Thursday, March 31, 2016

Human Embryonic Stem Cell Lines with Lesions in FOXP3 and NF1

Hui Zhu, Barry Behr, Vikrant V. Reddy, Mark Hughes, Yuqiong Pan, Julie Baker

Human embryonic stem cells (hESCs) are derived from the inner cell mass (ICM) of blastocyst staged embryos. Spare blastocyst staged embryos were obtained by in vitro fertilization (IVF) and donated for research purposes. hESCs carrying specific mutations can be used as a powerful cell system in modeling human genetic disorders. We obtained preimplantation genetic diagnosed (PGD) blastocyst staged embryos with genetic mutations that cause human disorders and derived hESCs from these embryos. We applied laser assisted micromanipulation to isolate the inner cell mass from the blastocysts and plated the ICM onto the mouse embryonic fibroblast cells. Two hESC lines with lesions in FOXP3 and NF1 were established. Both lines maintain a typical undifferentiated hESCs phenotype and present a normal karyotype. The two lines express a panel of pluripotency markers and have the potential to differentiate to the three germ layers in vitro and in vivo. The hESC lines with lesions in FOXP3 and NF1 are available for the scientific community and may serve as an important resource for research into these disease states.


Nanoscale devices for linkerless long-term single-molecule observation

Niccolò Banterle, Edward A Lemke

Total internal reflection fluorescence microscopy (TIRFM) can offer favorably high signal-to-noise observation of biological mechanisms. TIRFM can be used routinely to observe even single fluorescent molecules for a long duration (several seconds) at millisecond time resolution. However, to keep the investigated sample in the evanescent field, chemical surface immobilization techniques typically need to be implemented. In this review, we describe some of the recently developed novel nanodevices that overcome this limitation enabling long-term observation of free single molecules and outline their biological applications. The working concept of many devices is compatible with high-throughput strategies, which will further help to establish unbiased single molecule observation as a routine tool in biology to study the molecular underpinnings of even the most complex biological mechanisms.


Optical forces exerted on a graphene-coated dielectric particle by a focused Gaussian beam

Yang Yang, Zhe Shi, Jiafang Li, and Zhi-Yuan Li

In this paper, we derive the analytical expression for the multipole expansion coefficients of scattering and interior fields of a graphene-coated dielectric particle under the illumination of an arbitrary optical beam. By using this arbitrary beam theory, we systematically investigate the optical forces exerted on the graphene-coated particle by a focused Gaussian beam. Via tuning the chemical potential of the graphene, the optical force spectra could be modulated accordingly at resonant excitation. The hybridized whispering gallery mode of the electromagnetic field inside the graphene-coated polystyrene particle is more intensively localized than the pure polystyrene particle, which leads to a weakened morphology-dependent resonance in the optical forces. These investigations could open new perspectives for dynamic engineering of optical manipulations in optical tweezers applications.


Wednesday, March 30, 2016

Manipulation metallic nanoparticle at resonant wavelength using engineered azimuthally polarized optical field

Guanghao Rui, Xiaoyan Wang, Bing Gu, Qiwen Zhan, and Yiping Cui

In this work, we proposed a novel strategy to manipulate the behavior of the metallic nanoparticle under the resonant condition by using engineered azimuthally polarized optical field. Through optimizing the spatial phase distribution of the illumination, the optical force can be tailored to support stable optical trapping while avoiding trap destabilization caused by optical overheating effect simultaneously. Besides, the resonant particle can be stably trapped at predefined location in 3 dimensional space, or revolves around the beam axis with characteristics that can be holistically controlled in terms of both trajectory and rotation direction. The technique demonstrated in this work may open up new avenues for optical manipulation.


In vivo quantitative Raman-pH sensor of arterial blood based on laser trapping of erythrocytes

Man man Lin, Bin Xu, huilu yao, Aiguo Shen and Jiming Hu

We report on a continuous and non-invasive approach in vivo to monitor arterial blood pH based on laser trapping and Raman detection of single live erythrocytes. A home-built confocal laser tweezers Raman system (LTRS) is applied to trace the live erythrocytes under different pH values of extracellular environment to record their corresponding Raman changes in vitro and in vivo. The analysis results in vitro show that when the extracellular environment pH changes from 6.5 to 9.0, two Raman intensity ratio (R1603, 1616=I1603/I1616) of single erythrocytes decreases regularly, what is more, there has a good linear relationship between these two variables, and the linearity is 0.985, which is also verified successfully via in vivo Raman measurements. These results demonstrate that the Raman signal of single live erythrocytes is possible as a marker of extracellular pH value. This in vivo and quantitative Raman-pH sensor of arterial blood will be an important candidate for monitoring the acid-base status during treatment of ill patients and some major surgery because of its continuous and non-invasive characters.


In vivo acoustic and photoacoustic focusing of circulating cells

Ekaterina I. Galanzha, Mark G. Viegas, Taras I. Malinsky, Alexander V. Melerzanov, Mazen A. Juratli, Mustafa Sarimollaoglu, Dmitry A. Nedosekin & Vladimir P. Zharov

In vivo flow cytometry using vessels as natural tubes with native cell flows has revolutionized the study of rare circulating tumor cells in a complex blood background. However, the presence of many blood cells in the detection volume makes it difficult to count each cell in this volume. We introduce method for manipulation of circulating cells in vivo with the use of gradient acoustic forces induced by ultrasound and photoacoustic waves. In a murine model, we demonstrated cell trapping, redirecting and focusing in blood and lymph flow into a tight stream, noninvasive wall-free transportation of blood, and the potential for photoacoustic detection of sickle cells without labeling and of leukocytes targeted by functionalized nanoparticles. Integration of cell focusing with intravital imaging methods may provide a versatile biological tool for single-cell analysis in circulation, with a focus on in vivo needleless blood tests, and preclinical studies of human diseases in animal models.


Visible Raman excitation laser induced power and exposure dependent effects in red blood cells

Sunita Ahlawat, Nitin Kumar, Abha Uppal and Pradeep Kumar Gupta

We present results of Raman spectroscopic studies carried out on optically trapped red blood cells with Raman excitation wavelength in Q-band region of the hemoglobin (Hb) absorption spectrum. The results obtained suggest that when exposed to the Raman excitation laser the RBCs get deoxygenated due to photo-dissociation of oxygen from hemoglobin. For smaller exposure durations (5 s) the level of deoxygenation increases with an increase in power. However, for longer exposure durations the deoxygenated hemoglobin in the cells gets irreversibly oxidized to form a low spin ferric derivative of hemoglobin. The rate of oxidation depends upon the initial level of deoxygenation; higher the initial level of deoxygenation, higher is the rate of oxidation. However, the RBCs deoxygenated via oxygen deprivation (i.e. N2 purging) were found to be very stable against any laser induced effect. These observations suggests that in case of laser induced deoxygenation of RBCs the free oxygen generated by photo-dissociation acts as the oxidizing agent and leads to oxidative damage of the RBCs.


Multiplexed single-molecule force spectroscopy using a centrifuge

Darren Yang, Andrew Ward, Ken Halvorsen & Wesley P. Wong
We present a miniature centrifuge force microscope (CFM) that repurposes a benchtop centrifuge for high-throughput single-molecule experiments with high-resolution particle tracking, a large force range, temperature control and simple push-button operation. Incorporating DNA nanoswitches to enable repeated interrogation by force of single molecular pairs, we demonstrate increased throughput, reliability and the ability to characterize population heterogeneity. We perform spatiotemporally multiplexed experiments to collect 1,863 bond rupture statistics from 538 traceable molecular pairs in a single experiment, and show that 2 populations of DNA zippers can be distinguished using per-molecule statistics to reduce noise.


Tuesday, March 29, 2016

Photoacoustics of single laser-trapped nanodroplets for the direct observation of nanofocusing in aerosol photokinetics

Johannes W. Cremer, Klemens M. Thaler, Christoph Haisch & Ruth Signorell

Photochemistry taking place in atmospheric aerosol droplets has a significant impact on the Earth’s climate. Nanofocusing of electromagnetic radiation inside aerosols plays a crucial role in their absorption behaviour, since the radiation flux inside the droplet strongly affects the activation rate of photochemically active species. However, size-dependent nanofocusing effects in the photokinetics of small aerosols have escaped direct observation due to the inability to measure absorption signatures from single droplets. Here we show that photoacoustic measurements on optically trapped single nanodroplets provide a direct, broadly applicable method to measure absorption with attolitre sensitivity. We demonstrate for a model aerosol that the photolysis is accelerated by an order of magnitude in the sub-micron to micron size range, compared with larger droplets. The versatility of our technique promises broad applicability to absorption studies of aerosol particles, such as atmospheric aerosols where quantitative photokinetic data are critical for climate predictions.


Using Optical Tweezers to Characterize Physical Tethers at Membrane Contact Sites: Grab It, Pull It, Set It Free?

Imogen Sparkes

Compartmentalisation is a defining feature of eukaryotic life. Effective communication between organelles is essential for cell maintenance, growth and response to external stimuli. Static snapshots provided through ultrastructural studies of preserved tissue highlight that certain organelles are in intimate contact at membrane contact sites (MCSs), also referred to as inter-organellar tethering sites. However, live cell imaging indicates that these interactions are not necessarily stable with organelles frequently “colliding,” moving in unison and then separating. This dramatic intracellular “waltz” between organelles with ever changing partners (organelles) indicates that the molecular factors controlling MCSs are highly regulated. Key questions therefore relate to defining which organelles physically interact, deciphering the molecular components that control MCS formation, and ultimately deciphering the specific functional role that the interaction provides to the cell (Figure 1).


Assessment of red blood cell deformability in type 2 diabetes mellitus and diabetic retinopathy by dual optical tweezers stretching technique

Rupesh Agrawal, Thomas Smart, João Nobre-Cardoso, Christopher Richards, Rhythm Bhatnagar, Adnan Tufail, David Shima, Phil H. Jones & Carlos Pavesio

A pilot cross sectional study was conducted to investigate the role of red blood cells (RBC) deformability in type 2 diabetes mellitus (T2DM) without and with diabetic retinopathy (DR) using a dual optical tweezers stretching technique. A dual optical tweezers was made by splitting and recombining a single Nd:YAG laser beam. RBCs were trapped directly (i.e., without microbead handles) in the dual optical tweezers where they were observed to adopt a “side-on” orientation. RBC initial and final lengths after stretching were measured by digital video microscopy, and a Deformability index (DI) calculated. Blood from 8 healthy controls, 5 T2DM and 7 DR patients with respective mean age of 52.4yrs, 51.6 yrs and 52 yrs was analysed. Initial average length of RBCs for control group was 8.45 ± 0.25 μm, 8.68 ± 0.49 μm for DM RBCs and 8.82 ± 0.32 μm for DR RBCs (p < 0.001). The DI for control group was 0.0698 ± 0.0224, and that for DM RBCs was 0.0645 ± 0.03 and 0.0635 ± 0.028 (p < 0.001) for DR group. DI was inversely related to basal length of RBCs (p = 0.02). DI of RBC from DM and DR patients was significantly lower in comparison with normal healthy controls. A dual optical tweezers method can hence be reliably used to assess RBC deformability.


Quantum model of cooling and force sensing with an optically trapped nanoparticle

B. Rodenburg, L. P. Neukirch, A. N. Vamivakas, and M. Bhattacharya

Optically trapped nanoparticles have recently emerged as exciting candidates for tests of quantum mechanics at the macroscale and as versatile platforms for ultrasensitive metrology. Recent experiments have demonstrated parametric feedback cooling, nonequilibrium physics, and temperature detection, all in the classical regime. Here we provide the first quantum model for trapped nanoparticle cooling and force sensing. In contrast to existing theories, our work indicates that the nanomechanical ground state may be prepared without using an optical resonator; that the cooling mechanism corresponds to nonlinear friction; and that the energy loss during cooling is nonexponential in time. Our results show excellent agreement with experimental data in the classical limit, and constitute an underlying theoretical framework for experiments aiming at ground state preparation. Our theory also addresses the optimization of, and the fundamental quantum limit to, force sensing, thus providing theoretical direction to ongoing searches for ultraweak forces using levitated nanoparticles.


Acoustic force mapping in a hybrid acoustic-optical micromanipulation device supporting high resolution optical imaging

Gregor Thalhammer, Craig McDougall, Mike P MacDonald and Monika Ritsch-Marte

Many applications in the life-sciences demand non-contact manipulation tools for forceful but nevertheless delicate handling of various types of sample. Moreover, the system should support high-resolution optical imaging. Here we present a hybrid acoustic/optical manipulation system which utilizes a transparent transducer, making it compatible with high-NA imaging in a microfluidic environment. The powerful acoustic trapping within a layered resonator, which is suitable for highly parallel particle handling, is complemented by the flexibility and selectivity of holographic optical tweezers, with the specimens being under high quality optical monitoring at all times. The dual acoustic/optical nature of the system lends itself to optically measure the exact acoustic force map, by means of direct force measurements on an optically trapped particle. For applications with (ultra-)high demand on the precision of the force measurements, the position of the objective used for the high-NA imaging may have significant influence on the acoustic force map in the probe chamber. We have characterized this influence experimentally and the findings were confirmed by model simulations. We show that it is possible to design the chamber and to choose the operating point in such a way as to avoid perturbations due to the objective lens. Moreover, we found that measuring the electrical impedance of the transducer provides an easy indicator for the acoustic resonances.


Wednesday, March 23, 2016

5-mg suspended mirror driven by measurement-induced backaction

Nobuyuki Matsumoto, Kentaro Komori, Yuta Michimura, Gen Hayase, Yoichi Aso, and Kimio Tsubono

Quantum mechanics predicts superpositions of position states even for macroscopic objects. Recently, the use of a quasifreely suspended mirror combined with a laser was proposed to prepare such states [H. Müller-Ebhardt et al., Phys. Rev. Lett. 100, 013601 (2008)]. One of the key milestones towards this goal is the preparation of the mechanical oscillator mainly driven by measurement-induced backaction in the quantum regime. Here we describe the observation of backaction acting on a suspended 5-mg mirror in the classical regime. Furthermore, its quantum component is estimated to be larger than the thermal fluctuating force due to internal damping of the suspension, by a factor of 1.4±0.2 at 325 Hz.


Combined optical micromanipulation and interferometric topography (COMMIT)

Mohammad Sarshar, Thompson Lu, and Bahman Anvari

Optical tweezers have emerged as a prominent light-based tool for pico-Newton (pN) force microscopy in mechanobiological studies. However, the efficacy of optical tweezers are limited in applications where concurrent metrology of the nano-sized structures under interrogation is essential to the quantitative analysis of its mechanical properties and various mechanotransduction events. We have developed an all-optical platform delivering pN force resolution in parallel with nano-scale structural imaging of the biological sample by combining optical tweezers with interferometric quantitative phase microscopy. These capabilities allow real-time micromanipulation and label-free measurement of sample’s nanostructures and nanomechanical responses, opening avenues to a wide range of new research possibilities and applications in biology.


Investigation of Particle Harmonic Oscillation Using Four-Core Fiber Integrated Twin-Tweezers

H. Zhao; G. Farrell ; P. Wang ; L. Yuan

We present a numerical model for micro-scale particle trapping and transverse harmonic oscillation, for a twin-tweezers, based on the use of four-core fiber (FCF). The optical fiber tweezers is realized by appropriately shaping the fiber facet into a truncated diamond pyramid. The proposed FCF forms a dual optical tweezers, which are symmetrical with respect to the fiber's central axis. The trapping forces and the oscillation frequency of particle are calculated in a range of different motion processes. Simulated results show that the dual optical tweezers based on the FCF can be treated as a particle oscillator which could be used in the applications of fiber sensing, biomedicine, and other bio-related fields.


Optical trapping calculations for hollow metallic nanoparticles

Ebrahim Madadi

The special features of noble metals make them versatile and able to be used as handles in optical trapping. The trapping of various metal nanostructures has been investigated in the literature. In this paper, the optical trapping of hollow gold and silver nanoparticles is studied as a function of the trapping depth. It is shown that the trapping of hollow nanoparticles stiffens with the cavity size. It is shown that the trapping strength and the trapping efficiency enhance five-fold and two-fold, respectively.


Wednesday, March 16, 2016

Characterization of Individual Magnetic Nanoparticles in Solution by Double Nanohole Optical Tweezers

Haitian Xu, Steven Jones, Byoung-Chul Choi, and Reuven Gordon

We study individual superparamagnetic Fe3O4 (magnetite) nanoparticles in solution using a double nanohole optical tweezer with magnetic force setup. By analysis of the trapping optical transmission signal (step height, autocorrelation, the root mean square signal and the distribution with applied magnetic field), we are able to measure the refractive index, magnetic susceptibility, remanence and size of each trapped nanoparticle. The size distribution is found to agree well with scanning electron microscopy measurements, and the permeability, magnetic susceptibility and remanence values are all in agreement with published results. Our approach demonstrates the versatility of the optical tweezer with magnetic field setup to characterize nanoparticles in fluidic mixtures, with potential for isolation of desired particles and pick-and-place functionality.


Nanothermometry using optically trapped erbium oxide nanoparticle

Susil Baral, Samuel C. Johnson, Arwa A. Alaulamie, Hugh H. Richardson
A new optical probe technique using a laser-trapped erbium oxide nanoparticle (size ~150 nm) is introduced that can measure absolute temperature with a spatial resolution on the size of the trapped nanoparticle. This technique (scanning optical probe thermometry) is used to collect a thermal image of a gold nanodot prepared with hole-mask colloidal lithography. A convolution analysis of the thermal profile shows that the point spread function of our measurement is a Gaussian with a FWHM of 165 nm. We attribute the width of this function to clustering of Er2O3 nanoparticles in solution. The scanning optical probe thermometer is used to measure the temperature where vapor nucleation occurs in degassed water (555 K), confirming that a nanoscale object heated in water will superheat the surrounding water to the spinodal decomposition temperature. Subsequently, the temperature inside the vapor bubble rises to the melting point of the gold nanostructure (~1300) where a temperature plateau is observed. The rise in temperature is attributed to inhibition of thermal transfer to the surrounding liquid by the thermal insulating vapor cocoon.


Tuesday, March 15, 2016

The photon angular momentum controversy: Resolution of a conflict between laser optics and particle physics

Elliot Leader

The claim some years ago, contrary to all textbooks, that the angular momentum of a photon (and gluon) can be split in a gauge-invariant way into an orbital and spin term, sparked a major controversy in the Particle Physics community, exacerbated by the realization that many different forms of the angular momentum operators are, in principle, possible. A further cause of upset was the realization that the gluon polarization in a nucleon, a supposedly physically meaningful quantity, corresponds only to the gauge-variant gluon spin derived from Noether's theorem, evaluated in a particular gauge. On the contrary, Laser Physicists have, for decades, been happily measuring physical quantities which correspond to photon orbital and spin angular momentum evaluated in a particular gauge. This paper reconciles the two points of view, and shows that it is the gauge invariant version of the canonical angular momentum which agrees with the results of a host of laser optics experiments.


Transition from a Linear to a Harmonic Potential in Collective Dynamics of a Multifilament Actin Bundle

Jörg Schnauß, Tom Golde, Carsten Schuldt, B. U. Sebastian Schmidt, Martin Glaser, Dan Strehle, Tina Händler, Claus Heussinger, and Josef A. Käs

Attractive depletion forces between rodlike particles in highly crowded environments have been shown through recent modeling and experimental approaches to induce different structural and dynamic signatures depending on relative orientation between rods. For example, it has been demonstrated that the axial attraction between two parallel rods yields a linear energy potential corresponding to a constant contractile force of 0.1 pN. Here, we extend pairwise, depletion-induced interactions to a multifilament level with actin bundles, and find contractile forces up to 3 pN. Forces generated due to bundle relaxation were not constant, but displayed a harmonic potential and decayed exponentially with a mean decay time of 3.4 s. Through an analytical model, we explain these different fundamental dynamics as an emergent, collective phenomenon stemming from the additive, pairwise interactions of filaments within a bundle.


Protein folding trajectories can be described quantitatively by one-dimensional diffusion over measured energy landscapes

Krishna Neupane, Ajay P. Manuel & Michael T. Woodside

Protein folding features a diffusive search over a multidimensional energy landscape in conformational space for the minimum-energy structure1. Experiments, however, are usually interpreted in terms of a one-dimensional (1D) projection of the full landscape onto a practical reaction coordinate. Although simulations have shown that folding kinetics can be described well by diffusion over a 1D projection2, 3, 1D approximations have not yet been fully validated experimentally. We used folding trajectories of single molecules held under tension in optical tweezers to compare the conditional probability of being on a transition path4, calculated from the trajectory5, with the prediction for ideal 1D diffusion over the measured 1D landscape6, calculated from committor statistics7, 8. We found good agreement for the protein PrP (refs 9,10) and for one of the structural transitions in a leucine-zipper coiled-coil11, but not for a second transition in the coiled-coil, owing to poor reaction-coordinate quality12. These results show that 1D descriptions of folding can indeed be good, even for complex tertiary structures. More fundamentally, they also provide a fully experimental validation of the basic physical picture of folding as diffusion over a landscape.


Disorder-mediated crowd control in an active matter system

Erçağ Pinçe, Sabareesh K. P. Velu, Agnese Callegari, Parviz Elahi, Sylvain Gigan, Giovanni Volpe & Giorgio Volpe

Living active matter systems such as bacterial colonies, schools of fish and human crowds, display a wealth of emerging collective and dynamic behaviours as a result of far-from-equilibrium interactions. The dynamics of these systems are better understood and controlled considering their interaction with the environment, which for realistic systems is often highly heterogeneous and disordered. Here, we demonstrate that the presence of spatial disorder can alter the long-term dynamics in a colloidal active matter system, making it switch between gathering and dispersal of individuals. At equilibrium, colloidal particles always gather at the bottom of any attractive potential; however, under non-equilibrium driving forces in a bacterial bath, the colloids disperse if disorder is added to the potential. The depth of the local roughness in the environment regulates the transition between gathering and dispersal of individuals in the active matter system, thus inspiring novel routes for controlling emerging behaviours far from equilibrium.


Monday, March 14, 2016

Mechanical oscillations enhance gene delivery into suspended cells

Z. L. Zhou, X. X. Sun, J. Ma, C. H. Man, A. S. T. Wong, A. Y. Leung & A. H. W. Ngan

Suspended cells are difficult to be transfected by common biochemical methods which require cell attachment to a substrate. Mechanical oscillations of suspended cells at certain frequencies are found to result in significant increase in membrane permeability and potency for delivery of nano-particles and genetic materials into the cells. Nanomaterials including siRNAs are found to penetrate into suspended cells after subjecting to short-time mechanical oscillations, which would otherwise not affect the viability of the cells. Theoretical analysis indicates significant deformation of the actin-filament network in the cytoskeleton cortex during mechanical oscillations at the experimental frequency, which is likely to rupture the soft phospholipid bilayer leading to increased membrane permeability. The results here indicate a new method for enhancing cell transfection.


Optical tweezers study of red blood cell aggregation and disaggregation in plasma and protein solutions

Kisung Lee ; Matti Kinnunen ; Maria D. Khokhlova ; Evgeny V. Lyubin ; Alexander V. Priezzhev ; Igor Meglinski ; Andrey A. Fedyanin

Kinetics of optical tweezers (OT)-induced spontaneous aggregation and disaggregation of red blood cells (RBCs) were studied at the level of cell doublets to assess RBC interaction mechanics. Measurements were performed under in vitro conditions in plasma and fibrinogen and fibrinogen + albumin solutions. The RBC spontaneous aggregation kinetics was found to exhibit different behavior depending on the cell environment. In contrast, the RBC disaggregation kinetics was similar in all solutions qualitatively and quantitatively, demonstrating a significant contribution of the studied proteins to the process. The impact of the study on assessing RBC interaction mechanics and the protein contribution to the reversible RBC aggregation process is discussed.


Changes in radiation forces acting on a Rayleigh dielectric sphere by use of a wavefront-folding interferometer

Mengwen Guo and Daomu Zhao

We consider a class of fields generated by passing an isotropic Gaussian Schell-model beam through a wavefront-folding interferometer. The output field has various intensity profiles for different phase differences, including the central peak and doughnut shapes. The radiation force on a Rayleigh dielectric particle produced by the highly focused fields is investigated. Numerical results demonstrate that the new fields can be used to trap high-index particles at the focus for the specular case and nearby the focus for the anti-specular case. It is further revealed that the position, the range of particle sizes and the low limit of correlation length for stable trapping could be modulated by adjusting the phase difference.


Graphene based plasmonic tweezers

Jung-Dae Kim, Yong-Gu Lee

Conventional plasmonic tweezers with the ability to attract and immobilize nearby sub-diffraction limit sized particles can only enhance the trapping efficiency by changing the shape of the metal nanostructures. There are several problems with conventional plasmonic tweezers. First, trapped particles can easily escape from the trap by disturbances coming from the heat absorption of the metallic surfaces. These disturbances prevent prolonged observation of the trapped particles. Second, observation of the particles becomes a challenge because the opaqueness of the metal blocks the illumination pathways. These problems can be solved by using graphene, which has high transmittance and thermal conductivity. The carrier density of the graphene is tuned by externally controlling the Fermi level through the gate voltage. Tuning the carrier density alters the local field enhancement factor far beyond the capabilities provided by other metal-based plasmonic structures. In this paper, we have shown that particles can be trapped by graphene nanoholes with larger forces than gold nanoholes. The trapping forces on gold and graphene nanoholes were compared to illustrate the benefit of graphene nanoholes. Furthermore, various trapping modes of a particle under various geometries and configurations of graphene nanoholes is discussed.


Self-alignment of glass fiber nanospike by optomechanical back-action in hollow-core photonic crystal fiber

S. Xie, R. Pennetta, and P. St. J. Russell

A topic of great current interest is the harnessing and enhancement of optical tweezer forces for trapping small objects of different sizes and shapes at relatively small powers. Here we demonstrate the stable trapping, inside the core of a hollow-core photonic crystal fiber (HC-PCF), of a mechanically compliant fused silica nanospike, formed by tapering a single-mode fiber (SMF). The nanospike is subwavelength in diameter over its ∼50  μm insertion length in the HC-PCF. Laser light, launched into the SMF core, adiabatically evolves into a mode that extends strongly into the space surrounding the nanospike. It then senses the presence of the hollow core, and the resulting optomechanical action and back-action results in a strong trapping force at the core center. The system permits lens-less, reflection-free, self-stabilized, and self-aligned coupling from SMF to HC-PCF with a demonstrated efficiency of 87.8%. The unique configuration also provides an elegant means of investigating optomechanical effects in optical tweezers, especially at very low pressures.


Friday, March 11, 2016

Comparison of plasmonic structures in terms of temperature increase under equivalent maximal trapping forces

Yong-Jun Yang and Yong-Gu Lee

Plasmonic optical trapping is a new approach that can potentially overcome some of the limitations associated with conventional optical trapping. Plasmonic tweezers generate heat because of the absorption of light at the surface of metals, and this is one of the contributions to the failure of stable trapping. Heating problems and the trapping forces tend to differ with the geometry of the plasmonicstructures. Nanodisk structures can generally deliver stronger trapping forces than nanohole structures. However, the nanodisk structures also lead to greater heat generation, which can cause the medium to boil and eventually produce bubbles that can potentially push trapped particles away from the trap. Concentrated local heat can also melt the plasmonic features or instantaneously vaporize the medium. In this paper, we have closely examined this heat generation problem for two typical plasmonicstructures, nanodisks and nanoholes, and provided a detailed analysis. For identical force generations, it is shown that the nanohole structures exhibit less heat generation.


Mechanisms of backtrack recovery by RNA polymerases I and II

Ana Lisica, Christoph Engel, Marcus Jahnel, Édgar Roldán, Eric A. Galburt, Patrick Cramer, and Stephan W. Grill

During DNA transcription, RNA polymerases often adopt inactive backtracked states. Recovery from backtracks can occur by 1D diffusion or cleavage of backtracked RNA, but how polymerases make this choice is unknown. Here, we use single-molecule optical tweezers experiments and stochastic theory to show that the choice of a backtrack recovery mechanism is determined by a kinetic competition between 1D diffusion and RNA cleavage. Notably, RNA polymerase I (Pol I) and Pol II recover from shallow backtracks by 1D diffusion, use RNA cleavage to recover from intermediary depths, and are unable to recover from extensive backtracks. Furthermore, Pol I and Pol II use distinct mechanisms to avoid nonrecoverable backtracking. Pol I is protected by its subunit A12.2, which decreases the rate of 1D diffusion and enables transcript cleavage up to 20 nt. In contrast, Pol II is fully protected through association with the cleavage stimulatory factor TFIIS, which enables rapid recovery from any depth by RNA cleavage. Taken together, we identify distinct backtrack recovery strategies of Pol I and Pol II, shedding light on the evolution of cellular functions of these key enzymes.


Synchronization of colloidal rotors through angular optical binding

S. H. Simpson, L. Chvátal, and P. Zemánek

A mechanism for the synchronization of driven colloidal rotors via optical coupling torques is presented and analyzed. Following our recent experiments [Brzobohatý et al., Opt. Express 23, 7273 (2015)], we consider a counterpropagating optical beam trap that carries spin angular momentum, but no net linear momentum, operating in an aqueous solvent. The angular momentum carried by the beams causes the continuous low-Reynolds-number rotation of spheroidal colloids. Due to multiple scattering, the optical torques experienced by these particles depend on their relative orientations, while the effect of hydrodynamic interaction is negligible. This results in frequency pulling, which causes weakly dissimilar spheroids to synchronize their rotation rates and lock their relative phases. The effect is qualitatively captured by a coupled dipole model and quantitatively reproduced by T-matrix calculations. For pairs of rotors, the relative torque Δτ is shown to vary with relative phase Δϕ according to Δτ≈Asin(2Δϕ+δ)+B for constants A,B,δ, so the resulting motion is governed by the well-known Adler equation. We show that this behavior can be preserved for larger numbers of particles. The application of these phenomena to the inertial motion of particles in vacuum could provide a route to the sympathetic cooling of mesoscopic particles.


Picosecond optical vortex pulse illumination forms a monocrystalline silicon needle

Fuyuto Takahashi, Katsuhiko Miyamoto, Hirofumi Hidai, Keisaku Yamane, Ryuji Morita & Takashige Omatsu

The formation of a monocrystalline silicon needle by picosecond optical vortex pulse illumination was demonstrated for the first time in this study. The dynamics of this silicon needle formation was further revealed by employing an ultrahigh-speed camera. The melted silicon was collected through picosecond pulse deposition to the dark core of the optical vortex, forming the silicon needle on a submicrosecond time scale. The needle was composed of monocrystalline silicon with the same lattice index (100) as that of the silicon substrate, and had a height of approximately 14 μm and a thickness of approximately 3 μm. Overlaid vortex pulses allowed the needle to be shaped with a height of approximately 40 μm without any changes to the crystalline properties. Such a monocrystalline silicon needle can be applied to devices in many fields, such as core–shell structures for silicon photonics and photovoltaic devices as well as nano- or microelectromechanical systems.


Plasmonic trapping and tuning of a gold nanoparticle dimer

Zhe Shen and Lei Su

We demonstrate theoretically the trapping and manipulating of a gold nanoparticle dimer, using surface plasmon excited by a focused linearly-polarized laser beam on a silver film. We use both finite-difference time-domain force analysis and Maxwell stress tensor to show that the gold nanoparticle dimer can be trapped by a virtual probe pair. A formula is derived to represent the plasmonic field, suggesting that the gap between the two gold nanoparticles in the dimer can be controlled, for example, by tuning the excitation-laser wavelength. We further test our theory by successfully trapping nanoparticle dimers formed by nanospheres and nanorods. The controllable gap in between the nanoparticles can lead to tunable localized surface plasmon resonances, and this may find new exciting applications in plasmonic sensing or in lab-on-a-chip devices.


Wednesday, March 9, 2016

Optical Nanoparticle Sorting Elucidates Synthesis of Plasmonic Nanotriangles

María Ana Huergo, Christoph Matthias Maier, Marcos Federico Castez, Carolina Vericat, Spas Nedev, Roberto C. Salvarezza, Alexander S. Urban, and Jochen Feldmann

We investigate the optical and morphological properties of gold nanoparticles grown by reducing a gold salt with Na2S. Lasers are tuned to the observed plasmon resonances, and the optical forces exerted on the nanoparticles are used to selectively print individual nanoparticles onto a substrate. This enables us to combine dark-field spectroscopy and scanning electron microscopy to compare the optical properties of single nanoparticles with their morphology. By arresting the synthesis at different times, we are able to investigate which type of nanoparticle is responsible for the respective resonances. We find that thin Au nanotriangles are the source of the observed near infrared (NIR) resonance. The initial lateral growth of these triangles causes the plasmon resonance to redshift into the NIR, whereas a subsequent thickening of the triangles and a concomitant truncation lead to a blueshift of the resonance. Furthermore, we find that the nanotriangles produced have extremely narrow line widths (187 ± 23 meV), show nearly isotropic scattering, and are stable for long periods of time. This shows their vast potential for applications such as in vivo imaging and bio(chemical) sensing. The method used here is generally applicable to other syntheses, and shows how complex nanostructures can be built up on substrates by selectively printing NPs of varying plasmonic resonances.


Synthetic Capillaries to Control Microscopic Blood Flow

K. Sarveswaran, V. Kurz, Z. Dong, T. Tanaka, S. Penny & G. Timp

Capillaries pervade human physiology. The mean intercapillary distance is only about 100 μm in human tissue, which indicates the extent of nutrient diffusion. In engineered tissue the lack of capillaries, along with the associated perfusion, is problematic because it leads to hypoxic stress and necrosis. However, a capillary is not easy to engineer due to its complex cytoarchitecture. Here, it is shown that it is possible to create in vitro, in about 30 min, a tubular microenvironment with an elastic modulus and porosity consistent with human tissue that functionally mimicks a bona fide capillary using “live cell lithography”(LCL) to control the type and position of cells on a composite hydrogel scaffold. Furthermore, it is established that these constructs support the forces associated with blood flow, and produce nutrient gradients similar to those measured in vivo. With LCL, capillaries can be constructed with single cell precision—no other method for tissue engineering offers such precision. Since the time required for assembly scales with the number of cells, this method is likely to be adapted first to create minimal functional units of human tissue that constitute organs, consisting of a heterogeneous population of 100–1000 cells, organized hierarchically to express a predictable function.


Motion of optically heated spheres at the water-air interface

Antoine Girot, Noémie Danne, Alois Würger, Thomas Bickel, Kuan Fang Ren, Jean-Christophe Loudet, and Bernard Pouligny

A micrometer-sized spherical particle classically equilibrates at the water-air interface in partial wetting configuration, causing about no deformation to the interface. In condition of thermal equilibrium, the particle just undergoes faint Brownian motion, well visible under a microscope. We report experimental observations when the particle is made of a light-absorbing material and is heated up by a vertical laser beam. We show that, at small laser power, the particle is trapped in on-axis configuration, similarly to 2-dimensional trapping of a transparent sphere by optical forces. Conversely, on-axis trapping becomes unstable at higher power. The particle escapes off the laser axis and starts orbiting around the axis. We show that the laser-heated particle behaves as a micro-swimmer with velocities on the order of several 100 µm/s with just a few milliWatts of laser power.


Effects of cytosine modifications on DNA flexibility and nucleosome mechanical stability

Thuy T. M. Ngo, Jejoong Yoo, Qing Dai, Qiucen Zhang, Chuan He, Aleksei Aksimentiev & Taekjip Ha

Cytosine can undergo modifications, forming 5-methylcytosine (5-mC) and its oxidized products 5-hydroxymethylcytosine (5-hmC), 5-formylcytosine (5-fC) and 5-carboxylcytosine (5-caC). Despite their importance as epigenetic markers and as central players in cellular processes, it is not well understood how these modifications influence physical properties of DNA and chromatin. Here we report a comprehensive survey of the effect of cytosine modifications on DNA flexibility. We find that even a single copy of 5-fC increases DNA flexibility markedly. 5-mC reduces and 5-hmC enhances flexibility, and 5-caC does not have a measurable effect. Molecular dynamics simulations show that these modifications promote or dampen structural fluctuations, likely through competing effects of base polarity and steric hindrance, without changing the average structure. The increase in DNA flexibility increases the mechanical stability of the nucleosome and vice versa, suggesting a gene regulation mechanism where cytosine modifications change the accessibility of nucleosomal DNA through their effects on DNA flexibility.


Tuesday, March 8, 2016

Subnanometre enzyme mechanics probed by single-molecule force spectroscopy

Benjamin Pelz, Gabriel Žoldák, Fabian Zeller, Martin Zacharias & Matthias Rief

Enzymes are molecular machines that bind substrates specifically, provide an adequate chemical environment for catalysis and exchange products rapidly, to ensure fast turnover rates. Direct information about the energetics that drive conformational changes is difficult to obtain. We used subnanometre single-molecule force spectroscopy to study the energetic drive of substrate-dependent lid closing in the enzyme adenylate kinase. Here we show that in the presence of the bisubstrate inhibitor diadenosine pentaphosphate (AP5A), closing and opening of both lids is cooperative and tightly coupled to inhibitor binding. Surprisingly, binding of the substrates ADP and ATP exhibits a much smaller energetic drive towards the fully closed state. Instead, we observe a new dominant energetic minimum with both lids half closed. Our results, combining experiment and molecular dynamics simulations, give detailed mechanical insights into how an enzyme can cope with the seemingly contradictory requirements of rapid substrate exchange and tight closing, to ensure efficient catalysis.


Characterization of periodic cavitation in optical tweezers

Viridiana Carmona-Sosa, José Ernesto Alba-Arroyo, and Pedro A. Quinto-Su
Microscopic vapor explosions or cavitation bubbles can be generated repeatedly in optical tweezers with a microparticle that partially absorbs at the trapping laser wavelength. In this work we measure the size distribution and the production rate of cavitation bubbles for microparticles with a diameter of 3 μm using high-speed video recording and a fast photodiode. We find that there is a lower bound for the maximum bubble radius 𝑅max∼2  μm which can be explained in terms of the microparticle size. More than 94% of the measured 𝑅max are in the range between 2 and 6 μm, while the same percentage of the measured individual frequencies 𝑓𝑖 or production rates are between 10 and 200 Hz. The photodiode signal yields an upper bound for the lifetime of the bubbles, which is at most twice the value predicted by the Rayleigh equation. We also report empirical relations between 𝑅max, 𝑓𝑖, and the bubble lifetimes.


Extremely strong bipolar optical interactions in paired graphene nanoribbons

Wanli Lu, Huajin Chen, Shiyang Liu, Jian Zi and Zhifang Lin

Graphene is an excellent multi-functional platform for electrons, photons, and phonons due to exceptional electronic, photonic, and thermal properties. When combining its extraordinary mechanical characteristics with optical properties, graphene-based nanostructures can serve as an appealing platform for optomechanical applications at the nanoscale. Here, we demonstrate, using full-wave simulations, the emergence of extremely strong bipolar optical forces, or, optical binding and anti-binding, between a pair of coupled graphene nanoribbons, due to the remarkable confinement and enhancement of optical fields arising from the large effective mode indices. In particular, the binding and anti-binding forces, which are about two orders of magnitude stronger than that in metamaterials and high-Q resonators, can be tailored by selective excitation of either the even or the odd optical modes, achievable by tuning the relative phase of the lightwaves propagating along the two ribbons. Based on the coupled mode theory, we derive analytical formulae for the bipolar optical forces, which agree well with the numerical results. The attractive optical binding force Fby and the repulsive anti-binding force Fay exhibit a remarkably different dependence on the gap distance g between the nanoribbons and the Fermi energy EF, in the forms of Image ID:c5cp06581j-t1.gif and Fay ∝ 1/E2F. With EF dynamically tunable by bias voltage, the bipolar forces may provide a flexible handle for active control of the nanoscale optomechanical effects, and also, might be significant for optoelectronic and optothermal applications as well.


Surface forces between colloidal particles at high hydrostatic pressure

D. W. Pilat, B. Pouligny, A. Best, T. A. Nick, R. Berger, and H.-J. Butt

It was recently suggested that the electrostatic double-layer force between colloidal particles might weaken at high hydrostatic pressure encountered, for example, in deep seas or during oil recovery. We have addressed this issue by means of a specially designed optical trapping setup that allowed us to explore the interaction of a micrometer-sized glass bead and a solid glass wall in water at hydrostatic pressures of up to 1 kbar. The setup allowed us to measure the distance between bead and wall with a subnanometer resolution. We have determined the Debye lengths in water for salt concentrations of 0.1 and 1 mM. We found that in the pressure range from 1 bar to 1 kbar the maximum variation of the Debye lengths was <1 nm for both salt concentrations. Furthermore, the magnitude of the zeta potentials of the glass surfaces in water showed no dependency on pressure.


Maxwell stress induced optical torque upon gold prolate nanospheroid

Jiunn-Woei Liaw, Ying-Syuan Chen, Mao-Kuen Kuo

This study theoretically analyzes the surface traction on an elongated Au prolate nanospheroid to examine the resultant optical torque exerted by an optical tweezers. The multiple multipole method is applied to evaluate quantitatively the electromagnetic field induced by a linearly polarized plane wave illuminating a nanospheroid, then obtaining the surface traction in terms of Maxwell stress tensor. The optical torque is calculated by the surface integral of the cross product of position vector and traction over the nanospheroid’s surface. Our results show that two pairs of positive and negative traction zones at the two apexes of the nanospheroid play a critical role. Furthermore, the resultant optical torque is wavelength-dependent. If the wavelength is shorter than the longitudinal surface plasmon resonance (LSPR) of the nanospheroid, the optical torque rotates the long axis of nanospheroid perpendicular to the polarization direction of the incident wave. In contrast, if the wavelength is longer than the LSPR the long axis is pushed parallel to the polarization direction. The turning point with a null torque, between the perpendicular and parallel modes, is at the LSPR. The optical performance of Au nanospheroid is equivalent to that of Au NR with the same volume and aspect ratio, but the LSPR of Au NR is little red-shifted from that of an equivalent prolate spheroid.


Monday, March 7, 2016

Nonconservative dynamics of optically trapped high-aspect-ratio nanowires

Wen Jun Toe, Ignacio Ortega-Piwonka, Christopher N. Angstmann, Qiang Gao, Hark Hoe Tan, Chennupati Jagadish, Bruce I. Henry, and Peter J. Reece

We investigate the dynamics of high-aspect-ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the dynamics reveals a broad spectral resonance of the order of kHz with peak properties that are strongly dependent on the input trapping power. A dynamical model incorporating linear restoring optical forces, a nonconservative asymmetric coupling between translational and rotational degrees of freedom, viscous drag, and white noise provides an excellent fit to experimental observations. A persistent low-frequency cyclical motion around the equilibrium trapping position, with a frequency distinct from the spectral resonance, is observed from the time series data.


Nonlinear properties of gaseous optical mediums in a context of ball lightning explanation

V.P. Torchigin, , A.V. Torchigin
We consider a dependence of the refractive index of gases on the intensity of the conventional white light in a context of an analysis of properties of nonlinear optical mediums where a self-confined light radiation is possible. This is connected with the fact that a behavior of the self-confined light in a form of thin spherical layer of strongly compressed air where the intensive white light is circulating in all possible directions and a behavior of Ball Lightning in the terrestrial atmosphere are identical.


Influence of optical forces on nonlinear optical frequency conversion in nanoscale waveguide devices

Zhen-xing Wu, Wei Luo, Shi-han Tang, Fei Xu, and Yan-qing Lu

We theoretically investigate the influence of optical gradient forces on nonlinear frequency conversion in a typical nanoscale optomechanical system, which consists of two parallel, suspended waveguides. The waveguides deform with the input power and the phase-matching wavelength changes along the waveguides. Utilizing the spread of deformations collectively allows phase matching over a wider range of pump wavelengths. The third harmonic phase-matching wavelength shift can be as large as 3.6 nm/mW when the waveguide length is 100 μm and the initial gap is 150 nm. It is analogous to chirping the poling period of quasi-phase-matched devices to extend their bandwidths, and allows broad third harmonic to be generated for uses such as biological spectroscopy. Finally, we discuss the conversion efficiency and the optimal phase-matching wavelength with a single-frequency pump.


Measuring gene expression in single bacterial cells: recent advances in methods and micro-devices

Xu Shi, Weimin Gao, Jiangxin Wang, Shih-Hui Chao, Weiwen Zhang & Deirdre R. Meldrum

Populations of bacterial cells that grow under the same conditions and/or environments are often considered to be uniform and thus can be described by ensemble average values of their physiologic, phenotypic, genotypic or other parameters. However, recent evidence suggests that cell-to-cell differences at the gene expression level could be an order of magnitude greater than previously thought even for isogenic bacterial populations. Such gene expression or transcriptional-level heterogeneity determines not only the fate of individual bacterial cells in a population but could also affect the ultimate fate of the population itself. Although techniques for single-cell gene expression measurement in eukaryotic cells have been successfully implemented for a decade or so, they have only recently become available for single bacterial cells. This is due to the difficulty of efficient lysis of most bacterial cells, as well as short half-life time (low stability) of bacterial mRNA. In this article, we review the recent progress and challenges associated with analyzing gene expression levels in single bacterial cells using various semi-quantitative and quantitative methods. In addition, a review of the recent progress in applying microfluidic devices to isolate single bacterial cells for gene expression analysis is also included.


Friday, March 4, 2016

An opto-mechanical coupled-ring reflector driven by optical force for lasing wavelength control

M. Ren, H. Cai, L. K. Chin, J. G. Huang, Y. D. Gu, K. Radhakrishnan, W. Ser and A. Q. Liu

In this paper, an opto-mechanical coupled-ring reflector driven by optical gradient force is applied in an external-cavity tunable laser. A pair of mutually coupled ring resonators with a free-standing arc serves as a movable reflector. It obtains a 13.3-nm wavelength tuning range based on an opto-mechanical lasing-wavelength tuning coefficient of 127 GHz/nm. The potential applications include optical network, on-chip optical trapping, sensing, and biology detection.


Measuring the charge density of a tapered optical fiber using trapped microparticles

Kazuhiko Kamitani, Takuya Muranaka, Hideaki Takashima, Masazumi Fujiwara, Utako Tanaka, Shigeki Takeuchi, and Shinji Urabe

We report the measurements of charge density of tapered optical fibers using charged particles confined in a linear Paul trap at ambient pressure. A tapered optical fiber is placed across the trap axis at a right angle, and polystyrene microparticles are trapped along the trap axis. The distance between the equilibrium position of a positively charged particle and the tapered fiber is used to estimate the amount of charge per unit length of the fiber without knowing the amount of charge of the trapped particle. The charge per unit length of a tapered fiber with a diameter of 1.6 μm was measured to be 2+3−1×10−11 C/m.


Rib waveguides for trapping and transport of particles

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

Rib waveguides are investigated as an alternative to strip waveguides for planar trapping and transport of microparticles. Microparticles are successfully propelled along the surface of rib waveguides and trapped in the gap between opposing rib waveguides. The trapping capabilities of waveguide end facets formed by a single and opposing waveguide geometries are investigated. The slab beneath a rib waveguide continues to guide light after the end facet of a rib waveguide. Thus particles can be trapped in wider gaps formed by opposing rib waveguides than with strip waveguides. Rib waveguides were found more efficient in trapping a collection of particles in the gap and particles could be moved to different locations in the gap by changing the relative power in the two opposing rib waveguides. Numerical simulations are used to show that the trapping efficiency on the surface of rib and strip waveguides is comparable. The simulations also confirm the advantage of opposing rib waveguides for trapping particles in wide gaps. The low sidewalls of rib waveguides give low propagation losses and make it easy to integrate rib waveguides with other functions in a lab-on-a-chip where particle trapping and transport is required.


Integration of conductive reduced graphene oxide into microstructured optical fibres for optoelectronics applications

Yinlan Ruan, Liyun Ding, Jingjing Duan, Heike Ebendorff-Heidepriem & Tanya M. Monro

Integration of conductive materials into optical fibres can largely expand functions of fibre devices including surface plasmon resonator/metamaterial, modulators/detectors, or biosensors. Some early attempts have been made to incorporate metals such as tin into fibres during the fibre drawing process. Due to the restricted range of materials that have compatible melting temperatures with that of silica glass, the methods to incorporate metals along the length of the fibres are very challenging. Moreover, metals are nontransparent with strong light absorption, which causes high fibre loss. This article demonstrates a novel but simple method for creating transparent conductive reduced graphene oxide film onto microstructured silica fibres for potential optoelectronic applications. The strongly confined evanescent field of the suspended core fibres with only 2 μW average power was creatively used to transform graphene oxide into reduced graphene oxide with negligible additional loss. Existence of reduced graphene oxide was confirmed by their characteristic Raman signals, shifting of their fluorescence peaks as well as largely decreased resistance of the bulk GO film after laser beam exposure.


Grasping and manipulation of a micro-particle using multiple optical traps

Chien Chern Cheah, Quang Minh Ta, Reza Haghighi
In existing control techniques for optical tweezers, a target particle is directly trapped and manipulated by a single laser beam. However, a typical force generated by an optical trap is extremely small (on the order of piconewtons) and thus it is not sufficient to manipulate a large cell or object. Besides, the feasibility of optical manipulation also depends on the physical properties of the specimen. An opaque object or object with the same refractive index as the fluid media may not be trapped directly by the laser beam. Therefore, current control techniques for optical tweezers cannot be utilized to manipulate various types of cells or objects, including untrappable or large ones. In this paper, robotic control techniques are developed for optical tweezers to achieve grasping and manipulation of a microscopic particle, which is beyond the capability of a single optical trap. First, multiple laser beams are generated, and each laser beam is utilized to trap and drive one grasping particle to form a desired shape around the target particle. A grasping formation of trapped particles is thus generated to hold the target particle. Then the target particle is manipulated to a desired position by controlling the motorized stage. The proposed control strategy is particularly suitable for manipulation of large particles, or even untrappable cells or objects. Rigorous mathematical formulations have been developed to analyze the control system for grasping and manipulation of the microscopic particle. Experimental results are presented to illustrate the performance of the proposed grasping and manipulation techniques.


Tuesday, March 1, 2016

Nanomechanics of Suspended Fibroblast by Point-like Anchors Reveals Cytoskeleton Formation

Sabato Fusco, Pasquale Memmolo, Lisa Miccio, Francesco Merola, Martina Mugnano, Antonio Paciello, Pietro Ferraro and Paolo Antonio Netti

In an attempt to better elucidate the material-cytoskeleton crosstalk during the initial stage of cell adhesion, here we report how suspended cells anchored to point-like bonds are able to assemble their cytoskeleton when subjected to mechanical stress. The combination of holographic optical tweezers and digital holography gives cells footholds for the adhesion and mechanical stimulation and, at the same time, acts as a label-free, force-revealing system over time, detecting the cell nanomechanical response in the pN range. To confirm the formation of cytoskeleton structures after the stimulation, a fluorescence image system was added as a control. The strategy here proposed portends broad applicability to investigate the correlation between the forces applied to the cells and their cytoskeleton assembly process in this or other complex configurations with multiple anchor points.


Single Potassium Niobate Nano/Microsized Particles as Local Mechano-Optical Brownian Probes

Flavio Mor, Andrzej Sienkiewicz, Arnaud Magrez, Laszlo Forro and Sylvia Jeney

Perovskite alkaline niobates, due to their strong nonlinear optical properties, including birefringence and capability to produce second-harmonic generation (SHG) signals, attract a lot of attention as potential candidates for applications as local nano/microsized mechano-optical probes. Here, we report on an implementation of photonic force microscopy (PFM) to explore the Brownian motion and optical trappability of monocrystalline potassium niobate (KNbO3) nano/microsized particles having sizes within the range of 50 to 750 nm. In particular, we exploit the anisotropic translational diffusive regime of the Brownian motion to quantify thermal fluctuations and optical forces of singly-trapped KNbO3 particles within the optical trapping volume of a PFM microscope. We also show that, under near-infrared (NIR) excitation of the highly focused laser beam of the PFM microscope, a single optically-trapped KNbO3 particle reveals a strong SHG signal mani- fested by a narrow peak (λem = 532 nm) at half the excitation wavelength (λex = 1064 nm). Moreover, we demonstrate that the thus induced SHG emission can be used as a local light source that is capable to optically excite molecules of an organic dye, Rose Bengal (RB), which adhere to the particle surface, through the mechanism of luminescence energy transfer (LET).


Single-Molecule Biophysics: The light side of the force

Aakash Basu, Taekjip Ha

A combination of two single-molecule techniques has revealed new tertiary interactions in the TPP riboswitch.


Spider Silk Peptide Is a Compact, Linear Nanospring Ideal for Intracellular Tension Sensing

Michael D. Brenner, Ruobo Zhou, Daniel E. Conway, Luca Lanzano, Enrico Gratton, Martin A. Schwartz, and Taekjip Ha

Recent development and applications of calibrated, fluorescence resonance energy transfer (FRET)-based tension sensors have led to a new understanding of single molecule mechanotransduction in a number of biological systems. To expand the range of accessible forces, we systematically measured FRET versus force trajectories for 25, 40, and 50 amino acid peptide repeats derived from spider silk. Single molecule fluorescence-force spectroscopy showed that the peptides behaved as linear springs instead of the nonlinear behavior expected for a disordered polymer. Our data are consistent with a compact, rodlike structure that measures 0.26 nm per 5 amino acid repeat that can stretch by 500% while maintaining linearity, suggesting that the remarkable elasticity of spider silk proteins may in part derive from the properties of individual chains. We found the shortest peptide to have the widest range of force sensitivity: between 2 pN and 11 pN. Live cell imaging of the three tension sensor constructs inserted into vinculin showed similar force values around 2.4 pN. We also provide a lookup table for force versus intracellular FRET for all three constructs.