and rapidly at low cost. Second, in many cases, POT suffers from thermal effects (Marangoni convection and thermophoresis). Here, we propose an alternative approach using a nano-structured material that can enhance the optical force and be applied to optical tweezers. This material is metal-free black silicon (MFBS), the plasma etched nano-textured Si. We demonstrate that MFBS-based optical tweezers can efficiently manipulate small particles by trapping and binding. The advantages of MFBS-based optical tweezers are: (1) simple fabrication with high uniformity over wafer-sized areas, (2) free from thermal effects detrimental for trapping, (3) switchable trapping between one and two - dimensions, (4) tight trapping because of no detrimental thermal forces. This is the NON-PLASMONIC optical tweezers.


Improved generation of periodic optical trap arrays using noniterative algorithm

Anita Dalal; Aniket Chowdhury; Raktim Dasgupta; Shovan Kumar Majumder

In a holographic optical tweezers setup, although the use of noniterative algorithms can result in the fast generation of multiple traps array, the performance of these algorithms is often inferior compared to iterative types of algorithms. Particularly in the case of symmetric trap arrays, the performance of noniterative algorithms is very poor. Suitability of the use of a noniterative superposition algorithm for generating symmetric trap arrays has been investigated after introducing small position disorders for the individual traps. It could be seen that the introduction of small disorders in the positions of the individual traps can significantly improve the quality of the generated trap array pattern over the case when an ideal symmetric pattern is targeted.


Tuesday, November 7, 2017

Manufacturing with light - micro-assembly of opto-electronic microstructures

Shuailong Zhang, Yongpeng Liu, Yang Qian, Weizhen Li, Joan Juvert, Pengfei Tian, Jean-Claude Navarro, Alasdair W Clark, Erdan Gu, Martin D. Dawson, Jonathan M. Cooper, and Steven L. Neale

Optical micromanipulation allows the movement and patterning of discrete micro-particles within a liquid environment. However, for manufacturing applications it is desirable to remove the liquid, leaving the patterned particles in place. In this work, we have demonstrated the use of optoelectronic tweezers (OET) to manipulate and accurately assemble Sn62Pb36Ag2 solder microspheres into tailored patterns. A technique based on freeze-drying technology was then developed that allows the assembled patterns to be well preserved and fixed in place after the liquid medium in the OET device is removed. After removing the liquid from the OET device and subsequently heating the assembled pattern and melting the solder microspheres, electrical connections between the microspheres were formed, creating a permanent conductive bridge between two isolated metal electrodes. Although this method is demonstrated with 40 µm diameter solder beads arranged with OET, it could be applied to a great range of discrete components from nanowires to optoelectronic devices, thus overcoming one of the basic hurdles in using optical micromanipulation techniques in a manufacturing micro-assembly setting.


KiloHertz Bandwidth, Dual-Stage Haptic Device Lets You Touch Brownian Motion

Tianming Lu; Cécile Pacoret; David Hériban; Abdenbi Mohand-Ousaid; Stéphane Régnier; Vincent Hayward

This paper describes a haptic interface that has a uniform response over the entire human tactile frequency range. Structural mechanics makes it very difficult to implement articulated mechanical systems that can transmit high frequency signals. Here, we separated the frequency range into two frequency bands. The lower band is within the first structural mode of the corresponding haptic device while the higher one can be transmitted accurately by a fast actuator operating from conservation of momentum, that is, without reaction forces to the ground. To couple the two systems, we adopted a channel separation approach akin to that employed in the design of acoustic reproduction systems. The two channels are recombined at the tip of the device to give a uniform frequency response from DC to one kHz. In terms of mechanical design, the high-frequency transducer was embedded inside the tip of the main stage so that during operation, the human operator has only to interact with a single finger interface. In order to exemplify the type of application that would benefit from this kind of interface, we applied it to the haptic exploration with microscopic scales objects which are known to behave with very fast dynamics. The novel haptic interface was bilaterally coupled with a micromanipulation platform to demonstrate its capabilities. Operators could feel interaction forces arising from contact as well as those resulting from Brownian motion and could manoeuvre a micro bead in the absence of vision.


Mesoscopic model for DNA G-quadruplex unfolding

A. E. Bergues-Pupo, I. Gutiérrez, J. R. Arias-Gonzalez, F. Falo & A. Fiasconaro

Genomes contain rare guanine-rich sequences capable of assembling into four-stranded helical structures, termed G-quadruplexes, with potential roles in gene regulation and chromosome stability. Their mechanical unfolding has only been reported to date by all-atom simulations, which cannot dissect the major physical interactions responsible for their cohesion. Here, we propose a mesoscopic model to describe both the mechanical and thermal stability of DNA G-quadruplexes, where each nucleotide of the structure, as well as each central cation located at the inner channel, is mapped onto a single bead. In this framework we are able to simulate loading rates similar to the experimental ones, which are not reachable in simulations with atomistic resolution. In this regard, we present single-molecule force-induced unfolding experiments by a high-resolution optical tweezers on a DNA telomeric sequence capable of adopting a G-quadruplex conformation. Fitting the parameters of the model to the experiments we find a correct prediction of the rupture-force kinetics and a good agreement with previous near equilibrium measurements. Since G-quadruplex unfolding dynamics is halfway in complexity between secondary nucleic acids and tertiary protein structures, our model entails a nanoscale paradigm for non-equilibrium processes in the cell.


Optical trapping of otoliths drives vestibular behaviours in larval zebrafish

Itia A. Favre-Bulle, Alexander B. Stilgoe, Halina Rubinsztein-Dunlop & Ethan K. Scott

The vestibular system, which detects gravity and motion, is crucial to survival, but the neural circuits processing vestibular information remain incompletely characterised. In part, this is because the movement needed to stimulate the vestibular system hampers traditional neuroscientific methods. Optical trapping uses focussed light to apply forces to targeted objects, typically ranging from nanometres to a few microns across. In principle, optical trapping of the otoliths (ear stones) could produce fictive vestibular stimuli in a stationary animal. Here we use optical trapping in vivo to manipulate 55-micron otoliths in larval zebrafish. Medial and lateral forces on the otoliths result in complementary corrective tail movements, and lateral forces on either otolith are sufficient to cause a rolling correction in both eyes. This confirms that optical trapping is sufficiently powerful and precise to move large objects in vivo, and sets the stage for the functional mapping of the resulting vestibular processing.


How should the optical tweezers experiment be used to characterize the red blood cell membrane mechanics?

Julien Sigüenza, Simon Mendez, Franck Nicoud

Stretching red blood cells using optical tweezers is a way to characterize the mechanical properties of their membrane by measuring the size of the cell in the direction of the stretching (axial diameter) and perpendicularly (transverse diameter). Recently, such data have been used in numerous publications to validate solvers dedicated to the computation of red blood cell dynamics under flow. In the present study, different mechanical models are used to simulate the stretching of red blood cells by optical tweezers. Results first show that the mechanical moduli of the membranes have to be adjusted as a function of the model used. In addition, by assessing the area dilation of the cells, the axial and transverse diameters measured in optical tweezers experiments are found to be insufficient to discriminate between models relevant to red blood cells or not. At last, it is shown that other quantities such as the height or the profile of the cell should be preferred for validation purposes since they are more sensitive to the membrane model.


Negative force on free carriers in positive index nanoparticles

Mohammad Habibur Rahaman and Brandon A. Kemp

We theoretically demonstrate the reversal of optical forces on free charge carriers in positive refractive index nanostructures. Though optical momentum in positive refractive index materials is necessarily parallel to the local energy flow, reversal of optical momentum transfer can be accomplished by exploiting the geometry and size of subwavelength particles. Using the Mie scattering theory and separation of optical momentum transfers to the bound and free charges and currents, we have shown that metal nanoparticles can exhibit strong momentum transfer to free carriers opposite to the direction of incident electromagnetic waves. This can be explained for small particles in terms of a reversal of Poynting power inside the material resulting in a negative net force on free carriers in small particles. Two-dimensional simulations further illuminate this point by demonstrating the effect of incident wave polarization.


Friday, November 3, 2017

An Optical Tweezers Platform for Single Molecule Force Spectroscopy in Organic Solvents

Jacob W. Black, Maria Kamenetska, and Ziad Ganim

Observation at the single molecule level has been a revolutionary tool for molecular biophysics and materials science, but single molecule studies of solution-phase chemistry are less widespread. In this work we develop an experimental platform for solution-phase single molecule force spectroscopy in organic solvents. This optical-tweezer-based platform was designed for broad chemical applicability and utilizes optically trapped core–shell microspheres, synthetic polymer tethers, and click chemistry linkages formed in situ. We have observed stable optical trapping of the core–shell microspheres in ten different solvents, and single molecule link formation in four different solvents. These experiments demonstrate how to use optical tweezers for single molecule force application in the study of solution-phase chemistry.


Determination of size and refractive index of single gold nanoparticles using an optofluidic chip

Y. Z. Shi, S. Xiong, L. K. Chin, J. B. Zhang, W. Ser, J. H. Wu, T. N. Chen, Z. C. Yang, Y. L. Hao, and A. Q. Liu

We report a real-time method to determine the size, i.e. diameter, and refractive index of single gold nanoparticles using an optofluidic chip, which consists of a quasi-Bessel beam optical chromatography. The tightly focused (∼ 0.5 μm) quasi-Bessel beam with low divergence (NA ∼ 0.04) was used to trap sub-100 nm gold nanoparticles within a long trapping distance of 140 μm. In the experiment, 60 to 100 nm gold nanoparticles were separated efficiently with at least 18 μm. The diameter and refractive index (real and imaginary) of single gold nanoparticles were measured at high resolutions with respect to the trapping distance, i.e. 0.36 nm/μm, 0.003/μm and 0.0016/μm, respectively.


Effect of laser radiation power on laser trapping of light-absorbing microparticles in air

A. P. Porfirev, S. A. Fomchenkov

We investigate the effects of changing the power of a Gaussian laser beam on the motion of light-absorbing microparticles trapped in the beam region. Laser trapping of such particles was due to the action of so-called photophoretic forces. In addition, we demonstrate the possibility of controlled movement of trapped carbon nanoparticle agglomerations, both in the direction of propagation of the laser beam and in the opposite direction.


Combinatorial Particle Patterning

Clemens von Bojnicic-Kninski, Roman Popov, Edgar Dörsam, Felix F. Loeffler, Frank Breitling, Alexander Nesterov-Muelle

The unique properties of solid particles make them a promising element of micro- and nanostructure technologies. Solid particles can be used as building blocks for micro and nanostructures, carriers of monomers, or catalysts. The possibility of patterning different kinds of particles on the same substrate opens the pathway for novel combinatorial designs and novel technologies. One of the examples of such technologies is the synthesis of peptide arrays with amino acid particles. This review examines the known methods of combinatorial particle patterning via static electrical and magnetic fields, laser radiation, patterning by synthesis, and particle patterning via chemically modified or microstructured surfaces.


Rotation and Negative Torque in Electrodynamically Bound Nanoparticle Dimers

Nishant Sule, Yuval Yifat, Stephen K. Gray, and Norbert F. Scherer

We examine the formation and concomitant rotation of electrodynamically bound dimers (EBD) of 150 nm diameter Ag nanoparticles trapped in circularly polarized focused Gaussian beams. The rotation frequency of an EBD increases linearly with the incident beam power, reaching mean values of ∼4 kHz for relatively low incident powers of 14 mW. Using a coupled-dipole/effective polarizability model, we reveal that retardation of the scattered fields and electrodynamic interactions can lead to a “negative torque” causing rotation of the EBD in the direction opposite to that of the circular polarization. This intriguing opposite-handed rotation due to negative torque is clearly demonstrated using electrodynamics-Langevin dynamics simulations by changing particle separations and thus varying the retardation effects. Finally, negative torque is also demonstrated in experiments from statistical analysis of the EBD trajectories. These results demonstrate novel rotational dynamics of nanoparticles in optical matter using circular polarization and open a new avenue to control orientational dynamics through coupling to interparticle separation.

Thursday, November 2, 2017

Observation of radiation pressure induced deformation of high-reflective reflector

Yukun Yuan, Chunyang Gu, Yue Cao, Shiling Wang and Feng Zhou Fang

In this paper, radiation pressure induced deformation of series of thin aluminum reflector is analyzed theoretically and experimentally. Theory of quantum mechanics and material mechanics are applied in 2-D simulations and exhibits good coordination with experiment results. The original laser source used in experiment is Gauss-distributed and has been shaped and expanded, resulting in the flattened light to avoid over heating or even ablation of the irradiated reflector, which can also bring a major deformation. The aluminum reflectors are fabricated by a high precision machine tool into a thickness of 100μm, 200μm, 300μm with a surface roughness of 8 nm in Ra, and then coated with high-reflective(HR) coatings and mounted on a thick 3D printing base made of polylactic acid(PLA). In the experimental process, a vacuum chamber is employed to distinguish the effect of thermal convection. The results shows that radiation pressure induced deformation has an obvious negative correlation with the reflector thickness. The time-deformation curve of the reflector reaches 2.4 μm peak negative displacement at most the moment laser beam is acting when under vacuum circumstance, and soon raises up to over 12 μm positive displacement if the reflector is continuously irradiated. Subsequent analysis shows that such negative displacement is induced by radiation pressure and the positive displacement is caused by thermal expansion of the PLA base.


Feedback-tracking microrheology in living cells

Kenji Nishizawa, Marcel Bremerich, Heev Ayade, Christoph F. Schmidt, Takayuki Ariga and Daisuke Mizuno

Living cells are composed of active materials, in which forces are generated by the energy derived from metabolism. Forces and structures self-organize to shape the cell and drive its dynamic functions. Understanding the out-of-equilibrium mechanics is challenging because constituent materials, the cytoskeleton and the cytosol, are extraordinarily heterogeneous, and their physical properties are strongly affected by the internally generated forces. We have analyzed dynamics inside two types of eukaryotic cells, fibroblasts and epithelial-like HeLa cells, with simultaneous active and passive microrheology using laser interferometry and optical trapping technology. We developed a method to track microscopic probes stably in cells in the presence of vigorous cytoplasmic fluctuations, by using smooth three-dimensional (3D) feedback of a piezo-actuated sample stage. To interpret the data, we present a theory that adapts the fluctuation-dissipation theorem (FDT) to out-of-equilibrium systems that are subjected to positional feedback, which introduces an additional nonequilibrium effect. We discuss the interplay between material properties and nonthermal force fluctuations in the living cells that we quantify through the violations of the FDT. In adherent fibroblasts, we observed a well-known polymer network viscoelastic response where the complex shear modulus scales as G* ∝ (−iω)3/4. In the more 3D confluent epithelial cells, we found glassy mechanics with G* ∝ (−iω)1/2 that we attribute to glassy dynamics in the cytosol. The glassy state in living cells shows characteristics that appear distinct from classical glasses and unique to nonequilibrium materials that are activated by molecular motors.


Cell volume change through water efflux impacts cell stiffness and stem cell fate

Ming Guo, Adrian F. Pegoraro, Angelo Mao, Enhua H. Zhou, Praveen R. Arany, Yulong Han, Dylan T. Burnette, Mikkel H. Jensen, Karen E. Kasza, Jeffrey R. Moore, Frederick C. Mackintosh, Jeffrey J. Fredberg, David J. Mooney, Jennifer Lippincott-Schwartz, and David A. Weitz

Cells alter their mechanical properties in response to their local microenvironment; this plays a role in determining cell function and can even influence stem cell fate. Here, we identify a robust and unified relationship between cell stiffness and cell volume. As a cell spreads on a substrate, its volume decreases, while its stiffness concomitantly increases. We find that both cortical and cytoplasmic cell stiffness scale with volume for numerous perturbations, including varying substrate stiffness, cell spread area, and external osmotic pressure. The reduction of cell volume is a result of water efflux, which leads to a corresponding increase in intracellular molecular crowding. Furthermore, we find that changes in cell volume, and hence stiffness, alter stem-cell differentiation, regardless of the method by which these are induced. These observations reveal a surprising, previously unidentified relationship between cell stiffness and cell volume that strongly influences cell biology.


A semi-analytical model of a near-field optical trapping potential well

Mohammad Asif Zaman, Punnag Padhy, and Lambertus Hesselink

A semi-analytical model is proposed to describe the force generated by a near-field optical trap. The model contains fitting parameters that can be adjusted to resemble a reference force-field. The model parameters for a plasmonic near-field trap consisting of a C-shaped engraving are determined using least squares regression. The reference values required for the regression analysis are calculated using the Maxwell stress tensor method. The speed and accuracy of the proposed model are compared with the conventional method. The model is found to be significantly faster with an acceptable level of accuracy.


Poynting theorem in terms of beam shape coefficients and applications to axisymmetric, dark and non-dark, vortex and non-vortex, beams

Gérard Gouesbet

Electromagnetic arbitrary shaped beams may be described by using expansions over a set of basis functions, with expansion coefficients containing sub-coefficients called beam shape coefficients which encode the structure of the beam. In this paper, the Poynting theorem is expressed in terms of these beam shape coefficients. Special cases (axisymmetric, dark and non-dark beams) are thereafter considered, as well as specific applications to paradigmatic examples, from trivial cases (plane waves and spherical waves) to the more sophisticated case of vortex beams.


Wednesday, November 1, 2017

Modeling and calibrating nonlinearity and crosstalk in back focal plane interferometry for three-dimensional position detection

Peng Cheng, Sissy M. Jhiang, and Chia-Hsiang Menq

Back focal plane (BFP) interferometry is frequently used to detect the motion of a single laser trapped bead in a photonic force microscope (PFM) system. Whereas this method enables high-speed and high-resolution position measurement, its measurement range is limited by nonlinearity coupled with crosstalk in three-dimensional (3-D) measurement, and validation of its measurement accuracy is not trivial. This Letter presents an automated calibration system in conjunction with a 3-D quadratic model to render rapid and accurate calibration of the laser measurement system. An actively controlled three-axis laser steering system and a high-speed vision-based 3-D particle tracking system are integrated to the PFM system to enable rapid calibration. The 3-D quadratic model is utilized to correct for nonlinearity and crosstalk and, thus, extend the 3-D position detection volume of BFP interferometry. We experimentally demonstrated a 12-fold increase in detection volume when applying the method to track the motion of a 2.0 μm laser trapped polystyrene bead.


Tailoring optical pulling force on gain coated nanoparticles with nonlocal effective medium theory

X. Bian, D. L. Gao, and L. Gao

We study the optical scattering force on the coated nanoparticles with gain core and nonlocal plasmonic shell in the long-wavelength limit, and demonstrate negative optical force acting on the nanoparticles near the symmetric and/or antisymmetric surface plasmon resonances. To understand the optical force behavior, we propose nonlocal effective medium theory to derive the equivalent permittivity for the coated nanoparticles with nonlocality. We show that the imaginary part of the equivalent permittivity is negative near the surface resonant wavelength, resulting in the negative optical force. The introduction of nonlocality may shift the resonant wavelength of the optical force, and strengthen the negative optical force. Two examples of Fano-like resonant scattering in such coated nanoparticles are considered, and Fano resonance-induced negative optical force is found too. Our findings could have some potential applications in plasmonics, nano-optical manipulation, and optical selection.


Kinesin rotates unidirectionally and generates torque while walking on microtubules

Avin Ramaiya, Basudev Roy, Michael Bugiel, and Erik Schäffer

Cytoskeletal motors drive many essential cellular processes. For example, kinesin-1 transports cargo in a step-wise manner along microtubules. To resolve rotations during stepping, we used optical tweezers combined with an optical microprotractor and torsion balance using highly birefringent microspheres to directly and simultaneously measure the translocation, rotation, force, and torque generated by individual kinesin-1 motors. While, at low adenosine 5′-triphosphate (ATP) concentrations, motors did not generate torque, we found that motors translocating along microtubules at saturating ATP concentrations rotated unidirectionally, producing significant torque on the probes. Accounting for the rotational work makes kinesin a highly efficient machine. These results imply that the motor’s gait follows a rotary hand-over-hand mechanism. Our method is generally applicable to study rotational and linear motion of molecular machines, and our findings have implications for kinesin-driven cellular processes.


Optical Trap Assisted Nanopatterning: Process Parallelization and Dynamic Structure Generation

Johannes Strauss, Marcus Baum, Ilya Alexeev, Michael Schmidt

In this publication we present a novel setup for the Optical Trap Assisted Nanopatterning
(OTAN) technology. The setup allows process parallelization and thus higher throughput in this inventive and flexible direct-nanopatterning technology. We have determined the stiffness of the optical traps and compared the obtained result with the single beam OTAN parameters. Furthermore we estimate the increase in throughput for the parallelized approach in comparison to the conventional system.


Multiple Particles 3-D Trap Based on All-Fiber Bessel Optical Probe

Yaxun Zhang, Xiaoyun Tang, Yu Zhang, Zhihai Liu, Enming Zhao, Xinghua Yang, Jianzhong Zhang, Jun Yang, Libo Yuan

We propose and demonstrate an all-fiber Bessel optical tweezers for multiple microparticles (yeast cells) three-dimensional (3-D) trap. To the best knowledge of us, it is the first time to achieve the 3-D stable noncontact multiple microparticles optical traps with long distance intervals by using a single all-fiber probe. The Bessel beam is produced by splicing coaxially a single-mode fiber and a step index multimode fiber. The convergence of the output Bessel beam is performed by molding the tip of the multimode fiber into a special semiellipsoid shape. The effective trapping range of the all-fiber probe is 0 to 60 μm, which is much longer than normal single fiber optical tweezers probes. The all-fiber Bessel optical probe is convenient to integrate and suitable for the lab on the chip. The structure of this fiber probe is simple, high precision, low cost, and small size, which provides new development for biological cells experiment and operation.