Tuesday, January 31, 2012

Radiation forces acting on a Rayleigh dielectric sphere produced by highly focused elegant Hermite-cosine-Gaussian beams

Zhirong Liu and Daomu Zhao

We derive the analytical expression for the propagation of elegant Hermite-cosine-Gaussian (EHcosG) beams through a paraxial ABCD optical system and use it to study the radiation forces produced by highly focused this kind of beams acting on a Rayleigh dielectric sphere. Owing to the characteristics of focused EHcosG beams our analysis shows that it can be expected to simultaneously trap and manipulate dielectric spheres with low-refractive index at the focus point, and spheres with high-refractive index nearby the focus point. Finally, we discuss the stability conditions for effective trapping and manipulating the particle.


Light spin forces in optical traps: comment on “Trapping metallic Rayleigh particles with radial polarization”

Ignacio Iglesias and Juan José Sáenz

An incomplete modeling of the scattering forces on a Rayleigh particle without taking into account the light spin forces in “Trapping metallic Rayleigh particles with radial polarization” by Q. Zhan, leads to erroneous statements on the advantages of using radial polarization to trap metallic particles.


Biophotonic techniques for manipulation and characterization of drug delivery nanosystems in cancer therapy

E. Spyratou, M. Makropoulou, E.A. Mourelatou, C. Demetzos

Reactive oxygen species (ROS) are usually involved in two opposite procedures related to cancer: initiation, progression and metastasis of cancer, as well as in all non-surgical therapeutic approaches for cancer, including chemotherapy, radiotherapy and photodynamic therapy. This review is concentrated in new therapeutic strategies that take advantage of increased ROS in cancer cells to enhance therapeutic activity and selectivity. Novel biophotonic techniques for manipulation and characterization of drug delivery nanosystems in cancer therapy are discussed, including optical tweezers and atomic force microscopy. This review highlights how these techniques are playing a critical role in recent and future cancer fighting applications. We can conclude that Biophotonics and nanomedicine are the future for cancer biology and disease management, possessing unique potential for early detection, accurate diagnosis, dosimetry and personalized treatment of biomedical applications targeting cancer.


Colloids in one dimensional random energy landscapes

Richard D. L. Hanes, Cécile Dalle-Ferrier, Michael Schmiedeberg, Matthew C. Jenkins and Stefan U. Egelhaaf

Individual colloidal particles have been studied experimentally in a one dimensional random potential with energies that follow a Gaussian distribution. This rough, noise-like potential has been realised using a holographic optical set-up, which allows the width of the distribution to be varied. For different widths, the particle trajectories were followed and the particle dynamics characterised by, for example, the mean square displacement, non-Gaussian parameter, van Hove function, time-dependent diffusion coefficient and residence time distribution. The values obtained for these observables are consistent with the static properties of the system, in particular the barrier height distribution, which was obtained by a detailed characterisation of the tweezer-like set-up. The dynamics display three distinct behaviours: at short times normal diffusion, subsequently an extended regime of localisation within the different minima of the potential and finally a very slow approach towards long-time diffusive behaviour, for which diffusion coefficients consistent with theoretical predictions have been found.


Efficient transportation of nano-sized particles along slotted photonic crystal waveguide

Pin-Tso Lin and Po-Tsung Lee

We design a slotted photonic crystal waveguide (S-PhCW) and numerically propose that it can efficiently transport polystyrene particle with diameter as small as 50 nm in a 100 nm slot. Excellent optical confinement and slow light effect provided by the photonic crystal structure greatly enhance the optical force exerted on the particle. The S-PhCW can thus transport the particle with optical propulsion force as strong as 5.3 pN/W, which is over 10 times stronger than that generated by the slotted strip waveguide (S-SW). In addition, the vertical optical attraction force induced in the S-PhCW is over 2 times stronger than that of the S-SW. Therefore, the S-PhCW transports particles not only efficiently but also stably. We anticipate this waveguide structure will be beneficial for the future lab-on-chip development


Quantification of local matrix deformations and mechanical properties during capillary morphogenesis in 3D

Ekaterina Kniazeva, John W. Weidling, Rahul Singh, Elliot L. Botvinick, Michelle A. Digman, Enrico Gratton and Andrew J. Putnam

Reciprocal mechanical interactions between cells and the extracellular matrix (ECM) are thought to play important instructive roles in branching morphogenesis. However, most studies to date have failed to characterize these interactions on a length scale relevant to cells, especially in three-dimensional (3D) matrices. Here we utilized two complementary methods, spatio-temporal image correlation spectroscopy (STICS) and laser optical tweezers-based active microrheology (AMR), to quantify endothelial cell (EC)-mediated deformations of individual ECM elements and the local ECM mechanical properties, respectively, during the process of capillary morphogenesis in a 3D cell culture model. In experiments in which the ECM density was systematically varied, STICS revealed that the rate at which ECs deformed individual ECM fibers on the microscale positively correlated with capillary sprouting on the macroscale. ECs expressing constitutively active V14-RhoA displaced individual matrix fibers at significantly faster rates and displayed enhanced capillary sprouting relative to wild-type cells, while those expressing dominant-negative N19-RhoA behaved in an opposite fashion. In parallel, AMR revealed a local stiffening of the ECM proximal to the tips of sprouting ECs. By quantifying the dynamic physical properties of the cell-ECM interface in both space and time, we identified a correlation linking ECM deformation rates and local ECM stiffening at the microscale with capillary morphogenesis at the macroscale.


Thursday, January 26, 2012

Quantifying cellular differentiation by physical phenotype using digital holographic microscopy

Kevin J. Chalut, Andrew E. Ekpenyong, Warren L. Clegg, Isabel C. Melhuish and Jochen Guck

Although the biochemical changes that occur during cell differentiation are well-known, less known is that there are significant, cell-wide physical changes that also occur. Understanding and quantifying these changes can help to better understand the process of differentiation as well as ways to monitor it. Digital holographic microscopy (DHM) is a marker-free quantitative phase microscopy technique for measuring biological processes such as cellular differentiation, alleviating the need for introduction of foreign markers. We found significant changes in subcellular structure and refractive index of differentiating myeloid precursor cells within one day of differentiation induction, and significant differences depending on the type of lineage commitment. We augmented our results by showing significant changes in the softness of myeloid precursor cell differentiation within one day using optical stretching, a laser trap-based marker-free technique. DHM and optical stretching therefore provide consequential parameterization of cellular differentiation with sensitivity otherwise difficult to achieve. Therefore, we provide a way forward to quantify and understand cell differentiation with minimal perturbation using biophotonics.


Bimolecular integrin–ligand interactions quantified using peptide-functionalized dextran-coated microparticles

Jessie E. P. Sun, Justin Vranic, Russell J. Composto, Craig Streu, Paul C. Billings, Joel S. Bennett, John W. Weisel and Rustem I. Litvinov

Integrins play a key role in cell–cell and cell–matrix interactions. Artificial surfaces grafted with integrin ligands, mimicking natural interfaces, have been used to study integrin-mediated cell adhesion. Here we report the use of a new chemical engineering technology in combination with single-molecule nanomechanical measurements to quantify peptide binding to integrins. We prepared latex beads with covalently-attached dextran. The beads were then functionalized with the bioactive peptides, cyclic RGDFK (cRGD) and the fibrinogen γC-dodecapeptide (H12), corresponding to the active sites for fibrinogen binding to the platelet integrin αIIbβ3. Using optical tweezers-based force spectroscopy to measure non-specific protein–protein interactions, we found the dextran-coated beads nonreactive towards fibrinogen, thus providing an inert platform for biospecific modifications. Using periodate oxidation followed by reductive amination, we functionalized the bead-attached dextran with either cRGD or H12 and used the peptide-grafted beads to measure single-molecule interactions with the purified αIIbβ3. Bimolecular force spectroscopy revealed that the peptide-functionalized beads were highly and specifically reactive with the immobilized αIIbβ3. Further, the cRGD- and H12-functionalized beads displayed a remarkable interaction profile with a bimodal force distribution up to 90 pN. The cRGD–αIIbβ3 interactions had greater binding strength than that of H12–αIIbβ3, indicating that they are more stable and resistant mechanically, consistent with the platelet reactivity of RGD-containing ligands. Thus, the results reported here describe the mechanistic characteristics of αIIbβ3–ligand interactions, confirming the utility of peptide-functionalized latex beads for the quantitative analysis of molecular recognition.


A Structural Basis for Sustained Bacterial Adhesion: Biomechanical Properties of CFA/I Pili

Magnus Andersson, Oscar Björnham, Mats Svantesson, Arwa Badahdah, Bernt Eric Uhlin, Esther Bullitt

Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrheal disease worldwide. Adhesion pili (or fimbriae), such as the CFA/I (colonization factor antigen I) organelles that enable ETEC to attach efficiently to the host intestinal tract epithelium, are critical virulence factors for initiation of infection. We characterized the intrinsic biomechanical properties and kinetics of individual CFA/I pili at the single-organelle level, demonstrating that weak external forces (7.5 pN) are sufficient to unwind the intact helical filament of this prototypical ETEC pilus and that it quickly regains its original structure when the force is removed. While the general relationship between exertion of force and an increase in the filament length for CFA/I pili associated with diarrheal disease is analogous to that of P pili and type 1 pili, associated with urinary tract and other infections, the biomechanical properties of these different pili differ in key quantitative details. Unique features of CFA/I pili, including the significantly lower force required for unwinding, the higher extension speed at which the pili enter a dynamic range of unwinding, and the appearance of sudden force drops during unwinding, can be attributed to morphological features of CFA/I pili including weak layer-to-layer interactions between subunits on adjacent turns of the helix and the approximately horizontal orientation of pilin subunits with respect to the filament axis. Our results indicate that ETEC CFA/I pili are flexible organelles optimized to withstand harsh motion without breaking, resulting in continued attachment to the intestinal epithelium by the pathogenic bacteria that express these pili.


Membrane Tension Maintains Cell Polarity by Confining Signals to the Leading Edge during Neutrophil Migration

Andrew R. Houk, Alexandra Jilkine, Cecile O. Mejean, Rostislav Boltyanskiy, Eric R. Dufresne, Sigurd B. Angenent, Steven J. Altschuler, Lani F. Wu, Orion D. Weiner

Little is known about how neutrophils and other cells establish a single zone of actin assembly during migration. A widespread assumption is that the leading edge prevents formation of additional fronts by generating long-range diffusible inhibitors or by sequestering essential polarity components. We use morphological perturbations, cell-severing experiments, and computational simulations to show that diffusion-based mechanisms are not sufficient for long-range inhibition by the pseudopod. Instead, plasma membrane tension could serve as a long-range inhibitor in neutrophils. We find that membrane tension doubles during leading-edge protrusion, and increasing tension is sufficient for long-range inhibition of actin assembly and Rac activation. Furthermore, reducing membrane tension causes uniform actin assembly. We suggest that tension, rather than diffusible molecules generated or sequestered at the leading edge, is the dominant source of long-range inhibition that constrains the spread of the existing front and prevents the formation of secondary fronts.


Monday, January 23, 2012

3D trapping and manipulation of micro-particles using holographic optical tweezers with optimized computer-generated holograms

Tao Tao, Jing Li, Qian Long, and Xiaoping Wu

A multi-plane adaptive-additive algorithm is developed for optimizing computer-generated holograms for the reconstruction of traps in three-dimensional (3D) spaces. This algorithm overcomes the converging stagnation problem of the traditional multi-plane Gerchberg-Saxton algorithm and improves the diffraction efficiency of the holograms effectively. The optimized holograms are applied in a holographic optical tweezers (HOT) platform. Additionally, a computer program is developed and integrated into the HOT platform for the purpose of achieving the interactive control of traps. Experiments demonstrate that the manipulation of micro-particles into the 3D structure with optimized holograms can be carried out effectively on the HOT platform.


Optical-force-induced artifacts in scanning probe microscopy

Dana C. Kohlgraf-Owens, Sergey Sukhov, and Aristide Dogariu

In the practice of near-field scanning probe microscopy, it is typically assumed that the distance regulation is independent of the optical signal. However, we demonstrate that these two signals are entangled due to the inherent action of optically induced force. This coupling leads to artifacts in both estimating the magnitude of optical fields and recording topographic maps.


Damping of acoustic vibrations of single gold nanoparticles optically trapped in water

Paul V. Ruijgrok, Peter Zijlstra, Anna L. Tchebotareva, and Michel Orrit

We combine ultrafast pump-probe spectroscopy with optical trapping, to study homogeneous damping of the acoustic vibrations of single gold nanospheres (80 nm diameter) and nanorods (25 nm diameter by 60 nm length) in water. We find a significant particle-to-particle variation in damping times. Our results indicate that vibrational damping occurs not only by dissipation into the liquid, but also by damping mechanisms intrinsic to the particle. Our experiment opens the study of mechanisms of intrinsic mechanical dissipation in metals at frequencies 1-1000 GHz, a range that has been difficult to access thus far.


Friday, January 20, 2012

The Ribosome Modulates Nascent Protein Folding

Christian M. Kaiser, Daniel H. Goldman, John D. Chodera, Ignacio Tinoco Jr., Carlos Bustamante

Proteins are synthesized by the ribosome and generally must fold to become functionally active. Although it is commonly assumed that the ribosome affects the folding process, this idea has been extremely difficult to demonstrate. We have developed an experimental system to investigate the folding of single ribosome-bound stalled nascent polypeptides with optical tweezers. In T4 lysozyme, synthesized in a reconstituted in vitro translation system, the ribosome slows the formation of stable tertiary interactions and the attainment of the native state relative to the free protein. Incomplete T4 lysozyme polypeptides misfold and aggregate when free in solution, but they remain folding-competent near the ribosomal surface. Altogether, our results suggest that the ribosome not only decodes the genetic information and synthesizes polypeptides, but also promotes efficient de novo attainment of the native state.


Towards an integrated optical single aerosol particle lab

Marcel Horstmann, Karl Probst and Carsten Fallnich 

We present a manipulation and characterization system for single airborne particles which is integrated onto a microscope slide. Trapped particles are manipulated by means of radiation pressure and characterized by cavity enhanced Raman spectroscopy. Optical fibers are used to deliver the trapping laser light as well as to collect the Raman scattered light, allowing for a flexible usage of the device. The system features a sample chamber which is separated from an aerosol-flooded injection chamber by means of a light guiding glass-capillary. The coupling of this device with an aerosol optical tweezers setup to selectively load its trapping sites is demonstrated. Finally, a route towards chip-integrated handling and processing of multiple particles is shown and the first results are presented.


Plasmonic Coherent Drive of an Optical Trap

A. Cuche, O. Mahboub, E. Devaux, C. Genet, and T. W. Ebbesen

We demonstrate that optical trapping can be driven by delocalized surface plasmon modes resonantly excited within a standing wave trap. Dynamical modifications are shown to be determined by the near-field symmetry of the plasmonic modes with negligible thermal effect. With low trapping powers and polarization control, remarkable stiffness enhancements are recorded, the larger the smaller the particle. The results can be simply modeled accounting for a coherent interaction between the plasmon field and the Gaussian standing wave of the trap.


Retrieval of the Complex Refractive Index of Aerosol Droplets from Optical Tweezers Measurements

Rachael E.H. Miles, Jim S. Walker, Daniel R. Burnham and Jonathan P Reid

The cavity enhanced Raman scattering spectrum recorded from an aerosol droplet provides a unique fingerprint of droplet radius and refractive index, assuming that the droplet is homogeneous in composition. Aerosol optical tweezers are used in this study to capture a single droplet and a Raman fingerprint is recorded using the trapping laser as the source for the Raman excitation. We report here the retrieval of the real part of the refractive index with an uncertainty of ±0.0012 (better than ±0.11 %), simultaneously measuring the size of the micrometre sized liquid droplet with a precision of better than 1 nm (<±0.05 % error). In addition, the equilibrium size of the droplet is shown to depend on the laser irradiance due to optical absorption, which elevates the droplet temperature above that of the ambient gas phase. Modulation of the illuminating laser power leads to a modulation in droplet size as the temperature elevation is altered. By measuring induced size changes of <1 nm, we show that the imaginary part of the refractive index can be retrieved even when less than 10×10-9 with an accuracy of better than ± 0.5×10-9. The combination of these measurements allows the complex refractive index of a droplet to be retrieved with high accuracy, with the possibility of making extremely sensitive optical absorption measurements on aerosol samples and the testing of frequently using mixing rules for treating aerosol optical properties. More generally, this method provides an extremely sensitive approach for measuring refractive indices, particularly under solute supersaturation conditions that cannot be accessed by simple bulk-phase measurements.


The Measurement of Displacement and Optical Force in Multi-Optical Tweezers

LING Lin, GUO Hong-Lian, HUANG Lu, QU E, LI Zhao-Lin, LI Zhi-Yuan

We set up a system of multiple optical tweezers based on a spatial light modulator, and measured the displacement and optical force of the trapped particles simultaneously. All of the trapped particles can be clearly imaged in three dimensions by several CCDs. The displacement is obtained by calculating the gray weighted centroid in the trapped particle's image. The stiffness of the trapped particles in the optical traps is measured by oscillating the sample stage in a triangular wave based on Stokes fluid dynamics. The optical force of each trapped particle can be calculated by the measured displacement and stiffness.


Wave-guided optical waveguides

D. Palima, A. R. Bañas, G. Vizsnyiczai, L. Kelemen, P. Ormos, and J. Glückstad
This work primarily aims to fabricate and use two photon polymerization (2PP) microstructures capable of being optically manipulated into any arbitrary orientation. We have integrated optical waveguides into the structures and therefore have freestanding waveguides, which can be positioned anywhere in the sample at any orientation using optical traps. One of the key aspects to the work is the change in direction of the incident plane wave, and the marked increase in the numerical aperture demonstrated. Hence, the optically steered waveguide can tap from a relatively broader beam and then generate a more tightly confined light at its tip. The paper contains both simulation, related to the propagation of light through the waveguide, and experimental demonstrations using our BioPhotonics Workstation. In a broader context, this work shows that optically trapped microfabricated structures can potentially help bridge the diffraction barrier. This structure-mediated paradigm may be carried forward to open new possibilities for exploiting beams from far-field optics down to the subwavelength domain.


Beam optics approach to the ray optics model for the optical trapping efficiency of optical tweezers

SungHyun Kim, HyunIk Kim, HyeongJoon Jun, HyunJi Kim and Cha-Hwan Oh

The optical trapping efficiency of optical tweezers can be explained by the ray optics model when the particle size is much larger than the wavelength of laser light. The Optical trapping efficiency in the ray optics model does not depend on the particle size. However, The most experimental results usually show that the optical trapping efficiency decreases as the particle size decreases. In this study, we suggest a beam optics approach in order to take into account the particle size dependence effect in the ray optics model.


Molecular motors for DNA translocation in prokaryotes

Jean-François Allemand, Berenike Maier, Douglas E. Smith
DNA transport is an essential life process. From chromosome separation during cell division or sporulation, to DNA virus ejection or encapsidation, to horizontal gene transfer, it is ubiquitous in all living organisms. Directed DNA translocation is often energetically unfavorable and requires an active process that uses energy, namely the action of molecular motors. In this review we present recent advances in the understanding of three molecular motors involved in DNA transport in prokaryotes, paying special attention to recent studies using single-molecule techniques. We first discuss DNA transport during cell division, then packaging of DNA in phage capsids, and then DNA import during bacterial transformation. 

Thursday, January 19, 2012

Optimized free-form optical trapping systems

Andreas Oeder, Sebastian Stoebenau, and Stefan Sinzinger

We report a comprehensive process for designing and prototyping new and optimized optical trapping systems. A combination of traditional lens design strategies, simulation of optical forces, and high-end ultraprecision machining of optical free-form surfaces is applied to the realization of a highly specialized optical trapping system. The resulting compact and lightweight optical modules potentially open new classes of applications for optical manipulation. As an example we present a customized 3D trapping module made of a single piece of polymethylmethacrylate, with a large working distance of 650 μm.


Three powerful research tools from single cells into single molecules: AFM, laser tweezers, and raman spectroscopy

Wu, Y., Liu, K., Song, K., Pan, S.

By using three physical techniques (atomic force microscopy (AFM), laser tweezers, and Raman spectroscopy), many excellent works in single-cell/molecule research have been accomplished. In this review, we present a brief introduction to the principles of these three techniques, and their capabilities toward single-cell/ molecule research are highlighted. Afterward, the advances in single-cell/molecule research that have been facilitated by these three techniques are described. Following this, their complementary assets for single-cell/molecule research are analyzed, and the necessity of integrating the functions of these three techniques into one instrument is proposed.


Motion analysis of optically trapped particles and cells using 2D Fourier analysis

Martin Verner Kristensen, Peter Ahrendt, Thue Bjerring Lindballe, Otto Højager Attermann Nielsen, Anton P. Kylling,Henrik Karstoft, Alberto Imparato, Leticia Hosta-Rigau, Brigitte Stadler, Henrik Stapelfeldt, and Søren Rud Keiding
Motion analysis of optically trapped objects is demonstrated using a simple 2D Fourier transform technique. The displacements of trapped objects are determined directly from the phase shift between the Fourier transform of subsequent images. Using end- and side-view imaging, the stiffness of the trap is determined in three dimensions. The Fourier transform method is simple to implement and applicable in cases where the trapped object changes shape or where the lighting conditions change. This is illustrated by tracking a fluorescent particle and a myoblast cell, with subsequent determination of diffusion coefficients and the trapping forces.


Backward transport of nanoparticles in fluidic flow

Chong Xu, Hongxiang Lei, Yao Zhang, and Baojun Li

We demonstrate a backward transport of polystyrene (PS) particles (713-nm in diameter) in a pressure-driven fluidic flow using an optical fiber with a diameter of 710 nm. When a light of 980-nm wavelength was launched into the fiber in the opposite direction of the flow, the PS particles near the fiber were attracted onto the fiber and transported along the propagation direction of the light. The relationship between the velocity of the transported PS particles and the velocity of the flow at different input optical powers was investigated. Numerical analyses on both the optical field and the fluid field were carried out. The particle-size dependence of backward transport capability has also been investigated.

Efficient optical trapping using small arrays of plasmonic nanoblock pairs

Yoshito Tanaka and Keiji Sasaki

We report that a small two-dimensional array of gold nanoblock pairs separated by a nanometric gap significantly improves the performance of optical trapping compared to a single nanoblock pair. The array of 4 × 4 pairs suppresses the Brownian motion of a trapped 1 μm diameter particle by a factor of six compared to the single pair. In addition, the array enables particle trapping for a longer period of time. These results are essential for biological applications where intense optical irradiation is a concern.


Wednesday, January 18, 2012

Beyond Millikan: The Dynamics of Charging Events on Individual Colloidal Particles

Filip Beunis, Filip Strubbe, Kristiaan Neyts, and Dmitri Petrov

By measuring the stable charge on oil drops in air, Millikan demonstrated the discrete nature of electric charge. We extend his approach to the charge on solid-liquid interfaces, and focus on the dynamics of the discrete fluctuations. Our measurements are accurate and fast enough to observe changes of one elementary charge. Experiments over thousands of seconds yield information about the fast dynamics of electrochemical reactions, relevant for physicochemical and biological systems. As an example, we study (dis)charging processes on colloidal particles in a nonpolar liquid.


Nonlinear Proportional Plus Integral Control of Optical Traps for Exogenous Force Estimation

D. G. Cole and J. G. Pickel

This article explores nonlinear proportional plus integral (PI) feedback for controlling the position of an object held in an optical trap. In general, nonlinearities in the spatial dependence of the optical force complicate feedback control for optical traps. Nonlinear PI control has been shown to provide all of the benefits of integral control: disturbance rejection, servo tracking, and force estimation. The controller also linearizes the closed-loop system. More importantly, the nonlinear controller is shown to be equivalent to an estimator of the exogenous force. The ability of nonlinear PI control tolower the measurement SNR is evaluated and compared to the variational open-loop case. A simulation demonstrating the performance of thenonlinear PI control is presented.


Folding energy landscape of the thiamine pyrophosphate riboswitch aptamer

Peter C. Anthony, Christian F. Perez, Cuauhtémoc García-García, and Steven M. Block

Riboswitches are motifs in the untranslated regions (UTRs) of RNA transcripts that sense metabolite levels and modulate the expression of the corresponding genes for metabolite import, export, synthesis, or degradation. All riboswitches contain an aptamer: an RNA structure that, upon binding ligand, folds to expose or sequester regulatory elements in the adjacent sequence through alternative nucleotide pairing. The coupling between ligand binding and aptamer folding is central to the regulatory mechanisms of thiamine pyrophosphate (TPP) riboswitches and has not been fully characterized. Here, we show that TPP aptamer folding can be decomposed into ligand-independent and -dependent steps that correspond to the formation of secondary and tertiary structures, respectively. We reconstructed the full energy landscape for folding of the wild-type (WT) aptamer and measured perturbations of this landscape arising from mutations or ligand binding. We show that TPP binding proceeds in two steps, from a weakly to a strongly bound state. Our data imply a hierarchical folding sequence, and provide a framework for understanding molecular mechanism throughout the TPP riboswitch family.

The integration of a micropipette in a closed microfluidic chip with optical tweezers for investigations of single cells: erratum

Ahmed Alrifaiy and Kerstin Ramser

In July 2011 a new concept of a closed microfluidic system equipped with a fixed micropipette, optical tweezers and a UV-Vis spectrometer was presented [Biomed. Opt. Express 2, 2299 (2011)]. Figure 1 showed falsely oriented mirrors. To clarify the design of the setup, this erratum presents a correct schematic.


Raman microspectroscopy detects epigenetic modifications in living Jurkat leukemic cells

Mathilde Poplineau, Aurélie Trussardi-Régnier, Teddy Happillon, Jean Dufer, Michel Manfait, Philippe Bernard,Olivier Piot & Frank Antonicelli

Classical biochemical and molecular methods for discerning cells with epigenetic modifications are often biologically perturbing or even destructive. We wondered whether the noninvasive laser tweezer Raman spectroscopy technique allowed the discrimination of single living human cells undergoing epigenetic modifications. Human Jurkat leukemic cells were treated with inhibitors of histone deacetylases (trichostatin A and MS-275). Epigenetic changes were monitored through histone electrophoresis, nuclear image cytometry and laser tweezer Raman spectroscopy. Treatment of Jurkat cells with histone deacetylase inhibitors increased histone acetylation and induced chromatin organization changes. Characteristic vibrations, issued from laser tweezer Raman spectroscopy analyses, mostly assigned to DNA and proteins allowed discerning histone deacetylase inhibitor-treated cells from control with high confidence. Statistical processing of laser tweezer Raman spectroscopy data led to the definition of specific biomolecular fingerprints of each cell group. This original study shows that laser tweezer Raman spectroscopy is a label-free rapid tool to identify living cells that underwent epigenetic changes.


Friday, January 13, 2012

Complex field-stabilised nematic defect structures in Laguerre–Gaussian optical tweezers

Tine Porenta, Miha Ravnik and Slobodan Zumer

Novel optical field-stabilised defect structures are presented in nematic liquid crystals as produced by Laguerre–Gaussian optical beams. Our modelling study is based on the phenomenological free energy approach. To mimic generic experiments, Laguerre–Gaussian optical beams of various helical indices are applied to a nematic cell. The symmetry of these field-stabilised nematic structures is directly determined by the beam indices. We show that the structures can also be tuned by the beam intensity and the strength of the nematic elastic constant. Finally, local intensity-induced control of the nematic order via absorption of the beam is demonstrated as a complementary mechanism for producing and tuning the nematic field-stabilised structures.


Radiation forces of a dielectric medium plate induced by a Gaussian beam

Donghua Li, Jixiong Pu, Xiqing Wang

Based on the angular spectrum method, we investigate the radiation forces on a finite-size dielectric medium plate induced by a Gaussian beam. The formulas for the radiation force along longitudinal and transverse direction are derived, and numerically calculation is performed. It is shown that for the finite-size dielectric medium plate, the radiation forces exerted by the Gaussian beam is dependent upon the angle and position of a single ray striking on the plate and the intensity of light. Our numerical results indicate that if we choose the appropriate parameters there will be enough transverse forces to overcome the gravity force, making the plate move upwards.

Optically Trapped Gold Nanoparticle Enables Listening at the Microscale

Alexander Ohlinger, Andras Deak, Andrey A. Lutich, and Jochen Feldmann

We explore a new application of optical tweezers for ultrasensitive detection of sound waves in liquid media. Position tracking of a single gold nanoparticle confined in a three-dimensional optical trap is used to readout acoustic vibrations at a sound power level down to -60  dB, causing a ∼90  μeV increase in kinetic energy of the nanoparticle. The unprecedented sensitivity of such a nanoear is achieved by processing the nanoparticle’s motion in the frequency domain. The concept developed here will enable us to access the interior of biological microorganisms and micromechanical machines not accessible by other microscopy types.


Friday, January 6, 2012

Application of plasmonic bowtie nanoantenna arrays for optical trapping, stacking, and sorting

Brian James Roxworthy, Kaspar D Ko, Anil Kumar, Kin Hung Fung, Edmond K.C. Chow, Gang Logan Liu, Nicholas Fang, and Kimani Christopher Toussaint

We present the use of Au bowtie nanoantenna arrays (BNAs) for highly efficient, multipurpose particle manipulation with unprecedented low input power and low-numerical aperture (NA) focusing. Optical trapping efficiencies measured are up to 20x the efficiencies of conventional high-NA optical traps and are among the highest reported to date. Empirically obtained plasmonic optical trapping "phase diagrams" are introduced to detail the trapping response of the BNAs as a function of input power, wavelength, polarization, particle diameter and BNA array spacing (number density). Using these diagrams, parameters are chosen, employing strictly the degrees-of-freedom of the input light, to engineer specific trapping tasks including: (1) dexterous, single-particle trapping and manipulation, (2) trapping and manipulation of 2 and 3-dimensional particle clusters, and (3) particle sorting. The use of low input power densities (power and NA) suggests that this bowtie nanoantenna trapping system will be particularly attractive for lab-on-a-chip technology or biological applications aimed at reducing specimen photodamage.


Particle separation in fluidic flow by optical fiber

Hongxiang Lei, Yao Zhang, and Baojun Li

We report a separation of two different size particles in fluidic flow by an optical fiber. With a light of 1.55 μm launched into the fiber, particles in stationary water were massively trapped and assembled around the fiber by a negative photophoretic force. By introducing a fluidic flow, the assembled particles were separated into two different downstream positions according to their sizes by the negative photophoretic force and the dragging force acted on the particles. The intensity distribution of light leaked from the fiber and the asymmetry factor of energy distribution have been analysed as crucial factors in this separation. Poly(methyl methacrylate) particles (5-/10-μm diameter), SiO2 particles (2.08-/5.65-μm diameter), and SiO2particles (2.08-μm diameter) mixed with yeast cells were used to demonstrate the effectiveness of the separation. The separation mechanism has also been numerical simulated and theoretical interpreted.


Directed assembly of optically bound matter

Michael D. Summers, Richard D. Dear, Jonathan M. Taylor, and Grant A.D. Ritchie

We present a study of optically bound matter formation in a counter-propagating evanescent field, exploiting total internal reflection on a prism surface. Small ensembles of silica microspheres are assembled in a controlled manner using optical tweezers. The structures and dynamics of the resulting optically bound chains are interpreted using a simulation implementing generalized Lorentz-Mie theory. In particular, we observe enhancement of the scattering force along the propagation direction of the optically bound colloidal chains leading to a microscopic analogue of a driven pendulum which, at least superficially, resembles Newton’s cradle.


Thursday, January 5, 2012

Published Papers Statistics for 2011

Here is the long awaited results for the year of 2011 for published papers on optical tweezers, micromanipulation and trapping, in this blog (a total of 455 posts for 2011).

The top Journals (more than 2% hits) are:
Optics Express                             11%
Journal of Optics                            6%
Optics Letters                                5%
Nano Letters                                  4%
Lab on a Chip                                3%
Applied Physics Letters                  3%
Soft Matter                                    3%
Biophysical Journal                         2%
Physical Review Letters                  2%
Applied Optics                               2%
Nature Photonic                              2%
Proc. of the Nat. Acad. of Sci.         2%
Review of Scientific Instruments       2%

Below is also a tag cloud (from the words found in the title and abstracts for 2011):