Thursday, May 28, 2015

Optical force on diseased blood cells: Towards the optical sorting of biological matter

Juan Sebastian Totero Gongora, Andrea Fratalocchi

By employing a series of massively parallel ab-initio simulations, we study how optical forces act on biological matter subject to morphological disease. As a representative case study, we here consider the case of Plasmodium falciparum on red blood cells (RBC) illuminated by a monochromatic plane wave. Realistic parameters for the geometry and the refractive index are then taken from published experiments. In our theoretical campaign, we study the dependence of the optical force on the disease stage for different incident wavelengths. We show that optical forces change significantly with the disease, with amplitude variation in the hundreds of pN range. Our results open up new avenues for the design of new optical systems for the treatment of human disease.


Vibration-assisted optical injection of a single fluorescent sensor into a target cell

Hengjun Liu, Hisataka Maruyama, Taisuke Masuda, Fumihito Arai

In this paper, we propose the selective adhesion and rapid injection of a fluorescent sensor into a target cell via the optical control of zeta potential and local vibration stimulus using optical tweezers. A multi-fluorescent sensor, which can respond to both temperature and pH, was encapsulated in anionic lipid layers containing a photochromic material (spiropyran) via the layer-by-layer method. The zeta potential of the lipid layers containing spiropyran was adjusted from negative to positive by photo-isomerization of spiropyran using UV illumination. A single sensor was manipulated by optical tweezers and transferred to a cell surface, thereafter adhering selectively to the cell surface under UV illumination without excess sensor adhesion. We then drove the focal point of the optical tweezers to move up and down circularly near the sensor, mimicking a vibration on the sensor or rapid injection. The surface zeta potential of the liposome layers was measured using a zeta potential analyzer. The fluorescence resonance energy transfer (FRET) method was used to observe the changes in contact area between the adhered sensor and cell membrane before and after vibration. Holographic optical tweezers (HOT) and laser confocal microscopy were used to manipulate the single sensor and to capture fluorescent images. The results showed that the vibration applied on the sensor could push down the sensor, inducing a downward displacement. This displacement caused a corresponding deformation of the cell membrane, which increased the contact area between the sensor and the cell membrane. Without vibration, the sensor was injected into the cytoplasm in 5 h at an injection rate of 40%. By applying the vibration stimulus, we succeeded in the rapid injection of the sensor in 30 min at an injection rate of 80%.


Kinds of optically induced force derived from laws of conservation of the momentum and energy

V.P. Torchigin, A.V. Torchigin

Optically induced forces (OIF) applied to a transparent optical medium are analyzed. We deliberately do not use any assumptions about a nature of optically induced force and analyze thought experiments where only notions and laws of conservation are used. Two unambiguous but contradictory thought experiments are used as a ground of an analysis. It is shown that only two kinds of well-known OIF is sufficient to match contradictory results of the experiments. This is the kind of density force known in electrostatics that is responsible for the density force arising in an inhomogeneous dielectric located in an electrical field. This is the Abraham density force arising at propagation of a light pulse in an optical medium. The force is located in the regions of the optical medium where leading and trailing edges of a light pulse are propagating. OIF in a homogeneous optical medium located in an inhomogeneous electrical field is equal to zero at a steady-state. This result contradicts to that obtained by means of the widely used approach based on the Lorentz density force.


Direct Measurements of the Optical Cross Sections and Refractive Indices of Individual Volatile and Hygroscopic Aerosol Particles

B. J. Mason, M. I. Cotterell, T. C. Preston, A. J. Orr-Ewing, and J. P. Reid

We present measurements of the evolving extinction cross sections of individual aerosol particles (spanning 700–2500 nm in radius) during the evaporation of volatile components or hygroscopic growth using a combination of a single particle trap formed from a Bessel light beam and cavity ring-down spectroscopy. For single component organic aerosol droplets of 1,2,6-hexanetriol, polyethylene glycol 400, and glycerol, the slow evaporation of the organic component (over time scales of 1000 to 10 000 s) leads to a time-varying size and extinction cross section that can be used to estimate the refractive index of the droplet. Measurements on binary aqueous–inorganic aerosol droplets containing one of the inorganic solutes ammonium bisulfate, ammonium sulfate, sodium nitrate, or sodium chloride (over time scales of 1000 to 15 000 s) under conditions of changing relative humidity show that extinction cross-section measurements are consistent with expectations from accepted models for the variation in droplet refractive index with hygroscopic growth. In addition, we use these systems to establish an experimental protocol for future single particle extinction measurements. The advantages of mapping out the evolving light extinction cross-section of an individual particle over extended time frames accompanied by hygroscopic cycling or component evaporation are discussed.


Tuesday, May 26, 2015

Elastic theory for the deformation of a spherical dielectric biological object under electro-optical trapping

Md. Mozzammel Haque

The shear modulus of a dielectric spherical particle is investigated using a combination of triangular (or square) electrodes and a single-beam optical tweezer. The electronic response of a spherical dielectric particle is dominated by the local interactions with the trapping beams. Positive dielectrophoresis on a dielectric particle works at high frequencies. By measuring the geometrical parameters of the sphere as a function of the applied voltage, the elasticity of sphere is determined theoretically from the maximum applied voltage when a particle escapes to the electrode from trapping centre. To check the validity and efficiency of the elasticity derived mathematically, similar experimental results obtained by other techniques have been studied. The method is suggested to be a general tool to distinguish healthy and diseased cells.


Extended linear detection range for optical tweezers using a stop at the back focal plane of the condenser

S Masoumeh Mousavi, Akbar Samadi, Faegheh Hajizadeh and S Nader S Reihani

Optical tweezers are indispensable micro-manipulation tools. It is known that optical tweezers are force rather than position sensors due to the shorter linear range of their position detection system. In this paper, we have shown for the first time, that positioning an optical stop at the BFP of the condenser can overcome this problem by extending the linear detection range. This method would be valuable for the force spectroscopy applications of optical tweezers.


Creating and probing of a perfect vortex in situ with an optically trapped particle

Mingzhou Chen, Michael Mazilu, Yoshihiko Arita, Ewan M. Wright, Kishan Dholakia

We experimentally create a ‘perfect’ vortex beam, which has a uniform ring profile and fixed radius. In contrast with other vortex fields, the intensity profile is independent of its topological charge. We then correct this field in situ using a single trapped particle as a probe. This results in a constant angular velocity for the particle regardless of its position on the beam circumference.


An optical tweezer for complex plasmas

Jan Schablinski, Frank Wieben and Dietmar Block

This paper describes the experimental realization of an optical trap for microparticles levitating in the plasma sheath. Single particles can be trapped in a laser beam comparable to optical tweezers known from colloidal suspensions. The trapping mechanism is discussed and two applications of the system are shown.


Monday, May 25, 2015

Tomographic phase microscopy with 180° rotation of live cells in suspension by holographic optical tweezers

Mor Habaza, Barak Gilboa, Yael Roichman, and Natan T. Shaked

We present a new tomographic phase microscopy (TPM) approach that allows capturing the three-dimensional refractive index structure of single cells in suspension without labeling, using 180° rotation of the cells. This is obtained by integrating an external off-axis interferometer for wide-field wave front acquisition with holographic optical tweezers (HOTs) for trapping and micro-rotation of the suspended cells. In contrast to existing TPM approaches for cell imaging, our approach does not require anchoring the sample to a rotating stage, nor is it limited in angular range as is the illumination rotation approach. Thus, it allows noninvasive TPM of suspended live cells in a wide angular range. The proposed technique is experimentally demonstrated by capturing the three-dimensional refractive index map of yeast cells, while collecting interferometric projections at an angular range of 180° with 5° steps. The interferometric projections are processed by both the filtered back-projection method and the diffraction theory method. The experimental system is integrated with a spinning disk confocal fluorescent microscope for validation of the label-free TPM results.


The study of spermatozoa and sorting in relation to human reproduction

James Boon Yong Koh, Marcos

In this review article, we seek to provide a link between our understanding of the spermatozoa on the physical aspects and applications involving assisted reproduction, so that future research in this field can be better poised for improving current procedures. A brief discussion is included regarding the difference in the fluid mechanics of a Newtonian and viscoelastic fluid medium. A review is then done on the current microfluidic sorting techniques applicable to spermatozoa, which includes the albumin gradient separation, fluorescence activated flow cytometry, electrophoresis, dielectrophoresis, countercurrent distribution, movement of motile sperms, accumulation at walls, and optical trapping. Common preparation methods for spermatozoa used in assisted reproduction are also introduced. A number of other general particle manipulation methods which could potentially be incorporated for sperm sorting are also discussed.


Direct measurements of growing amorphous order and non-monotonic dynamic correlations in a colloidal glass-former

K. Hima Nagamanasa, Shreyas Gokhale, A. K. Sood & Rajesh Ganapathy

The transformation of flowing liquids into rigid glasses is thought to involve increasingly cooperative relaxation dynamics as the temperature approaches that of the glass transition. However, the precise nature of this motion is unclear, and a complete understanding of vitrification thus remains elusive. Of the numerous theoretical perspectives devised to explain the process, random first-order theory (RFOT) is a well-developed thermodynamic approach, which predicts a change in the shape of relaxing regions as the temperature is lowered. However, the existence of an underlying ‘ideal’ glass transition predicted by RFOT remains debatable, largely because the key microscopic predictions concerning the growth of amorphous order and the nature of dynamic correlations lack experimental verification. Here, using holographic optical tweezers, we freeze a wall of particles in a two-dimensional colloidal glass-forming liquid and provide direct evidence for growing amorphous order in the form of a static point-to-set length. We uncover the non-monotonic dependence of dynamic correlations on area fraction and show that this non-monotonicity follows directly from the change in morphology and internal structure of cooperatively rearranging regions. Our findings support RFOT and thereby constitute a crucial step in distinguishing between competing theories of glass formation.


Composite SERS-based satellites navigated by optical tweezers for single cell analysis

Inna Stetciura, Alexey Yashchenok, Admir Masic, Evgeny Lyubin, Olga Inozemtseva, Maria Drozdova, Elena Markvichova, Boris N. Khlebtsov, Andrey A. Fedyanin, Gleb Sukhorukov , Dmitry Alexandrovich Gorin and Dmitry Volodkin

Here we have designed composite SERS active micro-satellites possessing a dual role working as: i) effective probes for cellular composition and ii) optically movable and easily detectable markers. The satellites were synthesized by the layer-by-layer assisted decoration of silica microparticles with metal (gold or silver) nanoparticles and astralen in order to ensure satellite SERS-based microenvironment probing and satellite recognition, respectively. A combination of optical tweezers and Raman spectroscopy can be used to navigate the satellites to a certain cellular compartment followed by the cellular uptake and also to probe the intracellular composition. The developed approach may serve in future as a tool for the single cell analysis focusing on both extracellular and intracellular studies with nanometer precision due to the multilayer surface design.


Kinetically coupled folding of a single HIV-1 glycoprotein 41 complex in viral membrane fusion and inhibition

Junyi Jiao, Aleksander A. Rebane, Lu Ma, Ying Gao, and Yongli Zhang

HIV-1 glycoprotein 41 (gp41) mediates viral entry into host cells by coupling its folding energy to membrane fusion. Gp41 folding is blocked by fusion inhibitors, including the commercial drug T20, to treat HIV/AIDS. However, gp41 folding intermediates, energy, and kinetics are poorly understood. Here, we identified the folding intermediates of a single gp41 trimer-of-hairpins and measured their associated energy and kinetics using high-resolution optical tweezers. We found that folding of gp41 hairpins was energetically independent but kinetically coupled: Each hairpin contributed a folding energy of ∼−23 kBT, but folding of one hairpin successively accelerated the folding rate of the next one by ∼20-fold. Membrane-mimicking micelles slowed down gp41 folding and reduced the stability of the six-helix bundle. However, the stability was restored by cooperative folding of the membrane-proximal external region. Surprisingly, T20 strongly inhibited gp41 folding by actively displacing the C-terminal hairpin strand in a force-dependent manner. The inhibition was abolished by a T20-resistant gp41 mutation. The energetics and kinetics of gp41 folding established by us provides a basis to understand viral membrane fusion, infection, and therapeutic intervention.


Radiation pressure on plane dielectric surfaces

V.P. Torchigin, A.V. Torchigin

It is shown on the basis of unambiguous thought and real experiments regarding the pressure produced by light on a plane boundary of optical medium that the Abraham force takes part in the production of the pressure. As a result, all hitherto known information obtained by using another approach based on the Lorentz force should be corrected. Radiation pressures produced by a continuous light wave and a light pulse on the simplest plane dielectric surfaces are presented. The pressure produced by a continuous travelling light wave incident from ree space on a semi-infinite dielectric is negative and is equal -W0(n-1)(n + 1) where W0 is the energy density of light in free space, n is the refractive index. The pressure produced by the leading edge of a light pulse at the entrance to the dielectric is positive and equal W0(n-1)(n + 1). Pressures produced by a continuous light wave and light pulse on a semi-infinite dielectric with anti-reflection λ/4 coating are equal W0(1-n) and W0(1-1/n), respectively.


Friday, May 22, 2015

Chiral discrimination in optical binding

Kayn A. Forbes and David L. Andrews

The laser-induced intermolecular force that exists between two or more particles in the presence of an electromagnetic field is commonly termed “optical binding.” Distinct from the single-particle forces that are at play in optical trapping at the molecular level, the phenomenon of optical binding is a manifestation of the coupling between optically induced dipole moments in neutral particles. In other, more widely known areas of optics, there are many examples of chiral discrimination—signifying the different response a chiral material has to the handedness of an optical input. In the present analysis, extending previous work on chiral discrimination in optical binding, a mechanism is identified using a quantum electrodynamical approach. It is shown that the optical binding force between a pair of chiral molecules can be significantly discriminatory in nature, depending upon both the handedness of the interacting particles and the polarization of the incident light, and it is typically several orders of magnitude larger than previously reported.


Joule heating monitoring in a microfluidic channel by observing the Brownian motion of an optically trapped microsphere

Toon Brans, Filip Strubbe, Caspar Schreuer, Stijn Vandewiele, Kristiaan Neyts and Filip Beunis

Electric fields offer a variety of functionalities to Lab-on-a-Chip devices. The use of these fields often results in significant Joule heating, affecting the overall performance of the system. Precise knowledge of the temperature profile inside a microfluidic device is necessary to evaluate the implications of heat dissipation. This article demonstrates how an optically trapped microsphere can be used as a temperature probe to monitor Joule heating in these devices. The Brownian motion of the bead at room temperature is compared with the motion when power is dissipated in the system. This gives an estimate of the temperature increase at a specific location in a microfluidic channel. We demonstrate this method with solutions of different ionic strengths, and establish a precision of 0.9 K and an accuracy of 15%. Furthermore it is demonstrated that transient heating processes can be monitored with this technique, albeit with a limited time resolution.


Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations

Ryan D. Davis, Sara Lance, Joshua A. Gordon, and Margaret A. Tolbert

A novel optical trapping technique is described that combines an upward propagating Gaussian beam and a downward propagating Bessel beam. Using this optical arrangement and an on-demand droplet generator makes it possible to rapidly and reliably trap particles with a wide range of particle diameters (∼1.5–25 μm), in addition to crystalline particles, without the need to adjust the optical configuration. Additionally, a new image analysis technique is described to detect particle phase transitions using a template-based autocorrelation of imaged far-field elastically scattered laser light. The image analysis allows subtle changes in particle characteristics to be quantified. The instrumental capabilities are validated with observations of deliquescence and homogeneous efflorescence of well-studied inorganic salts. Then, a novel collision-based approach to seeded crystal growth is described in which seed crystals are delivered to levitated aqueous droplets via a nitrogen gas flow. To our knowledge, this is the first account of contact-induced phase changes being studied in an optical trap. This instrument offers a novel and simple analytical technique for in situ measurements and observations of phase changes and crystal growth processes relevant to atmospheric science, industrial crystallization, pharmaceuticals, and many other fields.


Microfluidics for Research and Applications in Oncology

Parthiv Chaudhuri, Majid Ebrahimi Warkiani, Tengyang Jing, Kenry Kenry and Chwee Teck Lim

Cancer is currently one of the top non-communicable human diseases and continual research and developmental efforts are being made to better understand and manage this disease. More recently, with improved understanding in cancer biology as well as advancement made in microtechnology and rapid prototyping, microfluidics is increasingly being explored and even validated for use in the detection, diagnosis and treatment of cancer. With inherent advantages such as small sample volume, high sensitivity and fast processing time, microfluidics is well-positioned to serve as a promising platform for applications in oncology. In this review, we look at recent advances in the use of microfluidics - from basic research such as understanding cancer cell phenotypes as well as metastatic behaviors to applications such as detection, diagnosis, prognosis and drug screening. We then conclude with a future outlook on this promising technology.


Stick-slip motion of surface point defects prompted by magnetically controlled colloidal-particle dynamics in nematic liquid crystals

Michael C. M. Varney, Qiaoxuan Zhang, and Ivan I. Smalyukh

We explore the dynamics of topological point defects on surfaces of magnetically responsive colloidal microspheres in a uniformly aligned nematic liquid crystal host. We show that pinning of the liquid crystal director to a particle surface with random nanostructured morphology results in unexpected translational dynamics of both particles and topological point defects on their surfaces when subjected to rotating magnetic fields. We characterize and quantify the “stick-slip” motion of defects as a function of field rotation rates as well as temperature, demonstrating the roles played by the competition of elastic forces, surface anchoring, and magnetic torques on the sphere as well as random-surface-mediated pinning of the easy axis of the nematic director on colloidal microspheres. We analyze our findings through their comparison to similar dynamic processes in other branches of science.


Thursday, May 14, 2015

Untethered photonic sensor for wall pressure measurement

Maurizio Manzo and Tindaro Ioppolo

In this Letter, we study a novel untethered photonic wall pressure sensor that uses as sensing element a dome-shaped micro-scale laser. Since the sensor does not require any optical or electrical cabling, it allows measurements where cabling tends to be problematic. The micro-laser is made by a mixture of Trimethylolpropane Tri(3-mercaptopropionate), commercial name THIOCURE and Polyethylene (glycol) Diacrylate (PEGDA) mixed with a solution of rhodamine 6G. Two different volume ratios between the THIOCURE and the PEGDA are studied, since different ratios lead to different mechanical properties. In addition, two different sensor configurations are presented: (i) sensor coupled to a membrane, that allows differential wall pressure measurement and (ii) sensor without membrane that allows absolute wall pressure measurement. The sensitivity plots are presented in the paper for both sensor configurations and polymer ratios.


Chirality in Optical Trapping and Optical Binding

David S. Bradshaw , Kayn A. Forbes , Jamie M. Leeder and David L. Andrews
Optical trapping is a well-established technique that is increasingly used on biological substances and nanostructures. Chirality, the property of objects that differ from their mirror image, is also of significance in such fields, and a subject of much current interest. This review offers insight into the intertwining of these topics with a focus on the latest theory. Optical trapping of nanoscale objects involves forward Rayleigh scattering of light involving transition dipole moments; usually these dipoles are assumed to be electric although, in chiral studies, magnetic dipoles must also be considered. It is shown that a system combining optical trapping and chirality could be used to separate enantiomers. Attention is also given to optical binding, which involves light induced interactions between trapped particles. Interesting effects also arise when binding is combined with chirality.


Wednesday, May 13, 2015

A Molecular Tuning Fork in Single-Molecule Mechanochemical Sensing

Shankar Mandal, Deepak Koirala, Sangeetha Selvam, Chiran Ghimire and Prof. Hanbin Mao

The separate arrangement of target recognition and signal transduction in conventional biosensors often compromises the real-time response and can introduce additional noise. To address these issues, we combined analyte recognition and signal reporting by mechanochemical coupling in a single-molecule DNA template. We incorporated a DNA hairpin as a mechanophore in the template, which, under a specific force, undergoes stochastic transitions between folded and unfolded hairpin structures (mechanoescence). Reminiscent of a tuning fork that vibrates at a fixed frequency, the device was classified as a molecular tuning fork (MTF). By monitoring the lifetime of the folded and unfolded hairpins with equal populations, we were able to differentiate between the mono- and bivalent binding modes during individual antibody-antigen binding events. We anticipate these mechanospectroscopic concepts and methods will be instrumental for the development of novel bioanalyses.

Femtosecond Optical Trap-assisted Nanopatterning Through Microspheres by a Single Ti:Sapphire Oscillator

Aleksander Shakhov , Artyom Astafiev , Dmytro Plutenko , Oleg M. Sarkisov , Anatoly I. Shushin , and Victor Andreevich Nadtochenko

A new approach to fabricating the various range of patterns using femtosecond optical trap assisted nanopatterning is presented. Here we report how a single Gaussian laser beam from a 55 fs 80 MHz 780 nm Ti:Sapphire oscillator trapping dielectric microspheres near surfaces can be used to enable near-field direct-write subwavelength ~λ/6 (~130 nm) 2D nanopartening of polymer surface. We discuss the stability conditions for effective manipulation of the particle by the pulsed beam. A Klein-Kramers and Brownian motion models were used to analyze the positional accuracy of femtosecond tweezers. We studied effects of the microsphere size, pulsed laser energy and light polarization, spacing between objective focal plane and polymer surface on the pattern size experimentally and theoretically. Microspheres with a diameter of about 1 μm provide the smallest patterns. The experimental results are reasonably matched by Generalized Lorentz Mie theory.


Photon momentum transfer in inhomogeneous dielectric mixtures and induced tractor beams

Cheng-Wei Qiu, Weiqiang Ding, M.R.C. Mahdy, Dongliang Gao, Tianhang Zhang, Fook Chiong Cheong, Aristide Dogariu, Zheng Wang and Chwee Teck Lim

The determination of optical force as a consequence of momentum transfer is inevitably subject to the use of the proper momentum density and stress tensor. It is imperative and valuable to consider the intrinsic scheme of photon momentum transfer, particularly when a particle is embedded in a complex dielectric environment. Typically, we consider a particle submerged in an inhomogeneous background composed of different dielectric materials, excluding coherent illumination or hydrodynamic effects. A ray-tracing method is adopted to capture the direct process of momentum transfer from the complex background medium, and this approach is validated using the modified Einstein–Laub method, which uses only the interior fields of the particle in the calculation. In this way, debates regarding the calculation of the force with different stress tensors using exterior fields can be avoided. Our suggested interpretation supports only the Minkowski approach for the optical momentum transfer to the embedded scatterer while rejecting Peierls's and Abraham's approaches, though the momentum of a stably moving photon in a continuous background medium should be considered to be of the Abraham type. Our interpretation also provides a novel method of realizing a tractor beam for the exertion of negative force that offers an alternative to the use of negative-index materials, optical gain, or highly non-paraxial or multiple-light interference.


Optical trapping of the anisotropic crystal nanorod

Paul B. Bareil and Yunlong Sheng

We observed in the optical tweezers experiment that some anisotropic nanorod was stably trapped in an orientation tiled to the beam axis. We explain this trapping with the T-matrix calculation. As the vector spherical wave functions do not individually satisfy the anisotropic vector wave equation, we expand the incident and scattered fields in the isotropic buffer in terms of E→, and the internal field in the anisotropic nanoparticle in terms of D→, and use the boundary condition for the normal components of D→ to compute the T-matrix. We found that when the optical axes of an anisotropic nanorod are not aligned to the nanorod axis, the nanorod may be trapped stably at a tilted angle, under which the lateral torque equals to zero and the derivative of the torque is negative.


Monday, May 11, 2015

Cytoplasmic dynein regulates its attachment to microtubules via nucleotide state-switched mechanosensing at multiple AAA domains

Matthew P. Nicholas, Florian Berger, Lu Rao, Sibylle Brenner, Carol Cho, and Arne Gennerich

Cytoplasmic dynein is a homodimeric microtubule (MT) motor protein responsible for most MT minus-end–directed motility. Dynein contains four AAA+ ATPases (AAA: ATPase associated with various cellular activities) per motor domain (AAA1–4). The main site of ATP hydrolysis, AAA1, is the only site considered by most dynein motility models. However, it remains unclear how ATPase activity and MT binding are coordinated within and between dynein’s motor domains. Using optical tweezers, we characterize the MT-binding strength of recombinant dynein monomers as a function of mechanical tension and nucleotide state. Dynein responds anisotropically to tension, binding tighter to MTs when pulled toward the MT plus end. We provide evidence that this behavior results from an asymmetrical bond that acts as a slip bond under forward tension and a slip-ideal bond under backward tension. ATP weakens MT binding and reduces bond strength anisotropy, and unexpectedly, so does ADP. Using nucleotide binding and hydrolysis mutants, we show that, although ATP exerts its effects via binding AAA1, ADP effects are mediated by AAA3. Finally, we demonstrate “gating” of AAA1 function by AAA3. When tension is absent or applied via dynein’s C terminus, ATP binding to AAA1 induces MT release only if AAA3 is in the posthydrolysis state. However, when tension is applied to the linker, ATP binding to AAA3 is sufficient to “open” the gate. These results elucidate the mechanisms of dynein–MT interactions, identify regulatory roles for AAA3, and help define the interplay between mechanical tension and nucleotide state in regulating dynein motility.


Fiber-optic SERS microfluidic chip based on light-induced gold nano-particle aggregation

Haitao Liu, Jiansheng Liu, Shaopeng Li, Luoyang Chen, Hongwen Zhou, Jinsong Zhu, Zheng Zheng

A novel optofluidic surface-enhanced Raman scattering (SERS) chip was specially designed and fabricated using polydimethylsiloxane (PDMS) and embedded with normal silica multi-mode optical fibers. Unlike in a conventional Raman detection configuration where an angle of 90° is commonly adopted, here the orientations of the excitation fiber and the collection fiber was set at such an obtuse angle so that the light beam from the excitation fiber can illuminate the endface, but is not within the acceptance angle of the collection fiber. It was found that with the laser irradiating on the endface of the collection fiber in the sample solution, the Raman scattering intensity continued to grow and a level about 30-times than its initial intensity was observed, which was understood by light-induced gold nano-particle aggregation. The effects of fibers' coupling angles, positions and laser irradiation power on the aggregation were investigated.


Surfing along Filopodia: A Particle Transport Revealed by Molecular-Scale Fluctuation Analyses

Felix Kohler, Alexander Rohrbach

Filopodia perform cellular functions such as environmental sensing or cell motility, but they also grab for particles and withdraw them leading to an increased efficiency of phagocytic uptake. Remarkably, withdrawal of micron-sized particles is also possible without noticeable movements of the filopodia. Here, we demonstrate that polystyrene beads connected by optical tweezers to the ends of adherent filopodia of J774 macrophages, are transported discontinuously toward the cell body. After a typical resting time of 1–2 min, the cargo is moved with alternating velocities, force constants, and friction constants along the surface of the filopodia. This surfing-like behavior along the filopodium is recorded by feedback-controlled interferometric three-dimensional tracking of the bead motions at 10–100 kHz. We measured transport velocities of up to 120 nm/s and transport forces of ∼70 pN. Small changes in position, fluctuation width, and temporal correlation, which are invisible in conventional microscopy, indicate molecular reorganization of transport-relevant proteins in different phases of the entire transport process. A detailed analysis implicates a controlled particle transport with fingerprints of a nanoscale unbinding/binding behavior. The manipulation and analysis methods presented in our study may also be helpful in other fields of cellular biophysics.


T Cells Have a Light Touch

Michael L. Dustin

There is great interest in the force generated by chemotacting T cells because force is proposed to be a key ingredient in the triggering of the T cell antigen receptor (TCR) and forming of a functional immunological synapse. This has been a challenging problem because methods typically used to measure forces with stromal cells cannot be applied to relatively less adherent and rapidly moving lymphocytes. Yang et al. (1) have successfully measured the light touch of a chemotacting lymphocyte with a tool previously reserved for molecular studies—the optical or laser trap.


Friday, May 8, 2015

A Dynamic Model of Chemoattractant-Induced Cell Migration

Hao Yang, Xue Gou, Yong Wang, Tarek M. Fahmy, Anskar Y.-H. Leung, Jian Lu, Dong Sun

Cell migration refers to a directional cell movement in response to chemoattractant stimulation. In this work, we developed a cell-migration model by mimicking in vivo migration using optically manipulated chemoattractant-loaded microsources. The model facilitates a quantitative characterization of the relationship among the protrusion force, cell motility, and chemoattractant gradient for the first time (to our knowledge). We verified the correctness of the model using migrating leukemia cancer Jurkat cells. The results show that one can achieve the ideal migrating capacity by choosing the appropriate chemoattractant gradient and concentration at the leading edge of the cell.


Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers

Giuseppe Pesce, Giulia Rusciano, Gianluigi Zito, and Antonio Sasso

Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simultaneous measurements of electrophoretic and dielectrophoretic forces. We first determine the electric charge carried by a single bead by keeping it in a predictable uniform electric field produced by two parallel planar electrodes, then, we examine same bead’s response in proximity to a tip electrode. In this case, besides electric forces, the bead simultaneously experiences non-negligible dielectrophoretic forces produced by the strong electric field gradient. The stochastic and deterministic motions of the trapped bead are theoretically and experimentally analysed in terms of the autocorrelation function. By fitting the experimental data, we are able to extract simultaneously the spatial distribution of electrophoretic and dielectrophoretic forces around the tip. Our approach can be used for determining actual, total force components in the presence of high-curvature electrodes or metal scanning probe tips.


Red blood cell as an adaptive optofluidic microlens

L. Miccio, P. Memmolo, F. Merola, P. A. Netti & P. Ferraro

The perspective of using live cells as lenses could open new revolutionary and intriguing scenarios in the future of biophotonics and biomedical sciences for endoscopic vision, local laser treatments via optical fibres and diagnostics. Here we show that a suspended red blood cell (RBC) behaves as an adaptive liquid-lens at microscale, thus demonstrating its imaging capability and tunable focal length. In fact, thanks to the intrinsic elastic properties, the RBC can swell up from disk volume of 90 fl up to a sphere reaching 150 fl, varying focal length from negative to positive values. These live optofluidic lenses can be fully controlled by triggering the liquid buffer’s chemistry. Real-time accurate measurement of tunable focus capability of RBCs is reported through dynamic wavefront characterization, showing agreement with numerical modelling. Moreover, in analogy to adaptive optics testing, blood diagnosis is demonstrated by screening abnormal cells through focal-spot analysis applied to an RBC ensemble as a microlens array.


Automatic real time evaluation of red blood cell elasticity by optical tweezers

Diógenes S. Moura, Diego C. N. Silva, Ajoke J. Williams, Marcos A. C. Bezerra, Adriana Fontes and Renato E. de Araujo

Optical tweezers have been used to trap, manipulate, and measure individual cell properties. In this work, we show that the association of a computer controlled optical tweezers system with image processing techniques allows rapid and reproducible evaluation of cell deformability. In particular, the deformability of red blood cells (RBCs) plays a key role in the transport of oxygen through the blood microcirculation. The automatic measurement processes consisted of three steps: acquisition, segmentation of images, and measurement of the elasticity of the cells. An optical tweezers system was setup on an upright microscope equipped with a CCD camera and a motorized XYZ stage, computer controlled by a Labview platform. On the optical tweezers setup, the deformation of the captured RBC was obtained by moving the motorized stage. The automatic real-time homemade system was evaluated by measuring RBCs elasticity from normal donors and patients with sickle cell anemia. Approximately 150 erythrocytes were examined, and the elasticity values obtained by using the developed system were compared to the values measured by two experts. With the automatic system, there was a significant time reduction (60 × ) of the erythrocytes elasticity evaluation. Automated system can help to expand the applications of optical tweezers in hematology and hemotherapy.


Thursday, May 7, 2015

Cavity Cooling a Single Charged Levitated Nanosphere

J. Millen, P. Z. G. Fonseca, T. Mavrogordatos, T. S. Monteiro, and P. F. Barker

Optomechanical cavity cooling of levitated objects offers the possibility for laboratory investigation of the macroscopic quantum behavior of systems that are largely decoupled from their environment. However, experimental progress has been hindered by particle loss mechanisms, which have prevented levitation and cavity cooling in a vacuum. We overcome this problem with a new type of hybrid electro-optical trap formed from a Paul trap within a single-mode optical cavity. We demonstrate a factor of 100 cavity cooling of 400 nm diameter silica spheres trapped in vacuum. This paves the way for ground-state cooling in a smaller, higher finesse cavity, as we show that a novel feature of the hybrid trap is that the optomechanical cooling becomes actively driven by the Paul trap, even for singly charged nanospheres.


Trapping and assembling of particles and live cells on large-scale random gold nano-island substrates

Zhiwen Kang, Jiajie Chen, Shu-Yuen Wu, Kun Chen, Siu-Kai Kong, Ken-Tye Yong & Ho-Pui Ho

We experimentally demonstrated the use of random plasmonic nano-islands for optical trapping and assembling of particles and live cells into highly organized pattern with low power density. The observed trapping effect is attributed to the net contribution due to near-field optical trapping force and long-range thermophoretic force, which overcomes the axial convective drag force, while the lateral convection pushes the target objects into the trapping zone. Our work provides a simple platform for on-chip optical manipulation of nano- and micro-sized objects, and may find applications in physical and life sciences.

Saturation Vapor Pressures and Transition Enthalpies of Low-Volatility Organic Molecules of Atmospheric Relevance: From Dicarboxylic Acids to Complex Mixtures

Merete Bilde, Kelley Barsanti, Murray Booth, Christopher D. Cappa, Neil M. Donahue, Eva U. Emanuelsson, Gordon McFiggans, Ulrich K. Krieger, Claudia Marcolli, David Topping, Paul Ziemann, Mark Barley, Simon Clegg, Benjamin Dennis-Smither, Mattias Hallquist, Åsa M. Hallquist, Andrey Khlystov, Markku Kulmala, Ditte Mogensen, Carl J. Percival, Francis Pope, Jonathan P. Reid, M. A. V. Ribeiro da Silva, Thomas Rosenoern, Kent Salo, Vacharaporn Pia Soonsin, Taina Yli-Juuti, Nønne L. Prisle, Joakim Pagels, Juergen Rarey, Alessandro A. Zardini, and Ilona Riipinen

Aerosol particles are important constituents of the atmosphere. They impact modern society through their effects on visibility, human health, and global climate. Despite this great importance, they continue to represent a challenge to scientists due to their complexity. Atmospheric aerosols have both natural and anthropogenic sources and consist of both organic and inorganic molecules. Organic compounds constitute 20−90% of the atmospheric aerosol particle mass, depending on the location. The term primary aerosol particle is used to describe particles that are emitted directly into the atmosphere as particles. These primary particles are transformed in the atmosphere through the continuous exchange between the gas and particle phases via evaporation and condensation. Additionally, a large fraction of the organic particulate mass results from condensation of vapors that are produced by chemical reactions in the gas phase and is termed secondary organic aerosol (SOA). To predict the temporal and spatial distribution of aerosols, particularly SOA, it is necessary to understand the fundamental parameters that govern the distribution of organic compounds between the gas and particle phases. Key thermodynamic properties describing the equilibrium gas to particle partitioning of organic compounds are the saturation vapor pressures and the enthalpies of vaporization and sublimation.


Dynamic buckling of actin within filopodia

Natascha Leijnse, Lene B Oddershede & Poul M Bendix

Filopodia are active tubular structures protruding from the cell surface which allow the cell to sense and interact with the surrounding environment through repetitive elongation-retraction cycles. The mechanical behavior of filopodia has been studied by measuring the traction forces exerted on external substrates.1 These studies have revealed that internal actin flow can transduce a force across the cell surface through transmembrane linkers like integrins. In addition to the elongation-retraction behavior filopodia also exhibit a buckling and rotational behavior. Filopodial buckling in conjunction with rotation enables the cell to explore a much larger 3-dimensional space and allows for more complex, and possibly stronger, interactions with the external environment.2 Here we focus on how bending of the filopodial actin dynamically correlates with pulling on an optically trapped microsphere which acts like an external substrate attached to the filopodial tip. There is a clear correlation between presence of actin near the tip and exertion of a traction force, thus demonstrating that the traction force is transduced along the actin shaft inside the filopodium. By extending a filopodium and holding it while measuring the cellular response, we also monitor and analyze the waiting times for the first buckle observed in the fluorescently labeled actin shaft.


Optical pulling of airborne absorbing particles and smut spores over a meter-scale distance with negative photophoretic force

Jinda Lin, Adam G. Hart and Yong-qing Li

We demonstrate optical pulling of single light-absorbing particles and smut spores in air over a meter-scale distance using a single collimated laser beam based on negative photophoretic force. The micron-sized particles are pulled towards the light source at a constant speed of 1–10 cm/s in the optical pulling pipeline while undergoing transverse rotation at 0.2–10 kHz. The pulled particles can be manipulated and precisely positioned on the entrance window with an accuracy of ∼20 μm, and their chemical compositions can be characterized with micro-Raman spectroscopy.


Tuesday, May 5, 2015

Concept for laser-assisted nano removal beyond the diffraction limit using photocatalyst nanoparticles

S. Takahashi, Y. Horita, F. Kaji, Y. Yamaguchi, M. Michihata, K. Takamasu

A new concept for the laser-assisted removal of material is proposed for achieving nanoscale correction in next-generation functional microstructures such as nanostructured photoresist surfaces and micro 3-D objects fabricated using microstereolithography. This proposed method is characterized by the entrapment of TiO2 photocatalyst nanoparticles by a remotely controlled radiation force, which allows not only for remote processing using the inherent properties of light, but also a fine process resolution that goes beyond the limits of diffraction focusing. Both theoretical and experimental analyses are used to verify the basic feasibility of this proposed concept.


Probing Mechanical Properties of Jurkat Cells under the Effect of ART Using Oscillating Optical Tweezers

Samaneh Khakshour, Timothy V. Beischlag, Carolyn Sparrey, Edward J. Park

Acute lymphoid leukemia is a common type of blood cancer and chemotherapy is the initial treatment of choice. Quantifying the effect of a chemotherapeutic drug at the cellular level plays an important role in the process of the treatment. In this study, an oscillating optical tweezer was employed to characterize the frequency-dependent mechanical properties of Jurkat cells exposed to the chemotherapeutic agent, artesunate (ART). A motion equation for a bead bound to a cell was applied to describe the mechanical characteristics of the cell cytoskeleton. By comparing between the modeling results and experimental results from the optical tweezer, the stiffness and viscosity of the Jurkat cells before and after the ART treatment were obtained. The results demonstrate a weak power-law dependency of cell stiffness with frequency. Furthermore, the stiffness and viscosity were increased after the treatment. Therefore, the cytoskeleton cell stiffness as the well as power-law coefficient can provide a useful insight into the chemo-mechanical relationship of drug treated cancer cells and may serve as another tool for evaluating therapeutic performance quantitatively.


Optomechanics of random media

S. Gentilini and C. Conti

Using light to control the movement of nanostructured objects is a great challenge. This challenge involves fields like optical tweezing, Casimir forces, integrated optics, biophysics, and many others. However, when the complexity of the light-activated devices increases, disorder unavoidably occurs and induces a number of effects, such as multiple-scattering, diffusion, and the localization of light. We show that these effects radically enhance the mechanical effect of light. We determine theoretically the link between optical pressure and the light diffusion coefficient and unveil that optical forces and their statistical fluctuations reach a maximum at the onset of the photon localization. Disorder may thus be exploited for increasing the mechanical action of light on complex objects.


Diagnostic Tools for Lab-on-Chip Applications Based on Coherent Imaging Microscopy

Merola, F.; Memmolo, P. ; Miccio, L. ; Bianco, V. ; Paturzo, M. ; Ferraro, P.

Today, fast and accurate diagnosis through portable and cheap devices is in high demand for the general healthcare. Lab-on-chips (LoCs) have undergone a great growth in this direction, supported by optical imaging techniques more and more refined. Here we present recent progresses in developing imaging tools based on coherent imaging microscopy that can be very useful when applied into biomicrofluidics. In some cases, the optical tweezers (OT) technique is combined with digital holography (DH), thus offering the possibility to manipulate, analyze, and measure fundamental parameters of different kinds of cells. This approach can open the route for rapid and high-throughput analysis in label-free microfluidic devices and for prognostic based on cell examination, thus allowing advancements in biomedical science.


Probing DNA interactions with proteins using a single-molecule toolbox: inside the cell, in a test tube and in a computer

Adam J. M. Wollman, Helen Miller, Zhaokun Zhou and Mark C. Leake

DNA-interacting proteins have roles in multiple processes, many operating as molecular machines which undergo dynamic meta-stable transitions to bring about their biological function. To fully understand this molecular heterogeneity, DNA and the proteins that bind to it must ideally be interrogated at a single molecule level in their native in vivo environments, in a time-resolved manner, fast enough to sample the molecular transitions across the free-energy landscape. Progress has been made over the past decade in utilizing cutting-edge tools of the physical sciences to address challenging biological questions concerning the function and modes of action of several different proteins which bind to DNA. These physiologically relevant assays are technically challenging but can be complemented by powerful and often more tractable in vitro experiments which confer advantages of the chemical environment with enhanced detection signal-to-noise of molecular signatures and transition events. In the present paper, we discuss a range of techniques we have developed to monitor DNA–protein interactions in vivo, in vitro and in silico. These include bespoke single-molecule fluorescence microscopy techniques to elucidate the architecture and dynamics of the bacterial replisome and the structural maintenance of bacterial chromosomes, as well as new computational tools to extract single-molecule molecular signatures from live cells to monitor stoichiometry, spatial localization and mobility in living cells. We also discuss recent developments from our laboratory made in vitro, complementing these in vivo studies, which combine optical and magnetic tweezers to manipulate and image single molecules of DNA, with and without bound protein, in a new super-resolution fluorescence microscope.