Thursday, October 29, 2015

Determining the size and refractive index of microspheres using the mode assignments from Mie resonances

Thomas C. Preston and Jonathan P. Reid

A new method for determining the radius and refractive index of microspheres using Mie resonances is presented. Previous methods have relied on searching multidimensional space to find the radius and refractive index that minimize the difference between observed and calculated Mie resonances. For anything but simple refractive index functions, this process can be very time consuming. Here, we demonstrate that once the mode assignment for the observed Mie resonances is known, no search is necessary, and the radius and refractive index of best-fit can be found immediately. This superior and faster way to characterize microspheres using Mie resonances should supplant previous fitting algorithms. The derivation and implementation of the equations that give the parameters of best-fit are shown and discussed. Testing is performed on systems of physical interest, and the effect of noise on measured peak positions is investigated.


Visualizing surface plasmon polaritons by their gradient force

Junghoon Jahng, Faezeh Tork Ladani, Ryan Muhammad Khan, Xiaowei Li, Eun Seong Lee, and Eric Olaf Potma

A new method is presented for visualizing the electric field distributions associated with propagating surface-plasmon-polariton (SPP) modes directly in the near-field. The method is based on detecting the photo-induced gradient force exerted by the evanescent field onto a sharp and polarizable tip. Using a photo-induced force microscope (PiFM), images of propagating SPPs are obtained on flat gold surfaces.


Tweezers controlled resonator

Samuel Kaminski, Leopoldo L. Martin, and Tal Carmon

We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activating it as a microresonator by bringing it close to a tapered-fiber coupler. Our tweezers facilitated the tuning of the coupling from the under-coupled to the critically-coupled regime while the quality-factor [Q] is 12 million and the resonator’s size is at the 80 μm scale.


Improved axial trapping with holographic optical tweezers

Russell Pollari and Joshua N. Milstein

Conventional optical tweezers suffer from several complications when applying axial forces to surface-tethered molecules. Aberrations from the refractive-index mismatch between an oil-immersion objective’s medium and the aqueous trapping environment both shift the trap centre and degrade the trapping strength with focal depth. Furthermore, interference effects from back-scattered light make it difficult to use back-focal-plane interferometry for high-bandwidth position detection. Holographic optical tweezers were employed to correct for aberrations to achieve a constant axial stiffness and modulate artifacts from backscattered light. Once the aberrations are corrected for, the trap height can be precisely determined from either the back-scattered light or Brenner’s formula.


Wednesday, October 28, 2015

Absorption Spectroscopy of Single Optically Trapped Gold Nanorods

Zhongming Li, Weizhi Mao, Mary Sajini Devadas, and Gregory V. Hartland

Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calculations allows us to determine the dimensions of the nanorod and estimate the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron–surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron–surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.


Scaling of optical forces on Au-PEG core-shell nanoparticles

Donatella Spadaro, Antonella Iatì, Maria Grazia Donato, Pietro Giuseppe Gucciardi, Rosalba Saija, Anurag Cherlakola, Stefano Scaramuzza, Vincenzo Amendola and Onofrio Marago

Optical trapping of hybrid core-shell gold-polymer particles is studied. Optical forces are measured for different gold core size and polymer shell thickness, revealing how a polymer shell increases the trapping efficiency with respect to the bare gold nanoparticles. Data are in agreement with calculations of optical trapping based on electromagnetic scattering theory in the T-matrix approach. The scaling behaviour of optical forces with respect to the ratio between polymer layer thickness and the whole particle radius is found and discussed.


An Optically Driven Bi-Stable Janus Rotor with Patterned Metal Coatings

Yiwu Zong, Jing Liu, Rui Liu, Honglian Guo, Mingcheng Yang, Zhiyuan Li, and Ke Chen

Bi-stable rotation is realized for a gold-coated Janus colloidal particle in an infrared optical trap. The metal coating on the Janus particles are patterned by sputtering gold on a monolayer of closely packed polystyrene particles. The Janus particle is observed to stably rotate in an optical trap. Both the direction and the rate of rotation can be experimentally controlled. Numerical calculations reveal that the bi-stable rotation is the result of spontaneous symmetry breaking induced by the uneven curvature of the coating patterns on the Janus sphere. Our results thus provide a simple method to construct large quantities of fully functional rotary motors for nano- or micro-devices.


Targeted binding of nucleocapsid protein transforms the folding landscape of HIV-1 TAR RNA

Micah J. McCauley, Ioulia Rouzina, Kelly A. Manthei, Robert J. Gorelick, Karin Musier-Forsyth, and Mark C. Williams

Retroviral nucleocapsid (NC) proteins are nucleic acid chaperones that play a key role in the viral life cycle. During reverse transcription, HIV-1 NC facilitates the rearrangement of nucleic acid secondary structure, allowing the transactivation response (TAR) RNA hairpin to be transiently destabilized and annealed to a cDNA hairpin. It is not clear how NC specifically destabilizes TAR RNA but does not strongly destabilize the resulting annealed RNA–DNA hybrid structure, which must be formed for reverse transcription to continue. By combining single-molecule optical tweezers measurements with a quantitative mfold-based model, we characterize the equilibrium TAR stability and unfolding barrier for TAR RNA. Experiments show that adding NC lowers the transition state barrier height while also dramatically shifting the barrier location. Incorporating TAR destabilization by NC into the mfold-based model reveals that a subset of preferential protein binding sites is responsible for the observed changes in the unfolding landscape, including the unusual shift in the transition state. We measure the destabilization induced at these NC binding sites and find that NC preferentially targets TAR RNA by binding to specific sequence contexts that are not present on the final annealed RNA–DNA hybrid structure. Thus, specific binding alters the entire RNA unfolding landscape, resulting in the dramatic destabilization of this specific structure that is required for reverse transcription.


Tuesday, October 27, 2015

Stochastic heating of a single Brownian particle by charge fluctuations in a radio-frequency produced plasma sheath

Christian Schmidt and Alexander Piel

The Brownian motion of a single particle in the plasma sheath is studied to separate the effect of stochastic heating by charge fluctuations from heating by collective effects. By measuring the particle velocities in the ballistic regime and by carefully determining the particle mass from the Epstein drag it is shown that for a pressure of 10 Pa, which is typical of many experiments, the proper kinetic temperature of the Brownian particle remains close to the gas temperature and rises only slightly with particle size. This weak effect is confirmed by a detailed model for charging and charge fluctuations in the sheath. A substantial temperature rise is found for decreasing pressure, which approximately shows the expected scaling with p−2. The system under study is an example for non-equilibrium Brownian motion under the influence of white noise without corresponding dissipation.


NF2/Merlin mediates contact-dependent inhibition of EGFR mobility and internalization via cortical actomyosin

Christine Chiasson-MacKenzie, Zachary S. Morris, Quentin Baca, Brett Morris, Joanna K. Coker, Rossen Mirchev, Anne E. Jensen, Thomas Carey, Shannon L. Stott, David E. Golan, and Andrea I. McClatchey

The proliferation of normal cells is inhibited at confluence, but the molecular basis of this phenomenon, known as contact-dependent inhibition of proliferation, is unclear. We previously identified the neurofibromatosis type 2 (NF2) tumor suppressor Merlin as a critical mediator of contact-dependent inhibition of proliferation and specifically found that Merlin inhibits the internalization of, and signaling from, the epidermal growth factor receptor (EGFR) in response to cell contact. Merlin is closely related to the membrane–cytoskeleton linking proteins Ezrin, Radixin, and Moesin, and localization of Merlin to the cortical cytoskeleton is required for contact-dependent regulation of EGFR. We show that Merlin and Ezrin are essential components of a mechanism whereby mechanical forces associated with the establishment of cell–cell junctions are transduced across the cell cortex via the cortical actomyosin cytoskeleton to control the lateral mobility and activity of EGFR, providing novel insight into how cells inhibit mitogenic signaling in response to cell contact.


Energy Landscape of Alginate-Epimerase Interactions Assessed by Optical Tweezers and Atomic Force Microscopy

Armend Gazmeno Håti, Finn Lillelund Aachmann, Bjørn Torger Stokke, Gudmund Skjåk-Bræk, Marit Sletmoen

Mannuronan C-5 epimerases are a family of enzymes that catalyze epimerization of alginates at the polymer level. This group of enzymes thus enables the tailor-making of various alginate residue sequences to attain various functional properties, e.g. viscosity, gelation and ion binding. Here, the interactions between epimerases AlgE4 and AlgE6 and alginate substrates as well as epimerization products were determined. The interactions of the various epimerase–polysaccharide pairs were determined over an extended range of force loading rates by the combined use of optical tweezers and atomic force microscopy. When studying systems that in nature are not subjected to external forces the access to observations obtained at low loading rates, as provided by optical tweezers, is a great advantage since the low loading rate region for these systems reflect the properties of the rate limiting energy barrier. The AlgE epimerases have a modular structure comprising both A and R modules, and the role of each of these modules in the epimerization process were examined through studies of the A- module of AlgE6, AlgE6A. Dynamic strength spectra obtained through combination of atomic force microscopy and the optical tweezers revealed the existence of two energy barriers in the alginate-epimerase complexes, of which one was not revealed in previous AFM based studies of these complexes. Furthermore, based on these spectra estimates of the locations of energy transition states (xβ), lifetimes in the absence of external perturbation (τ0) and free energies (ΔG#) were determined for the different epimerase–alginate complexes. This is the first determination of ΔG# for these complexes. The values determined were up to 8 kBT for the outer barrier, and smaller values for the inner barriers. The size of the free energies determined are consistent with the interpretation that the enzyme and substrate are thus not tightly locked at all times but are able to relocate. Together with the observed different affinities determined for AlgE4-polymannuronic acid (poly-M) and AlgE4-polyalternating alginate (poly-MG) macromolecular pairs these data give important contribution to the growing understanding of the mechanisms underlying the processive mode of these enzymes.


Optical force on toroidal nanostructures: Toroidal dipole versus renormalized electric dipole

Xu-Lin Zhang, S. B. Wang, Zhifang Lin, Hong-Bo Sun, and C. T. Chan

We study the optical forces acting on toroidal nanostructures. A great enhancement of optical force is unambiguously identified as originating from the toroidal dipole resonance based on the source representation, where the distribution of the induced charges and currents is characterized by the three families of electric, magnetic, and toroidal multipoles. On the other hand, the resonant optical force can also be completely attributed to an electric dipole resonance in the alternative field representation, where the electromagnetic fields in the source-free region are expressed by two sets of electric and magnetic multipole fields based on symmetry. The confusion is resolved by conceptually introducing the irreducible electric dipole, toroidal dipole, and renormalized electric dipole. We demonstrate that the optical force is a powerful tool to identify toroidal response even when its scattering intensity is dwarfed by the conventional electric and magnetic multipoles.


Monday, October 26, 2015

Efficient mass transport by optical advection

Veerachart Kajorndejnukul, Sergey Sukhov & Aristide Dogariu

Advection is critical for efficient mass transport. For instance, bare diffusion cannot explain the spatial and temporal scales of some of the cellular processes. The regulation of intracellular functions is strongly influenced by the transport of mass at low Reynolds numbers where viscous drag dominates inertia. Mimicking the efficacy and specificity of the cellular machinery has been a long time pursuit and, due to inherent flexibility, optical manipulation is of particular interest. However, optical forces are relatively small and cannot significantly modify diffusion properties. Here we show that the effectiveness of microparticle transport can be dramatically enhanced by recycling the optical energy through an effective optical advection process. We demonstrate theoretically and experimentally that this new advection mechanism permits an efficient control of collective and directional mass transport in colloidal systems. The cooperative long-range interaction between large numbers of particles can be optically manipulated to create complex flow patterns, enabling efficient and tunable transport in microfluidic lab-on-chip platforms.


Tethered cells in fluid flows—beyond the Stokes' drag force approach

Johan Zakrisson, Krister Wiklund, Ove Axner and Magnus Andersson

Simulations of tethered cells in viscous sub-layers are frequently performed using the Stokes' drag force, but without taking into account contributions from surface corrections, lift forces, buoyancy, the Basset force, the cells' finite inertia, or added mass. In this work, we investigate to what extent such contributions, under a variety of hydrodynamic conditions, influence the force at the anchor point of a tethered cell and the survival probability of a bacterium that is attached to a host by either a slip or a catch bond via a tether with a few different biomechanical properties. We show that a consequence of not including some of these contributions is that the force to which a bond is exposed can be significantly underestimated; in general by ~32–46%, where the influence of the surface corrections dominate (the parallel and normal correction coefficients contribute ~5–8 or ~23–26%, respectively). The Basset force is a major contributor, up to 20%, for larger cells and shear rates. The lift force and inertia contribute when cells with radii >3 μm have shear rates >2000 s−1. Buoyancy contributes significantly for cells with radii >3 μm for shear rates <10 s−1. Since the lifetime of a bond depends strongly on the force, both the level of approximation and the biomechanical model of the tether significantly affect the survival probability of tethered bacteria. For a cell attached by a FimH–mannose bond and an extendable tether with a shear rate of 3000 s−1, neglecting the surface correction coefficients or the Basset force can imply that the survival probability is overestimated by more than an order of magnitude. This work thus shows that in order to quantitatively assess bacterial attachment forces and survival probabilities, both the fluid forces and the tether properties need to be modeled accurately.


IRSp53 senses negative membrane curvature and phase separates along membrane tubules

Coline Prévost, Hongxia Zhao, John Manzi, Emmanuel Lemichez, Pekka Lappalainen, Andrew Callan-Jones & Patricia Bassereau

BAR domain proteins contribute to membrane deformation in diverse cellular processes. The inverted-BAR (I-BAR) protein IRSp53, for instance, is found on the inner leaflet of the tubular membrane of filopodia; however its role in the formation of these structures is incompletely understood. Here we develop an original assay in which proteins are encapsulated in giant unilamellar vesicles connected to membrane nanotubes. Our results demonstrate that I-BAR dimers sense negative membrane curvature. Experiment and theory reveal that the I-BAR displays a non-monotonic sorting with curvature, and expands the tube at high imposed tension while constricting it at low tension. Strikingly, at low protein density and tension, protein-rich domains appear along the tube. This peculiar behaviour is due to the shallow intrinsic curvature of I-BAR dimers. It allows constriction of weakly curved membranes coupled to local protein enrichment at biologically relevant conditions. This might explain how IRSp53 contributes in vivo to the initiation of filopodia.


Atmospherically relevant core–shell aerosol studied using optical trapping and Mie scattering

S. H. Jones, M. D. King and A. D. Ward

Solid core–liquid shell aerosols have been trapped in a counter-propagating optical trap confirming potential core–shell morphology in the atmosphere. Mie spectroscopy can be used to measure the core radius and film thickness to 0.5 and 1 nm precision respectively and to measure the wavelength dependent refractive indices of silica (core) and oleic acid (shell).


Wednesday, October 21, 2015

Cooling and manipulation of a levitated nanoparticle with an optical fiber trap

Pau Mestres, Johann Berthelot, Marko Spasenović, Jan Gieseler, Lukas Novotny and Romain Quidant

Accurate delivery of small targets in high vacuum is a pivotal task in many branches of science and technology. Beyond the different strategies developed for atoms, proteins, macroscopic clusters, and pellets, the manipulation of neutral particles over macroscopic distances still poses a formidable challenge. Here, we report an approach based on a mobile optical trap operated under feedback control that enables cooling and long range 3D manipulation of a silica nanoparticle in high vacuum. We apply this technique to load a single nanoparticle into a high-finesse optical cavity through a load-lock vacuum system. We foresee our scheme to benefit the field of optomechanics with levitating nano-objects as well as ultrasensitive detection and monitoring.


Design of a simple, low-cost, computer-controlled, dual-beam optical tweezer system

C.J. Firby, K.N. Smith, S.R. Gilroy, A. Porisky, A.Y. Elezzabi

We present the design of a simple optical tweezer system. Our system modifies a simple compound microscope to provide one stationary and one steerable trap. Vertical integration of the optical components results in a device with a small footprint that is both compact and portable. Motorized mounting systems are constructed to achieve precise trap motion in three dimensions. Control and image acquisition are performed via an intuitive computer interface. Common and readily obtainable components were incorporated into the apparatus to reduce cost and complexity. The system was used to successfully trap and manipulate yeast cells, and by tuning the laser power, trapping can easily be extended to a wide range of biological and dielectric samples.


Rotation of two trapped microparticles in vacuum: observation of optically mediated parametric resonances

Yoshihiko Arita, Michael Mazilu, Tom Vettenburg, Ewan M. Wright, and Kishan Dholakia

We demonstrate trapping and rotation of two mesoscopic particles in vacuum using a spatial-light-modulator-based approach to trap more than one particle, induce controlled rotation of individual particles, and mediate interparticle separation. By trapping and rotating two vaterite particles, we observe intensity modulation of the scattered light at the sum and difference frequencies with respect to the individual rotation rates. This first demonstration of optical interference between two microparticles in vacuum leads to a platform to potentially explore optical binding and quantum friction effects.


Columnar deformation of human red blood cell by highly localized fiber optic Bessel beam stretcher

Sungrae Lee, Boram Joo, Pyo Jin Jeon, Seongil Im, and Kyunghwan Oh
A single human red blood cell was optically stretched along two counter-propagating fiber-optic Bessel-like beams in an integrated lab-on-a-chip structure. The beam enabled highly localized stretching of RBC, and it induced a nonlinear mechanical deformation to finally reach an irreversible columnar shape that has not been reported. We characterized and systematically quantified this optically induced mechanical deformation by the geometrical aspect ratio of stretched RBC and the irreversible stretching time. The proposed RBC mechanism can realize a versatile and compact opto-mechanical platform for optical diagnosis of biological substances in the single cell level.


Tuesday, October 20, 2015

Dwell-Time Distribution, Long Pausing and Arrest of Single-Ribosome Translation through the mRNA Duplex

Ping Xie

Proteins in the cell are synthesized by a ribosome translating the genetic information encoded on the single-stranded messenger RNA (mRNA). It has been shown that the ribosome can also translate through the duplex region of the mRNA by unwinding the duplex. Here, based on our proposed model of the ribosome translation through the mRNA duplex we study theoretically the distribution of dwell times of the ribosome translation through the mRNA duplex under the effect of a pulling force externally applied to the ends of the mRNA to unzip the duplex. We provide quantitative explanations of the available single molecule experimental data on the distribution of dwell times with both short and long durations, on rescuing of the long paused ribosomes by raising the pulling force to unzip the duplex, on translational arrests induced by the mRNA duplex and Shine-Dalgarno(SD)-like sequence in the mRNA. The functional consequences of the pauses or arrests caused by the mRNA duplex and the SD sequence are discussed and compared with those obtained from other types of pausing, such as those induced by “hungry” codons or interactions of specific sequences in the nascent chain with the ribosomal exit tunnel.


Dynamics of biconcave vesicles in a confined shear flow

Zheng Yuan Luo, Bo Feng Bai

To study the dynamics of red blood cells (RBCs) in confined shear flows is essential to understand the flow behavior of RBCs in capillaries and in microfluidics, especially when the length scale of the flow is comparable to the RBC size. However, previous studies are focused on RBC dynamics in unbounded shear flows, and only a limited number of cases concern RBC dynamics in confined shear flows by using two-dimensional (2D) vesicle model. Here, we develop a more native-mimicking three-dimensional (3D) model to investigate RBC dynamics in confined shear flows. The present model can reproduce experimental data of the deformation of healthy RBCs under the stretching of optical tweezers. Using the validated model, we find that RBCs in a 3D confined shear flow can exhibit breathing, swinging and tumbling motion. Particularly, breathing and swinging motion were observed in previous 3D simulations of unbounded shear flows, but in previous 2D simulations of confined shear flows only tumbling and tank-treading motion were observed. In confined shear flows, the deformation of RBCs is significantly promoted by increasing the confinement (i.e., the ratio of the RBC size to the channel size), thus breathing motion is preferable over tumbling motion. By increasing confinement alone, the dynamical state of RBCs can transit from the tumbling to the breathing via an intermittent state (i.e., breathing with a weak tumbling). These results provide new insights into the dynamics of RBCs suspended in confined shear flows, and can be helpful for further studies on the dynamics of RBC suspensions in capillaries and in microfluidics.


Antibody-mediated disruption of the mechanics of CS20 fimbriae of enterotoxigenic Escherichia coli

Bhupender Singh, Narges Mortezaei, Bernt Eric Uhlin, Stephen J. Savarino, Esther Bullitt & Magnus Andersson

Preventive vaccines against enterotoxigenic Escherichia coli (ETEC) are being developed, many of which target common fimbrial colonization factors as the major constituent, based on empirical evidence that these function as protective antigens. Particularly, passive oral administration of ETEC anti-fimbrial antibodies prevent ETEC diarrhea. Little is, however, known regarding the specific mechanisms by which intestinal antibodies against ETEC fimbriae function to prevent disease. Using coli surface antigen 20 (CS20) fimbriae as a model ETEC colonization factor, we show using force spectroscopy that anti-fimbrial antibodies diminish fimbrial elasticity by inhibiting their natural capacity to unwind and rewind. In the presence of anti-CS20 antibodies the force required to unwind a single fimbria was increased several-fold and the extension length was shortened several-fold. Similar measurements in the presence of anti-CS20 Fab fragments did not show any effect, indicating that bivalent antibody binding is required to reduce fimbrial elasticity. Based on these findings, we propose a model for an in-vivo mechanism whereby antibody-mediated disruption of the biomechanical properties of CS20 fimbriae impedes sustained adhesion of ETEC to the intestinal mucosal surface. Further elucidation of the role played by intestinal antibodies in mechanical disruption of fimbrial function may provide insights relevant to ETEC vaccine development.


Controlling Cooperativity in β-Cyclodextrin–DNA Binding Reactions

P. S. Alves, O. N. Mesquita, and M. S. Rocha

We have investigated the interaction between the native neutral β-cyclodextrin (CD) and the DNA molecule by performing single-molecule stretching experiments with optical tweezers. In particular, we have monitored the changes of the mechanical properties of the CD–DNA complexes as a function of the CD concentration in the sample. By using a quenched disorder statistical model, we were also capable to extract important physicochemical information (equilibrium binding constants, cooperativity degree) of such interaction from the mechanical data. In addition, we have found that the interaction occurs by two different mechanisms, first with the formation of relatively large CD clusters along the double helix, which thereafter can locally denature the DNA molecule by forming hydrogen bonds with the base pairs that eventually flip out. A prediction of our quenched disorder model was that cooperativity could be controlled by adjusting the surface charge of β-CD molecules. This prediction is confirmed in the present work.


Friday, October 16, 2015

Direct measurements of forces induced by Bloch surface waves in a one-dimensional photonic crystal

Daniil A. Shilkin, Evgeny V. Lyubin, Irina V. Soboleva, and Andrey A. Fedyanin

An experimental study of the interaction between a single dielectric microparticle and the evanescent field of the Bloch surface wave in a one-dimensional (1D) photonic crystal is reported. The Bloch surface wave-induced forces on a 1 μm polystyrene sphere were measured by photonic force microscopy. The results demonstrate the potential of 1D photonic crystals for the optical manipulation of microparticles and suggest a novel approach for utilizing light in lab-on-a-chip devices.


Wide-field three-dimensional optical imaging using temporal focusing for holographically trapped microparticles

Roman Spesyvtsev, Helen A. Rendall, and Kishan Dholakia

A contemporary challenge across the natural sciences is the simultaneous optical imaging or stimulation of small numbers of cells or colloidal particles organized into arbitrary geometries. We demonstrate the use of temporal focusing with holographic optical tweezers in order to achieve depth-resolved two-photon imaging of trapped objects arranged in arbitrary three-dimensional (3D) geometries using a single objective. Trapping allows for the independent position control of multiple objects by holographic beam shaping. Temporal focusing of ultrashort pulses provides the wide-field two-photon depth-selective activation of fluorescent samples. We demonstrate the wide-field depth-resolved illumination of both trapped fluorescent beads and trapped HL60 cells in suspension with full 3D positioning control. These approaches are compatible with implementation through scattering media and can be beneficial for emergent studies in colloidal science and particularly optogenetics, offering targeted photoactivation over a wide area with micrometer-precision depth control.


Localized optical manipulation in optical ring resonators

Haotian Wang, Xiang Wu, and Deyuan Shen

We propose a tunable optical trapping system for nanoparticles based on generating standing wave by coupling two coherent beams into a ring resonator in opposite directions, respectively. The distributions of the mode field excited in three types of the ring-resonators-based trapping systems (microring, microdisk and slot ring) and the corresponding optical forces on the nanoparticles are calculated numerically. By the stability analysis in all directions, the smallest size of the particles could be stably trapped under the Brownian motion in the microring resonator is 61.2 nm when the input power is 10 mW, and the azimuthal orientations of the trapped particles are depended on the phase difference between the two input beams. On the other hand, the appearance of high order radial modes in the microdisk resonator enables a tunable radial trapping. To improve the trapping capability for the smaller particles, we utilize the slot ring resonator to make full use of the optical power and the trapping size could be minimized to ~29 nm when the input power is also set as 10 mW.


Bohr's 'Light and Life' revisited

H M Nussenzveig

I revisit Niels Bohr's famous 1932 'Light and Life' lecture, confronting it with current knowledge. Topics covered include: life origin and evolution, quantum mechanics and life, brain and mind, consciousness and free will, and light as a tool for biology, with special emphasis on optical tweezers and their contributions to biophysics. Specialized knowledge of biology is not assumed.


Thursday, October 15, 2015

Optical trapping and control of a dielectric nanowire by a nanoaperture

Mehdi Shafiei Aporvari, Fardin Kheirandish, and Giovanni Volpe

We demonstrate that a single sub-wavelength nanoaperture in a metallic thin film can be used to achieve dynamic optical trapping and control of a single dielectric nanowire. A nanoaperture can trap a nanowire, control its orientation when illuminated by a linearly polarized incident field, and rotate the nanowire when illuminated by a circularly polarized incident field. Compared to other designs, this approach has the advantage of a low-power driving field entailing low heating and photodamage.


Transportation of Multiple Biological Cells through Saturation-controlled Optical Tweezers in Crowded Microenvironments

Chen, H. ; Wang, C. ; Li, X. ; Sun, D.

Transportation of biological cells has attracted increased attention in bioscience and nanomedicine. Existing approaches to achieve automated multi-cell transportation are generally based on numerous over-strict conditions or assumptions, including static and clean environments, complex theoretical convergence cond itions, omitting tool kinematics, and off-line calibrations. This paper presents a novel approach for the automated transportation of multiple cells by using robotically controlled holographic optical tweezers. First, a swarming controller was developed with easily satisfied convergence conditions. The offset between centers of the cell and optical tweezers was constrained by saturation control to maintain the cells in the optically trapping area. An artificial first-order kinematic model of the tweezers was considered in the controller design to reduce controller oscillation. Second, a solution to the collision avoidance of random-moving obstacles was developed to remove the assumption of static or clean environments. Finally, an automated method based on the drag force model and gradient descent optimization was presented to calibrate cell dynamics online. Experiments on yeast cells were performed to verify the effectiveness of the proposed approach.


Raman Fingerprinting of Single Dielectric Nanoparticles in Plasmonic Nanopores

Sarp Kerman, Chang Chen, Yi Li, Wim VanRoy, Liesbet Lagae and Pol Van Dorpe

Plasmonic nano-apertures are commonly used for the detection of small particles such as nanoparticles and proteins by exploiting electrical and optical techniques. Plasmonic nanopores are metallic nano-apertures sitting on a thin membrane with a tiny hole. It has been shown that plasmonic nanopores with a given geometry identify internal molecules using Surface Enhanced Raman Spectroscopy (SERS). However, label-free identification of a single dielectric nanoparticle requires a highly localized field comparable to the size of the particle. Additionally, the particle’s Brownian motion can jeopardize the amount of photons collected from a single particle. Here, we demonstrate that the combination of optical trapping and SERS can be used for the detection and identification of 20 nm polystyrene nanoparticles in plasmonic nanopores. This work is anticipated to contribute to the detection of small bioparticles, optical trapping and nanotribology studies.


Hot Brownian thermometry and cavity-enhanced harmonic generation with nonlinear optical nanowires

Bennett E. Smith, Paden B. Roder, Xuezhe Zhou, Peter J. Pauzauskie

The non-linear response of nanoscale materials is affected by phase-matching conditions that are critically dependent on temperature. It has remained a persistent challenge to measure the temperature of individual nonlinear optical nanostructures in situ. Here, we measure the temperature of individual KNbO3 nanowires in an optical trap through analysis of the Brownian dynamics. Additionally, we show Fabry-Pérot-type cavity enhancement of second harmonic generation in one-dimensional potassium niobate nanowires (KNNW) using a tunable, continuous wave (CW), near-infrared (NIR) trapping laser. A second co-aligned CW NIR laser is used to extend the range of visible emission through sum frequency generation.


Tuesday, October 13, 2015

Energy, momentum and propagation of non-paraxial high-order Gaussian beams in the presence of an aperture

Alexander B Stilgoe, Timo A Nieminen and Halina Rubinsztein-Dunlop

Non-paraxial theories of wave propagation are essential to model the interaction of highly focused light with matter. Here we investigate the energy, momentum and propagation of the Laguerre–, Hermite– and Ince–Gaussian solutions (LG, HG, and IG) of the paraxial wave equation in an apertured non-paraxial regime. We investigate the far-field relationships between the LG, HG, and IG solutions and the vector spherical wave function (VSWF) solutions of the vector Helmholtz wave equation. We investigate the convergence of the VSWF and the various Gaussian solutions in the presence of an aperture. Finally, we investigate the differences in linear and angular momentum evaluated in the paraxial and non-paraxial regimes. The non-paraxial model we develop can be applied to calculations of the focusing of high-order Gaussian modes in high-resolution microscopes. We find that the addition of an aperture in high numerical aperture optical systems does not greatly affect far-field properties except when the beam is significantly clipped by an aperture. Diffraction from apertures causes large distortions in the near-field and will influence light–matter interactions. The method is not limited to a particular solution of the paraxial wave equation. Our model is constructed in a formalism that is commonly used in scattering calculations. It is thus applicable to optical trapping and other optical investigations of matter.


Lateral chirality-sorting optical forces

Amaury Hayat, J. P. Balthasar Mueller, and Federico Capasso

The transverse component of the spin angular momentum of evanescent waves gives rise to lateral optical forces on chiral particles, which have the unusual property of acting in a direction in which there is neither a field gradient nor wave propagation. Because their direction and strength depends on the chiral polarizability of the particle, they act as chirality-sorting and may offer a mechanism for passive chirality spectroscopy. The absolute strength of the forces also substantially exceeds that of other recently predicted sideways optical forces.


Thursday, October 8, 2015

Detection of eccentricity in silver nanotubes by means of induced optical forces and torques

R M Abraham Ekeroth and M F Lester

In previous works, we have conducted an exhaustive study about optical properties of metallic realistic two-dimensional (2D) nanotubes, using an experimental-interpolated dielectric function. In the case of non-homogeneous metallic shells, we suggested (in a theoretical form) a procedure to detect the non-uniformity of shells in parallel, disperse and randomly oriented long nanotubes (2D system). This detection is based exclusively on the plasmonic properties of the response. Here we consider exact calculations of forces and torques, exerted by light on these kinds of nanostructures, illustrating the mechanical effects of plasmonic excitations with one example of silver shell under p-polarized incidence. This study continues with the methodology implemented in the previous paper, for homogeneous nanotubes. The features of the electromagnetic interaction in these structures, from the point of view of mechanical magnitudes, make it possible to conceive new possible interesting applications. Particularly, we point out some results regarding detection of eccentricity in nanotubes in vacuum (when Brownian movement is not taken into account). We interpret the optical response of the realistic shells in the framework of plasmon hybridization model (PHM), which is deduced from a quasi-static approximation. Our integral formalism provides for retardation effects and possible errors is only due to its numerical implementation.


A measurement of the maximal forces in plasmonic tweezers

Jung-Dae Kim, Jun-Hee Choi and Yong-Gu Lee

Plasmonic tweezers that are designed to trap nanoscale objects create many new possibilities for single-molecule targeted studies. Numerous novel designs of plasmonic nanostructures are proposed in order to attain stronger forces and weaker laser intensity. Most experiments have consisted only of immobilization observations—that is, particles stick when the laser is turned on and fall away when the laser is turned off. Studies of the exertable forces were only theoretical. A few studies have experimentally measured trap stiffness. However, as far as we know, no studies have addressed maximal forces. In this paper, we present a new experimental design in which the motion of the trapped particle can be monitored in either parallel or orthogonal directions to the plasmonic structure's symmetric axis. We measured maximal trapping force through such monitoring. Although stiffness would be useful for force-calibration or immobilization purposes, for which most plasmonic tweezers are used, we believe that the maximal endurable force is significant and thus, this paper presents this aspect.


Optical manipulation in optofluidic microbubble resonators

HaoTian Wang, Xiang Wu
An optical manipulation system based on optofluidic microbubble resonators (MBR) is proposed. As the high-Q whispering gallery modes (WGMs) are excited in an MBR, the buildup of the field intensity inside the resonator is large enough to trap nanoscale particles. The optical gradient forces generated by the WGMs with different radial orders are investigated numerically. The negative effect of the resonance detuning induced by the particles is taken into account to investigate the optical gradient forces exerting on the particles. By the stability analysis, the WGMs with high radial orders show a better trapping stability under Brownian motion since most of the optical fields reside within the water core.


Measurement of particle motion in optical tweezers embedded in a Sagnac interferometer

Ivan Galinskiy, Oscar Isaksson, Israel Rebolledo Salgado, Mathieu Hautefeuille, Bernhard Mehlig, and Dag Hanstorp

We have constructed a counterpropagating optical tweezers setup embedded in a Sagnac interferometer in order to increase the sensitivity of position tracking for particles in the geometrical optics regime. Enhanced position determination using a Sagnac interferometer has previously been described theoretically by Taylor et al. [Journal of Optics 13, 044014 (2011)] for Rayleigh-regime particles trapped in an antinode of a standing wave. We have extended their theory to a case of arbitrarily-sized particles trapped with orthogonally-polarized counter-propagating beams. The working distance of the setup was sufficiently long to optically induce particle oscillations orthogonally to the axis of the tweezers with an auxiliary laser beam. Using these oscillations as a reference, we have experimentally shown that Sagnac-enhanced back focal plane interferometry is capable of providing an improvement of more than 5 times in the signal-to-background ratio, corresponding to a more than 30-fold improvement of the signal-to-noise ratio. The experimental results obtained are consistent with our theoretical predictions. In the experimental setup, we used a method of optical levitator-assisted liquid droplet delivery in air based on commercial inkjet technology, with a novel method to precisely control the size of droplets.


Wednesday, October 7, 2015

Optical trapping and manipulation of light-absorbing particles by means of a Hermite–Gaussian laser beam

A. P. Porfirev and R. V. Skidanov

This paper describes simultaneous optical trapping and displacement of several nonspherical light-absorbing microparticles in air by means of a focused Hermite–Gaussian laser beam (laser mode TEM10, radiation wavelength 457 nm). Agglomerations of carbon nanoparticles are displaced in three dimensions by distances of the order of a millimeter. The structure of the Hermite–Gaussian beam used here made it possible to displace microparticles simultaneously along two parallel trajectories in space.


Measuring Pushing and Braking Forces Generated by Ensembles of Kinesin-5 Crosslinking Two Microtubules

Yuta Shimamoto, Scott Forth, Tarun M. Kapoor

The proper organization of the microtubule-based mitotic spindle is proposed to depend on nanometer-sized motor proteins generating forces that scale with a micron-sized geometric feature, such as microtubule overlap length. However, it is unclear whether such regulation can be achieved by any mitotic motor protein. Here, we employ an optical-trap- and total internal reflection fluorescence (TIRF)-based assay to show that ensembles of kinesin-5, a conserved mitotic motor protein, can push apart overlapping antiparallel microtubules to generate a force whose magnitude scales with filament overlap length. We also find that kinesin-5 can produce overlap-length-dependent “brake-like” resistance against relative microtubule sliding in both parallel and antiparallel geometries, an activity that has been suggested by cell biological studies but had not been directly measured. Together, these findings, along with numerical simulations, reveal how a motor protein can function as an analog converter, “reading” simple geometric and dynamic features in cytoskeletal networks to produce regulated force outputs.


Slow water transport in MgSO4 aerosol droplets at gel-forming relative humidities

Chen Cai, See-Hua Tan, Hong-Nan Chen, Jia-Bi Ma, Yang Wang, Jonathan P Reid and Yun Hong Zhang

The effect of gel formation on the mass transfer of water during evaporation or condensation from magnesium sulphate droplets is studied using an aerosol optical tweezers coupled with Raman spectroscopy. In particular, the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies. Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol. Desorption of water continues over a long time at very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of the water adsorption isotherm. The observations can be rationalized when considering the possible phase transformation of gel structure at very low RH. Finally, the influences of the duration of the drying time (RH ≤ 10%) on the kinetics of condensation during hydration are investigated. Apparent diffusion coefficients of water molecules in the gel are obtained, showing little dependence on the water activity and droplet composition, and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.


Optical trapping of a dielectric-covered metallic microsphere

Shubo Cheng, Shaohua Tao, Conghua Zhou and Liang Wu

In this paper we propose a ray-optics model of single-beam gradient trap for a dielectric-covered metallic microsphere and calculate the optical forces exerted by a single incident optical ray striking the microsphere. The axial trapping forces experienced by the copper and silver microspheres covered with polystyrene are analyzed, respectively. The results show that, unlike the case for a purely dielectric Mie particle, the trapping positions for a metallic microsphere covered with dielectric depend on the thickness of the dielectric shell. In the experiments copper particles covered with polymer are trapped in the bright region rather than in the dark region of a focused vortex beam and rotate owing to the orbital angular momentum of the beam. The experimental results are in good agreement with the theoretical analysis.


Tuesday, October 6, 2015

Internal optical forces in plasmonic nanostructures

T. V. Raziman and Olivier J. F. Martin

We present a computational study of the internal optical forces arising in plasmonic gap antennas, dolmen structures and split rings. We find that very strong internal forces perpendicular to the propagation direction appear in these systems. These internal forces show a rich behaviour with varying wavelength, incident polarisation and geometrical parameters, which we explain in terms of the polarisation charges induced on the structures. Various interesting and anomalous features arise such as lateral force reversal, optical pulling force, and circular polarisation-induced forces and torques along directions symmetry-forbidden for orthogonal linear polarisations. Understanding these effects and mastering internal forces in plasmonic nanostructures will be instrumental in implementing new functionalities in these nanophotonic systems.


Measuring Local Viscosities near Plasma Membranes of Living Cells with Photonic Force Microscopy

Felix Jünger, Felix Kohler, Andreas Meinel, Tim Meyer, Roland Nitschke, Birgit Erhard, Alexander Rohrbach

The molecular processes of particle binding and endocytosis are influenced by the locally changing mobility of the particle nearby the plasma membrane of a living cell. However, it is unclear how the particle’s hydrodynamic drag and momentum vary locally and how they are mechanically transferred to the cell. We have measured the thermal fluctuations of a 1 μm-sized polystyrene sphere, which was placed in defined distances to plasma membranes of various cell types by using an optical trap and fast three-dimensional (3D) interferometric particle tracking. From the particle position fluctuations on a 30 μs timescale, we determined the distance-dependent change of the viscous drag in directions perpendicular and parallel to the cell membrane. Measurements on macrophages, adenocarcinoma cells, and epithelial cells revealed a significantly longer hydrodynamic coupling length of the particle to the membrane than those measured at giant unilamellar vesicles (GUVs) or a plane glass interface. In contrast to GUVs, there is also a strong increase in friction and in mean first passage time normal to the cell membrane. This hydrodynamic coupling transfers a different amount of momentum to the interior of living cells and might serve as an ultra-soft stimulus triggering further reactions.


Real-time observation of the initiation of RNA polymerase II transcription

Furqan M. Fazal, Cong A. Meng, Kenji Murakami, Roger D. Kornberg & Steven M. Block

Biochemical and structural studies have shown that the initiation of RNA polymerase II transcription proceeds in the following stages: assembly of the polymerase with general transcription factors and promoter DNA in a ‘closed’ preinitiation complex (PIC); unwinding of about 15 base pairs of the promoter DNA to form an ‘open’ complex; scanning downstream to a transcription start site; synthesis of a short transcript, thought to be about 10 nucleotides long; and promoter escape. Here we have assembled a 32-protein, 1.5-megadalton PIC derived from Saccharomyces cerevisiae, and observe subsequent initiation processes in real time with optical tweezers. Contrary to expectation, scanning driven by the transcription factor IIH involved the rapid opening of an extended transcription bubble, averaging 85 base pairs, accompanied by the synthesis of a transcript up to the entire length of the extended bubble, followed by promoter escape. PICs that failed to achieve promoter escape nevertheless formed open complexes and extended bubbles, which collapsed back to closed or open complexes, resulting in repeated futile scanning.


Biomedical Optics Express feature issue introduction: optical trapping applications (OTA)

Peter Reece and Steven Neale
This feature issue of Biomedical Optics Express presents studies which were the focus of the fourth OTA Topical Meeting that was held on 12–15 April 2015 in Vancouver, Canada.


Monday, October 5, 2015

Magnetically Self-Assembled Colloidal Three-Dimensional Structures as Cell Growth Scaffold

Gašper Kokot, Špela Zemljič Jokhadar, Urška Batista, and Dušan Babič

Understanding the chemical and physical conditions for cell growth is important from biological and medical aspects. Many tissues and cell types (e.g., epithelial cells and neurons) naturally grow on surfaces that span in three-dimensions and offer structural or mechanical support. The scaffold surface has to promote adhesion and cell proliferation as well as support their weight and retain its structural integrity. Here, we present a flexible method that uses self-assembly of micrometer superparamagnetic particles to produce appropriate scaffold surfaces with controllable general appearance in three dimensions, such as oriented membranes, branched structure, or void network. As a proof of principle, the Chinese hamster ovary epithelial cell line was successfully grown for several days on inclined membranes. Robustness of the oriented membrane architecture was probed with optical tweezers. We measured the magnetic force holding one particle in a self-assembled upright hexagonal sheet and modeled it as a sum of pair interaction forces between spatially arrested static dipoles.


Automated Pairing Manipulation of Biological Cells With a Robot-Tweezers Manipulation System

Mingyang Xie; Yong Wang; Gang Feng; Dong Sun

With an increased demand for various cell-based clinical applications and drug discovery, an enable technology that can automatically locate and pair biological cells from different groups, with high precision and throughput, is highly demanded. This paper presents a novel approach to achieving such cell manipulation using an automatically controlled holographic optical tweezers system, where a robotically controlled optical tweezers functions as a special manipulator to transfer cells automatically. The proposed cell pairing approach utilizes the concept of concentric circles for topology design and the artificial potential field functions for controller development. The significance of the proposed method lies in that the preassignment of cell destinations is not needed, the interdistance amongst the paired cells is controllable, and grouping scalability is not limited. Experiments are performed to demonstrate the effectiveness of the proposed approach.


Optical trapping and orientation of Escherichia coli cells using two tapered fiber probes

Jianbin Huang, Xiaoshuai Liu, Yao Zhang, and Baojun Li

We report on the optical trapping and orientation of Escherichia coli (E. coli) cells using two tapered fiber probes. With a laser beam at 980 nm wavelength launched into probe I, an E. coli chain consisting of three cells was formed at the tip of probe I. After launching a beam at 980 nm into probe II, the E. coli at the end of the chain was trapped and oriented via the optical torques yielded by two probes. The orientation of the E. coli was controlled by adjusting the laser power of probe II. Experimental results were interpreted by theoretical analysis and numerical simulations.


Surface imaging beyond the diffraction limit with optically trapped spheres

Lars Friedrich & Alexander Rohrbach
Optical traps play an increasing role in the bionanosciences because of their ability to apply forces flexibly on tiny structures in fluid environments. Combined with particle-tracking techniques, they allow the sensing of miniscule forces exerted on these structures. Similar to atomic force microscopy (AFM), but much more sensitive, an optically trapped probe can be scanned across a structured surface to measure the height profile from the displacements of the probe. Here we demonstrate that, by the combination of a time-shared twin-optical trap and nanometre-precise three-dimensional interferometric particle tracking, both reliable height profiling and surface imaging are possible with a spatial resolution below the diffraction limit. The technique exploits the high-energy thermal position fluctuations of the trapped probe, and leads to a sampling of the surface 5,000 times softer than in AFM. The measured height and force profiles from test structures and Helicobacter cells illustrate the potential to uncover specific properties of hard and soft surfaces.


Multi-dimensional single-spin nano-optomechanics with a levitated nanodiamond

Levi P. Neukirch, Eva von Haartman, Jessica M. Rosenholm & A. Nick Vamivakas

Considerable advances made in the development of nanomechanical and nano-optomechanical devices have enabled the observation of quantum effects, improved sensitivity to minute forces and provided avenues to probe fundamental physics at the nanoscale. Concurrently, solid-state quantum emitters with optically accessible spin degrees of freedom have been pursued in applications ranging from quantum information science to nanoscale sensing. Here, we demonstrate a hybrid nano-optomechanical system composed of a nanodiamond (containing a single nitrogen–vacancy centre) that is levitated in an optical dipole trap. The mechanical state of the diamond is controlled by modulation of the optical trapping potential. We demonstrate the ability to imprint the multi-dimensional mechanical motion of the cavity-free mechanical oscillator into the nitrogen–vacancy centre fluorescence and manipulate the mechanical system's intrinsic spin. This result represents the first step towards a hybrid quantum system based on levitating nanoparticles that simultaneously engages optical, phononic and spin degrees of freedom.


Saturday, October 3, 2015

An overview of micro-force sensing techniques

Yuzhang Wei, Qingsong Xu

Due to the trend of miniaturization of devices, micromanipulation has been a hot topic in the last two decades. Unlike the macro world, the micro object is easy to be damaged if the contact force is not reliably detected and controlled. Hence, micro-force sensing is of great importance in micromanipulation, microassembly, medical applications, biomedical applications, materials science, dimension measurements and MEMS/NEMS for protecting micro-parts and micro-gripper from being damaged and ensuring the success of the manipulation process. This paper presents a survey of the recent methods of micro-force sensing. The working principle, detection accuracy, advantage and disadvantage of seven widely used force sensing methods are presented. Typical applications of each method in micro-assembly and micromanipulation are discussed. In addition, the comparisons among different kinds of force sensing approaches have been addressed. Moreover, another five promising micro-force sensing methods, which are confined to special component measurements or not widely used, are briefly introduced. Furthermore, two popular types of commercial micro-force sensors are listed to provide a guideline of selection for a specific application. The presented state-of-the-art overview is helpful to those engaged in micro-force sensing area to know the recent development and research tendency on micro-force sensing.


Plasmonic Nanopores for Trapping, Controlling Displacement, and Sequencing of DNA

Maxim Belkin, Shu-Han Chao, Magnus P. Jonsson, Cees Dekker, and Aleksei Aksimentiev

With the aim of developing a DNA sequencing methodology, we theoretically examine the feasibility of using nanoplasmonics to control the translocation of a DNA molecule through a solid-state nanopore and to read off sequence information using surface-enhanced Raman spectroscopy. Using molecular dynamics simulations, we show that high-intensity optical hot spots produced by a metallic nanostructure can arrest DNA translocation through a solid-state nanopore, thus providing a physical knob for controlling the DNA speed. Switching the plasmonic field on and off can displace the DNA molecule in discrete steps, sequentially exposing neighboring fragments of a DNA molecule to the pore as well as to the plasmonic hot spot. Surface-enhanced Raman scattering from the exposed DNA fragments contains information about their nucleotide composition, possibly allowing the identification of the nucleotide sequence of a DNA molecule transported through the hot spot. The principles of plasmonic nanopore sequencing can be extended to detection of DNA modifications and RNA characterization.


Trapping and manipulation of microparticles using laser-induced convection currents and photophoresis

E. Flores-Flores, S. A. Torres-Hurtado, R. Páez, U. Ruiz, G. Beltrán-Pérez, S. L. Neale, J. C. Ramirez-San-Juan, and R. Ramos-García

In this work we demonstrate optical trapping and manipulation of microparticles suspended in water due to laser-induced convection currents. Convection currents are generated due to laser light absorption in an hydrogenated amorphous silicon (a:Si-H) thin film. The particles are dragged towards the beam's center by the convection currents (Stokes drag force) allowing trapping with powers as low as 0.8 mW. However, for powers >3 mW trapped particles form a ring around the beam due to two competing forces: Stokes drag and thermo-photophoretic forces. Additionally, we show that dynamic beam shaping can be used to trap and manipulate multiple particles by photophotophoresis without the need of lithographically created resistive heaters.


Relevance of the Drag Force during Controlled Translocation of a DNA–Protein Complex through a Glass Nanocapillary

Roman D. Bulushev, Sanjin Marion, and Aleksandra Radenovic

Combination of glass nanocapillaries with optical tweezers allowed us to detect DNA–protein complexes in physiological conditions. In this system, a protein bound to DNA is characterized by a simultaneous change of the force and ionic current signals from the level observed for the bare DNA. Controlled displacement of the protein away from the nanocapillary opening revealed decay in the values of the force and ionic current. Negatively charged proteins EcoRI, RecA, and RNA polymerase formed complexes with DNA that experienced electrophoretic force lower than the bare DNA inside nanocapillaries. Force profiles obtained for DNA-RecA in our system were different than those in the system with nanopores in membranes and optical tweezers. We suggest that such behavior is due to the dominant impact of the drag force comparing to the electrostatic force acting on a DNA–protein complex inside nanocapillaries. We explained our results using a stochastic model taking into account the conical shape of glass nanocapillaries.