Single-Molecule Mechanics in Ligand Concentration Gradient

Balázs Kretzer, Bálint Kiss, Hedvig Tordai, Gabriella Csík, Levente Herényi and Miklós Kellermayer

Single-molecule experiments provide unique insights into the mechanisms of biomolecular phenomena. However, because varying the concentration of a solute usually requires the exchange of the entire solution around the molecule, ligand-concentration-dependent measurements on the same molecule pose a challenge. In the present work we exploited the fact that a diffusion-dependent concentration gradient arises in a laminar-flow microfluidic device, which may be utilized for controlling the concentration of the ligand that the mechanically manipulated single molecule is exposed to. We tested this experimental approach by exposing a λ-phage dsDNA molecule, held with a double-trap optical tweezers instrument, to diffusionally-controlled concentrations of SYTOX Orange (SxO) and tetrakis(4-N-methyl)pyridyl-porphyrin (TMPYP). We demonstrate that the experimental design allows access to transient-kinetic, equilibrium and ligand-concentration-dependent mechanical experiments on the very same single molecule.

DOI

Generation and modulation of terahertz gradient force in the interactions of two-color laser pulses with magnetized plasmas editors-pick

Xiao-Bo Zhang, Xin Qiao, Ai-Xia Zhang, and Ju-Kui Xue

Terahertz (THz) waves, as far-infrared light, offer new opportunities for the optical trapping and manipulation of single cells, in contrast to the other light sources. We present an efficient scheme to flexibly control multiple THz field distribution patterns generated by the laser–plasma interaction in a magnetized plasma. An analytical THz radiation field and two-dimensional particle-in-cell simulation are constructed to verify the feasibility of the scheme. Modulation of the THz gradient force and the energy flux by an asymmetrical THz field is investigated for the purpose of trapping and manipulating particles and cells. In particular, the stabilities of flexibly controlled THz radiation are investigated carefully in the form of the strong and short laser and super-strong magnetic field induced significant spatial structure instabilities and frequency instabilities of terahertz radiation.

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Nanoplastic Analysis by On-line Coupling of Raman Microscopy and Field-Flow Fractionation Enabled by Optical Tweezers

Christian Schwaferts, Vanessa Sogne, Roland Welz, Florian Meier, Thorsten Klein, Reinhard Niessner, Martin Elsner, Natalia P. Ivleva

Nanoplastic pollution is of emerging environmental concern, but current analytical approaches are facing limitations in this size range. However, the coupling of nanoparticle separation with chemical characterization bears potential to close this gap. Here, we realize the hyphenation of particle separation / characterization (field-flow fractionation (FFF), UV and multi angle light scattering (MALS)) with subsequent chemical identification by on-line Raman microspectroscopy (RM). The problem of low Raman scattering was overcome by trapping particles with 2D optical tweezers. This setup enabled RM to identify particles of different materials (polymers and inorganic) in the size range from 200 nm to 5 µm, with concentrations in the order of 1 mg/L (109 particles L-1). The hyphenation was realized for asymmetric flow FFF (AF4) and centrif-ugal FFF (CF3), which separate particles based on different properties. This technique shows potential for application in nanoplastic analysis, as well as many other fields of nanomaterials.

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From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping

Theodoros D. Bouloumis and Síle Nic Chormaic

Optical tweezers are a very well-established technique that have developed into a standard tool for trapping and manipulating micron and submicron particles with great success in the last decades. Although the nature of light enforces restrictions on the minimum particle size that can be efficiently trapped due to Abbe’s diffraction limit, scientists have managed to overcome this problem by engineering new devices that exploit near-field effects. Nowadays, metallic nanostructures can be fabricated which, under laser illumination, produce a secondary plasmonic field that does not suffer from the diffraction limit. This advance offers a great improvement in nanoparticle trapping, as it relaxes the trapping requirements compared to conventional optical tweezers although problems may arise due to thermal heating of the metallic nanostructures. This could hinder efficient trapping and damage the trapped object. In this work, we review the fundamentals of conventional optical tweezers, the so-called plasmonic tweezers, and related phenomena. Starting from the conception of the idea by Arthur Ashkin until recent improvements and applications, we present the principles of these techniques along with their limitations. Emphasis in this review is on the successive improvements of the techniques and the innovative aspects that have been devised to overcome some of the main challenges.

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Observation of processive telomerase catalysis using high-resolution optical tweezers

Eric M. Patrick, Joseph D. Slivka, Bramyn Payne, Matthew J. Comstock & Jens C. Schmidt

Telomere maintenance by telomerase is essential for continuous proliferation of human cells and is vital for the survival of stem cells and 90% of cancer cells. To compensate for telomeric DNA lost during DNA replication, telomerase processively adds GGTTAG repeats to chromosome ends by copying the template region within its RNA subunit. Between repeat additions, the RNA template must be recycled. How telomerase remains associated with substrate DNA during this critical translocation step remains unknown. Using a single-molecule telomerase activity assay utilizing high-resolution optical tweezers, we demonstrate that stable substrate DNA binding at an anchor site within telomerase facilitates the processive synthesis of telomeric repeats. The product DNA synthesized by telomerase can be recaptured by the anchor site or fold into G-quadruplex structures. Our results provide detailed mechanistic insights into telomerase catalysis, a process of critical importance in aging and cancer.

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Measurement of viscoelastic properties of the cellular cytoplasm using optically trapped Brownian probes

Rahul Vaippully, Vaibavi Ramanujan, Saumendra Bajpai and Basudev Roy

Calibration of optically trapped particles in-vivo has been complicated given the frequency depen-dence and spatial inhomogeneity of the cytoplasmic viscosity, and the requirement of accurateknowledge of the medium refractive index. Further, it has been demonstrated that the mediumviscosity is dependent upon the measurement probe leading to reliability issues for measurementswith even micrometer sized particles. Here, we employ a recent extension of Jeffery's model ofviscoelasticity in the microscopic domain to fit the passive motional power spectra of micrometer-sized optically trapped particles embedded in a viscoelastic medium. We find excellent agreementbetween the 0 Hz viscosity in MCF7 cells and the typical values of viscosity in literature, between2 to 16 mPa sec expected for the typical concentration of proteins inside the cytoplasmic solvent.This bypasses the dependence on probe size by relying upon small thermal displacements. Ourmeasurements of the relaxation time also match values reported with magnetic tweezers, at about0.1 sec. Finally, we calibrate the optical tweezers and demonstrate the efficacy of the techniqueto the study of in-vivo translational motion.

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A PZT-assisted single particle loading method for dual-fiber optical trap in air

Guangzong Xiao, Tengfang Kuang, Wei Xiong, Xiang Han, Hui Luo

Efficiently loading particle into optical trap in air is always a challenge, especially for the small enclosed trapping chamber. We propose and demonstrate a novel particle-loading method by optimally selecting the vibration mode of the PZT system. The forces exerted on the particle and its dynamic characteristics in the loading process are investigated. The minimum detachment velocity and the optimal capturable detachment velocity ranges are defined and discussed. We point out that the optimal vibration mode should be selected to ensure that the minimum detachment velocity is tiny lower than minimum capturable detachment velocity. Then the detachment velocity can be enhanced into the optimal capturable detachment velocity range through changing the actuating voltage for the selected vibration mode. Our experiment prove that the method can achieve controllable and highly efficient particle loading into the optical trap in air.

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Radiation forces of hypergeometric-Gaussian type-II beams acting on a Rayleigh dielectric sphere

Gui Jin, Lirong Bian, Li Huang, Bin Tang

We drive the analytical expressions for the propagation of hypergeometric-Gaussian typed-II (HyGG-II) beam passing through a paraxial ABCD optical system. The radiation forces of a strongly focused HyGG-II beam exerting on a Rayleigh dielectric sphere are theoretically and numerically investigated under the dipole approximation. The influences of optical parameters on the radiation forces are also discussed in detail. The results demonstrate that the HyGG-II beams can hold the micro particles with both high and low refractive index. Finally, we analyze the necessary conditions for stably trapping the particle.

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Tunable emission and optical trapping of upconverting LiYF4:Yb,Er nanocrystal

L. Anbharasi, E. A. Bhanu Rekha, V. R. Rahul, Basudev Roy, M. Gunaseelan, S. Yamini, Venkata N.K.B. Adusumalli, Debashrita Sarkar, Venkataramanan Mahalingam, J. Senthilselvana

Present work investigates about hydrothermally prepared LiYF4:Yb,Er nanocrystals with novel tunable emission and optical trapping results. Optical trapping effect of LiYF4:Yb,Er upconversion nanocrystal under 980 nm diode laser interaction is reported and a single nanoparticle trapping is demonstrated. Optical trapping force of 10.8 fN is exerted on LiYF4:Yb,Er upconversion nanocrystal of size ~238 nm at laser power 50 mW and the result is in agreement with the reported values. At 980 nm excitation, tunable green to red upconversion emission is achieved by calcination. It depends on orthorhombic and tetragonal phases that confirmed by XRD. The nanocrystals calcined at 450 and 600 °C resulted in a decay time of 26 and 19 µs respectively and the upconversion emission mechanism is explained. FESEM and HRTEM examinations showed the hydrothermally synthesized nanocrystals are in trapezohedral, pyramidal, bi-pyramidal and tetragonal morphologies. The differential scanning calorimetry revealed the tetragonal phase formation temperature is lower at 450 °C compared to the reported value of 750 °C. The UV–VIS-NIR spectra of LiYF4:Yb,Er showed high absorption at 380, 480, 520 and 660 nm due to Er3+ ion and the broad absorption from 900 to 1000 nm centered at 980 nm is owing to Yb3+ ion. The crystalline phase, morphology, upconversion emission and optical trapping behaviors of LiYF4:Yb,Er nanocrystal are explained elaborately.

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Optical Micromachines for Biological Studies

Philippa-Kate Andrew, Martin A. K. Williams and Ebubekir Avci

Optical tweezers have been used for biological studies since shortly after their inception. However, over the years research has suggested that the intense laser light used to create optical traps may damage the specimens being studied. This review aims to provide a brief overview of optical tweezers and the possible mechanisms for damage, and more importantly examines the role of optical micromachines as tools for biological studies. This review covers the achievements to date in the field of optical micromachines: improvements in the ability to produce micromachines, including multi-body microrobots; and design considerations for both optical microrobots and the optical trapping set-up used for controlling them are all discussed. The review focuses especially on the role of micromachines in biological research, and explores some of the potential that the technology has in this area.

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Step-wise elimination of α-mitochondrial nucleoids and mitochondrial structure as a basis for the strict uniparental inheritance in Cryptococcus neoformans

Yoshiki Nishimura, Toshiharu Shikanai, Susumu Kawamoto & Akio Toh-e

In most sexual eukaryotes, mitochondrial (mt) DNA is uniparentally inherited, although the detailed mechanisms underlying this phenomenon remain controversial. The most widely accepted explanations include the autophagic elimination of paternal mitochondria in the fertilized eggs and the active degradation of paternal mitochondrial DNA. To decode the precise program for the uniparental inheritance, we focused on Cryptococcus neoformans as a model system, in which mtDNA is inherited only from the a-parent, although gametes of a- and α-cells are of equal size and contribute equal amounts of mtDNA to the zygote. In this research, the process of preferential elimination of the mitochondria contributed by the α-parent (α-mitochondria) was studied by fluorescence microscopy and single cell analysis using optical tweezers, which revealed that α-mitochondria are preferentially reduced by the following three steps: (1) preferential reduction of α-mitochondrial (mt) nucleoids and α-mtDNA, (2) degradation of the α-mitochondrial structure and (3) proliferation of remaining mt nucleoids during the zygote development. Furthermore, AUTOPHAGY RELATED GENE (ATG) 8 and the gene encoding mitochondrial endonuclease G (NUC1) were disrupted, and the effects of their disruption on the uniparental inheritance were scrutinized. Disruption of ATG8 (ATG7) and NUC1 did not have severe effects on the uniparental inheritance, but microscopic examination revealed that α-mitochondria lacking mt nucleoids persisted in Δatg8 zygotes, indicating that autophagy is not critical for the uniparental inheritance per se but is responsible for the clearance of mitochondrial structures after the reduction of α-mt nucleoids.

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Control of quantum dot emission by colloidal plasmonic pyramids in a liquid crystal

Haridas Mundoor, Enid M. Cruz-Colón, Sungoh Park, Qingkun Liu, Ivan I. Smalyukh, and Jao van de Lagemaat

We study the plasmon-enhanced fluorescence of a single semiconducting quantum dot near the apex of a colloidal gold pyramid spatially localized by the elastic forces of the liquid crystal host. The gold pyramid particles were manipulated within the liquid crystal medium by laser tweezers, enabling the self-assembly of a semiconducting quantum dot dispersed in the medium near the apex of the gold pyramid, allowing us to probe the plasmon-exciton interactions. We demonstrate the effect of plasmon coupling on the fluorescence lifetime and the blinking properties of the quantum dot. Our results demonstrate that topological defects around colloidal particles in liquid crystal combined with laser tweezers provide a platform for plasmon exciton interaction studies and potentially could be extended to the scale of composite materials for nanophotonic applications.

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Optical Trapping and Manipulating with a Silica Microring Resonator in a Self-Locked Scheme

Victor W. L. Ho, Yao Chang, Yang Liu, Chi Zhang, Yuhua Li, Roy R. Davidson, Brent E. Little, Guanghui Wang, and Sai T. Chu

Based on the gradient force of evanescent waves in silica waveguides and add-drop micro-ring resonators, the optical trapping and manipulation of micro size particles is demonstrated in a self-locked scheme that maintains the on-resonance system even if there is a change in the ambient temperature or environment. The proposed configuration allows the trapping of particles in the high Q resonator without the need for a precise wavelength adjustment of the input signal. On the one hand, a silicon dioxide waveguide having a lower refractive index and relatively larger dimensions facilitates the coupling of the laser with a single-mode fiber. Furthermore, the experimental design of the self-locked scheme reduces the sensitivity of the ring to the environment. This combination can trap the micro size particles with a high stability while manipulating them with high accuracy.

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Optically Levitated Nanodumbbell Torsion Balance and GHz Nanomechanical Rotor

Jonghoon Ahn, Zhujing Xu, Jaehoon Bang, Yu-Hao Deng, Thai M. Hoang, Qinkai Han, Ren-Min Ma, and Tongcang Li

Levitated optomechanics has great potential in precision measurements, thermodynamics, macroscopic quantum mechanics, and quantum sensing. Here we synthesize and optically levitate silica nanodumbbells in high vacuum. With a linearly polarized laser, we observe the torsional vibration of an optically levitated nanodumbbell. This levitated nanodumbbell torsion balance is a novel analog of the Cavendish torsion balance, and provides rare opportunities to observe the Casimir torque and probe the quantum nature of gravity as proposed recently. With a circularly polarized laser, we drive a 170-nm-diameter nanodumbbell to rotate beyond 1 GHz, which is the fastest nanomechanical rotor realized to date. Smaller silica nanodumbbells can sustain higher rotation frequencies. Such ultrafast rotation may be used to study material properties and probe vacuum friction.

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Zinc Oxide @ Silica Core/Shell Microspheres for Single-Molecule Force Microscopy in Aqueous and Non-Aqueous Solvents

Qixuan Yu, Ya-Na Chen, Jacob W. Black, Ziad Ganim

Optical tweezers provide a platform for both manipulating and probing the chemistry of a single polymer molecule tethered between dielectric microspheres. It has been challenging to adapt this technology to organic solvents, in part due to the limited availability of optically trappable materials possessing the necessary diameter and refractive index contrast. Here we report on the development of broadly accessible optical trapping in aqueous and organic solvents that utilizes zinc oxide-silica core-shell microspheres (beads). The addition of a silica shell allows otherwise highly scattering zinc oxide nanoparticles to be stably trapped and readily functionalized. Trapping was observed in water, chloroform, tetrahydrofuran, and ethyl acetate. We demonstrate how these beads can be used to measure the force-extension curves of DNA and poly(methyl methacrylate) respectively utilizing antibody/antigen complementation or strain-promoted azide/alkyne cycloaddition to form linkages in situ. In the latter, a strong, contiguous chain of covalent bonds is formed between the microspheres; therefore, UV bond photolysis was used to count the number of rupture steps and control for single-molecule link formation. In addition to being trappable in many solvents, ZnO@SiO2 core-shell beads can be used as solid support during harsh synthetic conditions and can be readily prepared in the presence of atmospheric oxygen.

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Kerker‐Type Intensity‐Gradient Force of Light

Xiaohao Xu Manuel Nieto‐Vesperinas Cheng‐Wei Qiu Xiaoshuai Liu Dongliang Gao Yao Zhang Baojun Li

The intensity gradient of light represents the most important property for optical tweezers to manipulate small particles, which is known to produce a conservative optical force that is either attractive or repulsive. Here, it is shown that Kerker interference, the interplay between electric and magnetic dipoles induced in nanoparticles, permits the intensity gradient to exert a nonconservative optical force, in the case of a standard optical trap created with linearly or elliptically polarized Gaussian beams. The Kerker‐type intensity‐gradient force has an “anisotropic” directionality, and it tends to repel particles away from the beam axis. Such repulsive effects can greatly sensitize the particle trapping behavior of optical tweezers to the particle size. Utilizing these peculiar properties, all‐optical sorting of Si nanoparticles is theoretically demonstrated, with tunable size‐selection criterion and accuracy.

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Trapping metallic particles using focused Bloch surface waves

Yifeng Xiang, Xi Tang, Yanan Fu, Fenya Lu, Yan Kuai, Changjun Min, Junxue Chen, Pei Wang, Joseph. R. Lakowicz, Xiaocong Yuan and Douguo Zhang

Metallic particles are promising for applications in various areas, including optical sensing, imaging and electric field enhancement-induced optical and thermal effects. The ability to trap or transport these particles stably will be important in these applications. However, while traditional optical tweezers can trap metallic Rayleigh particles easily, it is difficult to trap metallic mesoscopic/Mie particles because of the strong scattering forces that come from the far-field trapping laser beam. Here we demonstrate that metallic particles can be trapped stably using focused Bloch surface waves that propagate in the near-field region of a dielectric multilayer structure with a photonic band gap. Focused Bloch surface waves can be excited efficiently using an annular beam with azimuthal polarization and a high-numerical-aperture objective. Numerical simulations were performed to calculate the optical forces loaded on a gold particle by focused Bloch surface waves and the results were consistent with those of the experimental observations.

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Optical tweezers-based characterisation of gold core–satellite plasmonic nano-assemblies incorporating thermo-responsive polymers†

Fei Han, Thomas Armstrong, Ana Andres-Arroyo, Danielle Bennett, Alex Soeriyadi, Ali Alinezhad Chamazketi, Padmavathy Bakthavathsalam, Richard D. Tilley, J. Justin Gooding and Peter J. Reece

We report on the characterisation of the optical properties and dynamic behaviour of optically trapped single stimuli-responsive plasmonic nanoscale assemblies. Nano-assemblies consist of a core–satellite arrangement where the constituent nanoparticles are connected by the thermoresponsive polymer, poly(DEGA-co-OEGA). The optical tweezers allow the particles to be held isolated in solution and interrogated using dark-field spectroscopy. Additionally, controlling the optical trapping power provides localised heating for probing the thermal response of the nanostructures. Our results identify a number of distinct core–satellite configurations that can be stably trapped, which are verified using finite element modelling. Laser heating of the nanostructures through the trapping laser yields irreversible modification of the arrangement, as observed through the scattering spectrum. We consider which factors may be responsible for the observed behaviour in the context of the core–satellite geometry, polymer–solvent interaction, and the bonding of the nanoparticles.

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Silicon microparticles as handles for optical tweezers experiments

T. A. Moura, U. M. S. Andrade, J. B. S. Mendes, and M. S. Rocha

We study the behavior of silicon microparticles in a 1064 nm Gaussian-beam optical tweezers, showing that this semiconductor can be used to perform different types of optical manipulation experiments. Depending on the focal position and the laser power used, the particles can present an oscillatory dynamics in the tweezers or can be stably 3D-trapped with a trap stiffness that allows the application of femtoNewton forces with accuracy. A new, to the best of our knowledge, interpretation based on the photoexcitation of electrons in the valence band is proposed to explain the oscillations, and the quantities associated with such dynamics (e.g., amplitude, period, etc.) were characterized as a function of relevant parameters to optical tweezers setups.

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Simultaneous sensing and imaging of individual biomolecular complexes enabled by modular DNA–protein coupling

Mario J. Avellaneda, Eline J. Koers, David P. Minde, Vanda Sunderlikova & Sander J. Tans

Many proteins form dynamic complexes with DNA, RNA, and other proteins, which often involves protein conformational changes that are key to function. Yet, methods to probe these critical dynamics are scarce. Here we combine optical tweezers with fluorescence imaging to simultaneously monitor the conformation of individual proteins and their binding to partner proteins. Central is a protein–DNA coupling strategy, which uses exonuclease digestion and partial re-synthesis to generate DNA overhangs of different lengths, and ligation to oligo-labeled proteins. It provides up to 40 times higher coupling yields than existing protocols and enables new fluorescence-tweezers assays, which require particularly long and strong DNA handles. We demonstrate the approach by detecting the emission of a tethered fluorescent protein and of a molecular chaperone (trigger factor) complexed with its client. We conjecture that our strategy will be an important tool to study conformational dynamics within larger biomolecular complexes.

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A function of profilin in force generation during malaria parasite motility independent of actin binding

Catherine A. Moreau, Katharina A. Quadt, Henni Piirainen, Hirdesh Kumar, Saligram P. Bhargav, Léanne Strauss, Niraj H. Tolia, Rebecca C. Wade, Joachim P. Spatz, Inari Kursula, Friedrich Frischknecht

During transmission of malaria-causing parasites from mosquito to mammal, Plasmodium sporozoites migrate at high speed within the skin to access the bloodstream and infect the liver. This unusual gliding motility is based on retrograde flow of membrane proteins and highly dynamic actin filaments that provide short tracks for a myosin motor. Using laser tweezers and parasite mutants, we previously suggested that actin filaments form macromolecular complexes with plasma-membrane spanning adhesins to generate force during migration. Mutations in the actin-binding region of profilin, a near ubiquitous actin-binding protein, revealed that loss of actin binding also correlates with loss of force production and motility. Here we show that different mutations in profilin, not affecting actin binding in vitro, still generate lower force during Plasmodium sporozoite migration. Lower force generation inversely correlates with increased retrograde flow suggesting that, like in mammalian cells, the slow-down of flow to generate force is the key underlying principle governing Plasmodium gliding motility.

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Reducing photodamage in optical trapping of individual cells in living zebrafish

Panpan Yu, Yifan Liu, Qian Zhao, Ziqiang Wang, Yinmei Li and Lei Gong

We propose to reduce the photodamage in optical trapping of living cells by using azimuthally polarized beams (APBs). APBs achieve higher axial trapping efficiency and lower photodamage than linearly polarized Gaussian beam when used for optical trapping of individual red blood cells (RBCs). In particular, we further achieve reducing photodamage in optical trapping of individual RBCs in living Zebrafishes. Our work is expected to benefit in-vivo optical trapping and the study of living cells.

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Imaging unlabeled proteins on DNA with super-resolution

Anna E C Meijering, Andreas S Biebricher, Gerrit Sitters, Ineke Brouwer, Erwin J G Peterman, Gijs J L Wuite, Iddo Heller

Fluorescence microscopy is invaluable to a range of biomolecular analysis approaches. The required labeling of proteins of interest, however, can be challenging and potentially perturb biomolecular functionality as well as cause imaging artefacts and photo bleaching issues. Here, we introduce inverse (super-resolution) imaging of unlabeled proteins bound to DNA. In this new method, we use DNA-binding fluorophores that transiently label bare DNA but not protein-bound DNA. In addition to demonstrating diffraction-limited inverse imaging, we show that inverse Binding-Activated Localization Microscopy or ‘iBALM’ can resolve biomolecular features smaller than the diffraction limit. The current detection limit is estimated to lie at features between 5 and 15 nm in size. Although the current image-acquisition times preclude super-resolving fast dynamics, we show that diffraction-limited inverse imaging can reveal molecular mobility at ∼0.2 s temporal resolution and that the method works both with DNA-intercalating and non-intercalating dyes. Our experiments show that such inverse imaging approaches are valuable additions to the single-molecule toolkit that relieve potential limitations posed by labeling.

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Modified plasmonic response of dimer nanoantennas with nonlocal effects: From near-field enhancement to optical force

Hancong Wang, Kaixi Chen, Jia Pan, Shihao Huang, Jinyang Lin, Wenming Xie, Xuhong Huang

The metallic nanoparticle dimer is a fundamental model system for enhancing and tuning localized surface plasmon resonances. In the past, it had been found that the far- and near-field optical properties of dimer antennas can be regulated by many parameters (e.g., gap, size, orientation, materials, and surrounding medium). In recent years, the quantum mechanical effects such as nonlocal screening and electron tunneling have been achieved when the gap distance in a dimer approaches 1 nm and subnanometer. In this communication, both the near-field enhancement and optical force in dimer are fully investigated and compared between classical and nonlocal models. Compared with classical theory, we found that both the resonant wavelength and peak intensity have smaller changes in nonlocal model when geometrical or material parameters changes. Besides, the extent of parameter-induced spectral changes is slightly different between near-field enhancement and optical force. These results make possible the quantitative analysis of nonlocal effects in surface-enhanced spectroscopy, nanoantennas, refractive-index sensing, surface-enhanced optical force, and quantum plasmonics.

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OTSLM toolbox for Structured Light Methods

Isaac C.D.Lenton, Alexander B.Stilgoe, Timo A.Nieminen, Halina Rubinsztein-Dunlop

We present a new Matlab toolbox for generating phase and amplitude patterns for digital micro-mirror device (DMD) and liquid crystal (LC) based spatial light modulators (SLMs). This toolbox consists of a collection of algorithms commonly used for generating patterns for these devices with a focus on optical tweezers beam shaping applications. In addition to the algorithms provided, we have put together a range of user interfaces for simplifying the use of these patterns. The toolbox currently has functionality to generate patterns which can be saved as a image or displayed on a device/screen using the supplied interface. We have only implemented interfaces for the devices our group currently uses but we believe that extending the code we provide to other devices should be fairly straightforward. The range of algorithms included in the toolbox is not exhaustive. However, by making the toolbox open sources and available on GitHub we hope that other researchers working with these devices will contribute their patterns/algorithms to the toolbox.

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Self-Trapped Nanoparticle Binding via Waveguide Mode

Ivan D. Toftul, Danil F. Kornovan, Mihail I. Petrov

In this paper, we study a stable optomechanical system based on a nanoparticle chain coupled to a waveguide mode. Under the plane wave excitation, the nanoparticles form a stable self-organized periodic array along the waveguide axis through the transverse binding effect. We show that, owing to the long-range interaction between the nanoparticles, the trapping potential for each nanoparticle in the chain increases linearly with the system size, making the formation of long chains more favorable. We show that, for an optical nanofiber platform, the binding energy for two nanoparticles is in the range of 9–13 kT, reaching the value of 110 kT when the chain size is increased to 20 nanoparticles. We also suggest the geometry of the two counter-propagating plane waves excitation, which will allow trapping the nanoparticles close to the optical nanofiber, providing efficient interaction between the nanoparticles and the nanofiber.

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On chip optical tractor beam by surface plasmon polariton

M.R.C. Mahdy, Tianhang Zhang, Saikat Chandra Das, Hamim Mahmud Rivy

Since the discovery, surface wave tractor beams have drawn less attention compared to free-propagating optical tractor beams. For Mie-range objects, this article stands for one of the very first proposals of on chip SPP (surface plasmon polariton) based optical tractor beam where two SPP waves are excited by using gratings made of grooves and meta-surfaces. Such beam can pull dielectric particles within certain size ranges when it is non-paraxial enough. Our SPP tractor beam set-up enables on chip object transportation and radius based optical sorting which could further facilitate the on-chip manipulation and analysis of nanoparticles and biomolecules.

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Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet

Ningning Song, Renxian Li, Han Sun, Jiaming Zhang, Bojian Wei, Shu Zhang, F.G. Mitri

Based on the generalized Lorenz–Mie theory (GLMT) and the angular spectrum decomposition method (ASDM), we calculate the optical radiation force exerted on a lossless dielectric sphere of arbitrary size illuminated by an Airy light-sheet. The beam shape coefficients (BSCs) of the Airy light-sheet are calculated using the vector angular spectrum decomposition and vector spherical wave functions methods. The optical radiation force acting on the spherical particle is obtained by the integral of Maxwell’s stress tensor. The transverse (Fy) and longitudinal (Fz) forces are numerically computed. Two kinds of polarization (TE and TM) are considered for the Airy light-sheet, and the negative longitudinal optical (pulling) force is particularly emphasized. The influence of the transverse scale parameter w0 and attenuation parameter γ of the Airy light-sheet on the force is discussed. The results of the present theory are verified using the dipole approximation method in which the gradient force has been also computed for a Rayleigh sphere. The numerical results show that when the transverse scale parameter w0 and attenuation parameter γ increase, the transverse and longitudinal forces decrease. Furthermore, the force caustic (i.e., maximum) shifts to the direction of y < 0 as the transverse scale parameter w0 increases. As the dimensionless size parameter of the sphere ka increases (where k is the wavenumber and a is the radius), the resonance peaks of the optical forces become larger. The results of this paper are of practical significance for the development of Airy light-sheet based optical manipulation technologies.

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Making Permanent Optical Matter of Plasmonic Nanoparticles by in Situ Photopolymerization

Zhenzhen Chen, Fan Nan, Zijie Yan

Laser-induced self-organization of colloidal metal nanoparticles holds great promise in building artificial photonic structures, yet the self-organized nanoparticles (i.e., optical matter) disassemble immediately without the optical field. Here we report an approach for in situ construction of permanent mesoscale structures from optically bound nanoparticles. Metal nanoparticles are trapped by optical tweezers and self-organize into various optical matter structures, which are selectively immobilized by photocurable hydrogels upon additional ultraviolet light illumination. Making permanent optical matter of plasmonic nanoparticles will benefit bottom-up assembly of photonic materials and devices.

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Direct Observation of Liquid Crystal Droplet Configurational Transitions using Optical Tweezers

Jake Shechter, Noe Atzin, Ali Mozaffari, Rui Zhang, Ye Zhou, Benjamin Strain, Linda M. Oster, Juan J. de Pablo, Jennifer L. Ross

Liquid crystals (LCs) are easily influenced by external interactions, particularly at interfaces. When rod-like LC molecules are confined to spherical droplets, they experience a competition between interfacial tension and elastic deformations. The configuration of LCs inside a droplet can be controlled using surfactants that influence the interfacial orientation of the LC molecules in the oil-phase of an oil in water emulsion. Here we used the surfactant sodium dodecyl sulfate (SDS) to manipulate the orientation of 5CB molecules in a polydisperse emulsion, and examined the configuration of the droplets as a function of SDS concentration. We triggered pronounced morphological transitions by altering the SDS concentration while observing an individual LC droplet held in place using an optical tweezer. We compared the experimental configuration changes to predictions from simulations. We observed a hysteresis in the SDS concentration that induced the morphological transition from radial to bipolar and back, as well as a fluctuations in the configuration during the transition.

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Photonic Crystal Optical Tweezers with High Efficiency for Live Biological Samples and Viability Characterization

Peifeng Jing, Jingda Wu, Gary W. Liu, Ethan G. Keeler, Suzie H. Pun & Lih Y. Lin
We propose and demonstrate a new optical trapping method for single cells that utilizes modulated light fields to trap a wide array of cell types, including mammalian, yeast and Escherichia coli cells, on the surface of a two-dimensional photonic crystal. This method is capable of reducing the required light intensity and thus minimizing the photothermal damage to living cells, thereby extending cell viability in optical trapping and cell manipulation applications. To this end, a thorough characterization of cell viability in optical trapping environments was performed. This study also demonstrates the technique using spatial light modulation in patterned manipulation of live cell arrays over a broad area.

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Particle trapping and beaming using a 3D nanotip excited with a plasmonic vortex

Kai Liu, Nicolò Maccaferri, Yuefeng Shen, Xueyun Li, Remo Proietti Zaccaria, Xuejin Zhang, Yuri Gorodetski, and Denis Garoli

Recent advances in nanotechnology have prompted the need for tools to accurately and noninvasively manipulate individual nano-objects. Among the possible strategies, optical forces have been widely used to enable nano-optical tweezers capable of trapping or moving a specimen with unprecedented accuracy. Here, we propose an architecture consisting of a nanotip excited with a plasmonic vortex enabling effective dynamic control of nanoparticles in three dimensions. The structure illuminated by a beam with angular momentum can generate an optical field that can be used to manipulate single dielectric nanoparticles. We demonstrate that it is possible to stably trap or push the particle from specific points, thus enabling a new, to the best of our knowledge, platform for nanoparticle manipulation.

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Sample preparation for Raman microspectroscopy

I. J. Jahn, L. Lehniger, K. Weber, D. Cialla-May, J. Popp

Raman spectroscopy and its variants allow for the investigation of a wide range of biological and biomedical samples, i. e. tissue sections, single cells and small molecules. The obtained information is on a molecular level. By making use of databases and chemometrical approaches, the chemical composition of complex samples can also be defined. The measurement procedure is straight forward, however most often sample preparation protocols must be implemented. While pure samples, such as high purity powders or highly concentrated chemicals in aqueous solutions, can be directly measured without any prior sample purification step, samples of biological origin, such as tissue sections, pathogens in suspension or biofluids, food and beverages often require pre-processing steps prior to Raman measurements. In this book chapter, different strategies for handling and processing various sample matrices for a subsequent Raman microspectroscopic analysis were introduced illustrating the high potential of this promising technique for life science and medical applications. The presented methods range from standalone techniques, such as filtration, centrifugation or immunocapture to innovative platform approaches which will be exemplary addressed. Therefore, the reader will be introduced to methods that will simplify the complexity of the matrix in which the targeted molecular species are present allowing direct Raman measurements with bench top or portable setups.

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The Biogenesis of SRP RNA Is Modulated by an RNA Folding Intermediate Attained during Transcription

Shingo Fukuda, Shannon Yan, Yusuke Komi, Mingxuan Sun, Ronen Gabizon, Carlos Bustamante

The signal recognition particle (SRP), responsible for co-translational protein targeting and delivery to cellular membranes, depends on the native long-hairpin fold of its RNA to confer functionality. Since RNA initiates folding during its synthesis, we used high-resolution optical tweezers to follow in real time the co-transcriptional folding of SRP RNA. Surprisingly, SRP RNA folding is robust to transcription rate changes and the presence or absence of its 5′-precursor sequence. The folding pathway also reveals the obligatory attainment of a non-native hairpin intermediate (H1) that eventually rearranges into the native fold. Furthermore, H1 provides a structural platform alternative to the native fold for RNase P to bind and mature SRP RNA co-transcriptionally. Delays in attaining the final native fold are detrimental to the cell, altogether showing that a co-transcriptional folding pathway underpins the proper biogenesis of function-essential SRP RNA.

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Cooling of a levitated nanoparticle to the motional quantum ground state

Uroš Delić, Manuel Reisenbauer, Kahan Dare, David Grass, Vladan Vuletić, Nikolai Kiesel, Markus Aspelmeyer

Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allows us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises of 108 atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses.

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Simultaneous optical trapping and imaging in the axial plane: a review of current progress

Yansheng Liang, Shaohui Yan, Zhaojun Wang, Runze Li, Yanan Cai, Minru He, Baoli Yao and Ming Lei

Optical trapping has become a powerful tool in numerous fields such as biology, physics, chemistry, etc. In standard optical trapping systems, trapping and imaging shares the same objective lens, confining the region of observation to the focal plane. To capture optical trapping processes occurring in other plane, especially the axial plane, many methods have been proposed to achieve this goal. Here, we review the methods of acquiring the axial-plane information from which axial plane trapping is observed and discuss their advantages and limitations. To overcome the limitations existing in these methods, we developed an optical tweezer system that allows for simultaneous optical trapping and imaging technique. The versatility and usefulness of the system in axial-plane trapping and imaging is demonstrated by investigating its trapping performance with various optical fields, including Bessel, Airy and snake-like beams. The potential applications of the proposed technique are suggested to several fields, including optical pulling, longitudinal optical binding, tomographic phase microscopy and superresolution microscopy.

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Processive extrusion of polypeptide loops by a Hsp100 disaggregase

Mario J. Avellaneda, Kamila B. Franke, Vanda Sunderlikova, Bernd Bukau, Axel Mogk & Sander J. Tans

The ability to reverse protein aggregation is vital to cells1,2. Hsp100 disaggregases such as ClpB and Hsp104 are proposed to catalyse this reaction by translocating polypeptide loops through their central pore3,4. This model of disaggregation is appealing, as it could explain how polypeptides entangled within aggregates can be extracted and subsequently refolded with the assistance of Hsp704,5. However, the model is also controversial, as the necessary motor activity has not been identified6,7,8 and recent findings indicate non-processive mechanisms such as entropic pulling or Brownian ratcheting9,10. How loop formation would be accomplished is also obscure. Indeed, cryo-electron microscopy studies consistently show single polypeptide strands in the Hsp100 pore11,12. Here, by following individual ClpB–substrate complexes in real time, we unambiguously demonstrate processive translocation of looped polypeptides. We integrate optical tweezers with fluorescent-particle tracking to show that ClpB translocates both arms of the loop simultaneously and switches to single-arm translocation when encountering obstacles. ClpB is notably powerful and rapid; it exerts forces of more than 50 pN at speeds of more than 500 residues per second in bursts of up to 28 residues. Remarkably, substrates refold while exiting the pore, analogous to co-translational folding. Our findings have implications for protein-processing phenomena including ubiquitin-mediated remodelling by Cdc48 (or its mammalian orthologue p97)13 and degradation by the 26S proteasome14.

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Desialylation Of O-Glycans On Glycoprotein Ibα Drives Receptor Signaling And Platelet Clearance

Yingchun Wang, Wenchun Chen, Wei Zhang, Melissa M. Lee-Sundlov, Caterina Casari, Michael C. Berndt, Francois Lanza, Wolfgang Bergmeier, Karin M. Hoffmeister, X. Frank Zhang, Renhao Li

During infection neuraminidase desialylates platelets and induces their rapid clearance from circulation. The underlying molecular basis, particularly the role of platelet glycoprotein (GP)Ibα therein, is not clear. Utilizing genetically altered mice we report that the extracellular domain of GPIbα, but neither von Willebrand factor nor ADAM17 (a disintegrin and metalloprotease 17), is required for platelet clearance induced by intravenous injection of neuraminidase. Lectin binding to platelets following neuraminidase injection over time revealed that the extent of desialylation of O-glycans correlates with the decrease of platelet count in mice. Injection of α2,3-neuraminidase reduces platelet counts in wild-type but not in transgenic mice expressing only a chimeric GPIbα that misses most of its extracellular domain. Neuraminidase treatment induces unfolding of the O-glycosylated mechanosensory domain in GPIbα as monitored by single-molecule force spectroscopy, increases the exposure of the ADAM17 shedding cleavage site in the mechanosensory domain on the platelet surface, and induces ligand-independent GPIb-IX signaling in human and murine platelets. These results suggest that desialylation of O-glycans of GPIbα induces unfolding of the mechanosensory domain, subsequent GPIb-IX signaling including amplified desialylation of N-glycans, and eventually rapid platelet clearance. This new molecular mechanism of GPIbα-facilitated clearance could potentially resolve many puzzling and seemingly contradicting observations associated with clearance of desialylated or hyposialylated platelets.

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Orientation Dependent Interaction and Self‐Assembly of Cubic Magnetic Colloids in a Nematic Liquid Crystal

Devika Venkuzhy Sudhakaran,  Ravi Kumar Pujala,  Surajit Dhara

Spherical microparticles dispersed in nematic liquid crystals have been extensively investigated in the past years. Here, experimental studies are reported on the elastic deformation, colloidal interaction, and self‐assembly of hematite microcubes with homeotropic surface anchoring in a nematic liquid crystal. It is demonstrated that the colloidal interaction and self‐assembly of cubic colloids are orientation dependent. In a notable departure from the conventional microspheres, the microcubes stabilize diverse structures, such as bent chains, branches, kinks, and closed‐loops. The microcubes reorient under external rotating magnetic field, thereby experiencing an elastic torque in the medium, which allows us to measure the magnetic moment through competition between elastic and magnetic torques. The findings envisage that the faceted magnetic colloids in liquid crystals are potential for developing new functional magnetic materials with specific morphologies.

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