Thursday, July 28, 2011

Optical manipulation and self-assembly of nematic colloids: colloidal crystals and superstructures

Igor Muševič
Optical manipulation of colloidal particles in nematic liquid crystals is far more complex than manipulation in water-based colloids. Owing to the long-range nature of the orientational ordering, their elasticity and topological conservation laws, almost any kind of object can be trapped and manipulated in liquid crystals. Furthermore, local heating due to the absorption of light can be used to create microdroplets of the isotropic phase, which interact strongly with colloidal particles. This leads to a broad variety of colloidal assemblies in liquid crystals, which cannot be observed in isotropic solvents: colloidal wires, assembled by entangled topological defects, superstructures in mixtures of large and small colloidal particles and a broad variety of two-dimensional nematic colloidal crystals. In all cases, the colloidal binding energy is several orders of magnitude stronger compared with water-based colloids. The large variety of colloidal superstructures, which can be assembled in nematic colloids, makes them highly interesting for application in photonic materials.


Nanopore Translocation Dynamics of a Single DNA-Bound Protein

Andre Spiering, Sebastian Getfert, Andy Sischka, Peter Reimann, and Dario Anselmetti

We study the translocation dynamics of a single protein molecule attached to a double-stranded DNA that is threaded through a solid-state nanopore by optical tweezers and an electric field (nanopore force spectroscopy). We find distinct asymmetric and retarded force signals that depend on the protein charge, the DNA elasticity and its counterionic screening in the buffer. A theoretical model where an isolated charge on an elastic, polyelectrolyte strand is experiencing an anharmonic nanopore potential was developed. Its results compare very well with the measured force curves and explain the experimental findings that the force depends linearly on the applied electric field and exhibits a small hysteresis during back and forth translocation cycles. Moreover, the translocation dynamics reflects the stochastic nature of the thermally activated hopping between two adjacent states in the nanopore that can be adequately described by Kramers rate theory.


Hybrid optical tweezers for dynamic micro-bead arrays

Yoshio Tanaka, Shogo Tsutsui, Mitsuru Ishikawa, and Hiroyuki Kitajima

Dynamic micro-bead arrays offer great flexibility and potential as sensing tools in various scientific fields. Two optical trapping techniques, the GPC method using a spatial light modulator and a mechanical scanning method using galvano mirrors, are combined in a hybrid optical tweezers system to handle dynamic micro-bead arrays. This system provides greater versatility while the GPC method creates massive micro-bead arrays in a 2D space, where the trapped beads can be manipulated smoothly and very quickly in a 3D space using the mechanical scanning method. Four typical examples are demonstrated in real time.


Tuesday, July 26, 2011

Two-dimensional optical thermal ratchets based on Fibonacci spirals

Ke Xiao, Yael Roichman, and David G. Grier

An ensemble of symmetric potential energy wells arranged at the vertices of a Fibonacci spiral can serve as the basis for an irreducibly two-dimensional thermal ratchet. Periodic rotation of the potential energy landscape through a three-step cycle drives trapped Brownian particles along spiral trajectories through the pattern. Which spiral is selected depends on the angular displacement at each step, with transitions between selected spirals arising at rational proportions of the golden angle. Fibonacci spiral ratchets therefore display an exceptionally rich range of transport properties, including inhomogeneous states in which different parts of the pattern induce motion in different directions. Both the radial and angular components of these trajectories can undergo flux reversal as a function of the scale of the pattern or the rate of rotation.


Radiation pressure cross sections and optical forces over negative refractive index spherical particles by ordinary Bessel beams

Leonardo A. Ambrosio and Hugo E. Hernández-Figueroa

When impinged by an arbitrary laser beam, lossless and homogeneous negative refractive index (NRI) spherical particles refract and reflect light in an unusual way, giving rise to different scattered and internal fields when compared to their equivalent positive refractive index particles. In the generalized Lorenz–Mie theory, the scattered fields are dependent upon the Mie scattering coefficients, whose values must reflect the metamaterial behavior of an NRI scatterer, thus leading to new optical properties such as force and torque. In this way, this work is devoted to the analysis of both radial and longitudinal optical forces exerted on lossless and simple NRI particles by zero-order Bessel beams, revealing how the force profiles are changed whenever the refractive index becomes negative.


Direct Measurements of Torque During Holliday Junction Migration

Scott Forth, Christopher Deufel, Smita S. Patel and Michelle D. Wang
DNA experiences torsional stress resulting from the activities of motor enzymes and bound proteins. The mechanisms by which this torsional stress is dissipated to maintain DNA structural integrity are not fully known. Here, we show that a Holliday junction can limit torsion by coupling rotation to translocation and torque to force. The torque required to mechanically migrate through individual junctions was found to be an order of magnitude smaller than that required to melt DNA. We also directly show that substantially more torque was required to migrate through even a single-base sequence heterology, which has important implications for the activity of junction-migrating enzymes.


Monday, July 25, 2011

Trapping and guiding microparticles with morphing autofocusing Airy beams

Peng Zhang, Jai Prakash, Ze Zhang, Matthew S. Mills, Nikolaos K. Efremidis, Demetrios N. Christodoulides, and Zhigang Chen
We observe optical trapping and manipulation of dielectric microparticles using autofocusing radially symmetric Airy beams. This is accomplished by exploiting either the inward or outward transverse acceleration associated with their chirped wavefronts. We experimentally demonstrate, for the first time to our knowledge, that such Airy beams morph into nondiffracting Bessel beams in their far-field. Furthermore, the ability of guiding and transporting microparticles along the primary rings of this class of beams is explored.


Spin angular momentum transfer from TEM00 focused Gaussian beams to negative refractive index spherical particles

Leonardo A. Ambrosio and Hugo E. Hernández-Figueroa
We investigate optical torques over absorbent negative refractive index spherical scatterers under the influence of linear and circularly polarized TEM00 focused Gaussian beams, in the framework of the generalized Lorenz-Mie theory with the integral localized approximation. The fundamental differences between optical torques due to spin angular momentum transfer in positive and negative refractive index optical trapping are outlined, revealing the effect of the Mie scattering coefficients in one of the most fundamental properties in optical trapping systems.


Temperature and Confinement Effect on Interparticle Force in Nematic Colloids

Noboru Kondo, Yasutaka Iwashita & Yasuyuki Kimura
We experimentally studied the interparticle force between two colloidal particles with point defects in a nematic liquid crystal. The force F at various temperatures was measured using optical tweezers and with a free release method. The effective elastic constant was evaluated from F using electrostatic analogy of nematic field. At respective temperatures, that is in good agreement with the splay constant obtained by dielectric measurement. The dependence of F on cell thicknesses was also studied in a wedge-type cell. In a thinner cell, the magnitude of F becomes smaller, and F becomes short-ranged.


Efficient extension of the trapping lifetime of single atoms in an optical tweezer by laser cooling

Jun He, Baodong Yang, Tiancai Zhang and Junmin Wang
Optical tweezers have become powerful tools for the confinement and manipulation of neutral atoms, molecules, mesoscopic biological molecules and living cells. In our experiment, a single caesium atom was prepared in a large-magnetic-gradient magneto-optical trap (MOT). It was then efficiently transferred back and forth between the MOT and a 1064 nm microscopic optical tweezer. The atomic transfer between the MOT and the tweezer can be employed to measure the trapping lifetime and the energy distribution of the single atom in the tweezer. In order to extend the trapping lifetime, laser cooling is used to decrease the atom's kinetic energy. The trapping lifetime was extended from ~75 to ~130 s by applying a 10 ms laser cooling phase just after the single atom is transferred into the tweezer.


Thursday, July 21, 2011

How to integrate a micropipette into a closed microfluidic system: absorption spectra of an optically trapped erythrocyte

Ahmed Alrifaiy and Kerstin Ramser

We present a new concept of integrating a micropipette within a closed microfluidic system equipped with optical tweezers and a UV-Vis spectrometer. A single red blood cell (RBC) was optically trapped and steered in three dimensions towards a micropipette that was integrated in the microfluidic system. Different oxygenation states of the RBC, triggered by altering the oxygen content in the microchannels through a pump system, were optically monitored by a UV-Vis spectrometer. The built setup is aimed to act as a multifunctional system where the biochemical content and the electrophysiological reaction of a single cell can be monitored simultaneously. The system can be used for other applications like single cell sorting, in vitro fertilization or electrophysiological experiments with precise environmental control of the gas-, and chemical content.


Condensation Prevails over B-A Transition in the Structure of DNA at Low Humidity

Silvia Hormeño, Fernando Moreno-Herrero, Borja Ibarra, José L. Carrascosa, José M. Valpuesta and J. Ricardo Arias-Gonzalez

B-A transition and DNA condensation are processes regulated by base sequence and water activity. The constraints imposed by interhelical interactions in condensation compromise the observation of the mechanism by which B and A base-stacking modes influence the global state of the molecule. We used a single-molecule approach to prevent aggregation and mechanical force to control the intramolecular chain association involved in condensation. Force-extension experiments with optical tweezers revealed that DNA stretches as B-DNA under ethanol and spermine concentrations that favor the A-form. Moreover, we found no contour-length change compatible with a cooperative transition between the A and B forms within the intrinsic-force regime. Experiments performed at constant force in the entropic-force regime with magnetic tweezers similarly did not show a bistable contraction of the molecules that could be attributed to the B-A transition when the physiological buffer was replaced by a water-ethanol mixture. A total, stepwise collapse was found instead, which is characteristic of DNA condensation. Therefore, a low-humidity-induced change from the B- to the A-form base-stacking alone does not lead to a contour-length shortening. These results support a mechanism for the B-A transition in which low-humidity conditions locally change the base-stacking arrangement and globally induce DNA condensation, an effect that may eventually stabilize a molecular contour-length reduction.


Mechanical Properties of High-GC Content DNA with A-Type Base-Stacking

Silvia Hormeño, Borja Ibarra, José L. Carrascosa, José M. Valpuesta, Fernando Moreno-Herrero and J. Ricardo Arias-Gonzalez

The sequence of a DNA molecule is known to influence its secondary structure and flexibility. Using a combination of bulk and single-molecule techniques, we measure the structural and mechanical properties of two DNAs which differ in both sequence and base-stacking arrangement in aqueous buffer, as revealed by circular dichroism: one with 50% G·C content and B-form and the other with 70% G·C content and A-form. Atomic force microscopy measurements reveal that the local A-form structure of the high-G·C DNA does not lead to a global contour-length decrease with respect to that of the molecule in B-form although it affects its persistence length. In the presence of force, however, the stiffness of high-G·C content DNA is similar to that of balanced-G·C DNA as magnetic and optical tweezers measured typical values for the persistence length of both DNA substrates. This indicates that sequence-induced local distortions from the B-form are compromised under tension. Finally, high-G·C DNA is significantly harder to stretch than 50%-G·C DNA as manifested by a larger stretch modulus. Our results show that a local, basepair configuration of DNA induced by high-G·C content influences the stretching elasticity of the polymer but that it does not affect the global, double-helix arrangement.


Three-Dimensional to Two-Dimensional Crossover in the Hydrodynamic Interactions between Micron-Scale Rods

R. Di Leonardo, E. Cammarota, G. Bolognesi, H. Schäfer, and M. Steinhart

Moving micron-scale objects are strongly coupled to each other by hydrodynamic interactions. The strength of this coupling decays with the inverse particle separation when the two objects are sufficiently far apart. It has been recently demonstrated that the reduced dimensionality of a thin fluid layer gives rise to longer-ranged, logarithmic coupling. Using holographic tweezers we show that microrods display both behaviors interacting like point particles in three dimensions at large distances and like point particles in two dimensions for distances shorter then their length. We derive a simple analytical expression that fits our data remarkably well and further validate it with finite element analysis.


Wednesday, July 20, 2011

Optical recoil of asymmetric nano-optical antenna

Jung-Hwan Song, Jonghwa Shin, Hee-Jin Lim, and Yong-Hee Lee

We propose nano-optical antennas with asymmetric radiation patterns as light-driven mechanical recoil force generators. Directional antennas are found to generate recoil force efficiently when driven in the spectral proximity of their resonances. It is also shown that the recoil force is equivalent to the Poynting vector integrated over a closed sphere containing the antenna structures.


Tuesday, July 19, 2011

The effect of immersion oil in optical tweezers

Ali Mahmoudi, S. Nader, and S. Reihani

Optimized optical tweezers are of great importance for biological micromanipulation. In this paper, we present a detailed electromagnetic-based calculation of the spatial intensity distribution for a laser beam focused through a high numerical aperture objective when there are several discontinuities in the optical pathway of the system. For a common case of 3 interfaces we have shown that 0.01 increase in the refractive index of the immersion medium would shift the optimal trapping depth by 3–4μm (0.2–0.6μm) for aqueous (air) medium. For the first time, We have shown that the alteration of the refractive index of the immersion medium can be also used in aerosol trapping provided that larger increase in the refractive index is considered.


Calcium modulates force sensing by the von Willebrand factor A2 domain

Arjen J. Jakobi, Alireza Mashaghi, Sander J. Tans & Eric G. Huizinga

von Willebrand factor (VWF) multimers mediate primary adhesion and aggregation of platelets. VWF potency critically depends on multimer size, which is regulated by a feedback mechanism involving shear-induced unfolding of the VWF-A2 domain and cleavage by the metalloprotease ADAMTS-13. Here we report crystallographic and single-molecule optical tweezers data on VWF-A2 providing mechanistic insight into calcium-mediated stabilization of the native conformation that protects A2 from cleavage by ADAMTS-13. Unfolding of A2 requires higher forces when calcium is present and primarily proceeds through a mechanically stable intermediate with non-native calcium coordination. Calcium further accelerates refolding markedly, in particular, under applied load. We propose that calcium improves force sensing by allowing reversible force switching under physiologically relevant hydrodynamic conditions. Our data show for the first time the relevance of metal coordination for mechanical properties of a protein involved in mechanosensing.


Friday, July 15, 2011

Resolution of the Abraham-Minkowski debate: Implications for the electromagnetic wave theory of light in matter

B. A. Kemp

A century has now passed since the origins of the Abraham-Minkowski controversy pertaining to the correct form of optical momentum in media. Experiment and theory have been applied at both the classical and quantum levels in attempt to resolve the debate. The result of these efforts is the identification of Abraham’s kinetic momentum as being responsible for the overall center of mass translations of a medium and Minkowski’s canonical or wave momentum as being responsible for translations within or with respect to a medium. In spite of the recent theoretical developments, much confusion still exists regarding the appropriate theory required to predict experimental outcomes and to develop new applications. In this paper, the resolution of the longstanding Abraham-Minkowski controversy is reviewed. The resolution is presented using classical electromagnetic theory and logical interpretation of experiments disseminated over the previous century. Emphasis is placed on applied physics applications: modeling optical manipulation of cells and particles. Although the basic interpretation of optical momentum has been resolved, there is still some uncertainly regarding the complete form of the momentum continuity equation describing electromagnetics. Thus, while a complete picture of electrodynamics has still yet to be fully interpreted, this correspondence should help clarify the state-of-the-art view.


Chiral particles in a circularly polarised light field: new effects and applications

D V Guzatov and Vasilii V Klimov

Cross sections of scattering, absorption, and light pressure for a chiral spherical particle in a circularly polarised light field are studied for different values of the radius, dielectric constant, permeability, and parameter of the chiral particle. The conditions are found under which the cross sections of absorption and light pressure differ significantly when nanoparticles are exposed to light with different polarisations, which can be used to improve synthesis of chiral nanoparticles of complex structure.


Thursday, July 14, 2011

Novel optical techniques for measurements of light extinction, scattering and absorption by single aerosol particles

R.E.H. Miles, A.E. Carruthers, J.P. Reid

Aerosol particles play important roles in a broad range of scientific disciplines, from atmospheric chemistry and physics, to the delivery of fuels for combustion and drugs to the lungs, and extending to industrial processes such as spray drying. Measurements of the light extinction, scattering and absorption by ensembles of aerosol particles can be used to non-intrusively characterise aerosol particle samples. However, such measurements often lead to ambiguity in interpreting the properties and processes occurring on individual particles. In this review, recent developments in the use of laser based techniques to isolate and manipulate single particles and to characterise them will be highlighted. In particular, the use of cavity ring down spectroscopy, Bessel beams and optical tweezers for investigating light extinction, scattering and absorption, respectively, will be considered. The prospects for using optical techniques to interrogate the fundamental processes occurring in aerosol at the single particle level are discussed.


Mechanical Unfolding of the Beet Western Yellow Virus −1 Frameshift Signal

Katherine H. White, Marek Orzechowski, Dominique Fourmy, and Koen Visscher

Using mechanical unfolding by optical tweezers (OT) and steered molecular dynamics (SMD) simulations, we have demonstrated the critical role of Mg2+ ions for the resistance of the Beet Western Yellow Virus (BWYV) pseudoknot (PK) to unfolding. The two techniques were found to be complementary, providing information at different levels of molecular scale. Findings from the OT experiments indicated a critical role of stem 1 for unfolding of the PK, which was confirmed in the SMD simulations. The unfolding pathways of wild type and mutant appeared to depend upon pH and nucleotide sequence. SMD simulations support the notion that the stability of stem 1 is critical for −1 frameshifting. The all-atom scale nature of the SMD enabled clarification of the precise role of two Mg2+ ions, Mg45 and Mg52, as identified in the BWYV X-ray crystallography structure, in −1 frameshifting. On the basis of simulations with “partially” and “fully” hydrated Mg2+ ions, two possible mechanisms of stabilizing stem 1 are proposed. In both these cases Mg2+ ions play a critical role in stabilizing stem 1, either by directly forming a salt bridge between the strands of stem 1 or by stabilizing parallel orientation of the strands in stem 1, respectively. These findings explain the unexpected drop in frameshifting efficiency to null levels of the C8U mutant in a manner consistent with experimental observations.


Real-time detection of changes in the electrophoretic mobility of a single cell induced by hyperosmotic stress

Pau Mestres and Dmitri Petrov

Living cells survive environmentally stressful conditions by initiating a stress response. We monitored changes in the electrophoretic mobility (EPM) of single, optically trapped yeast cells under hyperosmotic stress conditions using optical tweezers combined with a position detector. We studied the dynamics of the EPM stress response for cells at different phases of the cell cycle.


Three-dimensional imaging of liquid crystal structures and defects by means of holographic manipulation of colloidal nanowires with faceted sidewalls

David Engström, Rahul P. Trivedi, Martin Persson, Mattias Goksör, Kris A. Bertness and Ivan I. Smalyukh

We use nanowires with faceted sidewalls for mapping of the patterns of three-dimensional orientational order and defect structures. In chiral nematics, the nanowires follow the local average orientation of rod-shaped molecules. When spatially translated by use of holographic optical tweezers in three dimensions, they mediate direct nondestructive visualization of the helicoidal ground-state structures, edge and screw dislocations, and kinks, as well as enable non-contact manipulation of these defects. We probe interactions of faceted nanowires with different defects and demonstrate their spontaneous self-alignment along the cores of singular defect lines.


Wednesday, July 13, 2011

Revisit on dynamic radiation forces induced by pulsed Gaussian beams

Li-Gang Wang and Hai-Shui Chai

Motivated by the recent optical trapping experiments using ultra-short pulsed lasers [Opt. Express 18, 7554 (2010); Appl. Opt. 48, G33 (2009)], in this paper we have re-investigated the trapping effects of the pulsed radiation force (PRF), which is induced by a pulsed Gaussian beam acting on a Rayleigh dielectric sphere. Based on our previous model [Opt. Express 15, 10615 (2007)], we have considered the effects arisen from both the transverse and axial PRFs, which lead to the different behaviors of both velocities and displacements of a Rayleigh particle within a pulse duration. Our analysis shows that, for the small-sized Rayleigh particles, when the pulse has the large pulse duration, it might provide the three-dimensional optical trapping; and when the pulse has the short pulse duration, it only provides the two-dimensional optical trapping with the axial movement along the pulse propagation. When the particle is in the vacuum or in the situation with the very weak Brownian motion, the particle can always be trapped stably due to the particle’s cumulative momentum transferred from the pulse, and only in this case the trapping effect is independent of pulse duration. Finally, we have predicted that for the large-sized Rayleigh particles, the pulse beam can only provide the two-dimensional optical trap (optical guiding). Our results provide the important information about the trapping mechanism of pulsed tweezers.


Tuesday, July 12, 2011

Cell Cytoskeleton and Tether Extraction

B. Pontes, N.B. Viana, L.T. Salgado, M. Farina, V. Moura Neto and H.M. Nussenzveig

We perform a detailed investigation of the force × deformation curve in tether extraction from 3T3 cells by optical tweezers. Contrary to conventional wisdom about tethers extracted from cells, we find that actin filaments are present within them, so that a revised theory of tether pulling from cells is called for. We also measure steady and maximum tether force values significantly higher than previously published ones for 3T3 cells. Possible explanations for these differences are investigated. Further experimental support of the theory of force barriers for membrane tube extension is obtained. The potential of studies on tether pulling force × deformation for retrieving information on membrane-cytoskeleton interaction is emphasized.


Magnus force effect in optical manipulation

Gabriella Cipparrone, Raul Josue Hernandez, Pasquale Pagliusi, and Clementina Provenzano

The effect of the Magnus force in optical micromanipulation has been observed. An ad hoc experiment has been designed based on a one-dimensional optical trap that carries angular momentum. The observed particle dynamics reveals the occurrence of this hydrodynamic force, which is neglected in the common approach. Its measured value is larger than the one predicted by the existing theoretical models for micrometric particles and low Reynolds number, showing that the Magnus force can contribute to unconventional optohydrodynamic trapping and manipulation.


Brownian Fluctuations and Heating of an Optically Aligned Gold Nanorod

P. V. Ruijgrok, N. R. Verhart, P. Zijlstra, A. L. Tchebotareva, and M. Orrit

We present the first quantitative measurements of the torque exerted on a single gold nanorod in a polarized three-dimensional optical trap. We determined the torque both by observing the time-averaged orientation distribution and by measuring the dynamics of the rotational Brownian fluctuations. The measurements are in good agreement with calculations, where the temperature profile around the hot nanorod gives rise to a reduced, effective viscosity. The maximum torque on a 60  nm×25  nm nanorod was 100  pN·nm, large enough to address single-molecule processes in soft and biological matter.


Monday, July 11, 2011

Single-molecule studies of viral DNA packaging

Douglas E Smith

Assembly of many dsDNA viruses involves packaging of DNA molecules into pre-assembled procapsids by portal molecular motor complexes. Techniques have recently been developed using optical tweezers to directly measure the packaging of single DNA molecules into single procapsids in real time and the forces generated by the molecular motor. Three different viruses, phages phi29, lambda, and T4, have been studied, revealing interesting similarities and differences in packaging dynamics. Single-molecule fluorescence methods have also been used to measure packaging kinetics and motor conformations. Here we review recent discoveries made using these new techniques.


Friday, July 8, 2011

Light Microscopy with Doughnut Modes: A Concept to Detect, Characterize, and Manipulate Individual Nanoobjects

Tina Züchner, Dr. Antonio Virgilio Failla, Prof. Dr. Alfred J. Meixne

Higher order laser modes, mainly called doughnut modes (DMs) have use in many different branches of research, such as, bio-imaging, material science, single-molecule microscopy, and spectroscopy. The main reason of their increasing importance is that recently, the techniques to generate well-defined DMs have been refined or rediscovered. Although their potential is still not fully utilized, their specifically polarized field distribution gives rise to a wide field of applications. They are contributing to complete our fundamental knowledge of the optical properties of single emitting species, such as molecules, nanoparticles, or quantum dots, offering insight into the three-dimensional dipole or particle orientation in space. The perfect zero intensity in the focus center qualifies some DMs for stimulated emission depletion (STED) microscopy. For the same reason, they have been suggested for trapping and tweezing applications.


Thursday, July 7, 2011

Numerical investigation of passive optical sorting of plasmon nanoparticles

M. Ploschner, M. Mazilu, T. Čižmár, and K. Dholakia

We explore the passive optical sorting of plasmon nanoparticles and investigate the optimal wavelength and optimal beam shape of incident field. The condition for optimal wavelength is found by maximising the nanoparticle separation whilst minimising the temperature increase in the system. We then use the force optical eigenmode (FOEi) method to find the beam shape of incident electromagnetic field, maximising the force difference between plasmon nanoparticles. The maximum force difference is found with respect to the whole sorting region. The combination of wavelength and beam shape study is demonstrated for a specific case of gold nanoparticles of radius 40nm and 50nm respectively. The optimum wavelength for this particular situation is found to be above 700nm. The optimum beam shape depends upon the size of sorting region and ranges from plane-wave illumination for infinite sorting region to a field maximising gradient force difference in a single point.


Wednesday, July 6, 2011

Near-IR Laser-Triggered Target Cell Collection Using a Carbon Nanotube-Based Cell-Cultured Substrate

Takao Sada, Tsuyohiko Fujigaya, Yasuro Niidome, Kohji Nakazawa, and Naotoshi Nakashima

Unique near-IR optical properties of single-walled carbon nanotube (SWNTs) are of interest in many biological applications. Here we describe the selective cell detachment and collection from an SWNT-coated cell-culture dish triggered by near-IR pulse laser irradiation. First, HeLa cells were cultured on an SWNT-coated dish prepared by a spraying of an aqueous SWNT dispersion on a glass dish. The SWNT-coated dish was found to show a good cell adhesion behavior as well as a cellular proliferation rate similar to a conventional glass dish. We discovered, by near-IR pulse laser irradiation (at the laser power over 25 mW) to the cell under optical microscopic observation, a quick single-cell detachment from the SWNT-coated surface. Shockwave generation from the irradiated SWNTs is expected to play an important role for the cell detachment. Moreover, we have succeeded in catapulting the target single cell from the cultured medium when the depth of the medium was below 150 μm and the laser power was stronger than 40 mW. The captured cell maintained its original shape. The retention of the genetic information of the cell was confirmed by the polymerase chain reaction (PCR) technique. A target single-cell collection from a culture medium under optical microscopic observation is significant in wide fields of single-cell studies in biological areas.


Tuesday, July 5, 2011

Direct and inverted nematic dispersions for soft matter photonics

I Muševič, M Škarabot and M Humar

General properties and recent developments in the field of nematic colloids and emulsions are discussed. The origin and nature of pair colloidal interactions in the nematic colloids are explained and an overview of the stable colloidal 2D crystalline structures and superstructures discovered so far is given. The nature and role of topological defects in the nematic colloids is discussed, with an emphasis on recently discovered entangled colloidal structures. Applications of inverted nematic emulsions and binding force mechanisms in nematic colloids for soft matter photonic devices are discussed.


Reconfigurable Knots and Links in Chiral Nematic Colloids

Uroš Tkalec, Miha Ravnik, Simon Čopar, Slobodan Žumer, Igor Muševič
Tying knots and linking microscopic loops of polymers, macromolecules, or defect lines in complex materials is a challenging task for material scientists. We demonstrate the knotting of microscopic topological defect lines in chiral nematic liquid-crystal colloids into knots and links of arbitrary complexity by using laser tweezers as a micromanipulation tool. All knots and links with up to six crossings, including the Hopf link, the Star of David, and the Borromean rings, are demonstrated, stabilizing colloidal particles into an unusual soft matter. The knots in chiral nematic colloids are classified by the quantized self-linking number, a direct measure of the geometric, or Berry’s, phase. Forming arbitrary microscopic knots and links in chiral nematic colloids is a demonstration of how relevant the topology can be for the material engineering of soft matter.

Exploitation of physical and chemical constraints for three-dimensional microtissue construction in microfluidics

Deepak Choudhury, Xuejun Mo, Ciprian Iliescu, Loo Ling Tan, Wen Hao Tong, and Hanry Yu

There are a plethora of approaches to construct microtissues as building blocks for the repair and regeneration of larger and complex tissues. Here we focus on various physical and chemical trapping methods for engineering three-dimensional microtissue constructs in microfluidic systems that recapitulate the in vivo tissue microstructures and functions. Advances in these in vitro tissue models have enabled various applications, including drug screening, disease or injury models, and cell-based biosensors. The future would see strides toward the mesoscale control of even finer tissue microstructures and the scaling of various designs for high throughput applications. These tools and knowledge will establish the foundation for precision engineering of complex tissues of the internal organs for biomedical applications.


Monday, July 4, 2011

Studies on erythrocytes in malaria infected blood sample with Raman optical tweezers

Raktim Dasgupta, Ravi Shanker Verma, Sunita Ahlawat, Abha Uppal, and Pradeep Kumar Gupta

Raman spectroscopy was performed on optically trapped red blood cells (RBCs) from blood samples of healthy volunteers (h-RBCs) and from patients suffering from P. vivax infection (m-RBCs). A significant fraction of m-RBCs produced Raman spectra with altered characteristics relative to h-RBCs. The observed spectral changes suggest a reduced oxygen-affinity or right shifting of the oxygen-dissociation curve for the intracellular hemoglobin in a significant fraction of m-RBCs with respect to its normal functional state.


Sunday, July 3, 2011

Published Paper Statistics (first half of 2011)

Here is the results for the first half of 2011 for published papers on optical tweezers, micromanipulation and trapping.

The top Journals (more than 2% hits) are:

  1. Journal of Optics   10.8%
  2. Optics Express   9.0%
  3. Optics Letters   4.5%
  4. Soft Matter   4.5%
  5. Lab on a Chip   3.1%
  6. Applied Optics   2.7%
  7. Applied Physics Letters   2.7%
  8. Nature Photonics   2.7%
  9. Biophysical Journal   2.2%
  10. Europhysics Letters   2.2%
  11. Plos One   2.2%

Below is a tag cloud for the corresponding words found in the title and abstracts for the period:

Friday, July 1, 2011

Direct observation of the full transition from ballistic to diffusive Brownian motion in a liquid

Rongxin Huang, Isaac Chavez, Katja M. Taute, Branimir Lukić, Sylvia Jeney, Mark G. Raizen and Ernst-Ludwig Florin

At timescales once deemed immeasurably small by Einstein, the random movement of Brownian particles in a liquid is expected to be replaced by ballistic motion. So far, an experimental verification of this prediction has been out of reach due to a lack of instrumentation fast and precise enough to capture this motion. Here we report the observation of the Brownian motion of a single particle in an optical trap with 75 MHz bandwidth and sub-ångström spatial precision and the determination of the particle’s velocity autocorrelation function. Our observation is the first measurement of ballistic Brownian motion of a particle in a liquid. The data are in excellent agreement with theoretical predictions taking into account the inertia of the particle and hydrodynamic memory effects.


Millikelvin cooling of an optically trapped microsphere in vacuum

Tongcang Li, Simon Kheifets & Mark G. Raizen

Cooling of micromechanical resonators towards the quantum mechanical ground state in their centre-of-mass motion has advanced rapidly in recent years. This work is an important step towards the creation of ‘Schrödinger cats’, quantum superpositions of macroscopic observables, and the study of their destruction by decoherence. Here we report optical trapping of glass microspheres in vacuum with high oscillation frequencies, and cooling of the centre-of-mass motion from room temperature to a minimum temperature of about 1.5 mK. This new system eliminates the physical contact inherent to clamped cantilevers, and can allow ground-state cooling from room temperature. More importantly, the optical trap can be switched off, allowing a microsphere to undergo free-fall in vacuum after cooling. This is ideal for studying the gravitational state reduction, a manifestation of the apparent conflict between general relativity and quantum mechanics. A cooled optically trapped object in vacuum can also be used to search for non-Newtonian gravity forces at small scales, measure the impact of a single air molecule and even produce Schrödinger cats of living organisms.


Experimental study of the optical forces exerted by a Gaussian beam within the Rayleigh range

L Ferrara, E Baldini, P Minzioni, F Bragheri, C Liberale, E Di Fabrizio and I Cristiani
Numerical tools for the evaluation of optical forces exerted by non-focused Gaussian beams are becoming increasingly important for the design of integrated devices dedicated to cell manipulation without physical contact. Here we consider two methods for the evaluation of optical forces based on a ray-optics approach and we compare them with experimental results. We show that optical forces can be calculated with good accuracy also within the Rayleigh range of the Gaussian beam and for a wide range of particle sizes.


Biological mechanisms, one molecule at a time

Ignacio Tinoco Jr and Ruben L. Gonzalez Jr

The last 15 years have witnessed the development of tools that allow the observation and manipulation of single molecules. The rapidly expanding application of these technologies for investigating biological systems of ever-increasing complexity is revolutionizing our ability to probe the mechanisms of biological reactions. Here, we compare the mechanistic information available from single-molecule experiments with the information typically obtained from ensemble studies and show how these two experimental approaches interface with each other. We next present a basic overview of the toolkit for observing and manipulating biology one molecule at a time. We close by presenting a case study demonstrating the impact that single-molecule approaches have had on our understanding of one of life's most fundamental biochemical reactions: the translation of a messenger RNA into its encoded protein by the ribosome.