## Tuesday, May 31, 2011

### Shaping the future of manipulation

K Dholakia & T Čižmár

Optical forces can be used to manipulate biological and colloidal material in a non-contact manner. This forms the foundation of a wealth of exciting science, particularly in the fields of physics, biology and soft condensed matter. Although the standard Gaussian single-beam trap remains a very powerful tool, shaping the phase and amplitude of a light field provides unusual light patterns that add a major new dimension to research into particle manipulation. This Review summarizes the impact and emerging applications of shaped light in the field of optical manipulation.

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### Plasmon nano-optical tweezers

Mathieu L Juan, Maurizio Righini & Romain Quidant

Conventional optical tweezers, formed at the diffraction-limited focus of a laser beam, have become a powerful and flexible tool for manipulating micrometre-sized objects. Extending optical trapping down to the nanometre scale would open unprecedented opportunities in many fields of science, where such nano-optical tweezers would allow the ultra-accurate positioning of single nano-objects. Among the possible strategies, the ability of metallic nanostructures to control light at the subwavelength scale can be exploited to engineer such nano-optical traps. This Review summarizes the recent advances in the emerging field of plasmon-based optical trapping and discusses the details of plasmon tweezers along with their potential applications to bioscience and quantum optics.

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### Tweezers with a twist

The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

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### A light touch

Since the discovery of the optical gradient force in 1970 and the first use of laser beams to manipulate microscopic and atomic systems in 1986, optical manipulation has proved to be a versatile optical tool for uncovering mysteries throughout many fields of science.

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### How it all began

Since the first discovery of optical gradient and scattering forces in 1970, optical tweezers have helped unveil many mysteries and given deeper insights in many areas of science. Arthur Ashkin, the father of optical tweezers, recalls some 'eureka' moments and shares his viewpoint of the field with Nature Photonics.

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### Optical tweezers study life under tension

Furqan M Fazal & Steven M Block

Optical tweezers have become one of the primary weapons in the arsenal of biophysicists, and have revolutionized the new field of single-molecule biophysics. Today's techniques allow high-resolution experiments on biological macromolecules that were mere pipe dreams only a decade ago.

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### Using laser light to trap and explore the cloud-forming properties of single aerosol particles

Jim Walker, Jon Wills, Jonathan Reid

The atmospheric radiative forcing of aerosols is one of the greatest sources of uncertainty in our current understanding of global climate change (Solomon et al., 2007). Aerosols perturb the radiation balance by directly scattering and absorbing radiation (a direct effect), and by influencing the albedo and lifetime of clouds (an indirect effect). In order to improve our ability to quantify the impact of aerosols on the climate system, the thermodynamic properties and kinetics of change of aerosol particles need to be fully resolved.

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### Simulation of near-field optical manipulator using the combination of a near-field scanning optical microscope probe and an atomic force microscope metallic probe

Binghui Liu, Lijun Yang, and Yang Wang

We propose a physical model to calculate the trapping force on a nanoparticle trapped by the system using the combination of a near-field scanning optical microscope (NSOM) probe and an atomic force microscope (AFM) metallic probe. Such a near-field trap is produced by evanescent illumination from the NSOM probe and light scattering at the tip of the AFM metallic probe. By using the Maxwell stress tensor through the electric field distribution obtained with the three-dimensional finite difference time domain (3-D FDTD) method, the dependence of the trapping force on the system parameters is discussed, and trapping properties including near-field distribution, trapping position, and the role of other forces versus trapping force are revealed. The results indicate that a particle down to tens of nanometers in size can be trapped toward the tip of an AFM probe with a lower laser intensity (∼1040 W/mm2) than that required by conventional optical manipulators (∼10^5 W/mm2).

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### Light-assisted templated self assembly using photonic crystal slabs

Camilo A. Mejia, Avik Dutt, and Michelle L. Povinelli
We explore a technique which we term light-assisted templated self-assembly. We calculate the optical forces on colloidal particles over a photonic crystal slab. We show that exciting a guided resonance mode of the slab yields a resonantly-enhanced, attractive optical force. We calculate the lateral optical forces above the slab and predict that stably trapped periodic patterns of particles are dependent on wavelength and polarization. Tuning the wavelength or polarization of the light source may thus allow the formation and reconfiguration of patterns. We expect that this technique may be used to design all-optically reconfigurable photonic devices.

## Monday, May 30, 2011

### Dynamic force spectroscopy on the binding of monoclonal antibodies and tau peptides

Carolin Wagner, David Singer, Olaf Ueberschär, Tim Stangner, Christof Gutsche, Ralf Hoffmann and Friedrich Kremer
Optical tweezers-assisted dynamic force spectroscopy (DFS) is employed to investigate specific receptor/ligand interactions on the level of single binding events. Here, the specific binding of two anti-human tau monoclonal antibodies (mAbs), HPT-110 and HPT-104, to synthetic tau-peptides with different phosphorylation patterns is analyzed. The specificity of HPT-110 to the tau-peptide containing a phosphorylation at Ser235 and of HPT-104 to the tau-peptide containing a phosphorylation at Thr231 is confirmed. Additionally, our approach allows for a detailed characterization of the unspecific interactions that are observed between HPT-104 and the peptide phosphorylated only at Ser235 and between HPT-110 and the peptide phosphorylated only at Thr231. By analyzing the measured rupture-force distributions it is possible to separate unspecific from specific interactions. Thereby for the latter characteristic parameters like the lifetime of the bond without force τ0, the characteristic length xts and the free energy of activation ΔG are determined. The results are in accordance with conventional ELISA tests but offer a much more refined insight.

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### Cell Technology Employing Femtosecond Laser Pulses

M. M. Rakityansky, M. B. Agranat, S. I. Ashitkov, A. V. Ovchinnikov, M. L. Semenova, S. A. Sergeev, D. S. Sitnikov and I. N. Shevelev
Practical advantages of using femtosecond laser pulses for manipulations in cell surgery were demonstrated. The use of femtosecond laser pulses enables precision punching of the zona pellucida of the embryo without damaging its cells. With the help of femtosecond laser tweezers/scalpel, auxillary laser hatching was performed and a technique of optical biopsy of mammalian embryo was developed, which enabled non-contact sampling of embryonic material for preimplantation diagnostics. Our findings suggest that about 90% embryos retained the ability to develop at least to the blastula stage after this manipulation.

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### Concentration Independent Modulation of Local Micromechanics in a Fibrin Gel

Maxwell A. Kotlarchyk, Samir G. Shreim, Martha B. Alvarez-Elizondo, Laura C. Estrada, Rahul Singh, Lorenzo Valdevit, Ekaterina Kniazeva, Enrico Gratton, Andrew J. Putnam, Elliot L. Botvinick

Methods for tuning extracellular matrix (ECM) mechanics in 3D cell culture that rely on increasing the concentration of either protein or cross-linking molecules fail to control important parameters such as pore size, ligand density, and molecular diffusivity. Alternatively, ECM stiffness can be modulated independently from protein concentration by mechanically loading the ECM. We have developed a novel device for generating stiffness gradients in naturally derived ECMs, where stiffness is tuned by inducing strain, while local mechanical properties are directly determined by laser tweezers based active microrheology (AMR). Hydrogel substrates polymerized within 35 mm diameter Petri dishes are strained non-uniformly by the precise rotation of an embedded cylindrical post, and exhibit a position-dependent stiffness with little to no modulation of local mesh geometry. Here we present the device in the context of fibrin hydrogels. First AMR is used to directly measure local micromechanics in unstrained hydrogels of increasing fibrin concentration. Changes in stiffness are then mapped within our device, where fibrin concentration is held constant. Fluorescence confocal imaging and orbital particle tracking are used to quantify structural changes in fibrin on the micro and nano levels respectively. The micromechanical strain stiffening measured by microrheology is not accompanied by ECM microstructural changes under our applied loads, as measured by confocal microscopy. However, super-resolution orbital tracking reveals nanostructural straightening, lengthening, and reduced movement of fibrin fibers. Furthermore, we show that aortic smooth muscle cells cultured within our device are morphologically sensitive to the induced mechanical gradient. Our results demonstrate a powerful cell culture tool that can be used in the study of mechanical effects on cellular physiology in naturally derived 3D ECM tissues.

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### The bidirectional depolymerizer MCAK generates force by disassembling both microtubule ends

Yusuke Oguchi, Seiichi Uchimura, Takashi Ohki, Sergey V. Mikhailenko & Shin’ichi Ishiwata

During cell division the replicated chromosomes are segregated precisely towards the spindle poles. Although many cellular processes involving motility require ATP-fuelled force generation by motor proteins, most models of the chromosome movement invoke the release of energy stored at strained (owing to GTP hydrolysis) plus ends of microtubules. This energy is converted into chromosome movement through passive couplers, whereas the role of molecular motors is limited to the regulation of microtubule dynamics. Here we report, that the microtubule-depolymerizing activity of MCAK (mitotic centromere-associated kinesin), the founding member of the kinesin-13 family, is accompanied by the generation of significant tension—remarkably, at both microtubule ends. An MCAK-decorated bead strongly attaches to the microtubule side, but readily slides along it in either direction under weak external loads and tightly captures and disassembles both microtubule ends. We show that the depolymerization force increases with the number of interacting MCAK molecules and is ~1 pN per motor. These results provide a simple model for the generation of driving force and the regulation of chromosome segregation by the activity of MCAK at both kinetochores and spindle poles through a ‘side-sliding, end-catching’ mechanism.

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### Optical trapping at low numerical aperture

S. Stallinga

A theory of optical trapping at low Numerical Aperture (NA) is presented. The theory offers an analytical description of the competition between the stabilizing gradient and destabilizing scattering force. The trade-off can be characterized by a single dimensionless trapping parameter, which increases with bead size to wavelength ratio $a/\lambda$ and refractive index contrast $m$ and decreases with NA. The gradient force dominates for small trapping parameters, the scattering force for large trapping parameters. The potential well depth, maximum forces and trap stiffness as a function of the three parameters ($a/\lambda$, $m$, NA) can be mapped onto universal functions of the trapping parameter. These functions do not depend on any free parameter. The universal well depth and maximum force curves match with numerical results based on the exact multipole expansion of the optical trapping force. The paraxial limit of low NA is relevant for compact optical tweezers based on Optical Pickup Units known from optical data storage.

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## Thursday, May 26, 2011

### The RSC chromatin remodelling ATPase translocates DNA with high force and small step size

George Sirinakis, Cedric R Clapier, Ying Gao, Ramya Viswanathan, Bradley R Cairns and Yongli Zhang

ATP-dependent chromatin remodelling complexes use the energy of ATP hydrolysis to reposition and reconfigure nucleosomes. Despite their diverse functions, all remodellers share highly conserved ATPase domains, many shown to translocate DNA. Understanding remodelling requires biophysical knowledge of the DNA translocation process: how the ATPase moves DNA and generates force, and how translocation and force generation are coupled on nucleosomes. Here, we characterize the real-time activity of a minimal RSC translocase ‘motor’ on bare DNA, using high-resolution optical tweezers and a ‘tethered’ translocase system. We observe on dsDNA a processivity of ~35 bp, a speed of ~25 bp/s, and a step size of 2.0 (±0.4, s.e.m.) bp. Surprisingly, the motor is capable of moving against high force, up to 30 pN, making it one of the most force-resistant motors known. We also provide evidence for DNA ‘buckling’ at initiation. These observations reveal the ATPase as a powerful DNA translocating motor capable of disrupting DNA–histone interactions by mechanical force.

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### Optical forces and trapping potentials of a dual-waveguide trap based on multimode solid-core waveguides

M. M. van Leest, F. Bernal Arango, J. Caro

We propose a novel design of the dual-waveguide trap for trapping and Raman identification of microscopic particles and biological objects in a fluid. The device is based on two embedded Si3N4 waveguides launching counterpropagating beams into the fluidic channel of a lab-on-chip. For waveguides with a square cross-section of 1 μm2, a 5 μm gap between them and a 785 nm operation wavelength, we perform finite-difference time-domain simulations of the beam profiles and the trapping forces acting on polystyrene beads (diameter 0.2-1.4 μm). The forces reach values up to 16 pN/W for a bead diameter of 1.4 μm, indicating that the trap is very suitable for trapping of particles in a fluidic environment. This is confirmed by the trapping potentials deduced from the force curves. The design of waveguides and chip is completely compatible with glass-based microfluidic technology, thus enabling mass production and widespead application, contrary to previous approaches.

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### Combined optical tweezers and laser dissector for controlled ablation of functional connections in neural networks

Francesco Difato, Marco Dal Maschio, Emanuele Marconi,Giuseppe Ronzitti, Alessandro Maccione, Tommasso Fellin, Luca Berdondini, Evelina Chieregatti, Fabio Benfenati, and Axel Blau

Regeneration of functional connectivity within a neural network after different degrees of lesion is of utmost clinical importance. To test pharmacological approaches aimed at recovering from a total or partial damage of neuronal connections within a circuit, it is necessary to develop a precise method for controlled ablation of neuronal processes. We combined a UV laser microdissector to ablate neural processes in vitro at single neuron and neural network level with infrared holographic optical tweezers to carry out force spectroscopy measurements. Simultaneous force spectroscopy, down to the sub-pico–Newton range, was performed during laser dissection to quantify the tension release in a partially ablated neurite. Therefore, we could control and measure the damage inflicted to an individual neuronal process. To characterize the effect of the inflicted injury on network level, changes in activity of neural subpopulations were monitored with subcellular resolution and overall network activity with high temporal resolution by concurrent calcium imaging and microelectrode array recording. Neuronal connections have been sequentially ablated and the correlated changes in network activity traced and mapped. With this unique combination of electrophysiological and optical tools, neural activity can be studied and quantified in response to controlled injury at the subcellular, cellular, and network level.

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### Single-Molecule Protein Unfolding and Translocation by an ATP-Fueled Proteolytic Machine

Marie-Eve Aubin-Tam, Adrian O. Olivares, Robert T. Sauer, Tania A. Baker and Matthew J. Lang

All cells employ ATP-powered proteases for protein-quality control and regulation. In the ClpXP protease, ClpX is a AAA+ machine that recognizes specific protein substrates, unfolds these molecules, and then translocates the denatured polypeptide through a central pore and into ClpP for degradation. Here, we use optical-trapping nanometry to probe the mechanics of enzymatic unfolding and translocation of single molecules of a multidomain substrate. Our experiments demonstrate the capacity of ClpXP and ClpX to perform mechanical work under load, reveal very fast and highly cooperative unfolding of individual substrate domains, suggest a translocation step size of 5–8 amino acids, and support a power-stroke model of denaturation in which successful enzyme-mediated unfolding of stable domains requires coincidence between mechanical pulling by the enzyme and a transient stochastic reduction in protein stability. We anticipate that single-molecule studies of the mechanical properties of other AAA+ proteolytic machines will reveal many shared features with ClpXP.

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### ClpX(P) Generates Mechanical Force to Unfold and Translocate Its Protein Substrates

Rodrigo A. Maillard, Gheorghe Chistol, Maya Sen, Maurizio Righini, Jiongyi Tan, Christian M. Kaiser, Courtney Hodges, Andreas Martin and Carlos Bustamante

AAA+ unfoldases denature and translocate polypeptides into associated peptidases. We report direct observations of mechanical, force-induced protein unfolding by the ClpX unfoldase from E. coli, alone, and in complex with the ClpP peptidase. ClpX hydrolyzes ATP to generate mechanical force and translocate polypeptides through its central pore. Threading is interrupted by pauses that are found to be off the main translocation pathway. ClpX's translocation velocity is force dependent, reaching a maximum of 80 aa/s near-zero force and vanishing at around 20 pN. ClpX takes 1, 2, or 3 nm steps, suggesting a fundamental step-size of 1 nm and a certain degree of intersubunit coordination. When ClpX encounters a folded protein, it either overcomes this mechanical barrier or slips on the polypeptide before making another unfolding attempt. Binding of ClpP decreases the slip probability and enhances the unfolding efficiency of ClpX. Under the action of ClpXP, GFP unravels cooperatively via a transient intermediate.

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### Adhesion through Single Peptide Aptamers

Marie-Eve Aubin-Tam, David C. Appleyard, Enrico Ferrari, Valeria Garbin, Oluwatimilehin O. Fadiran, Jacquelyn Kunkel, and Matthew J. Lang

Aptamer and antibody mediated adhesion is central to biological function and is valuable in the engineering of “lab on a chip” devices. Single molecule force spectroscopy using optical tweezers enables direct nonequilibrium measurement of these noncovalent interactions for three peptide aptamers selected for glass, polystyrene, and carbon nanotubes. A comprehensive examination of the strong attachment between antifluorescein 4−4−20 and fluorescein was also carried out using the same assay. Bond lifetime, barrier width, and free energy of activation are extracted from unbinding histogram data using three single molecule pulling models. The evaluated aptamers appear to adhere stronger than the fluorescein antibody under no- and low-load conditions, yet weaker than antibodies at loads above 25 pN. Comparison to force spectroscopy data of other biological linkages shows the diversity of load dependent binding and provides insight into linkages used in biological processes and those designed for engineered systems.

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### Nucleation of colloidal crystals on configurable seed structures

M. Hermes, E. C. M. Vermolen, M. E. Leunissen, D. L. J. Vossen, P. D. J. van Oostrum, M. Dijkstra and A. van Blaaderen

Nucleation is an important stage in the growth of crystals. During this stage, the structure and orientation of a crystal are determined. However, short time- and length-scales make nucleation poorly understood. Micrometer-sized colloidal particles form an ideal model system to study nucleation due to more experimentally accessible time- and length-scales and the possibility to manipulate them individually. Here we report experiments and simulations on nucleation in the bulk of a hard-sphere fluid, initiated by seed structures configured using optical tweezers. We find that the defect topology of the critical nucleus determines the crystal morphology. From the growth of the crystals beyond the critical nucleus size, new insights into the role of defects in crystal growth were gained that are incompatible with the assumption of equilibrium growth. These results explain the complex crystal morphologies observed in experiments on hard spheres.

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## Wednesday, May 25, 2011

### Doppler velocimetry on microparticles trapped and propelled by laser light in liquid-filled photonic crystal fiber

M. K. Garbos, T. G. Euser, O. A. Schmidt, S. Unterkofler, and P. St.J. Russell

Laser Doppler velocimetry is used to measure very accurately the velocity and position of a microparticle propelled and guided by laser light in liquid-filled photonic crystal fiber. Periodic variations in particle velocity are observed that correlate closely with modal beating between the two lowest order guided fiber modes.

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## Tuesday, May 24, 2011

### Force spectroscopy reveals multiple "closed states" of the muscle thin filament

Vijay S. Rao, Amy M. Clobes and William H. Guilford

Tropomyosin (Tm) plays a critical role in regulating contraction of striated muscle. The three-state model of activation posits that Tm exists in three positions on the thin filament: "blocked" in the absence of calcium when myosin cannot bind, "closed" when calcium binds troponin and Tm partially covers the myosin binding site, and "open" after myosin binding forces Tm completely off neighboring sites. However, we recently showed that actin filaments decorated with phosphorylated Tm are driven by myosin with greater force than bare actin filaments. This result cannot be explained by simple steric hindrance and suggests that Tm may have additional effects on actin-myosin interactions. We therefore tested the hypothesis that Tm and its phosphorylation state affect the rate at which single actin-myosin bonds form and rupture. Using a laser trap we measured the time necessary for the first bond to form between actin and rigor HMM, and the load-dependent durations of those bonds. Measurements were repeated in the presence of sub-saturating myosin-S1 to force Tm from the closed to the open state. Maximum bond lifetimes increased in the open state, but only when Tm was phosphorylated. While the frequency with which bonds formed was extremely low in the closed state, when a bond did form it took significantly less time to do so than with bare actin. These data suggest there are at least two closed states of the thin filament, and that Tm provides additional points of contact for myosin.

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### Optimization of computer-generated holograms for dynamic optical manipulation with uniform structured light spots

Jing Bu, Guanghui Yuan, Yuyang Sun, Siwei Zhu, and Xiaocong Yuan

An optimized iterative technique combining the merits of conventional Gerchber-Saxton (G-S) and adaptive-additive (A-A) algorithms to design multilevel computer-generated holograms for the creation of a desirable structured intensity pattern for multiple optical manipulation is theoretically adopted. Optical trap arrays are demonstrated with the help of liquid crystal spatial light modulator and a microscopic optical tweezer system. Additionally, continuous locked-in transport and deflection of microparticles with the generated optical lattice is proven experimentally. The proposed method possesses apparent high efficiency, high uniformity, and dynamic and reconfigurable advantages.

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## Monday, May 23, 2011

### Automated Transportation of Single Cells Using Robot-Tweezer Manipulation System

Songyu Hu and Dong Sun
Manipulation of biological cells becomes increasingly important in biomedical engineering to address challenge issues in cell–cell interaction, drug discovery, and tissue engineering. Significant demand for both accuracy and productivity in cell manipulation highlights the need for automated cell transportation with integrated robotics and micro/nano manipulation technologies. Optical tweezers, which use highly focused low-power laser beams to trap and manipulate particles at micro/nanoscale, have emerged as an essential tool for manipulating single cells. In this article, we propose to use a robot-tweezer manipulation system to solve the problem of automatic transportation of biological cells, where optical tweezers function as special robot end effectors. Dynamics equation of the cell in optical tweezers is analyzed. A closed-loop controller is designed for transporting and positioning cells. Experiments are performed on live cells to demonstrate the effectiveness of the proposed approach in effective cell positioning.

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## Friday, May 20, 2011

### Observation of microsphere movement driven by optical pulse

Hanyang Li, Yundong Zhang, Jin Li, and Liangsheng Qiang

A laser beam was coupled into a tapered optical fiber tip with a diameter of about 2.6 μm, and a 46 μm diameter microsphere was propelled by the outgoing optical pulse. With the change of pulse energy from 1.35 to 7.22 μJ, the calculated average velocity of the driven microsphere varied from 0.38 to 10.68 cm/s. The scanning electron microscope images of the fiber tip show that there is no thermal damage during the experiment.

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## Monday, May 16, 2011

### Laser-induced thermophoresis of individual particles in a viscous liquid

Ross T. Schermer, Colin C. Olson, J. Patrick Coleman, and Frank Bucholtz

This paper presents a detailed investigation of the motion of individual micro-particles in a moderately-viscous liquid in direct response to a local, laser-induced temperature gradient. By measuring particle trajectories in 3D, and comparing them to a simulated temperature profile, it is confirmed that the thermally-induced particle motion is the direct result of thermophoresis. The elevated viscosity of the liquid provides for substantial differences in the behavior predicted by various models of thermophoresis, which in turn allows measured data to be most appropriately matched to a model proposed by Brenner. This model is then used to predict the effective force resulting from thermophoresis in an optical trap. Based on these results, we predict when thermophoresis will strongly inhibit the ability of radiation pressure to trap nano-scale particles. The model also predicts that the thermophoretic force scales linearly with the viscosity of the liquid, such that choice of liquid plays a key role in the relative strength of the thermophoretic and radiation forces.

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### Near-axial rotation of nanorods by focused laser beams using dual-beam method

Sun-Uk Hwang, Yong-Gu Lee, Yong-Jin Kim and Gyu-Chul Yi

A dual-beam method for the near-axial rotation of dielectric nanorods was devised. The method uses two laser beams, where a focused Gaussian beam holds the object in the beam axis while a focused Laguerre–Gaussian beam rotates the object. The near-axial rotation of ZnO nanorods using this method was then experimentally demonstrated, and the radial offset distance of the rotating nanorod from the beam axis was quantified via a video tracking method.

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## Thursday, May 12, 2011

### Survey on indirect optical manipulation of cells, nucleic acids, and motor proteins

Ashis Gopal Banerjee, Sagar Chowdhury, Satyandra K. Gupta, Wolfgang Losert

Optical tweezers have emerged as a promising technique for manipulating biological objects. Instead of direct laser exposure, more often than not, optically-trapped beads are attached to the ends or boundaries of the objects for translation, rotation, and stretching. This is referred to as indirect optical manipulation. In this paper, we utilize the concept of robotic gripping to explain the different experimental setups which are commonly used for indirect manipulation of cells, nucleic acids, and motor proteins. We also give an overview of the kind of biological insights provided by this technique. We conclude by highlighting the trends across the experimental studies, and discuss challenges and promising directions in this domain of active current research.

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### Single-cell isolation using a DVD optical pickup

A. Kasukurti, M. Potcoava, S.A. Desai, C. Eggleton, and D. W. M. Marr

A low-cost single-cell isolation system incorporating a digital versatile disc burner (DVD RW) optical pickup has been developed. We show that these readily available modules have the required laser power and focusing optics to provide a steady Gaussian beam capable of optically trapping micron-sized colloids and red blood cells. Utility of the pickup is demonstrated through the non-destructive isolation of such particles in a laminar-flow based microfluidic device that captures and translates single microscale objects across streamlines into designated channel exits. In this, the integrated objective lens focusing coils are used to steer the optical trap across the channel, resulting in the isolation of colloids and red blood cells using a very inexpensive off-the-shelf optical component.

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### Bacterial Chemotaxis in an Optical Trap

Tuba Altindal, Suddhashil Chattopadhyay, Xiao-Lun Wu

An optical trapping technique is implemented to investigate the chemotactic behavior of a marine bacterial strain Vibrio alginolyticus. The technique takes the advantage that the bacterium has only a single polar flagellum, which can rotate either in the counter-clock-wise or clock-wise direction. The two rotation states of the motor can be readily and instantaneously resolved in the optical trap, allowing the flagellar motor switching rate to be measured under different chemical stimulations. In this paper the focus will be on the bacterial response to an impulsive change of chemoattractant serine. Despite different propulsion apparati and motility patterns, cells of V. alginolyticusapparently use a similar response as Escherichia coli to regulate their chemotactic behavior. Specifically, we found that the switching rate of the bacterial motor exhibits a biphasic behavior, showing a fast initial response followed by a slow relaxation to the steady-state switching rate . The measured can be mimicked by a model that has been recently proposed for chemotaxis in E. coli. The similarity in the response to the brief chemical stimulation in these two different bacteria is striking, suggesting that the biphasic response may be evolutionarily conserved. This study also demonstrated that optical tweezers can be a useful tool for chemotaxis studies and should be applicable to other polarly flagellated bacteria.

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### Optical Tweezers for Synchrotron Radiation Probing of Trapped Biological and Soft Matter Objects in Aqueous Environments

Silvia Santucci , Dan Cojoc , Heinz Amenitsch , Benedetta Marmiroli , Barbara Sartori , Manfred Burghammer , Sebastian Schoeder , Emanuela Di Cola , Michael Reynolds , and Christian F. Riekel
Investigations of single fragile objects manipulated by optical forces with high brilliance X-ray beams may initiate the development of new research fields such as protein crystallography in an aqueous environment. We have developed a dedicated optical tweezers setup with a compact, portable and versatile geometry for the customary manipulation of objects for synchrotron radiation applications. Objects of few microns up to few 10th of microns size can be trapped for extended periods of time. The selection and positioning of single objects out of a batch of many can be performed semi-automatically by software routines. The performance of the setup has been tested by wide-angle and small-angle X-ray scattering experiments on single optically trapped starch granules, using a synchrotron radiation microbeam. We demonstrate here for the first time the feasibility of microdiffraction on optically trapped protein crystals. Starch granules and insulin crystals were repeatedly raster-scanned at about 50 ms exposure/raster-point up to the complete loss of the structural order. Radiation damage in starch granules results in the appearance of low-angle scattering due to the breakdown of the polysaccharide matrix. For insulin crystals, order along the densely packed [110] direction is preferentially maintained until complete loss of long-range order.

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## Friday, May 6, 2011

### Position clamping in a holographic counterpropagating optical trap

Richard Bowman, Alexander Jesacher, Gregor Thalhammer,Graham Gibson, Monika Ritsch-Marte, and Miles Padgett

Optical traps consisting of two counterpropagating, divergent beams of light allow relatively high forces to be exerted along the optical axis by turning off one beam, however the axial stiffness of the trap is generally low due to the lower numerical apertures typically used. Using a high speed spatial light modulator and CMOS camera, we demonstrate 3D servocontrol of a trapped particle, increasing the stiffness from 0.004 to 1.5μNm−1. This is achieved in the “macro-tweezers” geometry [Thalhammer, J. Opt. 13, 044024 (2011); Pitzek, Opt. Express 17, 19414 (2009)], which has a much larger field of view and working distance than single-beam tweezers due to its lower numerical aperture requirements. Using a 10×, 0.2NA objective, active feedback produces a trap with similar effective stiffness to a conventional single-beam gradient trap, of order 1μNm−1 in 3D. Our control loop has a round-trip latency of 10ms, leading to a resonance at 20Hz. This is sufficient bandwidth to reduce the position fluctuations of a 10μm bead due to Brownian motion by two orders of magnitude. This approach can be trivially extended to multiple particles, and we show three simultaneously position-clamped beads.

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## Thursday, May 5, 2011

### Dynamics of two trapped Brownian particles: Shear-induced cross-correlations

J. Bammert, L. Holzer and W. Zimmermann

The dynamics of two Brownian particles trapped by two neighboring harmonic potentials in a linear shear flow is investigated. The positional correlation functions in this system are calculated analytically and analyzed as a function of the shear rate and the trap distance. Shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are linear in the shear rate, asymmetric in time, and occur for one particle as well as between both particles. Moreover, the shear rate enters as a quadratic correction to the well-known correlations of random displacements along parallel spatial directions. The correlation functions depend on the orientation of the connection vector between the potential minima with respect to the flow direction. As a consequence, the inter-particle cross-correlations between orthogonal fluctuations can have zero, one or two local extrema as a function of time. Possible experiments for detecting these predicted correlations are described.

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## Wednesday, May 4, 2011

### Characterization of Semiconductor Nanowires Using Optical Tweezers

Peter J. Reece, Wen Jun Toe, Fan Wang, Suriati Paiman, Qiang Gao, H. Hoe Tan, and C. Jagadish

We report on the optical trapping characteristics of InP nanowires with dimensions of 30 (±6) nm in diameter and 2–15 μm in length. We describe a method for calibrating the absolute position of individual nanowires relative to the trapping center using synchronous high-speed position sensing and acousto-optic beam switching. Through Brownian dynamics we investigate effects of the laser power and polarization on trap stability, as well as length dependence and the effect of simultaneous trapping multiple nanowires.

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### Radiation force of highly focused Lorentz-Gauss beams on a Rayleigh particle

Yunfeng Jiang, Kaikai Huang, and Xuanhui Lu

The radiation force of highly focused Lorentz-Gauss beams (LG beam) on a dielectric sphere in the Rayleigh scattering regime is theoretically studied. The numerical results show that the Lorentz-Gauss beam can be used to trap particles with the refractive index larger than that of the ambient. The radiation force distribution has been studied under different beam widths of the Lorentz part. The trapping stability under different conditions is also analyzed.

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