Friday, January 15, 2010

Active-passive calibration of optical tweezers in viscoelastic media

Mario Fischer, Andrew C. Richardson, S. Nader S. Reihani, Lene B. Oddershede, and Kirstine Berg-Sørensen

In order to use optical tweezers as a force measuring tool inside a viscoelastic medium such as the cytoplasm of a living cell, it is crucial to perform an exact force calibration within the complex medium. This is a nontrivial task, as many of the physical characteristics of the medium and probe, e.g., viscosity, elasticity, shape, and density, are often unknown. Here, we suggest how to calibrate single beam optical tweezers in a complex viscoelastic environment. At the same time, we determine viscoelastic characteristics such as friction retardation spectrum and elastic moduli of the medium. We apply and test a method suggested [M. Fischer and K. Berg-Sørensen, J. Opt. A, PureAppl. Opt. 9, S239 (2007)], a method which combines passive and active measurements. The method is demonstrated in a simple viscous medium, water, and in a solution of entangled F-actin without cross-linkers.

Quantum limited particle sensing in optical tweezers

Jian Wei Tay, Magnus T. L. Hsu, and Warwick P. Bowen

Particle sensing in optical tweezers systems provides information on the position, velocity, and force of the specimen particles. The conventional quadrant detection scheme is applied ubiquitously in optical tweezers experiments to quantify these parameters. In this paper, we show that quadrant detection is nonoptimal for particle sensing in optical tweezers and propose an alternative optimal particle sensing scheme based on spatial homodyne detection. A formalism for particle sensing in terms of transverse spatial modes is developed and numerical simulations of the efficacies of both quadrant and spatial homodyne detection are shown. We demonstrate that 1 order of magnitude improvement in particle sensing sensitivity can be achieved using spatial homodyne over quadrant detection.

Cascade optical chromatography for sample fractionation

Alex Terray, Joseph D. Taylor, and Sean J. Hart

Optical chromatography involves the elegant combination of opposing optical and fluid drag forces on colloidal samples within microfluidic environments to both measure analytical differences and fractionate injected samples. Particles that encounter the focused laser beam are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. In our recent devices particles are pushed into a region of lower microfluidic flow, where they can be retained and fractionated. Because optical and fluid forces on a particle are sensitive to differences in the physical and chemical properties of a sample, separations are possible. An optical chromatography beam focused to completely fill a fluid channel is operated as an optically tunable filter for the separation of inorganic, polymeric, and biological particle samples. We demonstrate this technique coupled with an advanced microfluidic platform and show how it can be used as an effective method to fractionate particles from an injected multicomponent sample. Our advanced three-stage microfluidic design accommodates three lasers simultaneously to effectively create a sequential cascade optical chromatographic separation system.

Microengineered Platforms for Cell Mechanobiology

Deok-Ho Kim, Pak Kin Wong, Jungyul Park, Andre Levchenko, and Yu Sun

Mechanical forces play important roles in the regulation of various biological processes at the molecular and cellular level, such as gene expression, adhesion, migration, and cell fate, which are essential to the maintenance of tissue homeostasis. In this review, we discuss emerging bioengineered tools enabled by microscale technologies for studying the roles of mechanical forces in cell biology. In addition to traditional mechanobiology experimental techniques, we review recent advances of microelectromechanical systems (MEMS)-based approaches for cell mechanobiology and discuss how microengineered platforms can be used to generate in vivo–like micromechanical environment in in vitro settings for investigating cellular processes in normal and pathophysiological contexts. These capabilities also have significant implications for mechanical control of cell and tissue development and cell-based regenerative therapies.

Thursday, January 14, 2010

Towards airborne optofluidics: Optical manipulation techniques for airborne particles

McGloin, D., Guillon, M., Rudd, D., Burnham, D.R., Summers, M.D., Firmin, J., Butler, J.R., Wills, J.B., Mitchem, L.,Meresman, H., Reid, J.P., Sheridan, A.

This paper details progress towards the possibility of creating integrated optical devices capable of manipulating and analyzing airborne particles in the form of aerosols. We also describe work designed to look at the possibility of controlling optical cavities created using liquid aerosols using light.


Optoelectronic Tweezers as a Tool for Parallel Single-Cell Manipulation and Stimulation

Valley, J.K.; Ohta, A.T.; Hsan-Yin Hsu; Neale, S.L.; Jamshidi, A.; Wu, M.C.;

Optoelectronic tweezers (OET) is a promising approach for the parallel manipulation of single cells for a variety of biological applications. By combining the manipulation capabilities of OET with other relevant biological techniques (such as cell lysis and electroporation), one can realize a true parallel, single-cell diagnostic and stimulation tool. Here, we demonstrate the utility of the OET device by integrating it onto single-chip systems capable of performing in-situ, electrode-based electroporation/lysis, individual cell, light-induced lysis, and light-induced electroporation.

Phototransistor-based optoelectronic tweezers for dynamic cell manipulation in cell culture media

Hsan-yin Hsu, Aaron T. Ohta, Pei-Yu Chiou, Arash Jamshidi, Steven L. Neale and Ming C. Wu

Optoelectronic tweezers (OET), based on light-induced dielectrophoresis, has been shown as a versatile tool for parallel manipulation of micro-particles and cells (P. Y. Chiou, A. T. Ohta and M. C. Wu, Nature, 2005, 436, 370–372).1 However, the conventional OET device cannot operate in cell culture media or other high-conductivity physiological buffers due to the limited photoconductivity of amorphous silicon. In this paper, we report a new phototransistor-based OET (Ph-OET). Consisting of single-crystalline bipolar junction transistors, the Ph-OET has more than 500× higher photoconductivity than amorphous silicon. Efficient cell trapping of live HeLa and Jurkat cells in Phosphate Buffered Saline (PBS) and Dulbecco's Modified Eagle's Medium (DMEM) has been demonstrated using a digital light projector, with a cell transport speed of 33 µm/sec, indicating a force of 14.5 pN. Optical concentration of cells and real-time control of individually addressable cell arrays have also been realized. Precise control of separation between two cells has also been demonstrated. We envision a new platform for single cell studies using Ph-OET.

Tuesday, January 12, 2010

Dynamic position and force measurement for multiple optically trapped particles using a high-speed active pixel sensor

M. Towrie, S. W. Botchway, A. Clark, E. Freeman, R. Halsall, A. W. Parker, M. Prydderch, R. Turchetta, A. D. Ward, and M. R. Pollard

A high frame rate active pixel sensor designed to track the position of up to six optically trapped objects simultaneously within the field of view of a microscope is described. Thesensor comprises 520×520 pixels from which a flexible arrangement of six independent regions of interest is accessed at a rate of up to 20 kHz, providing the capability to measure motion in multiple micron scale objects to nanometer accuracy. The combinedcontrol of both the sensor and optical traps is performed using unique, dedicated electronics (a field programmable gate array). The ability of the sensor to measure the dynamic position and the forces between six optically trapped spheres, down to femtonewton level, is demonstrated paving the way for application in the physical and life sciences.

Monday, January 11, 2010

The SAH domain extends the functional length of the myosin lever

Thomas G. Baboolal, Takeshi Sakamoto, Eva Forgacs,Howard D. White, Scott M. Jackson, Yasuharu Takagi,Rachel E. Farrow, Justin E. Molloy, Peter J. Knight, James R. Sellers and Michelle Peckham

Stable, single alpha-helix (SAH) domains are widely distributed in the proteome, including in myosins, but their functions are unknown. To test whether SAH domains can act as levers, we replaced four of the six calmodulin-binding IQ motifs in the levers of mouse myosin 5a (Myo5) with the putative SAH domain ofDictyostelium myosin MyoM of similar length. The SAH domain was inserted between the IQ motifs and the coiled coil in a Myo5 HMM construct in which the levers were truncated from six to two IQ motifs (Myo5–2IQ). Electron microscopy of this chimera (Myo5–2IQ-SAH) showed the SAH domain was straight and 17 nm long as predicted, restoring the truncated lever to the length of wild-type (Myo5–6IQ). The powerstroke (of 21.5 nm) measured in the optical trap was slightly less than that for Myo5–6IQ but much greater than for Myo5–2IQ. Myo5–2IQ-SAH moved processively along actin at physiological ATP concentrations with similar stride and run lengths to Myo5–6IQ in in-vitro single molecule assays. In comparison, Myo5–2IQ is not processive under these conditions. Solution biochemical experiments indicated that the rear head did not mechanically gate the rate of ADP release from the lead head, unlike Myo5–6IQ. These data show that the SAH domain can form part of a functional lever in myosins, although its mechanical stiffness might be lower. More generally, we conclude that SAH domains can act as stiff structural extensions in aqueous solution and this structural role may be important in other proteins.

Friday, January 8, 2010

Alignment, Rotation, and Spinning of Single Plasmonic Nanoparticles and Nanowires Using Polarization Dependent Optical Forces

Lianming Tong, Vladimir D. Miljkovic and Mikael Kall

We demonstrate optical alignment and rotation of individual plasmonic nanostructures with lengths from tens of nanometers to several micrometers using a single beam of linearly polarized near-infrared laser light. Silver nanorods and dimers of gold nanoparticles align parallel to the laser polarization because of the high long-axis dipole polarizability. Silver nanowires, in contrast, spontaneously turn perpendicular to the incident polarization and predominantly attach at the wire ends, in agreement with electrodynamics simulations. Wires, rods, and dimers all rotate if the incident polarization is turned. In the case of nanowires, we demonstrate spinning at an angular frequency of 1 Hz due to transfer of spin angular momentum from circularly polarized light.

Self-induced back-action optical trapping of dielectric nanoparticles

Mathieu L. Juan, Reuven Gordon, Yuanjie Pang, Fatima Eftekhari & Romain Quidant

Optical trapping has widely affected both the physical and life sciences. Past approaches to optical trapping of nanoscale objects required large optical intensities, often above their damage threshold. To achieve more than an order of magnitude reduction in the local intensity required for optical trapping, we present a self-induced back-action (SIBA) optical trap, where the trapped object has an active role in enhancing the restoring force. We demonstrate experimentally trapping of a single 50 nm polystyrene sphere using a SIBA optical trap on the basis of the transmission resonance of a nanoaperture in a metal film. SIBA optical trapping shows a striking departure from previous approaches, which we quantify by comprehensive calculations. The SIBA optical trap enables new opportunities for non-invasive immobilization of a single nanoscale object, such as a virus or a quantum dot.

Thursday, January 7, 2010

Rotational dynamics of optically trapped nanofibers

Antonio Alvaro Ranha Neves, Andrea Camposeo, Stefano Pagliara, Rosalba Saija,Ferdinando Borghese, Paolo Denti, Maria Antonia Iatì, Roberto Cingolani, Onofrio M. Maragò, and Dario Pisignano

We report on the experimental evidence of tilted polymer nanofiber rotation, using a highly focused linear polarized Gaussian beam. Torque is controlled by varying trapping power or fiber tilt angle. This suggests an alternative strategy to previously reported approaches for the rotation of nano-objects, to test fundamental theoretical aspects. We compare experimental rotation frequencies to calculations based on T-Matrix formalism, which accurately reproduces measured data, thus providing a comprehensive description of trapping and rotation dynamics of the linear nanostructures.

Ultralow power trapping and fluorescence detection of single particles on an optofluidic chip

S. Kühn, B. S. Phillips, E. J. Lunt, A. R. Hawkins and H. Schmidt

The development of on-chip methods to manipulate particles is receiving rapidly increasing attention. All-optical traps offer numerous advantages, but are plagued by large required power levels on the order of hundreds of milliwatts and the inability to act exclusively on individual particles. Here, we demonstrate a fully integrated electro-optical trap for single particles with optical excitation power levels that are five orders of magnitude lower than in conventional optical force traps. The trap is based on spatio-temporal light modulation that is implemented using networks of antiresonant reflecting optical waveguides. We demonstrate the combination of on-chip trapping and fluorescence detection of single microorganisms by studying the photobleaching dynamics of stained DNA in E. coli bacteria. The favorable size scaling facilitates the trapping of single nanoparticles on integrated optofluidic chips.

Tuesday, January 5, 2010

Quantification of droplet deformation by electromagnetic trapping

P. C. F. Møller and L. B. Oddershede

A tightly focused laser beam exerting a trapping force on an object also exerts deformation forces on the object. If the object is relatively easy to deform, as is, e.g., a low surface tension droplet, the resulting deformation is easily detectable even at moderate laser powers. The observed deformation is analytically explained by a model, which quantitatively predicts the deformation of any micron-sized drop where the only restoring force is the surface tension. Theoretical tools are also provided to include the effect of elasticity of the shell and bulk of the trapped object, this being particularly important for deformations of cells. This deformation effect of electromagnetic radiation is important to consider while trapping soft materials and it can be used to determine physical characteristics of soft materials.

Optimized optical trapping of gold nanoparticles

Faeghe Hajizadeh and S.Nader S.Reihani

Metallic nanoparticles are of significant interest due to their particular optical and biological applications. Gold nanoparticles are proven to be excellent candidate for in vivo micro-manipulation using Optical Tweezers. This manuscript reports on stable 3-D trapping of 9.5-254nm gold nanospheres using substantially decreased laser power. The lower limit is ∼2 times smaller than previous record. 5.4nm gold nanospheres were trapped for only 2-3 seconds. For the first time, our experimental data verify the volume corrected Rayleigh model for particles smaller than 100nm in diameter. Measuring the maximum applicable force for gold nanoparticles, we have shown that a few tens of milli-Watts of laser power can produce pico-Newton range forces.

Janus Nematic Colloids driven by light

M. Conradi, M. Zorko, and I. Muševič

We present an experimental analysis of different equilibrium orientations and light driven transformations of Janus particles in the nematic liquid crystal 5CB. Depending on the preparation technique of homeotropic (DMOAP) and planar (Au) hemispheres we have observed two types of director field configurations: dipolar-like in the case of Au/DMOAP capped colloids and boojum-like in the case of DMOAP/Au capped colloids. Using the manipulation of Au/DMOAP capped colloids with laser tweezers we report on light driven irreversible orientational transformations into Saturn-ring and a novel, boojum-ring configuration. On the contrary, boojum-like DMOAP/Au capped colloids can act as rotators when exposed to the laser filed. Observed rotation is continuous around an axis perpendicular to the laser beam axis, with the frequency increasing linearly with the laser power.

Friday, January 1, 2010

Published Papers Statistics for 2009

Here is the results for the year of 2009 for published papers on optical tweezers, micromanipulation and trapping.

The top Journals (more than 2% hits) are:
  1. Optics Express 12.6%
  2. Physical Review Letters 5.6%
  3. Biophysical Journal 4.4%
  4. Applied Optics 3.8%
  5. Applied Physics Letters 3.2%
  6. Nano Letters 3.2%
  7. Optics Letters 3.2%
  8. Physical Review E 3.2%
  9. Journal of Optics A 2.9%
  10. Physical Review A 2.9%
  11. Lab on a Chip 2.6 %
  12. Proceedings of the National Academy of Sciences 2.6%

Below is also a tag cloud from the words found in the title and abstracts for 2009: