Saturday, February 28, 2015

Manipulating ellipsoidal micro-particles by femtosecond vortex tweezers

Changxia Liu, Zhongyi Guo, Yan Li, Xinshun Wang and Shiliang Qu

We study the manipulation of the ellipsoidal micro-particles by the femtosecond vortex tweezer experimentally and theoretically. In our setup the micro-particles can be rotated stably by means of optical torque induced by the femtosecond optical whirl. In order to give insight into observed effects, we establish two adequate theoretical models: one is based upon assumption of full absorption and the other uses the ray tracing method. The numerical simulations agree well with the experimental results.

The lensing effect of trapped particles in a dual-beam optical trap

Steffen Grosser, Anatol W. Fritsch, Tobias R. Kießling, Roland Stange, and Josef A. Käs

In dual-beam optical traps, two counterpropagating, divergent laser beams emitted from opposing laser fibers trap and manipulate dielectric particles. We investigate the lensing effect that trapped particles have on the beams. Our approach makes use of the intrinsic coupling of a beam to the opposing fiber after having passed the trapped particle. We present measurements of this coupling signal for PDMS particles, as well as a model for its dependence on size and refractive index of the trapped particle. As a more complex sample, the coupling of inhomogeneous biological cells is measured and discussed. We show that the lensing effect is well captured by the simple ray optics approximation. The measurements reveal intricate details, such as the thermal lens effect of the beam propagation in a dual-beam trap. For a particle of known size, the model further allows to infer its refractive index simply from the coupling signal.


The Aqueous Stability of a Mars Salt Analog: Instant Mars

D. L. Nuding, R. D. Davis, R. V. Gough and M. A. Tolbert

Due to their stability in low temperature conditions, aqueous salt solutions are the favored explanation for potential fluid features observed on present-day Mars. A salt analog was developed to closely match the individual cation and anion concentrations at the Phoenix landing site as reported by the Wet Chemistry Laboratory instrument. ’Instant Mars’ closely replicates correct relative concentrations of magnesium, calcium, potassium, sodium, perchlorate, chloride, and sulfate ions. A Raman microscope equipped with an environmental cell probed liquid water uptake and loss by Instant Mars particles in a Mars relevant temperature and relative humidity (RH) environment. Our experiments reveal that Instant Mars particles can form stable, aqueoussolutions starting at 56 ± 5% RH between 235–243 K and persist as a metastable, aqueous solution at or above 13 ± 5% RH. Particle levitation using an optical trap examined the phase state and morphology of suspended Instant Marsparticles exposed to changing water vapor conditions at room temperature. Levitation experiments indicate that water uptake began at 42 ± 8% RH for Instant Mars particles at 293 K. As RH is decreased at 293 K, the aqueous Instant Mars particles transition into a crystalline solid at 18 ± 7% RH. These combined results demonstrate that Instant Mars can take up water vapor from the surrounding environment and transition into a stable, aqueous solution. Furthermore, this aqueous Instant Mars solution can persist as a metastable, supersaturated solution in low RH conditions.

Doing the Waltz with Light

Monika Ritsch-Marte

Light with orbital angular momentum can be compared to a bundle of spaghetti held together firmly and twisted in the middle. Such optical spiral waves have numerous applications. For example, they can be used to selectively sort particles with optical tweezers. In microscopy, they enable edge enhancement, and in quantum cryptography, spiral waves allow for more information to be entangled.


Wednesday, February 25, 2015

Transitional behavior in hydrodynamically coupled oscillators

S. Box, L. Debono, D. B. Phillips, and S. H. Simpson

In this article we consider the complete set of synchronized and phase-locked states available to pairs of hydrodynamically coupled colloidal rotors, consisting of spherical beads driven about circular paths in the same, and in opposing senses. Oscillators such as these have previously been used as coarse grained, minimal models of beating cilia. Two mechanisms are known to be important in establishing synchrony. The first involves perturbation of the driving force, and the second involves deformation of the rotor trajectory. We demonstrate that these mechanisms are of similar strength, in the regime of interest, and interact to determine observed behavior. Combining analysis and simulation with experiments performed using holographic optical tweezers, we show how varying the amplitude of the driving force perturbation leads to a transition from synchronized to phase-locked states. Analogies with biological systems are discussed, as are implications for the design of biomimetic devices.


Single-molecule kinetics and footprinting of DNA bis-intercalation: the paradigmatic case of Thiocoraline

Joan Camunas-Soler, Maria Manosas, Silvia Frutos, Judit Tulla-Puche, Fernando Albericio and Felix Ritort

DNA bis-intercalators are widely used in molecular biology with applications ranging from DNA imaging to anticancer pharmacology. Two fundamental aspects of these ligands are the lifetime of the bis-intercalated complexes and their sequence selectivity. Here, we perform single-molecule optical tweezers experiments with the peptide Thiocoraline showing, for the first time, that bis-intercalation is driven by a very slow off-rate that steeply decreases with applied force. This feature reveals the existence of a long-lived (minutes) mono-intercalated intermediate that contributes to the extremely long lifetime of the complex (hours). We further exploit this particularly slow kinetics to determine the thermodynamics of binding and persistence length of bis-intercalated DNA for a given fraction of bound ligand, a measurement inaccessible in previous studies of faster intercalating agents. We also develop a novel single-molecule footprinting technique based on DNA unzipping and determine the preferred binding sites of Thiocoraline with one base-pair resolution. This fast and radiolabelling-free footprinting technique provides direct access to the binding sites of small ligands to nucleic acids without the need of cleavage agents. Overall, our results provide new insights into the binding pathway of bis-intercalators and the reported selectivity might be of relevance for this and other anticancer drugs interfering with DNA replication and transcription in carcinogenic cell lines.


An Optically Controlled Microscale Elevator Using Plasmonic Janus Particles

Spas Nedev, Sol Carretero-Palacios, Paul Kühler, Theobald Loehmuller, Alexander S. Urban, Lindsey JE Anderson, and Jochen Feldmann

In this article, we report how Janus particles, comprised of a silica sphere with a gold half shell, can not only be stably trapped by optical tweezers, but also displaced controllably along the axis of the laser beam through a complex interplay between optical and thermal forces. Scattering forces orient the asymmetric particle, while strong absorption on the metal side induces a thermal gradient, resulting in particle motion. An increase in the laser power leads to an upwards motion of the particle, while a decrease leads to a downwards motion. We study this reversible axial displacement, including a hysteretic jump in the particle position that is a result of the complex pattern of a tightly focused laser beam structure above the focal plane. As a first application we simultaneously trap a spherical gold nanoparticle and show that we can control this distance between the two particles inside the trap. This photonic micron-scale “elevator” is a promising tool for thermal force studies, remote sensing as well as optical and thermal micromanipulation experiments.


Surprises and anomalies in acoustical and optical scattering and radiation forces

Philip L. Marston

Experiments on radiation torques and negative radiation forces by various researchers display how the underlying wave-field geometry influences radiation forces. Other situations strongly influenced by wave-field geometry include high-order caustics present in light-scattering patterns of objects as simple as oblate drops of water or oblate bubbles of air in water. Related theoretical and experimental investigations are considered. Acoustic scattering enhancements associated with various guided waves are also examined. These include guided waves having negative group velocities and guided wave radiating wavefronts having a vanishing Gaussian curvature.


Tuesday, February 24, 2015

Entangling the motion of two optically trapped objects via time-modulated driving fields

Mehdi Abdi and Michael J Hartmann
We study entanglement of the motional degrees of freedom of two tethered and optically trapped microdisks inside a single cavity. By properly choosing the position of the trapped objects in the optical cavity and driving proper modes of the cavity, it is possible to equip the system with linear and quadratic optomechanical couplings. We show that a parametric coupling between the fundamental vibrational modes of two tethered microdisks can be generated via a time-modulated input laser. For a proper choice of the modulation frequency, this mechanism can drive the motion of the microdisks into an inseparable state in the long time limit via a two-mode squeezing process. We numerically confirm the performance of our scheme for current technology and briefly discuss an experimental setup that can be used for detecting this entanglement by employing the quadratic coupling. We also comment on the perspectives for generating such entanglement between the oscillations of optically levitated nanospheres.


Continuous Allosteric Regulation of a Viral Packaging Motor by a Sensor that Detects the Density and Conformation of Packaged DNA

Zachary T. Berndsen, Nicholas Keller, Douglas E. Smith

We report evidence for an unconventional type of allosteric regulation of a biomotor. We show that the genome-packaging motor of phage ϕ29 is regulated by a sensor that detects the density and conformation of the DNA packaged inside the viral capsid, and slows the motor by a mechanism distinct from the effect of a direct load force on the motor. Specifically, we show that motor-ATP interactions are regulated by a signal that is propagated allosterically from inside the viral shell to the motor mounted on the outside. This signal continuously regulates the motor speed and pausing in response to changes in either density or conformation of the packaged DNA, and slows the motor before the buildup of large forces resisting DNA confinement. Analysis of motor slipping reveals that the force resisting packaging remains low (<1 pN) until ∼70% and then rises sharply to ∼23 pN at high filling, which is a several-fold lower value than was previously estimated under the assumption that force alone slows the motor. These findings are consistent with recent studies of the stepping kinetics of the motor. The allosteric regulatory mechanism we report allows double-stranded DNA viruses to achieve rapid, high-density packing of their genomes by limiting the buildup of nonequilibrium load forces on the motor.


Label-Free Free-Solution Nanoaperture Optical Tweezers for Single Molecule Protein Studies

Ahmed A Al Balushi, Abhay Kotnala, Skyler Wheaton, Ryan M. Gelfand, Yashaswini Rajashekara and Reuven Gordon

Nanoaperture optical tweezers are emerging as useful label-free, free-solution tools for the detection and identification of biological molecules and their interactions at the single molecule level. Nanoaperture optical tweezers provide a low-cost, scalable, straight-forward, high-speed and highly sensitive (SNR ~ 33) platform to observe real-time dynamics and to quantify binding kinetics of protein-small molecule interactions without the need to use tethers or labeling. Such nanoaperture-based optical tweezers, which are 1000 times more efficient than conventional optical tweezers, have been used trap and isolate single DNA molecules and to study proteins like p53, which has been claimed to be in mutant form for 75% of human cancers. More recently, nanoaperture optical tweezers have been used to probe the low-frequency (in the single digit wavenumber range) Raman active modes of single nanoparticles and proteins. Here we review recent developments in the field of nanoaperture optical tweezers and how they have been applied to protein-antibody interactions, protein – small molecule interactions including single molecule binding kinetics, and protein-DNA interactions. In addition, recent works on the integration of nanoaperture optical tweezers at the tip of optical fiber and in microfluidic environment are presented.


Universal axial fluctuations in optical tweezers

Marco Ribezzi-Crivellari, Anna Alemany, and Felix Ritort

Optical tweezers (OTs) allow the measurement of fluctuations at the nanoscale, in particular fluctuations in the end-to-end distance in single molecules. Fluctuation spectra can yield valuable information, but they can easily be contaminated by instrumental effects. We identify axial fluctuations, i.e., fluctuations of the trapped beads in the direction of light propagation, as one of these instrumental effects. Remarkably, axial fluctuations occur on a characteristic timescale similar to that of conformational (folding) transitions, which may lead to misinterpretation of the experimental results. We show that a precise measurement of the effect of force on both axial and conformational fluctuations is crucial to disentangle them. Our results on axial fluctuations are captured by a simple and general formula valid for all OT setups and provide experimentalists with a general strategy to distinguish axial fluctuations from conformational transitions.


Monday, February 23, 2015

The charged linker of the molecular chaperone Hsp90 modulates domain contacts and biological function

Markus Jahna, Alexandra Rehn, Benjamin Pelz, Björn Hellenkamp, Klaus Richter, Matthias Rief, Johannes Buchner, and Thorsten Hugel

The heat shock protein 90 (Hsp90) is a dimeric molecular chaperone essential in numerous cellular processes. Its three domains (N, M, and C) are connected via linkers that allow the rearrangement of domains during Hsp90’s chaperone cycle. A unique linker, called charged linker (CL), connects the N- and M-domain of Hsp90. We used an integrated approach, combining single-molecule techniques and biochemical and in vivo methods, to study the unresolved structure and function of this region. Here we show that the CL facilitates intramolecular rearrangements on the milliseconds timescale between a state in which the N-domain is docked to the M-domain and a state in which the N-domain is more flexible. The docked conformation is stabilized by 1.1 kBT (2.7 kJ/mol) through binding of the CL to the N-domain of Hsp90. Docking and undocking of the CL affects the much slower intermolecular domain movement and Hsp90’s chaperone cycle governing client activation, cell viability, and stress tolerance.


Optical assembly of bio-hybrid micro-robots

Álvaro Barroso, Shirin Landwerth, Mike Woerdemann, Christina Alpmann, Tim Buscher, Maike Becker, Armido Studer and Cornelia Denz

The combination of micro synthetic structures with bacterial flagella motors represents an actual trend for the construction of self-propelled micro-robots. The development of methods for fabrication of these bacteria-based robots is a first crucial step towards the realization of functional miniature and autonomous moving robots. We present a novel scheme based on optical trapping to fabricate living micro-robots. By using holographic optical tweezers that allow three-dimensional manipulation in real time, we are able to arrange the building blocks that constitute the micro-robot in a defined way. We demonstrate exemplarily that our method enables the controlled assembly of living micro-robots consisting of a rod-shaped prokaryotic bacterium and a single elongated zeolite L crystal, which are used as model of the biological and abiotic components, respectively. We present different proof-of-principle approaches for the site-selective attachment of the bacteria on the particle surface. The propulsion of the optically assembled micro-robot demonstrates the potential of the proposed method as a powerful strategy for the fabrication of bio-hybrid micro-robots.


Periodic Trajectories Obtained With an Active Tractor Beam Using Azimuthal Polarization: Design of Particle Exchanger

Carretero, L. ; Acebal, P. ; Garcia, C. ; Blaya, S.
We analytically calculate the forces generated on the near field by a focused azimuthally polarized Hermite–Gauss beam after passing a complex mask formed by two annular pupils. The resultant optical tractor beam shows two transport channels that move trapped objects upstream or downstream along the conveyor. From the analysis of the phase diagrams, we theoretically demonstrate that, depending on illumination intensity, the 3-D behavior of nanoparticles in this conveyor shows a limit cycle between transport channels. This limit cycle appears as a consequence of diffraction that produces spatially limited optical forces. We theoretically demonstrate the possibility of using the limit cycle to design a particle exchanger between channels.


Molecular interference of fibrin's divalent polymerization mechanism enables modulation of multiscale material properties

Ashley C. Brown, Stephen R. Baker, Alison M. Douglas, Mark Keating, Martha B. Alvarez-Elizondo, Elliot L. Botvinick, Martin Guthold, Thomas H. Barker

Protein based polymers provide an exciting and complex landscape for tunable natural biomaterials through modulation of molecular level interactions. Here we demonstrate the ability to modify protein polymer structural and mechanical properties at multiple length scales by molecular ‘interference’ of fibrin's native polymerization mechanism. We have previously reported that engagement of fibrin's polymerization ‘hole b’, also known as ‘b-pockets’, through PEGylated complementary ‘knob B’ mimics can increase fibrin network porosity but also, somewhat paradoxically, increase network stiffness. Here, we explore the possible mechanistic underpinning of this phenomenon through characterization of the effects of knob B-fibrin interaction at multiple length scales from molecular to bulk polymer. Despite its weak monovalent binding affinity for fibrin, addition of both knob B and PEGylated knob B at concentrations near the binding coefficient, Kd, increased fibrin network porosity, consistent with the reported role of knob B-hole b interactions in promoting lateral growth of fibrin fibers. Addition of PEGylated knob B decreases the extensibility of single fibrin fibers at concentrations near its Kd but increases extensibility of fibers at concentrations above its Kd. The data suggest this bimodal behavior is due to the individual contributions knob B, which decreases fiber extensibility, and PEG, which increase fiber extensibility. Taken together with laser trap-based microrheological and bulk rheological analyses of fibrin polymers, our data strongly suggests that hole b engagement increases in single fiber stiffness that translates to higher storage moduli of fibrin polymers despite their increased porosity. These data point to possible strategies for tuning fibrin polymer mechanical properties through modulation of single fiber mechanics.


Friday, February 20, 2015

Nonequilibrium Fluctuations for a Single-Particle Analog of Gas in a Soft Wall

Dong Yun Lee, Chulan Kwon, and Hyuk Kyu Pak

We investigate the motion of a colloidal particle driven out of equilibrium by a time-varying stiffness of the optical trap that produces persistent nonequilibrium work. Measurements of work production for repeated cycles composed of the compression and expansion processes for the optical potential show huge fluctuations due to thermal motion. Using a precise technique to modulate the stiffness in time, we accurately estimate the probability distributions of work produced for the compression and expansion processes. We confirm the fluctuation theorem from the ratio of the two distributions. We also show that the average values of work for the two processes comply with the Jarzynski equality. This system has an analogy with a gas in a breathing soft wall. We discuss about its applicability to a heat engine and an information engine operated by feedback control.


Imaging of a linear diode bar for an optical cell stretcher

K. B. Roth, K. B. Neeves, J. Squier, and D. W. M. Marr

We present a simplified approach for imaging a linear diode bar laser for application as an optical stretcher within a microfluidic geometry. We have recently shown that these linear sources can be used to measure cell mechanical properties; however, the source geometry creates imaging challenges. To minimize intensity losses and simplify implementation within microfluidic systems without the use of expensive objectives, we combine aspheric and cylindrical lenses to create a 1:1 image of the source at the stretcher focal plane and demonstrate effectiveness by measuring the deformation of human red blood cells and neutrophils.


Particles replaced axially in an optical trap

Murat Muradoglu, Chun Yat Lau, and Tuck Wah Ng

Flow-based measures to automate optical trapping have significant limitations. A scheme is advanced here where a spherical bead is first located in a trap, and a second bead below the focus point is selectively drawn into the trap to replace the original particle. Experimentation conducted showed that it was possible to do so with little perturbation of other surrounding particles. Simulations done allowed for a clearer description of the exchange mechanism.


Control of cytoplasmic dynein force production and processivity by its C-terminal domain

Matthew P. Nicholas, Peter Höök, Sibylle Brenner, Caitlin L. Wynne, Richard B. Vallee & Arne Gennerich

Cytoplasmic dynein is a microtubule motor involved in cargo transport, nuclear migration and cell division. Despite structural conservation of the dynein motor domain from yeast to higher eukaryotes, the extensively studied S. cerevisiae dynein behaves distinctly from mammalian dyneins, which produce far less force and travel over shorter distances. However, isolated reports of yeast-like force production by mammalian dynein have called interspecies differences into question. We report that functional differences between yeast and mammalian dynein are real and attributable to a C-terminal motor element absent in yeast, which resembles a ‘cap’ over the central pore of the mammalian dynein motor domain. Removal of this cap increases the force generation of rat dynein from 1 pN to a yeast-like 6 pN and greatly increases its travel distance. Our findings identify the CT-cap as a novel regulator of dynein function.


Thursday, February 19, 2015

Does DNA Exert an Active Role in Generating Cell-Sized Spheres in an Aqueous Solution with a Crowding Binary Polymer?

Kanta Tsumoto, Masafumi Arai, Naoki Nakatani, Shun N. Watanabe and Kenichi Yoshikawa

We report the spontaneous generation of a cell-like morphology in an environment crowded with the polymers dextran and polyethylene glycol (PEG) in the presence of DNA. DNA molecules were selectively located in the interior of dextran-rich micro-droplets, when the composition of an aqueous two-phase system (ATPS) was near the critical condition of phase-segregation. The resulting micro-droplets could be controlled by the use of optical tweezers. As an example of laser manipulation, the dynamic fusion of two droplets is reported, which resembles the process of cell division in time-reverse. A hypothetical scenario for the emergence of a primitive cell with DNA is briefly discussed.


Measurement of the gold–gold bond rupture force at 4 K in a single-atom chain using photon-momentum-based force calibration

D T Smith and J R Pratt

We present instrumentation and methodology for simultaneously measuring force and displacement at the atomic scale at 4 K. The technique, which uses a macroscopic cantilever as a force sensor and high-resolution, high-stability fiber-optic interferometers for displacement measurement, is particularly well-suited to making accurate, traceable measurements of force and displacement in nanometer- and atomic-scale mechanical deformation experiments. The technique emphasizes accurate co-location of force and displacement measurement and measures cantilever stiffness at the contact point in situ at 4 K using photon momentum. We present preliminary results of measurements made of the force required to rupture a single atomic bond in a gold single-atom chain formed between a gold flat and a gold tip. Finally, we discuss the possible use of the gold–gold bond rupture force as an intrinsic force calibration value for forces near 1 nN.


Design of a high-quality optical conjugate structure in optical tweezers

Chunguang Hu, Ran An, Chengwei Zhang, Hai Lei, Xiaodong Hu, Hongbin Li, and Xiaotang Hu

We propose an approach to realize a high-quality optical conjugate of a piezo-driven mirror (PM) in optical tweezers. Misalignments between the optical beam and the steering center of the PM are analyzed mathematically. The decentrations in different directions cause different changes, either a position change of the conjugate plane or a spot variation of the beam during PM steering. On the other hand, these misalignment-introduced problems provide the information to check the assembling errors. Thus a wanted conjugate plane of the PM can be effectively and precisely achieved according to the detection signals. This approach is also available to deal with multifactor coupling error. At the end, the procedure for error analysis is given by testing homebuilt optical tweezers.


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

Stephanie Helen Jones, Andrew D. Ward and Martin King

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


Tuesday, February 17, 2015

Spectrally resolved resonant propulsion of dielectric microspheres

Yangcheng Li, Alexey V. Maslov, Nicholaos I. Limberopoulos, Augustine M. Urbas and Vasily N. Astratov 

Use of resonant light forces opens up a unique approach to high-volume sorting of microspherical resonators with much higher uniformity of resonances compared to that in coupled-cavity structures obtained by the best semiconductor technologies. In this work, the spectral response of the propulsion forces exerted on polystyrene microspheres near tapered microfibers is directly observed. The measurements are based on the control of the detuning between the tunable laser and internal resonances in each sphere with accuracy higher than the width of the resonances. The measured spectral shape of the propulsion forces correlates well with the whispering-gallery mode resonances in the microspheres. The existence of a stable radial trap for the microspheres propelled along the taper is demonstrated. The giant force peaks observed for 20-μm spheres are found to be in a good agreement with a model calculation demonstrating an efficient use of the light momentum for propelling the microspheres.


Particles at fluid–fluid interfaces: From single-particle behavior to hierarchical assembly of materials

Bum Jun Park and Daeyeon Lee

Particles ranging in size from a few nanometers to tens of micrometers have a strong tendency to adsorb at interfaces between two immiscible fluids (e.g., water and oil or air). The driving force for this strong interfacial attachment is a reduction in interfacial area, and thus, interfacial energy. To design and engineer the structure and properties of materials constructed by such colloidal systems, it is imperative to understand the behavior of particles at fluid interfaces at the single-particle level and to establish the relationship between the microscopic behavior of interfacial particles and the bulk properties of particle-laden interfaces. In this article, we present background information on the behavior of particles at fluid–fluid interfaces and highlight recent advances in understanding the effects of particle shape and surface wettability on the behavior of particles at the interfaces. We also discuss recent advances in using interfacial attachment to direct the assembly of nanomaterials to create hierarchical structures with designed properties.


Interaction of G-Quadruplexes in the Full-Length 3′ Human Telomeric Overhang

Jibin Abraham Punnoose , Yunxi Cui , Deepak Koirala , Philip M. Yangyuoru , Chiran Ghimire , Prakash Shrestha , and Hanbin Mao

The 3′ human telomeric overhang provides ample opportunities for the formation and interaction of G-quadruplexes, which have shown impacts on many biological functions including telomerase activities in the telomere region. However, in the few investigations on DNA constructs that approach to the full length of the human telomeric overhang, the presence of higher-order quadruplex–quadruplex interactions is still a subject of debate. Herein, we employed dynamic splint ligation (DSL) to prepare a DNA construct, 5′-(TTAGGG)24 or 24G, which has the length comparable to the full stretch of 3′ human telomeric overhang. Using mechanical unfolding assays in laser tweezers, we observed a minor population (∼5%) of higher-order interactions between G-quadruplexes, while the majority of the quadruplexes follow the bead-on-a-string model. Analyses on the noninteracting G-quadruplexes in the 24G construct showed features similar to those of the stand-alone G-quadruplexes in the 5′-(TTAGGG)4 (4G) construct. As each 24G construct contains as many as six G-quadruplexes, this method offers increased throughput for the time-consuming mechanical unfolding experiments of non-B DNA structures.


Tracking heavy water (D2O) incorporation for identifying and sorting active microbial cells

David Berry, Esther Mader, Tae Kwon Lee, Dagmar Woebken, Yun Wang, Di Zhu, Marton Palatinszky, Arno Schintlmeister, Markus C. Schmid, Buck T. Hanson, Naama Shterzer, Itzhak Mizrahi, Isabella Rauch, Thomas Decker, Thomas Bocklitz, Jürgen Popp, Christopher M. Gibson, Patrick W. Fowler, Wei E. Huang, and Michael Wagner
Microbial communities are essential to the function of virtually all ecosystems and eukaryotes, including humans. However, it is still a major challenge to identify microbial cells active under natural conditions in complex systems. In this study, we developed a new method to identify and sort active microbes on the single-cell level in complex samples using stable isotope probing with heavy water (D2O) combined with Raman microspectroscopy. Incorporation of D2O-derived D into the biomass of autotrophic and heterotrophic bacteria and archaea could be unambiguously detected via C-D signature peaks in single-cell Raman spectra, and the obtained labeling pattern was confirmed by nanoscale-resolution secondary ion MS. In fast-growing Escherichia coli cells, label detection was already possible after 20 min. For functional analyses of microbial communities, the detection of D incorporation from D2O in individual microbial cells via Raman microspectroscopy can be directly combined with FISH for the identification of active microbes. Applying this approach to mouse cecal microbiota revealed that the host-compound foragers Akkermansia muciniphila and Bacteroides acidifaciens exhibited distinctive response patterns to amendments of mucin and sugars. By Raman-based cell sorting of active (deuterated) cells with optical tweezers and subsequent multiple displacement amplification and DNA sequencing, novel cecal microbes stimulated by mucin and/or glucosamine were identified, demonstrating the potential of the nondestructive D2O-Raman approach for targeted sorting of microbial cells with defined functional properties for single-cell genomics.


Sunday, February 15, 2015

Effect of multiple scattering to optical forces on a sphere near an optical waveguide

Jian Xu, Wei-Ping Zang, and Jian-Guo Tian

We have investigated the effect of multiple scattering to optical forces on a particle in the evanescent field produced by an optical waveguide. Considering the multiple scattering between the sphere and the waveguide, we extend the formalism based on transition matrix and reflection matrix to calculate the optical forces on a sphere near an optical waveguide. Numerical results show that the influence that multiple scattering has on the optical forces can’t be ignored, especially when the structure resonance of the particle arises. Moreover, the effect of multiple scattering to optical forces is also studied in detail on the condition that the distance between the sphere and the waveguide is within the effective operating distance.


Distinct mechanisms regulating mechanical force-induced Ca2+ signals at the plasma membrane and the ER in human MSCs

Tae-Jin Kim, Chirlmin Joo, Jihye Seong, Reza Vafabakhsh, Elliot L Botvinick, Michael W Berns, Amy E Palmer, Ning Wang, Taekjip Ha, Eric Jakobsson, Jie Sun, Yingxiao Wang

It is unclear that how subcellular organelles respond to external mechanical stimuli. Here, we investigated the molecular mechanisms by which mechanical force regulates Ca2+ signaling at endoplasmic reticulum (ER) in human mesenchymal stem cells. Without extracellular Ca2+, ER Ca2+ release is the source of intracellular Ca2+ oscillations induced by laser-tweezer-traction at the plasma membrane, providing a model to study how mechanical stimuli can be transmitted deep inside the cell body. This ER Ca2+ release upon mechanical stimulation is mediated not only by the mechanical support of cytoskeleton and actomyosin contractility, but also by mechanosensitive Ca2+ permeable channels on the plasma membrane, specifically TRPM7. However, Ca2+ influx at the plasma membrane via mechanosensitive Ca2+ permeable channels is only mediated by the passive cytoskeletal structure but not active actomyosin contractility. Thus, active actomyosin contractility is essential for the response of ER to the external mechanical stimuli, distinct from the mechanical regulation at the plasma membrane.


Giant Gradient Force for Nanoparticle Trapping in Coupled Graphene Strips Waveguides

Bofeng Zhu, Guobin Ren, Yixiao Gao, Yang Yang, Martin J. Cryan and Shuisheng Jian

We conduct both analytical and numerical investigations of the giant gradient force for nanoparticle trapping in the coupled graphene strips waveguides system. An analytical model based on coupled slab waveguides has been adopted in the analysis of mode performance and gradient force, and good agreement is obtained with numerical simulations. Both theoretical modelling and numerical simulations have shown that the gradient force can be as high as 8 nN/μm???mW at a gap size of 10 nm, which is at least one order of magnitude higher than the previously reported hybrid plasmonic waveguide. Meanwhile, the giant gradient force leads to an ultrahigh trapping force and potential of 1.5×106 fN/W and 2.4×103 kBT/W, which are three orders of magnitude larger than the hybrid plasmonic waveguides. This enables the efficient trapping of nanoparticles with diameters as small as 2 nm. This coupled graphene strips system opens up new possibilities of tunable nanoscale mechanical devices and various potential applications such as manipulating bio molecules


Optofluidic tunable manipulation of microparticles by integrating graded-index fiber taper with a microcavity

Yuan Gong, Chenlin Zhang, Qun-Feng Liu, Yu Wu, Huijuan Wu, Yunjiang Rao, and Gang-Ding Peng

We propose and demonstrate optofluidic tunable manipulation of polystyrene microparticles based on the combination of a graded-index fiber (GIF) taper and a microcavity. The tunability on the manipulation length is experimentally explored by changing the balance between the optical force and the microfluidic flow force, as well as by tuning the focus of light emitting from the GIF taper via adjusting the length of an air microcavity. By optimizing the geometric shape of the GIF taper, as well as the flow rate and laser power, a manipulation length of 177 μm is achieved, more than 4 times longer than the state-of-the-art optical fiber tweezers. This method has advantages of high flexibility, ease of fabrication and use, integration with microfluidics and has the potential for optofluidic sensing applications.


Wednesday, February 11, 2015

Photophoretic trapping of airborne particles using ultraviolet illumination

Brandon Redding, Steven C. Hill, Dimitri Alexson, Chuji Wang, and Yong-Le Pan

We demonstrate photophoretic trapping of micron-sized absorbing particles in air using pulsed and continuous-wave (CW) ultraviolet laser illumination at wavelengths of 351 nm and 244 nm. We compared the particle trapping dynamics in two trapping geometries consisting of a hollow optical cone formed by light propagating either with or against gravity. This comparison allowed us to isolate the influence of the photophoretic force from the radiative pressure and the convective forces. We found that the absorbing spherical particles tested experienced a positive photophoretic force, whereas the spatially irregular, non-spherical particles tested experienced a negative photophoretic force. By using two trapping geometries, both spherical and non-spherical absorbing particles could be trapped and held securely in place. The position of the trapped particles exhibited a standard deviation of less than 1 µm over 20 seconds. Moreover, by operating in the UV and deep-UV where the majority of airborne materials are absorptive, the system was able to trap a wide range of particle types. Such a general purpose optical trap could enable on-line characterization of airborne particles when coupled with interrogation techniques such as Raman spectroscopy.


Radiation forces of highly focused radially polarized hollow sinh-Gaussian beams on a Rayleigh metallic particle

Zhou Zhang, Hua-Feng Xu, Jun Qu & Wei Huang

Based on the vector diffraction theory, the tight focusing properties of radially polarized hollow sinh-Gaussian (HsG) beams are theoretically studied. It is found that the radially polarized HsG beams can form a longitudinally polarized sub-wavelength focal spot. Moreover, the radiation forces acting on a Rayleigh metallic particle are calculated for the case where the radially polarized HsG beams are applied. Compared with the use of conventional Gaussian beams, the high-order radially polarized HsG beams can largely enhance the radial trap stiffness and broaden the axial trap distance. The influence of the beam order m on the focusing properties and trap stiffness is investigated in detail.


Viscoelastic Properties of Levan-DNA Mixtures Important in Microbial Biofilm Formation as Determined by Micro- and Macrorheology

Biljana Stojković, Simon Sretenovic, Iztok Dogsa, Igor Poberaj, David Stopar

We studied the viscoelastic properties of homogeneous and inhomogeneous levan-DNA mixtures using optical tweezers and a rotational rheometer. Levan and DNA are important components of the extracellular matrix of bacterial biofilms. Their viscoelastic properties influence the mechanical as well as molecular-transport properties of biofilm. Both macro- and microrheology measurements in homogeneous levan-DNA mixtures revealed pseudoplastic behavior. When the concentration of DNA reached a critical value, levan started to aggregate, forming clusters of a few microns in size. Microrheology using optical tweezers enabled us to measure local viscoelastic properties within the clusters as well as in the DNA phase surrounding the levan aggregates. In phase-separated levan-DNA mixtures, the results of macro- and microrheology differed significantly. The local viscosity and elasticity of levan increased, whereas the local viscosity of DNA decreased. On the other hand, the results of bulk viscosity measurements suggest that levan clusters do not interact strongly with DNA. Upon treatment with DNase, levan aggregates dispersed. These results demonstrate the advantages of microrheological measurements compared to bulk viscoelastic measurements when the materials under investigation are complex and inhomogeneous, as is often the case in biological samples.


The Manipulation and Combustion of Carbon-Based Micro Particles by Optical Tweezers

Sheng-ji Li & Xue-feng Huang

This article presents the manipulation and combustion of carbon-based micro particles (polystyrene, active carbon) by optical tweezers. Trap, ignition, and combustion of micro particles are well demonstrated. For polystyrene in water, polystyrene micro particles of 2.9 µm are trapped with increasing laser power. The polystyrene takes reaction and combusts as the laser power of 520 mW. For active carbon in air, both single active carbon and active carbon bundle can be trapped, dragged, oxidized, ignited, and combusted. The drag speed and burning rate of single active carbon of 7.0 µm are 103.7 µm/s and 14.0 µm/s as the minimum ignition power of 3.2 mW. Combustions of single active carbon and bundle are flameless at minimum ignition power. As the power is further enhanced, strenuous oxidation and combustion flame can be observed. For active carbon bundle of 215.7 µm, combustion process sustains 0.72 s as the laser power of 90 mW.


Monday, February 9, 2015

First principles approach to the Abraham–Minkowski controversy for the momentum of light in general linear non-dispersive media

Tomás Ramos, Guillermo F Rubilar and Yuri N Obukhov

We study the problem of the definition of the energy–momentum tensor of light in general moving non-dispersive media with linear constitutive law. Using the basic principles of classical field theory, we show that for the correct understanding of the problem, one needs to carefully distinguish situations when the material medium is modeled either as a background on which light propagates or as a dynamical part of the total system. In the former case, we prove that the (generalized) Belinfante–Rosenfeld (BR) tensor for the electromagnetic field coincides with the Minkowski tensor. We derive a complete set of balance equations for this open system and show that the symmetries of the background medium are directly related to the conservation of the Minkowski quantities. In particular, for isotropic media, the angular momentum of light is conserved despite of the fact that the Minkowski tensor is non-symmetric. For the closed system of light interacting with matter, we model the material medium as a relativistic non-dissipative fluid and we prove that it is always possible to express the total BR tensor of the closed system either in the Abraham or in the Minkowski separation. However, in the case of dynamical media, the balance equations have a particularly convenient form in terms of the Abraham tensor. Our results generalize previous attempts and provide a first principles basis for a unified understanding of the long-standing Abraham–Minkowski controversy without ad hoc arguments.


Fano Resonance-Induced Negative Optical Scattering Force on Plasmonic Nanoparticles

Huajin Chen, Shiyang Liu, Jian Zi, and Zhifang Lin

We demonstrate theoretically that Fano resonance can induce a negative optical scattering force acting on plasmonic nanoparticles in the visible light spectrum when an appropriate manipulating laser beam is adopted. Under the illumination of a zeroth-order Bessel beam, the plasmonic nanoparticle at its Fano resonance exhibits a much stronger forward scattering than backward scattering and consequently leads to a net longitudinal backward optical scattering force, termed Fano resonance-induced negative optical scattering force. The extinction spectra obtained based on the Mie theory show that the Fano resonance arises from the interference of simultaneously excited multipoles, which can be either a broad electric dipole mode and a narrow electric quadrupole mode, or a quadrupole and an octupole mode mediated by the broad electric dipole. Such Fano resonance-induced negative optical scattering force is demonstrated to occur for core–shell, homogeneous, and hollow metallic particles and can therefore be expected to be universal for many other nanostructures exhibiting Fano resonance, adding considerably to the flexibility of optical micromanipulation on the plasmonic nanoparticles. More interestingly, the flexible tunability of the Fano resonance by particle morphology opens up the possibility of tailoring the optical scattering force accordingly, offering an additional degree of freedom to optical selection and sorting of plasmonic nanoparticles.


Design of Portable Nanosensor for Easy Breast Tomography

Ali Rostami, Ahmad Salmanogli, Farshad Farhadnia, Mahbobeh Dolatyari and Ghassem Rostami

In this study, a portable nanosensor for early and easily detection of carcinoma tumor was designed and simulated. The nanosensor consisted of deposited nanoparticles with a regular distance from each other. It has highly sensitivity to the alteration of electromagnetic fields, which are scattered from different tissues (normal and tumor). With respect to the fact that normal tissue permittivity differs from the permittivity of tumors, the interaction of electromagnetic wave with them lead to different results in the case of electrical field and its gradient profile. It means that the non-uniformity will be occurred and this case is a meaningful signal for detection. Nevertheless, it is obvious that, due to tissue absorption coefficient, scattered photons will be very negligible; and a small fraction of the photons reach to the detector. Hence, in this paper, a sensor based on nanoparticles is proposed, which has enough sensitivity to pick up scattered photons, amplify, and detect them. It should be noted that, the roles of the nanosensor between body surface and detector are the signal amplification and sampling via nanoparticle plasmonic effect. It means that the designed nanoparticles sample the scattered waves and amplify them in the near-field. At last, our design and simulation results show that the digitized signals could be easily and clearly detected by force or temperature detectors. So, the easy breast tomography will be carried out with no need to clinics and its equipment.


Sunday, February 8, 2015

Improved isolation strategies allowed the phenotypic differentiation of two Nitrospira strains from widespread phylogenetic lineages

Boris Nowka, Sandra Off, Holger Daims, Eva Spieck

The second step of nitrification, the oxidation of nitrite to nitrate, is vital for the functioning of the nitrogen cycle, but our understanding of the ecological roles of the involved microorganisms is still limited. The known diversity of Nitrospira, the most widely distributed nitrite-oxidizing bacteria, has increased remarkably by analyses of 16S rRNA and functional gene sequences. However, only few representatives could be brought into laboratory cultures so far. In this study, two Nitrospira from activated sludge were isolated using novel approaches together with established methods. Highly enriched ‘Candidatus N. defluvii’ was separated from concomitant heterotrophs by taking advantage of its resistance against ampicillin and acriflavine. Beside this member of lineage I, a novel species of lineage II, named N. lenta, was initially enriched at 10°C and finally purified by using optical tweezers. The tolerance to elevated nitrite levels was much higher in N. defluvii than in the more fastidious N. lenta and was accompanied by pronounced biofilm formation. Phylogenetic classification of twelve additional enrichments indicated that Nitrospira lineage I is common in extreme and moderate ecosystems like lineage II. The new cultures will help to explore physiological and genomic differences affecting niche separation between members of this highly diverse genus.


Sorting of integral membrane proteins mediated by curvature-dependent protein-lipid bilayer interaction

Bojan Božič, Sovan Lal Das and Saša Svetina

Cell membrane proteins, both bound and integral, are known to preferentially accumulate at membrane locations with curvatures favorable to their shape. This is mainly due to the curvature dependent interaction between membrane proteins and their lipid environment. Here we analyze the effects of the protein-lipid bilayer interaction energy due to mismatch between the protein shape and the principal curvatures of the surrounding bilayer. The role of different macroscopic parameters that define the interaction energy term is elucidated in relation to recent experiment in which the lateral distribution of a membrane embedded protein potassium channel KvAP is measured on a giant unilamellar lipid vesicle (reservoir) and a narrow tubular extension – a tether – kept at constant length. The dependence of the sorting ratio, defined as the ratio between the areal density of the protein on the tether and on the vesicle, on the inverse tether radius is influenced by the strength of the interaction, the intrinsic shape of the membrane embedded protein, and its abundance in the reservoir. It is described how the values of these constants can be extracted from experiments. The intrinsic principal curvatures of a protein are related to the tether radius at which the sorting ratio attains its maximum value. The estimate of the principal intrinsic curvature of the protein KvAP, obtained by comparing the experimental and theoretical sorting behavior, is consistent with the available information on its structure.


Nanophotonic Force Microscopy: Characterizing Particle–Surface Interactions Using Near-Field Photonics

Perry Schein, Pilgyu Kang, Dakota O’Dell, and David Erickson

Direct measurements of particle–surface interactions are important for characterizing the stability and behavior of colloidal and nanoparticle suspensions. Current techniques are limited in their ability to measure pico-Newton scale interaction forces on submicrometer particles due to signal detection limits and thermal noise. Here we present a new technique for making measurements in this regime, which we refer to as nanophotonic force microscopy. Using a photonic crystal resonator, we generate a strongly localized region of exponentially decaying, near-field light that allows us to confine small particles close to a surface. From the statistical distribution of the light intensity scattered by the particle we are able to map out the potential well of the trap and directly quantify the repulsive force between the nanoparticle and the surface. As shown in this Letter, our technique is not limited by thermal noise, and therefore, we are able to resolve interaction forces smaller than 1 pN on dielectric particles as small as 100 nm in diameter.


A microfluidic chamber to study the dynamics of muscle contraction specific molecular interactions

Horia Nicolae Roman, David Juncker, and Anne-Marie Lauzon

In vitro motility and laser trap assays are commonly used for molecular mechanics measurements. However, chemicals cannot be added during measurements because they create flows that alter the molecular mechanics. Thus, we designed a microfluidic device that allows the addition of chemicals without creating bulk flows. Biocompatibility of the components of this device was tested. A micro-channel chamber was created by photolithography with the patterns transferred to polydimethylosiloxane (PDMS). The PDMS chamber was bound to a polycarbonate membrane which itself was bound to a molecular mechanics chamber. The micro-channels assured rapid distribution of the chemicals over the membrane whereas the membrane assured efficient delivery to the mechanics chamber while preventing bulk flow. The biocompatibility of the materials was tested by comparing the velocity (νmax) of propulsion by myosin of fluorescently labeled actin filaments to that of the conventional assay; no difference in νmax was observed. To estimate total chemical delivery time, labeled bovine serum albumin was injected in the channel chamber and TIRF was used to determine the time to reach the assay surface (2.7±0.1 s). Furthermore, the standard distance of a trapped microsphere calculated during buffer diffusion using the microfluidic device (14.9±3.2 nm) was not different from that using the conventional assay (15.6±5.3 nm, p=0.922). Finally, νmax obtained by injecting adenosine triphosphate (ATP) in the micro-channel chamber (2.37±0.48 µm/s) was not different from that obtained when ATP was delivered directly to the mechanics chamber (2.52±0.42 µm/s, p=0.822). This microfluidic prototype validates the design for molecular mechanics measurements.


Friday, February 6, 2015

Testing the Maxwell-Boltzmann distribution using Brownian particles

Jianyong Mo, Akarsh Simha, Simon Kheifets, and Mark G. Raizen

We report on shot-noise limited measurements of the instantaneous velocity distribution of a Brownian particle. Our system consists of a single micron-sized glass sphere held in an optical tweezer in a liquid in equilibrium at room temperature. We provide a direct verification of a modified Maxwell-Boltzmann velocity distribution and modified energy equipartition theorem that account for the kinetic energy of the liquid displaced by the particle. Our measurements confirm the distribution over a dynamic range of more than six orders of magnitude in count-rate and five standard deviations in velocity.


Site specific supramolecular heterogeneous catalysis by optically patterned soft oxometalate–porous organic framework (SOM–POF) hybrid on a chip

Preethi Thomas, Cuiying Pei, Basudev Roy, Subhrokoli Ghosh, Santu Das, Ayan Banerjee, Teng Ben, Shilun Qiu and Soumyajit Roy

We have designed a supramolecularly bound multi-component catalytic material based on a soft oxometalate (SOM) and a porous organic framework (POF) material, which shows high catalytic conversion efficiency. We have also used this material for site directed supramolecular heterogeneous catalysis with a yield even higher than in the bulk, and with micron-level precision by controllably depositing the material on a glass substrate, making a reactor chip, using a thermo-optical tweezers. Various SOM–POF composites have been prepared in dispersion phase and patterned using thermo-optic tweezers and their catalytic activities have been compared with a benchmark molecular catalyst [PMo12]. This work can lead to further explorations for hybrid materials that are formed out of well-defined molecular level precursors which can be controllably micro-patterned to simultaneously catalyze targeted reactions.


Dissecting Cooperative Communications in a Protein with a High-Throughput Single-Molecule Scalpel

Dr. Zhongbo Yu, Yunxi Cui, Sangeetha Selvam, Chiran Ghimire and Prof. Hanbin Ma

Miscued communication often leads to misfolding and aggregation of the proteins involved in many diseases. Owing to the ensemble average property of conventional techniques, detailed communication diagrams are difficult to obtain. Mechanical unfolding affords an unprecedented perspective on cooperative transitions by observing a protein along a trajectory defined by two mutated cysteine residues. Nevertheless, this approach requires tedious sample preparation at the risk of altering native protein conformations. To address these issues, we applied click chemistry to tether a protein to the two dsDNA handles through primary amines in lysine residues as well as at the N terminus. As a proof of concept, we used laser tweezers to mechanically unfold and refold calmodulin along 36 trajectories, maximally allowed by this strategy in a single batch of protein preparation. Without a priori knowledge of the particular residues to which the double-stranded DNA handles attach, we used hierarchical cluster analysis to identify 20 major trajectories, according to the size and the pattern of unfolding transitions. We dissected the cooperativity into all-or-none and partially cooperative events, which represent strong and weak high-order interactions in proteins, respectively. Although the overall cooperativity is higher within the N or C lobe than that between the lobes, the all-or-none cooperativity is anisotropic among different the unfolding trajectories and becomes relatively more predominant when the size of the protein segments increases. The average cooperativity for all-or-none transitions falls within the expected range observed by ensemble techniques, which supports the hypothesis that unfolding of a free protein can be reconstituted from individual trajectories.


Three-Dimensional Optical Trapping of a Plasmonic Nanoparticle using Low Numerical Aperture Optical Tweezers

Oto Brzobohatý, Martin Šiler, Jan Trojek, Lukáš Chvátal, Vítězslav Karásek, Aleš Paták, Zuzana Pokorná, Filip Mika & Pavel Zemánek

It was previously believed that larger metal nanoparticles behave as tiny mirrors that are pushed by the light beam radiative force along the direction of beam propagation, without a chance to be confined. However, several groups have recently reported successful optical trapping of gold and silver particles as large as 250 nm. We offer a possible explanation based on the fact that metal nanoparticles naturally occur in various non-spherical shapes and their optical properties differ significantly due to changes in localized plasmon excitation. We demonstrate experimentally and support theoretically three-dimensional confinement of large gold nanoparticles in an optical trap based on very low numerical aperture optics. We showed theoretically that the unique properties of gold nanoprisms allow an increase of trapping force by an order of magnitude at certain aspect ratios. These results pave the way to spatial manipulation of plasmonic nanoparticles using an optical fibre, with interesting applications in biology and medicine.


Thursday, February 5, 2015

Effects of Lipid Composition and Solution Conditions on the Mechanical Properties of Membrane Vesicles

Nobuhiko Kato, Akihiko Ishijima, Takehiko Inaba, Fumimasa Nomura, Shuichi Takeda and Kingo Takiguchi

The mechanical properties of cell-sized giant unilamellar liposomes were studied by manipulating polystyrene beads encapsulated within the liposomes using double-beam laser tweezers. Mechanical forces were applied to the liposomes from within by moving the beads away from each other, which caused the liposomes to elongate. Subsequently, a tubular membrane projection was generated in the tip at either end of the liposome, or the bead moved out from the laser trap. The force required for liposome transformation reached maximum strength just before formation of the projection or the moving out of the bead. By employing this manipulation system, we investigated the effects of membrane lipid compositions and environment solutions on the mechanical properties. With increasing content of acidic phospholipids, such as phosphatidylglycerol or phosphatidic acid, a larger strength of force was required for the liposome transformation. Liposomes prepared with a synthetic dimyristoylphosphatidylcholine, which has uniform hydrocarbon chains, were transformed easily compared with liposomes prepared using natural phosphatidylcholine. Surprisingly, bovine serum albumin or fetuin (soluble proteins that do not bind to membranes) decreased liposomal membrane rigidity, whereas the same concentration of sucrose showed no particular effect. These results show that the mechanical properties of liposomes depend on their lipid composition and environment.


An integrated optofluidic device for single-cell sorting driven by mechanical properties

T. Yang, P. Paiè, G. Nava, F. Bragheri, R. Martinez Vazquez, P. Minzioni, M. Veglione, M. Di Tano, C. Mondello, R. Osellame and I. Cristiani

We present a novel optofluidic device for real-time sorting on the basis of cell mechanical properties, measured by optical stretching. The whole mechanism, based on optical forces, does not hamper the viability of the tested cells, which can be used for further analysis. The device effectiveness is demonstrated by extracting a sample population enriched with highly metastatic cells from a heterogeneous cell mixture.


Direct laser manipulation reveals the mechanics of cell contacts in vivo

Kapil Bambardekar, Raphaël Clément, Olivier Blanc, Claire Chardès, and Pierre-François Lenne

Cell-generated forces produce a variety of tissue movements and tissue shape changes. The cytoskeletal elements that underlie these dynamics act at cell–cell and cell–ECM contacts to apply local forces on adhesive structures. In epithelia, force imbalance at cell contacts induces cell shape changes, such as apical constriction or polarized junction remodeling, driving tissue morphogenesis. The dynamics of these processes are well-characterized; however, the mechanical basis of cell shape changes is largely unknown because of a lack of mechanical measurements in vivo. We have developed an approach combining optical tweezers with light-sheet microscopy to probe the mechanical properties of epithelial cell junctions in the early Drosophilaembryo. We show that optical trapping can efficiently deform cell–cell interfaces and measure tension at cell junctions, which is on the order of 100 pN. We show that tension at cell junctions equilibrates over a few seconds, a short timescale compared with the contractile events that drive morphogenetic movements. We also show that tension increases along cell interfaces during early tissue morphogenesis and becomes anisotropic as cells intercalate during germ-band extension. By performing pull-and-release experiments, we identify time-dependent properties of junctional mechanics consistent with a simple viscoelastic model. Integrating this constitutive law into a tissue-scale model, we predict quantitatively how local deformations propagate throughout the tissue.


Study on particle size dependence of axial trapping efficiency

Wonwook Lee, Hyunji Kim, and Cha-Hwan Oh

An optical tweezers system using laser beams with a Gaussian intensity profile and doughnut intensity profiles made by hollow core optical fiber and axicon lenses, respectively, was constructed. The axial trapping efficiencies for the three intensity profiles were measured and compared with each other. The particle size dependence of axial trapping efficiencies in the range of the particle diameter from 1 to 20 μm were analyzed by using the modified ray optics model [Appl. Opt. 33, 1735 (1994)].


Wednesday, February 4, 2015

The role of myosin-II in force generation of DRG filopodia and lamellipodia

Wasim A. Sayyad, Ladan Amin, Paolo Fabris, Erika Ercolini & Vincent Torre

Differentiating neurons process the mechanical stimulus by exerting the protrusive forces through lamellipodia and filopodia. We used optical tweezers, video imaging and immunocytochemistry to analyze the role of non-muscle myosin-II on the protrusive force exerted by lamellipodia and filopodia from developing growth cones (GCs) of isolated Dorsal Root Ganglia (DRG) neurons. When the activity of myosin-II was inhibited by 30 μM Blebbistatin protrusion/retraction cycles of lamellipodia slowed down and during retraction lamellipodia could not lift up axially as in control condition. Inhibition of actin polymerization with 25 nM Cytochalasin-D and of microtubule polymerization with 500 nM Nocodazole slowed down the protrusion/retraction cycles, but only Cytochalasin-D decreased lamellipodia axial motion. The force exerted by lamellipodia treated with Blebbistatin decreased by 50%, but, surprisingly, the force exerted by filopodia increased by 20-50%. The concomitant disruption of microtubules caused by Nocodazole abolished the increase of the force exerted by filopodia treated with Blebbistatin. These results suggest that; i- Myosin-II controls the force exerted by lamellipodia and filopodia; ii- contractions of the actomyosin complex formed by filaments of actin and myosin have an active role in ruffle formation; iii- myosin-II is an essential component of the structural stability of GCs architecture.


Intracellular and extracellular forces drive primary cilia movement

Christopher Battle, Carolyn M. Ott, Dylan T. Burnette, Jennifer Lippincott-Schwartz, and Christoph F. Schmidt

Primary cilia are ubiquitous, microtubule-based organelles that play diverse roles in sensory transduction in many eukaryotic cells. They interrogate the cellular environment through chemosensing, osmosensing, and mechanosensing using receptors and ion channels in the ciliary membrane. Little is known about the mechanical and structural properties of the cilium and how these properties contribute to ciliary perception. We probed the mechanical responses of primary cilia from kidney epithelial cells [Madin–Darby canine kidney-II (MDCK-II)], which sense fluid flow in renal ducts. We found that, on manipulation with an optical trap, cilia deflect by bending along their length and pivoting around an effective hinge located below the basal body. The calculated bending rigidity indicates weak microtubule doublet coupling. Primary cilia of MDCK cells lack interdoublet dynein motors. Nevertheless, we found that the organelles display active motility. 3D tracking showed correlated fluctuations of the cilium and basal body. These angular movements seemed random but were dependent on ATP and cytoplasmic myosin-II in the cell cortex. We conclude that force generation by the actin cytoskeleton surrounding the basal body results in active ciliary movement. We speculate that actin-driven ciliary movement might tune and calibrate ciliary sensory functions.


Force-dependent transition in the T-cell receptor β-subunit allosterically regulates peptide discrimination and pMHC bond lifetime

Dibyendu Kumar Das, Yinnian Feng, Robert J. Mallis, Xiaolong Li, Derin B. Keskin, Rebecca E. Hussey, Sonia K. Brady, Jia-Huai Wang, Gerhard Wagner, Ellis L. Reinherz, and Matthew J. Lang
The αβ T-cell receptor (TCR) on each T lymphocyte mediates exquisite specificity for a particular foreign peptide bound to a major histocompatibility complex molecule (pMHC) displayed on the surface of altered cells. This recognition stimulates protection in the mammalian host against intracellular pathogens, including viruses, and involves piconewton forces that accompany pMHC ligation. Physical forces are generated by T-lymphocyte movement during immune surveillance as well as by cytoskeletal rearrangements at the immunological synapse following cessation of cell migration. The mechanistic explanation for how TCRs distinguish between foreign and self-peptides bound to a given MHC molecule is unclear: peptide residues themselves comprise few of the TCR contacts on the pMHC, and pathogen-derived peptides are scant among myriad self-peptides bound to the same MHC class arrayed on infected cells. Using optical tweezers and DNA tether spacer technology that permit piconewton force application and nanometer scale precision, we have determined how bioforces relate to self versus nonself discrimination. Single-molecule analyses involving isolated αβ-heterodimers as well as complete TCR complexes on T lymphocytes reveal that the FG loop in the β-subunit constant domain allosterically controls both the variable domain module’s catch bond lifetime and peptide discrimination via force-driven conformational transition. In contrast to integrins, the TCR interrogates its ligand via a strong force-loaded state with release through a weakened, extended state. Our work defines a key element of TCR mechanotransduction, explaining why the FG loop structure evolved for adaptive immunity in αβ but not γδTCRs or immunoglobulins.


Nd:YAG Near-Infrared Luminescent Nanothermometers

Antonio Benayas, Blanca del Rosal, Alberto Pérez-Delgado, Karla Santacruz-Gómez, Daniel Jaque, Gustavo Adolfo Hirata and Fiorenzo Vetrone

In this work, the thermal sensing capability of Nd3+-doped Y3Al5O12 nanoparticles fabricated by combustion synthesis is reported. Under excitation at 808 nm, the relative intensity of the two spectrally isolated luminescence peaks located at around 940 nm (corresponding to a 4F3/2 [RIGHTWARDS ARROW]4I9/2 transition of the Nd3+ ions) is found to be markedly temperature-dependent allowing for ratiometric luminescence nanothermometry. The potential use of neodymium-doped yttrium aluminum garnet nanoparticles in nanothermometry has been successfully tested in a variety of systems including integrated microelectronics, optofluidic devices, and subtissue ex vivo experiments.


Monday, February 2, 2015

Debye series analysis of optical force induced by an axicon-generated Bessel beam

Renxian Li, Lixin Guo & Chunying Ding

Debye series expansion is employed to the analysis of radiation pressure force exerted on a spherical particle induced by an axicon-generated Bessel beam. The beam shape coefficients for Bessel beams are calculated using analytical expressions obtained by the integral localized approximation, and the scattering coefficients are expanded using Debye series. The effect of different parameters (beam center location, radius of particle, half-cone angle, etc.) on radiation pressure force is studied. The contributions of different scattering processes on radiation pressure force are isolated for better understanding of physical mechanism of various features of radiation pressure force.


Slow leakage of Ca-dipicolinic acid from individual Bacillus spores during initiation of spore germination

Shiwei Wang, Peter Setlow and Yong-qing Li

When exposed to nutrient or non-nutrient germinants, individual Bacillus spores can return to life through germination followed by outgrowth. Laser tweezers Raman spectroscopy, and either differential interference contrast or phase contrast microscopy were used to analyze the slow dipicolinic acid (DPA) leakage (normally ∼20% of spore DPA) from individual spores that takes place prior to the lag time, Tlag, when spores begin rapid release of remaining DPA. Major conclusions from this work with Bacillus subtilis spores were: 1) slow DPA leakage from wild-type spores germinating with nutrients did not begin immediately after nutrient exposure, but only at a later heterogeneous time T1; 2) the period of slow DPA leakage (ΔTleakage = Tlag - T1) was heterogeneous among individual spores, although the amount of DPA released in this period was relatively constant; 3) increases in germination temperature significantly decreased T1 times, but increased values of ΔTleakage; 4) upon germination with L-valine for 10 min followed by addition of D-alanine to block further germination, all germinated spores had T1 times less than 10 min, suggesting that T1 is the time when spores become committed to germinate; 5) elevated levels of SpoVA proteins involved in DPA movement in spore germination decreased T1 and Tlag times, but not the amount of DPA released in ΔTleakage; 6) lack of the cortex-lytic enzyme CwlJ increased DPA leakage during germination due to longer ΔTleakage times in which more DPA was released; and 6) there was slow DPA leakage early in germination of B. subtilis spores by the non-nutrients CaDPA and dodecylamine and in nutrient germination of Bacillus cereus and Bacillus megaterium spores. Overall, these findings have identified and characterized a new early event in Bacillus spore germination.


Cross-type optical separation of elastic oblate capsules in a uniform flow

Cheong Bong Chang, Wei-Xi Huang and Hyung Jin Sung

The dynamic behavior of an elastic capsule with an initially oblate spheroidal shape during cross-type optical separation was numerically investigated. The penalty immersed boundary method was adopted for the fluid-membrane interaction, and the optical force calculation was conducted by using the ray optics method including the ray-surface intersection algorithm. The oblate elastic capsule of b/a = 0.5 with different surface Young's moduli and different initial inclination angles was considered. The oblate capsule with higher surface Young's moduli was less deformed, and was more migrated for each initial inclination angle. Unlike the oblate rigid particle, the initially inclined capsules with moderate inclination angles were similarly migrated since the oblate elastic capsule was deformed during rotation near the laser beam axis. The oblate capsules can be separated according to the surface Young's modulus, except for nearly non-inclined capsules. As the fluid velocity decreased, the migration distance increased. The maximum deformation parameter was insensitive to the fluid velocity. Furthermore, a new dimensionless number (Sec ) was introduced to predict the migration distance of the oblate elastic capsule.


Sunday, February 1, 2015

Radiation force exerted on a sphere by focused Laguerre–Gaussian beams

Huachao Yu and Weilong She

The generalized Lorenz–Mie theory is employed to calculate the force exerted on a sphere by focused Laguerre–Gaussian beams. The key parameters of the theory, namely, the multipole coefficients of the beams, are exactly derived from the beams’ angular spectra in terms of some auxiliary coefficients. Several recurrence formulas, which can improve the calculation of the auxiliary coefficients and accordingly the force, are also derived. According to the calculated force, the trapping performances of the beams are investigated in the Mie regime. It is found that low(high)-azimuthal-order beams usually have advantages in the radial trapping of the high(low)-refractive-index sphere and the axial trapping of the low(high)-refractive-index sphere. The influences of the parameters of the beams, lens, and sphere on the trapping performance are also investigated.


Multilevel-Based Topology Design and Cell Patterning With Robotically Controlled Optical Tweezers

Xiao Yan; Dong Sun

This paper presents the use of robotically controlled optical tweezers to move a group of biological cells into a desired region for required patterning. A multilevel-based topology is designed to present different cell patterning in the desired region. A potential function-based controller is developed to control the cells in forming the required patterning as well as achieving rotation and scaling of the desired patterning. A pattern regulatory control force is additionally designed and added to the cell patterning controller for particularly addressing the local minima problem that causes the cells to stop at the undesired positions. The stability of the controlled system is analyzed using a direct Lyapunov approach. Experiments are performed with a cell manipulation system equipped with holographic optical tweezers to demonstrate the effectiveness of the proposed approach.


Upconversion Particle as a Local Luminescent Brownian Probe: A Photonic Force Microscopy Study

Flavio M. Mor, Andrzej Sienkiewicz , László Forró , and Sylvia Jeney

Near-infrared (NIR) light sensitive lanthanide-doped NaYF4 upconversion particles (UCPs) are gaining increasing attention as local probes in biomedical applications. Here, we implemented a photonic force microscope (PFM) to manipulate and study the optical properties of trapped single UCPs, β-NaYF4:Yb,Er. In particular, we focused on the mechanisms of the optical trapping of nonspherical UCPs of different sizes, in the range 0.5–2 μm, as well as on their upconversion photoluminescence (UCL) properties under excitation with the strongly focused laser beam (λ = 1064 nm) of the PFM, operating at power densities up to 14.7 MW cm–2. A careful analysis of UCL under such conditions points to three emission peaks at 469, 503.6, and 616.1 nm, which were enhanced by the high laser power density. The analysis of Brownian motion was used to quantify the thermal fluctuations of the particle inside the optical trap as well as the particle sizes and optical forces acting in the two dimensions perpendicular to the optical axis. A steep dependence of UCL as a function of the particle diameter was found for UCPs having sizes smaller than the focal spot (∼900 nm) of the NIR laser.


Probing linear and nonlinear microrheology of viscoelastic fluids

J. R. Gomez-Solano and C. Bechinger

Bulk rheological properties of viscoelastic fluids have been extensively studied in macroscopic shearing geometries. However, little is known when an active microscopic probe is used to locally perturb them far from the linear-response regime. Using a colloidal particle dragged periodically by scanning optical tweezers through a viscoelastic fluid, we investigate both, its linear and nonlinear microrheological response. With increasing particle velocity, we observe a transition from constant viscosity to a thinning regime, where the drag force on the probe becomes a nonlinear function of the particle velocity. We demonstrate that this transition is only determined by the ratio of the fluid's equilibrium relaxation time and the period of the driving.