Tuesday, August 30, 2016

Trapping metallic particles under resonant wavelength with 4π tight focusing of radially polarized beam

Wenjing Cui, Feng Song, Feifei Song, Dandan Ju, and Shujing Liu

Here we propose a new method for trapping the resonant metallic particles with the 4π tight focusing (high numerical-aperture (NA)) system, which is illuminated by radial polarization light. Numerical simulations have indicated the maximum total optical force is 16.1pN while with nearly zero scattering force under axis trapping, which keeps the gradient force predominant. Furthermore, the distribution of total force is centrosymmetric and odd. We also gain stable 3D trap with an equilibrium point along z axis and r axis as in normal optical tweezers. What’s more, we obtain the nearly pure longitudinal field. The maximum transverse intensity is only 2.3 × 10−3 and the transverse spot size reaches 0.36λ, which is below Abbe’s diffraction limit.


Symmetry breaking as a general design principle of oscillation-based methods for fixation and manipulation of nano-objects

V.L. Popov, R. Wetter

We present various examples of man-made and biological nanoscale actuators based on oscillations. In most cases it is the interplay of oscillation and friction which produces the driving effect. The basic idea of all such actuators is the same: an asymmetry in the oscillation causes a net directed motion. Introducing asymmetry in different components of the system (internal force, substrate, form of periodic actuation, etc.) leads to different types of drives. This symmetry concept is used to categorize and discuss the basic principles of nanoscale actuation and for formulating general principles of design guidelines that can be used to develop new concepts of nanoscale actuators.
Taking into account the general principle of symmetry breaking, a new concept for a high precision actuator suggested recently by the authors is discussed and its practical realization is outlined. This novel drive type consists of a sphere that is rolling back- and forth while being pushed on a movable substrate. The sphere acts as the drive and the substrate acts as the runner. A varying normal force leads to varying indentation depth and contact area during rolling. Together with the inertia of the runner, this asymmetry enables accurate control of the runner displacement. In theory, the actuator works with less wear because slip is completely omitted.


Grating-flanked plasmonic coaxial apertures for efficient fiber optical tweezers

Amr A. E. Saleh, Sassan Sheikhoelislami, Steven Gastelum, and Jennifer A. Dionne

Subwavelength plasmonic apertures have been foundational for direct optical manipulation of nanoscale specimens including sub-100 nm polymeric beads, metallic nanoparticles and proteins. While most plasmonic traps result in two-dimensional localization, three-dimensional manipulation has been demonstrated by integrating a plasmonic aperture on an optical fiber tip. However, such 3D traps are usually inefficient since the optical mode of the fiber and the subwavelength aperture only weakly couple. In this paper we design more efficient optical-fiber-based plasmonic tweezers combining a coaxial plasmonic aperture with a plasmonic grating coupler at the fiber tip facet. Using full-field finite difference time domain analysis, we optimize the grating design for both gold and silver fiber-based coaxial tweezers such that the optical transmission through the apertures is maximized. With the optimized grating, we show that the maximum transmission efficiency increases from 2.5% to 19.6% and from 1.48% to 16.7% for the gold and silver structures respectively. To evaluate their performance as optical tweezers, we calculate the optical forces and the corresponding trapping potential on dielectric particles interacting with the apertures. We demonstrate that the enahncement in the transmission translates into an equivalent increase in the optical forces. Consequently, the optical power required to achieve stable optical trapping is significantly reduced allowing for efficient localization and 3D manipulation of sub-30 nm dielectric particles.


Thursday, August 25, 2016

Transfer of orbital angular momentum from asymmetric Laguerre-Gaussian beams to dielectric microparticles

A. A. Kovalev, V. V. Kotlyar, A. P. Porfirev

Transfer of the orbital angular momentum to dielectric microparticles optically trapped using recently discovered asymmetric Laguerre-Gaussian (LG) laser beams is demonstrated. We experimentally show that at a fixed topological charge, increasing beam asymmetry parameter leads to increasing velocity of the polystyrene beads moving along a crescent-shaped path.


Probing the Kinetic Anabolism of Poly-Beta-Hydroxybutyrate in Cupriavidus necator H16 Using Single-Cell Raman Spectroscopy

Zhanhua Tao, Lixin Peng, Pengfei Zhang, Yong-Qing Li and Guiwen Wang

Poly-beta-hydroxybutyrate (PHB) can be formed in large amounts in Cupriavidus necator and is important for the industrial production of biodegradable plastics. In this investigation, laser tweezers Raman spectroscopy (LTRS) was used to characterize dynamic changes in PHB content—as well as in the contents of other common biomolecule—in C. necator during batch growth at both the population and single-cell levels. PHB accumulation began in the early stages of bacterial growth, and the maximum PHB production rate occurred in the early and middle exponential phases. The active biosynthesis of DNA, RNA, and proteins occurred in the lag and early exponential phases, whereas the levels of these molecules decreased continuously during the remaining fermentation process until the minimum values were reached. The PHB content inside single cells was relatively homogenous in the middle stage of fermentation; during the late growth stage, the variation in PHB levels between cells increased. In addition, bacterial cells in various growth phases could be clearly discriminated when principle component analysis was performed on the spectral data. These results suggest that LTRS is a valuable single-cell analysis tool that can provide more comprehensive information about the physiological state of a growing microbial population.


Laser Refrigeration of Ytterbium-Doped Sodium–Yttrium–Fluoride Nanowires

Xuezhe Zhou, Bennett E. Smith, Paden B. Roder, Peter J. Pauzauskie

Sodium yttrium fluoride (β-NaYF4) nanowires (NWs) with a hexagonal crystal structure are synthesized using a low-cost hydrothermal process and are shown to undergo laser refrigeration based on an upconversion process leading to anti-Stokes (blueshifted) photoluminescence. Single-beam laser trapping combined with forward light scattering is used to investigate cryophotonic laser refrigeration of individual NWs through analysis of their local Brownian dynamics.


Friday, August 19, 2016

Binding and Fusion of Extracellular Vesicles to the Plasma Membrane of Their Cell Targets

Ilaria Prada and Jacopo Meldolesi

Exosomes and ectosomes, extracellular vesicles of two types generated by all cells at multivesicular bodies and the plasma membrane, respectively, play critical roles in physiology and pathology. A key mechanism of their function, analogous for both types of vesicles, is the fusion of their membrane to the plasma membrane of specific target cells, followed by discharge to the cytoplasm of their luminal cargo containing proteins, RNAs, and DNA. Here we summarize the present knowledge about the interactions, binding and fusions of vesicles with the cell plasma membrane. The sequence initiates with dynamic interactions, during which vesicles roll over the plasma membrane, followed by the binding of specific membrane proteins to their cell receptors. Membrane binding is then converted rapidly into fusion by mechanisms analogous to those of retroviruses. Specifically, proteins of the extracellular vesicle membranes are structurally rearranged, and their hydrophobic sequences insert into the target cell plasma membrane which undergoes lipid reorganization, protein restructuring and membrane dimpling. Single fusions are not the only process of vesicle/cell interactions. Upon intracellular reassembly of their luminal cargoes, vesicles can be regenerated, released and fused horizontally to other target cells. Fusions of extracellular vesicles are relevant also for specific therapy processes, now intensely investigated.


Optical pulling force on a magneto-dielectric Rayleigh sphere in Bessel tractor polarized beams

F.G. Mitri, R.X. Li, R. Yang, L.X. Guo, C.Y. Ding

The optical radiation force induced by Bessel (vortex) beams on a magneto-dielectric subwavelength sphere is investigated with particular emphasis on the beam polarization and orderl(or topological charge). The analysis is focused on identifying the regions and some of the conditions to achieve retrograde motion of the sphere centered on the axis of wave propagation of the incident beam, or shifted off-axially. Exact non-paraxial analytical solutions are established, and computations for linear, circular, radial, azimuthal and mixed polarizations of the individual plane wave components forming the Bessel (vortex) beams by means of the angular spectrum decomposition method (ASDM) illustrate the theory with particular emphasis on the tractor (i.e. reversal) behavior of the force. This effect results in the pulling of the magneto-dielectric sphere against the forward linear momentum density flux associated with the incoming waves. Should some conditions related to the choice of the beam parameters as well as the permittivity and permeability of the sphere be met, the optical force vanishes and reverses sign. Moreover, the beam polarization is shown to affect differently the axial negative pulling force for either the zeroth- or the first-order Bessel beam. When the sphere is centered on the beam′s axis, the axial force component is always negative for the zeroth-order Bessel beam except for the radial and azimuthal polarization configurations. Nonetheless, for the first-order Bessel beam, the axial force is negative for the radial polarization case only. Additional tractor beam effects arise when the sphere departs from the center of the beam. It is also demonstrated that the tractor beam effect arises from the force component originating from the cross-interaction between the electric and magnetic dipoles. Potential applications are in particle manipulation, optical levitation, tractor beam tweezers, and other emergent technologies using polarized Bessel beams on a small (Rayleigh) magneto-dielectric particle.


Rewritable three-dimensional holographic data storage via optical forces

Ali K. Yetisen, Yunuen Montelongo and Haider Butt

The development of nanostructures that can be reversibly arranged and assembled into 3D patterns may enable optical tunability. However, current dynamic recording materials such as photorefractive polymers cannot be used to store information permanently while also retaining configurability. Here, we describe the synthesis and optimization of a silver nanoparticle doped poly(2-hydroxyethyl methacrylate-co-methacrylic acid) recording medium for reversibly recording 3D holograms. We theoretically and experimentally demonstrate organizing nanoparticles into 3D assemblies in the recording medium using optical forces produced by the gradients of standing waves. The nanoparticles in the recording medium are organized by multiple nanosecond laser pulses to produce reconfigurable slanted multilayer structures. We demonstrate the capability of producing rewritable optical elements such as multilayer Bragg diffraction gratings, 1D photonic crystals, and 3D multiplexed optical gratings. We also show that 3D virtual holograms can be reversibly recorded. This recording strategy may have applications in reconfigurable optical elements, data storage devices, and dynamic holographic displays.


Single cell ionization by a laser trap: a preliminary study in measuring radiation dose and charge in BT20 breast carcinoma cells

Michele Kelley, Ying Gao, and Daniel Erenso

In this work, a preliminary study in the application of a laser trap for ionization of living carcinoma cells is presented. The study was conducted using BT20 breast carcinoma cells cultured and harvested in our laboratory. Each cell, for a total of 50 cells, was trapped and ionized by a high intensity infrared laser at 1064 nm. The threshold radiation dose and the resultant charge from the ionization for each cell were determined. With the laser trap serving as a radiation source, the cell underwent dielectric breakdown of the membrane. When this process occurs, the cell becomes highly charged and its dielectric susceptibility changes. The charge creates an increasing electrostatic force while the changing dielectric susceptibility diminishes the strength of the trapping force. Consequently, at some instant of time the cell gets ejected from the trap. The time inside the trap while the cell is being ionized, the intensity of the radiation, and the post ionization trajectory of the cell were used to determine the threshold radiation dose and the charge for each cell. The measurement of the charge vs ionization radiation dose at single cell level could be useful in the accuracy of radiotherapy as the individual charges can collectively create a strong enough electrical interaction to cause dielectric breakdown in other cells in a tumor.


Wednesday, August 17, 2016

The Contribution of the C-Terminal Tails of Microtubules in Altering the Force Production Specifications of Multiple Kinesin-1

Mitra Shojania Feizabadi

The extent to which beta tubulin isotypes contribute to the function of microtubules and the microtubule-driven transport of molecular motors is poorly understood. The major differences in these isotypes are associated with the structure of their C-terminal tails. Recent studies have revealed a few aspects of the C-terminal tails’ regulatory role on the activities of some of the motor proteins on a single-molecule level. However, little attention is given to the degree to which the function of a team of motor proteins can be altered by the microtubule’s tail. In a set of parallel experiments, we investigated this open question by studying the force production of several kinesin-1 (kinesin) molecular motors along two groups of microtubules: regular ones and those microtubules whose C-terminals are cleaved by subtilisin digestion. The results indicate that the difference between the average of the force production of motors along two types of microtubules is statistically significant. The underlying mechanism of such production is substantially different as well. As compared to untreated microtubules, the magnitude of the binding time of several kinesin-1 is almost three times greater along subtilisin-treated microtubules. Also, the velocity of the group of kinesin molecules shows a higher sensitivity to external loads and reduces significantly under higher loads along subtilisin-treated microtubules. Together, this work shows the capacity of the tails in fine-tuning the force production characteristics of several kinesin molecules.


Comparison of Methods for Predicting the Compositional Dependence of the Density and Refractive Index of Organic–Aqueous Aerosols

Chen Cai, Rachael E. H. Miles, Michael I. Cotterell, Aleksandra Marsh, Grazia Rovelli, Andrew M. J. Rickards, Yun-hong Zhang, and Jonathan P. Reid

Representing the physicochemical properties of aerosol particles of complex composition is of crucial importance for understanding and predicting aerosol thermodynamic, kinetic, and optical properties and processes and for interpreting and comparing analysis methods. Here, we consider the representations of the density and refractive index of aqueous–organic aerosol with a particular focus on the dependence of these properties on relative humidity and water content, including an examination of the properties of solution aerosol droplets existing at supersaturated solute concentrations. Using bulk phase measurements of density and refractive index for typical organic aerosol components, we provide robust approaches for the estimation of these properties for aerosol at any intermediate composition between pure water and pure solute. Approximately 70 compounds are considered, including mono-, di- and tricarboxylic acids, alcohols, diols, nitriles, sulfoxides, amides, ethers, sugars, amino acids, aminium sulfates, and polyols. We conclude that the molar refraction mixing rule should be used to predict the refractive index of the solution using a density treatment that assumes ideal mixing or, preferably, a polynomial dependence on the square root of the mass fraction of solute, depending on the solubility limit of the organic component. Although the uncertainties in the density and refractive index predictions depend on the range of subsaturated compositional data available for each compound, typical errors for estimating the solution density and refractive index are less than ±0.1% and ±0.05%, respectively. Owing to the direct connection between molar refraction and the molecular polarizability, along with the availability of group contribution models for predicting molecular polarizability for organic species, our rigorous testing of the molar refraction mixing rule provides a route to predicting refractive indices for aqueous solutions containing organic molecules of arbitrary structure.


Label-free analysis of mononuclear human blood cells in microfluidic flow by coherent imaging tools

David Dannhauser, Domenico Rossi, Pasquale Memmolo, Filippo Causa, Andrea Finizio, Pietro Ferraro, Paolo A. Netti

The investigation of the physical properties of peripheral blood mononuclear cells (PBMC) is of great relevance, as they play a key role in regulating human body health. Here we report the possibility to characterize human PBMC in their physiological conditions in a microfluidic-based measurement system. A viscoelastic polymer solution is adopted for 3D alignment of individual cells inflow. An optical signature (OS) acquisition of each flowing cell is performed using a wide angle light scattering apparatus. Besides, a quantitative phase imaging (QPI) holographic system is employed with the aim (i) to check the position in flow of individual cells using a holographic 3D cell tracking method; and (ii) to estimate their 3D morphometric features, such as their refractive index (RI). Results obtained by combining OS and QPI have been compared with literature values, showing good agreement. The results confirm the possibility to obtain sub-micrometric details of physical cell properties in microfluidic flow, avoiding chemical staining or fluorescent labelling.


Mechanical Properties of the Tumor Stromal Microenvironment Probed In Vitro and Ex Vivo by In Situ-Calibrated Optical Trap-Based Active Microrheology

Jack R. Staunton, Wilfred Vieira, King Leung Fung, Ross Lake, Alexus Devine, Kandice Tanner

One of the hallmarks of the malignant transformation of epithelial tissue is the modulation of stromal components of the microenvironment. In particular, aberrant extracellular matrix (ECM) remodeling and stiffening enhances tumor growth and survival and promotes metastasis. Type I collagen is one of the major ECM components. It serves as a scaffold protein in the stroma contributing to the tissue’s mechanical properties, imparting tensile strength and rigidity to tissues such as those of the skin, tendons, and lungs. Here we investigate the effects of intrinsic spatial heterogeneities due to fibrillar architecture, pore size and ligand density on the microscale and bulk mechanical properties of the ECM. Type I collagen hydrogels with topologies tuned by polymerization temperature and concentration to mimic physico-chemical properties of a normal tissue and tumor microenvironment were measured by in situ-calibrated Active Microrheology by Optical Trapping revealing significantly different microscale complex shear moduli at Hz-kHz frequencies and two orders of magnitude of strain amplitude that we compared to data from bulk rheology measurements. Access to higher frequencies enabled observation of transitions from elastic to viscous behavior that occur at ~200–2750 Hz, which largely was dependent on tissue architecture well outside the dynamic range of instrument acquisition possible with SAOS bulk rheology. We determined that mouse melanoma tumors and human breast tumors displayed complex moduli ~5–1000 Pa, increasing with frequency and displaying a nonlinear stress–strain response. Thus, we show the feasibility of a mechanical biopsy in efforts to provide a diagnostic tool to aid in the design of therapeutics complementary to those based on standard histopathology.


Highly tunable plasmonic nanoring arrays for nanoparticle manipulation and detection

M Sergides, V G Truong and S Nic Chormaic

The advancement of trapping and detection of nano-objects at very low laser powers in the near-infra-red region (NIR) is crucial for many applications. Singular visible-light nano-optics based on abrupt phase changes have recently demonstrated a significant improvement in molecule detection. Here, we propose and demonstrate tunable plasmonic nanodevices, which can improve both the trapping field enhancement and detection of nano-objects using singular phase drops in the NIR range. The plasmonic nanostructures, which consist of gaps with dimensions 50 nm × 50 nm connecting nanorings in arrays is discussed. These gaps act as individual detection and trapping sites. The tunability of the system is evident from extinction and reflection spectra while increasing the aperture size in the arrays. Additionally, in the region where the plasmonic nano-array exhibits topologically-protected, near-zero reflection behaviour, the phase displays a rapid change. Our experimental data predict that, using this abrupt phase changes, one can improve the detection sensitivity by 10 times compared to the extinction spectra method. We finally report experimental evidence of 100 nm polystyrene beads trapping using low incident power on these devices. The overall design demonstrates strong capability as an optical, label-free, non-destructive tool for single molecule manipulation where low trapping intensity, minimal photo bleaching and high sensitivity is required.


Tuesday, August 16, 2016

Confocal Raman Microscopy of Hybrid Supported Phospholipid Bilayers within Individual C18-Functionalized Chromatographic Particles

Jay Preston Kitt and Joel M. Harris

Measuring lipid-membrane partitioning of small molecules is critical to predicting bioavailability and investigating molecule-membrane interactions. A stable model membrane for such studies has been developed through assembly of a phospholipid monolayer on n-alkane-modified surfaces. These hybrid-bilayers have recently been generated within n-alkyl-chain (C18) modified porous silica and used in chromatographic retention studies of small molecules. Despite their successful application, determining the structure of hybrid-bilayers within chromatographic silica is challenging, because they reside at buried interfaces within the porous structure. In this work, we employ confocal Raman microscopy to investigate the formation and temperature-dependent structure of hybrid-phospholipid bilayers in C18-modified, porous-silica chromatographic particles. Porous silica provides sufficient surface area within a confocal probe volume centered in an individual particle to readily measure, with Raman microscopy, the formation of an ordered hybrid-bilayer of 1,2-dimyristoyl-sn-glycero-3-phospho¬choline (DMPC) with the surface C18 chains. The DMPC surface density was quantified from the relative Raman scattering intensities of C18 and phospholipid acyl chains and found to be ~40% of a DMPC vesicle membrane. By monitoring Raman spectra acquired versus temperature, the bilayer main phase transition was observed to be broadened and shifted to higher temperature compared to a DMPC vesicle, in agreement with differential scanning calorimetry (DSC) results. Raman scattering of deuterated phospholipid was resolved from protonated C18-chain scattering, showing that the lipid-acyl and C18 chains melt simultaneously in a single phase transition. The surface-density of lipid in the hybrid bilayer, the ordering of both C18 and lipid acyl chains upon bilayer formation, and decoupling of C18 methylene C H vibrations by deuterated lipid acyl chains all suggest an interdigitated acyl-chain structure. The simultaneous melting of both layers is also consistent with an interdigitated structure, where immobility of surface-grafted C18-chains decreases the cooperativity and increases the melting temperature compared to a vesicle bilayer.


Load-induced enhancement of Dynein force production by LIS1–NudE in vivo and in vitro

Babu J. N. Reddy, Michelle Mattson, Caitlin L. Wynne, Omid Vadpey, Abdo Durra, Dail Chapman, Richard B. Vallee & Steven P. Gross

Most sub-cellular cargos are transported along microtubules by kinesin and dynein molecular motors, but how transport is regulated is not well understood. It is unknown whether local control is possible, for example, by changes in specific cargo-associated motor behaviour to react to impediments. Here we discover that microtubule-associated lipid droplets (LDs) in COS1 cells respond to an optical trap with a remarkable enhancement in sustained force production. This effect is observed only for microtubule minus-end-moving LDs. It is specifically blocked by RNAi for the cytoplasmic dynein regulators LIS1 and NudE/L (Nde1/Ndel1), but not for the dynactin p150Glued subunit. It can be completely replicated using cell-free preparations of purified LDs, where duration of LD force production is more than doubled. These results identify a novel, intrinsic, cargo-associated mechanism for dynein-mediated force adaptation, which should markedly improve the ability of motor-driven cargoes to overcome subcellular obstacles.


Superposition of nonparaxial vectorial complex-source spherically focused beams: Axial Poynting singularity and reverse propagation

F. G. Mitri

In this work, counterintuitive effects such as the generation of an axial (i.e., long the direction of wave motion) zero-energy flux density (i.e., axial Poynting singularity) and reverse (i.e., negative) propagation of nonparaxial quasi-Gaussian electromagnetic (EM) beams are examined. Generalized analytical expressions for the EM field's components of a coherent superposition of two high-order quasi-Gaussian vortex beams of opposite handedness and different amplitudes are derived based on the complex-source-point method, stemming from Maxwell's vector equations and the Lorenz gauge condition. The general solutions exhibiting unusual effects satisfy the Helmholtz and Maxwell's equations. The EM beam components are characterized by nonzero integer degree and order (n,m), respectively, an arbitrary waist w0, a diffraction convergence length known as the Rayleigh range zR, and a weighting (real) factor 0≤α≤1 that describes the transition of the beam from a purely vortex (α=0) to a nonvortex (α=1) type. An attractive feature for this superposition is the description of strongly focused (or strongly divergent) wave fields. Computations of the EM power density as well as the linear and angular momentum density fluxes illustrate the analysis with particular emphasis on the polarization states of the vector potentials forming the beams and the weight of the coherent beam superposition causing the transition from the vortex to the nonvortex type. Should some conditions determined by the polarization state of the vector potentials and the beam parameters be met, an axial zero-energy flux density is predicted in addition to a negative retrograde propagation effect. Moreover, rotation reversal of the angular momentum flux density with respect to the beam handedness is anticipated, suggesting the possible generation of negative (left-handed) torques. The results are particularly useful in applications involving the design of strongly focused optical laser tweezers, tractor beams, optical spanners, arbitrary scattering, radiation force, angular momentum, and torque in particle manipulation, and other related topics.


Guiding and nonlinear coupling of light in plasmonic nanosuspensions

Trevor S. Kelly, Yu-Xuan Ren, Akbar Samadi, Anna Bezryadina, Demetrios Christodoulides, and Zhigang Chen

We demonstrate two different types of coupled beam propagation dynamics in colloidal gold nanosuspensions. In the first case, an infrared (IR) probe beam (1064 nm) is guided by a low-power visible beam (532 nm) in a gold nanosphere or in nanorod suspensions due to the formation of a plasmonic resonant soliton. Although the IR beam does not experience nonlinear self-action effects, even at high power levels, needle-like deep penetration of both beams through otherwise highly dissipative suspensions is realized. In the second case, a master/slave-type nonlinear coupling is observed in gold nanoshell suspensions, in which the nanoparticles have opposite polarizabilities at the visible and IR wavelengths. In this latter regime, both beams experience a self-focusing nonlinearity that can be fine-tuned.


Optical pulling using evanescent mode in sub-wavelength channels

Tongtong Zhu, M. R. C. Mahdy, Yongyin Cao, Haiyi Lv, Fangkui Sun, Zehui Jiang, and Weiqiang Ding

Optical evanescent wave in total internal reflection has been widely used in efficient optical manipulation, where the object is trapped by the intrinsic intensity gradient of the evanescent wave while transported by the scattering force along the orthogonal direction. Here, we propose a distinct optical manipulation scheme using the attenuated modes in subwavelength optical channels, where both the trapping and transportation forces are along the channel direction. We create such a mode in a sub-wavelength photonic crystal waveguide and quantitatively obtain the net pushing and pulling forces, which can overcome the Brownian motion within a critical length. Due to the presence of the physical channel, subwavelength trapping on the transverse direction is natural, and manipulation along bend trajectories is also possible without the assistance of the self-acceleration beams provided a channel is adopted. This optical manipulation method can be extended to any other channels that support attenuation mode, and may provide an alternate way for flexible optical manipulation.


Thursday, August 11, 2016

A Plasmonic Optophoresis for Manipulating, In-situ Position Monitoring, Sensing, and 3D trapping of Micro/Nanoparticles

Mostafa Ghorbanzadeh, Mohammad Moravvej-Farshi; Sara Darbari

An optophoresis system based on surface plasmons in a simple microfluidic environment is proposed, which introduces new functionalities, including three dimensional trapping of par-ticles, bidirectional manipulation, controllable sorting, and in-situ sensing. Its operating principle is based on plasmonic field, which is excited by the Kretschmann configuration on a gold stripe. In this system, trapping, manipulating, and sorting mech-anisms are based on the balance of two oppositely exerted scat-tering forces induced by two counter-propagating surface plas-mons. Moreover, for detecting particle’s intrinsic properties and in-situ position monitoring, we take advantage of the particle’s lens-like behavior and stripe’s edge effects. The proposed low power functional system that can be simply and inexpensively integrated into lab-on-a-chip devices is a promising candidate for biological applications and developing integrated optical manipu-lation chips.

Single Particle and PET-based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy

Jesper Tranekjær Jørgensen, Kamilla Norregaard, Pengfei Tian, Poul Martin Bendix, Andreas Kjaer & Lene B. Oddershede

Plasmonic nanoparticle-based photothermal cancer therapy is a promising new tool to inflict localized and irreversible damage to tumor tissue by hyperthermia, without harming surrounding healthy tissue. We developed a single particle and positron emission tomography (PET)-based platform to quantitatively correlate the heat generation of plasmonic nanoparticles with their potential as cancer killing agents. In vitro, the heat generation and absorption cross-section of single irradiated nanoparticles were quantified using a temperature sensitive lipid-based assay and compared to their theoretically predicted photo-absorption. In vivo, the heat generation of irradiated nanoparticles was evaluated in human tumor xenografts in mice using 2-deoxy-2-[F-18]fluoro-D-glucose (18F-FDG) PET imaging. To validate the use of this platform, we quantified the photothermal efficiency of near infrared resonant silica-gold nanoshells (AuNSs) and benchmarked this against the heating of colloidal spherical, solid gold nanoparticles (AuNPs). As expected, both in vitro and in vivo the heat generation of the resonant AuNSs performed superior compared to the non-resonant AuNPs. Furthermore, the results showed that PET imaging could be reliably used to monitor early treatment response of photothermal treatment. This multidisciplinary approach provides a much needed platform to benchmark the emerging plethora of novel plasmonic nanoparticles for their potential for photothermal cancer therapy.


Structured light: Optomechanical tomography

Etienne Brasselet

The understanding that light carries momentum along its direction of propagation dates back to Kepler's astronomical observations of comet tails facing away from the Sun as the mechanical consequence of radiation pressure. Still, the collinearity between the direction that light propagates in and the force it exerts on matter is not a given. An example is the case of a photon impinging on a perfect mirror. The ensuing optical force is always directed perpendicular to the surface of the mirror. In other words, the optical force has in general both longitudinal and transverse contributions with respect to the propagation direction of the incident light. A more intriguing situation occurs when the optical momentum is no longer directed along the propagation direction of light. For instance, this may happen in evanescent waves. These are formed by so-called total internal reflection off an interface between two distinct dielectric transparent media when light propagates from a dense to a rare refractive medium. As their name suggests, evanescent waves have an intensity that vanishes exponentially with the distance from the interface while they propagate along the interface in a direction parallel to the incidence plane.


Precise control and measurement of solid-liquid interfacial temperature and viscosity with dual-beam femtosecond optical tweezers in condensed phase

Dipankar Mondal, Paresh Mathur and Debabrata Goswami

We present a novel method of microrheology based on femtosecond optical tweezers, which in turn enables us to directly measure and control in situ temperature at microscale volumes at solid-liquid interface. A noninvasive pulsed 780 nm trapped bead spontaneously responds to its changes in environment induced by a co-propagating 1560 nm pulsed laser due to mutual energy transfer between solvent molecules and trapped bead. Strong absorption of the hydroxyl group by the 1560 nm laser creates local heating in individual and binary mixtures of water and alcohols. “Hot Brownian motion” of the trapped polystyrene bead gets reflected in the corner frequency deduced from power spectrum. Changes in corner frequency values enable us to calculate the viscosity as well as temperature at the solid-liquid interface. We show that these experimental results can also be theoretically ratified.


Experimental setup for studying dynamics of the calcium interaction in cells

E. Yu. Loktionov, M. G. Mikhaylova, D. S. Sitnikov

A calcium cell signaling system is one of the first, which were formed in the course of evolution of systems. The understanding of calcium binding–uncaging dynamics is crucial in studies of corresponding intracellular processes. By now, a great number of calcium-dependent processes have been investigated. However, works that fully consider these processes are absent. This is specified in many respects by the instrumental abilities. In this work, requirements for the experimental setup intended for comprehensive studies of calcium interaction dynamics are briefly formulated, its block diagram is described, and the results of test experiments are presented.


Wednesday, August 10, 2016

Theoretical and experimental studies on optical trapping using radially polarized beams

Zhehai Zhou, Yuling Zhang, Lianqing Zhu

Optical trapping using radially polarized beams is studied theoretically and experimentally. First the geometric ray model is introduced to calculate the trapping efficiencies, and simulation results using three kinds of pupil apodization functions are presented to disclose the influences of pupil apodization functions of incident beams on trapping efficiencies, which indicates the better trapping performances can be achieved by modulating the polarization and amplitude distributions of incident beams using designated pupil filters. Furthermore, the optical tweezers using radially polarized beams are built up based on an inverted microscope and a spatial light modulator (SLM), where better trapping efficiencies can be achieved. Yeast cells about 10μm in diameter are trapped and manipulated using the optical tweezers and the cells can be trapped stably and shifted along the tracks of the focusing spot, which can be programmed by a computer. In addition, the values of trap stiffness for different pupil apodization functions are measured at different laser powers based on the Boltzmann statistics method, which indicates the AL-types pupil apodization function is a better choice for general trapping and manipulation of cells.


Scattering detection of a solenoidal Poynting vector field

Shima Fardad, Alessandro Salandrino, Akbar Samadi, Matthias Heinrich, Zhigang Chen, and Demetrios N. Christodoulides

The Poynting vector 𝑆 plays a central role in electrodynamics as it is directly related to the power and the momentum carried by an electromagnetic wave. In the presence of multiple electromagnetic waves with different polarizations and propagation directions, the Poynting vector may exhibit solenoidal components which are not associated to any power flow. Here, we demonstrate theoretically and experimentally that the presence of such solenoidal components has physical consequences, and it is not a mere artifact of the gauge invariance of 𝑆. In particular, we identify a simple field configuration displaying solenoidal components of 𝑆 and theoretically show that a judiciously designed scatterer can act as a “Poynting vector detector” which when immersed in such field distribution would experience a transverse optical force orthogonal to the incidence plane. We experimentally validate our theoretical predictions by observing a pronounced asymmetry in the scattering pattern of a spherical nanoparticle.


Enantioselective Amplification on Circularly Polarized laser-Induced Chiral Nucleation from a NaClO3 Solution Containing Ag Nanoparticles

Hiromasa Niinomi, Teruki Sugiyama, Miho Tagawa, Kenta Murayama, Shunta Harada and Toru Ujihara

We demonstrate that statistically-significant chiral bias in NaClO3 chiral crystallization can be provoked by inducing nucleation via the optical trapping of Ag nano-aggregates using continuous wave visible circularly polarized laser (λ= 532 nm). The laser was focused at the interface between air and an unsaturated NaClO3 aqueous solution containing Ag nanoparticles. The “dominant” enantiomorph was switchable by changing the handedness of the incident circularly polarized laser, indicating the chiral bias is enantioselective. Moreover, it has been found that the resulting crystal enantiomeric excess (CEE) reached approximately 25 %. The CEE is much higher than the typical enantiomeric excess (EE) in the asymmetric photosynthesis of organic compounds ranging from 0.5 to 2%. The efficient induction of the nucleation and the large chiral bias implies the contribution of localized surface plasmon resonance of the Ag nanoaggregates on chiral nucleation. Our method has a potential to offer the benefit for studies on the spatiotemporal nucleation control, optical resolution of chiral compounds and biohomochirality.

Nonlinear force dependence on optically bound micro-particle arrays in the evanescent fields of fundamental and higher order microfibre modes

Aili Maimaiti, Daniela Holzmann, Viet Giang Truong, Helmut Ritsch & Síle Nic Chormaic

Particles trapped in the evanescent field of an ultrathin optical fibre interact over very long distances via multiple scattering of the fibre-guided fields. In ultrathin fibres that support higher order modes, these interactions are stronger and exhibit qualitatively new behaviour due to the coupling of different fibre modes, which have different propagation wave-vectors, by the particles. Here, we study one dimensional longitudinal optical binding interactions of chains of 3 μm polystyrene spheres under the influence of the evanescent fields of a two-mode microfibre. The observation of long-range interactions, self-ordering and speed variation of particle chains reveals strong optical binding effects between the particles that can be modelled well by a tritter scattering-matrix approach. The optical forces, optical binding interactions and the velocity of bounded particle chains are calculated using this method. Results show good agreement with finite element numerical simulations. Experimental data and theoretical analysis show that higher order modes in a microfibre offer a promising method to not only obtain stable, multiple particle trapping or faster particle propulsion speeds, but that they also allow for better control over each individual trapped object in particle ensembles near the microfibre surface.


Single fiber optical trapping of a liquid droplet and its application in microresonator

Zhihai Liu, Yunhao Chen, Li Zhao, Yu Zhang, Yong Wei, Zongda Zhu, Jun Yang, Libo Yuan

We propose and demonstrate an optical trapping of a liquid droplet and its application based on an annular core microstructured optical fiber. We grind and polish the annular core fiber tip to be a special frustum cone shape to make sure the optical force large enough to trap the liquid droplet non-intrusively. The axial and transverse optical trapping forces are simulated. In addition, we investigate the whispering gallery modes resonance characteristic of the trapped liquid droplet as the example of applications. The whispering gallery modes spectrum is sensitive to the size of the micro liquid droplet. Due to the simple construction and flexible manipulation, the fiber-based optical trapping technology for micro liquid droplets trapping, manipulating, and controlling has great application penitential in many fields, such as physics, biology, and interdisciplinary studies.


Monday, August 8, 2016

Optomechanics based on angular momentum exchange between light and matter

H Shi and M Bhattacharya

The subject of optomechanics involves interactions between optical and mechanical degrees of freedom, and is currently of great interest as an enabler of fundamental investigations in quantum mechanics, as well as a platform for ultrasensitive measurement devices. The majority of optomechanical configurations rely on the exchange of linear momentum between light and matter. We will begin this tutorial with a brief description of such systems. Subsequently, we will introduce optomechanical systems based on angular momentum exchange. In this context, optical fields carrying polarization and orbital angular momentum will be considered, while for the mechanics, torsional and free rotational motion will be of relevance. Our overall aims will be to supply basic analyses of some of the existing theoretical proposals, to provide functional descriptions of some of the experiments conducted thus far, and to consider some directions for future research. We hope this tutorial will be useful to both theorists and experimentalists interested in the subject.


Walker-A Motif Acts to Coordinate ATP Hydrolysis with Motor Output in Viral DNA Packaging

Damian delToro, David Ortiz, Mariam Ordyan, Jean Sippy, Choon-Seok Oh, Nicholas Keller, Michael Feiss, Carlos E. Catalano, Douglas E. Smith

During the assembly of many viruses, a powerful ATP-driven motor translocates DNA into a preformed procapsid. A Walker-A “P-loop” motif is proposed to coordinate ATP binding and hydrolysis with DNA translocation. We use genetic, biochemical, and biophysical techniques to survey the roles of P-loop residues in bacteriophage lambda motor function. We identify 55 point mutations that reduce virus yield to below detectable levels in a highly sensitive genetic complementation assay and 33 that cause varying reductions in yield. Most changes in the predicted conserved residues K76, R79, G81, and S83 produce no detectable yield. Biochemical analyses show that R79A and S83A mutant proteins fold, assemble, and display genome maturation activity similar to wild-type (WT) but exhibit little ATPase or DNA packaging activity. Kinetic DNA cleavage and ATPase measurements implicate R79 in motor ring assembly on DNA, supporting recent structural models that locate the P-loop at the interface between motor subunits. Single-molecule measurements detect no translocation for K76A and K76R, while G81A and S83A exhibit strong impairments, consistent with their predicted roles in ATP binding. We identify eight residue changes spanning A78-K84 that yield impaired translocation phenotypes and show that Walker-A residues play important roles in determining motor velocity, pausing, and processivity. The efficiency of initiation of packaging correlates strongly with motor velocity. Frequent pausing and slipping caused by changes A78V and R79K suggest that these residues are important for ATP alignment and coupling of ATP binding to DNA gripping. Our findings support recent structural models implicating the P-loop arginine in ATP hydrolysis and mechanochemical coupling.


Controllable light capsules employing modified Bessel-Gauss beams

Lei Gong, Weiwei Liu, Qian Zhao, Yuxuan Ren, Xingze Qiu, Mincheng Zhong & Yinmei Li

We report, in theory and experiment, on a novel class of controlled light capsules with nearly perfect darkness, directly employing intrinsic properties of modified Bessel-Gauss beams. These beams are able to naturally create three-dimensional bottle-shaped region during propagation as long as the parameters are properly chosen. Remarkably, the optical bottle can be controlled to demonstrate various geometries through tuning the beam parameters, thereby leading to an adjustable light capsule. We provide a detailed insight into the theoretical origin and characteristics of the light capsule derived from modified Bessel-Gauss beams. Moreover, a binary digital micromirror device (DMD) based scheme is first employed to shape the bottle beams by precise amplitude and phase manipulation. Further, we demonstrate their ability for optical trapping of core-shell magnetic microparticles, which play a particular role in biomedical research, with holographic optical tweezers. Therefore, our observations provide a new route for generating and controlling bottle beams and will widen the potentials for micromanipulation of absorbing particles, aerosols or even individual atoms.


Optical manipulation of small particles on the surface of a material

Nayan Kumar Paul and Brandon A Kemp

Optical trapping and manipulation of dielectric particles on the surface of a dielectric prism using TE plane waves are demonstrated in the Rayleigh scattering regime. The interference of four counter propagating evanescent waves forms a standing wave on the planar surface and the trapping is realized based on the gradient force. Two mirrors are used to manipulate the trapped particles in any arbitrary direction on the surface. The required trapping potential and the irradiance within the Rayleigh scattering regime are computed. A hypothesis is developed to pull the particles at a maximum force toward the surface for further demonstration of this configuration. The standing wave on the surface of the prism using TE Gaussian beams are demonstrated for practical illustration of this study.


Friday, August 5, 2016

Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers

Bryan R. Bzdek, Liam Collard, James E. Sprittles, Andrew J. Hudson and Jonathan P. Reid

We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependent monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed.


Colloidal heat engines: a review

Ignacio A. Martínez, Édgar Roldán, Luis Dinis and Raúl A. Rica

Stochastic heat engines can be built using colloidal particles trapped using optical tweezers. Here we review recent experimental realizations of microscopic heat engines. We first revisit the theoretical framework of stochastic thermodynamics that allows to describe the fluctuating behavior of the energy fluxes that occur at mesoscopic scales, and then discuss recent implementations of the colloidal equivalents to the macroscopic Stirling, Carnot and steam engines. These small-scale motors exhibit unique features in terms of power and efficiency fluctuations that have no equivalent in the macroscopic world. We also consider a second pathway for work extraction from colloidal engines operating between active bacterial reservoirs at different temperatures, which could significantly boost the performance of passive heat engines at the mesoscale. Finally, we provide some guidance on how the work extracted from colloidal heat engines can be used to generate net particle or energy currents, proposing a new generation of experiments with colloidal systems.


Sliding sleeves of XRCC4–XLF bridge DNA and connect fragments of broken DNA

Ineke Brouwer, Gerrit Sitters, Andrea Candelli, Stephanie J. Heerema, Iddo Heller, Abinadabe J. Melo de, Hongshan Zhang, Davide Normanno, Mauro Modesti, Erwin J. G. Peterman & Gijs J. L. Wuite

Non-homologous end joining (NHEJ) is the primary pathway for repairing DNA double-strand breaks (DSBs) in mammalian cells1. Such breaks are formed, for example, during gene-segment rearrangements in the adaptive immune system or by cancer therapeutic agents. Although the core components of the NHEJ machinery are known, it has remained difficult to assess the specific roles of these components and the dynamics of bringing and holding the fragments of broken DNA together. The structurally similar XRCC4 and XLF proteins are proposed to assemble as highly dynamic filaments at (or near) DSBs2. Here we show, using dual- and quadruple-trap optical tweezers combined with fluorescence microscopy, how human XRCC4, XLF and XRCC4–XLF complexes interact with DNA in real time. We find that XLF stimulates the binding of XRCC4 to DNA, forming heteromeric complexes that diffuse swiftly along the DNA. Moreover, we find that XRCC4–XLF complexes robustly bridge two independent DNA molecules and that these bridges are able to slide along the DNA. These observations suggest that XRCC4–XLF complexes form mobile sleeve-like structures around DNA that can reconnect the broken ends very rapidly and hold them together. Understanding the dynamics and regulation of this mechanism will lead to clarification of how NHEJ proteins are involved in generating chromosomal translocations3, 4.


Quantum coherent tractor beam effect for atoms trapped near a nanowaveguide

Mark Sadgrove, Sandro Wimberger & Síle Nic Chormaic

We propose several schemes to realize a tractor beam effect for ultracold atoms in the vicinity of a few-mode nanowaveguide. Atoms trapped near the waveguide are transported in a direction opposite to the guided mode propagation direction. We analyse three specific examples for ultracold 23Na atoms trapped near a specific nanowaveguide (i.e. an optical nanofibre): (i) a conveyor belt-type tractor beam effect, (ii) an accelerator tractor beam effect, and (iii) a quantum coherent tractor beam effect, all of which can effectively pull atoms along the nanofibre toward the light source. This technique provides a new tool for controlling the motion of particles near nanowaveguides with potential applications in the study of particle transport and binding as well as atom interferometry.


Dynamic motions of DNA molecules in an array of plasmonic traps

Jun-Hee Choi, Jung-Dae Kim and Yong-Gu Lee

DNA molecules can undergo dynamic motion by fluidic and plasmonic forces. The latter, very weak compared to the former, can alter the trajectory of DNA molecules from their original fluid direction. For analyzing the trajectory of DNA molecules, it is easier to view the force exerting plasmonic trap as a potential well. The potential well can be placed periodically forming a kinetically biased landscape. The motion of DNA molecules in this is the main subject of this study. In this paper, a scaled-up mock-up of DNA molecules and the unit-cell of the periodic potential landscape is fabricated by 3D printing and they are used to experimentally obtain the trajectory data. The data is then inputted into a computer simulation to check the trajectory of DNA molecules along the entire array of plasmonic traps. The novel analysis method provided in this paper can be used in the design of an array of plasmonic traps.


Thursday, August 4, 2016

Optical Manipulation of Single Magnetic Beads in a Microwell Array on a Digital Microfluidic Chip

Deborah Decrop, Toon Brans, Pieter Gijsenbergh, Jiadi Lu, Dragana Spasic, Tadej Kokalj, Filip Beunis, Peter Goos, Robert Puers, and Jeroen Lammertyn

The detection of single molecules in magnetic microbead microwell array formats revolutionized the development of digital bio-assays. However, retrieval of individual magnetic beads from these arrays has not been realized until now despite having great potential for studying captured targets at the individual level. In this paper, optical tweezers were implemented on a digital microfluidic platform for accurate manipulation of single magnetic beads seeded in a microwell array. Successful optical trapping of magnetic beads was found to be dependent on Brownian motion of the beads, suggesting a 99% chance of trapping a vibrating bead. A tailor-made experimental design was used to screen the effect of bead type, ionic buffer strength, surfactant type and concentration on the Brownian activity of beads in microwells. Using the optimal conditions the manipulation of magnetic beads was demonstrated by their trapping, retrieving, transporting and repositioning to a desired microwell on the array. The presented platform combines the strengths of digital microfluidics, digital bioassays and optical tweezers, resulting in a powerful dynamic microwell array system for single molecule and single cell studies.


Light-Mediated Manufacture and Manipulation of Actuators

Dong-Dong Han, Yong-Lai Zhang, Jia-Nan Ma, Yu-Qing Liu, Bing Han, Hong-Bo Sun

Recent years have seen a considerable growth of research interests in developing novel technologies that permit designable manufacture and controllable manipulation of actuators. Among various fabrication and driving strategies, light has emerged as an enabler to reach this end, contributing to the development of actuators. Several accessible light-mediated manufacturing technologies, such as ultraviolet (UV) lithography and direct laser writing (DLW), are summarized. A series of light-driven strategies including optical trapping, photochemical actuation, and photothermal actuation for controllable manipulation of actuators is introduced. Current challenges and future perspectives of this field are discussed. To generalize, light holds great promise for the development of actuators.


Effects of non-Gaussian Brownian motion on direct force optical tweezers measurements of the electrostatic forces between pairs of colloidal particles

Allan Raudsepp, Martin A.K. Williams, Simon B. Hall

Measurements of the electrostatic force with separation between a fixed and an optically trapped colloidal particle are examined with experiment, simulation and analytical calculation. Non-Gaussian Brownian motion is observed in the position of the optically trapped particle when particles are close and traps weak. As a consequence of this motion, a simple least squares parameterization of direct force measurements, in which force is inferred from the displacement of an optically trapped particle as separation is gradually decreased, contains forces generated by the rectification of thermal fluctuations in addition to those originating directly from the electrostatic interaction between the particles. Thus, when particles are close and traps weak, simply fitting the measured direct force measurement to DLVO theory extracts parameters with modified meanings when compared to the original formulation. In such cases, however, physically meaningful DLVO parameters can be recovered by comparing the measured non-Gaussian statistics to those predicted by solutions to Smoluchowski's equation for diffusion in a potential.


Characteristics of the orbital rotation in dual-beam fiber-optic trap with transverse offset

Xinlin Chen, Guangzong Xiao, Kaiyong Yang, Wei Xiong, and Hui Luo

The orbital rotation is an important type of motion of trapped particles apart from translation and spin rotation. It could be realized by introducing a transverse offset to the dual-beam fiber-optic trap. The characteristics (e.g. rotation perimeter and frequency) of the orbital rotation have been analyzed in this article. We demonstrate the influences of offset distance, beam waist separation distance, light power, and radius of the microsphere by both experimental and numerical work. The experiment results, i.e. orbital rotation perimeter and frequency as functions of these parameters, are consistent with the theoretical model in the present work. The orbital rotation amplitude and frequency could be exactly controlled by varying these parameters. This controllable orbital rotation can be easily applied to the area where microfluidic mixing is required.


Wednesday, August 3, 2016

Damage induced in red blood cells by infrared optical trapping: an evaluation based on elasticity measurements

Marcos A. S. de Oliveira ; Diógenes S. Moura ; Adriana Fontes ; Renato E. de Araujo

We evaluated the damage caused to optically trapped red blood cells (RBCs) after 1 or 2 min of exposure to near-infrared (NIR) laser beams at 785 or 1064 nm. Damage was quantified by measuring cell elasticity using an automatic, real-time, homemade, optical tweezer system. The measurements, performed on a significant number (hundreds) of cells, revealed an overall deformability decrease up to ∼104%∼104% after 2 min of light exposure, under 10 mW optical trapping for the 785-nm wavelength. Wavelength dependence of the optical damage is attributed to the light absorption by hemoglobin. The results provided evidence that RBCs have their biomechanical properties affected by NIR radiation. Our findings establish limits for laser applications with RBCs.


DNA Twist Stability Changes with Magnesium(2+) Concentration

Onno D. Broekmans, Graeme A. King, Greg J. Stephens, and Gijs J. L. Wuite

To understand DNA elasticity at high forces (F>30  pN), its helical nature must be taken into account, as a coupling between twist and stretch. The prevailing model, the wormlike chain, was previously extended to include this twist-stretch coupling. Motivated by DNA’s charged nature, and the known effects of ionic charges on its elasticity, we set out to systematically measure the impact of buffer ionic conditions on twist-stretch coupling. After developing a robust fitting approach, we show, using our new data set, that DNA’s helical twist is stabilized at high concentrations of the magnesium divalent cation. DNA’s persistence length and stretch modulus are, on the other hand, relatively insensitive to the applied range of ionic strengths.


Holographic optical assembly and photopolymerized joining of planar microspheres

L. A. Shaw, S. Chizari, R. M. Panas, M. Shusteff, C. M. Spadaccini, and J. B. Hopkins

The aim of this research is to demonstrate a holographically driven photopolymerization process for joining colloidal particles to create planar microstructures fixed to a substrate, which can be monitored with real-time measurement. Holographic optical tweezers (HOT) have been used to arrange arrays of microparticles prior to this work; here we introduce a new photopolymerization process for rapidly joining simultaneously handled microspheres in a plane. Additionally, we demonstrate a new process control technique for efficiently identifying when particles have been successfully joined by measuring a sufficient reduction in the particles’ Brownian motion. This technique and our demonstrated joining approach enable HOT technology to take critical steps toward automated additive fabrication of microstructures.


Integrated plasmonic nanotweezers for nanoparticle manipulation

Giovanni Magno, Aurore Ecarnot, Christophe Pin, Vy Yam, Philippe Gogol, Robert Mégy, Benoit Cluzel, and Béatrice Dagens

We numerically demonstrate that short gold nanoparticle chains coupled to traditional SOI waveguides allow conceiving surface plasmon-based nanotweezers. This configuration provides for jumpless control of the trapping position of a nano-object as a function of the excitation wavelength, allowing for linear repositioning. This novel feature can be captivating for the conception of compact integrated optomechanical nanoactuators.


Trapping two types of particles by modified circular Airy beams

Yunfeng Jiang, Zili Cao, Hehong Shao, Wanting Zheng, Bixin Zeng, and Xuanhui Lu

The radiation force of modified circular Airy beams (MCAB) exerted on both a high-refractive-index particle and a low-refractive-index particle are analyzed in this paper. Our results show that the two kinds of particles can be simultaneously stably trapped by MCAB at different positions. Compared with the common circular Airy beams (CAB) with the same parameters, trapping forces on the two kinds of particles are greatly increased because of the enhanced abruptly autofocusing property and the appearance of hollow region in MCAB. The trapping forces can be modulated by varying parameters of MCAB, and it is important to choose appropriate parameters to trap particles in practice.


Tuesday, August 2, 2016

Micromanipulation of InP lasers with optoelectronic tweezers for integration on a photonic platform

Joan Juvert, Shuailong Zhang, Iain Eddie, Colin J. Mitchell, Graham T. Reed, James S. Wilkinson, Anthony Kelly, and Steven L. Neale

The integration of light sources on a photonic platform is a key aspect of the fabrication of self-contained photonic circuits with a small footprint that does not have a definitive solution yet. Several approaches are being actively researched for this purpose. In this work we propose optoelectronic tweezers for the manipulation and integration of light sources on a photonic platform and report the positional and angular accuracy of the micromanipulation of standard Fabry-Pérot InP semiconductor laser die. These lasers are over three orders of magnitude bigger in volume than any previously assembled with optofluidic techniques and the fact that they are industry standard lasers makes them significantly more useful than previously assembled microdisk lasers. We measure the accuracy to be 2.5 ± 1.4 µm and 1.4 ± 0.4° and conclude that optoelectronic tweezers are a promising technique for the micromanipulation and integration of optoelectronic components in general and semiconductor lasers in particular.


On-chip clonal analysis of glioma stem cell motility and therapy resistance

Daniel Gallego-Perez, Lingqian Chang, Junfeng Shi, Junyu Ma, Sung-Hak Kim, Xi Zhao, Veysi Malkoc, Xinmei Wang, Mutsuko Minata, Kwang Joo Kwak, Yun Wu, Gregory Lafyatis, Wu Lu, Derek J. Hansford, Ichiro Nakano, and Ly James Lee

Enhanced glioma stem cell (GSC) motility and therapy resistance are considered to play key roles in tumor cell dissemination and life-threatening recurrence. As such, a better understanding of the mechanisms by which these cells disseminate and withstand therapy could lead to more efficacious treatments for this devastating disease. Here we introduce a novel micro/nanotechnology-based chip platform for performing live-cell interrogation of patient-derived GSC cultures with single-clone resolution. On-chip analysis revealed marked inter-tumoral differences (> 10-fold) in single-clone motility profiles between two populations of patient-derived GSCs, which correlated well with results from supporting tumor xenograft experiments and gene expression analyses. Further chip-based examination of the more aggressive GSC population revealed pronounced inter-clonal variations in motility capabilities (up to ~4-fold) as well as gene expression profiles at the single-cell level. Chip-supported therapy resistance studies with a chemotherapeutic agent (i.e., Temozolomide) and an oligo RNA (anti-miR363) revealed a sub-population of CD44-high GSCs with strong anti-apoptotic behavior as well as enhanced motility capabilities. The living cell interrogation chip platform described herein enables thorough and large-scale live monitoring of heterogeneous cancer cell populations with single-cell resolution which is not achievable by any other existing technology, and thus has the potential to provide new insights into the cellular and molecular mechanisms modulating glioma stem cell dissemination and therapy resistance.


Characterization of cold atmospheric plasma inactivation of individual bacterial spores using Raman spectroscopy and phase contrast microscopy

Shiwei Wang, Christopher J. Doona, Peter Setlow and Yong-qing Li

Raman spectroscopy and phase-contrast microscopy were used to examine calcium dipicolinate (CaDPA) levels and rates of nutrient and non-nutrient germination of multiple individual Bacillus subtilis spores treated with cold atmospheric plasma (CAP). Major results for this work are: 1) >5 logs of spores deposited on glass surfaces were inactivated by CAP treatment for 3 min, while deposited spores placed inside an impermeable plastic bag were inactivated only ∼2 logs in 30 min. 2) >80% of the spores treated for 1-3 min with CAP were non-culturable, and retained CaDPA in their core, while >95% of spores treated with CAP for 5-10 min lost all CaDPA. 3) Raman measurements of individual CAP-treated spores without CaDPA showed differences from spores that germinated with L-valine in terms of nucleic acids, lipids, and proteins. 4) 1-2 min CAP treatment killed 99% of spores, but these spores still germinated with nutrients or exogenous CaDPA, albeit more slowerly and to a lesser extent than untreated spores, while spores CAP-treated for >3 min that retained CaDPA did not germinate via nutrients or CaDPA. However, even after 1-3 min of CAP-treatment, spores germinated normally with dodecylamine. These results suggest that exposure to the present CAP configuration severely damages spore's inner membrane and key germination proteins, such that the treated spores either lose CaDPA or can neither initiate nor complete germination with nutrients or CaDPA. Analysis of the various CAP components indicated that UV photons contributed minimally to spore inactivation, while charged particles and reactive oxygen species contributed significantly.


Hydrogel–colloid interfacial interactions: a study of tailored adhesion using optical tweezers

Amir Sheikhi and Reghan J. Hill

Dynamics of colloidal particles adhering to soft, deformable substrates, such as tissues, biofilms, and hydrogels play a key role in many biological and biomimetic processes. These processes, including, but not limited to colloid-based delivery, stitching, and sorting, involve microspheres exploring the vicinity of soft, sticky materials in which the colloidal dynamics are affected by the fluid environment (e.g., viscous coupling), inter-molecular interactions between the colloids and substrates (e.g., Derjaguin–Landau–Verwey–Overbeek (DLVO) theory), and the viscoelastic properties of contact region. To better understand colloidal dynamics at soft interfaces, an optical tweezers back-focal-plane interferometry apparatus was developed to register the transverse Brownian motion of a silica microsphere in the vicinity of polyacrylamide (PA) hydrogel films. The time-dependent mean-squared displacements are well described by a single exponential relaxation, furnishing measures of the transverse interfacial diffusion coefficient and binding stiffness. Substrates with different elasticities were prepared by changing the PA crosslinking density, and the inter-molecular interactions were adjusted by coating the microspheres with fluid membranes. Stiffer PA hydrogels (with bulk Young's moduli ≈1–10 kPa) immobilize the microspheres more firmly (lower diffusion coefficient and position variance), and coating the particles with zwitterionic lipid bilayers (DOPC) completely eliminates adhesion, possibly by repulsive dispersion forces. Remarkably, embedding polyethylene glycol-grafted lipid bilayers (DSPE–PEG2k–Amine) in the zwitterionic fluid membranes produces stronger adhesion, possibly because of polymer–hydrogel attraction and entanglement. This study provides new insights to guide the design of nanoparticles and substrates with tunable adhesion, leading to smarter delivery, sorting, and screening of micro- and nano-systems.


Effect of temperature and electric field on 2D nematic colloidal crystals stabilised by vortex-like topological defects

K. P. Zuhail and Surajit Dhara

We report experimental studies on 2D colloidal crystals of dimers stabilized by vortex-like defects in planar nematic and π/2 twisted nematic cells. The dimers are prepared and self-assembled using a laser tweezer. We study the effect of temperature and electric field on the lattice parameters of the colloidal crystals. The lattice parameters vary with the temperature in the nematic phase and a discontinuous structural change is observed at the nematic to smectic-A phase transition. In the nematic phase, we observed a large change in the lattice parameters (≃30%) by applying an external electric field perpendicular to the plane of the 2D crystals. The idea and the active control of the lattice parameters could be useful for designing tunable colloidal crystals.


Monday, August 1, 2016

Two-dimensional assemblies of nematic colloids in homeotropic cells and their response to electric fields

Yuta Tamura and Yasuyuki Kimura

Micrometer-sized colloidal particles dispersed in nematic liquid crystals interact with each other through anisotropic interactions induced by orientational deformation of the nematic field. In the case of so-called dipole nematic colloids, their interaction is of the dipole–dipole type. Two-dimensional, non-close-packed colloidal assemblies having various characteristics were fabricated using optical tweezers by exploiting the attraction between anti-parallel dipole nematic colloids in homeotropically aligned nematic cells. Structures comprising polygons, squares, and tetrahedra were built using equal-sized particles, and hexagonal structures were built using particles of two sizes. As the nematic field is sensitive to electric fields, the response of the fabricated assemblies toward an alternating electric field was also studied. All assemblies exhibited homogeneous reversible shrinkage, and their shrinkage rates were dependent on the structure. The maximum shrinkage rate in the linear dimension of the assemblies was over 20% at 5 Vrms for a hexagon comprising tetrahedral units.


An optical tweezer in asymmetrical vortex Bessel-Gaussian beams

V. V. Kotlyar, A. A. Kovalev and A. P. Porfirev

We study an optical micromanipulation that comprises trapping, rotating, and transporting 5-μm polystyrene microbeads in asymmetric Bessel-Gaussian (BG) laser beams. The beams that carry orbital angular momentum are generated by means of a liquid crystal microdisplay and focused by a microobjective with a numerical aperture of NA = 0.85. We experimentally show that given a constant topological charge, the rate of microparticle motion increases near linearly with increasing asymmetry of the BG beam. Asymmetric BG beams can be used instead of conventional Gaussian beam for trapping and transferring live cells without thermal damage.


Optical two-beam traps in microfluidic systems

Kirstine Berg-Sørensen

An attractive solution for optical trapping and stretching by means of two counterpropagating laser beams is to embed waveguides or optical fibers in a microfluidic system. The microfluidic system can be constructed in different materials, ranging from soft polymers that may easily be cast in a rapid prototyping manner, to hard polymers that could even be produced by injection moulding, or to silica in which waveguides may either be written directly, or with grooves for optical fibers. Here, we review different solutions to the system and also show results obtained in a polymer chip with DUV written waveguides and in an injection molded polymer chip with grooves for optical fibers.


Stochastic Dynamic Trapping in Robotic Manipulation of Micro-Objects Using Optical Tweezers

Xiao Yan; Chien Chern Cheah; Quang Minh Ta; Quang-Cuong Pham

Various automatic manipulation techniques have been developed for manipulating micro-objects using optical tweezers. Because of the small trapping force of optical traps and increase in kinetic energy during manipulation, a trapped object may not remain trappable, especially in the presence of random Brownian perturbation. However, there is no theoretical analysis so far to help understand the effects of dynamic motion and Brownian forces on the trappability problem of optical tweezers. This paper investigates the optical manipulation of micro-objects under random perturbations. Here, we provide for the first time a theoretical and experimental analysis of the dynamic trapping problem from stochastic perspectives. We derive the relationship between trapping probability and maximum manipulation velocity. A controller with appropriate velocity bound is then proposed to ensure that the system is bound and stable. The experimental results confirm the accuracy of our theoretical analysis and illustrate the necessity and usefulness of the proposed controller.