Thursday, April 30, 2015

Measuring Cohesion between Macromolecular Filaments One Pair at a Time: Depletion-Induced Microtubule Bundling

Feodor Hilitski, Andrew R. Ward, Luis Cajamarca, Michael F. Hagan, Gregory M. Grason, and Zvonimir Dogic

In the presence of nonadsorbing polymers, colloidal particles experience ubiquitous attractive interactions induced by depletion forces. Here, we measure the depletion interaction between a pair of microtubule filaments using a method that combines single filament imaging with optical trapping. By quantifying the dependence of filament cohesion on both polymer concentration and solution ionic strength, we demonstrate that the minimal model of depletion, based on the Asakura-Oosawa theory, fails to quantitatively describe the experimental data. By measuring the cohesion strength in two- and three- filament bundles, we verify pairwise additivity of depletion interactions for the specific experimental conditions. The described experimental technique can be used to measure pairwise interactions between various biological or synthetic filaments and complements information extracted from bulk osmotic stress experiments.


Variabilities and uncertainties in characterising water transport kinetics in glassy and ultraviscous aerosol

Andrew M. J. Rickards, Young-Chul Song, Rachael E. H. Miles, Thomas C. Preston and Jonathan P. Reid

We present a comprehensive evaluation of the variabilities and uncertainties present in determining the kinetics of water transport in ultraviscous aerosol droplets, alongside new measurements of the water transport timescale in sucrose aerosol. Measurements are performed on individual droplets captured using aerosol optical tweezers and the change in particle size during water evaporation or condensation is inferred from shifts in the wavelength of the whispering gallery mode peaks at which spontaneous Raman scattering is enhanced. The characteristic relaxation timescale (τ) for condensation or evaporation of water from viscous droplets following a change in gas phase relative humidity can be described by the Kohlrausch–Williams–Watts function. To adequately characterise the water transport kinetics and determine τ, sufficient time must be allowed for the particle to progress towards the final state. However, instabilities in the environmental conditions can prevent an accurate characterisation of the kinetics over such long time frames. Comparison with established thermodynamic and diffusional water transport models suggests the determination of τ is insensitive to the choice of thermodynamic treatment. We report excellent agreement between experimental and simulated evaporation timescales, and investigate the scaling of τ with droplet radius. A clear increase in τ is observed for condensation with increase in drying (wait) time. This trend is qualitatively supported by model simulations.


Geometry-guided colloidal interactions and self-tiling of elastic dipoles formed by truncated pyramid particles in liquid crystals

Bohdan Senyuk, Qingkun Liu, Ephraim Bililign, Philip D. Nystrom, and Ivan I. Smalyukh

The progress of realizing colloidal structures mimicking natural forms of organization in condensed matter is inherently limited by the availability of suitable colloidal building blocks. To enable new forms of crystalline and quasicrystalline self-organization of colloids, we develop truncated pyramidal particles that form nematic elastic dipoles with long-range electrostaticlike and geometry-guided low-symmetry short-range interactions. Using a combination of nonlinear optical imaging, laser tweezers, and video microscopy, we characterize colloidal pair interactions and demonstrate unusual forms of self-tiling of these particles into crystalline, quasicrystalline, and other arrays. Our findings are explained using an electrostatics analogy along with liquid crystal elasticity and symmetry breaking considerations, potentially expanding photonic and electro-optic applications of colloids.


Wide-range Axial Position Measurement for Jumping Behavior of Optically Trapped Microsphere Near Surface Using Chromatic Confocal Sensor

Shin-ichi Ueda, Masaki Michihata, Terutake Hayashi & Yasuhiro Takaya

When a microsphere is trapped near a surface by single-beam gradient force trapping, the standing wave is generated between the microsphere and the surface, where abrupt motion along the optical axis (jumping) is observed corresponding to displacement of the surface. This jumping distance is on the order of a few hundred nanometers. In the vicinity of the surface, intensity of retro-reflected light is increased so that the averaged position of the jumping is shifted up on the order of several micrometers. Therefore wide-range and high-resolution position measurement technique is required. In this article, we proposed to apply a chromatic confocal sensor to measure the axial position of the microsphere in the standing wave. It was experimentally validated that the position of the microsphere could be measured with a resolution of 10 nm and a measuring range of 3 µm.


Wednesday, April 29, 2015

Mechanical force releases nascent chain–mediated ribosome arrest in vitro and in vivo

Daniel H. Goldman, Christian M. Kaiser, Anthony Milin, Maurizio Righini, Ignacio Tinoco Jr., Carlos Bustamante

Protein synthesis rates can affect gene expression and the folding and activity of the translation product. Interactions between the nascent polypeptide and the ribosome exit tunnel represent one mode of regulating synthesis rates. The SecM protein arrests its own translation, and release of arrest at the translocon has been proposed to occur by mechanical force. Using optical tweezers, we demonstrate that arrest of SecM-stalled ribosomes can indeed be rescued by force alone and that the force needed to release stalling can be generated in vivo by a nascent chain folding near the ribosome tunnel exit. We formulate a kinetic model describing how a protein can regulate its own synthesis by the force generated during folding, tuning ribosome activity to structure acquisition by a nascent polypeptide.


Examining kinesin processivity within a general gating framework

Johan O L Andreasson, Bojan Milic, Geng-Yuan Chen, Nicholas R Guydosh, William O Hancock, Steven M Block

Kinesin-1 is a dimeric motor that transports cargo along microtubules, taking 8.2-nm steps in a hand-over-hand fashion. The ATP hydrolysis cycles of its two heads are maintained out of phase by a series of gating mechanisms, which lead to processive runs averaging ~1 μm. A key structural element for inter-head coordination is the neck linker (NL), which connects the heads to the stalk. To examine the role of the NL in regulating stepping, we investigated NL mutants of various lengths using single-molecule optical trapping and bulk fluorescence approaches in the context of a general framework for gating. Our results show that, although inter-head tension enhances motor velocity, it is crucial neither for inter-head coordination nor for rapid rear-head release. Furthermore, cysteine-light mutants do not produce wild-type motility under load. We conclude that kinesin-1 is primarily front-head gated, and that NL length is tuned to enhance unidirectional processivity and velocity.


Single-virus force spectroscopy unravels molecular details of virus infection

Andreas Herrmann and Christian Sieben

Virus infection is a multistep process that has significant effects on the structure and function of both the virus and the host cell. The first steps of virus replication include cell binding, entry and release of the viral genome. Single-virus force spectroscopy (SVFS) has become a promising tool to understand the molecular details of those steps. SVFS data complemented by biochemical and biophysical, including theoretical modeling approaches provide valuable insights into molecular events that accompany virus infection. Properties of virus–cell interaction as well as structural alterations of the virus essential for infection can be investigated on a quantitative level. Here we review applications of SVFS to virus binding, structure and mechanics. We demonstrate that SVFS offers unexpected new insights not accessible by other methods.


Force-dependent persistence length of DNA–intercalator complexes measured in single molecule stretching experiments

R. F. Bazoni, C. H. M. Lima, E. B. Ramos and M. S. Rocha

By using optical tweezers with an adjustable trap stiffness, we have performed systematic single molecule stretching experiments with two types of DNA–intercalator complexes, in order to investigate the effects of the maximum applied forces on the mechanical response of such complexes. We have explicitly shown that even in the low-force entropic regime the persistence length of the DNA–intercalator complexes is strongly force-dependent, although such behavior is not exhibited by bare DNA molecules. We discuss the possible physicochemical effects that can lead to such results. In particular, we propose that the stretching force can promote partial denaturation on the highly distorted double-helix of the DNA–intercalator complexes, which interfere strongly in the measured values of the persistence length.


Tuesday, April 28, 2015

Radiation forces on a Rayleigh particle by highly focused radially polarized beams modulated by DVL

Ruili Zhang, Ziyang Chen, Jixiong Pu, and P. H. Jones

The intensity and the radiation forces acting on a Rayleigh particle near the focus of completely coherent radially polarized beams whose phase are modulated by a devil’s vortex-lens (DVL) are studied. The influence of the structure of a DVL on the radiation force distribution is analyzed. It is found by numerical simulations that the modulated beams show a clear advantage over the unmodulated highly focused radially polarized beams, as the modulated beam can simultaneously trap and manipulate the multiple Rayleigh particles, while the unmodulated beam can trap only one particle under the same condition.


Rotational cavity optomechanics

M. Bhattacharya

We theoretically examine the optomechanical interaction between a rotating nanoparticle and an orbital angular momentum–carrying optical cavity mode. Specifically, we consider a dielectric nanosphere rotating uniformly in a ring-shaped optical potential inside a Fabry–Perot resonator. The motion of the particle is probed by a weak angular lattice created by introducing two additional degenerate Laguerre–Gaussian cavity modes carrying equal and opposite orbital angular momenta. We demonstrate that the rotation frequency of the nanoparticle is imprinted on the probe optical mode, via the Doppler shift, and thus may be sensed experimentally using homodyne detection. We show analytically that the effect of the optical probe on the particle rotation vanishes in the regime of linear response, resulting in an accurate frequency measurement. We also numerically characterize the degradation of the measurement accuracy when the system is driven in the nonlinear regime. Our results are relevant to rotational Doppler velocimetry and to studies of rotational Brownian motion in a periodic lattice.


Time-average forces over Rayleigh particles by superposition of equal-frequency arbitrary-order Bessel beams

Leonardo André Ambrosio and Mariana de Matos Ferreira

We investigate optical forces exerted by suitable superpositions of equal-frequency Bessel beams—frozen waves—over dielectric, magnetodielectric, and negative-index Rayleigh particles. Frozen waves are capable of providing virtually any desired longitudinal intensity pattern by adequately superposing arbitrary-order Bessel beams, serving as potential beams in optical trapping, atom guiding, optical bistouries, and so on. Analytical expressions for gradient and scattering forces experienced by very small particles are deduced, and our numerical examples reveal that both low- and high-index dielectric and magnetodielectric particles can be efficiently trapped and manipulated. Our results indicate that such wave fields could actually provide three-dimensional traps in multiple transverse planes.


Electromagnetic trapping of chiral molecules: orientational effects of the irradiating beam

David S. Bradshaw and David L. Andrews

The photonic interaction generally responsible for the electromagnetic trapping of molecules is forward-Rayleigh scattering, a process that is mediated by transition electric dipoles connecting the ground electronic state and virtual excited states. Higher order electric and magnetic multipole contributions to the scattering amplitude are usually negligible. However, on consideration of chiral discrimination effects (in which an input light of left-handed circular polarization can present different observables compared to right-handed polarization, or molecules of opposite enantiomeric form respond differently to a set circular polarization), the mechanism must be extended to specifically accommodate transition magnetic dipoles. Moreover, it is important to account for the fact that chiral molecules are necessarily nonspherical, so that their interactions with a laser beam will have an orientational dependence. Using quantum electrodynamics, this article quantifies the extent of the energetic discrimination that arises when chiral molecules are optically trapped, placing particular emphasis on the orientational effects of the trapping beam. An in-depth description of the intricate ensemble-weighted method used to incorporate the latter is presented. It is thus shown that, when a mixture of molecular enantiomers is irradiated by a continuous beam of circularly polarized light, a difference arises in the relative rates of migration of each enantiomer in and out of the most intense regions of the beam. As a consequence, optical trapping can be used as a means of achieving enantiomer separation.


Computational toolbox for optical tweezers in geometrical optics

Agnese Callegari, Mite Mijalkov, A. Burak Gököz, and Giovanni Volpe 

Optical tweezers have found widespread application in many fields, from physics to biology. Here, we explain in detail how optical forces and torques can be described within the geometrical optics approximation, and we show that this approximation provides reliable results in agreement with experiments for particles whose characteristic dimensions are larger than the wavelength of the trapping light. Furthermore, we provide an object-oriented software package implemented in MATLAB for the calculation of optical forces and torques in the geometrical optics regime: Optical Tweezers in Geometrical Optics (OTGO). We provide all source codes for OTGO as well as documentation and code examples—e.g., standard optical tweezers, optical tweezers with elongated particles, the windmill effect, and Kramers transitions between two optical traps—necessary to enable users to effectively employ it in their research. 

Monday, April 27, 2015

Frequency Modulated Microrheology

Matthew Myles Shindel and Eric M Furst

Coupling analog frequency modulation (FM) to the driving stimulus in active microrheology measurements conducted with optical tweezers effectively parallelizes numerous single-frequency experiments. Consequently, frequency modulated microrheology (FMMR) can efficiently characterize the dynamic stress response of complex fluids over several frequency decades in a single experiment. The time required to complete an FMMR measurement scales with the lowest frequency probed, improving throughput over the serial frequency sweep approach. The ease of implementation, straight-forward data analysis and rapidity of FMMR offer particular utility toward applications such as characterization of non-equilibrium materials, automated microrheology instrumentation, high-throughput screening of biomaterials and (bio)pharmaceutical formulations, and in-situ monitoring of chemical and biochemical reaction processes.


Stability of aerosol droplets in Bessel beam optical traps under constant and pulsed external forces

Grégory David, Kıvanç Esat, Sebastian Hartweg, Johannes Cremer, Egor Chasovskikh and Ruth Signorell

We report on the dynamics of aerosol droplets in optical traps under the influence of additional constant and pulsed external forces. Experimental results are compared with simulations of the three-dimensional droplet dynamics for two types of optical traps, the counter-propagating Bessel beam (CPBB) trap and the quadruple Bessel beam (QBB) trap. Under the influence of a constant gas flow (constant external force), the QBB trap is found to be more stable compared with the CPBB trap. By contrast, under pulsed laser excitation with laser pulse durations of nanoseconds (pulsed external force), the type of trap is of minor importance for the droplet stability. It typically needs pulsed laser forces that are several orders of magnitude higher than the optical forces to induce escape of the droplet from the trap. If the droplet strongly absorbs the pulsed laser light, these escape forces can be strongly reduced. The lower stability of absorbing droplets is a result of secondary thermal processes that cause droplet escape.


The Kinesin-8 Kip3 Switches Protofilaments in a Sideward Random Walk Asymmetrically Biased by Force

Michael Bugiel, Elisa Böhl, Erik Schäffer

Molecular motors translocate along cytoskeletal filaments, as in the case of kinesin motors on microtubules. Although conventional kinesin-1 tracks a single microtubule protofilament, other kinesins, akin to dyneins, switch protofilaments. However, the molecular trajectory—whether protofilament switching occurs in a directed or stochastic manner—is unclear. Here, we used high-resolution optical tweezers to track the path of single budding yeast kinesin-8, Kip3, motor proteins. Under applied sideward loads, we found that individual motors stepped sideward in both directions, with and against loads, with a broad distribution in measured step sizes. Interestingly, the force response depended on the direction. Based on a statistical analysis and simulations accounting for the geometry, we propose a diffusive sideward stepping motion of Kip3 on the microtubule lattice, asymmetrically biased by force. This finding is consistent with previous multimotor gliding assays and sheds light on the molecular switching mechanism. For kinesin-8, the diffusive switching mechanism may enable the motor to bypass obstacles and reach the microtubule end for length regulation. For other motors, such a mechanism may have implications for torque generation around the filament axis.


A lab-on-a-chip for hypoxic patch clamp measurements combined with optical tweezers and spectroscopy- first investigations of single biological cells

Ahmed Alrifaiy, Johan Borg, Olof A Lindahl and Kerstin Ramser

The response and the reaction of the brain system to hypoxia is a vital research subject that requires special instrumentation. With this research subject in focus, a new multifunctional lab-on-a-chip (LOC) system with control over the oxygen content for studies on biological cells was developed. The chip was designed to incorporate the patch clamp technique, optical tweezers and absorption spectroscopy. The performance of the LOC was tested by a series of experiments. The oxygen content within the channels of the LOC was monitored by an oxygen sensor and verified by simultaneously studying the oxygenation state of chicken red blood cells (RBCs) with absorption spectra. The chicken RBCs were manipulated optically and steered in three dimensions towards a patch-clamp micropipette in a closed microfluidic channel. The oxygen level within the channels could be changed from a normoxic value of 18% O 2 to an anoxic value of 0.0-0.5% O 2. A time series of 3 experiments were performed, showing that the spectral transfer from the oxygenated to the deoxygenated state occurred after about 227 ± 1 s and a fully developed deoxygenated spectrum was observed after 298 ± 1 s, a mean value of 3 experiments. The tightness of the chamber to oxygen diffusion was verified by stopping the flow into the channel system while continuously recording absorption spectra showing an unchanged deoxygenated state during 5400 ± 2 s. A transfer of the oxygenated absorption spectra was achieved after 426 ± 1 s when exposing the cell to normoxic buffer. This showed the long time viability of the investigated cells. Successful patching and sealing were established on a trapped RBC and the whole-cell access (Ra) and membrane (Rm) resistances were measured to be 5.033 ± 0.412 M Ω and 889.7 ± 1.74 M Ω respectively.


Friday, April 24, 2015

Focus issue introduction: optical cooling and trapping

Antonio A. R. Neves, Philip H. Jones, Le Luo, and Onofrio M. Maragò

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This Focus Issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint Focus Issue features 32 papers, including both experimental and theoretical works, which span this wide range of activities.


Optical cooling and trapping: introduction

Antonio A. R. Neves, Philip H. Jones, Le Luo, and Onofrio M. Maragò

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This focus issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint focus issue features 33 papers, including both experimental and theoretical works, which span this wide range of activities.


Cancellation of non-conservative scattering forces in optical traps by counter-propagating beams

Shawn Divitt, Loïc Rondin, and Lukas Novotny

Non-conservative forces in optical tweezers generate undesirable behavior, such as particle loss due to radiation pressure and the preclusion of the thermodynamic equilibrium. Here, we rigorously derive criteria for the elimination of non-conservative forces, and describe how these criteria can be met by a large class of counter-propagating, focused optical beams.


Liquid-Liquid Phase Separation in Mixed Organic/Inorganic Single Aqueous Aerosol Droplets

David Stewart , Chen Cai , James Nayler , Thomas C Preston , Jonathan Philip Reid , Ulrich Krieger , Claudia Marcolli , and Yun-Hong Zhang

Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient-force optical trap and the evolving size, refractive index (RI) and morphology characterised by cavity enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase separated structure inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-di-acids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at marginally higher RH (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02).


Thursday, April 23, 2015

Single-molecule chemo-mechanical unfolding reveals multiple transition state barriers in a small single-domain protein

Emily J. Guinn, Bharat Jagannathan & Susan Marqusee

A fundamental question in protein folding is whether proteins fold through one or multiple trajectories. While most experiments indicate a single pathway, simulations suggest proteins can fold through many parallel pathways. Here, we use a combination of chemical denaturant, mechanical force and site-directed mutations to demonstrate the presence of multiple unfolding pathways in a simple, two-state folding protein. We show that these multiple pathways have structurally different transition states, and that seemingly small changes in protein sequence and environment can strongly modulate the flux between the pathways. These results suggest that in vivo, the crowded cellular environment could strongly influence the mechanisms of protein folding and unfolding. Our study resolves the apparent dichotomy between experimental and theoretical studies, and highlights the advantage of using a multipronged approach to reveal the complexities of a protein’s free-energy landscape.


Engineering of a superhelicase through conformational control

Sinan Arslan, Rustem Khafizov, Christopher D. Thomas, Yann R. Chemla, Taekjip Ha

Conformational control of biomolecular activities can reveal functional insights and enable the engineering of novel activities. Here we show that conformational control through intramolecular cross-linking of a helicase monomer with undetectable unwinding activity converts it into a superhelicase that can unwind thousands of base pairs processively, even against a large opposing force. A natural partner that enhances the helicase activity is shown to achieve its stimulating role also by selectively stabilizing the active conformation. Our work provides insight into the regulation of nucleic acid unwinding activity and introduces a monomeric superhelicase without nuclease activities, which may be useful for biotechnological applications.


Step-by-step guide to the realization of advanced optical tweezers

Giuseppe Pesce, Giorgio Volpe, Onofrio M. Maragó, Philip H. Jones, Sylvain Gigan, Antonio Sasso, and Giovanni Volpe

Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have become an indispensable tool for contactless manipulation of micro- and nanoparticles. Nowadays OT are employed in a myriad of applications demonstrating their importance. While the basic principle of OT is the use of a strongly focused laser beam to trap and manipulate particles, more complex experimental setups are required to perform novel and challenging experiments. With this article, we provide a detailed step-by-step guide for the construction of advanced optical manipulation systems. First, we explain how to build a single-beam OT on a homemade microscope and how to calibrate it. Improving on this design, we realize a holographic OT, which can manipulate independently multiple particles and generate more sophisticated wavefronts such as Laguerre–Gaussian beams. Finally, we explain how to implement a speckle OT, which permits one to employ random speckle light fields for deterministic optical manipulation.


Isolation and identification of bacteria by means of Raman spectroscopy

Susanne Pahlow, Susann Meisel, Dana Cialla-May, Karina Weber, Petra Rösch, Jürgen Popp

Bacterial detection is a highly topical research area, because various fields of application will benefit from the progress being made. Consequently, new and innovative strategies which enable the investigation of complex samples, like body fluids or food stuff, and improvements regarding the limit of detection are of general interest. Within this review the prospects of Raman spectroscopy as a reliable tool for identifying bacteria in complex samples are discussed.
The main emphasis of this work is on important aspects of applying Raman spectroscopy for the detection of bacteria like sample preparation and the identification process. Several approaches for a Raman compatible isolation of bacterial cells have been developed and applied to different matrices. Here, an overview of the limitations and possibilities of these methods is provided. Furthermore, the utilization of Raman spectroscopy for diagnostic purposes, food safety and environmental issues is discussed under a critical view.


Principal-component analysis of particle motion

H. Y. Chen, Raphaël Liégeois, John R. de Bruyn, and Andrea Soddu

We demonstrate the application of principal-component analysis (PCA) to the analysis of particle motion data in the form of a time series of images. PCA has the ability to resolve and isolate spatiotemporal patterns in the data. Using simulated data, we show that this translates into the ability to separate individual frequency components of the particle motion. We also show that PCA can be used to extract the fluid viscosity from images of particles undergoing Brownian motion. PCA thus provides an efficient alternative to more traditional particle-tracking methods for the analysis of microrheological data.


Wednesday, April 22, 2015

Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal

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

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is ∼4.5×10−3C/m2 for 20-nm nanocolloids, and two orders of magnitude lower, i.e., ∼2.3×10−5C/m2, for 1−μm colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.


Optical trapping and manipulation of micrometer and submicrometer particles

Mark Daly, Marios Sergides and Síle Nic Chormaic
Subwavelength features in conjunction with light-guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping. Lately, the focus of particle trapping has shifted from the microscale to the nanoscale. This few orders of magnitude change is driven, in part, by the needs of life scientists who wish to better manipulate smaller biological samples. Devices with subwavelength features are excellent platforms for shaping local electric fields for this purpose. A major factor that inhibits the manipulation of submicrometer particles is the diffraction-limited spot size of free-space laser beams. As a result, technologies that can circumvent this limit are highly desirable. This review covers some of the more significant advances in the field, from the earliest attempts at trapping using focused Gaussian beams, to more sophisticated hybrid plasmonic/metamaterial structures. In particular, examples of emerging optical trapping configurations are presented.


Switchable optical cage by use of coated axicons for optical trapping

Qiang Song, Jing Zhu, Shiwang Tang, Baoxi Yang, Huijie Huang

We present a novel method to generate switchable optical trapping by together tight focusing of two noncoherent cylindrical vector beams, which were produced by two pairs of axicons with polarizing films. The simulation result shows that the optical trapping shape is controllable varying from three-dimensional cage to channel by smoothly tuning the distances between axicons. We verify the cylindrical vector characteristic of the output beams from proposed setup based on vector diffraction theory. Finally, gradient forces of the optical trapping were calculated to demonstrate the potential application of this system in the field of micro particle manipulation.


Oxygen governs gonococcal microcolony stability by enhancing the interaction force between type IV pili

Lena Dewenter, Thorsten E. Volkmann and Berenike Maier

The formation of small bacterial clusters, called microcolonies, is the first step towards the formation of bacterial biofilms. The human pathogen Neisseria gonorrhoeae requires type IV pili (T4P) for microcolony formation and for surface motility. Here, we investigated the effect of oxygen on the dynamics of microcolony formation. We found that an oxygen concentration exceeding 3 μM is required for formation and maintenance of microcolonies. Depletion of proton motive force triggers microcolony disassembly. Disassembly of microcolonies is actively driven by T4P retraction. Using laser tweezers we showed that under aerobic conditions T4P–T4P interaction forces exceed 50 pN. Under anaerobic conditions T4P–T4P interaction is severely inhibited. We conclude that oxygen is required for gonococcal microcolony formation by enhancing pilus–pilus interaction.


Direct observation of structure-function relationship in a nucleic acid–processing enzyme

Matthew J. Comstock, Kevin D. Whitley, Haifeng Jia, Joshua Sokoloski, Timothy M. Lohman, Taekjip Ha, Yann R. Chemla

The relationship between protein three-dimensional structure and function is essential for mechanism determination. Unfortunately, most techniques do not provide a direct measurement of this relationship. Structural data are typically limited to static pictures, and function must be inferred. Conversely, functional assays usually provide little information on structural conformation. We developed a single-molecule technique combining optical tweezers and fluorescence microscopy that allows for both measurements simultaneously. Here we present measurements of UvrD, a DNA repair helicase, that directly and unambiguously reveal the connection between its structure and function. Our data reveal that UvrD exhibits two distinct types of unwinding activity regulated by its stoichiometry. Furthermore, two UvrD conformational states, termed “closed” and “open,” correlate with movement toward or away from the DNA fork.


Friday, April 17, 2015

Higher order microfibre modes for dielectric particle trapping and propulsion

Aili Maimaiti, Viet Giang Truong, Marios Sergides, Ivan Gusachenko & Síle Nic Chormaic

Optical manipulation in the vicinity of optical micro- and nanofibres has shown potential across several fields in recent years, including microparticle control, and cold atom probing and trapping. To date, most work has focussed on the propagation of the fundamental mode through the fibre. However, along the maximum mode intensity axis, higher order modes have a longer evanescent field extension and larger field amplitude at the fibre waist compared to the fundamental mode, opening up new possibilities for optical manipulation and particle trapping. We demonstrate a microfibre/optical tweezers compact system for trapping and propelling dielectric particles based on the excitation of the first group of higher order modes at the fibre waist. Speed enhancement of polystyrene particle propulsion was observed for the higher order modes compared to the fundamental mode for particles ranging from 1 μm to 5 μm in diameter. The optical propelling velocity of a single, 3 μm polystyrene particle was found to be 8 times faster under the higher order mode than the fundamental mode field for a waist power of 25 mW. Experimental data are supported by theoretical calculations. This work can be extended to trapping and manipulation of laser-cooled atoms with potential for quantum networks.


Photonic nanojets in optical tweezers

Antonio Alvaro Ranha Neves

Photonic nanojets have been brought into attention ten years ago for potential application in ultramicroscopy, because of its sub-wavelength resolution that can enhance detection and interaction with matter. For these novel applications under development, the optical trapping of a sphere acts as an ideal framework to employ photonic nanojets. In the present study, we generated nanojets by using a highly focused incident beam, in contrast to traditional plane waves. The method inherits the advantage of optical trapping, especially for intracellular applications, with the microsphere in equilibrium on the beam propagation axis and positioned arbitrarily in space. Moreover, owing to optical scattering forces, when the sphere is in equilibrium, its center shifts with respect to the focal point of the incident beam. However, when the system is in stable equilibrium with a configuration involving optical tweezers, photonic nanojets cannot be formed. To overcome this issue, we employed double optical tweezers in an unorthodox configuration involving two collinear and co-propagating beams, the precise positioning of which would turn on/off the photonic nanojets, thereby improving the applicability of photonic nanojets.


Rational Design of a Cytotoxic Dinuclear Cu2 Complex That Binds by Molecular Recognition at Two Neighboring Phosphates of the DNA Backbone

Thomas Jany, Alexander Moreth, Claudia Gruschka, Andy Sischka, Andre Spiering, Mareike Dieding, Ying Wang, Susan Haji Samo, Anja Stammler, Hartmut Bögge, Gabriele Fischer von Mollard, Dario Anselmetti, and Thorsten Glaser

The mechanism of the cytotoxic function of cisplatin and related anticancer drugs is based on their binding to the nucleobases of DNA. The development of new classes of anticancer drugs requires establishing other binding modes. Therefore, we performed a rational design for complexes that target two neighboring phosphates of the DNA backbone by molecular recognition resulting in a family of dinuclear complexes based on 2,7-disubstituted 1,8-naphthalenediol. This rigid backbone preorganizes the two metal ions for molecular recognition at the distance of two neighboring phosphates in DNA of 6–7 Å. Additionally, bulky chelating pendant arms in the 2,7-position impede nucleobase complexation by steric hindrance. We successfully synthesized the CuII2 complex of the designed family of dinuclear complexes and studied its binding to dsDNA by independent ensemble and single-molecule methods like gel electrophoresis, precipitation, and titration experiments followed by UV–vis spectroscopy, atomic force microscopy (AFM), as well as optical tweezers (OT) and magnetic tweezers (MT) DNA stretching. The observed irreversible binding of our dinuclear CuII2 complex to dsDNA leads to a blocking of DNA synthesis as studied by polymerase chain reactions and cytotoxicity for human cancer cells.


Trapping of Rayleigh spheroidal particles by highly focused radially polarized beams

Manman Li, Shaohui Yan, Baoli Yao, Ming Lei, Yanlong Yang, Junwei Min, and Dan Dan

The optical forces and intrinsic optical torque of a highly focused radially polarized beam on a Rayleigh spheroidal particle are calculated with the dipole approximation. Numerical results show that the maximal trapping forces depend strongly on the orientation of the particle, and the torque is always perpendicular to the plane containing the major axis of the spheroid and the optical axis. As a result of optical mechanical and torque equilibrium, the spheroidal particle will stay at the focus with its major axis of the spheroid parallel to the optical axis.


Thursday, April 16, 2015

A Laser Trapping-Spectroscopy Study on Mass Transfer Processes Across a Single Micro-Droplet/Air Interface

M.A. Jiang, Shoji Ishizaka, Terufumi Fujiwara, Yuan Gao

Clouds regulate the earth's energy balance by reflecting and scattering solar radiation and by absorbing the earth's infrared radiation. The fundamental knowledge about mass transfer processes across a micro-droplet/air interface is very important to give mathematical equations that describe the growth process of clouds for climate models. So far, experimental studies on the condensation growth of water droplets have been conducted by using either an aerosol flow tube or a vibrating orifice aerosol generator. However, the mass accommodation coefficients evaluated by such techniques are very scattered and, therefore, the detailed mechanisms of condensation growth of micrometer-sized water droplets are still controversial. The primary reason for this is difficulties in observing the growth processes of single water droplets in air. In this study, we demonstrate a novel approach for in situ observation of the evaporation and condensation processes of single water droplets levitated in air by means of a laser trapping technique.


Biodegradable PLGA Nanoparticles Loaded with Hydrophobic Drugs: Confocal Raman Microspectroscopic Characterization

Hu Yan, Yi-Fan Hou, Peng-Fei Niu, Ke Zhang, Tatsuya Shoji, Yasuyuki Tsuboi, Fang-Yao Yao, Li-Min Zhao and Junbiao Chang

Poly(lactic-co-glycolic acid) (PLGA) nanoparticles with bicyclol (5%) and 3-n-butyl-6-bromophthalid (Br-NBP) (3%) were prepared by an emulsification-solvent evaporation technique. The PLGA nanoparticles were for the first time successfully characterized by a laser trapping/confocal Raman spectroscopic technique only using individual PLGA nanoparticles. This technique allowed us to selectively obtain Raman spectra of optically trapped PLGA nanoparticles (~ 10 nanoparticles) in solution. The Raman spectrum of PLGA nanoparticles loaded with hydrophobic drugs showed that these drugs were certainly incorporated in the nanoparticles.


Optical forces experienced by arbitrary-sized spherical scatterers from superpositions of equal-frequency Bessel beams

Leonardo André Ambrosio and Michel Zamboni-Rached

Radiation pressure cross sections over arbitrary-sized spherical scatterers are evaluated considering, as wave fields, recently developed frozen waves, which are a suitable superposition of equal-frequency Bessel beams, of arbitrary order. The so-called beam-shape coefficients are computed within the framework of the generalized Lorenz–Mie theory and with the aid of integral localized approximation. It is numerically shown that, under the paraxial regime, frozen waves could be designed to efficiently trap biological cells, viruses, bacteria, and so on, along multiple radial planes and at specific axial locations. Our results reinforce frozen waves as potential laser beams in optical trapping and manipulation.


Front-to-Rear Membrane Tension Gradient in Rapidly Moving Cells

Arnon D. Lieber, Yonatan Schweitzer, Michael M. Kozlov, Kinneret Keren

Membrane tension is becoming recognized as an important mechanical regulator of motile cell behavior. Although membrane-tension measurements have been performed in various cell types, the tension distribution along the plasma membrane of motile cells has been largely unexplored. Here, we present an experimental study of the distribution of tension in the plasma membrane of rapidly moving fish epithelial keratocytes. We find that during steady movement the apparent membrane tension is ∼30% higher at the leading edge than at the trailing edge. Similar tension differences between the front and the rear of the cell are found in keratocyte fragments that lack a cell body. This front-to-rear tension variation likely reflects a tension gradient developed in the plasma membrane along the direction of movement due to viscous friction between the membrane and the cytoskeleton-attached protein anchors embedded in the membrane matrix. Theoretical modeling allows us to estimate the area density of these membrane anchors. Overall, our results indicate that even though membrane tension equilibrates rapidly and mechanically couples local boundary dynamics over cellular scales, steady-state variations in tension can exist in the plasma membranes of moving cells.


Different Raman spectral patterns of primary rat pancreatic β cells and insulinoma cells

Qiu-Li Zhou ; Xi Rong ; Fang Wei ; Rui-Qiong Luo ; Hong Liu

As a noninvasive and label-free analytical technique, Raman spectroscopy has been widely used to study the difference between malignant cells and normal cells. Insulinomas are functional β-cell tumors of pancreatic islet cells. They exhibit many structural and immunohistochemical features in common with normal pancreatic β cells; thus, they are typically difficult to distinguish under the microscope, especially in vivo. We investigated insulinoma and primary rat pancreatic β-cell populations using Raman spectroscopy. The details of the optical heterogeneity between these two populations were determined based on different Raman regions primarily involving nucleic acid and protein contents, which are the most distinct cellular contents in these two types of cells. Using principal component analysis–linear discriminant analysis, these two cell types can be readily separated. The results of this work indicate that Raman spectroscopy is a promising tool for the noninvasive and label-free differentiation of insulinoma cells and normal pancreatic β cells.


Wednesday, April 15, 2015

The Mechanochemical Cycle of Mammalian Kinesin-2 KIF3A/B under Load

Johan O.L. Andreasson, Shankar Shastry, William O. Hancock, Steven M. Block

The response of motor proteins to external loads underlies their ability to work in teams and determines the net speed and directionality of cargo transport. The mammalian kinesin-2, KIF3A/B, is a heterotrimeric motor involved in intraflagellar transport and vesicle motility in neurons. Bidirectional cargo transport is known to result from the opposing activities of KIF3A/B and dynein bound to the same cargo, but the load-dependent properties of kinesin-2 are poorly understood. We used a feedback-controlled optical trap to probe the velocity, run length, and unbinding kinetics of mouse KIF3A/B under various loads and nucleotide conditions. The kinesin-2 motor velocity is less sensitive than kinesin-1 to external forces, but its processivity diminishes steeply with load, and the motor was observed occasionally to slip and reattach. Each motor domain was characterized by studying homodimeric constructs, and a global fit to the data resulted in a comprehensive pathway that quantifies the principal force-dependent kinetic transitions. The properties of the KIF3A/B heterodimer are intermediate between the two homodimers, and the distinct load-dependent behavior is attributable to the properties of the motor domains and not to the neck linkers or the coiled-coil stalk. We conclude that the force-dependent movement of KIF3A/B differs significantly from conventional kinesin-1. Against opposing dynein forces, KIF3A/B motors are predicted to rapidly unbind and rebind, resulting in qualitatively different transport behavior from kinesin-1.


Optical Binding Force between Two Chiral Spheres by an Incident On-axis Gaussian Beam

Yuanyuan Zhu, Zhensen Wu, , Zhengjun Li, Qingchao Shang

According to the electromagnetic scattering of two spheres, the incident on-axis Gaussian beam is expanded in terms of spherical vector wave functions (SVWFs), and the beam shape coefficients are obtained by applying the localized approximation method. Using the addition theorem, the interaction scattering fields of two chiral spheres and the internal fields are also expanded in terms of SVWFs. Based on the continuous tangential boundary conditions, the scattered field coefficients are derived analytically. Utilizing the Maxwell's stress tensor integration technique, the optical binding force between two chiral spheres is formulated explicitly. Numerical simulations of the binding force are carried out. The effects of the beam width and the radius of the sphere on the force are analyzed. The numerical results are compared with the results from references.


Chirality-Selective Optical Scattering Force on Single-Walled Carbon Nanotubes

Susan E. Skelton Spesyvtseva, Satoru Shoji, and Satoshi Kawata

We show that optical forces acting on carbon nanotubes are substantially enhanced when the optical wavelength is tuned to resonances in the electronic band structure. Using a tunable laser source, we experimentally demonstrate a resonant optical scattering force on single-walled carbon nanotubes and, by tuning the wavelength, exploit this force to achieve chirality enrichment of four chiralities of nanotubes. Our results represent a significant step towards optical manipulation and sorting of carbon nanotubes based on their chiral vector.


Two spheres translating in tandem through a colloidal suspension

Indira Sriram and Eric M. Furst

Using laser tweezers, two colloidal particles are held parallel to a uniformly flowing suspension of similarly sized bath particles at an effective volume fraction ϕeff=0.41. The local deformation in the bath suspension is imaged by confocal microscopy, and, concurrently, the drag forces exerted on both the leading and the trailing probe particles are measured as a function of probe separation and velocity. The bath structure changes in response to the velocity and separation of the probes. A depleted region between probes is observed at sufficiently high velocities. Both probes experience the same drag force and the drag force increases with probe separation. The results indicate that bath-probe and probe-probe hydrodynamic interactions contribute microstructure and drag force and that drag exerted by direct bath-probe collisions is reduced compared to an isolated probe.


Monday, April 13, 2015

A simple and direct reading flow meter fabricated by two-photon polymerization for microfluidic channel

Yi-Jui Liu, Juin-Yi Yang, Yung-Mau Nie, Chun-Hung Lu, Eric Dowkon Huang, Chow-Shing Shin, Patrice Baldeck, Chih-Lang Lin

This study introduces an innovative micromachine that enables precise measurement of the flow rate at the micron scale (μl/min) in microfluidic channel. It is fabricated by the state-of-the-art technique of two-photon polymerization and consists of: a rod-spring, a water-drop-shaped frame, and an indicator situated inside a microfluidic channel. The indicator is deflected by the flowing fluid while restrained by the spring to establish an equilibrium indication according to the flow rate. In the practical tests, the flow successfully agitates the spring in a proper deflection. The relationship between the flow rate and the deflection angle was calibrated.


Generation of controllable rotating petal-like modes using composited Dammann vortex gratings

Junjie Yu, Changhe Zhou, Wei Jia, Jun Wu, Linwei Zhu, Yancong Lu, Changcheng Xiang, and Shubin Li

A new type of diffractive optical element, called a composited Dammann vortex grating (CDVG), is proposed for generation of multiple equal-energy controllable rotating petal-like modes extra cavity. As an example, it is shown that a petal-like mode is well generated for each nonzero diffraction order by a binary pure-phase 1×7 CDVG. Mode decomposition is digitally implemented by a programmable spatial light modulator (SLM), and the experimental results show that those generated petal-like patterns are in high mode purity (∼90%) for all six different nonzero orders. Also, controllable rotating petal-like modes are demonstrated when the CDVG is digitally implemented by the programmable SLM, which provides the possibility to quantitatively control the rotation rate of this type of optical tweezers. Furthermore, tunable petal-like modes are also demonstrated experimentally by introducing a vortex incident field with different topological charges.


Fabrication of a Material Assembly of Silver Nanoparticles Using the Phase Gradients of Optical Tweezers

Zijie Yan, Manas Sajjan, and Norbert F. Scherer

Optical matter can be created using the intensity gradient and electrodynamic (e.g., optical binding) forces that nano- and microparticles experience in focused optical beams. Here we show that the force associated with phase gradient is also important. In fact, in optical line traps the phase gradient force is crucial in determining the structure and stability of optical matter arrays consisting of Ag nanoparticles (NPs). NP lattices can be repeatedly assembled and disassembled simply by changing the sign of the phase gradient. The phase gradient creates a compressive force (and thus a stress) in the optically bound Ag NP lattices, causing structural transitions (a stress response) from 1D “chains” to 2D lattices, and even to amorphous structures. The structural transitions and dynamics of driven transport are well described by electrodynamics simulations and modeling using a drift-diffusion Langevin equation.


Induced deflagellation of Isochrysis microalgae in a near-infrared optical trap

Veneranda G. Garces, Oscar Salazar-Oropeza, Beatriz Cordero-Esquivel, and Kevin A. O’Donnell

We report on the observation of an in situ deflagellation of an Isochrysis microalgae cell with a near-infrared laser beam. In particular, a cell is caught in an optical trap and is later observed to lose its flagella; the time required is termed the deflagellation time. The dependence of the average deflagellation time on cell growth rate phase is studied. For cells in the initial growth rate phase, this time (∼26  s) is considerably longer than observed in the stationary phase (∼16  s). The cellular transmittance is observed to fall with age, suggesting that the increased optical absorption and scattering may be related to the more rapid deflagellation. The dependence of deflagellation time on laser trapping power is consistent with a single-photon process. No significant difference was found in the deflagellation time for cells kept either in the dark or under light. The deflagellated cells were viable after showing the reappearance of red chlorophyll autofluorescence and cell division. Our observations reported here provide insight into the photostimulus produced by near-infrared light in Isochrysis and other living organisms.


Friday, April 10, 2015

Active one-particle microrheology of an unentangled polymer melt studied by molecular dynamics simulation

A. Kuhnhold and W. Paul

We present molecular dynamics simulations for active one-particle microrheology of an unentangled polymer melt. The tracer particle is forced to oscillate by an oscillating harmonic potential, which models an experiment using optical tweezers. The amplitude and phase shift of this oscillation are related to the complex shear modulus of the polymer melt. In the linear response regime at low frequencies, the active microrheology gives the same result as the passive microrheology, where the thermal motion of a tracer particle is related to the complex modulus. We expand the analysis to include full hydrodynamic effects instead of stationary Stokes friction only, and show that different approaches suggested in the literature lead to completely different results, and that none of them improves on the description using the stationary Stokes friction.


Three-dimensional laser trapping on the base of binary radial diffractive optical element

Roman Skidanov, Denis Kachalov, Svetlana Khonina, Alexey Porfirev & Vladimir Pavelyev

A radially symmetric binary diffractive optical element to generate an optical bottle beam is designed, fabricated, and characterized. Analysis of the numerical simulation and experimental research results shows that the fabricated element is well suited for solving three-dimensional (3D) laser trapping problems.


Anomalous optical forces on radially anisotropic nanowires

H. L. Chen, L. Gao

Full-wave electromagnetic scattering theory and Maxwell stress tensor integration techniques have been established to study the optical force on the radially anisotropic nanowires. The optical forces on the isotropic nanowires are dependent on the size of the nanowire and the wave vector in the media with the Rayleigh’s law. However, the optical forces on the anisotropic nanowires have the anomalous behaviors under non-Rayleigh vanishing condition and non-Rayleigh diverging condition. Therefore, the optical forces on the anisotropic nanowires may be enhanced or reduced by tuning the anisotropic parameters. These results may promote the potential applications in the field of nanotechnology.


Design of a robust unified controller for cell manipulation with a robot-aided optical tweezers system

Xiangpeng Li, Hao Yang, Jianjun Wang, Dong Sun

With the advantages of non-physical contact, high precision, and efficiency, optical tweezers have been increasingly used to manipulate biological cells in various biomedical applications. When trapping a cell with optical tweezers, the cell must be located within the optical trap. The lack of an efficient control technique that can automatically control cell motion while consistently locating such cell within the optical trap causes the trapped cell to escape easily, thus resulting in the failure of the manipulation task. Therefore, the development of a unified controller that can manipulate both cell trapping and cell motion simultaneously while possessing robustness to environmental disturbances is urgently needed. In this paper, we develop a novel unified controller that manipulates cell positioning and cell trapping simultaneously. First, we establish a geometric model to confine the cell within a local region around the optical trap. The connection between the cell and the optical tweezers is formulated by using the concept of cell–tweezers (C–T) coalition. Second, we develop a controller based on a defined potential field function to drive the C–T coalition to the desired state while avoiding collisions with other obstacles in the environment. Finally, we perform experiments of transferring yeast cells to demonstrate the effectiveness of the proposed approach.


Thursday, April 9, 2015

Theoretical study of optical torques for aligning Ag nanorods and nanowires

Jiunn-Woei Liaw, Wei-Jiun Lo, Wu-Chun Lin, Mao-Kuen Kuo

The optical torque that is exerted on an Ag nanorod (NR) by the irradiance of a linearly polarized laser was studied theoretically. The multiple multipole method was used to calculate the induced electromagnetic field, and then the surface traction in terms of Maxwell׳s stress tensor was integrated over the NR׳s surface. The maps of surface traction on the Ag NR in two (parallel and perpendicular) modes of alignment in various wavelength regimes were discussed. The turning point between the two modes was demonstrated to coincide with the longitudinal surface plasmon resonance (LSPR) of the Ag NR. For example, the optical torques that are induced by different lasers (1064 nm, 633 nm and 532 nm) on Ag NR (with a radius 10 nm and an aspect ratio of 3) in water were analyzed to demonstrate the wavelength-dependent performance. The NR was aligned parallel and perpendicular to the polarization of the 1064 nm and 532 nm lasers, respectively. A laser with a wavelength of 633 nm, which was close to the LSPR, induced a null torque, and caused severe plasmonic heating. In contrast, over the entire spectrum, only the parallel mode of dielectric NRs was observed. The optical torque on Ag NR is two orders of magnitude greater than that of a high-k dielectric NR of equal size. For a highly elongated Ag NR (including nanowire), the perpendicular mode, rather than the parallel one, is induced even irradiated by a 1064 nm NIR laser, because its LSPR wavelength exceeds 1064 nm.


Controllable Patterning of Different Cells Via Optical Assembly of 1D Periodic Cell Structures

Hongbao Xin, Yuchao Li andBaojun Li

Flexible patterning of different cells into designated locations with direct cell–cell contact at single-cell patterning precision and control is of great importance, however challenging, for cell patterning. Here, an optical assembly method for patterning of different types of cells via direct cell–cell contact at single-cell patterning precision and control is demonstrated. Using Escherichia coli and Chlorella cells as examples, different cells are flexibly patterned into 1D periodic cell structures (PCSs) with controllable configurations and lengths, by periodically connecting one type of cells with another by optical force. The patterned PCSs can be flexibly moved and show good light propagation ability. The propagating light signals can be detected in real-time, providing new opportunities for the detection of transduction signals among patterned cells. This patterning method is also applicable for cells of other kinds, including mammalian/human cells.


Line optical traps formed by LC SLM

Aleksandra Mayorova, Alexander Korobtsov, Svetlana Kotova, Nikolay Losevsky, Sergey Samagin

The methods for generation of optical traps in the form of line segments by means of liquid crystal modulators of two types are addressed in the paper, they are a multi-pixel modulator HOLOEYE HEO 1080P and a tunable liquid crystal focusing device (4-channel modulator) developed by the authors. The numerical and experimental assessment of the capture forces of the generated optical trap is fulfilled. The description of manipulation experiments with microobjects, including biological ones, carried out by the optical traps with the intensity distribution in the form of segments is offered.


Simultaneous 3D visualization and position tracking of optically trapped particles using optical diffraction tomography

Kyoohyun Kim, Jonghee Yoon, and YongKeun Park

Precise tracking of three-dimensional (3D) positions of objects, often associated with optical tweezers, is important for the study of biophysics and cell biology. Although various approaches for 3D particle tracking have been proposed, most are limited in resolution and axial localization for objects of complex geometry. Holographic tomography systems circumvent these problems and offer improved capability in localization of objects over current methods. Here, we present a combined system employing optical diffraction tomography and holographic optical tweezers capable of simultaneous 3D visualization of the shapes and tracking positions of trapped microscopic samples. We demonstrated the capability of the present combined system using optically trapped silica beads and biological cells.


Micro-Bubble Generated by Laser Irradiation on an Individual Carbon Nanocoil

Yanming Sun, Lujun Pan, Yuli Liu, Tao Sun

We have investigated the micro-bubbles generated by laser induction on an individual carbon nanocoil (CNC) immerged in deionized water. The photon energy of the incident focused laser beam is absorbed by CNC and converted to thermal energy, which efficiently vaporizes the surrounding water, and subsequently a micro-bubble is generated at the laser location. The dynamics behavior of bubble generation, including its nucleation, expansion and steady-state, has been studied experimentally and theoretically. We have derived equations to analyze the expansion process of a bubble based on classical heat and mass transfer theories. The conclusion is in good agreement with the experiment. CNC, which acts as a realistic micro-bubble generator, can be operated easily and flexibly.


Monday, April 6, 2015

Optical separation of heterogeneous size distributions of microparticles on silicon nitride strip waveguides

Saara A. Khan, Yu Shi, Chia-Ming Chang, Catherine Jan, Shanhui Fan, Audrey K. Ellerbee, and Olav Solgaard

We demonstrate two complementary optical separation techniques of dielectric particles on the surface of silicon nitride waveguides. Glass particles ranging from 2 μm to 10 μm in diameter are separated at guided powers below 40 mW. The effects of optical, viscous, and frictional forces on the particles are modeled and experimentally shown to enable separation. Particle interactions are investigated and shown to decrease measured particle velocity without interfering with the overall particle separation distribution. The demonstrated separation techniques have the potential to be integrated with microfluidic structures for cell sorting.


Physical interaction between peroxisomes and chloroplasts elucidated by in situ laser analysis

Kazusato Oikawa, Shigeru Matsunaga, Shoji Mano, Maki Kondo, Kenji Yamada, Makoto Hayashi, Takatoshi Kagawa, Akeo Kadota, Wataru Sakamoto, Shoichi Higashi, Masakatsu Watanabe, Toshiaki Mitsui, Akinori Shigemasa, Takanori Iino, Yoichiroh Hosokawa & Mikio Nishimura

Life on earth relies upon photosynthesis, which consumes carbon dioxide and generates oxygen and carbohydrates. Photosynthesis is sustained by a dynamic environment within the plant cell involving numerous organelles with cytoplasmic streaming. Physiological studies of chloroplasts, mitochondria and peroxisomes show that these organelles actively communicate during photorespiration, a process by which by-products produced by photosynthesis are salvaged. Nevertheless, the mechanisms enabling efficient exchange of metabolites have not been clearly defined. We found that peroxisomes along chloroplasts changed shape from spherical to elliptical and their interaction area increased during photorespiration. We applied a recent femtosecond laser technology to analyse adhesion between the organelles inside palisade mesophyll cells of Arabidopsis leaves and succeeded in estimating their physical interactions under different environmental conditions. This is the first application of this estimation method within living cells. Our findings suggest that photosynthetic-dependent interactions play a critical role in ensuring efficient metabolite flow during photorespiration.


Linked topological colloids in a nematic host

Angel Martinez, Leonardo Hermosillo, Mykola Tasinkevych, and Ivan I. Smalyukh
Geometric shape and topology of constituent particles can alter many colloidal properties such as Brownian motion, self-assembly, and phase behavior. Thus far, only single-component building blocks of colloids with connected surfaces have been studied, although topological colloids, with constituent particles shaped as freestanding knots and handlebodies of different genus, have been recently introduced. Here we develop a topological class of colloids shaped as multicomponent links. Using two-photon photopolymerization, we fabricate colloidal microparticle analogs of the classic examples of links studied in the field of topology, the Hopf and Solomon links, which we disperse in nematic fluids that possess orientational ordering of anisotropic rod-like molecules. The surfaces of these particles are treated to impose tangential or perpendicular boundary conditions for the alignment of liquid crystal molecules, so that they generate a host of topologically nontrivial field and defect structures in the dispersing nematic medium, resulting in an elastic coupling between the linked constituents. The interplay between the topologies of surfaces of linked colloids and the molecular alignment field of the nematic host reveals that linking of particle rings with perpendicular boundary conditions is commonly accompanied by linking of closed singular defect loops, laying the foundations for fabricating complex composite materials with interlinking-based structural organization.


Automated Translational and Rotational Control of Biological Cells With a Robot-Aided Optical Tweezers Manipulation System

Xie, M. ; Mills, J.K. ; Wang, Y. ; Mahmoodi, M. ; Sun, D.

Research and biomedical applications in cell surgery require transportation and rotation of biological cells. In these cell manipulation tasks, the cell of interest must be translated and oriented properly such that the desired component, such as the polar body or other organelles, can be imaged with optical microscopy. This paper presents a holographic optical tweezers (HOT) based system to carry out automated translational control in the plane, and rotational control about one rotational axes of a suspended cell. Based on the proposed general equations of motion of the cell, held in an optical trap, two controllers, one for cell translational and one for rotational control, are developed to translate and orient the cells to the desired position and orientation in a sequential manner. Experiments are performed to demonstrate the effectiveness of the proposed approach.


Thursday, April 2, 2015

Optical trapping of silver nanoplatelets

E. Messina, M. G. Donato, M. Zimbone, R. Saija, M. A. Iatì, L. Calcagno, M. E. Fragalà, G. Compagnini, C. D’Andrea, A. Foti, P. G. Gucciardi, and O. M. Maragò

Optical trapping of silver nanoplatelets obtained with a simple room temperature chemical synthesis technique is reported. Trap spring constants are measured for platelets with different diameters to investigate the size-scaling behaviour. Experimental data are compared with models of optical forces based on the dipole approximation and on electromagnetic scattering within a T-matrix framework. Finally, we discuss applications of these nanoplatelets for surface-enhanced Raman spectroscopy.


Force-induced on-rate switching and modulation by mutations in gain-of-function von Willebrand diseases

Jongseong Kim, Nathan E. Hudson, and Timothy A. Springer

Mutations in the ultralong vascular protein von Willebrand factor (VWF) cause the common human bleeding disorder, von Willebrand disease (VWD). The A1 domain in VWF binds to glycoprotein Ibα (GPIbα) on platelets, in a reaction triggered, in part, by alterations in flow during bleeding. Gain-of-function mutations in A1 and GPIbα in VWD suggest conformational regulation. We report that force application switches A1 and/or GPIbα to a second state with faster on-rate, providing a mechanism for activating VWF binding to platelets. Switching occurs near 10 pN, a force that also induces a state of the receptor−ligand complex with slower off-rate. Force greatly increases the effects of VWD mutations, explaining pathophysiology. Conversion of single molecule kon (s−1) to bulk phase kon (s−1M−1) and the kon and koff values extrapolated to zero force for the low-force pathways show remarkably good agreement with bulk-phase measurements.


Mechano-chemical kinetics of DNA replication: identification of the translocation step of a replicative DNA polymerase

José A. Morin, Francisco J. Cao, José M. Lázaro, J. Ricardo Arias-Gonzalez, José M. Valpuesta, José L. Carrascosa, Margarita Salas and Borja Ibarra

During DNA replication replicative polymerases move in discrete mechanical steps along the DNA template. To address how the chemical cycle is coupled to mechanical motion of the enzyme, here we use optical tweezers to study the translocation mechanism of individual bacteriophage Phi29 DNA polymerases during processive DNA replication. We determine the main kinetic parameters of the nucleotide incorporation cycle and their dependence on external load and nucleotide (dNTP) concentration. The data is inconsistent with power stroke models for translocation, instead supports a loose-coupling mechanism between chemical catalysis and mechanical translocation during DNA replication. According to this mechanism the DNA polymerase works by alternating between a dNTP/PPi-free state, which diffuses thermally between pre- and post-translocated states, and a dNTP/PPi-bound state where dNTP binding stabilizes the post-translocated state. We show how this thermal ratchet mechanism is used by the polymerase to generate work against large opposing loads (∼50 pN).


Development of a graded index microlens based fiber optical trap and its characterization using principal component analysis

J. Nylk, M. V. G. Kristensen, M. Mazilu, A. K. Thayil, C. A. Mitchell, E. C. Campbell, S. J. Powis, F. J. Gunn-Moore, and K. Dholakia

We demonstrate a miniaturized single beam fiber optical trapping probe based on a high numerical aperture graded index (GRIN) micro-objective lens. This enables optical trapping at a distance of 200μm from the probe tip. The fiber trapping probe is characterized experimentally using power spectral density analysis and an original approach based on principal component analysis for accurate particle tracking. Its use for biomedical microscopy is demonstrated through optically mediated immunological synapse formation.