Tuesday, March 31, 2015

Engineering of slow Bloch modes for optical trapping

L. Milord, E. Gerelli, C. Jamois, A. Harouri, C. Chevalier, P. Viktorovitch, X. Letartre and T. Benyattou
In the present paper, we propose an approach based on slow Bloch mode microcavity that enables the optical trapping of small nanoparticles over a broad surface. A specific design based on a double-period photonic crystal is presented. It enables an easy coupling using a wide free-space Gaussian beam and the cavity Q factor can be tuned at will. Moreover, the microcavity mode is mainly localized within the photonic crystal holes, meaning that each hole of the microcavity behaves as efficient nanotweezers. Experimental studies have shown that 200 nm and 100 nm particles can be trapped within the microcavity, in a spatial region that corresponds to the size of one hole (200 nm wide). The experimental trap stiffness has been extracted. It shows that this approach is among the most performant ones if we take into account the size of the cavity.


Entrainment of heterogeneous glycolytic oscillations in single cells

Anna-Karin Gustavsson, Caroline B. Adiels, Bernhard Mehlig & Mattias Goksör

Cell signaling, gene expression, and metabolism are affected by cell-cell heterogeneity and random changes in the environment. The effects of such fluctuations on cell signaling and gene expression have recently been studied intensively using single-cell experiments. In metabolism heterogeneity may be particularly important because it may affect synchronisation of metabolic oscillations, an important example of cell-cell communication. This synchronisation is notoriously difficult to describe theoretically as the example of glycolytic oscillations shows: neither is the mechanism of glycolytic synchronisation understood nor the role of cell-cell heterogeneity. To pin down the mechanism and to assess its robustness and universality we have experimentally investigated the entrainment of glycolytic oscillations in individual yeast cells by periodic external perturbations. We find that oscillatory cells synchronise through phase shifts and that the mechanism is insensitive to cell heterogeneity (robustness) and similar for different types of external perturbations (universality).


Micro particle launcher/cleaner based on optical trapping technology

Zhihai Liu, Peibo Liang, Yu Zhang, Yaxun Zhang, Enming Zhao, Jun Yang, and Libo Yuan

Efficient and controllable launching function of an optical tweezers is a challenging task. We present and demonstrate a novel single fiber optical tweezers which can trap and launch (clean) a target polystyrene (PS) microsphere (diameter~10μm) with independent control by using two wavelengths beams: 980nm and 1480nm. We employ 980nm laser beam to trap the target PS microsphere by molding the fiber tip into a special tapered-shape; and we employ 1480nm laser beam to launch the trapped PS microsphere with a certain velocity by using the thermophoresis force generated from the thermal effect due to the high absorption of the 1480nm laser beams in water. When the launching force is smaller than the trapping force, the PS microsphere will be trapped near the fiber tip, and the launching force will blow away other PS microspheres in the workspace realizing the cleaning function; When the launching force is larger than the trapping force, the trapped PS microsphere will be launched away from the fiber tip with a certain velocity and towards a certain direction, realizing the launching function. The launching velocity, acceleration and the distance can be measured by detecting the interference signals generated from the PS microsphere surface and the fiber tip end-face. This PS microsphere launching and cleaning functions expanded new features of single fiber optical tweezers, providing for the possibility of more practical applications in the micro manipulation research fields.


Friday, March 27, 2015

Dark-hollow optical beams with a controllable shape for optical trapping in air

A.P. Porfirev and R.V. Skidanov

A technique for generating dark-hollow optical beams (DHOBs) with a controllable cross-sectional intensity distribution is proposed and studied both theoretically and experimentally. Superimposed Bessel beams were used to generate such DHOBs. Variation of individual beam parameters enables the generation of Bessel-like non-diffracting beams. This technique allows the design of transmission functions for elements that shape both non-rotating and rotating DHOBs. We demonstrate photophoresis-based optical trapping and manipulation of absorbing air-borne nanoclusters with such beams.


Generation of microswimmers from passive Brownian particles in a spherically aberrated optical trap

Argha Mondal, Basudev Roy, and Ayan Banerjee

We induce spontaneous motion that is both directed and complex in micron-sized asymmetric Brownian particles in a spherically aberrated optical trap to generate microswimmers. The aberrated optical trap is prepared in a slightly modified optical tweezers configuration where we use a refractive index mismatched cover slip leading to the formation of an annular intensity distribution near the trap focal plane. Asymmetric scattering from a micro-particle trapped in this annular trap gives rise to a net tangential force on the particle causing it to revolve spontaneously in the intensity ring. The rate of revolution can be controlled from sub-Hz to a few Hz by changing the intensity of the trapping light. Theoretical simulations performed using finite-difference time-domain method verify the experimental observations. We also experimentally demonstrate simultaneous spin and revolution of a micro-swimmer which shows that complex motion can be achieved by designing a suitable shape of a micro-swimmer in the optical potential.


Thursday, March 26, 2015

Hamiltonian curl forces

M. V. Berry , Pragya Shukla

Newtonian forces depending only on position but which are non-conservative, i.e. whose curl is not zero, are termed ‘curl forces’. They are non-dissipative, but cannot be generated by a Hamiltonian of the familiar isotropic kinetic energy + scalar potential type. Nevertheless, a large class of such non-conservative forces (though not all) can be generated from Hamiltonians of a special type, in which kinetic energy is an anisotropic quadratic function of momentum. Examples include all linear curl forces, some azimuthal and radial forces, and some shear forces. Included are forces exerted on electrons in semiconductors, and on small particles by monochromatic light near an optical vortex. Curl forces imply restrictions on the geometry of periodic orbits, and non-conservation of Poincaré's integral invariant. Some fundamental questions remain, for example: how does curl dynamics generated by a Hamiltonian differ from dynamics under curl forces that are not Hamiltonian?


Optical transportation and controllable positioning of nanospheres using a microfiber

Yanjun Hu, Ying Li, Yonghe Deng and Ping Peng

We experimentally demonstrate an optical transportation and controllable positioning of polystyrene nanospheres using a 3 μm diameter microfiber. By placing the microfiber in a microfluidic channel and injecting a 980 nm laser light into the fiber, nanospheres suspended in the water were stably trapped to the microfiber and delivered along the direction of light propagation. Furthermore, by increasing the velocity of the fluid in the opposite direction of the laser light, it was found that, once the fluid velocity increased to 6 μm/s, spheres stopped their forward progress and halted on the microfiber, so the controllable positioning of spheres along the microfiber was realized.


Laser Trapping of Colloidal Metal Nanoparticles

Anni Lehmuskero, Peter Johansson, Halina Rubinsztein-Dunlop, Lianming Tong, and Mikael Käll

Optical trapping using focused laser beams (laser tweezers) has been proven extremely useful for contact-less manipulation of a variety of small objects, including biological cells, organelles within cells and a wide range of other dielectric micro/nano objects. Colloidal metal nanoparticles have drawn increasing attention in the field of optical trapping because of their unique interactions with electromagnetic radiation, caused by surface plasmon resonance effects, enabling a large number of nano-optical applications of high current interest. Here we try to give a comprehensive overview of the field of laser trapping and manipulation of metal nanoparticles based on results reported in the recent literature. We also discuss and describe the fundamentals of optical forces in the context of plasmonic nanoparticles, including effects of polarization, optical angular momentum and laser heating effects, as well as the various techniques that have been used to trap and manipulate metal nanoparticles. We conclude by suggesting possible directions for future research.


Wednesday, March 25, 2015

Divalent cations and molecular crowding buffers stabilize G-triplex at physiologically relevant temperatures

Hong-Xin Jiang, Yunxi Cui, Ting Zhao, Hai-Wei Fu, Deepak Koirala, Jibin Abraham Punnoose, De-Ming Kong & Hanbin Mao

G-triplexes are non-canonical DNA structures formed by G-rich sequences with three G-tracts. Putative G-triplex-forming sequences are expected to be more prevalent than putative G-quadruplex-forming sequences. However, the research on G-triplexes is rare. In this work, the effects of molecular crowding and several physiologically important metal ions on the formation and stability of G-triplexes were examined using a combination of circular dichroism, thermodynamics, optical tweezers and calorimetry techniques. We determined that molecular crowding conditions and cations, such as Na+, K+, Mg2+ and Ca2+, promote the formation of G-triplexes and stabilize these structures. Of these four metal cations, Ca2+ has the strongest stabilizing effect, followed by K+, Mg2+, and Na+ in a decreasing order. The binding of K+ to G-triplexes is accompanied by exothermic heats, and the binding of Ca2+ with G-triplexes is characterized by endothermic heats. G-triplexes formed from two G-triad layers are not stable at physiological temperatures; however, G-triplexes formed from three G-triads exhibit melting temperatures higher than 37°C, especially under the molecular crowding conditions and in the presence of K+ or Ca2+. These observations imply that stable G-triplexes may be formed under physiological conditions.


Radiative force on atoms from the view of photon emission

Zhuo Song, Yonggang Peng, and Yujun Zheng

In this Letter, we present a possible methodology to directly “read” the force on an atom via the photons emitted from the atom. In this methodology, the mean radiative force on an atom exerted by external fields can be expressed as a function of the average number of emitted photons 〈N〉 and its derivatives via the generating function approach developed by us recently.


From laser ultrasonics to optical manipulation

Tomaž Požar, Aleš Babnik, and Janez Možina
During the interaction of a laser pulse with the surface of a solid object, the object always gains momentum. The delivered force impulse is manifested as propulsion. Initially, the motion of the object is composed of elastic waves that carry and redistribute the acquired momentum as they propagate and reflect within the solid. Even though only ablation- and light-pressure-induced mechanical waves are involved in propulsion, they are always accompanied by the ubiquitous thermoelastic waves. This paper describes 1D elastodynamics of pulsed optical manipulation and presents two diametrical experimental observations of elastic waves generated in the confined ablation and in the radiation pressure regime.


Active diffusion positions the nucleus in mouse oocytes

Maria Almonacid, Wylie W. Ahmed, Matthias Bussonnier, Philippe Mailly, Timo Betz, Raphaël Voituriez, Nir S. Gov & Marie-Hélène Verlhac

In somatic cells, the position of the cell centroid is dictated by the centrosome. The centrosome is instrumental in nucleus positioning, the two structures being physically connected. Mouse oocytes have no centrosomes, yet harbour centrally located nuclei. We demonstrate how oocytes define their geometric centre in the absence of centrosomes. Using live imaging of oocytes, knockout for the ​formin 2 actin nucleator, with off-centred nuclei, together with optical trapping and modelling, we discover an unprecedented mode of nucleus positioning. We document how active diffusion of actin-coated vesicles, driven by ​myosin Vb, generates a pressure gradient and a propulsion force sufficient to move the oocyte nucleus. It promotes fluidization of the cytoplasm, contributing to nucleus directional movement towards the centre. Our results highlight the potential of active diffusion, a prominent source of intracellular transport, able to move large organelles such as nuclei, providing in vivo evidence of its biological function.


Nonlinear response and stability of a 2D rolling semi-cylinder during optical lift

Daniel G. Schuster Jr., Mario W. Gomes, Alexandra B. Artusio-Glimpse, Grover A. Swartzlander Jr.

In this paper, the response is found for a semi-cylindrical rod rocking on a level surface while subjected to forces from radiation pressure and gravity. Changes in the oscillation frequency of the rod as a function of light intensity are determined for both a mirrored and non-mirrored rod. The simulation results show that the equilibrium positions for the mirrored and non-mirrored rod exhibit a classic pitchfork and cusp catastrophe type bifurcation at critical laser intensities, respectively. By linearizing the systems equations of motion and sinusoidally modulating the laser intensity, the mathematical model for the rocking semi-cylinder could be transformed in the standard form of Mathieu’s equation. Inspired by the stability regions of the vertically oscillating inverted pendulum, a region of laser modulation parameters was determined, which could stabilize orientations of the lens which were unstable with constant laser intensity. Lastly, a comparison between the bifurcation point and change in natural frequency as functions of intensity between a previous analytical derivation and the full nonlinear model also showed that they agree closely for laser intensities near and below the critical intensity.


Tuesday, March 24, 2015

Influence of GHz electric fields on the mechanical properties of a microtubule

S. S. Setayandeh, A. Lohrasebi

The effects of external GHz electric fields on the mechanical properties of a microtubule (MT) have been modeled through the application of a molecular dynamics simulation method. To explore the properties of the MT, two different systems each consisting of a pair of dimers were exposed to an 0.03 V/nm electric field with a frequency ranging between 1 to 10 GHz. It was found that the Young’s modulus of each system, which is related to the flexibility of the MT, was lower at some frequencies and higher at others in comparison with normal biological conditions. Hence, the application of such an electric field with a frequency in this range may affect MT function, which could have positive or negative effects on cell health. Positive effects include its potential use in cancer treatment, where the application of such a field could lead to a decrease in MT rigidity, similar to the effect of Taxol on MTs. Negative effects include unwanted changes to the mechanical properties of MTs (e.g., disturbing the cell division process and in turn increasing the risk of cancer) upon the application of such a field.


Devil’s lens optical tweezers

Jixiong Pu and P. H. Jones

We demonstrate an optical tweezers using a laser beam on which is imprinted a focusing phase profile generated by a Devil’s staircase fractal structure (Cantor set). We show that a beam shaped in this way is capable of stably trapping a variety of micron- and submicron-sized particles and calibrate the optical trap as a function of the control parameters of the fractal structure, and explain the observed variation as arising from radiation pressure exerted by unfocused parts of the beam in the region of the optical trap. Experimental results are complemented by calculation of the structure of the focus in the regime of high numerical aperture.


Analytical approach of ordinary frozen waves for optical trapping and micromanipulation

Leonardo André Ambrosio and Michel Zamboni-Rached

The optical properties of frozen waves (FWs) are theoretically and numerically investigated using the generalized Lorenz-Mie theory (GLMT) together with integral localized approximation. These waves are constructed from a suitable superposition of equal-frequency ordinary Bessel beams and are capable of providing almost any desired longitudinal intensity profile along their optical axis, thus being of potential interest in applications in which intensity localization may be used advantageously, such as in optical trapping and micromanipulation systems. In addition, because FWs are composed of nondiffracting beams, they are also capable of overcoming the diffraction effects for longer distances when compared to conventional (ordinary) beams, e.g., Gaussian beams. Expressions for the beam-shape coefficients of FWs are provided, and the GLMT is used to reconstruct their intensity profiles and to predict their optical properties for possible biomedical optics purposes.


Non-spherical gold nanoparticles trapped in optical tweezers: shape matters

Oto Brzobohatý, Martin Šiler, Jan Trojek, Lukáš Chvátal, Vítězslav Karásek, and Pavel Zemánek

We present the results of a theoretical analysis focused on three-dimensional optical trapping of non-spherical gold nanoparticles using a tightly focused laser beam (i.e. optical tweezers). We investigate how the wavelength of the trapping beam enhances trapping stiffness and determines the stable orientation of nonspherical nanoparticles in the optical trap which reveals the optimal trapping wavelength. We consider nanoparticles with diameters being between 20 nm and 254 nm illuminated by a highly focused laser beam at wavelength 1064 nm and compare our results based on the coupled-dipole method with published theoretical and experimental data. We demonstrate that by considering the non-spherical morphology of the nanoparticle we can explain the experimentally observed three-dimensional trapping of plasmonic nanoparticles with size higher than 170 nm. These results will contribute to a better understanding of the trapping and alignment of real metal nanoparticles in optical tweezers and their applications as optically controllable nanosources of heat or probes of weak forces and torques.


Monday, March 23, 2015

Asymmetric Unwrapping of Nucleosomes under Tension Directed by DNA Local Flexibility

Thuy T.M. Ngo, Qiucen Zhang, Ruobo Zhou, Jaya G. Yodh, Taekjip Ha
Dynamics of the nucleosome and exposure of nucleosomal DNA play key roles in many nuclear processes, but local dynamics of the nucleosome and its modulation by DNA sequence are poorly understood. Using single-molecule assays, we observed that the nucleosome can unwrap asymmetrically and directionally under force. The relative DNA flexibility of the inner quarters of nucleosomal DNA controls the unwrapping direction such that the nucleosome unwraps from the stiffer side. If the DNA flexibility is similar on two sides, it stochastically unwraps from either side. The two ends of the nucleosome are orchestrated such that the opening of one end helps to stabilize the other end, providing a mechanism to amplify even small differences in flexibility to a large asymmetry in nucleosome stability. Our discovery of DNA flexibility as a critical factor for nucleosome dynamics and mechanical stability suggests a novel mechanism of gene regulation by DNA sequence and modifications.


Theory and simulations of toroidal and rod-like structures in single-molecule DNA condensation

Ruggero Cortini, Bertrand R. Caré, Jean-Marc Victor and Maria Barbi

DNA condensation by multivalent cations plays a crucial role in genome packaging in viruses and sperm heads, and has been extensively studied using single-molecule experimental methods. In those experiments, the values of the critical condensation forces have been used to estimate the amplitude of the attractive DNA-DNA interactions. Here, to describe these experiments, we developed an analytical model and a rigid body Langevin dynamics assay to investigate the behavior of a polymer with self-interactions, in the presence of a traction force applied at its extremities. We model self-interactions using a pairwise attractive potential, thereby treating the counterions implicitly. The analytical model allows to accurately predict the equilibrium structures of toroidal and rod-like condensed structures, and the dependence of the critical condensation force on the DNA length. We find that the critical condensation force depends strongly on the length of the DNA, and finite-size effects are important for molecules of length up to 105 μm. Our Langevin dynamics simulations show that the force-extension behavior of the rod-like structures is very different from the toroidal ones, so that their presence in experiments should be easily detectable. In double-stranded DNA condensation experiments, the signature of the presence of rod-like structures was not unambiguously detected, suggesting that the polyamines used to condense DNA may protect it from bending sharply as needed in the rod-like structures.


Organic Component Vapor Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers

Chen Cai, David J. Stewart, Jonathan P. Reid, Yun-hong Zhang, Peter Ohm, Cari S. Dutcher, and Simon L. Clegg

Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semivolatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapor pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for a homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol, or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterization of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <±2.5% to discriminate between different thermodynamic treatments.


Microgels and Nanoparticles: Where Micro and Nano Go Hand in Hand

Beatriz H. Juarez/ Luis M. Liz-Marzán

The integration of different types of materials in a single hybrid system allows the combination of multiple functionalities, which can even be used in conjunction with each other. This strategy has been exploited in nanoscale systems for the creation of so-called smart nanomaterials. Within this category, the combination of inorganic nanoparticles with stimuli-responsive microgels is of very high interest because of the wide variety of potential applications. We present here a short overview of this type of materials in which the nano- and micro-scales get nicely integrated, with a great potential to expand the range of technological applications. We focus mainly on the integration of metal nanoparticles, either by themselves or in combination with semiconductor and magnetic nanoparticles. Various examples of the synergic properties that can be obtained are described, as well as the possibility to extract useful information when optical tweezers are used to manipulate single particles. We expect that this review will stimulate additional research in this field.


Sunday, March 22, 2015

Overview of single-cell elastic light scattering techniques

Matti Kinnunen; Artashes Karmenyan

We present and discuss several modern optical methods based on elastic light scattering (ELS), along with their technical features and applications in biomedicine and life sciences. In particular, we review some ELS experiments at the single-cell level and explore new directions of applications. Due to recent developments in experimental systems (as shown in the literature), ELS lends itself to useful applications in the life sciences. Of the developed methods, we cover elastic scattering spectroscopy, optical tweezer-assisted measurement, goniometers, Fourier transform light scattering (FTLS), and microscopic methods. FTLS significantly extends the potential analysis of single cells by allowing monitoring of dynamical changes at the single-cell level. The main aim of our review is to demonstrate developments in the experimental investigation of ELS in single cells including issues related to theoretical “representations” and modeling of biological systems (cells, cellular systems, tissues, and so on). Goniometric measurements of ELS from optically trapped single cells are shown and the importance of the experimental verification of theoretical models of ELS in the context of biomedical applications is discussed.


Substrate-dependent cell elasticity measured by optical tweezers indentation

Muhammad S. Yousafzai, Fatou Ndoye, Giovanna Coceano, Joseph Niemela, Serena Bonin, Giacinto Scoles, Dan Cojoc

In the last decade, cell elasticity has been widely investigated as a potential label free indicator for cellular alteration in different diseases, cancer included. Cell elasticity can be locally measured by pulling membrane tethers, stretching or indenting the cell using optical tweezers. In this paper, we propose a simple approach to perform cell indentation at pN forces by axially moving the cell against a trapped microbead. The elastic modulus is calculated using the Hertz-model. Besides the axial component, the setup also allows us to examine the lateral cell–bead interaction. This technique has been applied to measure the local elasticity of HBL-100 cells, an immortalized human cell line, originally derived from the milk of a woman with no evidence of breast cancer lesions. In addition, we have studied the influence of substrate stiffness on cell elasticity by performing experiments on cells cultured on two substrates, bare and collagen-coated, having different stiffness. The mean value of the cell elastic modulus measured during indentation was 26±9 Pa for the bare substrate, while for the collagen-coated substrate it diminished to 19±7 Pa. The same trend was obtained for the elastic modulus measured during the retraction of the cell: 23±10 Pa and 13±7 Pa, respectively. These results show the cells adapt their stiffness to that of the substrate and demonstrate the potential of this setup for low-force probing of modifications to cell mechanics induced by the surrounding environment (e.g. extracellular matrix or other cells).


Cell-Scaffold Adhesion Dynamics Measured in First Seconds Predicts Cell Growth on Days Scale – Optical Tweezers Study

Rok Podlipec and Janez Štrancar

Understanding the cell–biomaterial interface from the very first contact is of crucial importance for their successful implementation and function in damaged tissues. However, the lack of bio- and mechano-analytical methods to investigate and probe the initial processes on the interface, especially in 3D, raises the need for applying new experimental techniques. In our study, optical tweezers combined with confocal fluorescence microscopy were optimized to investigate the initial cell–scaffold contact and to investigate its correlation with the material-dependent cell growth. By the optical tweezers-induced cell manipulation accompanied by force detection up to 100 pN and position detection by fluorescence microscopy, accurate adhesion dynamics and strength analysis was implemented, where several attachment sites were formed on the interface in the first few seconds. More importantly, we have shown that dynamics of cell adhesion on scaffold surfaces correlates with cell growth on the days scale, which indicates that the first seconds of the contact could markedly direct further cell response. Such a contact dynamics analysis on 3D scaffold surfaces, applied for the first time, can thus serve to predict scaffold biocompatibility.


In vivo X-ray elemental imaging of single cell model organisms manipulated by laser-based optical tweezers

Eva Vergucht, Toon Brans, Filip Beunis, Jan Garrevoet, Maarten De Rijcke, Stephen Bauters, David Deruytter, Michiel Vandegehuchte, Ine Van Nieuwenhove, Colin Janssen, Manfred Burghammer & Laszlo Vincze

We report on a radically new elemental imaging approach for the analysis of biological model organisms and single cells in their natural, in vivo state. The methodology combines optical tweezers (OT) technology for non-contact, laser-based sample manipulation with synchrotron radiation confocal X-ray fluorescence (XRF) microimaging for the first time. The main objective of this work is to establish a new method for in vivo elemental imaging in a two-dimensional (2D) projection mode in free-standing biological microorganisms or single cells, present in their aqueous environment. Using the model organism Scrippsiella trochoidea, a first proof of principle experiment at beamline ID13 of the European Synchrotron Radiation Facility (ESRF) demonstrates the feasibility of the OT XRF methodology, which is applied to study mixture toxicity of Cu-Ni and Cu-Zn as a result of elevated exposure. We expect that the new OT XRF methodology will significantly contribute to the new trend of investigating microorganisms at the cellular level with added in vivo capability.


Friday, March 20, 2015

Microfluidics-based single cell analysis reveals drug-dependent motility changes in trypanosomes

Axel Hochstetter, Eric Stellamanns, Siddharth Deshpande, Sravanti Uppaluri, Markus Engstler and Thomas Pfohl

We present a single cell viability assay, based on chemical gradient microfluidics in combination with optical micromanipulation. Here, we used this combination to in situ monitor the effects of drugs and chemicals on the motility of the flagellated unicellular parasite Trypanosoma brucei; specifically, the local cell velocity and the mean squared displacement (MSD) of the cell trajectories. With our method, we are able to record in situ cell fixation by glutaraldehyde, and to quantify the critical concentration of 2-deoxy-D-glucose required to completely paralyze trypanosomes. In addition, we detected and quantified the impact on cell propulsion and energy generation at much lower 2-deoxy-D-glucose concentrations. Our microfluidics-based approach advances fast cell-based drug testing in a way that allows us to distinguish cytocidal from cytostatic drug effects, screen effective dosages, and investigate the impact on cell motility of drugs and chemicals. Using suramin, we could reveal the impact of the widely used drug on trypanosomes: suramin lowers trypanosome motility and induces cell-lysis after endocytosis.


Singlet-Oxygen Generation from Individual Semiconducting and Metallic Nanostructures During Near-Infrared Laser Trapping

Bennett E. Smith , Paden B Roder , Jennifer L Hanson , Sandeep Manandhar , Arun Devaraj , Daniel Perea , Woo-Joong Kim , A. L. David Kilcoyne , and Peter J. Pauzauskie

Photodynamic therapy has been used for several decades in the treatment of solid tumors through the optical generation of chemically reactive singlet-oxygen molecules (1O2). Recently, nanoscale metallic and semiconducting materials have been reported to act as photosensitizing agents with additional diagnostic and therapeutic functionality. To date there have been no reports of observing the generation of singlet-oxygen at the level of single nanostructures, particularly at near-infrared (NIR) wavelengths. Here we demonstrate that NIR laser-tweezers can be used to observe the formation of singlet-oxygen produced from individual silicon and gold nanowires via use of a commercially available reporting dye. The laser trap also induces 2-photon photoexcitation of the dye following a chemical reaction with singlet oxygen. Corresponding 2-photon emission spectra confirms the generation of singlet oxygen from individual silicon nanowires at room temperature (30°C), suggesting a range of applications for investigating semiconducting and metallic nanoscale materials for solid tumor photoablation.


Measurement of small light absorption in microparticles by means of optically induced rotation

O. V. Angelsky, A. Ya. Bekshaev, P. P. Maksimyak, A. P. Maksimyak, and S. G. Hanson

The absorption parameters of micro-particles have been associated with the induced spin exerted upon the particle, when embedded in a circularly polarized coherent field. The induced rotational speed is theoretically analyzed, showing the influence of the beam parameters, the parameters of the particle and the tribological parameters of the surrounding fluid. The theoretical findings have been adequately confirmed in experiments.


Complex rotational dynamics of multiple spheroidal particles in a circularly polarized, dual beam trap

Oto Brzobohatý, Alejandro V. Arzola, Martin Šiler, Lukáš Chvátal, Petr Jákl, Stephen Simpson, and Pavel Zemánek

We examine the rotational dynamics of spheroidal particles in an optical trap comprising counter-propagating Gaussian beams of opposing helicity. Isolated spheroids undergo continuous rotation with frequencies determined by their size and aspect ratio, whilst pairs of spheroids display phase locking behaviour. The introduction of additional particles leads to yet more complex behaviour. Experimental results are supported by numerical calculations.


Thursday, March 19, 2015

Optical trapping in secondary maxima of focused laser beam

Martin Šiler, Pavel Zemánek

Single beam optical tweezers hold particles behind the focal plane due to the high gradients of optical intensity present in a focused laser beam. However, description of this optical field based on a vectorial theory of diffraction reveals that the high intensity focal area is accompanied by several secondary maxima on the optical axis as well as by a structure of rings away of the optical axis. Such a structure can be found in beams exhibiting spherical aberrations as well as in beams where aberration is corrected. Here, we discuss possibility to use these secondary maxima of aberration-corrected beams as the optical traps. We present the properties of such traps created by objective lenses of various numerical apertures that are focusing plane waves.


Signatures of material and optical chirality: Origins and measures

David S. Bradshaw, Jamie M. Leeder, Matt M. Coles, David L. Andrews

Chirality in materials and light is of abiding interest across a broad range of scientific disciplines. This article discusses present and emerging issues in relation to molecular and optical chirality, also including some important developments in chiral metamaterials. Quantifying the chirality of matter or light leads to issues concerning the most appropriate measures, such as a helicity parameter for specific chiral chromophores and technical measures of light chirality. An optical helicity and chirality density depend on a difference between the numbers of left- and right-handed photons in a beam. In connection with circularly polarised luminescence, adoption of the Stokes parameter to spontaneous emission from chiral molecules invites critical attention. Modern spectroscopic techniques are often based on the different response arising from left-handed circularly polarised light compared to right-handed light. This dissimilarity can be exploited as a foundation for the separation of chiral molecules, promising new avenues of application.


Optical Forces on Silver Homogeneous Nanotubes: Study of Shell Plasmonic Interaction

R. M. Abraham Ekeroth, M. F. Lester

In previous works (Abraham et al., Plasmonics 6(3):435–444 (2011); Abraham Ekeroth and Lester, Plasmonics 7(4):579–587, (2012); Abraham Ekeroth and Lester, Plasmonics 8:1417–1428 (2013)), we performed an exhaustive study about optical properties of metallic realistic nanotubes, hollow or with dielectric cores. Based on rigorous calculations, involving experimental-interpolated dielectric functions, we pointed out the importance of using an adequate size-corrected dielectric function in homogeneous bidimensional metallic shells when their thicknesses are about several nanometers. In this paper, we compute optical forces induced by electromagnetic plane waves on these kind of nanostructures. We focus the study under p polarisation, in order to observe plasmonic-related behaviour. The optical forces are calculated by the Maxwell’s stress tensor without any kind of approximation. We show three examples of mechanical effects on silver thin shells. The characteristics of the electromagnetic interaction in these structures, from the point of view of forces, allow us to comprehend the problem of the plasmonic interaction in the shell in a new way. We show numerically, for the first time, the nature of bonding/antibonding of surface plasmons in nanotubes made of realistic materials, in a way independent of approximations related to scale. The behaviour of the realistic silver shells is compared with the features deduced from the plasmon hybridization model, which are predicted from a quasi-static approximation of electromagnetic response. Our results conceive a full retarded problem and can only contain numerical errors. In addition, we compare rigorous calculations for the optical forces with those ones obtained from the far field approach, when specified for shell geometry.


Optical Injection of Gold Nanoparticles into Living Cells

Miao Li, Theobald Lohmüller, and Jochen Feldmann

The controlled injection of nanoscopic objects into living cells with light offers promising prospects for the development of novel molecular delivery strategies or intracellular biosensor applications. Here, we show that single gold nanoparticles from solution can be patterned on the surface of living cells with a continuous wave laser beam. In a second step, we demonstrate how the same particles can then be injected into the cells through a combination of plasmonic heating and optical force. We find that short exposure times are sufficient to perforate the cell membrane and inject the particles into cells with a survival rate of >70%.


Wednesday, March 18, 2015

Microrheology with Optical Tweezers: Measuring the relative viscosity of solutions ‘at a glance’

Manlio Tassieri, Francesco Del Giudice, Emma J. Robertson, Neena Jain, Bettina Fries, Rab Wilson, Andrew Glidle, Francesco Greco, Paolo Antonio Netti, Pier Luca Maffettone, Tihana Bicanic & Jonathan M. Cooper

We present a straightforward method for measuring the relative viscosity of fluids via a simple graphical analysis of the normalised position autocorrelation function of an optically trapped bead, without the need of embarking on laborious calculations. The advantages of the proposed microrheology method are evident when it is adopted for measurements of materials whose availability is limited, such as those involved in biological studies. The method has been validated by direct comparison with conventional bulk rheology methods, and has been applied both to characterise synthetic linear polyelectrolytes solutions and to study biomedical samples.


Optical trapping assembling of clusters and nanoparticles in solution by CW and femtosecond lasers

Hiroshi Masuhara, Teruki Sugiyama, Ken-ichi Yuyama, Anwar Usman

Laser trapping of molecular systems in solution is classified into three cases: JUST TRAPPING, EXTENDED TRAPPING, and NUCLEATION and GROWTH. The nucleation in amino acid solutions depends on where the 1064-nm CW trapping laser is focused, and crystallization and liquid–liquid phase separation are induced by laser trapping at the solution/air surface and the solution/glass interface, respectively. Laser trapping crystallization is achieved even in unsaturated solution, on which unique controls of crystallization are made possible. Crystal size is arbitrarily controlled by tuning laser power for a plate-like anhydrous crystal of l-phenylalanine. The α- or γ-crystal polymorph of glycine is selectively prepared by changing laser power and polarization. Further efficient trapping of nanoparticles and their following ejection induced by femtosecond laser pulses are introduced as unique trapping phenomena and finally future perspective is presented.


Forces due to pulsed beams in optical tweezers: linear effects

Nathaniel du Preez-Wilkinson, Alexander B. Stilgoe, Thuraya Alzaidi, Halina Rubinsztein-Dunlop, and Timo A. Nieminen

We present a method for the precise calculation of optical forces due to a tightly-focused pulsed laser beam using generalized Lorenz–Mie theory or the T-matrix method. This method can be used to obtain the fields as a function of position and time, allowing the approximate calculation of weak non-linear effects, and provides a reference calculation for validation of calculations including non-linear effects. We calculate forces for femtosecond pulses of various widths, and compare with forces due to a continuous wave (CW) beam. The forces are similar enough so that the continuous beam case provides a useful approximation for the pulsed case, with trap parameters such as the radial spring constant usually differing by less than 1% for pulses of 100 fs or longer. For large high-index (e.g., polystyrene, with n = 1.59) particles, the difference can be as large as 3% for 100 fs pulses, and up to 8% for 25 fs pulses. A weighted average of CW forces for individual spectral components of the pulsed beam provides a simple improved approximation, which we use to illustrate the physical principles responsible for the differences between pulsed and CW beams.


Identification of individual biofilm-forming bacterial cells using Raman tweezers

Ota Samek ; Silvie Bernatová ; Jan Ježek ; Martin Šiler ; Mojmir Šerý ; Vladislav Krzyžánek ; Kamila Hrubanová ; Pavel Zemánek ; Veronika Holá ; Filip Růžička

A method for in vitro identification of individual bacterial cells is presented. The method is based on a combination of optical tweezers for spatial trapping of individual bacterial cells and Raman microspectroscopy for acquisition of spectral “Raman fingerprints” obtained from the trapped cell. Here, Raman spectra were taken from the biofilm-forming cells without the influence of an extracellular matrix and were compared with biofilm-negative cells. Results of principal component analyses of Raman spectra enabled us to distinguish between the two strains of Staphylococcus epidermidis. Thus, we propose that Raman tweezers can become the technique of choice for a clearer understanding of the processes involved in bacterial biofilms which constitute a highly privileged way of life for bacteria, protected from the external environment.


Tuesday, March 17, 2015

An in-vacuo optical levitation trap for high-intensity laser interaction experiments with isolated microtargets

C. J. Price, T. D. Donnelly, S. Giltrap, N. H. Stuart, S. Parker, S. Patankar, H. F. Lowe, D. Drew, E. T. Gumbrell and R. A. Smith

We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments. The system uses a focused, vertically propagating continuous wave laser beam to capture and manipulate micro-targets by photon momentum transfer at much longer working distances than commonly used by optical tweezer systems. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution. Optical trapping of 10 ± 4 μm oil droplets in vacuum was demonstrated, over timescales of >1 h at extended distances of ∼40 mm from the final focusing optic. The stability of the levitated droplet was such that it would stay in alignment with a ∼7 μm irradiating beam focal spot for up to 5 min without the need for re-adjustment. The performance of the trap was assessed in a series of high-intensity (1017 W cm−2) laser experiments that measured the X-ray source size and inferred free-electron temperature of a single isolated droplet target, along with a measurement of the emitted radio-frequency pulse. These initial tests demonstrated the use of optically levitated microdroplets as a robust target platform for further high-intensity laser interaction and point source studies.


Temporal response of three-dimensional biological cells to high-frequency optical jumping tweezers

Lingyao Yu; Yunlong Sheng

We analyzed the temporal responses of biological cells in the jumping optical tweezers for tugging, wiggling, and stretching the cells in the time-sharing regime with the finite-element method. We showed that the jumping of local stress and local strain is independently omnipresent on the recovery time of the viscoelastic material and the jumping frequency of the load. We demonstrated that the elongation of a three-dimensional (3-D) viscoelastic object under a jumping load cannot be evaluated using the one-dimensional spring-dashpot material model without considering its 3-D structure.


Transition-Path Probability as a Test of Reaction-Coordinate Quality Reveals DNA Hairpin Folding Is a One-Dimensional Diffusive Process

Krishna Neupane, Ajay P. Manuel, John Lambert, and Michael T. Woodside

Chemical reactions are typically described in terms of progress along a reaction coordinate. However, the quality of reaction coordinates for describing reaction dynamics is seldom tested experimentally. We applied a framework for gauging reaction-coordinate quality based on transition-path analysis to experimental data for the first time, looking at folding trajectories of single DNA hairpin molecules measured under tension applied by optical tweezers. The conditional probability for being on a reactive transition path was compared with the probability expected for ideal diffusion over a 1D energy landscape based on the committor function. Analyzing measurements and simulations of hairpin folding where end-to-end extension is the reaction coordinate, after accounting for instrumental effects on the analysis, we found good agreement between transition-path and committor analyses for model two-state hairpins, demonstrating that folding is well-described by 1D diffusion. This work establishes transition-path analysis as a powerful new tool for testing experimental reaction-coordinate quality.


Torque Spectroscopy for the Study of Rotary Motion in Biological Systems

Jan Lipfert, Maarten M. van Oene, Mina Lee, Francesco Pedaci, and Nynke H. Dekker

To understand the mechanistic basis of cellular function, immense efforts are undertaken to investigate the many different molecules that constitute a cell, aiming to probe both individual molecules as well as their interactions with others. Our understanding of the molecular basis of, for example, genome processing (including transcription, translation, and replication), the cytoskeleton and its dynamics, membrane assembly and composition, and cellular motion has grown tremendously in recent decades. Underlying the dynamics of many of these interactions are highly specialized enzymatic processes that facilitate specific chemical reactions. When these reactions are coupled to mechanical motion, the enzymes that perform the mechanochemical couplings are referred to as molecular machines, because they transduce chemical energy into mechanical work. DNA and RNA polymerases and helicases, protein translocases, kinesins and myosins, etc., are well-known examples of such molecular machines. Many such machines employ forces to execute linear motion, but it is also possible for a molecular machine to generate torques and to execute rotary motion. Indeed, in processes as distinct as bacterial swimming and the copying of DNA during replication, rotational motion and accompanying torques play key roles.


Knot theory realizations in nematic colloids

Simon Čopara, Uroš Tkale, Igor Muševič, and Slobodan Žumer
Nematic braids are reconfigurable knots and links formed by the disclination loops that entangle colloidal particles dispersed in a nematic liquid crystal. We focus on entangled nematic disclinations in thin twisted nematic layers stabilized by 2D arrays of colloidal particles that can be controlled with laser tweezers. We take the experimentally assembled structures and demonstrate the correspondence of the knot invariants, constructed graphs, and surfaces associated with the disclination loop to the physically observable features specific to the geometry at hand. The nematic nature of the medium adds additional topological parameters to the conventional results of knot theory, which couple with the knot topology and introduce order into the phase diagram of possible structures. The crystalline order allows the simplified construction of the Jones polynomial and medial graphs, and the steps in the construction algorithm are mirrored in the physics of liquid crystals.


Monday, March 16, 2015

Generating stable tractor beams with dielectric metasurfaces

Carl Pfeiffer and Anthony Grbic

Propagation-invariant beams that pull objects towards a light source are commonly known as tractor beams. Here, an efficient, linearly polarized tractor beam with improved stability is introduced. The beam consists of a superposition of transverse-electric and transverse-magnetic polarized Bessel beams of orders m=+1 and m=−1. It is shown that this beam can stably pull a wide range of dielectric microparticles arbitrarily long distances, independent of ambient conditions. Next, a straightforward method of generating these high-performance beams is proposed. A Si metasurface transforms an incident linearly polarized Gaussian beam into the desired tractor beam. Full-wave simulations demonstrate that it is possible for this simple geometry to pull a polystyrene sphere a distance equal to the nondiffracting range of the Bessel beam. The simplicity of the setup and the robust performance of the proposed tractor beam significantly enhance the ability to manipulate matter with light.


Diffusible Crosslinkers Generate Directed Forces in Microtubule Networks

Zdenek Lansky, Marcus Braun, Annemarie Lüdecke, Michael Schlierf, Pieter Rein ten Wolde, Marcel E. Janson, Stefan Diez

Cytoskeletal remodeling is essential to eukaryotic cell division and morphogenesis. The mechanical forces driving the restructuring are attributed to the action of molecular motors and the dynamics of cytoskeletal filaments, which both consume chemical energy. By contrast, non-enzymatic filament crosslinkers are regarded as mere friction-generating entities. Here, we experimentally demonstrate that diffusible microtubule crosslinkers of the Ase1/PRC1/Map65 family generate directed microtubule sliding when confined between partially overlapping microtubules. The Ase1-generated forces, directly measured by optical tweezers to be in the piconewton-range, were sufficient to antagonize motor-protein driven microtubule sliding. Force generation is quantitatively explained by the entropic expansion of confined Ase1 molecules diffusing within the microtubule overlaps. The thermal motion of crosslinkers is thus harnessed to generate mechanical work analogous to compressed gas propelling a piston in a cylinder. As confinement of diffusible proteins is ubiquitous in cells, the associated entropic forces are likely of importance for cellular mechanics beyond cytoskeletal networks.


In situ laser-imprinted surface realignment of a nematic liquid crystal

Giorgio Mirri, Miha Skarabot and Igor Musevic

We present a new method for the in-plane realignment of nematic liquid crystals in already fully assembled cells with uni-directionally rubbed polyimide as an aligning layer. We use nematic liquid crystals (NLCs) with a relatively high nematic-isotropic transition temperature and we focus the IR laser beam of the laser tweezers selectively onto one or the other of the inner interfaces. The heat generated by the IR absorption locally melts the liquid crystal and creates an isotropic island with well-defined molecular anchoring at the nematic-isotropic interface. By scanning the laser beam along a pre-defined line, the moving isotropic-nematic interface leaves behind a well oriented LC domain, with LC molecules aligned at 45° to the rubbing direction. If we in addition move the sample with respect to this scanning line, we are able to selectively realign micro-domains of the liquid crystal with respect to the original alignment induced by the PI rubbing. The realignment can be performed independently on each LC-glass interface, thereby producing predefined domains with customized and controllable alignment within an otherwise uniformly aligned cell.


Designing a Plasmonic Optophoresis System for Trapping and Simultaneous Sorting/Counting of Micro- and Nano-particles

Ghorbanzadeh, M.; Moravvej-Farshi, M.; Darbari, S.

We are proposing a plasmonic-based optophoresis system that can trap and simultaneously sort and count metallic and dielectric micro- and nano-particles, in a simple microfluidic system. The operating principles of the proposed system are based on the particles intrinsic properties that modulate the in-duced optical force and the transmitted power. Particle manipu-lations, in this system, are based on the near-field optical forces excreted by leaky surface plasmons modes, excited on a gold stripe. Simulations show that the maximum potential depth sensi-tivity to the trapped PS/Au particles’ radius is ~ 0.09/0.03 (kBT / nm). The maximum transmission sensitivity in response to a change in radii of trapped Au and PS spheres are both ~0.01% per nm. Moreover, it is also shown that a minute change of ±1% in a refractive index of a 250-nm trapped dielectric particle re-sults in ±0.26 kBT and ∓0.13% variations in the potential depth and transmission, respectively. Furthermore, the proposed sys-tem that can be implemented simply and inexpensively, benefits from its small footprint for integration into lab-on-a-chip devices and low power consumption, with promising potentials for bio-logical applications.


Thursday, March 12, 2015

Solid friction between soft filaments

Andrew Ward, Feodor Hilitski, Walter Schwenger, David Welch, A. W. C. Lau, Vincenzo Vitelli, L. Mahadevan & Zvonimir Dogic

Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments1, 2. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments’ overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. Our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.


Optical tweezers: a non-destructive tool for soft and biomaterial investigations

A. Magazzú, D. Spadaro, M. G. Donato, R. Sayed, E. Messina, C. D’Andrea, A. Foti, B. Fazio, M. A. Iatí, A. Irrera, R. Saija, P. G. Gucciardi, O. M. Maragó

Optical tweezers are a key technique for trapping and contactless manipulation of particles at the micro- and nanoscale that can exert and sense forces from hundreds of piconewton down to few femtonewton. In their simplest implementation, they are based on a single laser beam tightly focused to a high-intensity diffraction-limited spot. Here, after reviewing the general theoretical background on optical forces, we focus on their calibration and show a comparison between frequency and time domain methods. Then, we show novel measurements and calculations of optical forces on gold nanoparticles discussing their size scaling behavior. Finally, we describe recent applications of chiral optical trapping to soft materials, and integration of optical tweezers with Raman spectroscopy for ultra-sensitive spectroscopy of biomolecules in liquids.


Laser optical separation of chiral molecules

David S. Bradshaw and David L. Andrews

The optical trapping of molecules with an off-resonant laser beam involves a forward-Rayleigh scattering mechanism. It is shown that discriminatory effects arise on irradiating chiral molecules with circularly polarized light; the complete representation requires ensemble-weighted averaging to account for the influence of the trapping beam on the distribution of molecular orientations. Results of general application enable comparisons to be drawn between the results for two limits of the input laser intensity. It emerges that, in a racemic mixture, there is a differential driving force whose effect, at high laser intensities, is to produce differing local concentrations of the two enantiomers.


Nanomechanical force transducers for biomolecular and intracellular measurements: is there room to shrink and why do it?

Donald J Sirbuly, Raymond W Friddle, Joshua Villanueva and Qian Huang

Over the past couple of decades there has been a tremendous amount of progress on the development of ultrasensitive nanomechanical instruments, which has enabled scientists to peer for the first time into the mechanical world of biomolecular systems. Currently, work-horse instruments such as the atomic force microscope and optical/magnetic tweezers have provided the resolution necessary to extract quantitative force data from various molecular systems down to the femtonewton range, but it remains difficult to access the intracellular environment with these analytical tools as they have fairly large sizes and complicated feedback systems. This review is focused on highlighting some of the major milestones and discoveries in the field of biomolecular mechanics that have been made possible by the development of advanced atomic force microscope and tweezer techniques as well as on introducing emerging state-of-the-art nanomechanical force transducers that are addressing the size limitations presented by these standard tools. We will first briefly cover the basic setup and operation of these instruments, and then focus heavily on summarizing advances in in vitro force studies at both the molecular and cellular level. The last part of this review will include strategies for shrinking down the size of force transducers and provide insight into why this may be important for gaining a more complete understanding of cellular activity and function.


Monday, March 9, 2015

Optical transport, lifting and trapping of micro-particles by planar waveguides

Øystein Ivar Helle, Balpreet Singh Ahluwalia, and Olav Gaute Hellesø

Optical waveguides can be used to trap and transport micro-particles. The particles are held close to the waveguide surface by the evanescent field and propelled forward. We propose a new technique to lift and trap particles above the surface of the waveguides. This is made possible by a gap between two opposing, planar waveguides. The field emitted from each of the waveguide ends diverge fast, away from the substrate and into the cover-medium. By combining two fields propagating at an angle upwards and coming from opposite sides of a gap, particles can be stably lifted and trapped at the crossing of the two fields. Thus, particles are transported by waveguides leading to a gap, where they are lifted away from the substrate and trapped. The experiments are supported by numerical simulations of the forces on the micro-particles. Fluorescence imaging is used to track the particles in 3D with a precision of 50 nm.


A nanotweezer system for evanescent wave excited surface enhanced Raman spectroscopy (SERS) of single nanoparticles

Lingbo Kong, Changwon Lee, Christopher M. Earhart, Bernardo Cordovez, and James W. Chan

We experimentally demonstrate the integration of near-field optical tweezers with surface enhanced Raman scattering (SERS) spectroscopy by using the optical evanescent wave from a silicon nitride waveguide to trap single shell-isolated metallic nanoparticles (NPs) and simultaneously excite SERS signals of Raman reporter molecules adsorbed on the surface of the trapped metallic NPs. Both evanescent wave excited Stokes and anti-Stokes SERS spectra of waveguide trapped single silver (Ag) NPs were acquired, which were compared to their far-field SERS spectra. We investigated the trapping of bare and shell-isolated metallic NPs and determined that the addition of a shell to the metallic NPs minimized particle-induced laser damage to the waveguide, which allowed for the stable acquisition of the SERS spectra. This work realizes a new nanophotonic approach, which we refer to as near-field light scattering Raman (NLS-Raman), for simultaneous near-field optical trapping and SERS characterization of single metallic NPs.


Ångström-Precision Optical Traps and Applications

Thomas T. Perkins

Single-molecule optical-trapping experiments are now resolving the smallest units of motion in biology, including 1-base-pair steps along DNA. This review initially concentrates on the experimental problems with achieving 1-Å instrumental stability and the technical advances necessary to overcome these issues. Instrumental advances are complemented by insights in optical-trapping geometry and single-molecule motility assay development to accommodate the elasticity of biological molecules. I then discuss general issues in applying this measurement capability in the context of precision measurements along DNA. Such enhanced optical-trapping assays are revealing the fundamental step sizes of increasingly complex enzymes, as well as informative pauses in enzymatic motion. This information in turn is providing mechanistic insight into kinetic pathways that are difficult to probe by traditional assays. I conclude with a brief discussion of emerging techniques and future directions.


Reconstructing Folding Energy Landscapes by Single-Molecule Force Spectroscopy

Michael T. Woodside and Steven M. Block

Folding may be described conceptually in terms of trajectories over a landscape of free energies corresponding to different molecular configurations. In practice, energy landscapes can be difficult to measure. Single-molecule force spectroscopy (SMFS), whereby structural changes are monitored in molecules subjected to controlled forces, has emerged as a powerful tool for probing energy landscapes. We summarize methods for reconstructing landscapes from force spectroscopy measurements under both equilibrium and nonequilibrium conditions. Other complementary, but technically less demanding, methods provide a model-dependent characterization of key features of the landscape. Once reconstructed, energy landscapes can be used to study critical folding parameters, such as the characteristic transition times required for structural changes and the effective diffusion coefficient setting the timescale for motions over the landscape. We also discuss issues that complicate measurement and interpretation, including the possibility of multiple states or pathways and the effects of projecting multiple dimensions onto a single coordinate.


Saturday, March 7, 2015

Optical trapping of interfaces at ultra-low interfacial tension

Aletta Adriana Verhoeff, Francois Lavergne, Denis Bartolo, Dirk G. A. L. Aarts and Roel P. A. Dullens

We achieve active control of interfacial phenomena by optically trapping the interface using the gradient forces of a strongly focussed laser beam parallel to the interface. We illustrate our technique in a phase separated colloid-polymer mixture by distorting the interface in a very controlled way. The static structure of the manipulated interface as well as its dynamic relaxation behaviour are analysed. Both the statics and dynamics can be related to the capillary wave height-height correlations using the fluctuation dissipation theorem up to surprisingly large deformations of the interface. To underline the novelty and potential of our approach we also show multiple interface distortions and the controlled snap-off of liquid droplets.


Characterizing the interaction between DNA and GelRed fluorescent stain

F. A. P. Crisafuli, E. B. Ramos, M. S. Rocha

We have performed single-molecule stretching and dynamic light-scattering (DLS) experiments to characterize the interaction between the DNA molecule and the fluorescent stain GelRed. The results from single-molecule stretching show that the persistence length of DNA–GelRed complexes increases as the ligand concentration increases up to a critical concentration, then decreases for higher concentrations. The contour length of the complexes, on the other hand, increases monotonically as a function of GelRed concentration, suggesting that intercalation is the main binding mechanism. To characterize the physical chemistry of the interaction, we used the McGhee–von Hippel binding isotherm to extract physicochemical data for the interaction from the contour length data. Such analysis enabled us to conclude that the GelRed stain is, in fact, a bis-intercalator. In addition, DLS experiments were performed to study the changes of the effective size of the DNA–GelRed complexes, measured as the hydrodynamic radius, as a function of ligand concentration. We observed qualitative agreement between the results obtained from the two techniques by comparing the behavior of the hydrodynamics radius and the radius of gyration, because the latter quantity can be expressed as a function of mechanical properties determined from the stretching experiments.


Condensation transition and forced unravelling of DNA-histone H1 toroids: a multi-state free energy landscape

A H Mack, D J Schlingman, R D Salinas, L Regan and S G J Mochrie

DNA is known to condense with multivalent cations and positively charged proteins. However, the properties and energetics of DNA superstructures, such as chromatin, are poorly understood. As a model system, we investigate histone H1 condensation of DNA with tethered particle motion and force-extension measurements. We show that after the addition of H1 to DNA, a concentration dependent lag time is followed by the DNA spontaneously condensing. The trigger for this condensation phase transition can be modeled as sufficient H1s having bound to the DNA, providing insight into the 30 nm fiber condensation upon H1 binding. Furthermore, optical tweezers force-extension measurements of histone H1 condensed DNA reveals a sequence of state transitions corresponding to the unwinding of superhelical turns. We determine the complete, experimental, multi-state free energy landscape for the complex using Crooks fluctuation theorem. The measured force-versus-extension and free energy landscape are compared to predictions from a simple, theoretical model. This work encourages the theoretical description of DNA/protein structure and energetics and their role in chromatin and other, more complex, systems.


Dynamics of Membrane Tethers Reveal Novel Aspects of Cytoskeleton-Membrane Interactions in Axons

Anagha Datar, Thomas Bornschlögl, Patricia Bassereau, Jacques Prost, Pramod A. Pullarkat

Mechanical properties of cell membranes are known to be significantly influenced by the underlying cortical cytoskeleton. The technique of pulling membrane tethers from cells is one of the most effective ways of studying the membrane mechanics and the membrane-cortex interaction. In this article, we show that axon membranes make an interesting system to explore as they exhibit both free membrane-like behavior where the tether-membrane junction is movable on the surface of the axons (unlike many other cell membranes) as well as cell-like behavior where there are transient and spontaneous eruptions in the tether force that vanish when F-actin is depolymerized. We analyze the passive and spontaneous responses of axonal membrane tethers and propose theoretical models to explain the observed behavior.


Thursday, March 5, 2015

Local Heat Activation of Single Myosins Based on Optical Trapping of Gold Nanoparticles

Mitsuhiro Iwaki, Atsuko H. Iwane, Keigo Ikezaki, and Toshio Yanagida

Myosin is a mechano-enzyme that hydrolyzes ATP in order to move unidirectionally along actin filaments. Here we show by single molecule imaging that myosin V motion can be activated by local heat. We constructed a dark-field microscopy that included optical tweezers to monitor 80 nm gold nanoparticles (GNP) bound to single myosin V molecules with nanometer and submillisecond accuracy. We observed 34 nm processive steps along actin filaments like those seen when using 200 nm polystyrene beads (PB) but dwell times (ATPase activity) that were 4.5 times faster. Further, by using DNA nanotechnology (DNA origami) and myosin V as a nanometric thermometer, the temperature gradient surrounding optically trapped GNP could be estimated with nanometer accuracy. We propose our single molecule measurement system should advance quantitative analysis of the thermal control of biological and artificial systems like nanoscale thermal ratchet motors.


Plasmonically Enhanced Chiral Optical Fields and Forces in Achiral Split Ring Resonators

M. H. Alizadeh and Björn M. Reinhard

The two enantiomers (mirror images) of a biomolecule can show drastically different behaviors, requiring the development of sensitive approaches for their identification and separation. Plasmonic nanostructures have shown promise for enhancing the sensitivities of chiral spectroscopies, but the generation of chiral near fields with a specific handedness in the spatial domain surrounding the plasmonic structures remains a challenge. Here we demonstrate that achiral bianisotropic structures, which couple the electric and magnetic fields, can achieve high enhancements of optical chirality in an extended spatial region. Magneto-electric coupling in such structures facilitates electrically excited magnetic resonances in the near IR and optical regimes, which in turn can result in highly enhanced optical chirality in an extended region of space. We apply this concept to achiral double split ring resonators (DSRRs) and demonstrate their potential in generating enhanced chiral fields and forces. Also, the behavior of optical chirality density gradient and chirality flux in such structures is examined, and it is shown that plasmonically generated chiral forces may pave the way to a new class of chiral biosensors


Maxwell stress on a small dielectric sphere in a dielectric

Vitaly V. Datsyuk and Oleg R. Pavlyniuk

Electrically induced normal pressure and tangential stress at the surface of a small dielectric sphere (or cavity) in a dielectric are calculated using the Minkowski, Einstein-Laub, Abraham, and Lorentz forms of the Maxwell stress tensor. Only the Lorentz tensor is in agreement with the following observations: (1) A spherical cavity in a dielectric transforms into a sharp-edge plate perpendicular to the electric field; (2) a liquid drop placed in a medium with a slightly lower refractive index is stretched along the electric field; and (3) there is a torque on a small birefringent sphere. These phenomena cannot be explained by the conventional theory using the Minkowski stress tensor. For example, the Minkowski stress tensor predicts lateral compression of a spherical cavity in a dielectric.


Precision Assembly of Complex Cellular Microenvironments using Holographic Optical Tweezers

Glen R. Kirkham, Emily Britchford, Thomas Upton, James Ware, Graham M. Gibson, Yannick Devaud, Martin Ehrbar, Miles Padgett, Stephanie Allen, Lee D. Buttery & Kevin Shakesheff

The accurate study of cellular microenvironments is limited by the lack of technologies that can manipulate cells in 3D at a sufficiently small length scale. The ability to build and manipulate multicellular microscopic structures will facilitate a more detailed understanding of cellular function in fields such as developmental and stem cell biology. We present a holographic optical tweezers based technology to accurately generate bespoke cellular micro-architectures. Using embryonic stem cells, 3D structures of varying geometries were created and stabilized using hydrogels and cell-cell adhesion methods. Control of chemical microenvironments was achieved by the temporal release of specific factors from polymer microparticles positioned within these constructs. Complex co-culture micro-environmental analogues were also generated to reproduce structures found within adult stem cell niches. The application of holographic optical tweezers-based micromanipulation will enable novel insights into biological microenvironments by allowing researchers to form complex architectures with sub-micron precision of cells, matrices and molecules.


Wednesday, March 4, 2015

Force-induced remodelling of proteins and their complexes

Yun Chen, Sheena E Radford, David J Brockwell

Force can drive conformational changes in proteins, as well as modulate their stability and the affinity of their complexes, allowing a mechanical input to be converted into a biochemical output. These properties have been utilised by nature and force is now recognised to be widely used at the cellular level. The effects of force on the biophysical properties of biological systems can be large and varied. As these effects are only apparent in the presence of force, studies on the same proteins using traditional ensemble biophysical methods can yield apparently conflicting results. Where appropriate, therefore, force measurements should be integrated with other experimental approaches to understand the physiological context of the system under study.


Precise revolution control in three-dimensional off-axis trapping with single Laguerre-Gaussian beam

Tomoko Otsu, Taro Ando, Yu Takiguchi, Yoshiyuki Ohtake, Haruyoshi Toyoda and Hiroyasu Itoh

We report the precise study of orbital angular momentum transfer from Laguerre-Gaussian (LG) beams to submicrometer-sized dielectric spheres. Stable three-dimensional off-axis trapping of the spheres was achieved and confirmed by the constant behavior of the sphere’s revolution radius against the trapping LG-beam power. By measuring the revolutions of the sphere that was trapped and revolved in mid-water, we confirmed excellent linearity between the revolution rate and the trapping light free from the hydrodynamic coupling with walls of a sample chamber. This result suggests a torque source of precise controllability, which will contribute to the study of biological molecules, nonequilibrium statistical physics, and micromachines.


Binding of a pair of Au nanoparticles in a wide Gaussian standing wave

Lukáš Chvátal, Oto Brzobohatý, Pavel Zemánek

We present theoretical results related to the optical binding of two nanoparticles (NPs) in a standing wave created by a retro-reflected wide Gaussian beam. Recent experimental results demonstrated that this geometry enables easy confinement and spatial self-arrangement of NPs. Since the NPs are not usually of the same size, we investigate the influence of variations in NPs size on their stable spatial confinement in this type of optical trap.


Prediction of metallic nano-optical trapping forces by finite element-boundary integral method

Xiao-Min Pan, Kai-Jiang Xu, Ming-Lin Yang, and Xin-Qing Sheng

The hybrid of finite element and boundary integral (FE-BI) method is employed to predict nano-optical trapping forces of arbitrarily shaped metallic nanostructures. A preconditioning strategy is proposed to improve the convergence of the iterative solution. Skeletonization is employed to speed up the design and optimization where iteration has to be repeated for each beam configuration. The radiation pressure force (RPF) is computed by vector flux of the Maxwell’s stress tensor. Numerical simulations are performed to validate the developed method in analyzing the plasmonic effects as well as the optical trapping forces. It is shown that the proposed method is capable of predicting the trapping forces of complex metallic nanostructures accurately and efficiently.


Tuesday, March 3, 2015

Direct measurement of axial optical forces

Gregor Thalhammer, Lisa Obmascher, and Monika Ritsch-Marte

Direct measurement of optical forces based on recording the change of momentum between the in- and outgoing light does not have specific requirements on particle size or shape, or on beam shape. Thus this approach overcomes many of the limitations of force measurements based on position measurements, which require frequent calibration. In this work we validate the achievable accuracy for direct force measurements in the axial direction for a single beam optical tweezers setup, based on numerical simulations and experimental investigations of situations, where the true force is known. We find that for typical experimental situations a good accuracy with an error of less than 1 % of the maximum force can be achieved, independent of particle size or refractive index, provided that the total amount of light scattered in the backward direction is also taken into account, which is easy to accomplish experimentally. Due to the inherent particle shape independence of the direct force measurement method, these findings support that it provides accurate results for 3D force measurements for particles of arbitrary shape.


Effect of the object 3D shape on the viscoelastic testing in optical tweezers

Lingyao Yu and Yunlong Sheng

Viscoelastic testing of biological cells has been performed with the optical tweezers and stretcher. Historically, the cells were modeled by the spring-dashpot network or the power-law models, which can however characterize only the homogeneous, isotropic viscoelastic material, but not the 3D cell itself. Our mechanical and finite element analyses show that the cell elongations are different significantly for different cell 3D shapes in the creep testing. In the dynamic testing the loss tangent, which is measurable directly in the experiment, is not sensitive to the cell shape. However, the stress-strain hysteresis loop still depends on the cell 3D shape.


Plasmonic optical trapping of soft nanomaterials such as polymer chains and DNA: micro-patterning formation

Tatsuya Shoji, Yasuyuki Tsuboi

Localized surface plasmons exert a strong radiation force on nanoparticles in the vicinity of noble metal nanostructures, resulting in optical trapping. Such plasmon-based optical trapping is one of the hot topics in the field of nanophotonics and can be applied to molecular manipulation techniques. In this review paper, we describe the plasmon-based optical trapping of polymer chains and DNA. In addition, we describe the future outlook for this trapping method.


Molecular Weight Characterization of Globular Proteins using Optical Nanotweezers

Skyler Wheaton and Reuven Gordon

We trap a set of molecular weight standard globular proteins using a double nanohole optical trap. The root mean squared variation of the trapping laser transmission intensity gives a linear dependence with the molecular weight, showing the potential for analysis of globular proteins. The characteristic time of the autocorrelation of the trapping laser intensity variations scales with a -2/3 power dependence with the volume of the particle. A hydrodynamic laser tweezer model is used to explain these dependencies. Since this is a single particle technique that operates in solution and can be used to isolate an individual particle, we believe that it provides an interesting alternative to existing analysis methods and shows promise to expand the capabilities of protein related studies to the single particle level.


Monday, March 2, 2015

Red Blood Cell Aging During Storage, Studied Using Optical Tweezers Experiment

Justyna Czerwinska, Stefan Michael Wolf, Hanieh Mohammadi, Sylvia Jeney

This paper presents experimental and numerical studies of erythrocyte stretching, with a focus on the aging of red blood cells in an in vitro environment during storage. The experimental studies were performed using optical tweezers. The laser beam was used to pull and stretch a cell sedimented on a flat surface. A force calibration was obtained via a comparison of the experimental data with results from finite element simulations of the cell stretching. The experiments were performed using blood samples from blood bank donations made by three donors. The experiments were performed over 21 days of storage, and the estimate erythrocyte membrane shear modulus during this period increased from 2.5 to 13 μN/m.


A balance between membrane elasticity and polymerization energy sets the shape of spherical clathrin coats

Mohammed Saleem, Sandrine Morlot, Annika Hohendahl, John Manzi, Martin Lenz & Aurélien Roux

In endocytosis, scaffolding is one of the mechanisms to create membrane curvature by moulding the membrane into the spherical shape of the clathrin cage. However, the impact of membrane elastic parameters on the assembly and shape of clathrin lattices has never been experimentally evaluated. Here, we show that membrane tension opposes clathrin polymerization. We reconstitute clathrin budding in vitro with giant unilamellar vesicles (GUVs), purified adaptors and clathrin. By changing the osmotic conditions, we find that clathrin coats cause extensive budding of GUVs under low membrane tension while polymerizing into shallow pits under moderate tension. High tension fully inhibits polymerization. Theoretically, we predict the tension values for which transitions between different clathrin coat shapes occur. We measure the changes in membrane tension during clathrin polymerization, and use our theoretical framework to estimate the polymerization energy from these data. Our results show that membrane tension controls clathrin-mediated budding by varying the membrane budding energy.


Ribosome Excursions during mRNA Translocation Mediate Broad Branching of Frameshift Pathways

Shannon Yan, Jin-Der Wen, Carlos Bustamante, Ignacio Tinoco Jr.

Programmed ribosomal frameshifting produces alternative proteins from a single transcript. −1 frameshifting occurs on Escherichia coli’s dnaX mRNA containing a slippery sequence AAAAAAG and peripheral mRNA structural barriers. Here, we reveal hidden aspects of the frameshifting process, including its exact location on the mRNA and its timing within the translation cycle. Mass spectrometry of translated products shows that ribosomes enter the −1 frame from not one specific codon but various codons along the slippery sequence and slip by not just −1 but also −4 or +2 nucleotides. Single-ribosome translation trajectories detect distinctive codon-scale fluctuations in ribosome-mRNA displacement across the slippery sequence, representing multiple ribosomal translocation attempts during frameshifting. Flanking mRNA structural barriers mechanically stimulate the ribosome to undergo back-and-forth translocation excursions, broadly exploring reading frames. Both experiments reveal aborted translation around mutant slippery sequences, indicating that subsequent fidelity checks on newly adopted codon position base pairings lead to either resumed translation or early termination.


Kinesin-8 Motors Improve Nuclear Centering by Promoting Microtubule Catastrophe

Matko Glunčić, Nicola Maghelli, Alexander Krull, Vladimir Krstić, Damien Ramunno-Johnson, Nenad Pavin, and Iva M. Tolić

In fission yeast, microtubules push against the cell edge, thereby positioning the nucleus in the cell center. Kinesin-8 motors regulate microtubule catastrophe; however, their role in nuclear positioning is not known. Here we develop a physical model that describes how kinesin-8 motors affect nuclear centering by promoting a microtubule catastrophe. Our model predicts the improved centering of the nucleus in the presence of motors, which we confirmed experimentally in living cells. The model also predicts a characteristic time for the recentering of a displaced nucleus, which is supported by our experiments where we displaced the nucleus using optical tweezers.